Strength of Materials, Vol. 42, No. 3, 2010

EVALUATION OF RESIDUAL STRESSES IN PVD-COATINGS. PART 1. REVIEW O. B. Soroka

UDC 621.539.4

We present a review of the results of investigations on residual stresses in plasma-vacuum coatings obtained by the physical vapor deposition methods. The data on the character of residual stresses and factors influencing their values and distribution are analyzed. The works are considered that investigate the effect of residual stresses on the physico-mechanical characteristics of the substrate– coating system. Recommendations on the further studies of residual stresses in PVD-coatings and the improvement in their level control are presented. Keywords: vacuum plasma coatings, residual stresses, thermal and structural components of stresses, reference potential, intermediate and buffer interlayers, multilayer coating, nanostructural and nanolayer coatings, discontinuous coating. Introduction. One of the peculiar features of modified surface layers of structural materials is the occurrence of residual stresses therein. Residual stresses in functional coatings affect the performance characteristics and life of parts and tools with coatings, as well as the strength characteristics of both the coating itself and the substrate–coating system. The account of these stresses is required, since the residual stresses promote or inhibit fracture processes, depending on the sign, distribution and mode of loading. Therefore, a strong need exists to correctly estimate the level of residual stresses, to reveal the reasons for their occurrence, to establish the manufacturing factors and parameters of the surface architecture that influence their value. The use of coatings obtained by the physical vapor deposition (PVD) method that has started in the sixties of the last century, is gaining further acceptance, particularly for parts and tools operating under conditions of heavy loads and high temperatures. The goal of the work is to review published investigations involving the defining of the physical nature and value of residual stresses in PVD-coatings, the revealing of the characteristics of the coating formation process and the parameters of the surface architecture that influence these stresses, and also the establishing of the relationship between the strength and wear resistance of the substrate–coating system and the level of residual stresses. We hope that this review will make it possible to state the problems and ways for further investigations on the residual stresses in PVD-coatings and to find the methods for attaining such “useful” level of these stresses that will agree with the maximum mechanical properties. Residual Stresses in PVD-Coatings: Value and Distribution, Physical Nature, and Dependence on Manufacturing Parameters. The occurrence of residual stresses is due to different factors, such as the difference in the coefficients of thermal expansion for the substrate and coating materials; the presence of the captured atoms of gas; the condensate imperfection, etc. Assumptions on the physical nature of residual stresses in PVD-coatings are somewhat different. In [1], using the X-ray method, it was determined that residual compressive stresses in steel-based TiN-coatings reach the values of -4 GPa at room temperature, whereas at the deposition temperature ( 475°C) they decrease to zero and are converted into tensile stresses with increasing temperature, which is indicative of their thermal character. As a result of the theoretical calculation [2] of the thermal residual stresses in stainless steel- and nickel-based TiN-coatings, the respective values of -1. 9 and -1. 22 GPa were obtained. The authors claim that these data are in agreement with those obtained by the experimental X-ray diffraction technique. Pisarenko Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kyiv, Ukraine. Translated from Problemy Prochnosti, No. 3, pp. 66 – 78, May – June, 2010. Original article submitted March 10, 2009. 0039–2316/10/4203–0287 © 2010 Springer Science + Business Media, Inc.

