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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.
New interpretation approaches for seizure load tests on the translatory oscillation tribometer (SRV)
(2016)
When looking in detail at analyses of the tribological load-carrying capacity of high-performance lubricants, it becomes apparent that the stop of the machine of an exclusive evaluation of the development of the coefficient of friction cannot provide any sufficient criteria for determining the occurrence of adhesive failure. This is due on the one hand to the increasing complexity of lubricant formulae and sophisticated additive packages, and on the other hand to the increasing frictional power capacity of modern drive and control concepts in the design of tribometers. For this reason, it is urgently needed to examine the adhesive processes and their detection in more detail with the help of appropriate tribological values and criteria. The evolution of the friction can be coupled with the stroke, contact resistant and/or noise emission. Besides from new criteria for adhesive failure, which were compiled by the work group for the relevant ISO, DIN and ASTM standards, this contribution discusses the inclusion of additional parameters. Among other things, it provides guidance on what evidence the continuous recording of the original coefficient of friction and position signals, the electrical contact resistance, temperature, noise emission, frictional power input of the machine and further derived parameters help to identify adhesive failures or events.
To investigate the influence of metal sulphides in brake pads, powder mixtures with up to three components were tribologically stressed in a pin-on-disc device under different levels of relative humidity. The tests revealed that each metal sulphide causes significant differences in resulting coefficients of friction as well in their sensitivity to humidity, so that only one solid lubricant cannot stabilise the friction behaviour during braking in different environments.
The magnetic plasma confinement system of the nuclear fusion experiment “Wendelstein 7-X”, which recently produced the first plasma, consists of 20 planar and 50 non-planar superconducting coils. To keep them precisely in position, these coils contact each other at special support elements. The supports are operated in high vacuum at a temperature of about 4 K, and when the magnetic field is ramped up, forces up to 1500 kN and sliding motion of some mm occurs. Because mechanical disturbances may cause severe failures, only low friction without any stick-slip behaviour is tolerable. In pre-tests a PVD-MoS2-coating had proven to be the optimum solution. It was tested with downsized samples in reciprocating motion in liquid nitrogen (T = 77 K) and liquid helium (T = 4,2 K). In long-term tests in liquid nitrogen this coating showed stable sliding for up to 14.900 friction cycles without coating failure. However, in liquid helium an unexpected stick-slip effect occurred, whose origin could not be clarified so far.
The polymer chemical structure of the polyimide has a major influence on the tribological behaviour. The addition of graphite in PI2 has a beneficial effect in hydrogen on the friction and wear. The low friction of graphite is associated with a lubricant film in hydrogen. The influence of hydrogen on graphite is more effective than humidity. CNTs have a similar effect to that of graphite in PEEK composites. TiO2 particles improve significantly the wear rate both in vacuum and hydrogen environment. In LH2 friction and wear decrease for unfilled polymers. Friction of graphite filled composites increases slightly and wear rate is stable.
The martensitic steels X30CrMoN15-1 and 100Cr6 did not show any hydrogen enhanced wear in sliding friction tests against alumina balls in hydrogen gas environment. However, depending on the purity of the environment, unexpected high wear occurred at the alumina ball. The results of a tests series with a broad variation of test parameters are presented.
In a first test series, the friction and wear properties of 6 polymeric candidate materials for application in LNG tribosystems were tested. The most favorable properties showed PTFE composite, filled with carbon fibers and PEEK, which showed low and stable friction and a wear coefficient in the order of 10-7 mm3N-1m-1. PTFE containing PEEK composites seem to be also appropriate for LNG applications. Admixtures of graphite as solid lubricant result in lower wear. Contrary, a lubricating mechanism of MoS2 was not observed. The PI-materials showed comparatively high friction and medium wear.
The steam technology has attracted attention for recovering energy from waste heat in transportation systems. 40% of the calorific value of the fuel is wasted through the tailpipe. The use of waste heat recovery systems (WHRS) in cars and especially in trucks corresponds to possible energy savings in terms of less fuel consumption in the range of 7 – 18 %. This relates not only to the savings of fuel but also to the non-production of green house gas CO2. Steam expander form tribosystems, which can´t be lubricated with oils, as hot steam hydro-cracks these fluids. Suited tribomaterials as well as their tribo-oxidatively reaction layers must be first steam degradation resistant and for the tribotesting of such candidate material, tribological testing devices for a hot steam environment are needed. Not much is known so far about the tribological properties of any material system running under pressurized hot steam conditions. For these reasons and for being able to evaluate tribological properties in hot steam, BAM has developed and built a new and sealed tribometer for hot steam allowing evaluating tribological properties up to 700°C and 10 bars in gaseous atmospheres like hot steam or ethanol.
Experimental results on the wear of self-mated aluminum oxide and MgO-ZrO2 couples show that friction as well as wear is largely determined by the above mentioned hydro-thermal conditions. In this system the presence of water and its amount available in the system either in liquid or in gaseous form plays a key role for wear behavior and can beneficial. Ethanol and/or water-ethanol mixtures can form beneficial tribofilms, also on specific diamond-like carbon films.