MATERIALS HIENGE& ENGIEERING ELSEVIER Materials Science and Engineering A 444(2007)58-63 www.elsevier.com/locate/msea Pressureless sintering of large-scale fine structural alumina matrix ceramic guideway materials Liu changxia * Zhang Jianhua , Sun Junlong a. Zhang Xihua a department of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China Department of Materials Science and Engineering, Shandong University, Jinan 250061, Shandong Province, PR China Received 17 April 2006: accepted 10 August 2006 Abstract In the present work, diopside is introduced in alumina matrix ceramic materials as a sintering aid. Large-scale fine structural alumina matrix ceramic guideway materials are fabricated by the technology of pressureless sintering, during which transient liquid phase sintering comes into being and new phases such as 3Al2O3 2SiOz, CaO-Al2O3 2SiO2 and Cao.6Al2O3 are produced by the chemical reactions taking place among alumina and the additives. The densification rate of the guideway composites as a function of diopside content is discussed. The hardness, the racture toughness and the bending strength of the guideway products are tested. Meanwhile, the effects of diopside on mechanical propertie fracture mechanism and porosity of large-scale fine structural alumina matrix ceramic guideway materials are analyzed C 2006 Elsevier B. V. All rights reserved Keywords: Alumina; Diopside; Guideway; Large-scale fine structural 1. Introduction matrix ceramic guideway materials. Diopside(MgCa(SiO3 )2) ompared to other addit The function of machine tool guideway is to support and for large-scale fine structural alumina matrix ceramic guideway uide the action elements such as worktable, tool rest and so on. products, their large dimension and complicated shape make it For another use of holding the kinematic accuracy, it is widely difficult to fabricate these ceramic materials by the process of applied on many kinds of machine tools. For example, metal hot-pressing. So pressureless sintering technology was chosen utting machine tool, boring lathe, miller, drill machine, planer, and transient liquid phase sintering came into being during the grinder, numerical control machine tool, three-coordinate gaug- process of sintering ing machine, etc. Compared to other guideway materials, e. g. In this paper, large-scale fine structural alumina matrix cast iron(quenched include), steel, plastics, and polymeric com- ceramic guideway materials with the different content of diop- pounds, fine alumina matrix ceramic materials has the virtues of side, acting as a sintering aid during the sintering process, are high hardness, good chemical inertness, high wear resistance, fabricated by the pressureless sintering technology in a fur- low coefficient of thermal expansion and friction coefficient. nace without any atmosphere protection. Being composed of However, the brittleness of pure alumina limits its potential SiO2, Cao and MgO phases, diopside decreases the temper applications, and excellent mechanical properties are obtained ature of pressureless sintering, and during which new phases always companied with a cost increase on account of the expen- of 3Al2O3.2SiO2, CaOAl2O3 2SiO2 and Cao- are sive second phases [1-9]. It generally costs more when fabri- formed. The effects of diopside on mechanical properties, degree cating ceramic guideway materials owing to its large dimen- of porosity and microstructure of large-scale fine structural alu sion. So low-cost also may be regarded as a priority when mina matrix ceramic guideway materials are discussed researchers design the compositions of large-scale fine alumina 2. Experimental procedure Corresponding authors. Tel. +86 531 8226 6727. Commercial AlzO3 powder of high purity(99.99%)and small hester5371@gmail.com(LChangxia) grain size(0.5-1 um)was used as the starting materials. Diop zhang(a (Z Jianhua) side(MgCa (SiO3)2), which is composed of Sio2(55 wt %), front matter 2006 Elsevier B V. All rights reserved
