# Martens Hardness of CAD/CAM Resin-Based Composites
## 4 Discussion  
The hypothesis that different CAD/CAM resin-composite materials show no similarities regarding indentation depth (h\({}_{\text{r}}\)), Martens hardness (H\({}_{\text{M}}\)), indentation hardness (H\({}_{\text{HT}}\)), indentation hardness (H\({}_{\text{HT}}\)), indentation modulus (E\({}_{\text{HT}}\)), the elastic part of indentation work (n\({}_{\text{HT}}\)), and indentation  
Figure 3: Martens hardness (H\({}_{\text{M}}\)), indentation modulus (E\({}_{\text{HT}}\)), and indentation creep (C\({}_{\text{HT}}\)) scaled to percentage of maximum value. Materials ordered by increasing amount of inorganic filler content. Figure 2: Force–indentation depth curves of the tested composite materials. creep (C\({}_{\text{IT}}\)) could be partly confirmed. The novelty of this study is that the data were used not only to evaluate the material hardness but also to differentiate the elastic and viscoelastic surface parameters. In addition, possible clinical consequences of the results and applications were discussed. Figure 4 shows the force-indentation depth curves, which highlight individual parameters investigated in this study. In comparison to research on the Martens hardness of CAD/CAM resin-based materials, the published H\({}_{\text{M}}\) values for the PICN material (VE) are distinctly higher (1524-1555 N/mm\({}^{2}\)) compared to the results of this investigation (1143 N/mm\({}^{2}\)) [24; 25]. Differences here were due to the transverse contraction number required for the calculations. With respect to the resin-composite materials with a filler content of more than 70%, the H\({}_{\text{M}}\) values found in the literature (667-1089 N/mm\({}^{2}\)) are in line or above the results of this study (588-771 N/mm\({}^{2}\)) [24; 25; 26; 27]. The HM values reported in the literature for composites with a filler content below 70% are distinctly below (BC*: 151 N/mm\({}^{2}\)) [24] or in line (477-573 N/mm\({}^{2}\)) [25; 27] with the values obtained in this investigation (411-603 N/mm\({}^{2}\)). The ET values found in the literature (2.5-30 kN/mm\({}^{2}\)) are lower or in line with the results of this study (9-25 kN/mm\({}^{2}\)) [24]. The CIT values obtained in this study (3.2-5.1%) were slightly higher compared to those of the literature (2.6-3.4%) [26; 27]. Creep and therefore C\({}_{\text{IT}}\) values are characterized by the short horizontal parts of a depth curve at peak force. E\({}_{\text{IT}}\) values are determined by the slope of the ascending part of the curve. The curve of VE indicates low C\({}_{\text{IT}}\) and high E\({}_{\text{IT}}\) values, which indicates a low susceptibility to creep and high resistance against elastic deformation. The curve of KA in comparison shows a longer horizontal movement at peak force and a less steep slope of the ascending curve, indicating higher C\({}_{\text{IT}}\) and lower E\({}_{\text{IT}}\) values. In this study, the H\({}_{\text{M}}\) values started at about 400 N/mm\({}^{2}\) for the composite with the lowest filler content and were almost twice as high for the composite with the highest filler content. Three times higher values were even identified for PICN, although the inorganic weight content was slightly comparable to that of the highly filled composite. These results confirm previous research that indicated at a positive correlation between inorganic filler content and surface hardness for resin-based composites [28; 29; 30]. However, our results also confirm the special position of PICN [24; 31]. As expected, a comparable behavior was also observed for results of h\({}_{\text{r}}\), with an approximately 25% lower indentation depth for the highly filled composite or  
Figure 4: Showcase force–indentation depth curve. W\({}_{\text{plastic/elastic}}\) = plastic/elastic indentation work; dF/dh = contact stiffness S; F\({}_{\text{max}}\) = maximum force; h\({}_{\text{max}}\) = maximum indentation depth; h\({}_{\text{r}}\) = depth at contact stiffness tangent. even 40% for the PICN.