- Email: [email protected]
Application of compression lubricant as final porosity controller in the sintering of tungsten powders
Int. Journal of Refractory Metals and Hard Materials 66 (2017) 21–25
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Application of compression lubricant as final porosity controller in the sintering of tungsten powders Ahmad Ghaderi Hamidi Department of Metallurgy and Materials Engineering, Hamedan University of Technology, 65155, Iran
a r t i c l e
i n f o
Article history: Received 17 July 2016 Received in revised form 31 December 2016 Accepted 20 January 2017 Available online 04 February 2017 Keywords: Lubricant Infiltrable tungsten Pore coarsening
a b s t r a c t Sintering behavior of two tungsten powders (1.2 μm and 6 μm) was studied for preparing infiltrable porous skeleton. Both powders were compressed by mechanical press (MP) and cold isostatic press (CIP) with and without stearic acid respectively as compaction lubricant. Results showed that presence of solid lubricant powder in addition of its essential effect on soundness of parts, depending on its size and distribution, could mainly affect sintered microstructure. Stearic acid as compaction lubricant in addition of decreasing friction between particles during the compaction, has acted as spacing particles between primary powder particles. In the cases that lubricant particles are much bigger than tungsten particles a big pore remained after evaporation of lubricant. During the sintering, big pores became bigger due to coarsening mechanism and formed an interconnected network of pores and on the other hand small pores shrank or even disappeared due to densification. By exact controlling of the size of tungsten powder and lubricant powder, infiltrable tungsten skeletons with 80% of theoretical density were produced successfully at low sintering temperatures such as 1500 °C. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Powder metallurgy is the predominant production process for tungsten alloys and composites. Compression of tungsten powder can be carried out via cold isostatic press (CIP) or mechanical press (MP) [1]. Uniform pressure in CIP process results in a high uniformity for green density and consequently dimensional stability during sintering. In the case of mechanical pressing, tungsten powder is formed in a uniaxial solid die and severe die wall friction can yield to density inhomogeneity or cracks [2]. Lubricants in solid powder form are mixed with primary powders and reduce the friction between pressed compact and die wall. They provide more uniform density of compacts and prevent ejection defects. Lubricant particles evaporate or decompose at sintering early stages and leave equivalent voids [3]. Size distribution of pores plays a key role in evolution of consolidation. A homogeneous initial pore structure gives a higher sintered density [4]. Densification and coarsening are two apparently counter process during sintering. Although total energy reduction via surface decrease is the drive force for both phenomena, but during coarsening the pores grow while during densification the pores shrink [5]. Relatively large size initial pores enlarge during sintering and regions of initially high packing (small pores) shrink. Initial pores larger than half the primary particle often form big pores [6]. Agglomeration or poor consolidation may lead to this defects [7]. Consequently big pores are unavoidable after sintering of a compact with bimodal porosity [8]. Maximum densification is aim
E-mail address: [email protected].
http://dx.doi.org/10.1016/j.ijrmhm.2017.01.005 0263-4368/© 2017 Elsevier Ltd. All rights reserved.
of most sintering treatments but some sintered parts such as infiltrable tungsten skeletons must have controlled open porosity. Characteristics of final channels is controlled by primary powder, compaction and sintering parameters [9,10]. Growth of pores due to coarsening has been considered as a defect forming mechanism and never has been used as constructive mechanism for porosity control. In this research, the effect of lubricant induced porosity on sintering densification was studied. 2. Material and methods Tungsten powders with two different particle size distributions were used. A tungsten powder with average particle size of 6 μm was one of them. This particle size is normally used for fabrication of tungsten skeletons. Besides that, a fine tungsten powder with average particle size of 1.2 μm was selected for studying behavior of a fine powder. SEM micrographs and particle size distributions of powders are shown in Figs. 1 and 2 respectively. The powder with average particle size of 1.2 μm and the powder with that of 6 μm will be referred as fine and medium powders respectively in whole text. Without lubricants, soundless green tungsten parts was not producible by mechanical pressing. 1.5 wt% of stearic acid powder was added as compression lubricant. The mixture of tungsten and lubricant powders were ball milled for 90 min and passed through 100-mesh sieve twice. Therefore homogenous mixtures of tungsten and lubricant particles were obtained and all lubricant particles had broken down to small flakes. Lubricated powders were compressed in a steel matrix according to ASTM B331 standard test method for Compressibility of Metal
22
A. Ghaderi Hamidi / Int. Journal of Refractory Metals and Hard Materials 66 (2017) 21–25
Fig. 1. SEM micrographs of the initial tungsten powders, a. Fine, b. Medium tungsten powder.
