GaN's electronic and optical properties have been studied extensively for several decades, said Zeng, the lead author of the APL article, but virtually no studies have been done of its tribological properties, that is, its resistance to the mechanical wear imposed by reciprocated sliding.
"Our group is the first to investigate the wear performance of GaN," said Zeng. "We have found that its wear rate approaches that of diamonds, the hardest material known."
Wear rate is expressed in negative cubic millimeters of Newton meters (Nm). The rate for chalk, which has virtually no wear resistance, is on the order of 10-2 mm3/Nm, while that of diamonds is between 10-9 and 10-10, making diamonds eight orders of magnitude more wear resistant than chalk. The rate for GaN ranges from 10-7 to 10-9, approaching the wear resistance of diamonds and three to five orders of magnitude more wear resistant than silicon (10-4).
The Lehigh researchers measured the wear rate and friction coefficients of GaN using a custom microtribometer to perform dry sliding wear experiments.
The range in wear resistance, the researchers said, is caused by several factors, including environment, crystallographic direction and, especially, humidity.
"The first time we observed the ultralow wear rate of GaN was in winter," said Zeng. "These results could not be replicated in summer, when the material's wear rate increased by two orders of magnitude."