Ph Colomban, M. Wey Examination of the monolith shows a poor infiltrated with a zirconia gel first increases up to egion below the upper surface. After 10 cycles of 1200C and then decreases(Fig. 6(a)). This agrees infiltration-thermal treatment at 1000 C, the open with the lack of shrinkage up to 1300C. the infil porosity is typically 24% using simple capillarity tration yield is a maximum for materials infil- intrusion and 20% using pressure assisted infiltra- trated with aluminium s-butoxide pyrolysed at tion. Full densification is obtained after a 1400c 1200 C ( Fig. 6(b)). On the other hand, the highest thermal treatment for 1 h. Figure 5(c) shows that yield is observed for materials infiltrated with elting from the gel pyrolysis zirconium propoxide pyrolysed below 1100.C are homogeneously dispersed and form particles This phenomenon can be related to the high reac with 0.2 um mean diameter which corresponds tivity of aluminium s-butoxide versus water. The well to the size of interparticle voids. Consequently surface of alumina porous bodies heated below the flexural strength measured at room tempera- 1200oC easily retains water molecules. Uncon ture reaches 550 MPa trolled hydrolysis leads to pore clogging and hence The evolution of the pore volume distribution as decreases the infiltration yield. Comparison of the function of size for various thermal treatments dilatometric traces of the same monolith shows shows that the mean pore size of a monolith post that the linear shrinkage after 1400oC thermal treatment is reduced from 12 to 5% after only one 1300120 cycle of infiltration with zirconium i-propoxide btained 1300C. Figure 7 gives a comparison between the pore volume distribution measured for a compos- ite without and after eight cycles of zirconium 1400c i-propoxide infiltration-hydrolysis-polycondensa tions and 1200 C firing The lack of voids gener ated by matrix cracking in the post-infil pore diameter (pm) o1 alumina matrix composite is straightforward in the pore diameter distribution(Fig. 7) 3.3 Optimization of the post-infiltrated interphase Post- infiltration with a liquid precursor of a very refractory composition allows the prevention of 4.00 interparticle diffusion and hence matrix shrinkage by the formation of an interphase acting as a 200 diffusion barrier. However, this barrier cannot continue because of the low yield of ceramization 1200 Fig. 4 shows cracks between zirconia aggregates Pyrolysis temperature") (0.05 mm)and between scales of aggregates Additional infiltration must be made with a pre- cursor which strengthens the interparticle bond Attempts have been made with titanium alkoxide infiltration. However, titanium ethoxide(propox ide, butoxide,.)infiltration activates the sintering of an alumina monolith and segmentation of the matrix occurs in the composite. Consequently the mechanical properties remain poor. The net-shape temperature (nil expansion/shrinkage)on the dilatometric trace occurs at 1050C and the final linear shrinkage of 1400oC heat-treated rutile pre 1000 13001400 cursor infiltrated monolith reaches 14% instead of PYROLYSIS TEM PERATURElC) 2% for a pure AKP50 monolith. The origin of Fig. 6. Evolution of pore diameter distribution as a function this sintering activation can be found in the easy of sintering temperature for a monolith after four cycles diffusion of titanium in alumina zirconium i-propoxide post-infiltration and in situ hydrolysis- Figure 8 shows the dilatometric trace of alumino- polycondensation (a); cumulative porosity variation for an alumina monolith post-infiltrated with aluminum s-butoxide silicate gel prepared by hydrolysis-polycondensa- (b) and zirconium propoxide()and pyrolysed at various tion of aluminium-silicon ester in vacuo. Marked temperatures( 1: first cycle, 2: second cycle, 3 third cycle, expansion occurs above 1200.C. This phenomenon1480 Ph. Colomban, A4. Wey Examination of the monolith shows a porous region below the upper surface. After 10 cycles of infiltration-thermal treatment at lOOO”C, the open porosity is typically 24% using simple capillarity intrusion and 20% using pressure-assisted infiltra￾tion. Full densification is obtained after a 1400°C thermal treatment for 1 h. Figure 5(c) shows that the oxide particles resulting from the gel pyrolysis are homogeneously dispersed and form particles with O-2 pm mean diameter which corresponds well to the size of interparticle voids. Consequently the flexural strength measured at room tempera￾ture reaches 550 MPa. The evolution of the pore volume distribution as a function of size for various thermal treatments shows that the mean pore size of a monolith post￾Pore diameter (pm) o+--- ’ 1 I I # I 1000 1200 1400 Pyrolysis temperature?C) C) ‘T Q 1000 1100 1200 1300 1400 PYROLYSIS TEMPERATUREW) Fig. 6. Evolution of pore diameter distribution as a function of sintering temperature for a monolith after four cycles of zirconium i-propoxide post-infiltration and in situ hydrolysis￾polycondensation (a); cumulative porosity variation for an alumina monolith post-infiltrated with aluminum s-butoxide (b) and zirconium propoxide (c) and pyrolysed at various temperatures (1: first cycle, 2: second cycle, 3: third cycle, . ..). infiltrated with a zirconia gel first increases up to 1200°C and then decreases (Fig. 6(a)). This agrees with the lack of shrinkage up to 1300°C. The infil￾tration yield is a maximum for materials infil￾trated with aluminium s-butoxide pyrolysed at 1200°C (Fig. 6(b)). On the other hand, the highest yield is observed for materials infiltrated with zirconium propoxide pyrolysed below 1100°C. This phenomenon can be related to the high reac￾tivity of aluminium s-butoxide versus water. The surface of alumina porous bodies heated below 1200°C easily retains water molecules. Uncon￾trolled hydrolysis leads to pore clogging and hence decreases the infiltration yield. Comparison of the dilatometric traces of the same monolith shows that the linear shrinkage after 1400°C thermal treatment is reduced from 12 to 5% after only one cycle of infiltration with zirconium i-propoxide. A net-shape consolidation is obtained up to 1300°C. Figure 7 gives a comparison between the pore volume distribution measured for a compos￾ite without and after eight cycles of zirconium i-propoxide infiltration-hydrolysis-polycondensa￾tions and 1200°C firing. The lack of voids gener￾ated by matrix cracking in the post-infiltrated alumina matrix composite is straightforward in the pore diameter distribution (Fig. 7). 3.3 Optimization of the post-infiltrated interphase precursor Post-infiltration with a liquid precursor of a very refractory composition allows the prevention of interparticle diffusion and hence matrix shrinkage by the formation of an interphase acting as a diffusion barrier. However, this barrier cannot continue because of the low yield of ceramization: Fig. 4 shows cracks between zirconia aggregates (co.05 mm) and between scales of aggregates. Additional infiltration must be made with a pre￾cursor which strengthens the interparticle bond. Attempts have been made with titanium alkoxide infiltration. However, titanium ethoxide (propox￾ide, butoxide, . ..) infiltration activates the sintering of an alumina monolith and segmentation of the matrix occurs in the composite. Consequently the mechanical properties remain poor. The net-shape temperature (nil expansion/shrinkage) on the dilatometric trace occurs at 1050°C and the final linear shrinkage of 1400°C heat-treated rutile pre￾cursor infiltrated monolith reaches 14% instead of 12% for a pure AKPSO monolith. The origin of this sintering activation can be found in the easy diffusion of titanium in alumina. Figure 8 shows the dilatometric trace of alumino￾silicate gel prepared by hydrolysis-polycondensa￾tion of aluminium-silicon ester in vacua. Marked expansion occurs above 1200°C. This phenomenon