Handbook Properties and Selection Nonferrous Alloys and Spl Purpose Mtls (1992) WW Part 12 pptx

Even finer diamond powder is used in combination with tungsten carbide for specialized applications such as polishing plane surfaces of hard metal tools or finishing the rolls for Sendzi

Molybdenum Boride Cermets. The molybdenum borides MoB and Mo2B have less thermal stability than the previously discussed metal borides, but their electrical properties, hardness, and wear resistance are very good When cemented with nickel, these cermets have excellent corrosion resistance, for example, to dilute sulfuric acid (Ref 79) Nickel-bonded molybdenum boride exhibits interesting behavior in two areas: First, if the composition corresponds to the compound molybdenum-nickel boride (Mo2NiB2), if the cermet contains Mo2B in addition to Mo2NiB2, or if a low-melting, intermetallic binder containing chromium boride and nickel is used, cutting tool materials can be produced from the composition that are comparable to commercial WC tool tips for machining brass, aluminum, and cast iron (Ref 90, 91) Second, the Mo2NiB2-type composition has thermal expansion characteristics that closely match those of the refractory metals and a favorable melting temperature; these properties make it ideal for use as a high-temperature braze for molybdenum and tungsten, without risk of excessive grain growth or embrittlement of the primary metal structure (Ref

79, 92) When used in rod form with shielded arc welding equipment, this cermet is suitable for brazing electronic components in applications such as vacuum tubes and magnetrons

Recently, a molybdenum boride cemented with an iron-base binder phase alloyed with nickel and chromium has shown promise as a cutting tool material (Ref 93) This cermet exhibits good mechanical properties coupled with excellent wear and corrosion resistance In specific tool applications, such as extrusion dies for hot copper and tools for can making, this boride cermet has performed better than cemented carbides The role of nickel in the Mo2FeB2 cermet and the effect of varying its content up to 10 wt% in the Fe-5B-44.4Mo composition have also been investigated, mainly as part of a study

of the corrosion resistance potential of the material (Ref 94) The nickel enters only into the iron-base binder phase, which changes with increasing nickel content from ferritic to martensitic to austenitic The martensitic binder phase at 2.5% Ni gives the cermet a transverse rupture strength of 2.24 GPa (325 ksi) and a hardness of 86.9 HRA

References cited in this section

75 L Kaufman, E.C Clougherty, and J.B Berkowitz-Mattuck, Oxidation Characteristics of Hafnium and

Zirconium Diboride, Trans AIME, Vol 239 (No 4), 1967, p 458-466

76 R Kieffer and F Benesovsky, Hartmetalle, Springer-Verlag, 1965, p 475-479

77 R Steinitz, Borides Part B: Fabrication, Properties and Applications, in Modern Materials, Vol 2,

Academic Press, 1960, p 191-224

78 C.E Halcombe, Jr., "Slip Casting of Zirconium Diboride," Report Y-1819, U.S Atomic Energy Commission, 28 Feb 1972

79 J.L Everhart, New Refractory Hard Metals, Mater Methods, Vol 40 (No 2), Aug 1954, p 90-92

80 J.D Latva, Selection and Fabrication of Ceramics and Intermetallics, Met Prog., Vol 82 (No 4), Oct 1962,

p 139-144, 180, 186

81 L Kaufman and E.V Clougherty, Investigation of Boride Compounds for High Temperature Applications,

Metals for the Space Age, Springer-Verlag, 1965, p 722-758

82 L Kaufman and E.V Clougherty, "Investigation of Boride Compounds for Very High Temperature Applications," Report RTD-TDR-63-4096, Part 1, U.S Air Force Materials Laboratory, Dec 1963

83 E.V Clougherty, R.L Pober, and L Kaufman, Synthesis of Oxidation Resistant Metal Diboride

Composites, Trans AIME, Vol 242 (No 6), 1968, p 1077-1082

84 E.V Clougherty et al., "Research and Development of Refractory Oxidation Resistant Diborides," Report

AFSC-ML-TR-68-190, U.S Air Force Materials Laboratory, Part 1, Oct 1968; Part 2, Vol 1-7, Nov June 1970; Part 3, May 1970

1969-85 H.M Greenhouse, R.F Stoops, and T.S Shevlin, A New Carbide-Base Cermet Containing TiC, TiB2 and

CoSi, J Am Ceram Soc., Vol 37 (No 5), 1954, p 203-206

86 E.T Montgomery et al., "Preliminary Microscopic Studies of Cermets at High Temperatures," U.S Air

Force Report WADC-TR-54-33, Part 1, April 1955, Part 2, Feb 1956

87 Gradient Ceramic/Metals Made by Advanced Methods, Adv Mater Proc., Vol 132 (No 4), Oct 1987, p 20

88 S.J Sindeband, Properties of Chromium Boride and Sintered Chromium Boride, Trans AIME, Vol 185,

Feb 1949, p 198-202

89 I Binder and D Moskowitz, "Cemented Borides," PB 121346, Office of Technical Services, U.S Department of Commerce, 1954-1955

90 R Steinitz and I Binder, New Ternary Boride Compounds, Powder Metall Bull., Vol 6 (No 4), Feb 1953,

93 K Takagi, S Ohira, T Ide, T Watanabe, and Y Kondo, New P/M Iron-Containing Multiple Boride Base

Hard Alloy, in Modern Developments in Powder Metallurgy, Vol 16, Metal Powder Industries Federation,

1985, p 153-166

94 K Takagi, M Komai, T Ide, T Watanabe, and Y Kondo, Effect of Ni on the Mechanical Properties of Fe,

Mo Boride Hard Alloys, Int J Powder Metall., Vol 23 (No 3), 1987, p 157-161

Other Refractory Cermets

The nitrides, carbonitrides, and silicides of certain transition metals have gained importance for specific uses in operations involving high temperatures The main mode of application for these refractory cermets, however, is in the form of coatings, such as TiN and TiC-TiN in various ratios for high-speed cutting tools or MoSi2 for surface protection of molybdenum against high-temperature oxidation In a very few cases, these compounds are used as solids, either in the pure state or cemented with a lower-melting metallic phase

Carbonitride- and Nitride-Based Cermets. Titanium nitrides and titanium carbonitrides have been found suitable for use as the hard phase for tool materials (Ref 95) The best binder is an alloy of 70Ni-30Mo, and optimum hardness, in the 1000

to 2000 HV range, is obtained with 10 wt% binder The hardness increases progressively with the TiC component of the solid solution The same trend prevails for the hardness of a cermet containing 14 wt% binder: The values increase from about 1400 to 1900 HV for the straight cemented TiC composition Transverse rupture strength does not follow any trend; the best values reach about 1300 MPa (188 ksi) for a 10 wt% binder composition with a 72-to-18 TiN-TiC ratio and a 14 wt% binder material with a 69-to-17 TiN-TiC ratio This compares with 1070 and 1275 MPa (155 ksi and 185 ksi), respectively, for the straight TiC cermets with 10 and 14 wt% binder The hardness of titanium nitride alone cemented with 10% of the 70Ni-30Mo alloy has a hardness level of about 1050 HV and a transverse rupture strength of about 785 MPa (115 ksi) Titanium carbonitride cermets for tool applications are discussed in greater detail in the section "Titanium Carbonitride Cermets" in this article

Combinations of nitrides and borides, with or without metallic binder, can also be fabricated into tools A mixture of 60 wt% tantalum nitride (TaN) and 40 wt% ZrB2 has been hot pressed into tool bits that have performed very well at very high cutting speeds (Ref 96)

Nitride products based on the metalloids boron and silicon, like their carbide counterparts, have gained some significant commercial uses since their early development in the 1950s and 1970s, respectively The normal hexagonal crystal lattice

of boron nitride (BN) can be converted to a cubic crystal form by reacting boron powder with nitrogen at a minimum temperature of 1650 °C (3000 °F) while simultaneously applying pressure in excess of 7000 MPa (1000 ksi) with the aid

of special press tools adopted from the manufacture of synthetic diamond The product is extremely hard and is considered to be one of the best electrical insulators known, especially at high temperatures up to about two-thirds of its melting point, that is, in the vicinity of 2730 °C (4950 °F) (Ref 45, 97)

Cermets exhibiting excellent cutting performance have been achieved by bonding carefully graded particles of the superhard cubic boron nitride with cobalt or similar hard metal binders Hot pressing is the preferred method of powder consolidation, and tool bits made in this manner outperform tungsten carbide tips by a factor of two-to-one and better (Ref 98)

The nitride of silicon and its combination with different oxides, notably Al2O3 (known as the SiAlONs), as well as the different silicon ceramics based on silicon carbide, belong to the increasingly important new class of refractory materials known as structural ceramics Additives of these cermets are nonmetallic and serve mainly to control the sintering mechanism They do not contribute to a strengthening of the hard particle structure in the sense of a metallic binder In fact, they cause a weakening of the grain-boundary network at high temperature in many systems Therefore, these silicon ceramics are considered to lie outside the material classification for cermets

Silicide Cermets. The metallic silicides have found commercial use only in isolated instances This is due chiefly to the extreme brittleness of these compounds and to the concomitant problems encountered when they are fabricated into solid objects Because of its outstanding high-temperature oxidation resistance, and its favorable coefficients of thermal expansion and electrical resistance, molybdenum disilicide (MoSi2) is an important material for heating elements Poor resistance to mechanical and thermal shock is the major deficiency of molybdenum disilicide and limits the applications

of this material to simple cylindrical or rectangular shapes Additions of metallic elements to remedy this handicap have been only partially successful, and MoSi2 cermets with nickel, cobalt, and platinum binder metals are still too brittle for fabrication into complex shapes (Ref 99) High-temperature bearings have been made experimentally by infiltrating molten silver into hard matrices containing MoSi2, tungsten disilicide (WSi2), or vanadium disilicide (VSi2); these bearings have shown good antifriction behavior against steels at elevated temperatures (Ref 100)

Graphite- and Diamond-Containing Cermets. Materials that contain a combination of carbon in the form of graphite or diamond with metals constitute a border region for cermets and are usually not designated as such However, because the carbon and metallic components are most often intimately mixed and uniformly distributed in the microstructure, they are pertinent to this discussion

Graphite-metal combinations for electrical contact applications basically fall into two types of materials For metallic brushes used in motors and generators, the metallic phase consists of copper or bronze; in the case of sliding contacts involving relatively low rubbing speeds and light contact pressure, the metallic phase is silver In brushes, the graphite particle content may spread over a wider range, from 5 to 70 wt% A typical binary composition contains 70% Cu and 30% graphite To improve wear and bearing properties, many brushes also contain up to 10% Sn and/or Pb and up to 12%

Zn (Ref 101) The graphite content in the silver contact composition generally ranges between 2 and 50 wt%

Graphite-containing metallic friction materials for brake linings and clutch facings have a predominantly metallic matrix

to utilize a high thermal conductivity This property permits rapid energy absorption, making this type of material suitable for service under a more severe wear and temperature environment than that which is possible for organic, resin-bonded asbestos friction elements The most important contribution of a cermet-type lining material in aircraft brakes probably has been an increased energy capacity without additional weight or the use of a larger unit (Ref 102) The friction coefficient of these cermets is tailored to the requirements of the particular application, principally by varying the ratio of

a friction-producing ceramic to the graphite, which acts as a solid lubricant The metallic matrix phase is essentially a bearing alloy containing 60 to 75 wt% Cu and 5 to 10% each of tin, lead, zinc, and/or iron Graphite content falls within the 5 to 10% range, and the ceramic, mainly SiO2 with the possibility of some Al2O3 additions, amounts to 2 to 7% (Ref 103)

