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Oxidation behaviour of Al2O3–TiC–Co composites at 800–1000 °C in air
Corrosion Science 53 (2011) 4058–4064
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Oxidation behaviour of Al2O3–TiC–Co composites at 800–1000 °C in air Ruixia Shi ⇑, Kang Li, Aiyu Zhang, Yongqiang Cao, Ping Yang School of Materials Science and Engineering, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Received 14 February 2011 Accepted 5 August 2011 Available online 22 August 2011 Keywords: A. Ceramic matrix composites C. Oxidation C. Kinetic parameters B. SEM B. X-ray diffraction
a b s t r a c t The oxidation behaviour of nanometre and micrometre sized Al2O3–TiC–Co composites is investigated at 800–1000 °C in air for 25 h. The oxidation resistance of nanometre sized samples is better than of micrometre sized. Phase compositions and microstructures were studied by XRD and SEM. The values of general rate constant k and oxidation exponent n are dependent on oxidation temperature and composites. The oxidation kinetics followed a rate that is slightly faster than the parabolic-rate law at 800–1000 °C. The activation energy of the nanometre sized is higher than of micrometre sized in the range of 800– 1000 °C. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Al2O3-based ceramic matrix composites are widely used in many applications, especially as excellent cutting tools [1–4]. Whereas, the intrinsic drawbacks of Al2O3-based cutting tools, such as lower strength, lower fracture toughness and lower thermal shock resistance usually make them more susceptible to fracture when machining, leading to a short tool life [5]. Studies have shown that the addition of TiC phase to alumina matrix can improve toughness, hardness and also thermal shock resistance [1,6–9]. Al2O3–TiC composites (denoted as AT) are widely used as cutting tools due to their attractive mechanical properties [10]. And AT composite is often manufactured by hot pressing of Al2O3 and TiC powder mixture [11,12]. However, inhomogeneous blend of starting powders, grain coarsening during sintering and weak interfaces between the ceramic particles deteriorate mechanical properties of AT composite fabricated by this method. Incorporating some cobalt into the AT by coating technology can improve the mixing homogeneity of powders and the mechanical properties of composite [13]. A novel Al2O3–TiC–Co composite (denoted as ATC) was fabricated in our study. It is important to evaluate the oxidation resistance of the ATC composite because it works under high temperature as a cutting tool. Numerous investigations about the oxidation resistance of ceramic matrix composites have been reported [14–20]. In the present work, the high-temperature oxidation behaviour and kinetics of nanometre and micrometre sized ATC composites sintered at 1650 °C in static air is investigated in detail.
2.1. Materials
⇑ Corresponding author. Tel.: +86 531 82765983. E-mail address: [email protected] (R. Shi). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.08.008
Nanometre sized ATC composites were fabricated from the mixed powders of cobalt-coated A12O3 and TiC. A12O3 (average particle size about 40 nm) and TiC (average particle size about 130 nm) were coated with cobalt film (3.96 vol.%) by chemical deposition, respectively. The coated powders with the mass ratio of 70:30 (A12O3:TiC) were then homogenized by ultrasonic dispersion as the starting powders (provided by copartners of Ningbo Lingri Surface Engineering Co., Ltd.,). Micrometre sized ATC mixed powders were prepared by the same technology. However the sizes of A12O3 (average particle size about 4 lm) and TiC (average particle size about 2 lm) powders were different. Then the starting powders were hot-pressed in vacuum at 1650 °C for 30 min under a pressure of 30 MPa (Model HIGH MULTI 5000, FUJI DENPA, Japan). Sintered specimens were cut into bars. The final dimensions of these samples were 3 4 36 mm and 4 2 36 mm for measuring flexural strength and fracture toughness, respectively. The mechanical properties and densities of samples are listed in Table 1. 2.2. Oxidation measurement The oxidation tests were conducted in a static air furnace at 800, 900 and 1000 °C. The specimens in size 3 4 10 mm were first ultrasonically cleaned and dried, and then weighed with a precision of 0.0001 g before and after the oxidation test. The exact dimensions were measured in order to calculate the surface area. For the oxidation tests, samples were first placed in a corundum crucible, and then the crucible was moved into the furnace. Each
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R. Shi et al. / Corrosion Science 53 (2011) 4058–4064 Table 1 Mechanical properties, relative densities and composition of ATC composites. Composites
Particle size
Al2O3 (vol.%)
TiC (vol.%)
Cobalt (vol.%)
Sintering temp. (°C)
Hardness (HRA)
Flexure strength (MPa)
Fracture toughness (MPa m1/2)
Relative density (%)
ATC
Nanometre sized Micrometre sized
71.28
24.76
3.96
1650
92.7
782
7.81
99.4
71.28
24.76
3.96
1650
91.7
559
7.43
94.8
ATC
Fig. 1. weight gain curves of samples during isothermal oxidation in air at: (a) 800 °C, (b) 900 °C and (c) 1000 °C.
