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Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 °C
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CERAMICS INTERNATIONAL
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Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C Houan Zhangn, Jianxian Lv, Yihui Wu, Siyong Gu, Yu Huang, Ying Chen The Key Laboratory for Power Metallurgy Technology and Advanced Materials of Xiamen, Xiamen University of Technology, Xiamen 361024, PR China Received 5 March 2015; received in revised form 28 July 2015; accepted 4 August 2015
Abstract (Mo,W)Si2–Si3N4 composite coating on pure molybdenum (Mo) substrate was prepared by using in situ diffusion of tungsten (W), nitrogen (N) and silicon (Si) elements. The microstructures and phase compositions were investigated using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). A coarse columnar crystal structure was observed in the original dense (Mo, W)Si2–Si3N4 layer in the SEM micrographs. The transition layers of (Mo,W)5Si3 and (Mo,W)3Si were formed between the coating and Mo substrate during a oxidation process at 1600 1C in air. The growth kinetics of (Mo,W)5Si3 transition layer in (Mo,W)Si2–Si3N4 coating obeyed a parabolic rate law and its growth rate constant was 7.22 10 10 cm2/s. The strong hindering effect of W element in coating on the Si diffusion led to a longer antioxidation life of the (Mo,W)Si2–Si3N4 coating on Mo substrate at1600 1C. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Electron microscopy; C. Diffusion; D. Silicides; Oxidation
1. Introduction Molybdenum (Mo) is widely used in industrial applications because of its advanced properties, such as high melting point of 2617 1C, outstanding high-temperature strength and creep strength, good heat resistance, and low coefficient of thermal expansion. However, its applications at high temperatures are limited due to its extremely poor oxidation resistance in air above 600 1C [1,2]. Molybdenum disilicide (MoSi2) has an excellent high-temperature oxidation resistance, which makes it an attractive coating material to protect Mo [3,4], Ni [5], Nb [6], ZrB2 [7] and C/C composite [8,9], etc. used in an oxidative atmosphere at higher temperatures. The excellent oxidation resistance of MoSi2 coatings is due to the formation of an adherent, dense, continuous and self-healing SiO2 film on the surface, which protects the coating material from further oxidation. However, due to mismatch of coefficient of thermal expansion (CTE) between coating (8.5 10 6/K) and Mo n Correspondence to: School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, Fujian, PR China. Tel./fax: þ86 592 6291045. E-mail address: [email protected] (H. Zhang).
substrate (5.8 10 6/K) [10], cracks will be formed along the grain boundaries of MoSi2 coating perpendicular to the Mo substrate in the cooling-down process from the service temperature to room temperature. As a result, the failure of coatings is accelerated. Hsieh et al. [11] observed that the CTE of MoSi2– (30–35) vol% Si3N4 composites produced by hot isostatic pressing was close to that of Mo at a temperature range of 1000–1500 1C. Nagae et al. [12] and Yoon et al. [13] reported that the Mo2N coating could be easily formed by ammonia nitridation of Mo substrates at 1100 1C and Si3N4 phase appeared at crystal boundaries of MoSi2. Our recent research [14] showed that MoSi2–Si3N4 composite coating on Mo could be made by in situ diffusion of Mo ammonia nitriding followed by siliconizing, and the coating was crack-free during the cyclic oxidation process at 1450 1C in the ambient air atmosphere. This work confirmed that the addition of Si3N4 into MoSi2 coating decreased the mismatch of CTE between the coating and Mo substrate. Yoon et al. [15,16] and Byun et al. [17] studied the diffusion and growth mechanism of intermediate silicides in MoSi2/Mo and Mo5Si3/Mo system. They found Mo5Si3 layer and Mo3Si layer formed by diffusion reaction of the released silicon (Si) and Mo substrate in order, and indicated that Si was the dominant diffusion element in
http://dx.doi.org/10.1016/j.ceramint.2015.08.012 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: H. Zhang, et al., Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.012
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Fig. 1. XRD patterns of the (Mo,W)Si2–Si3N4 coating on Mo substrate surface by in situ diffusion reaction.
