The invention relates to making continuous coating on the surface of a part.
The field of application of the invention is more particularly making a protective coating on parts made of a material that is sensitive to corrosion by oxidation and/or to forming a coating for sealing parts made of a refractory porous material.
The materials concerned are typically carbon, graphite, and thermostructural composite materials such as carbon/carbon (C/C) composites and ceramic matrix composite with carbon fiber reinforcement (e.g. C/SiC composites) or with ceramic fiber reinforcement (e.g. SiC/SiC), in which case carbon can be present in an interphase between ceramic fibers and the ceramic matrix.
Depending on the intended use, forming continuous coatings on the outside surfaces of parts made of such materials is necessary in order to ensure they behave well in an oxidizing medium at high temperature, and/or in order to seal their surfaces, given that materials such as carbon, graphite, and above all thermostructural composites, all possess internal porosity to a greater or lesser extent.
A coating layer may be formed by a gaseous technique, possibly combined with a liquid technique (a combined technique).
The gaseous technique consists in chemical vapor infiltration or deposition. A gas containing at least one precursor for the material of the coating that is to be made is brought into contact with the part under conditions of temperature and pressure that encourage the formation of said material by decomposition of the precursor or by reaction between a plurality of components thereof. With chemical vapor infiltration, the coating material is likewise deposited within the pores of the material of the part, and a surface coating is formed after the pores have been filled in, at least in the vicinity of the surface.
The liquid technique consists in coating the surface of the part in a liquid that is a precursor of the coating material, typically a resin, possibly together with a solvent and a filler. After cross-linking, the precursor is transformed by applying heat treatment. The coating is finished off by using a gaseous technique.
In order to form the coating, the part is usually placed on one or more supports with limited contact between the part and the or each support.
For example, with a coating made by chemical vapor deposition or infiltration, the material of the resulting coating 1 is deposited both on the part 2 and on the supports 3, as shown very diagrammatically in FIG. 1A, and in more detail in FIG. 1B. When the part is separated from the support, the rupture which occurs leaves an unprotected zone 4 (FIG. 1C) in particular when the material of the part, such as graphite, is not as strong as the material of the coating such as silicon carbide (SiC).
Such a defect is unacceptable since it leaves an access path for oxygen in the ambient medium to attack the material of the part.
The solution that is generally adopted consists in making a second coating layer 5 (FIG. 1D) by putting the part back on the support after turning it over.
Making two deposits in succession represents substantial extra cost. In addition, the same risk of damage is still possible when separating the part from the support, after the second deposition operation.
A similar difficulty is encountered when the coating of the part needs is to be performed by a liquid technique over the entire outside surface.