Hardy Mohrbacher

Laboratory testing of displacement and load induced fretting

Abstract A conceptually new simulation system for the laboratory investigation of fretting wear is described. Fretting vibrations are generated either directly by oscillating a linear relative displacement device of constant stroke between the contacting bodies or indirectly by oscillating the applied contact load resulting in a cyclic radial expansion of the contact zone. The principles of both actuation mechanisms are outlined in detail, indicating the precision and performance range of the simulation system. A data acquisition and evaluation strategy has been developed for the on-line characterization of the mechanical contact response. It is based on the measurement of the contact displacement, tangential force and normal load. Typical experimental results obtained under different testing conditions are presented for hard coatings such as TiN and diamond deposited on flat samples and vibrating against corundum counterbody balls.

Oxidational wear of TiN coatings on tool steel and nitrided tool steel in unlubricated fretting

The fretting wear behaviour of PVD TiN coatings against corundum has been investigated in unlubricated contacts. Analysis of the debris indicated that titanium oxides of different composition are being generated. The volumetric wear on the TiN coatings is shown to be directly proportional to the dissipated friction energy measured on-line during the fretting tests. Furthermore, the effect of oxide debris present in the vibrating contact on the velocity accommodation and, consequently, on the coefficient of friction is discussed. The hardness of some of the ASP 23 tool steel substrates was increased with a plasma nitriding treatment prior to the coating deposition. It was found that the coefficient of friction and the fretting wear rate of the TiN coatings remain unaltered by the plasma nitriding treatment. After perforation of the TiN coating, however, wear proceeds faster on nitrided tool steel substrates.

The influence of humidity on the fretting behaviour of PVD TiN coatings

Abstract The influence of the relative humidity (RH) in ambient air on the friction and wear behaviour of PVD TiN coatings subjected to contact vibrations against corundum and bearing steel (100Cr6) counterbodies has been investigated. The fretting experiments were performed in the gross-slip regime on TiN coatings produced by three different PVD processes. The results indicate two basic friction characteristics. At low relative humidity (RH 80%). A transition from high to low friction was observed during the course of experiments performed in atmospheres of medium relative humidity. The duration of the high friction phase in such transitions was found to depend on fretting parameters such as the normal force and the vibration frequency. This humidity dependence of the friction force was found for both counterbody materials. The size of the damaged surface area as well as the volumetric wear on the TiN coatings were found to be largest at low relative humidity. Fretting damage occurs over a smaller area but extends more into the depth at high relative humidity. The size of the fretted surface area induced on TiN is larger for Cr-steel than for corundum counterbodies.

Comparative measurement of residual stress in diamond coatings by low-incident-beam-angle-diffraction and micro-Raman spectroscopy

Two experimental techniques for the quantitative measurement of residual stress in thin polycrystalline diamond coatings have been developed. The x-ray low-incident-beam-angle-diffraction (LIBAD) allows one to measure the lattice strain with well-defined in-depth information, while micro-Raman spectroscopy permits one to accurately measure the frequencies of the zone-center optical phonons of diamond which are related to the lattice strain. The interpretation of the measured information in terms of residual stress is outlined for both techniques. The residual stress data obtained by either method in thin CVD diamond coatings were found to be in excellent agreement. The sign and magnitude of the balanced biaxial stress in the coating plane depend mainly on the substrate material used for the diamond deposition. Compressive stress was present in diamond coatings deposited on WC-Co substrates, whereas tensile stress was found in those on SiAlON substrates.

Low amplitude oscillating sliding wear on chemically vapour deposited diamond coatings

Abstract The tribological behaviour under low amplitude oscillatory sliding of diamond coatings obtained by chemical vapour deposition (CVD), also called fretting wear, was investigated. In a ball-on-flat configuration, CVD diamond coatings deposited on flat WC-Co substrates are worn against CVD-diamond-coated silicon nitride balls as well as against uncoated corundum balls. The influence of fretting parameters such as the contact load, displacement amplitude and test duration on the wear of the CVD diamond coatings deposited on flat WC-Co substrates has been studied. A criterion was derived to distinguish between different fretting regimes. Damage mechanisms such as elastic asperity interaction, abrasive sliding, particle ploughing and adhesive interaction are related to the fretting regimes. A correlation between the wear volume generated on CVD-diamond-coated WC-Co and the dissipated friction energy has been established. Based on this correlation, a prediction of the wear damage induced on CVD diamond coatings by low amplitude oscillatory sliding is shown to be possible when the contact conditions are known.

