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Oxidation of WC-TiC-TaC-Co hard materials at relatively low temperature
Int. Journal of Refractory Metals and Hard Materials 48 (2015) 134–140
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Oxidation of WC-TiC-TaC-Co hard materials at relatively low temperature Shiwei Huang a, Ji Xiong a,⁎, Zhixing Guo a, Weicai Wan a, Limei Tang a, Hua Zhong a, Wei Zhou a, Bitong Wang b a b
School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, PR China School of Material Science and Engineering, Sichuan University, Chengdu 610065, PR China
a r t i c l e
i n f o
Article history: Received 8 May 2014 Accepted 2 August 2014 Available online 9 August 2014 Keywords: Hard materials Oxidation Microstructure XPS
a b s t r a c t The oxidation behavior of WC-TiC-TaC-Co hard materials was investigated at 300–900 °C. The results indicate that the oxidation reactions have been occurred as low as 350 °C. The mass gain is slight at 350–600 °C but increases significantly above 600 °C. The Co binder is preferential oxidized to CoWO4 due to WC solution. Besides, WC and (W, Ti, Ta)C are oxidized to WO3 and mixture of TiO2 and TaxOy, respectively, until 400 °C. Lots of defects on the surface of WO3 accelerate substrate oxidation. On the contrary, the compact layer of the mixture prevents the inner oxidation and significantly enhances the oxidation resistance. A gradual increase in oxidation temperature causes a rapid growth of Co oxide and also promotes the deterioration of WO3. In contrast, the compact layer of the solid solution shows higher thermal stability. © 2014 Elsevier Ltd. All rights reserved.
Contents Introduction . . . . . . . . . . . . . . . . . . . . . Experimental procedures . . . . . . . . . . . . . . . Material preparation . . . . . . . . . . . . . . . Oxidation tests . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . Characterization of as-prepared cemented carbides . Mass gains during oxidation . . . . . . . . . . . . Surface morphology of as-oxidized cemented carbides Oxidation mechanism . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . .
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Introduction WC-Co cemented carbides have been widely used for cutting tools because of their high melting point, high strength, high hardness and wear resistance [1]. More excellent properties of cemented carbides tool are needed for high speed, more effective and long life. Therefore, refractory metal carbides such as TiC and TaC are added to improve either strength or hardness [2–10]. Besides, the oxidation resistance of these cemented carbides is significantly improved [11,12]. It has been proven that the temperature of interface during the cutting process ⁎ Corresponding author. Tel.: +86 13880085119; fax: +86 28 85196764. E-mail address: [email protected] (J. Xiong).
http://dx.doi.org/10.1016/j.ijrmhm.2014.08.002 0263-4368/© 2014 Elsevier Ltd. All rights reserved.
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can vary between 600 °C and 1000 °C [13–17]. Thus, the oxidation of WC-TiC-TaC-Co cemented carbides happened in the cutting condition. Moreover, it was confirmed that the surface oxidation causes a significant degradation of strength and hardness and decreases the life of cutting tools [18,19]. Thus, it is important to discuss the oxidation behavior of WC-TiC-TaC-Co cemented carbides. Many investigations have dealt with the effect of binder composition, binder content, grain size, the heating rate and the oxygen concentrate of atmosphere on the oxidation of WC-based cemented carbides [19–25]. A survey on oxidation behavior of WC-Co cemented carbides was carried out by Basu and Sarin [19] in flowing Ar-O2 mixture gas at 600 °C, 700 °C and 800 °C. They noticed that the oxidation rate increased rapidly with the increased oxygen content in the atmosphere.
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Table 1 Characteristics of raw materials. Raw materials
FSSS (μm)
Total carbon (wt.%)
Manufacturer
WC (W, Ti)C Co TaC
1 2.18 1.17 1.36
5.90 9.80 6.21
Xiamen Golden Egret Special Alloy Co. Ltd., China Changsha Wing Hing High-TechNew Materials Co., Ltd., China Nanjing Hanrui Cobalt Co., Ltd. Zhuzhou Cemented Carbide Group Co., Ltd.
