Low temperature alpha alumina thin film coating for cutting tool application by AC inverted magnetron sputtering technique

This work studies the feasibility of depositing alpha phase of alumina at low temperatures by AC inverted magnetron sputtering technique for cutting tool applications.;Alpha alumina has been deposited by CVD and PVD techniques previously by other researchers. In CVD technique the high temperature restricts the use of temperature sensitive substrates. PVD technique was previously used to deposit gamma alumina which was later heat treated to obtain alpha phase of alumina at a temperature of 700°C. Chromium oxide was used as a template layer to grow alpha phase of alumina but low deposition rates restricted the thickness to 150 nm.;This work mainly focuses on the depositing alpha phase of alumina at low temperatures using a chromium oxide template layer with high deposition rates using a new PVD process called AC inverted magnetron sputtering technique. The effect of deposition parameters such as power, partial pressure of oxygen, bias and thickness on the film properties were studied. Various characterization techniques were used to evaluate the coatings. Tribological and machining tests were carried out to evaluate the coating performance. Deposition temperature was measured as 350°C. Deposition of alpha alumina was approached in three different ways. In the first approach alpha alumina was deposited directly without any aid of substrate heating or template layer. In this approach, we were able to grow alpha alumina up to a thickness of 100 nm beyond which there is a transformation of phase from alpha to gamma. The second approach involved depositing alpha alumina using optical emission spectroscopy to monitor and to control the oxygen flow rate. This technique yielded gamma phase and the process was not stable and repeatable. In the third approach, a chromium oxide template layer was used to grow alpha alumina at low temperatures. Thin films of alpha-Cr2O3 were deposited by AC inverted cylindrical magnetron sputtering technique at 350°C. The hardness measurement showed lower hardness values of 16 GPa on glass and 30 GPa on stainless steel. Pure and mixed alumina coatings were deposited on this chromium oxide layer by AC inverted magnetron sputtering technique at 350°C. XRD and SAED results showed that pure alpha alumina coatings could be grown at 6 kW and 50% partial pressure of oxygen for a deposition time of 2 hours. The tribological properties of the alpha alumina coatings showed that the maximum hardness obtained was 20 GPa for the optimized parameter of 5 kW and 50% partial pressure of oxygen. The coefficient of friction (COF) was lower for pure alpha alumina coatings because of their low roughness. PVD alpha alumina coated tools were compared with CVD alpha alumina and PVD-TiAlN coated tools. In wet machining, the PVD-TiAlN coated tools showed the best performance. Future work will include a multilayered coating of chromium oxide and alpha alumina which may improve the performance of the PVD alumina coated tools.