Availableonlineatwww.sciencedirect.com Science Direct CERAMICS INTERNATIONAL ELSEVIER Ceramics International 34(2008)2129-2135 www.elsevier.com/locate/ceramint Critical zirconia amount to enhance the strength of alumina Wei-Hsing Tuan", Jiang-Rung Chen, Chang-Ju Ho Department of Materials Science and Engineering, National Taiwan University. Taipei 106. Te ised form 24 April 2007; accepted 20 August 2007 Available online 29 September 2007 Abstract A small amount of zirconia particles(<5 vol %)is added into alumina in the present study. The grain size of alumina is reduced; the strength of alumina is therefore enhanced. Though the theoretical analysis demonstrates that an addition of l vol. of fine zirconia particles is sufficient to prohibit the coarsening of alumina grains; the experimental measurements indicate that a minimum amount of 2 vol %o is required to reduce the coarsening of alumina matrix and its size distribution. This discrepancy is due to the separation between the zirconia particles and alumina grain boundaries, which takes place when the alumina grain size increases above a critical value Keywords: A. Sintering: B. Microstructure-final: C. Strength; D. Al2O3; D. ZrO2 1. ntroduction inert to the ceramic matrix during sintering at elevated temperature. Among the composite systems investigated Ceramics are brittle in nature. Such brittleness limits many alumina-zirconia systems have attracted wide attention. The pplications involving using ceramics as engineering compo- solubility between alumina and zirconia is low(2000 ppm at nents. To improve the toughness of ceramics, there has been 1450C(9), and the engineering potential of the composites is luch work in the last several decades. Since an increase of high. In the present study, the alumina-zirconia system is also strength also improves the possibility of brittle ceramics to used as a model system to explore the rule of microstructure survive external impacts, increasing the strength of ceramic is design for strength enhancement. also an important task. The first study on the Al2O3-ZrOz system employed a Several relationships have been derived to describe the minimum amount of 2 vol. ZrO2 particles to enhance the strength, o, of brittle ceramics [1, 2]. One relationship is that toughness of Al2O3 [5]. The study demonstrated that the proposed by griffith: toughness of Al2O3 could be enhanced only when the ZrO ontent was higher than 5 vol % o. Many studies were then K (1) performed to confirm this approach [10-14]. Most studies have added more than 5 vol. ZrO2 into Al2O3. They have demonstrated that both the toughness and strength of alumina constant, and C is the flaw size. The above relationship demon- are improved, nevertheless, the sinterability of the ceramic strates that the strength of a ceramic depends strongly on its matrix is reduced when a large amount of second phase is added faw size and grain size. [15]. Furthermore, the cost of the composites is also increased The reduction of flaw size can be achieved by controlling In order to cope with the drop of sintering activity and the ncrease of cost, a small amount, <5 vol %, of second phase is each processing step carefully [3]. The decrease of grain size therefore of interest. Such an approach has received little attention in previous studies. One limited study indicated that 8). The inhibitors can be oxides [5] non-oxides 4, 6 or the properties were not improved by using a small amount, metallic particles [7, 8]. These particles should be relatively <5 vol % of particles [5]. The dilemma on designing ceramic matrix composite is thus apparent. On the one hand, as the Corresponding author. Tel: +886 2 23659800: fax: +886 2 23634562 second phase content is low, the sintering activity is little tuan @ccms. ntu. edu. tw(W-H. Tuan affected. But the properties improvement is suspected to be also 2-8842/34.00@ 2007 Elsevier Ltd and Techna Group S.r.L. All rights reserved 10.016.ceramist200708.013
Critical zirconia amount to enhance the strength of alumina Wei-Hsing Tuan *, Jiang-Rung Chen, Chang-Ju Ho Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan Received 23 July 2006; received in revised form 24 April 2007; accepted 20 August 2007 Available online 29 September 2007 Abstract A small amount of zirconia particles (5 vol.%) is added into alumina in the present study. The grain size of alumina is reduced; the strength of alumina is therefore enhanced. Though the theoretical analysis demonstrates that an addition of 1 vol.% of fine zirconia particles is sufficient to prohibit the coarsening of alumina grains; the experimental measurements indicate that a minimum amount of 2 vol.% is required to reduce the coarsening of alumina matrix and its size distribution. This discrepancy is due to the separation between the zirconia particles and alumina grain boundaries, which takes place when the alumina grain size increases above a critical value. # 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Sintering; B. Microstructure-final; C. Strength; D. Al2O3; D. ZrO2 1. Introduction Ceramics are brittle in nature. Such brittleness limits many applications involving using ceramics as engineering components. To improve the toughness of ceramics, there has been much work in the last several decades. Since an increase of strength also improves the possibility of brittle ceramics to survive external impacts, increasing the strength of ceramic is also an important task. Several relationships have been derived to describe the strength, s, of brittle ceramics [1,2]. One relationship is that proposed by Griffith: s ¼ KIC Y