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MoB composite coating for Mo substrate
International Journal of Refractory Metals & Hard Materials 68 (2017) 60–64
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
High temperature oxidation and microstructure of MoSi2/MoB composite coating for Mo substrate
MARK
Yi Wanga, Jianhui Yana,b, Dezhi Wangc,d,e,⁎ a Hunan Provincial Key Defense Laboratory of High Temperature Wear-resisting Materials and Preparation Technology, Hunan University of Science and Technology, Xiangtan 411201, China b State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China c Key Laboratory of Ministry of Education for Non-ferrous Materials Science and Engineering, Central South University, Changsha 410083, China d Key Laboratory of Hunan Province for Metallurgy and Material Processing of Rare Metals, Central South University, Changsha 410083, China e School of Materials Science and Engineering, Central South University, Changsha 410083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: MoSi2/MoB composite coating Oxidation Microstructure Mo5Si3 Mo substrate
The two-layer MoSi2/MoB composite coatings were developed using the halide activated pack cementation (HAPC) method on Mo substrate. Oxidation resistance property and microstructural evolution of the coatings at high temperatures were investigated. During oxidation exposure, the coatings exhibited a good oxidation resistance property. The mass gains of the coated specimens oxidized at 1200 °C for 100 h and at 1300 °C for 80 h were 0.270 and 0.499 mg/cm2, respectively. Compared with the monolithic MoSi2 coatings, the transformation of MoSi2 phase in the MoSi2/MoB composite coatings was more sluggish at elevated temperatures. The growth rate constant of the Mo5Si3 layer in the composite coatings was two orders of magnitude lower than that of the Mo5Si3 layer in the monolithic coatings at 1300 °C. The microstructural degradation of MoSi2 in the composite coatings at high temperatures was slowed by the introduced MoB layer. The MoB layer in the composite coatings is useful to prolong the service life of MoSi2 coatings at high temperatures.
1. Introduction Refractory metal molybdenum and its alloys are considered as a superior structural material for applications at elevated temperatures up to 1500 °C due to their outstanding properties [1]. However, their poor oxidation resistance in air above 600 °C has severely restricted the utilization of Mo and its alloys [2]. Thus, the oxidation protection is very necessary for their applications in oxygen-containing environment. Currently, various anti-oxidation coatings have been developed to improve the high temperature oxidation resistance of these alloys. Molybdenum disilicide (MoSi2), which can form a protective glass silica (SiO2) layer at its surface in oxidative environment at high temperatures, is a promising coating material for the protection of molybdenum and molybdenum-based alloys [3–8]. However, the diffusion between MoSi2 coating and Mo substrate at elevated temperatures greatly weaken its oxidation resistance property. The studies in the MoSi2/Mo and Mo5Si3/Mo diffusion couples indicated that Si was the dominant diffusion element in MoSi2, Mo5Si3 and Mo3Si phases [9–11]. Oxidation forms rapidly when MoSi2 phase completely degrades into intermediate silicides (Mo5Si3 and/or Mo3Si phases) since the silicides do not form a protective silica layer. Thus, the lifetime of MoSi2 coating ⁎
on Mo substrate depended on the degradation rate of MoSi2 phase. Slowing the degradation rate of MoSi2 phase is a significant way to enhance the lifetime of MoSi2 coating. To improve the oxidation resistance of MoSi2 coating, varied silicide composite coatings were developed recently. A MoSi2/SiC-Mo2C gradient composite coating on Mo was prepared by in situ reaction [12]. It was found that the Mo2C could delay the diffusion rate of Si. The addition of W element postponed the diffusion rate of Si toward Mo substrate, and increased antioxidation lifetime of (Mo,W)Si2-Si3N4 composite coating at high temperatures [13]. However, the study on blocking the diffusion at high temperatures and prolonging the service lifetime has not been sufficiently investigated. Therefore, further retarding the degradation rate of MoSi2 phase to achieve a better antioxidation lifetime of the coating at elevated temperatures is still interesting topic. In the previous work, we found that MoB could act the role of the diffusion blocking between MoSi2 coating and Mo substrate at high temperatures [14], but there are still many issues need to be further considered, such as how the introduction of MoB influence the growth of intermediate silicide (Mo5Si3 phase) in the coating and the oxidation resistance property of the coating. Therefore, the objective of this work is to investigate
Corresponding author at: Key Laboratory of Ministry of Education for Non-ferrous Materials Science and Engineering, Central South University, Changsha 410083, China. E-mail address: [email protected] (D. Wang).
http://dx.doi.org/10.1016/j.ijrmhm.2017.06.008 Received 22 February 2017; Received in revised form 29 May 2017; Accepted 28 June 2017 Available online 01 July 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 68 (2017) 60–64
Y. Wang et al.
oxidation behavior of MoSi2/MoB composite coating at elevated temperatures and assess the oxidation resistance property of the coating by exploring the growth of Mo5Si3 in the coating and monolithic MoSi2 coating.
