ICMCTF2009 Session E3-1: Tribology of Nanostructured and Amorphous Films

Coatings from diamond-like carbon (DLC) have been proven to be an excellent choice for wear reduction in many technical applications. Also, a multitude of attempts for adaption to the MedTech field has been undertaken. However, data gained from realistic in-vitro test setups are in short supply despite being quintessential for application development; the prediction of layer stability and failure mechanisms in the human body environment are important tasks still to be researched.

In our team’s efforts to develop DLC implant coatings with predictably stable layer adhesion in the human body, a new type of simulator has been developed for testing of spinal disk implants. Test results gained from this setup using coated and uncoated spinal disk replacements are presented and compared with conventional tribotests. The relevant differences and possible factors leading to implant failure are discussed. Requirements on the DLC and interfacial layers in the bio-tribo environment are suggested.

Amorphous phosphorus doped carbon (C_xP_y) thin films have been considered as a new tribological coating material with unique electrical properties. However, the material could not find practical applications so far since C_xP_y rapidly oxidizes, hydrolyzes, and delaminates when in contact with air. Recently, we demonstrated that phosphorus carbide (CP_x , x≤0.15) thin solid films with a short range fullerene-like structure can be deposited by magnetron sputtering. Thus, the introduction of P atoms in the graphene structure induces the formation of bent and interlinked graphene planes^1,2. In this work we compare the uptake of water of amorphous phosphorus-carbide (a-CP_x) films, with fullerene-like phosphorus-carbide (FL-CP_x) and amorphous carbon (a-C) films. Films with thickness in the range 100-300 nm were deposited onto silicon, quartz, and gallium orthophosphate substrates by reactive DC magnetron sputtering. The film mi crostructure was characterized by X-ray photoelectron spectroscopy, and transmission electron microscopy and diffraction. A piezoelectric crystal microbalance placed in a vacuum chamber was used to measure film water adsorption³. Results indicate that the amount of adsorbed water is highest for the pure a-C films and that the FL-CP_x films adsorbed less water than a-CP_x. To provide additional insight into the atomic structure of defects in the FL-CP_x, a-CP_x, and a-C compounds, we performed first-principles calculations within the framework of Density Functional Theory. Emphasis was put on the energy cost for formation of vacancy defects and dangling bonds in relaxed systems³. Cohesive energy comparison reveals that the energy cost for formation for dangling bonds in different structural configurations is considerable higher in FL-CP_x than for the amorphous films. The simulations thus confirm the experimental results that dangling b onds are less likely in FL-CP_x than in a-CP_x and a-C films.

¹A. Furlan, G.K. Gueorguiev, Zs. Czigány, H. Högberg, S. Stafström, and L. Hultman, Phys. Stat. Solidi RRL 2 (2008) 191.

²G. K. Gueorguiev, A. Furlan, H. Högberg, S. Stafström, and, L. Hultman, Chem. Phys. Lett. 426 (2006) 374.

³E. Broitman, G. K. Gueorguiev, A. Furlan, N. T. Son, A.J. Gellman, S. Stafström, and L. Hultman, Thin Solid Films, in press (2008).

The Repetitive High Energy Pulse Power (RHEEP) facility at Sandia National Laboratories was used to generate ion-beams of 10 J/cm² energy. Ablating targets with the high energy ion-beams makes this facility a unique tool for low-stress multilayer thin film deposition. In the first part of this study, Mo-based films were grown on Si wafers, while keeping the substrate at 300°C during the deposition process with the intent of investigating MoS₂ films with improved tribological characteristics. Friction and wear measurements were performed using a Si₃N₄ ball in dry nitrogen and air with 50% relative humidity. Characterization of the films was performed on cross-sections of wear scars suitable for TEM analyses prepared using focused ion beam (FIB) microscopy. Results showed that dual target ablation of MoS₂ and Ti or V onto heated substrates permitted self-assembly of Mo species into 50 nm size boulders in the film structure. Consequen tly, unlike pure MoS₂ which oxidizes in humid air, the nanocomposites exhibited extremely low wear even when tested in air with 50%RH. Thus, we demonstrated that the nanostructure comprising of relatively hard particles of Mo metal in a matrix of softer MoS₂ provided a low friction and highly wear-resistant film that is also resistant to moisture.

By ablating different multiple metal targets, the RHEEP facility also demonstrated the capability for synthesizing a wide range of metal nanolaminate films, providing opportunities to understand fundamental mechanisms driving strength, stiffness and wear characteristics of this category of films. The metal laminate systems we investigated include Mo-Ir, Mo-Ti, and Mo-W. In these cases, nanolaminate films consisted of at least 100 alternating material layers, each layer less than 10 nm thick. Films were deposited on Silicon and Al₂O₃ substrates. The mechanical response of these nanolaminates was in vestigated using instrumented indentation. The role of layer thickness and shear modulus difference on the strength and wear characteristics of the nanolaminate films will be discussed.

¹Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.