40TH ANNIVERSARY C-H bonds remaining in the PCs Above 1000oC, B-Sic 280 Hi-Nicalon grains, free carbon aggregates and a poorly organised in tergranular phase comprising of silicon and carbon atoms are formed [37, 38]. The presence of the free carbon ag- 220 gregates is considered to hinder grain growth so allowing 200 the Sic grains to remain small during fibre production 180 which takes place at around 1300C as indicated in Table 160 NLM202 The radiation cross-linking process, although costly, was successful in reducing the oxygen content of fibres 100 produced from PCS precursors to 0.5 wt. and gave rise 15 to the Hi-Nicalon fibre produced by Nippon Carbon [39, Failure Stress(GPa)at Lo= 25mm 40] Figure& The second generation fibres showed increased Youngs moduli When the radiation process was applied to the Ptc compared to the first generation fibres[42] precursor fibres, it was found that, although the oxygen content fell, it was only reduced to around 5 wt % The explanation was that the Ti addition to the PCs, so as to to graft the zirconium onto the PCS polymer contained make the PTC polymer, was achieved by adding a titanium less oxygen than was the case with titanium alkoxide alkoxide, Ti(OR)4, which added oxygen by another route [41]. Two types of fibre were produced. The Tyranno ZM than that of oxygen cross-linking. This alkoxide grouping fibre was cured by oxidation and was commercially pro formed links between the PCS molecules through Ti-o duced and an experimental Tyranno ZE fibres cured by bonds which were stable up to 700C. Above this temper- electron irradiation, which was not commercialised. The ature Ti-C began to be formed. Intermediate species such former fibre was produced with an oxygen concentration as TiC,O, were produced with the formation of SiC Oy. of 10 wt. and also 7.8 wt.%. The stability of the inter The result was that, although cross-linking occurred by granular phase which contained zirconium was said to be the process described above for the PCS fibres, there re- improved, compared to the titanium containing intergran- mained a significant presence of oxygen in the Tyranno ular phase as weight loss began at 1600 C for the Tyranno LOX-E fibre which Ube Industries produced by this ZM fibre which was 200oC higher than for the Tyrannd method. Fig. 7 shows the atomic carbon to silicon ratios LOX-M. The Tyranno ZM fibre found a market, for some and oxygen contents for the first and second generation time, in filters for engines running on sulphur rich diesel fibres Ube Industries took the decision not to commercially produce the Tyranno LOX-E fibre as the irradiation pro- 5. Mechanical behaviour of second generation cess was expensive and the improvement in properties Sic fibres not sufficiently adequate. The company produced other The reduction of the oxygen content in the fibres, made fibres with the intention of reducing the oxygen content by irradiating PCS, produced Hi-Nicalon fibres which by changing the metal added to the PCS polymer and the were approximately 35% stiffer than the first generation titanium was replaced by zirconium. This substitution re- fibres, as can be seen from Fig 8. However the increase duced the oxygen concentration as the compound used was not nearly as significant for the Tyranno LOX-E fibre which, unlike the Hi-Nicalon, was not developed as commercial second generation fibre. There was no change in appearance of the fracture surfaces between the first and econd generation fibres which remained devoid of any obvious signs of an ordered microstructure, as seen in the scanning electron microscope Strength retention at high temperatures of the Hi- Nicalon fibre was improved, when compared to the first generation fibres [42]. These second generation fibres re LOx-M Oxidation curing mained linearly elastic up to 1350C which was 100C higher than the first generation fibres and room strength NLM was retained up to around 1200oC in both argon and air, although the Tyranno LOX-E fibres, heated in air, showed 0 2 4 6 8 10 12 14 a fall in strength at temperatures lower than 1000oC. This o % wt was not observed in argon, suggesting a sensitivity to car- Figure7 The carbon/silicon atomic ratios and oxygen contents in the first bon oxidation at the surface of the fibres. Such sensitivity and second generation SiC fibres [42]. could be seen in the Hi-Nicalon fibres but at higher tem- 83040TH ANNIVERSARY C–H bonds remaining in the PCS. Above 1000◦C, β-SiC grains, free carbon aggregates and a poorly organised in￾tergranular phase comprising of silicon and carbon atoms are formed [37, 38]. The presence of the free carbon ag￾gregates is considered to hinder grain growth so allowing the SiC grains to remain small during fibre production which takes place at around 1300◦C as indicated in Table III. The radiation cross-linking process, although costly, was successful in reducing the oxygen content of fibres produced from PCS precursors to 0.5 wt.