MoSi2, Mo5Si3 and Mo3Si phases. In our study [14], MoSi2– CrSi2–Si3N4 coating showed a better high temperature oxidation resistance than MoSi2–Si3N4 coating, which was attributed to chromium (Cr) element inhibiting the diffusion of Si and the formation of volatile MoO3. Above researches indicated that hindering the diffusion of silicon between the Mo substrate and the MoSi2 based coating was the key to increase the service life of the coating. Theoretical calculations on crystal binding energy of MoSi2 and WSi2 by Bhattacharyya et al. [18] were 1847.8 kJ/mol and 2058.1 kJ/mol, respectively, which indicated Si atom diffusion was more difficult in the W–Si bond than the Mo–Si bond. The diffusion layer of W/MoSi2 couple was thinner than that of Mo/ MoSi2 couple in Ref. [19], suggesting a hindering effect of W on the diffusion of Si. W element has an analogous crystal structure to Mo and its silicide [20], which provides a possibility to synthesize the (Mo,W)Si2–Si3N4 composite coating by in-situ diffusion of W, nitrogen (N) and Si elements as the formation of MoSi2–CrSi2– Si3N4 coating. However, there is no paper to report the oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on Mo substrate. Therefore, the objective of this paper is to discuss the effect of the W element on the high temperature oxidation resistance of (Mo,W) Si2–Si3N4 composite coating. 2. Experimental procedures Mo metal rods (99.95% purity) were cut into discs of Φ15 mm 3 mm and polished successively, up to a mirror-like surface using a colloidal silica suspension of 0.05 μm and a diamond paste of 1 μm. The polished discs were ultrasonically cleaned in alcohol and placed in an Al2O3 crucible inside a horizontal aluminum oxide tube furnace. The processes of (Mo, W)Si2–Si3N4 composite coating by in situ diffusion reaction was as followed. Firstly, pure Mo discs were pack tungstenized at 1200 1C for 10 h in Ar atmosphere. The powder pack consisted of 20 wt% W (tungsten source, 99.95% purity), 5 wt% NaF (activator) and balance 75 wt% alumina (inert filler). Secondly, ammonia nitridation of the samples was carried out at a reaction temperature of 1100 1C for 2 h. The purity and the flow rate of NH3 gas were 99.999% and 100 ml/min, respectively. Finally, the
silicide coating was applied by pack siliconizing at 1000 1C for 2 h in Ar atmosphere. The last powder pack consisted of 10 wt% Si (silicon source, 99.9% purity), 5 wt% NaF (activator) and 85 wt% alumina (inert filler). High temperature oxidation test was conducted at 1600 1C in air for a maximum duration of 360 h. Samples were placed in the hot zone of a furnace in a high-purity alumina crucible. The samples oxidized for a period of time were taken out from the furnace, followed by cooling to room temperature. The mass change during the high temperature oxidation was calculated by weighing a balance with a resolution of 0.1 mg. Microstructures and surface morphology of the coatings were carried out using scanning electron microscopy (SEM; S3400-N Hitachi). Simultaneously, the chemical compounds of the transition layers were analysis by the energy dispersive spectroscopy (EDS; 250 X-Max Oxford). A corrosive liquid (15 vol% HF þ 30 vol% HNO3 þ 55 vol% distilled H2O) was used to etch the cross-section of (Mo,W)Si2–Si3N4 composite coating for 30 s. The phase composition before and after oxidation was determined by X-ray diffraction (XRD; X' Pert PRO PANalytical), equipped with graphite monochromator using Cu Kα radiation. 3. Results and discussion XRD patterns of the Mo substrate surface by in situ diffusion reaction of sequential tungstenizing, ammonia nitrogenizing and siliconizing are shown in Fig. 1. It could be seen that the coating surface consisted of (Mo,W)Si2 and Si3N4 phases. Formation of (Mo,W)Si2 and Si3N4 phases could be divided into three stages: (1) During the pack tungstenizing, Mo–W infinite solid solution was formed on the basis of the Mo–W phase diagram. (2) During the ammonia nitrogenizing, the nitrides of W and Mo such as Mo2N and WN were synthesized in the coating because of the nitrogen diffusing reaction. (3) During the siliconizing, the reaction of W, Mo and N with Si resulted in the formation of MoSi2, WSi2 and Si3N4 in the coating as Si diffused into the nitridation layer. MoSi2 and WSi2 had the similar long-range order structures and the adjacent lattice parameters. When some Mo atoms in MoSi2 crystals were replaced by W atoms, (Mo, W)Si2 solid solutions were formed. These formation steps could be represented by the following equations: Moþ W-MoðWÞ solid solution
ð1Þ
2MoðWÞþ 3N-Mo2 N þ 2WN
ð2Þ
4Mo2 N þ 8WN þ 25Si-8ðMo; WÞSi2 þ 3Si3 N4
ð3Þ
Fig. 2(a) illustrated the cross-section morphology of the (Mo, W)Si2–Si3N4 composite coating and elements distribution of Mo, Si, W and N. The thickness of the coating layer tightly integrated with the substrate was measured about 250 μm, and the coating surface was divided into two layers: porous layer in the surface (about 35 μm) and dense layer in the middle. The formation of porous layer was thought to be due to the aggregation of a large number of Kirkendall voids [15]. The microstructures of the etched cross-section of (Mo,W)Si2–Si3N4
Please cite this article as: H. Zhang, et al., Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.012
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Fig. 2. Microstructures of (Mo,W)Si2–Si3N4 coating. (a) cross-section morphology and elements distribution; (b) columnar crystal structure of the selected area in (a) after etched.
