Strength of Materials, Vol. 45, No. 3, May, 2013

FATIGUE STRENGTH OF METALS WITH HARDENING COATINGS (REVIEW) A. G. Trapezon, B. A. Lyashenko,

UDC 621.792.4

and M. O. Lysenkov1 Investigations on the fatigue strength of structural materials with coatings are analyzed. The results of validation of both some theoretical approaches and numerous experimental data are presented for materials with coatings obtained by ion nitriding methods or by a physical vapor deposition method. Keywords: fatigue limit, hardening coatings, residual stresses, ion nitriding, physical vapor deposition. Introduction. The life and service life of engineering structures can be extended by the surface hardening of components using coatings applied by different methods: electroplating; chemicothermal treatment; gas-thermal sputtering; electric spark doping; laser thermal treatment; chemical vapor deposition (CVD). The process of physical vapor deposition (PVD) includes the methods of ion nitriding (IN), ion implantation (II), and gas vapor condensation. Most of these methods are alternative, i.e., the same material can be applied by different ways, and in doing so, the properties of both the base and coating may vary within wide ranges. The use of certain means of surface hardening requires relatively high temperatures and long-term treatment. As a rule, high temperatures adversely affect the properties of the parts to be hardened [1]. The most important characteristic determining the service life of structures is the number of cycles to failure being the main fatigue strength characteristic, i.e., the material or component ability to withstand alternating stresses. The opinion has been formed in the practice that coatings should inevitably reduce the fatigue limit [2]. However, in a number of investigations, including those carried out at the Pisarenko Institute of Problems of Strength of the National Academy of Sciences of Ukraine, the results were obtained that showed no evidence of this. Thus, using, as an example, nitride coatings deposited on titanium alloys by PVD technique, the possibility of increasing the fatigue limit s -1 due to the optimization of the process parameters was shown [3–5]. Nevertheless, in gas turbine building, a reduction in the fatigue limit by 10 to 15% is considered to be allowable when erosion- and corrosion-resistant coatings are applied [6], and in the theory and practice of helicopter GTEs, the fatigue limit s -1 values are expected to be reduced by tens of percent [7]. Based on the above, the problem of practical selection of techniques, types and conditions of surface hardening involving coatings seams to be pressing. Its solution can simplify the generalized analysis of the results of investigations on this subject matter, especially those performed recently. Here, the experimental results can be of primary applied importance since attempts of theoretical predicting the properties and behavior of materials with coatings cannot always lead to the goal, in view of the imperfection of computational models that allow one, at best, only to define the reference points from the expected strength characteristics or from the selection of the technology and the ways for its optimization. General problems of the theory of heat physical and physicochemical phenomena accompanying the application of coatings are given in [8–11], practical issues of producing plasma ion assisted deposition coatings are presented in reviews [12–14]. Pisarenko Institute of Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine ( [email protected]). Translated from Problemy Prochnosti, No. 3, pp. 42 – 57, May – June, 2013. Original article submitted September 10, 2012. 1

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Theoretical Approaches. The fundamental theory for practical evaluation of the strength properties of materials with coatings is not sufficiently developed due to the impossibility of a full taking into account the variety of real factors influencing the strength. The simplest approach is based on the additivity relationship [15, 16]: s = s s - v(s s - s c ),

(1)

where s, s s , and s c are the stresses in the substrate/coating system, in the substrate and coating at fixed strains, respectively, and v is the volume content of the coating. Relationship (1) is best satisfied at a sufficiently high degree of adhesion between the substrate and coating. It follows from (1) that s > s s if s c > s s , i.e., it can be assumed that the hardening of the composition is possible in the case of high values of the coating strength exceeding the strength of the substrate. Starting from this, the investigations aimed, in particular, at obtaining high-strength film elements as an analogue of coatings, are promising. High values of film strength are realized by dispersing their structure and can be presented analytically in the form of the empirical relationship s Y = s * + kd - n ,

(2)

where s Y is the yield stress or flow stress, s * and k are the parameters of the given material, d is the grain size, and n = 0. 5 to 1.0 (n is the constant). For n = 0.5, relationship (2) is known as the Hall–Petch relation that is more or less valid for metals. In the non-equilibrium state, films essentially contain all the defects of a crystalline lattice and have a high dispersion structure [17]. Therefore, experimental data for polycrystalline films can be described by the relationship [18]: s u = s * + kh -0. 5 .

