**3.3 Nomarski optical microscopy**

Nomarski optical microscopy is another technique used to study degraded surfaces. **Figure 7** shows the micrographs of the surfaces of the samples just after sintering (at 0 h) and after 80 h of LTD time of exposure.

The effects of degradation appear as surface roughness by volume due to phase transformation. After 80 h, changes are already observed on the surface, where an increase in roughness is observed. It is true that the irregularities, as well as the phase transformation, are lower than those observed in monolithic zirconia samples, such as, for example, Y-TZP [17]. Being the yttria in both materials the dopant of zirconia, the addition of alumina in the composition is what makes the aging of zirconia difficult.
