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General aspects and limits of conventional ultrafine WC powder manufacture and hard metal production
Int. • (~[ReJmctoo: Metals & Hard Materials 13 (1995) 281 296 Elsevier Science Limited Printed in Great Britain 0263-4368/95/$9.50
ELSEVIER
General Aspects and Limits of Conventional Ultrafine WC Powder Manufacture and Hard Metal Production W. D. Schubert, A. Bock & B. Lux Institute for Chemical Technology of Inorganic Materials, Technical University Vienna, A-1060 Vienna, Austria
Abstract: Ultrafine WC/Co hard metals (average WC grain sizes -<0"5/~m) can
be successfully and reliably obtained by conventional hard metal manufacturing techniques, in this paper, some of the crucial aspects of conventional powder manufacture, powder milling and liquid phase sintering are discussed. Conventional ultrafine WC powder manufacture is based on the production of tungsten powder by hydrogen reduction of tungsten oxides and subsequent carburization. Alternatively, direct carburization can be carried out. However, inherent to the powder processing techniques used and the particle growth mechanisms involved (oxide precursors used, reduction and carburization history), there exists a lower limit beyond which-finer WC powders cannot be produced. This limit lies in the particle size range of 50-150 nm (0-05-0.15
/~m).
Powder milling is carried out to obtain an even dispersion of the Co binder in the ultrafine WC matrix. The more uniform the phase distribution (WC, Co, grain growth inhibitor) within the green powder compact, the more uniform will be the material transport during sintering, and hence the uniformity of the WC grain growth/growth inhibition during sintering. Enhanced WC grain growth occurs early in the sintering cycle, even below the temperature at which the liquid phase is formed. This growth can be largely restricted by the addition of VC. However, effective grain growth inhibition has to take place already during this early period of solid-state sintering. The 'early' availability of the grain growth inhibitor at the WC/Co interface can, therefore, determine the degree of growth inhibition. Ultrafine hard metals are in particular prone to discontinuous grain growth of the WC. Different reasons for this local growth mode are propounded relating to both the chemical as well as the geometrical departures from uniformity in the green powder compact. While it is still not possible to predict exactly an ultimate WC grain size limit, below which WC grain growth can no longer be restricted, even with proper inhibitor additions, experimental evidence indicates that this average WC grain size limit lies in the range of 200-300 nm. This limit is inherent to the existing conventional processing techniques (powder manufacture, milling, liquid phase sintering) and the WC growth mechanisms involved and can be overcome only by establishing a completely new route in hard metal manufacture.
1
While submicron t WC/Co grades still predominate today's fine-grained tool market, there
INTRODUCTION
Over the years, there has been a discernible tendency towards using finer and finer grained materials in the hard metal industry to exploit the advantages of their high toughness, strength and rigidity coupled with high hardness and wear resistance.
+In this paper two terms are used in describing the fineness of WC powder raw materials and the WC grain size in the sintered alloy: the term 'submicron' is used for powder grades (alloys) with an average particle (grain) size in the range 0-5-1/~m. The term 'ultrafine' describes grades with an average particle (grain) size <0-5 ~m (500 nm). 281
W. D. Schubert, A. Bock, B. Lux
282
is increasing interest in using even finer grades for certain specific applications. ~,2 The advantages of using such ultrafine* hard metals over submicron grades is illustrated in Fig. 1, which gives a comparison between these grades for a WC/10 %wt Co system. A pronounced increase in hardness from 1780 Hv 3 to 1925 Hv 3 is evident at a comparably high bending strength (3800 MPa). In fact, hardness values up to 2200 Hv30 (6 %wt Co) and bending strength values of up to 5000 MPa (15 %wt Co) have been successfully achieved. (Lassner, E. & Zeiler, B., 1993, pers. comm.) ~ However, these ultrafine grades are extremely sensitive to processing conditions and are even more prone to carbide grain growth during consolidation. Further, due to the high reactivity of such ultrafine carbide powders during the liquid phase sintering, both powder raw material quality (in terms of homogeneity, purity and chemical stability) and the addition of appropriate grain growth inhibitors are crucial factors in achieving the required superior quality and reliability. In this paper, the problems involved in the manufacture of such highly sensitive ultrafine alloys are discussed, with special emphasis on: • Present day concepts in industrial ultrafine hard metal manufacture
Submicron WC/10 wt% Co hardmetal
• Conventional ultrafine WC powder production • Aspects involved in ultrafine WC powder quality assurance • Ultrafine WC-Co alloy sintering (densification, grain growth) 2 PRESENT DAY CONCEPTS IN INDUSTRIAL ULTRAFINE HARD METAL MANUFACTURE
There are basically two approaches for obtaining ultrafine hard metal structures by the conventional hard metal manufacturing route. Both are designed to obtain an evenly dispersed ultrafine WC/Co powder mix, which is subsequently granulated, pressed or extruded, and then liquid-phase sintered. • The first method is based on the use of an ultrafine WC powder grade (average WC particle size <-0.5 ~ m). This powder is processed in the same or similar way as other fine grained carbide powders, that is, the powder is ball milled or attritor milled together with a standard fine cobalt grade to optimize the blending of the carbide with the binder.
Ultrafine WC/10 wt% Co hardmetai
fracture surface (10 000 X)
Fig. 1.
1 ~m
Hardness
Hv 3
= 1780 kg/mm 2
Hv 3
= 1925 kg/mm 2
TRS Coercivity Magn. sat.
050 iHc 4~
= 3780 MPa = 24.9 kA/m = 18.2 }JTm3/kg
~5o iHc 4rc~
= 3890 MPa = 35.1 kA/m = 18.7 laTm3/kg
Fracture surfaces of submicron and ultrafine WC/10 %wt Co hard metal (refer to WC powders used, see Figs 6(a) and 6(b)).
Ultrafine WC powder manufacture and hard metal production Although powder milling is commonly regarded as a comminution operation it is of minor consequence in the powder milling of such extremely fine powders. Any particular mill and mill charge combination have a limiting grain size, beyond which no further comminution will occur) This is commonly in the range of 0.5-1/~m for the carbide, so no comminution will occur in the case of even finer particles. The main reason for this behaviour is that, with decreasing particle size, enhanced particle agglomeration will occur. Such particle agglomerates will then be separated into smaller units during powder milling, but will subsequently reform, generating heat. • The second method is based on a standard sized WC powder grade (e.g. 1.5 lure), which is heavily milled under such conditions that a further comminution will occur. This concept demands a fundamental 'know how' of the powder milling process in order to prevent the occurrence of pronounced local grain growth due to a nonuniform size distribution of the WC particles. There is, however, an absolute limit in comminution, which depends on the physical properties of the material. This limit seems to be in the 50-150 nm particle size range. The addition of so-called grain growth inhibitors (VC, Cr3C2,TaC; see Section 5) is an important prerequisite of ultrafine hard metal sintering to retain the ultrafine carbide powder structure in the consolidated material. It is the 'state of the art' today to add at least some of the inhibitor necessary for growth restriction during powder manufacture ('powder engineering'). This inhibitor addition can be done as early as during tungsten metal powder formation, or it can be carried out prior to metal powder carburization. Both of these methods have their weak points and technical limits. To obtain high-quality proagglomerate
particle ~i'nterparticle forces[ Iintergranular forces~j . . . .
grain
Fig. 2. Build up of tungsten metal agglomerates as obtained by hydrogen reduction of tungsten oxides (schematic).
283
ducts in a reliable manner, it is essential for the producer, with the help of an in-depth understanding of all processes involved in the manufacture, to search out these weak points in order to reduce or even eliminate their detrimental effects on the final product.
3
CONVENTIONAL ULTRAFINE WC POWDER PRODUCTION
Ultrafine WC powder manufacture is based on the production of tungsten powder by hydrogen reduction of tungsten oxides and subsequent carburization. Alternatively, direct carburization of tungsten oxides can be carried out. 4,5 The starting materials in both cases are either tungstic acid (H2WO4) or tungsten blue and yellow oxides originating from APT (ammonium paratungstate). The aim in both ultrafine tungsten metal and tungsten carbide powder manufacture is the production of a pure, uniform, fine and loosely agglomerated powder that can subsequently be easily dispersed during (W/C) powder mixing, WC powder deagglomeration and (WC/Co) powder milling. However, depending on exactly how they were manufactured (oxide raw materials used, reduction and carburization history) such powders can significantly vary with respect to their physical consitution.
3.1
Tungsten metal production
A typical peculiarity of ultrafine tungsten powders obtained by hydrogen reduction of tungsten oxides is their pseudomorphic appearance in the oxide raw material. These sponge-like metal agglomerates (pseudomorphs) form due to the extremely dry reduction conditions prevalent during the manufacture of ultrafine tungsten powders. They are made up of extremely fine metal grains (20-50 nm) held together by strong intergranular and interparticular forces (chemical and physical bondings), as shown schematically in Fig. 2. The physical constitution of the agglomerates (both on a macro- and microscopic scale) can be loose or dense and can be strongly influenced by the reduction conditions as well as by the chemical and physical nature of the oxide precursor used. 6 (Fig. 3.) The ease with which these agglomerates can be broken down during subsequent deagglomeration
284
W. D. Schubert, A. Bock, B. Lux
(a)
Co) I0 pm
Fig. 3.
Tungsten metal powder agglomerated in APT pseudomorphs (SEM 2000 x ), obtained from: (a) APT decomposed at 300°C (loose and dense agglomerates); (b) APT decomposed at 700°C (only loose agglomerates).
(a)
(b) 10 ~m
Fig. 4.
