Sol-gel control of the matrix net-shape sintering preform is dried stepwise in a controlled humidity invariant fibre preform promotes the segmentation tmosphere at selected temperatures between 40 of the matrix. a typical example is shown in Fig. 2. and 120oC. The drying time is 15 h This phenomenon lowers the mechanical strength Monoliths are processed without the fibre pre- The improvement to the thermomechanical prop form plate, and the powder particles are retained erties can be achieved if (i) net-shape shrinkage is by the sole filter paper, Dried monoliths with 60% obtained, (ii) the porosity is decreased and (iii)the of the theoretical density for a-alumina can be mechanical strength of the bulk matrix is increased without the formation of a strong fibre- The infiltrated preform and monoliths are the matrix interface mally treated at selected temperatures between At temperatures ranging from 1000 to 1400C 1000 and 1400C in a reducing argon atmosphere the sintering of an alumina submicron powder takes for I h in a carbon resistance furnace place by solid-state diffusion at the particle con- tacts. The interposition of an inert second phase 2. 4 Microstructure and mechanical between particles would prevent, or reduce, the formation of the contact for solid-state diffusion Monolith shrinkage was recorded versus tempera- The interparticle voids could also affect shortening ture using an Adamel Lhomargy DI24 apparatus of the interparticle distance during sintering (Instrument SA, 91 Longjumeau, France) with an The high green density of the monolith sample alumina rod and support(heating/cooling rate:(60% of the theoretical density) indicates a highly 5C/min). The pore size and distribution are optimized packing of the AKP50 particles. The meritics Pore Sizer 9,, infiltration using a Micro- open porosity of monoliths thermally treated at mcasured by merc according to the astM 1000oC ranges from 34 to 36% and that of vol. 1201, C699-1983 notice. The validity of the posites with the 3D preform(carbon fibre volume measurement has been verified by calibration of fraction: 28%)ranges from 35 to 38%. This the mercury/composite surface angle by compari- cates that the aK P50 particle packing inside the son with the pore volume determined using N2 preform is rather similar to that in the monolith adsorption/desorption plots carried out with a Figure 3(a) compares the pore volume distribution Micromeritics ASAP2000 instrument Comparable in the monolith alumina sample with that in the results are obtained for a surface angle equal to composite heated at various temperatures. only 145, this value being intermediate between the one family of pores(mean diameter -005 um) is angle values commonly used for Hg/C(155%)and observed in the monolith alumina sample. this Hg/oxide(130%) interfaces. The open porosity has pore range corresponds well to the voids between been calculated according to Archimedes'method. adjacent AKP50 particles. It is noted that the pore The flexural strength was recorded by a three- volume ranges up to 0-01 um for the monolith point bending test using specimens of 40 mm X8 alumina sample heated below 1200oC, according mm x 2 mm at a cross-head speed of 0. 1 mm/min to its shrinkage On the other hand, three kinds of at room temperature and at 1200oc(1300c) porosity are observed in the composite under argon, with I h stabilization at the tempera- ture of measurement. The tensile strength was ( The first family, centred near 0-05-0.1 um, recorded at room temperature on a dumbbell is assigned to the interparticle voids as ecimen machined in a 120 mm x 40 mm observed in the monolith alumina sample mm composite using a 10 um deformation gaug The small shift toward high values indi cates that the packing is slightly disturbed by the 3d fibre network 3 Results and Discussion (ii a broad distribution from 0.2 to 10 um can be assigned to the voids between fibres 3.1 Matrix shrinkage and its influence on (infiltrated powder can be lacking fror mechanical properties bundles, as evidenced by microscopy) and Dilatometric traces of the slip cast monoliths from to voids between fibres and matrix particles AKP50 suspensions show that sintering starts the latter voids increasing with increase in at 1000 C and ends at 1450 C 2 (The bending thermal treatment temperature ultimate strength can reach 400 MPa for the (iia bimodal distribution between 10 and 20 monolith specimen sintered at 1400C). The nil um Is assigned to cracks originating from dilatation/shrinkage point is observed at1l00°C constricted shrinkage, as shown in Fig. 2 Consequently, microcracking and a fbre-matrix Increasing the sintering temperature led to nar gap are observed in the composite heated above rowing of the pore size in the monolith, resulting 1100C, and the presence of the 3d geometric from particle centre shortening and pore collapsingSol-gel control of the matrix net-shape sintering 1477 preform is dried stepwise in a controlled humidity atmosphere at selected temperatures between 40 and 120°C. The drying time is 15 h. Monoliths are processed without the fibre pre￾form plate, and the powder particles are retained by the sole filter paper. Dried monoliths with 60% of the theoretical density for a-alumina can be achieved. The infiltrated preform and monoliths are ther￾mally treated at selected temperatures between 1000 and 1400°C in a reducing argon atmosphere for 1 h in a carbon resistance furnace. 