Effect of Si3N4 Addition on Oxidation Resistance of ZrB2-SiC Composites

Abstract

Abstract:The oxidation behavior of ZrB 2 -20 vol % SiC and ZrB 2 -20 vol % SiC-5 vol % Si 3 N 4 composites prepared by hot-pressing and subjected to isothermal exposure at 1200 or 1300 • C for durations of 24 or 100 h in air, as well as cyclic exposure at 1300 • C for 24 h, have been investigated. The oxidation resistance of the ZrB 2 -20 vol % SiC composite has been found to improve by around 20%-25% with addition of 5 vol % Si 3 N 4 during isothermal or cyclic exposures at 1200 or 1300 • C. This improvement in oxidation resistance has been attributed to the formation of higher amounts of SiO 2 and Si 2 N 2 O, as well as a greater amount of continuity in the oxide scale, because these phases assist in closing the pores and lower the severity of cracking by exhibiting self-healing type behavior. For both the composites, the mass changes are found to be higher during cyclic exposure at 1300 • C by about 2 times compared to that under isothermal conditions.

High-Temperature Environmental Degradation Behavior of Ultrahigh-Temperature Ceramic Composites

“…The results of isothermal exposure for 100 h at 1200 C and 1300 C as depicted in Fig. 6 have shown a stable regime being achieved in the case of the HS after about 24 h, whereas similar observation was recorded for ZS and ZSS after 54 h at 1200 C but earlier at 1300 C [29,46,47]. This study has also shown that under isothermal conditions, the HS has superior oxidation resistance compared to the investigated ZrB 2 -based composites, considering that its mass change is found to be the least at both 1200 C and 1300 C.…”

“…In fact, no definite trend emerges, because of simultaneous mass gain due to formation of ZrO 2 and SiO 2 along with vaporization of B 2 O 3 as well as formation and escape of CO. For example, during exposure of ZrB 2 -SiC and HfB 2 -SiC composites for the first 1 h, both gain and loss in mass have been observed in these composites after various intervals, as shown in Fig. 4 [29,47,49]. The results depicted in this figure show net mass loss in the HS composite at the 10th minute of exposure at 1200 C as well as at both 20th and 30th minute of exposure at 1300 C. In contrast, net mass gain has been observed on exposure of the ZS composite at both 1200 C and 1300 C, which may be attributed to dominance of the formation of ZrO 2 and SiO 2 .…”

“…Whereas the presence of SiC is found to be beneficial for facilitating the formation of protective borosilicate glass in the oxide scale, the ZrC reduces the oxidation resistance by formation of carbon monoxide which escapes by leaving a scale of porous ZrO 2 . In contrast, the addition of 5 vol.% Si 3 N 4 to the ZrB 2 -20 vol.% SiC composite has been found to promote the formation of protective borosilicate scale and thereby enhance the oxidation resistance [47].…”

“…The results of isothermal oxidation tests for 24 h at 1200 and 1300 C, as depicted in Fig. 5, have shown a stable regime with negligible mass change only in the case of the ZSS being exposed at the former temperature [47]. The stable regime could be either due to the formation of a continuous and protective B 2 O 3 -SiO 2 scale or due to a dynamic equilibrium between mass gain and loss.…”

“…Phase identification by XRD analyses has shown the presence of tetragonal (t) and monoclinic (m) ZrO 2 in the oxide scales of the ZrB 2 -based composites [29,46,47]. In addition, ZrSiO 4 has been also found in the oxide scales of all the ZrB 2 -based composites with the exception of ZS exposed at 1200 C. The peaks of Si 2 N 2 O have been also detected in the XRD pattern of the oxide scale formed on the ZSS composites.…”