Ion-plated aluminium/aluminium oxide coatings using a pulsed oxygen process

Pulsed oxygen was utilized in a thermionically assisted triode ion plating process to produce dense, hard and highly adherent aluminium/aluminium oxide coatings on steel substrates. The coatings were deposited in a 0.1 Pa argon discharge while oxygen gas was injected at various pulse rates into the chamber. The influence of the oxygen flow rates, the pulse rates and the substrate bias on the structure and crystal phase of the coating were investigated using scanning and transmission electron microscopy, X-ray diffraction and Rutherford backscattering techniques. The analysis showed that the resultant coating structure consist of a mixed state of crystalline and non-crystalline phases depending on the oxygen flow rate and on the substrate bias at a constant oxygen pulse rate. Analysis showed that the non-crystalline phase approached stoichiometry for Al₂O₃ at oxygen flow rates of 7.5 Torr l s^-1 and a substrate bias of 2 kV and that there was a substantial increase in hardness.