L.U.J.T. Ogbuji/ Journal of the European Ceramic Society 23(2003)613-617 composites involves conversion of the interphase to CO leaving no solid residue, SiC/BN/ SIC pesting is more complex. It seems to involve the synergistic oxidation of constituents in the interphase area. Whether Sio2 forms first via SiC oxidation or it is B,O3 via bn oxi- lation(depending on local conditions), the net result is that the compliant bn interphase is removed or eplaced by stiff, brittle silica which "cements"the fibers down and hence destroys composite behavior. The likely reaction sequence in SiC/BN SiC pesting is sum- marized in Table I. Where the Bn interphase is inter ected by a crack open to the surface it is oxidized locally to B2O3. The boria may be volatilized by hydro- lysis with ambient moisture if the temperature is low, brittle fracture zone fiber pull-out leaving a trench around the fiber; or it may react with Fig. 1. Normal or intrinsic mode of pest damage in SiC/BN/SiC. the surrounding SiC to form borosilicate.( Compare, for instance, Fig. 7a and b in ref 6. Hydrolysis of the bor- from a carbon-rich fiber or from sizing char yield In the silicate by ambient moisture results in a cementitious fast flame of a burner rig(which simulates a jet engine plug of silica in place of the BN the carbon layer rapidly burns off, leaving an annular 4 The foregoing describes what we may call the"nor- trench around the bn as a pathway for deep ingress of al"pest mode of SiC/BN/SiC: degradation starts at the ambient. The same reactions occur as before e surface/edge and proceeds inwards at a rate limited(sequence 0-3 in Fig. 2) but in this case it is exacerbated by intrinsic oxidation resistance of the BN. The fracture by the 10% moisture in the combustion product of the surface looks like a picture frame, as shown in Fig. 1, hydrocarbon fuel, by h flame velocity, and by the with a border of pested material(characterized by brit- huge surface area of bn exposed to ambient tle/flat fracture)around a shrinking core of undamaged attack. 12.3 The bn surface area is the biggest facto material (characterized by composite behavior and The interphase may be likened to a thin sheet of pape fibrous fracture, with copious fiber pull-out). Evans and and the effect of an underlying layer of fugitive carbo co-workers reported that mode of pesting in SiC/SiC and Sic/glass composites, and illustrated the picture- frame effect with a SiC/MAS (magnesium aluminosili- aLambientd D cate) minicomposite in which the sample was found to exhibit"a time-dependent(residual)strength". o Mor- scher and co-workers have reported the same phenom enon in SiC/BN/ SiC composites as well as in SiC/BN/ SiC and SiC/C/SiC minicomposites and correlated sample lure to growth of the rested zone at the borders From our work on SiC/BN SiC composites it has become obvious that a second and more pernicious mode of pest occurs in these composites. The details 0 have been presented elsewhere. This mode is driven by SN Carbon cvi-Sic lm SiC Fiber BN extrinsic factors at work in the interphase region as a Borosilicate(B content increases with depth) whole. It occurs in SIC/BN/SiC materials where a con- Fig. 2. Severe/extrinsic mode of Sic/BN/SiC pest, which is assisted by tinuous or skeletal film of carbon forms under the bn. free carbon able i Oxidative degradation of the Sic/BNSiC interphase I Oxidation of bn to BO3 This is favored over SiC oxidation except in very low oxygen partial pressures [Ref. 4. B2O3 dissolves fiber and cvi-SiC to form borosilicate liquid Borosilicate has a very low-melting eutectic composition(near 400C) 3 Borosilicate is hydrolyzed by HO, releasing B(OH)(g)[Ref 7] H2O is from combustion of hydrocarbon Boron loss increases borosilicate viscosity. 