Distortion Driven Fracture Analysis in Hard Si - C - N Coatings

The fracture behaviour of Si-C-N coatings was investigated through Vickers’s micro-indentation focusing on the distortion of the geometric indentation. A transition from ductile pile-up dominated petal like morphology to mixed-mode ductile-brittle fracture and finally to brittle chipping and delamination was examined. Phenomenon like radial crack initiation, crack joining and shear band assisted plastic flow were correlated with three and four petal distortion geometries. Fracture toughness and mixed-mode energy release rates were quantified using effective rack length and Griffith based criteria. Novel distortion corrected parameters like distortion corrected fracture index (DCFI), plastic fracture composition parameter (Π_pf), indentation geometry instability factor (IGIF), petal to chip transitions parameter (PCTP), shear band assisted fracture parameter (SBFP) and mixed mode indentation fracture severity index (IF_mm) were introduced to capture the interplay between plastically flow crack growth and indentation geometry. Drucker-Prager pressure sensitive plasticity showed hydrostatic stress assisted toughening beneath the indenter, rationalizing the observed asymmetric damage zones and delayed crack propagation. The heterogeneities in the nanocrystaline growth distribution were again correlated to the pop-in events occurring in nanoindentation and overall toughness of the coatings. The results provide a framework for the design of damage tolerant SiCN coatings and it can be found that Si-C-N which can be framed as a structurally disordered, amorphous- nanocrystalline ceramic, the damage tolerance arises from configurational and structural disorder rather than crystallographic failure bringing in a wider aspect of physics related to entropy driven disordered matter.