Microstructural Aspects of Cemented Carbides

Microstructural Aspects of Cemented Carbides

6 Microstructural Aspects of Cemented Carbides Microstructural studies of cemented carbides are of great significance for understanding their propert...

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6 Microstructural Aspects of Cemented Carbides

Microstructural studies of cemented carbides are of great significance for understanding their properties and engineering performances. There are many factors which have direct bearing on the microstructure of cemented carbides. They are" 9 Basic chemical composition of hard and binder phase 9 Shape, size and its distribution of WC particles ~ Degree of intersolubility of carbides 9 Excess or deficiency of carbon 9 Variation in composition due to diffusion or segregation 9 Processing technology, e.g., milling, carburizing, and sintering including purity of raw materials 9 Coatings, if any, on cemented carbides Optical microscopy can be used successfully for quality control and for microstructural evaluations in scientific studies for coarse grained hard metals.t 1] Modem grades of cemented carbides very often consist of a carbide grain size < l lxm. In addition to that, in most technical grades, pores, impurities, binder phase regions, and carbide grains cannot often be adequately studied due to insufficient resolution of optical microscopy. This has led to the introduction of the thin foil electron microscopy 166

Microstructural Aspects of Cemented Carbides 167 technique which has been used extensively for studying deformation and fracture behaviour and the nature of precipitates in the binder phase of WCCo alloys.t21t31 Quantitative optical microscopy in cemented carbides was first introduced by Gurland,t4] and, later on, Exner and Fischmeister f51[61 extended it to electron microscopy.

1.0

REFRACTORY CARBIDE PHASES

Tungsten carbide has a highly anisotropic structure and therefore develops anisotropic crystal shapes during growth which can be described as flat triangular prisms with truncated edges.tl9] In technical hard metals, this shape is not fully developed due to coalescence and impingement with other crystals. However, most of the crystal sections observed in polished cross section can easily be interpreted by this equilibrium configuration.[3][19] Two crystallographic planes play an important role in the formation of tungsten carbide interfaces" the {10]-0} prismatic plane and the {0001 } basal plane, which are the main facets of WC crystals. Crystal defects are also observed in WC but the density is fairly low. This appears due to the deformation of the WC crystals during milling, and partly due to the residual stresses during sintering, which are accommodated by plastic deformation. Almond et al.tl2] studied the origin of WC substructure and the effect of processing on the microstructure of WC/Co hard metals. According to them, the milling of WC powders with cobalt reduces the polycrystallinity of WC powder and introduces a high dislocation density into some grains. In the sintered structure, WC grains retain their dislocation substructure. On raising the sintering temperature, the dislocation density decreases. Heavy ball milling introduces a high dislocation density into WC powders, destroys polycrystallinity and produces fines. In the sintered structure, dislocation densities are higher and there are also less dislocation-free grains than in the sintered structure from a conventionally milled powder. The WC grain size and distribution are also observed to be increased in the sintered structure from heavily milled powders. A report on textured WC-6Co cemented carbides (2 ~tm average grain size), such that C-axis of WC was perpendicular to the base of the pressing die, has been made by Luyckx. [71] The idea was to see its improved effects on mechanical properties.

168 Cemented Tungsten Carbides The microstructure of (W, Mo)C-Co hard metal was found to depend on Mo/W ratio, such that, with increasing Mo content, a faceted carbide grain structure changed to a more irregular shaped globular, to an acicular and platelet carbide grain. [79] During grain growth, an epitaxial growth on the coarser grained carbide nuclei irrespective of their composition takes place. The driving force of solution precipitation mechanism was primarily dictated by the difference in carbide grain size. Typical TEM pictures of WC-10Co cemented carbide sintered at 1425~ in hydrogen are shown in Fig. 1 which show straight faceted WC grains along with dislocations and also WC/WC interface.

Figure 1. TEM micrographs of WC-10 Co cemented carbide (sintered at 1425~ in H2) showing (a) straight faceted WC grain, (b) dislocations in WC and (c) WC/WC interface. (Cont. next page)

Microstructural Aspects of Cemented Carbides 169

Figure 1 cont.

