The effect of coating cracking on the indentation response of thin hard-coated systems

Abstract

The effect of coating cracking during depth-sensing indentation testing (e.g. nanoindentation testing) on systems comprising thin hard coatings (e.g. TiN) on a less stiff, hard substrate (e.g. steel) has been investigated in order to establish whether, at such small scales, the occurrence of annular through-thickness cracks around indentation sites can be detected from load–displacement plots and whether any accompanying localized debonding of the coating from the substrate can also be identified. Experiments were performed on thin TiN and NbN monolayers, and compositionally modulated TiN/NbN and TiN/ZrN deposited onto hard steel or stainless steel substrates. Above a critical load ( P crit) in the range 80–500 mN, all systems exhibited annular through-thickness cracks around indentation sites. The cracking was found to be accompanied by a discrete change in gradient of the loading portion of the load–displacement curve. Also, analysis of the unloading curves beyond this point showed that elastic recovery was controlled by the elastic modulus of the underlying substrate material alone. The cracking mechanism has been characterized using scanning electron microscopy, coupled with optical profilometry to map the detailed z-displacement of the surface — a procedure critical to understanding whether coating–substrate debonding had occurred. Some cracks were found to show a spiral morphology and the effect of indentation loading rate was investigated to see if appearance of these cracks was loading-rate sensitive. Our results highlight the problems inherent in assessing both the interfacial failure and the interfacial fracture toughness of thin film coated systems at the small contact scales (⩽1 μm) associated with crack genesis where it is difficult to confirm the extent of sub-surface cracks by conventional means. We have also demonstrated that both load–displacement and load–displacement squared plots are necessary if a more complete understanding of system behaviour is to be gained. Also, a simple fracture mechanics model for the annular cracking behaviour seen in such systems has been derived and used to estimate a value of G i c (the interfacial work of fracture) as 180 J m −2 for the TiN/ZrN-stainless steel system.