This paper presents the results obtained by reactive radiofrequency sputter deposition of aluminium films onto XC 70 steel in a nitrogen atmosphere. The effect of humidity and temperature on ageing was determined by the such scratch test via the critical load. AES analyses of the Al coating have shown that the aluminium was completely bonded to nitrogen after ageing, whereas prior to ageing some aluminium was present in the metallic state. A similar observation was made in the case of samples heated in vacuum after deposition. Thus it was concluded that the increase in the critical load arising from the ageing treatment of the samples resulted from an increase in the coating hardness because of the bonding of aluminium to nitrogen.