40TH ANNIVERSARY Figure 11 High Resolution Transmission Electron Microscopy image of a Hi-Nicalon fibre, showing B-SiC grains of about 10 nm, surrounded by poorly a organised Si-C phase, and turbostratic carbon[42] Table IV. The microstructure can be seen to have consisted Ti compounds were found indicating that the grains must of areas of well ordered SiC surrounded by Si and C atoms be separated by a Si-C-Ti-O phase which were not completely crystallized into B-SiC grains. It became clear, therefore, that the oxygen content The Hi-Nicalon fibre contained a higher ratio of carbon was the key factor in controlling the microstructures atoms to silicon atoms than in the first generation fibres and ultimately the mechanical properties of the polymer as the absence of oxygen did not allow the excess carbon derived SiC fibres. The oxygen cured, first generation to be oxidized. The free carbon can be seen in Fig. Il and fibres, the Nicalon 200 and Tyranno LOX-M fibres, was composed of 4 to 10 distorted layers with some aggre- possessed similar grain sizes, of about 2 nm, and gates measuring up to 5 nm. The small amount of oxygen properties. However the Hi-Nicalon and Tyranno LOX-E boundaries. The Hi-Nicalon fibre would be composed by different behaviours. The reduction in oxygen content weight of 85% SiC, 11%C and 4% of SiCo86O.29 if the to 5 wt. in the Tyranno LOX-E fibre did not produc oxicarbide phase is taken to be composed of SiO Cl significant change when compared to the first generation with three carbon atoms to every one oxygen atom. It fibres however the Hi-Nicalon with only 0.5 wt %o can be was calculated that the non-crystallized Sic in the Hi- seen to be a very different fibre Nicalon fibre represented around 26% by weight of the Although bulk Sic can be used to 1600oC, the oxygen fibre [46]. The absence of a significant amount of oxygen rich SiC fibres became too brittle to be handled when removed the amorphous phase present in the first gener- heated above 1500C and were reduced to a powder ation fibres and allowed larger Sic grains to form at the at 1800oC by the degradation of the silicon oxicarbide higher manufacturing temperature used. Their size how- intergranular phase [47]. Thermodynamic analyses have ever was limited by the excess free carbon in the fibre. This shown that the solid solution SiCa OB, coexisting with microstructure explains the observed increase in Youngs SiOz and C, were unstable above 1150oC in an inert modulus of these fibres when compared to first generation atmosphere [48]. Its decomposition produced CO and fibres be seen from Table iv Sio and free carbon. The Sio gas reacted with the free The microstructure of the Tyranno LOX-E fibres was carbon to give SiC and CO [23]. The absence of an very similar to that of the first generation Tyranno LOX- oxicarbide intergranular phase in the Hi-Nicalon fibre M fibres with SiC grains having a mean size of approx clearly helped its high temperature stability and the larger imately 2 nm, although some larger grains up to 5 nm percentage of crystalline Sic accounted for its higher were seen. The free carbon aggregates were around 1- Youngs modulus when compared to the first generation 3 nm and, as with the first generation Tyranno fibres, no fibres and the still oxygen rich Tyranno LOX-E fibre40TH ANNIVERSARY Figure 11 High Resolution Transmission Electron Microscopy image of a Hi-Nicalon fibre, showing β-SiC grains of about 10 nm, surrounded by poorly a organised Si–C phase, and turbostratic carbon [42]. Table IV. The microstructure can be seen to have consisted of areas of well ordered SiC surrounded by Si and C atoms which were not completely crystallized into β-SiC grains. The Hi-Nicalon fibre contained a higher ratio of carbon atoms to silicon atoms than in the first generation fibres as the absence of oxygen did not allow the excess carbon to be oxidized. The free carbon can be seen in Fig. 11 and was composed of 4 to 10 distorted layers with some aggre￾gates measuring up to 5 nm. The small amount of oxygen in these fibres was presumed to be present at the SiC/SiC boundaries. The Hi-Nicalon fibre would be composed by weight of 85% SiC, 11% C and 4% of SiC0.86O0.29 if the oxicarbide phase is taken to be composed of SiOxC1−x/2 with three carbon atoms to every one oxygen atom. It was calculated that the non-crystallized SiC in the Hi￾Nicalon fibre represented around 26% by weight of the fibre [46]. The absence of a significant amount of oxygen removed the amorphous phase present in the first gener￾ation fibres and allowed larger SiC grains to form at the higher manufacturing temperature used. Their size how￾ever was limited by the excess free carbon in the fibre. This microstructure explains the observed increase in Young’s modulus of these fibres when compared to first generation fibres, as can be seen from Table IV. The microstructure of the Tyranno LOX-E fibres was very similar to that of the first generation Tyranno LOX￾M fibres with SiC grains having a mean size of approx￾imately 2 nm, although some larger grains up to 5 nm were seen. The free carbon aggregates were around 1– 3 nm and, as with the first generation Tyranno fibres, no Ti compounds were found indicating that the grains must be separated by a Si–C–Ti–O phase. It became clear, therefore, that the oxygen content was the key factor in controlling the microstructures and ultimately the mechanical properties of the polymer derived SiC fibres. The oxygen cured, first generation fibres, the Nicalon 200 and Tyranno LOX-M fibres, possessed similar grain sizes, of about 2 nm, and properties. However the Hi-Nicalon and Tyranno LOX-E fibres had distinctly different microstructures and showed different behaviours. The reduction in oxygen content to 5 wt.% in the Tyranno LOX-E fibre did not produce a significant change when compared to the first generation fibres however the Hi-Nicalon with only 0.5 wt.% can be seen to be a very different fibre. Although bulk SiC can be used to 1600◦C, the oxygen rich SiC fibres became too brittle to be handled when heated above 1500◦C and were reduced to a powder at 1800◦C by the degradation of the silicon oxicarbide intergranular phase [47]. Thermodynamic analyses have shown that the solid solution SiCαOβ, coexisting with SiO2 and C, were unstable above 1150◦C in an inert atmosphere [48]. Its decomposition produced CO and SiO and free carbon. The SiO gas reacted with the free carbon to give SiC and CO [23]. The absence of an oxicarbide intergranular phase in the Hi-Nicalon fibre clearly helped its high temperature stability and the larger percentage of crystalline SiC accounted for its higher Young’s modulus when compared to the first generation fibres and the still oxygen rich Tyranno LOX-E fibre. 832