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

Figure 26 Fourier transform from EELFS analysis of a wear crater, TiCrN coating on nitrided T15 steel: (a) cutting time ¼ 30 sec; (b) cutting time ¼ 180 sec; (c) cutting time ¼ 2100 sec Copyright 2004 by Marcel Dekker, Inc All Rights Reserved ˚ transform The first peak is now located at a distance R1¼1.7 A, while the ˚ These peaks correspond approxisecond peak is at a distance R2¼2.65 A mately to the length of C–Fe and Fe–Fe bonds (RCþRFe¼0.51þ1.26¼ ˚ ˚ 1.77 A; RFeþRFe¼1.26þ1.26¼2.52 A) The spectrum shown in Fig 26c is typical 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 protective role of the coating is most efficient when the effects of the work of cutting can be localized in the near-surface region of the coating [43] Current coating technologies achieve this goal by modifying the energy distribution (from the tool surface into the chip), and by promoting the selforganization of the tool This is done in two ways: (1) by surface engineered and self-lubricated coatings for low and moderate speed machining, and (2) by the use of hard or superhard coatings, that can act as thermal barriers and form very stable ‘‘tribo-ceramics’’ at the surface during high-speed cutting 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 hard coating, not directly onto the steel substrate but rather onto an engineered sublayer, so that a gradual change in properties at the coating–substrate interface, i.e., a functionally graded material, is realized This sublayer can be obtained by different technologies, e.g ion nitriding Usually such coatings will then include both a nitrided sublayer and a hard PVD coating The nitrided sublayer has two roles It prevents intensive plastic deformation of the substrate (HSS or DCPM) and cracking of the PVD coating that might be caused by deformation of the underlying substrate, while at the same time, it provides an additional thermal barrier [43] The advantages of HSS cutting tools with surface-engineered coatings are shown schematically in Fig 27 However the structure of the nitrided sublayer must be optimized in duplex coatings to achieve the best tool life The duration and temperature of the process are the most important parameters in ion nitriding [38] The ion current density should not be high, preferably about 3 A mÀ2 The experimental data given below were obtained when the surface temperature during nitriding was about 500–5308C At this temperature, rapid Copyright 2004 by Marcel Dekker, Inc All Rights Reserved 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 bombardment leads to the formation of a defective structure in the surface layers, which enhances nitrogen diffusion During the fist 10–20 min of nitriding, a saturated solid solution of N is formed After 30 min of nitriding a supersaturated solid solution of N is obtained at the surface (Fig 28a) The most pronounced changes in the lattice parameter and line broadening 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 little effect on either the lattice parameter or the line broadening Nitrides are observed after about 2–4 hr (The formation of a nitride using X-ray Figure 28 The time dependence of the structural characteristics and properties of the ion nitrided sublayer of a surface-engineered coating: (1) M2 HSS; (2) D2 tool steel; (3.1) nitrided layer of a cutting tool; (3.2) un-nitrided layer of the die steel Copyright 2004 by Marcel Dekker, Inc All Rights Reserved diffraction can be detected when the concentration of the nitride is approximately 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, both the e and g0 (W,Fe) 4N phases are detected After 4 hr of nitriding, the nitrides can be clearly detected by optical metallography as a network of thin, needle- or lath-shaped particles It is known that the presence of tungsten, molybdenum and chromium in the solid solution of the steel can lead to the formation of a high density of fine nitrides with a marked increase in the hardness When the nitriding time is increased to 2 hr or more, mixed (Cr, W, Mo) nitrides will also nucletlate These nitrides are very finely dispersed and hence are difficult to detect by X-ray diffraction, but they contribute significantly to the increased hardness (Fig 28d) The coefficient of plasticity of a nitrided M2 steel changes according to the data shown in Fig 28e This coefficient (determined from an indentation test) is highest (52%) when the hardness is low, and conversely decreases (to 48%) when the hardness is high (The Palmquist toughness for nitrided steels cannot be used to give a meaningful measure of the fracture resistance as the depth of the nitrided layer changes as nitriding proceeds.) The plasticity of the nitrided layer is sensitive to the microstructure When there are no nitrides in the layer, the plasticity coefficient is proportional to the nitrogen saturation The N content in this zone can be characterized by the lattice parameter of the a-phase (Fig 28a) As the nitrogen concentration (and lattice parameter) in the surface layer rises, there is a corresponding decrease in the plasticity, and vice versa A low plasticity is correlated with an increased lattice deformation of the solid solution, associated with the dissolution of N into the iron lattice, as shown by the line broadening of the (2 1 1) reflection of the nitrided martensite (Fig 28b) In addition, some influence on the plastic properties is exerted by residual stresses, which are formed in the surface layer during nitriding (Fig 28c) The residual stresses are high when the nitrogen content in the nitrided layer increases and extensive precipitation occurs on cooling The volume of the surface layer increases on nitriding and as a result compressive residual stresses are formed This effect is typical for M2 grade steels High compressive stresses in the nitrided layer of a M2 steel lead to increased hardness and plasticity, and inhibit cutting edge-flaking during the tool life It is important that the level and sign of stresses formed in the nitrided layer are similar to those in the adhesion sublayer of multilayer coatings then, the stress gradient between the nitrided substrate and the coating is low and the adhesion is improved The service properties of the nitrided layer also have a high structural sensitivity The longest tool life of nitrided HSS steels is obtained with an a-solid solution structure and is at least double that of un-nitrided tools The tool life increases with Copyright 2004 by Marcel Dekker, Inc All Rights Reserved 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 surface layer 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 nature of the coating–substrate interface is also of great importance The adhesion of the coating is one of the principal factors (together with the thermal stability) determining the tool life The interface must be free from brittle compounds (such as oxides, nitrides, etc.) formed in the hardening process or during interaction with the environment Several studies suggest that the surface of the tool should be polished to remove surface nitrides formed after the ion treatment [50] Surface cleaning is also effective when ion etching is used, but the etching must be performed very carefully The cutting edges of a sharp tool should not be rounded, the surface roughness should not increase and the tool dimensions should be kept to a close tolerance All this is the subject of technological optimization, but with care, excellent results can be achieved [45] a Friction and Wear Behavior and the Features of Self-Organizing of Surface-Engineered Coatings A surface-engineered coating can act as a ‘‘protective screen’’ at the surface of a cutting tool (Fig 27) During steady-state wear, a gradual, but controlled wear of the coating takes place All these advantages became even more obvious when surface-engineered coatings are applied Tests done at increased cutting speeds (90 m=min)(for for HSS tools) enhance all the thermal processes associated with cutting Under these conditions, the heat-insulating effect of a hard TiN coating is diminished, the protective function of the coating is reduced, plastic deformation of the steel substrate can occur, and the stability of cutting is disrupted All these trends can be seen in the data presented in Figs 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 the zone 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 into processes other than surface damage, i.e compatibility of the tool and workpiece is realized to a great degree The coating plays the role of a protective screen for the contact surfaces It should be emphasized that the successful fulfillment of this function, however, is possible only when the external thermo-mechanical effects are localized in the coating layer Studies of coating wear have shown that the intensive self-organizing process observed during cutting only occurs when a surface-engineered coating was used Copyright 2004 by Marcel Dekker, Inc All Rights Reserved coatings becomes questionable due to their brittleness During the machining of several types of alloys (e.g stainless steels or nickel-based alloys), unstable conditions can dominate and surface damaging mechanisms become prevalent In this case, the ability of a thin surface layer to protect the surface, well as dissipate most of the energy generated during cutting, thereby minimizing the cracking of the tool, becomes critically important This is a practical application of the universal principle of dissipative heterogeneity [47] For the most demanding cutting applications a third type of coating— the self-lubricated hard coating—has been developed A typical example of this type of development is the multi-layer coating, TiAlN–MoS2, with two energy-dissipating mechanisms built into the microstructure [48] The first is associated with the formation of an oxygen-containing secondary structure (SS-I) that readily forms at the surface of the hard coating (TiAlN) and plays the role of a