外文翻譯--涂有si-mosi2+氧化保護(hù)涂層的碳sic復(fù)合材料的氧化性和力學(xué)性能

1、<p><b>　　中文4040字</b></p><p><b>　　附錄 外語(yǔ)文獻(xiàn)原文</b></p><p><b>　　英文原文</b></p><p>　　Oxidation behavior and mechanical properties of C/SiC composite

2、s with Si-MoSi2 oxidation protection coating</p><p>　　YONGDONG XU, LAIFEI CHENG, LITONG ZHANG, HONGFENG YING,</p><p>　　WANCHENG ZHOU</p><p>　　State Key Laboratory of Solidi?cation P

3、rocessing, Northwestern Polytechnical University,Xi’an, Shaanxi 710072, People’s Republic of China</p><p>　　A new kind of oxidation protection coating of Si-MoSi2 was developed for three dimensional carbon ?

4、ber reinforced silicon carbide composites which could be serviced upto 1550 ℃. The overall oxidation behavior could be divided into three stages: (i)500 ℃<T <800 ℃, the oxidation mechanism was considered to be cont

5、rolled by the chemical reaction between carbon and oxygen; (ii) 800 ℃<T <1100 ℃, the oxidation of the composite was controlled by the diffusion of oxygen through the micro-cracks, and; (i</p><p>　　1. I

6、ntroduction</p><p>　　Carbon ?ber reinforced silicon carbide composite(C/SiC) is a kind of promising thermal structure composites for use in applications requiring high strength,low density, and high fracture

7、 toughness at elevated temperatures in aero-engines and aerospace[1]. However, a severe problem is that the carbon ?ber and in-terfacial layer of the composite are easy to be attacked by oxygen in air at the temperatures

8、 as low as 370℃[2, 3]. The main reason is that there are many microcracks on the silicon carbi</p><p>　　There are many issues which must be considered in the development of a successful oxidation protection

9、system for carbon-based composite materials [2, 3].The critical factors for oxidation protection system of carbon-based composite materials are: (i) oxygen diffusion through the coating micro-cracks; (ii) volatility of c

10、oating materials at high temperature; (iii) oxidation of coating by diffusion of oxygen through the coating layer, and; (iv) the chemical and mechanical compatibility between co</p><p>　　2. Experimental proc

11、edure</p><p>　　2.1. Preparation of three dimensional carbon/silicon carbide composites</p><p>　　PAN-based carbon ?ber was employed and each yarn contained 3000 ?laments. The fabric preform was p

12、repared by three dimensional braided method, and supplied by Nanjing Institute of Glass Fiber (People Republic of China).The ?ber volume fraction was 40%.In the present experiment, isothermal/isobaric chemical vapor in?l

13、tration (ICVI) was employed to prepare C/SiC composite which has been described elsewhere in details [13-15]. In ICVI process, a thin layer of pyrolytic carbon was pre-deposited on t</p><p>　　2.2. Oxidation

14、tests</p><p>　　The oxidation tests were conducted at 400~1500℃in air. The thermal shock experiments were conducted as follows: the samples were placed into the furnace at 1550℃ for 15 min, then drawn out fro

15、m the furnace placed into the boiling water of 100℃ immediately.The oxidation weight change of the samples were measured with an analytical balance (sensitivity: 0.1 mg).</p><p>　　2.3. Mechanical properties

16、measurement </p><p>　　Flexural strength was measured with three-point-bending method with samples 3×4×40 mm in size and the curves of loading stress vs. displacement were recorded. The loading rate

17、 for ?exural strength was 0.5 mm·min-1. The mechanical properties was conducted on an Instron-1195 machine at room temperature.</p><p>　　2.4. Microstructure observation and surface analysis</p>&

18、lt;p>　　The morphology of the specimens was characterized by a scanning electron microscope (SEM, JEOL 840).The microstructure of the texture was studied with a transmission electron microscope (TEM, Philips EM400).<

19、;/p><p>　　3. Results and Discussion</p><p>　　3.1. Oxidation behavior of C/SiC composite</p><p>　　The carbon ?ber has a substantially lower thermal expansion coef?cient than any of cera

20、mic materials. The occurrence of residual mechanical stress generated by thermal expansion mis-match and the lack of plasticity of SiCmatrix at the medium temperature (~900℃) are such that the matrix micro-cracks can not

