Microcracking of Monoclinic HfO₂

Abstract

Elasticity, internal friction, and thermal expansion were measured at temperatures up to 1500°C to characterize the micro‐cracking behavior of HfO2. Crack‐free HfO2 showed consistent, typical changes in these properties with temperature. Micro‐cracked HfO2 showed hysteresis in the elastic moduli and thermal expansion on thermal cycling. Internal friction increased with microcrack healing. Repeated thermal cycling and high‐temperature anneals markedly changed the microcrack system and a time‐ and atmosphere‐dependent aging effect in microcrack formation was noted.

The effect of microcracking upon the Poisson's ratio for brittle materials

“…No second phases were observed in asfired specimens of AI2Oa [8,22,23], Nb2Os [6,7], Eu203 [2], or HfO2 [24,25], to within the resolution of X-ray diffraction. However, small amounts of MgO and Y203 are present in the YMgxCrx_xO3 specimens [26].…”

“…2 and 3), each data point in the two figures corresponds to a specimen of a given grain size having a particular Y and v. For both materials, the highest SThe Young's modulus and Poisson's ratio was, for all of the data, corrected for volume fraction porosity [4]. Most specimens had volume fraction porosities ~< 0.05 [2,6,7,22,24,25,36], so that the magnitude of the porosity correction was not generally large. /2 w experimentally determined critical grain size for the onset of microcracking is 2/~m for HfO 2 [24,25] 280.0 0.28 [36] moduli and Poisson's ratio occur for the smallest grain size, with both Poisson's ratio and modulus decreasing with increasing grain size.…”

“…Most specimens had volume fraction porosities ~< 0.05 [2,6,7,22,24,25,36], so that the magnitude of the porosity correction was not generally large. /2 w experimentally determined critical grain size for the onset of microcracking is 2/~m for HfO 2 [24,25] 280.0 0.28 [36] moduli and Poisson's ratio occur for the smallest grain size, with both Poisson's ratio and modulus decreasing with increasing grain size. Although all of the Gd203 specimens (Fig.…”

“…2 and 3 show room temperature elasticity data for HfO2 [24,25] and Gd203 [22] specimens of varying grain size. Both HfO2 and Gd203 microcrack due to thermal expansion anisotropy, so that above a critical grai n size w both materials microcrack.…”

“…annea/ time and temperature 82 degree of texturing of the hot-pressed HfO 2 specimens was not determined [24,25]. *Both specimens were sintered.…”