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Various hardness test methods have been used to determine the polymer materials resistance to local plastic deformation in scientific community. The most commonly used method is Vickers microindentation. However, it is of crucial importance to fully understand the influence of the indentation load, since its value influences the obtained result. In this paper, two commercial PMMA dental reline resins in untreated and microwave treated condition have been tested. Vickers microindentation with different loads has been used to assess the materials microhardness: 30, 50, 70, 200, 300 and 500 gf. One way ANOVA statistical analysis followed by Tukey's test was used to determine the statistical differences between various groups. Finally, three models that quantitatively describe load-dependence of the measured Vickers hardness values were used: Meyer's law, PSR and modified PSR model. It was found that the optimal load for determining PMMA dental reline resins in both untreated and microwave treated conditions is 300 gf. This value may be regarded as loading independent hardness, or HLIH. At lower loads, a more or less pronounced indentation size effect was noticed, while at higher loads, forked crack development leads to an unreliable indentation diagonal measurement and therefore an unreliable microhardness result. The most adequate load-dependence model was found to be modified PSR, that takes into consideration the surface stresses induced by specimen preparation by grinding.

Indentation Size Effect in Autopolymerized and Microwave Post Treated Poly(methyl methacrylate) Denture Reline Resins