ffiffiffi C p (1) In the above equation, KIC is the toughness, Y a geometrical constant, and C is the flaw size. The above relationship demonstrates that the strength of a ceramic depends strongly on its flaw size and grain size. The reduction of flaw size can be achieved by controlling each processing step carefully [3]. The decrease of grain size can be reached through the addition of second phase particles [4–8]. The inhibitors can be oxides [5], non-oxides [4,6] or metallic particles [7,8]. These particles should be relatively inert to the ceramic matrix during sintering at elevated temperature. Among the composite systems investigated, alumina–zirconia systems have attracted wide attention. The solubility between alumina and zirconia is low (2000 ppm at 1450 8C [9]), and the engineering potential of the composites is high. In the present study, the alumina-zirconia system is also used as a model system to explore the rule of microstructure design for strength enhancement. The first study on the Al2O3–ZrO2 system employed a minimum amount of 2 vol.% ZrO2 particles to enhance the toughness of Al2O3 [5]. The study demonstrated that the toughness of Al2O3 could be enhanced only when the ZrO2 content was higher than 5 vol.%. Many studies were then performed to confirm this approach [10–14]. Most studies have added more than 5 vol.% ZrO2 into Al2O3. They have demonstrated that both the toughness and strength of alumina are improved; nevertheless, the sinterability of the ceramic matrix is reduced when a large amount of second phase is added [15]. Furthermore, the cost of the composites is also increased. In order to cope with the drop of sintering activity and the increase of cost, a small amount, <5 vol.%, of second phase is therefore of interest. Such an approach has received little attention in previous studies. One limited study indicated that the properties were not improved by using a small amount, <5 vol.%, of particles [5]. The dilemma on designing ceramic matrix composite is thus apparent. On the one hand, as the second phase content is low, the sintering activity is little affected. But the properties improvement is suspected to be also www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 34 (2008) 2129–2135 * Corresponding author. Tel.: +886 2 23659800; fax: +886 2 23634562. E-mail address: tuan@ccms.ntu.edu.tw (W.-H. Tuan). 0272-8842/$34.00 # 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2007.08.013
213 W-H. Tuan et al /Ceramics international 34(2008 )2129-2135 small (it will be demonstrated that this may not be the case).On the other hand, when the second phase content is high, the mechanical properties are improved. However, the cost increase associated with the material hinders the application potential of In the present study, a small amount of zro2 particles, c5 vol %, is added into Al,O3. The microstructure of the 90 opposites is carefully investigated. The minimum required amount of second phase is suggested in terms of achieving strength enhancement 2. Experimental An alumina (TM-DAR, dso=140 nm, Taimei Chem. Ce Ltd, Nagno-ken, Japan) powder was ball milled together with -5 vol zirconia powder (Tz-3Y, ZrO2 +3 mol %o Y203, 150=230 nm, Tosoh Co., Japan)in ethyl alcohol for 24 h, using 10 mm diameter zirconia balls as grinding media. The slurry of the powder mixtures was dried with a rotary evaporator, and ZrO, content/vol. then the dried lumps were crushed and passed through a plastic ieve Powder compacts with dimensions of 7 mm x 6 mm Fig. 1. Green and sintered density of the Al2O3/ZrO2 composites. 50 mm were formed by uniaxially pressing at 44 MPa. Sintering was carried out in a box furnace at 1600C for 1 h 3. Results in air with heating rate of 5C/min. a number of vith a diameter of 6 mm in diameter were prepared for a kinetic The density of the green powder compacts increases slightly study with a dilatometer(SETSYS 1600, TMA, Setaram Co., from 59.5% to 60.5% with the ZrO2 content varies from zero Caluire, France). The heating rate for the dilatometer was also five vol %(see Fig. 1). The densities of the sintered Al,O/ZrO 5°C/min. composites are all higher than 99%0. Fig. 2 shows the XRD The sintered rectangular bars were ground longitudinally patterns of the Al2O3 and Al2O3/5% ZrO2 composite. XRD with a 325 grit resin-bonded diamond wheel at a depth of 5 um analysis shows only Al2O3 and ZrO2 phases in the sintered per pass. The final dimensions of the specimens were Al2O,/ZrO2 composites. Fig 3(a)shows the linear shrinkage of 3 mm x 4 mm x 36 mm. The strength of the specimens was the Al2O/ZrO2 composites during sintering. The presence of determined using a four-point bend test carried out at ambient the ZrO2 particles delays the shrinkage of Al2O3 in the conditions. The upper and lower spans were 10 mm and 30 mm, temperature range from 1000 to 1400C. However, the respectively. The rate of loading was 0.5 mm/min. The fracture temperature at the peak densification rate(the second peak toughness was determined by the single-edge-notched-beam in Fig. 3b)remains the same. The densification for all the (SENB )technique. The notch was generated by cutting with a composites is almost complete as the temperature reaches diamond saw. The width of the notch was approximately 1450C Fig. 4 shows typical SEM images of the specimens. These Phase identification was performed on sintered and micrographs demonstrate that the addition of Zro2 particles fractured surfaces by X-ray diffractometry(XRD) with Cu Ko radiation. The relative phase content of zirconia was estimated by using the method proposed by Evans et