The effect of TiN interlayers on the indentation behavior of diamond-like carbon films on alloy and compound substrates

Abstract

Diamond-like carbon (DLC) films have been deposited onto a polished, Cr alloy substrate, with and without the presence of a physical vapor deposition (PVD) TiN intermediate layer, using a saddle-field, fast-atom beam source at 40°C. The process has been utilized to deposit DLC films ranging in thickness from 0.5 to 2.0 υm. The thicker of these films exhibited great difficulty in maintaining adequate adhesion to the Cr alloy substrates, presumably due to the increase in intrinsic (growth) stresses. To investigate the influence of substrate properties on the measurement of elastic modulus and hardness, PVD has been used to deposit intermediate layers of TiN, 3.0 υm thick, between the substrates and the DLC films. In addition, a second technique, that of r.f. electron-beam PVD, has been employed to synthesize DLC onto an uncoated substrate of the tool steel, ASP23. Delicate ultralow-load indentation measurements have been enabled by the utilization of a commercially available nanoindenter to probe the properties of deposited films as a function of indentation load at each of four loads: 8, 20, 80 and 120 mN. These techniques have demonstrated that the TiN interlayer possesses both higher elastic modulus and higher hardness compared with the DLC coatings, having values of approximately 300 and 20 GPa, respectively, at an indentation depth of 350 nm. The hardness of the DLC films ranges from 13 to 16 GPa and varies with indentation depth; hardness and modulus values begin to approach those of the underlying substrate or interlayer with increasing indentation depths. At a constant load of 20 mN, the 2.0 υm thick DLC coating over the TiN interlayer shows the greatest hardness of the three thicknesses investigated, perhaps due to the greatercompressive growth stresses normally associated with thicker coatings.