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Nanocrystalline materials: A study of WC-based hard metals
Progressm Materrals ScienceVol. 42,pp 3 II -320.1997 0 1997 Elsevier Science Ltd. All rwhts reserved
Pergamon
PII: SOO79-6425(97)00021-2
Printed in &eat Bntam 0079~6425/97 $32.00
NANOCRYSTALLINE MATERIALS: A STUDY OF WC-BASED HARD METALS S. Berger, R. Porat and R. Rosen Department
of Materials Engineering, Technion, Israel Institute of Technology: Haifa, -32000, Israel CONTENTS
1. INTRODUCTION 1.1. Nanocrystalline materials, general 1.2. Nanocrystalline WC-based hard metals 2. EXPERIMENTAL 2. I. Materials 2.2. Milling 2.3. Sintering 3. RESULTS 3. I. Thermal expansion measurements by dilatometer 3.2. Mechanical and physical properties 4. DISCUSSION 4. I. Sintering mechanism 4.2. Mechanical and physical properties AND CONCLUSIONS 5. SUMMARY ACKNOWLEDGEMENTS REFERENCES
311 311 312 312 312 313 313 313 313 313 316 316 318 319 319 319
1. INTRODUCTION 1.1. Nanocrystalline Materials, General The first papers reporting the manufacture of nanocrystalline materials appeared a little more than a decade ago.““) Since then an ever-increasing number of scientists and researchers have investigated the processing and properties of such materials and, today, there is a journal devoted exclusively to this subject. Any material that contains grains or particles in the size range from 1 to 100 nm is considered a nanocrystalline material. They may exist in various forms, such as powders, layers, agglomerates, dispersed and composite materials. The major microstructural difference between nanocrystalline materials and conventional polycrystalline materials is the grain/grain-boundary volume fraction. In the latter, the grain size is of the order of tens of micrometres and the fraction of atoms in the grain boundary regions is negligible, compared with their fraction in the bulk. In nanocrystalline materials, up to 50% of the atoms are located in the grain boundaries.‘4’ Consequently, the properties of these materials are predominantly determined and controlled by the grain boundaries. Since the properties of nanostructured materials are determined by a complex interplay among the small grains and the interfaces between them, some basic questions still remain to be answered for nanocrystalline materials. For example, What is the correlation between the orientation and the atomic arrangement in the grains and at the grain boundaries? What is the nature of line and surface defects in 311
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nanocrystals? Are there size-dependent structural phase transitions? What are the kinetics of grain growth and the mechanism of grain-growth inhibition? Although the answers to these questions are not yet complete, the fact is that nanocrystalline materials have some unique physical and mechanical properties. For example, the hardness increases with the reduction of grain size,(‘)although the Hall-Petch relationship does not hold for very small grains and even some softening has been observed for certain materials.@*” Ductility also increases with decreasing grain size and some inherently brittle materials, such as ceramics, become superplastic when the grain size is in the nano scale.(8~g)Diffusion in nanocrystalline materials is much faster”” and therefore powders with nano-sized particles can be sintered at lower temperatures. There are other properties, too, magnetic, optical and thermal, that are positively affected by the reduction of the grain size to the nano scale.“‘-“’
1.2. Nanocrystalline WC-based Hard Metals Cemented carbide cutting tools are usually produced from micrometre-sized powders of tungsten and cobalt. (14’In order to improve the mechanical properties of these cutting tools, especially their toughness for a certain hardness, it is desired to produce them from finer powders; for example, from submicrometre-sized powders with a grain size of up to 0.6 pm, or ultra-fine powders with a grain size of up to 0.3 pm. A further reduction of the grain size of the sintered product may be attained by using powders of nanometre grain size. Nanocrystalline ceramic materials were found to have higher hardness, fracture toughness and ductility and are sintered at lower temperatures than coarse-grained powders.“’ It is expected that the properties of cemented carbide cutting tools can be also improved by lowering the grain size to the nanometre scale. Cemented carbide tools are manufactured by means of powder metallurgy from a mixture of WC, Co and grain-growth inhibiting carbide and oxide powders. The recent advancement in the manufacturing of nano-sized powders gave a considerable impetus to the production of submicrometre-grain-sized products(‘5) and opened up prospects for producing nano-sized products as well. There are many ways to produce nano-sized WC/Co powder, some of them commercially. To mention a few, there is the spray conversion processu6) and long time milling.“” Recently, an intensive research project was initiated to study the sintering mechanism of nanometre- and submicrometre-sized WC and Co mixtures with addition of VC grain-growth inhibitor and to compare them with the sintering behaviour of a regular, micrometre-sized powder having the same composition. This paper promulgates the results of the study.