Table 1 Oxidation rate constant at different temperature for monolithic MoSi2 coating and MoSi2/ MoB composite coating. ko (mg2/cm4·h)
Coating type
1200 °C
2. Experimental Monolithic MoSi2 coating MoSi2/MoB composite coating
Mo was used as substrates for coating experiments. Samples with the dimension of Ø18 mm × 2 mm were obtained by electric-discharge machining. Subsequently, the surfaces and edges of the samples were polished, then ultrasonically cleaned in ethanol and dried. The MoSi2/ MoB composite coating was obtained by a two-step pack cementation method. Firstly, an inner MoB layer was prepared through boronizing process at 1000 °C for 10 h in alumina crucible. The pack mixture consisted of 0.8 wt% B, 5 wt% NaF (activator) and balance Al2O3 (inter filler). Subsequently, a MoSi2 layer on the upper part of the initially prepared MoB layer by siliconizing process in the mixture of 20 wt% Si, 5 wt% NaF (activator) and balance Al2O3 (inter filler) at 1000 °C for 10 h. After boronizing followed siliconizing in argon atmosphere, a MoSi2/MoB composite coating was achieved on Mo substrate. As the contrast, the other samples with a monolithic MoSi2 coating were prepared by siliconizing to probe the effort of diffusion blocking of MoB layer. Oxidation test of the coated samples was carried out in a box type furnace at 1200 °C and 1300 °C in ambient air. The samples were put into the box type furnace and heated up at first oxidation period. At the designated time, the samples were taken out and cooled to room temperature in air for weight. Then, the samples were directly put into the hot zone of the furnace again for the next oxidation period. Weights of the samples were measured on an electronic balance with sensitivity of ± 0.1 mg. Cumulative weight changes of the samples were calculated and reported as a function of the oxidation time. The morphology and cross-section microstructure of the coated samples were examined by using a scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDS) and electron probe microanalyzer (EMPA) with wavelength-dispersive spectrometer (WDS) was used to determine chemical composition of the coatings. The phase constitution of the coatings in the surface was identified by using an Xray diffraction (XRD) measurement.
1300 °C −4
5.42 × 10 7.80 × 10− 4
1.31 × 10− 3 3.14 × 10− 3
50 h was less than 0.16 mg/cm2. The maximum gain of 0.356 mg/cm2 in weight for the MoSi2 coated TZM during 6 times cyclic oxidation at 1300 °C in air was else reported somewhere [16]. Thus, the as-prepared composite coating showed a good oxidation resistant protective ability at high temperatures and it could provide long-term protection for Mo. Furthermore, the oxidation rate constant could be obtained by Eq. (1), which shows the relationships between squared mass change per unit area of coated Mo and oxidation time:
Δm 2 ⎛ ⎞ = ko t ⎝ S ⎠
(1)
where Δ m is the mass change of coated Mo before and after oxidation, S and ko are the surface area and oxidation rate constant of coated Mo, respectively, and t is oxidation time. The values of oxidation rate constant at different temperature were acquired by calculating the slopes of straight lines, which gained by Δm 2
( )
fitting data points of S and t. The values were listed in Table 1. Oxidation rate varied at different oxidation temperature. The results showed the oxidation rate constant increased with the increase of oxidation temperature. The oxidation rate constants of composite coating at 1200 °C and 1300 °C were 7.80 × 10− 4 and 3.14 × 10− 3 mg2/ (cm4·h), respectively. The value at 1200 °C closes to that reported by Chakraborty et al. (about 4.23 × 10− 4 mg2/(cm4·h) which calculated by the data of the paper [15]). The surface XRD patterns of coated Mo oxidized at 1200 °C for 100 h are shown in Fig. 2. The tetragonal MoSi2 (JCPDS card no. 410612) and tetragonal Mo5Si3 (JCPDS card no. 34-0371) phases were detected in both coatings (a and b in Fig. 2). The tetragonal SiO2 (JCPDS card no. 82-0512) was observed by the local magnified XRD patterns (c and d in Fig. 2). The SiO2 film covered the outer surface of the coatings after oxidation and provided a good oxidation resistant protection. Based on the results of XRD analysis, the coating reacted with oxygen in air during oxidation according to Eq. (2).
3. Results and discussion Fig. 1 shows the mass change of coated Mo samples during isothermal oxidation test. A weight gain was observed during the oxidation of coated Mo samples. The results showed that the mass change of the Mo sample coated MoSi2/MoB composite coating was 0.270 and 0.499 mg/cm2 after isothermal oxidation at 1200 °C for 100 h and 1300 °C for 80 h, respectively. Chakraborty et al. [15] reported that the mass change of the MoSi2 coated TZM after oxidation at 1200 °C for
5MoSi2 (s) + 7O2 (g) = Mo5 Si3 (s) + 7SiO2 (s)
(2)
Fig. 3 shows the cross-sectional microstructural BSE images of the silicide-based coatings after oxidation. The microstructure of MoSi2/ MoB composite coating and monolithic MoSi2 coating evolved after oxidation exposure. Continuous SiO2 layer formed on the surface of the
Fig. 1. Mass changes of the coated samples oxidized at 1200 °C (a) and 1300 °C (b) in air.
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International Journal of Refractory Metals & Hard Materials 68 (2017) 60–64
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Fig. 2. XRD patterns of the surface of the coated Mo at 1200 °C in air. a- monolithic MoSi2 coating; b- MoSi2/MoB composite coating; c and d are local magnified patterns of a and b, respectively.