They further found that an increase in Co content improved the oxidation behavior. Similar results was confirmed by Aristizabal et al. [20] in comparison with the oxidation behavior of WC-Co and WC-Ni-CoCr cemented carbides at experiment temperatures of 750 °C, 800 °C and 850 °C. Voitovich studied the oxidation of WC-Co, WC-Ni and WC-Co-Ni hard metals at 500–800 °C [21] and concluded that WC-Co hard metal had higher oxidation resistance compared with WC-Ni hard metal. In addition, some literatures reported the oxidation of cemented carbides with addition of other carbides and/or nitrides [26–31]. The effect of TiC and (Ta,Nb)C on the oxidation resistance was carried out by Barbatti et al. [26] at 600 °C and 800 °C. The researcher found that WC-10Co containing TiC and/or (Ta,Nb)C has a superior oxidation resistance. Besides, the oxidation kinetics of WC-31%(Ti, Ta, Nb)C-9%Co sintered carbides were studied by Campo at 492 °C–800 °C [27]. It was also found that the temperature range for the kinetics inversion can be modified by the presence of other elements in the WC-Co samples, in comparison with experimental results in pure WC-Co samples. Most of the papers focused on the oxidation at working temperature, approximately from 500 °C to 1000 °C. However, in fact, the oxidation reactions occurred at lower temperature, and the characteristic of oxide played an important role in the oxidation of substrate. Thus, it was necessary to study the oxidation behavior of WC-TiC-TaC-Co cemented carbides at relative low temperature. The aim of this paper was to study the oxidation behavior of WC-Co hard metal with TiC and TaC additions. This study was carried out at 350 °C, 400 °C, 450 °C and 500 °C to seek the evolution process and give an interpretation on oxidation mechanism of WC-TiC-TaC-Co cemented carbides. Experimental procedures
10wt%Co cemented carbide balls with a diameter of 6 mm were used as the milling bodies, the ball to powder weight ratio was selected to be 10:1 and the milling speed was 56 r/min. After the powders were pressed to green bodies. The specimens were sintered in vacuum furnace of pilot scale at 1440 °C for 1.5 h. Coercive force and magnetic saturation were tested by YSK-III and MCoM-3 instruments (Hunan, China), hardness was measured using an ARK-600 Rockwell hardness tester (AKashi, Japan) and density was measured using a BS224S balance (Satorius, 0.1 mg sensitive quantity, China) by Archimedes method. Oxidation tests Before the oxidation tests, the samples with dimensions of 6 mm × 5 mm × 5 mm were surface ground and polished to 1 μm then cleaned in acetone by ultrasonic cleaning instrument. The weight of original and oxidized specimens was measured using an electronic balance with an accuracy of 0.1 mg. After heating to target temperature, the specimens were put in the middle position of the chamber electric furnace. After 2 h oxidation process, the oxidized sample was taken out of the furnace and air-cooled to room temperature. Oxidation tests were performed in air for 2 h at the temperatures from 300 °C to 900 °C, respectively. The microstructures of samples were evaluated using an S-4800 scanning electron microscope (SEM; Hitachi Company, Japan). The compositions analysis of materials were conducted by using the energy dispersive spectroscopy (EDS) model OXFORD IE-250 attached to the SEM. X-ray photoelectron spectroscopy (XPS) was used for phase analysis. HSC chemistry software was applied to calculate the Gibbs free energy of oxidation reactions by the “reaction equations” module. All reactions were recorded per 20 °C from 20 °C to 1000 °C.
Material preparation
Results and discussion
Commercial powder of WC, TaC, Co and (W, Ti)C solid solution was used as raw material, and the characteristics of these powders were listed in Table 1. The composition of cemented carbides was 73.72 wt.%WC, 4.28 wt.%TiC, 12 wt.%TaC and 10 wt.%Co. Raw powders were ball milled in ethanol in stainless steel lined mills for 72 h. WC-
Characterization of as-prepared cemented carbides The microstructures of WC-TiC-TaC-Co cemented carbides are shown in Fig. 1. The figures present homogeneous microstructures with fine grain size. The irregular round gray phase is the (W, Ti, Ta)C solid solution. In addition, the white phase is WC grain. The black area locating in the WC/WC grain boundaries and WC/(W, Ti, Ta)C phase boundaries is Co binder. The composition differences affect the morphology and behavior of oxidized samples. It is not observed that TaC and TiC hard phases segregation to the WC/WC grain boundaries and WC/binder phase boundaries. TaC and WC possess high solubility in TiC [32]; thus, TaC dissolve preferential in the solid solution (W, Ti)C, and the inward diffusion force of Ti drives Table 2 Low magnification microstructure and mechanical properties of cemented carbide.
Fig. 1. SEM-BSD images of the original cemented carbide.
Properties
WC-TiC-TaC-Co
Low magnification microstructures Density (g/cm3) Hardness (HRA) Coercive force (kA/m) Relative magnet saturation (%) Transverse rupture strength (MPa)
A02B00C00 13.1 91.7 17.56 8.78 2200
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Fig. 2. Mass gain of investigated sample for different temperature.
the Ta diffusion [33]. Thus, the Ta and Ti are evenly distributed in the solid solution. The properties of WC-TiC-TaC-Co cemented carbides are list in Table 2. The polished samples showed only type A porosity with a concentration of A02; hence, the good densification of materials was
Fig. 4. EDS analysis of oxygen content in three phase for different temperature. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
achieved. The average grain size of WC is about 1 μm, and the size of solid solution is about 2 μm and larger than that of WC. Larger grain size of (W, Ti)C and TaC raw material may explain this phenomenon.
Fig. 3. SEM-SE micrographs of oxidized surface for different temperature: (a) 300 °C, (b) 350 °C, (c) 400 °C, (d) 450 °C, (e) 500 °C.
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Fig. 5. EDS analysis of the material surface for 300 °C oxidation. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
Mass gains during oxidation The mass gain is shown in Fig. 2 at different oxidation temperature (300–900 °C). Obviously, the gain increases with the oxidation temperature increasing. A slight mass gain is observed at 350–600 °C, and the increased temperature promotes the mass gain. A significant mass gain appears at the temperature above 600 °C because of the rapid oxidation.
