General introduction about the ball milling technique used to make nanocomposites and its parameters that affect the technique.

1. Mechanical Alloying/BALL MILLING
techniques – a basic introduction
By
Mr.M.Naveen Rooba Doss, M.Tech
Nanotechnology

2. Contents
• Mechanical Alloying
• Historical Perspective
• Process variables
• Mechanism of Alloying
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3. Mechanical Alloying
• Mechanical alloying
(MA) is a powder
processing technique
that allows production
of homogeneous
materials starting from
blended elemental
powder mixtures.
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Ref: www.crcpress.com, as on
26/7/18

4. Why Mechanical Alloying?
• To produce
advanced
materials.
• Theme:
– To synthesize at
non-equilibrium
condition.
– Achieved by
energizing and
quenching.
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Fig 2: Basic concept of energize and quench

5. Attributes of Mechanical Alloying
i. Production of fine dispersion of second phase (usually oxide) particles
ii. Extension of solid solubility limits
iii. Refinement of grain sizes down to nanometer range
iv. Synthesis of novel crystalline and quasicrystalline phases
v. Development of amorphous (glassy) phases
vi. Disordering of ordered intermetallics
vii. Possibility of alloying of difficult to alloy elements
viii. Inducement of chemical (displacement) reactions at low temperatures
ix. Scaleable process
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6. Historical Perspective
• John Benjamin and his colleagues at the Paul D. Merica
Research Laboratory of the International Nickel
Company (INCO) developed the process around 1966.
• To produce a nickel-base superalloy.
• For gas turbine application.
• To achieve high-temperature strength of oxide
dispersion and the intermediate-temperature strength
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7. Continued….
• INCO had developed a process for manufacturing graphitic
aluminum alloys by injecting nickel-coated graphite particles into a
molten aluminum bath by argon sparging.
• metal powder particles could be fractured by subjecting them to
heavy plastic deformation.
• Use of special chemicals could be employed to produce finer
particles.
• by preventing cold welding, suggesting that at some stage cold
welding could be as rapid as fracturing.
• The reactivity of the element also had to be considered.
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8. Finally, Benjamin Decided to produce
composites following
• using a high energy mill to favor plastic deformation required for
– cold welding
– reduce the process times.
• using a mixture of elemental and master alloy powders
– to reduce the activity of the element
• eliminating the use of surface-active agents
– To avoid producing finer pyrophoric powder
– To avoid contamination of the powder
• relying on a constant interplay between welding and fracturing to
yield a powder
– to obtain a refined internal structure.
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9. NOTE
Benjamin, was referred to as ``milling/mixing'',
but Mr. Ewan C. MacQueen, a patent attorney
for INCO coined the term mechanical alloying to
describe the process in the first patent
application, and this term has now come to stay
in the literature.
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10. Process and the Variables
• Raw materials
• Types of Mills
• Process Variables
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11. Raw material
• Should be in the range of 1-200 micrometers.
• Should be smaller in size than the milling balls.
• Size reduces exponentially with time during
process.
• Raw materials - pure metals, master alloys,
prealloyed powders, and refractory compounds.
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12. Nature of raw materials
• Initially, atleast 15% volume of ductile
components present in mixture.
• Later, fully brittle components have been
successfully milled.
• Mixtures of solid and liquid components have
also been milled. Ex: Ni powder and Liquid Ga
– Only solids – Dry grinding
– Solid and liquid – Wet grinding
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13. Wet Grinding compared to Dry Grinding
• Finer ground products
– Solvent particles get adsorbed on the surface.
– Lowers surface energy.
• Less agglomerated condition
• Rate of amorphization is faster
• Disadvantage:
– Increased contamination
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14. Types of Mills
• SPEX shaker mills
• Planetary ball mills
• Attritor mills
• Commercial mills
• New designs
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15. Mills - Difference
• Capacity
• Efficiency
• Way of operation
• Cooling and Heating arrangements
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16. SPEX SHAKER MILLS
• Sample : 10-20g
• Common Setup has
– One vial
– Grinding balls
• Securely clamped and swung back & forth
several thousand times a minute.
• Coupled with lateral motion of vial- the
motion forms a shape like 8.
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17. Continued…
• Velocity of balls: 5 m/s,
hence high energy
milling.
• Vial Materials
– hardened steel,
– alumina,
– tungsten carbide,
– zirconia,
– stainless steel,
– silicon nitride,
– agate, plastic, and
– methacrylate.
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Fig 3: Top- SPEX ball mill, Bottom – Vial
and components

18. Planetary Ball Mills
• Planet like movement of vials
• Movement of vials:
– Vials are arranged on a rotating support disk.
– also rotate around their own axes in opposite
direction of disk.
• Centrifugal acts producing two effects
– Impact effect
– Friction effect
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19. Planetary Mill
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Fig 4: (a) Planetary ball mill, (b) movement of balls

