Cold gas dynamic spray is increasingly used for dimensional repair in the aerospace sector as it is capable of producing dense, oxide-free deposits of significant thickness and with good levels of adhesion and inherent mechanical strength. There is significant interest in extending the application of cold spray deposits to include structural, load-bearing repairs. However, particularly for high strength aluminium alloys, cold spray deposits can exhibit high levels of porosity and micro-cracks, leading to mechanical properties that are inadequate for most load bearing applications. In this work, heat treatment was investigated as a potential means of improving the properties of a cold sprayed Al alloy C355 deposit. C355 alloy deposits were produced using two process gas temperatures (350°C and 500°C) and three gas pressures (40, 50 and 60 bar) using a commercially available HPCS system. Microstructural analysis of the coatings revealed that the optimal microstructure (ca. 1% porosity) was obtained at 500°C and 60 bar. Therefore, coatings produced with process conditions of 500°C and 60 bar were heat treated at 175, 200, 225, 250°C for 4h in air and the evolution of the microstructure and microhardness was analysed. The results show that heat treatment at 225°C can decrease porosity (<0.2%) and retain high hardness (105 HV0.05 vs 130 HV0.05 as-sprayed). Further investigation was performed on as-sprayed and 225°C heat treated deposits. The results show that this heat treatment can halve residual stress (-50 MPa vs -100 MPa as-sprayed), and improve tensile properties (UTS). Therefore, this work has demonstrated that the heat treatment of C355 cold sprayed deposits at 225°C can significantly improve their properties.

The application of aluminum coatings onto steel for corrosion mitigation is governed by standards specifying surface cleanliness and roughness prior to coating, and minimum coating bond strength. Controlling the surface preparation and spray parameters to achieve the specified surface condition and coating bond strength is challenging, particularly for manual on-site work. In this research, the process parameters were varied and the effect on surface quality and coating adhesion determined. It was found that blasting at angles as low as 30° from the surface, and varying stand-off distances up to 100 mm from the optimum, produced an acceptable surface; whilst spray angles of 60° to 90° and stand-off distances up to 50 mm from the optimum produced acceptable coatings with adhesion above 20 MPa. Adhesion appeared unaffected by a limited amount of remaining mill scale, but was reduced to ≈15 MPa when the surface chloride content was increased from 2 to 20 μg cm-2.

The photocatalytic properties of titania are widely known with applications including solar water disinfection, solar hydrogen production and as anti-bacterial surfaces. Herein, we report the use of suspension flame spraying for the deposition of conformal TiO 2 coatings. This process is carried out using aqueous titania suspension (ca. 5 wt%) injected into an acetylene flame (ca. 3100 °C), via an atomiser. Coatings deposited were adherent, majority anatase phase and retained their nanostructure after coating. In this study the acetylene/oxygen ratio was altered to determine the effect on the photoactivity of titania coatings produced, with photoactivity determined by photocurrent generation under illumination. All coatings produced were shown to be photoactive with a marked difference in photocurrent observed between coatings produced in the varying flame conditions. Photocurrent generation increased by up to 3.7 times with decreasing fuel ratio. This difference was attributed to the build-up of reduced titania species in the coating.

It has been observed that VC-WC-Co hardmetals have a better wear resistance than conventional WC-Co hardmetals. This work represents a first attempt to add VC to thermal spray powders in order to determine whether this would increase the wear resistance of HVOF hardmetal coatings. Standard WC-Co and WC-CoCr thermal spray powders were compared to two experimental VC-WC-Co powders in terms of size distribution, morphology and phase composition. Coatings were produced from the powders using the TAFA JP-5000 High Pressure High Velocity Oxy-Fuel (HP/HVOF) process. The microstructure, phase composition, hardness, dry solid particle erosion and three body abrasion of the coatings were compared. It was found that due to the non-homogeneous dispersion of VC particles in the powders, the experimental coatings were not optimised. Nevertheless they showed some advantage over the conventional coatings, particularly in low angle erosion tests. This result is regarded as promising, and it is believed that a homogeneous dispersion of VC particles in the coatings would result in a significant increase in wear resistance. In order to test this, the manufacture of improved powders and optimised coating microstructures would be necessary as a next step.

WC-Co and WC-CoCr coatings were deposited with the JP-5000 liquid fuel HP/HVOF system using various thermal spray powder types. The microstructure, microhardness, deposition rate and wear resistance of the coatings were characterized. The results show that these coatings provide significantly more protection from dry three-body abrasion than from dry sand erosion, when compared to mild steel. They also provide more advantage at low angles of erosion than at high angles of erosion. Furthermore, the coating composition was found to have a significant effect on the wear rates, with WC-CoCr providing the best wear resistance even after taking the higher cost of the powder into account. The powder manufacturing route had only a secondary effect on the wear rates, except in the case of fused and crushed powder, which produced an inferior coating.