Surface morphology of as-oxidized cemented carbides Fig. 3 shows the SEM micrographs of oxidized samples. EDS analysis of the Co binder phase, WC and (W, Ti, Ta)C is carried out, and oxygen content is shown in Fig. 4. The oxygen content increases with the oxidation temperature increasing. Actually, the EDS indicates that there is no oxygen on the surface of material at 300 °C, as shown in Fig. 5. The oxygen is detected on the binder at 350 °C, and it indicates that the binder is oxidized obviously at this temperature (Fig. 3(b)). The white Co oxide grows remarkably with temperature increasing in the grain boundary of WC/WC and the phase boundary of WC/solid solution because the ceramic skeleton prevents flank grown of Co oxide. EDS (Fig. 6) analysis demonstrates that the Co oxide mainly contains W, Co and O, and the content of W and Co is approximately 1:1. due to the high solubility of WC in Co [34]; thus, it could be identified that the Co oxide mainly consists of CoWO4 [24]. The low amount of Ti in Co binder is detected due to dissolution of TiC in Co and rapid growth of CoWO4. Moreover, the growth of CoWO4 is promoted by
the increase of temperature. Eventually, the CoWO4 shows floccose white morphology. In contrast to the oxidation of Co binder phase, the oxidation characteristic is found on WC and the solid solution at 400 °C (Fig. 3(c)). At this temperature, the oxygen is detected on WC and the solid solution, as shown in Fig. 4. Irregular gray-white WC oxide grain network is observed. According to the result of XPS in Fig. 7, WC grain is oxidized to WO3 [35]. A high number of defects such as porosity, voids and cracks are embed in WO3 phase, especially at 500 °C in high magnification. Moreover, substrate–oxide interface zones may contain some complex WOx oxides due to the low oxygen concentration [26]. These defects on surface resulted from the production of volatile gases such as CO and CO2 in the oxidation reactions. In addition, the volatile gases flow is accelerated due to the increase of temperature, thus elevating the compressive stresses of WO3 and promotes deterioration (Fig. 3(e)). Furthermore, the substrate oxidation reactions are promoted by the oxygen supply from the atmosphere through the cracks and gaps of the WO3 scale. An gray compact layer is found on the surface of the solid solution substrate. The position of Ti2p peak on 458.45 eV and 464.05 eV can corroborate the existence of TiO2 (Fig. 8) [36,37]. An energy shift of the Ta4f peak from Ta-C bonds at 23.2–23.4 eV and 25.1–25.3 eV to a binding energy of 25.75 eV and 27.7 eV is observed in Fig. 9 [38]. It reveals the occurrence of TaC in (W, Ti, Ta)C oxidation reaction; thus, the layer is related to TiO2 and TaxOy. The morphology of the solid solution oxide is less influenced by the increase of temperature because the compact and stable oxidation layer prevents the oxygen diffusion into the interior and inhibits the further oxidation.
Fig. 6. EDS analysis of Co binder phase. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
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Fig. 7. The O1s spectra of oxidized sample at 500 °C for 2 h.
Fig. 9. The Ta4f spectra of oxidized sample at 500 °C for 2 h. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
Oxidation mechanism Based on the above experimental results, it can be concluded that the initial oxidation temperature in three phases of WC-TiC-TaC-Co cemented carbides is different. The oxidation morphology of WC grains and Co binder phase differs sensitively with the increase of oxidation temperature, while little difference is observed as to the solid solution. Four or more independent oxidation processes can be involved in the oxidation of WC-TiC-TaC-Co materials, each of them owns specific activation barrier. In other words, the difficulty of the oxidation reaction of each phase varies due to the change of composition. Moreover, the final product of oxidation reaction may be the result of several simple reactions in chain; thus, the specific activation barrier of intermediate product can also determine occurrence of final product. Furthermore, the oxidation reaction rate is determined either by the chemical reactions or the oxygen concentrate at the reacting interface. The main component in cemented carbides is WC, which is oxidized to WO3. The low oxygen concentrate in the internal scale is responsible for the occurrence of the WOx, for example, WO2. The Co binder is oxidized to CoO or Co3O4, and considering the solution of WC, CoWO4 is formed. However, the addition of TiC promotes the formation of TiO2 and the addition of TaC promotes the formation of tantalum oxides, for example, the typical Ta2O5. These oxides is formed according to the following equations: WC þ 2O2 ðgÞ→WO2 þ CO2 ðgÞ
Fig. 8. The Ti2p spectra of oxidized sample at 500 °C for 2 h.
