MICROSTRUCTURAL ANALYSIS OF THERMAL SPRAY COATINGS BY ELECTRON MICROSCOPY

Plasma and High Velocity Oxy-Fuel (HVOF) spraying are two of the most common thermal spray processes. In both methods, heated particles are sprayed through a flame (either plasma or oxygen fuelled), before being projected at high velocity towards a substrate. Upon impact they flatten, forming splats, whose accumulation results in the formation of a coating. Such processes are used to achieve surface modification of substrates. However, understanding of the mechanisms of splat formation and the splatsubstrate interactions is limited by the difficulty in performing high resolution analysis of the splat-substrate interface. This thesis has investigated NiCr splats thermally sprayed onto both aluminium and stainless steel substrates, where the substrates were subject to pre-treatments to vary surface chemistry and roughness. The aim of the work is to enhance knowledge of the formation of splats by studying and characterising in detail the morphology and microstructure of splats, along with the structures of the splat-substrate interfaces. Features of interest include the grain structures of the splat and substrate, the presence, type and location of pores and oxides and the degree and type of contact between the splat and substrate. Localized substrate melting was observed, along with elemental interdiffusion between the splat and substrate. These observations led to the development of models describing splat formation. A range of substrate pre-treatments were used, such as etching, grinding, thermal or boiling treatments to create different substrate surface chemistry and roughness. The influence of substrate condition on splat characteristics was studied and their influence on splat formation was discussed. The substrate condition strongly influenced splat morphology. For example, boiling Al substrates prior to spraying promoted hydroxide formation, which inhibited splat adhesion. In contrast, heating the substrate during spraying significantly reduced splashing of the splats and promoted substrate melting. A high substrate surface roughness, induced by treatments such as grinding or grit blasting, strongly disrupted the spreading of the impacting particles, creating very irregularly-shaped splats with large pores. Spraying conditions also strongly influence splat formation: plasma sprayed splats are fully molten and characterized by frequent splashing and are strongly influenced by surface chemistry, while HVOF sprayed splats comprise partially molten particles that deform the substrate upon impact. These studies resulted in ten publications that constitute the submitted thesis.