Mechanical properties of polymer-infiltrated-ceramic-network materials
Introduction
The use of dental CAD/CAM-systems in combination with CAD/CAM-machinable materials enables the esthetic demands of prosthetic restorations to be achieved. Besides esthetic outcomes, complementary superior physical properties are needed for permanent dental restorations. Regarding non-metal CAD/CAM materials for permanent dental restorations there are currently two main groups, ceramics and composites [1]. The ceramic group is subdivided into polycrystalline and glass ceramics. The composites are subdivided in macro-, micro-, hybrid-filled- or nano-composites [2]. Ceramics tend to be more rigid and brittle, while composites are more compliant, soft and experience high wear. The ideal goal for restorative dentistry would be to replace lost tooth substance by a restorative material with tooth like structure and matching physical properties. Toward this objective a novel material that attempts to emulate the properties of natural teeth in its structure and physical properties was developed and named polymer-infiltrated-ceramic-network material (PICN). The goal is to achieve a material with enhanced mechanical characteristics, compared to conventional restorative materials like ceramics and composites.
PICNs can be classified as interpenetrating phase composites (IPC). IPCs have a three-dimensional interconnected geometry. A ceramic interpenetrating material was introduced to dentistry more than two decades ago, known as the In-Ceram system (Vita Zahnfabrik, Bad Saeckingen, Germany). In-Ceram materials are based on a porous ceramic structure with a glass interpenetrating phase [3]. The combination of ceramic-metal IPC has been reported by Horvitz et al. [4], who outlined the advantages of the ceramic matrix (Al2O3) as, for example, enhanced wear resistance, high temperature strength etc., and the fact that the metallic phase improves toughness by crack bridging.
Due to the presence of two connected phases within IPCs, crack propagation is generally limited because of interfacial crack deflection. The phase with the higher strain to failure enhances the fracture resistance by bridging the cracks introduced to the other phase [4], [5], [6].
Compared to In-Ceram the brittle glass phase is replaced by a polymer to form the PICN. In contrast to traditional composites which consist of one continuous phase filled with inorganic particles, PICN consists of two continuous interpenetrating networks. One network is a ceramic material (feldspar, light gray areas in Fig. 1) and the other a polymer (commonly used methacrylates for dental applications, dark gray areas in Fig. 1).
The anticipated improvements of the novel PICN CAD/CAM materials are reduced brittleness, rigidity and hardness coupled with improved flexibility, fracture toughness and better machinability compared to ceramics. Further improvements would be to maintain the wear “kindness” toward opposing teeth such as occurs with composites along with enamellike material attrition. The long-term aim is to imitate the mechanical behavior of a natural tooth. A literature summary of the physical properties of human dentin and enamel is listed in Table 1.
The main aim within the scope of this paper was the comparison of elastic modulus and hardness of PICNs, dense ceramic and polymer, employing three-point-bending and Vickers indentation. Also, the flexural strength of the mentioned materials above was compared. Additionally, the microstructure deformation and cracking behavior around indented areas of PICNs were observed by SEM.
Section snippets
Manufacturing of PICN
In the first step a porous pre-sintered feldspar ceramic with adjustable densities was produced. In the second step the porous ceramic network structure was filled with resin. The ceramic powder was initially compressed into blocks then sintered to a porous network. Different porosities of the ceramic could be achieved by manipulating the initial ceramic particle size and utilizing different firing temperatures. The ceramic network had to be conditioned by a coupling agent prior to resin
Flexural strength and elastic modulus
The flexural strengths of the materials tested are shown in Table 3, Table 4. With increasing density, the flexural strength of the three porous ceramics rises from 9.2 to 28.8 MPa. The flexural strength values of PICNs are inversely related to ceramic density with the highest value of 159.9 MPa (see Table 3, Table 4 and Fig. 2). In Fig. 2 linear equations were used to fit the measured flexural strength data. The 100% value (x-axis) of 103.4 MPa corresponds to the flexural strength of the dense
Discussion
The present study was mainly conducted to define the mechanical properties of novel polymer-infiltrated-ceramic-network materials (PICN). Comparing the results to natural teeth, a very promising step toward imitating natural teeth has been realized with PICNs, which was also reported by He and Swain [13], [14]. These investigators found that mechanical properties of PICNs were similar to those of human dentin and enamel. For example, an ISE (Indentation size effect) for elastic modulus and
Conclusion
Based on the results of the present study, the following conclusions were drawn:
- 1.
The mechanical properties (flexural strength and strain at failure) of feldspar ceramic can be enhanced by infiltration of a second phase (polymer) into porous ceramic precursor.
- 2.
The ratio between porous ceramic and polymer content influences the mechanical properties especially flexural strength (in the range of 131.1–159.9 MPa), elastic modulus (16.4–28.1 GPa), hardness (1.1–2.1 GPa) and strain at failure (0.5–1%) of
Acknowledgments
The authors would like to thank K. Kaufman and E. Bojemüller (Vita Zahnfabrik, Bad Saeckingen, Germany) for their support and discussions about interpenetrating phase materials and Glynny Kieser for editing the document.
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