ILAS 2019: LASER SURFACE TEXTURING


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


. Process development also plays a key role in


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


surfaces is usually around a few tens of nanometres, any debris that has piled up from laser processing becomes detrimental towards achieving friction reduction. Termal 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. Tere are other constraints that may have


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-


well-controlled surface micro-cavities… can dramatically decrease coefficients of friction


It has been shown that


desired takt times (the average time between manufacturing two consecutive pieces) can typically be only a few seconds, which is a strict constraint. Tis implies that a moderate to high rotational speed of the part or laser beam (or both), and a high average laser power,


limited the implementation of laser surface texturing so far as a widely-used technological solution to reduce friction in current components. Tese are mainly related to


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


An example of an industrial turn-key solution for laser surface texturing


www.lasersystemseurope.com | @lasersystemsmag


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. Te resulting peak power experienced by the material at the microscopic level is therefore extremely high, reaching the gigawatt level per pulse. Tis 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,


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 (leſt) and aſter (right) careful process optimisation. Te 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 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.


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.


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


ISSUE 42 • SPRING 2019 LASER SYSTEMS EUROPE 23


Oxford lasers


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