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High temperature oxidation behavior of Ti3SiC2-based material in air
Acta mater. 49 (2001) 4347–4353 www.elsevier.com/locate/actamat
HIGH TEMPERATURE OXIDATION BEHAVIOR OF Ti3SiC2BASED MATERIAL IN AIR Z. SUN1, Y. ZHOU1† and M. LI2 1
Ceramic and Composite Department, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China and 2State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China ( Received 10 October 2000; received in revised form 8 June 2001; accepted 8 June 2001 )
Abstract—The oxidation behavior of Ti3SiC2-based material in air has been studied from 900°C to 1200°C. The present work showed that the growth of the oxide scale on Ti3SiC2-based material obeyed a parabolic law from 900°C to 1100°C, while at 1200°C it followed a linear rule. The oxide scale was generally composed of an outer layer of coarse-grained TiO2 (rutile) and an inner layer of fine-grained TiO2 and SiO2 (tridymite) above 1000°C. A discontinuous coarse-grained SiO2 layer was observed within the outer coarse-grained TiO2 layer on the samples oxidized at 1100°C and 1200°C. Marker experiments showed that the oxidation process was controlled by the inward diffusion of oxygen, outward diffusion of titanium and CO or SiO, and that internal oxidation predominated. The TiC content in Ti3SiC2 was deleterious to the oxidation resistance of Ti3SiC2. 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ti3SiC2-based material; Oxidation
1. INTRODUCTION
Good oxidation resistance of structural materials is the primary criterion for their high-temperature applications. The oxidation resistance is dependent on a protective oxide scale formed on the surface of the matrixes, such as α-Al2O3, Cr2O3 and SiO2. Recently, a ternary layered material, Ti3SiC2, has been demonstrated to be a good candidate for hightemperature applications owing to its unique combination of the merits of both metals and ceramics. The low density, good thermal conductivity [1], high strength at elevated temperatures [2], excellent thermal shock resistance and machinability with conventional tools [1], just to name a few, make Ti3SiC2 and Ti3SiC2-based ceramics attractive for diverse hightemperature applications. To be used at high temperatures, it is important to understand the oxidation behavior of Ti3SiC2. The oxidation of Ti3SiC2 and its composites, however, has not been well understood because the fabrication of Ti3SiC2 bulk materials is difficult [1, 3–11]. Only a few researchers have investigated the oxidation behavior of Ti3SiC2 or Ti3SiC2-
† To whom all correspondence should be addressed. Tel.: +86-24-2384-3531x55180; fax: +86-24-2389-1320. E-mail address: [email protected] (Y. Zhou)
based material, but there is scatter in the reported data [3–6, 12]. Racault et al. [3] studied the oxidation of Ti3SiC2 powders under flowing oxygen. They showed that the oxidation rate of Ti3SiC2 was slower than that of TiC; the Ti3SiC2 powder almost totally oxidized to TiO2 (rutile) and SiO2 (cristobalite) at temperatures between 1050°C and 1250°C. Tong et al. [4] investigated the oxidation of monolithic Ti3SiC2 and Ti3SiC2/SiC composite at 1000°C in flowing air for 10 hours. They demonstrated that the oxidation resistance of Ti3SiC2/SiC composite was better than that of monolithic Ti3SiC2. Barsoum and El-Raghy [12] reported parabolic oxidation behavior of Ti3SiC2 bulk samples consisting of 2 vol.% TiC from 900°C to 1400°C for 500 minutes. The calculated activation energy was 320 and 370 kJ·mol⫺1 for Ti3SiC2 materials synthesized from lower and higher level of purities of initial powders, respectively. The oxide scale above 1000°C consisted of two layers: an outer layer of pure TiO2 (rutile) and the inner layer of a mixture of SiO2 and TiO2. Feng et al. [5] investigated the oxidation of polycrystalline Ti3SiC2 bulk material (containing 2 mol.% TiC) at temperatures between 800°C and 1100°C for 100 minutes. They reported the parabolic oxidation in the temperature range of 800°C to 950°C and non-parabolic oxidation from 950°C to 1100°C with the corresponding calculated
1359-6454/01/$20.00 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 1 ) 0 0 2 4 7 - 6
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activation energy of 137.7 and 312.5 kJ·mol⫺1, respectively. Radhakrishnan et al. [6] investigated the oxidation behavior for polycrystalline Ti3SiC2 (containing 2 vol.% TiSi2) at 1000°C in air for 50 hours. They demonstrated that the oxidation of Ti3SiC2 obeyed a linear law and Ti3SiC2 is not a good oxidation resistant material at 1000°C. We present here a rather detailed investigation on the oxidation of Ti3SiC2-based material and report some interesting phenomena that have not been found in previous works. The results will shed some light on the oxidation mechanism of Ti3SiC2-based material and is beneficial to the technological use of this material.
