Effect of AlN addition on the reaction sintering of Al2TiO5 composites fabricated by spark plasma sintering

ABSTRACT Fully dense Al2TiO5–Al2O3–TiN (ATN) composites were fabricated by reactive sintering using spark plasma sintering at 1400°C for 5 min under 100 MPa in vacuum. An equimolar ratio of Al2O3:TiO2 was used as the starting powder, while the addition of 0–36 mol% AlN was investigated. The thermodynamic calculation indicates that the initial reaction was that of TiO2 and AlN, forming TiN and Al2O3, and then the remaining TiO2 reacted with Al2O3 to produce Al2TiO5. With the increase in AlN precursor, Al2TiO5 gradually decreased, while Al2O3 and TiN increased. The lattice parameters of Al2TiO5 were enlarged with AlN addition, implying the incorporation of N atoms in the Al2TiO5 unit cell. The addition of AlN effectively produced fully densified bodies with small grain size, and microcrack-free, which therefore enhanced the mechanical properties of ATN composites. At 36 mol% AlN addition, the composite shows Vickers hardness and fracture toughness of 16.26 ± 1.61 GPa and 5.20 ± 0.46 MPa.m1/2, respectively.


Introduction
Aluminum titanate (Al 2 TiO 5 ) ceramics are well known for their low thermal expansion and excellent thermal shock resistance, which are the most important characteristics for components used in thermal cycle conditions [1]. Al 2 TiO 5 can be employed in various hightemperature applications, such as crucibles for melting non-ferrous alloys, parts for combustion engines, exhaust pot liners, and diesel particulate filters (DPF) [2][3][4]. Al 2 TiO 5 ceramics are typically synthesized by the solid-state reaction of Al 2 O 3 and TiO 2 powders, both of which are widely available and inexpensive precursors.
Al 2 TiO 5 has an orthorhombic crystal structure, with Al 3+ and Ti 4+ cations randomly occupied by octahedral sites surrounded by six oxygen ions. This type of crystal structure possesses a highly anisotropic thermal expansion coefficient, and as a result, microcracking initiates at the grain boundary upon cooling during the manufacturing process. These microcracks are a key factor in the low thermal expansion of Al 2 TiO 5 ceramics [1]. However, microcracks in an Al 2 TiO 5 body inevitably lead to a relatively low mechanical strength and limit Al 2 TiO 5 ceramics from achieving widespread use. Another major issue is that monolithic Al 2 TiO 5 is unstable at the temperature below 1280°C because it decomposes into Al 2 O 3 and TiO 2 [1].
The thermal stability of Al 2 TiO 5 can be improved by forming a solid solution using additives such as MgO and Fe 2 O 3 [5][6][7][8][9][10][11][12]. The substitution of the elements from these oxides at the cation site (Al 3+ or Ti 4+ sites) results in an increase in the free energy of thermal decomposition of Al 2 TiO 5 . Fabrication of Al 2 TiO 5 composites is one of the promising methods used to enhance the phase stability and strength. Some of the additives, including SiO 2 and ZrO 2 [9,[12][13][14] as well as minerals like talc, feldspar, and kaolin [15,16], not only form solid solutions with Al 2 TiO 5 but also react to create the dispersed phases with an effect of reducing grain size. Obtaining a microstructure of fine grain size is an important way to reduce microcracking at the grain boundary of Al 2 TiO 5 . Spark plasma sintering (SPS) technique has been extensively used for reactive sintering of nanostructured bulk materials because it provides a rapid heating rate that enables for the densification with limited grain growth [17]. Therefore, SPS is an effective technique to densify Al 2 TiO 5 composites with controlled grain size. Furthermore, many researchers have reported that reaction sintering to form the Al 2 TiO 5 -Al 2 O 3 composite significantly enhances the mechanical properties due to the second-phase reinforcement [18][19][20][21].
A review of the literature revealed that there were few reports of addition of nitride compounds to Al 2 TiO 5 ceramics. It was reported that Al 2 TiO 5 with a solid solution of nitrogen was prepared from a mixture of AlN, Al 2 O 3 and TiO 2 [22]. Since the Ti-N bond is a stronger covalent bond than the Ti-O bond, introducing an N atom into the Al 2 TiO 5 structure improves the phase stability and enhances the mechanical properties of Al 2 TiO 5 ceramics. It was also revealed that the displacement reaction between AlN and TiO 2 resulted in the formation of Al 2 O 3 and TiN phases [22]. TiN is known for its high hardness and good corrosion resistance. Therefore, incorporating this phase into Al 2 TiO 5 could be a promising strategy to improve the mechanical properties.
To the best of our knowledge, no research has previously been conducted on the effects of AlN addition content on the phase formation as well as the physical and mechanical properties of Al 2 TiO 5 ceramics. Thus, the aim of this research was to investigate the fabrication of Al 2 TiO 5 composites with the addition of AlN. Al 2 TiO 5 composites were prepared using one molar ratio of Al 2 O 3 and TiO 2 powders. The addition of different AlN contents, ranging from 0 to 36 mol%, was examined for the first time. The Al 2 TiO 5 composites were then produced by the reaction sintering using spark plasma sintering (SPS) at 1400°C with a heating rate of 200°C min −1 and a holding time of 5 min. The rapid heating rate and short holding time by SPS are the important keys to achieve fine microstructure of Al 2 TiO 5 composites. The influences of AlN addition were discussed in relation to the phase composition, lattice parameters, microstructures, and mechanical properties.

