The last two articles dealt with the mechanical, thermal and electrical properties of boron nitride ceramics, in particular our HeBoSint® SL-N 300. Today we are focussing on chemical resistance and relative density.
Corrosion: When we think of corrosion, we usually only think of metals, but corrosion describes the chemical attack of a medium on the respective material. Wear, on the other hand, refers to wear of the material due to tribological stress.
Types of corrosion: In general, there are four types of corrosion in technical ceramics: surface corrosion, selective corrosion, corrosion of defects and intergranular corrosion along the grain boundaries. However, in practice it is usually not possible to clearly differentiate between these, which is why we would like to take a closer look at the influencing factors of the application, the material and their influence on possible corrosion.
Application temperature: With regard to high-temperature corrosion, the thermal weak point of the respective material must also be considered. As soon as a melting or decomposition temperature of the respective phase is reached, the probability of a corresponding chemical reaction and thus corrosion increases.
With this in mind, the SL-N 300 material is very well suited for use in metallic castings. For metallic melts, the working temperatures are usually in the range of 1400 to 1600 °C. This shows that the temperature resistance of the two main phases of HeBoSint® SL-N 300, boron nitride and SiAlON, can withstand a possible chemical attack well, as their temperature resistance is well above the melting temperature of the metals. Most oxide ceramics can already be ruled out for such applications.
Reaction partners: In order to be able to assess possible corrosion, the compounds and elements involved must be analysed more closely. It has been shown, for example, that base metals in particular tend to oxidise. The absence of oxygen in non-oxide ceramics is therefore advantageous.
However, oxide ceramics show an advantage in the presence of excess oxygen. Here, non-oxide ceramics tend to oxidise even at low temperatures, which is why the HeBoSint® SL-N 300 is resistant up to approx. 900°C in air. However, if the surrounding atmosphere is inert, e.g. argon or nitrogen, a reaction can only be expected at temperatures above 1800°C. This is due to the nitride character of the material. This can be explained by the nitride character of SL-N 300.
Relative density/porosity: The reaction surface is also decisive for corrosion. This in turn is strongly influenced by porosity. This means that materials with low or no open porosity offer as little surface area as possible for chemical attack. In the field of boron nitride ceramics, HeBoSint® SL-N 300 is a material with little to no open porosity, which can represent a corrosive advantage in view of the aforementioned.
As described, corrosion is always influenced by several boundary conditions, which is why the combination of several properties usually leads to the selection of the most suitable material.
Conclusion and outlook
In our three-part HeBoSint® SL-N 300 series, we have analysed the mechanical, thermal, electrical and now also chemical properties. A strong character in every respect. A closer look at the individual properties reveals how they interact and influence each other. With this material, we have succeeded in creating a good balance and placing a composite ceramic on the market that fulfils the highest demands.
If you would like to find out more or would like advice, our technical sales team will be happy to help. We will be happy to advise you and work with you to find out whether our HeBoSint® SL-N 300 could be the right solution for you.
Link to technical advice: https://www.henze-bnp.de/service/technischer-vertrieb.php
