Microstructures and Properties of Submicrometer Carbides Obtained by Conventional Sintering

Microstructures and Properties of Submicrometer Carbides Obtained byConventional Sintering

Eduardo Soares,‡,§,† Luis F. Malheiros,‡ Joaquim Sacramento,§,¶ Manuel A. Valente,§

and Filipe J. Oliveirak

‡CEMUC/FEUP, Faculty of Engineering, University of Porto, Porto, Portugal

§DURIT – Metalurgia Portuguesa do Tungstenio Lda, Albergaria-a-Velha, Portugal

¶ESTGA – School of Technology and Management, University of Aveiro, Agueda, Portugal

kCICECO – Glass and Ceramics Engineering Department, University of Aveiro, Aveiro, Portugal

Submicrometer carbides (0.2–0.8 lm) are technologically

important new materials because they can reach very high

hardness with good mechanical resistance and excellent hard-

ness/toughness combinations, making them especially effectivefor the wear tools market. Sintering of these engineering mate-

rials requires a careful process control to reach the maximum

densification without damaging irreversibly the grain size, fun-

damental for reaching the expected properties. Especially forlarger wear tools, which are more complex to produce, further

know-how regarding sintering optimization is required to

improve their quality and consistency. In this work, gradesfrom WC 0.6 lm and WC 0.4 lm powders, combined with

varying contents of 0.7 lm Co powder (from 2 to 12 wt%),

were prepared by water technology to reach high-quality pow-

der mixtures. Thermal analyses were performed to obtain thepowders shrinkage profile and sintering temperatures. Pressed

powder samples were vacuum- and sinterHIP-sintered at differ-

ent temperatures on industrial furnaces and their relevant prop-

erties were analyzed. Some practical aspects, like compactionpressures, carbon content influence on sintered properties were

also addressed. Low Co (<6 wt%) grades presented excep-

tional high hardness levels (>2200HV30) and hardness/tough-ness ratios (2200HV30/8.7 MPa·m1/2) although requiring

special sintering cycles at higher pressures. The data collected

in this work will be further used to optimize the sintering and

maximize densification of these special grades using HIP –Hot Isostatic Pressing technology.

I. Introduction

THE WC-Co alloys are metal-ceramic composite materialscomposed of a hard phase of tungsten carbide grains

dispersed in a metallic Co binder. Hardmetal powders densi-fication results from a combined solid- and liquid-state sin-tering process of compacted powders.1 The main drivingforce for sintering is the interfacial energy reduction of thesystem achieved by reducing the free surfaces and interfacesarea on the green compact.2

Carbon balance (see Fig. 1 obtained from Thermo-Calc®)is a critical parameter for hardmetals as it determines the

liquid-phase temperature range, solubility of W in Co, andthe appearance of deleterious third phases, such as g phase(which is a carbon deficient phase either as M6C or M12C,with the possible combinations as Co3,2W2,8C to Co2W4Cand Co6W6C) or free carbon (carbon excess). The magneticmoment (r) of the binder phase is used as an indirectmethod to estimate the carbon content of sintered hardmet-als as r decreases in the three-phase region g-WC-Codue to the reduction of the magnetic phase when cobaltis incorporated in the nonmagnetic g phase, and itincreases in the two-phase domain WC-Co with the solubil-ity decrease of tungsten carbide in cobalt. The magneticmoment of pure Co is 16.1 lT·m3·kg�1, and for thetwo-phase region WC-Co, is usually in the range 12–15.6 lT·m3·kg�1.

Coercive force (HC) is another magnetic property conven-tionally used for quality control and characterizationof hardmetals. The HC increases for hardmetals withfiner WC grains and heterogeneous Co distribution; there-fore, it is used primarily to estimate the grain size and Codistribution.1

In the last decades, numerous research works started deal-ing with the reduction of the WC grain size to a submicrom-eter scale to produce WC-Co tools with enhanced properties,such as higher hardness and rupture strength for lowertoughness decrease, resulting in improved wear behaviorwhen compared with the conventional WC-Co grades. Hard-ness increase is associated with the well-known Hall-Petchrelation, taking into account, the difficulty in creating dislo-cations in smaller grains and their motion hindrance at grainboundaries. Rupture strength improvement can also be asso-ciated with the possibility of having lower sized criticaldefects as a result of using finer powders.2–6

