Excellent adhered thick diamond-like carbon coatings by optimizing hetero-interfaces with sequential highly energetic Cr and C ion treatment

Highlights: • A novel interlayer design and interface engineering of thick DLC coatings is presented. • The critical load is improved to 73 N for a 50 µm thick DLC coatings. • The DLC coatings have a high hardness of >18 GPa and an excellent tribological properties. • The effects of the sequential energetic ion bombardments at the interfaces are revealed by TEM. Diamond-like carbon (DLC) coatings have attracted much attention due to their excellent hardness, low friction, and superior corrosion resistance. Unfortunately, the poor adhesion caused by internal stress limits the typical thickness to below 3–5 μm. This paper presents a novel interlayer design and interface engineering and the induced excellent adhesion of thick diamond-like carbon (DLC) coatings on a high speed steel substrate. The interlayer has fundamentally a graded Cr/CrC_x/CrC/DLC structure, but various treatments to the hetero-interfaces have been conducted including sequential energetic ion bombardments with Cr at the substrate/Cr interface using high-power impulse magnetron sputtering (HiPIMS) and with C at the CrC/DLC interface using an anode layer ion source. It has been observed that the critical load has been improved from 18 N for a single Cr interlayer to 77 N for a Cr/CrCx/CrC interlayer for thick DLC coatings of 13 μm. Using the same design, a very high critical load of 73 N has been achieved on the ultra-thick DLC coating of 50 μm. This interlayer design has allowed the deposition of a DLC coating that is not only thick but also hard with excellent tribological properties. The DLC coatings have a high hardness of >18 GPa, a low friction coefficient of 0.12 and a low wear rate of (1.7 ± 0.2) × 10⁻¹⁵ m³/N·m. This paper discusses the effect of the ion bombardment of the interlayer on the coating adhesion and the strengthening mechanisms.