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Effects of high energy milling on densification behaviour of Mo–Si powder mixtures during pressureless sintering
Intermetallics 10 (2002) 873–878 www.elsevier.com/locate/intermet
Effects of high energy milling on densification behaviour of Mo–Si powder mixtures during pressureless sintering Thomas Schuberta,*, Alexander Bo¨hma, Bernd Kiebacka, Matthias Achtermannb, Roland Schollc a
Fraunhofer-Institut fu¨r Fertigungstechnik und Angewandte Materialforschung, Pulvermetallurgie und Verbundwerkstoffe, 01277 Dresden, Winterbergstraße 28, Germany b GFE Metalle und Materialien GmbH, 90431 Nu¨rnberg, Ho¨fener Straße 45, Germany c H.C. Starck GmbH, 79721 Laufenburg, Postfach 13 46, Germany Received 14 June 2002; accepted 17 June 2002
Abstract The present paper reports on basic investigations of the controlled reaction sintering of MoSi2-based materials, including particle reinforced composites. It is shown, that certain amounts of pre-formed silicide phase in the powder mixtures are useful to achieve high densities by pressureless sintering of green compacts. This partial phase formation of aMoSi2 can already take place during the mechanical treatment of the elemental powder mixtures by milling. In this context the role of volume changes linked with the phase formations is discussed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Molybdenum silicides; A. Composites; C. Mechanical alloying and milling; C. Sintering
1. Introduction MoSi2 is a promising material for high temperature applications. Due to its high melting temperature, good oxidation resistance combined with low density, good electrical and thermal conductivity this material may tolerate higher service temperatures in the field of energy machines, gas turbine parts, heat shields and tiles. Significant progress has been made in the last years in both the scientific and technological development of this intermetallic alloy [1,2]. Nevertheless some of the properties still have to be improved to establish silicides for structural components, e.g. fracture toughness at room temperature and strength at high temperature. The manufacturing of dense parts with a complex shape is still complicated today. The reinforcement of MoSi2 by hard ceramic particles, such as SiC, Si3N4, ZrO2, has proven to be an effective approach to improve the mechanical behaviour [2]. Usually manufacturing of * Corresponding author. Tel.: +49-351-2537-346; fax: +49-3512537-399. E-mail address: [email protected] (T. Schubert).
dense parts without any glass-phase requires pressure assisted powder metallurgical techniques, such as hot pressing or hot isostatic pressing of MoSi2 powders or powders produced by mechanical alloying, reactive or XD processing [3]. In addition a powder metallurgical technique to produce dense intermetallic materials by pressureless sintering was studied for several intermetallic systems [4–7]. This technique includes a mechanical treatment (high energy milling) of an elemental powder mixture hence allowing a controlled reaction sintering of the constituents in contrary to an uncontrolled reaction of coarse elemental powders (SHS). This paper gives new results on this less expensive technique for the production of complex-shaped dense parts of single phase or reinforced MoSi2 parts by pressureless sintering.
2. Experimental The preparation of single phase or particle reinforced MoSi2 started from elemental powder mixtures with an atomic ratio of 33.3% Mo and 66.7% Si which were
0966-9795/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(02)00083-3
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T. Schubert et al. / Intermetallics 10 (2002) 873–878
milled together in a planetary ball mill ‘‘Pulverisette 600 (Fa. Fritsch, Idar-Oberstein, Germany). To observe the process during milling in situ the gas pressure and temperature measuring system—GTM (Fa. Fritsch, IdarOberstein, Germany) was used. By measuring temperature in the milling container, an ‘‘integral’’ thermal signal is obtained reflecting the effects of friction, impact and phase formation processes. Abrupt and minute changes in temperature are recorded by the highly sensitive measurement of the gas pressure within the milling container. The high energy milling for several minutes or hours was done under an argon atmosphere using steel containers with balls of steel or zirconia. Reinforcements were added as fine SiC- or ZrO2-particles at the end of the milling procedure. Small samples for studying shrinkage behaviour were compacted by uniaxial die pressing. To monitor the influence of mechanical pre-treatment on sintering, different compacts were sintered in a dilatometer (DIL 402 E: Fa. Netzsch, Selb, Germany) under argon atmosphere. The microstructure of the samples was characterised by metallography and X-ray diffraction.
