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Densification behaviour of pure molybdenum powder by spark plasma sintering
Int. Journal of Refractory Metals & Hard Materials 28 (2010) 550–557
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Densification behaviour of pure molybdenum powder by spark plasma sintering R. Ohser-Wiedemann *, U. Martin, H.J. Seifert, A. Müller Institute of Materials Science, TU Bergakademie Freiberg, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany
a r t i c l e
i n f o
Article history: Received 18 December 2009 Accepted 15 March 2010
Keywords: Molybdenum Spark plasma sintering Densification Microstructure
a b s t r a c t Pure molybdenum was sintered with SPS under various temperatures, external pressures and heating rates. The microstructure of the specimens representing the different sintering conditions was investigated by classical metallographic methods. The relative density, the microhardness and the chord length distribution were measured. Linear shrinkage, depending on time or temperature, was calculated from piston travel, which was recorded during sintering process. These results show that the main part of consolidation takes place during fast heating up. The densification behaviour is controlled mainly by sintering temperatures and applied pressure. The molybdenum powder was successfully consolidated by SPS in very short times. A relative density of 95% was reached by sintering temperatures of 1600 °C and external pressure of 67 MPa. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Pure molybdenum (Mo) is an important refractory metal used for a wide scale of engineering applications, due to advantageous properties. Some examples are ribbons and wires for lighting technology, semiconductor base plates for power electronics, electrodes for glass melting, parts for high-temperature furnaces, spraying wires for automotive industry and sputter targets or evaporation sources for coating technology. Components from pure molybdenum or its alloys are produced either by powder metallurgy or by melting processes. Generally, powder metallurgical route is preferred since a fine-grained microstructure is obtained, which improve the mechanical properties of final products considerably. Additionally, the powder route is the exclusive possibility for production of molybdenum–copper–alloys or for doping molybdenum with high-temperature resistant oxides [1]. Spark plasma sintering (SPS) is a new technology for compaction of metallic or ceramic powders. It is a short-time sintering process, where powder particles are compacted by uniaxial pressing and heating simultaneously. The heating results from a pulsed electric field and a high heating rate of more than 100 K/min can be achieved. Compared to conventional sintering methods applying external pressure like Hot Pressing (HP) or Hot-Isostatic-Pressing (HIP), densification by SPS is extremely fast. Thus, the sintering temperatures can be lower which limits the grain growth.
* Corresponding author. Tel.: +49 3731 39 2647; fax: +49 3731 39 3657. E-mail address: [email protected] (R. Ohser-Wiedemann). 0263-4368/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2010.03.003
This allows the sintering of nanosized or metastable powders close to the theoretical density with slight grain growth or retention of metastability as well as cleaned grain boundaries. Further advantages are sintering of powders without any additives, no need of cold compaction and less sensitivity to initial powders characteristics. SPS is therefore an economical alternative to conventional sintering [1–9]. However, the densification mechanism during SPS process is unclear as yet [8]. It is assumed that small spark discharges appear between individual powder particles causing locally high temperatures in few minutes. These high temperatures induce vaporization or melting of the powder surface, destruction of oxide layers and raising sinter activity. Necks between the powder particles are formed. This and the high compressive force lead to the sintering of the particles [2,3,5,10]. However, the existence of spark plasma in SPS process is not clearly demonstrated, in particular when non-conductive powders are used. However, the fundamental effect of electric current on mass transport has been clearly shown, elucidated in [8]. The electrical current affects also the diffusion kinetics of reaction between different phases, for example between Mo and Si, see [6]. Early densification stages of nanocrystalline ceramics by SPS were discussed by Chaim [7], where the author identified two atomistic mechanisms. Nanocrystalline ceramics with low yield stress were mainly compacted by plastic deformation. Ceramics with high yield stress consolidate dominantly by collective grain rotation and sliding. This is a hint on the important role of applied pressure during spark plasma sintering. First studies on short-time sintering of molybdenum powders by Plasma Pressure Compaction (P2C) are reported in [9,11,12]. Similar to the SPS process, the powders were filled in a graphite die. The heating by P2C was carried out at first with pulsed electric
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current up to 1100 °C, followed by heating with constant current up to the final sintering temperature. Microsized powders (average size 47 lm) were consolidated at 1650 °C, 48 MPa, for 1–2 min, up to a relative density of 98%. ‘‘Nanosized” powders (average size 0.1 lm) were sintered at lower temperatures (1400 °C, 48 MPa, 3 min) up to a relative density of 97%. The smaller grain size of nanocrystalline samples leads to a higher microhardness (2.95 GPa) in comparison to the microcrystalline samples (2.16 GPa). Clean grain boundaries coupled with uniform grain structure were taken as evidence for the occurrence of surface cleaning during plasma activation stage. Furthermore, Mo–Si–B multiphase alloys were successfully consolidated by SPS at 1200 °C and an applied pressure of 40 MPa [13]. Unfortunately, the relative densities after sintering are not specified. The motivation of the present work was to estimate the best SPS conditions for consolidation of pure molybdenum powder and to achieve information about the densification behaviour of molybdenum during this process, compared with short-time sintering by P2C and isothermal sintering.
2. Experimental Pure molybdenum powders (99.95 wt%) with particle size of 3– 5 lm (produced by Plansee Metall GmbH Austria) were consolidated by SPS. Before consolidation, the powder contains an oxygen impurity of 400 ppm, as indicated by the producer of the material. The powder particles (Fig. 1) are of spherical shape, some of them include small pores. The particles agglomerates and tend to form chains. The powders contain a fraction of fine particles with sizes <1 lm. The powders were filled without any additives in cylindrical graphite dies either of 20 or 40 mm in diameter. The sintering runs in a FCT-HP D 25 spark plasma sinter equipment, manufactured by FCT Systeme GmbH (Germany). The compaction of all batches starts with raising the external pressure up to 29, 57 and 67 MPa, respectively, followed by heating up to temperatures between 850 and 2000 °C under vacuum. The heating rate was diversified between 130 and 360 K/min, and the holding time at maximum temperature was 3 min for all batches. The pulse pattern of the electric current was adjusted on 2:1 (10 ms pulse time, 5 ms pause time). The compaction pressure decreased during cooling step. Fig. 2 gives an example of two typical SPS runs. Table 1 contains sintering parameters of all batches. The densities of all consolidated bulk molybdenum specimens were measured by the Archimedes technique [14,15]. The relative
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Fig. 2. Typical SPS runs with different heating rates.
