Effect of microstructure on mechanical and tribological properties of TiAlSiN nanocomposite coatings deposited by modulated pulsed power magnetron sputtering

Abstract

TiAlSiN nanocomposite coatings were deposited in a closed field unbalanced magnetron sputtering system by reactive sputtering from Ti0.475Al0.475Si0.05 targets using modulated pulsed power magnetron sputtering (MPPMS) under a floating substrate bias. The ratio of the nitrogen flow rate to the total gas flow rate (fN2) was varied from 0 to 40%. The application of MPPMS as sputtering sources was aimed at generating a high ionization degree of the sputtered material and a high plasma density by using a pulsed high power approach. When fN2=0%, an amorphous-like structure Ti0.479Al0.454Si0.066 coating was deposited with a hardness of 10GPa. When nitrogen was added, an optimized nanocomposite structure of nc-TiAlN/a-Si3N4 formed in the TiAlSiN coating deposited at fN2=10%, in which 5–10nm TiAlN nanocrystallites were embedded in a 2–3nm thick amorphous Si3N4 matrix. As the fN2 was increased up to 40%, the elementary composition of the coatings remained almost the same, but the grain size of nanocrystallites approached to 10–20nm and the AlN phase gradually precipitated. A maximum hardness (H) of 33.2GPa, a hardness to the elastic modulus (E) ratio of 0.081 and an H3/E*2 ratio of 0.19GPa were found in the coating deposited at fN2=10%. The friction coefficient of the TiAlSiN coatings was around 0.8–0.9 as sliding against a Si3N4 counterpart under a normal load of 0.5N. A wear rate of 2.0×10−5mm3N−1m−1 was measured in the TiAlSiN coatings deposited at fN2=20–40%. As only a low residual stress is found in the TiAlSiN coatings, we consider the complete phase separation is responsible for the enhanced mechanical and tribological properties of the nc-TiAlN/a-Si3N4 nanocomposite coatings.