21、 be avoided.The cracking temperature (Tc) is de?ned as the formation temperature of micro-cracks in SiC matrix, and is lower than the deposition temperature (Td) of SiC matrix. The oxidation th</p><p>　　Figu

22、re 1 Oxidation curve of the coated C/SiC composite (annealing time 1 h).</p><p>　　Figure 2 Micro-cracks in SiC substrate Figure 3 Oxidation of CVI SiC matrix.</p><p>　　of C/SiC composite

23、.</p><p>　　the micro-cracks can be mechanically closed and sealed by oxidation products. The range between oxidation threshold and cracking temperature is very dangerous and sealant must be employed. Fig. 1

24、shows the oxidation behavior of the coated C/SiC composite. The oxidation pro?le could be classi?ed into three parts. The ?rst part is located at the temperature range from 500 to 800℃. The oxidation weight loss was incr

25、eased with the increase of temperature. For the coated C/SiC composite, the oxidatio</p><p>　　The oxidation behaviors of 3D C/SiC composites were similar to those of 2D C/SiC composites and could be illustra

26、ted by the model proposed by Lamouroux[4]. The carbon ?ber and CVI SiC matrix are known to exhibit coef?cients of thermal expansion that are signi?cantly different. The T300 carbon ?ber is an anisotropic material and cha

27、racterized by two CTEs: a radial CTE (-0.1-1.1×10-6 ℃-1) and a longitu-dinal CTE (7.0×10-6℃-1). The CTE of CVI SiC matrix is 4.8×10-6℃-1[17]. Hence, there are many mic</p><p><b>　　(1)<

28、;/b></p><p>　　Where e0 is the width of micro-crack at room temperature, T is the test temperature, and T0 is the temperature at which the micro-crack width is zero. The decrease of width of micro-cracks r

29、esulted in the varying of oxidation mechanism under a Fick and a Knudsen regime [18, 19]. Therefore, the oxidation mechanism of the composite was controlled by the diffusion of oxygen in the micro-cracks. The oxygendiffu

30、sion control implies a crack geometry effect especially regarding the width and depth of t</p><p>　　3.2. Structure characterization of oxidationprotection coating of C/SiC composite</p><p>　　An

31、important issue concerns the selection of the primary oxygen barriers. It is usually emphasized that the adherent insitu oxide could be formed in order to getter the oxygen at the free surface of the composites.The silic

32、on-based ceramics have the best thermal expansion comparability and exhibit the lowest oxidation rates. Therefore, they are often selected for protection of carbon-based composites. It is reported that the thin amorphous

33、 SiO2 scale has low oxygen diffusion coef?cient and can b</p><p><b>　　(2)</b></p><p><b>　　(3)</b></p><p>　　MoO3 has relatively low melting point (800 ℃) and

34、good ?uidity, as a result, it could seal the micro-cracks of oxidation coating and enhance the oxidation resisance of the composite. Usually,Mo is a kind of adjusting element of network in the glass, and has no ability t

35、o form glass. But at the condition of presence of SiO2,Mo and SiO2 could form a kind of glass with high resisance of both crystallization and oxygen diffusion. In the glass system,Mo exists in the form of [MoO3] [22].<

36、;/p><p>　　3.3. Effect of oxidation on the mechanical properties of coated composite </p><p>　　It was observed that the coated C/SiC composites exhibited oxidation weight gain during the whole oxida

37、tion.At the initial stage of oxidation, the lineal relation- ship is observed between oxidation weight gain and temperature. The increase rate was decreased when the oxidation time was more than 20 h. The weight gain rea

38、ched the Maximum value when the oxidation time was equal to 50 h, and then decreased with the increase of oxidation time. Correspondingly, the ?exural strength was slightly decr</p><p>　　SEM microstructure&l

39、t;/p><p>　　Figure 6 Relationship between oxidation and Figure 7 Hole on the oxidation protection flexural strength of coated C/SiC composite</p><p>　　point of mechanical properties degradatio

40、n for coated C/SiC composites.The oxidati- on weight gain at the initial stage is attributed to the oxidation reactions of silicon and MoSi2 presented by chemical reaction 2 and 3.However, the product of MoO3 is easily t

41、o escape from the oxidation coating because of its high vapor pressure at high temp- eratures. Accordingly, the oxidation weight gain rate is decreased with the tempera- tures.As the oxidation of silicon processes, the c

