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

Effect of AlN addition on the reaction sintering of Al2TiO5 composites fabricated by spark plasma... JOURNAL OF ASIAN CERAMIC SOCIETIES https://doi.org/10.1080/21870764.2023.2186008 Effect of AlN addition on the reaction sintering of Al TiO composites 2 5 fabricated by spark plasma sintering a,b c d e,f Mettaya Kitiwan , Duangduen Atong , Fumio Endo and Takashi Goto a b Department of Physics, School of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand; Devices and Systems for Energy and Environment Research Unit, School of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand; National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), Thailand d e Science Park, Pathum Thani, Thailand; Institute for Materials Research, Tohoku University, Sendai, Japan; New Industry Creation Hatchery Center, Tohoku University, Sendai, Japan; State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, China ABSTRACT ARTICLE HISTORY Received 16 September 2022 Fully dense Al TiO –Al O –TiN (ATN) composites were fabricated by reactive sintering using 2 5 2 3 Accepted 24 February 2023 spark plasma sintering at 1400°C for 5 min under 100 MPa in vacuum. An equimolar ratio of Al O :TiO was used as the starting powder, while the addition of 0–36 mol% AlN was investi- 3 2 KEYWORDS gated. The thermodynamic calculation indicates that the initial reaction was that of TiO and Aluminum titanate; ceramic AlN, forming TiN and Al O , and then the remaining TiO reacted with Al O to produce Al 2 3 2 2 3 2 composites; reactive TiO . With the increase in AlN precursor, Al TiO gradually decreased, while Al O and TiN sintering; spark plasma 5 2 5 2 3 increased. The lattice parameters of Al TiO were enlarged with AlN addition, implying the sintering 2 5 incorporation of N atoms in the Al TiO unit cell. The addition of AlN effectively produced fully 2 5 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 1/2 Vickers hardness and fracture toughness of 16.26 ± 1.61 GPa and 5.20 ± 0.46 MPa.m , respectively. 1. Introduction unstable at the temperature below 1280°C because it Aluminum titanate (Al TiO ) ceramics are well known decomposes into Al O and TiO [1]. 2 3 2 2 5 for their low thermal expansion and excellent thermal The thermal stability of Al TiO can be improved by 2 5 shock resistance, which are the most important char- forming a solid solution using additives such as MgO acteristics for components used in thermal cycle con- and Fe O [5–12]. The substitution of the elements 2 3 3+ 4+ ditions [1]. Al TiO can be employed in various high- from these oxides at the cation site (Al or Ti sites) 2 5 temperature applications, such as crucibles for melt- results in an increase in the free energy of thermal ing non-ferrous alloys, parts for combustion engines, decomposition of Al TiO . Fabrication of Al TiO com- 2 5 2 5 exhaust pot liners, and diesel particulate filters (DPF) posites is one of the promising methods used to [2–4]. Al TiO ceramics are typically synthesized by enhance the phase stability and strength. Some of 2 5 the solid-state reaction of Al O and TiO powders, the additives, including SiO and ZrO [9,12–14] as 2 2 2 3 2 both of which are widely available and inexpensive well as minerals like talc, feldspar, and kaolin [15,16], precursors. not only form solid solutions with Al TiO but also 2 5 Al TiO has an orthorhombic crystal structure, with react to create the dispersed phases with an effect of 2 5 3+ 4+ Al and Ti cations randomly occupied by octahedral reducing grain size. Obtaining a microstructure of fine sites surrounded by six oxygen ions. This type of crystal grain size is an important way to reduce microcracking structure possesses a highly anisotropic thermal at the grain boundary of Al TiO . Spark plasma sinter- 2 5 expansion coefficient, and as a result, microcracking ing (SPS) technique has been extensively used for initiates at the grain boundary upon cooling during the reactive sintering of nanostructured bulk materials manufacturing process. These microcracks are a key because it provides a rapid heating rate that enables factor in the low thermal expansion of Al TiO ceramics for the densification with limited grain growth [17]. 2 5 [1]. However, microcracks in an Al TiO body inevitably Therefore, SPS is an effective technique to densify Al 2 5 lead to a relatively low mechanical strength and limit TiO composites with controlled grain size. Al TiO ceramics from achieving widespread use. Furthermore, many researchers have reported that 2 5 Another major issue is that monolithic Al TiO is reaction sintering to form the Al TiO -Al O composite 2 5 2 3 2 5 CONTACT Mettaya Kitiwan mettaya.ki@kmitl.ac.th Department of Physics, School of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand © 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of The Korean Ceramic Society and The Ceramic Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent. 2 M. KITIWAN ET AL. significantly enhances the mechanical properties due a number, and the number denoted the amount of to the second-phase reinforcement [18–21]. AlN precursor. A review of the literature revealed that there were The phase analysis of sintered specimens was per- few reports of addition of nitride compounds to Al formed using X-ray diffraction (XRD; Ultima IV, Rigaku, TiO ceramics. It was reported that Al TiO with a solid Japan) in a 2θ range of 10–90° with a scanning step of 5 2 5 solution of nitrogen was prepared from