Fig. 2. Particle size analyzing of the initial tungsten powders, a. Fine, b. Medium tungsten powder.
Powder in uniaxial Compaction. Pressure of mechanical compression was about 500 MPa. On the other hand, Fine and medium tungsten powders were compressed via cold isostatic press (CIP) at pressures of 254, 495 and 663 MPa without any lubricant. All specimens were sintered under a flow of dry hydrogen in a tube furnace. Sintering cycles also included delubrication and reduction components each for 1 h at 400 and 1000 °C respectively. Densities of green and sintered specimens were determined by water immersion method according to ASTM B328 standard. Eventually all porous tungsten specimens were infiltrated with molten copper at 1250 °C for 30 min under hydrogen atmosphere. Scanning Electron Microscope (SEM) was employed for in detail study of microstructure. 3. Results and discussion 3.1. Compression Fracture surface of mechanically compressed green specimen of medium tungsten powder shown in Fig. 3. Primary powder is mixed with stearic acid flakes as lubricant. The milling and sieving of primary Table 1 Green and sintered densities of compacted from two tungsten powders after mechanical pressing with lubricant.
Fig. 3. SE-SEM micrograph of fractured section of mechanically compressed medium tungsten powder; some stearic acid flakes are marked.
Sintering temperature Fine powder Medium powder
Green density
Sintered density
54 72
1400 °C 58 72.3
1500 °C 76 72.8
A. Ghaderi Hamidi / Int. Journal of Refractory Metals and Hard Materials 66 (2017) 21–25
23
3.2. Sintering of medium tungsten powder
Fig. 4. Green and sintered densities of two tungsten powder after compaction by cold isostatic press without lubricant and sintering at 1500 °C for 2 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Green and sintered densities of mechanically and CIP compressed specimens are presented in Table 1 and Fig. 4 respectively. Compacts of medium tungsten powder had little densification (b1%) but fine tungsten MP compacts had significant densification (N40%). Surface diffusion and grain boundary diffusion are active mechanisms for materials transportation during sintering of tungsten at low temperatures (b1600 °C) [11]. In the case of surface diffusion, migration of atoms initiate from one point of surface and terminate to another point of that and just strengthen necks with no densification. On the contrary, grain boundary diffusion is controlling mechanism of tungsten densification in temperature range of 1100–1500 °C [12]. As a result, due to lower density of grain conjunctions, little densifications were expected for compacts of medium tungsten powder. Despite the slight densification, newly formed necks are obvious in Fig. 5. SEM micrograph of medium tungsten MP compact after sintering at 1400 °C and 1500 °C and infiltration are illustrated in Fig. 6. An increase in sintering temperature from 1400 °C to 1500 °C has no significant effect on densification (b 1%) but thickened the necks and developed strong junctions. It demonstrates that although increasing of sintering temperature does not have such improvement on densification but it can strengthen the skeleton and eliminate effects of sharp notches. As it is shown in Fig. 7, microstructure of a CIP compact from medium tungsten powder after sintering at 1500 °C and infiltration is same as MP specimen (Fig. 6b). As a consequence, lubricant particles had dimensions as of medium tungsten powder and pore size distribution and contiguity of particles for both compression processes (MP&CIP) are same and presence of lubricant hasn't disturbed homogeneity of specimen. 3.3. Sintering of fine tungsten powder
Fig. 5. SE-SEM micrograph from fracture surface of medium tungsten powder after CIP compaction at 663 MPa and sintering at 1500 °C.
mixture has controlled final lubricant particles size. The lubricant particles had same particle size as medium tungsten powders and therefore were a few times bigger than fine tungsten particles.
In spite of medium tungsten powder, sintering mechanisms was much more active for fine tungsten powder. Fine powder had much more specific area. Furthermore had great number of joints between particles that formed grain boundaries as facilitated diffusion path and also distances for material transportation were so shorter. Therefore green compacts of fine tungsten powder showed a superior densification. SEM micrograph from fracture surface of fine tungsten powder after CIP compaction at 491 MPa and sintering at 1500 °C is demonstrated in Fig. 8. Sintering has progressed up to 92% of theoretical density and microstructure is homogeneous. Because of close pores, this structure could not be used for infiltration. Micrograph of fine tungsten MP
Fig. 6. SEM micrograph from Copper infiltrated medium tungsten powder after mechanical compaction; a: sintering at 1400 °C, b: sintering at 1500 °C.