2. EXPERIMENTAL 2.1. Materials
Three types of powder mixture were investigated that had approximately the same composition but different grain sizes. The first powder, type N (nano), composed of 84.3 wt% WC + 14.7 wt% Co + 1.0 wt% VC, was prepared by spray conversion processing. (16’The average grain size of this powder was about 30 nm. The second powder, type S (submicrometre), contained 84.0 wt% WC + 15.0 wt% Co + 1.O wt% VC. The
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average grain size of this powder was 0.62 urn prior and 0.49 urn after milling. The third powder, type R (regular), was composed of 84.0 wt% WC + 15.0 wt% Co + 1.O wt% VC. The average grain of this powder was 1.85 urn prior and 0.97 urn after milling. Powders S and R were prepared by conventional reduction and carborization processes.(‘4) 2.2. Milling All the powders were milled in acetone and 2.8 wt% paraffin with a powder-to-ballweight ratio of 10: 1 and a powder-to-acetone weight ratio of about 2: 1. The milling was performed at a horizontal rotation velocity of 55 rev min-’ for 20 h. Milling conditions were the same for all the powders. After milling the powders were dried at 45°C in air and pelletized to small spherical shape with about 200 mm diameter. As mentioned above, milling caused the reduction of grain size, except for the type N powder.
2.3. Sintering Green compacts in the shape of small cylinders were prepared in a hydraulic pressure at 190 MPa. The specimens were 10 mm long and 5.6 mm in diameter. The green density of the N, S and R types of material was 6.4, 7.58 and 7.83 g cm-3, respectively. Sintering experiments were performed in a Linseis absolute differential dilatometer. After inserting the specimen into the vacuum chamber of the dilatometer, it was heated with a rate of 10°C min-’ up to 1420°C where the specimen was held for 95 min. Sintering was also performed in a regular sintering furnace in vacuum at 1420°C for 95 min.
3. RESULTS 3.1. Thermal Expansion Measurements by Dilatometer The results of the thermal expansion measurements obtained by means of the dilatometer for the three types of specimen are shown in Fig. l(a), (b) and (c). The three curves, which exhibit the shrinkage of the specimens versus time, are quite similar. Shrinkage starts slowly, followed by an increasing rate and slows down again at the high-temperature region. The differences between the three curves are the temperatures at which shrinkage begins and the temperature range where shrinkage is fast. The specimen of powder N starts to shrink at about 600°C and the fast shrinkage is between 900 and 1200°C. Specimen S starts to shrink at 700°C and the fast shrinkage shifts to between 1200 to 1300°C. The powder of specimen R is the latest of the three and it starts to shrink only at about 800°C; however, its fast-shrinkage region is very narrow, between 1250 and 1300°C. Repeated experiments with the three powders gave the same results as those shown in Fig. 1. 3.2. Mechanical and Physical Properties
The following mechanical and physical properties of the three types of specimen were determined: hardness, transverse rupture strength (three-point bending), fracture toughness (indentation testing), density (Archimedes’ tests) and grain size (magnetic measurements). Table 1 summarizes the results of these tests.
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Table 1 shows that, after sintering, the three types of specimen reach almost the same density. It is clearly shown that the final grain size depends on the grain size prior to sintering and the change in grain size is greater as the initial powder grain size is smaller.
-I 0
200
400
600 800 1000 1200 1400 1600 Temperature [“Cl
0
200
400
600 800 1000 1200 1400 1600 Temperature [“Cl
0
200
400
600 800 1000 1200 1400 1600 Temperature [“Cl
Fig. 1. Thermal expansion diagrams of: (a) specimen N, (b) specimen S and (c) specimen R.
3300 2150 3000
13.83
13.86
TRS (N mm _ ‘)
13.87
P (g cm-‘)
1.35
0.95
0.62
d (pm)
94
92.7
82.5
SMS (%)
197
288
90.2
91.1
92.6
&A
1380
1470
1678
Hv (kg mm-‘)
magnetic saturahon; H,--coercivity
H,
456
“p-density; TRS-transverse rupture strength; d-average grain-size diameter; SMS-specific A hardness; H-Vickers hardness; &--fracture toughness.
Specimen of type N 1420 Specimen of type S 1420 Specimen of type R 1420
Sintering temperature (“C)
Table 1. Physical and mechanical properties” of cemented carbides
force; HR,-Rockwell
15.28
13.23
11.79
Klc (MPa ml”)
vl
w
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Progress in Materials Science
It is also evident that the nano size of the grains could not be preserved during sintering and the sintered product contains submicrometre-sized grains for both N and S types of specimen. Table 1 also shows that the hardness increases and fracture toughness decreases with decreasing grain size. The maximum TRS value is obtained for the specimen with the smallest grain size.