Cermets composed of diamond, varying in size from coarse splinters to fine dust inside a metal matrix, are used for grinding, lapping, sawing, cutting, dressing, and trueing tools The size of the diamond is important for the efficiency of the tool; although finish improves as the grain or grit size becomes finer, the cutting speed is slower For dressing tools, 5

to 35 diamond splinters are embedded per carat with a size of approximately 1 to 2.5 mm (0.04 to 0.1 in.) For rough grinding, the grit size is in the range of 0.15 to 0.5 mm (0.006 to 0.02 in,); for fine polishing, it falls between 0.05 and 0.15 mm (0.002 and 0.006 in.) Even finer diamond powder is used in combination with tungsten carbide for specialized applications such as polishing plane surfaces of hard metal tools or finishing the rolls for Sendzimir-type mills Typical compositions of these tools contain 12 to 16 wt% diamond dust embedded in a tungsten carbide matrix cemented with 13% Co (Ref 104)

Other metallic bonding substances are based on copper, iron, nickel, molybdenum, or tungsten Examples for copper matrices are bronzes with 10 to 20% Sn or 2 to 4% Be, which can be strengthened by precipitation hardening, and a 47Cu-47Ag-6Co alloy Bonding metals suitable for somewhat higher-temperature service include iron-nickel, iron-nickel-chromium, and iron-tin-antimony-lead alloys; Permalloy; and nickel alloys containing 2 to 8% Be Refractory metal-base matrices are alloys of the molybdenum-copper, molybdenum-cobalt, or tungsten-nickel-copper types and tungsten-nickel-iron heavy alloys (Ref 104) In general, the bond materials must be selected with consideration of lowest possible processing temperatures to avoid the possible transformation of the diamond to graphite

References cited in this section

45 B.C Weber and M.A Schwartz, Container Materials for Melting Reactive Metals, in Cermets, Reinhold,

1960, p 154-158

95 R Kieffer, P Ettmayer, and M Freudhofmeier, About Nitrides and Carbonitrides and Nitride-Based

Cemented Hard Alloys, in Modern Developments in Powder Metallurgy, Vol 5, Plenum Press, 1971, p

201-214

96 F.C Holtz and N.M Parikh, Developments in Cutting Tool Materials, Eng Dig., Vol 28 (No 1), 1967, p

73, 75, 99

97 Borazon Man Made Material Is Hard as Diamond, Mater Methods, Vol 45 (No 5), 1957, p 194, 196

98 N.J Pipkin, D.C Roberts, and W.I Wilson, Amborite A Remarkable New Cutting Material from De

Beers, Ind Diamond Rev., June 1980, p 203-206

99 R Kieffer and F Benesovsky, Hartmetalle, Springer-Verlag, 1965, p 487-489

100 R.H Baskey, An Investigation of Seal Materials for High Temperature Applications, Trans Am Soc Lub Eng., Vol 3 (No 1), 1960, p 116-123

101 F.V Lenel, Powder Metallurgy, Metal Powder Industries Federation, 1980, p 556

102 R.H Heron, Friction Materials A New Field for Ceramics and Cermets, Ceram Bull., Vol 34 (No 12),

1955, p 295-298

103 F.V Lenel, Powder Metallurgy, Metal Powder Industries Federation, 1980, p 485

104 C.G Goetzel, Treatise on Powder Metallurgy, Vol 2, Interscience, 1950, p 171-174

Superabrasives and Ultrahard Tool Materials

T.J Clark, G.E Superabrasives; and R.C DeVries, G.E Corporate Research and Development Center (Retired)

Introduction

THE PRINCIPAL superhard materials are found as phases in the boron-carbon-nitrogen-silicon family of elements (Fig 1) Of these, the superhard materials of commercial interest include silicon nitride (Si3N4), silicon carbide (SiC), boron carbide (B4C), diamond, and cubic boron nitride (CBN) Silicon nitride provides the base composition for the important category of SiAION ceramics, which are used in structural applications (see the article "Structural Ceramics" in this

Volume) and as high-speed cutting tool materials (see the article "Ceramics" in Machining, Volume 16 of ASM Handbook, formerly 9th Edition Metals Handbook)

Fig 1 The carbon-boron-nitrogen-silicon composition tetrahedron showing the principal known superhard materials: the diamond form of

carbon, cubic BN, SiC, and B4C Polycrystalline aggregates of diamond and SiC as well as Si3N4 are also commercially available

The carbides of the metalloids boron and silicon (B4C and SiC in Fig 1) are also of considerable industrial significance and enjoy such diverse applications as superhard tools and electrical resistor heating elements These compounds are processed and used both with or without metallic binder phases When these two metalloid carbides are used without a metallic binder phase, the resultant material most likely falls within the material group of ceramics If silicon carbide (SiC) and boron carbide (B4C) are used with a metallic binder phase, then the resultant material is considered a cermet (see the article "Cermets" in this Volume)

This article focuses exclusively on the superhard materials consisting of either diamond or CBN The other commercially

significant materials in Fig 1 are discussed in the above-mentioned articles of ASM Handbook Additional information on

the superhard nitrides and carbides can be found in Ref 1 and 2 Information on possible new hard materials is available

in Ref 3

The focus of this article is further restricted to synthesized diamond and CBN The latter does not occur in nature, and the former commands 90% of the industrial diamond market These materials will be treated in terms of the forms in common use: diamond or CBN grains (looser or bonded) and sintered polycrystalline diamond or CBN tools

References

1.Ceram Bull., Vol 67 (No 6), 1988

2.P Schwarzkopf and R Kieffer, Refractory Hard Materials, Macmillan, 1953

3.A.Y Liu and M.L Cohen, Prediction of New Low Compressibility Solids, Science, Vol 245, 1989, p 841-842

Synthesis of Diamond and Cubic Boron Nitride

The basic objective in the synthesis of diamond and CBN is to transform a crystal structure from a soft hexagonal form to

a hard cubic form In the case of carbon, for example, hexagonal carbon (graphite) would be transformed into cubic carbon (diamond) Synthetic CBN and diamond are produced either as crystalline grains or as sintered polycrystalline products

The synthesis of CBN or diamond grit can be achieved by static high-pressure high-temperature (HPHT) processing or by dynamic (explosive) techniques The HPHT method, despite high equipment investment costs, is the predominant technique for producing synthetic diamond and CBN In addition, diamond is also synthesized under metastable conditions (see the section "Low-Pressure Synthesis of Superhard Coatings" in this article)

High-Pressure High-Temperature Synthesis. The bulk of synthetic CBN and diamond is made by subjecting hexagonal carbon

or boron nitride to high temperatures and high pressures with large special-purpose presses or with the commonly used mechanical device known as the uniaxial belt (Ref 4) By the simultaneous application of heat and pressure, hexagonal carbon or boron nitride can be transformed into a hard cubic form This requires strenuous pressures and temperatures, as illustrated in the graphite-diamond and hexagonal BN-cubic carbon nitride equilibrium diagrams (Fig 2, 3)

Fig 2 Pressure-temperature diagram showing the stability regions of diamond and graphite and the role of the solvent/catalyst in lowering

the synthesis conditions

Fig 3 Equilibrium diagram for HBN and CBN

It is possible to directly convert graphite to diamond, but very high pressures are required, and the properties of the resultant product are difficult to control In commercial practice, the required conditions for diamond synthesis can be reduced by the use of solvent/catalysts such as nickel, iron, cobalt, and manganese or alloys of these metals (Ref 5, 6) Figure 4 shows an example of a metal-carbon system at 5.7 GPa (57 kb), where a stable diamond plus liquid region exists Even with solvent/catalysts it is necessary to simultaneously sustain a pressure of about 5 GPa (50 kb)and a temperature

of about 1500 °C (2700 °F), for periods ranging from minutes to hours, to make the variety of products in common use today

Fig 4 Nickel-carbon system at 5.7 GPa (57 kb) showing the stability regions of diamond (d) and graphite (g) in equilibrium with liquid (l +

d, l + g) a, austenite Source: Ref 7

Conditions are similar for the synthesis of CBN, but the reactants are usually alkali, alkaline earth metals, or compounds Cubic boron nitride can be grown from a variety of solvent/catalysts, including metal systems similar to those used for diamond synthesis (Ref 8) Because the pressure-temperature conditions for the conversion of hexagonal boron nitride (HBN) to CBN are less severe than those for the conversion of graphite to diamond, some sintered polycrystalline products are synthesized by the direct process under static conditions; however, most commercial monocrystalline CBN is made by a solvent/catalyst process

Explosive Shock Synthesis. The direct conversion of graphite to diamond, or HBN to CBN, can be done on a commercial scale using explosive shock techniques (Ref 9) The process is relatively simple but produces only fine-grain materials, which are principally used as polishing powders or as possible source materials for sintering into polycrystalline products

Low-Pressure Synthesis of Superhard Coatings. The history of diamond synthesis under metastable conditions assisted, chemical vapor deposition, or physical vapor deposition coating processes) goes back at least to the late 1950s and perhaps even earlier The efforts of Russian (Ref 10) and Japanese (Ref 11) scientists in the period from 1975 to 1985 made this technique feasible for limited commercial applications The potential exists to make films or sheets of polycrystalline and single-crystal diamond at temperatures of about 900 °C (1650 °F) and at pressures of less than 1 atmosphere (0.1 MPa) A limited amount of information exists (Ref 12) on grinding or machining applications of these materials Some films have been made for x-ray windows, speaker diaphragms, and wear surfaces

(plasma-Synthesis of Polycrystalline Diamond and Polycrystalline Cubic Boron Nitride. It is possible also to produce polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN) by sintering (or binding) many individual crystals of diamond or CBN together to produce a larger polycrystalline mass It is commercial practice to enhance the rate of sintering by the addition of a metal second phase (Ref 13) In addition, the whole mass must again be maintained in the cubic region of the respective temperature-pressure phase diagram to prevent the hard cubic crystals from reverting to the

soft hexagonal form By such high-temperature high-pressure sintering techniques, it is possible to obtain a mass of diamond or CBN in which randomly oriented crystals are combined to produce a large isotropic mass

An immense range of polycrystalline products can be made of diamond or CBN Changes in grain size, the second phase employed, the degree of sintering, the particle size distribution, and the presence or absence of inert ceramic, metallic, or non-metallic fillers are examples of factors that have profound effects on the mechanical, physical, and thermal properties

of the final product By careful formulation it is possible to tailor material properties for particular applications

References cited in this section

4 H.T Hall, Ultra-High-Pressure, High-Temperature Apparatus: The "Belt," Rev Sci Instrum., Vol 31 (No

9 P.S DeCarli, Method of Making Diamond, U.S Patent 3,238,019, March 1966; and P.S DeCarli and J.C

Jamieson, Formation of Diamond by Explosive Shock, Science, Vol 133, 1966, p 1821-1822

10 B.V Spitsyn, L.L Bouilov, and B.V Derjaguin, Vapor Growth of Diamond on Diamond and Other

Surfaces, J Cryst Growth, Vol 52, 1981, p 219-226

11 S Matsumoto, Y Sato, M Tsutsumi, and N Setaka, Growth of Diamond Particles from Methane-Hydrogen

Gas, J Mater Sci., Vol 17, 1982, p 3106-3112

12 B Lux and R Haubner, Low Pressure Synthesis of Superhard Coatings, Int J Refract Met Hard Mater.,

Fig 5 Arrangement of carbon atoms in diamond and graphite The arrows indicate the transformation of graphite to diamond at HPHT

conditions and the reverse transformation at low pressures and high temperatures (LPHT)

