3042 Y Bao, PS. Nicholson /Journal of the European Ceramic Sociery 28(2008)3041-3048 the fired density can be 80% theoretical. Thus reaction-bonded- AlPO4 was coated onto the mullite/alumina fibers(Nextel M mullite with less than 20%o porosity and less than 3% shrinkage 720, 3M, St. Paul, MN) by a layer-by-layer electrostatic should not be problem if the SiOz comes from the Si precursor, method. Desized fibers were pre-treated with 0.5 wt%cationic i.e., fiber-reinforced, reaction-bonded-mullite composites can be polyelectrolyte solution(polydiallyldimethylammonium chlo- realistically fabricated by pressureless sintering free of macro ride, Aldrich, M W. 400,000-500,000)to induce a positive racks. The mullite-formation temperature is-1500oC, how- surface charge. The latter attracts the negatively-charged AlPO4 ever, the strength of the mullite/alumina(Nextel 720)fiber nano-particles to pro duce the coating. The coated fibers were degrades severely on heat-treatment >1300C. Thus the mul- heat-treated at 1100 C. AlPO4 was coated for 10 cycles lite matrix sintering temperature must be modified to s1300"C (10 wt% gain) to give an acceptable thickness. The coated for fiber strength retention. Recently, rare earth oxides added to fibers were unidirectionally mounted in a rectangular, plastic RBM reduced the mullite-formation temperature to 1350C. holder(25 mm x mm x 3 mm), the back of which was attached o mixed-rare-earth-oxides(MREO) were added to the rbm a metal plate as cathode to draw particles through the fiber mixture to form mullite <1300oC II preform and accomplish electrophoretic-infiltration-deposition In this paper, AlPO4-coated, Nextel 720 fiber/reaction- (EPID). 2, 3 The inter-electrode distance was 2 cm and EPID bonded mullite composites are fabricated by constant-current was conducted at a constant current of 0.07 mA/cm2. After depo- electrophoretic-infiltration-deposition( EPID)and subsequently sition, the composite was cold isostatically pressed at 140MPa then dried in an atmosphere-controlled closed container to avoid cracking. Uncoated fibers were also infiltrated with rBM matrix 2. Experimental for The green composites were heated to 1175C for 10 h in air Reaction-bonded mullite was prepared with submicron TM- to convert the Si to SiO2 then sintered at 1300C for 2h. Their DAR alumina powder(Taimei Chemicals, Tokyo, Japan)and thermal stability was determined by heat-treatment for 100 h at micron Si metal powder(Atlantic Equipment Engineers, Ner- 1300C. The fired density and open porosity were measured by genfield, NJ)which was pre-ball-milled in ethanol to a surface Archimedes method in water. area >20 m /g(to promote oxidation during sintering). Table 1 Four-point bend testing was performed at 0.10 mm/min in lists composition of mixed-rare-earth-oxides (MREO, Lan- a screw-driven, ultra-hard compression machine(Model 10053 thanide oxide, Molycorp, Fairfield, NJ). These were added &10055, Wykeham Farrance Engineering Ltd, UK) using an as sintering and mullite-formation aids. A mullite precursor alumina fixture with outer span, 20 mm, and inner span, 10 mm. (Siral 28M, SASOL GmbH, Hamburg, German), pre-sintered The sample thickness was 2.0-2.5 mm. Fracture at 1300C for 2 h to form pure mullite, was ground and added observed by SEM and the degree of fiber-pullout determined as mullite-promotion seeds. The molar ratio of Al: Si was set to en samples were tested at room temperature and five at elevated that of mullite temperatures. Anhydrous polyethyleneimine dispersant(PEl, M.W10,000, acid was added to stabilize the RBM-precursor, ethano/ 3. Results and discussion suspension. The optimal addition was determined via the elec- Fig. 1 shows the dta curve for 32 wt% MREo, 22 wt% trophoretic mobility value for the RBM precursors with a Al2O3 and 46 wt% Sio mixture. The endothermic peak around ta potential analyzer(ZetaPALS, Brookhaven Instruments, 1200 C is due to eutectic liquid-phase formation. Mullite phase Holtsville NY). The electrokinetic sonic amplitude(ESA)was can be promoted by formation of the MREO-Al2O3-SiOz eute measured on the mixed suspension(ESA-8000, Matec Applied tic liquid phase. Fig. 2 tracks the phase evolution in the RBM Sciences, Hopkinton, MA). RBM pellets were also uniaxi- matrix versus sintering temperature for a mixture containing ally pressed then cold isostatically pressed at 140 MPa and 7.5 wt% MREO. Mullite appears at 1270 C, and is the major eated to 1175-1200C for 10h in air to oxidize the Si phase at 1300 C. Traces of alumina and silica remain.The sil- to Sio2. Finally they were sintered at 1250-1350C for 2h to study mullite phase formation and shrinkage. Shrinkage was calculated from the change of the diameter of the pel- Table I 16 Composition of the mixed-rare-earth-oxides(MrEO) provided by Molycorp Oxide Concentration(wt%) PrO1 Temperature(C) Fig 1. DTA curve for 32 wt% MREO, 22 wt%AlO3 and 46 wt%SiOz mixture.3042 Y. Bao, P.S. Nicholson / Journal of the European Ceramic Society 28 (2008) 3041–3048 the fired