4. DISCUSSION 4.1. Sintering Mechanism The thermal expansion measurements carried out in the dilatometer for the three types of powder suggest that the process of densification is a complex one and is composed of different stages. Comparison between Figs l(a), I(b) and I(c) clearly show that the grain size of the powder has a great effect on its sintering behaviour. The smaller the grain size of a powder specimen, the lower is the temperature where shrinkage begins; the change of shrinkage is more abrupt and occurs at higher temperature and within a narrower temperature range for specimens with larger initial grain size. To analyse further the kinetics of the densification mechanism during sintering, the first time derivatives of the shrinkage were calculated by numerical differentiation. The results are shown in Fig. 2, which exhibit the variation of shrinkage rate with temperature of specimens N, S and R, respectively. Comparison of these three diagrams shows that the shrinkage rate increases with increasing temperature up to a certain maximum value, after which the shrinkage rate decreases with increasing temperature up to the melting point. The temperature at which the maximum shrinkage rate is obtained decreases with decreasing grain size; namely, 1200°C for specimen N, 1250°C for specimen S, and 1310°C for specimen R. The presence of a maximum value of shrinkage rate indicates that the shrinkage of this material is controlled by at least two parameters. It is suggested that these parameters are the flow of the material to pores and the density and distribution of pores in the specimen. When the density of pores in the material is high the densification depends mainly on the flow of the material and therefore the shrinkage increases with increasing temperature. When the density of pores reduces to a value that limits the flow of material, then the shrinkage is controlled by the pore density and the shrinkage rate is expected to decrease with decreasing density of the pores. In the specimen with micrometre-sized grains (type R), the material flow begins at a relatively high temperature and controls the shrinkage process up to the eutectic point of 131O”C, where melt flow inside pores and the density of the pores reduce to a state where the density and distribution of pores control the shrinkage. In specimen S, where the grains are submicrometre-sized, the flow of material begins at slightly lower temperature but the change in shrinkage with temperature is more enhanced than in specimen R. This result indicates that the two specimens have the same type of grain boundaries and phase boundaries where matter begins to flow, but in specimen S the density of grain boundaries is higher and therefore the flow of matter is enhanced, leading to higher shrinkage than in specimen R. The enhanced flow of matter leads to enhanced reduction in pore density and consequently the control on shrinkage by the density and distribution of pores is obtained at a lower temperature (about 1250°C) where solid-state processes prevail. In specimen N, where the initial grain size was 30 nm, the flow of matter begins at an even lower temperature because of the different type of
Nanocrystalline
materials: WC-based hard metals
-5+.‘,I’,.,“‘,.“,,,.,“.I 400 600
800
1000
1200
1400
1600
1400
1600
Temperature [“Cl
;
_If~
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___._-___:____
-1
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:
,-
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Temperature
1200
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Fig. 2. Variation of shrinkage rate with temperature of: (a) specimen N, (b) specimen S and (c) specimen R.
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grain and phase boundaries than in the specimens with submicrometreand micrometre-sized grains. Consequently, the flow of matter is enhanced at lower temperature and the control on shrinkage is changed from the flow of matter type to density and distribution of pores type at a lower temperature of about 1200°C. The distinct difference between the curves in Fig. 2 is the shape and width of the peak. The peak of S and R type specimens is narrow and the curve is smooth, while in specimen N the peak is wide and the curve is undulating. It is suggested that the densification of the specimen with nanometre-sized grains consists of several stages in a large temperature range, while in the specimens with submicrometre- and micrometre-sized grains the densification consists of a continuous stage within a narrow temperature range. In order to determine the eutectic point and its grain-size dependence for the three powders, differential thermal analysis (DTA) experiments were carried out. The results showed that the eutectic point is about the same for the three types of specimen, 1310°C. Therefore in specimen N most of the densification occurs in the solid state, while in the R type material most of the densification takes place in the liquid. The sintering mechanism of the S type material is more similar to that of the R type material than to that of the N type material concerning the shape of the shrinkage rate-temperature curves. On the other hand, most of the densification of the S type powder takes place in the solid state and in this aspect it is more similar to the N powder than to the R powder. The effect of initial grain size on the mechanism of grain growth and final microstructure after sintering remains to be studied. 4.2. Mechanical and Physical Properties From Table 1 one can see that the change in the average grain size due to sintering is more pronounced as the initial size of the grains is smaller. In specimen N the grain size increased by two orders of magnitude as a consequence of sintering, while in specimen R the increase was only about 35%. This result can be explained by the initial thermodynamic stability of the specimens. As the grain size is smaller, more grain boundaries are present in the material. The atoms in the grain boundaries are in higher energy state compared with those inside the ordered lattice. Consequently, increasing the density of grain boundaries results in an increase in the free energy of the system. Grain growth reduces the free energy by reducing the density of grain boundaries, and the growth of grains is faster as the change in free energy is larger. The use of grain-growth inhibitors such as VC was found to be successful only in specimen R and not very contributive in specimen S and especially in specimen N. Grain-growth inhibitors such as VC should dissolve first in the cobalt in order to prevent grain growth. In the micrometre-sized material the dissolution occurs at about 1242°C and therefore grain growth is indeed inhibited. In submicrometre- and especially in nanometre-sized grains the growth occurs at much lower temperatures than the dissolution temperature and therefore no grain-growth inhibition was obtained. It is of prime importance to continue the search for grain-growth inhibitors for submicrometre- and nanometre-sized grains. Archimedes’ type density measurements show that the three types specimen reach the same final density of about 13.86 g cmm3, which is close to the theoretical value. The reason is that all three were sintered at the same temperature. Preliminary experiments indicate that the nano-grained specimen can be sintered to full density at a lower temperature and this fact may have implication on the mechanical properties.
