"Not all properties may benefit from microstructural refinement, so due caution needs to be exercised in materials design," writes the National Institute of Standards and Technology's (NIST) Brian Lawn in the January issue of Journal of Materials Research.* An expert on brittle materials, Lawn advises that past experience is not always a useful guide for predicting material properties and performance when film thicknesses, grain sizes and other characteristic dimensions shrink toward molecular proportions. At this level, materials designers must reckon with interatomic force laws that are obscured at larger scales, from micrometers (millionths of a meter) on up.
"Generally in brittle materials, strength (resistance to crack initiation) increases and toughness (resistance to crack propagation) decreases as characteristic scaling dimensions diminish," Lawn concludes from his work to refine ceramics used in biomechanical applications such as dental crowns and orthopedic implants. At the nanoscale, tiny cracks require more load to spread them, but have little resistance to extension once they start and are, therefore, more likely to spread catastrophically. Depending on the application in mind, the decrease in fracture toughness may more than offset initial gains in strength, or the ability to withstand stresses that squeeze, stretch or twist the material.
This poses challenges for designers who choose to build minuscule devices and tiny systems with ceramics because of the light weight, high strength and hardness. Lawn says contact points in devices with moving parts will require especially close attention. As the size of contacts decreases, he notes, stresses will become more concentrated, "increasing the potential for irreversible damage and premature failure at ever-lower critical loads."
*B.R. Lawn, "Fracture and Deformation in Brittle Solids: A Perspective on the Issue of Scale," J. Mater. Res., Vol. 19, No. 1,Jan. 2004.