Figure 9-1 Potential transient during the galvanostatic CED process with current density of 0.4 mA/cm2 utilised in the present study
The PEO treatment in the present study was the same as that described in Chapter 8, the
154
voltage transient of the PEO process exhibited similar behaviour to that shown in Figure 8-1 and is therefore, not presented here. The potential transient of the galavanostatic CED process conducted in the present study is presented in Figure 9-1. As can be seen, once the CED process started, the potential shifted rapidly in the positive direction at a rate of 1.56 V/s from -4.6 V vs. SCE to about -3.4 V vs. SCE within 1 second. Afterwards, the potential increased much slower at a rate of <0.02 V/s to -3.1 V vs. SCE, and finally stabilised around -2.86 V vs. SCE, indicating that the HA deposition finally reached a steady state. Due to the deposition of the HA layer, the total coating thickness increased, which drove the potential to more noble values, as suggested by Shi et al.[98].
9.3. Coating Morphology
The surface of the PEO coating exhibited a smooth white appearance, and after the CED treatment, island-like features could be observed with a naked eye. The surface morphologies of the coated samples are shown in Figure 9-2. The PEO coating was produced using the same parameters as discussed in Chapter 8, and no inconsistence was found in surface morphology of the coatings produced here and those presented in Chapter 8.. Nevertheless, the PEO coating morphology is also presented in this chapter for the sake of comparison. Similar with the results presented in Chapter 8, crater-like porous microstructures can be observed on the surface of the PEO coating (Figure 9-2(a)) with cracks appeared around the craters (Figure 9-2(b)). Such morphologies could not be observed any more after the CED treatment. Instead, the sample surface featured island-like structures (Figure 9-2(c)). Higher magnification SEM image showed that the island-like structure was actually clusters of needle- and plate-shaped crystals, as shown in Figure 9-2(d). This observation was different with the potentiostatic CED coating presented in Chapter 8, where only needle-like crystals were observed (Figure 8-2). Therefore, both one- and two-dimensional growth of HA crystals after the nucleation could be envisaged according to the models proposed by Eliaz [216], and Dorozhkin [217]. Moreover, the large unfilled space between the crystal dendrites exhibited by the potentiostatic CED coating (Figure 8-2) could no longer be identified in the galavanostatic CED coating (Figure 9-2). As a result, the defects on the coating surface were reduced by the CED treatment, which would facilitate the passivation of the sample, as consistent with the analysis of Figure 9-1.
155
Figure 9-2 Surface morphologies of (a),(b) PEO coating and (c),(d) PEO coating following HA deposition.
The cross-sectional SEM images of the PEO coated samples before and after CED treatment are shown in Figure 9-3. Similar to the results presented in Figure 8-2, two different regions could be identified within the PEO coating based on the difference of porosity, as marked in Figure 9-3(a). The PEO coating appears to be bonded well with the substrate, even though there is a small region of de-bonding marked as ‘Crack’ in Figure 9-3(a). Nevertheless, the compact region itself is continuous. Examination of the cross-sectional morphology of the PEO coating after CED treatment revealed that the HA layer was deposited on top of the PEO coating, as shown between the two dashed lines in Figure 9-3(b). From the cross sectional image, it could be determined that the PEO coating of 21.24±2.9 µm is covered by a CED layer of a thickness of 1.50±0.23 µm. This thin CED layer could cause several effects. On one hand, the CED layer itself appears much more compact compared with the porous PEO coating (Figure 9-3). On the other hand, the pores within the PEO coating are partly filled
156
after CED treatment, resulting in a finer porosity, as determined from Figure 9-3. It could be predicted that the compact coating would inhibit the penetration of corrosive medium towards the substrate, thus improving the corrosion resistance of the substrate. Again, such observations provide further explanation to the potential transient behaviour during the CED process (Figure 9-1).
Figure 9-3 Cross sectional morphologies of PEO coatings before (a) and after (b) CED treatment
Apart from the positive effect of reduced defects, CED treatment also induced detrimental effects to the PEO coating. In detail, the continuity of the compact region within the PEO coating as discussed above was compromised; as a result, the two regions of the PEO coating could not be observed any more. Yet worse, some areas of delamination of the coatings could be determined, as shown in Figure 9-3(b). Such delamination must be raised during the CED process considering the much better bonding exhibited by the single PEO coating, as shown in Figure 9-3(a). In the CED process, considerable amount of H2 gas was generated at the interface between the substrate and PEO coating. Such gas was initially accumulated underneath the PEO coating because of the continuity of the compact PEO region and the hydrogen pressure was increased gradually, causing local delamination of the PEO coating from the substrate. When the pressure was high enough, the hydrogen gas would be liberated out of the sample surface and such phenomenon had been observed throughout the CED process. During the CED treatment, the gaps between the coating and substrate were filled with electrolyte. Such process would compromise the increasing potential transient of the CED process presented in Figure 9-1. Moreover, such delamination
157
could possibly deteriorate the corrosion resistance of the coated samples.
The XRD patterns of the PEO coated samples before and after CED treatment are presented in Figure 9-4. By comparing the two patterns, it was clear that randomly oriented HA crystals have been formed during the CED treatment. The mean HA crystallite size of 77.7 nm could be calculated according to the Scherrer equation. Such crystallite size is significantly larger than that observed in Chapter 8, which might be attributed to a longer crystal growth time allowed by the CED treatment (10 minutes longer here than that applied in Chapter 8). Moreover, after comparing the patterns shown in Figure 9-4 with the standard diffraction pattern of perfect HA crystal, it was found that all the peaks associated with the HA crystals were shifted to the positions of higher 2θ angles. For example, the strongest HA peak at 2θ=26.042o in Figure 9-4 should be positioned at 2θ=25.897o for the perfect HA crystal. As a hexagonal packed crystal, the inter-lattice spacing of the HA crystals could be calculated by [229]: of HA crystal. The shifts of the X-ray diffraction peaks indicated that the HA crystals deposited in the presented study were strained, and a smaller inter-lattice spacing could be predicted according to the Braggers Law. According to Equation ( 9.1 ), smaller 𝑎 and 𝑐 could be predicted compared with the perfect crystals. Therefore, compressive stress was imposed to the HA crystals deposited in the CED process. Such compressive stress may be attributed to the substitution of OH- with other cations, possibly F-. Actually, such substitution could readily occur on thermodynamic grounds (ΔE=-0.4…-0.6) kJ/mol) [230]. After incorporation of F-, the lattice parameters are changed accordingly. Since F- (1.32 Å) is smaller than OH- (1.68 Å), such substitution would result in the contraction in the a-axis [230]. Since F only substituted a small fraction of the total OH groups, the crystals were still identified as HA rather than fluorapatite from the XRD patterns (Figure 9-4).
158
Figure 9-4 XRD patterns of the PEO coated samples before and after CED treatment