3. Results and discussion 3.1. Mechanical alloying Fig. 1 shows a typical measurement of the GTM during milling of elemental Mo and Si powders. The temperature in the container is rising gradually from room temperature to about 45 C caused by friction and impact processes
during milling. Periodical breaks of 15 min in the milling process were necessary to keep the temperature below a maximum value of about 60 C. The increase of the temperature in the container gave a corresponding signal of the sensitive gas pressure sensor. Furthermore, a sharp pressure peak of about 320 kPa was monitored after a milling time of about 45 min reflecting exothermal reactions in the powder mixture. As was shown in additional experiments the incubation period, i.e. the milling time up to this peak, strongly depends on milling parameters and powder composition. This incubation time is increased by a lower milling intensity or by addition of any reinforcements (e.g. particles of SiC, ZrO2). The XRD spectrum of the reacted powder mixture (Fig. 2) compared to the initial stage identifies aMoSi2 as the main crystalline phase. Therefore, the abrupt rise in gas pressure can be attributed to the extremely rapid release of heat by the exothermic phase formation of aMoSi2 from the elements Mo and Si (Mo+2Si! MoSi2). A maximum gas temperatures of about 500 C can be estimated under ideal adiabatic conditions on the basis of the measured gas pressures in the milling container. Such a mechanically-induced rapid phase formation of MoSi2 was already reported in [8]. In addition a higher resolution of the monitored gas pressure signal (Fig. 3) shows the multistage nature of this process with a reaction time of some seconds. This result suggests that the heat released by the exothermic reaction of the first quantity of the mechanically activated Mo–Si powder mixture propagates and ignites further unreacted quantities until about 60–80 vol.% of the Mo–Si powder mixture has been converted to
Fig. 1. Gas pressure and temperature measurements during the milling of a Mo–Si mixture monitored by the GTM.
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Fig. 2. X-ray diffraction patterns recorded from the Mo–Si powder: only blended (A), milled for 1 h (B).
Fig. 3. Higher resolution of the recorded gas pressure within the container during the rapid mechanically-induced phase formation.
aMoSi2. This is analogous to the well-known self-propagating high-temperature synthesis (SHS). 3.2. Pressureless sintering It is known from earlier work at IFAM in Dresden, that silicide parts with low porosity can be manufactured by pressureless sintering [4,9]. This route consists of a high energy milling of an elemental powder mixture hence allowing a controlled reaction of the fine dispersed constituents during sintering in contrary to uncontrolled
reaction of coarse elemental powders. In addition, it was shown that both single-phase silicide material and particle reinforced silicide composites can be prepared by this technique. The starting densities (Fig. 4) of the mechanically activated but unreacted powder bodies were lower (< 50% TD (theoretical density in relation to the final phase composition)) compared with green compacts made of partially reacted powders (about 55% TD). However, the porosities of all green compacts were in the same range of about 40– 45% because of the differences in the phase compositions
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of the compacted powder mixtures. The density of the elemental powder mixture and the density of the intermetallic phase MoSi2 are 4.62 and 6.25 g/cm3, respectively. Pressureless sintering of the green bodies was carried out at temperatures up to 1600 C in argon/hydrogen. The final densities after sintering are also presented in
Fig. 4. The partly reacted powders result in higher densities after sintering compared to mechanically highly activated but still elemental powder mixtures. Therefore, closed porosity of the sintered parts is achieved. The phase formation Mo+2Si!MoSi2 is accompanied by a volume reduction of about 27%. This causes
Fig. 4. Comparison of green densities (cold compacted with 500 MPa) and sintering densities (1600 C/1 h) of different mechanically activated Mo– Si powder mixtures.
Fig. 5. The estimated linear shrinkage due to the volume loss during the formation of different silicides from the pure elements.