Diamond oxidizes in air above about 600 °C (1100 °F) and back converts into a poorly graphitized form (as indicated by the reverse arrow in Fig 5) upon heating in the absence of air The reaction rate for the transformation back into graphite

is dependent on conditions, but it is a significant factor at temperatures about 750 °C (1400 °F) in many practical applications These phenomena impose critical limitations on the use and fabrication of bonded-abrasive tools

Diamond is chemically inert to inorganic acids, but upon heating it reacts readily with carbide-forming elements such as iron, nickel, cobalt, tantalum, tungsten, titanium, vanadium, boron, chromium, zirconium, and hafnium Controlled reactivity is important in forming metal bonds, but that same reactivity can limit the use of diamonds in cutting and grinding applications

The thermal conductivity of some near-perfect diamond crystals can be as high as 5× that of copper at room temperature Less-perfect materials still have a high conductivity, and this has to be taken into consideration before use in many applications In terms of electrical conductivity, diamond is an electrical insulator unless doped with boron or, as in some commercial materials, mixed with a metal phase

Diamond is the hardest practical material known (Table 1) The hardness of single-crystal diamond varies as a function of orientation, but this is important only in single-point tools and in the polishing of gemstones, diamond microtome blades, and diamond surgical knives

Table 1 Properties of selected hard materials

strength

Coefficient of thermal expansion

Thermal conductivity

g/cm 3 lb/in. 3

Hardness,

HK

GPa 10 6 psi

Cubic boron nitride 3.48 0.126 4500 7 1 5.6 3.1 1400 3.3

Silicon carbide (SiC) 3.21 0.116 2700 1.3 0.19 4.5 2.5 42 0.10

Alumina oxide (Al2O3) 3.92 0.142 2100 3 0.435 8.6 4.8 33 0.08

Most natural diamonds are essentially octahedral in shape as grown Irregularly shaped fragments can be obtained by crushing and selection Synthesized diamond can be reproducibly grown as cubes, cubooctahedrons, and octahedrons (Fig 6) The cubooctahedral shapes are generally preferred for stone sawing, but they are not always appropriate for grinding

Fig 6 Shapes of synthesized diamond abrasive grains Varying proportions of cube (100) and octahedral faces (111) predominate, but (110)

and (113) shapes are often present as well

Synthesized diamond is available in the size range from submicron to about one centimeter The latter are specialty items for single-point tools; heat sinks, microtomes, surgical blades, and other applications Table 2 shows the most popular sizes for many applications For still larger sizes it is more practical to use sintered polycrystalline materials

Table 2 Sizes of diamond and CBN grains for grinding applications

U.S mesh size ranges Superabrasive Bond

Properties of Cubic Boron Nitride

The crystal structure of CBN can be derived from that of diamond (Fig 5) by substituting boron and nitrogen for carbon

on alternate sites in the diamond lattice The resulting zinc blende structure differs from diamond in having no center of symmetry and a different cleavage plane (110) The soft hexagonal form of boron nitride (HBN) has the same relationship

to graphite that CBN has to diamond Cubic boron nitride may also exhibit a back conversion to a hexagonal structure that

is analogous to the back conversion of diamond into graphite

Cubic boron nitride is nominally boron nitride (that is, B:N = 1:1), with a band gap of about 6.6 eV, and thus should be colorless However, it is usually amber in color and behaves like an extrinsic semiconductor Cubic boron nitride can be doped as both p- and n-type It is most likely to be boron-rich when obtained from conventional processes A black form

is also commercially available Cubic boron nitride can include solvent/catalyst materials and HBN from the synthesis process An extremely tough microcrystalline form is also available that is useful in metal and vitreous bonds for heavy-duty applications

Cubic boron nitride is more resistant both to oxidation and to back conversion into a graphitelike form than is diamond It can be heated to 1300 to 1400 °C (2350 to 2550 °F) before its protective oxide layer no longer prevents further degradation Back conversion is not significant until temperatures reach about 1700 °C (3100 °F)

Because the reactivity of CBN with iron-, cobalt-, and nickel-base alloys is much less than that of diamond, CBN fills an important gap in the use of ultrahard materials for the machining of these metals Cubic boron nitride reacts with strong nitride and boride formers such as titanium, tantalum, zirconium, hafnium, chromium, tungsten, silicon, and aluminum Under controlled conditions some of these elements can be used as bonding materials The reactivity of the oxidized surface of boron nitride with alkalis and alkaline earths can be used in making vitreous bonds, but it also can lead to degradation by borate formation in the presence of these reactants

Theoretically, the thermal conductivity of CBN is slightly more than half that of diamond Cubic boron nitride also is about half as hard as diamond, but it is about twice as hard as any other material In contrast to the four (111) cleavage

planes of diamond, CBN cleaves on six (110) planes and therefore is intrinsically more friable than diamond on a grain basis

single-The preferred growth form for CBN from most solvent systems is a (111) truncated tetrahedron, with some development

of cube forms The size range for CBN grains is from the submicron level to about 1

2mm (0.02 in.) Grains larger than 1

mm (0.04 in.) are not usually grown

Properties of Sintered Polycrystalline Diamond

Sintered PCD, which was developed in 1970 (Ref 13), is a unique material produced by liquid-phase sintering at HPHT conditions It is characterized by diamond-to-diamond bonding This sintering process made possible the production of pieces much larger than 1 mm (0.04 in.) with isotropic properties The commercial product is useful for cutting tools, drill bits, wear surfaces, and wire dies

Compared to a single crystal, a sintered polycrystalline material is essentially isotropic with respect to wear and cleavage; therefore, its practical toughness in use is improved Whereas a single crystal is fragile with respect to catastrophic failure

by cleavage, a polycrystalline material may chip locally but has no extensive cleavage plane Wire-drawing dies of sintered PCD outlast single crystals because they do not cleave in hoop tension and because they maintain roundness and dimensions

Sintered polycrystalline diamond blanks are made in situ in an HPHT apparatus, which imparts some limitations to size

and shape Round tool shapes are most common, but squares, triangles, and other shapes are also available The sintered diamond blanks are produced in both supported and unsupported configurations In supported structures, cemented tungsten carbide (WC-Co) provides additional strength and a brazeable surface for tool fabrication A combination of chemical and mechanical bonding exists between the diamond and the substrate by virtue of the transport of cobalt through the diamond layer during HPHT production Unsupported polycrystalline diamond can be mounted in tools by more conventional diamond-bonding techniques Subsequent finishing operations are involved in all cases to make tools with tight dimensional and angular tolerances from the blanks Finishing operations, depending on the tools, can include laser cutting, electrodischarge machine cutting, grinding, lapping, and polishing steps

Polycrystalline diamond with a metallic second phase has a microstructure of diamond grains with the metallic phase mostly at the grain boundaries (Fig 7) Both phases are continuous, and the metal phase can be removed chemically Within the grains it is very common to see deformation twin bands that were produced during the HPHT sintering These bands are visible in a polished section because they are more wear resistant than the surrounding material

Fig 7 Microstructure of sintered polycrystalline diamond (a) Diamond with second phase at the grain boundaries 225× (b) Detailed

structure of diamond-to-diamond bonding at grain boundary

The sintered diamond contains about 5 to 10 vol% of metal phase and, when made of synthetic diamonds, also may include metals and graphite from the original crystal growth process Because diamond predominates, the chemical

reactivity and stability for oxidation, back conversion, and wetting/bonding will be similar to those for synthetic diamond alone The metal phase is an added complication, however, with respect to thermal stability It can enhance graphitization (back conversion), and it can also contribute to degradation above about 700 °C (1300 °F) by thermal stresses that are due

to the large differences in the thermal expansion coefficients of diamond and metals It is advisable not to overheat these tools during bonding and brazing Without the metal phase, the material is more thermally stable Some sintered material

is made with better thermal expansion matching of the bonding phase (such as silicon carbide) with diamond to minimize thermal degradation at the expense of strength

Reference cited in this section

13 R.H Wentorf, Jr and W.A Rocco, Diamond Tools for Machining, U.S Patent 3,745,623, July 1973

Properties of Sintered Polycrystalline Cubic Boron Nitride

The microstructure of sintered PCBN is shown in Fig 8 The major phase is CBN with a metallic second phase from the liquid sintering process With respect to chemical composition and chemical reactivity (oxidation, back conversion, and wetting/bonding), CBN is the predominant material

Fig 8 Microstructure of sintered PCBN

Sintered PCBN can be heated to at least 700 °C (1300 °F) before thermal degradation occurs The maximum thermal conductivity of sintered PCBN lies in the range of 2.5 to 9.0 W/cm · °C, depending on the processing conditions used to make the samples (Ref 14)

Sintered PCBN is less tough than sintered polycrystalline diamond As with the diamond version, it has the advantage of isotropic wear and hardness rather than the catastrophic cracking and cleavage that characterize the single crystals in heavy-duty applications The sizes and shapes available are similar to those for diamond

Reference cited in this section

14 F.R Corrigan, Thermal Conductivity of Polycrystalline Cubic Boron Nitride in Compacts, High Pressure Science and Technology, Vol 1, Plenum Publishing, 1979, p 994-999

Superabrasive Grains

Superabrasive grains are commercially available in a range of sizes, shapes, and qualities (Table 2) Diamond or CBN grains can be used as loose abrasives, as bonded abrasives in grinding wheels and hones, and as bonded abrasives in single-point applications such as turning tools, dressers, and scribes

Loose Abrasive Grains

Lapping and polishing constitute two major applications of both natural and synthetic loose abrasive grains Of the synthetic abrasive powders, alumina and silicon carbide are the most widely used in lapping and polishing operations Silicon carbide is harder than alumina (Table 1) and fractures more easily, thereby providing new cutting edges and extending the useful life of the abrasive

Synthetic Lapping Abrasive. Silicon carbide, which has sharp edges for cutting, is used for lapping hardened steel or cast iron, particularly when an appreciable amount of stock is to be removed Fused alumina is also sharp, but it is tougher than silicon carbide and breaks down less readily Fused alumina is generally more suitable than silicon carbide for lapping soft steels or nonferrous metals

Boron carbide (B4C), one of the superhard materials shown in Fig 1, has a hardness of about 2800 HV and is an excellent abrasive for lapping However, because it costs 10 to 25 times as much as silicon carbide or fused alumina, boron carbide

is usually used only for lapping dies and gages, which is often done by hand and in small quantities, using little abrasive

An example of such a use of boron carbide is in the production of synthetic sapphire for electronic applications The raw material cost is expensive, justifying a high abrasive-processing cost

Diamond is also used as an abrasive for lapping metals It is available as a paste or a slurry Table 3 lists typical sizes of powders used for lapping applications Fine-mesh diamond and diamond micron powders are the abrasives most often used in lapping Depending on the needs of the purchaser, these powders can be provided in several types that differ in aggressiveness (the sharpness of cutting points and edges), shape, and toughness

Table 3 Size ranges of micron diamond powders for grinding and polishing

Polishing. As with lapping, aluminum oxide and silicon carbide are widely used synthetic abrasives for polishing They are harder, more uniform, longer lasting, and easier to control than most natural abrasives Aluminum oxide grains are very angular and are particularly useful in polishing tougher metals, such as alloy steels, high-speed steels, and malleable and wrought iron Silicon carbide is usually used in polishing low-strength metals, such as aluminum and copper It is also applied in polishing hard, brittle materials, such as carbide tools, high-strength steels, and chilled and gray irons Polishing with loose-grain diamond is more common for nonmetallic workpieces (like granite) than it is for metals When done on metals, however, the same fine mesh size diamond and diamond micron powders that are used for lapping are employed

