《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_porous interphase by sol-gel

Availableonlineatwww.sciencedirect.com SCIENCE E噩≈S Journal of the European Ceramic Society 23(2003)1207-1213 www.elsevier.com/locate/jeurceramsoc Sol-gel preparation and thermo-mechanical properties of porous xAlO3-ySiO coatings on SiC Hi-Nicalon fibres Martine Verdenelli, Stephane Parola Fernand Chassagneux, Jean-Marie Letoffe, Henri vincent. Jean-Pierre Scharff. Jean bouix Laboratoire des Multimateriaux ef Interfaces UMR CNRS 5615, Universite Claude bernard Lyon 1, 69622 Villeurbanne cedex, france Received 15 June 2002: received in revised form 30 August 2002: accepted 7 September 2002 mixed aluminium silica oxides were elaborated by the sol-gel process from aluminium tri-sec-butoxide and tetraethylorthosilicate on Hi-Nicalon fibres. The porosity was generated by addition of a surfactant, namely cetyl- trimethylammonium bromide(CTAB). SiC Hi-Nicalon fibres were coated by the dip-coating technique. After annealing in air(500- 1200C)crack-free coatings were observed, with a thickness in the range 100-1000 nm. The fibres were tensile tested and results were analysed by the Weibull statistic. They showed good mechanical properties compared to the commercial fibres. The systems were characterized by thermal gravimetry, differential scanning calorimetry, X-ray diffraction, BET and scanning electron micro- scopy. The powders obtained in the same conditions as the coatings were highly porous with surface areas in the range 540-150 m-/ g depending on the annealing temperature(400-1000oC) C 2002 Elsevier Science Ltd. All rights reserved Keywords: AlO3; Composites; Porosity; SiC fibres; Sol-gel processes; Sio 1. ntroduction of controling the porosity by adding various templates were of a great interest for our purpose. Several templates Fibre-reinforced ceramic matrix composites (CMCs) can be used to create the porosity in the oxide, the chelat are of great interest for applications under high thermal ing ligand acetylacetonate(acac-)and the ionic surfactant conditions or mechanical constraints(e. g. vehicles, aero- cetyltrimethylammonium bromide C16H33N(CH3)3Br space). The concept of porous and/or oxide interphase for (CTAB) for respectively micro-and meso-porosity are the deflection of matrix cracks has been known for several reported here. Silica doped alumina systems were selec- years(Fig. 1).-However, it has never been demonstrated ted for their high temperature of crystallization and whether a such interphase could work or not. More melting, in order to maintain the porosity even at high recently it was reported that oxide coatings can act as a temperature. Two binary mixtures are reported here, 3 diffusion barrier and provide, for example, suitable pro- Al2O 2 SiO(1)and a 10%(in weight) SiO2 doped tection against oxidation or corrosion reactions. -I In this alumina(2), which, according to the equilibrium phase work, the elaboration and characterization of oxide inter- diagram should give respectively the mullite phase phase with controlled porosity on Hi-Nicalon SiC fibres Al6Si2O13, and a solid solution xAl2O3-ySiO2 which re reported together with their mechanical behavior leads to a mixture of two phases a-Al2O3(corundum Several methods can be used to elaborate thin films and the mullite on fibres, including chemical vapour deposition(CVD puttering and the sol-gel process. Among the numerous advantages of the sol-gel process compared to conven- 2. Experimental procedure tional methods, the low temperature and the possibility 2. 1. Sol preparation Corresponding author. Tel. + 33-472-448-167: fax: 33-472 431-568 All experiments were performed under an inert atmo E-mail address: stephane. parola(@ univ-lyonl fr(S. Parola) sphere using standard Schlenk techniques. 2-Propanol 0955-2219/03/S. see front matter C 2002 Elsevier Science Ltd. All rights reserved. PII:S0955-2219(02)00296-0