oxidation experiment composed of heating for 1–5 h in the furnace at the setting temperature and cooling in air for 10 min. 2.3. Material characterization The densities of the sintered samples were measured using Archimedes principle and the fluid employed is water. Phase identification was made by X-ray diffractometer (D8 Advance, Germany) with monochromated Cu Ka radiation. The microstructures of samples were analyzed using a scanning electron microscope (SEM, Hitachi S-2500) equipped with an EDS system for elemental analysis (Link-ISIS300 Oxford). 3. Results 3.1. Oxidation behaviour The oxidation weight gain experiments of nanometre and micrometre sized ATC composites sintered at 1650 °C were carried
Fig. 2. X-ray diffraction patterns of the surfaces of nanometre sized ATC composites oxidized at different temperature.
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R. Shi et al. / Corrosion Science 53 (2011) 4058–4064
nanometre sized samples at 800–1000 °C. The weight gain of nanometre and micrometre sized composites both increased greatly at 1000 °C and were 31.03 and 9.62 times of one at 800 °C, respectively. The weight gain difference between nanometre and micrometre sized composites decreased with the oxidation temperature going up and increased with the extension of oxidation time. 3.2. Characterization
Fig. 3. X-ray diffraction patterns of the surfaces of micrometre sized ATC composites oxidized at different temperature.
out at 800–1000 °C in air from 1 to 25 h. Fig. 1 shows the primary oxidation kinetics of the as-sintered ATC composites at 800– 1000 °C. The composites had specific weight gain on the order of 0.036–0.706 mg/cm2 at 800 °C, 0.340–3.214 mg/cm2 at 900 °C and 0.786–6.795 mg/cm2 at 1000 °C between 1 and 25 h. The weight gain of micrometre sized samples are more than ones of
Figs. 2 and 3 present the XRD patterns of nanometre and micrometre sized ATC composites oxide scales formed on the samples after primary oxidation at 800–1000 °C in air for 25 h, respectively. It can be seen that the oxide scales of nanometre sized ATC composites formed on the samples are composed of Co3O4 at 800– 1000 °C. However, the amount of Co3O4 at 800 and 900 °C was much less than that of Co3O4 at 1000 °C and lots of tetragonal TiO2 (rutile) formed at 900 °C. The XRD pattern from the oxidized surface shown in Fig. 3 reveals Co3O4 peaks at 800 °C and only tetragonal TiO2 (rutile) on oxidation at 900 and 1000 °C in air for 25 h. In addition, there may be other oxides below at the junction of a thick TiO2 layer and the composite, but this could not be detected by scanning the surface. Surface morphologies of nanometre sized samples oxidized in air at 800–1000 °C for 25 h are shown in Fig. 4. As can be seen from Fig. 4 that some bigger holes appeared on oxidized surface at 800 °C (Fig. 4(b)) and prism-shaped oxides emerged at 900 °C (Fig. 4(c)). EDS patterns indicated that the prism-shaped oxides
Fig. 4. SEM morphologies of nanometre sized ATC composites oxidized in air for 25 h at: (a) before oxidation, (b) 800 °C, (c) 900 °C and (d) 1000 °C.
R. Shi et al. / Corrosion Science 53 (2011) 4058–4064
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Fig. 5. TEM photograph of triangle grain boundary of ATC composite sintered at 1650 °C (a) and EDS pattern (b).
Fig. 6. SEM morphologies of oxidized micrometre sized ATC composites at: (a) before oxidation, (b) 800 °C, (c) 900 °C and (d) 1000 °C.
were mainly composed of titanium and oxygen, which is in accord with XRD results. Consequently, it can be inferred that the prismshaped oxides were TiO2 (rutile). In addition, it is a remarkable fact
that a large amount of worm-like products mainly appeared on the grain boundaries and evenly distributed on the oxidized surface at 1000 °C (Fig. 4(d)). EDS analysis showed that the worm-like
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Fig. 7. SEM morphologies of micrometre sized ATC composites oxidized in air for 25 h at: (a) 800 °C and (b) 900 °C.
Table 2 oxidation reactions and calculation results of thermodynamics of ATC composites. Oxidation temp. (K)
Oxidation reaction
DG (kJ mol1)
T = 1073
TiC þ 2O2 ¼ TiO2 þ CO2 3Co þ 2O2 ¼ Co3 O4 TiC þ 2O2 ¼ TiO2 þ CO2 3Co þ 2O2 ¼ Co3 O4 TiC þ 2O2 ¼ TiO2 þ CO2 3Co þ 2O2 ¼ Co3 O4
1189.843 464.049 1193.312 424.319 1196.780 384.590
T = 1173 T = 1273
products were mainly composed of cobalt and oxygen, which is also in agreement with XRD results. Therefore it can be concluded that the worm-like products were Co3O4. The TEM analysis (Fig. 5) showed that a majority of cobalt flowed and aggregated at triangle grain boundary during and after sintering period [13]. Therefore the Co3O4 emerged mainly on the grain boundaries when cobalt was oxidized (Fig. 4d). Fig. 6 shows the surface morphologies of micrometre sized ATC composites oxide scales formed at 800– 1000 °C for 25 h. Some large and communicating voids and some needle-leaf shape particles appeared on oxidized surface at
Fig. 8. Logarithmic plots of weight gain, DW=S; vs time, t, for ATC composites oxidized at (a) 800 °C, (b) 900 °C and (c) 1000 °C. The exponent, n, is obtained from the slope.
