The influence of Er doping of Al–N sputtered coatings on their mechanical properties

Thin films of AlN(Er) were deposited on M2 steel substrates using r.f. reactive magnetron sputtering starting from an aluminium target in Ar/N2 mixtures. Er doping was achieved by partially covering the aluminium target with an adequate number of erbium pieces. All depositions were achieved with a discharge power of 600 W, PN2/Ptot=0.5 and a total pressure of 0.74 Pa. The Al/(N+Al) ratio in the as-deposited films varied between 0.48 and 0.51 while Erbium contents ranging from 0.2 to 3.5 at.% were achieved. X-ray diffraction showed that the AlN(Er) films with Erbium contents up to 1.4 at.% have a crystalline structure with a preferential orientation on the (002) planes. However, as a consequence of lattice distortion and/or defect generation originated by the incorporation of Er atoms in the AlN structure, the films became amorphous for higher Er contents. The highest hardness values were obtained for the crystalline AlN(Er) films (up to 31 GPa at 1.2 at.% of Er) while a significant decrease of the hardness was observed when the films became amorphous (22.5 GPa at 3.5 at.%). The mean critical loads for cohesive failure measured by scratch test were quite similar for all the coatings (varying from 7 to 8 N) revealing the brittle character of the deposited films. The mean critical loads for adhesive failure were in the range of 12 N for low Er content to 22 N for the higher Er content. Concerning the possible use of AlN(Er) films in tools applications, these films have a suitable hardness but a poor adhesion to steel alloys when compared with layers usually used for mechanical protection.