In spraying, sometimes only to allow or only need to spray a material on the parts. As in the copper matrix coating on copper and its alloys, only allowed to be used for copper and copper alloy powder spraying; if the first spraying a layer of nickel aluminum but is not desirable, because the thermal conductivity of copper matrix, painted on the nickel coated aluminum, not self-adhesive coating. But in most cases, the coating of a single material can not meet the requirements, often need to use a variety of materials, composite coating system. Commonly used composite coating systems include: multilayer composite coating, powder coating, composite powder coating.
1 multilayer composite coating and mixed powder coating
In the high temperature working environment, it is often necessary to use high temperature stability and heat resistance of high temperature oxide coating, such as alumina, zirconia, magnesium and magnesium as a coating material. But it has many shortcomings: (1) combined with the strength of oxide and metal is low; (2) oxide coatings often contain 10 to 15% holes, and which is part of the through hole, the medium can produce effect through the stomata and the substrate; (3) oxide expansion coefficient is low, does not match with the metal. Thermal shock performance is not good. Therefore, it is widely used in the oxide coating to apply the underlying method to make up for these shortcomings. In some cases, the bottom layer is used to prevent the interaction between the surface layer and the substrate. For example, in order to prevent the tungsten coating and the graphite matrix to generate tungsten carbide, the first coating on the graphite substrate as the bottom.
On the bottom of the requirements are: (1) it and the matrix and subsequent coating with good; (2) coating density, good oxidation resistance; (3) between the expansion coefficient between the substrate and the subsequent coating, in order to reduce the thermal expansion coefficient caused by different stress. Commonly used materials are Mo, Ni-Al and other self bonding materials. The application of Ni-A1 is more extensive, it has the advantages: it can bond with a variety of materials, the combination of high strength, the coating itself is dense, good oxidation resistance. But it does not apply to copper alloy and other materials.
In order to improve the thermal shock resistance of ceramic thermal barrier coatings, usually with multilayer composite coating; metal matrix to gradually transition to the ceramic surface, the composite layer, the three layer is the underlying metal, ceramic and metal transition layer and ceramic layer, metal ceramic layer can be used in three ways. The first is the metal and ceramic powder mixture spraying, this method is relatively simple, but also feasible. But because of the difference of the two kinds of powder proportion, mixed powder in the powder feeding process becomes uneven, and the two kinds of powder melting point, specific heat, thermal conductivity difference, to make these two kinds of powder are fully melted, it must strictly control the size of two kinds of powder respectively. The second kind is to send two kinds of metal and ceramic powder into flame flow through two powder feeding ports, respectively. The upper reaches of the high melting point and low thermal conductivity of the ceramic powder from the flame flow into, in order to increase the contact time of powder and high temperature area, so as to obtain sufficient heating; easily melting metal powder from the flame flow downstream into the metal powder, can prevent overheating and evaporation. The third is the use of the complex powder, such as nickel coated alumina, etc.. Each of the alumina powder in the powder is coated with a layer of nickel, so the coating composition is relatively uniform and constant, and does not generate segregation. The disadvantage is that the cost of powder is higher.
Recently, the thermal barrier coatings have been used in the thermal barrier coating. For the manufacture of powder feeding system with a plurality of powder feeding systems, the two groups of nickel cobalt chromium aluminium yttrium and zirconium oxide are divided into continuous changing step coating layer. The thermal shock resistance and oxidation resistance of the coating system are better than those of Ni-A1 (or Ni-Cr), Ni-A1 (or Ni-Cr) +ZrO2 and ZrO2 three layers coating systems, which can be used in the more important aviation engine components.
2 composite powder coating
The appearance of the composite powder significantly expands the application range of the coating material, and improves the performance of the coating. It has the following unique properties:
(1) according to the needs, using different fabrication methods, coatings were prepared with different performance requirements, especially with metal or alloy made of non metal ceramic composite powder coating, it is difficult to reach other methods;
(2) the unity of the heterogeneity of the single particle and the uniformity of the whole powder. The resultant surface coating has a uniform comprehensive physical and chemical properties. Therefore, in the process of storage and use, will not appear segregation tendency, so as to avoid the defects of the coating quality caused by uneven;
(3) the core material is protected by coating or coating powder, can avoid or reduce the thermal spraying process, oxidation loss, caused by the high temperature flame effect element of carbon loss and volatilization phenomenon, so as to obtain high performance coatings;
(4) select the appropriate composite powder was made, so that the flame temperature of the thermal spraying powder, chemical reaction can occur between the components, each component of the powder melting point than to generate a higher melting point intermetallic compounds, accompanied by a lot of heat release, the powder and the substrate surface auxiliary heating, dense the coating;
(5) compared with the spraying composite powder, the coating performance is excellent, and the spraying rate and deposition efficiency are much higher, so the labor productivity can be improved.
