Titanium is irreplaceable in many industrial applications including high strength-efficient and corrosion resistant structures [1, 2, 3]. It is a nonmagnetic and a low-density element (approximately 60% density of steel) that can be easily strengthened by proper alloying, heat treatment and deformation processing [1]. Mechanical components used in aerospace and chemical industries are typically made of titanium alloyed with aluminum and vanadium and a two-phase structure. Those alloys have very high strength-to-weight ratios and have been preferred as the materials for special applications at very low temperatures from -196° to -269°C (-320° to -452°F)[2]. Also, the high-temperature properties of titanium alloys are unique and further enhance application of this metal.

However, in many situations, surface properties of components made of titanium alloys have to be improved to provide the tribological properties needed for reducing friction between various rotating parts, reducing wear or for further enhancement of anti-corrosion properties. Those properties can be increased by thermochemical treatment such as nitriding[4]. Figure 1 shows the wear resistance of nitrided titanium alloy compared to the wear resistance of nitrided stainless steel [4, 5].

The general idea behind forming thermochemical layers is to fortify the surface of titanium, imparting performance qualities such as hardness, dry-lubrication and corrosion resistance or cleanliness of the surface. The treatment can greatly extend equipment lifetimes.

Plasma/ion nitriding is a very efficient method for forming hard surface layers in ferrous as well as titanium alloys[6]. It is carried out in a vacuum-type vessel where the glow discharge is generated between the cathode, which is the treated object, and vessel wall, which is the anode (Figure 2). Note the cathodic portion of the glow discharge covering the parts and central anode in Figure 2b. Direct current (DC) plasma used for this type of treatments contains approximately equal concentration of positive ions and electrons as well as a large number of excited but neutral species[6]. Energy of nitrogen N 2 +, N+ ions and neutrals bombarding the cathode reaches 200 eV and is sufficient for heating it to a desired temperature and causing minor sputtering of its surface needed for activation.

Figure 2a (right). Plasma/ion nitriding of extruder screws as seen through the port window. (Courtesy of Advanced Heat Treat Corp. Monroe, Michigan) Figure 2b (left). Plasma/ion nitriding of large machine block as seen through the port window. (Courtesy of Advanced Heat Treat Corp. Monroe, Michigan)

The mechanism of plasma nitriding is very complex; some of the ions are implanted into the surface and the others just lose their charge and supply active nitrogen atoms also reacting with titanium. Chemisorbed or implanted nitrogen diffuses into titanium forming the layer.

Nitrogen is one of the three interstitials (nitrogen, carbon and hydrogen when they are in solid solution), which is the most potential element in surface strengthening of titanium. In addition, titanium nitrides are essential in making the surface hard for both titanium and titanium alloys products [1, 5].

When a part made of titanium alloy is nitrided, the treated surface has a gold color characteristic for titanium nitride, TiN. This nitride is always present in the surface and it has a very high hardness reaching 2000 HV. It is the first portion of the nitrided layer but the zones underneath also contain Ti 2N and Ti2AlN nitrides and a solid solution diffusion zone with a slowly diminishing hardness and thickness reaching 0.1 mm[3].

Examples of Applications: Space

Aerospace applications comprise a majority of titanium usage. The recent space mission of the Mars Rover[7] required application of plasma/ion nitriding for proper surface engineering of some of its components. The titanium parts were nitrided to help reduce any risk for contamination from Earth – the parts needed to be some of the cleanest parts ever produced.

Figure 3. Engineers and technicians insert 39 sample tubes into the belly of the rover. Each tube is sheathed in a gold-colored cylindrical enclosure to protect it from contamination. (Courtesy NASA/JPL-Caltech)

As referenced in Figure 3, the Mars Rover contained gold-colored cylindrical parts. The gold color demonstrates that the nitriding process resulted in the presence of titanium nitride TiN and that treatment was very uniform. The nitride is also free of contaminations with oxygen and carbon which, when present, alter color and properties of the surface.

Armament

Figure 4. Gun components made of the Ti6Al4V alloy after plasma/ion nitriding. (Courtesy of Advanced Heat Treat Corp., Waterloo, Iowa)

The weight of guns carried by soldiers or guns installed on aircraft has paramount importance in increasing the efficiency of operations performed by the military. Titanium alloys used for making certain components of those guns offer unique opportunities, not only by reducing their weight, but also by improving their durability since nitrided titanium alloys have better wear and corrosion resistance than nitrided stainless steels[4, 5].

Summary

Titanium alloys applications are growing and the many benefits of this metal will be used to its full capacity, including properties at cryogenic and elevated temperatures. This will be more likely when a proper surface hardening technique, eliminating poor tribological performance of the metal is applied. This technique allows also for formation of the cleanest surface possible. Nitriding is the process that significantly enhances those properties.

References

  1. J. Foltz and M. Gram, “Introduction to Titanium and Its Alloys”, ASM Handbook, ASM International Vol. 4E, Heat Treating of Nonferrous Alloys, Volume Editor, G. E. Totten, 2016, pp. 481-497.
  2. R. R. Boyer & J. Foltz, “Effect of Heat Treatment on Mechanical Properties of Titanium Alloys”, ASM Handbook, ASM International Vol. 4E, Heat Treating of Nonferrous Alloys, Volume Editor, G. E. Totten, 2016, pp. 555-572.
  3. T. Perless, “Vanadium A Green Metal Critical to Aerospace and Clean Energy”, Aerospace & Defense Technology, August, 2020, pp. 12-16.
  4. E. Rolinski, “Surface properties of plasma nitrided titanium alloys”, Materials Science and Engineering, 108 (1989) 77-44.
  5. E. Rolinski, “Nitriding of Titanium Alloys”, ASM Handbook, ASM International Vol. 4E, Heat Treating of Nonferrous Alloys, Volume Editor, G. E. Totten, 2016, pp. 604-621.
  6. E. Rolinski, “Plasma Assisted Nitriding and Nitrocarburizing of Steel and other Ferrous Alloys”, Chapter 11 in Thermochemical Surface Engineering of Steels, Ed. E. J. Mittemeijer and M. A. J. Somers, Pub. Woodhead Publishing, 2014, pp 413-449.
  7. TECH BRIEFS Magazine SAE, Special Mars 2020 Issue.

This article was written by Dr. Edward Rolinski, Senior Scientist; Bill Cowell, VP of Operations; and Mikel Woods, President; Advanced Heat Treat Corp. (Waterloo, IA). For more information, visit here .