Supersonic plasma sprayed MoSi 2 –ZrB 2 antioxidation coating for SiC

Abstract

A dense and crack free MoSi₂–ZrB₂ coating was prepared on the SiC coated carbon/carbon (C/C) composites using a supersonic plasma spraying technique. The coated composites were characterised using scanning electron microscopy and X-ray diffraction. The analysis shows that the as prepared MoSi₂–ZrB₂ coating, with a thickness of ∼100 μm, combines well with the inner SiC coating. After oxidation at 1200°C for 72 h in air, the mass loss of MoSi₂–ZrB₂/SiC coated C/C is only 4.24 wt-%, which is significantly lower than that of MoSi₂/SiC coated C/C (11.35 wt-%). The excellent oxidation protective ability of the MoSi₂–ZrB₂ coating is mainly attributed to ZrSiO₄ produced by dispersive ZrO₂ particles and SiO₂ in the coating at high temperature.

Keywords

C/C composites MoSi2–ZrB2 coating Oxidation

Introduction

Carbon/carbon (C/C) composites have potential applications in the field of aerospace due to their attractive advantages including low density and excellent thermophysical properties at elevated temperatures.¹ Unfortunately, C/C composites are susceptible to oxidation above 500°C, which limits their applications in high temperatures and oxygen containing environments.² Coating technology is an effective approach to protect C/C composites against oxidation.³ Usually, a SiC coating is synthesised on the surface of C/C composites as the bonding layer owing to its excellent antioxidation property and good compatibility with C/C. SiC also plays a positive role in relieving the mismatch of the coefficient of thermal expansion (CTE) between the C/C substrate and the outer ceramic coatings.^4–6

Acting as the external protective coating of the C/C substrate, MoSi₂ is a promising material that possesses excellent properties such as modest density (6.24 g cm^− 3), high melting point (2030°C) and enhanced oxidation resistance at above 1500°C, even in some aggressive environments.⁷ It has been widely used in heating elements and oxidation protective coating on SiC coated C/C composites.^8,9 Nevertheless, MoSi₂ has a problem of pest degradation below 700°C (Ref. 10 and shows a very low creep resistance below 1200°C due to a brittle to ductile transition at 900–1000°C,¹¹ which will induce crack and other defects as the temperature changes and thus further damaging the substrate. Therefore, it is important to improve the oxidation resistance property of MoSi₂ at ∼1200°C to broaden its application.

Second phase doping is a logical way of improving the cohesion of the MoSi₂ coating and obstructing the propagation of cracks in the coating. ZrB₂ has the potential for use as the second phase due to its extremely high melting point and stability in extreme environment.¹² Moreover, during the oxidation process at high temperature, ZrO₂ is produced by the oxidation of ZrB₂, which will react with SiO₂, and the oxidation product of MoSi₂, to form ZrSiO₄.¹³ ZrSiO₄ exhibits a series of attractive properties like low oxygen diffusion coefficient, high melting point and chemical stability in oxidative and corrosive environments.¹⁴ It also has a CTE value close to that of SiC and could reduce the probability of generating cracks in the MoSi₂ coating. Moreover, the glass zircon could provide good oxidation protection for the C/C substrate at high temperature. At present, many techniques have been applied on the deposition of MoSi₂ and ZrB₂ coating, such as pack cementation,¹⁵ slurry painting¹⁶ and plasma spraying.¹⁷ Among the techniques, supersonic plasma spraying is a highly efficient method to deposit refractory material coating for commercial production. In this process, the accelerated melted particles were impinged on the surface of the substrate to form a compact coating.¹⁸

In this work, a SiC inner coating was prepared on the C/C composites by pack cementation, and then a ZrB₂ modified MoSi₂ outer coating was prepared by supersonic plasma spraying. The effect of ZrB₂ addition on the microstructure and oxidation protective property of the MoSi₂ coating was investigated. The oxidation mechanism of the SiC/MoSi₂–ZrB₂ coating was also discussed.

Experimental

Specimen preparation

Acting as the substrate, the bulk 2.5D C/C composites with a density of 1.75 g.cm^− 3 were cut into small cubes with dimensions of 10 × 10 × 10 mm. Then, the cubic specimens were hand-abraded with 80 grit SiC paper in order to remove the sharp edges and cleaned with absolute ethyl alcohol in the ultrasonic bath and dried at 80°C for 2 h.

