The Relationship between Microstructure and Fracture Toughness of Thermal Spray Coatings

In this work, two coatings, HVOF-sprayed Cr3C2-NiCr and plasma-sprayed HfO2-Si, were characterized and tested via micro and macroscale test methods. Comparison of two Cr3C2-NiCr coatings revealed the role of carbide dissolution in promoting the formation of brittle nanocrystalline and amorphous matrix phases. A higher starting matrix content appears to prevent oversaturation of Cr and C, thereby reducing the occurrence of the more brittle matrix phases, and consequently increasing coating and interface fracture toughness. The adhesion mechanism was identified to be mechanical bonding occurring at the nanometer scale where the coating matrix solidified around the nanoscale structures of the substrate surface. The employment of microscale methods allowed for the unique measurement of intrinsic fracture behavior in both systems. In particular, micropillar splitting appeared to provide accurate results, when compared with SEVNB and microindentation, for Si-rich and HfO2-rich single phases as well as the composite microstructure of the Cr3C2-NiCr coating. Microcantilevers, on the other hand, proved more difficult to implement as specimens suffered from modeling difficulties rooted in dimensional considerations which led to an apparent overestimation in the toughness of Si-rich and HfO2-rich phases. DCB experiments of a Cr3C2-NiCr coating on a grit-blasted steel substrate revealed substantial R-curve behavior stemming from the interfacial asperities and the oscillatory nature of the stress field at the interface. Back-calculation of wake zone traction-separation relations fell within literature ranges for monolithic ceramics. Interface microindentation of a Cr3C2-NiCr coating on a grit-blasted steel substrate combined with image analysis of the resultant crack geometries probed the variability associated with stochastic interface features unique to thermal spray coatings. Analytical modeling of interface crack geometries through a crack deflection model combined with inherent interface toughness, deduced through observed crack deflection tendencies, showed excellent agreement with direct measurement of interface toughness via SENB and DCB specimens. The results of the modeling effort provide insights into tailoring the mechanical behavior by manipulation of interface features and the possible limitations of this approach.