An SiO2 film was formed on the surface of (Mo,W)Si2–Si3N4 coating during oxidation in air, as shown in Fig. 4(a). A few pores and cracks were observed on the surface of the coating, which could be healed by adherent SiO2 at a high temperature. The gas escaping of volatile oxides of W, Mo and N elements [10,23] resulted in the formation of pore and the weight loss of the coating. XRD patterns of the surface of (Mo,W)Si2–Si3N4 coated sample oxidized after 3 h and 144 h are illustrated in Fig. 4(b). It could be seen that the original (Mo,W)Si2 phase in coating surface had been transformed to (Mo,W)5Si3, which was attributed to the reaction occurred as the following: 5ðMo; WÞSi2 þ 7O2 -ðMo; WÞ5 Si3 þ 7SiO2 Fig. 3. Plots of mass change of (Mo,W)Si2–Si3N4 coated samples oxidized at 1600 1C in air.
composite coating are shown in Fig. 2(b). The dense (Mo,W) Si2–Si3N4 layer exhibited a columnar crystal structure, which was perpendicular to the Mo substrate. Similar structures had been reported in the MoSi2 coating prepared with CVD [21] and WSi2 coating prepared with the pack siliconizing method [22]. A thin transition layer (about 3 μm) was observed between the coating and Mo substrate, and the composition of the layer was (Mo,W)5Si3 phase as indicated in EDS. The mass variation of (Mo,W)Si2–Si3N4 coated samples at 1600 1C in air is illustrated in Fig. 3. There was a stable stage from 96 to 288 h during oxidation process where the mass change was small. Then an accelerated oxidation behavior was observed in the (Mo,W)Si2–Si3N4 coating until the coating failed after 360 h. As a comparison, oxidation properties of MoSi2–CrSi2–Si3N4 and MoSi2–Si3N4 coatings at the same conditions in Ref. [14] are also presented in Fig. 3. The oxidation of MoSi2–CrSi2–Si3N4 and MoSi2–Si3N4 coatings accelerated at about 96 h and 72 h. The antioxidant life of (Mo,W)Si2–Si3N4 coating at 1600 1C in air was about 3.75 times and 5 times longer than that of MoSi2–CrSi2– Si3N4 and MoSi2–Si3N4 coatings, respectively.
ð4Þ
SiO2 formed in the reaction appeared in amorphous form on the surface of coating. With the oxidation time prolonging, the peak intensity of (Mo,W)5Si3 phase increased, indicating that the oxidation process of the coating was promoted. As the oxidation time increased, the microstructures of (Mo, W)Si2–Si3N4 coating changed. Fig. 5 showed the backscattered electron (BSE) images of the cross-section morphology of the coating after oxidation at 1600 1C for various periods of time. When oxidized for 48 h, the coating exhibited three layers, which were marked as I, II, and III from the coating surface to the inside in Fig. 5(a). Results of EDS analysis suggested that I layer was mainly composed of SiO2 and (Mo,W)5Si3. II layer was composed of (Mo,W)Si2, while III layer was a transition layer and composed of (Mo,W)5Si3 with a lower atomic fraction of W. (Mo,W)5Si3 in I layer was formed by the reaction (4). However, the formation of (Mo,W)5Si3 in III layer at the interface of (Mo,W)Si2/Mo was mainly attributed to the Si diffusion. Referring to the literature [17], the growth of (Mo, W)5Si3 transition layer could be explained as four steps: (1) the (Mo,W)Si2 in II layer decomponsed into (Mo,W)5Si3 and Si; (2) the released Si diffused to the interface (Mo,W)5Si3/Mo; (3) with the interaction between the released Si and Mo substrate, the Mo5Si3 was formed; (4) Mo5Si3 solutionized into (Mo, W)5Si3 in step (1), then a layer of (Mo,W)5Si3 with a lower atomic fraction of W was finally formed. These formation steps
Please cite this article as: H. Zhang, et al., Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.012
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Fig. 4. Micrograph (a) and XRD patterns (b) of the surface of (Mo,W)Si2– Si3N4 coating oxidized at 1600 1C for 3 h in air.
