Jiang et al. Polymer derived nitride matrix composites 105 0.14 100 0.12 0.10 95 0.08 90 0.04 0.02 904006008001000 0.00 0.00.20.40.60.81.01.214 Temperature(℃) Displacement(mm) 1 Thermogravimetric analysis curve of cured PBSz in 2 Load-displacement curves 2. 5D SiO2/Si3N/-BN NH3 atmosphere pretreated to prevent the reaction between the fibre and of the residual solvent or low molecular weight silazane the matrix according to literature , Second, the fabric and borazine 2 compounds before 400C.The was infiltrated with hybrid precursor in vacuum. Third, gain from 400 to 600 C was due to the nitridation effect the fabric filled with precursor cured at 100-200'C for 3- by reactive ammonia atmosphere. However, in the 6 h in an inert atmosphere. Finally, the cured fabrics hybrid precursor, the Si-H bond decreased for the were pyrolysed at 800C in flowing anhydrous ammonia, reaction between PHPS and PBZ, so the nitridation which helped reducing the residual carbon in the final effect and weight gain are not obvious. The weight loss ceramic matrix, hence obtaining the 2: 5D SiO2Si3N after 800 C was small and the char yield was very high (~86wt-% were repeated for four times to densify the composites During the pretreatment before polymer infiltration, Properties of composites ome fibre fabrics were precoated and the corresponding The properties of the 2 5D Sio2/Si3N4-BN composites composites were denoted B, whereas the composites are shown in Table 1 and the deviations have been without fibre precoating were denoted A attached to the mean values of the density, longitude Composites characterisation flexural strength and modulus. The density of the specimen a was a little larger than that of the specimen The density of the composites after four infiltration- B after the same infiltration-cure-pyrolysis cycles cure-pyrolysis cycles was measured according to However, the sI len b had a higher longitude flexural Archimedes'principle. The pyrolysis of the matrix was strength and modulus. The load-displacement curves of studied by thermogravimetric analysis (TGA)under an the composites are illustrated in Fig. 2. The load of the perature to 1000 C. The longitude flexural strength was until a catastrophic failur r with the displacement a low energy measured using a three point bending test machine absorbing ability, whereas the (WDW-100) with specimen dimensions of 36x 4 brittle failure behaviour with hi acture load and 3 mm, a bending span of 30 mm and a crosshead speed displacement, showing an improved deformation resis- of 0.5 mm min. For mechanical properties tests, at tant abilit least three specimens were measured for each composite. A scanning electron microscope (JSM-5600LV) was Microstructure of composites used to observe the morphology of the fracture surfaces. Figure 3 illustrates the SEM images of the fracture surface of the 2. 5D SiO2Si3Na-BN. For specimen A Results the interface between the fibres and the matrix was Properties of precursor undistinguishable as a result of the strong adhesion and no fibre pull-out was observed on the fracture surface The as received precursor was a pale yellow liquid with whereas specimen B showed relatively weak interface osity of 10-35 mPa s and a density of bonding with distinct fibre pull-out and fracture, 1.07 g cm. After some heat treatment under nitro- exhibiting the effectiveness of fibre reinforcement gen atmosphere, the precursor could be changed into ighly cross-linked PBSZ. Figure I shows the TGA curve of PBSZ, which exhibits a weight loss in the Dis scussion beginning followed by a little weight gain and a weight Figure 3 revealed that the precursor efficiently infiltrated loss again. The weight loss was due to the volatilisation into the interyarn and intrayarn porosity of the silica Table 1 Properties of 2 5D SiO2wSi3Na-BN composites Specimen Pyrolysis cycle ensity, g cm Longitude flexural strength, MPa Elastic modulus, GPa A 1·70+002 463+20 173+10 1-68+002 1295+70 254+15 Materials Science and Technology 2007 VOL 23 No 7 881pretreated to prevent the reaction between the fibre and the matrix