C Kaya et al. Journal of the European Ceramic Society 22(2002)2333-2342 2335 VErsical KA21, pH: 9, Allied Colloids, UK) in order to were bevelled using 6-um diamond paste on a soft cloth create a strong negative surface charge on the fibre sur- in order to minimise possible flaw sources. The tensile face and to improve the wetting behaviour of the fibres. surfaces were then polished to a 0. 25 um finish using This process maximises the mutual electrostatic attrac- diamond paste. Room and high-temperature four-point tion between the fibre surface and the positively charged flexure strength tests were performed on an Instron NdPO4 particles(as determined by electrophoretic mobi- Testing machine fitted with a furnace which has tung- lity measurements)in the coating suspension. During the sten mesh elements enabling tests to be carried out at coating process under vacuum, the fibre mats were temperatures up to 1500C. The pushrods and fixtures immersed in the NdPO4 suspension under sonication are all made from a tungsten-zirconium-moly bdenum causing the positively charged NdPO4 particles to adhere alloy Specimens to be tested at elevated temperatures to the negatively charged fibre surface. The immersed were held at the test temperature for at least an hour fibre mats were removed from the sol and dried in air for prior to testing to allow the system to equilibrate and to one day, and then finally fired at 600C for 0.5 h to ensure that the specimen was at this temperature. Flex- strengthen the coating layer on the fibre surface. ural tests were carried out at a constant displa rate of 0.5 mm/min using a test fixture having outer and 2.3. Composite processing inner spans of 40 and 20 mm, respectively. The span(S) to specimen thickness(h) ratio was chosen to be sufi An in-situ electrophoretic deposition(EPD) cell was cient large(S/h>10) to avoid large shear delamination used to infiltrate the NdPO4-coated mullite fibre mats forces with matrix material. Detailed information about the development and application of this technique can be 2.5. Thermal cycling tests found elsewhere. 7. 29,30 Coated fibre mats were attached to a stainless steel plate acting as deposition electrode, Thermal cycling tests on samples of prismatic shape, which was connected to the negative pole of a power suitable for flexural strength tests, were conducted using supply. Another stainless steel plate served as the counter a computer-controlled rig. A cycling program was electrode. After the fibre preform was placed in the sol, applied to induce heating and cooling between 25 and e system was vacuum degassed to remove any entrap 150oC and an isothermal hold time of 15 min. These ed air. The cell electrodes were connected to a0-60 v tests carried out in laboratory air. The applied d. c power supply. A stable mullite aqueous suspension cycles resulted in a heating/cooling rate of about 900C (pH=3), as described in Section 2.1, was used. EPD was per min at the initial stages, which gradually reduced to performed under constant voltage conditions (10 Vd.c) approximately 500C/min as the temperature approa- using a deposition time of l min An electrode separation ched the limits of the cycle. Temperature was measured distance of 15 mm was used in all experiments and eight by two different thermocouples attached to the samples coated fibre mats were individually deposited. Under After thermal cycling the samples were visually the applied electric field, the very fine mullite particles inspected for the presence of macroscopic surface possessing a net positive surface charge, as determined damage, such as delamination, chipping or fibre protru rom the electrophoretic mobility data, migrated sion. Selected thermal cycled specimens were tested at towards the negative electrode. The particles infiltrated room temperature in flexure strength configuration, as the fibre tows and deposited until a sufficient matrix described above thickness, which enveloped the fibre tows, was achieved Eight electrophoretically deposited fibre mats were then 2.6. Microstructural characterisation placed in a high-load pressure filtration(PF)assembly to form the green body (in the form of flat cylinders of 70 Mullite powder and sintered samples were analysed mm diameter). The PF procedure has been described in using X-ray diffraction(CuK, radiation and nickel filter detail elsewhere. The EPD-infiltrated and pressure fil- to remove the Cu kp peak, Philips X'Pert, Germany) trated green body specimens were subsequently dried operated at 40 keV and 30 mA. The diffractometer under humidity-controlled atmosphere for I day and left scanned from 5 to 80 with a scan step of 0.020 20 and a in normal air for another day before being pressureless count time of 2 s per step. Microstructural examinations sintered at 1200oC for 3 h in air on sintered monolithic mullite and fibre-reinforced composites were also carried out using a high-resolution 2. 4. Flexural tests at room and elevated temperatures Field Emission Gun SEM(FEG SEM FX-4000, Jeol Ltd Japan). The interfacial mi Flexural test bars were cut from the sintered discs NdPO4 interphase and the mullite matrix and between using an Accutom 5 high-speed, precision diamond saw, the NdPO4 interphase and the mullite fibres was exam- and then both surfaces were ground to be parallel using ined using a Jeol 4000 FX TEM operating at 400 keV, a 40-um diamond, resin bonded disc. All sharp edges and equipped with an energy dispersive X-Ray analysis(Versical KA21, pH: 9, Allied Colloids, UK) in order to create a strong negative surface charge on the fibre sur￾face and to improve the wetting behaviour of the fibres. This process maximises the mutual electrostatic attrac￾tion between the fibre surface and the positively charged NdPO4 particles (as determined by electrophoretic mobi￾lity measurements) in the coating suspension.During the coating process under vacuum, the fibre mats were immersed in the NdPO4 suspension under sonication, causing the positively charged NdPO4 particles to adhere to the negatively charged fibre surface.The immersed fibre mats were removed from the sol and dried in air for one day, and then finally fired at 600
