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Spark plasma sintering of ZrO2–Al2O3 nanocomposites at low temperatures aided by amorphous powders
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Spark plasma sintering of ZrO2–Al2O3 nanocomposites at low temperatures aided by amorphous powders Xiqing Xu∗, Tiyue Tao, Xin Zhang, Zhanzhi Cao, Di Huang, Hongchan Liang, Yinglu Hu School of Materials Science and Engineering, Chang'an University, Xi'an, 710061, China
ARTICLE INFO
ABSTRACT
Keywords: Spark plasma sintering Amorphous powders Sinterability ZrO2–Al2O3 Nanocomposites
In present work, ZrO2-5 wt% Al2O3 and ZrO2-10 wt% Al2O3 nanocomposites are fabricated through spark plasma sintering. Al2O3–ZrO2 amorphous powders and polycrystal Al2O3 powders and are doped in the polycrystalline ZrO2 powders, respectively. When doped with amorphous powders, the sintering of ZrO2–Al2O3 nanocomposites is promoted, and ZrO2-5 wt% Al2O3 and ZrO2-10 wt% Al2O3 nanocomposites with relative densities of 99% are obtained after spark plasma sintering at 1200 °C; however, when sintering of polycrystalline ZrO2 and polycrystalline Al2O3 powders, the relative densities are merely 93%. The enhanced sinterability is due to the metastability and phase transformation of the amorphous powders, which act as sintering aids. The nanocomposites with near-theoretical density show refined microstructure with homogenous mixture of ZrO2 and Al2O3 grains, which further leads to excellent mechanical properties. This article provides new ideas for low-temperature sintering of nanocomposites via using doping amorphous powders.
1. Introduction Low-temperature sintering is a vital issue in ceramics, which is beneficial to microstructural refinement and therefore enhanced mechanical properties [1–3]. Some other advantages are also brought about, for example, some unwanted reactions of constituents can be prevented [4], high-temperature melting can be avoided [5], and certain functional properties can be attained [6,7]. Therefore, several methods have been conducted to achieve low temperature sintering, including spark plasma sintering [8,9], liquid-phase sintering [10], hot pressing [11], sintering with aids [12], and sintering assisted by phase transition [13–15]. Phase-transition-assisted sintering is an important technique in the field of low-temperature sintering. The phase transition from metastable to stable phase takes place in sintering and promotes the mass diffusion and particle rearrangement, which contributes to low-temperature densification into volumes without serious grain growth. Series of ceramics have been fabricated through this technique, accompanied with transition from metastable to stable phases, such as Al2O3 from gamma to alpha phase [13], Y2O3 from cubic to monoclinic phase [14] and TiO2 from anatase to rutile phase [15]. Al2O3–ZrO2(Y2O3) is one of the most important ceramic composites with wide applications. Given the excellent ambient and high temperature properties, such as high toughness, high strength, good wear
∗
resistance, and biocompatibility, it is mainly used as cutting tools, bearings, high-temperature engine components, and knee replacement prostheses. Al2O3–ZrO2 composites with fine grains are generally prepared by sintering or SPS of nanocrystaline powders, and the sintering temperatures are generally above 1450 °C [16–18]. In our earlier works, Al2O3–ZrO2 composites were prepared through hot pressing [19] and pressureless sintering [20] of Al2O3–ZrO2 amorphous powders at low temperatures, benefited from the transformation of ZrO2 and Al2O3. However, in Al2O3–ZrO2 system, the powders can be obtained in amorphous state only near the eutectic composition due to the easy crystallization, which highly constrains the volume fraction of Al2O3 or ZrO2 in the products. Actually, for ZrO2–Al2O3, one of the most important ceramics, the microstructures and properties highly rely on the fraction of different components in a broad range [21,22]. In this condition, we point out that, the amorphous Al2O3–ZrO2 powders with eutectic composition can be added into the polycrystal powders, and act as sintering aids, to prepare ZrO2–Al2O3 nanocomposites distinct from eutectic composition. In this work, Al2O3–ZrO2 amorphous powders and polycrystal Al2O3 powders were doped into ZrO2 powders in spark plasma sintering, respectively, and the influence of different additives on sintering was studied. The dopant of amorphous powders enhanced the sintering, mainly led by the phase transformation of amorphous powders. Through spark plasma sintering aided by amorphous powders, ZrO2-
Corresponding author. E-mail address: [email protected] (X. Xu).