solid lubricant The second is associated with the thin MoS2 lubricating layer A second example of a similar technology is the use of nano-composite nc-TiN–BN coatings [49] These coatings give good results at moderate cutting speeds Following the earlier discussion, it seems likely that a high-alloyed Ti–B–O secondary structure of the first type (SS I, see above) and B2O3 both form The boron oxide plays the role of a liquid lubricant at the temperatures of cutting [50] The most important phase of the self-organizing process is associated with the running-in stage of wear During this stage of self-organization, the wear process gradually stabilizes and finally transforms to a stable (or normal) stage [7] It is very important to prevent surface damage and promote intensive self-organization at the surface during the running-in stage of wear using the phenomenon of screening [4,7] The less surface damage at the beginning of the normal stage of wear, the longer will be the tool life (Fig 2) Hard coatings are brittle and susceptible to extensive surface damage during this running-in stage Frequently, much of the hard coating is already destroyed at this phase, prior to the start of the stable (normal) stage of wear, 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 often leads to a dramatic decline in the wear resistance of the coating For this reason, 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 the most important goals for wear resistant coatings, especially at low and moderate cutting speeds, and for handling hard-to-machine materials where adhesive wear dominates This can be achieved by applying self-lubricated, multi-layer coatings These structures have many complex microstructural features that contribute to energy dissipation [e.g the TiAlN–MoS2 (or Copyright 2004 by Marcel Dekker, Inc All Rights Reserved Figure 30 Tool life of end mills with advanced coatings Machined material, 1040 steel Parameters of cutting: speed (m=min): 21; depth (mm): 3.0; width (mm): 5; feed (mm=flute): 0.028; cutting with coolant MoST) coatings [51,52], discussed earlier] One of the most effective commercial coatings of this type is the multi-layered TiAlN=WC-C hard lubricant coating developed by Balzers [53] The main advantage of this coating 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 good tribological properties at elevated temperatures All these oxides contain crystallographic shear planes with low shear strengths at high temperature [44] They are promising materials as solid lubricants for elevated temperature applications, and can be deposited by PVD methods The service performance of multi-layered coatings with an anti-friction top layer is characterized by the wear curves shown in Fig 31 The top (antifrictional) layer leads to a decrease in flank wear as soon as the running-in stage is completed, and the tool life is significantly increased (Figs 2 and 31) Unfortunately, not every mode of the running-in phase leads to the optimum self-organization [4,47], because damaging modes are also possible, especially during cutting Thus, the goal of friction control is to prevent serious surface damage at the running-in stage and transform the tribosystem from its initial state into a self-organizing mode If this can be achieved, Copyright 2004 by Marcel Dekker, Inc All Rights Reserved Table 6 Physico-chemical Propeties of Z-DOL [58,59] Property Molecular mass Average number of units in the molecule Molecular dimensions of SAM Density Thickness of epilamon layer Load-carrying capacity Maximum service temperature Value 2,194 12 5 nm 1,560 kg=m3 5–2,500 nm 3 GPa 723 K can be deposited by dipping the part into a boiling solution The physicochemical properties of Z-DOL are shown in Table 6 The thin film consists of a close-packed molecular mono-layer, that provides an even coating to a rough tool surface This coating has a high adsorption ability and due to its low thickness it also has high adhesion to the substrate and penetrates into pores The surface energy of oils contained in the typical coolant regularly used 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 latter film 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 seizure and wear during the initial stages of the tool service Studies of surface-engineered 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 friction characteristics are improved at the service temperature (5008C, Fig 23) The tool life data (Fig 31a) reflect a very low pattern of surface damage at the running-in stage of wear, leading to a marked improvement in the overall tool performance 3 ‘‘Smart’’, Multi-layered Wear Resistant Coatings Similar problems of friction control at service conditions leading to surface damage arise when the wear process changes from the normal to the ‘‘avalanche-like’’ stage As noted above, cutting tools made of HSS usually operate under conditions of adhesive wear, where seizure might occur, accompanied by a rapid increase in the wear intensity [56] Prolongation of the normal