Sol-gel preparation and thermo-mechanical properties of porous xAl2O3–ySiO2 coatings on SiC Hi-Nicalon fibres Martine Verdenelli,Stephane Parola*,Fernand Chassagneux,Jean-Marie Le´toffe´, Henri Vincent,Jean-Pierre Scharff,Jean Bouix Laboratoire des Multimate´riaux et Interfaces UMR CNRS 5615, Universite´ Claude Bernard Lyon 1, 69622 Villeurbanne cedex, France Received 15 June 2002; received in revised form 30 August 2002; accepted 7 September 2002 Abstract Porous thin films of mixed aluminium silica oxides were elaborated by the sol-gel process from aluminium tri-sec-butoxide and tetraethylorthosilicate on Hi-Nicalon fibres. The porosity was generated by addition of a surfactant,namely cetyl￾trimethylammonium bromide (CTAB). SiC Hi-Nicalon fibres were coated by the dip-coating technique. After annealing in air (500– 1200
C) crack-free coatings were observed,with a thickness in the range 100–1000 nm. The fibres were tensile tested and results were analysed by the Weibull statistic. They showed good mechanical properties compared to the commercial fibres. The systems were characterized by thermal gravimetry,differential scanning calorimetry,X-ray diffraction,BET and scanning electron micro￾scopy. The powders obtained in the same conditions as the coatings were highly porous with surface areas in the range 540–150 m2 / g depending on the annealing temperature (400–1000
C). # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Al2O3; Composites; Porosity; SiC fibres; Sol-gel processes; SiO2 1. Introduction Fibre-reinforced ceramic matrix composites (CMCs) are of great interest for applications under high thermal conditions or mechanical constraints (e.g. vehicles,aero￾space). The concept of porous and/or oxide interphase for the deflection of matrix cracks has been known for several years (Fig. 1).15 However,it has never been demonstrated whether a such interphase could work or not. More recently it was reported that oxide coatings can act as a diffusion barrier and provide,for example,suitable pro￾tection against oxidation or corrosion reactions.611 In this work,the elaboration and characterization of oxide inter￾phase with controlled porosity on Hi-Nicalon SiC fibres are reported together with their mechanical behavior. Several methods can be used to elaborate thin films on fibres,including chemical vapour deposition (CVD), sputtering and the sol-gel process. Among the numerous advantages of the sol-gel process compared to conven￾tional methods,12 the low temperature and the possibility of controling the porosity by adding various templates were of a great interest for our purpose. Several templates can be used to create the porosity in the oxide,the chelat￾ing ligand acetylacetonate (acac) and the ionic surfactant cetyltrimethylammonium bromide C16H33N(CH3)3Br (CTAB) for respectively micro- and meso-porosity are reported here. Silica doped alumina systems were selec￾ted for their high temperature of crystallization and melting,in order to maintain the porosity even at high temperature. Two binary mixtures are reported here,3 Al2O3–2 SiO2 (1) and a 10% (in weight) SiO2 doped alumina (2),which,according to the equilibrium phase diagram should give respectively the mullite phase Al6Si2O13,and a solid solution xAl2O3–ySiO2 which leads to a mixture of two phases a-Al2O3 (corundum) and the mullite. 2. Experimental procedure 2.1. Sol preparation All experiments were performed under an inert atmo￾sphere using standard Schlenk techniques. 2-Propanol 0955-2219/03/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(02)00296-0 Journal of the European Ceramic Society 23 (2003) 1207–1213 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Tel.: +33-472-448-167; fax: +33-472- 431-568. E-mail address: stephane.parola@univ-lyon1.fr (S. Parola).

