What are the physical connections between tribological contacts and what impacts do they have on abrasion and wear? What improvements can be derived from knowing this and what can be done to solve specific problems such as the right materials to select? These are the kind of questions occupying the Steinbeis Transfer Center for Tribology Application and Practice at the Baden-Württemberg Cooperative State University in Karlsruhe. The center offers a variety of services relating to wear testing, damage analysis and surface measurement, as well as seminars on these topics. The results of their analysis provide a basis for working out physical chains of effects and these, in turn, provide a basis for possible improvements. Such aspects are reflected in the emphasis of work at the Tribology Steinbeis Transfer Center. One recent project allowed the center to provide its customers with a measurement technique for use with the extensive, non-destructive tribological characterization of tribochemical reaction layers.
The experts at the Steinbeis Center investigated the friction of a rolling contact made out of two cylindrical steel rollers with a hardness of 670 HV (HV is the Vickers Pyramid Number). One roll in the contraform contact setup is as a drive input roll, the other was a drive output roll. The function of the setup was to transfer torque or power. A relative movement of the two cylinder steel rolls leads directly result in wear (through slippage), for avoiding that the friction coefficient during the entire period of operation had to be sufficiently high. Another requirement relating to the friction coefficient is that it remain uniform along the whole length of the cylinder roll. This is because inconsistencies would result in an off-center point of load and leads to a torque to torque around the upper axle with subsequent losses of the function. According to the experts’ observations, the structural density of the reaction layer was an important factor, since the friction coefficient was inversely proportional to it.
The tribochemical layers are produced by oxidation processes and are composed of various iron oxides and iron hydroxides. Exposed to light, they look like discoloration, mainly in different shades of brown. The formation of various oxides and hydroxides depends on the availability of oxygen and on the temperature. The layers have different physical properties relating to factors such as hardness (structural density), and these properties can cause topical differences in friction coefficients. Analytical methods used until now such as micro-hardness measurements, focus ion beam (FIB) cutting, and secondary neutral mass spectrometry (SNMS) are not suitable for entire rollers when characterizing the tribological effect of reaction layers without destruction it.
To determine a quantified value for the structural density – an important factor when it comes to friction coefficients – the team of Steinbeis experts found the photothermy because it allows for measurements without damage materials. This method uses a modulated, fiber-coupled diode laser which is expanded and directed on the surface of the sample. To measure rays of heat emitted from the surface (thermal waves), an infrared detector is used. The propagation of the thermal waves inside the material depends on the thermal properties of the material and can be characterized by the thermal diffusivity. This is inversely proportional to the product of the material density r and the specific heat capacity cp. In turn, this product is proportional to density. This indirect method makes it possible to work out differences in density based on different degrees of hardness, slippage/shifting, inconsistencies, load, or layering errors. The measurements that can be used are the amplitude and phases difference between the laser and the thermal waves that were measured.
Prof. Dr.-Ing. Dietmar Schorr, director of the Steinbeis Transfer Center for Tribology Application and Practice and his team have for the first time succeeded in characterizing tribochemical reaction layers over large areas, with a non-desctructively method. This was achieved by using photothermy, taking photothermic measurements of three used rollers and three new ones. By taking measurements along the length of the rollers at specific laser penetration depths, the measurements allowed the team to identify significant differences between new parts and parts that had already been used. Furthermore, it was possible to take quantifiable measurements of different reaction layers, which correlates with the discolorations over the length of the roller. These colors could only be used as an indication for the tribochemical layers, however, and they said nothing about the complex structures nor the thickness of reaction layers, which as a rule was between 100 and 300 nm.
The Steinbeis analysis was therefore able to provide clients with a method that can be used to test the impact of influencing factors on reaction layers, and thus friction coefficients, substantiated by quantifiable measurements. This makes it possible to make specific improvements in product performance and reduce downtimes.
Prof. Dr.-Ing. Dietmar Schorr
Steinbeis Transfer Center Tribology Application and Practice (Karlsruhe)
dietmar.schorr@stw.de