Journal of Materials Science & Technology  2020 , 41 (0): 149-158 https://doi.org/10.1016/j.jmst.2019.09.028

Research Article

Precipitation mechanism and microstructural evolution of Al2O3/ZrO2(CeO2) solid solution powders consolidated by spark plasma sintering

Wanjun Yu, Yongting Zheng*, Yongdong Yu

National Key Laboratory of Science and Technology on Advanced Composites in Special Environments and Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, China

Corresponding authors:   *Corresponding author. E-mail address: zhengyt@hit.edu.cn (Y. Zheng).

Received: 2019-06-29

Revised:  2019-08-24

Accepted:  2019-09-12

Online:  2020-03-15

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

It is difficult to synthesize Al2O3/ZrO2 solid solution because of its low solubility under equilibrium solidification conditions. In this work, a new combustion synthesis combined with water atomization (CS-WA) method was developed to prepare supersaturated Al2O3/ZrO2(CeO2) solid solution powders. The ultra-high cooling rate supplied by CS-WA greatly extends solid solubility of Al2O3 in ZrO2. The precipitation mechanism of solid solution was investigated by systematic heat treatments. The initial temperature of the metastable phase decomposed into Al2O3 and ZrO2 is 1050 °C, and it could be completely precipitated at 1400 °C in 0.5 h. The precipitated ZrO2 particles were uniformly dispersed in Al2O3 matrix and grew into submicron scale at annealing temperature of 1450 °C. Subsequently, together with detailed microstructure, phase composition, as well as mechanical properties were collaboratively outlined to discuss spark plasma sintering (SPS) behavior. The solid solution precipitated Al2O3 matrix and ZrO2 particles during the SPS process. Partial ZrO2 particles were uniformly distributed within Al2O3 matrix, while the residuary ZrO2 located at the grain boundaries and formed special transgranular/intergranular structure. The average size of nanoscale transgranular ZrO2 particles was only ∼11.5 nm. The compact ZrO2 toughened Al2O3 nano ceramic (N-ZTA) exhibits excellent mechanical properties. This work provides a guidance to produce nanostructured ZTA with high performance.

Keywords: Solid solution ; Precipitation ; Spark plasma sintering ; ZTA ; Transgranular/intergranular structure

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Wanjun Yu, Yongting Zheng, Yongdong Yu. Precipitation mechanism and microstructural evolution of Al2O3/ZrO2(CeO2) solid solution powders consolidated by spark plasma sintering[J]. Journal of Materials Science & Technology, 2020, 41(0): 149-158 https://doi.org/10.1016/j.jmst.2019.09.028

1. Introduction

On account of the impressive mechanical, thermal and physical properties, the nanostructured composite ceramics have attracted worldwide attention over the past few decades and are widely used in structures, biomedical and other advanced engineering fields [[1], [2], [3], [4], [5], [6], [7]]. Especially for the system of ZrO2 toughened Al2O3 composite ceramic (ZTA), which possesses excellent comprehensive properties, such as high strength and toughness, intrinsic thermal stability, good creep resistance and superior antioxidation [[8], [9], [10], [11], [12], [13], [14]]. However, due to the agglomeration of nanoparticles and inevitable grain growth during the sintering process, the conventional technologies, such as mechanical mixing method [2,3,7,10,11,13,15], liquid phase method [12,[16], [17], [18], [19]] and combustion synthesis method [20], are difficult to fabricate ZTA ceramic bulk with ultra-fine nanostructure. Therefore, it is urgent to invent new processing route to optimize nanostructures.

Recently, fine and uniform nanostructured composite ceramics were successfully prepared by solid phase transition methods (SPTM) of amorphous crystallization and solid solution precipitation [[21], [22], [23], [24], [25], [26]]. SPTM is an in-situ synthesis methods and involves the following stages. Firstly, the high temperature melting technologies, such as flame, laser, arc and so on, are applied to melt raw materials. Subsequently, the molten product rapidly solidifies into non-equilibrium amorphous or solid solution. Finally, the metastable phase undergoes solid phase transition and transforms into composite ceramics material after anneal or sintering treatment. Due to the multiphase nanostructures originating from homogeneous metastable phases, the composite ceramics obtained by SPTM possess fine and uniform microstructure.