Experimental procedures
The raw materials for this research were Al 2 O 3 (alpha phase; 0.18 μm, Sumitomo, Japan), TiO 2 (rutile phase; 0.1-0.3 μm, Kanto Chemical, Japan), and AlN (0.05 μm, Fuji Wako Pure Chemical Corporation, Japan). The powder mixture was prepared with an Al 2 O 3 :TiO 2 ratio of 1:1 in mol%, while the addition of AlN was 0, 6, 10, 20, 28, and 36 mol% of the total mixture of Al 2 O 3 and TiO 2 . The powders were ball-milled for 24 h using ZrO 2 balls in ethanol. After being dried and sieved, the powders were put into a graphite die that had an inner diameter of approximately 10 mm and sintered using SPS (SPS-210 LX, Fuji Electronic Industrial, Japan) at a sintering temperature of 1400°C, a holding time of 5 min, and a heating rate of 200°C min −1 in vacuum. An uniaxial pressure of 100 MPa was applied during sintering. After sintering, the sample without AlN addition was given the named AT, and the composites with AlN addition were given the named ATN plus a number, and the number denoted the amount of AlN precursor.
The phase analysis of sintered specimens was performed using X-ray diffraction (XRD; Ultima IV, Rigaku, Japan) in a 2θ range of 10-90° with a scanning step of 0.02°. All samples were thoroughly ground in a mortar and sieved through 44-µm mesh screen to ensure homogeneity and avoid preferred orientation. The quantitative analysis of each crystalline phase was performed based on Rietveld method [23] using PDXL software. The ICDD's Powder Diffraction File (PDF) was used as database for Rietveld's method. The unit cell lattice parameter was determined from the XRD peak position in a 2θ range of 10-130°. To obtain accurate lattice parameter determination, the sample was finely ground and mixed with silicon powder (SRM640e) as an internal standard. The apparent density of the sintered specimen was measured using the Archimedes method. The microstructure was observed using scanning electron microscopy (SEM; Hitachi S-3400, Japan) equipped with an energy-dispersive X-ray spectrometer (EDS; EAX Inc., USA).
The hardness and fracture toughness were measured by an indentation method under a load (P) of 19.6 N using a Vickers hardness tester (HM-221, Mitutoyo, Japan). The mechanical test was conducted with 10 indentations for each specimen to obtain the average value of its standard deviation. The hardness was calculated using the Vickers hardness formula as shown in Equation (1), and the fracture toughness was calculated using Miyoshi equation [24] as shown in Equation (2).
where d is the average of two diagonal lengths of Vickers indentation, c is the half-length of the crack formed at the corners of indentation, and E is Young's modulus of composites obtained by the rule of mixture [1,25]. Figure 1 shows the XRD patterns of AT and ATN composites after sintering by SPS at 1400°C. The corresponding phase compositions of AT and ATN composites were analyzed by Rietveld methods, and the results are summarized in Table 1. The XRD pattern of AT specimen revealed that it mainly consisted of Al 2 TiO 5 phase, which was formed by the solid-state reaction as described in Equation (3). The minor peaks of Al 2 O 3 and TiO 2 were also found in AT specimen. The sintered AT sample was determined to contain 77.5 wt % Al 2 TiO 5 , 20.1 wt% Al 2 O 3 , and 2.4 wt% TiO 2 . According to previous research, the reaction sintering of Al 2 TiO 5 from equimolar ratio of Al 2 O 3 and TiO 2 powders without any additives resulted in a composite that included unreacted precursors [26][27][28][29][30]. The percentage of completed reactions depended on many factors, such as particle size, sintering temperature, sintering time, and sintering method [26][27][28][29][30].

