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Effect of rare-earth Lu2O3 on the wear resistance of alumina ceramics for grinding media
Powder Technology 303 (2016) 27–32
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Effect of rare-earth Lu2O3 on the wear resistance of alumina ceramics for grinding media Tingting Wu a, Jian Zhou a, Bolin Wu a,b,⁎, Junchang Liu b a b
State-Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China College of Material Science and Engineering, Guilin University of Technology, Guilin, Guangxi 541004, PR China
a r t i c l e
i n f o
Article history: Received 19 January 2016 Received in revised form 4 September 2016 Accepted 7 September 2016 Available online 09 September 2016 Keywords: Alumina ceramics Grain refinement Lutetium oxide Wear resistance Grinding media
a b s t r a c t Wear resistance of grinding media is crucial for the quality of powders. The purpose of this work is to improve the wear resistance of grinding medium in an Al2O3-CaCO3-SiO2-MgO-Lu2O3 (ACSML) system. The effect of Lu2O3 content on bulk density and wear rate is discussed. The phase composition and microstructure of this material are analyzed. The results show that adding a trace amount of Lu2O3 to alumina can evidently improve wear resistance by grain refinement and enhancing density. The wear rate of grinding medium is as low as 0.00044‰, and the wear resistance has been improved by 31% than the sample without Lu2O3. However, excessive Lu2O3 can lead to deterioration of wear resistance, which due to grain growth and existence of Al5Lu3O12. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The processing of ball milling is very important in many industries like chemical, mineral, metallurgy, silicate, pharmaceutical, pesticide and food. Grinding media are indispensable material during ball milling. The wear resistance of grinding media is crucial to product quality. There is a strict limit to the content of contaminant (contaminant is unwanted material that polluted the sample) in some products, especially in food and pharmaceutical industry. Control of contaminant is a key factor in guarantee for drug safety. The United States Pharmacopeia divides impurities into three categories and the lowest content of three is b0.0005% [1]. Besides, in silicate industry, demands for high-purity alumina ceramic have increased with developing technology, such as insulator used for high pressure and extra-high voltage systems, electric component with low dielectric loss and good surface finish, ceramic tube used for lamps, and precision components in aerospace domain. The contaminant content is the key issue in preparing high-purity alumina ceramic, too. However, grinding media are one of the main pollution sources. So it is very important to improve the purity and wear resistance of the grinding media. The current research of grinding media focuses on ball size [2], shape design, material choice [3], the kinetics of grinding media wear, effect of grain size and porosity, wear mechanism, etc. Numerous studies have shown that polycrystalline alumina with sub-micrometer grains has good wear resistance [4]. In order to optimize wear resistance, the ⁎ Corresponding author. E-mail address: [email protected] (B. Wu).
http://dx.doi.org/10.1016/j.powtec.2016.09.018 0032-5910/© 2016 Elsevier B.V. All rights reserved.
alumina grain size should be small, regardless of the alumina content [5]. The wear resistance of polycrystalline alumina materials not only has a strong dependence on the microstructure and on the average grain size [6,7], but also is relevant to the chemical composition of the ceramic [8,9]. Besides, there are a lot of researches on the effect of rare earth on the performance of ceramics. The researchers discovered that rare earth oxides can reduce the sintering temperature [10], refine grains [11,12], increase the size of grain boundary grooves [13], and improve density [14,15], strength [16], hardness [17], thermo stability [18], creep resistance [19], etc. Nevertheless, the effect of rare earth on wear resistance mechanism has not been systematically investigated, especially for high alumina doped with Lu2O3. In the present study, we investigate the effect of Lu2O3 on wear resistance of high alumina ceramic (N98 wt%). Density, wear rate, phase composition and microstructure of this material are investigated. 2. Experimental High alumina ceramic was prepared in an Al2O3-CaCO3-SiO2-MgOLu2O3 (ACSML) system. Samples were prepared using a commercial alumina powder (N99.8% purity and a mean particle size of 0.65 μm). The alumina powder was mixed with magnesia, calcium carbonate, silicon dioxide and lutetium nitrate hydrate. Mixed additions of SiO2 and MgO allow a good densification and a good homogeneity of the microstructure [20,21]. The sum of Al2O3 and Lu2O3 was 99 wt%. The content of CaCO3 and MgO is the same that is 0.33 wt%. The content of SiO2 is 0.34 wt%. When the Lu2O3contents were 0, 0.0001, 0.001, 0.01, 0.1 and 1 wt%, the samples were referred to as Samples 1, 2, 3, 4, 5, and 6. The
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powders were ball-milled for 24 h, and then shaped to obtain green compacts (sphere of 30 mm in diameter) by cold isostatic pressing at 100 MPa. They were sintered at 1475 °C for 1 h in air. After these processes, bulk density and wear rate were tested. Bulk density was measured by the Archimedes method. Wear rate was tested in accordance with Chinese Building Materials Industry Standard JC/T 848.1-2010 (alumina grinding ball). The process is as follows: Weigh a sample (M1) and measure diameter (Dx). Then put about 1 kg samples and 1 L unfiltered water into a polyurethane pot, and then be milled for 24 h. The speed of ball mill is 80 rpm. Dry it and weigh again (M2). Wear rate is calculated by the equation: W ¼ KDðM1 −M2 =M 1 Þ
ð1Þ
In this equation, W is wear rate (‰); K is a constant (4.17 × 10−4 mm−1); D is the mean diameter (mm) of samples; M1 is the weight of before wear (g); and M2 is the weight of after wear (g). Based on the industry standard, wear rate of alumina grinding ball (alumina content is about 99 wt%) should be b0.15‰. Phase composition of samples was initially identified by comparing the X-ray powder diffraction (Cu Kα) patterns with the standard chart. The test was carried out in an “X'Pert PRO” multi-purpose X-ray diffractometer (PANalytical B.V., Almelo, Netherlands) with 40 kV voltage and 40 mA current. X-ray patterns were taken by measuring 2θ from 5° to 90°, at a step size of 0.02° and a dwell time of 5 s per step. The results of the powder diffraction patterns were analyzed with X'Pert High Score Plus software. The microstructure of the sintered samples was characterized using field emission scanning electron microscope (FESEM) equipped with energy dispersive spectroscopy (EDS), model S-4800, Hitachi, Japan makes. The samples were coated with golden prior to examination. In order to explore the mechanism, an additional experiment was done. Put Lu2O3 powder to coat on single-crystal alumina. And then, they were sintered at 1600 °C for 3 h. The interface between Al2O3 and Lu2O3 was observed by FESEM. 3. Experimental results 3.1. Wear rate and bulk density Fig. 1 shows the relationship among Lu2O3 content, wear rate, and bulk density. The curves of wear rate and bulk density vary parabolically