(3)

This expression corresponds formally to the Hall–Petch relation if the grain size in (2) for n = 0.5 is replaced with the film thickness h. According to the data of [18], this replacement is valid only within a narrow range of thicknesses. In [19], as applied to the yield stress of microlayer Fe–Cu compositions, the range of h =10 to 100 mm is established; for h =10 to 50 mm, the relationship s = s * + kh -1 is satisfied better. Due to the absence of the physically substantiated theory and in view of a rather ambiguous physical meaning of the parameters s * and k, relationships (2) and (3) are of little use for the practice but can be useful for estimation calculations. On the Primary Verification of Theoretical Statements. The data on the fatigue strength of condensed films are given in [20] using Ni as an example where it was found that similar to bulk metals [21], the relationship s -1 » (0.3–0.4) s u is satisfied for films, too. It is shown that the fatigue limit s -1 of films is significantly higher than that of bulk specimens. Therefore, a high cyclic strength of condensed films is defined by a high level of their static strength. Moreover, an experimental investigation was performed on the fatigue strength of armco iron and copper coated by Ni and Al films, respectively. The assumption following from (1) that application of high-strength films enhances s -1 of the substrate–coating system was confirmed. In this case, tensile residual stresses s res act in coatings, which does not agree with well established notions about the necessity of providing purely compressive residual stresses s res in order to increase s -1 of the substrate–coating system. The competitive mutual interaction between s -1 of the coating and the compressive residual stresses s res can result in either an increase, as in this case, or decrease in s -1 of the system depending on the quantitative values of s res and s -1 of the coating. The conclusions non-traditional in this sense also follow from the results of [22] where the cyclic strength of titanium VT1-0 with ceramic thin-film TiN coatings was investigated. Despite the high compressive stresses s res acting in TiN coatings (s res =1200 to 1000 MPa depending on the film thickness h = 5 to 10 mm), the value of s -1 of the system was increased insignificantly. The explanation of this fact was given in [23] where a computational model for cyclic bending also taking into account s res was proposed based on the additivity relation. The analysis of results according to this model shows that the hardening through the use of coatings is possible if the thermal expansion coefficients a s and a c of the substrate and coating, respectively, are satisfied by the condition a s > a c . In particular, such condition is realized for sure if the substrate is metal and the coating is ceramics (nitrides, carbides, 285