Materials Science and Engineering A 444 (2007) 58–63 Pressureless sintering of large-scale fine structural alumina matrix ceramic guideway materials Liu Changxia a,∗, Zhang Jianhua a,∗, Sun Junlong a, Zhang Xihua b a Department of Mechanical Engineering, Shandong University, Jinan 250061, Shandong Province, PR China b Department of Materials Science and Engineering, Shandong University, Jinan 250061, Shandong Province, PR China Received 17 April 2006; accepted 10 August 2006 Abstract In the present work, diopside is introduced in alumina matrix ceramic materials as a sintering aid. Large-scale fine structural alumina matrix ceramic guideway materials are fabricated by the technology of pressureless sintering, during which transient liquid phase sintering comes into being and new phases such as 3Al2O3·2SiO2, CaO·Al2O3·2SiO2 and CaO·6Al2O3 are produced by the chemical reactions taking place among alumina and the additives. The densification rate of the guideway composites as a function of diopside content is discussed. The hardness, the fracture toughness and the bending strength of the guideway products are tested. Meanwhile, the effects of diopside on mechanical properties, fracture mechanism and porosity of large-scale fine structural alumina matrix ceramic guideway materials are analyzed. © 2006 Elsevier B.V. All rights reserved. Keywords: Alumina; Diopside; Guideway; Large-scale fine structural ceramics 1. Introduction The function of machine tool guideway is to support and guide the action elements such as worktable, tool rest and so on. For another use of holding the kinematic accuracy, it is widely applied on many kinds of machine tools. For example, metal cutting machine tool, boring lathe, miller, drill machine, planer, grinder, numerical control machine tool, three-coordinate gauging machine, etc. Compared to other guideway materials, e.g. cast iron (quenched include), steel, plastics, and polymeric compounds, fine alumina matrix ceramic materials has the virtues of high hardness, good chemical inertness, high wear resistance, low coefficient of thermal expansion and friction coefficient. However, the brittleness of pure alumina limits its potential applications, and excellent mechanical properties are obtained always companied with a cost increase on account of the expensive second phases [1–9]. It generally costs more when fabricating ceramic guideway materials owing to its large dimension. So low-cost also may be regarded as a priority when researchers design the compositions of large-scale fine alumina ∗ Corresponding authors. Tel.: +86 531 8226 6727. E-mail addresses: hester5371@gmail.com (L. Changxia), jhzhang@sdu.edu.cn (Z. Jianhua). matrix ceramic guideway materials. Diopside (MgCa (SiO3)2) just has the virtue of low-cost compared to other additives. As for large-scale fine structural alumina matrix ceramic guideway products, their large dimension and complicated shape make it difficult to fabricate these ceramic materials by the process of hot-pressing. So pressureless sintering technology was chosen and transient liquid phase sintering came into being during the process of sintering. In this paper, large-scale fine structural alumina matrix ceramic guideway materials with the different content of diopside, acting as a sintering aid during the sintering process, are fabricated by the pressureless sintering technology in a furnace without any atmosphere protection. Being composed of SiO2, CaO and MgO phases, diopside decreases the temperature of pressureless sintering, and during which new phases of 3Al2O3·2SiO2, CaO·Al2O3·2SiO2 and CaO·6Al2O3 are formed. The effects of diopside on mechanical properties, degree of porosity and microstructure of large-scale fine structural alumina matrix ceramic guideway materials are discussed. 2. Experimental procedure Commercial Al2O3 powder of high purity (99.99%) and small grain size (0.5–1m) was used as the starting materials. Diopside (MgCa (SiO3)2), which is composed of SiO2 (55 wt.%), 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.08.040
L Changxia et al. Materials Science and Engineering A 444(2007)58-63 Compos 80k ositions of alumina matrix ceramic guideway materials Compositions(wt %) 3·25 100%Al2O3 √cao·eA0 AAAA 5% diopside +95% 10%o diopside 15% diopside +85% Al2O3 Cao (24 wt %) and Mgo(18 wt %) was used as additive. The content of diopside ranged from 2 to 15 wt %o as listed in Table 1 (The suffix in ADo, AD2, ADs, ADIo and ADis represents the weight content of diopside For example, ADo means the weight 26(°) content of diopside is zero.) Firstly, the raw materials were blended with each other ig. 1. X-ray diffraction phase analysis of ADs specimen. according to certain proportions and ball milled for 60h in an alcohol medium to obtain a homogeneous mixture. Secondly, the were studied on fracture surfaces and polished surfaces by scan- slurry was dried in vacuum and screened. Thirdly, the green bod- ning electron microscopy(HITACHI S-570) ies of guideway (1800 mm x 150 mm x 50 mm)were shaped by slip casting process and then dense green bodies of guideway are 3, Results and discussion obtained by using isostatic cool pressing technology in rubber molds. Lastly, pressureless sintering was used to sinter these 3. 