The sintering process also plays an essential role in optimization of microstructure of nano ceramics. The long sintering time and high sintering temperature usually result in abnormal growth of grain, which is extremely detrimental to the formation of ultra-fine nanostructures. To overcome this problem, numerous new sintering technologies have been developed in the past few decades. In particular, spark plasma sintering (SPS) have been receiving more and more attention in the modern era of ceramic science because of the fast sintering transformation, efficient heating and high-speed diffusion [13,[27], [28], [29]]. Compared with other sintering technologies, SPS exhibits more advantages in sintering time and sintering temperature, which is attributed to the fact that applied electric field directly heats the mold and powders.

Nevertheless, the preparation of ZTA ceramic by spark plasma sintering of Al2O3/ZrO2 solid solution powders had never been reported, which is mainly due to the following reasons. Firstly, the different valence states and similar ionic radius of Al3+ and Zr4+ lead to low solid solubility. According to the binary phase diagram of Al2O3/ZrO2, the solid solubility of Al2O3 in ZrO2 did not exceed 1 mol.% at room temperature [30]. Secondly, the above melting technologies are difficult to held a long time at ultra-high temperature, which induces inadequate diffusion and forms heterogeneous melt, consequently resulting in difficulty in preparing high-quality Al2O3/ZrO2 solid solution. Noteworthy, the relevant reports reveal that the solid solubility of Al2O3 in ZrO2 can be extended by improving cooling rate [[30], [31], [32]]. In previous work [33], the Al2O3/ZrO2 solid solution powders was successfully fabricated by a novel combustion synthesis combined with water atomization (CS-WA) method. The CS-WA supplied an ultra-high cooling rate and long mutual-dissolution time, which were favorable for the formation of solid solution phase. In this paper, CeO2 phase was added into binary Al2O3/ZrO2 system to further improve the fracture toughness of the ceramic material. Firstly, the Al2O3/ZrO2(CeO2) (namely AZC) ternary solid solution powders were prepared by CS-SC method. Subsequently, the synthesized powders were densified by SPS. During the sintering process, the non-equilibrium powders precipitated ultra-fine nanoparticles and formed compact ZrO2 toughened Al2O3 nano ceramics (namely N-ZTA). Moreover, the solid solution precipitation mechanism, SPS behavior, mechanical properties and toughening mechanism were investigated systematically.

2. Experimental

2.1. Synthesis of AZC solid solution powders

The powders of Zr(NO3)4 (Zibo Rongruida Micro Materials Plant, China) and Al (3-5 μm, purity of 99.9%, Northeast Light Alloy Co., Ltd., China) were used as oxidant and reductant, respectively. The combustion reaction based on Eq. (1) released amounts of heat to melt the raw materials for preparing AZC powders. The appropriate amounts of Al2O3 (3-5 μm, purity of 99.99%, Henan Fanrui Composite Materials Research Institute Corporation, China), ZrO2 (3-5 μm, purity of 99.99%, Henan Fanrui Composite Materials Research Institute Corporation, China) and CeO2 (3-5 μm, purity of 99.99%, Henan Fanrui Composite Materials Research Institute Corporation, China) ceramic powders were added to control the temperature of system and adjust the component ratio of products.

20Al+3Zr(NO3)4→10Al2O3+3ZrO2+6N2 (1)

The temperature of system was estimated by simple thermodynamic method, which is higher than the melting point of ZrO2 (>2950 K) to melt raw materials. CS-WA generated a high pressure to eject molten products through heating N2 generated in Eq. (1). In this work, the temperature and pressure of reaction system were designed as 4000 K and 5 MPa, respectively.

The mole ratio of ZrO2:Al2O3 is 25:75 in composite ceramic system. Moreover, 4 mol.% CeO2 in relation to the ZrO2 content was added to stabilize t-ZrO2. After drying, stoichiometric powders of Al, Zr(NO3)4, Al2O3, ZrO2 and CeO2 were mechanical mixed for 4 h using zirconia milling balls at a speed of 200 rpm. Subsequently, the mixed powders were poured into the SHS reactor. The 80A DC current was used to ignite combustion reaction. The raw materials were rapidly melted by reaction exotherm and then held for 30 s to fully interfuse. Under the effect of N2 pressure, the homogeneous melt was sprayed into water from the nozzle, and rapidly solidified into solid solution powders. The thermal conduction and heat convection between micron droplets and water generate ultra-high cooling rate during the solidification process, which greatly improves the solid solubility of Al2O3 in ZrO2.