borides, etc.). In this case, despite the tensile residual stresses s res in the substrate, the excess of the fatigue limit s -1 of the composition over that of the base metal would be expected. The operability of this model taking into account the parameters of the coating structure was confirmed experimentally for a number of thicknesses of thin-film nitride coatings obtained by the PVD process that makes it possible to govern the structure [18–23]. The data on the positive effect of oxide films, i.e., thin coatings, on the strength characteristics of crystals (the Rosco effect) are given in [24]. Experimental Investigations. Various methods of applying coatings are considered in numerous literature sources. A huge set of the known experimental results prevents the demonstration of their large variety in full measure within the framework of one paper and the satisfactory ordering by level of efficiency of the appropriate technology processes. Nevertheless, as can be judged from the analysis of the considered results, in recent time there has been a tendency to use the methods of PVD, ion nitriding, ion implantation – both separate or combined. Therefore, when reviewing the latest experimental results, we will limit ourselves to only the above methods of surface hardening. Ion Nitriding. Ion-vacuum nitriding is a method offering considerable opportunity for producing diffusion layers of a desired structure since the process of diffusion saturation is controllable and can be optimized depending on the specific engineering requirements. Diffusion layers with an ion phase (nitride) area or nitrided layers with no nitrided zone can be prepared by adjusting the composition of ionized gases and glow charge intensity. The nitride area is characterized by a high density and good adhesion with the base metal [25]. The problems concerning the fatigue of materials after nitridation are quite pressing, as evidenced by a great number of publications on this subject matter. Experimental investigations are mainly performed for corrosionresistant steels, with great attention also paid to titanium alloys. AISI 316L austenitic corrosion-resistant steel is widely used in nuclear and chemical industries for manufacturing structural elements subjected to cyclic strains. In [26], the influence of low-temperature (approximately at 400°C) plasma nitriding on the crystallographic structure and fatigue characteristics of the above steel was studied. The influence of nitriding on the growth of <100> and <111> texture components and disappearance of <110> texture components was established using the method of reflection electron diffraction. A set of low-cycle fatigue tests at room temperature showed a considerable increase in the fatigue strength after nitridation. The problem of the influence of tempering on the cyclic strength of 4140 steel after nitridation is considered in work [27]. The effect of the tempering temperature and time of nitridation (A) on the fatigue strength was studied. Specimens with hardness of HRC 30–32, 33–34, 35–36 were tempered at 550 to 620°C. The nitridation was performed at 530°C for 4 to16 h. It was found that the specimens tempered at 550 and subjected to nitridation for 4 to 16 h had the highest value of s -1 (820 to 840 MPa). Here, the layer thickness was 0.16 to 0.3 mm at the surface hardness of HV from 772 to 778 and the core hardness of HRC from 35 to 36. The specimens tempered at a temperature of 290°C and nitridation for 4 h had the lowest value of s -1 (720 MPa). The plasma nitridation is likely to allow the “healing” of defects. In work [28] the influence of the plasma nitridation on the fatigue strength of steel AISI 4140 having some fatigue damages is investigated. Non-treated and nitrided specimens were subjected to cyclic loading up to a certain level followed by the second nitridation for 0.5 h, after which the fatigue tests were carried out. The process applied to non-treated specimens was found to increase the fatigue life of the specimens that were plasma-nitrided for 0.5 h. However, for nitrided specimens, no considerable improvement in the fatigue life was observed since the surface layer impedes the diffusion during the second nitridation. The fatigue strength increases due to the appearance of residual compressive stresses in the surface layer, and the increase in hardness improves the fatigue characteristics of steels [29]. Also investigated in work [29] were the properties of ion-nitrided steel AISI 4340 after different temperature-time conditions of quenching and tempering. The influence of thickness of the nitridation layer on the increase in the fatigue characteristics (up to 90%) was found. The study of the fatigue mechanism has shown that the fatigue fracture is due to the formation of a fish-eye like structure in the process of cyclic testing in the sub-surface layer on non-metallic inclusions. For 38KhMYuA chromium steel, the condition was established for the nitridation process that provides the maximum increase in the fatigue strength [30]. Comparative fatigue tests were performed at the number of cycles of 5 × 10 6 for smooth standard specimens of 7.5 mm in diameter in pure bending with rotation using a testing machine 286

