The Future of Plasma-Based Surface Engineering Techniques in Tribology (Keynote)

Over the past twenty years or so, there have been major steps forward both in the understanding of tribological mechanisms and in the development of new coating and treatment techniques to better “engineer” surfaces to achieve reductions in wear and friction. Particularly in the coatings tribology field, improved techniques and theories which enable us to study and understand the mechanisms occurring at the “nano”, “micro” and “macro” scale have allowed considerable progress to be made in (for example) understanding contact mechanisms and the influence of “third bodies” [1–5]. Over the same period, we have seen the emergence of the discipline which we now call “Surface Engineering”, by which, ideally, a bulk material (the ‘substrate’) and a coating are combined in a way that provides a cost-effective performance enhancement of which neither would be capable without the presence of the other. It is probably fair to say that the emergence and recognition of Surface Engineering as a field in its own right has been driven largely by the availability of “plasma”-based coating and treatment processes, which can provide surface properties which were previously unachievable. In particular, plasma-assisted (PA) physical vapour deposition (PVD) techniques, allowing wear-resistant ceramic thin films such as titanium nitride (TiN) to be deposited on a wide range of industrial tooling, gave a step-change in industrial productivity and manufactured product quality, and caught the attention of engineers due to the remarkable cost savings and performance improvements obtained. Subsequently, so-called 2nd- and 3rd-generation ceramic coatings (with multilayered or nanocomposite structures) have recently been developed [6–9], to further extend tool performance — the objective typically being to increase coating hardness further, or extend hardness capabilities to higher temperatures.

Bearing Performance Limits With Grease Lubrication

This paper has examined the influence of bearing design and operation in controlling lubricant supply to the contact zone. Grease lubricated contacts are liable to starvation and as a result the film thickness is reduced, this can result in surface damage or premature bearing failure. It is of obvious importance to know when starvation occurs and the effect of grease type, bearing design and operation on lubrication replenishment. The aim therefore is to develop a starvation parameter capable of predicting the operating limits for a particular bearing/grease system. A number of bearing design parameters are examined in the paper, these include cage design, ball spin and bearing size. Ball spin and cage effects can be efficient mechanisms for maintaining the lubricant supply to the track. Increased bearing size, line contact geometries and high load result in reduced lubricant replenishment of the contact. Using this analysis it will be possible to establish operating limits for families of bearings.

Investigation of Sliding Friction Pairs With Electro-Pulse Sprayed Micro-Coats

Electro-pulse spraying (EPS) is the coating technology of “electric explosion of conductive materials” when high-voltage and powerful impulse flows through a wire conductor. Object of our investigation — tribological properties of sliding pairs with copper micro-coats made by EPS after one time explosion. Small-grained dense structure coat with evaluated thickness about 4–6 ?m was obtained. Tribological tests, performed at marginal lubrication with multi-stage load, shows that using EPS-specimens the value of friction coefficient is lower as control version. At instantaneous setting of load for long-term running the copper films adopts well to the change of load. The wear of friction pairs according to worn mass show that EPS-specimens worn 79% less than CV-specimens. The investigations point out that copper micro-coats have better tribological properties comparing to control version of friction pairs.

Temperature-Dependent Tribological Improvements in Low-Energy Nitrogen Ion Implanted Vanadium-Titanium Alloys

A detailed tribological characterization of low-energy, nitrogen implanted V5 at. %Ti alloy is presented. Samples were nitrogen-implanted at an accelerating voltage of 1.2 kV and 1 mA/cm2, up to a dose of 1E19 ions/cm2. The tribological properties of the alloys: microhardness, friction coefficient and wear resistance, have improved after ion implantation and this improvement increases as the implantation temperature increases. The microstructure of the alloys were analysed by transmission electron microscopy. A direct correlation between structural modifications of the nitrogen implanted layer and the improvement in their tribological properties is obtained. For samples implanted at 848 K a nanocomposite layer where the reinforcement particles are TiN precipitates forms. TiN precipitation appears as the responsible of the improvement in the tribological properties.

