C Kaya et al. Journal of the European Ceramic Society 22(2002)2333-2342 2339 counts/s 出A 15-0776 Mullite, syn I Al6si2013 2Theta ak U Fig. 5.(a)X-ray(CuKa)diffraction or the sintered matrix produced from hydrothermal sised mullite/s wt. conia powder showing also the stan ttern for JCPDS card no: 15-0776). The peaks labelled Z are for Fig. 4. SEM micrographs of mullite fibre-reinforced mullite matrix micrograph of the mullite i represent orthe ye presence of very fine composites with NdPOa interface fabricated by electrophoretic pores(as arrowed) in both inter-or trans-granular locations after drying and (b)sintered microstructure after pressureless sintering at 1200C for 3 h. The sintered composite contains approximately 35 Fig. 6a shows stress-deflection curves of the compo voL. fibre and has a relative density of 86. 4%TD sites at room temperature and at 1300C, as obtained in the four-point flexure strength test. The shape of the 3.4. Thermomechanical performance curves suggests that the composite exhibits"pseudo- plastic"deformation and damage tolerant behaviour The flexure strength values as a function of test tem- which should result from the presence of the optimised perature are given in Table 2. As can be seen from the interphase material(NdPO4) table, the maximum strength value (235 MPa) is The SEM micrographs of fracture surfaces tested at obtained at room temperature, but strength data are not room and elevated(1300C)temperatures are given in affected significantly by the test temperature up to Fig 6b and c, respectively, both indicating the presence 1300oC, which provides a strength value of 230 MPa of damage-tolerant behaviour with fibre pull-o lengths. Although mullite fibres of the type used he may suffer from significant strength loss at temperatures Table 2 higher than 1200C, the composites produced in this Four-point flexural strength of mullite fibre-reinforced mullite matrix work do not show dramatic strength decrease at composites with NdPOa interphase as a function of test temperature 1300C. This behaviour may result from two factors the nature of the interphase and the mullite matrix Test temperature Four-point fiexural microstructure. As shown in Figs. 2b and 3b and c, the strength(MPa) NdPO, interphase material is very dense but there is no 235±32 reaction with fibre or matrix. therefore there is no reac- tion product in the fibre/matrix zone. Secondly, as 230±41 shown in Fig. 5b, the mullite matrix has some residual porosity particularly at the inter-fibre tow regions, thus3.4. Thermomechanical performance The flexure strength values as a function of test tem￾perature are given in Table 2.As can be seen from the table, the maximum strength value (235 MPa) is obtained at room temperature, but strength data are not affected significantly by the test temperature up to 1300
C, which provides a strength value of 230 MPa. Fig.6a shows stress-deflection curves of the compo￾sites at room temperature and at 1300
C, as obtained in the four-point flexure strength test.The shape of the curves suggests that the composite exhibits ‘‘pseudo￾plastic’’ deformation and damage tolerant behaviour, which should result from the presence of the optimised interphase material (NdPO4). The SEM micrographs of fracture surfaces tested at room and elevated (1300
C) temperatures are given in Fig.6b and c, respectively, both indicating the presence of damage-tolerant behaviour with long fibre pull-out lengths.Although mullite fibres of the type used here may suffer from significant strength loss at temperatures higher than 1200
C,33 the composites produced in this work do not show dramatic strength decrease at 1300
C.This behaviour may result from two factors: the nature of the interphase and the mullite matrix microstructure.As shown in Figs.2b and 3b and c, the NdPO4 interphase material is very dense but there is no reaction with fibre or matrix, therefore there is no reac￾tion product in the fibre/matrix zone.Secondly, as shown in Fig.5b, the mullite matrix has some residual porosity particularly at the inter-fibre tow regions, thus Table 2 Four-point flexural strength of mullite fibre-reinforced mullite matrix composites with NdPO4 interphase as a function of test temperature Test temperature (
C) Four-point flexural strength (MPa) RT 23532 1000 23417 1200 23333 1300 23041 Fig.4.SEM micrographs of mullite fibre-reinforced mullite matrix composites with NdPO4 interface fabricated by electrophoretic deposition and pressure filtration indicating; (a) green microstructure after drying and (b) sintered microstructure after pressureless sintering at 1200
C for 3 h.The sintered composite contains approximately 35 vol.% fibre and has a relative density of 86.4%TD. Fig.5.(a) X-ray (CuKa) diffraction pattern for the sintered mullite matrix produced from hydrothermally synthesised mullite/5 wt.% zir￾conia powder showing also the standard pattern for the 3:2 mullite (JCPDS card no: 15–0776).The peaks labelled Z are for monocilinic zirconia and the other peaks represent orthorhombic mullite.(b) SEM micrograph of the mullite matrix showing the presence of very fine pores (as arrowed) in both inter- or trans-granular locations. C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342 2339