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 metal

and 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

M. Verdenelli et al /Journal of the European Ceramic Society 23(2003)1207-1213 1209 alkoxides was recently reviewed by Turova and co- workers. 3 Acetylacetone was used as chelating agent to stabilize the aluminium precursor and to prevent pre- cipitation. The best results were obtained with a molar 00°C ratio AI(OR)3: acacH=1: I A cationic surfactant(CTAB) was used to create the mesoporosity in the inorganic network Self assembly of the surfactant due to electrostatic interactions with the 1300°C inorganic precursors allow the formation of rod-like micelles 20-23 Association of the cationic head of the surfactant with anionic aluminosilicates lead to lamellar 1200°C phase which tend to form hexagonal mesophase as polymerisation of the silicates proceeds. Further elim l100°C ination of the surfactant during thermal annealing gen- erates the porosity in the inorganic network. 3. 2. Characterization of the powders 2 theta(deg) X-ray diffraction (XRD), thermal (TGA, SDTA DSC)and thermo-mechanical (TMA) analysis were the powders issued from the complete hydrolysis of the solution. The results of the X-ray dif- fraction experiments are presented in the Fig. 2. The 1400°C sample 1(Fig. 2a) started to crystallise between 1000 and 1100C, in the pure mullite Al6Si2O13 phase Crys- tallisation for the sample 2(Fig. 2b) started higher, between 1200 and 1300C. The main phase was the 300°C A-Al2O3(corundum)and the secondary phase appeared to be the mullite. TGa and dSc were performed to investigate the behavior during the thermal treatments 1200°C TGA and DSC analysis performed on samples 1 and 2 showed approximately the same behavior with the pre- 60 60 70 cursors complete decomposition in the same tempera 2 theta(deg) ture range(Fig 3a, b) The TGA(Fig. 3a) showed complete elimination of the organics between 100 and 600 oC with about 70- Fig. 2. X-ray difiraction patterns on the powders issued from com- plete hydrolysis of the precursors for samples 1(a) and 2(b)and 80%loss of mass. The DSC results(Fig. 3b)showed the heated at various temperatures (O: mullite, * al-Al2O3 endothermic melting point of the CtAB at 100C fol lowed by the melting point of the aluminium acety % exo 50100150200250300350400450C Fig 3. TGA/SDTA (a)and DSC(b) for samples I and 2

alkoxides was recently reviewed by Turova and co￾workers.13 Acetylacetone was used as chelating agent to stabilize the aluminium precursor and to prevent pre￾cipitation. The best results were obtained with a molar ratio Al(OR)3:acacH=1:1. A cationic surfactant (CTAB) was used to create the mesoporosity in the inorganic network. Self assembly of the surfactant due to electrostatic interactions with the inorganic precursors allow the formation of rod-like micelles.2023 Association of the cationic head of the surfactant with anionic aluminosilicates lead to lamellar phase which tend to form hexagonal mesophase as polymerisation of the silicates proceeds. Further elim￾ination of the surfactant during thermal annealing gen￾erates the porosity in the inorganic network. 3.2. Characterization of the powders X-ray diffraction (XRD),thermal (TGA,SDTA, DSC) and thermo-mechanical (TMA) analysis were performed on the powders issued from the complete hydrolysis of the solution. The results of the X-ray dif￾fraction experiments are presented in the Fig. 2. The sample 1 (Fig. 2a) started to crystallise between 1000 and 1100
C,in the pure mullite Al6Si2O13 phase. Crys￾tallisation for the sample 2 (Fig. 2b) started higher, between 1200 and 1300
C. The main phase was the a-Al2O3 (corundum) and the secondary phase appeared to be the mullite. TGA and DSC were performed to investigate the behavior during the thermal treatments. TGA and DSC analysis performed on samples 1 and 2 showed approximately the same behavior with the pre￾cursors complete decomposition in the same tempera￾ture range (Fig. 3a,b). The TGA (Fig. 3a) showed complete elimination of the organics between 100 and 600