42、ontents of both Si </p><p><b>　　(4)</b></p><p>　　Figure 8 Relationship between thermal Figure 9 Failure behavior of coated C/SiC</p><p>　　shock and flexural stren

43、gth</p><p>　　If the partial pressure reached the critical value, the gaseous product will escape out and damage the glass layer of SiO2-MoO3. A hole was formed in the oxidation coat- ing and became the trans

44、port tunnel for oxygen(Fig. 7). Hence, the oxidation process was enhanced and resulted in the oxidation of C interfacial layer andcarbon ?ber. Above all, it could be considered that the time of hole formation is the fail

45、ure point of oxidation coating.The effect of thermal shock (100 ℃~1550 ℃) on the ?exur</p><p>　　4. Conclusions</p><p>　　1. A new kind of oxidation protection coating of Si-MoSi2 was developed fo

46、r three dimensional carbon ?ber reinforced silicon carbide composites which could be serviced upto 1550 ℃. The oxidation protection coating had a three-layer structure: the out layer is oxidation layer of silica glass, t

47、he media layer is Si＋MoSi2 layer, and the inside layer is SiC layer.</p><p>　　2. The oxidation behaviors were investigated for the coated C/SiC composite at the temperature ranged from 500~1400 ℃. The overal

48、l oxidation behavior could be divided into three stages: (i) 500 ℃<T <800℃, the oxidation mechanism was considered to be controlled by the chemical reaction between carbon and oxygen; (ii) 800℃< T <1100 ℃, th

49、e oxidation of the composite was controlled by the diffusion of oxygen through the micro-cracks, and; (iii) T >1100 ℃, the oxidation of SiC became signi?cant and w</p><p>　　3. The coated C/SiC composites

50、 exhibited excellent oxidation resistance and thermal shock resistance.After the composites annealed at 1550℃for 50 h in air and 1550℃~100 ℃ thermal shock for 50 times, the ?exural strength was maintained by 85% and 80%

51、respectively.</p><p>　　4. The relationship between oxidation weight change and ?exural strength revealed the criteria for protection coating was that the maximum point of oxidation weight gain was the failur

52、e starting point for oxidation protection coating.</p><p>　　Acknowledgements </p><p>　　This research work has been supported by National Natural Scienti?c Foundation (NNSF), Nation Aviation Scie

53、nti?c Foundation (NASF) and Nation DefenseScienti?c Foundation (NDSF).</p><p><b>　　譯文</b></p><p>　　涂有Si-MoSi2 氧化保護(hù)涂層的碳/SiC復(fù)合材料的氧化性和力學(xué)性能</p><p>　　凝固處理國(guó)家重點(diǎn)實(shí)驗(yàn)室，西北工業(yè)大學(xué)，西安，陜西

54、710072,中華人民共和國(guó)</p><p>　　研制了一種新型的可用于三維碳纖維增強(qiáng)SiC復(fù)合材料（C/SiC）的抗氧化保護(hù)涂層，它的工作溫度可高達(dá)1550℃。整個(gè)氧化過(guò)程可分為三個(gè)階段:(1)500℃<T<800℃，氧化機(jī)理被認(rèn)為是由碳與氧之間的化學(xué)反應(yīng)控制；(2)800℃<T< 1100℃，復(fù)合材料的氧化由氧通過(guò)微裂紋的擴(kuò)散控制；(3)T>1100℃，SiC的氧化變得明顯，受氧

55、氣通過(guò)SiC層的擴(kuò)散控制。微觀結(jié)構(gòu)分析顯示,氧化保護(hù)涂層有三層結(jié)構(gòu):最外面一層是玻璃態(tài)SiO2、中間層是SiCMoSi2層,內(nèi)層是SiC層。被包覆的C/SiC復(fù)合材料表現(xiàn)出了優(yōu)越的抗氧化性和抗熱震性。復(fù)合材料分別于空氣中1550℃退火50 h和1550℃～100℃間熱沖擊50次后,抗彎強(qiáng)度分別保持了85%和80%。氧化重量變化和抗彎強(qiáng)度之間的關(guān)系,表明了氧化保護(hù)涂料的關(guān)鍵是氧化涂層增重的最大值點(diǎn)就是失效的開始點(diǎn)。</p>

