Thermochemical Treatment of Titanium and Titanium Alloys

The research described in the present thesis mainly concerns the thermochemical treatment of titanium and titanium alloys primarily by the use of the gas systems CO and CO₂. Titanium and its alloys suffer from the disadvantages of poor tribological properties, which can be overcome by introducing interstitial elements such as oxygen (oxidizing), carbon (carburizing) and nitrogen (nitriding) into the α-Ti lattice to enhance the strength/hardness. The adherent compound layer(s) such as oxides, carbides and nitrides lower the friction coefficient and increase the wear resistance of titanium; moreover, the hardened case formed through thermal diffusion of interstitials extends the service life of titanium components. Compared to the more recently developed techniques such as plasma, laser and PVD (Physical Vapor Deposition) coating for surface hardening of titanium, the old-fashioned gaseous methods have been developed more than 70 years ago, but are still of great interest to researchers, largely due to the low-cost devices and the absence of restrictions to the component size and shape. The study on “oxidation” of titanium has been conducted since 1950, while oxidizing of titanium for the purpose of surface hardening mainly received attention at the end of last century; similarly for the other interstitial species nitrogen and carbon. Most of the investigations focused on binary system, i.e. Ti-O, Ti-C and Ti-N. The present doctoral thesis investigates the ternary system, i.e. Ti-C-O, and also the quarternary system (Ti-C-O-N) using a duplex treatment.

Several aspects concerning the thermochemical treatment of titanium and titanium alloys within this Ph.D. work were addressed:
- The effect of temperature, medium and method on surface hardening of Ti-6Al-4V.
- The hardening response and carbo-oxidizing behavior of Ti-6Al- 4Vwith different initial microstructures.
- The influence of oxygen partial pressure and carbon activity on the surface hardening and evolution of surface compound layers of Ti-6Al-4V.
- The role of post nitriding on surface hardening of Ti-6Al-4V subjected to various prior carbo-oxidizing treatments.
- The hardening response and carbo-oxidizing behavior of pure titanium and various titanium alloys subjected to carbo-oxidizing in different media.
In order to investigate these points, the thermochemical treatments were followed by X-ray diffraction (XRD) to determine the surface phase composition of compound layers. In some cases, successive layer removal combined with XRD was applied to examine the phase distribution in depth. After sample preparation, metallography and electron microscopy investigations were performed on cross-sections. The chemical composition was determined by energy dispersive spectrometer (EDS), and electron probe microanalyzer (EPMA) was used for qualitative and quantitative analysis of light elements. Vickers hardness measurements were performed to examine the indepth hardness profile.

The main results of this Ph.D. work showcase the widely different types of treatments and the diversity of gaseous thermochemical methods that can be applied for surface engineering of titanium alloys. Selection of appropriate temperatures and media should be reconsidered for surface hardening of titanium alloys, because different alloys have different responses to surface treatment. The lamellar structure was revealed to be oxidation resistant, which is consistent with the observation by other researchers. Moreover, in the present work it is reported that the interlamellar β phase yields an effective barrier to penetration of rutile into the diffusion zone. A post nitriding treatment prior to the carbo-oxidizing treatment was found to enhance the hardening effect through dissolution of the firstly formed compound layer(s), formation of a nitride layer and by enhancing the interstitial diffusion in the hardened case. The advantage of post nitriding treatment largely depends on the prior condition of carbo-oxidizing. For example, a low-temperature carbo-oxidized sample has a better response in terms of surface hardening than a high-temperature treated sample. The formation and evolution of TiC_xO1-_x are one of the primary topics throughout the entire research.