brittle coating; ductile substrate; microcrack formation; indentation; temperature
A temperature-dependent fracture toughness model for brittle coating/ductile substrate systems under indentation is proposed based on microcrack formation theory. Numerous microcracks are generated from each corner of indentation impression and merge together to form radial cracks due to the tension of residual stresses in the coating/substrate system during the unloading period. The fracture toughness of the coating/substrate systems is determined such that the crack tip opening displacement (CTOD) is used to measure the total growth of a microcrack in tensile direction and the dislocation movement is associated with the crack propagation. The temperature effect is modeled in terms of the Arrhenius-type equation and rate controlling theory. Both the basic indentation pressure and composite hardness approaches are used to obtain the model parameters but the latter provides more reasonable results. The WC-10Co4Cr coating/1018 steel substrate system, prepared via high velocity oxygen fuel (HVOF) spraying, is analyzed using this model and the obtained fracture toughness shows increasing with temperature in a nonlinear manner.