A basic principle during the formation of thermal spray coatings is the phenomenon of “adiabatic shear instability” which influences splat formation, bonding or oxidation of intersplat regions, grain refinement and dynamic recrystallization as result of thermal softening gaining over work hardening upon the impact of accelerated particles of the spray feedstock. Deposits of uniform composition, low porosity and suitable bonding to the substrate are obtained by proper adjusting the window of deposition parameters with respect to the occurrence of adiabatic shear instabilities. However, a lack of knowledge on how low-ductile intermetallic compounds and hard oxide particles behave at high strain rates obtained during the thermal spray process has to be stated.

The present work shows the interfacial features of several coatings obtained at typical spray parameters. The examples include coatings formed from combinations of ductile or hard particles on hard or ductile substrates [1-4]. Electron backscatter diffraction (EBSD) was used to show the influence of adiabatic loading on the microstructural evolution. Kikuchi pattern quality maps (Fig. 1a-d) provide information about dynamic recrystallization and the occurrence of unchanged spherical particles, fragmented shells as well as particles agglomerated by sintering. EBSD kernel average misorientation (KAM) is effective in resolving grain regions subjected to deformation. KAM maps of the next and next-next neighbor pixels (Fig. 2a-d) show shear bands and dynamic recrystallization to a different degree in high-ductile aluminium (a) and low-ductile iron aluminide (b); hard layers of zirconia (c) and alumina (d) confirm expectedly marginal misorientation, but clearly indicate that the impinging particles induce different alterations of the steel substrates subjected to different pretreatments. Shear bands have been formed on the hills of the laser-cut structure; dynamic recrystallization occurred on top of the highly disturbed sandblasted surface.

1. R. Drehmann et al. (2014) Investigation of bonding mechanisms of cold gas-sprayed Al coatings on Al2O3. Mater. Sci. Eng. Techn. 45 (6): 476–485.

2. N. Cinca et al. (2015) Influence of spraying parameters on cold gas spraying of iron aluminide intermetallics. Surf. Coat. Techn. 268: 99–107.

3. P. Sokołowski et al. (2015) Advanced microscopic study of suspension plasma sprayed zirconia coatings with different microstructures. J. Therm. Spray Techn. DOI: 10.1007/s11666-015-0310-7.

4. T. Lampke et al. (2011) Alumina coatings obtained by thermal spraying and plasma anodizing — a comparison. Surf. Coat. Techn. 206: 2012–2016.

Figures:

Fig. 1 Kikuchi pattern quality maps of cold gas spray aluminium on aluminium nitride (a); and cold gas spray iron aluminide (b), suspension plasma spray zirconia (c) and air plasma spray alumina (d) on steel substrates with different pretreatment

Fig. 2 Corresponding KAM maps of cold gas spray aluminium on aluminium nitride (a); and cold gas spray iron aluminide (b), suspension plasma spray zirconia (c) and air plasma spray alumina (d) on steel substrates with different pretreatment; separations between coating and substrate are inserted for clarity, KAM between 0 and 5° according to the legend applicable for (a-d)

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EMC Abstracts - https://emc-proceedings.com/abstract/adiabatic-shear-loading-in-thermal-spray-coatings-studied-by-ebsd/