Processing these powders with consistent quality at indus-trial scale presents several technological challenges due to theagglomeration and oxidation tendency which hinders fulldensification of the parts besides other defects and heteroge-neities which avoid obtaining the expected mechanicalcharacteristics.7,8

The sintering reactivity is higher for these powders, givingrise to faster shrinkage rates at lower sintering temperatureswith the drawback of a pronounced grain growth even atlow temperatures.9,10 Grain growth control is therefore acritical factor on submicrometer grades production7,9,11

which imposes the addition of grain growth inhibitors (vana-dium, chromium, and tantalum carbides) to the WC-Co mix-tures. This solution is quite efficient when dealing with fine(1–2 lm) or micrograin (0.7–0.8 lm) powders and it is indus-trially used without major restraints.12,13 However, whendealing with finer powders (� 0.6 lm), higher amounts of

T. Besmann—contributing editor

Manuscript No. 30224. Received August 23, 2011; approved November 10, 2011.Work supported by DURIT and Portuguese Science Foundation (FCT) under grant

BDE15663.†Author to whom correspondence should be addressed. e-mail: eduardo.soares@

durit.pt.

951

J. Am. Ceram. Soc., 95 [3] 951–961 (2012)

DOI: 10.1111/j.1551-2916.2011.05005.x

© 2011 The American Ceramic Society

Journal

inhibitors are required with the deleterious effect of shiftingthe maximum shrinkage rate to higher temperatures anddelaying the degasification processes of the compacts, result-ing in entrapped gases and final porosity. Their solubility(Cr3C2) and precipitation (VC) on the binder may alsoreduce alloy strength and toughness and delay the powdersshrinkage during sintering and so, their use should becarefully controlled.4,14

Reaching dense and high-quality products from these newmaterials with the expected mechanical properties requires abalanced control of powder processing, inhibitors content,and sintering process.

Recent laboratory scale state-of-the-art research on sin-tering near nanogrades are based on new technologies,such as Microwave or Spark Plasma Sintering, with highheating rates and short dwelling times to avoid high-tem-perature exposure.15–18 Despite the efforts, the industrialapplication of these technologies is still unpractical due toconstraints regarding parts geometry and size as well asdue to equipment restrictions (i.e., large size high-frequencymagnetrons).

The most conventional sintering equipments are based onvacuum sintering or vacuum-pressure sintering (sinterHIP).Due to its flexibility and reduced costs, vacuum sintering isstill a state-of-the-art technology while sinterHIP is presentlythe most important sintering process in hardmetal fabricationusing argon gas at pressures in the range 2–20 MPa. Hot-Isostatic-Pressing (HIP) furnaces with graphite elements mayalso be used for postsintering treatments at low (5 MPa) tohigh (150 MPa) pressures to produce parts with highermechanical resistance.19

At short term, the production of high-quality submicrome-ter-grained WC-Co materials for wear tools depends there-fore on a deeper knowledge of properties and sinteringmechanism when using conventional technologies.

In this work, submicrometer hardmetals of WC powderswith 0.6 and 0.4 lm and varying contents of Co were pre-pared by water technology. This technology has proved to bevery efficient in preparing high-quality WC-Co powder mix-tures and facilitates the sintering mechanisms as alreadydescribed in a previous work.8 In a first step, powder com-paction curves were determined. Powder samples were thenprepared with different C contents to evaluate its effect onthe sintered properties and define optimized carbon contentfor subsequent batches. Thermal analysis was performed tocharacterize the powders sintering behavior and so, sampleswere sintered at two temperatures (1370°C and 1460°C) invacuum and sinterHIP furnaces using conventional industrialsintering cycles. Finally, metallurgical and physical propertieswere collected and analyzed.

II. Experimental Procedure

(1) MaterialsHigh-purity (99.80 wt%) WC 0.6 lm and WC 0.4 lm pow-ders, doped with 0.5 wt% VC and 0.6 wt% Cr3C2, suppliedby H.C. Starck, and HMP 0.7 lm Co powders, 99.65 wt%purity, from Umicore (Brussels, Belgium), were used as rawmaterials.

