Thermal Spray of Cemented Carbide Coatings in Off-Angle Spraying: Correlations Between Process, Coating Features/Characteristics and Performance

Abstract

Oxidation or decomposition of thermally sprayed coatings during deposition may noticably impair the coating features and characteristics, i.e. the surface integrity of coated components, including coating porosity, phase, microhardness, elastic modulus and toughness, etc., having a profound influence on the final performance of components. Further degradations along with enhanced oxidation of the coatings were observed for an off-angle spraying scheme in industrial practice, where inconsistent results were previously reported for spray angle-dependent degradation of various thermal spray coatings, with the mechanism not being fully understood with the less controlled surface integrity. An improved off-angle HVOF thermal spray has been developed for cemented carbide coatings, using pretreated WC-Ni feedstock powders covered with a nanoscale capsulizing nickel layer. A comprehensive study on the formation of multiple surface integrity parameters and the correlative interactions between them is thus facilitated with controllable surface integrity generation using the capsulized powders. Decarburization is mainly attributed to the exposure of pristine WC grains causing enhanced porosity in inter-splats regions, and then microhardness depends mainly on porosity and partly on phase composition, while the elastic modulus depends on intra-splats cohesion and the indentation fracture toughness on inter-splats cohesion, respectively. Abrasive wear from mild to severe regime transition could be interpreted by the inter-splats and intra-splats cohesion correlation. It has been demonstrated by the improved off-angle thermal spray that a manufacturing process could be designed and optimized on the identified unique correlations between the processes, surface integrity and the final performance.