- Email: [email protected]
High frequency induction heated sintering of nanostructured Al2O3–ZrB2 composite produced by MASHS technique
Available online at www.sciencedirect.com
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 9217–9224 www.elsevier.com/locate/ceramint
High frequency induction heated sintering of nanostructured Al2O3–ZrB2 composite produced by MASHS technique B. Jamal Abbasin, M. Zakeri, S.A. Tayebifard Materials and Energy Research Center, P.O. Box: 31787/316, Karaj, Iran Received 18 October 2013; received in revised form 25 January 2014; accepted 30 January 2014 Available online 6 February 2014
Abstract Nanostructured Al2O3–ZrB2 composite was synthesized using an alternative route called Mechanically Activated Self-propagating Hightemperature Synthesis (MASHS). It was demonstrated that Al2O3–ZrB2 composite with nanometric structure could be produced via a very fast combustion front in contrast with the SHS process. Ball milling for 5 h with ball to powder weight ratio of 5:1 was found to be the optimum condition for activation of the reactants. Using the high frequency induction heated sintering method, the composite was consolidated within 3 min. The relative density of the sintered sample was 99% of the theoretical density at an applied pressure of 20 MPa and temperature of 1800 1C. The average hardness and fracture toughness were 19.5 GPa and 4.1 70.3 MPa m1/2, respectively. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Nanocomposite; Zirconium diboride; Self-propagating high temperature synthesis; High frequency induction heated sintering
1. Introduction Zirconium diboride (ZrB2) has attracted substantial interests because of its extreme chemical and physical properties, such as, high melting point, superior hardness and low electrical resistance [1]. The addition of zirconium diboride to alumina is expected to result in a high mechanical strength similar to titanium diboride–alumina composite which has shown excellent mechanical properties such as strength, hardness, fracture toughness, and impact resistance [2]. Adding ZrB2 has advantage over titanium diboride, because it does not have many stable intermediate phases, which makes the ZrB2 reinforced composites more preferable despite having a higher density than the titanium diboride. Furthermore, incorporating ZrB2 to the ceramic can accomplish the self-lubricating purpose, because at high temperatures (caused by friction or any other reasons), boride can form oxide lubricating film dissoluted in other elements, which can be employed for green manufacturing [3–8]. Moderate amounts of ZrB2 (20 wt% or n
Corresponding author. Tel.: þ98 912 2143667; fax: þ 98 261 6201888. E-mail address: [email protected] (B. Jamal Abbasi).
http://dx.doi.org/10.1016/j.ceramint.2014.01.141 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
higher) dispersed in the alumina matrix make it electrically conducting which makes the composite machinable by electro discharge machining (EDM), resulting in cheaper and more precise machining processes [9,10]. The self-propagating, high-temperature synthesis (SHS) process is an advanced method which has recently been used extensively for preparing refractory materials such as borides, carbides, silicides, nitrides, and various composite materials [11,12]. SHS of the composite powder is more energy efficient than conventional processes. Moreover, defects induced during the SHS process by the high rate of heating and cooling should reduce the sintering temperature of the composite. The dispersion of the zirconium diboride is likely to be more homogenous when it is prepared in-situ by the SHS process than procedures when the two components are mixed and milled [13]. Recently, a new variation of the SHS process was proposed [14,15]: Mechanically Activated Self-propagating High-temperature Synthesis (MASHS). This process is a combination of two steps: the first is a mechanical activation where reactant powders are co-milled for a short time at given energy and frequency of shocks; the second is an SHS reaction. The use of mechanical activation prior to the SHS
9218
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
process can result in the formation of nanostructured materials [16]. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties [17]. More attention has been paid to the application of nanomaterials that take advantage of their high strength, hardness, excellent ductility and toughness [18]. However, the grain size in sintered material becomes much larger than that in pre-sintered powders due to fast grain growth during the conventional sintering processes. Therefore, even though the initial particle size is less than 100 nm, the grain size increases rapidly up to 2 mm or larger during the conventional sintering process [19]. So, controlling grain growth during sintering is one of the keys to the commercial success of nanostructured materials. In this regard, the high frequency induction heated sintering (HFIHS) method which can make dense materials within 3 min, has been shown to be effective [20,21]. This paper presents the results on the MASHS process which is applied to produce Al2O3–ZrB2 composite powder. The resulted powder has been used to produce dense Al2O3– ZrB2 composite by using the high frequency induction heated sintering method.
1 mm
Sample Temperature monitoring point
10 mm
5 mm
Sample holder Fig. 1. Schematic of sample in the SHS reactor. Temperature monitoring point is placed at the bottom of the pellet.
2.2. Cold compaction process Powder mixtures were uniaxially pressed in a stainless steel die at about 300 MPa without a binder into a cylindrical compact. The initial density of the green compacts was estimated from the mass and geometry to be about 70% of their theoretical density.
2.3. Ignition process
2. Experimental procedures 2.1. Mechanical activation Zirconium oxide (98.5 wt%, Merck), B2O3 (99.5 wt%, Merck), and reducing agent, aluminum (99.0 wt%, Fluka), were used as starting materials. The powders were mixed in accordance with the stoichiometry given by Eq. (1): ZrO2 þ 3B2 O3 þ 10Al ¼ 3ZrB2 þ 5Al2 O3
Tungsten coil
ð1Þ
Powder mixtures were subjected to mechanochemical activation in a planetary ball mill (PM400 type with four grinding stations, Retsch Company, Germany) at approximately room temperature and with cup speed of 750 rounds per minute (RPM). A distribution of 20, 15 and 10 mm stainless steel balls was used in all experiments. Four BPRs (Ball to Powder weight Ratio), 5:1, 10:1, 15:1 and 20:1, were used. The powder mixture and balls were charged into a stainless steel cup (250 mL) in argon atmosphere. The powder was milled for different periods of time (5, 10 and 15 h). At the end of each milling run, the vial was emptied in a glove box under argon atmosphere. Iron (Fe) contamination of milled powders was measured by atomic absorption spectroscopy (AAS). A Hitachi 170-50 (Japan) flame atomic absorption spectrometer equipped with hollow cathode lamps was used for the analyses. The instrumental parameters were adjusted according to the manufacturer's recommendations. Fe hollow cathode lamp operating at 248.3 nm was used as the radiation source. The lamp current was set at 15 mA. The flame composition was propane–butane (gas pressure 0.1 kg/cm2), and air (gas pressure 1.5 kg/cm2).
