Thermoplastic modification of polycrystalline copper surface microstructure by nonmelting nanosecond laser pulses

Abstract

Mechanisms of surface relief formation on bulk polycrystalline oxygen-free copper samples under single and multiple UV nanosecond laser pulses (355 nm, 10 ns) in air are investigated. The laser spot size ∼100 μm is several times larger than the average size ∼40 μm of crystal grains. Incident fluences are limited by ∼1 J/cm2 to avoid melting of copper. Confocal laser microscopy, scanning electron microscopy, and transmission electron microscopy are utilized to reveal the deformation processes leading to surface relief development. It is found that irradiation uncovers the grain boundaries on the exposed surface. Dislocation subboundaries and packets of nanoscale plates of deformation twins near the grain boundaries are observed in a subsurface layer of 20–100 nm thick. According to our theoretical analysis and simulations, the observed surface modification is caused by nonuniform plastic deformation of crystallites induced by nonmelting laser heating. Continuum modeling of such heating shows that the shockwave effects are negligible since the generated acoustic waves have a pressure of less than 100 atm. We demonstrate that the solid copper goes to the plastic deformation regime if the temperature rise exceeds ∼250 °C. Atomistic simulation of polycrystals reveals anisotropy of grain deformation and asynchronous plastic response due to their different stiffnesses depending on lattice orientation, as well as the residual stresses and growth of surface roughness after each heating–cooling cycle. The obtained results are important for understanding the physical mechanisms of surface degradation of copper mirrors under laser induced thermocycling.