contributed the extensive formation of Kirkendall voids. Moreover, a thin Mo2B layer in the interface of MoB/Mo was detected by EPMA analysis. To further investigate the transformation of microstructure of MoSi2/MoB composite coating after oxidation, the line-scanning of cross-section elemental distributions was characterized by EPMA. Fig. 4 reveals the variations of Mo, Si, B and O. It can be observed that the elements distribute consecutively. Along the direction from the surface of the coating to the substrate, the coating and substrate can be divided into seven varied parts, marked as 1, 2, 3, 4, 5, 6 and 7. Based on the quantitative analysis of EPMA, SiO2, MoSi2, Mo5Si3, MoB, Mo2B, Mo and Mo5Si3 corresponded to 1–7 layers in Fig. 4, respectively. Due to the oxidation reaction between MoSi2 and O2, SiO2 (corresponding to 1 in Fig. 4) and Mo5Si3 (corresponding to 7 in Fig. 4) layers formed on the surface of MoSi2. The transformation of MoSi2 and the Si diffusion resulted in the formation of Mo5Si3 layer below MoSi2 (corresponding to the third layer in Fig. 4). In addition, the transformation of MoB and the B diffusion resulted in the formation of Mo2B layer under MoB (corresponding to the fifth layer in Fig. 4). Fig. 5 shows the relationship between the thickness of Mo5Si3 layer in coated Mo and square root of time at 1300 °C. Their growth kinetics obeyed a parabolic rate law, which reported in the diffusion couple study of MoSi2-based composites [9,18,20], as shown in Eq. (3):
coatings, blocked oxygen in air and led to the transformation of the oxidation behavior of coatings from a rapid linear oxidation to a slow parabolic oxidation. The SiO2 film reduced the oxidation rate and provided a good protection for Mo. The light gray phase layer, which was identified as Mo5Si3 phase by EDS and EPMA analyses, could be observed beneath the continuous SiO2 layer. Light gray phase, which distributed in MoSi2 layer randomly, was also identified as Mo5Si3 phase. The diffusion rate of Si is faster than Mo in the Mo-Si system. As reported by Prasad and Paul [17], the diffusion coefficients of Si and Mo through Mo5Si3 phase at 1250 °C and 1300 °C are 1.05 × 10− 11, 1.02 × 10− 13 cm2/s and 2.0 × 10− 11, 1.94 × 10− 13 cm2/s, respectively. The diffusion coefficients of Si through MoSi2 phase at 1200 °C and 1300 °C are 1.59 × 10− 10 and 3.15 × 10− 10 cm2/s, respectively. The fast diffusion of Si causes the formation of Mo5Si3 phase. The results show that the diffusion rates of Si are different in the coatings. Most MoSi2 in the monolithic coating converted into intermediate silicides (Mo5Si3 and Mo3Si) (Fig. 3a). However, only partial MoSi2 in the composite coating developed into intermediate silicide (Fig. 3b). The Si diffusion in the coatings caused the microstructure evolution from MoSi2 developed into intermediate silicides (Mo5Si3 and/or Mo3Si). The evolution of microstructure was also reported elsewhere [9,18]. The growth rate of Mo3Si layer is slower than that of Mo5Si3 layer under the condition of the growth of Mo5Si3 layer (Fig. 3a), which was also reported in other studies [9,10,18–20]. A certain amount of Kirkendall voids were observed in the coatings. And it was obvious that the number of the voids in the monolithic MoSi2 coating was more than that in the composite coating. The tremendous disparity in the diffusivities of Mo and Si and the consequent unequal mass flow in the coatings resulted in the development of such voids [20]. The fast diffusion of Si in the monolithic MoSi2 coating not only caused the rapid development of intermediate silicides (Mo5Si3 and Mo3Si), but
x 2 = kp t
(3)
where x is the thickness of the Mo5Si3 layer, kp is the parabolic growth rate constant of the Mo5Si3 layer, and t is annealing time. The growth rate of Mo5Si3 layer in monolithic MoSi2 coating is faster than that in MoSi2/MoB composite coating in this work, especially when the duration time exceeds 20 h. As shown from Fig. 5, two periods of the growth of Mo5Si3 layer are observed in the composite Fig. 3. Cross-sectional BSE images of the coatings on Mo after oxidation at 1200 °C for 100 h. a- MoSi2; b- MoSi2/ MoB.
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Y. Wang et al.
of Mo5Si3 layer in the composite coating was far below that in the monolithic MoSi2 coating. Combined with the isothermal phase diagram of the Mo-Si-B ternary system [21] and our previous work [14], Mo5SiB2 (T2) phase formed in the coating due to diffusion reaction. Moreover, the diffusivity for Si in the T2 phase is far below that in the Mo5Si3 [22]. Thus, the growth of Mo5Si3 in thickness was arrested after introducing MoB layer. In other words, the diffusion of Si in the composite coating was postponed availably. Compared with monolithic MoSi2 coating, MoSi2/MoB composite coating can provide a better oxidation protection for Mo. 4. Conclusions MoSi2/MoB composite coating was prepared by a two-step pack cementation method. During oxidation at high temperatures, the coating exhibited a good oxidation resistance. The weight gain rates of the oxidized coated samples at 1200 °C and 1300 °C are 0.270 and 0.499 mg/cm2, respectively. Oxidation rate increases with the increase of oxidation temperature. Oxidation rate constants of the coating at 1200 °C and 1300 °C are 7.80 × 10− 4 and 3.14 × 10− 3 mg2/cm4·h, respectively. The growth of Mo5Si3 layer in the composite coating was very slow, which was two orders of magnitude lower than that in the monolithic MoSi2 coating. The introduction of MoB layer effectively controlled the Si diffusion in the coating and endued MoSi2/MoB composite coating with a good oxidation protective for Mo substrate at high temperatures.
Fig. 4. Elements concentration profiles measured by EMPA in the composite coating oxidized at 1200 °C.
Acknowledgements The financial support of National Nature Science Foundation of China (Grant No. 51475161), the Research Foundation of Education Bureau of Hunan Province (Grant No. 15A059), and Scientific Research Project of Hunan University of Science and Technology (Grant No. E51511) are gratefully acknowledged. The work is also supported by State Key Laboratory of Powder Metallurgy of Central South University. We also thank Advanced Materials Synthesis and Application Technology Laboratory (Hunan University of Science and Technology), the Key Laboratory of Non-ferrous Materials Science and Engineering (Central South University), Ministry of Education, for research facilities.
Fig. 5. Relationship between the thicknesses of Mo5Si3 layer in coated Mo and the square root of time at 1300 °C.