The pellets were placed vertically inside a stainless steel reactor and the SHS process was ignited by an electrically heated tungsten coil accurately placed at a distance of 1.0 mm on top of the pellet. All experiments were conducted in a high purity argon (99.998%) atmosphere. Temperature profiles of the advancing front of the combustion were recorded by a line scanner pyrometer with a resolution time of 0.01 s. The schematic of sample in the SHS reactor has been shown in Fig. 1. Temperature monitoring point was placed at the bottom of the pellet to avoid any interference resulted by tungsten coil heat.
2.4. Structural investigation X-ray diffraction (XRD) profiles were recorded by Siemens diffractometer (30 kV and 25 mA) with Cu Kα1 radiation (l.5404 Å). Recorded XRD patterns were used for calculation of crystallite size and strain. Mean crystallite size and microstrain were calculated on the basis of the Rietveld refinement method by using the X0 Pert high score plus software (version 2.0) (developed by Panalytical BV Company, Almelo, Netherlands). In this method, peak profile fitting, size broadening and strain broadening were calculated on the basis of the following equations [22]: Gik ¼ γ
C0:5 C 0:5 0 ½1þ C 0 X 2ik 1 þ ð1 γÞ 1 0:5 exp C 1 X 2ik Hk π Hk π
H k ¼ ðU tan 2 θ þ V tan θ þ WÞ0:5
ð2Þ ð3Þ
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
180 λ Di ¼ π ðW i W std Þ0:5 ηi ¼
½ðU i U std Þ ðW i W std Þ0:5 ð1=100Þ½180=π4ð2 ln 2Þ0:5
ð4Þ
ð5Þ
where Gik is pseudo-Voigt function, C0 ¼ 2, C1 ¼ 4 ln 2, Hk is full width at half maximum of the Kth Bragg reflection, γ is shape parameter, Xik ¼ (2θi 2θk)/Hk. Di, ηi, λ, U and W are crystallite size function, strain function, wavelength, strain parameter and size parameter of peak profile, respectively. In the size and strain functions, i and std refer to the experimental and standard samples, respectively. In this project Al2O3–ZrB2 composite annealed at 1400 1C for 4 h, was used as standard material to diminish the instrumental broadening. The morphology and particle size of samples were examined using a Philips (XL30) SEM operating at 30 kV. Microstructural characterization of milled powder was carried out with a Philips EM208 TEM operating at 200 kV. TEM samples were prepared by ultrasonically dispersing the powders in methanol. One drop of this suspension was then placed on a copper grid for TEM observation.
9219
until the consolidation rate was negligible, as can be conferred by observing the shrinkage of the sample. Sample shrinkage was measured in real time by a linear gauge, measuring the vertical displacement. It is worth to mention that, thermal expansion of the graphite die and the sample was dismissed. Temperature data was collected by a pyrometer focusing on the surface of the graphite die. Depending on heating rate, electrical and thermal conductivities of the compact, a difference in temperature between the surface and the center of the sample exists. At the end of the process, the induced current was turned off and the sample was allowed to cool down into room temperature. Relative density of the sintered samples was measured by the Archimedes method. Microstructural information was obtained from products, which had been fractured and polished. Microstructural analysis of products was made through scanning electron microscopy (SEM). The Vickers hardness of sintered samples was measured by performing indentations at a 5 kg load and a dwell time of 15 s. 3. Results and discussions 3.1. Synthesis of Al2O3–ZrB2 nanocomposite
2.5. High frequency induction heated sintering The synthesized powders were placed in a graphite die (outside diameter, 50 mm; inside diameter, 25 mm; height, 50 mm) and then introduced into the high-frequency induction heated sintering system. Schematic diagram of this method is shown in Fig. 2. The system was first vacuumed and a uniaxial pressure of 20 MPa was applied. An induced current (frequency of about 66 kHz, 30 kW) was then activated and maintained
Fig. 2. Schematic diagram of apparatus for high-frequency induction heated sintering.
Fig. 3 shows the XRD patterns of as-milled samples with different milling conditions. As the patterns include only reactant peaks, it can be concluded that no reaction has been occurred during the milling process. The milling process resulted in broadening and intensity decrease of the XRD peaks. In these patterns, B2O3 reflections are absent, due to the large mass absorption coefficient (MAC) of ZrO2 and Al [23] in comparison to the very small MAC of B2O3 (Table 1). Fe content of as-milled powder was measured by the AAS method. The results of this measurement were presented in Table 2. In all experiments Fe impurity is less than 0.35 wt%. XRD patterns of MASHS samples with different milling conditions and SHS sample are shown in Fig. 4. As seen in these patterns, in samples with BPR of 15:1 and 20:1, there is a small peak of ZrO2. By increasing the energy level of the
Fig. 3. XRD patterns of as milled powders before synthesis: (a) BPR5/5 h, (b) BPR10/5h, (c) BPR15/5 h, (d) BPR15/10 h, (e) BPR15/15 h and (f) BPR20/5 h.
9220
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
Table 1 Mass absorption coefficients of the reactants and products. Phase
B2O3
ZrO2
Al
ZrB2
Al2O3
MAC (cm2 g 1)
8.26
104.15
48.67
111.07
30.91
Table 2 Fe impurity of as-milled powders measured by the AAS method. Mechanical condition
Fe (wt%)
BPR 5/5 h BPR 10/5 h BPR15/5 h BPR15/10 h BPR15/15 h BPR20/5 h
0.04 0.07 0.16 0.17 0.33 0.21
Fig. 5. Evolution of the surface temperature inside the ignition zone: thermograms of the ignition zone for the SHS and the MASHS (BPR5/5 h milled sample) processes-ignition time (tig), ignition temperature (Tig) and combustion temperature (Tc).
Table 3 Ignition delay (tig t0), ignition temperature and combustion temperature for different mechanical activation conditions.
Fig. 4. XRD patterns of (a) not activated, (b) BPR5/5 h, (c) BPR10/5 h, (d) BPR15/5 h, (e) BPR15/10 h, (f) BPR15/15 h, (g) BPR20/5 h milled and synthesized samples in SHS reactor.
reactants via mechanical activation, reactivity increases, resulted in a higher conversion of reactants to side products. Aluminum borate (Al18B4O33) has been observed to be among SHS reaction products of Al, B2O3, and TiO2 reaction. The presence of Al18B4O33 is believed to be partially responsible for unique microstructural properties of SHS synthesized Al2O3/TiB2 composites. High energy milling could activate the formation of minor phases of aluminum borate and excess ZrO2 has remained unchanged. In order to compare both processes (i.e. SHS and MASHS), experiments were carried out under the same conditions. Fig. 5 shows two typical SHS and MASHS temperature profiles. It is clear from Fig. 5 that (1) ignition is achieved in a much shorter time for the MASHS samples and (2) the ignition average temperature is slightly lower than the one recorded for the SHS reaction. The quantitative results are presented in Table 3. In general MASHS Combustion temperature (Tc) is higher than SHS sample and by increasing the milling time and BPR, maximal combustion temperature decreases in MASHS samples.