ð1Þ
WC þ 5=2O2 ðgÞ→WO3 þ CO2 ðgÞ
ð2Þ
TiC þ 2O2 ðgÞ→TiO2 þ CO2 ðgÞ
ð3Þ
TaC þ 9=4O2 ðgÞ→1=2Ta2 O5 þ CO2 ðgÞ
ð4Þ
Co þ 1=2O2 ðgÞ→CoO
ð5Þ
Co þ 2=3O2 ðgÞ→1=3Co3 O4
ð6Þ
WC þ Co þ 3O2 ðgÞ→CoWO4 þ CO2 ðgÞ
ð7Þ
ðW; Ti; TaÞC þ 19=4O2 ðgÞ→WO3 þ TiO2 þ 1=2Ta2 O5 þ CO2 ðgÞ
ð8Þ
Fig. 10 shows the Gibbs free energy change of the reactions as a function of temperature. In accordance with thermodynamic calculations, reaction (6) owns a positive ΔG below 400 °C, so this reaction is not expect to proceed, while reaction (7) owns the lowest ΔG at 20–800 °C. Hence, the Co binder and WC dissolving in Co are preferential oxidized
Fig. 10. The Gibbs free energy change of related oxidation reactions as a function of temperature. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
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to CoWO4. However, as known, all W, Ti and Ta belong to elements which possess high electron/atom ratios and high valency, for example, W6 +, the covalent bond of Ta-C, Ti-C and W-C, is too strong to be destroyed due to lack of energy from the external environmental condition. Moreover, the activation energy of the solid solution is higher than that of WC because of solution of TiC and TaC in WC. Therefore, the thermostability of hard phase is higher than that of Co binder phase. According to the Arrhenius equation, k ¼ k0 expð−Q =RT Þ
ð9Þ
where k is oxidation rate, k0 is a constant value (frequency factor), Q is the activation energy for the thermally activated oxidation process, R is the perfect gas constant and T is the temperature. The reaction rate follows an exponential increase with the temperature increasing, while at the same temperature, the reaction rate is rapidly increased with the activation energy decreased. As found in Refs. [20], these Q-values decreased for WC-xCo samples with x = 6, 10, 15, respectively, between 650 °C and 800 °C. Moreover, the oxidation kinetics of hard materials with the addition of other elements, such as Ti and Ta, is studied [21,31]. It is found that the oxidation rate is decreased compared with the unmodified samples. Eq. (9) is applied, taking the same k0, R and T into account. It can be concluded that the addition of TiC and TaC enlarge the activation energy (Q) of the overall reaction. Furthermore, the little change of the activation energy can affect the oxidation rate significantly. However, the influence of temperature on the activation energy is negligible. The oxygen gradient is established due to the growth of the oxide scales. Three phases own different oxygen concentrate depending on the location of the oxidation interface. The Co oxide grows, and it is raised on the surface of sample; thus, the reaction interface possesses sufficient oxygen to further oxidation. Meanwhile, the oxygen concentrate of the WO3–WC oxidation interface is lower than that of the surface because the oxygen arrives through the pores and cracks of the WO3 phase slowly. However, the compact TiO2 and TaxOy layer inhibits oxygen diffusion into the substrate–oxide interface, and it means that the oxygen of solid solution reaction interface is shortage for the consumption of oxidation reaction. Conclusions In the study, the oxidation behavior of WC-TiC-TaC-Co hard materials has been investigated. The main findings are concluded as follows. (1). The oxidation reactions have been occurred at low temperature range, as low as 350 °C. The mass gain is slight at 350–600 °C and increases significantly above 600 °C. (2). The Co binder is preferential oxidized to CoWO4 due to WC solution. The morphology of CoWO4 changes from reticular to floccose with the temperature increasing. (3). WC is oxidized to WO3 until 400 °C. And lot of defects on the surface of WO3 such as porosity, voids and cracks accelerate substrate oxidation. (4). The mixture of TiO2 and TaxOy is the oxidation product of (W, Ti, Ta)C while the temperature approaches 400 °C. The compact layer of the mixture prevents the further oxidation and significantly enhances the oxidation resistance. (5). A gradual increase in oxidation temperature causes a rapid growth of Co oxide and also promotes the deterioration of WO3. In contrast, the compact layer of the solid solution shows higher thermal stability. Acknowledgments The study is financially supported by National Science and Technology Major Project (no. 2013ZX04009011), Fundamental Research Funds for
