Orientation-Dependent Dynamic Mechanical Behavior At The Micro-Scale Of Transparent Aluminum Oxynitride

Aluminum oxynitride（AlON）spinel transparent ceramic with excellent transparency and outstanding mechanical properties to rival sapphire,has been regarded as an ideal material for optical elements to be used in extreme conditions,such as infrared/visible domes,aircraft windows,and transparent amors.These applications involve rapidly applied loads at small-scale where dynamic or high strain rate effects become relevant.In addition,all the above-mentioned applications require AlON to be machined to a high surface quality,which normally associates with the machining processes involving high-speed grinding and dynamic wear at micro-scale.Therefore,understanding the dynamic mechanical response at micro-scale of AlON is a key issue for designing high performance protective structures.Ceramic plasticity under dynamic loading plays an important role in the impact performance,however direct characterization of the dynamic plasticity of ceramics is rather difficult since their brittle nature leads to fragmentation.In the present study,orientation dependence of dynamic deformation behavior(（?）～10² s^-1)of aluminum oxynitride（AlON）was investigated by nano-impact technique.A distinct crystallographic orientation dependency of the dynamic hardness was observed for（010）,（101）and（111）crystals.The（111）orientation is the hardest and the（010）orientation is the softest among the three selected planes.In addition,the nano-impact on the（111）plane exhibits a decreasing H_D from 22.71 to 17.22 GPa with increasing acceleration force from 10 m N to 50 m N.The similar reductions from 22.57 to 17.04GPa for the（101）orientation,and from 22.31 to 16.92 GPa for the（010）orientation can also be found.In each crystallographic orientation,the variation of dynamic hardness values reveals the strong indentation size effects,which can be well described using the strain gradient theory associating with the geometrically necessary dislocations and statistically stored dislocations.In the residual indentation characterization after dynamic deformation of AlON,numerous cracks were generated around and below the indentation,with a few of them attributed to cleavage fracture.There were also abundant dislocations present in the region below the indentation,indicating that plasticity in AlON during dynamic deformation was still controlled by dislocations,and no evidence of deformation twinning was observed.It is noteworthy that in the area with the severest strain at the tip of the residual indentation,there were numerous localized amorphous regions with sizes around several nanometers,which accounted for the dynamic hardness reduction under high strain rate conditions.