A completely new field of research was opened up with the establishment of the Collaborative Research Center (CRC) 1368: Oxygen-free production. The aim of the CRC is to gain a fundamental understanding of the processes and mechanisms in manufacturing, assembly and handling technology processes with the technically complete exclusion of oxygen.
The suppression of oxidation promises many advantages. In future, for example, the process forces required to break up the oxide layers could be avoided and tool wear significantly reduced.
The oxygen-free atmosphere is adequate for an extremely high vacuum and is generated in the special research area using silane-doped argon gas. As a result, the oxygen partial pressure in the process environment reaches the extremely low value of 10-17 bar.
Production without oxygen
Eight institutes at Leibniz Universität Hannover, four institutes at Clausthal University of Technology, the Chair of Data Management in Mechanical Engineering at Paderborn University and the Laser Zentrum Hannover are working together in the Collaborative Research Center 1368. The host university is Leibniz Universität Hannover. A total of almost 50 scientists are conducting research in 19 sub-projects. They have already completed more than four years of fundamental research. The second funding period of the Collaborative Research Center began in 2024 and will last another four years.
This article summarizes the research results of the IMPT – Institute of Micro Production Technology from the first funding period and provides an outlook on the second funding period.
Over the past four years, IMPT scientists have focused on identifying the fundamental relationships between wear processes in tribological systems in the silane-doped argon atmosphere and quantifying them using model tests. In addition, the atmosphere-dependent changes in diffusion and adhesion effects were investigated.
Findings on macroscopic effects
To investigate macroscopic friction and wear mechanisms, relevant points of action were simulated by means of analogy tests (ball-on-disc) under controlled conditions in air, in argon and in the silane-doped argon atmosphere. Oxygen-affine materials, such as copper, aluminium and the titanium alloy Ti-6Al-4V, were investigated. Material pairings of the same material were selected in order to be able to initially observe the effects in isolation (copper with copper, aluminum with aluminum, etc.).
The ball-on-disc investigations were carried out using the Universal Microtribometer from Bruker, which was extended to include a high-temperature chamber. This allowed investigations to be carried out at temperatures of up to 1000 °C in order to accelerate diffusion processes. Frictional forces were recorded and the coefficient of friction determined. In addition, the wear on the friction pairs was determined and the changes in the mechanical properties of the respective test specimens were characterized via identification (Triboindenter Ti900, Hysitron).
The materials Cu and Ti-6Al-4V showed an initial increase in the coefficient of friction (CoF) after removal of the native oxide layers due to the increased adhesion tendency. With increasing temperature, there was a sudden decrease in CoF, which could be attributed to the formation of new, tribologically relevant layers. Although increased adhesion was observed, the wear volume was significantly lower in silane-doped argon atmosphere compared to air or argon atmosphere. Properties such as hardness and modulus of elasticity increased significantly for copper and Ti-6Al-4V samples in silane-doped argon atmosphere compared to air atmosphere, while only a small increase was observed for aluminum.
In addition, various analytical methods – scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) – were used to investigate the microstructure and chemical composition of the samples at surfaces and interfaces. For copper, the formation of η-Cu3Si and other copper silicides was observed in silanated atmospheres, leading to a passivating effect. A tribologically relevant layer was also formed in titanium samples, while Si-containing layers could not be detected in aluminum samples. In cooperation with scientists from the Collaborative Research Centre, the microstructure and mechanical properties of arc-sprayed copper coatings were also analyzed. The results showed that the oxide-free, arc-sprayed copper layers exhibited improved wetting behavior, which leads to a significant reduction in layer porosity. The improved wetting behavior resulted in an increase in the adhesive strength and the apparent modulus of elasticity.
Findings on oxide layer thicknesses and deoxidation mechanisms
Due to their brittle nature, oxide layers have a significant influence on the further processing of copper. As a result, it is often necessary to remove them. In order to be able to assess the influence of the oxide layers, their mechanical properties were examined as a function of the layer thickness and the manufacturing process.
