An explanation for laser-induced spallation effect in a-C:H films: Altered phase evolution route caused by hydrogen doping

The laser-induced spalling effect has been recognized as a unique phenomenon for amorphous carbon (a-C) films during laser processing. In this work, the origin of spalling effect was investigated by ablating two different types of a-C film: hydrogenated a-C (a-C:H) and nonhydrogenated a-C with an Nd-yttrium aluminum garnet laser system. Comparisons of ablating results demonstrated that the spalling effect only occurred in a-C: H rather than nonhydrogenated a-C. Laser heating simulation indicated that the temperature distributions in both films after laser pulse are similar with a high temperature gradient in depth direction. Annealing test results, Raman spectra and nanoindentation show that with the increase in annealing temperature, a-C film transforms into grassy carbon directly, while a-C:H experiences two subprocess under heating: the hydrogen mobilization and rearrangement of C-C network at a relatively low temperature range resulting in a denser C-C network and raised film density; the graphitization at high temperature which would lower the film density. We propose that the reason of laser-induced spalling effect in a-C: H might depend on two aspects: (1) the heat source like laser pulse which could produce a high temperature gradient in depth direction within ultrashort time and (2) the unique evolution process of film microstructure under heating. Based on above model, the spalling effect is ascribed to the concentrated stress caused by different structure evolution subprocess at different depth in a-C: H during the laser irradiation. It is remarkable that the conclusions deduced from our model are proven to be in good agreement with our experimental results and the previous articles reported by others. (C) 2011 American Institute of Physics. [doi:10.1063/1.3528223]