Deformation of a hard coating on ductile substrate system during nanoindentation: Role of the coating microstructure

Deformation and fracture of a columnar-grained, ∼1.3-μm-thick TiN coating on a stainless steel substrate was investigated using a spherical tipped conical indenter of 5-μm nominal tip radius. Structural analysis, performed with the support of focused ion beam (FIB) milling and imaging techniques, revealed that the microstructure of the TiN coating had a strong influence on the deformation behavior of the coating. Intergranular sliding in the coating, as well as plastic flow in the ductile substrate, was found to be the predominant processes during the indentation. Neither plastic deformation, in the form of plastic flow, within the coating nor delamination of the interface was observed. Coating deformation was observed to be controlled by the intergranular shear cracking and thus by the interfacial columbic frictional stress between columnar grains. An indentation-energy based model was developed, which deconvolutes the coating behavior from that of the substrate, allowing quantification of the intergranular sliding stress.