56、<p><b>　　1.引言</b></p><p>　　碳纖維增強(qiáng)SiC復(fù)合材料(C/SiC)是一種用于應(yīng)用在航空發(fā)動(dòng)機(jī)和宇宙航行等高溫環(huán)境中使用的需要高強(qiáng)度、低密度、高斷裂韌性的有前途的熱結(jié)構(gòu)復(fù)合材料[1]。然而，一個(gè)嚴(yán)重的問題是,碳纖維和復(fù)合材料界面層在溫度低至370℃很容易受到空氣中氧氣的影響[2,3]。最主要的原因是在SiC基體上有許多由于SiC基體和碳纖維間熱膨脹系數(shù)的

57、不協(xié)調(diào)導(dǎo)致的微裂紋。氧化對(duì)材料弱化和力學(xué)性能有重要影響。據(jù)報(bào)道,在溫度為900℃的介質(zhì)中氧化處理，當(dāng)氧化失重2%時(shí)，拉伸強(qiáng)度下降了近40%[4]。到目前為止,已經(jīng)發(fā)展了兩類保護(hù)碳纖維和界面層方法。第一種方法是界面層的氧化保護(hù),例如分層合成C(B)界面和BN內(nèi)界面層[5]。顯然,這種方法的效果是有限的,因?yàn)榻缑鎸油ǔＪ欠浅１?約102nm)。第二種方法是用氧化保護(hù)涂層包覆復(fù)合復(fù)合材料,這被認(rèn)為是一個(gè)很有前途的方法。</p>

58、<p>　　在為碳基復(fù)合材料開發(fā)一種成功的抗氧化保護(hù)系統(tǒng)的過(guò)程中，必須考慮許多問題[2,3]。碳基復(fù)合材料氧化保護(hù)系統(tǒng)關(guān)鍵因素是；(1)氧氣擴(kuò)散通過(guò)涂層微裂紋擴(kuò)展；(2)涂料在高溫環(huán)境下的揮發(fā)性；(3)氧氣通過(guò)鍍層擴(kuò)散氧化涂層；(4)涂層和基體間的化學(xué)和機(jī)械相容性。復(fù)合材料氧化保護(hù)涂層通常通過(guò)化學(xué)氣相沉積(CVD)制備，并已見報(bào)道[11]。涂層是一個(gè)三明治結(jié)構(gòu)，可以分為三個(gè)層次。內(nèi)層是CVDSiC層，中間層是含硼(或B4C B

59、N)層，外層是CVD SiC層。在高溫下、含硼層和SiC氧化形成玻璃態(tài)的B2O3-SiO2能阻礙復(fù)合材料微裂紋的擴(kuò)展[2,12]。由于玻璃態(tài)的硼在高溫時(shí)擁有高的蒸汽壓，因此這種材料的使用溫度比1200℃更低。為了提高氧化保護(hù)涂料的工作溫度，開發(fā)新型涂料體系是十分必要的。本文是前期發(fā)表文章的延續(xù)；目的是：（1）開發(fā)一種新型的可在1200℃以上使用的C /SiC復(fù)合材料的抗氧化保護(hù)涂料；（2）研究C / SiC復(fù)合材料涂層氧化行為；(3)探

60、討氧化對(duì)復(fù)合材料的力學(xué)性能和失效的影響。</p><p><b>　　2.實(shí)驗(yàn)過(guò)程</b></p><p>　　2.1 制備的三維碳/SiC復(fù)合材料</p><p>　　聚丙烯腈基炭纖維,每束采用3000根細(xì)絲纏繞。預(yù)制塊由三維編織的方法制備,由南京玻璃纖維(中華人民共和國(guó))提供。纖維的體積分?jǐn)?shù)是40%。在當(dāng)前的研究中,等溫化學(xué)氣滲透(ICVI

61、)用來(lái)制備C/SiC復(fù)合材料已在其它文獻(xiàn)報(bào)道[13]。在ICVI工藝中，一層薄的熱解碳和丁烷在850℃前預(yù)先沉積在碳纖維表面上作為界面層使復(fù)合材料致密化。MTS（CH3SiCl3）通過(guò)氫氣泡帶入反應(yīng)室用于沉積SiC。典型的情況是被用于1100℃,含氫10%，壓力為幾個(gè)KPa的條件下沉積。氬作為保護(hù)氣體用于減緩沉積化學(xué)反應(yīng)速率。復(fù)合材料的致密完成后，將C /SiC復(fù)合材料于1500 ~ 1600℃的真空環(huán)境中，在硅鉬合金(Si-Mo)熔體

