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Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders
Int. Journal of Refractory Metals and Hard Materials 29 (2011) 692–697
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders Rui-xia Shi a,⁎, Jianrong Wang a, Jia Li a, Yan-sheng Yin b a b
School of Materials Science and Engineering, University of Jinan, Jinan 250022, China Institute of Marine Materials Science and Engineering, Shanghai Maritime University, Shanghai 201306, China
a r t i c l e
i n f o
Article history: Received 20 December 2010 Accepted 11 May 2011 Keywords: Oxidation behavior Kinetics Al2O3–TiC–Co Composites Cobalt-coated
a b s t r a c t The oxidation behavior of hot-pressed Al2O3–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al2O3–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al2O3–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental procedures
Alumina ceramic matrix composites are widely used in many applications [1,2]. Studies have shown that the incorporation of some hard phases into alumina matrix can improve toughness, hardness and also thermal shock resistance [3–6]. Titanium carbide (TiC), a common hard cermet powder, is extensively utilized in Al2O3/TiC [7–9]. Al2O3/TiC composite with good mechanical properties has a wide application in cutting tools [10]. While for some fields demanding higher performance materials, the toughness of Al2O3/TiC limits its potential applications. So many researches have been carried out for further improving its properties [11–13]. Cobalt is a well-known and widely used ductile metal. It possesses a unique set of thermal physical properties, e.g. higher thermal conductivity and lower Young's modulus, which contributes to the improvement of thermal shock resistance of Al2O3–TiC–Co composites (ATC) [14]. The ATC composites were hot-pressed from the cobalt coating powders obtained by a newly developed coating technique, which was expected to improve the mechanical and thermal shock resistance [15]. In this paper, the oxidation resistance and mechanism of micro-sized ATC composites therefore are investigated.
2.1. Material preparation and mechanical properties
⁎ Corresponding author. E-mail address: [email protected] (R. Shi). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.05.003
A12O3 (average particle size about 4 μm) and TiC (average particle size about 2 μm) were coated with cobalt film by chemical deposition, respectively. The chemical deposition was performed in a water bath with a magnetic stirrer. A summary of the solution compositions in the water bath is given in Table 1. Al2O3 or TiC powders were first ultrasonically dispersed into distilled water and then added into the water bath. After 15 min of the temperature of the solution attaining 70 °C, CoSO4·7H2O began to be reduced by NaH2PO2·H2O to form a film of cobalt on the surface of Al2O3 and TiC powders. The whole chemical deposition reaction lasted about 10 min and ended when the original red solution became colorless. During the whole coating process, the bath was stirred with a magnetic stirrer to reduce powder settling. After being washed with distilled water, the resulting coated powders, with the weight ratio of 70:30 (Al2O3:TiC), were then homogenized by ball milling as the starting powders [16] (provided by copartners of Ningbo Lingri Surface Engineering Co. Ltd.). Then the starting powders were hot-pressed in vacuum (2 × 10 − 4Pa) between 1500 and 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 mm × 4 mm × 36 mm and 4 mm × 2 mm × 36 mm for measuring flexural strength and fracture toughness, respectively. The mechanical properties and relative densities of samples are listed in Table 2.
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Table 1 Solution composition for chemical deposition. Component
Concentration (g/l)
Function
CoSO4·7H2O NaH2PO2·H2O KnaC6H4O6·4H2O H3BO3 PdCl2
20–28 21 147 31 Trace
Main salt Reduce Complexing agent Stabilizer Activator
Operating conditions pH Temperature
8–10 Temperature 70 ± 0.5 °C
2.2. Oxidation experimental The oxidation tests were performed in a static air furnace. The specimens in size 3 mm×4 mm×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 oxidation experiment composed of heating for 2 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. The phase composition after oxidation was identified on the surface using x-ray diffraction (XRD) (D8 ADVANCE, Bruker) with Cu Kα radiation. The oxidized surfaces were investigated using a scanning electron microscope (SEM, Hitachi S-2500) equipped with an EDS system for elemental analysis (Link-ISIS300 Oxford). 3. Results and discussion 3.1. Oxidation behaviors
Fig. 1. Plots of change in mass against time, obtained from isothermal oxidation tests carried out on ATC for a duration of 25 h at (a) 800 °C and (b)1000 °C.
