A material’s resistance to cracking—referred to as “toughness” by materials scientists—is measured customarily by taking a sample of the material, slightly notching one edge, and pulling on the ends repetitively to see if the tensile stress causes the notch to grow into a crack. Bulk silicon always has passed this test. But, argued the NIST team, in real-world MEMS devices the stresses are likely to be much more complicated.
To test this, they used an alternate method: pressing the top of test crystals with tiny tungsten-carbide spheres about 3 mm in diameter at pressures below the silicon’s breaking point. Simply pressing down hard on the crystal for days at a time caused no detectable cracks—arguing against the corrosion theory. On the other hand, using half the pressure but cycling the test hundreds of thousands of times revealed a gradually increasing pattern of surface damage at the indentation site—clear indication of mechanical fatigue. The NIST team, which included a researcher from the University of Extremadura in Spain, theorizes that the critical element in their experiments is the addition of shear stress (causing the crystal planes to slide against each other), a component missing in conventional tensile strength tests but not uncommon in real-world applications.
The NIST experiments demonstrated fatigue effects in silicon at the comparatively large scale of hundred of micrometers. The next step is to determine if the same mechanisms operate at the submicrometer level. ###
* S. Bhowmick, J.J. Meléndez-Martínez and B.R. Lawn. Bulk silicon is susceptible to fatigue. Applied Physics Letters 91, 201902. Published online 13 November 2007.
Contact: Michael Baum firstname.lastname@example.org 301-975-2763 National Institute of Standards and Technology (NIST)
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