Structural Design and Tribological Behavior of Graphite-like Carbon Multilayer Films on 15-5PH Stainless Steel

Martensitic precipitation-hardened stainless steel 15-5PH (0Cr15Ni5Cu4Nb) is used for some critical mechanical moving parts in marine equipment, including connecting rods and shafts, which function in the seawater environment for a long time, and are subjected to the dual effects of friction and corrosion. The tribological properties of these moving parts in seawater environment significantly affect the safety and durability of marine engineering equipment. An effective way to improve their tribological properties involves preparing a protective layer of antifriction and wear-resistant coatings on the surface of the friction pair. However, conventional organic protective coatings can not meet the corresponding requirements because seawater is a strong corrosive medium. In this study, to address the problem of corrosion-wear of 15-5PH in seawater environment, Cr / GLC multilayer films with modulation periods of 940, 375, and 234 nm (marked as Cr / GLC-S1, Cr / GLC-S2, and Cr / GLC-S3, respectively) were deposited on 15-5PH stainless steel using DC magnetron sputtering. The structure and tribological properties of the graphite-like carbon films were investigated using scanning electron microscopy, Raman spectroscopy, and an MFT-5000 multifunctional tribometer, among other instruments. From the results, the surfaces of the graphite-like carbon multilayer films with different modulation periods present obvious cauliflower-like particle morphology. As the modulation period decreased, the size of the cauliflower-like particles gradually decreased, the film became denser, and the surface roughness of the multilayer films increased. With a decrease in the modulation period, the content of sp(2) bond increased gradually, the degree of graphitization intensified; and the adhesion, hardness, and elastic modulus of the graphite-like carbon films increased gradually, resulting in excellent mechanical properties. Under dry friction, the Cr / GLC-S2 film with a moderate modulation period exhibited better tribological properties, and its wear form was abrasive. However, the Cr / GLC-S1 film with larger modulation period and Cr / GLC-S3 film with smaller modulation period exhibited different degrees of brittle spalling at high loads, resulting in direct contact between the substrate and the couple ball, which resulted in a higher wear rate and poor tribological properties. Under an artificial seawater environment, owing to the lubrication of seawater and the formation of transfer film, the friction coefficients of the three types of graphite-like multilayer films were significantly lower than those of dry friction. The tribological properties of Cr / GLC-S1 and Cr / GLC-S2 were better at medium and low loads. In addition, the wear form remained predominantly abrasive wear because the thicker GLC surface layer produced a good solid lubrication effect, and the formation of penetrating corrosion channels in the film was delayed, resulting in slight wear. However, the three types of multilayer films exhibited different degrees of brittle spalling at high loads. The Cr / GLC-S3 film with a small modulation period was worn out, a large area of the substrate was exposed, and the film was seriously damaged, losing its protective effect on the substrate. Because of the thinner depth of the GLC and Cr sublayers, the longitudinal growth gap formed in the deposition process quickly developed into longitudinal cracks under the interaction of the reciprocating load and corrosion of seawater. Longitudinal cracks quickly penetrated the entire film into the substrate, forming penetrating corrosion channels. Seawater along these etching channels penetrate through the entire film to the matrix, and the combination of the substrate and interlayer is weakened, resulting in serious spalling. Therefore, the design of a reasonable modulation period is the key to improving the tribologicalproperties of GLC films under different working conditions. These results provide a reference for the practical application of graphite-like carbon films in marine protection.