a mixture of 0.02°. All samples were thoroughly ground in a mortar AlN, Al O and TiO [22]. Since the Ti-N bond is and sieved through 44-µm mesh screen to ensure 2 3 2 a stronger covalent bond than the Ti-O bond, introdu- homogeneity and avoid preferred orientation. The cing an N atom into the Al TiO structure improves the quantitative analysis of each crystalline phase was per- 2 5 phase stability and enhances the mechanical proper- formed based on Rietveld method [23] using PDXL ties of Al TiO ceramics. It was also revealed that the software. The ICDD’s Powder Diffraction File (PDF) 2 5 displacement reaction between AlN and TiO resulted was used as database for Rietveld's method. The unit in the formation of Al O and TiN phases [22]. TiN is cell lattice parameter was determined from the XRD 2 3 known for its high hardness and good corrosion resis- peak position in a 2θ range of 10–130°. To obtain tance. Therefore, incorporating this phase into Al TiO accurate lattice parameter determination, the sample 2 5 could be a promising strategy to improve the mechan- was finely ground and mixed with silicon powder ical properties. (SRM640e) as an internal standard. The apparent den- To the best of our knowledge, no research has pre- sity of the sintered specimen was measured using the viously been conducted on the effects of AlN addition Archimedes method. The microstructure was observed content on the phase formation as well as the physical using scanning electron microscopy (SEM; Hitachi and mechanical properties of Al TiO ceramics. Thus, S-3400, Japan) equipped with an energy-dispersive 2 5 the aim of this research was to investigate the fabrica- X-ray spectrometer (EDS; EAX Inc., USA). tion of Al TiO composites with the addition of AlN. Al The hardness and fracture toughness were mea- 2 5 2 TiO composites were prepared using one molar ratio sured by an indentation method under a load (P) of of Al O and TiO powders. The addition of different 19.6 N using a Vickers hardness tester (HM-221, 2 3 2 AlN contents, ranging from 0 to 36 mol%, was exam- Mitutoyo, Japan). The mechanical test was conducted ined for the first time. The Al TiO composites were with 10 indentations for each specimen to obtain the 2 5 then produced by the reaction sintering using spark average value of its standard deviation. The hardness plasma sintering (SPS) at 1400°C with a heating rate of was calculated using the Vickers hardness formula as −1 200°C min and a holding time of 5 min. The rapid shown in Equation (1), and the fracture toughness was heating rate and short holding time by SPS are the calculated using Miyoshi equation [24] as shown in important keys to achieve fine microstructure of Al Equation (2). TiO composites. The influences of AlN addition were H ¼ 1:854P=d (1) discussed in relation to the phase composition, lattice parameters, microstructures, and mechanical 1=2 3=2 K ¼0:018ðE=HÞ P=c (2) properties. IC where d is the average of two diagonal lengths of Vickers indentation, c is the half-length of the crack 2. Experimental procedures formed at the corners of indentation, and E is Young’s The raw materials for this research were Al O (alpha 2 3 modulus of composites obtained by the rule of mixture phase; 0.18 μm, Sumitomo, Japan), TiO (rutile phase; [1, 25]. 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 O :TiO 2 3 2 3. Results and discussion 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 O Figure 1 shows the XRD patterns of AT and ATN com- 2 3 and TiO . The powders were ball-milled for 24 h using posites after sintering by SPS at 1400°C. The corre- ZrO balls in ethanol. After being dried and sieved, the sponding phase compositions of AT and ATN powders were put into a graphite die that had an inner composites were analyzed by Rietveld methods, and diameter of approximately 10 mm and sintered using the results are summarized in Table 1. The XRD pattern SPS (SPS-210 LX, Fuji Electronic Industrial, Japan) at of AT specimen revealed that it mainly consisted of Al a sintering temperature of 1400°C, a holding time of TiO phase, which was formed by the solid-state reac- −1 5 min, and a heating rate of 200°C min in vacuum. tion as described in Equation (3). The minor peaks of An uniaxial pressure of 100 MPa was applied during Al O and TiO were also found in AT specimen. The 2 3 2 sintering. After sintering, the sample without AlN addi- sintered AT sample was determined to contain 77.5 wt tion was given the named AT, and the composites with % Al TiO , 20.1 wt% Al O , and 2.4 wt% TiO . 2 5 2 3 2 AlN addition were given the named ATN plus According to previous research, the reaction sintering JOURNAL OF ASIAN CERAMIC SOCIETIES 3 Figure 1. XRD patterns of AT and ATN composites after sintering by SPS at 1400°C. Table 1. Phase compositions and contents of AT and ATN composites after sintering by SPS at 1400°C. Content (mass%) Statistical indices for Rietveld refinement Samples Al TiO Al O TiN TiO R * S** 2 5 2 3 2 wp AT 77.5 20.1 0 2.4 13.54 2.74 ATN6 69.4 30.6 0 0 10.67 2.18 ATN10 43.0 52.3 4.7 0 10.21 2.71 ATN20 38.1 55.0 7.0 0 9.81 2.61 ATN28 25.0 61.8 13.2 0 9.99 2.68 ATN36 12.3 68.6 19.1 0 10.12 2.72 Residual weighted profile; **Goodness of fit. of Al TiO from equimolar ratio of Al O and TiO the peak intensity of TiN phase increased with increas- 2 5 2 3 2 powders without any additives resulted in ing AlN addition. The contents of TiN phase are 4.7, 7.0, a composite that included unreacted precursors [26– 13.2, and 19.1 wt% for