In titanium turning operations where speed and feed parameters are held constant, tool life improvements of 200% have been recorded after a change to submicron carbide cutting tool mater
Trang 1Fig 35 Proprietary 55° parallelogram carbide insert
An undesirable by-product of thread machining is the V-shaped chip removed from the workpiece Indexable inserts with proprietary chip control geometries molded into the top rake surface of the cutting edge are becoming available, and they control and break the chips with varying levels of success In the more common thread forms, multitooth thread-chasing inserts are available as a means of reducing the number of passes required to complete a thread, thus improving productivity (Fig 36) An increasingly popular option available for many thread forms is the cresting insert, which machines the full thread form Noncresting inserts machine the root and flanks but not the crest of the thread
Fig 36 A multitooth thread-chasing insert
Thread milling, a thread machining method that is useful when turning is not possible, is performed on multiaxis computer numerical control machines capable of helical interpolation (Fig 37) A disadvantage of thread milling is that the thread form it produces is slightly imperfect because of the inability of the cutting tool to clear the helical angle of the thread form as it exits the part However, the threads are sufficiently accurate for all but the most demanding applications
Fig 37 Thread milling with indexable inserts
Grooving. There are three different grooving insert styles in common use:
• 90° V-bottom (Fig 38)
• Proprietary stand-up 55° parallelogram (Fig 35)
Trang 2• Stand-up (on-edge) triangle
The V-bottom system is the most suitable for deep grooving because the cutting edge of the insert is wider than the body and is directly supported by the toolholder The compact design and proprietary clamping method of the 55° parallelogram system maximize rigidity in shallow grooving The on-edge triangle system offers three cutting edges on each insert, as opposed to two in the other common grooving systems Chip control is a major concern in grooving, and products are becoming available that control and/or break chips with varying levels of success
Fig 38 90° V-bottom (dogbone) grooving inserts and toolholders
Cutoff. The early carbide cutoff tools consisted of carbide inserts brazed onto steel shanks As in the case of carbide
turning inserts, efforts to eliminate tool re-sharpening costs and to improve performance led to the development of mechanically held replaceable cutoff inserts These inserts are available in a variety of styles, but most have a vee shape
in the top or bottom surface, which is gripped by the steel holder for rigidity Most cutoff inserts have a single cutting edge and are held either by clamping or by wedging directly into the holder Chip control is available in either molded or ground geometries (Fig 39)
Trang 3Fig 39 Cutoff inserts and toolholders
Machining Applications
Uncoated Straight WC-Co Grades. Despite the advent of coated cemented carbide tools in the late 1970s, uncoated straight WC-Co grades still find a place in many machining operations Unalloyed gray cast iron is probably the most common workpiece material machined with WC-Co grades, but other materials such as high-temperature alloys, austenitic stainless steels, nonferrous alloys, and nonmetals are often machined with C1 and/or C2 grades The recommended speed and feed ranges for these materials are listed in Table 9 The higher-cobalt C1 grades are also often used on difficult-to-machine workpieces, such as chilled cast iron and heat-treated steels, where cutting tool strength and shock resistance are important High-temperature alloys and austenitic stainless steels are machined with C2 grades, typically in positive-rake, sharp-edge geometries at lower surface speeds
Table 9 Recommended speed and feed ranges for uncoated straight WC-Co grades
0.10- 0.014
0.10- 0.015
0.08- 0.010
0.003-91-183
300-600
0.30
0.08- 0.012
0.003-Gray cast iron
0.05- 0.006
0.002-91-137
300-450
0.25
0.05- 0.010
0.15- 0.016
0.006-84-137
275-450
0.51
0.13- 0.020
0.08- 0.012
0.003-68-122
225-400
0.38
0.08- 0.015
60-140
0.25
0.13- 0.010
0.005-23-61 75-200
0.08-0.30
0.012
0.08- 0.008
0.003-18-43 60-140
0.05-0.23
0.009
0.20- 0.020
0.008-91-366
300-1200
0.30
0.13- 0.012
Trang 40.005-(a) Depth of cut greater than 2.5 mm (0.100 in.)
Uncoated alloyed carbide grades (C5 to C8 carbide classifications) are primarily used in machining steels in the speed and feed ranges listed in Table 10 Uncoated alloyed carbides are well suited to a number of machining applications They are widely used when machines do not have sufficient horsepower to utilize the high metal removal capabilities of advanced coated tools Uncoated carbides also find application in brazed form tools, which are typically made with highly specialized geometries mirroring the part being machined
Table 10 Recommended speed and feed ranges for uncoated alloyed carbide grades
0.30- 0.025
0.13- 0.015
0.20- 0.020
0.008-91-152
300-500
0.30
0.08- 0.012
0.20- 0.020
0.008-91-137
300-450
0.51
0.13- 0.020
0.30- 0.025
0.012-91-122
300-400
0.51
0.13- 0.020
175-300
0.51
0.25- 0.020
0.010-76-106
250-350
0.41
0.10- 0.016
0.004-Alloy steels
125-250
0.38
0.15- 0.015
0.006-46-91
150-300
0.30
0.08- 0.012
0.25- 0.025
0.010-91-152
300-500
0.51
0.10- 0.020
Uncoated alloyed carbides are also employed in the machining of special part configurations with thin wall sections and tight tolerances These parts cannot be subjected to high forces during machining The CVD-coated tools, which are honed prior to coating, are not effective in these applications, for which high positive, sharp-edged, uncoated alloyed carbides are a better choice The recently developed PVD-coated inserts with sharp edges may prove effective in such applications
Coated carbide grades provide more abrasion resistance to the tool and permit the use of higher machining speeds and feeds This is illustrated in the tool life diagram shown in Fig 40, which compares the performance of a C5 grade with and without TiC-TiCN-TiN coating in turning SAE 1045 steel
Trang 5Fig 40 Tool life comparison of a coated and an uncoated carbide tool Constant tool life (15 min) plot for an
uncoated and a TiC-TiCN-TiN-coated C5 grade in turning SAE 1045 steel The depth of cut was 2.5 mm (0.100 in.)
