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Advanced and new grades of WC and binder powder – their properties and application
International Journal of Refractory Metals & Hard Materials 18 (2000) 87±102
Advanced and new grades of WC and binder powder ± their properties and application Gerhard Gille a
a,*
, J. Bredthauer a, B. Gries a, B. Mende a, W. Heinrich
b
H.C. Starck GmbH & Co. KG, Im Schleeke 78-91, Postfach 2540, 38642 Goslar, Germany b Kennametal Hertel AG, Eckersdorfer Straûe 10, 95489 Mistelgau, Germany Received 23 November 1999; accepted 9 December 1999
Abstract This paper deals with properties and applications of advanced WC and new (Fe/Ni/Co) powders. Special attention is focused on (1) ®ne-grained and highly sinteractive (Fe/Ni/Co) powder and (2) ultra®ne WC powder with linear WC intercepts averaging 100± 130 nm in structures of WC±10 wt% Co hardmetals. The quality, individual features and properties of the new powders are shown to be a key factor in achieving excellent and improved properties and performance of hardmetals and diamond tools. A number of correlations are established between powder properties, sintering behaviour and hardmetal structures and properties. In addition to other well-known parameters, the lattice microstructure, as well as the mixed-crystal phases of the new powders, are shown to be essential for optimized sintering, structure formation and improved properties of the tool materials. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Although more than 70 years old, the WC±Co hardmetals continue to gain importance for cutting, mining and chipless forming tools, as well as for highperformance construction and wear parts. For the modern hardmetals, a broad range of WC grades with particle sizes between 0.40 and 35 lm are used together with carbides such as (Ta,Nb)C, TiC, (W, Ti, Ta)C and special grades of Co powder. These continuously improved or newly developed powder intermediates have been essential to the improved performance of modern hardmetal tools and wear parts. Co metal currently dominates the market as a binder because of some unique properties. However, there are also detrimental properties of this binder metal, mainly due to its cph lattice structure. Newly developed (Fe/Ni/ Co) alloys could be an alternative for some hardmetal applications where improved fatigue strength and toughness are required. These new binder alloys combine a mixed crystal structure of specially selected (Fe:Ni:Co) ratios with some advantageous physical
*
Corresponding author. Tel.: +49-05321-751217; fax: +49-05321751192.
properties that are very important for processing this powder in hardmetals or diamond tools. In particular, well-known problems such as porosity and inhomogeneous structures can be avoided if these powders are used instead of mechanical mixtures of conventional Fe-, Ni- and Co-powders. The shift towards ®ner and ®ner hardmetals has continued for more than 20 years. To produce the required submicron and ultra®ne, high-performance WC powder is a great challenge for the powder producer. To produce ®ner WC powder, manufacturers must operate at lower temperatures compared to the conventional technology. Lower temperatures can result, however, in higher impurity contents because some of the raw material trace elements are not volatilized signi®cantly [1]. In addition, the WC lattice is more disordered with a higher content of defects such as stacking faults, dislocations and vacancies if the temperature of WC synthesis is reduced for ®ner grain sizes. Using a new process, an ultra®ne WC can be synthesized with a substantially improved lattice structure than ultra®ne WC powder available in the market today. The perfect lattice structure and the high purity of the new 0.1 lm WC powder are two of the most important reasons for the excellent sintering behaviour of the powder, the homogeneous and ®ne grained structure of the
0263-4368/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 0 0 ) 0 0 0 0 2 - 0
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hardmetal and the high hardness, and coercivity of the ®nished product.
2. New alternative (Fe/Ni/Co) binder The properties of WC-hardmetals are determined by the carbides, as well as the binder metals and can be widely varied through WC-content, WC-grain size and alloying additions such as TiC and (Ta/Nb)C. The carbides are responsible for properties such as hardness and wear resistance, but need to be bonded by metals or alloys to provide toughness and strength in the composite material. More than 90% of all WC-hardmetals utilize Co as the preferred binder metal with contents between 3% and 30% by weight. The reasons for the dominant role of Co are some unique properties of Co and the Co±W±C ternary system. It is well known that the solubility of WC in Co is not only high but also strongly varies depending upon the temperature. This is closely connected to the excellent wetting between WC and molten Co, as well as to favourable properties of the Co±W±C binder metal. In addition, properties such as the high hardness, yield stress, toughness and strength of Co with its cph structure and allotropic cph/ fcc phase transformation are behind the dominating use of Co in hardmetals. Only in special hardmetal applications, requiring high corrosion or oxidation resistance, are binders other than Co used, e.g. (Ni,Cr)based alloys. The history of hardmetals is ®lled with attempts to substitute Co as the binding metal. These investigations
are motivated not only by the temporarily high and strongly ¯uctuating prices of Co-metal powder, but there are also serious technical reasons why researchers are trying to improve hardmetals properties using alternative binders. In the early days of hardmetal history, especially in the German hard metal industry during and after the Second World War some eorts were made to produce hardmetals with Fe-based binders. These hardmetals never entered the market successfully and disappeared quickly due to detrimental properties or diculties in producing stable qualities. In the late 1970s, Prakash made the ®rst, well-founded investigations on a broad range of (Fe/Ni/Co)-alloys and Table 1 Corrosion rate (MDD) in 1% organic acid solutions
Formic acid Acetic acid Maleic acid/CH3 OH Maleic acid/H2 O
WC±9.5% Co
WC±9.5% (Co/Ni/Fe)
245 178 297 188
109 125 125 125
Table 2 Oxidation resistance: weight increase [mg=cm2 ] at 700°C Annealing time (h) 10
20
30
WC±7.5 % (Ti, Ta, Nb) C±6% Co
90
150
175
WC±7.5 % (Ti, Ta, Nb) C±6% Co (Co/Ni/Fe)
37
75
100
Fig. 1. Particle size distribution by MALVERN MASTER SIZER 1002 and SEM micrographs of a prealloyed (60Co/20Ni/20Fe) powder.
G. Gille et al. / International Journal of Refractory Metals & Hard Materials 18 (2000) 87±102
showed that hardmetals with such Fe-rich binders have improved properties such as higher hardness, abrasive wear resistance, toughness and strength compared to Co bonded hardmetals [2,3]. Based on this fundamental research, some hardmetal grades for wood cutting tools and wear parts were developed and launched to the market in the late 1990s [4]. In contrast to the grades with Fe-rich binders having bcc-structures or structures near to the bcc/fcc-phase transformation area, Heinrich et al. developed Co-rich binders with fcc-structures and considerable improved corrosion and oxidation restistance (see Tables 1 and 2). These (Co/Ni/Fe) bonded hardmetals with an fcc structure also have improved toughness and fatigue strength [5,6]. The hardness at and above room tem-
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perature, as well as the Yonug's modulus, are slightly decreased compared to WC±Co hardmetals. Until now, one reason preventing a broader application of (Fe/Ni/Co) binders was the lack of a suitable powder. The hardmetal producer had to mix Co powders quali®ed for hardmetals with the conventionally available Fe- and Ni- carbonyl powder which are relatively coarse and dicult to mill. To overcome this new prealloyed (Fe/Ni/Co) powders were developed and some of their properties are shown in Fig. 1 and Table 3. The new (Fe/Ni/Co) powders have some typical properties: 1. They are loosely agglomerated with agglomerate dimensions of around 20 lm and primary particle sizes of 1 lm or below.
