Effect of vacuum deposited coatings on the fatigue strength of commercial-grade titanium

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A. G. Trapezon UDC 669.15:539.43 We present the results of experimental and numerical-analytic investigations of thin hardening films and discuss some methodological problems encountered in this case. We propose the relations for the evaluation of fracture stresses under cyclic bending and residual stresses taking into account the specific features of thin coatings, as well as a computational model for the prediction of cyclic strength depending on the thickness and type of the coatings. The accumulated experimental data confirm the indicated dependences.

Section: In View Of the Introduced Parameters Of Transformation Relamentioning

confidence: 79%

“…For a fixed level of strains in any outer layer (e.g., in the zone of substrate-coating interface) regarded as constant over the thickness of the coating for sufficiently thin coatings, by virtue of Hooke's law, relation (16) implies that…”

Section: In View Of the Introduced Parameters Of Transformation Relamentioning

confidence: 99%

Methodological problems in the investigation of thin hardening films

Trapezon

2007

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“…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.…”

Section: Relationship Between Residual Stresses and The Mechanical Chmentioning

confidence: 97%

Evaluation of residual stresses in PVD-coatings. Part 1. Review

Soroka

2010

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 substratecoating 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 res...

“…Therefore, this result should be taken into account when the experimental values of σ b determined using formula (10), which holds for a one-sided coating, have to be used for a beam element with a double-sided coating, e.g., for a specimen shown in Fig. 1.…”

mentioning

confidence: 99%

To the method of accelerated evaluation of fatigue of metals with hardening coatings

Trapezon

2009

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A. G. Trapezon UDC 669.15:539.43 Within the framework of the proposed scheme of an accelerated evaluation of strength of base material-coating compositions under cyclic loading, we show the necessity of its comprehensive realization by experimental and theoretical methods. The use of a high-frequency method for this purpose is substantiated by the example of the proposed experimental procedure. On the basis of the relationships for calculating residual stresses obtained in this work, a calculation model for predicting cyclic strength has been developed. We show its applicability for a preliminary evaluation of the durability of the base material-coating systems and for refinement of the hardening coating technology.Introduction. Methods of depositing hardening coatings are finding an increasing use for improving the load-carrying capacity of structural elements. The experience showed that the fatigue strength of the base material-coating composition is the most important quality criterion for hardening coatings. Therefore, accelerated evaluation of this characteristic is considered to be an immediate and decisive task, particularly at the stage of developing and refining the processes of formation and deposition of coatings. A special feature of the base material-coating compositions is the presence of residual stresses (σ res ) in the coating and in the upper layer of the base material (base), which may play an important role in hardening (or softening) of the load-bearing structural elements. Consideration of σ res when evaluating cyclic strength, just as strength in general, is therefore necessary as one of the prerequisites for ensuring the required level of the service properties of coated structural elements. The coating thickness is an important factor; its magnitude influences both the σ res value and the strength characteristics, which depend on the structural and mechanical parameters of the coating. The latter are directly related to the specific features of the coating technology: the process duration, temperature, chemical composition, etc. It is apparent that when developing a base-coating system with the aim to optimize it by considering all the problem factors, an experimental method will be the most reliable for checking the obtained results. Thus, when determining the fatigue strength of materials with thin-film coatings on specimens subjected to flexural vibrations, with some initial conditions being fulfilled [1], consideration of σ res , specific features of the technological process, surface and structural effects takes place automatically and the fatigue strength is presented in the form of various values of endurance limits (σ −1 ). Experimental determination of σ −1 requires much time and money. For this reason, along with the problem of accelerating this process, an alternative task is to construct appropriate calculation models.It is evident that the calculation methods do not allow one to consider fully the great number of factors that influence the σ −1 value. Thus, for instance, ...