| Currently,one of the important characteristics of ultrafast laser proc essing technology is wide coverage and rapid development.Especi ally in the field of consumer electronics,semiconductor and aerospace industry,ultrafast laser has become an effective tool to process special components.However,during the process of ultrafast laser interacting with materials,it is very easy to happen thermal accumulation damage and thermal shock damage in the local range of target,which will affect the performance and service-life of machined components.Particularly,during the ultrafast laser processing hard brittle materials,the photothermal effect and thermal shock effect easily lead to the appearance of microcracks,remelting layer and other processing damage,which seriously restricts the application of ultrafast laser micromachining technology in consumer electronics,semiconductor,aerospace and other fields with high requirements for device reliability.Therefore,in order to research the damage problem during ultrafast laser irrration,analyzing the damage mechanism and law of materials,exploring modulation methods of the damage behavior,and developing laser micro-damage processing technology,which are of great theoretical and practical significance.In this paper,the sapphire crystal is processed by ultrafast laser,the damage behavior of sapphire crystal surface under ultrafast laser irradiation is focused.The results are as follows:(1)The types and evaluation method of sapphire crystal surface damage behavior under ultrafast laser irradiation are studied.The interaction mechanism between ultrafast laser and sapphire crystal is exp ounded.Experimental platform of ultrafast laser Gaussian beam processing is built.It is found that the surface damage behavior of sapphire crystal mainly includes the remelting layer formation,microcrack generation and phase transformation under ultrafast laser irradiation.The damage threshold of sapphire crystal is 6.74J/cm~2(pulse duration 250 fs and wavelength 1030nm),and the critical point of sapphire crystal damage energy is identified under the condition of specific wavelength and pulse width.(2)The molecular dynamics model of interaction between ultrafast laser and sapphire crystal is established,aiming at analyzing the evolution process of ultrafast micro particles under ultrafast laser irradiation.The influence of laser parameters on the splashing particles is analyzed qualitatively.The damage threshold of sapphire crystal with different pulse width is predicted.Based on the molecular dynamics simulation results,the strategy of controlling the splashing particles and remelting layer formation by focusing spot size or input energy is proposed.(3)Based on the molecular dynamics model of interaction between ultrafast laser and sapphire crystal,the variation of atomic number density and pressure with different irradiation direction is analyzed.The propagation velocity of laser-induced stress wave is calculated,and the anisotropy of stress wave propagation on the sapphire crystal is confirmed.The relationship between the propagation velocity of stress wave,the generation of crack and crystal orientation is discussed,crystal orientation effect of the crack generation is found,and the laser parameters and crystal orientation matching control strategy of sapphire crystal crack is proposed.(4)The experimental study on sapphire crystal surface damage behavior under ultrafast laser irradiation is carried out.The experimental platform of ultrafast laser Bessel beam processing is built.Compared with Gaussian beam,Bessel beam is used to reduce the focus spot size and inhibit the formation o f remelting layer.The number of cracks can be changed from 0 to 3 by pulse width,and the length of crack growth can be controlled by energy of single pulse.The ultrafast laser linear and curvilinear cleaving of sapphire crystal with high speed and micro-damage is realized,and the maximum cutting speed is 500mm/s.It provides a method of cutting sapphire wafer with good controllability and high efficiency. |