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Fig. 1. Dependence of the residual compressive stresses s res (¿) in the TiAlN-coating and its nanohardness H (¢) on the absolute value of the negative reference potential U [11]. The statement that residual stresses in PVD-coatings contain the thermal and structural components is widespread and convincing [3–8]. The thermal component of the residual stresses is caused by the difference in the thermal expansion coefficients for the substrate and coating materials: during the cooling from deposition to room temperature, the substrate and coating attain different values of strain, which results in the occurrence of residual stresses. The structural stresses are the consequence of the process of the increasing of the coating layer and are caused by strong imperfections of the condensate. As was found by the authors who investigated the residual stresses in PVD-coatings, their sign and value are dependent on the deposition conditions, such as the reference potential of the substrate, the current intensity of an arc evaporator; the substrate temperature; nitrogen pressure in the vacuum chamber. As shown in [3], during the layer deposition, the substrate temperature increases, resulting in the decrease of the value of residual stresses. The authors relate this to the increase in the surface atomic mobility that promotes the formation of a more perfect structure of the coating and the decrease of the stacking fault concentration. This fact also confirms the prevailing effect of the structural mechanism for the occurrence of residual stresses (according to the temperature mechanism, the stresses shall be increased with increasing temperature). As shown in [4, 9–11], the value of the reference potential is responsible for the ion flux density and energy affecting the grain size, composition, morphology, texture and lamination of a coating – the parameters by which the level of residual stresses is, in turn, determined (Fig. 1). In [12], it is noted that residual stresses occur due to a decrease in spacing between growing crystallites. In the above work, an attempt was made to create the model for the occurrence of residual stresses based on the statement that under conditions of ion bombardment, the transition from residual tensile stresses to compressive stresses is performed with the increasing ion flux. The parameter is introduced that characterizes the ratio of the number of ions to that of atoms, whose increase is accompanied by the transition from the positive to negative values of residual stresses. It is shown that the greater the ion flux energy, the smaller the ratio of the number of ions to that of atoms at which tensile stresses are transformed into compressive stresses. As a rule, considerable compressive stresses in the coating plane are inherent in PVD-coatings. In [13] it is shown that for Ti(CN)-coatings, the stresses measured by the method of free bending of a thin plate are -2 to -3 GPa, and the temperature stresses are 10 to 20% of this value. In works [6–8], it is also argued that in coatings the stresses reach -5 GPa and are defined by the thermal and structural components, the value and sign of which are dependent on the base material and displacement potential, with the structural component dominating over the thermal one. The authors [7, 14] argue that in the coating, the stresses that are due to the structural component, in contrast to those that are due to the thermal component, are independent of the base material and, at the same time, are dependent on the coating thickness [15–17], decreasing with its increase. 288

