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Mechanical properties of Al2O3\Ti composites fabricated by spark plasma sintering
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 4637–4643 www.elsevier.com/locate/ceramint
Mechanical properties of Al2O3\Ti composites fabricated by spark plasma sintering S. Meir, S. Kalabukhov, N. Frage, S. Hayunn Department of Materials Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel Received 25 November 2014; received in revised form 1 December 2014; accepted 1 December 2014 Available online 9 December 2014
Abstract Al2O3\Ti composites presenting a range of metal to ceramic ratios were fabricated using SPS technology and subsequently characterized. A titanium hydride that decomposes in the 600–700 1C temperate range was used as the source of Ti. The composite densification process was initiated at 825 1C, and reached a maximum densification rate at 1150 1C. Microstructural analysis revealed the homogenous distribution of submicron-sized Al2O3 grains and micron-sized Ti grains. Thus, the fine microstructure of the composites provided improvements in hardness, Young's moduli and flexure strengths. The composites displayed lower resistance to fracture than did pure alumina, with cracks largely propagating around the alumina grains, which indicates that the ceramic–metal interface is weak. Finally, the formation of a Ti–Al–O solid solution at the metal/ceramic interface was detected by scanning electron microscopy and X-ray diffraction. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Al2O3\Ti composites; Cermet; Homogeneous; Micron
1. Introduction The fracture toughness of ceramics can be improved by the incorporation of ductile metal particles into the brittle ceramic matrix. For example, composites comprising alumina and metallic particles, such as aluminum, nickel or chromium, display improvements in specific strength, fracture toughness and wear resistance, relative to the ceramic alone [1–3]. In the case of alumina/titanium composites, only limited information is available. While MasGuindal et al. [4] reported on nano-structured alumina–titanium composite fabrication by self-propagating high temperature synthesis (SHS), the physical and mechanical properties of the composites were not discussed. Edalati et al. [5] described how titanium-based nano-composites containing 20 wt% Al2O3 with elevated hardness value were generated by applying severe plastic deformation to the powder mixtures. Two other reports [6,7] discussed the fabrication of Al2O3/Ti composites using the spark plasma sintering (SPS) approach. Atiyah et al. [6] did not reach full n
Corresponding author. Tel.: þ972 8 6428742; fax: þ 972 8 6428744. E-mail address: [email protected] (S. Hayun).
http://dx.doi.org/10.1016/j.ceramint.2014.12.008 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
densification of the specimens at 1500 1C under 17 MPa and, moreover, did not consider the mechanical properties of the composites. The mechanical properties of the alumina–titanium composite with 72.5 wt% Al2O3 were, however, presented in Ref. [7], where the hardness and flexural strength of the composites were reported as being about 150 GPa and 350 MPa, respectively. Furthermore, one of the goals of this earlier study was to achieve adequate particle homogeneity distribution, which in turn can effect favorably on the mechanical properties [8]. It had been established that addition of 3 wt% of Dolapix CE64 surfactant combined with milling zirconia balls in polyethylene containers at 150 r.p.m. for 24 h led to homogeneous particle distribution. It should be noted that coarse titanium particles (about 150 μm in diameter) were used [7]. When fine titanium particles were used for composite fabrication, an additional problem arose related to the fact that a thin oxide layer covers such particles. This layer affects the sintering behavior of the powder mixture and promotes the formation of uncontrolled and unfavorable phases at the alumina/ titanium interface. This reaction, including its formation kinetics and thermodynamics, has been discussed in detail in studies where
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interactions between alumina and titanium layers were investigated [9–14]. In these studies, almost all possible phases of the Al–Ti–O ternary phase diagrams were identified at the interface using X-ray diffraction and electron probe-based analysis of the reactive layers. Such phases included TiAl, Ti3Al intermetallic compounds, titanium oxide and a Ti–Al–O solid solution. The presence of these phases at the metal/ceramic interface in composites weakens grain boundaries and leads to decreased mechanical properties. To prevent the massive formation of these undesired phases, titanium hydride powder (instead of Ti powder) was used in the present work as a starting material for mixture preparation. Titanium hydride and alumina display very close specific density values (about 3.9 g/cm3) and can be blended to achieve a homogenous mixture with relative ease. As hydride decomposition takes place at a relatively low temperature under the reducing atmosphere