A.R. Boccaccini et al. Materials Characterization 54(2005)75-83 2.2. Thermal shock tests applied loads investigated were both below and above the fracture load, which was taken from results in the The thermal shock tests involved heating the literature on the same composite [12] amples in air in a muffle furnace at a pre-determined temperature(650C) for 10 min and then dropping 2.6. Microstructural characterisation them abruptly into an unstirred water bath maintained at room temperature(20C). The thermal gradient SEM of polished sections was conducted in an was chosen on the basis of previous studies [12, 18] in attempt to characterise the microstructural damage order to ensure that the microstructural damage induced in the specimens. Since the main energy induced in the samples could be attributed only to dissipation mechanism that occurs during fracture of the effect of thermal shock and not to other environ- these materials is the phenomenon of fibre pull-out mental influences such as, for example, fibre/matrix [10, 12], whose occurrence an nsion can be interface or SiC fibre oxidation. After each thermal appreciated visually, SEM observations of fracture shock test, the samples were dried in an oven at 100 surfaces were carried out C for I h. They were carefully inspected, visually, for the appearance of any macroscopic damage, such as 2.7. Impact test delamination, chipping or fibre protrusion. Samples in form of test bars were impacted with an 2.3. Thermal cycling energy of 4 J. This value of energy was adopted on the basis of impact test data obtained in previous For the thermal cycling tests, the samples were studies [19]. a pendulous apparatus with a mass and alternated quickly between high temperature(700C) arm especially designed to deliver the necessary and room temperature for different number of cycles energy to ensure the same operative conditions was (up to 1000 cycles). A computer program controlled used. A custom-made data acquisition system was the movement of the sample holder. Details of the adapted to the impact tester for observing if signifi equipment used for this test can be found elsewhere cant variations of energy consumed by the samples [10]. The time at high temperature was set at 15 min, occurred during impact. More detailed information of with 5 min being allowed for the sample to reach the these experiments is given elsewhere [19] target temperature of 700C, while the time at room temperature was fixed at 5 min. Thus, a complete 2.8. Measurement of the thermal expansion coefficient thermal cycle lasted 20 min. After a determined number of cycles, the samples were inspected care The thermal expansion coefficient (ax) of compo- fully for evidence of any macroscopic damage, such ite samples before and after thermomechanical as a change of the surface colour, delamination, loading was determined using a high temperature protrusion of fibres, spalling and/or chipping dilatometer (NETZSCH GmbH TMA 402). The samples were cut with a diamond saw to an 2. 4. Thermal aging approximate length of 23 mm. Before carrying out the experiments, the dimensions of the samples were The thermal aging experiments involved heating measured using a vernier with a precision of 0.05 samples at 600 and 700C in a fumace in an air mm. The measurements were carried out in normal atmosphere for long periods of time (up to 100 h). atmosphere between room temperature(20C)and After the thermal exposure, the samples were cooled 750C with a heating rate of 5 K min. At least two down slowly inside the fumace measurements were performed for each specimen and the results were averaged. The values were calculated 2.5. Mechanical pre-stressing for temperature intervals: 20-300, 20-400 and 20- 00C. The accuracy of the measured values was Selected samples were subjected to mechanical Ax=+lx10K. A high reproducibility of results loading in a three-point bending configuration. The was confirmed2.2. Thermal shock tests The thermal shock tests involved heating the samples in air in a muffle furnace at a pre-determined temperature (650 8C) for 10 min and then dropping them abruptly into an unstirred water bath maintained at room temperature (20 8C). The thermal gradient was chosen on the basis of previous studies [12,18] in order to ensure that the microstructural damage induced in the samples could be attributed only to the effect of thermal shock and not to other environ￾mental influences such as, for example, fibre/matrix interface or SiC fibre oxidation. After each thermal shock test, the samples were dried in an oven at 100 8C for 1 h. They were carefully inspected, visually, for the appearance of any macroscopic damage, such as delamination, chipping or fibre protrusion. 2.3. Thermal cycling For the thermal cycling tests, the samples were alternated quickly between high temperature (700 8C) and room temperature for different number of cycles (up to 1000 cycles). A computer program controlled the movement of the sample holder. Details of the equipment used for this test can be found elsewhere [10]. The time at high temperature was set at 15 min, with 5 min being allowed for the sample to reach the target temperature of 700 8C, while the time at room temperature was fixed at 5 min. Thus, a complete thermal cycle lasted 20 min. After a determined number of cycles, the samples were inspected care￾fully for evidence of any macroscopic damage, such as a change of the surface colour, delamination, protrusion of fibres, spalling and/or chipping. 2.4. Thermal aging The thermal aging experiments involved heating samples at 600 and 700 8C in a furnace in an air atmosphere for long periods of time (up to 100 h). After the thermal exposure, the samples were cooled down slowly inside the furnace. 2.5. Mechanical pre-stressing Selected samples were subjected to mechanical loading in a three-point bending configuration. The applied loads investigated were both below and above the fracture load, which was taken from results in the literature on the same composite [12]. 2.6. Microstructural characterisation SEM of polished sections was conducted in an attempt to characterise the microstructural damage induced in the specimens. Since the main energy￾dissipation mechanism that occurs during fracture of these materials is the phenomenon of fibre pull-out [10,12], whose occurrence and extension can be appreciated visually, SEM observations of fracture surfaces were carried out. 2.7. Impact test Samples in form of test bars were impacted with an energy of ~4 J. This value of energy was adopted on the basis of impact test data obtained in previous studies [19]. A pendulous apparatus with a mass and arm especially designed to deliver the necessary energy to ensure the same operative conditions was used. A custom-made data acquisition system was adapted to the impact tester for observing if signifi￾cant variations of energy consumed by the samples occurred during impact. More detailed information of these experiments is given elsewhere [19]. 2.8. Measurement of the thermal expansion coefficient The thermal expansion coefficient (a) of compo￾site samples before and after thermomechanical loading was determined using a high temperature dilatometer (NETZSCH GmbH TMA 402). The samples were cut with a diamond saw to an approximate length of 23 mm. Before carrying out the experiments, the dimensions of the samples were measured using a vernier with a precision of 0.05 mm. The measurements were carried out in normal atmosphere between room temperature (20 8C) and 750 8C with a heating rate of 5 K min1 . At least two measurements were performed for each specimen and the results were averaged. The values were calculated for temperature intervals: 20–300, 20–400 and 20– 500 8C. The accuracy of the measured values was Da=F1
107 K1 . A high reproducibility of results was confirmed. A.R. Boccaccini et al. / Materials Characterization 54 (2005) 75–83 77