Recubrimientos en base carbono tipo DLC para proteger aceros en situaciones de desgaste y corrosión

Machine components are designed to withstand various mechanical stresses. However, surface degradation due to wear or corrosion occurs more frequently than failures caused by mechanical stress. Such surface damage can create stress concentrations, potentially leading to catastrophic failures, production downtime and economic losses. Surface engineering addresses these problems by enhancing properties such as hardness, wear and corrosion resistance, or reducing the friction coefficient. This is achieved through physical or chemical surface modifications or by applying coatings with tailored properties.
Amorphous carbon coatings, usually known as Diamond-like Carbon or DLC, are recognized for their excellent properties, including high surface hardness, an outstanding tribological behavior, i.e. very low friction coefficient and good wear resistance, as well as chemical stability and corrosion resistance. Furthermore, doping these coatings with different elements enables properties adjustments to meet diverse application requirements. However, they face challenges such as surface defects that may compromise corrosion resistance and stability under extreme conditions, as well as adhesion issues on certain substrates, especially when the hardness difference is significant. Combining techniques, such as nitriding prior to DLC deposition, can significantly improve its adhesion and wear resistance. However, further optimization is required to overcome these limitations, especially for applications demanding simultaneous high wear and corrosion resistance.
The main goal of this thesis was to generate scientific and technological knowledge about different types of DLC coatings, whether deposited as a single layer, as part of multilayer systems or in combination with other surface treatments. Several plasma-assisted techniques and different precursors were employed to develop coatings tailored to industrial needs, aiming to reduce energy consumption and emissions, extend the service life of mechanical components and minimize costs associated with maintenance shutdowns.
For this purpose, DLC coatings were deposited on AISI 4140 steel samples using Physical Vapor Deposition (PVD) techniques such as cathodic arc discharge and magnetron sputtering, the latter also applied in a particular case to generate a multilayer coating with DLC as the top layer. Chemical Vapor Deposition (CVD) techniques, including Plasma-based Ion Implantation and Deposition (PII\&D) and Plasma-Assisted CVD (PA-CVD), were also employed. Coatings were characterized using SEM with EDS, X-ray diffraction, Raman spectroscopy, and XPS. Mechanical properties were evaluated via nanoindentation, the adhesion was assessed by means of indentation and scratch tests, and the wear performance was determined with Pin-on-Disk and abrasion tests with rubber wheel and dry sand. Corrosion resistance was evaluated through potentiodynamic techniques using a NaCl solution as electrolyte.
The results showed that DLC coatings significantly reduce the friction coefficient, reaching values as low as 0.05, and minimizing wear under sliding conditions, with wear reductions reaching up to a couple of orders of magnitude, although the humidity of the environment influenced its performance. The multilayer coating with a-C:H:Cr as top layer exhibited excellent resistance to sliding and abrasive wear when applied on a previously nitrided substrate, which also enhanced corrosion resistance. Additionally, thick a-C:H and a-C:H:Si coatings produced via PA-CVD showed superior adhesion. The softer a-C:H presented better performance in sliding wear, while the harder a-C:H:Si demonstrated outstanding resistance to abrasive wear and corrosion, the latter thanks to its electrically insulating nature that allows it to act as an effective barrier against electrolyte.