that exists within the SPS apparatus, this approach allows for avoiding oxidation of the Ti particles. Composites presenting different degrees of Ti content ranging from pure alumina to pure Ti were fabricated using the SPS approach. Their mechanical and physical properties were tested and are discussed here. 2. Experimental procedures 2.1. Starting materials and experimental setup Alumina (Ceralox, high purity SPA-0.5, 0.5 mm) and titanium hydride (Alfa Aesar, 99% metal basis, 325 mesh) powders were blended in a planetary ball mill inside a polyethylene container for three days with ethanol and alumina milling balls. Powder mixtures of Al2O3 with 0, 20, 40, 60, 80 and 100 wt% titanium hydride were inserted into a graphite die/punch setup (inner diameter, 20 mm; outer diameter, 40 mm). The die was covered with a 20 mm-thick layer of graphite wool to improve thermal insulation. The die was placed into the SPS apparatus (HP D5/1, FCT System. Rauenstein, Germany) equipped with a 50 kN uniaxial press. The applied pressure was increased to 40 MPa over the first three minutes of the process, was raised to 64 MPa after an additional ten minutes and held constant until the end of the process. The sintering procedure was conducted at 1300 1C under a vacuum of about 1 Torr and 64 MPa of uniaxial pressure. The heating rate was 25 1C/min and the holding time at 1300 1C was 30 min (Fig. 1). 2.2. Characterization Phase composition was determined by X-ray diffraction (XRD) using a Rigaku RINT 2100 (Tokyo, Japan) diffractometer with Cu Kα radiation. The operating parameters were 40 kV and 40 mA, with a 2θ step size of 0.021. The XRD patterns were analyzed using a whole pattern fitting approach with the MDI Jade 2010 software (MDI, Livermore, CA). The surface areas of the raw and mixed powders were measured by the Brunauer–Emmett–Teller (BET) method using a Micromeritics ASAP 2020 (Micromeritics, Norcross, GA) instrument. Ten-point adsorption isotherms of nitrogen were collected in the 0.05–0.30 relative pressure range (P/P0, where
Fig. 1. SPS regime used for composite sintering.
P0 is the saturation pressure) at 196 1C. Prior to analysis, the alumina sample was vacuum-degassed at 400 1C for 4 h and the TiH2-containing samples were vacuum-degassed at 100 1C for 4 h to avoid any hydride decomposition. Simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) were carried out using a Netzsch STA 449 F3 system (Netzsch, Selb, Germany). The sample was heated at a rate of 10 1C/min in an alumina crucible from 30 to 1000 1C under argon. A buoyancy correction was introduced by subtracting the baseline value collected upon running a blank TGA/DSC assay with an empty alumina crucible. Microstructural analysis was conducted on polished specimens using a scanning electron microscopy (SEM, JEOL-35, Tokyo, Japan) equipped with the Noran energy dispersive spectrometer (EDS). Samples for metallographic characterization were prepared using a standard procedure that included a final stage of polishing with 3 mm diamond paste. The velocities of the longitudinal C l and shear C s acoustic waves were measured by a pulse-echo technique using a 5 MHz probe. The elastic modulus was derived from the ultrasonic velocity data and density values, measured by the liquid displacement method in deionized water. SPS-processed composite hardness was determined using a Buehler-Micromet 2100 micro-hardness tester (Lake Bluff, IL) with a Vickers indenter under 20 N loads. The lengths of the cracks that appeared at corners of the Vickers indentations were used for evaluating fracture toughness (Kc). Empirical equations for fracture toughness estimation depend on the nature of the cracks [15]. Two common types of cracks (Palmqvist and half penny) were considered. To identify the nature of the crack system, a polishing procedure was performed after indention. Flexural strength was determined using a three point bending test on 1.5 mm 2 mm 10 mm bars that were polished after cutting using an LRX Plus apparatus (Lloyd Instruments, Fareham Hants, U.K.). Five specimens for each sample were tested. 3. Results and discussion SEM analysis of TiH2 powder (Fig. 2) and Al2O3 þ TiH2 mixtures after 72 h of mixing (Fig. 2b) indicated that TiH2
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Fig. 2. SEM micrographs of TiH2 as received (a), and of alumina and TiH2 powders after mixing (b).
Fig. 3. Typical densification and densification rate curves as a function of time for the powder mixture containing 80 wt% Al2O3.
particles were crushed and homogeneously dispersed within the Al2O3 powder. The measured surface areas of the initial powders were 8.76 7 0.02 and 0.047 0.01 m2/g for Al2O3 and TiH2, respectively. The specific surface area of the 60 wt% Al2O3 mixture was 5.48 7 0.02 m2/g. Assuming a spherical shape for the TiH2 particles and taking into account the surface area of the initial Al2O3 powder, the average TiH2 particle size in the mixture was estimated to be about 3 μm. The calculated particle size is in a good agreement with the values observed by SEM (Fig. 2b).
Fig. 4. Gas emission from a specimen during sintering.
