Influence of Plasma-NiTRIDING on the ADHESION OF DLC FILMS

Abstract

DLC films have special properties, which are of great interest in fields of tribology, e. g. the high hardness and the low coefficient of friction. These properties are limited because of shortcomings in adhesion of DLC films on steel. So far, metallic adhesive layers like titanium and chromium are established options for the improvement of the adhesion of DLC films on steel. However, it is hardly investigated how plasma nitrided surfaces increase the adhesion by itself.Beginning for this research were the different surface modifications, which can occur through plasma nitriding. Three different surface modifications were investigated in detail: Compound layer-free (α), γ'-nitride compound layer and ε-nitride compound layer. These surface modifications differ in their composition, crystal structure, bond type, as well as their solubility and diffusion velocity of carbon. By means of a two-stage nitriding process, it was ensured that the hardness depth profile of all three surface modifications were similar. Thus, the "eggshell" effect should have no significant influence on the research. Afterwards, the nitrided specimens were DLC coated by PACVD. Tests, executed on a scratch test rig, indicate that plasma nitriding can improve the adhesion of DLC films. LC2 values of up to 60 N could be achieved. Typical values of LC2 DLC films with metallic adhesive layers are about 25 N. During the investigations, it became apparent that adhesion in this system is strongly influenced by other factors, such as environmental conditions and polishing processes.

Hot die forging with nitrided and thermally stabilized DLC coated tools

“…They determined that an increased sp³ content increases the hardness, but also reduces the toughness and thereby the adhesion of the coating [10]. Another promising approach to increase coating adhesion is prior plasma nitriding, which results in a less precipitous hardness gradient [11]. Due to the diamond bond's low thermal stability, the field of applications of DLC coatings is limited, i.e.…”

Metal-Containing Diamond-Like Carbon Coatings with Increased Temperature Stability for Wear Protection of Nitrided Hot Forging Dies

“…According to Kipp et. al, compound layers exhibit an increased support effect, since the lower hardness gradient reduces coating spalling, also known as eggshell effect [14].…”