Table 3 Physical and chemical parameters of prealloyed (Co/Ni/Fe) powder
a
Composition of prealloyed powder
Co/Ni/Fe 60/20/20
Co/Ni/Fe 40/30/30
Co/Ni/Fe 15/15/70
Fe (%) Ni (%) Co (%) O (%) C (%) FSSS-value (lm)
19.32 20.27 60.32 0.525 0.002 1.73
30.22 29.88 39.70 0.640 0.002 0.85
71.30 14.95 11.15 1.980
Grain size distributiona d10 (lm) d50 (lm) d90 (lm) BET-value (m2 /g) XRD-analysis
5.96 14.9 28.9 1.98 fcc, bcc
2.68 6.85 15.2 2.16 fcc, bcc
14.8 25.0 39.0 1.49 fcc, bcc
2.44
MALVERN MASTER SIZER 1002.
Fig. 2. X-ray diraction pattern of a new mixed crystal (60Fe/20Ni/20Fe) powder compared with a mechanically mixed powder of the same composition.
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2. They are prealloyed or mixed crystals, which means they are mixtures of all metal constituents on an atomic level. The structure can be varied between bcc, fcc and hcp depending on the (Fe:Ni:Co) ratio. One example which demonstrates this is given in Fig. 2. In comparison to a mechanical mixture of Co, Ni- and Fe-carbonyl powder comprising the same (Fe:Ni:Co) ratio the new powder shows a pure fcc-structure with lattice constants shifted, according to the rule of mixture. 3. The (Fe/Ni/Co) powders can be produced in a wide range of compositions. The oxygen content is related to their high speci®c surface area relatively low, see Table 3. 4. The powders are highly sinteractive and can be used in both hardmetals (liquid phase sintering), as well as diamond tool applications (solid state sintering).
The sintering behaviour of the new mixed crystal (Fe/ Ni/Co) powder is shown in Figs. 3±5 for hardmetals with 9.5% and 20% binder and compared with that of mechanically mixed powder of same composition. The most important features of Figs. 3±5 may be summarized as: · Especially in the case of 20% binder, the high sinteractivity of the ®ne grained (60Co/20Ni/20Fe) powder is proven by a shrinkage rate starting at a lower temperature and spreading over a wider temperature range, compared to hardmetals with a mechanical mixture of Fe-, Ni- and Fe-powders, see Fig. 4. · According to the ®ner grain size (primary particle) and the higher speci®c surface area (BET-value) of the (Fe/Ni/Co) powder, the hardmetals bonded with this binder show higher weight losses than the hardmetals with the mechanically mixed powder. This is
Fig. 3. Shrinkage rate, DSC-signal, TG mass loss and mass spectroscopy of WC (0.5 lm) ± 9.5% (60Co/20Ni/20Fe) hardmetals. Comparison between mixed crystal and mechanically mixed (60Co/20Ni/20Fe) powder.
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Fig. 4. Shrinkage rate, DSC-signal, TG mass loss and mass spectroscopy of WC (2.5 lm) ± 20% (60Co/20Ni/20Fe) hardmetals. Comparison between mixed crystal and mechanically mixed (60Co/20Ni/20Fe) binder powder.
indicated by the thermal gravimetry, as well as by the mass spectrometry analysis of released gases, see Figs. 3±5. The water m 18 is completely removed at 450± 500°C. The adhesive oxygen or oxides on particle surfaces are forming CO2 and CO with carbon from WC. The removal of CO2 is completed at 720°C and that of CO at 1050°C which may be explained according to the Boudouard equilibrium between CO2 and CO. After reducing all oxides, especially those on the surfaces of WC particles, the shrinkage starts [7]. Based on the improved sintering behaviour, the porosity of vacuum sintered WC-hardmetals with 9% (75Fe/15Ni/10Co) mixed crystal binder is also considerably improved compared to that of hardmetals with mechanically mixed Fe-, Co- and Ni powder, see Fig. 6. While a hardmetal with a mechanically mixed, Fe-rich powder ends up with a A06±A08 porosity, the hard-
metal bonded with the prealloyed powder results in a hardmetal with A00±A02 porosity. The homogeneity of the structure can be improved too by using the new mixed crystal powder, see Fig. 7. In the example shown in Fig. 7, pressure-sintering was applied to the hardmetals. Porosity was
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Fig. 5. TG mass loss and mass spectroscopy of WC (0.5 lm) ± 9.5% (60Co/20Ni/20Fe) hardmetals. Comparison between mixed crystal and mechanically mixed (60Co/20Ni/20Fe) binder powder.
hardening. Therefore, one of the disadvantages of the Co-rich (Co/Ni/Fe) binder could be lower hardness at and above room temperatures, as given in Fig. 8. Because the fcc lattice structure causes lower hardness and yield stress, the use of mixed crystal, instead of mechanically mixed binder, can improve high temperature performance only slightly. The improved homogeneity of the structure (see Fig. 7), however, improves the toughness and fatigue properties as shown by the ®rst results with the mixed crystal binder. Besides the application as binder for hardmetals, the new mixed crystal powder can be used as a binder for diamond tools. In this case, solid state and not a liquid phase sintering is applied to bound the hard particles with the binder metal. By applying a solid state sintering technique below the melting point of the binder metals, the homogeneous distribution of all metal components is considerably reduced compared to a molten binder
process. Therefore, a ®ne grained and mixed crystal powder is especially advantageous of solid state sintering. Powder properties such as grain size, grain size distribution and lattice structure have a greater in¯uence on the structure of the sintered material compared to hardmetals densi®ed by liquid phase sintering. The grain size of the binding metal in the diamond tool is especially important because it correlates closely the hardness, toughness and cutting performance of the tool. According to the Hall±Petch relation, the hardness increases with the inverse square root of the grain size, however, a ®ner grained structure also must be highly dense too and nearly free of pores. Therefore, the starting powder should be ®ne grained, easy to mix with the diamond powder, homogeneous in composition and structure, but also sinteractive to densify at temperatures as low as possible to prevent considerable grain growth.
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Fig. 6. Porosity of vacuum sintered WC (0.8 lm)±9% Co and WC (0.8 lm) ± 9% (75Fe/15Ni/10Co) hardmetals.
Fig. 7. Structure micrographs (SEM) of pressure ± sintered WC (2.5 lm) ± 20% (60Co/20Ni/20Fe) hardmetals. Comparison between prealloyed and mechanically mixed binder powder.
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Fig. 8. Hot hardness of WC-hardmetals with Co and (Co/Ni/Fe) mixed crystal binders.
In Fig. 9 the hardness curves of hot pressed samples based on mechanically mixed and prealloyed (75Fe/ 15Ni/10Co) powder show the much higher hardnesses of the prealloyed powder. Although there is a slight decrease after the hardness maximum of HRB 118 at 580±620°C hot pressing temperatures, the hardnesses at higher temperatures are still around 10 units higher than that of mechanically mixed powder. This is mainly due to the ®ner grained structure of hot pressed samples with mixed crystal powder. Another requirement of the diamond tool producer is a high but stable hardness within a broad range of hot pressing
temperatures. This is ful®lled very well by the (Fe/Co/ Ni) mixed crystal powder, see Fig. 9. 3. Ultra®ne WC powder Since launching the ®rst submicron hardmetals in the late 1970s the tendency towards ®ner and ®ner hardmetals has continued [8±10]. The main interest in hardmetals with these ®ner grain sizes derives from the understanding that hardness and wear resistance increase with decreasing WC grain sizes. This under-
Fig. 9. Hardness and density of hot pressed samples based on mechanically mixed and prealloyed (75Fe/15Ni/10Co) powder at varying on the hot pressing temperatures.