friction and wear stage, however, is quite feasible, even if seizure is a problem This can be achieved by applying an Copyright 2004 by Marcel Dekker, Inc All Rights Reserved follows: (1) elements forming stable protective surface films under frictional conditions [4], e.g O, N, and Cl; non-metals (e.g., B, C, Si) forming compounds with good tribological properties when they interact with base materials and elements in the environment; and (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, with good anti-frictional properties, and a low coefficient of thermal conductivity; and (d) metals (Ag, Cu) known to have a low coefficient of friction, and low mutual solubility when in contact with steel, nickel and titanium alloys (Fig 33) [63] (2) In addition, the study was extended to study surfaces subjected to treatments with: – four types of anti-friction alloys used to improve conditions of sliding friction, viz Zn þ Al(9%) þ Cu(2%), Cu þ Pb(12%) þ Sn(8%), Pb þ Sn (1%) þ Cu (3%) and Al þ Sn(20%) þ Cu(1%) þ Si(0.5%) [28]; – Zr þ N, W þ C, W þ N, Ti þ N, Al þ O, to create layers with a high wear and oxidation resistance The wear of these coatings was studied while turning 1045 carbon steels at a cutting speed of 70 m=min, a cutting depth of 0.5 mm and a feed rate of 0.28 mm=rev with and without a coolant The flank wear of tetragonal, indexable HSS inserts with multi-layered coatings was studied; when the flank wear exceeds 0.3 mm, the cutting tool loses its serviceability [3] The effectiveness of ion modification was determined by comparing the cutting time 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 additional step of ion modification Adhesion was determined using the scratch method Friction coefficients were determined with the aid of a specially designed adhesiometer shown in Fig 5 The results of these tests, summarized in Table 7, demonstrate to a large extent that the influence of the implanted elements on the tool life is determined by the cutting conditions The operational temperature during Copyright 2004 by Marcel Dekker, Inc All Rights Reserved Table 7 (Continued) N of group (subgroup) Material Babbitt BK2 GOST 1320–74 (Russia) Al–Sn–Cu AO20–1 GOST 14113–69 (Russia) Element composition Coefficient of PVD-coating adhesion to modified surface base Pb þ Sn (1.5%) 0.35 0.6 — Al þ Sn (20%) þ Cu (1%) þ Si (0.5%) 0.3 0.4 — 3.0 4.0 0.53 0.4 1.33 — 2.5 — — — Surface modified by ion mixing of wear resistant elements 5 AlþO 0.4 TiþN 0.6 ZrþN — WþN 0.4 WþC 0.4 Durability coefficient on cutting Without coolant With coolant a reduction in the adhesion of the tool surface to the processed material and, at the same time, an increased adhesion of the hard PVD-coating to the modified base material The data from Table 7 show that a class of anti-frictional alloys, widely used to improve the conditions of sliding friction [28,62], can double the tool life However, this method of increasing the tool life, i.e one that primarily depends on a reduction in the strength of the adhesion bonds between the tool and workpiece, is not the most efficient, as the adhesion of the coating to the modified surface was found to be rather low This precludes their usage, as de-cohesion of a coating cannot be tolerated in practical applications Implanting elements such as indium, silver and nitrogen enhances the tool 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 observation that indium and silver show little interaction with iron, and find use as solidstate lubricants (Fig 33) Nitrogen implantation probably leads to the formation of an amorphous film with improved tribological characteristics [65] Ion modification of the tool surface with the other elements studied led to unstable or negative effects, i.e a reduction in tool life and=or poor adhesion between the hard coating and the substrate Copyright 2004 by Marcel Dekker, Inc All Rights Reserved The most beneficial element in this study was indium The life of the tool 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 and indium-modified surface of the tool was sufficient to ensure a reliable tool performance Indium is a surface-active metal and usually displays a low tribological compatibility with traditionally machined alloys based on steel, nickel, and titanium [63] Because of this, the wear peculiarities of In-containing coatings have been comprehensively investigated [64] Scanning electron microscopy and x-ray microanalysis were used to study surface-engineered cutting tools, composed of an ion-doped HSS surface, nitrided by a glow discharge technique, with a hard PVD coating over the In-modified layer (Fig 34a) Figure 34 shows the microstructure of a 58 angle lap specimen (including the surface-engineered coating), taken in the SEM with the back-scattered electron signal which is sensitive to the mean atomic number Separate layers of the multi-layered coating (dark for TiN and gray for the In-rich sublayer) can be seen in the back-scattered electron image The thickness of this zone is about 