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Materials Tribology

Materials Tribology in Mechanical Design

Most mechanical assemblies have one or more solid interfaces that can be critical to the function of the assembly. When there is a relative motion between these solids friction and wear occur. For most systems, it is desirable to have low friction and low wear to make these systems either work better, require less energy, or last longer. Some systems require high friction (i.e., tires) and some even high wear (i.e., crayons), however most efforts are focused on developing low wear and low friction applications.

In design of a mechanical system, the properties of the materials selected by the engineer are as imperative as the design itself. Often, design engineers will use reference charts or other published and non-published data to select a material for application specific properties. As an engineer tries to optimize a design, accuracy of these properties become of crucial importance. This can be challenging when designing tribological components as friction coefficients and wear rates are not simply material properties. Friction and wear behavior of materials depend on many parameters and conditions, including:

material pairing
contact geometry
applied normal load
contact pressures
relative sliding speed
material surface topography and roughness
environment
temperature
chemical interactions
sliding direction (unidirectional, reciprocating, random, etc.)

These system specific variations in tribological properties require scientists to study material pairings in very specific experimental conditions to provide reliable data. Even worse, many mechanical assemblies have to work in multiple environments such as a watercraft that must operate in air and submerged, or spacecraft components that must operate in both terrestrial and the various space environments [1]. Some components must provide reliable wear and friction against multiple counter materials.

Solid Lubricants
Although extremely low friction coefficients and wear rates can be achieved with fluids and greases, fluids and greases are often undesirable, and sometimes unacceptable, in many systems [2]. Fluid lubricants often have a narrow temperature range in which they can operate. Fluid lubricants require seals and filtration systems to keep proper function. In some cases, they require specific sliding speeds to generate hydrodynamic lubrication. Greases are also affected by their operating environment, including temperature and pressure. Fluids and greases can easily migrate or get contaminated.

In the cases where a fluid or grease lubricant is no longer ideal, solid lubricants are an excellent solution. Solid lubricants provide many added benefits including:

a broad operating temperature range
new environmental capabilities (use in vacuum and other environments)
the lubricant remains in the contact and is self-replenishing
solid lubricant can be applied as a coating

Though a solid lubricant can offer many advantageous properties, they do have one major downfall that scientists and engineers must accept or try to overcome: solid lubricants wear. This wear manifests as design problems in that the lubricant has a finite lifetime and generates wear debris. For obvious reasons, it is desirable to increase the lifetime of a tribological component to as long as possible (or at least until other parts of the system will fail first). The generation of wear debris is a less obvious problem, but in many systems, such as biological implants, the wear debris composition and morphology is more important than the wear rates and friction coefficients.

Many materials have been used as solid lubricants [2]. Low shear strength metals and thin metallic films, such as gold, silver and lead have been used as solid lubricants [3]. These materials are inert and act as a sacrificial component. Lamellar solids such as Graphite, MoS2, WS2, talc, and boric acid are popular as extremely low friction solid lubricants; low interaction energy between lamellar layers allow these materials to shear easily [4]. These materials often exhibit extreme dependence on environmental constituents such as water [5].
Diamond-like carbon (DLC) coatings show extremely low friction and wear, especially in dry and vacuum conditions [6]. However, temperature and oxidation are limiting factors for conventional DLC’s. Some advanced DLC’s attempt to alleviate these concerns by adding dopants.

Polymers and polymeric composites are frequent candidates for solid lubrication systems [7-10]. Common tribological polymer matrices include polyether ether ketone (PEEK), polyimide (PI), polyamide imide (PAI), Polyethylene and Polytetrafluoroethylene (PTFE). PEEK, PI, and PAI have desirable mechanical and thermal properties with moderate wear behaviors but have higher friction coefficient. PTFE and similar fluoropolymers have low friction coefficients but suffer from poor wear. Using volume fractions of fillers, this wear can be reduced [8, 11-46].

Tribology: Perspective of a Design Engineer

Motivation: Development of Low-Friction, Ultra-Low-Wear Materials for Dry Sliding
Traditional solid lubricants for dry sliding each have their benefits and limiting properties. Engineers are pushing designs to last longer, be more efficient, and operate in more extreme environments. This means that solid lubricants must have lower wear rates, lower friction coefficients and be robust enough to operate in chemical, physical and thermal extremes. Although commonly overlooked, in many cases it is important for materials on both sides of the interface to be low wearing. Sometimes, in low friction systems, when one material is low wear it is at the expense of another material. Dry solid lubrication systems that form transfer films can often prevent this problem and promote low friction and low wear of both entities in the contact.