Bonded-Abrasive Grains

The principal use of bonded-abrasive grains is in grinding wheels The primary metals commonly ground with diamond

or CBN are shown in Table 4 Of these applications, the most important worldwide is the grinding of cemented tungsten carbide for producing machine tools, wear surfaces, and dimensioned parts and for resharpening tool blanks Resin-bonded diamond grinding wheels have become the accepted standard for this application Cemented tungsten carbide, made by sintering compacted mixtures of tungsten carbide particles with cobalt powders, is a hard, tough, wear-resistant material suitable for use in metal-cutting tools These same characteristics make it difficult to grind but an ideal workpiece for resin-bonded diamond grinding wheels Generally, a more friable diamond is best suited for these applications because friable diamond is capable of regenerating cutting edges and points

Table 4 Metals typically ground or machined with superabrasives and ultrahard tool materials

Grind with Machine with Metal types Hardness Examples of

designations or applications

Principal alloying elements

Cr, Mo, Ni, V Yes No Yes No

Cast iron

Gray iron >180 HB Engine blocks, flywheels,

crankshafts

White iron >450 HB Ni-Hard (rolls) C, Ni, Si, Cr Yes No Yes No

Ductile iron >200 HB Crankshafts, exhaust

Cobalt-base superalloys >35 HRC Stellite, AiResist, Haynes Cr, W Yes No Yes No

Iron-base superalloys >35 HRC A-286, Incoloy Cr, Ni, Mo Yes No Yes No

Hardfacing materials

Carbide/Oxide-base

materials

>35 HRC UCAR LA-2, LC-4 Al2O3, Cr2O3 WC No Yes No Yes

Metal-base materials >35 HRC Stellite, Hastelloy Mo, Ni, Cr, Co, Fe Yes No Yes No

Aluminum alloys

Sand or permanent cast

alloys

Mold cast alloys

Die cast alloys 65-125 HB A360, 380, 390 Si, Cu, Zn No No No Yes

Cemented tungsten carbide

All tool and die grades 84-95

HRA

All presintered tool and die

grades

(a) Can be machined or ground if the equipment and operating conditions are suitable for superabrasives

One limitation to the economical use of diamond as the superabrasive of choice is its solubility in iron, nickel, cobalt, and alloys based on these metals Cubic boron nitride is preferred for the grinding of these metals (Table 4), and CBN grains for grinding iron, nickel, cobalt, and their alloys are generally in the 60 to 400 mesh size range (250 to 38 μm)

Grinding wheels are available in a wide variety of sizes and shapes Selection of the proper wheel for a given application is critical The grinding wheel manufacturers have years of experience and can provide help as needed Krar and Ratterman (Ref 15) also give guidelines that can be of help They indicate that one should first choose the best bond for the application, then specify, in order, wheel diameters and widths, superabrasive mesh size, and concentrations If properly done, this procedure will ensure good wheel life, good material removal rates, and the required workpiece surface finish

Once the grinding wheels have been fabricated, it is good practice to true them to establish the desired shape; they should then be dressed to ensure good protrusion of the abrasive grains These operations are covered in detail in Ref 16 and in

the article "Superabrasives" in Machining, Volume 16 of ASM Handbook, formerly 9th Edition Metals Handbook

Bonds. Several commercial bonds are available for grinding wheels The most common are resin systems, vitreous systems, metal systems, and electroplated systems All are suitable for use with superabrasive

Resin Bonds. The diamond types synthesized for use in resin bond grinding wheels are shown in Fig 9(a) This type of diamond is friable and thus is capable of regenerating cutting edges and points It is also well suited to the scratching action required for material removal in the grinding of hard materials such as cemented tungsten carbide Depending on the application, diamond concentrations can range from 50 to 150 (12.5 to 37.5 vol% of superabrasive) Most commercially available wheels have a 75 to 100 concentration (18.75 to 25 vol% of superabrasive)

Fig 9 Commercially available diamond grains used in various applications (a) Friable diamond grains especially tailored for resin bond

grinding wheels (b) Diamond grains tailored for use in metal bond grinding wheels These grains are typically in the 80 to 400 mesh size (350 to 38 μm) range (c) Synthesized diamond grains for use in diamond saw blade applications, such as for the sawing of marble, granite, and concrete These grains are in the 20 to 60 mesh size (850 to 250 μm) range

Nearly all of the common resin bonds are thermosetting resins, and most of the thermosetting resins are phenolic resins Resin powders and a solvent, such as furfural, are mixed with superabrasive particles and a filler, such as silicon carbide, and then placed in a mold containing the metal core The resin mixture is cured in the hot press mold at pressures of 35 to

105 MPa (5 to 15 ksi) temperatures of at least 150 °C (300 °F) for times ranging from 30 min to 2 h Before use, the wheels are trued to eliminate chatter and to ensure the proper form After trueing, it is necessary to dress the wheels to ensure abrasive protrusion for free cutting Resin bond wheels are commercially available in a large range of sizes The wheels can be used wet or dry and are free cutting, but they have relatively short life and poor form-holding characteristics

Phenol-Aralkyl Bonds. Recently, work has been made public concerning new phenol-aralkyl bond systems for diamond abrasives (Ref 17) This class of resins can be used for making grinding wheels in the same equipment used for ordinary phenolic resin wheels The new formulations are claimed to provide significantly improved wear life, cooler cutting, and

a superior workpiece surface finish

Thermoplastic resins, such as polyimides, are of interest as bond systems for heavy-duty grinding wheels They are characterized by higher temperature stability limits than those of the phenolic resins However, these resins do soften at high temperature; this can allow the superabrasive particles to move within the softened bond, and grains may be lost prematurely from the wheels Special rough coatings have been devised to anchor the abrasive grains in such bonds (Fig 10), and these coatings have proved effective

Fig 10 Diamond of the type shown in Fig 9(a), but with a special spiked nickel coating Cubic boron nitride can be coated in a similar

fashion

Vitreous bond systems are generally tailored from glass or ceramic formulations Vitreous bonds are finding increasing application with CBN abrasive grains; they are also useful for diamond grain wheels Most vitreous bond systems are proprietary materials used for the production grinding of steel, cast iron, and superalloys To be suitable for use with superabrasives, the bonds must have the proper wear characteristics, be formable at moderate temperatures and pressures, and be chemically compatible with the superabrasive grains Some vitreous bonds meet these criteria with diamond but are too reactive with CBN; these applications require the use of protective metal coatings on the CBN A significant reaction between the bond and the CBN abrasive can produce gaseous by-products that cause excessive porosity in the bond; this can lead to a loss of abrasive particle material and a weakening of the bond Properly made vitreous bonds have several advantages: ease of conditioning, free-cutting characteristics, reduced frictional heat, excellent surface finish capabilities, consistently accurate geometry, and long wheel life (Ref 16)

Metal bond systems are used with superabrasive grinding wheels in applications such as glass and ceramic grinding Figure 9(b) shows typical diamond grains used in metal bonds for grinding The grains are stronger than those used in resin bonds (Fig 9a), but they are not, as strong as the grains used in saw blade applications for stone and concrete (Fig 9c) It

is common practice to use softer metals such as bronze for metal bond grinding wheels These metals wear away during use at a rate that ensures both crystal protrusion at the wear surface and free-cutting action The two basic processes for the fabrication of metal bond wheels are hot pressing and cold pressing followed by sintering Processing temperatures range from 600 °C (1100 °F) to greater than 1100 °C (2000 °F), pressures from about 14 to 140 MPa (2 to 20 ksi), and times at temperature from about 15 min to over 1 h Superabrasive concentrations generally vary from 50 to 100 (12.5 or

25 vol% of superabrasive) These bonds are relatively tough, and they have long life and good form-holding characteristics For glass and ceramic grinding, the mesh size ranges from 60 to 400 (250 to 38 μm)

Electroplated bond systems are available for grinding wheel fabrication Superabrasive grains are bonded to wheel cores by electrodeposition of nickel or a nickel alloy Normally, the layer of superabrasive is tacked down by immersing the core

as a cathode into a bed of the superabrasive crystals in a plating solution; the wheel is then removed to a fresh bath for final plating The final product has a single layer of superabrasive crystals with good particle exposure Such wheels can

be fabricated into complex forms and will hold those forms well for the life of the wheel The wheels are free cutting but have a relatively short life because they possess only a monolayer of crystals

Coatings. Superabrasive grains are often coated before being incorporated into the bond systems The coatings are generally of metals, specifically nickel, cobalt, copper, and titanium The coatings serve several purposes, depending on the superabrasive and the bond Many of the resin bonds wet metals better than they wet superabrasives A good example

of this is a phenolic bond with nickel-coated synthetic diamond as compared with the same bond with uncoated synthetic diamond The bond with the nickel-coated diamond is stronger aiding retention of the protruding grains In addition, metal coatings can slow the transfer of heat from the cutting points of the grains to the resin bond delaying the onset of charring and degradation of the bond and extending the life of the grinding wheel Coatings can also act as barriers to

chemical reactions, such as those that occur between some vitreous bonds and CBN Detrimental reactions can be eliminated by thin coatings of titanium on the superabrasive surfaces

While a number of processes can be used for coating superabrasives (for example, chemical vapor deposition, physical vapor deposition, plasma spraying, and sputter), most commercial coatings are prepared by electroplating techniques Electrolytic coatings can be applied using a standard or modified Watts bath (Ref 18) Autocatalytic (electroless) coatings are also common and can be applied with baths that require no passage of electric current from external power sources (Ref 19) Autocatalytic coatings can generally be distinguished by the presence of phosphorus from the hypophosphites used as reducing agents This phosphorus can slightly embrittle the coating, which often improves its performance

Copper coatings have been designed for superabrasives in resin bond wheels that are used for dry grinding, and the copper-coated wheels are more effective for these applications that those with nickel coatings or uncoated crystals Copper coatings are normally applied at a 60 wt% concentration Nickel coatings are more effective in wet grinding with resin bond grinding wheels; they are commercially available at 30 and 56 wt% concentrations To simplify inventories, some shops prefer to use nickel coatings for all applications, wet or dry The dry grinding performance of wheels with nickel-coated superabrasives is definitely not as good as that of wheels with copper coatings, but it may be acceptable The reverse situation, that is, using copper-coated grains in wet grinding applications, gives poor results and is not recommended

Coated grains are not commonly found in metal bond grinding wheels or in electro-plated wheels There is nothing to restrict their use for special applications, however

References cited in this section

15 S.F Krar and E Ratterman, Super-abrasives Grinding and Machining With CBN and Diamond,

McGraw-Hill, 1990

16 B Nailor, "Trueing Parameters for Conditioning Vitrified Bond CBN Wheel," Paper presented at Advancements in Abrasives, The 27th International Abrasive Engineering Conference, Bloomingdale, IL, Sept 1989

17 G.I Harris, "Phenol-aralkyl Resin Bonded Wheels," Paper presented at the Industrial Diamond Association [ges]Ultra-Hard Materials Seminar, Toronto, Sept 1989

18 N.V Parthasaradhy, Practical Electroplating Handbook, Prentice-Hall, 1989, p 183-186

19 F.A Lowenheim, Electroplating Fundamentals of Surface Finishing, McGraw-Hill, 1978, p 391-400

Ultrahard Tool Materials

Ultrahard tool materials of sintered polycrystalline diamond (PCD) or PCBN are commercially available in many shapes, sizes, and compositions Depending on their type, they are used for cutting, drilling, milling, dressing, and as wear surfaces

