Introduction 

Ball milling is generally known as a method to mix, blend, shape and reduce particle sizes. It can also be used as a mechanical synthesis method to produce materials that are milled to extremely fine powders. Several factors, like milling temperature, type of mill, milling speed etc. need to be considered when using ball milling as a synthesis method. Ball milling can be thought as ecologically friendly synthesis method as it doesn't need any solvents in the process, although solvents can be used. 

Ball milling is one of many synthesis methods used to produce solid-state materials. What separates ball milling from the rest is the easy usability, cheapness, different techniques and various advantages during ball milling process. However, ball milling has its own disadvantages as well. One of the biggest disadvantages of ball milling is the possible contaminations during milling process. Thus, the control over ball milling process is important as several factors affect the end-product. In addition to all theory on this Wiki page about ball milling, two real life syntheses are briefly presented as well. 

Ball milling process

Ball milling is made possible by a cylindrical mill chamber and balls inside the chamber. These balls are usually very hard materials, like tungsten, silicon carbide, hardened steel etc. These small balls are made to rotate inside the chamber. Already powderous material will be made into nanosized material. However, ball mills can be used to get different sort of materials (brittle, ductile, hard etc.)^{[1, p.23–26]}The basic principle of the process is similar with each ball mill. Slight variations with the mills, for example using magnets during milling, enable different end results. Figure 1 presents the basic ball milling process.^{[1, p.26–38][2]}

Figure 1. Basic ball milling process of tumbler ball mill.^[2] (License: CC BY-NC-ND 4.0).

Ball milling process via mechanical alloying (MA), mechanical disordering (MD), and mechanical milling (MM) are different techniques used to synthesize and fabricate materials.^{[1, p.48–83]} MA was originally developed to produce Oxide Dispersion Superalloys (ODS) in powder metallurgy. It is a solid-state reaction between diffusion couples, also known as powders of the reacting material (i.e., different metals or compounds), where the end-product is usually homogenous powder with metastable phase due to material transfer in room temperature. The phase can also be in equilibrium if the conditions are "hard". Nowadays it has an important role in the preparation of materials with enhanced mechanical and physical properties, or altogether preparing new phases or engineering materials. For example, intermetallic compounds, amorphous glassy alloys, metallic glassy alloys, nano crystalline and nanocomposite are some of the prepared materials in addition of ODS.^{[1, p.6–7, 23–26][3]}

Mechanical milling (MM) term is linked to grain size reduction of material powder which happens due to lattice defects during ball milling process, but it can also express crystal-metastable phase transformations. With MM, material transfer is not needed to for homogenization. Mechanical disordering (MD), previously known as mechanical grinding (MG), reflects mechanical crystal-to-metastable transformations during ball milling process. MD refers to destruction of long-range order in material to produce disordered alloys or amorphous phase. MM and MD have shorter processing time since the powders used in those techniques are already alloyed.   ^{[1, p.23–26][4]}

Control of ball milling process

Several factors affect the end-product and its properties during MA, MM or MD ball milling techniques. For example, milling temperature, time, and speed are some of these main factors. Figure 2 shows presentation of these main factors. Selection of ball mills is discussed in High-energy and low-energy ball mills -chapter.^{[1, p.48–49]}

Figure 2. Main factors that affect the end-product and its properties during ball milling.^{[1, p.48–49]} (Figure: Aki Saarnio).

Shape of the milling vial 

Internal shape of the ball mill can be flat-ended or concave-ended (round-ended). Using flat-ended vial takes less time to obtain single phase (homogeneous) end-product. The balls can roll around in the end of concave-ended vial, unlike in flat-ended vial where the balls hit the end of the vial, thus resulting in unprocessed powders and longer milling time.^{[1, p.51–52][4]}

Impurities

Contaminations are considered as one of the biggest problems of ball milling process. Impurities can occur from organic contaminations, gas contaminations, and solid contaminations. They can affect the end-product and/or milling tools. Organic contaminations are commonly the result of using process control agent (PCA), i.e. ethanol and boric acid, during milling process. PCA is used to prevent powder agglomeration when milling ductile materials. Most of the PCAs have low melting and boiling point which can result in their decomposition during excessive high-energy milling forming undesired compounds (i.e., carbides) that can change the end-product properties. However, such compounds is thought to not have negative effects to the properties as they are homogeneously dispersed into the powder matrix. End-products should still be subjected to drying and surface activation processes before any further work. Also, the milling tools can be contaminated by the PCAs if they are not properly washed, cleaned and dried.^{[1, p.52–53]}

