Materials Science of Mechanical Contacts
From bearings, over rails, to tooth or hip implants – the number of examples where materials are exposed to mechanical contact loads is as countless as the number of materials used under such conditions. The materials science of mechanical contacts is versatile and challenging. The loads decay with distance from the surface and with that the amount of plastic deformation. They can generate short but significant local increments in temperature. Both effects result in a microstructural gradient. In the case of cyclic loads the microstructure never reaches a stable condition but evolves in time. Often a connecting liquid medium like a lubricant or a biofilm is involved that leads to the formation of a reaction layer at the surface, the composition and structure of which can have an enormous influence on the overall lifetime of the part. Materials that can bear high cyclic contact loads for a longer time usually are comprised of a complex nanocrystalline multi-phase microstructure and thus understanding these phenomena often requires joint crystallographic and chemical characterization at the nanometer scale.
The research group “Materials Science of Mechanical Contacts” headed by Dr. Michael Herbig aims at deepening our understanding of the materials science phenomena associated with intense joint mechanical and environmental contacts. This is key to revealing a broad range of phenomena required for improving crucial engineering components such as bearings, rails, hip implants, extrusion tools, boring heads, cutting inserts or dental fillings.
The contact between two solid bodies subjected to high forces under harsh environmental conditions and large numbers of repetitions involves complex materials science phenomena (Fig. 1): Plastic deformation can lead to fatigue, grain refinement and precipitate decomposition. Frictional heat can cause diffusion, phase transformation, recovery or recrystallization. The presence of air, lubricants or body fluids at the contact point causes oxidation, tribolayers, or even corrosion or hydrogen embrittlement. These processes usually occur simultaneously in service and cannot be tracked in-situ. The analysis of such phenomena requires combined chemical and structural characterization down to the atomic scale [1-3].
One main focus of this research group is the study of white-etching-cracks (WECs, Fig. 2), which are primarily known to cause failure in bearings and rails, but which are in reality ubiquitous in high carbon steel applications subjected to intense mechanical contacts. This failure mode causes billions of euros costs worldwide each year. The group follows a diversified approach to yield a breakthrough on this long-standing challenge: Specimen failure under controlled laboratory conditions is generated using a customized rolling contact fatigue machine built in-house that simulates test conditions similar to ball bearings but on self-designed alloys. Both, lubrication and loading conditions as well as the electric current flow through the bearing (which is of importance as electric discharge events have been associated with the presence of WECs) can be controlled with this instrument. These specimens are compared to samples that failed during service using state-of-the-art microscopy. Individual phenomena are investigated separately where possible. Dedicated experiments are conducted to investigate the mechanisms of precipitate decomposition by deformation, heat and electricity. A further project aims at the direct observation of solute hydrogen by atom probe tomography (APT) in deformed pearlite to shed light on the role of hydrogen in the process of cementite decomposition. The fracture toughness of white-etching-layers in rails is determined by cantilever bending tests in the scanning electron microscope.
The research activities on steels are complemented by research on hip implants where corrosion and wear debris at the contact point between the stem and head leads to adverse tissue reactions, requiring the explantation of 2.5% of all prostheses. Here, correlative transmission electron microscopy and APT give access to the complex body/implant interactions (Fig. 4).
The group employs a broad variety of characterization techniques available at MPIE to get a full picture understanding on all length scales - from macroscopic rolling contact fatigue tests on whole bearings, over mesoscale investigations in the scanning electron microscope (SEM), down to the atomistic scale using transmission electron microscopy (TEM) and atom probe tomography (APT). A key component of this group is the ability to perform both of these high-end techniques on one and the same sample (correlative TEM / APT, Fig. 4) . This gives access to combined 3D structural and compositional information on near atomic scale which is often the only way to answer a long-standing materials scientific question.
The research group ”Materials Science of Mechanical Contacts” acknowledges funding by the Bundesministerium für Bildung und Forschung (BMBF) and by the German Research Council (DFG).