鳥塚 史郎

Ceramic TiB_2 composites with up to 40 wt% ZrO_2 or 2 mol%Y_2O_3-ZrO_2 were fabricated by hot isostatic pressing (HIP) with glass encapsulation. The effect of addition on the mechanical properties and microstructure of TiB_2 HIP'ed bodies was investigated. The addition of ZrO_2 or 2 mol%Y_2O_3-ZrO_2 was very effective in improving the 3-point bending strength of TiB_2. The 3-point bending strengths of TiB_2 containing 10 wt% ZrO_2 and 10-20 wt% (2 mol%Y_2O_3-ZrO_2) were 1160-1200 MPa, which were about three times higher than that of monolithic TiB_2 (450 MPa). The 3-point bending strength was dependent on the monoclinic ZrO_2 ratio (monoclinic ZrO_2/monoclinic ZrO_2+tetragonal ZrO_2) and the monoclinic ZrO_2 content in the HIP'ed bodies. To obtain HIP'ed bodies with strengths higher than 1000 MPa, the ratio of monoclinic ZrO_2 must be less than 60% and the monoclinic ZrO_2 content must be less than 10 wt% in HIP'ed bodies. Vickers hardness and fracture toughness were also affected by the addition of ZrO_2 and 2 mol%Y_2O_3-ZrO_2. The Vickers hardness of HIP'ed bodies decreased with increasing ZrO_2 and 2 mol%Y_2O_3-ZrO_2 content. On the other hand, the fracture toughness increased linearly with increasing ZrO_2 and 2 mol%Y_2O_3-ZrO_2 content. The fracture toughness of TiB_2 containing 30 wt% ZrO_2 was 8.2 MPa・m^<1/2>. This toughning was caused by crack deflection, however, stress induced transformation was not observed. TEM observation showed no grain boundary phase which was resulted from the reaction between TiB_2 and ZrO_2. However, EDS analysis showed that Ti and Zr existed in the ZrO_2 and TiB_2 grains, respectively, indicating that diffusion occured between TiB_2 and ZrO_2.

The method for determining free silicon in sintered Si_3N_4 by laser Raman spectrometry has been studied. In this study, it was found that the amount of free silicon can be determined from the Raman peak intensity ratio I.521 cm^<-1>/I.206 cm^<-1>, in which the peak of 521 cm^<-1> is assigned to free silicon and that of 206 cm^<-1>, to skeleton vibration of β-Si_3N_4. The results obtained by using this method are as follows : (1) The increase in sintering temperature and time tends to increase the amount of free silicon. (2) The variation of free silicon content at the surface part from place to place is greater than that at the inside part. This method makes it possible to measure the amount of free silicon in β-Si_3N_4 easily, and possibly gives instructive information such as the condition for avoiding precipitation of free silicon.

Full dense TiB₂ sintered body containing ZrO₂ between 0wt % and 40wt % was fabricated by glass encapsulation HIP method. Three point bending strength of TiB₂ increased drastically by adding 10wt % ZrO₂, compared to that of monolithic TiB₂. Three point bending strength and Vickers hardness of TiB₂-10wt %ZrO₂ sintered body were 1.17GPa and 23.4GPa, respectively. Such an amount of ZrO₂ was effective to fabricate high strength and high hardness of TiB₂ sintered body.<BR>However, three point bending strength decreased with increasing ZrO₂ content. On the other hand, fracture toughness increased linearly with increasing ZrO₂ content. TiB₂-30wt%ZrO₂ sintered body exhibited a high fracture toughness of 8.2MPam^1/2. The amount of monoclinic ZrO₂ increased with increasing ZrO₂ content. This increase resulted in the decrease in strength, while the fracture toughness increased.