62、中浸泡0.5 h。</p><p><b>　　2.2 氧化實(shí)驗(yàn)</b></p><p>　　氧化實(shí)驗(yàn)是在400 ~ 1500℃條件下于空氣中進(jìn)行的。熱沖擊實(shí)驗(yàn)過(guò)程如下:樣品放置在燒結(jié)爐中于1550℃下保溫15min，然后從爐抽取出立即放入100℃沸水中煮。用分析測(cè)定樣品氧化重量的變化(精度：0.1mg)。</p><p><b>　

63、　2.3力學(xué)性能測(cè)量</b></p><p>　　用三點(diǎn)抗彎法測(cè)量3×4×40 mm樣品的抗彎強(qiáng)度并得出應(yīng)力應(yīng)變曲線，都予以記錄?？箯潖?qiáng)度加載速率為0.5mm?min-1。力學(xué)性能用Instron-1195機(jī)器于室溫下測(cè)定。</p><p>　　2.4微觀結(jié)構(gòu)觀察和斷口分析</p><p>　　用掃描電子顯微鏡(SEM，JEOL840

64、)觀察試樣形貌。用透射電子顯微鏡(TEM、飛利浦EM400）觀察斷口的微觀結(jié)構(gòu)。</p><p><b>　　3.結(jié)果和討論</b></p><p>　　3.1 C/SiC復(fù)合材料的氧化機(jī)理</p><p>　　碳纖維具有比任何的陶瓷類材料都要低的熱膨脹系數(shù)。在900℃介質(zhì)中，由于熱膨脹不協(xié)調(diào)和SiC基體缺乏塑性變形產(chǎn)生的殘余機(jī)械應(yīng)力導(dǎo)致的微裂

65、紋是不可避免的。裂解溫度(Tc)的定義是SiC基體中微裂紋形成的溫度,低于SiC基體的沉積溫度(Td)。碳-碳復(fù)合材料的氧化臨界溫度(Tt)是370℃[2、3、16]。這個(gè)溫度可通過(guò)加入難溶顆粒作為抑制劑得到改善。涂層本身的保護(hù)范圍內(nèi)由涂層裂解溫度和涂層使用限制溫度決定。在這個(gè)溫度范圍內(nèi),微裂紋可以通過(guò)氧化涂料機(jī)械閉合和密封。閾值之間范圍內(nèi)氧化溫度和開裂是非常危險(xiǎn)的，必須采用密封膠。在氧化臨界溫度和裂解溫度范圍內(nèi)是非常危險(xiǎn)的，因此這時(shí)候

66、有必要使用密封劑。</p><p>　　圖1 包覆的C/SiC復(fù)合材料的氧化曲線</p><p>　　圖1為C /SiC復(fù)合材料涂層氧化過(guò)程。氧化過(guò)程可以分為三個(gè)部分。第一部分是位于500~800℃溫度范圍。隨著溫度的升高，氧化失重逐漸增加。對(duì)于包覆的C /SiC復(fù)合材料，由為SiC基體中加入了復(fù)合材料作為抑制劑，氧化的臨界溫度提高到了500℃。第二部分位為800 ~ 1100℃的溫度范圍

67、。隨著溫度的升高，氧化失重逐漸減少。第三部分是溫度高于1100℃的部分。圖像表明氧化重量增加而不是氧化重量減少,隨著氧化溫度的升高，氧化重量增加逐漸增加。</p><p>　　氧化行為的三維C /SiC復(fù)合材料和二維C /SiC復(fù)合材料氧化過(guò)程相似可以 </p><p>　　圖2 C/SiC復(fù)合材料中SiC基體的微裂紋SEM 圖3：CVI制備SiC基體氧化曲線</p&g

68、t;<p>　　由Lamouroux[4]提出的模型得到證明。眾所周知，碳纖維、和CVI制備的SiC基體的熱膨脹系數(shù)有明顯的差異。T300碳纖維是一種各向異性材料，通過(guò)兩個(gè)CTE表征：一個(gè)徑向CTE（-0.1-1.1×10-6 ℃-1）和一個(gè)縱向CTE(7.0×10-6℃-1).CVI制備SiC基體的CTE是4.8×10-6℃-1[17]。因此,在SiC基體上有許多由于熱應(yīng)力產(chǎn)生的垂直于纖維軸

