Availableonlineatwww.sciencedirect.col RECTO State ELSEVIER Solid State Sciences 5(2003)125-131 ciences On the preparation of bio-, nano-and structural ceramics and composites by spark plasma sintering Mats Nygren* Zhijian Shen Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University 10691 Stockholm, Sweden Dedicated to Sten Andersson for his scientific contribution to Solid State and Structural Chemistry bstract park plasma sintering(SPS) is a comparatively new technique. It allows very fast heating and cooling rates, very short holding times, and the possibility to obtain fully dense samples at comparatively low sintering temperatures, typically a few hundred degrees lower than in normal hot pressing. During recent years, a wide variety of materials, e.g., ceramics, composites, cermets, metals and alloys, have been successfully compacted by the SPs process. This and other processing techniques that use direct current and electrically conducting pressure dies are briefly described, but the focus of our presentation are on how the kinetics of densification and grain growth can be manipulated by the use of the SPs technique so as to yield various materials that hold significant fundamental and technological interest. e 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved Keywords: Sintering; Grain growth; Nano-materials; Laminates; Composites; FGM; SPS 1. Introducti sintering process. This procedure is often referred to in the literature as a"single pulse cycle process", with a typical The idea to compact metallic materials by an electro- 30-60 ms pulse current of x1000 A intensity during 60- discharge process was originally proposed in the 1960s [1 90 s In a few cases the pulsed dC procedure is repeated Based on this concept, three sintering processes have been during the sintering process, and in this case the procedure developed and commercialised during recent years, they Is referred to as a"multiple pulse cycle process";(ii)In the are named"Spark Plasma Sintering"(SPS)[21,"Plasma electroconsolidation process, various types of currents(DC, Activated Sintering(PAS)[3 and"Electroconsolidation pulsed DC, or AC)can be used, i. e, the system is designed [4]. These processes are similar to conventional hot pressing to be flexible in terms of power supply. These processes have in that the precursors are loaded in a die and a uniaxial now been developed beyond the production of small objects pressure is applied during the sintering. However, instead of with simple shapes, as continuous production of compacts of using an external heat source, a current(DC, pulsed DC, or complex geometry and of pieces with diameters larger than AC)is allowed to pass through the electrically conducting 150 mm has been achieved. Despite the fact that a uniaxial pressure die and, in appropriate cases, also through the pressure is applied, green bodies of complex geometry can sample. This implies that the die itselfacts as heat source and be exposed to a "pseudo-isostatic"pressure when embedded characteristics of the three processes are as follows: () In as pressure-transmitting medium inside the de ates that act that the sample is heated from both outside and inside. The in free-flowing electrically conducting particul the SPs process a pulsed dC (3.3 ms pulses of 0.5 to 10 Common to these three processes is the possibility of kA intensity)is applied from the beginning to the end of using very fast heating rates(up to 600 C/min or more) the sintering cycle; (i)In the PAS process a pulsed DC is and very short holding times(minutes), and the ability to normally applied at room temperature for a short period of obtain fully dense samples at comparatively low sintering time, and then a continuous DC during the remainder of the temperatures, typically a few hundred degrees lower than in normal hot pressing. Four factors that contribute to the fast densification process can be discerned: (i)Rapid heat transfer;(ii) The application of a mechanical pressure exceeding that used in normal hot pressing processes 1293-2558/03A-see front matter o 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved. i:10.1016/1293-2558(02)00086-9
Solid State Sciences 5 (2003) 125–131 www.elsevier.com/locate/ssscie On the preparation of bio-, nano- and structural ceramics and composites by spark plasma sintering Mats Nygren ∗, Zhijian Shen Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, 106 91 Stockholm, Sweden Received 21 June 2002; accepted 24 August 2002 Dedicated to Sten Andersson for his scientific contribution to Solid State and Structural Chemistry Abstract Spark plasma sintering (SPS) is a comparatively new technique. It allows very fast heating and cooling rates, very short holding times, and the possibility to obtain fully dense samples at comparatively low sintering temperatures, typically a few hundred degrees lower than in normal hot pressing. During recent years, a wide variety of materials, e.g., ceramics, composites, cermets, metals and alloys, have been successfully compacted by the SPS process. This and other processing techniques that use direct current and electrically conducting pressure dies are briefly described, but the focus of our presentation are on how the kinetics of densification and grain growth can be manipulated by the use of the SPS technique so as to yield various materials that hold significant fundamental and technological interest. 