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Task IA: Coating Heat Treatments

Introduction

The University of Pittsburgh is collaborating with the University of Connecticut to develop thermal barrier coatings and metallic coatings with superior reliability and durability. In order to determine pretreatments of platinum aluminide bond coats that may be useful to improve TBC performances, platinum aluminide coatings on the substrate CMSX4 with different surface conditions are being exposed to a number of different preoxidation treatments. The surfaces of the exposed specimens are being examined by using optical microscopy and scanning electron microscopy. In the previous semi-annual report it was shown that as processed platinum aluminide coatings developed voids at oxide-coating interfaces for 2 hour exposures at 900°, 1000°, 1100° and 1200°C in air, pure oxygen and in an argon-hydrogen gas mixture that established an oxygen pressure of about 10-8 atm. In the current report results will be presented for platinum aluminide coatings exposure to additional surface modification and preoxidation treatments.

Results

A platinum aluminide coating in the as processed condition was given a special pretreatment by Howmet. Specimens of this coating were then exposed in dry air for: A) 2 hours at 1080°C(1975°F), B) 16 hours at 1080°C(1975°F), C) 16 hours at 1093°C(2000°F), D) 16 hours at 1121°C(2050°F). Scanning micrographs showing the surfaces of these exposed specimens are presented in Figure 11. No voids are evident. Some rosette shaped oxide is evident, Figures 1A and 1B

Grit blasted platinum aluminide bond coats were exposed for 12 hours at 1100°C in the argon-hydrogen gas mixture and in dry air. Scanning micrographs of the exposed surfaces are presented in Figure 12. No voids are evident in these specimens.

The surfaces of platinum aluminide coatings have also been given a media finish where the times in this finishing treatment were 40 and 120 minutes. The resulting specimens were then exposed for 2 hours at 1100°C in the argon-hydrogen gas mixture and in air. The surfaces of the exposed specimens are shown in Figure 13. Some voids were evident in the specimens exposed in air, but were fewer voids evident beneath the oxide scales on specimens exposed in the argon-gas mixture.

The results that have been obtained show that by using different surface preparations and heat treatments the formation of voids beneath the alumina scales during preoxidation can be inhibited and in some cases prevented. The cause of this void formation is still not understood.

Future Work

At present the following surface preparations and pretreatments are proposed for the platinum aluminide bond coats prior to TBC deposition.

All of the above samples would have TBCs deposited. The following specimens would be tested without TBCs.

Pretreatment Conditions Heavy media finish + 2hrs @ 1100°C in air Heavy media finish + 2hrs @ 1100°C in Ar-H Light media finish + 2hrs @ 1100°C in air Light media finish + 2hrs @ 1100°C in Ar-H2 Hand Polish + 2hrs @ 1100°C in Ar-H Hand polish

Heavy media finish +2hrs @ 900°C in dry air

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