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Low-temperature microwave sintering of TiN–SiC nanocomposites

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Low-temperature microwave sintering of TiN–SiC nanocomposites
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  Low-temperature microwave sintering of TiN–SiCnanocomposites Dmytro Demirskyi  • Andrey Ragulya Received: 22 August 2011/Accepted: 21 December 2011/Published online: 6 January 2012   Springer Science+Business Media, LLC 2012 Abstract  Densification kinetics study during microwavesintering of titanium nitride-based nanocomposite has beenconducted. A series of TiN–SiC compositions with 1, 3,5 wt% of silicon carbide were microwave sintered at rel-atively low sintering temperatures (900–1,300   C) for0–30 min. The SiC content influenced on heating unifor-mity and final density and grain-size achieved. Densifica-tion process during microwave sintering obeyed themechanism of grain-boundary diffusion with activationenergy of 235 kJ mol - 1 . Microwave sintering resulted infine microstructure ( * 300 nm) and hence high values of micro hardness ( * 20 GPa). Introduction Titanium nitride, owing to its combination of properties(high melting point, high hardness, low electrical resistiv-ity) [1, 2], and chemical and metallurgical stability (high corrosion resistance to acid and alkaline solutions), ispotentially very useful in demanding engineering applica-tions. However, owing to low sintering activities inmicrocrystalline powders [3], TiN has been primarily usedas a coating, rather than as a monolithic ceramic material,or used in composites both as matrix and reinforcementphases [4, 5]. Difficulties associated with densification of nitrides andcarbides arise because they possess high surface energy butlow self-diffusion constant. Therefore, such sinteringresistant materials are usually densified by liquid phasesintering [3]. The other possible route to promote self-diffusion is to decrease in particle size used [6]. It wasreported that decrease in the particle size up to * 50 nmwill result in an increase in self-diffusion coefficient bythree orders of magnitude. Thus, the decrease in initialpowder size also results in the decrease of the initial [7] andfinal [8] sintering temperatures.Since diffusion for densification is enhanced underapplied pressure, titanium nitride was successfully densi-fied by hot-pressing and hot isostatic pressing [9, 10]. The pressure applied combined with applied electric field (thespark plasma sintering techniques) was also successfullyused to consolidate nitrides [11–14]. The other technique that uses external alternating field during sintering processis microwave sintering [15]. Microwave sintering is rela-tively new method for processing of ceramics, metals, andhigh-melting point compounds, which provides fast andvolumetric heating [16]. Use of microwave irradiation wassuccessfully used for synthesis and sintering of nanocrys-talline TiN and its composites at relatively low tempera-tures [17–20]. The aim of this article is to investigate into sinteringkinetics of the TiN-based nanocomposite during micro-wave sintering by studying the densification kinetics. Materials and methods Preparation processThe starting materials used in this experiments were TiNnanoparticles (the average particle size of 80 nm,O 2 \ 1.6 wt% (Hefei Kaier NanoTech & Development D. Demirskyi ( & )    A. RagulyaFrantsevich Institute for Problems in Material Science,3 Krzhizhanovsky str., 03680 Kyiv-142, Ukrainee-mail: dxd43@ipms.kiev.uaA. Ragulyae-mail: ragulya@ipms.kiev.ua  1 3 J Mater Sci (2012) 47:3741–3745DOI 10.1007/s10853-011-6224-y  Co., Ltd.)) as matrix, and the SiC powder (particle size of 55–60 nm, O 2 \ 1.2 wt%, (IPMS, NASU)) as reinforce-ment phase; no sintering aids were used in the presentstudy. The TiN/SiC nanocrystalline (nc) composite powderwas obtained by wet-chemical mixing in alcohol, with lowcalcinations temperatures ( * 100   C) to remove moisture.The composite nanopowders were uniaxially pressed in acarbon steel die at 100 MPa to form rectangular specimensand subsequently compacted in a cold isostatic press at250 MPa .These green bodies were microwave sintered in 2.45-GHz, 2-kW power multimode microwave applicator attemperatures 900–1,300   C for 0–30 min. To avoid addi-tional errors in electromagnetic field distribution andtemperature measurements [20], the temperature wasmonitored using infrared pyrometer—Raytek MaraphonSeries (working temperature range 350–2,000   C) fromsamples’ surface and recorded in situ by a computer. Afterthe sintering temperature was reached, the isothermalholding was applied. Throughout the sintering, nitrogen gasflow was maintained at 2 L min - 1 .Properties and microstructure measurementsThe shrinkage, weight loss, and the density of sinteredsamples