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Sintering; Grain growth; Nano-materials; Laminates; Composites; FGM; SPS 1. Introduction The idea to compact metallic materials by an electrodischarge process was originally proposed in the 1960s [1]. Based on this concept, three sintering processes have been developed and commercialised during recent years; they are named “Spark Plasma Sintering” (SPS) [2], “Plasma Activated Sintering” (PAS) [3] and “Electroconsolidation” [4]. These processes are similar to conventional hot pressing in that the precursors are loaded in a die and a uniaxial pressure is applied during the sintering. However, instead of using an external heat source, a current (DC, pulsed DC, or AC) is allowed to pass through the electrically conducting pressure die and, in appropriate cases, also through the sample. This implies that the die itself acts as heat source and that the sample is heated from both outside and inside. The characteristics of the three processes are as follows: (i) In the SPS process a pulsed DC (3.3 ms pulses of 0.5 to 10 kA intensity) is applied from the beginning to the end of the sintering cycle; (ii) In the PAS process a pulsed DC is normally applied at room temperature for a short period of time, and then a continuous DC during the remainder of the * Corresponding author. E-mail address: mats@inorg.su.se (M. Nygren). sintering process. This procedure is often referred to in the literature as a “single pulse cycle process”, with a typical 30–60 ms pulse current of ∼1000 A intensity during 60– 90 s. In a few cases the pulsed DC procedure is repeated during the sintering process, and in this case the procedure is referred to as a “multiple pulse cycle process”; (iii) In the electroconsolidation process, various types of currents (DC, pulsed DC, or AC) can be used, i.e., the system is designed to be flexible in terms of power supply. These processes have now been developed beyond the production of small objects with simple shapes, as continuous production of compacts of complex geometry and of pieces with diameters larger than 150 mm has been achieved. Despite the fact that a uniaxial pressure is applied, green bodies of complex geometry can be exposed to a “pseudo-isostatic” pressure when embedded in free-flowing electrically conducting particulates that act as pressure-transmitting medium inside the die. Common to these three processes is the possibility of using very fast heating rates (up to 600 ◦C/min or more) and very short holding times (minutes), and the ability to obtain fully dense samples at comparatively low sintering temperatures, typically a few hundred degrees lower than in normal hot pressing. Four factors that contribute to the fast densification process can be discerned: (i) Rapid heat transfer; (ii) The application of a mechanical pressure exceeding that used in normal hot pressing processes; 1293-2558/03/$ – see front matter 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. doi:10.1016/S1293-2558(02)00086-9
M Noren, Z Shen/Solid State Sciences 5(2003)125-131 (iii) The use of fast heating and cooling rates; (vi) The use of a pulsed direct current, implying that the samples are also △-3YZo2 exposed to an electric field It is generally accepted that application of mechanical pressure is helpful in removing pores from compacts and 0.8 enhancing diffusion. The heat transfer from the die to the 8 compact is very efficient in these processes, because the die 9 medium when used)acts as heating element, so that the ther- 20.7 mmm mal energy necessary for compaction is efficiently transmit- ted to the sample. However, it is frequently argued that the improved densification rates stem mostly from the use of DC pulses of high energy. Thus it was originally claimed by the process inventors that the pulses generate spark and 400 001400 even plasma discharges between the powder particles, which Temperature(C) is why the processes were named spark plasma sintering and plasma activated sintering. During the initial part of the sin- △-3Y-Z2 of cleaned and activated surfaces is expected to enhance he grain boundary diffusion which, together with the pro- posed spark discharges and/or plasma processes, will pro- and grain growth [2, 3). Murir and Smaltzrield argue that the 2 0.002 application of an electric field enhances grain growth and erefore also densification 5] 001 Whether plasma is generated has not yet been confirmed. specially when non-conducting ceramic powders are com- 0.000 pacted. It has, however, been experimentally verified that the densification is enhanced by the use of DC pulses [6], and 200 this family of techniques is accordingly named pulsed elec Temperature (C tric current sintering(PECS)[7-9]or electric pulse assisted consolidation(EPAC)[10] Fig. I Recorded sintering curves for Zno, ZrO(doped with 3% yttria), and Various materials have been compacted by spark plasma a-A1,03 powders, using a heating rate of 100oC/min Relative density(a) sintering. The number of publications on this topic has in- and