Environment and time dependent hardness in zirconia
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Environment and time dependent hardness in zirconia A. S. Pereiraa) and J. A. H. da Jornada Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91500-Porto Alegre-RS, Brasil (Received 14 April 1993; accepted 7 January 1994)
The microhardness of monoclinic ZrO 2 single-crystals was measured in different environments: air, water, and toluene. An indentation creep process at room temperature was observed for the measurements in moist media pointing for a water-activated plastic relaxation mechanism. This effect is discussed employing the models previously proposed to explain similar behaviors in ZrO 2 and other nonmetallic materials. A possible correlation with the conditions for the nucleation in phase transitions is proposed.
Zirconia (ZrO2) based ceramics are very attractive materials mainly because of the substantial improvements in their toughness, which can be provided by the transformation toughening process.1 For such a reason there is a great interest in understanding the mechanisms involved in the phase transitions between the possible allotropic variations of ZrO 2 , as well as in the determination of its mechanical properties. Several studies have shown that ZrO 2 -containing ceramics can have significant changes in properties such as strength, toughness, and hardness1"4 when submitted to low-temperature treatments. These effects are usually correlated to changes in the phase composition and are strongly dependent on the environmental conditions, mainly the presence of water, although the mechanisms acting in such processes are not yet totally clarified.5"8 Another interesting set of results, which also reflects the influence of low-temperature-activated processes on the properties of zirconia, is provided by studies of the phase stability under high pressure. It has been found that thermal treatments at moderate temperatures can change the conditions for the nucleation of new phases.9"11 As these phase transformations are martensitic in nature, some relaxation mechanism of possible stress generation centers could occur in such conditions. In this work, in order to get more information about such low-temperature-activated processes, we studied the dependence of Vickers microhardness with respect to indentation time and environmental conditions for ZrO 2 single-crystals at room temperature. The indentations were performed under three different environments: humid air (about 60% of relative humidity), toluene, and distilled water. These environments were chosen to investigate the possible influence of water on the hardness
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with the Departamento de Materials, Escola de Engenharia da UFRGS, Porto Alegre, RS, Brasil. J. Mater. Res., Vol. 9, No. 5, May 1994
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values, since the presence of water has been often proposed to explain the variation of mechanical properties in zirconia-based ceramics and other materials.6'7'12"15 Toluene was used because it is a nonaqueous solvent, which has been successfully employed as an anhydrous inert me
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