3. RESULTS
3.1. Kinetic analysis
2. EXPERIMENTAL PROCEDURES
Figure 1(a) shows the square of weight gain per unit area as a function of time in the temperature range from 900°C to 1100°C in air for 20 hours. From the linear fit in Fig. 1(a), it can be concluded that the oxidation of Ti3SiC2-based material from 900°C to 1100°C is in excellent agreement with a parabolic rate law. The parabolic rate constant, Kp, increased from 4.9×10⫺9 kg2 m⫺4 s⫺1 at 900°C to 2.1×10⫺7 kg2 m⫺4 s⫺1 at 1100°C. The present results agreed well with previous works [5, 12, 14]. Weight gain at 1200°C, however, was much faster and the oxidation obeyed a linear rule as shown in Fig. 1(b). The linear oxidation constant was 5.67×10⫺9 kg2 m⫺4 s⫺1. The results suggested that the oxidation mechanism of Ti3SiC2-based
The Ti3SiC2 based samples used in this work were TSCZS610 which was fabricated by the in-situ hot pressing/solid–liquid reaction process [11]. Briefly, the material was made according to the following procedure. Ti, Si and graphite powders were mixed and milled in a polypropylene jar for ten hours. After ball milling, the mixture was cold pressed in a graphite die with a diameter of 50 mm. The in-situ hot pressing/solid–liquid reaction was conducted under a flowing argon atmosphere in a furnace using graphite as a heating element. The bulk Ti3SiC2-based material, TSCZS610, was in-situ synthesized by hotpressing at 1600°C under a pressure of 40 MPa for 60 min. The Ti3SiC2 content in the hot pressed material was 88 wt%, calculated by the Rietveld method [13]. The second phase in TSCZS610 was mainly TiC [11]. Rectangular samples with the dimensions of 10×3×4 mm3 were cut from the Ti3SiC2 based material by electrical discharge machining. The surfaces were polished before examination. The continuous-isothermal-mass-change measurements were performed from 900°C to 1200°C in air for 20 hours. The sample was suspended in a thermobalance (mtb 10–8, Setaram, France) with a Pt wire. To study the adhesive property of the oxide scale, cyclic oxidation experiments were performed in a furnace controlled automatically. The samples were held at 1100°C in a furnace for one hour and then cooled down to room temperature in air for 10 minutes, which was defined as one cycle. The total time in furnace at 1100°C is 88 hours. After the tests, the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The phase composition of the oxide scale was identified by X-ray diffraction (Rigaku D/max-rA diffractometer, Japan). The surface morphology and chemical composition of the oxide scale were investigated using an S-360 scanning electron microscope (Cambridge Instruments, UK) equipped with an energy dispersive spectroscopy (EDS) system.
Fig. 1. (a) Square of weight gain per unit area versus time for Ti3SiC2-based material oxidized at the temperature range from 900°C to 1100°C; (b) weight gain per unit area as a function of time for Ti3SiC2-based material oxidized at 1200°C.
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material changed when temperature increased from below 1100°C to 1200°C. The temperature dependence of the parabolic rate constants for Ti3SiC2-based material from 900°C to 1100°C is shown in Fig. 2, which also includes the results of the previous study [14]. The discrepancy between the present used material TSCZS610 and the previous material, TSCZS510, was that the latter material contained higher Ti3SiC2 content (93 wt%). The activation energy for TSCZS610 and TSCZS510 was calculated to be 253 kJ·mol⫺1 and 343 kJ·mol⫺1, respectively. The lower activation energy for TSCZS610 means that generally the TiC content in Ti3SiC2 is deleterious to the oxidation resistance of Ti3SiC2 material. Note that TSCZS610 exhibits lower oxidation rate than TSCZS510 at 1100°C. It is known that TiC is not resistant to oxidation [15], therefore, the more TiC content in Ti3SiC2, the larger weight gain would be expected. The discrepancy here is not clear at present and it is under further investigation. Cyclic oxidation was performed in order to study the adhesive property of the as-formed oxide scale to Ti3SiC2-based material. Figure 3 shows the weight gain as a function of time for the cyclic oxidation at 1100°C; the former result of TSCZS510 is also included in Fig. 3. No weight loss was detected and the weight gain would simply increase if the experiment were not interrupted. It is therefore concluded that the oxide scale formed on Ti3SiC2-based material is adherent and resistant to thermal cycling. It is also noted that TSCZS610 demonstrated better cyclic oxidation resistance than TSCZS510, coinciding with its better isothermal oxidation resistance at 1100°C.
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Fig. 3. Weight gain per unit area versus time for Ti3SiC2-based material cycled from 1100°C to room temperature 88 times. The total time for the cyclic oxidation is 102.7 hours.
3.2. Morphology, structure and composition of oxide scale The phase composition and surface morphology of the oxide surface were analyzed by X-ray diffraction and scanning electron microscopy (SEM). Figure 4 shows the X-ray diffraction patterns of the oxide sur-
Fig. 2. The temperature dependence of the parabolic rate constant for the oxidation of Ti3SiC2-based material.