shrinkage rate(b). Note that the densification is completed within a few creased dramatically during the last few years. It is not our minutes aim to give a comprehensive review of these technique and their application in this article. We will rather focus 1250C, respectively. As seen in this figure, the main part our presentation on how the kinetics of densification and of the densification occurs within a period of 3 min, whereas grain growth can be manipulated by the use of one of these conventional hot-press and pressure-less sintering processes techniques, namely SPS, to yield: (i) Dense compacts con- typically require hours of heat treatment at substantially taining nano-sized grains; () Fully compacted bodies con- higher temperatures to yield fully densified compacts taining metastable constituent(s); (iii) Laminated structures The normalized shrinkage rate is plotted versus the (iv) very rapid formation of tough interlocking microstru temperature in Fig. Ib. The shrinkage rate curves exhibit tures in Si3 N4-based ceramics maximums around 700. 1100 and 1150C. for ZnO. ZrO and Al2O3, respectively and these maximum rates are one or two orders of magnitude faster than those observed during 2. Preparation of nano-ceramics conventional pressure-less sintering [11, 12] As shown in a recent article concerning SPS processing Shrinkage curves of the oxides Zno, ZrO(doped with of Al2O3, the kinetics of grain growth is quite fast at high 3 mol%Y203), and Al2O3, compacted by SPs under a sintering temperatures, for both gain-boundary migration pressure of 50 MPa and a heating rate of 100C/min are and grain-boundary diffusion are enhanced. It is, however, shown in Fig. la. Using these sintering conditions, shrinkage possible to determine a temperature"window, within which starts at600, 800 and 900C for ZnO, ZrO2 and Al2O3, fully dense compacts can be obtained without or with only and fully dense compacts are obtained at 850, N 1100, and very limited grain growth [ 12]
126 M. Nygren, Z. Shen / Solid State Sciences 5 (2003) 125–131 (iii) The use of fast heating and cooling rates; (vi) The use of a pulsed direct current, implying that the samples are also exposed to an electric field. It is generally accepted that application of mechanical pressure is helpful in removing pores from compacts and enhancing diffusion. The heat transfer from the die to the compact is very efficient in these processes, because the die itself (and the electrically conductive pressure-transmitting medium when used) acts as heating element, so that the thermal energy necessary for compaction is efficiently transmitted to the sample. However, it is frequently argued that the improved densification rates stem mostly from the use of DC pulses of high energy. Thus it was originally claimed by the process inventors that the pulses generate spark and even plasma discharges between the powder particles, which is why the processes were named spark plasma sintering and plasma activated sintering. During the initial part of the sintering process, the spark and/or plasma discharges are said to clean the surfaces of the powders from adsorbed species such as CO2 and H2O. In subsequent stages, the presence of cleaned and activated surfaces is expected to enhance the grain boundary diffusion which, together with the proposed spark discharges and/or plasma processes, will promote transfer of material and thus facilitate densification and grain growth [2,3]. Murir and Smaltzrield argue that the application of an electric field enhances grain growth and therefore also densification [5]. Whether plasma is generated has not yet been confirmed, especially when non-conducting ceramic powders are compacted. It has, however, been experimentally verified that the densification is enhanced by the use of DC pulses [6], and this family of techniques is accordingly named pulsed electric current sintering (PECS) [7–9] or electric pulse assisted consolidation (EPAC) [10]. Various materials have been compacted by spark plasma sintering. The number of publications on this topic has increased dramatically during the last few years. It is not our aim to give a comprehensive review of these techniques and their application in this article. We will rather focus our presentation on how the kinetics of densification and grain growth can be manipulated by the use of one of these techniques, namely SPS, to yield: (i) Dense compacts containing nano-sized grains; (ii) Fully compacted bodies containing metastable constituent(s); (iii) Laminated structures; (iv) Very rapid formation of tough interlocking microstructures in Si3N4-based ceramics. 