- Email: [email protected]
Evolution of microstructural feature and oxidation behavior of LaB6-modified MoSi2-SiC coating
Journal of Alloys and Compounds 753 (2018) 703e716
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Evolution of microstructural feature and oxidation behavior of LaB6-modified MoSi2-SiC coating Changcong Wang a, Kezhi Li a, *, Caixia Huo a, Qinchuan He a, Xiaohong Shi a, b, ** a b
State Key Laboratory of Solidification Processing, Carbon/carbon Composites Research Center, Northwestern Polytechnical University, Xi'an, 710072, China Shenzhen Research Institute of Northeastern Polytechnical Uinversity, Shenzhen, 518057, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 January 2018 Received in revised form 14 April 2018 Accepted 23 April 2018 Available online 24 April 2018
LaB6-modified MoSi2-SiC (MSL) oxidation resistant coating was designed and successfully fabricated on SiC-coated carbon/carbon (C/C) composites using efficient supersonic atmospheric plasma spraying (SAPS) method. The MSL coating protected the C/C composites from static oxidation at 1773 K for 100 h with a mass loss of 6.52 mg cm2 and dynamic ablation about 2023 K for six cycles with a mass loss of 29.86 mg cm2, which was superior to the MoSi2-SiC (MS) coating without La-doping. The presence of rare earth La element promoted the liquid phase sintering of MoSi2 and B element facilitated the hightemperature stability of SiO2-rich glass scale, resulting in the formation of La-Si-O-B compound scale with a low volatility and oxygen permeability cover the coating surface, which contributed to improving the resistant to oxidative response of the MSL coating. © 2018 Elsevier B.V. All rights reserved.
Keywords: Carbon/carbon composites Static isothermal oxidation Dynamic cyclic ablation Compound scale
1. Introduction Carbon/carbon (C/C) composites have caught much attention for the demand of thermal-structural components in the aerospace fields because of their unique performances such as low density, high specific strength, resistance to thermal shock, wear, chemical attack [1e3]. The broader introduction of C/C composites can lead to weight reduction, longer service lifetime, more design flexibility and cost economy. Regrettably, their intrinsic weakness of oxidative sensitivity hinders its applications in the high-temperature aerobic environment [4,5]. The degradation of C/C composites is controlled by the transport of gaseous molecules to react with matrix carbon and formed volatile gaseous products [6,7]. Therefore, the C/C composites should be protected when exposed to aerobic environments. Surface coatings are usually applied to protect C/C composites against long-term static oxidation and short-term cyclic ablation in a dynamic scour environment of oxyacetylene flame [2,4,5,8e10]. Intermetallic molybdenum disilicide (MoSi2) with a melting point of 2303 K was the focus of many studies on oxidative protection for
* Corresponding author. ** Corresponding author. State Key Laboratory of Solidification Processing, Carbon/carbon Composites Research Center, Northwestern Polytechnical University, Xi'an, 710072, China. E-mail addresses: [email protected] (K. Li), [email protected] (X. Shi). https://doi.org/10.1016/j.jallcom.2018.04.267 0925-8388/© 2018 Elsevier B.V. All rights reserved.
multiple materials due to their excellent high-temperature stability and low oxygen permeability [11,12]. However, the MoSi2 combined the dual characteristics of metal (>1273 K, high-temperature plasticity) and ceramic (<1273 K, low-temperature brittleness), which caused by cracking and peeling off the coating due to the structural transformation of MoSi2 in the process of high-low temperature cyclic oxidation [13]. Therefore, low-temperature toughening and high-temperature reinforcement are the challenges to be controlled urgently in the engineering application of MoSi2 structural material. In addition, the “pesting” oxidation of MoSi2 in lowmedium temperatures (<1173 K) was another critical factor hindering its application [14e16]. The good oxidation resistance of MoSi2 in the temperature range of 1273e2173 K was displayed due to the formation of SiO2 continuous glass protective film with selfhealing ability [17e19]. Nevertheless, at the medium and low temperatures of 673e1273 K, poor fluidity of the vitreous silica phase was difficult to produce a continuous dense protective film to prevent oxygen from infiltration. Meanwhile, the formed MoO3 with porous structure was easily transformed into a powder and exfoliated from the surface of MoSi2 under the effect of interfacial stress, and then catastrophic oxidation occurred [20,21]. All of the above challenges indicated that the MoSi2-based materials for use at elevated temperatures were restricted and it was worth exploring the potential of a new heterogeneous systems regarding the increase in compactness of MoSi2 and reduce in evaporation of gaseous products.
704
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
To enhance the low-temperature toughness and hightemperature strength of MoSi2 and reduce the evaporation of gaseous products, many efforts have been made on rare earth elements (RE ¼ La, Y, Ce etc.) to MoSi2 to form a polyphase solid solution and refine the crystalline structure [22e24]. It was reported that the rare earth compound coatings had been successfully prepared on MoSi2 and a good match between substrate and coatings was proved during variable temperature cycles process at temperatures above 1773 K without apparent delamination [25,26]. Doping of rare earth oxide (La2O3/Y2O3) into Mo-Si-B also showed encouraging results concerning the oxidation-resistant ability in 1073e1273 K and mechanical properties [27,28]. However, the mechanism of rare earth elements on silica scale forming process was not well revealed in detail owing to the sophisticated molecular structure of thermally grown oxides glass layer. Enlightened by this, in present work, attempts were made to study the microstructural feature and oxidation behavior of MoSi2-SiC coating with the addition of LaB6 in variable proportions at the selected conditions of static air and dynamic oxygen acetylene erosion. The Lacontaining oxide glass scale were characterized to understand the mechanisms in silica-based amorphous glass layer, supported by thermodynamic considerations.
area mass change rate (n1) were evaluated as the following Eqs. (1) and (2).
Dw1 ¼ n1 ¼
m0 mt S1
m0 mt S1 $t1
(1)
(2)
where m0 is the original mass of the tested specimen; mt is the mass of the tested specimen after oxidation for a certain time; S1 is the total surface area of the specimen; t1 is the oxidation time. In addition, the non-isothermal oxidation behavior of the specimens was also studied using the thermogravimetric (TG) analyzer from 573 K to 1873 K at the heating rate of 10 K min1. 2.3. Dynamic thermal shock test To measure the thermal corrosion resistance of sprayed coatings in a dynamic environment, cyclic thermal shock test from 2023 K to ambient temperature was conducted under vertical oxyacetylene torch. During the process of cyclic thermal shock, the pressure and
2. Experimental procedure 2.1. Materials and coating preparation Two-dimensional (2D) C/C composites with a density of 1.75 g cm3 were prepared using a commonly method of isothermal chemical vapor infiltration (ICVI) [29], which were used as substrates in this study. The C/C specimens were hand-abraded with SiC sandpapers (200 grit, 400 grit), and then cleaned ultrasonically with ethanol for 2 h and dried at 353 K for 24 h. SiC buffer coating was preferentially prepared by pack cementation, and the details of which were described elsewhere [30]. Next, the LaB6-modified MoSi2-SiC coating was deposited on SiCcoated C/C composites using supersonic atmospheric plasma spraying (SAPS) method. Powder compositions were as follows: 5e15 wt.% LaB6, 25 wt.% SiC and 60e70 wt.% MoSi2, which were dry-mixed and roller milled to guarantee uniformity. The obtained mixed powders were agglomerated by a spray tower drier to enhance the fluidity of particles for spraying, and then sprayed to form the desired coatings with the power of 50e55 kW. To accelerate the particles melting, a feedstock nozzle with an internal diameter of 5.5 mm was selected, and the distance between nozzle and specimen was kept at 100 mm. The feed rate was controlled at 4.5 r min1. For comparing, the MoSi2-SiC coating without LaB6 doping was obtained using the same technique. To simplify the discussion, the sprayed coating with LaB6 content of 0 wt.%, 5 wt.%, 10 wt.% and 15 wt.% were identified as MS, MSL5, MSL10 and MSL15, respectively. 2.2. Oxidation tests Static isothermal oxidation behavior of the specimens (10 mm 10 mm 10 mm) was evaluated at 1773 K in a constant temperature furnace. Prior to the oxidation experiment, the corundum crucibles were pre-heated at 1773 K until no discernible mass change. Afterwards, the specimens were placed in corundum crucibles for a designated time, and they were removed and cooled down to indoor temperature in air. The mass of the oxidized specimens was obtained by weighing in an electric balance with a sensitive of ±0.1 mg. The relationship between the mass changes of specimens and oxidation duration was described. Throughout the oxidation process, the per unit area mass change (Dw1) and per unit
Fig. 1. Surface SEM images of the sprayed coatings at different magnifications: (a1, a2) MS coating; (b1, b2) MSL5 coating; (c1, c2) MSL10 coating; (d1, d2) MSL20 coating.
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
flux of O2 were 0.4 MPa and 0.86 m3 h1, and those of C2H2 were 0.095 MPa and 0.65 m3 h1, respectively. The distance between oxyacetylene gun and specimen was adjusted to 10e15 mm to ensure the surface temperature of specimen reach 2023 K. The ablation time at 2023 K was 4 s in each cycle under oxyacetylene torch. The cyclic thermal shock test was carried out with a duration of 10 s in this study. After that, the oxyacetylene flame was removed, and the specimens (F30 mm 10 mm) were cooled down naturally before the next thermal cycle until 6 cycles were reached. The per unit area mass change (Dw2) and per unit area mass change rate (n2) were represented by following Eqs. (3) and (4).
Dw2 ¼ n2 ¼
m0 mt S2
m0 mt S2 $t2
(3)
(4)
where m0 and mt are the mass of tested specimens before and after thermal shock test, respectively; S2 is the area of contact surface; t2 is the number of thermal cycles. During the dynamic thermal shock test, three specimens formed a group, and the final experimental data was the average of three specimens. 2.4. Characterization Microstructural features and morphologies of the specimens
705
were analyzed by scanning electron microscopy (SEM, JSM6460) and transmission electron microscope (TEM, JEM3010), which were equipped with EDS. Roughness values of different specimens were measured using a 3D confocal laser microscope (Optelics C130). Coefficient of thermal expansion (CTE) from 1273 K to 2073 K was evaluated by a DIL402E Dilatometer in He ambience with a heating rate of 10 K min1. Hardness and elastic modulus of the sprayed coatings were measured using micron indentation techniques (MHT-1, NANOVEA) with a quadrangular pyramid diamond indenter on the wellpolished cross-section of sprayed coatings. During the test, the maximum pressure of the indenter was set to 2 N, the pouring rate was 4 N min1, and the creep time was 10 s at maximum load. The location of indentation center was selected as the middle of coating cross-section. Ten different locations were tested to acquire the mean of the hardness and elastic modulus according to Oliver and Pharr principle [31]. The fracture toughness (KIC) of sprayed coatings was characterized by indentation fracture, and the type and size were same as that of the upper hardness test, which was calculated using the following Eq. (5).
KIC ¼ 0:016
1=2 E 1000,P H ð1 þ aÞ3=2
(5)
where E and H are elastic modulus and hardness of the coating, respectively. Symbol a is half the length of the indentation diagonal, and P is the applied load. Porosity of the sprayed coatings was tested using the
Fig. 2. Roughness comparative results of the MS, MSL5, MSL10, MSL15 coatings (a); three-dimensional (3D) surface topography of the MS coating (b) and MSL10 coating (c).
