Microstructure evolution and high-temperature tribological behavior of AlCrBN coatings with various B contents

The boron content affects the microstructures and high-temperature tribological performance of AlCrBN coatings. AlCrBN coatings with various B contents (0 at.%, 1.5 at.%, 2.9 at.%, 4.8 at.%,) were synthesized by multi-arc ion plating. The microstructure, mechanical and high-temperature tribological properties were investigated. TEM results show that addition of 1.5 at.% of B results in solid solution (Al, Cr, B)N and a small amorphous a-BNx within the (Al, Cr, B)N columnar structure. Further addition of B contents of 2.9 at.% and 4.8 at.% leads to the composite structure comprising of nano-grains surround by amorphous a-BNx. As the B content increases, the amount of nano-grains decreases, and the thickness of the amorphous layer increases. The hardness measurements also indicate that the AlCrBN coating with 1.5 at.% B content had the highest hardness of 38.3 GPa due to the solution strengthening. On the other hand the AlCrBN coating with 2.9 at.% B content had the best toughness with higher values of H/E (0.073) and H3/E*2 (0.169 GPa). High-temperature tribological measurements showed that the friction coefficients decreased with temperature for all such AlCrBN coatings. At 400 °C, AlCrBN coatings with B contents of 0 at.%, 1.5 at.%, and 2.9 at.% exhibited low wear rates due to their high hardness, while AlCrBN coating with a B content of 4.8 at.% showed the lower wear rate at 800 °C due to the formation of a continuous B2O3/B(OH)3 film on the coatings.