40TH ANNIVERSARY lower than 10-10 s-, which reduced as the temperature the methyl groups present in the PCS and PTC poly increased mers. The presence of oxygen in the precursor fibres Similar mechanical data have been reported for the lab- induced the out-gassing of carbon oxides between 400 oratory produced fibres produced from high-molecular- and 600 C at the beginning of the pyrolysis so reduc- weight PCS, both in tension and in creep[18] ing the final carbon content of the ceramic fibre most The creep data given in this paper, which come from important was the understanding that the presence of the authors'laboratory, have been obtained using ma- the oxygen produced an amorphous phase in which the hines dedicated to the high temperature testing of small small Sic grains and the free carbon aggregates were diameter filaments These machines have been described embedded elsewhere [19] and are capable of conducting tensile, re- The first fibre of this generation to be studied by XRD laxation, creep and, if necessary, fatigue tests on very and TEM was the Nicalon NLM-102 fibre with a 16 wt% small diameter filaments over a wide range of temper- content [17]. This study was followed by others on the atures. Other research teams have conducted creep tests same type of fibre and gave greater information on the using dead weights supported by the fibres. An alternative bonding of the oxygen atoms with neighbouring silicon and rapid way of ranking the creep or relaxation behaviour and carbon atoms [21, 22]. These studies revealed that of ceramic fibres is to put them into the form of a loop the fibre consisted of very small B-SiC grains with an around a former and to heat treat them. If the fibre re- average size, as calculated from the XRD patterns using mains perfectly elastic during the test its form returns to the Sherrer method, of 1.7 nm in size and these were its original, straight form on cooling and removal from embedded in an oxygen rich amorphous silicate phase the former. However if any anelastic behaviour occurs the The B-Sic grains were the only crystalline phase de- fibre retains a curvature. The degree of the induced cur- tected by XRD and electron diffraction. The early grade vature is taken as a mean of ranking the probable creep was later replaced by what was called the ceramic grade behaviour of the fibre compared to that of other fibres Nicalon NLM-202 fibre, the composition of which dif- [20] fered slightly from the earlier grade as it had a lower oxygen content of 12%. Studies on this fibre, which be- came the standard and most widely studied first gener- tion fibre, by TEM, allowed the structure of the free 3.1. Compositions and microstructures carbon to be determined and the mean composition of of first generation fibres the amorphous phase to be proposed [23]. Originally the Fig. 5 shows a typical failure surface of a first genera- oxygen was presumed to be included in an amorphous tion Sic fibre. There is no sign of any granular texture silica phase. Further studies showed that only a fraction so that the fibre appears to be glassy and possibly amor- of the oxygen was in the form of SiO2 and the rest was phous, as was originally concluded. However, wide angle present in the fibre in the form of a ternary phase SiO, Cy X-ray diffraction(XRD) studies and later examination by [21]. The fibre therefore seen to be composed of a transmission electron microscopy (TEM) showed that the continuum of SiO, C, tetrahedra with x +y=4.The first generation fibres were nano-crystalline with sizes of Nicalon 200 grade fibre with a 1l wt. oxygen content B-Sic grains generally being in the range 1.7 to 2 nm was determined as being composed of SiC4 tetrahedra although some variation occurred in early fibres due, pre- formed into small crystallites of 1.4 nm with a diamond sumably, to optimisation by the manufacturers in the du- like structure separated by two Sio, Cy tetrahedra (x+ ration of pyrolysis [17]. O), whereas the 100 grade fibres with 16 wt. of oxygen The compositions by weight percentages and densi- had a lower crystallinity. The Nicalon 200 grade fibre was s of the fibres are given in Table Il, together with the composed of 55 wt% B-SiC grains, 40% of an intergran carbon-to-silicon atomic weight ratios. It is clear that the ular phase with a mean composition of SiO,, 15 Co85 and first generation fibres were far from being stoichiomet- 5% of randomly oriented free carbon aggregates, I nm ric silicon carbide and that they contained an excess of in size. Coo2 lattice fringe images showed small stacks oxygen and carbon. This observation was the beginning of two fringes around 0.7 nm in size suggesting that the of an understanding of the easons why the fibres had basic structural unit(BSU) was a face-to-face associa- such different characteristics from those of bulk Sic. Al- tion of aromatic rings, called dicoronenes, in which the though the manufacture of these fibres from PCS and hydrogen-to-carbon atomic ratio is 0.5. With such a model PTC was analogous to that of carbon fibres made from for the microstructure of the fibre, a porosity level of PAN, an important difference quickly became evident, more than 2% was calculated [23]. Other authors pro which was that the oxygen, used to crosslink and re posed that the intergranular phase should be written as der infusible the precursors before pyrolysis, introduced Sio, Cl-xp which suggests that the composition could into the PAN fibres, is entirely removed at high temper- vary continuously from SiC to SiOz as the oxygen concen- atures but that used to stabilise the pcs and ptc fibres ation varied [24]. This gave a composition by weight of remains. The excess carbon in the SiC fibres came from 56% SiC, 10%C and 34% SiO.1 Co4440TH