Interface design for metal-ceramic bonding

Abstract

Adhesion of thin ceramic films to metallic substrates is of key importance in engineering applications where a coating failure could compromise the safe operation of a certain component, or result in an accelerated degradation of the overall performance and therefore not meet the specifications. Besides corrosion and wear, typical engineering materials are usually subjected to thermomechanical variations. For this reason, the goal of this thesis is to use helium implantation or laser treatment to design a substrate-integrated thermally or mechanically functional interface to improve the performance and durability of ceramic coatings on metals. Alumina, yttria-fully-stabilized zirconia, and boron carbide thin films (360-550 nm thick) were deposited on Inconel 600 and Ti6Al4V substrates by electron-beam physical vapour deposition. Additionally, helium implantation or ultraviolet nanosecond pulse laser were used to treat the alloy surface prior to ceramic thin film deposition, generating modified layers of depths ranging from 300 nm to 1.5 μm. The microstructure, nanomechanical and fracture behaviours of the metal-ceramic pairs were analyzed using a diverse range of techniques in order to assess the properties of the constituents and their interface, and how these are affected by the substrate treatments. Helium implantation was found to decrease the reduced modulus of the base substrates and increase the hardness of Inconel 600, however it also modified the residual stress of the coatings generating higher tensile states compared to coatings deposited on unimplanted substrates, leading to increased delamination, cracking and weaker adhesion. The nanoindentation pop-out phenomenon was successfully linked to delamination of boron carbide coatings, and mechanical twinning was identified as a main contributing mechanism to implanted Inconel deformation. Apart from three specific cases, helium was found to impact negatively on metal-ceramic bonding. The highly rough surface topography generated by laser treatment prevented a full coating coverage of the surface for thinner films, and thus affected the ability to extract meaningful nanomechanical properties. Specimens with metallic interlayers displayed a cross-sectional layered morphology as a result of preferential laser ablation instead of melting. Hence, the use of this treatment to enhance coating/substrate bonding is advised against in its current form. Direct laser treatment of Ti6Al4V resulted in a decrease in its reduced modulus and hardness, which was related to the formation of a martensitic phase. Unusual {10¯11} compression twins, typical of high strain deformation of Ti6Al4V, were identified in the modified region and linked to the shock wave generated by the laser pulse.