Factors governing the mechanical behaviour of CrSiN coatings: Combined nanoindentation testing and transmission electron microscopy

A direct link between the microstructure and mechanical properties of CrSiN coatings with varying Si contents was established by examining their microstructural evolution and deformation behaviour, enabled by cross-sectional transmission electron microscopy. A transformation from a columnar-grained structure, in the binary compound, to a nanocomposite structure, consisting of CrN nanocrystallites embedded in an amorphous matrix, was observed in the coating having a Si content of 28.5 at.%. The columnar-structured coatings were observed to deform by grain boundary shear sliding, whilst cracks occurred in nanocomposite coatings. Moreover, the coating having a Si content of 14.3 at.% exhibited the highest hardness as a result of grain boundary strengthening facilitated by Si3N4 segregation, together with solid solution strengthening of Si in CrN, the Hall–Petch effect for the finer CrN grains and a high compressive residual stress. A further increase of Si content to 28.5 at.% led to a reduction in hardness due to the high volume fraction of the amorphous phase, together with the presence of cracks in the as-deposited coating. The findings provide deeper insight into the origin of the mechanical behaviour of CrSiN coatings.