706
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
Archimedes drainage method. Firstly, the specimens with sprayed coating were chemically etched by aqua regia solution at 393e423 K, then the peeled coating was subjected to low power ultrasonic cleaning and placed in air at 343 for drying. They were weighed using an analytical electronic balance, denoted as m0. Secondly, the peeled coating was boiled in deionized water for 2e3 h so that the pores inside the coating were filled with water, and the floating mass of m1 and the wet mass of m2 were weighed on a special balance. The porosity (p) was calculated using Eq. (6).
r¼
m2 m0 m2 m1
coating with various doping content of LaB6 are presented in Fig. 2. It can be observed that average values of surface roughness are 25.89 mm of MS coating, 20.46 mm of MSL5 coating, 17.23 mm of MSL10 coating and 19.24 mm of MSL15 coating (Fig. 2(a)), which demonstrate the reduction of 33.45% in MSL10 coating by comparing with MS coating. Obviously, the fluctuation of the surface structure is consistent with the morphologies observed under SEM. Fig. 3 shows the cross-section BSE images of the specimens with
(6)
where m0, m1, m2 are the mass after drying, the mass in the water, the mass containing deionized water, respectively. 3. Results and discussion 3.1. Microstructural features and morphologies of the sprayed coating Fig. 1 compares the surface morphologies of sprayed MS, MSL5, MSL10 and MSL15 coatings. Representative sprayed coating characteristics with molten and semi-molten particles intertwined are revealed, but their microstructural features are varied due to the different doping amount of LaB6. Specifically, with the doping amount of LaB6 increased from 0 wt.% to 10 wt.%, the sprayed coatings as a whole became flatter (Fig. 1(a1, b1, c1)) and compact along with some small pores by stacking of semi-molten particles (Fig. 1(a2, b2, c2)). Further increasing the doping amount of LaB6, large un-melted granules gradually accumulated on the coating surface, resulting in a significant growth in defects (Fig. 1(d1)) and decrease in compactness of coating (Fig. 1(d2)). Three-dimension (3D) surface profiles and corresponding surface roughness of the
Fig. 4. Comparison of porosity on four different coatings: MS coating; MSL5 coating; MSL10 coating; MSL15 coating.
Fig. 3. Cross-sectional BSE images of the sprayed coatings: (a) MS coating; (b) MSL5 coating; (c) MSL10 coating; (d) MSL15 coating.
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
MS, MSL5, MSL10, MSL15 coatings, both of which exhibits a distinct bilayer structure. For the MS-coated specimen, the interface between exterior MS layer and interior SiC layer appeared many cracks and holes, as displayed in Fig. 3(a). However, owing to the rare earth La element was incorporated, the number of defects in the coating and at the junction was dramatically reduced. In particular, when the doping amount of LaB6 was 10 wt.%, the overall compactness and connectivity of sprayed coating were optimized (Fig. 3(bed)). Meanwhile, the porosity of sprayed coating was measured using Archimedes principle, and the corresponding results are shown in Fig. 4. Increasing the doping amount of LaB6 from 0 wt.% to 10 wt.%, the porosity of sprayed coating decreased from 15.88% (MS) to 7.89% (MSL10), indicating that the introduction of rare earth La element has a positive effect on the improvement of coating compactness, while the excessive doping of LaB6 has side effects on the denseness of sprayed coating. Based on the exhibited results, doping 10 wt.% LaB6 into the coating is a relatively suitable choice in this study.
707
To more clearly observe the distribution of rare earth La element within the coating, TEM is adopted to further characterization, and the corresponding TEM images of MSL10 coating are exhibited in Fig. 5. It was clearly that white phase A and black phase B were interactively distributed together, as shown in Fig. 5(a), and the black phase was mainly concentrated in the phase boundary of white phase. The EDS results (Fig. 5(e and f)) revealed that the chemical composition of phases A and B respectively were 56.23Mo-43.77Si (At. %) and 62.49La-37.51B (At. %), inferring that the phase A was MoSi2 and the phase B was LaB6. Meanwhile, SAED patterns of phases A and B are shown in Fig. 5(c and d). Two sides of the diffraction parallelogram for the phase A were identified to be (211) and (0e11) reflections in [222] zone spots of MoSi2. While for the phase B, they were identified to be (320) and (222) reflections in [46-2] zone spots of LaB6. These clear diffraction spots suggested the single crystal structure of the phases in the coating with few defects. Moreover, Fig. 5(b) demonstrates the high-resolution transmission electron microscope (HRTEM) image of the interface
Fig. 5. TEM images of the MSL10 coating to observing the distribution of rare element La: (a) bright field image; (b) HRTEM image of representative interface between MoSi2 and LaB6; (c, d) the SAED patterns of (a) confirming presence of MoSi2 and LaB6; (e, f) the EDS results of spots A and B in (a).
708
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
between MoSi2 and LaB6. The interplanar spacings of MoSi2 and LaB6 respectively were measured to be 0.142 nm and 0.115 nm, consistent with that of the (211) plane of MoSi2 and (320) plane of LaB6. Based on the above analysis, it can be deduced that the rare earth La element inclines to be distributed at the phase boundaries or defects within the coating, then causing the changes in physical, chemical or interface energies of the grain boundaries and the precipitation of new phase, which eventually lead to the variation in microstructure and performance of modified coating. Previous studies have proved that the rare earth compounds have a positive influence on the mechanical properties of MoSi2based materials [22e28]. In this study, the effect of LaB6 on the hardness and elastic modulus of MoSi2-based coatings was investigated using micro-indentation method and the results are shown in Fig. 6. From Fig. 6(a), it can be seen that the penetration depth of MSL10 coating (3.62 ± 0.25 mm) was significantly lower than that of MS coating (2.79 ± 0.18 mm) under the same positive pressure load. The comparison of hardness and elastic modulus of MS coating and MSL10 coating are exhibited in Fig. 6(b). Specifically, prior to introducing LaB6 in the coating, the hardness and elastic modulus of MS coating were 9.75 ± 1.52 GPa and 148.96 ± 12.58 GPa, respectively. After the incorporation of LaB6 in the coating, the hardness and elastic modulus of MSL10 coating were increased to 13.52 ± 1.68 GPa and 180.64 ± 14.31 GPa, which separately increased by 38.67% and 21.27% compared to MS coating. It is well known that the hardness and elastic modulus of ceramic coating have an important impact on their toughness, and the coatings with greater hardness and elastic modulus usually possess better toughness [32]. Therefore, it is reasonable to speculate that the toughness of sprayed MS coating can be enhanced by introducing LaB6, which is also beneficial for the high-temperature performance of MSL coating. 3.2. Isothermal static oxidation behavior of the sprayed coating Isothermal static oxidation can be considered as a mild oxygen corrosion process due to the absence of dynamic disturbances. Under this condition, the relationship between the mass change of the specimens and oxidation duration was revealed in Fig. 7(a and b). After oxidation at 1773 K for 100 h, the per unit area mass loss of MS coating and MSL10 coating respectively were 15.45 mg cm2 and 6.52 mg cm2 (Fig. 7(a)), and the corresponding mass loss rate separately were 0.15 mg cm2 h1 and 0.06 mg cm2 h1 (Fig. 7(b)). This result showed that the MSL10 coating possessed a better oxidation resistance than the MS coating due to the incorporation of LaB6 in the MSL coating. Meanwhile, during the mass gain process, the maximum mass gain of the MSL10 coating (5.21 mg cm2) was almost twice that of the MS coating (2.84 mg cm2), indicating that the presence of LaB6 promoted the formation rate of oxides during the oxidation process, thereby gaining mass in a relatively short period of time. The oxidation process of the specimens with MS coating and MSL10 coating was monitored using thermogravimetric (TG) experiments in a wide temperature range from 573 K to 1873 K, and the results are shown in Fig. 7(c). The overall process of oxidation can be divided into three stages, and each stage represents various oxidation behavior and self-healing mechanisms of cracks with the increase of oxidation temperature. At the temperature from 573 K to 1173 K (stage A), there was no significant mass loss in MSL10coated specimen, while the mass of MS-coated specimen was gradually reduced, which attributed to the formation of B2O3 to filling in cracks and blocking oxygen diffusion. The formation of B2O3 was accompanied by a 250% volume expansion [33], which could fill in crack also. In addition, the produced SiO2 was difficult to effectively heal cracks due to its large viscosity in this
Fig. 6. Representative loads vs. displacement curves of MS coating and MSL10 coating under the same positive pressure (a); hardness and elastic modulus comparative results of MS coating and MSL10 coating (b).
temperature range. During the temperature from 1173 K to 1473 K (stage B), the MSL10-coated specimen began to lose mass gradually, and the mass loss rate of MS-coated specimen was also faster than the previous stage A. With the increase of oxidation temperature, the oxidation rate of the coating accelerated and the number of cracks in the coating increased. The generated cracks were not enough to healed by B2O3 and SiO2 due to low production of B2O3 and insufficient fluidity of SiO2, while the generated B2O3 could delay the propagation of cracks to some extent. Further increasing the temperature from 1473 K to 1873 K (stage C), the oxidation of MoSi2 and SiC would form SiO2 with a low viscosity at this time, which exhibited self-healing properties to close down both the superficial and penetrative cracks in the coating. Meanwhile, the generated B2O3 and La2O3 could be dissolved in the SiO2-rich scale to form polyphase silicates (B-La-Si-O scale), which had much lower volatility and oxygen permeability [24,34], thereby inhibited the volatilization of B2O3. However, for MS-coated specimen, with the increase of oxidation temperature, the thickness of formed single SiO2 scale might decrease gradually due to the volatilization of gaseous products (MoO3, CO2, CO, SiO). Therefore, the mass loss rate of MS-coated specimen slowly increased, while the mass of MSL10-coated specimen was almost constant. In brief, the presence of La element and B element in the coating has a positive effect on the oxidation protection of the MSL coating in a wide temperature domain. Fig. 8 shows the surface morphologies of the specimens with MS
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
709
Fig. 7. Isothermal (a, b) and non-isothermal (c) oxidation kinetics curves of MS and MSL10 coatings: (a) per unit area mass change (mg cm2) vs oxidation time (h); (b) per unit area mass change rate (mg cm2 h1) vs oxidation time (h); (c) TG results from 573 K to 1873 K.
and MSL10 coatings after oxidation at 1773 K for 100 h. From Fig. 8(a1), it was readily observed that the glass scale with numerous cracks covered the surface of MS coating, and many particles were randomly distributed in the glass scale to aggregate into larger protrusions. Enlargement of these protrusions revealed that the build-up of particles caused more cracks and stomatal formation because of the produced SiO2-rich glass was difficult to effectively heal them, as shown in Fig. 8(a2). Compared with MS coating, a crack-free MSL10 coating with many flocculating products was observed (Fig. 8(b1)), inferring that the introduction of LaB6 promoted the directional growth of partial oxides. The white phase and gray phase tightly wrapped together (Fig. 8(b2)), which respectively were detected by EDS (Fig. 8(c and d)) as Si-rich oxide (containing elements of Si, Mo, O) and La-Si-B rich oxide (containing elements of Si, La, O, B), indicating that the formation of LaSi-O-B compound scale on the coating surface, which provided an effective protection for C/C composites. Moreover, many wrinkles were also discovered on the MSL10 coating surface (Fig. 8(b2)), which attributed to the compressive stress affected the oxidation behavior by eliminating the coating cracking during the cooling of the coated specimen. Therefore, LaB6-modified MoSi2-SiC coating can be considered as a promising oxidation resistant coating for C/C composites. 3.3. Dynamic cycling hot corrosion behavior of the sprayed coating Previous thermal shock experiments were mainly performed in
static air in a high-temperature furnace, where the gas environment could be regarded as a thermal equilibrium [29,35], while the erosion of dynamic combustion gas close to practical application environment was usually overlooked. However, in this research, a high-temperature dynamic gas erosion (about 2023 K) was constructed to simulate the service capability of the coating in practical application, and the cyclic thermal shock test from 2023 K to room temperature was proceeded under the vertical oxyacetylene flame. Fig. 9 compares the per unit area mass change (or per unit area mass change rate) of the specimens with MS and MSL10 coatings during cyclic thermal shock test. With respect to MS coating, after six thermal cycles from 2023 K to room temperature, the per unit area mass loss and per unit area mass loss rate were 48.97 ± 4.52 mg cm2 and 8.16 ± 0.48 mg cm2 t1, respectively. Compared with MS coating, the per unit area mass loss and per unit area mass loss rate were severally reduced by 39.02% to 29.86 ± 4.28 mg cm2 and 39.09% to 4.97 ± 0.42 mg cm2 t1 after six thermal cycles under the identical condition. Meanwhile, neither coating showed apparent mass gain during the whole thermal shock process (Fig. 9), while the mass loss rate of MSL10 coating was significantly lower than that of un-doped MS coating. The above results directly indicated that the addition of LaB6 improved the thermal shock resistance of MS coating in the dynamic combustion environment. The surface and cross-section morphologies after cyclic thermal shock test are revealed in Fig. 10. Observation under low magnification micrographs found that there was no apparent spalling on
710
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
Fig. 8. Surface SEM images of the MS coating (a1, a2) and MSL10 coating (b1, b2) after oxidation for 100 h at 1773 K, and the EDS results of spots C and D in (b2).