Mechanical condition
tig t0 (s)
Tig (1C)
Tc (1C)
None (Classical SHS) BPR 5/5 h BPR 10/5 h BPR15/5 h BPR15/10 h BPR15/15 h BPR20/5 h
4.4 2.2 2.5 2.6 2.5 2.1 2.3
1048 945 948 952 937 912 932
1796 1888 1870 1867 1842 1820 1835
Although the temperature profile shape is similar for the SHS and MASHS processes, the time of ignition is significantly shorter for the MASHS samples. The same trend is also observed for the ignition temperature: Tig (MASHS) is 1007 20 K lower than Tig (SHS). These results are in agreement with earlier observations on the Mo–Si [24] and Fe–Al [14] systems. The same behavior has been reported in the reaction involving mechanically activated SiO2 and Al [24]. The MASHS sample particles include very small crystallites in the range of nanometer. Table 4 shows the results of crystallite size, calculated by the Rietveld refinement method. As seen in this table, a nanostructured powder was obtained in all milling conditions. The mean crystallite size of MASHS samples is less than 40 nm. The higher the BPR and milling time, the smaller mean crystallite size was obtained. Due to the presence of excess ZrO2 in samples with BPR 10, 15 and 20 and a negligible difference between mean crystallite size of MASHS samples, the 5 h milled sample with BPR 5:1 was selected as the optimum condition of Al2O3–ZrB2 nanocomposite synthesis. To confirm the Rietveld results, the microstructure of MASHS sample (BPR5/5 h milled) was studied by TEM.
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
9221
Table 4 Calculation of mean crystallite size of Al2O3–ZrB2 composite after synthesis. Parameters
Activation time (h)
BPR 5
Phase Mean crystallite size (nm)
5 10 15
10
15
20
Not activated sample
Al2O3
ZrB2
Al2O3
ZrB2
Al2O3
ZrB2
Al2O3
ZrB2
Al2O3
ZrB2
28 – –
37 – –
33 – –
35 – –
32 27 20
32 31 28
25 – –
29 – –
113
108
Fig. 6. TEM image of Al2O3–ZrB2 powder fabricated by MASHS (BPR 5/5 h milled): (A) bright field image and selected area diffraction pattern (left bottom corner) and (B) energy dispersive X-ray (EDX) analysis of the marked grains.
As seen in Fig. 6A, a nanostructure with the mean crystallite size less than 40 nm was obtained for this powder that is in consistency with the Rietveld results. Sharp rings in the SAD pattern (left bottom corner of Fig. 6A) also confirm the
nanocrystallity of this powder. According to EDX analysis of marked grains (Fig. 6B.) the composition of Grains 1 and 2 can be conferred as Al2O3 and ZrB2, respectively. B element cannot be detected by this method.
9222
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
3.2. High frequency induction heated sintering The variations of shrinkage and temperature versus heating time during the sintering of Al2O3–ZrB2 composite produced by MASHS (5 h milled with BPR 5:1) and the SHS process
Fig. 7. Variation of temperature and shrinkage displacement with heating time during high frequency induction heated sintering of Al2O3–ZrB2 composite synthesized by MASHS and classical SHS techniques.
using HFIHS under 20 MPa pressure and 90% output of total power capacity (30 kW) are shown in Fig. 7. The sintering process took 3 min and maximum temperature reached as high as 1800 1C. Thermal expansion contribution was not accounted in this figure. In both cases, application of induced current resulted in shrinkage of the samples due to consolidation. The shrinkage initiation temperature is found to be 1530 1C and 1280 1C for SHS and MASHS sample, respectively. The relative density of the sintered sample was measured 99% for MASHS sample and 97% for SHS sample by the Archimedes method. Although the consolidation mechanism under induced current heating and pressure is unclear, the accelerated consolidation by HFIHS may be due to a combination of electrical discharge, resistance heating and pressure application effects. Despite the relatively short time, the samples sintered by the HFIHS method were exposed to high temperatures and pressures. Meanwhile, the effect of the electrical induced current on rapid consolidation could also be explained by the rapid heating rate due to Joule heating, and the intrinsic contribution of the current to mass transport [25]. Fig. 8 shows SEM (secondary electron) images of (A and B) the as-synthesized powder, (C and D) the specimen sintered by HFIHS. The as-synthesized powder is round in shape and
Fig. 8. SEM images of Al2O3–ZrB2 composite produced by (A) SHS, (B) MASHS (BPR5/5 h milled) before sintering; (C) SHS and (D) MASHS samples after sintering by HFIHS.
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
9223
very small crystallites in the range of nanometer. Fe contamination of as-milled powder was measured via the AAS method and found to be less than 0.35 wt% in every sample. Due to the presence of excess ZrO2 in samples with BPR 10, 15 and 20 and a negligible difference between mean crystallite sizes of MASHS samples, the 5 h milled sample with BPR 5:1 was selected as the optimum condition of Al2O3–ZrB2 nanocomposite synthesis. Using the HFIHS method, both MASHS and SHS samples were sintered. The relative density of MASHS and SHS sintered samples were 99% and 97% of their theoretical density, respectively. The average hardness and fracture toughness of MASHS samples were 19.5 GPa and 4.17 0.3 MPa m1/2, respectively. These fracture toughness and hardness of the nanostructured Al2O3–ZrB2 were higher than those of microstructured composite. Fig. 9. Vickers hardness indentation and median crack propagating in Al2O3–ZrB2 composite.
