Activation Sintering

8/12/2019 Activation Sintering

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TablaTypical Sintering Temperatures for Activated Sinteringand Liquid Phase Sintering of Refractory Metals

155011001460

>1350

efractoryelement

Molybdenum

Meltingpoint,OC

Tungsten carbideThe t ungs ten carbide-cobalt eutectic

34102610

Activated sintering

AdditiveNickel

PalladiumNickel

Pal ad um-

temperature i s 1 3 2 O o C

making it possible to lower the sinteringtemperature substantially, a great deal of workhas been carried out in to the activated sinteringof tungsten and other refractory metals, par-ticularly with additions of Group VIII tran-sition metals. Much of this work has involvedthe use of platinum group metals which havebeen shown to be very effective as sinteringactivators. This paper reviews the publishedwork on th e effect of the platinum gro up metalson the activated sintering of tungsten and otherrefractory metals, in particular on the kineticsand mechanisms of sintering. The propertiesand micro structure of the sintered materials arealso examined. Finally, the scientific basis forthe beneficial effect of the platinum groupmetals in the activated sintering of the re-fractory metals is examined in terms of currenttheories and phenomenological models.Tungsten

Much of the early work on the activatedsintering of tungsten was carried out byBrophy, Hayden and co-workers 3-7). Theirinitial work focused on the sintering of tungstenpowder coated with nickel. They found tha t, onsintering at I IOOOC, he tungsten underwentrapid densification to more than 90 per centtheoretical density. Moreover, they found thatthe amount of nickel required to promote thisaccelerated sintering was roughly equivalent toa nickel coating thickness of about I atommonolayer 3). Nickel coatings thicker than this

Sinteringtemperature,OC

1100110012001200

Liquid phase sintering

AdditiveNickelCopperNickel

Cobalt

Sinteringtemperature,OC

did not produce any further enhancement; in-deed, there was a tendency for the sintering rateto decrease from the optimum. They also foundthat densification occurred in two stages, thesecond stage coinciding with th e onset of graingrowth in the tungsten (4).

In an attempt to clarify the mechanism ofactivated sintering, which they had earlierattributed to the activating metal acting as acarrier phase for the diffusion of tungsten to th einterparticle necks, Hayden and Brophyexamined the influence of ruthenium,rhodium, platinum and palladium additions onthe kinetics of sintering in the temperaturerange 850 o IIOOOC 7).

As in their previous work, the platinumgroup metal sintering additives were added tothe tungsten powder in the form of aqueoussolutions of salts chlorides and nitrates) in therequisite amount; this was dried at I ~ oO C ndprereduced in hydrogen at 80o0C to form ametallic coating on the tungsten powder.Sintering was carried out under hydrogen.

For all the platinum group m etals examined,Hayden and Brophy found that a minimumlevel of platinum group metal was required topromote full activation, see Figure I , as hadbeen observed in the case of nickel, and thatlarger amounts did not produce any furtherenhancement. Interestingly, palladium was themost effective element; this is clearly illustratedin Figure 2 which shows the temperaturedependence of shrinkage for each platinum

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g o 1 l 'o 2.00.002

310 4AMOUNT OF A DDI T I V E , we igh t per c e n tFig. 1 During the activated sintering oftungsten the linear shrinkage is dependentupon the activator, and the amount added;a minimum level b e i i equiredtopromotefull aetivation. Data from Hayden andBrophy (7).Sintered for 1 hourPalladium, 95OOCRuthenium, 1100O CPlatinum, 1 100O CRhodium, l l O O C

0.1.

:05.2 0.02-5 0 01

-4WzK-

group m etal additive after sintering for I hour.Ruthenium was the least effective element.Significantly, the authors found that palladiumwas better than nickel in promoting densi-fication. For example, the densities of samplessintered at 110o0C or 30 minutes and 16 hourswere 93.5 and 99.5 per cen t, respectively, in thecase of palladium in tungsten compared to 92and 98 per cent, respectively, for nickel intungsten. Untreated tungsten would only bepresintered at this temperature.

