**1. Introduction**

Due to their clearly deviating properties, resin-based CAD/CAM (computer-aided design/computer-aided manufacturing) composites are an interesting clinical alternative to dental ceramics [1]. Similar to direct resin-based composites, resin-based CAD/CAM composites consist of inorganic fillers embedded in an organic polymer matrix, commonly using silanes as coupling agents. Their mechanical properties such as modulus of elasticity or flexural strength are improved due to the standardized polymerization process under industrial conditions compared to chair-side light-curing polymerization [2]. A variation of resin-based composites is the so-called polymer-infiltrated ceramic network (PICN), which comprises a structure-sintered ceramic matrix and a reinforcing polymer network (ceramic content: 86 wt%; polymer content: 14 wt%). Resin-based CAD/CAM composites and resin-infiltrated ceramic networks are used for inlays, onlays, and veneers, as well as tooth- and implant-retained crowns. Some composites are even approved for bridges and for use in patients suffering from bruxism.

One key benefit of these resin-based materials—as advertised by many manufacturers —is the dentine-like modulus of elasticity of approximately 10–30 GPa. Although composites do not reach the high aesthetics of ceramics, they are commonly regarded as less hard and brittle, and they cause less wear and stress in antagonistic teeth [3]. These qualities may be beneficial for the rehabilitation of patients suffering from parafunctions such as bruxism. Energy-dissipation capabilities might also be increased by the utilization of resinbased CAD/CAM composites with a low modulus of elasticity [4–7]. Implant-supported

**Citation:** Rosentritt, M.; Hahnel, S.; Schneider-Feyrer, S.; Strasser, T.; Schmid, A. Martens Hardness of CAD/CAM Resin-Based Composites. *Appl. Sci.* **2022**, *12*, 7698. https:// doi.org/10.3390/app12157698

Academic Editor: Mary Anne Melo

Received: 11 July 2022 Accepted: 28 July 2022 Published: 30 July 2022

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restorations, with their lower tactility and elasticity of the osseointegrated implants, might benefit from less stress build-up during normal mastication. For example, there is evidence for improved implant osseointegration with low-modulus titanium implants [8,9]. This phenomena is mostly attributed to the so-called stress-shielding effect, which is caused by the differences of the elastic moduli between implant and bone. The mismatch leads to an insufficient transfer of force and therefore inadequate stimulation of bone remodeling [10]. It is suggested that the stimulation of bone growth may be enhanced by reducing or adjusting the elastic modulus of the restorative material.

Yet, with respect to the mechanical properties of CAD/CAM resin-based composites, previous research suggests a fairly inhomogeneous class of materials [11]. This is mostly attributed to different types, sizes, and amounts of inorganic fillers (approximately 60–85 wt%), as well as the organic matrix [12]. The significant differences in CAD/CAM resin materials, e.g., flexural strength (150–330 MPa) and modulus of elasticity (10.3–30.0 GPa), may have impacts under clinical conditions. To properly evaluate the available materials and perhaps even choose certain materials for specific clinical indications, detailed information on their mechanical behavior is essential. One method for evaluating elastic and viscoelastic behavior is indentation hardness testing. Surface hardness is defined as the resistance against plastic and therefore permanent deformation by indentation. Hardness is commonly measured with methods such as Vickers, Rockwell or Brinell hardness testing. However, indentation testing encompasses more than just permanent deformation, as elastic or even viscoelastic components can also be determined by the measurement. These properties can be measured using instrumented indentation testing, also called Martens hardness (HM) testing. HM is derived from the applied force (F) divided by the indentation surface (As), which is a function of the indentation depth (h) (Equation (1)).

$$\mathbf{H}\_{\rm M} = \frac{\mathbf{F}}{\mathbf{A}\_{\rm S}(\mathbf{h})} \tag{1}$$

Furthermore, the constant measurement of force and indentation depth provides a force–indentation depth curve, as well as the fundamentals for additional analysis.

The indentation modulus (EIT), which is determined in the compression mode, is related but not identical to the modulus of elasticity, which is determined in the flexure mode [13]. The elastic part of the indentation (expressed by ηIT) could help in the assessment of the use of resin-based CAD/CAM composites for use as stress-breakers for implant-supported restorations. The time-dependent response to the indentation of a viscoelastic material [14] can be expressed as indentation creep (CIT), expressing the relative increase of strain under constant force application, e.g., due to the rearrangement of polymer chains. As the deformation caused by creep is of plastic character, CIT can help to estimate the long-term dimensional and mechanical stability of a material [15–18]. Materials that significantly differ in these properties could therefore be used for different applications.

The hypothesis of this study was 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). The obtained results can help to estimate the energy-conversion behavior and therefore the clinical performance of the significantly different materials under masticatory loads, as well as their stress-breaking capabilities.
