J. Martinez.Fernandez, G N Morscher Journal of the European Ceramic Society 20(2000)2627-2636 Retained strength at room temperature data for C-HN and BN-HN minicomposites Incomposite Temperature Applied stress Time at stress Retained of as-produced (°C) strength(MPa) strength C-HN 000000 337 337 17 BN-HN 400 1630 7867507758 The average ultimate strength(of the fibers if fully loaded) for as-produced C-HN=2000 MPa and for as-produced BN-HN=2100 MPa. higher rupture temperatures resulted in lower retained though the carbon interphase was removed by oxida- strength(Table 1) tion, resulting in a gap between the fiber and matrix At 1200 C the formation of Sio, is more evident on 33. Microstructural characterization the surface of the CVI SiC matrix and in the gap that was formerly the C interphase(Fig. 7). Two different Figs. 6 and 7 show the range of observed micro- regions are noted in this sample. The higher magnifica structural features observed on fracture surfaces after tion insets in Fig. 7 show that the crack had propagated for C-Hn minicomposite specimens tested between 700 through part of the sample prior to failure, where the and 1200C. Fig. 6 shows a micrograph of a typical matrix region is clearly oxidized. Part of the failure fracture surface for a stress-rupture test run at 700C. matrix crack surface was not oxidized. Apparently, the Due to the fibrous nature of these minicomposites frac- non-through thickness matrix crack propagated ire often occurs at different planar levels, although at through an uncracked section of matrix at ultimate each"level"the matrix fracture surfaces were flat. For minicomposite failure. However, the C interphase was short-term rupture(t<0. 1 h) there was some fiber pull- removed and fibers were strongly bonded to the matrix out. For longer rupture time at 700oC, some regions of in this region, presumably from oxidation through the minicomposite fracture surface showed relatively another matrix crack and along the vacated interphase long pullout lengths(Fig. 6). However, this increase in channel. A SiO2 scale did not cover the fiber fracture length was not uniform throughout the sample, ever surfaces because the fibers did not fail until mini omposite ultimate failure The fracture surfaces of the stress-rupture test per formed at 950C(not shown) contain little if any fiber pull out even for very short-term rupture because the 00m oxide reaction product nearly fills the gap left by the vacated interphase. Bonding between the fiber and matrix was obviously strong. 4. Discussion 4.1. Stress-rupture: comparison with previous data In Fig 3a, the stress-rupture data at 700oC is com- pared with previous data from NIC minicomposites with a carbon interphase. In both cases the mini- composite rupture is due to fiber degradation, as the Fig. 6. Typical fracture surface of C-HN minicomposite for stress matrix-fiber bonding associated with SiOz formation is ruptures at 700C(1183 MPa, 5.9 h) negligible. For this reason the better properties of C-higher rupture temperatures resulted in lower retained strength (Table 1). 3.3. Microstructural characterization Figs. 6 and 7 show the range of observed micro￾structural features observed on fracture surfaces after for C-HN minicomposite specimens tested between 700 and 1200C. Fig. 6 shows a micrograph of a typical fracture surface for a stress-rupture test run at 700C. Due to the ®brous nature of these minicomposites frac￾ture often occurs at di€erent planar levels, although at each ``level'' the matrix fracture surfaces were ¯at. For short-term rupture (t<0.1 h) there was some ®ber pull￾out. For longer rupture time at 700C, some regions of the minicomposite fracture surface showed relatively long pullout lengths (Fig. 6). However, this increase in length was not uniform throughout the sample, even though the carbon interphase was removed by oxida￾tion, resulting in a gap between the ®ber and matrix. At 1200C the formation of SiO2 is more evident on the surface of the CVI SiC matrix and in the gap that was formerly the C interphase (Fig. 7). Two di€erent regions are noted in this sample. The higher magni®ca￾tion insets in Fig. 7 show that the crack had propagated through part of the sample prior to failure, where the matrix region is clearly oxidized. Part of the failure matrix crack surface was not oxidized. Apparently, the non-through thickness matrix crack propagated through an uncracked section of matrix at ultimate minicomposite failure. However, the C interphase was removed and ®bers were strongly bonded to the matrix in this region, presumably from oxidation through another matrix crack and along the vacated interphase channel. A SiO2 scale did not cover the ®ber fracture surfaces because the ®bers did not fail until mini￾composite ultimate failure. The fracture surfaces of the stress-rupture test per￾formed at 950C (not shown) contain little if any ®ber pull out even for very short-term rupture because the oxide reaction product nearly ®lls the gap left by the vacated interphase. Bonding between the ®ber and matrix was obviously strong. 4. Discussion 4.1. Stress±rupture: comparison with previous data In Fig. 3a, the stress±rupture data at 700C is com￾pared with previous data from NIC minicomposites with a carbon interphase. In both cases the mini￾composite rupture is due to ®ber degradation, as the matrix±®ber bonding associated with SiO2 formation is negligible. For this reason the better properties of C± Table 1 Retained strength at room temperature data for C±HN and BN±HN minicomposites Minicomposite Temperature ( C) Applied stress (MPa) Time at stress (h) Retained strength (MPa) % of as-produced strengtha C±HN 700 328 50 1389 69 700 1099 358 1114 56 950 590 94 683 34 950 655 337 732 37 950 655 337 766 38 1200 439 3 512 26 1200 439 17 544 27 1200 439 162 491 25 BN±HN 700 996 400 2260 100 816 951 324 1411 67 950 1019 252 1630 77 1200 328 882 525 25 1200 385 965 792 38 a The average ultimate strength (of the ®bers if fully loaded) for as-produced C±HN=2000 MPa and for as-produced BN±HN=2100 MPa. Fig. 6. Typical fracture surface of C±HN minicomposite for stress ruptures at 700C (1183 MPa, 5.9 h). J. Marti nez-FernaÂndez, G.N. Morscher / Journal of the European Ceramic Society 20 (2000) 2627±2636 2631