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Materials Transactions, Vol. 44, No. 3 (2003) pp. 401 to 406 #2003 The Japan Institute ofMetals

Oxidation of – Carbide Composite at 1073 to 1773 K

Takayuki Narushima1, Takashi Goto2, Makoto Maruyama*1, Haruo Arashi3;*2 and Yasutaka Iguchi1

1Department of Metallurgy, Tohoku University, Sendai 980-8579, Japan 2Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan 3Department of Machine Intelligence and System Engineering, Tohoku University, Sendai 980-8579, Japan

The oxidation behavior ofB 4C–(25–60 mol%)SiC composites prepared by arc-melting was investigated in the temperature range of1073 to 1773 K using a thermogravimetric technique. Liquid borosilicate, SiO2 and were identified as oxidation products by X-ray diffraction and Raman . Mass gain was observed during oxidation at 1073 K, while mass loss due to the vaporization ofboron in liquid borosilicate was observed at temperatures of1273 K and higher. In situ Raman spectra ofthe surfaceofB 4C–SiC composites indicated that the silica concentration in the liquid borosilicate increased with increasing SiC content in the composite. Micro-Raman spectroscopy showed that carbon was enriched in the borosilicate layer close to the oxide/composite interface. The parabolic rate constants for B4C– 50 mol%SiC composites at 1073 K were proportional to ambient partial pressures ranging between 30 and 100 kPa. The diffusion of oxygen through the liquid borosilicate layer could be the rate-controlling process. The increase ofSiC content in the B 4C–SiC composites improved the oxidation resistance in both the mass gain and mass loss regions.

(Received September 24, 2002; Accepted January 24, 2003) Keywords: oxidation, Raman spectroscopy, , , silica, boron oxide, kinetics, carbon

1. Introduction 2. Experimental Procedure

Boron carbide B4C has been used for engineering B4C and SiC powders were mixed at specific composi- materials such as wear parts and because ofits tions, and then pressed into 15 mm diameter disks with 5 mm high hardness and excellent wear resistance. Recently, the thickness. The disks were arc-melted in an atmosphere boron carbide has been expected to be a high temperature with a tungsten electrode. The B4C–SiC composites after arc- thermoelectric material1,2) and a first-wall material for a melting were cut into plates (10 mm 10 mm 1mm). The fusion reactor3) due to its high figure ofmerit and low atomic plates were polished with paste to 1 mm. SiC content number. Oxidation resistance ofthe boron carbide is an in the composites ranged between 25 and 60 mol%. The important issue for its practical applications at high tem- quasi-binary B4C–SiC system contains a eutectic composi- peratures. The oxidation behavior has been reported by tion at SiC content of45–50 mol%, 19) and the solid 4–10) several research groups. Liquid B2O3 was detected as the ofboth B 4C in SiC and SiC in B4C are less than 20) oxidation product. The liquid B2O3 layer can not effectively 2 mass%. protect boron carbide because ofits low viscosity and high The mass change ofthe composite due to oxidation was vapor pressure at high temperatures.11) It is well known that measured continuously using an electrobalance. The speci- SiO2 formed on silicon-based materials by thermal oxidation men temperature was increased to a specific value in the is an excellent protective layer for oxidation.12) Telle flowing argon gas, and then oxygen gas or oxygen–argon gas reported that B4C–SiC composites synthesized by hot- mixture was introduced to initiate oxidation. The gases used pressing exhibited better oxidation resistance than single in the present experiments were dried by passing them 13) 14) 13,15–18) phase B4C. Liquid B2O3 and borosilicate have through CaCl2 and P2O5. The total gas pressure was 100 kPa. been reported as oxidation products ofcomposites in the The oxygen partial pressures ranged between 30 and 100 kPa B4C–SiC–C system. The oxide layers on the composites were in oxygen–argon atmospheres. The oxidation temperatures analyzed by X-ray diffraction,13,14,17) X-ray photoelectron varied from 1073 to 1773 K. The longest oxidation time was spectroscopy13,17) and IR absorption spectroscopy.18) How- 86.4 ks. After the oxidation, the specimen was cooled in ever, the relationship between the oxidation rate and oxide flowing argon gas. layer has not been clearly understood in the B4C–SiC system. In situ Raman spectra ofthe surfaceofB 4C–SiC In the present work, the oxidation behavior ofB 4C–SiC composites during the oxidation were measured by a back- composites was investigated by thermogravimetry and scattering method (T64000, Jovin-Yvon).21,22) An argon Raman spectroscopy, and the oxidation mechanism ofthe laser was used as an incident beam at 514.5 nm. The B4C–SiC composites was discussed. measurement ofscattered light was carried out with a triple monochromator and a multi-channel detector. The measuring time for each in situ Raman spectrum ranged from 100 to 300 s. The oxide films on the specimens after the oxidation experiments were analyzed at room temperature by micro- *1Graduate Student, Tohoku University. Present address: Furukawa Raman spectroscopy (T64000, Jovin-Yvon) and X-ray Electric Co., Ltd., Fukui 913-8588, Japan. diffraction (CuK , Ni-filtered, RAD-C, Rigaku). *2Deceased. 402 T. Narushima, T. Goto, M. Maruyama, H. Arashi and Y. Iguchi