The PCD or PCBN in ultrahard tool blanks often is bonded to a cemented carbide substrate (Fig 11), which allows brazing to tool shanks or to indexable inserts for use in standard toolholders Solid PCD and PCBN can also be used as inserts

Fig 11 Polycrystalline diamond with substrates (a) Typical fully round PCD tool blank This type of blank is brazed or mechanically

clamped to extend the usable cutting edge (b) Typical square PCD tool blank with a long straight cutting edge that is ideal for many applications (c) Typical triangular PCD tool blank that is useful in single-point turning applications, either as tools or as brazed-in tips on carbide tools

The types of metals typically machined with ultrahard tool materials are summarized in Table 4 More detailed

information on tool fabrication and applications is available in the article "Ultrahard Tool Materials" in Machining, Volume 16 of ASM Handbook, formerly 9th Edition Metals Handbook

Polycrystalline diamond tool blanks are useful in the machining of nonferrous and nonmetallic materials (Tables 4 and 5) and are commercially available in a variety of shapes and sizes (Fig 12) An important variable for the end user is the average grain size, which is separated into three grades in Fig 13: fine (average diamond grain size, 4 μm), medium (5 μm), and coarse (25 μm) As shown in Fig 13, differences in grain size can cause variations in abrasion resistance,

grindability, and workpiece surface finish As a result of these differences, the areas of preferred application are different for the three grades:

PCD grade Application

Fine grain Applications requiring good surface finishes; woodworking

Medium grain General purpose applications (aluminum alloys with <16% Si)

Coarse grain Heavy-duty applications; milling applications, interrupted cuts (aluminum alloys with >16% Si)

Table 5 PCD tool applications

Fig 12 Various configurations and sizes of PCD tool blanks Round shapes are available in standard sizes up to about 34 mm (1.34 in.) in

diameter

Fig 13 Variation in tool performance with average grain size in PCD tool blanks (a) Abrasion resistance (b) Grindability (c) Surface finish

Surface finish is also dependent on other factors such as feed rate, tool geometry, and workpiece condition

The techniques for using PCD tool blanks are not very different from those for using conventional ceramic blanks Where possible, these guidelines should be followed:

• Use a positive rake

• Maintain a sharp cutting edge

• Use the largest nose radius possible

• Use a rigid machine set-up

• Minimize tool overhang

• Use a flood coolant whenever possible

When first machining a new material, the starting conditions suggested in Table 6 can be used Slight modifications may give improved results, depending on the particular configuration Polycrystalline diamond tool blanks can be used dry; the high thermal conductivity of the diamond layer removes and distributes heat generated at the cutting edge However, tool performance is generally improved by the use of coolants (Ref 15) Water-soluble oil emulsions, such as those used in conventional machining with cemented tungsten carbide tools, are adequate if properly applied to the rake surface They reduce frictional heating and the formation of built-up edges while providing good chip flow

Table 6 General starting conditions for PCD cutting tools

Plastics and composites 300-1005 1000-3300 0.1-0.3 0.004-0.012

Sintered tungsten carbide 20-40 65-130 0.15-0.25 0.006-0.010

Polycrystalline cubic boron nitride (PCBN) tool blanks are useful in the machining of iron, steel, and cobalt- and nickel-base alloys (Tables 4 and 7) This makes them complementary to rather than competitive with the PCD tool blanks They are generally not recommended for use with superalloys or steels that have hardness of less than 35 and 45 HRC, respectively The causes and solutions of common problems encountered when using PCBN tools are listed in Table 8

Table 7 PCBN tool applications

Application

Hard cast iron

Ni-Hard

Alloy cast iron

Chilled cast iron

Nodular cast iron

Soft cast iron

Gray cast iron

Table 8 Common problems encountered with PCBN tool blanks

Change speed to recommended rates: For hardened ferrous materials (>45 HRC), 70-130 m/min (230-430 ft/min) For soft gray cast iron (200 HB), 450-915 m/min (1500-3000 ft/min)

Feed rate too light (thin chip cannot dissipate heat; tool rubbing)

Use a minimum feed rate of 0.1 mm/rev (0.004 in./rev)

Tool Geometry. The general guidelines for the use of PCBN tools (Ref 15) are similar but not identical to those for PCD tools Negative-rake PCBN tools should be used wherever possible because they can withstand high cutting forces

The lead or side cutting-edge angle should be as large as possible when using PCBN tools; it should only rarely be less than 15° A large lead angle spreads the cut over a wide section of the cutting edge, resulting in a thinner chip, which in turn reduces loading on the tool blank or insert The reduced loading allows the feed per revolution to be increased without increasing the chances of cutting edge chippage In addition, a large lead angle helps reduce notching at the depth-of-cut line; notching can occur in an overly hard workpiece, or as the presence of scale on a workpiece

Sharp corners on cutting tools concentrate stresses and can cause premature load failures Honing a radius on the edge and chamfering the cutting edge are two available methods for overcoming this problem A chamfer of 15° with a width of 0.2

mm (0.008 in.) is recommended for most roughing operations Honing the edge is slightly is suggested for finishing operations (Ref 15)

Starting Feeds and Speeds. Good starting conditions are listed in Table 9 for several materials commonly machined with PCBN tools While feeds and speeds are dependent on workpiece properties, the conditions given in Table 9 generally

produce satisfactory result In the speed ranges shown, the higher speeds are for finishing operations It is recommended that cutting fluids be used whenever possible

Table 9 General starting conditions for PCBN cutting tools

Classification Material Type

m/min ft/min mm/rev in./rev

Hardened ferrous materials

Cold-sprayed materials Hardfacing materials 105- 350-500 0.1-0.33

0.004-Reference cited in this section

15 S.F Krar and E Ratterman, Super-abrasives Grinding and Machining With CBN and Diamond,

CERAMICS are nonmetallic, inorganic engineering materials processed at a high temperature The general term

"structural ceramics" refers to a large family of ceramic materials used in an extensive range of applications Included are both monolithic ceramics and ceramic-ceramic composites Chemically, structural ceramics include oxides, nitrides, borides, and carbides Many processing routes are possible for structural ceramics and are important because the microstructure, and therefore the properties, are developed during processing

General properties and uses of structural ceramics are reviewed first Ceramic processing is described and the relationship

of processing, microstructure, and properties presented Specific structural ceramic materials, including composites, are presented This article concludes with a discussion of future direction and problems with structural ceramics

Uses and General Properties of Structural Ceramics

Industrial uses, required properties, and examples of specific applications for structural ceramics are summarized in Table

1 These applications take advantage of the temperature resistance, corrosion resistance, hardness, chemical inertness, thermal and electrical insulating properties, wear resistance, and mechanical properties of the structural ceramic materials Combinations of properties for specific applications are summarized in Table 1 Ceramics offer advantages for structural

applications because their density is about one-half the density of steel, and they provide very high stiffness-to-weight ratios over a broad temperature range The high hardness of structural ceramics can be utilized in applications where mechanical abrasion or erosion is encountered The ability to maintain mechanical strength and dimensional tolerances at high temperature makes them suitable for high-temperature use For electrical applications, ceramics have high resistivity, low dielectric constant, and low loss factors that when combined with their mechanical strength and high-temperature stability make them suitable for extreme electrical insulating applications

Table 1 Industry, use, properties, and applications for structural ceramics

Fluid handling Transport and control of

aggressive fluids

Resistance to corrosion, mechanical erosion, and abrasion

Mechanical seal faces, meter bearings, faucet valve plates, spray nozzles, micro-filtration membranes

Mineral processing

power generation

Handling ores, slurries, pulverized coal, cement clinker, and flue gas neutralizing compounds

Hardness, corrosion resistance, and electrical insulation

Pipe linings, cyclone linings, grinding media, pump components, electrostatic precipitator insulators

Wire manufacturing Wear applications and surface

finish

Hardness, toughness Capstans and draw blocks, pulleys and

sheaves, guides, rolls, dies

Pulp and paper High-speed paper

stiffness-to-Bearings and bushings, close tolerance fittings, extrusion and forming dies, spindles, metal-forming rolls and tools, coordinate- measuring machine structures

Thermal processing Heat recovery, hot-gas cleanup,

general thermal processing

Thermal stress resistance, corrosion resistance, and dimensional stability at extreme temperatures

Compact heat exchanges, heat exchanger tubes, radiant tubes, furnace components, insulators, thermocouple protection tubes, kiln furniture

Internal combustion

engine components

Engine components High-Temperature resistance,

wear resistance, and corrosion resistance

Exhaust port liners, valve guides, head faceplates, wear surface inserts, piston caps, bearings, bushing, intake manifold liners

Specific properties of ceramics compared with other materials are discussed in the section "Properties and Applications of Structural Ceramics" in this article The text by Kingery (Ref 1) should be consulted for a general discussion of the properties as related to composition and microstructure

Reference cited in this section

1 W.D Kingery, H.K Bowen, and D.R Uhlmann, Introduction to Ceramics, 2nd ed., John Wiley & Sons,

1976

Processing of Structural Ceramics

The processing steps for producing structural ceramics are shown in the flow chart given in Fig 1 These steps can be grouped into four general categories:

• Raw material preparation

• Forming and fabrication

• Thermal processing

• Finishing

These categories are also indicated on Fig 1 Only a brief overview of ceramic processing can be included here For specific details see the text by Reed (Ref 2)

Fig 1 Flow chart for ceramic processing

Raw material preparation includes material selection, ceramic-body preparation, mixing and milling, and the addition of processing additives such as binders Material selection is important because structural ceramics require high-quality starting materials that can be described as industrial inorganic chemicals For example, the Al2O3 powder for alumina ceramics is usually obtained as calcined alumina from the Bayer process, which uses bauxite as the starting material Zirconia is obtained from industrial sources that process zircon (ZrSiO4) to produce ZrO2 of 99% purity Silicon carbide, SiC, is produced by the Acheson process in which silica, SiO2, and coke are placed in an arc furnace and reacted at 2200

to 2500 °C (4000 to 4500 °F) Silicon nitride, Si3N4, is produced by reacting silicon with nitrogen Chemical techniques are used to produce powders where extremely high purity and very fine particle sizes are required A detailed description

of material selection for structural ceramics is included in the reference text edited by Somiya (Ref 3) The assurance of final product quality starts with well-defined and strict material acceptance criteria

Ceramic body preparation consists of combining the collection of materials necessary for the final body composition For oxide systems, the starting materials are generally mixed in aqueous systems and milled to obtain the specified particle-size distribution for the body If necessary, organic binders are added after the milling and mixing This results in a slurry

or slip, which is the starting material for forming and fabrication of the component

Forming and Fabrication. Structural ceramics are formed from either powders, stiff pasters, or slurries The slurry from the preparation procedures is converted to an agglomerated flowable powder by spray drying or to a stiff paste by filter pressing Structural components are formed by pressing of powders, extrusion of stiff pastes, or by slip casting of slurries

In some cases, pre-sinter machining (green machining) is required

Forming Structural Ceramics From Powders. Pressing operations are used to make consolidated ceramics starting from a powder Complex shapes are made to net shape in large volumes by dry pressing in uniaxial double-acting presses using specific tooling made for each part No further shaping of these components is usually required prior to the thermal processing step Faucet valve plates, pipe linings, and grinding media are made by pressing

Isostatic pressing is used for larger components and where extensive pre-sinter machining is required Powders are placed

in flexible tooling and pressure is hydraulically applied in all directions forming a part that is machined to net shape prior

to sintering Electrostatic precipitator insulators, sodium-vapor lamp tubes, and spark plugs are examples of products formed by isopressing