References coating. Moreover, the growth rate of Mo5Si3 layer under the line segment 2 (solid line) is much less than that under the line segment 1(broken line). The extrapolated curves of thickness of Mo5Si3 layer do not pass through zero. The thickness of Mo5Si3 layer in the both coatings at the duration of 0 h is greater than zero, which is explained as the growth of Mo5Si3 layer in coatings develops during heating and cooling stages. To assess the parabolic growth rate constant by calculating slope of curve precisely, the Eq. (3) was adjusted appropriately followed as:
(x − x 0 )2 = k p t
[1] B.V. Cockeram, Measuring the fracture toughness of molybdenum-0.5 pct titanium0.1 pct zirconium and oxide dispersion-strengthened molybdenum alloys using standard and subsized bend specimens, Metall. Mater. Trans. A 33 (2002) 3685–3707. [2] M.Z. Alam, B. Venkataraman, B. Sarma, D.K. Das, MoSi2 coating on Mo substrate for short-term oxidation protection in air, J. Alloys Compd. 487 (2009) 335–340. [3] A. Mueller, G. Wang, R.A. Rapp, E.L. Courtright, Deposition and cyclic oxidation behavior of a protective (Mo,W)(Si,Ge)2 coating on Nb-base alloys, J. Electrochem. Soc. 139 (1992) 1266–1275. [4] Y.L. Jeng, E.J. Lavernia, Processing of molybdenum disilicide, J. Mater. Sci. 29 (1994) 2557–2571. [5] J.K. Yoon, G.H. Kim, J.Y. Byun, J.K. Lee, H.S. Yoon, K.T. Hong, Effect of Cl/H input ratio on the growth rate of MoSi2 coatings formed by chemical vapor deposition of Si on Mo substrates from SiCl4-H2 precursor gases, Surf. Coat. Technol. 172 (2003) 176–183. [6] K. Krnel, D. Sciti, E. Landi, A. Bellosi, Surface modification and oxidation kinetics of hot-pressed AlN–SiC–MoSi2 electroconductive ceramic composite, Appl. Surf. Sci. 210 (2003) 274–285. [7] H. Zhang, S. Gu, Preparation and oxidation behavior of MoSi2–CrSi2–Si3N4 composite coating on Mo substrate, Int. J. Refract. Met. Hard 41 (2013) 128–132. [8] M. Samadzadeh, C. Oprea, H. Karimi Sharif, T. Troczynski, Comparative studies of the oxidation of MoSi2 based materials: low-temperature oxidation (300–900 °C), Int. J. Refract. Met. Hard 66 (2017) 11–20. [9] J.K. Yoon, J.K. Lee, K.H. Lee, J.Y. Byun, G.H. Kim, K.T. Hong, Microstructure and growth kinetics of the Mo5Si3 and Mo3Si layers in MoSi2/Mo diffusion couple, Intermet 11 (2003) 687–696. [10] H.A. Chatilyan, S.L. Kharatyan, A.B. Harutyunyan, Diffusion annealing of Mo/ MoSi2 couple and silicon diffusivity in Mo5Si3 layer, Mater. Sci. Eng. A 459 (2007) 227–232.
(4)
The values of x0 in monolithic MoSi2 coating and MoSi2/MoB composite coating are 2.61, 0.17 (line segment 1 in Fig. 5) and 14.88 μm (line segment 2 in Fig. 5), respectively. The parabolic growth rate constants in the monolithic and composite coatings are 9.57 × 10− 11, 3.78 × 10− 11 (line segment 1 in Fig. 5) and 5.15 × 10− 13 cm2/s (line segment 2 in Fig. 5), respectively. As shown from the values, the kp in MoSi2/MoB composite coating is lower than that in monolithic MoSi2 coating during the whole time, only the difference of kp in the two coatings at the duration time within 20 h is very small. With the increase of the time, the growth rate of Mo5Si3 layer in the composite coating dramatically decreased. The parabolic growth rate constant of Mo5Si3 layer in the composite coating is two orders of magnitude lower than that in the monolithic MoSi2 coating. The growth 63
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[17] S. Prasad, A. Paul, Growth mechanism of phases by interdiffusion and atomic mechanism of diffusion in the molybdenum–silicon system, Intermet 19 (2011) 1191–1200. [18] J.K. Yoon, G.H. Kim, J.Y. Byun, J.S. Kim, C.S. Choi, Simultaneous growth mechanism of intermediate silicides in MoSi2/Mo system, Surf. Coat. Technol. 148 (2001) 129–135. [19] J.K. Yoon, J.Y. Byun, G.H. Kim, J.S. Kim, C.S. Choi, Growth kinetics of three Mosilicide layers formed by chemical vapor deposition of Si on Mo substrate, Surf. Coat. Technol. 155 (2002) 85–95. [20] J.Y. Byun, J.K. Yoon, G.H. Kim, J.S. Kim, C.S. Choi, Study on reaction and diffusion in the Mo–Si system by ZrO2 marker experiments, Scr. Mater. 46 (2002) 537–542. [21] T. Hayashi, K. Ito, H. Numakura, Reaction diffusion of MoSi2 and Mo5SiB2, Intermet 13 (2005) 93–100. [22] J.H. Perepezko, R. Sakidja, Oxidation-resistant coatings for ultra-high-temperature refractory Mo-based alloys, JOM 62 (2010) 13–19.
[11] S.L. Kharatyan, H.A. Chatilyan, G.S. Galstyan, Growth kinetics of Mo3Si layer in the Mo5Si3/Mo diffusion couple, Thin Solid Films 516 (2008) 4876–4881. [12] J. Liu, Q. Gong, Y. Shao, D. Zhuang, J. Liang, In-situ fabrication of MoSi2/SiC–Mo2C gradient anti-oxidation coating on Mo substrate and the crucial effect of Mo2C barrier layer at high temperature, Appl. Surf. Sci. 308 (2014) 261–268. [13] H. Zhang, J. Lv, Y. Wu, S. Gu, Y. Huang, Y. Chen, Oxidation behavior of (Mo,W) Si2–Si3N4 composite coating on molybdenum substrate at 1600 °C, Ceram. Int. 41 (2015) 14890–14895. [14] Y. Wang, D. Wang, J. Yan, Preparation and characterization of MoSi2/MoB composite coating on Mo substrate, J. Alloys Compd. 589 (2014) 384–388. [15] S.P. Chakraborty, S. Banerjee, I.G. Sharma, A.K. Suri, Development of silicide coating over molybdenum based refractory alloy and its characterization, J. Nucl. Mater. 403 (2010) 152–159. [16] S. Majumdar, I.G. Sharma, S. Raveendra, I. Samajdar, P. Bhargava, In situ chemical vapour co-deposition of Al and Si to form diffusion coatings on TZM, Mater. Sci. Eng. A 492 (2008) 211–217.
