Gallium nitride almost as wear-resistant as diamond
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.”
To determine how the higher summer humidity was affecting GaN’s wear performance, the researchers put their tribometer in a glove box that can be backfilled with either nitrogen or humid air.
“We observed that as we increased the humidity inside the glove box, we also increased the wear rate of GaN,” said Zeng.
An author of the paper, Tansu said the group’s discovery of GaN’s hardness and wear performance could have a dramatic effect on the electronic and digital device industries. In a device such as a smartphone, he said, the electronic components are housed underneath a protective coating of glass or sapphire. This poses potential compatibility problems which could be avoided by using GaN.
“The wear resistance of GaN,” said Tansu, “gives us the opportunity to replace the multiple layers in a typical semiconductor device with one layer made of a material that has excellent optical and electrical properties and is wear-resistant as well.
“Using GaN, you can build an entire device in a platform without multiple layers of technologies. You can integrate electronics, light sensors and light emitters and still have a mechanically robust device. This will open up a new paradigm for designing devices. And because GaN can be made very thin and still strong, it will accelerate the move to flexible electronics.”
In addition to its unexpectedly good wear performance, said Zeng, GaN also has a favorable radiation hardness, which is an important property for the solar cells that power space vehicles. In outer space, these solar cells encounter large quantities of very fine cosmic dust, along with x-rays and gamma rays, and thus require a wear-resistant coating, which in turn needs to be compatible with the cell’s electronic circuitry. GaN provides the necessary hardness without introducing compatibility issues with the circuitry.
The Lehigh group has begun collaborating with Bruce E. Koel, a surface chemistry expert and professor of chemical and biological engineering at Princeton University, to gain a better understanding of the interaction of GaN and water under contact. Koel was formerly a chemistry professor and vice president for research and graduate studies at Lehigh.
The paper can be found here: https://dx.doi.org/10.1063/1.4960375.
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