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Nb2O5 doping on densification, microstructure and wear resistance of alumina
Ceramics International 45 (2019) 18205–18209
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of Nb2O5 and MgO/Nb2O5 doping on densification, microstructure and wear resistance of alumina
T
Cheng Chen, Wei Li∗ School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, 200237, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Al2O3 Density Microstructure Wear resistance Nb2O5 MgO
In this paper, the influence of Nb2O5 single-doping and MgO/Nb2O5 co-doping on the densification, microstructure and wear resistance of Al2O3 has been investigated. The results show that Nb2O5 single-doping can increase the density of alumina effectively, but excessive Nb2O5 will lead to abnormal grain growth. Comparing with Nb2O5 single-doping, MgO/Nb2O5 co-doping can further increase the density and suppress the abnormal grain growth of the alumina. Meanwhile, the co-doped samples also show much lower wear rates. Typically, when 500 ppm MgO/1000 ppm Nb2O5 co-doped, the alumina ceramics has a minimum wear rate of 0.01‰, which is about 1/5 of the alumina ceramics with 1000 ppm Nb2O5 single-doped. The inhibiting effect of codoping on abnormal grain growth and the reasons for the decrease of wear rate are also discussed.
1. Introduction Ceramics are ideal materials for wear-resistant media due to their high hardness, high temperature resistance, chemical inertness, and wear resistance. Many kinds of ceramics, including ZrO2 [1–3], Si3N4 [4,5], SiC [4,6,7] and Al2O3 [8–13], have been applied to make into parts that require repeated friction in industrial production. Meanwhile, more and more investigations have been carried out to improve their performance. Y.J. He et al. [14] studied the wear resistance of Y-TZP ceramics and ADZ ceramics. They found that the wear resistance of ADZ ceramics was improved by 4–10 times compared with Y-TZP ceramics. This increase in wear resistance is due to the fact that Al2O3, which is the second phase of the intercrystalline phase, increases the elastic modulus and hardness of the system, while higher hardness is beneficial for the improvement of wear resistance [4,15]. M. Belmonte et al. [16] reduced the wear rate by adding SiC to Al2O3. The addition of SiC changed the wear mechanism from intergranular fracture to plastic deformation, thereby improving wear resistance. Alexandra Kovalčíková et al. [17] studied the Si3N4, Al2O3, ZrO2 and SiC ceramic spheres and found that the elongated SiC grains can improve the fracture toughness, inhibit the propagation of cracks, and form an interlocking network to prevent the grains from being pulled out. T.E. Fischer et al. [18] explored the effect of toughness on the wear resistance of Y2O3doped ZrO2 ceramics. The experimental results showed that the wear resistance of ZrO2 increases significantly with the increase of toughness. For a long time, alumina ceramics have been widely used as
∗
grinding balls in industrial production due to their low price. However, these grinding balls in the market are commonly made of 95 alumina (Al2O3 content is ∼95 wt%) with poor wear resistance and become more and more difficult to meet the needs of modern industrial development now. In recent years, many investigations have focused on improving the performance of the Al2O3 grinding balls. These efforts can be broadly divided into two categories. One is to prepare aluminabased composites by doping with some high strength or toughness materials. C.P. Doǧan et al. [19] added 34 vol% SiC to high purity alumina to prepare alumina-based composites, which greatly increased wear resistance. M. Kuntz et al. [20] researched ZTA composites by using Al2O3 as the matrix and ZrO2 as the second phase. By the addition of ZrO2, the strength, toughness and the wear resistance of the composite were improved. R. Kumar et al. [21] reported that the hardness and density of Al2O3eTiC composites increased with the rise of sintering temperature, and the associated wear resistance also increased. Another is to investigate the grinding mediums with higher Al2O3 content. Tingting Wu et al. [22,23] studied the effect of Lu2O3,Y2O3 on the wear resistance of ceramics with high alumina content (> 97 wt%). They found that an appropriate amount of Lu2O3 or Y2O3 could improve wear resistance by grain refinement and enhancing density. Unfortunately, few studies on the wear properties of alumina ceramics with purity higher than 99% have been reported up to now. In this paper, alumina ceramics with high purity (Al2O3 content > 99.7 wt%) was prepared by doping different contents of Nb2O5 or MgO/Nb2O5. The effects of the dopants on the densification,
Corresponding author. E-mail address: [email protected] (W. Li).
