Surface Chemistry and Morphology of Aluminium

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Oxide layer composition of different etched surfaces

The oxide layer present on the surface of aluminium reacts with atmospheric moisture to form a variety of hydroxides and oxyhydroxides, as discussed in section 2.3. This results in three resolvable oxygen peaks when conducting XPS analysis, one due to an Al-OH bond, one due to the Al=O bond in AlOOH and one due to chemically adsorbed water [38]. Sulphate and silicon oxide contaminants have not been taken into account during fitting of the O 1s spectra, but the levels were too low to affect the fitting of peaks to the O 1s spectra. As can be seen from Figure 4.2, the BWI samples had equal proportions of O:OH and no adsorbed water, indicating a pseudoboehmitic surface was formed, as expected. This result was used to define the peak widths and separation of the oxide and hydroxide peaks.

Surface chemistry pretreatment, AA5052

The substrates were changed from AA5005 to AA5052 at this point, from this point forward all substrates discussed are AA5052 unless stated otherwise. After initial investigations of a range of chemical pretreatments, the six treatments summarised in Table 4.4, and fully explained in section 3.1, were selected for further investigation and thermal spraying. Boehmitising was selected as it produced a relatively basic aluminium oxy-hydroxide surface with a known chemistry and known bonding groups on the surface. Etching with AcidBrite was chosen as a pretreatment due to the consistent acidic surface it produced, significantly different to the boehmitised surface. AcidBrite also produced a rough surface for comparison to the polished surface, surface roughness of all the pretreatments sprayed are characterised in section 4.2.

Oxide layer composition and thickness of selected

pretreatments The change to AA5052 substrates necessitated further XPS to not only verify the elemental composition as reported above, but also to determine the effect of the pretreatments on the aluminium oxide species present on the substrate surfaces. The results for the six selected pretreatments on AA5052 are presented in Figure 4.7. Whereas the AA5005 substrates were prepared fresh and then immediately placed in high vacuum, the AA5052 substrates were prepared, then stored in a desiccator for a period of about 24 hours before being placed in the high vacuum of the XPS. This was the same procedure used for spraying substrates, where pretreatments were performed 24 hours before spraying, during which time they were stored in a desiccator, thus these results represent as accurately as possible the surface chemistries of the substrates as they were coated.

Surface morphology

Upon surface modification the samples were placed on atomic force microscopy (AFM) sample stubs and the surface morphology imaged by AFM. A representative area was selected, and four sub samples of 50µm squares were taken to evaluate the surface roughness. As can be seen from Figure 4.8 and Figure 4.9, thermal treatment of the surfaces had no significant effect on morphology of the surface, which was confirmed by the Ra values presented in Table 4.7. The polished and boehmitised surfaces are both very smooth, with boehmitising only resulting in a slight increase in roughness of the surface. In contrast, etched substrates had a very rough surface with significant potential for mechanical keying to the surface for increased adhesion of polymer splats.

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Table of Contents :

List of Figures
List of Tables
List of Abbreviations
1 Introduction
2 Literature Review
- 2.1 Surface chemistry and morphology of aluminium
- 2.2 Adhesion of organic compounds to aluminium surfaces
- 2.3 Surface chemistry of aluminium by XPS
- 2.4 Thermal spray of polymers
  - 2.4.1 Thermal spray processes
  - 2.4.2 Microstructure of polymer coatings
  - 2.4.3 Spray variables
  - 2.4.4 Substrate variables
  - 2.4.5 Polymer single splats
  - 2.4.6 Plasma spray of polymers
  - 2.4.7 Combustion spray of polymers
  - 2.4.8 Spray of polymers with filler particles
- 2.5 Impact and spreading of single splats during thermal spray
- 2.6 PEEK
- 2.7 Aims and methodology of this thesis
3 Experimental Methods
- 3.1 Substrate selection
- 3.2 Surface preparation
  - 3.2.1 Polishing and degreasing
  - 3.2.2 Etching
  - 3.2.3 Boehmitising
  - 3.2.4 Thermal oxidation
- 3.3 Surface characterisation techniques
  - 3.3.1 Atomic force microscopy
  - 3.3.2 Scanning electron microscopy
  - 3.3.3 X-ray photoelectron spectroscopy
4 Surface Chemistry and Morphology of Aluminium
- 4.1 Surface chemistry
  - 4.1.1 Surface composition of treated substrates
  - 4.1.2 Oxide layer composition of different etched surfaces
  - 4.1.3 Surface acidity measurements
  - 4.1.4 Surface chemistry pretreatment, AA
  - 4.1.5 Oxide layer composition and thickness of selected pretreatments
- 4.2 Surface morphology
5 Single Splat Experimental Methodology
- 5.1 Thermal spray torches
- 5.2 PEEK powder
- 5.3 Depositing splats on substrates
  - 5.3.1 Single splat deposition
- 5.4 Substrate mounting and temperature control
- 5.5 Imaging single splats
- 5.6 Image analysis
6 Qualitative Splat Analysis
7 Plasma Spray of PEEK Single Splats
- 7.1 Effect of surface chemistry and morphology on plasma sprayed PEEK splats
  - 7.1.1 Number of splats
  - 7.1.2 Splat circularity
  - 7.1.3 Mean area of a splat
  - 7.1.4 Splat perimeter
  - 7.1.5 Splat Feret diameter
- 7.2 Effect of substrate temperature on plasma splat properties
- 7.3 Plasma splats discussion and summary
8 HVAF Spray of PEEK Single Splats
- 8.1 Effect of surface chemistry and morphology on HVAF sprayed PEEK splats
  - 8.1.1 Number of splats
  - 8.1.2 Splat circularity
  - 8.1.3 Average area of a splat
  - 8.1.4 Splat perimeter
  - 8.1.5 Splat Feret diameter
- 8.2 The effect of substrate temperature on HVAF splat properties
- 8.3 Kinetic energy conversion to thermal energy on particle impact
- 8.4 HVAF discussion summary
9 Mechanism of Splat Formation
- 9.1 Particles in Flight
- 9.2 Particle Impact
- 9.3 Splat Spreading
- 9.4 Post Impact Effects
10 Conclusions
11 References