Kosuke Kawahara

Development of Laser Turning Using Femtosecond Laser Ablation

Ablation using femtosecond-lasers has taken much attention for micromachining with the advantages of efficient ultra-thin layer peeling without undesirable thermal effects for both opaque and transparent materials. In this work, we have proposed the femtosecond-laser turning based on the fact that femtosecond-laser ablation is a promising technique for excellent surface finishing techniques. The effect of the machining conditions such as focusing condition, energy of laser pulse and plural scanning, on the thickness of peeling layer for various kinds of materials (PMMA, silica glass, silicon, copper and aluminum) have been investigated. As a result, the important information for the laser turning as a practical application of the femtosecond lasers were obtained.

Effect of Nanotexturing on Increase in Elastohydrodynamic Lubrication Oil Film Thickness

The effects of nanotexturing on oil film thickness and shape under pointcontact elasto-hydrodynamic lubrication (EHL) conditions were experimentally investigated. A disk-onball friction tester with an optical interferometer was used to measure oil film thickness and to observe the oil film shape. Periodic groove structures with a spiral, perpendicular, or parallel shape and with various groove depths and distances were formed by irradiation of a femtosecond laser onto the surface of steel balls. These nanotextured balls were tested under a load of 20 N and at rotational speeds from 1.0 to 3.0 m/s. Most of the balls with nanotexturing had a thicker oil film than those without texturing. The groove depth and angle were the key parameters determining the thickness of the oil film as they controlled the amount of side leakage of oil from the contact point. Optimization of these parameters resulted in an oil film that was almost twice as thick as that on the ball without texturing.

Hydrodynamic Performance Produced by Nanotexturing in Submicrometer Clearance With Surface Roughness

Surface texturing is a promising way to expand the hydrodynamic lubrication regime and thereby modify the tribological properties of sliding surfaces. Spiral-groove textures in particular have attracted much attention over the past several decades because they produce a thicker lubrication film in the gap. However, no research has been reported on the effect of periodic texturing with a several 100 nm depth on hydrodynamic performance in submicrometer clearance with surface roughness. The purpose of the study reported here was to investigate the effect of such nanotexturing on hydrodynamic performance. This was done by conducting ring-on-disk friction tests, focusing on the existence of surface roughness in the narrow clearance. The samples were rings with various degrees of surface roughness and disks with spiral-groove textures produced by femtosecond laser processing. The friction coefficients experimentally obtained were plotted as a Stribeck curve and compared with a theoretical one calculated using a Reynolds equation formulated from two physical models, the Patir–Cheng average flow model and a sinusoidal wave model. The results showed that surface roughness did not affect the friction coefficient in the hydrodynamic lubrication regime. However, the hydrodynamic lubrication regime gradually shrank with an increase in surface roughness, and mild transitions to the mixed lubrication regime were observed at higher rotational speeds. The minimum clearances reached at the transition speed were almost the same, about 200–300 nm, for all experiments regardless of surface roughness.