T. Schubert et al. / Intermetallics 10 (2002) 873–878
a linear shrinkage of about 10%, which can hamper the sintering process by local destruction of particle contacts or by formation of new pores. Presumably some of these pores reach a size, which does not allow healing at the conditions chosen for sintering, and a high residual porosity remains in these sintered bodies. The partial phase formation prior to the sintering process reduces this detrimental effect. The investigations were focussed on MoSi2, but the results suggest an universal approach for several other intermetallic materials, too. For further optimisation of the material properties it is also necessary to consider the possible volume changes during the silicide phase
877
formation in dependence of the chemical composition. Fig. 5 shows the estimated linear shrinkage caused by the increase of the density during the phase formation of different silicides from the elements. Therefore, a partial phase formation prior to the sintering should be helpful in most cases analogous to the preparation of highly dense MoSi2. This investigation suggests, that the shrinkages caused by phase formations with the remaining elemental powder during sintering should be preferably less than 5%. In addition to the preparation of single-phase MoSi2, the simultaneous introduction of particulate reinforcements (e.g. SiC, ZrO2) during the milling process allows
Fig. 6. Optical micrograph of a hot consolidated MoSi2/15%SiC composite (left: MoSi2 matrix in polarised light; right: distribution of SiC particles bright field).
Fig. 7. Dilatometer plot of differently activated Mo–Si green compacts with additions of 20 vol.% ZrO2 during the sintering process (1600 C/1 h).
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T. Schubert et al. / Intermetallics 10 (2002) 873–878
the powder-metallurgical production of silicide composites with improved properties. This was successfully demonstrated up to particle contents of about 20 vol.% (e.g. silicon carbide). The optical micrographs in Fig. 6 reveal the homogenous distribution of the small SiC particles (< 1 mm) in a fine grained silicide matrix with closed porosity. Furthermore, the results reveal, that the use of partially transformed Mo–Si mixtures is necessary for the consolidation of oxide reinforced MoSi2 (e.g. with ZrO2) by pressureless sintering. Compacts of milled but still elemental powder mixtures with additions of ZrO2 particles show an unexpected swelling behaviour in the temperature range of 700–1000 C (Fig. 7). Swelling effects of up to about 25% reduce the density of the body drastically and therefore also its mechanical stability. Thus pressureless sintering is impossible. The green compact with partially preformed MoSi2 phase shows the typical shrinkage behaviour leading to a sintered part with high density and closed porosity. Additional investigations have to be performed to understand this sintering behaviour of Mo–Si mixtures with an oxide reinforcement.
The incubation period strongly depends on milling parameters and powder composition. During subsequent pressureless sintering of green powder bodies the partly reacted powders result in higher densities by 5–10% compared to mechanically highly activated, but still elemental powders. This may be explained by volume changes taking place during formation of aMoSi2. This reaction causes a linear shrinkage of about 10%, which can hinder the sintering process by local destruction of particle contacts or by formation of new pores. The partial powder transformation reduces this detrimental effect. In addition it was also demonstrated that the use of partly transformed powder mixtures is essentially in case of oxide reinforced MoSi2 composites.
Acknowledgements The authors are grateful for the financial support by Deutschen Forschungsgemeinschaft.
References 4. Conclusions Starting from elemental Mo and Si powders various powder mixtures were prepared by a mechanical milling technique monitored by the GTM (gas pressure and temperature measuring) system. The results show that the aMoSi2 phase can be synthesised by an mechanically induced exothermic reaction during milling. This partial phase formation results in a powder mixture consisting of aMoSi2 and residual elemental Mo and Si. The phase formation can be monitored by the extremely rapid increase of the gas pressure within the milling vial.
[1] [2] [3] [4] [5]
[6] [7] [8] [9]
Petrovic JJ, Vasudevan AK. Mater Sci Eng A261 1999:1. Petrovic JJ. Intermetallics 2000;8:1175. Stoloff NS. Mater Sci Eng 1999;A261:169. Scholl R, Bo¨hm A, Kieback B. Mater Sci Eng A261 1999:204. Scholl R, Ju¨ngling T, Bu¨ttner T, Kieback B. Proc of World Congress on Powder Metallurgy and Particulate Materials, Washington DC 1996;4:143. Scholl R, Ju¨ngling T, Kieback B. Mater Res Soc Symp Proc 1995;364:931. Bo¨hm A, Kieback B. Zeitschrift fu¨r Metallkunde 1998;89:90. Patankar SN, Xiao SQ, Lewandowski JJ, Heuer AH. J Mater Res 1993;8:1311. Project No. 03 M 3059: National German Project of the Federal Ministry for Education and Research.
