Sintering definition

Sintering is a term used in many industries. It's used to manufacture ceramics, refractory metals, spark plugs, cemented carbides, self-lubricating bearings, electrical contacts, insulators, oxidized nuclear fuel, structural materials, magnetic materials, aerospace and medical components, and has many more potential applications in the future. Therefore, there are many different definitions across different industries. The German definition of sintering is: "Sintering, as a heat treatment method, involves heating atoms to give them sufficient energy to migrate, preventing particle adhesion in the powder. This bonding increases strength and reduces system energy." The glossary in ASTM B 243-09a, in Section 3.3.1, emphasizes the explanation: "Increasing the bonding of powders or compacts by heating to below the melting point of the main component."

Sintering Theory

Sintering theory lags far behind the development and complexity of sintering practice. The German book "Sintering Theory and Practice" fills this gap well, explaining various mechanisms of sintering, with Pease and West providing a comprehensive explanation.

Sintering reduces the surface energy of powder materials by forming bonds between powder particles, thereby reducing the specific surface area. With increased interparticle bonding, the pore structure of the material changes significantly, leading to substantial improvements in properties such as strength, ductility, corrosion resistance, electrical conductivity, and magnetic permeability. These changes are significant in industrial applications, including MIM (Metal Injection Molding). Furthermore, in MIM, the significant elimination of porosity leads to substantial shrinkage of the parts; therefore, variables throughout the process must be kept under control to achieve process repeatability that maintains the dimensions of the sintered parts within the required tolerances.

Atomic Diffusion Mechanism

Atoms are constantly in motion and vibration, even in the solid state. This motion, coupled with the need to reduce free surface area to lower particle surface free energy, leads to bonding between particles. Temperature enhances this reaction tendency. When the free surface area disappears, the resulting vacancies may move through the pores via grain boundaries. Grain growth reduces the number of grain boundaries, further lowering the energy, and then the entire system is further homogenized through atomic diffusion. If this process continues indefinitely, the two spheres in the model will eventually merge into one sphere, reaching the lowest energy state.

Sintering theory assumes that the effect of the sintering driving force on atomic diffusion is known, and that sintering takes place under isothermal conditions between two spheres of the same size, starting in point contact. Under these conditions, atomic diffusion may occur through surface diffusion and bulk diffusion mechanisms.

Figure 7.6 shows the different stages and mass transfer mechanisms of the two-sphere sintering model during the sintering process. During sintering, spherical phases come into contact to form sintering necks, which then grow. The surface diffusion mechanism mainly involves material flow along the particle surface, namely evaporation and condensation (E-C), surface diffusion (SD), and volume diffusion (VD). In all these cases, atoms move along the particle surface and eventually reach the contact point between the two particles. This increases the bond strength between the two particles without changing the distance between them, thus leading to the formation and growth of sintering necks between the two particles.

As the sintering neck continues to grow in the later stages of sintering, the centers of the two particles begin to approach each other and shrink. At this point, bulk diffusion dominates, through plastic flow (PF), viscous flow, grain boundary diffusion (GB), and volume diffusion (VD), causing atoms within the two particles to grow at the sintering neck and eliminate porosity within the material. Only bulk diffusion leads to material shrinkage.

A brief overview of each diffusion mechanism is as follows:

(1) Evaporation and condensation. Materials with high vapor pressure or those that react with the sintering atmosphere to form volatile substances may undergo evaporation and condensation during sintering. NaCl, TiO2, H2O, and Si,N are typical examples of materials that exhibit this phenomenon.

(2) Viscous Flow. Viscous flow is a common sintering mechanism for amorphous materials. The viscosity of most amorphous materials decreases with increasing temperature, and external pressure promotes this trend. Since the material lacks grain boundaries, the amorphous structure is filled with defects, much like a liquid, so bonding continues until the amorphous structure disappears.

(3) Surface Diffusion. The surface of solid crystals is filled with defects, and even with state-of-the-art polishing techniques, the surface is not smooth. Surface diffusion involves atoms moving from one defect site to another, eventually reaching the contact point between two particles, forming a contact and creating a sintering neck, thus reducing the surface energy between the two particles. Surface diffusion results in a lower viscous activation energy compared to volume diffusion. Smaller actual powder particles can bond together at low temperatures, helping to maintain the shape of loosely bound powder bodies, such as in slurry casting or powder injection molding.

(4) Volume Diffusion. Volume diffusion occurs when atoms migrate from the interior of crystalline materials through interstitial defects (such as dislocations and vacancies). Volume diffusion can lead to sintering neck formation and material densification.

(5) Grain boundary diffusion. Grain boundaries consist of numerous defects arising from orientation mismatches between adjacent grains. These grain boundaries serve as pathways for atomic migration to close pores or for vacancies in pores to move outwards to the surface. Grain boundary diffusion has a lower activation energy than volume diffusion and is therefore a mass transfer mechanism that occurs before volume diffusion.

(6) Plastic flow. Plastic flow occurs due to dislocation climb causing vacancies to disappear. Whether this occurs without external force is controversial. Under isothermal sintering conditions, this process is considered transient; under external pressure, plastic flow is the dominant diffusion mechanism.