G. Gille et al. / International Journal of Refractory Metals & Hard Materials 18 (2000) 87±102
standing, together with advances in the performance of machine tools and wear parts, has resulted in considerable improvements, especially in cutting tools used in advanced applications such as microdrills for drilling holes in PCB's, endmills or drills for cutting cast iron, steel castings or hardened steels and wood cutting tools. To meet the requirements of the hardmetal industry and to follow trend towards ®ner grain sized tools, a new WC powder was developed using a completely new process. Powder characteristics and the resulting hardmetal properties are shown in Table 4 and Figs. 10, 11. By considering powder properties such as FSSSvalue dFSSS or particle size distribution alone, some misunderstandings may be caused because these properties are in¯uenced by the measuring method itself. The best and most objective way to characterize the powder is to measure the hardmetal properties, such as hardness and coercivity, as well as the hardmetal structure [11]. Fineness and particle size distribution of the structure, as measured by the linear intercept or a point counting method [12], are most closely connected to hardness and other properties of the sintered hardmetal, as documented later in this paper (see Section 4). Application-related properties, such as hardness and Palmqivst toughness Kc , give a realistic evaluation of the powder. The comparison between the new WC 0.1 L and the ®nest, commercially available WC grades DS 40 and DS 60 is made mainly on the basis of the hardmetal structure and properties, see Figs. 10, 11 and Table 4. The structure analysis, which was made with a SEM high-resolution electron microscope and a special etching technique, was dicult due to the extreme ®neness. For the ®rst time, an arithmetic mean linear intercept of 0.1 lm 100 nm was measured for a WC±10 wt% Co structure. Most people consider 100 nm as the border
95
between the nano- and microstructured world. Connected with this ®ne grained structure, the hardness HV30 of WC 0.1 L±10 wt% Co hardmetals surpasses 2000. This high hardness was achieved with a VC/Cr3 C2 dopant content of only 1 wt% in total. Making a ®ne grained WC powder is only the ®rst step to end up successfully with a ®ne grained hardmetal structure. In addition to mixing and milling, sintering is a second essential step in transforming powder properties into hardmetals properties. Obviously, there are some interrelations between powder properties and the sintering behaviour which become more pronounced as the powder becomes ®ner. In particular, the densi®cation which is obtained in the stadium of solid state-sintering increases and is intensi®ed considerably with decreasing WC grain size and increasing interfaces between WC particles and Co binder [7,13]. Fig. 12 shows some of the most important characteristics and peculiarities of sintering a WC 0.1 L±10 wt% Co hardmetal. Compared with the WC DS 60 hardmetal, the WC 0.1 L hardmetal with the same VC, Cr3 C2 doping shows the following characteristics: · Shrinkage starts at a lower temperature and the maximum of shrinkage rate is shifted from 1260°C for the WC DS 60±10 wt% Co hardmetal to 1200° for the WC 0.1 L±10 wt% Co hardmetal. · The shrinkage rate curve is wider and more ¯at than that of the WC DS 60 hardmetal. This corresponds to a general tendency of shifting the shrinkage to lower temperatures with decreasing WC grain sizes [7]. · The mass loss is higher for the WC 0.1 L hardmetal due to the higher content of oxygen and oxides which have to be released as CO2 and CO. The removal of these oxides as gases is, however, completely ®nished before the high rate shrinkage starts and the binder melts.
Table 4 Properties of WC DS 60, WC DS 40 and WC 0.1 L powder (above)a and hardmetals based on WC 0.1 L and WC DS 40 powder doped with VC and Cr3 C2 (below)
a
Powder
d10 b
d50 b
d90 b
Dd c
dFSSS
d50 d
WC DS 60 WC DS 40 WC 0.1 L
0.34 0.23 0.14
0.57 0.44 0.26
1.36 1.02 0.56
1.02 0.79 0.42
0.55 0.48 0.33
0.22 0.17 0.10
Hardmetal composition
q g=cm3
Hc [kA/m]
4 prs lTm3 =kg
HV30 at 20°C
Kc [N/mm]
WC WC WC WC WC
14.83 14.46 14.43 14.30 13.95
53.3 45.4 35.8 41.8 35.8
10.2 17.2 19.0 21.9 27.7
2280 2043 1870 1910 1700
370 530 810 570 725
0.1 L±6 wt% Co 0.1 L±10 wt% Co DS 40±10 wt% Co 0.1 L±12 wt% Co 0.1 L±15 wt% Co
All values in lm. SEDIGRAPH 5000 D. c Dd d90 ÿ d10 . d d50 50% value of intercept distribution. b
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Fig. 10. Properties of WC 0.1 L, WC DS 40 and WC DS 60 powder and structures of hardmetals using these WC powders.
An answer to the relationship between sintering behaviour and the homogenous and very ®ne grained structure of WC 0.1 L hardmetals is given in Fig. 13. For all lots of WC 0.1 L powder type the combination of crystallite size and lattice distortion, both measured
by X-ray diraction, is within the marked area. The data show that the combination of low crystallite size and low lattice distortion is typical for this new WC powder type. Investigations of commercially available, ultra®ne grained WC powder show that the combinations of
G. Gille et al. / International Journal of Refractory Metals & Hard Materials 18 (2000) 87±102
97
Fig. 11. Relationship between coercivity and magnetic saturation for WC 0.1 L±10 wt% Co and WC DS 40±10 wt% Co hardmetals.
Fig. 12. Shrinkage rate, DSC-signal, TG mass loss and TG mass loss rate for WC 0.1 L±10 wt% Co and WC DS 60±10 wt% Co hardmetals.
98
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Fig. 13. Crystallite size and lattice distortion of WC 0.1 L and conventional ultra®ne WC powder. Measurements made by X-ray diraction.
crystallite size and lattice distortion are found outside the WC 0.1 L powder type area. This means that for a given crystallite size or lattice distortion, the combined distortion or crystallite size is greater for commercially available WC powder than for a WC 0.1 L powder type. The lattice distortion measured by X-ray diraction is a measure of stacking fault and dislocation densities within the cph lattice of WC. The higher these densities, the higher the lattice distortions. A highly distorted lattice however, shows a higher solution pressure and therefore the rate of solution and reprecipitation of such WC grains is much higher during the liquid phase-sintering than that of more perfect WC grains. One of the important peculiarities of the WC 0.1 L powder is their low lattice distortion which explains some of the favourable features such as stable sintering behaviour, and structure ®neness and homogeneity without detrimental qualities such as abnormal or selective grain growth.