Effects of high energy milling on densification behaviour of Mo–Si powder mixtures during pressureless sintering Thomas Schuberta,*, Alexander Bo¨hma, Bernd Kiebacka, Matthias Achtermannb, Roland Schollc a
Fraunhofer-Institut fu¨r Fertigungstechnik und Angewandte Materialforschung, Pulvermetallurgie und Verbundwerkstoffe, 01277 Dresden, Winterbergstraße 28, Germany b GFE Metalle und Materialien GmbH, 90431 Nu¨rnberg, Ho¨fener Straße 45, Germany c H.C. Starck GmbH, 79721 Laufenburg, Postfach 13 46, Germany Received 14 June 2002; accepted 17 June 2002
Abstract The present paper reports on basic investigations of the controlled reaction sintering of MoSi2-based materials, including particle reinforced composites. It is shown, that certain amounts of pre-formed silicide phase in the powder mixtures are useful to achieve high densities by pressureless sintering of green compacts. This partial phase formation of aMoSi2 can already take place during the mechanical treatment of the elemental powder mixtures by milling. In this context the role of volume changes linked with the phase formations is discussed. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Molybdenum silicides; A. Composites; C. Mechanical alloying and milling; C. Sintering
1. Introduction MoSi2 is a promising material for high temperature applications. Due to its high melting temperature, good oxidation resistance combined with low density, good electrical and thermal conductivity this material may tolerate higher service temperatures in the field of energy machines, gas turbine parts, heat shields and tiles. Significant progress has been made in the last years in both the scientific and technological development of this intermetallic alloy [1,2]. Nevertheless some of the properties still have to be improved to establish silicides for structural components, e.g. fracture toughness at room temperature and strength at high temperature. The manufacturing of dense parts with a complex shape is still complicated today. The reinforcement of MoSi2 by hard ceramic particles, such as SiC, Si3N4, ZrO2, has proven to be an effective approach to improve the mechanical behaviour [2]. Usually manufacturing of * Corresponding author. Tel.: +49-351-2537-346; fax: +49-3512537-399. E-mail address: [email protected] (T. Schubert).
dense parts without any glass-phase requires pressure assisted powder metallurgical techniques, such as hot pressing or hot isostatic pressing of MoSi2 powders or powders produced by mechanical alloying, reactive or XD processing [3]. In addition a powder metallurgical technique to produce dense intermetallic materials by pressureless sintering was studied for several intermetallic systems [4–7]. This technique includes a mechanical treatment (high energy milling) of an elemental powder mixture hence allowing a controlled reaction sintering of the constituents in contrary to an uncontrolled reaction of coarse elemental powders (SHS). This paper gives new results on this less expensive technique for the production of complex-shaped dense parts of single phase or reinforced MoSi2 parts by pressureless sintering.
2. Experimental The preparation of single phase or particle reinforced MoSi2 started from elemental powder mixtures with an atomic ratio of 33.3% Mo and 66.7% Si which were
0966-9795/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(02)00083-3
874
T. Schubert et al. / Intermetallics 10 (2002) 873–878
milled together in a planetary ball mill ‘‘Pulverisette 600 (Fa. Fritsch, Idar-Oberstein, Germany). To observe the process during milling in situ the gas pressure and temperature measuring system—GTM (Fa. Fritsch, IdarOberstein, Germany) was used. By measuring temperature in the milling container, an ‘‘integral’’ thermal signal is obtained reflecting the effects of friction, impact and phase formation processes. Abrupt and minute changes in temperature are recorded by the highly sensitive measurement of the gas pressure within the milling container. The high energy milling for several minutes or hours was done under an argon atmosphere using steel containers with balls of steel or zirconia. Reinforcements were added as fine SiC- or ZrO2-particles at the end of the milling procedure. Small samples for studying shrinkage behaviour were compacted by uniaxial die pressing. To monitor the influence of mechanical pre-treatment on sintering, different compacts were sintered in a dilatometer (DIL 402 E: Fa. Netzsch, Selb, Germany) under argon atmosphere. The microstructure of the samples was characterised by metallography and X-ray diffraction.