Forming Structural Ceramics From Stiff Pastes. Stiff pastes are used to form structural ceramics by extrusion In extrusion the plastic mass is forced through a die at high pressure, which determines the shape of the component Rods and tubes are usually formed by extrusion

Forming Structural Ceramics From Slurries or Slips. Slurries, or slips, are used to form structural components by slip casting

In slip casting a porous mold, usually plaster, is filled with slip Capillary action draws the water from the slip into the mold, which forms a solid layer at the slip-mold interface When the required wall thickness is reached, the remaining slip

is poured from the mold Thermocouple protection tubes are made by slip casting

Pre-Sinter Machining. Structural components, when required, are machined to final unfired dimensions after the forming operations described above Conventional machining techniques such as turning, milling, and drilling are used, and in many cases the machining is done on numerically controlled machines

Thermal processing for structural ceramics is done either at ambient pressure or with added pressure in the case of hot pressing or hot isostatic pressing (HIP) The final microstructure is developed during thermal processing by sintering, vitrification, or reaction bonding

Sintering takes place by volume, surface, or grain-boundary diffusion and is a solid-state process During sintering the pores are removed, the piece is densified, and grain growth occurs if desired for the particular ceramic being processed Sintering is used for high-purity oxide systems

Vitrification involves the presence of a liquid phase during thermal processing The liquid phase provides faster diffusion paths and holds the piece together by capillary action during processing This results in an amorphous or glassy phase

being present in the final microstructure The final microstructure is created by vitrification for system with less than 99% pure oxide, porcelains, and Si3Ni4 with sintering additives

In some cases the thermal processing is aided by adding external pressure during sintering The pressure can be applied uniaxially in hot pressing or isostatically in hot isostatic pressing Covalent materials such as silicon carbide and silicon nitride, and composite systems usually undergo hot pressing Pressure can also be used to suppress the decomposition of materials (such as in the gas-pressure sintering of Si3Ni4) Hot pressing is also used in the processing of spinels

The microstructure is developed by reaction bonding for some covalent structural ceramics such as silicon carbide and silicon nitride For example, silicon carbide components are formed by mixing together very fine SiC coated with fine carbon, which is exposed to silicon above its melting point The molten silicon and the carbon react to form silicon carbide in place which bonds the SiC grains together

Finishing. Additional processing is required where tolerances are tighter than can be achieved by sintering or where a surface must be extremely flat or polished Diamond grinding is used to provide tight dimensional tolerances Lapping using abrasive slurries, extremely flat surfaces, and polishing by slurry abrasion will achieve a fine surface finish

References cited in this section

2 I.S Reed, Introduction to the Principles of Ceramic Processing, John Wiley & Sons, 1988

3 Advanced Technical Ceramics, S Somiya, Ed., Academic Press, 1989

Properties and Applications of Structural Ceramics

Alumina Ceramics. Aluminum oxide, Al2O3 (often referred to as alumina), is perhaps the material most commonly used in the production of technical ceramics The reasons for its wide acceptance are many; alumina has a high hardness, excellent wear and corrosion resistance, and low electrical conductivity It is also fairly economical to manufacture, involving low-cost alumina powders

Alumina ceramics actually include a family of materials, typically having alumina contents from 85 to ≥99% Al2O3, the remainder being a grain-boundary phase The different varieties of alumina stem from diverse application requirements For example, 85% alumina ceramics such as milling media are used in applications requiring high hardness, yet they are economical Aluminas having purities in the 90 to 97% range are often found in electronic applications as substrate materials, due to the low electrical conductivity The grain-boundary phase in these materials also allows for a strong bond between the ceramic and the metal conduction paths for integrated circuits High-purity alumina (>99%) is often used in the production of translucent envelopes for sodium-vapor lamps

The microstructure and resulting properties of alumina ceramics greatly depend on the percentage of alumina present For example, high-purity aluminas typically have a fairly simple microstructure of equiaxed alumina grains (Fig 2), whereas

as 96% alumina ceramic will have a more complicated microstructure consisting of alumina grains (often elongated in shape) surrounded by a grain-boundary phase (Fig 3) Depending on processing, this grain-boundary phase may be amorphous, crystalline, or both The properties of this family of materials vary widely, as shown in Table 2

Table 2 Properties of various alumina ceramics

Fracture toughness, MPa m

(ksi in)

Hardness, GPa (10 6 psi)

Elastic modulus, GPa (10 6 psi)

Thermal conductivity, W/m · K (Btu/ft · h · °F)

Linear coefficient of thermal expansion, ppm/°C (ppm/°F)

(46)

3-4 (2.8-3.7)

9 (1.3)

221 (32)

16.0 (9.24)

7.2 (4)

(49) (2.8-3.7) (1.5) (40) (9.65) (4.5)

(51)

3-4 (2.8-3.7)

12 (1.7)

296 (43)

22.4 (12.9)

8.2 (12.9)

(52)

3-4 (2.8-3.7)

11 (1.6)

303 (44)

24.7 (14.3)

8.2 (4.6)

99.5 3.89 379

(55)

3-4 (2.8-3.7)

14 (2.0)

372 (54)

35.6 (20.6)

8.0 (4.4)

99.9 3.96 552

(80)

3-4 (2.8-3.7)

15 (2.2)

386 (56)

38.9 (22.5)

8.0 (4.4)

Source: Coors Ceramic Company

Fig 2 Scanning electron micrograph of a high-purity Al2O3 The sample has been thermally etched to reveal the grain boundaries Note the

equiaxed grain morphology and lack of any intergranular phase

Fig 3 Scanning electron micrograph of a typical 96% Al2O3 ceramic The sample has been thermally etched to reveal the grain boundaries

The intergranular phase was also removed during etching Note the tabular morphology of some of the alumina grains

Aluminum titanate, Al2TiO5, is a ceramic material that has recently received much attention because of its good thermal shock resistance Aluminum titanate has an orthorhombic crystal structure, which results in a very anisotropic thermal

expansion The coefficient of thermal expansion (CTE) normal to the c-axis of the orthorhombic crystal is -2.6 × 10-6/°C (-1.4 × 10-6/°F) whereas the CTE parallel to the c-axis is about 11 × 10-6/°C (6.1 × 10-6/°F) The resulting thermal expansion coefficient for a polycrystalline material is very low (0.7 × 10-6/°C, or 0.4 × 10-6/°F) as shown in Table 3

Table 3 Physical properties of various ceramics

Material Bulk density,

g/cm 3

Flexure strength, MPa (ksi)

Fracture toughness, MPa m

(ksi in)

Hardness, GPa (10 6 psi)

Elastic modulus, GPa

Thermal conductivity, W/m · K (Btu/ft · h · °F)

Linear coefficient of thermal expansion, ppm/°C

(ppm/°F)

Aluminum titanate 3.10 25

(3.6)

5 (0.7)

1.0 (0.6)

0.7 (0.4)

Sintered SiC 3.10 550

(80)

4 (3.6)

29 (4.2)

400 (58)

110.0 (63.6)

4.4 (2.4)

Reaction-bond SiC 3.10 462

(67)

3-4 (2.7-3.6)

25 (3.6)

393 (57)

125.0 (72.2)

4.3 (2.4)

Silicon nitride 3.31 906

(131)

6 (5.5)

15 (2.2)

311 (45)

15.0 (8.7)

3.0 (1.7)

Boron carbide 2.50 350

(51)

3-4 (2.7-3.6)

29 (4.2)

350 (51)

The excellent thermal shock resistance of aluminum titanate derives from this considerable thermal expansion anisotropy During cooling from the densification temperature, the aluminum titanate grains shrink more in one direction than the other, which results in small microcracks developing in the microstructure as the grains actually pull away from each other Subsequent thermal stresses (either by fast cooling or heating) are thereby dissipated by the opening and closing of the microcracks Unfortunately, a consequence of the microcracks is that aluminum titanate does not have particularly high strength (25 MPa, or 3 ksi) However, the microcracks do impart very low thermal conductivity, making it an excellent candidate for thermal insulation devices

The excellent thermal shock resistance of aluminum titanate offers the potential for many applications For example, aluminum titanate has found uses as funnels and ladles in the foundry industry (aluminum, magnesium, zinc, and iron do not wet aluminum titanate) The automotive industry is also investigating aluminum titanate for exhaust port liners and exhaust manifolds

Silicon carbide, SiC, is ceramic material that has been in existence for decades but has recently found many applications in advanced ceramics There are actually two families of silicon carbide, one known as direct-sintered SiC, and the other known as reaction-bonded SiC (also referred to as silconized SiC) In direct-sintered SiC, submicrometer SiC powder is compacted and sintered at temperatures in excess of 2000 °C (3600 °F), resulting in a high-purity product Reaction-bonded SiC, on the other hand, is processed by forming a porous shape comprised of SiC and carbon-powder particles The shape is then infiltrated with silicon metal; the silicon metal acts to bond the SiC particles

The properties of the two families of SiC are similar in some ways and quite different in others Both materials have very high hardnesses (27 GPa, or 3.9 × 106 psi), high thermal conductivities (typically 110 W/m · K), and high strengths (500 MPa, or 73 ksi) However, the fracture toughness of both materials is generally low, of the order of 3 to 4 MPa m (2.7

to 3.6 ksi in) The major differences are found in wear and corrosion resistance While both are very good in each

category, direct-sintered SiC has a greater ability to withstand severely corrosive and erosive environments (the limiting factor for reaction-bonded SiC is the silicon metal)

Applications for SiC ceramics are typically in the areas where wear and corrosion are problems For example, SiC is often found as pump seal rings and automotive water-pump seals Silicon carbide's high thermal conductivity also allows them

to be used as radiant heating tubes in metallurgical heat-treatment furnaces

Silicon Nitride. An intense interest in silicon nitride (Si3N4) ceramics has emerged over the past few decades The motivation for such interest lies in the automotive industry, where use of ceramic components in engines would greatly improve operating efficiency Silicon nitride offers great potential in these applications because of its excellent high-temperature strength of 900 MPa (130 ksi) at 1000 °C (1830 °F), high fracture toughness of 6 to 10 MPa m (5.5 to 9 ksi

in), and good thermal shock resistance It also has very good oxidation resistance, a property of particular importance in automotive applications

The automotive components of interest are turbocharger rotors, pistons, piston liners, and valves The greatest application

of Si3N4, however, is as a cutting-tool material in metal-machining applications, where machining rates can be dramatically increased due to the high-temperature strength of Si3N4

Boron carbide, B4C, is another material that is just now finding applications The chief advantages of B4C are its exceptionally high hardness (29 GPa, or 4.2 × 106 psi) and low density (2.50 g/cm3, or 0.09 lb/in.3) However manufacturing B4C is difficult because of the high temperatures necessary to effect densification (>>2000 °C, or 3600

°F) Thus in most cases B4C is densified with pressure, as in hot pressing This limits the complexity of shapes possible without excessive grinding and machining

A disadvantage of B4C is the high cost of the powders and subsequent processing As such, B4C has found use only in applications that demand the unique properties of B4C, namely military armor

SiAlON is an acronym for silicon-aluminum-oxynitride SiAlON is fabricated in several ways, but is typically made by reacting Si3N4 with Al2O3 and AIN at high temperatures SiAlON is a generic term for the family of compositions that can

be obtained by varying the quantities of the original constituents The advantages of SiAlONs are their low thermal expansion coefficient (2 to 3 × 10-6/°C, or 1 to 1.7 × 10-6/°F) and good oxidation resistance

The array of potential applications is similar to that of Si3N4, namely automotive components and machine tool bits However, the chemistry of SiAlON is complex, and reproducibility is a major to becoming more commercially

successful The processing of SiAlONs and their use as cutting-tool materials are discussed in more detail in Machining, Volume 16 of ASM Handbook, formerly 9th Edition Metals Handbook

