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Self-Lubricating Behavior of Graphite-Reinforced Composites

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Tribology for Scientists and Engineers

Abstract

Lubricants are extensively used between contacting surfaces to reduce friction and wear. Typically, liquid lubricants are used to achieve low friction and wear. However, these lubricants are not effective in elevated temperature applications or vacuum environments. For these reasons, solid lubricants are utilized to meet these operational needs, where liquid lubrication is impractical. Solid lubricants are only effective as long as they are present in the tribo-interface. Therefore, it is desirable to provide a constant supply of solid lubricant material to the contacting surface. This is often achieved by incorporating solid lubricants as a second phase in the base material. These composite materials have the ability to achieve low friction and wear at the contact surfaces without any external supply of lubrication during sliding. Metal matrix composites reinforced with lamellar solid lubricant particles such as graphite are being used as self-lubricating materials for various engineering applications. In this chapter, the tribological behavior of metal and ceramic matrix composites reinforced with graphite particles has been reviewed. More specifically, copper–graphite, nickel–graphite, magnesium–graphite, silver–graphite, aluminum–graphite, silicon nitride–graphite, and alumina–graphite composites are studied. The influence of various environmental and mechanical parameters on the friction coefficient and wear rate is discussed. It was found that the amount of graphite released on the worn surface forms a thin transfer film on the contact surfaces. This transfer film reduces the overall friction coefficient and wear rate. The presence of the graphite-based transfer film increases the seizure resistance and enables the contacting surfaces to run under boundary lubrication without galling. The formation and retention of this transfer film on the sliding surface as well as its composition, area fraction, thickness, and hardness are important factors in controlling the friction and wear behavior of the material. The effectiveness of the transfer film also depends on the nature of the sliding surface, the test condition, environment, and the graphite content in the composite.

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Author information

Authors and Affiliations

  1. Department of Industrial Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI, 53201, USA

    Pradeep L. Menezes & Michael R. Lovell

  2. Department of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI, 53201, USA

    Carlton J. Reeves

  3. Department of Materials Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI, 53201, USA

    Pradeep K. Rohatgi

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  1. Pradeep L. Menezes
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  2. Carlton J. Reeves
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  3. Pradeep K. Rohatgi
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  4. Michael R. Lovell
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Correspondence to Pradeep L. Menezes .

Editor information

Editors and Affiliations

  1. Department of Industrial Engineering, University of Wisconsin, Milwaukee, Wisconsin, USA

    Pradeep L. Menezes

  2. Department of Mechanical Engineering, University of Wisconsin, Milwaukee, Wisconsin, USA

    Michael Nosonovsky

  3. Department of Marine Engineering, Texas A&M University, Galveston, Texas, USA

    Sudeep P. Ingole

  4. Department of Mechanical Engineering, Indian Institute of Science, Bangalore, India

    Satish V. Kailas

  5. Department of Industrial Engineering, University of Wisconsin, Milwaukee, Wisconsin, USA

    Michael R. Lovell

Appendices

Questions

  1. 1.

    What is a composite and describe two types of composites?

  2. 2.

    How does a self-lubricating matrix composite operate and what are the two stages?

  3. 3.

    Why is copper–graphite used as a self-lubricating metal matrix composite and what are some of its applications?

  4. 4.

    What are the dominant wear mechanisms during copper–graphite contact strip applications?

  5. 5.

    What is the effect of sliding speed on copper–graphite composites and how does this influence friction and wear?

  6. 6.

    What are the dominant wear mechanisms in the magnetic levitation transportation system?

  7. 7.

    In nickel–graphite composites, what compounds form to enhance the tribological properties when combining Ni, Cr, W, and Fe with graphite and molybdenum disulfide at high temperatures?

  8. 8.

    What contributed to the low- and high-temperature performance of the nickel alloy-based graphite–MoS2-reinforced composite?

  9. 9.

    What are the mechanical properties that make magnesium alloys attractive? What are some of the drawbacks? What applications are magnesium alloys primarily used in? Provide an example of a magnesium alloy.

  10. 10.

    When using silver–graphite composites in electrical contact applications, the friction and wear behavior under sliding with electrical current flow can become quite complex when compared to that under purely mechanical sliding conditions. Briefly explain this behavior.

  11. 11.

    What are the drawbacks to aluminum matrix composites and what is the benefit to adding graphite to these composites?

  12. 12.

    What is the effect on the coefficient of friction when adding intercalated NiCl2 graphite to silicon nitride and alumina ceramic matrices and what causes these phenomena?

Answers

  1. 1.

    Composite materials are a special class of materials and are made from two or more constituent components with significantly different physical or chemical properties, which remain separate and distinct at the macroscopic or microscopic scale within the finished structure. In the case of two constituent components, the material with the highest volume fraction is considered as the matrix, whereas the other is the reinforcement material that modifies the properties of the matrix. In metal matrix composites, the matrix is made of a metal and the reinforcement may be a different metal or another material, such as a ceramic or organic compound. Similarly, in ceramic matrix composites, the matrix is composed of silicon nitride, alumina, or another type of ceramic material, and the reinforcement may be a different ceramic, metallic, or solid lubricant compound.