densities were calculated based on the theoretical density of 10.28 g cm 3, listed in [1]. The microstructure was studied by optical microscopy as well as by scanning electron microscopy at cross sections (sections parallel to the acting force). The preparation steps were grinding, mechanical polishing up to 3 lm diamond abrasive, followed by short electrolytic polishing and chemical etching using Murakami’s reagent (a solution mixture of 2.5 g potassium hydroxide, 2.5 g potassium ferricyanide and 50 ml of distilled water). For the batches 3.1–3.7, the grain size was estimated by conventional metallographic methods. In our setting, the grain size distribution is defined as the chord length distribution, which was measured at least as five measuring fields per cross sections using the software ‘‘a4i Analysis” of aquinto AG. The hardness of bulk molybdenum specimens was measured at cross sections either by nanohardness tester (CSM Instruments) or by microhardness tester (LECO Instrumente GmbH). The nanoindentations were generated using a BERKOVICH-indenter, which was loaded with a maximum load of 30 mN. The indentations (20 in samples with high density, 40 in samples with low density) were individually located inside of grains, away from grain boundaries and pores. Basing on the recorded load-displacement-curve, the indentation hardness and the indentation modulus were calculated as described in [16]. The microhardness HV0.1 was measured with a VICKERS indenter. The X-ray diffraction (XRD) spectra of the carburized outer zone were recorded with URD65 (Seifert FPM) powder diffractometer using Cu–Ka radiation and a graphite monochromator in the diffracted beam. The PDF data base [17] was used for qualitative phase analysis. Quantitative pole figures were obtained by measuring with the XRD system D8-Discover from Bruker AXS using Co– Ka radiation. 3. Experimental results 3.1. Density
Fig. 1. Molybdenum powder before consolidation.
The measured densities and the calculated relative densities of all sintered samples are listed in Table 1. Fig. 3 shows the relative densities depending on sintering temperatures and pressures. The correlation between relative density and temperature follows the well-known sigmoid course of heating up during isothermal sintering. A relative density of 95% and higher can be reached if the sintering temperature excites 1600 °C and the external pressure amounts 57 MPa or more. In the experiments with low external pressure of 29 MPa, the relative density does not exceed 95% even at high temperatures. This indicates the applied pressure as an important factor for the densification during the SPS process. The heating rates, studied in the present article, do not influence the
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Table 1 Sintering parameters and results of density and nanohardness measurements. Batch
Heating rate (K/ min)
Final sintering temperature (°C)
Pressure (MPa)
Final density (g/ cm3)
Final relative density (%)
Indentation hardness (MPa)
Indentation modulus (GPa)
1.1 1.2 1.3 2.1 2.2 2.3 2.4 3.1 3.2 3.3 3.4 3.5 3.6 3.7
130 130 130 200 130 360 360 200 325 200 280 340 360 200
1600 1800 1900 1400 1600 1800 1900 850 1100 1400 1600 1800 1900 2000
29 29 29 57 57 57 57 67 67 67 67 67 67 67
9.19 9.47 9.50 9.30 9.70 9.80 9.80 5.96 7.18 9.24 9.77 9.84 9.85 9.88
89.4 92.1 92.4 90.5 94.4 95.3 95.3 58.0 69.0 89.9 95.0 95.7 95.8 96.1
1766 ± 194 1646 ± 185 1810 ± 138 1765 ± 194 2009 ± 169 1934 ± 88 2025 ± 143 – 1504 ± 279 1665 ± 220 2085 ± 99 2152 ± 96 2151 ± 84 1751 ± 91
214 ± 22 280 ± 43 227 ± 21 214 ± 22 246 ± 16 281 ± 30 288 ± 26 – 123 ± 13 233 ± 28 260 ± 21 289 ± 29 291 ± 31 288 ± 31
Fig. 3. Relative density of spark plasma sintered molybdenum vs. sintering temperature, compared with other published data.
density of molybdenum samples, significantly. This is in accordance with the densification behaviour of copper powders with SPS [10]. Finally, it is observed that the density of spark plasma sintered bulk molybdenum is primarily controlled by external pressure and temperature. Our results are in good agreement with the P2C results in [9,11,12], where a relative density of 98% was measured at 1650 °C and at 48 MPa external pressure. Additionally, we compare the densities obtained by SPS with those of isothermal sintering. The isothermal powder metallurgical route starts with hydraulic or isostatic pressing of the molybdenum powder into rods and plates of various geometries and dimensions. The sintering process is carried out in furnaces at high temperatures (typically in the range of 1800–2200 °C) and in hydrogen atmosphere over long times (2–3 h) to get densities about 90% of the theoretical density [1,18–24]. For higher densities, hot rolling, extrusion or forging at temperatures in the range 1200–1500 °C are subsequently used [1]. Fig. 3 contains relative densities published in [19], which were estimated after 1 h isothermal sintering. For temperatures higher than 1400 °C, the comparison shows that the relative densities of SPS are comparable or higher than relative densities generated by isothermal sintering, although the sintering time is very short at SPS. At lower temperatures isothermal sintered molybdenum bodies becomes denser caused by the higher green density due to higher compaction pressure. 3.2. Nanohardness In spite of marginal loading during nanoindentation, porosity has also strong influence on the values of indentation hardness
Fig. 4. Results of nanohardness measurements depending on sintering temperature and relative density.
and indentation modulus (Fig. 4, Table 1). Sintered bodies with low density apparently exhibit lower hardness and modulus. If pores occur beneath grains, the indentation process is overlapped by elastic recovery, plastic deformation or crack propagation. In order to exclude the influence of porosity, measured hardness values of each sample were divided by their relative density. In this case, the indentation hardness’s of all investigated samples are in the same range, independent on sintering conditions. The average indentation hardness is 2057 ± 152 MPa, which corresponds to results of hardness measurement of P2C sintered molybdenum powders in [12]. Furthermore, the indentation modulus depends on the porosity. At relative densities up to 95%, an average value of 297 ± 12 GPa can be calculated which shows an excellent agreement with Youngs modulus of 305 ± 2 GPa, measured with an ultrasonic scanner at different sintered molybdenum samples, and with values from literature for example in [1,24]. The sintering temperature of 2000 °C leads to a softening of the material caused by the large grain size, but keeping the indentation modulus constant. 3.3. Microstructure Fig. 5 gives an impression of microstructure development depending on raising sintering temperature at constant external pressure of 67 MPa. The cross sections exhibit relatively large pores and the observed porosity does not agree with that measured by Archimedes technique. The difference is a consequence of preparation by electrolytic polishing. Caused by the low hardness of
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Fig. 5. Scanning electron micrographs of the microstructure of molybdenum at different sintering temperatures, SPS at 67 MPa pressure.