with the increase of Lu2O3 content. With the increase of Lu2O3 content, the bulk density curve increases first and then decreases, but the wear rate curve decreases first and then increases. When Lu2O3 content is b0.01%, the density and wear resistance of samples are better than that of Lu2O3-free sample. Wear rate of 99% alumina grinding ball should be b0.15‰ according to Chinese Building Materials Industry Standard. The wear rate of Sample 1 without Lu2O3 is 0.00064‰. Wear rates of samples fall sharply with the increasing of Lu2O3 content. The wear rate of Sample 4 reaches the minimum (0.00044‰). The wear resistance of Sample 4 has been improved by 31% than the sample without Lu2O3 and been enhanced by 284 times over a product with good resistance on the market (its wear rate is 0.125‰.). However, the wear rate of Sample 5 rises rapidly and is over than that of Sample 1. The results indicate that trace amounts of Lu2O3 can effectively improve the wear resistance and bulk density of alumina ceramic. The appropriate adding quantity is 0.001–0.01 wt%. Excessive Lu2O3 will result in poor wear resistance of the alumina ceramic. 3.2. Phase composition Sample 1 is Lu2O3-free sample. The wear resistance of Sample 4 is the best. The wear resistance starts becoming bad from Sample 5. And the wear resistance of Sample 6 is the worst. So the Samples 1, 4, 5, and 6 were chosen for testing. The phase composition of Samples 1, 4, 5, and 6 was analyzed by XRD. Fig. 2 shows the XRD results. In Samples 1, 4, and 5, Al2O3 is the main crystal phase and MgAl2O4 is the secondary phase. In Sample 6, Al5Lu3O12 appears in addition to Al2O3 and MgAl2O4. 3.3. Microstructure Samples 1, 4, 5, and 6 were cut and polished for FESEM testing. The microstructures of the cross-section are shown in Fig. 3. Submicron alumina ceramic exhibits substantially higher wear resistance compared to the larger grain size of alumina [22] (the grain size of submicron alumina ceramic is between 100 nm and 1 μm). In Fig. 3, average grain sizes of the four samples are b 1 μm. They belonged to submicron alumina ceramic. With the increasing of Lu2O3 content, the average grain size decreases firstly and then increases. A comparison among four samples shows that the average grain size of Sample 4 with 0.01 wt% Lu2O3 is the smallest and the microstructure
Fig. 1. Variation of the wear resistance rate and bulk density of alumina ceramics prepared with different concentrations of Lu2O3.
T. Wu et al. / Powder Technology 303 (2016) 27–32
Fig. 2. Powder X-ray diffraction patterns of alumina ceramics with different Lu2O3 concentrations. Sample 1: 0%; Sample 4: 0.01%; Sample 5: 0.1%; Sample 6: 1%.
is the densest. Dopants can alter the boundary, lattice and surface diffusivities, leading to a change in boundary [23,24]. Besides, the densification kinetic was consistent with grain-boundary diffusion being the controlling mechanism during densification [11]. Large rare-earth cations segregate strongly to the grain boundaries in Al2O3, which can create drag forces on boundary motion and block the diffusion of ions along grain boundaries, leading to reduced grain-boundary diffusivity and inhibiting grain growth. It is beneficial to form dense structure [10,11,25]. However, decreased grain size results in the number of grain boundary increased. The large rare-earth ions are not enough to cover all grain boundaries, causing individual abnormal grain growth. In Sample 4, the populations of small rounded grains are very high. There is only a very small amount of larger plate-like grains. From the histogram, it can be seen that 50% of grains are about 0.3 μm. Small
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grains can fill the voids between grains, leading to dense structure. This is consistent with the result of bulk density (Fig. 1). When Lu2O3 content is 0.1 wt% (Sample 5), the populations of coarse-grains increase and the average grain size is bigger than Samples 1 and 4. Sunil Kumar C. Pillai et al. [24] reported that when the impurity (impurity are unwanted compounds (atom, ion, molecule) that are present into the crystal lattice) content at the grain boundary attained a critical level, liquid phases of thermodynamically stable thickness could form and induced a sudden increase in the mobility of grain boundaries that were wetted by the liquid film at higher temperature thus resulting in grain growth. The Lu2O3 content still increases to 1 wt% (Sample 6), the proportion of large grains is shown to be quite substantial, which is obvious from the microstructure. V. Ucar et al. [26] believed that the large grain size and the higher porosity percentage caused large particles coming off during the wear process. So wear resistance has been degraded. Generally, the microstructures of the samples are characterized by mixed grain sizes with some differences in the range and distribution. The grain sizes of all the samples are bimodal with some grains that are small and some that are quite large. Compared to the four histograms and combined with the result of wear rate, it is indicated that the ceramic whose grain size appears to be an approximately normal distribution maybe has good wear resistance. Meanwhile, adding a trace amount of Lu2O3 is helpful to refine grain. Fine grains, dense structure, and grain size presented a normal distribution are beneficial to improve wear resistance. 3.4. Element distribution In order to study the effect of element distribution on wear resistance of the alumina ceramic, Samples 4, 5, and 6 were analyzed by EDS. The results are shown in Fig. 4. In the three samples, there are many Mg-rich regions due to the formation of MgAl2O4. The distributions of Si and Lu elements are relatively homogeneous in Samples 4 and 5, but the enrichment appears in Sample 6. A big amount of Lu enrichment is because of the formation of Al5Lu3O12. It's worth noting that the three positions of Si enrichment are the same with Lu. Meanwhile, a
Fig. 3. FESEM images of samples with different Lu2O3 concentrations. Sample 1: 0%; Sample 4 with the best wear resistance: 0.01%; Sample 5: 0.1%; Sample 6: 1%. The wear resistance of Samples 5 and 6 is worse than that of Lu2O3-free sample.