Fig. 9. Variation in per unit area mass change (a) (or per unit area mass change rate (b)) with the increase of thermal cycles of the MS and MSL10 coatings under vertical oxyacetylene torch.
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
711
Fig. 10. Surface (a1, a2, b1, b2) and cross-section (a3, b3) SEM images of the MS and MSL10 coated specimens after thermal shock test: (a1-a3) MS coating; (b1-b3) MSL10 coating.
the surface of MS and MSL10 coatings (Fig. 10(a1, b1)), while the evacuation channels of numerous pores with various sizes were produced owing to the escape of some gaseous products (B2O3, MoO3, CO, CO2, etc.) with a low melting point from oxide glass scale. By comparing Fig. 10(a2) with Fig. 10(b2), the MS coating presented an obvious porous surface structure, while the surface of MSL10 coating remained dense with the coverage of oxides scale. After the incorporation of LaB6 in the coating, the MoSi2 granules connected with the melting degree became more larger and the defects dramatically shrunk as demonstrated in Fig. 10(b2). Some small particles containing La element were randomly distributed between oxide scale and MSL grains. Among them, the rare earth La element played a dual role including promote the creation of big grains and separate the LaB6 small particles as well as heal the defects. As shown in Fig. 10(a3, b3), a loose cross-section structure with some cracks and pores was observed in MS-coated specimen, while the cross-section of MSL10-coated specimen still reminded a dense feature without obvious cracks. The formation of numerous cracks in the MS coating was mainly caused by the thermal mismatch during thermal cycling process, and the generated SiO2-
rich glass scale was difficult to heal them for a short period of time. After the introduction of LaB6 in the MS coating, the La and B elements promoted the formation rate and fluidity of SiO2-rich glass scale to produce La-Si-O-B compound scale, then the cracks caused by thermal mismatch was healed timely, thereby the MSL10-coated specimen exhibited a compact structure without visible cracks (Fig. 10(b3)). The generation of La-Si-O-B compound scale served as an oxygen barrier to improve the oxidation resistance of MSL10 coating. For the purpose of further studying the formation mechanism and microstructure of small particles (marked in Fig. 10(b2)), TEM observations of these particles are shown in Fig. 11. The size of these small particles distributed in tens of nanometers to hundreds of nanometers (Fig. 10(b2)). The explicit boundaries between particles were detected (Fig. 11 (a, b)), and homogeneous areas were distributed inside the particles. The representative high-resolution TEM image of area C (marked in Fig. 11(b)) revealed four kinds of phases: La2O3, La2SiO5, La(BO2)3 and amorphous phase, as shown in Fig. 11(c), indicating that a diffusion process and dissolution of La into SiO2 scale might occur at elevated temperatures. The formation
712
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
Fig. 11. TEM images of small particles distributed in MSL10 coating (a, b); HRTEM image of representative interface between these particles (c); the SAED pattern of (b) confirming presence of La2O3, La2SiO5, La(BO2)3 and amorphous phase.
of amorphous phase covering layer also accelerated this process owing to the higher diffusion rate of La in liquid than that of solid [36]. Moreover, the La-containing oxides were encapsulated by amorphous layer according to the selected area electron diffraction (SAED) pattern (Fig. 11(d)). Among them, the semi-melted La(BO2)3 and La2SiO5 dispersed to these particles (Fig. 11(c)). These Lacontaining oxides were solid solution of silica, in which La occupied the position of Si atoms. This replacement could inevitably improve the melting point of silica (compared with pure silica, La-
Fig. 12. Changes of the standard Gibbs free energies of reactions (7e10) at different temperatures calculated by Fact-Sage software.
containing solid solution possessed a higher melting point [37]), which were more prone to occur at grain boundaries of these particles. These areas became the highly active areas of the oxide scale during oxidation and reduced the evaporation of pure silica at elevated temperatures. 3.4. Oxidation protective mechanism of the sprayed coating In order to explore the protective mechanism of MSL coating,
Fig. 13. Coefficients of thermal expansion (CTE) of the MS coating and MSL10 coating.
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
the reactions and phase transformation in the oxidation process should be carefully considered. The following reactions might occur during the static and dynamic oxidation experiments.
2MoSi2ðsÞ þ 7O2ðgÞ/2MoO3ðgÞ þ 4SiO2ðlÞ
(7)
21 O2ðgÞ/La2O3ðlÞ þ 6B2O3ðgÞ 2
(8)
2LaB6ðsÞ þ
SiCðsÞ þ 2O2ðgÞ/SiO2ðlÞ þ CO2ðgÞ SiO2ðlÞ þ La2O3ðlÞ/La2SiO5ðlÞ
(9) (10)
The standard Gibbs free energy for the above Eqs. (7)e(10) are shown in Fig. 12. It is well known that the lower Gibbs free energy of reaction suggests the more obvious trend of the reaction [38]. Based on the results of Fig. 12, it can be inferred that the chemical reactions (7) and (8) play the leading roles, suggesting that the performance enhancement of MSL coating is chiefly attributed to the formation of compact La-Si-O-B compound scale with a suitable viscosity and better stability in extremely hot environment relative to pure SiO2 scale. In addition, the generation of low-pointing oxides (MoO3, B2O3) with a high vapor pressure and volatility plays a dual role: accelerate the enrichment of Si toward coating surface
713
and development of SiO2-rich scale [21,39,40], and destroy the integrity of La-Si-O-B scale. Moreover, thermal stress derived from thermal mismatch between coatings and substrate is crucial to the service life of applying coating during oxidation process, which can be described as follows [41].
DT,E s ¼ Da, 1n
(11)
where DT is difference between the testing temperature and cooling temperature, Da is the difference of CTE between coating and matrix, E and n are the Young's modulus and Poisson's ratio of coating materials, respectively. In accordance with the above Eq. (11), when the variables of DT, E and n are considered to be same, the greater thermal stress can be caused by larger CTE mismatch. The CTE results of MSL10 and MS coatings in the temperature range of 1273e2023 K are shown in Fig. 13. The difference in CTE between MSL10 coating and substrate is lower than that of MS coating, so the larger thermal stress can be preserved in the MS coating, thereby resulting in the formation of cracks. Meanwhile, the hightemperature stability and crack healing capacity of single SiO2 scale are all inferior to the La-Si-O-B compound scale, therefore the MSL10 coating presents a better oxidation resistance than MS coating.
Fig. 14. Calculated phase diagrams of multiple systems: (a) binary La2O3-SiO2 system [42,43]; (b) binary La2O3-B2O3 system [44,45]; (c) ternary SiO2-B2O3-La2O3 system [46, 47].
714
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
Proverbially, lanthanum has a better affinity with oxygen than silicon, so the lanthanum atoms are more likely to spread to the coating surface to form La2O3 because of selectively oxidation, while the single La2O3 cannot effectively block the oxygen diffusion. The oxygen partial pressure at the interface of coating/oxide layer gradually increased to the equilibrium one of Si-SiO2. Therefore, the Si and La are simultaneously oxidized, and the La2O3 dissolved in the SiO2 to form a compound glass scale, which further protects the coating from degradation. In order to theoretically determine the possibility of La-containing compound scale existence, assumed that LaB6, MoSi2 and SiC have been fully converted to La2O3, SiO2 and B2O3, the calculated phase diagrams of binary La2O3-SiO2 system [42,43], La2O3-B2O3 system [3,45] and ternary SiO2-B2O3-La2O3 system [46] are shown in Fig. 14. During the static oxidation at 1773 K and dynamic ablation (about 2023 K), the La2SiO5 phase can be formed in theory (Fig. 14(a)), while the phases within shadow area are in solid state, which hardly produce a liquid oxide scale of SiO2 and La2O3. Therefore, the formed liquid oxides might contain B element. As shown in the phase diagram of La2O3-B2O3 (Fig. 14(b)), under the theoretical ratio of the produced B2O3 and La2O3 (6:1), the product of LaB3O6 is most likely to be generated. However, as a matter of fact, the actual ratio of B2O3 and La2O3 is lower than theoretical value, so the content line moves left to form LaBO3 due to the volatilization of B2O3. Meanwhile, LaBO3 as well as the un-
volatilized B2O3 are in a liquid state. The solid phases of SiO2 and La2O3 are inclined to dissolve in these liquids because the liquid phase promotes faster mass transport than solid phase. With the increase of the temperature and the extension of oxidation time, element B gradually volatilizes and the La2O3 may reacts with SiO2 to form La2SiO5 (Fig. 14(c)). The more B element the coating has, the more LaBO3 and B2O3 produce, resulting in the greater probability of reaction between La2O3 and SiO2. Obviously, the element of B derived from raw materials inevitably involved, which facilitate the formation of La-Si-O-B compound, as shown in the triangular phase diagram of ternary SiO2-B2O3-La2O3 (Fig. 14(c)). Whatever, amorphous SiO2 as the main phase in glass scale directly determines the oxidation rate of MSL coating. Meanwhile the structure mode of SiO2 have been changed by doping lanthanum, as shown in Fig. 15. Glassy SiO2 has a network tetrahedral structure of SiO44, in which the lanthanum and boron atoms occupy part of the silicon atoms and other vacancies. The dispersion of La2O3 and B2O3 can reduce the viscosity of SiO2 glassy at high temperatures through forming a La-Si-O-B compound glass scale, which confers a self-repairing capability to the oxide glass scale, rather than a porous layer, thereby improving the oxidation resistance. Moreover, the good oxidation response is relevant to the low oxygen permeability of LaSi-O-B compound glass scale compared with pure SiO2 glass, which result in less formation and loss of oxides, thus ensure the
Fig. 15. Structure mode of SiO2 and is that for SiO2 dissolving La-B based oxides.