References agglomerated. The average grain size of the sintered composite is not much larger than that of the initial powder and seems to be several microns, indicating the absence of significant grain growth during sintering. This retention of grain size was attributed to the fast heating rate and relatively short-term exposure of the powders to the high temperature. Fig. 9 shows Vickers hardness indentation in Al2O3–ZrB2 sintered composites. The calculated average hardness of five measurements on MASHS and SHS samples were 19.5 and 16.5 GPa, respectively. Indentations with sufficiently high loads produced median cracks around the indent. From the length of these cracks, the fracture toughness can be determined using two expressions. The first expression, proposed by Anstis et al. [26] is 1=2 3=2 E P K IC ¼ 0:016 ð6Þ H C where E is Young's modulus, H is the indentation hardness, P is the indentation load, and C is the trace length of the crack measured from the center of the indentation. The second expression proposed by Niihara et al. [27] is c 3=2 K IC ¼ 0:023 H v a1=2 ð7Þ a where c is the trace length of the crack measured from the center of the indentation, a is the half of average length of two indent diagonals, and Hv is hardness. The toughness values calculated using the two methods for MASHS and SHS samples were 4.1 7 0.3 and 2.37 0.3 MPa m1/2, respectively. These fracture toughness and hardness values of nanostructured Al2O3–ZrB2 (MASHS sample) were higher than those of microstructured Al2O3–ZrB2 [28] due to the refinement of the grain size. 4. Conclusion Al2O3–ZrB2 nanocomposite was successfully synthesized by MASHS technique. The MASHS sample particles included
[1] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory diborides of zirconium and hafnium, J. Am. Ceram. Soc. 90 (2007) 1347–1364. [2] J. Liu, P.D. Ownby, Enhanced mechanical properties of alumina by dispersed titanium diboride particulate inclusions, J. Am. Ceram. Soc. 74 (1991) 241–243. [3] B. Li, J. Deng, Addition of Zr–O–B compounds to improve the performances of alumina matrix ceramic materials, J. Alloys Compd. 473 (2009) 190–194. [4] C. Liu, J. Sun, X. Zhang, Y. Ma, N. Qi, Pressureless sintering of alumina matrix ceramic materials improved by Al–Ti–B master alloys and diopside, Ceram. Int. 35 (2009) 1327–1333. [5] C. Liu, J. Zhang, J. Sun, X. Zhang, Addition of Al–Ti–B master alloys to improve the performances of alumina matrix ceramic materials, Ceram. Int. 33 (2007) 1319–1324. [6] D. Jianxin, C. Tongkun, L. Lili, Self-lubricating behaviors of Al2O3/TiB2 ceramic tools in dry high-speed machining of hardened steel, J. Eur. Ceram. Soc. 25 (2005) 1073–1079. [7] F. Monteverde, A. Bellosi, S. Guicciardi, Processing and properties of zirconium diboride-based composites, J. Eur. Ceram. Soc. 22 (2002) 279–288. [8] B. Jamal Abbasi, M. Zakeri, S.A. Tayebifard, Mechanochemical synthesis of Al2O3–ZrB2–ZrO2 nanocomposite powder, Mater. Res. Bull. 49 (2014) 672–676. [9] S.K. Mishra, S.K. Das, V. Sherbacov, Fabrication of Al2O3–ZrB2 in situ composite by SHS dynamic compaction: a novel approach, Compos. Sci. Technol. 67 (2007) 2447–2453. [10] S. Postrach, J. Pötschke, Pressureless sintering of Al2O3 containing up to 20 vol% zirconium diboride (ZrB2), J. Eur. Ceram. Soc. 20 (2000) 1459–1468. [11] A.G. Merzhanov, The chemistry of self-propagating high-temperature synthesis, J. Mater. Chem. 14 (2004) 1779–1786. [12] H.C. Yi, J.J. Moore, Self-propagating high-temperature (combustion) synthesis (SHS) of powder-compacted materials, J. Mater. Sci. 25 (1990) 1159–1168. [13] S.K. Mishra, S.K. Das, P. Ramachandrarao, D. Yu Belov, S. Mamyan, Synthesis of zirconium diboride–alumina composite by the self-propagating, high-temperature synthesis process, Metall. Mater. Trans. A 34 (2003) 1979–1983. [14] F. Charlot, F. Bernard, E. Gaffet, D. Klein, J.C. Niepce, In situ timeresolved diffraction coupled with a thermal i.r. camera to study mechanically activated SHS reaction: case of Fe–Al binary system, Acta Mater. 47 (1999) 619–629. [15] V. Gauthier, C. Josse, F. Bernard, E. Gaffet, J.P. Larpin, Synthesis of niobium aluminides using mechanically activated self-propagating
9224
[16] [17] [18] [19] [20]
[21]
[22]
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224 high-temperature synthesis and mechanically activated annealing process, Mater. Sci. Eng. A 265 (1999) 117–128. P. Mossino, Some aspects in self-propagating high-temperature synthesis, Ceram. Int. 30 (2004) 311–332. F.H. Froes, O.N. Senkov, E.G. Baburaj, Synthesis of nanocrystalline materials – an overview, Mater. Sci. Eng. A 301 (2001) 44–53. M.A. Meyers, A. Mishra, D.J. Benson, Mechanical properties of nanocrystalline materials, Progress Mater. Sci. 51 (2006) 427–556. J. Jung, S. Kang, Sintered (Ti,W)C carbides, Scr. Mater. 56 (2007) 561–564. S.W. Kim, K.A.-R. Khalil, High-frequency induction heat sintering of mechanically alloyed alumina–yttria-stabilized zirconia nano-bioceramics, J. Am. Ceram. Soc. 89 (2006) 1280–1285. H.C. Kim, I.J. Shon, Z.A. Munir, Rapid sintering of ultra-fine WC-10 wt % Co by high-frequency induction heating, J. Mater. Sci. 40 (2005) 2849–2854. H. Rietveld, A profile refinement method for nuclear and magnetic structures, J. Appl. Crystall. 2 (1969) 65–71.