C for 0.5 h to strengthen the coating layer on the fibre surface. 2.3. Composite processing An in-situ electrophoretic deposition (EPD) cell was used to infiltrate the NdPO4-coated mullite fibre mats with matrix material.Detailed information about the development and application of this technique can be found elsewhere.7,29,30 Coated fibre mats were attached to a stainless steel plate acting as deposition electrode, which was connected to the negative pole of a power supply.Another stainless steel plate served as the counter electrode.After the fibre preform was placed in the sol, the system was vacuum degassed to remove any entrap￾ped air.The cell electrodes were connected to a 0–60 V d.c. power supply. A stable mullite aqueous suspension (pH=3), as described in Section 2.1, was used. EPD was performed under constant voltage conditions (10 V d.c.) using a deposition time of 1 min.An electrode separation distance of 15 mm was used in all experiments and eight coated fibre mats were individually deposited.Under the applied electric field, the very fine mullite particles possessing a net positive surface charge, as determined from the electrophoretic mobility data, migrated towards the negative electrode.The particles infiltrated the fibre tows and deposited until a sufficient matrix thickness, which enveloped the fibre tows, was achieved. Eight electrophoretically deposited fibre mats were then placed in a high-load pressure filtration (PF) assembly to form the green body (in the form of flat cylinders of 70 mm diameter).The PF procedure has been described in detail elsewhere.29 The EPD-infiltrated and pressure fil￾trated green body specimens were subsequently dried under humidity-controlled atmosphere for 1 day and left in normal air for another day before being pressureless sintered at 1200
C for 3 h in air. 2.4. Flexural tests at room and elevated temperatures Flexural test bars were cut from the sintered discs using an Accutom 5 high-speed, precision diamond saw, and then both surfaces were ground to be parallel using a 40-mm diamond, resin bonded disc.All sharp edges were bevelled using 6-mm diamond paste on a soft cloth in order to minimise possible flaw sources.The tensile surfaces were then polished to a 0.25 mm finish using diamond paste.Room and high-temperature four-point flexure strength tests were performed on an Instron Testing machine fitted with a furnace which has tung￾sten mesh elements enabling tests to be carried out at temperatures up to 1500
C.The pushrods and fixtures are all made from a tungsten-zirconium-molybdenum alloy.Specimens to be tested at elevated temperatures were held at the test temperature for at least an hour prior to testing to allow the system to equilibrate and to ensure that the specimen was at this temperature.Flex￾ural tests were carried out at a constant displacement rate of 0.5 mm/min using a test fixture having outer and inner spans of 40 and 20 mm, respectively.The span (S) to specimen thickness (h) ratio was chosen to be suffi- cient large (S/h>10) to avoid large shear delamination forces. 2.5. Thermal cycling tests Thermal cycling tests on samples of prismatic shape, suitable for flexural strength tests, were conducted using a computer-controlled rig.A cycling program was applied to induce heating and cooling between 25 and 1150
C and an isothermal hold time of 15 min.These tests were carried out in laboratory air.The applied cycles resulted in a heating/cooling rate of about 900
C per min at the initial stages, which gradually reduced to approximately 500
C/min as the temperature approa￾ched the limits of the cycle.Temperature was measured by two different thermocouples attached to the samples. After thermal cycling the samples were visually inspected for the presence of macroscopic surface damage, such as delamination, chipping or fibre protru￾sion.Selected thermal cycled specimens were tested at room temperature in flexure strength configuration, as described above. 2.6. Microstructural characterisation Mullite powder and sintered samples were analysed using X-ray diffraction (CuKa radiation and nickel filter to remove the Cu Kb peak, Philips X’Pert, Germany), operated at 40 keV and 30 mA.The diffractometer scanned from 5 to 80
with a scan step of 0.02
2y and a count time of 2 s per step.Microstructural examinations on sintered monolithic mullite and fibre-reinforced composites were also carried out using a high-resolution Field Emission Gun SEM (FEG SEM FX-4000, Jeol Ltd.Japan).The interfacial microstructure between the NdPO4 interphase and the mullite matrix and between the NdPO4 interphase and the mullite fibres was exam￾ined using a Jeol 4000 FX TEM operating at 400 keV, and equipped with an energy dispersive X-Ray analysis C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342 2335