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Fig. 4. Element distributions of Mg, Si, and Lu in samples with different Lu2O3 concentrations. Sample 4: 0.01%; Sample 5: 0.1%; Sample 6: 1%.
tiny amount of Mg enrichment also appears in the three positions. Besides, the distribution of Ca element is homogeneous in three samples (not shown in the Fig. 4). The results show that when Lu2O3 content is b0.1%, the distribution of Lu element is homogeneous in the alumina ceramic and it does not cause the distribution change of other constituents. However, adding excessive Lu2O3 will lead to the enrichments of Si and Mg elements, causing non uniform distribution of components in the alumina ceramic. Then it causes poor performance of wear resistance.
lutetium complex oxides and Al2O3 is poor. Moreover, those complex oxides and Lu2O3 present a loose structure after high heat sintering (Locations A and B). The crystal growth direction is uncertainty. The crystals are grown together messily. The results indicated that aluminum-lutetium complex oxides clutter on the grain boundaries. They cannot closely combine with alumina grains, which lead to bad wear resistance.
3.5. Results of the high temperature reaction model of Lu2O3 with Al2O3
In this study, we discovered that a trace amount of Lu2O3 addition can improve the wear resistance of alumina ceramic. A number of researchers [5,28,29] indicated that the ceramics with a smaller crystal size demonstrated a higher level of physical properties. Evans had pointed that the brittleness of ceramic is the major reason of wear [30,31]. The manifestation of wear is fracture. Compared with coarsergrained ceramic, increased amount of grain boundary lead to that the fracture path will be more tortuous in fine-grained ceramic. It is not conducive to the propagation of cracks, which blocks effectively fracture of ceramic. And then the wear resistance of ceramic is improved. It is consistent with the experimental result. Trace amounts of Lu2O3 can refine grain size. It is reported that lutetium ions were found to segregate along the grain boundaries to form a continuous segregation layer. The grain boundary diffusion of Al3+ was restricted by the segregation and grain growth was inhibited [32–34]. Meanwhile, the density of the ceramic is improved. The grain size distribution of sample with best wear resistance appears approximately to be normal distribution. The different grain sizes matched well. That is another reason for improving the density. Moreover, Iftekhar et al. [35] reported that the thermal expansion coefficient of silicate glasses could decrease by adding rare earth ions. So the stresses created by the glass phase at grain boundaries are easy to become compressive. In alumina ceramic, if the stresses are compressive, then the abrasive wear resistance will be
In ceramics, the combination of interface between different phases has a strong influence on wear resistance. A single-crystal alumina is used to simulate an alumina grain in ceramic. The interface between Al2O3 and Lu2O3 after sintering was analyzed by FESEM and EDS. Fig. 5 shows the result. As shown in Fig. 5, Location A is Lu2O3. Location D is single-crystal alumina. Location B and C belong to the reaction area. As can be seen from the figure, there are some aluminum-lutetium complex oxides formed on the surface of single-crystal alumina. Compared Location B with C, the atom percent of Al is basically the same and that of Lu is different. The atom percent of Lu is lower nearby single-crystal alumina. In PDF of ICDD, the aluminum-lutetium complex oxides include Al2Lu4O9, AlLuO3, and Al5Lu3O12. It is indicated that the high-aluminum lowlutetium compound (Al5Lu3O12) should be formed in alumina ceramic. This is well matched with the XRD result. The aluminum-lutetium complex oxides are scattered on the surface of single-crystal alumina. Their shapes are spherical approximately. This phenomenon indicates the aluminum-lutetium complex oxides have low surface wettability with Al2O3. However, high surface wettability is prerequisite to good performance of interfacial bonding [27]. It can be concluded that the interfacial bonding between the aluminum-
4. Discussion
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Fig. 5. FESEM image and composition of the interface between Al2O3 and Lu2O3.
