(Fig. 3-a). The roughness of this surface was measured to be about 7.49 mm. The typical defects observed in the sprayed coatings are vanished after laser cladding. The porosity ratio of the laser clad coating was measured to be about 4.2%, while this ratio in the YSZ layer was about 11.2%. This is another evidence that the pores can be eliminated by laser cladding treatment. However, still a continuous network of segmented cracks is visible on the surface. These cracks probably were generated because of the small molten pool, fast cooling and localized temperature gradient which generate residual stresses after laser cladding [28,29]. At a higher magnification (Fig. 3-b), it is observed that the cracks are perpendicular to the surface. It has been reported that these cracks increase thermal shocks resistance and are expected to be beneficial for accommodating the oxidation and the mismatch stresses [30].
Fig. 4 shows the XRD patterns of the APSed YSZ and the laser clad layer. Pattern (a) belongs to the YSZ which indicates the presence of t′-ZrO2 crystal structure, a common phenomenon after plasma spraying of YSZ. As seen in pattern (b) (The XRD pattern of the Al2O3 laser clad layer), the dense layer contained t′-ZrO2 and the rhombohedral phase of Al2O3. This pattern confirms the formation of in situ Al2O3-YSZ composite with the thickness of about 45 mm on the top of the sprayed YSZ. This composite probably is formed due to the partial melting of the YSZ layer and mixing with molten alumina during laser cladding. To prove formation of the Al2O3/ZrO2 composite layer, Grazing Incidence X-ray Diffraction was utilized (Fig. 4-c).