Zirconia coatings oxidation resistance

Zirconium oxide is fused with alurnina in electric-arc furnaces to make alumina—zirconia abrasive grains for use in grinding wheels, coated-abrasive disks, and belts (104) (see Abrasives). The addition of zirconia improves the shock resistance of brittle alurnina and toughens the abrasive. Most of the baddeleyite imported is used for this appHcation, as is zirconia produced by burning zirconium carbide nitride. 

Merck in Japan has recently patented [16] a process for the production of water and weather-resistant pearlescent pigments produced by coating mica with hydrous zirconia. This is in many ways similar to processes operated in the titanium oxide industry and mentioned previously. The zirconium hydroxide aids dispersion and gives better compatibility with the polymer matrix that it is incorporated in. 

Two methods for the evaporation of precursors may be employed - resistance heating and electron beam collision. The first method employs a simple alumina crucible that is heated by a W filament. Temperatures as high as 1,800°C may be reached inside the chamber, which is enough for some metals or metal salts to vaporize. Deposition rates for this method are 1-20 A s . The use of an electron beam to assist in the precursor evaporation results in temperatures on the order of 3,000°C, being more suited for the deposition of refractory metals/alloys and metal oxides such as alumina, titania, and zirconia. Since the temperature of the chamber interior is much higher than the walls, the gas-phase ions/atoms/molecules condense on the sidewalls as well as the substrate this may lead to film contamination as the nonselective coating flakes off the chamber walls. 

Thin films and coatings can be fabricated by vapor deposition [i.e., chemical vapor deposition (CVD) and electrochemical vapor deposition (EVD)], sputtering, sol-gel processing, and electrophoretic depositionElectrochemical vapor deposition, a thin-film technique, is used to form thin ( 40 pm) layers of dense yttria-stabilized zirconia in the seal-less tubular solid oxide fuel celP ". Thin layers of stabilized zirconia are required in this application to keep the internal resistance and the operating temperature of the electrochemical device as low as possible. 

Several factors are important with regard to cracking within the TBC or along the TBC-bond-coat interface. These include the stress state in the zirconia layer, the microstructure of the bond coat, the thickness of the TGO, the stress state in the TGO, and the fracture resistance of the various interfaces between the bond coat and the TGO. It was apparently first pointed out by Miller and it is now generally accepted that oxidation of the bond coat is a critical factor controlling the lives of EB-PVD TBCs. It is now well established that the ability of a bond coat to form an a-alumina layer with negligible transient oxidation, and the adherence of the alumina to the bond coat, are critical factors in controlling the durability of TBCs. 

To avoid classical corrosion mechanisms, ceramics and composite materials have also been tested (some of these materials are also common components in heterogeneous catalysts). An alumina reactor for SCWO was proposed among ceramic materials only a few aluminas and zirconias did not corrode severely, whereas SiC or BN lost up to 90% by weight under SCWO conditions in the presence of HCl. The combination of steel and ceramic coatings should theoretically provide high-pressure stability and improved corrosion resistance, but only slight improvements were reported for stainless steel SS316 coated with sol-gel-prepared Ti, Zr or Hf oxides, stainless steel SUS-304 with TiN or Ni alloys and ceramics. Often the adhesion of the ceramic layer on the steel surface is not sufficient. 

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