J Mater sci(2007)42:763-771 The BMAS/SiC (Tyranno) fibre composite as rapid at low temperatures could be due to slower received had a carbon-rich interface typically 20 nm kinetics of oxidation. However as the temperature is thick, a typical thickness also noted by Plucknett et al. increased, two effects could contribute to reducing the [14. The thickness of the carbon-rich layer does effect however depend on hot pressing temperature and time [28, 29]and thicknesses as high as 250 nm have been (a) Any residual glassy phases in the matrix could noted 10]. The carbon exists as an amorphous phase at begin to soften. The extent to which these may lower manufacturing temperatures but if manufactur- flow to fill gaps depends on the viscosity. As ing temperatures are higher than 1, 250C graphitic temperature is increased, viscosity is reduced and carbon formation has been seen [10, 11. The EDS viscous fow can occur analyses of the interfacial layer shows it to consist of C (D) As the temperature is increased further oxidation (42%),O(13%)and Si (23%)which suggests a of the silicon carbide could occur to produce more mixture of C and Sioz. Interestingly for the BMAs SiOz and further carbon(pipe line diffusion) composite the interfacial layer also contained some Ti The results examined for the anneals at the higher which can only have come from the Tyranno fibre temperatures (>1,000C) revealed that the thermal suggesting that the formation of the interfacial layer diffusivity is changed by very little as a result of these had occurred by oxidation of the fibre via one of the heat treatments. The thickness of the interface how two reactions proposed by Eq (1)or(2) ever is much thicker than that of the as-received The most dramatic results during the heat treat- composites, amounting to some -45 nm after the 30 h ments are for the heat treatments in the temperature heat treatment at 1, 200C. Examination by EDs of range 700-900C where significant decreases in diffu- various regions of the interface after a 30 h heat sivity and thermal expansion coefficient are noted. treatment reveals it to contain Si, O and C possibly as TEM micrographs clearly show the development of a result of reaction. The interfacial layer contained Ti all gap at the interface after heat treatments in this the way through thus supporting the view that the temperature range. This is clearly due to the oxidation reaction products were the result of oxidation of the of the carbon to leave a silicon oxide-rich region SiC fibre. Interestingly the presence of Ba, Al,Mg was forming the bridges between matrix and fibre. Pluck- also noted near the matrix interface suggesting some nett et al. [14] reported similar behaviour and stated diffusion from the matrix also. The effect of these heat that the bridges they observed were silica. The degra- treatments on the diffusivity is difficult to quantify dation of the C layer occurs at a temperature lower except that for the higher temperature heat treatments than that at which the fibre oxidises. Two reactions are there is a slight increase in thermal diffusivity after the possible heat treatment at 1. 200oC of some 4% after 30 h. may be postulated for this C+O2→CO (a) Interfacial effects or more probably (b) Crystallisation of residual glassy phases, and (c) Changes in phase structure of the phases in the 2C+O2→2CO matrix Changes in the interface will affect thermal diffu 3 Irrespective of whether the oxidation of C occurs via sIvIty if any contact resistance is reduced. Although the ne formation of CO or CO2 the mechanism whereby differences in thickness of the interface were noted, the reaction occurs is worth investigation. Oxygen this is difficult to quantify. There is a reduction in the diffusion from the BMAS matrix is unlikely at these glassy phase content which could increase the diffusive low temperatures. The interfaces themselves may act ity. According to Winter et al. [30] crystallisation starts as pipe oxidation channels providing a route for gas at 900-950C with the formation of a magnesium transport from the exposed ends of the fibres. aluminosilicate of high-quartz type structure. Osum The reduction in oxygen content of the interfacial lite starts to crystallise at around 970C. The other layer suggests that this mechanism may be a contrib- phases in the matrix begin to crystallise at around utory factor although not necessarily the only one. The 1,000C. This recrystallisation in the residual glasses is largest change in diffusivity occurs after the 700C heat possible at around those temperatures. However, treatment. It is questioned why degradation is most crystallisation of those phases is a time-dependent rapid at 700C. The rate of oxidation of C increases process and with increasing heat treatment, the with temperature and the fact that degradation is less percentage of the crystallised phases is increased 2 SpringerThe BMAS/SiC (Tyranno) fibre