Residual stress and adhesion of thermal spray coatings: Microscopic view by solidification and crystallisation analysis in the epitaxial CoNiCrAlY single splat

Abstract

A new approach is proposed to achieve an in-depth understanding of crystallisation, residual stress and adhesion in epitaxial splats obtained by Combustion Flame Spray. Modelling of the fundamental process mechanisms is achieved with the help of experimental observations providing details with a sub-micrometre spatial resolution. At this scope, High Angular Resolution Electron Backscatter Diffraction and Transmission Electron Microscopy analysis are employed to provide insights into crystallisation and residual stress levels, while FIB-milled microcantilever beam bending is used for fracture strength measurements in the case of single splats. A comparison to fully-developed coatings is achieved by employing the X-ray Diffraction sin 2 ψ technique and pull-off methods for residual stress and fracture strength, respectively. The methodology is applied to metallic CoNiCrAlY material sprayed onto a Ni-based superalloy substrate. The establishment of different crystallisation regions: epitaxial and polycrystalline, is the result of variations in the heat flux direction at the solidification front. Significant dislocation density is also reported, indicating the relevance of impact dynamics and plastic deformation mechanisms. The comparison with fully-developed coatings suggests a reduction in inter-splat bonding at splat overlapping.

Effect of Microstructural Modifications on the Corrosion Resistance of CoCrFeMo0.85Ni Compositionally Complex Alloy Coatings

“…[22] on the same alloy class show that only FCC, σ and µ phases are the predicted stable phases at room temperature for the specific composition under analysis and a BCC stability region appears only for mole fractions of Mo higher than~0.4 and high temperatures (>1500 • C). The appearance of BCC phase, at room temperature, in the coatings produced by using thermal spray in this work would thus suggest that a metastable BCC phase is generated at high temperature in Mo-rich regions (created by local-solute segregation during solidification) and then retained after the material has solidified, due to the extremely high solidification rate characteristic of the thermal spray technique [23]. For the MA coating, a small peak corresponding to the BCC phase of the CCA alloy appears overlapped with a shoulder containing contributions from the BCC CCA phase and σ/µ phases (Figure 4).…”

Hydrogen in Aluminium-Coated Steels Exposed to Synthetic Seawater

“…As the coating process introduces mechanical stresses (due to high cooling rates and thermal expansion mismatch between the aluminium and steel), there may be deformation-related defects (e.g., dislocation arrays, interparticle boundaries, etc.) in these coatings [56]. Because of the above, the amount of hydrogen in TSA-coated steel is significantly more than expected.…”

Corrosion Behaviour of a Cr2O3 Coating on Mild Steel in Synthetic Mine Water

“…The Cr 2 O 3 coating exhibited cracks (Figure 3(b)) due to residual stress during solidification [31] and cooling of the coating [32]. Additionally, there were pores observed between the top and bonding layers (Figures 3(b) and 3(c)) resulting from unmelted or partially melted particles, attributed to the difference in melting points of the powders (Cr 2 O 3 : 2435 °C [33] and NiCrAlY: 1356 °C [31], as reported by Abbasi et al [11], Nu et al [4], and Gerald et al [34].…”