M. Nygren, Z Shen/Solid State Sciences 5(2003)125-131 HAp TZP 1卜m Fig. 4. SEM micrographs depicting the microstructures of an SPSed sample of a composite containing 50 vol% HAP and 50 vol% yttria doped zirconia. the first time, a series of hydroxyapatite ceramic composites To produce FGMs, powder metallurgy is the most popula containing a continuous framework of nano-sized tetrago- processing procedure, where the critical issue is to suppress nal zirconia grains have been fully densified and mechan- diffusion and reactions between the laminated layers during ically tested [17] The samples were prepared at 1150C, sintering in order to retain the desired composition gradation using a holding time of 5 min, a pressure of 50 MPa and a in the final component. SPs is an effective process for rapid heating and cooling rate of 200'C/min. X-ray powder dif- sintering and sinter-bonding of a wide range of materials fraction studies combined with SEM and HRTEM investiga- with laminated structures. We have demonstrated that it tions showed that no decomposition reactions had occurred. possible to consolidate laminated structures of TIN/Al2O3 Fig 4 is an SEM micrograph showing the microstructure of and structures containing(TIN)x(Al2O3)1-x layers with x a cross section of a composite containing 50 vol% HAp. The ranging from I to O by the SPS process. An SEM micrograph obtained composite materials are 5-7 times stronger and 4- of such a laminated structure is seen in Fig. 5. A notable 7 times tougher than monolithic hydroxyapatite ceramics, feature of this figure is the clean interface between the ating that they have great potential for a wide variety ayers of different compositions. The entire processing time of clinical applications required to obtain densified bodies of these compositions is less than 1/10 of that required by conventional sintering processes, indicating SPS is also a cost-effective processing 4. Preparation of laminated structures technique [19, 20]. Studies of other types of laminated structures are in progress including The worldwide interest in functionally graded materials (FGM)surged when research efforts in Switzerland and (i) Laminated structures of silicon nitride based materials in Japan met with success in the late 1980s. Initially, the Because the composition of each laminates is different, FGM concept was proposed to minimise the stress caused post heat treatment of these materials provide us with by thermal expansion mismatch between surface thermal unique possibilities to investigate the kinetics of forma barriers and matrix materials. Nowadays, when the FGM tion of various silicon oxynitride phases research field has expanded very much, FGM is not only (i) Cemented carbide based laminates composed of layers referred to as a new category of materials but also as a novel with different cobalt concentration and /or different wc technology that offers possibilities to incorporate various grain sIzes. functions into one single component. During the last decade, (ii)Cemented carbide compacts with diamond particles various FGMs have been developed for applications in the corporated in the outermost WC/Co layer. field of thermal barriers, graded abrasive tools, functional, (iv) Cemented carbide and steel compacts furnished with an energy conversion and biomedical components [ 18] abrasive top layer of TiN and Al2O3, respectivelyM. Nygren, Z. Shen / Solid State Sciences 5 (2003) 125–131 129 Fig. 4. SEM micrographs depicting the microstructures of an SPSed sample of a composite containing 50 vol% HAP and 50 vol% yttria doped zirconia. the first time, a series of hydroxyapatite ceramic composites containing a continuous framework of nano-sized tetrago￾nal zirconia grains have been fully densified and mechan￾ically tested [17]. The samples were prepared at 1150 ◦C, using a holding time of 5 min, a pressure of 50 MPa and a heating and cooling rate of 200 ◦C/min. X-ray powder dif￾fraction studies combined with SEM and HRTEM investiga￾tions showed that no decomposition reactions had occurred. Fig. 4 is an SEM micrograph showing the microstructure of a cross section of a composite containing 50 vol% HAp. The obtained composite materials are 5–7 times stronger and 4– 7 times tougher than monolithic hydroxyapatite ceramics, indicating that they have great potential for a wide variety of clinical applications. 4. Preparation of laminated structures The worldwide interest in functionally graded materials (FGM) surged when research efforts in Switzerland and in Japan met with success in the late 1980s. Initially, the FGM concept was proposed to minimise the stress caused by thermal expansion mismatch between surface thermal barriers and matrix materials. Nowadays, when the FGM research field has expanded very much, FGM is not only referred to as a new category of materials but also as a novel technology that offers possibilities to incorporate various functions into one single component. During the last decade, various FGMs have been developed for applications in the field of thermal barriers, graded abrasive tools, functional, energy conversion and biomedical components [18]. To produce FGMs, powder metallurgy is the most popular processing procedure, where the critical issue is to suppress diffusion and reactions between the laminated layers during sintering in order to retain the desired composition gradation in the final component. SPS is an effective process for rapid sintering and sinter-bonding of a wide range of materials with laminated structures. We have demonstrated that it is possible to consolidate laminated structures of TiN/Al2O3 and structures containing (TiN)x(Al2O3)1−x layers with x ranging from 1 to 0 by the SPS process. An SEM micrograph of such a laminated structure is seen in Fig. 5. A notable feature of this figure is the clean interface between the layers of different compositions. The entire processing time required to obtain densified bodies of these compositions is less than 1/10 of that required by conventional sintering processes, indicating SPS is also a cost-effective processing technique [19,20]. Studies of other types of laminated structures are in progress including: (i) Laminated structures of silicon nitride based materials. Because the composition of each laminates is different, post heat treatment of these materials provide us with unique possibilities to investigate the kinetics of forma￾tion of various silicon oxynitride phases. (ii) Cemented carbide based laminates composed of layers with different cobalt concentration and/or different WC grain sizes. (iii) Cemented carbide compacts with diamond particles incorporated in the outermost WC/Co layer. (iv) Cemented carbide and steel compacts furnished with an abrasive top layer of TiN and Al2O3, respectively