On the origin, growth and application of ripples

Abstract

The origin and growth of a self organizing rippled nanostructure have been studied on slightly etched high temperature steel using 240 fs laser pulses with a wavelength of 800 nm. SEM, AFM and confocal microscopy have been used for analyses. A relatively low energy input results in a regular structure with a wavelength in the range 300 - 500 nm and with an orientation perpendicu-lar to the polarization direction of the laser light. In twinned areas nearby on the same specimen only "pre ripples" with a much smaller wavelength of about 150 nm and an amplitude of about 10 nm are observed. Surprisingly these "pre ripples" have a different orientation. It is proposed that energy absorption within the matrix is strongly influenced by crystal orientation differences and that "pre ripples" can be observed only within a certain energy input range. With increasing energy in-put regular ripples initiate, here on lines of small intergranular carbides in an area with "pre rip-ples". When the energy input is further enhanced the regular rippled pattern is disturbed and finally the ripples transform into disordered structures. Surfaces with a microstructure due to laser abla-tion and a superimposed rippled texture have been used successfully for creating hydrophobic sur-faces.