As already described in earlier chapters, many grades of WC-Co cemented carbides contain cubic carbides, e.g., TiC, TaC and NbC. In the over all microstructure, in contrast to tungsten carbide, these appear in rounded shape. Because of differential diffusion effect, they often show a 'cored' structure on etching. Figure 2 shows the microstructure of WCMC-Co cemented carbides showing round cubic monocarbide. Different possible configurations of WC grain boundaries with or without a continuous cobalt film, or with segregated cobalt phase have been reported. [531154] Lattice fringe imaging [52]and high resolution electron microscopyt5al confirm the contiguity of some grain boundaries which do not contain any cobalt. Vicens et al. [451 classified two types of grain boundaries between two WC crystals: one as twist grain boundaries (where the crystals get rotated around (1010) and are parallel to prismatic plane), and the other as asymmetric tilt boundaries (where the grain boundaries get rotated around (1120)). The orientation relations between WC and cobalt at WC/Co interface have still not been fully studied. Earlier work on STEM by Sharma et al. [371 and on field-ion atom probes [241 confirmed the presence of high cobalt concentration at the carbide grain boundaries, which indicated the presence of approximately 20 A thick cobalt rich layers. However, on the other hand Hagege et al.,t141 after TEM studies, observed that at least some of the grain boundaries

170

Cemented Tungsten C a r b i d e s

provide a direct transition between contiguous WC crystals, which, pair wise, exhibit relatively good lattice coincidence and are able to accommodate a small mismatch of low index atomic planes by the defect structure of the grain boundary.

Figure 2. Optical micrograph of WC-8 TiC-12 Ta(Nb)C-9 Co cemented carbide (WC grain size -- 3 I.tm), magnification X1500 (courtesy Dr. H. Pastor, CERMeP, Grenoble).

2.0

BINDER PHASE

Cobalt is used most extensively as the bonding metal with WC based hard metals. This is present in the microstructure as a continuous thin film separating the carbide particles and is normally associated with high dislocation density and stacking faults.t72jt73] At least for the hard metals sufficiently rich in cobalt, a cobalt skeleton forms in the sintered products in addition to carbide skeleton. Hinnuber and Rudiger [46] explained that the high anisotropy of the hexagonal WC crystals leads to lower solid/solid interfacial energy along certain planes than the solid/liquid one, and thus leads to a direct contact between carbide grains.

Microstructural Aspects of Cemented Carbides 171 In WC-Co alloys, the binder phase is actually a dilute cobalt alloy containing tungsten and carbon as solutes and can exist in either of two allotropic forms~hcp or fcc. Both tungsten and carbon stabilize the fcc phase, [741169]which is the high temperature form, by reducing the transformation temperature. [65]t66] Carbon is more effective than tungsten. In most cemented carbides, the Co-W-C binder is present as a mixture of fcc and hcp structures. The ratio of the two is determined in bulk alloys by prior processing treatment and composition. The relative ease of interchangeability of the two forms is related to the low stacking fault energy of dilute cobalt alloys. For example, a small parallel array of stacking faults in the fcc structure represents a finite amount of hcp phase in the form of thin lamela.E64~ Zhengi,ts01 after quenching just after sintering the WC-11Co cemented carbide, found the retention of fcc cobalt in contrast to the as sintered structure, which had both fcc and hcp forms. Mixing during milling has significant influence on the distribution of cobalt. Insufficient milling results in a large cobalt pool in the microstructure and may cause porosity.t281E291 Cobalt distribution is also strongly dependent on carbon content, which seems to control its redistribution during heating to the sintering temperature.t211 A typical TEM picture of the binder phase showing stacking faults in WC-10Co cemented carbide sintered at 1425~ in hydrogen is exhibited in Fig. 3.

Figure 3. TEM micrograph of WC-10 Co cemented carbide (sintered at 1425~ in H2) showing stacking faults in binder phase.

