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Microstructure and mechanical properties of ZTA composites fabricated by oscillatory pressure sintering
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Microstructure and mechanical properties of ZTA composites fabricated by oscillatory pressure sintering ⁎
Tianbin Zhua, Zhipeng Xiea, , Yao Hana, Shuang Lib a b
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing, China School of Resources and Environmental Engineering, Shandong University of Technology, Zibo, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Oscillatory pressure sintering (OPS) Microstructure Mechanical properties ZTA composites
The influence of sintering temperature on the microstructure and mechanical properties of Al2O3−20 wt% ZrO2 composites fabricated by oscillatory pressure sintering (OPS) was investigated by means of X-ray diffraction, scanning electron microscopy, three-point bending test and Vickers indentation. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The optimum oscillatory pressure sintering temperature was found to be 1600 °C; almost fully dense materials (99.94% of theoretical density) with homogeneous microstructure could be achieved. The highest flexural strength, fracture toughness and hardness of such composites reached 1145 MPa, 5.74 MPa m1/2 and 19.08 GPa when sintered at 1600 °C, respectively. Furthermore, the oscillatory pressure sintering temperature could be decreased by more than 50 °C as compared with the HP method, OPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.
1. Introduction ZTA composites (known as zirconia toughened alumina ceramics) are widely used for structural applications such as cutting tools, bearings, dies, and prosthetic components due to their excellent mechanical properties [1,2]. Such composites possess some combined merits, namely the good chemical stability and superior wear resistance of αAl2O3 and high fracture strength and toughness of t-ZrO2 [3,4]. Generally, ZTA composites are to disperse some t-ZrO2 (often up to 20 vol %) into the α-Al2O3 matrix. Therefore, the existence of t-ZrO2 would lead to the improved fracture toughness and strength of the α-Al2O3 matrix owing to the occurrence of the stress-induced phase transformation (t-ZrO2→m-ZrO2). Such tetragonal to monoclinic phase transformation results in different toughening mechanisms of ZTA composites, mainly including stress-induced transformation toughening and microcracks toughening [5,6]. In addition, the influence of residual stresses in such composites has been reported by some researchers [7,8]. To enhance the mechanical properties of ZTA composites, much work, mainly including the optimization of ZrO2 addition, the incorporation of various additives and the development of different sintering techniques, has been performed in the past 20 years. For example, Sommer et al. [9,10] reported that the flexural strength of ZTA composites was related highly with ZrO2 content and a relatively high
⁎
flexural strength of 1288 MPa was obtained for the Al2O3−24 vol% ZrO2 composites. Furthermore, many investigators focused on the incorporation of various additives in ZTA composites, such as Cr2O3 [11,12], MgO [13], TiO2 [14], CeO2 [15], Ta2O5 [16], Nb2O5 [17], BN [18], single-wall and multi-wall carbon nanotubes (SWCNTs and MWCNTs) [19,20], and graphene platelets (GPLs) [21], etc. It was reported [16] that an apparent increase of 27.6% was achieved for the flexural strength of ZTA composites with 0.36 vol% Ta2O5 as compared to the pure ZTA composites, and the corresponding improvement of 25.9% was for their Vickers hardness. Also, Liu et al. [21] found that the addition of 0.81 vol% GPLs into ZTA composites resulted in a 40% increase in fracture toughness; the pull-out of GPLs, crack bridging and crack deflection were considered as the main toughening mechanisms. As is well known, the mechanical properties of ZTA composites can be significantly improved by decreasing the porosity and grain sizes and by enhancing the homogeneity of the phase dispersion [22], which is strongly influenced by the used sintering technique. In the past three decades, the pressureless sintering (PS) [23,24], hot pressing (HP) [9,10], hot isostatic pressing (HIP) [25], spark plasma sintering (SPS) [22,26], and microwave sintering (MS) [27,28] were developed to fabricate ZTA composites. In comparison with PS and MS, the pressureassisted sintering techniques (HP, HIP and SPS) exhibited more advantages, namely effectively reducing the sintering temperature, shortening the soaking time and improving the mechanical properties
Corresponding author. E-mail address: [email protected] (Z. Xie).
http://dx.doi.org/10.1016/j.ceramint.2017.09.204 Received 11 August 2017; Received in revised form 7 September 2017; Accepted 25 September 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhu, T., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.09.204
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of ZTA composites. Nevertheless, the pressure-assisted sintering processes mentioned-above were performed in the form of a constant static pressure. Much recently, a new oscillatory pressure sintering (OPS) technique was developed by introducing an oscillatory loading with controlled frequency and value in the sintering process [29]. Xie et al. [29–31] found that the 3Y-TZP and Si3N4 ceramics fabricated by OPS had higher density and superior mechanical properties as compared with the conventional sintering methods. In the present work, Al2O3−20 wt% ZrO2 composites were fabricated by the OPS method with commercially available Al2O3-ZrO2 composite powders as starting materials. The main aim of the present work was the optimization of sintering temperature, which could yield the maximum density close to the theoretical density of such composites, with emphasis on controlling their grain growth and enhancing their mechanical properties. Moreover, the results were compared to specimens obtained by conventional HP method under a similar sintering schedule.
