Preparation and oxidation property of ZrB₂-MoSi₂/SiC coating on carbon/carbon composites

ZHANG Wu-zhuang, ZENG Yi, GBOLOGAH Lemuel, XIONG Xiang, HUANG Bai-yun

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Received 10 November 2010; accepted 16 February 2011

Abstract: To improve the oxidation resistance of carbon/carbon composites, ZrB₂-MoSi₂/SiC coating on the carbon/carbon substrate was prepared. The inner coating of SiC was prepared by pack cementation and the outer coating of ZrB₂-MoSi₂ was prepared by slurry painting. The phase compositions and microstructures of the coating were characterized by XRD and SEM, respectively. The preparation and the high temperature oxidation property of the coated composites were investigated. The results show that the outer coating of carbon/carbon composites is composed of ZrB₂, MoSi₂ and SiC phases. The mass losses of the ZrB₂-MoSi₂/SiC coated samples with SiC nano-whiskers after 30 h and 10 h of oxidation at 1 273 K and 1 773 K were, respectively, 5.3% and 3.0%. The ZrB₂-MoSi₂/SiC coated samples exhibit self-sealing performance and good oxidation resistance at high temperature.

Key words: carbon/carbon composites; ZrB₂-MoSi₂; SiC; coating; oxidation

1 Introduction

Carbon/carbon (C/C) composites have many advantages over traditional materials, such as low density, high specific strength and retention of mechanical properties at high temperatures. However, C/C composites oxidize rapidly when they are exposed to an oxidizing atmosphere at high temperatures. Therefore, much attention has been given to the development of methods for oxidation protection of carbon materials [1]. One of such approaches  most  widely  used  is  to  apply  coatings  to  the  article  surface [2], such as SiC [3], MoSi₂ [4] and ZrSiO₄ [5]. The oxidation protection properties of these coatings are different from the fabrication technologies such as pack cementation, chemical vapor deposition (CVD) and slurry painting. Slurry painting has a lot of advantages including its easy processes, low costs, etc. MoSi₂ is promising candidate material for high-temperature applications because of the self-sealing performance at high-temperature [6]. ZrB₂ has excellent anti-oxidation property due to B₂O₃ glaze which is one kind of oxides of ZrB₂ at 1 273 K [7]. The SiC ceramic also has outstanding anti-oxidation properties because of the self-sealing performance of SiO₂ formed from oxidation of SiC. However, not much literature has been reported for the oxidation performance of ZrB₂-MoSi₂ coating prepared by slurry painting. In the present work, the preparation and the oxidation property of ZrB₂-MoSi₂ coating on the C/C composites were investigated. In order to relieve the thermal expansion mismatch between the outer coating and C/C composites, the inter SiC transition layer was prepared by pack cementation.

2 Experimental

The C/C composites used were three-dimensional materials with a density of 1.78 g/cm³. The specimens were polished using a SiC paper, cleaned with ethanol and oven dried at 393 K. The inner coating of SiC was prepared by pack cementation. The sintering temperature and holding time were 2 073 K and 2 h, respectively. The pack chemistry was as follows: 60%-80% (mass fraction) Si, 10%-20% carbon, and 1%-5% Al₂O₃. The ZrB₂-MoSi₂ layer was prepared by painting the slurry mixture comprising of 50%Si, 25%ZrB₂, 25%Mo powders and silica sol on the surface of SiC coated substrates, and then dried. Since Si element would evaporate in the sintering procedure, two methods were taken to control the Si content. The first one was to control the mole ratio of Si to Mo in the mixture larger than that of MoSi₂ (2:1). The second one was to decrease the evaporation of Si from the coating by wrapping substrates in a parchment paper on which the Si and silica sol were painted. The samples treated by the first and second methods were labeled A and B, respectively. Then the two kinds of samples were placed in a graphite crucible with a graphite lid and heated in an argon atmosphere at 1 773 K for 15 min to obtain ZrB₂-MoSi₂ coating. The phase composition of the prepared coatings was determined with X-ray diffractometer (XRD). The morphologies of the coatings were analyzed using scanning electronic microscope (SEM). The balance with an accuracy of 0.1 mg was used to study the mass changes of samples after oxidation.