Fig. 4. X-ray diffraction patterns of the oxide surfaces on the Ti3SiC2-based samples oxidized from 900°C to 1200°C for 20 hours.
face for the TSCZS610 samples oxidized from 900°C to 1200°C for 20 hours. It is noted that the oxides formed at 900°C and 1000°C are mainly TiO2 (rutile) with trace amount of SiO2 (tridymite). Reflections from Ti3SiC2 are also observable at 900°C, indicating that the oxide scale is very thin (little specific weight gain was observed in Fig. 1(a)). At 1100°C and 1200°C, only reflections from TiO2 are observed. Figure 5(a), (b), (c) and (d) shows the surface morphology of the oxide scale at 900°C, 1000°C, 1100°C and 1200°C, respectively. It is seen that the grain size increased with increasing temperature and different surface morphology was also observed. Dense oxide scale with well-shaped crystallites was observed for the sample oxidized at 900°C. At 1100°C, dense oxide scale was also observed, however, there were some finely structured regions embedded in large TiO2 grains. At 1200°C, the oxide surface consisted of large TiO2 crystallites with well-shaped facets and
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Fig. 5. Surface morphology of oxide scales after Ti3SiC2-based material exposure in air for 20 hours at (a) 900°C, (b) 1000°C, (c) 1100°C and (d) 1200°C.
finely structured materials embedded in the large crystals. The results of EDS X-ray microanalysis for the large crystallites and the finely structured materials in Fig. 5(d) are presented in Fig. 6(a) and (b), respectively. The large crystallites contain Ti and
O (Fig. 6(a)), while the finely structured materials contain Ti, O and Si elements (Fig. 6 (b)). It is then concluded that the large grains in Fig. 5(d) consist of only TiO2. As for the case of finely structured material there are two possibilities since no SiO2 phase in the oxide scales of 1100°C and 1200°C was observed from the X-ray diffraction analysis results and the EDS data here are not definitive. One possibility is that the finely structured materials are only amorphous SiO2 and the Ti signal in EDS (Fig. 6 (b)) comes from underlying TiO2; The other possibility is that the finely structured materials are composed of TiO2 and SiO2 with the latter being too low to be detected by the X-ray diffraction analyzer. 3.3. Morphology, structure and composition of subsurface zone
Fig. 6. EDS X-ray spectra for (a) large crystallites and (b) finely structured material in Fig. 5(d).
Figure 7(a), (b) and (c) shows the back scattered electron image of the cross-section of the scale after oxidation. Combined with the X-ray microanalysis, the oxide scale at below 1000°C contained one layer of a mixture of TiO2 and SiO2. At 1100°C and 1200°C, the oxide layer was stratified. Figure 8(a), (b) and (c) shows the results of EDS analysis for the scale shown in Fig. 7(c) for the bright large grainedlayer on the left, dark grained-layer in the middle and dark grey grained-layer near the matrix, respectively.
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Fig. 8. EDS X-ray spectra for (a) bright layer on the left (b) dark layer within the bright layer (c) dark grey layer on the right.
grained TiO2 and SiO2. When TiAl oxidized at 900°C [16], an Al2O3 layer formed in the border region of the outer and inner layer, hence good oxidation resistance was observed at 900°C. Unlike TiAl, the SiO2 layer within the TiO2 outer layer cannot protect the matrix from further oxidation because of its discontinuous character. Fig. 7. Back scattered electron image of cross-section of the oxide scales after exposure in air for 20 hours at (a) 1000°C, (b) 1100°C, (c) 1200°C for 20 h.
The presence of Au was owing to the spraying for SEM analysis. Based on the EDS results, it can be concluded that the bright layer on the left was TiO2; the discontinuous dark layer within TiO2 was primarily SiO2, and the dark grey part near the matrix was a mixture of fine-grained TiO2 and SiO2. The SiO2 content in the dicontinuous dark layer was much higher than in the fine-grained mixing layer as demonstrated by EDS analysis. This dark layer is then referred to as the so-called SiO2 layer to distinguish it from the fine-grained mixture. Thus the oxide layers at 1100°C and 1200°C can be described as an outer layer of coarse-grained TiO2 sandwiching a so-called SiO2 layer, and an inner layer of a mixture of fine-
4. DISCUSSION
4.1. Transport in the oxidation process A marker experiment was conducted to study the diffusion process in the condition of isothermal oxidation at 1200°C for 100 hours. In brief, an inert marker of Pt with a diameter of 0.3 mm was welded onto a Ti3SiC2 sample by argon arc welding. The sample was then put in a furnace with MoSi2 as a heating element, where it was isothermally oxidized at 1200°C for 100 hours. Figure 9 shows the backscattered electron image of the corresponding crosssection scale in which the inert marker (the bright round dot at the top of Fig. 9) was in the outer coarsegrained TiO2 layers. From the marker’s position in the oxide layer shown in Fig. 9 it is concluded that the outer coarse-
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Fig. 9. Back scattered electron image of cross-section of the scale from marker experiment.
