Abstract
The effect of thermal aging in air on a Nextel(TM) 720 aluminosilicate fibre reinforced alumina matrix material (N72O/AI2O3) has been investigated. Samples were aged at 1100oC for up to 4000 hours as well as for 200 hours at 1100°C, 1200°C, 1300°C, 1400°C and 1480°C. On completion of the thermal aging treatments, the microstructures of the samples were characterised, principally using scanning and transmission electron microscopy. The mechanical properties of the material, flexural strength, Young's modulus and relative toughness, after aging were investigated using three point flexural testing. The as-received material was found to contain many voids and a large quantity of cracking that are believed to arise from in-complete matrix infiltration and green body production, respectively, during manufacture. It was found that the material does not meet the original proposed design criteria for this class of material. Initial results indicate that the 1100°C thermal aging treatment for up to 2000 hours has no detectable effect on the microstructure or properties of the material. After aging at 1100°C for 4000 hours, changes were detected in the material suggesting that prolonged thermal exposure of the material does have an effect on its properties, specifically a reduction in sample thickness indicating that the matrix may have densified slightly and a small increase in modulus and loss of aluminium from the fibre. In contrast, much shorter exposures to higher temperatures lead to significant changes to the microstructure, principally in terms of the reduction in porosity and grain growth in the matrix regions and an embrittlement of the material from an aging temperature of 1300°C, such that the material behaved as a monolithic ceramic after aging at 1480°C. Aging at 1200°C and above was found to cause a progressive decrease in the material thickness indicating a densification of the material. The fibre architecture was found to restrict densification in the plane of the fibre reinforcement. The mechanical properties of the material aged for 200 hours at 1200°C appear unaffected by the thermal aging. The aging of material at 1300°C was found to increase the Young's modulus to a maximum value after aging at 1400°C. Aging at 1480°C appeared to cause a slight decrease in the Young's modulus of the material. Aging of the material at 1300°C and above was found to cause a continuing reduction in the flexural strength of the material until a minimum value was reached after aging at 1480°C. A change in the microstructure of the fibre was initially observed after aging at 1300°C and was more pronounced after aging at 1400°C and 1480"C. A progressive growth of elongated alumina grains in the fibres was observed to occur as the meta-stable aluminium-rich mullite transformed to a silicon-rich mullite within the fibre. After aging at 1480°C the fibre was also observed to contain significant quantities of porosity. Furthermore, the fibre reinforcement appears to have lost aluminium, possibly to the matrix. The results of this investigation have found that the material is stable for aging periods of 2000 hours at 1100°C and for up to 200 hours at 1200°C. Whilst aging regimes of over 2000 hours at 1100°C may be acceptable, evidence has been found to suggest that the material is changing.