A new method for evaluating the tensile adhesive strength of thermal sprayed coatings has been developed using a test specimen that incorporates an artificially introduced circumferential crack to control the stress intensity factor at the crack tip along the interface between the coating and the substrate. FEM-analysis is carried out to calculate a correction factor and the stress intensity factor for the test specimen. When the results of tensile test are sorted out using the stress intensity factor at which failure occurred, constant values should be obtained regardless of changes in geometry such as crack length and the diameter of test specimen. The method was first applied to air plasma spray (APS) coatings and then to HVOF sprayed coatings. Effects of substrate preparation such as surface roughness and preheating temperature on the resultant coating adhesion were studied experimentally.

For thermal sprayed coatings, compactness of their constituent particles is required in many applications, e.g. to obtain impermeable anticorrosion coating in marine use. We investigated key factors to improve compactibility of deposited particles in HVOF sprayed coatings by condition measurements of spray particles. The results revealed that plastic deformability of the sprayed particles as well as their molten fraction was important to obtain the dense VHOF coatings.

A new method to capture and evaluate the condition of thermal sprayed particles has been developed by using an agar gel target. HVOF sprayed Hastelloy C particles were collected by a gel target placed at the substrate position, i.e., 380mm downstream from the spray nozzle exit. In the surface layer of the target, a large number of fine particles were observed, whereas in the deeper part, globular particles were trapped. The ratio of particles in the surface layer with respect to the deeper part changed by spraying parameters. Furthermore, particles in the target were separated by cutting the gel, collected after resolving the agar, and then observed by SEM. Particles collected from the target's surface layer were fine particles (under 10µm) and fragments of dendritic crystals. These collected from the deeper part were mostly unmelted particles, some of which exposed dendrites. Results obtained by other techniques such as splat observation and in-flight diagnostics are compared with these results. It was concluded that by capturing thermal sprayed particles with a gel target, it is possible to visualize and quantify the melting condition of HVOF sprayed particles.

316L stainless steel powder was sprayed by a high-pressure HVOF process. Effects of powder size and the pressure in the combustion chamber on the velocity and temperature of sprayed particles were studied by using an optical instrument firstly at the substrate position. A strong negative correlation between the particle diameter and temperature was found whereas the correlation between the diameter and the velocity was not significant. The pressure in the combustion chamber affected the velocity of sprayed particles significantly whereas the particles' temperature remained largely unchanged. In-situ curvature measurement was employed in order to sturdy the process of stress generation during HVOF spraying. From the measured curvature changes, the intensity of peening action and the resultant compressive stress by HVOF sprayed particles were found to increase with the kinetic energy of the sprayed particles. The results were further used to estimate the stress distribution within the coatings.

Direct and quantitative observation of the stress generation during HVOF spray is carried out by measuring the curvature of substrates in-situ during spraying. A high pressure HVOF gun is used to spray SUS316L, Hastelloy C and WC-12%Co powder onto SUS316L substrates. The observed curvature data indicate that there are 3 regimes of stress evolution during the HVOF spray: (1) generation of compressive stress on the substrate surface at the beginning of spraying, (2) stress buildup in the coating during spraying, and (3) superposition of stress due to the mismatch in the thermal expansivity between the coating and the substrate as the specimen cools down to the room temperature after fabrication. Compressive stress ranging from 70 to 400 MPa is observed in the second regime during the HVOF spray; the value depending on the powder materials and spray conditions. Microstructural observation reveals that a significant portion of the coatings consists of poorly molten particles. Beneath the coatings formed by the HVOF process, a thin layer of increased hardness exists within the substrate.