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Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
High temperature oxidation and microstructure of MoSi2/MoB composite coating for Mo substrate
MARK
Yi Wanga, Jianhui Yana,b, Dezhi Wangc,d,e,⁎ a Hunan Provincial Key Defense Laboratory of High Temperature Wear-resisting Materials and Preparation Technology, Hunan University of Science and Technology, Xiangtan 411201, China b State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China c Key Laboratory of Ministry of Education for Non-ferrous Materials Science and Engineering, Central South University, Changsha 410083, China d Key Laboratory of Hunan Province for Metallurgy and Material Processing of Rare Metals, Central South University, Changsha 410083, China e School of Materials Science and Engineering, Central South University, Changsha 410083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: MoSi2/MoB composite coating Oxidation Microstructure Mo5Si3 Mo substrate
The two-layer MoSi2/MoB composite coatings were developed using the halide activated pack cementation (HAPC) method on Mo substrate. Oxidation resistance property and microstructural evolution of the coatings at high temperatures were investigated. During oxidation exposure, the coatings exhibited a good oxidation resistance property. The mass gains of the coated specimens oxidized at 1200 °C for 100 h and at 1300 °C for 80 h were 0.270 and 0.499 mg/cm2, respectively. Compared with the monolithic MoSi2 coatings, the transformation of MoSi2 phase in the MoSi2/MoB composite coatings was more sluggish at elevated temperatures. The growth rate constant of the Mo5Si3 layer in the composite coatings was two orders of magnitude lower than that of the Mo5Si3 layer in the monolithic coatings at 1300 °C. The microstructural degradation of MoSi2 in the composite coatings at high temperatures was slowed by the introduced MoB layer. The MoB layer in the composite coatings is useful to prolong the service life of MoSi2 coatings at high temperatures.
1. Introduction Refractory metal molybdenum and its alloys are considered as a superior structural material for applications at elevated temperatures up to 1500 °C due to their outstanding properties [1]. However, their poor oxidation resistance in air above 600 °C has severely restricted the utilization of Mo and its alloys [2]. Thus, the oxidation protection is very necessary for their applications in oxygen-containing environment. Currently, various anti-oxidation coatings have been developed to improve the high temperature oxidation resistance of these alloys. Molybdenum disilicide (MoSi2), which can form a protective glass silica (SiO2) layer at its surface in oxidative environment at high temperatures, is a promising coating material for the protection of molybdenum and molybdenum-based alloys [3–8]. However, the diffusion between MoSi2 coating and Mo substrate at elevated temperatures greatly weaken its oxidation resistance property. The studies in the MoSi2/Mo and Mo5Si3/Mo diffusion couples indicated that Si was the dominant diffusion element in MoSi2, Mo5Si3 and Mo3Si phases [9–11]. Oxidation forms rapidly when MoSi2 phase completely degrades into intermediate silicides (Mo5Si3 and/or Mo3Si phases) since the silicides do not form a protective silica layer. Thus, the lifetime of MoSi2 coating ⁎
on Mo substrate depended on the degradation rate of MoSi2 phase. Slowing the degradation rate of MoSi2 phase is a significant way to enhance the lifetime of MoSi2 coating. To improve the oxidation resistance of MoSi2 coating, varied silicide composite coatings were developed recently. A MoSi2/SiC-Mo2C gradient composite coating on Mo was prepared by in situ reaction [12]. It was found that the Mo2C could delay the diffusion rate of Si. The addition of W element postponed the diffusion rate of Si toward Mo substrate, and increased antioxidation lifetime of (Mo,W)Si2-Si3N4 composite coating at high temperatures [13]. However, the study on blocking the diffusion at high temperatures and prolonging the service lifetime has not been sufficiently investigated. Therefore, further retarding the degradation rate of MoSi2 phase to achieve a better antioxidation lifetime of the coating at elevated temperatures is still interesting topic. In the previous work, we found that MoB could act the role of the diffusion blocking between MoSi2 coating and Mo substrate at high temperatures [14], but there are still many issues need to be further considered, such as how the introduction of MoB influence the growth of intermediate silicide (Mo5Si3 phase) in the coating and the oxidation resistance property of the coating. Therefore, the objective of this work is to investigate
Corresponding author at: Key Laboratory of Ministry of Education for Non-ferrous Materials Science and Engineering, Central South University, Changsha 410083, China. E-mail address: [email protected] (D. Wang).
http://dx.doi.org/10.1016/j.ijrmhm.2017.06.008 Received 22 February 2017; Received in revised form 29 May 2017; Accepted 28 June 2017 Available online 01 July 2017 0263-4368/ © 2017 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 68 (2017) 60–64
Y. Wang et al.
oxidation behavior of MoSi2/MoB composite coating at elevated temperatures and assess the oxidation resistance property of the coating by exploring the growth of Mo5Si3 in the coating and monolithic MoSi2 coating.
Table 1 Oxidation rate constant at different temperature for monolithic MoSi2 coating and MoSi2/ MoB composite coating. ko (mg2/cm4·h)
Coating type
1200 °C
2. Experimental Monolithic MoSi2 coating MoSi2/MoB composite coating
Mo was used as substrates for coating experiments. Samples with the dimension of Ø18 mm × 2 mm were obtained by electric-discharge machining. Subsequently, the surfaces and edges of the samples were polished, then ultrasonically cleaned in ethanol and dried. The MoSi2/ MoB composite coating was obtained by a two-step pack cementation method. Firstly, an inner MoB layer was prepared through boronizing process at 1000 °C for 10 h in alumina crucible. The pack mixture consisted of 0.8 wt% B, 5 wt% NaF (activator) and balance Al2O3 (inter filler). Subsequently, a MoSi2 layer on the upper part of the initially prepared MoB layer by siliconizing process in the mixture of 20 wt% Si, 5 wt% NaF (activator) and balance Al2O3 (inter filler) at 1000 °C for 10 h. After boronizing followed siliconizing in argon atmosphere, a MoSi2/MoB composite coating was achieved on Mo substrate. As the contrast, the other samples with a monolithic MoSi2 coating were prepared by siliconizing to probe the effort of diffusion blocking of MoB layer. Oxidation test of the coated samples was carried out in a box type furnace at 1200 °C and 1300 °C in ambient air. The samples were put into the box type furnace and heated up at first oxidation period. At the designated time, the samples were taken out and cooled to room temperature in air for weight. Then, the samples were directly put into the hot zone of the furnace again for the next oxidation period. Weights of the samples were measured on an electronic balance with sensitivity of ± 0.1 mg. Cumulative weight changes of the samples were calculated and reported as a function of the oxidation time. The morphology and cross-section microstructure of the coated samples were examined by using a scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDS) and electron probe microanalyzer (EMPA) with wavelength-dispersive spectrometer (WDS) was used to determine chemical composition of the coatings. The phase constitution of the coatings in the surface was identified by using an Xray diffraction (XRD) measurement.