2.2. Heat treatment of AZC solid solution powders

A series of heat treatments were used to investigate the mechanism of solid solution precipitation. Heat treatments were performed in a muffle furnace under air atmosphere at selected temperature for 1 h with a heating rate of 10 °C/min. The selected starting temperature of heat treatments is 550 °C, and increasing by 100 °C each time until to 1450 °C, and named as AZC-550, AZC-650, AZC-750, AZC-850, AZC-950, AZC-1050, AZC-1150, AZC-1250, AZC-1350 and AZC-1550, respectively. In addition, the effect of annealing time on precipitation mechanism is also performed at 1400 °C for 0.5 h, 1 h, 2 h, 4 h, 6 h and 12 h. According to the increase of annealing time, the samples are named AZC-1400-0.5 h, AZC-1400-1 h, AZC-1400-2 h, AZC-1400-4 h, AZC-1400-6 h, AZC-1400-12 h, respectively.

2.3. Sintering densification of AZC solid solution powders

The AZC solid solution powders were ball milled in a planetary ball mill with a ball-to-powder weight ratio of 5:1. Milling was performed in zirconia grinding tank, using zirconia balls, at a speed of 500 rpm for 2 h. The ground powders were placed in a cylindrical graphite die with an inner diameter of 40 mm and then introduced into SPS furnace. Subsequently, the powders were heated under vacuum from room temperature to 1000 °C with a rate of 60 °C/min and applied pressure keeping at 24 MPa, and then heated to 1400 °C with a rate of 100 °C/min under vacuum and the pressure increased form 24 MPa to 40 MPa. After holding for 30 min, the powders were cooled to 800 °C with a rate of 20 °C /min, and finally cooled to room temperature with a rate of 30 °C /min. During the sintering process, the high temperature triggered solid solution precipitation. As a consequence, the AZC powders precipitated ultra-fine nanostructure and formed compact N-ZTA ceramic.

2.4. Characterization

The phase compositions of AZC powders and N-ZTA ceramic were confirmed by X-ray diffraction (XRD, D/MAX-RB, Rigaku, Japan) with CuKα radiation. The morphologies of AZC powders were observed by scanning electron microscopy (SEM, JSM-6390, Japan). A universal laser instrument (Mastersizer 2000) was applied to measure particle size of AZC powders. The microstructure of AZC powders and N-ZTA ceramic were observed by backscatter electron (BSE, JSM-6390, Japan). In order to characterize the cross-sectional microstructure of the AZC powders, the preparation of BSE specimens comprised the following procedures. Firstly, appropriate amount of AZC powders and modified acrylate adhesive were homogenously mixed and then placed in the mold. After curing, the mechanical thinning and polishing were used to split the AZC powders. Finally, the specimens were treated by spray-gold. A systematic transmission electron microscopy (TEM, Talos F200x, FEI, USA) investigation was carried out for observing precipitated nanostructure and elemental distribution of N-ZTA ceramic. The flexural strength was determined by the three-point bending tests (Instron-1186, USA), in which the size of test specimens was 3 mm × 4 mm × 22 mm. A cross-head speed of 0.5 mm/min and a span of 16 mm were applied for strength test. The fracture toughness was evaluated by a single-edge notched beam (SENB) with a cross-head speed 0.05 mm/min and a span of 16 mm on a universal testing machine (Instron-1186, USA). The test specimens were obtained via following steps. Firstly, the sintered sample was cut into prismatic bars of 2 mm × 4 mm × 22 mm. Then, a notch with tip radius less than 20 μm was fabricated on the centre part of 2 mm × 22 mm surface by traditional notching method. The hardness was measured using a Vickers hardness testing machine (HVS-5, Huayin Experimental Instrument Co., Ltd., China) with the applied loading of 5 kg and the loading time of 15 s. The flexural strength, fracture toughness and Vickers hardness of N-ZTA were tested five times to get average values.

