**2. Materials and Methods**

Eleven resin-based CAD/CAM materials (n = 6 per material) were investigated using instrumented indentation testing according to ISO 14577-1 [19]. Table 1 provides an overview over the tested materials, as well as some of their properties (modulus of elasticity E (GPa), filler content wt. (%), and fracture strength FS (MPa)) for the better interpretation of the results of this study. Rectangular specimens (10 × 10 × 2 mm) were produced in CAD/CAM dental milling machine (98 milling blank, Inlab MC X5, Dentsply Sirona, Germany) and polished (1000 grit sandpaper, Tegramin 25, Struers, Germany).


**Table 1.** Materials, manufacturers, abbreviations (Abbr.), modulus of elasticity (E), filler content (wt.), fracture strength (FS) according to manufacturer's specifications or literature: <sup>a</sup> [20], <sup>b</sup> [21], <sup>c</sup> [22], and <sup>d</sup> [11]). Filler content classification: low-fill <sup>≤</sup> 74% wt. <sup>≤</sup> compact.

Testing was carried out with a universal hardness-testing machine (ZwickiLine Z2.5, ZwickRoell, Germany; see Figure 1).

**Figure 1.** (**a**) ZwickiLine Z2.5 and (**b**) schematic test procedure with maximum indentation depth (hmax) at application of maximum force (Fmax) and residual indentation depth (hmin) after stress relaxation.

The Martens hardness (HM) is the ratio of the maximum force to the associated contact area (N/mm2). Other material parameters, such as indentation modulus, indentation creep, and plastic and elastic work of deformation, can be characterized from a force–indentation depth curve. In this study, force, depth and time during the indentation of the diamond pyramid were continuously recorded. The contact area under load was calculated from the maximum indentation depth. The indentation depth was constantly monitored at a loading speed of 0.5 mm/min to a maximum force of Fmax = 10 N using a Vickers indenter and dwell-time of 10 s. Unloading was performed at 0.1 mm/min. The recorded force– indentation depth curves were used to calculate indentation depth (hr), Martens hardness (HM), indentation hardness (HIT), indentation modulus (EIT), the elastic part of indentation work (ηIT), and indentation creep (CIT) as defined in ISO 14577-1. The Poisson's ratio of the diamond indenter was set to ν<sup>i</sup> = 0.07, and that of the resin-based composite materials was set to ν<sup>s</sup> = 0.3 [23]. The Young's modulus of the indenter was Ei = 1140 GPa.

Calculations and statistical analyses were performed using SPSS 25.0 for Windows (IBM, Armonk, NY, USA). The normal distribution of data was controlled using the Shapiro– Wilk test. Means and standard deviations were calculated and analyzed using ANOVA

and the Bonferroni test for post-hoc analysis. Pearson correlations were calculated. The level of significance was set to α = 0.05.

#### **3. Results**

The Shapiro–Wilk test confirmed the normal distribution of the tested parameters. The one-way ANOVA revealed significant differences (*p* < 0.001) within the parameters. Table 2 shows mean results and statistical Bonferroni post-hoc comparison. Force–indentationcurves of the investigated materials are shown in Figure 2.

**Table 2.** Material, abbreviation (Abbr.), mean and standard deviation (in brackets) for indentation depth (hr), Martens hardness (HM), indentation hardness (HIT), indentation modulus (EIT), elastic part of indentation work (ηIT), and indentation creep (CIT). Identical superscript letters indicate column-wise non-significant (Bonferroni post hoc test, *p* > 0.05) differences between the materials.


The mean Martens hardness (HM) ranged from 410.8 ± 55.8 N/mm<sup>2</sup> (KA) to 1143.4 ± 124.7 N/mm<sup>2</sup> (VE). The ANOVA showed significant (*<sup>p</sup>* ≤ 0.001) differences between the results (Table 2). The residual indentation depth (hr) was between 13.9 ± 0.7 μm (VE) and 23.0 ± 1.4 μm (KA), with significant differences between the results (ANOVA: *<sup>p</sup>* ≤ 0.001). The indentation hardness (HIT) varied between 666.5 ± 88.2 N/mm2 (KA) and 1834.2 ± 198.0 N/mm<sup>2</sup> (VE). The ANOVA confirmed significant (*<sup>p</sup>* ≤ 0.001) differences between the results. The indentation modulus (EIT) ranged from 8.8 ± 1.4 kN/mm2 (KA) to 25.3 ± 3.0 kN/mm2 (VE), with significant differences between the results (ANOVA: *p* ≤ 0.001). The mean elastic part of indentation (ηIT) varied between 40.2 ± 0.5% (VO) and 50.4 ± 4.3% (LU) (Figure 3). The ANOVA confirmed significant differences between the mean values (*p* ≤ 0.001). The indentation creep (CIT) ranged from 3.2 ± 0.1% (VE) to 5.1 ± 0.4% (KC). The ANOVA showed significant (*p* ≤ 0.001) differences between the values.

A highly significant (*p* < 0.01, Pearson correlation >0.576) correlation between the materials and HM, HIT or EIT was identified. Negative correlations were established for hr (−0.623), and CIT (−0.584). No correlation could be determined for ηIT (−0.151, *p* = 0.227). A significant (*p* < 0.001) impact of the material was found on HM (η<sup>2</sup> = 0.914), HIT (η<sup>2</sup> = 0.867), EIT (η<sup>2</sup> = 0.910), CIT (η<sup>2</sup> = 0.771), ηIT (η<sup>2</sup> = 0.544), and hr (η<sup>2</sup> = 0.814).

**Figure 2.** Force–indentation depth curves of the tested composite materials.

**Figure 3.** Martens hardness (HM), indentation modulus (EIT), and indentation creep (CIT) scaled to percentage of maximum value. Materials ordered by increasing amount of inorganic filler content.