4. Structure±hardness relationship Powder manufacturers and the hardmetal industry alike are interested in ®nding straightforward relationships between powder and hardmetal properties, however, there are many steps between the initial powder and the ®nished hardmetal, such as milling, pressing and sintering. They clearly overlap with each other in their eects on the resulting hardmetal properties, but there is no doubt that the properties of the powder have a sig-
ni®cant in¯uence on hardmetal properties. The structure of the sintered hardmetal is a suitable bridge between the powder properties on the one side and the mechanical properties of the hardmetal on the other side of the river to be crossed. Hardness is a good example of how this can work because it is one of the most important, application-related properties, and it is also as an easy to measure and interpret property. As shown in Fig. 14, the hardness of 120 hardmetals with dierent Co-contents and WC grain sizes can be very well correlated with the mean linear intercepts dWC of these hardmetals [14]. The structure was analyzed by means of a modern high resolution SEM with a ®eld emission electron beam source [12]. In addition to the results in [7,13,14], Fig. 14 also comprises the newest results on the WC 0.1 L hardmetals. The dependencies of hardness on the mean linear intercept dWC can be well ®tted by power law functions with high correlation coecients varying between 0.980 and 0.988. Otherwise it is clearly to see in Fig. 14 that the hardness dependence on dWC does not ®t a Hall-Petch relation. A theoretical model helps to ®nd relationship between hardness HV30 and the two most important structure parameters, the mean intercept dWC and the volume fraction of Co VCo HV30 f VCo dWC
ÿ1=m
2 1824 1 ÿ 1:65VCo 0:92VCo dWC
ÿ0:194
:
According to this model one straight line correlates hardness HV30 , mean linear intercept dWC and Co volume fraction VCo of 120 hardmetals within a wide spread
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Fig. 14. Hardness HV30 depending on WC arithmetic mean linear intercept dWC (above) and inverse square root of dWC (below).
range of parameter values (dWC 0.10±7.80 lm, VCo 0.10±0.38) with a correlation factor ÿK 2 0:9872, see Fig. 15. Based on data available at that time Merz et al. [15] established an empirical relationship between hardness, dWc and the mean free path in Co phase pCo dWc VCo = 1 ÿ VCo )) 2 =dWC HV30 877 pCo
ÿ1=5
877
1 ÿ VCo VCo
0:4 dWC
ÿ0:2
:
Because of in®nite hardness for vanishing Co-contents VCo , this relation could not be true for binderless hardmetals or hardmetals with very low binder contents. However, the relation ®ts the hardness values very well
for normal Co-contents in whole range of WC grain sizes. Disregarding the discrepancy at very low volume fraction of Co, this empirical relation also corresponds ÿ1=m well with the above relation HV30 f VCo dWC . Conspicuously, both relations have nearly the same exponent for the dWC dependency of HV30 : 1/m 0.194, which is nearly 0.20. Finally, a relationship between hardness and grain size of the starting WC powder dFSSS can be found if a relation between the mean linear intercept dWC and dFSSS is known. Such a relation is shown in Fig. 16 for WC±10 wt % Co hardmetals. Combining the relationship between HV30 and dWC and that between dWC and dFSSS for WC±10 wt % Co hardmetals ®nally results in
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Fig. 15. Hardness HV30 of 120 hardmetals as a function of dWC (above) and VCo with m 5.155.
Fig. 16. Relationship between arithmetic mean linear intercept of structure dWC and powder grain size dFSSS for 10 WC±10 wt% Co hardmetals.
HV30 1603 dFSSS
ÿ0:1568
which ®ts very well the experimental values as given in [13]. In all the three relationships dWC and dFSSS are assumed in lm. 5. Concluding remarks Sometimes the question arises how far the tendency to ®ner and ®ner WC grains could or should go. The answers are very dierent and depend on the sector of
hardmetal industry the answer is coming from. The presented investigations have shown the possibility to synthesize a real 0.1 lm powder where 0.1 lm is the arithmetic mean linear intercept of hardmetal structure. The dependence of hardness on Co Volume fraction VCo (see Table 4) may be extrapolated to a hardness of 2.700 HV30 for a binderless WC skeleton. These values correspond very well with extrapolations of hardness ± WC grain size dependencies as given in Fig. 17 for binderless WC-hardmetals (extrapolation for disappearing Co content). In Fig. 17 are also shown the new areas of hardness which may be realized for hardmetals
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Fig. 17. Hardness of WC±Co hardmetals versus WC grain size and Co content (above). Hardness of binderless WC-hardmetals in dependence on WC powder grain size (below).
by applying the new 0.1 lm WC powder with dierent Co-binder contents. In the lower part of Fig. 17 it is indicated that an alternative technology is necessary to produce the new 0.1 lm WC powder grade. But the question arises, can the extreme hardness be used in a tool or wear part because of low toughness. The answer can only be given by the hardmetal industry itself. Also for the new (Fe/Ni/Co) mixed crystal powder, only a close cooperation with the hardmetal or diamond tool industry can give a well founded and concluding evaluation. As a powder producer H.C. Starck is ready to hear from the tool industry the promising (Fe:Ni:Co) compositions they selected for their special application.
Based on that information, alternative powder can be produced and applied with a broad range of compositions, as well as advantageous physical and chemical properties for each special case of application. Acknowledgements The authors would like to thank Dr. Leitner from IKTS/FhG (Dresden) for conducting the thermoanalytical measurements and Dr. Roebuck from NPL (Teddinghon) for the structure analysis of the extremely ®ne grained WC 0.1 L±Co hardmetals.
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References [1] Lassner E, Schubert WD. Tungsten. New York: Kluwer Academic Publishers/Plenum Press; 1999. [2] Prakash L. Doctoral Thesis, University Karlsruhe, 1979. [3] Prakash L. In: Prococeedings of 12th International Plansee Seminar. Reutte, vol. 2, 1993. pp. 80±109. [4] DE 29617040 U1. Gebrauchsmuster: WC-Hartlegierung. [5] WO 99/10549. PCT Patent: A cermet having a binder with improved plasticity, a method for the manufacture and use thereof. [6] WO 99/10551. PCT Patent: A pick-style tool with a cermet insert having a Co±Ni±Fe-binder. [7] Gille G, Leitner G, Roebuck B. In: Proceedings of the European Conference on Advances in Hard Materials Production, Stockholm, 1996. pp. 195±210. [8] Egami A, Kusaka M, Machida M. In: Proceedings of 12th Internnational Plansee Seminar, Reutte, vol. 2, 1989. pp. 53±70.
[9] Kassel D, Schaaf G, Dreyer K. In: Proceedings of the 12th International Tungsten Symposium, Goslar, 1996. pp. 24±37. [10] Daub HW, Dreyer K, Happe A, Holzhauer H, Kassel D. Pulvermetallurgie in Wissenschaft und Praxis Bd. 11, Oberursel, 1995. pp. 285±306. [11] Conner C, Carroll D. et al. In: Proceedings of the 12th International Tungsten Symposium, Goslar, 1996. pp. 171±80. [12] Roebuck B. Int J Refractory Metals & Hard Materials 1995;13:265. [13] Gille G, Szesny B, Leitner G. In: Proceedings of the 14th International Plansee Seminar, Reutte, vol. 2, 1997. pp. 139±67. [14] Gille G, Schwier G, Szesny B. Pulvermetallurgie in Wissenschaft und Praxis Bd. 14, Oberursel, 1998. pp. 194±227. [15] Merz A, Pompe W, Gille G. In: Proceedings of the Fourth European Powder Metallurgy Conference, Grenoble, 1975. pp. 581±87.