were measured by vernier/micrometer caliper,electronic balance and Archimedes displacement method(ASTM B963-08), respectively. The theoretical density of composites was calculated according to the rule of mix-tures. Phase identification was performed by X-ray dif-fraction (XRD) method on (Dron 3) with Cu K  a  radiation at40 kV and 50 mA at room temperature, with a scanningspeed of 1   per minute. The morphology, element contentof the sintered samples was observed by scanning electronmicroscopy (SEM, Model: Hitachi S-3500 N). Microh-ardness was determined by Vickers hardness tester (PMT-3), a load of 200 g was employed at a dwell time of 10 s.The fracture toughness was calculated by the length of thecracks srcinating from the edges of the indentation marks,using method described in-depth by Niihara et al. [21]. Results and discussion Microwave heating processThe general temperature–time profiles of sintering experi-ments of TiN-3 wt% SiC samples in multimode micro-wave applicator are presented in Fig. 1. As pyrometerstarts recording of temperature from 350   C, it may take upto 200 s to heat up green sample in the microwave furnace,heating to set temperature was made possible by control-ling the power (0.4–0.6 kW); the microwave generatorpower level was suitably reduced to get constant temper-ature for soaking period. For all sintering experimentsconducted, the heating rate of 200   C min - 1 was main-tained. The increase in SiC content generally resulted inlowering the microwave generators’ power applied duringheating process, because SiC is one of the best absorbers of microwaves, and is usually used as an external absorber topromote microwave heating at low temperatures [15].Sintering kineticsSamples with identical green density ( * 62  ±  1% fromtheoretical) were used for sintering experiments. Everysintering run was repeated at least three times; while theaverage values of relative densities were used for furthercalculations.For studying densification kinetics of microwave sin-tering, the densification parameter  v  as defined below [22]was used: v  ¼  q s    q 0 q te    q 0 ¼  kt  m ;  ð 1 Þ where,  q 0 ,  q s , and  q te  correspond to density of pressed,density of current sample at given temperature and soakingtime, and density of fully dense sample, respectively;  k   isthe linear parameter; and  m  is the time exponent of sin-tering equation. Parameters  k   and  m  of equation can becalculated as parameters of line’s equation for sinteringdata presented in logarithmic coordinates.Previous studies [22, 23] suggest the usage of densifi- cation parameter  v  is more accurate in comparison withcommon  dL/L  * t   kinetics, as it allows us to comparesintering process of given materials with various parame-ters applied. In present study, the SiC content and soakingconditions were such parameters. Fig. 1  Time–temperature profile of experiments during microwavesintering of TiN-3 wt% SiC nanocomposite using 2.45-GHz multi-mode applicator3742 J Mater Sci (2012) 47:3741–3745  1 3  Densification kinetics results for microwave sintering of TiN–SiC composites with various SiC content are pre-sented in Fig. 2 in logarithmic coordinates for 1,200   C.The SiC content does not influence significantly on thenature of curves obtained while dwell time applied, but,more importantly, it affects the final density of compositeafter microwave sintering. The final density for compositeswith 5% wt of SiC is by 4% higher than those obtained for1 wt% and 3 wt%. This may be addressed to the fact thatimplantation of ceramic powder may raise the so-called‘‘shielding effect’’ limit, that is where conductors liketitanium nitride become reflectors for microwaves andfurther densification or heating is negligible [24].Densification kinetics for TiN-3 wt% SiC compositeswith various soaking temperatures applied (1,000, 1,100,and 1,200   C) are presented in Fig. 3, and the resultingdependence of densification parameter  v  in logarithmicform in Fig. 4. As seen from Figs. 3 and 4, there are areas of linearity for experimental data obtained. It is clear that,such data may be approximated using straight line equa-tion. Resulting parameters  k   and  n  were calculated and areused for evaluation of and activation energy of densifica-tion process during microwave sintering [23]. What is moreimportant, the value curve’s slopes which can be calculatedby least squares method is identical to the theoretical val-ues that are predicted for mechanism of grain–boundarydiffusion  m * 0.33 [25, 26]. The form of Eq. 1 allows one to calculate activationenergy of densification process [22]. Thus, by plotting log( v 1/m T t  - 1 ) against reciprocal temperature (Fig. 5), weshall obtain linear parameter  k   in form  k   =  k  0  exp ( - Q a  / RT), where  k  0  is the pre-exponential factor, and  Q a  is the Fig. 2  Sintering