molybdenum (see above), a deformation layer is generated by mechanical grinding and polishing on the cross sections surface. This deformation layer covers the real microstructure. Additionally, pores can be filled during grinding and mechanical polishing. To remove the deformation layer, electrolytic polishing is necessary. The polishing conditions have to be chosen in such a way that the deformation layer is removed completely. However, it is unavoidable then to enlarge existing pores strongly during this preparation, whereas the etching extends the pores in minor way. The single preparation steps are demonstrated in Fig. 6. After mechanical grinding and polishing the surface appears without contrast, it exhibits small pores and several scratches. At higher magnification the deformation layer is detectable clearly. After electrolytic polishing the surface is clean, the scratches are removed, grain boundaries become apparent, but the pores are strongly enlarged. In contrast, only small material erosion takes place during etching. The first compact sample was produced with maximum sintering temperature of 850 °C and pressure of 67 MPa. Under these conditions a body of high porosity was obtained, where the shapes of origin powder particles are well preserved (Fig. 5). The original shape of particles retains up to sintering temperature of 1100 °C, even if the densification is higher. This is in good agreement with isothermal sintering of molybdenum, where a grain structure at 1100 °C does not appear [19]. At temperatures of 1400 °C or higher, the original shapes of the powder particles get lost and the formation of a polyhedral micro-
structure is observed. The sample still exhibits open porosity, the pores are irregularly shaped and exist mainly between the grains. The average grain size is larger than the initial average grain size. Further increasing of temperature initiates a strong grain growth, which was not expected in such a manner at short-time sintering. For sintering temperatures higher than 1600 °C, the porosity is lower than 5%, i.e. closed porosity appears. The pore size decreases and pores exhibit a more and more spherical shape. Pores are disconnected from grain boundaries during grain growth and are located mainly inside the grains. At the highest temperatures, an agglomeration of pores at grain boundary triples can be observed. Plane grain faces and grain boundary angles of about 120° are observed, which is typical for polycrystalline metals, since the surface stress of the most grain boundaries is similar. In contrast to isothermal sintering of molybdenum in [19], no abnormal grain growth was observed. To investigate accurately the relationship between grain growth and temperature, the chord length distribution of the grains was measured in samples sintered with 67 MPa. Fig. 7 shows the relative frequency of chord length depending on sintering temperature. A continuous grain growth starts at temperatures higher than 1400 °C. This is closely related to a broadening of the grain spectrum. Also, during isothermal sintering this effect occurs, but in contrast, it depends on sintering time [10]. When plotting the mean chord length vs. the sintering temperature in a log–log scale, two straight lines of different slope can be adapted to the date points (Fig. 8). The straight lines intersect at the initial temperature
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Fig. 6. Preparation steps of cross sections: (a) mechanical grinded and polished; (b) electrolytic polished; (c) etched and (d) deformation layer after mechanical grinding and polishing. Hardness indentation marks the equal position on sample surface.
Fig. 7. Grain growth of molybdenum by SPS process. Fig. 9. Grain size, normalized by initial grain size, as a function of the sintering temperature of our experiments compared with those obtained for isothermal sintering.
Fig. 8. Log–log plot of grain growth (67 MPa pressure) vs. sintering temperature.
for the grain growth. For pure molybdenum the initial temperature is about 1330 °C, which corresponds to a homologous temperature of 0.56 Tm. Fig. 9 shows the grain size as a function of the sintering temperature of our experiments compared with those obtained for isothermal sintering [19,20]. The data are normalized by the initial
grain size. Up to about 1700 °C the grain sizes of SPS samples are smaller than those of isothermal sintering. This effect is due to the shorter sintering times of SPS. Finally, the starting temperature of grain growth was calculated from the slope of the curves for isothermal sintering (as in Fig. 8). The grain growth starts at temperatures between 950 and 1035 °C, which are lower than the SPS starting temperature of 1330 °C. The huge difference is a consequence of the fast heating up in the SPS process. The contact of molybdenum powder with the graphite die and sheets leads to diffusion of carbon inside the samples, although the spark plasma sintering is a short time process. The micrograph (Fig. 10) shows a sharp transition between carburized region and uninfluenced molybdenum body. The thickness of the layer depends on temperature and can reach 200 lm at 2000 °C. The layer grows in a parabolic manner, which allows a rough estimation of the activation energy of carbon diffusion in molybdenum. We give an estimate of 180 kJ mol 1, which is in agreement with the values of literature, e.g. in [25]. The carburization kinetics is comparable with the boriding kinetics in molybdenum under SPS conditions, investigated in [26].
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Fig. 11. Experimental and corrected piston travel and the corresponding baseline during sintering of molybdenum powder under a pressure of 67 MPa. The piston travel, caused by the applied pressure before sintering, was subtracted. Fig. 10. Carburized region after sintering at 1800 °C.
In the carburized layer, the microhardness rises up to 1300 HV 0.1 and cracks are created. The carbon concentration amounts ca. 7 wt% which is consistent with the carbon content of Mo2C-carbide. The results of X-ray phase analyses detected, that the carburized region was completely converted in the hexagonal Mo2Ccarbide. The measured diffraction pattern shows a preferred orientation of the (0 0 2) reflection. Texture measurements verify occurrence of a (0 0 1) fibre texture modified by a coarse grain structure. The fibre texture is caused by columnar grain growth, which was observed by light microscopy.
3.4. Densification behaviour The force adjustment of our SPS equipment is starting with a load of 5 kN. At this point, the motion of piston is recorded during whole sintering process. The so called ‘‘relative piston travel” contains information about linear intrinsic shrinkage of every sample. For interpretation of the records, the experimental data must be corrected by subtracting the contribution of piston motion caused by compression during applied pressure before sintering starts (see Fig. 2). This is necessary, because this first compaction depends on the applied pressure as well as on the die filling behaviour of powder varying with every batch. Furthermore, the thermal expansion of sample, die and graphite sheets must be subtracted. For this correction, which is explained in [6], baselines were determined experimentally. The baselines were recorded with heating rate of 200 K/min, maximal temperature of 2000 °C and pressure of 67 and 57 MPa, respectively. Each baseline can be described by a polynomial, and this is valid for all experimental data recorded at the same pressure and with the same die. Fig. 11 shows the experimental and corrected piston travel and the corresponding baseline during sintering of molybdenum powder under a pressure of 67 MPa. Fig. 12 gives examples of corrected piston travels depending on different heating rates vs. sintering time. The curves of all samples are very similar in their shape but they differ in their slopes depending on the heating rates. On the curves one can observe typical points (in Fig. 12 marked by arrows) where the slope changes considerably. These points are visible in particular for higher sintering temperatures (cf. Fig. 12, 1800 °C). By these points, each curve can be divided into parts in the following denoted with Roman numerals. Parts I–III are related to the heating up, the flat part IV is controlled by holding time at sintering temperature. The large
Fig. 12. Corrected piston travel vs. sintering time for different heating rates (pressure of 67 MPa).