20. Continued….
• Linear velocity high
• Frequency of impacts is low
• Grinding vials and balls materials:
– agate, silicon nitride,
– sintered corundum,
– zirconia, chrome steel,
– Cr-Ni steel, tungsten carbide,
– and plastic polyamide.
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21. Attritor Ball Mills
• Vertical Drum with series of Impellers
• Set progressively at right angles to each other, the
impellers energize the ball charge.
• This leads to powder size reduction because of
– impact between balls,
– between balls and container wall, and
– between balls, agitator shaft, and impellers.
• Size reduction also appears to take place by interparticle
collisions and by ball sliding.
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22. Continued….
• Velocity is much low.
• Shearing and Impact forces
act.
• Grinding tanks or
containers
– stainless steel or
– stainless steel coated inside
with
• alumina, silicon carbide,
• silicon nitride, zirconia,
• rubber, and polyurethane.
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Fig 5: Attritor ball mill

23. Commercial Mills
• Scaled up for mass production
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24. New designs
• Rod mills
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Fig 6: Cut section of a rod mill
Ref:www.mine-engineer.com/mining/rodmill.htm as on 26/7/2018

25. Continued
• Vibrating Frame Mills
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Fig 7: Vibratory frame mill setup(left) and removed vial (right)
Ref: www.siebtechnik.com/en/vibrating-mill/ as on 26/7/2018

26. Process Variables
• Milling container
• Milling speed
• Milling time
• Grinding medium
• Ball-to-powder weight ratio
• Extent of filling the vial
• Milling atmosphere
• Process control agents
• Temperature of milling
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27. Milling Container
• Due to impact :
– Container surface may break.
– Gets incorporated into the powder.
• Based on material:
– Same as milling : chemistry may alter
– Different : contamination
• Shape of container:
– Flat end bottom (milling rate relatively higher)
– Round end bottom
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28. Milling Speed
• Maximum speed – maximum energy
– But design will affect the extent of speed
• Critical Speed - Speed above which the grinding balls
gets pinned to the surface.
– Speed just lower than critical must be employed
• More speed - High Temperature
– Aids diffusion for homogeneous
– Aids decomposition of metastable phases.
– Aids contamination
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29. Milling Time
• Time setting based on:
– To achieve steady state between fracturing and cold welding
• Time varies depending on:
– Type of mill
– Intensity of milling
– Ball to powder ratio
– Temperature rise
• Longer Milling time:
– Contamination
– Formation of undesirable phases
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30. Grinding Medium
• Common Types:
– Hardened steel, tool steel, hardened chromium steel,
tempered steel, stainless steel, WC-Co, and bearing
steel.
• Most cases – made of same material to avoid
cross contamination
• Grinding ball diameter
– Larger : only crystalline phase
– Smaller: amorphous phase, solid solution produced.
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31. Continued…
• Material gets cold welded to surface
• Advantage:
– prevents contamination
– Prevents excessive wear of grinding balls.
• Disadvantage:
– More thickness: it will lead to heterogeneous
phase
– Unable to detach powder and yield will be low
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32. Use of Different sized grinding balls
• Less coating on surface
• High yield
• Possible explanation: difference in shearing
forces exerted.
• Don’t follow particular track and more
randomized movement.
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33. Ball to powder ratio
• Also known as Charge ratio
• Varied from 1:1 to 220:1
– SPEX mill - 10:1
– Attritor mill – 100:1
• Higher the BPR – shorter time for milling.
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34. Extent of filling the Vial
• Enough space must be available
• Generally 50% is left empty.
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35. Milling Atmosphere
• Milling containers
– Either evacuated
– Filled with inert gases: helium, argon
• Nitrogen not filled as it will react, unless if
Nitrides are required.
• Presence of air – oxides formation favored if
contents are reactive in nature.
• Presence of hydrogen- hydrides formation.
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36. Process Control Agent(PCA)
• Final size, shape, and purity of the powder particles is determined
by PCA.
• Examples: Benzene, graphite, methanol etc, refer Table 5 in book
• Selection based:
– Nature of the powder being milled
– The purity of the Final product desired
• Amount of PCA depends on:
– cold welding characteristics of the powder particles,
– chemical and thermal stability of the PCA, and
– amount of the powder and grinding medium used.
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37. Continued….
• Not harmfull but increase strength and high
hardness.
• PCA
– decompose during milling,
– interact with the powder and
– Form compounds,
– and get incorporated in the form of inclusions
and/or dispersoids into the powder particles
during milling.
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38. Reference
Mechanical alloying and milling by
C.Suryanarayana
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