Coating failure analysis based on microfracture beneath point indentation

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Hard coatings are increasingly used to improve the tribological performance and corrosion resistance of advanced engineering components, for example, gas turbine blades and combustion chambers. Generally, coating failures occur in two modes: (1) interfacial failure or delamination such that the coating layer spalls off the substrate; and (2) coating fracture. In either way, the coating loses its function of protection, thus the component life may be reduced significantly. While many studies have focused on coating/substrate interfacial delamination, the proposed research is to investigate coating fracture based on the mechanics of point-indentation microfracture. The point-indentation technique was originally developed to characterize a material's hardness or resistance to yield. Later, it was also found to be an effective means to characterize fracture toughness of brittle materials. Since most coating materials are brittle, it is expected that this approach can be applied for coating strength evaluation. In this research, indentation tests will be conducted on various coatings (varying thickness and materials) to investigate their cracking behaviors, combining with the Scanning Electron Microscope (SEM) observation. An analytical approach to the mechanics of point-indentation microfracture of coatings will be developed. The effects of coating thickness and the interface on the cracking pattern will be concerned. Two steps will be involved in the analytical solution: (1) determine the stress field beneath the loaded indenter in a coating in terms of the solution of an elastic half-space subjected to a normal point load, because among the three principal normal stresses, the tensile stress field will predetermine the ultimate history of brittle fracture in an elastically loaded coating, and (2) quantify the scale of the micro-cracking in terms of the applied load and thus derive an expression for the stress intensity factor in terms of the indenter load and characteristic crack dimension. The ultimate goal of this research will develop an indentation based method to evaluate the fracture toughness of coatings through crack length and geometry measurements.