1482 Ph. Colomban, M. Wey Fig. 10(a), and the use of four zirconium propox- might not be sensitive to the thermal cycling, but ide infiltrations followed by one infiltration with the low rigidity of their matrix prevents their use aluminium-silicon ester appeared an effective under tensile-compressive cycling as established combination both for the fabrication of the com for the SiC matrix composit posites(handling in air)and for strength. It is post-infiltrated oxide matrix composite and CVi noticed that the strength at room temperature is Sic matrix composite exhibit the same mechanical ower than that at high temperatures. Self-healing strengths could occur at high temperature between the aluminosilicate interphases derived from the(alu minium) silicon alkoxide precursor at high tem- 4 Conclusions peratures. A typical tensile stress-strain plot recorded at room temperature is given in This novel processing route using the liquid infil- b)and the hysteresis indicates the load tration cycles, i.e. first using a suspension of a ing-unloading cycle. The same phenomenon has submicron alumina powder, then using an alk- been observed for SiC matrix composites prepared oxide (or any liquid precursor) offers several using the pyrolysis of infiltrated polysilazane pre- advantages: controlling the matrix shrinkage cursor in the 3D composite. B, I9 On the other hand, during sintering to avoid matrix segmentation in the Sic matrix composites processed using the 3D preform, controlling the interparticle chemical CVI route do not exhibit such hysteresis. The diff- bonding to strengthen the interparticle bonding erence can be related to the meso/microporosity of without shrinkage and simplifying the process the matrix in the composites derived from pyrol posites have a ysed metal-organic reagent from which the solids higher strength at high temperatures is believed to yield is low(25-30% for alkoxides). The homo- be the reaction between the aluminosilicate inter geneity of this meso/micro-porosity can help to phase derived from the alkoxide by the last post ccommodate, by microcracking, the thermal infiltration and the primary alumina matrix, the stress from thermal expansion mismatch but low- matrix shrinkage being simultaneously hindered ers the Youngs modulus. For instance, the tensile by the initial post-infiltration of a zirconia inter Young's modulus of post-infiltrated alumina granular pha matrix composites, prepared from the same 3D fibre preform, ranges between 20 and 30 GPa whereas that of the SiC matrix composite pro- Acknowledgement cessed by the CVI route is 75 GPa. Thus, the mechanical properties of CVI processed compo- Thanks are due to the Societe Europeenne de sites are significantly sensitive to thermal cycling. Propulsion(SEP), Le Haillan, 33165 Saint Medard Composites prepared from liquid oxide precursor en Jalles, for supporting M. Wey financially D 4Al+4Zr 4 Zr+ AlOSi 4 Zr +AlOS 3⊙ 60}1000 8o/ 4 p4zr+TEoS. 401 A293 1000 1200 T Fig. 10. Room-temperature flexural strength versus deformation of a composite before( CAlo, )and after post-infiltration and in situ hydrolysis-polycondensation and subsequent sintering at various temperatures: four infiltrations with zirconium i-propoxide (4Zr), with additional TEOS (4Zr+TEOS) or aluminium-silicon ester (4Zr+AlOSi) post-infiltration or alternatively, zirconium i- propoxide and aluminium (4A1+47r)post -infiltrations have been suc ively made. Black circles correspond recorded at the temperatur y the abscissa, in argon atmosphere, after one hour stabilization at high temperature for 4Zr+AlOSi post-infiltrated es. The corresponding room-temperature tensile strain-stress plot is given for a 4Zr+ AlOSi 1000.C(three loading/unloading cycles have been made before the rupture)1482 Ph. Colomban, A4. Wey Fig. 10(a), and the use of four zirconium propox￾ide infiltrations followed by one infiltration with aluminium-silicon ester appeared an effective combination both for the fabrication of the com￾posites (handling in air) and for strength. It is noticed that the strength at room temperature is lower than that at high temperatures. Self-healing could occur at high temperature between the aluminosilicate interphases derived from the (alu￾minium) silicon alkoxide precursor at high tem￾peratures. A typical tensile stress-strain plot recorded at room temperature is given in Fig. 10(b) and the hysteresis indicates the load￾ing-unloading cycle. The same phenomenon has been observed for SIC matrix composites prepared using the pyrolysis of infiltrated polysilazane pre￾cursor in the 3D composite.8,‘g On the other hand, the Sic matrix composites processed using the CVI route do not exhibit such hysteresis. The diff￾erence can be related to the meso/microporosity of the matrix in the composites derived from pyrol￾ysed metal-organic reagent from which the solids yield is low (-25-30% for alkoxides). The homo￾geneity of this mesa/micro-porosity can help to accommodate, by microcracking, the thermal stress from thermal expansion mismatch but low￾ers the Young’s modulus. For instance, the tensile Young’s modulus of post-infiltrated alumina matrix composites, prepared from the same 3D fibre preform, ranges between 20 and 30 GPa, whereas that of the SIC matrix composite pro￾cessed by the CVI route is 75 GPa.20 Thus, the mechanical properties of CVI processed compo￾sites are significantly sensitive to thermal cycling. Composites prepared from liquid oxide precursor 160 F 0 4A1+4Zr a 720. : ; 60 1000 1200 VW might not be sensitive to the thermal cycling, but the low rigidity of their matrix prevents their use under tensile-compressive cycling as established for the Sic matrix composites.20 Nevertheless, post-infiltrated oxide matrix composite and CVI SIC matrix composite exhibit the same mechanical strengths. 4 Conclusions This novel processing route using the liquid infil￾tration cycles, i.e. first using a suspension of a submicron alumina powder, then using an alk￾oxide (or any liquid precursor) offers several advantages: controlling the matrix shrinkage during sintering to avoid matrix segmentation in 3D preform, controlling the interparticle chemical bonding to strengthen the interparticle bonding without shrinkage and simplifying the process. The mechanism whereby composites have a higher strength at high temperatures is believed to be the reaction between the aluminosilicate inter￾phase derived from the alkoxide by the last post￾infiltration and the primary alumina matrix, the matrix shrinkage being simultaneously hindered by the initial post-infiltration of a zirconia inter￾granular phase. Acknowledgement Thanks are due to the SociCtC Europeenne de Propulsion (SEP), Le Haillan, 33165 Saint MCdard en Jalles, for supporting M. Wey financially. 80 b 4 Zr +AIOSi GO_ tooo”c .2 .4 .6 Strain(o& Fig. 10. Room-temperature flexural strength versus deformation of a composite before (CAl,Os) and after post-infiltration and in situ hydrolysis-polycondensation and subsequent sintering at various temperatures: four infiltrations with zirconium i-propoxide (4Zr), with additional TEOS (4Zr+TEOS) or ahuninium-silicon ester (4Zr+AlOSi) post-infiltration or alternatively, zirconium i￾propoxide and aluminium s-butoxide (4A1+4Zr) post-infiltrations have been successively made. Black circles correspond to data recorded at the temperature given by the abscissa, in argon atmosphere, after one hour stabilization at high temperature for 4Zr+AlOSi post-infiltrated composites. The corresponding room-temperature tensile strain-stress plot is given for a 4Zr+AlOSi composite smtered at 1000°C (three loading/unloading cycles have been made before the rupture)