M. Verdenelli et al /Journal of the European Ceramic Society 23(2003)1207-1213 121 Fig. 6. Scanning electron micrographs of the surface of the coated fibres annealed at 500oC speed v, the density p and the viscosity n of the sols Hi-Nicalon fibres, coated with a 0.5 um film with both following Eq.(1), where g is the gravity acceleration systems and uncoated fibres, treated at 600 and 1200C (9. ms-)and k is a correction factor(k=0.). 12 for I h in air were tensile tested. Results are reported in Z=k[(nD)/(pg)22 ( fibre treated at 1200 C in the air(2170 MPa)while it tained relatively high for a fibre coated with This was demonstrated mainly on macroscopic sub he oxide(2970 MPa)compared to the results obtained strates. In our case the substrates were microscopic, on the commercial fibre(3000 MPa). The tensile mod- with a particular shape (fibre). Theoretical and few ulus of the coated fibres after thermal annealing at experimental works were previously reported concern- 1200C(270 GPa) was very similar to the commercial ng deposition on microscopic fibres taking into account fibres(270 GPa) while it was lower for the uncoated the diameter of the fibre and the surface tension of the fibres (230 GPa). These observations evidenced the solvent which become much more influential than at the protective role of the coatings towards oxidation macroscopic level. 9. 24 In any case it has been shown that reactions. the experimental values do not fit with the predicted ones. The evolution of the viscosity of the alkoxides solution could explain this difference. Another para Table 2 meter that could be considered is the chemical reactivity ible imental mechanical properties of the commercial Hi-Nicalon SiC Experi of the sol and/ or the precursors at the surface of the substrate. However, this hypothesis would necessitate Hi-Nicalon fibre some further investigation Fibre diameter(um) 3.4. Characterization of the mechanical behaviour Tensile modulus(GPa) Tensile strength(MPa) 3000 Density (g cm-) 2.74 Table 2 summarizes the thermo-mechanical pro 1.l operties Elongation(%) measured on the commercial sic hi-Nicalon fibres Thermal expansion coefficient (C-) 4.6x10-6speed , the density and the viscosity  of the sols following Eq. (1),where g is the gravity acceleration (9.806 m.s2 ) and k is a correction factor (k=0.1).12 l ¼ k½ ð Þ  = ð Þ g 1=2 ð1Þ This was demonstrated mainly on macroscopic sub￾strates. In our case the substrates were microscopic, with a particular shape (fibre). Theoretical and few experimental works were previously reported concern￾ing deposition on microscopic fibres taking into account the diameter of the fibre and the surface tension of the solvent which become much more influential than at the macroscopic level.9,24 In any case it has been shown that the experimental values do not fit with the predicted ones. The evolution of the viscosity of the alkoxides solution could explain this difference. Another para￾meter that could be considered is the chemical reactivity of the sol and/or the precursors at the surface of the substrate. However,this hypothesis would necessitate some further investigations. 3.4. Characterization of the mechanical behaviour Table 2 summarizes the thermo-mechanical properties measured on the commercial SiC Hi-Nicalon fibres. Hi-Nicalon fibres,coated with a 0.5 mm film with both systems and uncoated fibres,treated at 600 and 1200
C for 1 h in air were tensile tested. Results are reported in Table 3. The tensile strength fell drastically down for a fibre treated at 1200
C in the air (2170 MPa) while it was maintained relatively high for a fibre coated with the oxide (2970 MPa) compared to the results obtained on the commercial fibre (3000 MPa). The tensile mod￾ulus of the coated fibres after thermal annealing at 1200
C (270 GPa) was very similar to the commercial fibres (270 GPa) while it was lower for the uncoated fibres (230 GPa). These observations evidenced the protective role of the coatings towards oxidation reactions. Fig. 6. Scanning electron micrographs of the surface of the coated fibres annealed at 500
C. Table 2 Experimental mechanical properties of the commercial Hi-Nicalon SiC fibres Properties Hi-Nicalon fibre Fibre diameter (mm) 14 Tensile modulus (GPa) 270 Tensile strength (MPa) 3000 Density (g cm3 ) 2.74 Elongation (%) 1.1 Thermal expansion coefficient (
C1 ) 4.6106 M. Verdenelli et al. / Journal of the European Ceramic Society 23 (2003) 1207–1213 1211