Sintering of nanophase alumina powder and alumina composites reinforced with carbon nanotubes, and creep of ceramic matrix composites

Phase transformations of the nanophase alumina (γ-phase, the mean diameter: 23 nm) were studied by heat treatment of the nanophase alumina powder at 1000∼1250°C in air, and followed with x-ray diffraction. An activation energy for the phase transformation (γ to α) of 595 ± 49 kJ/mole was calculated from times when the amount of α-phase was estimated to be about 40%. Only large γ phase particles (>44 nm) transformed to α phase, as determined by x-ray line broadening measurements of particle size. Two types of powder, one as received (γ-phase) and the other heated at 1300°C for 7 minutes to transform γ-phase into α, were used to conduct sintering experiments. Pressure-less sintering was performed at temperatures from 1250°C to 1550°C in air, and hot pressing was performed at 1150∼1450°C, 40 to 80 MPa in Ar atmosphere. The γ-phase compacts showed poor sinterability. In hot pressing of nanophase alumina, the densification rate was increased by using powder mixed with γ- and α-phase.; Alumina and alumina matrix composites containing 5 to 20 volume percent of MWNT were fabricated by hot pressing in Ar atmosphere at 1300°C for 1h with 60 MPa of applied pressure. A powder of α-phase alumina, with 44 nm mean diameter, and arc-derived MWNT were used for these experiments. The 5 vol. % MWNT helps the densification of the composite, but more MWNT slows further densification. The hardness of the composite decreased with increasing volume content of the MWNT. The hardness of the composite containing 20 vol. % of MWNT was 13.6 GPa. The fracture toughness for the composite containing 10 vol. % of the MWNT was 4.2 MPa.m0.5; an improvement of 24% when compared with that of the monolithic alumina (3.4 MPa.m0.5).; Creep properties of two commercial ceramic matrix composites, GE AS 720, and Dow Corning SYLRAMIC S200 were measured at 5 to 80 MPa of compressive stress and 1050∼1400°C. Creep rates in AS 720 composites were dependent on the direction of load with respect to the apparent fiber orientation. The creep rate in the composite was higher when the fiber was oriented parallel to the load direction. An apparent activation energy for creep of 340 kJ/mole was found from the slope of the graph log (dϵ/dt) vs. 1/T at 50 MPa. The stress exponent 0.99 ± 0.08, close to 1, was calculated from the slope of the graph log (dϵ/dt) vs. log σ. Therefore, the creep of the AS 720 is probably controlled by a viscous flow mechanism. For SYLRAMIC S200, an activation energy of 88 kJ/mole was found, and the stress exponent was the unusually low value of 0.5∼0.6 at 1300°C.