3.1. Sintering behavior Evaluation of the consolidation of the Al2O3–TiH2 powder mixture during the SPS process was carried out using the curve of punches displacement as a function of time and its derivative (Fig. 3). The data presented in Fig. 3 correspond to the sintering of a mixture containing 80 wt% Al2O3. Densification started at 825 1C, with the maximum densification rate being reached at 1120 1C (Fig. 3, peak 2). During holding at the sintering temperature, the densification rate slowly decreased, reaching zero after 10 min of soaking. Nevertheless, the relative density of the specimen at this point was about 96% of the density of Al2O3. Almost complete densification was achieved after an additional 25 min of soaking. The apparent densification, taking up to 15 min (Fig. 3, dL/dt, peak 1), is usually related to compaction of the powder
Fig. 5. Thermo-gravimetric analysis and differential scanning calorimetric measurements of TiH2 powder.
by applied pressure, which rises gradually to a maximal value of 64 MPa (Fig. 1). When comparing the dL/dt curve to similar curves from previous studies on Al2O3/TiC composites [16], the shape of peak 1 was quite unusual. In the case where only mechanical compaction occurred, the densification rate should
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parallel the speed of the applied force and remain constant. The consolidation behavior around 670 1C (peak 1) indicates that an additional process affects displacement of the punches, namely hydrogen released from the powder causes negative die displacement. Examination of the process-recorded data revealed variations in the gaseous pressure within the SPS chamber (Fig. 4). The amount of the gas released as a result
of TiH2 decomposition correlated with the fraction of TiH2 in the mixture. To clarify this phenomenon, thermo-gravimetric analysis of TiH2 decomposition was conducted. The decomposition process occurred gradually in two steps (Fig. 5). The first step of decomposition began at about 466 1C, while the second was initiated at 578 1C. The total mass loss was 4 wt% after heating up to 1000 1C; this value corresponds to the hydrogen content of the hydride. The sintering behaviors of the Al2O3/Ti composites presenting various compositions are presented in Fig. 6. The densification process of the Al2O3/Ti composites strongly depended on the composition of the mixture (Fig. 6). Pure titanium reached full density at about 850 1C, while pure alumina only reached 95% density at 1300 1C. Fully dense alumina was obtained after 35 min of holding at this temperature. The results presented in Fig. 6 are thus important for the fabrication of graded alumina– titanium composites by SPS. The measured densities of the final composites were slightly higher than those calculated based on densities of Ti and Al2O3 alone (Table 1). These differences can be attributed to changes in phase composition during the SPS process.
3.2. Phase composition and microstructure Fig. 6. Composite specimen compression as a function of temperature.
Table 1 Calculated and measured densities of the SPS-processed composites. Al2O3 (wt%)
Calculated density (g/cm3)
Measured density (g/cm3)7 0.02
0 20 40 60 80 100
4.50 4.38 4.26 4.15 4.05 3.95
4.55 4.48 4.39 4.25 4.11 3.95
XRD analysis of the SPS-processed composites revealed that the composites consisted of Al2O3 and α-Ti solid solutions. No peaks of TiH2 were detected (Fig. 7a). Nevertheless, the position peaks corresponding to α-Ti were shifted from those positions indicative of pure titanium (Fig. 7b). Reasons for such shifts include alumina decomposition and dissolution of alumina and oxygen in titanium. The results of EDS analysis (Fig. 8) support this suggestion, with the formation of α-Ti[Al,O] solid solution being detected [12]. The microstructure of the composites indicates homogeneous distribution of the phases (Fig. 9). The dark gray spots in Fig. 9A and B are the result of uneven gold deposition during sample preparation for SEM characterization.
Fig. 7. (A) XRD patterns of titanium and the four alumina–titanium composites, processed by SPS. (B) Comparison of the titanium peaks, with and without alumina.
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Fig. 8. (A) SEM image with EDS line. (B) EDS counts of aluminum and titanium along the line shown in (A).
Fig. 9. SEM micrographs of SPS-processed composites of various alumina contents, wt% (a) 20, (b) 40, (c) 60 and (d) 80 Al2O3. White grains correspond to titanium.
3.3. Mechanical properties The measured Young's modulus and hardness (Fig. 10) increased monotonically with alumina content. For instance, the hardness value of the composite containing 80 wt% Al2O3 was about 19.7 GPa and is higher (15 GPa) than the hardness value reported previously [7] for a composite of the same
composition. This difference may be related to the coarse Ti particles used in the earlier study, which were two orders of magnitude larger than those used here. Relatively high hardness values were observed for the titanium-rich composites. This may be related to the formation of the Τi–Al–O solid solution. The flexural strength values (Fig. 10) displayed maximum (767 7 23 MPa) for the 80 wt% Al2O3 composite. This value
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is more than twice that was reported in an earlier study [7]. The fracture toughness of the composites, KC, was calculated from the lengths of the cracks that appeared at the corners of the Vickers indentation. Optical micrographs of the indentation vicinity collected after subsequent polishing (Fig. 11b)
Fig. 10. Hardness and Young's modulus as a function of alumina content.