grained TiO2 layer grows by the outward diffusion of Ti, while the inner layer of the fine-grained mixture of TiO2 and SiO2 grows by the inward diffusion of oxygen. Therefore, the diffusion-controlled oxidation process of Ti3SiC2 should include the inward diffusion of oxygen and the simultaneous outward diffusion of titanium and carbonaceous species. It is also noted that the width of the inner mixture layer of TiO2 and SiO2 is larger than the outer TiO2 layer, which suggests that the intertal oxidation predominates. 4.2. Proposed mechanism for the formation of the socalled SiO2 layer The oxidation process of Ti3SiC2 can be expressed as the following total reaction equation: Ti3SiC2 ⫹ 5O2(g) ⫽ 3TiO2 ⫹ SiO2 ⫹ 2CO(g) (1)
or Ti3SiC2 ⫹ SiO2 ⫹ 4O2(g) ⫽ 3TiO2 ⫹ 2SiO(g) ⫹ 2CO(g)
(2)
wherever oxygen pressure is insufficient, and followed by the reaction SiO(g) ⫹ 1/2O2(g) ⫽ SiO2
(3)
wherever oxygen pressure is sufficient. The formation of the so-called SiO2 layer within the outer TiO2 at 1100°C and 1200°C was supposed as follows. The oxidation rate at 1100°C and 1200°C was very high, therefore, a two-layer scale formed after a short oxidation time: an outer layer of coarsegrained TiO2 and an inner layer of a mixture of finegrained TiO2 and SiO2. The oxygen pressure in the inner layer is much lower than that in the outer layer due to the presence of SiO2. Low oxygen pressure supports the formation of SiO rather than SiO2 [17]. Thus, the oxidation takes place according to equation
(2). The as-formed SiO gas will diffuse outward. During the outward diffusion process, SiO gas will be transformed into solid SiO2 where the oxygen pressure is sufficiently high to support equation (3). It is known that the oxygen pressure in the outer TiO2 layer is much higher than that in the inner mixture layer, thus SiO2 precipitated in the outer coarsegrained TiO2 layer, forming the so-called SiO2 layer. However, if SiO happened to diffuse along large channels, such as cracks, it could come out of the oxide scale forming SiO2 in the oxide surface rather than turning into SiO2 in the coarse-grained TiO2 layer. The discontinuous character of the so-called SiO2 layer and the presence of SiO2 in the oxide surface mentioned in the preceding section could be explained from this point of view. This conclusion could also be confirmed by the higher level of discontinuous character of the so-called SiO2 layer in the scale formed at 1200°C than that at 1100°C. More cracks or defects formed when oxidized at 1200°C, thus diffusion mode might be changed from lattice diffusion at below 1100°C to crack diffusion at 1200°C, hence much higher oxidation rate was observed at 1200°C, i.e. linear oxidation, and therefore less SiO2 precipitated in the outer TiO2 layer. If the oxidation temperature was so high, for example above 1200°C, that the concentration gradient of oxygen pressure in the scale was not steep, oxidation at the scale/matrix interface occurred according to equation (1), forming SiO2 instead of SiO. At low temperatures, the scale was thin because of the lower oxidation rate. Thus the oxygen concentration gradient was almost uniform in the scale and oxidation also occurred according to equation (1). In those conditions, the so-called SiO2 layer would not be observed in the oxide scale. Therefore, the socalled SiO2 layer seemed to have a direct relation with the oxygen pressure at the scale/matrix interface. It is seen from the above analysis that no protective SiO2 layer formed at the matrix/scale interface, which was attributed to the low Si content in Ti3SiC2. The minimum silicon levels needed to form a SiO2 protective layer on Ti-Si alloys are high, of the order of 40% and possibly 45% [18]. The silicon content in Ti3SiC2 is 16.7 mol.%, which is much lower than that criteria value. Therefore, it is not surprising that continuous, protective SiO2 scale is not observed at the Ti3SiC2/scale interface. 5. CONCLUSIONS
The oxidation kinetics of Ti3SiC2-based material containing 12 wt.% TiC at 900°C苲1100°C in air were parabolic with an activation energy of 253 kJ·mol⫺1, supporting a diffusion-controlled oxidation process. The oxidation at 1200°C, however, is a linear rule, which suggested that the oxidation mechanism changed when the oxidation temperature increased to 1200°C. The oxidation mechanism might change from a diffusion-controlled process at below 1100°C
SUN et al.: HIGH TEMPERATURE OXIDATION
to an oxidation reaction-controlled process at above 1200°C where diffusion is no longer the rate-limiting step. The oxide scales consisted of one layer of the mixture of TiO2 and SiO2 when oxidized at 900°C and 1000°C; whereas it consisted of an outer layer of coarse-grained TiO2, within which was sandwiched a so-called SiO2 layer, and an inner part of a finegrained mixture of TiO2 and SiO2 when oxidized at 1100°C and 1200°C. The formation of the so-called SiO2 layer in the outer TiO2 was attributed to the outward diffusion of SiO, which precipitated as a SiO2 solid wherever the oxygen pressure is sufficiently high to support the transformation reaction. TiC content in Ti3SiC2 bulk material is deleterious to the oxidation resistance of Ti3SiC2. Acknowledgements—This work was supported by the National Science Foundation of China under Grant No. 59772021, National Outstanding Young Scientist Foundation under Grant No. 5992508, “863” project, and High-tech Bureau of the Chinese Academy of Sciences.
REFERENCES 1. Barsoum, M. W. and El-Raghy, T., J. Am. Ceram. Soc., 1996, 79, 1953.