The oxidation mass gain experiments of micro-sized ATC composites sintered at various temperatures were carried out at 200 °C, 400 °C, 600 °C, 800 °C and 1000 °C from 2 h to 25 h. There were no oxidation mass gains when the samples were oxidized at 200 °C, 400 °C and 600 °C in air for 25 h. The plots of mass gain per unit surface area as a function of time at 800 °C and 1000 °C are showed in Fig. 1. ATC composites had specific mass gains on the order of 0.141–10.633 mg/cm2 at 800 °C and 1.176–19.158 mg/cm2 at 1000 °C after 25 h oxidation. There were less mass gains at 800 °C than ones at 1000 °C. However, the mass gains decreased with sintering temperature increase both at 800 °C and 1000 °C, which was attributed to different relative densities of composites (Table 1). The composites with lower relative densities possess the more mass gains. The presence of more pores provides channels for oxygen diffusion and accelerates the oxidation process because small pores in the scale provide short-circuit paths for oxygen transportation to the oxide interface. The composites sintered at 1500 °C and 1650 °C had the most and the least mass gains, respectively, both at 800 °C and 1000 °C. Fig. 2 shows plots of oxidation rate as a function of time of ATC composites oxidized at 800 °C and 1000 °C. It can be seen
from Fig. 2 that oxidation rates of ATC composites sintered at different temperatures reached the highest values at 2 h but decreased quickly during initial oxidation (before 3 h) both at 800 °C and 1000 °C. Then the oxidation rates kept steady. In addition, the oxidation rates of samples sintered at 1650 °C kept the lowest values during whole oxidation process, which mainly was attributed to its higher relative density (Table 1). The average oxidation rates of composites were lower at 800 °C than those at 1000 °C. The oxidation resistance of samples sintered at 1650 °C is the best both at 800 °C and 1000 °C. So the oxidized surfaces of samples sintered at 1650 °C were analyzed. 3.2. Characterization and discussion Fig. 3 shows XRD patterns of exposed surface of samples oxidized from 200 °C to 1000°C in air for 25 h. It can be seen that there were no oxidation reactions from 200 °C to 600 °C. The XRD pattern from the oxidized surface shown in Fig. 3 reveals Co3O4 peaks at 800 °C. The oxidized surface of the ATC composite has shown only tetragonal TiO2
Table 2 Mechanical properties 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 ATC
Micro-sized Nano-sized
71.28 71.28
24.76 24.76
3.96 3.96
1650 1650
91.7 92.7
559 782
7.43 7.81
94.8 99.4
694
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Fig. 2. Oxidation rate as a function of time of ATC composites oxidized at (a) 800 °C and (b) 1000 °C.
(rutile) on oxidation at 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.
Fig. 4 shows the plots of Gibbs free energy ΔG of the possible oxidation reactions versus temperature. The values of Gibbs free energy ΔG for all reactions are found to be negative from 200 °C to 1000 °C which indicates that they are thermodynamically possible to take place. From Fig. 4, it is evident that TiO possesses the lowest free energy of formation as compared to TiO2, CoO and Co3O4 and, hence, is the most stable. As the free energies of formation of TiO and Co3O4 are lower than that of TiO2 and CoO at all temperatures, this would mean that TiO and Co3O4 would be preferable to form. However, TiO would be easy to be oxidized and to form TiO2 with temperature increase. Fig. 5 shows surface morphologies of samples oxidized from 200 °C to 1000 °C in air for 25 h. According to XRD results there are no oxidation products when ATC composites were oxidized from 200 °C to 600 °C after 25 h. However, it can be seen that some small voids appeared on the surface of composite oxidized at 200 °C (Fig. 5(b)). The quantity of small voids increased and irregular products appeared when samples were oxidated at 400 °C (Fig. 5(c)). Then more voids and some round-like products emerged on the surface of composite oxidized at 600 °C (Fig. 5(d)). These mainly were attributed to the thermal behaviors. In addition, maybe there was very small quantity of oxidized products that could not be detected by XRD methods. Some large and communicating voids and some needle-leaf shape particles appeared on oxidized surface at 800 °C (Fig. 5(e) and Fig. 6 (a)). EDS (Fig. 6(b)) showed that the needle-leaf shape particles are mainly composed of O, Ti, Co and Al. However, the content of Al was much lower than that of original sample. Compared with the surface oxidized at 1000 °C (Fig. 5(f)) it can be inferred that the needle-leaf shape particles may be the early form of rutile TiO2. It can be observed that the surface oxidized is gray and rough and possessed completely by TiO2 after oxidation at 1000 °C for 25 h (Fig. 5(f)), as it is clear from the X-ray images shown in Fig. 3. In addition, escape of CO2 or CO led to the formation of porosities in the oxide layer due to the oxidation of TiC. The tetragonal TiO2 (rutile) is very stable [17] and decomposes only at high temperature. Therefore, TiO2 can form oxidation scale on surface of composites to protect from oxidation. So the further oxidation process took place only when oxygen diffuses through TiO2 oxidation scale. The TiO2 formed at elevated temperature not only reduces the sticky wear but also works as solid lubricant which can degrade friction coefficients and improve wear resistance of cutting tool when ATC composites are used as cutting tools [18].
