COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 61(2001)607-614 Oxidation behaviour of a multi-layered ceramic-matrix composite(SiC)r/C/(sibC)m J. P. Viricelle, P. Goursat*, D. Bahloul-Hourlier S.P.C.T.S., U.R.A. CNRS 6638, Universite de Limoges, 123, Av. Albert Thomas, 87060 Limoges Cedex, france Received 2 July 1999; accepted 6 December 2000 Abstract The oxidation behaviour of a 2.5D multi-layered ceramic-matrix composite (Sic)/C/(SiBC)m was investigated in a dry atmo- sphere O2(20 vol %)CO2(5 voL % He and in the presence of water vapour H20 (2.3 vol %) The matrix denoted (SiBC)m constituted of three phases: silicon carbide, boron carbide and a SibC phase. The aim of boron incorporation is to improve the oxidation resistance of the composite by boron oxide or borosilicate formation. The transformations were followed by thermo- gravimetry during isothermal experiments of 20 h exposure in the range 600-1200 C and the changes of the specific area of the ples were measured by krypton adsorption at 77K. The results of this study show that effective protection occurs in the range 650-900oC and is mainly related to the oxidation of boron carbide. At higher temperatures, boron oxide is no longer protectiv because of its volatilisation and consumption by reaction with water vapour. However, the two other constituents of the matrix, C and SiBC, lead to self-healing behaviour by both borosilicate and silica formation. C 2001 Published by Elsevier Science Ltd Keywords: Ceramic-matrix composite; Oxidation 1. ntroduction boron-containing composites [12, 13 revealed that the formation of a borosilicate layer was efficient in limiting Ceramic-matrix composites(CMCs) reinforced with oxidation. Some new composites have therefore beer continuous Sic-based fibers exhibit attractive properties developed on the basis of a(SiC)/C/(SiC)m structure or thermostructural applications, including low den- but with the incorporation of boron compounds in the sity, high strength and non-brittle mechanical beha- matrix [14]. In this paper, a study of the oxidation viour. This last property is controlled by the presence of behaviour of a 2. 5D multilayered matrix composite with an interphase between the fibers and the matrix. The silicon carbide fibers and a carbon interphase is pre- interphase, which often consists of pyrocarbon, allows fiber debonding and crack deflection with energy dis- sipation [1-3]. Nevertheless, a critical aspect of the tance of the interphase [4-7. The erosion of the carbo- naceous structure by oxidation alters the properties of The experimental techniques used for the composite the medium and thus affects the durability of the cera- haracterisation were optical microscopy (Olympus mic material [8]. An answer consists in replacing the Mo61)and scanning electron microscopy (Philips pyrocarbon interphase by a boron nitride Bn inter- XL30) for morphology and architecture observations, phase [9-11]. However, modelling of the oxidation of a and microanalysis and X-ray diffraction (Philips (SiC)f/C/( SiC)m composite [4, 5] has shown that self- DW1130)for the determination of the composition and healing behaviour consisting in crack closure by silica nature of the phases. Specific area measurements formation occurred above 1000C, and other studies on (Micromeritics ASAP2000) were performed with iso- thermal krypton adsorption experiments at 77 K in s Corresponding author. Fax: +33-05-55-45-75-86 order to follow the variation of the reactive area after E-mail address: goursat(@ unilim fr(P Goursat) oxidation treatments of the samples 0266-3538/01/S.see front matter C 2001 Published by Elsevier Science Ltd. All rights reserved PII:S0266-3538(00)00243-8