al. [16] The final density of the specimens was determined by the Archimedes method. The solubility between the materials used the present study was low; the relative density of the sintered composites was estimated by using the theoretical density of 3.98 g/cm'for Al2O3 and 6.05 g/cm' for ZrO2. Polished surfaces for microstructure observation were prepared by 2 AL-O zrO grinding and polishing with diamond paste to 6 um and with silica suspension to 0.05 um. The polished specimens were thermally etched at 1450C for 0.5 h to reveal the grain boundaries of matrix grains. Microstructure characterization was conducted using scanning electron microscopy (SEM) mage analysis was conducted on SEM micrographs to determine the average size of Al2O3 grains and their size Two Theta/Degree distribution Fig. 2. XRD patterns of pure Al2O3 and AlO/5% ZrO2 composite
small (it will be demonstrated that this may not be the case). On the other hand, when the second phase content is high, the mechanical properties are improved. However, the cost increase associated with the material hinders the application potential of the composites. In the present study, a small amount of ZrO2 particles, 5 vol.%, is added into Al2O3. The microstructure of the composites is carefully investigated. The minimum required amount of second phase is suggested in terms of achieving strength enhancement. 2. Experimental An alumina (TM–DAR, d50 = 140 nm, Taimei Chem. Co. Ltd., Nagno-ken, Japan) powder was ball milled together with 0–5 vol.% zirconia powder (TZ-3Y, ZrO2 + 3 mol.% Y2O3, d50 = 230 nm, Tosoh Co., Japan) in ethyl alcohol for 24 h, using 10 mm diameter zirconia balls as grinding media. The slurry of the powder mixtures was dried with a rotary evaporator, and then the dried lumps were crushed and passed through a plastic sieve. Powder compacts with dimensions of 7 mm 6 mm 50 mm were formed by uniaxially pressing at 44 MPa. Sintering was carried out in a box furnace at 1600 8C for 1 h in air with heating rate of 5 8C/min. A number of disc samples with a diameter of 6 mm in diameter were prepared for a kinetic study with a dilatometer (SETSYS 1600, TMA, Setaram Co., Caluire, France). The heating rate for the dilatometer was also 5 8C/min. The sintered rectangular bars were ground longitudinally with a 325 grit resin-bonded diamond wheel at a depth of 5 mm per pass. The final dimensions of the specimens were 3 mm 4 mm 36 mm. The strength of the specimens was determined using a four-point bend test carried out at ambient conditions. The upper and lower spans were 10 mm and 30 mm, respectively. The rate of loading was 0.5 mm/min. The fracture toughness was determined by the single-edge-notched-beam (SENB) technique. The notch was generated by cutting with a diamond saw. The width of the notch was approximately 0.3 mm. Phase identification was performed on sintered and fractured surfaces by X-ray diffractometry (XRD) with Cu Ka radiation. The relative phase content of zirconia was estimated by using the method proposed by Evans et al. [16]. The final density of the specimens was determined by the Archimedes method. The solubility between the materials used in the present study was low; the relative density of the sintered composites was estimated by using the theoretical density of 3.98 g/cm3 for Al2O3 and 6.05 g/cm3 for ZrO2. Polished surfaces for microstructure observation were prepared by grinding and polishing with diamond paste to 6 mm and with silica suspension to 0.05 mm. The polished specimens were thermally etched at 1450 8C for 0.5 h to reveal the grain boundaries of matrix grains. Microstructure characterization was conducted using scanning electron microscopy (SEM). Image analysis was conducted on SEM micrographs to determine the average size of Al2O3 grains and their size distribution. 3. Results The density of the green powder compacts increases slightly from 59.5% to 60.5% with the ZrO2 content varies from zero to five vol.% (see Fig. 1). The densities of the sintered Al2O3/ZrO2 composites are all higher than 99%. Fig. 2 shows the XRD patterns of the Al2O3 and Al2O3/5% ZrO2 composite. XRD analysis shows only Al2O3 and ZrO2 phases in the sintered Al2O3/ZrO2 composites. Fig. 3(a) shows the linear shrinkage of the Al2O3/ZrO2 composites during sintering. The presence of the ZrO2 particles delays the shrinkage of Al2O3 in the temperature range from 1000 to 1400 8C. However, the temperature at the peak densification rate (the second peak in Fig. 3b) remains the same. The densification for all the composites is almost complete as the temperature reaches 1450 8C. Fig. 4 shows typical SEM images of the specimens. These micrographs demonstrate that the addition of ZrO2 particles Fig. 1. Green and sintered density of the Al2O3/ZrO2 composites. Fig. 2. XRD patterns of pure Al2O3 and Al2O3/5% ZrO2 composite. 2130 W.-H. Tuan et al. / Ceramics International 34 (2008) 2129–2135
