Effect of sintering temperature on Ba0-Al2o Sio, glass ceramic coating for carbon fibre o reinforced silicon carbide matrix composites X. H Zheng , Y.G. Du, ). Y. Xiao, W.J. Zhang and C. Y Liang Bao-Al2O3-SiO2(BAS)glass ceramic coatings were prepared on carbon fibre reinforced silicon carbide matrix composites(C/SiC)by in situ sintering. The effect of sintering temperature on the final phase and microstructure of the coatings were studied. The results show that higher degree of crystallisation in the Bas glass is achieved at higher sintering temperatures and more crystalloids are melted by the residual glass with low viscosity at the same time. Considering these two factors, the crystallinity of Bas glass ceramic should have a peak value at a specific sintering temperature. The sample sintered at 1473 K has an obvious transition layer of 40 um in thickness between the coating and the composite, which enhances the adhesion strength between them. The oxidation resistance of C/SiC composites with BAS protective coatings was characterised using isothermal oxidation tests at 1773 K for 15 min. The weight loss of the composites with BAS coatings is decreased with the increase of the sintering temperature Keywords: Composite materials, Glass ceramics, Coatings, Sintering Introduction phases was studied. The phase composition and micro- structure of coatings sintered at different temperatures Carbon fibre reinforced silicon carbide(C/SiC) compo- were characterised and their oxidation resistance was ites have potential use as the key engine components in tested aeronautics and space applications. Precursor infiltra tion and pyrolysis(PIP)is an efficient and low cost method to prepare C/SiC composites, but defects in the Experimental procedure composites made by PIP are inevitably. In addition, the The C/SiC composites used in the present work were thermal mismatch between the carbon fibre and silicon made by the PIP method in the National University of arbide also causes defects in composites. These defects Defense Technology in China. The samples used in the ll cause the rapid oxidation of composites in oxygen present work were 10x 4 mm in size. After being rich atmospheres above 673 K. As a result, the applica- cleaned with water and ethanol, the samples were dried tions of C/Sic composites within oxidising atmospheres at 423K The BAs glass ceramic coatings were prepared by Applying resistant coatings to the surface of carbon situ reaction method with BAS glass as starting composites is an effective way to protect the composites materials on the surface of the C/SiC composites at from oxidation. Glass ceramic is composed of crystal designated temperatures. The starting composition of phase and glass phase. At high temperatures, the high the bAs glass was: 37BaO-29SiO3-24A12O3-6B2O3- melting point crystalline phase provides strength for the 2ZrO2-2Co203(wt-%). Reagent grade powders were coating and helps to retain its shape, while the glass will weighed, mixed together and then melted in a platinum melt and flow into cracks in the coating and composite. crucible at 1823 K for 2 h. The melt was quenched in Bao-AlO3-Sio2(BAS) glass ceramic has the highest distilled water to form glass frits, which were subse melting point of 2033 K among glass ceramic materials, quently milled in distilled water for 2 h to obtain glas which makes it an attractive candidate for protective powder. The mean particle size of the fine glass powder coating materials. The main objective of the present was 2-3 um. An organic solvent was designed: 10 wt-% research is the preparation and testing of bas glass tirbutyl citrate, 79. 2 wt-% butyl carbitol, 2.8 wt-% ethyl paper,the effect of sintering temperature on the final The bas glass powder(70- o- 1.4-butyrolactone was mixed into the organic solvent(30 wt-%) to create a suspending glass paste. The glass paste was applied to all sides of the C/ Materials Engineering, National University of SiC composites with brush and then the assembly was Defense Technology, Changsha 410073, China dried in a furnace at 423 K for 20 min. The resulting corrEspondingauthoremailzheng_nudt@163.com samples covered by green glass coatings were fired at a 2008 Institute of Materials, Minerals and Mining accepted 9 May 2008 Do10.1179/174328408X323122 Materials Science and Technology 2008 VOL 24 No 11 1399Effect of sintering temperature on BaO–Al2O3– SiO2 glass ceramic coating