Thermal spray WC-NiCr coatings generally requires grinding processing to meet the surface finish requirements. The cost associated with grinding can potentially be reduced through the deposition of finer (– 30 + 5 μm) feedstock rather than the more conventional commercial (– 45 + 15 or – 53 + 20 μm) feedstock. Additionally, such a fine powder is likely to require lower energy spray parameters, resulting in less heat input to the substrate, which could be beneficial in application on heat sensitive substrates. However, the spray parameters need to be optimised to mitigate increased degradation of coatings due to unacceptable brittleness caused by decarburisation and oxidation, and to produce defect-free dense coatings. In the present work, a theoretical model to optimise spray parameters was developed, which suggested an oxygen-to-fuel ratio slightly more than 3.3 and a shorter barrel to avoid decarburisation in coatings. In total four parameter sets suggested by the theoretical model were selected to spray the fine-cut powder using a 100 mm long barrel. Scanning electron microscopy, X-ray diffraction, microhardness, and 3Dprofilometer were used to analyse the produced coatings. The coatings deposited using optimised parameters exhibited the best performance in terms of low porosity, inter-splat cracks, brittleness, and roughness. Coating deposited at lower kerosine and oxygen flow rates, with reduced stand-off distance, was denser, crack-free, and ductile. Hence, the fine-cut powder can be used to produce a finer as-sprayed finish, thereby demonstrating the potential in reducing grinding efforts. Additionally, successful deposition of coatings using low energy parameters, making this an attractive option for thermally sensitive substrates.

The metal finishing process of electrolytic hard chrome (EHC) plating has been identified as a source of environmental pollution in most industrialized countries like Australia, Europe and USA. The key driver for the technology replacement is that the EHC plating process uses hexavalent chromium, which is a known carcinogen. Our previous research has identified that cold spray nanostructured tungsten carbide cobalt (WC-Co) coatings can be a suitable alternative to provide a functional coating in wear applications. This work explores at another similar technology- Kinetic Metallization for deposition of WC-Co coatings. In this work, the objective is to characterize the residual stress profile of these WC-Co coatings that are deposited by the latest KM systems. These coating systems are used in critical applications such as landing gear pistons and axle journals, hydraulic rods, engine shaft journals, and numerous other external surfaces that operate under high cyclic loading conditions. As such, the residual stress developed during the KM coating process has a significant influence on the fatigue properties of the components. Thus, knowledge of stresses and their linkage with other properties and production parameters is essential for the quality control of these critical structures.

A study was conducted into the use of HVOF coatings on hydraulic cylinder rods in marine environments subject to both biofouling and corrosion. The study comprised a laboratory phase for candidate material selection and HVOF process optimisation. This was followed by long term field testing of the most promising candidates, which were subsequently assessed based on corrosion, adhesion, surface roughness and hydraulic sealing performance. Two candidates, nickel based WC cermet coatings, WC–10Ni5Cr and WC–18Hastelloy C, showed excellent results, and are considered as candidates for the application.

High entropy alloys (HEAs) are a new class of alloys that have multiple principal elements in an equiatomic ratio and that exhibit novel phase structures. HEAs are known for their high temperature microstructural stability, enhanced oxidation and wear resistance properties. Apart from bulk material consolidation methods such as casting and sintering, HEAs can also be deposited as a surface coating. In this work, thermal sprayed HEA coatings are investigated as an alternative bond coat material for a thermal barrier coating system. Nanostructured HEAs that were based on AlCoCrFeNi and MnCoCrFeNi were prepared by ball milling and then plasma sprayed. Splat studies were assessed to optimize the appropriate thermal spray parameters and spray deposits were prepared. Subsequently, the microstructure and mechanical properties of two HEAs coatings of different composition were characterized and compared to conventional plasma spray NiCrAlY bond coats.

This paper discusses the development of optimized processing windows for spraying high quality metal carbide-based coatings using advanced particle diagnostic technology. The cermet coatings are produced via the high velocity oxygen fuel spray process (HVOF) and are intended to be used in severe service applications such as marine hydraulics. To develop suitable coating process parameters, the traditional “trial and error” method is not technically robust, is costly and time consuming. Instead we investigated the use of real-time monitoring of parameters associated with the HVOF flame jets and particles using advanced in-flight particle diagnostics. Subsequently, coatings can be produced with knowledge concerning the molten particle size, temperature and velocity profile. The analytical results allow identification of optimized coating process windows, which translate to coatings of lower porosity and improved mechanical performance.

This study investigates relationships between in-flight particle characteristics and the microstructural properties of air plasma sprayed YSZ. Particle velocity and temperature were measured at different stand-off distances and contour plots were created. Coating porosity and hardness were measured and plotted over the same area. It is shown that in-flight particle characteristics are strongly affected by particle size distribution. Smaller particles in trajectories above the torch central axis were found to be 10 m/sec slower on average than larger particles in the center and lower section of the plume. The section of the plume with the highest temperatures and velocities is about 2 mm below the central axis and is shown to generate dense, hard layers with less than 7% porosity and hardness values on the order of 700 HV. Reynolds numbers determined from in-flight particle data also provide an indication of microstructural properties; where Reynolds numbers exceed 1000, coating porosity is less than 5%.

Thermal spraying produces coatings with relatively rough surfaces compared to other deposition methods. In this work, NdFeB coatings were deposited on stainless steel by plasma spraying at various standoff distances. Some of the coatings were also annealed. Surface roughness profiles of as-sprayed and heat-treated coatings were measured by contact profilometry and analyzed via statistical methods. The effect of standoff distance and annealing on roughness is discussed along with the significance of measurement direction and evaluation length.