1300 °C −4
5.42 × 10 7.80 × 10− 4
1.31 × 10− 3 3.14 × 10− 3
50 h was less than 0.16 mg/cm2. The maximum gain of 0.356 mg/cm2 in weight for the MoSi2 coated TZM during 6 times cyclic oxidation at 1300 °C in air was else reported somewhere [16]. Thus, the as-prepared composite coating showed a good oxidation resistant protective ability at high temperatures and it could provide long-term protection for Mo. Furthermore, the oxidation rate constant could be obtained by Eq. (1), which shows the relationships between squared mass change per unit area of coated Mo and oxidation time:
Δm 2 ⎛ ⎞ = ko t ⎝ S ⎠
(1)
where Δ m is the mass change of coated Mo before and after oxidation, S and ko are the surface area and oxidation rate constant of coated Mo, respectively, and t is oxidation time. The values of oxidation rate constant at different temperature were acquired by calculating the slopes of straight lines, which gained by Δm 2
( )
fitting data points of S and t. The values were listed in Table 1. Oxidation rate varied at different oxidation temperature. The results showed the oxidation rate constant increased with the increase of oxidation temperature. The oxidation rate constants of composite coating at 1200 °C and 1300 °C were 7.80 × 10− 4 and 3.14 × 10− 3 mg2/ (cm4·h), respectively. The value at 1200 °C closes to that reported by Chakraborty et al. (about 4.23 × 10− 4 mg2/(cm4·h) which calculated by the data of the paper [15]). The surface XRD patterns of coated Mo oxidized at 1200 °C for 100 h are shown in Fig. 2. The tetragonal MoSi2 (JCPDS card no. 410612) and tetragonal Mo5Si3 (JCPDS card no. 34-0371) phases were detected in both coatings (a and b in Fig. 2). The tetragonal SiO2 (JCPDS card no. 82-0512) was observed by the local magnified XRD patterns (c and d in Fig. 2). The SiO2 film covered the outer surface of the coatings after oxidation and provided a good oxidation resistant protection. Based on the results of XRD analysis, the coating reacted with oxygen in air during oxidation according to Eq. (2).
3. Results and discussion Fig. 1 shows the mass change of coated Mo samples during isothermal oxidation test. A weight gain was observed during the oxidation of coated Mo samples. The results showed that the mass change of the Mo sample coated MoSi2/MoB composite coating was 0.270 and 0.499 mg/cm2 after isothermal oxidation at 1200 °C for 100 h and 1300 °C for 80 h, respectively. Chakraborty et al. [15] reported that the mass change of the MoSi2 coated TZM after oxidation at 1200 °C for
5MoSi2 (s) + 7O2 (g) = Mo5 Si3 (s) + 7SiO2 (s)
(2)
Fig. 3 shows the cross-sectional microstructural BSE images of the silicide-based coatings after oxidation. The microstructure of MoSi2/ MoB composite coating and monolithic MoSi2 coating evolved after oxidation exposure. Continuous SiO2 layer formed on the surface of the
Fig. 1. Mass changes of the coated samples oxidized at 1200 °C (a) and 1300 °C (b) in air.
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Fig. 2. XRD patterns of the surface of the coated Mo at 1200 °C in air. a- monolithic MoSi2 coating; b- MoSi2/MoB composite coating; c and d are local magnified patterns of a and b, respectively.
