Zheng et aL. Effect of sintering temperature on Ba0-AL203-SiOa glass ceramic coating transition layer 20 8留 10 1001150120 50 Distance /um 4 Thermal oxidation curves (1773 k of coated samples fired at different temperatures 3 Energy dispersive spectroscopy line profiles of Ba, AI Si,O and C from composite to coating fired at 1473 K materials were absent from the samples. However, the average weight loss of these samples is only 7-62% transition layer from the composite to the coating The transition layer is important for the protective However, other elements in the coating such as Al and Si efficiency of BAS coatings. On the one hand, it has a change minimally. The areas around the carbon fibres graded composition which reduced the thermal mis- are as bright as the coating and EDS patterns show that match between the coating and composite Lowering the this area has a high Ba content. The diffusion of Ba mismatch can result in the reduction of the susceptibility makes the coating bind tightly with the composite. The of glass ceramic coatings to cracks On the other hand, samples with coatings sintered at different temperatures the transition layer has good adhesion strength to both were hoisted and fell from 2 m high. This test was the coating and composite because it is formed by the repeated for 8 times. After that, the coating sintered at diffusion of coating materials. Both of the effects do 1323 K was broken and fell off the surface of comp improve the protective capabilit meanwhile, the coatings sintered at 1323 and 1473 K Glass with low viscosity seals the defects in the surface adhesion strength of the interface between the composite composite. However, glass with too low viscosity may and the coating is excellent after processing at higher flow easily down from the surface of the composite. If temperatures above 1323 K. oxidised for longer periods, the e surrace coating will Oxidation tests protective ability may be lost entirely. The coatings Isothermal oxidation tests were carried out at 1773 K sintered at 1473 K, which have more glass phase, are for 30 min. After oxidation, the surfaces of the samples easier to flow off the samples at high temperatures, such prepared at 1123 K were cracked, and some coatings as 1773 K. Consequently, the higher sintering tempera flaked off. Whereas, the coatings prepared at 1323 and tures at 1473 K are impeded for the stability of glass 1473 K were glassy and fat after testing. As Fig. 1 ceramic coatings at high temperatures. Considering the indicates, the coating prepared at 1123 K has more protective capability and the stability at high tempera hexacelsian phase. The hexagonal celsian phase has a tures, the optimal temperature for heat treatment is high coefficient of thermal expansion (CTE) 1323 K in the present work To improve the oxidation (8 10K ). Consequently, the CTE of this coating resistance of the glass ceramic coating, the temperature is much higher than that of the composite (2- dependant viscosity of the coating should be adjusted to 3 K). During the cooling process, thermal an optimal value. stresses cause the formation of cracks in the outer the cracks and oxidises the composite. The cte of conclusions monoclinic celsian is 3 x10-6K, which is close to that bAS glass ceramic is an appropriate candidate material of composite. As a result, samples fired at 1323 and to protect C/SiC composites from oxidation at high 1473 K can retain tightly on the C/Sic composite after temperatures. With the increase of sintering tempera- oxidation tests. Figure 4 shows the weight loss of the ture the crystallisation of bas glass ceramic has a peak samples fired at different temperatures. The weight loss value. Meanwhile, the diffusion of the elements in BAS of the samples decreases with the increase of processing glass ceramic is favourable and a transition layer is temperature. The weight loss of the samples prepared at formed between the coating and the composite. This 1123 K is the highest, 21-88%. The coating fired at transition layer improves the adhesion strength of the 1323 K retained its surface morphology after oxidation coating to the composite and reduces the thermal stress testing, with only minor microcracks and a weight loss in the coating. The oxidation resistance of bas glass of 15.25%. During the course of oxidation, the coatings ceramic coated C/SiC composites is improved by the prepared at 1473 K flowed badly and some coating increase in the sintering temperature. However, the Materials Science and Technology 2008 VOL 24 No 11 1401transition layer from the composite to the coating. However, other elements in the coating such