This series on trends and developments in heat-treatment technology concludes.

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 edition 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 2,700-3,200 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-carbonitride procedure can also produce substantially high surface-hardness values in the range of 2,800-3,500 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 (because of the split to cleaning procedures) and will also produce a very low coefficient of friction.

 

Plasma-Assisted Carburizing

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 that 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 period for the time of amortization has been completed, the process-gas consumption is, in reality, a minimal procedure cost. This is simply because the process gas necessary for carburizing under plasma conditions is only what is necessary at the surface. The usual practice of carburizing (even low-pressure carburizing) uses enough hydrocarbon gas to fill the process chamber, as opposed to the plasma-assisted procedure, which uses only the hydrocarbon gas necessary 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 or excessive heat from the process unit because the process unit is water-cooled and no residual heat is introduced into the working area.

The workpiece surface is improved because of the ionic bombardment of the work surface. This means that the surface is being atomically shot blasted (if one can use that term), which will erode the machining lines on the metal component.

 

Conclusion

Plasma utilization for thermal process energy opens the door to many of the surface treatments of steel used today. Because it is a low-pressure process, there is no risk of surface oxidation. If the elements selected for the particular procedure that is to be conducted can be transformed into a vapor within the process chamber, then most of the surface treatments can be conducted. The “secret” of the surface treatments is the plasma generation system.

Plasma surface treatments offers the following benefits to metallurgical process users:

  • An energy-efficient thermal metallurgical processing system
  • A low cost of the process gases because one is only using the process gas at the component surface and not filling the process vessel. 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 for plasma-assisted carburizing) has the ability to conduct the surface diffusion chemistry at a higher process temperature than what has been previously known (and served its purpose well).
  • The process control and repeatability of operating conditions can be accurately monitored and process data stored. (This is of course available now 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 for the formed-case austenitizing of plasma-assisted carburizing procedures.