enhanced [36]. Fine grains, uniform structure, and grain size presented a normal distribution are beneficial to improve wear resistance. However, excessive Lu2O3 will induce grain growth, reduce density, and deteriorate wear resistance. On the one hand, the XRD result shows that Al5Lu3O12 is formed in sample with excessive Lu2O3 (Sample 6). The phase diagram of Lu2O3 and Al2O3 [37] shows that forming temperature of Al5Lu3O12 is above 1700 °C. In the ACSML system, the sintering temperature is only 1475 °C. According to the result of element distribution, the enrichments of Si and Mg elements appear at the position of Lu enrichment. It is speculated that 1 wt% sintering aids (CaCO3, MgO, and SiO2) react with the compounds that contained only a trace of K+, and Na+ in Al2O3 powder to form liquid phase during sintering [38]. The liquid phase promotes the formation of Al5Lu3O12. Al5Lu3O12 leads to the enrichments of Si and Mg elements, causing non uniform distribution of components in the alumina ceramic. The contents of Si4 + and Mg2+ are higher in the glass phase near Al5Lu3O12, which leads to that most of the other glass phase far from Al5Lu3O12 contains higher content of Ca2 +. The thermal expansion coefficient of high-CaO glass (9.5 × 10−6 °C−1) is much that of Al2O3 in certain crystallographic directions (αa = 8.6 × 10− 6 °C− 1 and αc = 9.5 × 10−6 °C− 1) [39]. Thus, the interface between the glass phase and Al2O3 grains can experience tensile stresses that may result in a weaker boundary. In addition, aluminum-lutetium complex oxides clutter on the grain boundaries and cannot closely combine with alumina grains. On the other hand, when impurity content at the grain boundary attained a critical level, liquid
phases of thermodynamically stable thickness could form and induced a sudden increase in the mobility of grain boundaries thus resulting in grain growth [24]. The rapidly moving of grain boundaries makes that pores do not have time to eliminate, which causes the densification of material to reduce. These are bad for wear resistance. 5. Conclusion Adding trace amounts of Lu2O3 can significantly improve the wear resistance of high-alumina ceramic. The main conclusions can be drawn as follows. (1) Adding trace amounts of Lu2O3 to alumina ceramic can improve the wear resistance. The wear rate is as low as 0.00044‰. The optimum quantity is about 0.001–0.01 wt%. (2) Adding trace amounts of Lu2O3 can inhibit grain growth during sintering to refine the grain and improve microstructure. Fine grains, uniform structure, and grain size presented a normal distribution are beneficial to improve wear resistance. (3) Excessive Lu2O3 will induce the grain growth, reduce density, and deteriorate wear resistance. Excess Lu2O3 can react with Al2O3 to form Al5Lu3O12. Al5Lu3O12 clutters on the grain boundaries and cannot closely combine with alumina grains. Moreover, the enrichment phenomenon causes inhomogeneous structure. Those lead to bad wear resistance.
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Acknowledgements This work was supported by grants from the National Natural Science Foundation of China Project (51172049), State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (WUT, China) (No. 2015-KF-4 and 2016-KF-4), and Guangxi Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials (No. 13AA-1), the Central Universities (WUT: 20141g0164 and 20131f0061201322ZX01). Special thanks are due to Professor Jian Zhou and Bolin Wu for their kind help in experimental instruction. References [1] Z. Zhang, N. Wei, S. Lu, Common principle in establishing the limitation, Chin. J. New Drugs 17 (2008) 1457–1460. [2] S. Razavi-Tousi, J. Szpunar, Effect of ball size on steady state of aluminum powder and efficiency of impacts during milling, Powder Technol. 284 (2015) 149–158. [3] I.E.R. Madrid, B.Á. Rodríguez, O. Bustamante, O.J.R. Baena, J.M. Menéndez-Aguado, Ceramic ball wear prediction in tumbling mills as a grinding media selection tool, Powder Technol. 268 (2014) 373–376. [4] A. Krell, E. Pippel, J. Woltersdorf, W. Burger, Subcritical crack growth in Al2O3 with submicron grain size, J. Eur. Ceram. Soc. 23 (2003) 81–89. [5] C. Doğan, J. Hawk, Role of composition and microstructure in the abrasive wear of high-alumina ceramics, Wear 225 (1999) 1050–1058. [6] R. Davidge, P. Twigg, F. Riley, Effects of silicon carbide nano-phase on the wet erosive wear of polycrystalline alumina, J. Eur. Ceram. Soc. 16 (1996) 799–802. [7] A. Franco, S. Roberts, Controlled wet erosive wear of polycrystalline alumina, J. Eur. Ceram. Soc. 16 (1996) 1365–1375. [8] W.M. Rainforth, The sliding wear of ceramics, Ceram. Int. 22 (1996) 365–372. [9] A.Y. Badmos, D.G. Ivey, Characterization of structural alumina ceramics used in ballistic armour and wear applications, J. Mater. Sci. 36 (2001) 4995–5005. [10] Q. Guanming, L. Xikum, Q. Tai, Z. Haitao, Y. Honghao, M. Ruiting, Application of rare earths in advanced ceramic materials, J. Rare Earths 25 (2007) 281–286. [11] J. Fang, A.M. Thompson, M.P. Harmer, H.M. Chan, Effect of yttrium and lanthanum on the final-stage sintering behavior of ultrahigh-purity alumina, J. Am. Ceram. Soc. 80 (1997) 2005–2012. [12] Y. Yao, T. Qiu, B. Jiao, C. Shen, The effects of rare earth oxide on the properties of alumina ceramics, Vac. Electron. (2004) 28–31. [13] G. West, J. Perkins, M. Lewis, Characterisation of fine-grained oxide ceramics, J. Mater. Sci. 39 (2004) 6687–6704. [14] Y. Qiuhong, Z. Zhijiang, X. Jun, Z. Hongwei, D. Jun, Effect of La2O3 on microstructure and transmittance of transparent alumina ceramics, J. Chin. Rare Earth Soc. 6 (2005) 72–75. [15] Y. Yijun, Q. Tai, J. Baoxiang, S. Chunying, Effect of Y2O3, La2O3, Sm2O3 on behaviors of alumina ceramics, J. Chin. Rare Earth Soc. 23 (2005) 158–161. [16] H. Jilin, L. Xin, D. Changze, Z. Xinxing, H. Chuanyue, W. Shumei, Influence of additives on properties of low temperature sintering 95 alumina ceramics, Chin. Ceram. 2 (2012) 003. [17] D. Yichao, Z. Zhufa, Effect of Y3+ on alumina ceramics, Foshan Ceram. 17 (2007) 8–11.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Effect of rare-earth Lu2O3 on the wear resistance of alumina ceramics for grinding media Tingting Wu a, Jian Zhou a, Bolin Wu a,b,⁎, Junchang Liu b a b
State-Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China College of Material Science and Engineering, Guilin University of Technology, Guilin, Guangxi 541004, PR China
a r t i c l e
i n f o
