E. Laarz et al / Journal of the European Ceramic Society 21(2001)1027-1035 homogeneous and agglomerate-free microstructures of much dependent on the added amount of titanium 2O TiC composites were obtained at both 0. 25 and nitride. All nano composites contained micron-sized 0.5 wt. PAA content(Fig. 7b) TIN agglomerates, in amounts increasing with increas- Composites containing TiC and Ti(C, N) whiskers ing TiN content. In the samples containing 5 vol. exhibited high sintered densities, though slightly lower TiNnano the particles and particle agglomerates were than the particle reinforced composites(Table 3). With well dispersed and homogeneously distributed in the an addition of 0.5 wt. PAA, the resulting micro- matrix phase. The particles are located both within the structure show well-dispersed phases but also traces of alumina grains and at the grain boundaries(Fig. 9a) porosity(Fig 8). The whiskers have a preferred orien- However, the alumina grain coarsening during sintering tation perpendicular to the hot-press axis. Obvious was pronounced. At 10 and 20 vol. TINnano, the alu defects in the microstructure are due to fibre pull-out mina grain coarsening was suppressed; Tin particles during polishing of the samples. XPS measurements formed large agglomerates at the alumina grain bound indicated a high amount of free carbon on the whisker aries, thus acting as grain growth inhibitors(Fig. 9b and surfaces. The graphite might weaken the bonding c). Based on these results it is expected that using up to between whisker and alumina matrix, thus facilitating 5 vol. TINnano particles in combination with fibre pull-out. Possibly, the hydrophobic graphite also reduced sintering temperature will result in a micro- promotes the formation of air bubbles, which are diffi- structure with smaller TiN agglomerates and less exces cult to remove during slurry processing sive alumina grain growth Composites containing TiNnano particles were fully densified (Table 3)and exhibited microstructures very Table 3 Compositions of sintered samples and measured density after hot The aim of this study has been to develop an aqueous pressing colloidal processing route suitable for Al2O3-TIN/TIC Compositi Amount of PAA added Final density with respect to dry l2O3(wt.%) Alox 3 Al, 25 vol %o TIC 4 Al,Or25 vol% TiC 0.5 5 AlOr-25 vol% TIN 0.25 6 Al,Or-25 vol% 7 Al,Or5 vol% 8 AlOr10 vol. 10Al2O3-20vol.% 0000000098 n Ti(C, N)whiskers I1 Al,O325 vol% Fig 8. SEM micrograph of the sample cross-section perpendicular to the pressing direction of hot-pressed Al2Or-20 vol Ti(C, N)whisker composite(0.5 wt% PAA) 18 Fig. 7. SEM micrographs of the sample cross-section perpendicular to the pressing direction:(a)Al2Or-25 vol TiN composite (0.5 wt% PAA); b)Al2Or25 vol TiC composite(0.5 wt. PAA)             0
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