Weight silicon nitrides

As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature appHcations the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m and 2—4 g/m, respectively. The kinetics of the oxidation process have been iavestigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

Submicrometer-size droplets of methyldichlorosilane were produced at cryo-scopic temperatures and reacted with ammonia to give high-molecular-weight solid silazane derivatives in a rapid reaction. On calcination these precursors ended as spherical mixed silicon nitrides of a- and p-Si3N4 (70). 

Volatility can be a problem during the thermal conversion of polysilazanes to silicon nitride. The efiBciency of the process not only depends on losses due to stripping of excessive side groups but also decreases because of losses due to polymer volatility. Therefore, the polysilazanes must have suflSciently high molecular weights to minimize such losses. 

Mixed oxide, Al203-Si02, bond phases are generally the next most corrosion resistant to acids including low quantities of HF. Silicon nitride and silicon oxynitride bonded silicon carbides perform similarly to mixed oxide bonds. Table 19-2 shows typical weight loss and retained strength values for these products in contact with common acidic solutions for the times and temperatures shown. ... 

Four examples were given in the patent, all using supercritical propane with polysilane and aluminum isopropoxide, so-called precursor ceramic materials. In one instance scanning electron microscopy was used to demonstfate that polysil me was deposited as a smooth surtace film on the alumina fibers and fine silicon nitride whiskers. A silicon carbide material with internal pore openings as small as 10 microns was penetrated by supercritical propane laden with aluminum isopropoxide. Weight gains of up to 43% can be obtained depending on the density of the initial host ceramic. 

Fig. 1. Specific yield strength (0.2 % proof stress in compression per unit weight density at 10 4s 1 strain rate in compression) as a function of temperature for the D022 phase Al3Nb [112, 113], the Heusler-type phase Co2TiAl [67], the Laves phasesTiCr15Si05 andTaFcAl [67,114], the two-phase alloy NbNiAl-NiAl with 15 vol.% NiAl in the Laves phase NbNiAl [67,114], and the hexagonal D8g phase Ti5Si3 [100] in comparison to the superalloy MA 6000 (in tension) [115] and the hot-pressed silicon nitride HPSN (upper limit of flexural strength) [116],...

Commercial silicon nitride bearing balls with a diameter of 31.75 and 4.761 mm respectively were used for this investigation. Of the first kind 60 balls (two sets A, C) and of the second kind 30 balls (set B) were tested. The balls were weighed and their diameters were measured. The measurement uncertainties of the diameter were 1 pm, i.e. they are determined by the precision of the micrometre screw. The density of the ball materials (determined by the ratio of weight/volume) is 3244 2 kg/m. These data demonstrate a high quality and reproducibility of manufacture of the balls. 

Figure 31. Weight loss of silicon nitride materials with different Si02 content in the grain boundary (200 h corrosion in H2SO4 at 60°C).

Figure 33. Weight loss of silicon nitride materials under hydrothermal conditions at 210°C.

Chlorine is very corrosive to virtually all metals at high temperatures but not to the binary ceramic compounds tungsten carbide, hexagonal boron nitride, and silicon nitride. WC shows no reaction up to 700°C. BN suffers a weight decrease of 0.55 mg/cm after 40 h at 700°C. Si3N4 remains inert even after 500 h of exposure to chlorine at a temperature of up to 900°C. 

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