Elastic-plastic properties of Li2B4O7 determined by nanoindentation

Abstract

Relevance. Crystalline and amorphous lithium tetraborate (Li2B4O7) has a wide range of practical applications due to its physical properties. The knowledge of the mechanical characteristics of the surface layers of these materials, which are studied by nanoindentation, is necessary for optimising the technological processes for obtaining “optically perfect” samples.

Aim. A comparative study of the mechanical properties and deformation mechanisms of glassy and crystalline Li2B4O7 samples in a wide range of applied loads.

Methodology. The elastic-plastic properties of crystalline and glassy lithium tetraborate were investigated using the multiple-loading cyclic nanoindentation method. The samples were measured at maximum loading forces of 50, 100, 150, 200, 250, 300, 350, 400, 450, and 500 mN. Four measurements (in the form of a 2x2 matrix) were made at a distance of 50 µm from each other on each sample and at each load. Young’s modulus, hardness, and Poisson’s ratio of Li2B4O7 glass were also calculated using the Makishima-Mackenzie theory.

Results. The load-displacement curves and graphs of the dependence of the average contact pressure on the displacement of the diamond Berkovich indenter were obtained, which have a “smooth” shape and no anomalies associated with “pop-in” or “pop-out” effects. The indentation modulus (Young’s modulus E) and hardness H of the studied samples were obtained from the experimental P-h load-displacement diagrams. The measured values mainly depend on the applied load or the contact depth of the indenter penetration into the crystalline and vitreous lithium tetraborate.

Conclusions. Both the hardness and Young’s modulus of Li2B4O7 glass are lower than those of the single crystal, indicating a lower resistance of amorphous lithium tetraborate to elastic and plastic deformations. The obtained experimental values of hardness and Young’s modulus of Li2B4O7 glass correlate well with the results of the calculation within the framework of the Makishima-Mackenzie theory. Multiple-loading cyclic nanoindentation leads to deformation densification of glassy Li2B4O7 due to changes in the angles and lengths of chemical bonds, which leads to a decrease in the free volume in the medium-order structure of glass, as well as a change in the coordination of Boron atoms relative to Oxygen, i.e., the transformation of three-coordinated Boron into four-coordinated Boron