Fig. 1. Schematic of the applied pressure during the soaking stage at target temperature.
software (Image-Pro Plus 6.0) from field-emission scanning electron microscopy (FESEM; Leo-1530, Zeiss, Oberkochen, Germany) images of the polished and thermally etched surfaces. FESEM was also used to observe the fracture surfaces of the specimens. The phase compositions of Al2O3-ZrO2 powders and sintered specimens were confirmed by Xray diffractometry (XRD, D8 advance, Bruker, Germany; using Ni filtered, Cu Kα radiation at a scanning rate of 6 deg/min). In addition, the hardness of all the specimens was measured using Vickers indentation with an applied loading of 10 kg and a soaking time of 15 s 12 indentations were used for each specimen and the results were statistically analyzed. Also, the elastic modulus (E) of the specimens was determined via the nondestructive dynamic elastic modulus tester (based on the acoustic method). Fracture toughness (KIC) was evaluated by a single-edge notched beam (SENB) test with a span of 16 mm using test bars of 4 mm × 2 mm × 22 mm (the width and depth of the notch were ~0.16 mm and ~2 mm, respectively); the loading was continuously applied at a constant speed of 0.05 mm/min until the fracturing of the specimen. Six specimens were used to determine the average KIC value. Flexural strength (σ) was measured by three-point bending method using test bars of 3 mm × 4 mm × 36 mm, in which the tensile surface of the bars was polished down to 1 µm; the tests were performed at ambient temperature using a universal testing machine with a loading rate of 0.5 mm/min and a span of 30 mm. The measured σ value was the average of 22 specimens. Moreover, the flexural strength of the specimens was studied statistically according to Weibull distribution analysis [32,33]:
2. Experimental The Al2O3-ZrO2 composite powders (ZTA-008G, Shandong Sinocera Xinmeiyu Alumina Co., Ltd., Shandong, China) were selected as the starting materials. The chemical compositions of the as-received powders were as follows: Al2O3 + ZrO2 + Y2O3- > 99.9 wt%, Y2O3−3.07 wt%, ZrO2−20.02 wt%, Fe2O3-≤0.01 wt%, SiO2≤0.01 wt%, and NaO-≤0.02 wt%. Their specific surface area was 14.58 m2/g. In addition, the composite powders consisted of the quasisphere like particles, in which the average sizes of Al2O3 and ZrO2 were 128 nm and 54 nm, respectively. The α-Al2O3 and t-ZrO2 phases were detected as main crystalline phases with the aid of X-ray diffraction (XRD), where a small amount of m-ZrO2 phase was also verified. The sintering experiments were performed on a self-developed oscillatory pressure sintering (OPS) machine, whose detailed presentation can be found in our previous work [31]. With respect to specimens fabricated by OPS, the as-received Al2O3-ZrO2 powders were put into a graphite die with the inner diameter of 100 mm and then prepressed under a pressure of 10 MPa. Subsequently, the powders were heated under vacuum from ambient temperature to 500 °C with a constant heating rate of 25 °C/min and applied pressure keeping at 10 MPa, and then heated to 1100 °C with a heating rate of 15 °C/min under vacuum and applied pressure increasing from 10 MPa to 20 MPa. After soaking at 1100 °C for 30 min (with the aim of eliminating the volatiles in the powders), the powders were continuously heated to the target sintering temperature with a heating rate of 10 °C/min under an argon atmosphere while the applied pressure increased gradually from 20 MPa to 30 MPa. When the temperature was increased to the target temperature, an oscillatory pressure (PO) was added on the basis of the constant pressure (PC), as shown in Fig. 1. Hence, the total pressure (PT) of 27.5–32.5 MPa was achieved during the soaking period at final sintering temperature. After holding for 60 min, the PO was removed while PC was maintained until the temperature was decreased to 900 °C with a cooling rate of 15 °C/min. In this work, the selected target sintering temperatures included 1500 °C, 1550 °C, 1600 °C and 1650 °C; the corresponding sintered specimens were designated as A-1500, A-1550, A-1600 and A-1650, respectively. For comparison, the as-received Al2O3-ZrO2 powders were also used to prepare ZTA composites via the conventional hot pressing (HP); the prepared specimen was marked as A-HP. Specifically, the specimen AHP was prepared on the OPS machine under a similar sintering schedule except that a constant pressure of 32.5 MPa was applied at 1600 °C for 60 min without oscillations. The relative density of the sintered ZTA composites was measured by Archimedes' principle taking 4.29 g/cm3 as their theoretical density (achieved by the rule of mixtures assuming a density of 6.10 g/cm3 for ZrO2 and 3.98 g/cm3 for Al2O3). The mean grain sizes of Al2O3 and ZrO2 (at least 300 grains) were measured using an image analysis
ln ln[1/(1 − Pn )] = m ln σn − m ln σ0
(1)
The failure probability estimator is:
Pn = (i − 0.5)/ N
(2)
where m is the Weibull modulus, i is the ranking, N is the number of specimens tested, σn is the nth flexural strength value, and σ0 is the characteristic flexural strength. 3. Results and discussion Fig. 2 presents the XRD patterns of ZTA composites after sintering at various temperatures. The phases were confirmed with respect to reference peak positions from the Inorganic Crystal Structure Database (ICSD). Apparently, irrespectively of the used sintering temperature (1500–1650 °C), only α-Al2O3 (ICSD #52 647) and t-ZrO2 (ICSD #85 322) phases were detected. It was indicated that m-ZrO2 (ICSD #62 993) phase in the starting powders transformed completely into t-ZrO2 phase in the sintering process, and did not appeared again during the cooling process. In this work, the presence of t-ZrO2 phase in ZTA composites would lead to an enhancement of flexural strength and fracture toughness of such composites, due mainly to the occurrence of 2