1. X-ray diffraction phase analysis dense green bodies in a furnace in air. The green bodies of guideway were heated up to 1500C according to a tempera- The X-ray diffraction phase analysis of ADs specimen is ture gradient [10], as listed in Table 2. nown in Fig. 1. It is clear from Fig. l that there exist in AD The sintered guideway pro mens by an inside diameter slicer F Standout into speci- the Al203, 3A12032SiO2, CaOAl2032SiO2 and CaO-6A1203 phases. Theoretically there should exist MgO AlO3 in the com- ing, finish grinding with diamond wheels and polishing posites, but XRD analysis does not show the occurrence of MgOAl2O3. This is because the content of diopside is only point-bending mode was used to measure the bending strength 5 wt% in the composites, and that of MgO is 18 wt% in diop- on an electronic universal experimental instrument (WD-10) side, so the percent of MgO in the composites is only 0.9 wt% with a span of 20 mm at a crosshead speed of 0.5 mm/min. and thus the amount of MgO Al2O3, produced by the reaction Twelve specimens, chosen form one guideway product with taking place between MgO and Al2O3, is too small to be detected the same compositions, were used for measuring the bending by XRd analysis strength in air at room temperature. The bending strength was The sintering temperature C)is higher than the melt calculated by the following formula [11] 3PL phase sintering comes into being and the following chemical [- 2bh2 (1) reactions, yielding mullite, anorthite and Ca0-6A1203, may occur during the sintering process where or is bending strength(MPa), P load(N)under which the samples break, b and h width and height(mm), respectively, and 3Al2O3+2SiO2-3Al2O3-2SiO2 Vickers hardness was measured on polished surface with CaO+Al2O3+2SiO2→CaO·Al2O32SiO a load of 9 8N for 5s with a micro-hardness tester (MH-6). 6A103+Ca0, CaO 6A10 Fracture toughness measurement was performed using indenta- tion method with a hardness tester(Hv-120), and results were Based on thermodynamic analysis of the reactions, we can obtained by the formula proposed by Cook and Lawn [12]. XRD get the following rankings[13] hases after sintering. The microstructures of the specimens△G<0.△G<0.△G<0 0-300300-600600-900900-15001500 1500-10001000-5050 Speed of heating up and cooling (C/h) 10 3 h holding Open the furnace door
L. Changxia et al. / Materials Science and Engineering A 444 (2007) 58–63 59 Table 1 Compositions of alumina matrix ceramic guideway materials Specimens Compositions (wt.%) AD0 100% Al2O3 AD2 2% diopside + 98% Al2O3 AD5 5% diopside + 95% Al2O3 AD10 10% diopside + 90% Al2O3 AD15 15% diopside + 85% Al2O3 CaO (24 wt.%) and MgO (18 wt.%), was used as additive. The content of diopside ranged from 2 to 15 wt.% as listed in Table 1. (The suffix in AD0, AD2, AD5, AD10 and AD15 represents the weight content of diopside. For example, AD0 means the weight content of diopside is zero.) Firstly, the raw materials were blended with each other according to certain proportions and ball milled for 60 h in an alcohol medium to obtain a homogeneous mixture. Secondly, the slurry was dried in vacuum and screened. Thirdly, the green bodies of guideway (1800 mm × 150 mm × 50 mm) were shaped by slip casting process and then dense green bodies of guideway are obtained by using isostatic cool pressing technology in rubber molds. Lastly, pressureless sintering was used to sinter these dense green bodies in a furnace in air. The green bodies of guideway were heated up to 1500 ◦C according to a temperature gradient [10], as listed in Table 2. The sintered guideway products were cut into specimens by an inside diameter slicer. Standard test pieces (3 mm × 4 mm × 36 mm) were obtained through rough grinding, finish grinding with diamond wheels and polishing. Threepoint-bending mode was used to measure the bending strength on an electronic universal experimental instrument (WD-10) with a span of 20 mm at a crosshead speed of 0.5 mm/min. Twelve specimens, chosen form one guideway product