Hard materials such as chromium nitride are used as wear and corrosion protection coatings in a wide range of applications, including metal cutting. Now, researchers at Singapore's Agency for Science, Technology and Research (A*STAR) have discovered exactly how such materials behave when used in high-stress situations, paving the way to producing even better coatings.

One way to improve a material's resistance to wear is to increase its hardness. This depends mainly on the force it can withstand before it starts to permanently deform. In most crystalline materials, this deformation occurs when defects, known as dislocations, start to move through a material’s crystal structure.

Previous research has shown that it is difficult to break crystals that are extremely small. Shiyu Liu, of A*STAR's Institute of Manufacturing Technology, and colleagues investigated this property to study how coatings based on chromium nitride might deform.

The researchers first made microscopic pillars of the material, roughly 380 nanometers in diameter. Then, they compressed them using a diamond flat punch in a scanning electron microscope at temperatures of up to 500 degrees Celsius and studied the results.

Scanning electron microscope images show pristine (left) and compressed (right) micropillars of chromium aluminum nitride/silicon nitride nanocomposite. Image credit: AIP Publishing.They found that if the chromium nitride-based coatings are made with very fine grains—each roughly 10 nanometers in diameter, with each grain separated by a thin grain boundary phase—the force required to deform such materials increased dramatically. Deformation began at stresses very much higher than expected—and close to the theoretical maximum value based on calculations.

Liu's team showed that the increase happened when the grains became so small that they did not contain dislocations, so the applied forces had to be sufficiently large to form new dislocations within the grains. It had long been thought that the thin grain boundary phase was the primary factor in determining the material’s properties. The results show that the formation of a fine-grained microstructure could provide a ceramic coating with enhanced hardness and fracture toughness.

The team plans to use the results in advanced manufacturing and engineering applications, such as protective coatings in high-speed machining tools for titanium and nickel-based alloys