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Journal of Materials Science

, Volume 47, Issue 19, pp 6835–6848 | Cite as

Influence of visco-elastic binder properties on ram extrusion of a hardmetal paste

  • H. Ferstl
  • R. Barbist
  • S. L. Rough
  • D. I. WilsonEmail author
Article

Abstract

The influence of the viscous and elastic components of a visco-elastic binder phase on the extrusion behaviour of a tungsten carbide–cobalt hardmetal paste was investigated by studying the paste over a range of temperatures, 30–42 °C, where the binder changed from a semi-solid gel to a viscous liquid. The extrusion behaviour of the paste was studied by ram extrusion and quantified by the Benbow–Bridgwater (BB) method. The elastic and viscous components of the binder were studied separately by means of oscillatory and steady shear techniques in a controlled stress rheometer using rough parallel plates. The paste behaviour fitted the four parameter BB model well: the initial bulk yield stress parameter, σ0, increased linearly with the binder elastic modulus, G′, while both the velocity dependence parameters α and β were linearly related to the binder steady shear viscosity. Analysis of paste-wall slip parameters gave estimated slip layer thicknesses of 2–5 particle diameters.

Keywords

Shear Rate Steady Shear Solid Volume Fraction Wall Slip Binder Phase 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

List of symbols

Roman

A

Pre-exponential factor in Eq. (6) (Pa s)

C

Carreau relaxation time constant (s)

d10

10 % of the population are smaller than this diameter (m)

d50

50 % of the population are smaller than this diameter (m)

d90

90 % of the population are smaller than this diameter (m)

d[4,3]

Volume moment (De Brouckere) mean diameter (m)

D

Die land diameter (m)

D0

Barrel diameter (m)

E

Activation energy in Eq. (6) (J mol−1)

G

Elastic shear modulus (Pa)

\( G_{\text{B}}^{\prime } \)

Elastic shear modulus of unloaded binder (Pa)

G

Viscous shear modulus (Pa)

[k]

Intrinsic viscosity (−)

K

Herschel–Bulkley consistency (Pa sλ)

L

Die land length (m)

m

Benbow–Bridgwater die entry velocity index (−)

n

Benbow–Bridgwater wall slip velocity index (−)

p

Carreau power law index (−)

Pex

Extrusion pressure (Pa)

P1

Component of extrusion pressure associated with die entry flow (Pa)

P2

Component of extrusion pressure associated with die land flow (Pa)

R

Universal gas constant (J mol−1 K−1)

R2

Correlation coefficient (−)

T

Absolute temperature (K)

V

Mean extrudate velocity (m s−1)

Greek

α

Benbow–Bridgwater extensional deformation velocity coefficient (Pa s m  mm )

β

Benbow–Bridgwater wall slip velocity coefficient (Pa s n  mn )

\( \dot{\gamma } \)

Shear rate (s−1)

\( \dot{\gamma }_{\text{app}} \)

Apparent shear rate (s−1)

δ

Slip layer thickness (m)

η

Shear viscosity (Pa s)

ηapp

Apparent viscosity (Pa s)

ηB

Viscosity of binder (Pa s)

ηr

Relative shear viscosity (−)

ηw

Effective viscosity (Pa s)

η0

Viscosity at zero shear rate (Pa s)

η

Viscosity at infinite shear rate (Pa s)

λ

Herschel–Bulkley flow behaviour index (−)

σ0

Benbow–Bridgwater initial bulk yield stress (Pa)

τ

Shear stress (Pa)

τ0

Benbow–Bridgwater initial wall shear yield stress (Pa)

τB

Shear yield stress of binder phase (Pa)

τC

Shear stress at crossover (Pa)

τel

Shear stress at elastic limit (Pa)

τw

Wall shear stress (Pa)

τy

Shear yield stress (Pa)

ϕ

Solids volume fraction (−)

ϕmax

Maximum solids volume fraction (−)

Acronyms

BB

Benbow–Bridgwater

BET

Brunauer, Emmett and Teller

Co

Cobalt

CPVC

Critical powder volume concentration

DSC

Differential scanning calorimetry

HB

Herschel–Bulkley

LVE

Linear visco-elastic

PIM

Powder injection moulding

SEM

Scanning electron microscopy

VE

Visco-elastic

WC

Tungsten carbide

Notes

Acknowledgements

Project funding from the Cambridge European Trust and Ceratizit Austria is gratefully acknowledged, as are ellipsometry measurements by Dr A. Potthoff (IKTS Dresden) and laser diffraction testing by J. Kathrein (Plansee SE).

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Copyright information

© Springer Science+Business Media, LLC 2012

Authors and Affiliations

  • H. Ferstl
    • 1
    • 2
  • R. Barbist
    • 2
  • S. L. Rough
    • 1
  • D. I. Wilson
    • 1
    Email author
  1. 1.Department of Chemical Engineering & BiotechnologyUniversity of CambridgeCambridgeUK
  2. 2.CERATIZIT Austria GmbHReutteAustria

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