indicated that the nature of the cracks corresponded to a Palmqvist crack system (Fig. 12). As previously suggested [15], KC values were calculated according to Eq. (1) 0:4 0:5 K C ¼ 0:0264ðH V aÞ E=H V l ð1Þ where HV is the Vickers hardness, E is Young's modulus, a is the half length of the Vickers indentation diagonal line, and l is the length of the crack from the tip of the indentation. Unfortunately, the composites displayed lower resistances to fracture than did pure alumina (Fig. 11). The reason for this may be clarified by analysis of the crack propagation path. Accordingly, the SEM image of the polished and etched crosssection of the specimen containing 40 wt% Al2O3, with a crack that originated from a Vickers indentation was considered. The crack propagation clearly shows that the crack crossed the titanium matrix and appeared around the alumina grains (Fig. 13). This could indicate that the ceramic–metal interface is weaker than the Al2O3 particles are, as reported previously [11]. The composites, consisting of components with different coefficients of thermal expansion (CTE), can exhibit spontaneous micro-cracking upon cooling. Crack propagation along the grain boundaries depends on grain size [17], with a “critical” grain size below which cracks are not observed. As previously proposed [17], “critical” grain size is given by Eq. (2) dcrit ¼
Fig. 11. Flexural strength of the specimens as a function of alumina content.
14:4γ EΔα2 max ΔT 2
ð2Þ
where E is the larger Young's modulus (E), Δαmax is the maximum difference between the CTEs of the individual components, ΔT is the difference between the sintering and room temperatures, and γ is the surface energy of the newly formed surfaces. Using the value of Young's modulus for alumina (410 MPa), a Δαmax of 1.5 10 6/1C, a ΔT equal to 1250 1C and a γ of 1 J/m2, the critical grain size was calculated to be 11 μm. This value is higher than the grain size observed in the SPS-fabricated composites, yet is significantly lower than the grain size (150 μm) of the composites fabricated by GutierrezGonzalez et al. [7]. The micro-cracks that were observed in the composites and which present relatively low mechanical properties could be attributed to coarse microstructure.
Fig. 12. Optical micrographs of a Vickers imprint before (a) and after (b) polishing. The gap between the crack and the imprint indicates a Palmqvist type crack.
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References
Fig. 13. Crack propagation path in a composite with 40 wt% Al2O3.
4. Summary Dense Al2O3/Ti composites, fabricated using TiH2, were sintered using SPS at 1300 1C for 30 min. Microstructural analysis revealed the homogenous distribution of titanium and alumina grains in the composites. Young's modulus and hardness values increased monotonically, while fracture toughness decreased with the magnitude of the alumina fraction in the composites. The cracks that originated from a Vickers indentation easily crossed the titanium matrix, propagated mostly around the alumina grains, reflective of the ceramic– metal interface being weak. Nevertheless, the fine microstructure of the composites prevents spontaneous micro-cracking upon cooling. Acknowledgments This work was partially supported by the Israel Ministry of Defense (Grant no. 4440400102) and the FP7-PEOPLE-2012CIG Grant no. 321838-EEEF-GBE-CNS. S. Meir thanks the Kreitman School of Advanced Graduate Studies, and Ben-Gurion University of the Negev for a postdoctoral scholarship.
[1] B.D. Flinn, M. Ruhle, A.G. Evans, Toughening in composites of Al2O3 reinforced with Al, Acta Metall. Mater. 37 (1989) 3001–3006. [2] C. Marcin, P. Katarzyna, Processing, microstructure and mechanical properties of Al2O3–Cr nanocomposites, J. Eur. Ceram. Soc. 27 (2007) 1273–1279. [3] V. Udhayabanu, K.R. Ravi, K. Murugan, D. Sivaprahasam, B.S. Murty, Development of Ni–Al2O3 in-situ nanocomposite by reactive milling and spark plasma sintering, Metall. Mater. Trans. A 42 (2011) 2085–2093. [4] M.J. Mas-Guindal, E. Benko, M.A. Rodrıguez, Nanostructured metastable cermets of Ti–Al2O3 through activated SHS reaction, J. Alloy. Compd. 