could be represented by the following equations: 5ðMo; WÞSi2 -ðMo; WÞ5 Si3 þ 7Si
ð5Þ
35 7 Moþ 7Si- Mo5 Si3 3 3
ð6Þ
7 7ð1 xÞ 7 Mo5 Si3 þ ðMo; WÞ5 Si3 ðMox ; W1 x Þ5 Si3 3 3ð2x 1Þ 3ð2x 1Þ ð7Þ where x was the atomic fraction of Mo, and x was larger than 1 x. Some pores emerged in the III layer were located parallel to the interface of (Mo,W)5Si3/(Mo,W)Si2, which was attributed to the intrinsic Si diffusion inducing a Kirkendall effect [24]. As the oxidation time increased, the thickness of III layer increased while the thickness of II layer decreased, and a (Mo,W)3Si phase (now marked as IV) was formed at the (Mo,W)5Si3/Mo interfaces. The growth of (Mo,W)3Si layer (IV) was similar to the growth of (Mo,W)5Si3 transition layer at interface of (Mo, W)Si2/Mo, as described in Ref. [17]. The pores observed in III layer were filled with SiO2, which was due to the good flowability and self-healing property of SiO2 at high temperature. When oxidized for 288 h, the distribution of the (Mo,W) Si2 layer became discontinuous and cracks appeared in II and III layers in Fig. 5(c), and the mass change began to decrease significantly as shown in Fig. 3. It was found in Fig. 5(d) that a
very thin and discontinuous (Mo,W)Si2 layer (II layer) and a large number of micro-pores appeared in the interfaces of (Mo, W)3Si/Mo. As these micro-pores grew till to be connected with each other, the coating peeled off from the surface of Mo substrate. Fig. 6 showed the linear regression fit to the plots for grown thickness of (Mo,W)5Si3 transition layer (III layer in Fig. 5)) in (Mo,W)Si2–Si3N4 coating, Mo5Si3 transition layer in MoSi2– CrSi2–Si3N4 and MoSi2–Si3N4 coating by oxidation at 1600 1C in air. Each point in Fig. 6 was the average value of 10 measurements. Their growth kinetics obeyed a parabolic rate law. The data fitting agreed with the results reported in Refs. [6,16,24]. The linear regression fitting of the coating layers suggested that the growth rate constant of the (Mo,W)5Si3 transition layer in (Mo,W)Si2–Si3N4 coating was 7.22 10 10 cm2/s, and the growth rate constants of Mo5Si3 transition layer in MoSi2–CrSi2–Si3N4 coating and MoSi2–Si3N4 coating were 2.64 10 9 cm2/s and 3.47 10 9 cm2/s, respectively. A similar value of growth rate constant of Mo5Si3 transition layer in MoSi2 coating has been reported as 3.54 10 9 cm2/s [15]. The growth rate constant of the (Mo,W)5Si3 transition layer in (Mo,W)Si2– Si3N4 coating was about twenty times lower than that of Mo5Si3 layer in MoSi2 coating. Zmii et al. [25] also reported that the transition rate at 1700 1C for molybdenum silicide on tungsten from MoSi2 to (Mo,W)5Si3 was about twenty times slower than that from MoSi2 to Mo5Si3. Meanwhile, the growth rate constant of the (Mo,W)5Si3 transition layer in (Mo,W)Si2–Si3N4 coating was about 3.65 times lower than that of Mo5Si3 layer in MoSi2– CrSi2–Si3N4 coating, and 4.81 times lower than that of MoSi2– Si3N4 coating. The above ratios of growth rate constants of the transition layers were very close to the relative length value of antioxidant life for their corresponding coatings as shown in Fig. 3. It indicated the antioxidant life of the MoSi2 based coatings on Mo substrate was closely related to the growth of Mo5Si3 or (Mo, W)5Si3 transition layer. The lower growth rate constant of the (Mo, W)5Si3 transition layer in (Mo,W)Si2–Si3N4 coating exhibited the addition of W element had a strong hindering effect to the diffusion of Si between the coating and Mo substrate. 4. Conclusions Microstructural characteristics of (Mo,W)Si2–Si3N4 coating on Mo substrate oxidized at an temperature of 1600 1C in air were investigated. A coarse columnar crystal structure was observed in original dense (Mo,W)Si2–Si3N4 layer. The antioxidant life of (Mo,W)Si2–Si3N4 coating on Mo substrate at 1600 1C in air reached 360 h. During oxidation process, a SiO2 film was formed on the surface of coating, and the transition layers of (Mo,W)5Si3 and (Mo,W)3Si were formed between the coating and Mo substrate by Si diffusion. The growth kinetics of (Mo,W)5Si3 transition layer obeyed a parabolic rate law. The growth rate constant of the (Mo,W)5Si3 transition layer in (Mo, W)Si2–Si3N4 coating was 7.22 10 10 cm2/s, which was lower than that of Mo5Si3 transition layer in MoSi2–CrSi2–Si3N4 coating and MoSi2–Si3N4 coating. The addition of W element in the coating exhibited a strong hindering effect on the diffusion of Si between the coating and Mo substrate.
Please cite this article as: H. Zhang, et al., Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.012
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Fig. 5. BSE images of cross-section morphology of (Mo,W)Si2–Si3N4 coated samples after oxidation at 1600 1C in air for various times: (a) 48 h, (b) 144 h, (c) 288 h and (d) 360 h. The amplification micrographs of the selected area were presented in (b) and (d).
and Technology project of Fujian Provincial of China (No. 2014H0046), the Scientific Research Fund of Xiamen City of Fujian Provincial of China (No. 3502Z20143036) and the Education Department Science and Technology Project of Fujian Provincial of China (No. JB13149).
