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Minimising friction using laser surface texturing

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Dr Daniel Arnaldo del Cerro, laser applications engineer at Oxford Lasers, describes how global energy consumption could be cut back through laser surface texturing to minimise the friction of moving mechanical components

Friction between sliding surfaces has a significant impact on our everyday life. It is the reason you can steer the wheels of your car to keep it on the road, why you can walk on a pavement without slipping, and why you can warm your hands by rubbing them together. Conversely, lack of friction is the reason why your car slides when driving on an icy or wet surface.

Perhaps less evident, but no less significant, is that a proportion of the fuel costs for your car can be attributed to friction. The relative movement of the pistons inside the cylinder liners of your car engine produces friction. This friction reduces the efficiency of the engine through energy loss, and also contributes to the gradual deterioration of these moving parts. 

Engines, pumps and compressors are just a few examples of where friction between sliding surfaces has a detrimental impact on performance. This has a huge impact in key sectors such as transportation, power generation and manufacturing, not just because of reductions in efficiency, but also because wear can cause the catastrophic failure of key components, which can be difficult to predict, contributes to additional machine downtime, and results in significant cost to the industries concerned.

According to a recent study by Holmberg and Erdemir (2017)1, up to 20 per cent of the world’s energy consumption is wasted overcoming friction, with an additional 3 per cent then being used to replace damaged parts due to wear – these are astonishing numbers! Therefore, engineers are faced with the challenge of improving designs in an effort to reduce friction and the resulting dramatic energy losses.

Laser surface texturing is a manufacturing process with fine surface etch control that can provide a solution. 

The surface finish of sliding parts plays a key role in controlling the friction they experience. It has been shown that well-controlled surface micro-cavities of varying geometry – for example dimples or rectangular pockets – with lateral dimensions ranging between 10 to 100µm, and shallow depth profiles of up to a few tens of micrometres, can dramatically decrease coefficients of friction.

The range of desirable feature sizes makes laser surface texturing an ideal tool for producing them, as the laser beam trajectory and other processing parameters – power, spot size, pulse frequency and scan speed – can be adjusted while scanning across the workpiece, enabling a variety of surface textures to be produced with sufficient accuracy, down to the single-unit micrometre level. This ability to adjust parameters during scanning is particularly relevant in friction reduction, as the optimal surface texture for reducing friction changes with the relative speed of a moving part. A piston, for example, accelerates and decelerates rapidly as it moves between two points, reaching its highest speed mid-stroke2, therefore different surface textures are needed along the piston’s trajectory to ensure optimal friction reduction is achieved. Laser surface texturing on piston rings has previously been demonstrated to reduce the fuel consumption of an engine by up to 4 per cent3.

An example of an industrial turn-key solution for laser surface texturing. (Image: Oxford Lasers)

There is, however, a severe geometrical constraint: as the film thickness of the lubricant being used between two sliding surfaces is usually around a few tens of nanometres, any debris that has piled up from laser processing becomes detrimental towards achieving friction reduction. Thermal management of the laser etching process is therefore critical. 

An ultra-short pulsed laser source seems to be an adequate choice then, due to the increased accuracy and cleaner process that results from the reduced thermal load to the irradiated substrates. An additional final mechanical postprocess step capable of removing undesired material would otherwise need to be applied, if employing conventional laser sources with longer pulse duration in the nanosecond or longer regime.

There are other constraints that may have limited the implementation of laser surface texturing so far as a widely-used technological solution to reduce friction in current components. These are mainly related to processing time, as a result of the relatively large areas that need to be processed using a sub-millimetre form tool, but also to the associated difficulties in handling beam delivery onto tight spaces, such as the inner part of a cylindrical piece. 

With respect to processing time, industry-desired takt times (the average time between manufacturing two consecutive pieces) can typically be only a few seconds, which is a strict constraint. This implies that a moderate to high rotational speed of the part or laser beam (or both), and a high average laser power, are needed. Fortunately, the constant development of diode-pumped solid-state laser sources in the pico- and femtosecond regimes, which are nowadays capable of reaching sufficient energy per pulse of tens of microjoules – even at high repetition rates of 1MHz – has helped overcome these barriers, enabling their use as reliable tools for high-volume industrial applications.

Despite a few tens of microjoules sounding like a modest amount of pulse energy, this is confined by the focusing lens to small spot sizes of around 100µm or less in diameter, during timescales of only hundreds of picoseconds. The resulting peak power experienced by the material at the microscopic level is therefore extremely high, reaching the gigawatt level per pulse. This is similar to the maximum power that a rather large nuclear power station can generate.

Figure 1 shows microscope images of a laser surface texture suitable for friction reduction, machined on a cylindrical part by a cost-effective, five-axes sub-nanosecond pulsed laser system prototype that was designed and built at Oxford Lasers4. 

Figure 1: A laser surface texture for friction reduction on a cylindrical segment (left), and a close up of the generated dimples (right)

Process development also plays a key role in achieving suitable surface textures for friction reduction. Careful selection of an adequate laser source and corresponding process parameters, therefore, becomes crucial to obtain the desired results. Figure 2 shows confocal microscope images and corresponding cross-section profiles of representative dimples created with the same laser source, before (left) and after (right) careful process optimisation. The optimised dimpled texture shows very limited material accumulation around the dimple perimeter, which results in a maximum reduction of friction coefficients of about 25 per cent4.

Figure 2: confocal microscope images (top) and corresponding cross-section profiles (bottom) of representative dimples created with the same laser source, before (left) and after (right) careful process optimisation

Besides being able to generate surface features with the required accuracy and speed, there are further practicalities associated with implementation of laser surface texturing in industrial production scenarios. Automation, part loading/unloading and registration have to be carefully taken into account to avoid incurring additional processing time overheads. In addition, intelligent rotary laser head designs, including position sensors and/or machine vision, that are capable of operating in non-clean production environments, have to be properly designed and integrated, to ensure that all the requirements related to this demanding application can be met. 

The author would like to acknowledge the partial financial support for this work from Innovate UK under grant 102713.

References

[1] Holmberg K, Erdemir A. Influence of tribology on global energy consumption, costs and emissions. Friction 5: 263-284 (2017).

[2] Vlădescu, SC, Ciniero, A, Tufail, K, Gangopadhyay, A, & Reddyhoff, T. Optimization of pocket geometry for friction reduction in piston–liner contacts. Tribology Transactions, 61(3): 522-531 (2018).

[3] Etsion I, Sher E. Improving fuel efficiency with laser surface textured piston rings. Tribology International 42: 542–547 (2009).

[4] Arnaldo del Cerro D, Pelletier E, Karnakis D, Juste K and Cunha A. Towards industrial implementation of laser surface texturing as a tool for enhancing wear resistance and friction reduction on sliding surfaces. ILAS 2019: The 6th Industrial Laser Applications Symposium 20 to 21 March 2019, Crewe, UK.

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