Journal of the European Ceramic Society 17(1997)299-5 Printed in Great b itin ea s richt reserved PII:s0955-2219(96)00131-8 0955-221997517 Tape-Cast Alumina-Zirconia Laminates Processing and mechanical Properties T. Chartier &t. rouxel LMCTS, URA CNRS 320, ENSCI, 47 Av. Albert Thomas, 87065 Limoges Cedex, France (Received 15 September 1995; revised version received 19 June 1996; accepted 15 July 1996) abstract aim is to improve both the toughness and the strength. This type of composite allows the associ Alumina-zirconia laminar ceramics made from lay- ation of two reinforcing mechanisms, the first act ers of different compositions with diferent stacking ing at the scale of the microstructure, inside the sequences were fabricated by tape casting and com- layers, due to the stress-induced transformation of pared. A phosphate ester dispersant was optimized zirconia particles. the second acting at a macro in MEK/EtOH and an optimum formulation of scopic scale, due to the interfaces between the lay organic components for tape casting was defined. ers. In these laminar structures, residual stresses The fracture resistance, toughness and elastic prop- develop during cooling from the sintering temper erties were characterized. A significant improvement ature because of the differences in thermal expa of both the fracture resistance and the toughness, sions between layers of different compositions from 380 t0 560 Pa and from 3.7 to8 MPavm The sign and the magnitude of these stresses may respectively, was gained between the pressed alu- be adjusted through the compositions, the stacking mina monolith and the tupe-cast AlO -ZrO, comm- sequences, and also through the layer thicknesses posites. The improvement was tentatively related to Hence, it is possible to develop high compressive The presence of residual stresses at both the micro- stresses in thin layers whereas the tensile stresses copic scale( phase transformation toughening )and remain low in the associated thick layers at the macroscopic scale( interface effects). C1996 Tape casting, -4 which is extensively used for Elsevier Science limited electronic ceramics. is well suited to fabricate homogeneous wide and thin ceramic sheets that can be reinforced by zirconia particles. Multilayer 1 Introduction systems are made by stacking green sheets, lami nating, removing the organic components and sin- Engineering applications require improved mechan- tering. Tape casting involves the dispersion of th cal properties of ceramics, particularly fracture ceramic powder in a solvent (typically organic) toughness. A noticeable increase in toughness can with the aid of a dispersant, followed by the addi obtained through ceramic composites. At the tion of binders and plasticizers to ensure the cohe- microscopic scale, composites can be roughly sion, fexibility and workability of the green tape classified into particle-, platelet-and whisker-rein- when the solvent is evaporated forced materials. At the macroscopic scale, com s paper posite materials can be arranged according to nated AlO3-ZrO, composites by tape casting and various configurations, among which are lami- the optimization of the dispersion which is a cru nated composites, consisting of alternate layers cial step. The influence of organic compounds on with different compositions. These structures pro- properties of green tapes, cracking sensitivity, den ide the opportunity for tailoring the properties sity and thermocompression ability will also be by stacking layers of different compositions in a reported. The room temperature fracture char suitable sequence. It is then possible to produce teristics (strength, toughness, fracture path)and functionally gradient ceramics to meet specific elastic properties were investigated to give an requirements- insight into the complex nature of the mechanical The laminated ceramic composites which are behaviour of ceramic laminates and to illustrate studied here consist of alternate layers of alumina the advantages of the composite materials over reinforced by various amount of zirconia. The monoliths. 299Printed in Great Britain. All rights reserved PII: SO955-2219(96)00131-8 0955-22191971Sl7.00 Tape-Cast Alumina-Zirconia Laminates: Processing and Mechanical Properties T. Chartier & T. Rouxel LMCTS, URA CNRS 320, ENSCI, 47 Av. Albert Thomas, 87065 Limoges Cedex, France (Received 15 September 1995; revised version received 19 June 1996; accepted 15 July 1996) Abstract Alumina-zirconia laminar ceramics made from lay￾ers of dtflerent compositions with dtyerent stacking sequences were fabricated by tape casting and com￾pared. A phosphate ester dispersant was optimized in MEWEtOH and an optimum formulation of organic components for tape casting was defined. The fracture resistance, toughness and elastic prop￾erties were characterized. A signt$cant improvement of both the fracture resistance and the toughness, from 380 to 560 MPa and from 3.7 to 8 MPadm respectively, was gained between the pressed alu￾mina monolith and the tape-cast A120J-Zr02 com￾posites. The improvement was tentatively related to the presence of residual stresses at both the micro￾scopic scale (phase transformation toughening) and at the macroscopic scale (interface eflects). 0 1996 Elsevier Science Limited. 1 Introduction Engineering applications require improved mechan￾ical properties of ceramics, particularly fracture toughness. A noticeable increase in toughness can be obtained through ceramic composites. At the microscopic scale, composites can be roughly classified into particle-, platelet- and whisker-rein￾forced materials. At the macroscopic scale, com￾posite materials can be arranged according to various configurations, among which are lami￾nated composites, consisting of alternate layers with different compositions. These structures pro￾vide the opportunity for tailoring the properties by stacking layers of different compositions in a suitable sequence. It is then possible to produce functionally gradient ceramics to meet specific requirements. I-9 The laminated ceramic composites which are studied here consist of alternate layers of alumina reinforced by various amount of zirconia.” The aim is to improve both the toughness and the strength. This type of composite allows the associ￾ation of two reinforcing mechanisms, the first act￾ing at the scale of the microstructure, inside the layers, due to the stress-induced transformation of zirconia particles, the second acting at a macro￾scopic scale, due to the interfaces between the lay￾ers. In these laminar structures, residual stresses develop during cooling from the sintering temper￾ature because of the differences in thermal expan￾sions between layers of different compositions. The sign and the magnitude of these stresses may be adjusted through the compositions, the stacking sequences, and also through the layer thicknesses. Hence, it is possible to develop high compressive stresses in thin layers whereas the tensile stresses remain low in the associated thick layers. Tape casting,’ ‘-I4 which is extensively used for electronic ceramics, is well suited to fabricate homogeneous wide and thin ceramic sheets that can be reinforced by zirconia particles. Multilayer systems are made by stacking green sheets, lami￾nating, removing the organic components and sin￾tering. Tape casting involves the dispersion of the ceramic powder in a solvent (typically organic) with the aid of a dispersant, followed by the addi￾tion of binders and plasticizers to ensure the cohe￾sion, flexibility and workability of the green tape when the solvent is evaporated. This paper describes the processing of lami￾nated A&O,-Zr02 composites by tape casting and the optimization of the dispersion which is a cru￾cial step. The influence of organic compounds on properties of green tapes, cracking sensitivity, den￾sity and thermocompression ability will also be reported. The room temperature fracture charac￾teristics (strength, toughness, fracture path) and elastic properties were investigated to give an insight into the complex nature of the mechanical behaviour of ceramic laminates and to illustrate the advantages of the composite materials over monoliths. 299