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
minimum degradation to the underlying carbon matrix from the static and dynamic oxidizing atmosphere. 4. Conclusions LaB6-modified MoSi2-SiC (MSL) oxidation-resistant coating has been designed and successfully fabricated on C/C composites by an efficient method of SAPS. The microstructural evolution and oxidative protection mechanism were investigated in detail. The compactness and micro-hardness of sprayed MSL coating were obviously improved compared to MS coating. Moreover, the MSL coating protected the C/C composites from static oxidation at 1773 K for 100 h with a mass loss of 6.52 mg cm2 and dynamic ablation about 2023 K for six cycles with a mass loss of 29.86 mg cm2, which was superior to the MoSi2-SiC (MS) coating without La-doping. The presence of rare earth La element promoted the liquid phase sintering of MoSi2 and B element facilitated the high-temperature stability of SiO2-rich glass scale, resulting in the formation of La-Si-O-B compound scale with a low volatility and oxygen permeability cover the coating surface, which contributed to improving the resistant to oxidative response of the MSL coating. This study provided an oxidation-resistant coating system, which was a good solution to seal the cracks and bubbles caused by CTE mismatch and gases. Acknowledgments This work has been supported by the National Natural Science Foundation of China under Grant No. 51772247 and 5172780072, the Basic Research Project in Shenzhen City of China under Grant No. JCYJ20160531172042899 and the Creative Research Foundation of Science and Technology on Thermostructural Composite Materials Laboratory under Grant No. 6142911050217. References [1] G. Kou, L.J. Guo, H.J. Li, Effect of copper on the heat erosion mechanism of carbon/carbon composites, J. Alloys Compd. 723 (2017) 1132e1141. [2] L. Zhuang, Q.G. Fu, H.J. Li, SiCnw/PyC core-shell networks to improve the bonding strength and oxyacetylene ablation resistance of ZrB2-ZrC coating for C/C-ZrB2-ZrC-SiC composites, Carbon 124 (2017) 675e684. [3] Y.J. Jia, H.J. Li, X.Y. Yao, Z.G. Zhao, J.J. Sun, Effect of LaB6 content on the gas evolution and structure of ZrC coating for carbon/carbon composites during ablation, Ceram. Int. 43 (2017) 3601e3609. [4] C.C. Wang, K.Z. Li, X.H. Shi, J. Sun, Q.C. He, C.X. Huo, Self-healing YSZ-La-Mo-Si heterogeneous coating fabricated by plasma spraying to protect carbon/carbon composites from oxidation, Compos. B Eng. 125 (2017) 181e194. [5] L. Li, H.J. Li, X.M. Yin, H.J. Lin, Q.L. Shen, X.Y. Yao, T. Feng, Q.G. Fu, Microstructure evolution of SiC-ZrB2-ZrC coating on C/C composites at 1773 K under different oxygen partial pressures, J. Alloys Compd. 687 (2016) 470e479. [6] X.C. Jin, X.L. Fan, C.S. Lu, T.J. Wang, Advances in oxidation and ablation resistance of high and ultra-high temperature ceramics modified or coated carbon/carbon composites, J. Eur. Ceram. Soc. 38 (2018) 1e28. [7] J.Y. Liu, Q.J. Tang, Y. Wang, The study of inspection on SiC coated carboncarbon composite with subsurface defects by lock-in thermography, Compos. Sci. Technol. 72 (2012) 1240e1250. [8] J.P. Zhang, Q.G. Fu, J.L. Qu, R.M. Yuan, H.J. Li, Blasting treatment and chemical vapor deposition of SiC nanowires to enhance the thermal shock resistance of SiC coating for carbon/carbon composites in combustion environment, J. Alloys Compd. 666 (2016) 77e83. [9] C.C. Wang, K.Z. Li, X.H. Shi, Q.C. He, C.X. Huo, High-temperature oxidation behavior of plasma-sprayed ZrO2 modified La-Mo-Si composite coatings, Mater. Des. 128 (2017) 20e33. [10] T. Feng, H.J. Li, M.H. Hu, H.J. Lin, L. Li, Oxidation and ablation resistance of the ZrB2-CrSi2-Si/SiC coating for C/C composites at high temperature, J. Alloys Compd. 662 (2016) 302e307. [11] Z.Y. Cai, S.N. Liu, L.R. Xiao, Z. Fang, W. Li, B. Zhang, Oxidation behavior and microstructural evolution of a slurry sintered Si-Mo coating on Mo alloy at 1650 oC, Surf. Coating. Technol. 324 (2017) 182e189. [12] Y.C. Wang, D. Su, H.M. Ji, X.L. Li, Z.H. Zhao, H.J. Tang, Gradient structure high emissivity MoSi2-SiO2-SiOC coating for thermal protective application, J. Alloys Compd. 703 (2017) 437e447. [13] H.A. Zhang, D.Z. Wang, S.P. Chen, X.Y. Liu, Toughening of MoSi2 doped by La2O3 particles, Mater. Sci. Eng., A 345 (2003) 118e121.
715
[14] C.C. Wang, K.Z. Li, X.H. Shi, C.X. Huo, Q.C. He, Y.D. Zhang, Effect of spraying power on oxidation resistance and mechanical properties of plasma sprayed La-Mo-Si coating, Surf. Coating. Technol. 311 (2017) 138e150. [15] L. Wang, Q.G. Fu, F.L. Zhao, Improving oxidation resistance of MoSi2 coating by reinforced with Al2O3 whiskers, Intermetallics 94 (2018) 106e113. [16] F.X. Niu, Y.X. Wang, I. Abbas, S.L. Fu, C.G. Wang, A MoSi2-SiOC-Si3N4/SiC antioxidation coating for C/C composites prepared at relatively low temperature, Ceram. Int. 43 (2017) 3238e3245. [17] J. Sun, Q.G. Fu, L.P. Guo, L. Wang, Silicide coating fabricated by HAPC/SAPS combination to protect niobium alloy from oxidation, ACS Appl. Mater. Interfaces 8 (2016) 15838e15847. [18] L.Y. Cao, Z. Bai, J.F. Huang, H.B. Ouyang, J.P. Wu, Ablation properties of Cf/CSiC-MoSi2 composites: effects of hydrothermal penetration temperature, J. Alloys Compd. 703 (2017) 45e55. [19] H.J. Li, T. Feng, Q.G. Fu, H. Wu, X.T. Shen, Oxidation and erosion resistance of MoSi2-CrSi2-Si/SiC coated C/C composites in static and aerodynamic oxidation environment, Carbon 48 (2010) 1636e1642. [20] J. Sun, Q.G. Fu, L.P. Guo, Y. Liu, C.X. Huo, H.J. Li, Effect of filler on the oxidation protective ability of MoSi2 coating for Mo substrate by halide activated pack cementation, Mater. Des. 92 (2016) 602e609. [21] F. Yu, D.G. Gu, Y.F. Zheng, Y.L. Luo, X.Y. Li, H. Chen, L.C. Guo, Influence of MoO3 on boron aluminosilicate glass-ceramic coating for enhancing titanium hightemperature oxidation resistance, J. Alloys Compd. 729 (2017) 453e462. [22] N.A. Nasiri, N. Patra, D. Horlait, D.D. Jayaseelan, W.E. Lee, Thermal properties of rare-earth monosilicates for EBC on Si-based ceramic composites, J. Am. Ceram. Soc. 99 (2016) 589e596. [23] J.Y. Gao, W. Jiang, Effect of La2O3 addition to modification of grain-boundary phase in MoSi2, J. Alloys Compd. 476 (2009) 667e670. [24] C.C. Wang, K.Z. Li, X.H. Shi, C.X. Huo, Q.C. He, Comparison of microstructure and oxidation behavior of La-Mo-Si coatings deposited by two approaches, Ceram. Int. 43 (2017) 10848e10860. [25] W.J. Tobler, W. Durisch, Plasma-spray coated rare-earth oxides on molybdenum disilicide high temperature stable emitters for thermophotovoltaics, Appl. Energy 85 (2008) 371e383. [26] Y. Wang, D.Z. Wang, W. Zhu, H.Y. Liu, X.Q. Zan, Microstructure and properties of hot-rolled 2.0wt.% MoSi2/rare earth oxide doped molybdenum alloys, Int. J. Refract. Metals Hard Mater. 31 (2012) 152e156. [27] S. Majumdar, B. Gorr, H.J. Christ, D. Schliephake, M. Heilmaier, Oxidation mechanisms of lanthanum-alloyed Mo-Si-B, Corrosion Sci. 88 (2014) 360e371. [28] Y. Wang, D.Z. Wang, H.Y. Liu, W. Zhu, X.Q. Zan, Preparation and characterization of sintered molybdenum doped with MoSi2/La2O3/Y2O3 composite particle, Mater. Sci. Eng., A 558 (2012) 497e501. [29] Y. Liu, Q.G. Fu, F.L. Zhao, G.D. Sun, H.J. Li, Internal friction vs. thermal shock in C/C composites, Compos. B Eng. 106 (2016) 59e65. [30] L. Li, H.J. Li, H.J. Lin, L. Zhuang, S.L. Wang, T. Feng, X.Y. Yao, Q.G. Fu, Comparison of the oxidation behaviors of SiC coatings on C/C composites prepared by pack cementation and chemical vapor deposition, Surf. Coating. Technol. 302 (2016) 56e64. [31] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564e1583. [32] Y.H. Chu, H.J. Li, Q.G. Fu, L.H. Qi, L. Li, Bamboo-shaped SiC nanowiretoughened SiC coating for oxidation protection of C/C composites, Corrosion Sci. 70 (2013) 11e16. [33] J. Li, R.Y. Luo, C. Lin, Y.H. Bi, Q. Xiang, Oxidation resistance of a gradient selfhealing coating for carbon/carbon composites, Carbon 45 (2007) 2471e2478. [34] J. Pang, W. Wang, C.G. Zhou, Microstructure evolution and oxidation behavior of B modified MoSi2 coating on Nb-Si based alloys, Corrosion Sci. 105 (2016) 1e7. [35] J.P. Zhang, Q.G. Fu, Y.J. Wang, Interface design and HfC additive to enhance the cyclic ablation performance of SiC coating for carbon/carbon composites from 1750 C to room temperature under vertical oxyacetylene torch, Corrosion Sci. 123 (2017) 139e146. [36] Y.J. Jia, H.J. Li, X.Y. Yao, J.J. Sun, Z.G. Zhao, Long-time ablation protection of carbon/carbon composites with different-La2O3-content modified ZrC coating, J. Eur. Ceram. Soc. 38 (2018) 1046e1058. [37] H.A. Zhang, S.Y. Gu, N.P. Xie, Effect of La2O3 on the wear behavior of MoSi2 at high temperature, J. Rare Earths 29 (2011) 370e373. [38] Y. Cui, A. Li, B. Li, X. Ma, R. Bai, W. Zhang, M. Ren, J. Sun, Microstructure and ablation mechanism of C/C-SiC composites, J. Eur. Ceram. Soc. 34 (2014) 171e177. [39] S.P. Sun, X.P. Li, H.J. Wang, Y. Jiang, D.Q. Yi, Adsorption of oxygen atom on MoSi2 (110) surface, Appl. Surf. Sci. 382 (2016) 239e248. [40] 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) 335e340. [41] J.P. Zhang, Q.G. Fu, J.L. Qu, H.S. Zhou, N.K. Liu, Surface modification of carbon/ carbon composites and in-situ grown SiC nanowires to enhance the thermal cycling performance of Si-Mo-Cr coating under parallel oxyacetylene torch, Corrosion Sci. 111 (2016) 667e674. [42] S.S. Kim, J.Y. Park, T.H. Sanders, Thermodynamic modeling of the miscibility gaps and the metastability in the R2O3-SiO2 systems (R¼La, Sm, Dy, Er), J. Alloys Compd. 321 (2001) 84e90. [43] L. Li, Z.J. Tang, W.Y. Sun, P.L. Wang, Phase diagram prediction of the Al2O3-
716
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
SiO2-La2O3 system, J. Mater. Sci. Technol. 15 (1999) 439e443. [44] E.M. Levin, C.R. Robbins, J.L. Waring, Immiscibility and the system lanthanum oxide-boric oxide, J. Am. Ceram. Soc. 44 (1961) 87e91. n, Glass formation and structure[45] S. Iftekhar, J. Grins, P.N. Gunawidjaja, M. Ede property-composition relations of the RE2O3-Al2O3-SiO2 (RE¼La, Y, Lu, Sc)
systems, J. Am. Ceram. Soc. 94 (2011) 2429e2435. goue €ta, D. Cauranta, O. Maje rusa, T. Charpentierb, T. Lerougea, [46] H. Tre L. Cormier, Exploration of glass domain in the SiO2-B2O3-La2O3 system, J. Non Cryst. Solids 476 (2017) 158e172.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Evolution of microstructural feature and oxidation behavior of LaB6-modified MoSi2-SiC coating Changcong Wang a, Kezhi Li a, *, Caixia Huo a, Qinchuan He a, Xiaohong Shi a, b, ** a b
State Key Laboratory of Solidification Processing, Carbon/carbon Composites Research Center, Northwestern Polytechnical University, Xi'an, 710072, China Shenzhen Research Institute of Northeastern Polytechnical Uinversity, Shenzhen, 518057, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 January 2018 Received in revised form 14 April 2018 Accepted 23 April 2018 Available online 24 April 2018
LaB6-modified MoSi2-SiC (MSL) oxidation resistant coating was designed and successfully fabricated on SiC-coated carbon/carbon (C/C) composites using efficient supersonic atmospheric plasma spraying (SAPS) method. The MSL coating protected the C/C composites from static oxidation at 1773 K for 100 h with a mass loss of 6.52 mg cm2 and dynamic ablation about 2023 K for six cycles with a mass loss of 29.86 mg cm2, which was superior to the MoSi2-SiC (MS) coating without La-doping. The presence of rare earth La element promoted the liquid phase sintering of MoSi2 and B element facilitated the hightemperature stability of SiO2-rich glass scale, resulting in the formation of La-Si-O-B compound scale with a low volatility and oxygen permeability cover the coating surface, which contributed to improving the resistant to oxidative response of the MSL coating. © 2018 Elsevier B.V. All rights reserved.
