Laser-induced surface functionalization to mitigate cavitation erosion

Balaji Baskar and Guillermo Aguilar (Texas A&M University)

Abstract:

Cavitation is defined as the hydrodynamic process of vaporization of liquid and bubble formation when the local pressure rapidly drops below the saturated vapor pressure of the liquid. While cavitation may be used favorably in some applications like heat exchangers, biomedical surgeries, intraocular pressure sensing, etc., usually, the erosion damage associated with the collapse of cavitation bubbles close to solid surfaces, makes it undesirable in the field of ocean and naval engineering. For example, cavitation-erosion of brass (caused by shockwaves and high-speed jets) can reach up to 120 µm depths in a period of approximately 30 h [1]. 

The most researched way to mitigate cavitation erosion is through coatings. All types of coatings (metallic, intermetallic, metal ceramic, silicide, and non-metallic materials) have been widely used for cavitation erosion resistance [2], but they all have high associated costs, especially nickel and titanium alloys. More than cost, sometimes coatings may lead to material discontinuity (impurities, defects) between the substrate and the coating, favoring mechanical failure or corrosion. Fluorinated or non-stick coatings are used as an alternative but are toxic and undergo degradation after many cavitation events [3]. Hence, there has been more focus on laser-based alternatives like peening and cladding to improve material properties and resistance to cavitation erosion [4, 5]. However, there is very little research on ultrafast laser modification of surfaces and how these nanotextured surfaces can dampen the cavitation bubble dynamics as opposed to changing only the material properties. Our research group specializes in laser-based nano-structuring methods for mitigating cavitation erosion in materials commonly used in hydraulic systems.

By nano-structuring metallic surfaces with laser-induced periodic surface structures (LIPSS), the effects of cavitation erosion were observed and compared to a bare untreated surface [6]. Cavitation events were generated using an in-house nanosecond laser-induced cavitation setup that consists of a high energy nanosecond pulsed laser focused inside a liquid causing avalanche ionization and dielectric breakdown to form plasma and a subsequent vapor bubble. The bubble grows and collapses to form secondary bubbles that damage the surface at different standoff distances. This is repeated for millions of cycles to record the overall cavitation damage on the surfaces with different features. The bubble growth and collapse dynamics for each cavitation pulse were recorded by a high-speed camera and the eroded area and volume were quantified using Atomic Force Microscopy (AFM). Based on the experiments, a significant difference in eroded volume between both surfaces was observed, with LIPSS surfaces leading to significantly less damage. This may be attributed to the lower quantity of remnant bubbles attached to the LIPSS-modified surface between cavitation events, which reduces the number of nucleation sites for subsequent cavitation pulses [7]. Therefore, laser nanotexturing can be a viable solution for mitigating erosion due to scalability and ease of manufacturing.

References

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