MUI-6000. The specimens were subjected to heat treatment (HT) (normalization), followed by the nitridation under five different conditions. It was found that the cyclic behavior of the nitridation is more efficient and makes it possible to increase the fatigue limit by 21% as compared to the classical condition of the nitridation process and by 37% as compared to the fatigue limit of specimens that have passed a HT only. The results of fatigue characteristics of 42CrMo4 chromium steel subjected to ion nitriding are presented in [31] where the influence of the layer of accompanying chemical compounds on the fatigue fracture resistance is investigated. This resistance, in its turn, depends on the hardening and the stress state (of the applied and residual stresses), which changes under cyclic loading. In the field of high-cycle fatigue (>10 5 cycles), an increase in the fatigue limit by 30% is achieved. It is noted that a great sensitivity of the layer to overloads severely restricts the use of the nitridation for low-cycle fatigue. Carbon Steels. The investigation of the effect of influence of ion nitriding on the fatigue and deformation of SAE1045 steel was performed on smooth specimens at room temperature [32]. Both the monotonic and cyclic loading was studied which was compared with the loading of the initial material. To assess the stability of behavior of nitrided specimens under cyclic loading at constant amplitudes of deformation, a composite model was used. Monotonic static tension curves of nitrided specimens are close to the initial ones, however, their plasticity decreases from 50 down to 9%, the strength of the carburized specimens increases as compared to the initial strength (by a factor of approximately 2), with the plasticity decreasing down to about 20%. For a low number of cycles, the specimen ranking in order of increasing the cyclic strength is: carburized; initial; implanted (however, at the number of cycles of more than 10 4 , the nitrided specimens exhibited the longest life, next are the carburized ones, with the initial specimens being well behind). The factors influencing the strength properties are as follows: surface nucleation of cracks, multiaxial stress state and residual stresses. Noteworthy is the increase in the number of cycles of implanted specimens at large amplitudes (it manifests itself at the amplitudes of more than 5 × 10 -3 ). The authors [33] investigated the effect of nitridation combined with the dispersion hardening on the fatigue strength and tribological properties of 1045 medium-carbon steel. The specimens were subjected to nitridation in the ferrite or austenite region in the temperature range of 580 to 630°C for 2–4 h, austenization at 740°C for 1 h, followed by quenching in water and aging at 100°C for 1 h. Due to the dispersion hardening, the wear rate of the nitrided steel decreased by a factor of four and reached the value of approximately 0.053 mm/min, with the fatigue strength of the material increasing considerably. Therefore, it is better to perform nitridation in the ferrite region, in this case the thickness of the hardened layer should not be large. Titanium Alloys. Work [34] presents results of the investigation of the fatigue strength after the nitridation of a high-strength Ti-based alloy. It is shown that the fatigue strength of nitrided pure Ti increased as a result of the increase in the fatigue strength of its substrate. This is found to be due to a decrease in the value of the stress field occurring in the adherent layer during the substrate slip. However, the above increase is limited for nitrided titanium alloys of high strength (e.g., nitrided alloys Ti–6A–4V and SP-700) because the adherent layer is subjected to an additional intense tensile stress due to the difference in the Young moduli between the layer and substrate. However, other investigations show that the fatigue strength decreases for alloy Ti–15Mo–5Zr–3Al [35]. Smooth specimens were tested under different conditions of nitridation in bending with rotation. The depth of the nitrided layer varied from 130 to 200 mm. It was found that the nitridation reduces the fatigue strength of the material as compared to that for the annealed material due to the acceleration of the crack nucleation process in the nitrided layer. The life of nitrided alloy VT6 was investigated in [36]. Diffusion layers formed during nitridation of a sheet material of VT6 was found to reduce its life by a factor of five to seven under conditions of repeated static tension. It was shown that its cyclic life can be recovered either by the high-temperature annealing in argon or by removing (with the measured chemical etching) the subsurface part of the nitrided layer. Based on this, in the analysis of the service properties of titanium, it is possible to use the thickness of the embrittled layer d embr formed as a result of its interaction with gases as an integral characteristic of the surface state. Thus, to recover the cyclic life of nitrided alloy VT6 up to the level of the base metal, it is necessary to remove a surface layer of the thickness of more than (2.1 to 2.3) d embr , and a layer of the thickness of (2.7 to 2.9) d embr to achieve the level of the repeated static endurance exceeding the endurance of the base metal by 15 to 20%. 287