Powders were initially controlled concerning their carbonand oxygen content (LECO CS 245 equipment, LECO,St. Joseph, MI), surface area (BET – Brunauer–Emmett–Teller method, in a Micromiretics Gemini 2370 V5, Micro-miretics, Norcross, GA), FSSS grain size (Fisher Sub-SieveSizer, Fisher Scientific, Pittsburg, PA), and particle size dis-tribution (Malvern Nano ZetaSizer, Malvern, Worcestershire,UK). The measurement of submicrometer WC grain size is adifficult task due to the powders agglomeration tendency anddue to the lower limit detection range of the traditionallyused technologies for conventional WC powders. As a goodpractice, the results obtained from different methods shouldbe compared with particular relevance to the results achievedby BET. Table I summarizes the powders’ main propertiesand Fig. 2 presents their SEM images.

(2) Grades PreparationThe WC-Co powder mixtures referenced according toTable II were prepared. Small (2 kg) powder batches wereattritor-milled in water with 2% paraffin wax in a stainlesssteel vial with hardmetal arm sleeves and 5 mm hardmetalballs of a submicrometric grade as grinding media. Millingtime optimization and water processing know-how resultedfrom previous work.8 Small amounts of amorphous carbonlamp powder from Degussa were added prior to milling, aspresented in Table II, to adjust carbon balance.

In an initial experiment, for determining the effect of car-bon variation over the two-phase WC-Co stoichiometricrange on the mechanical and magnetic properties, powdersamples from grades SG7 and SG24 were collected with dif-ferent carbon content by adding increasing amounts (from 0to 0.4 wt%) of amorphous carbon.

All slurries were vacuum dried, powders de-agglomeratedwith a 350-lm sieve and stored under a vacuum protectiveatmosphere.

(3) Compaction TestsThe effect of Co content (3.5 and 12 wt% Co) and pressurewas tested by compacting cylindrical powder samples(ø20 mm) at 50 MPa followed by compaction at six different

Fig. 1. Equilibrium phase diagram for W-C-Co system at 1450°C.

Table I. Raw Powders Grain Size, Specific Surface Area,Carbon, and Oxygen Contents

Content (wt%)

Powders

WC 0.4 lm WC 0.6 lm Co

Total carbon 6.13 ± 0.02 6.13 ± 0.02 0.30 ± 0.05Oxygen 0.27% 0.20% 0.90%Particle sizeBET surfacearea (m2/g)

2.64 ± 0.02 1.68 ± 0.02 4.18 ± 0.02

Fisher grainsize dFSSS

†0.51 ± 0.03 0.64 ± 0.03 0.71 ± 0.03

d50‡ (lm) 0.35 0.58 1.75d10‡ (lm) 0.16 0.28 0.55d90‡ (lm) 0.97 1.27 3.9Δd§ (lm) 0.81 0.99 3.45†Fisher Sub-Sieve Sizer.‡Laser scattering, Malvern Nano Zeta Sizer.§Δd = d90-d10.

952 Journal of the American Ceramic Society—Soares et al. Vol. 95, No. 3

pressures (from 50 to 350 MPa) in a rubber mold using aNational Forge CIP (Cold Isostatic Press). Green densitywas then measured for each specimen.

(4) Thermal AnalysisCylindrical samples from grades with 3.5 and 12 wt% Co,with ø4 mm 9 8 mm, were uniaxially compacted at150 MPa, a typical pressure for industrial fine WC grades,and subjected to dilatometric tests. Relative shrinkage andshrinkage rate were measured at room temperature up to1460°C at a heating rate of 10°C/min in vacuum; a home-built thermodilatometer with graphite furnace and elementswas used. Differential thermal analysis (DTA) was also per-formed to obtain the liquid-phase formation temperaturesusing a Setaram LABSYS at room temperature up to 1500°Cat a heating rate of 5°C/min in vacuum. The DTA analysiswas only performed for the higher Co content powders toobtain representative heat flow peaks.

(5) SinteringPowders from grades SG24 and SG7, with different carboncontents, were initially sintered in vacuum at 1460°C to eval-uate the effect of carbon on their physical and metallurgical

characteristics. The magnetic moment value which resulted inbetter properties was then defined as the quality control spec-ification to approve the following batches. Then, after uniax-ial compaction at 150 MPa, samples from all batches weredewaxed and sintered at 1370°C and 1460°C, in vacuum andsinterHIP, in industrial furnaces at a heating rate of 5°C/minand dwelling time of 60 min.

