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参考文献

1. Introduction▲
2. Experimental▲
3. Results and discussion▲
4. Conclusions▲
图表▲

Rare Metals2009年第5期

Microstructure of a Mo-Si-C-N multi-layered anti-oxidation coating on carbon/carbon composites by fused slurry

JEON Jaeho

Department of Materials Technology, Korea Institute of Machinery & Materials

作者简介：LAI Zhonghong E-mail: zhhlai@hit.edu.cn;

收稿日期：23 June 2008

基金：supported by the Foundation of Heilongjiang Province (No. LC04C16);the Foundation of Aerospace Technology (No. HTJSZC-0506);the Post-Doctor Foundation of Heilongjiang Province, China;

Microstructure of a Mo-Si-C-N multi-layered anti-oxidation coating on carbon/carbon composites by fused slurry

Abstract：

A Mo-Si-C-N multi-layered anti-oxidation coating was in-situ fabricated by introducing nitrogen atmosphere during the fused sintering of Mo-Si slurry pre-layer on carbon/carbon composites. The phase composition and microstructure of the Mo-Si-C-N coating were characterized by X-ray diffractometry, optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The Mo-Si-C-N coating exhibited a three-layered structure. Besides the MoSi2/Si main-layer and the SiC bonding-layer, a surface layer of about 10 μm in thickness was synthesized on the coating surface. The surface layer mainly consisted of SiC nanowires and contained some Si3N4 and Si phases. SiC nanowires of 10 to 200 nm in diameter presented a terrace and distortion structure. Transmission electron microscopy indicated that the SiC nanowires grew along the preferred <111> direction. During oxidation test, SiC nanowires transmuted into SiO2 glass, which can play an important role in improving the oxidation resistance of C/C composites.

Keyword：

C/C composites; coating; microstructures; anti-oxidation;

Received： 23 June 2008

1. Introduction

The silicide coatings,such as MoSi₂ and SiC,have been widely used to protect carbon/carbon(C/C)composites against oxidation due to their excellent high-temperature oxidation resistance and self-sealing ability [ 1, 2, 3, 4] .However a higher fabrication temperature was required for the coating containing Mo Si₂due to its high melting point(2030°C) [ 5, 6] .In our previous research,a Mo-Si-C coating composed of Mo Si₂/Si main-layer and Si C bonding-layer was synthesized by fused slurry using Mo and Si powder as raw materials at 1420°C in vacuum [ 7] .Compared with other synthetic methods,the fused slurry is a simple coating technique with low cost.However,the above Mo-Si-C fused slurry coating only provides oxidation protection for C/C composites at 1370°C.In order to improve the oxidation resistance of a Mo-Si-C fused slurry coating,a Mo-Si-C-N coating was synthesized in nitrogen atmosphere using the same Mo-Si slurry and the anti-oxidizing temperature of the coated C/C composites was expanded to 1450°C(for 20h) [ 8] .In this paper,the microstructure of the Mo-Si-C-N coating,especially that of the surface layer,was investigated in detail before and after oxidation test,and the anti-oxidation mechanism was discussed.

2. Experimental

Two-dimensional C/C composites with a density of 1.77g/cm³ were used as substrate material for the multi-layered coating.The substrate specimens with a size of 5 mm×5mm×15 mm were polished using Si C abrasive paper and ultrasonically cleaned in alcohol before coating.

Mo,Si,and Al powders(their characteristics are shown in Table 1)were used as raw materials of two types of slurry namely,6wt.%Al-Si slurry and 35wt.%Mo-Si slurry.The slurry binder and solvent were polyvinyl-butyral(PVB)and methylethyl ketone(MEK),respectively.Fig.1 presents the fabrication procedure of the coating.First,the substrate specimen was repeatedly dipped into the Al-Si slurry to form Al-Si pre-layer.Subsequently,the specimens with Al-Si pre-layer were coated with Mo-Si layer by cyclically dipping with the Mo-Si slurry.Finally,the C/C composites covered with both Al-Si and Mo-Si pre-layers was sintered at 1500°C in nitrogen atmosphere to form silicide coating via in-situ reaction.The Al-Si pre-layer is introduced to prevent liquid Si from infiltrating too much into the C/C substrate during sintering,which results in the decrease of mechanical properties of C/C composites [ 9] .