Friction and Wear Behavior of Silicon Carbonitride Processed From the Polymer-Derived Ceramic Route

Polymer derived ceramics (PDC’s) are processed from liquid organometallic precursors by cross-linking the polymers into infusible solids, followed by controlled pyrolysis. No previous work regarding their tribology has been reported. Further, the synthesis of PDC’s as thin films, and the role that the nanostructure plays on the mechanical properties has not been reported. The objective of this research was to evaluate the fundamental tribological behavior of polymer derived SiCN in both bulk and thin film form. Friction and wear evaluations were made on bulk materials and thin films using a Si3N4 ballon-disk linear wear tester at various contact pressures and in different environments that contained various amounts of humidity. The micro/nanostructure was characterized by FTIR, microRaman, and scanning electron microscopy. Bulk SiCN gave a low friction coefficient and good wear resistance in humid environments but showed significant fracture and gouging in dry environments at higher contact pressures. Although there is ambiguity regarding the tribology of the thin films there seems to be a dependence upon the nitrogen content within the materials derived from the polymeric stage. The future work will focus on optimizing processing conditions of thin films and investigating the role that nitrogen plays in both bulk and thin film SiCN materials.

First-Principles Study of Superlow Friction Between Hydrogenated Diamond Surfaces

Attractive interaction between two clean diamond(001) slabs turns repulsive upon the hydrogenation of surfaces. Even under high loading forces, this repulsive interaction prevents the sliding surfaces from being closer to each other. As a result, calculated lateral force variation generated during sliding has small magnitude under high constant loading forces. Superlow friction observed earlier between diamond like carbon coated surfaces can be understood by the steady repulsive interaction between sliding surfaces, as well as strong and stiff carbon-carbon and carbon-hydrogen bonds which do not favor energy dissipation. In ambient conditions, the steady repulsive interaction is, however, destroyed by oxygenation of hydrogenated surface.

Design, Analysis and Fabrication of a Six-Legged Walking Machine

This paper presents the steps involved in the design, analysis and fabrication of a six-legged walking machine. The suggested machine uses the basic four bar chain mechanism and slotted lever type quick return mechanism with some modifications. The synthesis of the link lengths is based on Freudenstein’s (algebraic) three accuracy points. The velocity and acceleration analysis of oscillating links explains the effect of change in the crank’s angular velocity and acceleration on all other links. The suggested machine is capable of moving forward and backward, turning right and left and rotating around a point on the commands given.

Micro/Nano-Tribological Behavior of Octadecyltrichlorosilane on Silicon as a Coating for MEMS

Despite progresses achieved in the technology of MEMS, the tribological problem continues being an unresolved matter. Wear and stick-slip phenomena are many times the origin of failure of these devices. The application of self-assembled monolayers (SAMs) in liquid phase seems to be a solution to this problems. SAMs of octadecyltrichlorosilane (CH3(CH2)17SiCl3, OTS) were attached to Si(100) oxidized in liquid phase. Contact angle measurements were used for characterizing the grade of hydrophobicity. The topography of the coating was obtained with an Atomic Force Microscopy (AFM) in semicontact mode. The images showed the presence of particles related to the polymerization of the precursor molecule during the formation process of the SAMs. Creating the film of lubricant in vapour phase would avoid this undesirable effect. Tribological tests were carried out with a microtribometer in linear reciprocating movement with a ball of 2 mm of diameter (100Cr6 and Si3N4) and load of some milinewtons. Results were compared with those obtained for silicon oxidized without any coating. The coefficient of friction (COF) and wear (substrate and ball) were studied under different test conditions.

Packing Structure of OTS on Silicon Surfaces: A Computational Approach

Optimal packing structure of Octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) adsorbed on a SiO2 surface with a Si (100) substrate was studied performing molecular dynamics (MD) computational simulations. Molecular substitution, substitution pattern and molecular orientation of the OTS molecules on the SiO2 (100) are the main factors studied in order to determine the optimal packing structure taking into account energetic balance. We have used the optimal packing structure to study other properties usually used to characterize SAMs as molecular and system tilt angles, film thickness and gauche defects. These properties and monolayer stability were studied performing MD simulations in a temperature range from 100 K to 600 K and we found that results obtained agree with those from experimental measurements. We found that OTS films are stable up to 500 K. The optimal structure obtained could be used in further MD simulations studies in order to determine tribological properties of OTS-SiO2 systems.

PVD-Based Microstructuring Surface Techniques for Tribological Applications

Combined techniques of Physical Vapour Deposition (PVD), laser ablation and UV-Photolithography have been set up to produce well defined surface textures able to increase the seizure resistance of high loaded lubricated systems. Using these new techniques, different predefined surface textures, following rectangular grid and zigzag stripped patterns have been generated. The microstructured surfaces developed have been characterised with confocal microscopy, optical and scanning electron microscopy. Ball-on-disc tribological tests under progressively increased load have been carried out using mineral oil as lubricant to determine the influence of surface microtextures on seizure resistance. The influence of shape and size of texture patterns on the tribological performance of the surface have been also studied.