W-H. Tie 2131 Fig. 6 shows the flexural strength of the Al,O3/ZrO2 omposites as a function of Zro2 content. The flexural strength of the composites increases with the increase of ZrO2 ontent. Apart from the dependence on the ZrO2 content, the strength as expected also shows strong dependence on the size of Al2O3 grains (see Fig. 7). Fig. 8 shows the fracture toughness of the Al2O3/ZrO2 composites as a function of ZrO2 content. The XRD analysis shows that around 30% of the Zro2 particles on the fracture surface of the Al2O3/5% Zro composites is transformed from tetragonal to monoclinic phase, indicating that transformation toughening is active -·1%Z02 2% Zr0, 3%Z02 4. Discussion 490 Zro 5% Zro Two peaks, a small peak at 1160C and a larger one at 8009001000110012001300140015001600 1400C, are found in the sintering kinetic curves(see Fig 3b) The first peak can be related to the presence of Al2O3 agglomerates due to the fact that intra-agglomerate pores are usually small [17]. The addition of more than 2 vol. ZrO2 ·3%ZrO particles reduces the height of the first peak, suggesting that the 4 Zro Al2O3 agglomerates are dispersed due to the presence of zro ==2%0Z 5%Zr0 particles. The reduction of agglomerate can contribute to the reduction of flaw size [3]: the strength of the alumina Through microstructure observation shown in Fig. 4 indicates that the ZrO2 particles act as effective grain growth inhibitors to Al2O3. The size of alumina grains reduces from 5.5 um (for pure alumina) to 2.2 um (for Al2O3/5% ZrO2 5 composite). Except the Al2O3/1% ZrO2 composite, the size scatter of the Al2O3 grains in the composites is also reduced Most Zro2 particles are well separated within the Al2O3 matrix. Since the coarsening of ZrO2 particles can then take place only through the diffusion within the Al2O3 matrix, this is a relatively slow process. Therefore, the size of zirconia particles is more or less the same in all the Al2O3/ZrO composites, indicating that the coarsening of well-dispersed 900100011001200130014001500160 ZrO, inclusions is limited. Such limited co related to the low ZrO2 content used in the present study. The Fig 3.(a)Percent shrinkage and (b)densification rate as function of tempera- coarsening of ZrO2 particles is found only in the ZrO ture for various Al2O3 /ZrO2 composites. agglomerate(see Fig 4e). For a composite containing mono-sized inclusions, the grain reduces the size of Al2O3 grains( see Table 1). In the Al2O3/1% size decreases with the increase of particle content(Zener ZrO2 composite (see Fig. 4b), most ZrO2 particles are effect). The nearest neighbor distance (or mean free path), embedded within the AlO3 grains. For the composites between randomly distributed particles can be estimated with containing more than 2 vol. ZrO2, the particles are mainly the following relationship proposed by Westmacott et al. [18] at the boundaries of the Al2O3 matrix grains(see Fig. 4c-f) The and Kock [19]; size of ZrO2 particles is about 0.3 um which is more or less the same in all the Al2O3/ZrO2 composites. Nevertheless, some ZrO2 agglomerates are found occasionally(see Fig. 4e). The ZrO2 grains in the agglomerate are larger than the isolated ZrO2 particles. Fig. 5 shows the size distribution of Al2O3 grains for where d is the size of particle and F is the volume fraction. The the Al2O3 and Al2O3/ZrO2 specimens. When the Zro2 content calculated values for the distance between nearest neighboring is higher than 2 vol %, the addition of a small amount of Zro2 ZrO2 particles in Al2O3 matrix are shown in Table 1. The table reduces not only the average size of Al2O3 grains but also their indicates that the mean free path between the Zro2 particles in distribution. In the Al2O3/Zro2 composite with 1% ZrO2, all the Al2O3/Zro, composites is shorter than the correspondin several large grains are still present(Figs. 4b and 5) size of the Al2O3 grains
reduces the size of Al2O3 grains (see Table 1). In the Al2O3/1% ZrO2 composite (see Fig. 4b), most ZrO2 particles are embedded within the Al2O3 grains. For the composites containing more than 2 vol.% ZrO2, the particles are mainly at the boundaries of the Al2O3 matrix grains (see Fig. 4c–f). The size of ZrO2 particles is about 0.3 mm which is more or less the same in all the Al2O3/ZrO2 composites. Nevertheless, some ZrO2 agglomerates are found occasionally (see Fig. 4e). The ZrO2 grains in the agglomerate are larger than the isolated ZrO2 particles. Fig. 5 shows the size distribution of Al2O3 grains for the Al2O3 and Al2O3/ZrO2 specimens. When the ZrO2 content is higher than 2 vol.%, the addition of a small amount of ZrO2 reduces not only the average size of Al2O3 grains but also their distribution. In the Al2O3/ZrO2 composite with 1% ZrO2, several large grains are still present (Figs. 4b and 5). Fig. 6 shows the flexural strength of the Al2O3/ZrO2 composites as a function of ZrO2 content. The flexural strength of the composites increases with the increase of ZrO2 content. Apart from the dependence on the ZrO2 content, the strength as expected also shows strong dependence on the size of Al2O3 grains (see Fig. 7). Fig. 8 shows the fracture toughness of the Al2O3/ZrO2 composites as a function of ZrO2 content. The XRD analysis shows that around 30% of the ZrO2 particles on the fracture surface of the Al2O3/5% ZrO2 composites is transformed from tetragonal to monoclinic phase, indicating that transformation toughening is active in the composites. 4. Discussion Two peaks, a small peak at 1160 8C and a larger one at 1400 8C, are found in the sintering kinetic curves (see Fig. 3b). The first peak can be related to the presence of Al2O3 agglomerates due to the fact that intra-agglomerate pores are usually small [17]. The addition of more than 2 vol.