Gas contaminations (i.e. oxygen and nitrogen) can be present because most of the milling vials don't have sealing causing metal oxide phases. Thus, powders should be handled and sealed together with balls to inside the mill vial in a box filled with an inert gas atmosphere (argon or helium). Nitrogen gas is not recommended as some materials (i.e., zirconium Zr) are very sensitive towards it.^{[1, p.53]}

Solid contaminations of foreign materials can be caused by milling tools. Materials from milling tools can introduced to the powders during milling process because of frictions between milling balls, or between the milling ball(s) and milling vial internal wall. Amount of solid contaminations is depending on the difference of strength and hardness between milling tools and powders, and also on milling medium, time and speed.^{[1, p.55]}

Milling media

Milling media consists of the balls and vials. Selection milling media materials, and also shapes and sizes, is important. Milling media should have large surface area to provide proper contact with the material being milled and they should also be as heavy as possible to reduce powder particle sizes with sufficient energy. The following milling media properties need to be considered: hardness, specific gravity, brittleness, balls' sizes, and balls' shapes.^{[1, p.56–57][4]}

Hardness is the most important characteristic when deciding on milling media. Harder milling medias are more efficient and take less time to get fine and homogenous end-product but they can also contaminate the powders with foreign materials as the balls wear during the milling process.^{[1, p.56][4]}

Specific gravity is affected by ball density and diameter. High density and large diameter would give better end-results as they most likely generate higher impact forces onto the milled powders.^{[1, p.57][4]}

Brittleness reflects how easily a material will break. Breaking down brittle materials is easier than breaking down ductile materials. Choosing right milling media ensures successful break down.^{[1, p.57][4]}

Surface area is directly affected by the balls' sizes. Capacity of ball mill increases as ball size decreases, and also the ball-to-ball contact increases as the balls decreases in size because more balls can accommodate the mill. Using different ball sizes can improve the milling efficiency. Balls that are responsible for refining the powders should as small as possible. Largest balls should be just heavy enough to grind and break down the largest and hardest particles.^{[1, p.57][4]}

Shape of milling media can be rods or barrels in addition to the balls. Balls have greater surface area which makes them better for particle size reduction. Motion of balls can be divided into three different ball mill zones: attrition zone, cataracting zone and inactive zone. These are explained in Table 1.^{[1, p.57, 59]}

Table 1. Explanation of attrition, cataracting and inactive zones.^{[1, p.59]}

Milling speed

Milling speed is one of the most important factors when controlling ball milling process, and it is defined by the number of circulations of the mill a particle makes per mill revolution. Higher milling speed leads to higher kinetic energy. Milling speed provides the needed energy for refining, solid-state reaction, solid-state reduction, solid-state mixing and phase transformations. Too high milling speeds can lead to increased contamination by milling tools due to high wear. Maximum speed is also limited by increasing vial temperature and pinning of balls to the inner walls of vial. At lower speeds the balls are predicted to fall on to the powders and not pinning to the walls. Speed needs to be controlled such a way that kinetic energy is high enough for powders but not so high that the balls pin of the inner walls.^{[1, p.60–70][4]}

Milling time

Ball milling type is dependent on the type of ball mill used. High-energy ball mills have faster solid-state reaction, phase transformations and particle size reductions than low-energy ball mills due to higher kinetic energy. Using MA, MM and MD techniques allows the possibility to monitor the progress of solid-state reaction, solid-state mixing, phase transformations and degree of mechanical mixing. Most of the published studies have focused on how ball milling time affects the end-product's structural, morphological, chemical, physical, mechanical and thermal stability properties. Mechanically induced solid-state reaction (MISSR) can be monitored by different characterization methods, such as Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Differential Scanning Calorimeter (DSC) and Powder X-Ray Diffraction (XRD).^{[1, p.70–72][4]}

Milling atmosphere

Milling atmosphere (gas atmosphere) inside the milling vial can be under normal air atmosphere, vacuum or inert gas. It is one of the most important factors along side with milling speed. Oxygen can cause impurities during MA technique as was already stated earlier. Fine powders have rather large surface areas causing them to be highly reactive with oxygen, hydrogen and nitrogen. Reactive ball milling can utilize this to produce i.e., oxidized end-products.^{[1, p.72–75][4]}