(6) Physical and Microstructural CharacterizationAll sintered samples were characterized according to hard-metal industrial quality control parameters: coercive force(Foerster koerzimat CS 1.096, Auburn Hills, MI), magneticmoment (LDJ model 702 magnetic multimeter), density(according to Archimedes’ method, following ISO3369),hardness (according to ISO 3878), and Palmqvist indentationtoughness (according to ISO 28079).

Polished and etched samples were observed by opticalmicroscopy (LEICA DM2500, LEICA, Wetzlar, Germany)and SEM (Hitachi-SU-70 FE-SEM, HITACHI, Krefeld,Germany). The presence of micro (A type) and macro (Btype) porosity was detected according to ISO 4505.

Grain size was determined following the linear interceptmeasurements for SG- and TG-sintered samples with 3.5 and12 wt% Co, according to ISO 4499-2:2008.

III. Results and Discussion

(1) Compaction BehaviorAfter obtaining a good quality powder mixture, compactionis the starting point for optimized sintering of submicrometerhardmetals. Due to their abrasiveness and small grain size,all hardmetal powders are generally considered difficult-to-compact materials. It is also well-known that the finer thepowder mixtures, the higher the compaction forces for avoid-ing extremely low green strengths and very high shrinkageratios (leading to components distortion and/or large finalporosities), and a better control of dimensional tolerances.Besides WC grain size, Co content also plays an importantrole. The compaction study was performed over samples witha quite common Co content (12 wt%) of many industrialhardmetal grades and a lower Co content (3.5 wt%) which ismore addressed to specific applications. These contents alsocorrespond to the maximum and almost the minimum valuesof the range studied and have served as an indicator for thebehavior of the intermediate Co content grades.

Figure 3 shows the effect of compaction pressure on therelative green density of four of the grades studied. Theslight higher relative densities achieved by SG7 and TG7grades may be related to their lower Co content. It is clear,having in account the obtained curves that, up to 55% rela-tive density, there is an apparent linear relationship betweenthe applied pressure and the compacts green density. At the

(a)

(b)

(c)

Fig. 2. SEM images of the raw materials used: (a) WC 0.6 lm;(b) WC 0.4 lm; and (c) Co 0.7 lm.

Table II. Composition and Milling Process Variables ofWC-Co Grades

Reference

Composition Milling

WC

grain size

(lm)

Co

(wt%)

Theoretical

density

(g/cm3)

Milling

time

(h:min)

Vol%

solids

C

added

(wt%)

SG4 0.6 2 15.31 4:45 15 0.08SG7 3.5 15.15 4:00 0.17SG11 5.5 14.92 3:30 0.19SG24 12 14.25 3:00 0.22TG4 0.4 2 15.25 5:30 14 0.09TG7 3.5 15.07 5:00 0.20TG11 5.5 14.85 4:45 0.22TG24 12 14.20 4:00 0.25

March 2012 Conventional Sintering of Submicron Carbides 953

highest pressure, we registered, however, a small deviationfrom linearity, toward smaller density values, an effect thatis more relevant for the coarser WC grades (SG). The pres-sure to be used should thus be as large as possible; how-ever, as one of the main aims of the work was to evaluatethe sintering and the properties under industrial processingconditions, all samples were compacted at a maximum pres-sure of 150 MPa. This may cause some problems of dimen-sional stability in larger pieces as the relative densities canbe as low as 47%, thus yielding linear contractions higherthan 21% (50% in volume) to reach a minimum of 98%densification.

(2) Effect of Carbon Content on Powder SinterabilityA group of samples was specifically prepared to presentafter sintering increasingly higher carbon contents, andconsequently, different magnetic moment values. Type Amicroporosity (<10 lm), coercive force, hardness, and KIC

were specifically analyzed. Carbon content influences primar-ily the WC-Co eutectic temperature and therefore theamount of liquid phase in equilibrium as well as the W solu-bility in Co. The graphics from Fig. 4 represent the variationof type A microporosity and coercive force as a function ofr for SG grades with 3.5 and 12 wt% Co. The data show

the effect of a balanced carbon control of the submicrometercarbide powders on their microporosity reduction. For the3.5 wt% Co SG grade, the microporosity volume fractionwas almost eliminated around 13.5 lT·m3·kg�1, whereas forthe 12 wt% Co grade, the microporosity was already totallyeliminated at 13 lT·m3·kg�1. Especially for submicrometerhardmetals, the carbon content must be carefully controllednot only for avoiding the precipitation of g phase or freecarbon but also to reduce microporosity.