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The small differences in the final grain size of the three types of specimen do not justify the pronounced difference in their hardness as presented in Table 1. The significantly higher hardness of the N type material, compared with that of the other two, probably can be explained on the basis of the differences in their microstructure such as density of defects, crystal geometry, and type of grain boundaries. This is a very important observation and needs further study. The highest value of fracture toughness of the R type material is a result of the material’s low hardness.
5. SUMMARY
AND CONCLUSIONS
1. Powder grain size strongly affects the sintering behaviour of WC/Co powders. As the grain size decreases the onset of shrinkage shifts to lower temperatures, it spans more in the solid-state region, and its rate is more gradual and less abrupt with increasing temperature. 2. The thermal contraction curves of the three types of specimen show that at a certain temperature the shrinkage rate changes its trend from increasing to decreasing with increasing temperature. 3. The temperature of this deflection point decreases with decreasing grain size. This behaviour is explained by the change in parameter that controls the shrinkage. It is suggested that these parameters are the flow of the material to pores and the density and distribution of pores in the specimen. As long as the density of pores and their volume distribution are high, the shrinkage rate increases with temperature and is controlled by the transfer of matter in the material. When the density and distribution of the pores decrease below a critical value the shrinkage turns to be controlled by this parameter and it therefore decreases with increasing temperature. 4. Grain-growth inhibitors such as VC are only effective during the sintering of the micrometre-sized powder, but not in the case of the powders with submicrometre- and nanometre-sized grains. 5. The hardness and fracture toughness of the sintered specimens depends on the initial powder grain size prior to sintering. The specimen with initial nanometre-sized grains exhibits a significant increase in hardness, accompanied with a decrease in fracture toughness. ACKNOWLEDGEMENTS The authors wish to thank Nanodyne Inc., NJ, U.S.A. for supplying the ‘Nanocarb’ TM WC-Co powders in the framework of a mutual project with Iscar Ltd. The authors are also grateful to Iscar Ltd for preparing and sintering the powders and for characterization of the physical properties of the powders. The help of Mrs Leiderman is highly appreciated.
REFERENCES 1. H. Gleiter. hog.
Maw.
Sci. 33, 223 (1989).
2. R. Birringer, H. Gleiter, H.-P. Klein and P. Marquadt, Phys. Lert. 102A, 365 (1984). 3. H. Gleiter and P. Marquardt, Z. Merallkde 75, 264 (1984). 4. H. Gleiter, in Proc. of the 2nd Riser Iniernational Symposium on Metallurgy and Materials Science (N. Hansen
et al. eds), p. 15. Risa National Laboratory, Roskilde (1981).
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5. R. W. Siegel and H. Hahn, in Current Trends in Physics of Materials (M. Yousouff ed.), p. 403. World Scientific, Singapore (1987). 6. R. W. Siegel, H. Hahn, S. Ramasamy, L. Zongquan, L. Ting and R. Gronsky, J. Phys. CS 49,681 (1988). 7. A. H. Chokshi, A. Rosen, J. Karch and H. Gleiter, Scripta Metall. 23, 1679 (1989). 8. J. Karch, R. Birringer and H. Gleiter, Nature 330, 556 (1987). 9. K. R. Venkatachari and R. Rai. J. Am. Ceram. Sot. 69. 135 (1986). 10. R. Birringer, H. Hahn, H. Ho&r, J. Karch and H. Gleiter, Difect kzd Dr@sion Forum 59, 17 (1988). 11. R. Birringer, U. Herr and H. Gleiter, Suppl. Trans. Jpn Inst. Met. 27, 43 (1986). 12. S. Veprek, Z. Iqbal, H. R. Oswald and A. P. Webb, J. Phys. C14, 295 (1981). 13. J. Rupp and R. Birringer, Phys. Rev. B36, 7888 (1987). 14. F. V. Lenel, Powder Metallurgy Principles and Applications, p. 383. Metal Powder Industries Federation, Princeton, NJ (1980). IS. P. Ettmayer, Annu. Rev. Mater. Sci. 19, 145 (1989). 16. L. E. McCandlish, B. H. Kear and S. Bhatia, International Patent WO91/07244 (1991). 17. W. Schlump and H. Grewe, in Proc. Conf. on New Materials by Mechanical Alloying Techniques, Claw-Hirasu (1988).