https://doi.org/10.1016/j.ceramint.2019.10.160 Received 8 August 2019; Received in revised form 30 September 2019; Accepted 17 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xiqing Xu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.160
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5 wt% Al2O3 and ZrO2- 10 wt% Al2O3 nanocomposites with full dense were fabricated, and the microstructures, as well as mechanical properties were investigated. 2. Experimental procedures 2.1. Raw powders Amorphous powders in the composition of Al2O3-40 mol% ZrO2 were synthetized by sol-gel method, and calcinated at 800 °C for 2 h, whose detailed procedures were reported in our earlier work [19]. These amorphous powders were then added into 3Y-ZTP powders (Tosoh, Japan) to serve as the second phase and sintering aids. As 9 wt% of amorphous powders with composition of Al2O3-40 mol% ZrO2 were added into 91 wt % of polycrystalline ZrO2 powders, the resulting composition was ZrO25wt%Al2O3, and this sample is marked by Z5A-PA, hereafter. Similarly, ZrO2-10 wt%Al2O3 powders were obtained by added 18 wt% of Al2O340 mol% ZrO2 amorphous powders and 82 wt% polycrystalline ZrO2 powders, and marked by Z10A-PA. In comparison, commercial polycrystalline Al2O3 powders (Sumitomo, Japan) were also added into the 3YZTP powders as the second phase. When 5 wt% or 10 wt%of polycrystalline Al2O3 powders with were added into polycrystalline ZrO2 powders, ZrO2-5wt%Al2O3 or ZrO2-10 wt%Al2O3powders were obtained, and marked by Z5A-PP or Z10A-PP. To obtain ZrO2-5wt%Al2O3 or ZrO2-10 wt %Al2O3 nanoceramics with different dopants, the detailed composition of raw powders was displayed in Table 1. As the powders were homogenously mixed through ball milling with ethanol media, they were sieved via 80-mesh after drying in air. The raw powders were characterized through scanning electron microscope (SEM, Model S4800, Hitachi, Japan) to investigate their morphologies and particle sizes. The phase composition was identified by X-ray diffraction analysis (XRD, Cu Kα, D/Max-2500v/pc, Rigaku, Japan).
Fig. 1. The pressure and temperature schedules in spark plasma sintering.
hardness (HV) and fracture toughness (KIC) were measured through indentation method using a microhardness machine (HXD-1000TM, Shanghai), in which 10 N was applied on the polished surface for 15 s. HV and KIC can be calculated based on the following equation [24,25],
HV = 1.854
KIC = 0.016
P d2 E HV
(2) 1/2
P L3/2
(3)
where P was the loading of 10 N, d meant the average diagonal length of the indentation, L was the crack length initiated from the indent edge, and E prensented the Young's modulus, which were determined as 214 GPa and 228 GPa for ZrO2-5wt%Al2O3 and ZrO2-10 wt%Al2O3 based on mixing rule [26]. Each value of HV or KIC was taking average value of 20 indentations.
2.2. Spark plasma sintering The powders were sintered using a spark plasma sintering apparatus (DR. SINTER, SPS-2050 systems, Fuji Electronic Industrial, Japan) in vacuum with the partial pressure less than 4 Pa. For each sample, the powders were added into a cylindrical graphite die with internal diameter of 20 mm using 0.3 mm-thick graphite papers for wrapping the powder to prevent adhesion and contamination between the specimen and the tooling. As the powders were pressed at 10 MPa, the graphite die was placed into the SPS apparatus. After loading of 75 MPa, the apparatus was heated to 1200 °C with heating rate of 100 °C/min. After the temperature and pressure were maintained for 5 min, the apparatus was cooled to ambient temperature, and the loading was released to 10 MPa. The pressure and temperature schedules in SPS are illustrated in Fig. 1. Bulk densities of Al2O3–ZrO2 nanocomposites after SPS were measured by Archimedes' method [23], each measurement was taking the average value of 10 specimens. The theoretical densities of ZrO2-5wt %Al2O3 and ZrO2-10 wt%Al2O3 are estimated as 5.93 g/cm3 and 5.77 g/cm3 on the basis of the true density of α-Al2O3 and t-ZrO2. Microstructures were investigated through SEM (Model S4800, Hitachi, Japan). The samples for SEM were polished and thermally etched for 30 min at 900 °C to make the grain boundaries clear. Vickers
3. Results 3.1. Powders characterization Fig. 2 shows the XRD patterns of the raw powders. The purchased ZrO2 powders exhibit broad diffraction peaks identified as t-ZrO2 according to PDF#79-1768, without any detectable m-ZrO2. In the XRD patterns of purchased Al2O3 powders, strong diffraction peaks of αAl2O3 (PDF #42-1468) are shown, suggesting the well crystallization of Al2O3 powders. In the Al2O3–ZrO2 powders prepared by sol-gel method, only broad diffraction maxima are exhibited with no reflections for crystallization, indicating the amorphous phase. SEM images were taken on the raw powders, as shown in Fig. 3. The polycrystal zirconia powders are spherical in shape with slight agglomeration, and the average particle size is about 30 nm (Fig. 3(a)). The polycrystalline alumina powders show both nonspherical and spherical particles in Fig. 3(b), with average grain size of about 190 nm. In Fig. 3(c), the amorphous powders show morphology with round shape and soft particle contacts, and the powders are 25 nm in scale with a narrow size distribution.
