2003 ME Graduate Student Conference

2004 ME Graduate Student Conference April 17, 2004

SYNTHESIS, CHARACTERIZATION AND TRIBOLOGICAL BEHAVIOR OF NITROGEN-DOPED CHROMIUM DIAMONDLIKE CARBON NANOCOMPOSITE THIN FILMS

Sunil Pal M.S. Candidate

Advisor: Dr. E.I.Meletis

ABSTRACT In the present study, nitrogen is introduced to Nanostructured Metal-Diamondlike Carbon (Me- modify the Cr-DLC films by promoting the formation of DLC) thin films exhibit unique mechanical, chemical and CrN and/or the C network by forming –C-N- sp3 bonds. electronic properties and thus, possess great potential for Since the binding energies of CrC and CrN are close, there applications in tribology, electronics, biomedical and is good probability that both types of nanoparticles will be information technology. The tribological behavior is mainly forming enhancing the properties in that nanocomposite. dependent on the materials surface characteristics and thus, Previous results in the literature show that CrN possesses these films have tremendous potential to modify and high hardness and wear resistance thus its incorporation into improve the surface properties and optimize the materials the DLC structure is expected to improve the film’s wear and frictional behavior. Additionally, it is desirable to performance. The carbide and nitride phases are present in keep beneficial bulk material properties, such as strength the form of nanoparticulate phases. Furthermore, N and toughness. incorporation in the carbon network of the matrix is Previous studies show that DLC thin films possess expected to soften the film (reduce elastic modulus) excellent properties related to friction, wear, corrosion, and allowing more flexibility and conformance while under biocompatibility. However, these films have certain loading. limitations such as low thermal stability, low toughness and Synthesis of the films is accomplished by using adhesion. To alleviate these drawbacks, Me-DLC films Intensified Plasma-assisted Processing, a technique that have been introduced. In the past, work has been carried out combines plasma-assisted, reactive physical vapor with carbide formers such as Si, Nb, Ti, Ta and W and non- deposition along with chemical vapor deposition. carbide formers such as Cu, Au, Ag incorporated into DLC Mechanical properties of the films are assessed by films. These developments have been reviewed by Klages Microhardness testing. Film characterization in terms of and Memming1. A system with high potential is Cr-DLC. microstructure, structure, composition and chemical state of Previous studies in our laboratory have shown that Cr-DLC components is carried out by Transmission Electron films can be tailored to have equal wear resistance to that of Microscopy (TEM) and X-ray Photoelectron Spectroscopy DLC. These films are nanocomposites since they consist of (XPS). Tribological behavior is assessed by pin-on-disc nanoparticles (either Cr or Cr carbide) dispersed in an experiments. amorphous DLC matrix.

1 The first films developed were undoped Cr-DLC films with a gradient in the Cr content. A Cr-rich layer (bond layer) was first deposited on the Si substrate, Fig. 1, followed by granular Cr-DLC amorphous layer, Fig. 2. Compositional analysis, Fig. 3, showed that the Cr-rich bond layer composed of ~80% Cr. XPS surface analysis, Fig. 4, revealed a top layer composition of 20%Cr. Thus, the present films exhibit a gradient in the Cr content ranging from 80%Cr (interface with substrate) to 20%Cr at the top surface. Thus, the former layer provides high adhesion to the substrate whereas the top DLC-rich layer provides low friction and high wear resistance. Synthesis of N-doped films is currently under way.

Fig. 3 Compositional analysis showing a Cr-rich lower film layer.

Fig. 1 Cross sectional TEM Image of Cr-DLC film showing layers with Cr-content gradient. Inset is the electron diffraction pattern from the bottom Cr layer.

Fig. 4 XPS spectrum from the top Cr-DLC film surface showing a composition of 20:80 Cr:C.

ACKNOWLEDGMENTS This research is supported by the National Science Foundation (NSF) and the Board of Regents of the State of Louisiana under the contract No. NSF/LEQSF Fig. 2 Cross sectional TEM of the substrate/Cr Film interface also showing details of the film (2001-04) RII-03. The characterization was performed microstructure. in the Materials Characterization Center at Louisiana State University supported by the NSF grants DMR- 9871417 and DMR-0116757.

2 REFERENCES 1. C.P.Klages, R.Memming, Materials Science Forum 52- 53 (1989) 609. 2. Microstructure and Tribological Behavior of Cr-DLC Nanocomposite Films, V. Singh, J.C. Jiang and E.I. Meletis, Proceedings MRS, Vol. 697, P7.6.1-P7.6.6, 2002.