Earlier in [18], the values of residual stresses in TiN-, Ti(CN)- and (Ti, Al)N-coatings on titanium alloys were obtained in the range from -1 to -5 GPa, which decreased with the increasing thickness of the coating and their value was dependent on the parameters of conditions for its deposition (temperature, reaction gas pressure). It was found in [19] that, depending on the ratio of boron and nitrogen atoms in the coating composition, the residual stresses in boron nitride coatings were from -5 to -1 GPa. The residual compressive stresses for Ti(CN)-coatings on a specimen of niobium alloy were from -2 to -3 GPa [20]. According to the data in [21], the residual compressive stresses in the TiN coating deposited by reactive sputtering in the absence of oxygen were from -7. 8 GPa. In the above work it was shown that with the increasing oxygen concentration, these stresses in TiOxNn-films decrease and are transformed into weakly tensile stresses. This is caused by the diminishing of the lattice parameter due to the substitution of nitrogen by oxygen atoms and the formation of an amorphous phase. The X-ray studies of residual stresses in Ti(CN)-coatings show [16] that their values are in the range from -4. 6 to -5. 9 GPa, are dependent on the coating thickness and increase with the increasing reference potential. As revealed by the calculations using the flexible sample method, with the increasing reference potential, the residual stresses in TiN-coatings increase from -4. 2 to -7. 2 GPa [14]. In [15], the residual stresses in TiN-coatings were determined to be -4 GPa. According to data from [22], their value in TiN- and (Ti, Al)N-coatings reaches -15 GPa. In [23], using the X-ray diffraction (XRD) method, the values of residual stresses were determined to be at a level of -8 GPa, while reaching -17 GPa in (Ti, Cr)N-coatings, which, in the author’s opinion, is caused by different directions of crystallite growth in coatings. In [17], as shown by measurements of residual stresses with the sin 2 y-method, these are -5. 6 GPa. Layers of the multilayer TiN/(Ti, Al)N-coating are also in the state of compression with the stress level in the range from -2. 7 to -4. 2 GPa for (Ti, Al)N and from -8 to -10.1 GPa for TiN. The fact that the stress value in thinner layers of the multilayer TiN-coating is higher than that in a monolayer is explained by that the residual stresses decrease with the increasing coating thickness. This is attributable to the step-by-step improvement in the structure of subsequent (grown by deposition) layers [24]. In [25], it is obtained that residual stresses for nanostructural CrN/NbN-coatings increase from -4. 4 GPa at the substrate potential of -75 V to -9. 5 GPa at -150 V. The values and distribution of residual stresses in a 3 mm-thick TiN-coating are obtained based on the finite-element simulation in [26], where considerable compressive stresses almost uniformly distributed over the coating thickness (-3. 2 GPa) in the coating plane (along the interface) are found to be balanced by tensile stress in the substrate (250 MPa). The tensile stresses are also acting in the coating in the direction normal to the surface, the maximum value of which (500 MPa) is observed in the coating near the interface. According to calculations, their value increases with the increase in the coating thickness and decreases with the increase in the hardness of the base material (Fig. 2). The distribution of residual compressive stresses is obtained using the method of a flexible sample whose deflection was measured after a step-by-step removal of coating layers due to chemical corrosion. The stresses are found to increase in the direction from the interface attaining the maxima in the center of the coating followed by a decrease near the surface [12]. Works [6, 13] also show that the tensile stresses in the substrate and shear stresses on the interface surface are balanced by considerable residual compressive stresses in the coatings. As determined by the analysis of the stress-strain state on the interface surface [13], the shear stresses are 0.1 to 0.2 GPa. According to the data in [6], the tensile stresses in the substrate that correspond to considerable compressive stresses (-5 GPa) in the coating are 100 to 200 MPa. As found in [2], the estimated values of the stresses in the substrate of a stainless steel reach 1.5 GPa and 305 MPa at the stresses in the coating of -1. 9 GPa and -380 MPa, respectively. In addition, the value of tensile stresses from 30 to 70 MPa in the substrate with a vacuum plasma deposited coating was obtained in [27] by way of the calculation. The distribution of residual stresses in a 3 mm-thick hard-alloy plate–(Ti,Al)N-coating system was obtained by the authors of [28] based on the X-ray studied. It is shown that the residual compressive stresses, as in [12], increase from the zero values on the interface surface and reach the maximum in the center of the coating with a subsequent decrease to -1. 2 GPa near the surface. These compressive stresses in the coating are balanced by tensile stresses in the interfacial region of the substrate, while reaching the maximum (400 GPa) at a depth of about 1.5 mm from the interface (Fig. 3). 289

a

b

Fig. 2. Distribution of residual stresses [26] obtained based on the finite-element simulation in 3 mm-thick TiN-coating in the plane of the coating (along the interface) (a), and in the direction normal to the surface (b).

Fig. 3. Distribution of residual stresses over the depth of a 3 mm-thick (Ti, Al)N-coating and in the near-surface areas in lateral faces 1 and 2 of the hard-alloyed plate [28]. Relationship between Residual Stresses and the Mechanical Characteristics in a Substrate–Coating System. The presence of considerable residual compressive stresses favorably affects the characteristics of the substrate–coating system, such as the fatigue limit [29] and yield strength [30]. According to the data of [31], the tensile strength value correlates with residual stresses. The relationship between the strength of a hard alloy in bending and the value of residual stresses in a TiN-coating is noted in work [32]. A relatively high level of these stresses is shown to ensure a high strength, minimum chipping of the cutting tool edges and the best characteristics during grinding and milling. It is the relatively high level of residual compressive stresses that is associated with the enhanced crack growth resistance, wear resistance and corrosion resistance of surfaces modified by vacuum plasma coatings [33–35]. As noted in [36], a high level of residual compressive stresses (is -0. 8 GPa and about -3 GPa for TiN- and TiCN-coatings, respectively) retards the processes of crack formation in coatings during the operation of the cutting tool, especially, under intermittent cutting conditions. In the authors’ opinion [37], the increase in the residual compressive stresses to -660 to -720 MPa after the deposition of a vacuum plasma coating, preliminarily hardened by ultrasound, on a helicopter engine compressor blade of alloy VT8M, is responsible for the decrease of the alternating operating tensile stresses and retards the process of fatigue crack nucleation at blade edges. As a result, the fatigue limit of blades increases by 19%. 290