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the Central Universities (no. 2013SCU04A30), Sichuan Science and Technology Project (no. 2013GZX0146) and Chengdu Science and Technology Project (no. 11DXYB096JH-027). The authors are grateful to Chengdu Mingwu Technology Corp., LTD. of China for supply of materials. References [1] Upadhyaya GS. Materials science of cemented carbides—an overview. Mater Des 2001;22:483–9. [2] Östberg G, Buss K, Christensen M, Norgren S, Andrén H-O, Mari D, et al. Effect of TaC on plastic deformation of WC-Co and Ti(C, N)-WC-Co. Int J Refract Met Hard Mater 2006;24:135–44. [3] Weidow J, Zackrisson J, Jansson B, Andrénet H-O. Characterisation of WC–Co with cubic carbide additions. Int J Refract Met Hard Mater 2009;27:244–8. [4] Soleimanpour AM, Abachi P, Simchi A. Microstructure and mechanical properties of WC-10Co cemented carbide containing VC or (Ta, Nb) C and fracture toughness evaluation using different models. Int J Refract Met Hard Mater 2012;31:141–6. 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Oxidation of WC-TiC-TaC-Co hard materials at relatively low temperature Shiwei Huang a, Ji Xiong a,⁎, Zhixing Guo a, Weicai Wan a, Limei Tang a, Hua Zhong a, Wei Zhou a, Bitong Wang b a b
School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, PR China School of Material Science and Engineering, Sichuan University, Chengdu 610065, PR China
a r t i c l e
i n f o
Article history: Received 8 May 2014 Accepted 2 August 2014 Available online 9 August 2014 Keywords: Hard materials Oxidation Microstructure XPS
a b s t r a c t The oxidation behavior of WC-TiC-TaC-Co hard materials was investigated at 300–900 °C. The results indicate that the oxidation reactions have been occurred as low as 350 °C. The mass gain is slight at 350–600 °C but increases significantly above 600 °C. The Co binder is preferential oxidized to CoWO4 due to WC solution. Besides, WC and (W, Ti, Ta)C are oxidized to WO3 and mixture of TiO2 and TaxOy, respectively, until 400 °C. Lots of defects on the surface of WO3 accelerate substrate oxidation. On the contrary, the compact layer of the mixture prevents the inner oxidation and significantly enhances the oxidation resistance. A gradual increase in oxidation temperature causes a rapid growth of Co oxide and also promotes the deterioration of WO3. In contrast, the compact layer of the solid solution shows higher thermal stability. © 2014 Elsevier Ltd. All rights reserved.
Contents Introduction . . . . . . . . . . . . . . . . . . . . . Experimental procedures . . . . . . . . . . . . . . . Material preparation . . . . . . . . . . . . . . . Oxidation tests . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . Characterization of as-prepared cemented carbides . Mass gains during oxidation . . . . . . . . . . . . Surface morphology of as-oxidized cemented carbides Oxidation mechanism . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . .
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Introduction WC-Co cemented carbides have been widely used for cutting tools because of their high melting point, high strength, high hardness and wear resistance [1]. More excellent properties of cemented carbides tool are needed for high speed, more effective and long life. Therefore, refractory metal carbides such as TiC and TaC are added to improve either strength or hardness [2–10]. Besides, the oxidation resistance of these cemented carbides is significantly improved [11,12]. It has been proven that the temperature of interface during the cutting process ⁎ Corresponding author. Tel.: +86 13880085119; fax: +86 28 85196764. E-mail address: [email protected] (J. Xiong).
http://dx.doi.org/10.1016/j.ijrmhm.2014.08.002 0263-4368/© 2014 Elsevier Ltd. All rights reserved.
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can vary between 600 °C and 1000 °C [13–17]. Thus, the oxidation of WC-TiC-TaC-Co cemented carbides happened in the cutting condition. Moreover, it was confirmed that the surface oxidation causes a significant degradation of strength and hardness and decreases the life of cutting tools [18,19]. Thus, it is important to discuss the oxidation behavior of WC-TiC-TaC-Co cemented carbides. Many investigations have dealt with the effect of binder composition, binder content, grain size, the heating rate and the oxygen concentrate of atmosphere on the oxidation of WC-based cemented carbides [19–25]. A survey on oxidation behavior of WC-Co cemented carbides was carried out by Basu and Sarin [19] in flowing Ar-O2 mixture gas at 600 °C, 700 °C and 800 °C. They noticed that the oxidation rate increased rapidly with the increased oxygen content in the atmosphere.
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Table 1 Characteristics of raw materials. Raw materials
FSSS (μm)
Total carbon (wt.%)
Manufacturer
WC (W, Ti)C Co TaC
1 2.18 1.17 1.36
5.90 9.80 6.21
Xiamen Golden Egret Special Alloy Co. Ltd., China Changsha Wing Hing High-TechNew Materials Co., Ltd., China Nanjing Hanrui Cobalt Co., Ltd. Zhuzhou Cemented Carbide Group Co., Ltd.