Analysis of the sintering kinetics in term s ofthe process controlling mechanism in theircarrier phase model of activated sintering-which applies also to liquid phase sintering-showed that for all the platinum group metalsexamined, the sintering rate was not depende ntupon composition, but was proportional to thecube root of time, except for rhodium in a lowshrinkage regime. This time dependence wasinterpreted in terms of the diffusion controlledtransport of tungsten in the interface betweenthe tungsten and the platinum group metal

coating layer. I n the case of rhodium there is atransition in the rate controlling process, fromthe dissolution of tungsten in the rhodium layerat low shrinkages to interface diffusion at largeshrinkages. Ta ble I1 summarises these results,the slope, S being the time dependence of thesintering curves.Also shown in Table I1 are the calculatedactivation energies for each platinum groupmetal additive. The se lie in the range 86 to I 14kcal/mol, which the authors believed to becomparable to the activation energy fortungsten grain boundary self-diffusion.Th e effectiveness of the platinum metals andnickel in promoting enhanced sintering oftungsten were found to be in the order:

Pd > Ni > Rh > Pt > RuTh e reason for the platinum group metals beingsuch effective activators for the sintering oftungsten was not established in this work,although it was suggested that it may be linkedto their relatively high ( 10 to 20 per cent)solubility for tungsten and the ir low solubilityin tungsten.

Subsequently Hayden and Brophy extendedtheir work o n platinum group metal activators

Rhod ium , ./50.005 (lWt. ), ' //

0.002i1 .800 900 * l o o 0 1100TEMPERATURE CFig. 2 The dependence of linear shrink-age upon temperature during the activatedsintering of tungsten. This shows that aftersintering for 1 hour, palladium is the mosteffective activator (7)



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Table I1Summary of the Effect of Platinum Group Metal Additiveson the Sintering of Tungsten (Data from References 7 and 3)

AdditivePalladiumRutheniumPlatinumRhodium a)Rhodium (b lNickel

Slope S0.330.390.330.50.330.5

Controlof processDiffusionDiffusionDiffusionSolutionDiffusionSolution

Activation energy,kcal/mol

8611492859868

a) Rhodium : ow shrinkage regimeIb l Rhodium : high sh rinkage regime

to the sintering of tungsten with iridium addi-tions 8). In contrast to the other platinummetals, they found that iridium actuallydecreased the rate of densification of tungsten,the effect reaching a minimum value at about 2wt. pe r cent iridium, larger additions having nofurther effect. The measured activation energyof 133 kcal/mol was close to that for volumeself-diffusion of tungsten I 35 kcal/mol).

Further work on the activated sintering oftungsten by palladium and nickel additions wascarried out by Toth and Lockington, who alsofound that there were optimum concentrationsof both palladium and nickel for maximumdensification during sintering at 10ooOC 9).Calculations showed these optim um c oncentra-tions to correspond approximately to a mono-layer of the activating element o n the tungstensurface, as also found earlier by Brophy,Shepherd and Wulff 3). Once again, palladiumwas found to be more effective than nickel,especially at and below a tempe rature of 95Ooc.Toth and Lockington found the time depend-ence of the densification to be 0.5 for bothpalladium and nickel, in contrast t o the value of0.33 for palladium found by Brophy 7). T h eapparent activation energies were lower, 62.5kcal/mol compared to 86 kcal/mol forpalladium and 50.6 kcal/mol as against 68kcal/mol for nickel. M icroprobe analysis of thefracture surfaces of sintered spec imens showedsegregation of the activating elements on grainboundary surfaces. The authors concluded thatBrophys model for a ctivated sintering was not

applicable; rather, they favoured a mechanismin which the surface diffusion of tungsten onthe activator surface is the controlling step;both are shown schematically in Figure 3.The influence of a wide range of transitionmetal additions, including all the platinumgroup metals, on the sintering of tungsten attemperatures between 1000 and 2000OC wasstudied by Samsonov and Jakowlev (10).Theyfound, in agreement with earlier findings, thatadditions of Group VIII elements-includingthe platinum group metals-promoted densi-fication of tungsten, with the exception ofosmium which was neutral. Iridium had a smallbeneficial effect at the highest temperaturestudied, 2o0o0C, which is not inconsistent withthe earlier work of Hayden and Brophy (8),since extrapolation of their Arrhenius plotspredicts a transition from a detrimental to abeneficial effect at temperatures above about14moC. The effectiveness of the platinumgroup metals in enhancing sintering was foundto be in the order:

Ru < Rh < Pdand 0s< Ir < Ptwith the upper row of elements being superiorto the lower row. This is shown in Table 111,which also gives the measured values of com-pressive strength, hardness and grain size. Onthis basis, nickel appears t o be slightly moreeffective as an activator than palladium, in con-trast to the earlier work, but this is based onresults obtained at higher sintering tem-peratures than those of the earlier studies.