the ATN10, ATN20, ATN28, and 30]. The percentage of completed reactions depended ATN36 specimens, respectively. It was reported that on many factors, such as particle size, sintering tem- the reaction between TiO and AlN proceeded in perature, sintering time, and sintering method [26–30]. many steps including reduction of TiO , formation of Although the formation of Al TiO could be enhanced intermediated Al Ti O compounds, and nitridation of 2 5 x y 5 at higher temperature and longer sintering time, these mixed oxide [31]. The progress of these steps finally would cause grain growth and decreased mechanical resulted in the formation of TiN, Al O , and N products 2 3 2 strength. as shown in Equation (4) [31]. For ATN composites, with increasing AlN content, Al O þTiO ! Al TiO (3) 2 3 2 2 5 the peak intensity of Al TiO gradually decreased but 2 5 that of Al O phase conversely increased, while AlN 2 3 1:5TiO þ2AlN ! 1:5TiNþ Al O þ0:25N (4) 2 2 3 2 and TiO phases were not detected. Furthermore, a small peak indicating the presence of TiN phase At 1400°C, the Gibbs free energy of the reaction in −1 was first discovered in ATN10 composite, and then Equation (4) is −179 kJ mol [32], whereas that of Al 2 4 M. KITIWAN ET AL. −1 TiO formation in Equation (3) is −1.319 kJ mol [26]. Therefore, the reaction between TiO and AlN initially occurred due to thermodynamically favorable condi- tions. After that, the Al TiO was produced through 2 5 the interaction of the remaining TiO with both the Al 2 2 O precursor and Al O product from Equation (4). It 3 2 3 should be noted that no TiO phase was identified in any ATN composites, suggesting that the decomposi- tion reaction of Al TiO did not occur during cool down. 2 5 It was observed that the peaks of Al TiO shifted to 2 5 a lower angle, but the peaks of Al O and TiN were 2 3 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 TiO shifts to 2 5 a lower angle with increase in AlN content. It is possi- ble that the incorporation of N atoms with a larger ionic radius into the O site in Al TiO structure leads 2 5 Figure 3. Dependence of density on AlN addition for ATN to an increase in the lattice constant. Figures 2(b) and composites fabricated by SPS at 1400°C. (c) showed the change of a, b, and c lattice parameters of Al TiO . As AlN addition increased from 0 to 10 mol 2 5 %, the lattice constant values increased rapidly, Figure 3 shows the dependence of density on AlN whereas when the AlN content was increased further addition for ATN composites after sintering. The theo- to 36 mol%, the a-axis and b-axis values showed only retical density (TD) of Al TiO , Al O , TiO , and TiN are 2 5 2 3 2 −3 −3 −3 −3 slightly increase, while the c-axis was nearly constant. It 3.70 g cm , 3.98 g cm , 4.25 g cm , and 5.40 g cm , was assumed that the solid solution of N in Al TiO respectively [1,25]. The AT composites (0 mol% AlN) 2 5 could be rapidly formed with the addition of AlN at 6– had a density close to the theoretical density of Al 10 mol% after that the N solid solubility limit of Al TiO TiO due to the majority of its composition being the 2 5 was almost reached at 20–36 mol% AlN. The expansion low-density phase Al TiO . With increase in the AlN 2 5 of lattice parameters of Al TiO was in accordance with content, the density tended to increase rapidly due 2 5 the study of nitrogen-containing aluminum titanate to the development of the high-density Al O and 2 3 reported by Perera and Bowden [22]. The Ti-Al TiN phases. When the AlN content was higher than -O-N composites produced by the reaction sintering 20 mol%, the density of composite exceeded the of AlN, Al O , and TiO at 1400–1470°C showed value of the theoretical density of Al O . When com- 2 3 2 3 2 a phase isostructural to orthorhombic Al TiO but pared to the theoretical density calculated from the 2 5 had an expanded unit cell dimension due to the repla- compositions in Table 1, AT, ATN6, and ATN10 showed cement of oxygen by nitrogen in the unit cell [22]. In relative densities in the range of 96–99% TD, while addition, the XRD patterns of ATN specimens revealed ATN20, ATN28, and ATN36 exhibited the densities of a peak position that shifted to match with the peaks of fully densified bodies with relative densities of approxi- aluminum titanium oxide nitride phase (PDF no. 00- mately 100%TD. Reaction sintering of ATN composites 042-1279, space group Cmcm(63), orthorhombic struc- involves an exothermic interaction between AlN and ture). Therefore, these XRD results could confirm the TiO to form TiN, with energy released during the solid solution of N atoms in Al TiO structure. process. For the composite sintered with higher 2 5 Figure 2. (a) The peak of the (230) plane of Al2TiO5 in at and ATN composites, and (b and c) relationship between AlN addition and the lattice parameters of Al2TiO5. JOURNAL OF ASIAN CERAMIC SOCIETIES 5 content of AlN, the additional heat generated within the sample leads to an increase in local temperature which therefore enhances the mass transport and pro- motes 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 TiO with 2 5 distributed phases of unreacted Al O and TiO . In 2 3 2 addition, it is obvious that the reaction sintering of Al TiO without additive caused many microcracks to 2 5 evolve in the body. On the other hand, the ATN com- posites showed a fine microstructure with no micro- cracks. 