454 (2008) 352–358. [5] K. Edalati, H. Iwaokaa, Z. Horitaa, M. Konnoc, T. Satoc, Unusual hardening in Ti/Al2O3 nanocomposites produced by high-pressure torsion followed by annealing, Mater. Sci. Eng. A 529 (2011) 435–444. [6] A.A. Atiyah, S.B.H. Farid, D.N. Abdulamer, Fabrication of ceramic– metal functionally graded materials, Eng. Technol. J. 31 (2013) 513–525. [7] C.F. Gutierrez-Gonzalez, E. Fernandez-Garcia, A. Fernandez, R. Torrecillas, S. Lopez-Esteban, Processing, spark plasma sintering, and mechanical behavior of alumina/titanium composites, J. Mater. Sci. 49 (2014) 3823–3830. [8] J.S. Moya, S. Lopez-Esteban, C. Pecharromán, The challenge of ceramic/ metal microcomposites and nanocomposites, Prog. Mater. Sci. 52 (2007) 1017–1090. [9] G.P. Kelkar, A.H. Carim, Phase equilibria in the Ti–Al–O system at 945 1C and analysis of Ti/Al2O3 reactions, J. Am. Ceram. Soc. 78 (1995) 572–576. [10] G. Braichotte, G. Cizeron, Sintering of (aluminaþ titanium) powder mixtures and elaboration of the corresponding cermets, J. Mater. Sci. 24 (1989) 3123–3136. [11] R.E. Tressler, T.L. Moore, R.L. Crane, Reactivity and interface characteristics of titanium–alumina composites, J. Mater. Sci. 8 (1973) 151–161. [12] K. Das, P. Choudhury, S. Das, The Al–O–Ti (aluminum–oxygen– titanium) system, J. Phase Equilib. 23 (2002) 525–536. [13] Y. Chen, D.J. Young, B. Gleeson, A new Ti-rich ternary phase in the Ti–Al–O system, Mater. Lett. 22 (1995) 125–129. [14] X.L. Li, R. Hillel, F. Teyssandier, S.K. Choi, F.J.J. Van loo, Reactions and phase relations in the Ti–Ai–O system, Acta Metall. Mater. 40 (11) (1992) 3149–3157. [15] K.A. Niihara, Fracture mechanics analysis of indentation-induced Palmqvist cracks in ceramics, J. Mater. Sci. Lett. 2 (1983) 221–223. [16] S. Meir, S. Kalabukhov, S. Hayun, Low temperature spark plasma sintering of Al2O3–TiC composites, Ceram. Int. 40 (2014) 12187–12192. [17] J.J. Cleveland, R.C. Bradt, Grain size/microcracking relations for pseudobrookite oxides, J. Am. Ceram. Soc. 61 (1978) 478–481.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 4637–4643 www.elsevier.com/locate/ceramint
Mechanical properties of Al2O3\Ti composites fabricated by spark plasma sintering S. Meir, S. Kalabukhov, N. Frage, S. Hayunn Department of Materials Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel Received 25 November 2014; received in revised form 1 December 2014; accepted 1 December 2014 Available online 9 December 2014
Abstract Al2O3\Ti composites presenting a range of metal to ceramic ratios were fabricated using SPS technology and subsequently characterized. A titanium hydride that decomposes in the 600–700 1C temperate range was used as the source of Ti. The composite densification process was initiated at 825 1C, and reached a maximum densification rate at 1150 1C. Microstructural analysis revealed the homogenous distribution of submicron-sized Al2O3 grains and micron-sized Ti grains. Thus, the fine microstructure of the composites provided improvements in hardness, Young's moduli and flexure strengths. The composites displayed lower resistance to fracture than did pure alumina, with cracks largely propagating around the alumina grains, which indicates that the ceramic–metal interface is weak. Finally, the formation of a Ti–Al–O solid solution at the metal/ceramic interface was detected by scanning electron microscopy and X-ray diffraction. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Al2O3\Ti composites; Cermet; Homogeneous; Micron
1. Introduction The fracture toughness of ceramics can be improved by the incorporation of ductile metal particles into the brittle ceramic matrix. For example, composites comprising alumina and metallic particles, such as aluminum, nickel or chromium, display improvements in specific strength, fracture toughness and wear resistance, relative to the ceramic alone [1–3]. In the case of alumina/titanium composites, only limited information is available. While MasGuindal et al. [4] reported on nano-structured alumina–titanium composite fabrication by self-propagating high temperature synthesis (SHS), the physical and mechanical properties of the composites were not discussed. Edalati et al. [5] described how titanium-based nano-composites containing 20 wt% Al2O3 with elevated hardness value were generated by applying severe plastic deformation to the powder mixtures. Two other reports [6,7] discussed the fabrication of Al2O3/Ti composites using the spark plasma sintering (SPS) approach. Atiyah et al. [6] did not reach full n
Corresponding author. Tel.: þ972 8 6428742; fax: þ 972 8 6428744. E-mail address: [email protected] (S. Hayun).
