Superhard Ceramic Oxides
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Superhard
Ceramic Oxides J.E. Lowther
Abstract Oxide-based ceramic materials are rapidly proving to have exciting potential for application as hard coatings. The most commonly known oxide material is silica, with a well-known variety of polytypes. Many other oxides, especially those involving metals, are now also proving to be important for a variety of applications where, because of the chemical role played by metal d electrons, large oxygen coordinations can be sustained. Properties of a variety of oxides—from silica to metal-based oxides—are discussed, and the role of predictive computer modeling is shown to be valuable in guiding the search for potentially new superhard oxides. Keywords: ceramic oxides, elastic properties, superhard coating materials.
Introduction New superhard materials are constantly being suggested and investigated1,2 in the hope that many of these will have the potential to rival diamond, which prevails as the hardest of all known materials. But there is also a need for new materials with properties that can surpass diamond under certain extreme conditions. Unlike carbides and nitrides, oxides offer a unique chemical character for potential refractory applications, mainly because of their low reactivity with the atmosphere. Moreover, some of the ceramic oxide materials are moldable and have beneficial mechanical properties such as high hardness, good chemical resistance, high tensile strength, and good fracture toughness. The use of such ceramic materials in hip-joint prostheses has been reported to produce lower wear rates in total hip arthroplasties.3 Silica (SiO2) with its many polytypes is often thought of as the simplest of oxides. Under compression, the relative volume per molecular unit of SiO2 decreases, indicating compressibility and enhanced hardness. Two of the higher-density structures of SiO2 have Si atoms in sixfold coordination, which yields high hardness; one of these is stishovite. Normally, Si is unstable at such a high coordination, and thus producing a stable high-compression phase may be difficult. In contrast to metal oxides, the flexibility of the metal d-bonding character enhances metal coordination possibilities, and very high oxygen coordinations are now observed under compression. Zirconia (ZrO2) is another ceramic mate-
MRS BULLETIN/MARCH 2003
rial with various polytypes; in some of these, Zr is observed to have ninefold oxygen coordination. Stabilization of the oxide structures is presently the main challenge facing the rapidly emerging field of refractory ceramic oxides. To date, SiO2 in the hightemperature and high-pressure stishovite form has not been stabilized, and with modest temperature changes, it rapidly reverts back to quartz. ZrO2, on the other hand, has two high-temperature phases (tetragonal and cubic) that can be stabilized with the addition of modest amounts of other chemical impurities that are thought to affect the oxygen sublattice. Aside from SiO2 and ZrO2, many other oxides, such as HfO2 and RuO2, have been considered for potential use a
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