Table 2 Values of sample high before sintering. Batch
Final height after sintering lf (mm)
Final corrected piston travel Dlf (mm)
Height before sintering, l0 (mm)
3.1 3.2 3.3 3.4 3.5 3.6 3.7
7.74 6.43 4.18 3.89 4.62 4.65 3.71
0.37 0.97 2.06 2.54 3.79 3.86 3.43
8.11 7.40 6.24 6.43 8.41 8.51 7.14
slopes in parts II and III indicate that the consolidation takes place mainly during fast heating up of SPS. Linear shrinkage Dl/l0 can be calculated basing on the corrected piston travel, where Dl is corrected piston travel depending on sintering time and temperature, respectively. The sample height l0 before sintering starts (in our investigation after the cold pressing step) can not be estimated exactly. It can be approximated by the sum of the final sample height lf (after sintering) and the final corrected piston travel Dlf (values see Table 2). If this linear shrinkage plotted vs. temperature during heating up, the curves of all samples, compacted with the same pressure, cover each other with slight deviations (Fig. 13). As pointed out in Section 3.1, the densification of molybdenum by SPS process depends strongly on the acting temperature, generated by resistance heating in the powder.
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Fig. 13. Corrected piston travel of different sintered batches (pressure of 67 MPa) vs. temperature during heating up.
The parts I up to III marked in Fig. 12 can be observed again and their transitions can now be linked to characteristic values of temperature, shrinkage, porosity and mean grain size, as demonstrated in Fig. 13. The reason for the small shift of the curves of Fig. 13 is not clear. 4. Discussion As the densification process is thermally activated, a sigmoid progression of sintering curve was expected, comparable with such of isothermal sintering [27,28], where in our curves the part I exhibits a linear relationship between Dl/l0 and temperature and a sharp transition to second part starting at 900 or 950 °C, respectively. This transition temperature is consistent with 0.4 Tm of Mo, the temperature, where appreciable volume diffusion starts in solid materials. The end of this part is related to a small shrinkage of 5% and a high porosity of 40%. The densification rate is low. As shown in Section 3.3, grain growth and change in particle shape were not observed. Part II is characterised by the highest densification rate of sintering process. The shrinkage achieves more than 30% and the porosity decreases to 15%, where the micrographs do not show any change in particle shape up to 1100 °C. Nevertheless, the density increases and the number and sizes of contact areas between particles are enlarged. Furthermore, the particle strength decreases further, which makes particle rotation and sliding easier. A phenomenological description of the consolidation process during parts I and II is an open problem. The grain growth starts with the beginning of the third part at 1330 °C. At first the porosity amounts 10%, which marks the transition to closed porosity. During the third part the porosity reduces to 5% and the densification rate decreases continuously. The micrographs (Fig. 5, 1600 °C) show that the pores are rounded and the grain boundaries are separated from pores. Both processes are driven mainly by diffusion. In part IV, where sintering temperature is constant, the densification is comparable to isothermal sintering process. The material transport is supported by the applied pressure and by the electric field, which was demonstrated in [6,8]. 5. Conclusions The results of this investigation demonstrate that molybdenum powder can be successfully consolidated by spark plasma sintering in very short times. A relative density of 95% is achieved at sintering temperatures of 1600 °C and an external pressure of 67 MPa. It can be concluded that the density is primarily controlled by exter-
nal pressure and temperature. In comparison with values from the literature, the densities obtained by SPS exceed those by isothermal sintering. The grain growth during spark plasma sintering was mainly controlled by the sintering temperature. The grain growth starts at 1350 °C, which corresponds to a homologous temperature of 0.56 Tm. A strong grain growth occurs at temperatures higher than 1600 °C. Due to the shorter sintering times of SPS the grain sizes of our samples are smaller than those of isothermal sintering. Nanohardness and indentation modulus was estimated by nanohardness testing. It was shown that the porosity strongly influences both values in spite of the marginal loads used at this test. An average indentation hardness of 2057 ± 152 MPa and an average indentation modulus of 297 ± 12 GPa were estimated, which are in agreement with values for bulk molybdenum. Based on the recorded relative piston travel, the linear shrinkage of molybdenum powder during SPS process can be calculated. The shrinkage of all batches, compacted with the same external pressure, follow the same curve independent of heating rate. The main part of compaction takes place during fast heating up by SPS. In summary, our investigation shows that the best results of molybdenum powder spark plasma sintering are achieved at 1600 °C and 67 MPa. The optimal sintering temperature corresponds to that of Plasma Pressure Compaction, but a higher external pressure is necessary for SPS. Acknowledgments The authors are grateful for the financial support of this work by Dr. Erich Krüger Stiftung and for the abandonment of the molybdenum powder by Dr. A. Hoffmann, Plansee Metall GmbH Reutte. References [1] Plansee high performance materials, Molybdenum – material properties and applications. The Official Website of the Plansee Group, [accessed 15.3.2010]. [2] Tokita M. Mechanism of spark plasma sintering and its application to ceramics. Nyu Seramikkusu 1997;10:43–54. Alternative: SPS SYNTEX INC, What’s SPS?. The Official Website of the SPS SYNTEX INC, [accessed 15.3.2010]. [3] Omuri M. Sintering, consolidation, reaction and crystal growth by the spark plasma system (SPS). Mater Sci Eng A 2000;287:183–8. [4] Chen W, Anselmi-Tamburini U, Garay JE, Groza JR, Munir ZA. Fundamental investigations on the spark plasma sintering/synthesis process. I. Effect of dc pulsing on reactivity. Mater Sci Eng A 2005;394:132–8. [5] Anselmi-Tamburini U, Gennari S, Garay JE, Munir ZA. Fundamental investigations on the spark plasma sintering/synthesis process. II. Modelling of current and temperature distributions. Mater Sci Eng A 2005;394:139–48. [6] Anselmi-Tamburini U, Garay JE, Munir ZA. Fundamental investigations on the spark plasma sintering/synthesis process. III. Current effect on reactivity. Mater Sci Eng A 2005;407:24–30. [7] Chaim R. Densification mechanism in spark plasma sintering of nanocrystalline ceramics. Mater Sci Eng A 2007;443:25–32. [8] Munir ZA, Anselmi-Tamburini U, Ohyanagi M. The effect of electric field and pressure on the synthesis and consolidation of materials: a review of the spark plasma sintering method. J Mater Sci 2006;41:763–77. [9] Orrú R, Licheri R, Locci AM, Cincotti A, Cao G. Consolidation/synthesis of materials by electric current activated/assisted sintering. Mater Sci Eng R 2009;63:127–287. [10] Zhang ZH, Wang FC, Wang L, Li SK. Ultrafine-grained copper prepared by spark plasma sintering process. Mater Sci Eng A 2008;476:201–5. [11] Srivatsan TS, Ravi BG, Nauka AS, Riester L, Petraroli M, Sudarshan TS. The microstructure and hardness of molybdenum powders consolidated by plasma pressure compaction. Powder Technol 2001;114:136–44. [12] Srivatsan TS, Ravi BG, Petraroli M, Sudarshan TS. The microhardness and microstructural characteristics of bulk molybdenum samples obtained by consolidating nanopowders by plasma pressure compaction. Int J Refract Met Hard Mater 2002;20:181–6. [13] Yamauchi A, Yoshimi K, Kurokawa K, Hanada S. Synthesis of Mo–Si–B in situ composites by mechanical alloying. J Alloys Compd 2007;434–435:420–3. [14] Impermeable sintered metal materials and hardmetals – determination of density [ISO 3369: 1975]. German version DIN ISO 3369; 1990. [15] Sintered metal materials, excluding hardmetals – permeable sintered metal materials – determination of density, oil content and open porosity [ISO 2738: 1999]. German version EN ISO 2738; 2000.