http://dx.doi.org/10.1016/j.ceramint.2014.12.008 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
densification of the specimens at 1500 1C under 17 MPa and, moreover, did not consider the mechanical properties of the composites. The mechanical properties of the alumina–titanium composite with 72.5 wt% Al2O3 were, however, presented in Ref. [7], where the hardness and flexural strength of the composites were reported as being about 150 GPa and 350 MPa, respectively. Furthermore, one of the goals of this earlier study was to achieve adequate particle homogeneity distribution, which in turn can effect favorably on the mechanical properties [8]. It had been established that addition of 3 wt% of Dolapix CE64 surfactant combined with milling zirconia balls in polyethylene containers at 150 r.p.m. for 24 h led to homogeneous particle distribution. It should be noted that coarse titanium particles (about 150 μm in diameter) were used [7]. When fine titanium particles were used for composite fabrication, an additional problem arose related to the fact that a thin oxide layer covers such particles. This layer affects the sintering behavior of the powder mixture and promotes the formation of uncontrolled and unfavorable phases at the alumina/ titanium interface. This reaction, including its formation kinetics and thermodynamics, has been discussed in detail in studies where
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interactions between alumina and titanium layers were investigated [9–14]. In these studies, almost all possible phases of the Al–Ti–O ternary phase diagrams were identified at the interface using X-ray diffraction and electron probe-based analysis of the reactive layers. Such phases included TiAl, Ti3Al intermetallic compounds, titanium oxide and a Ti–Al–O solid solution. The presence of these phases at the metal/ceramic interface in composites weakens grain boundaries and leads to decreased mechanical properties. To prevent the massive formation of these undesired phases, titanium hydride powder (instead of Ti powder) was used in the present work as a starting material for mixture preparation. Titanium hydride and alumina display very close specific density values (about 3.9 g/cm3) and can be blended to achieve a homogenous mixture with relative ease. As hydride decomposition takes place at a relatively low temperature under the reducing atmosphere that exists within the SPS apparatus, this approach allows for avoiding oxidation of the Ti particles. Composites presenting different degrees of Ti content ranging from pure alumina to pure Ti were fabricated using the SPS approach. Their mechanical and physical properties were tested and are discussed here. 2. Experimental procedures 2.1. Starting materials and experimental setup Alumina (Ceralox, high purity SPA-0.5, 0.5 mm) and titanium hydride (Alfa Aesar, 99% metal basis, 325 mesh) powders were blended in a planetary ball mill inside a polyethylene container for three days with ethanol and alumina milling balls. Powder mixtures of Al2O3 with 0, 20, 40, 60, 80 and 100 wt% titanium hydride were inserted into a graphite die/punch setup (inner diameter, 20 mm; outer diameter, 40 mm). The die was covered with a 20 mm-thick layer of graphite wool to improve thermal insulation. The die was placed into the SPS apparatus (HP D5/1, FCT System. Rauenstein, Germany) equipped with a 50 kN uniaxial press. The applied pressure was increased to 40 MPa over the first three minutes of the process, was raised to 64 MPa after an additional ten minutes and held constant until the end of the process. The sintering procedure was conducted at 1300 1C under a vacuum of about 1 Torr and 64 MPa of uniaxial pressure. The heating rate was 25 1C/min and the holding time at 1300 1C was 30 min (Fig. 1). 2.2. Characterization Phase composition was determined by X-ray diffraction (XRD) using a Rigaku RINT 2100 (Tokyo, Japan) diffractometer with Cu Kα radiation. The operating parameters were 40 kV and 40 mA, with a 2θ step size of 0.021. The XRD patterns were analyzed using a whole pattern fitting approach with the MDI Jade 2010 software (MDI, Livermore, CA). The surface areas of the raw and mixed powders were measured by the Brunauer–Emmett–Teller (BET) method using a Micromeritics ASAP 2020 (Micromeritics, Norcross, GA) instrument. Ten-point adsorption isotherms of nitrogen were collected in the 0.05–0.30 relative pressure range (P/P0, where
Fig. 1. SPS regime used for composite sintering.
P0 is the saturation pressure) at 196 1C. Prior to analysis, the alumina sample was vacuum-degassed at 400 1C for 4 h and the TiH2-containing samples were vacuum-degassed at 100 1C for 4 h to avoid any hydride decomposition. Simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) were carried out using a Netzsch STA 449 F3 system (Netzsch, Selb, Germany). The sample was heated at a rate of 10 1C/min in an alumina crucible from 30 to 1000 1C under argon. A buoyancy correction was introduced by subtracting the baseline value collected upon running a blank TGA/DSC assay with an empty alumina crucible. Microstructural analysis was conducted on polished specimens using a scanning electron microscopy (SEM, JEOL-35, Tokyo, Japan) equipped with the Noran energy dispersive spectrometer (EDS). Samples for metallographic characterization were prepared using a standard procedure that included a final stage of polishing with 3 mm diamond paste. The velocities of the longitudinal C l and shear C s acoustic waves were measured by a pulse-echo technique using a 5 MHz probe. The elastic modulus was derived from the ultrasonic velocity data and density values, measured by the liquid displacement method in deionized water. SPS-processed composite hardness was determined using a Buehler-Micromet 2100 micro-hardness tester (Lake Bluff, IL) with a Vickers indenter under 20 N loads. The lengths of the cracks that appeared at corners of the Vickers indentations were used for evaluating fracture toughness (Kc). Empirical equations for fracture toughness estimation depend on the nature of the cracks [15]. Two common types of cracks (Palmqvist and half penny) were considered. To identify the nature of the crack system, a polishing procedure was performed after indention. Flexural strength was determined using a three point bending test on 1.5 mm 2 mm 10 mm bars that were polished after cutting using an LRX Plus apparatus (Lloyd Instruments, Fareham Hants, U.K.). Five specimens for each sample were tested. 3. Results and discussion SEM analysis of TiH2 powder (Fig. 2) and Al2O3 þ TiH2 mixtures after 72 h of mixing (Fig. 2b) indicated that TiH2
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Fig. 2. SEM micrographs of TiH2 as received (a), and of alumina and TiH2 powders after mixing (b).