172 Cemented Tungsten Carbides From the early history of cemented carbides, cobalt has been the traditional and predominant binder metal for WC, although nickel and iron are cited as alternatives. The use of cobalt in preference to iron and nickel is believed to be due to higher solubility of WC in cobalt. Secondly, cobalt has super comminution characteristics and hence a better distribution is achieved during milling, which subsequently, during sintering, leads to a desirable microstructure. Dawihlt621 suggested that nickel bonded alloys equivalent to cobalt bonded cemented carbides can be obtained by using nickel oxides as the starting powder, since it has comminution behaviour more akin to cobalt. Nickel addition in cobalt stabilizes the relatively ductile fcc cobalt phase, preventing it from transforming to the less ductile hcp phase during deformation. This has been taken into account in developing abrasion resistant cemented carbides.t611 The role of additives in cobalt binder has been investigated by a number of authors. Lisovsky et al.t641 in their study on the effect of RE on WC-11 Co cemented carbide, observed the binder composition as 71Co, 20.5Re, 8W and the balance C. The addition was found to considerably reduce the stacking fault energy in the cobalt and contributed in flCo ~ eco martensitic transformation. Chenguangt631 studied the effect of rare earth (RE) addition on WC8Co and WC-14TiC-8Co cemented carbides and found that the volume fraction of fcc cobalt (60%) in straight alloy increased to as much as 97% after RE addition. In addition, the melting point of binder was decreased by about 30~ It was noticed that RE had no effect on the distribution of cobalt in WC-TiC-Co cemented carbide. A similar effect on cobalt was also noticed by Zhonglin.t781

3.0

ETA PHASE

The carbon deficiency in WC-Co alloys leads to the formation of a third phase (rl phase) other than the carbide and binder phase. 'Eta' phase is a ternary compound of tungsten, cobalt and carbon. It can exist in two forms, either MrC carbide ranging from Co3.2W2.8C to Co2W4C,[24] or MIEC carbide of fixed composition Co6W6C.[25] Both M6C and M~2C are indistinguishable physically and M6C may represent a metastable form which can undergo in-situ decomposition (M6C --~ ~ + MIEC + WC) by a fairly sluggish reaction. [251 However, in commercial tungsten carbide

Microstructural Aspects of Cemented Carbides 173 alloys with a relatively fast cooling rate, presence of M6C is more probable than that of M12C.[26] With minor carbon deficiency levels, 'r I' phase is not produced at the sintering temperature but forms on subsequent cooling (i.e., WC + liq. -~ WC + 1"1+ liq.), and in doing so occurs as isolated concentrated areas in which considerable volumes of WC and cobalt binder phase are locally consumed during its growth. Rounded partially dissolved WC grains are often associated with such 'rl' phase areas. Further cooling into the solidus range can produce peritectic decomposition of this '11' phase (liq. + 11 ---) WC + 13), and the final quantity of '11' phase retained at room temperature is determined by cooling rate and carbon content. [27] Very minor carbon deficiency levels, for example, lead to the complete disappearance of 'q' phase on cooling, whereas '11' phase is retained at higher carbon deficiency levels irrespective of the cooling rate. Finally, with fairly extreme carbon deficiency levels '11' phase is also present at the sintering temperature and tends to be retained after cooling. The morphologies of the 'I"1' phase range from finely dispersed particles at low carbon deficiency to large areas of massive '1"1' in highly carbon deficient alloy (Figs. 4 and 5). Final '1"1' phase distributions and morphologies are therefore controlled by the carbon content developed during sintering and cooling. The carbon content, dependent on the initial carbon and oxygen content, is also controlled by the carbon activity in the sintering furnace atmosphere. During resintering, eta-phase changes chemically to cobalt and WC after being recarburized. [561 The C-deficient regions attract cobalt which flows until eta-phase has formed and remains trapped until a carbon balance is restored. Carbide grain growth occurring in enlarged cobalt lakes is ascribed to the recrystallization of tungsten from solution during recarburization of eta-phase. Roux [56] successfully demonstrated the above features by fine TEM studies. The possibility of substitution of cobalt binder by other iron group metals has been highlighted in Ch. 5. Nickel is said to be more prone than cobalt to the formation of 'eta' phase, where Ni-W-C system is deficient in carbon.[6ol When cobalt was completely replaced by an Fe-Ni binder, iron showed a tendency to form 'eta' phase. [591 Addition of free carbon to Fe-WC alloys exceeding the stoichiometric amount required for tungsten carbide can inhibit the formation of large grains of the brittle 'eta' phase.[48][49] Moskowitz et al. [47] found that, for iron substitution, presence of 'eta' or graphite phase is possible due to variation in carbon content in the binder. Cemented carbide is free of both 'eta' and graphite when the carbon content of iron is between 1.4-3.0%. [50]

174 Cemented Tungsten Carbides

Figure 4. Microstructure of cemented carbide (85.5 WC-12 Co-3.5 Cr) showing well dispersed eta phase, (WC grain size 31.tm), Murakami etching (X1500) (courtesy Dr. H. Pastor, CERMeP, Grenoble).