69、的微裂紋(Fig. 2)。在溫度為500 ~800℃氧化過(guò)程,這些微裂紋扮演了一個(gè)供氧內(nèi)擴(kuò)散的通道。在這一階段,氧化的機(jī)理被認(rèn)為是由碳與氧之間的化學(xué)反應(yīng)控制,氧化率應(yīng)遵循阿倫尼烏斯方程。很明顯,溫度的增加導(dǎo)致氧化速率加快。隨著溫度增加到800 ~ 1100℃,裂紋的寬度(e(T)）變窄，這個(gè)溫度稱為功能溫度。</p><p><b>　　(1)</b></p><p>

70、;　　e0是在室溫下裂紋的寬度,T是測(cè)試溫度,T0是裂紋寬度為零時(shí)的溫度。菲克魯生定律認(rèn)為裂紋寬度的減少會(huì)引起氧化機(jī)理的變化[18、19]。因此,復(fù)合材料的氧化機(jī)理受氧氣在微裂紋中的擴(kuò)散控制。氧擴(kuò)散受微裂紋幾何形狀的影響，特別是有關(guān)微裂紋的寬度和深度。在溫度高于1100℃時(shí),微裂紋的寬度由于SiC基體熱膨脹變得非常狹窄，SiC氧化變得明顯，并受氧通過(guò)SiC層擴(kuò)散的控制。此外，大量氧化的SiC最終會(huì)覆蓋微裂紋。因此，氧化的機(jī)理受氧在SiC

71、層的擴(kuò)散控制。在圖3中，CVI制備的SiC基體的氧化過(guò)程,驗(yàn)證了復(fù)合材料氧化模型。在800℃以下，SiC基體的氧化重量基本沒有變化。在800 ~ 1400℃的溫度范圍內(nèi)、氧化速率隨著溫度的升高而迅速提高。</p><p>　　3.2 C/SiC復(fù)合材料氧化保護(hù)涂層的結(jié)構(gòu)表征</p><p>　　一個(gè)重要問題是有關(guān)氧氣主要障礙的選擇。它通常是強(qiáng)調(diào)形成的原位吸附氧可吸附在復(fù)合材料自由表面上。硅

72、基陶瓷擁有最好的熱膨脹相容性且具有最低氧</p><p>　　圖6 涂層C /SiC復(fù)合材料氧化 圖7 氧化保護(hù)層孔洞</p><p>　　和抗彎強(qiáng)度關(guān)系曲線 </p><p>　　化速率。因此，它們經(jīng)常被用于碳基復(fù)合材料的保護(hù)。據(jù)報(bào)道,一定量薄的無(wú)定形二氧化硅有低的氧擴(kuò)散系數(shù)

73、，可通過(guò)加入其他氧化物控制玻璃的粘度來(lái)改善。如圖4所示,掃描電鏡觀察結(jié)果表明，氧化保護(hù)涂層包括三層：(1)最外層是玻璃態(tài)二氧化硅氧化層；(2)中間層是Si＋MoSi2層；(3)內(nèi)層是SiC層。由于Si＋MoSi2層的氧化，玻璃態(tài)二氧化硅層氧化層非常光滑。在Si＋MoSi2層，MoSi2顆粒分散在硅基體中。如圖5 b、c、d所示為中間層的TEM形貌和衍射圖形。進(jìn)一步觀察用TEM觀察玻璃態(tài)二氧化硅顆粒顯示該層主要由二氧化硅和少量的鉬元素組成

74、(圖5)。硅、二硅化鉬的氧化反應(yīng)反應(yīng)式如下[20，21]：</p><p><b>　　(2)</b></p><p><b>　　(3)</b></p><p>　　MoO3 有相對(duì)低的熔點(diǎn)(800℃)和優(yōu)越的流動(dòng)性,以致它能密封的氧化涂層的裂紋，提高復(fù)合材料的抗氧化性。通常，Mo是一種玻璃網(wǎng)絡(luò)中的調(diào)節(jié)元素，并不能單獨(dú)形