% ZrO2 particles reduces the height of the first peak, suggesting that the Al2O3 agglomerates are dispersed due to the presence of ZrO2 particles. The reduction of agglomerate can contribute to the reduction of flaw size [3]; the strength of the alumina is therefore enhanced. Through microstructure observation shown in Fig. 4 indicates that the ZrO2 particles act as effective grain growth inhibitors to Al2O3. The size of alumina grains reduces from 5.5 mm (for pure alumina) to 2.2 mm (for Al2O3/5% ZrO2 composite). Except the Al2O3/1% ZrO2 composite, the size scatter of the Al2O3 grains in the composites is also reduced. Most ZrO2 particles are well separated within the Al2O3 matrix. Since the coarsening of ZrO2 particles can then take place only through the diffusion within the Al2O3 matrix, this is a relatively slow process. Therefore, the size of zirconia particles is more or less the same in all the Al2O3/ZrO2 composites, indicating that the coarsening of well-dispersed ZrO2 inclusions is limited. Such limited coarsening can be related to the low ZrO2 content used in the present study. The coarsening of ZrO2 particles is found only in the ZrO2 agglomerate (see Fig. 4e). For a composite containing mono-sized inclusions, the grain size decreases with the increase of particle content (Zener effect). The nearest neighbor distance (or mean free path), l, between randomly distributed particles can be estimated with the following relationship proposed by Westmacott et al. [18] and Kock [19]; l ¼ p 6 1=2 d F1=2 (2) where d is the size of particle and F is the volume fraction. The calculated values for the distance between nearest neighboring ZrO2 particles in Al2O3 matrix are shown in Table 1. The table indicates that the mean free path between the ZrO2 particles in all the Al2O3/ZrO2 composites is shorter than the corresponding size of the Al2O3 grains. Fig. 3. (a) Percent shrinkage and (b) densification rate as function of temperature for various Al2O3/ZrO2 composites. W.-H. Tuan et al. / Ceramics International 34 (2008) 2129–2135 2131
2132 W.H. Tuan et al /Ceramics international 34(2008)2129-2135 (b) 之 tm知 (D) Fig 4. SEM images of the(a) Al2O3, (b)Al203/1% ZrO2(c)Al203/2% ZrO2,(d)Al2O3/3% ZrO2,(e)Al203/4%0 ZrO2, and(f)Al203/5% ZrO2 composites.Al It is thus of interest to estimate the minimum amount of particle(dark particle). As the size of particle is the same as that econd phase particles needed to prohibit the growth of matrix of matrix grains, the particle content is around 50 vol % grains. By assuming that one particle is enough to prohibit the Though the microstructure should be stable throughout the movement of one grain boundary, then A NG(G=size of matrix sintering process, such high particle content can, not only grains). The amount of particle needed to prohibit the growth of prohibit the grain growth but also limit the sintering activity of matrix grains can be estimated by re-arranging Eq. (2)as, the ceramic matrix grains. Therefore, the amount of the second phase should be lower than 50 vol %o /2 As the particle size is reduced to one tenth that of matrix grains, Fig. 9(b), a volume fraction of 0.5 vol %o is obtained by using Eq. (3). If the d/G ratio is further decreased to 0.01, the For the microstructure shown in Fig. 9(a), each grain amount of second phase can then be reduced to 0.005 vol %. boundary of matrix grains(white grains) is pinned by one The analysis implies that the amount of l vol %o particle may be Table I Microstructure characteristics of the Al2O,/ZrO2 composites Mean free path between of Al2O3 grains(um) ZrO2 particles(um) Al,O1% ZrO2 Al,O2% ZrO2 1.2/40% 2O3/4%Zr 1038 AL,O,5% ZrO 0.76/35% b Calculated by using Eq. (1); the size of ZrO2 particle M0.3 um
It is thus of interest to estimate the minimum amount of second phase particles needed to prohibit the growth of matrix grains. By assuming that one particle is enough to prohibit the movement of one grain boundary, then l G (G = size of matrix grains). The amount of particle needed to prohibit the growth of matrix grains can be estimated by re-arranging Eq. (2) as, F1=2 ¼ p 6 1=2 d G (3) For the microstructure shown in Fig. 9(a), each grain boundary of matrix grains (white grains) is pinned by one particle (dark particle). As the size of particle is the same as that of matrix grains, the particle content is around 50 vol.%. Though the microstructure should be stable throughout the sintering process, such high particle content can, not only prohibit the grain growth but also limit the sintering activity of the ceramic matrix grains. Therefore, the amount of the second phase should be lower than 50 vol.%. As the particle size is reduced to one tenth that of matrix grains, Fig. 9(b), a volume fraction of 0.5 vol.% is obtained by using Eq. (3). If the d/G ratio is further decreased to 0.01, the amount of second phase can then be reduced to 0.005 vol.%. The analysis implies that the amount of 1 vol.% particle may be Fig. 4. SEM images of the (a) Al2O3, (b) Al2O3/1% ZrO2, (c) Al2O3/2% ZrO2, (d) Al2O3/3% ZrO2, (e) Al2O3/4% ZrO2, and (f) Al2O3/5% ZrO2 composites. An agglomerate of ZrO2 in (e) is indicated with arrow. Table 1 Microstructure characteristics of the Al2O3/ZrO2 composites Average size of Al2O3 grains (mm) Standard deviation/ coefficient of variationa Mean free path between ZrO2 particlesb (mm) Al2O3 5.5 2.5/45% – Al2O3/1% ZrO2 3.8 1.9/50% 2.2 Al2O3/2% ZrO2 3.0 1.2/40% 1.5 Al2O3/3% ZrO2 3.0 1.2/40% 1.3 Al2O3/4% ZrO2 2.6 1.0/38% 1.1 Al2O3/5% ZrO2 2.2 0.76/35% 1.0 a Coefficient of variation = standard deviation/average value. b Calculated by using Eq. (1); the size of ZrO2 particle 0.3 mm. 2132 W.-H. Tuan et al. / Ceramics International 34 (2008) 2129–2135
