Low-Temperature Thermochemical Surface Hardening of Titanium Alloys through Oxidizing and Vacuum Treatment

Abstract

This study investigates a two-step surface hardening process for CP titanium and Ti6Al4V, improving wear resistance while preserving the metallic luster of titanium. The process consists of: (a) gaseous oxidation forming a thin, dense, and adherent oxide layer with a shallow oxygen diffusion zone; and (b) a high-vacuum treatment that dissolves the oxide layer and promotes further inward oxygen diffusion, with the oxide acting as an oxygen reservoir. Systematic experiments were conducted on CP titanium using CO ₂ and N ₂ /N ₂ O gas mixtures at 500–700 °C for 16–81 h. The CO ₂ -based route appeared beneficial for industrial adoption due to its low equivalent oxygen partial pressure. A selected condition (CO ₂ at 650 °C for 16 h, followed by vacuum at 680 °C for 16 h) was extensively characterized using SEM, GDOES, XRD, spectrophotometry, micro-hardness indentation, ball-on-disk wear testing, and corrosion testing. No loss in light reflectivity was observed after treatment. Surface hardness exceeded 750 HV0.005 for CP titanium and 700 HV0.005 for Ti6Al4V. Sliding wear tests showed an approximately 3.5-fold reduction in wear volume, while corrosion resistance was maintained. The relatively low processing temperatures reduce the risk of dimensional changes and grain growth, making this method attractive for industrial titanium components exposed to loaded sliding.