Analysis of the indentation size effect in brittle materials from nanoindentation load–displacement curve

In this paper, we investigated the relationships between crack initiation/propagation, stress induced transformation and indentation size effect (ISE) using Universal hardness. Preparing three kinds of specimens, 8 mol%Y2O3–ZrO2 single crystal (single 8Y-FSZ), 12 mol%CeO2–ZrO2 polycrystals (12Ce-TZP) and fused quartz, we were able to research the above relationships separately. The load dependence of universal hardness in the three specimens was studied using nanoindentation equipment and found that single 8Y-FSZ and 12Ce-TZP showed the ISE. To reveal the origin of the ISE in these materials, we analyzed the relationship between load–displacement curve and Universal hardness ISE. Adapting the second-order differential (d2P/dh2) to the load–displacement curve showed that a small displacement change (micro multiple pop-ins) would occur in each deformation process of the single 8Y-FSZ and 12Ce-TZP. The origin of the micro multiple pop-ins is considered to be crack initiation/propagation in single 8Y-FSZ and stress induced transformation in 12Ce-TZP, respectively, and these micro multiple pop-ins should result in the Universal hardness ISE.