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The topics of friction, lubrication and wear are becoming increasingly important against the background of environmental and climate protection, energy efficiency and electric mobility. The research focus of the department of surface technology is therefore the development of tribological functional coatings. On the one hand, this includes novel friction-reducing hard material coatings to increase energy efficiency and the service life of transmission components, especially for rolling bearings and gears. On the other hand, tribological functional coatings are also used for coating high-performance cutting and forming tools. For cutting tools, the development goals are dry machining, hard machining, tools for workpiece materials that are difficult to machine, and increasing the cutting path and metal removal rate. For forming tools, the focus is on dry machining, increasing tool life and workpiece quality.

Increasing the energy efficiency and service life of gearbox components

Increasing the energy efficiency and service life of gear components by using surfaces that minimize friction and wear is an important future topic for climate protection. This applies both to primary energy generation and to the industrial, commercial and mobility sectors. The great importance of tribology for climate protection is impressively illustrated in the current study “Tribology in Germany: cross-sectional technology for reducing CO2 emissions and conserving resources” by the Gesellschaft für Tribologie e.V. (2019).

Nahaufnahme einer Präzisionsprüfmaschine, die auf eine metallische Oberfläche drückt.
New requirements for tribology and coating technology

For example, this study shows that a compact and therefore lightweight design of electric motors requires high speeds in the range of 10,000 to 30,000 rpm. This results in numerous tribological challenges for the reduction gears required. In addition to minimizing friction and wear and the use of new low-viscosity lubricants, triboacoustic emissions also play a major role in such gearboxes.

This is because these emissions are often in a frequency range that is unpleasant for people and are easier to hear due to the significantly lower overall noise emissions of electric vehicles compared to combustion engines. In the future, friction, lubricant film formation, wear and the acoustic emissions of high-speed transmission components must therefore be controlled by various measures such as friction-reducing hard coatings or targeted microstructuring of the surfaces of the machine elements used.

In the area of hard material coating for high-performance cutting tools, the topics of dry machining, machining of difficult-to-cut workpiece materials such as titanium or cobalt-chrome alloys from the medical sector, as well as increasing the cutting path and metal removal rate, are of great importance. For the coating of forming tools (cold forging), the focus is on dry machining and increasing tool life. If cooling lubricants are dispensed with completely, subsequent cleaning processes are no longer necessary. New tool coatings that enable dry machining with the same tool life and workpiece quality therefore make a direct contribution to environmental protection and resource efficiency.

PVD-based hard coatings and functionalized surfaces in tribological applications

The Surface Technology department focuses on the application of tribological hard coating systems using the PVD magnetron sputtering process. Magnetron sputtering is an industrially used physical vapor deposition process (PVD). In collaboration with project partners, hard coatings deposited using the PVD-Arc process are also being investigated. In general, the environmental compatibility of physical vapour deposition processes is relatively good, despite the complex vacuum system technology required, as no or only a few environmentally harmful waste products are produced during arc evaporation in the PVD arc process and during cathode sputtering in magnetron sputtering of the mostly metallic target materials.

PVD processes enable the deposition of a very wide range of metallic and non-metallic coating materials. Reactive PVD processes can also be used to incorporate light elements such as nitrogen, carbon, oxygen, but also boron or hydrogen into the coatings, so that in combination with metallic target materials such as titanium, chromium, vanadium, molybdenum, zirconium, tungsten, aluminum, etc., the corresponding nitrides, carbides, oxides or borides can also be deposited. In this way, an almost unlimited number of different hard materials can be deposited using PVD processes. These include, in particular, purely covalently bonded hard materials such as diamond or diamond-like amorphous carbon layers.   

In the field of friction-minimizing coatings for rolling bearing rings and gears, the Surface Technology department is working both on the further development of hydrogen-containing amorphous carbon coatings (a-C:H) and on the development of new types of self-lubricating hard coating systems based on various PVD solid lubricant coatings in combination with transition metal nitride coatings (TiAlN, CrAlN, ...). In addition to the coating composition and layer structure, the tribology can also be influenced by setting defined surface topographies in the micrometer range. The microstructuring of the coatings or substrates prior to coating is carried out in close cooperation with the Laboratory for Micro Machining, as well as with partner institutes from the field of laser processing.