composite as received had a carbon-rich interface typically 20 nm thick, a typical thickness also noted by Plucknett et al. [14]. The thickness of the carbon-rich layer does however depend on hot pressing temperature and time [28, 29] and thicknesses as high as 250 nm have been noted [10]. The carbon exists as an amorphous phase at lower manufacturing temperatures but if manufactur￾ing temperatures are higher than 1,250 C graphitic carbon formation has been seen [10, 11]. The EDS analyses of the interfacial layer shows it to consist of C (~42%), O (~13%) and Si (23%) which suggests a mixture of C and SiO2. Interestingly for the BMAS composite the interfacial layer also contained some Ti which can only have come from the Tyranno fibre suggesting that the formation of the interfacial layer had occurred by oxidation of the fibre via one of the two reactions proposed by Eq. (1) or (2). The most dramatic results during the heat treat￾ments are for the heat treatments in the temperature range 700–900 C where significant decreases in diffu￾sivity and thermal expansion coefficient are noted. TEM micrographs clearly show the development of a gap at the interface after heat treatments in this temperature range. This is clearly due to the oxidation of the carbon to leave a silicon oxide-rich region forming the bridges between matrix and fibre. Pluck￾nett et al. [14] reported similar behaviour and stated that the bridges they observed were silica. The degra￾dation of the C layer occurs at a temperature lower than that at which the fibre oxidises. Two reactions are possible C þ O2 ! CO2 ð3Þ or more probably 2C þ O2 ! 2CO ð4Þ Irrespective of whether the oxidation of C occurs via the formation of CO or CO2 the mechanism whereby the reaction occurs is worth investigation. Oxygen diffusion from the BMAS matrix is unlikely at these low temperatures. The interfaces themselves may act as pipe oxidation channels providing a route for gas transport from the exposed ends of the fibres. The reduction in oxygen content of the interfacial layer suggests that this mechanism may be a contrib￾utory factor although not necessarily the only one. The largest change in diffusivity occurs after the 700 C heat treatment. It is questioned why degradation is most rapid at 700 C. The rate of oxidation of C increases with temperature and the fact that degradation is less rapid at low temperatures could be due to slower kinetics of oxidation. However as the temperature is increased, two effects could contribute to reducing the effect; (a) Any residual glassy phases in the matrix could begin to soften. The extent to which these may flow to fill gaps depends on the viscosity. As temperature is increased, viscosity is reduced and viscous flow can occur. (b) As the temperature is increased further oxidation of the silicon carbide could occur to produce more SiO2 and further carbon (pipe line diffusion). The results examined for the anneals at the higher temperatures (>1,000 C) revealed that the thermal diffusivity is changed by very little as a result of these heat treatments. The thickness of the interface how￾ever is much thicker than that of the as-received composites, amounting to some ~45 nm after the 30 h heat treatment at 1,200 C. Examination by EDS of various regions of the interface after a 30 h heat treatment reveals it to contain Si, O and C possibly as a result of reaction. The interfacial layer contained Ti all the way through thus supporting the view that the reaction products were the result of oxidation of the SiC fibre. Interestingly the presence of Ba, Al, Mg was also noted near the matrix interface suggesting some diffusion from the matrix also. The effect of these heat treatments on the diffusivity is difficult to quantify except that for the higher temperature heat treatments there is a slight increase in thermal diffusivity after the heat treatment at 1,200 C of some 4% after 30 h. Various reasons may be postulated for this, these are (a) Interfacial effects, (b) Crystallisation of residual glassy phases, and (c) Changes in phase structure of the phases in the matrix. Changes in the interface will affect thermal diffu￾sivity if any contact resistance is reduced. Although the differences in thickness of the interface were noted, this is difficult to quantify. There is a reduction in the glassy phase content which could increase the diffusiv￾ity. According to Winter et al. [30] crystallisation starts at 900–950 C with the formation of a magnesium aluminosilicate of high-quartz type structure. Osumi￾lite starts to crystallise at around 970 C. The other phases in the matrix begin to crystallise at around 1,000 C. This recrystallisation in the residual glasses is possible at around those temperatures. However, crystallisation of those phases is a time-dependent process and with increasing heat treatment, the percentage of the crystallised phases is increased. 123 770 J Mater Sci (2007) 42:763–771