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Eingeladener Vortrag
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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 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.
The wear rates of self-mated alumina couples show that friction as well as wear is largely determined by the above-mentioned hydro-thermal conditions. The presence of water and its amount available in the surrounding system either in liquid or in gaseous form plays a key role for friction and wear behavior and can be beneficial for the tribological profile of steam degradation resistant materials. Hot steam enhances the tribo-chemical formations of oxides and hydroxides on MgO-ZrO2 alumina and antimony impregnated carbon.
Downsizing (power-to-weight ratio) and higher speeds lead to a rise in Hertzian contact pressures in combination with an increase in surface or oil temperatures. Under such conditions, commonly used bearing steels, such as 100Cr6, reach their limits, creating a demand for alternative slip-rolling resistant steel alloys. The present work therefore compares the slip-rolling performance of various steel types with Maraging- and PM-type steel alloys such as e.g. CSS-42L™, ASP2012, BIMAX42+, in the Hertzian contact pressure range up to P0max of 4 GPa. Through-hardened 100Cr6H (AISI 52100), case-hardened 20MnCr5 (AISI 5120H) and nitrogen alloyed Croni-dur30 (AMS 5898) still compete in terms of slip-rolling and wear resistance and load carrying capacity, whereas Maraging- and PM-type steel alloys offer superior strength and toughness properties.
Research and development of WC grades is 90 years ahead of niobium carbide-based hard materials, which showed already as light-weight and unexplored material a very high potential for many technical applications, especially for wear protection and machining. NbC evoluted recently from lab scale to pilot scale, especially in areas where established WC-based materials are causing economic, environmental or technical concerns. Benchmark trials of NbC-based hard metals indicated already a favorable performance in machining of steel components.
The process window and range of functional properties of NbC is wide, because micro-hardness, hot hardness, sliding wear resistance, elastic modulus and toughness can be tailored by the C/Nb ratio, because Nb4C3, Nb6C5 and other short and long range ordered phases occur in the region of homogeneity of NbCx (0,75≤ x ≤1.0) in the binary Phase diagram, and by the type of binder. Further improvements enable the addition of secondary carbides. Supporting results from literature and different grades with varying C/Nb ratio or binder types were illuminated. Thus, the NbC system offers to producers a wider process window, than WC. The impact of these tailored properties on sliding wear and cutting performance was illuminated by tribological and machining results. NbC can be synthesized by carbothermal conversion of Nb2O5 or be metallurgically grown in melts and leached out as coarse NbC cubes. NbC provides comparably lower friction in many relevant tribo-contacts and displays low wear. NbC and Nb2O5 have so far no REACH classification related to human toxicology and are not listed as substances of very high concern contrary to WO3 and Co3O4.
Hydraulic oils are well established formulations developed around the pump test. Requirements for energy efficiency and also environmentally acceptable properties increased the demand for development. In the same time, system pressure and the used of servo-hydraulic vanes increased bringing specific formulations and materials to their limit.
The vane pump is a key component in hydraulic systems and many other tribosystems in hydraulic circuits operate under different tribological operating conditions. The concept for ranking of hydraulic oils consists in using existing SRV-based ASTM test methods for evaluation of friction, wear and extreme pressure properties: D6425, friction and wear of oils (homologue to DIN 51834-2), D7421, extreme pressure properties of oils and D7755, wear volumes of ball and disks (homologue to DIN 51834-3. The tribological properties are assessed under the regime of mixed/boundary lubrication The SRV-based concept of tribological profile generates the wear volumes on two triboelements (specimen) and the extreme pressure properties as well as the evolution of friction in the wear and extreme pressure load step test. Thus, the benchmark and validation of hydraulic oils bear on a wider range of tribological properties. Round robin test results using the aforementioned testing concept will be shown. Another outcome is tribological set limits for inclusion in hydrau¬lic oil speci¬fications.