Fabricating a Novel Intragranular Microstructure for Al2O3/GdAlO3 Ceramic Composites

In order to make the embryonic form of intragranular structure, the Al2O3/GdAlO3 system was selected due to its excellent mechanical properties. Gd2O3 and Al(NO3)3·9H2O were used as the starting materials. A co-precipitation method was used for the preparation of fine ceramics and applied to synthesize the nano-powder of GdAlO3 firstly. Then, the nano-powder of GdAlO3 was mixed with the precipitates by the second co-precipitation method. After drying and calcination, the compound powder with eutectic composition (77 mol % Al3+—23 mol % Gd3+) was fast sintered by using the spark plasma sintering technique. The results revealed that the phases of the sintered samples were Al2O3 and GdAlO3. The phases showed a homogeneous and interlaced distribution. All the matrix grains were submicron. The sizes of the intragranular structures were between 50 nm and 150 nm. Therefore, the intragranular structure displayed a novel mixture of nanometer–submicron and submicron–submicron types. The different intragranular structures all changed the fracture modes of Al2O3 grains from intergranular fracture to transgranular fracture.


Introduction
Compared with traditional ceramics, ceramic nanocomposites have better properties of fracture strength, fracture toughness, creep resistance, and wear resistance [1]. Niihara thought ceramic nanocomposites could be divided into three categories: intragranular nanocomposite, intergranular nanocomposite, and nano/nano composite [2]. The intragranular microstructure is unique as one nanophase exists inside the matrix grain of the other phase. Therefore, the unique microstructure enables the nanocomposite ceramic to draw much attention in the research field.
Nowadays, the main nanocomposite ceramics in the non-rare-earth system are Al 2 O 3 /SiC(n), Si 3 N 4 / SiC(n), and ZrO 2 /Al 2 O 3 (n) (n stands for the nanoparticles in the above chemical compounds) et al. [3,4]. While, for the Al 2 O 3 /Re 3 Al 5 O 12 (Re stands for the rare-earth elements) system, the Al 2 O 3 /Y 3 Al 5 O 12 (YAG) composite ceramic is mainly focused on [5,6]. Liquid-coating method, reaction-sintering process, compound-powder method, and suspension-disperse-mixing method are the main synthetic methods to fabricate the intragranular microstructures [7]. The matrix grains with micron sizes are reported in the above mentioned systems. For the Al 2 O 3 /GdAlO 3 system, excellent flexural strength and thermal stability at high temperature have been reported. Waku et al. found the Al 2 O 3 /GdAlO 3 system displayed plastic deformation at 1873 K owing to dislocation motion, as in metals [8]. Ohashi et al. investigated the microstructures and orientation relationships of the Al 2 O 3 /GdAlO 3 eutectic fibers fabricated by the micro-pulling down method [9]. Ma Then, Gd 2 O 3 was dissolved in nitric acid to produce Gd(NO 3 ) 3 .
Al(NO 3 ) 3 and Gd(NO 3 ) 3 were mixed in the homogeneous aqueous solution. For the co-precipitation method, the above salt solution and ammonia were both added into distilled water simultaneously in the back titration while pH value was kept between 8 and 9 till the end of the titration. The reactions between the above nitrate solutions and the ammonia are as follows.
In order to avoid the aggregation of small particles, ultrasonic fibrations were used during the co-precipitation reaction. The power of the ultrasonic instrument (Kunshan Ultrasonic Instruments Co, Ltd KQ-100DB, Kunshan, China) was selected as 100 W and the ultrasonic frequency was selected as 40 kHz. Then, the gelatinous precipitate was filtered and washed several times with water and ethanol, respectively. After drying at 120 • C for 24 h, the precipitates were calcined in air for 2 h, at a temperature ranging from 800 to 1200 • C, at a heating rate of 10 • C/min and cooled with the furnace.
The mixture process (routine 2) was as follows. The starting materials of Al(NO 3 ) 3 ·9H 2 O and Gd 2 O 3 were weighed to achieve the final eutectic ratio (77 mol % Al 3+ -23 mol % Gd 3+ ). The amount of GdAlO 3 powder was selected to represent 2 vol % of the ceramic composite. Considering the dispersion of the GdAlO 3 powder and the precipitation reaction, the pH value was kept between 9 and 10 [15]. At the same time, the nanoparticles of GdAlO 3 from routine 1 were added during the co-precipitation reaction for routine 2. In order to gain a well dispersion of GdAlO 3 nanoparticles, the ultrasonic and mechanical agitation were used during the reaction. After drying and calcination at 1100 • C, the compound powder was loaded in the graphite mold.
According to phase diagram of Al 2 O 3 -Gd 2 O 3 [16], the temperature for the coexistence of GdAlO 3 and Al 2 O 3 phases is near 1600 • C. Therefore, the spark plasma sintering (SPS-3.20-MK-V, Sumitomo Coal Mining Co., Ltd., Kyoto, Japan) was conducted at 1600 • C for 3 min at a heating rate of 100 • C/min, at the pressure maintaining of 30 MPa and cooled with the furnace.
Phases of the calcined powder and the sintered sample were identified by an X-ray diffractometer (XRD, RIGAKU D/Max-rB, Tokyo, Japan) with Cu Ka radiation (0.1542 nm). The accelerating voltage was 40 kV with a tube current of 40 mA. The morphology of the calcined powder and the microstructure of the composite ceramic were examined by using a scanning electron microscope (FE-SEM, S-4800, HITACHI, Tokyo, Japan).

