une 1999 High-Temperature Oxidation of Boron Nitride: Boron Nitride Layers in Composites 1475 Grind one face BN 10 -100μmsic CVD o8 SiC/BN--2 hr -10μmBN SiC/BN--50 h Borosilicate glass 0.2 Oxidize 11 0005101620253035 Reglon a Distance from Sic/borosilicate interface, um Region b Fig. 4. Interpreted RBS data of CVD SiC with 2 um BN after oxi- Regis dation at 900]C for 2 h and 50 h. in a borosilicate liquid follows approximately ideal solution behavior and, thus, would have a high activity in this situa- tion. Note, the weight loss is not linear-apparently the diffu- sion of B,O3 out of the glass influences the kinetics. After 50 h, the RBS information(Fig. 4)shows little B,O, in the oxi- dation product, supporting this interpretation ount, grind and polish SEM examination of the surface after 50 h shows a smooth econd face to examine featureless glass. Figure 5 shows a cross section of this glass xtent of oxidation The average thickness of this product(-8 um)is substantially greater than that expected from oxidation of pure SiC in oxy Fig. 2. Preparation of layered samples. gen(<0.5 um). This indicates that boron-enhanced SiC oxi- dation is occurring simultaneously with the oxidation of BN B,O3 from the borosilicate. As discusse TGA results for the Bn deposited at <1000 C are shown in Fig. rates in SiO, and, hence, enhance oxidation rates. s ansport 3. Note, there is an initial rapid weight gain and then a slower Figure 6 illustrates the reaction kinetics for a high weight loss. After 2 h, a thick glassy borosilicate layer is ob temperature(1900C)CVD BN deposited on CVD SiC. The served on the coupon environment is the same as that for Fig. 3. The initial rapid In order to determine the relative amounts of B2O3 in the weight loss is likely an experimental artifact In the first 40 h, lass, RBS measurements are taken. Results shown in Fig 4 a weight gain occurs due to BN and SiC oxidation. This period indicate that, after 2 h, the oxidation product is a borosilicate of weight gain is also observed for the low-deposition liquid with a substantial quantity of B2O3 temperature BN(Fig. 3), but the rate for the latter is much After the initial weight gain, the specimen slowly loses faster because of the higher oxidation rates of low-deposition- weight. Thermochemically, this can only be due to volatilize temperature BN. Figure 6 also shows that, after h, a tion of B2O3 as HBO2(g) from reactions with residual water vapor. This is expected, because the chemical activity of B2O3 吕016 012 008 0510162025303540 Fig. 5. Cross section of CVD SiC with 2 um BN after oxidation atTGA results for the BN deposited at <1000°C are shown in Fig. 3. Note, there is an initial rapid weight gain and then a slower weight loss. After 2 h, a thick glassy borosilicate layer is ob￾served on the coupon. In order to determine the relative amounts of B2O3 in the glass, RBS measurements are taken. Results shown in Fig. 4 indicate that, after 2 h, the oxidation product is a borosilicate liquid with a substantial quantity of B2O3. After the initial weight gain, the specimen slowly loses weight. Thermochemically, this can only be due to volatiliza￾tion of B2O3 as HBO2(g) from reactions with residual water vapor. This is expected, because the chemical activity of B2O3 in a borosilicate liquid follows approximately ideal solution behavior14 and, thus, would have a high activity in this situa￾tion. Note, the weight loss is not linear—apparently the diffu￾sion of B2O3 out of the glass influences the kinetics. After 50 h, the RBS information (Fig. 4) shows little B2O3 in the oxi￾dation product, supporting this interpretation. SEM examination of the surface after 50 h shows a smooth, featureless glass. Figure 5 shows a cross section of this glass. The average thickness of this product (∼8 mm) is substantially greater than that expected from oxidation of pure SiC in oxy￾gen (<0.5 mm).15 This indicates that boron-enhanced SiC oxi￾dation is occurring simultaneously with the oxidation of BN and vaporization of B2O3 from the borosilicate. As discussed in the introduction, small amounts of boron enhance transport rates in SiO2 and, hence, enhance oxidation rates.5 Figure 6 illustrates the reaction kinetics for a high￾temperature (∼1900°C) CVD BN deposited on CVD SiC. The environment is the same as that for Fig. 3. The initial rapid weight loss is likely an experimental artifact. In the first 40 h, a weight gain occurs due to BN and SiC oxidation. This period of weight gain is also observed for the low-deposition￾temperature BN (Fig. 3), but the rate for the latter is much faster because of the higher oxidation rates of low-deposition￾temperature BN.8 Figure 6 also shows that, after ∼60 h, a Fig. 2. Preparation of layered samples. Fig. 3. TGA kinetics of a CVD SiC coupon coated with 2 mm of low-deposition-temperature (<1000°C) CVD BN oxidized in O2/20 ppm H2O at 900°C. Fig. 4. Interpreted RBS data of CVD SiC with 2 mm BN after oxi￾dation at 900°C for 2 h and 50 h. Fig. 5. Cross section of CVD SiC with 2 mm BN after oxidation at 900°C for 50 h. June 1999 High-Temperature Oxidation of Boron Nitride: Boron Nitride Layers in Composites 1475