2.4 Microstructure and mechanical characterization Monolith shrinkage was recorded versus tempera￾ture using an Adamel Lhomargy D124 apparatus (Instrument SA, 91 Longjumeau, France) with an alumina rod and support (heating/cooling rate: SUmin). The pore size and distribution are measured by mercury infiltration using a Micro￾meritics Pore Sizer 9310 according to the ASTM vol. 1201, C699-1983 not:ice. The validity of the measurement has been verified by calibration of the mercury/composite surface angle by compari￾son with the pore volume determined using N, adsorption/desorption plaits carried out with a Micromeritics ASAP2000 instrument. Comparable results are obtained for a surface angle equal to 145”, this value being intermediate between the angle values commonly us.ed for Hg/C (155’) and Hg/oxide (130”) interfaces.. The open porosity has been calculated according to Archimedes’ method. The flexural strength was recorded by a three￾point bending test using specimens of 40 mm X 8 mm X 2 mm at a cross-hecad speed of 0.1 mm/min at room temperature and at 1200°C (1300°C) under argon, with 1 h stabilization at the tempera￾ture of measurement. The tensile strength was recorded at room temperature on a dumbbell specimen machined in a 120 mm X 40 mm X 8 mm composite using a 10 ,um deformation gauge. 3 Results and Discussion 3.1 Matrix shrinkage and its influence on mechanical properties Dilatometric traces of the slip cast monoliths from AKPSO suspensions show that sintering starts at 1000°C and ends at l,450°C.i2 (The bending ultimate strength can reach 400 MPa for the monolith specimen sintered at 1400°C). The nil dilatation/shrinkage point is observed at 1100°C. Consequently, microcracking and a fibre-matrix gap are observed in the composite heated above llOO”C, and the presence of the 3D geometric invariant fibre preform promotes the segmentation of the matrix. A typical example is shown in Fig. 2. This phenomenon lowers the mechanical strength. The improvement to the thermomechanical prop￾erties can be achieved if (i) net-shape shrinkage is obtained, (ii) the porosity is decreased and (iii) the mechanical strength of the bulk matrix is increased without the formation of a strong fibre￾matrix interface. At temperatures ranging from 1000 to 14OO”C, the sintering of an alumina submicron powder takes place by solid-state diffusion at the particle con￾tacts. The interposition of an inert second phase between particles would prevent, or reduce, the formation of the contact for solid-state diffusion. The interparticle voids could also affect shortening of the interparticle distance during sintering. The high green density of the monolith sample (60% of the theoretical density) indicates a highly optimized packing of the AKPSO particles. The open porosity of monoliths thermally treated at 1000°C ranges from 34 to 36% and that of com￾posites with the 3D preform (carbon fibre volume fraction: 28%) ranges from 35 to 38%. This indi￾cates that the AKPSO particle packing inside the preform is rather similar to that in the monolith. Figure 3(a) compares the pore volume distribution in the monolith alumina sample with that in the composite heated at various temperatures. Only one family of pores (mean diameter -0.05 pm) is observed in the monolith alumina sample. This pore range corresponds well to the voids between adjacent AKPSO particles. It is noted that the pore volume ranges up to 0.01 pm for the monolith alumina sample heated below 12OO”C, according to its shrinkage. On the other hand, three kinds of porosity are observed in the composite: (i) The first family, centred near 0.0550.1 pm, is assigned to the interparticle voids as observed in the monolith alumina sample. The small shift toward high values indi￾cates that the packing is slightly disturbed by the 3D fibre network. (ii) A broad distribution from 0.2 to 10 pm can be assigned to the voids between fibres (infiltrated powder can be lacking from bundles, as evidenced by microscopy) and to voids between fibres and matrix particles, the latter voids increasing with increase in thermal treatment temperature. (iii) A bimodal distribution between 10 and 20 pm is assigned to cracks originating from constricted shrinkage, as shown in Fig. 2. Increasing the sintering temperature led to nar￾rowing of the pore size in the monolith, resulting from particle centre shortening and pore collapsing