Coating preparation

The inner SiC coating was prepared as a linking layer on C/C specimens by pack cementation. Details of the procedure were reported in our previous work.¹⁹ The outer 80MoSi₂–20ZrB₂ (vol.-%) coatings were prepared by supersonic plasma spraying. The MoSi₂ and ZrB₂ raw powder was dry milled together for 1 h. Then, the polyvinyl alcohol (PVA) solution (PVA/water = 2–3 wt-%) was poured into the milled powders with the mass ratio of 1:1, and the mixed turbid liquid was evaporated at ∼120°C using a centrifugal rotary evaporator. The remaining powder was sieved by a 180 grit griddle for use in spraying.

The sieved powder was sprayed on the SiC coated C/C composites with a mixture of second gas H₂ (4 L min^− 1), primary gas Ar (75 L min^− 1) and carrier gas Ar (10 L min^− 1). The power applied in supersonic plasma spraying was 50 kW, the powder feedrate was 20 g min^− 1 and the distance between the nozzle and specimens was ∼100 mm.

Characterisation

The isothermal oxidation tests of the prepared specimens were carried out in an electric furnace at 1200°C in air. The specimens were directly placed into the furnace and then taken out at the designated time. After the specimens were cooled to room temperature, their mass was measured by an electric balance with a sensitivity of ± 0.1 mg. Mass loss percentages were calculated according to equation (1) 1 where m₁ is the sample mass before oxidation, m₂ is the sample mass after oxidation and ΔW% is the mass loss percentage.

Microstructure and composition of the surface and cross-section of the double layer coating before and after the oxidation were analysed by a scanning electron microscope (SEM, JSM-6460, JEOL Ltd, Mitaka, Japan) equipped with an energy dispersive spectrometer (EDS). The crystalline structure of the coatings was analysed by an X-ray diffraction instrument (XRD, X'Pert PRO, PANalytical, Almelo, The Netherlands).

Results and discussion

Microstructure of as prepared coatings

Figure 1 shows the SEM of the MoSi₂–ZrB₂ spray powder, and its average diameter is ∼50 μm. The powder is prepared through a rotary evaporation process, and a single particle consists of several smaller particles bound by PVA. The powder is spherical with some defects. Figure 2 shows the XRD patterns of the as sprayed MoSi₂–ZrB₂ coating and its raw powder, from which it can be seen that the powder before spraying is composed of MoSi₂ and ZrB₂. The sprayed coating mainly consists of ZrB₂, MoSi₂, Mo₅Si₃ and ZrO₂. SiC in the bonding layer was also detected, which is due to the rough interface between the SiC layer and the sprayed coating. Mo₅Si₃ and ZrO₂ were formed by the oxidation of raw materials during the supersonic plasma spraying process as the temperature of the plasma spray nozzle is ∼10 000°C, which is much higher than the oxidation temperature of MoSi₂ and ZrB₂. In the process of heating and speeding, the raw powder, MoSi₂ and ZrB₂ were partly oxidised according to equations (2)–(4) 2 3 4 Then, these products were impinged on the surface of the SiC coated C/C substrate. MoO₃ and B₂O₃ were not detected due to the low volatilisation temperature of MoO₃ (above 800°C) and B₂O₃ (above 1000°C). The high melting point of ZrO₂ makes it a good antioxidation material. In the temperature range of 1100–1400°C, compared with MoSi₂, Mo₅Si₃ has a superior creep resistance.²⁰ Therefore, the as formed Mo₅Si₃ and ZrO₂ in the MoSi₂–ZrB₂ coating should have a positive effect on its oxidation resistance.