with the same compositions, were used for measuring the bending strength in air at room temperature. The bending strength was calculated by the following formula [11]: σf = 3PL 2bh2 (1) where σf is bending strength (MPa), P load (N) under which the samples break, b and h width and height (mm), respectively, and L is span (mm). Vickers hardness was measured on polished surface with a load of 9.8 N for 5 s with a micro-hardness tester (MH-6). Fracture toughness measurement was performed using indentation method with a hardness tester (Hv-120), and results were obtained by the formula proposed by Cook and Lawn [12]. XRD (D/max-2400) analysis was undertaken to identify the crystal phases after sintering. The microstructures of the specimens Fig. 1. X-ray diffraction phase analysis of AD5 specimen. were studied on fracture surfaces and polished surfaces by scanning electron microscopy (HITACHI S-570). 3. Results and discussion 3.1. X-ray diffraction phase analysis The X-ray diffraction phase analysis of AD5 specimen is shown in Fig. 1. It is clear from Fig. 1 that there exist in AD5 the Al2O3, 3Al2O3·2SiO2, CaO·Al2O3·2SiO2 and CaO·6Al2O3 phases. Theoretically there should exist MgO·Al2O3 in the composites, but XRD analysis does not show the occurrence of MgO·Al2O3. This is because the content of diopside is only 5 wt.% in the composites, and that of MgO is 18 wt.% in diopside, so the percent of MgO in the composites is only 0.9 wt.% and thus the amount of MgO·Al2O3, produced by the reaction taking place between MgO and Al2O3, is too small to be detected by XRD analysis. The sintering temperature (1500 ◦C) is higher than the melting point of diopside (1300–1390 ◦C). Hence transient liquid phase sintering comes into being and the following chemical reactions, yielding mullite, anorthite and CaO·6Al2O3, may occur during the sintering process: 3Al2O3 + 2SiO2 → 3Al2O3·2SiO2 (2) CaO + Al2O3 + 2SiO2 → CaO · Al2O3·2SiO2 (3) 6Al2O3 + CaO → CaO · 6Al2O3 (4) Based on thermodynamic analysis of the reactions, we can get the following rankings [13]: Gθ T2 < 0, Gθ T3 < 0, Gθ T4 < 0 Table 2 Temperature gradient of pressureless sintering process (115 h) [10] Temperature (◦C) 20–300 300–600 600–900 900–1500 1500 1500–1000 1000–50 50 Speed of heating up and cooling (◦C/h) 10 20 30 40 3 h holding 20 50 Open the furnace door
Changxia et al. Materials Science and Engineering A 444(2007)58-63 diopside content (wt% Fig. 2. Relation of diopside content and relative density. Fig. 3. Relation of diopside content and hardness. wher,△G2,△G;and△ GT, represent the Gibbs free energy of the Eqs. (2)-(4), respectively. XRD analysis shows that mul- lite, anorthite and Cao-6Al2O3 actually exist in specimen of 300 Ds as previously mentioned. The existing of mullite in large scale fine structural alumina matrix ceramic guideway materials may be helpful to make the composite has a high potential for ear resistance applications [14] 3.2. Densification of the composites 50 Fig. 2 shows the relation of diopside content and relative den sity of the guideway composites pressureless sintered at 1500C diposide content (wt % in air for 3 h. It is clear from the figure that the relative density Fig. 4. Relation of diopside content and bending strength. tive density of the composite increases sharply(up to 96%)as the addition amount of diop- and the hardness is enhanced. while the hardness is reduced as of the composites increases with the content of diopside increas- addition effect is in turn becoming dominant(10-15 wt% The effects of diopside content on the bending strength of such as bending strength, fracture toughness and hardness of the the guideway composites are similar with that on hardness composites increased with the increasing of the relative density The bending strength increases with increasing the amount of diopside before 10 wt. reaching its maximum values of to improve the mechanical properties of large-scale fine struc- 341 MPa, and then decreases after 10 wt % The fracture tough- tural alumina matrix ceramic guideway materials, which will be ness increases sharply as diopside content is raised from 0 discussed in the following section 10wt% and reaches its maximum value of 4.62 MPam/.then decreases from 10 to 15 wt%0 3.3. Mechanical properties It is obvious that the fabricated large-scale fine structural alumina matrix ceramic guideway materials, pressureless sin- The relations of diopside content and hardness, bending tered at 1500C for 3h in air, exhibit significant improve strength, fracture toughness of pressureless sintered large-scale fine structural alumina matrix ceramic guideway materials are shown in Figs. 3-5, respectively. As seen from Fig 3, the hard- ness and bending strength of pressureless sintered pure alumina is, respectively, 5.6 GPa and 113 MPa, and the fracture tough- ness is 3. 