06) 1964–1967www.elsevier.com/locate/matlet

Materials Letters 60 (20

Oxidation protective behavior of SiC/Si–MoSi2 coatingfor different graphite matrix

Juan Zhao a,b, Lang Liu a,⁎, Quangui Guo a, Jingli Shi a, Gengtai Zhai a

a Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, PR Chinab Graduate School, Chinese Academy of Sciences, Beijing, 100039, PR China

Received 19 October 2005; accepted 16 December 2005Available online 11 January 2006

Abstract

A SiC/Si–MoSi2 oxidation protective coating was prepared on different graphite matrix, and the oxidation behavior of SiC/Si–MoSi2 coatedon different graphite matrix was also studied. Results showed that the porosity and pore radius of graphite materials had marked effect on theoxidation behavior ability for carbon materials. Appropriate porosity and pore radius are the necessary factors to realize the excellent oxidationresistance in air, and the reason for the different oxidation behavior of coating on different carbon matrix was also analyzed.© 2005 Elsevier B.V. All rights reserved.

Keywords: Graphite; Coating; Oxidation

Table 1Characters of three types of graphite materials

Graphite ρ/g/cm3 Open porosity/ %

1# 1.89 12.62# 1.80 15.63# 1.60 18.8

Table 2The distribution of pore radius of three types of graphite materials

Pore radius(nm)

(%) Volume in interval of carbon materials

1# 2# 3#

2 ∼ 5 10.23 4.64 2.435 ∼ 10 12.23 2.53 4.8610 ∼ 50 50.48 14.3 9.4050 ∼ 100 9.03 8.68 11.40100 ∼ 500 11.35 48.00 2.71

1. Introduction

Carbon and graphite are a lightweight, high strength andmodulus class of materials that can be used in high tem-perature [1,2]. However, it is oxidized dramatically above500 °C, which limits its broad applications. Therefore,many studies have been performed to improve the oxidationresistance of carbon-based materials in the past 60 years[3].

Oxidation resistance coatings are the logical choice forprotecting carbon material at high temperature. Many ceramicsare promising candidate materials for oxidation protectiveapplications. However, due to the mismatch of thermalexpansion between ceramics and carbon materials, manyceramics can not be used directly to coat the carbon materialssurface. Amulti-layer coating is to be a feasible way to solve thisproblem. So far, many multi-layer coatings such as SiC/mullite[4], SiC–Al2O3–mullite [5] and SiC/borosilicate glass [6] havebeen achieved by several researches, with the aim of improvingoxidation/corrosion resistance of carbon materials at thetemperature above 1300 °C. On the other hand, the oxidationbehavior of the coating prepared on the different carbon matrix

⁎ Corresponding author.E-mail address: ndyang@163.com (L. Liu).

0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2005.12.072

can be quite different, and few researchers have reported theoxidation behavior of multi-layer coatings prepared on thedifferent carbon matrix.

In this study, a new SiC/Si–MoSi2 oxidation protectivecoating was prepared on three kinds of graphite matrix, and

500 ∼ 1000 3.82 17.6 10.311000 ∼ 4000 2.71 56.80Total 97.14 98.46 97.91

Fig. 1. XRD patterns of the coatings: (a) the coating on 1# graphite; (b) thecoating on 2# graphite; (c) the coating on 3# graphite.

1965J. Zhao et al. / Materials Letters 60 (2006) 1964–1967

investigated it's oxidation protective ability for differentgraphite matrix. Graphite substrates were firstly infiltratedwith silicon, and then coated with Si–MoSi2 by a simpleslurry method. The structure and oxidation resistant propertyin air at 1673 K of the as-received coating were alsoinvestigated.

Fig. 2. Microstructure of cross-section of coating.

2. Experimental

2.1. Coating preparation

Three kinds of graphite were used in this research. The char-acteristics of the graphite are summarized in Table 1. 1# graphitewas under construction at the authors' institute by using hot-pressingmethod.BaofengCarbonCorporation provided 2# graph-ite. 3# was a type of anode graphite provided by Jilin CarbonCorporation. In addition, the distribution of pore radius of threetypes of graphite is showed in Table 2. Three types of graphitewere cut into small specimens with a size of 15×15×30 mm.Before coating, the specimens were hand-polished and ultra-sonically cleaned with distilled water. The inner SiC layer wasachieved by silicon infiltration. Graphite covered 500 μm Sislurry were heated at 1500 °C for 2 h in vacuum furnace to forminner SiC. The outer coating was made by a simple slurrymethod. In the preparation of Si–Mo slurry, high purity Si andMo powders with average sizes of 7 and 3 μm respectively wereweighed to the desired composition of Si–40 wt.% Mo andblended with water and PVA binder in a ball mill for about 1 h.The solid-to-liquid ratio of the slurry was adjusted to produce aviscosity suitable for the application of slurry on the specimens.The SiC pre-coated specimens were dipped into the resultingslurry. After drying, the sintering of as-coated specimens wasperformed at 1420 °C for 2 h in a vacuum furnace.

2.2. Oxidation tests

The samples were heated at 1673 K in a corundum tubefurnace in air to investigate the isothermal oxidation behavior.

a: 1# graphite; b: 2# graphite; c: 3# graphite.

Fig. 3. Surface micrograph of as-received coating before oxidation. a: 1# graphite; b: 2# graphite; c: 3# graphite.

1966 J. Zhao et al. / Materials Letters 60 (2006) 1964–1967

The morphology and crystalline structure of the double-layercoating were analyzed by scanning electron microscopy (SEM),X-ray diffraction (XRD) and energy dispersive spectroscopy(EDS).

