Do Organic Materials Make Metals Brittle?

The environment may play a larger role in how and why metals bend and fracture than previously thought.

In the paper "On the role of surface stress in environment-assisted fracture," Exponent's Ronald Latanision and coauthors explore how organic molecules affect metal surface plasticity and fracture. They expound on a recently discovered effect in metal cutting called organic monolayer embrittlement (OME), the process by which metal surfaces are rendered brittle by long-chain organic adsorbates — molecules that adhere to the surface of metals in open environments. 

Latanision and his coauthors review cutting experiments with aluminum-containing self-assembled monolayers (SAMs) that show how OME is controlled by surface stress induced by adsorbates. This new discovery could change the way metals are cut and machined in the future.

The authors also describe a cantilever technique to measure surface stress, demonstrate the strong effect of SAM molecule chain length on surface stress, characterize how dislocation activity at crack-tips is affected by adsorbate-induced surface stress, and describe large improvements in machining processes enabled by controlled environment-assisted fracture.

From the publication: "We make the case that surface stress, due to adsorbates, likely influences all environmentally assisted cracking (EAC) phenomena, warranting a revisit of extant models of EAC."