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Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Densification behaviour of pure molybdenum powder by spark plasma sintering R. Ohser-Wiedemann *, U. Martin, H.J. Seifert, A. Müller Institute of Materials Science, TU Bergakademie Freiberg, Gustav-Zeuner-Str. 5, 09599 Freiberg, Germany
a r t i c l e
i n f o
Article history: Received 18 December 2009 Accepted 15 March 2010
Keywords: Molybdenum Spark plasma sintering Densification Microstructure
a b s t r a c t Pure molybdenum was sintered with SPS under various temperatures, external pressures and heating rates. The microstructure of the specimens representing the different sintering conditions was investigated by classical metallographic methods. The relative density, the microhardness and the chord length distribution were measured. Linear shrinkage, depending on time or temperature, was calculated from piston travel, which was recorded during sintering process. These results show that the main part of consolidation takes place during fast heating up. The densification behaviour is controlled mainly by sintering temperatures and applied pressure. The molybdenum powder was successfully consolidated by SPS in very short times. A relative density of 95% was reached by sintering temperatures of 1600 °C and external pressure of 67 MPa. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Pure molybdenum (Mo) is an important refractory metal used for a wide scale of engineering applications, due to advantageous properties. Some examples are ribbons and wires for lighting technology, semiconductor base plates for power electronics, electrodes for glass melting, parts for high-temperature furnaces, spraying wires for automotive industry and sputter targets or evaporation sources for coating technology. Components from pure molybdenum or its alloys are produced either by powder metallurgy or by melting processes. Generally, powder metallurgical route is preferred since a fine-grained microstructure is obtained, which improve the mechanical properties of final products considerably. Additionally, the powder route is the exclusive possibility for production of molybdenum–copper–alloys or for doping molybdenum with high-temperature resistant oxides [1]. Spark plasma sintering (SPS) is a new technology for compaction of metallic or ceramic powders. It is a short-time sintering process, where powder particles are compacted by uniaxial pressing and heating simultaneously. The heating results from a pulsed electric field and a high heating rate of more than 100 K/min can be achieved. Compared to conventional sintering methods applying external pressure like Hot Pressing (HP) or Hot-Isostatic-Pressing (HIP), densification by SPS is extremely fast. Thus, the sintering temperatures can be lower which limits the grain growth.
* Corresponding author. Tel.: +49 3731 39 2647; fax: +49 3731 39 3657. E-mail address: [email protected] (R. Ohser-Wiedemann). 0263-4368/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2010.03.003
This allows the sintering of nanosized or metastable powders close to the theoretical density with slight grain growth or retention of metastability as well as cleaned grain boundaries. Further advantages are sintering of powders without any additives, no need of cold compaction and less sensitivity to initial powders characteristics. SPS is therefore an economical alternative to conventional sintering [1–9]. However, the densification mechanism during SPS process is unclear as yet [8]. It is assumed that small spark discharges appear between individual powder particles causing locally high temperatures in few minutes. These high temperatures induce vaporization or melting of the powder surface, destruction of oxide layers and raising sinter activity. Necks between the powder particles are formed. This and the high compressive force lead to the sintering of the particles [2,3,5,10]. However, the existence of spark plasma in SPS process is not clearly demonstrated, in particular when non-conductive powders are used. However, the fundamental effect of electric current on mass transport has been clearly shown, elucidated in [8]. The electrical current affects also the diffusion kinetics of reaction between different phases, for example between Mo and Si, see [6]. Early densification stages of nanocrystalline ceramics by SPS were discussed by Chaim [7], where the author identified two atomistic mechanisms. Nanocrystalline ceramics with low yield stress were mainly compacted by plastic deformation. Ceramics with high yield stress consolidate dominantly by collective grain rotation and sliding. This is a hint on the important role of applied pressure during spark plasma sintering. First studies on short-time sintering of molybdenum powders by Plasma Pressure Compaction (P2C) are reported in [9,11,12]. Similar to the SPS process, the powders were filled in a graphite die. The heating by P2C was carried out at first with pulsed electric
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current up to 1100 °C, followed by heating with constant current up to the final sintering temperature. Microsized powders (average size 47 lm) were consolidated at 1650 °C, 48 MPa, for 1–2 min, up to a relative density of 98%. ‘‘Nanosized” powders (average size 0.1 lm) were sintered at lower temperatures (1400 °C, 48 MPa, 3 min) up to a relative density of 97%. The smaller grain size of nanocrystalline samples leads to a higher microhardness (2.95 GPa) in comparison to the microcrystalline samples (2.16 GPa). Clean grain boundaries coupled with uniform grain structure were taken as evidence for the occurrence of surface cleaning during plasma activation stage. Furthermore, Mo–Si–B multiphase alloys were successfully consolidated by SPS at 1200 °C and an applied pressure of 40 MPa [13]. Unfortunately, the relative densities after sintering are not specified. The motivation of the present work was to estimate the best SPS conditions for consolidation of pure molybdenum powder and to achieve information about the densification behaviour of molybdenum during this process, compared with short-time sintering by P2C and isothermal sintering.