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Fig. 3of tungsten particles which have been coated with a metallic activator.These two models show different representations of the sinteringa) The model of Brophy, Hayden andW N 3)as tungsten d i f f u s i n g throughthe carr ier activator) phase, away fromthe line joining the centres of adjacentparticles, to be redeposited elsewhere onthe particles as indicated by the arrows.b) The model of Toth a nd Lockington(9),where dissolution of tungsten at theactivator-tungsten interface is followedby volume diffusion outwards throughthe activator layer and subsequent sur-face diffusion, this being the rate con-trolling step. Diffusion through theactivator layer to the contact point bet-ween adjacent particles results in the for-mation of sintering necks

TUNGSTEN

These results show that the stronger activatorsalso enhance the associated grain growth in thefinal stage of sintering. The higher densitieslower porosity) achieved are also reflected in

higher values of compressive strength andhardness.

Samsonov and Jakowlev summarised theirfindings in terms of the position of theactivating element in the Periodic Table,

Figure 4. The arrows indicate an increasingdegree of activation. They interpreted theseresults in terms of the electron structure of theactivators and tungsten; an increase of thestable d-bonds in the system lowers the freeenergy, activating the sintering process inwhich diffusion is accelerated by the activatorsfor which tungsten acts as an electron donor.

More recently, German and his co-workers

Tabla 111Dependence of the Properties of Tungsten on the Activating Element

at the Optimum Concentration and Sintering Temperature (10)Activator,

weightper cent

wFe (0.5- 1.O)Ni (0.2-0.4)Ru (1.0)Rh (0.5)

CO (0.3-0.4)

Pd (0.3-0.4)0 s (1.0)Ir (1.0)Pt (1.0)

Sinteringtemperature,

OC200016001600

400- 16001600- 1800

16001400- 1600

20002000

1800-2000

Density,g/cm316.1

17.4- 17.9517.4- 17.9518.1- 18.4

17.417.8

18.1 -18.3516.116.7

17.6- 17.9

Compressivestrength,kg/mm*

8092-9683-8791 -95

7771

83-908098

88-93

Hardness,kg/mm2

181310-390277-282280-306

309290

290-300181238

305-330

Tungstengrain

size, pm5-7

12-1520-25100- 120

15-2010-1 520-25

5-75.5-720-30

Platinum Metals Rev. 1986, 30, (4) 188


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have investigated the activated sintering oftungsten in more detail I I-13). erman andHam I I ) confirmed that palladium is the bestmetallic activator for the sintering of tungstenin the range 1100 to 140o0C, as shown inFigure 5 . This shows that, for both palladiumand nickel, enhancement of sintering starts atapproximately I monolayer thickness of addi-tive and peaks at a thickness of 4 monolayers.Sintering in a moist hydrogen atmosphere wasfound to be detrimental to palladium activation

TI

Fig. 4 Trends in the activated sinteringoftungsten are related to the position of theactivating element in the Periodic Table, asproposed by Samsonov and Jakowlev (10).l e arrows indicate increasing degrees ofsctivation

PALLAOIUM,wetght per cent0 01 0 1

NICKEL.Welght per cent0.01 0.1 1

EOUIVALENT MONOLAYER THICKNESS

Fig. 5 The effect of the thicknees of thepalladium or Rickel activators upon thelinear shrinkage of tungsten powdersintered at 1200 and 130OOC in ryhydrogen is shown, d e r German and Ham(11)Tungsten+palladium, 130OOCA Tungsten nickel, 1300OCv Tungsten+palladium, 120OOCTungsten+nickel, 1200OC

. for one tungsten powder, but beneficial for asecond. The apparent activation energy islowered on sintering in a moist atmosphere.

