Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2001-08-27
2004-01-06
Smith, Matthew (Department: 2825)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S762000, C438S763000, C438S785000, C438S793000, C438S794000, C428S620000
Reexamination Certificate
active
06673709
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the formation on a metal substrate of a protective coating of the aluminide type incorporating at least one reactive element.
The field of application of the invention is that of the production or repair of metal components which, because of their use at high temperatures and in an oxidizing medium, must be provided with a protective coating.
The invention is especially, but not exclusively, applicable to gas turbine components, in particular to components of the hot parts of turbojets.
To optimize their operation, it is endeavoured to make gas turbines, especially turbojets, operate at the highest possible temperatures.
The components exposed to these temperatures are usually made of a refractory metal alloy, or superalloy, based on nickel or cobalt.
In order to improve their high-temperature behaviour, in particular their corrosion and oxidation resistance, it is well known to form a protective coating on the superalloy metal substrate.
Among the constituent materials of such a protective coating, aluminide-type coatings, which especially allow the development of a protective alumina film on their surface, are commonly used.
Aluminization by cementation is the technique used most often to form aluminide-type coatings. This technique generally consists in placing the metal substrate in a closed chamber containing a cementation agent and in raising the assembly to a temperature usually between 900°C. and 1150° C.
The aluminide-type coatings can be used by themselves, or in combination with an external coating forming a thermal barrier, such as a ceramic coating. In the latter case, the aluminide-type coating constitutes a bond coat between the substrate and the external coating, attachment of the latter being favoured by the presence of the alumina film forming an adhesion layer.
To increase the lifetime of the alumina-film-generating aluminide and to limit its deterioration by spalling it is known to incorporate into the aluminide-type coating at least one reactive element usually chosen from the group consisting of zirconium, yttrium, hafnium and the lanthanides.
Such a reactive element reinforces the diffusion barrier function with respect to elements of the metal substrate which are liable to affect the alumina film, and it therefore favours the integrity and the persistence of the latter. The presence of the reactive element also results in a reduction in the rate of oxidation of the metal substrate and prevents the segregation, which is highly undesirable, of sulphur at the interface with a ceramic external coating.
Various processes have been proposed for forming an aluminide-type coating incorporating a reactive element.
A first type of known process consists in alloying or combining separately the reactive element with one or more constituents of the coating and in forming the latter by a process involving physical deposition on the metal substrate.
For example, reference may be made to the document U.S. Pat. No. 4,055,705 which describes the formation of a bond coat by the plasma spraying or sintering of NiCrAlY or depositing it using another physical technique. Reference may also be made to the document FR 96/15257 which describes the deposition, by electrophoresis, or in the form of a paint with a thermally degradable or volatile binder, of an MCrAlY (M being Ni and/or Co and/or Fe) alloy powder on a metal substrate. Electroplating an alloy containing a metal of the platinum group is then carried out before heat treatment and possible aluminization. Reference may also be made to the document U.S. Pat. No. 5,824,423 which, although envisaging the initial deposition of a reactive element on a metal substrate by physical vapour deposition followed by aluminization, preferably indicates the formation of a bond coat by the plasma spraying of an MAlY (M being Ni and/or Co and/or Fe) pre-alloyed powder.
These types of known processes require a further step of adding the reactive element to an alloy. This may require major investment.
Reference may also be made to the documents SU 1 527 320 and SU 541 896 which describe the application to the surface of a metal substrate of a suspension containing aluminium and zirconium powders and a binder, such as a varnish in solution, in order to obtain a protective coating after drying and heat treatment.
However, the handling of elements such as zirconium in divided form is particularly difficult because of the high risk of spontaneous reaction with the air.
A second type of known process consists in forming an aluminium coating incorporating a reactive element by chemical vapour deposition (CVD). Reference may be made to the document U.S. Pat. No. 5,503,874 which describes the alternating deposition of an aluminium layer and a metal oxide layer, such as yttrium oxide, zirconium oxide, chromium oxide or hafnium oxide, from organometallic precursors. A heat treatment allows the oxide to be reduced by the aluminium. Reference may also be made to the document U.S. Pat. No. 5,989,733 which describes the formation of a coating by the chemical vapour deposition of the elements Al, Si, Hf and possibly Zr, or another reactive element, preceded or followed by the electroplating of Pt, in order to obtain a modified nickel aluminide.
These types of known processes require the use of a chemical vapour deposition plant, which is expensive both in terms of investment and maintenance.
A third type of known process makes use of the aluminization technique, but by modifying it with the incorporation of the reactive element into the cementation agent. Reference may be made to the document FR 2 511 396 which proposes the use of a cementation agent containing aluminium, an aluminium alloy, an activator salt and a reactive element.
SUBJECT AND SUMMARY OF THE INVENTION
It is an object of the invention to provide a process allowing an aluminide-type coating incorporating at least one reactive element to be formed on a metal substrate in a simple and inexpensive manner.
This object is achieved by the fact that, according to the invention, the process comprises the steps which consist in:
introducing the said reactive element to the surface of the metal substrate in the form of a powder of the oxide of the reactive element; and
then forming the aluminide-type coating.
Introducing the reactive element in the form of a powder of the oxide of this element makes it possible to avoid difficulties in handling a powder of the reactive element.
The reactive element may be introduced to the surface of the metal substrate by coating with a composition containing the powder mixed with a liquid, or by spraying such a composition, or by spraying the powder on the substrate so that it becomes encrusted in its surface, or else by electrophoresis.
The process according to the invention is noteworthy in that, despite introducing the reactive element in pulverulent form, an aluminide-type coating is obtained whose microstructure and effectiveness are completely comparable to those of the similar coatings of the prior art, whereas the method of implementation of the process proves to be particularly advantageous.
This is because the process does not require expensive equipment to be installed or maintained.
The reactive element is furthermore introduced as close as possible to the metal substrate, thereby optimizing the efficiency between mass of reactive element involved and doping of the coating thus formed.
In addition, it is possible for the mass of reactive element introduced to be controlled precisely and over a very wide range.
Furthermore, the process allows the reactive element to be introduced into localized regions of the surface of the substrate, for example for the purpose of repairing a protective coating. This is not possible with the processes of the prior art, in which the reactive element is deposited in the gas phase or incorporated in a cementation agent.
The aluminide-type coating may be formed by aluminization after introducing the reactive element to the surface of the substrate. No modification to t
Jaslier Yann
Martinez Alain
Ntsama Etoundi Marie-Christine
Oberlaender Guillaume
Lee Grandvill D.
Smith Matthew
SNECMA Moteurs
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