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J. P. Viricelle et al. Composites Science and Technology 61(2001)607-614 The oxidation of the material was recorded by ther gravimetry using a thermobalance (Setaram B60) Matrix 3 SiC(5) The weight changes were continuously monitored. A Matrix 1 SiBC( 2) platinum wire and an alumina crucible were used to hold the sample in the furnace. The experiments were Matrix 3 SiC(4) performed with a total flow rate of 1. 8 1/h either in a dry atmosphere consisting of oxygen (20 vol %) carbon Matrix 1 SiBC(1) dioxide (5 vol %)and helium or a wet atmosphere obtained by the bubbling of the previous one in a water Matrix 3 Sic(3 bath regulated at a constant temperature(20C). The Matrix 2 BC(2) resulting water vapour content was about 2.3 voL% Carbon dioxide was used to control the activity as it is Matrix 3 SiC(2) produced by the oxidation reaction. Moreover, this gas is present in the combustion atmosphere in real appli Matrix 2 BC(1) cations of composite materials. Increasing-temperature Matrix 3 Sic(1) experiments with a low heating rate of 2C/min until 1200 C have been conducted in order to identify the Carbon interphase various steps during oxidation. Isothermal kinetic curves were then obtained in the range 600-1200%C for a duration of 20 h. The temperature ramp(10C/min) Fig 1. Schematic illustration of the composite architecture was conducted under dry helium until the desired tem- perature was reached. The chosen atmosphere for the experiments was then introduced and this constituted SiC (5) the starting time of the isotherm ∈SiBC(2 SC(4) ∈SiBC(1) 3. Material SiC (3) ←B4C(2 The composite architecture is of crucial importance for the oxidation behaviour The material is a 2 5D network of Nicalon NL202 fibers coated with a pyr- carbon interphase of approximately 0.1 um thick ness. The multilayered matrix is then deposited by Fig. 2. Optical micrograph(x 180)of the composite, external layers chemical vapour infiltration or deposition depending on the layer. The sequence of the deposit is sum marised in Fig. 1. It can be seen that the matrix is composed of three different constituents: a phase containing silicon, boron and carbon noted SiBC and referred to as matrix 1, a phase containing boron and sc(5)一 carbon noted B. C, called matrix 2, and a third phase containing silicon and carbon noted SiC, matrix 3 The samples used in this study( 8 mm x8 mm x 4 SiC (4 mm)were obtained by machining larger plates perpen dicularly to the 2D fiber plane. In the four lateral cut faces( 8 mm x4 mm), the fibers and the interphase are thus unprotected by the matrix which was deposited before machining, as shown in Fig. 2. The sequence of the matrix layers inside the yarn network depends on SiC (2 the interyarn porosity which is progressively filled and losed during infiltration cycles. The layers SiC(1)+ Fig 3. Scanning electron micrograph in the bulk of the composite. B,C(1) have a total thickness of about I um. The sequence SiC(2)+(BC(2)fills the intrayarn porosity osition of the boron-carbide phase, matrix Figs 3 and 4). The specific surface area of the samples 2, B4C(2)and of the SiBC phase, matrix 1, SiBC(1)and is very low, about 0.02 m/g, indicating that there is SiBC( been determined by microanalysis. The no microporosity. The total macroporosity is about oxidation behaviour of samples Ith a composition 10% close to that of matrix 2 has been studied and presented
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J. P. Viricelle et al. Composites Science and Technology 61(2001)607-614 C/SiC(1)/B4C(1) SiC(3) 1020304050607080 SiC (5) Fig. 4. Scanning electron mi h in the bulk of the composite 70 elsewhere [15]. The atomic composition of matrix I is 一C very homogeneous throughout the layers SiBC(1) and SiBC(2)(Figs. 5 and 6). X-ray diffraction patterns(Cu, Ka) performed on particular samples where only matrix I has been deposited shows a poor crystallisation state Fig. 7)and a structure based on silicon carbide. The pyrolysis of these samples at 1500C in helium leads to the segregation of the constituents and crystallisation of licon carbide boron carbide and carbon (Fig. 8) More detailed investigations on the SiBC structure have been performed by high-resolution electron microscopy [14] and reveal that it is composed of Sic crystallites surrounded by an amorphous phase. Boron may be Point (au) present either in Sic lattice or as an amorphous boron Fig. 6. Composition profile of SiBC(2) layer. carbide phase. No trace of crystallised boron carbide has been observed. it is also known that the maximum solid solubility of boron in silicon carbide is about 0.2 wt%[16], and that boron can be incorporated either in Matrix 1 silicon or carbon sites [17]. However, by considering the boron content of matrix 1, the presence of an amor- -"--"Carbon phous boron carbide phase seems to