Introduction

One of the driving forces for high-quality PM product manufacturing is the characteristics and quality of the raw materials, i.e., the powder particles (PPs). Their characteristics and quality influence the powder-forming processes (compaction, sintering, and post-sintering operations) and, by consequence, the formed part quality. This section (Powder Characterization, Production, and Treatments) describes the main topics of particle characterization, production, and appropriate treatments to improve the particle quality before subsequent powder-forming operations.

Powder Characterization

Physical Properties

Also known as morphological properties, physical properties refer to the particle size, shape, and structural characteristics. Their assessment is based on specific methods/tools and analytical equipment as detailed below.

Particle Size and Particle Size Distribution

Polygranular powder mixtures are frequently used in industrial applications that are a combination of particles of different size (from a few nanometers up to maximum of a few hundred microns) for optimal self-packing (Fig. 1).
Particle size means the one-dimensional value of the geometrical parameters describing the particle shape which is projected into areas, inscribed or circumscribed to a circle whose diameter represents the particle size (Table 1).

Also, other types of diameter are used to determine particle size, including:

• Stoke diameter – the free falling diameter of a particle in the laminar flow region
• Drag diameter – diameter of a sphere having the same resistance to motion as a particle in a fluid of the same viscosity and the same speed
• Equivalent light scattering diameter – diameter of a sphere giving the same intensity of light scattering as that of a particle, obtained by the light scattering method
• Sieve diameter – the diameter of the smallest grid in a sieve that the particle will pass through

Powder mixtures can contain particles from a few nanometers up to a maximum of a few hundred microns that are divided into fractions. The quantity of particles by volume, weight, number, or surface area within each fraction represents the particle size distribution (usually Gaussian) which is simultaneously measured, in general, with the particle size, on the same equipment (e.g., laser diffractometer), under ISO conditions and terms (Fig. 2). It is important when considering a reported particle size distribution to note whether it is based on particle weight, volume, or number within each fraction. If it is a single component powder (as distinct from a blend of powders of different composition), volume and weight have the same meaning. If it is a mixture of powders of different composition, and therefore density, weight distribution can be misleading and therefore volume distribution is more meaningful. Particle number distribution, and by association surface area measurement, can be misleading as it gives an overrepresentation of the fine particles in terms of their actual volume contribution; however, it can be a good indication of potential sintering efficiency, since this is dominated by the fine particles. In practice, there are four types of particle size distributions according to the distribution mathematical functions: numerical, length, surface, and volumetric distribution. Annex 1 to Annex 6 present a selection of ISO standards of the common measurement methods (Table 2).

Particle Shape

Microparticles and nanoparticles can be imaged with respect to particle shape, described by geometrical parameters. The main criterion for identifying different particle shapes is based on the ratio between the particle dimensions: length (l), width (w), and height (h). For each powder shape, there are specific shape descriptors (Table 3).
The nanotechnology age cannot exist without high-tech microscopy. In the 1600s, the first objects microscopically analyzed by Francesco Stelluti were bees (Stephan 2000). The next big leap forward was the invention of the electron microscope in the 1930s. Scanning electron microscopy (SEM) evolved into environmental SEM (ESEM) which does not require conductive coatings, allowing ceramic particles and hydrated powders to be visualized without preparation and particle corrosion to be analyzed in real time. In parallel was the evolving transmission electron microscope (TEM) and its offspring scanning TEM (STEM) for improved resolution, high-resolution TEM (HR-TEM) and computed tomography TEM (CT-TEM). Since the 1980s, scanning probe microscopy (SPM) was a turning point, followed by the atomic force microscope (AFM), scanning tunneling microscope (STM), ballistic electron emission microscope (BEEM), chemical force microscope (CFM), feature-oriented scanning probe microscope (FOSPM), and scanning thermal microscopy (SThM). All these microscopy advancements have meant that microparticle and nanoparticle characterization has come of age (Fig. 3).

Specific Surface Area (SSA) and Internal Structure of Powder Particles

Powder mixtures often contain agglomerated and/or aggregated particles, especially with nanoparticles and/or nanostructured particles, so particle size determination is not so accurate. A more accurate morphological parameter used to quantify these powder agglomerates is specific surface area (SSA) which measures the total surface (including connected and terminating open pores) of a material per unit of mass [m2/g], solid or bulk volume [m2/cm3], or cross-sectional area (Fig. 4).
Due to various pore sizes (standardized by IUPAC) and shapes, a specific parameter distinguishes nanoparticles from coarser particles, namely, the volume-specific surface area (VSSA) that takes into consideration the SSA and the density of the powder material. Thus, nanoparticles are characterized by VSSA  ≥ 60 (m2/cm3). The methods for porosity/SSA/VSSA determination are presented in Table 4.

SSA is related to the porosity of the particles, both pore size and shape. According to the International Union of Pure and Applied Chemistry (IUPAC) standards, pores are divided in three main groups from the point of view of the size (Sing et al. 1985).