2. Preparation of nano-ceramics Shrinkage curves of the oxides ZnO, ZrO2 (doped with 3 mol% Y2O3), and Al2O3, compacted by SPS under a pressure of 50 MPa and a heating rate of 100 ◦C/min are shown in Fig. 1a. Using these sintering conditions, shrinkage starts at ∼ 600, ∼ 800 and 900 ◦C for ZnO, ZrO2 and Al2O3, and fully dense compacts are obtained at ∼ 850, ∼ 1100, and (a) (b) Fig. 1. Recorded sintering curves for ZnO, ZrO2 (doped with 3% yttria), and α-Al2O3 powders, using a heating rate of 100 ◦C/min. Relative density (a) and shrinkage rate (b). Note that the densification is completed within a few minutes. ∼ 1250 ◦C, respectively. As seen in this figure, the main part of the densification occurs within a period of 3 min, whereas conventional hot-press and pressure-less sintering processes typically require hours of heat treatment at substantially higher temperatures to yield fully densified compacts. The normalized shrinkage rate is plotted versus the temperature in Fig. 1b. The shrinkage rate curves exhibit maximums around 700, 1100, and 1150 ◦C, for ZnO, ZrO2 and Al2O3, respectively and these maximum rates are one or two orders of magnitude faster than those observed during conventional pressure-less sintering [11,12]. As shown in a recent article concerning SPS processing of Al2O3, the kinetics of grain growth is quite fast at high sintering temperatures, for both gain-boundary migration and grain-boundary diffusion are enhanced. It is, however, possible to determine a temperature “window”, within which fully dense compacts can be obtained without or with only very limited grain growth [12]
M. Nygren, Z Shen/Solid State Sciences 5(2003)125-131 a 1F阶 1N m Fig. 2. SEM micrographs depicting the nano-sized microstructures of fully densified a-Al2O3(a)and yttria doped Zro?(b) a 1Nm SEM micrographs depicting the microstructures of an SPSed sample of the composition Ba2Alr Si12-rN16-rO2- with x 2.0(a), and of the sar act heat treated ex situ at 1700C for 6 hours(b), yielding an almost monophasic sample of the Ba-S phase The critical temperature above which the grain growth very important. The microstructures of SPSed compacts rate becomes appreciable is largely determined by the of Al2 O3 and Zro(doped with 3% yttria) are depicte properties of the precursor powders, e.g., their particle size, in Fig. 2. The a-Al2O3 precursor powder had an average reactivity, degree of agglomeration, etc, but also by the particle size around 400 nm, and the corresponding data applied heating rate and pressure. The use of well de- for the doped zirconia powder was 60 nm. As seen in agglomerated nano-sized precursor powders seems to be Fig 3, very homogeneous microstructures were obtained
M. Nygren, Z. Shen / Solid State Sciences 5 (2003) 125–131 127 Fig. 2. SEM micrographs depicting the nano-sized microstructures of fully densified α-Al2O3 (a) and yttria doped ZrO2 (b). Fig. 3. SEM micrographs depicting the microstructures of an SPSed sample of the composition Ba2AlxSi12−xN16−xO2−x with x ≈ 2.0 (a), and of the same compact heat treated ex situ at 1700 ◦C for 6 hours (b), yielding an almost monophasic sample of the Ba-S phase. The critical temperature above which the grain growth rate becomes appreciable is largely determined by the properties of the precursor powders, e.g., their particle size, reactivity, degree of agglomeration, etc., but also by the applied heating rate and pressure. The use of well deagglomerated nano-sized precursor powders seems to be very important. The microstructures of SPSed compacts of Al2O3 and ZrO2 (doped with 3% yttria) are depicted in Fig. 2. The α-Al2O3 precursor powder had an average particle size around 400 nm, and the corresponding data for the doped zirconia powder was 60 nm. As seen in Fig. 3, very homogeneous microstructures were obtained
M. Nygren, Z Shen/Solid State Sciences 5 (2003)125-13 with an average grain size around 500 nm for alumina, and Ba2Alx Si12-xN16-xO2-x with x a 2.0, abbreviated Ba-s less than 100 nm for zirconia. These alumina and zirconia phase below. Appropriate powder mixtures of Si3N4, AIN compacts had high flexural strengths, typically around 800 Al2O3, and Ce2(CO3 )3/BaCO3 were loaded into the pres- and 1000 MPa, respectively. By optimising the sintering sure die and pre-heated at 1000C during 2 min in order to parameters we succeeded in consolidating various nano-or allow the carbonates to decompose, then heated at a rate of sub-micron sized oxide powders, e. g, ZnO, Al2O3, ZrO2, 200C/min to 1400-1600C, using a pressure of 50 MPa, YAG, BaTiO3, to full density with a grain growth factor 1550C), and that post-heat treatment at low ), is recognized for its excellent bone bonding ability modification, Si6-zAl2O- N8-z iso-structural with B-Si3N4, thopaedic implants because they are too brittle, and accord- and other phases. For geometric reasons, supported by ingly it is necessary to improve the mechanical properties of experimental findings, it has been argued that M cations with these components before they can be used as implants for a radius larger than a I A cannot be accommodated in the load-bearing applications. Pure HAp is rather easily sintered a-sialon structure. It is difficult to understand why, however, to high densities and has been reported to be fully densified because the available interstices are much larger, e. g, havi at a temperature as low as 800C, but higher temperatures a lateral extent of about 2.6A are required for densification in composite oxide/HAp sys- SPS allows very rapid heating rates, and in the experiment tems. The temperatures needed depend on the type and vol- described below we have used a rate of 200C/min and a ume of the reinforcing material used In the zirconium ox- pressure of 50 MPa. Such fast heating prevents the formation ide/HAp system sintering temperatures