Thermal oxidation of titanium-based cold spray coatings for biological and wear-related applications

Abstract

The combination of strength, toughness and ductility makes metallic materials more preferable for engineering applications than ceramics and polymers. But the surface properties of metallic materials are insufficient for some of these applications. Thus, surface modification of metallic materials enables the keeping bulk properties with better surface facilities. In this thesis, the aim is the surface modification of metallic materials (Co-Cr alloy and 316L stainless steel) via cold gas dynamic spraying (CS) system. CS enables the spraying of feedstock powders to the substrate with supersonic velocities. The collusion of powders with the substrate leads to plastic deformation of powders and generates the coating formation over the substrate. Via the CS process, it is possible to deposit metallic, ceramic, polymer and composite coatings on different kinds of substrates. The application of the CS process on metallic materials generally purposes to improve the wear, corrosion and biological performance of these materials. In the scope of the thesis, the surface modification of Co-Cr alloy and S316L Stainless steel handled by Titanium-based CS coatings. With the native oxide film forming on the Titanium and its alloys, these metals show high corrosion resistance and biocompatibility. Nevertheless, the thickness of the native oxide film is approximately 10nm and bad at wear applications. Thus, there is a need for a second treatment to make the oxide film more mechanically and chemically more stable. Thermal oxidation (TO) is a very cheap and practical method for Titanium-based materials' surface modification. TO enables the formation of relatively thick and protective oxide layers on Titanium-based materials. In the scope of the thesis, the feasibility of TO on CS'ed Titanium-based coatings is examined. Microstructural and mechanical characterisation and biological tests carried on the coatings manufactured the sequential application of CS and TO processes. In the first section of the thesis, the feedstock was prepared by 92wt.% Titanium and 8wt.% Aluminium deposited on the ASTM F75 Co-Cr alloy via CS process. After that, TO applied these coatings at 600°C for 60h in the air atmosphere. The heating and cooling rate was kept very slow to minimise thermal stresses during the TO step. The results of microstructural examinations after the TO process proved that the outermost surface of the Titanium-based coating was covered by oxide film with a thickness of 3µm. The phase analysis showed that the oxide layer consists of the Rutile phase of TiO2 and a minor amount of Al2O3. The coating showed better wear performance than the Co-Cr alloy during the wear tests conducted under 4N in dry sliding conditions. Furthermore, the bioactivity test conducted in simulated body fluid for 28 days proved that the bioactivity of the coating is way better than the Co-Cr substrate. In the second study of the thesis, the same feedstock and substrate as the first study were used. As a difference from the first study, the microstructural changes in the inner section of the coating were evaluated, and mechanical characterisation was done in more detail. During microstructural examinations, it was proved that the enrichment of Titanium with Oxygen took place beneath the oxide layer during the TO process. Furthermore, the reaction between neighbouring Titanium and Aluminium particles led to Ti-Al intermetallic at the inner coating section. All these reactions occurred during the TO process, making the section beneath the oxide layer harder than the deposited structure. The adhesion tests conducted under compression and shear loads proved that the oxide layer is well-adherent to the coating. The hardness of the oxide layer was measured as 1200HV via with depth-sensing method. At reciprocating wear tests conducted under 1-2-3N in dry and serum conditions, the coating produced by sequential application of CS and TO yielded higher wear resistance than the untreated Co-Cr substrate. In the third section of the thesis, a work was initiated to examine the role of Zinc addition to the feedstock on the wear performance of the coating. For this purpose, CS feedstock was modified with different amounts of Zinc powders (2.5-5-10 wt.%). The microstructural examinations conducted on CS'ed coatings demonstrated that increment in zinc amount causes a decrement in the deposition efficiency during the CS process. The Zinc Oxide (ZnO) phase was detected along with TiO2 and Al2O3 during the phase analyses conducted after the TO process. The accumulation of zinc particles in the different parts of the coating is also observed during the microstructural examinations. These sections of the coating are called "Zinc-rich regions". The phase change during/after the TO process was simulated by FactSage software for Zinc-rich and Zinc-poor regions. The main phase for zinc-reach regions after the TO process was simulated as ZnO, while the Zinc-poor regions were simulated as TiO2, consistent with the XRD results. The wear tests conducted under 3N load in dry sliding conditions show that the coefficient of friction (COF) value can be decreased to 0.12 by adding 5wt.% Zinc to the feedstock. In the final section of the thesis, the feedstock is reinforced with Silver to improve the biological performance of the coating. Silver is a well-known antibacterial metallic agent for biological applications. The feedstock prepared with 5wt.% Silver reinforcement was successfully deposited on 316L stainless steel via the CS process. The phase analysis conducted after the TO process proved that Silver still standing on the metallic form. Microstructural examinations have been shown that the Silver was not distributed homogenously around the coating. For mechanical evaluation, wear tests were applied under 1N load in dry and serum conditions to both coating and 316L substrate. The coating produced via sequential application of the CS and TO process yielded superior performance in both dry and serum conditions compared to the 316L substrate. Bioactivity tests proved that the coating was more bioactive than 316L stainless steel. The antibacterial efficiency of the coating was verified by the disc diffusion type test against Staphylococcus aureus (S. aureus) bacteria, and the antibacterial characteristics of the coating were supported by the release test of Silver ions. In this scope, the coating fabricated with 5wt.% reinforced feedstock could be a choice not only for the in-vivo applications but also the in-vitro applications where the hygiene of touch surfaces is essential such as door handles, touch plates, bed rails, and call buttons the hospitals. In summary, with the combination of CS and TO processes, it is possible to modify surfaces of the metallic materials with Titanium-based coatings. The outermost surface of the coating is covered by an oxide layer mainly consisting of the Rutile form of TiO2. The hardness of the oxide layer is approximately 1300HV, and this layer is well-adherent to the coating. The tests conducted under different loads and in different conditions showed that the coatings yielded superior wear performance than the Co-Cr and 316L substrates. Furthermore, the bioactivity and antibacterial tests results indicate that these coatings can also be used in biological applications.