https://doi.org/10.1016/j.ceramint.2019.04.189 Received 27 March 2019; Received in revised form 22 April 2019; Accepted 22 April 2019 Available online 22 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 18205–18209
C. Chen and W. Li
Table 1 Doping amount of Nb2O5 and MgO in the samples. Numbers
Nb2O5
MgO
A1 A2 A3 A4 B1 B2 B3 B4
500 ppm 1000 ppm 1500 ppm 2000 ppm 500 ppm 1000 ppm 1500 ppm 2000 ppm
0 0 0 0 500 ppm 500 ppm 500 ppm 500 ppm
microstructure and wear resistance were investigated systematically. 2. Experimental procedure In this experiment, high purity Al2O3 powder (Al2O3 content > 99.95 wt%) was used as raw material, and Nb2O5 and MgO were used as dopants. The doping amounts are shown in Table 1. The Al2O3 powder was dispersed in deionized water, and Nb2O5 and MgO powders were added. The suspension was dried at 75 °C for 24 h. The dried powders were milled, filtered and pressed into cylindrical samples with the diameter of ∼16 mm and height of ∼8 mm. The samples were placed in a muffle furnace and sintered at 1550 °C for 4 h. Then, the samples were placed in a 500 ml jars, and three times of crude alumina powder and 200 ml of water were added. After sealing, the samples were milled at a speed of 45 r/min. After 48 h, the samples were taken out, dried and weighed. The density of the samples was measured by the Archimedes method. The wear rate calculation formula was W=(M1-M2)/ M1 × T × 1000‰. In the formula: M1 - the quality of the samples before being milled, g; M2 - the quality of the samples after being milled, g; T - the milling time of the samples, h; The theoretical density of alumina is 3.98 g/cm3. The average grain size was calculated by the lineal intercept method [24]. The microstructure of the samples was observed by a scanning electron microscope (Hitachi, Japan, TM-300 scanning electron microscope). The Xray diffraction (XRD) patterns of the sintered samples were examined by XRD analysis with a Bruker D8 Advance X-ray diffractometer (Karlsruhe, Germany; CuKα radiation generated at 40 kV and 40 mA), with 2θ ranging from 10° to 80° and a scanning speed of 6°/min.
Fig. 1. Density curves of the Al2O3 samples doped with different content of Nb2O5 or MgO/Nb2O5.
Fig. 2. XRD diffraction pattern of Al2O3 samples doped with different content of Nb2O5: (a) 500 ppm (b) 1000 ppm (c) 1500 ppm and (d) 2000 ppm and MgO/ Nb2O5: (e) 500 ppm/500 ppm (f) 500 ppm/1000 ppm (g) 500 ppm/1500 ppm and (h) 500 ppm/2000 ppm.
3. Results and discussion Fig. 1 shows the density curves of the Al2O3 samples doped with different content of Nb2O5 or MgO/Nb2O5. In both cases the density of the samples increases first and then decreases slowly as the doping content keeps increasing, and the highest density is obtained when 1500 ppm Nb2O5 is added. These results imply that suitable content of Nb2O5 doping is favorable for promoting the densification of Al2O3 ceramics. On the other hand, in the case of the same Nb2O5 doping content, the density of the samples co-doped with MgO/Nb2O5 is obviously higher than that with only Nb2O5 doped. For example, the highest density of the MgO/Nb2O5 co-doped sample is about 3.94 g/ cm3, while the Nb2O5 doped sample is only about 3.89 g/cm3. This finding means that 500 ppm MgO is beneficial for the densification of the Nb2O5 doped Al2O3 ceramics. Fig. 2 is X-ray diffraction patterns of Al2O3 doped with different content of Nb2O5. All the samples exhibit the single corundum phase (JCPDS file NO.10–0173), and the diffraction peak of the second phase does not appear. However, as the solid solution limitation of Nb2O5 in the Al2O3 is very low [25,26], the formation of the second phase usually unavoidable. Fig. 2's results probably mean that the content of the
second phase is lower than the detection limit of XRD. Fig. 3 illustrates the SEM micrograph of the samples doped with different content of Nb2O5. As shown in Fig. 3(a), when 500 ppm Nb2O5 doped, the sample exhibits the typical equiaxial morphology with a grain size of about 2 μm. Some isolated pores also exist, which is consistent with the low density of the sample. When Nb2O5 content increases to 1000 ppm, the morphology of the sample keeps unchanged as the grain size grows slowly and the pore number decreases. However, when Nb2O5 content increases to 1500 ppm, the morphology changes rapidly. Abnormal grain growth happens and the average grain size increases to about 16 μm. In addition, the pore size and number keep decreasing. Further increase of the Nb2O5 content does not cause significant changes in morphology and grain size. A similar situation regarding the rapid grain growth of the alumina ceramics has also been reported by Hsu's early investigations [27]. Although the real reason of this phenomenon is still unclear, it might be attributed to the forming of the liquid phase. As is known to all, the solubility of Nb2O5 in Al2O3 is
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Fig. 3. SEM pictures of the Al2O3 samples doped with different content of Nb2O5: (a) 500 ppm (b) 1000 ppm(c) 1500 ppm (d) 2000 ppm.