3. Results and discussion

3.1. Powder characterization and precipitation mechanism

3.1.1. Phase composition and microstructure

Fig. 1 presents the XRD pattern of synthesized AZC solid solution powders. The narrow diffraction peaks of T-ZrxAlyCezOm reveal that the solid solution phase has good crystalline quality. Therefore, it can be inferred that the high-temperature melt directly crystallizes to form solid solution phases at the ultra-high cooling rate, which extends the solid solubility of Al2O3 in ZrO2 greatly. Table 1 represents the lattice parameter of AZC solid solution powders. Due to smaller ions radius of Al3+ (0.068 nm) than Zr4+ (0.086 nm) [32], the lattice constant of t-ZrO2 phase would change when Al3+ occupy substitutional sites in lattice which results in the right shift of the diffraction peaks of t-ZrO2. In addition, substitution reaction generates large amounts of oxygen vacancies because of the necessity of maintaining charge balance. The metastable γ-Al2O3 phase are also observed in the XRD spectrum of the powders, which may be ascribed to the rapid solidification process. However, the weak peaks indicate that Al2O3 is almost dissolved in the t-ZrO2 lattice. According to previous works [30,33], the N2 released by Eq. (1) could react with the Al and ZrO2 (Eq. (2)) and generates by-product ZrN. Thereby, a small amount of ZrN phase is also found in products. Due to the solution of Ce atoms and the low content, the diffraction peaks of CeO2 are not observed in the XRD patterns. In short, the extreme non-equilibrium solidification supplied by CS-WA is favorable for the formation of solid solution with higher solid solubility.

Al+3AlN+3ZrO2→2Al2O3+3ZrN (2)

Fig. 1.   XRD pattern of the synthesized AZC solid solution powders.

Table 1   Lattice parameter of solid solution powders.

SamplePeak 1 (2θ)Peak 2 (2θ)Peak 3 (2θ)a (mm)b (mm)c (mm)
Pure t-ZrO229.73°49.28°59.30°0.3650.3650.531
AZC30.32°50.83°60.37°0.3610.3610.517

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Fig. 2(a) displays the morphologies of the synthesized AZC powders. The powders have regular spherical morphologies and high specific surface. A small amount of fibers is also observed in the synthesized AZC powders, as indicated by the red arrow in Fig. 2(a). The formation of fiber can be explained by following reason: the melt is accelerated by N2 pressure in the reactor, which attain a high speed. According quantitative estimation (Eq. (3)), the speed of molten drops reaches 12.6 m/s. During the ejection process, the high-speed drops were stretched by the resistance of the water, which forms fiber-like product.

V=$\frac{4m}{πρtd2}$ (3)

where V is the speed of molten drops (m/s); m is the mass of the ejected powder (kg); ρ is the density of the melt (4.2 × 10-3 kg/m3); t is spray time (s); d is the size of nozzle (1 × 10-3 m).

Fig. 2.   Powder morphologies (a), particle-size distribution (b), cross-sectional microstructure (c) and EDS spectra (d) of the synthesized AZC solid solution powders.

The particle-size distribution demonstrates that the synthesized powders possess uniform particle size, and the surface area mean particle size reaches 6.81 μm, as shown in Fig. 2(b). The inner microstructure of synthesized AZC powders was observed by BSE (Fig. 2(c)). The synthesized powders exhibit uniform inner microstructure without dendritic crystal, cell crystal and second phase, indicating that no segregation or phase decomposition occur during the rapid solidification process. The AZC powders possess strong Zr and Al peaks, as shown in Fig. 2(d), which proves that Al atoms are bound to Zr atoms. Combining with strong peaks of ∼30.32°, ∼35.26°, ∼50.83° and ∼60.37° in Fig. 1, it can be concluded that the Al2O3 is dissolved in the t-ZrO2 lattice. In fact, the melt burst out from the nozzle and sprayed to water, which was broken into micron droplets. Due to high heat conductivity and convective heat transfer the micron droplets were rapidly solidified under ultra-high cooling rate, leading to the migration rate of solid/liquid interface is higher than short-range diffusion rate of solute and the molten droplets directly crystallize to form singe solid solution phase.