XRD Patterns of the Powder and Sintered Sample
The XRD patterns of the precursor powders of GdAlO 3 calcined at different temperatures are shown in Figure 1. As shown in Figure 1, there was no distinctive diffraction peak of the precursor powder calcined at 800 • C. When the calcination temperature got to 900 • C, the diffraction peaks were made up of GdAlO 3 , α-Al 2 O 3 , Gd 2 O 3 , and Gd 4 Al 2 O 9 . It can be known that hydroxide in the precursor underwent decomposition and the perovskite-type GdAlO 3 began to crystallize. The reaction equations are shown as follows.
2Al(OH) 3 Further calcining at 1000 • C led to an increase in the intensity of the GdAlO 3 peaks while the intensities of the diffraction peaks of α-Al 2 O 3 , Gd 2 O 3 , and Gd 4 Al 2 O 9 decreased. Coal Mining Co., Ltd., Kyoto, Japan) was conducted at 1600 °C for 3 min at a heating rate of 100 °C/min, at the pressure maintaining of 30 MPa and cooled with the furnace. Phases of the calcined powder and the sintered sample were identified by an X-ray diffractometer (XRD, RIGAKU D/Max-rB, Tokyo, Japan) with Cu Ka radiation (0.1542 nm). The accelerating voltage was 40 kV with a tube current of 40 mA. The morphology of the calcined powder and the microstructure of the composite ceramic were examined by using a scanning electron microscope (FE-SEM, S-4800, HITACHI, Tokyo, Japan).

XRD Patterns of the Powder and Sintered Sample
The XRD patterns of the precursor powders of GdAlO3 calcined at different temperatures are shown in Figure 1. As shown in Figure 1, there was no distinctive diffraction peak of the precursor powder calcined at 800 °C. When the calcination temperature got to 900 °C, the diffraction peaks were made up of GdAlO3, α-Al2O3, Gd2O3, and Gd4Al2O9. It can be known that hydroxide in the precursor underwent decomposition and the perovskite-type GdAlO3 began to crystallize. The reaction equations are shown as follows.
Gd2O3 + Al2O3 = 2GdAlO3 (7) Further calcining at 1000 °C led to an increase in the intensity of the GdAlO3 peaks while the intensities of the diffraction peaks of α-Al2O3, Gd2O3, and Gd4Al2O9 decreased.
When the calcination temperature reached up to 1100 °C, the phase of the calcined powder was only GdAlO3. It indicates that pure GdAlO3 whose precursor was synthesized via co-precipitation can be obtained after calcination at 1100 °C. For 1200 °C, the diffraction peaks of GdAlO3 had no change. Based on the kinetics and thermodynamics [17], when the stoichiometric ratios for Al and Gd were close, the transition phase of Gd4Al2O9 could be easily formed in the calcination process. When the stoichiometric ratio for Al and Gd was close to the eutectic ratio, the transition phase of Gd3Al5O12 could be easily formed in the calcination process [15]. 20 30 40 50 60 70 Intensity / arb.unit  When the calcination temperature reached up to 1100 • C, the phase of the calcined powder was only GdAlO 3 . It indicates that pure GdAlO 3 whose precursor was synthesized via co-precipitation can be obtained after calcination at 1100 • C. For 1200 • C, the diffraction peaks of GdAlO 3 had no change. Based on the kinetics and thermodynamics [17], when the stoichiometric ratios for Al and Gd were close, the transition phase of Gd 4 Al 2 O 9 could be easily formed in the calcination process. When the stoichiometric ratio for Al and Gd was close to the eutectic ratio, the transition phase of Gd 3 Al 5 O 12 could be easily formed in the calcination process [15].
The XRD patterns of the sample with eutectic composition sintered by SPS is shown in Figure 2. It shows that Al 2 O 3 /GdAlO 3 composite ceramic is successfully fabricated and no impurity phase is found.
The XRD patterns of the sample with eutectic composition sintered by SPS is shown in Figure 2. It shows that Al2O3/GdAlO3 composite ceramic is successfully fabricated and no impurity phase is found.  Figure 3a shows the SEM micrographs of the precursor powders calcined at 1100 °C. Primary nanoparticles are observed and they are slightly aggregated due to the drying and calcination processes. Figure 3b shows the SEM micrographs of compound powders prepared from routine 2.

