Over the past two years, we have often reported on our new hot press and new materials.
But how exactly does the production process actually work? In this blog post, we want to take a closer look at the hot pressing process.
Sintering technology is a generally known process for compacting powdery materials with the aid of high temperatures. Diffusion processes lead to the approximation of particles, which results in an increasing compaction of the material or shrinkage of the component and a reduction of pores.
The beginnings of hot pressing date back more than 100 years. As early as 1912, a patent proposal described sintering powdered boron or other fusible materials, such as tungsten and refractory carbides, in a direct current passage in such a way that the heat exerts pressure on the mass to be sintered[1].
If this sintering takes place under high temperatures and external mechanical pressure, we speak of hot pressing; the process often takes place in an inert gas atmosphere.
As an alternative to hot pressing, where the applied pressure is unidirectional or bidirectional, hot isostatic pressing (HIP) involves the application of isostatic pressure, which is usually applied with the help of a process gas. In the case of boron nitride, for example, which has anisotropic properties due to its platelet-like structure, sintered bodies with almost 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 by means of mechanical pressing.
The advantage of hot pressing over pressureless sintering lies in the higher achievable compaction and lower porosity of the sintered material, which means that more homogeneous structures with better performance, e.g. strength, hardness and wear resistance, can be produced.
Metals such as iron and steel powder, copper, aluminum and bronze alloys, precious metals and also hard metals and, above all, technical ceramics such as aluminum oxide Al2O3, silicon nitride Si3N4 and aluminum nitride AlN are sintered. These are materials for which shaping can take place independently of the actual sintering process. Sintering is always necessary for technical ceramics, as the starting materials are in powder form. For metals, sintering is more of a special process, as these materials tend to be cast and then forged.
Hot pressing is typically used for materials whose sintering activity is too low in the pressureless sintering process and therefore do not densify sufficiently (boron carbide B4C, titanium diboride TiB2, boron nitride BN ), i.e. materials for which the property requirements are higher or which cannot be produced without external pressure. Hot pressing allows even higher densities to be achieved 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).
Powder production Pulver preparation - Pre-densification - Shaping process and sintering
First of all, starting powders are required that have to be specially prepared. Mixing and grinding is one of the most important process steps in order to produce a powder mixture with the best possible statistical distribution of sometimes very different powder components and to obtain primary particles that are as fine as possible and suitable for sintering This process significantly influences the quality of the resulting sintered body.
In order to guarantee the best quality, it is necessary to characterize the powder mixture with regard to particle size distribution, flow behaviour, homogeneity or tap density.
This is followed by the hot pressing process. In the case of our hexagonal boron nitride, this must be pre-compacted in the first step without the effect of temperature. The reason for this is the low bulk density. Now the actual compaction process begins, which transforms the loose powder into a more stable and less porous body.
Due to the low bulk density, a porosity of approx. 50 % can be assumed in the pre-compacted state. To prevent oxidation at around 900 °C during the hot pressing process, the material is heated under nitrogen.
The pressing itself is divided into three phases: the heating phase, the isothermal dwell phase at the respective sintering temperature and the cooling phase. All these phases are strictly controlled and follow exact temperature curves.
The densification follows this pattern
The cooling phase is followed by demolding and cleaning as well as the first visual inspection of the sintered body for cracks or abnormalities.
During the subsequent quality control, characteristics such as density, open porosity, mechanical strength and electrical properties are determined. In addition, the sintered structure can be examined using a scanning electron microscope and suitable detectors to assess the homogeneity and chemical composition of the material.
After approval, HeBoSint® components can be manufactured for our customers from this sintered body using sawing and conventional machining methods.
The design of a hot press is complex and highly specialized. Each system and its components are adapted to the respective application. The design of the heating chamber, the heart of the hot press, as well as the pressing tool consisting of the pressing punch and die are individual. In addition to these factors, the pressure and temperature control system and the process gas play a decisive role in successful compaction. The design requires extensive know-how and many years of expertise in order to produce high-quality non-oxide ceramics. In addition to boron nitride ceramics, other materials can be produced on the Henze BNP hot press. If you are interested in our hot pressing capacity, we will be happy to check your request individually.
- Unique heat recovery system
- 2200°C maximum temperature
- Maximum component height 300mm
- Maximum component diameter 470mm
- Other dimensions possible for other materials
Sources:
[1] Dr. Richard Kieffer und Dr. Werner Hotop, Pulvermetallurgie und Sinterwerkstoffe, 2. Auflage S. 139, Springer
[2] Werner SChatt, Klaus-Peter Wieters, Bernd Kieback, Pulvermetallurgie – Technologie und Werkstoffe, 2. Auflage, Springer
[3] H. Salmang, H. Scholze, Keramik, 7. Auflage, Rainer Telle
[4] Brevier Technische Keramik (keramverband.de)