2. Experimental Pure molybdenum powders (99.95 wt%) with particle size of 3– 5 lm (produced by Plansee Metall GmbH Austria) were consolidated by SPS. Before consolidation, the powder contains an oxygen impurity of 400 ppm, as indicated by the producer of the material. The powder particles (Fig. 1) are of spherical shape, some of them include small pores. The particles agglomerates and tend to form chains. The powders contain a fraction of fine particles with sizes <1 lm. The powders were filled without any additives in cylindrical graphite dies either of 20 or 40 mm in diameter. The sintering runs in a FCT-HP D 25 spark plasma sinter equipment, manufactured by FCT Systeme GmbH (Germany). The compaction of all batches starts with raising the external pressure up to 29, 57 and 67 MPa, respectively, followed by heating up to temperatures between 850 and 2000 °C under vacuum. The heating rate was diversified between 130 and 360 K/min, and the holding time at maximum temperature was 3 min for all batches. The pulse pattern of the electric current was adjusted on 2:1 (10 ms pulse time, 5 ms pause time). The compaction pressure decreased during cooling step. Fig. 2 gives an example of two typical SPS runs. Table 1 contains sintering parameters of all batches. The densities of all consolidated bulk molybdenum specimens were measured by the Archimedes technique [14,15]. The relative
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Fig. 2. Typical SPS runs with different heating rates.
densities were calculated based on the theoretical density of 10.28 g cm 3, listed in [1]. The microstructure was studied by optical microscopy as well as by scanning electron microscopy at cross sections (sections parallel to the acting force). The preparation steps were grinding, mechanical polishing up to 3 lm diamond abrasive, followed by short electrolytic polishing and chemical etching using Murakami’s reagent (a solution mixture of 2.5 g potassium hydroxide, 2.5 g potassium ferricyanide and 50 ml of distilled water). For the batches 3.1–3.7, the grain size was estimated by conventional metallographic methods. In our setting, the grain size distribution is defined as the chord length distribution, which was measured at least as five measuring fields per cross sections using the software ‘‘a4i Analysis” of aquinto AG. The hardness of bulk molybdenum specimens was measured at cross sections either by nanohardness tester (CSM Instruments) or by microhardness tester (LECO Instrumente GmbH). The nanoindentations were generated using a BERKOVICH-indenter, which was loaded with a maximum load of 30 mN. The indentations (20 in samples with high density, 40 in samples with low density) were individually located inside of grains, away from grain boundaries and pores. Basing on the recorded load-displacement-curve, the indentation hardness and the indentation modulus were calculated as described in [16]. The microhardness HV0.1 was measured with a VICKERS indenter. The X-ray diffraction (XRD) spectra of the carburized outer zone were recorded with URD65 (Seifert FPM) powder diffractometer using Cu–Ka radiation and a graphite monochromator in the diffracted beam. The PDF data base [17] was used for qualitative phase analysis. Quantitative pole figures were obtained by measuring with the XRD system D8-Discover from Bruker AXS using Co– Ka radiation. 3. Experimental results 3.1. Density
Fig. 1. Molybdenum powder before consolidation.
The measured densities and the calculated relative densities of all sintered samples are listed in Table 1. Fig. 3 shows the relative densities depending on sintering temperatures and pressures. The correlation between relative density and temperature follows the well-known sigmoid course of heating up during isothermal sintering. A relative density of 95% and higher can be reached if the sintering temperature excites 1600 °C and the external pressure amounts 57 MPa or more. In the experiments with low external pressure of 29 MPa, the relative density does not exceed 95% even at high temperatures. This indicates the applied pressure as an important factor for the densification during the SPS process. The heating rates, studied in the present article, do not influence the
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Table 1 Sintering parameters and results of density and nanohardness measurements. Batch
Heating rate (K/ min)
Final sintering temperature (°C)
Pressure (MPa)
Final density (g/ cm3)
Final relative density (%)
Indentation hardness (MPa)
Indentation modulus (GPa)
1.1 1.2 1.3 2.1 2.2 2.3 2.4 3.1 3.2 3.3 3.4 3.5 3.6 3.7
130 130 130 200 130 360 360 200 325 200 280 340 360 200
1600 1800 1900 1400 1600 1800 1900 850 1100 1400 1600 1800 1900 2000
29 29 29 57 57 57 57 67 67 67 67 67 67 67
9.19 9.47 9.50 9.30 9.70 9.80 9.80 5.96 7.18 9.24 9.77 9.84 9.85 9.88
89.4 92.1 92.4 90.5 94.4 95.3 95.3 58.0 69.0 89.9 95.0 95.7 95.8 96.1
1766 ± 194 1646 ± 185 1810 ± 138 1765 ± 194 2009 ± 169 1934 ± 88 2025 ± 143 – 1504 ± 279 1665 ± 220 2085 ± 99 2152 ± 96 2151 ± 84 1751 ± 91
214 ± 22 280 ± 43 227 ± 21 214 ± 22 246 ± 16 281 ± 30 288 ± 26 – 123 ± 13 233 ± 28 260 ± 21 289 ± 29 291 ± 31 288 ± 31
Fig. 3. Relative density of spark plasma sintered molybdenum vs. sintering temperature, compared with other published data.
density of molybdenum samples, significantly. This is in accordance with the densification behaviour of copper powders with SPS [10]. Finally, it is observed that the density of spark plasma sintered bulk molybdenum is primarily controlled by external pressure and temperature. Our results are in good agreement with the P2C results in [9,11,12], where a relative density of 98% was measured at 1650 °C and at 48 MPa external pressure. Additionally, we compare the densities obtained by SPS with those of isothermal sintering. The isothermal powder metallurgical route starts with hydraulic or isostatic pressing of the molybdenum powder into rods and plates of various geometries and dimensions. The sintering process is carried out in furnaces at high temperatures (typically in the range of 1800–2200 °C) and in hydrogen atmosphere over long times (2–3 h) to get densities about 90% of the theoretical density [1,18–24]. For higher densities, hot rolling, extrusion or forging at temperatures in the range 1200–1500 °C are subsequently used [1]. Fig. 3 contains relative densities published in [19], which were estimated after 1 h isothermal sintering. For temperatures higher than 1400 °C, the comparison shows that the relative densities of SPS are comparable or higher than relative densities generated by isothermal sintering, although the sintering time is very short at SPS. At lower temperatures isothermal sintered molybdenum bodies becomes denser caused by the higher green density due to higher compaction pressure. 3.2. Nanohardness In spite of marginal loading during nanoindentation, porosity has also strong influence on the values of indentation hardness
Fig. 4. Results of nanohardness measurements depending on sintering temperature and relative density.