as high as 1400C of transient M-rich oxide and oxynitride phases that often have been applied occurs in conventional sintering processes. Samples with an A universal problem in connection with preparation of overall a-sialon composition and with M=Ce, La, Sr and HAp/oxide based composites is that HAp reacts with the Eu could thus be fully densified at a temperature as low as added oxide during densification. HAp begins to lose water 1500C. Subsequent SEM, TEM and X-ray studies revealed around 600C, and an oxyhydroxyapatite is formed,see that the a-sialon phase was formed in all SPSed samples. Eq (1)below. In the presence of zirconia even more OH The main phases formed are thus: a-sialon, small amounts ions are lost due to the reaction between oxyhydroxyapatite of B-sialon and the 2IR polytypoid phase, SiAl6O2 N6, and and zirconia according to Eq (2)below in the Sr(Eu-containing samples a polytypoid phase of the composition Sr(Eu)Sig Al19ON31 [13]. Prolonged ex situ or CaIo(PO4)6(OH)(2-2x)Ox ase decompose into B-sialon and various nitrogen-rich Ca10(PO4)6(OH)(2-2x)0,+ZrO2 phases with overall compositions close to that of a-sialon The geometric argument discussed above is thus not valid. ->3 Ca3(PO4)2+ Cao(Zro2)+(1-x)H2O but the M-doped a-sialons with M= Ce, La, Sr and Eu are The decomposition of oxyhydroxyapatite in the presence thermodynamically unstable and decompose comparatively of zirconium oxide is reported to take place at a surpris- fast into more stable phases, and it is only possible to ingly low temperature(950C), taking into account that prepare a-sialon phases in these systems by applying very the oxyhydroxyapatite formed according to Eq (1)is stable fast heating rate [14] up to around 1400C in air. It goes without saying that a In conjunction with our work on the synthesis of new decomposition temperature of 950oC is well below that re- oxynitride compounds in the sialon system we have applied quired for densification of HAp/zirconia composites by con- a similar technique. We have thus prepared almost monopha- ventional processes such as pressure-less sintering and hot sic compacts of the n phase, Ce3 Sig-xAlxNI1-O4-r with pressing. By using the SPs technique, however, the delete x 1.75, abbreviated Ce-N phase below, and of the S phase, rious reactions described above could be avoided. Thus for
128 M. Nygren, Z. Shen / Solid State Sciences 5 (2003) 125–131 with an average grain size around 500 nm for alumina, and less than 100 nm for zirconia. These alumina and zirconia compacts had high flexural strengths, typically around 800 and 1000 MPa, respectively. By optimising the sintering parameters we succeeded in consolidating various nano- or sub-micron sized oxide powders, e.g., ZnO, Al2O3, ZrO2, YAG, BaTiO3, to full density with a grain growth factor 2. 3. Preparation of composites containing metastable constituents α-sialon having the formula of MxSi12−(m+n)Alm+nOnN16−n is a solid solution isostructural with the α-Si3N4 phase, formed by substituting Al and O for Si and N, respectively, and simultaneously incorporating metal cations (M) into the interstices present in the α-Si3N4 structure. This phase was regarded as thermodynamically stable until the early 1990s, when it was discovered that in certain systems the α-sialon phase is stable only at high temperature (typically T > 1550 ◦C), and that post-heat treatment at lower temperatures made it decompose into the β-sialon modification, Si6−zAlzOzN8−z iso-structural with β-Si3N4, and other phases. For geometric reasons, supported by experimental findings, it has been argued that M cations with a radius larger than ≈ 1 Å cannot be accommodated in the α-sialon structure. It is difficult to understand why, however, because the available interstices are much larger, e.g., having a lateral extent of about 2.6 Å. SPS allows very rapid heating rates, and in the experiment described below we have used a rate of 200 ◦C/min and a pressure of 50 MPa. Such fast heating prevents the formation of transient M-rich oxide and oxynitride phases that often occurs in conventional sintering processes. Samples with an overall α-sialon composition and with M = Ce, La, Sr and Eu could thus be fully densified at a temperature as low as 1500 ◦C. Subsequent SEM, TEM and X-ray studies revealed that the α-sialon phase was formed in all SPSed samples. The main phases formed are thus: α-sialon, small amounts of β-sialon and the 21R polytypoid phase, SiAl6O2N6, and in the Sr(Eu)-containing samples a polytypoid phase of the composition Sr(Eu)Si9Al19ON31 [13]. Prolonged ex situ or in situ heat treatment at 1700 ◦C or above made the α-sialon phase decompose into β-sialon and various nitrogen-rich phases with overall compositions close to that of α-sialon. The geometric argument discussed above is thus not valid, but the M-doped α-sialons with M = Ce, La, Sr and Eu are thermodynamically unstable and decompose comparatively fast into more stable phases, and it is only possible to prepare α-sialon phases in these systems by applying very fast heating rate [14]. In conjunction with our work on the synthesis of new oxynitride compounds in the sialon system we have applied a similar technique. We have thus prepared almost