Al2O3 and Si C

FRACTURE, STRENGTH AND DAMAGE TOLERANCE OF CERAMIC NANOCOMPOSITES

Objectives
The objectives of the project were to investigate ceramic “nanocomposites”, principally Al2O3 / SiC:
1. To identify strengthening and damage-tolerance mechanisms
2. To optimise design of nanocomposites
3. To optimise processing of nanocomposites
Substantial progress was made in all three of these areas. We now have a “recipe” for producing a sintered (rather than hot-pressed) Al2O3 / SiC “nanocomposite”, with improved strength, toughness, damage tolerance and wear resistance, and have made good progress in understanding the mechanisms of property improvement. Since the research program focused on the links between processing, microstructure, and mechanical properties, this report deals with processing issues first, followed by mechanical property investigations, finally dealing with the mechanisms linking the two via microstructure and leading to property improvements Processing Two aspects of processing were investigated: (a) the feasibility of using sintering to produce Al2O3 / SiC nanocomposites with enhanced properties, and (b) production of hot-pressed composites with very fine SiC particle size.

Sintering Previous investigations on Al2O3 / SiC nanocomposites by other workers suggested that it is difficult to obtain near fully dense products using the pressureless sintering route. However, the flexibility of this route (in terms of the geometrical complexity of manufactured products) and its relative costeffectiveness are compelling, so a main aim of the project was to investigate this method of fabrication. For a full report on this aspect of the project see [1].

Ultra-fine SiC composites. We have been able to fabricate a range of alumina/SiC nanocomposites with mean SiC particle sizes ranging from 12 nm to 120 nm. These were fabricated by two routes, the first a conventional powder processing and the second using a novel polymer precursor route where ultrafine SiC was generated by the pyrolysis of a polycarbosilane [2,3,4,5].

Publications
1. C.C Anya and S.G. Roberts, Pressureless sintering and elastic constants of Al2O3-SiC ‘nanocomposites’, J.Eur.Cer.Soc. in press.
2. B. Su and M. Sternitzke, A novel processing route for alumina/SiC nanocomposites by Si-polymer pyrolysis, 4th Europ. Ceram. Conv. (Ed. A. Bellosi), Vol. 4 p. 109-116 (1995).
3. L. Carroll, M. Sternitzke and B. Derby, Silicon carbide particle size effects in alumina based nanocomposites, Acta Mater. 44 (1996) 4543.
4. M. Sternitzke, E. Dupas, P. Twigg and B. Derby, Surface mechanical properties of alumina based nanocomposites, submitted to Acta Mater.
5. C.C. Anya and S.G. Roberts, Indentation fracture toughness & surface flaw analysis of sintered alumina/SiC ‘nanocomposites’ J.Eur. Cer.Soc. 16 (1996) 1107

http://www-sgrgroup.materials.ox.ac.uk/abstracts/J77542.pdf