R. Shi et al. / Corrosion Science 53 (2011) 4058–4064
800 °C (Figs. 6(b) and 7(a)) and prism-shaped oxides appeared at 900 °C (Figs. 6(c) and 7(b)). EDS showed that the needle-leaf particles and prism-shaped oxides were both mainly composed of titanium and oxygen. The needle-leaf particles contain some aluminium element but the content of aluminium was much lower than that of original sample. Compared with the microstructure of surface oxidized at 1000 °C (Fig. 5(d)) it can be inferred that the needle-leaf and prism-shaped oxides may be the different stage of rutile TiO2. In addition, escape of CO2 led to the formation of porosities in the oxide layer due to the oxidation of TiC. The presence of these pores provided channels for oxygen diffusion and accelerated the oxidation process because small pores in the scale provide short-circuit paths for oxygen transportation to the oxide interface. 4. Discussion
When oxidizing in air at high temperature, following reactions occurred for ATC composites, leading to oxide formation:
TiC þ 2O2 ¼ TiO2 þ CO2
ð1Þ
3Co þ 2O2 ¼ Co3 O4
ð2Þ
Table 2 showed the standard Gibbs free energy DG of the reactions at 800–1000 °C. The values of standard Gibbs free energy DG for both reactions are found to be negative at 800–1000 °C which indicates that they are thermodynamically possible to take place. Table 3 Oxidation parameters of ATC composites at 800–1000 °C. Oxidation temp. (°C)
Particle size
General rate constant, k (mgn cm2n s1)
Oxidation exponent (n)
800
Nanometre sized Micrometre sized Nanometre sized Micrometre sized Nanometre sized Micrometre sized
3.486 107 5.616 106 6.601 106 1.585 105 2.544 104 3.752 104
1.999 1.944 1.932 1.863 1.880 1.839
900 1000
4.2. Oxidation kinetics The data of variation of weight gain per unit area of the sample due to oxidation (DW=S) with exposure time (t) have been further analyzed to determine the kinetic parameters, namely, the general rate constant (k), oxidation exponent (n), and the parabolic rate constant (k) [21,22]. The relation between DW=S, n and k is expressed as:
ðDW=SÞn ¼ kt
Table 4 Rate constant kp (mg2 cm4 s1) of oxidation ATC composites at 800–1000 °C. Particle size
800 °C
900 °C
1000 °C
Nanometre sized Micrometre sized
1.422 106 2.769 105
2.003 104 5.739 104
1.370 103 2.565 103
ð3Þ
It is obvious that n and k can be determined from the slope of the plot showing the variation of the logarithm of weight gain per unit area verse the logarithm of time (Eq. (3)). The parabolic rate constant (kp) is related to the weight gain per unit surface area of specimen (DW=S) and exposure time (t) by:
ðDW=SÞ2 ¼ kp t þ c
4.1. Chemical thermodynamics analysis
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ð4Þ
The value of kp can be obtained graphically from the slope of the plot of variation of ðDW=SÞ2 with t (Eq. (4)). Using Eq. (3), the oxidation exponent n and general rate constant k have been determined from the slopes and intercepts, respectively, of the best-fit lines in Fig. 8 and are reported in Table 3. A value of n = 1 or 2 indicates a linear or parabolic nature, respectively. It can be seen that the k increased with increase in oxidation temperature both nanometre and micrometre sized ATC composites. The k values of nanometre sized composites are lower than ones of micrometre sized composites at 800–1000 °C. However, the difference of k values of both nanometre and micrometre sized ATC composites is getting less and less with oxidation temperature going up. The k values of micrometre sized ATC composites are 16.11, 2.40 and 1.47 times of nanometre sized composites at 800, 900 and 1000 °C, respectively. The values of nanometre and micrometre sized ATC composites n = 1.999 and n = 1.944 at 800 °C, n = 1.932 and n = 1.863 at 900 °C and n = 1.880 and n = 1.839 at 1000 °C obtained in the present research indicate that the oxidation kinetics was slightly faster than that predicted by the parabolic-rate law at 800–1000 °C. The parabolic-rate constants at 800–1000 °C have been determined using Eq. (4). The value of kp is the slope of the best-fit lines in the W2 vs t plots for different temperatures and has been determined graphically. The calculated values of kp for temperatures of 800–1000 °C are presented in Table 4. The value of kp showed a steady increase with the increase in the temperature of oxidation. The apparent activation energy (Q) has been determined from an Arrhenius-type equation,
Fig. 9. Arrhenius plot of lnkp vs 1/T for ATC composites sintered at 1650 °C (a) nanometre sized and (b) micrometre sized.
