## 1 Introduction  
The global market for dental materials is surpassing 10 billion dollars [1]. The main driving forces for this growth are improving the dentists' workflow and increasing patients' comfort. Therefore, remarkable investigations have been conducted to develop improved or new restorative dental materials with enhanced properties. Among these materials, resin-based composites, ceramics, glass-ceramics or polymer-infiltrated ceramics are of great importance. The microstructural characterization and determination of the mechanical properties of these materials are the first steps to understand their behavior in restorative dentistry [2, 3]. The resin-based composites such as glass ionomer cements and whisker-reinforced resins are among the main dental restorative materials. These composites are composed of a polymeric matrix and reinforcing inorganic fillers. The type, morphology, content and structure of the fillers and interface directly dictate the mechanical and aesthetic properties of the composites. The development of new fillers such as nano-whiskers and nanoparticles has resulted in considerable improvements of the composite properties [4, 5]. The key criteria for selecting a filler include the biocompatibility, mechanical and optical characteristics. Additionally, the dimensional changes, final density, fracture toughness and machinability resulting from the polymerization are determined by the polymer matrix, which commonly includes bisphenol A-glycidyl methacrylate (Bis-GMA), urethane dimethacrylate (UDMA), urethane trimethacrylate (UTMA) and ethoxylated bisphenol A-glycol dimethacrylate (Bis-EMA) [4; 5]. Nevertheless, the clinical performance of resin-based composites is still inferior to the performance of ceramic or glass-ceramic restorations considering the marginal adaptation, color match, anatomic form and mechanical strength [6]. A three-year clinical study revealed that these restorations have inferior aesthetic and wear resistance compared to all-ceramic restorations [7]. However, the use of ceramics/glass-ceramics in many applications suffers from their low fracture toughness and high susceptibility to slow crack growth [8]. The combined properties of these two groups of materials, such as Al2O3 ceramics and resin-based composites, could be a solution for overcoming the shortcomings of restorative materials. For example, combining [PERSON]'s modulus of resin-based composites, which is similar to the dentin Young's modulus, with the long-lasting strength, hardness and aesthetics of ceramics, would be ideal for a restorative material. The polymer-infiltrated ceramic (PIC) network may offer an alternative solution. The fabrication process of this type of material requires the following two steps: first, a porous pre-sintered ceramic network is produced and then, this network is infiltrated with a polymer by capillary action. The flexural strength, elastic modulus, hardness and strain at failure of PIC structures have been reported in previous studies [9; 10], showing similar properties to the tooth structure, which encourages further studies on these materials. PMMA is commonly used for prosthetic dental applications, including the fabrication of artificial teeth, denture bases, dentures, obturators, orthodontic retainers, temporary or provisional crowns and the repair of dental prostheses. Other applications of PMMA in dentistry include occlusal splints, printed or milled casts, dies for treatment planning and the embedding of tooth specimens for research purposes. The distinctive properties of PMMA, such as its low density, aesthetics, cost-effectiveness, ease of handling and tailorable physical and mechanical properties, make it a suitable and popular biomaterial for these applications. Several chemical modifications and mechanical reinforcement techniques using various types of fibers, nanoparticles and nanotubes have been reported to improve the properties (thermal behavior, water sorption, solubility, impact strength, flexural strength) of PMMA [10].