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1248 |
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Wednesday, June 04, 2008, Room 05 11:40 AM Characterization 2 Modern characterization offers a good possibility for proofing the reliability and the performance of thermal sprayed coatings |
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The metallographic characterization of thermal spray coating microstructures |
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Douglas Puerta* / IMR Test Labs, NY Fred Anderson / IMR Test Labs, NY Arthur Geary/ Metallography Consulting Services, CT |
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Within the past twenty years, there have been a number of significant advances in the area of metallographic preparation techniques. Traditional preparation methods which relied heavily on the use of silicon carbide (SiC) abrasive papers have been replaced by new methods which better utilize diamond as the abrasive for both grinding and polishing. As a result, microstructural features which were previously lost or masked during preparation are now readily visible. Unfortunately, the slow adoption of these new techniques has led to confusion and mis-interpretation within the gas turbine industry. Due to the sensitive nature of thermal spray coatings to metallographic preparation, different operators using different metallographic procedures can end up with varying results. As a result, decisions affecting spray booth parameters are frequently made based on an inaccurate evaluation. This problem is compounded by the fact that the gas turbine industry has long used photostandards for plasma spray coating analysis. Unfortunately, in many cases these photostandards were developed by engine manufacturers prior to the development of modern metallographic techniques. In this presentation, plasma spray coating families including ceramics, hardcoats, abradables, and metallic coatings will be examined to demonstrate the effects of metallographic preparation on coating features. Vacuum impregnation of low-viscosity epoxies containing fluorescent dyes will be used to highlight features inherent to each coating. Scanning electron microscopy (SEM) will allow for analysis of cryogenic fracture surfaces, from which voids and other microstructural features can be qualitatively observed. Particular attention will be paid to the bonding of individual splat particles and the formation of linear detachments (voids) or oxides at these interfaces. |