**3.4 Effect on mechanical properties**

After exposure to LTD, the structural quality of the material is evaluated, analyzing the hardness and Young's modulus, depending on the aging time. The tests were performed on the polished surfaces that were exposed to LTD. The values are shown in **Figure 8**.

*Design and Development of Zirconia-Alumina Bioceramics Obtained at Low Temperature… DOI: http://dx.doi.org/10.5772/intechopen.102903*

#### **Figure 7.**

*Nomarski microscopy images of ATZ composite in various exposure times LTD for MW1300\_10: (a) 0 h, (b) 80 h; and CS1500\_120: (c) 0 h and (d) 80 h.*

#### **Figure 8.**

*Hardness values for samples as a function of the LTD time of exposure.*

As can be seen, the hardness is affected differently by the sintering method used, with the sintered sample offering more resistance to wear by MW, which corroborates the results obtained in the other techniques. After sintering, before exposure to LTD, the samples show a hardness value of 20.3 and 19.5 GPa for MW1300\_10 and CS1500\_120, respectively. At 40 h, the hardness for CS1500\_120 drops sharply to 15.7 GPa. However, the sample MW1300\_10 decreases its hardness more slowly,

**Figure 9.**

*Young's module values for various LTD exposure of times for sintered ATZ composite by (a) MW1300\_10 and (b) CS1500\_120.*

following a less pronounced trend. After 200 h, the hardness values obtained are 13.8 and 11.9 GPa for MW1300\_10 and CS1500\_120, respectively. These results show that MW1300\_10 resists better the exposure to hydrothermal degradation.

**Figure 9** shows Young's modulus values for the composite densified by both sintering methods.

Like hardness, Young's modulus (E) is affected after exposure to LTD. The E values are modified in both samples, although for MW1300\_10 to a lesser extent than CS1500\_120. The first significant change in the E-value appears at 40 h for CS1500\_120, while in sample MW1300\_10 such change is not observed until after 80 h, corroborating the results obtained previously. After 200 h, the average values of Young's modulus are 237 and 170 GPa for MW1300\_10 and CS1500\_120, respectively.

So far, a number of studies have concentrated on the understanding of the LTD process and its effect on 3Y-TZP dental materials sintered *via* conventional methods [40]. The investigation has also explored the influence of processing conditions, the incorporation of other components such as Al2O3 and the addition of the stabilizer Y2O3 in the LTD [41, 42].

Nevertheless, no comprehensive study has been conducted on the influence of microwave sintering on ATZ materials exposed to LTD environments. Since the sintering process and its associated conditions are a critical factor to determine the LTD behavior of 3Y-TZP-based materials, it is very relevant to research the effect of microwave sintering techniques on this behavior.

Presenda et al. [17] corroborate the high resistance to LTD of the microwavesintered dense zirconia materials (3Y-TZP) as E and H remain almost unaltered; even after 140 h, E is still around 250 GPa and H is approximately 15 GPa. The starting material also has an important role in the microstructure evolution during sintering and, thus, in the LTD resistance. Microwave-sintered 3Y-TZP does not show any significant evidence of degradation after 200 h of exposure to LTD.
