Effect of vacuum degree on adhesion strength and corrosion resistance of magnetron sputtered aluminum coating on NdFeB magnet

A pure aluminum (Al) coating was deposited by direct current (DC) magnetron sputtering onto sintered NdFeB magnet coupons and glass slides at different vacuum degrees. The effect of vacuum degree on adhesion strength and corrosion resistance of the coating was investigated. The results showed that the adhesion strength between the Al coating and the substrate NdFeB magnet increased first and then decreased while the corrosion resistance of the Al coating increased with increasing vacuum degree. The Al coating formed at the low vacuum degree fractured within the coating when being peeled off because of a relatively high density of coating defects, while that formed at the high vacuum degree was mainly debonded along the coating/substrate interface. The decreased adhesion strength when the vacuum degree further increased could be attributed to excessive internal stress. The Al coating deposited at high vacuum degree was more corrosion resistant than that formed at low vacuum as the coating becomes less porous. Due to the presence of the micro defects, the corrosion behavior of the Al coating was accelerated on the substrate NdFeB magnet through the galvanic effect via these defects.