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. reac tive magnetron sputtering starting from an aluminium target in Ar/N-2 mixtu res. 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, P-N2/P-tot = 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 gen eration originated by the incorporation of Er atoms in the AIN structure, t he films became amorphous for higher Er contents. The highest hardness valu es were obtained for the crystalline AIN(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 c oatings (varying from 7 to 8 N) revealing the brittle character of the depo sited 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 t he possible use of AIN(Er) films in tools applications, these films have a suitable hardness but a poor adhesion to steel alloys when compared with la yers usually used for mechanical protection. (C) 2000 Elsevier Science S.A. All rights reserved.