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.

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.

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.