monophasic compacts of the N phase, Ce3Si8−xAlxN11−xO4−x with x ≈ 1.75, abbreviated Ce-N phase below, and of the S phase, Ba2AlxSi12−xN16−xO2−x with x ≈ 2.0, abbreviated Ba-S phase below. Appropriate powder mixtures of Si3N4, AlN, Al2O3, and Ce2(CO3)3/BaCO3 were loaded into the pressure die and pre-heated at 1000 ◦C during 2 min in order to allow the carbonates to decompose, then heated at a rate of 200 ◦C/min to 1400–1600 ◦C, using a pressure of 50 MPa, and finally quenched when the ultimate sintering temperature was reached. Fully dense compacts were obtained, and it was noticed that the main part of the nitride reactants, Si3N4 and AlN, was retained in the compacted bodies, while the oxide reactants formed a fairly homogeneously distributed liquid/glassy phase, see Fig. 3a. Ex situ post heat treatment of these compacts at 1700 ◦C for 10 hours yielded almost monophasic species as seen in Fig. 3b, which made structural refinement of these phases possible. The monophasic samples of these types of compact are hard to obtain by conventional solid-state reactions, because evaporation of various constituents from a porous body cannot be avoided in this temperature region [15,16]. Hydroxyapatite (Ca10(PO4)6(OH)2, abbreviated HAp below), is recognized for its excellent bone bonding ability. Densified HAp components cannot be used as dental or orthopaedic implants because they are too brittle, and accordingly it is necessary to improve the mechanical properties of these components before they can be used as implants for load-bearing applications. Pure HAp is rather easily sintered to high densities and has been reported to be fully densified at a temperature as low as 800 ◦C, but higher temperatures are required for densification in composite oxide/HAp systems. The temperatures needed depend on the type and volume of the reinforcing material used. In the zirconium oxide/HAp system sintering temperatures as high as 1400 ◦C have been applied. A universal problem in connection with preparation of HAp/oxide based composites is that HAp reacts with the added oxide during densification. HAp begins to lose water around 600 ◦C, and an oxyhydroxyapatite is formed, see Eq. (1) below. In the presence of zirconia even more OH− ions are lost due to the reaction between oxyhydroxyapatite and zirconia according to Eq. (2) below. Ca10(PO4)6(OH)2 → Ca10(PO4)6(OH) (1) (2−2x)Ox + xH2O Ca10(PO4)6(OH)(2−2x)Ox + ZrO2 → 3 Ca3(PO4)2 + CaO(ZrO2) + (1 − x)H2O (2) The decomposition of oxyhydroxyapatite in the presence of zirconium oxide is reported to take place at a surprisingly low temperature (∼950 ◦C), taking into account that the oxyhydroxyapatite formed according to Eq. (1) is stable up to around 1400 ◦C in air. It goes without saying that a decomposition temperature of 950 ◦C is well below that required for densification of HAp/zirconia composites by conventional processes such as pressure-less sintering and hot pressing. By using the SPS technique, however, the deleterious reactions described above could be avoided. Thus for
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, respectively
M. 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 tetragonal zirconia grains have been fully densified and mechanically 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 diffraction studies combined with SEM and HRTEM investigations 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 formation 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
M Noren, Z Shen/Solid State Sciences 5(2003)125-131 TiN vol% formed in situ during sintering. This implies that extended 100 holding times at high sintering temperature are necessary to ensure the formation of tough interlocking microstructures 跃强然 consisting of elongated grains. When a rapid heating rate is A203 used, the grain growth is further accelerated by enhanced dissolution of precursor particles or of the small grains formed in the early stages of the sintering process, yielding a super-saturated liquid with a composition far from ther modynamic equilibrium. The liquid immediately responds 50 to this super-saturation by preferentially expelling appro- priate constituents onto the crystallographic surfaces that will most easily accommodate them, implying that elongated a-and/or B-sialon grains form and yield interlocking mi- 签点了 crostructure and compacts with improved mechanical prop- erties [21, 22] This is illustrated in Fig. 6, which depicts the microstruc tures of two Si3N4 samples, both doped with 7.5 wt%Y203 签 and 2.5 wt% Al2O3, one compacted by SPS at 1500 and the other at 1800C. The heating rate was 200C/min and the D 100Nm pressure 50 MPa, and both compacts are fully densified In the compact quenched from 1500C most of the sub-micron sized nearly equiaxed precursor a-Si3N4 grains(85%)are Fig 5 SEM micrograph of a laminated structure of TiN and Al2 O3(a), and retained, whereas the one quenched from 1800C contains layers of the composition(TIN(Al203)1-x with elongated grains of monophasic B-Si3N4. Note that the mi- crostructure shown in Fig 6a is transformed into the one shown in Fig. 6b within 1.5 min. Thus, under the applied sin- 5. Rapid formation of tough interlocking tering conditions the phase transformation and grain