The problem on the influence of grain size – as a metallurgical factor – on the fatigue strength of nitrided pure titanium is considered in work [37]. The grain sizes of the titanium varied within 100 to 1800 mm. Smooth specimens were tested in bending with rotation at room temperature. It was noted that the fatigue strength of titanium increases during low-temperature nitridation when the grain growth is limited. The fatigue limit of the specimens increased with the increasing thickness of the nitrided surface layer. The comparison of the fatigue properties of nitrided pure iron and titanium was made in [38]. The influence of the nitridation on the fatigue strength of pure iron and titanium was studied experimentally. It was noted that for pure iron, it increased (a hardened surface layer under the layer of the compound formed during the nitridation suppressed crack propagation), whereas for nitrided pure titanium, it decreased (the crack that has occurred in the compound layer at a low stress level promoted a complete fracture of the brittle hardened layer). In the authors’ opinion, a decrease in the fatigue strength of pure titanium after nitridation is associated with grain growth at high temperatures. Coatings Produced by Vapor and Gas Condensation. Coatings based on refractory compounds, in particular, nitrides and carbides of metals, are widely used in different fields of modern engineering, for example, as protective coatings in aircraft and space industries, integrated and functional micro- and nanoelectronics, computer engineering, in medicine and pharmacology, agriculture, etc. [39–41]. It is known that composite coatings of TiN with a large area of intergranular and interlayer boundaries have high values of toughness, are resistant to “brittle” crack initiation and propagation, withstand efficiently to fracture under conditions of external complex-stress action [42]. It is assumed that TiN coatings having a nanostructured and multi-layered texture are capable of considerably extending the in-service life of products used in mechanical engineering [43, 44]. Corrosion-Resistant Steels. The influence of TiN coatings on the fatigue strength of steel AISI 316L is investigated in work [45]. The coating thickness was 1.4 mm. The coating was deposited by the physical vapor deposition (PVD) method. The presence of such coating on a steel substrate was found to ensure a considerable increase in the fatigue strength and fatigue limit (by 22%) as compared to the ones for steel without a coating. A microstructural analysis showed that in tensile and fatigue testing, the coating-substrate adherence remains satisfactory at low maximum alternating stresses (480 MPa), however at higher alternating stresses (510 MPa), delamination of the layer from the substrate was revealed during the fatigue tests. Moreover, the cracking of the substrate with a coating took place, which was primarily due to the cracking of the TiN coating with subsequent crack propagation into the substrate. The fatigue properties of a martensitic stainless steel 13Cr with the TiN layer thickness of about 2 mm were studied under plane bending loading [46]. The test results showed that with the selection of proper conditions for deposition of the material, a thin-film coating increased its fatigue strength. It was found that high hardness and residual compressive stresses in the coating provided an increase in the fatigue strength of the steel under study. The results of investigations on the assessment of the influence of single-layer and multilayer films of TiN on the tensile and fatigue characteristics of AISI 1045 steel were obtained in work [47]. Using a uniaxial tension-compression testing machine, the mechanical properties of AISI 1045 carbon steel (0.45% C) were determined under tension and at a cyclic load on specimens with and without TiN coatings. The coatings were single-layer and multilayer coatings with the thickness from 3 to 9 mm. The fracture mechanism was studied using a light and scanning microscope. It was found that the presence of surface TiN films does not influence the Young modulus, the yield stress and ultimate strength but lowers the fracture strain, percent elongation and reduction of area. The coating deposition increases the fatigue strength for >10 5 cycles, however, single-layer coatings have more favorable effect than the multilayer ones. A single TiN layer of 3 mm in thickness assists in obtaining the highest increase in the fatigue limit (~ 40 MPa). However, for quick-cutting and ball bearing steel coated using TiN sputtering at 350°C, the fatigue strength does not increase [48]. The tests were performed in cyclic bending at a frequency of f = 24 Hz. The results of studying the influence of vacuum deposited coatings on the mechanical properties of structural metals are presented in [49]. Alloys 38KhA, 40NKh2MA, 12Kh18N9T, ÉP975ID, VT1189 and other alloys coated by TiN, CrN, CrC of different thicknesses were investigated. It was found that vacuum deposited coatings based on carbides and nitrides do not practically change the mechanical properties of metals if the deposition temperature is 288

