Cu-Ag alloy wears itself well

Friction between metallic materials can cause severe damage, leading to surface wear and ultimately even failure. But although this plastic deformation is generally regarded as detrimental, it can have surprising advantages. Researchers from the University of Illinois at Urbana-Champaign, the Karlsruhe Institute of Technology, and Technische Universität Darmstadt have found such plastic deformation of the precipitate- containing, two-phase Cu90Ag10 alloy spontaneously leads to the creation of nanolayers at the surface that reduce wear [Ren, F., et al., Acta Materialia 72 (2014) 148-158, DOI: 10.1016/j.actamat.2014.03.060].

The phenomenon of plastic deformation triggering the formation of self-organized microstructures that actually improve wear resistance has been widely recognized for over a decade. The effect can be exploited through severe plastic deformation (SPD), in which the grain size in an alloy is reduced to the nanoscale, boosting the material’s strength. Similarly, friction can be used to induce a transition in ‘chameleon’ coatings that ultimately leads to a reduction in friction and wear. Another example of such self-adaptation is the spontaneous formation of tribolayers during wear, which act as solid lubricants. But while these phenomena are well-documented in elemental metals, there has been much less exploration of similar effects in multiphase alloys.

Now, however, the US and German team has found a novel self-adapting mechanism in Cu90Ag10 whereby chemically nanolayered structures form spontaneously under plastic deformation at the surface that reduce wear.

“We [have] observed that the presence of the chemical nanolayering can reduce wear by a factor 2 to 20,” Pascal Bellon of the University of Illinois at Urbana-Champaign told Materials Today.

Of crucial significance appears to be the initial size of the Ag precipitates. In alloys with larger precipitates, despite the reduction in hardness, alternating Ag- and Cu-rich nanolayers form under the sliding surface, which provide excellent wear resistance. The nanolayers remain stable as long as wear continues, the report finds.

The researchers believe that deformation-induced chemical layering, and the wear resistance it conveys, should be observable in other alloys. The alloys would have to contain precipitates of low-melting point, ductile metals like Ag, Sn, In or Bi or other low shear strength phases. The precipitates of these metals would have to be sufficiently small to form nanolayers during wear, but large enough not to homogenize. The phenomenon could, the researchers suggest, constitute a novel approach to creating metallic alloys with low wear rates.

“Novel materials for high wear performance could be designed by choosing their chemistry and initial microstructure so that their surface would spontaneously self-organize into nanolayers of alternating chemical make-up, resulting in improved wear resistance,” explains Bellon. “We believe that this approach can be implemented without too much problem.”

In fact, the researchers are already working with industry to exploit the results for specific wear resistant applications. While designing wear resistant materials is not a trivial undertaking, with many additional factors coming into play such as corrosion resistance, thermal stability and cost, they believe there are no critical limitations to the new approach.