Fig. 1 Mass change ofB 4C–50 mol%SiC composite due to oxidation in an oxygen atmosphere.

3. Results and Discussion

3.1 Mass change during oxidation Figure 1 shows the mass change due to the oxidation ofthe B4C–50 mol%SiC composite in an oxygen atmosphere at different temperatures. Mass gain was observed at 1073 K, Fig. 2 XRD patterns ofB 4C–SiC composites before and after oxidation in while mass loss occurred at temperatures of1273 K and an oxygen atmosphere for 86.4 ks. higher. The mass loss rates decreased with time and with decreasing temperature. Figure 2 demonstrates the XRD patterns ofthe surfaceofB 4C–SiC composites after oxida- tion in an oxygen atmosphere for 86.4 ks. SiO2 was detected on the B4C–SiC composites oxidized at 1473 and 1773 K. Liquid B2O3 and solid SiO2 could be formed by the oxidation ofB 4C and SiC in the composite, respectively. SiO2 might be soluble in liquid B2O3 up to around 60 mol% at 1073 K and would form liquid borosilicate.23) Ogawa et al. reported liquid borosilicate would be rapidly formed by the dissolu- 18) tion ofSiO 2 in liquid B2O3. Other research groups detected 13) the borosilicate on the B4C–SiC composites and B4C– SiC–C composites15,17,18) by X-ray photoelectron spectro- scopy and IR absorption spectroscopy. As described later, the liquid borosilicate was investigated by Raman spectroscopy in the present work. The mass loss observed in the oxidation ofB 4C–SiC composites at 1273 K and higher might be caused by vaporization ofB 2O3 in the liquid borosilicate. Figure 3 shows the surface of the B4C–50 mol%SiC compo- site oxidized at 1773 K for 86.4 ks. Pores were observed on Fig. 3 Scanning electron micrograph ofthe surfaceofB 4C–50 mol%SiC the surface, which were formed by the vaporization of B2O3. oxidized at 1773 K for 86.4 ks. The silica concentration in the liquid borosilicate increased by the vaporization ofB 2O3, and then a solid SiO2 phase formed in the liquid borosilicate when the SiO2 concentration 3.2 Observation of oxide layer by Raman spectroscopy exceeded the value.23) Figure 5 shows the in situ Raman spectra ofthe surfaceof 8) Mass losses due to the oxidation ofB 4C and the B4C–SiC the B4C–50 mol%SiC composite during oxidation in an composites in the present work are compared in Fig. 4. The oxygen atmosphere at 1073 K. The Raman spectra during addition ofSiC to B 4C was effective to decrease mass loss by heating to 1073 K and cooling to room temperature in an oxidation. The silica-rich borosilicate and solid SiO2 phases argon atmosphere are included in Fig. 5. The Raman peaks B in the oxide layer could suppress the formation and the and S due to liquid borosilicate24) were detected. The Raman 1 vaporization ofB 2O3. peak B around 800 cm was assigned to a breathing Oxidation ofBoron Carbide–Silicon Carbide Composite at 1073 to 1773 K 403