27 MPa m. The hardness of the guidewa increases as the diopside content is raised from0 to 10 wt %, and then decreases from 10 to 15 wt% o. There are two factors influ to les hg the hardness of composites, namely a hardness effect due ition of diopside and a densification effect of the com- posites. The hardness of the composites increases for the reason that the densification effect is stronger than the hardness effect otherwise the hardness of the composites decreases. In the first diposide content (wt % stage(0-10 wt. addition), the densification effect is dominant Fig. 5. Relation of diopside content and fracture toughness
60 L. Changxia et al. / Materials Science and Engineering A 444 (2007) 58–63 Fig. 2. Relation of diopside content and relative density. where, Gθ T2 , Gθ T3 and Gθ T4 represent the Gibbs free energy of the Eqs. (2)–(4), respectively. XRD analysis shows that mullite, anorthite and CaO·6Al2O3 actually exist in specimen of AD5 as previously mentioned. The existing of mullite in largescale fine structural alumina matrix ceramic guideway materials may be helpful to make the composite has a high potential for wear resistance applications [14]. 3.2. Densification of the composites Fig. 2 shows the relation of diopside content and relative density of the guideway composites pressureless sintered at 1500 ◦C in air for 3 h. It is clear from the figure that the relative density of AD0 is only 90.2%, and the relative density of the composites increases sharply (up to ≈96%) as the addition amount of diopside is raised from 0 to 15 wt.%. Obviously, the densification rate of the composites increases with the content of diopside increasing. Kim et al. [15] has observed that the mechanical properties such as bending strength, fracture toughness and hardness of the composites increased with the increasing of the relative density of sintered specimens. So addition of diopside may be helpful to improve the mechanical properties of large-scale fine structural alumina matrix ceramic guideway materials, which will be discussed in the following section. 3.3. Mechanical properties The relations of diopside content and hardness, bending strength, fracture toughness of pressureless sintered large-scale fine structural alumina matrix ceramic guideway materials are shown in Figs. 3–5, respectively. As seen from Fig. 3, the hardness and bending strength of pressureless sintered pure alumina is, respectively, 5.6 GPa and 113 MPa, and the fracture toughness is 3.27 MPa m1/2. The hardness of the guideway composites increases as the diopside content is raised from 0 to 10 wt.%, and then decreases from 10 to 15 wt.%. There are two factors influencing the hardness of composites, namely a hardness effect due to the addition of diopside and a densification effect of the composites. The hardness of the composites increases for the reason that the densification effect is stronger than the hardness effect, otherwise the hardness of the composites decreases. In the first stage (0–10 wt.% addition), the densification effect is dominant Fig. 3. Relation of diopside content and hardness. Fig. 4. Relation of diopside content and bending strength. and the hardness is enhanced, while the hardness is reduced as the hardness effect is in turn becoming dominant (10–15 wt.% addition). The effects of diopside content on the bending strength of the guideway composites are similar with that on hardness. The bending strength increases with increasing the amount of diopside before 10 wt.%, reaching its maximum values of 341 MPa, and then decreases after 10 wt.%. The fracture toughness increases sharply as diopside content is raised from 0 to 10 wt.% and reaches its maximum value of 4.62 MPa m1/2, then decreases from 10 to 15 wt.%. It is obvious that the fabricated large-scale fine structural alumina matrix ceramic guideway materials, pressureless sintered at 1500 ◦C for 3 h in air, exhibit significant improveFig. 5. Relation of diopside content and fracture toughness.