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Table 1.Characteristics of raw powders

Fig.1.Fabrication procedure of coating.

The phase compositions of the coating were identified by X-ray diffractometry(XRD,Rigaku,D/max-r B,Japan)using Cu K_αradiation and a scan speed of 2°/min with an operating voltage of 50 kV and a current of 40 mA.The morphology of the coating was characterized by means of optical microscopy(OM,Olympus,Japan),field emission scanning electron microscopy(FESEM,Hitachi S-4700,Japan),and transmission electron microscopy(TEM,Philips CM12,Netherlands).The elemental composition of the coating was determined by energy dispersive spectroscopy(EDS,EDAX,USA).

The surface chemistry of the coating was analyzed by X-ray photoelectron spectroscopy(XPS,PHI-5700 ESCA system,USA)at an operating pressure of less than1.33×10^-7 Pa.The samples were irradiated with Al(K_α)X-ray(1486.6 eV).The pass energy of 187.85 eV was used for survey scan(0-1100 e V)and elemental regional scan was acquired with a pass energy of 29.35 e V and 0.125e V/step.The main carbon peak was fixed to a binding energy of 284.6 eV.

3. Results and discussion

Fig.2 shows the XRD pattern of the coated C/C composites before oxidation.It can be seen that the coating consists of main phases of MoSi₂,SiC,and Si and a small quantity of Si₃N₄,Al₂O₃,AlN,and sialon phases.

Fig.3 presents the cross-sectional microstructure of the coated C/C specimens before oxidation.As shown in Fig.3(a),the coating exhibits a three-layered structure on the C/C composite substrate,namely,bonding layer,middle(main)layer,and surface layer.White particles disperse in the matrix of the main layer with a thickness of about 50μm.EDS results show that the white particle mainly contains Mo and Si elements and the molar ratio of Mo/Si approaches 1:2,as shown in Fig.3(b).Combining XRD with EDS analysis,the white particle is determined as Mo Si₂ phase while the light-grey matrix surrounding the MoSi₂ particles is Si phase.Also,Ref. [ 7] has confirmed the MoSi₂ phase in the coating to be in-situ reaction synthesized from Mo and Si elements during the sintering of the coating.The dentate dark-grey layer close to the C/C composite substrate is Si C bonding-layer identified by line-scanning(Fig.3(c)),in which the C content increases gradually from the coating to the substrate,while the Si content changes reversely.That is,the Si C bonding-layer results from the diffusion reaction of Si and C elements during sintering of the coating,which is in favor of reducing the thermal stress and enhancing the binding strength between the coating and the substrate.As an evidence of the outward diffusion of the carbon element,few small dark-grey grains distributed on Mo Si₂/Si main-layer have been confirmed as Si C phase as well(as arrowed in Figs.3(a)and 3(c)).

Fig.2.XRD pattern of the coated C/C composites.

Fig.3.Cross-section images of the coated C/C composites be-fore oxidation:(a)optical image;(b)EDS result of white parti-cles in Fig.3(a);(c)line-scanning image.

Fig.4 shows the magnified images of the surface-layer of the coating before oxidation.The surface-layer is about 10μm in thickness and exhibits wire-like texture(Fig.4(a)).In Figs.4(b)and 4(c),these wires are 10 to 200 nm in diameter,presenting obvious terrace and distortion structure.The TEM image and SAD(inset)analysis of the nanowires show clear microtwins structure and indicate that a nanowire is aβ-Si C single crystal grown along the<111>direction(Fig.5).The growth mechanism of SiC nanowires cannot be explained by a well-accepted vapor-liquid-solid(VLS)mechanism in this experiment,because no liquid droplet has been found on the top of the SiC nanowires,which is inherent for VLS mechanism [ 10] .On the other hand,the emergence of the terrace(Fig.4(c))and microtwins(Fig.5)structure of Si C nanowires implies that the growth mechanism of Si C nanowires accords to the vapor-solid(VS)theory.At first,the SiC formed via gas-reaction randomly deposited on the coating surface as crystal nuclei.Then the SiC nuclei grew along the preferred<111>direction,which accords with the lowest growth energy principle.However,the preferred growth of SiC nanowires would be stopped temporarily due to a fluctuation of gaseous Si C supersaturation;i.e.,the growth of SiC crystal was not continuous.When the SiC-gas supersaturation reached a critical value again,new SiC nuclei generated and grew afresh on the existing Si C nanowires.As a result,the growth-steps of Si C nanowires generated.