3. Results and discussion

3.1. Microstructure of the coating

Fig. 1 shows the XRD pattern of the coating surface on three kinds ofgraphite, respectively. Strong peaks about β-SiC, Silicon andMolybdenum disilicide were detected. It should be here stressed that

Fig. 4. Isothermal oxidation curves of the samples in air at 1400 °C. a: 1#

graphite; b: 2# graphite; c: 3# graphite.

the intensity ofβ-SiC of the coated 3#graphite was strongest, and that ofthe coated 1# graphite was weakest, indicating that SiC concentrationfor three kinds of graphite was different after silicon infiltration.

Fig. 2 shows microstructure of cross-section of the coated graphite.The outer layer with the thickness of about 80 μm for three kinds ofgraphite has the same structure, which consists of the white dispersephase and the grey matrix. By using EDS analysis, the white and greyphase can be distinguished as MoSi2 and Si, respectively. The SiC innerlayer was formed after silicon infiltration. The thickness of SiC layer for3# graphite is thickest and that of 1# graphite was thinnest, whichaccorded with XRD results. In addition, it should be pointed out thatafter silicon infiltration a composition-gradient SiC layer was formedonly for 2# graphite. The surface morphology of the coated graphite asseen in Fig. 3. For SiC/Si–MoSi2 coated 1

# and 3# graphite, cracks werefound to traverse the coating. In the case of coated 2# graphite, somesmall holes are visible, while no cracks are found. Therefore, a SiC/Si–MoSi2 coating without a gradient SiC inner layer tends to crack duringthe coating preparation procedure when the sample was rapidly cooledfrom sintering temperature to the room temperature, due to the thermal

Fig. 5. Surface morphologies of coating at 1673 k after oxidation 40 h.

1967J. Zhao et al. / Materials Letters 60 (2006) 1964–1967

stress caused by the difference in thermal expansion between the outercoating and the substrate.

3.2. Evaluation of oxidation resistance

Fig. 4 Shows the weight loss curves of the SiC/Si–MoSi2 coatedgraphite isothermally oxidized at 1673 K. For the coated 1# and 3#

graphite, about 18% and 12% level of weight loss was observed after10 h oxidation, indicating that the coated 1# and 3# graphite are unableto withstand oxygen attack for an oxidation test longer than 10 h. Incontrast, 2# graphite shows a better oxidation resistance. Fig. 5 show thesurface micrograph of 2# graphite after 40 h oxidation. The microcracksin the coating can be sealed by the formation of SiO2 glass. This SiO2

glass was the oxidation production of Si, MoSi2 and SiC and served as abarrier to oxygen diffusion and protected graphite from oxygen at-tacking. As a result, the oxidation behavior of SiC/Si–MoSi2 coated on2# graphite was improved. For the coated 1# and 3# graphite, the cracksmight not be sealed by SiO2 glass, since a weight loss due to oxidationwas observed. Based on above results, it can be concluded that theimprovement of the SiC/Si–MoSi2 coating properties for 2# graphitecan be attributed to the SiC gradient inner layer. Although SiC innerlayer for graphite was made at the same condition, a SiC gradient innerlayer was easily formed for 2# graphite and difficult to form for 1# and 3#

graphite. The reasons can be explained as following: in the process ofthe formation SiC inner layer, the infiltration of liquid Si into the poresof graphite and the formation of SiC by the chemical reaction of Si andCcoexist. Only when the infiltration rate of liquid Si is smaller than thechemical reaction rate, a SiC gradient layer forms [7]. Three kindsgraphite has the same chemical reaction rate, because the chemicalreaction rate of Si andC is a function of temperature. The infiltration rateof liquid Si can be expressed by the following equation [8]:

m ¼ dl=dt ¼ rgLV cosh=gi ð1Þwhere t is the infiltration time, l the length of infiltration at time t, r theequivalent radius of the pore, γLV the surface tension, θ the wettingangle and η the viscosity of the infiltration. In our experiment, thechemical reaction rate of Si and C is fixed, and γLV, θ and η are alsofixed, The rate of infiltration is proportional to the pore diameter asindicated by Eq. (1). Therefore, the infiltration rate of liquid Si is mainlydetermined by the pore size of the substrate. From above results, it canbe concluded that when the pore size of graphite is between 100∼500μm, the infiltration rate of liquid Si could be smaller than the chemicalreaction rate and a SiC gradient layer forms. This point was confirmedby the experimental results of Singh and Behrendt [9]. For 3 graphite,the pore size of graphite is comparatively large, the infiltration rate of

liquid Si could be higher than the chemical reaction rate and no gradientSiC layer forms. For 1# graphite, the pore size of graphite is too small,the pore radium could be totally blocked by the newly formed SiC andthe further infiltration of liquid Si could be prevented, which resulted inSiC existing on the surface of substrate. Therefore, to improve theoxidation resistance of SiC/Si–MoSi2 coated 3# graphite we need toreduce the size of pore radius by additional measure such as repeatedinfiltration phenolic resin into graphite substrate. For 1# graphite, SiCgradient inner layer might be formed by chemical vapor deposition(CVD). So, further research should be continued.

4. Conclusions

A SiC/Si–MoSi2 oxidation protective coating was preparedon different graphite matrix. The inner SiC layer was made bysilicon infiltration, and using a simple slurry method made theouter Si–MoSi2 layer. The porosity and pore radius of graphitematerials had marked effect on the oxidation behavior ability forcarbon materials. Appropriate porosity and pore radius are thenecessary factors to realize the excellent oxidation resistance inair.

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