Table 1 The fraction of different raw powders in different samples. Sample
Composition
ZrO2 powder
Al2O3 powder
Al2O3-40 mol% ZrO2 amorphous powder
Z5A-PP Z5A-PA Z10A-PP Z10A-PA
ZrO2-5wt%Al2O3 ZrO2-5wt%Al2O3 ZrO2-10 wt%Al2O3 ZrO2-10 wt%Al2O3
95 wt% 91 wt% 90 wt% 82 wt%
5 wt% 0 10 wt% 0
0 9 wt% 0 18 wt%
2
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than that in SPS of Z5A-PP, and the piston displacement exhibits a sigmoid curve with increasing temperature, in which the displacement initiates at 850 °C and turns stable in the vicinity of 1150 °C. Compared to Z5A-PP, the peak value shifts to a lower temperature at about 1100 °C for Z5A-PA. It was suggested that, the densification is slightly improved at low temperatures when amorphous powders are doped. In SPS of Z10A-PP (Fig. 4(c)), the densification behavior is similar to that of Z5A-PP with a quasi-parabolic curve, and the densification begins at about 1000 °C, and the maximum values of dx/dT take place at 1150 °C. However, the densification behavior is quite different in SPS of Z10A-PA (Fig. 4(d)), the sigmoid curve even turns stable below 1100 °C, and the derivative of displacement versus SPS temperature shows the largest value at 1000 °C, confirming much better sinterability of amorphous powders than polycrystalline ones. It is confirmed that, the doping of amorphous powders significantly benefited the sintering of Z5A and Z10A nanocomposites, compared to the doping of polycrystal Al2O3 powders. In our earlier study [19], Al2O3–ZrO2 nanocomposites were fabricated by hot pressing of polycrystal or amorphous powders, and amorphous powders were easier to densify into dense volumes than polycrystal powders. It was inferred that, the easier sintering of amorphous powders were attributed to the metastable state and finer particles of the amorphous powders. In another work [20], Al2O3–ZrO2 nanocomposites were prepared by pressureless sintering of powders in different crystallinity degrees. Powders in crystallinity of 25 vol% showed the best sinterability attributed to the crystallization assisted by the existing nucleation seeds. In this work, the easier sintering of Z5A-PA and Z10A-PA is also related to the metastability of amorphous powders. Compared to polycrystal powders, the amorphous powders are in high-energy state, and the surface and bulk diffusion are fast in sintering. Furthermore, the phase transformation of amorphous powder to crystalline phase in sintering also enhanced the mass diffusion and particle rearrangement in sintering, which is similar
Fig. 2. XRD patterns recorded from the powders.
3.2. Sinterablity The densification behaviors as function of temperature in SPS of ZrO2–Al2O3 nanocomposites using different raw powders are shown in Fig. 4. In the SPS of ZrO2-5wt%Al2O3 nanoceramics using polycrystalline ZrO2 and polycrystalline Al2O3 powders (Z5A-PP), the densification starts at about 950 °C, and shows a quasi-parabolic increase with increasing temperature (Fig. 4(a)). According to the derivative of displacement versus SPS temperature for Z5A-PP in Fig. 4(a), the maximum value of dx/dT is in the vicinity of 1150 °C for Z5A-PP. However, for SPS of ZrO2-10 wt%Al2O3 nanoceramics using polycrystalline ZrO2 and amorphous powders (Fig. 4(b)), the densification is much more obvious
Fig. 3. SEM photographs of the powders: (a) polycrystal zirconia powders; (b) polycrystalline alumina powders and (c) amorphous alumina-zirconia powders. 3
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Fig. 4. The piston displacement and derivative of displacement versus SPS temperature for ZrO2–Al2O3 nanocomposites in SPS using different raw powders: (a) Z5APP; (b) Z5A-PA; (c) Z10A-PP; (d) Z10A-PA.
to those in earlier reports [13–15]. Therefore, the amorphous powders act as sintering aids and promote the sintering in this work.
