The surface topography plays an important role in the design of a function-optimised surface. Therefore, the influence of topography with microsized structures produced by laser surface texturing (LST) is experimentally examined under lubricated sliding conditions. The structured specimens were made of AISI 51200 (DIN 100Cr6) hardened to about 800 HV. Concerning the requirements of tribological testing without any debris caused by the preprocessing, the structuring was carried out using a picosecond laser system (Trumpf TruMicro) with 6 ps pulse duration. A laboratory pin-on-disc tribometer (Plint TE-92 HS) was used for the tests, which were run under wet conditions with counterbodies made of bronze and steel at a nominal contact pressure of up to 4 MPa and sliding speeds between 0.04 and 2.0 m/s. Furthermore, start-stop cycles with accelerating and decelerating shares were used to simulate an automotive start-stop system. In the tribological experiments, a significant reduction of the friction coefficient was observed compared to sliding pairs without microstructured pin surfaces. Whereas no measureable wear occurred on the steel pins and discs, the bronze discs showed a significant amount of wear and the microstructures on the pin surfaces mated against bronze discs were almost completely filled with wear debris.
Since the transport sector accounts for a big share in anthropogenic CO2 emissions, there is a need to reduce fuel consumption in this area. In passenger cars, about one-third of the fuel energy is used to overcome friction. These friction losses can be broken down to braking and rolling resistance on one side and losses in the engine and transmission on the other side [
Besides the early work of Zum Gahr [
The structured specimens were made of AISI 51200 (DIN 100Cr6) hardened to about 800
As already shown theoretically in [
The influence of different pulse energies (and resulting peak fluence) and number of repetitions on the depth of the dimples is shown in Figure
The ablated depth of dimples in AISI 52100 as a function of the peak fluence and the number of repetitions per position.
The influence of peak laser fluence and the repetitions on the ablated squared diameter area on the surface are shown in Figure
Squared diameter of the ablated dimples as a function of the peak laser fluence, LIPSS, at the edge of a dimple structure.
As already noted in [
Like shown in Figure
SEM pictures of dimples showing rough surfaces due to “Laser Induced Periodic Surface Structures” (LIPSS). (a) Structure 1. (b) Structure 2. (c) Structure 3.
The variable dimple parameters in this work were the structured area, the dimple diameter, and depth. These structures were ablated with similar framework conditions like the preliminary tests. The exemplary structures for the tribological tests shown in Figure
The structured steel pins were tribologically evaluated in unidirectional sliding contact using a laboratory pin-on-disc tribometer (Plint TE-92 HS) with counterbodies made of bearing bronze (155 HV,
Figure
Friction coefficient versus sliding speed for pins with 3 different structure sizes and a ground reference pin mated against (a, c) bronze and (b, d) steel during Stribeck velocity ramps at normal loads of (a, b) 60 N and (c, d) 120
60 N
60 N
120 N
120 N
The positive effect of a microstructured surface was also evident during the start-stop tests. Figure
Friction coefficient versus sliding speed for ground and structured pins mated against (a) bronze and (b) steel during the acceleration ramp of the start-stop cycles at a normal load of 60
60 N, acceleration
60 N, acceleration
After the complete test procedure with overall 4 velocity ramps and 100 start-stop cycles at normal loads of 60 and 120 N no measureable wear occurred for the steel pins as well as the steel discs. On the other hand the bronze discs showed a significant linear wear in the range from 5.4
The differences in wear behavior were also clearly visible on the surfaces of the pins after the tests. Figure
SEM pictures of microstructured pins after tribological tests in contact with (a, c) bronze and (b, d) steel discs. (a, b) Structure 1 and (c, d) structure 3 (arrow indicates the sliding direction).
(a) Topography and (b) surface profile of pins with structure 3 after a tribological test in contact with bronze.
The results of the tribological tests clearly illustrate that, especially under mixed lubrication conditions, the optimized dimples can lead to a significant reduction of friction. The positive effect of the dimples can mainly be attributed to three mechanisms. In the low wear regime dimples can result in a positive hydrodynamic effect which results in an increased thickness of the oil film. The effectiveness of a specific dimple geometry thereby strongly depends on the experimental conditions (e.g., load, sliding speed, viscosity of the lubricant
The authors declare that there is no conflict of interests regarding the publication of this paper.