The indentation size effect and its role in microhardness measurements of two viscoelastic materials under different loads and times

Background: Vickers and knoop hardness tests have been used to determine the viscoelastic materials resistance to local plastic deformation in scientific community. The existence of the indentation size effect (ISE) implies that, if hardness is used as a materials selection characterization, it is clearly insufficient to cite a single hardness number, since using a load-dependent hardness number in material characterization may result in some unreliable conclusions. However, it is of crucial importance to fully understand the influence of the indentation load for different time, since its value influences the obtained result. Aim of Study: The aim of this study was to evaluate the effect of indentation size under different loads and times on microhardness values of two viscoelastic restorative materials by using Vickers and Knoop microhardness tests. Material and Methods: In this paper, two commercial viscoelastic restorative materials; resin composite and resin modified glass ionomer (RMGIC), have been tested. Vickers and Knoop micro-indentation with different loads (100, 200 and 300 g) and different times (10, 20 and 30 seconds) has been used to assess the materials microhardness. Results: Two-way ANOVA statistical analysis followed by Tukey’s test was used to determine the statistical differences between various groups. There are statistically significant differences (p £ 0.05) at different indentation loads for both of VHN and KHN of composite and only VHN of resin modified GIC while, KHN of RMGIC was none significant (load-independent hardness). It was found that the optimal load and time for determining hardness in both tested viscoelastic materials are difficult to determined. This value may be regarded as loading independent hardness (H_LIH). At lower loads, a more or less pronounced indentation size effect was noticed, while at higher loads and time, an unreliable indentation measurement and therefore an unreliable microhardness result.