W.-H. Than et al. /Ceramics International 34(2008)2129-2135 2133 3%Zr0. --4%o ZrO2 5%Zr0 grain sIze/m log(grain size)/m Fig. 5. Size distribution of Al2O3 grains in the Al2O/ZrO2 composites. Fig. 7. Flexural strength of the Al2O,ZrO2 composites as function of Al2O3 grain size. more than enough to prohibit the grain growth of Al O3 matrix. However, as the d/G value is too low, it may encourage the separation of the particle and grain boundary, as demonstrated in the Al2O3/l vol %o ZrO2 composite(see Fig. 4b). In the composite, most Zro2 particles are separated from the gr boundaries of Al2O3 grains. The ZrO2 particles with such low amount can no longer prohibit the growth of matrix grains; arse grains are thus surv Fig 5). In order to prohibit the movement of grain boundary, the particle is preferably located at the grain boundary as the case shown in Fig. 9(a)and (b). The case as demonstrated in Fig 9(c) should be avoided. Therefore, the criteria for the separation of secondary particles and grain boundary should be investigated ZrO, content/vol% Fig. 8. Fracture toughness of the composites as function of zro, content. During sintering, with the help of a number of transport mechanisms the matrix grains can grow with the help from the movement of grain boundary. The velocity of the grain boundary movement is virtually the same as the grain growth rate. As the velocity of grain boundary, Ub, is much higher than that of particle, Up, the separation can take place(see Fig. 10) The following relationship defines the criteria for the separation to take place Ub>Un In order for the particle and grain boundary to move, the velocity is contributed by mobility, M, and force, F,as Fig. 6. Flexural strength of the composites as function of ZrO2 content. U=MF
more than enough to prohibit the grain growth of Al2O3 matrix. However, as the d/G value is too low, it may encourage the separation of the particle and grain boundary, as demonstrated in the Al2O3/1 vol.% ZrO2 composite (see Fig. 4b). In the composite, most ZrO2 particles are separated from the grain boundaries of Al2O3 grains. The ZrO2 particles with such low amount can no longer prohibit the growth of matrix grains; several coarse grains are thus survived after sintering (see Fig. 5). In order to prohibit the movement of grain boundary, the particle is preferably located at the grain boundary as the case shown in Fig. 9(a) and (b). The case as demonstrated in Fig. 9(c) should be avoided. Therefore, the criteria for the separation of secondary particles and grain boundary should be investigated. During sintering, with the help of a number of transport mechanisms the matrix grains can grow with the help from the movement of grain boundary. The velocity of the grain boundary movement is virtually the same as the grain growth rate. As the velocity of grain boundary, vb, is much higher than that of particle, vp, the separation can take place (see Fig. 10). The following relationship defines the criteria for the separation to take place: vb > vmax p (4) In order for the particle and grain boundary to move, the velocity is contributed by mobility, M, and force, F, as Fig. 6. Flexural strength of the composites as function of ZrO v ¼ MF (5) 2 content. Fig. 7. Flexural strength of the Al2O3/ZrO2 composites as function of Al2O3 grain size. Fig. 8. Fracture toughness of the composites as function of ZrO2 content. Fig. 5. Size distribution of Al2O3 grains in the Al2O3/ZrO2 composites. W.-H. Tuan et al. / Ceramics International 34 (2008) 2129–2135 2133
2134 W.H. Tuan et al /Ceramics international 34(2008)2129-2135 b) Fig. 10. Interaction between a moving grain boundary and a moving particl where 3 is the volume of controlling ions and n is a constant depending on the mechanism. The force on the particle is also provided by the grain boundary energy By using the relation ship to estimate the force on an isolated pore [20], the force on a particle is estimated as F=2rd (8) The velocity of particle, Up Up=MpFp comparing Eqs. (6)and (9), the following equation is Fig. 9. Interactions between grain boundary and wing(a) both matrix and the particles are the maller interboundary particles. The particles in 2TDeffs2 dn (10) boundaries due to the matrix grains are larger than For the grain boundary, its mobility is controlled by the The analysis above demonstrates that there is a critical diffusion across the grain boundary, D, the Boltzmanns value for the ratio of grain size over particle size. Since the constant, k, and the absolute temperature, T. The force on the grains grow rapidly as the density is higher than around 80% grain boundary is controlled by the grain boundary energy, Ygb, [20]. Along with the grain growth, the ratio of grain size over and grain size, G. The velocity of boundary, Ub, can then be particle size also increases. Since the coarsening of particles is limited as the second phase amount is low, the separation between the grain boundary and particle is virtually controlled 1b=16Fh、D (6) by the growth of matrix grains. From the microstructure observation, the critical size for the Al2O3 grain to separate For the particle, mass can transport along the interface or the from the ZrO2 particle is around 4 um. Si the ZrO, nterior of the particle or both. Therefore, an effective diffusion particles in all the Al2O/ZrO2 composite are similar in size. oefficient, Deff, to illustrate the mobility of the particle is used the increase of ZrO2 content increases the number of Zro instead as [20] particles More Zro2 particles at each grain boundary exert higher dragging force to the movement of grain boundary. The k 7 size of alumina grains is thus reduced with the increase of Ta ZrO, content