This investigation employs tensile adhesion tests (TAT) and tubular coating tensile (TCT) tests to measure the adhesion and cohesion bond strength of plasma sprayed YSZ coatings. Tensile adhesion testing measures the bond strength of the YSZ-bond coat system perpendicular to the spray direction, while tubular coating tensile testing measures the intersplat strength of the YSZ coating parallel to the spray direction. In both cases, the failure strength of the coatings can be approximated to a Weibull distribution, indicative of anisotropic behavior as verified by Knoop microhardness indentation tests. The average coating strength parallel to the spray direction is shown to be about 1.5 times greater than the bond strength perpendicular to the spraying direction.

Formation of voids is inevitable in plasma sprayed coatings, and the role of voids on coating properties has long been established. In fact, the void content within coatings is often adjusted by manipulating the process parameters to obtain coatings with desirable performance. Quantification of voids via image analysis allows the determination of not only the void content within a coating, but also the spatial distribution of the voids. Void content in plasma sprayed neodymium iron boron (Nd-Fe-B) coatings was varied from 1.8 to 8.2% by changing the standoff distance (SOD). Spatial distribution parameters, including near-neighbor distance (d min ), nearest-neighbor distance (d mean ), and nearest-neighbor angle (θ n ), were determined via the Dirichlet tessellation method. Coefficient of variation (COV) values of d min and d mean allow the determination of inhomogeneity and degree of clustering of the voids within a coating. The θ n values show the anisotropy behavior of the voids within plasma sprayed coatings. The influence of void content and its spatial distribution within the coatings on the microhardness and elastic modulus of the coatings was determined.

Polypropylene (PP) was flame sprayed onto rough mild steel substrates at room temperature (RT) that was preheated at 70 °C, 120 °C, and 170 °C. Single solidified droplets (splats) were collected and analysed to understand how processing variables influenced the thermal spray coating characteristics. The splat morphology was characterized in detail using optical and scanning electron microscopy (SEM). The splats exhibited a disk-like shape with a large central viscous core and a fully melted wide rim with a thin edge. The splat size increased with increasing substrate temperature. A unique flat microstructure was observed on the surface of the splat deposited onto the RT substrate, whereas a flowing pattern appeared on the splat surfaces deposited onto the preheated substrates and the pattern increased by increasing the substrate temperature. The results of this study revealed improved splat-substrate adhesion by heating the substrate from RT to 170 °C. On the basis of the result, the influence of substrate parameters on splat morphologies was employed to establish a relationship between the microstructural characteristics and processing variables of flame sprayed polymeric coatings.

Excimer laser annealing provides a rapid and efficient means for surface alloying and modification of ceramic materials. In this study, Alumina-13% Titania coatings were sprayed with a water-stabilized plasma spray gun. The coated surface was treated by Excimer laser having a wavelength of 248 nm and pulse duration of 24 ns. The surface structure of the treated coating was examined by field emission scanning electron microscope and X-ray diffraction (XRD). A detailed analysis of the effects of various laser parameters including laser energy density (fluence), pulse repetition rate (PRR), and number of pulses on the morphology and the microstructure of the coatings are presented.

The oxidation of a NiCr bond coat during air plasma spraying was controlled by designing a gas shroud system that attached to the plasma torch nozzle. Two nozzles, termed as “normal” and “high speed” nozzles examined the effect of nozzle internal design on the microstructure and phase structure of coatings. X-ray diffraction and SEM morphologies showed that the shroud system reduced the oxidation of NiCr particles during the spray process. Compared with conventional air plasma spraying, the argon gas shroud reduced the coating hardness because the volume fraction of partially melted particles increased. The high speed nozzle reduced the oxidation and hardness of NiCr coatings due to the increase of partially melted particles in the coatings.

Thermal spray coatings are comprised of millions of heated particles that are driven at high velocity to impact against a substrate; thereby building up to form a consolidated coating. Thus, investigating single solidified droplets contributes to fundamental understanding of coating evolution and their properties. In this study, Scanning Electron Microscopy (SEM) studied the splat morphology of flame sprayed ethylene methacrylic acid (EMAA) with respect to the stand-off distance when deposited onto glass and mild steel substrates. A splat shape transition from a “splash” to a “disc shape” was observed. The morphology of EMAA droplets can be described as a ‘splash splat’ when sprayed onto mild steel at room temperature, whereas a 35 cm stand-off distance produced a disk-shaped splat when the polymer was deposited onto a glass substrate.

Cold spray is a material deposition process that uses a high pressure, high velocity gas jet for the deformation and bonding of particles. However, deposition of brittle or hard materials such as ceramics has not been successful: unless they are co-deposited with a ductile matrix material. This paper examines the WC particle size and its influence on the deposition of Co-based cermets. Micro- and nano-structured powders with similar Co content were employed. Varying the WC particle size influenced significantly the deposition efficiency of the coating process. Micrometer-structured WC-Co feedstocks did not permit coating build up when processed under comparable or elevated thermal spray parameters used for the nanostructured WC-Co feedstocks. In addition, micrometer-structured WC-Co coatings exhibited a conjoint erosion and deposition effect on the surface. Fine WC particles (<1 μm) were observed near to the substrate interface and larger WC particles (1-2 μm) in the vicinity of the coating surface. These observations indicate the existence of a critical WC particle size for deposition by the cold spray method and that the size criteria arises due to the formation and cohesion mechanisms within the coating layer.