Surface morphology of MoSi₂–ZrB₂ spray powder

X-ray diffraction patterns of MoSi₂–ZrB₂ coating and its raw powder

Figure 3 shows the surface morphologies of MoSi₂ and MoSi₂–ZrB₂ coatings before the oxidation test. From Fig. 3a, the surface morphology of the MoSi₂ coating is rather homogenous without distinct pores or cracks. Figure 3b shows the enlarged view of the coating surface in Fig. 3a. There are plenty of well flattened lamellae along with some spherical features on the surface, which show a typical surface morphology of the coating prepared by supersonic plasma spraying.²¹ During the process of coating preparation, high speed powders were heated by the plasma flame. The melted powder impinged on the surface of the substrate, forming a flattened surface structure. From Fig. 3c, it can be seen that there are a lot of pores and small bulges on the surface of the MoSi₂–ZrB₂ coating caused by the rushing out of entrapped carrier gas, MoO₃ and B₂O₃.²² From the cross-section of the MoSi₂–ZrB₂ coated SiC–C/C composites (Fig. 3d), it can been seen the MoSi₂–ZrB₂ and SiC coatings are compact, and the interfaces between these two coatings are continuous and mutually penetrated, and no visible crack can be observed, indicating a good combination of these two coatings. The MoSi₂–ZrB₂ outer coating is ∼100 μm in thickness. The SiC bonding layer is 100–250 μm in thickness, and part of the coating was infiltrated into the C/C substrate due to the porous structure of the C/C substrate and good fluidity of the powder during pack cementation.

a surface morphology of MoSi₂ coating, b enlarged view of a, c surface morphology of MoSi₂–ZrB₂ coating and d cross-section BSE morphology of MoSi₂–ZrB₂ coated SiC–C/C composites

Oxidation resistant property of coatings

Figure 4 shows the isothermal oxidation test results of the samples with MoSi₂–ZrB₂ and pure MoSi₂ coatings at 1200°C in air. The sample with pure MoSi₂ as an outer coating losses mass in a short time and the mass losing ratio accelerated over time. After oxidation for 72 h, the mass loss percentage reaches 11.35 wt-%. The sample with MoSi₂–ZrB₂ coating exhibits better oxidation resistance. At first, there is a mass gain for the first 30 h following which the mass decreases slowly. After oxidation for 72 h, the mass loss reaches 4.24 wt-%, which is less than that of the sample with pure MoSi₂ coating. The oxidation test results indicate that a certain amount of ZrB₂ could greatly improve the oxidation protective property of MoSi₂ coatings at 1200°C in air 5 6

Oxidation curves of MoSi₂–ZrB₂ and pure MoSi₂ coatings at 1200°C in air

Figure 5 shows the surface XRD patterns of MoSi₂–ZrB₂ coating and MoSi₂ coating after oxidation at 1200°C in air for 72 h. It can be seen that the main phases of these two coatings are Mo₅Si₃ and some crystallographic SiO₂. A ZrSiO₄ phase exists only in the MoSi₂–ZrB₂ coating. The SiO₂ phase was formed by the oxidation of silicide, such as MoSi₂, Mo₅Si₃ and SiC according to equations (2), 3 , 5 and (6)), while the ZrSiO₄ phase was formed by the reaction of SiO₂ and ZrO₂ according to equation (7) 7 Figure 6 shows the surface morphologies of MoSi₂–ZrB₂ and MoSi₂ coatings after oxidation at 1200°C in air for 72 h. There are a lot of cracks on the surface of the MoSi₂ coating, as shown in Fig. 6a. During the oxidation process, the coating suffered abrupt temperature changes from 1200°C to room temperature, and the cracks formed under the thermal stress caused by CTE mismatch between the inner SiC (4.7 × 10^− 6 K^− 1) coating and the outer oxidative MoSi₂ (8.1 × 10^− 6 K^− 1) coating. Additionally, a brittle Mo₅Si₃ formed in the MoSi₂ coating after oxidation, making the coating apt to break. Usually, the formation of a SiO₂ glass layer on the surface of the MoSi₂ coating would prevent oxygen. However, at 1200°C, the viscosity of the SiO₂ glass is too high to flow and seal the defects in the coating. This phenomenon could be clearly observed from the top left corner of Fig. 6a in which the coating has a large size crack.

X-ray diffraction patterns of MoSi₂–ZrB₂ coating and MoSi₂ coating surface after oxidation at 1200°C in air

a surface morphology of MoSi₂ coating after oxidation for 72 h at 1200°C in air and b surface morphology of MoSi₂–ZrB₂ coating after oxidation for 72 h at 1200°C in air

In Fig. 6b, there are several cracks and broken bulges on the MoSi₂–ZrB₂ coating. Some pores can be observed inside the bulges. Those bulges are formed due to the gas release. The oxidation behaviour of the MoSi₂–ZrB₂ coating shows a slow mass gain at the first stage due to the formation of ZrO₂ according to equation (4). Then, ZrO₂ reacts with SiO₂ gradually to form ZrSiO₄, which is a mass conservation reaction. At this stage, the mass loss was attributed to the oxidation of the MoSi₂–ZrB₂, which is accompanied by the evaporation of MoO₃ and B₂O₃.