References
Fig. 6. Linear regression fits to the respective plots for grown thickness vs. the square root of oxidation times for (Mo,W)5Si3 transition layer in (Mo,W)Si2– Si3N4 coating, and Mo5Si3 transition layers in MoSi2–CrSi2–Si3N4 and MoSi2– Si3N4 coating by oxidized at 1600 1C in air.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51371155), the Key Science
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Please cite this article as: H. Zhang, et al., Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.012
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Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C Houan Zhangn, Jianxian Lv, Yihui Wu, Siyong Gu, Yu Huang, Ying Chen The Key Laboratory for Power Metallurgy Technology and Advanced Materials of Xiamen, Xiamen University of Technology, Xiamen 361024, PR China Received 5 March 2015; received in revised form 28 July 2015; accepted 4 August 2015
Abstract (Mo,W)Si2–Si3N4 composite coating on pure molybdenum (Mo) substrate was prepared by using in situ diffusion of tungsten (W), nitrogen (N) and silicon (Si) elements. The microstructures and phase compositions were investigated using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). A coarse columnar crystal structure was observed in the original dense (Mo, W)Si2–Si3N4 layer in the SEM micrographs. The transition layers of (Mo,W)5Si3 and (Mo,W)3Si were formed between the coating and Mo substrate during a oxidation process at 1600 1C in air. The growth kinetics of (Mo,W)5Si3 transition layer in (Mo,W)Si2–Si3N4 coating obeyed a parabolic rate law and its growth rate constant was 7.22 10 10 cm2/s. The strong hindering effect of W element in coating on the Si diffusion led to a longer antioxidation life of the (Mo,W)Si2–Si3N4 coating on Mo substrate at1600 1C. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Electron microscopy; C. Diffusion; D. Silicides; Oxidation
1. Introduction Molybdenum (Mo) is widely used in industrial applications because of its advanced properties, such as high melting point of 2617 1C, outstanding high-temperature strength and creep strength, good heat resistance, and low coefficient of thermal expansion. However, its applications at high temperatures are limited due to its extremely poor oxidation resistance in air above 600 1C [1,2]. Molybdenum disilicide (MoSi2) has an excellent high-temperature oxidation resistance, which makes it an attractive coating material to protect Mo [3,4], Ni [5], Nb [6], ZrB2 [7] and C/C composite [8,9], etc. used in an oxidative atmosphere at higher temperatures. The excellent oxidation resistance of MoSi2 coatings is due to the formation of an adherent, dense, continuous and self-healing SiO2 film on the surface, which protects the coating material from further oxidation. However, due to mismatch of coefficient of thermal expansion (CTE) between coating (8.5 10 6/K) and Mo n Correspondence to: School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, Fujian, PR China. Tel./fax: þ86 592 6291045. E-mail address: [email protected] (H. Zhang).
substrate (5.8 10 6/K) [10], cracks will be formed along the grain boundaries of MoSi2 coating perpendicular to the Mo substrate in the cooling-down process from the service temperature to room temperature. As a result, the failure of coatings is accelerated. Hsieh et al. [11] observed that the CTE of MoSi2– (30–35) vol% Si3N4 composites produced by hot isostatic pressing was close to that of Mo at a temperature range of 1000–1500 1C. Nagae et al. [12] and Yoon et al. [13] reported that the Mo2N coating could be easily formed by ammonia nitridation of Mo substrates at 1100 1C and Si3N4 phase appeared at crystal boundaries of MoSi2. Our recent research [14] showed that MoSi2–Si3N4 composite coating on Mo could be made by in situ diffusion of Mo ammonia nitriding followed by siliconizing, and the coating was crack-free during the cyclic oxidation process at 1450 1C in the ambient air atmosphere. This work confirmed that the addition of Si3N4 into MoSi2 coating decreased the mismatch of CTE between the coating and Mo substrate. Yoon et al. [15,16] and Byun et al. [17] studied the diffusion and growth mechanism of intermediate silicides in MoSi2/Mo and Mo5Si3/Mo system. They found Mo5Si3 layer and Mo3Si layer formed by diffusion reaction of the released silicon (Si) and Mo substrate in order, and indicated that Si was the dominant diffusion element in
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Please cite this article as: H. Zhang, et al., Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.012
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Fig. 1. XRD patterns of the (Mo,W)Si2–Si3N4 coating on Mo substrate surface by in situ diffusion reaction.
