Zheng et aL Effect of sintering temperature on Ba0-AL03-SiO glass ceramic coating a Hexagonal-BaAl, Si, O 77-0189 the glass ceramic coating sintered at 1473 K. As the sintering temperature increases, the crystals in the Monoclinic-BaAl i10 38-1450 ceramic phase of the coating grow more rapidly, which increases the percent of crystalloids in the coating. At the same time, the viscosity of the glass phase decreases and more crystalloids dissolve into the melt, which reduces the overall quantity of crystalloids in the coating. These two factors compete during sintering to determine the final crystallinity of glass ceramic coat- 1323K ngs. The optimum conditions are achieved at 1323K as evidenced the high degree of the crystallinity of the ngs fired at that temperature the sintering temperature, all the 1473K coatings contain two crystalline phases: hexacelsian and monoclinic celsian. During the heat treatment, the first phase to crystallise from bas glass is the 60 metastable, high temperature hexacelsian phase, rather than the stable, low temperature monoclinic celsian x-ray diffraction patterns of Bao-A120-Sio 2 glass phase. The rate of hexacelsian to monoclinic celsian eramic coatings fired at different temperatures transformation in BAs glass ceramic is very slug The transformation can be accelerated by increasing the sintering temperature, prolonging the sintering time, temperatures of 1123, 1323 and 1473 K in an argon usIng isostatic pressing" or adding proper nucleation rotective atmosphere to obtain the bas glass ceramic agents. In the present study, more monoclinic celsian phase is detected at higher temperatures The phase composition of the fired coatings was Morphology of glass ceramic coatings analysed by X-ray diffraction (XRD). The morphology After the sintering process, the coatings on the and microstructure of the glass ceramic coatings was composite samples were dense and crack free. In the characterised by scanning electron microscopy (SEM) sintering, BAs glass powders are softened at 1053 K, and the element distribution of the interface between the and densification are achieved. Then at temperatures coatings and composites was studied using SEM and above 1093 K, crystallisation occurs in the dense glass energy dispersive spectroscopy (EDS). The oxidation coating.Because the onset of cry stallisation occurs resistance of the coatings was tested using isothermal after the glass has fully densified, BAS glass ceramic oxidation tests at 1773 K for 30 min. The samples were coatings are dense by sintering weighed before and after the oxidation on an electronic balance with a sensitivity of +0-1 mg The cross-sections of the samples with glass ceram coatings are shown in Fig. 2. No obvious defects are detected in the interfaces between the coatings and Results and discussion composites. The sample fired at 1123K has a very clear Crystallographic component of coatings interface, while the sample fired at 1473K has a transition layer between the coating and the composite. During the sintering process, the glass in the protective The layer is about 40 um in thickness. Based on the coatings crystallises to form glass ceramic, and a dense, location of carbon fibres in the transition layer, a crack free coating is fabricated. Figure I shows the conclusion can be drawn that the transition layer is XRD patterns of the glass ceramic coatings prepared at formed by the diffusion of the surface phase into the different temperatures. The coatings prepared at differ- composites. To test this theory, a line profile for the ent temperatures show different degrees of crystallinity. coating was taken on. The position of the line profile is The surface coating of composites fired at 1323 K shows marked in Fig. 2, and the results of the test are shown in the highest degree of crystallinity, which falls sharply in Fig 3. The content of Ba increases gradually in the transition lay 50um 50 2 Morphologies of cross-section of celsian glass ceramic coatings fired at different temperatures 1400 Materials Science and Technol 2008vou24No11temperatures of 1123, 1323 and 1473 K in an argon protective atmosphere to obtain the BAS glass ceramic coatings. The phase composition of the fired coatings was analysed by X-ray diffraction (XRD). The morphology and microstructure of the glass ceramic coatings was characterised by scanning electron microscopy (SEM) and the element distribution of the interface between the coatings and composites was studied using SEM and energy dispersive spectroscopy (EDS). The oxidation resistance of the coatings was tested using isothermal oxidation tests at 1773 K for 30 min. The samples were weighed before and after the oxidation on an electronic balance with a sensitivity of ¡0?1 mg. Results and discussion Crystallographic component of coatings During the sintering process, the glass in the protective coatings crystallises to form glass ceramic, and a dense, crack free coating is fabricated. Figure 1 shows the XRD patterns of the glass ceramic coatings prepared at different temperatures. The coatings prepared at differ￾ent temperatures show different degrees of crystallinity. The surface coating of composites fired at 1323 K shows the highest degree of crystallinity, which falls sharply in the glass ceramic coating sintered at 1473 K. As the sintering temperature increases, the crystals in the ceramic phase of the coating grow more rapidly, which increases the percent of crystalloids in the coating. At the same time, the viscosity of the glass phase decreases and more crystalloids dissolve into the melt, which reduces the overall quantity of crystalloids in the coating. These two factors compete during sintering to determine the final crystallinity of glass ceramic coat￾ings. The optimum conditions are achieved at 1323 K as evidenced the high degree of the crystallinity of the coatings fired at that temperature. Regardless of the sintering temperature, all the coatings contain two crystalline phases: hexacelsian and monoclinic celsian. During the heat treatment, the first phase to crystallise from BAS glass is the metastable, high temperature hexacelsian phase, rather than the stable, low temperature monoclinic celsian phase.9 The rate of hexacelsian to monoclinic celsian transformation in BAS glass ceramic is very sluggish. The transformation can be accelerated by increasing the sintering temperature, prolonging the sintering time,10 using isostatic pressing11 or adding proper nucleation agents.12 In the present study, more monoclinic celsian phase is detected at higher temperatures. Morphology of glass ceramic coatings After the sintering process, the coatings on the composite samples were dense and crack free. In the sintering, BAS glass powders are softened at 1053 K, and densification are achieved. Then at temperatures above 1093 K, crystallisation occurs in the dense glass coating.13 Because the onset of crystallisation occurs after the glass has fully densified, BAS glass ceramic coatings are dense by sintering.14 The cross-sections of the samples with glass ceramic coatings are shown in Fig. 2. No obvious defects are detected in the interfaces between the coatings and composites. The sample fired at 1123 K has a very clear interface, while the sample fired at 1473 K has a transition layer between the coating and the composite. The layer is about 40 mm in thickness. Based on the location of carbon fibres in the transition layer, a conclusion can be drawn that the transition layer is formed by the diffusion of the surface phase into the composites. To test this theory, a line profile for the coating was taken on. The position of the line profile is marked in Fig. 2, and the results of the test are shown in Fig. 3. The content of Ba increases gradually in the 1 X-ray diffraction patterns of BaO–Al2O3–SiO2 glass ceramic coatings fired at different temperatures 2 Morphologies of cross-section of celsian glass ceramic coatings fired at different temperatures Zheng et al. Effect of sintering temperature on BaO–Al2O3–SiO2 glass ceramic coating 1400 Materials Science and Technology 2008 VOL 24 NO 11