As shown in [38], in the analysis of the slip contact based on the finite-element model involving the increasing normal loading over the surface of the high-speed cutting steel with a 2 mm-thick TiN-coating and the mean level of residual compressive stresses in the coating of -1 GPa, the presence of residual stresses results in the 10 to 50%-increase in the maximum tensile stresses on the surface behind the contact area and the increase in the compressive stresses in the coating directly before the contact, whereas the service compressive stresses in the base material decrease due to the presence of residual tensile stresses in the substrate. At the same time, it is found that, if the plastic deformation occurs in the substrate during operation, the relaxation of residual stresses takes place in the coating. The occurrence of the first cracks on the coating surface also results in the relaxation of these stresses; therefore, they do not influence the process of the subsequent crack formation any more. In [39] it is emphasized that the problem concerning the effect of residual stresses on the level of residual tensile stresses at which the coating does not crack, remains completely unclarified and requires further study. In the investigation of [40] it is shown that the shear stresses between crystallites of the TiN-coating of a columnar structure decrease with the increase in the residual stresses. This gives grounds to consider that the presence of high residual compressive stresses provides the strength of the above coatings. As is pointed out in [11, 41], the coating microhardness depends on the level of residual stresses. An increase in the residual compressive stresses from -1. 7 GPa, at the substrate potential of 40 V, to - 5 GPa at 70 to 200 V and a simultaneous decrease in the hardness of the (Ti, Al)N-coating (Fig. 1) is related to the different orientation of crystallites in the formed coating [11]. At the same time, the authors of [40] imply that the microhardness value is not a function of the residual stresses. Together with it, a high level of residual compressive stresses can result in the crack formation and growth along the interface, and, also, in the coating buckling (the loss of stability) with subsequent cracking [42, 43]. At the same time, it is pointed out that in the absence of service loads, for coatings on plane surfaces, the buckling takes place initially and then forces occur that are sufficient for the interface crack growth [42]. Moreover, the expressions to define the critical value of the residual stresses corresponding to the buckling are proposed in [42, 43]. In [44], it is shown that the field of residual stresses defines the path of brittle crack propagation in the case of both the local fracture under the action of an indenter and the complete spontaneous fracture of the coating. A spontaneous delamination of the coating from the substrate still prior to the beginning of the operation due to considerable compressive stresses is observed in [40], whereas in [41] it is reported about the decrease in the coating–substrate adhesion due to residual compressive stresses in the process of operation. The authors [45, 46] also state that due to too high level of residual stresses, coatings become more brittle and their adhesion with the base material loosens. In [28], the beginning of the crack formation in the tool base material is related to the presence of the residual tensile stresses in the surface area of the substrate, added by tensile stresses that occur in the process of the tool operation. Experimental investigations show that too high values of residual stresses in hard ceramic coatings can result in their local fracture and delamination, for example, either at the edge or on the surface of considerable roughness when the coating thickness exceeds the edge radius or the surface roughness [38]. At the same time, thin (with respect to the roughness parameters) coatings are less prone to fracture due to the action of residual stresses. In the authors’ opinion [25], the dependence of the adhesive strength on residual stresses is extreme and its maximum is observed at the residual stresses of -5. 4 GPa and the substrate potential of -95 V. It is found in [47] that there exists the level of residual stresses that is optimal from the standpoint of ensuring the load-carrying capacity of a coating–substrate system. Effect of the Surface Architecture on the Values of Residual Stresses. A “useful” level of residual stresses promotes the enhancement of the wear resistance of coatings, the prevention of their premature cracking and improvement in the mechanical characteristics of the substrate–coating system, whereas at a too high level, coatings become more brittle, premature delamination from the substrate takes place. Therefore, too high residual stresses should be lowered and kept at a “useful” level. It is possible to attain this by improving the process of coating deposition and the creation of the corresponding surface architecture. Among the improvements in the surface architecture are the formation of systems with intermediate and buffer interlayers and multilayer coatings including the micro- and nanostructural, as well as nanolayered ones. Thus, the diminishing of the residual stresses in a coating–substrate system is possible by means of thin metallic interlayers inserted between a hard PVD coating and the substrate [26, 40, 48–5]. The authors [26] managed to obtain 291