• Micropores: <2 nm. Zeolites, activated carbon and metal organic frameworks
• Mesopores: 2–50 nm. Mesoporous silica and activated carbon
• Macropores: >50 nm. Sintered metals and ceramics

Pore shapes are shown in Fig. 6 (Rouquerol et al. 1999). For industrial applications, the usual technique to measure porosity is gas absorption (Fig. 5) (Lowell and Shields 1991).
The standard SSA measurement method is BET absorption, developed by Brunauer, Emmett, and Teller (Brunauer et al. 1938), which is based on the following assumptions:
• The gas molecules are infinitely absorbed by the pore surfaces in multilayers (Fig. 7).
• Between layers there are no interactions.
• The Langmuir theory (Langmuir 1916) can be applied to each adsorbed layer.

Other Physical–Chemical Properties

Transitioning from the micron-level to the nano-level, particles change their properties due to the change in atomic arrangement, as shown in Fig. 8.
The main consequence of this atomic change is a greatly increasing chemical reactivity, with different effects on the environment and on human health. Parameters have been defined for complex characterization of nanoparticles.
Specifically:

• The crystallinity. For particles in the 1–100 nm range, SSA changes may determine allotropic transitions for the same powder.
• The redox potential, which can enhance or block electron transfer.
• The photocatalysis activity which is enhanced by the nano-features of the particles.
• The zeta potential which controls the colloidal stability especially when the nanoparticles are electrically charged.

Technological Properties

Automation of PM technology involves, among other steps, the mechanization of particle production/handling. Knowledge of particle technological properties allows the best control over their production, with benefits in subsequent processing steps. Specialized organizations in the field (ISO, MPIF, ASTM) provide standards for the technological property determination, as described below.

Apparent Density

The apparent density of a free-flowing mass of particles is calculated as follows:

where:
ρa – apparent density [g/cm3]
m – powder mass [g]
V – loose powder volume [cm3]

Hall/Carney funnels or Arnold meters are used for the apparent density determination for micron-sized particles. For submicron powders, the same measurements develop on different standards, based on the mobility and aerodynamic characteristics of these powders, using complex equipment such as an aerosol mass spectrometer (AMS), aerosol differential mobility analyzer (DMA), nano-DMA, and tapered element oscillating microbalance (TEOM). The factors affecting the apparent density are the chemical composition, particle size and distribution, particle shape, and manufacturing process. 

Tap Density

The tap density of a mass of particles is calculated as follows:

where:
rt – tap density [g/cm3]
m – powder mass [g]
V – tapped powder volume [cm3]

Also, depending on the particle size, different standards provide specific measurement procedures.

Flowability

This characteristic represents the powder property to flow readily, uniformly, and rapidly into a die or mold cavity, and it is evaluated by the flow rate parameter:

where:
f = powder flow rate [s/g]
t = required time by the powder mass to free-flow through the equipment [s]
m = powder mass [g], usually 50 g is used for the standard determination

The flow rate is influenced by the following factors: particle shape, size, distribution, humidity, and interparticle friction.

Segregation Susceptibility

Mixing and segregation (or demixing) phenomena have been researched from the point of view of the complex mechanisms and processes occurring. The most strongly influencing factors on the segregation susceptibility are particle size, shape, density, and chemical composition.

Compressibility and Compactibility

The compressibility, measured by green density, represents the ability of a mass of powder to decrease its volume under pressure. The green density is calculated by formula (4) and graphically represented by the compressibility curve (Fig. 9):

where:
rg = powder green density [g/cm3]
m = powder mass [g]
V = green compact volume [cm3]

The following factors influence the powder compressibility: particle shape, size distribution, hardness, and lubricant addition to the powder mixture. The powder compactibility, evaluated by the green strength, represents the ability of that powder to be compressed into a compact of a specified strength, and it is calculated by the Rattler test/value as follows:

where:
R = rattler value [%]
A = green compact weight before the test [g]
B = green compact weight after the test [g]

Powder Production

Introduction

The quality of a PM product is strongly dependent on the quality of the powder particles. Partially affected by the 2008 global financial crisis, the PM sector has regained its place on the industrial market with a reduced number of players. A significant reason for this recovery was the powder recycling and recovery policy accompanied by appropriate legislation and educational assessments.

Classification of Powder Processing Methods

Methods to process powder particles can be divided into the following criteria:

(a) The aggregation phase (Table 5)
(b) The structure of the particles, tailored by the reaction kinetics during their processing (Fig. 10). Thus, microstructured metallic particles are obtained by equilibrium processes (continuous lines) and amorphous/nanostructured powders by nonequilibrium processes (dot lines) (Froes et al. 1994, 1995).
(c) Reactions during processing: mechanical, chemical, and electrochemical methods
(d) The approaching route: the top-down and bottom-up methods for submicron powder production (Fig. 11)
(e) Low-gravity (G) methods: conducted on the Moon (0.125 G) or Mars (0.369 G) providing more advantageous powder particle processing technologies than on Earth (1.0 G) for space applications
(f) The production ranking: dividing the processing methods into primary and secondary routes (recycling, recovery)
(g) The material type (Tables 6 and 7)

Powder Production Methods

Metallic Powder Production

Mechanical methods for metal powder production involve solid/liquid comminution. In solid comminution, the initial solid particles keep the aggregation state during their processing by coarse comminution or milling.

