and Film
The images of the element ablated after being polished compared to the film cross-section is shown in figure 4.8. This shows how the metalicity of the film has been reduced after the laser ablation. In addition the films structure is altered, the presence of voids is much lower in the ablated sample indicating the melting of the film as the ablation takes place. This laser ablation was done at a high power and low speed as defined in the figures and corresponds to the four squares in figure 4.3. A matrix of the power and speeds of laser is shown in figure 4.6. This shows four different combinations of power and speed applied to the element with the top of the element ground and polished.
Within the laser tracks with a greater amount of ablation ( higher power and lower speed) the more the milky areas turn into dark oxidised areas with the metallic globules. This is seen also in the EDX measurements of the film. This indicates that the ablation has a large effect on the oxidation and composition of the melted regions with a large heat affected area. This means that the processing method chosen for the ablation treatment needs to effect the film as little as possible.
Figure 4.6: An optical image of squares of ablation after mounting and polishing the upper surface. The fluence of each is shown in figure 4.5.
Figure 4.7: Optical images of the cross section of the film before ablation at the same magnification as 4.8.
Figure 4.8: Optical images of the cross section of the film after ablation. The ablated sections cut across the lines of ablation with the beam having a slight overlap in each line
4.5 X-ray Diffraction of Samples
To investigate the phases present within the flame sprayed element in the prepared powder, before ablation and after ablation x-ray techniques have been employed as discussed within the methods section.
Table 4.3 shows the proportion of peaks matched and thus the relative quantity of various phases throughout the samples. There are four samples examined, the prepared powder used in the flame spray process, the flame sprayed element, the Laser ablated film, and a heavily laser ablated flake that has been melted with the laser and ablated then removed from the substrate to investigate the wider effect of the laser treatment.
Table 4.3: XRD results summary of the proportion of various compounds on various materials investigated.
Prepared powder Flame sprayed Laser ablated Laser ablated Compound Percentage of Phase element film flake
Metallic Ni 60 54.3 35 5.7
Alloy (Fe-Cr) 12 9.1 12 8
Oxides 3.2 19.0 24.7 68.5
Other traces bal bal bal bal
Within the initial prepared powder is a high proportion of metallic nickel and alloy, there is still some oxide present which is produced in the oxidation step to prepare the powder. These values match with the proportions of phases seen with Duffield’s work [2]. The element produced by the flame spray has a lower proportion of metallic nickel and alloy but a larger amount of oxides, this is to be expected from the flame spray process and again matches Duffield’s work who recorded an oxide proportion of≈20%.
NiO Ni NiO Spinel 111 200 111 311 Alloy A B C D 35 40 45 50 2 (Degrees)
Figure 4.9: XRD results shown with the miller indices indicated for various shown species. A:Prepared Pre-flame sprayed powder, B: Flame sprayed element, C: Laser ablated
element, D: Heavily laser ablated flake.
The laser ablated element oxidation increases again but less than the amount gained through the flame spray oxidation process. As a proportion, the amount of alloy increases and the proportion of Nickel decreases. The hypothesis drawn from this observation is that less alloy will be ablated in comparison to the nickel areas due to the lower melting point of Nickel. This follows for the oxide which also has a higher melting point than the nickel. In addition, the proportion of Nickel is excepted to decrease if the proportion of oxide increases as the Nickel is oxidised.
The matched peaks of the XRD can be seen in figure 4.9. This shows the four main components, the oxide with Miller indices [200] and [111], metallic and alloy peaks with a Miller indices of [111] and the spinel peak with [311]. These results match the results seen in Duffields thesis and match the positions of the peaks found by Duffield.
Notable changes depicted in the plot are the decrease in metallic peak and the increase of the oxide peak within the ablated and heavily ablated proportions. This gives evidence that the laser ablation treatment also increases oxidation of the film. This has consequences for the research as the electrical conductivity of the oxide regions is lower than the conductivity of the metallic regions. As a result the calibration and selection of the parameters for ablation have to take into account the reduction in thickness of the conductor and the change in conductivity of material due to laser ablation.