3. Results and discussion 3.1. Mechanical alloying Fig. 1 shows a typical measurement of the GTM during milling of elemental Mo and Si powders. The temperature in the container is rising gradually from room temperature to about 45 C caused by friction and impact processes
during milling. Periodical breaks of 15 min in the milling process were necessary to keep the temperature below a maximum value of about 60 C. The increase of the temperature in the container gave a corresponding signal of the sensitive gas pressure sensor. Furthermore, a sharp pressure peak of about 320 kPa was monitored after a milling time of about 45 min reflecting exothermal reactions in the powder mixture. As was shown in additional experiments the incubation period, i.e. the milling time up to this peak, strongly depends on milling parameters and powder composition. This incubation time is increased by a lower milling intensity or by addition of any reinforcements (e.g. particles of SiC, ZrO2). The XRD spectrum of the reacted powder mixture (Fig. 2) compared to the initial stage identifies aMoSi2 as the main crystalline phase. Therefore, the abrupt rise in gas pressure can be attributed to the extremely rapid release of heat by the exothermic phase formation of aMoSi2 from the elements Mo and Si (Mo+2Si! MoSi2). A maximum gas temperatures of about 500 C can be estimated under ideal adiabatic conditions on the basis of the measured gas pressures in the milling container. Such a mechanically-induced rapid phase formation of MoSi2 was already reported in [8]. In addition a higher resolution of the monitored gas pressure signal (Fig. 3) shows the multistage nature of this process with a reaction time of some seconds. This result suggests that the heat released by the exothermic reaction of the first quantity of the mechanically activated Mo–Si powder mixture propagates and ignites further unreacted quantities until about 60–80 vol.% of the Mo–Si powder mixture has been converted to
Fig. 1. Gas pressure and temperature measurements during the milling of a Mo–Si mixture monitored by the GTM.
T. Schubert et al. / Intermetallics 10 (2002) 873–878
875
Fig. 2. X-ray diffraction patterns recorded from the Mo–Si powder: only blended (A), milled for 1 h (B).
Fig. 3. Higher resolution of the recorded gas pressure within the container during the rapid mechanically-induced phase formation.
aMoSi2. This is analogous to the well-known self-propagating high-temperature synthesis (SHS). 3.2. Pressureless sintering It is known from earlier work at IFAM in Dresden, that silicide parts with low porosity can be manufactured by pressureless sintering [4,9]. This route consists of a high energy milling of an elemental powder mixture hence allowing a controlled reaction of the fine dispersed constituents during sintering in contrary to uncontrolled
reaction of coarse elemental powders. In addition, it was shown that both single-phase silicide material and particle reinforced silicide composites can be prepared by this technique. The starting densities (Fig. 4) of the mechanically activated but unreacted powder bodies were lower (< 50% TD (theoretical density in relation to the final phase composition)) compared with green compacts made of partially reacted powders (about 55% TD). However, the porosities of all green compacts were in the same range of about 40– 45% because of the differences in the phase compositions
876
T. Schubert et al. / Intermetallics 10 (2002) 873–878
of the compacted powder mixtures. The density of the elemental powder mixture and the density of the intermetallic phase MoSi2 are 4.62 and 6.25 g/cm3, respectively. Pressureless sintering of the green bodies was carried out at temperatures up to 1600 C in argon/hydrogen. The final densities after sintering are also presented in
Fig. 4. The partly reacted powders result in higher densities after sintering compared to mechanically highly activated but still elemental powder mixtures. Therefore, closed porosity of the sintered parts is achieved. The phase formation Mo+2Si!MoSi2 is accompanied by a volume reduction of about 27%. This causes
Fig. 4. Comparison of green densities (cold compacted with 500 MPa) and sintering densities (1600 C/1 h) of different mechanically activated Mo– Si powder mixtures.
Fig. 5. The estimated linear shrinkage due to the volume loss during the formation of different silicides from the pure elements.