For the grain boundary, its mobility is controlled by the diffusion across the grain boundary, D* , the Boltzmann’s constant, k, and the absolute temperature, T. The force on the grain boundary is controlled by the grain boundary energy, ggb, and grain size, G. The velocity of boundary, vb, can then be expressed as [20]; vb ¼ MbFb ¼ D kT ggb G (6) For the particle, mass can transport along the interface or the interior of the particle or both. Therefore, an effective diffusion coefficient, Deff, to illustrate the mobility of the particle is used instead as [20]; Mp ¼ DeffV kTdn (7) where V is the volume of controlling ions and n is a constant depending on the mechanism. The force on the particle is also provided by the grain boundary energy. By using the relationship to estimate the force on an isolated pore [20], the force on a particle is estimated as Fp ¼ 2pdggb (8) The velocity of particle, vp, is vp ¼ MpFp ¼ DeffV kTdn 2pdggb (9) By comparing Eqs. (6) and (9), the following equation is obtained: D 2pDeffV > G dn1 (10) The analysis above demonstrates that there is a critical value for the ratio of grain size over particle size. Since the grains grow rapidly as the density is higher than around 80% [20]. Along with the grain growth, the ratio of grain size over particle size also increases. Since the coarsening of particles is limited as the second phase amount is low, the separation between the grain boundary and particle is virtually controlled by the growth of matrix grains. From the microstructure observation, the critical size for the Al2O3 grain to separate from the ZrO2 particle is around 4 mm. Since the ZrO2 particles in all the Al2O3/ZrO2 composite are similar in size, the increase of ZrO2 content increases the number of ZrO2 particles. More ZrO2 particles at each grain boundary exert higher dragging force to the movement of grain boundary. The size of alumina grains is thus reduced with the increase of ZrO2 content. Fig. 9. Interactions between grain boundary and second phase particles; showing (a) both matrix and the particles are the same size and (b) much smaller interboundary particles. The particles in (c) fail to pin the grain boundaries due to the matrix grains are larger than a critical value. Fig. 10. Interaction between a moving grain boundary and a moving particle. 2134 W.-H. Tuan et al. / Ceramics International 34 (2008) 2129–2135
W.-H. Than et al. /Ceramics International 34(2008 )2129-21 The analysis above demonstrates that there is a critical References amount for the second phase to prohibit the coarsening of matrix grains. The critical amount of the second phase is [l] R.w.Rice. Microstructure dependence of mechanical behavior of cera- function of the ratio of grain size over particle size. In the mics, Treatise. Mater. Sci. Tech. 11(1977)199-381 present study, it demonstrates that the critical amount of second [2] W.H. Tuan, M.J. Lai, M.C. Lin, C.C. Chan, S.C. Chiu, The mechanical phase depends on the growth of matrix grains, provided that the formance of alumina as a function of grain size, Mater. Chem. Phys. 36 (1994)246-251 coarsening of particles is limited. For the present system under [3] W.H. Rhodes, Agglomerate and particle size effect on sintering yttria- the processing conditions used, the critical amount of ZrOz stabilized zirconia, J. Am. Ceram Soc. 64(1981)19-26 particles is 2 vol % The analysis also suggests that the decrease 14] G.C. Wei, P.F. Becher, Development of SiC-whisker-reinforced ceramics, of particle size can further encourage the separation of grain Am Ceram. Soc. Bull. 64(1985)298-304 [5 N. Claussen, J. Steeb, R.F. Fabst, Effect of induced microcracking on the boundary and particles. If the particles are preferable to be fracture toughness of ceramics, Am. Ceram Soc. Bull. 56(1977)559- swallowed by the matrix grains in order to leave behind wavy grain boundaries, as is the case for the Al_,/SiC nanocomp- [6] K Nihara, New design concept of structural cerar te [21], such target possibly can be achieved either by posites, J. Ceram. Soc. Jpn. 99(1991)974982 reducing the particle size or by increasing the grain size through 7 WH Tuan, R J Brook. The toughening of alumina with nickel inclusions. the rise of sintering temperature 8 T Sekino, T. Nakajima, S Ueda, K Niihara, Reduction and sintering of a ickel-dispersed-alumina composite and its properties, J. Am. Ceram. 5. Conclusions Soc.80(1977)1139-1148 [9] R. Majumdar, E Gilbart, R.J. Brook, Kinetics of densification of alumina- zirconia ceramics, Br. Ceram. Trans. J. 85(1993)156-160. There are several advantages of using a small amount [10] E.E. Lange, Transformation toughening, IV, fabrication, fracture toughness (<5 vol %)of ZrO2 particles as the strengthening agent to d strength of AL2Or-ZrO, composites, J Mater. Sci. 17(1982)247-254. Al,O3 ceramics [11] P.F. Becher, Slow crack growth behavior in transformation-toughened AlOJ-ZrO.(Y2O3)ceramics, J Am Ceram Soc. 66(1983)485-488 1. The addition of the fine ZrO2 particles reduces the amount of [12 M. Ruhle, A.G. Evans, R.M. McMeeking.PGCharalambides, Jw Hutchinson, Microcrack toughening in alumina/zirconia, Acta Metall. Al,O3 agglomerates. 25(1987)2701-2710 2. The addition of a small amount of ZrO2 particles can also [13] M. Ruhle, A.G. Evans, High toughness and ceramic-matri reduce the size of Al2O3 matrix grains composites, Prog. Mater. Sci. 33(1989)85-167 3. The strength of Al2O3 is enhanced due to the reduction of [14]R H.J. Hannink, P.M. Kelly, B.C. Muddle, Transformation toughening in aIn size zirconia-containing ceramics, J. Am. Ceram. Soc. 83(2000)461-487 4. The coarsening of particles within ceramic matrix is limited [15]0. Sudre, E.E. Lange, The effect of inclusions on densification: I,micro- ructural developments in an Al2O3 matrix containing a large volume as the particle content is lower. fraction of ZrO inclusion, J. Am. Ceram. Soc. 75(1992)519-524 5. The critical amount of particles to prohibit the coarsening of [16] P.A. Evans, R. Stevens, J.P. Binner, Quantitative X-ray diffraction analysis of matrix grains is a function of grain size over particle size [171 A Roosen, H. Hausner, Sintering kinetics of ZrO2 powders, Adv Ceram 12(1984)714-726. Acknowledgments [18] K.H. Westmacott, C.W. Fountain, RJ. Stirton, On the spacing of obstacles, Acta Metall. 