Pergamon
PII: SOO79-6425(97)00021-2
Printed in &eat Bntam 0079~6425/97 $32.00
NANOCRYSTALLINE MATERIALS: A STUDY OF WC-BASED HARD METALS S. Berger, R. Porat and R. Rosen Department
of Materials Engineering, Technion, Israel Institute of Technology: Haifa, -32000, Israel CONTENTS
1. INTRODUCTION 1.1. Nanocrystalline materials, general 1.2. Nanocrystalline WC-based hard metals 2. EXPERIMENTAL 2. I. Materials 2.2. Milling 2.3. Sintering 3. RESULTS 3. I. Thermal expansion measurements by dilatometer 3.2. Mechanical and physical properties 4. DISCUSSION 4. I. Sintering mechanism 4.2. Mechanical and physical properties AND CONCLUSIONS 5. SUMMARY ACKNOWLEDGEMENTS REFERENCES
311 311 312 312 312 313 313 313 313 313 316 316 318 319 319 319
1. INTRODUCTION 1.1. Nanocrystalline Materials, General The first papers reporting the manufacture of nanocrystalline materials appeared a little more than a decade ago.““) Since then an ever-increasing number of scientists and researchers have investigated the processing and properties of such materials and, today, there is a journal devoted exclusively to this subject. Any material that contains grains or particles in the size range from 1 to 100 nm is considered a nanocrystalline material. They may exist in various forms, such as powders, layers, agglomerates, dispersed and composite materials. The major microstructural difference between nanocrystalline materials and conventional polycrystalline materials is the grain/grain-boundary volume fraction. In the latter, the grain size is of the order of tens of micrometres and the fraction of atoms in the grain boundary regions is negligible, compared with their fraction in the bulk. In nanocrystalline materials, up to 50% of the atoms are located in the grain boundaries.‘4’ Consequently, the properties of these materials are predominantly determined and controlled by the grain boundaries. Since the properties of nanostructured materials are determined by a complex interplay among the small grains and the interfaces between them, some basic questions still remain to be answered for nanocrystalline materials. For example, What is the correlation between the orientation and the atomic arrangement in the grains and at the grain boundaries? What is the nature of line and surface defects in 311
312
Progress in Materials Science
nanocrystals? Are there size-dependent structural phase transitions? What are the kinetics of grain growth and the mechanism of grain-growth inhibition? Although the answers to these questions are not yet complete, the fact is that nanocrystalline materials have some unique physical and mechanical properties. For example, the hardness increases with the reduction of grain size,(‘)although the Hall-Petch relationship does not hold for very small grains and even some softening has been observed for certain materials.@*” Ductility also increases with decreasing grain size and some inherently brittle materials, such as ceramics, become superplastic when the grain size is in the nano scale.(8~g)Diffusion in nanocrystalline materials is much faster”” and therefore powders with nano-sized particles can be sintered at lower temperatures. There are other properties, too, magnetic, optical and thermal, that are positively affected by the reduction of the grain size to the nano scale.“‘-“’
1.2. Nanocrystalline WC-based Hard Metals Cemented carbide cutting tools are usually produced from micrometre-sized powders of tungsten and cobalt. (14’In order to improve the mechanical properties of these cutting tools, especially their toughness for a certain hardness, it is desired to produce them from finer powders; for example, from submicrometre-sized powders with a grain size of up to 0.6 pm, or ultra-fine powders with a grain size of up to 0.3 pm. A further reduction of the grain size of the sintered product may be attained by using powders of nanometre grain size. Nanocrystalline ceramic materials were found to have higher hardness, fracture toughness and ductility and are sintered at lower temperatures than coarse-grained powders.“’ It is expected that the properties of cemented carbide cutting tools can be also improved by lowering the grain size to the nanometre scale. Cemented carbide tools are manufactured by means of powder metallurgy from a mixture of WC, Co and grain-growth inhibiting carbide and oxide powders. The recent advancement in the manufacturing of nano-sized powders gave a considerable impetus to the production of submicrometre-grain-sized products(‘5) and opened up prospects for producing nano-sized products as well. There are many ways to produce nano-sized WC/Co powder, some of them commercially. To mention a few, there is the spray conversion processu6) and long time milling.“” Recently, an intensive research project was initiated to study the sintering mechanism of nanometre- and submicrometre-sized WC and Co mixtures with addition of VC grain-growth inhibitor and to compare them with the sintering behaviour of a regular, micrometre-sized powder having the same composition. This paper promulgates the results of the study.