kinetics during microwave sintering of nanocrystal-line TiN–SiC composites with various SiC contents at 1,200   C Fig. 3  Sintering kinetics during microwave sintering TiN-3 wt% SiCcomposite at various soaking temperatures used Fig. 4  Densification kinetics of TiN-3 wt% SiC nanocompositeduring microwave sintering at 1,000–1,200   C Fig. 5  Arrhenius plot of normalized densification parameter againstreciprocal temperature based on data of present investigationJ Mater Sci (2012) 47:3741–3745 3743  1 3  activation energy of the densification process. The value of an activation energy obtained in this manner is 235  ±  21kJ mol - 1 for all compositions used. Though there is someobvious mismatch between 1 and 3 wt% SiC compositions,it is less than 3% of the value for composition with 5 wt%SiC, which results in minor difference in pre-exponentialfactor ( k  0 ).The value of activation energy obtained 235  ±  21kJ mol - 1 is close to data previously obtained by conven-tional sintering process for pure TiN 230 kJ mol - 1 [3],which was attributed to grain–boundary diffusion; however,it is much less then values as obtained elsewhere: 460 kJmol - 1 [27]. The lowering of an activation energy for thedensification process may be caused by the fact that, fornanocrystalline materials, some coalescence and majordensification usually take place during heating and initialstages of sintering process [28]. The fact that grain–bound-aries and various defects usually are heated more preferablyduring microwave sintering [29] may also be offered as anexplanation. Also, in the present case, the local temperatureobserved in contact points between particles may be muchhigher than on specimen’s surface [30]. This mismatch intemperature may be responsible for mass-transport activa-tion and thus cause errors in calculation of activation energywhileusingArrheniusplot.Theotherpossibleexplanationisthe effect of surface diffusion that does not contribute todensification, but, in case of submicron and nanoparticles, itis active during initial stage of sintering before any mea-surable densification [31]. Alternatively, influence of SiC asthe secondary phase, and grain-growth during microwavesintering (final grain size was * 300  ±  27 nm for 1,300   C(Fig. 6b)) may affect activation energy obtained [32]. In order to confirm grain–boundary diffusion as main sinteringmechanism during microwave sintering of TiN–SiC com-posites, in-depth studies on grain-growth and densificationkinetics (for TiN–SiC and pure TiN) are considered as thenext step in a forthcoming research.Phase compositionThe XRD pattern of the 1,300   C sample revealed thatTiN, SiC, and Ti 3 O 5  was the major detectable oxide phase(Fig. 7). The XRD patterns for the 1,000   and 1,100   Csamples were similar (not shown), which indicates thatTi 3 O 5  was present at least since the intermediate sinteringstage. The intensities of the oxide phase also increasedfrom the 1,000   C sample to the 1,300   C sample, whichmay suggest that this phase crystallized out of amorphousTiO  x  as was reported by Castro and Ying previously [33].Mechanical propertiesOn average, a microhardness of 20  ±  4.2 GPa was deter-mined for sample sintered at 1,300   C, which is at the lowend of the range of known hardness data (21.5–26 GPa)[34, 35]. The  K  1c  values were comparatively low—3.6  ±  1.2 MPa m 1/2 and might be attributed to the fact thatdensities of  * 93.5  ±  1.2% from theoretical values where Fig. 6  Microstructures of fractured sections of TiN-5 wt% SiC samples sintered at  a  900 and  b  1,300   C (soaking time 10 min) Fig. 7  X-ray diffraction pattern of TiN-5 wt% SiC sample sintered at1,300   C3744 J Mater Sci (2012) 47:3741–3745  1 3  achieved during present investigation, and to the presenceand the coarsening of the secondary oxide phase. Concluding remarks TiN–SiC nanocrystalline composites were densified bymicrowave sintering at the temperature interval 900–1,300   C. The heating rate during sintering experiments wasidentical for all compositions used, and the resulting densifi-cation kinetics reveled grain-boundary diffusion as mainsintering mechanism with an activation energy of 235 ±  21 kJ mol - 1 .Thevaluesofmicrohardness( * 20 GPa)andfracture toughness obtained (3.6  ±  1.2 MPa m 1/2 ) might beinfluenced by the presence of minor oxide phase. Acknowledgements  This study was supported by STCU #4259.The authors thank Dr. M. Gadzira (IPMS NASU) for providingnanocrystalline SiC powder used in the present investigation. References 1. Pierson HO (1996) Handbook of refractory carbides and nitrides:properties characteristics processing and applications. NoyesPublications, New Jersey2. Munsten A, Sagel K, Schlamp G (1954) Nature 174:11543. Kuzenkova MA, Kislyi PS (1971) Powder Metall Metal Ceram10:1254. 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