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kp ¼ k0 expðQ =RTÞ
R. Shi et al. / Corrosion Science 53 (2011) 4058–4064
ð5Þ
where k0 is a constant, R is the Boltzmann constant and T is absolute temperature. Fig. 9 shows the variation of ln kp with reciprocal of the absolute temperature (1/T). The slope of the best-fit line in the plot of Fig. 9 was used to determine the value of Q/R. The activation energy of nanometre and micrometre sized ATC composites oxidation in the range of 800–1000 °C was found to be 142.21 and 114.84 kJ/mol, respectively, which maybe contribute the different oxidation products of nanometre and micrometre sized ATC composites. 5. Conclusions The oxidation behaviour of nanometre and micrometre sized ATC composites prepared by hot pressed sintering was investigated at temperatures of 800–1000 °C in air for 25 h. The following main results were obtained: (1) The oxide scales of nanometre sized ATC composites all formed Co3O4 at 800–1000 °C but the main oxide products were TiO2 (rutile) at 900 °C in air for 25 h. The XRD of micrometre sized composites reveals Co3O4 peaks only at 800 °C and tetragonal TiO2 (rutile) on oxidation both at 900 °C and 1000 °C in air for 25 h. (2) The general rate constant k increased and the difference of k values was getting less and less with increase in oxidation temperature both nanometre and micrometre sized ATC composites. And the k values of nanometre sized composites are lower than of micrometre sized at 800–1000 °C. (3) The values of oxidation exponent n decreased with oxidation temperature going up both nanometre and micrometre sized ATC composites. All the n values obtained indicate that the oxidation kinetics was slightly faster than that predicted by the parabolic-rate law at 800–1000 °C. (4) In the research the activation energy of nanometre and micrometre sized ATC composites oxidation in the range of 800–1000 °C was found to be 142.21 and 114.84 kJ/mol, respectively.
Acknowledgements This work is supported by the Natural Science Foundation of Shandong Province (Grant Nos. ZR2009FM072 and ZR2009FM068).
References [1] E. Mohammad Sharifi, F. Karimzadeh, M.H. Enayati, Mechanochemically synthesized Al2O3–TiC nanocomposite, J. Alloy. Compd. 491 (2010) 411–415. [2] C.H. Xu, C.Z. Huang, X. Ai, Mechanical property and cutting performance of yttrium-reinforced Al2O3/Ti(C, N) composite ceramic tool material, J. Mater. Eng. Perform. 10 (2001) 102–107. [3] L.K. Qiu, X.K. Li, Y. Peng, W.M. Ma, G.M. Qiu, Y.B. Sun, Types, performance and application of Al2O3 system ceramic cutting tool, J. Rare Earth 25 (2007) 322– 326. [4] Y.M. Ko, W.T. Kwon, Y.W. Kim, Development of Al2O3–SiC composite tool for machining application, Ceram. Int. 30 (2004) 2081–2086. [5] J. Zhao, X.L. Yuan, Y.H. Zhou, Cutting performance and failure mechanisms of an Al2O3/WC/TiC micro- nano-composite ceramic tool, Int. J. Refract. Met. Hard Mater. 28 (2010) 330–337. [6] T.Z. Si, N. Liu, Q.A. Zhang, X.Q. You, Thermal shock fatigue behaviour of TiC/ Al2O3 composite ceramics, Rare Met. 27 (2008) 308–314. [7] Y.F. Zhang, L.J. Wang, W. Jiang, L.D. Chen, G.Z. Bai, Microstructure and properties of Al2O3–TiC nanocomposites fabricated by spark plasma sintering from high-energy ball milled reactants, J. Eur. Ceram. Soc. 26 (2006) 3393– 3397. [8] J.H. Gong, H.Z. Miao, Z. Zhao, The influence of TiC-particle-size on the fracture toughness of Al2O3 30 wt.% TiC composites, J. Eur. Ceram. Soc. 21 (2001) 2377– 2381. [9] X.Q. You, T.Z. Si, N. Liu, P.P. Ren, Y.D. Xu, J.P. Feng, Effect of grain size on thermal shock resistance of Al2O3–TiC ceramics, Ceram. Int. 31 (2005) 33–38. [10] T.D. Xia, Z.A. Munir, Y.L. Tang, W.J. Zhao, T.M. Wang, Structure formation in the combustion synthesis of Al2O3-TiC composites, J. Am. Ceram. Soc. 83 (2000) 507–512. [11] Y. Wan, J.H. Gong, Influence of TiC particle size on the load-independent hardness of Al2O3–TiC composites, Mater. Lett. 57 (2003) 3439–3443. [12] T. Nagano, H. Kato, F. Wakai, Deformation of alumnia titanium carbide composite at elevated-temperatures, J. Am. Ceram. Soc. 74 (1991) 2258–2262. [13] R.X. Shi, J. Li, D.Z. Wang, Y.S. Yin, Mechanical properties and thermal shock resistance of Al2O3–TiC–Co Composites, J. Mater. Eng. Perform. 