Fig. 3. XRD patterns obtained from the oxide scales formed on ATC during isothermal oxidation tests in air at different temperatures for 25 h.
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It is obvious that n and k can be determined from the slope of the plot showing the variation of the logarithm of W with the logarithm of t (Eq. [1]). Differentiating Eq. [1] gives n−1 nW dW = dt = k:
ð2Þ
Substituting from Eq. [1] into Eq. [2], one obtains dW=dt = W = nt:
ð3Þ
Again, the parabolic rate constant is related to W by the relation 2
ðW Þ = kp t + c
Fig. 4. ΔG0 versus temperature for the oxidation of Co and TiC.
3.3. Oxidation kinetics The data of variation of weight gain per unit area of the sample due to oxidation (W) with 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 (kp) [19]. The relation between W, n and k is expressed as n
ðW Þ = kt
ð1Þ
ð4Þ
The value of kp can be obtained graphically from the slope of the plot of variation of with t (Eq. [4]). Using Eq. [1], the oxidation exponents and general rates constant have been determined from the slopes and intercepts, respectively, of the best-fit lines in Fig. 7 and are reported in Table 2. A value of n =1 or 2 indicates a linear or parabolic nature, respectively. It can be seen that the k decreased with sintering temperature increase both at 800 °C and 1000 °C, which attributes to the relative density increase with sintering temperature going up. Except for samples sintered at 1500 °C, the k value of composites sintered at same temperature showed a steady increase with the increase in the temperature of oxidation. Samples sintered at 1650 °C possessed the lowest k both at 800 °C and 1000 °C. The values of n= 1.945 at 800 °C and n =1.839 at 1000 °C of samples sintered at 1650 °C indicate that the oxidation rate is faster than that predicted by the parabolic-rate law at 800 °C and 1000 °C. This trend is more obvious from the plots of variation of oxidation rate with time for oxidation of ATC
Fig. 5. SEM surface morphology of oxidized micro-sized ATC composites at different temperatures: (a) original, (b) 200 °C, (c) 400 °C, (d) 600 °C, (e) 800°C and (f) 1000 °C.
696
R. Shi et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 692–697
Fig. 7. Logarithmic plots of weight gain, W, vs. time, t, for ATC oxidized at (a) 800 °C and (b) 1000 °C. The exponent, n, is obtained from the slope.
the absolute temperature (1/T). The slope of the best-fit line in the plot of Fig. 8 was used to determine the value of −Q/R. The activation energy of ATC composites oxidation in the range of 800 °C to 1000 °C was found to be 113.7 kJ/mol. Fig. 6. Sample of ATC composites oxidized at 800 °C (a) SEM and (b) EDS of point 1. Sample surface coated by Au.
at 800 °C and 1000 °C, as shown in Fig. 2(a) and (b), respectively. The rate of oxidation decreased quickly with the increase in time of exposure at 800 °C and 1000 °C, as the oxide scale acted as a diffusion barrier. The parabolic-rate constants at 800 °C and 1000 °C have been determined using Eq. [2]. 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 °C, 900 °C and 1000 °C are presented in Table 3. 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, kp ¼ ko expð−Q=RTÞ
ð5Þ
where ko is a constant, R is the Boltzmann constant and T is absolute temperature. Fig. 8 shows the variation of with reciprocal of
4. Conclusions The oxidation behavior of micro-sized ATC composites hotpressed at various temperatures is studied in the research. The oxidation experiments were carried out from 200 °C to 1000 °C. The oxidation reactions happened from 800 °C. The oxidation resistance of ATC composites increased with sintering temperature going up. The ATC composites sintered at 1650 °C possessed the best resistance to oxidation both at 800 °C and 1000 °C. The oxidized surface of the ATC
Table 3 Possible oxidation reactions and calculation results of thermodynamics of TiC. Oxidation temp./K
Possible oxidation reaction
ΔH/J
T = 673
TiC + 2O2 = TiO2 + CO2 2TiC + 3O2 = 2TiO + 2CO2
−1,152,632 13,872 −1,166,504 −1,457,000 −54,416 −1,402,584
TΔS/J
ΔG/J
R. Shi et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 692–697
Fig. 8. Arrhenius plot of lnkp vs. 1/T for ATC composites sintered at 1650 °C.
composite showed Co3O4 at 800 °C and tetragonal TiO2 (rutile) on oxidation at 1000 °C in air for 25 h, which can be explained from consideration of the chemical thermodynamics. The rate of oxidation decreased quickly with the increase in time of exposure at 800 °C and 1000 °C. The oxidation kinetics of ATC followed a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. Acknowledgements This work is supported by the Natural Science Foundation of Shandong Province (grant no. ZR2009FM072 and ZR2009FM068). References [1] Xu C, Ai X, Huang C. Fabrication and performance of an advanced ceramic tool material. Wear 2001;249:503–8.