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boundaries in Al2O3 matrix with dark contrast (Fig. 3); the microstructure was homogeneous in all specimens, and no abnormal growth of Al2O3 grains can be found. So, the presence of t-ZrO2 grains suppressed the abnormal growth of Al2O3 grains. Certainly, the grain sizes of Al2O3 and ZrO2 increased with the sintering temperature from 1500 °C to 1650 °C. In addition, their mean grain sizes were calculated, and the results are displayed in Table 1. With regard to specimens fabricated by OPS, when the sintering temperature increased from 1500 °C to 1650 °C, the mean grain sizes of Al2O3 became bigger from 622 nm to 1086 nm, and that of ZrO2 increased from 235 nm to 385 nm. By comparing specimens A-HP and A-1600, it can be found that the introduction of the oscillatory pressure inhibited the growth of Al2O3 (813 nm vs 872 nm) and ZrO2 (323 nm vs 348 nm) grains. In general, the driving force of the grain size was only related with the curvature radius of the grain boundary. Therefore, increased curvature radius might be obtained due to improved plastic deformation from the oscillatory pressure, which would lead to decreased migration rate of grain boundary. Consequently, the specimen A-1600 had smaller mean grain sizes of Al2O3 and ZrO2 as compared to the specimen A-HP. In addition, the flexural strength of all specimens was measured at room temperature by three-point bending test. Due to the stochastic nature of fracture in ZTA composites, their flexural strengths (σ) were statistically investigated according to the Weibull distribution analysis; the results are shown in Fig. 4 and Table 2. As expected, the specimen A-1500 possessed the lowest σ value of 867 MPa and the smallest Weibull modulus (m) of 8.70, which was attributed mainly to the existence of its higher residual porosity (0.5%). When the sintering temperature was 1600 °C, the highest σ value of 1145 MPa and the largest m value of 13.08 were achieved for the specimen A-1600 due to the almost perfectly full density and homogenous microstructure. With further increasing the sintering temperature to 1650 °C, both the σ and m values of the specimen decreased, namely 1042 MPa and 11.20; this can be explained by the obvious growth of Al2O3 and ZrO2 grains. It is generally accepted that grain refinement is favorable for enhancing flexural strength and hardness of a material according to a classic HallPetch relationship [34,35]. Correspondingly, the hardness change of specimens had a similar tendency to their flexural strength change. That is to say, the specimen A-1600 had the highest hardness of 19.08 GPa. In the case of the specimen A-HP, a flexural strength of 990 MPa and a hardness of 18.58 GPa were obtained. According to Table 2, it can be concluded that the sintering temperature had a crucial influence on the flexural strength of ZTA composites, and the oscillatory pressure sintering temperature could be decreased by more than 50 °C as compared to the conventional HP method. The fracture toughness (KIC) of all the specimens was measured by the SENB method, and the results are exhibited in Table 3. It was not difficult to find that the specimen A-1600 presented the highest KIC value of 5.74 MPa m1/2. As is well known, the KIC is positively correlated with the elastic modulus (E) and fracture surface energy (γ) of a material [36]. In order to explain the KIC change of the specimens, the E and γ changes will be considered in the following part. Concerning the E of specimens, it was commonly considered that it was only sensitive to the porosity in specimens, besides the measured temperature. In this work, a relatively high densification (not lower than 99.5%) was achieved for all specimens, so the influence of the porosity on the elastic modulus might be neglected. In order to confirm this idea, the E values of all specimens were measured by a nondestructive dynamic elastic modulus tester. As expected, the E value of ~360 GPa was obtained for all specimens irrespectively of the used sintering temperatures. Hence, the γ change will have a significant influence on the KIC. In practice, it was very difficult to determine directly the γ value of ZTA composites, so the γ change was analyzed indirectly with the aid of Vickers indentations. The lengths of cracks in the specimens after indenting were analyzed statically and listed in Table 3. It can be seen that the specimen A-1500 had the largest average crack length, and the smallest one was for the specimen A-1600. Generally,
Fig. 2. XRD patterns of specimens fabricated by oscillatory pressure sintering.
the phase transformation (t-ZrO2→m-ZrO2) when subjected to external mechanical loading. The density change of ZTA composites is shown in Table 1. The relative density of all the specimens was higher than 99.50%. As for specimens fabricated by OPS, their relative density increased continuously with the sintering temperature up to 1600 °C but decreased slightly up to 1650 °C. Particularly, the specimen A-1600 had the highest relative density of 99.94%, which was almost a fully dense material. In the case of the specimen A-HP, its relative density was similar to that of the specimen A-1500, but lower than that of other specimens (A-1550, A-1600 and A-1650). By comparing the relative density of specimens A-HP and A-1600, it was clear that an improvement of ~0.4% (seemed a rather minor change) was achieved for the specimen A-1600 even though the same sintering temperature was used; in reality, it was very difficult to remove the residual closed pores (~0.4%) in the final stage of sintering. Therefore, the introduction of oscillatory pressure was strongly helpful for the densification of ZTA composites. As mentioned in the experimental part, the oscillatory pressure was added when ZTA composites were heated to the target sintering temperature (namely during the soaking period). So, the particle rearrangement caused by the oscillatory pressure can be neglected owing to the achievement of a rather denser composites when heated to the target temperature (not lower than 1500 °C); however, the oscillatory pressure might offer an additional sintering driving force in the final stage of sintering, thus promoting grain boundary sliding and improving plastic deformation and diffusion that favored mass transport in comparison with the conventional HP. As a consequence, the sintering temperature of OPS could be decreased by 50–100 °C as compared to the conventional HP. The polished and thermally etched surfaces of specimens were observed with the aid of FESEM. Obviously, it can be seen that t-ZrO2 grains with light contrast were homogeneously dispersed at the grain
Table 1 The relative density and mean grain sizes of specimens.
Specimen 1500 Specimen 1550 Specimen 1600 Specimen 1650 Specimen
Relative density (%)
Mean grain size of Al2O3 (nm)
Mean grain size of ZrO2 (nm)
A-
99.50
622 ± 194
235 ± 80
A-
99.80
733 ± 242
250 ± 96
A-
99.94
813 ± 300
323 ± 105
A-
99.91
1086 ± 387
385 ± 147
A-HP
99.53
872 ± 303
348 ± 111
3
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Fig. 3. SEM images of polished and thermally etched surfaces of specimens fabricated by oscillatory pressure sintering: (A) specimen A-1500, (B) specimen A-1550, (C) specimen A-1600 and (D) specimen A-1650.
