Periodic Reporting for period 4 - TriboKey (Deformation Mechanisms are the Key to Understanding and Tayloring Tribological Behaviour)
Berichtszeitraum: 2023-03-01 bis 2024-08-31
The science of friction, wear and lubrication is called tribology. Friction and wear are responsible for more than 20 % of mankind’s primary energy usage. Therefore CO2 emissions could be significantly reduced if better tribological practices were being available. This could in large part be achieved by designing new materials to be employed in tribological systems. Such a strategic materials development is currently however not possible. This is in part true as the so-called elementary deformation processes acting in materials – and we are focusing on metals here – are not sufficiently known and understood.
The overall objective of this research therefore was to identify elementary deformation mechanisms acting in metals that are subjected to a tribological load, with the long term goal to be able to provide guide lines for alloy and microstructure development.
In order to achieve this goal, we mainly focued our attention on model materials like high-purity copper, as well as on copper alloys, as here these mechanisms can be studied. This focus on elementary mechanisms during the implementation of the action was extended to the field of tribologically induced chemical changes, mainly tribo-oxidation, which is also an important failure mechanism.
One of the challenges that such materials tribology research faces is that these deformation mechanisms are difficult to detect. This is mainly true as every tribological contact constitutes a so-called “buried interface”, meaning that it is normally not possible to directly observe the processes acting in the contact itself. Traditionally elementary mechanisms are therefore studied by interrupting a tribological experiment after a pre-defined sliding distance and the material is then investigated by electron microscopy.
The area of the materials that one is focusing its attention on hereby is not the surface itself, but the material under the surface, down to a subsurface depth of several micrometers. We made extensive use of this approach during this action. However, we also aimed at developing a completely new way of in situ probing the subsurface structure of metals subjected to a tribological load. To this end we designed and built a new tribometer that probes the subsurface structure of the metal in the contact by means of non-destructive testing methods, such as ultrasound and acoustic emission. With this new tribometer at hand, we are now able to study elementary deformation mechanisms for a range of copper-based model materials with the ultimate aim of being able to come up with design guidelines for alloy development. It needs to be mentioned that the latter was a very ambitious goal. While we made substantial progress in this direction during the actions, we are still working on achieving this final goal, even now, after the conclusion of the grant.
The overall objective of this research therefore was to identify elementary deformation mechanisms acting in metals that are subjected to a tribological load, with the long term goal to be able to provide guide lines for alloy and microstructure development.
In order to achieve this goal, we mainly focued our attention on model materials like high-purity copper, as well as on copper alloys, as here these mechanisms can be studied. This focus on elementary mechanisms during the implementation of the action was extended to the field of tribologically induced chemical changes, mainly tribo-oxidation, which is also an important failure mechanism.
One of the challenges that such materials tribology research faces is that these deformation mechanisms are difficult to detect. This is mainly true as every tribological contact constitutes a so-called “buried interface”, meaning that it is normally not possible to directly observe the processes acting in the contact itself. Traditionally elementary mechanisms are therefore studied by interrupting a tribological experiment after a pre-defined sliding distance and the material is then investigated by electron microscopy.
The area of the materials that one is focusing its attention on hereby is not the surface itself, but the material under the surface, down to a subsurface depth of several micrometers. We made extensive use of this approach during this action. However, we also aimed at developing a completely new way of in situ probing the subsurface structure of metals subjected to a tribological load. To this end we designed and built a new tribometer that probes the subsurface structure of the metal in the contact by means of non-destructive testing methods, such as ultrasound and acoustic emission. With this new tribometer at hand, we are now able to study elementary deformation mechanisms for a range of copper-based model materials with the ultimate aim of being able to come up with design guidelines for alloy development. It needs to be mentioned that the latter was a very ambitious goal. While we made substantial progress in this direction during the actions, we are still working on achieving this final goal, even now, after the conclusion of the grant.
As described above, we focused our attention on the elementary mechanisms of microstructural changes in materials subjected to a tribological load. To this end we subjected ultra-high purity copper and its alloys to mainly reciprocating tribological experiments. Our main counter body material was sapphire, as here we can be sure that sapphire does not take part in any microstructural changes nor in chemical processes taking place at the interface.
