Tribology International 37 (2004) 1031–1038 www.elsevier.com/locate/triboint Tribological behavior of plasma-enhanced CVD a-C:H films. Part II: multinanolayers P. Gupta, E.I. Meletis  Materials Science and Engineering Program, Mechanical Engineering Department, Louisiana State University, Baton Rouge, LA 70803, USA Abstract The tribological behavior of multinanolayered amorphous hydrogenated carbon (a-C:H) films was investigated. The multi- layered films were synthesized by depositing soft/hard a-C:H alternating nanolayers, using dc plasma-enhanced CVD in a CH 4 / Ar discharge. The soft and hard nanolayer components were produced by changing the CH 4 /Ar ratio. Multilayered films with the same or variable thickness of the soft and hard nanolayer were synthesized. The mechanical properties were studied by nanoin- dentation, intrinsic stresses by profilometry and tribological behavior by pin-on disk experiments. In general, the multilayered films exhibited lower intrinsic stresses, higher hardness and elastic modulus than the respected average values of their two con- stituent films. Furthermore, the multilayers were found to possess both, low friction and high wear resistance. The tribological properties were enhanced by decreasing interlayer thickness with the 50 nm/50 nm multinanolayered film exhibiting the best tri- bological performance. This behavior is attributed to a correlation between the thickness of the nanolayer constituent and proper- ties of the transfer layer. # 2004 Elsevier Ltd. All rights reserved. Keywords: Multinanolayers; Amorphous hydrogenated carbon; Plasma-enhanced CVD; Tribology; Nanoindentation 1. Introduction Amorphous hydrogenated carbon (a-C:H) films have been the focus of attention in the tribological field especially due to their attractive combination of low friction and high wear resistance along with chemical inertness. One major drawback of a-C:H films is their high internal compressive stresses that increase by increasing the sp 3 content. High intrinsic stresses reduce adhesion to the substrate preventing the growth of thick films [1]. Previous efforts to reduce intrinsic stresses in a-C:H involved thermal annealing, especially in ta-C [1]. The complete stress relief has shown to occur at 600–700 v C with minimal structural modifica- tions [2,3]. The problem with thermal annealing of a-C:H films is that they start graphitizing after 350–400 v C and loose their diamond-like properties [4]. An alternative route to reduce intrinsic stresses is to synthesize diamond-like carbon/diamond-like carbon multilayered (DDM) films. There have been attempts in this direction recently, using alternating nano layers of soft (low stress) and hard (high stress) a-C:H films to improve mechanical and tribological properties [5,6]. Knoblauch et al. [5] reported improved tribological properties in multilayered a-C:H films as compared to their monolithic films. Also, studies on amorphous car- bon (a-C) multilayers have shown reduction in intrinsic stress and improvement in mechanical and tribological properties [7,8]. In the above studies, the multilayered films synthesized have been of total thickness up to 500 nm. Thick films, however, are usually required for practical wear applications. Also, multilayered struc- tures, in general, have the potential to improve other critical properties such as toughness [9]. Thus, synthesis of a-C:H multilayers has the potential to further enhance performance of these films and promote their utilization in advanced applications. It has been well established that a-C:H film char- acteristics such as content of sp 3 bonded C can be con- trolled by processing parameters. Composite a-C:H multilayers have been synthesized previously by mod- ulating the bias voltage [5–8]. A different route was  Corresponding author. Tel.:+1-225-578-5806; fax:+1-225-578- 5924. E-mail address: meletis@lsu.edu (E.I. Meletis). 0301-679X/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2004.07.021