Change in the Morphology of the Surface of Ta–W Coating after Exposure to Cyclic Laser Pulses

Abstract

The effect of low-energy pulsed laser irradiation (laser LRS-150A, λ = 1.06 μm) on the surface of Ta–10% W coating produced by ion-plasma deposition in vacuum was investigated. The coating thickness was ~250 μm. With the help of the metallography method, it was shown that the coating average grain size was ~25 μm. The laser pulse energy was 30 J, pulse duration τ = 14 × 10^–3 s, and the interval between laser pulses was 7.5 s. The laser beam was focused on the sample surface into a spot with a diameter of 5 mm. The microrelief of the coating surface was investigated with the use of a TESCAN VEGA II scanning electron microscope. X-ray microanalysis in the scanning line of the surface and cross section of the coating and the substrate was carried out on an INCA Energy 250 energy dispersive spectrometer. It was shown that, after a multicycle laser irradiation (200 pulses), numerous rounded protrusions appeared on the surface of the coating. The diameter of the protrusions ranged from 25 to 225 μm and their maximum height was 30 μm. In the irradiation zone, redistribution of W occurred. The concentration of W in the region of the protrusions decreased substantially and increased in the ranges between the protrusions. It was concluded that the appearance of protrusions on the surface of irradiated surface could be due to the simultaneous occurrence of several interdependent processes, such as the loss of stability of the coating with the formation of alternating areas of tensile and compressive stresses, diffusion of tungsten into the regions of compressive stresses, and relaxation of internal stresses by grain-boundary sliding with extrusion of grains and their conglomerates on the surface.