S.M. Dong et al. Ceramics International 28(2002)899-905 fabricated by polymer impregnation/microwave pyr- all the samples. Fracture surface of each sample after olysis(PIMP) process using 2 D woven Tyranno sa bending test was also observed by sem to determine fiber preforms as the reinforcement. The effects of fiber the fracture behavior. Microstructural evolution of the coating and particulate loading during first cycle AHPCS derived matrix after PIMP process was char impregnation on fiber/matrix interaction and mechan- acterized by transmission electron microscopy (TEM) ical behaviors are also evaluated The thin film was prepared by the focused ion beam (FIB)method. 2. Experimental proced 2.3. Physical and mechanical measurements 2.. Materials All PIMP samples were cut and ground into nearly 4x220 mm rectangular bars to perform three-point Polymer precursor used for matrix formation was bending test at room temperature with an Instron 5581 allylperhydropolycarbosilane(AHPCS), which is regar- test machine. The cross-head speed was 0.5 mm/min ded as a polymer precursor of high ceramic yield [10]. and the span was 18 mm. During bending test, the Since this polymer precursor is very sensitive to the fracture behavior of the bars was in situ recorded by anhydrous and anaerobic environment to minimize the composites was measured by Archimedes's method the ambient atmosphere, it was handled under strictly optical microscope camera. The bulk density of potential for oxygen contamination. The maximum Push-out and push-back tests were performed on thin pyrolysis temperature obtained by microwave irradia- slices of each sample to evaluate the interfacial debond- tion at 37 GHz was approximately 1100oC. The heating ing strength(IDS)and interfacial frictional stress(IFS). time in each cycle was less than 5 min Detailed proces- respectively, using a load controlled micro-indentation sing technique has been described in literature [9] testing system. Those slices were double-face polished Two kinds of SiC/SiC composites were prepared their thickness being less than 100 um to allow the fibers according to PIMP process. The first one is non-coated to be pushed out. During this test, the slices were set on Tyranno SA fiber(Ube Industries Ltd, Japan) rein- a tungsten carbide holder with a 50 um width groove, forced SiC composite (TSA SiC). During impregnation and the load was applied on a single fiber end above the ind pyrolysis, the polymer precursor was used unloaded groove using a triangle diamond indenter. The max- no SiC particulate). Eight cycles of impregnation/pyr- imum load was IN and this load was modified accord- olysis processing were performed. The second is carbon ing to the value of IDS and IFS. The interfacial coated Tyranno SA fiber reinforced Sic composite displacement rate was 0.2 um/s. After push-out test, the (TSA/ C/SiC). In this composite, B-SiC particles(with protruding fibers were firstly observed by SEM, and an average diameter of 0.6 um) were added to the pre- then push-back test was conducted on those protruding cursor in the first cycle of impregnation and then the fibe material was submitted to microwave pyrolysis. This first cycle was followed by five cycles of PIMP with unloaded AHPCS. In the first PIMP cycle, polymer to 3. Results and disscussion filler(SiC) ratio was 50% to 50% by weight. All fiber reinforcements used in this experiment were 2-D woven 3. Microstructural evolution fabrics, and fiber volume fraction of the composites was about 30-35 vol % The fiber volume fraction for com- By contrast to the conventional polymer impregna posite TSA SiC is slightly higher than that in the second tion and pyrolysis(PIP)technique, the PIMP only material. Typical properties of Tyranno SA fiber used in requires very short pyrolysis time. Because the polymer nis experiment are listed in Table I to ceramic conversion is a complex process, such very short pyrolysis duration might affect the evolution of 2. 2. Microstructural observation microstructure. Fig. I shows a typical SEM micrograph Optical microscopy and scanning electron microscopy solidated parts dominate the cross section area. Some (SEM)were conducted on the polished cross-section of isolated large pores could be observed in inter-bundle Table l Properties of Tyranno SA fibers( Grade Im) SiC fiber C/Si atomic ratio Diameter(um) Density (g/cm) Filaments/yarn Tensile strength(GPa) Elastic modulus(GPa) Elongation% Tyranno SA 1.08fabricated by polymer impregnation/microwave pyr￾olysis (PIMP) process using 2 D woven Tyranno SA fiber preforms as the reinforcement. The effects of fiber coating and particulate loading during first cycle impregnation on fiber/matrix interaction and mechan￾ical behaviors are also evaluated. 