Laboratory milled tungsten powders, obtained from the 'as supplied' powders shown in Fig. 3. (a) Some agglomerates remain; (b) agglomerates completely disintegrated.
can be significantly different. As an example, Fig. 4 shows the differences in agglomerate breakdown during powder milling (ASTM No. B430 (1970) deagglomeration treatment) of the two tungsten metal powders, as seen earlier in Fig. 3. It is apparent that in one case some rather coarse agglomerates have survived the deagglomeration treatment, while in the other case the powder was completely disintegrated. It must, however, be kept in mind that during disintegration a certain degree of agglomeration (a
certain particle or agglomerate size) will always remain, due to the inherent chemical and physical forces acting between the individual particles and grains (Fig. 2). The more uniform the disintegration during deagglomeration, the more uniform will be the particle dispersion during subsequent blending with carbon black. The lower the degree of metal-metal contacts during carburization, the lesser will be the particle coarsening. It is thus evident that both reduction conditions and the use of appropriate oxide raw materials
Ultrafine WCpowder manufacture and hard metal production play a significant role in decisively influencing the fineness and uniformity of the carbide finally obtained.
4
4.1 3.2 Particle coarsening during carburization and limits in carbide particle size Carburization of ultrafine tungsten powders is carried out at rather low carburization temperatures (1200-1450°C) in order to restrict severe particle coarsening during the W~ WC transition. However, even then, a certain coarsening of the particles occurs as evidenced by a strong decrease in the specific surface areas of the powders, as shown in Fig. 5. In general, particle coarsening of such extremely fine metal structures during carburization and the mechanism involved still remain to be worked out systematically. It has, however, been observed that the addition of grain growth inhibitors (Cr3C2), a common practice in ultrafine carbide manufacture, can have an extraordinary influence on the micromorphological evolution of the final carbide particles obtained by promoting the occurrence of individual fine WC particles instead of coarse polycrystalline particle agglomerates. 7 Practical carburization experience indicates that there is a lower particle size range below which finer WC powders cannot be produced. This is true for both precursors used (HzWO4, APT) and, according to SEM (scanning electron microscope) imaging investigations, lies in the range 5 0-15 0 nm. This limit is inherent to the existing conventional processing technique itself and can be overcome only by establishing a completely new route in carbide production.
5
Limit inherent to conventional processing techniques i~1111
~
i ii
3 •
•
ca
0
I
0
2 4 6 10 12 14 BET-specific surface area of the W powder [m2/g]
16
Fig. 5. Changes in the BET specific surface area during carburization (W~WC) of different industrial powder grades. A maximum of about 4.5 m2/g is obtained, which is an inherent limit for the conventional powder processing.
285
ASPECTS INVOLVED IN ULTRAFINE WC POWDER QUALITY ASSURANCE Powder purity
High purity of the raw materials and the avoidance of any contamination of the powders during subsequent processing is mandatory for the reliable manufacture of defect-free high-strength WC/Co alloys. In particular, heterogeneous iron and nickel contaminations must be strictly avoided during WC powder manufacture, as they can lead to local melt formation and enhanced local grain growth during carburization even at the relatively low temperature of 1150°C. These contaminations can originate from furnace and boat materials. Powders must be low in sulfur and calcium to avoid CaS precipitation during sintering, which can severely lower the strength of the sintered alloys. ~ As seen from earlier studies, while these elements orginate mainly from carbon blacks, they could also stem from 'technically' pure tungsten and cobalt powder grades, s- ~0 4.2 Particle size and particle size distribution of ultrafine WC powder grades The control of particle size in the manufacture of hard metals is an important part of the quality assurance system. The hard metal producer, therefore, has based his system on the measurement of certain chemical and physical powder properties, which must be achieved in order to obtain reliable alloy properties. This system is easy to handle and works quite well in the case of standard alloys, but has significant limitations for ultrafine powders. A simple comparison between a submicron and an ultrafine WC powder grade reveals the scope of this problem. The submicron WC powder (Fig. 6(a)) consists of small individual particles which are loosely agglomerated. These agglomerates break down easily during the physical measurements as normally applied for control purposes (Fisher sub-sieve size (FSSS), laser scattering). Both FSSS particle size (0"92 ktm) and the mean particle size obtained by laser scattering (0.91 ktm) are in sufficient agreement with the only slightly lower mean size determined by SEM imaging analysis (0"8 ktm) (Fig. 7(a)).
W. D. Schubert, A. Bock, B. Lux
286
(a) submicron WC
(b) ultrafine WC 1 pm
Fig. 6.
Submicron (a) and ultrafine WC powder grade (b); both grades are characterized in Fig. 7 with regard to their mean particle size/size distribution.
~
20
, , (a) subTn]croll
l~ BET-grain size (0.5 ~m)
FSSs'graill
size (0.92 p.m
~ ~ -
SEM-grain size (0.8 ~ml
or. 10
0
jll :o
/
|~1~ Laser scattelmg
J i5
o
/
.--I-'7
0,01
"
!|1
0,i
'~.--~
1
. i0
WC grain size [g.rn] SF_,M-grain size (0.i.5 ~m)
3O
(b) u l t r a R n e
~,
'/
-
~,. O N
F~ 3S-grain size (0.3 ~m)
? 20
BET-gr in size (0.0-~~rn)
It //\
L
zo
.~
~.,[~
o
0.01
PhotoJ correlation s ectro~)py (0.25~m)
1 ~
Laser ecatterin ' (d50--0.63 gm)
~
0.1
I
10
WC grain size [~m] Fig. 7.
WC mean particle size and particle size distribution measurements obtained by laser scattering. FSSS, BET-surface area measurement and SEM image analysis on the submicron and ultrafine powders shown in Fig. 6.
Ultrafi'ne WC powder manufacture and hard metal production
287
The ultrafine WC powder consists of strongly agglomerated particles (Fig. 6(b)). Here, the FSSS size (0.25 /~m) and the mean particle size as obtained by laser scanning (0"63/~m) are in strong disagreement with the average size value as determined by SEM imaging (0.15/~m) (Fig. 7(b)). It is thus evident that this strong agglomeration tendency, which is inherent to the ultrafines, makes the measurement of classical particle size, and in particular, particle size distribution, of rather limited value for control purposes. 4.3 BET (Brunauer-Emmet-Teller) specific surface area measurements Most of the more recent papers on fine grained tungsten carbide powders include the specific surface area measurements in the characterization of the materials. Although this technique can give the closest agreement with image analysis] ~ a direct interpretation in terms of a calculated particle size (6/(BET-surface x density)) can result in misleadingly low size values in the case of spongelike ultrafine powders. 4.4
Particle irregularities
Coarse WC grains, present in an ultrafine carbide matrix, can readily act as seed crystals for local grain coarsening during subsequent sintering. As could be shown in a recent paper] 2 most of the ultrafine WC powder grades exhibit small amounts of coarse grains and/or rather dense looking agglomerates. These irregularities are often difficult to detect even through careful SEM imaging of the powders, due to the high overall agglomeration of the fine particles. In addition, all the usual physical measurement techniques fail to detect any inhomogeneity at all. It has, however, been established that a good characterization of even small amounts of these irregularities can be obtained through the SEM imaging of the residue left after selective dissolution of the fine grains. 13 Figure 8 shows such a coarse particle detected in an ultrafine WC powder grade which was prone to local grain growth during subsequent sintering. 4.5 Physical and chemical constitution of grain growth inhibitors during powder production Although the proper addition of grain growth inhibitors plays a key role in ultrafine WC powder
Fig. 8. SEM imageof the residualWC after selectivedissolutionof the finegrains.
and WC/Co alloy manufacture (see below), very little is actually understood about the physical and chemical constitution of the inhibitor during manufacture, in particular during WC powder production. Other than the gross chemical analysis of the powders (e.g. 5000 ppm of V, Ctot and Cfree), no further information is available on the chemical and physical states of these compounds. In view of the above limitations, it can be seen that the usual quality assurance controls used for standard powder grades are inadequate and often give misleading results when used for controlling the quality of such ultrafine grades. Apparently, the best solution then is to 'build in' the powder quality at different processing stages, rather than merely measure it at the final stage. 5 5.1
SINTERING OF ULTRAFINE HARD METALS The goal
The most crucial aspect of the sintering of ultrafine grades is retaining the ultrafine microstructure of the WC in the consolidated (dense) material. Only if this aim is both successfully and reliably achieved can a pronounced technical advantage (higher hardness, better wear resistance, high strength) be realized over the submicron alloys, which are slightly coarser, but less growth sensitive during the manufacturing process. In the following, the important aspects of sintering with reference to ultrafine WC powders will be discussed.
W. D. Schubert, A. Bock, B. Lux
288
5.2
In addition, it must be kept in mind that, during sintering of hard metals, chemical reactions take place which can bring about diffusion gradients and can interact with both processes involved during sintering, unless chemical equilibrium has been reached (dissolution of the carbide in the cobalt binder, possible formation of eta-phase and subsequent transformation, carbon adjustment). 14 The earliest stage at which we can expect this to take place is during the isothermal liquid phase sintering (see Fig. 9).
Driving force
The primary driving force for sintering of fine grained hard metals, as for any sintering system, is a reduction in the interface energy of the system. This is accomplished by reducing the area of the surfaces and interfaces of the compact, which happens mainly by a combination of two concurrent processes, namely: • Densification (replacement of the gas/solid interface by a lower-energy solid-solid or solid-liquid interface). • WC grain growth (decrease of the solid-solid or solid-liquid interface area and formation of low-energy prismatic interfaces).
5.3 Densification of ultrafine hard metals and early WC grain growth during densification Even though conventional hard metal sintering is based on liquid phase sintering, pronounced densification occurs even during solid state sintering by solid state diffusion, transport of particles and plastic flow of carbide binder areas.'5-~ 7 In the case of ultrafine alloys, up to 90% of the densification occurs during solid state sintering (below 1280°C~). Solid state densification increases with decreasing particle size (submicron ultrafine) (Fig. 10) as it is also enhanced by high gross carbon contents in the alloys.J5.19 It can be seen upon interrupting the sintering cycle at 1100 and 1200°C that, concurrent with this densification, pronounced grain growth also occurs (Fig. 11 ). This growth is accompanied by a significant change in the carbide grain shape from non-facetted to prismatic, thus indicating the formation of low-energy prismatic interfaces.