lower than the standard temperature of thermal treatment of the given metal. The maximum s -1 is reached for specimens with the optimal depth and degree of cold work hardening, and with the minimum roughness. In [50], the influence of thin TiC, TiN, NC coatings on the fatigue resistance of steel specimens (bending with rotation) was analyzed. The fatigue resistance s -1 was found to be the maximal for TiC coatings. Carbon Steels. The influence of plasma ion assisted deposition coatings of TiN on the fatigue strength of specimens of steel 20 was studied in work [51]. A plasma ion assisted deposition coating was deposited at both the temperatures of phase transformation and the temperatures that were different from the former ones. It was shown that the maximum fatigue strength of 20 steel specimens takes place in the case of a plasma ion assisted deposition coating at the phase transformation temperature (727°C). The low-cycle fatigue of steels with TiN coatings was studied in work [52]. A TiN coating was deposited by the chemical gas-phase deposition method. The influence of coatings on the value of s -1 was dependent on their thickness and adhesive strength. The specimens with a coating were distiguished from the ones with no coating by a higher fatigue limit s -1 in the region of low values of the strain amplitude, whereas in the region of high strains, the value of s -1 of the specimens with a coating was lower. Researchers [53] assume that the harder the film, the higher the fatigue of the material. In this work, the fatigue behavior of high-strength HT60 steels with films of titanium nitride deposited by the method of dynamic mixing was investigated. Starting from the obtained results, the influence of the film composition on the character of the steel fatigue was also discussed. The deposition of TiN and Ti2N films assists in increasing the life at low stresses. Paper [54] considers the problem of the influence of defects in a film coating on the fatigue strength of the steel coated with titanium nitride. The variation in the fatigue strength and life of steel specimens coated with a 3 to 5 mm-thick film of titanium nitride depending on defects in that film was determined using the method of bending with rotation with single-sided support. Defects were formed as a result of the 1.1 to 1.6% static tensile prestraining of the coated specimens. The experiments were performed in air and 3% water solution of sodium chloride. The presence of defects was found to reduce considerably the fatigue life of specimens as compared to the life of specimens without defects in the films. Thus, in air medium, the life is 90 to 75% of the nominal life, whereas in salt solution it is 70 to 50%. Titanium Alloys. The study presented in [55] deals with the influence of thin coatings on the fracture process in titanium alloys. In order to increase the fatigue limit s -1 of TiAl14V and Ti6246 alloys, the coatings of amorphous alloys of NiTi and SiC were prepared by the dynamic ion mixing technique. Both types of the substrates were tested at room temperature and in air medium. It was found that coatings change the deformation mechanism of the surface layer and retard the initiation of surface microcracks, resulting in a considerable increase in the value of s -1 which is dependent on the nature of coatings and the amplitudes of the applied cyclic loads. The influence of a multilayer TiN coating on the characteristics of fatigue resistance of GTE blades was also studied in work [56]. It was shown that s -1 of the blades having such coating is higher than the one after finish polishing. Moreover, they have a lower scatter of the life data. Authors [57] consider the influence of multilayer plasma ion assisted deposition coatings of TiN on the fatigue resistance of steel and titanium alloys. In contrast to the data given in [47], it was found that the multilayer coatings influence s -1 more favorably than the single layer ones. This is due to a structural nature of the coatings. The different influence of coatings on s -1 of steels and titanium alloys is attributed to the difference in the thermal expansion coefficients of the coating and substrate, and hence the occurrence of residual stresses that are different in modulus and sign. Corrosion-Resistant Steels with ZrN Coatings. The work [58] is devoted to the study of a stainless steel coated with different ZrN precipitates. Investigated were the fatigue properties of 316L stainless steel coated with three ZrNx films deposited by the physical vapor deposition technique of magnetron sputtering. The comparison was made between the properties of this steel and the ones of the steel with no coating. The excellent adhesion of this kind of films with the substrate together with the increased compression residual stresses give rise to a considerable increase in the cyclic strength of the base steel. A fractographic investigation has shown that ZrNx coatings exhibited 289

