Thrust 1 - Efficiency

Title

1D: Micro- and Nano-Texturing for Low-Friction Fluid Power Systems

Project Leader

Prof. William King (UIUC)

Statement of Project Goals

The goal of this project is to develop low-cost microstructured surfaces with significantly reduced coefficient of friction compared to surfaces with conventional surface finish.  The project aims to design, fabricate and characterize the effect of micro-textures on lubricated surfaces that are suitable for real world fluid power applications.  The focus is to enhance the performance of lubricated contacts by using micro-textures that lead to a significant reduction in the sliding friction between the surfaces compared to non-textured ones.  The focus is also on low-cost scaling of these surfaces to sizes and shapes appropriate to the industrial applications.

This is a new project, beginning August 2010.  It follows from a similar project of different goals, which graduated in August 2010.

Project's Role in Support of the Strategic Plan

Friction between lubricated surfaces is one of the main energy losses in fluid power systems.  Surfaces having reduced coefficient of friction will result in improved energy efficiency for today's fluid power systems.

Description and Explanation of Research Approach

Friction between lubricated sliding surfaces can be reduced when one or both surfaces have surface roughness or surface microstructures [1, 2].  The mechanism behind this friction reduction is based on the textures serving as lubricant reservoirs thereby ensuring constant supply of lubricant to the surfaces in relative motion.  Micro-texturing also leads to the creation of lift and hence increased separation between the surfaces.  For a fixed separation between two surfaces, micro-texturing of one or both of those surfaces can lead to reduced sliding friction.

Only a few published papers have investigated how microstructures affect sliding friction, either through small-scale laboratory tests [3-5] or numerical simulations [6-8].  These papers have shown the general feasibility of microstructures for friction reduction.  However, there has been little research on how to design microstructures for a specific application.  A gap remains in understanding how to choose the optimum parameters of the texture in order to obtain the best performance in terms of lift and friction.  There exist some numerical models [9]; however, they concentrate on low Reynolds number flows (Re < 1) and use the Reynolds equation to solve the fluid flows [14].  Thus one of the gaps to address is the engineering design rules and to do so over a larger range of Re.

Very little research has been done on how to manufacture microstructures in real materials and on a scale relevant to the fluid power industry.  Laser Surface Texturing (LST) has been proposed to make arrays of microscopic dimples directly into metal parts [10, 11].  The LST method is expensive, difficult to scale up and creation of dimples other than circular ones is difficult.  Thus the second gap to address is the manufacture of microtextured surfaces in industrially-relevant materials, sizes, and shapes.

Within the scope of this 2-year project, we aim to develop design rules for friction-reducing microstructures as relevant for the fluid power industry, fabricate microstructured surfaces and characterize their coefficient of friction under lubricated sliding conditions, and extrapolate these results to fluid power applications.

Figure 5: Metal surfaces fabricated with surface micro-textures. Left - 4" diameter metal disk with surface micro-textures. The fabrication process allows replication of sub-micron textures. Right - a metal roller having surface micro-textures [15].

References

1)     Dowson, D., Taylor, C. M., Godet, M., and Berthe, D., "Surface Roughness Effects in Lubrication", Proceedings of the 4th Leeds-Lyon Symposium on Tribology, Sep. 13-19, 1978

2)     Sun, D. C., and Chen, K. K., "First Effects of Stokes Roughness on Hydrodynamic Lubrication Technology," ASME J. Lubr. Technol., vol. 99, pp. 2-9, 1977

3)     H. L. Costa and I. M. Hutchings, "Hydrodynamic lubrication of textured steel surfaces under reciprocating sliding conditions," Tribology International, vol. 40, pp. 1227-1238, Aug 2007.

4)     U. Pettersson and S. Jacobson, "Influence of surface texture on boundary lubricated sliding contacts," Tribology International, vol. 36, pp. 857-864, Nov 2003

5)     M. Nakano, et al., "Tribological Properties of Patterns NiFe-Covered Si Surfacs," Tribology International, vol. 35, pp. 133-139, 2009

6)     Q. J. Wang and D. Zhu, "Virual Texturing: Modeling the Performanc of Lubricated Contacts of Engineered Surfaces," ASME Journal of Tribology, vol. 127, pp. 722-728, 2005

7)     Y. Kligerman, et al., "Improving Tribological Performance of Piston Rings by Partial Surface Texturing," ASME Journal of Tribology, vol. 127, pp. 632-638, 2005

8)     Marian et al., "Theoretical and experimental analysis of a partially textured thrust bearing with square dimples," Proc. IMechE, Part J: J. Engineering Tribology, vol. 221, 2007

9)     Dobrica et al., "Optimizing surface texture for hydrodynamic lubricated contacts using a mass-conserving numerical approach," Proc. IMechE, Part J: J. Engineering Tribology, vol. 224, 2010

10)  P. Andersson, et al., "Microlubrication effect by laser-textured steel surfaces," Wear, vol. 262, pp. 369-379, Feb 2007

11)  Geiger et al., "Influence of laser-produced microstructures on the tribological behaviour of ceramics", Surface and Coatings Technology, vol. 100-101, pp. 17-22, 1998

12)  Arghir, M., Roucou, N., Helene, M., and Frene, J., "Theoretical Analysis of the Incompressible Laminar Flow in a Macro-Roughness Cell," Journal of Tribology - Transactions of the ASME, vol. 125, pp 309-318, 2003

13)  Sahlin, F., Glavatskih, S. B., Almqvist, T., and Larsson, R., "Two-Dimensional CFD-Analysis of Micro-Patterned Surfaces in Hydrodynamic Lubrication," Journal of Tribology - Transactions of the ASME, vol. 127, pp 96-102., 2005

14)  Dobrica, M. B. and Fillon, M. "About the validity of Reynolds equation and inertia effects in textured sliders of infinite width," Proc. IMechE, Part J: J. Engineering Tribology, vol. 223, 69-78, 2009

15)  H. Cannon and W. P. King, "Casting metal microstructures from a flexible and reusable mold," Journal of Micromechanics and Microengineering, vol. 19, Sep 2009

16)  H. Cannon, et al., "Molding ceramic microstructures on flat and curved surfaces with and without embedded carbon nanotubes," Journal of Micromechanics and Microengineering, vol. 16, pp. 2554-2563, Dec 2006

17) H. Cannon and W. P. King, "Microstructured metal molds fabricated via investment casting," Journal of Micromechanics and Microengineering, 2009