Modeling and Simulation for Material Selection and Mechanical Design Part 13 pptx

schemati-However the structure of the nitrided sublayer must be optimized induplex coatings to achieve the best tool life.. The formation of a nitride using X-ray bom-Figure 28 The time

Figure 26 Fourier transform from EELFS analysis of a wear crater, TiCrNcoating on nitrided T15 steel: (a) cutting time¼ 30 sec; (b) cutting time ¼ 180 sec;(c) cutting time¼ 2100 sec.

transform The first peak is now located at a distance R1¼1.7 A˚, while thesecond peak is at a distance R2¼2.65 A˚ These peaks correspond approxi-mately to the length of C–Fe and Fe–Fe bonds (RCþRFe¼0.51þ1.26¼1.77 A˚; RFeþRFe¼1.26þ1.26¼2.52 A˚) The spectrum shown in Fig 26c istypical of the BCC-lattice of T15 high-speed steel.

B Tribological Properties and the Metallurgical Design of

Surface-Engineered Tools

The study of the wear resistance of coated tools demonstrates that the tective role of the coating is most efficient when the effects of the work ofcutting can be localized in the near-surface region of the coating [43] Cur-rent coating technologies achieve this goal by modifying the energy distribu-tion (from the tool surface into the chip), and by promoting the self-organization of the tool This is done in two ways: (1) by surface engineeredand self-lubricated coatings for low and moderate speed machining, and (2)

pro-by the use of hard or superhard coatings, that can act as thermal barriersand form very stable ‘‘tribo-ceramics’’ at the surface during high-speedcutting

1 Surface-Engineered or Duplex Coatings

The principal application of these coatings [45] is for cutting at low speeds,when HSS and DCPM tools are used It is desirable to deposit the hardcoating, not directly onto the steel substrate but rather onto an engineeredsublayer, so that a gradual change in properties at the coating–substrateinterface, i.e., a functionally graded material, is realized This sublayer can

be obtained by different technologies, e.g ion nitriding Usually such ings will then include both a nitrided sublayer and a hard PVD coating.The nitrided sublayer has two roles It prevents intensive plastic defor-mation of the substrate (HSS or DCPM) and cracking of the PVD coatingthat might be caused by deformation of the underlying substrate, while atthe same time, it provides an additional thermal barrier [43] The advantages

coat-of HSS cutting tools with surface-engineered coatings are shown cally inFig 27

schemati-However the structure of the nitrided sublayer must be optimized induplex coatings to achieve the best tool life The duration and temperature

of the process are the most important parameters in ion nitriding [38] Theion current density should not be high, preferably about 3 A m2 Theexperimental data given below were obtained when the surface tempera-ture during nitriding was about 500–5308C At this temperature, rapid

nitrogen diffusion occurs The dependence of the structure and properties

of an M2 tool steel on the nitriding time is shown in Fig 28 Ion bardment leads to the formation of a defective structure in the surfacelayers, which enhances nitrogen diffusion During the fist 10–20 min ofnitriding, a saturated solid solution of N is formed After 30 min of nitrid-ing a supersaturated solid solution of N is obtained at the surface(Fig 28a) The most pronounced changes in the lattice parameter and linebroadening of the (2 1 1) reflection occur after 0.5–2.0 hr of nitriding(Fig 28b) A further increase in the nitriding time from 2 to 4 hr has littleeffect on either the lattice parameter or the line broadening Nitrides areobserved after about 2–4 hr (The formation of a nitride using X-ray

bom-Figure 28 The time dependence of the structural characteristics and properties ofthe ion nitrided sublayer of a surface-engineered coating: (1) M2 HSS; (2) D2 toolsteel; (3.1) nitrided layer of a cutting tool; (3.2) un-nitrided layer of the die steel

diffraction can be detected when the concentration of the nitride isapproximately 5%.) The first nitride to be detected by x-ray diffraction

in this study is the e-phase (W,Fe) 2–3N, while after 4 hr of nitriding, boththe e and g0 (W,Fe) 4N phases are detected After 4 hr of nitriding, thenitrides can be clearly detected by optical metallography as a network ofthin, needle- or lath-shaped particles