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[2] Sharifi EM, Karimzadeh F, Enayati MH. Mechanochemically synthesized Al2O3–TiC nanocomposite. J Alloy Comd 2010;491:411–5. [3] Deng JX, Duan ZX, Yun DL, Zhang H, Ai X, Zhao J. Fabrication and performance of Al2O3/(W, Ti)C + Al2O3/TiC multilayered ceramic cutting tools. Mater Sci Eng A 2010;527:1039–47. [4] Zhu JF, Gao JQ, Wang F, Chen P. Fabrication of TiN/Al2O3 composite ceramic by in-situ reactions. Rare Metal Mater Eng 2008;37:789–92. [5] Huang SG, Vanmeensel K, Van der Biest O, Vleugels J. Pulsed electric current sintering and characterization of ultrafine Al2O3-WC composites. Mater Sci Eng A 2010;527:584–9. [6] Shi XL, Xu FM, Zhang ZJ, Dong YL, Tan Y, Wang L, et al. Mechanical properties of hot-pressed Al2O3/SiC composites. Mater Sci Eng A 2010;527:4646–9. [7] Kobyakov VP, Barinova TV, Ratnikov VI. Self-propagating high-temperature synthesis of Al2O3/TiC-based ceramic materials. Inorg Mater 2009;452:211–9. [8] Zimmermann K, Schneider GA, Bhattacharya Anup K, Hintze W. Surface modification of Al2O3/TiC cutting ceramics. J Am Ceram Soc 2007;90:3773–8. [9] Zhang YF, Wang LJ, Jiang W, Chen Li, Bai GZ. Microstructure and properties of Al2O3–TiC nanocomposites fabricated by spark plasma sintering from high-energy ball milled reactants. J Eur Ceram Soc 2006;26:3393–7. [10] Golkar G, Zebarjad SM, Khaki JV. Role of nano-size TiO2 particles on the crystallite size of microwave—combustion synthesized Al2O3/TiC composite. J Alloy Comd 2010;504:566–72. [11] Liu CX, Zhang JH, Zhang XH, Sun JL. Fabrication of Al2O3/TiB2/AlN/TiN and Al2O3/ TiC/AlN composites. Mater Sci Eng A 2007;465:72–7. [12] Deng JX, Cao TK, Ding ZL, Liu JH, Sun JL, Zhao JL. Tribological behaviors of hotpressed Al2O3/TiC ceramic composites with the additions of CaF2 solid lubricants. J Eur Ceram Soc 2006;26:1317–23. [13] Xia TD, Liu TZ, Zhao WJ, Ma BY, Wang TM. Self-propagating high-temperature synthesis of Al2O3–TiC–Al composites by aluminothermic reactions. J Mater Sci 2001;36:5581–4. [14] Shi RX, Yin YS, Li J, Wang DZ. Influence of cobalt phase on thermal shock resistance of Al2O3–TiC composites evaluated by indentation technique. Mater Res Bull 2008;43:2544–53. [15] Mao DS, Liu XH, Li J, Guo SY, Zhang XB, Mao ZY. A fine cobalt-toughened Al2O3–TiC ceramic and its wear resistance. J Mater Sci 1998;33:5677–82. [16] Huang LP, Li J. Properties of cobalt-reinforced Al2O3–TiC ceramic matrix composite made via a new processing route. Compos Pt A-Appl Sci Manuf 1999;30:615–8. [17] Araoyinbo AO, Fauzi Ahmad MN, Sreekantan S, Aziz A. Preparation and characterization of thin film TiO2 dip coated on non-conductive substrate prepared from tetraethyl orthotitanate precursor. Asian J Appl Sci 2010;3:35–43. [18] Deng JX, Ai X. Friction and wear behavior of Al2O3/TiB2 based ceramic materials at elevated temperature. J Chin Ceram Soc 1996;24(2):160–5. [19] Mitra R, Rao Rama VV. Elevated-temperature oxidation behavior of titanium silicide and titanium silicide-based alloy and composite. Mater Trans A 1998;29: 1665–75.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders Rui-xia Shi a,⁎, Jianrong Wang a, Jia Li a, Yan-sheng Yin b a b
School of Materials Science and Engineering, University of Jinan, Jinan 250022, China Institute of Marine Materials Science and Engineering, Shanghai Maritime University, Shanghai 201306, China
a r t i c l e
i n f o
Article history: Received 20 December 2010 Accepted 11 May 2011 Keywords: Oxidation behavior Kinetics Al2O3–TiC–Co Composites Cobalt-coated
a b s t r a c t The oxidation behavior of hot-pressed Al2O3–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al2O3–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al2O3–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental procedures
