J Mater Sci(2006)41:7425-7436 of such ceramics is technologically difficult, and fur- Experimental procedure thermore weak interfaces are known to compromise certain properties. The use of residual stress patterns Powder preparation associated with thermal expansion mismatch within ceramic laminate materials therefore offers an attrac- A high purity, submicron a-Al2O3 powder(Ceralox, tive alternative mechanism to improve fracture HPA-0.5) and a known volume percent of a-Al2O3 behaviour seed platelets(10-15 um diameter, ELF Atochem) Q In the present research, texture-reinforced oxide were used to prepare the TA layers. The oxide matrix amic laminates have been prepared by an alginate- precursor powder compositions and their sources are based, gel-cast, doctor-blade process, in which tape listed in Table 1. The rBao powder mixture was lay-ups to develop the laminated structure. Three attrition-milled in acetone with 3 mm-diameter high laminate systems were studied; all based on strongly purity Al2O, beads(99.9%, Union Process) for 6-8 h bonded interfaces between the matrix composition and at 500-600 rpm. After drying in a ventilation hood, the highly textured alumina(TA)interlayers. The control powder mixture was sieved with a 60-mesh sieve. The bonded and textured interlayers is independent of, and attrition milling was monitored by scanning electa of mechanical anisotropy by the presence of strongly particle size of the rBao powder before and af quite different from, the principle of reinforcement by microscopy(LEO 982)and laser particle size analysis weak-interface crack deflection, which is used to inhibit (HORIBA, LA-910) through-thickness crack propagation in many aligned ceramic composites. Gel casting and lamination In this paper, the texture and microstructure of the alumina interlayers were optimized by controlling the Water-based gel casting [2] was used to fabricate both volume fraction and alignment of alumina seed plate- the textured and texture-free layers. An in situ sol-gel lets in a precursor tape. Meanwhile, three types of reaction based on ion exchange between water-soluble matrix layer were used in three laminate systems: sodium alginate and polyvalent cations yields an 1. A reaction-bonded aluminum oxide (RBAO) insoluble, cation cross-linked alginate gel near-zero residual thermal stress in the fully sin- Nanalginate n/2Ca--nNa+ Can/2 alginate (1) tered, single-phase alumina/TA laminate 2. An alumina-toughened zirconia (ATZ), designed A 0.15 M Ca solution of calcium nitrate-4-hydrate for minimum tetragonal zirconia grain-size and (AR purity, Riedel-de Haen), served as the gelling with controlled residual tension in the zirconia solution. The seeded alumina layers were prepared by matrix layers of the laminate first mixing the alumina powder with alginate and a 3. A reaction-bonded(zircon-based) mullite(RBM), dispersant in distilled water by ball milling in a plastic designed for maximum mullite conversion and bottle for at least 12 h(99.9% alumina balls, 5 mn residual compression in the mullite matrix layers of diameter--Union Process). The alumina seed platelets the laminate were then incorporated and ball-milled for a further 5 h. The homogeneously mixed slip was transferred to This paper reports the processing methodology and a second plastic bottle to remove the alumina balls, and texture control for these three texture-reinforced then rolled slowly on the ball mill for about 2 h to ceramic laminates. It will be followed by a further remove trapped air bubbles. The de-gassed slips were report on microstructural development and mechanical poured into the tape caster and cast with the gelling behaviour of the three laminate systems. solution onto a Mylar sheet substrate(Dupont) which Table 1 Precursor powders for RBAO. ATZ and rBM ayer type Material Powder characteristic and source Composition(wt% RBAO 99.36%,45-90mm Miller Thermal. In AlO3 TZ3Y TSK, Y-stabilized tetragonal ZrO2(75.2)Y2O3(4.2) s Another zircon powde ZrO+a-AlO3, <100 n Al2O3(20) submicron Rami submicron (5 mm) was used for AlO3 Ceralox, HPA-05 2 Springerof such ceramics is technologically difficult, and fur￾thermore weak interfaces are known to compromise