and indentation modulus (Fig. 4, Table 1). Sintered bodies with low density apparently exhibit lower hardness and modulus. If pores occur beneath grains, the indentation process is overlapped by elastic recovery, plastic deformation or crack propagation. In order to exclude the influence of porosity, measured hardness values of each sample were divided by their relative density. In this case, the indentation hardness’s of all investigated samples are in the same range, independent on sintering conditions. The average indentation hardness is 2057 ± 152 MPa, which corresponds to results of hardness measurement of P2C sintered molybdenum powders in [12]. Furthermore, the indentation modulus depends on the porosity. At relative densities up to 95%, an average value of 297 ± 12 GPa can be calculated which shows an excellent agreement with Youngs modulus of 305 ± 2 GPa, measured with an ultrasonic scanner at different sintered molybdenum samples, and with values from literature for example in [1,24]. The sintering temperature of 2000 °C leads to a softening of the material caused by the large grain size, but keeping the indentation modulus constant. 3.3. Microstructure Fig. 5 gives an impression of microstructure development depending on raising sintering temperature at constant external pressure of 67 MPa. The cross sections exhibit relatively large pores and the observed porosity does not agree with that measured by Archimedes technique. The difference is a consequence of preparation by electrolytic polishing. Caused by the low hardness of
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Fig. 5. Scanning electron micrographs of the microstructure of molybdenum at different sintering temperatures, SPS at 67 MPa pressure.
molybdenum (see above), a deformation layer is generated by mechanical grinding and polishing on the cross sections surface. This deformation layer covers the real microstructure. Additionally, pores can be filled during grinding and mechanical polishing. To remove the deformation layer, electrolytic polishing is necessary. The polishing conditions have to be chosen in such a way that the deformation layer is removed completely. However, it is unavoidable then to enlarge existing pores strongly during this preparation, whereas the etching extends the pores in minor way. The single preparation steps are demonstrated in Fig. 6. After mechanical grinding and polishing the surface appears without contrast, it exhibits small pores and several scratches. At higher magnification the deformation layer is detectable clearly. After electrolytic polishing the surface is clean, the scratches are removed, grain boundaries become apparent, but the pores are strongly enlarged. In contrast, only small material erosion takes place during etching. The first compact sample was produced with maximum sintering temperature of 850 °C and pressure of 67 MPa. Under these conditions a body of high porosity was obtained, where the shapes of origin powder particles are well preserved (Fig. 5). The original shape of particles retains up to sintering temperature of 1100 °C, even if the densification is higher. This is in good agreement with isothermal sintering of molybdenum, where a grain structure at 1100 °C does not appear [19]. At temperatures of 1400 °C or higher, the original shapes of the powder particles get lost and the formation of a polyhedral micro-
structure is observed. The sample still exhibits open porosity, the pores are irregularly shaped and exist mainly between the grains. The average grain size is larger than the initial average grain size. Further increasing of temperature initiates a strong grain growth, which was not expected in such a manner at short-time sintering. For sintering temperatures higher than 1600 °C, the porosity is lower than 5%, i.e. closed porosity appears. The pore size decreases and pores exhibit a more and more spherical shape. Pores are disconnected from grain boundaries during grain growth and are located mainly inside the grains. At the highest temperatures, an agglomeration of pores at grain boundary triples can be observed. Plane grain faces and grain boundary angles of about 120° are observed, which is typical for polycrystalline metals, since the surface stress of the most grain boundaries is similar. In contrast to isothermal sintering of molybdenum in [19], no abnormal grain growth was observed. To investigate accurately the relationship between grain growth and temperature, the chord length distribution of the grains was measured in samples sintered with 67 MPa. Fig. 7 shows the relative frequency of chord length depending on sintering temperature. A continuous grain growth starts at temperatures higher than 1400 °C. This is closely related to a broadening of the grain spectrum. Also, during isothermal sintering this effect occurs, but in contrast, it depends on sintering time [10]. When plotting the mean chord length vs. the sintering temperature in a log–log scale, two straight lines of different slope can be adapted to the date points (Fig. 8). The straight lines intersect at the initial temperature
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Fig. 6. Preparation steps of cross sections: (a) mechanical grinded and polished; (b) electrolytic polished; (c) etched and (d) deformation layer after mechanical grinding and polishing. Hardness indentation marks the equal position on sample surface.
Fig. 7. Grain growth of molybdenum by SPS process. Fig. 9. Grain size, normalized by initial grain size, as a function of the sintering temperature of our experiments compared with those obtained for isothermal sintering.
Fig. 8. Log–log plot of grain growth (67 MPa pressure) vs. sintering temperature.
for the grain growth. For pure molybdenum the initial temperature is about 1330 °C, which corresponds to a homologous temperature of 0.56 Tm. Fig. 9 shows the grain size as a function of the sintering temperature of our experiments compared with those obtained for isothermal sintering [19,20]. The data are normalized by the initial
grain size. Up to about 1700 °C the grain sizes of SPS samples are smaller than those of isothermal sintering. This effect is due to the shorter sintering times of SPS. Finally, the starting temperature of grain growth was calculated from the slope of the curves for isothermal sintering (as in Fig. 8). The grain growth starts at temperatures between 950 and 1035 °C, which are lower than the SPS starting temperature of 1330 °C. The huge difference is a consequence of the fast heating up in the SPS process. The contact of molybdenum powder with the graphite die and sheets leads to diffusion of carbon inside the samples, although the spark plasma sintering is a short time process. The micrograph (Fig. 10) shows a sharp transition between carburized region and uninfluenced molybdenum body. The thickness of the layer depends on temperature and can reach 200 lm at 2000 °C. The layer grows in a parabolic manner, which allows a rough estimation of the activation energy of carbon diffusion in molybdenum. We give an estimate of 180 kJ mol 1, which is in agreement with the values of literature, e.g. in [25]. The carburization kinetics is comparable with the boriding kinetics in molybdenum under SPS conditions, investigated in [26].
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Fig. 11. Experimental and corrected piston travel and the corresponding baseline during sintering of molybdenum powder under a pressure of 67 MPa. The piston travel, caused by the applied pressure before sintering, was subtracted. Fig. 10. Carburized region after sintering at 1800 °C.
In the carburized layer, the microhardness rises up to 1300 HV 0.1 and cracks are created. The carbon concentration amounts ca. 7 wt% which is consistent with the carbon content of Mo2C-carbide. The results of X-ray phase analyses detected, that the carburized region was completely converted in the hexagonal Mo2Ccarbide. The measured diffraction pattern shows a preferred orientation of the (0 0 2) reflection. Texture measurements verify occurrence of a (0 0 1) fibre texture modified by a coarse grain structure. The fibre texture is caused by columnar grain growth, which was observed by light microscopy.
