Developments of Plasma Processing in Surface-Treating Technology
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This article reviews the developments that have taken place, particularly for surface treatments, within the heat-treatment processing industry.
Surface treatments were initially all carburizing followed by austenitizing, quench and temper. It was very soon recognized that the carburizing process was somewhat limited.
The first surface transformation deviation from carburizing was in 1903 with Adolph Machlet’s patent application for the process of gaseous nitriding (Fig. 2). In the late 1920s, two German metallurgists named Berghaus and Wienheldt started research into the surface treatment of steel by the utilization of plasma.
It became evident that the use of plasma technology could enhance the treatment of a steel surface if the steel had a pre-heat-treated core followed by nitriding, which could then be further treated to produce a hard abrasion-resistant surface.
The basic nitriding procedure – gaseous, salt or plasma-enhanced (Fig. 4) – also offered improved corrosion and fatigue resistance. From the simple thermochemical procedure of nitriding, it was soon discovered that the immediate nitride surface can be modified.
The advent of plasma processing technology for surface treatments resulted in many new opportunities for developing treatment processes. Figure 6 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.
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 is improved.
Other Advantages of Plasma-Assisted Processing
Plasma-assisted nitriding and nitrocarburizing will also improve both resistance to static and dynamic loads as well as have a significant improvement on corrosion resistance.
Plasma-Assisted Nitriding and Ferritic Nitrocarburizing
Another procedure that can be accomplished with plasma conditions (it should also be noted that similar results can be obtained by gaseous nitriding and by salt-bath nitriding) is that of post-oxidation to assist in the improvement of corrosion resistance. The plasma-assisted procedure is very simple to conduct and is performed in the following manner.
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. 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 the desired surface color.
Process mediums for post-oxidation include a controlled oxygen flow introduced into the process chamber. Alternatively, carbon dioxide or water vapor can be introduced. The temperature can be selected to match a specific tempering temperature.
Resulting structures from the plasma-assisted nitriding process or ferritic nitrocarburizing with post-oxidation are discussed here.
- A dense, fine-grained immediate oxide surface layer (magnetite, Fe₃0₄) will be formed.
- The immediate oxide surface layer will be chemically resistant and have a low coefficient of friction.
- The oxide surface layer will be determined by the selected process temperature.
Subsurface Compound Layer
- High surface hardness can be accomplished with final hardness values of 800-1,400 HV (depending on the chemical analysis of the material being treated and the ferritic nitrocarburizing process parameters).
- High wear resistance
- Improved fatigue resistance due to induced residual compressive stresses
- Diminishing hardness gradient in the diffusion layer and into the core hardness results
Plasma-Assisted Surface Depositions
The plasma-assisted surface depositions can be subdivided into two groups – CVD and PVD (Fig. 7).
Plasma-assisted surface-deposition (PASD) procedures are applied to the surface of a heat-treated component after the hardening and temper procedures have been completed. Plasma-assisted nitriding is completed with a minimal case depth.
So, instead of a component that has been heat treated followed by the diffusion treatment of plasma-assisted nitriding, we now have a deposition treatment. These procedures can be termed in a collective manner as thin-coating deposition technology.
Thin-Coating Deposition Technology
The basis of this technology is to deposit onto a hardened substrate material to produce extremely high surface-hardness values while at the same time retaining the prior metallurgy of the component subjected to this treatment. This technology will produce abrasive-resistant coatings that have been deposited onto a nitrided substrate that has been subjected to a protective method of precleaning and processing.
The procedure ultimately offers low coefficient-of-friction surfaces and good sliding surfaces. The deposited coating is well bonded to the nitrided substrate and necessitates very thin depositions.
In CVD (chemical vapor deposition), which is conducted under low-pressure conditions (medium-vacuum), the deposited coating is produced by chemically starting from a gaseous component at high temperatures.
PVD (physical vapor deposition) coating is derived from a metallic vapor that deposits the extremely hard coating onto the component being treated through cathodic contact with the process-furnace hearth. Typical applications of both CVD and PVD procedures are usually well suited for hard-wearing and abrasive process conditions.