Zirconia. Pure zirconia cannot be fabricated into a fully dense ceramic body using existing conventional processing techniques The 3 to 5% volume increase associated with the tetragonal-to-monoclinic phase transformation causes any pure ZrO2 body to completely destruct upon cooling from the sintering temperature Additives such as calcia (CaO), magnesia (MgO), yttria (Y2O3, or ceria (CeO2) must be mixed with ZrO2 to stabilize the material in either the tetragonal

or cubic phase Applications for cubic-stabilized ZrO2 (CSZ) include various oxygen-sensor devices (cubic ZrO2 has excellent ionic conductivity), induction heating elements for the production of optical fibers, resistance heating elements

in new high-temperature oxidizing kilns, and inexpensive diamond-like gemstones Partially-stabilized or stabilized ZrO2 systems will be discussed below

tetragonal-Toughened Ceramics. Decades ago, ceramics were characterized as hard, high-strength materials with excellent corrosion and electrical resistance in addition to high-temperature capability However, low fracture toughness limited its use in structural applications The birth of toughened ceramics coincided with industrial applications requiring high-temperature capability, high strength, and an improvement in fracture resistance over existing ceramic materials The primary driving force toward developing toughened ceramics was the promise of an all-ceramic engine Several of the materials discussed

in this section were or are being considered as ceramic-engine component materials

Zirconia-toughened alumina (ZTA) is the generic term applied to alumina-zirconia systems where alumina is considered the primary or continuous (70 to 95%) phase Zirconia particulate additions (either as pure ZrO2 or as stabilized ZrO2 from 5

to 30% represent the second phase (Fig 4) The solubility of ZrO2 in Al2O3 and Al2O3 in ZrO2 is negligible The ZrO2 is present either in the tetragonal or monoclinic symmetry ZTA is a material of interest primarily because it has a significant higher strength and fracture toughness than alumina

Fig 4 Scanning electron micrograph of high-purity, zirconia-toughened alumina showing dispersed zirconia phase (white) within an alumina

matrix

The microstructure and subsequent mechanical properties can be tailored to specific applications Higher ZrO2 contents lead to increased fracture toughness and strength values, with little reduction in hardness and elastic modulus, provided most of the ZrO2 can be retained in the tetragonal phase Strengths up to 1050 MPa (152 ksi) and fracture toughness values as high as 7.5 MPa m (6.8 ksi in) have been measured (Table 4) Wear properties in some applications may also improve due to mechanical property enhancement compared to alumina These types of ZTA compositions have been used in some cutting-tool applications

Table 4 Typical physical properties of various ceramics

Flexure strength Fracture toughness Hardness, Elastic modulus Material Bulk density,

Zirconia-toughened alumina was invented almost 15 years ago However, commercial success has been limited, partly due to the failure of industry to produce a low-cost ZTA with improved properties and its failure to identify markets allowing immediate penetration One exception has been the use of ZTA in some cutting-tool applications

Transformation-toughened zirconia is a generic term applied to stabilized zirconia systems in which the tetragonal symmetry

is retained as the primary zirconia phase The four most popular tetragonal phase stabilizers are CeO2, Y2O3, CaO, and MgO The use of these four additives results in two distinct microstructures MgO- and CaO-stabilized ZrO2 consist of 0.1

to 0.25 μm tetragonal precipitates within 50 to 100 μm cubic grains Firing usually occurs within the single cubic-phase field, and phase assemblage is controlled during cooling

Interest in CaO-stabilized ZrO2 has waned in recent years MgO-stabilized ZrO2 (Mg-PSZ), on the other hand, has enjoyed immense commercial success Its combination of moderate-high strength of 600 to 700 MPa (87 to 100 ksi), high fracture toughness of 11 to 14 MPa m (10 to 13 ksi in, and flaw tolerance enables the use of Mg-PSZ in the most demanding structural ceramic applications The elastic modulus is approximately 210 GPa (30 × 106 psi), and the hardness is approximately 12 to 13 GPa (1.7 to 1.9 × 106 psi) Among the applications for this material are extrusion nozzles in steel production, wire-drawing cap stands, foils for the paper-making industry, and compacting dies Among the toughened or high-technology ceramic materials, Mg-PSZ exhibits the best combination of mechanical properties and cost, for room- and moderate-temperature structural applications

Yttria-stabilized ZrO2 (Y-TZP) is a fine-grain, high-strength, and moderate-high fracture toughness material strength Y-TZPs are manufactured by sintering at relatively low sintering temperatures (1400 °C, or 2550 °F) Nearly 100% of the zirconia is in the tetragonal symmetry and the average grain size is approximately 0.6 to 0.8 μm The tetragonal phase in this microstructure is very stable Higher firing temperatures (1550 °C, or 2800 °F) result in a high-strength (1000 MPa, or 145 ksi), high fracture toughness (8.5 MPa m , or 7.7 ksi in), fine-grain material with excellent wear resistance The microstructure (Fig 5) consists of a mixture of 1 to 2 μm tetragonal grains (90 to 95%) and

High-4 to 8 μm cubic grains (5 to 10%) The tetragonal phase in this microstructure is more readily transformable than above due to the larger tetragonal grain size and a lower yttria content in the tetragonal phase, resulting in a tougher material

Fig 5 Scanning electron micrograph of a Y-TZP sample The larger 3 to 5 μm grains are cubic (~5%); the smaller 1 to 2 μm grains are

tetragonal (~95%)

Among the applications for Y-TZP are ferrules for fiber-optic assemblies Materials requirements include a very grain microstructure, grain-size control, dimensional control, excellent wear properties, and high strength The fine-grain microstructure and good mechanical properties lend the Y-TZP as a candidate material for knife-edge applications, including scissors, slitter blades, knife blades, scalpels, and so forth However, compared to Mg-PSZ, Y-TZP is more expensive, has a lower fracture toughness, and is not nearly as flaw tolerant

fine-There are some temperature limitations in these materials Mechanical strength of both Mg-PSZ and Y-TZP may start to deteriorate at temperatures as low as 500 °C (930 °F) Also, the Y-TZP ceramic is susceptible to severe degradation at temperatures between 200 to 300 °C (400 to 570 °F)

Composite Ceramics. The early success of ZTA and partially-stabilized zirconia systems provided the impetus to include toughened ceramics as a candidate for structural applications However, due to the limited maximum-temperature use of these materials, intense research was generated to determined other toughening mechanisms (besides transformation toughening and dispersed-phase toughening) and alternative toughened-ceramic systems

Silicon carbide whisker (SiC w )-reinforced alumina surfaced in the last decade as a potential ceramic-engine component material Composed of fine equiaxed alumina grains and needlelike SiC whiskers, this material exhibited promising fracture toughness (6.5 MPa m, or 5.9 ksi in) and strength (600 MPa, or 87 ksi) properties Al2O3-SiCw composites have been used quite successfully in cutting-tool applications These composites may also overcome the severe obstacles that currently prevent the use of ceramic materials in some aluminum can tooling applications

Conventional processing methods can be employed provided the whisker loading is less than approximately 8 vol% Composites with higher whisker loadings must be hot pressed, or sufficient liquid-glass-phase sintering must occur to fabricate fully dense bodies The former limits the fired billet size and requires extensive grinding after sintering The latter limits its high-temperature use

Silicon Nitride Matrix Composites. High-temperature degradation of the mechanical properties of Al2O3-SiCw composites and the excellent high-temperature strength, oxidation resistance, thermal shock resistance and fracture toughness of Si3N4

caused a recent thrust of interest in fabricating SiCw-reinforced Si3N4 The major phase, Si3N4, offers many favorable properties and the SiC whiskers provide significant improvement in the fracture toughness of the composite Whisker-reinforced Si3N4 is now being touted as the material of choice for hot-section ceramic-engine components, although production is currently limited to laboratory or pilot plant-size fabrication Processing difficulties, health issues, and raw material costs of all of the SiC whisker-reinforced composites have lessened the industrial impact of these materials and may prevent widespread acceptance and use in the near future

Future Directions and Problems

One of the primary disadvantages of ceramic materials is their brittle nature, characterized by a low fracture toughness Although significant improvements have been made to increase the fracture toughness, brittleness continues to keep ceramics from more widespread use

Ceramic Composites. One direction that shows promise is that of composite materials For example, silicon carbide whiskers have been incorporated into an aluminum oxide matrix, resulting in a composite with greatly improved toughness The toughening mechanism is probably a combination of whisker pullout and crack bridging, whereby SiC whisker effectively resists crack propagation Other types of ceramic-ceramic composites would include adding a transforming phase such as zirconia to a host matrix, allowing transformation toughening to improve the fracture toughness of ceramics

Metal-Ceramic Composites (Cermets). Another class of composites is metal-ceramic composites (cermets) In this case, a ductile metal phase is incorporated into the brittle ceramic In the event of a propagating crack, the crack interacts with the metal phase, and the metal then begins to plastically deform This deformation absorbs energy, acting to increase the toughness of the composite The development and commercial use of various metal-ceramic composites are the subject of the article "Cermets" in this Volume

Processing. Another area of importance is the science and technology of ceramic processing, both from an economic and performance sense Currently manufacturing ceramics is a labor- and capital-intensive industry, where products are often custom-made for customers Manufacturers are continually striving to increase productivity and reduce costs, very often through intense process engineering and optimization

Improved processing techniques should also enhance the performance of structural ceramic components, particularly with respect to reliability Currently ceramics tend to be very flaw sensitive, in that the strength depends on the size of flaw in the microstructure The flaw size in turn is usually determined by processing conditions In most ceramics, conventional processing results in a fairly broad flaw size distribution, which yields a broad strength distribution Since design engineers often need to know the average strength and strength deviation, a large standard deviation will limit the design strength of a component Therefore, improved processing techniques should reduce the spread in strengths and allow greater opportunities for ceramics in structural applications

• "Niobium-Titanium Superconductors" (the most widely used superconductor)

• "A15 Superconductors" (in which class the important material Nb3Sn lies)

• "Ternary Molybdenum Chalcogenides (Chevrel Phases)"

• "Thin-Film Materials"

• "High-Temperature Superconductors for Wires and Tapes"

Even with this broad view, however, only a brief flavor of the breadth of the superconducting state and its applications can be given here

At the beginning of the 1990s, the science and applications of superconductivity find themselves in an interesting state A vigorous industry has grown up around the applications of low-temperature niobium-base superconductors This includes

a superconducting electronics industry and a substantial industry producing superconducting magnets Few large laboratories are now without a superconducting magnet, whether used for physical property measurements, nuclear magnetic resonance (NMR) and other resonance experiments, or for investigations of superconductivity itself Magnetic resonance imaging (MRI) magnets for the NMR imaging of the whole human body are installed in thousands of hospitals worldwide, and enormous magnet assemblies for particle accelerators and plasma fusion experiments have been built

At present the largest superconducting device is the Tevatron, the 4.8 km (3 mile) circumference 1000 GeV proton accelerator at Fermilab near Chicago This consists of about 1000 6 m (20 ft) long superconducting magnets The proven success of this device was vital to the decision to construct the Superconducting Super Collider (SSC) The SSC, now beginning its construction phase near Dallas, will be about 80 km (50 miles) in circumference and contain about 10,000

20 m (65 ft) long superconducting magnets

The great vitality of the superconducting community has been enormously enhanced by the amazing and very unexpected discovery in early 1986 by Bednorz and Muller of high-temperature superconductivity in the rare earth cuprates (Ref 1)