3.4. Densification behaviour The force adjustment of our SPS equipment is starting with a load of 5 kN. At this point, the motion of piston is recorded during whole sintering process. The so called ‘‘relative piston travel” contains information about linear intrinsic shrinkage of every sample. For interpretation of the records, the experimental data must be corrected by subtracting the contribution of piston motion caused by compression during applied pressure before sintering starts (see Fig. 2). This is necessary, because this first compaction depends on the applied pressure as well as on the die filling behaviour of powder varying with every batch. Furthermore, the thermal expansion of sample, die and graphite sheets must be subtracted. For this correction, which is explained in [6], baselines were determined experimentally. The baselines were recorded with heating rate of 200 K/min, maximal temperature of 2000 °C and pressure of 67 and 57 MPa, respectively. Each baseline can be described by a polynomial, and this is valid for all experimental data recorded at the same pressure and with the same die. Fig. 11 shows the experimental and corrected piston travel and the corresponding baseline during sintering of molybdenum powder under a pressure of 67 MPa. Fig. 12 gives examples of corrected piston travels depending on different heating rates vs. sintering time. The curves of all samples are very similar in their shape but they differ in their slopes depending on the heating rates. On the curves one can observe typical points (in Fig. 12 marked by arrows) where the slope changes considerably. These points are visible in particular for higher sintering temperatures (cf. Fig. 12, 1800 °C). By these points, each curve can be divided into parts in the following denoted with Roman numerals. Parts I–III are related to the heating up, the flat part IV is controlled by holding time at sintering temperature. The large
Fig. 12. Corrected piston travel vs. sintering time for different heating rates (pressure of 67 MPa).
Table 2 Values of sample high before sintering. Batch
Final height after sintering lf (mm)
Final corrected piston travel Dlf (mm)
Height before sintering, l0 (mm)
3.1 3.2 3.3 3.4 3.5 3.6 3.7
7.74 6.43 4.18 3.89 4.62 4.65 3.71
0.37 0.97 2.06 2.54 3.79 3.86 3.43
8.11 7.40 6.24 6.43 8.41 8.51 7.14
slopes in parts II and III indicate that the consolidation takes place mainly during fast heating up of SPS. Linear shrinkage Dl/l0 can be calculated basing on the corrected piston travel, where Dl is corrected piston travel depending on sintering time and temperature, respectively. The sample height l0 before sintering starts (in our investigation after the cold pressing step) can not be estimated exactly. It can be approximated by the sum of the final sample height lf (after sintering) and the final corrected piston travel Dlf (values see Table 2). If this linear shrinkage plotted vs. temperature during heating up, the curves of all samples, compacted with the same pressure, cover each other with slight deviations (Fig. 13). As pointed out in Section 3.1, the densification of molybdenum by SPS process depends strongly on the acting temperature, generated by resistance heating in the powder.
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Fig. 13. Corrected piston travel of different sintered batches (pressure of 67 MPa) vs. temperature during heating up.
The parts I up to III marked in Fig. 12 can be observed again and their transitions can now be linked to characteristic values of temperature, shrinkage, porosity and mean grain size, as demonstrated in Fig. 13. The reason for the small shift of the curves of Fig. 13 is not clear. 4. Discussion As the densification process is thermally activated, a sigmoid progression of sintering curve was expected, comparable with such of isothermal sintering [27,28], where in our curves the part I exhibits a linear relationship between Dl/l0 and temperature and a sharp transition to second part starting at 900 or 950 °C, respectively. This transition temperature is consistent with 0.4 Tm of Mo, the temperature, where appreciable volume diffusion starts in solid materials. The end of this part is related to a small shrinkage of 5% and a high porosity of 40%. The densification rate is low. As shown in Section 3.3, grain growth and change in particle shape were not observed. Part II is characterised by the highest densification rate of sintering process. The shrinkage achieves more than 30% and the porosity decreases to 15%, where the micrographs do not show any change in particle shape up to 1100 °C. Nevertheless, the density increases and the number and sizes of contact areas between particles are enlarged. Furthermore, the particle strength decreases further, which makes particle rotation and sliding easier. A phenomenological description of the consolidation process during parts I and II is an open problem. The grain growth starts with the beginning of the third part at 1330 °C. At first the porosity amounts 10%, which marks the transition to closed porosity. During the third part the porosity reduces to 5% and the densification rate decreases continuously. The micrographs (Fig. 5, 1600 °C) show that the pores are rounded and the grain boundaries are separated from pores. Both processes are driven mainly by diffusion. In part IV, where sintering temperature is constant, the densification is comparable to isothermal sintering process. The material transport is supported by the applied pressure and by the electric field, which was demonstrated in [6,8]. 5. Conclusions The results of this investigation demonstrate that molybdenum powder can be successfully consolidated by spark plasma sintering in very short times. A relative density of 95% is achieved at sintering temperatures of 1600 °C and an external pressure of 67 MPa. It can be concluded that the density is primarily controlled by exter-
nal pressure and temperature. In comparison with values from the literature, the densities obtained by SPS exceed those by isothermal sintering. The grain growth during spark plasma sintering was mainly controlled by the sintering temperature. The grain growth starts at 1350 °C, which corresponds to a homologous temperature of 0.56 Tm. A strong grain growth occurs at temperatures higher than 1600 °C. Due to the shorter sintering times of SPS the grain sizes of our samples are smaller than those of isothermal sintering. Nanohardness and indentation modulus was estimated by nanohardness testing. It was shown that the porosity strongly influences both values in spite of the marginal loads used at this test. An average indentation hardness of 2057 ± 152 MPa and an average indentation modulus of 297 ± 12 GPa were estimated, which are in agreement with values for bulk molybdenum. Based on the recorded relative piston travel, the linear shrinkage of molybdenum powder during SPS process can be calculated. The shrinkage of all batches, compacted with the same external pressure, follow the same curve independent of heating rate. The main part of compaction takes place during fast heating up by SPS. In summary, our investigation shows that the best results of molybdenum powder spark plasma sintering are achieved at 1600 °C and 67 MPa. The optimal sintering temperature corresponds to that of Plasma Pressure Compaction, but a higher external pressure is necessary for SPS. Acknowledgments The authors are grateful for the financial support of this work by Dr. Erich Krüger Stiftung and for the abandonment of the molybdenum powder by Dr. A. Hoffmann, Plansee Metall GmbH Reutte. References [1] Plansee high performance materials, Molybdenum – material properties and applications. The Official Website of the Plansee Group,
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