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Thursday, May 24, 2012: 9:00 AM
Room 339 AB (Hilton Americas Houston )
An investigation of thermally activated transformations occurring to thermal sprayed (TS) coatings is presented in this paper, specifically for NiCr coatings. A rapid evolution of the microstructure, hereafter referred as thermal shakedown, has been measured via electrical resistivity monitoring using a four-point probe as a permanent change in the property value after the first heating cycle. This effect is also evident via dilatometry as a permanent elongation or contraction of the overall length of the coatings. Both effects are observed in the coating in its free standing form.
TS coatings are subjected during deposition to rapid solidification, quenching, strain hardening, residual stress, among others. Therefore, their microstructures are prone to exhibit metastable phases, inhomogeneous phase distribution, small grain sizes, etc., all these affect the atomic cell structures that recover by stress relief, recrystallization, martensitic transformations, homogenization, and sintering. Therefore, it is proposed that the thermal shakedown occurs due to the rapid annealing of these imperfections and phase transformations during the first cycle of heat exposure
Coatings were thermally cycled, with dimensional changes recorded in each heating, cooling, and various isothermal stages up to 550°C. Similarly, electrical resistivity was measured up to 127°C. Results are analyzed in the context of 'irreversible' aspects (e.g. phase changes, oxidation, stress relief) and their respective order of magnitudes.
TS coatings are subjected during deposition to rapid solidification, quenching, strain hardening, residual stress, among others. Therefore, their microstructures are prone to exhibit metastable phases, inhomogeneous phase distribution, small grain sizes, etc., all these affect the atomic cell structures that recover by stress relief, recrystallization, martensitic transformations, homogenization, and sintering. Therefore, it is proposed that the thermal shakedown occurs due to the rapid annealing of these imperfections and phase transformations during the first cycle of heat exposure
Coatings were thermally cycled, with dimensional changes recorded in each heating, cooling, and various isothermal stages up to 550°C. Similarly, electrical resistivity was measured up to 127°C. Results are analyzed in the context of 'irreversible' aspects (e.g. phase changes, oxidation, stress relief) and their respective order of magnitudes.