More than 50% of the metal cutting inserts currently sold in the United States are CVD coated These coatings are used in
a wide range of machining applications, including turning, milling, threading, and grooving Physical vapor deposited coatings can be applied to sharp insert edges without the deleterious effect of phase formation The sharp, tough PVD inserts are particularly well suited to milling applications; they have also been found to perform well in threading and grooving operations on difficult-to-machine materials such as high-temperature alloys and austenitic stainless steels
Submicron Grades for Aerospace Materials. Although new cutting tool materials such as Sialons and reinforced ceramics have provided great increases in machining productivity on nickel-base alloys, similar improvements have not occurred in the machining of titanium alloys However, submicron (fine-grain) carbide grades have shown the capacity to enhance productivity in both titanium alloys and the nickel-base materials In general, the strength or toughness of a carbide grade is inversely proportional to its abrasion resistance The microstructures associated with submicron carbide materials provide both strength and abrasion resistance In titanium turning operations where speed and feed parameters are held constant, tool life improvements of 200% have been recorded after a change to submicron carbide cutting tool materials
whisker-References
1 H Moissan, The Electrical Furnace, V Lenher, Trans., Chemical Publishing Company, 1904
2 E.K Storms, The Refractory Carbides, Academic Press, 1978
3 M Hansen and K Anderko, Constitution of Binary Alloys, McGraw-Hill, 1958
4 K Schroeter, U.S Patent 1,549,615, 1925
5 E.M Trent, Cutting Tool Materials, Metall Rev., Vol 13 (No 127), 1948, p 129-144
6 K.J.A Brookes, World Directory and Handbook of Hardmetals, 4th ed., International Carbide Data, 1987
7 E Lardner, Powder Metall., Vol 21, 1978, p 65
8 H.E Exner, Int Met Rev., Vol 24 (No 4), 1979, p 149-173
9 P.M McKenna, U.S Patent 3,379,503, 1968
10 P.M McKenna, Tool Materials Cemented Carbides, in Powder Metallurgy, J Wulff, Ed., 1942, p 454-469
11 H.D Hanes, D.A Seifert, and C.R Watts, Hot Isostatic Processing, Battelle Press, 1979, p 20-24
Trang 612 R.C Lueth, Advances in Hardmetal Production, in Proceedings of the Metal Powder Report Conference
(Luzern), Vol 2, MPR Publishing Services Ltd., 1983
13 P.A Dearnley, Met Technol., Vol 10, 1983
14 H Tanaka, Relationship Between the Thermal, Mechanical Properties and Cutting Performance of TiN-TiC
Cermet, in Cutting Tool Materials, Conference Proceedings, American Society for Metals, 1981, p 349-361
15 R.C Lueth, Fracture Mechanics of Ceramics, R.C Bradt et al., Ed, Plenum Press, 1974, p 791-806
16 J.L Chermant and F Osterstock, J Mater Sci., Vol 11, 1976, p 1939-1951
17 J.R Pickens and J Gurland, Mater Sci Eng., Vol 33, 1978, p 135-142
18 W.D Kingery, H.K Bowen, and D.R Uhlmann, Introduction to Ceramics, 2nd ed., John Wiley & Sons,
1960, p 828
19 C.S Ekmar, German Patent 2,007,427
20 W Schintlmeister, O Pacher, and K Pfaffinger, in Chemical Vapor Deposition, Fifth International
Conference, J.M Blocker, Jr et al., Ed., The Electrochemical Society Softbound Symposium Series,
Electrochemical Society, 1975, p 523
21 W Schintlmeister, O Pacher, K Pfaffinger, and T Raine, J Electrochem Soc., Vol 123, 1976, p 924-929
22 V.K Sarin and J.N Lindstorm, J Electrochem Soc., Vol 126, 1979, p 1281-1287
23 W Schintlmeister, W Wallgram, J Ganz, and K Gigl, Wear, Vol 100, 1984, p 153-169
24 B.N Kramer and P.K Judd, J Vac Sci Technol., Vol A3 (No 6), 1985, p 2439-2444
25 T.E Hale, Paper presented at the International Machine Tool Show Technical Conference, National Machine Tools Builders Association, 1982
26 H.E Hintermann, Wear, Vol 100, 1984, p 381-397
27 B.J Nemeth, A.T Santhanam, and G.P Grab, in Proceedings of the Tenth Plansee Seminar (Reutte/Tyrol),
Metallwerk Plansee A.G., 1981, p 613-627
28 A.T Santhanam, G.P Grab, G.A Rolka, and P Tierney, An Advanced Cobalt-Enriched Grade Designed to
Enhance Machining Productivity, in High Productivity Machining Materials and Processes, Conference
Proceedings, American Society for Metals, 1985, p 113-121
29 R.F Bunshah and A.C Raghuram, J Vac Sci Technol., Vol 9, 1972, p 1385
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31 D.T Quinto, G.J Wolfe, and P.C Jindal, Thin Solid Films, the International Conference on Metallic
Composition and Microstructure
Trang 7Ceramics are defined as any class of inorganic or nonmetallic products that are subjected to high temperature during manufacture or use (Ref 1) Typically, but not exclusively, a ceramic is a metallic oxide, boride, or carbide, or a mixture
or compound of such materials By this definition of ceramics, the following materials theoretically fall into the group of cermets:
• WC + Co (Tungsten carbide + cobalt)
• WC/TiC/TaC + Co (Tungsten carbide/titanium carbide/tantalum carbide + cobalt)
• TiC + Ni (Titanium carbide + nickel)
• Ti(C,N) + Ni/Mo (Titanium carbonitride + nickel/molybdenum)
However, the cutting tool industry considers only the titanium carbide and titanium carbonitride base materials to be cermets, while the tungsten carbide based materials are named cemented carbides Therefore, this article will concentrate
on cermets based on titanium carbide and titanium carbonitride
Titanium Carbide Cermets. The first attempts to apply titanium carbide in sintered, tungsten-free cutting tool materials were made in Germany in 1929, when titanium carbide/molybdenum carbide solid solutions with 15% nickel as binder were manufactured and applied for finish turning of steel (Ref 2) Acceptance was limited because of the low strength and high brittleness of this material However, the interest in titanium carbide continued, mainly due to the lower cost and availability of its raw material, titanium oxide (TiO2) Also, the higher hardness, melting point, and oxidation resistance of titanium carbide (TiC) compared to that of tungsten carbide (WC) promised greater potential
