Chemical Vapor Deposition (CVD) coating has been used for many years and several techniques have been developed, including:
- Pack cementation: the earliest method used for blades. In a powder mix, the surface of the part is enriched with aluminum under the formation of an aluminized surface layer.
- Out-of-pack (vapor phase): In this process, developed later, the powder mix and part are separated. This created the possibility for internal coating of cooling passages.
- Real CVD: Low-pressure system developed for the optimal coating of internal passages. In this system, the active aluminum compounds are produced outside the coating furnace and coating is performed under low pressure.
Two different aluminide coatings can be produced with the real CVD process: high-activity and low-activity coating. The distinction is caused by the difference in diffusion behavior:
- For high-activity coating with more than 50 at% Aluminum (Al) (=29 weight%), the coating is formed by inward diffusion of aluminum
- For low-activity coating with less than 50 at% Aluminum (Al) (=29 weight%), the coating is produced by outward diffusion of nickel.
The properties of this type of CVD coatings are highly dependent on the chemistry of the base material, especially for high-activity coatings.
To improve their properties, aluminide diffusion coatings are modified with other elements.
In Platinum (Pt)-modified coatings, Pt is first applied by electroplating followed by a diffusion treatment and a diffusion coating. This creates a PtAl-rich outer surface which is much more thermally stable and which provides better corrosion and oxidation properties.
There are two different types of PT-modified coatings:
- One-phase PtAl coating has an outer layer with only one PtAl phase. The Al content is low (22 - 25%) and is produced with a low activity aluminum diffusion process (mostly vapor phase). This type offers better oxidation resistance at higher temperatures and is very suitable as a bond coat for EB-PVD thermal barrier coatings.
- Two-phase PtAl coating has a mixed PtAl/NiAl outer layer with a higher Al content. The coating is mostly produced with a high- or medium-activity Al diffusion process. This type has better resistance to hot corrosion.
Silicon (Si) can be applied first using a diffusion process, or it can be co-deposited using a slurry or a diffusion process. This type of coating has better resistance to hot corrosion.
Chromium (Cr) can be applied using a separate CVD process followed by an aluminum diffusion coating. This mixed coating has better corrosion resistance.
In many locations worldwide, Sulzer has both the equipment and the expertise to coat your components for wear and corrosion resistance. We help make your equipment more reliable and improve its performance.