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2. Sun, Z., Zhou, Y. and Zhou, J., Phil. Mag. Lett., 2000, 80, 289. 3. Racault, C., Langlais, F. and Naslain, R., J. Mater. Sci., 1994, 29, 3384. 4. Tong, X., Okano, T., Iseki, T. and Yano, T., J. Mater. Sci., 1995, 30, 3087. 5. Feng, A., Orling, T. and Munir, Z. A., J. Mater. Res., 1999, 14, 925. 6. Radhakrishnan, R., Williams, J. J. and Akinc, M., J. Alloy. Comp., 1999, 286, 85. 7. Goto, T. and Hirai, T., Mater. Res. Bull., 1987, 22, 1195. 8. Lis, J., Miyamoto, Y., Pampuch, R. and Tanihata, K., Mater. Lett., 1995, 22, 163. 9. Racault, C., Langlais, F. and Bernard, C., J. Mater. Sci., 1994, 29, 5023. 10. Arunajatesan, S. and Carim, A., J. Am. Ceram. Soc., 1995, 78, 667. 11. Zhou, Y., Sun, Z., Chen, S. and Zhang, Y., Mat. Res. Innovat., 1998, 2, 142. 12. Barsoum, M. W. and El-Raghy, T., J. Electrochem. Soc., 1997, 144, 2508. 13. Young, R. A., The Rietveld Method. Oxford University Press, 1993. 14. Sun, Z., Zhou, Y. and Li, M., Corro. Sci., 2000, 43, 1095. 15. Tampieri, A. and Bellosi, A., J. Am. Ceram. Soc., 1992, 75, 1688. 16. Becker, S., Rahmel, A., Schorr, M. and Schutze, M., Oxid. Met., 1992, 38, 425. 17. Gulbransen, E. A., Andrew, K. F. and Brassart, F. A., J. Electrochem. Soc., 1966, 113, 834. 18. Luthra, K. L., Oxid. Met., 1991, 36, 475.
HIGH TEMPERATURE OXIDATION BEHAVIOR OF Ti3SiC2BASED MATERIAL IN AIR Z. SUN1, Y. ZHOU1† and M. LI2 1
Ceramic and Composite Department, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China and 2State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China ( Received 10 October 2000; received in revised form 8 June 2001; accepted 8 June 2001 )
Abstract—The oxidation behavior of Ti3SiC2-based material in air has been studied from 900°C to 1200°C. The present work showed that the growth of the oxide scale on Ti3SiC2-based material obeyed a parabolic law from 900°C to 1100°C, while at 1200°C it followed a linear rule. The oxide scale was generally composed of an outer layer of coarse-grained TiO2 (rutile) and an inner layer of fine-grained TiO2 and SiO2 (tridymite) above 1000°C. A discontinuous coarse-grained SiO2 layer was observed within the outer coarse-grained TiO2 layer on the samples oxidized at 1100°C and 1200°C. Marker experiments showed that the oxidation process was controlled by the inward diffusion of oxygen, outward diffusion of titanium and CO or SiO, and that internal oxidation predominated. The TiC content in Ti3SiC2 was deleterious to the oxidation resistance of Ti3SiC2. 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Ti3SiC2-based material; Oxidation
1. INTRODUCTION
Good oxidation resistance of structural materials is the primary criterion for their high-temperature applications. The oxidation resistance is dependent on a protective oxide scale formed on the surface of the matrixes, such as α-Al2O3, Cr2O3 and SiO2. Recently, a ternary layered material, Ti3SiC2, has been demonstrated to be a good candidate for hightemperature applications owing to its unique combination of the merits of both metals and ceramics. The low density, good thermal conductivity [1], high strength at elevated temperatures [2], excellent thermal shock resistance and machinability with conventional tools [1], just to name a few, make Ti3SiC2 and Ti3SiC2-based ceramics attractive for diverse hightemperature applications. To be used at high temperatures, it is important to understand the oxidation behavior of Ti3SiC2. The oxidation of Ti3SiC2 and its composites, however, has not been well understood because the fabrication of Ti3SiC2 bulk materials is difficult [1, 3–11]. Only a few researchers have investigated the oxidation behavior of Ti3SiC2 or Ti3SiC2-
† To whom all correspondence should be addressed. Tel.: +86-24-2384-3531x55180; fax: +86-24-2389-1320. E-mail address: [email protected] (Y. Zhou)
based material, but there is scatter in the reported data [3–6, 12]. Racault et al. [3] studied the oxidation of Ti3SiC2 powders under flowing oxygen. They showed that the oxidation rate of Ti3SiC2 was slower than that of TiC; the Ti3SiC2 powder almost totally oxidized to TiO2 (rutile) and SiO2 (cristobalite) at temperatures between 1050°C and 1250°C. Tong et al. [4] investigated the oxidation of monolithic Ti3SiC2 and Ti3SiC2/SiC composite at 1000°C in flowing air for 10 hours. They demonstrated that the oxidation resistance of Ti3SiC2/SiC composite was better than that of monolithic Ti3SiC2. Barsoum and El-Raghy [12] reported parabolic oxidation behavior of Ti3SiC2 bulk samples consisting of 2 vol.% TiC from 900°C to 1400°C for 500 minutes. The calculated activation energy was 320 and 370 kJ·mol⫺1 for Ti3SiC2 materials synthesized from lower and higher level of purities of initial powders, respectively. The oxide scale above 1000°C consisted of two layers: an outer layer of pure TiO2 (rutile) and the inner layer of a mixture of SiO2 and TiO2. Feng et al. [5] investigated the oxidation of polycrystalline Ti3SiC2 bulk material (containing 2 mol.% TiC) at temperatures between 800°C and 1100°C for 100 minutes. They reported the parabolic oxidation in the temperature range of 800°C to 950°C and non-parabolic oxidation from 950°C to 1100°C with the corresponding calculated
1359-6454/01/$20.00 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 5 4 ( 0 1 ) 0 0 2 4 7 - 6
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activation energy of 137.7 and 312.5 kJ·mol⫺1, respectively. Radhakrishnan et al. [6] investigated the oxidation behavior for polycrystalline Ti3SiC2 (containing 2 vol.% TiSi2) at 1000°C in air for 50 hours. They demonstrated that the oxidation of Ti3SiC2 obeyed a linear law and Ti3SiC2 is not a good oxidation resistant material at 1000°C. We present here a rather detailed investigation on the oxidation of Ti3SiC2-based material and report some interesting phenomena that have not been found in previous works. The results will shed some light on the oxidation mechanism of Ti3SiC2-based material and is beneficial to the technological use of this material.
