Study On Mechanical Behaviours Of Typical Infrared Monocrystalline Materials Under Nanoindentation

Currently, the constantly developing national defense industry includingmissile and aviation impels the application of infrared technology and correspondinginfrared materials. Due to the mechanical properties is one of the importantindicators of the evaluation and design of material systems, the study of materialmechanical properties for infrared optical materials is particularly important.This paper explores the mechanical behaviors of the cubic crystal systeminfrared single crystal Si, MgO, and the trigonal system single crystal Al2O3, such as,the elastic deformation, the elastic-plastic transformation, the plastic deformationand fracture under the condition of nanoindentation in order to discuss the influencefactors of the above mechanical behaviors, the sources of anisotropism in the chosencrystals, and the reason of the differences in mechanical properties of differentcrystal structure single crystals. Furthermore, powerful theoretical support in theapplication of infrared single crystals as well as other infrared materials in infraredwindows can be provided.Nanoindentation tests were made in single crystal Si(100), Si(110), Si (111),MgO(100), and the Al2O3(0001) crystal planes in order to measured the mechanicalparameters including hardness and elastic modulus and so forth. The surfacemorphologies after the indentation are also been characterized, so as the analysis ofthe change about the structure in indentation areas. The selected single crystalmaterials are all plastic materials, but plastic deformation occurs under the highpressured indenter tip. Raman spectrums of each indented point show structuralchanges under the action of pressure. Anisotropism is possessed by different crystalplanes of Si.Using ABAQUS finite element software simulated the nanoindentation tests ofthe selected materials. The impacts of head geometries, the material and so on werebeing measured and the performances of materials on the mechanical properties innanoindentation simulations were discussed. Degrees of elastic recovery related tothe ratio of elastic modulus and yield strength E/y, the greater the E/yis, thehigher materials pill-up after unloading. Degree of elastic recovery also related tothe contact area. The differences between the tests and simulations might begenerated by the changes of the contact area originated from the blunt of indentertip.First-principles calculations of the mechanics parameters for the selectedmaterials were made using Materials Studio. Elastic constants of single crystal materials (Cij), bulk modulus B, shear modulus G, Young’s modulus E and Poisson’sratio v were calculated, and the energy state structures, the electron density and soforth of the materials were analyzed. Calculated anisotropy coefficients A of singlecrystal Si, single crystal MgO and sapphire single crystal is1.527,1.540and0.872,respectively. Hence, anisotropism shown in the tests and simulations can beexplained. Furthermore, the material B/G values are less than1.75, lead thematerials possess the phenomenon of brittle fracture.