In this study, aluminium 5005 and stainless steel 304L substrate surfaces were modified by thermal and hydrothermal treatments. Ni-Cr single splats were deposited onto these substrates at room temperature using plasma spraying. The collected splats were characterised qualitatively and quantitatively by Scanning Electron Microscopy (SEM) and ImageJ software. A splat classification scheme was developed based around splash and disk-type splats. The proportions of the different splat types were found to vary significantly as a function of substrate pretreatment, especially when the pretreatment involved heating. The effect of surface roughness to splat formation and splat shape was investigated. It was observed that surface roughness was not a critical factor in splat morphology. The above substrate surfaces were also characterised by X-ray photoelectron spectroscopy (XPS) using in-situ heating in vacuum to determine the effect of thermal pretreatment on substrate surface chemistry and the oxide thickness. It was found that heat treatment promoted a change in the chemical structure of the oxide surface layer which was consistent with the dehydration of oxyhydroxide to oxide. Dehydration of the substrate surface layer improved the physical contact between the splat and substrate which enhanced the formation of disk-splats, decreased the number of pores evident in the splats and increased number of splats and their diameter.

Thermal spray of polymers has had limited investigation due to the narrow processing windows that are inherent to polymer powders, especially their low temperatures of thermal degradation. The polymer poly aryl ether ether ketone (PEEK) has a high thermal degradation temperature and high resistance to alkaline and acidic attack. These properties led to PEEK being selected for investigation. To minimise thermal degradation of the particles, the high velocity air fuel (HVAF) technique was used. To investigate the effect of substrate pre-treatment on single splat properties, single splats were collected on aluminium 5052 substrates with six different pretreatments. The single splats collected were imaged by scanning electron microscopy (SEM) and image analysis was performed with ImageJ, an open source scientific graphics package. On substrates held at 323°C it was found that substrate pretreatment had a significant effect on the circularity and area of single splats, and also on the number of splats deposited on the substrates. Increases in splat circularity, area, and the number of splats deposited on the surface were linked to the decrease in chemisorbed water on the substrate surface and the decrease of surface roughness. This proved that surface chemistry and roughness are crucial to forming single splats with good properties, which will lead to coatings of good properties.

Coatings of poly(ether-ether-ketone) (PEEK) have been produced using the high-velocity air fuel (HVAF) thermal spray technique. These coatings have been produced at 50 and 100 mm nozzle lengths and 200, 300, and 400 mm gun-to-substrate distances on stainless steel 304 substrates. The techniques used to characterize and determine the extent of thermal degradation of the PEEK coatings were valence-band XPS and FTIR-ATR. Valence-band XPS showed that, in general, minimal degradation of the PEEK occurred during the HVAF thermal spraying process. FTIR-ATR results showed that more surface degradation of the PEEK coating occurred at the 200 mm gun-to-substrate distance for both nozzle lengths than at the larger gun-to-substrate distances. Specifically, absorption bands appeared at 2918 and 2850 cm -1 , which correspond to alkane –CH 2 – asymmetric stretching modes. The resolution of the 672 cm -1 peak, which corresponds to C–H vibrations on the phenyl ring, increased from one to two peaks in the spectra of the 200 and 300 mm gun-to-substrate runs. This indicates a structural change in the phenyl ring, possibly indicating a change in the extent of crystallization of the PEEK polymer.

PEEK was sprayed with a Browning Aerospray HVAF thermal spray gun to enable the study of the wetting and interaction of single splats with an aluminium 5005 substrate. Single splats were obtained by exposing the substrate to the spray flame for 0.02 s by dropping a steel shutter with a 25 mm aperture milled in the centre. The single splats were then analysed through SEM (scanning electron microscopy) and FIB microscopy (focussed ion beam). Splat shape was found to be dependant on nozzle length, with a 100 mm nozzle resulting in more splashing, and a 450 mm spray distance providing more disc splats. PEEK splats do not wet the aluminium oxide surface well. Porosity occurs independently of nozzle length, in the form of cracks and pores in the splats, some cracks completely segmenting a splat.