Nitriding of Titanium and Titanium Alloys

Titanium is not in any sense of the word a wonder metal. Take the rigidity and strength of steel, combine it with the ductility of aluminum, the light weight of magnesium, the corrosion resistance of platinum, add creep resistance at high temperature, high impact resistance at sub-zero temperature, blend these together and then, and only then, might we employ the term "wonder metal." Titanium embodies some of these properties in certain respects; it is light weight, strong, corrosion resistant and, fortunately, abundant in nature.

Titanium and its alloys possess tensile strengths from 30,000 to 200,000 psi (200 to 1400 MPa) which are equivalent to those strengths found in most alloy steels. The density of titanium is only 56 percent that of steel and its corrosion resistance compares well with that of platinum. Of all the elements in the earth's crust, titanium is the ninth most plentiful.

Titanium and its alloys are attractive materials because of their superior strength-to-weight ratio and their resistance to corrosion. However, one of the main disadvantages of Ti and Ti-based alloys is their poor resistance to oxidation at high temperatures. The highest operating temperatures for components made of Ti-based alloys are limited to about 550°C.

Nitriding to enhance the wear, friction, fatigue and/or corrosion properties of production tools and components made of ferrous alloys is widely applied throughout industry and the high level of flexibility and reproducibility achieved by modern plasma surface treatments provides many opportunities for end users to improve performance and reduce life cycle costs. Since introducing the pulsed plasma into industrial heat treatment a broad range of applications for titanium alloys is also possible.

Nitriding is a thermo-chemical heat treatment process of introducing nitrogen into the outermost surface of parts and components. The process time is diffusion controlled. Due to this reason a compromise of nitriding temperature has to be found. To realize short cycle times the highest treatment temperature is of interest. High temperatures lower the hot strength of the material and results in distortions. Therefore high temperatures are only recommended for parts and components with a simple geometry. Typical layer thicknesses (TiN) for the most applications are in the range of 1–3 μm. Such layers generated by a diffusion process are showing surface hardnesses of 900 – 1100 HV0,05.

Due to a growing of the layer from the bulk material there is no danger of the layer spalling off as in case of PVD treatments. Furthermore a diffusion zone supports the TiN layer. The typical layer thickness of the diffusion zone is 20-40 μm. Due to the small layer thickness pulsed plasma nitriding results in an improvement of adhesive wear only. The lattice structure of TiN avoids adhesion in case of contact of two components made of titanium if one of these is nitrided. Also the wear in case of contact with other materials can be reduced by pulsed plasma nitriding.

An improvement of fatigue properties as in the case of steel nitriding isn’t observed. Actually, too high nitriding temperature of titanium alloys (Ti6V4) results in a loss of toughness. High temperatures which give thicker layers in shorter times are only recommended for applications where a primary improvement of wear is required. Nitriding causes a growth of the parts in the range of some μm because nitrogen is introduced into the surface. By changing the machining dimensions the mass growth can be compensated for Distortions are related to the treatment temperature, the time on temperature and of where the nitriding is required. In any case a uniform nitriding of all surfaces is recommended.

Plasma surface treatment is now well established as a flexible, cost effective alternative to salt bath and gaseous nitriding. The high level of flexibility and reproducibility achieved by pulsed plasma nitriding provides many opportunities for users to improve performance and reduce life cycle costs.

Pulsed Plasma Nitriding

With conventional dc plasma operation, overheating of thin sections can occur due to the voltages and current densities required to obtain a glow discharge that completely surrounds the workpiece. Localized heating can further concentrate the energy input from the plasma in such areas, leading to arcing, which damages the surface finish. Within deep slots or narrow openings, there is a possibility of workpiece melting via the ‘hollow cathode effect’: therefore the treatment of titanium or titanium alloys was restricted to very simple geometries, otherwise big problems with distortions happen.

The pulsed system solves the overheating problem by decoupling the heating and surface treatment functions, limiting the plasma energy input to that required to affect the metallurgical changes sought. This is achieved by replacing the steady state plasma with spiked current and voltage pulses.

Nitriding, with ion density determined by the voltage amplitude, is thus accomplished without significant heating from the plasma, temperature control being provided independently by resistance elements. The pulse duration, of the order of microseconds, reduces the heat input. Duty cycles during which the pulse is on for only 10-50% of the cycle do not appear to have any effect on nitrogen activity or nitriding time and the nature of the cycle can be varied during treatment to produce microstructures with the required features.

Gas Nitriding: Effect on Ti Alloy Microstructure

The microstructure of titanium alloys depends on the chemical composition of the alloys and the processing parameters of gas nitriding, as determined using a differential scanning calorimeter. There is a tendency toward grain growth with increases in temperature and time. This is new phase formation on the surface of these materials.