growth microstructures in Si3N4 ceramics progresses so fast that compacts consist- ing of sub-micron sized nearly equiaxed grains develop into It is commonly accepted that grain growth in covalent- microstructures consisting of interlocking elongated needles bonded Si3N4 based ceramics is a sluggish process, con- within a few minutes. The grain growth rate strongly de- trolled by diffusion, and favoured by the presence of a liq- pends on the heating rate applied and also on the amount uid and by proper nucleation sites, added extraneously or and composition of the added sintering aid. It is thus pos a Fig. 6. SEM micrographs depicting the microstructures of an SPSed Si3 N4 sample containing 7.5 wt%Y,O3 and 2.5 wt%Al]O, heated to 1500C(a), and to 1800C(b), using a heating rate of 200C/min and a pressure of 50 MPa
130 M. Nygren, Z. Shen / Solid State Sciences 5 (2003) 125–131 Fig. 5. SEM micrograph of a laminated structure of TiN and Al2O3 (a), and a structure containing layers of the composition (TiN)x (Al2O3)1−x with x ranging from 1 to 0 (b). 5. Rapid formation of tough interlocking microstructures It is commonly accepted that grain growth in covalentbonded Si3N4 based ceramics is a sluggish process, controlled by diffusion, and favoured by the presence of a liquid and by proper nucleation sites, added extraneously or formed in situ during sintering. This implies that extended holding times at high sintering temperature are necessary to ensure the formation of tough interlocking microstructures consisting of elongated grains. When a rapid heating rate is used, the grain growth is further accelerated by enhanced dissolution of precursor particles or of the small grains formed in the early stages of the sintering process, yielding a super-saturated liquid with a composition far from thermodynamic equilibrium. The liquid immediately responds to this super-saturation by preferentially expelling appropriate constituents onto the crystallographic surfaces that will most easily accommodate them, implying that elongated α-and/or β-sialon grains form and yield interlocking microstructures and compacts with improved mechanical properties [21,22]. This is illustrated in Fig. 6, which depicts the microstructures of two Si3N4 samples, both doped with 7.5 wt% Y2O3 and 2.5 wt% Al2O3, one compacted by SPS at 1500 and the other at 1800 ◦C. The heating rate was 200 ◦C/min and the pressure 50 MPa, and both compacts are fully densified. In the compact quenched from 1500 ◦C most of the sub-micron sized nearly equiaxed precursor α-Si3N4 grains (85%) are retained, whereas the one quenched from 1800 ◦C contains elongated grains of monophasic β-Si3N4. Note that the microstructure shown in Fig. 6a is transformed into the one shown in Fig. 6b within 1.5 min. Thus, under the applied sintering conditions the phase transformation and grain growth in Si3N4 ceramics progresses so fast that compacts consisting of sub-micron sized nearly equiaxed grains develop into microstructures consisting of interlocking elongated needles within a few minutes. The grain growth rate strongly depends on the heating rate applied and also on the amount and composition of the added sintering aid. It is thus posFig. 6. SEM micrographs depicting the microstructures of an SPSed Si3N4 sample containing 7.5 wt% Y2O3 and 2.5 wt% Al2O3 heated to 1500 ◦C (a), and to 1800 ◦C (b), using a heating rate of 200 ◦C/min and a pressure of 50 MPa
M. Nygren, Z Shen/Solid State Sciences 5(2003)125-131 13 sible to adjust the microstructures of the produced SiN Acknowledgements based ceramics by manipulating the grain growth kinetics and accordingly to tailor their mechanical properties Te wish to thank our colleagues dr. Zhe Zhao Dr. Mats Johnsson, Dr Jekabs Grins and Ms. Hong Peng for their contributions to the work presented in this article 6. Concluding remarks References Using conventional sintering processes, densification is ways accompanied by grain growth. We have demon [K Inoue, US Patent No. 3, 241, 956(1966) strated that the sPs process provides us with a unique op 22] M. Tokita, J. Soc. Powder Technol, Jpn. 30(1993)790 portunity to separate grain growth from densification. One []K. Yamazaki, S.H. Risbud, H. Aoyama, K Shoda, J. Mater. Proc outstanding feature of the SPS process is the short sintering ech65(1996955 times and low sintering temperatures needed to obtain fully [4 W.M. Goldberger, Mater. Tech. 10(1995)48. dense compacts. The use of extremely high heating and cool 5]ZA Munir, H. Schmalzried, J of Mater. Synth. Proc 1(1993)3 [6].S. Mishra, S.H. Risbud, A K. Mukherjee, J. Mater, Res. 13(1998) ing rates in conjunction with high pressures is certainly of importance in this connection. As discussed above, the oc- [7N Murayama, Bull. Ceram Soc. Jpn. 32(1997)445 currence of a plasma discharge is still debated, but it seems [8] M. Yoshimura, T. Ohji, M. Sando, K. Niihara, J. Mater. Sci. Lett. 17 to be widely accepted that an electric discharge process takes (1998)1389 9Y. Zhou, K. Hirao, M. Toriyama, H. Tanaka, J. Am. Ceram Soc. 83 place on a microscopic level. Since no current or only a very 000)654 weak one will pass through non-conducting samples, the dis- 10]RS Mishra, A K. Mukherjee, Mater. Sci. Eng. A 