The poor wettability of titanium carbide (TiC) with nickel (Ni) was improved drastically with the addition of molybdenum (Mo) or molybdenum carbide (Mo2C) to the nickel binder phase (Ref 3) The microstructure of such a composition is shown schematically in Fig 1 The core of the carbide phase consists of titanium carbide ( 1-phase), while the rim is enriched with molybdenum carbide ( 2-phase) (Ref 4 and 5) The molybdenum from the binder phase diffuses into the carbide phase and improves wettability by means of the metal binder The abrasion resistance of such a composition varies with the sintering temperature (Fig 2)
Fig 1 Schematic of cermet microstructure
Trang 8Fig 2 Flank wear of titanium carbide cermet sintered at different temperatures Machining parameters: feed,
0.28 mm/rev (0.011 in./rev); depth of cut, 2.5 mm (0.100 in.); speed, 106 m/min (350 sfm) Workpiece: 1045 steel (163 to 174 HB)
Cermets Based on Metal Carbonitrides. The development of cermets continued with the introduction of metal carbonitrides Titanium-molybdenum-carbon-nitrogen and titanium-tungsten-carbon-nitrogen compounds with metal binders, preferably consisting of nickel, molybdenum, cobalt, or a combination thereof, gained specific attention Considerable improvement was achieved when the carbonitride phase had a composition within the parameters described
in Fig 3
Trang 9Fig 3 Preferred compositions of titanium carbonitride cermets M, molybdenum and/or tungsten; z, number of
moles carbon and nitrogen divided by the number of moles titanium and M; z is variable between the limits
0.80 and 1.07 Source: Ref 6
The microstructure of such a material is shown in Fig 1 It is possible to observe a core/rim microstructure of the carbonitride phase, with the 1-phase of the core consisting of titanium carbonitride and the 2-phase of the rim being enriched with molybdenum carbide and/or tungsten carbide (Ref 4)
The nitrogen additions to the hard phase resulted in higher wear resistance (Fig 4) and reduced plastic deformation of the cutting edge (Fig 5) Additions of cobalt to the binder phase and of tantalum and/or niobium to the hard phase of complex metal carbonitrides also can improve the cutting performance of cermets The high-temperature properties of a complex metal carbonitride is compared with a titanium carbide cermet in Table 1
Table 1 Comparison of high-temperature properties of a TiC cermet and a complex carbonitride cermet
Transverse rupture strength at
900 °C
(1650 °F)
Composition of cermet Vickers hardness
at 1000 °C (2000 °F), kg/mm2
MPa ksi
Oxidation resistance at
1000 °C (2000 °F) weight gain, mg/cm2 · h
Thermal conductivity at
1000 °C (2000 °F), W/K6 · m
Trang 10Fig 4 Comparison of flank wear for two cermets and a cemented carbide when turning 4340 steel Source: Ref
6
Fig 5 Effect of titanium nitride addition on the plastic deformation of a cutting edge Workpiece, 4340 steel
(300 HB) Source: Ref 7
Properties and Grade Selection
The manufacturers of cutting tool materials treat the compositions and properties of their grades as proprietary The commercially available grades fall into two categories: the titanium carbide base cermets and the titanium carbonitride base cermets The titanium carbide base cermet grades are in the process of being replaced by the titanium carbonitride base cermet grades because of their higher wear resistance (Fig 4), hardness, and transverse rupture strength (Fig 6) Typical properties of titanium carbonitride cermets are shown in Table 2
Trang 11Table 2 Typical properties of titanium carbonitride cermets
(a) The ISO and U.S classification systems for cemented tungsten carbide can be used to designate
cermet grades The ISO and U.S classification system, are defined in the article "Cemented Carbides" in this Volume
(b) Cermet grades of this group are typically based on the system (Ti,Mo/W) (C,N) and are referred to
as molybdenum carbide toughened cermets
(c) Cermet grades of this group are typically based on the system (Ti, Mo/W, Nb, Ta) (C,N) and are
referred to as tantalum/niobium carbide toughened cermets
Fig 6 Relationship between hardness and modulus of rupture for various cutting materials Source: Ref 8
Cermets Compared With Cemented Carbides. Titanium carbide and titanium carbonitride cermets are more wear resistant than cemented carbides (Fig 4) and allow higher cutting speeds than tungsten carbides or coated carbides The following additional comparisons with cemented tungsten carbides are helpful in the successful application of titanium carbide and titanium carbonitride cermets
The hardness of titanium carbonitride cermets is approximately comparable to that of cemented carbides The influence of temperature on hardness follows similar patterns because the metal binder system and its volume determine this behavior
The strength of cemented carbides is about 15 to 25% higher than the strength of titanium carbonitride cermets As a result, feed rates and depth of cut have to be selected more conservatively, specifically in roughing operations, in comparison to cemented carbides
Trang 12The toughness range of titanium carbonitride cermets is smaller than that of cemented carbides, limiting the use of these cermets in heavy roughing applications However, the fracture toughness of TiC-base cermets may be in the same range as that of WC-Co, at equivalent binder volumes and carbide grain sizes
The temperature shock resistance of titanium carbonitride cermets is lower than that of cemented carbides and restricts the use of coolant in roughing applications Coolant is applicable in threading, grooving, and finish turning applications
Wear Resistance. The wear mechanisms of a cutting edge are closely related to cutting temperature (Fig 7) Low cutting temperatures produce pressure welding, which results in a built-up edge while high cutting temperatures enhance diffusion and oxidation processes Diffusion processes between the chip and the top rake surface of the cutting edge result
in crater wear, and oxidation reactions with the environment produce scaling of the cutting edge
Fig 7 Wear mechanisms of cutting tools
Oxidation, diffusion, and pressure welding are thermochemical processes which are roughly correlated to the formation enthalpy of the materials involved Figure 8 shows the free-formation enthalpy of various cutting tool materials Titanium nitride and titanium carbide, the base materials for cermets, considerably surpass tungsten carbide Built-up edge, scaling, and crater wear are greatly reduced with cermet cutting tools, and a wider temperature range and higher cutting speeds are possible with cermet cutting tools because of their higher resistance to diffusion and oxidation processes