3 Results and discussion

3.1 Phase composition and micro-structure of coating

Figure 1 shows the XRD patterns of the ZrB₂-MoSi₂/SiC coating. It is observed that ZrB_2, MoSi₂ and SiC are the main phases. The peak intensity of SiC in Fig. 1(b) is stronger than that of SiC in Fig. 1(a), suggesting that more SiC phases were formed in the coating after wrapping with painted parchment paper. The SiO₂ phases came from the silica sol.

Figure 2 shows the surface and cross-section SEM micrographs of ZrB₂-MoSi₂/SiC coatings. Figure 2(a) shows the surface SEM image of ZrB₂-MoSi₂/SiC coating of sample A. It is observed that there are a lot of cracks on the surface of the coating, which is due to the silica sol shrinkage and the crystal volume change when Si reacts with Mo [8]. Figure 2(b) shows the surface SEM image of ZrB₂-MoSi₂/SiC coating of sample B. It is noted that the coating is denser than that on sample A due to the formation of the whiskers and particles, as shown in Fig. 2(e). Figures 2(c) and (d) show the cross-section micrographs of SiC coating and ZrB₂-MoSi₂ coating, respectively. From Fig. 2(c), it is observed that there is good combination between the SiC coating and carbon substrate due to the infiltration of SiC into the fibers of the substrate which can enhance the thermal shock resistance. There is also no obvious interface between SiC coating and ZB₂-MoSi₂ coating (Fig. 2(d)), inferring that there is good physical compatibility between the inner and outer coatings. From Fig. 2(f), the nano- whiskers on ZrB₂-MoSi₂ coating on sample B can be observed. Results obtained from Fig. 3 indicate that the composition of the nano whisker is SiC.

Fig. 1 XRD patterns of ZrB₂-MoSi₂/SiC coatings: (a) Sample A; (b) Sample B

Ca and Al in Fig. 3 may be impurities in the Si powder. The diameters of the whiskers are 25-63 nm. Some cracks on the surface of the coating are full of SiC whiskers which can enhance the oxidation resistance of the coated C/C composites. The mechanism of crystal growth of the SiC nano-whisker can be attributed to the reactions [9]:

SiO₂(s)+Si(l)=2SiO(g)                        (1)

SiO(g)+2C(s)=SiC(s)+CO(g)                   (2)

SiO(g)+3CO(s)=SiC(s)+2CO₂(g)                (3)

SiO is obtained by the reaction between SiO₂ and Si on the parchment paper (reaction 1), and SiO reacts with the carbon to form SiC whiskers because of the high carbon potential in graphite crucible.

As noted in Fig. 2(e), not only the SiC whiskers were observed, but also the SiC particles were found, which is related to the different growth velocities of SiC crystals. The velocity of crystal growth of SiC can be attributed to the following equation [10]:

                    (4)

  where A is equilibrium constant of reaction; T is thermodynamic temperature; p_co is partial pressure of CO; Δε is activation energy. Reaction (4) shows that high partial pressure of CO is beneficial to the growth of SiC crystal. The SiC whiskers almost form from reaction (3) and the SiC particles form from reaction (2) [11-12]. Therefore, the higher partial pressure of CO accelerates the reaction of reaction (3), which, in turn, results in formation of more SiC whiskers. Formation of some SiC particles is due to the higher equilibrium constant of reaction (2) than reaction (3).

Fig. 2 Surface and cross-section SEM micrographs of ZrB₂-MoSi₂/SiC coating: (a) Surface micrograph of sample A; (b) Surface micrograph of sample B; (c) Cross-section micrograph of SiC coating; (d) Cross-section micrograph of ZrB₂-MoSi₂/SiC coating; (e) Particles and whisker of SiC; (f) SiC nano-whiskers

Fig. 3 EDS analysis of surface spot E in Fig. 2(f)

3.2 Oxidation properties analysis

3.2.1 Oxidation properties of coated samples at 1 273 K

Figure 4 shows the isothermal oxidation curves of the C/C composites with coatings in air at 1 273 K. Figure 5 shows the surface SEM micrographs of the ZrB₂-MoSi₂/SiC coating after oxidation at 1 273 K for  30 h. Figure 6 shows XRD patterns of ZrB₂-MoSi₂/SiC coating after oxidation at 1 273 K.