Sintering Stage

The sintering stage is characterized by changes from powder consolidation to the final formation of a dense object. Sintering processes vary depending on the application. For example, in press-sintered metal powder structures, powder particles are compressed together under high pressure, resulting in a large contact area before sintering and thus a high green density. Conversely, filters or self-lubricating bearings require specific pore sizes, preventing the parts from being fully densified. MIM represents a state similar to the sintering of loose powder, to which all sintering stages apply.

The first stage of sintering involves particle adhesion and rearrangement, beginning at points where van der Waals forces are weak. At high temperatures, particles undergo slight rotation or torsion to achieve a lower energy state relative to particle orientation.

The next stage of sintering involves the formation of sintering necks between particle contact points through the aforementioned process, leading to neck growth. This is the initial stage of sintering, where sintering necks begin to appear, but little or no densification occurs.

This stage is followed by the intermediate stage of sintering. In the intermediate stage, the sintering neck continues to grow, leading to densification of the part. At this point, the sintering neck no longer resembles its original shape, and the pores in the part begin to spherize, although they remain interconnected.

In the final stage of sintering, the interconnected open pores disappear, becoming isolated closed pores. Grain growth also occurs when the pores close, which slows down the diffusion process. Although the first few sintering stages are relatively rapid, the final stage of sintering gradually slows down as the density exceeds 95% and approaches 99%, depending on the forming system and sintering temperature.

Figure 7.7 shows a sintered SEM image of the iron-cobalt-vanadium pre-alloyed material. Neck growth and intergranular boundaries can be seen in the surface powder particle area, with some interconnected pores between these particles. A fully sintered region is visible in the background, indicating that as the sintering process progresses, the particle characteristics have been lost, and larger intragranular boundaries gradually form due to grain growth.

Figure 7.8(a) shows the microstructure of a cobalt-chromium alloy (F75) sintered at 1200℃. The image shows very small powder particles with a considerable number of pores, some of which are internal to the particles. Figure 7.8(b) shows the microstructure of the same alloy sintered at 1300℃. Compared to the sample sintered at 1200℃, the grains are significantly larger, the total porosity is significantly reduced, most pores are located within grain boundaries, and the size of individual pores is also larger. This is because sintering at these two temperatures leads to grain growth, pore migration along grain boundaries, and aggregation of pores within the grain boundaries.

 

 (a) Microstructure image of cobalt-chromium alloy (F75) sintered at 1200 °C

 

(b) Microstructure image of cobalt-chromium alloy (F75) sintered at 1300°C. 

Sintering Practice

Although there are many sintering models for different particle shapes, such as spherical, linear, conical, and biconical, sintering practice involves many particles. Not all particles are the same size; often, particle sizes vary. Nor are all particles spherical; often, particle shapes vary, depending on the powder preparation method. Heating conditions are also never isothermal. The presence of other elements further complicates the sintering process; these elements may originate from the powder preparation system or be added for specific reasons. Ternary or quaternary systems typically generate low-melting-point eutectics at temperatures lower than those of similar binary systems, leading to the presence of a liquid phase and influencing the sintering process.

When powder is consolidated into a shape, the contact formed between particles depends on the consolidation process, as well as the particle shape, size, and size distribution. The term "consolidation process" is a general term for all powder injection molding systems, applicable to powders that can be pressed in a mold or isostatically, powders that are slid-molded or cast from high to low temperatures, powders injected or molded with binders, loose powders in molds, etc. In the case of pressure consolidation, pressure induces plastic deformation under cold, warm, or hot conditions and enhances the contact between particles. In other processes without external pressure, they are filled into a near-ideal random packing model, approaching the tap density of the powder. The conditions in metal injection molding correspond to consolidation without external pressure.

Powder mixtures are frequently used to manufacture parts, but these mixtures are not necessarily homogeneous pre-alloyed materials. The interactions between these elements have a significant impact on the sintering process, depending on the nature of these interactions. Adding elemental powders is generally the most economical method for preparing sintered alloys; however, in some cases, only mixing intermediate alloy powders with elemental powders can produce specific alloys. When one elemental powder is added to another, the following possibilities may exist:

(1) When mixed uniformly, the two powder components are miscible, such as alloys formed from mixtures of elemental powders or mixtures of parent alloy elemental powders.

(2) The powder is soluble in the alloying additive, but the reverse is not true, which promotes sintering. Adding a small amount of nickel to tungsten can activate sintering, i.e., sintering tungsten at temperatures below 1400°C.

(3) The alloying additive is soluble in the matrix, but the matrix is ​​insoluble in the alloying additive. This causes the additive to dissolve in the matrix, but because the matrix is ​​insoluble in the additive, the voids and pores left by the alloying additive will expand. This situation needs to be avoided during the sintering process.

(4) The two components are incompatible. This is the case in composite materials, where the properties of both components are essential to the material. One example is oxide dispersion-strengthened alloys formed when Al₂O₃ is dispersed in a metal matrix, such as some Incoly materials.

The formation of the liquid phase during sintering requires separate discussion.