It is known that the presence of tungsten, molybdenum and chromium inthe solid solution of the steel can lead to the formation of a high density of finenitrides with a marked increase in the hardness When the nitriding time isincreased to 2 hr or more, mixed (Cr, W, Mo) nitrides will also nucletlate Thesenitrides are very finely dispersed and hence are difficult to detect by X-ray dif-fraction, but they contribute significantly to the increased hardness(Fig 28d).The coefficient of plasticity of a nitrided M2 steel changes according tothe data shown in Fig 28e This coefficient (determined from an indentationtest) is highest (52%) when the hardness is low, and conversely decreases (to48%) when the hardness is high (The Palmquist toughness for nitridedsteels cannot be used to give a meaningful measure of the fracture resistance

as the depth of the nitrided layer changes as nitriding proceeds.) The city of the nitrided layer is sensitive to the microstructure When there are nonitrides in the layer, the plasticity coefficient is proportional to the nitrogensaturation The N content in this zone can be characterized by the latticeparameter of the a-phase (Fig 28a) As the nitrogen concentration (and lat-tice parameter) in the surface layer rises, there is a corresponding decrease inthe plasticity, and vice versa A low plasticity is correlated with an increasedlattice deformation of the solid solution, associated with the dissolution of

plasti-N into the iron lattice, as shown by the line broadening of the (2 1 1) tion of the nitrided martensite (Fig 28b) In addition, some influence on theplastic properties is exerted by residual stresses, which are formed in the sur-face layer during nitriding (Fig 28c) The residual stresses are high when thenitrogen content in the nitrided layer increases and extensive precipitationoccurs on cooling The volume of the surface layer increases on nitridingand as a result compressive residual stresses are formed This effect is typicalfor M2 grade steels

reflec-High compressive stresses in the nitrided layer of a M2 steel lead toincreased hardness and plasticity, and inhibit cutting edge-flaking duringthe tool life It is important that the level and sign of stresses formed inthe nitrided layer are similar to those in the adhesion sublayer of multi-layer coatings then, the stress gradient between the nitrided substrateand the coating is low and the adhesion is improved The service properties

of the nitrided layer also have a high structural sensitivity The longest toollife of nitrided HSS steels is obtained with an a-solid solution structureand is at least double that of un-nitrided tools The tool life increases with

the nitrogen content in the layer, which, as noted earlier, can be monitored

by the change in the lattice parameter of the nitrided martensite(Fig 28f).After nitrides have precipitated, the tool life decreases as a result of flaking

at the cutting edge, caused by a decrease in the plasticity of the surfacelayer The formation of a residual compressive stress also plays some role

in flaking, as these stresses are highest with a N solid solution

In addition to the structure of the surface-engineered coating, the ure of the coating–substrate interface is also of great importance The adhe-sion of the coating is one of the principal factors (together with the thermalstability) determining the tool life The interface must be free from brittlecompounds (such as oxides, nitrides, etc.) formed in the hardening process

nat-or during interaction with the environment Several studies suggest thatthe surface of the tool should be polished to remove surface nitrides formedafter the ion treatment [50] Surface cleaning is also effective when ion etch-ing is used, but the etching must be performed very carefully The cuttingedges of a sharp tool should not be rounded, the surface roughness shouldnot increase and the tool dimensions should be kept to a close tolerance Allthis is the subject of technological optimization, but with care, excellentresults can be achieved [45]

a Friction and Wear Behavior and the Features of Self-Organizing ofSurface-Engineered Coatings A surface-engineered coating can act as a