Fig.4.Microstructures of the surface-layer of the coating be-fore oxidation:(a)backscattering electron image of cross-section;(b)and(c)SEM images of surface-section.

Fig.5.TEM and SAD of SiC nanowires.

Fig.6 shows the Si 2p XPS spectra of the coating surface before oxidation.The separated four peaks at 98.9,100.3,101.5,and 103.3 e V are identified as Si,Si C,Si₃N₄,and Si O₂ according to the bonding energy with Si [ 11, 12] .The formation of the by-product Si O₂ results from the oxygen-impurity in nitrogen atmosphere(purity of 99.5%).During the sintering of the coating,N₂can react with Si to synthesize Si₃N₄ at the temperature above the Si melting point.However,Si₃N₄ only form on the surface of the coating,because nitrogen difficultly diffuses into the interior of the melting Si.

Fig.6.Si 2p XPS spectra of the surface of the coated C/C specimens before oxidation.

Based on the above results,a Mo-Si-C-N(Si₃N₄/Si C/Si-Mo Si₂/Si-Si C)multi-layer coating on C/C composites is in-situ synthesized.Ref. [ 8] has reported that C/C composites covered with the above Mo-Si-C-N coating exhibit the steady mass loss of 2.0-2.3 wt.%after 3 h oxidation at1450°C.

Fig.7 presents the cross-section and surface images of the coated C/C composites after oxidation at 1450°C.The coating maintains a three-layered structure after oxidation(Fig.7(a)).The morphologies of Mo Si₂/Si main-layer and Si C bonding-layer show no changes.It means that the severe oxidation do not happen in the main-layer and the bonding-layer.It is worth noting that a smooth glass film was found on the exterior surface of the coating after oxidation(Fig.7(b)).Fig.8 exhibits Si 2p XPS spectra of the glass film.Comparing Fig.8 with Fig.6,it can be seen that the content of SiC obviously decreases while SiO₂ replaces Si C as the main phase in the coating surface after oxidation.It implied that a lot of SiC nanowires transmuted into SiO₂glass via oxidation reaction.A large amount of SiO₂ would effectively heal the cracks as the diffusion channel of oxygen in the coating due to its good liquidity and low oxygen diffusion coefficient,which benefits to improve the oxidation resistance of the coated C/C composites at the oxidation temperature.

Fig.7.Cross-section(a)and surface(b)images of the coated C/C composites after oxidation.

Fig.8.Si 2p XPS spectra of the surface of the coated C/C specimens after oxidation.

4. Conclusions

A Mo-Si-C-N multi-layered anti-oxidation coating was in-situ fabricated by introducing nitrogen atmosphere during the fused sintering of Mo-Si slurry pre-layers on C/C composites.The Mo-Si-C-N coating has a three-layered structure.Besides MoSi₂/Si main-layer and SiC bonding-layer,a surface layer of about 10μm in thickness was synthesized The surface layer mainly consists of Si C nanowires and contains some Si₃N₄ and Si phases.SiC nanowires of 10 to200 nm in diameter exhibit a terrace and distortion structure Transmission electron microscopy indicates that the Si C nanowires grow along the preferred<111>direction.During oxidation test Si C nanowires transmute into Si O₂ glass which can play an important role in improving the oxidation resistance of C/C composites.

Acknowledgements

This work was financially supported by the Foundation of Heilongjiang Province(No.LC04C16),the Foundation of Aerospace Technology(No.HTJSZC-0506),and the Post-Doctor Foundation of Heilongjiang Province,China.