SiC3 coatable materialsPosted on:
Thermic Edge Coatings (TEC) deposits a high purity silicon carbide coating on various materials. The cubic, SiC3, coating has excellent corrosion protective properties at low, medium and high temperature. The coating finds application in semiconductor industry, LED and solar production and aerospace.
Materials coated are graphite, carbon composites, various ceramics and some refractory metals. Other materials are not advised as those react with the gas fed into the reactor and/or have incompatible thermal expansion which results in cracking and delamination of the coating.. For example, titanium will react with hydrogen at high temperature (above 600 oC) and steel will react with the silicon compound used to form silicon carbide. To maintain the high standards of purity of the coating, materials should be of the highest purity or purified before coating.
Graphite is used in many high temperature applications for its chemical and mechanical stability and thermal and electrical characteristics. In semiconductor applications these characteristics and compatibility with silicon and other materials are used extensively. However, when graphite is used at high temperature or i.e. in plasma environments it is not chemically stable and can react with the gas environment to form compounds which interact with the process. Graphite is also a very porous material and can store compounds in the pores which can interact with the process.
To avoid interactions between the graphite surface and seal the porosity of the graphite it is coated with SiC3. In this way the characteristics of graphite can be combined with chemical stability of SiC3. Due to the SiC3 process the coating diffuses into the graphite surface and results in a very well adhering layer when the right graphite is selected. For high end applications such as semiconductor industry TEC chooses isostatic graphite qualities from a wide range of suppliers.
As an independent supplier TEC is able to select from a wide range of suitable graphite types from various suppliers such as GrafTech, Ibiden, Mersen, Morgan, SGL and Tokai Carbon. TEC can use various graphite machining workshops with experience in handling parts for semiconductor applications.
For some applications extruded graphite can be used and coated with SiC3. In most cases the thermal expansion of the graphite is lower than SiC3 coating but the porosity of the extruded graphite is high resulting is well adhering layers and the formation of a smooth transition from graphite to coating.
2. Carbon composites
A large group of materials based on carbon fibres are referred to as carbon composites. It covers a wide range of materials with various characteristics based on the production process. What the materials have in common is the carbon fibre which is moulded (short fibres) or woven (long fibres) into various structures and impregnated to densify the structure. The properties of the fibre vary very much in the length of the fibre and perpendicular to the fibre. Especially the thermal expansion differs and makes it very difficult to coat a composite structure without cracking and/or delamination.
TEC has gained some experience with coating carbon composite parts developed for high temperature applications. Due to the large difference in thermal expansion and variations in different directions cracks appear. A good example is given below. The width of the crack in horizontal direction is much larger than in the vertical direction indicating a different orientation of the fibre. Once the material is heated and close to the SiC3 deposition temperature most of the cracks will be closed and could protect the underlying material.
Another example which is very usable in high temperature furnaces is coating insulation material with SiC3. Even though the penetration depth of the coating is good it will mainly be present near the surface due to the very large surface area. An example of fibre based insulation material coated with SiC3 is shown below.
The most promising ceramics to be coated with SiC3 are the silicon based ceramics such as SiC, SiSiC, Si3N4 etc. The reason being that the thermal expansion of those materials fits very well with the SiC3 coating. The purpose of a top coating on those ceramics is to improve corrosion resistance and to block diffusion of impurities from the base material. In semiconductor industry SiSiC boats and other parts are mostly coated by the ceramics supplier themselves. In that case the coating prevents preferential attack and erosion of silicon from the base material.
So far alumina has proven difficult to coat as well as quartz due to its very low thermal expansion.
4. Refractory metals
Tungsten and molybdenum have been coated succesfully with SiC3 coating. Depending on the dimensions and shape up to 250 µm can be applied. The coating adheres very well and the system has a long-term stability at high temperature. The SiC3 coating can prevent oxidation of the underlying base material in oxidisng environments.