no delamination even after a severe plastic deformation of the coating-substrate system. A rough estimation of the strength assuming the validity of the mixture law (1) for describing the yield strength of specimens with a coating, showed that the strength of the films varies in the range of 22.5 to 34.6 GPa with a noticeable tendency toward the increase in the values with increasing nitrogen content in the compound. The calculation of the constants entering into the parametric relationship used to describe the stress-strain curves of the specimens with coatings and without them makes it possible to assess the increase in the fatigue life due to the presence of coatings. The specimens were tested at the stresses of about 435 to 480 MPa. It was found that the fatigue limit of the specimens with a coating increases by 6.6 to 9.1% as compared to that of the specimens with no coating. As shown by the fractographic analysis performed on fracture surfaces, the fracture process at low alternating stresses occurs primarily in the form of single crack propagation, whereas at higher stresses it occurs in the form of two crack propagation. The coatings are damaged on the fracture surface at both low and high alternating stresses. Longitudinal and circular cracks were found to occur on the surface of specimens with a coating at higher stresses. The results obtained testify that the fracture process in the specimens with a coating under cyclic loading starts primarily from the crack initiation on the coating surface and their propagation upon reaching the coating– substrate interface. Corrosion-Resistant Steels with a CrN Coating. The work [59] considers the influence of chrome plating on the endurance limits of AISI 4340 steel with Cr2C3–25NiCr and WC–10Ni coatings prepared by high-speed deposition using air/fuel mixture. It is noted the fatigue limit of a material with a coating is greatly dependent on the level of the internal residual stresses. Chromium coatings are used to ensure the protection from wear and corrosion, combined with the chemical resistance. In a number of cases, a decrease in the fatigue limit of the base metal and a poor (in terms of ecology) technology of producing such coatings presents a problem of their replacement. The influence of droplet-like defects on the fatigue strength of SUS 304 steel with a CrN film was considered in [60]. The fatigue tests were performed by the three-point bending of an austenitic steel with two types of coatings prepared by cathodic arc ion plating deposition. In order to provide a different distribution of droplets, the deposition was performed under two different conditions. It was shown that in specimens with a coating, the fatigue strength decreases irrespective of the deposition conditions. Fatigue cracks are initiated at the stresses below the fatigue limit of the material without a coating. The difference in the crack propagation in specimens with a different distribution of droplets manifests itself both at the stage of fatigue crack initiation and at the stage of its propagation. In the case of a lower density of droplets, a fatigue crack is initiated at early stages, with the rate of its propagation being much lower. For specimens with a higher density of droplets, the coalescence of fatigue cracks was observed. The difference in the fatigue life is associated with the difference between the time of crack initiation and the rate of their propagation. Work [61] presents the results of the experimental investigation into the influence of deep cryogenic treatment (holding in liquid helium) and plasma deposition of a chrome-nitride coating on the fatigue of AISI 302 stainless steel in circular bending. The fatigue limit of specimens (rods of 8 mm in diameter at a frequency of 40 Hz) was determined for the number of loading cycles of 3 × 10 5 . It was shown that the presence of coatings contributes to the increase of the fatigue limit by 18%, whereas the cryogenic treatment does not lead to a considerable change in the fatigue characteristics. Tool Steels. The relation between the fatigue and residual stresses in terms of the thickness of CVD CrN coatings was investigated in [62]. A continuous-solid 5 mm-thick layer of CrN was deposited on the surface of N11 tool steel and studied using the methods of X-ray analysis and nanoindentation. The hardness, modulus of elasticity, chemical composition, texture and lattice constant were determined, together with the distribution of residual stresses over the thickness. Low values of compressive stresses took place in the vicinity of the coating/substrate interface. The fatigue tests were performed using the four-point bending method. A fatigue crack was initiated in the bulk of the substrate material, which was due to low compressive residual stresses near the coating/substrate interface and the presence of non-metallic inclusions acting as discontinuities within the substrate. The size of these inclusions was adopted as the critical microstructural parameter influencing negatively the fatigue resistance of the studied specimens with a CrN coating. 290

Chromium Steels with CN Coatings. The carbonitriding or nitrocarburizing was considered in [63]. The paper investigated the influence of nitrocarburizing hardening of the surface of Cr–Mo–Cr specimens at 570°C tempered at 600°C on the fatigue resistance. Cylindrical specimens (smooth and notched specimens) of 10 mm in diameter were tested by the method of bending with rotation. The thickness of the carbon nitride film and size of the nitrogen diffusion zone were 18 mm and 0.5 mm, respectively. An increase in s -1 by 30% was observed for smooth specimens and by 90% for notched specimens. Cast Iron. The tests data for spheroidal graphite cast iron after carbonitriding are presented in [64]. The specimens of two grades of spheroidal graphite cast iron with a ferritic and pearlitic matrix structure were tested for fatigue in cyclic bending (rotation in supports with the applied bending moment). It was noted that a nitride layer of an increased hardness resulting from the carbonitriding occurs on the cast iron surface. Such hardening thermal chemical treatment of ferritic cast iron gives rise to an increase in the fatigue strength by 40% and 15% for pearlitic cast iron. CONCLUSIONS 1. As before, the problem of selecting the way of surface hardening is solved by the trial method based on experimental results relevant to quite specific process requirements. 2. The current state of the problem is caused by the absence of reliable theoretical grounds for the hardening by means of coatings, the development of which remains at early stage, i.e., at the stage of laboratory experiments on the verification of the proposed computational and experimental models. 3. The methods, such as ion nitriding, ion implantation, physical vapor deposition, have been proven to be promising since, in practice, in contrast to a number of other techniques, the above methods provide a constant reproducibility of the hardening effects by increasing – to one or another degree – the fatigue strength of steels, titanium alloys and cast iron. 4. The applied importance of the results in this review may consist in the critical analysis of the given data, whose direct use is permissible in practice only after their comparison with the other known results and, if possible, after their experimental verification, while keeping the initial process requirements, special features and types of the performed fatigue tests. REFERENCES 1. 2. 3. 4. 5. 6. 7.

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