In recent years the use of nitrided substrates for hard coatings has been widely reported. The majority of work refers to the coating of high alloy steels with a thin nitrided case, in order to achieve higher wear resistance and better adherence of the hard coating. These composite systems are mostly used as cutting tools. Beside that a lot of research work has been done on investigating tribological properties of DLC films, which were deposited on ‘hard’ substrates like ceramics. However, requirements for machine parts are quite different from those for tools. In addition to a hard, wear and fatigue resistant surface with good frictional characteristics, a tough, fracture resistant core is necessary. In the contrast to high alloy steels such as tool steels, hardened and tempered low alloy steels have high fracture toughness. Unfortunately, due to poor load carrying capacity of the steel substrate and very thin nature of the hard coating, hard-coated engineering low alloy steels would fail at high surface pressures, especially in the case of DLC coatings. On the other hand high hardness and internal compressive stresses of the case formed by plasma nitriding can lead to increased load carrying capacity as well as to improved coating adhesion, fatigue strength and tribological properties of coated parts. Undoubtedly, optimisation of the entire coating-substrate system is necessary. Properties obtained by the combination of plasma nitriding with hard-coating would allow function sharing between the core material, the hardened case and the surface, which is of special interest for application in complex stressed machine components.

The intention of the present work was to open the possibility of using hard DLC coatings in the field of machine elements. Therefore, samples made of AISI 4140 steel pre-treated by plasma nitriding and coated by a hydrogen-free hard carbon coating were investigated with respect to the microhardness, residual stress, scratch adhesion and the dry sliding wear resistance. Wear tests in which duplex treated pins were mated to hardened ball bearing steel discs were performed under unidirectional and reciprocating sliding. In order to determine the influence of the nitrided zone on the tribological properties of coating-substrate composite coating was deposited on hardened as well as on plasma nitrided samples, nitrided under different nitriding conditions.

The results of the investigation showed improved mechanical and tribological properties of plasma nitrided hard-coated specimens compared to un-coated and pre-hardened ones, observed for both unidirectional and reciprocating sliding. Furthermore, compound layer was found to act as an intermediate hard layer leading to superior tribological properties.

Recently, amorphous carbon (a-C) and carbon nitride (a-CN_x) films have attracted a great interest to be used as thin protective coatings for the improvement of the wear resistance magnetic hard disks and for IR optical surfaces.

The present study discusses the effects of the technological parameters on the structural and vibrational properties of amorphous carbon and carbon nitride films obtained by Plasma Enhanced Chemical Vapour Deposition.

The films were deposited on silicon and Corning glass substrates at room temperature by rf glow discharge decomposition of methane and nitrogen. The technological parameters were changed as follows: the rf-power between 1W and 200W, the bias voltage between -40V and -600V and the deposition pressure between 1Pa and 5Pa. In the amorphous carbon nitride series the gas mixture of methane and nitrogen was varied in order to control the stoichiometry of CN_x.

The compressive stress of the films ranged from 1Gpa and 5Gpa depending on both, the bias voltage and deposition pressure on the a-C and a-CN_x films. The films have been also analysed by Fourier-transform infrared spectroscopy, Raman spectroscopy and ellipsometry. The composition of the films of a-CN_x was studied with X-ray photoelectron spectroscopy.

Diamond-like carbon (DLC) films, also referred to as hydrogenated amorphous carbon, have superior physical and chemical properties such as high hardness, low coefficient of friction, high wear resistance and chemical inertness. However, the films have high residual compressive stress up to 10 GPa resulting in the limited coating thickness, beyond that the film is spontaneously delaminated from substrates. Especially, the poor adhesion on steels substrate is significantly limit the applications of the DLC films. Many attempts has been reported to improved the adhesion of the DLC films by choosing a suitable buffer layer and optimizing the pre-treatment of the substrate surface.

In the present work, we investigated the W buffer layer deposited by DC magnetron sputtering. SKD11 block of HRC 59 and rms roughness of 0.4µm was used as the substrate. Substrate was cleaned by Ar plasma at the bias voltage of -750V for 1h. W buffer layer was deposited by DC magnetron sputtering using Ar or mixture of Ar and CH₄ as the sputter gas. 2µm thick DLC film was then deposited by r.f.-PACVD using benzene as the precursor gas. The adhesion of the film was tested by conventional scratch method using CSEM REVETEST scratch tester. The composition and the structure of the buffer layer was investigated by RBS and high resolution TEM. We found that the adhesion of the film was strongly dependent on the properties and composition of the WC buffer layer. The result will be discussed in terms of the microstructure and mechanical properties of the buffer layer.