3. RESULTS
3.1. Kinetic analysis
2. EXPERIMENTAL PROCEDURES
Figure 1(a) shows the square of weight gain per unit area as a function of time in the temperature range from 900°C to 1100°C in air for 20 hours. From the linear fit in Fig. 1(a), it can be concluded that the oxidation of Ti3SiC2-based material from 900°C to 1100°C is in excellent agreement with a parabolic rate law. The parabolic rate constant, Kp, increased from 4.9×10⫺9 kg2 m⫺4 s⫺1 at 900°C to 2.1×10⫺7 kg2 m⫺4 s⫺1 at 1100°C. The present results agreed well with previous works [5, 12, 14]. Weight gain at 1200°C, however, was much faster and the oxidation obeyed a linear rule as shown in Fig. 1(b). The linear oxidation constant was 5.67×10⫺9 kg2 m⫺4 s⫺1. The results suggested that the oxidation mechanism of Ti3SiC2-based
The Ti3SiC2 based samples used in this work were TSCZS610 which was fabricated by the in-situ hot pressing/solid–liquid reaction process [11]. Briefly, the material was made according to the following procedure. Ti, Si and graphite powders were mixed and milled in a polypropylene jar for ten hours. After ball milling, the mixture was cold pressed in a graphite die with a diameter of 50 mm. The in-situ hot pressing/solid–liquid reaction was conducted under a flowing argon atmosphere in a furnace using graphite as a heating element. The bulk Ti3SiC2-based material, TSCZS610, was in-situ synthesized by hotpressing at 1600°C under a pressure of 40 MPa for 60 min. The Ti3SiC2 content in the hot pressed material was 88 wt%, calculated by the Rietveld method [13]. The second phase in TSCZS610 was mainly TiC [11]. Rectangular samples with the dimensions of 10×3×4 mm3 were cut from the Ti3SiC2 based material by electrical discharge machining. The surfaces were polished before examination. The continuous-isothermal-mass-change measurements were performed from 900°C to 1200°C in air for 20 hours. The sample was suspended in a thermobalance (mtb 10–8, Setaram, France) with a Pt wire. To study the adhesive property of the oxide scale, cyclic oxidation experiments were performed in a furnace controlled automatically. The samples were held at 1100°C in a furnace for one hour and then cooled down to room temperature in air for 10 minutes, which was defined as one cycle. The total time in furnace at 1100°C is 88 hours. After the tests, the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The phase composition of the oxide scale was identified by X-ray diffraction (Rigaku D/max-rA diffractometer, Japan). The surface morphology and chemical composition of the oxide scale were investigated using an S-360 scanning electron microscope (Cambridge Instruments, UK) equipped with an energy dispersive spectroscopy (EDS) system.
Fig. 1. (a) Square of weight gain per unit area versus time for Ti3SiC2-based material oxidized at the temperature range from 900°C to 1100°C; (b) weight gain per unit area as a function of time for Ti3SiC2-based material oxidized at 1200°C.
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material changed when temperature increased from below 1100°C to 1200°C. The temperature dependence of the parabolic rate constants for Ti3SiC2-based material from 900°C to 1100°C is shown in Fig. 2, which also includes the results of the previous study [14]. The discrepancy between the present used material TSCZS610 and the previous material, TSCZS510, was that the latter material contained higher Ti3SiC2 content (93 wt%). The activation energy for TSCZS610 and TSCZS510 was calculated to be 253 kJ·mol⫺1 and 343 kJ·mol⫺1, respectively. The lower activation energy for TSCZS610 means that generally the TiC content in Ti3SiC2 is deleterious to the oxidation resistance of Ti3SiC2 material. Note that TSCZS610 exhibits lower oxidation rate than TSCZS510 at 1100°C. It is known that TiC is not resistant to oxidation [15], therefore, the more TiC content in Ti3SiC2, the larger weight gain would be expected. The discrepancy here is not clear at present and it is under further investigation. Cyclic oxidation was performed in order to study the adhesive property of the as-formed oxide scale to Ti3SiC2-based material. Figure 3 shows the weight gain as a function of time for the cyclic oxidation at 1100°C; the former result of TSCZS510 is also included in Fig. 3. No weight loss was detected and the weight gain would simply increase if the experiment were not interrupted. It is therefore concluded that the oxide scale formed on Ti3SiC2-based material is adherent and resistant to thermal cycling. It is also noted that TSCZS610 demonstrated better cyclic oxidation resistance than TSCZS510, coinciding with its better isothermal oxidation resistance at 1100°C.
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Fig. 3. Weight gain per unit area versus time for Ti3SiC2-based material cycled from 1100°C to room temperature 88 times. The total time for the cyclic oxidation is 102.7 hours.
