Evaluation of elastoplastic properties of DLC coating on SKD61 steel by optical indentation microscopy

Diamond-like carbon (DLC) is well known as the unique properties of very hard, very low friction and low wear. That is why, the DLC coating has succeeded in commercializing on various field, including tools and/or diesʹ surface treatment. The life times of tools and dies are extended significantly. However, the mechanical properties of the DLC coating have not been understood well, yet. One of the authors (TM) has developed a novel method to measure the mechanical properties, such as Youngʹs modulus, through the in situ determination of a true contact area during an optically transparent tip indentation by observing the contact image with CCD camera. The system, we called “Optical Indentation Microscopy” (OIM), enables one to determine true indentation stress-strain property. With this OIM, we have measured the mechanical properties of DLC coated and uncoated SKD61 (JIS or X40CrMoV5-1, ISO) steel samples. DLC coating made by our bipolar pulse PBII system. The thickness of DLC coatings were about 7 µm. Berkovich shape tip for usual indentation test and spherical-corn shape diamond indenter tip (0.4 mm in radius) for OIM were used. The maximum load and depth were about 2.5 N and 1 µm, respectively. Usual indentation test with Berkovich tip revealed that the DLC coated sample is harder than uncoated one, and the load-total penetration depth plots in logarithm shows the data of the DLC coated sample can be analyzed as if the tip shape is spherical. From the result of OIM, Youngʹs modulus, yield stress and elastic limit of the DLC coated and uncoated SKD61 samples are 156 GPa and 201 GPa, 2.4 GPa and 2.6 GPa, and 2.2 GPa and 1.1 GPa, respectively. The DLC coating can improve the elastic limit of SKD61 by 100%. This effect of DLC coating is very useful to improve the lifetime of tools and dies.