density of Z5A-PP nanocomposite is smaller than 94%. However, the sample Z5A-PA shows dense microstructure with no porosity in Fig. 5(b), suggesting near-theoretical density after SPS at 1200 °C, whose relative density is up to 98.7%, and the average grain size is about 290 nm. Based on the backscattering electron imaging mechanism, the dark grains are Al2O3, and the light particles are ZrO2, as marked in Fig. 5(b). SEM images of Z10A-PP and Z10A-PA samples SPSed at 1200 °C are shown in Fig. 6(a) and (b). The sample Z10A-PP shows loose microstructure similar to sample Z5A-PP, and a large porosity of 8.2% is determined through Archimedes' method. In comparison, the image of sample Z10A-PA shows almost full dense microstructure with homogeneously and tightly mixed grains, and the relative density is up to
3.3. Microstructure SEM images of Al2O3-5 mol% ZrO2 nanocomposites through SPS of different raw powders at 1200 °C are shown in Fig. 5. In Fig. 5(a), the microstructure of sample Z5A-PP is not dense, with quantities of pores around the particles, agreeing with the relative density of 93.6% measured by Archimedes' method. For polycrystal powders, the mass diffusion and particle rearrangement are difficult at 1200 °C, and higher temperature is demanded to provide driving forces for sintering; therefore, the relative
Fig. 5. SEM images of ZrO2-5 mol% Al2O3 samples through SPS at 1200 °C using (a) polycrystalline ZrO2 and polycrystalline Al2O3 powders; (b) polycrystalline ZrO2 and amorphous powders. 4
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Fig. 6. SEM images of ZrO2-10 mol% Al2O3 samples through SPS at 1200 °C using (a) polycrystalline ZrO2 and polycrystalline Al2O3 powders; (b) polycrystalline ZrO2 and amorphous powders.
amorphous powders and polycrystal Al2O3 powders and are doped in the polycrystalline ZrO2 powders, respectively. The influence of raw powder conditions on the sinterability, microstructure and mechanical property are studied. When doped with amorphous powders, the sintering of ZrO2–Al2O3 nanocomposites is promoted, and ZrO2-5 wt% Al2O3 and ZrO2-10 wt% Al2O3 nanocomposites with relative densities above 99% are obtained after spark plasma sintering at 1200 °C; however, when sintering of polycrystalline ZrO2 and polycrystalline Al2O3 powders, the relative densities are merely 93%. The enhanced sinterablity is due to the metastablity and phase transformation of the amorphous powders, which act as sintering aids. The nanocomposites with near-theoretical density show refined microstructure with homogenous mixture of ZrO2 and Al2O3 grains, which further leads to excellent mechanical properties. Acknowledgments
Fig. 7. Vickers hardness and fracture toughness of different samples.
This work is supported by Natural Science Foundation of Province, No. 2019JQ-694, Fundamental Research Funds Central Universities, CHD300102319303, and National Students Innovative Entrepreneurial Training Plan Program of Province, S201910710300.
99.3%. The mean particle size is about 250 nm, which is close to that of Z5A-PA. In the report of Abdullah [27] and Nevarez-Rascon [28], ZrO2-10 wt % Al2O3 ceramics with near-theoretical density were obtained by pressreless sintering at 1500 °C for 2 h, and the mean grain size was above 500 nm. In this work, the doping of amorphous powders really benefited the sintering of ZrO2–Al2O3 nanocomposites at low temperatures in this work, which is beneficial to the refinement of microstructures.
Shaanxi for the College Shaanxi