The latter process is of particular importance during the sintering of ultrafine alloys. temperature
1400"C
~ " ~i' non-isothermal 1/ i s t ast ien t e r ~ i~i
1100'C
800"C
Fig. 9.
J
liquid
isothermal phase sintermg
f ~
~
Formation o liquid phase 1280-1310"C ~ n I 1 2 3 sintering time [hours]
0
Schematic illustration of a technical hard metal sintering cycle.
solid state densification
/'
100 8O 60
i
f
"formation of eutectic liquid
40
ultrafine grade 0.3 pm WC
20
submicron grade 0.7 lam WC
1000
1100
1200
1300
1400
temperature [°C] Fig. 10.
Densification of WC/10 %wt Co hard metals based on ultrafine and submicron WC (no grain growth inhibitor added; heating rate: 3°C/min).
Ultrafine WC powder manufacture and hard metal production
ll00°C
289
1200oc 1 lain
Fig. 11.
Fracture surfaces of ultrafine WC/10 %wt Co hard metal samples (no grain growth inhibitor added), sintered for 1 rain at 1100°C (left) and 1200°C (right).
"I )
II00°C
1200oc 1 Cm
Fig. 12.
Fracture surfaces of ultrafine WC/10 %wt Co hard metal samples (0.65 %wt VC added in the WC powder as grain growth inhibitor), sintered for I min at 1100°C (left) and 1200°C (right).
Closer WC particle packing by solid state solution-reprecipitation and a decrease of the frictional forces which enhance particle gliding 17have apparently promoted the particle rearrangement process. This particle rearrangement process is a prime factor in the extensive shrinkage of compacts occurring during this early period of sintering. 15 This early growth, however, can largely be restricted by a proper addition of VC (0.65 %wt) as grain growth inhibitor (Fig. 12). The decrease in the growth rate of the carbide is accompanied by a
lower densification rate, thus indicating that the mechanisms of both processes, densification and grain growth, are closely interlinked, although the driving forces for the processes have different origins. It can, therefore, be seen that a peculiarity of highly reactive fine and ultrafine hard metal structures is this early grain growth, which can occur concurrently with the densification of the alloy. It constitutes the starting point for further carbide coarsening during subsequent liquid phase sintering. The avoidance of such growth during this
290
W. D. Schubert, A. Bock, B. Lux
early period plays a crucial role in the successful manufacture of ultrafine structures. 5.4 Grain growth and grain growth inhibition in ultrafine hard metals
In practice, one distinguishes between 'continuous' (overall or normal) grain growth and 'discontinuous' (local or abnormal) grain growth. During continuous grain growth, the sizes of the individual grains are relatively uniform. During discontinuous grain growth, on the other hand, the differences in individual sizes increase due to the rapid growth of some of these grains.
5.4.1 Continuous grain growth Standard sized hard metals. The theory of continuous grain growth is based on the interfacial free energy being the driving force.18 Grain growth during liquid phase sintering has been phenomenologically treated as an Ostwald ripening process, m Smaller particles dissolve due to their higher dissolution potential (increased chemical potential), while coarser ones grow by material reprecipitation, thereby reducing the interface area of the system. Continuous grain growth in standard sized hard metals has been observed to proceed rather slowly as compared to other carbide/binder systems (VC/Co, NbC/Co, TaC/Co). This has been attributed to the extremely low WC/Co interface tension (< 10 -2 J/m 2) and the relatively high activation energy of
~
C
o
Ultrafine alloys. Although, basically, the main driving force for solution-reprecipitation is the same for fine and ultrafine alloys (it obviously increased with decreasing grain size), it seems likely that the mechanism involved in liquid phase sintering of standard sized carbides cannot be directly transferred to the growth behavior of ultrafine particles, which exhibit particle sizes from 50 to 200 nm and free binder paths in the range of 10-50 rim. 'Coalescence' of ultrafine WC particles by thin film (solid or liquid) migration or by thin filmaided solution-reprecipitation, as shown schematically in Fig. 13, seem to be probable alternative mechanisms for coarsening as observed during solid-state sintering. Grains of different sizes and orientation fuse in a single grain by a continuous process of directional growth and grain shaping. 22 Dissolution of extremely fine particles in the binder could be further enhanced by a decrease in the activation energy of the interracial reaction as
.~.,.:.-.~.'-:;~.~;~!~::i!~i~il;!~::!:.,.:.cobalt :. binder
off stoichiometrie 'i.:i!ii~!!iiii::ii!iiiii!::..ar !ii ::: ~ i i ! ea (eta-phase) :!iii~
the solution-reprecipitation step, which indicates an interface-reaction controlled mechanism, z° The growth rate of the carbide is increased remarkably by a high carbon content in the alloy (stoichiometric alloy) but can be significantly decreased by the addition of so-called grain growth inhibitors (see below). 13,17,21 In both cases, the influence of the additive (C, inhibitor) has been explained by interactions at the WC/Co interface. 16
interface ,.-...,.-.
.......i!!iii~ii~.:.~..........
.....%:.:::::~.!!ii!~::::..,:.,% " !i~: %s~iiiii!~s~i:.~-~i~::ii~):..:~i.~s~'oC/Cinterface formation of thin Co-film by grain boundary attack (WC/Co/WC interface)
WC solution / reprecipitation (a) due to differences in grain size]constitution (b) ~ue to .compositional differences
(a) coalescence by (solid) thin film migration (b) WC solution / reprecipitation enhanced by WC/Co interface diffusion
Fig. 13. Schematicrepresentation of possiblemechanismsfor WC grain coarseningduring solid-state sintering.
Ultrafine WCpowder manufacture and hard metal production
a result of the 'bond loosening', thus significantly increasing the growth rate even at low temperatures (near 1100°C). There is still no information available on WC grain size distribution during this early period of sintering and its further development during liquid phase sintering. In general, grain growth during solid state sintering and the possible interactions with the densification still remain to be worked out systematically. 19 5.4.2 Grain growth inhibition Sintering practice has shown that the continuous growth of submicron and even ultrafine WC powders during sintering can be largely restricted by the proper addition of grain growth inhibitors (Fig. 14). Theories on the mechanism of grain growth inhibition assume either an alteration of the interface energies or an interference of the .inhibitor with the interfacial dissolution/reprecipitation steps. 23,24 The additives are soluble in the cobalt binder and most apparently segregate at the WC/ Co interface during sintering. The inhibition mechanism involved is most likely the disturbance of crystal growth by the additive, by:
291
• blocking of active growth centers of the crystal (deposition at dislocations, edges) In general, whatever the mechanism involved may be, the inhibitor can be simply thought of as an additive that makes the growth tendency of the system (which consists of finely dispersed carbide grains in a cobalt matrix) more tolerant of differences in individual grain sizes. Among the various grain growth inhibitors (VC, Cr3C2, TaC, NbC), VC, by virtue of its chemical nature and high mobility even during solid state sintering, 2',23 has proved to be by far the most effective and reliable inhibitor in ultra/ fine alloys. Therefore, VC is most commonly used today, either alone or in combination with others (e.g. Cr3C 2 o r T a C ) ) '2 Problems inherent to the manufacture of ultrafine alloys. In the manufacture of ultrafine alloys, grain growth inhibition is a fundamental prerequisite for retaining the ultrafine microstructure. There are, however, two key points which render the manufacture of this class of alloys more difficult than submicron alloys:
• face-specific adsorption • face-oriented deposition ('interface alloying') or
without addition
• Grain growth inhibition of the ultrafines is extremely sensitive to the gross carbon content of the alloy. 2' High-carbon alloys (stoichiometric) promote grain growth even at
addition of 0.65 wt% VC
I0 pm Fig. 14.
Microstructures of ultrafine WC/10 %wt Co hard metals sintered at 1400°C for 1 h; comparison of WC grain growth with 0"65 %wt VC inhibitor and without inhibitor addition.
W. D. Schubert, A. Bock, B. Lux
292 low carbon (5.47 wt% C)
high carbon (5.53 wt% C)
I pm
Fig. 15. Fracturesurfacesof ultrafineWC/10 %wt Co hard metalsampleswithvaryinggross carboncontents.
high VC levels (Fig. 15) and, therefore, necessitate a stringent carbon control during alloy manufacture. • If grain growth inhibition is not handled properly, enhanced local grain growth occurs.
manufacture. Practical experience shows that this growth mode is more pronounced in the case of low-cobalt alloys (e.g. 6 %wt Co) rather than in the case of high cobalt alloys (> 10 %wt Co). Different modes of local grain growth can be observed in the sintering of ultrafine hard metals:
Based on what has already been said about the high reactivity of the ultrafines and their extreme readiness to coarsen, it seems evident that the availability of the growth inhibitor at the appropriate time is of crucial importance for a uniform, continuous grain growth inhibition. This uniform availability must be assured during powder and alloy manufacture, otherwise pronounced local grain growth will occur.
• Local giant WC growth (Fig. 16(a)) • Nest-like giant WC growth (Fig. 16(b)) • Nest-like growth of relatively uniformly sized WC (Fig. 16(c)) • Firmament-like WC growth (Fig. 16(d))
Proper addition of the grain growth inhibitor. There is still some discussion as to whether or not it is necessary to add the inhibitor as early as during powder manufacture, and thereby distributing it within the WC powder matrix, or if it is sufficient to add it directly to the WC/Co charge. It seems likely that this question is closely linked with the efficiency of the powder milling process, which in the end determines the phase distribution within the powder compact and thereby the 'early availability' of the additive. 5.4.3 Discontinuous WC grain growth Discontinuous grain growth and how to avoid it constitute the principal problems in ultrafine alloy
There are several possible reasons for local growth, but it seems probable that ultimately they all have their origin in a departure from perfect uniformity, either chemical or geometrical, present in the green powder compact. Two basic origins exist: • Coarser WC particles, which are already present in the green powder compact and act as seeds for rapid grain growth. • Irregularities (chemical/geometrical), which lead to the nucleation of abnormal grains during sintering, followed by preferred growth of these grains. 22 Typical examples for the first group are: • Coarse grained WC heterogeneities which originate during the powder manufacturing process (as could be shown in a recent paper'2), most ultrafine powder raw materials exhibit such heterogeneities).