free-Fig 8 Free-formation enthalpy of cutting materials at 260 °C (500 °F)
Trang 13Molybdenum Content. Abrasion resistance is affected by the molybdenum content in the nickel binder of titanium carbide cermets Figure 9 shows the flank wear as a function of the molybdenum content in the nickel binder, and Fig 10 compares the flank wear of a titanium carbide/molybdenum carbide solid solution material and titanium carbide with molybdenum and molybdenum carbide added to the nickel binder
Fig 9 Cermet tool life with varying molybdenum content Machining parameters: feed, 0.28 mm/rev (0.011
in./rev); depth of cut, 2.5 mm (0.100 in.); speed, 180 m/min (600 sfm); coolant workpiece: 1045 steel (163 to
174 HB) Tool: 80TiC-20Ni, Mo Source: Ref 3
Trang 14Fig 10 Comparison of wear of cermet with molybdenum in three different states: Mo2 C in solid solution with TiC, Mo2C in the binder phase, and Mo in the binder phase Machining parameters: feed, 0.28 mm/rev (0.011 in./rev); depth of cut, 1.5 mm (0.060 in.); speed, 365 m/min (1200 sfm); coolant Workpiece: AISI 1045 (192
to 201 HB) Source: Ref 3
Applications
Cermet cutting tools are suitable for the machining of steels, cast irons, cast steels, and nonferrous free-machining alloys,
as detailed in Table 3 Cermet cutting tools are capable of operating at higher cutting speeds than cemented or coated carbides, thus allowing better surface finishes with cermet cutting tools Some comparisons of surface finish are shown in Fig 11 and Fig 12
Table 3 Machining applications of cermet cutting tools
Rough and semifinish turning; milling, grooving, and threading
Carbon steels, alloy steels, cast irons; free-machining and nonferrous materials, such as aluminum, copper, brass, and bronze, with hardnesses
up to about 38 HRC
Molybdenum carbide
toughened cermet
Finish turning and milling Carbon steels, alloy steels, tool steels, stainless steels, and cast irons;
free-machining nonferrous materials, such as aluminum, copper, brass, and bronze, with hardnesses up to about 38 HRC
Fig 11 Comparison of surface finishes of cermet and cemented tungsten carbide tools Machining parameters:
cutting speed, 250 m/min (825 sfm); feed rate, 0.30 mm/rev (0.012 in./rev); depth of cut, 3.0 mm (0.12 in.); dry, no coolant Workpiece: 1045 steel
Trang 15Fig 12 Comparison of theoretical surface finish with actual surface produced with cermet tool Theoretical
value = f2/8R, where f = feed rate and R = tool nose radius Machining parameters: cutting speed, 200 m/min
(650 sfm); depth of cut, 2.0 mm (0.080 in.) Workpiece: 4135 steel
Cermet cutting tools are generally used in the form of indexable inserts Neither brazed tools nor solid cermet tools
have reached any commercial significance Cermet indexable inserts are manufactured by applying the basic process steps suitable for cemented carbide indexable inserts The similarity of the manufacturing process results not only in manufacturing costs comparable to uncoated cemented carbides, but also allows the manufacture of inserts in shapes and sizes customary for cemented carbide cutting tools Cutting tool manufacturers also provide various chip-breaker geometries
Turning. The major reason for the application of cermet cutting tools in turning is their long and consistent tool life over
a wide range of cutting speeds When turning ductile materials, the proper choice of a chip-breaker geometry is essential for a safe and reliable turning operation Tables 4, 5, and 6 present general machining parameters for turning various metals with cermet cutting tools
Trang 16Table 4 Machining parameters with cermet tools in the turning of various steels
Cutting speed, m/min (sfm) Feed rate, mm/rev (in./rev) Maximum
0.46 (0.018)
7.6 0.300
100
Semifinishing, finishing
150 (500)
300 (1000)
365 (1200)
0.20 (0.008)
0.33 (0.013)
0.41 (0.016)
3.8 0.150 Roughing 50 (160) 195 (640) 245 (800) 0.15
(0.006)
0.36 (0.014)
0.41 (0.016)
5.1 0.200
300
Semifinishing, finishing
120 (400)
245 (800) 300
(1000)
0.15 (0.006)
0.30 (0.012)
0.38 (0.015)
3.8 0.150 Roughing 45 (150) 120 (400) 213 (700) 0.15
(0.006)
0.30 (0.012)
0.38 (0.015)
3.8 0.150
400
Semifinishing, finishing
60 (200) 183 (600) 245 (800) 0.10
(0.004)
0.25 (0.010)
0.30 (0.012)
0.38 (0.015)
5.1 0.200
100
Semifinishing, finishing
183 (600)
245 (800) 275 (900) 0.25
(0.010)
0.30 (0.012)
0.38 (0.015)
2.5 0.100 Roughing 90 (300) 165 (540) 245 (800) 0.10
(0.004)
0.28 (0.011)
0.33 (0.013)
2.5 0.100
200
Semifinishing, finishing
90 (300) 183 (600) 245 (800) 0.10
(0.004)
0.25 (0.010)
0.30 (0.012)
2.0 0.080 Roughing 90 (300) 165 (540) 245 (800) 0.10
(0.004)
0.23 (0.009)
0.30 (0.012)
2.0 0.080
260
Semifinishing, finishing
90 (300) 183 (600) 245 (800) 0.10
(0.004)
0.23 (0.009)
0.30 (0.012)
2.0 0.080 Roughing 90 (300) 165 (540) 245 (800) 0.10
(0.004)
0.23 (0.009)
0.30 (0.012)
2.0 0.080
300
Semifinishing, finishing
90 (300) 183 (600) 245 (800) 0.10
(0.004)
0.23 (0.009)
0.30 (0.012)
0.46 (0.018)
5.1 0.200
150
Semifinishing, finishing
60 (200) 275 (900) 300
(1000)
0.20 (0.008)
0.33 (0.013)
0.41 (0.016)
3.8 0.150 Roughing 34 (110) 183 (600) 205 (675) 0.23
(0.009)
0.36 (0.014)
0.43 (0.017)
5.1 0.200
250
Semifinishing, finishing
45 (150) 245 (800) 275 (900) 0.20
(0.008)
0.30 (0.012)
0.41 (0.016)
3.8 0.150 Roughing 34 (110) 137 (450) 183 (600) 0.15
(0.006)
0.30 (0.012)
0.41 (0.016)
4.7 0.187
350
Semifinishing, finishing
45 (150) 183 (600) 245 (800) 0.15
(0.006)
0.28 (0.011)
0.38 (0.015)
3.8 0.150 Roughing 30 (100) 120 (400) 183 (600) 0.15
(0.006)
0.28 (0.011)
0.38 (0.015)
3.8 0.150
400
Semifinishing, finishing
45 (150) 183 (600) 245 (800) 0.10
(0.004)
0.20 (0.008)
0.25 (0.010)
0.36 (0.014)
5.6 0.220
150
Semifinishing, finishing
120 (400)
213 (700) 275 (900) 0.20
(0.008)
0.30 (0.012)
0.38 (0.015)
5.1 0.200 Roughing 73 (240) 146 (480) 195 (640) 0.15
(0.006)
0.30 (0.012)
0.33 (0.013)
4.2 0.165
200
Semifinishing, finishing
90 (300) 183 (600) 245 (800) 0.15
(0.006)
0.25 (0.010)
0.33 (0.013)
3.8 0.150 Roughing 50 (160) 120 (400) 170 (560) 0.10
(0.004)
0.25 (0.010)
0.30 (0.012)
