Wet chemical surface modifications of Titanium and Ti6Al4V alloy and their effect on the hydrothermal aging mechanisms and adhesion properties

Abstract

Titanium and its alloys are used in many areas of industry (aerospace, automobile, etc.). Adhesive bonding of titanium and titanium alloys is an industrial topic for approximately 40 years. However, development of new surface pre-treatments based on environmentally friendly and non toxic chemicals to improve the durability of titanium bonded joints have gained more interest in recent years. The main aims of this thesis is focused on the development and examination of non toxic and environmentally friendly pre-treatment of Ti6Al4V prior to adhesive bonding using a conversion coating and anodization process based on cerium (IV). The basic purpose of this employment is to investigate the influence of such surface treatments on the degradation mechanisms during aging of titanium oxide layer as well as on the shear strength and durability of Ti6Al4V/epoxy resin adhesive bonded joints system. This thesis also deals with the development and characterization of environmental friendly pre-treatment of pure titanium and Ti6Al4V before adhesive bonding using an etching process based on sulfuric acid. Such etching process was performed in order to generate microstructures and to increase roughness on the surface. Additionally, it was carried out to determine the hydrogen embrittlement behavior of pure titanium and Ti6Al4V alloy and its influence on the durability of bonded joints. The results show that surface treatments (conversion coating and anodization) in cerium (IV)-based solution lead to the formation of ultrathin cerium-based layers (< 10 nm) consisting of CeO2 and Ce2O3 on top of the amorphous titanium oxide. Aging tests under hot-wet conditions reveal that such layers protect titanium oxide films against hydrothermal effects whereby a kinetic stabilization is achieved, i.e. kinetic inhibition of transformation of amorphous titanium oxide into crystalline state. However, it was found that such ultrathin cerium-based oxide layers on Ti6Al4V surface are not a qualified basis layer for adhesive bonding applications due to their low cohesion strength and high affinity to carbon compounds from ambient air. The investigations of sulfuric acid etched samples reveal that a hydrogen-embrittled layer forms near the surface of pure titanium substrate whereby the durability of bonded joints is negatively affected. In the case of sulfuric acid etched Ti6Al4V, no hydrogen embrittlement is observed. The findings also show that sulfuric acid etching generates microstructures on both substrate surfaces and lead to an increase in surface roughness.