In hardmetal conventional compositions, g phase starts toform usually below 11.5 lT·m3·kg�1.1 However, for submi-crometer hardmetals and as referred by some authors,19 thepresence of Cr3C2 shifts the “carbon window” or the r rangefor lower levels due to its solubility in Co. This fact explainswhy we already found traces of C type porosity above14 lT·m3·kg�1 in these grades. On the other hand, no visibleg phase was found on the lower MS samples, although sometraces of W2C phase has been later identified by X-rayDiffraction.

Coercive force presented a continuous decrease over the rrange. This behavior is explained by the lower amount of Wdissolved in the binder, grain growth, and improved Co dis-persion favored by the carbon content increase. Coerciveforce reduction reached over 80 Oe for SG24 samples and50 Oe for SG7 over the two-phase domain.

Fig. 3. Compaction behavior of grades: (a) SG24 and TG24; (b)SG7; and TG7.

Fig. 4. Effect of carbon content (evaluated by the Co magneticmoment) on the SG (WC 0.6 lm) samples’ microporosity andcoercive force variation after sintering with Co magnetic moment of:(a) SG24 grade and (b) SG7 grade.


There are also relationships between r and mechanicalproperties, such as hardness and indentation fracture tough-ness. The evolution of these properties as a function of r ofthe binder phase is depicted from the graphics of Fig. 5 forsamples SG7 and SG24. There are obviously differences inthe absolute values of hardness and of indentation fracturetoughness as a result of the different cobalt contents; SG7grade is much harder but less tougher than SG24. For bothgrades, hardness drops with increasing r over the intervalmeasured due to the effect of carbon content on graingrowth. The same reasoning can be applied to Palmqvistfracture toughness measurements which exhibit an oppositevariation. Fracture toughness variations are fundamentallydependent on the homogeneous distribution and plasticdeformation of the binder phase which, in turn, depends onthe amount of W dissolved.1

Results from Figs. 4 and 5 determined an optimized rvalue of 13.5 ± 0.5 lT·m3·kg�1 as a target to the remainingspecimens to be produced. To reach this value, initial carbonadditions were carried out as pointed on Table II.

(3) Sintering KineticsThe DTA performed on the SG24 and TG24 powders,Fig. 6, allowed to identify the beginning of melting and the

maximum formation of liquid phase. Two endothermicpeaks, corresponding to the liquid phase maximum forma-tion, were clearly identified, respectively, at 1332°C forSG24 powder and 1318°C for TG24 powder, while thebeginning of melting occurred around 1320°C for SG24 and1310°C for TG24. The difference between liquid-phase tem-peratures can be better explained by the higher content ofgrain growth inhibitors VC and Cr3C2 of the WC 0.4 lmpowders which are known to reduce the eutectic meltingformation.20

The shrinkage and shrinkage rates were obtained for thepowders with 3.5 and 12 wt% Co, Figs. 7 and 8. In the caseof the higher Co content grades, SG24 and TG24, theshrinkage peaks start at temperatures much lower than theWC-Co liquid formation (around 1320°C according to DTA,Fig. 6). Densification over 90% was reached before meltingoccurs due to maximum shrinkage rates attained in the range1195°C–1275°C, with a pronounced peak at 1210°C forTG24 powder and 1240°C for SG24 powder, as can beobserved in Figs. 7(a) and (b), respectively. For the lower Cocontent powders, SG7 and TG7, the sintering kinetics isslower. Solid-state sintering contributes for densification ofmore than 84% for SG7 and 80% for TG7, with maximumsintering rates at 1210°C and 1250°C, respectively; similarresults were obtained for the 12 wt% Co powders. However,for these grades, the temperature range for the maximumshrinkage rates is wider, between 1195°C and 1400°C, as it isgraphically depicted by the dilatometric curves in Figs. 7(b)and 8(b). When comparing with conventional WC-Co grades,submicrometer carbides exhibit a considerably faster solid-state sintering,20 reaching higher densifications before theliquid-phase sintering step due to the increased reactivity ofthe finer powders.