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, Figure 5. Dependence of Vickers hardness on AlN content for ATN composites. The bar chart shows the variation content the amount of Al O (dark gray phase) as well as TiN 2 3 (right vertical axis) of the main phases in the composites. (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 contents of the main phases (right-vertical axis), i.e. Al distributed in the composites. As a result, the existence TiO , Al O , and TiN, are shown as a bar chart in the 5 2 3 of TiN in the composites limited the grain growth of Al background of Figure 5 to demonstrate the relation of TiO . Furthermore, for the ATN composites, the grain Vickers hardness on the produced phases in compo- size of each phase was observed to be smaller than 1 sites. The Vickers hardness of the AT composite (0 mol% µm. It is known that the Al TiO crystalline structure AlN) is 5.15 ± 0.54 GPa, which is consistent with pre- 2 5 has an intrinsic strong anisotropic thermal expansion vious literature [1]. The Vickers hardness remarkably which leads to a microcrack formation. This problem increased with increasing AlN addition and reached can be lessened by the reduction of grain size. Since the highest value of 16.26 ± 1.61 GPa at 36 mol% AlN. the small Al TiO grains exhibit low thermal expansion The Vickers hardness tended to increase when the 2 5 mismatch within the grains, the localized internal amount of Al TiO decreased. Furthermore, it is clearly 2 5 stress that generates microcracks is less pronounced. seen that the Vickers hardness increases with the It was reported that the critical grain size of Al TiO for increase in Al O and TiN. Therefore, the improvement 2 5 2 3 inhibiting microcracks is 1–2 µm [34]. This can be in hardness of ATN composites was mainly contributed explained that the ATN composites fabricated with from the increase in the content of Al O and TiN (H 2 3 V fine microstructure show lower tendency to form = 19–26 GPa and 18–21 GPa, respectively [25, 35]) microcracks in Al TiO grains. which are significantly harder than Al TiO (H = 5 2 3 2 5 V Figure 5 shows the Vickers hardness (H ) of ATN GPa) [1]. Additionally, porosity and density are also composites as a function of AlN addition content. The important factors affecting the hardness of composite. AT ATN10 ATN20 ATN36 Figure 4. BSE-SEM cross-sectional images of at and ATN composites. High magnification Low magnification 6 M. KITIWAN ET AL. the proportion of Al TiO decreased, while the content 2 5 of Al O and TiN increased. The initial reaction 2 3 between TiO and AlN produced TiN and Al O phases, 2 2 3 then the remaining TiO reacted with Al O to form 2 2 3 a fine grain Al TiO . The lattice parameters of Al TiO 2 5 2 5 were found to increase with AlN content, suggesting a solid solution of N atoms in the crystal structure of Al TiO . The microstructure observation indicated that the addition of AlN effectively produced Al TiO com- 2 3 posites with small grain size, fully densified microstruc- ture, 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 O and TiN content. At the highest 2 3 AlN addition of 36 mol%, the Al TiO composite shows 2 5 −3 Figure 6. Dependence of fracture toughness on AlN content a density of 4.2 g cm , a Vickers hardness of 16.26 ± for ATN composites. The bar chart shows the variation content 1.61 GPa, and a fracture toughness of 5.20 ± 0.46 MPa. (right-vertical axis) of the main phases in the composites. 1/2 m . Acknowledgments The relative density of composites fabricated with 20– 36 mol% AlN reached fully densified value, and the SEM This study was financially supported by the Thailand images revealed that the microstructure of these com- Research Fund (TRF) under grant number MRG6280028. posites was free of pores and microcracks. The absence of defects in the completely dense body contributed to Disclosure statement the enhancement of the hardness of ATN composites. Figure 6 shows the fracture toughness (K ) of ATN IC No potential conflict of interest was reported by the authors. composites as a function of AlN addition content. The contents of the main phases are also displayed as a bar Funding chart in the background to show the dependence of fracture toughness on the produced phases. Similar to The work was supported by the Thailand Research Fund Vickers hardness, the fracture toughness of ATN com- [MRG6280028] posites was also improved by the addition of AlN. The 1/ fracture toughness increased from 2.18 ± 0.22 MPa.m ORCID at 6 mol% AlN, and it appears to reach high values of 1/2 5.31 ± 0.42 and 5.20 ± 0.46 MPa.m at 28 and 36 mol Mettaya Kitiwan http://orcid.org/0000-0002-9474-9279 % AlN, respectively. In multiphase ceramics, the differ - ence of elastic modulus could induce the residual References stresses that cause the crack deflection [36]. The enhanced fracture toughness of ATN composite could [1] Bueno S, Baudín C. Aluminum Titanate, Structure and be due to the significant difference in elastic properties Properties. In: Pomeroy M, editor. Encyclopaedia of of Al O and TiN (E = 390 GPa and 260 GPa, respec- materials: technical Ceramics and Glasses. 2 3 Amsterdam: Elsevier. 2021. pp. 76–92. tively [25,37]) compared to Al TiO (E = 20 GPa) [1]. In 2 5 [2] Maitra S, Bhattacharya S, Sil G, et al. Aluminium tita- general, the fracture toughness of ceramics is inversely nate ceramics – a review. 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Effect of AlN addition on the reaction sintering of Al2TiO5 composites fabricated by spark plasma sintering

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....