Alumina ceramic matrix composites are widely used in many applications [1,2]. Studies have shown that the incorporation of some hard phases into alumina matrix can improve toughness, hardness and also thermal shock resistance [3–6]. Titanium carbide (TiC), a common hard cermet powder, is extensively utilized in Al2O3/TiC [7–9]. Al2O3/TiC composite with good mechanical properties has a wide application in cutting tools [10]. While for some fields demanding higher performance materials, the toughness of Al2O3/TiC limits its potential applications. So many researches have been carried out for further improving its properties [11–13]. Cobalt is a well-known and widely used ductile metal. It possesses a unique set of thermal physical properties, e.g. higher thermal conductivity and lower Young's modulus, which contributes to the improvement of thermal shock resistance of Al2O3–TiC–Co composites (ATC) [14]. The ATC composites were hot-pressed from the cobalt coating powders obtained by a newly developed coating technique, which was expected to improve the mechanical and thermal shock resistance [15]. In this paper, the oxidation resistance and mechanism of micro-sized ATC composites therefore are investigated.
2.1. Material preparation and mechanical properties
⁎ Corresponding author. E-mail address: [email protected] (R. Shi). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.05.003
A12O3 (average particle size about 4 μm) and TiC (average particle size about 2 μm) were coated with cobalt film by chemical deposition, respectively. The chemical deposition was performed in a water bath with a magnetic stirrer. A summary of the solution compositions in the water bath is given in Table 1. Al2O3 or TiC powders were first ultrasonically dispersed into distilled water and then added into the water bath. After 15 min of the temperature of the solution attaining 70 °C, CoSO4·7H2O began to be reduced by NaH2PO2·H2O to form a film of cobalt on the surface of Al2O3 and TiC powders. The whole chemical deposition reaction lasted about 10 min and ended when the original red solution became colorless. During the whole coating process, the bath was stirred with a magnetic stirrer to reduce powder settling. After being washed with distilled water, the resulting coated powders, with the weight ratio of 70:30 (Al2O3:TiC), were then homogenized by ball milling as the starting powders [16] (provided by copartners of Ningbo Lingri Surface Engineering Co. Ltd.). Then the starting powders were hot-pressed in vacuum (2 × 10 − 4Pa) between 1500 and 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 mm × 4 mm × 36 mm and 4 mm × 2 mm × 36 mm for measuring flexural strength and fracture toughness, respectively. The mechanical properties and relative densities of samples are listed in Table 2.
R. Shi et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 692–697
693
Table 1 Solution composition for chemical deposition. Component
Concentration (g/l)
Function
CoSO4·7H2O NaH2PO2·H2O KnaC6H4O6·4H2O H3BO3 PdCl2
20–28 21 147 31 Trace
Main salt Reduce Complexing agent Stabilizer Activator
Operating conditions pH Temperature
8–10 Temperature 70 ± 0.5 °C
2.2. Oxidation experimental The oxidation tests were performed in a static air furnace. The specimens in size 3 mm×4 mm×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 oxidation experiment composed of heating for 2 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. The phase composition after oxidation was identified on the surface using x-ray diffraction (XRD) (D8 ADVANCE, Bruker) with Cu Kα radiation. The oxidized surfaces were investigated using a scanning electron microscope (SEM, Hitachi S-2500) equipped with an EDS system for elemental analysis (Link-ISIS300 Oxford). 3. Results and discussion 3.1. Oxidation behaviors
Fig. 1. Plots of change in mass against time, obtained from isothermal oxidation tests carried out on ATC for a duration of 25 h at (a) 800 °C and (b)1000 °C.