MoSi2, Mo5Si3 and Mo3Si phases. In our study [14], MoSi2– CrSi2–Si3N4 coating showed a better high temperature oxidation resistance than MoSi2–Si3N4 coating, which was attributed to chromium (Cr) element inhibiting the diffusion of Si and the formation of volatile MoO3. Above researches indicated that hindering the diffusion of silicon between the Mo substrate and the MoSi2 based coating was the key to increase the service life of the coating. Theoretical calculations on crystal binding energy of MoSi2 and WSi2 by Bhattacharyya et al. [18] were 1847.8 kJ/mol and 2058.1 kJ/mol, respectively, which indicated Si atom diffusion was more difficult in the W–Si bond than the Mo–Si bond. The diffusion layer of W/MoSi2 couple was thinner than that of Mo/ MoSi2 couple in Ref. [19], suggesting a hindering effect of W on the diffusion of Si. W element has an analogous crystal structure to Mo and its silicide [20], which provides a possibility to synthesize the (Mo,W)Si2–Si3N4 composite coating by in-situ diffusion of W, nitrogen (N) and Si elements as the formation of MoSi2–CrSi2– Si3N4 coating. However, there is no paper to report the oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on Mo substrate. Therefore, the objective of this paper is to discuss the effect of the W element on the high temperature oxidation resistance of (Mo,W) Si2–Si3N4 composite coating. 2. Experimental procedures Mo metal rods (99.95% purity) were cut into discs of Φ15 mm 3 mm and polished successively, up to a mirror-like surface using a colloidal silica suspension of 0.05 μm and a diamond paste of 1 μm. The polished discs were ultrasonically cleaned in alcohol and placed in an Al2O3 crucible inside a horizontal aluminum oxide tube furnace. The processes of (Mo, W)Si2–Si3N4 composite coating by in situ diffusion reaction was as followed. Firstly, pure Mo discs were pack tungstenized at 1200 1C for 10 h in Ar atmosphere. The powder pack consisted of 20 wt% W (tungsten source, 99.95% purity), 5 wt% NaF (activator) and balance 75 wt% alumina (inert filler). Secondly, ammonia nitridation of the samples was carried out at a reaction temperature of 1100 1C for 2 h. The purity and the flow rate of NH3 gas were 99.999% and 100 ml/min, respectively. Finally, the
silicide coating was applied by pack siliconizing at 1000 1C for 2 h in Ar atmosphere. The last powder pack consisted of 10 wt% Si (silicon source, 99.9% purity), 5 wt% NaF (activator) and 85 wt% alumina (inert filler). High temperature oxidation test was conducted at 1600 1C in air for a maximum duration of 360 h. Samples were placed in the hot zone of a furnace in a high-purity alumina crucible. The samples oxidized for a period of time were taken out from the furnace, followed by cooling to room temperature. The mass change during the high temperature oxidation was calculated by weighing a balance with a resolution of 0.1 mg. Microstructures and surface morphology of the coatings were carried out using scanning electron microscopy (SEM; S3400-N Hitachi). Simultaneously, the chemical compounds of the transition layers were analysis by the energy dispersive spectroscopy (EDS; 250 X-Max Oxford). A corrosive liquid (15 vol% HF þ 30 vol% HNO3 þ 55 vol% distilled H2O) was used to etch the cross-section of (Mo,W)Si2–Si3N4 composite coating for 30 s. The phase composition before and after oxidation was determined by X-ray diffraction (XRD; X' Pert PRO PANalytical), equipped with graphite monochromator using Cu Kα radiation. 3. Results and discussion XRD patterns of the Mo substrate surface by in situ diffusion reaction of sequential tungstenizing, ammonia nitrogenizing and siliconizing are shown in Fig. 1. It could be seen that the coating surface consisted of (Mo,W)Si2 and Si3N4 phases. Formation of (Mo,W)Si2 and Si3N4 phases could be divided into three stages: (1) During the pack tungstenizing, Mo–W infinite solid solution was formed on the basis of the Mo–W phase diagram. (2) During the ammonia nitrogenizing, the nitrides of W and Mo such as Mo2N and WN were synthesized in the coating because of the nitrogen diffusing reaction. (3) During the siliconizing, the reaction of W, Mo and N with Si resulted in the formation of MoSi2, WSi2 and Si3N4 in the coating as Si diffused into the nitridation layer. MoSi2 and WSi2 had the similar long-range order structures and the adjacent lattice parameters. When some Mo atoms in MoSi2 crystals were replaced by W atoms, (Mo, W)Si2 solid solutions were formed. These formation steps could be represented by the following equations: Moþ W-MoðWÞ solid solution
ð1Þ
2MoðWÞþ 3N-Mo2 N þ 2WN
ð2Þ
4Mo2 N þ 8WN þ 25Si-8ðMo; WÞSi2 þ 3Si3 N4
ð3Þ
Fig. 2(a) illustrated the cross-section morphology of the (Mo, W)Si2–Si3N4 composite coating and elements distribution of Mo, Si, W and N. The thickness of the coating layer tightly integrated with the substrate was measured about 250 μm, and the coating surface was divided into two layers: porous layer in the surface (about 35 μm) and dense layer in the middle. The formation of porous layer was thought to be due to the aggregation of a large number of Kirkendall voids [15]. The microstructures of the etched cross-section of (Mo,W)Si2–Si3N4
Please cite this article as: H. Zhang, et al., Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.012
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Fig. 2. Microstructures of (Mo,W)Si2–Si3N4 coating. (a) cross-section morphology and elements distribution; (b) columnar crystal structure of the selected area in (a) after etched.
