Development of PVD coating processes informed by plasma diagnostics.

Physical vapour deposition technologies have been on the fast track of development for the last two decades due to their ability to meet demands for special materials and performance tools. The ever increasing complexity of the required coating microstructure and chemical composition can be achieved only by the development of PVD technology and in particular plasma sources for vapour generation that can provide the necessary tools.This thesis describes plasma diagnostic studies of plasma discharges, developments of plasma sources and deposition of CrN coatings. Initially the project investigated vacuum arc plasma discharges used in the Hauzer HTC1000/ABS industrially sized coater. The attention was concentrated to the plasma pretreatment by low energy (1200 - 3600 eV) Cr ion implantation into substrates, which contributed to an enhanced adhesion of subsequently deposited TiAIN coatings. Optical emission spectroscopy (OES), electrostatic probes, and time-of-flight (TOF) spectroscopy were used to study the interactions of the arc plasma with the gas atmosphere in the chamber. It was shown that increasing the pressure of Ar gas had a strong effect on the composition of the generated metal ion flux as the density of highly charged metal species reduced significantly to the benefit of gas ionisation. The mechanisms behind these observations are discussed and supported by further experiments. Based on the plasma diagnostic results, a novel two-stage pretreatment method was developed which allowed an enhanced adhesion due to faster sputter cleaning of the substrate surface and more efficient metal ion incorporation in the substrate material.In the final stages of the project a novel high power pulsed magnetron sputtering (HIPIMS) process utilising peak power densities of 3000 Wcm[-2] was investigated. OES studies showed the first evidence of doubly charged Cr and Ti ions generated by the HIPIMS discharge. Peak plasma densities of 10[13] cm[-3] were measured and, in the case of Cr, metal ions were found to constitute 30% of the total deposition flux to substrates. The influence of power on the plasma density, plasma composition and time evolution of the plasma was studied in detail using OES and electrostatic probes. The conditions for glow-to-arc transition were investigated. CrN coatings (thickness 2 mum) were deposited for the first time using HIPIMS of Cr in a nitrogen atmosphere. The microstructure observed in transmission electron microscopy cross sections was highly dense and droplet free and contributed to an excellent corrosion and wear resistance superior to 20 mum thick electroplated hard Cr, and CrN coatings deposited by arc and unbalanced magnetron sputtering. The HIPIMS discharge was used also for pretreatment of substrates with metal ions analogous to the one performed previously with arc discharge. High adhesion was achieved as indicated by the scratch test critical load value Lc = 85 N.Finally, at an intermediate stage of the PhD project, an alternative source providing metal ionisation was studied. It was based on a radio frequency (RF) powered coil that was inductively coupled to a magnetron sputtering discharge. Energy resolved mass spectroscopy and OES in a laboratory-sized version of the plasma source revealed elevated metal ion densities and high ion energies of the order of 60 eV. This source was upscaled, installed, and tested successfully in the industrially sized Hauzer coater. The ion-to-neutral ratio at the substrate position could be increased 5-fold for a similar increase in RF power.