Abstract: 

Diamond-like Carbon (DLC) coatings have been recognized as one of the most valuable engineering materials for various industrial applications including manufacturing, transportation, biomedical and microelectronics. Among its many properties, DLC stands out for good frictional behaviour combined with high surface hardness, offering an elevated protection against abrasive wear. DLC coatings are produced by energetic deposition methods that are in continuous
progress. The intensive dedication to improve current deposition methods with novel technologies such as High Power Impulse Magnetron Sputtering (HiPIMS) encourages the optimization of the tribomechanical properties of DLC coatings. Nevertheless, substantial challenges remain in improving deposition techniques. Compressive intrinsic stresses develop in dense films as a consequence of the intense bombardment during deposition and lead to the delimitation failure of the coatings in many industrial applications. Another factor limiting the widespread
application of DLC coatings is their thermal stability. DLC is very temperature-sensitive since its sp3-sp2 structure undergoes a graphitization process at high temperatures that deteriorates both hardness and coefficient of friction.

The aim of this thesis is to improve the tribomechanical properties of DLC coatings at high temperature. Innovative high power impulse magnetron sputtering (HiPIMS) deposition processes are introduced to optimize the performance of coatings. High temperature tribomechanical characterization is an unexplored area for DLC coatings that will be evaluated in this work with nanoindentation and pin-on-disk tests. In parallel to these studies, special attention will be given to improve the adhesion of coatings to steel substrates, as it is a problem of great industrial relevance.

In this FYA document, novel methods for depositing DLC coatings are presented. Films were grown using magnetron sputtering with a magnetic configuration optimized for C-based compounds applying DC, DC-Pulsed and HiPIMS plasma excitation modes. The influence of hydrogen concentration on the mechanical properties is studied as it is possible to tailor DLC properties from wear resistant and hard (H ⇡ 25GPa) to ultra-low friction (m ⇡ 0.1) coatings.

An adhesion enhancement process to improve the adhesion of DLC coatings to steel substrates using HiPIMS is also presented. Highly ionized metal flux generated with HiPIMS allows to efficiently remove contaminants of the surface and to incorporate metals into the substrate. This HiPIMS ion etching pretreatment was carried out with chromium and titanium in order to evaluate the influence of factors such as the ionization degree, the energy flux or the depth of incorporation. Adhesion enhancement is examined with Rockwell and nanoscratch testing, obtaining outstanding critical load values for those coatings prepared with HiPIMS pretreatment method.

After completing the tribomechanical characterization at room temperature, high temperature nanoindentation measurements are being carried out to study the evolution of various DLC coatings with temperature. Information about the range of temperature where DLC loses its excellent tribomechanical properties as well as the underlying physical processes involved still
remain unclear. New strategies to extend the temperature range of application will be the subject of further study as it constitutes a central topic of this thesis.