High rate deposition of thick CrN and Cr2N coatings using modulated pulse power (MPP) magnetron sputtering

As a variation of high power pulsed magnetron sputtering technique, modulated pulse power (MPP) magnetron sputtering can achieve a high deposition rate while at the same time achieving a high degree of ionization of the sputtered material with low ion energies. These advantages of the MPP technique can be utilized to obtain dense coatings with a small incorporation of the residual stress and defect density for the thick coating growth. In this study, the MPP technique has been utilized to reactively deposit thick Cr2N and CrN coatings (up to 55 μm) on AISI 440C steel and cemented carbide substrates in a closed field unbalanced magnetron sputtering system. High deposition rates of 15 and 10 μm per hour have been measured for the Cr2N and CrN coating depositions, respectively, using a 3 kW average target power (16.7 W/cm2 average target power density), a 50 mm substrate to target distance and an Ar/N2 gas flow ratio of 3:1 and 1:1. The CrN coatings showed a denser microstructure than the Cr2N coatings, whereas the Cr2N coatings exhibited a smaller grain size and surface roughness than those of the CrN coatings for the same coating thickness. The compressive residual stresses in the CrN and Cr2N coatings increased as the coating thickness increased to 30 μm and 20 μm, respectively, but for thicker coatings, the stress gradually decreased as the coating thickness increased. The CrN coatings exhibited an increase in the scratch test critical load as the thickness was increased. Both CrN and Cr2N coatings showed a decrease in the hardness and an increase in the sliding coefficient of friction as the coating thickness increased from 2.5 to 55 μm. However, the wear rate of the CrN coatings decreased significantly as the coating thickness was increased to 10 μm or higher. The 10–55 μm CrN coating exhibited low wear rates in the range of 3.5–5 × 10−7 mm3 N−1 m−1. To the contrary, the Cr2N coating exhibited relatively low wear resistance in that high wear rates in the range of 3.5 to 7.5 × 10−6 mm3 N−1 m−1 were observed for different thicknesses.

Research Highlights
► Reactive sputtering thick CrN and Cr2N coatings (55 μm) using the MPP technique.
► High deposition rates (10–15 μm/h) were achieved using the MPP technique.
► The thick CrN coatings showed excellent adhesion and tribological properties.
► The structure and properties of the thick CrN and Cr2N coatings were compared.