Effect of lowering GN temperature on compound layer microstructure formed on CP titanium
Commercially pure (CP) titanium due to its high corrosion resistance is especially well suited for applications in medicine as implants. However, the titanium suffers from fast wear, because of its relatively low hardness. One of the effective way to improve surface properties and protect the substrate could be gas nitriding (GN) treatment. It is well developed and widely applied process for titanium. GN treatment leads to the formation of thick nitriding affected zone (NAZ) consisting of a compound layer and a diffusive zone. The surface layer built of titanium nitrides exhibits high hardness. The diffusive zone is a layer produced as a result of diffusion of nitrogen atoms into the material. It is usually performed at high temperature (~1000oC) for long time. Such process conditions lead to the coarsening of the microstructure of the core material and as a consequence the decrease of mechanical properties of the treated part. Therefore, the aim of the presented work was to investigate the microstructure and conduct a phase analysis of the nitrided layer formed on CP Titanium Grade 2 during gas treatment performed at as low as possible processing temperature.
The gas nitriding of pure titanium at 680oC and 740oC for 6 hours allowed to obtain relatively thick nitriding affected zone (NAZ), which is increased from 0.9 to 1.6 µm, respectively. The TEM investigations revealed that it consist of two layers strongly differing in types of characteristic microstructure features. The near-surface one is formed by cubic -TiN phase, while the one below is of ”-Ti martensite. The former is built of nano-crystalline columnar grains with chevron shape tops, while the latter is built of large grains filled with parallel plates. The nitride layer formed at higher temperature is thicker but bears porosity, while the one formed at lower temperature is thinner but nearly fully dense. The lowering GN temperature to specified above values helped to preserve high specific strength of the core material and coat it with continuous nitride compound layers. Even the porosity, which presence was documented in the nitride layer grown at higher of the two temperatures, could be considered advantageous, as it secures better condition for growth of human tissue over implants.
Investigations were co-financed by the European Union from the European Social Fund
(Project No. WND-POWR.03.02.00-00-I043/16)