German and Munir (12) xtended this workto other Group VIII elements includingplatinum, and confirmed that enhanced sinter-ing commenced at about I monolayer thicknessand peaked at 4 monolayers. They found theeffectiveness of the activator to be in the order:

P d > N i > P t = Co>Fe>CuBelow 130o0C, ron was more effective thanplatinum and cobalt. In the case of palladiumand nickel, where sintering progressed to thesecond stage, extensive tungsten gr in growthwas observed. The onset of grain growth wasassociated with a decline in the shrinkage rate.The authors found a time dependence of theshrinkage for all activators, similar to thatfound by Toth and Lockington, favouring



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volume diffusion of tungsten through theactivator layer as the rate controllingmechanism. The addition of 0.4 weight percent palladium was found to increase theapparent grain boundary diffusion rate byabout 6 orders of magnitude, corresponding toa decreased activation energy. The measuredactivation energies decreased in the order ofincreasing activator effectiveness, palladiumhaving the lowest value. This was related to theelectron structure modifications as postulatedby Samsonov and Jakowlev IO), that is thetransition metals with unfilled d-shells are theoptimal activators for tungsten. Based on thisconcept, the authors suggest that palladium andnickel are optimum activators for all refractorymetal powders.

Later work by Li and German examined theproperties of palladium- and nickel-activatedtungsten sintered with optimum activator con-tent in the temperature range 1200 to 16ooOC13). Hardness levels were in the order

Pd > Ni > Co > Fe at lower temperatures, ascan be seen in Figure 6, but were closertogether at the higher temperatures. In the caseof transverse rupture strength, nickel-activatedtungsten was stronger than palladium-activatedtungsten, the strength decreasing with in-creasing sintering temperature above 140ooCdue to rapid grain coarsening. For the 0.43weight per cent palladium-activated material,the grain size increased from 4.5pm at 1200~Cto 18.opm at 140oOC and to 28.5pm at 1600OC.

Recent work on activated secondary re-crystallisation of heavily-drawn doped tungstenwire has provided additional evidence for theinfluence of the activators during sintering 14).In this work the tungsten wire was coated withpalladium, platinum or nickel prior to anneal-ing and the rate of secondary recrystallisationmeasured. The highest rate of recrystallisationwas found in the presence of palladium, fol-lowed by nickel and then platinum, graingrowth being induced at temperatures severalhundred degrees lower than uncoated tungstenwire. The process was controlled by the pene-tration of the activating elements into the wire.The diffusivities of these were found to be

much higher than in prerecrystallised tungsten,which is attributed to the high diffusivity pathsthrough an intergranular phase formed by theactivator which segregates to the grainboundaries. Auger electron spectroscopyrevealed this layer to be about 2nm thick forboth palladium and nickel. The measured dif-fusivities of the activators were in the order:

Pd > Ni > Pt > COStudies by Gessinger and Buxbaum on elec-

tron emission from thoriated tungsten cathodeshas shown that platinum can also activateenhanced diffusion of thorium to the surfacealong grain boundaries, enabling the tempera-ture limit for electron emission to be extended

8 0 al lad iurn

Unt rea ted

I I

I

K 4 0 0 -c)W

2

300-

W

9 200-EzW 100 .

P1200 1400 1600SlNTERlNG lEM PERATURE, 'C

Fig. 6 The hardness and the strength oftungsten after activated sintering withvarious activators is shown here as a func-tion of sintering temperature, from Li andGerman 1 )



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from 1950 to 215oK and the maximumemission current to be increased from 3 to 7.5A/cm2 IS). This work demonstrates thatplatinum group metals not only enhance thediffusion of tungsten, but can also enhance thediffusion of other elements in the tungstengrain boundaries.Molybdenum and Other RefractoryMetals

As with tungsten, several investigators haveshown that both palladium and nickel canenhance the sintering of molybdenum, for ex-ample see References 16 to 18. Further, moredetailed work on the activated sintering ofmolybdenum by platinum group metal addi-tions has been carried out by German and hisco-workers in th e U.S.A. 19, 20). Their workon the heterodiffusion modelling of tungstenwas extended to molybdenum where the effectof I3 transition metal additions includingrhodium, palladium, iridium and platinum wasexamined in the temperature range 1000 to135oOC 19). Again, they found that activationof sintering comm enced at activator concentra-tions equivalent to I monolayer thickness andreached the maximum effect at about 10monolayers thickness, although this plateaushifted to greater thicknesses with increasingsintering temperature. They confirmed thatpalladium was the best activator formolybdenum, with the degree of effectivenessbeing in the order:

Pd > Ni > Rh > Co > Pt > Au > FeAs in the case of tungsten, iridium was

detrimental to the sintering of molybdenum.The activation energy for sintering decreasedwith increasing effectiveness of the activator,that for palladium-activation being 280 k J/mol66.9 kcal/mol) compared to 405 kJ/mol 96.8kcal/mol) for untreated molybdenum. Thisdecrease in activation energy for palladium,

nickel and platinum was observed to be concen-tration depende nt, a rapid decrease occurring atabout I monolayer thickness and reaching aminimum value at the optimum concentrationof about 10 monolayers. T he authors concludedthat th e sintering process was in accord with the

grain boundary heterodiffusion modeldeveloped earlier for tungsten 21).

Later studies by German and Labombard(20) on palladium, nickel and platinumadditions to two different molybdenumpowders of th e same particle size sintered at lowtemperatures 1050 to I I 50C) confirmed theearlier findings, namely that palladium is thebest activator, followed by nickel and platinum,and that sintering behaviour conformed to theheterodiffusion model.

German and Munir also studied the activatedsintering of hafnium 22 ) and tantalum 23)with transition metals as part of their broaderinvestigation into the m echanisms, particularlythe d-electron exchange model proposed bySamsonov 10). In the case of hafnium, the ac-tivators were added to a thickness equivalent of4 monolayers. Isothermal sintering experimentsin the range 1050 to 145oOC showed densi-fication after I hour in the following order ofenhancement:

Ni > Pd > Co > FtCobalt and platinum were only beneficial attemperatures above about 130oOC. Unusually,a non-Arrhenius temperature dependence wasfound for the activated sintering, and this wasconfirmed by constant heating rate experimentswhich showed sh arp peaks for some activatorsat varying temperatures. These peaks occurredat about 1375OC for palladium and about1240OCfor nickel, for example, and are indica-tive of an optimum sintering temperature formaximum densification enhancement. It wasconcluded that the activated sintering of haf-nium does correlate with the electron stru cturemodel, although the activators only impart alimited benefit.

In their study of the activated sintering oftantalum in the range 1250 to 170ooC,Germanand M unir found only slight enhancement withplatinum, palladium and rhodium 23). T h epoor enhancem ent of palladium was particular-ly surprising, but examination of fractured su r-faces suggested that palladium enhanced onlysurface diffusion, not bulk diffusion.

T he use of palladium and nickel as activatorsin the sintering of chromium has been studied

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Fig. 7 This geometric model of theheterodiffueion controlled activated sinter-ing process shows the activator layer wet-ting the interparticle grain boundary, afterGerman and Munir (29)

at the Tokohu University in Japan 24). t wasfound that palladium enhanced sintering con-siderably, the degree of enhancement reachinga plateau at about 0.8 w t . per cent palladiumover the range 1050 o 120oOC. Above 120ooCon sintering for I hour, the extent of enhance-ment became suppressed; this was mainly an ef-fect of the higher density levels achieved at thehigher temperatures and a reflection of theretardation due to grain growth in the secondstage. In contrast, nickel had little activatingeffect. The relative behaviour of palladium andnickel was interpreted by the authors in termsof the mutual solubility criterion suggestedearlier by Hayden and Brophy 7). Palladium-chromium fulfils this requirement whereasnickel-chromium does not.

The activated sintering of rhenium andtungsten-rhenium mixtures has been studied byseveral investigators (25-28). Dushina andNevskaya found that on sintering rhenium for2 hours in the range 1300 to 20oooC, substan-tial enhancement of sintering occurred withpalladium contents of 0. to 0 . 5 weight per cent(25). Maximum enhancement was found in therange 0 . 2 to 0.4 wt. per cent palladium. Thisenhancement was observed to be accompanied

by substantial grain growth, grain sizes of 10 oI 5pm being observed compared to I to 2pm foruntreated rhenium. Sintering at 180o0C pro-duced densities of 92 per cent in 0.2 wt. percent palladium-activated rhenium and only 81per cent for untreated rhenium.