[23] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Co., Reading, Massachusetts, 1956. [24] C. Gras, E. Gaffet, F. Bernard, Combustion wave structure during the MoSi2 synthesis by mechanically-activated self-propagating hightemperature synthesis (MASHS): in situ time-resolved investigations, Intermetallics 14 (2006) 521–529. [25] J.E. Garay, U. Anselmi-Tamburini, Z.A. Munir, Enhanced growth of intermetallic phases in the Ni–Ti system by current effects, Acta Mater. 51 (2003) 4487–4495. [26] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, A critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurements, J. Am. Ceram. Soc. 64 (1981) 533–538. [27] K. Niihara, R. Morena, D.P.H. Hasselman, Evaluation of K Ic of brittle solids by the indentation method with low crack-to-indent ratios, J. Mater. Sci. Lett. 1 (1982) 13–16. [28] S.K. Mishra, P.K.P. Rupa, S.K. Das, V. Shcherbakov, Effect of titanium diluent on the fabrication of Al2O3–ZrB2 composite by SHS dynamic compaction, Compos. Sci. Technol. 67 (2007) 1734–1739.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 9217–9224 www.elsevier.com/locate/ceramint
High frequency induction heated sintering of nanostructured Al2O3–ZrB2 composite produced by MASHS technique B. Jamal Abbasin, M. Zakeri, S.A. Tayebifard Materials and Energy Research Center, P.O. Box: 31787/316, Karaj, Iran Received 18 October 2013; received in revised form 25 January 2014; accepted 30 January 2014 Available online 6 February 2014
Abstract Nanostructured Al2O3–ZrB2 composite was synthesized using an alternative route called Mechanically Activated Self-propagating Hightemperature Synthesis (MASHS). It was demonstrated that Al2O3–ZrB2 composite with nanometric structure could be produced via a very fast combustion front in contrast with the SHS process. Ball milling for 5 h with ball to powder weight ratio of 5:1 was found to be the optimum condition for activation of the reactants. Using the high frequency induction heated sintering method, the composite was consolidated within 3 min. The relative density of the sintered sample was 99% of the theoretical density at an applied pressure of 20 MPa and temperature of 1800 1C. The average hardness and fracture toughness were 19.5 GPa and 4.1 70.3 MPa m1/2, respectively. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Nanocomposite; Zirconium diboride; Self-propagating high temperature synthesis; High frequency induction heated sintering
1. Introduction Zirconium diboride (ZrB2) has attracted substantial interests because of its extreme chemical and physical properties, such as, high melting point, superior hardness and low electrical resistance [1]. The addition of zirconium diboride to alumina is expected to result in a high mechanical strength similar to titanium diboride–alumina composite which has shown excellent mechanical properties such as strength, hardness, fracture toughness, and impact resistance [2]. Adding ZrB2 has advantage over titanium diboride, because it does not have many stable intermediate phases, which makes the ZrB2 reinforced composites more preferable despite having a higher density than the titanium diboride. Furthermore, incorporating ZrB2 to the ceramic can accomplish the self-lubricating purpose, because at high temperatures (caused by friction or any other reasons), boride can form oxide lubricating film dissoluted in other elements, which can be employed for green manufacturing [3–8]. Moderate amounts of ZrB2 (20 wt% or n
Corresponding author. Tel.: þ98 912 2143667; fax: þ 98 261 6201888. E-mail address: [email protected] (B. Jamal Abbasi).
http://dx.doi.org/10.1016/j.ceramint.2014.01.141 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
higher) dispersed in the alumina matrix make it electrically conducting which makes the composite machinable by electro discharge machining (EDM), resulting in cheaper and more precise machining processes [9,10]. The self-propagating, high-temperature synthesis (SHS) process is an advanced method which has recently been used extensively for preparing refractory materials such as borides, carbides, silicides, nitrides, and various composite materials [11,12]. SHS of the composite powder is more energy efficient than conventional processes. Moreover, defects induced during the SHS process by the high rate of heating and cooling should reduce the sintering temperature of the composite. The dispersion of the zirconium diboride is likely to be more homogenous when it is prepared in-situ by the SHS process than procedures when the two components are mixed and milled [13]. Recently, a new variation of the SHS process was proposed [14,15]: Mechanically Activated Self-propagating High-temperature Synthesis (MASHS). This process is a combination of two steps: the first is a mechanical activation where reactant powders are co-milled for a short time at given energy and frequency of shocks; the second is an SHS reaction. The use of mechanical activation prior to the SHS
9218
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
process can result in the formation of nanostructured materials [16]. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties [17]. More attention has been paid to the application of nanomaterials that take advantage of their high strength, hardness, excellent ductility and toughness [18]. However, the grain size in sintered material becomes much larger than that in pre-sintered powders due to fast grain growth during the conventional sintering processes. Therefore, even though the initial particle size is less than 100 nm, the grain size increases rapidly up to 2 mm or larger during the conventional sintering process [19]. So, controlling grain growth during sintering is one of the keys to the commercial success of nanostructured materials. In this regard, the high frequency induction heated sintering (HFIHS) method which can make dense materials within 3 min, has been shown to be effective [20,21]. This paper presents the results on the MASHS process which is applied to produce Al2O3–ZrB2 composite powder. The resulted powder has been used to produce dense Al2O3– ZrB2 composite by using the high frequency induction heated sintering method.
1 mm
Sample Temperature monitoring point
10 mm
5 mm
Sample holder Fig. 1. Schematic of sample in the SHS reactor. Temperature monitoring point is placed at the bottom of the pellet.
2.2. Cold compaction process Powder mixtures were uniaxially pressed in a stainless steel die at about 300 MPa without a binder into a cylindrical compact. The initial density of the green compacts was estimated from the mass and geometry to be about 70% of their theoretical density.
2.3. Ignition process
2. Experimental procedures 2.1. Mechanical activation Zirconium oxide (98.5 wt%, Merck), B2O3 (99.5 wt%, Merck), and reducing agent, aluminum (99.0 wt%, Fluka), were used as starting materials. The powders were mixed in accordance with the stoichiometry given by Eq. (1): ZrO2 þ 3B2 O3 þ 10Al ¼ 3ZrB2 þ 5Al2 O3
Tungsten coil
ð1Þ
Powder mixtures were subjected to mechanochemical activation in a planetary ball mill (PM400 type with four grinding stations, Retsch Company, Germany) at approximately room temperature and with cup speed of 750 rounds per minute (RPM). A distribution of 20, 15 and 10 mm stainless steel balls was used in all experiments. Four BPRs (Ball to Powder weight Ratio), 5:1, 10:1, 15:1 and 20:1, were used. The powder mixture and balls were charged into a stainless steel cup (250 mL) in argon atmosphere. The powder was milled for different periods of time (5, 10 and 15 h). At the end of each milling run, the vial was emptied in a glove box under argon atmosphere. Iron (Fe) contamination of milled powders was measured by atomic absorption spectroscopy (AAS). A Hitachi 170-50 (Japan) flame atomic absorption spectrometer equipped with hollow cathode lamps was used for the analyses. The instrumental parameters were adjusted according to the manufacturer's recommendations. Fe hollow cathode lamp operating at 248.3 nm was used as the radiation source. The lamp current was set at 15 mA. The flame composition was propane–butane (gas pressure 0.1 kg/cm2), and air (gas pressure 1.5 kg/cm2).