Ultrafine WC powder manufacture and hard metal production
293
(b) Nest-like giant WC growth
(a) Local giant WC growth
(c) Nest-like growth of uniform sized WC
(d) Firmament like WC growth
I0 pm Fig. 16. Different modes of local grain growth as observed during sintering of ultrafine hard metals, Murakami etching. ( x 1000 original magnification.)
• Coarse WC particles which originate from (unwanted) blending of the ultrafines with coarser grained material (contamination during powder milling or spray drying). Both of these problems can be avoided by careful manufacturing. The reason for the chemical/geometrical irregularities is less transparent. Again several explanations are plausible: • Extremely fine agglomerates, which undergo more rapid grain growth than the balance of the material. 22 • Any non-uniform inhibitor availability within the compact.
• Insufficient amounts of inhibitor added (local or overall). • Local high carbon. • Off-stoichiometric grains or areas, which might bring about local chemical driving forces for growth (formation and transformation of eta-phase). As an example for the nucleation mode of coarse WC grain growth, in Fig. 17 the fracture surfaces of an alloy prone to local growth are shown. This alloy was interrupted during the heating up period in the sintering cycle at 1200°C (solid state) and 1350°C (liquid phase already present). Already at 1200°C irregularities in the
W. D. Schubert, A. Bock, B. Lux
294
1200°C
1350°C I pm
Fig. 17. Fracture surfaces of an ultrafine WC/10 %wt Co alloy (prone to local grain growth) sintered for 1 min at (a) 1200°C
(solid state) or (b) 1350°C (liquid state).
compact were observed. Several areas were present which differed from the matrix due to the higher degree of local densification (Fig. 17(a)). Above the liquidus line, sudden formation of rather coarse individual WC grains took place (Fig. 17(b)). Although it is not possible to state clearly the origin of this growth, it is likely that it occurred due to local irregularities within the otherwise uniform green powder compact. In general, discontinuous grain growth was always observed in the case of ultrafine WC powders, which, in the 'as supplied' condition, were inhomogeneous with strong and nonuniform particle agglomeration, inherent to the oxide raw material used. Although this inhomogeneous powder is significantly altered during milling, the uniformity of phase dispersion in the green compact is determined by both the powder raw material and the efficiency of the powder milling process. Thus, the better the phase distribution (WC, Co, inhibitor) during powder milling, the less the nature (dispersability) of the powder plays a crucial role (see also ultrafine WC powder manufacture). This 'perfect' dispersion of the powders is so important because it assures the uniform material transport during sintering, thereby avoiding local growth phenomena.
Today's practical limits in carbide grain size during conventional fiquid phase sintering. Based on our own experience with ultrafine hard metals, it is still impossible to state an absolute WC grain size below which, even at proper inhibitor additions, grain growth cannot be restricted. It seems
WC Co
dark
grey
1 pm
Fig. 18. Microstructure of an ultrafine WC/30 %wt Co hard metal obtained by solid state sintering (1200°C); K2CO3/KOH etching; SEM micrograph. (x 10000 original magnification).
likely, however, that such a natural limit exists. For classical liquid phase sintering of ultrafine grades this limit seems to be in the range 200-300 nm.
6
GETTING EVEN FINER
Solid state sintering of the compacts might push the limit down slightly, but is has not yet been established whether a sufficiently uniform binder
Ultrafine WC powder manufacture and hard metal production
submicron hard metals
I
295
nanophase hard metals
ultrafine hard metals
co'~nventional fine grained alloys
.......................................................................................................................................................................................................
alternative consolidation (flash sintering, forging, extrusion) reaction milling of elementary powders
conventional liquid phase sintering
--~
solid state sintering g
1
•
u
I
i
|
i
u
0.5
•
atomic structure n
•
0.1 0.05
WC grain size [pzn] (schematic) Fig. 19.
Today's situation for fine grained hard metal manufacture and new trends for nanophase materials.
distribution can be achieved to yield an adequate hardness/toughness property relationship. Figure 18 shows as an example the microstructure of a high-cobalt hard metal grade made by solid state sintering. Ultrafine cobalt powder grades, which have only recently been offered on the market, might make an excellent binder distribution possible. The questions have not yet been answered whether or not even finer structures can be obtained in the consolidated material and, if so, can they be obtained in a sufficiently reliable and uniform manner to be transformed into superior mechanical and technological properties. These issues cannot be satisfactorily settled until such alloys have been made and thoroughly tested. However, it is quite clear that alternative processes for powder manufacture and alloy consolidation are needed, as illustrated in Fig. 19. Uniform phase dispersion is one of the crucial aspects to be considered in order to meet the demand for product uniformity and reliability. New dispersion techniques have been proposed 25,26 and nanophase powder mixtures are already available on the market. 27 It has, however, still not been clearly demonstrated that they can actually bring about any significant technical and/or economic advantages. It should ultimately be kept in mind that the technical challenge is not in the production of nanophase particles per se,
but in a uniform retention of extremely fine microstructures in the consolidated materials.
ACKNOWLEDGEMENTS The present work is mainly based on the results obtained during a group project on 'The action of trace elements on tungsten reduction, carburization and cemented carbide sintering' in which the following companies participated: Krupp WIDIA, Mitsubishi Metal Corp., Philips Lighting/Maarheeze, Elmet/Lewiston USA, Sandvik Coromant, H.C. Starck, Sumitomo Electric, Teledyne Wah Chang and Wolfram Bergbau- und Htittengesellschaft. The supportive cooperation of all participating companies is gratefully acknowledged. The authors would also like to thank Mrs Dipl. Ing. C. Jelinek and Dr S. Venkateswaran for their critical correction of the manuscript.
REFERENCES 1. Fukatsu, T., Kobori, K. & Ueki, M., Int. J. Refract. Met. & Hard Mater., 10 (1991) 57-60. 2. Egami, E., Kusaka, T., Machida, M. & Kobayashi, K., 12th Int. Plansee Seminar, Reutte, Austria, Vol. 2, ed. H. Bildstein & H. Ortner. Verlaganstalt Tyrolia, Innsbruck, 1989, pp. 53-70.
296
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3. Lardner, E. & lggstrom, S., lOth hit. Plansee Seminar, Reutte, Austria, Vol. 1, eds H. Bildstein & H. Ortner. Verlaganstalt Tyrolia, Innsbruck, 1981, pp. 549-79. 4. Miyake, M., Hara, A., Sho, T. & Kawabata, Y., 5th Eur. Symp. Powder Met., 1978, Vol. 2, pp. 93-8. 5. Schubert, W. D. & Lassner, E., Int. J. Refract. Met. & Hard Mater., 10 (1991) 133-41. 6. Schubert, W. D. & Lassner, E., Int. J. Refract. Met. & Hard Mater., 10 (1991) 171-83. 7. Yang, J., PowderMet. hit., 18 (2)(1986) 62-4. 8. Uhrenius, B., Brandrup-Wognesen, H., Gustavsson, U., Nordgren, A., Lehtinen, B. & Manninen, H., 12th hTt. Plansee Seminar, Reutte, Austria, Vol. 2, ed. H. Bildstein & H. Ortner. Verlaganstalt Tyrolia, Innsbruck, 1989, pp. 71-96. 9. Schubert, W. D & Kiibel, E., 12th lilt. Plansee Seminar, Reutte, Austria, Vol. 2, eds H. Bildstein & H. Ortner. Verlaganstalt Tyrolia, Innsbruck, 1989, pp. 869-909. 10. Virag, A., Friedbacher, G., Grasserbauer, M., Schubert, W. D., Fryc, M. & Lux, B., Mikrochim. Acta, III (1988) 57-73. 11. Almond, E. A. & Roebuck, B., Int. J. Refract. Met. & Hard Mater., 6 (1987) 137-44. 12. Bock, A., Schubert, W. D. & Lux, B., Proc. of the Meeting on 'Advances in Hard Materials Production', MPR Publishing Services, Shrewsbury, 1992, Vol. 14, pp. 1-7. 13. Bock, A., Doctoral Thesis, Technical University, Vienna, 1992.
14. Huppmann, W. J., Z. Metallkde., 70 (12) (1979) 792-7. 15. Snowball, R. F. & Milner, D. R., Powder Met., 11 (1968) 23-40. 16. Meredith, B. & Milner, D. R., Powder Met.. 19 (1976) 38-45. 17. Exner, H. E., C'onf Proc. Adv. in HardMat. Prod., MPR Publishing Services, Shrewsbury, 1988, Vol. 13, pp. 1-5. 18. Wagner, C., Z. Elektrochemi., 65 (1961) 581. 19. Fischmeister, H. & Grimval, G., Sintering and Related Phenomena, ed. G. C. Kuczynski. Plenum Press, New York, 1980, pp. 119-49. 20. Exner, H. E. & Fischmeister, H., Arch. f.d. kisenhiittenwesen, 37 (2)(1966)417-26. 21. Suzuki, H., Fuke, Y. & Hayashi, K., J. Jap. Osc. Powder &PowderMetall., 19 (3)(1972) 106-12. 22. German, R. M., Liquid Phase Sintering, Plenum Press, New York and London, 1985. 23. Kim, D. & Accary, A., Mater. Sci. Res., 13 (1980) 235-44. 24. Bock, A., Schubert, W. D. & Lux, B., pmi, 24 (1)(1992) 20-6. 25. Grewe, H., 12th lilt. Plansee Seminar, Reutte. Austria, Vol. 2, eds H. Bildstein & H. Ortner. Verlaganstalt Tyrolia, Innsbruck, 1989, pp. 117-49. 26. McCandlish, L. E., Kear, B. H. & Kim, B. K., Mater. Sci. and Technol., 6 (1990) 953-7. 27. Beardsley, T., Scientific American, (Oct) (1992) 91-2.
