Lateral hardness and the scratch resistance of glasses in the Na2O-CaO-SiO2 system

We report on the lateral hardness of glasses from the Na2O-CaO-SiO2 (NCS) system, including CS and NS binaries. Quantitative data are provided on the work of lateral deformation, obtained through instrumented indentation by monitoring the lateral force as a function of normal load. Experiments were conducted in the regime of elastic-plastic deformation, that is, in the absence of microscopic cracking for normal loads of 50-70 mN. The scratch hardness is determined from the work of deformation per deformed volume. Parallel observations of the elastic properties are undertaken so as to reference scratching data to glass composition and structure. We find that structural densification and high rigidity favor higher scratch hardness. For the considered range of chemical compositions, the scratch hardness exhibits an approximately linear correlation with bulk modulus and packing density. It increases in the direction of compositions with higher content of CaO, and decreases with the content of Na2O. Furthermore, we find a strongly linear correlation between scratch hardness and the deviation of experimental compressibility from the mean-field estimate of the Makishima-Mackenzie model. This points to the importance of intermediate-range structural heterogeneity in the scratching process.