The oxidation mass gain experiments of micro-sized ATC composites sintered at various temperatures were carried out at 200 °C, 400 °C, 600 °C, 800 °C and 1000 °C from 2 h to 25 h. There were no oxidation mass gains when the samples were oxidized at 200 °C, 400 °C and 600 °C in air for 25 h. The plots of mass gain per unit surface area as a function of time at 800 °C and 1000 °C are showed in Fig. 1. ATC composites had specific mass gains on the order of 0.141–10.633 mg/cm2 at 800 °C and 1.176–19.158 mg/cm2 at 1000 °C after 25 h oxidation. There were less mass gains at 800 °C than ones at 1000 °C. However, the mass gains decreased with sintering temperature increase both at 800 °C and 1000 °C, which was attributed to different relative densities of composites (Table 1). The composites with lower relative densities possess the more mass gains. The presence of more pores provides channels for oxygen diffusion and accelerates the oxidation process because small pores in the scale provide short-circuit paths for oxygen transportation to the oxide interface. The composites sintered at 1500 °C and 1650 °C had the most and the least mass gains, respectively, both at 800 °C and 1000 °C. Fig. 2 shows plots of oxidation rate as a function of time of ATC composites oxidized at 800 °C and 1000 °C. It can be seen
from Fig. 2 that oxidation rates of ATC composites sintered at different temperatures reached the highest values at 2 h but decreased quickly during initial oxidation (before 3 h) both at 800 °C and 1000 °C. Then the oxidation rates kept steady. In addition, the oxidation rates of samples sintered at 1650 °C kept the lowest values during whole oxidation process, which mainly was attributed to its higher relative density (Table 1). The average oxidation rates of composites were lower at 800 °C than those at 1000 °C. The oxidation resistance of samples sintered at 1650 °C is the best both at 800 °C and 1000 °C. So the oxidized surfaces of samples sintered at 1650 °C were analyzed. 3.2. Characterization and discussion Fig. 3 shows XRD patterns of exposed surface of samples oxidized from 200 °C to 1000°C in air for 25 h. It can be seen that there were no oxidation reactions from 200 °C to 600 °C. The XRD pattern from the oxidized surface shown in Fig. 3 reveals Co3O4 peaks at 800 °C. The oxidized surface of the ATC composite has shown only tetragonal TiO2
Table 2 Mechanical properties 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 ATC
Micro-sized Nano-sized
71.28 71.28
24.76 24.76
3.96 3.96
1650 1650
91.7 92.7
559 782
7.43 7.81
94.8 99.4
694
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Fig. 2. Oxidation rate as a function of time of ATC composites oxidized at (a) 800 °C and (b) 1000 °C.
(rutile) on oxidation at 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.
Fig. 4 shows the plots of Gibbs free energy ΔG of the possible oxidation reactions versus temperature. The values of Gibbs free energy ΔG for all reactions are found to be negative from 200 °C to 1000 °C which indicates that they are thermodynamically possible to take place. From Fig. 4, it is evident that TiO possesses the lowest free energy of formation as compared to TiO2, CoO and Co3O4 and, hence, is the most stable. As the free energies of formation of TiO and Co3O4 are lower than that of TiO2 and CoO at all temperatures, this would mean that TiO and Co3O4 would be preferable to form. However, TiO would be easy to be oxidized and to form TiO2 with temperature increase. Fig. 5 shows surface morphologies of samples oxidized from 200 °C to 1000 °C in air for 25 h. According to XRD results there are no oxidation products when ATC composites were oxidized from 200 °C to 600 °C after 25 h. However, it can be seen that some small voids appeared on the surface of composite oxidized at 200 °C (Fig. 5(b)). The quantity of small voids increased and irregular products appeared when samples were oxidated at 400 °C (Fig. 5(c)). Then more voids and some round-like products emerged on the surface of composite oxidized at 600 °C (Fig. 5(d)). These mainly were attributed to the thermal behaviors. In addition, maybe there was very small quantity of oxidized products that could not be detected by XRD methods. Some large and communicating voids and some needle-leaf shape particles appeared on oxidized surface at 800 °C (Fig. 5(e) and Fig. 6 (a)). EDS (Fig. 6(b)) showed that the needle-leaf shape particles are mainly composed of O, Ti, Co and Al. However, the content of Al was much lower than that of original sample. Compared with the surface oxidized at 1000 °C (Fig. 5(f)) it can be inferred that the needle-leaf shape particles may be the early form of rutile TiO2. It can be observed that the surface oxidized is gray and rough and possessed completely by TiO2 after oxidation at 1000 °C for 25 h (Fig. 5(f)), as it is clear from the X-ray images shown in Fig. 3. In addition, escape of CO2 or CO led to the formation of porosities in the oxide layer due to the oxidation of TiC. The tetragonal TiO2 (rutile) is very stable [17] and decomposes only at high temperature. Therefore, TiO2 can form oxidation scale on surface of composites to protect from oxidation. So the further oxidation process took place only when oxygen diffuses through TiO2 oxidation scale. The TiO2 formed at elevated temperature not only reduces the sticky wear but also works as solid lubricant which can degrade friction coefficients and improve wear resistance of cutting tool when ATC composites are used as cutting tools [18].