ELSEVIER
General Aspects and Limits of Conventional Ultrafine WC Powder Manufacture and Hard Metal Production W. D. Schubert, A. Bock & B. Lux Institute for Chemical Technology of Inorganic Materials, Technical University Vienna, A-1060 Vienna, Austria
Abstract: Ultrafine WC/Co hard metals (average WC grain sizes -<0"5/~m) can
be successfully and reliably obtained by conventional hard metal manufacturing techniques, in this paper, some of the crucial aspects of conventional powder manufacture, powder milling and liquid phase sintering are discussed. Conventional ultrafine WC powder manufacture is based on the production of tungsten powder by hydrogen reduction of tungsten oxides and subsequent carburization. Alternatively, direct carburization can be carried out. However, inherent to the powder processing techniques used and the particle growth mechanisms involved (oxide precursors used, reduction and carburization history), there exists a lower limit beyond which-finer WC powders cannot be produced. This limit lies in the particle size range of 50-150 nm (0-05-0.15
/~m).
Powder milling is carried out to obtain an even dispersion of the Co binder in the ultrafine WC matrix. The more uniform the phase distribution (WC, Co, grain growth inhibitor) within the green powder compact, the more uniform will be the material transport during sintering, and hence the uniformity of the WC grain growth/growth inhibition during sintering. Enhanced WC grain growth occurs early in the sintering cycle, even below the temperature at which the liquid phase is formed. This growth can be largely restricted by the addition of VC. However, effective grain growth inhibition has to take place already during this early period of solid-state sintering. The 'early' availability of the grain growth inhibitor at the WC/Co interface can, therefore, determine the degree of growth inhibition. Ultrafine hard metals are in particular prone to discontinuous grain growth of the WC. Different reasons for this local growth mode are propounded relating to both the chemical as well as the geometrical departures from uniformity in the green powder compact. While it is still not possible to predict exactly an ultimate WC grain size limit, below which WC grain growth can no longer be restricted, even with proper inhibitor additions, experimental evidence indicates that this average WC grain size limit lies in the range of 200-300 nm. This limit is inherent to the existing conventional processing techniques (powder manufacture, milling, liquid phase sintering) and the WC growth mechanisms involved and can be overcome only by establishing a completely new route in hard metal manufacture.
1
While submicron t WC/Co grades still predominate today's fine-grained tool market, there
INTRODUCTION
Over the years, there has been a discernible tendency towards using finer and finer grained materials in the hard metal industry to exploit the advantages of their high toughness, strength and rigidity coupled with high hardness and wear resistance.
+In this paper two terms are used in describing the fineness of WC powder raw materials and the WC grain size in the sintered alloy: the term 'submicron' is used for powder grades (alloys) with an average particle (grain) size in the range 0-5-1/~m. The term 'ultrafine' describes grades with an average particle (grain) size <0-5 ~m (500 nm). 281
W. D. Schubert, A. Bock, B. Lux
282
is increasing interest in using even finer grades for certain specific applications. ~,2 The advantages of using such ultrafine* hard metals over submicron grades is illustrated in Fig. 1, which gives a comparison between these grades for a WC/10 %wt Co system. A pronounced increase in hardness from 1780 Hv 3 to 1925 Hv 3 is evident at a comparably high bending strength (3800 MPa). In fact, hardness values up to 2200 Hv30 (6 %wt Co) and bending strength values of up to 5000 MPa (15 %wt Co) have been successfully achieved. (Lassner, E. & Zeiler, B., 1993, pers. comm.) ~ However, these ultrafine grades are extremely sensitive to processing conditions and are even more prone to carbide grain growth during consolidation. Further, due to the high reactivity of such ultrafine carbide powders during the liquid phase sintering, both powder raw material quality (in terms of homogeneity, purity and chemical stability) and the addition of appropriate grain growth inhibitors are crucial factors in achieving the required superior quality and reliability. In this paper, the problems involved in the manufacture of such highly sensitive ultrafine alloys are discussed, with special emphasis on: • Present day concepts in industrial ultrafine hard metal manufacture
Submicron WC/10 wt% Co hardmetal
• Conventional ultrafine WC powder production • Aspects involved in ultrafine WC powder quality assurance • Ultrafine WC-Co alloy sintering (densification, grain growth) 2 PRESENT DAY CONCEPTS IN INDUSTRIAL ULTRAFINE HARD METAL MANUFACTURE
There are basically two approaches for obtaining ultrafine hard metal structures by the conventional hard metal manufacturing route. Both are designed to obtain an evenly dispersed ultrafine WC/Co powder mix, which is subsequently granulated, pressed or extruded, and then liquid-phase sintered. • The first method is based on the use of an ultrafine WC powder grade (average WC particle size <-0.5 ~ m). This powder is processed in the same or similar way as other fine grained carbide powders, that is, the powder is ball milled or attritor milled together with a standard fine cobalt grade to optimize the blending of the carbide with the binder.
Ultrafine WC/10 wt% Co hardmetai
fracture surface (10 000 X)
Fig. 1.
1 ~m
Hardness
Hv 3
= 1780 kg/mm 2
Hv 3
= 1925 kg/mm 2
TRS Coercivity Magn. sat.
050 iHc 4~
= 3780 MPa = 24.9 kA/m = 18.2 }JTm3/kg
~5o iHc 4rc~
= 3890 MPa = 35.1 kA/m = 18.7 laTm3/kg
Fracture surfaces of submicron and ultrafine WC/10 %wt Co hard metal (refer to WC powders used, see Figs 6(a) and 6(b)).
Ultrafine WC powder manufacture and hard metal production Although powder milling is commonly regarded as a comminution operation it is of minor consequence in the powder milling of such extremely fine powders. Any particular mill and mill charge combination have a limiting grain size, beyond which no further comminution will occur) This is commonly in the range of 0.5-1/~m for the carbide, so no comminution will occur in the case of even finer particles. The main reason for this behaviour is that, with decreasing particle size, enhanced particle agglomeration will occur. Such particle agglomerates will then be separated into smaller units during powder milling, but will subsequently reform, generating heat. • The second method is based on a standard sized WC powder grade (e.g. 1.5 lure), which is heavily milled under such conditions that a further comminution will occur. This concept demands a fundamental 'know how' of the powder milling process in order to prevent the occurrence of pronounced local grain growth due to a nonuniform size distribution of the WC particles. There is, however, an absolute limit in comminution, which depends on the physical properties of the material. This limit seems to be in the 50-150 nm particle size range. The addition of so-called grain growth inhibitors (VC, Cr3C2,TaC; see Section 5) is an important prerequisite of ultrafine hard metal sintering to retain the ultrafine carbide powder structure in the consolidated material. It is the 'state of the art' today to add at least some of the inhibitor necessary for growth restriction during powder manufacture ('powder engineering'). This inhibitor addition can be done as early as during tungsten metal powder formation, or it can be carried out prior to metal powder carburization. Both of these methods have their weak points and technical limits. To obtain high-quality proagglomerate
particle ~i'nterparticle forces[ Iintergranular forces~j . . . .
grain
Fig. 2. Build up of tungsten metal agglomerates as obtained by hydrogen reduction of tungsten oxides (schematic).
283
ducts in a reliable manner, it is essential for the producer, with the help of an in-depth understanding of all processes involved in the manufacture, to search out these weak points in order to reduce or even eliminate their detrimental effects on the final product.
3
CONVENTIONAL ULTRAFINE WC POWDER PRODUCTION
Ultrafine WC powder manufacture is based on the production of tungsten powder by hydrogen reduction of tungsten oxides and subsequent carburization. Alternatively, direct carburization of tungsten oxides can be carried out. 4,5 The starting materials in both cases are either tungstic acid (H2WO4) or tungsten blue and yellow oxides originating from APT (ammonium paratungstate). The aim in both ultrafine tungsten metal and tungsten carbide powder manufacture is the production of a pure, uniform, fine and loosely agglomerated powder that can subsequently be easily dispersed during (W/C) powder mixing, WC powder deagglomeration and (WC/Co) powder milling. However, depending on exactly how they were manufactured (oxide raw materials used, reduction and carburization history) such powders can significantly vary with respect to their physical consitution.
3.1
Tungsten metal production
A typical peculiarity of ultrafine tungsten powders obtained by hydrogen reduction of tungsten oxides is their pseudomorphic appearance in the oxide raw material. These sponge-like metal agglomerates (pseudomorphs) form due to the extremely dry reduction conditions prevalent during the manufacture of ultrafine tungsten powders. They are made up of extremely fine metal grains (20-50 nm) held together by strong intergranular and interparticular forces (chemical and physical bondings), as shown schematically in Fig. 2. The physical constitution of the agglomerates (both on a macro- and microscopic scale) can be loose or dense and can be strongly influenced by the reduction conditions as well as by the chemical and physical nature of the oxide precursor used. 6 (Fig. 3.) The ease with which these agglomerates can be broken down during subsequent deagglomeration
284
W. D. Schubert, A. Bock, B. Lux
(a)
Co) I0 pm
Fig. 3.
Tungsten metal powder agglomerated in APT pseudomorphs (SEM 2000 x ), obtained from: (a) APT decomposed at 300°C (loose and dense agglomerates); (b) APT decomposed at 700°C (only loose agglomerates).
(a)
(b) 10 ~m
Fig. 4.