Surface of Sintered Samples
The thickness of the sintered sample is about 2 mm and the diameter is about 10 mm. Figure 4 is the SEM micrograph of the polished surface of the sample sintered by SPS. It indicates that the microstructure of the ceramic composite is dense and made up of two phases. The average grain size of the microstructure is about 500 nm. According to the testing conditions, the brighter submicron phase is GdAlO3 and the darker submicron phase is α-Al2O3. It reveals that the homogeneous, interlaced and fine microstructure for Al2O3/GdAlO3 can be successfully prepared by the wet chemical process and the SPS technique.  Figure 3a shows the SEM micrographs of the precursor powders calcined at 1100 • C. Primary nanoparticles are observed and they are slightly aggregated due to the drying and calcination processes. Figure 3b shows the SEM micrographs of compound powders prepared from routine 2. The XRD patterns of the sample with eutectic composition sintered by SPS is shown in Figure 2. It shows that Al2O3/GdAlO3 composite ceramic is successfully fabricated and no impurity phase is found.  Figure 3a shows the SEM micrographs of the precursor powders calcined at 1100 °C. Primary nanoparticles are observed and they are slightly aggregated due to the drying and calcination processes. Figure 3b shows the SEM micrographs of compound powders prepared from routine 2.

Surface of Sintered Samples
The thickness of the sintered sample is about 2 mm and the diameter is about 10 mm. Figure 4 is the SEM micrograph of the polished surface of the sample sintered by SPS. It indicates that the microstructure of the ceramic composite is dense and made up of two phases. The average grain size of the microstructure is about 500 nm. According to the testing conditions, the brighter submicron phase is GdAlO3 and the darker submicron phase is α-Al2O3. It reveals that the homogeneous, interlaced and fine microstructure for Al2O3/GdAlO3 can be successfully prepared by the wet chemical process and the SPS technique. 5μm 5μm