The trend towards the increasing integration of additional functions such as corrosion protection, electrical, optical, acoustic or antimicrobial properties in functionalized surfaces of tools and machine elements is also continuously leading to new research topics. Other areas of research in the Surface Technology department include new metallic alloys produced using PVD processes, corrosion research and sol-gel coating processes.

Projects of Surface Technology

Tribological transfer mechanisms and large-area micro-contact simulation of solid lubricant provision from PVD layers for dry-running gear stages

In gear systems, liquid lubricants are used to reduce friction and wear, dissipate frictional heat and to protect the gears from corrosion.

However, there are applications where liquid lubricants cannot be used, such as in vacuum pumps or for gear systems that must remain operational at temperatures above 250 °C or below -50 °C. For such applications, novel self-lubricating MoS2:Ti/Ti(C,N) thin film systems were developed and deposited onto gears with various tooth profiles using PVD magnetron sputtering. The coated gears were tested on a gear test bench by our project partner. It was found that the dry sliding friction of the MoS2:Ti coatings decreased from ≈ 0.12 to ≈ 0.07 ± 0.01 if the Hertzian pressure pHertz was increased from 0.83 GPa to 2.0 GPa. With the developed coating system, gear lifetimes of 0.3 × to 1.0 × 105 rolling cycles were achieved at pHertz = 1.6 GPa. For pHertz = 2.0 GPa the lifetime of the coated gears reduced to 0.1 × to 0.3 × 105 rolling cycles.

 

 

Cooperation: WZL der RWTH Aachen 
Funding: DFG – SPP 2074: Fluidfreie Schmiersysteme mit hoher mechanischer Belastung
 

Tribological Potentials of PVD-Coated Micro-Textures – TriPot

In previous projects (SFB 747 “Micro Cold Forming” and SPP 1676 “Lubricant-Free Massive Forming”) friction and wear of forming tools were significantly reduced by micro-texturing and subsequent PVD hard coating of the tool surfaces.

In his new project empiric models are developed in order to predict friction and wear of micro-milled tool steel surfaces. The project aims to answer the following scientific questions: How do material pairing, contact pressure, and different surface topography parameters such as Sa, Sz, Sk, Spk, Svk, etc. influence the resulting coefficient of dry sliding friction and run-in behaviour? How do these parameters influence the basic friction and wear mechanisms of adhesion, elastic hysteresis, plastic deformation and abrasion? How to design optimized micro-textures and PVD coatings to meet given tribological requirements? Is it possible to predict the tribological properties of hypothetical micro-textures using these empirical models?
 

 

Cooperation: FB4 der Universität Bremen
Funding: DFG

Increase of the life time of rolling rings for welding wires by novel PVD hard coatings – ZIM EcoWire

In the steel works of the cooperating partner, steel billets are processed into steel wires on a rolling mill.

The rolling mill consists of 25 sequentially arranged pairs of rolling rings, each formed by an upper and a lower roll with a deep profile corresponding to the target diameter of the wire at the respective rolling stage. For certain steel alloys – particularly welding wire grades – significant wear occurs at the profiles, especially for the final rolling ring stages. This is due to the high wire throughput speed resulting from the small wire diameter at the final rolling stages. Until now, uncoated WC/Co carbide rolling rings have been used. Within the scope of the project, tailored PVD coatings are developed for the existing rolling rings and tested in the rolling mill.

 

 

Cooperation: Arcelor Mittal Hamburg und A+S Oberflächentechnik GmbH
Funding: AiF ZIM

Contact: 
Dr.-Ing. Andreas Mehner 
Tel.: +49 421 218 51377
E-Mail: mehner@iwt-bremen.de