The pellets were placed vertically inside a stainless steel reactor and the SHS process was ignited by an electrically heated tungsten coil accurately placed at a distance of 1.0 mm on top of the pellet. All experiments were conducted in a high purity argon (99.998%) atmosphere. Temperature profiles of the advancing front of the combustion were recorded by a line scanner pyrometer with a resolution time of 0.01 s. The schematic of sample in the SHS reactor has been shown in Fig. 1. Temperature monitoring point was placed at the bottom of the pellet to avoid any interference resulted by tungsten coil heat.
2.4. Structural investigation X-ray diffraction (XRD) profiles were recorded by Siemens diffractometer (30 kV and 25 mA) with Cu Kα1 radiation (l.5404 Å). Recorded XRD patterns were used for calculation of crystallite size and strain. Mean crystallite size and microstrain were calculated on the basis of the Rietveld refinement method by using the X0 Pert high score plus software (version 2.0) (developed by Panalytical BV Company, Almelo, Netherlands). In this method, peak profile fitting, size broadening and strain broadening were calculated on the basis of the following equations [22]: Gik ¼ γ
C0:5 C 0:5 0 ½1þ C 0 X 2ik 1 þ ð1 γÞ 1 0:5 exp C 1 X 2ik Hk π Hk π
H k ¼ ðU tan 2 θ þ V tan θ þ WÞ0:5
ð2Þ ð3Þ
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
180 λ Di ¼ π ðW i W std Þ0:5 ηi ¼
½ðU i U std Þ ðW i W std Þ0:5 ð1=100Þ½180=π4ð2 ln 2Þ0:5
ð4Þ
ð5Þ
where Gik is pseudo-Voigt function, C0 ¼ 2, C1 ¼ 4 ln 2, Hk is full width at half maximum of the Kth Bragg reflection, γ is shape parameter, Xik ¼ (2θi 2θk)/Hk. Di, ηi, λ, U and W are crystallite size function, strain function, wavelength, strain parameter and size parameter of peak profile, respectively. In the size and strain functions, i and std refer to the experimental and standard samples, respectively. In this project Al2O3–ZrB2 composite annealed at 1400 1C for 4 h, was used as standard material to diminish the instrumental broadening. The morphology and particle size of samples were examined using a Philips (XL30) SEM operating at 30 kV. Microstructural characterization of milled powder was carried out with a Philips EM208 TEM operating at 200 kV. TEM samples were prepared by ultrasonically dispersing the powders in methanol. One drop of this suspension was then placed on a copper grid for TEM observation.
9219
until the consolidation rate was negligible, as can be conferred by observing the shrinkage of the sample. Sample shrinkage was measured in real time by a linear gauge, measuring the vertical displacement. It is worth to mention that, thermal expansion of the graphite die and the sample was dismissed. Temperature data was collected by a pyrometer focusing on the surface of the graphite die. Depending on heating rate, electrical and thermal conductivities of the compact, a difference in temperature between the surface and the center of the sample exists. At the end of the process, the induced current was turned off and the sample was allowed to cool down into room temperature. Relative density of the sintered samples was measured by the Archimedes method. Microstructural information was obtained from products, which had been fractured and polished. Microstructural analysis of products was made through scanning electron microscopy (SEM). The Vickers hardness of sintered samples was measured by performing indentations at a 5 kg load and a dwell time of 15 s. 3. Results and discussions 3.1. Synthesis of Al2O3–ZrB2 nanocomposite
2.5. High frequency induction heated sintering The synthesized powders were placed in a graphite die (outside diameter, 50 mm; inside diameter, 25 mm; height, 50 mm) and then introduced into the high-frequency induction heated sintering system. Schematic diagram of this method is shown in Fig. 2. The system was first vacuumed and a uniaxial pressure of 20 MPa was applied. An induced current (frequency of about 66 kHz, 30 kW) was then activated and maintained
Fig. 2. Schematic diagram of apparatus for high-frequency induction heated sintering.
Fig. 3 shows the XRD patterns of as-milled samples with different milling conditions. As the patterns include only reactant peaks, it can be concluded that no reaction has been occurred during the milling process. The milling process resulted in broadening and intensity decrease of the XRD peaks. In these patterns, B2O3 reflections are absent, due to the large mass absorption coefficient (MAC) of ZrO2 and Al [23] in comparison to the very small MAC of B2O3 (Table 1). Fe content of as-milled powder was measured by the AAS method. The results of this measurement were presented in Table 2. In all experiments Fe impurity is less than 0.35 wt%. XRD patterns of MASHS samples with different milling conditions and SHS sample are shown in Fig. 4. As seen in these patterns, in samples with BPR of 15:1 and 20:1, there is a small peak of ZrO2. By increasing the energy level of the
Fig. 3. XRD patterns of as milled powders before synthesis: (a) BPR5/5 h, (b) BPR10/5h, (c) BPR15/5 h, (d) BPR15/10 h, (e) BPR15/15 h and (f) BPR20/5 h.
9220
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
Table 1 Mass absorption coefficients of the reactants and products. Phase
B2O3
ZrO2
Al
ZrB2
Al2O3
MAC (cm2 g 1)
8.26
104.15
48.67
111.07
30.91
Table 2 Fe impurity of as-milled powders measured by the AAS method. Mechanical condition
Fe (wt%)
BPR 5/5 h BPR 10/5 h BPR15/5 h BPR15/10 h BPR15/15 h BPR20/5 h
0.04 0.07 0.16 0.17 0.33 0.21
Fig. 5. Evolution of the surface temperature inside the ignition zone: thermograms of the ignition zone for the SHS and the MASHS (BPR5/5 h milled sample) processes-ignition time (tig), ignition temperature (Tig) and combustion temperature (Tc).
Table 3 Ignition delay (tig t0), ignition temperature and combustion temperature for different mechanical activation conditions.
Fig. 4. XRD patterns of (a) not activated, (b) BPR5/5 h, (c) BPR10/5 h, (d) BPR15/5 h, (e) BPR15/10 h, (f) BPR15/15 h, (g) BPR20/5 h milled and synthesized samples in SHS reactor.
reactants via mechanical activation, reactivity increases, resulted in a higher conversion of reactants to side products. Aluminum borate (Al18B4O33) has been observed to be among SHS reaction products of Al, B2O3, and TiO2 reaction. The presence of Al18B4O33 is believed to be partially responsible for unique microstructural properties of SHS synthesized Al2O3/TiB2 composites. High energy milling could activate the formation of minor phases of aluminum borate and excess ZrO2 has remained unchanged. In order to compare both processes (i.e. SHS and MASHS), experiments were carried out under the same conditions. Fig. 5 shows two typical SHS and MASHS temperature profiles. It is clear from Fig. 5 that (1) ignition is achieved in a much shorter time for the MASHS samples and (2) the ignition average temperature is slightly lower than the one recorded for the SHS reaction. The quantitative results are presented in Table 3. In general MASHS Combustion temperature (Tc) is higher than SHS sample and by increasing the milling time and BPR, maximal combustion temperature decreases in MASHS samples.