density can be 80% theoretical. Thus reaction-bonded￾mullite with less than 20% porosity and less than 3% shrinkage should not be problem if the SiO2 comes from the Si precursor, i.e., fiber-reinforced, reaction-bonded-mullite composites can be realistically fabricated by pressureless sintering free of macro cracks. The mullite-formation temperature is ∼1500 ◦C, how￾ever, the strength of the mullite/alumina (NextelTM 720) fiber degrades severely on heat-treatment >1300 ◦C. 9 Thus the mul￾lite matrix sintering temperature must be modified to ≤1300 ◦C for fiber strength retention. Recently, rare earth oxides added to RBM reduced the mullite-formation temperature to 1350 ◦C.10 So mixed-rare-earth-oxides (MREO) were added to the RBM mixture to form mullite <1300 ◦C.11 In this paper, AlPO4-coated, Nextel 720 fiber/reaction￾bonded mullite composites are fabricated by constant-current electrophoretic-infiltration-deposition (EPID) and subsequently pressureless-sintered. 2. Experimental Reaction-bonded mullite was prepared with submicron TM￾DAR alumina powder (Taimei Chemicals, Tokyo, Japan) and micron Si metal powder (Atlantic Equipment Engineers, Ner￾genfield, NJ) which was pre-ball-milled in ethanol to a surface area >20 m2/g (to promote oxidation during sintering). Table 1 lists composition of mixed-rare-earth-oxides (MREO, Lan￾thanide oxide, Molycorp, Fairfield, NJ). These were added as sintering and mullite-formation aids. A mullite precursor (Siral 28 M, SASOL GmbH, Hamburg, German), pre-sintered at 1300 ◦C for 2 h to form pure mullite, was ground and added as mullite-promotion seeds. The molar ratio of Al:Si was set to that of mullite. Anhydrous polyethyleneimine dispersant (PEI, M.W. 10,000, Polysciences, Warrington, PA), protonated with glacial acetic acid was added to stabilize the RBM-precursor, ethanol suspension. The optimal addition was determined via the elec￾trophoretic mobility value for the RBM precursors with a zeta potential analyzer (ZetaPALS, Brookhaven Instruments, Holtsville NY). The electrokinetic sonic amplitude (ESA) was measured on the mixed suspension (ESA-8000, Matec Applied Sciences, Hopkinton, MA). RBM pellets were also uniaxi￾ally pressed then cold isostatically pressed at 140 MPa and heated to 1175–1200 ◦C for 10 h in air to oxidize the Si to SiO2. Finally they were sintered at 1250–1350 ◦C for 2 h to study mullite phase formation and shrinkage. Shrinkage was calculated from the change of the diameter of the pel￾lets. Table 1 Composition of the mixed-rare-earth-oxides (MREO) provided by Molycorp Oxide Concentration (wt%) CeO2 49 La2O3 33 Nd2O3 13 Pr6O11 4 Other 1 AlPO4 was coated onto the mullite/alumina fibers (NextelTM 720, 3 M, St. Paul, MN) by a layer-by-layer electrostatic method.5 Desized fibers were pre-treated with 0.5 wt% cationic polyelectrolyte solution (polydiallyldimethylammonium chlo￾ride, Aldrich, M.W. 400,000–500,000) to induce a positive surface charge. The latter attracts the negatively-charged AlPO4 nano-particles to produce the coating. The coated fibers were heat-treated at 1100 ◦C. AlPO4 was coated for 10 cycles (∼10 wt% gain) to give an acceptable thickness. The coated fibers were unidirectionally mounted in a rectangular, plastic holder (25 mm × 5 mm × 3 mm), the back of which was attached to a metal plate as cathode to draw particles through the fiber preform and accomplish electrophoretic-infiltration-deposition (EPID).12,13 The inter-electrode distance was 2 cm and EPID was conducted at a constant current of 0.07 mA/cm2. After depo￾sition, the composite was cold isostatically pressed at 140 MPa then dried in an atmosphere-controlled closed container to avoid cracking. Uncoated fibers were also infiltrated with RBM matrix for comparison. The green composites were heated to 1175 ◦C for 10 h in air to convert the Si to SiO2 then sintered at 1300 ◦C for 2 h. Their thermal stability was determined by heat-treatment for 100 h at 1300 ◦C. The fired density and open porosity were measured by Archimedes’ method in water. Four-point bend testing was performed at 0.10 mm/min in a screw-driven, ultra-hard compression machine (Model 10053 & 10055, Wykeham Farrance Engineering Ltd., UK) using an alumina fixture with outer span, 20 mm, and inner span, 10 mm. The sample thickness was 2.0–2.5 mm. Fracture surfaces were observed by SEM and the degree of fiber-pullout determined. Ten samples were tested at room temperature and five at elevated temperatures. 3. Results and discussion Fig. 1 shows the DTA curve for 32 wt% MREO, 22 wt% Al2O3 and 46 wt% SiO2 mixture. The endothermic peak around 1200 ◦C is due to eutectic liquid-phase formation. Mullite phase can be promoted by formation of the MREO–Al2O3–SiO2 eutec￾tic liquid phase. Fig. 2 tracks the phase evolution in the RBM matrix versus sintering temperature for a mixture containing 7.5 wt% MREO. Mullite appears at 1270 ◦C, and is the major phase at 1300 ◦C. Traces of alumina and silica remain. The sil￾Fig. 1. DTA curve for 32 wt% MREO, 22 wt%Al2O3 and 46 wt%SiO2 mixture