The term fouling summarizes deposits generated by crystallization, corrosion or by chemical or biological reactions. In Germany alone fouling leads to economic losses in the range of several billion euros. Fouling regards for example surfaces of valves or heat exchangers in the chemical, pharmaceutical or food industry. Such fouling deposits cause a considerable rising of the thermal resistivity and reduce the efficiency e. g. of heat exchangers. Modified hydrogenated amorphous diamond-like carbon coatings (DLC, a-C:H) have the potential for considerable reduction or even for complete avoidance of fouling.
Coating properties and preparation method
DLC or a-C:H coatings can be modified by incorporation of suitable elements like silicon, oxygen or fluorine in such a way that the surface energy of the coatings decreases and the anti-adhesive properties increase. These modifications lead only to a moderate decrease of hardness and wear resistance of the coatings. Thereby these coatings are completely different to Teflon® which exhibits a low surface energy but also a low hardness and wear resistance. At the Fraunhofer IST modified a-C:H:X coatings were developed and prepared by Plasma-assisted Chemical Vapor Deposition (PACVD). These a-C:H:X coatings (a-C:H:Si – SICAN and a-C:H:Si:O – SICON®) were tested at the Institute for Chemical and Thermal Process Engineering (ICTV), Technical University Braunschweig with respect to their antifouling properties.
As model substance in the tests an aqueous solution of calcium sulfate (CaSO4) was used. The effect of the coatings was assessed comparing the so called induction times of coated and uncoated surfaces. After the induction time the deposition of fouling products begins and the thermal resistance (fouling resistance) increases measurably. With these modified a-C:H:X coatings it is possible to reach a considerable extension of the induction time in comparison to uncoated stainless steel surfaces. In the diagram the beneficial effect of the modified DLC coatings is clearly visible. The effect of coatings is especially pronounced if the CaSO4 solution is flowing. In the demonstrated case a thick deposit grew on the uncoated steel surface, whereas the coated surfaces remained even after several 100 hours nearly unaffected.
Fields of application
Similar effects of DLC coated surfaces could be achieved for other applications, for example for the treatment of milk (reduction of whey protein deposition) or for soot reduction in exhausts. The effects of the a-C:H:X coatings could be further improved if the coated steel substrates were electro polished before coating deposition.
Advantages of antifouling coatings
- Considerable increaseof induction times (more than factor 10) and increased service life
- Reduction of efforts for maintenance and cleaning
- High mechanical and chemical stability
- Reduced adhesion forces (easy-to-clean)
Future coating developments for antifouling applications will be focused to new application fields like protection of sensors from impairments by fouling and improvements of coatings for components of process technologies like heat exchangers, valves and on inner walls of pipes. Due to the limited thermal application range of DLC coatings (operating temperatures < 300 °C), alternative antifouling coatings are under development. Modifications of transition metal nitrides like chromium nitride (CrN) have promising properties to be a suitable approach.
- Preparation of modified DLC coatings and other coatings for user-specific applications (prototypes and small batches)
- Development of concepts for the optimization of technical components like heat exchangers (in cooperation with the Technical University Braunschweig, ICTV)
- Coating deposition process development and consulting in the selection of most appropriate coatings for specific applications