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TBC Network Meeting, Nottingham 15 th December, 2008

‘ Mechanical properties of CoNiCrAlY coatings’ Department of Mechanical, Materials and Manufacturing Engineering University of Nottingham S. Saeidi Supervisors: Dr K. Voisey Prof D.G. McCartney. TBC Network Meeting, Nottingham 15 th December, 2008. Aims & objectives.

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TBC Network Meeting, Nottingham 15 th December, 2008

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  1. ‘Mechanical properties of CoNiCrAlY coatings’Department of Mechanical, Materials and Manufacturing EngineeringUniversity of NottinghamS. SaeidiSupervisors: Dr K. Voisey Prof D.G. McCartney TBC Network Meeting, Nottingham 15th December, 2008

  2. Aims & objectives • Studying the mechanical behaviour of the CoNiCrAlY coating as a function of temperature • Young’s Modulus • Hardness → Yield strength • Studying the presence of different phases in a free standing CoNiCrAlY coating in a range of temperatures (equilibrium conditions)

  3. Top coat Superalloy Thermal barrier coatings (TBC’s) • TBC’s consist of: 1- Superalloy substrate 2- Al rich bond coat 3- Thermally grown oxide layer 4- Ceramic top coat • Final failure is associated with spallation of the top coat • Overall performance and life time of a TBC is closely related to the bond coat behaviour

  4. β MCrAlY bond coat (1) • MCrAlY’s (M=Co, Ni, or both) are one of the most widely use types of oxidation resistant coatings • Depending on composition, MCrAlY alloys typically exhibit a two phase microstructure of γ+β at operating temperatures • γ is a FCC phase and is a solid solution of Co, Ni, Cr, etc. • β is a BCC phase and can be described as (Co,Ni)Al

  5. Al2O3 βDepletion MCrAlY bond coat (2) • β phase acts as Al reservoir, thus cyclic oxidation life of the coating is directly related to the amount of β phase • Microstructure of MCrAlY’s depends not only on composition but also on the spraying process (e.g. HVOF, LPPS, etc.) • MCrAlY’s are capable of developing a thermodynamically stable, slow growing and adherent oxide scale (i.e. alumina) • The Al2O3 layer inhibits continued ingress of active oxygen and other species

  6. Overview of presentation • Young’s modulus of as-sprayed and annealed HVOF and VPS free standing coatings as a function of temperature • Hardness of HVOF and VPS free standing coatings as a function of annealing temperature • Yield strength of HVOF and VPS free standing coatings extracted from hardness values (as a function of annealing temperature)

  7. Annealing procedure • HVOF & VPS free standing coatings have been annealed in vacuum at the temperatures & times below followed by water quenching: • 4 hrs at 1100°C • 24 hrs at 1000°C • 90 hrs at 900°C • 840 hrs at 800°C

  8. Experimental methods • 3-point bending technique was used for Young’s modulus measurements (using a DMA machine) • Free standing coatings • Sample dimension of 2x15mm • Minimum of 3 runs for each set of temperatures • Vickers microhardness was used for hardness measurements (average of 20 indents with a 300gf load)

  9. Phases

  10. Young’s modulus as a function of annealing temperature (1) • Properties of thermally sprayed deposits differ from those of fully dense material • Features such as pores, un-melted particles, micro cracks, etc influence the mechanical properties of the coatings • Elastic modulus of isotropic, homogeneous material is related to the volume fraction of the phases present and their respective elastic moduli • Elastic modulus of anisotropic, non-homogeneous and porous material is affected my microstructural features.

  11. HVOF VPS Young’s modulus as a function of annealing temperature (2) • Annealing of the coatings enhances cohesion between the individual Lamella through diffusion and sintering effect • Porosity↓=> Modulus↑

  12. Hardness as a function of annealing temperature • Hardness has great dependence on microstructure • Hardness for HVOF > VPS • Hardness is related to temperature & speed of particles when they impact on substrate • T↓ & V ↑ for HVOF => hardness ↑ (compared to VPS) • Well distributed fine β particles in HVOF increase the hardness compared to VPS (no β particles) • Annealing => Hardness ↓ due to the coarsening of the β phase & stress relieving

  13. Yield strength as a function of temperature Sprayed by Plasma torch σy= 9.81HV/3

  14. Conclusions • Determined temperature dependence of ‘E’ of VPS & HVOF coating • Increase in temperature decreases the elastic modulus • Elastic modulus remains approximately constant than decreases with T (this fits some previous published work and appears to be characteristic of sprayed coatings) • Annealing increases the elastic modulus • Enhance cohesion between individual lamella • Hardness measured for HVOF & VPS as a function of annealing temperature • Annealing decreases hardness • Hardness decrease appears insensitive to annealing temperature • HVOF has a greater hardness compared to VPS • Due to lower T and higher particle velocity • Presence of β particles in HVOF and not in VPS

  15. Future Work • Nano indentation • Hardness determination of each phase • TEM • More precise composition of each phase (before and after annealing)

  16. Thank you

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