Could confinement layer free LSP, investigated by CLF scientists using DiPOLE10, be the industry applicable version we have been searching for?
Laser Shock Peening (LSP) is a technique used to increase a materials resistance to repeated exposure to external stresses (fatigue life) and improve the wear resistance of materials, which is important in industry for improving the operational life of systems. The technique uses a high-energy short pulse laser (like DiPOLE) focused on the surface of the metal to form a plasma, which is confined (normally using water). This process results in compressive residual stresses being introduced into the sample, which are part of what improve the fatigue life of the sample.
However, the requirement for a confinement layer in LSP can have a detrimental impact on the acceptance of LSP as a technique in industry. This is because a transparent material like water is used as a confinement layer around the sample to contain the formed plasma and was traditionally vital to introducing compressive stresses into the sample. For this confinement layer to do its job, it needs to uniformly coat the sample, like an even coat of paint on ceramics. The complex shapes of real-world metals you might want to perform LSP on makes uniform coating difficult to achieve, especially with a liquid like water. Therefore, the ability to perform LSP on a sample without the need for the uniform water layer would save precious time during LSP applications, making the technique more accessible for remote or on-the-job industry use.
Therefore, Dr Saumayabrata Banerjee and Dr Jacob Spear from the CLF used DiPOLE10 to investigate if it was possible to simplify the process of nanosecond-LSP (ns-LSP) by replacing the water confinement layer with ambient gas at atmospheric pressure. Therefore, instead of surrounding the sample with water, or putting it under extreme pressures like some LSP techniques, it is simply being left exposed to the open air around us and the pressure we feel. This eliminates the need for additional equipment to add the confinement layer or put the sample under additional pressure
Using STFC Proof of Concept Funding, the team tested out this new technique on tungsten (and one of its alloys) and compared the results to previous ns-LSP experiments they performed on tungsten with a confinement layer. They also compared their confinement layer free ns-LSP results to similar confinement layer free femtosecond-LSP experiments performed on aluminium, and found their technique appeared more effective at introducing compressive stresses to the sample.
The team demonstrated that ambient gas at atmospheric pressure can be used as a confinement layer in nanosecond time scale laser irradiation for the introduction of compressive stresses in tungsten and a tungsten alloy. To their knowledge, this is the first time experiments of this nature have been done with tungsten. This is an important result towards making ns-LSP accessible for industry applications, because it eliminates the difficult need for a uniform confinement layer.
Read the publication on confinement layer free ns-LSP.