be the more prob able hypothesis. From the mean chemical composition it is possible to calculate a theoretical molar composi tion by taking into account the main phases: SiC, B4C, C. The resulting molar composition shows an excess of carbon 102030405060708090 4. Results and discussion Prior to this study, the oxidation behaviour of each Fig. 7. X-ray diffraction pattern of matrix I SiBC constituent of the composite(fiber, matrices I and 2) had been investigated in similar conditions [18-20]. The occurs through the lateral faces( 8 mm x 4 mm). As a results obtained are necessary to understand the whole at, oxidation of our specimens is greatly enhanced composite behaviour. As indicated previously, the sam- compared to the final materials used in applications ples used for the composite oxidation study are coupons However, the interest is an elucidation of the phenom- (8 mm x8 mm x 4 mm) cut from larger plates. Thus, ena which facilitates their identification and under the access of oxygen in the bulk of the material mainl
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J. P. Viricelle et al. Composites Science and Technology 61(2001)607-614 △m/m(%) T(C) Heat-treated e(m.2m01 SiC 1200 B C -0.05 800°C "- Carbon 1000 4., 200400600800 Fig 8. X-ray diffraction l500° C in helium pattem of a matrix I SiBC sample heated Fig 9. Thermal behaviour of the composite during calcination in wet atmosphere He-O2(20%)-CO2(5%H-HyO(2.3%), with a heating rate 4.1. Oxidation in wet atmosphere/He-O2(20% )-C0 (5%-H2012.3% spreading over the surface of the sample. The result is a 4.1.1. Increasing temperature strong weight gain in range(ii)600-1050oC. The varia The thermogravimetric curve obtained during the tions of the rate at 670 and 800C are not accurately calcination of a sample heated in a wet atmosphere with explained. They may correspond to a change in the a linear rate of 2 C/min until 1200oC and maintained at kinetics of B.C oxidation(670C)and to the beginning this temperature for 10 h is reported in Fig 9. It can be of the oxidation of the other constituents(SiBC, Sic)at seen that the relative weight change is small: -0.3%< 800C which modify the viscosity of the oxide scale Am/m, +0.05. However, the variations of the rate of Effectively, for individual constituents, at 670 C, matrix weight change (loss or gain and change of the slope) are 1 (SiBC) and Sic oxidation is negligible. The decrease important. Four temperature ranges can be considered of the weight gain rate at 670@C may be a consequence of the decrease of the reactive area due to the formation 20-600C, which corresponds to a weight loss of the continuous boron oxide layer which seals the 2. 600-1050oC, where a strong weight gain occurs porosity. In the third range (iii)1050-1200oC, the with various rates corresponding to three domains weight loss is a result of B2O3 consumption, either by (600-670,670-800,800-1050°C), volatilisation or reaction with water vapour, and of the 050-1200oC, with a weight loss which goes on change of the viscosity of the borosilicate [15]. Conse 4. the isotherm at 1200 C, where a stabilisation quently, the oxide layer is no more protective and the luring the first hour of the isothermal treatment. weight loss corresponds to B2O3 removal, but also to the weight change is observed for nearly 3 h before the carbon interphase oxidation. The partial stabilisa a final and continuous loss tion occurring during the beginning of the isotherm at 1200C is explained by a more important contribution The interpretation of this complex behaviour may be of the oxidation of matrices I and 3 which forms a explained by considering the behaviour of each con- borosilicate and silica stituent which was studied previously [18-20]. As the composite is porous and the atmosphere in the thermo- 4.1.2. Isothermal treatments balance saturated with water vapour at the beginning of Isothermal experiments have been performed at var- the experiment, the weight loss occurring in the range ious temperatures in the range 600-1200oC. The relative 20-400C is due to its desorption and evaporation. weight changes measured by thermogravimetry during Then, between 400 and 600oC, the continuous weight isothermal dwells of 20 h are reported in Fig. 10. The loss is the result of the combustion