Laboratory milled tungsten powders, obtained from the 'as supplied' powders shown in Fig. 3. (a) Some agglomerates remain; (b) agglomerates completely disintegrated.
can be significantly different. As an example, Fig. 4 shows the differences in agglomerate breakdown during powder milling (ASTM No. B430 (1970) deagglomeration treatment) of the two tungsten metal powders, as seen earlier in Fig. 3. It is apparent that in one case some rather coarse agglomerates have survived the deagglomeration treatment, while in the other case the powder was completely disintegrated. It must, however, be kept in mind that during disintegration a certain degree of agglomeration (a
certain particle or agglomerate size) will always remain, due to the inherent chemical and physical forces acting between the individual particles and grains (Fig. 2). The more uniform the disintegration during deagglomeration, the more uniform will be the particle dispersion during subsequent blending with carbon black. The lower the degree of metal-metal contacts during carburization, the lesser will be the particle coarsening. It is thus evident that both reduction conditions and the use of appropriate oxide raw materials
Ultrafine WCpowder manufacture and hard metal production play a significant role in decisively influencing the fineness and uniformity of the carbide finally obtained.
4
4.1 3.2 Particle coarsening during carburization and limits in carbide particle size Carburization of ultrafine tungsten powders is carried out at rather low carburization temperatures (1200-1450°C) in order to restrict severe particle coarsening during the W~ WC transition. However, even then, a certain coarsening of the particles occurs as evidenced by a strong decrease in the specific surface areas of the powders, as shown in Fig. 5. In general, particle coarsening of such extremely fine metal structures during carburization and the mechanism involved still remain to be worked out systematically. It has, however, been observed that the addition of grain growth inhibitors (Cr3C2), a common practice in ultrafine carbide manufacture, can have an extraordinary influence on the micromorphological evolution of the final carbide particles obtained by promoting the occurrence of individual fine WC particles instead of coarse polycrystalline particle agglomerates. 7 Practical carburization experience indicates that there is a lower particle size range below which finer WC powders cannot be produced. This is true for both precursors used (HzWO4, APT) and, according to SEM (scanning electron microscope) imaging investigations, lies in the range 5 0-15 0 nm. This limit is inherent to the existing conventional processing technique itself and can be overcome only by establishing a completely new route in carbide production.
5
Limit inherent to conventional processing techniques i~1111
~
i ii
3 •
•
ca
0
I
0
2 4 6 10 12 14 BET-specific surface area of the W powder [m2/g]
16
Fig. 5. Changes in the BET specific surface area during carburization (W~WC) of different industrial powder grades. A maximum of about 4.5 m2/g is obtained, which is an inherent limit for the conventional powder processing.
285
ASPECTS INVOLVED IN ULTRAFINE WC POWDER QUALITY ASSURANCE Powder purity
High purity of the raw materials and the avoidance of any contamination of the powders during subsequent processing is mandatory for the reliable manufacture of defect-free high-strength WC/Co alloys. In particular, heterogeneous iron and nickel contaminations must be strictly avoided during WC powder manufacture, as they can lead to local melt formation and enhanced local grain growth during carburization even at the relatively low temperature of 1150°C. These contaminations can originate from furnace and boat materials. Powders must be low in sulfur and calcium to avoid CaS precipitation during sintering, which can severely lower the strength of the sintered alloys. ~ As seen from earlier studies, while these elements orginate mainly from carbon blacks, they could also stem from 'technically' pure tungsten and cobalt powder grades, s- ~0 4.2 Particle size and particle size distribution of ultrafine WC powder grades The control of particle size in the manufacture of hard metals is an important part of the quality assurance system. The hard metal producer, therefore, has based his system on the measurement of certain chemical and physical powder properties, which must be achieved in order to obtain reliable alloy properties. This system is easy to handle and works quite well in the case of standard alloys, but has significant limitations for ultrafine powders. A simple comparison between a submicron and an ultrafine WC powder grade reveals the scope of this problem. The submicron WC powder (Fig. 6(a)) consists of small individual particles which are loosely agglomerated. These agglomerates break down easily during the physical measurements as normally applied for control purposes (Fisher sub-sieve size (FSSS), laser scattering). Both FSSS particle size (0"92 ktm) and the mean particle size obtained by laser scattering (0.91 ktm) are in sufficient agreement with the only slightly lower mean size determined by SEM imaging analysis (0"8 ktm) (Fig. 7(a)).
W. D. Schubert, A. Bock, B. Lux
286
(a) submicron WC
(b) ultrafine WC 1 pm
Fig. 6.
Submicron (a) and ultrafine WC powder grade (b); both grades are characterized in Fig. 7 with regard to their mean particle size/size distribution.
~
20
, , (a) subTn]croll
l~ BET-grain size (0.5 ~m)
FSSs'graill
size (0.92 p.m
~ ~ -
SEM-grain size (0.8 ~ml
or. 10
0
jll :o
/
|~1~ Laser scattelmg
J i5
o
/
.--I-'7
0,01
"
!|1
0,i
'~.--~
1
. i0
WC grain size [g.rn] SF_,M-grain size (0.i.5 ~m)
3O
(b) u l t r a R n e
~,
'/
-
~,. O N
F~ 3S-grain size (0.3 ~m)
? 20
BET-gr in size (0.0-~~rn)
It //\
L
zo
.~
~.,[~
o
0.01
PhotoJ correlation s ectro~)py (0.25~m)
1 ~
Laser ecatterin ' (d50--0.63 gm)
~
0.1
I
10
WC grain size [~m] Fig. 7.
WC mean particle size and particle size distribution measurements obtained by laser scattering. FSSS, BET-surface area measurement and SEM image analysis on the submicron and ultrafine powders shown in Fig. 6.
Ultrafi'ne WC powder manufacture and hard metal production
287
The ultrafine WC powder consists of strongly agglomerated particles (Fig. 6(b)). Here, the FSSS size (0.25 /~m) and the mean particle size as obtained by laser scanning (0"63/~m) are in strong disagreement with the average size value as determined by SEM imaging (0.15/~m) (Fig. 7(b)). It is thus evident that this strong agglomeration tendency, which is inherent to the ultrafines, makes the measurement of classical particle size, and in particular, particle size distribution, of rather limited value for control purposes. 4.3 BET (Brunauer-Emmet-Teller) specific surface area measurements Most of the more recent papers on fine grained tungsten carbide powders include the specific surface area measurements in the characterization of the materials. Although this technique can give the closest agreement with image analysis] ~ a direct interpretation in terms of a calculated particle size (6/(BET-surface x density)) can result in misleadingly low size values in the case of spongelike ultrafine powders. 4.4
Particle irregularities
Coarse WC grains, present in an ultrafine carbide matrix, can readily act as seed crystals for local grain coarsening during subsequent sintering. As could be shown in a recent paper] 2 most of the ultrafine WC powder grades exhibit small amounts of coarse grains and/or rather dense looking agglomerates. These irregularities are often difficult to detect even through careful SEM imaging of the powders, due to the high overall agglomeration of the fine particles. In addition, all the usual physical measurement techniques fail to detect any inhomogeneity at all. It has, however, been established that a good characterization of even small amounts of these irregularities can be obtained through the SEM imaging of the residue left after selective dissolution of the fine grains. 13 Figure 8 shows such a coarse particle detected in an ultrafine WC powder grade which was prone to local grain growth during subsequent sintering. 4.5 Physical and chemical constitution of grain growth inhibitors during powder production Although the proper addition of grain growth inhibitors plays a key role in ultrafine WC powder
Fig. 8. SEM imageof the residualWC after selectivedissolutionof the finegrains.
and WC/Co alloy manufacture (see below), very little is actually understood about the physical and chemical constitution of the inhibitor during manufacture, in particular during WC powder production. Other than the gross chemical analysis of the powders (e.g. 5000 ppm of V, Ctot and Cfree), no further information is available on the chemical and physical states of these compounds. In view of the above limitations, it can be seen that the usual quality assurance controls used for standard powder grades are inadequate and often give misleading results when used for controlling the quality of such ultrafine grades. Apparently, the best solution then is to 'build in' the powder quality at different processing stages, rather than merely measure it at the final stage. 5 5.1
SINTERING OF ULTRAFINE HARD METALS The goal
The most crucial aspect of the sintering of ultrafine grades is retaining the ultrafine microstructure of the WC in the consolidated (dense) material. Only if this aim is both successfully and reliably achieved can a pronounced technical advantage (higher hardness, better wear resistance, high strength) be realized over the submicron alloys, which are slightly coarser, but less growth sensitive during the manufacturing process. In the following, the important aspects of sintering with reference to ultrafine WC powders will be discussed.
W. D. Schubert, A. Bock, B. Lux
288
5.2
In addition, it must be kept in mind that, during sintering of hard metals, chemical reactions take place which can bring about diffusion gradients and can interact with both processes involved during sintering, unless chemical equilibrium has been reached (dissolution of the carbide in the cobalt binder, possible formation of eta-phase and subsequent transformation, carbon adjustment). 14 The earliest stage at which we can expect this to take place is during the isothermal liquid phase sintering (see Fig. 9).
Driving force
The primary driving force for sintering of fine grained hard metals, as for any sintering system, is a reduction in the interface energy of the system. This is accomplished by reducing the area of the surfaces and interfaces of the compact, which happens mainly by a combination of two concurrent processes, namely: • Densification (replacement of the gas/solid interface by a lower-energy solid-solid or solid-liquid interface). • WC grain growth (decrease of the solid-solid or solid-liquid interface area and formation of low-energy prismatic interfaces).
5.3 Densification of ultrafine hard metals and early WC grain growth during densification Even though conventional hard metal sintering is based on liquid phase sintering, pronounced densification occurs even during solid state sintering by solid state diffusion, transport of particles and plastic flow of carbide binder areas.'5-~ 7 In the case of ultrafine alloys, up to 90% of the densification occurs during solid state sintering (below 1280°C~). Solid state densification increases with decreasing particle size (submicron ultrafine) (Fig. 10) as it is also enhanced by high gross carbon contents in the alloys.J5.19 It can be seen upon interrupting the sintering cycle at 1100 and 1200°C that, concurrent with this densification, pronounced grain growth also occurs (Fig. 11 ). This growth is accompanied by a significant change in the carbide grain shape from non-facetted to prismatic, thus indicating the formation of low-energy prismatic interfaces.