L Changxia et al. Materials Science and Engineering A 444(2007)58-63 85881115KX3,gK1.8um 85981115ky氵函3自m Fig. 6. SEM photomicrograph on fracture surface of pure Al2O3 Fig 8. SEM Photomicrograph on fracture surface of ADs specimen. ments in mechanical properties than pure Al2O3. Composite there appears abnormal growth. While in Figs. 7-9, composites with the addition of 10 wt diopside shows better compre- with the addition of diopside sintered under the same conditions hensive performances, the hardness, bending strength and the show a homogeneous distribution of alumina grains and addi- fracture toughness of the composite reach 13.5 GPa, 341 MPa tive particles. The fracture surfaces of alumina matrix ceramic and 4.1 MPam", respectively, which are enhanced by 141, guideway composites are relatively rough, and the roughness of 201.8 and 25.4%, respectively, with respect to pure Al2O3 pres- fracture surface increases with increasing the content of diopside cureless sintered under the same conditions in the composites(Figs. 7-9) The fracture mode of 3.4. Analysis of microstructures failure. Two weight percent addition of diopside makes the frac ture mode mainly transgranular failure, which is accompanied SEM photomicrograph on fracture surface of pure alumina with little intergranular failure(Fig. 7).Then the fracture mecha pressureless sintered at 1500 Cis shown in Fig. 6, those of AD2, nism becomes the combination of intergranular failure and trans- ADs and ADio are shown in Figs. 7-9, respectively. As seen granular failure with further addition of diopside(Figs. 8 and 9) from Fig. 6, the grain shapes of pure alumina are irregular and As seen from Figs. 8 and 9, the bondings of grains become 85981115KV'ibm 5g81115KyX8.8K3.8um Fig. 7. SEM photomicrograph on fracture surface of AD2 specimen. Fig 9. SEM photomicrograph on fracture surface of ADIo specimen
L. Changxia et al. / Materials Science and Engineering A 444 (2007) 58–63 61 Fig. 6. SEM photomicrograph on fracture surface of pure Al2O3. ments in mechanical properties than pure Al2O3. Composite with the addition of 10 wt.% diopside shows better comprehensive performances, the hardness, bending strength and the fracture toughness of the composite reach 13.5 GPa, 341 MPa and 4.1 MPa m1/2, respectively, which are enhanced by 141, 201.8 and 25.4%, respectively, with respect to pure Al2O3 pressureless sintered under the same conditions. 3.4. Analysis of microstructures SEM photomicrograph on fracture surface of pure alumina pressureless sintered at 1500 ◦C is shown in Fig. 6, those of AD2, AD5 and AD10 are shown in Figs. 7–9, respectively. As seen from Fig. 6, the grain shapes of pure alumina are irregular and Fig. 7. SEM photomicrograph on fracture surface of AD2 specimen. Fig. 8. SEM photomicrograph on fracture surface of AD5 specimen. there appears abnormal growth. While in Figs. 7–9, composites with the addition of diopside sintered under the same conditions show a homogeneous distribution of alumina grains and additive particles. The fracture surfaces of alumina matrix ceramic guideway composites are relatively rough, and the roughness of fracture surface increases with increasing the content of diopside in the composites (Figs. 7–9). The fracture mode of pure alumina is mainly intergranular failure. Two weight percent addition of diopside makes the fracture mode mainly transgranular failure, which is accompanied with little intergranular failure (Fig. 7). Then the fracture mechanism becomes the combination of intergranular failure and transgranular failure with further addition of diopside (Figs. 8 and 9). As seen from Figs. 8 and 9, the bondings of grains become Fig. 9. SEM photomicrograph on fracture surface of AD10 specimen