6.0 mm, so that the true depth of the modified (gray) layer is about 0.3 mm It is probably a Fe-layer containing implanted Ar (as a result of etching by Arþ after nitriding) and In The presence of W in the tool steel increases the intensity of the x-ray In Ka radiation and the background emission This matrix effect influences the apparent 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 an In-modified surface (ion implantation) 600Â magnification (a) Microstructure of the angle lap section of the multi-layered coating (SEM image); (b) distribution of elements along the II direction (x-ray microanalysis) Copyright 2004 by Marcel Dekker, Inc All Rights Reserved 5008C) during (Ti, Cr) N deposition will modify the as-implanted In profile, which is expected to be about 0.3 mm in depth [66] Following the x-ray microanalysis, the intensity ratios of the characteristic lines Lb=La for an In standard (99.99% purity) and the nitrided specimen were found to be 0.63 and 0.97, respectively Changes in the intensity of the characteristic x-ray fluorescence are frequently observed when pure elements and their chemical compounds are compared [67] In this study, clusters of In–N are thought to develop in the zone of In implantation SIMS data (Fig 35) demonstrated that the ratio of the In concentration in a free state or present as clusters was approximately 10:1 To explain how the implanted indium influences the tool life, the following factors were investigated: (1) the dependence of the friction coefficient on temperature; (2) the distinctive features of indium oxidation in the wear zone (as investigated by SIMS); and (3) the development of oxides on heating specimens with an In- modified surface Figure 35 Secondary ion mass spectra from the wear zone of the cutting tool (cutting time is 30 min) Copyright 2004 by Marcel Dekker, Inc All Rights Reserved Figure 36 Impact of test temperature on the frictional properties of surface modified HSS cutting tools The temperature dependence of the friction coefficient demonstrated that In improves the frictional properties of HSS (Fig 36), by acting as a lubricant and reducing the shear strength (t) of the adhesion bonds developed in the tribo-couples This factor, however, is probably insufficient to Copyright 2004 by Marcel Dekker, Inc All Rights Reserved Figure 37 Change in the shape of the In 3d5=2 line from the photoelectron spectrum taken from the HSS surface after ion implantation and oxidation at 823K for: (a) 0 min; (b) 5 min; (c) 15 min; and (d) 20 min Pressure of oxygen in the chamber ¼ 2.5 Â 10À6 Pa account for the twofold increase in the tool life of cutters having an In-modified surface Mass-spectrometric analysis of the wear zone (Fig 35) suggests that the role of In is more complicated Apart from metallic indium, the wear zone reveals the presence of indium oxide, coming from both In and In–N dissociation and reaction during the wear process X-ray photoelectron spectroscopy (XPS) was used to study the changes in the shape of the In 3d5=2 lines in the electron spectra after oxidation Fig 37a–d presents the spectra obtained before and after heating the specimens to 823K, with exposure times of 0, 0.5, 15, and 20 min, Copyright 2004 by Marcel Dekker, Inc All Rights Reserved respectively The position of the In 3d5=2 peak in the starting sample corresponds to a binding energy of 444.8 eV Deconvolution of the spectra from the oxidized sample gave an additional peak, initially located at about 445.7 eV and, after a 25-min exposure, at 445.8 eV These higher binding energies correspond to the formation of the oxide, In2O3 The relative intensity of this line compared to In 3d5=2 (the ratio of IIn2O3=IIn) was 23% in the initial state (Fig 37a), increasing to 41% after 25 min (Fig 37d) Figure 38 presents the change in the relative concentration of In2O3 on the surface of HSS specimens during heating at 423K, 623K, and 823K for times up to 25 min At 823K, oxidation of the implanted indium rises quickly and saturates after about a 25 min exposure The edge of a cutting tool runs at a temperature of about 773K (5008C) during normal operations These conditions suffice for oxidation of a fraction of the implanted indium However, not all the indium is oxidized, as a part remains dissolved in solid solution in the iron matrix As the hard overlay (TiCr) N-coating is worn away, typically at the transition from the normal to catastrophic wear stage [43], the In-modified layer becomes exposed at the friction surface This usually coincides with the Figure 38 Change in the relative concentration of In2O3 on the surface of a HSS specimen after In-implantation and oxidation at temperatures: (1) 423K, (2) 623K, and (3) 823K Copyright 2004 by Marcel Dekker, Inc All Rights Reserved point where the protective PVD-coating detaches from the contact face of the tool Under conditions of high load and high temperature, partial oxidation of In will probably start before the complete destruction of the PVDcoating Since normal