18 (2009) 414– 419. [14] P. Hu, G.L. Wang, Z. Wang, Oxidation mechanism and resistance of ZrB2–SiC composites, Corros. Sci. 51 (2009) 2724–2732. [15] X.K. Qian, X.D. He, Y.B. Li, Y. Sun, H. Li, D.L. Xu, Cyclic oxidation of Ti3AlC2 at 1000–1300 °C in air, Corros. Sci. 53 (2011) 290–295. [16] Z.M. Sun, Y.C. Zhou, M.S. Li, Oxidation behaviour of Ti3SiC-based ceramics at 900–1300 °C in air, Corros. Sci. 43 (2001) 1095–1109. [17] Z.J. Wu, Z. Wang, Q. Qu, G.D. Shi, Oxidation mechanism of a ZrB2–SiC–ZrC ceramic heated through high frequency induction at 1600 °C, Corros. Sci. 53 (2011) 2344–2349. [18] X.M. Yi, Akira Yamauchi, Kazuya Kurokawa, Tomohiro Akiyama, Oxidation of b-SiAlONs prepared by a combination of combustion synthesis and spark plasma sintering, Corros. Sci. 52 (2010) 1738–1745. [19] X.M. Hou, K.C. Chou, Model of oxidation of SiC microparticles at high temperature, Corros. Sci. 50 (2008) 2367–2371. [20] F. Monteverde, A. Bellosi, Oxidation behaviour of titanium carbonitride based materials, Corros. Sci. 44 (2002) 1967–1982. [21] R. Mitra, V.V.R.A.M.A. RAO, Elevated-temperature oxidation behaviour of titanium silicide and titanium silicide-based alloy and composite, Metall. Mater. Trans. A Phys. 29 (1998) 1665–1675. [22] G.B. Raju, B. Basu, Oxidation kinetics and mechanisms of hot-pressed TiB2– MoSi2 composites, J. Am. Ceram. Soc. 91 (2008) 3320–3327.
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Oxidation behaviour of Al2O3–TiC–Co composites at 800–1000 °C in air Ruixia Shi ⇑, Kang Li, Aiyu Zhang, Yongqiang Cao, Ping Yang School of Materials Science and Engineering, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Received 14 February 2011 Accepted 5 August 2011 Available online 22 August 2011 Keywords: A. Ceramic matrix composites C. Oxidation C. Kinetic parameters B. SEM B. X-ray diffraction
a b s t r a c t The oxidation behaviour of nanometre and micrometre sized Al2O3–TiC–Co composites is investigated at 800–1000 °C in air for 25 h. The oxidation resistance of nanometre sized samples is better than of micrometre sized. Phase compositions and microstructures were studied by XRD and SEM. The values of general rate constant k and oxidation exponent n are dependent on oxidation temperature and composites. The oxidation kinetics followed a rate that is slightly faster than the parabolic-rate law at 800–1000 °C. The activation energy of the nanometre sized is higher than of micrometre sized in the range of 800– 1000 °C. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Al2O3-based ceramic matrix composites are widely used in many applications, especially as excellent cutting tools [1–4]. Whereas, the intrinsic drawbacks of Al2O3-based cutting tools, such as lower strength, lower fracture toughness and lower thermal shock resistance usually make them more susceptible to fracture when machining, leading to a short tool life [5]. Studies have shown that the addition of TiC phase to alumina matrix can improve toughness, hardness and also thermal shock resistance [1,6–9]. Al2O3–TiC composites (denoted as AT) are widely used as cutting tools due to their attractive mechanical properties [10]. And AT composite is often manufactured by hot pressing of Al2O3 and TiC powder mixture [11,12]. However, inhomogeneous blend of starting powders, grain coarsening during sintering and weak interfaces between the ceramic particles deteriorate mechanical properties of AT composite fabricated by this method. Incorporating some cobalt into the AT by coating technology can improve the mixing homogeneity of powders and the mechanical properties of composite [13]. A novel Al2O3–TiC–Co composite (denoted as ATC) was fabricated in our study. It is important to evaluate the oxidation resistance of the ATC composite because it works under high temperature as a cutting tool. Numerous investigations about the oxidation resistance of ceramic matrix composites have been reported [14–20]. In the present work, the high-temperature oxidation behaviour and kinetics of nanometre and micrometre sized ATC composites sintered at 1650 °C in static air is investigated in detail.