Article history: Received 19 January 2016 Received in revised form 4 September 2016 Accepted 7 September 2016 Available online 09 September 2016 Keywords: Alumina ceramics Grain refinement Lutetium oxide Wear resistance Grinding media
a b s t r a c t Wear resistance of grinding media is crucial for the quality of powders. The purpose of this work is to improve the wear resistance of grinding medium in an Al2O3-CaCO3-SiO2-MgO-Lu2O3 (ACSML) system. The effect of Lu2O3 content on bulk density and wear rate is discussed. The phase composition and microstructure of this material are analyzed. The results show that adding a trace amount of Lu2O3 to alumina can evidently improve wear resistance by grain refinement and enhancing density. The wear rate of grinding medium is as low as 0.00044‰, and the wear resistance has been improved by 31% than the sample without Lu2O3. However, excessive Lu2O3 can lead to deterioration of wear resistance, which due to grain growth and existence of Al5Lu3O12. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The processing of ball milling is very important in many industries like chemical, mineral, metallurgy, silicate, pharmaceutical, pesticide and food. Grinding media are indispensable material during ball milling. The wear resistance of grinding media is crucial to product quality. There is a strict limit to the content of contaminant (contaminant is unwanted material that polluted the sample) in some products, especially in food and pharmaceutical industry. Control of contaminant is a key factor in guarantee for drug safety. The United States Pharmacopeia divides impurities into three categories and the lowest content of three is b0.0005% [1]. Besides, in silicate industry, demands for high-purity alumina ceramic have increased with developing technology, such as insulator used for high pressure and extra-high voltage systems, electric component with low dielectric loss and good surface finish, ceramic tube used for lamps, and precision components in aerospace domain. The contaminant content is the key issue in preparing high-purity alumina ceramic, too. However, grinding media are one of the main pollution sources. So it is very important to improve the purity and wear resistance of the grinding media. The current research of grinding media focuses on ball size [2], shape design, material choice [3], the kinetics of grinding media wear, effect of grain size and porosity, wear mechanism, etc. Numerous studies have shown that polycrystalline alumina with sub-micrometer grains has good wear resistance [4]. In order to optimize wear resistance, the ⁎ Corresponding author. E-mail address: [email protected] (B. Wu).
http://dx.doi.org/10.1016/j.powtec.2016.09.018 0032-5910/© 2016 Elsevier B.V. All rights reserved.
alumina grain size should be small, regardless of the alumina content [5]. The wear resistance of polycrystalline alumina materials not only has a strong dependence on the microstructure and on the average grain size [6,7], but also is relevant to the chemical composition of the ceramic [8,9]. Besides, there are a lot of researches on the effect of rare earth on the performance of ceramics. The researchers discovered that rare earth oxides can reduce the sintering temperature [10], refine grains [11,12], increase the size of grain boundary grooves [13], and improve density [14,15], strength [16], hardness [17], thermo stability [18], creep resistance [19], etc. Nevertheless, the effect of rare earth on wear resistance mechanism has not been systematically investigated, especially for high alumina doped with Lu2O3. In the present study, we investigate the effect of Lu2O3 on wear resistance of high alumina ceramic (N98 wt%). Density, wear rate, phase composition and microstructure of this material are investigated. 2. Experimental High alumina ceramic was prepared in an Al2O3-CaCO3-SiO2-MgOLu2O3 (ACSML) system. Samples were prepared using a commercial alumina powder (N99.8% purity and a mean particle size of 0.65 μm). The alumina powder was mixed with magnesia, calcium carbonate, silicon dioxide and lutetium nitrate hydrate. Mixed additions of SiO2 and MgO allow a good densification and a good homogeneity of the microstructure [20,21]. The sum of Al2O3 and Lu2O3 was 99 wt%. The content of CaCO3 and MgO is the same that is 0.33 wt%. The content of SiO2 is 0.34 wt%. When the Lu2O3contents were 0, 0.0001, 0.001, 0.01, 0.1 and 1 wt%, the samples were referred to as Samples 1, 2, 3, 4, 5, and 6. The
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powders were ball-milled for 24 h, and then shaped to obtain green compacts (sphere of 30 mm in diameter) by cold isostatic pressing at 100 MPa. They were sintered at 1475 °C for 1 h in air. After these processes, bulk density and wear rate were tested. Bulk density was measured by the Archimedes method. Wear rate was tested in accordance with Chinese Building Materials Industry Standard JC/T 848.1-2010 (alumina grinding ball). The process is as follows: Weigh a sample (M1) and measure diameter (Dx). Then put about 1 kg samples and 1 L unfiltered water into a polyurethane pot, and then be milled for 24 h. The speed of ball mill is 80 rpm. Dry it and weigh again (M2). Wear rate is calculated by the equation: W ¼ KDðM1 −M2 =M 1 Þ
ð1Þ
In this equation, W is wear rate (‰); K is a constant (4.17 × 10−4 mm−1); D is the mean diameter (mm) of samples; M1 is the weight of before wear (g); and M2 is the weight of after wear (g). Based on the industry standard, wear rate of alumina grinding ball (alumina content is about 99 wt%) should be b0.15‰. Phase composition of samples was initially identified by comparing the X-ray powder diffraction (Cu Kα) patterns with the standard chart. The test was carried out in an “X'Pert PRO” multi-purpose X-ray diffractometer (PANalytical B.V., Almelo, Netherlands) with 40 kV voltage and 40 mA current. X-ray patterns were taken by measuring 2θ from 5° to 90°, at a step size of 0.02° and a dwell time of 5 s per step. The results of the powder diffraction patterns were analyzed with X'Pert High Score Plus software. The microstructure of the sintered samples was characterized using field emission scanning electron microscope (FESEM) equipped with energy dispersive spectroscopy (EDS), model S-4800, Hitachi, Japan makes. The samples were coated with golden prior to examination. In order to explore the mechanism, an additional experiment was done. Put Lu2O3 powder to coat on single-crystal alumina. And then, they were sintered at 1600 °C for 3 h. The interface between Al2O3 and Lu2O3 was observed by FESEM. 3. Experimental results 3.1. Wear rate and bulk density Fig. 1 shows the relationship among Lu2O3 content, wear rate, and bulk density. The curves of wear rate and bulk density vary parabolically
