Coating processes – Coating by vapor – gas – or smoke – Metal coating
Reexamination Certificate
1999-04-21
2002-07-02
Chen, Bret (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Metal coating
C427S248100, C427S249160, C427S249170
Reexamination Certificate
active
06413585
ABSTRACT:
The present invention relates to:
thermochemically treating a carbon-containing material which can optionally have an open porosity, to generate a refractory carbide coating on the surface and within said material if it is porous, by pack-cementation (carbiding);
the use of specific alloys as a pack (cement) to thermochemically treat carbon-containing materials, which may optionally have an open porosity, in a halogenated atmosphere.
In a first aspect, the present invention proposes an efficient method of generating refractory carbide coatings on the external, and on the internal surfaces when they exist and are accessible, of carbon-containing materials. Forming this type of coating is of great importance in numerous fields since such coatings endow such carbon-containing materials with high resistance to wear, ablation, erosion, oxidation, and corrosion. Such coatings can also protect such carbon-containing materials from the diffusion of elements within them. Said coatings can also improve the moistening of carbon-containing materials by molten metals. In particular, the method of the invention has been developed for producing particularly effective thermal shields and barriers to diffusion.
The method of the invention is a pack-cementation (carbiding) method: it generates a refractory carbide on the surface (surface=external surface+, possibly, the internal surface) of a carbon-containing material from the carbon C of said heat treated material and from an element E supplied in the form of a pack (cement) in the reaction medium and transported in the form of a halide to the surface of said carbon-containing material. Said method of the invention jointly uses an E—M type donor pack alloy (more precisely xE-yM-zM′) and a solid activating compound with formula MX
n
; under conditions, notably of pressure, in which said element E can be transported (where compound MX
n
is sufficiently stable for halides MX
n
, in the solid and gas forms, to coexist with EX
n
, in the gas form). Said conditions and the nature of elements E, M, M′, X are defined below.
A pack-cementation method has already been described in French patent application FR-A-2 304 590. That method consists of treating the carbon-containing material to generate a coating of a carbide of a refractory metal at its surface:
at a temperature in the range 850° C. to 1250° C.;
at atmospheric pressure, in a hydrogen-containing atmosphere;
in the presence of a pack powder comprising an intimate mixture of the refractory metal (Ti, Zr, Hf, Ta, Nb) and a halide of that refractory metal (TiCl
4
, ZrCl
4
, etc.) or a halide which, in situ, can generate the halide of that refractory metal (ammonium halides (volatile), or cobalt, nickel, iron, or aluminium halides).
That method is carried out using the material to be coated in contact with (in) the pack powder which comprises a refractory diluent (alumina, magnesia) and chromium (catalyst), in addition to the refractory metal and the halide of that refractory metal, or a precursor thereof.
In the prior art method, the refractory metal (E) is transported by its own halide (EX
n
), introduced directly or generated in situ from a precursor of that halide (M′X
n
) of which the halogen is displaced (precursor M′X
n
does not remain solid, it is only used to generate EX
n
).
The method of the invention can be analyzed as an improvement over, or an optimization of, that prior art method. The method of the invention produces very good results with a very large range of carbon-containing materials which are non porous, or slightly porous, or even very porous (the prior art method does not enable suitable coatings to be produced in the internal portions of the treated parts, in the pores of such parts . . . the thickness of the deposit is observed to be non uniform, in particular because diffusion in the gas phase is too slow) and with a larger range of elements E including metalloids such as boron and silicon, in addition to metals such as titanium, zirconium, hafnium, tantalum, niobium and chromium. The very good results obtained with boron in particular should be mentioned. It should also be noted incidentally that the commercially available packs recommended for boriding steels do not enable boron to be transported to the surface of carbon-containing materials.
The prior art also describes the production of coatings in the internal and external portions of metal parts, by suppling a metal such as aluminium (FR-A-2 576 916 and FR-A-2 576 917). The method described in FR-A-2 576 916 employs a gas phase flowing between the inlet and outlet of a reactor. To transport the supplying metal (E), the method described in FR-A-2 576 917 uses a solid halide of that supplying metal (EX
n
=AlF
3
, CrF
2
or CrCl
2
) or a solid alkali halide (NaX, KX, for example) (in contrast to the method of the present invention which uses a halide of a metal M alloyed to E, of the type MX
n
). In any case, the methods of FR-A-2 576 916 and FR-A-2 576 917 were developed in a completely different context to that of the present invention. Those methods generate coatings which are not carbides. Generation of those coatings does not use carbon migration within the treated material.
The present invention proposes an efficient method of thermochemically treating non porous, slightly porous or even highly porous carbon-containing materials (in other words, carbon-containing materials which may optionally have open porosity) in a halogenated atmosphere (pack-cementation) to generate refractory carbide coatings at the surface of said materials (surface=external surface+optionally internal surface). Said method can produce coatings with a morphology that is regular (particularly in terms of thickness and nature of the phases) which in the case of porous materials can be measured in terms of uniformity (the ratio, expressed as a %, between the thickness of the coating over the central zone of the treated material and the thickness of the coating on the external surface of said treated material). Said method, controlled, can generate coatings with uniformity of more than 70% in some types of carbon-containing materials with a large open porosity. Such results could not be obtained using the methods of the prior art, where the uniformity of the coatings hardly ever exceed 10%.
The method of the invention comprises maintaining the carbon-containing material:
a) at a temperature in the range 700° C. to 1300° C.;
b) at a reduced pressure, in the range 0.1 kPa to 30 kPa, of hydrogen, a rare gas or a mixture of such gases;
c) in the presence of a donor pack constituted by at least one element E selected from titanium, zirconium, hafnium, tantalum, niobium, chromium, silicon, and boron, alloyed to an element M selected from aluminium, calcium, chromium, yttrium, and magnesium, and optionally alloyed to a moderator element M′; said moderator element M′ being necessarily used if E=M=Cr and then being other than chromium;
and a solid activating compound of low volatility at said treatment temperature, with formula MX
n
, where X consists of chlorine or fluorine, advantageously fluorine (and n, a whole number, corresponds to the valency of element M).
According to the invention, pack-cementation is carried out at reduced pressure using element E (to be transported and to react with the carbon of the material to generate the expected carbide) alloyed to an element M, on the one hand and a halide (chloride or fluoride, preferably fluoride) of the same element M, of low volatility, present in the solid form, on the other hand.
The pack used contains at least one element E alloyed with an element M. Said pack generally contains a single element E but a plurality of elements is not excluded. It is recommended that boron and silicon are used jointly, to generate a coating constituted by the carbides of these two elements, endowing the treated material with improved resistance to oxidation over a wider temperature range than that provided by a boron carbide. Said
Goujard Stephane
Madec Yves
Robin-Brosse Christian
Thebault Jacques
Vandenbulcke Lionel
Chen Bret
Societe Nationale d'Etude et de Construction de Moteurs d&a
Weingarten Schurgin, Gagnebin & Lebovici LLP
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