Keywords: Carbon/carbon composites Static isothermal oxidation Dynamic cyclic ablation Compound scale
1. Introduction Carbon/carbon (C/C) composites have caught much attention for the demand of thermal-structural components in the aerospace fields because of their unique performances such as low density, high specific strength, resistance to thermal shock, wear, chemical attack [1e3]. The broader introduction of C/C composites can lead to weight reduction, longer service lifetime, more design flexibility and cost economy. Regrettably, their intrinsic weakness of oxidative sensitivity hinders its applications in the high-temperature aerobic environment [4,5]. The degradation of C/C composites is controlled by the transport of gaseous molecules to react with matrix carbon and formed volatile gaseous products [6,7]. Therefore, the C/C composites should be protected when exposed to aerobic environments. Surface coatings are usually applied to protect C/C composites against long-term static oxidation and short-term cyclic ablation in a dynamic scour environment of oxyacetylene flame [2,4,5,8e10]. Intermetallic molybdenum disilicide (MoSi2) with a melting point of 2303 K was the focus of many studies on oxidative protection for
* Corresponding author. ** Corresponding author. State Key Laboratory of Solidification Processing, Carbon/carbon Composites Research Center, Northwestern Polytechnical University, Xi'an, 710072, China. E-mail addresses: [email protected] (K. Li), [email protected] (X. Shi). https://doi.org/10.1016/j.jallcom.2018.04.267 0925-8388/© 2018 Elsevier B.V. All rights reserved.
multiple materials due to their excellent high-temperature stability and low oxygen permeability [11,12]. However, the MoSi2 combined the dual characteristics of metal (>1273 K, high-temperature plasticity) and ceramic (<1273 K, low-temperature brittleness), which caused by cracking and peeling off the coating due to the structural transformation of MoSi2 in the process of high-low temperature cyclic oxidation [13]. Therefore, low-temperature toughening and high-temperature reinforcement are the challenges to be controlled urgently in the engineering application of MoSi2 structural material. In addition, the “pesting” oxidation of MoSi2 in lowmedium temperatures (<1173 K) was another critical factor hindering its application [14e16]. The good oxidation resistance of MoSi2 in the temperature range of 1273e2173 K was displayed due to the formation of SiO2 continuous glass protective film with selfhealing ability [17e19]. Nevertheless, at the medium and low temperatures of 673e1273 K, poor fluidity of the vitreous silica phase was difficult to produce a continuous dense protective film to prevent oxygen from infiltration. Meanwhile, the formed MoO3 with porous structure was easily transformed into a powder and exfoliated from the surface of MoSi2 under the effect of interfacial stress, and then catastrophic oxidation occurred [20,21]. All of the above challenges indicated that the MoSi2-based materials for use at elevated temperatures were restricted and it was worth exploring the potential of a new heterogeneous systems regarding the increase in compactness of MoSi2 and reduce in evaporation of gaseous products.
704
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
To enhance the low-temperature toughness and hightemperature strength of MoSi2 and reduce the evaporation of gaseous products, many efforts have been made on rare earth elements (RE ¼ La, Y, Ce etc.) to MoSi2 to form a polyphase solid solution and refine the crystalline structure [22e24]. It was reported that the rare earth compound coatings had been successfully prepared on MoSi2 and a good match between substrate and coatings was proved during variable temperature cycles process at temperatures above 1773 K without apparent delamination [25,26]. Doping of rare earth oxide (La2O3/Y2O3) into Mo-Si-B also showed encouraging results concerning the oxidation-resistant ability in 1073e1273 K and mechanical properties [27,28]. However, the mechanism of rare earth elements on silica scale forming process was not well revealed in detail owing to the sophisticated molecular structure of thermally grown oxides glass layer. Enlightened by this, in present work, attempts were made to study the microstructural feature and oxidation behavior of MoSi2-SiC coating with the addition of LaB6 in variable proportions at the selected conditions of static air and dynamic oxygen acetylene erosion. The Lacontaining oxide glass scale were characterized to understand the mechanisms in silica-based amorphous glass layer, supported by thermodynamic considerations.
area mass change rate (n1) were evaluated as the following Eqs. (1) and (2).
Dw1 ¼ n1 ¼
m0 mt S1
m0 mt S1 $t1
(1)
(2)
where m0 is the original mass of the tested specimen; mt is the mass of the tested specimen after oxidation for a certain time; S1 is the total surface area of the specimen; t1 is the oxidation time. In addition, the non-isothermal oxidation behavior of the specimens was also studied using the thermogravimetric (TG) analyzer from 573 K to 1873 K at the heating rate of 10 K min1. 2.3. Dynamic thermal shock test To measure the thermal corrosion resistance of sprayed coatings in a dynamic environment, cyclic thermal shock test from 2023 K to ambient temperature was conducted under vertical oxyacetylene torch. During the process of cyclic thermal shock, the pressure and
2. Experimental procedure 2.1. Materials and coating preparation Two-dimensional (2D) C/C composites with a density of 1.75 g cm3 were prepared using a commonly method of isothermal chemical vapor infiltration (ICVI) [29], which were used as substrates in this study. The C/C specimens were hand-abraded with SiC sandpapers (200 grit, 400 grit), and then cleaned ultrasonically with ethanol for 2 h and dried at 353 K for 24 h. SiC buffer coating was preferentially prepared by pack cementation, and the details of which were described elsewhere [30]. Next, the LaB6-modified MoSi2-SiC coating was deposited on SiCcoated C/C composites using supersonic atmospheric plasma spraying (SAPS) method. Powder compositions were as follows: 5e15 wt.% LaB6, 25 wt.% SiC and 60e70 wt.% MoSi2, which were dry-mixed and roller milled to guarantee uniformity. The obtained mixed powders were agglomerated by a spray tower drier to enhance the fluidity of particles for spraying, and then sprayed to form the desired coatings with the power of 50e55 kW. To accelerate the particles melting, a feedstock nozzle with an internal diameter of 5.5 mm was selected, and the distance between nozzle and specimen was kept at 100 mm. The feed rate was controlled at 4.5 r min1. For comparing, the MoSi2-SiC coating without LaB6 doping was obtained using the same technique. To simplify the discussion, the sprayed coating with LaB6 content of 0 wt.%, 5 wt.%, 10 wt.% and 15 wt.% were identified as MS, MSL5, MSL10 and MSL15, respectively. 2.2. Oxidation tests Static isothermal oxidation behavior of the specimens (10 mm 10 mm 10 mm) was evaluated at 1773 K in a constant temperature furnace. Prior to the oxidation experiment, the corundum crucibles were pre-heated at 1773 K until no discernible mass change. Afterwards, the specimens were placed in corundum crucibles for a designated time, and they were removed and cooled down to indoor temperature in air. The mass of the oxidized specimens was obtained by weighing in an electric balance with a sensitive of ±0.1 mg. The relationship between the mass changes of specimens and oxidation duration was described. Throughout the oxidation process, the per unit area mass change (Dw1) and per unit
Fig. 1. Surface SEM images of the sprayed coatings at different magnifications: (a1, a2) MS coating; (b1, b2) MSL5 coating; (c1, c2) MSL10 coating; (d1, d2) MSL20 coating.
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
flux of O2 were 0.4 MPa and 0.86 m3 h1, and those of C2H2 were 0.095 MPa and 0.65 m3 h1, respectively. The distance between oxyacetylene gun and specimen was adjusted to 10e15 mm to ensure the surface temperature of specimen reach 2023 K. The ablation time at 2023 K was 4 s in each cycle under oxyacetylene torch. The cyclic thermal shock test was carried out with a duration of 10 s in this study. After that, the oxyacetylene flame was removed, and the specimens (F30 mm 10 mm) were cooled down naturally before the next thermal cycle until 6 cycles were reached. The per unit area mass change (Dw2) and per unit area mass change rate (n2) were represented by following Eqs. (3) and (4).
Dw2 ¼ n2 ¼
m0 mt S2
m0 mt S2 $t2
(3)
(4)
where m0 and mt are the mass of tested specimens before and after thermal shock test, respectively; S2 is the area of contact surface; t2 is the number of thermal cycles. During the dynamic thermal shock test, three specimens formed a group, and the final experimental data was the average of three specimens. 2.4. Characterization Microstructural features and morphologies of the specimens
705
were analyzed by scanning electron microscopy (SEM, JSM6460) and transmission electron microscope (TEM, JEM3010), which were equipped with EDS. Roughness values of different specimens were measured using a 3D confocal laser microscope (Optelics C130). Coefficient of thermal expansion (CTE) from 1273 K to 2073 K was evaluated by a DIL402E Dilatometer in He ambience with a heating rate of 10 K min1. Hardness and elastic modulus of the sprayed coatings were measured using micron indentation techniques (MHT-1, NANOVEA) with a quadrangular pyramid diamond indenter on the wellpolished cross-section of sprayed coatings. During the test, the maximum pressure of the indenter was set to 2 N, the pouring rate was 4 N min1, and the creep time was 10 s at maximum load. The location of indentation center was selected as the middle of coating cross-section. Ten different locations were tested to acquire the mean of the hardness and elastic modulus according to Oliver and Pharr principle [31]. The fracture toughness (KIC) of sprayed coatings was characterized by indentation fracture, and the type and size were same as that of the upper hardness test, which was calculated using the following Eq. (5).
KIC ¼ 0:016
1=2 E 1000,P H ð1 þ aÞ3=2
(5)
where E and H are elastic modulus and hardness of the coating, respectively. Symbol a is half the length of the indentation diagonal, and P is the applied load. Porosity of the sprayed coatings was tested using the
Fig. 2. Roughness comparative results of the MS, MSL5, MSL10, MSL15 coatings (a); three-dimensional (3D) surface topography of the MS coating (b) and MSL10 coating (c).