with the increase of Lu2O3 content. With the increase of Lu2O3 content, the bulk density curve increases first and then decreases, but the wear rate curve decreases first and then increases. When Lu2O3 content is b0.01%, the density and wear resistance of samples are better than that of Lu2O3-free sample. Wear rate of 99% alumina grinding ball should be b0.15‰ according to Chinese Building Materials Industry Standard. The wear rate of Sample 1 without Lu2O3 is 0.00064‰. Wear rates of samples fall sharply with the increasing of Lu2O3 content. The wear rate of Sample 4 reaches the minimum (0.00044‰). The wear resistance of Sample 4 has been improved by 31% than the sample without Lu2O3 and been enhanced by 284 times over a product with good resistance on the market (its wear rate is 0.125‰.). However, the wear rate of Sample 5 rises rapidly and is over than that of Sample 1. The results indicate that trace amounts of Lu2O3 can effectively improve the wear resistance and bulk density of alumina ceramic. The appropriate adding quantity is 0.001–0.01 wt%. Excessive Lu2O3 will result in poor wear resistance of the alumina ceramic. 3.2. Phase composition Sample 1 is Lu2O3-free sample. The wear resistance of Sample 4 is the best. The wear resistance starts becoming bad from Sample 5. And the wear resistance of Sample 6 is the worst. So the Samples 1, 4, 5, and 6 were chosen for testing. The phase composition of Samples 1, 4, 5, and 6 was analyzed by XRD. Fig. 2 shows the XRD results. In Samples 1, 4, and 5, Al2O3 is the main crystal phase and MgAl2O4 is the secondary phase. In Sample 6, Al5Lu3O12 appears in addition to Al2O3 and MgAl2O4. 3.3. Microstructure Samples 1, 4, 5, and 6 were cut and polished for FESEM testing. The microstructures of the cross-section are shown in Fig. 3. Submicron alumina ceramic exhibits substantially higher wear resistance compared to the larger grain size of alumina [22] (the grain size of submicron alumina ceramic is between 100 nm and 1 μm). In Fig. 3, average grain sizes of the four samples are b 1 μm. They belonged to submicron alumina ceramic. With the increasing of Lu2O3 content, the average grain size decreases firstly and then increases. A comparison among four samples shows that the average grain size of Sample 4 with 0.01 wt% Lu2O3 is the smallest and the microstructure
Fig. 1. Variation of the wear resistance rate and bulk density of alumina ceramics prepared with different concentrations of Lu2O3.
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Fig. 2. Powder X-ray diffraction patterns of alumina ceramics with different Lu2O3 concentrations. Sample 1: 0%; Sample 4: 0.01%; Sample 5: 0.1%; Sample 6: 1%.
is the densest. Dopants can alter the boundary, lattice and surface diffusivities, leading to a change in boundary [23,24]. Besides, the densification kinetic was consistent with grain-boundary diffusion being the controlling mechanism during densification [11]. Large rare-earth cations segregate strongly to the grain boundaries in Al2O3, which can create drag forces on boundary motion and block the diffusion of ions along grain boundaries, leading to reduced grain-boundary diffusivity and inhibiting grain growth. It is beneficial to form dense structure [10,11,25]. However, decreased grain size results in the number of grain boundary increased. The large rare-earth ions are not enough to cover all grain boundaries, causing individual abnormal grain growth. In Sample 4, the populations of small rounded grains are very high. There is only a very small amount of larger plate-like grains. From the histogram, it can be seen that 50% of grains are about 0.3 μm. Small
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grains can fill the voids between grains, leading to dense structure. This is consistent with the result of bulk density (Fig. 1). When Lu2O3 content is 0.1 wt% (Sample 5), the populations of coarse-grains increase and the average grain size is bigger than Samples 1 and 4. Sunil Kumar C. Pillai et al. [24] reported that when the impurity (impurity are unwanted compounds (atom, ion, molecule) that are present into the crystal lattice) content at the grain boundary attained a critical level, liquid phases of thermodynamically stable thickness could form and induced a sudden increase in the mobility of grain boundaries that were wetted by the liquid film at higher temperature thus resulting in grain growth. The Lu2O3 content still increases to 1 wt% (Sample 6), the proportion of large grains is shown to be quite substantial, which is obvious from the microstructure. V. Ucar et al. [26] believed that the large grain size and the higher porosity percentage caused large particles coming off during the wear process. So wear resistance has been degraded. Generally, the microstructures of the samples are characterized by mixed grain sizes with some differences in the range and distribution. The grain sizes of all the samples are bimodal with some grains that are small and some that are quite large. Compared to the four histograms and combined with the result of wear rate, it is indicated that the ceramic whose grain size appears to be an approximately normal distribution maybe has good wear resistance. Meanwhile, adding a trace amount of Lu2O3 is helpful to refine grain. Fine grains, dense structure, and grain size presented a normal distribution are beneficial to improve wear resistance. 3.4. Element distribution In order to study the effect of element distribution on wear resistance of the alumina ceramic, Samples 4, 5, and 6 were analyzed by EDS. The results are shown in Fig. 4. In the three samples, there are many Mg-rich regions due to the formation of MgAl2O4. The distributions of Si and Lu elements are relatively homogeneous in Samples 4 and 5, but the enrichment appears in Sample 6. A big amount of Lu enrichment is because of the formation of Al5Lu3O12. It's worth noting that the three positions of Si enrichment are the same with Lu. Meanwhile, a
Fig. 3. FESEM images of samples with different Lu2O3 concentrations. Sample 1: 0%; Sample 4 with the best wear resistance: 0.01%; Sample 5: 0.1%; Sample 6: 1%. The wear resistance of Samples 5 and 6 is worse than that of Lu2O3-free sample.
