Sprayed coatings plasma process

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. 

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). 

There are two ways in which coatings can be applied thermomechanical processes (e.g. detonation gun, flame spraying and plasma spraying) and vapour phase deposition processes. The latter category can be subdivided into CVD (chemical vapour deposition) and PVD (physical vapour deposition). In the case of a CVD process, a chemical reaction takes place in an oven and as a result the coating material is formed and deposited on the object. Figures 11.7.9 and 11.7.10 are representations of two methods to apply coatings. 

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. 

These challenges notwithstanding, several attempts have been made in the past to use zirconia as a bond coat material to improve the performance of biocon-ductive hydroxyapatite coatings for hip endoprostheses. The microstructures of zirconia bond coat/HAp coating cross-sections were studied by 2D-SIMS imaging to obtain information on plasma spray-induced diffusional processes at the coating interfaces as well as the spatial distribution of minor and trace elements across the coating cross-sections (Heimann et al., 1998). 

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. 

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. 

Difficult-to-process composites can be readily produced by thermal spray forming, and plasma spray is the process of choice for the most reactive matrix materials. Particulate-, fiber-, and whisker-reinforced composites have all been produced and used in various applications. Particulate-reinforced wear-resistant coatings such as WC/Co, Cr3C2/NiCr, and TiC/NiCr are the most common applications. Figure 8 illustrates the diverse forms of composites that can be thermally spray formed. Whisker particulates can be incorporated into sprayed deposits using so-called engineered powders, mechanical blending, or by... 

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