July 2002 C-B-Si Coatings for S, Fiber-Reinforced Composites for Improved Oxidation Resistance l817 Table. Fabrication Conditions and Thicknesses of C-B-Si Coatings ating condition Thickness of Gas-flow rate(mmols-) Coating BCl3 SiCl CH4 H, N I0.298 0741.124461448 40-120 I0.149 0740.005.581448 R0.000 3720.004.171448100-200 (a) Sputter time()0.81.0 ference condition, R, included a carbon coating, the properties of which were described in a previous paper. Figure 2 shows depth profiles of coatings I and Il Coating thickness, determined by aES analysis, is shown in Table I Coating I had a boron-containing sublayer Coating Il contained a small amount of boron. However, the aes depth profile had the following problems, so that only relative changes in elemental composition were valid: (i)disagree 百E8 -- ment of the composition of the Si3 N4 fiber with the chemical analytical value;27,28 and(ii)miscalculation of 15 mol% of boron on the Si3N4 fiber, nevertheless nondetection of a boron peak in the AES spectra. These disagreements were explained by inaccu- facies of the default-calculation parameters and misreading of the (b)0002。0406 081.0 spectrum background by the calculation program. Therefore, the Sputter time(ks) depth profile on Fig. 2 was revised using the AES spectra as follows. Fig. 2. Depth profile of apparent elemental composition on(a)coating (1) For coating I, the surface consisted of carbon only, and boron, silicon, and nitrogen were detected at depths of 15, 30, and 60 nm, respectively. Boron had the maximum value, at a depth of 60 nm. The concentrations of silicon and nitrogen increased gradually and reached saturation at a depth of 120 nm, which the fiber side to the matrix side. The L2 presented the image of a nded to a ber interfa crystal lattice with 0.26 nm interlayer spacing. The NBED pattern (2) For coating Il, the surface consisted of carbon, and of the L2 layer was obscure spots, suggesting that L2 was silicon, and nitrogen were detected at depths of 15, 30, and 30 nm composed of disordered crystallites. The NBED patterns of bright respectively. Boron did not mark the obvious maximum value. layers LI and l3 lacked a lattice image and showed obscure rings, Silicon and nitrogen increased gradually, reaching saturation at a which polarized the brightness toward the direction of the fiber depth of 80 nm surface. Therefore, LI and L3 were composed of crystallites with The deposits in the CVd chamber after the preparation of very low crystallinity, oriented toward the fiber surface. EDS was coating I were examined using EPMA and XRD. The deposit sed on the fiber-matrix interface to obtain elemental information were >10 um thick, i.e., thick enough to make the peaks of the oron,although definitely contained in the sample, was not substrate graphite negligible on EPMA. Figure 3 shows changes in detected because of limitations in the present equipment. The peak ak intensity for each element along the longitudinal direction of intensities of carbon and silicon on the interface were about three the chamber. The temperature distribution in the chamber was mes and one-forth, respectively, that on the Si-N-C matrix; measured, using a thermocouple inserted into the chamber, under therefore, the interface consisted mainly of carbon and a small the flow of a carrier gas only. No deposit was observed below.8 amount of silicon. m(0. 8 m upstream from the center of the furnace). At-08 m boron and carbon were first detected. The boron attained a maximum value at-0.75 m. The carbon intensity exhibited a minimum at the boron maximum and then increased gradually on the uited a maximum at-03 m. Peaks of B,C were detected from 1500 ≌ peaks of 3C-SiC were detected from.5 to-0.1 m. The existence of carbon in the deposit was not confirmed using XRD. Because 0.8 the intrusion depth of the X-rays was greater than the thickness of 2 POB the deposits, the diffraction peaks of the substrate graphite over 06 lapped that of the deposit. The single-filament strength of the coated fibers is shown in 90.4 Table Il. The strength of the as-fabricated fiber was scattered among the fabrication lots of the fiber. Thus, a direct comparison of the strengths of the coated fibers was inadequate; the strength retention ratio was used. The retention ratio of each fiber was 77%121% 0.0 (2) Interface between Fiber and Matrin Position from center of furnace(m) The composite reinforced with Si3N4 fiber coated with coatin I is described here as composite I. Figure 4(a)shows the TEM Fig. 3. Change of relative intensity of boron, carbon, and silicon peaks image of the fiber-matrix interface of non-oxidized composite I EMPA analysis on the deposits in the coating chamber after fabrication coating I. Relative intensity he ratio of the peak intensity of ar The interface of this composite had a layered structure, composed lement on the deposit to that on the simple body of the element under the of a bright layer 5 nm thick (LI), a dark layer 10-15 m thick same measuring conditions. Origin of the horizontal axis is positioned on (2), and a bright layer 10-15 nm thick(3), in that order, from the center of the coating chamber. Negative direction is the upstream sidereference condition, R, included a carbon coating, the properties of which were described in a previous