contributed the extensive formation of Kirkendall voids. Moreover, a thin Mo2B layer in the interface of MoB/Mo was detected by EPMA analysis. To further investigate the transformation of microstructure of MoSi2/MoB composite coating after oxidation, the line-scanning of cross-section elemental distributions was characterized by EPMA. Fig. 4 reveals the variations of Mo, Si, B and O. It can be observed that the elements distribute consecutively. Along the direction from the surface of the coating to the substrate, the coating and substrate can be divided into seven varied parts, marked as 1, 2, 3, 4, 5, 6 and 7. Based on the quantitative analysis of EPMA, SiO2, MoSi2, Mo5Si3, MoB, Mo2B, Mo and Mo5Si3 corresponded to 1–7 layers in Fig. 4, respectively. Due to the oxidation reaction between MoSi2 and O2, SiO2 (corresponding to 1 in Fig. 4) and Mo5Si3 (corresponding to 7 in Fig. 4) layers formed on the surface of MoSi2. The transformation of MoSi2 and the Si diffusion resulted in the formation of Mo5Si3 layer below MoSi2 (corresponding to the third layer in Fig. 4). In addition, the transformation of MoB and the B diffusion resulted in the formation of Mo2B layer under MoB (corresponding to the fifth layer in Fig. 4). Fig. 5 shows the relationship between the thickness of Mo5Si3 layer in coated Mo and square root of time at 1300 °C. Their growth kinetics obeyed a parabolic rate law, which reported in the diffusion couple study of MoSi2-based composites [9,18,20], as shown in Eq. (3):
coatings, blocked oxygen in air and led to the transformation of the oxidation behavior of coatings from a rapid linear oxidation to a slow parabolic oxidation. The SiO2 film reduced the oxidation rate and provided a good protection for Mo. The light gray phase layer, which was identified as Mo5Si3 phase by EDS and EPMA analyses, could be observed beneath the continuous SiO2 layer. Light gray phase, which distributed in MoSi2 layer randomly, was also identified as Mo5Si3 phase. The diffusion rate of Si is faster than Mo in the Mo-Si system. As reported by Prasad and Paul [17], the diffusion coefficients of Si and Mo through Mo5Si3 phase at 1250 °C and 1300 °C are 1.05 × 10− 11, 1.02 × 10− 13 cm2/s and 2.0 × 10− 11, 1.94 × 10− 13 cm2/s, respectively. The diffusion coefficients of Si through MoSi2 phase at 1200 °C and 1300 °C are 1.59 × 10− 10 and 3.15 × 10− 10 cm2/s, respectively. The fast diffusion of Si causes the formation of Mo5Si3 phase. The results show that the diffusion rates of Si are different in the coatings. Most MoSi2 in the monolithic coating converted into intermediate silicides (Mo5Si3 and Mo3Si) (Fig. 3a). However, only partial MoSi2 in the composite coating developed into intermediate silicide (Fig. 3b). The Si diffusion in the coatings caused the microstructure evolution from MoSi2 developed into intermediate silicides (Mo5Si3 and/or Mo3Si). The evolution of microstructure was also reported elsewhere [9,18]. The growth rate of Mo3Si layer is slower than that of Mo5Si3 layer under the condition of the growth of Mo5Si3 layer (Fig. 3a), which was also reported in other studies [9,10,18–20]. A certain amount of Kirkendall voids were observed in the coatings. And it was obvious that the number of the voids in the monolithic MoSi2 coating was more than that in the composite coating. The tremendous disparity in the diffusivities of Mo and Si and the consequent unequal mass flow in the coatings resulted in the development of such voids [20]. The fast diffusion of Si in the monolithic MoSi2 coating not only caused the rapid development of intermediate silicides (Mo5Si3 and Mo3Si), but
x 2 = kp t
(3)
where x is the thickness of the Mo5Si3 layer, kp is the parabolic growth rate constant of the Mo5Si3 layer, and t is annealing time. The growth rate of Mo5Si3 layer in monolithic MoSi2 coating is faster than that in MoSi2/MoB composite coating in this work, especially when the duration time exceeds 20 h. As shown from Fig. 5, two periods of the growth of Mo5Si3 layer are observed in the composite Fig. 3. Cross-sectional BSE images of the coatings on Mo after oxidation at 1200 °C for 100 h. a- MoSi2; b- MoSi2/ MoB.
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of Mo5Si3 layer in the composite coating was far below that in the monolithic MoSi2 coating. Combined with the isothermal phase diagram of the Mo-Si-B ternary system [21] and our previous work [14], Mo5SiB2 (T2) phase formed in the coating due to diffusion reaction. Moreover, the diffusivity for Si in the T2 phase is far below that in the Mo5Si3 [22]. Thus, the growth of Mo5Si3 in thickness was arrested after introducing MoB layer. In other words, the diffusion of Si in the composite coating was postponed availably. Compared with monolithic MoSi2 coating, MoSi2/MoB composite coating can provide a better oxidation protection for Mo. 4. Conclusions MoSi2/MoB composite coating was prepared by a two-step pack cementation method. During oxidation at high temperatures, the coating exhibited a good oxidation resistance. The weight gain rates of the oxidized coated samples at 1200 °C and 1300 °C are 0.270 and 0.499 mg/cm2, respectively. Oxidation rate increases with the increase of oxidation temperature. Oxidation rate constants of the coating at 1200 °C and 1300 °C are 7.80 × 10− 4 and 3.14 × 10− 3 mg2/cm4·h, respectively. The growth of Mo5Si3 layer in the composite coating was very slow, which was two orders of magnitude lower than that in the monolithic MoSi2 coating. The introduction of MoB layer effectively controlled the Si diffusion in the coating and endued MoSi2/MoB composite coating with a good oxidation protective for Mo substrate at high temperatures.
Fig. 4. Elements concentration profiles measured by EMPA in the composite coating oxidized at 1200 °C.
Acknowledgements The financial support of National Nature Science Foundation of China (Grant No. 51475161), the Research Foundation of Education Bureau of Hunan Province (Grant No. 15A059), and Scientific Research Project of Hunan University of Science and Technology (Grant No. E51511) are gratefully acknowledged. The work is also supported by State Key Laboratory of Powder Metallurgy of Central South University. We also thank Advanced Materials Synthesis and Application Technology Laboratory (Hunan University of Science and Technology), the Key Laboratory of Non-ferrous Materials Science and Engineering (Central South University), Ministry of Education, for research facilities.
Fig. 5. Relationship between the thicknesses of Mo5Si3 layer in coated Mo and the square root of time at 1300 °C.