!ii~!i!!ii!i.I!i!"if!!i!!!iii!!ii!iiiii~!!iliiiiiiiiiillill!iii!ii!ii!!i!iiiiiiiiii !!! i! !iiiii!i!!ii! !i!i! ? !~

?ii

i~ i ~

i?

ii

i

~IIIIII!II~/i i?/??i! i iiiiiiiiiiiiii!~iiiiiii! iii iiii! iil i ilii i

Figure 5. Microstructure of cemented carbide showing strong eta phase, light Murakami etching (10 s), X100 (courtesy Dr. H. Pastor, CERMeP, Grenoble).

Microstructural Aspects of Cemented Carbides 175 CVD coated cemented carbides exhibit the presence of 'eta' phase, which nucleates on WC grains in contact with cobalt binder phase. It may contain both partly dissolved WC and cobalt grain. Both MI2C and M6C type phases have been reported to form during CVD of WC-2(TiWTaNb)C6Co cemented carbide at 1000~ t58J This phase was seen as totally featureless, although sometimes it contained a few dislocations.

4.0

PRECIPITATES

Precipitates generally observed in the binder phase of cemented carbides are non-metallic impurities, graphite, carbides, and intermetallic compounds precipitated during cooling or heat treatment in the solid state. According to Gruter, E321in technical WC-Co hard metals, a three phase region (11 + WC + liquid) exists, even if the carbon content corresponds to stoichiometric WC. In quenched alloys, this phase retains and lowers the magnetic saturation. But if the alloy is cooled slowly, the 13 phase reacts with the carbon supersaturated in the binder to form WC and C03W. Grewe et al. [33] suggest that the eta-carbide W6Co6Cprecipitates from the supersaturated solution after cooling from the sintering temperature initially as Co2W4 C, which changes composition to the more cobalt rich variations, Co3W3Cand C04W2C, and finally decays into CoaW. From the phase diagram of W-C-Co system, this intermetallic should not be present at any temperature. Local concentration gradient, however, can arise due to slow diffusion of tungsten in cobalt. It is more likely that the transformation of the cubic into the hexagonal phase is the reason for the formation of the intermetallic due to the sudden decrease of tungsten solubility in the binder phase.t~sJ Literature [2]/341 is available on the effect of heat treatment on the structure and properties of cemented carbides. The presence of C03W simultaneously with hexagonal cobalt modification in heat treated WC-Co alloys has been reported, t34H361 After heat treatment, hardness increases, but T.R.S. decreases due to the precipitation hardening and the decrease in ductility of the C03W containing binder phase.t341E351 A study of Nishigaki et al. [36Jon WC based cemented carbides with Co/Ni/Cr/A1 binder suggests that both hardness and toughness of hard metals can be increased by strengthening the binder phase through y' precipitation.

176 Cemented Tungsten Carbides 5.0

PORES

Though modem techniques of cemented carbide production provide high density hard metals, pores can hardly be avoided by normal sintering. A number of workers have studied the effects of porosity on mechanical properties. But the influence of micropores is less known since their influence is overshadowed by that of the macropores and large WC grains. However, these flaws play an important role in fracture initiation and are having considerable importance in quality control. Pores due to different causes have different appearances, and a full range of shapes and sizes may exist in poorly produced grades. Lardner et al.E281and Amberg et al. [29] reviewed the causes of residual porosity in sintered hard metals. The most important causes of the porosity are the following: 1. Impurities; either originally present in the ores or intentionally or unintentionally added during the cemented carbide production. 2. Inhomogeneities due to insufficient comminution and mixing of the carbides, cobalt, and lubricant. 3. Low carbon content leading to the formation of carbide, which prevents the binder phase from filling out the carbide skeleton. 4. Entrapped gases which are caused by poor sintering conditions. 5. Incorrect sintering temperature which leads to an incomplete filling out of the carbide skeleton by the binder phase. The common method of examining the porosity is by the means of optical microscopy, using the ISO norm 4505, [301 which is described in the chapter on 'Testing and Quality Control.' The 'A' porosity and 'B' porosity are mainly due to causes (1) and (5). 'B' porosities originate frequently from impurities resulting from processing and are not evenly distributed. Pores larger than 25 ~tm can occur for the same causes as 'B' porosity, but more likely are caused by factor (2). The effect of various trace elements on the formation of pores in sintered hard metals has also been reported.t671 The results obtained by secondary ion mass spectroscopy (SIMS) suggest that B-type pores are always lined by elements such as Si, Ca, AI, Mg. K. Schuler et al. 168] investigated the impurity-particles reaction and the formation of different