Advanced and new grades of WC and binder powder ± their properties and application Gerhard Gille a
a,*
, J. Bredthauer a, B. Gries a, B. Mende a, W. Heinrich
b
H.C. Starck GmbH & Co. KG, Im Schleeke 78-91, Postfach 2540, 38642 Goslar, Germany b Kennametal Hertel AG, Eckersdorfer Straûe 10, 95489 Mistelgau, Germany Received 23 November 1999; accepted 9 December 1999
Abstract This paper deals with properties and applications of advanced WC and new (Fe/Ni/Co) powders. Special attention is focused on (1) ®ne-grained and highly sinteractive (Fe/Ni/Co) powder and (2) ultra®ne WC powder with linear WC intercepts averaging 100± 130 nm in structures of WC±10 wt% Co hardmetals. The quality, individual features and properties of the new powders are shown to be a key factor in achieving excellent and improved properties and performance of hardmetals and diamond tools. A number of correlations are established between powder properties, sintering behaviour and hardmetal structures and properties. In addition to other well-known parameters, the lattice microstructure, as well as the mixed-crystal phases of the new powders, are shown to be essential for optimized sintering, structure formation and improved properties of the tool materials. Ó 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Although more than 70 years old, the WC±Co hardmetals continue to gain importance for cutting, mining and chipless forming tools, as well as for highperformance construction and wear parts. For the modern hardmetals, a broad range of WC grades with particle sizes between 0.40 and 35 lm are used together with carbides such as (Ta,Nb)C, TiC, (W, Ti, Ta)C and special grades of Co powder. These continuously improved or newly developed powder intermediates have been essential to the improved performance of modern hardmetal tools and wear parts. Co metal currently dominates the market as a binder because of some unique properties. However, there are also detrimental properties of this binder metal, mainly due to its cph lattice structure. Newly developed (Fe/Ni/ Co) alloys could be an alternative for some hardmetal applications where improved fatigue strength and toughness are required. These new binder alloys combine a mixed crystal structure of specially selected (Fe:Ni:Co) ratios with some advantageous physical
*
Corresponding author. Tel.: +49-05321-751217; fax: +49-05321751192.
properties that are very important for processing this powder in hardmetals or diamond tools. In particular, well-known problems such as porosity and inhomogeneous structures can be avoided if these powders are used instead of mechanical mixtures of conventional Fe-, Ni- and Co-powders. The shift towards ®ner and ®ner hardmetals has continued for more than 20 years. To produce the required submicron and ultra®ne, high-performance WC powder is a great challenge for the powder producer. To produce ®ner WC powder, manufacturers must operate at lower temperatures compared to the conventional technology. Lower temperatures can result, however, in higher impurity contents because some of the raw material trace elements are not volatilized signi®cantly [1]. In addition, the WC lattice is more disordered with a higher content of defects such as stacking faults, dislocations and vacancies if the temperature of WC synthesis is reduced for ®ner grain sizes. Using a new process, an ultra®ne WC can be synthesized with a substantially improved lattice structure than ultra®ne WC powder available in the market today. The perfect lattice structure and the high purity of the new 0.1 lm WC powder are two of the most important reasons for the excellent sintering behaviour of the powder, the homogeneous and ®ne grained structure of the
0263-4368/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 0 0 ) 0 0 0 0 2 - 0
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hardmetal and the high hardness, and coercivity of the ®nished product.
2. New alternative (Fe/Ni/Co) binder The properties of WC-hardmetals are determined by the carbides, as well as the binder metals and can be widely varied through WC-content, WC-grain size and alloying additions such as TiC and (Ta/Nb)C. The carbides are responsible for properties such as hardness and wear resistance, but need to be bonded by metals or alloys to provide toughness and strength in the composite material. More than 90% of all WC-hardmetals utilize Co as the preferred binder metal with contents between 3% and 30% by weight. The reasons for the dominant role of Co are some unique properties of Co and the Co±W±C ternary system. It is well known that the solubility of WC in Co is not only high but also strongly varies depending upon the temperature. This is closely connected to the excellent wetting between WC and molten Co, as well as to favourable properties of the Co±W±C binder metal. In addition, properties such as the high hardness, yield stress, toughness and strength of Co with its cph structure and allotropic cph/ fcc phase transformation are behind the dominating use of Co in hardmetals. Only in special hardmetal applications, requiring high corrosion or oxidation resistance, are binders other than Co used, e.g. (Ni,Cr)based alloys. The history of hardmetals is ®lled with attempts to substitute Co as the binding metal. These investigations
are motivated not only by the temporarily high and strongly ¯uctuating prices of Co-metal powder, but there are also serious technical reasons why researchers are trying to improve hardmetals properties using alternative binders. In the early days of hardmetal history, especially in the German hard metal industry during and after the Second World War some eorts were made to produce hardmetals with Fe-based binders. These hardmetals never entered the market successfully and disappeared quickly due to detrimental properties or diculties in producing stable qualities. In the late 1970s, Prakash made the ®rst, well-founded investigations on a broad range of (Fe/Ni/Co)-alloys and Table 1 Corrosion rate (MDD) in 1% organic acid solutions
Formic acid Acetic acid Maleic acid/CH3 OH Maleic acid/H2 O
WC±9.5% Co
WC±9.5% (Co/Ni/Fe)
245 178 297 188
109 125 125 125
Table 2 Oxidation resistance: weight increase [mg=cm2 ] at 700°C Annealing time (h) 10
20
30
WC±7.5 % (Ti, Ta, Nb) C±6% Co
90
150
175
WC±7.5 % (Ti, Ta, Nb) C±6% Co (Co/Ni/Fe)
37
75
100
Fig. 1. Particle size distribution by MALVERN MASTER SIZER 1002 and SEM micrographs of a prealloyed (60Co/20Ni/20Fe) powder.
G. Gille et al. / International Journal of Refractory Metals & Hard Materials 18 (2000) 87±102
showed that hardmetals with such Fe-rich binders have improved properties such as higher hardness, abrasive wear resistance, toughness and strength compared to Co bonded hardmetals [2,3]. Based on this fundamental research, some hardmetal grades for wood cutting tools and wear parts were developed and launched to the market in the late 1990s [4]. In contrast to the grades with Fe-rich binders having bcc-structures or structures near to the bcc/fcc-phase transformation area, Heinrich et al. developed Co-rich binders with fcc-structures and considerable improved corrosion and oxidation restistance (see Tables 1 and 2). These (Co/Ni/Fe) bonded hardmetals with an fcc structure also have improved toughness and fatigue strength [5,6]. The hardness at and above room tem-
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perature, as well as the Yonug's modulus, are slightly decreased compared to WC±Co hardmetals. Until now, one reason preventing a broader application of (Fe/Ni/Co) binders was the lack of a suitable powder. The hardmetal producer had to mix Co powders quali®ed for hardmetals with the conventionally available Fe- and Ni- carbonyl powder which are relatively coarse and dicult to mill. To overcome this new prealloyed (Fe/Ni/Co) powders were developed and some of their properties are shown in Fig. 1 and Table 3. The new (Fe/Ni/Co) powders have some typical properties: 1. They are loosely agglomerated with agglomerate dimensions of around 20 lm and primary particle sizes of 1 lm or below.
