Nitriding is a thermo-chemical heat treatment process 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 realise short cycle times the highest treatment temperature is of interest.
High temperatures lower the hot strength of the material and result in distortions. Therefore high temperatures are only recommended for parts and components with a simple geometry. In any case a direct temperature measurement on a real component or part is required. Fig. 1 shows the time and temperature dependence on the layer of TiN. Typical layer thicknesses for the most applications are in the range of 1 – 3 μm.
Fig. 1 Time and tempertaure dependence on the TiN – layer thickness
Fig.2 Nitrided Structure, etched in 2% HF ( IWT Bremen )
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. Specially too high nitriding temperature of titanium alloys (Ti6V4) results in a loss of toughness.
High temperatures which gives 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.
U.Huchel; S. Strämke
Pulsed Plasma Nitriding of Titanium and Titanium Alloys
Science and technology
Proceedings of the 10th World Conference on Titanium
p.935 – 939