C with about 70– 80% loss of mass. The DSC results (Fig. 3b) showed the endothermic melting point of the CTAB at 100
C fol￾lowed by the melting point of the aluminium acetyl￾Fig. 2. X-ray diffraction patterns on the powders issued from com￾plete hydrolysis of the precursors for samples 1 (a) and 2 (b) and heated at various temperatures (O:mullite, :a-Al2O3). Fig. 3. TGA/SDTA (a) and DSC (b) for samples 1 and 2. M. Verdenelli et al. / Journal of the European Ceramic Society 23 (2003) 1207–1213 1209

M. Verdenelli et al /Journal of the European Ceramic Society 23(2003)1207-1213 acetonate Al(acac)3 at 194C. Decomposition of the temperature(Fig. 5). The results were very similar for precursors occurred at about 210C both samples. They showed a swell which was due to the However, looking closely to the results one can see formation and evolving of the gas phase during the hat the dsc curve for sample 1 showed a broad peak decomposition of the organics around 250-300 oC. at 250 oC, which was spread over a larger range of Then, a contraction was observed for the two samples temperature than sample 2. SDTA results were con-(10-16% linear for, respectively, samples 1 and 2). The sistent with this observation, with an endothermic signal density of the residues was compatible with a very high at 230C, corresponding to the inflexion point of the porous volume, which was consistent with the SEM loss of mass observed in tga characterization. Above 800C, mainly closed macro- An exothermic reaction (AH=300 Jg)was pores were evidenced, with almost no contribution to observed in the range 300-500 oC for 1 and 300-400oc the surface area for 2 corresponding to an oxidation of the latest organic residues. One can also notice on the dsc or SDTA 3.3. Characterization of the coatings curves that the broad exothermic peak following the endothermic phenomena was shifted towards the The coatings elaborated on the fibres in our condi- higher temperature for sample 1 (up to 500C) than tions were usually adherent and crack-free. The thick ple 2. nesses were in the range 0.5-l um depending on the The surface areas were estimated by the Bet method dipping parameters. Cracks were observed when rapid and the results are reported in Fig 4. The surface area thermal annealing(300-500C) was applied. Therefore was relatively high for both systems, even at high tem- drying at 120 oC between the layers and thermal peratures(800 oC). Some differences can be noticed annealing with a rate of 5C/min were performed on between the two samples. The sample 1 (mullite) the samples to prevent cracking. The SEM character showed that the surface area increased between 400 and isations are presented on the 3A1203-2SiO2 composition 600C with a maximum of 510 m-g, while the max-(1)but similar results were observed with the oxides imum of 540 m-g was reached at 400C for the mixture(2). Figs 6 and 7 show typical scanning electron sample 2(Al2O3/mullite). The explanation could be that micrograph for the Al-Si-O coatings. The surface was ne organics were completely removed form the pores very homogeneous and smooth, without apparent defect earlier for sample 2(400C)than for sample 1(600C).(Fig 6a). For a fibre annealed at 500oC in the air, the For both systems the surface area remained high at distribution of the mesopores at the surface looked very 800C(310-370 m'g)and even at 1000C for the regular, and the size of the mesopores(50 nm)was oxides mixture 2(170 m2g). Above 1000C and nearly monodisperse(Fig. 6b). The observation of the 1200C for, respectively, 1 and 2, the surface de ecrease cross-section of a fibre annealed at 1200 oC for 1 h in drastically due to the crystallization of the respective the air showed that the oxide particles size was 50 nr oxides as shown