Table 2 Mechanical properties of specimens fabricated by different sintering methods.
Specimen 1500 Specimen 1550 Specimen 1600 Specimen 1650 Specimen
Flexural strength (MPa)
Vickers hardness (GPa)
Elastic modulus (GPa)
A-
867 ± 129
18.62 ± 0.40
361 ± 3
A-
1003 ± 99
18.91 ± 0.20
366 ± 2
A-
1145 ± 124
19.08 ± 0.35
371 ± 5
A-
1042 ± 111
18.39 ± 0.33
362 ± 3
A-HP
990 ± 114
18.58 ± 0.34
362 ± 4
Table 3 Length of cracks and fracture toughness of specimens.
Specimen Specimen Specimen Specimen Specimen
Fig. 4. The Weibull plot of flexural strength for ZTA composites fabricated by oscillatory pressure sintering.
the area of a crack is correlated positively with its length. Meanwhile, all specimens were indented under the same loading, so the driving force of crack extension was fixed. Consequently, the increased energy dissipation per unit area during crack extension was achieved for the specimen A-1600, thus resulting in its improved fracture surface energy. Furthermore, to better explain the improvement of the fracture surface energy of the specimen A-1600, its fracture surface and crack extension path were observed by means of FESEM. According to Fig. 5A, its fracture mode was dominantly transgranular, cleaving more than half of Al2O3 and ZrO2 grains, which could also be found in typical indentation crack path (see Fig. 5B). It was indicated that the grain
A-1500 A-1550 A-1600 A-1650 A-HP
Length of cracks (μm)
Fracture toughness (MPa m1/2)
385 ± 36 366 ± 42 320 ± 35 341 ± 27 358 ± 47
4.85 ± 0.41 5.07 ± 0.19 5.74 ± 0.22 5.38 ± 0.34 5.19 ± 0.10
boundaries of Al2O3/Al2O3 and ZrO2/ZrO2, and phase boundaries of Al2O3/ZrO2 were strengthened, which would increase energy dissipation during crack extension, thus contributing to higher fracture toughness. Furthermore, crack deflection, crack bridging and crack branching were also observed in specimens (Fig. 5B). So, the specimen A-1600 possessed the highest KIC value. Based on the above analysis, the optimum oscillatory pressure sintering temperature of 1600 °C in ZTA composites was favorable for suppressing porosity and elimination of residual closed pores, controlling grain growth and accelerating densification of the microstructure 4
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with increased mechanical properties. In comparison with the conventional HP method, the oscillatory pressure sintering temperature could be decreased by more than 50 °C. 4. Conclusions (1) The oscillatory pressure sintering temperature had a significant influence on the densification, microstructure and mechanical properties of ZTA composites. The optimum sintering temperature of 1600 °C was achieved, which yielded an almost fully dense material (99.94% of theoretical density) with homogeneous microstructure. Therefore, higher flexural strength of 1145 MPa, Weibull modulus of 13.08, hardness of 19.08 GPa, and fracture toughness of 5.74 MPa m1/2 with an average grain size of 813 nm for Al2O3 and 323 nm for ZrO2 resulted from ZTA composites sintered at 1600 °C for 60 min (2) In comparison with the ZTA composites fabricated by conventional hot pressing method, the oscillatory pressure sintering temperature could be decreased by more than 50 °C. The oscillations in the final stage of sintering helped grain boundary sliding, and promoted plastic deformation and diffusion that facilitated mass transport, thus favouring the densification of ZTA composites. Hence, the oscillatory pressure sintering method was proved to be a promising route to fabricate high performance ZTA composites. Acknowledgements This work was supported by the China Postdoctoral Science Foundation (2016M600087), the National Natural Science Foundation of China (51427802 and 51672147), and the National Key Research and Development Program of China (2017YFB0310400). References [1] J. Lalande, S. Scheppokat, R. Janssen, N. Claussen, Toughening of alumina/zirconia ceramic composites with silver particles, J. Eur. Ceram. Soc. 22 (2002) 2165–2171. [2] D. Casellas, M.M. Nagl, L. Llanes, M. Anglada, Fracture toughness of alumina and ZTA ceramics: microstructural coarsening effects, J. Mater. Process. Technol. 143 (2003) 148–152. [3] J. Chevalier, L. Gremillard, Ceramics for medical applications: a picture for the next 20 years, J. Eur. Ceram. Soc. 29 (2009) 1245–1255. [4] A. Kirsten, S. Begand, T. Oberbach, R. Telle, H. Fischer, Subcritical crack growth behavior of dispersion oxide ceramics, J. Biomed. Mater. Res. B 95 (2010) 202–210. [5] R.H.J. Hannink, P.M. Kelly, B.C. Muddle, Transformation toughening in zirconiacontaining ceramics, J. Am. Ceram. Soc. 83 (2000) 461–487. [6] X.J. Jin, Martensitic transformation in zirconia containing ceramics and its applications, Curr. Opin. Solid State Mater. Sci. 9 (2005) 313–318. [7] K. Fan, J.Y. Pastor, J. Ruiz-Hervias, J. Gurauskis, C. Baudin, Determination of mechanical properties of Al2O3/Y-TZP ceramic composites: influence of testing method and residual stresses, Ceram. Int. 42 (2016) 18700–18710. [8] J.F. Bartolomé, G. Bruno, A.H. DeAza, Neutron diffraction residual stress analysis of zirconia toughened alumina (ZTA) composites, J. Eur. Ceram. Soc. 28 (2008) 1809–1814.