L Changxia er al. / Materials Science and Engineering A 444(2007)58-63 seen from Figs. 10 and ll, there exist apparent air holes polished surface of large-scale fine structural lImina n ceramic guideway materials, which may be difficult to avoid in pressureless sintering technology. Air holes on polished face of ADlo specimen(Fig. 11)are much fewer than that in ADIo specimen(Fig. 10), which make it clear that the addition ers.Es of diopside decreases the number of air holes in large-scale fine structural alumina matrix ceramic guideway materials. 4. Conclusion Large-scale fine structural alumina matrix ceramic guideway materials are fabricated by pressureless sintering technology. Transient liquid phase sintering comes into being, during which new phases such as mullite, anorthite and Cao 6Al2O3 are pro- duced by the chemical reactions taking place among alumina and diopside. The addition of diopside obviously prompts the sinter- ing of large-scale fine structural alumina matrix ceramic guide- way materials, the relative density of the composites reaches Fig 10. SEM photomicrograph on polished surface of ADs specimen 96% as the content of diopside is 15 wt %. Based on this good densification rate, improved mechanical properties have been stronger when the fracture mode turns to the combination of ttained. The composite with the addition of 10 wt %o diopside transgranular failure and intergranular failure. There are some shows better comprehensive performances, and the hardness, classic features of the fracture surface in Figs. and 9, on the one bending strength and fracture toughness of the composite are hand, the grain boundaries are slowly being absorbed by diffu- 13. 5 GPa. 341 MPa and 4.1 M respectively The shapes of pure alumina grains, unevenly distributed, are on the surface what looks like a"cement"that has adhered to irregular and abnormal growth appears. Composites with the the surface of the new grain, and this"cement may be formed addition of diopside sintered under the same conditions show a by adding diopside, which produces liquid phases in the sinter- homogeneous distribution of alumina grains and additive parti- ing process. Thus, the composites represent a different kind of cles. The fracture mode of pure Al2O3 is mainly intergranular intergranular failure. A high strength cement binder consumes failure. While that of composites with addition of diopside is porosity, which results in the increased strength of the compos- the combination of transgranular failure and intergranular fail- ites, but has only marginal effect on fracture toughness. ure,which may bring on the increase in bending strength and Figs. 10 and 11, respectively, show the SEM photomicro- fracture toughness. Addition of diopside decreases the number graphs on polished surface of ADs and ADlo specimen. As of air holes in large-scale fine structural alumina matrix ceramic guideway materials, which may also contribute to the improve- ments in mechanical properties of the guideway composites Acknowledgements The work described in this paper is supported by the Ministry of Education, PR China(No. 03101)and the Outstanding Young Scientist Rewards of Shandong Province(No. 03BS103) References [1 S Jiao, M L. Jenkins, R W. Davidge, Acta Mater. 45(1)(1997)149-156. [2]SI Cha, K.T. Kim, K.H. Lee, C.B. Mo, S.H. Hong. Scripta Mater. 53(7) (2005)793-797 [3]S K.C. Pillai, B. Baron, M.J. Pomeroy, S. Hampshire, J. Eur. Ceram Soc. 24(2004)3317-3326 [4 J. Chandradass, M. Balasubramanian, Mater Sci Eng. A 408(1-2)(2005) 5 W. Nakao, M. Ono, S.K. Lee, et al., J. Eur. Ceram Soc. 25(16)(2005) 46262215KV 0K 20.gum [6 P C Ostertag, Mater Sci Eng. A 260(1-2)(1999)124-131 [7 E. Laarz, M. Carlsson, B. Vivien, et al., J. Eur. Ceram Soc. 21(8)(2001) Fig. 11. SEM photomicrograph on polished surface of ADIo specimen. [8] B. Zhang, F. Boey, Mater. Lett. 43(4)(2000)197-202
62 L. Changxia et al. / Materials Science and Engineering A 444 (2007) 58–63 Fig. 10. SEM photomicrograph on polished surface of AD5 specimen. stronger when the fracture mode turns to the combination of transgranular failure and intergranular failure. There are some classic features of the fracture surface in Figs. 8 and 9, on the one hand, the grain boundaries are slowly being absorbed by diffusion, and are almost gone. On the other hand, we can clearly see on the surface what looks like a “cement” that has adhered to the surface of the new grain, and this “cement” may be formed by adding diopside, which produces liquid phases in the sintering process. Thus, the composites represent a different kind of intergranular failure. A high strength cement binder consumes porosity, which results in the increased strength of the composites, but has only marginal effect on fracture toughness. Figs. 10 and 11, respectively, show the SEM photomicrographs on polished surface of AD5 and AD10 specimen. As Fig. 11. SEM photomicrograph on polished surface of AD10 specimen. seen from Figs. 10 and 11, there exist apparent air holes on the polished surface of large-scale fine structural alumina matrix ceramic guideway materials, which may be difficult to avoid in pressureless sintering technology. Air holes on polished surface of AD10 specimen (Fig. 11) are much fewer than that in AD10 specimen (Fig. 10), which make it clear that the addition of diopside decreases the number of air holes in large-scale fine structural alumina matrix ceramic guideway materials. 