AVS2011 Session TR-ThP: Tribology Focus Topic Poster Session

Diamond-like carbon (DLC) films are promising materials for dry-contact applications where resistance to surface damage or lubricating performance is required. In the present work, the structure of 3 and 10 nm Focus Cathodic Arc (FCA) carbon films and 3 and 10 nm plasma carbon films grown on Si(100) was studied by angle-resolved X-ray photoelectron spectroscopy (ARXPS). The concentration and distribution of sp² and sp³ carbon within the film was assessed with methods described elsewhere [i]. Atomic force microscopy (AFM) with a silicon tip was used to study the friction coefficient μ on the nanoscopic scale. It is defined by the Amonton law, µ=F_L/F_N, where F_N and F_L are the normal and the lateral force applied to a probe [ii]. The calculation of the normal and lateral force applied during the process of sliding the tip over the sample was done according to the methodology proposed by Carpick [iii] . The results suggest a relationship between the chemical structure and the tribological performance for each film tested. The films grown by FCA presented the best tribological performance (μ~0.02) compared to films grown by plasma (μ~0.06) indicating a direct link between the distribution of sp² and sp³ carbon and the nanoescale friction coefficient. Since during the friction tests there was no evidence of damage or wear on either the surface or the tip, the conventional interpretation for the origin of friction proposed by Bowden and Tabor [iv] grossly underestimates the energy loss in the sliding process. On the other hand, the energy loss can be quantitatively explained in terms of heat generation [v]. The tribological performance analysis indicates that the friction is related to the adhesion force between tip and sample. However, the friction coefficient values do not show significant changes as the strength of adhesion varies, indicating that the value of the friction coefficient depend on the contributions of atomic bonds at the surface.

[i] A. Herrera-Gomez, et al., "Structure of ultra-thin diamond-like carbon Films grown with Filtered cathodic arc on Si(001)”. Analytical Sciences 26, pp. 267 (2010).

[ii] G. Amonton, “De la resistance cause´e dans les machines”. Mem. Acad. R. A, 275 (1699).

[iii] R. W. Carpick, “Scratching the Surface: Fundamental Investigation of the Tribology with the Atomic Force Microscopy”. Chemical Reviews, 97, pp. 1163 (1997).

[iv] F. P. Bowden, and D. Tabor, “Friction and Lubrication of Solids”. Oxford University Press, pp. 52 (1964).

[v] J. Luo, Y. Hu, S. Wen (Editors), “Physics and Chemistry of Micro-Nanotribology.” ASTM-International, West Conshohocken, PA, USA. Chapter 9, pp. 167 (2009).

Microelectromechanical systems (MEMS) are critically-limited by interfacial phenomena such as friction and adhesion. One strategy to reduce friction between MEMS surfaces is to coat them with molecularly-thin self-assembled monolayer (SAM) coatings. Historically, silicon MEMS have been coated with silane-based SAMs, such as octadecyltrichlorosilane (OTS). However, continued progress in the development of MEMS may require new material systems to be employed. Therefore, in this study, we have investigated the frictional properties of octadecylphosphonic acid (ODP) monolayers deposited on aluminum oxide surfaces, across speed regimes. Measurements using an atomic force microscope (AFM) and separately using a nanoindenter-quartz crystal microbalance system were performed each with a microsphere-terminated probe. This allows for a comparative study between different velocity regimes using contacts with similar sizes, pressures, surface roughnesses, and interfacial chemistries. AFM colloidal probe friction measurements indicate that for a bare tip sliding on various substrates, ODP-coated alumina surfaces exert a lower friction force than either bare or OTS-coated alumina substrates. We also observed strong evidence of transfer of the ODP molecules to the tip when the tip is uncoated. The results presented in this study are significant contributions towards our goal of better understanding the frictional properties of phosphonate SAMs in pursuit of alternative MEMS materials.

The multilayer coatings can improve the corrosion and wear resistance of materials for biomedical applications. The tribocorrosion behavior of TiALN and TiAlPt_xN coatings and TiAlPt_xN-TiAlPt_x multilayers immersed in a corrosive environment was investigated. The coatings were deposited on 316L stainless steel and Ti6Al4V alloys by magnetron sputtering. The period thickness of multilayers was 300 nanometers and the total thickness was 3.6 microns. In order to evaluate the influence of the environment, the corrosion was studied using open circuit potential (OCP) measurements and potentiodynamic polarization techniques in saline and a Ringer ´s solutions. For the tribocorrosion test a counterbody of Alumina with 10 mm diameter was used. The loads used were from 1 to 5 N, the oscillating frequencies were 1Hz to 5 Hz. The electrochemical noise measurements were performed during, and after the sliding and scratch tests. The structure and composition of multilayers were studied by means of XRD, XPS and RBS techniques. It was found that the codeposition of Pt and TiAlN-TiAl multilayer can improve the wear-corrosion resistance of materials for biomedical applications.The tribocorrosion behavior results in terms of the coefficient of friction showed a dependence against the force and the sliding frequency.

Tribological properties play significant role on the performance of tool steel surfaces. Here, application of electron beam excited plasma nitriding and its effect on tribological properties is described. The technique eliminates the formation of the brittle and rough compound layer that is common in nitriding processes. The hardening process is done through diffusion of the plasma species in to the subsurface of the treated material without altering the initial surface finish. The applications of the process can be in areas of hard coating where adhesion of the coating material with the tool steel is of significant importance.

The experimental tool steel material is SKD 61 with a chemical composition of 0.36% C, 5.05% Cr, 1.21% Mo, 0.83% V, 0.92% Si, 0.43% Mn, 0.008% P, >0.001% S, Fe bal. The sample was heat treated, hardened and triple tempered to a hardness of 630 Hv. The sample was then treated in a nitrogen plasma produced by a beam current of 8 A under a working pressure of 0.4 Pa. The temperature was set at 500 degrees centigrade throughout the treatment time. The experimental set up includes bias terminals that reduce the ion density within the vicinity of the tool steel material. This is done to reduce nitriding due to ion and increase the chance of nitriding due to neutral species within the plasma. The cross sectional hardness distributions and wear measurements of the nitrided tool steels were examined to determine the mechanical and tribological surface properties. The surface has no trace of the compound layer that is usually observed in the ion nitriding processes. This is also confirmed from the X-ray Diffraction peaks, as there is no visible Fe₃N and Fe₄N peaks observed. These results are attractive as they open new areas of application especially in the coating industry where adhesion remains to be the limiting factor in lots of the hard coatings that protect cutting and forming tools against wear.