for carbon fibre reinforced silicon carbide matrix composites X. H. Zheng*, Y. G. Du, J. Y. Xiao, W. J. Zhang and C. Y. Liang BaO–Al2O3–SiO2 (BAS) glass ceramic coatings were prepared on carbon fibre reinforced silicon carbide matrix composites (C/SiC) by in situ sintering. The effect of sintering temperature on the final phase and microstructure of the coatings were studied. The results show that higher degree of crystallisation in the BAS glass is achieved at higher sintering temperatures and more crystalloids are melted by the residual glass with low viscosity at the same time. Considering these two factors, the crystallinity of BAS glass ceramic should have a peak value at a specific sintering temperature. The sample sintered at 1473 K has an obvious transition layer of 40 mm in thickness between the coating and the composite, which enhances the adhesion strength between them. The oxidation resistance of C/SiC composites with BAS protective coatings was characterised using isothermal oxidation tests at 1773 K for 15 min. The weight loss of the composites with BAS coatings is decreased with the increase of the sintering temperature. Keywords: Composite materials, Glass ceramics, Coatings, Sintering Introduction Carbon fibre reinforced silicon carbide (C/SiC) compo￾sites have potential use as the key engine components in aeronautics and space applications.1 Precursor infiltra￾tion and pyrolysis (PIP) is an efficient and low cost method to prepare C/SiC composites,2 but defects in the composites made by PIP are inevitably. In addition, the thermal mismatch between the carbon fibre and silicon carbide also causes defects in composites.3 These defects will cause the rapid oxidation of composites in oxygen rich atmospheres above 673 K. As a result, the applica￾tions of C/SiC composites within oxidising atmospheres are limited. Applying resistant coatings to the surface of carbon composites is an effective way to protect the composites from oxidation.4,5 Glass ceramic is composed of crystal phase and glass phase. At high temperatures, the high melting point crystalline phase provides strength for the coating and helps to retain its shape, while the glass will melt and flow into cracks in the coating and composite.6 BaO–Al2O3–SiO2 (BAS) glass ceramic has the highest melting point of 2033 K among glass ceramic materials,7 which makes it an attractive candidate for protective coating materials. The main objective of the present research is the preparation and testing of BAS glass ceramic coatings on C/SiC composites. In the present paper, the effect of sintering temperature on the final phases was studied. The phase composition and micro￾structure of coatings sintered at different temperatures were characterised and their oxidation resistance was tested. Experimental procedure The C/SiC composites used in the present work were made by the PIP method in the National University of Defense Technology in China.8 The samples used in the present work were 106564 mm in size. After being cleaned with water and ethanol, the samples were dried at 423 K. The BAS glass ceramic coatings were prepared by in situ reaction method with BAS glass as starting materials on the surface of the C/SiC composites at designated temperatures. The starting composition of the BAS glass was: 37BaO–29SiO2–24Al2O3–6B2O3– 2ZrO2–2Co2O3 (wt-%). Reagent grade powders were weighed, mixed together and then melted in a platinum crucible at 1823 K for 2 h. The melt was quenched in distilled water to form glass frits, which were subse￾quently milled in distilled water for 2 h to obtain glass powder. The mean particle size of the fine glass powder was 2–3 mm. An organic solvent was designed: 10 wt-% tirbutyl citrate, 79?2 wt-% butyl carbitol, 2?8 wt-% ethyl cellulose, 3 wt-% span-85, 5 wt-% 1,4-butyrolactone. The BAS glass powder (70 wt-%) was mixed into the organic solvent (30 wt-%) to create a suspending glass paste. The glass paste was applied to all sides of the C/ SiC composites with brush and then the assembly was dried in a furnace at 423 K for 20 min. The resulting samples covered by green glass coatings were fired at College of Aerospace & Materials Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author, email zheng_nudt@163.com
2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 26 February 2008; accepted 9 May 2008 DOI 10.1179/174328408X323122 Materials Science and Technology 2008 VOL 24 NO 11 1399