according to literature.6,7 Second, the fabric was infiltrated with hybrid precursor in vacuum. Third, the fabric filled with precursor cured at 100–200uC for 3– 6 h in an inert atmosphere. Finally, the cured fabrics were pyrolysed at 800uC in flowing anhydrous ammonia, which helped reducing the residual carbon in the final ceramic matrix, hence obtaining the 2?5D SiO2f/Si3N4– BN composites. The infiltration–cure–pyrolysis cycles were repeated for four times to densify the composites. During the pretreatment before polymer infiltration, some fibre fabrics were precoated and the corresponding composites were denoted B, whereas the composites without fibre precoating were denoted A. Composites characterisation The density of the composites after four infiltration– cure–pyrolysis cycles was measured according to Archimedes’ principle. The pyrolysis of the matrix was studied by thermogravimetric analysis (TGA) under an ammonia atmosphere (10uC min21 ) from room tem￾perature to 1000uC. The longitude flexural strength was measured using a three point bending test machine (WDW-100) with specimen dimensions of 36646 3 mm, a bending span of 30 mm and a crosshead speed of 0?5 mm min21 . For mechanical properties tests, at least three specimens were measured for each composite. A scanning electron microscope (JSM-5600LV) was used to observe the morphology of the fracture surfaces. Results Properties of precursor The as received precursor was a pale yellow liquid with a viscosity of 10–35 mPa s and a density of y1?07 g cm23 . After some heat treatment under nitro￾gen atmosphere, the precursor could be changed into highly cross-linked PBSZ. Figure 1 shows the TGA curve of PBSZ, which exhibits a weight loss in the beginning followed by a little weight gain and a weight loss again. The weight loss was due to the volatilisation of the residual solvent or low molecular weight silazane and borazine12 compounds before 400uC. The weight gain from 400 to 600uC was due to the nitridation effect by reactive ammonia atmosphere.11 However, in the hybrid precursor, the Si–H bond decreased for the reaction between PHPS and PBZ,10 so the nitridation effect and weight gain are not obvious. The weight loss after 800uC was small and the char yield was very high (y86 wt-%). Properties of composites The properties of the 2?5D SiO2f/Si3N4–BN composites are shown in Table 1 and the deviations have been attached to the mean values of the density, longitude flexural strength and modulus. The density of the specimen A was a little larger than that of the specimen B after the same infiltration–cure–pyrolysis cycles. However, the specimen B had a higher longitude flexural strength and modulus. The load–displacement curves of the composites are illustrated in Fig. 2. The load of the specimen A was almost linear with the displacement until a catastrophic failure, showing a low energy absorbing ability, whereas the composite B shows non￾brittle failure behaviour with higher fracture load and displacement, showing an improved deformation resis￾tant ability. Microstructure of composites Figure 3 illustrates the SEM images of the fracture surface of the 2?5D SiO2f/Si3N4–BN. For specimen A, the interface between the fibres and the matrix was undistinguishable as a result of the strong adhesion and no fibre pull-out was observed on the fracture surface whereas specimen B showed relatively weak interface bonding with distinct fibre pull-out and fracture, exhibiting the effectiveness of fibre reinforcement. Discussion Figure 3 revealed that the precursor efficiently infiltrated into the interyarn and intrayarn porosity of the silica 1 Thermogravimetric analysis curve of cured PBSZ in NH3 atmosphere Table 1 Properties of 2?5D SiO2f/Si3N4–BN composites Specimen Pyrolysis cycles Density, g cm23 Longitude flexural strength, MPa Elastic modulus, GPa A4 1.70¡0. 02 46. 3¡2.0 17.3¡1. 0 B4 1. 68¡0. 02 129. 5¡7. 0 25.4¡1. 5 2 Load–displacement curves of 2?5D SiO2f/Si3N4–BN composites A and B Jiang et al. Polymer derived nitride matrix composites Materials Science and Technology 2007 VOL 23 NO 7 881