Review of Ball Milling
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Review of Ball MillingMatthew King2/4/04
Ball Milling parametersDifferent Milling machines and their respective energiesOverview of contamination and methods of reducing itMilling ball motion in a planetary mill and the effects on collision energiesThe effect of temperature and milling ball energy on sample evolutionOverview of Mechanical Milling pre-alloyed intermetallicsOverview of Mechanical Alloying of elemental powdersSummary of Milling parametersBrief Conclusions
Overview of Ball Milling ParametersType of mill (planetary, attrition, vibratory, rod, tumbler, etc.)Speed of mill, relative speeds of pot rotation to disk revolution in a planetary mill
Composition, size, shape and surface of potDegree of filling potNumber, size(s), material (density, elasticity), and surface of milling balls
Weight, shape, size and composition of starting material
Macroscopic temperatures of pot, ball and powderMicroscopic Temperature at collision pointMilling atmosphereMilling time
- Different milling machinesFritsch planetary millerMedium-high energy research miller (
Power of different milling machines
ContaminationFresh atomic surfaces constantly created during mechanical milling, so contamination by O2 & N2 is a real problem:Argon commonly used as an inert environment (impurities in Argon can be a problem though)Can be useful mechanical milling invented as means of creating metals with uniform dispersion of oxide for strength.
Milling with a liquid surface agent can lower particles surface energy, allowing smaller particle sizes to develop. However, some becomes absorbed into the sample as a contaminant.
Other source of contamination is from pot/balls:Less foreign material tends to be introduced if sample/milling media are dissimilar materials.Effect of contamination tends to be more serious if sample/milling media are dissimilar materialsUsing milling media that are of the same material as the sample is the solution if contamination critical, but compensation to retain balance of alloy may be required.
Reducing cross-contaminationMilling often results in powder being welded to the pot, and diffused into the milling surfaces.
This can be minimised by:Having designated pots for a particular materialSkimming the milling surfaceRemoving chemically (not a good option if milling media/stuck-on sample are similar materials)Milling with sand both manufacturers recommend milling with high purity (quartz) sand to remove stuck-on materialMilling with meths/acetone
Trajectory of milling balls in a planetary millAim is generally to have milling balls cascading across pot and colliding with maximum energy with the opposite wall.
In reality, there is a high slip-factor between pot and balls (80%), so observed trajectories in a Fritsch mill are:25% reduction in energy transfer due to slipping
Trajectory of milling balls continued . . .Slip factor means higher rotation:revolution ratio needed for cascading motion:Power input approximately given by:A higher ball elasticity should increase the efficiency with which energy is transmitted to the powder. Too high a pot rotn:disk revn ratio leads to milling balls being pinned to side, so a balance is needed.Pot rotation in opposite direction to revolution of disk more efficient
Other parameters affecting the ball impactGreater Ball to Powder Ratio (BPR) generally only increases the collision frequency (decreases the time it takes for powders to evolve)
However, filling the pot above 50% reduces the milling efficiency
Having milling balls of varying sizes can have a chaotic effect on the ball trajectories, preventing tracks forming (making sure all the sample is milled uniformly), but this less of an issue with planetary ball millers
Heavier milling balls increase the impact energy of collisions
Cr on graph means BPR. R = ball size
Temperature ReachedMacroscopic Pot temperature:Maximum temperature rise of 100-200oC is generally accepted.Maximum depends on milling conditions & cooling mechanisms
Macroscopic ball temperature:Paper predicts milling balls will get much hotter (600oC) than pot:Max temperature linked to ball sizeHeat transfer from balls to pot relies on forced convectionMicroscopic powder collision temperature:Only weakly linked to ball temperatureCan be 100oC up to 1100oCTemperature pulses calculated to last ~10-5s
General effects of milling powdersGenerally, increasing the impact energy:Increases the amount of cold welding and fractureCollisions plastically deform the atomic structure, which:Introduces dislocations, vacancies, stacking faults, atomic disorderThereby increases strain, which is relieved by grain refinement (i.e. fracture). More energetic milling reduces the grain size.The stored energy of cold work introduced into the sample has been known to be up to 50% of enthalpy of fusion of a material
Effect of temperature:More energetic milling increases the temperature of the powder (which can result in recovery from disorder . . .)Vacancy density and fracture decrease as temperature increaseHigher macroscopic/microscopic temperatures promote recovery from disordered state and promote diffusion
Effect of Mechanically Milling pre-alloyed IntermetallicsCreates disordered nanocrystalline structure, with amorphous halos between grains.
