Experimental characterization of the inner surface in micro-drilling of spray holes: A comparison between ultrashort pulsed laser and EDM

• The inner surfaces of micro-drilled holes are analyzed by Shear Force Microscopy.
• Surface texture is illustrated by maximum peak-to-valley distance and periodicity.
• Laser ablated surfaces offer a smooth texture with a peculiar periodic structure.
• A comparative analysis is carried out between the highest energy laser and EDM.
• Rq derived from maps ranges between 220 and 560 nm for EDM and 50 and 100 nm for fs-laser.

In this research, the inner surface characteristics of micro-drilled holes of fuel injector nozzles were analyzed by Shear Force Microscopy (SHFM). The surface texture was characterized by maximum peak-to-valley distance and periodicity whose dimensions were related to the adopted energy. 180 µm diameter holes were drilled using ultrashort pulsed laser process using pulse energies within the range of 10–50 µJ. Laser ablated surfaces in the tested energy range offer a smooth texture with a peculiar periodic structure with a variation in height between 60 and 90 nm and almost constant periodicity. The Scanning Electron Microscopy (SEM) photograph of the Laser Induced Periodic Surface Structure (LIPSS) showed the co-existence of Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL). A comparative analysis was carried out between the highest laser pulse energy in the tested range energy laser drilling which enables the shortest machining time and micro-Electrical Discharge Machining (µ-EDM). On the contrary, results showed that surfaces obtained by electro-erosion are characterized by a random distribution of craters with a total excursion up to 1.5 µm with a periodicity of 10 µm. The mean-squared surface roughness (Rq) derived from the scanned maps ranges between 220 and 560 nm for µ-EDM, and between 50 and 100 nm for fs-pulses laser drilling.