Table 3 Physical and chemical parameters of prealloyed (Co/Ni/Fe) powder
a
Composition of prealloyed powder
Co/Ni/Fe 60/20/20
Co/Ni/Fe 40/30/30
Co/Ni/Fe 15/15/70
Fe (%) Ni (%) Co (%) O (%) C (%) FSSS-value (lm)
19.32 20.27 60.32 0.525 0.002 1.73
30.22 29.88 39.70 0.640 0.002 0.85
71.30 14.95 11.15 1.980
Grain size distributiona d10 (lm) d50 (lm) d90 (lm) BET-value (m2 /g) XRD-analysis
5.96 14.9 28.9 1.98 fcc, bcc
2.68 6.85 15.2 2.16 fcc, bcc
14.8 25.0 39.0 1.49 fcc, bcc
2.44
MALVERN MASTER SIZER 1002.
Fig. 2. X-ray diraction pattern of a new mixed crystal (60Fe/20Ni/20Fe) powder compared with a mechanically mixed powder of the same composition.
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2. They are prealloyed or mixed crystals, which means they are mixtures of all metal constituents on an atomic level. The structure can be varied between bcc, fcc and hcp depending on the (Fe:Ni:Co) ratio. One example which demonstrates this is given in Fig. 2. In comparison to a mechanical mixture of Co, Ni- and Fe-carbonyl powder comprising the same (Fe:Ni:Co) ratio the new powder shows a pure fcc-structure with lattice constants shifted, according to the rule of mixture. 3. The (Fe/Ni/Co) powders can be produced in a wide range of compositions. The oxygen content is related to their high speci®c surface area relatively low, see Table 3. 4. The powders are highly sinteractive and can be used in both hardmetals (liquid phase sintering), as well as diamond tool applications (solid state sintering).
The sintering behaviour of the new mixed crystal (Fe/ Ni/Co) powder is shown in Figs. 3±5 for hardmetals with 9.5% and 20% binder and compared with that of mechanically mixed powder of same composition. The most important features of Figs. 3±5 may be summarized as: · Especially in the case of 20% binder, the high sinteractivity of the ®ne grained (60Co/20Ni/20Fe) powder is proven by a shrinkage rate starting at a lower temperature and spreading over a wider temperature range, compared to hardmetals with a mechanical mixture of Fe-, Ni- and Fe-powders, see Fig. 4. · According to the ®ner grain size (primary particle) and the higher speci®c surface area (BET-value) of the (Fe/Ni/Co) powder, the hardmetals bonded with this binder show higher weight losses than the hardmetals with the mechanically mixed powder. This is
Fig. 3. Shrinkage rate, DSC-signal, TG mass loss and mass spectroscopy of WC (0.5 lm) ± 9.5% (60Co/20Ni/20Fe) hardmetals. Comparison between mixed crystal and mechanically mixed (60Co/20Ni/20Fe) powder.
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91
Fig. 4. Shrinkage rate, DSC-signal, TG mass loss and mass spectroscopy of WC (2.5 lm) ± 20% (60Co/20Ni/20Fe) hardmetals. Comparison between mixed crystal and mechanically mixed (60Co/20Ni/20Fe) binder powder.
indicated by the thermal gravimetry, as well as by the mass spectrometry analysis of released gases, see Figs. 3±5. The water m 18 is completely removed at 450± 500°C. The adhesive oxygen or oxides on particle surfaces are forming CO2 and CO with carbon from WC. The removal of CO2 is completed at 720°C and that of CO at 1050°C which may be explained according to the Boudouard equilibrium between CO2 and CO. After reducing all oxides, especially those on the surfaces of WC particles, the shrinkage starts [7]. Based on the improved sintering behaviour, the porosity of vacuum sintered WC-hardmetals with 9% (75Fe/15Ni/10Co) mixed crystal binder is also considerably improved compared to that of hardmetals with mechanically mixed Fe-, Co- and Ni powder, see Fig. 6. While a hardmetal with a mechanically mixed, Fe-rich powder ends up with a A06±A08 porosity, the hard-
metal bonded with the prealloyed powder results in a hardmetal with A00±A02 porosity. The homogeneity of the structure can be improved too by using the new mixed crystal powder, see Fig. 7. In the example shown in Fig. 7, pressure-sintering was applied to the hardmetals. Porosity was
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Fig. 5. TG mass loss and mass spectroscopy of WC (0.5 lm) ± 9.5% (60Co/20Ni/20Fe) hardmetals. Comparison between mixed crystal and mechanically mixed (60Co/20Ni/20Fe) binder powder.
hardening. Therefore, one of the disadvantages of the Co-rich (Co/Ni/Fe) binder could be lower hardness at and above room temperatures, as given in Fig. 8. Because the fcc lattice structure causes lower hardness and yield stress, the use of mixed crystal, instead of mechanically mixed binder, can improve high temperature performance only slightly. The improved homogeneity of the structure (see Fig. 7), however, improves the toughness and fatigue properties as shown by the ®rst results with the mixed crystal binder. Besides the application as binder for hardmetals, the new mixed crystal powder can be used as a binder for diamond tools. In this case, solid state and not a liquid phase sintering is applied to bound the hard particles with the binder metal. By applying a solid state sintering technique below the melting point of the binder metals, the homogeneous distribution of all metal components is considerably reduced compared to a molten binder
process. Therefore, a ®ne grained and mixed crystal powder is especially advantageous of solid state sintering. Powder properties such as grain size, grain size distribution and lattice structure have a greater in¯uence on the structure of the sintered material compared to hardmetals densi®ed by liquid phase sintering. The grain size of the binding metal in the diamond tool is especially important because it correlates closely the hardness, toughness and cutting performance of the tool. According to the Hall±Petch relation, the hardness increases with the inverse square root of the grain size, however, a ®ner grained structure also must be highly dense too and nearly free of pores. Therefore, the starting powder should be ®ne grained, easy to mix with the diamond powder, homogeneous in composition and structure, but also sinteractive to densify at temperatures as low as possible to prevent considerable grain growth.
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Fig. 6. Porosity of vacuum sintered WC (0.8 lm)±9% Co and WC (0.8 lm) ± 9% (75Fe/15Ni/10Co) hardmetals.
Fig. 7. Structure micrographs (SEM) of pressure ± sintered WC (2.5 lm) ± 20% (60Co/20Ni/20Fe) hardmetals. Comparison between prealloyed and mechanically mixed binder powder.
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Fig. 8. Hot hardness of WC-hardmetals with Co and (Co/Ni/Fe) mixed crystal binders.
In Fig. 9 the hardness curves of hot pressed samples based on mechanically mixed and prealloyed (75Fe/ 15Ni/10Co) powder show the much higher hardnesses of the prealloyed powder. Although there is a slight decrease after the hardness maximum of HRB 118 at 580±620°C hot pressing temperatures, the hardnesses at higher temperatures are still around 10 units higher than that of mechanically mixed powder. This is mainly due to the ®ner grained structure of hot pressed samples with mixed crystal powder. Another requirement of the diamond tool producer is a high but stable hardness within a broad range of hot pressing
temperatures. This is ful®lled very well by the (Fe/Co/ Ni) mixed crystal powder, see Fig. 9. 3. Ultra®ne WC powder Since launching the ®rst submicron hardmetals in the late 1970s the tendency towards ®ner and ®ner hardmetals has continued [8±10]. The main interest in hardmetals with these ®ner grain sizes derives from the understanding that hardness and wear resistance increase with decreasing WC grain sizes. This under-
Fig. 9. Hardness and density of hot pressed samples based on mechanically mixed and prealloyed (75Fe/15Ni/10Co) powder at varying on the hot pressing temperatures.
