NiCr was plasma sprayed at room temperature on aluminum and stainless steel substrates that had been hydrothermally treated in deionized water for 30 minutes. A major difference was observed in that splat formation occurred only on the stainless steel. A numerical model was developed to simulate the impact of molten nickel splats on the treated substrates. The simulation closely matched experimental results in terms of splat morphology, porosity, delamination, and specific locations of substrate melting. Moreover, it confirmed that splat morphology is influenced, not by oxide thickness, but rather by water release from the dehydration of oxyhydroxide at the outermost surface. The insulating layer of released water inhibits heat transfer from the splat to the substrate, which reduces solidification rates, resulting in further spreading and thinning of splats. These findings shed light on splat spreading and solidification and provide insights on the effect of substrate surface chemistry on thermal spray splat morphology.

In this study, single splats of polyether ether ketone were plasma sprayed onto aluminum substrates that had been boiled, etched, or polished and then thermally treated, except for one etched substrate, to remove water from the surface. Splat morphology was viewed in a scanning electron microscope and splat-substrate interfaces were examined using TEM and focused ion beam imaging. The results show that PEEK splats have a poor level of contact on aluminum substrates that were boiled and those that were etched but not thermally treated. In contrast, specimens that had undergone thermal treatment to minimize the presence of water on the substrate surface exhibited high levels of contact at the splat-substrate interface with significantly less porosity.

A study of plasma-sprayed NiCr splats shows that the morphology and characteristics of the splat-substrate interface depends on the substrate material. Splats on stainless steel tend to be regular and disc-shaped and localized melting of the substrate and intermixing and diffusion were observed at the interface. Splats on aluminum, on the other hand, were slightly more irregular and the contact at the splat-substrate interface was found to be porous and poor. For both types of splats, several oxide phases were observed. Their formation and the formation of the splats are discussed, although further study is required to fully identify the origins of the differences observed.

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.

High-velocity air fuel (HVAF) spraying was selected for spray trials of a Cr3C2-NiCr powder. To determine the effect of spray parameters on coating characteristics, particularly porosity and phase degradation, a statistical design of experiments was implemented. A wide range of statistical designs have been applied to the optimization of thermal spray coatings with a great deal of success. In this instance, a lack of prior knowledge and the need to assess many process-variable interactions efficiently led to the selection of a two-level full factorial design. High and low settings for each variable, including spray distance, traverse speed, and powder feedrate, were chosen based on the ranges typically used to spray similar materials. The resulting coatings were assessed for microhardness, porosity, residual stress, deposition efficiency, and phase transformation, after which several follow-up runs were conducted to explore trends brought to light by the initial factorial design.

WC-based coatings have found extensive application in industries requiring wear resistance. WC-12% Co coatings were applied with a variety of high speed combustion spray guns (including HVAF) with various spray parameters. This paper aims to investigate the influence of the particle velocity, the particle temperature and the spray atmosphere on the decarburisation reactions and phase changes that take place during spraying. XRD and SEM used BSE image processing to characterize and quantify these responses. A comparison between temperature and speed measurements as well as the XRD results showed that the decarburisation is influenced not only by the temperature but also by the residence time and the entrainment of oxygen. Paper includes a German-language abstract.