Coating processes – With post-treatment of coating or coating material – Chemical agent applied to treat coating
Reexamination Certificate
1999-07-16
2002-03-19
Meeks, Timothy (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Chemical agent applied to treat coating
C427S202000, C427S228000, C427S294000, C427S337000, C427S374400, C427S376200, C427S397700, C427S402000
Reexamination Certificate
active
06358565
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for making a protective coating containing silicon carbide, particularly a wear/tear, corrosion or abrasion protective coating on at least a portion of the surface of a substrate made from a material with a softening temperature that is above the melting temperature of silicon, where silicon is deposited on the portion of the surface of the substrate that is to be provided with a protective coating and where, under vacuum or in an inert atmosphere, the substrate is heated to a temperature above the melting point of silicon, brought to reaction with carbon that is contained in a porous coating, and, thereafter cooled down.
Such a method is known from the German Patent No. DE-A1 42 03 773, which corresponds to U.S. Pat. No. 5,318,799. According to the method known from this, to heat-treat the surface of an object, the surface to be coated is initially covered with a coat of at least one meltable material and a binding agent, and prepared in such a manner, then is exposed to a heat treatment at a temperature above 600° C. This heat treatment occurs under vacuum or in an inert atmosphere. During this process, the coat is transformed to an at least partially porous top coating with an outer crust. The temperature is then increased such that the softening temperature of the object is not violated, however the meltable effective material is heated above its melting point. This increased temperature is then maintained until the melted effective material is at least partially vaporized and in the process by diffusion has created a surface zone that forms a dense coating in the surface material of the object together with this surface material. Thereafter, the top coating is cooled down and removed from the object. With such a method, Sic coatings can be formed as protective coatings, where then Si powder and dust are applied to the object as a binding agent, a first heat treatment is performed at 700° C.-800° C. to carbonize the coat; thereafter, a heat treatment is carried out in a range of 1400° C. to 1700° C. for 1-2 hours to silicate the carbon structures. This creates an object with a Sic coating.
In addition, a vehicle's brake disk, or clutch disk respectively, is known from the German Utility Model No. DE-U1 296 10 498, where said disk is constructed of a C—C/SiC material, where the disk features an Sic coating. This Sic coating is formed by dip coating or made through vacuum impregnation.
Thin protective coatings made of SiC that are used according to the above-mentioned current state-of-the-art are brittle even at room temperature. Due to this brittleness, cracking and chipping occur such that the desired effect of this coating as a protective coating is lost. This brittleness can be observed especially when the object covered with such a SiC coating is exposed to fluctuating temperature cycles.
SUMMARY OF THE INVENTION
Based on this state-of-the-art, it is the objective of the present invention to cover an object with a protective coating that features the advantages of an SiC coating, which however, does not have the brittleness that is present with coatings according to the current state-of-the-art.
With the type of method referred to at the beginning, this in order to achieve a homogeneous protective coating of silicon carbide and free silicon, the surface of the object to be coated is initially provided with a porous carbon coating with a porosity in a range between 40 and 95%, and the porous carbon coating is covered with coating of silicon, where the ratio of the mass of the applied silicon to that of the carbon in the porous carbon coating is greater than 2.34. The substrate is heated to a temperature that is above the melting temperature of the silicon, to a maximum temperature of 1650° C. to avoid a boiling condition of the silicon. The substrate, provided with the coating that contains silicon carbide and free silicon, is cooled down to room temperature.
To form the coating subject to the invention, it is initially important to provide a porous carbon coating on the substrate to be coated. This porous carbon coating provides on the one hand the portion of carbon that will be brought to reaction with the silicon to form the subsequent protective coating containing the silicon carbide coating, and on the other hand, the rate of free silicon, that is important for the properties of the finished protective coating, can be set by selecting a porosity where a silicon surplus remains. Free silicon can then, on the one hand, be stored in the pores, and on the other hand, the rate of silicon can be set such that a thin coating that consists primarily of silicon remains on the surface of the finished protective coating. In any case, attention has to be paid to the fact that the applied silicon in relation to the carbon in the porous carbon coating, concerning the ratio of silicon to carbon in percent of mass is greater than 2.34. The structure of such a protective coating does not require organic binding agents that would otherwise compromise the purity or quality of the silicon through splitting off of decomposition products during the heating phase or through outgasing.
It has been found, that practically no wear occurs when using such a surface protective coating that contains a portion of free silicon, when such an object coated with such a surface coating is brought into abrasive contact with an organic layer, for example, with organic abrasive coatings of an abrasion unit. In addition, such a coating distinguishes itself through a very high thermal shock resistance, that is, R
1
≧500 K, where R
1
is defined as the ratio of the tensile stress (&sgr;) to E*&agr;, where E is the modulus of elasticity and &agr; the coefficient of thermal expansion of the protective coating.
To name a few preferred areas of applications, protective coatings that are formed according to the method subject to the invention are of particular advantage as wear/tear protective coatings on abrasion units such as brake disks, corrosion protective coatings on pipe-shaped heat exchanger elements or abrasion protective coatings on sliding ring packings.
It is apparent that such protective coatings do not require that the object to be coated exhibits a certain porosity, because the protective coating is applied to the object to be coated by using a porous carbon object or a porous carbon coating respectively. To enhance bonding between the substrate and the protective coating, it may be advantageous to adjust the roughness of the surface of the substrate to be coated, for example to a mean roughness of Ra=2. Precisely such a roughness value offers the advantage of a uniform and strong bond between the protective coating and the substrate.
Practically any substrate that has a temperature resistance to temperatures above the melting point of silicon can be coated with this coating. This includes ceramic substrates, substrates made of carbon, substrates made of silicon carbide, ceramic composite materials such as C/C, C/C—SiC, SiC/SiC, or metals such as tungsten, to name just the most important ones.
Suitable for the structures of the porous carbon coating are carbon felt, carbon mats, carbon weaves, carbon foils and/or carbon fleece that are placed on the substrate according to the desired thickness of the protective coating. The porosity can be specified by selecting a suitable type of the aforementioned carbon materials. The carbon coating made of carbon felt or mats is preferable, because these materials have the advantage of a rapid and complete reaction between C and SiC, particularly when the individual fibers are very thin or in an amorphous condition. Alternatively, the porous carbon coating can be created through pyrolysis of paper, wood, wood pulp and/or cardboard placed on the substrate, where the pyrolysis of these materials occurs in the furnace during the heating stage prior to melting the creating the reaction with the carbon to form silicon carbide.
The required silicon can be provided by applying parti
Henke Thilo
Krenkel Walter
Crockford Kirsten A.
Deutsches Zentrum fuer Luft-und Raumfahrt E.V.
Meeks Timothy
Milde Hoffberg & Macklin, LLP
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