STOICHIOMETRY OF UNBALANCED MAGNETRON-SPUTTERED AL-MG ALLOY COATINGS

Numerous Al-Mg coatings were prepared by unbalanced magnetron sputteri ng with ii range of substrate bias voltages. The coatings were deposit ed from circular planar targets, composed of sequential sectors of Al and Mg, bonded to a directly cooled copper backing plate. The chemical composition of the coatings was found to be significantly affected by preferential resputtering of Mg due to ion bombardment. X-ray diffrac tion indicated that all coatings were extremely fibre textured. Some d ramatic changes in cl-spacing of the coating were observed for only sm all increments in the bias voltage. This indicates a transition in cry stal structure induced by preferential resputtering. Extremely high am ounts of argon were also detected in the coatings which contained a Mg -rich cph structured phase. Very little argon was detected in any of t he Al-rich fee coatings, even with;high substrate bias voltages (-200 V). The burial of argon in the cph structured Al-Mg coatings is not pu rely an ion implantation phenomenon, but is controlled by microstructu ral entrapment or alloying effects. It is concluded that ion-assisted deposition of Al-Mg alloy coatings should be treated with some caution , because of these difficulties with the control of stoichiometry. (C) 1997 Elsevier Science S.A.