The effect of low Co volume fraction in slowing down thedensification kinetics is clear for both SG and TG WC pow-ders, as evidenced by Figs. 7 and 8, respectively. Figures 7(a)and 8(a) show that there is no skeleton effect for 12 wt% Copowders as, only above 90% relative density, densificationkinetics is substantially reduced. The opposite is true for the3.5 wt% Co powder.

This effect is related to the larger contact WC-Co areas atgreen state as well as with the dissolution of a higher amountof WC for higher Co content grades; these are two primarypremises of sintering mechanisms for submicrometer hard-metals (based on WC-Co dense agglomerates formationby viscous binder spreading at the solid state, mass trans-port, and pore elimination by solution and re-precipitationmechanism).

Fig. 5. Effect of carbon content (evaluated by the Co magneticmoment) of the SG (WC 0.6 lm) samples on hardness and fracturetoughness after sintering at 1460°C: (a) SG24 and (b) SG7 grades.

Fig. 6. Eutectic melting formation determined by DTA runs forSG24 and TG24 grades.


For lower Co powders, the WC-Co agglomerates forma-tion kinetics is slower as Co has to travel longer distancesto wet and cover different WC particles to form WC-Coagglomerates, slowing down the solid-state process. Onlyduring the latter stages of liquid-phase sintering, in aslower final densification, there is enough liquid to fill thereminiscent pores and sinter the WC-WC skeleton contactsnot covered by Co. This is more clearly visible for thefiner TG7 powder, where the formation of WC-Coagglomerates is even more difficult due to the higher WCsurface area, resulting in slower solid-state and liquid-phasedensification rates. A WC skeleton effect exists above 70%densification as depicted from Fig. 8(b) by the reduction ofthe densification rate.

(4) Densification and Physical PropertiesOne of the objectives of this work was to produce submi-crometer hardmetal samples with sintering technologies andthermal cycles usually employed for sintering conventionalgrades. The aim was to gain insight into their sinteringbehavior and to analyze possible process improvements lead-ing to better properties. So, four vacuum sintering and sinter-HIP cycles at temperatures of 1370°C and 1460°C,respectively, just above and quite above the liquid-phaseformation temperature (Fig. 6) were used to sinter the eightdifferent compositions referred in Table II.

The relative density of the WC-Co samples after sinteringunder four different cycles are compared in Fig. 9. Theresults confirm that the higher temperature (1460°C) is moreeffective in improving densification of the compacts duringthe liquid-phase sintering. Higher diffusion coefficients andbetter wettability of the WC grains by Co lead to well-filledpores and faster shrinkage rates. For a given temperature,the sinterHIP cycles yield denser materials than vacuum sin-tering. For example, by applying sinterHIP to all SG andTG grades with Co content above 5.5 wt% Co and also forthe SG7 grade, led to densification around 99%, whereas forTG7 grade only 98% densification was reached. Evenusing a sinterHIP cycle at the higher temperature, the verylow Co grades SG4 and TG4 only reached 95% and 93%densification, respectively. It is evident from this data thatthe lower is the Co content, the larger are the density differ-ences obtained with the four different cycles. According tothese results as well as the dilatometric data obtained, it isclear that the sintering of low Co powders is slower, requir-ing higher temperatures, higher dwelling times, and/or opti-mized cycles with new sintering approach to achieve fulldensification.

Results from coercive force measurements for all WC-Cosamples after sintering with the same thermal cycles arepresented in the graphics of Fig. 10. Coercive forcedecreases with Co content for both SG- and TG-sintered

Fig. 7. Relative shrinkage and shrinkage rate measured bydilatometric analysis for: (a) SG24 and (b) SG7 grades.

