AVS2010 Session TR-ThP: Tribology Focus Session Poster Session

TiAlN/WN multilayer thin films with a range of different compositions and number of layers were deposited by PVD magnetron sputtering onto a variety of metallic substrates in a mixture of Ar/N₂ atmospheres. The utilisation of different nitride-formers elements and nitrogen (as a reactive gas) allows the deposition of alternative coatings with improved properties (e.g. wear and corrosion resistance) to those observed in individual commercial nitrides. The structure and morphology of the coatings were studied by means of X-ray Diffraction and Scanning Electron Microscopy. The elemental compositions and the surface chemistry (to gain precise information on the bonding environment) of these coatings were obtained by XPS. Microhardness and scratch tests were used to evaluate the mechanical properties and the adhesion of TiAlN/WN thin films respectively. The corrosion resistance was studied by potentiodynamic polarisation experiments in saline solutions of 0.5 M. The tribological properties were studied through reciprocating sliding against ceramic counterparts and different loads. In this paper we report the results relating to the influence of deposition conditions such as deposition temperature, period size and nitrogen content on coating properties. The improvement of corrosion and wear resistance by the utilisation of a multilayered TiAlN/WN arrangement is also presented and discussed.

Understanding the mechanical properties of nanotubes is of significant practical and fundamental interest. Multiwalled nanotubes and nanoparticles of metal dichalcogenides such as WS₂ express unique mechanical and tribological characteristics.[1] The structure of WS₂ nanotubes consists of layers of covalently bound trigonal bipyramidal WS₂. The interaction between the layers is a van der Waals interaction between adjacent sulfur sheets. One of the intriguing aspects of these structures is the response of these layers under mechanical stress. Whereas some of the elastic constants of these unique structures have been addressed by experimental and theoretical work, the radial compression mode has not yet been studied. Relatively few studies of radial modulus of multiwalled carbon nanotubes have been made, and these also do not explicitly include the multilayered aspect of the structures. Here, we report an experimental and modeling study of this mode in the WS₂ nanotubes.

Three independent atomic force microscope (AFM) experiments were employed to measure the nanomechanical response, using both large (R=200 nm) and small (R=3-15 nm) probe tips. Two different analytical models were applied to analyze the results.[2] For a large AFM tip, a Hertzian model presuming an elliptical contact was applied. A shell continuum model is applied in the case of the sharper AFM tips. This model treats the nanotube wall as a thin, curved membrane. The results indicate that the derived modulus varies with nanotube diameter and compression depth

The modulus values derived from the analytical models were used as initial input for finite element analysis (FEA). The FEA model described the nanotubes as alternating high stiffness (representing the covalent shells) and low stiffness (representing the vdW gap) layers for the outer two shells, with a homogeneous inner core. This model was fit with the experimental results over the initial linear elastic region of the first few nm of deformation. Values obtained varied for different nanotube diameters, and compression depths, showing the importance of the inter-layer contact. In addition, first-principles calculations using density functional theory tight binding give qualitative agreement with a reversible collapse of the nanotubes, seen at larger deformations.

[1] L. Rapoport, et al, Nature 387, 791-93 (1997); I. Kaplan-Ashiri et al, Proc. Natl. Acad. Sci. USA. 103, 523 (2006); I. Kaplan-Ashiri et al, J. Phys. Chem C. 111 8432-8436 (2007)

The tribocorrosion property of a nc-TiN/a-Si₃N₄ nanocomposite deposited using DC and RF codeposition reactive magnetron sputtering technique on SS316L nitrated and Titanium Ti6Al4V alloy against ceramic and metallic balls was studied in comparison with substrate materials using a reciprocating tribotester in distilled water, 1% NaCl water solution and artificial saliva solution. The effects of load and reciprocating sliding speed on the tribocorrosion properties of the nanocomposite were investigated. The structure and composition of coatings were studied by means of XRD and XPS techniques respectively. Under the experimental conditions of the present study, the nc-TiN/a-Si₃N₄ nanocomposite showed excellent resistance against corrosion and lower wear rate compared with SS316L nitrated and Titanium substrates. Profilometer analysis shows that on both materials most of overall tribocorrosion damage is due to mechanical wear. The tests suggest that nc-TiN/a-Si₃N₄ nanocomposite is a promising biomaterial for applications where reciprocating conditions occur.