DLC coatings on steel substrates for the EFFIMEC project

As part of the EFFIMEC project, to increase surface performance, we produced DLC (Diamond Like Carbon) coatings on stainless steel substrates for Food and Packaging machinery.

The EFFIMEC project (EFFIMEC is an acronym for High-efficiency production systems for special mechanical components) aims to develop innovative process/product systems in the packaging machine sector and in the aerospace sector.

As far as the food packaging sector is concerned, the project focuses on the mechanical assemblies of critical tubular circuits of automatic machines.
The various components of automatic packaging and wrapping machines are subject to wear and tear, corrosion and possible organic contamination of surfaces.

This gives rise to the specific objectives we set ourselves as Ecor International:

Increasing surface performance, in terms of corrosion resistance and cleanability.

Improving the sustainability of the cleaning process of a system: reducing the frequency of washing, the quantity of chemicals and water. This is on a theoretical level, to be validated later through further studies.       

Let’s take a closer look at one of the development lines of the EFFIMEC project, the one that concerns the production of DLC (Diamond Like Carbon) coatings on stainless steel substrates (in particular on AISI 316L steel), used for the manufacture of automatic machines in the Food and Packaging sector.

The development of a DLC coating

This type of coating is usually produced using a technology called PECVD, Plasma Enhanced Chemical Vapor Deposition.    

The Research Centre Il Sentiero International Campus, founded in 2017 by Ecor International, is equipped with a machine that incorporates PVD and PECVD technologies. Thanks to this system, it is possible to deposit thin films (metallic and ceramic) with a thickness from hundreds of nanometres to tens of microns.

This is a chemical-physical process that uses a precursor, usually a gaseous hydrocarbon, through which the coating itself is produced in a high-vacuum chamber in which the steel substrates are arranged. DLC coatings made using PECVD technology also contain hydrogen atoms in their microstructure, bound to carbon atoms, the concentration of which can vary depending on the parameters used and the precursor gas used. This class of DLC coatings is usually referred to as a-C:H.

As part of the EFFIMEC project, a-C:H DLC coatings are produced using PECVD technology and acetylene as a gaseous precursor.   

Picture 1 shows a high-magnification detail of the surface of a-C:H-type DLC coating obtained by PECVD on a steel substrate with a surface finish of Ra<0.18;
the micrometric and sub-micrometric globular morphology of the coating microstructure can be appreciated.

Optimisation of a DLC coating

Another important aspect in the development and optimisation of a DLC coating concerns its adhesion to the substrate, which may vary depending on the nature of the material on which it is deposited: e.g. polymers, metal alloys etc.

In the case of coatings on stainless steels, such as those used for Food and Packaging or pharmaceutical machines, in order to improve the adhesion of the coating on the substrate, a series of metal-ceramic interlayers are realised to create interfaces with a high chemical affinity between the individual layers; these interlayers, usually made of transition metals, are also able to offer improved corrosion protection of the metal substrate.

Within the EFFIMEC project, transition metals with high corrosion resistance properties and high affinity for carbon such as titanium, tantalum and/or niobium will be used for the construction of the substrate adhesion/protection interlayers.     

Picture 2 shows in detail the PVD/PECVD system owned by Il Sentiero International Campus used for the production of DLC coatings.

Picture 2 – Ecor International’s PVD-PECVD system: a) closed deposition chamber + controller, b) detail of open deposition chamber

Functionalisation of surfaces through the application of DLC coatings:

The DLC coatings developed in the EFFIMEC project will be optimised according to the component they will be applied to and its specific operating conditions, in view of the specific objectives listed at the beginning.

The functionalisation of component surfaces by applying DLC coatings can be distinguished in:

1.         Surfaces with improved cleanability: DLC-coated 316L steel component systems with improved cleanability properties compared to the surface of steel as is.

2.         Surfaces with antibacterial properties: systems of 316L steel components coated with Ag-doped DLC will be manufactured to give antibacterial properties to surfaces more subject to possible organic contamination phenomena.

3.         Surfaces with improved resistance to tribocorrosion: martensitic steel/DLC-coated component systems will be realised to increase the average life of the component, improving its corrosion resistance properties.

Validation of developed solutions:

Coatings will be characterised through laboratory tests such as:
– nanoindentations,
– scratch tests,
– electron microscope microstructural analysis 
– tribocorrosion tests.

The EFFIMEC project also includes the design and construction of test benches capable of reproducing the real operating conditions of specific machine parts.

These benches will be used to study the effectiveness of the solutions developed, such as DLC coatings, and they will be integrated on components identified as critical as they are potentially subject to wear, corrosion and organic contamination.

Picture 3 shows a portion of a tube used in machines for packaging food fluids, such as milk or fruit juices, selected as a prototype industrial component, as it is subject to possible surface contamination by food fluids on whose surface a DLC coating has been applied.

This component will be compared to the same component without coating by studying on the test bench the cleanability of the surface after organic contamination, compared to the washing cycles adopted by protocol on industrial plants.

Picture 3 – Overview (left) and detail image (right) of a tube for the circulation of food fluids in food packaging machines, surface coated with DLC using PECVD technology