In their study on the sintering of rhenium,German and Munir found that at 10ooOC nlyplatinum enhanced sintering, while elementssuch as palladium, nickel, iron and cobaltinhibited densification (26). The enhancementeffect of platinum commenced at a thickness ofabout I monolayer, reaching a peak at about 2monolayers. At 140ooC both platinum andpalladium enhanced sintering, platinumbecoming effective at about I monolayer, risingto a plateau of maximum effectiveness at about4 monolayers. Palladium acts less rapidly,reaching that of platinum at about 10 mono-layers thickness, which suggests that palladiumwould be better than platinum at higher con-centrations. The results of this study wereconsidered to correlate with the electron struc-ture model reasonably well.

The study of palladium additions to co-reduced tungsten-rhenium powders byShnaiderman and Skorokhod again illustratesthe beneficial effect of palladium in activatedsintering of refractory metals, although in thisinstance palladium-rich alloy interlayers areformed at the grain boundaries 28) .Models of Activated Sintering

As we have seen, the activation of sinteringrefractory metal powders by transition metalelements has been interpreted in terms ofseveral models, which are generally qualitativein nature. The results of the many studies onseveral refractory metals have shown a reason-ably consistent pattern in that the most bene-ficial activators are palladium, nickel andplatinum, generally in that order. Since thesethree elements sit in the same column of thePeriodic Table, it is reasonable to assume thattheir role is related to their electronic structureand its ability to promote rapid diffusion of therefractory element. The time, temperature andactivator concentration dependencies are also

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MODEL E X P E R I M E N T A L CONSENSUSFig. 8 The position of the activator in the Periodic Table affects the densi-fication of the tungsten. There is good agreement between the predictions ofthe heterodiffusion model (29) and experimental consensus

similar for all the refractory metals studied,which suggests that there is a common basis fora generalised model 29).

The initial model postulated by Hayden andBrophy was based on a solution-precipitationapproach in which the relative solubilities of theactivator in the refractory element and therefractory metal in the activating elementshould be low and high, respectively 7). In h i smodel, illustrated schematically in Figure 3 a),the refractory element diffuses away from theinterparticle boundary and is redepositedelsewhere on the particle surface-as indicatedby the arrows in the Figure. Experimentally therate controlling step is found to be either refrac-tory metal diffusion at the interface with the ac-tivator layer, or refractory metal solution in theactivator layer.

Later, Samsonov and Jakowlev proposed thatactivated sintering was a consequence of theelectronic structure stabilisation of the refrac-tory metal caused by the additive metal (10).They based this approach on the argument thata metallic system containing partially filledd-subshells becomes more stable as the numberof d5 and d * O electron configurations increase.The refractory element acts as an electrondonor, and this ease of electron transfer gives

rise to the high solubility in the activatingelement.

A more recent proposal by German andMunir takes this model further (21) nd appliesthe Engel-Brewer theory 30) o the predictionof the activation energies for the diffusion ofrefractory metals through the activator layer. Inthis quantitative model the activator has a rolein providing enhanced grain boundary dif-fusion of the refractory metal. This is shownschematically in Figure 7, with the activatorlayer wetting the interparticle grain boundary.This s taken from Reference 29,where a moredetailed description of these models is given.The relative solubility criterion is a prerequisitefor enhanced diffusion of the refractory metal.Enhanced mass transport, and hence densi-fication, results from the lowering of the acti-vation energy for the refractory metal in theactivator.German and Munir have shown 29) thattheir calculated values of activation energy fordiffusion of molybdenum agree well withexperimentally determined values 19).Thesecalculations indicate that palladium, nickel andplatinum are the best activators, as shownexperimentally for several refractory metals.The predicted shrinkages for molybdenum also



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agree well with experiment, as shown in TableIV 29). Figure 8 shows the good agreementbetween their predictions and the experimentalconsensus for tungsten in terms of the positionof the activator in the Periodic Table. Thisclearly demonstrates the superiority of pal-ladium as an activator, with rhodium andplatinum also in significant positions.