24
A. Ghaderi Hamidi / Int. Journal of Refractory Metals and Hard Materials 66 (2017) 21–25
Fig. 7. SEM micrograph from Copper infiltrated medium tungsten powder after CIP compaction at 663 MPa and sintering at 1500 °C.
Fig. 9. SEM micrograph from Copper infiltrated fine tungsten powder after MP compaction and sintering at 1400 °C, big pores were induced by lubricant particles.
compact after sintering at 1400 °C and infiltration is illustrated in Fig. 9. In fine tungsten MP compacts, lubricant particles were a few times bigger than tungsten particles. Lubricant had come out during sintering and its space had remained empty. Big dark regions are these spaces. Big pores in a bimodal pore distribution do not shrink but grow and remain. By increasing of sintering temperature from 1400 °C to 1500 °C, small pores were eliminated and big pore became more round and smooth (Fig. 10). During the sintering, small pores shrank and pig pores expanded and total pores fraction decreased. The Microstructure of Fig. 10 is favorite microstructure for infiltrated tungsten composites. Production of such a composites from medium tungsten powder need to high sintering temperature that is 2200 °C [13] but in this research has formed by an innovative approach.
4. Conclusions
Fig. 8. SEM micrograph from fracture surface of fine tungsten powder after CIP compaction at 491 MPa and sintering at 1500 °C.
The medium tungsten powder with particle size of 6 μm had negligible densification during sintering at low temperatures (1400–1500 °C) with and without lubricant but sintering densification of the fine tungsten powder with particle size of 1.2 μm is sensitive to presence of lubricant. The fine powder without lubricant showed significant densification (40% of theoretical) and closed pores. Lubricant particles formed big pores in compacts of fine tungsten powder that did not disappeared by sintering. The lubricated fine powder had controlled
Fig. 10. SEM micrograph from Copper infiltrated fine tungsten powder after MP compaction and sintering at 1500 °C.
A. Ghaderi Hamidi / Int. Journal of Refractory Metals and Hard Materials 66 (2017) 21–25
sintering densification (20% of theoretical). Using the lubricant for fine powder and its sintering at 1500 °C developed a structure of interconnected open porosity that is desirable for subsequent infiltration. References [1] V.D. Barth, H.O. McIntire, Tungsten Powder Metallurgy, Technology Utilization Division, National Aeronautics and Space Administration (NASA), 1965. [2] E. Lassner, W.-D. Schubert, Tungsten, Properties, Chimestary, Technology of Element, Alloys, and Chemical Componds, Kluwer Academic/Plenum Publication, New York, 1999. [3] E. Klar, C.B. Thompson, Powder treatments and lubrication, Powder Metal Technologies and Applications, 71998, ASM Handbook 1998, pp. 745–758. [4] R.M. German, Coarsening in sintering: grain shape distribution, grain size distribution, and grain growth kinetics in solid-pore systems, Critical Reviews in Solid State and Materials Sciences 35 (4) (2010) 263–305. [5] R.M. German, Sintering Theory and Practice, Wiley, 1996.
