Plasma-spray-coating process

Kang, C.W. and Ng, H.W. (2006) Splat morphology and spreading behaviour due to oblique impact of droplets onto substrates in plasma spray coating process. Surf. 

Newer high velocity thermal spray coating processes produce coatings in compression rather than tension because of the shot peening effect of the supersonic particles on impact. This has permitted coating as thick as 12,500 p.m without delamination as compared to older processes limited to 1,250 p.m. The reduced residence time of particles at temperature minimises decomposition of carbides present in conventional d-c plasma. This improves wear and hardness (qv) properties. 

Figure 1.8. Schematic showing (a) plasma spray deposition process and (b) droplet formation, acceleration, surface impact, and coating formation.

In the field of metallic powder applications, a method of plasma spray coating suitable for biomedical materials has been developed using titanium and calcium phosphate composite powder. By means of the mechanical shock process, the appropriate composite powder was prepared, and plasma sprayed on Ti substrate under a low-pressure argon atmosphere. A porous Ti coating layer was obtained in which the surface and the inside of the pores were covered thinly with hydroxyapatite. This surface coating is expected to show excellent bone ingrowth and fixation with bone (21). 

Obviously the tertiary structure of the catalyst obtained by cold rolling is optimal and is least efficient for the plasma-sprayed coating. It should be stressed that by variation of the process parameters, also galvanically generated NiZn codeposits yield optimal performance, which is comparable to that of cold-rolled coatings (83). 

Chen, D.Y., Jordan, E.H., and Gell, M. (2010) The solution precursor plasma spray coatings influence of solvent type. Plasma Chem. Plasma Process., 30 (1), 111-119. 

Houben, J.M. (1988) Relation of the adhesion of plasma sprayed coatings to the process parameters size, velocity and heat content of the spray particles. Doctoral thesis/dissertation. TU Eindhoven, The Netherlands. 

Attempts have been made to produce hard, dense boron carbide coatings by air plasma and low pressure plasma spray (LPPS) processes, but all techniques have so far proved unsuccessful. 

In the past, although much effort has been expended to predict residual coating stresses by modeling the life expectancy of the TBCs, problems were encountered by the assumption of a continuum theory and the non-consideration of elastic finite elements, nonlinear processes, and the general fractal nature of plasma-sprayed coatings (Heimann, 2008). 

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