They further found that an increase in Co content improved the oxidation behavior. Similar results was confirmed by Aristizabal et al. [20] in comparison with the oxidation behavior of WC-Co and WC-Ni-CoCr cemented carbides at experiment temperatures of 750 °C, 800 °C and 850 °C. Voitovich studied the oxidation of WC-Co, WC-Ni and WC-Co-Ni hard metals at 500–800 °C [21] and concluded that WC-Co hard metal had higher oxidation resistance compared with WC-Ni hard metal. In addition, some literatures reported the oxidation of cemented carbides with addition of other carbides and/or nitrides [26–31]. The effect of TiC and (Ta,Nb)C on the oxidation resistance was carried out by Barbatti et al. [26] at 600 °C and 800 °C. The researcher found that WC-10Co containing TiC and/or (Ta,Nb)C has a superior oxidation resistance. Besides, the oxidation kinetics of WC-31%(Ti, Ta, Nb)C-9%Co sintered carbides were studied by Campo at 492 °C–800 °C [27]. It was also found that the temperature range for the kinetics inversion can be modified by the presence of other elements in the WC-Co samples, in comparison with experimental results in pure WC-Co samples. Most of the papers focused on the oxidation at working temperature, approximately from 500 °C to 1000 °C. However, in fact, the oxidation reactions occurred at lower temperature, and the characteristic of oxide played an important role in the oxidation of substrate. Thus, it was necessary to study the oxidation behavior of WC-TiC-TaC-Co cemented carbides at relative low temperature. The aim of this paper was to study the oxidation behavior of WC-Co hard metal with TiC and TaC additions. This study was carried out at 350 °C, 400 °C, 450 °C and 500 °C to seek the evolution process and give an interpretation on oxidation mechanism of WC-TiC-TaC-Co cemented carbides. Experimental procedures
10wt%Co cemented carbide balls with a diameter of 6 mm were used as the milling bodies, the ball to powder weight ratio was selected to be 10:1 and the milling speed was 56 r/min. After the powders were pressed to green bodies. The specimens were sintered in vacuum furnace of pilot scale at 1440 °C for 1.5 h. Coercive force and magnetic saturation were tested by YSK-III and MCoM-3 instruments (Hunan, China), hardness was measured using an ARK-600 Rockwell hardness tester (AKashi, Japan) and density was measured using a BS224S balance (Satorius, 0.1 mg sensitive quantity, China) by Archimedes method. Oxidation tests Before the oxidation tests, the samples with dimensions of 6 mm × 5 mm × 5 mm were surface ground and polished to 1 μm then cleaned in acetone by ultrasonic cleaning instrument. The weight of original and oxidized specimens was measured using an electronic balance with an accuracy of 0.1 mg. After heating to target temperature, the specimens were put in the middle position of the chamber electric furnace. After 2 h oxidation process, the oxidized sample was taken out of the furnace and air-cooled to room temperature. Oxidation tests were performed in air for 2 h at the temperatures from 300 °C to 900 °C, respectively. The microstructures of samples were evaluated using an S-4800 scanning electron microscope (SEM; Hitachi Company, Japan). The compositions analysis of materials were conducted by using the energy dispersive spectroscopy (EDS) model OXFORD IE-250 attached to the SEM. X-ray photoelectron spectroscopy (XPS) was used for phase analysis. HSC chemistry software was applied to calculate the Gibbs free energy of oxidation reactions by the “reaction equations” module. All reactions were recorded per 20 °C from 20 °C to 1000 °C.
Material preparation
Results and discussion
Commercial powder of WC, TaC, Co and (W, Ti)C solid solution was used as raw material, and the characteristics of these powders were listed in Table 1. The composition of cemented carbides was 73.72 wt.%WC, 4.28 wt.%TiC, 12 wt.%TaC and 10 wt.%Co. Raw powders were ball milled in ethanol in stainless steel lined mills for 72 h. WC-
Characterization of as-prepared cemented carbides The microstructures of WC-TiC-TaC-Co cemented carbides are shown in Fig. 1. The figures present homogeneous microstructures with fine grain size. The irregular round gray phase is the (W, Ti, Ta)C solid solution. In addition, the white phase is WC grain. The black area locating in the WC/WC grain boundaries and WC/(W, Ti, Ta)C phase boundaries is Co binder. The composition differences affect the morphology and behavior of oxidized samples. It is not observed that TaC and TiC hard phases segregation to the WC/WC grain boundaries and WC/binder phase boundaries. TaC and WC possess high solubility in TiC [32]; thus, TaC dissolve preferential in the solid solution (W, Ti)C, and the inward diffusion force of Ti drives Table 2 Low magnification microstructure and mechanical properties of cemented carbide.
Fig. 1. SEM-BSD images of the original cemented carbide.
Properties
WC-TiC-TaC-Co
Low magnification microstructures Density (g/cm3) Hardness (HRA) Coercive force (kA/m) Relative magnet saturation (%) Transverse rupture strength (MPa)
A02B00C00 13.1 91.7 17.56 8.78 2200
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Fig. 2. Mass gain of investigated sample for different temperature.
the Ta diffusion [33]. Thus, the Ta and Ti are evenly distributed in the solid solution. The properties of WC-TiC-TaC-Co cemented carbides are list in Table 2. The polished samples showed only type A porosity with a concentration of A02; hence, the good densification of materials was
Fig. 4. EDS analysis of oxygen content in three phase for different temperature. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
achieved. The average grain size of WC is about 1 μm, and the size of solid solution is about 2 μm and larger than that of WC. Larger grain size of (W, Ti)C and TaC raw material may explain this phenomenon.
Fig. 3. SEM-SE micrographs of oxidized surface for different temperature: (a) 300 °C, (b) 350 °C, (c) 400 °C, (d) 450 °C, (e) 500 °C.
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Fig. 5. EDS analysis of the material surface for 300 °C oxidation. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
Mass gains during oxidation The mass gain is shown in Fig. 2 at different oxidation temperature (300–900 °C). Obviously, the gain increases with the oxidation temperature increasing. A slight mass gain is observed at 350–600 °C, and the increased temperature promotes the mass gain. A significant mass gain appears at the temperature above 600 °C because of the rapid oxidation.
