Plasma spray techniques

Flame sprayed molybdenum articles have poor corrosion resistance, no doubt owing to the porosity of the coating. However, modern plasma spraying techniques produce a dense coating and this should lead to more widespread use of clad materials such as molybdenum clad steel where the clad product should have the same corrosion resistance as the solid material. 

S.D. Parukuttyamma et al., Yttrium aluminum garnet (YAG) films through a precursor plasma spraying technique. J. Am. Ceram. Soc. 84(8), 1906-1908 (2001). 

The plasma spray technique has been used for about 30 years. Figure 10.12 shows a simple schematic diagram of a plasma torch. Basically, a powder is fed into the plasma, which is used as a heater, and the molten product is sprayed onto the substrate to form just about any thickness. The coatings tend to be porous and of poor adhesion. The adhesion can be improved by increased surface roughness of the substrate, which can be produced by grit blasting. An interlayer bond coat of Ni-Al... 

Electrodeposition, diffusion, and thermal spraying techniques are widely in use to improve the surface of components for hot corrosion resistance. Particularly, thermal spray and vapor deposition techniques are the most efficient techniques for prolonging the life of components significantly [3]. On the other hand, plasma spraying technique basically enables spraying of a wide range of powdered materials. 

Laha T, Agarwal A, McKechnie T, Rea K, Seal S. Synthesis of hulk nanostructured aluminum alloy component through vacuum plasma spray technique. Acta Mater 2005 53 5429-38. 

Aluminum is preferentially leached out from NiaAls and NisAl by hot caustic solutions. The mixture of these compounds, applied by the plasma spraying technique, was found to be active for the hydrogen electrode process, the best results being observed at Ni/Al = 65/35 [33,34]. 

Extensive efforts have been made to produce ultra-hard boron carbide coatings using low pressure and inert gas shrouded air plasma spray techniques. A wide variety of test conditions were utilized (Table 2), but no deposition occurred. 

A plasma spray method is also a well-studied method for FGM fabrication. In this method, the source powder (metal and/or ceramic) is transported to the plasma jet by a torch nozzle. Then the source materials in a molten state by plasma are sprayed onto a substrate to form a coating. The composition gradient within the sprayed coating can be realized by changing the composition of the supply source. To date, various kinds of ceramic/metal, ceramic/ceramic and metal/metal FGMs (such as Zr02/NiCrAl, NiCr/Al and NiCrAlY/YSZ) have been fabricated by the plasma spray technique [40, 41]. 

At present, intense research is continuing worldwide to develop novel, more efficient electrode materials and interconnects so as to increase the electrochemical conversion rate (fuel efficiency), the active surface area and the electrical efficiency, and to reduce the operating temperature and system degradation. Advanced production technologies, induding the use of plasma spray techniques to fabricate the SOFCs, are also currently under development (Table 7.9). 

The plasma spray technique offers a straightforward and cost-effective means to spray deposits of metals and ceramics that are tens to hundreds of micrometers in thickness onto a variety of substrates in applications involving thermal-barrier or insulator coatings. Typical plasma-spray deposits are porous, with only 85-90% of theoretical density. 

Firstly, work has started to evaluate the use of sol-gel procedures on stainless steel. Early results are encouraging. Secondly the use of plasma spraying techniques has been tried on low carbon steels [69]. Researchers report that, although temperature control is both difficult and critical, the initial results obtained are good. 

The stack assembly procedure also affects the seal selection process. For instance, a laminate structure of anode, electrolytes, and cathode is formed by the plasma spraying technique. The microstructuring processing steps must be controlled so that the interfacial reactions and interdiffusion between those layers can... 

Different methods used to form coatings of ZrB /HfB will be discussed in this section. The techniques used are classified into three categories (1) Chemical vapor deposition, (2) Physical vapor deposition, and (3) Plasma spray technique. 

Recently, some innovative plasma-spray techniques have been employed to produce TBCs with lower thermal conductivities and more durable performance. The solution precursor plasma spray (SPPS) has been exploited to deposit TBCs, in which an aqueous chemical precursor feedstock, rather than YSZ powders, is injected into plasma jet [25,26]. The lack of large-scale splat boundaries effectively toughens the TBCs, making SPPS TBCs highly durable relative to APS TBCs. 

---