*2.1.5 Hybrid ceramics*

In recent years, all ceramic and composite restorations have been widely used because of their biocompatibility and esthetic features compared with metalceramic restorations (MCR) [49, 50]. A range of ceramic systems are commercially available like leucite, alumina, zirconia, and feldspar based ceramics [51]. Thus, many indirect composites categories, with various size of filler particles, are also used [52].

Recently, new ceramic/polymer materials, used in CAD/CAM technology, have been developed [53]. In fact, the use of mixed materials enhances crack propagation and reduces fracture stress. The protocol for glass–ceramic materials requires acid etching followed by silanization and the application of resin cement [54–56]. Silane

### *Advanced Ceramic Materials*

coupling agent is a molecule having two functional groups, which has the ability to form a durable bond between organic and inorganics materials [56, 57].

The adhesive cementation technique improves the clinical performance of allceramic dental restorations due to the ability of resin to penetrate the microporosities that are created by etching process [58, 59]. However, the indirect restorations with composite, such as resins, can be produced by the application of physical conditioning technique using air-particle abrasion and silanization to achieve optimum adhesive bond between different materials [60].

Recent studies [61] showed that hardness of hybrid ceramic materials was given from the ceramic content because the indenter was highly sensitive to making this portion. Moreover, there was a significant interaction in the interface between resin cements and hybrid ceramic materials. The longevity of restorations can be affected by storage because of the high concentration of the water and smaller molecules, which cause a reduction in free spaces between functional groups and polymer chains [62].

New hybrid ceramics filled and un-filled polyamide 12 (PA 12) were developed by a fused deposition modeling framework [63]. The proprieties of hybrid ceramics filled and un-filled polyamide 12 are summarized in **Table 3**. The highest tensile strength was recorded at 40% filled PA 12 as compared to un-filled PA 12. The highest tensile modulus was recorded at 35% filled PA 12 as compared to un-filled PA 12. Indeed, the highest impact strength was recorded at 35% filled PA 12 as compared to un-filled PA 12.

### **2.2 Based on processing methods**

In the last few decades, there have been remarkable advances in the mechanical properties and methods of fabrication of ceramic materials.

### *2.2.1 Casting*

Casting is based on the solidification of a fluid that has been poured or injected into a mold. The final product is also known as a casting. Thus, casting process consists of three steps: melting, casting, and recovery.

The biocompatibility effects of indirect exposure of base-metal dental casting alloys were analyzed [64].

The effects of the rare earth element lanthanum on the metal-ceramic bond strength of Co-Cr alloys prepared by casting were studied [65]. XRD and SEM analysis of the samples revealed the presence of dendritic microstructures with some defects and an island shaped intermetallic compounds rich in Cr and Mo. The increasing of the number of "La" leads to the increasing of the diffusion layer at the interface, the increasing of thickness of the native oxide layer, and to improving the wettability. In addition, the results showed that the debonded surfaces of the specimens exhibit mixed fracture modes (adhesive and cohesive failure).


### **Table 3.**

*Proprieties of hybrid ceramics filled and un-filled polyamide 12 [63].*

Similarly, Atwood et al. [66] modeled the surface contamination of dental titanium produced by casting. They showed that the contamination of the wedge sample was established to extended range from 30 to 120 mm. Hence, they concluded that the addition of micro- and nano-models revealed the predictions are shown to be in good agreement for the pattern of contamination. **Figure 3** shows the image of the mold metal interface, which was characterized by three layers: (i) irregular contact surface and with topography in the scale of 20 μm, (ii) globular structure, and (iii) dendritic structure.