of the carbon inter- specific area of each sample and its changes after the phase, which is directly accessible by oxygen through isothermal treatment have been measured by adsorption the lateral faces of the sample. Above 600 C, the sudden of krypton at 77 K and calculated by the B ET change of behaviour (loss/ gain)is attributed to the method. The results are reported in Table 1. Because of contribution of the oxidation of boron carbide(matrix the presence of many constituents and of the complex 2)[15]. It occurs with a weight gain and the formation microstructure, it is very difficult to model the phenom- of a liquid boron oxide protects the interphase by ena quantitatively. We can only perform a description
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J. P. Viricelle et al/ Composites Science and Technology 61(2001)607-614 and a qualitative explanation of the behaviour during and the protection becomes more effective. Microscopic each isothermal treatment observations of samples oxidised at 800 C show that all external surfaces are covered by a glassy oxide layer 4.1.2.1. 600-800C range. The kinetic curves(Fig. 10) (Fig. 11). Hence, the interphase is protected(no further confirm the important change of behaviour above carbon combustion) and the substantial weight gain 600C, described previously (Fig. 9). During the iso- corresponds mainly to B4 C oxidation, limited by oxygen therm at 600C, a continuous and quasi-linear weight diffusion though the oxide layer [15] loss is observed. The linearity confirms that the main These results are confirmed by the variation of the phenomenon is the carbon interphase combustion, and specific area(Table 1). An increase of the area is linked thus that there is no protection by boron carbide oxi- to the creation of porosity and thus to carbon con- dation at this temperature. Effectively, the linearity is sumption, whereas a decrease is the result of porosity characteristic of a chemical regime where the reactive closure mainly explained by oxide formation. The var interface(carbon/oxygen) is constant and corresponds iation measured at 600C(AS=So *8)clearly points out to the area of the annular pores created by carbon con- a significant consumption of carbon. This variation sumption [4]. At a slightly higher temperature 650@C, decreases for increasing temperatures(650, 700oC)but the weight change becomes very small. It is negligible remains large (AS=S *2 at 700C). This information during the first 8 h and then a slow weight loss(com- supports that the small weight change measured by pared to the rate at 600oC)is observed. As explained thermogravimetry at these temperatures corresponds to previously, two phenomena occur: carbon combustion a balance between the loss of carbon and the formation (Am<0)and boron carbide oxidation(Am>0). The of boron oxide. At 800C, the change in area is very measured variation is the result of the balance between small and negative, which confirms that boron oxide has the two reactions. With increasing temperatures up to sealed all the porosity created by the interphase degra- 800C, kinetics of boron carbide oxidation increases dation as it can be seen in Fig. 11 In the range 600-800oC, the composite behaviour is thus mainly explained by the reactions of two con stituents: the interphase and matrix 2(B.C). The oxi- dation of matrix 1 (SiBC)is very limited and negligible 900°C for matrix 3 (SiC) and Sic fibers for experiments of 20 h 600C Appears to be a critical temperature 1000c 4.1.2.2. 800-1200C range. The maximum weight gain is obtained at 800C. For increasing temperatures(900, 1000, 1200oC), the final weight gain decreases but the 650°C shape of the curves is quite different for each tempera ture(Fig. 10) At 800C, the main phenomenon is the oxidation of BC according to a diffusion limited regime in agree- ment with the results of matrix 2 oxidation [15]. The ⊥1L 020040060080010001200 tion of atmosphere He-O2(20%CO2(5%H-H0(2.3%) Table l Initial specific area(So) of uncoated samples and variations(AS)after treatment in a wet atmosphere for 20 h Temperature(°C (m2g-1) 0.025 0.01l 0.00 +0.270 Smear=0.017 Fig. Il. Optical micrograph (x90) of a sample oxidised at 800C without surface preparation)
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