~FeaturesTin Bronze Powders for FiltersNeil SharpeInternational Tin Research Institute

Figure 1. (a) A sintered bronze filter and (b) its micrographic features.

500 lim

Spherical Bronze 89/110.2-<l.315 mmOne hour, N2 : H2 = 1:1

A-<l .46% PO-<l.27% PO-<l.05% P

b

~ 2.5.J::.e;,ai 2.0

u3!!!a 1.5::la:~ 1.0.---t-::r~¥n:~~-t---1Ql

>CIlc:~ 0.5I-

740 760 780 800Sintering Temperature eC)

Figure 2. The strengthening effect of phos­phorus on a Cu-Sn 89/11 alloy.'

ing phosphorus concentrations. How­ever, a combination of high phosphorusconcentration and high sintering tem­perature results in high shrinkage and asignificant reduction inporevolume (over­sintering). The phosphorus promotessinter neck growth, intensifies the sinter­ing operation and, therefore, increasesshrinkage during isothermal sintering.

Qualitative energy-dispersive x-rayanalysis has identified an additionalproblem with increasing the phosphoruscontent-an increased presence of aternary (a + 0) + Cu2P eutectold," Thisphase has beel1 found preferentially atsinter necks where it can cause a localincrease in strength, but at the expenseof greater embrittlement. Further difficul­ties can arise at these higher phospho­rus contents because a low melting (a +0) + Cu2P ternary eutectic can also formlocally. Scanning electron microscopyhas shown that this eutectic can lead tothe presence of a liquid phase during theearlystages ofsintering.This liquidphasehas poor wetting properties and so re-

a

EFFECTS OF PHOSPHORUSPhosphorus is a very important addi­

tion to tin bronzes, promoting a uniformand fine-grained texture which increasesthe strength of the alloy. As well aspossessing deoxidizing properties, phos­phorus is crucial to tin-bronze powderproduction in that small additions (0.1­0.4%) extend the solidification tempera­ture range of tin-bronze and lower themelt viscosity. Further, with surface ten­sion, proper air-atomization conditionsare established.

The strengthening effectofphosphorusin spherical tin-bronze 89/11 at differentsintering temperatures was investigatedby Eckart-Werke using sintered blankswith phosphorus concentrations of 0.05wt.% (the typical concentration after adeoxidizing treatment), 0.27 wt.% and0.46 wt.%. The results (Figure 2) clearlyshow that the transverse rupture strengthofthe specimens increases with increas-

packing density is uniform. Unlike mostP/M components, filters are sinteredwithout pressure since pressure nega­tively influences porosity. Links areformed between the powder particles attheir points of contact by surface diffu­sion during the initial stages of sintering.Higher temperatures and longer sinter­ing times cause these links to grow intosinter necks, reducing the pore volumeand causing shrinkage. The driving forcefor this material transport is the reductionin free surface area, reducing the inter­facial energy and thereby increasing thefree enthalpy. Accordingly, the higherspecific surface area of finer powdersresults in greater shrinkage.

In a recent paper,' Neubing describedthe effect of the phosphorus concentra­tion and the powder size and size distri­bution on the properties of sinteredbronze filters.

Editor's Note: Thefollowing isadaptedfromanarticlewhichoriginallyappearedintheInternational TinResearchInstitute'spublication Tinand Its Uses(No. 152, 1987).

INTRODUCTIONEver since Osann first described the

rudiments of powder metallurgy (P/M) in1841, the technology has grown tremen­dously, both in terms of usage and so­phistication. At present, most P/M com­ponents are made by pressing metalpowder in a die, either unidirectionally orisostatically, and subsequently or simul­taneously heating the powder to pro­duce a coalesced mass. One of theadvantages of the technique is its abilityto impart uniqueproperties-forexample,controlled porosity-to filters and bear­ings. Powder metallurgy is suitable forproducing metal/non-metal mixtures,enabling the use of alloys which aredifficult to produce by conventional cast­ing techniques.

TIN BRONZE POWDERS

Based at GOntersthal, Bavaria, Eck­art-Werke produces and markets a rangeof powders which bear the tradenamesStandart, Stapa and Ecka. The latter, aspherical bronze (89/11) powder, isproduced in a wide variety offorms (e.g.,sheets, tubes, rods and cones) and isparticularly suitable for the manufactureof P/M filters (Figure 1). Useful for anumber of applications, the bronze filtersare permeable (according to ISO-4022/1977), possess nominal filter fineness ormaximum pore size (according to U.S.MIL-F-25682), are resistant to heat andcorrosion, and feature tensile and bend­ing strength.

Ecka spherical bronze 89/11 AK has acomposition of typically Cu-10.8%Sn­0.3%P, an alloy which provides a desir­able blend of hardness, ductility andcorrosion resistance. The raw materialsfor manufacturing this alloy are pureelectrolytic copper and pure tin (tin~99.95%). After melting, phosphorus isadded in the form of phosphorus-cop­per. Powder production is by air atomiza­tion, a technique which involves spray­ing the superheated liquid alloy througha nozzle by compressed air so that themetal stream breaks up to form very finedroplets. Solidification occurs rapidly dueto the high surface-area to volume ratioof the droplets and results in ho­mogeneous spherical particles.

Filters are manufactured from thebronze powder by loose sintering ingraphite or non-scaling steel molds.Vibration mold filling ensures that the

56 JOURNAL OF METALS. August 1988

3.