thermodynamically stable at low temperatures. A broad Raman peak S around 450 cm1 could be assigned to a bending or rocking ofB–O–B, B–O–Si and Si–O–Si bridging 24) bonds. In situ Raman spectra ofthe surfaces ofthe B 4C– 60 mol%SiC and B4C–25 mol%SiC composites during oxi- dation in an oxygen atmosphere at 1073 K are shown in Fig. 6. Furukawa and White24) suggested that the Raman peak S corresponded to silica in liquid borosilicate because the intensity ofthe Raman peak S increased with increasing silica concentration in the liquid borosilicate. Figure 7 demonstrates the change ofthe intensity ratio ofthe Raman peak S to the Raman peak B (IS=IB) with oxidation time at 1073 K. The value of IS=IB increased with increasing SiC contents in the composites. At constant SiC contents, the value of IS=IB increased with oxidation time, particularly up to 10 ks. This result suggests that B4C in the composites might be preferentially oxidized in the initial period. In addition to the Raman peaks ofthe liquid borosilicate, the Raman peaks ofcarbon were observed at 1360 and 1 Fig. 4 Mass loss ofB 4C and B4C–SiC composites due to oxidation. 1580 cm (see Figs. 5 and 6). Figure 8 shows the micro- Raman spectra in the borosilicate on the B4C–50 mol%SiC composite after oxidation in an oxygen atmosphere at 1073 K for 86.4 ks. The intensity of carbon peaks near the oxide/ composite interface was stronger than that of the gas/oxide interface. The carbon was enriched in the borosilicate layer near the oxide/composite interface because of lower oxygen partial pressure at the oxide/composite interface than that at the oxide/gas interface. Shimada et al. reported the formation ofcarbon in the initial stage ofoxidation ofHfCin the

Fig. 5 In situ Raman spectra ofB 4C–50 mol%SiC composite during heating, oxidation and cooling. vibration ofboroxy rings. 24–28) The boroxy rings are equilibrated with isolated BO3 units in the liquid borosili- cate.24) The increase ofRaman peak B during the cooling process might be caused by the formation of boroxy rings Fig. 6 In situ Raman spectra ofB 4C–60 mol%SiC and B4C–25 mol%SiC from isolated BO3 units. The boroxy rings would be composites during oxidation in an oxygen atmosphere at 1073 K. 404 T. Narushima, T. Goto, M. Maruyama, H. Arashi and Y. Iguchi

Fig. 9 Equilibrium oxygen partial pressures at the oxide/composite inter- face.

Fig. 7 Intensity ratio ofRaman peak S to the Raman peak B ( IS=IB) during oxidation ofB 4C–SiC composites in an oxygen atmosphere at 1073 K.

Si(s in SiC) þ O2(g) ¼ SiO2(s) ð2Þ

2B(s in B4C) þ 3/2O2(g) ¼ B2O3(l) ð3Þ The equilibrium oxygen partial pressures ofeqs. ( 1), (2) and (3), which were calculated using the thermodynamic data11) are shown in Fig. 9. The activities ofC, SiO 2 and B2O3 were assumed to be unity. The activity values ofSi and B decided by eqs. (4) and (5), respectively, were used in the calculation, e.g. 6:7 104 for Si and 0.19 for B at 1073 K. Si(s in SiC) þ C(s) ¼ SiC(s) ð4Þ

