Structure and mechanical properties of graded Cr/CrN/CrTiN coatings synthesized by close field unbalanced magnetron sputtering

Abstract

Graded Cr/CrN/CrTiN coatings were deposited onto M42 HSS tool steel substrates by close field unbalanced magnetron sputtering (CF-UBMS). The effects of varying Ti sputtering current, ITi, on the chemical composition, crystal structure, morphology and mechanical properties of the graded Cr/CrN/CrTiN coatings were investigated via XPS, XRD, AFM, electron microscopy techniques and nanoindentation tests. XPS and XRD results indicate that the CrTiN layer had formed as a solid solution nitride with fcc B1 type phase. The layers in the coatings were distinct and all the coatings exhibited a columnar microstructure, which decreased in size as the Ti content increased due to the increase in the Ti sputtering current. The mechanical properties of these coatings were determined by nanoindentation tests, which revealed an increase in hardness and modulus as the Ti content increased. However, the damage resistance of the coatings decreased with increasing Ti concentration. Furthermore, deformation behaviour of the coatings at high loads revealed a good resistance to plastic deformation and good adhesion strength for all coatings. These improvements, and the increase in hardness, are attributed to a combination of a decrease in grain size and a formation of solid solution nitride.