Figure 7 shows the cross-section BSE morphologies of MoSi₂ and MoSi₂–ZrB₂ coatings and the EDS patterns of MoSi₂–ZrB₂ after oxidation at 1200°C in air for 72 h. From the cross-section of the MoSi₂ coating shown in Fig. 7a, it can be observed that the SiC bonding layer separated from the C/C substrate, which has been oxidised severely. Some particles exist at the interface of the SiC bonding layer and the MoSi₂ oxidation layer, due to the slight pest degradation of MoSi₂ during the cooling and heating process. Additionally, there are some penetrating cracks caused by CTE mismatch between the inner and outer coatings. The horizontal cracks were induced by the thermal shock from 1200°C to room temperature to prove the poor cohesion of the MoSi₂ coating. These defects provided channels for the oxygen to react aggressively with the inner coating and damage the C/C substrate, leading to a mass loss of the MoSi₂ coating.

a cross-section BSE morphology of MoSi₂ coating after oxidation at 1200°C in air, b cross-section BSE morphology of MoSi₂–ZrB₂ coating after oxidation at 1200°C in air, c enlarged view of white area in b and d EDS patterns of spot A in c

Figure 7b shows the cross-section of the MoSi₂–ZrB₂ coating after oxidation, and Fig. 7c is the enlarged view of the white area in Fig. 7b. The SiC coating remains intact and combines with the C/C substrate closely. After oxidation, the outer MoSi₂–ZrB₂ coating combines with the SiC coating well, and no visible cracks are observed. Its thickness increases to ∼200 μm, which was mainly resulted from the formation of large pores dispersed evenly in the outer coating. The pores are mostly formed due to the volatilisation of B₂O₃ and MoO₃. From the XRD patterns (Fig. 5) and EDS (Fig. 7d) analysis, the white particles are ZrSiO₄, and the grey phase is SiO₂ mixed with some Mo₅Si₃. For the MoSi₂ coating, there are many cracks in the coating. The gaseous MoO₃ could flow out of the coating in time, and there is a discontinuous glass layer to obstruct the release of the gas. In the MoSi₂–ZrB₂ coating, there are not enough penetrating cracks to release the gas (B₂O₃ and MoO₃) in time. At the same time, high concentration of ZrSiO₄ particles produced in the coating acts as a barrier for gas to flow out of the coating. This caused a lot of big pores dispersed in the outer coating. Then, gradually, a small amount of gas in the pores was released and left the contact areas for oxygen, which induced a fast mass loss at ∼30 h during the oxidation test.

To clearly illustrate the oxidation mechanism of the MoSi₂ and MoSi₂–ZrB₂ coatings, the schematic diagrams of their oxidation process are shown in Figs. 8 and 9. In Fig. 8a, some SiO₂ glass covers the surface of the coating after oxidation for 72 h. A few vertical and horizontal cracks emerge and reach the inner SiC coating down to the C/C substrate as the oxidation continued (Fig. 8b). For the MoSi₂–ZrB₂ coating (Fig. 9a), some microcracks also propagated during the oxidation process (Fig. 9b). ZrO₂ particles will be produced and dispersed around ZrB₂ within the coating near the surface, and a SiO₂ glass layer also formed and covered the coating. These ZrO₂ particles are able to terminate the spreading of the cracks, restrict the generation of cracks and prevent the diffusion of oxygen, thus effectively protecting C/C composites from being consumed. The ZrSiO₄ particle layer will be formed due to the reaction between ZrO₂ and SiO₂, as shown in Fig. 9c. It also covers the surface of the coating and heals microcracks, which make the coating more integral. The ZrSiO₄ exhibits a low coefficient of oxygen diffusion and a similar CTE with SiC at 1200°C. They can not only decrease the diffusion of oxygen but also reduce the crack propagation resulting from thermal shock while cooling from 1200°C to room temperature. Therefore, ZrSiO₄ produced by dispersive ZrO₂ particles is significantly associated with the excellent oxidation protective ability of the MoSi₂–ZrB₂ coating.

