M.L. Antti et al. Journal of the European Ceramic Society 24(2004)565-578 pecimens were heat-treated in a box furnace at tem- the matrix was investigated as a function of heat-treat peratures of 500, 1000 and 1100C for times between 20 ment time and temperature, using a digital microhard- and 3240 h. The holes were drilled before heat-treat- ness indenter(Matsuzawa MXT-a)at a load of 100 g ment but the dimensions used for the stress calculation and a loading time of 15 s were measured afterwards. Both as-processed and ther- Test specimens before and after thermal exposure mally exposed test specimens were tensile tested either at were investigated with X-ray diffraction (XRD). The X- room or elevated temperature using a servohydraulic ray spectra were obtained between 10 and 90 20 in step mechanical testing machine (MTS 810, Eden Prairie, intervals of 0.03 20 at a rate of 1.5 s/step. An automatic MN, USA)equipped with hydraulically actuated grips divergence slit was used with a beam area of 12x 16 mm and a compact furnace with Sic heating elements. The which thus covered both matrix and fibre regions of the tensile tests were performed at a constant cross-head composite. High-temperature XRd has also been per- displacement rate of 10 um/s while the deformation of formed on pure Nextel 720 fibres, in a powder X-ray the specimen was measured over a 25 mm gauge section diffractometer(Philips Pw 1710) with a step of 0.03 20 using a strain gauge extensometer at ambient tempera- at a rate of 8 S/step 4 After crushing the fibres to a tures, and a low-contact force capacitance extensometer powder heating cycles up to 1400C were performed with at elevated temperatures a heating rate of 5C per minute and holding times of 10 The tested specimens were embedded in epoxy, min every 150C to perform appropriate angular scans. polished and examined using optical and scanning elec- Selected samples were also examined using Raman tron microscopy (SEM), along axes both transverse and spectroscopy. A Dilor xY 800 triple stage Raman parallel (LT and TW surfaces) to the loading direction microprobe (Y, Inc, Edison, NJ)and an Innova 308C (see the schematic diagram, Fig. 2). The fracture sur- Argon ion laser(Coherent, Inc, Santa Clara, CA, USA) faces of both fibre orientations were studied in the operating at 514.5 nm with a 300 mW output power SEM, both directly and embedded in epoxy and were used to record Raman spectra from the fibres and polished on the LW surface Density and porosity mea the matrix separately. The laser was focuse d onto areas surements were performed using the Archimedes of interest with an optical objective providing a spatial method as well as image analysis. The microhardness of resolution of 2 um Table I Properties of 0/90 fibre orientation Full section strength [MPa stiffness [GPa As-received A (RT) 0.34 0.26 As-received B (RT) 0.30 0.28 0.29 200 h at C(B) 0.29 100 h at C(B) 0.28 0.29 0.19 3240hatl000°C(B) 0.15 0.1 0.11 0 h at 1100°C(A) 0.11 0.17 0.25 0.15 0.39 l00 h at 1100°C(A)s Asrec test at 1000C(B) As-rec test at 1100C(A)Specimens were heat-treated in a box furnace at tem￾peratures of 500, 1000 and 1100
C for times between 20 and 3240 h. The holes were drilled before heat-treat￾ment but the dimensions used for the stress calculation were measured afterwards. Both as-processed and ther￾mally exposed test specimens were tensile tested either at room or elevated temperature using a servohydraulic mechanical testing machine (MTS 810, Eden Prairie, MN, USA) equipped with hydraulically actuated grips and a compact furnace with SiC heating elements. The tensile tests were performed at a constant cross-head displacement rate of 10 mm/s while the deformation of the specimen was measured over a 25 mm gauge section using a strain gauge extensometer at ambient tempera￾tures, and a low-contact force capacitance extensometer at elevated temperatures. The tested specimens were embedded in epoxy, polished and examined using optical and scanning elec￾tron microscopy (SEM), along axes both transverse and parallel (LT and TW surfaces) to the loading direction (see the schematic diagram, Fig. 2). The fracture sur￾faces of both fibre orientations were studied in the SEM, both directly and embedded in epoxy and polished on the LW surface. Density and porosity mea￾surements were performed using the Archimedes method as well as image analysis. The microhardness of the matrix was investigated as a function of heat-treat￾ment time and temperature, using a digital microhard￾ness indenter (Matsuzawa MXT-a) at a load of 100 g and a loading time of 15 s. Test specimens before and after thermal exposure were investigated with X-ray diffraction (XRD). The X￾ray spectra were obtained between 10 and 90
2
in step intervals of 0.03 2
at a rate of 1.5 s/step. An automatic divergence slit was used with a beam area of 1216 mm which thus covered both matrix and fibre regions of the composite. High-temperature XRD has also been per￾formed on pure Nextel 720 fibres, in a powder X-ray diffractometer (Philips PW 1710) with a step of 0.03 2
at a rate of 8 s/step.24 After crushing the fibres to a powder heating cycles up to 1400
C were performed with a heating rate of 5
C per minute and holding times of 10 min every 150
C to perform appropriate angular scans. Selected samples were also examined using Raman spectroscopy. A Dilor XY 800 triple stage Raman microprobe (JY, Inc, Edison, NJ) and an Innova 308C Argon ion laser (Coherent, Inc., Santa Clara, CA, USA) operating at 514.5 nm with a 300 mW output power were used to record Raman spectra from the fibres and the matrix separately. The laser was focused onto areas of interest with an optical objective providing a spatial resolution of 2 mm. Table 1 Properties of 0/90 fibre orientation Sample a/W Net-section strength [MPa] Full section stiffness [GPa] Hardness HV Strain to failure [%] As-received A (RT) 0.10 201 72 – 0.34 0.26 203 70 0.28 0.39 197 62 0.26 As-received B (RT) 0.10 204 68 204 0.34 0.30 179 62.5 0.28 0.42 191 54 0.29 200 h at 500
C (B) 0.10 203a 74 – 0.32 0.29 191 62 0.29 100 h at 1000
C (B) 0.10 180 72 213 0.28 0.29 172 66 0.24 0.42 149 57 0.19 3240 h at 1000
C (B) 0.10 114 80 323 0.15 0.29 107 74 0.12 0.42 102 66 0.11 20 h at 1100
C (A) 0.11 125 81 334 0.17 0.25 113 73 0.15 0.39 114 69 0.12 100 h at 1100
C (A)s 0.10 54.5 90 457 0.06 0.24 37 83 0.04 0.38 39 74 0.04 As-rec test at 1000
C (B) 0.09 210 68 – 0.35 0.29 171 61 0.24 0.40 186 56 0.24 As-rec test at 1100
C (A) 0.09 189 57 – 0.35 0.24 164 63 0.24 0.38 154 53 0.18 a Failed at grips. 568 M.-L. Antti et al. / Journal of the European Ceramic Society 24 (2004) 565–578