Mechanical condition
tig t0 (s)
Tig (1C)
Tc (1C)
None (Classical SHS) BPR 5/5 h BPR 10/5 h BPR15/5 h BPR15/10 h BPR15/15 h BPR20/5 h
4.4 2.2 2.5 2.6 2.5 2.1 2.3
1048 945 948 952 937 912 932
1796 1888 1870 1867 1842 1820 1835
Although the temperature profile shape is similar for the SHS and MASHS processes, the time of ignition is significantly shorter for the MASHS samples. The same trend is also observed for the ignition temperature: Tig (MASHS) is 1007 20 K lower than Tig (SHS). These results are in agreement with earlier observations on the Mo–Si [24] and Fe–Al [14] systems. The same behavior has been reported in the reaction involving mechanically activated SiO2 and Al [24]. The MASHS sample particles include very small crystallites in the range of nanometer. Table 4 shows the results of crystallite size, calculated by the Rietveld refinement method. As seen in this table, a nanostructured powder was obtained in all milling conditions. The mean crystallite size of MASHS samples is less than 40 nm. The higher the BPR and milling time, the smaller mean crystallite size was obtained. Due to the presence of excess ZrO2 in samples with BPR 10, 15 and 20 and a negligible difference between mean crystallite size of MASHS samples, the 5 h milled sample with BPR 5:1 was selected as the optimum condition of Al2O3–ZrB2 nanocomposite synthesis. To confirm the Rietveld results, the microstructure of MASHS sample (BPR5/5 h milled) was studied by TEM.
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
9221
Table 4 Calculation of mean crystallite size of Al2O3–ZrB2 composite after synthesis. Parameters
Activation time (h)
BPR 5
Phase Mean crystallite size (nm)
5 10 15
10
15
20
Not activated sample
Al2O3
ZrB2
Al2O3
ZrB2
Al2O3
ZrB2
Al2O3
ZrB2
Al2O3
ZrB2
28 – –
37 – –
33 – –
35 – –
32 27 20
32 31 28
25 – –
29 – –
113
108
Fig. 6. TEM image of Al2O3–ZrB2 powder fabricated by MASHS (BPR 5/5 h milled): (A) bright field image and selected area diffraction pattern (left bottom corner) and (B) energy dispersive X-ray (EDX) analysis of the marked grains.
As seen in Fig. 6A, a nanostructure with the mean crystallite size less than 40 nm was obtained for this powder that is in consistency with the Rietveld results. Sharp rings in the SAD pattern (left bottom corner of Fig. 6A) also confirm the
nanocrystallity of this powder. According to EDX analysis of marked grains (Fig. 6B.) the composition of Grains 1 and 2 can be conferred as Al2O3 and ZrB2, respectively. B element cannot be detected by this method.
9222
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
3.2. High frequency induction heated sintering The variations of shrinkage and temperature versus heating time during the sintering of Al2O3–ZrB2 composite produced by MASHS (5 h milled with BPR 5:1) and the SHS process
Fig. 7. Variation of temperature and shrinkage displacement with heating time during high frequency induction heated sintering of Al2O3–ZrB2 composite synthesized by MASHS and classical SHS techniques.
using HFIHS under 20 MPa pressure and 90% output of total power capacity (30 kW) are shown in Fig. 7. The sintering process took 3 min and maximum temperature reached as high as 1800 1C. Thermal expansion contribution was not accounted in this figure. In both cases, application of induced current resulted in shrinkage of the samples due to consolidation. The shrinkage initiation temperature is found to be 1530 1C and 1280 1C for SHS and MASHS sample, respectively. The relative density of the sintered sample was measured 99% for MASHS sample and 97% for SHS sample by the Archimedes method. Although the consolidation mechanism under induced current heating and pressure is unclear, the accelerated consolidation by HFIHS may be due to a combination of electrical discharge, resistance heating and pressure application effects. Despite the relatively short time, the samples sintered by the HFIHS method were exposed to high temperatures and pressures. Meanwhile, the effect of the electrical induced current on rapid consolidation could also be explained by the rapid heating rate due to Joule heating, and the intrinsic contribution of the current to mass transport [25]. Fig. 8 shows SEM (secondary electron) images of (A and B) the as-synthesized powder, (C and D) the specimen sintered by HFIHS. The as-synthesized powder is round in shape and
Fig. 8. SEM images of Al2O3–ZrB2 composite produced by (A) SHS, (B) MASHS (BPR5/5 h milled) before sintering; (C) SHS and (D) MASHS samples after sintering by HFIHS.
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224
9223
very small crystallites in the range of nanometer. Fe contamination of as-milled powder was measured via the AAS method and found to be less than 0.35 wt% in every sample. Due to the presence of excess ZrO2 in samples with BPR 10, 15 and 20 and a negligible difference between mean crystallite sizes of MASHS samples, the 5 h milled sample with BPR 5:1 was selected as the optimum condition of Al2O3–ZrB2 nanocomposite synthesis. Using the HFIHS method, both MASHS and SHS samples were sintered. The relative density of MASHS and SHS sintered samples were 99% and 97% of their theoretical density, respectively. The average hardness and fracture toughness of MASHS samples were 19.5 GPa and 4.17 0.3 MPa m1/2, respectively. These fracture toughness and hardness of the nanostructured Al2O3–ZrB2 were higher than those of microstructured composite. Fig. 9. Vickers hardness indentation and median crack propagating in Al2O3–ZrB2 composite.
