Nitrürlenmiş titaniyum alaşımların karakteristiği ve aşınma davranışları

Titanium and its alloys are used in the aerospace industry, chemical applications, architectural areas, sports equipment industry, biomedical applications, power plant industries, transportation, marine and automobile industry. Properties such as high strength, low density and enhanced corrosion resistance are the most important advantages of titanium alloys for these engineering applications. On the other hand, the most unfavorable feature of titanium is the poor tribological properties such as wear resistance; hence, the usage of titanium alloys is limited with wear related applications. Since gas nitriding is a very simple, economical and most used surface treatment method which has been used for many years, present study focuses on gas nitriding of titanium and its alloys conduced with the aim of improving the surface hardness and wear resistance by forming titanium nitride (TiN) layer and a nitrogen diffusion zone (NDZ) beneath it. In nitriding process, in addition to treatment time and temperature, chemical composition of titanium alloys could have significant influence on the characteristics of the nitrogen exposed surfaces. Therefore, it is needed to have further studies on different classes and grades of titanium alloys to investigate the effect of chemical composition on the characteristics of nitrided surfaces. This study focuses on characteristics of commercial purity grade titanium (Cp-Ti Grade 4) and titanium alloys (Ti6Al4V and Ti6Al7Nb) after gas nitriding at 950, 1120 and 1250 °C for 3, 7 and 12 h. The objective of this study can be summarized as investigation of microstructural features, nitriding kinetics (based on the weight changes and thickness of the grown layers), hardness and tribological performances of nitrogen exposed surfaces. As the result of the nitriding process, the surface appearances of the samples changed to golden color upon formation of a TiN layer. As a general trend higher nitriding temperatures and longer exposure times favored formation of thicker TiN layer. However, XRD analyses conducted on Cp-Ti revealed that nitriding temperature 950°C did not cause complete covering of the surface with TiN layer unlike titanium alloys. Microscopic examinations revealed the formation of two layers consisting of TiN and NDZ on Cp-Ti, while three layers, including TiN, Intermediate Zone (IZ) and NDZ, were developed on nitrogen-exposed surfaces of Ti6Al6V and Ti6Al7Nb alloys. Energy dispersive X-ray spectroscopy (EDX) equipped scanning electron microscopic (SEM) surveys showed that IZ has a wavy appearance and formed as a thin layer upon enrichment by aluminum. In the case of Cp-Ti, the interface between TiN layer and NDZ was straight without any evidence of changes in composition. Kinetic studies conducted on the nitrided samples revealed that presence of alloying elements in the substrate caused higher activation energy for diffusion of nitrogen. In this respect, higher nitrogen diffusion activation energy of titanium alloys as compared to Cp-Ti can be attributed to IZ formation as the result of repelling of aluminum atoms from the growing TiN layer. As the nitriding temperature and time increase, TiN layer tend to be more porous and exhibit poor adhesion characteristics. In general, TiN layers formed at nitriding temperature of 950 °C exhibited better adhesion performance than those formed at 1120 and 1250 °C. Results of the cross-sectional microhardness measurements and wear tests showed that nitriding process is very beneficial to enhance the hardness and wear resistance of the examined samples as compared to their as-received states. In accordance with adhesion characteristics of the TiN layer nitriding temperature of 950°C provided excellent wear resistance for the examined titanium alloys. The best wear performance of Cp-Titanium was obtained after nitriding at 1120 ºC for 7 hours.