3418 D. Sciti er al. / Joumal of the European Ceramic Society 26 (2006)3415-3423 350 (c) Fig 3. Microstructure of the ICI laminate: (a) polished cross-section; (b)detail showing the interface between adjacent layers; (c)enlarged view of the conductive layers; and(d) enlarged view of the insulating layers. Bright contrast particles belong to the MoSi2 phase. resolution of 0.01 um) connected to a personal computer to 170-180 um for the inner layers and 370 um for the outer layers drive highly precise displacements in order to scan the bar (see Table 3). lateral surface. i.e. from the bar edge under tension to the bar In Fig 3c and d, detailed views of the single layers are edge under compression reported. The bright contrast particles are molybdenum disili- cide and the grey regions consist of AIN and Sic phases, not 3. Results distinguishable(even by backscattered imaging)due to the very close atomic number. The MoSi2 particle distribution is homo- 3.1. Microstructure and properties geneous inside each layer. The mean MoSig diameter evaluated by image analysis on polished surface is in the range 0.9-1 um Highly dense materials were obtained after sintering at in both the I and C layers. Some aggregates of MoSiz particles 1850C, confirming the good sinterability of this non-oxide sys- are observed in the C layers due to the higher volumetric frac- tem. The relative densities of the BI and BC laminates were in the tion. Intergranular phases consisting of crystalline YAG and/or range 98-99%. The relative density of ICI laminate after sinter- amorphous silicate phases are present in all the systems. SEM ing was >97%. All the theoretical densities were calculated with observations evidence that the amount of secondary phases in I the rule of mixture, considering the starting nominal composi- layers is considerably lower than in C layers. At the same time, tions. The presence of MoSi2 particles is considered to be the key I layers present a higher level of porosity. These two factors are factor that improves the densification of this composite. MoSi2 again related to the Mosi content and confirm that a higher particles are in fact originally coated with an amorphous silica content of this phase is beneficial for improving the densifica- layer that enhances sintering kinetics acting as grain bound- tion. The microstructural features(not shown) of samples BI ary lubricant and favouring the formation of liquid phase in the and BC are similar to those reported for I and C layers. Despite AIN-Y203-SiO2 system. 6, I7 he lamination technique used, no traces of the junction between The main crystalline phases in the dense materials are the overlapping layers are observed, ie. these layered composites starting ones, as confirmed by previous studies. b, l/ Some resemble bulk materials. The mean MoSiz particle dimensions microstructural features of the ICI composite are illustrated are in the range 0.9-l um, likewise for I and C layers in Fig. 3a-d, showing a polished cross-section. Perfect adhe- As reported in Table 2, the BC laminate is a good conductor sion was found between the layers and no delamination was as a result of the interconnectivity of MoSi2 particles, whilst observed through the composite(Fig. 3b), very likely due to in the BI sample, the amount of electro-conductive phase is too the close similarity of composition of the layers containing the low to reach the percolation limit. As expected, the CTEs of the same starting phases. The final layers'thickness was around two materials are different as a result of the different starting3418 D. Sciti et al. / Journal of the European Ceramic Society 26 (2006) 3415–3423 Fig. 3. Microstructure of the ICI laminate: (a) polished cross-section; (b) detail showing the interface between adjacent layers; (c) enlarged view of the conductive layers; and (d) enlarged view of the insulating layers. Bright contrast particles belong to the MoSi2 phase. resolution of 0.01
m) connected to a personal computer to drive highly precise displacements in order to scan the bar lateral surface, i.e. from the bar edge under tension to the bar edge under compression. 3. Results 3.1. Microstructure and properties Highly dense materials were obtained after sintering at 1850 ◦C, confirming the good sinterability of this non-oxide sys￾tem. The relative densities of the BI and BC laminates were in the range 98–99%. The relative density of ICI laminate after sinter￾ing was >97%. All the theoretical densities were calculated with the rule of mixture, considering the starting nominal composi￾tions. The presence of MoSi2 particles is considered to be the key factor that improves the densification of this composite. MoSi2 particles are in fact originally coated with an amorphous silica layer that enhances sintering kinetics acting as grain bound￾ary lubricant and favouring the formation of liquid phase in the AlN–Y2O3–SiO2 system.16,17 The main crystalline phases in the dense materials are the starting ones, as confirmed by previous studies.16,17 Some microstructural features of the ICI composite are illustrated in Fig. 3a–d, showing a polished cross-section. Perfect adhe￾sion was found between the layers and no delamination was observed through the composite (Fig. 3b), very likely due to the close similarity of composition of the layers containing the same starting phases. The final layers’ thickness was around 170–180
m for the inner layers and 370
m for the outer layers (see Table 3). In Fig. 3c and d, detailed views of the single layers are reported. The bright contrast particles are molybdenum disili￾cide and the grey regions consist of AlN and SiC phases, not distinguishable (even by backscattered imaging) due to the very close atomic number. The MoSi2 particle distribution is homo￾geneous inside each layer. The mean MoSi2 diameter evaluated by image analysis on polished surface is in the range 0.9–1
m in both the I and C layers. Some aggregates of MoSi2 particles are observed in the C layers due to the higher volumetric frac￾tion. Intergranular phases consisting of crystalline YAG and/or amorphous silicate phases are present in all the systems. SEM observations evidence that the amount of secondary phases in I layers is considerably lower than in C layers. At the same time, I layers present a higher level of porosity. These two factors are again related to the MoSi2 content and confirm that a higher content of this phase is beneficial for improving the densifica￾tion. The microstructural features (not shown) of samples BI and BC are similar to those reported for I and C layers. Despite the lamination technique used, no traces of the junction between overlapping layers are observed, i.e. these layered composites resemble bulk materials. The mean MoSi2 particle dimensions are in the range 0.9–1
m, likewise for I and C layers. As reported in Table 2, the BC laminate is a good conductor as a result of the interconnectivity of MoSi2 particles, whilst in the BI sample, the amount of electro-conductive phase is too low to reach the percolation limit. As expected, the CTEs of the two materials are different as a result of the different starting