Surface of Sintered Samples
The thickness of the sintered sample is about 2 mm and the diameter is about 10 mm. Figure 4 is the SEM micrograph of the polished surface of the sample sintered by SPS. It indicates that the microstructure of the ceramic composite is dense and made up of two phases. The average grain size of the microstructure is about 500 nm. According to the testing conditions, the brighter submicron phase is GdAlO 3 and the darker submicron phase is α-Al 2 O 3 . It reveals that the homogeneous, interlaced and fine microstructure for Al 2 O 3 /GdAlO 3 can be successfully prepared by the wet chemical process and the SPS technique.  Figure 5a shows the Al2O3 matrix grain without intragranular structure. It can be known that the two phases combines well and there is no impurity phase at the interfaces. Figure 5b shows there are several nano-particles (~50 nm) of GdAlO3 phase in the submicron matrix grain of Al2O3 phase to form the novel intragranular structure of the nanometre-submicron type. Moreover, the intragranular structures are both observed in the matrix grains of Al2O3 phase in Figure 5c,d. The average nanoparticle sizes are about 100 and 150 nm in Figure 5c,d, respectively. The grain sizes of the intragranular structures in Figure 5c,d are relatively larger than those in Figure 5b. Furthermore, the amount of the nano-particles in Figure 5b is more than those in Figure 5c Figure 5a shows the Al 2 O 3 matrix grain without intragranular structure. It can be known that the two phases combines well and there is no impurity phase at the interfaces. Figure 5b shows there are several nano-particles (~50 nm) of GdAlO 3 phase in the submicron matrix grain of Al 2 O 3 phase to form the novel intragranular structure of the nanometre-submicron type. Moreover, the intragranular structures are both observed in the matrix grains of Al 2 O 3 phase in Figure 5c,d. The average nano-particle sizes are about 100 and 150 nm in Figure 5c,d, respectively. The grain sizes of the intragranular structures in Figure 5c,d are relatively larger than those in Figure 5b. Furthermore, the amount of the nano-particles in Figure 5b is more than those in Figure 5c,d. Thus, the intragranular structures of the submicron-submicron type in Figure 5c,d are presented.   Figure 5a shows the Al2O3 matrix grain without intragranular structure. It can be known that the two phases combines well and there is no impurity phase at the interfaces. Figure 5b shows there are several nano-particles (~50 nm) of GdAlO3 phase in the submicron matrix grain of Al2O3 phase to form the novel intragranular structure of the nanometre-submicron type. Moreover, the intragranular structures are both observed in the matrix grains of Al2O3 phase in Figure 5c,d. The average nanoparticle sizes are about 100 and 150 nm in Figure 5c,d, respectively. The grain sizes of the intragranular structures in Figure 5c,d are relatively larger than those in Figure 5b. Furthermore, the amount of the nano-particles in Figure 5b is more than those in Figure 5c The formation of the intragranular structures is discussed as follows. The formation of the intragranular structures was associated with the chemical environment and the dispersion of GdAlO 3 particles. When the GdAlO 3 particles were dispersed and surrounded by more Al 2 O 3 particles, it was beneficial to promote the diffusion and mass transfer for Al 2 O 3 phase by using the spark plasma sintering process. It enabled the Al 2 O 3 grains to have the relatively lower sintering temperature and faster migration velocity of crystal boundary. Then, with the enhanced growth of Al 2 O 3 grains, the GdAlO 3 nano-particles were probably swallowed up by the Al 2 O 3 grain for the case of Figure 5b. When the GdAlO 3 nano-particles were slightly aggregated, they were easily to form one larger grain during the sintering process for the cases of Figure 5c,d. When the GdAlO 3 nano-particles were surrounded by other more GdAlO 3 particles, the nanoparticles of GdAlO 3 phase tended to grow into one larger submicron grain of GdAlO 3 phase for the case of Figure 5a.

