All about sintering

Articles about the process of sintering, the latest innovations, and the future of technology.


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What is sintering?

Sintering is a process in which heat and pressure are used to create bulk items from a powder. The temperature used in sintering are relatively low and below the melting point of the material being sintered.

Sintering can be used with a variety of materials such as metals, ceramics, plastics, and semiconductors. The particles in the sintered powder can range in size from nanometers to micrometers.

How does sintering work?

The process of sintering comprises diffusion of atoms across the particles boundaries, effectively fusing them together and resulting in a solid piece.

Temperature and pressure are generally applied to cause this diffusion to take place. However, the temperature during the sintering process always stays below the melting point of the material.

In contrast to melting, the powder used in sintering never reaches a liquid phase. This makes sintering a great choice for materials that have a high melting point, such as tungsten and molybdenum.

Image credits: Sintering and Grain Growth. In: Ceramic Materials. Springer, New York, NY (2007)

Types of Sintering

Depending on whether pressure is applied during the process, sintering can be classified into two types:

1. Pressureless sintering (PLS)

2. Pressurized sintering

Pressureless Sintering

Pressureless sintering is conducted without any application of external pressure. This allows uniform densification of sintered material contrary to conventional hot pressing methods.

Pressureless sintering can further be classified into reaction sintering and atmospheric sintering.

Reaction sintering involves reaction between the green components to form the sintering bond.

Atmospheric sintering are done in a specialized atmosphere such as that of inert gases. Few examples of pressureless sintering methods includes thermal plasma sintering, microwave sintering and milliwave sintering.

Pressurized Sintering

Pressurized sintering can be further classified as solid compaction sintering and gas compaction sintering.

Few examples of pressurized sintering are hot pressing, hot isostatic pressing, super high-pressure sintering, spark plasma sintering and high pressure gas reacting sintering.

The figure below shows a taxonomy of sintering processes.

A taxonomy of sintering process

There are several different types of sintering, each with their own unique characteristics and applications.

  1. Cold Isostatic Pressing (CIP) Sintering: CIP sintering is a process where powders are placed in a sealed container and then subjected to high pressure. This process is often used to create dense, homogenous parts with complex shapes.
  2. Spark Plasma Sintering (SPS): SPS is a process where a spark is used to heat powders in a sealed container. This process is often used to create high-density parts with high strength and excellent electrical conductivity.
  3. Hot Isostatic Pressing (HIP) Sintering: HIP sintering is similar to CIP sintering, but the powders are heated to a high temperature before being subjected to high pressure. This process is often used to create high-density, homogenous parts with excellent mechanical properties.
  4. Field-Assisted Sintering: Field-assisted sintering uses an electric field to compact powders. This process is often used to create high-density, homogenous parts with excellent mechanical properties.
  5. Microwave Sintering: Microwave sintering uses microwave energy to heat powders. This process is often used to create high-density, homogenous parts with excellent mechanical properties and is faster than traditional sintering methods.
Reactive Sintering

Reactive sintering is a process that involves the addition of a reactive element to the powders being sintered. During the sintering process, the reactive element reacts with one or more of the other elements in the powder, forming a new compound that bonds the powders together. This process results in the formation of a solid object with unique properties that are not possible to achieve through traditional sintering methods.

There are several types of reactive sintering, each with their own unique characteristics and applications. Some common types of reactive sintering include:

  1. Self-propagating High-temperature Synthesis (SHS): SHS is a reactive sintering process where a mixture of powders is ignited, and the reaction proceeds through a self-sustaining exothermic reaction. This process is often used to create high-density, homogenous parts with excellent mechanical properties.
  2. Reactive hot pressing (RHP): RHP is a process where powders are placed in a sealed container and then subjected to high pressure and high temperature. This process is often used to create high-density, homogenous parts with excellent mechanical properties.
  3. Reactive liquid-phase sintering (RLPS): RLPS is a process where a liquid phase is added to the powders being sintered. The liquid reacts with the powders, forming a new compound that bonds the powders together. This process is often used to create high-density, homogenous parts with excellent mechanical properties.

Reactive sintering can be used to create parts with unique properties, such as high strength, excellent electrical and thermal conductivity, and improved wear resistance. It is used to sinter a wide range of materials, such as ceramics, metals, and alloys. Applications of reactive sintering include aerospace, biomedical, and electronic industries.

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