Environmental effects on ultra-high temperature creep behavior of directionally solidified oxide eutectic ceramics

Experimental studies were undertaken to assess the environmental effects on ultra-high temperature tensile creep behavior of directionally solidified Al2O3/Y3Al5O12(YAG) and Al2O3/GdAlO3(GAP) eutectic ceramics. Tensile creep deformation tests conducted under constant stress ranging 100-200 MPa at temperature range of 1673-1873 K at different environmental conditions consisting of air and at different water vapor pressure, pH2O, up to 0.6 MPa. These eutectic ceramics exhibited a stress exponent of about 4.9-11.6, indicative of tensile creep behavior characteristic of dislocation mechanism. The apparent activation energy for creep deformation was 737-984 kJ/mol of Al2O3/YAG and 957 kJ/mol of Al2O3/GAP in air, while it was 529-645 kJ/mol of Al2O3/YAG and 611 kJ/mol of Al2O3/GAP at pH2O = 0.06 MPa and 570-620 kJ/mol of Al2O3/YAG at pH2O = 0.6 MPa. The presence of moisture accelerated the creep rate by about 1.4-4 orders of magnitude for pH2O = 0.06 MPa, about five orders of magnitude for pH2O = 0.4 MPa and about 5-7 orders of magnitude for pH2O = 0.6 MPa as compared with that in air at 1773 K. Results of this study and reasonable estimates in changes of the water vapor pressure indicated accelerated creep behavior by the presence of moisture.