G. Gille et al. / International Journal of Refractory Metals & Hard Materials 18 (2000) 87±102
standing, together with advances in the performance of machine tools and wear parts, has resulted in considerable improvements, especially in cutting tools used in advanced applications such as microdrills for drilling holes in PCB's, endmills or drills for cutting cast iron, steel castings or hardened steels and wood cutting tools. To meet the requirements of the hardmetal industry and to follow trend towards ®ner grain sized tools, a new WC powder was developed using a completely new process. Powder characteristics and the resulting hardmetal properties are shown in Table 4 and Figs. 10, 11. By considering powder properties such as FSSSvalue dFSSS or particle size distribution alone, some misunderstandings may be caused because these properties are in¯uenced by the measuring method itself. The best and most objective way to characterize the powder is to measure the hardmetal properties, such as hardness and coercivity, as well as the hardmetal structure [11]. Fineness and particle size distribution of the structure, as measured by the linear intercept or a point counting method [12], are most closely connected to hardness and other properties of the sintered hardmetal, as documented later in this paper (see Section 4). Application-related properties, such as hardness and Palmqivst toughness Kc , give a realistic evaluation of the powder. The comparison between the new WC 0.1 L and the ®nest, commercially available WC grades DS 40 and DS 60 is made mainly on the basis of the hardmetal structure and properties, see Figs. 10, 11 and Table 4. The structure analysis, which was made with a SEM high-resolution electron microscope and a special etching technique, was dicult due to the extreme ®neness. For the ®rst time, an arithmetic mean linear intercept of 0.1 lm 100 nm was measured for a WC±10 wt% Co structure. Most people consider 100 nm as the border
95
between the nano- and microstructured world. Connected with this ®ne grained structure, the hardness HV30 of WC 0.1 L±10 wt% Co hardmetals surpasses 2000. This high hardness was achieved with a VC/Cr3 C2 dopant content of only 1 wt% in total. Making a ®ne grained WC powder is only the ®rst step to end up successfully with a ®ne grained hardmetal structure. In addition to mixing and milling, sintering is a second essential step in transforming powder properties into hardmetals properties. Obviously, there are some interrelations between powder properties and the sintering behaviour which become more pronounced as the powder becomes ®ner. In particular, the densi®cation which is obtained in the stadium of solid state-sintering increases and is intensi®ed considerably with decreasing WC grain size and increasing interfaces between WC particles and Co binder [7,13]. Fig. 12 shows some of the most important characteristics and peculiarities of sintering a WC 0.1 L±10 wt% Co hardmetal. Compared with the WC DS 60 hardmetal, the WC 0.1 L hardmetal with the same VC, Cr3 C2 doping shows the following characteristics: · Shrinkage starts at a lower temperature and the maximum of shrinkage rate is shifted from 1260°C for the WC DS 60±10 wt% Co hardmetal to 1200° for the WC 0.1 L±10 wt% Co hardmetal. · The shrinkage rate curve is wider and more ¯at than that of the WC DS 60 hardmetal. This corresponds to a general tendency of shifting the shrinkage to lower temperatures with decreasing WC grain sizes [7]. · The mass loss is higher for the WC 0.1 L hardmetal due to the higher content of oxygen and oxides which have to be released as CO2 and CO. The removal of these oxides as gases is, however, completely ®nished before the high rate shrinkage starts and the binder melts.
Table 4 Properties of WC DS 60, WC DS 40 and WC 0.1 L powder (above)a and hardmetals based on WC 0.1 L and WC DS 40 powder doped with VC and Cr3 C2 (below)
a
Powder
d10 b
d50 b
d90 b
Dd c
dFSSS
d50 d
WC DS 60 WC DS 40 WC 0.1 L
0.34 0.23 0.14
0.57 0.44 0.26
1.36 1.02 0.56
1.02 0.79 0.42
0.55 0.48 0.33
0.22 0.17 0.10
Hardmetal composition
q g=cm3
Hc [kA/m]
4 prs lTm3 =kg
HV30 at 20°C
Kc [N/mm]
WC WC WC WC WC
14.83 14.46 14.43 14.30 13.95
53.3 45.4 35.8 41.8 35.8
10.2 17.2 19.0 21.9 27.7
2280 2043 1870 1910 1700
370 530 810 570 725
0.1 L±6 wt% Co 0.1 L±10 wt% Co DS 40±10 wt% Co 0.1 L±12 wt% Co 0.1 L±15 wt% Co
All values in lm. SEDIGRAPH 5000 D. c Dd d90 ÿ d10 . d d50 50% value of intercept distribution. b
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Fig. 10. Properties of WC 0.1 L, WC DS 40 and WC DS 60 powder and structures of hardmetals using these WC powders.
An answer to the relationship between sintering behaviour and the homogenous and very ®ne grained structure of WC 0.1 L hardmetals is given in Fig. 13. For all lots of WC 0.1 L powder type the combination of crystallite size and lattice distortion, both measured
by X-ray diraction, is within the marked area. The data show that the combination of low crystallite size and low lattice distortion is typical for this new WC powder type. Investigations of commercially available, ultra®ne grained WC powder show that the combinations of
G. Gille et al. / International Journal of Refractory Metals & Hard Materials 18 (2000) 87±102
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Fig. 11. Relationship between coercivity and magnetic saturation for WC 0.1 L±10 wt% Co and WC DS 40±10 wt% Co hardmetals.
Fig. 12. Shrinkage rate, DSC-signal, TG mass loss and TG mass loss rate for WC 0.1 L±10 wt% Co and WC DS 60±10 wt% Co hardmetals.
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Fig. 13. Crystallite size and lattice distortion of WC 0.1 L and conventional ultra®ne WC powder. Measurements made by X-ray diraction.
crystallite size and lattice distortion are found outside the WC 0.1 L powder type area. This means that for a given crystallite size or lattice distortion, the combined distortion or crystallite size is greater for commercially available WC powder than for a WC 0.1 L powder type. The lattice distortion measured by X-ray diraction is a measure of stacking fault and dislocation densities within the cph lattice of WC. The higher these densities, the higher the lattice distortions. A highly distorted lattice however, shows a higher solution pressure and therefore the rate of solution and reprecipitation of such WC grains is much higher during the liquid phase-sintering than that of more perfect WC grains. One of the important peculiarities of the WC 0.1 L powder is their low lattice distortion which explains some of the favourable features such as stable sintering behaviour, and structure ®neness and homogeneity without detrimental qualities such as abnormal or selective grain growth.