From Fig. 4, it can be observed that the mass losses of ZrB₂-MoSi₂/SiC coated samples A and B after 30 h of oxidation at 1 273 K are 30.2% and 5.3%, respectively. This suggests that the oxidation protection for sample B is superior to that for sample A. The single SiC coated sample has a mass loss of 11.0% after oxidation for 5.5 h. Figure 5(b) shows that a ceramic glaze is formed on the surface of the coating.

There is new phase of ZrO₂ since the oxidation of ZrB₂ can form ZrO₂ and B₂O₃. However, the B₂O₃ phase is not detected. It is believed that the samples were quenched in air in the course of drawing the samples rapidly out of the furnace and that the liquid B₂O₃ was not able to crystallize before solidification. So the ceramic glaze on the coating can be considered B₂O₃  (Fig. 5(b)) [13]. The B₂O₃ glaze seals the holes and cracks due to the liquidity on the coating [7], which can enhance the oxidation resistance of the coated C/C composites. However, obvious cracks can be found in Fig. 5(a). The original cracks on the coating without SiC whiskers and particles covering was too wide for limited B₂O₃ glaze to be sealed. This may be the reason why the oxidation protection for sample B is superior to that for sample A.

Fig. 4 Isothermal oxidation curves of C/C composites with coatings in air at 1 273 K

Fig. 5 Surface SEM micrographs of coatings after oxidation at  1 273 K for 30 h: (a) Sample A; (b), (c) Sample B

From Fig. 6 and Fig. 1(b), the peaks of MoSi₂ decreased and that of SiO₂ increased after oxidation, which indicates that MoSi₂ was oxidized, producing SiO₂ and MoO₃ at 1 273 K [14]. However, the MoO₃ phase is not detected due to MoO₃ (melting point: 1 068 K) evaporation at this temperature which results in some minor holes on the coating (Fig. 5(b)). The evolution of CO₂ and CO from the SiC and substrate oxidation may result in the other holes. The whiskers disappeared and the coating became smooth (Fig. 5(c)), also suggesting that the SiC phase was oxidized. Hence, all of these reactions can be described as follows:

2ZrB₂(s) +5O₂(g)=2ZrO₂(s)+2B₂O₃(g)           (5)

2MoSi₂(s)+7O₂(g)=2MoO₃(g)+4SiO₂(s)          (6)

SiC(s)+2O₂(g)=SiO₂(s)+CO₂(g)                 (7)

2SiC(s)+3O₂(g)=2SiO₂(s)+2CO(g)              (8)

Fig. 6 XRD pattern ZrB₂-MoSi₂/SiC coating after oxidation at 1 273 K

3.2.2 Oxidation properties of coated samples at 1 773 K

Figure 7 shows isothermal oxidation curves of the coated C/C composites in air at 1 773 K. Figure 8 shows the surface and cross-section SEM micrographs of ZrB₂-MoSi₂/SiC coating after oxidation at 1 773 K for  10 h.

From Fig. 7, the mass loss of the single SiC coated samples is 9.2% after oxidation for 3 h, which suggests that the sample with a single coating of SiC has limited protection for substrates. It is also found that the mass losses of the samples A and B after 10 h of oxidation at   1 773 K are 3.6% and 3.0%, respectively. This suggests that ZrB₂-MoSi₂/SiC coated samples have excellent oxidation resistance due to the formation of a glassy layer which seals the original cracks and the coating becomes more smoother (Fig. 8(a)). In Fig. 8 (b), there are still good combination between the carbon substrate and coatings and there is no obvious interface between SiC coating and ZrB₂-MoSi₂ coating, which is the reason why the coated samples possess good thermal shock resistance during oxidation at 1 773 K. From Figs. 8(c) and (d), the samples A and B have similar micrographs after oxidation at 1 773 K, which results in the excellent anti-oxidation performance of the two samples.

