A.R. Boccaccini et al. Materials Characterization 54(2005)75-83 3. Results and discussion Negligible differences exist between the values measured for the different samples, except for the 3.I. Microstructure of the investigated material specimen subjected 20 times to thermal shock (AT=630C), indicating that this sample may have A SEM micrograph showing the microstructure of suffered a higher degree of microstructural damage a polished section of an as received sample is shown than the rest. The experimental values of the thermal in Fig. 1. It can be seen that the matrix material is expansion coefficient obtained for the temperature dense, without residual porosity, and the composite ranges 20-400 and 20-500C were very similar to exhibits a fairly regular fibre distribution wn In Fig Fig 3 shows the thermal expansion curve obtained 3.2. Evaluation of the thermal expansion behaviour expansion that characterises the glass transition Considering the values of thermal expansion temperature (Ty) of the glass matrix (525C for efficient pertaining to the duran)glass matrix dURAN glass)[17] is not apparent due to the and to the silicon carbide fibres(am and dominant influence of the fibres respectively ), and taking into account that the fibre ig. 4 shows the thermal expansion curves of both volume fraction is -0.4, the theoretical value of the the first and second measurements performed on the thermal expansion coefficient of the composite can be same sample which had been subjected to thermal calculated by a simple rule of mixtures: sho oc= aflf + amM measurement a“wave" between300and400°Ccan be observed. this is characteristic of materials that where Pr and Vm are the fibre and matrix volume possess internal stresses. These stresses are generated fractions, respectively. With the values of ar and om as result of the progressive number of thermal shocks given in Table 1, the thermal expansion coefficient [ 18]. Concerning the second measurement on the obtained from Eq. (1)is: ac-3. 19x10-6K- same sample, a different value of thermal expansion Fig. 2 shows the experimental values of the coefficient was obtained, which is very close to that thermal expansion coefficient measured in the temper- calculated by Eq. (1). Moreover, no"wave"is ature range of 20-300C for all samples investigated. observed in the curve, as shown in Fig. 4. This As can be this graph, and taking into suggests that during heating the sample up to 750C, account the average error of the measurements during the first measurement of the thermal expansion (+lx10K), the measured values are very close coefficient, an internal relaxation of stresses was to that calculated by the rule of mixtures(Eq(1). induced. Previous studies on the same material [12 have shown that thermal shocks of At=570C do not cause microstructural damage in this type of compo site, at least up to 21 cycles. However, thermal shocks with△T≥690° C produce serious microscopic degra- dation of the material in the form of microcracks delamination and oxidation of the carbon-rich inter face and of Nicalon SiC fibres. In the same previous study [12], microstructural damage was confirmed on samples subjected to thermal shock for 20 cycles with a thermal gradient equal to that utilised in this work (AT=630C). In that study, a forced-vibration technique and flexural strength tests were used to 50m detect microstructural damage. The damage detected was in the form of small microcracks In the Fig. 1 SEM micrograph of the polished section of the composit glass ial investigated. A regular distribution of Sic fibres in the matrix, without major delamination or mass changes matrix is observed and absence of porosit samples. This precludes the3. Results and discussion 3.1. Microstructure of the investigated material A SEM micrograph showing the microstructure of a polished section of an as received sample is shown in Fig. 1. It can be seen that the matrix material is dense, without residual porosity, and the composite exhibits a fairly regular fibre distribution. 3.2. Evaluation of the thermal expansion behaviour Considering the values of thermal expansion coefficient pertaining to the (DURANR) glass matrix and to the silicon carbide fibres (am and af, respectively), and taking into account that the fibre volume fraction is ~0.4, the theoretical value of the thermal expansion coefficient of the composite can be calculated by a simple rule of mixtures: aC ¼ afVf þ amVm ð1Þ where Vf and Vm are the fibre and matrix volume fractions, respectively. With the values of af and am given in Table 1, the thermal expansion coefficient obtained from Eq. (1) is: aC=3.19
106 K1 . Fig. 2 shows the experimental values of the thermal expansion coefficient measured in the temper￾ature range of 20–300 8C for all samples investigated. As can be seen in this graph, and taking into account the average error of the measurements (F1
107 K1 ), the measured values are very close to that calculated by the rule of mixtures (Eq. (1)). Negligible differences exist between the values measured for the different samples, except for the specimen subjected 20 times to thermal shock (DT=630 8C), indicating that this sample may have suffered a higher degree of microstructural damage than the rest. The experimental values of the thermal expansion coefficient obtained for the temperature ranges 20–400 and 20–500 8C were very similar to those shown in Fig. 2. Fig. 3 shows the thermal expansion curve obtained for the as-received sample. The quick increase of expansion that characterises the glass transition temperature (Tg) of the glass matrix (525 8C for DURANR glass) [17] is not apparent due to the dominant influence of the fibres. Fig. 4 shows the thermal expansion curves of both the first and second measurements performed on the same sample which had been subjected to thermal shock for 20 cycles (DT=630 8C). In the first measurement a bwaveQ between 300 and 400 8C can be observed. This is characteristic of materials that possess internal stresses. These stresses are generated as result of the progressive number of thermal shocks [18]. Concerning the second measurement on the same sample, a different value of thermal expansion coefficient was obtained, which is very close to that calculated by Eq. (1). Moreover, no bwaveQ is observed in the curve, as shown in Fig. 4. This suggests that during heating the sample up to 750 8C, during the first measurement of the thermal expansion coefficient, an internal relaxation of stresses was induced. Previous studies on the same material [12] have shown that thermal shocks of DT=570 8C do not cause microstructural damage in this type of compo￾site, at least up to 21 cycles. However, thermal shocks with DTz690 8C produce serious microscopic degra￾dation of the material in the form of microcracks, delamination and oxidation of the carbon-rich inter￾face and of NicalonR SiC fibres. In the same previous study [12], microstructural damage was confirmed on samples subjected to thermal shock for 20 cycles with a thermal gradient equal to that utilised in this work (DT=630 8C). In that study, a forced-vibration technique and flexural strength tests were used to detect microstructural damage. The damage detected was in the form of small microcracks in the glass matrix, without major delamination or mass changes of the samples. This precludes the possibility of a Fig. 1. SEM micrograph of the polished section of the composite material investigated. A regular distribution of SiC fibres in the glass matrix is observed and absence of porosity. 78 A.R. Boccaccini et al. / Materials Characterization 54 (2005) 75–83