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3.4. Mechanical properties The mechanical properties of the Z5A and Z10A nanoceramics prepared by SPS of different raw powders are investigated through Vickers indentation, and characterized by Vickers hardness (HV) and fracture toughness (KIC), as displayed in Fig. 7. It is apparent that, both Z5A and Z10A samples from polycrystalline ZrO2 and amorphous powders show superior mechanical property with high Vickers hardness and fracture toughness, compared to those from polycrystalline ZrO2 and polycrystalline Al2O3 powders. The Z5A-PA sample exhibited Vickers hardness of 13.7 GPa and fracture toughness of 5.3 MPa m1/2, and these values are 14.0 GPa and 5.1 MPa m1/2 for sample Z10A-PA, which are much higher than those reported in most Al2O3–ZrO2 composites [29,30]. It is reasonable that, the excellent mechanical properties are contributed by the high density and refined microstructure. 4. Conclusions In present work, ZrO2-5 wt% Al2O3 and ZrO2-10 wt% Al2O3 nanocomposites are fabricated through spark plasma sintering; Al2O3–ZrO2 5
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X. Xu, et al. additives, J. Am. Ceram. Soc. 74 (2010) 2011–2013. [13] M. Azar, P. Palmero, M. Lombardi, V. Garnier, L. Montanaro, G. Fantozzi, J. Chevalier, Effect of initial particle packing on the sintering of nanostructured transition alumina, J. Eur. Ceram. Soc. 36 (2008) 1121–1128. [14] R.P. Yavetskiy, V.N. Baumer, N.A. Dulina, YuI. Pazura, I.А. Petrusha, V.N. Tkach, А.V. Tolmachev, V.Z. Turkevich, An approach to Y2O3:Eu3+ optical nanostructured ceramics, J. Eur. Ceram. Soc. 32 (2012) 257–260. [15] K.N.P. Kumar, K. Keizer, A.J. Burggraaf, T. Okubo, H. Nagamoto, S. Morooka, Densification of nanostructured titania assisted by a phase transformation, Nature 358 (1992) 48–51. [16] J. Suffner, M. Lattemann, H. Hahn, G. Lars, C. Hess, I.G. Cano, S. Dosta, J.M. Guilemany, C. Musa, A.M. Locci, R. Licheri, R. Orrú, Giacomo Cao, Microstructure evolution during spark plasma sintering of metastable (ZrO2-3 mol% Y2O3)–20 wt% Al2O3 composite powders, J. Am. Ceram. Soc. 93 (2010) 2864–2870. [17] A. Rittidech, R. Somrita, T. Tunkasiri, Effect of addingY2O3 on structural and mechanical properties of Al2O3-ZrO2 ceramics, Ceram. Int. 39 (2013) S433–S436. [18] J. Freim, J. McKittrick, Modeling and fabrication of fine-grain alumina–zirconia composites produced from nanocrystalline precursors, J. Am. Ceram. Soc. 81 (1998) 1773–1780. [19] X. Xu, X. Xu, J. Liu, W. Hong, H. Du, F. Hou, Low-temperature fabrication of Al2O3ZrO2 (Y2O3) nanocomposites through hot pressing of amorphous powders, Ceram. Int. 42 (2016) 15065–15071. [20] X. Xu, J. Liu, M. Zhuang, A. Guo, H. Du, F. Hou, Nanograined Al2O3-ZrO2 (Y2O3) ceramics through pressureless sintering of powders with different degrees of crystallinity, Ceram. Int. 43 (2017) 16872–16879. [21] A. Nevarez-Rascon, A. Aguilar-Elguezabal, E. Orrantia, M.H. Bocanegra-Bernal, On the wide range of mechanical properties of ZTA and ATZ based dental ceramic
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Spark plasma sintering of ZrO2–Al2O3 nanocomposites at low temperatures aided by amorphous powders Xiqing Xu∗, Tiyue Tao, Xin Zhang, Zhanzhi Cao, Di Huang, Hongchan Liang, Yinglu Hu School of Materials Science and Engineering, Chang'an University, Xi'an, 710061, China
ARTICLE INFO
ABSTRACT
Keywords: Spark plasma sintering Amorphous powders Sinterability ZrO2–Al2O3 Nanocomposites
In present work, ZrO2-5 wt% Al2O3 and ZrO2-10 wt% Al2O3 nanocomposites are fabricated through spark plasma sintering. Al2O3–ZrO2 amorphous powders and polycrystal Al2O3 powders and are doped in the polycrystalline ZrO2 powders, respectively. When doped with amorphous powders, the sintering of ZrO2–Al2O3 nanocomposites is promoted, and ZrO2-5 wt% Al2O3 and ZrO2-10 wt% Al2O3 nanocomposites with relative densities of 99% are obtained after spark plasma sintering at 1200 °C; however, when sintering of polycrystalline ZrO2 and polycrystalline Al2O3 powders, the relative densities are merely 93%. The enhanced sinterability is due to the metastability and phase transformation of the amorphous powders, which act as sintering aids. The nanocomposites with near-theoretical density show refined microstructure with homogenous mixture of ZrO2 and Al2O3 grains, which further leads to excellent mechanical properties. This article provides new ideas for low-temperature sintering of nanocomposites via using doping amorphous powders.