The fretting behaviour of PVD TiN coatings in aqueous solutions

The friction and wear of physical vapour deposition TiN coatings and steel vibrating against corundum in distilled water, seawater, and sodium phosphate solutions have been investigated. The coefficient of friction measured on the TiN-coated surface remained at a level of around 0.2 whereas the uncoated steel substrate exhibited a significantly higher friction. The TiN coating appeared to degrade by oxidational wear. The progress of the wear damage was largely influenced by the transfer layer of titanium oxides formed on the corundum counterbody. The narrow transfer layer formed in distilled water caused a premature perforation of the TiN coating due to the reduction of the real contact area. The results showed that this unfavourable wear behaviour can be avoided by adding sodium phosphate to distilled water resulting in an accelerated removal of the transfer layer.

Friction mechanisms in hydrogenated amorphous carbon coatings

Abstract The friction behaviour of unlubricated contacts between a hydrogenated amorphous carbon coating and corundum counterbodies was investigated under reciprocating sliding conditions in ambient air. Tests were performed under various normal forces at controlled relative humidity. The coefficient of friction is shown to decrease as the normal contact force increases. At low (5%) and medium (50%) relative humidities, linear correlations between the coefficient of friction and the inverse of the hertzian contact pressure were observed. A physical interpretation for this behaviour is proposed by considering interfacial shear mechanisms accommodating the sliding motion.

Hard coatings under vibrational contact conditions

Contacting surfaces subjected to small-amplitude oscillating motions (fretting) can undergo severe damage. This paper is concerned with the investigation of a possible materials solution to this problem, i.e. the use of hard coatings. The different fretting modes and the relevant fretting variables and fretting regimes are explained in the Introduction. Then, selected results on the fretting of TiN, diamond-like carbon and diamond coatings are presented. The mechanical contact response is characterized with on-line analysis of the tangential force and the displacement. Ex situ characterization techniques are used to evaluate material modification in the contact. It is shown that the combined analysis of the mechanical contact response and the material modification in the contact allows us to quantify the damage and to understand the failure mechanisms of the coating material.

Frictional Behaviour of Diamond-like carbon and diamond coatings in oscillating sliding

Abstract It is generally accepted that diamond-like carbon and diamond coatings have good frictional properties. Here it is shown that the frictional behaviour of diamond-like carbon coatings is strongly dependent on the actual deposition process used and that the frictional behaviour of diamond coatings is markedly different, depending on the counterbody material. An arc-ion-plated amorphous carbon (a-C) coating in oscillating sliding against a corundum counterbody has self-lubricating properties, owing to the formation of debris with characteristics of a defected graphitic phase. However, a plasma-assisted, chemical vapour deposition (CVD) hydrogenated a-C coating in oscillating sliding against a corundum counterbody does not give rise to measurable debris formation and, in comparison with the a-C coating, has a higher coefficient of friction. Brittle, non-metallic counterbody materials, such as corundum and diamond, in oscillating sliding contact with hot filament CVD diamond coatings result in a low coefficient of friction, whereas metallic materials, such as WC-C and Cr steel, form transfer layers on the diamond coatings, resulting in high friction.

Micro-Raman spectroscopy for the characterization of wear induced surface modifications on hard coatings

Micro-Raman spectroscopy can be a helpful tool in determining wear induced surface modifications due to its ability to identify compounds and measure internal stress in an area with a diameter as low as 1 pm. Micro-Raman spectroscopy is used here to characterize surface reactions, transfer layers and internal stress relaxation induced by wear processes for four different hard coatings: PVD TiN, solar beam oxidized TiN, CVD diamond and diamond-like carbon coatings. The Raman spectroscopic investigation of partially worn coatings has resulted in very specific information complementary to results that can be obtained with commonly used surface characterization techniques.