‘‘protective screen’’ at the surface of a cutting tool (Fig 27) Duringsteady-state wear, a gradual, but controlled wear of the coating takes place.All these advantages became even more obvious when surface-engineeredcoatings are applied Tests done at increased cutting speeds (90 m=min)(forfor HSS tools) enhance all the thermal processes associated with cutting.Under these conditions, the heat-insulating effect of a hard TiN coating isdiminished, the protective function of the coating is reduced, plastic defor-mation of the steel substrate can occur, and the stability of cutting is disrup-ted All these trends can be seen in the data presented inFigs 24b and 29.Hardening an M2 steel by a surface-engineered coating can be employed

to counteract these effects The wear value is considerably lower and thezone of stable cutting process is significantly broader (Fig 29, curve 4).The best results are achieved when a substrate material (T15 HSS) having

a high heat resistance is used The dissipation of energy is channeled intoprocesses other than surface damage, i.e compatibility of the tool andworkpiece is realized to a great degree The coating plays the role of a pro-tective screen for the contact surfaces It should be emphasized that thesuccessful fulfillment of this function, however, is possible only when theexternal thermo-mechanical effects are localized in the coating layer Studies

of coating wear have shown that the intensive self-organizing process observedduring cutting only occurs when a surface-engineered coating was used

coatings becomes questionable due to their brittleness During themachining of several types of alloys (e.g stainless steels or nickel-basedalloys), unstable conditions can dominate and surface damaging mechan-isms become prevalent In this case, the ability of a thin surface layer toprotect the surface, well as dissipate most of the energy generated duringcutting, thereby minimizing the cracking of the tool, becomes criticallyimportant This is a practical application of the universal principle of dis-sipative heterogeneity [47].

For the most demanding cutting applications a third type of coating—the self-lubricated hard coating—has been developed A typical example ofthis type of development is the multi-layer coating, TiAlN–MoS2,with twoenergy-dissipating mechanisms built into the microstructure [48] The first isassociated with the formation of an oxygen-containing secondary structure(SS-I) that readily forms at the surface of the hard coating (TiAlN) andplays the role of a solid lubricant The second is associated with the thinMoS2 lubricating layer A second example of a similar technology is theuse of nano-composite nc-TiN–BN coatings [49] These coatings give goodresults at moderate cutting speeds Following the earlier discussion, it seemslikely that a high-alloyed Ti–B–O secondary structure of the first type (SS I,see above) and B2O3both form The boron oxide plays the role of a liquidlubricant at the temperatures of cutting [50]

The most important phase of the self-organizing process is associatedwith the running-in stage of wear During this stage of self-organization, thewear process gradually stabilizes and finally transforms to a stable (or nor-mal) stage [7] It is very important to prevent surface damage and promoteintensive self-organization at the surface during the running-in stage of wearusing the phenomenon of screening [4,7] The less surface damage at thebeginning of the normal stage of wear, the longer will be the tool life(Fig 2)

Hard coatings are brittle and susceptible to extensive surface damageduring this running-in stage Frequently, much of the hard coating is alreadydestroyed at this phase, prior to the start of the stable (normal) stage ofwear, where the wear rate can be lowered by an order of magnitude due

to the self-organizing of the system (Fig 2) The initial surface damage oftenleads to a dramatic decline in the wear resistance of the coating For this rea-son, a top layer with high anti-frictional properties is a critical component,and can be used to protect the surface of the hard coating This is one of themost important goals for wear resistant coatings, especially at low and mod-erate cutting speeds, and for handling hard-to-machine materials whereadhesive wear dominates This can be achieved by applying self-lubricated,multi-layer coatings These structures have many complex microstructuralfeatures that contribute to energy dissipation [e.g the TiAlN–MoS2 (or

MoST) coatings [51,52], discussed earlier] One of the most effective mercial coatings of this type is the multi-layered TiAlN=WC-C hard lubri-cant coating developed by Balzers [53] The main advantage of thiscoating is a very low initial wear rate, during the running-in stage of wear(Fig 2) that leads to a significant increase in the tool life (Fig 30) Recently,several oxides such as WO3,V2O5,and TiO2[54] were found to exhibit goodtribological properties at elevated temperatures All these oxides containcrystallographic shear planes with low shear strengths at high temperature[44] They are promising materials as solid lubricants for elevated tempera-ture applications, and can be deposited by PVD methods.