Fig. 8. Relative shrinkage and shrinkage rate measured bydilatometric analysis for: (a) TG24 and (b) TG7 grades.


samples, the latter exhibiting larger values for the same Cocontent. Besides, the higher temperature sintering originateshardmetals with lower coercivity values than the lower tem-perature cycle (for instance, for TG24: 330 Oe at 1370°Cand 283 Oe at 1460°C). For the same temperature and Cocontent, slightly larger coercive force values were obtainedwith sinterHIP than for samples sintered in vacuum.Higher temperature promotes the growth of the WC grains(which starts already at the solid state by diffusion), disso-lution, and thin film migration. Grain growth duringliquid-phase sintering stage may be considered as anOstwald-Ripening process, with the reduction of the totalWC/Co interface area as its driving force. Smaller WC par-ticles dissolve due to their higher dissolution potential andre-precipitate after diffusion through the binder on thecoarser grains.

To explain further the initial indications provided by thecoercive force values, linear intercept measurements ofthe WC grains were performed in SEM micrographs of themicrostructure of sintered samples at 1460°C and 1370°C bysinterHIP sintering. The data presented in Table III corrob-orate that for the same Co content, sintering at 1460°Cresults in a larger grain size than sintering at 1370°C, andconsequently, in lower coercive forces as determined. Thehigher grain size distribution, reflected by the Dd valuesobtained for sintering at 1460°C, testifies the grain coarsen-

ing. The effect of Co on grain growth and thus on the coer-cive force is also evidenced by the larger grain size of thehigher Co content samples, irrespectively of the raw WCgrain size.

(5) Mechanical PropertiesThe hardness and indentation fracture toughness of the SGand TG samples, Figs. 11 and 12, respectively, are functionsof the relative density reached, i.e., of the sintering tempera-ture and technology used, and of the Co content and WC

Fig. 9. Relative final density after different sintering cycles of: (a)SG (WC 0.6 lm) and (b) TG (WC 0.4 lm) grades.

Fig. 10. Coercive force measurements after sintering cycles of: (a)SG (WC 0.6 lm) and (b) TG (WC 0.4 lm) grades.

Table III. Arithmetic Mean Linear Intercept dWC of

SinterHIP Sintered Samples

Sample

1370°C 1460°C% grain growth

1370°C to 1460°Cd50 Dd† d50 Dd†

SG24 0.49 0.89 0.73 1.15 33SG7 0.42 0.68 0.66 0.91 36TG24 0.36 0.79 0.59 1.09 39TG7 0.25 0.71 0.45 0.93 44

†Dd = d90-d10.


grain size. Hence, as expected, the higher the Co content, thelower is the hardness of the hardmetal grades.

For a given Co content, higher temperatures yield sinteredsamples with lower hardness as a consequence of enhancedgrain growth, Table III. The larger grain size of SG gradesalso explains its lower hardness in relation with the TG sam-ples, for the same Co content. The sinterHIP cycles slightlyincrease the hardness as a result of the improved final rela-tive density as depicted in Fig. 9. The sintering cycles usedare clearly not sufficient for full densification of the 2 wt%Co powders, for both WC powders, as its hardness is lowerthan that for the 3.5 wt% Co grades.

The mechanical properties are strictly related to the densi-fication of the alloy (Fig. 9) and grain size (Fig. 10 andTable III).

Data for the Palmqvist fracture toughness are presented inFig. 12. Due to insufficient densification, data for the 2 wt%Co grades are not presented. For dense hardmetals, the frac-ture toughness is a function of Co content and WC grainsize, following a trend opposite to the hardness results. Thelarger the Co content, the higher is the fracture toughness.Moreover, for the same Co content and sintering cycle, SGsamples present higher fracture toughness. The resistance ofWC-Co materials for fracture propagation results essentiallyfrom their ductile phase and for the same Co content; grades

with a smaller grain size present a thinner intergranularmetallic films and thus indentation cracks propagation is eas-ier through them (intergranular fracture) or through the lessresistant WC grains (transgranular fracture).

Higher sintering temperatures display larger fracturetoughness values for both SG and TG grades, clearly indicat-ing that a better mechanical integrity is achieved, possiblydue to a better Co distribution and higher quality interphaseand intercarbide boundaries. When comparing vacuum sin-tering with sinterHIP technologies at the same temperature,only marginally better results were found for all the sinteredgrades by sinterHIP.

(6) Sintered MicrostructuresThe effects of the type and temperature of the sintering cycleon the microstructures of SG and TG hardmetal sampleswith 12 and 3.5 wt% Co are shown in Figs. 13–16.