Surface morphology of as-oxidized cemented carbides Fig. 3 shows the SEM micrographs of oxidized samples. EDS analysis of the Co binder phase, WC and (W, Ti, Ta)C is carried out, and oxygen content is shown in Fig. 4. The oxygen content increases with the oxidation temperature increasing. Actually, the EDS indicates that there is no oxygen on the surface of material at 300 °C, as shown in Fig. 5. The oxygen is detected on the binder at 350 °C, and it indicates that the binder is oxidized obviously at this temperature (Fig. 3(b)). The white Co oxide grows remarkably with temperature increasing in the grain boundary of WC/WC and the phase boundary of WC/solid solution because the ceramic skeleton prevents flank grown of Co oxide. EDS (Fig. 6) analysis demonstrates that the Co oxide mainly contains W, Co and O, and the content of W and Co is approximately 1:1. due to the high solubility of WC in Co [34]; thus, it could be identified that the Co oxide mainly consists of CoWO4 [24]. The low amount of Ti in Co binder is detected due to dissolution of TiC in Co and rapid growth of CoWO4. Moreover, the growth of CoWO4 is promoted by
the increase of temperature. Eventually, the CoWO4 shows floccose white morphology. In contrast to the oxidation of Co binder phase, the oxidation characteristic is found on WC and the solid solution at 400 °C (Fig. 3(c)). At this temperature, the oxygen is detected on WC and the solid solution, as shown in Fig. 4. Irregular gray-white WC oxide grain network is observed. According to the result of XPS in Fig. 7, WC grain is oxidized to WO3 [35]. A high number of defects such as porosity, voids and cracks are embed in WO3 phase, especially at 500 °C in high magnification. Moreover, substrate–oxide interface zones may contain some complex WOx oxides due to the low oxygen concentration [26]. These defects on surface resulted from the production of volatile gases such as CO and CO2 in the oxidation reactions. In addition, the volatile gases flow is accelerated due to the increase of temperature, thus elevating the compressive stresses of WO3 and promotes deterioration (Fig. 3(e)). Furthermore, the substrate oxidation reactions are promoted by the oxygen supply from the atmosphere through the cracks and gaps of the WO3 scale. An gray compact layer is found on the surface of the solid solution substrate. The position of Ti2p peak on 458.45 eV and 464.05 eV can corroborate the existence of TiO2 (Fig. 8) [36,37]. An energy shift of the Ta4f peak from Ta-C bonds at 23.2–23.4 eV and 25.1–25.3 eV to a binding energy of 25.75 eV and 27.7 eV is observed in Fig. 9 [38]. It reveals the occurrence of TaC in (W, Ti, Ta)C oxidation reaction; thus, the layer is related to TiO2 and TaxOy. The morphology of the solid solution oxide is less influenced by the increase of temperature because the compact and stable oxidation layer prevents the oxygen diffusion into the interior and inhibits the further oxidation.
Fig. 6. EDS analysis of Co binder phase. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
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Fig. 7. The O1s spectra of oxidized sample at 500 °C for 2 h.
Fig. 9. The Ta4f spectra of oxidized sample at 500 °C for 2 h. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
Oxidation mechanism Based on the above experimental results, it can be concluded that the initial oxidation temperature in three phases of WC-TiC-TaC-Co cemented carbides is different. The oxidation morphology of WC grains and Co binder phase differs sensitively with the increase of oxidation temperature, while little difference is observed as to the solid solution. Four or more independent oxidation processes can be involved in the oxidation of WC-TiC-TaC-Co materials, each of them owns specific activation barrier. In other words, the difficulty of the oxidation reaction of each phase varies due to the change of composition. Moreover, the final product of oxidation reaction may be the result of several simple reactions in chain; thus, the specific activation barrier of intermediate product can also determine occurrence of final product. Furthermore, the oxidation reaction rate is determined either by the chemical reactions or the oxygen concentrate at the reacting interface. The main component in cemented carbides is WC, which is oxidized to WO3. The low oxygen concentrate in the internal scale is responsible for the occurrence of the WOx, for example, WO2. The Co binder is oxidized to CoO or Co3O4, and considering the solution of WC, CoWO4 is formed. However, the addition of TiC promotes the formation of TiO2 and the addition of TaC promotes the formation of tantalum oxides, for example, the typical Ta2O5. These oxides is formed according to the following equations: WC þ 2O2 ðgÞ→WO2 þ CO2 ðgÞ
Fig. 8. The Ti2p spectra of oxidized sample at 500 °C for 2 h.
