Understanding Coating of Titanium Substrates by Cold Spray and Aerosol Deposition at the Nanoscale

Thermal spray methods involve melting and accelerating particles at high velocities towards a substrate. The impact of particles with a variety of sizes and shapes leads to deformation and flattening of particles and creates a coating on the surface. The method has been used for depositing metal oxides and bioceramics for producing bioactive coatings or creating other functional surfaces. However, imperfections can occur due to non-molten particles, insufficient particle velocity and oxidation of the substrate. It has been known that ceramic coatings under the influence of high-temperature thermal spray processes exhibit a phase transformation and recrystallization behaviour in conventional bioceramic coatings. The microstructural changes can reduce coating stability and increase the clinical failure of implants. These phenomena can cause significant variations in the overall micromechanical properties across the coating thickness. Therefore, new methods of ceramic and bioactive materials coatings such a cold spray and aerosol deposition (AD) method have been considered more recently. However, deposition processes using these methods need to be optimized. The scope of this research thesis encompasses molecular dynamics simulation (MD) and experiments for cold and room temperature spraying of bioactive particles. The simulations were conducted utilizing high-performance computational systems and using LAMMPS molecular dynamic software. To simulate the deposition of particles, state-of-the-art potential models were used to model the materials by molecular simulation. The bonding mechanisms of the important bioactive particles titanium, titania and hydroxyapatite (HA) at the nanoscale were investigated for cold spray and aerosol deposition. The results showed the effect of several control parameters in the kinetic coating techniques. These included the effects of velocity, particle size, particle shape, particle material and deposition density. The simulation results captured picoseconds ultrafast deformation of the particles, leading to unravelling the rich mechanical behaviour of various materials. The stress-strain plots and maps of temperature rose in the impact zone due to the impact of high-velocity particles, as produced in all the simulated materials. The temperature rise was found to be less than the melting point in the particle deposition all the simulations, titanium, HA and titania, on titanium substrate. The effect of the particle shape of the titanium nanoparticles sprayed by the CS method revealed differences in stress, strain and temperature rises, penetration and transformation depth of the substrate. Cubic and spherical particles were shown to result in higher stress and temperature levels than found in conical and cylindrical shaped particles of the same mass. A higher flattening ratio was also observed in cubes and spheres. Considering the higher degree of plasticity with cubic and spherical particles, the results suggest that these particle shapes might be optimal for producing larger areas of coating covered by the same volume of material. The effects of size and velocity were also studied for titanium spherical particles deposited on a titanium substrate with the two different particle sizes of 2nm and 20 nm and three different substrate sizes. The results showed that an increase in velocity during cold spraying of nanoparticles caused a range of extreme behaviour in the coated titanium, with higher stress, higher strain and temperature, and a higher degree of plastic deformation. The room temperature for the nanostructured ceramic coating mechanism was studied using MD simulations. The two biocompatible ceramics studied were hydroxyapatite and TiO2, both of which are commonly used in biomedical application. The impact and coating behaviours of nanoceramic spherical particles were studied for deposition on titanium substrates. The structural analysis revealed a possible explanation for the bonding mechanism between the deposited particles and the underlying substrate. In both HA and TiO2, specific plastic behaviour was captured at the nanoscale for ceramic particles before the failure point in the stress-strain analysis. Although the fracture behaviour of nanoparticles was not studied, it is assumed that the fracture points captured in stress-strain curves might result from the high strain rate and initial plastic deformation of particles. Further experiments were used to determine the potential of the AD method for the coating of TiO2 on a titanium substrate for biological applications. Focussed ion beam scanning electron microscopes (SEM-FIB) were used to characterise the AD coating. Further tests of hardness, roughness and bonding adhesion test were performed to study properties of the coating. Biological characterisation of the coated substrates was studied using conducting cell attachment and proliferation tests. The experimental results showed that the rough surface of AD titanium samples coated with titania could provide an excellent host for cells. The results also demonstrated that cell attachment and proliferation improved in the coated samples compared with uncoated titanium specimens. The rough surface was conducive to cellular activity with improved attachment and proliferation on the coated specimens. The characterization results demonstrated the successful deposition of a thin layer of Titania of 500nm-2μm thickness, which resulted in a rough surface with the average roughness of 425 nm, which was ~10 times coarser than that of the uncoated titanium. The rough morphology of the coating improved the adhesion between cell and ceramic surface. Therefore, the experimental evaluation of nanostructured ceramic coating with low-temperature methods suggested the application of aerosol deposition for bio-implants. The simulation results revealed the properties of coated samples and pointed to future directions for improving the efficiency of the coating by the cold spray family as well as aerosol deposition methods. The experimental part of the project in this thesis has used TiO2 coated on titanium samples that are made from aerosol deposition method in the research group of Dr Frank Gärtner at the Department of Materials Engineering, Helmut Schmidt University, Hamburg, Germany