com-The service performance of multi-layered coatings with an anti-frictiontop layer is characterized by the wear curves shown inFig 31.The top (anti-frictional) layer leads to a decrease in flank wear as soon as the running-instage is completed, and the tool life is significantly increased (Figs 2 and31) Unfortunately, not every mode of the running-in phase leads to theoptimum self-organization [4,47], because damaging modes are also possi-ble, especially during cutting Thus, the goal of friction control is to preventserious surface damage at the running-in stage and transform the tribosys-tem from its initial state into a self-organizing mode If this can be achieved,

Figure 30 Tool life of end mills with advanced coatings Machined material, 1040steel Parameters of cutting: speed (m=min): 21; depth (mm): 3.0; width (mm): 5; feed(mm=flute): 0.028; cutting with coolant

can be deposited by dipping the part into a boiling solution The chemical properties of Z-DOL are shown in Table 6 The thin film consists

physico-of a close-packed molecular mono-layer, that provides an even coating to arough tool surface This coating has a high adsorption ability and due to itslow thickness it also has high adhesion to the substrate and penetrates intopores The surface energy of oils contained in the typical coolant regularlyused for machining is higher than the surface energy of the Z-DOL film As

a result of the molecular interaction of the oil and Z-DOL film, the latterfilm is not sheared from the surface of the cutting tool during the first stages

of cutting The principal function of the top anti-frictional layer(Fig 32)is

to increase the adaptability of cutting tools with hard nitride coatings.The two surfaces are separated by a layer of oil that prevents seizureand wear during the initial stages of the tool service Studies of surface-engi-neered coatings (a PVD TiCrN hard coating and a top layer of Z-DOL)deposited on a HSS substrate in contact with a 1040 steel show that the fric-tion characteristics are improved at the service temperature (5008C,Fig 23).The tool life data (Fig 31a)reflect a very low pattern of surface damage atthe running-in stage of wear, leading to a marked improvement in the overalltool performance

3 ‘‘Smart’’, Multi-layered Wear Resistant Coatings

Similar problems of friction control at service conditions leading to face damage arise when the wear process changes from the normal to the

sur-‘‘avalanche-like’’ stage As noted above, cutting tools made of HSSusually operate under conditions of adhesive wear, where seizure mightoccur, accompanied by a rapid increase in the wear intensity [56] Prolon-gation of the normal friction and wear stage, however, is quite feasible,even if seizure is a problem This can be achieved by applying an

Table 6 Physico-chemical Propeties of Z-DOL [58,59]

Average number of units in the molecule 12

Thickness of epilamon layer 5–2,500 nm

(3) metals including:

(a) low-melting point elements (in particular In, Mg, Sn, Ga) used

as lubricants or anti-friction materials;

(b) metals with a hexagonal lattice and anti-frictional properties[62,63]

(c) metals (Al, Cr) that form stable oxide films during cutting, withgood anti-frictional properties, and a low coefficient of thermalconductivity; and

(d) metals (Ag, Cu) known to have a low coefficient of friction,and low mutual solubility when in contact with steel, nickel andtitanium alloys (Fig 33)[63]

In addition, the study was extended to study surfaces subjected totreatments with:

– four types of anti-friction alloys used to improve conditions ofsliding friction, viz Zn þ Al(9%) þ Cu(2%), Cu þ Pb(12%) þSn(8%), Pbþ Sn (1%) þ Cu (3%) and Al þ Sn(20%) þ Cu(1%) þSi(0.5%) [28];

– Zrþ N, W þ C, W þ N, Ti þ N, Al þ O, to create layers with ahigh wear and oxidation resistance