More recently, Miodownik has proposed aquantitative figure of merit for assessing thepotential of additive elements as activators 31).This parameter, 4, has been derived by com-bining the relevant heats of solution, surfaceenergies and the energy of vacancy formation inthe activator, and is based on the underlyingthermodynamic parameters that are responsiblefor the phase equilibria; the solubility criterionof the earlier models is an aspect of the latter:where AH,, AH, and AH, are thermodynamicfunctions related to solubility, segregation anddiffusion, respectively. Using calculated valuesof for the sintering of tungsten, Miodownikspredictions are correct for 12 ut of 14 ctivatorelements shown in Figure 8, the only dis-crepancies being manganese and gold. Onceagain palladium is predicted to be the mosteffective activator.

4 = AH, AH2 AH3

Properties of Sintered MaterialsWhile there has been a considerable number

of studies into the phenomenon of activatedsintering, relatively few studies have measuredthe properties of the sintered materials.Fracture is generally intergranular in nature,suggesting that the activator-rich grainboundaries are paths of easy fracture. Strengthand hardness are very density dependent andthe effectiveness of the activator on densi-fication clearly plays a major role. Thus, bothtype and concentration of the activatorinfluence sintered strength, as shown in TableI11 and Figure 6. Strengths as high as 1050MPahave been shown for palladium-activatedtungsten ( 1 0 ) .

No data have been presented for hightemperature creep properties, but the enhance-ment of grain boundary diffusion in the

Table I VComparison of Predicted andMeasured Shrinkage for theActivated Sintering of MolybdenumPowder (29)

2.2pm size: sintered 1 hour at 125OOCin hydrogen

Shrinkage, per centActivator Predicted Measured

Untreated MoPdNiPtcoFeCr

1.68.96.94.74 .63.5

~ 1 . 1

2.18.27.23.64.32.21 . 1

presence of activators would be expected topromote diffusional creep of the Coble type.

As stated above, the use of platinum groupactivators can promote enhanced electrical pro-perties: Gessinger and Buxbaum have utilisedthe increased grain boundary diffusivity inplatinum-activated thoriated-tungsten emittersto improve electron emission I 5 .Summary

This paper has reviewed the numerousstudies of the activated sintering of refractorymetals by transition metal additions and hasshown that the platinum group metals, and par-ticularly palladium and platinum, even atamounts of less than I per cent by weight, arevery effective in promoting densification a ttemperatures several hundred degrees lowerthan would otherwise be required. The modelsdeveloped to describe the phenomenon havebeen examined and those of German and Munir29) nd Miodownik 31) have been shown topredict the order of effectiveness remarkably

well. Significantly, palladium is predicted to bethe best activator element for several refractorymetals including tungsten and molybdenum, inaccord with experimental findings. Theenhanced densification that results from the useof platinum group metal activators leads toimproved strength properties.



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I

23

456789I 0

I 1

I2

13I4

I 5

16

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Oxidation Behaviour of Some Platinum AlloysFor a limited number of specialised applica-tions, such as for jewellery, the aesthetic ap-pearance of a material is a crucial factor. Clearlythe appearance must be pleasing at the time ofpurchase, and for precious metal items it isequally important that they should not losetheir appeal with use or the passage of time.Recently the results of a study sponsored by

Rustenburg Platinum Mines Limited into theoxidation behaviour of a number of commer-cially available platinum-rich and 18 carat goldalloys has been reported A. Wells and I. Le R.Strydom, J . Muter. Sci. Lett., 1986, 5 7),743-746). The reactivity of the alloys wasassessed by examining them in both the as-received condition and after heating at I~oOCfor 24 hours under a flow of oxygen.After this treatment it was observed that boththe gold alloys had undergone a colour

change, but no changes were perceived on thefour platinum alloys, which contained 5 weightper cent cobalt, 10 nd 15 palladium, and 7palladium plus 3 cobalt.Auger electron spectroscopy detected surfacesegregation of alloying elements on all samples,except for the alloy containing cobalt, an ele-ment for which the technique is insensitive. Ingeneral, the platinum alloys showed no signifi-cant changes as a result of the oxidation treat-ment, although minimal oxidation-enhancedcopper enrichment was observed on the surfaceof the platinum-15 palladium alloy, in which asmall amount of copper is incorporated.From this study it was concluded that theplatinum alloys examined were significantlyless environmentally reactive than the two goldalloys, under oxidising conditions at near am-bient temperatures.

Platinum Metals Rev. 1986, 30 4) 195

Activation Sintering

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