3.2. Morphology, structure and composition of oxide scale The phase composition and surface morphology of the oxide surface were analyzed by X-ray diffraction and scanning electron microscopy (SEM). Figure 4 shows the X-ray diffraction patterns of the oxide sur-
Fig. 2. The temperature dependence of the parabolic rate constant for the oxidation of Ti3SiC2-based material.
Fig. 4. X-ray diffraction patterns of the oxide surfaces on the Ti3SiC2-based samples oxidized from 900°C to 1200°C for 20 hours.
face for the TSCZS610 samples oxidized from 900°C to 1200°C for 20 hours. It is noted that the oxides formed at 900°C and 1000°C are mainly TiO2 (rutile) with trace amount of SiO2 (tridymite). Reflections from Ti3SiC2 are also observable at 900°C, indicating that the oxide scale is very thin (little specific weight gain was observed in Fig. 1(a)). At 1100°C and 1200°C, only reflections from TiO2 are observed. Figure 5(a), (b), (c) and (d) shows the surface morphology of the oxide scale at 900°C, 1000°C, 1100°C and 1200°C, respectively. It is seen that the grain size increased with increasing temperature and different surface morphology was also observed. Dense oxide scale with well-shaped crystallites was observed for the sample oxidized at 900°C. At 1100°C, dense oxide scale was also observed, however, there were some finely structured regions embedded in large TiO2 grains. At 1200°C, the oxide surface consisted of large TiO2 crystallites with well-shaped facets and
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Fig. 5. Surface morphology of oxide scales after Ti3SiC2-based material exposure in air for 20 hours at (a) 900°C, (b) 1000°C, (c) 1100°C and (d) 1200°C.
finely structured materials embedded in the large crystals. The results of EDS X-ray microanalysis for the large crystallites and the finely structured materials in Fig. 5(d) are presented in Fig. 6(a) and (b), respectively. The large crystallites contain Ti and
O (Fig. 6(a)), while the finely structured materials contain Ti, O and Si elements (Fig. 6 (b)). It is then concluded that the large grains in Fig. 5(d) consist of only TiO2. As for the case of finely structured material there are two possibilities since no SiO2 phase in the oxide scales of 1100°C and 1200°C was observed from the X-ray diffraction analysis results and the EDS data here are not definitive. One possibility is that the finely structured materials are only amorphous SiO2 and the Ti signal in EDS (Fig. 6 (b)) comes from underlying TiO2; The other possibility is that the finely structured materials are composed of TiO2 and SiO2 with the latter being too low to be detected by the X-ray diffraction analyzer. 3.3. Morphology, structure and composition of subsurface zone
Fig. 6. EDS X-ray spectra for (a) large crystallites and (b) finely structured material in Fig. 5(d).
Figure 7(a), (b) and (c) shows the back scattered electron image of the cross-section of the scale after oxidation. Combined with the X-ray microanalysis, the oxide scale at below 1000°C contained one layer of a mixture of TiO2 and SiO2. At 1100°C and 1200°C, the oxide layer was stratified. Figure 8(a), (b) and (c) shows the results of EDS analysis for the scale shown in Fig. 7(c) for the bright large grainedlayer on the left, dark grained-layer in the middle and dark grey grained-layer near the matrix, respectively.
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Fig. 8. EDS X-ray spectra for (a) bright layer on the left (b) dark layer within the bright layer (c) dark grey layer on the right.
grained TiO2 and SiO2. When TiAl oxidized at 900°C [16], an Al2O3 layer formed in the border region of the outer and inner layer, hence good oxidation resistance was observed at 900°C. Unlike TiAl, the SiO2 layer within the TiO2 outer layer cannot protect the matrix from further oxidation because of its discontinuous character. Fig. 7. Back scattered electron image of cross-section of the oxide scales after exposure in air for 20 hours at (a) 1000°C, (b) 1100°C, (c) 1200°C for 20 h.
The presence of Au was owing to the spraying for SEM analysis. Based on the EDS results, it can be concluded that the bright layer on the left was TiO2; the discontinuous dark layer within TiO2 was primarily SiO2, and the dark grey part near the matrix was a mixture of fine-grained TiO2 and SiO2. The SiO2 content in the dicontinuous dark layer was much higher than in the fine-grained mixing layer as demonstrated by EDS analysis. This dark layer is then referred to as the so-called SiO2 layer to distinguish it from the fine-grained mixture. Thus the oxide layers at 1100°C and 1200°C can be described as an outer layer of coarse-grained TiO2 sandwiching a so-called SiO2 layer, and an inner layer of a mixture of fine-
4. DISCUSSION
4.1. Transport in the oxidation process A marker experiment was conducted to study the diffusion process in the condition of isothermal oxidation at 1200°C for 100 hours. In brief, an inert marker of Pt with a diameter of 0.3 mm was welded onto a Ti3SiC2 sample by argon arc welding. The sample was then put in a furnace with MoSi2 as a heating element, where it was isothermally oxidized at 1200°C for 100 hours. Figure 9 shows the backscattered electron image of the corresponding crosssection scale in which the inert marker (the bright round dot at the top of Fig. 9) was in the outer coarsegrained TiO2 layers. From the marker’s position in the oxide layer shown in Fig. 9 it is concluded that the outer coarse-
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Fig. 9. Back scattered electron image of cross-section of the scale from marker experiment.
