M.L. Antti et al. Journal of the European Ceramic Society 24(2004)565-578 this was not possible to confirm with accuracy due the is possible that large voids \ s am therr just beneath the surface extreme fineness of the pores. Densification of the in these cases. In Table 4 are ed average hardnesses matrix was however indicated by an increase in micro- and standard deviations based arness ting these extremely low values. The scatter of the The microhardness of the matrix increased markedly values also increased significantly after thermal expo- with treatment temperature and time at 1000 and sure, indicating that the matrix became inhomogeneous 1 100C(see Table 4). At a few locations on samples This is also indicated in the micrograph in Fig 13 where treated at 1100C and at 1000C for 3240 h very low variations in porosity levels can be seen, the denser hardness values were obtained in association with a areas giving brighter reflection. Several indentations glassy appearance of the matrix and with the formation also indicated matrix embrittlement after heat treatment of cracks from the corners and edges of the indent. by the formation of indentation cracks emanating from Since SEM-EDS analysis of these locations did not indi- indent corners(Fig. 14). It can be noted that the cracks cate any significant deviation in chemical composition it only propagate perpendicular to the fibres and not Fig. Il. Fracture surfaces of as-received and heat-treated samples(a)As-received. (b) Heat-treated 100 h at 1000C.(c) Heat-treated 100 h at 1100C.(d) Heat-treated 100 h at 1100C showing sintered bundlethis was not possible to confirm with accuracy due the extreme fineness of the pores. Densification of the matrix was however indicated by an increase in micro￾hardness. The microhardness of the matrix increased markedly with treatment temperature and time at 1000 and 1100
C (see Table 4). At a few locations on samples treated at 1100
C and at 1000
C for 3240 h very low hardness values were obtained in association with a glassy appearance of the matrix and with the formation of cracks from the corners and edges of the indent. Since SEM–EDS analysis of these locations did not indi￾cate any significant deviation in chemical composition it is possible that large voids lay just beneath the surface in these cases. In Table 4 are listed average hardnesses and standard deviations based on 10 indentations omit￾ting these extremely low values. The scatter of the values also increased significantly after thermal expo￾sure, indicating that the matrix became inhomogeneous. This is also indicated in the micrograph in Fig. 13 where variations in porosity levels can be seen, the denser areas giving brighter reflection. Several indentations also indicated matrix embrittlement after heat treatment by the formation of indentation cracks emanating from indent corners (Fig. 14). It can be noted that the cracks only propagate perpendicular to the fibres and not Fig. 11. Fracture surfaces of as-received and heat-treated samples. (a) As-received. (b) Heat-treated 100 h at 1000
C. (c) Heat-treated 100 h at 1100
C. (d) Heat-treated 100 h at 1100
C showing sintered bundle. 574 M.-L. Antti et al. / Journal of the European Ceramic Society 24 (2004) 565–578