*2.1.3 Aluminum oxide ceramics reinforced with zirconium oxides*

Mechanical properties of alumina were improved by addition of ceramic composites, as reinforcing agents, like zirconia. Generally, these ceramic–ceramic composites present a great hardness as compared to the all composites. Although most ceramic second phases improve strength and hardness they modestly improve fracture toughness [28, 29]. Al2O3-SiC nanocomposite has been reported to have the


### **Table 2.**

*Mechanical properties of alumina [26].*

most improved properties [30]. Thus, it has been shown that SiC increases significantly the wear resistance of aluminum oxide.

Doğan and Hawk [31] revealed that the toughness of alumina with 34 vol%SiC increased from 3.4 to 4.6 MPa.m1/2. Similarly, Belmonte et al. [32] showed that the fracture toughness of the sample of alumina with 20 vol%SiC reached a value of 5.9 MPa.m1/2.

However, the zirconia system uses a mixture of zirconium oxide and aluminum oxide as a framework to achieve a marked increase in the flexural strength. Alumina constitute approximately two third of the structure and the remaining structure was composed of tetragonal zirconia. In addition, the proportion of glass phase covers 20–25% of the total structure. The increase over alumina is due to the zirconia particles that protect the structure against crack propagation. It has a very high strength of around 700 MPa and very poor translucency.

Tuan et al. [33] incorporated zirconia particles into alumina and reported that the fracture toughness was improved. For zirconia-toughened alumina including 10 vol% zirconia, they recorded that fracture strength and fracture toughness were 943 MPa and 11.8 MPa.m½, respectively. Toughness values of 10 MPa.m½ for 10 vol% zirconia [34] and 7.02 MPa.m½ for 50 vol% zirconia content have also been reported [35]. Zirconia is a bioinert ceramic and can suffer from low cellular attachment, which could be compensated when mixed with biopolymers [36]. Alumina-zirconia composites have received great attention in dentistry as promoted bioceramics due to their excellent biocompatibility [37].

In the last years, many recent studies were focused on the investigation of the tribological-mechanical behaviors and biosafety of alumina toughened zirconia (ATZ) composites [38–41]. Thus, the benefits of these composites are the combination of the properties of alumina and zirconia.

Daskalova et al. [42] studied the effect of surface modification by femtosecond laser on zirconia based ceramics for screening of cell-surface interaction. The X-ray diffraction analysis demonstrated preservation of the tetragonal phase of Zr ceramic materials for a particular fs-laser treatment conditions (see **Figure 1**).

Moreover, scaffolds design and fabrication are major areas in dentistry for tissue engineering applications that need controlled positioning of cells on solid substrates with predefined orientation. Hence, surface functionalization generated by defined surface structure was strongly depending on the quality and surface texture.

### *2.1.4 Zirconium oxides ceramics*

Zirconia (ZrO2) is a ceramic material which has been applied in the health field and distinguished by its high mechanical properties, biocompatibility, and chemical stability [43]. The polycrystal tetragonal zirconia, stabilized with yttria (3Y-TPZ) contains 3 mol% yttria oxide (Y2O3), was first applied in the field of medical. The 3Y-TPZ has been the most studied and utilized in dentistry [44]. Thus, the 3Y-TPZ was fabricated in small grains (0.2–0.5 mm in diameter), which minimizes the phenomenon of structural deterioration or destabilization in the presence of saliva, decreasing the subcritical crack growth [45]. **Figure 2** shows the SEM micrograph of the powder after sintering. However, for the formation of a great amount of monolclinic zirconia a powerful machining should be used because of the high compression applied by machining, leading to the formation of micro-cracks on the surface of material [3].

Similar to that of stainless steel, zirconia is characterized by good chemical stability, good biocompatibility, mechanical strength, toughness, and Young's modulus [46]. No adverse reactions have been found, when osteoblasts were seeded on zirconia and were able to proliferate and differentiate on it [47]. Zirconia ceramics are becoming a prevalent biomaterial in dentistry and dental implantology [48].

*Recent Advances in Ceramic Materials for Dentistry DOI: http://dx.doi.org/10.5772/intechopen.96890*

**Figure 1.** *XRD patterns of the surface of sample [42].*

**Figure 2.** *SEM micrograph of the sample obtained after sintering [45].*