References coating. Moreover, the growth rate of Mo5Si3 layer under the line segment 2 (solid line) is much less than that under the line segment 1(broken line). The extrapolated curves of thickness of Mo5Si3 layer do not pass through zero. The thickness of Mo5Si3 layer in the both coatings at the duration of 0 h is greater than zero, which is explained as the growth of Mo5Si3 layer in coatings develops during heating and cooling stages. To assess the parabolic growth rate constant by calculating slope of curve precisely, the Eq. (3) was adjusted appropriately followed as:
(x − x 0 )2 = k p t
[1] B.V. Cockeram, Measuring the fracture toughness of molybdenum-0.5 pct titanium0.1 pct zirconium and oxide dispersion-strengthened molybdenum alloys using standard and subsized bend specimens, Metall. Mater. Trans. A 33 (2002) 3685–3707. [2] M.Z. Alam, B. Venkataraman, B. Sarma, D.K. Das, MoSi2 coating on Mo substrate for short-term oxidation protection in air, J. Alloys Compd. 487 (2009) 335–340. [3] A. Mueller, G. Wang, R.A. Rapp, E.L. Courtright, Deposition and cyclic oxidation behavior of a protective (Mo,W)(Si,Ge)2 coating on Nb-base alloys, J. Electrochem. Soc. 139 (1992) 1266–1275. [4] Y.L. Jeng, E.J. Lavernia, Processing of molybdenum disilicide, J. Mater. Sci. 29 (1994) 2557–2571. [5] J.K. Yoon, G.H. Kim, J.Y. Byun, J.K. Lee, H.S. Yoon, K.T. Hong, Effect of Cl/H input ratio on the growth rate of MoSi2 coatings formed by chemical vapor deposition of Si on Mo substrates from SiCl4-H2 precursor gases, Surf. Coat. Technol. 172 (2003) 176–183. [6] K. Krnel, D. Sciti, E. Landi, A. Bellosi, Surface modification and oxidation kinetics of hot-pressed AlN–SiC–MoSi2 electroconductive ceramic composite, Appl. Surf. Sci. 210 (2003) 274–285. [7] H. Zhang, S. Gu, Preparation and oxidation behavior of MoSi2–CrSi2–Si3N4 composite coating on Mo substrate, Int. J. Refract. Met. Hard 41 (2013) 128–132. [8] M. Samadzadeh, C. Oprea, H. Karimi Sharif, T. Troczynski, Comparative studies of the oxidation of MoSi2 based materials: low-temperature oxidation (300–900 °C), Int. J. Refract. Met. Hard 66 (2017) 11–20. [9] J.K. Yoon, J.K. Lee, K.H. Lee, J.Y. Byun, G.H. Kim, K.T. Hong, Microstructure and growth kinetics of the Mo5Si3 and Mo3Si layers in MoSi2/Mo diffusion couple, Intermet 11 (2003) 687–696. [10] H.A. Chatilyan, S.L. Kharatyan, A.B. Harutyunyan, Diffusion annealing of Mo/ MoSi2 couple and silicon diffusivity in Mo5Si3 layer, Mater. Sci. Eng. A 459 (2007) 227–232.
(4)
The values of x0 in monolithic MoSi2 coating and MoSi2/MoB composite coating are 2.61, 0.17 (line segment 1 in Fig. 5) and 14.88 μm (line segment 2 in Fig. 5), respectively. The parabolic growth rate constants in the monolithic and composite coatings are 9.57 × 10− 11, 3.78 × 10− 11 (line segment 1 in Fig. 5) and 5.15 × 10− 13 cm2/s (line segment 2 in Fig. 5), respectively. As shown from the values, the kp in MoSi2/MoB composite coating is lower than that in monolithic MoSi2 coating during the whole time, only the difference of kp in the two coatings at the duration time within 20 h is very small. With the increase of the time, the growth rate of Mo5Si3 layer in the composite coating dramatically decreased. The parabolic growth rate constant of Mo5Si3 layer in the composite coating is two orders of magnitude lower than that in the monolithic MoSi2 coating. The growth 63
International Journal of Refractory Metals & Hard Materials 68 (2017) 60–64
Y. Wang et al.
[17] S. Prasad, A. Paul, Growth mechanism of phases by interdiffusion and atomic mechanism of diffusion in the molybdenum–silicon system, Intermet 19 (2011) 1191–1200. [18] J.K. Yoon, G.H. Kim, J.Y. Byun, J.S. Kim, C.S. Choi, Simultaneous growth mechanism of intermediate silicides in MoSi2/Mo system, Surf. Coat. Technol. 148 (2001) 129–135. [19] J.K. Yoon, J.Y. Byun, G.H. Kim, J.S. Kim, C.S. Choi, Growth kinetics of three Mosilicide layers formed by chemical vapor deposition of Si on Mo substrate, Surf. Coat. Technol. 155 (2002) 85–95. [20] J.Y. Byun, J.K. Yoon, G.H. Kim, J.S. Kim, C.S. Choi, Study on reaction and diffusion in the Mo–Si system by ZrO2 marker experiments, Scr. Mater. 46 (2002) 537–542. [21] T. Hayashi, K. Ito, H. Numakura, Reaction diffusion of MoSi2 and Mo5SiB2, Intermet 13 (2005) 93–100. [22] J.H. Perepezko, R. Sakidja, Oxidation-resistant coatings for ultra-high-temperature refractory Mo-based alloys, JOM 62 (2010) 13–19.
[11] S.L. Kharatyan, H.A. Chatilyan, G.S. Galstyan, Growth kinetics of Mo3Si layer in the Mo5Si3/Mo diffusion couple, Thin Solid Films 516 (2008) 4876–4881. [12] J. Liu, Q. Gong, Y. Shao, D. Zhuang, J. Liang, In-situ fabrication of MoSi2/SiC–Mo2C gradient anti-oxidation coating on Mo substrate and the crucial effect of Mo2C barrier layer at high temperature, Appl. Surf. Sci. 308 (2014) 261–268. [13] H. Zhang, J. Lv, Y. Wu, S. Gu, Y. Huang, Y. Chen, Oxidation behavior of (Mo,W) Si2–Si3N4 composite coating on molybdenum substrate at 1600 °C, Ceram. Int. 41 (2015) 14890–14895. [14] Y. Wang, D. Wang, J. Yan, Preparation and characterization of MoSi2/MoB composite coating on Mo substrate, J. Alloys Compd. 589 (2014) 384–388. [15] S.P. Chakraborty, S. Banerjee, I.G. Sharma, A.K. Suri, Development of silicide coating over molybdenum based refractory alloy and its characterization, J. Nucl. Mater. 403 (2010) 152–159. [16] S. Majumdar, I.G. Sharma, S. Raveendra, I. Samajdar, P. Bhargava, In situ chemical vapour co-deposition of Al and Si to form diffusion coatings on TZM, Mater. Sci. Eng. A 492 (2008) 211–217.
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