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Microstructure and mechanical properties of ZTA composites fabricated by oscillatory pressure sintering ⁎
Tianbin Zhua, Zhipeng Xiea, , Yao Hana, Shuang Lib a b
State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing, China School of Resources and Environmental Engineering, Shandong University of Technology, Zibo, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Oscillatory pressure sintering (OPS) Microstructure Mechanical properties ZTA composites
The influence of sintering temperature on the microstructure and mechanical properties of Al2O3−20 wt% ZrO2 composites fabricated by oscillatory pressure sintering (OPS) was investigated by means of X-ray diffraction, scanning electron microscopy, three-point bending test and Vickers indentation. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The optimum oscillatory pressure sintering temperature was found to be 1600 °C; almost fully dense materials (99.94% of theoretical density) with homogeneous microstructure could be achieved. The highest flexural strength, fracture toughness and hardness of such composites reached 1145 MPa, 5.74 MPa m1/2 and 19.08 GPa when sintered at 1600 °C, respectively. Furthermore, the oscillatory pressure sintering temperature could be decreased by more than 50 °C as compared with the HP method, OPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.
1. Introduction ZTA composites (known as zirconia toughened alumina ceramics) are widely used for structural applications such as cutting tools, bearings, dies, and prosthetic components due to their excellent mechanical properties [1,2]. Such composites possess some combined merits, namely the good chemical stability and superior wear resistance of αAl2O3 and high fracture strength and toughness of t-ZrO2 [3,4]. Generally, ZTA composites are to disperse some t-ZrO2 (often up to 20 vol %) into the α-Al2O3 matrix. Therefore, the existence of t-ZrO2 would lead to the improved fracture toughness and strength of the α-Al2O3 matrix owing to the occurrence of the stress-induced phase transformation (t-ZrO2→m-ZrO2). Such tetragonal to monoclinic phase transformation results in different toughening mechanisms of ZTA composites, mainly including stress-induced transformation toughening and microcracks toughening [5,6]. In addition, the influence of residual stresses in such composites has been reported by some researchers [7,8]. To enhance the mechanical properties of ZTA composites, much work, mainly including the optimization of ZrO2 addition, the incorporation of various additives and the development of different sintering techniques, has been performed in the past 20 years. For example, Sommer et al. [9,10] reported that the flexural strength of ZTA composites was related highly with ZrO2 content and a relatively high
⁎
flexural strength of 1288 MPa was obtained for the Al2O3−24 vol% ZrO2 composites. Furthermore, many investigators focused on the incorporation of various additives in ZTA composites, such as Cr2O3 [11,12], MgO [13], TiO2 [14], CeO2 [15], Ta2O5 [16], Nb2O5 [17], BN [18], single-wall and multi-wall carbon nanotubes (SWCNTs and MWCNTs) [19,20], and graphene platelets (GPLs) [21], etc. It was reported [16] that an apparent increase of 27.6% was achieved for the flexural strength of ZTA composites with 0.36 vol% Ta2O5 as compared to the pure ZTA composites, and the corresponding improvement of 25.9% was for their Vickers hardness. Also, Liu et al. [21] found that the addition of 0.81 vol% GPLs into ZTA composites resulted in a 40% increase in fracture toughness; the pull-out of GPLs, crack bridging and crack deflection were considered as the main toughening mechanisms. As is well known, the mechanical properties of ZTA composites can be significantly improved by decreasing the porosity and grain sizes and by enhancing the homogeneity of the phase dispersion [22], which is strongly influenced by the used sintering technique. In the past three decades, the pressureless sintering (PS) [23,24], hot pressing (HP) [9,10], hot isostatic pressing (HIP) [25], spark plasma sintering (SPS) [22,26], and microwave sintering (MS) [27,28] were developed to fabricate ZTA composites. In comparison with PS and MS, the pressureassisted sintering techniques (HP, HIP and SPS) exhibited more advantages, namely effectively reducing the sintering temperature, shortening the soaking time and improving the mechanical properties
Corresponding author. E-mail address: [email protected] (Z. Xie).
http://dx.doi.org/10.1016/j.ceramint.2017.09.204 Received 11 August 2017; Received in revised form 7 September 2017; Accepted 25 September 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhu, T., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.09.204
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of ZTA composites. Nevertheless, the pressure-assisted sintering processes mentioned-above were performed in the form of a constant static pressure. Much recently, a new oscillatory pressure sintering (OPS) technique was developed by introducing an oscillatory loading with controlled frequency and value in the sintering process [29]. Xie et al. [29–31] found that the 3Y-TZP and Si3N4 ceramics fabricated by OPS had higher density and superior mechanical properties as compared with the conventional sintering methods. In the present work, Al2O3−20 wt% ZrO2 composites were fabricated by the OPS method with commercially available Al2O3-ZrO2 composite powders as starting materials. The main aim of the present work was the optimization of sintering temperature, which could yield the maximum density close to the theoretical density of such composites, with emphasis on controlling their grain growth and enhancing their mechanical properties. Moreover, the results were compared to specimens obtained by conventional HP method under a similar sintering schedule.
