Planetary Milling

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Sintering, Grain Growth of Nano Particles and
Manufacturing of Bulk Nano Materials
Z. Zak Fang and H.Y. Sohn Department of
Metallurgical Engineering University of Utah
A Project Funded by US DOE, EERE, ITP, Material
Program, DE-FC36-04GO14141
Poster P22
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Background

• For WC-Co materials, WC grain size is one of the
  most import factors in controlling the properties
  of materials.
• Reducing WC grain size to nanoscale holds the
  promise of superior mechanical properties

• Nanocrystalline WC-Co materials could not only
  have significant increase in hardness but also
  possible higher strength and toughness.

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Objectives
Focus on the early stage of the sintering

• Study the grain growth of WC during early stage
  sintering , mainly during heat-up.

• Understand the effects on WC grain growth
• Initial grain sizes of powders
• Binder-cobalt
• Grain growth inhibitor-VC

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Results discussion
Morphology of as-milled WC-10 Co_10 nm powder
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Results discussion
Grain growth during heat-up

Clearly, the rapid grain growth during
the early stage of sintering must be understood
and controlled.
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Results discussion
Grain growth curves during heat-up and isothermal
hold were combined

The entire WC grain process consists of
two parts

• Early stage
• During heat-up , rapid grain growth
  somewhat depends on the temperature.

Part II
Part I
II. Latter stage During isothermal hold,
grain growth continuous gradually.
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Results discussion
Grain growth kinetics

The latter stage, during isothermal hold, can
be fitted well to interface controlled
solution-reprecipitation coarsening (n2)
But for the early stage, during heat-up, the
n value (n0.65) has no physical meaning.
The fitting of latter part of WC grain growth
This change of grain growth kinetics
suggests different grain growth mechanisms
between these two stages.
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Results discussion
Evolution of grain structure during heat-up

Both the calculation of grain growth kinetics
and the layered morphology evolution suggest new
grain growth mechanisms for the early stage of
sintering.
As-milled
800 oC
1000 oC
1100 oC
1200 oC
1300 oC
Morphology of WC-10Co_10nm powder during heat-up
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Results discussion
Effects of cobalt
Comparing WC to WC-Co 1. No significant change
in grain sizes even up to 1300?C. 2. Aggregates
are not yet faceted at 1300 oC
WC grain growth in pure WC powder
Evidently, the addition of Co significantly
facilitates WC grain growth.
Pure WC
WC-10Co
Pure WC powders at 1300?C
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Results discussion
Effects of grain growth inhibitor VC

• With the addition of VC
• 1. The rapid grain growth is inhibited
• 2. The transformation of aggregates to single
  crystals are delayed

WC grain growth in VC-doped powder
WC-10Co-1VC
WC-10Co
WC-10Co-1VC powders at 1300?C
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Key features of the UPRC process
Unique advantages of UPRHC

• Comparing to conventional HIPing, it is a much
  more rapid and energy efficient process.
• Comparing to other ultrahigh pressure techniques,
  it is a batch process. And it is a commercially
  viable high pressure technology.

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Summary

• Early stage grain growth is very important.
• Early stage part of the grain growth process is
  somewhat temperature dependent.
• The grain growth kinetics during heat-up is
  significantly different from that during
  isothermal hold.
• WC grain can grow via oriented coalescence of
  nano particles.
• The finer the initial grain size of powders, the
  smaller the final grain size.
• Co facilitates the transformation of nano WC
  particle from aggregates to single crystals.
• VC inhibits the coalescence process and slows
  down grain growth in the early stage.

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Thank you!
Questions?