T. Schubert et al. / Intermetallics 10 (2002) 873–878
a linear shrinkage of about 10%, which can hamper the sintering process by local destruction of particle contacts or by formation of new pores. Presumably some of these pores reach a size, which does not allow healing at the conditions chosen for sintering, and a high residual porosity remains in these sintered bodies. The partial phase formation prior to the sintering process reduces this detrimental effect. The investigations were focussed on MoSi2, but the results suggest an universal approach for several other intermetallic materials, too. For further optimisation of the material properties it is also necessary to consider the possible volume changes during the silicide phase
877
formation in dependence of the chemical composition. Fig. 5 shows the estimated linear shrinkage caused by the increase of the density during the phase formation of different silicides from the elements. Therefore, a partial phase formation prior to the sintering should be helpful in most cases analogous to the preparation of highly dense MoSi2. This investigation suggests, that the shrinkages caused by phase formations with the remaining elemental powder during sintering should be preferably less than 5%. In addition to the preparation of single-phase MoSi2, the simultaneous introduction of particulate reinforcements (e.g. SiC, ZrO2) during the milling process allows
Fig. 6. Optical micrograph of a hot consolidated MoSi2/15%SiC composite (left: MoSi2 matrix in polarised light; right: distribution of SiC particles bright field).
Fig. 7. Dilatometer plot of differently activated Mo–Si green compacts with additions of 20 vol.% ZrO2 during the sintering process (1600 C/1 h).
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T. Schubert et al. / Intermetallics 10 (2002) 873–878
the powder-metallurgical production of silicide composites with improved properties. This was successfully demonstrated up to particle contents of about 20 vol.% (e.g. silicon carbide). The optical micrographs in Fig. 6 reveal the homogenous distribution of the small SiC particles (< 1 mm) in a fine grained silicide matrix with closed porosity. Furthermore, the results reveal, that the use of partially transformed Mo–Si mixtures is necessary for the consolidation of oxide reinforced MoSi2 (e.g. with ZrO2) by pressureless sintering. Compacts of milled but still elemental powder mixtures with additions of ZrO2 particles show an unexpected swelling behaviour in the temperature range of 700–1000 C (Fig. 7). Swelling effects of up to about 25% reduce the density of the body drastically and therefore also its mechanical stability. Thus pressureless sintering is impossible. The green compact with partially preformed MoSi2 phase shows the typical shrinkage behaviour leading to a sintered part with high density and closed porosity. Additional investigations have to be performed to understand this sintering behaviour of Mo–Si mixtures with an oxide reinforcement.
The incubation period strongly depends on milling parameters and powder composition. During subsequent pressureless sintering of green powder bodies the partly reacted powders result in higher densities by 5–10% compared to mechanically highly activated, but still elemental powders. This may be explained by volume changes taking place during formation of aMoSi2. This reaction causes a linear shrinkage of about 10%, which can hinder the sintering process by local destruction of particle contacts or by formation of new pores. The partial powder transformation reduces this detrimental effect. In addition it was also demonstrated that the use of partly transformed powder mixtures is essentially in case of oxide reinforced MoSi2 composites.
Acknowledgements The authors are grateful for the financial support by Deutschen Forschungsgemeinschaft.
References 4. Conclusions Starting from elemental Mo and Si powders various powder mixtures were prepared by a mechanical milling technique monitored by the GTM (gas pressure and temperature measuring) system. The results show that the aMoSi2 phase can be synthesised by an mechanically induced exothermic reaction during milling. This partial phase formation results in a powder mixture consisting of aMoSi2 and residual elemental Mo and Si. The phase formation can be monitored by the extremely rapid increase of the gas pressure within the milling vial.
[1] [2] [3] [4] [5]
[6] [7] [8] [9]
Petrovic JJ, Vasudevan AK. Mater Sci Eng A261 1999:1. Petrovic JJ. Intermetallics 2000;8:1175. Stoloff NS. Mater Sci Eng 1999;A261:169. Scholl R, Bo¨hm A, Kieback B. Mater Sci Eng A261 1999:204. Scholl R, Ju¨ngling T, Bu¨ttner T, Kieback B. Proc of World Congress on Powder Metallurgy and Particulate Materials, Washington DC 1996;4:143. Scholl R, Ju¨ngling T, Kieback B. Mater Res Soc Symp Proc 1995;364:931. Bo¨hm A, Kieback B. Zeitschrift fu¨r Metallkunde 1998;89:90. Patankar SN, Xiao SQ, Lewandowski JJ, Heuer AH. J Mater Res 1993;8:1311. Project No. 03 M 3059: National German Project of the Federal Ministry for Education and Research.