ANNIVERSARY lower than 10−10 s−1, which reduced as the temperature increased. Similar mechanical data have been reported for the lab￾oratory produced fibres produced from high-molecular￾weight PCS, both in tension and in creep [18]. The creep data given in this paper, which come from the authors’ laboratory, have been obtained using ma￾chines dedicated to the high temperature testing of small diameter filaments. These machines have been described elsewhere [19] and are capable of conducting tensile, re￾laxation, creep and, if necessary, fatigue tests on very small diameter filaments over a wide range of temper￾atures. Other research teams have conducted creep tests using dead weights supported by the fibres. An alternative and rapid way of ranking the creep or relaxation behaviour of ceramic fibres is to put them into the form of a loop around a former and to heat treat them. If the fibre re￾mains perfectly elastic during the test its form returns to its original, straight form on cooling and removal from the former. However if any anelastic behaviour occurs the fibre retains a curvature. The degree of the induced cur￾vature is taken as a mean of ranking the probable creep behaviour of the fibre compared to that of other fibres [20]. 3.1. Compositions and microstructures of first generation fibres Fig. 5 shows a typical failure surface of a first genera￾tion SiC fibre. There is no sign of any granular texture so that the fibre appears to be glassy and possibly amor￾phous, as was originally concluded. However, wide angle X-ray diffraction (XRD) studies and later examination by transmission electron microscopy (TEM) showed that the first generation fibres were nano-crystalline with sizes of β-SiC grains generally being in the range 1.7 to 2 nm although some variation occurred in early fibres due, pre￾sumably, to optimisation by the manufacturers in the du￾ration of pyrolysis [17]. The compositions by weight percentages and densi￾ties of the fibres are given in Table II, together with the carbon-to-silicon atomic weight ratios. It is clear that the first generation fibres were far from being stoichiomet￾ric silicon carbide and that they contained an excess of oxygen and carbon. This observation was the beginning of an understanding of the reasons why the fibres had such different characteristics from those of bulk SiC. Al￾though the manufacture of these fibres from PCS and PTC was analogous to that of carbon fibres made from PAN, an important difference quickly became evident, which was that the oxygen, used to crosslink and ren￾der infusible the precursors before pyrolysis, introduced into the PAN fibres, is entirely removed at high temper￾atures but that used to stabilise the PCS and PTC fibres remains. The excess carbon in the SiC fibres came from the methyl groups present in the PCS and PTC poly￾mers. The presence of oxygen in the precursor fibres induced the out-gassing of carbon oxides between 400 and 600◦C at the beginning of the pyrolysis so reduc￾ing the final carbon content of the ceramic fibre. Most important was the understanding that the presence of the oxygen produced an amorphous phase in which the small SiC grains and the free carbon aggregates were embedded. The first fibre of this generation to be studied by XRD and TEM was the Nicalon NLM-102 fibre with a 16 wt.% content [17]. This study was followed by others on the same type of fibre and gave greater information on the bonding of the oxygen atoms with neighbouring silicon and carbon atoms [21, 22]. These studies revealed that the fibre consisted of very small β-SiC grains with an average size, as calculated from the XRD patterns using the Sherrer method, of 1.7 nm in size and these were embedded in an oxygen rich amorphous silicate phase. The β-SiC grains were the only crystalline phase de￾tected by XRD and electron diffraction. The early grade was later replaced by what was called the ceramic grade Nicalon NLM-202 fibre, the composition of which dif￾fered slightly from the earlier grade as it had a lower oxygen content of 12%. Studies on this fibre, which be￾came the standard and most widely studied first gener￾ation fibre, by TEM, allowed the structure of the free carbon to be determined and the mean composition of the amorphous phase to be proposed [23]. Originally the oxygen was presumed to be included in an amorphous silica phase. Further studies showed that only a fraction of the oxygen was in the form of SiO2 and the rest was present in the fibre in the form of a ternary phase SiOxCy [21]. The fibre was therefore seen to be composed of a continuum of SiOxCy tetrahedra with x + y = 4. The Nicalon 200 grade fibre with a 11 wt.% oxygen content was determined as being composed of SiC4 tetrahedra formed into small crystallites of 1.4 nm with a diamond￾like structure separated by two SiOxCy tetrahedra (x = 0), whereas the 100 grade fibres with 16 wt.% of oxygen had a lower crystallinity. The Nicalon 200 grade fibre was composed of 55 wt.% β-SiC grains, 40% of an intergran￾ular phase with a mean composition of SiO1.15C0.85 and 5% of randomly oriented free carbon aggregates, 1 nm in size. C002 lattice fringe images showed small stacks of two fringes around 0.7 nm in size suggesting that the basic structural unit (BSU) was a face-to-face associa￾tion of aromatic rings, called dicoronenes, in which the hydrogen-to-carbon atomic ratio is 0.5. With such a model for the microstructure of the fibre, a porosity level of more than 2% was calculated [23]. Other authors pro￾posed that the intergranular phase should be written as SiOxC1−x/2 which suggests that the composition could vary continuously from SiC to SiO2 as the oxygen concen￾tration varied [24]. This gave a composition by weight of 56% SiC, 10% C and 34% SiO1.12C0.44. 828