as Al and Si change minimally. The areas around the carbon fibres are as bright as the coating and EDS patterns show that this area has a high Ba content. The diffusion of Ba makes the coating bind tightly with the composite. The samples with coatings sintered at different temperatures were hoisted and fell from 2 m high. This test was repeated for 8 times. After that, the coating sintered at 1323 K was broken and fell off the surface of composite; meanwhile, the coatings sintered at 1323 and 1473 K kept dense and integrated. The result shows that the adhesion strength of the interface between the composite and the coating is excellent after processing at higher temperatures above 1323 K. Oxidation tests Isothermal oxidation tests were carried out at 1773 K for 30 min. After oxidation, the surfaces of the samples prepared at 1123 K were cracked, and some coatings flaked off. Whereas, the coatings prepared at 1323 and 1473 K were glassy and flat after testing. As Fig. 1 indicates, the coating prepared at 1123 K has more hexacelsian phase. The hexagonal celsian phase has a high coefficient of thermal expansion (CTE) (861026 K21 ).15 Consequently, the CTE of this coating is much higher than that of the composite (2– 361026 K21 ). During the cooling process, thermal stresses cause the formation of cracks in the outer coating and oxygen diffuses into the composite through the cracks and oxidises the composite. The CTE of monoclinic celsian is 361026 K21 , which is close to that of composite. As a result, samples fired at 1323 and 1473 K can retain tightly on the C/SiC composite after oxidation tests. Figure 4 shows the weight loss of the samples fired at different temperatures. The weight loss of the samples decreases with the increase of processing temperature. The weight loss of the samples prepared at 1123 K is the highest, 21?88%. The coating fired at 1323 K retained its surface morphology after oxidation testing, with only minor microcracks and a weight loss of 15?25%. During the course of oxidation, the coatings prepared at 1473 K flowed badly and some coating materials were absent from the samples. However, the average weight loss of these samples is only 7?62%. The transition layer is important for the protective efficiency of BAS coatings. On the one hand, it has a graded composition which reduced the thermal mis￾match between the coating and composite. Lowering the mismatch can result in the reduction of the susceptibility of glass ceramic coatings to cracks. On the other hand, the transition layer has good adhesion strength to both the coating and composite, because it is formed by the diffusion of coating materials. Both of the effects do improve the protective capability. Glass with low viscosity seals the defects in the surface effectively and prevents the diffusion of oxygen into the composite. However, glass with too low viscosity may flow easily down from the surface of the composite. If oxidised for longer periods, the surface coating will become too thin to protect the composite and its protective ability may be lost entirely. The coatings sintered at 1473 K, which have more glass phase, are easier to flow off the samples at high temperatures, such as 1773 K. Consequently, the higher sintering tempera￾tures at 1473 K are impeded for the stability of glass ceramic coatings at high temperatures. Considering the protective capability and the stability at high tempera￾tures, the optimal temperature for heat treatment is 1323 K in the present work. To improve the oxidation resistance of the glass ceramic coating, the temperature dependant viscosity of the coating should be adjusted to an optimal value. Conclusions BAS glass ceramic is an appropriate candidate material to protect C/SiC composites from oxidation at high temperatures. With the increase of sintering tempera￾ture, the crystallisation of BAS glass ceramic has a peak value. Meanwhile, the diffusion of the elements in BAS glass ceramic is favourable and a transition layer is formed between the coating and the composite. This transition layer improves the adhesion strength of the coating to the composite and reduces the thermal stress in the coating. The oxidation resistance of BAS glass ceramic coated C/SiC composites is improved by the increase in the sintering temperature. However, the 3 Energy dispersive spectroscopy line profiles of Ba, Al, Si, O and C from composite to coating fired at 1473 K 4 Thermal oxidation curves (1773 K) of coated samples fired at different temperatures Zheng et al. Effect of sintering temperature on BaO–Al2O3–SiO2 glass ceramic coating Materials Science and Technology 2008 VOL 24 NO 11 1401