Fig. 3. Typical densification and densification rate curves as a function of time for the powder mixture containing 80 wt% Al2O3.
particles were crushed and homogeneously dispersed within the Al2O3 powder. The measured surface areas of the initial powders were 8.76 7 0.02 and 0.047 0.01 m2/g for Al2O3 and TiH2, respectively. The specific surface area of the 60 wt% Al2O3 mixture was 5.48 7 0.02 m2/g. Assuming a spherical shape for the TiH2 particles and taking into account the surface area of the initial Al2O3 powder, the average TiH2 particle size in the mixture was estimated to be about 3 μm. The calculated particle size is in a good agreement with the values observed by SEM (Fig. 2b).
Fig. 4. Gas emission from a specimen during sintering.
3.1. Sintering behavior Evaluation of the consolidation of the Al2O3–TiH2 powder mixture during the SPS process was carried out using the curve of punches displacement as a function of time and its derivative (Fig. 3). The data presented in Fig. 3 correspond to the sintering of a mixture containing 80 wt% Al2O3. Densification started at 825 1C, with the maximum densification rate being reached at 1120 1C (Fig. 3, peak 2). During holding at the sintering temperature, the densification rate slowly decreased, reaching zero after 10 min of soaking. Nevertheless, the relative density of the specimen at this point was about 96% of the density of Al2O3. Almost complete densification was achieved after an additional 25 min of soaking. The apparent densification, taking up to 15 min (Fig. 3, dL/dt, peak 1), is usually related to compaction of the powder
Fig. 5. Thermo-gravimetric analysis and differential scanning calorimetric measurements of TiH2 powder.
by applied pressure, which rises gradually to a maximal value of 64 MPa (Fig. 1). When comparing the dL/dt curve to similar curves from previous studies on Al2O3/TiC composites [16], the shape of peak 1 was quite unusual. In the case where only mechanical compaction occurred, the densification rate should
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parallel the speed of the applied force and remain constant. The consolidation behavior around 670 1C (peak 1) indicates that an additional process affects displacement of the punches, namely hydrogen released from the powder causes negative die displacement. Examination of the process-recorded data revealed variations in the gaseous pressure within the SPS chamber (Fig. 4). The amount of the gas released as a result
of TiH2 decomposition correlated with the fraction of TiH2 in the mixture. To clarify this phenomenon, thermo-gravimetric analysis of TiH2 decomposition was conducted. The decomposition process occurred gradually in two steps (Fig. 5). The first step of decomposition began at about 466 1C, while the second was initiated at 578 1C. The total mass loss was 4 wt% after heating up to 1000 1C; this value corresponds to the hydrogen content of the hydride. The sintering behaviors of the Al2O3/Ti composites presenting various compositions are presented in Fig. 6. The densification process of the Al2O3/Ti composites strongly depended on the composition of the mixture (Fig. 6). Pure titanium reached full density at about 850 1C, while pure alumina only reached 95% density at 1300 1C. Fully dense alumina was obtained after 35 min of holding at this temperature. The results presented in Fig. 6 are thus important for the fabrication of graded alumina– titanium composites by SPS. The measured densities of the final composites were slightly higher than those calculated based on densities of Ti and Al2O3 alone (Table 1). These differences can be attributed to changes in phase composition during the SPS process.
3.2. Phase composition and microstructure Fig. 6. Composite specimen compression as a function of temperature.
Table 1 Calculated and measured densities of the SPS-processed composites. Al2O3 (wt%)
Calculated density (g/cm3)
Measured density (g/cm3)7 0.02
0 20 40 60 80 100
4.50 4.38 4.26 4.15 4.05 3.95
4.55 4.48 4.39 4.25 4.11 3.95
XRD analysis of the SPS-processed composites revealed that the composites consisted of Al2O3 and α-Ti solid solutions. No peaks of TiH2 were detected (Fig. 7a). Nevertheless, the position peaks corresponding to α-Ti were shifted from those positions indicative of pure titanium (Fig. 7b). Reasons for such shifts include alumina decomposition and dissolution of alumina and oxygen in titanium. The results of EDS analysis (Fig. 8) support this suggestion, with the formation of α-Ti[Al,O] solid solution being detected [12]. The microstructure of the composites indicates homogeneous distribution of the phases (Fig. 9). The dark gray spots in Fig. 9A and B are the result of uneven gold deposition during sample preparation for SEM characterization.