Results and discussion
Although the formation of Al 2 TiO 5 could be enhanced at higher temperature and longer sintering time, these would cause grain growth and decreased mechanical strength.
For ATN composites, with increasing AlN content, the peak intensity of Al 2 TiO 5 gradually decreased but that of Al 2 O 3 phase conversely increased, while AlN and TiO 2 phases were not detected. Furthermore, a small peak indicating the presence of TiN phase was first discovered in ATN10 composite, and then the peak intensity of TiN phase increased with increasing AlN addition. The contents of TiN phase are 4.7, 7.0, 13.2, and 19.1 wt% for the ATN10, ATN20, ATN28, and ATN36 specimens, respectively. It was reported that the reaction between TiO 2 and AlN proceeded in many steps including reduction of TiO 2 , formation of intermediated Al x Ti y O 5 compounds, and nitridation of mixed oxide [31]. The progress of these steps finally resulted in the formation of TiN, Al 2 O 3 , and N 2 products as shown in Equation (4) At 1400°C, the Gibbs free energy of the reaction in Equation (4) is −179 kJ mol −1 [32], whereas that of Al 2   (3) is −1.319 kJ mol −1 [26]. Therefore, the reaction between TiO 2 and AlN initially occurred due to thermodynamically favorable conditions. After that, the Al 2 TiO 5 was produced through the interaction of the remaining TiO 2 with both the Al 2 O 3 precursor and Al 2 O 3 product from Equation (4). It should be noted that no TiO 2 phase was identified in any ATN composites, suggesting that the decomposition reaction of Al 2 TiO 5 did not occur during cool down.

TiO 5 formation in Equation
It was observed that the peaks of Al 2 TiO 5 shifted to a lower angle, but the peaks of Al 2 O 3 and TiN were unchanged. Figure 2(a) shows an enlarged view of XRD patterns at the 2θ range of 32.0-34.5°. It can be seen that the peak in the (230) plane of Al 2 TiO 5 shifts to a lower angle with increase in AlN content. It is possible that the incorporation of N atoms with a larger ionic radius into the O site in Al 2 TiO 5 structure leads to an increase in the lattice constant. Figures 2(b) and (c) showed the change of a, b, and c lattice parameters of Al 2 TiO 5 . As AlN addition increased from 0 to 10 mol %, the lattice constant values increased rapidly, whereas when the AlN content was increased further to 36 mol%, the a-axis and b-axis values showed only slightly increase, while the c-axis was nearly constant. It was assumed that the solid solution of N in Al 2 TiO 5 could be rapidly formed with the addition of AlN at 6-10 mol% after that the N solid solubility limit of Al 2 TiO 5 was almost reached at 20-36 mol% AlN. The expansion of lattice parameters of Al 2 TiO 5 was in accordance with the study of nitrogen-containing aluminum titanate reported by Perera and Bowden [22]. The Ti-Al -O-N composites produced by the reaction sintering of AlN, Al 2 O 3 , and TiO 2 at 1400-1470°C showed a phase isostructural to orthorhombic Al 2 TiO 5 but had an expanded unit cell dimension due to the replacement of oxygen by nitrogen in the unit cell [22]. In addition, the XRD patterns of ATN specimens revealed a peak position that shifted to match with the peaks of aluminum titanium oxide nitride phase (PDF no. 00-042-1279, space group Cmcm(63), orthorhombic structure). Therefore, these XRD results could confirm the solid solution of N atoms in Al 2 TiO 5 structure. Figure 3 shows the dependence of density on AlN addition for ATN composites after sintering. The theoretical density (TD) of Al 2 TiO 5 , Al 2 O 3 , TiO 2 , and TiN are 3.70 g cm −3 , 3.98 g cm −3 , 4.25 g cm −3 , and 5.40 g cm −3 , respectively [1,25]. The AT composites (0 mol% AlN) had a density close to the theoretical density of Al 2 TiO 5 due to the majority of its composition being the low-density phase Al 2 TiO 5 . With increase in the AlN content, the density tended to increase rapidly due to the development of the high-density Al 2 O 3 and TiN phases. When the AlN content was higher than 20 mol%, the density of composite exceeded the value of the theoretical density of Al 2 O 3 . When compared to the theoretical density calculated from the compositions in Table 1, AT, ATN6, and ATN10 showed relative densities in the range of 96-99% TD, while ATN20, ATN28, and ATN36 exhibited the densities of fully densified bodies with relative densities of approximately 100%TD. Reaction sintering of ATN composites involves an exothermic interaction between AlN and TiO 2 to form TiN, with energy released during the process. For the composite sintered with higher  content of AlN, the additional heat generated within the sample leads to an increase in local temperature which therefore enhances the mass transport and promotes the densification [33]. Figure 4 shows the BSE-SEM cross-sectional images of the AT and ATN composites. The microstructure of AT specimen presents a major area of Al 2 TiO 5 with distributed phases of unreacted Al 2 O 3 and TiO 2 . In addition, it is obvious that the reaction sintering of Al 2 TiO 5 without additive caused many microcracks to evolve in the body. On the other hand, the ATN composites showed a fine microstructure with no microcracks. Only a few pores were observed in ATN10, while fully densified morphologies were achieved for ATN20 and ATN36, which was in good agreement with the relative density results. With the increase in AlN, the amount of Al 2 O 3 (dark gray phase) as well as TiN (white phase) was found to have increased. Depending on the displacement reaction in Equation (4), it is likely that when TiN began to form, it became uniformly distributed in the composites. As a result, the existence of TiN in the composites limited the grain growth of Al 2 TiO 5 . Furthermore, for the ATN composites, the grain size of each phase was observed to be smaller than 1 µm. It is known that the Al 2 TiO 5 crystalline structure has an intrinsic strong anisotropic thermal expansion which leads to a microcrack formation. This problem can be lessened by the reduction of grain size. Since the small Al 2 TiO 5 grains exhibit low thermal expansion mismatch within the grains, the localized internal stress that generates microcracks is less pronounced. It was reported that the critical grain size of Al 2 TiO 5 for inhibiting microcracks is 1-2 µm [34]. This can be explained that the ATN composites fabricated with fine microstructure show lower tendency to form microcracks in Al 2 TiO 3 grains. Figure 5 shows the Vickers hardness (H V ) of ATN composites as a function of AlN addition content. The contents of the main phases (right-vertical axis), i.e. Al 2 TiO 5 , Al 2 O 3 , and TiN, are shown as a bar chart in the background of Figure 5 to demonstrate the relation of Vickers hardness on the produced phases in composites. The Vickers hardness of the AT composite (0 mol% AlN) is 5.15 ± 0.54 GPa, which is consistent with previous literature [1]. The Vickers hardness remarkably increased with increasing AlN addition and reached the highest value of 16.26 ± 1.61 GPa at 36 mol% AlN. The Vickers hardness tended to increase when the amount of Al 2 TiO 5 decreased. Furthermore, it is clearly seen that the Vickers hardness increases with the increase in Al 2 O 3 and TiN. Therefore, the improvement in hardness of ATN composites was mainly contributed from the increase in the content of Al 2 O 3 and TiN (H V = 19-26 GPa and 18-21 GPa, respectively [25,35]) which are significantly harder than Al 2 TiO 5 (H V = 5 GPa) [1]. Additionally, porosity and density are also important factors affecting the hardness of composite.