quite small. When doping content is too high, the redundant Nb2O5 will enrich at grain boundaries and reacts with the Al2O3 to form AlNbO4 [27]. However, according to the Nb2O5eAl2O3 equilibrium diagram reported by Roth et al. [26], the melting point of AlNbO4 is about 1530 °C, so it is very easy to turn into liquid phase under the sintering temperature (1550 °C), which might finally cause the rapid grain growth of the samples. Fig. 4 illustrates the SEM micrograph of the samples with 500 ppm MgO and different content of Nb2O5 co-doped. All the samples exhibit the typical equiaxial morphology. As the content of Nb2O5 increases from 500 ppm to 2000 ppm, the average grain size grows slowly from
about 2.5 μm to about 3 μm. No abnormal grain growth happens. A few isolated pores could also be found in Fig. 4(a), which almost disappear in Fig. 4(b) and (c). Comparing Figs. 3 and 4, we could find that the abnormal rapid grains growth has effectively restrained by the doping of MgO. Our previous studies have shown that there are two possible reasons for this effective suppression [28,29]. One is the higher solid solubility caused by the charge compensation effect when Mg2+/Nb5+ is co-doped; another is that some eutectic melt might form and evaporate in the high temperature during the sintering process. Both of these processes resulted in the reduction of AlNbO4 phase and the suppression of abnormal grain growth.
Fig. 4. SEM pictures of the Al2O3 samples doped with 500 ppm MgO and different content of Nb2O5: (a) 500 ppm (b) 1000 ppm(c) 1500 ppm and (d)2000 ppm. 18207
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investigated. Appropriate amount of Nb2O5 is beneficial to promote densification and grain growth of Al2O3 ceramics. When the doping amount of Nb2O5 is 1500 ppm, the highest density is obtained while the abnormal grain growth also happens, which might due to the formation of the liquid phase at the grain boundary. When MgO/Nb2O5 co-doped, the densities can be further increased obviously and the abnormal grain growth can be inhibited effectively. Charge compensation effect and evaporation of co-melted crystals are the causes for the suppression of abnormal grain growth. All the MgO/Nb2O5 co-doped samples show much lower wear rate than the Nb2O5 single-doped ones, mainly because the former has much higher densities than the later. Meanwhile, the grain size also shows some inference on the wear rate. When 500 ppm MgO/1000 ppm Nb2O5 co-doped, the alumina ceramics obtain a minimum wear rate, which is as low as 0.01‰. References
Fig. 5. Wear rate curves of the Al2O3 samples doped with different content of Nb2O5 or MgO/Nb2O5.
Fig. 5 shows the wear rate curves of the samples doped with different content of Nb2O5 or Mg/Nb2O5. Obviously, we can find that the two wear rate curves are similar, that is, as the Nb2O5 content increases, the wear rate decreases first and then increases. The lowest wear rate is obtained when 1000 ppm Nb2O5 is added. For samples doped with Nb2O5 and MgO/Nb2O5, the minimum wear rates are 0.05‰ and 0.01‰ respectively. As we all know, there are many factors affecting wear rate, among which density and grain size are two important ones. Generally speaking, higher density means lower wear rate [22,30]. Comparing Figs. 1 and 5, it can be seen that the density has a significant effect on the wear rate. The higher density samples co-doped with MgO/Nb2O5 show much smaller wear rate than the lower density samples doped with Nb2O5. Typically, the average grain sizes of the 1000 ppm Nb2O5 and 1000 ppm Nb2O5/500 ppm MgO doped samples are similar, but the density of the former is lower than that of the latter, so the wear rate of the former is about five times that of the latter. However, in Figs. 1 and 5, we can also find that the changes of wear rate and density are not completely synchronized. For example, the highest density of Nb2O5 doped and MgO/Nb2O5 co-doped samples is obtained when the Nb2O5-doped content is 1500 ppm, while the lowest wear rate is obtained when the Nb2O5-doped content is 1000 ppm. Considering many previous investigations have shown that alumina ceramics with a larger grain size usually have a higher wear rate [22,30], it seems reasonable to speculate that the asynchrony between Figs. 1 and 5 might be also caused by the influence of the grain size because the grain size of the 1500 ppm Nb2O5 doped sample is indeed larger than that of the 1000 ppm Nb2O5 doped sample. For the Nb2O5 single-doped samples, the density increases slowly from 3.80 g/cm3 to 3.85 g/cm3 when the Nb2O5 content increases from 1000 ppm to 1500 ppm, while the average grain size grows rapidly from ∼ 3 μm to ∼16 μm. For the Nb2O5/MgO co-doped samples, although the growth of average grain size is not particularly significant when the Nb2O5 content increases from 1000 ppm to 1500 ppm, the increase of density can be almost neglected. In both cases, the effect of grain size on wear rate may appear, and it will not be “shielded” by the influence of density. Of course, this speculation needs further experimental verification. Further experimental demonstrations are under way now.