by powder X-ray diffraction and by the(Fig. 7a, b). The pore distribution evidenced on the sur exothermic drift starting at 650C in the SDTA. The face of this fibre was much less homogeneous with sizes surface area decreased less rapidly for 2 because of the of about 100-200 nm(Fig. 7c) highest crystallization temperature of alumina. The thicknesses of the coatings were estimated either The thermo-mechanical analysis(TMA)was used to directly from scanning electron micrographs on the fibres investigate the mechanical behavior as a function of the cross-sections or using laser interferometer. As previously mentioned, the dipping time was one of the most influ ential parameters on the thickness of the films. Usual thickness of the film can be correlated to the withe a300 80o 1000 1200 Termperature〔C Fig. 4. Surface area(BET) for both samples I(A)and 2(.)depend- ing on the temperature Fig. 5. Thermomechanical analysis for samples I and 2

acetonate Al(acac)3 at 194
C. Decomposition of the precursors occurred at about 210
C. However,looking closely to the results one can see that the DSC curve for sample 1 showed a broad peak at 250
C,which was spread over a larger range of temperature than sample 2. SDTA results were con￾sistent with this observation,with an endothermic signal at 230
C,corresponding to the inflexion point of the loss of mass observed in TGA. An exothermic reaction (
H=300 J g1 ) was observed in the range 300–500
C for 1 and 300–400
C for 2 corresponding to an oxidation of the latest organic residues. One can also notice on the DSC or SDTA curves that the broad exothermic peak following the endothermic phenomena was shifted towards the higher temperature for sample 1 (up to 500
C) than for sample 2. The surface areas were estimated by the BET method and the results are reported in Fig. 4. The surface area was relatively high for both systems,even at high tem￾peratures (800
C). Some differences can be noticed between the two samples. The sample 1 (mullite) showed that the surface area increased between 400 and 600
C with a maximum of 510 m2 g1 ,while the max￾imum of 540 m2 g1 was reached at 400
C for the sample 2 (Al2O3/mullite). The explanation could be that the organics were completely removed form the pores earlier for sample 2 (400
C) than for sample 1 (600
C). For both systems the surface area remained high at 800
C (310–370 m2 g1 ) and even at 1000
C for the oxides mixture 2 (170 m2 g1 ). Above 1000
C and 1200
C for,respectively, 1 and 2,the surface decreased drastically due to the crystallization of the respective oxides as shown by powder X-ray diffraction and by the exothermic drift starting at 650
C in the SDTA. The surface area decreased less rapidly for 2 because of the highest crystallization temperature of alumina. The thermo-mechanical analysis (TMA) was used to investigate the mechanical behavior as a function of the temperature (Fig. 5). The results were very similar for both samples. They showed a swell which was due to the formation and evolving of the gas phase during the decomposition of the organics around 250–300
C. Then,a contraction was observed for the two samples (10–16% linear for,respectively,samples 1 and 2). The density of the residues was compatible with a very high porous volume,which was consistent with the SEM characterization. Above 800
C,mainly closed macro￾pores were evidenced,with almost no contribution to the surface area. 3.3. Characterization of the coatings The coatings elaborated on the fibres in our condi￾tions were usually adherent and crack-free. The thick￾nesses were in the range 0.5–1 mm depending on the dipping parameters. Cracks were observed when rapid thermal annealing (300–500
C) was applied. Therefore drying at 120
C between the layers and thermal annealing with a rate of 5