The procedural process temperatures are higher than those applied to plasma-assisted nitriding and plasma-assisted ferritic nitrocarburizing. The temperatures employed for CVD and PVD are generally found in the region of 1450-1900°F (788-1038°C), so the process-vessel material must be chosen with great care to operate consistently and repeatedly at these high process temperatures.
Plasma-assited titanium-nitride deposition coatings are used for:
- Milling cutter tools
- Punches and dies
The resulting surface hardness after processing by plasma-assisted titanium nitriding can be in the region of 2000 HV, which will assist in wear resistance, corrosion resistance and erosion.
Alternative Plasma-Assisted Surface-Deposition Coatings
There are alternative plasma-assisted surface-deposition coatings that will produce higher surface-hardness values with successful bonding of the coating to the nitrided substrate material. The procedure makes use of a higher process temperature in the same plasma-assisted bell furnace, which will now be discussed.
Titanium Aluminum-Nitride Process (TiAlN)
This procedure makes use of the additional element of aluminum. This addition will most certainly improve the immediate surface hardness as well as oxidation stability of the coating. Surface-hardness values of the titanium aluminum nitride are generally found to be in the region of 2700-3200 HV.
Obviously, the procedure will extend the operational life of the component as well as improve the surface corrosion resistance. The higher surface hardness will most certainly assist in cutting speeds of tooling. Generally, the depositions of the TiAlN can vary in the region of 0.001-0.003 inch.
Titanium Carbonitriding (TiCN)
The titanium carbonitriding procedure can also produce substantially high surface-hardness values in the range of 2800-3500 HV.
The addition of the carbonitrides into the procedure substantially increases the surface-hardness value. This is obtained by introducing a hydrocarbon gas into the reactive process gas during the procedure. Once again, this allows for higher cutting speeds with excellent adhesion and will also produce a very low coefficient of friction.
Even the process of carburizing can be conducted by plasma energy. The procedure is once again conducted under low-pressure conditions, which ensures that the presence of oxygen is reduced to such an extent as there is no concern regarding the diffusion of oxygen into the surface grain boundaries. Although the capital investment of a plasma-assisted carburizing unit is high, the unit cost per components or per pound will depend on the principle of amortization that is employed by the company.
Once the amortization period has been completed, the process-gas consumption is (in reality) a minimal procedure cost. This is simply because, under plasma conditions, only the process gas necessary for carburizing is needed.
The usual practice of carburizing (even low-pressure carburizing) uses the amount of hydrocarbon gas necessary to fill the process chamber. Conversely, the plasma-assisted procedure utilizes only the hydrocarbon gas necessary directly at the component surface. The amount of the hydrocarbon process gas now becomes a minimal operational cost.
The process is clean and does not produce any toxic odors into the workshop. Also, due to water-cooling of the process unit, no residual heat is introduced into the working area. The workpiece surface is improved due to the ionic bombardment of the surface, which will erode the machining lines on the metal component.
The utilization of plasma for thermal-process energy opens the door to many of the surface treatments of steel that are used in today’s world. Because the process is low-pressure, there is no risk of surface oxidation. If the elements selected for the particular procedure can be transformed into a vapor within the process chamber, then most surface treatments can be conducted. The “secret” of the surface treatments is the plasma-generation system.
Here is a quick review of some of the things plasma surface treatments offer.
- An energy-efficient thermal metallurgical processing system
- A low process-gas cost because only the gas at the component surface is utilized. Instead of working in high-volume gas flows, the plasma system now offers the ability to utilize only milliliters of process gas.
- Plasma-assisted processing technology (particularly plasma-assisted carburizing) provides the ability to conduct the surface diffusion chemistry at a higher process temperature than what has been previously known.
- The process control and repeatability of operating conditions can be accurately monitored and process data stored. This is now available to the heat treater when purchasing new furnace processing equipment.
- If the unit is only used for plasma-assisted carburizing, the same principles of metallurgy are required as for the austenitizing/tempering plasma-assisted carburizing procedures.
For more information: Contact David Pye, Pye Metallurgical International Consulting, 911 Backspin Court, Newport News, Va.; tel: 1-757-968-1007; e-mail: firstname.lastname@example.org; web: www.heat-treatment-metallurgy.com.