Microstructural Aspects of Cemented Carbides 177 types of macropores. The authors reported that the reaction gases, CO and CO 2, are responsible for the development of such pores. 'C' porosities result from a too high carbon content in the powder and/or unsuitable dewaxing or sintering conditions. It may form during liquid phase sintering or precipitate during slow cooling. In the first case, large crack-like features are visible in the microstructure, while small black spots occur in the second case. t311

6.0

MICROSTRUCTURAL

PARAMETERS

Quantitative characterization of cemented carbide microstructure usually involves the measurement of: 9 Carbide grain size distribution and mean carbide size 9 Carbide contiguity 9 Binder mean free path 9 Volume fraction of individual phases One of the most convenient tools has been semi-automatic analysis based on magnified light optical micrographs (LOM) of etched specimens. Currently, automatic image analysis using scanning electron microscopes (SEM) is very common. Table 1 shows the basic differences between the methods. The ability to detect very thin binder ligaments and small carbide grains is considerably lower in LOM as compared to SEM. However, the high magnification used in SEM, in order to image the grain boundary network and the fine carbide grains, makes it difficult to obtain a representative result for vary coarse carbide grains. An authoritative discussion on microstructure of cemented carbides using SEM based automatic image analysis is made by Nordgren. [571

6.1

Grain Size

The grain size of carbide phases is the most carefully controlled variable besides carbon control in the production of cemented carbides. Right from ore processing to final sintering, each step of production has an influence on the final grain size of the sintered product. Up to milling, the changes of grain size and size distribution by different process parameters have already been discussed in previous chapters. Once the milled powder

178 Cemented Tungsten Carbides is produced, the only remaining operation that can effect the grain size is sintering. Table 1. Principal Differences Between Two Different Methods of WCCo Cemented Carbide Structure Characterization.

SEM

LOM

Microscopy

Back Scattered Electrons ~ atomic number contrast 9 Channeling contrast at low accelerating voltage

Reflection mode

Magnification

x500 to x 10000

x 1000, micrographs magnified to x2500

Specimen surface

9 Polished (for Co measurement) ~ Etched (for WC measurement)

Etched

Image analysis

Automatic detection of grain boundary network

Manual marking of intercept chords and phase boundaries

The state of the WC crystals in the milled WC-Co premix is also important for grain growth response. Cherniavsky[ 551 elaborated that two WC powders of similar size distributionm but milled (96 h) from two separate starting sizes obtained by controlling the original tungsten particle size and carburizing conditions, showed different responses during sintering. In the original case, the particle size was small, the growth kinetics were not fast, since, here the milling merely separated the pseudomorphic aggregate of fine particles. In the case of severely fragmented WC particles, although of similar size distribution, the growth kinetics were pretty fast. Although the main driving force for grain growth is reduction in interfacial energy, still the exact nature of the growth mechanism is yet to

Microstructural Aspects of Cemented Carbides 179 be understood. There are two distinct modes of grain growth in cemented carbides, [71 namely: 9 Coalescence process 9 Solution/reprecipitation process One school of thoughtt81E91 says that grain growth takes place predominantly due to coalescence, while according to others, E7J[51the solution/ reprecipitation is mostly responsible for grain growth. The mechanisms of the coalescence process are of the same nature as that of solid-state grain growth. It involves the formation of solid grains contact and the subsequent elimination of the contiguous boundaries by the movement of grain boundaries or by the relative rotation of the grains. As soon as the eutectic liquid is formed, the fine fraction of WC is mostly affected. Due to its high surface energy, there is continuous solution of fines and at the same time deposits by reprecipitation on the existing coarser WC grains. Solution/reprecipitation increases as the temperature increases. As the sintering temperature is reached, the fines in the powder disappear rapidly and the coarser particles tend to grow. Once this stage is over, and an average grain size is attained, further grain growth is slow. But the fine carbides are extremely sensitive to this effect and prone to exaggerated grain growth. The materials with a wide range of grain size are, therefore, more sensitive to grain growth because of the higher surface energy of fines. On the other hand, the materials with uniform grain size (> 3lxm) show little change in grain size over a wide range of sintering conditions. Composition of cemented carbides has also an influence on grain growth. Grain growth during sintering is dependent on carbon content because this alters the characteristics of liquid phase markedly. A slight increase in carbon content results in more rapid grain growth. ~1~][7~ Cobalt content also plays an important role in grain growth, in this type of composites, grain growth is strongly dependent on the amount of solvent phase available to allow the transfer of fine to coarse particles. This effect has been discussed by Exner and Fischmeister, t51 according to whom in hard metal containing cobalt above 10 mass%, solution/reprecipitation is the main mode of grain growth. At low cobalt content and especially with finer carbides, where carbide to carbide grain boundaries are more prevalent, other grain growth mechanisms become evident. This can lead to exaggerated grain growth, which is not generally encountered with higher cobalt content. Carbon control in WC powder as well as uniform distribution of cobalt in green compacts are also important to prevent the formation of anomalously large WC grain in WC-Co cemented carbides, f4~