  2. 2.

    In self-lubricating matrix composites, solid particle such as graphite and MoS2 are embedded in the matrix. Here, the formation of a thin film will occur by transferring the solid particles from the matrix to the tribo-surface during initial periods of sliding. The observed friction and wear behavior will, therefore, have two distinct stages: (a) transient state, while the thin solid lubricant film is being established, and (b) steady state, when a stable solid lubricant film (in the dynamical sense of being continuously replenished, i.e., “self-lubricating,” to make up for the wear loss) has formed.

  3. 3.

    Copper–graphite is used as a metal matrix composite because it combines the properties of copper (i.e., excellent thermal and electrical conductivities) and graphite (i.e., superior lubricity and a low thermal expansion coefficient). These matrix composites are optimal for electromechanical applications such as in electrical sliding contact applications use in brushes in electric motors and generators. In welding machines, the low voltage, high current densities, and sliding of critical components necessitate copper–graphite metal matrix composites with a very high specific electrical conductivity, good thermal conductivity, and low friction coefficient.

  4. 4.

    In the copper–graphite contact strips, arc erosion wear, oxidative wear, and adhesive wear were the dominant mechanisms during the electrical sliding process.

  5. 5.

    When the sliding speed exceeds a critical value, a transition of the friction and wear regime occurs. The formation of a lubricant layer on the contact surface is regarded as an important characteristic for enhanced tribological performance of copper–graphite composites. Due to a large strain gradient in the subsurface deformation zone, the graphite-rich lubricant layer can easily form on the sliding surface when the speed is lower than the critical value. At speeds exceeding the critical value, the formation of the lubricant layer is difficult due to the effects of delamination wear caused by the high strain rate. At speeds less than the critical value, the wear mechanism occurring tends to be mild wear caused by ratcheting, and at speeds exceeding the critical value, the wear mechanism is more severe induced by delamination wear.

  6. 6.

    Adhesive wear, abrasive wear, and electrical erosion wear are the dominant wear mechanisms during the electrical sliding wear processes in the magnetic levitation transportation system.

  7. 7.

    Chromium sulfide and tungsten carbide were formed in the composite, and they were responsible for lowering the friction and improving the wear resistance at elevated temperatures.

  8. 8.

    The Ni–Cr–W–Fe–C–MoS2 composite possessed excellent self-lubricating properties over a wide range of temperatures as a result of synergistic lubricating effect of graphite (C) and molybdenum disulfide (MoS2). The graphite contributed to the dominant role of lubrication at room temperature, while the sulfides were responsible for low friction at high temperatures.

  9. 9.

    Magnesium alloys are used in automobile and aerospace industries due to their low density, high specific strength, stiffness, good damping characteristic, excellent machinability, and ease of casting. Additionally, the poor corrosion and wear resistance of magnesium is a drawback that facilitates its primary use as an alloying agent to make aluminum–magnesium alloys. The most commonly used magnesium alloy is AZ91 alloy (consisting of 90 % magnesium, 9 % aluminum, and 1 % zinc). Magnesium alloy–graphite composites have found applications in pistons and space structural applications, thus expanding their versatility and use as self-lubricating metal matrix composites.

  10. 10.

    In electrical contact applications, the amount of material removed under sliding electrical contact is the sum of the contributions from purely mechanical wear in the absence of electrical current flow, arc erosion, and mechanical wear resulting from softening of the matrix by local heating caused by the electrical arcing. In a purely mechanical sliding condition without current flow, only mechanical wear would result.

  11. 11.

    Aluminum alloys suffer from low resistance to wear under poor lubricating conditions, and their severe seizure and galling under boundary lubrication conditions are major concerns for their use in high-performance tribological applications. With solid lubricant particle dispersion in the matrix of an aluminum alloy, this material can exhibit improved wear resistance and consequently become more suitable for high-performance tribological applications. The inclusion of solid lubricating particles in aluminum alloy matrices improves friction, wear, and anti-seizing properties. Self-lubricating aluminum alloy–graphite particulate composites have received attention because of their low friction and wear, reduced temperature rise in the tribo-interface, improved machinability, excellent anti-seizure effects, low thermal expansion, and high damping capacity.

  12. 12.

    In silicon nitride–graphite composites, the decrease in the coefficient of friction is a result of the formation of a graphite transfer layer consisting of materials from both contacting surfaces (i.e., consisting of graphite, iron oxide, and the silicates). The lack of a decrease in the coefficient of friction for the alumina–graphite composite can be explained by the presence of steel wear particles that inhibited the formation of the graphite transfer layer. The steel wear debris physically covered the graphite regions within the composite thus retarding the lubricating properties of the graphite.

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Menezes, P.L., Reeves, C.J., Rohatgi, P.K., Lovell, M.R. (2013). Self-Lubricating Behavior of Graphite-Reinforced Composites. In: Menezes, P., Nosonovsky, M., Ingole, S., Kailas, S., Lovell, M. (eds) Tribology for Scientists and Engineers. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-1945-7_11

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