C/min were performed on the samples to prevent cracking. The SEM character￾isations are presented on the 3Al2O3–2SiO2 composition (1) but similar results were observed with the oxides mixture (2). Figs. 6 and 7 show typical scanning electron micrograph for the Al–Si–O coatings. The surface was very homogeneous and smooth,without apparent defect (Fig. 6a). For a fibre annealed at 500
C in the air,the distribution of the mesopores at the surface looked very regular,and the size of the mesopores (50 nm) was nearly monodisperse (Fig. 6b). The observation of the cross-section of a fibre annealed at 1200
C for 1 h in the air showed that the oxide particles size was 50 nm (Fig. 7a,b). The pore distribution evidenced on the sur￾face of this fibre was much less homogeneous with sizes of about 100–200 nm (Fig. 7c). The thicknesses of the coatings were estimated either directly from scanning electron micrographs on the fibres cross-sections or using laser interferometer. As previously mentioned,6 the dipping time was one of the most influ￾ential parameters on the thickness of the films. Usually the thickness l of the film can be correlated to the withdrawal Fig. 4. Surface area (BET) for both samples 1 (~) and 2 () depend￾ing on the temperature. Fig. 5. Thermomechanical analysis for samples 1 and 2. 1210 M. Verdenelli et al. / Journal of the European Ceramic Society 23 (2003) 1207–1213

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-) 4.6x10-6

speed , 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

212 M. Verdenelli et al / Journal of the European Ceramic Society 23(2003)1207-1213 5 um 1 um 1 Fig. 7. Scanning electron micrographs of the cross-section(a, b)and the surface(c)of the coated fibres annealed at 1200C in the ai

Fig. 7. Scanning electron micrographs of the cross-section (a,b) and the surface (c) of the coated fibres annealed at 1200
C in the air. 1212 M. Verdenelli et al. / Journal of the European Ceramic Society 23 (2003) 1207–1213

M. Verdenelli et al / Journal of the European Ceramic Society 23(2003)1207-1213 Table 3 and SrZrO3 coatings on SiC and C-fibers. J. Mater. Sci., 1999, 34 Tensile tests the coated SiC fibres for systems I and 2 after 4031-4037 nnealing at 600 and 1200C 6. Parola, S. Verdenelli, M.. Sigala. C. Scharff, J. P. Velez. K. Veytizou, C, and Quinson, J F. Sol-gel coatings on non-oxide SIC Hi-Nicalon Coated Hi-Nicalon Hi- Nicalon treated at1200° C treated at1200° planar substrates and fibers: a protection barrier against oxida- In aIr tion and corrosion. J. Sol-Gel Sci. Tech (in press). 7. Hashishin, T, Murashita, J, Joyama, A and Kaneko, Y Tensile strength(MPa) 3000 2170 dation-resistant coating of carbon fibers with TiO, b ethod Weibull modulus 8. Aparicio, M. and Duran, A, Yttrium silicate coatings for oxida- tion protection of carbon-silicon carbide compo 9. Gundel, D. B, Taylor, P J and Wawner, F. E, Fabrication of The original mechanical properties were preserved thin oxide coatings on ceramic fibres by a sol-gel technique. J. after the deposition of the oxide. Moreover, at high arer.Sci,1994,29,1795-1800. temperatures in an oxidative atmosphere, the mechan 10. Karlin, S and Colomban, Ph, Micro-Raman study of Sic fibre. ical behaviours were much better with a coated fibre 11. Colomban, Ph. Bruneton, E, Lagrange, J. L and Mouchon, E than with the uncoated ones Sol-gel mullite matrixSiC and-mullite 2D woven fabric compo- sites interphase: elaboration and properties. Eur. Ceram. Soc., 4. Conclusion 12. Brinker. C. J and Scherer. G. W.. Sol-Gel Science: The Physics ad Chemistry of Sol-Gel Processing. Academic Press, San Diego, Thin films of mixed silicon/aluminium oxides on SiC 13. Turova, N Ya, Turevskaya, E. P, Kessler, V.G. and Yanovs- Hi-Nicalon fibres were successfully elaborated using the kaya, M. I, The Chemistry of Metal Alkoxides. Kluwer Academic Publishers. Norwell MA. 2001 oI-gel process. The coated fibres were stable in an oxi- 14. Bradley, D. C, Mehrotra, R C and Gaur, D. P, Metal alk- lative environment even at high temperatures(1200C) oxides. Academic Press. London. 