2.8 0.110
350
Semifinishing, 60 (200) 150 (500) 213 (700) 0.10 0.23 0.30 2.5 0.100
Trang 170.30 (0.012)
2.8 0.110
200
Semifinishing, finishing
90 (300) 183 (600) 230 (750) 0.15
(0.006)
0.25 (0.010)
0.30 (0.012)
2.5 0.100 Roughing 73 (240) 137 (450) 170 (560) 0.10
(0.004)
0.23 (0.009)
0.28 (0.011)
2.3 0.090
250
Semifinishing, finishing
75 (250) 167 (550) 213 (700) 0.10
(0.004)
0.23 (0.009)
0.28 (0.011)
2.0 0.080 Roughing 50 (160) 120 (400) 146 (480) 0.10
(0.004)
0.20 (0.008)
0.28 (0.011)
2.3 0.090
350
Semifinishing, finishing
60 (200) 150 (500) 183 (600) 0.10
(0.004)
0.20 (0.008)
0.28 (0.011)
2.0 0.080
Table 5 Machining parameters with cermet tools in the turning of cast iron
Cutting speed, m/min (sfm) Feed rate, mm/rev (in./rev) Depth of cut Hardness, HB Operation
Gray cast irons, malleable cast irons
Roughing 90 (300) 150 (500) 260 (850) 0.25 (0.010) 0.41 (0.016) 0.46 (0.018) 6.3 0.250
100
Finishing 150 (500) 275 (900) 365 (1200) 0.23 (0.009) 0.38 (0.015) 0.43 (0.017) 3.8 0.150 Roughing 60 (200) 146 (480) 245 (800) 0.23 (0.009) 0.36 (0.014) 0.41 (0.016) 4.7 0.187
180
Finishing 150 (500) 245 (800) 300 (1000) 0.20 (0.008) 0.33 (0.013) 0.38 (0.015) 3.2 0.125 Roughing 60 (200) 120 (400) 170 (560) 0.23 (0.009) 0.33 (0.013) 0.38 (0.015) 4.7 0.187
250
Finishing 60 (200) 150 (500) 213 (700) 0.20 (0.008) 0.30 (0.012) 0.36 (0.014) 3.2 0.125 Roughing 45 (150) 110 (360) 146 (480) 0.18 (0.007) 0.30 (0.012) 0.33 (0.013) 3.8 0.150
250
Finishing 120 (400) 150 (500) 245 (800) 0.18 (0.007) 0.25 (0.010) 0.33 (0.013) 3.2 0.125 Roughing 45 (150) 98 (320) 150 (500) 0.18 (0.007) 0.25 (0.010) 0.30 (0.012) 2.5 0.100
320
Finishing 160 (200) 120 (400) 183 (600) 0.15 (0.006) 0.20 (0.008) 0.25 (0.010) 2.5 0.100 Roughing 45 (150) 85 (280) 137 (450) 0.15 (0.006) 0.23 (0.009) 0.25 (0.010) 2.5 0.100
380
Finishing 160 (200) 90 (300) 120 (400) 0.13 (0.005) 0.15 (0.006) 0.20 (0.008) 2.5 0.100
Table 6 Machining parameters with cermet tools in the turning of nonferrous free-machining metals
Cutting speed, m/min (sfm) Feed rate, mm/rev (in./rev) Maximum
730 (2400)
0.23 (0.009)
0.51 (0.020)
0.56 (0.022)
7.6 0.300
100
Semifinishing, finishing
550 (1800)
610 (2000)
915 (3000)
0.20 (0.008)
0.46 (0.018)
0.51 (0.020)
6.3 0.250
(1300)
425 (1400)
670 (2200)
0.23 (0.009)
0.46 (0.018)
0.51 (0.020)
7.6 0.300
120
Grooving operations are associated with high heat and cutting pressure at the cutting edge Cermet cutting tools, because
of their high resistance to plastic deformation and their thermochemical stability, offer long tool life in comparison to cemented carbides Typical wear characteristics are shown in Fig 13
Trang 18Fig 13 Wear comparison between cemented carbide and cermet cutting tools in grooving of 4135 alloy steel
Cermet indexable inserts are applied in grooving operations to achieve close dimensional control and good surface finish
of the machined surfaces Figures 14 and 15 show cermet indexable inserts mounted on a toolholder and boring bar for grooving Table 7 contains guidelines for the median cutting speed and feed rate used when various materials are grooved with cermet cutting tools The use of coolant improves surface finish and tool life
Table 7 Machining parameters for the grooving of various steels with cermet tools
Feed rate, 0.05 to 0.13 mm/rev maximum (0.002 to 0.005 in./rev maximum)
Cutting speed, median Hardness, HB
m/min sfm Carbon steels
Trang 19Fig 14 Toolholder with cermet indexable inserts for grooving
Fig 15 Boring bars with cermet indexable inserts for grooving
Threading. The development of tougher cermet cutting materials resulted in the economical application of cermet cutting tools in single-point thread turning Threading requires tougher cutting materials because of the small tool nose radii of most standard thread forms and because of the rapid change of cutting forces at the entry and exit of the cut
The application of cermet indexable inserts in single-point turning extends tool life, in comparison to carbides and coated carbides (Fig 16) and allows higher cutting speeds Table 8 contains guidelines for the cutting speed in single-point turning of threads with cermet cutting tools, and Table 9 gives guidelines on the depth of cut and number of passes The first pass normally takes a deeper cut, and on each subsequent pass the depth of cut is reduced, as shown in Table 9 Coolant is applicable and results in improved surface finish and longer tool life
Trang 20Table 8 Cutting speeds for threading various steels with cermet tools
Cutting speed Hardness, HB
m/min sfm Carbon steels
0.18 (0.00 7)
0.15 (0.00 6)
0.07 (0.00 3)
0.03 (0.00 1)
0.18 (0.00 7)
0.15 (0.00 6)
0.10 (0.00 4)
0.05 (0.00 2)
0.03 (0.00 1)
0.18 (0.00 7)
0.15 (0.00 6)
0.15 (0.00 6)
0.10 (0.00 4)
0.05 (0.00 2)
0.03 (0.00 1)
0.18 (0.00 7)
0.15 (0.00 6)
0.15 (0.00 6)
0.10 (0.00 4)
0.07 (0.00 3)
0.05 (0.00 2)
0.03 (0.00 1)
0.18 (0.00 7)
0.15 (0.00 6)
0.13 (0.00 5)
0.13 (0.00 5)
0.10 (0.00 4)
0.07 (0.00 3)
0.05 (0.00 2)
0.03 (0.00 1)
0.20 (0.00 8)
0.15 (0.00 6)
0.13 (0.00 5)
0.13 (0.00 5)
0.10 (0.00 4)
0.10 (0.00 4)
0.07 (0.00 3)
0.05 (0.00 2)
0.03 (0.00 1)
0.20 (0.00 8)
0.18 (0.00 7)
0.15 (0.00 6)
0.15 (0.00 6)
0.13 (0.00 5)
0.10 (0.00 4)
0.10 (0.00 4)
0.08 (0.00 3)
0.05 (0.00 2)
0.03 (0.00 1)
Internal threads per 25 mm (1 in.)
0.15 (0.00 6)
0.13 (0.00 5)
0.12 (0.00 5)
0.10 (0.00 4)
0.07 (0.00 3)
0.03 (0.00 1)
18 0.0 0.00 0.8 0.03 8 0.18 0.15 0.15 0.13 0.10 0.10 0.05 0.03
Trang 218 3 9 5 (0.00
7)
(0.00 6)
(0.00 6)
(0.00 5)
(0.00 4)
(0.00 4)
(0.00 2)
(0.00 1)
0.15 (0.00 6)
0.15 (0.00 6)
0.13 (0.00 5)
0.10 (0.00 4)
0.10 (0.00 4)
0.08 (0.00 3)
0.05 (0.00 2)
0.03 (0.00 1)
0.18 (0.00 7)
0.15 (0.00 6)
0.13 (0.00 5)
0.13 (0.00 5)
0.10 (0.00 4)
0.08 (0.00 3)
0.07 (0.00 3)
0.05 (0.00 2)
0.03 (0.00 1)
0.18 (0.00 7)
0.18 (0.00 7)
0.15 (0.00 6)
0.13 (0.00 5)
0.10 (0.00 4)
0.10 (0.00 4)
0.08 (0.00 3)
0.07 (0.00 3)
0.05 (0.00 2)
0.03 (0.00 1) (a) Based on 4135 steel (180 to 220 HB) Harder workpiece or poor machine condition may require a
decrease in depth of cut per pass
Fig 16 Wear comparison between cemented carbide, coated carbide, and cermet cutting tools in the threading
of alloy steel Machining parameters: cutting speed, 130 m/min (430 sfm); six passes with coolant Workpiece:
4140 steel Pitch: 6 threads per 25 mm (1 in.)
