Designing multilayer diamond like carbon coatings for improved mechanical properties

New multilayer coatings were produced by incorporating alternating soft and hard DLC layers enabled by varying the bias voltage during deposition process while maintaining a constant hard-to-soft layer thickness ratio. These coatings were deposited onto a Cr/CrC_x graded layer by closed field unbalanced magnetron sputtering (CFUBMS). The cross-sectional analysis of the coatings showed that the multilayer coatings possess sharp interfaces between the soft and hard layers with the hard to soft layer thickness ratio (1:1.33) constant in all the coatings. Raman analysis uncovered the increasing sp³ character of the DLC coatings as a result of decreasing I_D/I_G ratio and increasing full width at half maximum (FWHM) values of the G band peak induced supposedly by an increase in bias voltage during hard layer deposition. Nanoindentation tests showed an increase in hardness of the DLC coatings which can be correlated with the increase in the sp³ content of the coatings as well as decreasing sp²-C cluster size, as calculated from the I_D/I_G ratio. Furthermore, the coatings exhibited excellent plastic deformation resistance and adhesion strength upon microindentation and scratch testing, respectively. Although further investigations are required to assess coating durability, the multilayer design could offer the DLC coatings with a rare opportunity to combine the high hardness with damage resistance with a constant bilayer thickness and without the need to introduce complex multilayer system.