Low magnification
High magnification  The relative density of composites fabricated with 20-36 mol% AlN reached fully densified value, and the SEM images revealed that the microstructure of these composites was free of pores and microcracks. The absence of defects in the completely dense body contributed to the enhancement of the hardness of ATN composites. Figure 6 shows the fracture toughness (K IC ) of ATN composites as a function of AlN addition content. The contents of the main phases are also displayed as a bar chart in the background to show the dependence of fracture toughness on the produced phases. Similar to Vickers hardness, the fracture toughness of ATN composites was also improved by the addition of AlN. The fracture toughness increased from 2.18 ± 0.22 MPa.m 1/ 2 at 6 mol% AlN, and it appears to reach high values of 5.31 ± 0.42 and 5.20 ± 0.46 MPa.m 1/2 at 28 and 36 mol % AlN, respectively. In multiphase ceramics, the difference of elastic modulus could induce the residual stresses that cause the crack deflection [36]. The enhanced fracture toughness of ATN composite could be due to the significant difference in elastic properties of Al 2 O 3 and TiN (E = 390 GPa and 260 GPa, respectively [25,37]) compared to Al 2 TiO 5 (E = 20 GPa) [1]. In general, the fracture toughness of ceramics is inversely proportional to the hardness. However, in the ATN composites, they revealed an increase in both Vickers hardness and fracture toughness. This result suggested that the ATN consisted in a multiphase, with refined grain size and defect-free state contributing to the superior mechanical properties of composites.

Conclusions
Al 2 TiO 5 composites were fabricated from Al 2 O 3 , TiO 2 , and AlN powder mixtures by reactive sintering using SPS at 1400°C for 5 min under a pressure of 100 MPa. With an increase in AlN addition from 0 to 36 mol%, the proportion of Al 2 TiO 5 decreased, while the content of Al 2 O 3 and TiN increased. The initial reaction between TiO 2 and AlN produced TiN and Al 2 O 3 phases, then the remaining TiO 2 reacted with Al 2 O 3 to form a fine grain Al 2 TiO 5 . The lattice parameters of Al 2 TiO 5 were found to increase with AlN content, suggesting a solid solution of N atoms in the crystal structure of Al 2 TiO 5 . The microstructure observation indicated that the addition of AlN effectively produced Al 2 TiO 3 composites with small grain size, fully densified microstructure, and microcrack-free grains. The density, Vickers hardness, and fracture toughness of composites were substantially improved with AlN content due to the development of Al 2 O 3 and TiN content. At the highest AlN addition of 36 mol%, the Al 2 TiO 5 composite shows a density of 4.2 g cm −3 , a Vickers hardness of 16.26 ± 1.61 GPa, and a fracture toughness of 5.20 ± 0.46 MPa. m 1/2 .