Novel route toβ‐SiAlON–SiO₂ceramic composites

Abstract

This paper reports a novel synthetic route to prepare dense b-SiAlON-SiO 2 ceramic composites. The stoichiometric b-Si 4 Al 2 O 2 N 6 extrudates prepared by the reaction sintering of a-Si 3 N 4 , a-Al 2 O 3 , AlN and Y 2 O 3 precursor mixture at 1675uC for 4 h were fine ground and mixed with different amounts of commercial fused silica (20, 40, 50, 60 and 80 wt-%) powder, dry pressed and sintered for 3-4 h at 1500-1750uC. These sintered materials were thoroughly characterised for bulk density, apparent porosity, water absorption capacity, phase composition, microstructure, hardness, dielectric constant and coefficient of thermal expansion. These characterisation results are presented and discussed in this paper.

Novel composites of $$\upbeta $$ β -SiAlON and radome manufacturing technology developed at ARCI, Hyderabad, for hypervelocity vehicles

“…The values in the range of 3.532−4.657 × 10 −6 • C −1 between 30 and 700 • C have been measured for β-Si 6−z Al z O z N 8−z for which z was varied from 1.5 to 4 [38]. Thus, the CTE values measured for SS40-1-SS40-5 are quite encouraging for these ceramic composites to be employed as radome materials [21,61,62]. Table 2 presents the basic chemical composition, BD, elastic modulus, flexural strength, CTE, dielectric permittivity and tan δ of SS40-5 material (developed at ARCI, Hyderabad) and of several other commercial radome materials [21].…”

“…The dielectric constant of mullite, which is expected to form due to the reactions between Al 2 O 3 and fused silica that occur during sintering process, is also lower (5.5) than that of β-Si 4 Al 2 O 2 N 6 at GHz frequency. These results indicate that the addition of 40 wt% SiO 2 into β-Si 4 Al 2 O 2 N 6 matrix can result in the reduction of the dielectric constant of SS-40 composite from 7.313 to 6.313, which was further reduced to 5.896 (for SS40-5) by the addition of 5 wt% Al 2 O 3 in the form of AlN powder [21,61]. Dielectric constant values in the range of 6.84-7.46 were reported for a gelcast β-Si 4 Al 2 O 4 N 6 .…”

“…Nevertheless, introducing more than 30% porosity by utilizing organic fugitive material in gelcast β-SiAlON radomes is not an easy thing, particularly to have products with required product yield. In view of these difficulties in synthesizing β-SiAlON composites with higher than 30 wt% silica, we have developed a simple method to prepare β-Si 4 Al 2 O 2 N 6 -SiO 2 ceramic composites with SiO 2 ranging 20-80 wt% [61]. In our process, initially, a β-Si 4 Al 2 O 2 N 6 extrudates (figure 15) were prepared and then ground them to the desired fineness.…”

“…In our process, initially, a β-Si 4 Al 2 O 2 N 6 extrudates (figure 15) were prepared and then ground them to the desired fineness. This fine ground powder can be mixed with different amounts of fused silica (0100 wt%) and they can be compacted following any one of the consolidation techniques including CDPP, GC, HAS and GCHAS, and can be sintered for 3-4 h at 1500-1750 • C to achieve the desired densification [61]. We found that among the different composites made from different compositions of β-Si 4 Al 2 O 2 N 6 and fused silica, the one formed out of 60 wt% β-Si 2 Al 2 O 2 N 6 + 40 wt% fused silica exhibited the best combination of sintered properties [61].…”

“…This fine ground powder can be mixed with different amounts of fused silica (0100 wt%) and they can be compacted following any one of the consolidation techniques including CDPP, GC, HAS and GCHAS, and can be sintered for 3-4 h at 1500-1750 • C to achieve the desired densification [61]. We found that among the different composites made from different compositions of β-Si 4 Al 2 O 2 N 6 and fused silica, the one formed out of 60 wt% β-Si 2 Al 2 O 2 N 6 + 40 wt% fused silica exhibited the best combination of sintered properties [61]. Owing to these reasons, a powder mixture containing 60 wt% β-Si 2 Al 2 O 2 N 6 + 40 wt% fused silica was chosen for consolidation by following GCHAS process with the help of 1-5 wt% as-purchased AlN powder as a setting agent.…”