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Fig. 4. Element distributions of Mg, Si, and Lu in samples with different Lu2O3 concentrations. Sample 4: 0.01%; Sample 5: 0.1%; Sample 6: 1%.
tiny amount of Mg enrichment also appears in the three positions. Besides, the distribution of Ca element is homogeneous in three samples (not shown in the Fig. 4). The results show that when Lu2O3 content is b0.1%, the distribution of Lu element is homogeneous in the alumina ceramic and it does not cause the distribution change of other constituents. However, adding excessive Lu2O3 will lead to the enrichments of Si and Mg elements, causing non uniform distribution of components in the alumina ceramic. Then it causes poor performance of wear resistance.
lutetium complex oxides and Al2O3 is poor. Moreover, those complex oxides and Lu2O3 present a loose structure after high heat sintering (Locations A and B). The crystal growth direction is uncertainty. The crystals are grown together messily. The results indicated that aluminum-lutetium complex oxides clutter on the grain boundaries. They cannot closely combine with alumina grains, which lead to bad wear resistance.
3.5. Results of the high temperature reaction model of Lu2O3 with Al2O3
In this study, we discovered that a trace amount of Lu2O3 addition can improve the wear resistance of alumina ceramic. A number of researchers [5,28,29] indicated that the ceramics with a smaller crystal size demonstrated a higher level of physical properties. Evans had pointed that the brittleness of ceramic is the major reason of wear [30,31]. The manifestation of wear is fracture. Compared with coarsergrained ceramic, increased amount of grain boundary lead to that the fracture path will be more tortuous in fine-grained ceramic. It is not conducive to the propagation of cracks, which blocks effectively fracture of ceramic. And then the wear resistance of ceramic is improved. It is consistent with the experimental result. Trace amounts of Lu2O3 can refine grain size. It is reported that lutetium ions were found to segregate along the grain boundaries to form a continuous segregation layer. The grain boundary diffusion of Al3+ was restricted by the segregation and grain growth was inhibited [32–34]. Meanwhile, the density of the ceramic is improved. The grain size distribution of sample with best wear resistance appears approximately to be normal distribution. The different grain sizes matched well. That is another reason for improving the density. Moreover, Iftekhar et al. [35] reported that the thermal expansion coefficient of silicate glasses could decrease by adding rare earth ions. So the stresses created by the glass phase at grain boundaries are easy to become compressive. In alumina ceramic, if the stresses are compressive, then the abrasive wear resistance will be
In ceramics, the combination of interface between different phases has a strong influence on wear resistance. A single-crystal alumina is used to simulate an alumina grain in ceramic. The interface between Al2O3 and Lu2O3 after sintering was analyzed by FESEM and EDS. Fig. 5 shows the result. As shown in Fig. 5, Location A is Lu2O3. Location D is single-crystal alumina. Location B and C belong to the reaction area. As can be seen from the figure, there are some aluminum-lutetium complex oxides formed on the surface of single-crystal alumina. Compared Location B with C, the atom percent of Al is basically the same and that of Lu is different. The atom percent of Lu is lower nearby single-crystal alumina. In PDF of ICDD, the aluminum-lutetium complex oxides include Al2Lu4O9, AlLuO3, and Al5Lu3O12. It is indicated that the high-aluminum lowlutetium compound (Al5Lu3O12) should be formed in alumina ceramic. This is well matched with the XRD result. The aluminum-lutetium complex oxides are scattered on the surface of single-crystal alumina. Their shapes are spherical approximately. This phenomenon indicates the aluminum-lutetium complex oxides have low surface wettability with Al2O3. However, high surface wettability is prerequisite to good performance of interfacial bonding [27]. It can be concluded that the interfacial bonding between the aluminum-
4. Discussion
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Fig. 5. FESEM image and composition of the interface between Al2O3 and Lu2O3.
