What Does Pack Cementation Mean?
Pack cementation is a batch process that has been used to produce corrosion- and wear-resistant coatings on inexpensive or otherwise inadequate substrates since 1920.
Pack cementation is a widely used chemical vapor deposition technique that confers oxidation resistance on ferrous alloys. Usually relatively expensive aluminum or binary alloys grade reagent is used during the pack process with aluminum as a source.
Pack cementation is the most widely used as it is inexpensive and well adapted to coating of small parts.
Corrosionpedia Explains Pack Cementation
Pack cementation is a coating process for corrosion and wear resistance. This process includes:
The traditional pack consists of four components:
- Substrate or parts to be coated
- Master alloy (Cr and/or Al, Cr and/or Si)
- Halide salt activator
- Relatively inert filler powder (Al2O3, SiO2, or SiC)
Pack cementation aluminizing is the most widely used coating process. Corrosion resistance of aluminide coatings can be increased by modification with chromium, platinum, or silicon. Chromium diffusion coatings can be used at lower temperatures.
In this process, the components to be coated are immersed in a powder mixture containing Al2O3 and aluminum particles. About 1-2 wt% of ammonium halide activators are added to this pack. This is then heated around 800-1000°C in argon or H2 atmosphere. At these temperatures, aluminum halides form, which diffuse through the pack and react on the substrate to deposit Al metal.
Variables that affect the coating process include:
- Substrate composition
- Powder bed composition
- Heat treating conditions
- Furnace atmosphere
The structure and composition of the coating depends on the substrate. Aluminide coatings lack ductility below 750°C. One of the major problems faced by aluminide coatings is thermomechanical fatigue, as cyclic strains induced by temperature gradients in the blades can lead to thermal fatigue cracks.
Pack cementation coating is a promising method for producing coatings on high-temperature ceramic materials for advanced gas turbine engines. Some advantages include:
- Non-line-of-sight process for uniform coatings on complex shapes
- Batch process can coat hundreds of parts simultaneously
- No unique equipment required to establish coating capability
- Technology can be easily transitioned to industrial production
Pack cementation and related diffusion coatings serve well for most aircraft engine applications. The trend for industrial and marine engines is more toward the use of overlay coatings because of the greater ease of designing these to meet a wide variety of corrosion conditions.