3.1.2. Precipitation mechanism

Fig. 3 demonstrates the evolution of phase structure of AZC solid solution powders after heating treatment at 550-1450 ºC. The diffraction peaks of ZrN decrease obviously with the increase of annealing temperature, which corresponds to oxidation of nitride. When the temperature rises to 650 °C, the diffraction peaks of ZrN disappear and the diffraction peaks of m-ZrO2 appear, which indicates that ZrN phase transforms into stable m-ZrO2 rather than t-ZrO2. Noticeable phase transformation was not observed after annealing at 750-950 °C. When the annealing temperature continues to increase to 1050 °C, a weak diffraction peak of α-Al2O3 appeared in the XRD spectra of AZC-1050, which verifies that Al3+ desorbs from metastable phase. However, the precipitation process is still slow at low annealing temperature. It should be pointed out that the diffraction peaks of γ-Al2O3 decrease obviously after annealing at 1050 °C, suggesting that the γ-Al2O3 convert into stable α-Al2O3. With the further increase of heat treatment temperature, the strong diffraction peaks of the α-Al2O3 are identified in the XRD patterns of AZC-1150. It was concluded that the deep precipitation is triggered at temperatures above 1150 °C and forms larger amount of Al2O3 phases. The diffraction peaks of ZrO2 and α-Al2O3 are gradually enhanced and become sharper after heat treated at 1250-1450 °C, as shown in Fig. 3(b). In addition, the Al atoms desorbed from the t-ZrO2 lattice resulting in the left shift of the diffraction peaks of t-ZrO2 and the formation of highly crystalline ZrO2 and Al2O3 during the whole precipitation process. On the other hand, the desorption of Al could optimize the lattice of t-ZrO2 and reduce distortion, which triggers phase transformation of tetragonal to monoclinic ZrO2 at annealing temperature above the phase transformation temperature (1173 °C). Close-view around the diffraction peaks of m-ZrO2 reveals that the tetragonal phase gradually transforms into the monoclinic phase with the increase of annealing temperature, as shown in Fig. 3(b). However, large amounts of t-ZrO2 are still retained after annealing at 1450 °C because of the addition of the CeO2.

Fig. 3.   XRD patterns of AZC solid solution powders after annealing at 550-1450 °C.

Fig. 4 demonstrates the cross-sectional microstructure evolution of AZC solid solution powders after annealing at 950-1450 °C for 1 h. The cross-sectional microstructure of AZC-950 are similar to that of AZC solid solution powders, as shown in Fig. 4(a). No decomposition is observed in the inner structure of AZC-950 sample, which is consistent with the above XRD analysis. When the annealing temperature rises to 1050 °C, the single solid solution phase decomposes into duplex structure in a very fine scale (Fig. 4(b)). However, the precipitation process is insignificant because of slow diffusion caused by low annealing temperature. More obvious two-phase structure is formed after annealing at 1150 °C (Fig. 4(c)). The Al-rich area and Zr-rich area are alternately arranged with fuzzy interface. The ZrO2 phases have arc-shaped morphologies and are uniformly distributed in particles. For the AZC-1250, the temperature supplies enough driving force to promote diffusion on a large scale, which leads to the growth of ZrO2 phases (Fig. 4(d)). Based on the principle of minimum energy, the arc-shaped ZrO2 experiences spheroidization process at 1350 °C, as shown in Fig. 4(e). The result of which is that spherical ZrO2 particles are uniformly dispersed in Al2O3 matrix phase. With further increase of annealing temperature, the regular ZrO2 particles grow into submicron scale.

Fig. 4.   BSE images of AZC solid solution powders after annealing at 950-1450 °C: (a) AZC-950; (b) AZC-1050; (c) AZC-1150; (d) AZC-1250; (e) AZC-1350; (f) AZC-1450.

