The role of grain boundary phase on contact damage accumulation in alumina ceramics was investigated using Hertzian indentation test. Specially, liquid-phase-sintered alumina ceramics with relatively coarse and elongated microstructures were processed.
The compositions of grain boundary phase were controlled using different additives. Magnesia (MgO), anorthite ($CaOㆍAl_2O_3ㆍ2SiO_2$), silica ($SiO_2$), or anorthite +magnesia were added to high-purity alumina, while maintaining geometrically similar grain size($\cong 5-6㎛$) and volume fractions($\cong 1 vol%$) of liquid phase. Each alumina ceramics were fabricated by pressureless sintering within the range of 1570~1620℃ for 0.5~2hrs.
All alumina ceramics with high-density(R.D.＞95%) showed different microstructures. First, MgO doped alumina(M) showed relatively fine and equiaxed microstructure. Second, Anorthite(A) or $SiO_2$(S) doped alumina represented relatively coarse and elongated microstructures. Third, Anorthite and MgO(AM) doped alumina was illustrated that it has relatively coarse and equiaxed microstructure. A and S alumina materials showed rising R-curve behavior due to the coarse and elongated microstructures.
The large differences of mechanical properties could not be detected by routine Vickers indentation tests. On the other hand, the Hertzian indentation tests detected those properties even though they showed the nearly same properties in Vickers indentation tests. The dominant damage modes shift from cone cracking to subsurface quasi-plasticity as the grain boundary phase is weakened, high residual stress acted, and the microstructure coarsened. Strength tests on damage specimens also revealed the grain boundary phase affects strength loss from induced damage. First, in contrast to the M ceramics, A and S ceramics revealed quasi-plasticity behavior instead of brittle fracture by cone cracking. Secondly, A and AM ceramics showed similar damage behaviors even if they have different microstructures. Finally, quite the contrary, A and S ceramics represented different damage behaviors even though they have similar microstructures. A ceramics showed quasi-plasticity and damage tolerance properties. These different damage behaviors of A and S ceramics were estimated by the calculation of residual stresses. As a result, A ceramics showed damage tolerance due to the lower compressive resisdual stress of the grain boundaries than S ceramics.
Thus, grain boundary phase acted as a principal factor in the damage characteristics of liquid-phase-sintered alumina.