15 30 45 60 75 90Sintering Time (minutes)

Figure 3. Radial shrinkage vs. sintering time.'

duces the effectiveness of the sinteringoperation. From these results, the au­thor concludes that the phosphoruscontent must be kept within tight toler­ances for effective performance.

THE EFFECT OF PARTICLESIZE AND SIZE DISTRIBUTION

The relationship between the perme­ability of sintered bronze filters, the par­ticle size distribution and the undersizecontent (particles smaller than minimumspecification) has been investigatedusing samples made from different cutsof spherical bronze 89/11 powder. Bymeasuring the flow rate and pressuredrop of air passed through these speci­men filters, the specific permeability wascalculated. The results show that differ­ent cuts have varying permeabilities, and,in particular, an increased undersizecontent lowers permeability substantially.

The effect of particle cut on the shearstrength of sintered blanks at phospho­rus concentrations of 0.26% and 0.36%was investigated with an SMK punchingdevice. Results show that the shearstrength is inversely proportional to theparticle sizes at both phosphorus con­centrations. The reason for this relation­ship was studied by measuring the sinterneck diameters by scanning electronmicroscopy since sinter necks are thesources of strength in sintered blanks.These measurements reveal that sinterneck diameters are approximately 40%of the powder particle diameters. How­ever, shear strength is not directly pro­portional to particle diameters becausesmaller particles have higher values ofparticles per unit volume, more sinternecks per unit volume and a higher blank

may be possible to observe the amor­phous-to-crystalline transition in wet,chemically derived ceramics by examin­ing the morphology of the powder sur­faces.

While the importance of indium sulfidein the preparation of green and red phos­phors is apparent, sulfide ceramics ingeneral are also of great interest in infra­red optical window technology for the 8­14 urn wavelength regime. Cubic-formLa2S3 (1) is a potential candidate mate­rial which possesses a higher meltingpoint and lower thermal expansion thanthe current material of interest, CaLa2S4'

Failure to retain the cubic form duringpresently employed synthesizing tech­niques for optical sulfide ceramics [e.g.,evaporative decomposition of solutions(EDS)] has left the material's opticalproperties uninvestigated.

At the University of Arizona, amor­phous rare-earth oxysulfide precursorshave been synthesized at room tem­perature using metalorganics. Theseprecursors have been shown to trans­form to the cubic form of La

2S3 at therelatively low temperature of 1,OOO°C inthe presence of H

2S.The reaction time

for the transformation is eight hours. Di­mensionally, the transformed powdersare on the order of 1-3 urn, Future workis directed towards densification of thepowders and investigation of transmis­sion properties.If you want more information on this SUbject,please circle reader service card number 55.

If you want more information on this subject,please circle reader service card number 56.

References

1. H.C. Neubing, Powder Metallurgy Interna­tional, 18 (4) (1986), p. 278.2. H. Frydrych, "Influence of Phosphorus onthe Properties and Structure of Sintered LeadTin Bronze," Metalurgia I Odlesnictwo (2)(1985), p. 11.

relationship between radial shrinkageand shear strength was found to belinear for blanks sintered over a range ofconditions and phosphorus contents.

Prashant N. KumtaUniversity of Arizona

In work at the University of Arizona,low-temperature chemical synthesis ofnon-oxide ceramics is being developed.Specifically, amorphous indium sulfide(In2S3) powders have been preparedusing the reaction between an acidified(pH 1.3) solution of indium chloride andsodium hydrosulfide (NaHS). Indiumsulfide is a semiconductor used for greenand red phosphors.

A variety of techniques have beenused to characterize the as-preparedpowders. Scanning electron microscopy(SEM) shows the as-prepared powdersto possess irregularly sized particulates(500 urn dimensions) and a conichoidalfracture surface containing some poros­ity due to loss of moisture during drying.The EDAX spectrum of the as-preparedpowders indicates the presence of in­dium and sulfur while x-ray diffraction(XRD) and transmission electron mi­croscopy (TEM) have confirmed theamorphous nature of the powders. Aglass transition temperature at 330°C,followed by a crystallization exotherm onheating at about 390°C, were obtainedusing differential scanning calorimetry(DSC). On further heating, the amor­phous powders transformed to crystal­line ~-ln2S3'

Micrographs show cleavage featurestypical of crystalline materials, in con­trast to the glassy surface that had beenobserved previously. This indicates thatin addition to using conventional tech­niques such as XRD, TEM and DSC, it

Using Powder Characterizationin the Development of New Materials

cross-sectional area. By assuming close­packing, the number of sintering necksperunitvolumewas calculated, enablingthe cross-sectional area to be deter­mined. The area is inversely propor­tional to the particle diameters. The in­creased strength with decreased par­ticle diameters was, therefore, a result ofthe higher blank cross-sectional area.

Sinter neck diameters increase duringsintering as particle volumes are reduced.Sintering would thus be expected toproduce a steady increase in shrinkage.This was actually found when atin-bronzepowder of 0.315-0.4 mm cut was sin­tered at 780°C in an H2 atmosphere fortimes of up to 90 minutes (Figure 3). The

Spherical Bronze, 0.315-0.4 mm

5780·C. H2

V~

~0

5/

/0

1

/

5

~-; 5.

~c:~ 4.enc;;'6~ 4.

5.

JOURNAL OF METALS. August 1988 57