The XRD spectrums reveal the phase composition of AZC solid solution powders annealed at 1400 °C for different soaking time, as shown in Fig. S1 in Supplementary Material. The peak intensity of m-ZrO2 increases gradually with increasing soaking time, which indicates the occurrence of t-m phase transformation during the heat treatment process. Therefore, the long isothermal treatment at high temperature is unfavorable for stabilization of tetragonal structure. In the case of AZC-1400-0.5 h, although the soaking time is only 0.5 h, the strong peak intensity of α-Al2O3 phase are detected. The precipitated Al2O3 phase in AZC-1400-0.5 h reaches 68.63 wt% based on the rough quantitative calculation, which is approximately equal to theoretical value of 66 wt%. This result indicates that the solid solution precipitation is fully carried out at 1400 °C in only 0.5 h time period. There is no additional change in diffraction peak intensity of α-Al2O3 with the further increase of soaking time.

3.2. Densification behavior during SPS

The temperature, pressure, displacement and shrinkage rate of powders during SPS process was discussed. The densification curve has two different variation tendencies, as shown in the blue profile of Fig. 5. One is the initial thermal expansion, which is attributed to the heating of the graphite punch and powders, consequently resulting in decline of displacement. The other one is the subsequent densification of AZC solid solution powders during the sintering process, which leads to the rise of displacement. Based on the hypothesis that the displacement increment caused by thermal expansion in unit time is a definite value, the sintering behavior of AZC solid solution powders could be investigated by variation of displacement and shrinkage rate.

Fig. 5.   Densification curves of AZC solid solution powders in the SPS (red curve: temperature; blue curve: displacement; green curve: shrinkage rate; black curve: pressure).

The sintering behavior of powders are classified into three stages according to the variation of shrinkage profile. In the early stage of sintering (I region in Fig. 5), the particles would undergo rearrangement process under external pressure, which leads to initial densification. However, due to lower shrinkage rate of particle rearrangement than thermal expansion rate of graphite mould and powders, the displacement declines with the increase of sintering temperature at 400-1180 °C. The subsequent stage is referred to as fast densification process, starting at ∼1180 °C (II region in Fig. 5). The enough driving forces of sintering offered by high temperature enables a large-scale mass transfer and diffusion between adjacent contact particles. Combining the increase of sintering pressure, the secondary particles rearrangement and plastic deformation take place in the compact, which leads to the gaps between the particles quickly removed and the powders densified rapidly. The shrinkage rate increases rapidly and reaches a peak at 1400 °C during heat process, as the green curve shown in Fig. 5. Simultaneously, the displacement curve also rises rapidly in fast densification region. The shrinkage rate decreases dramatically during the subsequent isothermal temperature. However, the variation of displacement is still noteworthy at the beginning of isothermal process. As the holding time exceeding 15 min, the sintering enters into final stage (III region in Fig. 5). The shrinkage rate approximately equal to 0 and the displacement profile turns to flatten. The nanostructures were fully precipitated by extending isothermal time to 30 min, and finally the high-density N-ZTA ceramic are obtained. Due to thermal contraction of system, the displacement further increases during the cooling process.

3.3. Microstructure and mechanical properties of N-ZTA

3.3.1. Phase composition and microstructure

The microstructure of N-ZTA composite ceramic is shown in Fig. 6 via BSE imaging. The metastable phase decomposes into Al2O3 and ZrO2 during the SPS process, leading to the formation of a typical duplex microstructure. Due to the higher crystallization velocity of Al2O3 than that of ZrO2, the newly precipitated ZrO2 particles are surrounded by Al2O3 matrix, which inhibits further growth of crystal. The ZrO2 particles reaches nanoscale and are uniformly distributed in Al2O3 matrix, as shown in Fig. 6. Compared with the sample annealed at 1350 °C, the precipitation process in N-ZTA composite ceramic is more complete. Simultaneously, the sintered ceramic has a finer structure than that of the powders annealed at 1450 °C.

Fig. 6.   BSE image of the N-ZTA ceramic.

