Characterization of Thermochemically Surface-Hardened Titanium by Light Optical Microscopy

Thermochemically treated titanium grades 2 and 5 were investigated by light optical microscopy and hardness indentation. Gaseous oxidation in oxygen and N2O containing atmospheres resulted in a diffusion zone of oxygen in solid solution in titanium with a hardness up to 1000 HV. A surface scale consisting of oxide can be present depending on the treatment conditions. A new type of carbo-oxidtion treatment was applied, where carbon and oxygen are simultaneously incorporated into the surface. This resulted in new microstructural features such as a deep zone of mixed interstitial solid solution, i.e., a diffusion zone, and surface regions consisting of a mixed interstitial compound (TiC_XO₁- X structure). Carbo-oxidation yields hardness values in excess of 2500 HV in the mixed interstitial compound and values up to 1500 HV in the diffusion zone. Simultaneously, with the surface hardening
treatment, core hardening of the material can be obtained.