Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2001-07-13
2004-05-04
Oltmans, Andrew L. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S242000, C148S280000, C427S190000, C427S191000, C427S192000, C427S205000, C427S405000, C427S419100, C427S419700
Reexamination Certificate
active
06730178
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a coating method (or bonding method) which is useful in coating a base material with an intermetallic compound or bonding base materials to each other with an intermetallic compound and relates to a coating apparatus therefor. More particularly, the present invention is concerned with a technology which can be applied to, for example, a corrosion resistant coating provided on a turbine blade of aircraft engine, a heat resistant, wear and abrasion resistant coating provided on an exhaust valve of automobile engine and a corrosion resistant, oxidation resistant coating provided on an exhaust system of garbage incinerator and further can be applied to a buildup welding for repairing parts thereof.
BACKGROUND OF THE INVENTION
It is inevitable to develop a light material of high heat resistance as a structural material for use in heat engines accommodated in next-generation automobiles, marine vessels, aerospace equipment and other transport means and also accommodated in thermomotor of energy plants. Intermetallic compounds such as aluminides expected as providing a heat resistant material of high strength are materials which can meet the above requirements, and investigations for putting the intermetallic compounds to practical use, including those on the composition, texture optimization, performance enhancement and processing method for obtaining a desired structure, are being conducted in a number of countries.
It is common practice to coat the surface of a metal material with an intermetallic compound in order to impart a corrosion resistance and a wear and abrasion resistance thereto. However, the principle and method of simply obtaining a coating of intermetallic compound have not yet been established. As the method for obtaining an intermetallic compound, there can be mentioned the diffusion process, the flame spraying process and the combustion synthesis process.
The diffusion process is a process that after another type of metal which can react with the base metal to thereby form an intermetallic compound is applied to the surface of a base material metal by, for example, vapor deposition, the base material is heated and maintained in an elevated temperature, and whereby a reaction layer is formed on the base material surface. In the diffusion process, the diffusion, namely ultralow-velocity mass transfer phenomenon is utilized. Therefore, although there is such an advantage that controlling of the coating thickness of reaction layer is easy, there occurs such a disadvantage that the period of time required for coating formation is as extremely long as several hours to tens of hours. Accordingly, the attained coating thickness is restricted to a small one such as tens of microns or less. Further, the processing is performed at high temperature, so that the effect thereof on the base material cannot be disregarded.
The flame spraying process is a method that, as disclosed in, for example, Japanese Patent Laid-open Publication No. 2000-96206, a layer of alloy containing a specified type of element is piled on a base material by vacuum plasma spraying and thereafter aluminum is thermal-sprayed over the deposited alloy layer so that a coating layer of intermetallic compound is formed by a metallurgical reaction accompanied by heat buildup of aluminum. It is advantageous in that extreme energy consumption is not required because an exothermic reaction can be induced at a relatively low temperature within a relatively short period of time, as compared with the process of directly melting an intermetallic compound at high temperature. However, this process necessitates a large heat source of, for example, plasma, electric arc or laser. The flame spraying process poses such an inherent problem that voids are likely occur in the coating because of the adherence between the coating layer and the base material and because of the entrainment of ambient gas at the flame spraying. Furthermore, the thickness of the coating layer formed by the above process only ranges from about tens of microns to hundreds of microns, even if the alloy layer and the aluminum layer are summed up, because of spraying workability and cost, etc. As a result, there is the danger of cracking or local exfoliation caused by the nonuniformity, brittleness, etc. of formed coating.
The self-propagating high temperature synthesis process, or combustion synthesis process is a process wherein a reaction of compound formation spontaneously proceeds with the generation of high heat of reaction within a short period of time. The process is characterized in that a high-melting-point ceramic or intermetallic compound can be easily synthesized. Since the principle of the combustion synthesis process was discovered by Meljarnov, et al. of the old Soviet Union in 1967, theoretical researches and application developments have been promoted (see Journal of the Metallurgical Society of Japan, vol. 32, No. 12, page 845). The combustion synthesis process is a process that a green compact of a mixture of different types of metals capable of forming an intermetallic compound is placed on a base material surface and the base material is heated under pressure so that a combustion synthesis reaction is induced to thereby accomplish not only formation of an intermetallic compound but also bonding of the intermetallic compound to the base material. The combustion synthesis process, because a self-exothermic reaction is utilized, has such an advantage that the synthesis of coating layer and the bonding thereof to the base material can be simultaneously performed within a short period of time. However, because of a balance between the exothermicity at the synthesis and the heat transfer to the base material, the thickness of the coating layer is limited to large one such as several millimeters or more.
Moreover, the intermetallic compound such as NiAl or TiAl can be utilized at high temperatures such as 800 to 1000° C. and exhibits superior properties as a high-temperature material. Therefore, development researches for the application thereof to engine parts, aerospace mechanical parts, etc. are being promoted, but, despite the excellent properties, ductility required for molding and working cannot be obtained to thereby inhibit the practical use thereof. The intermetallic compound has a high formation energy of about 100 kJ per mol, and the combination of metals is accompanied by an exothermic reaction of high temperature such as 1500 to 2000° C. to thereby enable spontaneous proceeding of a synthetic reaction. Therefore, the combustion synthesis process is one of energy saving not requiring any external heating at high temperature, but controlling of reaction propagation, temperature and texture is difficult. As a result, many problems remain unsolved on the application of intermetallic compound to parts requiring heat resistance. For example, in the conventional combustion synthesis process, different types of metals are mixed together and used in the form of a green compact, so that the different types of metals are simultaneously heated at the same temperature. Consequently, the melting point of one of the metals automatically becomes the reaction initiation temperature, so that the reaction initiation temperature, thus the exothermic temperature resulting from the reaction, and the melt depth of the base material cannot be controlled. In the event of a thin coating layer, it may occur that the bonding strength cannot be enhanced.
As apparent from the above, although the coating of base materials with the intermetallic compound can be effected by, for example, the diffusion process, the flame spraying process and the combustion synthesis process, numerous steps and much energy consumption are inevitable because of the property of the intermetallic compound that working thereof is extremely difficult. Thus, coated members (for example, buildup welded members) cannot be obtained with less energy consumption at low cost within a short period of time.
Therefore, it is an object o
Matsuura Kiyotaka
Miyamoto Yoshinari
Teramoto Toshio
JSR Corporation
Oltmans Andrew L.
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