2.1. Materials
⇑ Corresponding author. Tel.: +86 531 82765983. E-mail address: [email protected] (R. Shi). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.08.008
Nanometre sized ATC composites were fabricated from the mixed powders of cobalt-coated A12O3 and TiC. A12O3 (average particle size about 40 nm) and TiC (average particle size about 130 nm) were coated with cobalt film (3.96 vol.%) by chemical deposition, respectively. The coated powders with the mass ratio of 70:30 (A12O3:TiC) were then homogenized by ultrasonic dispersion as the starting powders (provided by copartners of Ningbo Lingri Surface Engineering Co., Ltd.,). Micrometre sized ATC mixed powders were prepared by the same technology. However the sizes of A12O3 (average particle size about 4 lm) and TiC (average particle size about 2 lm) powders were different. Then the starting powders were hot-pressed in vacuum at 1650 °C for 30 min under a pressure of 30 MPa (Model HIGH MULTI 5000, FUJI DENPA, Japan). Sintered specimens were cut into bars. The final dimensions of these samples were 3 4 36 mm and 4 2 36 mm for measuring flexural strength and fracture toughness, respectively. The mechanical properties and densities of samples are listed in Table 1. 2.2. Oxidation measurement The oxidation tests were conducted in a static air furnace at 800, 900 and 1000 °C. The specimens in size 3 4 10 mm were first ultrasonically cleaned and dried, and then weighed with a precision of 0.0001 g before and after the oxidation test. The exact dimensions were measured in order to calculate the surface area. For the oxidation tests, samples were first placed in a corundum crucible, and then the crucible was moved into the furnace. Each
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R. Shi et al. / Corrosion Science 53 (2011) 4058–4064 Table 1 Mechanical properties, relative densities and composition of ATC composites. Composites
Particle size
Al2O3 (vol.%)
TiC (vol.%)
Cobalt (vol.%)
Sintering temp. (°C)
Hardness (HRA)
Flexure strength (MPa)
Fracture toughness (MPa m1/2)
Relative density (%)
ATC
Nanometre sized Micrometre sized
71.28
24.76
3.96
1650
92.7
782
7.81
99.4
71.28
24.76
3.96
1650
91.7
559
7.43
94.8
ATC
Fig. 1. weight gain curves of samples during isothermal oxidation in air at: (a) 800 °C, (b) 900 °C and (c) 1000 °C.
oxidation experiment composed of heating for 1–5 h in the furnace at the setting temperature and cooling in air for 10 min. 2.3. Material characterization The densities of the sintered samples were measured using Archimedes principle and the fluid employed is water. Phase identification was made by X-ray diffractometer (D8 Advance, Germany) with monochromated Cu Ka radiation. The microstructures of samples were analyzed using a scanning electron microscope (SEM, Hitachi S-2500) equipped with an EDS system for elemental analysis (Link-ISIS300 Oxford). 3. Results 3.1. Oxidation behaviour The oxidation weight gain experiments of nanometre and micrometre sized ATC composites sintered at 1650 °C were carried
Fig. 2. X-ray diffraction patterns of the surfaces of nanometre sized ATC composites oxidized at different temperature.
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R. Shi et al. / Corrosion Science 53 (2011) 4058–4064
nanometre sized samples at 800–1000 °C. The weight gain of nanometre and micrometre sized composites both increased greatly at 1000 °C and were 31.03 and 9.62 times of one at 800 °C, respectively. The weight gain difference between nanometre and micrometre sized composites decreased with the oxidation temperature going up and increased with the extension of oxidation time. 3.2. Characterization
Fig. 3. X-ray diffraction patterns of the surfaces of micrometre sized ATC composites oxidized at different temperature.
out at 800–1000 °C in air from 1 to 25 h. Fig. 1 shows the primary oxidation kinetics of the as-sintered ATC composites at 800– 1000 °C. The composites had specific weight gain on the order of 0.036–0.706 mg/cm2 at 800 °C, 0.340–3.214 mg/cm2 at 900 °C and 0.786–6.795 mg/cm2 at 1000 °C between 1 and 25 h. The weight gain of micrometre sized samples are more than ones of
Figs. 2 and 3 present the XRD patterns of nanometre and micrometre sized ATC composites oxide scales formed on the samples after primary oxidation at 800–1000 °C in air for 25 h, respectively. It can be seen that the oxide scales of nanometre sized ATC composites formed on the samples are composed of Co3O4 at 800– 1000 °C. However, the amount of Co3O4 at 800 and 900 °C was much less than that of Co3O4 at 1000 °C and lots of tetragonal TiO2 (rutile) formed at 900 °C. The XRD pattern from the oxidized surface shown in Fig. 3 reveals Co3O4 peaks at 800 °C and only tetragonal TiO2 (rutile) on oxidation at 900 and 1000 °C in air for 25 h. In addition, there may be other oxides below at the junction of a thick TiO2 layer and the composite, but this could not be detected by scanning the surface. Surface morphologies of nanometre sized samples oxidized in air at 800–1000 °C for 25 h are shown in Fig. 4. As can be seen from Fig. 4 that some bigger holes appeared on oxidized surface at 800 °C (Fig. 4(b)) and prism-shaped oxides emerged at 900 °C (Fig. 4(c)). EDS patterns indicated that the prism-shaped oxides
Fig. 4. SEM morphologies of nanometre sized ATC composites oxidized in air for 25 h at: (a) before oxidation, (b) 800 °C, (c) 900 °C and (d) 1000 °C.