75、成玻璃。但在SiO2存在的條件下，Mo和SiO2 能形式一種具有結(jié)晶和氧擴(kuò)散能力的高阻抗玻璃。在玻璃系統(tǒng)中，鉬以MoO3的形式存在[22]。</p><p>　　3.3氧化作用對(duì)含涂層復(fù)合材料力學(xué)性能的影響</p><p>　　研究發(fā)現(xiàn)，含涂層的C /SiC復(fù)合材料在整個(gè)氧化過(guò)程中氧化增重。在最初的氧化階段、氧化增重和溫度之間呈現(xiàn)線性關(guān)系。當(dāng)氧化時(shí)間超過(guò)20小時(shí)時(shí)增長(zhǎng)速率減少。</p

76、><p>　　氧化時(shí)間達(dá)到50小時(shí)時(shí)，氧化重量達(dá)到最大值,然后隨著氧化時(shí)間的增加而降低。與之相對(duì)應(yīng)的，在50小時(shí)的氧化時(shí)間內(nèi)，抗彎強(qiáng)度稍有降低。然后，在50小時(shí)后，抗彎強(qiáng)度急劇降低,當(dāng)氧化時(shí)間達(dá)到70小時(shí)時(shí)，抗彎曲度為零(圖6)。顯然，氧化增重曲線最大值點(diǎn)是涂層C/SiC復(fù)合材料力學(xué)性能減弱起點(diǎn)。氧化增重在初始階段歸因于Si和MoSi2的氧化反應(yīng)，通過(guò)2和3兩個(gè)化學(xué)反應(yīng)完成。然而，產(chǎn)物MoO3由于高溫時(shí)擁有高蒸汽壓而

77、很容易逃離氧化涂層。因此，氧化增重率隨溫度而減小。隨著硅氧化的進(jìn)行，Si和MoSi2的含量減少，如果Si和MoSi2在小范圍內(nèi)存在，氧化將生成SiC。由于局部氧分壓低，SiC的氧化活性將改變[23]。</p><p>　　如果局部壓力達(dá)到臨界值，氣態(tài)產(chǎn)物將會(huì)逃溢對(duì)SiO2-MoO3玻璃層造成的傷害。氧化涂層中形成的孔洞變成了氧傳輸?shù)耐ǖ?圖7)。進(jìn)而,加速氧化過(guò)程,導(dǎo)致C界面層和碳纖維氧化。最重要的是孔洞的形成部

78、位是氧化涂層的失效點(diǎn)。圖8所示為熱沖擊（100 ℃～1550 ℃）對(duì)抗彎強(qiáng)度的影響。隨著熱沖擊次數(shù)的增加,抗彎強(qiáng)度稍有下降。在50次熱沖擊后，抗彎強(qiáng)度保持在420MPa，但涂層C/SiC復(fù)合材料的失效行為與復(fù)合材料相似(圖9)。研究結(jié)果表明:涂層C/SiC復(fù)合材料具有優(yōu)越的抗熱震性。</p><p><b>　　4 結(jié)論</b></p><p>　　1、研發(fā)了一種可用

79、于三維碳纖維增強(qiáng)SiC復(fù)合材料的新型Si-MoSi2 氧化保護(hù)涂層。該氧化保護(hù)涂層有三層結(jié)構(gòu)：外層是氧化硅玻璃層，中間層是Si-MoSi2層，內(nèi)層是SiC層。</p><p>　　2、研究了涂層C/SiC復(fù)合材料在500 ~ 1400℃溫度范圍內(nèi)的氧化性。整個(gè)氧化過(guò)程可分為三個(gè)階段：(1)500℃< T < 800℃，氧化機(jī)理被認(rèn)為是由碳與氧之間的化學(xué)反應(yīng)控制；(2)800℃< T < 1

80、100℃，復(fù)合材料的氧化由氧通過(guò)微裂紋的擴(kuò)散控制；(3)T > 1100℃，SiC的氧化變得明顯，受O2通過(guò)SiC層的擴(kuò)散控制。</p><p>　　3、被包覆的C /SiC復(fù)合材料表現(xiàn)出了優(yōu)越的抗氧化性和抗熱震性。復(fù)合材料分別于空氣中1550℃退火50 h和1550℃~ 100℃間熱沖擊50次后，抗彎強(qiáng)度分別保持了85%和80%。</p><p>　　4、氧化重量變化和抗彎強(qiáng)度之間