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JOURNAL OF ASIAN CERAMIC SOCIETIES https://doi.org/10.1080/21870764.2023.2186008 Effect of AlN addition on the reaction sintering of Al TiO composites 2 5 fabricated by spark plasma sintering a,b c d e,f Mettaya Kitiwan , Duangduen Atong , Fumio Endo and Takashi Goto a b Department of Physics, School of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand; Devices and Systems for Energy and Environment Research Unit, School of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand; National Metal and Materials Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), Thailand d e Science Park, Pathum Thani, Thailand; Institute for Materials Research, Tohoku University, Sendai, Japan; New Industry Creation Hatchery Center, Tohoku University, Sendai, Japan; State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, China ABSTRACT ARTICLE HISTORY Received 16 September 2022 Fully dense Al TiO –Al O –TiN (ATN) composites were fabricated by reactive sintering using 2 5 2 3 Accepted 24 February 2023 spark plasma sintering at 1400°C for 5 min under 100 MPa in vacuum. An equimolar ratio of Al O :TiO was used as the starting powder, while the addition of 0–36 mol% AlN was investi- 3 2 KEYWORDS gated. The thermodynamic calculation indicates that the initial reaction was that of TiO and Aluminum titanate; ceramic AlN, forming TiN and Al O , and then the remaining TiO reacted with Al O to produce Al 2 3 2 2 3 2 composites; reactive TiO . With the increase in AlN precursor, Al TiO gradually decreased, while Al O and TiN sintering; spark plasma 5 2 5 2 3 increased. The lattice parameters of Al TiO were enlarged with AlN addition, implying the sintering 2 5 incorporation of N atoms in the Al TiO unit cell. The addition of AlN effectively produced fully 2 5 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 1/2 Vickers hardness and fracture toughness of 16.26 ± 1.61 GPa and 5.20 ± 0.46 MPa.m , respectively. 1. Introduction unstable at the temperature below 1280°C because it Aluminum titanate (Al TiO ) ceramics are well known decomposes into Al O and TiO [1]. 2 3 2 2 5 for their low thermal expansion and excellent thermal The thermal stability of Al TiO can be improved by 2 5 shock resistance, which are the most important char- forming a solid solution using additives such as MgO acteristics for components used in thermal cycle con- and Fe O [5–12]. The substitution of the elements 2 3 3+ 4+ ditions [1]. Al TiO can be employed in various high- from these oxides at the cation site (Al or Ti sites) 2 5 temperature applications, such as crucibles for melt- results in an increase in the free energy of thermal ing non-ferrous alloys, parts for combustion engines, decomposition of Al TiO . Fabrication of Al TiO com- 2 5 2 5 exhaust pot liners, and diesel particulate filters (DPF) posites is one of the promising methods used to [2–4]. Al TiO ceramics are typically synthesized by enhance the phase stability and strength. Some of 2 5 the solid-state reaction of Al O and TiO powders, the additives, including SiO and ZrO [9,12–14] as 2 2 2 3 2 both of which are widely available and inexpensive well as minerals like talc, feldspar, and kaolin [15,16], precursors. not only form solid solutions with Al TiO but also 2 5 Al TiO has an orthorhombic crystal structure, with react to create the dispersed phases with an effect of 2 5 3+ 4+ Al and Ti cations randomly occupied by octahedral reducing grain size. Obtaining a microstructure of fine sites surrounded by six oxygen ions. This type of crystal grain size is an important way to reduce microcracking structure possesses a highly anisotropic thermal at the grain boundary of Al TiO . Spark plasma sinter- 2 5 expansion coefficient, and as a result, microcracking ing (SPS) technique has been extensively used for initiates at the grain boundary upon cooling during the reactive sintering of nanostructured bulk materials manufacturing process. These microcracks are a key because it provides a rapid heating rate that enables factor in the low thermal expansion of Al TiO ceramics for the densification with limited grain growth [17]. 2 5 [1]. However, microcracks in an Al TiO body inevitably Therefore, SPS is an effective technique to densify Al 2 5 lead to a relatively low mechanical strength and limit TiO composites with controlled grain size. Al TiO ceramics from achieving widespread use. Furthermore, many researchers have reported that 2 5 Another major issue is that monolithic Al TiO is reaction sintering to form the Al TiO -Al O composite 2 5 2 3 2 5 CONTACT Mettaya Kitiwan mettaya.ki@kmitl.ac.th Department of Physics, School of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok, Thailand © 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of The Korean Ceramic Society and The Ceramic Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent. 2 M. KITIWAN ET AL. significantly enhances the mechanical properties due a number, and the number denoted the amount of to the second-phase reinforcement [18–21]. AlN precursor. A review of the literature revealed that there were The phase analysis of sintered specimens was per- few reports of addition of nitride compounds to Al formed using X-ray diffraction (XRD; Ultima IV, Rigaku, TiO ceramics. It was reported that Al TiO with a solid Japan) in a 2θ range of 10–90° with a scanning step of 5 2 5 solution of nitrogen was prepared from a mixture of 0.02°. All samples were thoroughly ground in