Fig. 3. XRD patterns obtained from the oxide scales formed on ATC during isothermal oxidation tests in air at different temperatures for 25 h.
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It is obvious that n and k can be determined from the slope of the plot showing the variation of the logarithm of W with the logarithm of t (Eq. [1]). Differentiating Eq. [1] gives n−1 nW dW = dt = k:
ð2Þ
Substituting from Eq. [1] into Eq. [2], one obtains dW=dt = W = nt:
ð3Þ
Again, the parabolic rate constant is related to W by the relation 2
ðW Þ = kp t + c
Fig. 4. ΔG0 versus temperature for the oxidation of Co and TiC.
3.3. Oxidation kinetics The data of variation of weight gain per unit area of the sample due to oxidation (W) with 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 (kp) [19]. The relation between W, n and k is expressed as n
ðW Þ = kt
ð1Þ
ð4Þ
The value of kp can be obtained graphically from the slope of the plot of variation of with t (Eq. [4]). Using Eq. [1], the oxidation exponents and general rates constant have been determined from the slopes and intercepts, respectively, of the best-fit lines in Fig. 7 and are reported in Table 2. A value of n =1 or 2 indicates a linear or parabolic nature, respectively. It can be seen that the k decreased with sintering temperature increase both at 800 °C and 1000 °C, which attributes to the relative density increase with sintering temperature going up. Except for samples sintered at 1500 °C, the k value of composites sintered at same temperature showed a steady increase with the increase in the temperature of oxidation. Samples sintered at 1650 °C possessed the lowest k both at 800 °C and 1000 °C. The values of n= 1.945 at 800 °C and n =1.839 at 1000 °C of samples sintered at 1650 °C indicate that the oxidation rate is faster than that predicted by the parabolic-rate law at 800 °C and 1000 °C. This trend is more obvious from the plots of variation of oxidation rate with time for oxidation of ATC
Fig. 5. SEM surface morphology of oxidized micro-sized ATC composites at different temperatures: (a) original, (b) 200 °C, (c) 400 °C, (d) 600 °C, (e) 800°C and (f) 1000 °C.
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Fig. 7. Logarithmic plots of weight gain, W, vs. time, t, for ATC oxidized at (a) 800 °C and (b) 1000 °C. The exponent, n, is obtained from the slope.
the absolute temperature (1/T). The slope of the best-fit line in the plot of Fig. 8 was used to determine the value of −Q/R. The activation energy of ATC composites oxidation in the range of 800 °C to 1000 °C was found to be 113.7 kJ/mol. Fig. 6. Sample of ATC composites oxidized at 800 °C (a) SEM and (b) EDS of point 1. Sample surface coated by Au.
at 800 °C and 1000 °C, as shown in Fig. 2(a) and (b), respectively. The rate of oxidation decreased quickly with the increase in time of exposure at 800 °C and 1000 °C, as the oxide scale acted as a diffusion barrier. The parabolic-rate constants at 800 °C and 1000 °C have been determined using Eq. [2]. 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 °C, 900 °C and 1000 °C are presented in Table 3. 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, kp ¼ ko expð−Q=RTÞ
ð5Þ
where ko is a constant, R is the Boltzmann constant and T is absolute temperature. Fig. 8 shows the variation of with reciprocal of
4. Conclusions The oxidation behavior of micro-sized ATC composites hotpressed at various temperatures is studied in the research. The oxidation experiments were carried out from 200 °C to 1000 °C. The oxidation reactions happened from 800 °C. The oxidation resistance of ATC composites increased with sintering temperature going up. The ATC composites sintered at 1650 °C possessed the best resistance to oxidation both at 800 °C and 1000 °C. The oxidized surface of the ATC
Table 3 Possible oxidation reactions and calculation results of thermodynamics of TiC. Oxidation temp./K
Possible oxidation reaction
ΔH/J
T = 673
TiC + 2O2 = TiO2 + CO2 2TiC + 3O2 = 2TiO + 2CO2
−1,152,632 13,872 −1,166,504 −1,457,000 −54,416 −1,402,584
TΔS/J
ΔG/J
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Fig. 8. Arrhenius plot of lnkp vs. 1/T for ATC composites sintered at 1650 °C.
composite showed Co3O4 at 800 °C and tetragonal TiO2 (rutile) on oxidation at 1000 °C in air for 25 h, which can be explained from consideration of the chemical thermodynamics. The rate of oxidation decreased quickly with the increase in time of exposure at 800 °C and 1000 °C. The oxidation kinetics of ATC followed a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. Acknowledgements This work is supported by the Natural Science Foundation of Shandong Province (grant no. ZR2009FM072 and ZR2009FM068). References [1] Xu C, Ai X, Huang C. Fabrication and performance of an advanced ceramic tool material. Wear 2001;249:503–8.
