**4. Discussion**

The hypothesis that different CAD/CAM resin-composite materials show no similarities regarding indentation depth (hr), Martens hardness (HM), indentation hardness (HIT), indentation modulus (EIT), the elastic part of indentation work (ηIT), and indentation

creep (CIT) 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 HM values for the PICN material (VE) are distinctly higher (1524–1555 N/mm2) compared to the results of this investigation (1143 N/mm2) [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 HM values found in the literature (667–1089 N/mm2) are in line or above the results of this study (588–771 N/mm2) [24–27]. The HM values reported in the literature for composites with a filler content below 70% are distinctly below (BC\*: 151 N/mm2) [24] or in line (477–573 N/mm2) [25,27] with the values obtained in this investigation (411–603 N/mm2). The EIT values found in the literature (2.5–30 kN/mm2) are lower or in line with the results of this study (9–25 kN/mm2) [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].

**Figure 4.** Showcase force–indentation depth curve. Wplastic/elastic = plastic/elastic indentation work; dF/dh = contact stiffness S; Fmax = maximum force; hmax = maximum indentation depth; hr = depth at contact stiffness tangent.

Creep and therefore CIT values are characterized by the short horizontal parts of a depth curve at peak force. EIT values are determined by the slope of the ascending part of the curve. The curve of VE indicates low CIT and high EIT 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 CIT and lower EIT values. In this study, the HM values started at about 400 N/mm2 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 [28–30]. However, our results also confirm the special position of PICN [24,31]. As expected, a comparable behavior was also observed for results of hr, with an approximately 25% lower indentation depth for the highly filled composite or

even 40% for the PICN. However, there were also exceptions in the resin-based composite group, since, e.g., materials with the same filler content (BC and ES; approximately 72%) showed differences of up to 25%. Filler type and size, as well as polymer composition or the chemical bonding of the fillers, may also affect materials' surface properties and explain the differences in materials with similar filler contents [12,32–34]. A correlation between surface hardness and inorganic filler content [11] could also be observed for the materials investigated in this study. The relative differences in standard deviations can be attributed to the uneven filler distribution and the resulting different filler content on the material surface. Since the composition and topography of a material's surface have decisive influence on hardness measurements, results may vary accordingly. In addition, a different polymerization of the matrix due to a distinct manufacturing process can influence results [27]. Resin-composite materials are considered to be less hard and brittle and to cause less stress build-up in antagonistic teeth compared to ceramics. The present material's properties were within the range of data of human dentine from literature (indentation hardness of 0.4–1.1 GPa and indentation modulus of 12.2–22.9 GPa) [35–37]. As the elastic modulus also resembles that of dentin (approximately 15 GPa) [38], CAD/CAM composites could be considered when looking for a biomimetic approach for a dentine replacement [39].

With mean indentation creep CIT values between 3.2% and 5.1%, the analyzed resin composite materials were more resistant to creep compared to human dentine at 8.6 to 10.7% [37]. However, CIT is difficult to interpret in the context of dental materials and their clinical application, as the duration of teeth contact in a physiological masticatory cycle is only about 0.1 to 0.2 s [40], whereas the application time is 10 s during instrumented indentation testing. Assuming only intermittent tooth contact, e.g., while chewing or swallowing, as well as the natural energy-dissipation capabilities of hard dental tissues and the periodontal ligament, the differences in CIT seem to be negligible. However, clenching or bruxism, perhaps even in combination with reduced resiliency in implants or ankylosed teeth, could increase the significance of creep behavior because the magnitude and, especially, the duration of stress application may increase. In these cases, creep will be more relevant for the long-term stability and integrity of the restoration, as stress will also be induced at the intaglio surface [41,42]. This phenomena could lead to debonding, permanent deformation, and perhaps ultimately to an insufficient fit of the restoration. At the tooth–restoration interface, creep could lead to over-contouring or the formation of gaps, which could significantly reduce clinical performance. The energy-dissipating capabilities ("damping effects") are associated with the conversion of energy (storage and energy dissipation). The obtained ηIT values indicate the work that is converted into potentially stored elastic energy (welastic), whereas the other part of indentation work is mostly dissipated throughout plastic deformation or heat (wplastic).

Based on the current data, an indication-driven selection of the investigated materials could improve clinical performance. For example, the reduced resiliency and tactility of implants could be compensated for by a material that causes less stress build while being creep resistant. Such a material must therefore have low EIT and CIT values. The finite element analysis of inlay or partial crowns with higher elastic moduli points to higher stress build-up within the restoration while simultaneously causing less stress build-up in the cement layer and hard dental tissue [43–45]. Materials with high EIT and fracture strength values yet low CIT values could therefore show superior clinical performance when used for partial or inlay crowns. Materials with high CIT values should be considered with caution for permanent restorations. However, the ability to gradually deform under constant stress could be useful in cases where a certain self-balancing effect is desired. These materials could therefore be considered for long-term temporary crowns during pre-prosthodontic treatment, as the viscoelastic behavior could help to self-equilibrate the occlusion. The parameters presented in this study can be regarded as a relevant contribution to established parameters such as flexural strength, wear, and filler content.