Milling. The development of tougher titanium carbonitride compositions widened the application range of cermets in
milling The modern molybdenum carbide and tantalum/niobium carbide grades can endure the mechanical and temperature shock cycling and the variations in chip load associated with milling
Cermet cutting tools are applied when consistent tool life and good surface finish are required, and the cutting tool industry provides various styles of cermet indexable inserts for the most common milling cutter designs Tables 10 and 11 contain general guidelines for milling various steels and cast irons with cermet cutting tools
Trang 22Table 10 Machining parameters for the milling of various steels with cermet tools
Cutting speed, m/min (sfm) Feed rate, mm/rev (in./rev) Maximum
depth of cut Hardness, HB Operation
Low Median High Low Median High mm in Carbon steels
0.18 (0.007)
7.6 0.300
150-200
Semifinishing, finishing
150 (500)
300 (1000)
365 (1200)
0.05 (0.002)
0.10 (0.004)
0.13 (0.005)
6.3 0.250 Roughing 90 (300) 183 (600) 230 (750) 0.05
(0.002)
0.13 (0.005)
0.15 (0.006)
7.6 0.300
200-250
Semifinishing, finishing
137 (450)
275 (900) 335
(1100)
0.05 (0.002)
0.10 (0.004)
0.13 (0.005)
0.18 (0.007)
7.6 0.300
Austenitic
130-180
Semifinishing, finishing
75 (250) 150 (500) 183 (600) 0.05
(0.002)
0.10 (0.004)
0.13 (0.005)
5.1 0.200 Roughing 90 (300) 183 (600) 230 (750) 0.05
(0.002)
0.13 (0.005)
0.15 (0.006)
7.6 0.300
Ferritic 130-180
Semifinishing, finishing
90 (300) 200 (660) 245 (800) 0.05
(0.002)
0.10 (0.004)
0.13 (0.005)
5.1 0.200 Roughing 60 (200) 130 (430) 150 (500) 0.05
(0.002)
0.10 (0.004)
0.13 (0.005)
7.6 0.300
Martensitic
200-300
Semifinishing, finishing
60 (200) 137 (450) 183 (600) 0.05
(0.002)
0.08 (0.003)
0.10 (0.004)
0.18 (0.007)
7.6 0.300
200-250
Semifinishing, finishing
120 (400)
245 (800) 300
(1000)
0.05 (0.002)
0.08 (0.003)
0.10 (0.004)
5.1 0.200 Roughing 75 (250) 140 (460) 183 (600) 0.05
(0.002)
0.10 (0.004)
0.13 (0.005)
7.6 0.300
250-350
Semifinishing, finishing
90 (300) 183 (600) 230 (750) 0.05
(0.002)
0.08 (0.003)
0.10 (0.004)
5.1 0.200 Roughing 55 (180) 106 (350) 150 (500) 0.05
(0.002)
0.10 (0.004)
0.18 (0.005)
6.3 0.250
350-400
Semifinishing, finishing
75 (250) 137 (450) 150 (500) 0.05
(0.002)
0.08 (0.003)
0.10 (0.004)
0.18 (0.007)
7.6 0.300
100-150
Semifinishing, finishing
150 (500)
300 (1000)
365 (1200)
0.05 (0.002)
0.10 (0.004)
0.13 (0.005)
5.1 0.200 Roughing 90 (300) 183 (600) 245 (800) 0.05
(0.002)
0.15 (0.006)
0.18 (0.007)
7.6 0.300
150-200
Semifinishing, finishing
120 (400)
250 (820) 300
(1000)
0.05 (0.002)
0.10 (0.004)
0.13 (0.005)
5.1 0.200 Roughing 60 (200) 120 (400) 183 (600) 0.05
(0.002)
0.13 (0.005)
0.15 (0.006)
7.6 0.300
200-250
Semifinishing, finishing
75 (250) 150 (500) 183 (600) 0.05
(0.002)
0.10 (0.004)
0.13 (0.005)
5.1 0.200
Trang 23Table 11 Machining parameters for the milling of cast irons with cermet tools
Cutting speed, m/min (sfm) Feed rate, mm/rev (in./rev) Maximum
depth of cut Hardness, HB Operation
Gray cast irons
Roughing 60 (200) 150 (500) 245 (800) 0.05 (0.002) 0.13 (0.005) 0.18 (0.007) 7.6 0.300
150
Finishing 150 (500) 245 (800) 300 (1000) 0.05 (0.002) 0.10 (0.004) 0.13 (0.005) 5.1 0.200 Roughing 60 (200) 120 (400) 170 (560) 0.05 (0.002) 0.13 (0.005) 0.15 (0.006) 7.6 0.300
250
Finishing 90 (300) 150 (500) 245 (800) 0.05 (0.002) 0.10 (0.004) 0.13 (0.005) 5.1 0.200
References
1 E.C Van Schoick, Ed., Ceramic Glossary, The Ceramic Society, 1963
2 R Kieffer and F Benesovsky, Hartmetalle, Springer, 1965, p 8, 14, 250
3 M Humenik and D Moskowitz, U.S Patent 2,967,349, June 1959
4 A.H Heuer, J.S Sears, and N.J Zalucec, Analytical Electron Microscopy of Phase Separated Ti/Mo
Cemented Carbides and Carbonitrides, chapter 4 in Institute of Physics Conference Series No 75, Adam
Hilger, 1986
5 D Moskowitz and M Humenik, Jr., Modern Developments in Powder Metallurgy, Vol 3, Plenum Press,
1966
6 E Rudy, U.S Patent 3,971,656, May 1974
7 D Moskowitz, L.L Terner, and M Humenik, chapter 7 in Some Physical and Metalcutting Properties of
Titanium Carbonitride Base Materials, Institute of Physics Conference Series No 75, Adam Hilger, 1986
8 H Doi, Advanced TiC and TiC-TiN Base Cermets, chapter 6 in Institute of Physics Conference Series No
75, Adam Hilger, 1986
Trang 24Ceramics are inorganic, nonmetallic materials that are subjected to high temperature during synthesis or use (Ref 1) The use of ceramic tools in metal cutting, after a long period of slow growth, is beginning to increase with the advent of alloyed ceramics and ceramic-matrix composites, as well as with the advances in ceramic processing technology
Production Process
The production of ceramic tools involves the consolidation and sintering of powdered material The sintering provides the necessary densification of the consolidated powder and can be performed with or without the assist of pressure In pressureless sintering, the powder is first shaped into a green (unsintered) body, which is then sintered to achieve the necessary densification In hot pressing, the shaping is performed during sintering
Hot pressing involves heating the powder in a die, along with the simultaneous application of a high uniaxial pressure Although hot-pressed materials are more expensive, they usually have a finer grain size and a higher density and transverse rupture strength than cold-pressed products
Hot isostatic pressing is used to reduce the size of closed pores in high-performance ceramics This process exposes sintered or unsintered products to a hot pressurized gas The hot isostatic pressing of unsintered parts requires a gastight container for transmitting the gas pressure to the porous part
Compositions
The ceramics currently used in metal cutting are based on either alumina (Al2O3) or silicon nitride (Si3N4) Other ceramics (such as magnesia, yttria, zirconia, chromium oxide, and titanium carbide) are used as additives to aid sintering or to form composite ceramics with improved thermo-mechanical properties
Alumina-Base Tool Materials
Alumina was considered for certain machining applications as early as 1905, and patents based on this technology were issued in England and in Germany around 1912 However, the strength and toughness of these ceramic tools were inadequate for commercial applications During World War II, because of the strategic value of tungsten and the potential for increased machining rates, the Germans reconsidered ceramics as cutting tool materials (Ref 2) The pioneering work
of Ryschkewitsch on pure oxide ceramics lead to Degussit, the first commercially produced Al2O3 ceramic tool (Ref 3) A similar effort at the Institute of Chemical Technology in Moscow lead to the development of an Al2O3 ceramic, Microlith,