180 Cemented Tungsten Carbides Most of the impurities in the starting powders seem to have no notable effect on normal carbide crystal growth in WC-Co alloys or to reduce growth tendency. [13] However, very often the grain growth inhibition is exercised in practice by adding other transition metal carbides, especially those of V th group elements. [11] Grain refinement has been reported to occur with Li, Na, K, Ni, Sn, V, Mo, Cr, Si, and A1 while Ni, P, and C have reported to increase the tendency for normal grain growth. Impurities containing P are known to promote discontinuous growth. [15] Further discussion on fine grained cemented carbides will be discussed in a later chapter. Generally, two methods are used for the measurement of grain size on planar surface./16] In the Jeffries method, a fixed area A is superimposed on the image and the average area of each grain A is calculated from:

Eq. (1)

Total number of grains within fixed area + ~-= 1/2 grain intersected by boundary A

The Jeffries grain size is defined as ~/A,. The Heyn grain size is obtained by lineal analysis. A line is placed across the sample at random and the arithmetic mean linear intercept, or Heyn gains size 1, is derived from: Eq. (2)

-

L

1 =-N

where L is the total length of line across WC and N is the number of WC grains traversed. The size distributions of the tungsten carbide phase in sintered alloys is mainly determined by the milling conditions and the initial size distribution of the carbide powders.t171 Usually, the linear intercepts in the carbide grains show a distribution very close to a logarithmic Gaussian distribution. [18] The range of distribution widens due to the fragmentation of coarse powders while milling, or due to mixing of carbide powders of widely varying mean particle size. The size distribution widens and loses its log-normal shape when discontinuous grain growth occurs. Exner et al. [5] showed that when the distribution of carbide intercept lengths is plotted against logarithmically sizeA classes, a normal distribution is followed:

Eq. (3)

F(lnx) :

1 1 ,~ [ (lnx-ln ) ld(lnx) lno'x .exp - 2(lno' )

Microstrucmral Aspects of Cemented Carbides 181 where ,~ is the mean value, t~x is the standard deviation, and F(ln x) is the relative number of intercepts with sizes less than or equal to x. Figure 6 shows the variation of different microstructural parameters of sintered WC-Co cemented carbides and other modified compositions. The rationale for this has been described in Ch. 5. It is apparent that the average carbide grain sizes of composition A and D are similar. [44~ Similar types of investigations on WC-Co modified by TiN, Ti(C,N), Mo C, and Ni have been carried out by the same authors. [421143] The results on coarsening WC grains when TiC (20%) was introduced in WC-10Co cemented carbide, were in contrast to those of Zhenhui[391 who found grain refinement when added in a rather small quantity of 3-6%. It was observed that in the case of an additive being in TiC-WC solid solution form, the grain size of WC becomes finer with a decrease in the saturation degree. The effect of a supersaturated solid solution on coarsening during sintering was attributed to its decomposition, with subsequent dissolution of WC in cobalt and precipitation.

E2.0-

8u ~ .-ffl.2, w ~0.4 ,f ~ ~0.5 i~ ~0.3 C

o ~0.1 >,

1.0'

(c)

'0

.-~

0.6

C

8

0.2 Alloy A B A - WC 10 Co

C

D

B - WC 8TiC 1 2 ~ C - WC 8TiC6Co 6Ni D- WC 6TiC 2MozC 6(:o 6Ni

Figure 6. Different microstructural parameters of straight and complex tungsten carbide based hardmetals.t441

182 Cemented Tungsten Carbides A detailed systematic investigation on the role of TiC, TaC, and (W,Ti)C addition on the carbide grain size of WC-6Co (10.08 vol%) was reported by Buljan et a1.[38] Figure 7 shows that the most pronounced effect on grain growth inhibition was observed at low levels of additive (___1 vol%). Addition at higher levels had little effect on further grain growth inhibition, and, in the case of TiC some grain growth was noticed. Although the initial particle size of WC powder was very coarse (13.5 I.tm), attrition milling was adopted for preparation, followed by vacuum sintering (14 Pa).