14(1966)1628-1629 The present study was supported by the National Science [191 U.F. Kocks, On the spacing of dispersed obstacles, Acta Metall. 14(1966) Council through the contract numbers of NSC94-2216-E-002- 1629-1631 14 and NSC94-2216-E-002-016. Valuable comments given by [20]R.. Brook, Controlled grain growth, Treatise. Mater. Sci. Tech. 9(1976) Prof. Jay Shieh, Dept. of Mater. Sci. Eng, National Taiwan [21] M. Sternitzke, Review: structural ceramic nanocomposites, I. Eur.Ceram University, are helpful Soc.17(1997)1061-1082
The analysis above demonstrates that there is a critical amount for the second phase to prohibit the coarsening of matrix grains. The critical amount of the second phase is a function of the ratio of grain size over particle size. In the present study, it demonstrates that the critical amount of second phase depends on the growth of matrix grains, provided that the coarsening of particles is limited. For the present system under the processing conditions used, the critical amount of ZrO2 particles is 2 vol.%. The analysis also suggests that the decrease of particle size can further encourage the separation of grain boundary and particles. If the particles are preferable to be swallowed by the matrix grains in order to leave behind wavy grain boundaries, as is the case for the Al2O3/SiC nanocomposite [21], such target possibly can be achieved either by reducing the particle size or by increasing the grain size through the rise of sintering temperature. 5. Conclusions There are several advantages of using a small amount (<5 vol.%) of ZrO2 particles as the strengthening agent to Al2O3 ceramics. 1. The addition of the fine ZrO2 particles reduces the amount of Al2O3 agglomerates. 2. The addition of a small amount of ZrO2 particles can also reduce the size of Al2O3 matrix grains. 3. The strength of Al2O3 is enhanced due to the reduction of grain size. 4. The coarsening of particles within ceramic matrix is limited as the particle content is lower. 5. The critical amount of particles to prohibit the coarsening of matrix grains is a function of grain size over particle size. Acknowledgments The present study was supported by the National Science Council through the contract numbers of NSC94-2216-E-002- 014 and NSC94-2216-E-002-016. Valuable comments given by Prof. Jay Shieh, Dept. of Mater. Sci. & Eng., National Taiwan University, are helpful. References [1] R.W. Rice, Microstructure dependence of mechanical behavior of ceramics, Treatise. Mater. Sci. Tech. 11 (1977) 199–381. [2] W.H. Tuan, M.J. Lai, M.C. Lin, C.C. Chan, S.C. Chiu, The mechanical performance of alumina as a function of grain size, Mater. Chem. Phys. 36 (1994) 246–251. [3] W.H. Rhodes, Agglomerate and particle size effect on sintering yttriastabilized zirconia, J. Am. Ceram. Soc. 64 (1981) 19–26. [4] G.C. Wei, P.F. Becher, Development of SiC-whisker-reinforced ceramics, Am. Ceram. Soc. Bull. 64 (1985) 298–304. [5] N. Claussen, J. Steeb, R.F. Fabst, Effect of induced microcracking on the fracture toughness of ceramics, Am. Ceram. Soc. Bull. 56 (1977) 559– 562. [6] K. Niihara, New design concept of structural ceramic–ceramic nanocomposites, J. Ceram. Soc. Jpn. 99 (1991) 974–982. [7] W.H. Tuan, R.J. Brook, The toughening of alumina with nickel inclusions, J. Eur. Ceram. Soc. 6 (1990) 31–37. [8] T. Sekino, T. Nakajima, S. Ueda, K. Niihara, Reduction and sintering of a nickel-dispersed-alumina composite and its properties, J. Am. Ceram. Soc. 80 (1977) 1139–1148. [9] R. Majumdar, E. Gilbart, R.J. Brook, Kinetics of densification of alumina– zirconia ceramics, Br. Ceram. Trans. J. 85 (1993) 156–160. [10] F.F. Lange, Transformation toughening, IV, fabrication, fracture toughness and strength of Al2O3–ZrO2 composites, J. Mater. Sci. 17 (1982) 247–254. [11] P.F. Becher, Slow crack growth behavior in transformation-toughened Al2O3–ZrO2(Y2O3) ceramics, J. Am. Ceram. Soc. 66 (1983) 485–488. [12] M. Ruhle, A.G. Evans, R.M. McMeeking, P.G. Charalambides, J.W. Hutchinson, Microcrack toughening in alumina/zirconia, Acta Metall. 25 (1987) 2701–2710. [13] M. Ruhle, A.G. Evans, High toughness ceramics and ceramic–matrix composites, Prog. Mater. Sci. 33 (1989) 85–167. [14] R.H.J. Hannink, P.M. Kelly, B.C. Muddle, Transformation toughening in zirconia-containing ceramics, J. Am. Ceram. Soc. 83 (2000) 461–487. [15] O. Sudre, F.F. Lange, The effect of inclusions on densification: I, microstructural developments in an Al2O3 matrix containing a large volume fraction of ZrO2 inclusion, J. Am. Ceram. Soc. 75 (1992) 519–524. [16] P.A. Evans, R. Stevens, J.P. Binner, Quantitative X-ray diffraction analysis of polymorphic mixes of pure zirconia, Br. Ceram. Trans. J. 84 (1984) 39–43. [17] A. Roosen, H. Hausner, Sintering kinetics of ZrO2 powders, Adv. Ceram. 12 (1984) 714–726. [18] K.H. Westmacott, C.W. Fountain, R.J. Stirton, On the spacing of dispersed obstacles, Acta Metall. 14 (1966) 1628–1629. [19] U.F. Kocks, On the spacing of dispersed obstacles, Acta Metall. 14 (1966) 1629–1631. [20] R.J. Brook, Controlled grain growth, Treatise. Mater. Sci. Tech. 9 (1976) 331–364. [21] M. Sternitzke, Review: structural ceramic nanocomposites, J. Eur. Ceram. Soc. 17 (1997) 1061–1082. W.-H. Tuan et al. / Ceramics International 34 (2008) 2129–2135 2135