TEM investigation was performed to provide more detailed microstructure on nanoscale (Fig. 7). The solid solution precipitates matrix phase and reinforced phase during the SPS process, as shown in Fig. 7(a) and (b). According to the elemental maps (Fig. 8), the matrix phase and the reinforced phase are Al2O3 and ZrO2, respectively. The Al2O3 matrix reaches submicron scale with the average particle size of 560 nm and the ZrO2 particles have two different distribution states. Part of ZrO2 particles are surrounded and spatially isolated by Al2O3 matrix (transgranular ZrO2 particles), while the residuary ZrO2 particles locate at grain boundaries (intergranular ZrO2 particles), which forms special transgranular/intergranular structure. According to the Fig. 7(b), the intergranular ZrO2 particles grow up to ∼160 nm, which could be explained by following reasons: (1) the intergranular temperature is higher than transgranular temperature during the SPS process, which supplies enough driving force to nucleation and growth; (2) the crystal growth at grain boundaries is not restricted by Al2O3 matrix. Particularly notable is that the particle size of transgranular ZrO2 particles reach nanoscale because of the relatively low sintering temperature and the restriction of Al2O3 matrix (Fig. 7(c)). These ultra-fine nanoparticles have a highly uniform particle size and a vast majority (72%) of them are enriched in the range of 4-20 nm, and the average particle size is only 11.5 nm, as shown in Fig. 7(d). The sintered N-ZTA composite ceramic has incomparable ultra-fine nanostructure. Additionally, the EDS elemental mapping (Fig. 8) reveals that the Ce element is doped into ZrO2 lattice, which is consistent with above analysis.

Fig. 7.   BF-TEM (a), HAADF-STEM at high magnification (b), HAADF-STEM at low magnification (c) and particle-size distribution (d) of transgranular ZrO2 particles.

Fig. 8.   Corresponding EDS elemental mapping of Fig. 7(c): (a) O element; (b) Al element; (c) Zr element; (d) Ce element.

To elucidate the interfacial microstructure feature of N-ZTA ceramic, a high-resolution TEM (HR-TEM) image is demonstrated in Fig. 9(a). The microstructure of composite consists of Al2O3 matrix and embedded ZrO2 nanoparticles. The interface is well bonded without aggregation of heterogeneous atoms and dislocations, as shown in the Fig. 9(b), which indicates that nano-ZrO2 are highly coherent with matrix phase and consequently resulting in impressive mechanical properties. Fig. 9(c) and (d) depicts the magnified high-resolution images of II and III region in Fig. 9(a). It can be inferred that the Al2O3 matrix and ZrO2 nanoparticles are hexagonal and tetragonal structure, respectively according to the fast Fourier transform (FFT) patterns. Particularly notable is that the weak diffraction spots are also identified in Fig. 9(c). Such phenomenon may be attributed to the dissolution of Ce4+ into ZrO2 lattice. In addition, both of ZrO2 and Al2O3 phases are highly crystalline and have ordered structure, which is further proven that the solid solution is full precipitation at 1400 °C in only 0.5 h time period.

Fig. 9.   (a) HRTEM image of the N-ZTA ceramic and (b-d) the magnified high-resolution images of I, II and III in (a), respectively. The grain boundaries (GB) is marked by the blue triangles. Insets are the corresponding FFT patterns.

3.3.2. Mechanical properties and toughening mechanisms

The load-displacement curves in the measurement of fracture toughness and flexural strength are exhibited in Fig. S2(a). Both of the blue and red curves show typical brittle rupture feature. The tested sample instantaneously breaks after the applied load reaches the maximum. In addition, the N-ZTA composite ceramic has high flexural strength and fracture toughness of 701 ± 18 MPa and 8.43 ± 0.25 MPa m1/2, respectively. Fig. S2(b) shows SEM image of the Vickers indentation on the polished N-ZTA surface and the Vickers hardness reaches 18.55 ± 0.37 GPa. Compared with the ZTA composite ceramic prepared by other technologies [3,[10], [11], [12], [13],34], the N-ZTA composite ceramic has more excellent mechanical properties.

Fig. 10 shows the fracture surface of N-ZTA composite ceramic, which reveals that the fracture propagation mode is transgranular/intergranular fracture. The intergranular ZrO2 particles will play a pinning effect, which is favorable for crack deflection and crack bridging during the intergranular fracture. Simultaneously, many small pores are found in the fracture surface (blue arrows in Fig. 10), indicating that the particles are detached from the grain. Therefore, more energy is needed to overcome during the intergranular fracture process. On the other hand, intergranular ZrO2 particles also strengthen grain boundaries, which induces transgranular fracture (red arrow in Fig. 10). In particular, larger-size grains are marked because of its weak strength. When transgranular fracture occurs, the crack extends along the sub-boundaries of transgranular ZrO2 nanoparticles and Al2O3 matrix. The ultra-fine transgranular nanoparticles greatly increases the propagation path, resulting in high fracture toughness. Due to the pinning effect of transgranular ZrO2 nanoparticles, cleavage fracture is also found in fracture surface, as indicated by yellow arrows shown in Fig. 10, which further improves the toughness of N-ZTA ceramic.