1. Introduction Low-temperature sintering is a vital issue in ceramics, which is beneficial to microstructural refinement and therefore enhanced mechanical properties [1–3]. Some other advantages are also brought about, for example, some unwanted reactions of constituents can be prevented [4], high-temperature melting can be avoided [5], and certain functional properties can be attained [6,7]. Therefore, several methods have been conducted to achieve low temperature sintering, including spark plasma sintering [8,9], liquid-phase sintering [10], hot pressing [11], sintering with aids [12], and sintering assisted by phase transition [13–15]. Phase-transition-assisted sintering is an important technique in the field of low-temperature sintering. The phase transition from metastable to stable phase takes place in sintering and promotes the mass diffusion and particle rearrangement, which contributes to low-temperature densification into volumes without serious grain growth. Series of ceramics have been fabricated through this technique, accompanied with transition from metastable to stable phases, such as Al2O3 from gamma to alpha phase [13], Y2O3 from cubic to monoclinic phase [14] and TiO2 from anatase to rutile phase [15]. Al2O3–ZrO2(Y2O3) is one of the most important ceramic composites with wide applications. Given the excellent ambient and high temperature properties, such as high toughness, high strength, good wear
∗
resistance, and biocompatibility, it is mainly used as cutting tools, bearings, high-temperature engine components, and knee replacement prostheses. Al2O3–ZrO2 composites with fine grains are generally prepared by sintering or SPS of nanocrystaline powders, and the sintering temperatures are generally above 1450 °C [16–18]. In our earlier works, Al2O3–ZrO2 composites were prepared through hot pressing [19] and pressureless sintering [20] of Al2O3–ZrO2 amorphous powders at low temperatures, benefited from the transformation of ZrO2 and Al2O3. However, in Al2O3–ZrO2 system, the powders can be obtained in amorphous state only near the eutectic composition due to the easy crystallization, which highly constrains the volume fraction of Al2O3 or ZrO2 in the products. Actually, for ZrO2–Al2O3, one of the most important ceramics, the microstructures and properties highly rely on the fraction of different components in a broad range [21,22]. In this condition, we point out that, the amorphous Al2O3–ZrO2 powders with eutectic composition can be added into the polycrystal powders, and act as sintering aids, to prepare ZrO2–Al2O3 nanocomposites distinct from eutectic composition. In this work, Al2O3–ZrO2 amorphous powders and polycrystal Al2O3 powders were doped into ZrO2 powders in spark plasma sintering, respectively, and the influence of different additives on sintering was studied. The dopant of amorphous powders enhanced the sintering, mainly led by the phase transformation of amorphous powders. Through spark plasma sintering aided by amorphous powders, ZrO2-
Corresponding author. E-mail address: [email protected] (X. Xu).
https://doi.org/10.1016/j.ceramint.2019.10.160 Received 8 August 2019; Received in revised form 30 September 2019; Accepted 17 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xiqing Xu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.160
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5 wt% Al2O3 and ZrO2- 10 wt% Al2O3 nanocomposites with full dense were fabricated, and the microstructures, as well as mechanical properties were investigated. 2. Experimental procedures 2.1. Raw powders Amorphous powders in the composition of Al2O3-40 mol% ZrO2 were synthetized by sol-gel method, and calcinated at 800 °C for 2 h, whose detailed procedures were reported in our earlier work [19]. These amorphous powders were then added into 3Y-ZTP powders (Tosoh, Japan) to serve as the second phase and sintering aids. As 9 wt% of amorphous powders with composition of Al2O3-40 mol% ZrO2 were added into 91 wt % of polycrystalline ZrO2 powders, the resulting composition was ZrO25wt%Al2O3, and this sample is marked by Z5A-PA, hereafter. Similarly, ZrO2-10 wt%Al2O3 powders were obtained by added 18 wt% of Al2O340 mol% ZrO2 amorphous powders and 82 wt% polycrystalline ZrO2 powders, and marked by Z10A-PA. In comparison, commercial polycrystalline Al2O3 powders (Sumitomo, Japan) were also added into the 3YZTP powders as the second phase. When 5 wt% or 10 wt%of polycrystalline Al2O3 powders with were added into polycrystalline ZrO2 powders, ZrO2-5wt%Al2O3 or ZrO2-10 wt%Al2O3powders were obtained, and marked by Z5A-PP or Z10A-PP. To obtain ZrO2-5wt%Al2O3 or ZrO2-10 wt %Al2O3 nanoceramics with different dopants, the detailed composition of raw powders was displayed in Table 1. As the powders were homogenously mixed through ball milling with ethanol media, they were sieved via 80-mesh after drying in air. The raw powders were characterized through scanning electron microscope (SEM, Model S4800, Hitachi, Japan) to investigate their morphologies and particle sizes. The phase composition was identified by X-ray diffraction analysis (XRD, Cu Kα, D/Max-2500v/pc, Rigaku, Japan).
Fig. 1. The pressure and temperature schedules in spark plasma sintering.
hardness (HV) and fracture toughness (KIC) were measured through indentation method using a microhardness machine (HXD-1000TM, Shanghai), in which 10 N was applied on the polished surface for 15 s. HV and KIC can be calculated based on the following equation [24,25],
HV = 1.854
KIC = 0.016
P d2 E HV
(2) 1/2
P L3/2
(3)
where P was the loading of 10 N, d meant the average diagonal length of the indentation, L was the crack length initiated from the indent edge, and E prensented the Young's modulus, which were determined as 214 GPa and 228 GPa for ZrO2-5wt%Al2O3 and ZrO2-10 wt%Al2O3 based on mixing rule [26]. Each value of HV or KIC was taking average value of 20 indentations.