References agglomerated. The average grain size of the sintered composite is not much larger than that of the initial powder and seems to be several microns, indicating the absence of significant grain growth during sintering. This retention of grain size was attributed to the fast heating rate and relatively short-term exposure of the powders to the high temperature. Fig. 9 shows Vickers hardness indentation in Al2O3–ZrB2 sintered composites. The calculated average hardness of five measurements on MASHS and SHS samples were 19.5 and 16.5 GPa, respectively. Indentations with sufficiently high loads produced median cracks around the indent. From the length of these cracks, the fracture toughness can be determined using two expressions. The first expression, proposed by Anstis et al. [26] is 1=2 3=2 E P K IC ¼ 0:016 ð6Þ H C where E is Young's modulus, H is the indentation hardness, P is the indentation load, and C is the trace length of the crack measured from the center of the indentation. The second expression proposed by Niihara et al. [27] is c 3=2 K IC ¼ 0:023 H v a1=2 ð7Þ a where c is the trace length of the crack measured from the center of the indentation, a is the half of average length of two indent diagonals, and Hv is hardness. The toughness values calculated using the two methods for MASHS and SHS samples were 4.1 7 0.3 and 2.37 0.3 MPa m1/2, respectively. These fracture toughness and hardness values of nanostructured Al2O3–ZrB2 (MASHS sample) were higher than those of microstructured Al2O3–ZrB2 [28] due to the refinement of the grain size. 4. Conclusion Al2O3–ZrB2 nanocomposite was successfully synthesized by MASHS technique. The MASHS sample particles included
[1] W.G. Fahrenholtz, G.E. Hilmas, I.G. Talmy, J.A. Zaykoski, Refractory diborides of zirconium and hafnium, J. Am. Ceram. Soc. 90 (2007) 1347–1364. [2] J. Liu, P.D. Ownby, Enhanced mechanical properties of alumina by dispersed titanium diboride particulate inclusions, J. Am. Ceram. Soc. 74 (1991) 241–243. [3] B. Li, J. Deng, Addition of Zr–O–B compounds to improve the performances of alumina matrix ceramic materials, J. Alloys Compd. 473 (2009) 190–194. [4] C. Liu, J. Sun, X. Zhang, Y. Ma, N. Qi, Pressureless sintering of alumina matrix ceramic materials improved by Al–Ti–B master alloys and diopside, Ceram. Int. 35 (2009) 1327–1333. [5] C. Liu, J. Zhang, J. Sun, X. Zhang, Addition of Al–Ti–B master alloys to improve the performances of alumina matrix ceramic materials, Ceram. Int. 33 (2007) 1319–1324. [6] D. Jianxin, C. Tongkun, L. Lili, Self-lubricating behaviors of Al2O3/TiB2 ceramic tools in dry high-speed machining of hardened steel, J. Eur. Ceram. Soc. 25 (2005) 1073–1079. [7] F. Monteverde, A. Bellosi, S. Guicciardi, Processing and properties of zirconium diboride-based composites, J. Eur. Ceram. Soc. 22 (2002) 279–288. [8] B. Jamal Abbasi, M. Zakeri, S.A. Tayebifard, Mechanochemical synthesis of Al2O3–ZrB2–ZrO2 nanocomposite powder, Mater. Res. Bull. 49 (2014) 672–676. [9] S.K. Mishra, S.K. Das, V. Sherbacov, Fabrication of Al2O3–ZrB2 in situ composite by SHS dynamic compaction: a novel approach, Compos. Sci. Technol. 67 (2007) 2447–2453. [10] S. Postrach, J. Pötschke, Pressureless sintering of Al2O3 containing up to 20 vol% zirconium diboride (ZrB2), J. Eur. Ceram. Soc. 20 (2000) 1459–1468. [11] A.G. Merzhanov, The chemistry of self-propagating high-temperature synthesis, J. Mater. Chem. 14 (2004) 1779–1786. [12] H.C. Yi, J.J. Moore, Self-propagating high-temperature (combustion) synthesis (SHS) of powder-compacted materials, J. Mater. Sci. 25 (1990) 1159–1168. [13] S.K. Mishra, S.K. Das, P. Ramachandrarao, D. Yu Belov, S. Mamyan, Synthesis of zirconium diboride–alumina composite by the self-propagating, high-temperature synthesis process, Metall. Mater. Trans. A 34 (2003) 1979–1983. [14] F. Charlot, F. Bernard, E. Gaffet, D. Klein, J.C. Niepce, In situ timeresolved diffraction coupled with a thermal i.r. camera to study mechanically activated SHS reaction: case of Fe–Al binary system, Acta Mater. 47 (1999) 619–629. [15] V. Gauthier, C. Josse, F. Bernard, E. Gaffet, J.P. Larpin, Synthesis of niobium aluminides using mechanically activated self-propagating
9224
[16] [17] [18] [19] [20]
[21]
[22]
B. Jamal Abbasi et al. / Ceramics International 40 (2014) 9217–9224 high-temperature synthesis and mechanically activated annealing process, Mater. Sci. Eng. A 265 (1999) 117–128. P. Mossino, Some aspects in self-propagating high-temperature synthesis, Ceram. Int. 30 (2004) 311–332. F.H. Froes, O.N. Senkov, E.G. Baburaj, Synthesis of nanocrystalline materials – an overview, Mater. Sci. Eng. A 301 (2001) 44–53. M.A. Meyers, A. Mishra, D.J. Benson, Mechanical properties of nanocrystalline materials, Progress Mater. Sci. 51 (2006) 427–556. J. Jung, S. Kang, Sintered (Ti,W)C carbides, Scr. Mater. 56 (2007) 561–564. S.W. Kim, K.A.-R. Khalil, High-frequency induction heat sintering of mechanically alloyed alumina–yttria-stabilized zirconia nano-bioceramics, J. Am. Ceram. Soc. 89 (2006) 1280–1285. H.C. Kim, I.J. Shon, Z.A. Munir, Rapid sintering of ultra-fine WC-10 wt % Co by high-frequency induction heating, J. Mater. Sci. 40 (2005) 2849–2854. H. Rietveld, A profile refinement method for nuclear and magnetic structures, J. Appl. Crystall. 2 (1969) 65–71.
[23] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Publishing Co., Reading, Massachusetts, 1956. [24] C. Gras, E. Gaffet, F. Bernard, Combustion wave structure during the MoSi2 synthesis by mechanically-activated self-propagating hightemperature synthesis (MASHS): in situ time-resolved investigations, Intermetallics 14 (2006) 521–529. [25] J.E. Garay, U. Anselmi-Tamburini, Z.A. Munir, Enhanced growth of intermetallic phases in the Ni–Ti system by current effects, Acta Mater. 51 (2003) 4487–4495. [26] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, A critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurements, J. Am. Ceram. Soc. 64 (1981) 533–538. [27] K. Niihara, R. Morena, D.P.H. Hasselman, Evaluation of K Ic of brittle solids by the indentation method with low crack-to-indent ratios, J. Mater. Sci. Lett. 1 (1982) 13–16. [28] S.K. Mishra, P.K.P. Rupa, S.K. Das, V. Shcherbakov, Effect of titanium diluent on the fabrication of Al2O3–ZrB2 composite by SHS dynamic compaction, Compos. Sci. Technol. 67 (2007) 1734–1739.