Schematics of oxidation process of MoSi₂ coated SiC–C/C composites at 1200°C: a as prepared coating; b coating after oxidation

Schematics of oxidation process of MoSi₂–ZrB₂ coating at 1200°C in air: a as prepared MoSi₂–ZrB₂ coating; b coating during oxidation process; c coating after oxidation

Conclusion

Dense and crack free MoSi₂ and MoSi₂–ZrB₂ coatings were prepared by supersonic plasma spraying on the SiC–C/C composites.

The doping of ZrB₂ in the MoSi₂ coating could improve the oxidation resistant property of pure MoSi₂ based coating, and the mass loss was decreased from 11.35 to 4.24 wt-% after oxidation at 1200°C for 72 h.

The excellent oxidation protective ability is mainly attributed to a ZrSiO₄ glass being able to heal cracks and decrease oxygen diffusion. The glass was produced by SiO₂ and dispersive ZrO₂ particles, which could restrict the spread of microcracks.

Acknowledgements

This work has been supported by the National Natural Science Foundation of China under grant nos. 51221001 and 51222207 and the project supported by the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (grant no.12-BZ-2014).

References

Jacobson

N. S.

and Curry

D. M.

: ‘Oxidation microstructure studies of reinforced carbon/carbon’, Carbon, 2006, 44, 1142–1150. doi: 10.1016/j.carbon.2005.11.013

Strife

J. R.

and Sheehan

J. E.

: ‘Ceramic coatings for carbon–carbon metallic dispersions’, J. Am. Ceram. Soc., 1988, 67, 369–374.

Y. D.

, Cheng

L. F.

and Zhang

L. T.

: ‘Oxidation behavior and mechanical properties of C/SiC composites with Si–MoSi₂ oxidation protection coating’, J. Mater. Sci., 1999, 34, 6009–6014. doi: 10.1023/A:1004741014185

Huang

J. F.

, Zeng

X. R.

, Li

H. J.

, Xiong

X. B.

and Fu

Y. W.

: ‘Influence of the preparation temperature on the phase, microstructure and anti-oxidation property of a SiC coating for C/C composites’, Carbon, 2004, 42, 1517–1521. doi: 10.1016/j.carbon.2004.01.066

Z. B.

, Li

H. J.

, Shi

X. H.

, Fu

Q. G.

and Wu

: ‘Microstructure and oxidation resistance of SiC–MoSi₂ multi-phase coating for SiC coated C/C composites’, Prog. Nat. Sci., 2014, 24, 247–252. doi: 10.1016/j.pnsc.2014.05.005

Ren

X. R.

, Li

H. J.

and Fu

Q. G.

: ‘TaB₂–SiC–Si multiphase oxidation protective coating for SiC-coated carbon/carbon composites’, J. Eur. Ceram. Soc., 2013, 33, 2953–2959. doi: 10.1016/j.jeurceramsoc.2013.06.028

Dumont

A. L.

, Bonnet

J. P.

and Chartier

: ‘MoSi₂/Al₂O₃ FGM: elaboration by tape casting and SHS’, J. Eur. Ceram. Soc., 2001, 21, 2353–2360. doi: 10.1016/S0955-2219(01)00198-4

Fei

X. A.

, Niu

Y. R.

, Ji

, Huang

L. P.

and Zheng

X. B.

: ‘A comparative study of MoSi₂ coatings manufactured by atmospheric and vacuum plasma spray process’, Ceram. Int., 2011, 37, 813–817. doi: 10.1016/j.ceramint.2010.10.018

Zhang

Y. L.

, Li

H. J.

and Hu

Z. X.

: ‘C/SiC/MoSi₂–SiC–Si multilayer coating for oxidation protection of carbon/carbon composites’, Trans. Nonferr. Met. Soc. China, 2013, 23, 2118–2122. doi: 10.1016/S1003-6326(13)62705-3

10.

Ramasesha

S. K.

and Shobu

: ‘Oxidation of MoSi₂ and MoSi₂-based materials’, Bull. Mater. Sci., 1999, 22, 769–773. doi: 10.1007/BF02745603

11.