2.2. Spark plasma sintering The powders were sintered using a spark plasma sintering apparatus (DR. SINTER, SPS-2050 systems, Fuji Electronic Industrial, Japan) in vacuum with the partial pressure less than 4 Pa. For each sample, the powders were added into a cylindrical graphite die with internal diameter of 20 mm using 0.3 mm-thick graphite papers for wrapping the powder to prevent adhesion and contamination between the specimen and the tooling. As the powders were pressed at 10 MPa, the graphite die was placed into the SPS apparatus. After loading of 75 MPa, the apparatus was heated to 1200 °C with heating rate of 100 °C/min. After the temperature and pressure were maintained for 5 min, the apparatus was cooled to ambient temperature, and the loading was released to 10 MPa. The pressure and temperature schedules in SPS are illustrated in Fig. 1. Bulk densities of Al2O3–ZrO2 nanocomposites after SPS were measured by Archimedes' method [23], each measurement was taking the average value of 10 specimens. The theoretical densities of ZrO2-5wt %Al2O3 and ZrO2-10 wt%Al2O3 are estimated as 5.93 g/cm3 and 5.77 g/cm3 on the basis of the true density of α-Al2O3 and t-ZrO2. Microstructures were investigated through SEM (Model S4800, Hitachi, Japan). The samples for SEM were polished and thermally etched for 30 min at 900 °C to make the grain boundaries clear. Vickers
3. Results 3.1. Powders characterization Fig. 2 shows the XRD patterns of the raw powders. The purchased ZrO2 powders exhibit broad diffraction peaks identified as t-ZrO2 according to PDF#79-1768, without any detectable m-ZrO2. In the XRD patterns of purchased Al2O3 powders, strong diffraction peaks of αAl2O3 (PDF #42-1468) are shown, suggesting the well crystallization of Al2O3 powders. In the Al2O3–ZrO2 powders prepared by sol-gel method, only broad diffraction maxima are exhibited with no reflections for crystallization, indicating the amorphous phase. SEM images were taken on the raw powders, as shown in Fig. 3. The polycrystal zirconia powders are spherical in shape with slight agglomeration, and the average particle size is about 30 nm (Fig. 3(a)). The polycrystalline alumina powders show both nonspherical and spherical particles in Fig. 3(b), with average grain size of about 190 nm. In Fig. 3(c), the amorphous powders show morphology with round shape and soft particle contacts, and the powders are 25 nm in scale with a narrow size distribution.
Table 1 The fraction of different raw powders in different samples. Sample
Composition
ZrO2 powder
Al2O3 powder
Al2O3-40 mol% ZrO2 amorphous powder
Z5A-PP Z5A-PA Z10A-PP Z10A-PA
ZrO2-5wt%Al2O3 ZrO2-5wt%Al2O3 ZrO2-10 wt%Al2O3 ZrO2-10 wt%Al2O3
95 wt% 91 wt% 90 wt% 82 wt%
5 wt% 0 10 wt% 0
0 9 wt% 0 18 wt%
2
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than that in SPS of Z5A-PP, and the piston displacement exhibits a sigmoid curve with increasing temperature, in which the displacement initiates at 850 °C and turns stable in the vicinity of 1150 °C. Compared to Z5A-PP, the peak value shifts to a lower temperature at about 1100 °C for Z5A-PA. It was suggested that, the densification is slightly improved at low temperatures when amorphous powders are doped. In SPS of Z10A-PP (Fig. 4(c)), the densification behavior is similar to that of Z5A-PP with a quasi-parabolic curve, and the densification begins at about 1000 °C, and the maximum values of dx/dT take place at 1150 °C. However, the densification behavior is quite different in SPS of Z10A-PA (Fig. 4(d)), the sigmoid curve even turns stable below 1100 °C, and the derivative of displacement versus SPS temperature shows the largest value at 1000 °C, confirming much better sinterability of amorphous powders than polycrystalline ones. It is confirmed that, the doping of amorphous powders significantly benefited the sintering of Z5A and Z10A nanocomposites, compared to the doping of polycrystal Al2O3 powders. In our earlier study [19], Al2O3–ZrO2 nanocomposites were fabricated by hot pressing of polycrystal or amorphous powders, and amorphous powders were easier to densify into dense volumes than polycrystal powders. It was inferred that, the easier sintering of amorphous powders were attributed to the metastable state and finer particles of the amorphous powders. In another work [20], Al2O3–ZrO2 nanocomposites were prepared by pressureless sintering of powders in different crystallinity degrees. Powders in crystallinity of 25 vol% showed the best sinterability attributed to the crystallization assisted by the existing nucleation seeds. In this work, the easier sintering of Z5A-PA and Z10A-PA is also related to the metastability of amorphous powders. Compared to polycrystal powders, the amorphous powders are in high-energy state, and the surface and bulk diffusion are fast in sintering. Furthermore, the phase transformation of amorphous powder to crystalline phase in sintering also enhanced the mass diffusion and particle rearrangement in sintering, which is similar
Fig. 2. XRD patterns recorded from the powders.