The latter process is of particular importance during the sintering of ultrafine alloys. temperature
1400"C
~ " ~i' non-isothermal 1/ i s t ast ien t e r ~ i~i
1100'C
800"C
Fig. 9.
J
liquid
isothermal phase sintermg
f ~
~
Formation o liquid phase 1280-1310"C ~ n I 1 2 3 sintering time [hours]
0
Schematic illustration of a technical hard metal sintering cycle.
solid state densification
/'
100 8O 60
i
f
"formation of eutectic liquid
40
ultrafine grade 0.3 pm WC
20
submicron grade 0.7 lam WC
1000
1100
1200
1300
1400
temperature [°C] Fig. 10.
Densification of WC/10 %wt Co hard metals based on ultrafine and submicron WC (no grain growth inhibitor added; heating rate: 3°C/min).
Ultrafine WC powder manufacture and hard metal production
ll00°C
289
1200oc 1 lain
Fig. 11.
Fracture surfaces of ultrafine WC/10 %wt Co hard metal samples (no grain growth inhibitor added), sintered for 1 rain at 1100°C (left) and 1200°C (right).
"I )
II00°C
1200oc 1 Cm
Fig. 12.
Fracture surfaces of ultrafine WC/10 %wt Co hard metal samples (0.65 %wt VC added in the WC powder as grain growth inhibitor), sintered for I min at 1100°C (left) and 1200°C (right).
Closer WC particle packing by solid state solution-reprecipitation and a decrease of the frictional forces which enhance particle gliding 17have apparently promoted the particle rearrangement process. This particle rearrangement process is a prime factor in the extensive shrinkage of compacts occurring during this early period of sintering. 15 This early growth, however, can largely be restricted by a proper addition of VC (0.65 %wt) as grain growth inhibitor (Fig. 12). The decrease in the growth rate of the carbide is accompanied by a
lower densification rate, thus indicating that the mechanisms of both processes, densification and grain growth, are closely interlinked, although the driving forces for the processes have different origins. It can, therefore, be seen that a peculiarity of highly reactive fine and ultrafine hard metal structures is this early grain growth, which can occur concurrently with the densification of the alloy. It constitutes the starting point for further carbide coarsening during subsequent liquid phase sintering. The avoidance of such growth during this
290
W. D. Schubert, A. Bock, B. Lux
early period plays a crucial role in the successful manufacture of ultrafine structures. 5.4 Grain growth and grain growth inhibition in ultrafine hard metals
In practice, one distinguishes between 'continuous' (overall or normal) grain growth and 'discontinuous' (local or abnormal) grain growth. During continuous grain growth, the sizes of the individual grains are relatively uniform. During discontinuous grain growth, on the other hand, the differences in individual sizes increase due to the rapid growth of some of these grains.
5.4.1 Continuous grain growth Standard sized hard metals. The theory of continuous grain growth is based on the interfacial free energy being the driving force.18 Grain growth during liquid phase sintering has been phenomenologically treated as an Ostwald ripening process, m Smaller particles dissolve due to their higher dissolution potential (increased chemical potential), while coarser ones grow by material reprecipitation, thereby reducing the interface area of the system. Continuous grain growth in standard sized hard metals has been observed to proceed rather slowly as compared to other carbide/binder systems (VC/Co, NbC/Co, TaC/Co). This has been attributed to the extremely low WC/Co interface tension (< 10 -2 J/m 2) and the relatively high activation energy of
~
C
o
Ultrafine alloys. Although, basically, the main driving force for solution-reprecipitation is the same for fine and ultrafine alloys (it obviously increased with decreasing grain size), it seems likely that the mechanism involved in liquid phase sintering of standard sized carbides cannot be directly transferred to the growth behavior of ultrafine particles, which exhibit particle sizes from 50 to 200 nm and free binder paths in the range of 10-50 rim. 'Coalescence' of ultrafine WC particles by thin film (solid or liquid) migration or by thin filmaided solution-reprecipitation, as shown schematically in Fig. 13, seem to be probable alternative mechanisms for coarsening as observed during solid-state sintering. Grains of different sizes and orientation fuse in a single grain by a continuous process of directional growth and grain shaping. 22 Dissolution of extremely fine particles in the binder could be further enhanced by a decrease in the activation energy of the interracial reaction as
.~.,.:.-.~.'-:;~.~;~!~::i!~i~il;!~::!:.,.:.cobalt :. binder
off stoichiometrie 'i.:i!ii~!!iiii::ii!iiiii!::..ar !ii ::: ~ i i ! ea (eta-phase) :!iii~
the solution-reprecipitation step, which indicates an interface-reaction controlled mechanism, z° The growth rate of the carbide is increased remarkably by a high carbon content in the alloy (stoichiometric alloy) but can be significantly decreased by the addition of so-called grain growth inhibitors (see below). 13,17,21 In both cases, the influence of the additive (C, inhibitor) has been explained by interactions at the WC/Co interface. 16
interface ,.-...,.-.
.......i!!iii~ii~.:.~..........
.....%:.:::::~.!!ii!~::::..,:.,% " !i~: %s~iiiii!~s~i:.~-~i~::ii~):..:~i.~s~'oC/Cinterface formation of thin Co-film by grain boundary attack (WC/Co/WC interface)
WC solution / reprecipitation (a) due to differences in grain size]constitution (b) ~ue to .compositional differences
(a) coalescence by (solid) thin film migration (b) WC solution / reprecipitation enhanced by WC/Co interface diffusion
Fig. 13. Schematicrepresentation of possiblemechanismsfor WC grain coarseningduring solid-state sintering.
Ultrafine WCpowder manufacture and hard metal production
a result of the 'bond loosening', thus significantly increasing the growth rate even at low temperatures (near 1100°C). There is still no information available on WC grain size distribution during this early period of sintering and its further development during liquid phase sintering. In general, grain growth during solid state sintering and the possible interactions with the densification still remain to be worked out systematically. 19 5.4.2 Grain growth inhibition Sintering practice has shown that the continuous growth of submicron and even ultrafine WC powders during sintering can be largely restricted by the proper addition of grain growth inhibitors (Fig. 14). Theories on the mechanism of grain growth inhibition assume either an alteration of the interface energies or an interference of the .inhibitor with the interfacial dissolution/reprecipitation steps. 23,24 The additives are soluble in the cobalt binder and most apparently segregate at the WC/ Co interface during sintering. The inhibition mechanism involved is most likely the disturbance of crystal growth by the additive, by:
291
• blocking of active growth centers of the crystal (deposition at dislocations, edges) In general, whatever the mechanism involved may be, the inhibitor can be simply thought of as an additive that makes the growth tendency of the system (which consists of finely dispersed carbide grains in a cobalt matrix) more tolerant of differences in individual grain sizes. Among the various grain growth inhibitors (VC, Cr3C2, TaC, NbC), VC, by virtue of its chemical nature and high mobility even during solid state sintering, 2',23 has proved to be by far the most effective and reliable inhibitor in ultra/ fine alloys. Therefore, VC is most commonly used today, either alone or in combination with others (e.g. Cr3C 2 o r T a C ) ) '2 Problems inherent to the manufacture of ultrafine alloys. In the manufacture of ultrafine alloys, grain growth inhibition is a fundamental prerequisite for retaining the ultrafine microstructure. There are, however, two key points which render the manufacture of this class of alloys more difficult than submicron alloys:
• face-specific adsorption • face-oriented deposition ('interface alloying') or
without addition
• Grain growth inhibition of the ultrafines is extremely sensitive to the gross carbon content of the alloy. 2' High-carbon alloys (stoichiometric) promote grain growth even at
addition of 0.65 wt% VC
I0 pm Fig. 14.
Microstructures of ultrafine WC/10 %wt Co hard metals sintered at 1400°C for 1 h; comparison of WC grain growth with 0"65 %wt VC inhibitor and without inhibitor addition.
W. D. Schubert, A. Bock, B. Lux
292 low carbon (5.47 wt% C)
high carbon (5.53 wt% C)
I pm
Fig. 15. Fracturesurfacesof ultrafineWC/10 %wt Co hard metalsampleswithvaryinggross carboncontents.
high VC levels (Fig. 15) and, therefore, necessitate a stringent carbon control during alloy manufacture. • If grain growth inhibition is not handled properly, enhanced local grain growth occurs.
manufacture. Practical experience shows that this growth mode is more pronounced in the case of low-cobalt alloys (e.g. 6 %wt Co) rather than in the case of high cobalt alloys (> 10 %wt Co). Different modes of local grain growth can be observed in the sintering of ultrafine hard metals:
Based on what has already been said about the high reactivity of the ultrafines and their extreme readiness to coarsen, it seems evident that the availability of the growth inhibitor at the appropriate time is of crucial importance for a uniform, continuous grain growth inhibition. This uniform availability must be assured during powder and alloy manufacture, otherwise pronounced local grain growth will occur.
• Local giant WC growth (Fig. 16(a)) • Nest-like giant WC growth (Fig. 16(b)) • Nest-like growth of relatively uniformly sized WC (Fig. 16(c)) • Firmament-like WC growth (Fig. 16(d))
Proper addition of the grain growth inhibitor. There is still some discussion as to whether or not it is necessary to add the inhibitor as early as during powder manufacture, and thereby distributing it within the WC powder matrix, or if it is sufficient to add it directly to the WC/Co charge. It seems likely that this question is closely linked with the efficiency of the powder milling process, which in the end determines the phase distribution within the powder compact and thereby the 'early availability' of the additive. 5.4.3 Discontinuous WC grain growth Discontinuous grain growth and how to avoid it constitute the principal problems in ultrafine alloy
There are several possible reasons for local growth, but it seems probable that ultimately they all have their origin in a departure from perfect uniformity, either chemical or geometrical, present in the green powder compact. Two basic origins exist: • Coarser WC particles, which are already present in the green powder compact and act as seeds for rapid grain growth. • Irregularities (chemical/geometrical), which lead to the nucleation of abnormal grains during sintering, followed by preferred growth of these grains. 22 Typical examples for the first group are: • Coarse grained WC heterogeneities which originate during the powder manufacturing process (as could be shown in a recent paper'2), most ultrafine powder raw materials exhibit such heterogeneities).