a mortar AlN, Al O and TiO [22]. Since the Ti-N bond is and sieved through 44-µm mesh screen to ensure 2 3 2 a stronger covalent bond than the Ti-O bond, introdu- homogeneity and avoid preferred orientation. The cing an N atom into the Al TiO structure improves the quantitative analysis of each crystalline phase was per- 2 5 phase stability and enhances the mechanical proper- formed based on Rietveld method [23] using PDXL ties of Al TiO ceramics. It was also revealed that the software. The ICDD’s Powder Diffraction File (PDF) 2 5 displacement reaction between AlN and TiO resulted was used as database for Rietveld's method. The unit in the formation of Al O and TiN phases [22]. TiN is cell lattice parameter was determined from the XRD 2 3 known for its high hardness and good corrosion resis- peak position in a 2θ range of 10–130°. To obtain tance. Therefore, incorporating this phase into Al TiO accurate lattice parameter determination, the sample 2 5 could be a promising strategy to improve the mechan- was finely ground and mixed with silicon powder ical properties. (SRM640e) as an internal standard. The apparent den- To the best of our knowledge, no research has pre- sity of the sintered specimen was measured using the viously been conducted on the effects of AlN addition Archimedes method. The microstructure was observed content on the phase formation as well as the physical using scanning electron microscopy (SEM; Hitachi and mechanical properties of Al TiO ceramics. Thus, S-3400, Japan) equipped with an energy-dispersive 2 5 the aim of this research was to investigate the fabrica- X-ray spectrometer (EDS; EAX Inc., USA). tion of Al TiO composites with the addition of AlN. Al The hardness and fracture toughness were mea- 2 5 2 TiO composites were prepared using one molar ratio sured by an indentation method under a load (P) of of Al O and TiO powders. The addition of different 19.6 N using a Vickers hardness tester (HM-221, 2 3 2 AlN contents, ranging from 0 to 36 mol%, was exam- Mitutoyo, Japan). The mechanical test was conducted ined for the first time. The Al TiO composites were with 10 indentations for each specimen to obtain the 2 5 then produced by the reaction sintering using spark average value of its standard deviation. The hardness plasma sintering (SPS) at 1400°C with a heating rate of was calculated using the Vickers hardness formula as −1 200°C min and a holding time of 5 min. The rapid shown in Equation (1), and the fracture toughness was heating rate and short holding time by SPS are the calculated using Miyoshi equation [24] as shown in important keys to achieve fine microstructure of Al Equation (2). TiO composites. The influences of AlN addition were H ¼ 1:854P=d (1) discussed in relation to the phase composition, lattice parameters, microstructures, and mechanical 1=2 3=2 K ¼0:018ðE=HÞ P=c (2) properties. IC where d is the average of two diagonal lengths of Vickers indentation, c is the half-length of the crack 2. Experimental procedures formed at the corners of indentation, and E is Young’s The raw materials for this research were Al O (alpha 2 3 modulus of composites obtained by the rule of mixture phase; 0.18 μm, Sumitomo, Japan), TiO (rutile phase; [1, 25]. 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 O :TiO 2 3 2 3. Results and discussion 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 O Figure 1 shows the XRD patterns of AT and ATN com- 2 3 and TiO . The powders were ball-milled for 24 h using posites after sintering by SPS at 1400°C. The corre- ZrO balls in ethanol. After being dried and sieved, the sponding phase compositions of AT and ATN powders were put into a graphite die that had an inner composites were analyzed by Rietveld methods, and diameter of approximately 10 mm and sintered using the results are summarized in Table 1. The XRD pattern SPS (SPS-210 LX, Fuji Electronic Industrial, Japan) at of AT specimen revealed that it mainly consisted of Al a sintering temperature of 1400°C, a holding time of TiO phase, which was formed by the solid-state reac- −1 5 min, and a heating rate of 200°C min in vacuum. tion as described in Equation (3). The minor peaks of An uniaxial pressure of 100 MPa was applied during Al O and TiO were also found in AT specimen. The 2 3 2 sintering. After sintering, the sample without AlN addi- sintered AT sample was determined to contain 77.5 wt tion was given the named AT, and the composites with % Al TiO , 20.1 wt% Al O , and 2.4 wt% TiO . 2 5 2 3 2 AlN addition were given the named ATN plus According to previous research, the reaction sintering JOURNAL OF ASIAN CERAMIC SOCIETIES 3 Figure 1. XRD patterns of AT and ATN composites after sintering by SPS at 1400°C. Table 1. Phase compositions and contents of AT and ATN composites after sintering by SPS at 1400°C. Content (mass%) Statistical indices for Rietveld refinement Samples Al TiO Al O TiN TiO R * S** 2 5 2 3 2 wp AT 77.5 20.1 0 2.4 13.54 2.74 ATN6 69.4 30.6 0 0 10.67 2.18 ATN10 43.0 52.3 4.7 0 10.21 2.71 ATN20 38.1 55.0 7.0 0 9.81 2.61 ATN28 25.0 61.8 13.2 0 9.99 2.68 ATN36 12.3 68.6 19.1 0 10.12 2.72 Residual weighted profile; **Goodness of fit. of Al TiO from equimolar ratio of Al O and TiO the peak intensity of TiN phase increased with increas- 2 5 2 3 2 powders without any additives resulted in ing AlN addition. The contents of TiN phase are 4.7, 7.0, a composite that included unreacted precursors [26– 13.2, and 19.1 wt% for the ATN10, ATN20, ATN28, and 30]. The percentage