4. Structure±hardness relationship Powder manufacturers and the hardmetal industry alike are interested in ®nding straightforward relationships between powder and hardmetal properties, however, there are many steps between the initial powder and the ®nished hardmetal, such as milling, pressing and sintering. They clearly overlap with each other in their eects on the resulting hardmetal properties, but there is no doubt that the properties of the powder have a sig-
ni®cant in¯uence on hardmetal properties. The structure of the sintered hardmetal is a suitable bridge between the powder properties on the one side and the mechanical properties of the hardmetal on the other side of the river to be crossed. Hardness is a good example of how this can work because it is one of the most important, application-related properties, and it is also as an easy to measure and interpret property. As shown in Fig. 14, the hardness of 120 hardmetals with dierent Co-contents and WC grain sizes can be very well correlated with the mean linear intercepts dWC of these hardmetals [14]. The structure was analyzed by means of a modern high resolution SEM with a ®eld emission electron beam source [12]. In addition to the results in [7,13,14], Fig. 14 also comprises the newest results on the WC 0.1 L hardmetals. The dependencies of hardness on the mean linear intercept dWC can be well ®tted by power law functions with high correlation coecients varying between 0.980 and 0.988. Otherwise it is clearly to see in Fig. 14 that the hardness dependence on dWC does not ®t a Hall-Petch relation. A theoretical model helps to ®nd relationship between hardness HV30 and the two most important structure parameters, the mean intercept dWC and the volume fraction of Co VCo HV30 f VCo dWC
ÿ1=m
2 1824 1 ÿ 1:65VCo 0:92VCo dWC
ÿ0:194
:
According to this model one straight line correlates hardness HV30 , mean linear intercept dWC and Co volume fraction VCo of 120 hardmetals within a wide spread
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Fig. 14. Hardness HV30 depending on WC arithmetic mean linear intercept dWC (above) and inverse square root of dWC (below).
range of parameter values (dWC 0.10±7.80 lm, VCo 0.10±0.38) with a correlation factor ÿK 2 0:9872, see Fig. 15. Based on data available at that time Merz et al. [15] established an empirical relationship between hardness, dWc and the mean free path in Co phase pCo dWc VCo = 1 ÿ VCo )) 2 =dWC HV30 877 pCo
ÿ1=5
877
1 ÿ VCo VCo
0:4 dWC
ÿ0:2
:
Because of in®nite hardness for vanishing Co-contents VCo , this relation could not be true for binderless hardmetals or hardmetals with very low binder contents. However, the relation ®ts the hardness values very well
for normal Co-contents in whole range of WC grain sizes. Disregarding the discrepancy at very low volume fraction of Co, this empirical relation also corresponds ÿ1=m well with the above relation HV30 f VCo dWC . Conspicuously, both relations have nearly the same exponent for the dWC dependency of HV30 : 1/m 0.194, which is nearly 0.20. Finally, a relationship between hardness and grain size of the starting WC powder dFSSS can be found if a relation between the mean linear intercept dWC and dFSSS is known. Such a relation is shown in Fig. 16 for WC±10 wt % Co hardmetals. Combining the relationship between HV30 and dWC and that between dWC and dFSSS for WC±10 wt % Co hardmetals ®nally results in
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Fig. 15. Hardness HV30 of 120 hardmetals as a function of dWC (above) and VCo with m 5.155.
Fig. 16. Relationship between arithmetic mean linear intercept of structure dWC and powder grain size dFSSS for 10 WC±10 wt% Co hardmetals.
HV30 1603 dFSSS
ÿ0:1568
which ®ts very well the experimental values as given in [13]. In all the three relationships dWC and dFSSS are assumed in lm. 5. Concluding remarks Sometimes the question arises how far the tendency to ®ner and ®ner WC grains could or should go. The answers are very dierent and depend on the sector of
hardmetal industry the answer is coming from. The presented investigations have shown the possibility to synthesize a real 0.1 lm powder where 0.1 lm is the arithmetic mean linear intercept of hardmetal structure. The dependence of hardness on Co Volume fraction VCo (see Table 4) may be extrapolated to a hardness of 2.700 HV30 for a binderless WC skeleton. These values correspond very well with extrapolations of hardness ± WC grain size dependencies as given in Fig. 17 for binderless WC-hardmetals (extrapolation for disappearing Co content). In Fig. 17 are also shown the new areas of hardness which may be realized for hardmetals
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Fig. 17. Hardness of WC±Co hardmetals versus WC grain size and Co content (above). Hardness of binderless WC-hardmetals in dependence on WC powder grain size (below).
by applying the new 0.1 lm WC powder with dierent Co-binder contents. In the lower part of Fig. 17 it is indicated that an alternative technology is necessary to produce the new 0.1 lm WC powder grade. But the question arises, can the extreme hardness be used in a tool or wear part because of low toughness. The answer can only be given by the hardmetal industry itself. Also for the new (Fe/Ni/Co) mixed crystal powder, only a close cooperation with the hardmetal or diamond tool industry can give a well founded and concluding evaluation. As a powder producer H.C. Starck is ready to hear from the tool industry the promising (Fe:Ni:Co) compositions they selected for their special application.
Based on that information, alternative powder can be produced and applied with a broad range of compositions, as well as advantageous physical and chemical properties for each special case of application. Acknowledgements The authors would like to thank Dr. Leitner from IKTS/FhG (Dresden) for conducting the thermoanalytical measurements and Dr. Roebuck from NPL (Teddinghon) for the structure analysis of the extremely ®ne grained WC 0.1 L±Co hardmetals.
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References [1] Lassner E, Schubert WD. Tungsten. New York: Kluwer Academic Publishers/Plenum Press; 1999. [2] Prakash L. Doctoral Thesis, University Karlsruhe, 1979. [3] Prakash L. In: Prococeedings of 12th International Plansee Seminar. Reutte, vol. 2, 1993. pp. 80±109. [4] DE 29617040 U1. Gebrauchsmuster: WC-Hartlegierung. [5] WO 99/10549. PCT Patent: A cermet having a binder with improved plasticity, a method for the manufacture and use thereof. [6] WO 99/10551. PCT Patent: A pick-style tool with a cermet insert having a Co±Ni±Fe-binder. [7] Gille G, Leitner G, Roebuck B. In: Proceedings of the European Conference on Advances in Hard Materials Production, Stockholm, 1996. pp. 195±210. [8] Egami A, Kusaka M, Machida M. In: Proceedings of 12th Internnational Plansee Seminar, Reutte, vol. 2, 1989. pp. 53±70.
[9] Kassel D, Schaaf G, Dreyer K. In: Proceedings of the 12th International Tungsten Symposium, Goslar, 1996. pp. 24±37. [10] Daub HW, Dreyer K, Happe A, Holzhauer H, Kassel D. Pulvermetallurgie in Wissenschaft und Praxis Bd. 11, Oberursel, 1995. pp. 285±306. [11] Conner C, Carroll D. et al. In: Proceedings of the 12th International Tungsten Symposium, Goslar, 1996. pp. 171±80. [12] Roebuck B. Int J Refractory Metals & Hard Materials 1995;13:265. [13] Gille G, Szesny B, Leitner G. In: Proceedings of the 14th International Plansee Seminar, Reutte, vol. 2, 1997. pp. 139±67. [14] Gille G, Schwier G, Szesny B. Pulvermetallurgie in Wissenschaft und Praxis Bd. 14, Oberursel, 1998. pp. 194±227. [15] Merz A, Pompe W, Gille G. In: Proceedings of the Fourth European Powder Metallurgy Conference, Grenoble, 1975. pp. 581±87.