Study On The Structure And Properties Of Porous Ta Coatings And Ti-Ta Composites Prepared By Cold Spraying

With the massive increase in human aging,more and more patients require the use of prostheses in order to replace critical parts of their skeletal system.This leads to an ever increasing demand for orthopedic implants worldwide.Among many metallic biomaterials,Ti alloys have become the most widely used orthopedic implant materials in the medical industry due to its excellent mechanical properties,high corrosion resistance and good biocompatibility.However,lack of osteoinduction and potential release of harmful ion in Ti implants often lead to its short-term failure.Further,it may lead to allergic reactions,serious long-term health problems and even secondary surgery.Therefore,researchers always look forward for new solutions by adopting following two strategies.The first is surface modification of Ti alloys,where the surface characteristic of biomaterials is the key factor to determine the success of the implant;The other is to develop a suitable new implant material.Based on this,the first objective of this work was to prepare a suitable Ta coating on Ti alloy substrate.Ta has been reported to be better than Ti in vivo/vitro biological properties,which has attracted more and more attention from researchers in recent years.Currently,Ta coatings are mainly prepared by plasma spraying(PS),chemical or physical vapor deposition(CVD or PVD),and laser engineered net shaping(LENS（？）)However,these methods have the problems of high deposition temperature,easy oxidation,high cost and so on,which require high vacuum or high purity protective atmosphere.Compared with the above methods,cold spray is a more suitable and simpler method to prepare Ta coating.Further,very limited reports are available in the open literature on the corrosion performance of cold sprayed Ta coatings.In this thesis,Ta coatings were prepared for human bone hard tissue repair using cold spray technology,and their in vitro biological properties were evaluated.Moreover,a novel Ti-Ta composite bio-implant materials were prepared by cold spray additive manufacturing technology and preliminarily explored in this thesis.The main contents of this thesis are as follows:The influence of feedstock powder morphology on microstructure and mechanical properties of cold sprayed Ta deposits was studied.For this purpose,hydrogenation dehydrogenation(HDH)-1-Ta powder(with porous morphology),HDH-2-Ta powder(with quasi-spherical),and mechanical pulverization(MP)-Ta powder(with angular block morphology)were used.The results showed that the powder morphology has a significant effect on the deposition quality.The coating deposited by using HDH-1-Ta powder was the best with the relatively flat section,the lowest porosity,the highest deposition efficiency,the highest microhardness and the decent adhesion bonding strength,while HDH-2-Ta powder can be cold sprayed to obtain a unique rough/porous structure.In contrast,the deposition performance of MP-TA powder was the worst.The HDH-1-Ta powder showed fine cold spray ability which was associated with its porous structure.This led to its reduced hardness,which was more conducive to achieve excellent compressibility and compaction behavior and thus a denser deposition.The rough/porous Ta coatings and Ta/HA composite coatings were successfully fabricated by cold spray on Ti6Al4V substrate using different Ta powders.The results showed that this process has the potential to create rough/porous Ta coatings with lots of visible macropores.Ta/HA composite powders were prepared by mechanical milling porous Ta and agglomerated nanocrystalline hydroxyapatite(NC HA)powders.The asprepared composite coating was successfully deposited by cold spraying without oxidation,decomposition and phase transformation.Finally,the SBF soaking test showed that the rough/porous Ta coating and the Ta/HA composite coating has good bioactivity.Its rough/porous surface and island structure are favorable for nucleation of apatite,and the addition of HA in the composite coating is also favorable for SBF mineralization.Based on the formation of rough/porous Ta coatings,macroporous Ta coatings with different pore size have been developed by cold spray(CS)technology.The pore size increased with the increase of deposition passes,then the surface roughness also increased significantly.In vitro cell culture experiments showed that there was no significant difference in cell activity between macroporous CS-Ta coatings and TC4 control group,indicating CS-Ta coatings had good cytocompatibility.However,compared with TC4 sample,macroporous CS-Ta coatings could promote cell spreading.HBMSCs could penetrate and grow into the pores of CS-Ta-L coatings with pore size larger than 300 μm at the early stage,revealing its excellent osteoconductivity.In addition,macroporous CS-Ta coatings presented significantly enhanced osteodifferentiation when compared with Flat-Ta and TC4 in terms of the expression level of ALP activity,ECM mineralization as well as bone-related genes(ALP,OCN and OPN).In particular,CS-Ta-L exhibited significantly upregulated bone-related genes expression.This is mainly related to the special micro-nano structure in large pores and Ta2O5 composition.In order to explore the use of cold spray technology for the development of suitable new implant materials,novel Ti-Ta composites were successfully fabricated through cold spray additive manufacturing followed by a diffusion treatment using mechanically mixed Ti and Ta powders with different proportions.The microstructures and mechanical properties of the prepared Ti-Ta composites were explored for their possible use in biomedical industry.The results showed that the as-sprayed Ti-Ta composites were composed of uniformly distributed Ti and Ta splats,and an insufficient diffusion occurred during the heat treatment of Ti-Ta composites.The prepared composites were composed of unique triple structure,consisting of Ti-rich,Ta-rich and diffusion regions.The ultimate tensile strength of Ti-Ta composites firstly increased and then decreased with the increase of heat treatment temperature,which reached the peak value at 1000℃.The elastic modulus showed an opposite trend and it reached the lowest value at 1000℃.Overall,Ti-20Ta composite(heat treated at 1000℃ for 72 h)exhibited low elastic modulus(83.13 GPa)and high tensile strength(742 MPa).Therefore,cold spray technology seems to open up an opportunity for the rapid manufacturing of biocompatible implants.