4. Conclusion The densification, microstructure and wear resistance of Al2O3 ceramics doped with different content of Nb2O5 and MgO/Nb2O5 were
[1] Y. Fu, A.W. Batchelor, H. Xing, Y. Gu, Wear behaviour of laser-treated plasmasprayed ZrO2 coatings, Wear 210 (1997) 157–164. [2] G.B. Stachowiak, G.W. Stachowiak, P. Evans, Wear and friction characteristics of ion-implanted zirconia ceramics, Wear 241 (2000) 220–227. [3] W. Dongfang, L. Jian, M. Zhiyuan, Study of abrasive wear resistance of transformation toughened ceramics, Wear 165 (1993) 159–167. [4] P. Tatarko, M. Kašiarová, J. Dusza, J. Morgiel, P. Šajgalík, P. Hvizdoš, Wear resistance of hot-pressed Si3N4/SiC micro/nanocomposites sintered with rare-earth oxide additives, Wear 269 (2010) 867–874. [5] M.G. Gee, D. Butterfield, The combined effect of speed and humidity on the wear and friction of silicon nitride, Wear 162–164 (1993) 234–245. [6] O. Borrero-Lopez, A.L. Ortiz, F. Guiberteau, N.P. Padture, Improved sliding-wear resistance in situ-toughened silicon carbide, J. Am. Ceram. Soc. 88 (2005) 3531–3534. [7] O. Borrero-López, A.L. Ortiz, F. Guiberteau, N.P. Padture, Sliding-Wear-Resistant Liquid-Phase-Sintered SiC processed using α-SiC atarting powders, J. Am. Ceram. Soc. 90 (2007) 541–545. [8] H.J. Chen, W.M. Rainforth, W.E. Lee, The wear behaviour of Al2O3-SiC ceramic nanocomposites, Scripta Mater. 42 (2000) 555–560. [9] R. Singha Roy, D. Basu, A. Chanda, M.K. Mitra, Distinct wear characteristics of submicrometer-grained alumina in air and distilled water: a brief analysis on experimental observation, J. Am. Ceram. Soc. 90 (2007) 2987–2991. [10] C.P. Dogan, J.A. Hawk, Effect of grain boundary glass composition and devitrification on the abrasive wear of Al2O3, Wear 181–183 (1995) 129–137. [11] T.G. Durai, K. Das, S. Das, Wear behavior of nano structured Al (Zn)/Al2O3 and Al (Zn)–4Cu/Al2O3 composite materials synthesized by mechanical and thermal process, Mater. Sci. Eng., A 471 (2007) 88–94. [12] B. Kerkwijk, E. Mulder, H. Verweij, Zirconia–Alumina ceramic composites with extremely high wear resistance, Adv. Eng. Mater. 1 (1999) 69–71. [13] R. Günther, T. Klassen, B. Dickau, F. Gärtner, A. Bartels, R. Bormann, Advanced alumina composites reinforced with Nb-based alloys, Adv. Eng. Mater. 4 (2002) 121–125. [14] Y.J. He, A.J.A. Winnubst, D.J. Schipper, A.J. Burggraaf, H. Verweij, Effects of a second phase on the tribological properties of Al2O3 and ZrO2 ceramics, Wear 210 (1997) 178–187. [15] K.H. Zum Gahr, W. Bundschuh, B. Zimmerlin, Effect of grain size on friction and sliding wear of oxide ceramics, Wear 162–164 (1993) 269–279. [16] M. Belmonte, M.I. Nieto, M.I. Osendi, P. Miranzo, Influence of the SiC grain size on the wear behaviour of Al2O3/SiC composites, J. Eur. Ceram. Soc. 26 (2006) 1273–1279. [17] A. Kovalčíková, P. Kurek, J. Balko, J. Dusza, P. Šajgalík, M. Mihaliková, Effect of the counterpart material on wear characteristics of silicon carbide ceramics, Int. J. Refract. Metals Hard Mater. 