Reducing friction for
efficient motion of sliding
surfaces by Laser Surface
Texturing
Energy losses due to friction and wear of sliding surfaces and othertribological contacts account for 23 % of the world’s energy consumption.1
This figure justifies the need for technologies capable of mitigating
friction and wear. Accurate laser surface texturing of micro-metric sizedfeatures on sliding surfaces have shown a significant decrease of friction
coefficients, while improving the part’s service life due to an increased
wear resistance.2,3 However, there are some constraints that may act as abarrier for this technology to be implemented in industry. These are partly
related to the cost of the equipment, since the required quality of thesurface features usually implies the use of expensive ultra-short pulsed laser
sources. In addition, laser surface texturing speed may result in excessively
long processing times.

Oxford Lasers As one of the mostsuccessful spin outs from Oxford
University, Oxford Lasers have beenat the forefront of laser technology
for over 40 years. They design,develop and manufacture the full
spectrum of Laser Micro-MachiningTools & Laser High Speed Imaging
Systems. The authors of this study
are Daniel Arnaldo, Etienne Pelletierand Dimitris Karnakis. The authors
would like to acknowledge thepartial financial support fromInnovate UK under grant Nº102713.

In this study, we demonstrate a novel laser surface texturing method basedon a cost-effective sub-nanosecond (ns) near-infrared (NIR) fibre laser
source. This laser-based method shows friction reduction and an increased
CASE STUDY REDUCING FRICTION FOR EFFICIENT MOTION OF SLIDING SURFACES BY LASER SURFACETEXTURING

1

wear resistance, which in turn results in greater component efficiency anda longer service life (e.g., pistons). The surface pattern consists of arrays of
micrometric-sized dimples and slots, with controlled depths and widths in
the range of a few tens of micrometers (Figure 1).

Figure 1. Illustration of the laser surface pattern.

■ Measurements
The laser surface texturing process melts material, which may accumulate as
an edge rim around the dimples (Figure 1). The height of this molten layerrim (h1), whenever present, is critical towards the final application as there
might be a very tight manufacturing tolerance of less than 1 micrometer
maximum allowed height between sliding surfaces. In addition, the depthof the dimples (h2) is also relevant: in most of the cases it is necessary tomeasure topographies (heights) with an accuracy better than 1 micrometer(Figure 2).

Figure 2. Illustration of the laser texturing process.
h1 is the height of the molten layer. h2 is the depth of the dimple.

2

CASE STUDY REDUCING FRICTION FOR EFFICIENT MOTION OF SLIDING SURFACES BY LASER SURFACETEXTURING

Figure 3. Illustration of the maximum allowed height
between sliding surfaces in a piston.

In the past, Scanning Electron Microscopy and Optical Microscopy wereused to inspect the surface height profile in laser micromachined surfaces.
Nowadays, optical metrology techniques such as Confocal, Interferometry
and Focus Variation enable the evaluation and study of new laser
micromachining routines with much greater speed and sufficient resolution.
However, given the wide variety of materials and types of surfaces (polishedvs unpolished, smooth vs rough, high reflectivity vs low reflectivity materials,
among others), each optical technique is only suitable under certain
situations. This is one of the main reasons why the 3D optical profilometer
S lynx was chosen to carry out this work. This feature allows for quickly
evaluating all kinds of surfaces.
The samples used in this study consisted of grey cast iron cylinders (Figure 4),
which are widely employed in internal combustion engines, cylinder blocks,
pumps and compressors, among others. The cylinders were preconditioned
by a honing process and cut into sample segments of 30º each to allowfor inspection. After studying the optimum acquisition settings in eachtechnique, the Confocal technique with a magnification of 50X was foundto be the most suitable for this study.

Figure 4. Example of the sample used in this study.

CASE STUDY REDUCING FRICTION FOR EFFICIENT MOTION OF SLIDING SURFACES BY LASER SURFACETEXTURING3

Figure 5. Surface topography of a laser textured grey cast iron cylinder segment,
measured with the S lynx in Confocal mode and with a 10X magnification (left)
and 50X (right).

The novel laser texturing process is compared with a standard method inFigure 6. As can be seen, the new method shows a reduction in heightsof re-deposited material below 1 µm whereas in the Standard method thisvalue is two times larger. Further analyses also suggest a reduction on thecoefficient of friction of about 25%.

Figure 6. Top row: 3D topography and 2D profile obtained with the standard
laser texturing method. Bottom row: 3D topography and 2D profile
obtained with the new laser texturing method.

4CASE STUDY REDUCING FRICTION FOR EFFICIENT MOTION OF SLIDING SURFACES BY LASER SURFACETEXTURING

■ Conclusions
The 3D optical profilometer S lynx has been shown to be an accurate, fastand easy-to-use tool to investigate a novel laser surface texturing method.
The 3D optical profilometry allowed qualitative and quantitative analysis,in particular, Confocal technology has proven to be an effective technique
to investigate and characterize the depth and diameter of the dimples
and more importantly, the height of the re-deposited material aroundthe dimples during laser process development. The new method provides
an optimum surface with practically zero-heights and no re-depositedmaterial after laser process optimization, including a reduction on thecoefficient of friction (COF) of about 25%. Further studies may consider theuse of an automatic detection and analyses software where all the previous
parameters of interest can be extracted automatically (i.e., plug-in Hole,SensoPRO software, Sensofar).

■ Bibliography
1. Holmberg, K. & Erdemir, A. Influence of tribology on global energy consumption,costs and emissions. Friction 5, 263–284 (2017).
2. Vladescu, S., Olver, A., Pegg, I. & Reddyhoff, T. Combined friction and wear
reduction in reciprocating contact through laser surface texturing. Wear 358–359, 51–61 (2016).
3. Schneider, J., Braun, D. & Greiner, C. Laser textured surfaces for mixed lubrication:
influence of aspect ratio, textured area and dimple arrangement. Lubricants 5,32–46 (2017).

CASE STUDY REDUCING FRICTION FOR EFFICIENT MOTION OF SLIDING SURFACES BY LASER SURFACETEXTURING

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