Cold spraying is a promising process for fabricating functional coatings. Because of the solid-state particle deposition, the electrical and chemical properties of the coatings are similar to those of the bulk materials. Mechanical properties, on the other hand, differ from those of bulk materials due to severe plastic deformation of the particles. Residual stress may thus be an important variable to track during cold spraying although the formation mechanism is not entirely clear. In this study, the residual stress of metallic (copper) and ceramic (titania) coatings is measured during the cold spray process. The results are presented and discussed.

In this study, copper coatings are deposited on polyacetal substrates by low-power microwave plasma spraying and the coating formation mechanism is investigated. In the initial formation of the coating, molten copper particles are embedded on the substrate, creating conditions for excellent bonding as confirmed by adhesion strength measurements exceeding 40 MPa. The study also shows that adding hydrogen to the argon working gas improves oxide reduction, resulting in copper coatings with volume resistivity as low as 0.049 µΩm.

The main objective of this research is to evaluate the dynamic wetting of the substrate by thermally sprayed particles through direct observation on the freely fallen metallic droplets onto the substrate surface. Compared to normal static wetting, dynamic wetting can be characterized as the wetting phenomenon in which both material movement inside the splat and heat transfer from the splat into the substrate occur simultaneously. As the phenomenon is so complicated, there is a lack of an appropriate evaluation manner. In this study, the wetting behavior of freely fallen metallic droplets was systematically observed onto several kinds of substrate materials. Actually, concrete measurement values on the wetting angle of the droplets with an increasing of the substrate temperature were measured. It was found from the results that even the dynamic contact angle changed transitionally with an increase of the substrate temperature and corresponded well with the changing tendency in the disk splat fraction ratio of the thermally sprayed particle onto the substrate. From the result, it was confirmed that the flattening of thermal sprayed particles is essentially dominated by the dynamic wetting of the substrate by them.

In the present work, we have examined the role of active elements in controlling the extent of splashing of plasma sprayed splats. Splats of Copper (Cu) and Copper alloyed with Al, Zr were deposited by plasma spray or as free-falling droplet. The splat formation and splat-substrate interfaces were characterised using SEM and FIB. It was found that the presence of Zr, either in the splat material or the substrate, enhanced disk splat formation in free falling droplets. Similarly, the presence of aluminium in plasma sprayed Cu- 10%Al increased the portion of favourable disk-shaped splats compared to pure copper. It was also found that the disk-shaped splat proportion of Cu-10%Al at room temperature is nearly identical to that of Cu splat at 300°C, indicating that the addition of Al in the alloy leads to the improvement of splat formation, correlating with the improvement in the interface bonding with the substrate.

Aluminum nitride (AlN) ceramics is characterized with its high thermal conductivity and chemical stability. However, it was impossible to fabricate AlN coatings by conventional thermal spray processes directly from AlN feedstock powder due to thermal decomposition of AlN during spraying. In the last decade we were apple to fabricate the AlN coatings through the reactive plasma spraying process (RPS) in the atmospheric ambient. This study describes the way to fabricate high thermal conductivity plasma sprayed AlN coatings. The thermal conductivity of the AlN coatings was investigated by laser flash method. The as sprayed coating had very low thermal conductivity (2.43 W/m.K), compared to the AlN value. It is attributed to presences of high oxide content (Al 5 O 6 N, γ-Al 2 O 3 and α-Al 2 O 3 phases), low density (2.32 g/cm 3 ) and high porosity in the plasma sprayed coating (about 22%). Besides that, although the N 2 gas flow improved the nitride content, the thermal conductivity decreased gradually. It is related to the further increase of the coating porosity and decreasing its density with the N 2 gas. The influence of the process parameters on the thermal conductivity was investigated and to fabricate high thermal conductivity AlN coating adjusting the oxide content, the coating porosity and microstructure are the main factors. Very high thermal conductivity (about 95 W/m.K) atmospheric plasma sprayed AlN coating was fabricated. The coating consists of mainly AlN phase (more than 95% AlN), very small amount of oxide phases, low porosity (about 3%) with a sintered microstructure (nicked-shape sintered particles).

Cold spraying is a promising candidate for the metallization of carbon fiber reinforced polymer (CFRP) composites, but it requires the use of a protective interlayer deposited by either flame or plasma spray. This study evaluates the effect of different surface treatments on interlayer adhesion strength. CFRP samples were treated mechanically, chemically, and thermally and then a copper layer was deposited by atmospheric plasma spraying. Adhesion strength was measured by pullout testing and the results are compared with surface roughness measurements, SEM fracture surface images, FIB bisections of single Cu splats, and XPS analysis of the chemical affinity between the polymer and copper.