Home
Tags
No Tags
Surface Hardening of Titanium Alloys by Gas Phase Nitridation under Kinetic Control
| Title: | Surface Hardening of Titanium Alloys by Gas Phase Nitridation under Kinetic Control |
| Author: | Liu, Lizhi |
| Description: | This work describes a nitriding process for titanium and its alloys, which will improve the wear and corrosion resistance by forming a single phase Ti (N) solid solution that has the same lattice properties as the substrate, but not forming any nitrides on the surface. By kinetically controlling the chemical potential of nitrogen as diffusional interstitials, a large solubility can be achieved, which results in a super-hard surface. The nitrogen pressure, heat treatment temperature, and heat treatment time were investigated to form a desired diffusion profile without the formation of surface nitride compounds. This process can improve the wear resistance without the cost of reduced corrosion and fatigue resistance. A thermodynamic calculation has shown that the partial pressure of nitrogen acquired to avoiding the formation of nitrides is extremely low. A kinetic calculation, however, has indicated that a much higher nitrogen partial pressure can be afforded to nitride titanium alloys without forming nitrides. This kinetic calculation considers the impingement rate at the gas-solid interface, the sticking coefficient, and the interstitial diffusion coefficient. The conception of nitridation under kinetic control is verified in a laboratory-scale system, the design and construction of which is part of this work. Nitridation under well-defined and reproducible conditions was achieved by using a long fused silica tube as the reactor and sealing it with a hydrogen/oxygen torch. To clean the gas atmosphere in the tube prior to nitridation, titanium foil was integrated into the system as a getter material. After the titanium getter has cleaned the atmosphere from undesired impurities, the titanium or titanium alloy specimen was nitrided by exposing it to a well-controlled but very low nitrogen partial pressure, generated by a metal nitride/metal powder pack. With the Cr2 N/Cr powder pack, for example, nitrogen pressure from 10-4 Pa to 10-1 Pa can be achieved by adjusting the powder pack temperature. Thermodynamic and kinetic calculation indicated that this pressure range is suitable for nitridation under kinetic control. By carefully controlling the nitrogen pressure, the sample temperature, and treatment time, a thickness of hardened layer equal to 25 μm was achieved without the formation of nitrides. The surface microhardness is more than 10 GPa, The cross-section microhardness measurements showed the nitrogen concentration profile resulted from diffusion. Tensile testing indicated that nitridation under kinetic control increases the yield strength and reduces the ductility. |
| Permanent Link: |
http://rave.ohiolink.edu/etdc/view?acc_num=case1094223428
http://hdl.handle.net/2374.OX/16913 |
| Date: | 2005 |
Files in this item
| Files | Size | Format | View |
|---|---|---|---|
|
There are no files associated with this item. |
|||