Abstract
This work is concerned with the processing and properties of chromiuni reinforced alumina ceramics with the Cr particles in both the micro- and nano-scale ranges. The influence of processing and microstructure on the mechanical properties has been studied. Al2O3-20vol%Cr micro-composites have been fabricated using both sintering and hot pressing techniques. Sintering environment has a crucial influence on the microstructural development of the pressureless sintered Al2O3-Cr composites. It was found that too little or too much oxygen is detrimental to Al2O3/Cr interfacial bonding. Attempts have been made to improve the Al2O3/Cr interfacial bonding by sintering in a graphite powder bed in order to control the oxygen partial pressure. The fracture toughness of the composite with strengthened interfaces was the highest of all the sintered samples. However, the improvement is limited by the brittle fracture of Cr. This may be caused by the high carbon content associated with Cr particles in the composite. The ductility of Cr was higher in the hot-pressed Al2O3-Cr samples. The possibility of further toughening Al2O3 by Cr80Ni20 and Cr20Ni80 alloys with higher ductility was explored. It was shown that 20 wt% of Ni present in the alloying phase did not change the ductility, but when the Ni content increased to 80 wt% the crystal structure changed to fc.c., giving a inherently ductile metal. However, the large thermal mismatch between Al2O3 and Ni/Cr alloys led to a high density of microcracks at the interfaces. The composites with different metallic phases had similar' fracture toughness values as measured by double cantilever beam testing. Among the Al2O3-CrxNi1-x composites, the highest fracture toughness, 5.8 MPa m1/2, was achieved by the hot pressed Al2O3-Cr composite. This value is comparable to values measured for other alumina-metal systems. The poor bonding at the alumina/metal interface is the main limitation to toughening in these composites. Thus, it may not be possible to have a strongly bonded and ductile reinforcement. The pressureless sintered Al2O3-Cr composites with different particle sizes showed different thermal shock behaviour. The composite with fine Cr particles exhibited a thermal shock behaviour which is typical of engineering ceramics, but with an improved critical temperature difference compared to sintered Al2O3. The specimen with a larger Cr particle size showed gradual strength degradation with increasing temperature difference. The increased fracture toughness, low initial strength and low Young's modulus of the composite are the primary reasons for the greater strength retention following quenching. Although Al2O3 was toughened by Cr and Cr/Ni alloys, the strength of the micro-composites was not improved as the metal particles acted as large flaws. In order to reduce the flaw size nano-composites were investigated. Al2O3-5vol% Cr nanocomposites were fabricated using a chemical method. Optimisation of the processing procedure led to a desirable microstructure and significantly increased strength. Among the nanocomposites, the highest strength, 736+/-29 MPa, was achieved by hot pressing at 1450°C. The improved strength of the nanocomposites is the consequence of the microstructure refinement by homogeneously distributed nano-sized Cr particles. The nanocomposites are slightly tougher than the parent Al2O3, although the values of the fracture toughness are lower than those for the 20vol% micro-scale particle toughened Al2O3. Thus, a small degree of toughening and significant strengthening have been achieved by Al2O3-Cr nanocomposites.