287(2000)178. charge ought to originate from the electric field set up by the [II]L Gao, Z Shen, H. Miyamoto, M. Nygren, J. Am. Ceram Soc. 82 pulsed direct current used. The intensity of this discharge is 9)1061 presumably not only dependent on the intensity of the ap- [12] Z Shen, M. Johnsson, Z Zhao, M. Nygren, J. Am. Ceram Soc. 85 02)1921 plied pulses but also on factors such as particle size, pore [13]J. Grins, S, Esmaeilzadeh, G, Svensson, Z. Shen, J. Euro. Ceram. size, and relative density of the compact. Anyhow, it seems Soc.19(1999)2723 plausible that it is only during the initial part of a sinter- 14Z Shen, M. Nygren, J. Euro Ceram Soc. 21(2001)611 ing process, i.e., up to the point where the system reaches 5]J Grins, Z Shen, S. Esmaeilzadeh, P. Berastegui, J. Mater. Chem. II a"closed porosity"state, that such a discharge process can 2001)238 make a major contribution to the densification. From that [16]J. Grins, Z Shen, in manuscript. [17Z Shen, E. Adolfsson, M. Nygren, L Gao, H Kawaoka, K Nihar, stage on, grain boundary diffusion and grain boundary mi- Adv. Mater.13(2001)214 gration ought to be the rate determining processes. An elec- [18T Hirai, L Chen, Mater. Interg. 12(1999)51 tric discharge process would certainly clean the surfaces of 19]J. Zhang, Z. Huang, D Jiang, S. Tan, Z. Shen, M. Nygren, J.Am. he powders from adsorbed species and create various types Ceram Soc. 85(2002) of surface defects that will enhance the grain boundary dif- 20]Z Shen, N. Nygren, Key Eng Mater. 206-213(2002)2155 21]Z Shen, M. Nygren, J Mater. Chem. 11(2001)20 fusion [22 Z Shen, Z Zhao, H Peng, M. Nygren, Nature 417(2002)266
M. Nygren, Z. Shen / Solid State Sciences 5 (2003) 125–131 131 sible to adjust the microstructures of the produced Si3N4 based ceramics by manipulating the grain growth kinetics and accordingly to tailor their mechanical properties. 6. Concluding remarks Using conventional sintering processes, densification is always accompanied by grain growth. We have demonstrated that the SPS process provides us with a unique opportunity to separate grain growth from densification. One outstanding feature of the SPS process is the short sintering times and low sintering temperatures needed to obtain fully dense compacts. The use of extremely high heating and cooling rates in conjunction with high pressures is certainly of importance in this connection. As discussed above, the occurrence of a plasma discharge is still debated, but it seems to be widely accepted that an electric discharge process takes place on a microscopic level. Since no current or only a very weak one will pass through non-conducting samples, the discharge ought to originate from the electric field set up by the pulsed direct current used. The intensity of this discharge is presumably not only dependent on the intensity of the applied pulses but also on factors such as particle size, pore size, and relative density of the compact. Anyhow, it seems plausible that it is only during the initial part of a sintering process, i.e., up to the point where the system reaches a “closed porosity” state, that such a discharge process can make a major contribution to the densification. From that stage on, grain boundary diffusion and grain boundary migration ought to be the rate determining processes. An electric discharge process would certainly clean the surfaces of the powders from adsorbed species and create various types of surface defects that will enhance the grain boundary diffusion. Acknowledgements We wish to thank our colleagues Dr. Zhe Zhao, Dr. Mats Johnsson, Dr. Jekabs Grins and Ms. Hong Peng for their contributions to the work presented in this article. References [1] K. Inoue, US Patent No. 3,241,956 (1966). [2] M. Tokita, J. Soc. Powder Technol., Jpn. 30 (1993) 790. [3] K. Yamazaki, S.H. Risbud, H. Aoyama, K. Shoda, J. Mater. Proc. Tech. 65 (1996) 955. [4] W.M. Goldberger, Mater. Tech. 10 (1995) 48. [5] Z.A. Munir, H. Schmalzried, J. of Mater. Synth. & Proc. 1 (1993) 3. [6] R.S. Mishra, S.H. Risbud, A.K. Mukherjee, J. Mater. Res. 13 (1998) 86. [7] N. Murayama, Bull. Ceram. Soc. Jpn. 32 (1997) 445. [8] M. Yoshimura, T. Ohji, M. Sando, K. Niihara, J. Mater. Sci. Lett. 17 (1998) 1389. [9] Y. Zhou, K. Hirao, M. Toriyama, H. Tanaka, J. Am. Ceram. Soc. 83 (2000) 654. [10] R.S. Mishra, A.K. Mukherjee, Mater. Sci. Eng. A 287 (2000) 178. [11] L. Gao, Z. Shen, H. Miyamoto, M. Nygren, J. Am. Ceram. Soc. 82 (1999) 1061. [12] Z. Shen, M. Johnsson, Z. Zhao, M. Nygren, J. Am. Ceram. Soc. 85 (2002) 1921. [13] J. Grins, S. Esmaeilzadeh, G. Svensson, Z. Shen, J. Euro. Ceram. Soc. 19 (1999) 2723. [14] Z. Shen, M. Nygren, J. Euro. Ceram. Soc. 21 (2001) 611. [15] J. Grins, Z. Shen, S. Esmaeilzadeh, P. Berastegui, J. Mater. Chem. 11 (2001) 2358. [16] J. Grins, Z. Shen, in manuscript. [17] Z. Shen, E. Adolfsson, M. Nygren, L. Gao, H. Kawaoka, K. Niihar, Adv. Mater. 13 (2001) 214. [18] T. Hirai, L. Chen, Mater. Interg. 12 (1999) 51. [19] J. Zhang, Z. Huang, D. Jiang, S. Tan, Z. Shen, M. Nygren, J. Am. Ceram. Soc. 85 (2002). [20] Z. Shen, N. Nygren, Key Eng. Mater. 206–213 (2002) 2155. [21] Z. Shen, M. Nygren, J. Mater. Chem. 11 (2001) 204. [22] Z. Shen, Z. Zhao, H. Peng, M. Nygren, Nature 417 (2002) 266