Hot pressing

Over the past two years, we’ve often talked about our new hot press and new materials. But how exactly does the manufacturing process work? In this blog post, we’ll take a closer look at the hot-pressing process.

Hot pressing and sintering

Sintering is a well-known process for compacting powdered materials using high temperatures. Diffusion causes the particles to move closer together, resulting in increased compaction of the material, shrinkage of the component, and the elimination of pores.

The origins of hot pressing date back more than 100 years. As early as 1912, a patent application described a method for sintering powdered boron or other fusible materials—such as tungsten and high-melting-point carbides—in a direct current circuit in such a way that pressure is applied to the material being sintered as it is heated.[1]

If this sintering takes place at high temperatures and under external mechanical pressure, we refer to it as hot pressing; the process often occurs in an inert gas atmosphere.

In contrast to hot pressing, where the applied pressure is unidirectional or bidirectional, hot isostatic pressing (HIP) applies isostatic pressure, which is typically generated using a process gas. For example, in the case of boron nitride, which exhibits anisotropic properties due to its plate-like structure, sintered bodies with nearly isotropic characteristics can be produced.

In short, hot and hot isostatic pressing are characterized by the fact that the sintering process takes place during the shaping process through mechanical pressing.

The advantage of hot pressing over pressureless sintering lies in the higher degree of compaction that can be achieved and the lower porosity of the sintered material, which allows for the production of more homogeneous microstructures with improved performance characteristics, such as strength, hardness, and wear resistance.

Which material classes are better suited for sintering or hot pressing?

Metals such as iron and steel powders, copper, aluminum, and bronze alloys, precious metals, and hard metals—and especially technical ceramics such as aluminum oxide (Al₂O₃), silicon nitride (Si₃N₄), and aluminum nitride (AlN)—are sintered. Consequently, this applies to materials where shaping can take place independently of the actual sintering process. Sintering is always necessary for technical ceramics, as the raw materials are in powder form. For metals, sintering is more of a specialized process, as these materials are typically cast and then forged.

Hot pressing is typically used for materials whose sintering activity is too low during pressureless sintering, preventing them from compacting sufficiently (boron carbide B4C, titanium diboride TiB2, boron nitride BN). In other words, these are materials that require higher performance specifications or cannot be produced without external pressure. Hot pressing allows even higher densities to be achieved, for example, for materials such as silicon nitride (Si3N4) and aluminum nitride (AlN), which are otherwise produced using other heat treatment methods (e.g., gas pressure sintering).

What happens during hot pressing?

Powder production – Powder processing – Precompaction – Molding process and sintering

First, starting powders are required, which must be specially prepared. Mixing and grinding are among the most critical steps in this process, as they are essential for combining powder components—some of which vary greatly—into a powder mixture with the most uniform distribution possible, and for producing primary particles that are as fine and sinterable as possible. This process has a significant impact on the quality of the resulting sintered body.

To ensure the highest quality, it is necessary to characterize the powder mixture in terms of particle size distribution, flow properties, homogeneity, and bulk density.

Next comes the hot-pressing process. In the case of our hexagonal boron nitride, it must first be pre-compacted without applying heat. The reason for this is its low bulk density. Now the actual compaction process begins, which transforms the loose powder into a more stable, less porous body.

Due to the low bulk density, a porosity of approximately 50% can be assumed in the pre-compacted state. To prevent oxidation at approximately 900 °C during the hot-pressing process, the material is heated in a nitrogen atmosphere.

The pressing process itself is divided into three phases: the heating phase, the isothermal holding phase at the respective sintering temperature, and the cooling phase. All of these phases are carried out under strict control and according to precise temperature profiles.

The compression follows this pattern