44 (2014) 12–18. [18] T.E. Fischer, M.P. Anderson, S. Jahanmir, Influence of fracture toughness on the wear resistance of yttria-doped zirconium oxide, J. Am. Ceram. Soc. 72 (1989) 252–257. [19] C.P. Doǧan, J.A. Hawk, Influence of whisker reinforcement on the abrasive wear behavior of silicon nitride-and alumina-based composites, Wear 203–204 (1997) 267–277. [20] M. Kuntz, R. Krüger, The effect of microstructure and chromia content on the properties of zirconia toughened alumina, Ceram. Int. 44 (2018) 2011–2020. [21] R. Kumar, A.K. Chaubey, S. Bathula, K.G. Prashanth, A. Dhar, Al2O3-TiC composite prepared by spark plasma sintering process: evaluation of mechanical and tribological properties, J. Mater. Eng. Perform. 27 (2018) 997–1004. [22] T. Wu, J. Zhou, B. Wu, Y. Xiong, Effect of Y2O3 additives on the wet abrasion resistance of an alumina-based grinding medium, Wear 356–357 (2016) 9–16. [23] T. Wu, J. Zhou, B. Wu, J. Liu, Effect of rare-earth Lu2O3 on the wear resistance of alumina ceramics for grinding media, Powder Technol. 303 (2016) 27–32. [24] L.C. Stearns, M.P. Harmer, Particle-Inhibited Grain Growth in Al2O3-SiC: II, equilibrium and kinetic analyses, J. Am. Ceram. Soc. 79 (1996) 3020–3028. [25] E.M. Levin, C.R. Robbins, H.F. Mcmurdie, Phase diagrams for ceramists, Am. Ceram. Soc. 4 (1975) 379. [26] R.S. Roth, T. Negas, L.P. Cook, Phase diagrams for ceramists, The Am. Ceram. Soc. 4
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(1981) 117. [27] Y.F. Hsu, Influence of Nb2O5 additive on the densification and microstructural evolution of fine alumina powders, Mater. Sci. Eng., A 399 (2005) 232–237. [28] K. Zhang, L. Yuan, Y. Fu, C. Yuan, W. Li, Microwave dielectric properties of Al2O3 ceramics co-doped with MgO and Nb2O5, J. Mater. Sci. Mater. Electron. 26 (2015) 6526–6531.
[29] L. Yuan, H. Wang, H. Lin, W. Li, X. Li, J. Shi, Effect of MgO/La2O3 co-doping on the microstructure, transmittance and microwave dielectric properties of translucent polycrystalline alumina, Ceram. Int. 40 (2014) 2109–2113. [30] T. Wu, J. Zhou, B. Wu, W. Li, Effect of La2O3 content on wear resistance of alumina ceramics, J. Rare Earths 34 (2016) 288–294.
18209
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of Nb2O5 and MgO/Nb2O5 doping on densification, microstructure and wear resistance of alumina
T
Cheng Chen, Wei Li∗ School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, 200237, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Al2O3 Density Microstructure Wear resistance Nb2O5 MgO
In this paper, the influence of Nb2O5 single-doping and MgO/Nb2O5 co-doping on the densification, microstructure and wear resistance of Al2O3 has been investigated. The results show that Nb2O5 single-doping can increase the density of alumina effectively, but excessive Nb2O5 will lead to abnormal grain growth. Comparing with Nb2O5 single-doping, MgO/Nb2O5 co-doping can further increase the density and suppress the abnormal grain growth of the alumina. Meanwhile, the co-doped samples also show much lower wear rates. Typically, when 500 ppm MgO/1000 ppm Nb2O5 co-doped, the alumina ceramics has a minimum wear rate of 0.01‰, which is about 1/5 of the alumina ceramics with 1000 ppm Nb2O5 single-doped. The inhibiting effect of codoping on abnormal grain growth and the reasons for the decrease of wear rate are also discussed.