of completed reactions depended ATN36 specimens, respectively. It was reported that on many factors, such as particle size, sintering tem- the reaction between TiO and AlN proceeded in perature, sintering time, and sintering method [26–30]. many steps including reduction of TiO , formation of Although the formation of Al TiO could be enhanced intermediated Al Ti O compounds, and nitridation of 2 5 x y 5 at higher temperature and longer sintering time, these mixed oxide [31]. The progress of these steps finally would cause grain growth and decreased mechanical resulted in the formation of TiN, Al O , and N products 2 3 2 strength. as shown in Equation (4) [31]. For ATN composites, with increasing AlN content, Al O þTiO ! Al TiO (3) 2 3 2 2 5 the peak intensity of Al TiO gradually decreased but 2 5 that of Al O phase conversely increased, while AlN 2 3 1:5TiO þ2AlN ! 1:5TiNþ Al O þ0:25N (4) 2 2 3 2 and TiO phases were not detected. Furthermore, a small peak indicating the presence of TiN phase At 1400°C, the Gibbs free energy of the reaction in −1 was first discovered in ATN10 composite, and then Equation (4) is −179 kJ mol [32], whereas that of Al 2 4 M. KITIWAN ET AL. −1 TiO formation in Equation (3) is −1.319 kJ mol [26]. Therefore, the reaction between TiO and AlN initially occurred due to thermodynamically favorable condi- tions. After that, the Al TiO was produced through 2 5 the interaction of the remaining TiO with both the Al 2 2 O precursor and Al O product from Equation (4). It 3 2 3 should be noted that no TiO phase was identified in any ATN composites, suggesting that the decomposi- tion reaction of Al TiO did not occur during cool down. 2 5 It was observed that the peaks of Al TiO shifted to 2 5 a lower angle, but the peaks of Al O and TiN were 2 3 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 TiO shifts to 2 5 a lower angle with increase in AlN content. It is possi- ble that the incorporation of N atoms with a larger ionic radius into the O site in Al TiO structure leads 2 5 Figure 3. Dependence of density on AlN addition for ATN to an increase in the lattice constant. Figures 2(b) and composites fabricated by SPS at 1400°C. (c) showed the change of a, b, and c lattice parameters of Al TiO . As AlN addition increased from 0 to 10 mol 2 5 %, the lattice constant values increased rapidly, Figure 3 shows the dependence of density on AlN whereas when the AlN content was increased further addition for ATN composites after sintering. The theo- to 36 mol%, the a-axis and b-axis values showed only retical density (TD) of Al TiO , Al O , TiO , and TiN are 2 5 2 3 2 −3 −3 −3 −3 slightly increase, while the c-axis was nearly constant. It 3.70 g cm , 3.98 g cm , 4.25 g cm , and 5.40 g cm , was assumed that the solid solution of N in Al TiO respectively [1,25]. The AT composites (0 mol% AlN) 2 5 could be rapidly formed with the addition of AlN at 6– had a density close to the theoretical density of Al 10 mol% after that the N solid solubility limit of Al TiO TiO due to the majority of its composition being the 2 5 was almost reached at 20–36 mol% AlN. The expansion low-density phase Al TiO . With increase in the AlN 2 5 of lattice parameters of Al TiO was in accordance with content, the density tended to increase rapidly due 2 5 the study of nitrogen-containing aluminum titanate to the development of the high-density Al O and 2 3 reported by Perera and Bowden [22]. The Ti-Al TiN phases. When the AlN content was higher than -O-N composites produced by the reaction sintering 20 mol%, the density of composite exceeded the of AlN, Al O , and TiO at 1400–1470°C showed value of the theoretical density of Al O . When com- 2 3 2 3 2 a phase isostructural to orthorhombic Al TiO but pared to the theoretical density calculated from the 2 5 had an expanded unit cell dimension due to the repla- compositions in Table 1, AT, ATN6, and ATN10 showed cement of oxygen by nitrogen in the unit cell [22]. In relative densities in the range of 96–99% TD, while addition, the XRD patterns of ATN specimens revealed ATN20, ATN28, and ATN36 exhibited the densities of a peak position that shifted to match with the peaks of fully densified bodies with relative densities of approxi- aluminum titanium oxide nitride phase (PDF no. 00- mately 100%TD. Reaction sintering of ATN composites 042-1279, space group Cmcm(63), orthorhombic struc- involves an exothermic interaction between AlN and ture). Therefore, these XRD results could confirm the TiO to form TiN, with energy released during the solid solution of N atoms in Al TiO structure. process. For the composite sintered with higher 2 5 Figure 2. (a) The peak of the (230) plane of Al2TiO5 in at and ATN composites, and (b and c) relationship between AlN addition and the lattice parameters of Al2TiO5. JOURNAL OF ASIAN CERAMIC SOCIETIES 5 content of AlN, the additional heat generated within the sample leads to an increase in local temperature which therefore enhances the mass transport and pro- motes 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 TiO with 2 5 distributed phases of unreacted Al O and TiO . In 2 3 2 addition, it is obvious that the reaction sintering of Al TiO without additive caused many microcracks to 2 5 evolve in the body. On the other hand, the ATN com- posites showed a fine microstructure with no micro- cracks. 