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[2] Sharifi EM, Karimzadeh F, Enayati MH. Mechanochemically synthesized Al2O3–TiC nanocomposite. J Alloy Comd 2010;491:411–5. [3] Deng JX, Duan ZX, Yun DL, Zhang H, Ai X, Zhao J. Fabrication and performance of Al2O3/(W, Ti)C + Al2O3/TiC multilayered ceramic cutting tools. Mater Sci Eng A 2010;527:1039–47. [4] Zhu JF, Gao JQ, Wang F, Chen P. Fabrication of TiN/Al2O3 composite ceramic by in-situ reactions. Rare Metal Mater Eng 2008;37:789–92. [5] Huang SG, Vanmeensel K, Van der Biest O, Vleugels J. Pulsed electric current sintering and characterization of ultrafine Al2O3-WC composites. Mater Sci Eng A 2010;527:584–9. [6] Shi XL, Xu FM, Zhang ZJ, Dong YL, Tan Y, Wang L, et al. Mechanical properties of hot-pressed Al2O3/SiC composites. Mater Sci Eng A 2010;527:4646–9. [7] Kobyakov VP, Barinova TV, Ratnikov VI. Self-propagating high-temperature synthesis of Al2O3/TiC-based ceramic materials. Inorg Mater 2009;452:211–9. [8] Zimmermann K, Schneider GA, Bhattacharya Anup K, Hintze W. Surface modification of Al2O3/TiC cutting ceramics. J Am Ceram Soc 2007;90:3773–8. [9] Zhang YF, Wang LJ, Jiang W, Chen Li, Bai GZ. Microstructure and properties of Al2O3–TiC nanocomposites fabricated by spark plasma sintering from high-energy ball milled reactants. J Eur Ceram Soc 2006;26:3393–7. [10] Golkar G, Zebarjad SM, Khaki JV. Role of nano-size TiO2 particles on the crystallite size of microwave—combustion synthesized Al2O3/TiC composite. J Alloy Comd 2010;504:566–72. [11] Liu CX, Zhang JH, Zhang XH, Sun JL. Fabrication of Al2O3/TiB2/AlN/TiN and Al2O3/ TiC/AlN composites. Mater Sci Eng A 2007;465:72–7. [12] Deng JX, Cao TK, Ding ZL, Liu JH, Sun JL, Zhao JL. Tribological behaviors of hotpressed Al2O3/TiC ceramic composites with the additions of CaF2 solid lubricants. J Eur Ceram Soc 2006;26:1317–23. [13] Xia TD, Liu TZ, Zhao WJ, Ma BY, Wang TM. Self-propagating high-temperature synthesis of Al2O3–TiC–Al composites by aluminothermic reactions. J Mater Sci 2001;36:5581–4. [14] Shi RX, Yin YS, Li J, Wang DZ. Influence of cobalt phase on thermal shock resistance of Al2O3–TiC composites evaluated by indentation technique. Mater Res Bull 2008;43:2544–53. [15] Mao DS, Liu XH, Li J, Guo SY, Zhang XB, Mao ZY. A fine cobalt-toughened Al2O3–TiC ceramic and its wear resistance. J Mater Sci 1998;33:5677–82. [16] Huang LP, Li J. Properties of cobalt-reinforced Al2O3–TiC ceramic matrix composite made via a new processing route. Compos Pt A-Appl Sci Manuf 1999;30:615–8. [17] Araoyinbo AO, Fauzi Ahmad MN, Sreekantan S, Aziz A. Preparation and characterization of thin film TiO2 dip coated on non-conductive substrate prepared from tetraethyl orthotitanate precursor. Asian J Appl Sci 2010;3:35–43. [18] Deng JX, Ai X. Friction and wear behavior of Al2O3/TiB2 based ceramic materials at elevated temperature. J Chin Ceram Soc 1996;24(2):160–5. [19] Mitra R, Rao Rama VV. Elevated-temperature oxidation behavior of titanium silicide and titanium silicide-based alloy and composite. Mater Trans A 1998;29: 1665–75.