Nitrogen diffuses inward toward the metal substrate, forming a compound layer on the surface of the material. The compound layer is mainly composed of titanium nitrides Ti2N and TiN, followed by a diffusion zone that consists of an interstitial solution of nitrogen in the α or β titanium phases. After nitriding at temperatures below their β-transus temperatures, the titanium alloys have a homogeneous microstructure. This microstructure changes to inhomogeneous as the temperature increases above their β-transus temperature. The change in the microstructure is due to the phase transformations that occur during nitriding at increasing temperature.

Surface gas nitriding significantly increases the hardness of titanium alloys. This is due to the new phase formation on the surface of these materials. The titanium nitrides formed on the surface of the alloys have very high hardness that can reach values above 2,000 HV. The hardness decreases in the depth of the materials until it reaches the core microhardness. The hardness increases as the temperature and time of nitriding are increased.

One of the main purposes of the nitriding is to increase the surface hardness of the materials in order to improve their tribological properties. Increasing surface hardness does not necessarily improve the wear properties of titanium alloys significantly, which is one of the main obstacles to mechanical applications for these alloys.

The microhardness profiles of titanium alloys after nitriding, prepared on cross sections of the samples, give information not only about the microhardness behavior of the materials but also about the thickness of the nitrided layers. It can be assumed that the nitrided layer ends where the microhardness values reach the microhardness of the unsaturated core of the sample.

From the experimental study of the hardness evolution, the main results are the following. First, the evolution of the surface hardness of the titanium alloys studied in response to the nitriding temperature and time is similar for all alloys. The increase in surface hardness is mainly due to the new phases formed during nitriding, such as TiN and TiO₂. In addition, the hardness values of the surface layers are very high and they decrease through the diffusion zones to approach the base microhardness of the matrix in the unsaturated core. The microhardness increases with the increase of the time and temperature of nitriding.

The thickness of the nitrided layers, which can be estimated from the microhardness profiles, varies between 250 μm and 350 μm depending on the processing parameters of nitriding and the chemical composition of the alloys studied. Finally, nitriding at temperatures below the beta transus for titanium alloys is recommended for practical applications.

After nitriding, a thin nitriding layer appears on the surface of tribopart with the thickness of 5-20 μm, and with average microhardness of 1500 MPa. The thickness of the layer enriched by nitrogen achieves 0.1 to 0.15 mm, its microhardness arrives to 700-900 MPa. The temperature of nitriding must not exceed 980°C, since at higher temperatures the fragility of superficial layer increases rarely. Also, the temperature of nitriding must not be below 550°C, since the velocity of nitrogen diffusion into titanium strongly diminishes below this temperature.

As mentioned above, poor resistance to oxidation at high temperatures of Ti and Ti-based alloys is one of their main disadvantages. There have been many attempts to improve high-temperature oxidation resistance of titanium. Among the methods used for this purpose, surface alloying with silicon has been extensively studied. This type of alloying seems to be more suitable than bulk alloying because the silicon significantly modifies the mechanical properties of titanium. Besides a reduction in the oxidation rate, silicon has been shown to improve wear and creep resistance.

Although the exact mechanism of the oxidation of titanium alloyed with silicon isn’t fully understood, it is generally believed that silicon, which is present both in solid solution in retile and in small silica particles, plays several roles:

• Si decreases the depth of the oxygen penetration into the alloy substrate, which is in agreement with its β-forming nature,
• Si dissolved in the TiO₂ surface layer reduces the diffusion rate of oxygen atoms through this layer, and
• Si modifies the stress relaxation processes in the oxide layer and contributes to the formation of a more compact layer with a lower porosity.

Several methods are used for the surface modification of metals with silicon. Laser surface alloying, silicon-ion implantation, vapor-phase siliconizing and powder siliconizing have been the most extensively studied.

The laser surface alloying of titanium with silicon involves rapid melting of thin surface layer and simultaneous feeding of silicon powder. As a result, a layer of rapidly solidified (cooling rates of more than 10⁴ K/s) Ti–Si alloy with a very fine microstructure is formed.

The implantation of accelerated silicon ions into the surface of titanium is also able to improve its oxidation resistance. However, a large dose of ions and/or a high acceleration voltage can lead to the introduction of an excessive number of lattice defects. This enhances the diffusion, and the improvement of the oxidation resistance is not significant. The negative effect of lattice defects can be partially diminished with post-implantation annealing.

Wide applicability of both laser surface alloying and ion implantation for the treatment of components from Ti-based alloys seems to be limited, particularly because of their high cost. In addition, the reproducibility of the laser surface alloying appears to be unreliable.