Structures and Properties in Varying Phase Composition of PEO CoatingsHolly Avins, Sam Deschaine, Jasmine Duvall, Corey KwokFaculty Advisor: Dr. David BahrIndustrial Sponsor: Elgin Miller

Project Background

IBC Materials and Technologies, Inc. has developed a process for creating Plasma ElectrolyticOxidation (PEO) coatings on various aluminum alloy substrates. The purpose of the project wasto understand the effects of IBC’s proprietary process on the phase composition and mechanicalproperties of PEO coatings. The characterization was focused on the relationship betweenprocessing parameters and its effect on the alpha and gamma alumina volume fractions.Hardness, X-Ray Diffraction, and microscopy were performed to relate processing to properties.

Optical Microscopy

RecommendationsFurther research should be focused on comparisonsbetween phase composition and tribologicalproperties. Scratch testing using a pin on discmethod would be desired.

Conclusions

Average cell sizes werecompared with substratealloy grade and processparameter changes. Achange in processparameters and anincrease in substratealloy copper contentdirectly relate to largercells. Increased ironcontent in the substratealloy has a negativecorrelation with cell size.

MSE 430-440: Materials Processing and Design

Cells were analyzedaccording to ASTM E-562standards. Sets of 25points were randomlychosen and counted forintersections and numberof cells. This data wasthen used to calculate thevolume fractions and cellareas.

This work is sponsored by IBC Materials &Technologies, Lebanon, IN

The transparent nature ofthe PEO coating allows forspecialized microscopy.Optical micrographs takenunder polarized light show acellular structure (top) withinthe PEO coating. Evidencesuggest that the cells growin a columnar pattern(bottom). SEM testing fromprevious research showsthe cells contain highercopper and iron contentthan the surrounding walls.

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Figure 2: Optical micrographs wereoverlaid with a 100 µm spaced gridat a random offset.

HardnessA fixed load of 100g and50g were applied ontothe coating’s surfaceand cross sectionrespectively. Randompoints on the surfaceand points along thecenter of the crosssection were evaluated.

Figure 4: Micro-hardnessindentation procedure using theVickers method. Shown in red isthe plastically deformed regionwhich offsets data in close byindents.

Hardness

Figure 5: Hardness of the cross section was found to bemore indicative of the coating. No significant difference wasfound between IBC processes 1 and 2 with the exceptionbeing that process 1 samples had thicker coatings.

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Cell Hardness ComparisonLocation Load (g) Hardness (HV)

Cells 50 1434 ± 65Cell wall 50 1266 ± 201

Using the microscope attached to the micro-indenter, the cell structure was identified on thesurface of the coating. A proper load and pointswere selected such that indents would be able tosample the cell structure. It was found that thecells are significantly harder than the outside wallstructure. The results were confirmed using a 95%significance t-test.

Figure 3: Average cell radii arecompared by similarprocessing and substrate alloygrade.

X-Ray Diffraction

References

The mechanical properties of PEO coatings areprimarily dependent on the phase fraction ofalumina present. In understanding phase growth,the substrate composition, processing parameters,and nucleation must be considered.

Hardness testing has shown that coating strengthwas unaffected by the different PEO processesprovided. However, the cells display greaterhardness than the surrounding wall. This suggeststhat the cells are comprised of greater amounts ofalpha phase.

Cell size shows varying relationships with substratealloying elements. Cell size increases betweenprocesses 1 and 2, also it increases with thesubstrate’s copper content. Higher iron contentwithin the substrate will decrease cell size.

Substrate dependence of the cell size may be dueto iron and copper oxide nucleation. Theseinclusions provide a site for possible epitaxialgrowth through the coating [1][3].

[1] - Andersson, J. M. (2005). Controlling the Formation and Stability of Alumina Phases. Linköping, Sweden: Linköping: Linköpings Universitet.

[2] - IBC Coatings. (n.d.). Plasma Electrolytic Oxidation (PEO) | Micro Arc Oxidation | CeraTough™. Retrieved from IBC Coatings: http://www.ibccoatings.com/plasma-electrolytic-oxidation-peo-ceratough

[3] - Wenbin X., Zhiwei D., Ruyi C., Tonghe Z. (2006). Growth regularity of ceramic coatings formed by microarc oxidation on Al–Cu–Mg alloy. Thin Solid Films.

Alpha phase aluminum oxide is preferred overgamma phase for many reasons. Alpha Al2O3 ismuch harder, thermodynamically stable, andmore corrosion resistant. Conversely, gammaphase is thermodynamically unstable and haslow surface energy [1].

To form alpha alumina requires a phasetransformation from gamma phase attemperatures above 1000ºC. PEO applies ahigh voltage of over 100 V to the metal samplewhich creates a plasma at the surface. Anelectrostatic attraction is created between themetal substrate and the oxygen in the electrolytebath. This lowers the phase transformationtemperature and assists the nucleation growth ofalpha alumina [2].

Figure 1: Surface plasma interactingwith metal sample in electrolytebaths [2].

Coating Properties

Alloy Grade alpha% Hardness (HV) Cell Radius (µm)

Al 7050 A 6.5 1313 10.15Al 7050 B 0.0 1300 7.19Al 7075 C 2.7 1432 7.72Al 7075 D 41.4 1248 5.69

Al C355 C83 E 89.6 1524 9.16Al C355 C97 F 0.2 1487 6.41

Figure 6: XRD spectra for Al 7050 A identifying the multiple phasespresent with the coating. The presence of Al peaks representingthe substrate indicates the entire thickness of the coating wasanalyzed.

Figure 7: Cell radius is compared to the copper contentwithin the substrate (left). Both IBC processes show andincrease in the cell size as copper content increases. Theiron content within the substrate alloy leads to a decrease incell radius (right).

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