1.1

1.9 " A T a C

N

1.7

-O _ n

TiC W~TiC

~~ 0.9 N {3 ""

E

9--"

U3

~ 0 ' ~ 0.7 Z~

E o ~

1.5 0.5

C9

J

I

9

I

0,4 0.9 v/o Additive

LI

1.3 N

,,a , . .

E 1.1 O

Z

0.9

0.7 ../.~

=l_

l

i

I

l

0

2

4

6

8

10

12

v/o Additive Figure 7. Average grain size versus cubic carbide concentration for TiC, (W,Ti)C and TaC containing WC-6 Co cemented carbides (1560~ sintering for TiC containing and 1540~ for TaC containing alloys).psI

Microstructural Aspects of Cemented Carbides 183 The modification of binder phase in WC-Co cemented carbides has been described in an earlier chapter. Nishigaki et al. [75] observed a slight retardation of grain growth in WC-27(Co/Ni)-0.9Cr cemented carbide when 1.2% AI was introduced for 7' strengthening. Upadhyaya and Basut761 like Nishigaki et al.t751 did not find any effect of Co/Ni ratio in the binder on the WC grain size of sintered cemented carbides. Nickel-molybdenum alloy binder with WC exhibited grain refinement in the carbide phase during sintering.t771

6.2

Contiguity

Tungsten carbide particles in sintered WC-Co alloys, whether from a continuous skeleton or not, is still debatable, tlSl Based on Gurland' s theory of particle contact,Warren and Waldront71 suggested that in the majority of hard metals, the carbide phase exists as a continuous skeleton both during and after sintering. Several other workersE~91t3~l also support the presence of skeleton structure in WC-Co hard alloys. There are still proponents of the view that all carbide grains are separated from each other by thin cobalt layers, but a modified skeleton theory is now more generally accepted.t221 A quantitative measure of skeleton formation is contiguity, which is defined as the ratio of grain boundary surface (WC/WC interface) to total surface (WC/WC + WC/Co interface). [231 Contiguity is thus a measure of the degree of contacts between carbide grains. It can be expressed by the equation" 2(N~.)~,~ Eq. (4)

C~ - 2(NL)~ ~ + (N~.)~

where (Nt.)aa and (NL)a# are the average number of intercepts per unit length of test lines with the traces of carbide-carbide grain boundaries and carbide-binder interface. Alternately, the contiguity can be evaluated using mean values of carbide size and binder width.HI1

Eq.

(5)

Ca

-- I -- ~'-carbideVbinder

184 Cemented Tungsten Carbides where

t-carbid e - -

mean intercept carbide size,

t--c~rbid~ = mean free path of binder Vcarbide, Vbinder -- volume fractions of the different phases (Vcarbid e -- 1--Vbinder )

Contiguity varies with cobalt content, sintering time, and also with temperature, but not so pronouncedly (Figs. 8, 9, and 10). [5][7][23]17~ Contiguity decreases with the increase of binder content[Tl which is attributed to the fact that the probability of the spatial coincidence of carbides decreases with the increase of binder content. At the start of sintering, contiguity is high and attains an equilibrium contiguity which is determined by the relative values of the interfacial energy between the solid and the liquid (YSL),and the contiguous boundary energy (Yss).[7]tl~ The effect of temperature on equilibrium contiguity is relatively small, however a decrease in contiguity is observed with increase in temperature.[7l

1.0

0.8

o O.6 ~

C

oo 0.4

0.2

I

1

J

0.1 0.2 0.3 Coba[t Volume Fraction

Figure 8. WC contiguity versus cobalt volume fraction.

0.4

Microstructural Aspects of Cemented Carbides 185

Sinterlng 10 3. w

. . .

.,

104 i

Time, 9 10s .,

106

,

I

I

0.4 - 5.5

0.3

-5.0 o

r

z >,

-4.5

~

~

c

.~ 0.2

,J

C

4.0 c0

8

U

Nc 0.1 -

A

3.5

Jkllllll

I

1 lllllll

1,1

llilJll

1

!