An SiO2 film was formed on the surface of (Mo,W)Si2–Si3N4 coating during oxidation in air, as shown in Fig. 4(a). A few pores and cracks were observed on the surface of the coating, which could be healed by adherent SiO2 at a high temperature. The gas escaping of volatile oxides of W, Mo and N elements [10,23] resulted in the formation of pore and the weight loss of the coating. XRD patterns of the surface of (Mo,W)Si2–Si3N4 coated sample oxidized after 3 h and 144 h are illustrated in Fig. 4(b). It could be seen that the original (Mo,W)Si2 phase in coating surface had been transformed to (Mo,W)5Si3, which was attributed to the reaction occurred as the following: 5ðMo; WÞSi2 þ 7O2 -ðMo; WÞ5 Si3 þ 7SiO2 Fig. 3. Plots of mass change of (Mo,W)Si2–Si3N4 coated samples oxidized at 1600 1C in air.
composite coating are shown in Fig. 2(b). The dense (Mo,W) Si2–Si3N4 layer exhibited a columnar crystal structure, which was perpendicular to the Mo substrate. Similar structures had been reported in the MoSi2 coating prepared with CVD [21] and WSi2 coating prepared with the pack siliconizing method [22]. A thin transition layer (about 3 μm) was observed between the coating and Mo substrate, and the composition of the layer was (Mo,W)5Si3 phase as indicated in EDS. The mass variation of (Mo,W)Si2–Si3N4 coated samples at 1600 1C in air is illustrated in Fig. 3. There was a stable stage from 96 to 288 h during oxidation process where the mass change was small. Then an accelerated oxidation behavior was observed in the (Mo,W)Si2–Si3N4 coating until the coating failed after 360 h. As a comparison, oxidation properties of MoSi2–CrSi2–Si3N4 and MoSi2–Si3N4 coatings at the same conditions in Ref. [14] are also presented in Fig. 3. The oxidation of MoSi2–CrSi2–Si3N4 and MoSi2–Si3N4 coatings accelerated at about 96 h and 72 h. The antioxidant life of (Mo,W)Si2–Si3N4 coating at 1600 1C in air was about 3.75 times and 5 times longer than that of MoSi2–CrSi2– Si3N4 and MoSi2–Si3N4 coatings, respectively.
ð4Þ
SiO2 formed in the reaction appeared in amorphous form on the surface of coating. With the oxidation time prolonging, the peak intensity of (Mo,W)5Si3 phase increased, indicating that the oxidation process of the coating was promoted. As the oxidation time increased, the microstructures of (Mo, W)Si2–Si3N4 coating changed. Fig. 5 showed the backscattered electron (BSE) images of the cross-section morphology of the coating after oxidation at 1600 1C for various periods of time. When oxidized for 48 h, the coating exhibited three layers, which were marked as I, II, and III from the coating surface to the inside in Fig. 5(a). Results of EDS analysis suggested that I layer was mainly composed of SiO2 and (Mo,W)5Si3. II layer was composed of (Mo,W)Si2, while III layer was a transition layer and composed of (Mo,W)5Si3 with a lower atomic fraction of W. (Mo,W)5Si3 in I layer was formed by the reaction (4). However, the formation of (Mo,W)5Si3 in III layer at the interface of (Mo,W)Si2/Mo was mainly attributed to the Si diffusion. Referring to the literature [17], the growth of (Mo, W)5Si3 transition layer could be explained as four steps: (1) the (Mo,W)Si2 in II layer decomponsed into (Mo,W)5Si3 and Si; (2) the released Si diffused to the interface (Mo,W)5Si3/Mo; (3) with the interaction between the released Si and Mo substrate, the Mo5Si3 was formed; (4) Mo5Si3 solutionized into (Mo, W)5Si3 in step (1), then a layer of (Mo,W)5Si3 with a lower atomic fraction of W was finally formed. These formation steps
Please cite this article as: H. Zhang, et al., Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.012
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Fig. 4. Micrograph (a) and XRD patterns (b) of the surface of (Mo,W)Si2– Si3N4 coating oxidized at 1600 1C for 3 h in air.