4 The residue is Sio(s) The SiO, residue bonds fibers to the cvi-SiC. Holes in the sio indicate emission of gaseous productsi.e boron hydroxidescomposites involves conversion of the interphase to CO, leaving no solidresidue, SiC/BN/SiC pesting is more complex. It seems to involve the synergistic oxidation of all constituents in the interphase area. Whether SiO2 forms first via SiC oxidation4 or it is B2O3 via BN oxi￾dation5 (depending on local conditions), the net result is that the compliant BN interphase is removedor replacedby stiff, brittle silica which ‘‘cements’’ the fibers down and hence destroys composite behavior. The likely reaction sequence in SiC/BN/SiC pesting is sum￾marizedin Table 1. Where the BN interphase is inter￾sectedby a crack open to the surface, it is oxidized locally to B2O3. The boria may be volatilizedby hydro￾lysis with ambient moisture if the temperature is low, leaving a trench aroundthe fiber;6 or it may react with the surrounding SiC to form borosilicate. (Compare, for instance, Fig. 7a andb in ref. 6.) Hydrolysis of the bor￾osilicate by ambient moisture results in a cementitious plug of silica in place of the BN interphase.58 The foregoing describes what we may call the ‘‘nor￾mal’’ pest mode of SiC/BN/SiC: degradation starts at the surface/edge and proceeds inwards at a rate limited by intrinsic oxidation resistance of the BN. The fracture surface looks like a picture frame, as shown in Fig. 1, with a border of pested material (characterized by brit￾tle/flat fracture) arounda shrinking core of undamaged material (characterizedby composite behavior and fibrous fracture, with copious fiber pull-out). Evans and co-workers reportedthat mode of pesting in SiC/SiC andSiC/glass composites,9 andillustratedthe picture￾frame effect with a SiC/MAS (magnesium aluminosili￾cate) minicomposite in which the sample was foundto exhibit ‘‘a time-dependent (residual) strength’’.10 Mor￾scher andco-workers have reportedthe same phenom￾enon in SiC/BN/SiC composites6 as well as in SiC/BN/SiC andSiC/C/SiC minicomposites11 andcorrelatedsample failure to growth of the pestedzone at the borders. From our work on SiC/BN/SiC composites it has become obvious that a secondandmore pernicious mode of pest occurs in these composites. The details have been presentedelsewhere.5 This mode is driven by extrinsic factors at work in the interphase region as a whole. It occurs in SiC/BN/SiC materials where a con￾tinuous or skeletal film of carbon forms under the BN, from a carbon-rich fiber or from sizing char yield. In the fast flame of a burner rig (which simulates a jet engine) the carbon layer rapidly burns off, leaving an annular trench aroundthe BN as a pathway for deep ingress of the ambient. The same reactions occur as before (sequence 0–3 in Fig. 2) but in this case it is exacerbated by the 10% moisture in the combustion product of the hydrocarbon fuel, by the high flame velocity, and by the huge surface area of BN exposedto ambient attack.5,12,13 The BN surface area is the biggest factor. The interphase may be likenedto a thin sheet of paper andthe effect of an underlying layer of fugitive carbon Table 1 Oxidative degradation of the SiC/BN/SiC interphase Step Process Comments 1 Oxidation of BN to B2O3 This is favoredover SiC oxidation except in very low oxygen partial pressures [Ref. 4]. 2 B2O3 dissolves fiber and cvi-SiC to form borosilicate liquidBorosilicate has a very low-melting eutectic composition (near 400 C). 3 Borosilicate is hydrolyzed by H2O, releasing B(OH)4(g) [Ref. 7] H2O is from combustion of hydrocarbon. Boron loss increases borosilicate viscosity. 4 The residue is SiO2(s) The SiO2 residue bonds fibers to the cvi-SiC. Holes in the SiO2 indicate emission of gaseous products—i.e. boron hydroxides Fig. 1. Normal or intrinsic mode of pest damage in SiC/BN/SiC. Fig. 2. Severe/extrinsic mode of SiC/BN/SiC pest, which is assisted by free carbon. 614 L.U.J.T. Ogbuji / Journal of the European Ceramic Society 23 (2003) 613–617