grained TiO2 layer grows by the outward diffusion of Ti, while the inner layer of the fine-grained mixture of TiO2 and SiO2 grows by the inward diffusion of oxygen. Therefore, the diffusion-controlled oxidation process of Ti3SiC2 should include the inward diffusion of oxygen and the simultaneous outward diffusion of titanium and carbonaceous species. It is also noted that the width of the inner mixture layer of TiO2 and SiO2 is larger than the outer TiO2 layer, which suggests that the intertal oxidation predominates. 4.2. Proposed mechanism for the formation of the socalled SiO2 layer The oxidation process of Ti3SiC2 can be expressed as the following total reaction equation: Ti3SiC2 ⫹ 5O2(g) ⫽ 3TiO2 ⫹ SiO2 ⫹ 2CO(g) (1)
or Ti3SiC2 ⫹ SiO2 ⫹ 4O2(g) ⫽ 3TiO2 ⫹ 2SiO(g) ⫹ 2CO(g)
(2)
wherever oxygen pressure is insufficient, and followed by the reaction SiO(g) ⫹ 1/2O2(g) ⫽ SiO2
(3)
wherever oxygen pressure is sufficient. The formation of the so-called SiO2 layer within the outer TiO2 at 1100°C and 1200°C was supposed as follows. The oxidation rate at 1100°C and 1200°C was very high, therefore, a two-layer scale formed after a short oxidation time: an outer layer of coarsegrained TiO2 and an inner layer of a mixture of finegrained TiO2 and SiO2. The oxygen pressure in the inner layer is much lower than that in the outer layer due to the presence of SiO2. Low oxygen pressure supports the formation of SiO rather than SiO2 [17]. Thus, the oxidation takes place according to equation
(2). The as-formed SiO gas will diffuse outward. During the outward diffusion process, SiO gas will be transformed into solid SiO2 where the oxygen pressure is sufficiently high to support equation (3). It is known that the oxygen pressure in the outer TiO2 layer is much higher than that in the inner mixture layer, thus SiO2 precipitated in the outer coarsegrained TiO2 layer, forming the so-called SiO2 layer. However, if SiO happened to diffuse along large channels, such as cracks, it could come out of the oxide scale forming SiO2 in the oxide surface rather than turning into SiO2 in the coarse-grained TiO2 layer. The discontinuous character of the so-called SiO2 layer and the presence of SiO2 in the oxide surface mentioned in the preceding section could be explained from this point of view. This conclusion could also be confirmed by the higher level of discontinuous character of the so-called SiO2 layer in the scale formed at 1200°C than that at 1100°C. More cracks or defects formed when oxidized at 1200°C, thus diffusion mode might be changed from lattice diffusion at below 1100°C to crack diffusion at 1200°C, hence much higher oxidation rate was observed at 1200°C, i.e. linear oxidation, and therefore less SiO2 precipitated in the outer TiO2 layer. If the oxidation temperature was so high, for example above 1200°C, that the concentration gradient of oxygen pressure in the scale was not steep, oxidation at the scale/matrix interface occurred according to equation (1), forming SiO2 instead of SiO. At low temperatures, the scale was thin because of the lower oxidation rate. Thus the oxygen concentration gradient was almost uniform in the scale and oxidation also occurred according to equation (1). In those conditions, the so-called SiO2 layer would not be observed in the oxide scale. Therefore, the socalled SiO2 layer seemed to have a direct relation with the oxygen pressure at the scale/matrix interface. It is seen from the above analysis that no protective SiO2 layer formed at the matrix/scale interface, which was attributed to the low Si content in Ti3SiC2. The minimum silicon levels needed to form a SiO2 protective layer on Ti-Si alloys are high, of the order of 40% and possibly 45% [18]. The silicon content in Ti3SiC2 is 16.7 mol.%, which is much lower than that criteria value. Therefore, it is not surprising that continuous, protective SiO2 scale is not observed at the Ti3SiC2/scale interface. 5. CONCLUSIONS
The oxidation kinetics of Ti3SiC2-based material containing 12 wt.% TiC at 900°C苲1100°C in air were parabolic with an activation energy of 253 kJ·mol⫺1, supporting a diffusion-controlled oxidation process. The oxidation at 1200°C, however, is a linear rule, which suggested that the oxidation mechanism changed when the oxidation temperature increased to 1200°C. The oxidation mechanism might change from a diffusion-controlled process at below 1100°C
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to an oxidation reaction-controlled process at above 1200°C where diffusion is no longer the rate-limiting step. The oxide scales consisted of one layer of the mixture of TiO2 and SiO2 when oxidized at 900°C and 1000°C; whereas it consisted of an outer layer of coarse-grained TiO2, within which was sandwiched a so-called SiO2 layer, and an inner part of a finegrained mixture of TiO2 and SiO2 when oxidized at 1100°C and 1200°C. The formation of the so-called SiO2 layer in the outer TiO2 was attributed to the outward diffusion of SiO, which precipitated as a SiO2 solid wherever the oxygen pressure is sufficiently high to support the transformation reaction. TiC content in Ti3SiC2 bulk material is deleterious to the oxidation resistance of Ti3SiC2. Acknowledgements—This work was supported by the National Science Foundation of China under Grant No. 59772021, National Outstanding Young Scientist Foundation under Grant No. 5992508, “863” project, and High-tech Bureau of the Chinese Academy of Sciences.
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