BY: L. Mellor, S.P. Edwardson, W. Perrie, G. Dearden, K.G. Watkins

Laser Group, Department of Engineering, The University of Liverpool, Liverpool L69 3GQ, UK

A study has been carried out on the production of micron and sub-micron features on the surface of a range of metallic materials through laser illumination. Possible application areas for such features include the biotechnology, communications, and aerospace industries for applications such as control of surface wetting angle. Modification of the surfaces was attributed to the production of Surface Plasmon Polaritons (SPPs) along the sample surface, resulting in a ripple effect with features smaller than the wavelength of the incoming laser radiation. Periodic ripple structures were produced on the surfaces of two industrially relevant materials; M42 High Speed Steel, used in the manufacture of various cutting tools, and Ti-6Al-4V, used in aerospace engine and frame components. For this work a high power ultra fast laser system by Fianium was used. This system provided picosecond pulses at 1064nm with pulse energies up to 10mJ with selectable repetition rates from 200 kHz down to the single pulse level. The pitch of the periodic features produced with this system was typically 1mm with depths ranging from 100-500nm into the sample. The high repetition rate of the Fianium system allowed for the production of these surface features at traverse speeds up to 500mms^-1 allowing for rapid sample coverage. Alteration of the beam polarisation via a half wave plate allowed the orientation of the ripple structures to be controlled. Samples were textured with ripples in one orientation, then after altering the beam polarisation sections of the pre machined area were re-exposed to varying numbers of laser pulses. Re-exposed areas showed ripple growth corresponding to the new polarisation orientation between 5-10 pulses, below this it was found that under certain processing parameters it was possible to completely erase the ripple features. This gave us a high degree of control over the production of these ripple features allowing writing, over writing and erasing.

Textured surfaces were then analysed for changes to their water contact angle. Areas were textured with an increasing level of ripple coverage with initial results showing a steady reduction of the contact angle as the coverage increased. Maximum contact angle changes of up to 60⁰ in Ti-6Al-4V and 25⁰ in HSS occurred when complete ripple coverage was achieved. In the case of Ti-6Al-4V the resulting surface was highly hydrophilic giving rapid spreading of water with a contact angle of approximately 15⁰. Results such as these are a positive indication that gaining control over the production of these ripple structures at high enough speeds to make them industrially attractive could lead to some interesting applications.

Overlap patterns produced on a) Ti-6Al-4V; background pattern produced at 2.5mJ/pulse, 200kHz, 500mms^-1. Spot produced with polarisation rotated approximately 45⁰ at 2.5mJ with a single pulse b) HSS; background pattern produced at 2.5mJ/pulse, 200kHz, 500mms^-1. Spot produced with ten 1.2mJ pulses rotating beam polarisation by 60⁰ and c) effect of ripple patterns on the water contact angle of Ti-6Al-4V. Left side of the sample has been modified with ripple patterns and displays a high level of hydrophilicity.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here