This study deals with a plasma technique that combines two plasma spray torches to produce finely-structured zirconia coatings. Ideally, the deposition process path involves the vaporization of most of the particles injected in the plasma jet and the transport of the vapor to the substrate where it re-condenses. The arrangement of the plasma torches makes it possible to limit the deposition of non-completely evaporated particles onto the substrate. The experimental design of the vapor deposition process has been assisted by experimental characterization of the plasma temperature field and numerical simulations of the two plasma flow interactions and powder vaporization.

A good adhesion of plasma sprayed hydroxyapatite (HA) coating on Ti-based alloy is crucial for ensuring highly-reliable non cemented implants in the biomedical industry. In the present work, the laser shock adhesion test, namely LASAT, has been applied to investigate the interface strength of plasma sprayed HA coatings. This contact less method allowed a rapid assessment of the HA coating adhesion on simple coated plates. Varying the laser energy to impact the substrate and to generate the interface decohesion, a LASAT adhesion threshold can be determined for the highest laser fluence (J/m²) for which no debonding of the coating occurred. This qualitative and discerning LASAT procedure has been carried out on HA coatings to investigate the role of various interfaces on the adhesive property of the HA/Ti bond. According to the LASAT analysis, a surface roughness prepared with medium or coarse grit-blasting did not influence drastically the adhesion threshold while smooth pre-oxidized specimens LASAT threshold were near to those obtained with a Ti bond-coat. These thresholds also corresponded with the highest adhesion measured in this study. In addition, pre-heating treatment of substrates just prior to spraying up to 270°C did not exhibit a significant difference with grit-blasted HA/Ti interface. Further investigations (SEM, XRD) was also achieved to investigate the interface characteristics before and after the laser treatment. Sample cross-sections of laser shocked specimens were examined in detail, right at the impact location and within the debonding area to assess the fracture feature. This complementary materials analysis permitted to establish the relevance of the LASAT test as a fast and easy-to-use method devoted to the design or the control of highly adhesive HA coatings. Preliminary experiments to apply the LASAT method in liquid environment is described. Further work is on progress to implement an in situ adhesion testing of HA coating in simulated body fluid.

Coating production with reproducible properties within a range of values acceptable for the specific application requires a on-line control of the thermal spray process. Sprayed coatings present very often reproducibility problems due to spray parameter variations. In fact the quality of coatings is strongly linked to the temperature and velocity of particles at impact, and the temperature evolution of substrate and coating, before (preheating), during (spraying) and after (cooling). The particle temperature and velocity measurements require sophisticated devices rather expensive and not well adapted to work in the harsh environment of spray booths. Moreover in HVOF spraying, the particle surface temperature is generally below 2200K which make the measurements trickier than in plasma conditions. That is why, the Spray and Deposit Control (SDC) has been developed (collaboration between SPCTS laboratory of the University of Limoges and SNECMA Services), which allows to follow continuously both the light emitted by the hot particles (maximum intensity, mean trajectory and trajectory dispersion) and the substrate and coating temperature evolution during spraying. This simple and light system, fixed on the torch, has been tested in flame, plasma and HVOF spraying. The SDC measured parameters allow to develop easily process windows to obtain good working areas corresponding to specific coating parameters. In this study, the SDC on-line system is used in industrial conditions to optimize and control the HVOF spraying of carbide (WC-Co 17% wt) powders. A special attention is given to the possibilities of process parameters adjustment to regain the specific particle jet parameters and the good substrate temperature, in case of measured SDC parameters shifting. This study demonstrates the SDC system ability for an industrial on-line control of HVOF spraying, using process maps, to increase the consistency of coating properties. It is a step to a closed-loop control of the process through both the particle spray jet properties (mean trajectory and radiated flux distribution) and the target surface temperature.