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Organisationseinheit der BAM
Zero wear (Null Verschleiß)
(2013)
Ceramics are of increasing interest for application in many branches of technique. In order to improve the tribological performance of ceramic materials, composites on the basis of silicon carbide with different amounts of titanium carbide, boron carbide, titanium diboride and free carbon in the form of graphite were manufactured by a reaction sintering process.
The friction and wear behaviour of these composites was investigated with oscillating sliding motion in unlubricated tests at room temperature on different levels of relative humidity. Additional tests were performed under water lubrication, revealing the superior behaviour of composites for special test conditions. A major influence of the free carbon on friction or wear behaviour was not observed. The tribological behaviour in air as well as under water is mainly dependant on the amount of titanium phases.
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.
Self-mated magnesia stabilized zirconia (Mg-PSZ) ceramic sliding couples have been investigated at 100 N load (P0max= 1324 MPa) in oscillating sliding conditions in different humidity conditions in air and in hot steam. Temperatures have been varied up to 400 °C and pressures up to 6 bars. The results show that the wear behavior of MgO-ZrO2 under high Hertzian contact pressures is strongly dependent on temperature and is similar for both dry oscillating and oscillating in hot steam. However, although the evolution in wear rates on temperature is similar and the wear rates of MgO-ZrO2 plunged above 300 °C in hot steam and air by nearly three orders of magnitude, SEM micrographs revealed in hot steam at 400 °C smooth wear tracks. In contrast, hot steam enhanced the tribochemistry of self-mated alumina couples and reduced wear rates. Hot steam decreased the coefficients of friction of MgO-ZrO2 with increasing temperature, but not the wear rates.
The tribological behavior of different low infrared (IR) emission ceramic materials from the titanate group has been investigated at room temperature under gross slip fretting conditions against 100Cr6 ball bearing steel and α-alumina as the counterface materials. For all material pairs, friction and wear depended largely on the relative humidity of the environment. When paired with steel the low IR-emission ceramic disks showed higher wear resistance. Under dry conditions, almost no wear was found on the low IR-emission ceramic specimen. The high wear on the steel counter body is caused not only by tribo-oxidation, which is the main wear mechanism, but also by abrasion. In dry conditions a COF of 0.6 and in humid conditions a COF of 0.2 have been measured. When paired with α-alumina the results on wear are just opposite to those with steel counterbody and the low IR-emission ceramic materials show much lower wear resistance under all conditions. This is explained by the predominance of abrasion and the relatively large difference in hardness between the low IR-emission ceramic and α-alumina. In high humidity environments the results point to the formation of stable reaction layers causing lower friction and wear.
Untersucht wurden Verschleißpartikel als elektronentransparente Proben auf dünnen Trägerfolien mit einem hochauflösendem Rasterelektronenmikroskop und energiedispersiver Röntgenspektrometrie im Transmissionsmodus.
Die hohe laterale Auflösung dieser modernen Methoden ermöglicht eine morphologische Charakterisierung der Verschleißpartikel bis in den nm-Bereich. Die hochauflösende Elementanalyse erlaubt eine Interpretation der Zusammensetzung und damit Rückschlüsse auf die Verschleißpartikelbildung. Die tribologischen Untersuchungen sind an einem Modell-Verschleißtribometer nach DIN 51834-2 geschmiert, aber ohne Ölkreislauf, durchgeführt worden. Die Präparation der Verschleißpartikel über eine Entnahme und Übertragung einer geringen Schmierstoffmenge mit einer Mikropipette auf eine TEM-Trägerfolie ist mit einem geringen Aufwand möglich. Der präparative Aufwand ist relativ gering. Das benötigte Schmierstoffvolumen beträgt wenige Mikroliter (0,001 ml).
Die Untersuchung zeigt die geringe geometrische Größe der Verschleißpartikel und bestätigt indirekt die These der Reaktionsschichtbildung mit einer geringen Schichtdicke, die ständig verschleißt und sich stetig neu bildet. Die beschriebenen Verschleißprozesse sind in der Modell-Verschleiß-Prüfung mit sehr hohen Pressungen am Versuchsbeginn nachweisbar.