CVD coatings: from cutting tools
CVD has been a prime coating technique making a significant impact in the cutting tool and aerospace fields where the technique has been used to form numerous types of protective coatings. While many of the products served by this technology are mature, it will remain in usage for decades because of its versatility and its ability to provide an inexpensive means of molecular forming. This controlled deposition of matter on an atomic or molecular basis can deposit coatings uniformly on relatively complex shapes compared with other chemically non-reactive coating processes. Since no single material can function over the extreme operating conditions required for cutting tools and aerospace alloys, CVD has been used to enhance new material development where improved properties are needed. It provides the ability to protect metals from hostile environments. The protective coatings provide a solution for minimizing the effects of wear, erosion, corrosion, and high temperature oxidation. These coatings have been selected to complement the substrate material so that the combined composite system properties can satisfy a particular set of operating conditions. The dramatic changes achieved in application of this technology in the period from 1960 to 1990 will most likely be reduced to incremental improvements. The target will change from ‘one coating does it all’ to choosing specific coating designs to suit a particular set of conditions. This review will explore the historical development of protective coatings, and attempt to predict their future direction in the cutting tool and aerospace fields.
Coating for internal passages and cavities
In a protective atmosphere, a chemical compound of the metal is evaporated or created in gas form away from the component to be coated. The vapor is transported by a carrier gas onto and into the component. The metal is released when the chemical compound decomposes, either by a temperature increase or by a shift in the chemical reaction balance when the metal is absorbed by the component.
Advantages of CVD include:
Unlimited supply of coating metal, unrestricted coating thickness (theoretically)
Ability to coat internal cavities
Non-line-of-sight process: apply homogeneous coatings on intricate shapes
For gas turbine components, high-temperature CVD is commonly used to apply aluminum for an internal coating of hollow components. Components must be attached to a vapor distribution tool inside a protective atmosphere retort furnace. The coating thickness is controlled by time, temperature, and process gas flow.
The professional staff at Sulzer has the necessary training and equipment in selected locations in order to apply the various coating technologies to your components.
Pack cementation (also known as diffusion coating) is a process whereby components are packed in a powder mixture containing both the coating metal and chemicals that can form and carry the vapor for coating deposition. Pack cementation is a CVD process, although there is no visible flow of carrier gas.
As in other CVD processes, the carrier gas containing the metal is extremely sensitive to oxidation.
The coating heat treatments, which control thickness by length of time and temperature, must take place in a hydrogen-containing atmosphere inside a retort furnace.
Pack cementation is widely used for small turbine blades and vanes, as well as for the enrichment of MCrAlY-type overlay coatings with aluminum. MCrAlY coatings are alloys of M (Metal = nickel, cobalt, or iron, or a combination), Cr (chromium), Al (aluminum), and Y (yttrium).