Comparative interaction mechanisms for different laser systems with selected materials on titanium alloys

This paper describes further research into, and modelling of, the interaction mechanisms of various laser types with materials, including synthetic oil and silicone grease. A Q-switched Nd:YAG pulsed laser, a CW CO2 laser, and a pulsed KrF excimer laser (λ = 248 nm) were used in the study. In general, the materials studied were very absorptive in the ultraviolet, less absorptive in the infrared, and least absorptive in the near-infrared. For the excimer, photo-ablation takes place, while for the other two wavelengths thermal vapourization dominates. In the case of silicone grease, full volatilization is only obtained using the excimer. For the other two wavelengths, this is not the case, with a sticky residue remaining after treatment. Interaction with synthetic oil with its lower boiling point can be obtained for all three laser types. With the Nd:YAG the dominant heating mechanism is conduction from the substrate and a baking effect is observed with strong interaction between the contaminant and the substrate. For the two pulsed lasers, oxidation is substantially less, while for the continuous wave CO2, the interaction time is sufficiently long for oxidation and secondary reactions to take place if care is not taken to work below the reaction threshold.