paper.33 Figure 2 shows depth profiles of coatings I and II. Coating thickness, determined by AES analysis, is shown in Table I. Coating I had a boron-containing sublayer. Coating II contained a small amount of boron. However, the AES depth profile had the following problems, so that only relative changes in elemental composition were valid: (i) disagree￾ment of the composition of the Si3N4 fiber with the chemical analytical value;27,28 and (ii) miscalculation of 15 mol% of boron on the Si3N4 fiber, nevertheless nondetection of a boron peak in the AES spectra. These disagreements were explained by inaccu￾racies of the default-calculation parameters and misreading of the spectrum background by the calculation program. Therefore, the depth profile on Fig. 2 was revised using the AES spectra as follows. (1) For coating I, the surface consisted of carbon only, and boron, silicon, and nitrogen were detected at depths of 15, 30, and 60 nm, respectively. Boron had the maximum value, at a depth of 60 nm. The concentrations of silicon and nitrogen increased gradually and reached saturation at a depth of 120 nm, which corresponded to a coating–fiber interface. (2) For coating II, the surface consisted of carbon, and boron, silicon, and nitrogen were detected at depths of 15, 30, and 30 nm, respectively. Boron did not mark the obvious maximum value. Silicon and nitrogen increased gradually, reaching saturation at a depth of 80 nm. The deposits in the CVD chamber after the preparation of coating I were examined using EPMA and XRD. The deposits were 10 m thick, i.e., thick enough to make the peaks of the substrate graphite negligible on EPMA. Figure 3 shows changes in peak intensity for each element along the longitudinal direction of the chamber. The temperature distribution in the chamber was measured, using a thermocouple inserted into the chamber, under the flow of a carrier gas only. No deposit was observed below –0.8 m (0.8 m upstream from the center of the furnace). At –0.8 m, boron and carbon were first detected. The boron attained a maximum value at –0.75 m. The carbon intensity exhibited a minimum at the boron maximum and then increased gradually on the downstream side. Silicon was detected from –0.7 m and exhibited a maximum at –0.3 m. Peaks of B4C were detected from –0.7 to –0.6 m from XRD analysis of the deposits. Very weak peaks of 3C-SiC were detected from –0.5 to –0.1 m. The existence of carbon in the deposit was not confirmed using XRD. Because the intrusion depth of the X-rays was greater than the thickness of the deposits, the diffraction peaks of the substrate graphite over￾lapped that of the deposit. The single-filament strength of the coated fibers is shown in Table II. The strength of the as-fabricated fiber was scattered among the fabrication lots of the fiber. Thus, a direct comparison of the strengths of the coated fibers was inadequate; the strength retention ratio was used. The retention ratio of each fiber was 77%–121%. (2) Interface between Fiber and Matrix The composite reinforced with Si3N4 fiber coated with coating I is described here as composite I. Figure 4(a) shows the TEM image of the fiber–matrix interface of non-oxidized composite I. The interface of this composite had a layered structure, composed of a bright layer 5 nm thick (L1), a dark layer 10–15 nm thick (L2), and a bright layer 10–15 nm thick (L3), in that order, from the fiber side to the matrix side. The L2 presented the image of a crystal lattice with 0.26 nm interlayer spacing. The NBED pattern of the L2 layer was obscure spots, suggesting that L2 was composed of disordered crystallites. The NBED patterns of bright layers L1 and L3 lacked a lattice image and showed obscure rings, which polarized the brightness toward the direction of the fiber surface. Therefore, L1 and L3 were composed of crystallites with very low crystallinity, oriented toward the fiber surface. EDS was used on the fiber–matrix interface to obtain elemental information. Boron, although definitely contained in the sample, was not detected because of limitations in the present equipment. The peak intensities of carbon and silicon on the interface were about three times and one-forth, respectively, that on the Si-N-C matrix; therefore, the interface consisted mainly of carbon and a small amount of silicon. Table I. Fabrication Conditions and Thicknesses of C-B-Si Coatings Coating Coating condition Thickness of coating on Si3N4 fiber (nm) Gas-flow rate (mmol
s 1 ) Temperature BCl (K) 3 SiCl4 CH4 H2 N2 I 0.298 0.149 0.074 1.12 4.46 1448 40–120 II 0.149 0.223 0.074 0.00 5.58 1448 30–80 R 0.000 0.372 0.372 0.00 4.17 1448 100–200 Fig. 2. Depth profile of apparent elemental composition on (a) coating I and (b) coating II using AES. Fig. 3. Change of relative intensity of boron, carbon, and silicon peaks by EMPA analysis on the deposits in the coating chamber after fabrication of coating I. Relative intensity was the ratio of the peak intensity of an element on the deposit to that on the simple body of the element under the same measuring conditions. Origin of the horizontal axis is positioned on the center of the coating chamber. Negative direction is the upstream side. July 2002 C-B-Si Coatings for S3N4-Fiber-Reinforced Composites for Improved Oxidation Resistance 1817