Relationship of the R-curve with the microstructure of alumina ceramics

The influence of sintering temperature and soaking time period on R-curve behavior of Al2O3 ceramics have been investigated. Al2O3 samples were prepared by solid state sintering at 1500 degrees C, 1600 degrees C and 1700 degrees C for different soaking time periods of 3 h, 6 h, 9 h, 12 h, under an ambient atmosphere. However, longer soaking time periods of 18 h and 24 h were also applied for the samples sintered at 1500 degrees C. The fracture toughness of the sintered samples was determined from the cracks induced by the Vickers indentation method. The fracture toughness increases with the crack extension (R-curve behavior) for the samples having an average grain size limited to 7.3 mu m. R-curve behavior was not obeyed for the samples having a grain size more than 7.3 mu m. The R-curve nature has been predicted from the predominant intergranular crack extension mode as has been observed from microstructures of fracture surfaces of the samples. Evidence of crack deflection was found for the samples sintered at 1500 degrees C and 1600 degrees C for which relatively higher fracture toughnesses of 5.28 +/- 0.08 MPam(1/2) and 5.37 +/- 0.25 MPam(1/2) respectively, were determined. The samples sintered at 1700 degrees C have a relatively lower fracture toughness (4.86 +/- 0.21 MPam(1/2)). Transgranular fracture in the samples sintered at 1700 degrees C for a soaking time of 6 h and more containing larger grains is more pronounced. Evidence of crack deflection by studying the fracture surface and fracture surface roughness characteristics using Field Emission Scanning Electron Microscope (FESEM) and Atomic Force Microscope (AFM) respectively has been shown.