Analytical Electron Microscopy of Interfaces in Cathodic Arc Coatings

The aim of this work was to study the effects of changing the substrate bias and orientation on arc evaporated nitride coatings. The microstructure and microchemistry of the coatings were investigated with emphasis placed on study of the coating/substrate interface region which governs the adhesion of the coating - a property that is vital to the functional performance of the coating. The effect of the sputter cleaning process prior to deposition of the coating was investigated in some detail.

Analytical electron microscopy techniques were employed in the investigations in this thesis. High resolution EELS and EDX line profiles across the interface region were used in analysis of the coatings, as well as bright and dark field imaging, EDX mapping and diffraction techniques. A non-standard diffraction technique, whereby the angle of convergence was decreased to increase the angular resolution, was used in diffraction analysis of the interface region where the diffraction patterns were difficult to resolve. In analysis of EELS data, the use of edges from standard compounds to remove the background from edges in similar compounds was demonstrated.

The substrate pre-treatment of Ti(6%Al, 4%V) by polishing and Cr sputter cleaning resulted in modification of the microstructure at the substrate surface with the formation of small grains of b -Ti(6%Al, 4%V). Cr was implanted in the substrate during the sputter cleaning process resulting in an interface layer of graded composition; this layer was a bcc alloy of Cr and Ti. Etching of material from the substrate surface also occurred during the sputter cleaning process. Cr sputter cleaning of Ti resulted in both etching and implantation of material although there was net removal of material from the substrate. In contrast Ti sputter cleaning of TiN etched little of the substrate but deposited a layer of Ti, the thickness of which strongly depended upon the orientation of the substrate surface relative to the cathode surface.

Deposition of CrN on Ti(6%Al,4%V) resulted in the initial deposition of a layer of fine grained sub-stoichiometric Cr2N. This was followed by columnar growth of sub-stoichiometric CrN. Changing the substrate bias from -70V to -200V resulted in deposition of coatings with larger grains and fewer faults.

Increasing the angle of orientation between the substrate surface and the cathode surface led to the deposition of thinner coatings which were of a more uniform thickness. Due to gas scattering the effect of the orientation upon the thicknesses of coatings deposited was much less pronounced during the deposition process than during the sputter cleaning process. For the deposition of the CrN coatings a decrease in the Cr content of the coating and an increase in the grain size was observed as the angle of orientation increased.

The presence of droplets increased the surface roughness of the coatings by about three orders of magnitude and led to defects with voids and growth defects being observed. The deposition of material in the form of droplets also showed a strong dependence on the orientation of the substrate to the cathode with the number of droplets deposited decreasing as the angle of orientation increased.