706
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
Archimedes drainage method. Firstly, the specimens with sprayed coating were chemically etched by aqua regia solution at 393e423 K, then the peeled coating was subjected to low power ultrasonic cleaning and placed in air at 343 for drying. They were weighed using an analytical electronic balance, denoted as m0. Secondly, the peeled coating was boiled in deionized water for 2e3 h so that the pores inside the coating were filled with water, and the floating mass of m1 and the wet mass of m2 were weighed on a special balance. The porosity (p) was calculated using Eq. (6).
r¼
m2 m0 m2 m1
coating with various doping content of LaB6 are presented in Fig. 2. It can be observed that average values of surface roughness are 25.89 mm of MS coating, 20.46 mm of MSL5 coating, 17.23 mm of MSL10 coating and 19.24 mm of MSL15 coating (Fig. 2(a)), which demonstrate the reduction of 33.45% in MSL10 coating by comparing with MS coating. Obviously, the fluctuation of the surface structure is consistent with the morphologies observed under SEM. Fig. 3 shows the cross-section BSE images of the specimens with
(6)
where m0, m1, m2 are the mass after drying, the mass in the water, the mass containing deionized water, respectively. 3. Results and discussion 3.1. Microstructural features and morphologies of the sprayed coating Fig. 1 compares the surface morphologies of sprayed MS, MSL5, MSL10 and MSL15 coatings. Representative sprayed coating characteristics with molten and semi-molten particles intertwined are revealed, but their microstructural features are varied due to the different doping amount of LaB6. Specifically, with the doping amount of LaB6 increased from 0 wt.% to 10 wt.%, the sprayed coatings as a whole became flatter (Fig. 1(a1, b1, c1)) and compact along with some small pores by stacking of semi-molten particles (Fig. 1(a2, b2, c2)). Further increasing the doping amount of LaB6, large un-melted granules gradually accumulated on the coating surface, resulting in a significant growth in defects (Fig. 1(d1)) and decrease in compactness of coating (Fig. 1(d2)). Three-dimension (3D) surface profiles and corresponding surface roughness of the
Fig. 4. Comparison of porosity on four different coatings: MS coating; MSL5 coating; MSL10 coating; MSL15 coating.
Fig. 3. Cross-sectional BSE images of the sprayed coatings: (a) MS coating; (b) MSL5 coating; (c) MSL10 coating; (d) MSL15 coating.
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
MS, MSL5, MSL10, MSL15 coatings, both of which exhibits a distinct bilayer structure. For the MS-coated specimen, the interface between exterior MS layer and interior SiC layer appeared many cracks and holes, as displayed in Fig. 3(a). However, owing to the rare earth La element was incorporated, the number of defects in the coating and at the junction was dramatically reduced. In particular, when the doping amount of LaB6 was 10 wt.%, the overall compactness and connectivity of sprayed coating were optimized (Fig. 3(bed)). Meanwhile, the porosity of sprayed coating was measured using Archimedes principle, and the corresponding results are shown in Fig. 4. Increasing the doping amount of LaB6 from 0 wt.% to 10 wt.%, the porosity of sprayed coating decreased from 15.88% (MS) to 7.89% (MSL10), indicating that the introduction of rare earth La element has a positive effect on the improvement of coating compactness, while the excessive doping of LaB6 has side effects on the denseness of sprayed coating. Based on the exhibited results, doping 10 wt.% LaB6 into the coating is a relatively suitable choice in this study.
707
To more clearly observe the distribution of rare earth La element within the coating, TEM is adopted to further characterization, and the corresponding TEM images of MSL10 coating are exhibited in Fig. 5. It was clearly that white phase A and black phase B were interactively distributed together, as shown in Fig. 5(a), and the black phase was mainly concentrated in the phase boundary of white phase. The EDS results (Fig. 5(e and f)) revealed that the chemical composition of phases A and B respectively were 56.23Mo-43.77Si (At. %) and 62.49La-37.51B (At. %), inferring that the phase A was MoSi2 and the phase B was LaB6. Meanwhile, SAED patterns of phases A and B are shown in Fig. 5(c and d). Two sides of the diffraction parallelogram for the phase A were identified to be (211) and (0e11) reflections in [222] zone spots of MoSi2. While for the phase B, they were identified to be (320) and (222) reflections in [46-2] zone spots of LaB6. These clear diffraction spots suggested the single crystal structure of the phases in the coating with few defects. Moreover, Fig. 5(b) demonstrates the high-resolution transmission electron microscope (HRTEM) image of the interface
Fig. 5. TEM images of the MSL10 coating to observing the distribution of rare element La: (a) bright field image; (b) HRTEM image of representative interface between MoSi2 and LaB6; (c, d) the SAED patterns of (a) confirming presence of MoSi2 and LaB6; (e, f) the EDS results of spots A and B in (a).
708
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
between MoSi2 and LaB6. The interplanar spacings of MoSi2 and LaB6 respectively were measured to be 0.142 nm and 0.115 nm, consistent with that of the (211) plane of MoSi2 and (320) plane of LaB6. Based on the above analysis, it can be deduced that the rare earth La element inclines to be distributed at the phase boundaries or defects within the coating, then causing the changes in physical, chemical or interface energies of the grain boundaries and the precipitation of new phase, which eventually lead to the variation in microstructure and performance of modified coating. Previous studies have proved that the rare earth compounds have a positive influence on the mechanical properties of MoSi2based materials [22e28]. In this study, the effect of LaB6 on the hardness and elastic modulus of MoSi2-based coatings was investigated using micro-indentation method and the results are shown in Fig. 6. From Fig. 6(a), it can be seen that the penetration depth of MSL10 coating (3.62 ± 0.25 mm) was significantly lower than that of MS coating (2.79 ± 0.18 mm) under the same positive pressure load. The comparison of hardness and elastic modulus of MS coating and MSL10 coating are exhibited in Fig. 6(b). Specifically, prior to introducing LaB6 in the coating, the hardness and elastic modulus of MS coating were 9.75 ± 1.52 GPa and 148.96 ± 12.58 GPa, respectively. After the incorporation of LaB6 in the coating, the hardness and elastic modulus of MSL10 coating were increased to 13.52 ± 1.68 GPa and 180.64 ± 14.31 GPa, which separately increased by 38.67% and 21.27% compared to MS coating. It is well known that the hardness and elastic modulus of ceramic coating have an important impact on their toughness, and the coatings with greater hardness and elastic modulus usually possess better toughness [32]. Therefore, it is reasonable to speculate that the toughness of sprayed MS coating can be enhanced by introducing LaB6, which is also beneficial for the high-temperature performance of MSL coating. 3.2. Isothermal static oxidation behavior of the sprayed coating Isothermal static oxidation can be considered as a mild oxygen corrosion process due to the absence of dynamic disturbances. Under this condition, the relationship between the mass change of the specimens and oxidation duration was revealed in Fig. 7(a and b). After oxidation at 1773 K for 100 h, the per unit area mass loss of MS coating and MSL10 coating respectively were 15.45 mg cm2 and 6.52 mg cm2 (Fig. 7(a)), and the corresponding mass loss rate separately were 0.15 mg cm2 h1 and 0.06 mg cm2 h1 (Fig. 7(b)). This result showed that the MSL10 coating possessed a better oxidation resistance than the MS coating due to the incorporation of LaB6 in the MSL coating. Meanwhile, during the mass gain process, the maximum mass gain of the MSL10 coating (5.21 mg cm2) was almost twice that of the MS coating (2.84 mg cm2), indicating that the presence of LaB6 promoted the formation rate of oxides during the oxidation process, thereby gaining mass in a relatively short period of time. The oxidation process of the specimens with MS coating and MSL10 coating was monitored using thermogravimetric (TG) experiments in a wide temperature range from 573 K to 1873 K, and the results are shown in Fig. 7(c). The overall process of oxidation can be divided into three stages, and each stage represents various oxidation behavior and self-healing mechanisms of cracks with the increase of oxidation temperature. At the temperature from 573 K to 1173 K (stage A), there was no significant mass loss in MSL10coated specimen, while the mass of MS-coated specimen was gradually reduced, which attributed to the formation of B2O3 to filling in cracks and blocking oxygen diffusion. The formation of B2O3 was accompanied by a 250% volume expansion [33], which could fill in crack also. In addition, the produced SiO2 was difficult to effectively heal cracks due to its large viscosity in this
Fig. 6. Representative loads vs. displacement curves of MS coating and MSL10 coating under the same positive pressure (a); hardness and elastic modulus comparative results of MS coating and MSL10 coating (b).
temperature range. During the temperature from 1173 K to 1473 K (stage B), the MSL10-coated specimen began to lose mass gradually, and the mass loss rate of MS-coated specimen was also faster than the previous stage A. With the increase of oxidation temperature, the oxidation rate of the coating accelerated and the number of cracks in the coating increased. The generated cracks were not enough to healed by B2O3 and SiO2 due to low production of B2O3 and insufficient fluidity of SiO2, while the generated B2O3 could delay the propagation of cracks to some extent. Further increasing the temperature from 1473 K to 1873 K (stage C), the oxidation of MoSi2 and SiC would form SiO2 with a low viscosity at this time, which exhibited self-healing properties to close down both the superficial and penetrative cracks in the coating. Meanwhile, the generated B2O3 and La2O3 could be dissolved in the SiO2-rich scale to form polyphase silicates (B-La-Si-O scale), which had much lower volatility and oxygen permeability [24,34], thereby inhibited the volatilization of B2O3. However, for MS-coated specimen, with the increase of oxidation temperature, the thickness of formed single SiO2 scale might decrease gradually due to the volatilization of gaseous products (MoO3, CO2, CO, SiO). Therefore, the mass loss rate of MS-coated specimen slowly increased, while the mass of MSL10-coated specimen was almost constant. In brief, the presence of La element and B element in the coating has a positive effect on the oxidation protection of the MSL coating in a wide temperature domain. Fig. 8 shows the surface morphologies of the specimens with MS
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
709
Fig. 7. Isothermal (a, b) and non-isothermal (c) oxidation kinetics curves of MS and MSL10 coatings: (a) per unit area mass change (mg cm2) vs oxidation time (h); (b) per unit area mass change rate (mg cm2 h1) vs oxidation time (h); (c) TG results from 573 K to 1873 K.
and MSL10 coatings after oxidation at 1773 K for 100 h. From Fig. 8(a1), it was readily observed that the glass scale with numerous cracks covered the surface of MS coating, and many particles were randomly distributed in the glass scale to aggregate into larger protrusions. Enlargement of these protrusions revealed that the build-up of particles caused more cracks and stomatal formation because of the produced SiO2-rich glass was difficult to effectively heal them, as shown in Fig. 8(a2). Compared with MS coating, a crack-free MSL10 coating with many flocculating products was observed (Fig. 8(b1)), inferring that the introduction of LaB6 promoted the directional growth of partial oxides. The white phase and gray phase tightly wrapped together (Fig. 8(b2)), which respectively were detected by EDS (Fig. 8(c and d)) as Si-rich oxide (containing elements of Si, Mo, O) and La-Si-B rich oxide (containing elements of Si, La, O, B), indicating that the formation of LaSi-O-B compound scale on the coating surface, which provided an effective protection for C/C composites. Moreover, many wrinkles were also discovered on the MSL10 coating surface (Fig. 8(b2)), which attributed to the compressive stress affected the oxidation behavior by eliminating the coating cracking during the cooling of the coated specimen. Therefore, LaB6-modified MoSi2-SiC coating can be considered as a promising oxidation resistant coating for C/C composites. 3.3. Dynamic cycling hot corrosion behavior of the sprayed coating Previous thermal shock experiments were mainly performed in
static air in a high-temperature furnace, where the gas environment could be regarded as a thermal equilibrium [29,35], while the erosion of dynamic combustion gas close to practical application environment was usually overlooked. However, in this research, a high-temperature dynamic gas erosion (about 2023 K) was constructed to simulate the service capability of the coating in practical application, and the cyclic thermal shock test from 2023 K to room temperature was proceeded under the vertical oxyacetylene flame. Fig. 9 compares the per unit area mass change (or per unit area mass change rate) of the specimens with MS and MSL10 coatings during cyclic thermal shock test. With respect to MS coating, after six thermal cycles from 2023 K to room temperature, the per unit area mass loss and per unit area mass loss rate were 48.97 ± 4.52 mg cm2 and 8.16 ± 0.48 mg cm2 t1, respectively. Compared with MS coating, the per unit area mass loss and per unit area mass loss rate were severally reduced by 39.02% to 29.86 ± 4.28 mg cm2 and 39.09% to 4.97 ± 0.42 mg cm2 t1 after six thermal cycles under the identical condition. Meanwhile, neither coating showed apparent mass gain during the whole thermal shock process (Fig. 9), while the mass loss rate of MSL10 coating was significantly lower than that of un-doped MS coating. The above results directly indicated that the addition of LaB6 improved the thermal shock resistance of MS coating in the dynamic combustion environment. The surface and cross-section morphologies after cyclic thermal shock test are revealed in Fig. 10. Observation under low magnification micrographs found that there was no apparent spalling on
710
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
Fig. 8. Surface SEM images of the MS coating (a1, a2) and MSL10 coating (b1, b2) after oxidation for 100 h at 1773 K, and the EDS results of spots C and D in (b2).