1978 The protective role of the oxides was thus demon 15. Sanchez, C, Livage, J, Henry, M. and Babonneau, F, Chemical strated, as well as the good mechanical behavior of the odification of alkoxide precursors. J. Non-Cryst. Solids, 1988, fibre/coating system. These porous coatings are there 100,65-76 fore potential interphases for applications in Ceramic Matrix Composites and micro-composites SiC/Oxide/ S, Tin dioxide thin films from Sn(Iv) modified alkoxides- synthesis and structural characterization of Sn(OEt)2(acac)2 and Sic which are currently under investigation Sn(O)(oEt)o(acac)2. Polyhedron, 2000, 10, 2069-2075. 17. Velez, K, Quinson, J. F and Fenet, B, Modification study of Acknowledgements 18. Nass, R and Schmidt, H, Synthesis of an alumina coating from chelated aluminium alkoxide Non-Cryst. Solids, 1990, 12 The authors wish to thank the cnrs for fina support 19. Patankar, S.N., Weibull distribution as applied to ceramic fibres J. Mater.Sci.Let,1991,10,1176-1 20. Kresge, C.T., Leonowicz, M. E, Roth, w.J., Vartuli, J. C and Beck, J.S. Ordered mesoporous molecular sieves by a liquid- Referen crystal template mechanism. Nature, 1992, 359, 710-712. 21. Monnier, A, Schuth, F, Huo, Q, Kumar, D, Margolese, 1. Carpenter, H. W. and Bohlen, J. w, Fibers coatings for D, Maxwell, R. S, Stucky, G. D, Krishnamurty, M, Petr- matrix composites. Ceram. Eng. Sci. Proc., 1992. 13(7 ff, P, Firouzi, A, Janicke, M. and Chmelka, B. F, Coop- 6 H.W. Carpenter, J Bohlen and N.s. Steffier, Weak fr erative formation of inorganic-organic interfaces in the fiber coating with unfilled pores for toughening ceramic fiber synthesis of silicate mesostructures. Science, 1993, 261, 1299- matrix composites, US Patent No. 5.221. 578, 1992 1303. 2. Reig, P, Demazeau, G. and Naslain, R, KMg2AISi4O12 phyllo- 2. Murakata, T, Sato. S, Ohgawara, T, Watanabe, T and process using inorganic salts, surfactants as additives. J 3. Tressler, R. E, Recent developments in fibers and interphases for high temperature ceramic matrix composites Composites: Part A 23. Zhou, H.S. Kundu, D. and Honma, I, Synthesis of oriented 4. Cinibulk, M. K. and Hay, R.S., Ter magnetoplumbite 13 fiber-matrix interphase derived from sol-gel fiber coatings. J. Am. 24. Goucher, F.S. and Ward, H, A problem in viscosity: the thick- eram.Soc,1996.795),1233-1246 ness of liquid films formed on solid surfaces under dynami 5. Wurm. R, Dernovsek, O. and Greil, P. Sol-gel derived SrTiO ditions. Phil Mag. 6th Ser., 1922, 44, 1002-1014

The original mechanical properties were preserved after the deposition of the oxide. Moreover,at high temperatures in an oxidative atmosphere,the mechan￾ical behaviours were much better with a coated fibre than with the uncoated ones. 4. Conclusion Thin films of mixed silicon/aluminium oxides on SiC Hi-Nicalon fibres were successfully elaborated using the sol-gel process. The coated fibres were stable in an oxi￾dative environment even at high temperatures (1200
C). The protective role of the oxides was thus demon￾strated,as well as the good mechanical behavior of the fibre/coating system. These porous coatings are there￾fore potential interphases for applications in Ceramic Matrix Composites and micro-composites SiC/Oxide/ SiC which are currently under investigation. Acknowledgements The authors wish to thank the CNRS for financial support. References 1. Carpenter,H. W. and Bohlen,J. W.,Fibers coatings for ceramic matrix composites. Ceram. Eng. Sci. Proc.,1992, 13(7–8),238– 256 H.W. Carpenter,J. Bohlen and N.S. Steffier,Weak frangible fiber coating with unfilled pores for toughening ceramic fiber￾matrix composites,US Patent No. 5.221.578,1992. 2. Reig,P.,Demazeau,G. and Naslain,R.,KMg2AlSi4O12 phyllo￾siloxide as potential interphase material for ceramic matrix com￾posites. J. Mater. Sci.,1997, 32,4195–4200. 