1. Introduction Ceramics are ideal materials for wear-resistant media due to their high hardness, high temperature resistance, chemical inertness, and wear resistance. Many kinds of ceramics, including ZrO2 [1–3], Si3N4 [4,5], SiC [4,6,7] and Al2O3 [8–13], have been applied to make into parts that require repeated friction in industrial production. Meanwhile, more and more investigations have been carried out to improve their performance. Y.J. He et al. [14] studied the wear resistance of Y-TZP ceramics and ADZ ceramics. They found that the wear resistance of ADZ ceramics was improved by 4–10 times compared with Y-TZP ceramics. This increase in wear resistance is due to the fact that Al2O3, which is the second phase of the intercrystalline phase, increases the elastic modulus and hardness of the system, while higher hardness is beneficial for the improvement of wear resistance [4,15]. M. Belmonte et al. [16] reduced the wear rate by adding SiC to Al2O3. The addition of SiC changed the wear mechanism from intergranular fracture to plastic deformation, thereby improving wear resistance. Alexandra Kovalčíková et al. [17] studied the Si3N4, Al2O3, ZrO2 and SiC ceramic spheres and found that the elongated SiC grains can improve the fracture toughness, inhibit the propagation of cracks, and form an interlocking network to prevent the grains from being pulled out. T.E. Fischer et al. [18] explored the effect of toughness on the wear resistance of Y2O3doped ZrO2 ceramics. The experimental results showed that the wear resistance of ZrO2 increases significantly with the increase of toughness. For a long time, alumina ceramics have been widely used as
∗
grinding balls in industrial production due to their low price. However, these grinding balls in the market are commonly made of 95 alumina (Al2O3 content is ∼95 wt%) with poor wear resistance and become more and more difficult to meet the needs of modern industrial development now. In recent years, many investigations have focused on improving the performance of the Al2O3 grinding balls. These efforts can be broadly divided into two categories. One is to prepare aluminabased composites by doping with some high strength or toughness materials. C.P. Doǧan et al. [19] added 34 vol% SiC to high purity alumina to prepare alumina-based composites, which greatly increased wear resistance. M. Kuntz et al. [20] researched ZTA composites by using Al2O3 as the matrix and ZrO2 as the second phase. By the addition of ZrO2, the strength, toughness and the wear resistance of the composite were improved. R. Kumar et al. [21] reported that the hardness and density of Al2O3eTiC composites increased with the rise of sintering temperature, and the associated wear resistance also increased. Another is to investigate the grinding mediums with higher Al2O3 content. Tingting Wu et al. [22,23] studied the effect of Lu2O3,Y2O3 on the wear resistance of ceramics with high alumina content (> 97 wt%). They found that an appropriate amount of Lu2O3 or Y2O3 could improve wear resistance by grain refinement and enhancing density. Unfortunately, few studies on the wear properties of alumina ceramics with purity higher than 99% have been reported up to now. In this paper, alumina ceramics with high purity (Al2O3 content > 99.7 wt%) was prepared by doping different contents of Nb2O5 or MgO/Nb2O5. The effects of the dopants on the densification,
Corresponding author. E-mail address: [email protected] (W. Li).
https://doi.org/10.1016/j.ceramint.2019.04.189 Received 27 March 2019; Received in revised form 22 April 2019; Accepted 22 April 2019 Available online 22 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 18205–18209
C. Chen and W. Li
Table 1 Doping amount of Nb2O5 and MgO in the samples. Numbers
Nb2O5
MgO
A1 A2 A3 A4 B1 B2 B3 B4
500 ppm 1000 ppm 1500 ppm 2000 ppm 500 ppm 1000 ppm 1500 ppm 2000 ppm
0 0 0 0 500 ppm 500 ppm 500 ppm 500 ppm
microstructure and wear resistance were investigated systematically. 2. Experimental procedure In this experiment, high purity Al2O3 powder (Al2O3 content > 99.95 wt%) was used as raw material, and Nb2O5 and MgO were used as dopants. The doping amounts are shown in Table 1. The Al2O3 powder was dispersed in deionized water, and Nb2O5 and MgO powders were added. The suspension was dried at 75 °C for 24 h. The dried powders were milled, filtered and pressed into cylindrical samples with the diameter of ∼16 mm and height of ∼8 mm. The samples were placed in a muffle furnace and sintered at 1550 °C for 4 h. Then, the samples were placed in a 500 ml jars, and three times of crude alumina powder and 200 ml of water were added. After sealing, the samples were milled at a speed of 45 r/min. After 48 h, the samples were taken out, dried and weighed. The density of the samples was measured by the Archimedes method. The wear rate calculation formula was W=(M1-M2)/ M1 × T × 1000‰. In the formula: M1 - the quality of the samples before being milled, g; M2 - the quality of the samples after being milled, g; T - the milling time of the samples, h; The theoretical density of alumina is 3.98 g/cm3. The average grain size was calculated by the lineal intercept method [24]. The microstructure of the samples was observed by a scanning electron microscope (Hitachi, Japan, TM-300 scanning electron microscope). The Xray diffraction (XRD) patterns of the sintered samples were examined by XRD analysis with a Bruker D8 Advance X-ray diffractometer (Karlsruhe, Germany; CuKα radiation generated at 40 kV and 40 mA), with 2θ ranging from 10° to 80° and a scanning speed of 6°/min.