25
[6] J. Zheng, J.S. Reed, Effects of particle packing characteristics on solid-state sintering, J. Am. Ceram. Soc. 72 (5) (1989) 810–817. [7] J. Zhao, M.P. Harmer, Effect of pore distribution on microstructure development: I, matrix pores, J. Am. Ceram. Soc. 71 (2) (1988) 113–120. [8] H.E. Exner, Principles of Single Phase Sintering, Tel-Aviv Freund, 1979. [9] A.G. Hamidi, H. Arabi, S. Rastegari, A feasibility study of W-Cu composites production by high pressure compression of tungsten powder, Int. J. Refract. Met. Hard Mater. 29 (1) (2011) 123–127. [10] A.G. Hamidi, H. Arabi, S. Rastegari, Tungsten–copper composite production by activated sintering and infiltration, Int. J. Refract. Met. Hard Mater. 29 (4) (2011) 538–541. [11] N.C. Kothari, Sintering kinetics in tungsten powder, Journal of Less-Common Metals 5 (2) (1963) 140–150. [12] T. Vasilos, J.T. Smith, Diffusion mechanism for tungsten sintering kinetics, Journal of Applied Physics 1 (1) (1964). [13] S.W.H. Yih, C.T. Wang, Tungsten Sources, Metallurgy, Properties and Applications, Plenum Press, New York, 1979.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Application of compression lubricant as final porosity controller in the sintering of tungsten powders Ahmad Ghaderi Hamidi Department of Metallurgy and Materials Engineering, Hamedan University of Technology, 65155, Iran
a r t i c l e
i n f o
Article history: Received 17 July 2016 Received in revised form 31 December 2016 Accepted 20 January 2017 Available online 04 February 2017 Keywords: Lubricant Infiltrable tungsten Pore coarsening
a b s t r a c t Sintering behavior of two tungsten powders (1.2 μm and 6 μm) was studied for preparing infiltrable porous skeleton. Both powders were compressed by mechanical press (MP) and cold isostatic press (CIP) with and without stearic acid respectively as compaction lubricant. Results showed that presence of solid lubricant powder in addition of its essential effect on soundness of parts, depending on its size and distribution, could mainly affect sintered microstructure. Stearic acid as compaction lubricant in addition of decreasing friction between particles during the compaction, has acted as spacing particles between primary powder particles. In the cases that lubricant particles are much bigger than tungsten particles a big pore remained after evaporation of lubricant. During the sintering, big pores became bigger due to coarsening mechanism and formed an interconnected network of pores and on the other hand small pores shrank or even disappeared due to densification. By exact controlling of the size of tungsten powder and lubricant powder, infiltrable tungsten skeletons with 80% of theoretical density were produced successfully at low sintering temperatures such as 1500 °C. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Powder metallurgy is the predominant production process for tungsten alloys and composites. Compression of tungsten powder can be carried out via cold isostatic press (CIP) or mechanical press (MP) [1]. Uniform pressure in CIP process results in a high uniformity for green density and consequently dimensional stability during sintering. In the case of mechanical pressing, tungsten powder is formed in a uniaxial solid die and severe die wall friction can yield to density inhomogeneity or cracks [2]. Lubricants in solid powder form are mixed with primary powders and reduce the friction between pressed compact and die wall. They provide more uniform density of compacts and prevent ejection defects. Lubricant particles evaporate or decompose at sintering early stages and leave equivalent voids [3]. Size distribution of pores plays a key role in evolution of consolidation. A homogeneous initial pore structure gives a higher sintered density [4]. Densification and coarsening are two apparently counter process during sintering. Although total energy reduction via surface decrease is the drive force for both phenomena, but during coarsening the pores grow while during densification the pores shrink [5]. Relatively large size initial pores enlarge during sintering and regions of initially high packing (small pores) shrink. Initial pores larger than half the primary particle often form big pores [6]. Agglomeration or poor consolidation may lead to this defects [7]. Consequently big pores are unavoidable after sintering of a compact with bimodal porosity [8]. Maximum densification is aim
E-mail address: [email protected].
http://dx.doi.org/10.1016/j.ijrmhm.2017.01.005 0263-4368/© 2017 Elsevier Ltd. All rights reserved.
of most sintering treatments but some sintered parts such as infiltrable tungsten skeletons must have controlled open porosity. Characteristics of final channels is controlled by primary powder, compaction and sintering parameters [9,10]. Growth of pores due to coarsening has been considered as a defect forming mechanism and never has been used as constructive mechanism for porosity control. In this research, the effect of lubricant induced porosity on sintering densification was studied. 2. Material and methods Tungsten powders with two different particle size distributions were used. A tungsten powder with average particle size of 6 μm was one of them. This particle size is normally used for fabrication of tungsten skeletons. Besides that, a fine tungsten powder with average particle size of 1.2 μm was selected for studying behavior of a fine powder. SEM micrographs and particle size distributions of powders are shown in Figs. 1 and 2 respectively. The powder with average particle size of 1.2 μm and the powder with that of 6 μm will be referred as fine and medium powders respectively in whole text. Without lubricants, soundless green tungsten parts was not producible by mechanical pressing. 1.5 wt% of stearic acid powder was added as compression lubricant. The mixture of tungsten and lubricant powders were ball milled for 90 min and passed through 100-mesh sieve twice. Therefore homogenous mixtures of tungsten and lubricant particles were obtained and all lubricant particles had broken down to small flakes. Lubricated powders were compressed in a steel matrix according to ASTM B331 standard test method for Compressibility of Metal
22
A. Ghaderi Hamidi / Int. Journal of Refractory Metals and Hard Materials 66 (2017) 21–25
Fig. 1. SEM micrographs of the initial tungsten powders, a. Fine, b. Medium tungsten powder.