could be represented by the following equations: 5ðMo; WÞSi2 -ðMo; WÞ5 Si3 þ 7Si
ð5Þ
35 7 Moþ 7Si- Mo5 Si3 3 3
ð6Þ
7 7ð1 xÞ 7 Mo5 Si3 þ ðMo; WÞ5 Si3 ðMox ; W1 x Þ5 Si3 3 3ð2x 1Þ 3ð2x 1Þ ð7Þ where x was the atomic fraction of Mo, and x was larger than 1 x. Some pores emerged in the III layer were located parallel to the interface of (Mo,W)5Si3/(Mo,W)Si2, which was attributed to the intrinsic Si diffusion inducing a Kirkendall effect [24]. As the oxidation time increased, the thickness of III layer increased while the thickness of II layer decreased, and a (Mo,W)3Si phase (now marked as IV) was formed at the (Mo,W)5Si3/Mo interfaces. The growth of (Mo,W)3Si layer (IV) was similar to the growth of (Mo,W)5Si3 transition layer at interface of (Mo, W)Si2/Mo, as described in Ref. [17]. The pores observed in III layer were filled with SiO2, which was due to the good flowability and self-healing property of SiO2 at high temperature. When oxidized for 288 h, the distribution of the (Mo,W) Si2 layer became discontinuous and cracks appeared in II and III layers in Fig. 5(c), and the mass change began to decrease significantly as shown in Fig. 3. It was found in Fig. 5(d) that a
very thin and discontinuous (Mo,W)Si2 layer (II layer) and a large number of micro-pores appeared in the interfaces of (Mo, W)3Si/Mo. As these micro-pores grew till to be connected with each other, the coating peeled off from the surface of Mo substrate. Fig. 6 showed the linear regression fit to the plots for grown thickness of (Mo,W)5Si3 transition layer (III layer in Fig. 5)) in (Mo,W)Si2–Si3N4 coating, Mo5Si3 transition layer in MoSi2– CrSi2–Si3N4 and MoSi2–Si3N4 coating by oxidation at 1600 1C in air. Each point in Fig. 6 was the average value of 10 measurements. Their growth kinetics obeyed a parabolic rate law. The data fitting agreed with the results reported in Refs. [6,16,24]. The linear regression fitting of the coating layers suggested that the growth rate constant of the (Mo,W)5Si3 transition layer in (Mo,W)Si2–Si3N4 coating was 7.22 10 10 cm2/s, and the growth rate constants of Mo5Si3 transition layer in MoSi2–CrSi2–Si3N4 coating and MoSi2–Si3N4 coating were 2.64 10 9 cm2/s and 3.47 10 9 cm2/s, respectively. A similar value of growth rate constant of Mo5Si3 transition layer in MoSi2 coating has been reported as 3.54 10 9 cm2/s [15]. The growth rate constant of the (Mo,W)5Si3 transition layer in (Mo,W)Si2– Si3N4 coating was about twenty times lower than that of Mo5Si3 layer in MoSi2 coating. Zmii et al. [25] also reported that the transition rate at 1700 1C for molybdenum silicide on tungsten from MoSi2 to (Mo,W)5Si3 was about twenty times slower than that from MoSi2 to Mo5Si3. Meanwhile, the growth rate constant of the (Mo,W)5Si3 transition layer in (Mo,W)Si2–Si3N4 coating was about 3.65 times lower than that of Mo5Si3 layer in MoSi2– CrSi2–Si3N4 coating, and 4.81 times lower than that of MoSi2– Si3N4 coating. The above ratios of growth rate constants of the transition layers were very close to the relative length value of antioxidant life for their corresponding coatings as shown in Fig. 3. It indicated the antioxidant life of the MoSi2 based coatings on Mo substrate was closely related to the growth of Mo5Si3 or (Mo, W)5Si3 transition layer. The lower growth rate constant of the (Mo, W)5Si3 transition layer in (Mo,W)Si2–Si3N4 coating exhibited the addition of W element had a strong hindering effect to the diffusion of Si between the coating and Mo substrate. 4. Conclusions Microstructural characteristics of (Mo,W)Si2–Si3N4 coating on Mo substrate oxidized at an temperature of 1600 1C in air were investigated. A coarse columnar crystal structure was observed in original dense (Mo,W)Si2–Si3N4 layer. The antioxidant life of (Mo,W)Si2–Si3N4 coating on Mo substrate at 1600 1C in air reached 360 h. During oxidation process, a SiO2 film was formed on the surface of coating, and the transition layers of (Mo,W)5Si3 and (Mo,W)3Si were formed between the coating and Mo substrate by Si diffusion. The growth kinetics of (Mo,W)5Si3 transition layer obeyed a parabolic rate law. The growth rate constant of the (Mo,W)5Si3 transition layer in (Mo, W)Si2–Si3N4 coating was 7.22 10 10 cm2/s, which was lower than that of Mo5Si3 transition layer in MoSi2–CrSi2–Si3N4 coating and MoSi2–Si3N4 coating. The addition of W element in the coating exhibited a strong hindering effect on the diffusion of Si between the coating and Mo substrate.
Please cite this article as: H. Zhang, et al., Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.012
H. Zhang et al. / Ceramics International ] (]]]]) ]]]–]]]
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Fig. 5. BSE images of cross-section morphology of (Mo,W)Si2–Si3N4 coated samples after oxidation at 1600 1C in air for various times: (a) 48 h, (b) 144 h, (c) 288 h and (d) 360 h. The amplification micrographs of the selected area were presented in (b) and (d).
and Technology project of Fujian Provincial of China (No. 2014H0046), the Scientific Research Fund of Xiamen City of Fujian Provincial of China (No. 3502Z20143036) and the Education Department Science and Technology Project of Fujian Provincial of China (No. JB13149).
References
Fig. 6. Linear regression fits to the respective plots for grown thickness vs. the square root of oxidation times for (Mo,W)5Si3 transition layer in (Mo,W)Si2– Si3N4 coating, and Mo5Si3 transition layers in MoSi2–CrSi2–Si3N4 and MoSi2– Si3N4 coating by oxidized at 1600 1C in air.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51371155), the Key Science
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Please cite this article as: H. Zhang, et al., Oxidation behavior of (Mo,W)Si2–Si3N4 composite coating on molybdenum substrate at 1600 1C, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.012