Fig. 7 Isothermal oxidation curves of coated C/C composites in air at 1 773 K for 10 h

Figure 9 shows XRD patterns of ZrB₂-MoSi₂/SiC coating after oxidation at 1 773 K. Figure 10 shows the phase distribution of the coating and EDS analysis of the corresponding surface spots.

From Fig. 9, ZrSiO₄ phase formed was due to the reaction between ZrO₂ and SiO₂. In Fig. 10, there are two kinds of textures (shown as grey 1, white 2) on the coating after oxidation. By spot EDS analysis, the elements composing of grey 1 and white 2 are mainly Si, O and Si, O, Zr, respectively. Based on this fact together with the X-ray diffraction analysis in Fig. 9, it is believed that the grey phase 1 is composed of SiO₂, whereas the white phase 2 is composed of ZrSiO_4. From Fig10(a), it is observed that the ZrSiO₄ phase spreads around the glassy SiO₂, which can enhance the anti-oxidation performance based on the following facts: 1) glassy SiO₂ possesses low oxygen diffusion coefficient (10^{-13 g}/(cm?s) at 1 473 K) which can decline/reduce the velocity of oxygen through the coating [15]; 2) thermal expansion coefficient of ZrSiO₄ is close to that of SiC, which can eliminate the thermal expansion mismatch between the outer and inner coating. The ZrSiO₄ phase also has pining effect on the melting-point (2 550 °C) [5] in the case of the fact that the SiO₂ glaze vaporized fast.

Fig. 8 Surface (a, b) and cross-section (c, d) SEM micrographs of ZrB₂-MoSi₂/SiC coating after oxidation at 1 773 K for 10 h: (a), (c) Sample A; (b), (d) Sample B

Fig. 9 XRD pattern of ZrB₂-MoSi₂/SiC coating after oxidation

Fig. 10 Phase distribution of ZrB₂-MoSi₂/SiC coating (a) and EDS analyses (b, c) of corresponding surface spots

4 Conclusions

1) ZrB₂-MoSi₂/SiC coating on the carbon/carbon substrate was prepared successfully. The outer coating is composed of ZrB₂, MoSi_2, and SiC phases. There are lots of SiC nano-whiskers on ZrB₂-MoSi₂/SiC coating for sample B.

2) Single SiC coating has limited protection for carbon/carbon substrates. The ZrB₂-MoSi₂/SiC coated samples B shows good anti-oxidation ability at 1 273 K due to the SiC nano-whiskers covering the original cracks and the formation of B₂O₃ glaze after oxidation. The ZrB₂-MoSi₂/SiC coated samples exhibits self-sealing performance and oxidation resistance due to the formation of ZrSiO₄ and SiO₂ glaze at 1 773 K.

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ZrB₂-MoSi₂/SiC涂层C/C复合材料的制备及氧化性能

张武装，曾 毅，GBOLOGAH Lemuel，熊 翔，黄伯云

中南大学 粉末冶金国家重点实验室，长沙 410083

摘  要：为了提高C/C复合材料的抗氧化性，在C/C复合材料基体上制备了ZrB₂-MoSi₂/SiC涂层。采用包埋法制备SiC中间层，采用喷涂法制备ZrB₂-MoSi₂外涂层。用XRD和SEM分别分析、测试所制备涂层的物相组成和显微结构，研究涂层复合材料的高温抗氧化性能。结果表明：C/C复合材料的外涂层由ZrB₂、MoSi₂ 和SiC三相组成；在1 273 K和1 773 K下分别氧化30 h和10 h后ZrB₂-MoSi₂/SiC涂层试样的质量损失分别为5.3%和3.0%，涂层表面长有纳米SiC晶须。C/C复合材料ZrB₂-MoSi₂/SiC涂层具有自愈合特性和良好的高温抗氧化性能。

关键词：C/C复合材料；ZrB₂-MoSi₂；SiC；涂层；氧化

(Edited by YANG Hua)

Foundation item: Project (50721003) supported by the Innovation Community Foundation of National Natural Science of China; Project (2011CB605805) supported by the National Basic Research Program of China

Corresponding author: XIONG Xiang; Tel/Fax: +86-731-88836079; E-mail: Xiong228@sina.com

DOI: 10.1016/S1003-6326(11)60893-5