s res , GPa

hb , mm Fig. 4. Dependence of residual compressive stresses in a TiN-coating on the thickness hb of the buffer Ti interlayer [54]. Ti/TiN-coatings having the thickness of up to 16 mm with reduced residual stresses, as compared to TiN-coatings, and improved adhesive strength in the coating–substrate system, and also a higher wear resistance, especially, under conditions of corrosive and erosive wear. According to the data of [52], the insertion of a thin titanium interlayer makes it possible to lower the level of residual stresses, to ensure the adhesive strength and to improve the resistance to abrasive wear. The enhancement in the resistance under conditions of corrosive wear in a stainless steel-Ti/TiN-coating as compared to a single layer TiN-coating system is also explained by the diminishing of residual stresses and improvement of the substrate– coating adhesion ability [41]. As is noted in [53], the adhesion provided by interlayers, such as those of Al, Ti or Cu located between a hard (Ti, Al)N-coating and a steel substrate, is improved with the decreasing modulus of elasticity of the interlayer material. It is shown [55] that it is reasonable to locate a titanium interlayer, which came to be known as a buffer one, not only between the coating and substrate but also inside the TiN-coating itself. Residual stresses in the coating decrease with the increase in the buffer layer thickness (Fig. 4). As was noted earlier in [56], almost three-fold decrease of residual stresses in the TiN-coating is found when a layer of pure chrome is inserted between the substrate and coating. According to data of [26, 34, 48, 49], the multilayer architecture makes it possible to control the level of residual stresses in the coating and thus to obtain coatings of greater thickness without reducing the adhesive strength. As noted above, in [17] it is shown that residual stresses of TiN layers of a multilayer TiN/(Ti, Al)N-coating are much higher than those of a single TiN-coating. The conception of coatings with nano-sized grains [55–59] and multilayer coatings with micro- and nano-sized grains and nanothicknesses of layers [48, 49] is one of the most promising ways of improving functional coatings, including the improvement in terms of their stress state. For example, a lower level of residual stresses and a higher resistance to erosive wear of a nanostructural TiN-coating agree with the minimum size of crystallites (9 nm) [57]. A new conceptual approach to the formation of the architecture of surface layers involving the creation of surfaces that have a discrete topography [60] has been proposed and developed at the Pisarenko Institute of Problems of Strength of the National Academy of Sciences of Ukraine. In contrast to coatings obtained by the electric-spark doping technique and the methods of surface modification using highly concentrated heating sources, for PVDcoatings, in particular for coatings obtained by cathode-ion bombardment, a purposeful replacement of a continuous surface layer with a discontinuous (a fragmented) layer takes place [61. 62]. The discretization makes it possible to retain the barrier function of continuous coatings and, at the same time, to avoid the disadvantages of the above coatings, such as cohesive cracking and adhesive delamination. Moreover, it is possible not only to lower the level of residual stresses in the coating, but to obtain the desired “useful” values of stresses by varying the dimensions of the discrete areas [56, 63], which, in particular, enables the enhancement of wear resistance of hard-alloy cutting tool [64]. 292

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