To date, there is no universal answer to the engineer’s material needs for solid lubricants, but there are good, and even great, materials for specific applications or specific environments. There are a few obvious paths for developing new low-friction, low-wear materials for applications in various environments and mechanical systems:

fundamental research to find a new material set (exploring minerals, ceramics, polymers and inorganic carbon based materials to name a few)
developing composite materials to:

reduce wear of low friction materials
reduce friction of low wear materials
reduce friction and wear of a system
reduce environmental sensitivity to friction and wear

explore thin coatings, films and composites
explore the unique properties of nanomaterials in reducing wear

55. Lai, S.Q., et al., The friction and wear properties of polytetrafluoroethylene filled with ultrafine diamond. Wear, 2006. 260(4-5): p. 462-468.
56. McCook, N.L., et al., Epoxy, ZnO, and PTFE nanocomposite: friction and wear optimization. Tribology Letters, 2006. 22(3): p. 253-257.
57. McElwain, S.E., Wear Resistant PTFE Composites Via Nano-Scale Particles, in Rensselaer Polytechnic Institute, Troy, New York. 2006.
58. Santos, C.M., et al., Transfer of PTFE and PEEK-PTFE composites in tribological interfaces. Abstracts of Papers of the American Chemical Society, 2006. 231: p. -.
59. Burris, D.L. and W.G. Sawyer, Tribological behavior of PEEK components with compositionally graded PEEK/PTFE surfaces. Wear, 2007. 262(1-2): p. 220-224.
60. Mcelwain, S.E., et al., Effect of particle size on the wear resistance of alumina-filled PTFE micro- and nanocomposites. Tribology Transactions, 2008. 51(3): p. 247-253.
61. Burris, D.L., et al., A route to wear resistant PTFE via trace loadings of functionalized nanofillers. Wear, 2009. 267(1-4): p. 653-660.
62. Vail, J.R., D.L. Burris, and W.G. Sawyer, Multifunctionality of single-walled carbon nanotube-polytetrafluoroethylene nanocomposites. Wear, 2009. 267(1-4): p. 619-624.
63. Blanchet, T.A., S.S. Kandanur, and L.S. Schadler, Coupled effect of filler content and countersurface roughness on PTFE nanocomposite wear resistance. Tribology Letters, 2010. 40(1): p. 11-21.
64. Burris, D.L., et al., Polymeric nanocomposites for tribological applications. Macromolecular Materials and Engineering, 2007. 292(4): p. 387-402.
65. Clerico, M. and V. Patierno, Sliding wear of polymeric composites. Wear, 1979. 53(2): p. 279-&.
66. Sung, N.H. and N.P. Suh, Effect of fiber orientation on friction and wear of fiber reinforced polymeric composites. Wear, 1979. 53(1): p. 129-141.
67. Gong, D.L., et al., Effect of tribochemical reaction of polytetrafluoroethylene transferred film with substrates on its wear behavior. Wear, 1990. 137(2): p. 267-273.
68. Lancaster, J.K., Polymer-based bearing materials - role of fillers and fiber reinforcement in wear. Wear, 1972. 22(3): p. 412-&.

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Undergraduate Research Positions Available

Oportunity for hands on research experience.

Now Accepting Applications.

In the Tribology Laboratory, undergraduates will do experimental research focused on interfacial interactions of condensed matter. This includes studying the fundamental origins of friction, wear, surface deformation and adhesion on complex surfaces and materials ranging from cells to nanocomposites in environments ranging space to kilometers under water.

Active research includes analysis of materials that recently returned from the international space station, evaluating wear of dinosaur dental fossils, developing and patenting ultra-low wear polymer nanocomposites, studying and designing biocompatible and bio-inspired polymeric and hydrogel materials, and collaborating internationally on the physics of soft matter interactions. This research in tribology is at the intersection of mechanical engineering, materials science and surface physics.

Nanomechanical and Tribological Properties on Hadrosaurid Dinosaurs

Prof. Greg Sawyer, Greg Erickson and Brandon Krick measured nanomechanical and tribological properties on hadrosaurid (duck-billed dinosaur) dental fossils from the American Museum of Natural History. Using custom instruments, we measured tissue hardness and wear rates that were preserved in the 65 million year old tooth. These properties are preserved in fossilized teeth because apatite mineral content is the major determinant of dental tissue hardness. Measured tissue wear rates were used to simulate the formation of hadrosaurid tooth chewing surfaces using a 3-D wear simulation. The simulation results in a surface profile nearly identical to a naturally worn hadrosaurid dental battery. The model revealed how each tissue (of differing wear rates) contributed to the formation of sophisticated slicing and grinding features in these reptiles tens of millions of years before mammals evolved analogous chewing capacity. This capacity to measure wear-relevant properties preserved in fossils provides a new route to study biomechanics throughout evolution. See Journal papers:
Science, October 5, 2012, pp.98-101.

Experiments back from the International Space Station

Space Tribometers and Samples back for analysis

Materials on the International Space Station Experiments Space Tribometerd

Materials on the International Space Station Experiments (MISSE) Space Tribometers were the first ever active tribometers directly exposed to the Low Earth Orbit Environment

The Tribology Laboratory at Lehigh University is under construction

The lab as of May 2013

The lab as of July, 3rd 2013

The main laboratory is located in Lehigh's Packard Laboratory.

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“Space Tribology with Brandon Krick” Society of Tribologists and Lubrication Engineers Compass podcast hosted by Kara Lemar.
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