R. Shi et al. / Corrosion Science 53 (2011) 4058–4064
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Fig. 5. TEM photograph of triangle grain boundary of ATC composite sintered at 1650 °C (a) and EDS pattern (b).
Fig. 6. SEM morphologies of oxidized micrometre sized ATC composites at: (a) before oxidation, (b) 800 °C, (c) 900 °C and (d) 1000 °C.
were mainly composed of titanium and oxygen, which is in accord with XRD results. Consequently, it can be inferred that the prismshaped oxides were TiO2 (rutile). In addition, it is a remarkable fact
that a large amount of worm-like products mainly appeared on the grain boundaries and evenly distributed on the oxidized surface at 1000 °C (Fig. 4(d)). EDS analysis showed that the worm-like
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Fig. 7. SEM morphologies of micrometre sized ATC composites oxidized in air for 25 h at: (a) 800 °C and (b) 900 °C.
Table 2 oxidation reactions and calculation results of thermodynamics of ATC composites. Oxidation temp. (K)
Oxidation reaction
DG (kJ mol1)
T = 1073
TiC þ 2O2 ¼ TiO2 þ CO2 3Co þ 2O2 ¼ Co3 O4 TiC þ 2O2 ¼ TiO2 þ CO2 3Co þ 2O2 ¼ Co3 O4 TiC þ 2O2 ¼ TiO2 þ CO2 3Co þ 2O2 ¼ Co3 O4
1189.843 464.049 1193.312 424.319 1196.780 384.590
T = 1173 T = 1273
products were mainly composed of cobalt and oxygen, which is also in agreement with XRD results. Therefore it can be concluded that the worm-like products were Co3O4. The TEM analysis (Fig. 5) showed that a majority of cobalt flowed and aggregated at triangle grain boundary during and after sintering period [13]. Therefore the Co3O4 emerged mainly on the grain boundaries when cobalt was oxidized (Fig. 4d). Fig. 6 shows the surface morphologies of micrometre sized ATC composites oxide scales formed at 800– 1000 °C for 25 h. Some large and communicating voids and some needle-leaf shape particles appeared on oxidized surface at
Fig. 8. Logarithmic plots of weight gain, DW=S; vs time, t, for ATC composites oxidized at (a) 800 °C, (b) 900 °C and (c) 1000 °C. The exponent, n, is obtained from the slope.
R. Shi et al. / Corrosion Science 53 (2011) 4058–4064
800 °C (Figs. 6(b) and 7(a)) and prism-shaped oxides appeared at 900 °C (Figs. 6(c) and 7(b)). EDS showed that the needle-leaf particles and prism-shaped oxides were both mainly composed of titanium and oxygen. The needle-leaf particles contain some aluminium element but the content of aluminium was much lower than that of original sample. Compared with the microstructure of surface oxidized at 1000 °C (Fig. 5(d)) it can be inferred that the needle-leaf and prism-shaped oxides may be the different stage of rutile TiO2. In addition, escape of CO2 led to the formation of porosities in the oxide layer due to the oxidation of TiC. The presence of these pores provided channels for oxygen diffusion and accelerated the oxidation process because small pores in the scale provide short-circuit paths for oxygen transportation to the oxide interface. 4. Discussion
When oxidizing in air at high temperature, following reactions occurred for ATC composites, leading to oxide formation:
TiC þ 2O2 ¼ TiO2 þ CO2
ð1Þ
3Co þ 2O2 ¼ Co3 O4
ð2Þ
Table 2 showed the standard Gibbs free energy DG of the reactions at 800–1000 °C. The values of standard Gibbs free energy DG for both reactions are found to be negative at 800–1000 °C which indicates that they are thermodynamically possible to take place. Table 3 Oxidation parameters of ATC composites at 800–1000 °C. Oxidation temp. (°C)
Particle size
General rate constant, k (mgn cm2n s1)
Oxidation exponent (n)
800
Nanometre sized Micrometre sized Nanometre sized Micrometre sized Nanometre sized Micrometre sized
3.486 107 5.616 106 6.601 106 1.585 105 2.544 104 3.752 104
1.999 1.944 1.932 1.863 1.880 1.839
900 1000
4.2. Oxidation kinetics The data of variation of weight gain per unit area of the sample due to oxidation (DW=S) with exposure time (t) have been further analyzed to determine the kinetic parameters, namely, the general rate constant (k), oxidation exponent (n), and the parabolic rate constant (k) [21,22]. The relation between DW=S, n and k is expressed as:
ðDW=SÞn ¼ kt
Table 4 Rate constant kp (mg2 cm4 s1) of oxidation ATC composites at 800–1000 °C. Particle size
800 °C
900 °C
1000 °C
Nanometre sized Micrometre sized
1.422 106 2.769 105
2.003 104 5.739 104
1.370 103 2.565 103
ð3Þ