Fig. 9. Variation in per unit area mass change (a) (or per unit area mass change rate (b)) with the increase of thermal cycles of the MS and MSL10 coatings under vertical oxyacetylene torch.
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
711
Fig. 10. Surface (a1, a2, b1, b2) and cross-section (a3, b3) SEM images of the MS and MSL10 coated specimens after thermal shock test: (a1-a3) MS coating; (b1-b3) MSL10 coating.
the surface of MS and MSL10 coatings (Fig. 10(a1, b1)), while the evacuation channels of numerous pores with various sizes were produced owing to the escape of some gaseous products (B2O3, MoO3, CO, CO2, etc.) with a low melting point from oxide glass scale. By comparing Fig. 10(a2) with Fig. 10(b2), the MS coating presented an obvious porous surface structure, while the surface of MSL10 coating remained dense with the coverage of oxides scale. After the incorporation of LaB6 in the coating, the MoSi2 granules connected with the melting degree became more larger and the defects dramatically shrunk as demonstrated in Fig. 10(b2). Some small particles containing La element were randomly distributed between oxide scale and MSL grains. Among them, the rare earth La element played a dual role including promote the creation of big grains and separate the LaB6 small particles as well as heal the defects. As shown in Fig. 10(a3, b3), a loose cross-section structure with some cracks and pores was observed in MS-coated specimen, while the cross-section of MSL10-coated specimen still reminded a dense feature without obvious cracks. The formation of numerous cracks in the MS coating was mainly caused by the thermal mismatch during thermal cycling process, and the generated SiO2-
rich glass scale was difficult to heal them for a short period of time. After the introduction of LaB6 in the MS coating, the La and B elements promoted the formation rate and fluidity of SiO2-rich glass scale to produce La-Si-O-B compound scale, then the cracks caused by thermal mismatch was healed timely, thereby the MSL10-coated specimen exhibited a compact structure without visible cracks (Fig. 10(b3)). The generation of La-Si-O-B compound scale served as an oxygen barrier to improve the oxidation resistance of MSL10 coating. For the purpose of further studying the formation mechanism and microstructure of small particles (marked in Fig. 10(b2)), TEM observations of these particles are shown in Fig. 11. The size of these small particles distributed in tens of nanometers to hundreds of nanometers (Fig. 10(b2)). The explicit boundaries between particles were detected (Fig. 11 (a, b)), and homogeneous areas were distributed inside the particles. The representative high-resolution TEM image of area C (marked in Fig. 11(b)) revealed four kinds of phases: La2O3, La2SiO5, La(BO2)3 and amorphous phase, as shown in Fig. 11(c), indicating that a diffusion process and dissolution of La into SiO2 scale might occur at elevated temperatures. The formation
712
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
Fig. 11. TEM images of small particles distributed in MSL10 coating (a, b); HRTEM image of representative interface between these particles (c); the SAED pattern of (b) confirming presence of La2O3, La2SiO5, La(BO2)3 and amorphous phase.
of amorphous phase covering layer also accelerated this process owing to the higher diffusion rate of La in liquid than that of solid [36]. Moreover, the La-containing oxides were encapsulated by amorphous layer according to the selected area electron diffraction (SAED) pattern (Fig. 11(d)). Among them, the semi-melted La(BO2)3 and La2SiO5 dispersed to these particles (Fig. 11(c)). These Lacontaining oxides were solid solution of silica, in which La occupied the position of Si atoms. This replacement could inevitably improve the melting point of silica (compared with pure silica, La-
Fig. 12. Changes of the standard Gibbs free energies of reactions (7e10) at different temperatures calculated by Fact-Sage software.
containing solid solution possessed a higher melting point [37]), which were more prone to occur at grain boundaries of these particles. These areas became the highly active areas of the oxide scale during oxidation and reduced the evaporation of pure silica at elevated temperatures. 3.4. Oxidation protective mechanism of the sprayed coating In order to explore the protective mechanism of MSL coating,
Fig. 13. Coefficients of thermal expansion (CTE) of the MS coating and MSL10 coating.
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
the reactions and phase transformation in the oxidation process should be carefully considered. The following reactions might occur during the static and dynamic oxidation experiments.
2MoSi2ðsÞ þ 7O2ðgÞ/2MoO3ðgÞ þ 4SiO2ðlÞ
(7)
21 O2ðgÞ/La2O3ðlÞ þ 6B2O3ðgÞ 2
(8)
2LaB6ðsÞ þ
SiCðsÞ þ 2O2ðgÞ/SiO2ðlÞ þ CO2ðgÞ SiO2ðlÞ þ La2O3ðlÞ/La2SiO5ðlÞ
(9) (10)
The standard Gibbs free energy for the above Eqs. (7)e(10) are shown in Fig. 12. It is well known that the lower Gibbs free energy of reaction suggests the more obvious trend of the reaction [38]. Based on the results of Fig. 12, it can be inferred that the chemical reactions (7) and (8) play the leading roles, suggesting that the performance enhancement of MSL coating is chiefly attributed to the formation of compact La-Si-O-B compound scale with a suitable viscosity and better stability in extremely hot environment relative to pure SiO2 scale. In addition, the generation of low-pointing oxides (MoO3, B2O3) with a high vapor pressure and volatility plays a dual role: accelerate the enrichment of Si toward coating surface
713
and development of SiO2-rich scale [21,39,40], and destroy the integrity of La-Si-O-B scale. Moreover, thermal stress derived from thermal mismatch between coatings and substrate is crucial to the service life of applying coating during oxidation process, which can be described as follows [41].
DT,E s ¼ Da, 1n
(11)
where DT is difference between the testing temperature and cooling temperature, Da is the difference of CTE between coating and matrix, E and n are the Young's modulus and Poisson's ratio of coating materials, respectively. In accordance with the above Eq. (11), when the variables of DT, E and n are considered to be same, the greater thermal stress can be caused by larger CTE mismatch. The CTE results of MSL10 and MS coatings in the temperature range of 1273e2023 K are shown in Fig. 13. The difference in CTE between MSL10 coating and substrate is lower than that of MS coating, so the larger thermal stress can be preserved in the MS coating, thereby resulting in the formation of cracks. Meanwhile, the hightemperature stability and crack healing capacity of single SiO2 scale are all inferior to the La-Si-O-B compound scale, therefore the MSL10 coating presents a better oxidation resistance than MS coating.
Fig. 14. Calculated phase diagrams of multiple systems: (a) binary La2O3-SiO2 system [42,43]; (b) binary La2O3-B2O3 system [44,45]; (c) ternary SiO2-B2O3-La2O3 system [46, 47].
714
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
Proverbially, lanthanum has a better affinity with oxygen than silicon, so the lanthanum atoms are more likely to spread to the coating surface to form La2O3 because of selectively oxidation, while the single La2O3 cannot effectively block the oxygen diffusion. The oxygen partial pressure at the interface of coating/oxide layer gradually increased to the equilibrium one of Si-SiO2. Therefore, the Si and La are simultaneously oxidized, and the La2O3 dissolved in the SiO2 to form a compound glass scale, which further protects the coating from degradation. In order to theoretically determine the possibility of La-containing compound scale existence, assumed that LaB6, MoSi2 and SiC have been fully converted to La2O3, SiO2 and B2O3, the calculated phase diagrams of binary La2O3-SiO2 system [42,43], La2O3-B2O3 system [3,45] and ternary SiO2-B2O3-La2O3 system [46] are shown in Fig. 14. During the static oxidation at 1773 K and dynamic ablation (about 2023 K), the La2SiO5 phase can be formed in theory (Fig. 14(a)), while the phases within shadow area are in solid state, which hardly produce a liquid oxide scale of SiO2 and La2O3. Therefore, the formed liquid oxides might contain B element. As shown in the phase diagram of La2O3-B2O3 (Fig. 14(b)), under the theoretical ratio of the produced B2O3 and La2O3 (6:1), the product of LaB3O6 is most likely to be generated. However, as a matter of fact, the actual ratio of B2O3 and La2O3 is lower than theoretical value, so the content line moves left to form LaBO3 due to the volatilization of B2O3. Meanwhile, LaBO3 as well as the un-
volatilized B2O3 are in a liquid state. The solid phases of SiO2 and La2O3 are inclined to dissolve in these liquids because the liquid phase promotes faster mass transport than solid phase. With the increase of the temperature and the extension of oxidation time, element B gradually volatilizes and the La2O3 may reacts with SiO2 to form La2SiO5 (Fig. 14(c)). The more B element the coating has, the more LaBO3 and B2O3 produce, resulting in the greater probability of reaction between La2O3 and SiO2. Obviously, the element of B derived from raw materials inevitably involved, which facilitate the formation of La-Si-O-B compound, as shown in the triangular phase diagram of ternary SiO2-B2O3-La2O3 (Fig. 14(c)). Whatever, amorphous SiO2 as the main phase in glass scale directly determines the oxidation rate of MSL coating. Meanwhile the structure mode of SiO2 have been changed by doping lanthanum, as shown in Fig. 15. Glassy SiO2 has a network tetrahedral structure of SiO44, in which the lanthanum and boron atoms occupy part of the silicon atoms and other vacancies. The dispersion of La2O3 and B2O3 can reduce the viscosity of SiO2 glassy at high temperatures through forming a La-Si-O-B compound glass scale, which confers a self-repairing capability to the oxide glass scale, rather than a porous layer, thereby improving the oxidation resistance. Moreover, the good oxidation response is relevant to the low oxygen permeability of LaSi-O-B compound glass scale compared with pure SiO2 glass, which result in less formation and loss of oxides, thus ensure the
Fig. 15. Structure mode of SiO2 and is that for SiO2 dissolving La-B based oxides.
