Vapor phase aluminide (VPA)

This type of aluminide coating is also referred to as VPA or Above-The-Pack. At Bodycote, our Vapor Phase Aluminide (VPA) coating is an above-the-pack process where components are placed in a heated inert atmosphere, surrounded by CrAl donor material.  The donor material does not contact the parts directly.

During thermal processing, the aluminum in the donor material and halide activator vaporize in the presence of a carrier gas and condense onto the target parts.  It then further diffuses into the substrate and combines with nickel, forming a nickel aluminide.  The resulting coating contains both a diffused and additive layer.  In hot service, a durable oxide scale is formed, which protects the component from further oxidation.  This VPA process can also be used to coat the internal passages of parts, such as turbine blades.  Additionally, VPA can be combined with platinum plating to form platinum aluminides or with a change in donor material can be used for Vapor Phase Chromizing (VPC), both for hot corrosion resistance.

Advantages

• Cost-effective solution to increase hot oxidation and corrosion resistance on superalloys
• Relatively thin at approximately .001”-.003” additive layer
• Capable of coating internal passages
• Can be combined with other thermal barrier coating processes to further enhance protection
• Robust processing once developed
• Used on a wide variety of superalloys

Applications

• Typically, the first line of protection for increased oxidation and corrosion resistance for superalloys
• Utilised heavily on gas turbine components subjected to hot temperature applications (blades, vanes, shrouds, and combustors)

Process Details

Components are batch processed within large circular cans surrounded by donor material and an activator.  The donor material is held in separate baskets and does not contact components directly.  These cans are then loaded into a retort and placed in a furnace.  Inert gas is passed through both the cans and the retort while heating to a temperature suitable to vaporize donor material.  Donor vapor condenses on target parts and diffuses into substrate material.  Parts are subsequently vacuum heat treated to enhance properties further.