friction is characterized by minimal depth of damage of the contact surface [4], even a relatively thin modified layer can enhance the tool life Indium improves the frictional properties of the surface and reduces the sticking intensity over the friction surface In addition, an oxygen-containing amorphous In–O film, formed by interaction with the environment, is likely to enhance favorable friction conditions in the contact zone of a cutting tool Indium lies in the same group as Al in the periodic table, and probably forms oxygen-containing phases having a low coefficient of thermal conductivity These protect the tool surface, enhance the thermal conditions of cutting, and delay the onset of catastrophic wear Thus, the influence of In is twofold: on the one hand it acts as a metal lubricant; on the other, it forms protective oxygen-containing phases Indium enhances both the self-organization of the system and extends the stage of normal and stable wear, in accord with the principal laws of friction control [4] The data presented in Table 7 show that the highest wear resistance after the triple surface treatment is achieved when transition metals together with nitrogen are used to modify the surface by ion mixing Compounds such as TiN, ZrN, WN, WC, Al2O3 do not appear to form, the implanted ions remaining in solid solution The best wear resistance is shown by a 1 mm thick layer modified with Ti and N (Fig 39a and c) At the same time, implantation can lead to amorphization of the surface layer (see Fig 40) The decrease of the peaks intensity at Fourier transform at remote interatomic distances shows that this modifies the wear mechanism due to a delay in surface crack propagation [68] The diffusion of the implanted nitrogen into the chip and the reverse flux of oxygen into the tool surface lead to a partial replacement of implanted nitrogen by oxygen during cutting (Fig 39c) The rapid formation of a protective secondary structure takes place (Fig 41), since the initial structure of the surface after mixing is similar to the structure of the films formed at the friction surface as a result of the self-organizing process Ion mixing can produce thin surface layers with a fine, so-called, nanocrystalline structure [68] As noted above, the secondary structures have a similar microstructure, an amorphous supersaturated solid solution of oxygen (coming from the environment) having been formed by reaction with the metal component of the tool material [4,55] Ion mixing enhances this process, which naturally evolves in the tribosystem during the self-organizing stage and results in the formation of a stable secondary structure In the final stage of wear, oxygen from the environment penetrates through the numerous pores and cracks in the PVD coating to the surface of the Copyright 2004 by Marcel Dekker, Inc All Rights Reserved formed on the near-surface layers These secondary structures (SSs) delay the transformation to the avalanche-like stage of tool wear, and the tribosystem can again revert to a stable state (i.e to a normal pattern of wear) From our point of view, this is the most beneficial effect of the modified Ti þ N layer on the wear behavior The behavior of multi-layer coatings illustrates an important principle that can be used to design effective materials for cutting tools The more energy dissipation channels that can be built into the microstructure for the transitional (non-steady) stage of tool wear, the longer will be the tool life These channels can operate simultaneously in the same stage of the wear (e.g during the running-in stage), but also subsequently, using multi-layer coatings, when the wear process transforms from one stage to another After completion of the first stage of wear and exhaustion of the corresponding channel of energy dissipation in the top layers, the next layer of a multilayered coating can be used to control the ‘‘avalanche-like’’ wear with alternative channels of energy dissipation [37] In this way, a multi-layered, ‘‘smart’’ coating can be developed where each layer fulfills a given function at a definite stage of wear (Fig 32), leading to high serviceability over a wide range of operating conditions This concept has been widely used for protective coatings, corrosion control [70], and, as shown above, this concept can be extended to wear resistant coatings At the stable stage of wear, the coating must have adequate strength and toughness, and a stable SS In the unstable stage(s), the coating must have sufficient energy dissipation channels to prevent surface damage A multi-layer coating that includes an adaptive top layer, composed either of oxides with favorable friction properties (such as WO3, V2O5) or a self-lubricated layer, a working layer of a superhard or self-protecting coating, and an anti-frictional sublayer form the basis for future ‘‘smart’’ coatings 4 SuperHard (nano-composite=superlattice) and Self-Protecting Coatings An alternative way of improving the performance of cutting tools relies upon