The aim of the present thesis is to develop novel surface coating techniques for titanium alloys in order to produce the desired coatings against both oxidation and wear attacks.
A novel packed powder diffusion coating (PPDC) technique was developed to produce composite coatings on Ti alloy substrates with high bonding strength. The PPDC process produces thick and compact composite coating consisting of TiAl3 intermetallic matrix, NiAl3 compound as reinforcement and minor residual Al. The composite coating not only shows significantly increased hardness and wear resistance at dry sliding conditions at room temperature, but also provides effective protection of the substrate from cyclic oxidation in air up to 1000 oC. Its oxidation behavior at high temperatures is compatible with the Superni 75 superalloy. This indicates the potential to partially replace the more expensive and heavy Ni-base superalloys with Ti alloys to be used as heat-resistant materials. The thickness of the coating varies from 50 µm to a few millimeters with the different temperatures, times and the Ni contents in the masteralloy source.
The effects of masteralloy composition, PPDC treatment temperature and time on the thickness, microstructure and mechanical properties of composite coating, hardness in particular, were comprehensively investigated. Optimized PPDC treatment parameters were finalized. This technique is valid to pure Ti and various Ti alloys.
Microstructure and phase analysis of the PPDC coatings produced on pure Ti and Ti-6Al-4V was conducted using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was indicated that the composite coatings on both pure Ti and Ti alloy are the same, being a compact structure comprising of TiAl3 matrix with a dispersive distribution of NiAl3 particles and a minor residual Al.
An outward diffusion model was proposed to understand the formation mechanism of the composite coatings on Ti alloys. During the PPDC treatment, Ti atoms diffuse from the substrate into the surrounding packed powder and react with the Al, forming TiAl3 aluminide. Ni particles in the powder mixture play the key role, not only in formation of NiAl3, which further improve the wear resistance and oxidation resistance, but also promote the diffusion of Ti atoms and reaction between Ti and Al, even though the actual mechanism is still unclear. In addition, although the diffusion of Al atoms into the Ti substrate is relatively much slower, such diffusion is responsible for the formation of diffusion bond, resulting in high bonding strength between the composite coating and the substrate. Obviously, the present PPDC approach is innovatively different from the conventional pack-aluminizing process for steels and Ni-base superalloys, where Al atoms diffuse into the substrates and then modify the surface chemical composition of the substrates.
In order to investigate the effects of coating structure and Ni content on the diffusion behavior of Ti and Al, different diffusion couples were prepared through deposition of metals on solid substrates. After heating of these diffusion couples to promote diffusion, it was observed that diffusion of both Al and Ti atoms from the Kinetic Metallization (KM) coatings to the Ti and Al substrates is always much slower than the diffusion from the substrates to the coatings. This indicates that the actual diffusivity of Ti in Al or Al in Ti has little effect on the formation of PPDC coatings, but the coating structure significantly affects the diffusion rate of atoms. Because the inter-particles boundaries in both the KM coatings and the PPDC coatings are loosely bonded as compared with the bulk metals, these boundaries act as the fast diffusion channels to promote the formation of intermetallic. In addition, it was also observed that the additions of Ni can significantly increase the diffusivity of Ti in the TiAl3 coating.
Another novel surface treatment technique developed is a practical, low-cost and environmental friendly gas nitriding process for various titanium alloys. It uses either the powder mixture of 5 wt.% Mg and Al2O3 or the mixture of 60 wt.% Cu and Al2O3 to purify the industrial purity nitrogen. Thus, the contact of high purity nitrogen with the surface of Ti alloy components leads to the formation of compact nitride layers. Compared with the currently available gas nitriding process for Ti alloys using ultrahigh purity nitrogen, the new method significantly reduces the overall costs. After nitriding at 950 oC for 24 hours using Mg purification powder, a 40 µm thick nitride layer can be produced on pure Ti, which resulted in a surface microhardness of HV0.1 1960 that is about 10 times higher than that of the substrate. Optical microscopy, X-ray diffraction and TEM results showed that the nitrides layer on pure Ti comprised of two sublayers, the outmost layer is TiN followed by the adjacent Ti2N layer. Such nitride layer can provide effective protection for Ti alloy substrates to resist wear and corrosion attacks.