Evaluation of fracture toughness of as plasma sprayed alumina–13 wt.% titania coatings by micro-indentation techniques

The fracture toughness of the as plasma sprayed alumina–13 wt.% titania (AT-13) coatings was evaluated using micro-indentation techniques. Indentations with smaller loads of 0.49 N show features such as partially melted particles to be extremely hard and brittle. However, the matrix made up of bi-modulus microstructure of splats was much tougher due to operative toughening mechanism for instance, crack bridging, which effectively retarded propagation of the cracks. Log plots of crack length (C) versus load (P) for loads varying from 1.96 to 9.8 N occurred with a slope of 0.65 ± 0.095 signifying sub-surface median or palmqvist type cracks to be rampant. The as sprayed microstructure made of pores; partially melted particles and splats were found to be anisotropic with regard to indentation toughness. Cracks were found to initiate and propagate easily in direction parallel to the coating growth directions than along the splat boundaries. Accordingly the bonding between the splats caused by plasma spray process could be deemed to be superior. The preferential directionality developed in the microstructure due to the imposed heat flow conditions essentially has weakened the microstructure.