the deposition of multi-component compounds Recent improvements in the lifetime of cutting tools have been achieved by the development of titanium aluminum nitride (Ti,Al) N coatings (see Fig 30) The results of milling tests with TiAlN coatings have demonstrated that the wear behavior is improved by lowering the running-in wear and increasing the duration of the period of normal wear This can be achieved when all the interactions between the tool and workpiece are localized in a thin surface layer, i.e a striking demonstration of ‘‘tool–workpiece’’ compatibility Films such as TiAlN with a Ti=Al ratio of 1.0 [71,72] display a unique combination of Copyright 2004 by Marcel Dekker, Inc All Rights Reserved Table 8 Oxidation Stability of the Compound (PVD, CVD coating) [74] Coating TiC Ti(C,N) TiN ZrN CrN CrC TiAlN(50:50) TiC þ Al2O3 Loss of oxidation stability (max working, T 8C) 400 450 550 500=600 650 700 850 1,200 properties, viz a high hardness at elevated temperature together with thermal and chemical stability (i.e., stability to diffusion, dissolution into the chip and oxidation stability) Considerably, more heat is dissipated via chip removal An extremely important advantage of (Ti, Al) N coatings is their oxidation stability up to 850–9258C [73], due to the formation of stable oxide films (a mixture of rutile and alumina) [74,75], see Table 8 Stable tribo-ceramic films (SS-II) can then be formed on the surface during cutting [76] and limit the diffusion of the coating material into the workpiece These compounds are probably a mixture of alumina and rutile, as found in the oxidation of titanium aluminides [77,78] Another recent development is the application of compounds that ensure stable friction and wear under high-speed=high-stress cutting conditions Two methods have been developed for this application The first is based on the use of advanced multi-component coatings, i.e the so-called ‘‘superhard’’ coatings, with a room temperature hardness in excess of 40 GPa, and excellent oxidation resistance up to 10008C Under high-speed machining conditions the surface of the tool can reach 10008C, so the coating should be stable at this temperature [68] The development of nano-composite coatings with very fine grains (about 10 nm or less) illustrates the potential of this class of material The mechanical behavior of nano-composite materials may be controlled by the response of the grain boundary, because the number of atoms in the grain can be comparable to that in the boundary regions Grain boundary sliding can replace dislocation climb and glide as the dominant plastic deformation mechanism These materials can be prepared only by methods that simultaneously ensure a high rate of nucleation and a low rate of growth Magnetron sputtering and filtered arc deposition can be used for the production of nano-crystalline films [42,68] Copyright 2004 by Marcel Dekker, Inc All Rights Reserved In both methods, a highlyionized plasma ensures rapid crystal nucleation on the one hand and very fast cooling rates on the other The kinetic energy of the bombarding ions is transferred into very small volumes, of atomic dimensions, and the cooling rate of the film is high [68] These are highly non-equilibrium processes The most familiar type of superhard coating is nc-MeN=a-nitride, where Me ¼ Ti, W, V, Zr or other transition metals and a-nitride is an amorphous Si3N4 [79] The high hardness is associated with the formation of isolated nanocrystals of the nitride phase dispersed in an amorphous matrix, so that dislocation motion and grain boundary sliding are suppressed The second type of superhard nano-composite coating, nc-MeN=metal [68,80], relies on a combination of soft and hard materials such as Cu, Ni, Y, Ag, and Co with TiN (or other nitrides) [81] Superhard nano-composite films have a high hot hardness (beneficial for flank wear), a high resistance against crack formation, and increased thermal and chemical stability (beneficial for crater wear resistance), as shown by their high oxidation stability up to 11508C [82] As noted before, the unique properties of these coatings, in particular the nc-TiN=aSi3N4 or TiAlSiN coatings [83], are associated with the small dimensions of the nanocrystals (1–10 nm) Strong segregation effects can lead to a thermodynamic stabilization of the grain boundaries, with a high energy of activation for grain coarsening [79] Coatings such as nc-TiN=a-Si3N4; nc-TiAlSiN, and nc-TiN-ncBN [83–87] are promising materials for cutting tool application TiAlN coatings could also be superhard [68] and show excellent wear resistance at high-speed cutting Superlattice or multi-layer coatings with a superlattice period ranging from 5 to 10 nm have also been developed for cutting tool applications The bi-layers in these superlattice structures can be metal layers, nitrides, carbides, or oxides of different materials or combinations of these compounds such as TiAlN=NbN; TiAlN=CrN or TiAlN=VN [88–90] The mechanism of hardening in these