Ultrafine WC powder manufacture and hard metal production
293
(b) Nest-like giant WC growth
(a) Local giant WC growth
(c) Nest-like growth of uniform sized WC
(d) Firmament like WC growth
I0 pm Fig. 16. Different modes of local grain growth as observed during sintering of ultrafine hard metals, Murakami etching. ( x 1000 original magnification.)
• Coarse WC particles which originate from (unwanted) blending of the ultrafines with coarser grained material (contamination during powder milling or spray drying). Both of these problems can be avoided by careful manufacturing. The reason for the chemical/geometrical irregularities is less transparent. Again several explanations are plausible: • Extremely fine agglomerates, which undergo more rapid grain growth than the balance of the material. 22 • Any non-uniform inhibitor availability within the compact.
• Insufficient amounts of inhibitor added (local or overall). • Local high carbon. • Off-stoichiometric grains or areas, which might bring about local chemical driving forces for growth (formation and transformation of eta-phase). As an example for the nucleation mode of coarse WC grain growth, in Fig. 17 the fracture surfaces of an alloy prone to local growth are shown. This alloy was interrupted during the heating up period in the sintering cycle at 1200°C (solid state) and 1350°C (liquid phase already present). Already at 1200°C irregularities in the
W. D. Schubert, A. Bock, B. Lux
294
1200°C
1350°C I pm
Fig. 17. Fracture surfaces of an ultrafine WC/10 %wt Co alloy (prone to local grain growth) sintered for 1 min at (a) 1200°C
(solid state) or (b) 1350°C (liquid state).
compact were observed. Several areas were present which differed from the matrix due to the higher degree of local densification (Fig. 17(a)). Above the liquidus line, sudden formation of rather coarse individual WC grains took place (Fig. 17(b)). Although it is not possible to state clearly the origin of this growth, it is likely that it occurred due to local irregularities within the otherwise uniform green powder compact. In general, discontinuous grain growth was always observed in the case of ultrafine WC powders, which, in the 'as supplied' condition, were inhomogeneous with strong and nonuniform particle agglomeration, inherent to the oxide raw material used. Although this inhomogeneous powder is significantly altered during milling, the uniformity of phase dispersion in the green compact is determined by both the powder raw material and the efficiency of the powder milling process. Thus, the better the phase distribution (WC, Co, inhibitor) during powder milling, the less the nature (dispersability) of the powder plays a crucial role (see also ultrafine WC powder manufacture). This 'perfect' dispersion of the powders is so important because it assures the uniform material transport during sintering, thereby avoiding local growth phenomena.
Today's practical limits in carbide grain size during conventional fiquid phase sintering. Based on our own experience with ultrafine hard metals, it is still impossible to state an absolute WC grain size below which, even at proper inhibitor additions, grain growth cannot be restricted. It seems
WC Co
dark
grey
1 pm
Fig. 18. Microstructure of an ultrafine WC/30 %wt Co hard metal obtained by solid state sintering (1200°C); K2CO3/KOH etching; SEM micrograph. (x 10000 original magnification).
likely, however, that such a natural limit exists. For classical liquid phase sintering of ultrafine grades this limit seems to be in the range 200-300 nm.
6
GETTING EVEN FINER
Solid state sintering of the compacts might push the limit down slightly, but is has not yet been established whether a sufficiently uniform binder
Ultrafine WC powder manufacture and hard metal production
submicron hard metals
I
295
nanophase hard metals
ultrafine hard metals
co'~nventional fine grained alloys
.......................................................................................................................................................................................................
alternative consolidation (flash sintering, forging, extrusion) reaction milling of elementary powders
conventional liquid phase sintering
--~
solid state sintering g
1
•
u
I
i
|
i
u
0.5
•
atomic structure n
•
0.1 0.05
WC grain size [pzn] (schematic) Fig. 19.
Today's situation for fine grained hard metal manufacture and new trends for nanophase materials.
distribution can be achieved to yield an adequate hardness/toughness property relationship. Figure 18 shows as an example the microstructure of a high-cobalt hard metal grade made by solid state sintering. Ultrafine cobalt powder grades, which have only recently been offered on the market, might make an excellent binder distribution possible. The questions have not yet been answered whether or not even finer structures can be obtained in the consolidated material and, if so, can they be obtained in a sufficiently reliable and uniform manner to be transformed into superior mechanical and technological properties. These issues cannot be satisfactorily settled until such alloys have been made and thoroughly tested. However, it is quite clear that alternative processes for powder manufacture and alloy consolidation are needed, as illustrated in Fig. 19. Uniform phase dispersion is one of the crucial aspects to be considered in order to meet the demand for product uniformity and reliability. New dispersion techniques have been proposed 25,26 and nanophase powder mixtures are already available on the market. 27 It has, however, still not been clearly demonstrated that they can actually bring about any significant technical and/or economic advantages. It should ultimately be kept in mind that the technical challenge is not in the production of nanophase particles per se,
but in a uniform retention of extremely fine microstructures in the consolidated materials.
ACKNOWLEDGEMENTS The present work is mainly based on the results obtained during a group project on 'The action of trace elements on tungsten reduction, carburization and cemented carbide sintering' in which the following companies participated: Krupp WIDIA, Mitsubishi Metal Corp., Philips Lighting/Maarheeze, Elmet/Lewiston USA, Sandvik Coromant, H.C. Starck, Sumitomo Electric, Teledyne Wah Chang and Wolfram Bergbau- und Htittengesellschaft. The supportive cooperation of all participating companies is gratefully acknowledged. The authors would also like to thank Mrs Dipl. Ing. C. Jelinek and Dr S. Venkateswaran for their critical correction of the manuscript.
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3. Lardner, E. & lggstrom, S., lOth hit. Plansee Seminar, Reutte, Austria, Vol. 1, eds H. Bildstein & H. Ortner. Verlaganstalt Tyrolia, Innsbruck, 1981, pp. 549-79. 4. Miyake, M., Hara, A., Sho, T. & Kawabata, Y., 5th Eur. Symp. Powder Met., 1978, Vol. 2, pp. 93-8. 5. Schubert, W. D. & Lassner, E., Int. J. Refract. Met. & Hard Mater., 10 (1991) 133-41. 6. Schubert, W. D. & Lassner, E., Int. J. Refract. Met. & Hard Mater., 10 (1991) 171-83. 7. Yang, J., PowderMet. hit., 18 (2)(1986) 62-4. 8. Uhrenius, B., Brandrup-Wognesen, H., Gustavsson, U., Nordgren, A., Lehtinen, B. & Manninen, H., 12th hTt. Plansee Seminar, Reutte, Austria, Vol. 2, ed. H. Bildstein & H. Ortner. Verlaganstalt Tyrolia, Innsbruck, 1989, pp. 71-96. 9. Schubert, W. D & Kiibel, E., 12th lilt. Plansee Seminar, Reutte, Austria, Vol. 2, eds H. Bildstein & H. Ortner. Verlaganstalt Tyrolia, Innsbruck, 1989, pp. 869-909. 10. Virag, A., Friedbacher, G., Grasserbauer, M., Schubert, W. D., Fryc, M. & Lux, B., Mikrochim. Acta, III (1988) 57-73. 11. Almond, E. A. & Roebuck, B., Int. J. Refract. Met. & Hard Mater., 6 (1987) 137-44. 12. Bock, A., Schubert, W. D. & Lux, B., Proc. of the Meeting on 'Advances in Hard Materials Production', MPR Publishing Services, Shrewsbury, 1992, Vol. 14, pp. 1-7. 13. Bock, A., Doctoral Thesis, Technical University, Vienna, 1992.
14. Huppmann, W. J., Z. Metallkde., 70 (12) (1979) 792-7. 15. Snowball, R. F. & Milner, D. R., Powder Met., 11 (1968) 23-40. 16. Meredith, B. & Milner, D. R., Powder Met.. 19 (1976) 38-45. 17. Exner, H. E., C'onf Proc. Adv. in HardMat. Prod., MPR Publishing Services, Shrewsbury, 1988, Vol. 13, pp. 1-5. 18. Wagner, C., Z. Elektrochemi., 65 (1961) 581. 19. Fischmeister, H. & Grimval, G., Sintering and Related Phenomena, ed. G. C. Kuczynski. Plenum Press, New York, 1980, pp. 119-49. 20. Exner, H. E. & Fischmeister, H., Arch. f.d. kisenhiittenwesen, 37 (2)(1966)417-26. 21. Suzuki, H., Fuke, Y. & Hayashi, K., J. Jap. Osc. Powder &PowderMetall., 19 (3)(1972) 106-12. 22. German, R. M., Liquid Phase Sintering, Plenum Press, New York and London, 1985. 23. Kim, D. & Accary, A., Mater. Sci. Res., 13 (1980) 235-44. 24. Bock, A., Schubert, W. D. & Lux, B., pmi, 24 (1)(1992) 20-6. 25. Grewe, H., 12th lilt. Plansee Seminar, Reutte. Austria, Vol. 2, eds H. Bildstein & H. Ortner. Verlaganstalt Tyrolia, Innsbruck, 1989, pp. 117-49. 26. McCandlish, L. E., Kear, B. H. & Kim, B. K., Mater. Sci. and Technol., 6 (1990) 953-7. 27. Beardsley, T., Scientific American, (Oct) (1992) 91-2.