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

Abstract

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, ...

Effect of the deposition and thickness parameters of titanium nitride (TiN) coatings on the fatigue strength

“…A quantitative assessment of fatigue strength with account of the above effect can be provided by calculation using the earlier proposed formula for a double-sided coating [9]:…”

“…The discrepancy between experimental (σ −1 ) and calculated (σ a ) values are relatively high in the above cases: δ = 10.6 and 8.7%, respectively, which may indicate certain drawbacks of formula (1), which ignores, e.g., crack propagation effects in the base-coating composition. Relation (1) has been constructed based on the crack initiation onset concept [9], i.e., it postulates that starting from the instant of fatigue crack initiation in the base metal [at stress level σ −1( ) b ] fatigue life of the composition is, in general, controlled by elastic properties and dimensions of the base and coating components. In practice, fatigue durability after crack initiation depends as well on the crack propagation behavior, in particular, crack propagation rate with possible crack retardation or arrest.…”