A rapid phase of new discovery quickly produced several new classes of high-temperature superconductors (Ref 2) Enormously important issues of basic physics are posed by the existence of superconductivity at temperatures as high as

125 K and magnetic fields of greater than 50 T (500 kG) At the same time, the potential for applications is enormous Before considering these issues further, however, it is instructive to go back to the beginning at superconductivity and trace the development of its technology This overview will provide a foundation for understanding the basic science and potential applications of superconductivity

References

1.J.G Bednorz and K.A Muller, Z Phys., Vol B64, 1986, p 189

2.J.C Philips, Physics of High T c Superconductors, Academics Press, 1989

Historical Development

The superconducting state was an unexpected outcome of the low-temperature researches of a group led by Kamerlingh Onnes at the University of Leiden (Holland) in 1911 Onnes discovered that mercury lost all resistance when cooled to about 4 K Two years later, he came to Chicago to report to the third International Conference of Refrigeration (1913) At this time he reviewed the recent research of the Leiden group (Ref 3) This article is quite astonishing, and only extensive quotations can convey the breadth of Onnes's conception of the possibilities of the superconducting state Onnes commences by describing his initial 1911 experiments on mercury and then proceeds to rapidly sketch whole segments of the technology of superconducting magnets:

Mercury has passed into a new state, which on account of its extraordinary electrical properties may be called the superconductive state The behavior of metals in this state gives rise to new fundamental questions as to the mechanism

of electrical conductivity

It is therefore of great importance that tin and lead were found to become superconductive also Tin has its step-down point

at 3.8° K, a somewhat lower temperature than that of the vanishing point of mercury The vanishing point of lead may be put at 6° K Tin and lead being easily workable metals, we can now contemplate all kinds of electrical experiments with apparatus without resistance

The extraordinary character of this state can be well elucidated by its bearing on the problem of producing intense magnetic fields with the aid of coils without iron cores Theoretically it will be possible to obtain a field as intense as

we wish by arranging a sufficient number of amperewindings round the space where the field has to be established This is the idea of Perrin, who made the suggestion of a field of 100,000 gauss being produced over a fairly large space in this way He pointed out that by cooling the coil by liquid air the resistance of the coil and therefore the electric work to maintain the field could be diminished In order to get a field of 100,000 gauss in a coil with an internal space of 1 cm radius, with copper as metal, and cooled by liquid air 100 kilowatt would be necessary The electric supply, as Fabry remarks, would give no real difficulty, but it would arise from the development of Joule-heat in the small volume of coil, the dimensions of which are measured by centimeters, to the amount of 25 kilogram-calories per second, which in order to

be carried off by evaporation of liquid air would require about 0.4 liter of liquid air per second, let us say about 1500 liters

of liquid air per hour

But greatest difficulty, as Fabry points out, resides in the impossibility of making the small coil give off the relatively enormous quantity of Joule-heat to be liquefied gas The dimensions of the coil to make the cooling possible must be much larger, by which at the same time the electric work and the amount of liquefied gas required becomes greater in the same proportion The cost of carrying out Perrin's plan even with liquid air might be about comparable to that of building a cruiser

We should no more get a solution by cooling with liquid helium as long as the coil doest not become superconductive.The problem which seems hopeless in this way enters a quite new phase when a superconductive wire can be used Joule-heat comes not more into play, not even at very high current densities, and an exceedingly great number of amperewindings can be located in a very small space without in such a coil heat being developed A current of 1000 amp/mm2 density was sent through a mercury wire, and of 460 amp/mm2 density through a lead wire, without appreciable heat being developed in either

There remains of course the possibility that a resistance is developed in the superconductor by the magnetic field If this were the case, the Joule-heat depending on this resistance would have been withdrawn One of the first things to be investigated as soon as the appliances, which are arranged for making the projected researches on magnetism at helium-temperatures, will be ready, will be this magnetic resistance We shall see that this plays no role for fields below say 1000 gauss

The insulation of the wire was obtained by putting silk between the windings, which being soaked by the liquid helium brought the windings as much as possible into contact with the bath The coil proved to bear a current of 0.8 ampere without losing its superconductivity There may have been bad places in the wire, where heat was developed which could not be withdrawn and which locally warmed the wire above the vanishing point of resistance

I think it will be possible to come to a higher current density if we secure a better heat conduction from the bad places

in the wire to the liquid helium in a coil of bare lead wire wound on a copper tube the current will take its way, when the whole is cooled to 1.5° K, practically exclusively through the windings of the superconductor If the projected contrivance succeeds and the current through the coil can be brought to 8 amperes we shall approach to a field of 10,000 gauss The solution of the problem of obtaining a field of 100,000 gauss could then be obtained by a coil of say 30 centimeters in diameter and the cooling with helium would require a plant which could be realized in Leiden with a relatively modest financial support When all outstanding questions will have been studied and all difficulties overcome, the miniature coil referred to may prove to be the prototype of magnetic coils without iron, by which in future much stronger and at the same time much more extensive fields may be realized then are at present reached in the interferrum of the strongest electromagnets As we may trust in an accelerated development of experimental science this

future ought not to be far away.

What a description! Many of the points essential to the development of a proper magnet technology were sketched out by Onnes already in 1913 His vision of powerful magnets, the problem of heat removal from compact windings, the attractive economic feasibility of superconducting as opposed to resistive magnets, operation at current densities of 1000 A/mm2 and temperatures down t 1.5 K all of these are crucial aspects of our present superconducting magnet technology Elsewhere in the same article he describes the melting of superconducting wires following an abrupt transition from the superconducting to the normal state and perhaps prefigures modern composite conductor manufacture by considering the properties of a resistive constantan wire coated with a superconducting layer of tin

Returning to Onnes's own words, it is only the last sentence that strike a false note An accelerated progress to applications was reasonable to dream about but it did not happen The reason is clearly delineated in a footnote to Onnes's paper: The passage of the electric current through the superconducting wire easily produced a magnetic field of about 0.05 T (500 G), which though weak was sufficiently strong to quench the superconducting state of a type I superconductor such as lead, tin, or mercury Sadly, more than 20 years passed before there was much understanding of this issue

In the early 1930s the thermodynamic aspects of the superconducting-to-normal transition were established by Meissner and Ochsenfeld by Gorter and Casimir (Ref 4) As we know now, the crucial step to applications would have been to identify how to make the transition from a (low-field) type I superconductor to a (high-field) type II superconductor (see the article "Principles of Superconductivity" in this Section for an explanation of these terms) This work was in fact underway

The systematic effects of alloying lead with indium, tellurium, and similar solutes were carried out by Shubnikov's group

in Kharkov (Ukrainian Republic) in the period 1935 to 1937, and the basic thermodynamic aspects of the transition the

appearance of a lower (Hc1) and an upper critical field (Hc2) in the alloys in place of the single small critical field (Hc) of pure lead were all identified These were crucial observations Tragically, Shubnikov's work remained unappreciated by the scientific community as a whole In 1937 Shubnikov was falsely denounced and sent to a labor camp, dying in prison

in 1945 As a political prisoner his work could not be cited by his fellow Soviet scientists, and the scanty accounts of his early researches that had appeared in the Western literature were ignored An alternative erroneous hypothesis the filamentary sponge model, which inherently regarded the high-field superconducting properties as being associated with microscopic metallurgical inhomogeneities was then used to explain the occasional reports of high-field superconductivity (Ref 4)

Further advances had to wait until the 1950s when Soviet theoreticians Ginzburg a Landau addressed the phenomenology

of the superconducting-to-normal transition in a magnetic field It is very striking to recall that event then, Ginzburg and Landau rejected as unphysical those solutions that predicted type II superconductivity It was left to a persistent student of Landau's (Abrikosov) to explore the "unphysical" type II state This work was published only in 1957 (Ref 5)

It took the serendipitous experiments of Kunzler et al in late 1960 (Ref 6) to convince the experimentalists that type II

superconductivity could indeed realize Onnes's 1913 dreams Kunzler's experiment showed that a prototype wire of

Nb3Sn could carry a supercurrent of more than 105 A/cm2 in a field of 8.8 T (88.8 kG) Compared to copper, which might operate (resistively) at 103 A/cm2, the advantages of superconductors for high-field magnets became widely appreciated

A rapid advance to applications proceeded during the 1960s, culminating in a wide range of applications in both magnetic-field and electronic devices (Ref 7, 8, 9)

high-During the 1960s, 1970s, and 1980s there was a continued interest in the search for new superconductors However,

higher Tc materials were hard to find The A15 compound Nb3Sn held the record of 18 K in 1960, and no advance beyond the 23 K of Nb3Ge was obtained after 1973 In extensive reviews of the field in 1986 (75 years after the discovery of

Onnes), virtually no attention was paid to the prospect of developing materials having higher Tc (Ref 7, 8, 9) The community had run out of collective ideas

Fortunately, however, one group at least, that of Muller in Switzerland, was still pursuing higher Tc materials After several years of unsuccessful efforts, their researches were crowned with success A mixed-phase ceramic of La-Ba-Cu-O

exhibited a Tc onset of about 40 K (Ref 1) Within a very short time Tc had been raised to 92 K (YBa2Cu3O7-x), 110 K (Bi2Sr2Ca2Cu3Ox), and 125 K (Tl2Ba2Ca2Cu3Ox) (Ref 2) Because major expectations for exciting new physics and

applications lie with these materials, we confidently expect that the next edition of Metals Handbook will require a

comprehensive rewrite of the present introduction

References cited in this section

1 J.G Bednorz and K.A Muller, Z Phys., Vol B64, 1986, p 189

2 J.C Philips, Physics of High Tc Superconductors, Academics Press, 1989

3 H Kamerlingh Onnes, Comm Physical Lab Leiden Suppl., No 34b, 1913

4 T Berlincourt, Type II Superconductivity: Quest for Understanding, IEEE Trans on Magn., Vol 23 (No 2),

March 1987, p 403-412

5 A.A Abrikosov, Sov Phys JETP, Vol 5, 1957, p 1174

6 G Kunzler, Recollection of Events Associated With the Discovery of High Field-High Current

Superconductivity, IEEE Trans Magn., Vol 23 (No 2), March 1987, p 396-402

7 Phys Today Spec Issue: Supercond., Vol 39 (No 3), March 1986, p 22-80

8 Kamerlingh Onnes Symposium on the Origins of Applied Superconductivity 75th Anniversary of the

Discovery of Superconductivity, IEEE Trans on Magn., Vol 23 (No 2), March 1987, p 354-415

9 Superconducting Devices, S Ruggiero and D Rudman, Ed., Academic Press, 1990

• References 2 and 3 for a general overview of superconductivity and its applications

• Reference 4 for a midlevel introduction to the theory of superconductivity

• Reference 5 for a comprehensive survey of filamentary superconductors in magnet applications

• References 6 and 7 for technical information on superconducting materials and applications

• References 8 and 9 for an advanced treatment of superconductivity theory

The breadth of this article is further restricted to focus primarily on the principles of superconductivity as they relate to applications As a result, details of the quantum theory and thermodynamics of superconductivity will be largely left to the references The few equations that are described use the International System of Units (SI)

Table 1 Approximate superconducting properties of selected superconducting materials

Coherence length (ξ )

nm

Critical current density

0.03 0.01(c) >200(c) 1000(c) 2-3(c)

1 (at 77 K, 0 T)(d)

(a) Thermodynamic critical field at 0 K

(b) Measured with field parallel to the c-axis

(c) Measured with field parallel to the a-b plane

(d) Epitaxial thin film, current in the a-b plane

Fig 1 Periodic table of the elements showing the large number of elements known to have superconducting transitions