certain properties. The use of residual stress patterns associated with thermal expansion mismatch within ceramic laminate materials therefore offers an attrac￾tive alternative mechanism to improve fracture behaviour. In the present research, texture-reinforced oxide ceramic laminates have been prepared by an alginate￾based, gel-cast, doctor-blade process, in which tape lay-ups to develop the laminated structure. Three laminate systems were studied; all based on strongly bonded interfaces between the matrix composition and highly textured alumina (TA) interlayers. The control of mechanical anisotropy by the presence of strongly bonded and textured interlayers is independent of, and quite different from, the principle of reinforcement by weak-interface crack deflection, which is used to inhibit through-thickness crack propagation in many aligned ceramic composites. In this paper, the texture and microstructure of the alumina interlayers were optimized by controlling the volume fraction and alignment of alumina seed plate￾lets in a precursor tape. Meanwhile, three types of matrix layer were used in three laminate systems: 1. A reaction-bonded aluminum oxide (RBAO), designed for minimum sintering shrinkage and a near-zero residual thermal stress in the fully sin￾tered, single-phase alumina/TA laminate. 2. An alumina-toughened zirconia (ATZ), designed for minimum tetragonal zirconia grain-size and with controlled residual tension in the zirconia matrix layers of the laminate. 3. A reaction-bonded (zircon-based) mullite (RBM), designed for maximum mullite conversion and residual compression in the mullite matrix layers of the laminate. This paper reports the processing methodology and texture control for these three texture-reinforced ceramic laminates. It will be followed by a further report on microstructural development and mechanical behaviour of the three laminate systems. Experimental procedure Powder preparation A high purity, submicron a-Al2O3 powder (Ceralox, HPA-0.5) and a known volume percent of a-Al2O3 seed platelets (10–15 lm diameter, ELF Atochem) were used to prepare the TA layers. The oxide matrix precursor powder compositions and their sources are listed in Table 1. The RBAO powder mixture was attrition-milled in acetone with 3 mm-diameter high￾purity Al2O3 beads (99.9%, Union Process) for 6–8 h at 500–600 rpm. After drying in a ventilation hood, the powder mixture was sieved with a 60-mesh sieve. The particle size of the RBAO powder before and after attrition milling was monitored by scanning electron microscopy (LEO 982) and laser particle size analysis (HORIBA, LA-910). Gel casting and lamination Water-based gel casting [2] was used to fabricate both the textured and texture-free layers. An in situ sol-gel reaction based on ion exchange between water-soluble sodium alginate and polyvalent cations yields an insoluble, cation cross-linked alginate gel: Nanalginate þ n=2Ca2þ ! nNaþ þ Can=2 alginate ð1Þ A 0.15 M Ca2+ solution of calcium nitrate-4-hydrate (AR purity, Riedel-deHae¨n), served as the gelling solution. The seeded alumina layers were prepared by first mixing the alumina powder with alginate and a dispersant in distilled water by ball milling in a plastic bottle for at least 12 h (99.9% alumina balls, 5 mm diameter—Union Process). The alumina seed platelets were then incorporated and ball-milled for a further 5 h. The homogeneously mixed slip was transferred to a second plastic bottle to remove the alumina balls, and then rolled slowly on the ball mill for about 2 h to remove trapped air bubbles. The de-gassed slips were poured into the tape caster and cast with the gelling solution onto a Mylar sheet substrate (Dupont) which Table 1 Precursor powders for RBAO, ATZ and RBM layers * Another zircon powder (~5 mm) was used for comparison Layer type Material Powder characteristic and source Composition (wt%) RBAO Al Globular, 99.36%, 45–90 mm, Miller Thermal, Inc. 20–40 Al2O3 Submicron, Ceralox, HPA-0.5 60–80 ATZ TZ3Y20A TSK, Y-stabilized tetragonal ZrO2 + a-Al2O3, < 100 nm ZrO2 (75.2) Y2O3 (4.2) Al2O3 (20) RBM ZrSiO4* submicron, Rami Submicron, Ceralox, HPA-0.5 54.5 Al2O3 45.5 7426 J Mater Sci (2006) 41:7425–7436 123