4. Conclusion Large-scale fine structural alumina matrix ceramic guideway materials are fabricated by pressureless sintering technology. Transient liquid phase sintering comes into being, during which new phases such as mullite, anorthite and CaO·6Al2O3 are produced by the chemical reactions taking place among alumina and diopside. The addition of diopside obviously prompts the sintering of large-scale fine structural alumina matrix ceramic guideway materials, the relative density of the composites reaches ≈96% as the content of diopside is 15 wt.%. Based on this good densification rate, improved mechanical properties have been attained. The composite with the addition of 10 wt.% diopside shows better comprehensive performances, and the hardness, bending strength and fracture toughness of the composite are 13.5 GPa, 341 MPa and 4.1 MPa m1/2, respectively. The shapes of pure alumina grains, unevenly distributed, are irregular and abnormal growth appears. Composites with the addition of diopside sintered under the same conditions show a homogeneous distribution of alumina grains and additive particles. The fracture mode of pure Al2O3 is mainly intergranular failure. While that of composites with addition of diopside is the combination of transgranular failure and intergranular failure, which may bring on the increase in bending strength and fracture toughness. Addition of diopside decreases the number of air holes in large-scale fine structural alumina matrix ceramic guideway materials, which may also contribute to the improvements in mechanical properties of the guideway composites. Acknowledgements The work described in this paper is supported by the Ministry of Education, PR China (No. 03101) and the Outstanding Young Scientist Rewards of Shandong Province (No. 03BS103). References [1] S. Jiao, M.L. Jenkins, R.W. Davidge, Acta Mater. 45 (1) (1997) 149–156. [2] S.I. Cha, K.T. Kim, K.H. Lee, C.B. Mo, S.H. Hong, Scripta Mater. 53 (7) (2005) 793–797. [3] S.K.C. Pillai, B. Baron, M.J. Pomeroy, S. Hampshire, J. Eur. Ceram. Soc. 24 (2004) 3317–3326. [4] J. Chandradass, M. Balasubramanian, Mater. Sci. Eng. A 408 (1–2) (2005) 165–168. [5] W. Nakao, M. Ono, S.-K. Lee, et al., J. Eur. Ceram. Soc. 25 (16) (2005) 3649–3655. [6] P.C. Ostertag, Mater. Sci. Eng. A 260 (1–2) (1999) 124–131. [7] E. Laarz, M. Carlsson, B. Vivien, et al., J. Eur. Ceram. Soc. 21 (8) (2001) 1027–1035. [8] B. Zhang, F. Boey, Mater. Lett. 43 (4) (2000) 197–202
L Changxia et al. Materials Science and Engineering A 444(2007)58-63 19 L. Tian Y. Zhou, T.C. Lei, Ceram. Int. 22(6)(1996)451-456. [13] D L. Ye, J.H. Hu, Handbook of the Thermodynamic Data of Inorganic [10 X H. Zhang, C.X. Liu, J H. Zhang, Mater. Sci. Forum 471-472(2004) Substances, Metallurgical Industry Press, Peking, China, 2002, pp. 57 [11] Z. H Jin, J Q. Gao, G. H. Qiao, Engineering Ceramics, Publishing House [14] Eugene Medvedovski, Ceram. Int 32(4)(2006)369-375 of Xian Jiao Tong University, Xian, China, 2000. [15] H.w. Kim, Y.H. Koh, H E. Kim, J. Am. Ceram. Soc. 83(11)(2000) [12]R F Cook, B R. Lawn, J Am Ceram Soc. 66(11)(1983)200-201
L. Changxia et al. / Materials Science and Engineering A 444 (2007) 58–63 63 [9] L. Tian, Y. Zhou, T.C. Lei, Ceram. Int. 22 (6) (1996) 451–456. [10] X.H. Zhang, C.X. Liu, J.H. Zhang, Mater. Sci. Forum 471–472 (2004) 821–824. [11] Z. H. Jin, J. Q. Gao, G. H. Qiao, Engineering Ceramics, Publishing House of Xian Jiao Tong University, Xian, China, 2000. [12] R.F. Cook, B.R. Lawn, J. Am. Ceram. Soc. 66 (11) (1983) 200–201. [13] D.L. Ye, J.H. Hu, Handbook of the Thermodynamic Data of Inorganic Substances, Metallurgical Industry Press, Peking, China, 2002, pp. 57– 1061. [14] Eugene Medvedovski, Ceram. Int. 32 (4) (2006) 369–375. [15] H.W. Kim, Y.H. Koh, H.E. Kim, J. Am. Ceram. Soc. 83 (11) (2000) 2863–2865
