Cross-section analysis

After the surface characterisation, the cross-sections of the tested samples were characterised using Raman and Cr³⁺ fluorescence spectroscopy. The micrograph of the cross-section of an alumina sample is shown in Figure 6.26 (a). A large number of cracks were observed below the surface. In addition, a plastic zone beneath the impacted region was observed, as highlighted in Figure 6.26 (a). This plastic zone

Change of peak width / cm-1

Al2O3 10% ZTA 15% ZTA

was easily observed mainly due to the extensive material pull-out, as shown in Figure 6.26 (b). Because the materials were highly deformed and a large microcrack network existed in the plastically deformed area, the materials were easily pulled out during polishing.

Figure 6.26 (a) SEM micrographs of the cross-section of an alumina specimen, the circle highlights the plastic zone formed by the impact; (b) materials pull-out on the plastic zone.

The cross section of the 10% and 15% ZTA samples are shown in Figure 6.27. A significant difference in the cross sections between the alumina and ZTAs is evident.

No material pull out in the plastic zone was observed underneath the impact surface except for two to three large cracks forming a cone shape in the ZTA cross-sections.

In addition, the extent of crack propagation was much suppressed in the ZTA samples compared to that in the alumina sample, which suggests that the identical impact speed caused much reduced damage.

Figure 6.27 SEM micrographs of the cross-section of the 10% and 15% ZTA specimens

The Cr³⁺ fluorescence maps are shown in Figures 6.26 and 6.27,the peak shift map is shown in Figure 6.28. For the alumina sample, due to the polishing-induced

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10% ZTA 15% ZTA

(a) (b)

material pull out in the plastic zone, the compressive residual stress built up during the impact was released, therefore a lower negative peak shift was observed in the plastic zone compared to the other area underneath the impact surface. For the ZTA samples, no material pull out happened, suggesting that there was no significant fracture beneath the impact area, but plastic deformation could still exist. From the fluorescence mapping results, the both ZTA samples showed obvious signs of tensile stress areas beneath the impact surface, in which strong positive peak shift were observed. In addition, the 15% ZTA showed a larger and deeper area with tension. Comparing the stress maps with the SEM micrographs of the ZTA samples, it was found that, although no materials deformation or large degree of cracking was observed, plastic deformations still happened to form a plastic zone and induced the zirconia phase transformation, the latter generated a significant effect on the residual stress state of the impact affected regions.

Figure 6.28 R1 fluorescence peak shift map on the cross-sections of the specimen:

alumina, 10% ZTA and 15% ZTA, the dashed line circle highlights the position of the plastic zone in Figure 6.26.

The peak broadening of the cross-sections were mapped and are shown in Figure 6.29. It should be noted that the scale bars for the alumina and the ZTA samples are different due to the significantly different levels of the peak width change occurring during impact. The alumina sample showed the highest degree of peak broadening within the plastic zone, which was about 33 cm^-1increase in peak width. The peak broadening region in the alumina sample was found to overlap with the plastic zone and compressively stressed region shown in Figure 6.28. The 10% and 15% ZTA samples also showed regions with obvious peak broadening, confirming the existence of plastic zone, whilst their highest values were only about 11 cm^-1 and 8

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cm^-1 respectively, which is much less compared to that observed in the alumina samples. The dislocations may have been able to induce the zirconia phase transformation, which inhibited further damage in the materials. The much lower level of peak broadening observed in the ZTA samples beneath the impact surface is consistent with the results observed on the impact surfaces, and therefore, further proved that the much reduced damage in the ZTA samples after impact may be derived from the impact triggered zirconia phase transformation. In addition, with the higher zirconia content, the protection from the zirconia phase transformation also increased.

Figure 6.29 R1 fluorescence peak width change mapping on the cross-sections of the specimen: alumina, 10% ZTA and 15% ZTA.

To further confirm that the reduced level of the residual stress and dislocation density change observed from the ZTA samples originated from the zirconia phase transformation, the concentration of the transformed monoclinic zirconia phases on the cross-sections of the ZTA samples were mapped. The results are shown in Figure 6.30. Because the Cr³⁺ fluorescence map results showed that the residual stress and dislocation density change was concentrated on the region about 0-600 µm underneath the surface, the zirconia phase mapping was focused on this area of

Al₂O₃

10% ZTA 15% ZTA

Change of peak width / cm-1

Change of peak width / cm-1

interest. Very significant zirconia phase transformation was observed and the highest amount was about 45% for the 10% ZTA and 50% for the 15% ZTA. The phase transformation area was larger for the latter as well. The results confirmed the presence of zirconia phase transformation in the impact-affected region.

Figure 6.30 m-ZrO₂ concentration map on the cross-sections of the 10% and 15% ZTA specimens.