Year

2014

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

Monolayer TiN, TiCN and multilayer TiN/Ti and TiCN/Ti coatings have been successfully deposited on the surface of Ti substrate and Ti6Al4V alloy substrate using the filtered arc deposition system (FADS). The samples’ thicknesses are about 3.5‐4.5 μm. Characterisation of these coatings on different substrates in terms of texture, mechanical properties and fracture behaviour have been carried out. The fracture behaviour is supported by a finite element analysis which has provided results of shear/Von Mises stresses and strain energy density.

Nano‐indentation tests have been performed using Berkovich indenter on all the samples to detect the mechanical properties such as Young’s modulus and hardness. The Young’s modulus and hardness of Ti substrate coated with a monolayer TiN with the thickness about 3.5 μm was 340 GPa and 32 GPa, respectively. For multilayer TiN/Ti coated sample, the corresponding values decreased a little because of the soft Ti interlayer between TiN coatings. Compared the samples coated on Ti substrate to Ti6Al4V alloy substrate, the Young’s modulus and hardness of the samples coated on Ti6Al4V alloy were slightly higher because of the contribution of the harder Ti6Al4V alloy.

The Young’s modulus and hardness of the monolayer TiCN reached 398 GPa and 36 GPa, respectively. The values of Young’s modulus and hardness of multilayer TiCN/Ti coated Ti substrate decreased a little because of the softer Ti layer. The Young’s modulus and hardness values of TiCN and TiCN/Ti coatings deposited on Ti6Al4V alloy were higher compared to that of the corresponding Ti substrate.

Surface topography and chemical composition of coatings were characterised by atomic force microscopy (AFM) and X‐ray diffraction (XRD), respectively. The lattice constants of the coatings were also characterised by X‐ray diffraction. For the bare Ti and Ti6Al4V alloy substrate after polishing, the roughnesses were 0.98 nm and 1.50 nm, respectively. The roughness of all the monolayer coated samples and multilayer coated samples have increased to about 20 nm and 10 nm, respectively. The roughness of TiCN‐based coating was higher than TiN‐based coating due to the mixed reacted gas of N2 and CH4, which made the coating material structure more complex. All the XRD spectra of monolayer TiN and TiCN coatings showed FCC structure with (111) preferred orientation. Other peaks, such as (200) and (220) could be detected on multilayer coatings. The Ti in the multilayer TiN/Ti or TiCN/Ti coatings could stop the growth of the columnar crystal structure. This phenomenon indicated that (200) and (220) are other orientations at the early stage of the growth of TiN or TiCN crystals.

The corrosion of uncoated commercially pure Ti and Ti6Al4V alloy substrate and all the samples with coating were performed on a CHI electrochemical analyser in Ringer’s solution at room temperature. The potentiodynamic polarisation curves showed that all the samples with coating had better corrosion resistance than bare Ti and Ti6Al4V substrate. The samples coated with multilayer TiN/Ti or TiCN/Ti had lower log current density and higher corrosion potential than their corresponding monolayer coatings.

Nano‐indentation tests were used to characterise fracture modes of the coatings. A spherical diamond indenter with a radius of 5 μm was employed in the tests. The maximum load for each sample was up to 160, 260 and 360 mN. For monolayer TiN or TiCN coatings, a series pop‐ins could be detected, which suggested that severe crack could be generated underneath the coating. But for all the multilayer TiN/Ti and TiCN/Ti coatings, few pop‐in events could be seen or even none could be found. TEM observations were carried out to elucidate the deformation mechanisms of the coatings. Inter‐columnar, inclined and lateral cracks were found to be the dominant crack modes in the monolayer TiN and TiCN coatings while small bending crack and radial crack were the dominant crack modes in the multilayer coatings. High resolution TEM (HRTEM) image inside the Ti layer of multilayer TiCN/Ti coating indicated that grain size of the Ti layer less than 10 nm. Very few edge dislocations were observed in HRTEM image as dislocations can be easily absorbed at the grain boundaries once they were generated.

To qualitatively analyse the fracture behaviour during the indention process, FEM analysis was conducted using ANSYS/LS‐DYNA to simulate the nano‐indentation experiments conducted in the present study. Distribution of shear stress, Von Mises stress and strain energy density were used to analyse the crack behaviour of monolayer TiCN and multilayer TiCN/Ti coatings on Ti substrate. The strain energy density was found to follow the distribution of shear stress. It has shown that this type of simulation is an effective way to explain the fracture behaviour of coatings.

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