3.0

10 100 Sintering" Time, h

1

F'igure 9. Dependence of contiguity and continuity for WC-25 Co hardmetal sintered at 1400"C on sintering period.1701

!

I

'

I

0.5

6.0 0.4 5.5

U

= 0.3

U Z

5.0 "5

._

C

C 0 U

C

C

4.5 8

0.2 Nr 4.0

0.1

3.S I

.1

_|

13so 140o i4so Sintering Temperature, ~

Figure 10. Dependence of contiguity and continuity on sintering temperature for WC-25 Co hardmetals (sintering time 1 h).lT01

186 Cemented Tungsten Carbides The study of contiguity in cemented carbides is important as it effects the mechanical properties and gives an insight into the sintering mechanism. Exner and FischmeisterE5] attributed the decrease in contiguity mainly to grain agglomeration and grain boundary migration in the agglomerates, whereas Lee et al. [7~ showed that the decrease of contiguity is due mainly to continuous cobalt penetration of WC-WC grain boundaries. Deshmukh and Gurland [1~ studied stereological measurements in an attempt to study observed changes during sintering and found that various competing processes make it difficult to arrive at a simple conclusion. They observed considerable grain growth, but a decrease of contiguity and grain contacts during isothermal sintering. The average contact length per contact increased with an increase in the sintering period (Fig. 11). These results were in agreement with those previously reported in literature. [5]t7~

.

0.40

D

9

.

L

.

..

.

., . | .

Somple 4A (25 wt% Co-WC) Sintering Temp - 1450 ~ z~ Contiguity o Contoct Length/Contoct

E .I-I U

>.~

8

O

6

0 U c-

e . - .

:D

.4-1 ~

0.30 0'-

O tO

4

o 4,a o

O E: O tO

0.20 -

.,i.a

I

L___

0.1

......

I

10 S interlng Time, h 1

Figure 11. The change of contiguity and length of contact between WC grains with sintering period at 1450~ in WC-25 Co cemented carbide.l~0)

Microstructural Aspects of Cemented Carbides 187 6.3

Mean Binder Free Path

The mean binder free path is a measure of the thickness of the cobalt layer which depends on both the cobalt content and the particle size. It is defined by the arithmetic mean of the distance from one carbide/binder interface to the other, measured in the binder phase. Usually the mean binder free path is calculated from the following equation [5][2~ using an experimentally determined value of mean carbide grain size (dwc), contiguity of carbide phase (C), and volume fraction binder phase (Vco).

Eq. (6)

dc ~ =

l_~dw c Vco Vwc

I+C

where dco and Vwc are mean binder free path and volume fraction of carbide phase respectively. Roebuck et al.tl6l reported that a reasonably accurate estimate of the mean binder free path in two phase WC/Co hard metals can be obtained by this equation. In most of the cases, dco is determined by the above equation because they are usually too small to resolve in the microscope. For low binder fraction, it is possible to detect binder pools as individual objects on polished cross sections, and thus to evaluate the binder pool areas. However, at high binder fractions, the binder appears as a more or less continuous phase with few individual binder pools. Sometimes coercive force measurement is used as the indirect measure of the mean binder free path. It has been found that a direct and theoretically defined linear correlation exists between the reciprocal mean binder free path and the coercive force.IS] Due to microscope resolution limitation, complete knowledge about the distribution of binder free path is still lacking. Carbon content and milling seem to have a strong influence on the binder phase distribution. Carbon content seems to control the redistribution of the cobalt during heating to sintering temperature.t21] On the other hand, insufficient mixing during milling may result in a large cobalt pool and influence the distribution of the binder phase, t~71

188 CementedTungstenCarbides 6.4

Volume Fraction of Individual Phases

Due to the high contrast between WC and cobalt, the detection of phase boundaries can be made with high accuracy on a scanning electron microscope. It was found that for most of the hard metal compositions, at least for straight WC-Co, the measured values of binder phase tally vary well with the nominal composition.t571 The introduction of cubic carbide in WC-Co cemented carbides increases the demands on noise free, high quality imaging of the specimen surface. In such a case, due to the high solubility of WC into the ),-phase, the nominal values cannot be directly compared with values for sintered cemented carbides. This effect is more if the volume fraction of cubic carbide (T-phase) is high.

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