in 1943 In the United States, the work in this area began as early as 1935, but no progress was made until 1945, after extensive testing of Degussit
In the 1960s, the development of hot pressing and hot isostatic pressing resulted in stronger and more reliable parts Ceramic cutting tools were based on sintered or hot-pressed polycrystalline -Al2O3 (white ceramic) with a variety of sintering aids and compositions (Ref 4) These tools were primarily used for the high-speed finish machining of cast iron and steel for the automotive industry or for the slow-speed machining of extremely hard (and difficult-to-machine) cast or forged alloy steels in the steel roll industry These ceramics were basically fine-grain (<5 m) Al2O3-base materials with magnesia as a sintering aid and grain growth inhibitor They were also alloyed with suboxides of titanium or chromium to form solid solutions For example, in 1960, General Electric Company developed an AlO-TiO ceramic made by cold
Trang 25pressing and liquid-phase sintering (Ref 5) This ceramic is characterized by a reasonably fine grain size (3 to 5 m) and
a uniform microstructure The TiO constitutes about 10% A hardness of 93 to 94 HRA and a transverse rupture strength
of about 550 MPa (80 ksi) were achieved The low strength and toughness of these Al2O3-base ceramics limited their use
to speed finish machining or to the high removal rate machining of chill cast iron rolls on extremely rigid, power (<450 kW, or 600 hp), high-precision lathes (Ref 6)
high-Since the 1960s, the development of alloyed ceramics and ceramic-matrix composites has resulted in ceramic tool materials with improved thermo-mechanical properties The three main Al2O3-base tool materials are Al2O3-TiC, Al2O3-ZrO2, and Al2O3 reinforced with silicon carbide (SiC) whiskers Other Al2O3-base ceramics have additives of TiN, TiB2, Ti(C,N), and Zr(C,N)
Alumina and Titanium Carbide. Improvements in the thermo-mechanical properties occurred in the late 1960s when Japanese researchers added titanium carbide (TiC) to an Al2O3 matrix This alloyed ceramic is a dispersion-strengthened ceramic that contains 25 to 40 vol% TiC as a dispersed particulate phase Alumina-titanium carbide is often called a black ceramic composite due to its color, which results from the presence of titanium carbide Hot pressing is the typical production process
Figure 1 shows an electron micrograph of hot-pressed Al2O3, and Fig 2 shows an optical micrograph of a hot-pressed
Al2O3-TiC ceramic The addition of TiC greatly increases thermal conductivity, presumably through the formation of a more conductive intergranular phase (Ref 7) The Al2O3-TiC ceramics also exhibit a higher hardness and toughness than single-phase Al2O3 (Ref 8, 9, 10)
Fig 1 Scanning electron micrograph of hot-pressed Al2 O 3 Polished and etched 5000× Courtesy of Kennametal Inc
Trang 26Fig 2 Micrograph of a hot-pressed Al2 O 3 -TiC ceramic tool material 1500× Courtesy of Kennametal Inc
Alumina-zirconia (Al2O3-ZrO2) is an alloyed ceramic with higher fracture toughness and thermal shock resistance than monolithic Al2O3 However, the degree of toughening of Al2O3-ZrO2 ceramics decreases with increasing temperature (Ref
11, 12) The hardness of the ceramic is also reduced with increasing ZrO2
The toughening of Al2O3 with ZrO2 exploits a specific crystallographic change (martensitic-type transformation) that is an energy-absorbing mechanism The presence of metastable tetragonal ZrO2 provides the potential for transformation under stress into a stable monoclinic structure The transformation acts as a stress absorber and prevents, even when cracks exist, further cracking by the twinning of the monoclinic phase
Figure 3 is a micrograph of a fracture surface of an alumina-zirconia alloy The zirconia particles are concentrated predominantly at the alumina grain boundaries Although the fracture is intergranular, the presence of these particles is believed to provide additional toughness before failure can occur by fracture
Fig 3 Micrograph of a fracture surface of an alumina-zirconia ceramic (Al2 O 3 + 8% ZrO 2 ) showing the concentration of zirconia particles (the bright edges) at the alumina grain boundaries 3000× Courtesy of Carboloy Inc
An Al2O3-ZrO2 ceramic with traces of tungsten carbide was introduced by the Carboloy Systems Department of General Electric in the early 1980s The high toughness of this alloy is attributed to rapid freezing from the melt, which results in a dendritic freezing structure and superior grinding performance The three popular compositions contain 10, 25, and 40% ZrO2; the remainder is Al2O3 The 40% ZrO2 composition is close to the eutectic The higher-ZrO2 compositions are less hard, but tougher
Silicon carbide whisker reinforced alumina (Al2O3-SiCw) is the newest Al2O3-base tool material The incorporation of SiC whiskers (25 to 45 vol%) into an Al2O3 matrix with subsequent hot pressing results in a composite with significantly improved toughness Whisker reinforcement produces a twofold increase in fracture toughness relative
to monolithic Al2O3 (Ref 13, 14) Figure 4 shows the microstructure of SiC whisker reinforced Al2O3
Trang 27Fig 4 Microstructure of SiC whisker reinforced Al2O3 composite tool material (a) 2100× (b) 5000× Courtesy
of Kennametal Inc
The whiskers, which are small fibers of single-crystal SiC about 0.5 to 1 m (20 to 40 in.) in diameter and 10 to 80 m (400 to 3200 in.) long, have a higher thermal conductivity and a lower coefficient of thermal expansion than Al2O3 This improves thermal shock resistance
The SiC whiskers in the alumina matrix also improve fracture toughness Although the details of the micromechanisms for improved fracture toughness of Al2O3-SiC whiskers have not been clearly established, a plausible mechanism for the toughness is based on current knowledge of the general behavior of composite materials As a crack propagates through the ceramic matrix, bonds between the Al2O3 matrix and the whiskers are broken However, the SiC whiskers, because of their inherently high tensile strength (7 GPa, or 1 × 106 psi), remain essentially intact Consequently, whisker pullout occurs as a result of the separation of the matrix from the whiskers Interfacial shear stress resisting the whisker pullout absorbs a substantial amount of this fracture energy and inhibits crack propagation For improved fracture toughness, a strong metallurgical bond between the fibers and the matrix is not desirable; a strong bond would cause the whiskers to fail along with the matrix This is the case with SiC whisker reinforced Al2O3 because the bond between the matrix and the fibers is not particularly strong For optimum results, it is recommended that the fracture energy of the interface not exceed 10% of the fracture energy of the Al2O3 matrix
Silicon Nitride Base Tool Materials
