J. Martinez.Fernandez, G N Morscher Journal of the European Ceramic Society 20(2000)2627-2636 product would be activity in order to capture and digitize the waveform. matrix are not fully bonded as evidenced by the occurrence This approach provides greater accuracy in sorting out of fiber pullout(Fig. 7)and the differences between fatigue gage events and correlating individual events with phy- and stress rupture are not as significant as at 950%C. sical sources. 2 Fig. Al shows a schematic of the tensile test. For most tests, only two wide band(50 kHz to 2 4.4. Retained strength MHz) sensors(Model B1080, Digital Wave Corpora tion)were attached with alligator clips to the epoxy. For The retained strengths at room temperature of C-hn two experiments, four wide band sensors were attached minicomposites that did not fail during the rupture test to the epoxy, two wide-band and two resonant fre- are listed for C-HN and BN-HN' minicomposites in quency(Model"Pico", Physical Acoustics Corporation) Table 1. For rupture conditions at 700 and 900oC, the (the same as used in the earlier studies). The data was etained strength of BN-HN minicomposites are sig collected on a Digital Wave(Englewood, CO) Fracture nificantly better than for C-HN minicomposites. This is Wave Detector. After the test, the same software used most likely due to the slower recession of bn which to collect the data was used to analyze the data. For enables the fibers to be separated from the SiC matrix. experiments with only two sensors, 100 to 200 events Higher temperatures are required to form enough oxide were recorded. For experiments with four sensors, up to reaction product, for BN-HN minicomposites, in order to 600 events were recorded because the resonant fre- fill the gap between the fibers and matrix. In comparison, quency sensors were much more sensitive, especially to the fibers in C-hn minicomposites are observed to be in lower frequency AE. intimate contact with the matrix shortly after the C layer Post-test analysis consisted first of determining the was removed by oxidation at 700C(e.g. Fig. 6). After speed of sound through the sample in order to locate the 1200C rupture conditions, the vacated interphases are sources of the aE events and to sort out events which completely filled by the glass reaction products for both occurred outside of the gage section. Pencil-lead breaks ystems and the retained strength data for C-Hn and had been performed prior to a tensile test on the epoxy BN-HN minicomposites are nearly identical of an undamaged specimen outside of the sensors, so that the sound waves produced by the fracture of the pencil-lead traveled from one sensor to the next, and the 5. Conclusions ae data was saved on a separate file. The maximum difference in time of arrival, Atr from one sensor to the The stress-rupture properties of HN fiber-reinforced next was determined from the first peak of the sound CVI-SiC minicomposites with carbon interphase were wave(extensional wave) received on both sensors from tudied at temperatures ranging from 700 to 1200oC The stress-rupture lives of C-HN are by far superior to C-NIC, presumably due to the lack of fiber decom- position/reaction during CVI SiC processing that occurs for NIC fibers. This study demonstrates that C-HN minicomposites have worse mechanical properties at 700 and 950C than the BN-Hn minicomposites pre- viously studied, due to the removal of carbon inter phases and the ease with which fibers bond to the natrix. Therefore, BN interphases are more envir onmentally stable in the intermediate temperature regime. At 1200C, the improvement of the BN inter- phase over the carbon interphase was not quite as sig nificant as at the lower temperatures; the stress-rupture and fatigue properties predominantly being controlled by the fiber properties. This study also shows that Ae emission can be used on this system as a reliable quan titative method to monitor damage in this system Appendix. Acoustic emission detection and analysis of minicomposite tensile tests The AE set-up was similar to previous studies. 21 22 However, wide-band sensors were used to detect AE Fig. Al. Schematic of room temperature tensile test.product would be expected. At 700C, the ®ber and matrix are not fully bonded as evidenced by the occurrence of ®ber pullout (Fig. 7) and the di€erences between fatigue and stress rupture are not as signi®cant as at 950C. 4.4. Retained strength The retained strengths at room temperature of C±HN minicomposites that did not fail during the rupture test are listed for C±HN and BN±HN2 minicomposites in Table 1. For rupture conditions at 700 and 900C, the retained strength of BN±HN minicomposites are sig￾ni®cantly better than for C±HN minicomposites. This is most likely due to the slower recession of BN which enables the ®bers to be separated from the SiC matrix. Higher temperatures are required to form enough oxide reaction product, for BN±HN minicomposites, in order to ®ll the gap between the ®bers and matrix. In comparison, the ®bers in C±HN minicomposites are observed to be in intimate contact with the matrix shortly after the C layer was removed by oxidation at 700C (e.g. Fig. 6). After 1200C rupture conditions, the vacated interphases are completely ®lled by the glass reaction products for both systems and the retained strength data for C±HN and BN±HN minicomposites are nearly identical. 5. Conclusions The stress±rupture properties of HN ®ber-reinforced CVI±SiC minicomposites with carbon interphase were studied at temperatures ranging from 700 to 1200C. The stress-rupture lives of C±HN are by far superior to C±NIC, presumably due to the lack of ®ber decom￾position/reaction during CVI SiC processing that occurs for NIC ®bers. This study demonstrates that C±HN minicomposites have worse mechanical properties at 700 and 950C than the BN±HN minicomposites pre￾viously studied, due to the removal of carbon inter￾phases and the ease with which ®bers bond to the matrix. Therefore, BN interphases are more envir￾onmentally stable in the intermediate temperature regime. At 1200C, the improvement of the BN inter￾phase over the carbon interphase was not quite as sig￾ni®cant as at the lower temperatures; the stress±rupture and fatigue properties predominantly being controlled by the ®ber properties. This study also shows that AE emission can be used on this system as a reliable quan￾titative method to monitor damage in this system. Appendix. Acoustic emission detection and analysis of minicomposite tensile tests The AE set-up was similar to previous studies.21,22 However, wide-band sensors were used to detect AE activity in order to capture and digitize the waveform. This approach provides greater accuracy in sorting out gage events and correlating individual events with phy￾sical sources.22 Fig. A1 shows a schematic of the tensile test. For most tests, only two wide band (50 kHz to 2 MHz) sensors (Model B1080, Digital Wave Corpora￾tion) were attached with alligator clips to the epoxy. For two experiments, four wide band sensors were attached to the epoxy, two wide-band and two resonant fre￾quency (Model ``Pico'', Physical Acoustics Corporation) (the same as used in the earlier studies). The data was collected on a Digital Wave (Englewood, CO) Fracture Wave Detector. After the test, the same software used to collect the data was used to analyze the data. For experiments with only two sensors, 100 to 200 events were recorded. For experiments with four sensors, up to 600 events were recorded because the resonant fre￾quency sensors were much more sensitive, especially to lower frequency AE. Post-test analysis consisted ®rst of determining the speed of sound through the sample in order to locate the sources of the AE events and to sort out events which occurred outside of the gage section. Pencil-lead breaks had been performed prior to a tensile test on the epoxy of an undamaged specimen outside of the sensors, so that the sound waves produced by the fracture of the pencil-lead traveled from one sensor to the next, and the AE data was saved on a separate ®le. The maximum di€erence in time of arrival,
tx, from one sensor to the next was determined from the ®rst peak of the sound wave (extensional wave) received on both sensors from Fig. A1. Schematic of room temperature tensile test. 2634 J. Marti nez-FernaÂndez, G.N. Morscher / Journal of the European Ceramic Society 20 (2000) 2627±2636