Different fretting tests of thin steel wires under different conditions (loads, strokes, number of cycles and mean pressures) were performed. The wires used had 0.45 mm in diameter and the material was a cold-drawn eutectoid carbon steel (0.8% C) with a tensile strength over 2800 MPa. The tests were carried out with 90° "crossed cylinders" configuration varying the stroke, normal load and testing time (number of cycles). Afterwards the volumetric wear was obtained using surface profilometry with diamond stylus.
With a classical study of the coefficient of wear with respect to the sliding distance, the results show that the wear coefficient increases with normal load and stroke, consequently, for the same sliding distance, the wear produced was higher when the load and/or stroke is increased. In this work, the coefficient of wear is decreasing continuously as the test is running. In order to analyze better the wear behaviour, another method, in which the volumetric wear is compared with respect to the Archard's loading factor, is proposed. As result of this method, two distinctive behaviours were detected; one corresponds to the running-in period with a more aggressive wear behaviour and the second one to the stable steady state period. In all the tests the running-in period was similar with regard to the wear rates but in the stable period the wear rates were increasing with higher loads or strokes.
The influence of temperature and counterbody material on the tribological properties of a-C:H
coatings deposited on Cronidur 30 steel has been investigated in a lubricated ball on disk
contact situation with alpha-alumina and silicon nitride as counterbodies. The results show, that the wear volumes of the systems increase exponentially with increasing temperature, for alpha-alumina more than for silicon nitride. Two different wear mechanisms seem to have a
major influence: First, the abrasive action due to materials hardness and second, the tribo-oxidation when silicon nitride is counter material.
Three different contact geometries, ball on disk and two different settings with crossed cylinders have been investigated with regard to the agreement of the results of the tribological quantities determined by each method. The material combination was α-alumina as upper specimen and steel 100Cr6 as lower specimen. For each of the contact geometries 5 independent measurements were carried out and evaluated independently. The agreement between the single results of coefficient of friction and the coefficient of system wear as well as between the different groups and conditions are good. Largest scatter of the results were found in the dry air regime where both friction and wear depend strongly on the relative humidity. However, comparison of the absolute values of the coefficient of wear between the ball on disk contact geometry on one side and the 2 crossed cylinder contact geometries on the other side reveals a significant difference in that the ball on disk results are lower by about a factor of two. The results document also a high degree of repeatability of single wear tests.
An extended study on tribological properties of carbon nitride coatings under oscillating sliding conditions has been carried out. The coatings were prepared by reactive magnetron sputtering with graphite target using nitrogen as reactive gas in an argon/nitrogen atmosphere during the deposition process to deposit carbon nitride, CNx. Coating thickness was about 2 µm. A 200300 nm thick Ti interlayer was used to enhance adhesion. The nitrogen flow during the deposition was varied to control the amount of nitrogen in the coating. Maximum amount of N-content was about 28 at%, achieved at a maximum N2 flow rate. Other deposition parameters were target to substrate distance, substrate temperature during deposition and bias voltage. The results show a large variation of hardness depending mainly on the nitrogen content and bias voltage. Higher bias voltage produced harder coatings as did lower nitrogen content. The tribological properties were tested unlubricated and lubricated by paraffin oil under gross slip fretting conditions at room temperature against alumina as counter material. Most coatings, but not all, showed a more or less pronounced dependence of wear on relative humidity with wear rates from 10-5 to 10-6 mm3/Nm. The best coatings with regard to a low wear rate and the least dependence upon relative humidity were carbon nitride coatings deposited at elevated substrate temperatures up to 350 °C. The wear resistance of these coatings was increased under the most severe conditions by about one order of magnitude. The results are discussed on the basis of a transfer layer formation. The results show clearly that wear is not significantly correlated to the coating hardness which itself is strongly dependant on the nitrogen content. However, under lubricated conditions with paraffin oil, wear rates were generally very low with wear coefficients at about 10-8 mm3/Nm.
The influence of temperature and counterbody material on the tribological properties of a-C:H coatings deposited on Cronidur 30 steel has been investigated in a lubricated ball on disk contact situation. The results show, that the wear volumes of the system increase exponentially with increasing temperature. Two different wear mechanisms seem to have a major influence: First, the abrasive action due to materials hardness and second, the tribooxidation when silicon nitride is counter material.