Silicon nitride recently has attracted much attention as a tool material because of its unique combination of excellent high-temperature mechanical properties and resistance to oxidation and thermal shock Both transverse rupture strength and fracture toughness of Si3N4 are higher than for Al2O3-TiC Moreover, because poor thermal-shock resistance is a major cause of failure of Al2O3-TiC tools, the difference in thermal properties is significant The thermal conductivity of
Si3N4 is approximately double that of Al2O3-TiC, while the coefficient of thermal expansion is around one-half that of
Al2O3-TiC These thermal properties result in a much lower sensitivity to temperature changes and improved shock resistance
thermal-The difficulty of utilizing these properties arises from the problems involved in sintering pure Si3N4 In Si3N4, the volume diffusivity is not large enough to offset the densification retardation effects of surface diffusion and volatilization phenomena; as a result, sintering is difficult The covalent bond is believed to be the reason for such a low volume diffusivity Therefore, dense Si3N4-base material can be obtained only by alloying Si3N4 with additives that promote sintering The properties of these Si3N4 alloys, especially at high temperatures, are to a large extent controlled by the additives As a result, there is not just a single Si3N4 material, because the properties of Si3N4 alloys depend on the additives
Various metal oxides and nitrides have been found to be effective sintering aids The sintering additives can be divided into three classes The first class exhibits extended solid-solution formation with -Si3N4 or -Si3N4; the second class does not The third class is made up of combinations of the additives from the first and second class and yields a mixed
Trang 28behavior (Table 1) Consequently, by using soluble additives, it is possible to manufacture single-phase alloys; using insoluble additives necessarily yields multiple-phase alloys with either crystalline or amorphous phases
Table 1 Sintering additives for Si 3 N 4
Systems with extended solid-solution formation
Al 2 O 3 , AlN, SiO 2 Si6-xAlxOxN8-x -SiAlON
BeO,Be 3 N 2 , SiO 2 Si6-x, BexO2xN8-2x -SiBeON
Al 2 O 3 , AlN, SiO 2 , BeO, Be 3 N 2 Si6-x-yBexAlyO2x+y -SiAlBeOn
Al 2 O 3 , AlN, metal oxide, metal nitride, M = Li, Ca, Mg, Y Mx(Si, Al) 12 (O,N) 16 -SiAlON
Systems without extended solid-solution formation
MgO or MgO, Mg 3 N 2 , SiO 2 No quaternary phases SiMgON
Y 2 O 3 or Y 2 O 3 , SiO 2 Quaternary phases SiYON
Mixed systems
Al 2 O 3 , AlN, SiO 2 , Y 2 O 3 -SiAlON + Y
SiAlON generally refers to a system made up of Al2O3 and Si3N4 The SiAlONs are derived from the structure of
-Si3N4 (hexagonal crystal structure with ABAB stacking) having compositions of Si 6-xAlxOxN8-x (O x 4.2) The SiAlONs are derived from the -Si3N4 structure (hexagonal crystal structure with ABCDABCD stacking) The solid-solution formation is purely substitutional; silicon is replaced by aluminum and nitrogen by oxygen The mechanism of densification of SiAlON is a transient liquid-phase sintering
-Alumina is a desirable additive for cutting tool applications because its chemical inertness limits wear (such as crater wear) by chemical reactions This benefit associated with Al2O3 can be demonstrated by considering the free energy of formation, which is a good indicator of chemical inertness Figure 5 compares the free energy of formation as a function
of temperature for various materials The more negative the free energy, the more chemically inert or corrosion resistant the material should be In Fig 5, the free energy of Si3N4 falls within the range of cemented carbides, while Al2O3 has the most negative free energy Therefore, an Al2O3 additive should improve the wear resistance by making the resultant alloy more chemically inert
Trang 29Fig 5 Standard free energies of formation ( F°) of Al2 O 3 , Al 2 O 3 -TiC, Si 3 N 4 , SiC, Co, Fe, and cemented carbides
at various temperatures Source: Ref 16
A mixture of two different classes of oxide additives is sometimes used for the densification of Si3N3 This is done to densify the material at a lower temperature and to yield more refractory-type secondary phases Alumina is used most often, so the systems studied usually include a mixed alloy of -SiAlON, such as SiAlMgON or SiAlYON
The SiAlZrON system also gains interest because of the possibility of the martensitic transformation toughening of Si3N4
by ZrO2, which has been demonstrated in Al2O3 ceramics If only ZrO2 is added to Si3N4, an undesirable chemical reaction without densification occurs (Ref 17) Further additions of Al2O3 and AlN yield a dense material consisting of -SiAlON and ZrO2 (Ref 18)
If yttria (Y2O3) is used, the material is Y2O3-stabilized silicon aluminum oxynitride (SiAlYON), which is isostructural with -Si3N4 Because of the similarity in crystal structure, '-SiAlON has physical and mechanical properties similar to those of Si3N4, and because of its Al2O3 additive, '-SiAlON has additional chemical inertness
To produce '-SiAlON, a mixture of Al2O3 ( 13%), Si3N4 ( 77%), Y2O3 ( 10%), and AlN is used as the starting material During sintering, this mixture produces a larger volume of lower-viscosity liquid than in the synthesis of Y2O3-stabilized Si3N4 (and its surface silica); therefore, '-SiAlON can be fully densified by pressureless sintering The powder mix for '-SiAlON is first ball milled, then preformed by cold isostatic pressing, and subsequently sintered at a maximum temperature of 1800 °C (3300 °F) under isothermal conditions for approximately 1 h before it is allowed to cool slowly The microstructure of '-SiAlON consists of ' grains cemented by a glass phase A scanning electron micrograph of a polished and etched sample is shown in Fig 6
Trang 30Fig 6 Scanning electron micrograph of a SiAlON ceramic tool 10,000× Courtesy of Kennametal Inc
Rapid cooling from the processing temperature produces a microstructure of -SiAlON grains with an intergranular glassy phase If Y2O3 is the sintering aid, a portion of this glassy phase can be converted to crystalline yttrium-aluminum-garnet (YAG) by heat treating and slow cooling However, most sintered SiAlONs contain some residual glass phase, particularly at grain-boundary triplepoints Like the Si3N4, sintering aid systems, the properties of the SiAlONs are dependent on the type and amount of sintering aid employed and the processing route followed during part fabrication
Properties
Ceramics provide a desirable tool material because their good hot hardness (Fig 7) and oxidation resistance reduce the amount of tool wear at high cutting temperatures These properties allow ceramic tools to be used in the high-speed machining (>300 m/min, or 1000 sfm) of even difficult-to-machine metals