As fabricated material fibre microetructure

The greater density and strength o f the composite allowed a better quality o f finish to be achieved during SEM specimen preparation, and a 'tree ring' effect was observed towards the carbon core o f fibres in some specimens; this is shown in fig.5.11, which is a high magnification view o f the carbon c o re / silicon carbide sheath region o f a fibre. The pyrolytic carbon layer, which is applied t o the carbon core prior to deposition o f the CVD SiC layers can also be seen in fig.5.11, an d has a thickness o f approximately 1.5pm.

Radial pores were observed in the o u ter SiC layer o f the fibre, the pore size being <5pm. The pores appeared to be confined to the outer region o f the fibre, clearly seen in backscattered electron imaging (fig.5.12). Fig.5.13 is a TEM view o f the SiC sheath around the carbon core o f the fibre, showing the columnar grain structure. The faulted appearance o f the grains is typical o f silicon carbide, particularly grown by C V D, and is due to stacking faults and twins in the 3 C silicon carbide polytype, and the presence of other polytypes o f SiC. The creep behaviour o f SCS-6 has been attributed to either free silicon [59] or free carbon [73] in low concentrations, but no evidence o f either was observed in this work.

fíg.5.13 SiC sheath, showing radial columnar grain structure

5X3 A s fabricated material - interface microstructure

The fibre/m atrix interface was studied using baclcscattered electron imaging and X-ray m apping in the SEM, and bright field imaging, electron diffraction and EDS in the TEM . Back scattered imaging o f the interface region showed that specimens containing A ^ O y Y20 3 pressed at 1700°C had a thin reaction lay e r between the fibre and matrix (fig.5.14), whereas t h e MgO containing sample pressed a t the same temperature had a thicker reaction lay er (fig3 .15). The interface region w as mapped using X-rays, for carbon, nitrogen, oxygen, silicon and sintering additives, showing the reaction zone between the matrix a n d the outer layer for samples where th e zone was thick enough. Fig. 3.16 shows the X -ray maps obtained for 5wt% M gO additive SRBSN pressed at 1700°C. H ie m agnesium m ap shows a trace o f magnesium in the fibre sheath; this is due to the background radiation and the strong silicon peak (1.739 keV) producing spurious

K g J .1 7 shows similar m ap for a YjOj/AIjOj sam ple pressed at 1700°C in which there w as less detectable reaction between the fibre and m atrix. A feature o f both the X-ray m ap s and the backscattered electron images is the silicon rich zone between the outer layers o f the fibre and in the reaction zone. There is a corresponding reduction in the carbon concentration, indicated by a dark region between the carbon rich layers and (particularly in the M gO additive sample) in the reaction zone.

fig J . 17 Backscattered image and corresponding X -ray maps for carbon, silicon and nitrogen for a YjOj/AIjC^ SRBSN sample pressed at 1700°C.

A view o f the outer fibre layers is shown in fig.5.18. T h e inner layer has separated from the silicon carbide sheath, which was often seen when specim en were mechanically tested (see chapter 6). The twin carbon layers can be seen, the o uter layer reaction zone being clearly defined. The silicon rich layer between the carbon layers can be seen as a well defined boundary, and can be compared to the backscattered SEM images, where there is a sharp division between the inner and outer layers.

F ig J . 19 show s the outer layers at higher magnification, with silicon n itride grains attached to the o u ter layer. The carbon layers and the reaction zone have sm all particles within them, th e density o f particles being higher in the reaction zone. T h e particles were found to be crystalline as they showed diffraction contrast when the specim en was tilted. Diffraction patterns from the carbon rich layers show the existence o f silicon carbide in the form o f fine crystallites (diffraction rings formed), with an enrichment o f silicon carbide in the reaction zone (stronger diffraction rings than the unreacted part o f the layers). The diffraction patterns also reveal the presence o f graphitic carbon, with a pr eferred orientation, the patterns in the outer layer being more pronounced. The silicon carbide particles distributed within the unreacted part o f the layers were produced during the final chemical vapour deposition o f carbon on the surface of the fibres. D iffusion o f silicon from the matrix has enriched the silicon carbide concentration in the reaction zone.

fig.5.19 Outer fibre layers, showing reaction layer, outer carbon layer, silicon rich zone, inner carbon layer and diffraction patterns.

5 3 E FF E C T O F OX IDATIO N ON M IC R O ST R U C T U R E

Samples o f the 6%Y203/2%A1203 additive composite were subjected to oxidation at 1000°C, 1200°C and 1400°C for 100 and 1000 hours, sectioned, polished and examined in the SEM (the mechanical behaviour and fracture surfaces o f the samples are described in chapter 6). 100 hours oxidation at 1000°C, 1200°C and 1400°C appeared to have little visible effect on the carbon rich layers; exam ples o f these are shown in fig.5.20,5.21 and

fig J .20 SiC/SRBSN sample oxidised for 100 hours at 1000°C.

fig5.22 SiCTSRBSN ¡ample oxidised for 100 hours at 1400°C

F o r oxidation times o f 1000 hours, damage to the composite w as m ore obvious; fig.5.23 show s the interfacial region in a sample oxidised for 1000 hours at 1000°C. The carbon rich layers have been removed, however both die silicon rich zone b etw een the layers and the reaction zone between the outer layer and the matrix remain. T he cen tral carbon core is a lso removed during prolonged oxidation, a s shown in fig.5.24, w hich is p art o f the same