Fig. 1. Schematic of the applied pressure during the soaking stage at target temperature.
software (Image-Pro Plus 6.0) from field-emission scanning electron microscopy (FESEM; Leo-1530, Zeiss, Oberkochen, Germany) images of the polished and thermally etched surfaces. FESEM was also used to observe the fracture surfaces of the specimens. The phase compositions of Al2O3-ZrO2 powders and sintered specimens were confirmed by Xray diffractometry (XRD, D8 advance, Bruker, Germany; using Ni filtered, Cu Kα radiation at a scanning rate of 6 deg/min). In addition, the hardness of all the specimens was measured using Vickers indentation with an applied loading of 10 kg and a soaking time of 15 s 12 indentations were used for each specimen and the results were statistically analyzed. Also, the elastic modulus (E) of the specimens was determined via the nondestructive dynamic elastic modulus tester (based on the acoustic method). Fracture toughness (KIC) was evaluated by a single-edge notched beam (SENB) test with a span of 16 mm using test bars of 4 mm × 2 mm × 22 mm (the width and depth of the notch were ~0.16 mm and ~2 mm, respectively); the loading was continuously applied at a constant speed of 0.05 mm/min until the fracturing of the specimen. Six specimens were used to determine the average KIC value. Flexural strength (σ) was measured by three-point bending method using test bars of 3 mm × 4 mm × 36 mm, in which the tensile surface of the bars was polished down to 1 µm; the tests were performed at ambient temperature using a universal testing machine with a loading rate of 0.5 mm/min and a span of 30 mm. The measured σ value was the average of 22 specimens. Moreover, the flexural strength of the specimens was studied statistically according to Weibull distribution analysis [32,33]:
2. Experimental The Al2O3-ZrO2 composite powders (ZTA-008G, Shandong Sinocera Xinmeiyu Alumina Co., Ltd., Shandong, China) were selected as the starting materials. The chemical compositions of the as-received powders were as follows: Al2O3 + ZrO2 + Y2O3- > 99.9 wt%, Y2O3−3.07 wt%, ZrO2−20.02 wt%, Fe2O3-≤0.01 wt%, SiO2≤0.01 wt%, and NaO-≤0.02 wt%. Their specific surface area was 14.58 m2/g. In addition, the composite powders consisted of the quasisphere like particles, in which the average sizes of Al2O3 and ZrO2 were 128 nm and 54 nm, respectively. The α-Al2O3 and t-ZrO2 phases were detected as main crystalline phases with the aid of X-ray diffraction (XRD), where a small amount of m-ZrO2 phase was also verified. The sintering experiments were performed on a self-developed oscillatory pressure sintering (OPS) machine, whose detailed presentation can be found in our previous work [31]. With respect to specimens fabricated by OPS, the as-received Al2O3-ZrO2 powders were put into a graphite die with the inner diameter of 100 mm and then prepressed under a pressure of 10 MPa. Subsequently, the powders were heated under vacuum from ambient temperature to 500 °C with a constant heating rate of 25 °C/min and applied pressure keeping at 10 MPa, and then heated to 1100 °C with a heating rate of 15 °C/min under vacuum and applied pressure increasing from 10 MPa to 20 MPa. After soaking at 1100 °C for 30 min (with the aim of eliminating the volatiles in the powders), the powders were continuously heated to the target sintering temperature with a heating rate of 10 °C/min under an argon atmosphere while the applied pressure increased gradually from 20 MPa to 30 MPa. When the temperature was increased to the target temperature, an oscillatory pressure (PO) was added on the basis of the constant pressure (PC), as shown in Fig. 1. Hence, the total pressure (PT) of 27.5–32.5 MPa was achieved during the soaking period at final sintering temperature. After holding for 60 min, the PO was removed while PC was maintained until the temperature was decreased to 900 °C with a cooling rate of 15 °C/min. In this work, the selected target sintering temperatures included 1500 °C, 1550 °C, 1600 °C and 1650 °C; the corresponding sintered specimens were designated as A-1500, A-1550, A-1600 and A-1650, respectively. For comparison, the as-received Al2O3-ZrO2 powders were also used to prepare ZTA composites via the conventional hot pressing (HP); the prepared specimen was marked as A-HP. Specifically, the specimen AHP was prepared on the OPS machine under a similar sintering schedule except that a constant pressure of 32.5 MPa was applied at 1600 °C for 60 min without oscillations. The relative density of the sintered ZTA composites was measured by Archimedes' principle taking 4.29 g/cm3 as their theoretical density (achieved by the rule of mixtures assuming a density of 6.10 g/cm3 for ZrO2 and 3.98 g/cm3 for Al2O3). The mean grain sizes of Al2O3 and ZrO2 (at least 300 grains) were measured using an image analysis
ln ln[1/(1 − Pn )] = m ln σn − m ln σ0
(1)
The failure probability estimator is:
Pn = (i − 0.5)/ N
(2)
where m is the Weibull modulus, i is the ranking, N is the number of specimens tested, σn is the nth flexural strength value, and σ0 is the characteristic flexural strength. 3. Results and discussion Fig. 2 presents the XRD patterns of ZTA composites after sintering at various temperatures. The phases were confirmed with respect to reference peak positions from the Inorganic Crystal Structure Database (ICSD). Apparently, irrespectively of the used sintering temperature (1500–1650 °C), only α-Al2O3 (ICSD #52 647) and t-ZrO2 (ICSD #85 322) phases were detected. It was indicated that m-ZrO2 (ICSD #62 993) phase in the starting powders transformed completely into t-ZrO2 phase in the sintering process, and did not appeared again during the cooling process. In this work, the presence of t-ZrO2 phase in ZTA composites would lead to an enhancement of flexural strength and fracture toughness of such composites, due mainly to the occurrence of 2
