Modification of the surface properties of Ti by explosion of laser-plasma containing Coulomb particles

Abstract

This paper continues the study of the effects on a Ti surface caused by the explosion of a laser-plasma containing Coulomb particles. (Generation of such a system and particle self-organization in laser-material interaction (L-M I), was described in: S Lugomer and G Bitelli, Vacuum, 47 , 13 (1996). The laser-plasma explosion in an O 2 atmosphere, and the characteristic surface morphology by accelerated Coulomb particles, was reported in S Lugomer and G Bitelli, Vacuum, 47 , 271 (1996).) Modification of the surface properties of titanium by the Coulomb particles in the exploding plasma was studied by Vickers microhardness tests, X-ray diffraction and Raman backscattering. A large jump of microhardness from the background value of 288 Kp/mm 2 to ≈ 700–7200 Kp/mm 2 was observed at the places of gassy Ti-oxide; the jump is smaller (to ≈450–700 Kp/mm 2) on the other oxidized zones. Finally, a decrease of microhardness on the pure Tisurface occurs, because of plasma thermal effects (annihilation of dislocations in the surface zone). Raman backscattering studies have shown the existence of a partially crystalline Ti-oxide and also an amorphous (glassy) one. The contribution of the crystalline phases, anatase and the rutile (the low- and the high-temperature phases), changes with the number of laser pulses, i.e. the low-temperature phase is transferred into the high-temperature one, by repetitive laser pulsing. The broad background band (Bose peak) was interpreted on the basis of the fraction model using the Yakubo's relation for fitting the real spectra. The phonon-fracton crossover frequency ω CO1 , was found to be sensitive to the O 2 pressure for P < 5 atm, and quite insensitive for P ≥ 5 atm. Correlation of the results of Raman backscattering and the X-ray diffraction, indicates that the laser plasma explosion in O 2 generates the glassy matrix of Ti-oxide, into which the crystallites of TiO 2 (the anatase and the rutile phases, of size ranging from <10 A ̊ to tens of nm) are embedded.