Stability of layered coatings with outer layer made of ceramics

Abstract

Introduction. The factors responsible for the performance of coatings in various elements of modern metallurgical equipment [6, 7] (such as the combustion chamber and air tuyere of a blast furnace) include: external conditions (temperature and stress in structural elements, composition and intensity of corrosive components in the gas flow); internal conditions (composition of the coating, material of the substrate, coating technology). The reactions to external and internal factors include thermal fatigue, physical and chemical development and formation of interlayers and, as a consequence, local bulging of coatings; erosive damage and cracking; change in the behavior of the interaction between the components of the gas flow and the materials of the layered coating (hydrogenation) and the oxidation processes, which give rise to new surface layers. One of the possible causes of bulging may be the surface microbuckling of the coating. The condensed metal coatings currently used for protection purposes are capable of resisting the erosive action of the gas flow. Note that the more functions the coating performs, the more layers it consists of and the higher the probability that it will lose stability. This, in turn, may lead to failure of such elements of metallurgical equipment, with all that ensues. It is of interest to study the surface instability of layered coatings in tuyeres and the effect of the basic mechanical, physical, and chemical processes in the blast furnace on this process. The three-dimensional stability of layered coatings with variable structure was addressed in [4, 8–13] where a general problem-solving approach was developed and some specific results were discussed. In [3, 14], the effect of high temperature is incorporated in buckling problems by correcting the values of the mechanical characteristics of the layers at set temperature of the coating. Following [3, 14], we will study (using a plane strain assumption) the effect of the outer ceramic layer on the stability of a layered coating in the operating conditions of the air tuyere of a blast furnace. We will use a piecewise-homogeneous medium model for linear elastic bodies. We will give a problem statement, outline a problem-solving method, and solve specific surface-buckling problems for layered coatings with outer layer made of ceramics. Small subcritical strains in the composite under biaxial thermomechanical loading will be assumed. 1. Problem Formulation. Governing Equations. The air tuyere of a blast furnace can be modeled by a layered