Molecular dynamics-based study of the effects of grain size and temperature on the nanoscratching groove characteristics of grating polycrystalline layered Al films

Abstract Echelle gratings, a specialized type of diffraction grating, feature a periodic groove pattern that exhibits excellent light-splitting capabilities. These gratings are characterized by a low density of lines per millimeter and a significant blaze angle. The primary substrate for echelle gratings in mechanical scratching is an aluminum film with a layered structure, which is determined by the coating process. However, the unique interlayer structure of the layered aluminum film, combined with the challenges posed by deep and high-precision triangular diffraction grooves, complicates the control of the scratching process. This article aims to investigate the material removal process and the thermal field-assisted scratching mechanism in the nano-scratching of polycrystalline layered aluminum films, with a particular emphasis on the material removal mechanism involved in nano-scratching. It discusses the influence of the thermal field and grain size on the mechanical response and the material removal process, while thoroughly examining their effects on the accuracy of groove formation. The results indicate that as the temperature increases, both the frictional and normal forces exhibit a decreasing trend. Additionally, a reduction in grain size corresponds to smaller frictional and normal forces. This decrease in scratching force contributes to minimizing the deformation of the workpiece during the scratching process, which is advantageous for precise control of groove shape and enhances the scratching quality of large-area gratings. As the temperature increases, the number of atoms removed from the polycrystalline layered aluminum film rises, leading to an enhanced material removal rate. Consequently, the accuracy of the diffraction grating groove also improves. Concurrently, the presence of delaminated grain boundary interfaces impedes the downward propagation of defects such as dislocations and can accommodate these dislocations. As temperature rises, the total length of dislocation lines decreases. Elevated temperatures promote the transformation of the crystal structure into an amorphous state. The grain boundaries and interlayer grain boundaries at the delamination restrict the movement of dislocations. The deformation behavior suggests that grain boundaries significantly contribute to the suppression of strain and stress propagation, resulting in a gradient distribution at the layered grain boundary interface, which further impedes stress transmission. Stress and strain are concentrated not only in the contact area between the tool tip and the substrate but also within the grain boundaries and their adjacent regions.