Coating Morphology Characterisation

Since the properties of PEO coated magnesium are directly determined by the morphology, it is critical to observe the coating morphology development under different treatment conditions. It is a prerequisite to reveal the relationship between the morphology, processing parameters and final coating properties.

4.5.1 Coating Thickness Measurements

The coating thickness is of interest here because of the following reasons. On one hand, the corrosion resistance and mechanical properties which are important for biomedical application are influenced by the coating thickness; on the other hand, coating thickness reflects the PEO process efficiency. For a given processing time and applied voltage, a greater coating thickness suggests higher process efficiency. In the present study, the thickness of the PEO coatings was analysed using an Electrometer 355 Coating Thickness Gauge equipped with N4 standard anodisers probe with an accuracy of ±1 µm. The probe utilises a relatively high frequency signal (up to several mega-Hertz) to generate an alternating electric field in the substrate beneath the coating. The field causes eddy currents to circulate in the substrate which in turn induce associated magnetic fields. These fields interact with the probe and cause electrical impedance changes that are dependent on the coating thickness. Before performing the measurement, the thickness gauge was zeroed by pressing the probe against a well-polished sample surface made of the same material as the substrate. Then the gauge was calibrated using dielectric films of known thickness. About 20 measurements were taken from each coated sample. The results of the measurements were statistically analysed, and the arithmetic average is taken as the coating thickness.

4.5.2 Coating Morphology Observation by Scanning Electron Microscopy

Scanning electron microscopy (SEM) is a widely used technique in various areas like materials, physics, biology, etc.. In SEM, the electron beam generated by a biased filament is focused by electromagnetic lens and directed towards the sample, where the high energy electrons will interact with the atoms of the specimen, emitting different kinds of signals. Of the signals, secondary electrons (SE) are very sensitive to characteristics of surface morphology such as roughness, porosity, cracks, etc.; as a result, the interpretation of SE image is of significance to reveal surface morphologies. Apart from SE images, the

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elementary composition within the sample surface can also be evaluated by collecting characteristic EDX spectra using a detector attached to the SEM.

In the present study, the plain surface and cross sectional morphologies of the coatings were observed using JEOL JSM-6400 and/or FEI Inspect F SEM instruments operated at an acceleration voltage of 15-20 kV. The chemical composition of the coatings was evaluated by EDX attachments (Oxford instruments) to the electron microscopies. To prepare the cross-sectional specimens, the coated magnesium discs were firstly cut into halves using the IsoMet 5000 precision saw mentioned in Section 4.2. However, the cutting speed was reduced to 1.5 mm/min to eliminate the risk of damaging the coating. Then the sample was cold mounted using an epoxy resin (MetPrep Ltd.) before being subjected to grinding and polishing. The samples were firstly ground using SiC abrasive papers of upto 4000 grit. Then a polish cloth of 1 μm was used for polishing. Since magnesium is a relative soft material, just soapy water was used during the polishing for the purpose of lubrication. It also prevents the temperature increase, eliminating the oxidation of magnesium substrate.

For surface plane SEM observation, the samples were stuck on an aluminium stub (Ø30 x 10 mm) using electrical conductive carbon tape. Both the cold mounted cross sectional samples and the surface plane samples were sputter coated with carbon to eliminate the charging effects under electron bombmartment during the SEM observation.

4.5.3 Coating Phase Characterisation by XRD

The phase composition of the coating was characterised using X-ray diffraction method. The basics of this technique rely on the fact that crystals contain periodic arrangements of atoms.

When the incident X-ray beam interacts with a crystal, it is reflected by different atomic planes.

When the reflected beams are in phase, they will be amplified (constructive diffraction), otherwise they will be dismissed (destructive diffraction). The schematic of the XRD principle is illustrated in Figure 4-2. Then the relationship between the crystal lattice plane spacing, wavelength of incident X-ray and the incident angle follows the Bragg’s Law:

2𝑑𝑠𝑖𝑛𝜃 = 𝑛𝜆 ( 4.1 )

Where 𝑑 is the crystal lattice plane spacing, 𝜃 is the incident angle and 𝝀 is the incident X-ray wavelength. This equation clearly shows the relationship between the diffraction pattern observed when X-ray is diffracted through the crystal lattice and the atomic plane

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spacing.

Figure 4-2 Schematic illustration of XRD principle (the black dots represent atoms)

Equation (4.1) guarantees specific diffraction patterns for each phase; therefore, XRD is widely used for phase identification. In the present project, the XRD experiment was performed on a Siemens D5000 X-ray diffractometer operated at 40 kV and 30 mA with Cu Kα radiation (wavelength λ=0.154 nm). The samples were scanned under the normal coupled θ-2θ geometry in the range of 2θ from 15º to 85º, at a step size of 0.02º, with dwell time of 2 s/step. The obtained diffraction patterns were analysed using Bruker EVA software.

4.5.4 Residual Stress of the Coatings by XRD

Residual stress is built up within the PEO coating because of (1) the steep temperature gradient during the PEO process and (2) the difference of the molar volume between the substrate and its oxide. Depending on the type (tensile or compressive) and magnitude of the residual stress, the mechanical properties as well as corrosion performance of the material will be influenced. It is generally realised that compressive residual stress is beneficial for the wear properties, while tensile stress is usually detrimental for both mechanical properties and corrosion performance, as it could easily cause cracking, especially in the corrosive environment. Therefore, it is critical to quantify the type and magnitude of the residual stress.

In the present study, the residual stresses in the PEO coatings were evaluated using XRD. In this measurement, the strain in the crystal lattice is measured, assuming a linear elastic distortion of the crystal. The inter-planer spacing of an unstressed material produces a characteristic diffraction pattern, as stated in Section 4.5.3. When the material is under stress, elongation and contraction will be produced within the crystal lattice, therefore inter-planar spacing of the (hkl) lattice planes would be changed causing a shift in diffraction peaks. The

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magnitude of the shift (strain) could be calculated by comparing the inter-planar spacing with and without stress defined by Equation (4.1). By solving the generalised Hooke’s law, the stress generating the strain can be calculated through the following equation:

𝜎 = 𝐸

(1 + 𝑣)𝑠𝑖𝑛²𝜓∙𝑑_𝜓− 𝑑₀

𝑑₀ ( 4.2 )

Where 𝜎 is the direct in-plane residual stress, 𝐸 and 𝑣 are the Elastic’s modulus and Possion’s ratio of the material under investigation, respectively; 𝑑𝜓 is the crystal plane spacing of the stressed crystal at the tilt angle 𝜓. 𝑑0 is the unstressed crystal lattice spacing, which can be obtained from the X-ray diffraction pattern of the unstressed crystal powder.

In the present study, the measurement was performed at the diffracted peak corresponding with the (422) crystal plane of MgO at 2𝜽=127.28° because of its high sensivity to strain. The test was conducted on the same X-ray diffractrometer mentioned in Section 4.4.3 in the 2𝜽 range of 125^o to 130^o at different 𝜓 angles (-45^o, -33.75^o, -22.5^o, -11.25^o, 0^o, 11.25^o, 22.5^o, 33.75^o). The final results are analysed using a Bruker stress software package .