Mechanical and tribological properties of CrAlN-Ag self-lubricating films

This paper reports on the investigation of the mechanical and tribological properties of reactively sputtered CrAlN-Ag nanocomposite films that were co-sputtered from three individual targets of Cr, Al, and Ag onto Si (111) and stainless steel SS-440C substrates. The deposition parameters included: power to the Cr and Al targets of PCr = 200 W and PAl = 160 W; substrate temperature T = 300 °C; substrate bias voltage Vb = - 130 V; and, power to the Ag target PAg = 0 to 20 W. The elemental composition of the films was deduced from X-ray Photoelectron Spectroscopy and the Ag content was found to increase with the increase in PAg. The crystal structure and film architecture were evaluated using X-ray diffraction and transmission electron microscopy. The films were found to consist of nanocrystals of Ag embedded in a solid solution of CrAlN with a pattern typical of the NaCl structure. The CrAlN grain size was found to decrease with increasing Ag content in the film. The hardness and elastic modulus of each sample were measured by nanoindentation. The hardness increased initially for a very small increase in Ag content (PAg = 5 W) and then decreased substantially with further increases in PAg. Finally, the coatings were rubbed against alumina balls using a ball-on-disk tribotester. Characterization of the wear tracks were performed by optical profilometry. A significant decrease in the friction and wear coefficients was achieved with CrAlN single and multi-phase films compared to the CrN-based counterpart. For example, a low friction coefficient of 0.23 and a wear rate of 3.0 × 10- 8 mm3/N m. were obtained when the silver content was in the 8% range. © 2007 Elsevier B.V. All rights reserved