enhanced [36]. Fine grains, uniform structure, and grain size presented a normal distribution are beneficial to improve wear resistance. However, excessive Lu2O3 will induce grain growth, reduce density, and deteriorate wear resistance. On the one hand, the XRD result shows that Al5Lu3O12 is formed in sample with excessive Lu2O3 (Sample 6). The phase diagram of Lu2O3 and Al2O3 [37] shows that forming temperature of Al5Lu3O12 is above 1700 °C. In the ACSML system, the sintering temperature is only 1475 °C. According to the result of element distribution, the enrichments of Si and Mg elements appear at the position of Lu enrichment. It is speculated that 1 wt% sintering aids (CaCO3, MgO, and SiO2) react with the compounds that contained only a trace of K+, and Na+ in Al2O3 powder to form liquid phase during sintering [38]. The liquid phase promotes the formation of Al5Lu3O12. Al5Lu3O12 leads to the enrichments of Si and Mg elements, causing non uniform distribution of components in the alumina ceramic. The contents of Si4 + and Mg2+ are higher in the glass phase near Al5Lu3O12, which leads to that most of the other glass phase far from Al5Lu3O12 contains higher content of Ca2 +. The thermal expansion coefficient of high-CaO glass (9.5 × 10−6 °C−1) is much that of Al2O3 in certain crystallographic directions (αa = 8.6 × 10− 6 °C− 1 and αc = 9.5 × 10−6 °C− 1) [39]. Thus, the interface between the glass phase and Al2O3 grains can experience tensile stresses that may result in a weaker boundary. In addition, aluminum-lutetium complex oxides clutter on the grain boundaries and cannot closely combine with alumina grains. On the other hand, when impurity content at the grain boundary attained a critical level, liquid
phases of thermodynamically stable thickness could form and induced a sudden increase in the mobility of grain boundaries thus resulting in grain growth [24]. The rapidly moving of grain boundaries makes that pores do not have time to eliminate, which causes the densification of material to reduce. These are bad for wear resistance. 5. Conclusion Adding trace amounts of Lu2O3 can significantly improve the wear resistance of high-alumina ceramic. The main conclusions can be drawn as follows. (1) Adding trace amounts of Lu2O3 to alumina ceramic can improve the wear resistance. The wear rate is as low as 0.00044‰. The optimum quantity is about 0.001–0.01 wt%. (2) Adding trace amounts of Lu2O3 can inhibit grain growth during sintering to refine the grain and improve microstructure. Fine grains, uniform structure, and grain size presented a normal distribution are beneficial to improve wear resistance. (3) Excessive Lu2O3 will induce the grain growth, reduce density, and deteriorate wear resistance. Excess Lu2O3 can react with Al2O3 to form Al5Lu3O12. Al5Lu3O12 clutters on the grain boundaries and cannot closely combine with alumina grains. Moreover, the enrichment phenomenon causes inhomogeneous structure. Those lead to bad wear resistance.
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Acknowledgements This work was supported by grants from the National Natural Science Foundation of China Project (51172049), State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (WUT, China) (No. 2015-KF-4 and 2016-KF-4), and Guangxi Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials (No. 13AA-1), the Central Universities (WUT: 20141g0164 and 20131f0061201322ZX01). Special thanks are due to Professor Jian Zhou and Bolin Wu for their kind help in experimental instruction. References [1] Z. Zhang, N. Wei, S. Lu, Common principle in establishing the limitation, Chin. J. New Drugs 17 (2008) 1457–1460. [2] S. Razavi-Tousi, J. Szpunar, Effect of ball size on steady state of aluminum powder and efficiency of impacts during milling, Powder Technol. 284 (2015) 149–158. [3] I.E.R. Madrid, B.Á. Rodríguez, O. Bustamante, O.J.R. Baena, J.M. Menéndez-Aguado, Ceramic ball wear prediction in tumbling mills as a grinding media selection tool, Powder Technol. 268 (2014) 373–376. [4] A. Krell, E. Pippel, J. Woltersdorf, W. Burger, Subcritical crack growth in Al2O3 with submicron grain size, J. Eur. Ceram. Soc. 23 (2003) 81–89. [5] C. Doğan, J. Hawk, Role of composition and microstructure in the abrasive wear of high-alumina ceramics, Wear 225 (1999) 1050–1058. [6] R. Davidge, P. Twigg, F. Riley, Effects of silicon carbide nano-phase on the wet erosive wear of polycrystalline alumina, J. Eur. Ceram. Soc. 16 (1996) 799–802. [7] A. Franco, S. Roberts, Controlled wet erosive wear of polycrystalline alumina, J. Eur. Ceram. Soc. 16 (1996) 1365–1375. [8] W.M. Rainforth, The sliding wear of ceramics, Ceram. Int. 22 (1996) 365–372. [9] A.Y. Badmos, D.G. Ivey, Characterization of structural alumina ceramics used in ballistic armour and wear applications, J. Mater. Sci. 36 (2001) 4995–5005. [10] Q. Guanming, L. Xikum, Q. Tai, Z. Haitao, Y. Honghao, M. Ruiting, Application of rare earths in advanced ceramic materials, J. Rare Earths 25 (2007) 281–286. [11] J. Fang, A.M. Thompson, M.P. Harmer, H.M. Chan, Effect of yttrium and lanthanum on the final-stage sintering behavior of ultrahigh-purity alumina, J. Am. Ceram. Soc. 80 (1997) 2005–2012. [12] Y. Yao, T. Qiu, B. Jiao, C. Shen, The effects of rare earth oxide on the properties of alumina ceramics, Vac. Electron. (2004) 28–31. [13] G. West, J. Perkins, M. Lewis, Characterisation of fine-grained oxide ceramics, J. Mater. Sci. 39 (2004) 6687–6704. [14] Y. Qiuhong, Z. Zhijiang, X. Jun, Z. Hongwei, D. Jun, Effect of La2O3 on microstructure and transmittance of transparent alumina ceramics, J. Chin. Rare Earth Soc. 6 (2005) 72–75. [15] Y. Yijun, Q. Tai, J. Baoxiang, S. Chunying, Effect of Y2O3, La2O3, Sm2O3 on behaviors of alumina ceramics, J. Chin. Rare Earth Soc. 23 (2005) 158–161. [16] H. Jilin, L. Xin, D. Changze, Z. Xinxing, H. Chuanyue, W. Shumei, Influence of additives on properties of low temperature sintering 95 alumina ceramics, Chin. Ceram. 2 (2012) 003. [17] D. Yichao, Z. Zhufa, Effect of Y3+ on alumina ceramics, Foshan Ceram. 17 (2007) 8–11.
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