coatings is associated with the restriction of dislocation motion across an interface or within the layer itself, due to the suppression of the normal dislocation source and multiplication effects encountered in bulk materials [88–90] If we summarize all the known data of superhard coatings and ionmixed structures and compare them to the properties of dissipative structures, it is apparent that the self-organization in these systems at extremely non-equilibrium conditions of the coating deposition process can be associated with the formation of a stable, nano-structured material [91,92] A novel material (in the form of a thin film) is created at the surface whose characteristics are very similar to the extreme properties of the dissipative, secondary structures associated with friction The material is both Copyright 2004 by Marcel Dekker, Inc All Rights Reserved very hard and as chemically stable as SS-II, while at the same time, its structure is similar to the amorphous state of SS-I Thus, it is possible to create an ‘‘artificial’’ material, possessing unique, but previously unattainable properties, and a new level of tool performance under the extreme conditions of high-speed cutting This is an exciting yet practical realization of the underlying principles of friction control Further improvements in the use of coatings for high-speed machining will depend on a better understanding of the stable tribo-ceramics that form at the surface of superhard nano-composites To achieve this research goal, the elements of the coating composition that have an ability to act synergistically must be investigated All known commercial coatings (e.g TiN, TiCrN, TiAlN) and even ‘‘state-of-the-art’’, superhard coatings probably generate only tribo-ceramics such as rutile or mixtures of rutile and alumina or rutile and chromia that possess limited stability at high-speed cutting The generation of a stable continuous film of alumina on the surface could be a goal for the development of new coatings for high-speed cutting applications It has been proven indirectly that a thin layer of alumina was formed on the surface of PVD TiAlN coatings, and multi-layered coatings with excellent properties have been reported [93] Recently, some companies (e.g Bulzers, CemeCon) have started to deposit an aluminum-rich layer (65–75 at.%) in the coating to ensure the formation of a protective alumina layer on cutting [94] The formation of an alumina-like SS-II on the surface of the coating during cutting might enhance the tool life For this type of coating, other alloying components in the hard coating might act synergistically to promote the formation of stable triboceramics A promising composition of this type is based on TiAlCrN coatings [95] It is known that certain ternary TiAlCr alloys form a very stable alumina layer during high-temperature oxidation, in contrast to TiAl alloys where the oxide that forms is non-protective [96] Several authors have reported on the beneficial properties of these coatings for high-temperature applications [97] TiAlCrYN and TiAlN=CrN superlattice coatings show excellent oxidation resistance [90,98,99] while the former demonstrated promising wear resistance at elevated temperature [100] The addition of Y drastically reduces the grain size [101] and leads to superhard coatings In ternary TiAlCr alloys, chromium forces the other elements to act synergistically, and forms a protective alumina film at the surface The development of the next generation of coatings for high-speed machining could combine the properties of superhard coatings, as outlined above, with an ability to generate a protective alumina layer during cutting (the principle of self-protection) An alternative technology is the use of stable ceramic coatings (e.g alumina or zirconia) [102–104] These ceramics are the most stable and wear resistant materials for high-speed cutting applications Unfortunately, these Copyright 2004 by Marcel Dekker, Inc All Rights Reserved ceramics are brittle, but when they are used as thin films in coatings, this problem can be mitigated In this chapter, we have described how current coating technologies might be exploited to develop and bring new tribological tool materials to the marketplace, based on the concept of a functionally graded microstructure We believe 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Marcel Dekker, Inc All Rights Reserved  ... the chip), and by promoting the selforganization of the tool This is done in two ways: (1) by surface engineered and self-lubricated coatings for low and moderate speed machining, and (2) by... most important goals for wear resistant coatings, especially at low and moderate cutting speeds, and for handling hard-to-machine materials where adhesive wear dominates This can be achieved by applying... [4], e.g O, N, and Cl; non-metals (e.g., B, C, Si) forming compounds with good tribological properties when they interact with base materials and elements in the environment; and (3) metals including: