M. Verdenelli et al / Journal of the European Ceramic Society 23(2003)1207-1213 porous coating crack free coatings. The coatings were slowly heated at about 120C to eliminate the solvent. Multilayered films were obtained by repeating those operations. The ceratum best quality for the films was obtained when three layers manx were deposited. Finally, annealing of the samples fibre between 200 and 1200C were performed in air. During matn matrix crack the final thermal treatments, the temperature was raise at a rate of 5C/min and the fibres were heated for 2 h before slowly cooling down to room temperature 23. Characterization Fig. I. The concept of porous interphase for the reinforcement of CMCs Thermogravimetric analysis (TGa)was carried out using a Mettler-Toledo TGA/SDTA /851. DSC mea and aluminium tri-sec-butoxide(Aldrich) were distilled surements were performed with a Mettler- Toledo DSC prior to use. Table I lists the compositions of the solu- 820. A TMA/ SDTA 840 Mettler Toledo system was tions of precursors. Aluminium tri-sec-butoxide, used for Thermomechanical analysis. All TGA, DSC tetraethylorthosilicate (TEOs, Prolabo) and cetyl- and TMA experiments were carried out in the air. For trimethylammonium bromide (CTAB, Aldrich) were phase analysis by X-ray powder diffraction (XRD),a dissolved separately in 2-propanol. The TEOS/2-propa- standard Philips Pw 1840 diffractometer with CuKal nol solution was then added to the aluminium pre- was used. Data were collected by step-scanning from 10 cursor. Acetylacetone(Aldrich) which act as chelating to 70(20) with a step size of 0. 020%(20)and I s count ligand-o was used to stabilize the aluminium pre- ing time at each step. Specific surface areas were deter- cursor towards hydrolysis reactions, 18 and to create mined by the BET method at 77 K using N2 as the micro-porosity. The calculated volume of the CTAB adsorptive agent. The morphology of the coated fibres solution was then added. The resulting solution was was observed with a scanning electron microscope stirred for one hour to obtain the Al/Si/O solution used (SEM, Hitachi $800, 15 kv). The cross sections of the for the coating. The solutions were filtered with I um samples were prepared by cutting coated fibres with filters prior to deposition. sharp edge. The tensile strengths of the monofilaments were measured with an Adamel dY22 testing machine 2. 2. Powders elaboration and sol-gel coatings of Sic The crosshead speed was 0. 1 mm min, the load cell fibres was 500 cN and the gauge lengths were 10 mm. Fifty monofilaments were tested for each sample. Before The powders used for the BET measurements and mechanical testing, the monofilament diameter was powder X-ray diffraction were prepared by complete determined by laser interferometry. The results of the hydrolysis of the solutions of precursors with a ratio tensile tests were analysed by the Weibull statistic. (OR)n, filtration, drying and the mal annealing of the powder. The Hi-Nicalon SiC fibres, manufactured by Nippon 3. Results and discussion Carbon (Japan) were selected as substrate. Commercial fibres were desized for 30 min in air at 600C before 3. 1. Elaboration of the films on the SiC fibres deposition. This treatment creates a thermally evolved Sio, coating to enhance the adhesion of the sol-gel The coatings of the fibres were performed using dip- coating. The single fibres were mounted on a specific coating technique. The starting materials for the oxides support before deposition and dip-coated in the solu- were metal alkoxides, namely aluminium tri-sec but tion of precursors, maintained for 5 min and drawn out oxide and tEos (tetraethylorthosilicate). The pre ertically with a withdrawal speed of 300 mm. min-. cursors were mixed with a molar ratio of Al: i=3: 1 or Thermal treatments were optimized in order to obtain 10.5: 1 The aluminium and silicon precursors do not behave Table the same way toward hydrolysis reactions. Aluminium Compositions of the solutions (mol) used for the deposition of the alkoxide are very easily hydrolysable while silicon alk oxides oxides necessitate acid or basic conditions. An alter- Al(OBu°)3 Et)a acach CTAB native in order to prepare a solution with precursors having a more similar reactivity is to decrease the 0.095 0.25 hydrolysis rate of the most hydrolysable species through chemical modification 13-18 Such modification of metaland aluminium tri-sec-butoxide (Aldrich) were distilled prior to use. Table 1 lists the compositions of the solu￾tions of precursors. Aluminium tri-sec-butoxide, tetraethylorthosilicate (TEOS,Prolabo) and cetyl￾trimethylammonium bromide (CTAB,Aldrich) were dissolved separately in 2-propanol. The TEOS/2-propa￾nol solution was then added to the aluminium pre￾cursor. Acetylacetone (Aldrich) which act as chelating ligand1316 was used to stabilize the aluminium pre￾cursor towards hydrolysis reactions17,18 and to create the micro-porosity. The calculated volume of the CTAB solution was then added. The resulting solution was stirred for one hour to obtain the Al/Si/O solution used for the coating. The solutions were filtered with 1 mm filters prior to deposition. 2.2. Powders elaboration and sol-gel coatings of SiC fibres The powders used for the BET measurements and powder X-ray diffraction were prepared by complete hydrolysis of the solutions of precursors with a ratio h=20 (h=[H2O][M(OR)n],filtration,drying and ther￾mal annealing of the powder. The Hi-Nicalon SiC fibres,manufactured by Nippon Carbon (Japan) were selected as substrate. Commercial fibres were desized for 30 min in air at 600
C before deposition. This treatment creates a thermally evolved SiO2 coating to enhance the adhesion of the sol-gel coating. The single fibres were mounted on a specific support before deposition and dip-coated in the solu￾tion of precursors,maintained for 5 min and drawn out vertically with a withdrawal speed of 300 mm. min1 . Thermal treatments were optimized in order to obtain crack free coatings. The coatings were slowly heated at about 120
C to eliminate the solvent. Multilayered films were obtained by repeating those operations. The best quality for the films was obtained when three layers were deposited. Finally,annealing of the samples between 200 and 1200
C were performed in air. During the final thermal treatments,the temperature was raised at a rate of 5
C/min and the fibres were heated for 2 h before slowly cooling down to room temperature. 2.3. Characterization Thermogravimetric analysis (TGA) was carried out using a Mettler-Toledo TGA/SDTA/851e . DSC mea￾surements were performed with a Mettler-Toledo DSC 820. A TMA/SDTA 840 Mettler Toledo system was used for Thermomechanical analysis. All TGA,DSC and TMA experiments were carried out in the air. For phase analysis by X-ray powder diffraction (XRD),a standard Philips PW 1840 diffractometer with CuKa1 was used. Data were collected by step-scanning from 10 to 70
(2
) with a step size of 0.020
(2
) and 1 s count￾ing time at each step. Specific surface areas were deter￾mined by the BET method at 77 K using N2 as adsorptive agent. The morphology of the coated fibres was observed with a scanning electron microscope (SEM,Hitachi S800,15 kV). The cross sections of the samples were prepared by cutting coated fibres with a sharp edge. The tensile strengths of the monofilaments were measured with an Adamel DY22 testing machine. The crosshead speed was 0.1 mm min1 ,the load cell was 500 cN and the gauge lengths were 10 mm. Fifty monofilaments were tested for each sample. Before mechanical testing,the monofilament diameter was determined by laser interferometry. The results of the tensile tests were analysed by the Weibull statistic.19 3. Results and discussion 3.1. Elaboration of the films on the SiC fibres The coatings of the fibres were performed using dip￾coating technique. The starting materials for the oxides were metal alkoxides,namely aluminium tri-sec but￾oxide and TEOS (tetraethylorthosilicate). The pre￾cursors were mixed with a molar ratio of Al:Si=3:1 or 10.5:1. The aluminium and silicon precursors do not behave the same way toward hydrolysis reactions. Aluminium alkoxide are very easily hydrolysable while silicon alk￾oxides necessitate acid or basic conditions. An alter￾native in order to prepare a solution with precursors having a more similar reactivity is to decrease the hydrolysis rate of the most hydrolysable species through chemical modification.1318 Such modification of metal Table 1 Compositions of the solutions (mol) used for the deposition of the porous oxides Sample Al(OBus )3 Si(OEt)4 acacH CTAB 1 6 2 6 1.5 2 1 0.095 1 0.25 Fig. 1. The concept of porous interphase for the reinforcement of CMCs. 1208 M. Verdenelli et al. / Journal of the European Ceramic Society 23 (2003) 1207–1213