Fig. 2. Particle size analyzing of the initial tungsten powders, a. Fine, b. Medium tungsten powder.
Powder in uniaxial Compaction. Pressure of mechanical compression was about 500 MPa. On the other hand, Fine and medium tungsten powders were compressed via cold isostatic press (CIP) at pressures of 254, 495 and 663 MPa without any lubricant. All specimens were sintered under a flow of dry hydrogen in a tube furnace. Sintering cycles also included delubrication and reduction components each for 1 h at 400 and 1000 °C respectively. Densities of green and sintered specimens were determined by water immersion method according to ASTM B328 standard. Eventually all porous tungsten specimens were infiltrated with molten copper at 1250 °C for 30 min under hydrogen atmosphere. Scanning Electron Microscope (SEM) was employed for in detail study of microstructure. 3. Results and discussion 3.1. Compression Fracture surface of mechanically compressed green specimen of medium tungsten powder shown in Fig. 3. Primary powder is mixed with stearic acid flakes as lubricant. The milling and sieving of primary Table 1 Green and sintered densities of compacted from two tungsten powders after mechanical pressing with lubricant.
Fig. 3. SE-SEM micrograph of fractured section of mechanically compressed medium tungsten powder; some stearic acid flakes are marked.
Sintering temperature Fine powder Medium powder
Green density
Sintered density
54 72
1400 °C 58 72.3
1500 °C 76 72.8
A. Ghaderi Hamidi / Int. Journal of Refractory Metals and Hard Materials 66 (2017) 21–25
23
3.2. Sintering of medium tungsten powder
Fig. 4. Green and sintered densities of two tungsten powder after compaction by cold isostatic press without lubricant and sintering at 1500 °C for 2 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Green and sintered densities of mechanically and CIP compressed specimens are presented in Table 1 and Fig. 4 respectively. Compacts of medium tungsten powder had little densification (b1%) but fine tungsten MP compacts had significant densification (N40%). Surface diffusion and grain boundary diffusion are active mechanisms for materials transportation during sintering of tungsten at low temperatures (b1600 °C) [11]. In the case of surface diffusion, migration of atoms initiate from one point of surface and terminate to another point of that and just strengthen necks with no densification. On the contrary, grain boundary diffusion is controlling mechanism of tungsten densification in temperature range of 1100–1500 °C [12]. As a result, due to lower density of grain conjunctions, little densifications were expected for compacts of medium tungsten powder. Despite the slight densification, newly formed necks are obvious in Fig. 5. SEM micrograph of medium tungsten MP compact after sintering at 1400 °C and 1500 °C and infiltration are illustrated in Fig. 6. An increase in sintering temperature from 1400 °C to 1500 °C has no significant effect on densification (b 1%) but thickened the necks and developed strong junctions. It demonstrates that although increasing of sintering temperature does not have such improvement on densification but it can strengthen the skeleton and eliminate effects of sharp notches. As it is shown in Fig. 7, microstructure of a CIP compact from medium tungsten powder after sintering at 1500 °C and infiltration is same as MP specimen (Fig. 6b). As a consequence, lubricant particles had dimensions as of medium tungsten powder and pore size distribution and contiguity of particles for both compression processes (MP&CIP) are same and presence of lubricant hasn't disturbed homogeneity of specimen. 3.3. Sintering of fine tungsten powder
Fig. 5. SE-SEM micrograph from fracture surface of medium tungsten powder after CIP compaction at 663 MPa and sintering at 1500 °C.
mixture has controlled final lubricant particles size. The lubricant particles had same particle size as medium tungsten powders and therefore were a few times bigger than fine tungsten particles.
In spite of medium tungsten powder, sintering mechanisms was much more active for fine tungsten powder. Fine powder had much more specific area. Furthermore had great number of joints between particles that formed grain boundaries as facilitated diffusion path and also distances for material transportation were so shorter. Therefore green compacts of fine tungsten powder showed a superior densification. SEM micrograph from fracture surface of fine tungsten powder after CIP compaction at 491 MPa and sintering at 1500 °C is demonstrated in Fig. 8. Sintering has progressed up to 92% of theoretical density and microstructure is homogeneous. Because of close pores, this structure could not be used for infiltration. Micrograph of fine tungsten MP
Fig. 6. SEM micrograph from Copper infiltrated medium tungsten powder after mechanical compaction; a: sintering at 1400 °C, b: sintering at 1500 °C.
