Effect of chromium on the microstructure and mechanical properties of amorphous carbon coatings

Amorphous carbon (a-C) films, known for exhibiting an attractive combination of mechanical and physical properties, have been extensively studied. However, the inherent residual stresses affecting their adhesion to substrates often prevent the growth of thicker films and limit the usage of carbon films. Carbide forming metals such as W, Ti, Cr and Al, when incorporated into the carbon network, can help to stabilise the film structure, reduce compressive residual stresses and improve performance. This thesis focuses on the microstructure, composition, and mechanical property characterization under contact loading of amorphous carbon based films containing chromium. The films were fabricated using an unbalanced magnetron sputtering deposition system, with four C and two Cr targets and Ar gas. M2 steel, with two different hardness values (Rockwell 20 and 60), was used for the substrate, with roughly half of samples deposited on each substrate type. The Cr concentration in the films of the samples (from 0% up to ~48at.%) was controlled by varying the current (up to 3A) on the Cr targets. The microstructure and composition of the carbon were characterised by transmission electron microscopy (TEM), energy-dispersive x-ray spectrometer (EDS), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The hardness and elastic modulus of the coatings were analysed with nanoindentation tests using both a Hysitron Triboscope and a UMIS 2000 nanoindentation system. The latter indenter was used to study the deformation behaviour of the coating-substrate composites, with a maximum applied load up to 500mN. The indented areas were cross-sectioned and examined using a focused ion beam microscope (FIB). Raman and XPS analysis suggested that all the films had a low sp3/sp2 ratio. The results showed that both pure a-C, and carbon films with very low Cr contents, exhibited the highest hardness and elastic modulus, while a-C with medium Cr content (~16 at.%) were found to have the lowest hardness and modulus values. Incorporation of more Cr results in, possibly, the presence of chromium carbide phase and led to an increase in hardness compared to the medium-Cr samples. Films deposited on the softer substrate exhibited significant cracking on contact loading compared to the coatings on the harder substrates.This thesis focuses on the microstructure, composition, and mechanical property characterization under contact loading of amorphous carbon based films containing chromium. The films were fabricated using an unbalanced magnetron sputtering deposition system, with four C and two Cr targets and Ar gas. M2 steel, with two different hardness values (Rockwell 20 and 60), was used for the substrate, with roughly half of samples deposited on each substrate type. The Cr concentration in the films of the samples (from 0% up to ~48at.%) was controlled by varying the current (up to 3A) on the Cr targets. The microstructure and composition of the carbon were characterised by transmission electron microscopy (TEM), energy-dispersive x-ray spectrometer (EDS), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The hardness and elastic modulus of the coatings were analysed with nanoindentation tests using both a Hysitron Triboscope and a UMIS 2000 nanoindentation system. The latter indenter was used to study the deformation behaviour of the coating-substrate composites, with a maximum applied load up to 500mN. The indented areas were cross-sectioned and examined using a focused ion beam microscope (FIB). Raman and XPS analysis suggested that all the films had a low sp3/sp2 ratio. The results showed that both pure a-C, and carbon films with very low Cr contents, exhibited the highest hardness and elastic modulus, while a-C with medium Cr content (~16 at.%) were found to have the lowest hardness and modulus values. Incorporation of more Cr results in, possibly, the presence of chromium carbide phase and led to an increase in hardness compared to the medium-Cr samples. Films deposited on the softer substrate exhibited significant cracking on contact loading compared to the coatings on the harder substrates