ð1Þ
WC þ 5=2O2 ðgÞ→WO3 þ CO2 ðgÞ
ð2Þ
TiC þ 2O2 ðgÞ→TiO2 þ CO2 ðgÞ
ð3Þ
TaC þ 9=4O2 ðgÞ→1=2Ta2 O5 þ CO2 ðgÞ
ð4Þ
Co þ 1=2O2 ðgÞ→CoO
ð5Þ
Co þ 2=3O2 ðgÞ→1=3Co3 O4
ð6Þ
WC þ Co þ 3O2 ðgÞ→CoWO4 þ CO2 ðgÞ
ð7Þ
ðW; Ti; TaÞC þ 19=4O2 ðgÞ→WO3 þ TiO2 þ 1=2Ta2 O5 þ CO2 ðgÞ
ð8Þ
Fig. 10 shows the Gibbs free energy change of the reactions as a function of temperature. In accordance with thermodynamic calculations, reaction (6) owns a positive ΔG below 400 °C, so this reaction is not expect to proceed, while reaction (7) owns the lowest ΔG at 20–800 °C. Hence, the Co binder and WC dissolving in Co are preferential oxidized
Fig. 10. The Gibbs free energy change of related oxidation reactions as a function of temperature. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
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to CoWO4. However, as known, all W, Ti and Ta belong to elements which possess high electron/atom ratios and high valency, for example, W6 +, the covalent bond of Ta-C, Ti-C and W-C, is too strong to be destroyed due to lack of energy from the external environmental condition. Moreover, the activation energy of the solid solution is higher than that of WC because of solution of TiC and TaC in WC. Therefore, the thermostability of hard phase is higher than that of Co binder phase. According to the Arrhenius equation, k ¼ k0 expð−Q =RT Þ
ð9Þ
where k is oxidation rate, k0 is a constant value (frequency factor), Q is the activation energy for the thermally activated oxidation process, R is the perfect gas constant and T is the temperature. The reaction rate follows an exponential increase with the temperature increasing, while at the same temperature, the reaction rate is rapidly increased with the activation energy decreased. As found in Refs. [20], these Q-values decreased for WC-xCo samples with x = 6, 10, 15, respectively, between 650 °C and 800 °C. Moreover, the oxidation kinetics of hard materials with the addition of other elements, such as Ti and Ta, is studied [21,31]. It is found that the oxidation rate is decreased compared with the unmodified samples. Eq. (9) is applied, taking the same k0, R and T into account. It can be concluded that the addition of TiC and TaC enlarge the activation energy (Q) of the overall reaction. Furthermore, the little change of the activation energy can affect the oxidation rate significantly. However, the influence of temperature on the activation energy is negligible. The oxygen gradient is established due to the growth of the oxide scales. Three phases own different oxygen concentrate depending on the location of the oxidation interface. The Co oxide grows, and it is raised on the surface of sample; thus, the reaction interface possesses sufficient oxygen to further oxidation. Meanwhile, the oxygen concentrate of the WO3–WC oxidation interface is lower than that of the surface because the oxygen arrives through the pores and cracks of the WO3 phase slowly. However, the compact TiO2 and TaxOy layer inhibits oxygen diffusion into the substrate–oxide interface, and it means that the oxygen of solid solution reaction interface is shortage for the consumption of oxidation reaction. Conclusions In the study, the oxidation behavior of WC-TiC-TaC-Co hard materials has been investigated. The main findings are concluded as follows. (1). The oxidation reactions have been occurred at low temperature range, as low as 350 °C. The mass gain is slight at 350–600 °C and increases significantly above 600 °C. (2). The Co binder is preferential oxidized to CoWO4 due to WC solution. The morphology of CoWO4 changes from reticular to floccose with the temperature increasing. (3). WC is oxidized to WO3 until 400 °C. And lot of defects on the surface of WO3 such as porosity, voids and cracks accelerate substrate oxidation. (4). The mixture of TiO2 and TaxOy is the oxidation product of (W, Ti, Ta)C while the temperature approaches 400 °C. The compact layer of the mixture prevents the further oxidation and significantly enhances the oxidation resistance. (5). A gradual increase in oxidation temperature causes a rapid growth of Co oxide and also promotes the deterioration of WO3. In contrast, the compact layer of the solid solution shows higher thermal stability. Acknowledgments The study is financially supported by National Science and Technology Major Project (no. 2013ZX04009011), Fundamental Research Funds for
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the Central Universities (no. 2013SCU04A30), Sichuan Science and Technology Project (no. 2013GZX0146) and Chengdu Science and Technology Project (no. 11DXYB096JH-027). The authors are grateful to Chengdu Mingwu Technology Corp., LTD. of China for supply of materials. References [1] Upadhyaya GS. Materials science of cemented carbides—an overview. Mater Des 2001;22:483–9. [2] Östberg G, Buss K, Christensen M, Norgren S, Andrén H-O, Mari D, et al. Effect of TaC on plastic deformation of WC-Co and Ti(C, N)-WC-Co. Int J Refract Met Hard Mater 2006;24:135–44. [3] Weidow J, Zackrisson J, Jansson B, Andrénet H-O. Characterisation of WC–Co with cubic carbide additions. Int J Refract Met Hard Mater 2009;27:244–8. [4] Soleimanpour AM, Abachi P, Simchi A. Microstructure and mechanical properties of WC-10Co cemented carbide containing VC or (Ta, Nb) C and fracture toughness evaluation using different models. Int J Refract Met Hard Mater 2012;31:141–6. 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