It is obvious that n and k can be determined from the slope of the plot showing the variation of the logarithm of weight gain per unit area verse the logarithm of time (Eq. (3)). The parabolic rate constant (kp) is related to the weight gain per unit surface area of specimen (DW=S) and exposure time (t) by:
ðDW=SÞ2 ¼ kp t þ c
4.1. Chemical thermodynamics analysis
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ð4Þ
The value of kp can be obtained graphically from the slope of the plot of variation of ðDW=SÞ2 with t (Eq. (4)). Using Eq. (3), the oxidation exponent n and general rate constant k have been determined from the slopes and intercepts, respectively, of the best-fit lines in Fig. 8 and are reported in Table 3. A value of n = 1 or 2 indicates a linear or parabolic nature, respectively. It can be seen that the k increased with increase in oxidation temperature both nanometre and micrometre sized ATC composites. The k values of nanometre sized composites are lower than ones of micrometre sized composites at 800–1000 °C. However, the difference of k values of both nanometre and micrometre sized ATC composites is getting less and less with oxidation temperature going up. The k values of micrometre sized ATC composites are 16.11, 2.40 and 1.47 times of nanometre sized composites at 800, 900 and 1000 °C, respectively. The values of nanometre and micrometre sized ATC composites n = 1.999 and n = 1.944 at 800 °C, n = 1.932 and n = 1.863 at 900 °C and n = 1.880 and n = 1.839 at 1000 °C obtained in the present research indicate that the oxidation kinetics was slightly faster than that predicted by the parabolic-rate law at 800–1000 °C. The parabolic-rate constants at 800–1000 °C have been determined using Eq. (4). The value of kp is the slope of the best-fit lines in the W2 vs t plots for different temperatures and has been determined graphically. The calculated values of kp for temperatures of 800–1000 °C are presented in Table 4. The value of kp showed a steady increase with the increase in the temperature of oxidation. The apparent activation energy (Q) has been determined from an Arrhenius-type equation,
Fig. 9. Arrhenius plot of lnkp vs 1/T for ATC composites sintered at 1650 °C (a) nanometre sized and (b) micrometre sized.
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kp ¼ k0 expðQ =RTÞ
R. Shi et al. / Corrosion Science 53 (2011) 4058–4064
ð5Þ
where k0 is a constant, R is the Boltzmann constant and T is absolute temperature. Fig. 9 shows the variation of ln kp with reciprocal of the absolute temperature (1/T). The slope of the best-fit line in the plot of Fig. 9 was used to determine the value of Q/R. The activation energy of nanometre and micrometre sized ATC composites oxidation in the range of 800–1000 °C was found to be 142.21 and 114.84 kJ/mol, respectively, which maybe contribute the different oxidation products of nanometre and micrometre sized ATC composites. 5. Conclusions The oxidation behaviour of nanometre and micrometre sized ATC composites prepared by hot pressed sintering was investigated at temperatures of 800–1000 °C in air for 25 h. The following main results were obtained: (1) The oxide scales of nanometre sized ATC composites all formed Co3O4 at 800–1000 °C but the main oxide products were TiO2 (rutile) at 900 °C in air for 25 h. The XRD of micrometre sized composites reveals Co3O4 peaks only at 800 °C and tetragonal TiO2 (rutile) on oxidation both at 900 °C and 1000 °C in air for 25 h. (2) The general rate constant k increased and the difference of k values was getting less and less with increase in oxidation temperature both nanometre and micrometre sized ATC composites. And the k values of nanometre sized composites are lower than of micrometre sized at 800–1000 °C. (3) The values of oxidation exponent n decreased with oxidation temperature going up both nanometre and micrometre sized ATC composites. All the n values obtained indicate that the oxidation kinetics was slightly faster than that predicted by the parabolic-rate law at 800–1000 °C. (4) In the research the activation energy of nanometre and micrometre sized ATC composites oxidation in the range of 800–1000 °C was found to be 142.21 and 114.84 kJ/mol, respectively.
Acknowledgements This work is supported by the Natural Science Foundation of Shandong Province (Grant Nos. ZR2009FM072 and ZR2009FM068).
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