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boundaries in Al2O3 matrix with dark contrast (Fig. 3); the microstructure was homogeneous in all specimens, and no abnormal growth of Al2O3 grains can be found. So, the presence of t-ZrO2 grains suppressed the abnormal growth of Al2O3 grains. Certainly, the grain sizes of Al2O3 and ZrO2 increased with the sintering temperature from 1500 °C to 1650 °C. In addition, their mean grain sizes were calculated, and the results are displayed in Table 1. With regard to specimens fabricated by OPS, when the sintering temperature increased from 1500 °C to 1650 °C, the mean grain sizes of Al2O3 became bigger from 622 nm to 1086 nm, and that of ZrO2 increased from 235 nm to 385 nm. By comparing specimens A-HP and A-1600, it can be found that the introduction of the oscillatory pressure inhibited the growth of Al2O3 (813 nm vs 872 nm) and ZrO2 (323 nm vs 348 nm) grains. In general, the driving force of the grain size was only related with the curvature radius of the grain boundary. Therefore, increased curvature radius might be obtained due to improved plastic deformation from the oscillatory pressure, which would lead to decreased migration rate of grain boundary. Consequently, the specimen A-1600 had smaller mean grain sizes of Al2O3 and ZrO2 as compared to the specimen A-HP. In addition, the flexural strength of all specimens was measured at room temperature by three-point bending test. Due to the stochastic nature of fracture in ZTA composites, their flexural strengths (σ) were statistically investigated according to the Weibull distribution analysis; the results are shown in Fig. 4 and Table 2. As expected, the specimen A-1500 possessed the lowest σ value of 867 MPa and the smallest Weibull modulus (m) of 8.70, which was attributed mainly to the existence of its higher residual porosity (0.5%). When the sintering temperature was 1600 °C, the highest σ value of 1145 MPa and the largest m value of 13.08 were achieved for the specimen A-1600 due to the almost perfectly full density and homogenous microstructure. With further increasing the sintering temperature to 1650 °C, both the σ and m values of the specimen decreased, namely 1042 MPa and 11.20; this can be explained by the obvious growth of Al2O3 and ZrO2 grains. It is generally accepted that grain refinement is favorable for enhancing flexural strength and hardness of a material according to a classic HallPetch relationship [34,35]. Correspondingly, the hardness change of specimens had a similar tendency to their flexural strength change. That is to say, the specimen A-1600 had the highest hardness of 19.08 GPa. In the case of the specimen A-HP, a flexural strength of 990 MPa and a hardness of 18.58 GPa were obtained. According to Table 2, it can be concluded that the sintering temperature had a crucial influence on the flexural strength of ZTA composites, and the oscillatory pressure sintering temperature could be decreased by more than 50 °C as compared to the conventional HP method. The fracture toughness (KIC) of all the specimens was measured by the SENB method, and the results are exhibited in Table 3. It was not difficult to find that the specimen A-1600 presented the highest KIC value of 5.74 MPa m1/2. As is well known, the KIC is positively correlated with the elastic modulus (E) and fracture surface energy (γ) of a material [36]. In order to explain the KIC change of the specimens, the E and γ changes will be considered in the following part. Concerning the E of specimens, it was commonly considered that it was only sensitive to the porosity in specimens, besides the measured temperature. In this work, a relatively high densification (not lower than 99.5%) was achieved for all specimens, so the influence of the porosity on the elastic modulus might be neglected. In order to confirm this idea, the E values of all specimens were measured by a nondestructive dynamic elastic modulus tester. As expected, the E value of ~360 GPa was obtained for all specimens irrespectively of the used sintering temperatures. Hence, the γ change will have a significant influence on the KIC. In practice, it was very difficult to determine directly the γ value of ZTA composites, so the γ change was analyzed indirectly with the aid of Vickers indentations. The lengths of cracks in the specimens after indenting were analyzed statically and listed in Table 3. It can be seen that the specimen A-1500 had the largest average crack length, and the smallest one was for the specimen A-1600. Generally,
Fig. 2. XRD patterns of specimens fabricated by oscillatory pressure sintering.
the phase transformation (t-ZrO2→m-ZrO2) when subjected to external mechanical loading. The density change of ZTA composites is shown in Table 1. The relative density of all the specimens was higher than 99.50%. As for specimens fabricated by OPS, their relative density increased continuously with the sintering temperature up to 1600 °C but decreased slightly up to 1650 °C. Particularly, the specimen A-1600 had the highest relative density of 99.94%, which was almost a fully dense material. In the case of the specimen A-HP, its relative density was similar to that of the specimen A-1500, but lower than that of other specimens (A-1550, A-1600 and A-1650). By comparing the relative density of specimens A-HP and A-1600, it was clear that an improvement of ~0.4% (seemed a rather minor change) was achieved for the specimen A-1600 even though the same sintering temperature was used; in reality, it was very difficult to remove the residual closed pores (~0.4%) in the final stage of sintering. Therefore, the introduction of oscillatory pressure was strongly helpful for the densification of ZTA composites. As mentioned in the experimental part, the oscillatory pressure was added when ZTA composites were heated to the target sintering temperature (namely during the soaking period). So, the particle rearrangement caused by the oscillatory pressure can be neglected owing to the achievement of a rather denser composites when heated to the target temperature (not lower than 1500 °C); however, the oscillatory pressure might offer an additional sintering driving force in the final stage of sintering, thus promoting grain boundary sliding and improving plastic deformation and diffusion that favored mass transport in comparison with the conventional HP. As a consequence, the sintering temperature of OPS could be decreased by 50–100 °C as compared to the conventional HP. The polished and thermally etched surfaces of specimens were observed with the aid of FESEM. Obviously, it can be seen that t-ZrO2 grains with light contrast were homogeneously dispersed at the grain
Table 1 The relative density and mean grain sizes of specimens.
Specimen 1500 Specimen 1550 Specimen 1600 Specimen 1650 Specimen
Relative density (%)
Mean grain size of Al2O3 (nm)
Mean grain size of ZrO2 (nm)
A-
99.50
622 ± 194
235 ± 80
A-
99.80
733 ± 242
250 ± 96
A-
99.94
813 ± 300
323 ± 105
A-
99.91
1086 ± 387
385 ± 147
A-HP
99.53
872 ± 303
348 ± 111
3
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Fig. 3. SEM images of polished and thermally etched surfaces of specimens fabricated by oscillatory pressure sintering: (A) specimen A-1500, (B) specimen A-1550, (C) specimen A-1600 and (D) specimen A-1650.