Now, after the conclusion of the action, several main conclusions can be drawn: First, there is one absolutely universal deformation mechanism: A rotation of the crystal under a tribological load around the so-called transvers direction, meaning orthogonal to the sliding direction. Second, there is a fascinating interplay between the movement of dislocations and mechano-chemical changes in materials subjected to a tribological load. For example, oxygen can diffuse into a material along the core of dislocation, then reach supersaturated concentration and lead to oxides far away from thermodynamic equilibrium. Third, tribological systems are very sensitive to the precise starting conditions. This is something that has to be kept in mind when interpreting the reproducibility of tribological experiments. With the use of machine learning however, one can predict where high friction and wear will occur on a specific surface. All in all, one can say that the grant has demonstrated that besides the microstructure, one also has to consider mechanical and mechano-chemical factors. At the same time, this complexity in my opinion should not be daunting, but a step towards more degrees of freedom for a strategic alloy, microstructure and surface design has come much closer.
Now, after the conclusion of the action, several main conclusions can be drawn: First, there is one absolutely universal deformation mechanism: A rotation of the crystal under a tribological load around the so-called transvers direction, meaning orthogonal to the sliding direction. Second, there is a fascinating interplay between the movement of dislocations and mechano-chemical changes in materials subjected to a tribological load. For example, oxygen can diffuse into a material along the core of dislocation, then reach supersaturated concentration and lead to oxides far away from thermodynamic equilibrium. Third, tribological systems are very sensitive to the precise starting conditions. This is something that has to be kept in mind when interpreting the reproducibility of tribological experiments. With the use of machine learning however, one can predict where high friction and wear will occur on a specific surface. All in all, one can say that the grant has demonstrated that besides the microstructure, one also has to consider mechanical and mechano-chemical factors. At the same time, this complexity in my opinion should not be daunting, but a step towards more degrees of freedom for a strategic alloy, microstructure and surface design has come much closer.
All the results described above go way beyond the state of the art. At the same time we will choose three aspects that not only go beyond the state of the art, but were also unexpected.
The first is the use of twin boundaries as “markers” for subsurface processes. We were able to identify three separate, but congruent elementary mechanisms acting during the very early stages of deformation: A simple shear, a highly localized shear and a rotation of the lattice above a dislocation self-organization phenomenon. This is completely new knowledge and most definitely goes beyond the state of the art.
Second, we were able to build a new tribometer, which allows to test single and bicrystalline samples in different, highly defined sliding and therefore crystallographic directions, which allows us to measure the difference in friction forces as a function of crystallography. This was something which in the past was only possible in ultra-high vacuum conditions, and we are now able to do so in ambient and controlled atmosphere.
Third, classical common knowledge about tribologically enhanced oxidation is that due to sliding there is a temperature increase in the contact – concept of flash temperature – and this temperature increase is responsible for the accelerated oxidation. With very systematic experiments we could show that this classical picture is wrong. It is the diffusion of oxygen and metal ions along lattice defects that is rate controlling. We could show the decoration of for example dislocations with oxygen with near atomic scale resolution.
Finally, we were able to connect all our tribometers used during the action directly to an electronic lab notebook, so that now, after the end of the action, all this research is performed in a completely FAIR way, which is a capability that would not have been possible without the grant and will allow fascinating new insights in the future as it facilitates the strategic use of machine learning on tribological data.
The first is the use of twin boundaries as “markers” for subsurface processes. We were able to identify three separate, but congruent elementary mechanisms acting during the very early stages of deformation: A simple shear, a highly localized shear and a rotation of the lattice above a dislocation self-organization phenomenon. This is completely new knowledge and most definitely goes beyond the state of the art.
Second, we were able to build a new tribometer, which allows to test single and bicrystalline samples in different, highly defined sliding and therefore crystallographic directions, which allows us to measure the difference in friction forces as a function of crystallography. This was something which in the past was only possible in ultra-high vacuum conditions, and we are now able to do so in ambient and controlled atmosphere.
Third, classical common knowledge about tribologically enhanced oxidation is that due to sliding there is a temperature increase in the contact – concept of flash temperature – and this temperature increase is responsible for the accelerated oxidation. With very systematic experiments we could show that this classical picture is wrong. It is the diffusion of oxygen and metal ions along lattice defects that is rate controlling. We could show the decoration of for example dislocations with oxygen with near atomic scale resolution.
Finally, we were able to connect all our tribometers used during the action directly to an electronic lab notebook, so that now, after the end of the action, all this research is performed in a completely FAIR way, which is a capability that would not have been possible without the grant and will allow fascinating new insights in the future as it facilitates the strategic use of machine learning on tribological data.