Nyutu

E. K.

, Kmetz

M. A.

and Suib

S. L.

: ‘Formation of MoSi₂–SiO₂ coatings on molybdenum substrates by CVD/MOCVD’, Surf. Coat. Technol., 2006, 200, 3980–3986. doi: 10.1016/j.surfcoat.2005.02.212

12.

H. J.

, Yao

X. Y.

and Zhang

Y. L.

: ‘Anti-oxidation properties of ZrB₂ modified silicon-based multilayer coating for carbon/carbon composites at high temperatures’, Trans. Nonferr. Met. Soc. China, 2013, 23, 2094–2099. doi: 10.1016/S1003-6326(13)62701-6

13.

Gao

, Zhang

and Fu

J. Y.

: ‘Oxidation of zirconium diboride–silicon carbide ceramics under an oxygen partial pressure of 200 Pa: formation of zircon’, Corros. Sci., 2010, 52, 3297–3303. doi: 10.1016/j.corsci.2010.06.004

14.

Q. G.

, Zhang

J. P.

, Zhang

Z. Z.

and Li

H. J.

: ‘SiC–MoSi₂/ZrO₂–MoSi₂ coating to protect C/Ccomposites against oxidation’, Trans. Nonferr. Met. Soc. China, 2013, 23, 2113–2117. doi: 10.1016/S1003-6326(13)62704-1

15.

Ren

X. R.

, Li

H. J.

and Chu

Y. H.

: ‘Preparation of oxidation protective ZrB₂–SiC coating by in-situ reaction method on SiC-coated carbon/carbon composites’, Surf. Coat. Technol., 2014, 247, 61–67. doi: 10.1016/j.surfcoat.2014.03.017

16.

Zhang

W. Z.

, Zeng

, Gbologah

, Xiong

and Huang

B. Y.

: ‘Preparation and oxidation property of ZrB₂–MoSi₂/SiC coating on carbon/carbon composites’, Trans. Nonferr. Met. Soc. China, 2011, 21, 1538–1544. doi: 10.1016/S1003-6326(11)60893-5

17.

Zhang

Y. L.

, Hu

Z. X.

and Li

H. J.

: ‘Ablation resistance of ZrB₂–SiC coating prepared by supersonic atmosphere plasma spraying for SiC-coated carbon/carbon composites’, Ceram. Int., 2014, 40, 14749–14755. doi: 10.1016/j.ceramint.2014.06.064

18.

Liu

Y. B.

, Wang

Q. S.

, Ma

, Wang

, Zheng

K. N.

, Wei

S. H.

and Wu

: ‘Anti-ablation properties of MoSi₂–W multilayer coating system deposited by atmospheric plasma spraying’, Mater. Res. Innov., 2014, 18, 1017–1022.

19.

Q. G.

, Li

H. J.

, Shi

X. H.

, Li

K. Z.

and Sun

G. D.

: ‘Silicon carbide coating to protect carbon/carbon composites against oxidation’, Scr. Mater., 2005, 52, 923–927. doi: 10.1016/j.scriptamat.2004.12.029

20.

, Li

H. J.

, Ma

and Fu

Q. G.

: ‘MoSi₂-based oxidation protective coatings for SiC-coated carbon–carbon composites prepared by supersonic plasma spraying’, J. Eur. Ceram. Soc., 2010, 30, 3267–3270. doi: 10.1016/j.jeurceramsoc.2010.06.007

21.

Kassner

, Siegert

, Hathiramani

, Vassen

and Stoever

: ‘Application of suspension plasma spraying (SPS) for manufacture of ceramic coatings’, J. Therm. Spray Technol., 2008, 17, 115–123. doi: 10.1007/s11666-007-9144-2

22.

Silvestroni

and Sciti

: ‘TEM analysis, mechanical characterization and oxidation resistance of a highly refractory ZrB₂ composite’, J. Alloys Compd, 2014, 602, 346–355. doi: 10.1016/j.jallcom.2014.02.133

Supersonic plasma sprayed MoSi 2 –ZrB 2 antioxidation coating for SiC–C/C composites

Abstract

Keywords

Introduction

Experimental

Specimen preparation

Coating preparation

Characterisation

Results and discussion

Microstructure of as prepared coatings

Oxidation resistant property of coatings

Conclusion

Acknowledgements

References