Unraveling of the laser drilling of carbon/carbon composites: Ablation mechanisms, shape evolution, and damage evaluation

Laser machining is promising in shaping the brittle carbon/carbon composites (C/Cs) with deep holes, sharp edges, or thin walls. However, there are still many unknowns relating to the laser ablation of carbon materials, and the existing theory and practice is insufficient to guide the industrial machining of C/Cs. Herein the laser drilling of C/Cs was experimentally conducted and numerically modeled to probe into the mechanisms responsible for the material removal, surface formation, and damage evaluation. Firstly, the intrinsic correlations among the anisotropic hole feature, the fiber yarn alignment and the steady-state thermal conduction are revealed. The detailed characterizations of the ablated surface and the recast layer clearly prove that sublimation of the graphitic carbon dominates the material removal process under laser ablation. Furthermore, it is proposed that the greater portion of crystalized graphene layers enables the lower ablation rate of the pyrocarbon matrix than the carbon fibers. Secondly, the combination of the experimental and simulated results unravels that the continuously evolved surface slope and the redeposited recast layer are the decisive factors in the laser-carbon interaction, which affect the efficient absorption coefficient of the laser and result in the nonlinear drilling rate and the self-limiting of the drilling. Finally, the roles of the laser heating and the subsequent rapid cooling in damage initiation and propagation are identified: nanoscale splitting of the pyrocarbon occurs due to the growth and realignment of the graphene layers upon laser heating, and the tensile thermal stress induced by the cooling drives the further growth of high-density but discrete microcracks from these splitting sites. The load bearing capability of the carbon fibers, however, is retained in this severe thermal shock. As a result, the laser drilling induces only a slight degradation of the mechanical strength of the C/Cs.