Milling environment

Milling environment is an important factor that refers to ball milling mode: wet or dry. Wet mode utilizes the already mentioned PCAs that prevents excessive cold welding during the milling process. However, higher concentrations of PCAs can lead to slower grain-size refinement.^{[1, p.75–76][4]}

Ball-to-powder weight ratio

Ball-to-powder weight ratio (BPR) ranges between 10:1 and 20:1. Higher capacity mills usually have higher BPR. With higher BPR less milling time is needed due to increased weight proportion of balls. This leads to higher energy transfer to powders as the number of collisions increases.^{[1, p.76–80][4]}

Milling temperature

Temperature factor is very important with MA and MD techniques as it affects the processes and the end-product structure. It can fasten the required milling time to acquire the wanted end-product. Milling temperature can either be increased or decreased to get wanted material phase.^{[1, p.80–82][4]}

High-energy and low-energy ball mills

Ball mills can be classified into high-energy ball mills and low-energy ball mills by their rotation speed. Selection of certain ball mill depends on used technique; MA, MM or MD. However, some ball mills can be utilized for all three of these techniques. Table 2 presents the most common ball mills and their energy types.^{[1, p.26][4]}

Table 2. Classification of different ball mill types.^{[1, p.26–41][4][5]}

Planetary, tumbler, attrition and vibration ball mills are illustrated in Figure 3.^[6]

Figure 3. Images of planetary, tumbler, attrition and vibration ball mills.^[6] (License: CC BY 4.0).

Examples of ball milling synthesis methods in journal publications

Two different examples ball milling synthesis methods are presented. The first example uses high-energy ball milling for two-stage mechanical alloying (MA).^[7]Second example uses low-energy ball milling for synthesization of three-dimensional (3D) network.^[8]

Example of high-energy ball milling

Tikhov et al. study ternary Al-alloy Al-Cu-Fe as basis to develop ceramometal catalyst. Higher surface area makes the catalyst more effective. Catalyst is desired to be stable in reaction media which is displayed in spinels, in this case by CuFe₂O₄. This spinel needs to be preceded by formation of copper-iron alloy before aluminum is introduced. This is not possible by conventional processing methods due to no mutual solubility. Mechanical alloying (MA) is used as it is widely used to prepare Al-Cu-Fe alloys, and porous nano composite ceramometals.^[7]

Tikhov et al. work presents the synthesis of 33Al-45Cu-22Fe alloy powder by two-stage mechanical alloying with APF-type planetary ball mill in air. First-stage of high-energy ball milling consists of Cu and Fe powders with atomic ratio of 67:33. The ball mill was equipped with two 1l steel-milling vials and balls with diameter of 5 mm were used. BPR was 20:1 and the ball acceleration speed was 200 m s^-2.^[7]

Cu-Fe powders were milled total for 90 minutes with 15 minute increments. After first stage aluminium powder was added into vials to maintain atomic ratio of 33Al:45Cu:22Fe. Second stage milling lasted for 24 minutes. 30 seconds before stage one and two was completed 2 ml of ethanol was added to the vials to prevent powders sticking to vials and balls.^[7]

The following reaction happened at first stage to form a metastable solid solution^[7]:

At second stage, metastable solid solution was further synthesized to 33Al-45Cu-22Fe^[7]:

Figure 4 illustrates the preparation steps of high-energy ball milling procedure. During characterization oxide phase with spinel structure was observed  by Mössbauer spectroscopy and TEM.^[7]

Figure 4. Preparation and SEM and TEM images of first-stage and second-stage steps.^[7] (License: CC BY 4.0).

Example of low-energy ball milling

Wang et al. used low-energy ball milling to create a basis for three dimensional (3D) TiN_NP/Ni structure. Mechanical properties of ceramic particles reinforced metal composites (CPRMMCs) can be further improved by modifying distribution of reinforcements. Wang et al. demonstrated that modifying Ni matrix composite with TiN nanoparticles can improve the mechanical properties.^[8]

Ni and TiN nanoparticle powders were milled in stainless-steel vial under inert Argon Ar gas. Milling time was 3 hours with the speed of 200 rpm. BPR was 10:1 and total mass for each powder was 100 g.^[8]

Similar reaction equation between Ni and TiN_NP, as in the first high-energy ball milling reaction, happens. This forms one powder that is then sintered resulting in TiN_NP/Ni structure.^[8]

References