Use of Different Process Gases for Manufacturing Isolating Alumina Coatings by Flame Spraying with Cords

AbstractBesides conventional industrial demands, thermally sprayed coatings are increasingly used for innovative products. Such an application is the additive manufacturing of electrical components in automotive engineering. In particular, heating units are currently manufactured by a combination of various spray technologies. At present, simpler spraying processes like flame spraying are investigated with regard to their suitability as a future cost-effective alternative for fabricating isolating alumina coatings. In the present study, alumina cords were flame-sprayed using compressed air and argon as atomizing gases. The results demonstrate finely dispersed microstructures and a more regular and partially even higher surface and volume resistivity compared to past investigations in the literature as well as conventionally plasma-sprayed coatings despite a significantly reduced coating thickness. The content of alpha phase is clearly higher than for plasma-sprayed coatings, regardless of the atomizing gas used. Moreover, flame-sprayed coatings using argon reveal a higher resistivity in comparison to coatings sprayed with air. While the atomizing gas is found to mainly influence the ideal stand-off distance, the phase composition is not changed severely. In addition to the phase composition and kinematics, it can finally be concluded that humidity plays a major role in the coating properties.

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Use of Different Process Gases for Manufacturing Isolating Alumina Coatings by Flame Spraying with Cords

10.31399/asm.cp.itsc2021p0648 ◽

2021 ◽

Author(s):

M. Hauer ◽

M. Meyer ◽

D. Billieres ◽

C. Bricquet ◽

F. Gerstgrasser ◽

...

Keyword(s):

Volume Resistivity ◽

Alpha Phase ◽

Flame Spraying ◽

Flame Spray ◽

Alumina Coatings ◽

Spray Coatings ◽

Aluminum Hydroxides ◽

Fine Microstructure ◽

Plasma Sprayed ◽

Aluminum Substrates

Abstract This study assesses the quality of flame-sprayed alumina coatings produced from recently developed alumina cord using argon and compressed air as atomizing gases. Coatings of different thicknesses were deposited on aluminum substrates and then analyzed using optical microscopy, X-ray diffraction, and resistivity measurements. The coatings, particularly those sprayed with argon, had fine microstructure and higher surface and volume resistivity than flame-spray coatings made from alumina cord in the past. They were also found to have higher alpha phase content than plasma-sprayed coatings, regardless of the atomizing gas used. The effect of humidity and the possible formation of aluminum hydroxides are also addressed.

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Damage Mechanisms in Plasma-Sprayed Alumina Coatings

10.1115/imece2000-2690 ◽

2000 ◽

Author(s):

Rodney W. Trice ◽

David W. Prine ◽

Katherine T. Faber

Keyword(s):

Crack Nucleation ◽

Alumina Coatings ◽

Damage Mechanisms ◽

Cyclic Compression ◽

Loading Direction ◽

Transmission Electron ◽

Small Cracks ◽

Plasma Sprayed Coatings ◽

Plasma Sprayed ◽

Sprayed Coatings

Abstract Al2O3 coatings produced by plasma spray have been considered for wear resistance and corrosion protection. Mechanical investigations of these coatings are often performed when the coating is still on the metallic substrate, hiding the intrinsic response of the coatings and the lamellae that make up their microstructure. The development of a compression test for stand-alone coatings will be described. Cyclic compression loading with monotonically increased peak stresses was employed to study the deformation and damage of the coatings. Transmission electron microscopy and acoustic emission were also used to identify damage mechanisms that ultimately lead to failure. It is proposed that numerous defects that exist in plasma-sprayed coatings, including porosity and microcracks, serve as sites for crack nucleation and/or propagation. As these small cracks extend subcritically under an applied stress that ultimately propagate parallel to the loading direction along inter-lamella boundaries. With increasing stress, these cracks ultimately link resulting in catastrophic failure.

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Alumina Coatings by Plasma Spraying of Monosize Sapphire Particles

Thermal Spray 1998: Proceedings from the International Thermal Spray Conference ◽

10.31399/asm.cp.itsc1998p0791 ◽

1998 ◽

Author(s):

L.C. Erickson ◽

T. Troczynski ◽

H.M. Hawthorne ◽

H. Tai ◽

D. Ross

Keyword(s):

High Hardness ◽

Processing Parameters ◽

Raw Material ◽

Splat Morphology ◽

Alumina Coatings ◽

Spray Coatings ◽

Dense Microstructure ◽

Plasma Sprayed Coatings ◽

Plasma Sprayed ◽

Sprayed Coatings

Abstract In an effort to obtain a series of plasma sprayed coatings of controlled microstructure, three mono-sized sapphire powders were deposited using an axial injection torch in which the plasma gas composition and nozzle diameter were the only processing parameters varied. The effects of changes in these parameters on the resultant coating splat morphology, porosity, angular crack distribution and hardness are reported. The uniform, dense microstructure and high hardness of 14 GPa - a level usually only associated with chromia thermal spray coatings - of the best alumina coatings results from use of the combination of tightly controlled processing conditions and mono-disperse precursor powders. The microstructural quality of plasma sprayed coatings and, hence, their properties can be improved significantly by minimizing variations in processing and raw material parameters.

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