Fig. 1. Density curves of the Al2O3 samples doped with different content of Nb2O5 or MgO/Nb2O5.
Fig. 2. XRD diffraction pattern of Al2O3 samples doped with different content of Nb2O5: (a) 500 ppm (b) 1000 ppm (c) 1500 ppm and (d) 2000 ppm and MgO/ Nb2O5: (e) 500 ppm/500 ppm (f) 500 ppm/1000 ppm (g) 500 ppm/1500 ppm and (h) 500 ppm/2000 ppm.
3. Results and discussion Fig. 1 shows the density curves of the Al2O3 samples doped with different content of Nb2O5 or MgO/Nb2O5. In both cases the density of the samples increases first and then decreases slowly as the doping content keeps increasing, and the highest density is obtained when 1500 ppm Nb2O5 is added. These results imply that suitable content of Nb2O5 doping is favorable for promoting the densification of Al2O3 ceramics. On the other hand, in the case of the same Nb2O5 doping content, the density of the samples co-doped with MgO/Nb2O5 is obviously higher than that with only Nb2O5 doped. For example, the highest density of the MgO/Nb2O5 co-doped sample is about 3.94 g/ cm3, while the Nb2O5 doped sample is only about 3.89 g/cm3. This finding means that 500 ppm MgO is beneficial for the densification of the Nb2O5 doped Al2O3 ceramics. Fig. 2 is X-ray diffraction patterns of Al2O3 doped with different content of Nb2O5. All the samples exhibit the single corundum phase (JCPDS file NO.10–0173), and the diffraction peak of the second phase does not appear. However, as the solid solution limitation of Nb2O5 in the Al2O3 is very low [25,26], the formation of the second phase usually unavoidable. Fig. 2's results probably mean that the content of the
second phase is lower than the detection limit of XRD. Fig. 3 illustrates the SEM micrograph of the samples doped with different content of Nb2O5. As shown in Fig. 3(a), when 500 ppm Nb2O5 doped, the sample exhibits the typical equiaxial morphology with a grain size of about 2 μm. Some isolated pores also exist, which is consistent with the low density of the sample. When Nb2O5 content increases to 1000 ppm, the morphology of the sample keeps unchanged as the grain size grows slowly and the pore number decreases. However, when Nb2O5 content increases to 1500 ppm, the morphology changes rapidly. Abnormal grain growth happens and the average grain size increases to about 16 μm. In addition, the pore size and number keep decreasing. Further increase of the Nb2O5 content does not cause significant changes in morphology and grain size. A similar situation regarding the rapid grain growth of the alumina ceramics has also been reported by Hsu's early investigations [27]. Although the real reason of this phenomenon is still unclear, it might be attributed to the forming of the liquid phase. As is known to all, the solubility of Nb2O5 in Al2O3 is
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Fig. 3. SEM pictures of the Al2O3 samples doped with different content of Nb2O5: (a) 500 ppm (b) 1000 ppm(c) 1500 ppm (d) 2000 ppm.
