Plasma spray-coating techniques

In the past few decades, plasma spray-coating techniques have been developed to cover orthopedic implants with protective and/or bioactive coatings. As introduced in Chapter 1, the plasma spray-coating method employs high temperature plasma jet to melt and spray a feedstock material onto a substrate to form a coating. The feedstock materials for plasma spray can be in the forms of solid, liquid or suspension [29,30]. For the fabrication of nanocoating on orthopedic implants, the commonly used solid... 

Thickness of APS (atmospheric plasma spraying) coating can be selected between 50 and 250 pm, depending on application however, novel deposition techniques such as suspension or solution plasma-spraying allow coatings with thickness <10 pm. 

The electronic conductivity depends on the crystalline structure, preparation technique, and porosity of the material. For example, the LSM samples prepared by plasma spray coating exhibit conductivity between 50 and 200 S/cm as compared to 40 85 S/cm for the sintered samples at 1000°C. When the porosity increases from 10 to 41%, a decrease in conductivity from 148 to 45 S/cm is observed for LSM samples at 800°C. The CTE for LaMnOs (undoped) is slightly higher than that of the YSZ electrolyte. 

Drawbacks of these processes are capital cost and limited flexibility. Use of plasma spray deposition techniques requires sufficiently smooth surface of the underlying layer to be coated. Preformed perforated alloy sheets are thus preferably used for these processes. If the alloy is to be processed by powder metallurgy methods, fine-grained alloy powder is more favorable to produce substrate with small pore size and surface roughness. On the other hand, the resulting enhanced specific surface area may yield, after a certain operating time to Cr depletion in the porous substrate. This might induce non-protective breakaway oxidation if the critical Cr threshold is reached and subsequent breakdown in electrochemical performance. 

Hard facing of various components in the aircraft gas-turbine engine and in industrial apphcations for textile machinery parts, oil and gas machinery parts, paper-shtting knives, etc, is estimated at 1 x 10 in 1995 with an estimated growth rate of 5% annually. The mix is approximately 45% aerospace apphcations, 55% industrial apphcations. Additionally, repair coatings for gas-turbine blades and vanes is estimated at 500 x 10 . These coatings are primarily deposited by plasma spray, arc-wire, HVOF, and detonation gun 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. 

Interconnects are formed into the desired shape using ceramic processing techniques. For example, bipolar plates with gas channels can be formed by tape casting a mixture of the ceramic powder with a solvent, such as trichloroethylene (TCE)-ethanol [90], Coating techniques, such as plasma spray [91] or laser ablation [92] can also be used to apply interconnect materials to the other fuel cell components. 

In contrast to plasma spraying, low pressure plasma CVD does not require any remote handling technique. However, there is yet no experience with large scale applications of this method, particularly in metallic vessels. This is the first goal towards which future studies have to be directed. Very little is also known about the adherence of the coatings deposited by low pressure plasma CVD and their resistance against thermal shock. The choice of the best material for such coatings is presently open to discussion. 

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