Fracture Surface of Sintered Samples
Based on the above discussion, the synthesized Al 2 O 3 /GdAlO 3 ceramic composite has the novel microstructure as shown in Figure 6a. The novel microstructure is different from the traditional intragranular microstructure and intergranular microstructure. The volume fractions for Al 2 O 3 phase and GdAlO 3 phase are close and the well alternative distribution of the submicron matrix grains is formed. In the matrix grains of Al 2 O 3 phase, nanoparticles with sizes of 50-150 nm in the intragranular microstructures are present. Up to now, the traditional intragranular microstructures of Figure 6b for Al 2 O 3 /YAG are mainly reported [18,19]. The common sizes of the matrix grains for the traditional intragranular microstructures are micron. The nanoparticles of YAG phase often locate in the matrix grains of Al 2 O 3 phase and the larger particles of YAG phase locate at the grain boundares of Al 2 O 3 phase, as shown in Figure 6b. The formation of the intragranular structures is discussed as follows. The formation of the intragranular structures was associated with the chemical environment and the dispersion of GdAlO3 particles. When the GdAlO3 particles were dispersed and surrounded by more Al2O3 particles, it was beneficial to promote the diffusion and mass transfer for Al2O3 phase by using the spark plasma sintering process. It enabled the Al2O3 grains to have the relatively lower sintering temperature and faster migration velocity of crystal boundary. Then, with the enhanced growth of Al2O3 grains, the GdAlO3 nano-particles were probably swallowed up by the Al2O3 grain for the case of Figure 5b. When the GdAlO3 nano-particles were slightly aggregated, they were easily to form one larger grain during the sintering process for the cases of Figure 5c,d. When the GdAlO3 nano-particles were surrounded by other more GdAlO3 particles, the nanoparticles of GdAlO3 phase tended to grow into one larger submicron grain of GdAlO3 phase for the case of Figure 5a.
Based on the above discussion, the synthesized Al2O3/GdAlO3 ceramic composite has the novel microstructure as shown in Figure 6a. The novel microstructure is different from the traditional intragranular microstructure and intergranular microstructure. The volume fractions for Al2O3 phase and GdAlO3 phase are close and the well alternative distribution of the submicron matrix grains is formed. In the matrix grains of Al2O3 phase, nanoparticles with sizes of 50-150 nm in the intragranular microstructures are present. Up to now, the traditional intragranular microstructures of Figure 6b for Al2O3/YAG are mainly reported [18,19]. The common sizes of the matrix grains for the traditional intragranular microstructures are micron. The nanoparticles of YAG phase often locate in the matrix grains of Al2O3 phase and the larger particles of YAG phase locate at the grain boundares of Al2O3 phase, as shown in Figure 6b. Generally, the fracture mode of Al2O3 grains is intergranular fracture [20,21]. It reveals that the intragranular structure tended to induce the transgranular fracture of Al2O3 grains in Figure 5d. Since the average coefficient of volume thermal expansion for GdAlO3 (~31.8 × 10 −6 /°C) is higher than that of Al2O3 (~21.9 × 10 −6 /°C) [17,22], the GdAlO3 nanoparticles in the intragranular structure may have larger volume contraction than that of Al2O3 matrix during the cooling process following the sintering densification. Thus, the GdAlO3 nanoparticles pulled the Al2O3 matrix for the GdAlO3/Al2O3 system at room temperature. The residual tensile stress field generated and tended to form microcracks near the interfaces of the intragranular structures. If the cracks nucleated near the intragranular structures, the stress field of the crack tip would interact with the residual tensile stress field and other microstructure defects as the external force loaded on the ceramic composite. When the shear stress value exceeded the cleavage strength of the ceramic composite, the cleavage crack would propagate along the specific crystal plane [23]. If the cracks nucleated far from the intragranular structures, the crack would be captured by the intragranular structure as it propagated [24]. Due to the interactions of residual stress and defects, the direction of crack propagation was deflected and the crack went through the Al2O3 matrix grain along the specific crystal plane. As described above, the cleavages formed for the cases of  Generally, the fracture mode of Al 2 O 3 grains is intergranular fracture [20,21]. It reveals that the intragranular structure tended to induce the transgranular fracture of Al 2 O 3 grains in Figure 5d. Since the average coefficient of volume thermal expansion for GdAlO 3 (~31.8 × 10 −6 / • C) is higher than that of Al 2 O 3 (~21.9 × 10 −6 / • C) [17,22], the GdAlO 3 nanoparticles in the intragranular structure may have larger volume contraction than that of Al 2 O 3 matrix during the cooling process following the sintering densification. Thus, the GdAlO 3 nanoparticles pulled the Al 2 O 3 matrix for the GdAlO 3 /Al 2 O 3 system at room temperature. The residual tensile stress field generated and tended to form microcracks near the interfaces of the intragranular structures. If the cracks nucleated near the intragranular structures, the stress field of the crack tip would interact with the residual tensile stress field and other microstructure defects as the external force loaded on the ceramic composite. When the shear stress value exceeded the cleavage strength of the ceramic composite, the cleavage crack would propagate along the specific crystal plane [23]. If the cracks nucleated far from the intragranular structures, the crack would be captured by the intragranular structure as it propagated [24]. Due to the interactions of residual stress and defects, the direction of crack propagation was deflected and the crack went through the Al 2 O 3 matrix grain along the specific crystal plane. As described above, the cleavages formed for the cases of Figure 5b-d. Meanwhile, Figure 5b-d also present the cleavage patterns for the Al 2 O 3 matrix grains. The cleavage patterns are mainly consisted of some parallel cleavage steps that are made up of the intersections of different cleavage planes.

Summary
In order to prepare the intragranular structures for Al 2 O 3 /GdAlO 3 composite ceramic, GdAlO 3 powder was synthesized at the calcination temperature of 1100 • C for 2 h by co-precipitation method. The nanocomposite ceramic of Al 2 O 3 /GdAlO 3 with intragranular structures was successfully obtained by the chemical process and spark plasma sintering technique. The sizes of all the matrix grains were kept submicron. By the above techniques, the intragranular structure that contains GdAlO 3 nanoparticles in the Al 2 O 3 grains can be prepared for the Al 2 O 3 /GdAlO 3 system, even suitable for Al 2 O 3 /ReAlO 3 systems. The novel intragranular structures present two types: nanometre-submicron and submicron-submicron. The intragranular structures have changed the fracture mode of the Al 2 O 3 phase and induced the transgranular fracture instead of intercrystalline fracture due to the residual stress. Furthermore, the features of cleavage in the Al 2 O 3 grains display parallel cleavage steps.