If the increase in the free energy (associated with grain boundaries and atomic defects) reaches a critical level, the formation of a meta-stable phase becomes possible (e.g solid solution, amorphous, or simply nanocrystalline)Low temperatures generally favour amorphous phase, as:Lower recovery rateMore disorder (e.g. smaller grain size) produced
Can be a maximum temperature for amorphous phase formation.
Mechanical Alloying in ductile/ductile systemsIn ductile/ductile systems:Lamellar structure developsLayers get thinner and thinnerThermal diffusion occursAlloy is formed
Mechanical Alloying in ductile/brittle systemsIn ductile/brittle systems alloying:Brittle powder fragments when hitBrittle powder embedded in ductile materialIf brittle material soluble in ductile material, short range diffusion of the brittle material into the ductile and an alloy can form
Mechanical Alloying in brittle/brittle systemsBoth brittle powders fragment when hitWhen powder size reaches limit of comminution, material can become more ductile, allowing conglomeration and cohesion.Diffusion may then form alloy (e.g. Si/Ge)In brittle/brittle systems, alloying may occur:Temperature generally critical. If the temperature is below a certain limit, diffusion will be too limited to allow alloy formation
Effect of Mechanically Alloying powder mixtureMechanical alloying often results in a (fully mixed) amorphous (or partially amorphous) phase, which can then be annealed to form the crystalline intermetallic.However, in different milling conditions, the intermetallic may be formed directly.Predicting which phase will predominate is difficult:There is evidence that softer (less energetic) conditions favour amorphisation over intermetallic formation. However, the effects of temperature on this trend cannot be ignoredContradictory evidence for effect of temperature existsThe evolution of particle size generally follows the trend opposite:
Summary of Milling parametersMechanical Milling: Likely optimum conditions for creating disordered (nano-crystalline then amorphous) intermetallics:Low temperature milling favours smaller average grain size (more defects, less re-arrangement) but broader distribution.Greater ball size and BPR results in lower grain sizes, more defects and faster grain-size reductionGreater pot width and higher ratio of pot rotation to disk revolution (opposite directions) is more energetic (until milling balls stick to side). Filling the pot too full can inhibit milling
Mechanical Alloying:The process depends on how ductile the constituent powders are.Softer milling favours amorphous phase formation over intermetallicContradictory evidence for the effect of temperature on phase
Contamination should be controlled by use of pure Argon, minimising milling times and by use of milling media of the same material as the sample if possible
ConclusionsThe physics of the process is very complicated, and is dependant on large numbers of parameters, making comparisons between research groups difficult.More research is needed to understand ball milling enough to allow accurate prediction of material phases prior to experiment.However, mechanisms for mechanical milling and alloying are broadly agreed upon in the literature, allowing general effect of milling parameters to be predicted. Mechanical Milling provides a means of producing materials that cannot be produced by other means, and so this is a very exciting research area.
ReferencesAbdellaoui, The Physics of Mechanical Alloying in a modified horizontal rod mill: mathematical treatment, Acta Materialia 44(2) 725-734Atzmon, In situ Thermal Observation of Explosive Compound-Formation Reaction during mechanical Alloying, Physical Review Letters 64 (4)Begin, Nanocrystalline oxides synthesized by mechanical alloying J Physique III France 1997 473-482Burgio, Mechanical Alloying of the Fe-Zr system. Correlation between input energy and end products, Il Nuovo Cimento 13 D,(4) p459Chattopadhyay, A mathematical analysis of milling mechanics in a planetary ball mill, Materials Chemistry and Physics 68 84-94Froes, Synthesis of Intermetallics by Mechanical Alloying, Materials Science and Engineering A192/193 612-623Gaffet, Planetary Ball-Milling an experimental parameter phase-diagram, Materials Science