Fig. 10.   SEM micrographs of fracture surface for N-ZTA composite ceramic: (a, b) high magnification; (c, d) low magnification.

Fig. 11 demonstrates XRD patterns of AZC powders after annealing at 1400 °C and N-ZTA composite ceramic. The dissolution of Ce4+ in ZrO2 lattice and the ultra-fine particle are favorable for the stabilization of t-ZrO2 phase. Although the nitride is oxidized and there is no stress bonding during the heat treatment, most ZrO2 phase of AZC-1400-0.5 h still retain tetragonal (Fig. 11(a)). Therefore, it can be inferred that nearly all ZrO2 phases in N-ZTA ceramic are tetragonal structure. In the XRD pattern of polished surface (Fig. 11(b)), the ZrO2 are consisted of monoclinic structure and tetragonal structure. The strong diffraction peak of the m-ZrO2 phase indicates the occurrence of stress-induced phase transformation. Generally, larger t-ZrO2 grains have a greater propensity to undergo the phase transformation to the equilibrium monoclinic structure. Therefore, the transgranular ZrO2 nanoparticles still keep tetragonal structure because of ultra-small particle size, while the submicron sized intergranular ZrO2 particles undergo phase transformation induced by the external stress. Moreover, the comparable intensity of m-ZrO2 and t-ZrO2 are also found in Fig. 11(c), suggesting the submicron ZrO2 particles will also experience a t-m phase transformation when fracture occur, which leads to 3-5% volume expansion. As a consequence, cracks tend to close based on compressive stress, which greatly enhances the toughness of N-ZTA ceramic. Combining the nano-toughening and phase transformation toughening, the N-ZTA composite ceramic exhibits impressive fracture toughness. The relative intensity of Al2O3 in N-ZTA enhances significantly when compared with that of AZC solid solution powders, which further indicates the sintering ignites solid solution precipitation.

Fig. 11.   XRD patterns of AZC powders and N-ZTA composite ceramic: (a) AZC-1400-0.5 h; (b) polished surface of N-ZTA; (c) fracture surface of N-ZTA.

4. Conclusion

The AZC solid solution powders were successfully synthesized by a novel combustion synthesis combined with water atomization method. The ultra-high cooling rate greatly extends solid solubility of Al2O3 in ZrO2. Al3+ desorbs from solid solution phase starting at 1050 °C during annealing process. As the annealing temperature increases, the Al2O3 content gradually increases and t-ZrO2 transforms into the m-ZrO2. The ZrO2 particles are uniformly dispersed in Al2O3 matrix. When annealing temperature increases to 1350 °C, the arc-shaped ZrO2 phase experience spheroidization process and finally grow into submicron scale at 1450 °C. The solid solution is full precipitation at 1400 °C in only 0.5 h time period. Subsequently, the synthesized powders are densified by SPS. The metastable phase decomposes into Al2O3 and ZrO2 during SPS process, and ZrO2 particles are uniformly distributed in Al2O3 matrix. Partial ZrO2 are surrounded and spatially isolated by Al2O3 matrix, while the residuary ZrO2 locate at grain boundaries, which forms special transgranular/intergranular microstructure. The transgranular ZrO2 particles reach nanoscale with average particle size of 11.5 nm. The interface of Al2O3 and ZrO2 is well bonded without aggregation of heterogeneous atoms and dislocations. Due to the dissolution of Ce4+ in ZrO2 lattice and the ultra-small particle size, nearly all ZrO2 phases in N-ZTA ceramic still retain tetragonal. Combining nano-toughening and phase transformation toughening, the strength, toughness and hardness of N-ZTA composite ceramic reach 701 ± 18 MPa, 8.43 ± 0.25 MPa m1/2 and 18.55 ± 0.37 GPa, respectively.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos. 91016014 and 51872062).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2019.09.028.


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