(a) Coarse comminution (cold stream) processing (Fig. 12) is recommended in the case of large brittle material feed (≈ 20 mm max.) that needs to be reduced to as fine as 30 mm by crushing/grinding. Similar processes develop in ball/vertical roller/hammer/vibration mills.
(b) Milling. As an extension to comminution, submicron particles and nanoparticles are produced, with/without new structural phase synthesis, depending on the energy developed inside the mill by means of the milling media, which are usually balls (Fig. 13).

Figure 14 shows mechanical milling (MM) with no mass transfer for homogenization. Figure 15 shows mechanical alloying (MA) with mass transfer phenomena. Reactive mechanical alloying (RM), cryomilling (CM), mechanical disordering (MD), and others are alternatives of the milling process.
The particles obtained depend on the mechanical alloying processing parameters: milling time, tool material, milling ambient, milling temperature, milling speed, ball-to-powders ratio, milling atmosphere, and milling tools.

Liquid comminution or atomization consists of dispersing a molten metal stream into droplets by means of a fluid (liquid/gas) or solid support (Fig. 16).

The droplets solidify into particles of various morphologies, which are controlled by the processing parameters, as shown in Fig. 17.

Chemical and Electrolytic Methods Oxide reduction via a reducing atmosphere is a frequently used industrial process to manufacture fine (<150 mm) and low density (0.5–3 g/cm3) metallic powders (Hoganas, Ruthner, and Lurgi (www.hoganas.com; Ruthner 2012; Danninger et al. 1990) (Fig. 18).

The carbonyl process provides better quality processed metal particles (chemical purity, apparent density 4.1–4.5 g/cm3, particle size 2.4–8 mm; Ni, Fe, Co, Cr, W, Mo) in comparison to oxide reduction (Fig. 19).

The electrolysis method produces dendritic particles with higher purity and greater sinterability than the other processing routes (Fig. 20).

Physical–Chemical Methods Gas phase precipitation generates submicron metallic powders from metal compounds with a transient gas phase toward solid powders.

Ceramic Powder Production

Advanced ceramic applications require specific microstructural characteristics: small defect size, homogeneous dispersion of the structural phases, uniform phase composition of the grains boundary, and ability to work at room temperature and high temperatures. Some of the processing methods for ceramic powders are listed in Table 7 above.

• Mechanical methods (comminution/milling route). This approach is more efficient for ceramic powders than for metallic powders, due to the inherent brittleness of ceramics (Table 6 and Shigeyuki and Rustum 2000). This method is a pathway to grain size refinement of the ceramic end product, increasing the specific surface area by decreasing particle size, with structural changes occurring at room temperature.

• Thermal decomposition method (chemical vapor deposition (CVD) route). Short reaction time and low decomposition temperature are the main advantages of the CVD method for producing high purity powders of thin coatings, with tailored properties related to the application using gas/vapor phase reactants.

• Precipitation or hydrolysis method (solgel route). This method provides good control of powder chemical purity, porosity, crystal size, particle size distribution, and other physical–chemical properties (Vegad 2007).

• Hydrothermal method (hydrothermal (HT) synthesis route). This method, and its alternatives, aims to crystallize anhydrous ceramic powders directly from solution, producing powders with outstanding properties: submicron grain size, narrow particle size distribution, weak agglomeration effect, crystalline or amorphous structure, and very high reactivity with benefits for sinterability.

• Other combined methods (spray pyrolysis (SP) route). This method allows powder processing from microparticles to nanopowders with precise morphology (spherical, hollow, porous, fibrous) and precise chemical composition (ceramic and ceramic-based composites).

Powder Treatments

Subsequent to the powder production stage, for specific cases, powder treatments allow improvements in their properties.

Mechanical Treatments

The aims of these treatments are:

• Powder cleaning to remove superficial solid or gaseous residues by a degassing process (cold/hot, static/dynamic methods).
• Powder grinding for comminution. This can be carried out at the powder production facility for online correction of the particle size and distribution, during the processing stage.
• Powder blending and mixing (in wet or dry conditions) to prepare homogeneous powder mixtures for the compaction stage.

Heat Treatments

Some powder properties can be improved by means of heat treatments. A frequently used process is annealing in hydrogen or other reducing atmosphere, which enables:

• Improvement of the powder chemical purity (Fig. 21)
• Reducing the work hardening effect on metal powders caused by the milling process
• Morphological changes of the powder particles
• Mechanical (strength, ductility) and structural (grain size, impurity segregation) beneficial changes resulting from optimal powder synthesis (Fig. 9)

Concluding Comments: Powder Characterization, Production, and Treatments

The above discussion on powder characterization, production, and treatments is intended to give guidance for PM technologists to design the optimal infrastructure for ideal powder research, development, and production facilities, optimized with respect to forming and sintering outcomes (Fig. 22).