2. Experimental procedure 2.1. Materials Polymer precursor used for matrix formation was allylperhydropolycarbosilane (AHPCS),which is regar￾ded as a polymer precursor of high ceramic yield [10]. Since this polymer precursor is very sensitive to the ambient atmosphere,it was handled under strictly anhydrous and anaerobic environment to minimize the potential for oxygen contamination. The maximum pyrolysis temperature obtained by microwave irradia￾tion at 37 GHz was approximately 1100
C. The heating time in each cycle was less than 5 min. Detailed proces￾sing technique has been described in literature [9]. Two kinds of SiC/SiC composites were prepared according to PIMP process. The first one is non-coated Tyranno SA fiber (Ube Industries Ltd.,Japan) rein￾forced SiC composite (TSA/SiC). During impregnation and pyrolysis,the polymer precursor was used unloaded (no SiC particulate). Eight cycles of impregnation/pyr￾olysis processing were performed. The second is carbon coated Tyranno SA fiber reinforced SiC composite (TSA/C/SiC). In this composite, b-SiC particles (with an average diameter of 0.6 mm) were added to the pre￾cursor in the first cycle of impregnation and then the material was submitted to microwave pyrolysis. This first cycle was followed by five cycles of PIMP with unloaded AHPCS. In the first PIMP cycle,polymer to filler (SiC) ratio was 50% to 50% by weight. All fiber reinforcements used in this experiment were 2-D woven fabrics,and fiber volume fraction of the composites was about 30–35 vol.%. The fiber volume fraction for com￾posite TSA/SiC is slightly higher than that in the second material. Typical properties of Tyranno SA fiber used in this experiment are listed in Table 1. 2.2. Microstructural observation Optical microscopy and scanning electron microscopy (SEM) were conducted on the polished cross-section of all the samples. Fracture surface of each sample after bending test was also observed by SEM to determine the fracture behavior. Microstructural evolution of the AHPCS derived matrix after PIMP process was char￾acterized by transmission electron microscopy (TEM). The thin film was prepared by the focused ion beam (FIB) method. 2.3. Physical and mechanical measurements All PIMP samples were cut and ground into nearly 4220 mm rectangular bars to perform three-point bending test at room temperature with an Instron 5581 test machine. The cross-head speed was 0.5 mm/min and the span was 18 mm. During bending test,the fracture behavior of the bars was in situ recorded by optical microscope camera. The bulk density of the composites was measured by Archimedes’s method. Push-out and push-back tests were performed on thin slices of each sample to evaluate the interfacial debond￾ing strength (IDS) and interfacial frictional stress (IFS), respectively,using a load controlled micro-indentation testing system. Those slices were double-face polished, their thickness being less than 100 mm to allow the fibers to be pushed out. During this test,the slices were set on a tungsten carbide holder with a 50 mm width groove, and the load was applied on a single fiber end above the groove using a triangle diamond indenter. The max￾imum load was 1N and this load was modified accord￾ing to the value of IDS and IFS. The interfacial displacement rate was 0.2 mm/s. After push-out test,the protruding fibers were firstly observed by SEM,and then push-back test was conducted on those protruding fibers. 3. Results and disscussion 3.1. Microstructural evolution By contrast to the conventional polymer impregna￾tion and pyrolysis (PIP) technique,the PIMP only requires very short pyrolysis time. Because the polymer to ceramic conversion is a complex process,such very short pyrolysis duration might affect the evolution of microstructure. Fig. 1 shows a typical SEM micrograph of the polished cross-section. Actually,well-con￾solidated parts dominate the cross section area. Some isolated large pores could be observed in inter-bundle Table 1 Properties of Tyranno SA fibers (Grade II) SiC fiber C/Si atomic ratio Diameter (mm) Density (g/cm3 ) Filaments/yarn Tensile strength (GPa) Elastic modulus (GPa) Elongation% Tyranno SA 1.08 10 3.02 800 2.8 420 0.7 900 S.M. Dong et al. / Ceramics International 28 (2002) 899–905