quite small. When doping content is too high, the redundant Nb2O5 will enrich at grain boundaries and reacts with the Al2O3 to form AlNbO4 [27]. However, according to the Nb2O5eAl2O3 equilibrium diagram reported by Roth et al. [26], the melting point of AlNbO4 is about 1530 °C, so it is very easy to turn into liquid phase under the sintering temperature (1550 °C), which might finally cause the rapid grain growth of the samples. Fig. 4 illustrates the SEM micrograph of the samples with 500 ppm MgO and different content of Nb2O5 co-doped. All the samples exhibit the typical equiaxial morphology. As the content of Nb2O5 increases from 500 ppm to 2000 ppm, the average grain size grows slowly from
about 2.5 μm to about 3 μm. No abnormal grain growth happens. A few isolated pores could also be found in Fig. 4(a), which almost disappear in Fig. 4(b) and (c). Comparing Figs. 3 and 4, we could find that the abnormal rapid grains growth has effectively restrained by the doping of MgO. Our previous studies have shown that there are two possible reasons for this effective suppression [28,29]. One is the higher solid solubility caused by the charge compensation effect when Mg2+/Nb5+ is co-doped; another is that some eutectic melt might form and evaporate in the high temperature during the sintering process. Both of these processes resulted in the reduction of AlNbO4 phase and the suppression of abnormal grain growth.
Fig. 4. SEM pictures of the Al2O3 samples doped with 500 ppm MgO and different content of Nb2O5: (a) 500 ppm (b) 1000 ppm(c) 1500 ppm and (d)2000 ppm. 18207
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investigated. Appropriate amount of Nb2O5 is beneficial to promote densification and grain growth of Al2O3 ceramics. When the doping amount of Nb2O5 is 1500 ppm, the highest density is obtained while the abnormal grain growth also happens, which might due to the formation of the liquid phase at the grain boundary. When MgO/Nb2O5 co-doped, the densities can be further increased obviously and the abnormal grain growth can be inhibited effectively. Charge compensation effect and evaporation of co-melted crystals are the causes for the suppression of abnormal grain growth. All the MgO/Nb2O5 co-doped samples show much lower wear rate than the Nb2O5 single-doped ones, mainly because the former has much higher densities than the later. Meanwhile, the grain size also shows some inference on the wear rate. When 500 ppm MgO/1000 ppm Nb2O5 co-doped, the alumina ceramics obtain a minimum wear rate, which is as low as 0.01‰. References
Fig. 5. Wear rate curves of the Al2O3 samples doped with different content of Nb2O5 or MgO/Nb2O5.
Fig. 5 shows the wear rate curves of the samples doped with different content of Nb2O5 or Mg/Nb2O5. Obviously, we can find that the two wear rate curves are similar, that is, as the Nb2O5 content increases, the wear rate decreases first and then increases. The lowest wear rate is obtained when 1000 ppm Nb2O5 is added. For samples doped with Nb2O5 and MgO/Nb2O5, the minimum wear rates are 0.05‰ and 0.01‰ respectively. As we all know, there are many factors affecting wear rate, among which density and grain size are two important ones. Generally speaking, higher density means lower wear rate [22,30]. Comparing Figs. 1 and 5, it can be seen that the density has a significant effect on the wear rate. The higher density samples co-doped with MgO/Nb2O5 show much smaller wear rate than the lower density samples doped with Nb2O5. Typically, the average grain sizes of the 1000 ppm Nb2O5 and 1000 ppm Nb2O5/500 ppm MgO doped samples are similar, but the density of the former is lower than that of the latter, so the wear rate of the former is about five times that of the latter. However, in Figs. 1 and 5, we can also find that the changes of wear rate and density are not completely synchronized. For example, the highest density of Nb2O5 doped and MgO/Nb2O5 co-doped samples is obtained when the Nb2O5-doped content is 1500 ppm, while the lowest wear rate is obtained when the Nb2O5-doped content is 1000 ppm. Considering many previous investigations have shown that alumina ceramics with a larger grain size usually have a higher wear rate [22,30], it seems reasonable to speculate that the asynchrony between Figs. 1 and 5 might be also caused by the influence of the grain size because the grain size of the 1500 ppm Nb2O5 doped sample is indeed larger than that of the 1000 ppm Nb2O5 doped sample. For the Nb2O5 single-doped samples, the density increases slowly from 3.80 g/cm3 to 3.85 g/cm3 when the Nb2O5 content increases from 1000 ppm to 1500 ppm, while the average grain size grows rapidly from ∼ 3 μm to ∼16 μm. For the Nb2O5/MgO co-doped samples, although the growth of average grain size is not particularly significant when the Nb2O5 content increases from 1000 ppm to 1500 ppm, the increase of density can be almost neglected. In both cases, the effect of grain size on wear rate may appear, and it will not be “shielded” by the influence of density. Of course, this speculation needs further experimental verification. Further experimental demonstrations are under way now.
4. Conclusion The densification, microstructure and wear resistance of Al2O3 ceramics doped with different content of Nb2O5 and MgO/Nb2O5 were
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