Table 2 Mechanical properties of specimens fabricated by different sintering methods.
Specimen 1500 Specimen 1550 Specimen 1600 Specimen 1650 Specimen
Flexural strength (MPa)
Vickers hardness (GPa)
Elastic modulus (GPa)
A-
867 ± 129
18.62 ± 0.40
361 ± 3
A-
1003 ± 99
18.91 ± 0.20
366 ± 2
A-
1145 ± 124
19.08 ± 0.35
371 ± 5
A-
1042 ± 111
18.39 ± 0.33
362 ± 3
A-HP
990 ± 114
18.58 ± 0.34
362 ± 4
Table 3 Length of cracks and fracture toughness of specimens.
Specimen Specimen Specimen Specimen Specimen
Fig. 4. The Weibull plot of flexural strength for ZTA composites fabricated by oscillatory pressure sintering.
the area of a crack is correlated positively with its length. Meanwhile, all specimens were indented under the same loading, so the driving force of crack extension was fixed. Consequently, the increased energy dissipation per unit area during crack extension was achieved for the specimen A-1600, thus resulting in its improved fracture surface energy. Furthermore, to better explain the improvement of the fracture surface energy of the specimen A-1600, its fracture surface and crack extension path were observed by means of FESEM. According to Fig. 5A, its fracture mode was dominantly transgranular, cleaving more than half of Al2O3 and ZrO2 grains, which could also be found in typical indentation crack path (see Fig. 5B). It was indicated that the grain
A-1500 A-1550 A-1600 A-1650 A-HP
Length of cracks (μm)
Fracture toughness (MPa m1/2)
385 ± 36 366 ± 42 320 ± 35 341 ± 27 358 ± 47
4.85 ± 0.41 5.07 ± 0.19 5.74 ± 0.22 5.38 ± 0.34 5.19 ± 0.10
boundaries of Al2O3/Al2O3 and ZrO2/ZrO2, and phase boundaries of Al2O3/ZrO2 were strengthened, which would increase energy dissipation during crack extension, thus contributing to higher fracture toughness. Furthermore, crack deflection, crack bridging and crack branching were also observed in specimens (Fig. 5B). So, the specimen A-1600 possessed the highest KIC value. Based on the above analysis, the optimum oscillatory pressure sintering temperature of 1600 °C in ZTA composites was favorable for suppressing porosity and elimination of residual closed pores, controlling grain growth and accelerating densification of the microstructure 4
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with increased mechanical properties. In comparison with the conventional HP method, the oscillatory pressure sintering temperature could be decreased by more than 50 °C. 4. Conclusions (1) The oscillatory pressure sintering temperature had a significant influence on the densification, microstructure and mechanical properties of ZTA composites. The optimum sintering temperature of 1600 °C was achieved, which yielded an almost fully dense material (99.94% of theoretical density) with homogeneous microstructure. Therefore, higher flexural strength of 1145 MPa, Weibull modulus of 13.08, hardness of 19.08 GPa, and fracture toughness of 5.74 MPa m1/2 with an average grain size of 813 nm for Al2O3 and 323 nm for ZrO2 resulted from ZTA composites sintered at 1600 °C for 60 min (2) In comparison with the ZTA composites fabricated by conventional hot pressing method, the oscillatory pressure sintering temperature could be decreased by more than 50 °C. The oscillations in the final stage of sintering helped grain boundary sliding, and promoted plastic deformation and diffusion that facilitated mass transport, thus favouring the densification of ZTA composites. Hence, the oscillatory pressure sintering method was proved to be a promising route to fabricate high performance ZTA composites. Acknowledgements This work was supported by the China Postdoctoral Science Foundation (2016M600087), the National Natural Science Foundation of China (51427802 and 51672147), and the National Key Research and Development Program of China (2017YFB0310400). References [1] J. Lalande, S. Scheppokat, R. Janssen, N. Claussen, Toughening of alumina/zirconia ceramic composites with silver particles, J. Eur. Ceram. Soc. 22 (2002) 2165–2171. [2] D. Casellas, M.M. Nagl, L. Llanes, M. Anglada, Fracture toughness of alumina and ZTA ceramics: microstructural coarsening effects, J. Mater. Process. Technol. 143 (2003) 148–152. [3] J. Chevalier, L. Gremillard, Ceramics for medical applications: a picture for the next 20 years, J. Eur. Ceram. Soc. 29 (2009) 1245–1255. [4] A. Kirsten, S. Begand, T. Oberbach, R. Telle, H. Fischer, Subcritical crack growth behavior of dispersion oxide ceramics, J. Biomed. Mater. Res. B 95 (2010) 202–210. [5] R.H.J. Hannink, P.M. Kelly, B.C. Muddle, Transformation toughening in zirconiacontaining ceramics, J. Am. Ceram. Soc. 83 (2000) 461–487. [6] X.J. Jin, Martensitic transformation in zirconia containing ceramics and its applications, Curr. Opin. Solid State Mater. Sci. 9 (2005) 313–318. [7] K. Fan, J.Y. Pastor, J. Ruiz-Hervias, J. Gurauskis, C. Baudin, Determination of mechanical properties of Al2O3/Y-TZP ceramic composites: influence of testing method and residual stresses, Ceram. Int. 42 (2016) 18700–18710. [8] J.F. Bartolomé, G. Bruno, A.H. DeAza, Neutron diffraction residual stress analysis of zirconia toughened alumina (ZTA) composites, J. Eur. Ceram. Soc. 28 (2008) 1809–1814.
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