CVD System

Diffusion coatings to improve the high temperature durability of superalloys by providing a thin adherent oxide layer for oxidation protection of the parent alloy.

APPLICATIONS

Improved operating efficiency via higher operating temperatures for gas turbine components results in a corresponding decrease in the high temperature durability of the components. Significant advances have been made in high temperature nickel and cobalt based superalloys. Diffusion coatings can further improve the high temperature durability of these alloys, by providing a thin adherent oxide layer for oxidation protection of the parent alloy. Some of these components are made more reliable by applying aluminide coatings to the surface, while others require aluminide or chromide coatings for better corrosion protection. Others also require an external thermal barrier coating to be applied over an underlying bond coating containing aluminides. The chemical composition of these coatings are adjusted to be compatible with the parent alloys and to meet the particular oxidation or corrosion problems associated with the parts in service.

COATING PROCESSES

PACK COATING:

Coatings can be applied by pack coating processes, where the compounds containing Al or Cr are placed inside a powder mixture of aluminum oxide and an activator to produce a metal halide vapor. These vapors will deposit on the surface of the work piece.

VAPOR PHASE COATING:

A more advanced method is to process parts in a vapor phase coating system, where the workpiece is placed above the donor materials, so the surface of the parts will not be contaminated by contact with the powder. Vapor Phase Aluminizing (VPA) and Vapor Phase Chromizing (VPC) are typical coating processes.

CHEMICAL VAPOR DEPOSITION (CVD):

The most advanced method of coating is to produce the elements to be applied either outside or inside the coating chamber by reacting donor material with halide forming gasses, controlling the rate of deposition by adjusting the temperature and gas flow rates over or through the work piece.

VPA Coating Box

COATING FURNACE SYSTEM

A typical coating furnace system is comprised of an electrically heated furnace; a reaction chamber (retort); vacuum system; hydrogen and argon delivery system; halide gas generator (for CVD); fume neutralization system (scrubber or bubbler) -- all controlled by a microprocessor based automated control system. The furnace systems can be either bell type (a moveable furnace that gets lowered over the fixtured parts or coating box(s)) or a top loading system (fixtured parts or coating box(s) get lowered into a stationary furnace).