2. EXPERIMENTAL 2.1. Materials
Three types of powder mixture were investigated that had approximately the same composition but different grain sizes. The first powder, type N (nano), composed of 84.3 wt% WC + 14.7 wt% Co + 1.0 wt% VC, was prepared by spray conversion processing. (16’The average grain size of this powder was about 30 nm. The second powder, type S (submicrometre), contained 84.0 wt% WC + 15.0 wt% Co + 1.O wt% VC. The
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313
average grain size of this powder was 0.62 urn prior and 0.49 urn after milling. The third powder, type R (regular), was composed of 84.0 wt% WC + 15.0 wt% Co + 1.O wt% VC. The average grain of this powder was 1.85 urn prior and 0.97 urn after milling. Powders S and R were prepared by conventional reduction and carborization processes.(‘4) 2.2. Milling All the powders were milled in acetone and 2.8 wt% paraffin with a powder-to-ballweight ratio of 10: 1 and a powder-to-acetone weight ratio of about 2: 1. The milling was performed at a horizontal rotation velocity of 55 rev min-’ for 20 h. Milling conditions were the same for all the powders. After milling the powders were dried at 45°C in air and pelletized to small spherical shape with about 200 mm diameter. As mentioned above, milling caused the reduction of grain size, except for the type N powder.
2.3. Sintering Green compacts in the shape of small cylinders were prepared in a hydraulic pressure at 190 MPa. The specimens were 10 mm long and 5.6 mm in diameter. The green density of the N, S and R types of material was 6.4, 7.58 and 7.83 g cm-3, respectively. Sintering experiments were performed in a Linseis absolute differential dilatometer. After inserting the specimen into the vacuum chamber of the dilatometer, it was heated with a rate of 10°C min-’ up to 1420°C where the specimen was held for 95 min. Sintering was also performed in a regular sintering furnace in vacuum at 1420°C for 95 min.
3. RESULTS 3.1. Thermal Expansion Measurements by Dilatometer The results of the thermal expansion measurements obtained by means of the dilatometer for the three types of specimen are shown in Fig. l(a), (b) and (c). The three curves, which exhibit the shrinkage of the specimens versus time, are quite similar. Shrinkage starts slowly, followed by an increasing rate and slows down again at the high-temperature region. The differences between the three curves are the temperatures at which shrinkage begins and the temperature range where shrinkage is fast. The specimen of powder N starts to shrink at about 600°C and the fast shrinkage is between 900 and 1200°C. Specimen S starts to shrink at 700°C and the fast shrinkage shifts to between 1200 to 1300°C. The powder of specimen R is the latest of the three and it starts to shrink only at about 800°C; however, its fast-shrinkage region is very narrow, between 1250 and 1300°C. Repeated experiments with the three powders gave the same results as those shown in Fig. 1. 3.2. Mechanical and Physical Properties
The following mechanical and physical properties of the three types of specimen were determined: hardness, transverse rupture strength (three-point bending), fracture toughness (indentation testing), density (Archimedes’ tests) and grain size (magnetic measurements). Table 1 summarizes the results of these tests.
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Table 1 shows that, after sintering, the three types of specimen reach almost the same density. It is clearly shown that the final grain size depends on the grain size prior to sintering and the change in grain size is greater as the initial powder grain size is smaller.
-I 0
200
400
600 800 1000 1200 1400 1600 Temperature [“Cl
0
200
400
600 800 1000 1200 1400 1600 Temperature [“Cl
0
200
400
600 800 1000 1200 1400 1600 Temperature [“Cl
Fig. 1. Thermal expansion diagrams of: (a) specimen N, (b) specimen S and (c) specimen R.
3300 2150 3000
13.83
13.86
TRS (N mm _ ‘)
13.87
P (g cm-‘)
1.35
0.95
0.62
d (pm)
94
92.7
82.5
SMS (%)
197
288
90.2
91.1
92.6
&A
1380
1470
1678
Hv (kg mm-‘)
magnetic saturahon; H,--coercivity
H,
456
“p-density; TRS-transverse rupture strength; d-average grain-size diameter; SMS-specific A hardness; H-Vickers hardness; &--fracture toughness.