4B(s in B4C) þ C(s) ¼ B4C(s) ð5Þ

The activities ofSiC and B 4C were assumed to be unity because oflow solid solubilities ofB 4C in SiC and SiC in B4C. The formation of carbon at the oxide/composite interface during the oxidation of B4C–SiC composites seems to be thermodynamically possible under the conditions, CO partial pressure >101 Pa, at 1073 K. It was reported that the intensity ratio between Raman 1 1 peaks at 1360 cm and 1580 cm (I1360=I1580) was 1.2 for amorphous carbon,31) and the inverse ofthe crystallite size 32) was proportional to I1360=I1580. The value of I1360=I1580 evaluated from the Raman spectra in Fig. 8 was above 1, Fig. 8 Micro-Raman spectra in the borosilicate on B4C–50 mol%SiC meaning that the carbon could be amorphous or crystalline composite after oxidation in an oxygen atmosphere at 1073 K for 86.4 ks. with a small crystallite size (3 nm). temperature range of753 to 873 K. 29) They assumed that 3.3 Oxidation mechanism carbon could be present beneath the HfO2 layer or contained Figure 10 shows the relationships between mass gain (M) in the HfO2 layer. Recently, the presence ofcarbon in silica and oxidation time (t) for the B4C–50 mol%SiC composite at layer after oxidation of silicon carbide at 973 K in - 1073 K in oxygen–argon atmospheres. The gradient ofeach containing atmospheres has been shown.30) These previous curve changed around 1 ks. A high oxidation rate at t < 1 ks reports and the present result suggest that the oxygen partial may be caused by the preferential oxidation of B4C in the pressure at the oxide/composite interface was lower than the composites because liquid B2O3-rich borosilicate has high equilibrium oxygen partial pressure ofeq. ( 1). oxygen permeability and may not be protective for oxidation above 1050 K.16) In the present work, the gradient at t > 1 ks C(s) þ 1/2O (g) ¼ CO(g) ð1Þ 2 in Fig. 10 is around 0.5, and the oxidation rate seemed to be The oxidation reactions ofSi and B were represented by eqs. described by the parabolic law shown in eq. (6). (2) and (3), respectively. Oxidation ofBoron Carbide–Silicon Carbide Composite at 1073 to 1773 K 405

Fig. 10 Mass change ofB 4C–50 mol%SiC composite due to oxidation in Fig. 12 Comparison ofthe present parabolic rate constants ofB 4C– oxygen–argon atmospheres at 1073 K. 50 mol%SiC composite with those ofB 4C, SiC and B4C–SiC composite reported by other research groups.

M2 ¼ k t ð6Þ p Figure 12 shows the parabolic rate constants obtained in where kp is the parabolic rate constant. Figure 11 shows the the present work comparing with those ofB 4C, SiC and B4C– 4,14,33) 14) effects ofambient oxygen partial pressure on the parabolic SiC composites reported in literatures. The reported rate constant for the B4C–50 mol%SiC composite at 1073 K. and present kp values ofB 4C–SiC composites had values 4) 33) The parabolic rate constant was proportional to the ambient between those ofB 4C and SiC. The borosilicate would oxygen partial pressure. This dependence implies that the provide more effective protection for oxidation than liquid rate-controlling process ofthe oxidation may be the diffusion B2O3 because ofits high viscosity and low oxygen perme- 16) ofoxygen molecules through the liquid borosilicate layer and ability. the oxidation reaction proceeded at the oxide/composite interface. Other research groups13,16–18) also reported that the 4. Conclusions oxygen diffusion in the oxide layer could be the rate- controlling process ofcomposites in the B 4C–SiC–C system. The oxidation behavior ofB 4C–(25–60 mol%)SiC com- posites were investigated in oxygen–argon atmospheres at 1073 to 1773 K. The following results were obtained: (1) Liquid borosilicate, solid SiO2 and carbon were detected as the oxidation products by X-ray diffraction and in situ Raman spectroscopy. The silica concentra- tion in the liquid borosilicate layer increased with increasing SiC content in the composite. Carbon enriched in the borosilicate layer near the oxide/ composite interface. (2) Mass gain due to the formation of liquid borosilicate was observed at 1073 K, while mass loss caused by the vaporization ofboron oxide in liquid borosilicate was observed at temperatures of1273 K and higher. The addition ofSiC to B 4C was effective to improve the oxidation resistance both in mass gain and mass loss regions. This was caused by the increase ofsilica concentration in the liquid borosilicate layer and formation of solid SiO2. (3) The parabolic rate constants for oxidation of the composites were proportional to the ambient oxygen partial pressure at 1073 K, and the rate-controlling Fig. 11 Effect ofambient oxygen partial pressure on the parabolic rate process ofoxidation could be the diffusion ofoxygen constant for oxidation of B4C–50 mol%SiC composite at 1073 K. molecules through the liquid borosilicate layer. 406 T. Narushima, T. Goto, M. Maruyama, H. Arashi and Y. Iguchi

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