Fig. 7. (A) XRD patterns of titanium and the four alumina–titanium composites, processed by SPS. (B) Comparison of the titanium peaks, with and without alumina.
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Fig. 8. (A) SEM image with EDS line. (B) EDS counts of aluminum and titanium along the line shown in (A).
Fig. 9. SEM micrographs of SPS-processed composites of various alumina contents, wt% (a) 20, (b) 40, (c) 60 and (d) 80 Al2O3. White grains correspond to titanium.
3.3. Mechanical properties The measured Young's modulus and hardness (Fig. 10) increased monotonically with alumina content. For instance, the hardness value of the composite containing 80 wt% Al2O3 was about 19.7 GPa and is higher (15 GPa) than the hardness value reported previously [7] for a composite of the same
composition. This difference may be related to the coarse Ti particles used in the earlier study, which were two orders of magnitude larger than those used here. Relatively high hardness values were observed for the titanium-rich composites. This may be related to the formation of the Τi–Al–O solid solution. The flexural strength values (Fig. 10) displayed maximum (767 7 23 MPa) for the 80 wt% Al2O3 composite. This value
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is more than twice that was reported in an earlier study [7]. The fracture toughness of the composites, KC, was calculated from the lengths of the cracks that appeared at the corners of the Vickers indentation. Optical micrographs of the indentation vicinity collected after subsequent polishing (Fig. 11b)
Fig. 10. Hardness and Young's modulus as a function of alumina content.
indicated that the nature of the cracks corresponded to a Palmqvist crack system (Fig. 12). As previously suggested [15], KC values were calculated according to Eq. (1) 0:4 0:5 K C ¼ 0:0264ðH V aÞ E=H V l ð1Þ where HV is the Vickers hardness, E is Young's modulus, a is the half length of the Vickers indentation diagonal line, and l is the length of the crack from the tip of the indentation. Unfortunately, the composites displayed lower resistances to fracture than did pure alumina (Fig. 11). The reason for this may be clarified by analysis of the crack propagation path. Accordingly, the SEM image of the polished and etched crosssection of the specimen containing 40 wt% Al2O3, with a crack that originated from a Vickers indentation was considered. The crack propagation clearly shows that the crack crossed the titanium matrix and appeared around the alumina grains (Fig. 13). This could indicate that the ceramic–metal interface is weaker than the Al2O3 particles are, as reported previously [11]. The composites, consisting of components with different coefficients of thermal expansion (CTE), can exhibit spontaneous micro-cracking upon cooling. Crack propagation along the grain boundaries depends on grain size [17], with a “critical” grain size below which cracks are not observed. As previously proposed [17], “critical” grain size is given by Eq. (2) dcrit ¼
Fig. 11. Flexural strength of the specimens as a function of alumina content.
14:4γ EΔα2 max ΔT 2
ð2Þ
where E is the larger Young's modulus (E), Δαmax is the maximum difference between the CTEs of the individual components, ΔT is the difference between the sintering and room temperatures, and γ is the surface energy of the newly formed surfaces. Using the value of Young's modulus for alumina (410 MPa), a Δαmax of 1.5 10 6/1C, a ΔT equal to 1250 1C and a γ of 1 J/m2, the critical grain size was calculated to be 11 μm. This value is higher than the grain size observed in the SPS-fabricated composites, yet is significantly lower than the grain size (150 μm) of the composites fabricated by GutierrezGonzalez et al. [7]. The micro-cracks that were observed in the composites and which present relatively low mechanical properties could be attributed to coarse microstructure.
Fig. 12. Optical micrographs of a Vickers imprint before (a) and after (b) polishing. The gap between the crack and the imprint indicates a Palmqvist type crack.
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References
Fig. 13. Crack propagation path in a composite with 40 wt% Al2O3.
4. Summary Dense Al2O3/Ti composites, fabricated using TiH2, were sintered using SPS at 1300 1C for 30 min. Microstructural analysis revealed the homogenous distribution of titanium and alumina grains in the composites. Young's modulus and hardness values increased monotonically, while fracture toughness decreased with the magnitude of the alumina fraction in the composites. The cracks that originated from a Vickers indentation easily crossed the titanium matrix, propagated mostly around the alumina grains, reflective of the ceramic– metal interface being weak. Nevertheless, the fine microstructure of the composites prevents spontaneous micro-cracking upon cooling. Acknowledgments This work was partially supported by the Israel Ministry of Defense (Grant no. 4440400102) and the FP7-PEOPLE-2012CIG Grant no. 321838-EEEF-GBE-CNS. S. Meir thanks the Kreitman School of Advanced Graduate Studies, and Ben-Gurion University of the Negev for a postdoctoral scholarship.
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