CrN–Ag nanocomposite coatings: Tribology at room temperature and during a temperature ramp

Abstract

5 µm thick CrN–Ag composite layers with 22 at.% Ag were deposited by reactive magnetron co-sputtering on 440C stainless steel substrates. Increasing the growth temperature from T s = 500 to 600 to 700 °C leads to Ag segregation within the CrN matrix and the formation of embedded lamellar Ag aggregates with increasing size, < 10 5, 9 × 10 6, and 7 × 10 7 nm 3, respectively. Ball-on-disk tests against 100Cr6 steel, followed by optical profilometry and energy dispersive spectroscopy, indicate that the Ag grains for T s = 500 °C are too small to facilitate an effective lubricious surface layer, resulting in a friction coefficient μ = 0.58 and a composite coating wear rate of 3.8 × 10 − 6 mm 3/Nm that are nearly identical to those measured for pure CrN with μ = 0.64 and 3.6 × 10 − 6 mm 3/Nm. The T s = 600 °C coating exhibits a Ag concentration which is 15% higher within than outside the wear track, and acts as a lubricious layer that reduces μ to 0.47 and yields a 16× and 2.4× lower wear rate for coating and counterface, respectively. T s = 700 °C leads to a dramatic increase in surface roughness and an associated increase in friction, μ = 0.85, and wear, 9.9 × 10 − 6 mm 3/Nm. Replacing the steel counterface with an alumina ball results in the lowest μ = 0.50 for T s = 500 °C, attributed to the presence of Ag and the relatively low hardness of 16.5 GPa for this particular coating. In contrast, friction and wear increase dramatically for T s = 600 °C, which is attributed to a breakdown of the lubricious Ag layer by the harder counterface. The transient friction coefficient μ t during experiments with continuously increasing testing temperature T t = 25–700 °C initially decreases for all samples, attributed to drying of the environment and an effective softening of both coating and counterface. For the T s = 500 °C coating, a temperature activated solid lubricant transport yields a lubricious Ag surface layer and a very low μ t = 0.05 at T t ~ 500 °C. All coatings exhibit an increasing μ t for T t > 500 °C, which is attributed to oxidative degradation.