24
A. Ghaderi Hamidi / Int. Journal of Refractory Metals and Hard Materials 66 (2017) 21–25
Fig. 7. SEM micrograph from Copper infiltrated medium tungsten powder after CIP compaction at 663 MPa and sintering at 1500 °C.
Fig. 9. SEM micrograph from Copper infiltrated fine tungsten powder after MP compaction and sintering at 1400 °C, big pores were induced by lubricant particles.
compact after sintering at 1400 °C and infiltration is illustrated in Fig. 9. In fine tungsten MP compacts, lubricant particles were a few times bigger than tungsten particles. Lubricant had come out during sintering and its space had remained empty. Big dark regions are these spaces. Big pores in a bimodal pore distribution do not shrink but grow and remain. By increasing of sintering temperature from 1400 °C to 1500 °C, small pores were eliminated and big pore became more round and smooth (Fig. 10). During the sintering, small pores shrank and pig pores expanded and total pores fraction decreased. The Microstructure of Fig. 10 is favorite microstructure for infiltrated tungsten composites. Production of such a composites from medium tungsten powder need to high sintering temperature that is 2200 °C [13] but in this research has formed by an innovative approach.
4. Conclusions
Fig. 8. SEM micrograph from fracture surface of fine tungsten powder after CIP compaction at 491 MPa and sintering at 1500 °C.
The medium tungsten powder with particle size of 6 μm had negligible densification during sintering at low temperatures (1400–1500 °C) with and without lubricant but sintering densification of the fine tungsten powder with particle size of 1.2 μm is sensitive to presence of lubricant. The fine powder without lubricant showed significant densification (40% of theoretical) and closed pores. Lubricant particles formed big pores in compacts of fine tungsten powder that did not disappeared by sintering. The lubricated fine powder had controlled
Fig. 10. SEM micrograph from Copper infiltrated fine tungsten powder after MP compaction and sintering at 1500 °C.
A. Ghaderi Hamidi / Int. Journal of Refractory Metals and Hard Materials 66 (2017) 21–25
sintering densification (20% of theoretical). Using the lubricant for fine powder and its sintering at 1500 °C developed a structure of interconnected open porosity that is desirable for subsequent infiltration. References [1] V.D. Barth, H.O. McIntire, Tungsten Powder Metallurgy, Technology Utilization Division, National Aeronautics and Space Administration (NASA), 1965. [2] E. Lassner, W.-D. Schubert, Tungsten, Properties, Chimestary, Technology of Element, Alloys, and Chemical Componds, Kluwer Academic/Plenum Publication, New York, 1999. [3] E. Klar, C.B. Thompson, Powder treatments and lubrication, Powder Metal Technologies and Applications, 71998, ASM Handbook 1998, pp. 745–758. [4] R.M. German, Coarsening in sintering: grain shape distribution, grain size distribution, and grain growth kinetics in solid-pore systems, Critical Reviews in Solid State and Materials Sciences 35 (4) (2010) 263–305. [5] R.M. German, Sintering Theory and Practice, Wiley, 1996.
25
[6] J. Zheng, J.S. Reed, Effects of particle packing characteristics on solid-state sintering, J. Am. Ceram. Soc. 72 (5) (1989) 810–817. [7] J. Zhao, M.P. Harmer, Effect of pore distribution on microstructure development: I, matrix pores, J. Am. Ceram. Soc. 71 (2) (1988) 113–120. [8] H.E. Exner, Principles of Single Phase Sintering, Tel-Aviv Freund, 1979. [9] A.G. Hamidi, H. Arabi, S. Rastegari, A feasibility study of W-Cu composites production by high pressure compression of tungsten powder, Int. J. Refract. Met. Hard Mater. 29 (1) (2011) 123–127. [10] A.G. Hamidi, H. Arabi, S. Rastegari, Tungsten–copper composite production by activated sintering and infiltration, Int. J. Refract. Met. Hard Mater. 29 (4) (2011) 538–541. [11] N.C. Kothari, Sintering kinetics in tungsten powder, Journal of Less-Common Metals 5 (2) (1963) 140–150. [12] T. Vasilos, J.T. Smith, Diffusion mechanism for tungsten sintering kinetics, Journal of Applied Physics 1 (1) (1964). [13] S.W.H. Yih, C.T. Wang, Tungsten Sources, Metallurgy, Properties and Applications, Plenum Press, New York, 1979.