Research On The Crack Generation And Propagation Mechanism Of Sapphire Indentation Surface Under Different Frequency Loading Conditions

As an advanced functional material,sapphire is widely used in many fields such as aerospace,optics,and medical due to its excellent physical properties and chemical stability.However,sapphire may be damaged due to the loading under different conditions,including manufacturing processing and the service processing of its parts.The fracture of sapphire can be divided into three stages: crack generation,crack stable propagation and crack unstable propagation.Therefore,the researches on the crack generation and crack propagation of sapphire under different loading environments have important significance for guiding the optimization of sapphire processing parameters and improving the working performance of sapphire parts in different situations.In this paper,quasi-static loading tests,cyclic impact loading tests with low-frequency(20 Hz),cyclic impact loading tests with medium-frequency(200 Hz)and cyclic impact loading tests with high-frequency(20000 Hz)have been performed on A-plane,C-plane,M-plane and R-plane sapphire respectively.The crack generation and crack propagation on the indentation surface of sapphire with different crystal planes have been studied by establishing the mechanical model of indentation,analyzing force signal and investigating indentation morphology.Firstly,based on the contact model of spherical indenter and sapphire specimen,a mechanical model of the spherical indentation process has been established to predict the critical load at which four kinds of crystal plane sapphire produce radial cracks respectively.The accuracy of the mechanical model has been verified by the results of the quasi-static loading tests.Secondly,cyclic impact loading tests with low-frequency and medium-frequency have been performed on the sapphire of four kinds of crystal planes.Compared with the results of the quasi-static loading tests,it is found that the critical load at which radial cracks occur on each crystal plane sapphire is affected by loading frequency.Loading frequency has the greatest influence on the reliability of the contact formula used in the mechanical model for M-plane sapphire,and the increase of loading frequency leads to a decrease in the accuracy of the mechanical model for predicting the critical load at which radial cracks generate of each crystal plane sapphire.Finally,cyclic impact loading tests with high-frequency have been performed on four kinds of crystal plane sapphire.It is found that the response of each crystal plane sapphire can be mainly divided into two stages: failure stage and fatigue stage.The crack propagation direction in the indentation surface of four kinds of crystal planes is not affected by the loading frequency.The cracks propagate along the direction of the corresponding slip system,and a new explanation for the crack propagation mechanism of R-plane sapphire has been proposed.After high-frequency cyclic impact,the surface morphology of each crystal plane sapphire varies greatly affected by crack propagation.R-plane sapphire produces a conical crack that makes the residual surface convex and flat,while A-plane,C-plane and M-plane sapphire have better plastic deformation ability than R-plane sapphire.