The wear of these coatings was studied while turning 1045 carbonsteels at a cutting speed of 70 m=min, a cutting depth of 0.5 mm and a feedrate of 0.28 mm=rev with and without a coolant The flank wear of tetrago-nal, indexable HSS inserts with multi-layered coatings was studied; when theflank wear exceeds 0.3 mm, the cutting tool loses its serviceability [3] Theeffectiveness of ion modification was determined by comparing the cuttingtime to reach a specified depth of wear of tools with multi-layered coatings(i.e., those having both surface-engineered coatings and ion modification)with identical surface-engineered coatings prepared without the additionalstep of ion modification Adhesion was determined using the scratchmethod Friction coefficients were determined with the aid of a speciallydesigned adhesiometer shown inFig 5

The results of these tests, summarized in Table 7, demonstrate to alarge extent that the influence of the implanted elements on the tool life isdetermined by the cutting conditions The operational temperature during

a reduction in the adhesion of the tool surface to the processedmaterial and, at the same time, an increased adhesion of the hardPVD-coating to the modified base material.

The data from Table 7 show that a class of anti-frictional alloys, widelyused to improve the conditions of sliding friction [28,62], can double the toollife However, this method of increasing the tool life, i.e one that primarilydepends on a reduction in the strength of the adhesion bonds between the tooland workpiece, is not the most efficient, as the adhesion of the coating to themodified surface was found to be rather low This precludes their usage, asde-cohesion of a coating cannot be tolerated in practical applications.Implanting elements such as indium, silver and nitrogen enhances thetool life by a factor of 2–3 (see Table 7) for a range of cutting conditions(with and without cooling) These results are consistent with the observationthat indium and silver show little interaction with iron, and find use as solid-state lubricants (Fig 33) Nitrogen implantation probably leads to the for-mation of an amorphous film with improved tribological characteristics [65].Ion modification of the tool surface with the other elements studied led tounstable or negative effects, i.e a reduction in tool life and=or poor adhe-sion between the hard coating and the substrate

Table 7 (Continued)

N of group

Element composition

Coefficient of PVD-coating adhesion to modified surface base

Durability coefficient

on cutting Without coolant

With coolant

Babbitt BK2 GOST 1320–74 (Russia)

Al–Sn–Cu AO20–1 GOST 14113–69 (Russia)

The most beneficial element in this study was indium The life of thetool was found to be a maximum, with or without the use of a coolant(see Table 7) At the same time, the adhesion between the coating andindium-modified surface of the tool was sufficient to ensure a reliable toolperformance Indium is a surface-active metal and usually displays a low tri-bological compatibility with traditionally machined alloys based on steel,nickel, and titanium [63] Because of this, the wear peculiarities of In-con-taining coatings have been comprehensively investigated [64].

Scanning electron microscopy and x-ray microanalysis were used tostudy surface-engineered cutting tools, composed of an ion-doped HSS sur-face, nitrided by a glow discharge technique, with a hard PVD coating overthe In-modified layer(Fig 34a).Figure 34 shows the microstructure of a 58angle lap specimen (including the surface-engineered coating), taken in theSEM with the back-scattered electron signal which is sensitive to the meanatomic number Separate layers of the multi-layered coating (dark for TiNand gray for the In-rich sublayer) can be seen in the back-scattered electronimage The thickness of this zone is about 6.0 mm, so that the true depth ofthe modified (gray) layer is about 0.3 mm It is probably a Fe-layer contain-ing implanted Ar (as a result of etching by Arþafter nitriding) and In Thepresence of W in the tool steel increases the intensity of the x-ray In Karadiation and the background emission This matrix effect influences theapparent emission volume of In Ka radiation and degrades the accuracy

of measurement of the In distribution In addition, surface heating (up to

Figure 34 Microstructure of the multi-layered HSS-base (Ti, Cr)N coating with anIn-modified surface (ion implantation) 600 magnification (a) Microstructure ofthe angle lap section of the multi-layered coating (SEM image); (b) distribution ofelements along the II direction (x-ray microanalysis)