*Article* **Martens Hardness of CAD/CAM Resin-Based Composites**

**Martin Rosentritt , Sebastian Hahnel \* , Sibylle Schneider-Feyrer, Thomas Strasser and Alois Schmid**

Department of Prosthetic Dentistry, UKR University Hospital Regensburg, 93042 Regensburg, Germany; martin.rosentritt@ukr.de (M.R.); sibylle.schneider-feyrer@ukr.de (S.S.-F.); thomas.strasser@ukr.de (T.S.); alois.schmid@ukr.de (A.S.)

**\*** Correspondence: sebastian.hahnel@ukr.de; Tel.: +49-941-944-11900

**Abstract:** (1) Background: The properties of CAD/CAM resin-based composites differ due to differences in their composition. Instrumented indentation testing can help to analyze these differences with respect to hardness, as well as energy-converting capabilities due to viscoelastic behavior. (2) Methods: Eleven materials were investigated using instrumented indentation testing. Indentation depth (hr), Martens hardness (HM), indentation hardness (HIT), indentation modulus (EIT), the elastic part of indentation work (ηIT), and indentation creep (CIT) were investigated, and statistical analysis was performed using one-way ANOVA, Bonferroni post-hoc test, and Pearson correlation (α = 0.05). (3) Results: All of the investigated parameters revealed differences between the analyzed materials. Besides the differences in hardness-associated parameters (hr, HM, and HIT), instrumented indentation testing demonstrated differences in energy-converting properties. The subsequent one-way ANOVA revealed significant differences (*p* < 0.001). A significant (*p* < 0.01, Pearson correlation >0.576) correlation between the materials and HM, HIT, or EIT was identified. (4) Conclusions: Due to the differences found in the energy-converting properties of the investigated materials, certain CAD/CAM resin-based composites could show superior stress-breaking capabilities than others. The consequential reduction in stress build-up may prove to beneficial, especially for implant-retained restorations or patients suffering from parafunctions.

**Keywords:** CAD/CAM; resin composite; hardness; instrumented indentation testing