3.2. Sinterablity The densification behaviors as function of temperature in SPS of ZrO2–Al2O3 nanocomposites using different raw powders are shown in Fig. 4. In the SPS of ZrO2-5wt%Al2O3 nanoceramics using polycrystalline ZrO2 and polycrystalline Al2O3 powders (Z5A-PP), the densification starts at about 950 °C, and shows a quasi-parabolic increase with increasing temperature (Fig. 4(a)). According to the derivative of displacement versus SPS temperature for Z5A-PP in Fig. 4(a), the maximum value of dx/dT is in the vicinity of 1150 °C for Z5A-PP. However, for SPS of ZrO2-10 wt%Al2O3 nanoceramics using polycrystalline ZrO2 and amorphous powders (Fig. 4(b)), the densification is much more obvious
Fig. 3. SEM photographs of the powders: (a) polycrystal zirconia powders; (b) polycrystalline alumina powders and (c) amorphous alumina-zirconia powders. 3
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Fig. 4. The piston displacement and derivative of displacement versus SPS temperature for ZrO2–Al2O3 nanocomposites in SPS using different raw powders: (a) Z5APP; (b) Z5A-PA; (c) Z10A-PP; (d) Z10A-PA.
to those in earlier reports [13–15]. Therefore, the amorphous powders act as sintering aids and promote the sintering in this work.
density of Z5A-PP nanocomposite is smaller than 94%. However, the sample Z5A-PA shows dense microstructure with no porosity in Fig. 5(b), suggesting near-theoretical density after SPS at 1200 °C, whose relative density is up to 98.7%, and the average grain size is about 290 nm. Based on the backscattering electron imaging mechanism, the dark grains are Al2O3, and the light particles are ZrO2, as marked in Fig. 5(b). SEM images of Z10A-PP and Z10A-PA samples SPSed at 1200 °C are shown in Fig. 6(a) and (b). The sample Z10A-PP shows loose microstructure similar to sample Z5A-PP, and a large porosity of 8.2% is determined through Archimedes' method. In comparison, the image of sample Z10A-PA shows almost full dense microstructure with homogeneously and tightly mixed grains, and the relative density is up to
3.3. Microstructure SEM images of Al2O3-5 mol% ZrO2 nanocomposites through SPS of different raw powders at 1200 °C are shown in Fig. 5. In Fig. 5(a), the microstructure of sample Z5A-PP is not dense, with quantities of pores around the particles, agreeing with the relative density of 93.6% measured by Archimedes' method. For polycrystal powders, the mass diffusion and particle rearrangement are difficult at 1200 °C, and higher temperature is demanded to provide driving forces for sintering; therefore, the relative
Fig. 5. SEM images of ZrO2-5 mol% Al2O3 samples through SPS at 1200 °C using (a) polycrystalline ZrO2 and polycrystalline Al2O3 powders; (b) polycrystalline ZrO2 and amorphous powders. 4
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Fig. 6. SEM images of ZrO2-10 mol% Al2O3 samples through SPS at 1200 °C using (a) polycrystalline ZrO2 and polycrystalline Al2O3 powders; (b) polycrystalline ZrO2 and amorphous powders.
amorphous powders and polycrystal Al2O3 powders and are doped in the polycrystalline ZrO2 powders, respectively. The influence of raw powder conditions on the sinterability, microstructure and mechanical property are studied. When doped with amorphous powders, the sintering of ZrO2–Al2O3 nanocomposites is promoted, and ZrO2-5 wt% Al2O3 and ZrO2-10 wt% Al2O3 nanocomposites with relative densities above 99% are obtained after spark plasma sintering at 1200 °C; however, when sintering of polycrystalline ZrO2 and polycrystalline Al2O3 powders, the relative densities are merely 93%. The enhanced sinterablity is due to the metastablity and phase transformation of the amorphous powders, which act as sintering aids. The nanocomposites with near-theoretical density show refined microstructure with homogenous mixture of ZrO2 and Al2O3 grains, which further leads to excellent mechanical properties. Acknowledgments
Fig. 7. Vickers hardness and fracture toughness of different samples.
This work is supported by Natural Science Foundation of Province, No. 2019JQ-694, Fundamental Research Funds Central Universities, CHD300102319303, and National Students Innovative Entrepreneurial Training Plan Program of Province, S201910710300.
99.3%. The mean particle size is about 250 nm, which is close to that of Z5A-PA. In the report of Abdullah [27] and Nevarez-Rascon [28], ZrO2-10 wt % Al2O3 ceramics with near-theoretical density were obtained by pressreless sintering at 1500 °C for 2 h, and the mean grain size was above 500 nm. In this work, the doping of amorphous powders really benefited the sintering of ZrO2–Al2O3 nanocomposites at low temperatures in this work, which is beneficial to the refinement of microstructures.
Shaanxi for the College Shaanxi
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