Hot isostatic pressed silicon nitride

Hakulinen, M., 1985, Residual strength of ground hot isostatically pressed silicon nitride, /. Mater. Sci., Vol. 20, pp. 1049-1060. 

Since the 1970s, the search for improved materials has led to a better understanding of the role of additives in the densification and microstructural development of silicon nitride-based ceramics and the consequences for final properties [1, 6]. Improvements in powder manufacture and forming techniques and the development of alternative firing processes has led to a complete family of materials including RBSN, HPSN, sintered silicon nitride (SSN), sintered reaction-bonded silicon nitride (SRBSN), gas-pressure sintered silicon nitride (GPSSN), hot isostatically pressed silicon nitride (HIPSN) and silicon nitride alloys or solid solutions termed SiAlONs, based on their elemental components. 

Comparison of silicon nitrides with carbon additions prepared by hot isostatic pressing and pressureless sintering... 

Concerning carbon nanotube-reinforced silicon nitride matrices, only a few reports have so far been published [19]. In this case, hot isostatic pressing has been used for composite processing. The carbon nanotubes remained in the microstructure only under low pressures (2 MPa) they connect the silicon nitride grains and produce a 15-37% improvement of the mechanical properties as compared with other carbon-filled samples (Fig. 19.11). Increase of pressure... 

In general, covalently bonded materials are difficult to sinter because of their inherently low value of self-diffusivity. However, using microwave heating, silicon nitride with 20 wt.% yttria-doped zirconia has been sintered at 1400°C in a nitrogen atmosphere at a pressure 0.1 MPa. In contrast, via a conventionally heated hot isostatic press, a sintering temperature of 1850°C and a nitrogen pressure of about 180 MPa were required to densify the same silicon nitride/zirconia composition as completely as was achieved by microwave heating at 1400°C. ... 

I. Tanaka, G. Pezzotti, T. Okamoto, Y. Miyamoto and M. Koizumi, Hot Isostatic Press Sintering and Properties of Silicon Nitride at Elevated Temperatures , J. Am. Ceram. Soc., 74, 1992, 752-59. 

Balazsi et al. [72] carried out extensive experimental work to analyse the effectiveness of the SPS method compared to conventional hot isostatic pressing for silicon nitride composites with 6 wt.% MWCNTs. For spark plasma sintered samples, different sintering conditions such as temperature and holding time were investigated to find the most suitable processing parameters. It was shown that dense samples with improved mechanical properties were achieved at comparatively lower sintering temperatures by SPS [72]. At least a 100% increase in the modulus of elasticity (around 300GPa) was achieved for spark plasma sintered samples compared to hot isostatic pressed samples. 

Table 11.1.1 shows the features of various ceramic materials [5]. Ceramic materials include silicon nitride (Si3N4), zirconia (Zr02), silicon carbide (SiC) and alumina (AI2O3). Mechanical strength is in the order alumina < silicon carbide < silicon nitride < zirconia, and corrosion resistance is in the order silicon nitride < zirconia < silicon carbide = alumina. Silicon nitride and zirconia are sintered by hot isostatic pressing (HIP). 

In order to achieve fuUy dense silicon nitride monolithic bodies, it is necessary to employ either the hot pressing (HP) or hot isostatic pressing (HIP) of sihcon nitride powder, with the addition of metal oxides (for a previous review, see Maccagno, 1989). At temperatures above 1550°C, these additives form with contaminant silicon dioxide films present around individual silicon nitride grains a liquid silicon oxynitride phase in which the silicon nitride may be readily dissolved. 

Enhanced parabolic oxidation rates of SiC and Si3N4 due to contamination from furnace tubes used in oxidation experiments have also been observed (Choi et al., 1989 Opila, 1995 Ogbuji and Opila, 1995 Fox, 1998). This enhancement is also attributed to Na impurities from impurities in alumina tubes (Opila, 1995). In one case (Fox, 1998), the oxidation rates of SiC and Si3N4, both very pure CVD materials, were found to follow parabolic kinetics, but at essentially the same rate. It was proposed that impurities from the alumina furnace tubes modified the structure of the silicon oxynitride so much that it no longer offered any additional barrier to oxygen transport than the silica scale. Backhaus-Ricault and Go-gotsi (1995) have also shown that silicon oxynitride is absent in hot isostatically pressed additive-free silicon nitride. This result may also be due to minor amounts of impurities resulting from the HIP process. 

Silicon nitride balls tested in this study were procured from specialist manufacturers. They were manufactured by a hot isostatically pressed (HIP) method. Ball blanks were ground and polished to 12.7 mm diameter in this case by a manufacturer standardised procedures were adopted to ensure consistent quality of material and geometry. The... 

Sintered and isostatically hot-pressed SiC materials, as well as silicon nitride (Si3N4) are playing an important role in the development of ceramic components (see Fig. 35) for motor vehicle engines and gas turbines [288-293]. Real technical success with sintered SiC components in the field of high temperature gas turbines has not yet been achieved [294,295]. However, SiC and Si3N4 gas turbine parts are in field tests and strong development efforts continue in several countries. 

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