Characterization of Thin MoO3 Layers Obtained by Laser Bonding

Over the last years, a technology process similar to the laser cladding finds increasingly broader applications for laser marking of different materials, mainly metals. In the literature it could be found as "laser bonding". The method consists of laser sintering of pre-coated powder material, forming a layer with assigned graphical and topological design. Unlike the conventional laser-cladding-technology, the starting material is a complex composite of materials with different physical and chemical properties. Another difference is that enormous temperature gradients as well as huge temperature rate of change during the laser forming are usually registered. The presented work includes preliminary results of laser bonding investigations on stainless steel. The initial deposited material is powder of pure MoO3 and the laser beam has been fixed over the treated surface. A continuous-wave (cw) 30 W CO2 laser is used. The main goal of the research is to study the layer forming dynamics, phase transitions in the deposited material and adhesion with substrate. Samples of laser bonded MoO3 are prepared on stainless steel at different technological conditions. The samples have been analyzed by micro-X-Ray scanning electron microscope (SEM). Morphological and micro-chemical analyses of the coatings have been carried out. Raman spectroscopy is used to identify the phase states of MoO3 in different regions of the coatings. A finite element model (FEM) of the layer forming dynamics is created and technologically relevant results are presented. The model indicates a temperature increasing beyond 2,000 K in a spot of 150-200 �m within 200-300 �s, leading to gradients and temperature speeds of 107 K/m and 107 K/s, respectively. Under further heating MoO3 melts, getting transferred to an amorphous phase, and undergoing solidification nearly without re-crystallization. It has been shown that in the region with highest temperature a melting of steel and mutual diffusion of MoO3 into the melt pool is observed. Computer model shows that under specific conditions in the melt pool surface hydraulic waves can be excited, which is experimentally confirmed.