% and gave rise to the Hi-Nicalon fibre produced by Nippon Carbon [39, 40]. When the radiation process was applied to the PTC precursor fibres, it was found that, although the oxygen content fell, it was only reduced to around 5 wt.%. The explanation was that the Ti addition to the PCS, so as to make the PTC polymer, was achieved by adding a titanium alkoxide, Ti(OR)4, which added oxygen by another route than that of oxygen cross-linking. This alkoxide grouping formed links between the PCS molecules through Ti–O bonds which were stable up to 700◦C. Above this temper￾ature Ti–C began to be formed. Intermediate species such as TiCxOy were produced with the formation of SiCxOy. The result was that, although cross-linking occurred by the process described above for the PCS fibres, there re￾mained a significant presence of oxygen in the Tyranno LOX-E fibre which Ube Industries produced by this method. Fig. 7 shows the atomic carbon to silicon ratios and oxygen contents for the first and second generation fibres. Ube Industries took the decision not to commercially produce the Tyranno LOX-E fibre as the irradiation pro￾cess was expensive and the improvement in properties not sufficiently adequate. The company produced other fibres with the intention of reducing the oxygen content by changing the metal added to the PCS polymer and the titanium was replaced by zirconium. This substitution re￾duced the oxygen concentration as the compound used Figure 7 The carbon/silicon atomic ratios and oxygen contents in the first and second generation SiC fibres [42]. Figure 8 The second generation fibres showed increased Young’s moduli compared to the first generation fibres [42]. to graft the zirconium onto the PCS polymer contained less oxygen than was the case with titanium alkoxide [41]. Two types of fibre were produced. The Tyranno ZM fibre was cured by oxidation and was commercially pro￾duced and an experimental Tyranno ZE fibres cured by electron irradiation, which was not commercialised. The former fibre was produced with an oxygen concentration of 10 wt.% and also 7.8 wt.%. The stability of the inter￾granular phase which contained zirconium was said to be improved, compared to the titanium containing intergran￾ular phase, as weight loss began at 1600◦C for the Tyranno ZM fibre which was 200◦C higher than for the Tyranno LOX-M. The Tyranno ZM fibre found a market, for some time, in filters for engines running on sulphur rich diesel. 5. Mechanical behaviour of second generation SiC fibres The reduction of the oxygen content in the fibres, made by irradiating PCS, produced Hi-Nicalon fibres which were approximately 35% stiffer than the first generation fibres, as can be seen from Fig. 8. However the increase was not nearly as significant for the Tyranno LOX-E fibre which, unlike the Hi-Nicalon, was not developed as a commercial second generation fibre. There was no change in appearance of the fracture surfaces between the first and second generation fibres which remained devoid of any obvious signs of an ordered microstructure, as seen in the scanning electron microscope. Strength retention at high temperatures of the Hi￾Nicalon fibre was improved, when compared to the first generation fibres [42]. These second generation fibres re￾mained linearly elastic up to 1350◦C which was 100◦C higher than the first generation fibres and room strength was retained up to around 1200◦C in both argon and air, although the Tyranno LOX-E fibres, heated in air, showed a fall in strength at temperatures lower than 1000◦C. This was not observed in argon, suggesting a sensitivity to car￾bon oxidation at the surface of the fibres. Such sensitivity could be seen in the Hi-Nicalon fibres but at higher tem- 830