9.5 Tribologie und Verschleißschutz
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Eingeladener Vortrag
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The performance of titanium alloy (Ti6Al4V) surfaces was investigated in lubricated reciprocating sliding tribological tests (RSTT). Special emphasis was laid on the effect of surface nanostructures in area of contact on the respective friction and wear behaviour. These so-called laser-induced periodic surface structures (LIPSS, ripples) were produced on the titanium alloy surface upon scan processing in air by an ultrashort pulsed femtosecond (fs) laser. As lubricant served two types of base oils, a pure polyalcylene-glycol, and an SAE 0W30 oil containing only antioxidants and temperature stabilizers. Tribological tests were carried out on polished as well as LIPSS covered areas using both types of base oil. A test metrics was established, combining the additive 2-ethylhexyl-zincdithiophosphate (ZDDP) or the ionic liquid [P6,6,6,14] [DEHP] (98% purity) with the respective base oils. The test metrics also considered the orientation of motion with respect to the orientation of the structures formed on the surface. Results are presented which show that the interplay between LIPSS and the local chemistry formed by the respective additives is beneficial for the tribological behaviour of the titanium alloy. Certain combinations of base oil, additive and LIPSS reduced friction and wear significantly in the tribological contact.
Elastomer seals are mounted as barrier seals in lid systems of containers designed for transport and disposal of negligible heat generating radioactive waste and as auxiliary seals in spent fuel storage and transportation casks (dual purpose casks (DPC)). When the behavior of mounted seals under normal and hypothetical accident conditions of disposal and transport is to be simulated, a comprehensive knowledge of their complex mechanical properties at every state of aging is necessary. In previous works, BAM’s efforts in experimental investigations on specimen artificially aged at different temperatures and times and the implementation of the found results in finite element material models were presented. Additionally, our approaches to reproduce the aging process itself and to extrapolate the results of artificially accelerated aging to longer times were presented. Numerical simulations have shown that the behavior of the seal during mounting and one-sided pressurizing and the resulting performance values such as leakage rate strongly depend on the coefficient of friction (COF) between flange and seal. The friction coefficient, in turn, depends on the aging state of the elastomer material as several publications suggest (see below).
Dynamic COF between an exemplary ethylene propylene diene rubber (EPDM) material and a stainless steel ball were determined by using a self-designed linear oscillation tribometer. Unaged and artificially aged EPDM specimen stored for 30 days and 100 days at a temperature of 150 °C were tested. A stainless steel ball (d=10 mm) is brought in contact with the specimen’s surface und loaded by normal forces of 2.5 N, 5 N, 10 N and 20 N. During a reciprocating movement of the EPDM sheet, the horizontal force/friction force is continuously measured, and the COF can be derived.
It is well known that friction is a complex phenomenon especially in soft materials. It cannot be excluded that the measured friction force is influenced by additional force components, resulting from the ball’s grooving through the elastomer’s surface. This force depends on the penetration depth of the ball and on the resistance of the elastomer in its different states of aging. The latter results from microstructural changes i.e., chain scission and additional crosslinking that occur during aging which in turn influence the softening or hardening of the material.
A finite element (FE) ABAQUS® model was developed to reproduce the measurement process. It should help to better understand the physical mechanisms and to quantify the percentage of measured forces resulting from real friction on the one hand and forces resulting from unintended side effects that could falsify the result on the other hand. The behavior of the elastomer in its different states of aging is reproduced by a FE material model already presented in previous works of BAM.
The friction at the interfaces between pieces of wood and steel is of crucial importance for the impact and energy absorption in impact limiters of transport casks for radioactive material. Here, the friction coefficient for the combinations wood-wood and wood-steel was measured in the temperature range between -40°C and 90°C. Results show decreasing friction with increasing temperature, ranging from 0.43 at -40°C to 0.22 for 90°C for wood-steel combinations and from 0.3 at -40°C to 0.24 at 90°C for a wood-wood combination.
The addition of WC and TiC in a nickel bonded NbC cermet was investigated regarding the tribological behavior under dry oscillating sliding at different relative humidities φ and the cutting performance in dry cylindrical turning of C45E at different cutting speeds. NbC-12Ni-12WC-14TiC shows similar coefficient of friction f and wear rate compared to NbC-12Ni at φ = 2 %, but no pronounced decrease in f with increasing humidity. When turning C45E, NbC-12Ni-12WC-14TiC shows lower material removal VW compared to other NbC-based cermets, which can be attributed to inhomogeneities in the cutting material and a low hardness.
Verschleißfeste Kohlenstoffschichten mit niedriger Reibung sind hervorragend für Anwendungen in der Erdgas- und Wasserstofftechnologie geeignet. Einige Varianten versagen jedoch in inerter Umgebung, was für die praktische Anwendung sehr nachteilig ist. Im Vortrag werden Beispiele aus Modellversuchen für verschiedene Varianten, auch für Flüssigmethan-Umgebung gezeigt.
Holz wird vielfach in Stoßdämpfern von Transportbehältern für radioaktives Material eingsetzt. Dafür wird oftmals Fichtenholz verwendet. Die Reibung zwischen den dabei entstehenden Grenzflächen ist von entscheidender Bedeutung. Die Ergebnisse hierzu zeigen eine gleichmäßige, mit steigender Temperatur abfallende Reibung.
European tribological research organisations unify their capabilities for the European Tribology Centre (https://www.i-tribomat.eu/). At its core is a database, which data are made comparable and trusted via harmonised procedures for their generation. This contribution will focus on wear volume data.
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 in the Hertzian contact pressure range up to P0max of 4 GPa.