Coating processes – Spray coating utilizing flame or plasma heat – Continuous feed solid coating material
Patent
1993-03-30
1994-10-18
Beck, Shrive
Coating processes
Spray coating utilizing flame or plasma heat
Continuous feed solid coating material
427453, 427540, 427576, 427579, 427580, 427600, 427189, B05D 512
Patent
active
053566742
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a process for applying a coating of ceramic material to a substrate by plasma spraying wherein the material is added to the plasma jet and includes a chemical compound, one constituent of which is a non-metallic element from the group N, C, B or from the main groups VI or VII of the Periodic Table which decomposes at least partially before reaching the melting point and is present in the solid phase in the applied state.
Such a process is known from DE-OS 30 24 611. Herein, iron spinel and cobalt spinel are applied by plasma spraying at low torch power. In view of the low torch power, it is to be assumed that only the iron spinel will melt as it has a low melting point, while the cobalt spinel will only be embedded in the molten iron spinel. Furthermore, decomposition products of the cobalt spinel will presumably be present in spite of the low torch power.
With plasma spraying at low torch power, the bonding of the applied coating to the substrate is not optimum and the internal solidity of the outer coating is also limited. Furthermore, the coating yield is also very low for when the material to be sprayed is hurled in the still solid state by the plasma jet against the substrate, an impact reflection occurs at the substrate and hence only a small part of the material to be sprayed remains adhering to the substrate.
The object underlying the invention is, therefore, to so improve a process of the generic kind that the chemical compound contained in the material can be applied stoichiometrically, i.e., undecomposed to the substrate and forms an impermeable, adherent and stable coating.
SUMMARY OF THE INVENTION
This object is accomplished in accordance with the invention in a process of the kind described at the beginning in that in addition to the material to be sprayed, the non-metallic element is carried along in the plasma jet in a free form not bound to a foreign element. In this way, by addition of the element, the reaction balance of the compound tending to decompose is shifted to the undecomposed compound and so this compound can be heated to sufficiently high tempertures in the plasma jet and hence form on contacting the substrate an impermeable, adherent and stable coating which, in addition, contains the chemical compound essentially in undecomposed form.
It has proven particularly advantageous within the scope of the inventive process for a laminar jet generated by a plasma torch with a nozzle generating a laminar jet to be used as a plasma jet as this laminar jet offers the best preconditions for the material to be sprayed to be heated constantly and uniformly and for an adequate time in order to achieve as uniform a coating as possible on the substrate.
It is particularly expedient for the length of the laminar jet to be at least 60 mm from the nozzle on. Even better results are obtained with 80 mm and very good results with a laminar jet length of 100 mm. Optimum results are to be achieved with a laminar jet length of 150 mm.
It has, furthermore, proven particularly expedient within the scope of the inventive solution for the latter to be carried out in a vacuum as problems otherwise arise from impurities or interactions owing to the atmosphere surrounding the plasma jet.
It is preferable to use as the plasma jet that of a d.c. plasma torch with which a plasma jet having as constant a temperature as possible and as constant a flow profile as possible can be generated.
To shift the reaction balance in the direction towards the undecomposed chemical compound, it is particularly advantageous for the non-metallic element to be carried along in the plasma jet in dissociated or atomic form.
In the examples of the inventive process described so far, no details were given as to how the non-metallic element is conducted in the plasma jet. It is expedient for the non-metallic element to be conducted, after its introduction, in the core region near the axis of the plasma jet so there is as intensive an interaction as possible with the plasma jet s
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Henne Rudolf
Kabs Michael
Schiller Guenter
Schnurnberger Werner
Weber Winfried
Beck Shrive
Deutsche Forschungsanstalt fuer Luft-Raumfahrt e.V.
Lipsitz Barry R.
Utech Benjamin L.
W. C. Heraeus GmbH
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