Specimen of type N 1420 Specimen of type S 1420 Specimen of type R 1420
Sintering temperature (“C)
Table 1. Physical and mechanical properties” of cemented carbides
force; HR,-Rockwell
15.28
13.23
11.79
Klc (MPa ml”)
vl
w
316
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It is also evident that the nano size of the grains could not be preserved during sintering and the sintered product contains submicrometre-sized grains for both N and S types of specimen. Table 1 also shows that the hardness increases and fracture toughness decreases with decreasing grain size. The maximum TRS value is obtained for the specimen with the smallest grain size.
4. DISCUSSION 4.1. Sintering Mechanism The thermal expansion measurements carried out in the dilatometer for the three types of powder suggest that the process of densification is a complex one and is composed of different stages. Comparison between Figs l(a), I(b) and I(c) clearly show that the grain size of the powder has a great effect on its sintering behaviour. The smaller the grain size of a powder specimen, the lower is the temperature where shrinkage begins; the change of shrinkage is more abrupt and occurs at higher temperature and within a narrower temperature range for specimens with larger initial grain size. To analyse further the kinetics of the densification mechanism during sintering, the first time derivatives of the shrinkage were calculated by numerical differentiation. The results are shown in Fig. 2, which exhibit the variation of shrinkage rate with temperature of specimens N, S and R, respectively. Comparison of these three diagrams shows that the shrinkage rate increases with increasing temperature up to a certain maximum value, after which the shrinkage rate decreases with increasing temperature up to the melting point. The temperature at which the maximum shrinkage rate is obtained decreases with decreasing grain size; namely, 1200°C for specimen N, 1250°C for specimen S, and 1310°C for specimen R. The presence of a maximum value of shrinkage rate indicates that the shrinkage of this material is controlled by at least two parameters. It is suggested that these parameters are the flow of the material to pores and the density and distribution of pores in the specimen. When the density of pores in the material is high the densification depends mainly on the flow of the material and therefore the shrinkage increases with increasing temperature. When the density of pores reduces to a value that limits the flow of material, then the shrinkage is controlled by the pore density and the shrinkage rate is expected to decrease with decreasing density of the pores. In the specimen with micrometre-sized grains (type R), the material flow begins at a relatively high temperature and controls the shrinkage process up to the eutectic point of 131O”C, where melt flow inside pores and the density of the pores reduce to a state where the density and distribution of pores control the shrinkage. In specimen S, where the grains are submicrometre-sized, the flow of material begins at slightly lower temperature but the change in shrinkage with temperature is more enhanced than in specimen R. This result indicates that the two specimens have the same type of grain boundaries and phase boundaries where matter begins to flow, but in specimen S the density of grain boundaries is higher and therefore the flow of matter is enhanced, leading to higher shrinkage than in specimen R. The enhanced flow of matter leads to enhanced reduction in pore density and consequently the control on shrinkage by the density and distribution of pores is obtained at a lower temperature (about 1250°C) where solid-state processes prevail. In specimen N, where the initial grain size was 30 nm, the flow of matter begins at an even lower temperature because of the different type of
Nanocrystalline
materials: WC-based hard metals
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grain and phase boundaries than in the specimens with submicrometreand micrometre-sized grains. Consequently, the flow of matter is enhanced at lower temperature and the control on shrinkage is changed from the flow of matter type to density and distribution of pores type at a lower temperature of about 1200°C. The distinct difference between the curves in Fig. 2 is the shape and width of the peak. The peak of S and R type specimens is narrow and the curve is smooth, while in specimen N the peak is wide and the curve is undulating. It is suggested that the densification of the specimen with nanometre-sized grains consists of several stages in a large temperature range, while in the specimens with submicrometre- and micrometre-sized grains the densification consists of a continuous stage within a narrow temperature range. In order to determine the eutectic point and its grain-size dependence for the three powders, differential thermal analysis (DTA) experiments were carried out. The results showed that the eutectic point is about the same for the three types of specimen, 1310°C. Therefore in specimen N most of the densification occurs in the solid state, while in the R type material most of the densification takes place in the liquid. The sintering mechanism of the S type material is more similar to that of the R type material than to that of the N type material concerning the shape of the shrinkage rate-temperature curves. On the other hand, most of the densification of the S type powder takes place in the solid state and in this aspect it is more similar to the N powder than to the R powder. The effect of initial grain size on the mechanism of grain growth and final microstructure after sintering remains to be studied. 4.2. Mechanical and Physical Properties From Table 1 one can see that the change in the average grain size due to sintering is more pronounced as the initial size of the grains is smaller. In specimen N the grain size increased by two orders of magnitude as a consequence of sintering, while in specimen R the increase was only about 35%. This result can be explained by the initial thermodynamic stability of the specimens. As the grain size is smaller, more grain boundaries are present in the material. The atoms in the grain boundaries are in higher energy state compared with those inside the ordered lattice. Consequently, increasing the density of grain boundaries results in an increase in the free energy of the system. Grain growth reduces the free energy by reducing the density of grain boundaries, and the growth of grains is faster as the change in free energy is larger. The use of grain-growth inhibitors such as VC was found to be successful only in specimen R and not very contributive in specimen S and especially in specimen N. Grain-growth inhibitors such as VC should dissolve first in the cobalt in order to prevent grain growth. In the micrometre-sized material the dissolution occurs at about 1242°C and therefore grain growth is indeed inhibited. In submicrometre- and especially in nanometre-sized grains the growth occurs at much lower temperatures than the dissolution temperature and therefore no grain-growth inhibition was obtained. It is of prime importance to continue the search for grain-growth inhibitors for submicrometre- and nanometre-sized grains. Archimedes’ type density measurements show that the three types specimen reach the same final density of about 13.86 g cmm3, which is close to the theoretical value. The reason is that all three were sintered at the same temperature. Preliminary experiments indicate that the nano-grained specimen can be sintered to full density at a lower temperature and this fact may have implication on the mechanical properties.