3. Tressler,R. E.,Recent developments in fibers and interphases for high temperature ceramic matrix composites. Composites: Part A, 1999, 30,429–437. 4. Cinibulk,M. K. and Hay,R. S.,Textured magnetoplumbite fiber-matrix interphase derived from sol-gel fiber coatings. J. Am. Ceram. Soc.,1996, 79(5),1233–1246. 5. Wurm,R.,Dernovsek,O. and Greil,P.,Sol-gel derived SrTiO3 and SrZrO3 coatings on SiC and C-fibers,J. Mater. Sci.,1999, 34, 4031–4037. 6. Parola,S.,Verdenelli,M.,Sigala,C.,Scharff,J.P.,Velez,K. Veytizou,C.,and Quinson,J.F. Sol-gel coatings on non-oxide planar substrates and fibers: a protection barrier against oxida￾tion and corrosion. J. Sol-Gel Sci. Tech. (in press). 7. Hashishin,T.,Murashita,J.,Joyama,A. and Kaneko,Y.,Oxi￾dation-resistant coating of carbon fibers with TiO2 by sol-gel method. J. Ceram. Soc. Jap. Int. Ed.,1998, 106,4–8. 8. Aparicio,M. and Dura´n,A.,Yttrium silicate coatings for oxida￾tion protection of carbon-silicon carbide composites. J. Am. Ceram. Soc.,2000, 83(6),1351–1355. 9. Gundel,D. B.,Taylor,P. J. and Wawner,F. E.,Fabrication of thin oxide coatings on ceramic fibres by a sol-gel technique. J. Mater. Sci.,1994, 29,1795–1800. 10. Karlin,S. and Colomban,Ph.,Micro-Raman study of SiC fibre￾oxide matrix reaction. Composites Part B,1998, 29B,41–50. 11. Colomban,Ph.,Bruneton,E.,Lagrange,J. L. and Mouchon,E., Sol-gel mullite matrix-SiC and-mullite 2D woven fabric compo￾sites interphase: elaboration and properties. J. Eur. Ceram. Soc., 1996, 16,301–314. 12. Brinker,C. J. and Scherer,G. W., Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. Academic Press,San Diego, 1990. 13. Turova,N.Ya.,Turevskaya,E. P.,Kessler,V. G. and Yanovs￾kaya,M. I., The Chemistry of Metal Alkoxides. Kluwer Academic Publishers,Norwell MA,2001. 14. Bradley,D. C.,Mehrotra,R. C. and Gaur,D. P., Metal Alk￾oxides. Academic Press,London,1978. 15. Sanchez,C.,Livage,J.,Henry,M. and Babonneau,F.,Chemical modification of alkoxide precursors. J. Non-Cryst. Solids,1988, 100,65–76. 16. Verdenelli,M.,Parola,S.,Hubert-Pfalzgraf,L. G. and Lecocq, S.,Tin dioxide thin films from Sn(IV) modified alkoxides— synthesis and structural characterization of Sn(OEt)2(acac)2 and Sn4(O)2(OEt)10(acac)2. Polyhedron,2000, 10,2069–2075. 17. Velez,K.,Quinson,J. F. and Fenet,B.,Modification study of aluminium sec-butoxide by acrylic acid. J. Sol-gel Sci. Technol, 1999, 16,201–208. 18. Nass,R. and Schmidt,H.,Synthesis of an alumina coating from chelated aluminium alkoxides. J. Non-Cryst. Solids,1990, 121, 329–333. 19. Patankar,S. N.,Weibull distribution as applied to ceramic fibres. J. Mater. Sci. Let.,1991, 10,1176–1181. 20. Kresge,C. T.,Leonowicz,M. E.,Roth,W. J.,Vartuli,J. C. and Beck,J. S.,Ordered mesoporous molecular sieves by a liquid￾crystal template mechanism. Nature,1992, 359,710–712. 21. Monnier,A.,Schu¨th,F.,Huo,Q.,Kumar,D.,Margolese, D.,Maxwell,R. S.,Stucky,G. D.,Krishnamurty,M.,Petr￾off,P.,Firouzi,A.,Janicke,M. and Chmelka,B. F.,Coop￾erative formation of inorganic-organic interfaces in the synthesis of silicate mesostructures. Science,1993, 261,1299– 1303. 22. Murakata,T.,Sato,S.,Ohgawara,T.,Watanabe,T. and Suzuki, T.,Control of pore size distribution of silica gel through sol-gel process using inorganic salts,surfactants as additives. J. Mater. Sci.,1992, 27,1567–1574. 23. Zhou,H. S.,Kundu,D. and Honma,I.,Synthesis of oriented meso-structure silica functional thin film. J. Eur. Ceram. Soc., 1999, 19,1361–1364. 24. Goucher,F. S. and Ward,H.,A problem in viscosity: the thick￾ness of liquid films formed on solid surfaces under dynamic con￾ditions. Phil. Mag. 6th Ser.,1922, 44,1002–1014. Table 3 Tensile tests on the coated SiC fibres for systems 1 and 2 after annealing at 600 and 1200
C SiC Hi-Nicalon Hi-Nicalon treated at 1200
C in air Coated Hi-Nicalon treated at 1200
C in air Tensile strength (MPa) 3000 2170 2970 Tensile modulus (GPa) 270 230 270 Weibull modulus 6.9 5.5 4.8 M. Verdenelli et al. / Journal of the European Ceramic Society 23 (2003) 1207–1213 1213