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, Figure 5. Dependence of Vickers hardness on AlN content for ATN composites. The bar chart shows the variation content the amount of Al O (dark gray phase) as well as TiN 2 3 (right vertical axis) of the main phases in the composites. (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 contents of the main phases (right-vertical axis), i.e. Al distributed in the composites. As a result, the existence TiO , Al O , and TiN, are shown as a bar chart in the 5 2 3 of TiN in the composites limited the grain growth of Al background of Figure 5 to demonstrate the relation of TiO . Furthermore, for the ATN composites, the grain Vickers hardness on the produced phases in compo- size of each phase was observed to be smaller than 1 sites. The Vickers hardness of the AT composite (0 mol% µm. It is known that the Al TiO crystalline structure AlN) is 5.15 ± 0.54 GPa, which is consistent with pre- 2 5 has an intrinsic strong anisotropic thermal expansion vious literature [1]. The Vickers hardness remarkably which leads to a microcrack formation. This problem increased with increasing AlN addition and reached can be lessened by the reduction of grain size. Since the highest value of 16.26 ± 1.61 GPa at 36 mol% AlN. the small Al TiO grains exhibit low thermal expansion The Vickers hardness tended to increase when the 2 5 mismatch within the grains, the localized internal amount of Al TiO decreased. Furthermore, it is clearly 2 5 stress that generates microcracks is less pronounced. seen that the Vickers hardness increases with the It was reported that the critical grain size of Al TiO for increase in Al O and TiN. Therefore, the improvement 2 5 2 3 inhibiting microcracks is 1–2 µm [34]. This can be in hardness of ATN composites was mainly contributed explained that the ATN composites fabricated with from the increase in the content of Al O and TiN (H 2 3 V fine microstructure show lower tendency to form = 19–26 GPa and 18–21 GPa, respectively [25, 35]) microcracks in Al TiO grains. which are significantly harder than Al TiO (H = 5 2 3 2 5 V Figure 5 shows the Vickers hardness (H ) of ATN GPa) [1]. Additionally, porosity and density are also composites as a function of AlN addition content. The important factors affecting the hardness of composite. AT ATN10 ATN20 ATN36 Figure 4. BSE-SEM cross-sectional images of at and ATN composites. High magnification Low magnification 6 M. KITIWAN ET AL. the proportion of Al TiO decreased, while the content 2 5 of Al O and TiN increased. The initial reaction 2 3 between TiO and AlN produced TiN and Al O phases, 2 2 3 then the remaining TiO reacted with Al O to form 2 2 3 a fine grain Al TiO . The lattice parameters of Al TiO 2 5 2 5 were found to increase with AlN content, suggesting a solid solution of N atoms in the crystal structure of Al TiO . The microstructure observation indicated that the addition of AlN effectively produced Al TiO com- 2 3 posites with small grain size, fully densified microstruc- ture, 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 O and TiN content. At the highest 2 3 AlN addition of 36 mol%, the Al TiO composite shows 2 5 −3 Figure 6. Dependence of fracture toughness on AlN content a density of 4.2 g cm , a Vickers hardness of 16.26 ± for ATN composites. The bar chart shows the variation content 1.61 GPa, and a fracture toughness of 5.20 ± 0.46 MPa. (right-vertical axis) of the main phases in the composites. 1/2 m . Acknowledgments The relative density of composites fabricated with 20– 36 mol% AlN reached fully densified value, and the SEM This study was financially supported by the Thailand images revealed that the microstructure of these com- Research Fund (TRF) under grant number MRG6280028. posites was free of pores and microcracks. The absence of defects in the completely dense body contributed to Disclosure statement the enhancement of the hardness of ATN composites. Figure 6 shows the fracture toughness (K ) of ATN IC No potential conflict of interest was reported by the authors. composites as a function of AlN addition content. The contents of the main phases are also displayed as a bar Funding chart in the background to show the dependence of fracture toughness on the produced phases. Similar to The work was supported by the Thailand Research Fund Vickers hardness, the fracture toughness of ATN com- [MRG6280028] posites was also improved by the addition of AlN. The 1/ fracture toughness increased from 2.18 ± 0.22 MPa.m ORCID at 6 mol% AlN, and it appears to reach high values of 1/2 5.31 ± 0.42 and 5.20 ± 0.46 MPa.m at 28 and 36 mol Mettaya Kitiwan http://orcid.org/0000-0002-9474-9279 % AlN, respectively. In multiphase ceramics, the differ - ence of elastic modulus could induce the residual References stresses that cause the crack deflection [36]. The enhanced fracture toughness of ATN composite could [1] Bueno S, Baudín C. Aluminum Titanate, Structure and be due to the significant difference in elastic properties Properties. In: Pomeroy M, editor. Encyclopaedia of of Al O and TiN (E = 390 GPa and 260 GPa, respec- materials: technical Ceramics and Glasses. 2 3 Amsterdam: Elsevier. 2021. pp. 76–92. tively [25,37]) compared to Al TiO (E = 20 GPa) [1]. In 2 5 [2] Maitra S, Bhattacharya S, Sil G, et al. Aluminium tita- general, the fracture toughness of ceramics is inversely nate ceramics – a review. 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Journal

Journal of Asian Ceramic SocietiesTaylor & Francis

Published: Apr 3, 2023

Keywords: Aluminum titanate; ceramic composites; reactive sintering; spark plasma sintering

References