**4.2 Surface ccharacteristics at Mach 4.5 flow using the micrometre gauge and SEM**

The micrometre measurements also recorded losses in thickness over successive runs. Four points were used across the disc at 0 mm, 8 mm, 16 mm, and 24 mm from the centre as shown in **Figure 10**. The measured thickness across the four

**Figure 10.**

*Scanning locations on graphite surface. (a) Graphite specimen (top view). (b) Graphite specimen (side view not to scale).*

#### **Figure 11.**

*Variation in thickness across disc along paths perpendicular to the surface at various radial positions using the micrometre gauge after 1, 2, 4, 8, 12 and 16 heated-with-flow runs.*

*Plasma Preheating Technology for Ablation Studies of Hypersonic Reentry Vehicles DOI: http://dx.doi.org/10.5772/intechopen.100129*

**Figure 12.**

*SEM images after the second heated with flow run in proximity of points A, C and E. (a) In the proximity of point A. (b) In the proximity of point C. (c) In the proximity of point E.*

**Figure 13.**

*SEM images after the fourth heated with flow run in proximity of points A, C and E. (a) In the proximity of point A. (b) In the proximity of point C. (c) In the proximity of point E.*

locations is shown in **Figure 11**. The anvil and spindle of the micrometre (where the micrometre contacted the disc) were 6.5 mm in diameter. The disc deformation and the ablating surface meant that the contact from the micrometre was unlikely to accurately measure the local thickness at each point, but more likely measured a general thickness at the location in the vicinity of each point. **Figure 11** shows the general trend resulting from the ablation of the disc. The thickness from the micrometre measurements in **Figure 11** shows a general agreement with that obtained from the measuring arm [34]. The laser sheet visualisation confidently identifies that the material loss is not at a consistent rate along a radius of the disc, but that the trend of the material loss rate is relatively consistent over the duration

**Figure 14.**

*SEM images after the eighth heated with flow run in proximity of points A, B, C, D and E. (a) In the proximity of point A. (b) In the proximity of point B. (c) In the proximity of point C. (d) In the proximity of point D. (e) In the proximity of point E.*

*Plasma Preheating Technology for Ablation Studies of Hypersonic Reentry Vehicles DOI: http://dx.doi.org/10.5772/intechopen.100129*

#### **Figure 15.**

*SEM images after the twelfth heated with flow run in proximity of points A, B, C, D and E. (a) In the proximity of point A. (b) In the proximity of point B. (c) In the proximity of point C. (d) In the proximity of point D. (e) In the proximity of point E.*

of the experiments. The experiments show a significant spatial variation in thickness loss for the graphite test material over the disc radius though the spatial variation was still largely axisymmetric.

Tools such as the Scanning Electron Microscope (SEM) was used for surface characteristics and the results from microscopy are presented in **Figures 12**–**18**. Five positions in total were chosen at 4.5 mm increments, shown as A, B, C, D, and E in **Figure 10**. This resulted in position E being 7 mm from the outside edge of the disc (6 mm from the chamfer used to retain the disc). During experiments, the actual points scanned in the SEM were not exactly the same; each scan was a representative area in close proximity to the points described by A, B, C, D and E.

#### **Figure 16.**

*SEM images after the sixteenth heated with flow run in proximity of points A, B, C, D and E. Back of disc. (a) In the proximity of point A. (b) In the proximity of point B. (c) In the proximity of point C. (d) In the proximity of point D. (e) In the proximity of point E.*
