Measuring the fracture resistance of hard coatings

A way of characterizing cracking in a hard coating is described. Microscale double cantilever beams have been made by focused ion beam milling and compressed in situ using a nanoindenter. The method can account for frictional effects and is demonstrated first on single crystals of SiC and GaAs of known toughness, before studying cracking in CrN-based hard coatings. It is found that ultra-fine grained CrAlN/Si3N4 coatings have a toughness approximately twice that of a conventional CrN coating. Although grain-size effects are still unclear, in situ observations directly show crack interactions with particles of Cr and voids in the film.