Model oxide layers with defined layer thicknesses were applied to the samples thermally and by means of cathode sputtering and characterized. The samples were then deoxidized using a cold plasma deoxidation unit provided by the SFB and an open air plasma (Plasmatreat GmbH) using various process gases. Suitable process parameters for the deoxidation were determined as a function of the oxide layer thickness, and the surfaces of the samples were analyzed after deoxidation.
The analyses revealed a significantly stronger wear behavior as well as a reduction in hardness and modulus of elasticity with increasing oxide layer thickness compared to the deoxidized samples. The process forces required to achieve plastic deformation were found to be dependent on the thickness of the oxide layer and decreased with decreasing thickness of the oxide layer. Based on the knowledge gained, more precise predictions can be made in future about the material behavior of Cu/Cu2O layers under mechanical stress, for example.
Findings on nanoscale effects
In order to investigate the adhesion tendency of different materials in isolation and without interfering macroscopic and microscopic effects, free-standing, functionalized, flat indenter tips made of silicon were first produced at the IMPT using photolithography and reactive ion depth etching (DRIE) in batch production. The manufactured tips were installed in the triboindenter and pressed onto polished, deoxidized or natively oxidized sample surfaces with a defined surface pressure without relative movement and relieved after a defined contact time. The resulting pull-off forces after relief were recorded and analyzed. This allows the tendency for adhesion between two materials to be measured.
The tests were also carried out in a silane-doped argon atmosphere after the removal of the native oxide layer. The triboindenter was placed in a glove box from M. Braun Inertgas-Systeme GmbH. The tests showed that deoxidized Cu sample surfaces trigger increased pull-off forces after unloading. These could not be observed in the Cu samples with native oxide. The measurement of the forces required to separate the adhesive bonds provided insights into the contact adhesion of macro- to nanomechanical structures.
Development of integrated sensor technology for temperature detection at the point of action
The generation of frictional heat in connection with the surrounding atmosphere has a significant influence on the wear of tools. A sensor and contacting approach for ball-on-disc tests was developed to determine friction-induced temperature increases in the contact zone as a function of the atmosphere. Component-inherent temperature sensors with a diameter of 250 µm were produced on Al2O3 balls with a diameter of 6 mm and contacted on machinable ceramic ball holders made of BN+AlN using a newly developed laser direct structuring process (LDS).
The measurement set-up for the friction test was also extended in order to be able to use the temperature sensors, which were then tested in air and a silane-doped argon atmosphere with different oxygen partial pressures, at different ambient temperatures and process parameter combinations for the copper/copper material pairing.
The results initially showed the functionality of the new temperature sensors and also a significant dependence of the friction-induced temperatures on the atmosphere, the process parameters and the ambient temperature. The investigations of the friction-induced contact temperatures showed an increased temperature rise in a silane-doped argon atmosphere. The temperature development in a normal atmosphere correlates strongly with the ambient temperature. These findings are used to predict wear as a function of temperature and atmosphere.
Objective of the current funding period of the Collaborative Research Center 1368
In the second funding period, the findings from the first funding period of the CRC 1368 serve as a basis for the investigation, development and provision of tool and work piece coatings. The focus is now on coatings already established for production applications. The IMPT will investigate whether these coatings can be applied under conditions corresponding to extreme high vacuum (XHV). In addition, the IMPT will develop new coatings through the use of an atmospheric pressure plasma that can be used by the other scientists in the Collaborative Research Center. Materials such as silicon carbide, titanium and aluminum nitride as well as diamond-like carbon (DLC) are being considered. The tribological, mechanical, corrosion and diffusion properties of these coatings are examined in detail in relevant temperature ranges.
In addition, the influence of the silane concentration in the argon mixture on the formation of tribologically advantageous Si/SiO2-containing top layers is being investigated. The friction-reducing and diffusion-blocking layers that can be produced on deoxidized surfaces are also being investigated and how the bond between coatings and base material can be improved in order to increase the layer quality.
In this way, further findings on production in an XHV-adequate atmosphere and the usability of tool coatings are being gathered in large steps.
