Surface coatings, found on everything from painted walls to computer chips, often build up residual stresses during deposition, drying, or mechanical loading. If the stresses become large enough, the film can fracture, which typically leads to a disordered crack pattern. But under the right conditions, regular patterns like the ones in the optical microscope images shown on page 19 can appear.

Joël Marthelot (now at MIT), Benoît Roman, and José Bico of ESPCI ParisTech, along with their colleagues from the Saint-Gobain Corp and the University of Santiago, have taken a close look at the formation of such cracks. The material they studied was so-called spin-on glass (SOG)—a silicate coating commonly used for electronic components—deposited on a silicon substrate. What they discovered was a new fracture mechanism that has implications for understanding the stability of thin films and offers the possibility of novel applications.1 

Although unusual, the crack patterns aren’t...

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