In general, sintering at higher temperatures resulted inhigher quality microstructures concerning Co distribution,although with the drawback of a pronounced grain growth(as previously discussed). This is valid not only for SG andTG grades but also for both Co contents. In the SEMmicrographs of Figs. 13(a) and (b), abnormal grain growth isidentified for SG24 hardmetal grade, with large WC grains

Fig. 11. Hardness measurements after sintering cycles of: (a) SG(WC 0.6 lm) and (b) TG (WC 0.4 lm) grades.

Fig. 12. Palmqvist toughness measurements after sintering cycles of:(a) SG (WC 0.6 lm) and (b) TG (WC 0.4 lm) grades.


reaching up to about 1.5–2 lm. The distribution of WCgrains for the same grade sintered at 1370°C was muchlower, within a narrow range of 200–600 nm, with a smallerpercentage of abnormally large WC grains. Higher Co con-tents correspond to larger volume of liquid phase thatenhances dissolution and re-precipitation processes, responsi-ble for both faster densification and for the grain growtheffect as well as for the presence of abnormal WC grains. Asevidenced by the SEM micrographs of Figs. 13–16, hardmet-

al grades with Co content higher than 3.5 wt% could be suc-cessfully sintered with low porosity level and good Codistribution at both temperatures and sintering technologies.Nevertheless, a higher density of Co pooling areas wasdetected for TG24 sample sintered in vacuum at 1370°C.

For low Co grades, sintering at 1460°C resulted generallyin an improved Co distribution and lower porosities (<A04for SG7 or A06 for TG7). For these grades, sintering withsinterHIP resulted in more homogeneous microstructures,

Fig. 13. SEM microstructures of SG24 samples sintered by: (a) sinterHIP at 1460°C; (b) vacuum at 1460°C; (c) sinterHIP at 1370°C; and(d) vacuum at 1370°C.

Fig. 14. SEM microstructures of SG7 samples sintered by: (a) sinterHIP at 1460°C; (b) vacuum at 1460°C; (c) sinterHIP at 1370°C; and (d)vacuum at 1370°C.


independently of the temperature. On the other hand, sinter-ing at 1370°C resulted in worse Co distribution and B-typemacroporosity, especially for vacuum sintering and TG7samples, as is testified by the SEM micrograph in Fig. 16(d).

IV. Conclusions

This article presents an overview of the densification processof submicrometer carbides with varying contents of Co

binder based on current industrial technologies and conven-tional sintering cycles. High compaction pressures(>200 MPa) lead to green densities above or near 50%. Withdilatometric analyses, shrinkages peaks were found at thesolid state between 1200°C and 1250°C, with over 90% solid-phase sintering for 12 wt% Co grades and 80% for 3.5 wt%Co grades. The conventional industrial technology is still aviable way for producing such fine hardmetal grades by opti-mizing compaction pressures and thermal cycles in terms of

Fig. 15. SEM microstructures of TG24 samples sintered by: (a) sinterHIP at 1460°C; (b) vacuum at 1460°C; (c) sinterHIP at 1370°C; and(d) vacuum at 1370°C.

Fig. 16. SEM microstructures of TG7 samples sintered by: (a) sinterHIP at 1460°C; (b) vacuum at 1460°C; (c) sinterHIP at 1370°C; and(d) vacuum at 1370°C.


intensifying densification during solid-phase sintering tocontrol grain growth.

These new grades require a careful control of the carboncontent; slight carbon content deviations can lead to abruptproperty variations.

Even though conventional sintering cycles were used,much harder grades than current fine-grained hardmetals inproduction were obtained, especially in the case of TGgrades where values of 1700HV30 for 12 wt% Co and over2200HV30 for 3.5 wt% Co were reached.

Based on this data, work will be performed regarding thedesigning of balanced sintering cycles, maximizing the solid-state sintering step, and also using Hot Isostatic Pressing(HIP), particularly for the grades with 3.5 wt% and espe-cially 2 wt% Co for which very high hardness close to orhigher than 2400HV30 are expected.

Acknowledgment

The authors gratefully acknowledge the contributions of their collaboratorsand co-workers from DEMM – University of Porto, DECV – University ofAveiro, and DURIT.

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Microstructures and Properties of Submicrometer Carbides Obtained by Conventional Sintering

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