High-temperature polymer ceramic

Novel coatings formed by electrophoretic deposition of ceramics, glasses, conducting polymers, and high-temperature polymers... 

Materials classes that were tested included ceramics, nickel-based and cobalt-based alloys, refractory metals and alloys, reactive metals and alloys, noble metals and alloys, and high-temperature polymers, a total of 26 materials. Test periods varied between 37.5 and 47.5 hours. None of the materials was found to be suitable for all test conditions, and most exhibited moderate (equivalent to between 10 and 200 mil per year) to severe (>2()0 mil per year) corrosion. Titanium and titanium alloys (Nb/Ti and Ti-21S) exhibited the best performance, showing only slight corrosion in the presence of excess sodium hydroxide. Under acidic conditions, titanium showed increased rates of corrosion, apparently from attack by sulfuric acid and hydrochloric acid. Both localized pitting and wall thinning were observed. 

Metals are normally ductile, and plastic deformation can be considerable. This is a valuable property of metals and is used, for example, to fabricate dish shapes of one sort or another by impressing a die into a sheet of metal. The metal deforms plastically and retains the dished shape when the load on the die is removed. Ceramics are regarded as brittle, and normally the same fabrication process applied to a ceramic would shatter it. However, at high temperatures, many ceramic materials can deform plastically, and at low temperatures many metals become brittle and lose ductility. Thus, it is convenient to treat these two apparently dissimilar crystalline materials together. Polymers, which contain crystalline and amorphous regions, are considered in Section 10.1.11. 

Service temperature limitations must be considered in the use of composites, not only in the selection of polymer and process, but sometimes in the selection of the reinforcement as weU. Composites cannot generally perform as weU as metals or ceramics in very high temperature appHcations, but they can be made fire resistant to meet most constmction and transportation codes. 

Those basic matrix selection factors are used as bases for comparing the four principal types of matrix materials, namely polymers, metals, carbons, and ceramics, listed in Table 7-1. Obviously, no single matrix material is best for all selection factors. However, if high temperatures and other extreme environmental conditions are not an issue, polymer-matrix materials are the most suitable constituents, and that is why so many current applications involve polymer matrices. In fact, those applications are the easiest and most straightforward for composite materials. Ceramic-matrix or carbon-matrix materials must be used in high-temperature applications or under severe environmental conditions. Metal-matrix materials are generally more suitable than polymers for moderately high-temperature applications or for modest environmental conditions other than elevated temperature. 

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