Coatings by thermal spray

Thermal spray is a technology that allows the development of large parts, which cannot be produced by powder metallurgy for technical and economical reasons, with hardmetal characteristics via the application of thick hardmetal coatings. Thermally sprayed hardmetal coatings are predominantly mechanically bonded to the substrate having a typical thickness within the range 100-500 μm. This chapter reviews different spray processes, with focus on the currently most important high-velocity oxy-fuel and high-velocity air fuel processes, and the properties of the different feedstock powders in a historical context. The properties of the most important hard phases, namely, WC and Cr3C2 and TiC as an alternative, are discussed. Commercially available hardmetal compositions are reviewed. Since the spray process influences the chemical and phase compositions, coating formation and microstructures are discussed. The most important methods of coating characterization, such as microstructural analysis; X-ray diffraction for the study of phase composition; measurements of hardness, Young's modulus and indentation fracture; and thermophysical properties are presented. Corrosion properties, the influence of heat treatments on coating microstructure and the oxidation in service are reviewed. Wear protection (abrasion, erosion and sliding wear resistance) and potential new applications are also discussed