The advent of plasma processing technology for surface treatments resulted in many new opportunities for developing treatment processes. Figure 6 is a display of the ferritic nitrocarburizing process and its resulting metallurgy in relation to the compound zone and its resulting metallurgy. Figure 7 is a sketch of the carbonitride case formation. Because of the wide range of process-control options for plasma-based nitriding processes such as plasma-assisted ferritic nitrocarburizing, resulting metallurgy can be developed for specific applications.
Precleaning
The procedure for precleaning within the confines of the plasma process vessel is known as sputter cleaning. A simple analogy of what sputter cleaning accomplishes is that of atomic “shot blasting.” As a result of this procedure, and using less than 5% (maximum) of argon plus the balance of hydrogen, the surface finish of the component being treated will improve.
Other Advantages of Plasma-Assisted Processing
Plasma-assisted nitriding and nitrocarburizing will improve both resistance to static and dynamic loads. It also has a significant improvement on corrosion resistance.
Plasma-Assisted Nitriding and Ferritic Nitrocarburizing
Another procedure that can be accomplished with plasma conditions is that of post-oxidation to assist in the improvement of corrosion resistance. It should also be noted that similar results can be obtained by gaseous nitriding and by salt-bath nitriding. The procedure is very simple to conduct and is performed in the following manner (and referring to plasma-assisted processing).
Once the nitriding or ferritic nitrocarburizing procedure has been completed, the process vessel MUST be purged with nitrogen to ensure that all residual hydrogen has been extracted from the process vessel. Thereafter, an oxygen-bearing gas or liquid is introduced into the process vessel in a controlled manner. The process temperature can be selected in relation to the surface finish, and resulting surface color can be accomplished.
Process mediums for post-oxidation include a controlled oxygen flow introduced into the process chamber. Alternatively, carbon dioxide can be introduced into the process chamber. Another medium is water vapor. Temperature that can be selected to match a specific tempering temperature.
The resulting structures from the process of plasma-assisted nitriding or ferritic nitrocarburizing with post-oxidation are the following.
Oxide Layer
- Immediate oxide surface layer = magnetite (Fe₃0₄)
- A dense fine-grained immediate oxide surface layer will be formed.
- The immediate oxide surface layer will be chemically resistant and with a low coefficient of friction.
- The oxide surface layer will be determined by the oxide treatment process temperature selected.
Subsurface Compound Layer
- High surface hardness can be accomplished with final hardness values of 800 HV up to 1,400 HV (depending on the analysis of the material being treated and the ferritic nitrocarburizing process parameters).
- High wear resistance
Diffusion Layer
- Improved fatigue resistance due to induced residual compressive stresses
- Diminishing hardness gradient in the diffusion layer and into the core hardness results