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The small differences in the final grain size of the three types of specimen do not justify the pronounced difference in their hardness as presented in Table 1. The significantly higher hardness of the N type material, compared with that of the other two, probably can be explained on the basis of the differences in their microstructure such as density of defects, crystal geometry, and type of grain boundaries. This is a very important observation and needs further study. The highest value of fracture toughness of the R type material is a result of the material’s low hardness.
5. SUMMARY
AND CONCLUSIONS
1. Powder grain size strongly affects the sintering behaviour of WC/Co powders. As the grain size decreases the onset of shrinkage shifts to lower temperatures, it spans more in the solid-state region, and its rate is more gradual and less abrupt with increasing temperature. 2. The thermal contraction curves of the three types of specimen show that at a certain temperature the shrinkage rate changes its trend from increasing to decreasing with increasing temperature. 3. The temperature of this deflection point decreases with decreasing grain size. This behaviour is explained by the change in parameter that controls the shrinkage. It is suggested that these parameters are the flow of the material to pores and the density and distribution of pores in the specimen. As long as the density of pores and their volume distribution are high, the shrinkage rate increases with temperature and is controlled by the transfer of matter in the material. When the density and distribution of the pores decrease below a critical value the shrinkage turns to be controlled by this parameter and it therefore decreases with increasing temperature. 4. Grain-growth inhibitors such as VC are only effective during the sintering of the micrometre-sized powder, but not in the case of the powders with submicrometre- and nanometre-sized grains. 5. The hardness and fracture toughness of the sintered specimens depends on the initial powder grain size prior to sintering. The specimen with initial nanometre-sized grains exhibits a significant increase in hardness, accompanied with a decrease in fracture toughness. ACKNOWLEDGEMENTS The authors wish to thank Nanodyne Inc., NJ, U.S.A. for supplying the ‘Nanocarb’ TM WC-Co powders in the framework of a mutual project with Iscar Ltd. The authors are also grateful to Iscar Ltd for preparing and sintering the powders and for characterization of the physical properties of the powders. The help of Mrs Leiderman is highly appreciated.
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5. R. W. Siegel and H. Hahn, in Current Trends in Physics of Materials (M. Yousouff ed.), p. 403. World Scientific, Singapore (1987). 6. R. W. Siegel, H. Hahn, S. Ramasamy, L. Zongquan, L. Ting and R. Gronsky, J. Phys. CS 49,681 (1988). 7. A. H. Chokshi, A. Rosen, J. Karch and H. Gleiter, Scripta Metall. 23, 1679 (1989). 8. J. Karch, R. Birringer and H. Gleiter, Nature 330, 556 (1987). 9. K. R. Venkatachari and R. Rai. J. Am. Ceram. Sot. 69. 135 (1986). 10. R. Birringer, H. Hahn, H. Ho&r, J. Karch and H. Gleiter, Difect kzd Dr@sion Forum 59, 17 (1988). 11. R. Birringer, U. Herr and H. Gleiter, Suppl. Trans. Jpn Inst. Met. 27, 43 (1986). 12. S. Veprek, Z. Iqbal, H. R. Oswald and A. P. Webb, J. Phys. C14, 295 (1981). 13. J. Rupp and R. Birringer, Phys. Rev. B36, 7888 (1987). 14. F. V. Lenel, Powder Metallurgy Principles and Applications, p. 383. Metal Powder Industries Federation, Princeton, NJ (1980). IS. P. Ettmayer, Annu. Rev. Mater. Sci. 19, 145 (1989). 16. L. E. McCandlish, B. H. Kear and S. Bhatia, International Patent WO91/07244 (1991). 17. W. Schlump and H. Grewe, in Proc. Conf. on New Materials by Mechanical Alloying Techniques, Claw-Hirasu (1988).