Sven Scheppokat

RBAO composites containing TiN and TiNTiC

Abstract TiC and TiN were tested as candidate materials for particle reinforcement of reaction bonded aluminium oxide (RBAO). As part of the reaction bonding process Al-containing green bodies have to be heated in air in order to oxidize the Al; therefore the oxidation behaviour of any added particles is of critical importance. Previous studies have shown that neither TiC nor TiN is sufficiently oxidation-resistant to withstand the heating cycle necessary to completely oxidize the Al in the process. Therefore, two alternative approaches were investigated. First, TiC and TiN were retained in incompletely reacted RBAO, resulting in an Al 2 O 3 - TiC TiN composite with alloy and possibly intermetallic phases. These metal phases were obtained from Al and Ti Zr which formed when Al reduced TiO 2 and, to a lesser extent, ZrO 2 . Second, the reaction of AlN and TiO 2 to form Al 2 O 3 and TiN at temperatures above 1350 °C was investigated. The resulting material contained Al 2 O 3 , ZrO 2 , TiN and a nitrogen-containing aluminium titanate.

Characterisation of an oxidation layer on reaction bonded mullite/zirconia composites by indentation

Abstract An oxidised outer layer that was formed on reaction bonded mullite/zirconia composites and the difference in the mechanical properties between this layer and the bulk material were investigated using an ultra micro indentation system (UMIS) combined with XRD and high resolution SEM. It was shown that the bulk consisted of mullite and tetragonal zirconia as well as some residual alumina and SiC. The outer layer consisted of mullite and larger grain-sized zirconia as well as a small amount of zircon. About 60% of the zirconia was transformed from tetragonal to monoclinic phase in the layer. It was found that the bulk had higher hardness but lower toughness than the outer layer. Indentation testing revealed the presence of residual compressive stress in the outer layer, which was attributed to the different degrees of t - to m -ZrO 2 transformation, different fractions of mullite, and different coefficients of thermal expansion in the layer versus the bulk.

Reaction Bonding of Three-Layer Alumina-Based Composites

Three-layer alumina-based composites with Al2O3-containing mullite as the outer layer and ZrO2-containing alumina as the inner layer were fabricated by die-pressing and reaction bonding. The effects of the outer-layer compositions on the densification behavior of three-layer composites were investigated. The existence of residual stresses in the layers was verified using indentation methods. Compared with single-layer ceramics, three-layer composites exhibit an improved fracture toughness and an excellent damage resistance due to the presence of the compressive stresses in the outer layers.