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
minimum degradation to the underlying carbon matrix from the static and dynamic oxidizing atmosphere. 4. Conclusions LaB6-modified MoSi2-SiC (MSL) oxidation-resistant coating has been designed and successfully fabricated on C/C composites by an efficient method of SAPS. The microstructural evolution and oxidative protection mechanism were investigated in detail. The compactness and micro-hardness of sprayed MSL coating were obviously improved compared to MS coating. Moreover, the MSL coating protected the C/C composites from static oxidation at 1773 K for 100 h with a mass loss of 6.52 mg cm2 and dynamic ablation about 2023 K for six cycles with a mass loss of 29.86 mg cm2, which was superior to the MoSi2-SiC (MS) coating without La-doping. The presence of rare earth La element promoted the liquid phase sintering of MoSi2 and B element facilitated the high-temperature stability of SiO2-rich glass scale, resulting in the formation of La-Si-O-B compound scale with a low volatility and oxygen permeability cover the coating surface, which contributed to improving the resistant to oxidative response of the MSL coating. This study provided an oxidation-resistant coating system, which was a good solution to seal the cracks and bubbles caused by CTE mismatch and gases. Acknowledgments This work has been supported by the National Natural Science Foundation of China under Grant No. 51772247 and 5172780072, the Basic Research Project in Shenzhen City of China under Grant No. JCYJ20160531172042899 and the Creative Research Foundation of Science and Technology on Thermostructural Composite Materials Laboratory under Grant No. 6142911050217. References [1] G. Kou, L.J. Guo, H.J. Li, Effect of copper on the heat erosion mechanism of carbon/carbon composites, J. Alloys Compd. 723 (2017) 1132e1141. [2] L. Zhuang, Q.G. Fu, H.J. Li, SiCnw/PyC core-shell networks to improve the bonding strength and oxyacetylene ablation resistance of ZrB2-ZrC coating for C/C-ZrB2-ZrC-SiC composites, Carbon 124 (2017) 675e684. [3] Y.J. Jia, H.J. Li, X.Y. Yao, Z.G. Zhao, J.J. Sun, Effect of LaB6 content on the gas evolution and structure of ZrC coating for carbon/carbon composites during ablation, Ceram. Int. 43 (2017) 3601e3609. [4] C.C. Wang, K.Z. Li, X.H. Shi, J. Sun, Q.C. He, C.X. Huo, Self-healing YSZ-La-Mo-Si heterogeneous coating fabricated by plasma spraying to protect carbon/carbon composites from oxidation, Compos. B Eng. 125 (2017) 181e194. [5] L. Li, H.J. Li, X.M. Yin, H.J. Lin, Q.L. Shen, X.Y. Yao, T. Feng, Q.G. Fu, Microstructure evolution of SiC-ZrB2-ZrC coating on C/C composites at 1773 K under different oxygen partial pressures, J. Alloys Compd. 687 (2016) 470e479. [6] X.C. Jin, X.L. Fan, C.S. Lu, T.J. Wang, Advances in oxidation and ablation resistance of high and ultra-high temperature ceramics modified or coated carbon/carbon composites, J. Eur. Ceram. Soc. 38 (2018) 1e28. [7] J.Y. Liu, Q.J. Tang, Y. Wang, The study of inspection on SiC coated carboncarbon composite with subsurface defects by lock-in thermography, Compos. Sci. Technol. 72 (2012) 1240e1250. [8] J.P. Zhang, Q.G. Fu, J.L. Qu, R.M. Yuan, H.J. Li, Blasting treatment and chemical vapor deposition of SiC nanowires to enhance the thermal shock resistance of SiC coating for carbon/carbon composites in combustion environment, J. Alloys Compd. 666 (2016) 77e83. [9] C.C. Wang, K.Z. Li, X.H. Shi, Q.C. He, C.X. Huo, High-temperature oxidation behavior of plasma-sprayed ZrO2 modified La-Mo-Si composite coatings, Mater. Des. 128 (2017) 20e33. [10] T. Feng, H.J. Li, M.H. Hu, H.J. Lin, L. Li, Oxidation and ablation resistance of the ZrB2-CrSi2-Si/SiC coating for C/C composites at high temperature, J. Alloys Compd. 662 (2016) 302e307. [11] Z.Y. Cai, S.N. Liu, L.R. Xiao, Z. Fang, W. Li, B. Zhang, Oxidation behavior and microstructural evolution of a slurry sintered Si-Mo coating on Mo alloy at 1650 oC, Surf. Coating. Technol. 324 (2017) 182e189. [12] Y.C. Wang, D. Su, H.M. Ji, X.L. Li, Z.H. Zhao, H.J. Tang, Gradient structure high emissivity MoSi2-SiO2-SiOC coating for thermal protective application, J. Alloys Compd. 703 (2017) 437e447. [13] H.A. Zhang, D.Z. Wang, S.P. Chen, X.Y. Liu, Toughening of MoSi2 doped by La2O3 particles, Mater. Sci. Eng., A 345 (2003) 118e121.
715
[14] C.C. Wang, K.Z. Li, X.H. Shi, C.X. Huo, Q.C. He, Y.D. Zhang, Effect of spraying power on oxidation resistance and mechanical properties of plasma sprayed La-Mo-Si coating, Surf. Coating. Technol. 311 (2017) 138e150. [15] L. Wang, Q.G. Fu, F.L. Zhao, Improving oxidation resistance of MoSi2 coating by reinforced with Al2O3 whiskers, Intermetallics 94 (2018) 106e113. [16] F.X. Niu, Y.X. Wang, I. Abbas, S.L. Fu, C.G. Wang, A MoSi2-SiOC-Si3N4/SiC antioxidation coating for C/C composites prepared at relatively low temperature, Ceram. Int. 43 (2017) 3238e3245. [17] J. Sun, Q.G. Fu, L.P. Guo, L. Wang, Silicide coating fabricated by HAPC/SAPS combination to protect niobium alloy from oxidation, ACS Appl. Mater. Interfaces 8 (2016) 15838e15847. [18] L.Y. Cao, Z. Bai, J.F. Huang, H.B. Ouyang, J.P. Wu, Ablation properties of Cf/CSiC-MoSi2 composites: effects of hydrothermal penetration temperature, J. Alloys Compd. 703 (2017) 45e55. [19] H.J. Li, T. Feng, Q.G. Fu, H. Wu, X.T. Shen, Oxidation and erosion resistance of MoSi2-CrSi2-Si/SiC coated C/C composites in static and aerodynamic oxidation environment, Carbon 48 (2010) 1636e1642. [20] J. Sun, Q.G. Fu, L.P. Guo, Y. Liu, C.X. Huo, H.J. Li, Effect of filler on the oxidation protective ability of MoSi2 coating for Mo substrate by halide activated pack cementation, Mater. Des. 92 (2016) 602e609. [21] F. Yu, D.G. Gu, Y.F. Zheng, Y.L. Luo, X.Y. Li, H. Chen, L.C. Guo, Influence of MoO3 on boron aluminosilicate glass-ceramic coating for enhancing titanium hightemperature oxidation resistance, J. Alloys Compd. 729 (2017) 453e462. [22] N.A. Nasiri, N. Patra, D. Horlait, D.D. Jayaseelan, W.E. Lee, Thermal properties of rare-earth monosilicates for EBC on Si-based ceramic composites, J. Am. Ceram. Soc. 99 (2016) 589e596. [23] J.Y. Gao, W. Jiang, Effect of La2O3 addition to modification of grain-boundary phase in MoSi2, J. Alloys Compd. 476 (2009) 667e670. [24] C.C. Wang, K.Z. Li, X.H. Shi, C.X. Huo, Q.C. He, Comparison of microstructure and oxidation behavior of La-Mo-Si coatings deposited by two approaches, Ceram. Int. 43 (2017) 10848e10860. [25] W.J. Tobler, W. Durisch, Plasma-spray coated rare-earth oxides on molybdenum disilicide high temperature stable emitters for thermophotovoltaics, Appl. Energy 85 (2008) 371e383. [26] Y. Wang, D.Z. Wang, W. Zhu, H.Y. Liu, X.Q. Zan, Microstructure and properties of hot-rolled 2.0wt.% MoSi2/rare earth oxide doped molybdenum alloys, Int. J. Refract. Metals Hard Mater. 31 (2012) 152e156. [27] S. Majumdar, B. Gorr, H.J. Christ, D. Schliephake, M. Heilmaier, Oxidation mechanisms of lanthanum-alloyed Mo-Si-B, Corrosion Sci. 88 (2014) 360e371. [28] Y. Wang, D.Z. Wang, H.Y. Liu, W. Zhu, X.Q. Zan, Preparation and characterization of sintered molybdenum doped with MoSi2/La2O3/Y2O3 composite particle, Mater. Sci. Eng., A 558 (2012) 497e501. [29] Y. Liu, Q.G. Fu, F.L. Zhao, G.D. Sun, H.J. Li, Internal friction vs. thermal shock in C/C composites, Compos. B Eng. 106 (2016) 59e65. [30] L. Li, H.J. Li, H.J. Lin, L. Zhuang, S.L. Wang, T. Feng, X.Y. Yao, Q.G. Fu, Comparison of the oxidation behaviors of SiC coatings on C/C composites prepared by pack cementation and chemical vapor deposition, Surf. Coating. Technol. 302 (2016) 56e64. [31] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564e1583. [32] Y.H. Chu, H.J. Li, Q.G. Fu, L.H. Qi, L. Li, Bamboo-shaped SiC nanowiretoughened SiC coating for oxidation protection of C/C composites, Corrosion Sci. 70 (2013) 11e16. [33] J. Li, R.Y. Luo, C. Lin, Y.H. Bi, Q. Xiang, Oxidation resistance of a gradient selfhealing coating for carbon/carbon composites, Carbon 45 (2007) 2471e2478. [34] J. Pang, W. Wang, C.G. Zhou, Microstructure evolution and oxidation behavior of B modified MoSi2 coating on Nb-Si based alloys, Corrosion Sci. 105 (2016) 1e7. [35] J.P. Zhang, Q.G. Fu, Y.J. Wang, Interface design and HfC additive to enhance the cyclic ablation performance of SiC coating for carbon/carbon composites from 1750 C to room temperature under vertical oxyacetylene torch, Corrosion Sci. 123 (2017) 139e146. [36] Y.J. Jia, H.J. Li, X.Y. Yao, J.J. Sun, Z.G. Zhao, Long-time ablation protection of carbon/carbon composites with different-La2O3-content modified ZrC coating, J. Eur. Ceram. Soc. 38 (2018) 1046e1058. [37] H.A. Zhang, S.Y. Gu, N.P. Xie, Effect of La2O3 on the wear behavior of MoSi2 at high temperature, J. Rare Earths 29 (2011) 370e373. [38] Y. Cui, A. Li, B. Li, X. Ma, R. Bai, W. Zhang, M. Ren, J. Sun, Microstructure and ablation mechanism of C/C-SiC composites, J. Eur. Ceram. Soc. 34 (2014) 171e177. [39] S.P. Sun, X.P. Li, H.J. Wang, Y. Jiang, D.Q. Yi, Adsorption of oxygen atom on MoSi2 (110) surface, Appl. Surf. Sci. 382 (2016) 239e248. [40] 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) 335e340. [41] J.P. Zhang, Q.G. Fu, J.L. Qu, H.S. Zhou, N.K. Liu, Surface modification of carbon/ carbon composites and in-situ grown SiC nanowires to enhance the thermal cycling performance of Si-Mo-Cr coating under parallel oxyacetylene torch, Corrosion Sci. 111 (2016) 667e674. [42] S.S. Kim, J.Y. Park, T.H. Sanders, Thermodynamic modeling of the miscibility gaps and the metastability in the R2O3-SiO2 systems (R¼La, Sm, Dy, Er), J. Alloys Compd. 321 (2001) 84e90. [43] L. Li, Z.J. Tang, W.Y. Sun, P.L. Wang, Phase diagram prediction of the Al2O3-
716
C. Wang et al. / Journal of Alloys and Compounds 753 (2018) 703e716
SiO2-La2O3 system, J. Mater. Sci. Technol. 15 (1999) 439e443. [44] E.M. Levin, C.R. Robbins, J.L. Waring, Immiscibility and the system lanthanum oxide-boric oxide, J. Am. Ceram. Soc. 44 (1961) 87e91. n, Glass formation and structure[45] S. Iftekhar, J. Grins, P.N. Gunawidjaja, M. Ede property-composition relations of the RE2O3-Al2O3-SiO2 (RE¼La, Y, Lu, Sc)
systems, J. Am. Ceram. Soc. 94 (2011) 2429e2435. goue €ta, D. Cauranta, O. Maje rusa, T. Charpentierb, T. Lerougea, [46] H. Tre L. Cormier, Exploration of glass domain in the SiO2-B2O3-La2O3 system, J. Non Cryst. Solids 476 (2017) 158e172.