|
2239 |
|
Tuesday, May 04, 2010, Sophia Room 12:00 PM Properties 1 |
|
Thermal shakedown of thermal spray coatings |
|
Alfredo Valarezo / SUNY Stony Brook University
Center for Thermal Spray Research
Materials Science & Engineering Department
, USA Miryan Bejarano / Stony Brook University, USA David Welch/ Stony Brook University, USA Sanjay Sampath/ Stony Brook University, USA Gopal Dwivedi*/ Stony Brook University, USA |
|
Recent investigation has revealed a rapid annealing of imperfections and phase transformations occurring in the microstructure of thermal sprayed coatings during the first cycle of heat exposure. This behavior named here as thermal shakedown was measured by dilatometry as a permanent elongation or contraction of the overall length of the coatings in their free standing form. Pure materials such as Al, Ni, Ag as well as alloys e.g. NiCr, MCrAlY and Inconel deposited by various processes and spray parameters have all presented this recovery behavior with different magnitudes. Being thermal spray coatings, structures subjected to rapid solidification, quenching, strain hardening, residual stress, among others, 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, homogenization, and sintering. Coatings were thermally cycled, with dimensional changes recorded in each heating, cooling, isothermal stages. Results are analyzed in the context of 'irreversible' aspects (e.g. phase changes, oxidation, sintering, stress relief) and their respective order of magnitudes. The importance of thermal shakedown is discussed in correlation to the thermal mismatch stresses in the coatings when attached to the substrate. |