Wear, corrosion, and fatigue failures are the most important mechanisms that lead to material degradation, which eventually causes failure of many gears. Those failures involve mechanical and/or chemical interaction of the material component considered with loads imposed by the environment. Hence, material performance and service life rely, to a high degree in many cases, on the properties of a material in its surface region.
Accordingly, engineering the intrinsic properties of the surface region is desired and can be realized by the application of various thermochemical treatments (TCT). TCT modifies surface composition as well as microstructure of the component, and it is dominated by nitriding/nitrocarburizing as well as carburizing [1].
Plasma Nitriding
Plasma-assisted methods offer significant environmental benefits and have been used for many purposes of surface engineering for ferrous and titanium alloys [2, 3]. Plasma nitriding is carried out in a glow discharge with the workpiece being the cathode and the vacuum vessel wall being the anode. (See Figure 1)
The main advantage of plasma methods versus gas methods is the activation of the most difficult alloys such as stainless steels, nickel, and titanium alloys by ion bombardment leading to surface sputtering and removal of the passive layers. Plasma processes are carried out under soft vacuum with a low partial pressure of nitrogen; therefore, the layers produced are never too rich in nitrogen and/or carbon, not brittle, which makes them perfectly suited for multiple applications, especially those involving mechanical stresses, contact and bending fatigue, as well as wear. Because of the nature of plasma processes, they are the main methods for surface treating low-density powder metallurgy (PM) sintered products. In plasma nitriding, a simple mechanical masking causing interruption of the plasma shield around the part allows for selective hardening of the gears and an easy protection of certain areas, for example, threads and blind holes from the hardening. Masking in conventional gas nitriding and nitrocarburizing (FNC) is difficult. It requires labor-intensive galvanic plating with copper, which has to be stripped off after the treatment and parts have to be baked off to release hydrogen trapped in the steel, which may cause embrittlement if left in the structure.
Treatments with post oxidizing are designated for significant improvement of the corrosion and cavitation resistance of treated parts, especially those subjected at the same time to surface wear either by abrasion or adhesion. Allover treatment of the gears is carried out in the atmosphere of ammonia and other gases controlled by the superior software and instrumentation of the modern furnaces, allowing for any phases from the Fe-N-C phase diagram. The treatment can be combined with the post oxidizing and sealing steps to enhance further properties of the gears.
Surface Hardening of Stainless Steels
A development in recent years is low-temperature surface hardening of stainless steels. The process can be applied to austenitic, duplex, martensitic, and precipitation hardenable steels to improve their already-high corrosion resistance with the simultaneous significant increase of hardness, wear, and bending/contact fatigue properties. The process can be carried out using plasma as well as the gas methods at temperatures not exceeding 400°C (752°F). Those processes include low-temperature nitriding, carburizing, and combinations of both that are applied to small gears working in the environmentally difficult conditions. Those gears, because of their size, require thin yet durable surface layers produced in the corrosion-resistant steels.
Titanium Gears
In regards to titanium, the nitriding process is mainly used for improving tribological properties of the alloy, although high corrosion resistance of titanium is also often increased at the same. The titanium has a great chemical affinity not only for nitrogen but also for oxygen and carbon. Those elements — N, C, O, and H — form thermodynamically stable compounds within a broad range of temperatures used for nitriding. These elements are likely to occur in the processing atmospheres, and they play an important role in the growth of the nitrided layer. Significant solubility of nitrogen in titanium hexagonal lattice allows for forming compound and diffusion layers with excellent adherence to the base metal and with a very high hardness and comparatively smooth transition to the core.
Nitrided titanium has a yellow/gold color characteristic of titanium nitride TiN, which always forms in the surface during nitriding. Presence of TiN enhances the aesthetic appearance of nitrided titanium gears. Compared to ferrous alloys, nitriding of titanium alloys is difficult in the sense of the requirements for clean atmospheres as well as for much higher temperatures needed for the process. Plasma/ion nitriding of titanium alloys is carried out at temperatures exceeding 680°C.
Environmental Benefits
Application of ion/plasma nitriding and nitrocarburizing also has many benefits in respect to the environmental cleanness of the process. This has been an important factor, especially in densely populated industrial centers, and the American heat treating industry is paying more attention to environmental regulations. Plasma nitriding and nitrocarburizing are environmentally friendly processes since they use very little amounts of gases such as nitrogen and hydrogen. For example, a plasma nitriding vessel with the working volume of 30 m3 requires no more than 0.5 m3/hour. Those processing gases, once deactivated and deionized, can go through the same sequence of ionization and activation multiple times. In conventional processes such as gas nitriding and nitrocarburizing (FNC), ammonia, once cracked, is lost forever from the nitriding process. Therefore, a similar-sized gas nitriding system may require 50 m3/hour of undissociated ammonia in addition to similar quantities of nitrogen.
References
- Thermochemical Surface Engineering of Steels, Ed. E. J. Mittemeijer and M. A. J. Somers, 2014, Woodhead Publishing, 1-792.
- E. Rolinski, Plasma-assisted nitriding and nitrocarburizing of steels and other ferrous alloys, Chapter in Thermochemical Surface Engineering of Steels, 413-457.
- E. Rolinski, Nitriding of titanium alloys, ASM Metals Handbook, in editing.
