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

5 mu m 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 degrees C leads to Ag segregation within the CrN matrix and the formation of embedded lamellar Ag aggregates with increasing Size, < 10(5), 9 X 10(6), and 7 x 10(7) nm(3), respectively. Ball-on-clisk tests against 100Cr6 steel, followed by optical profilometry and energy dispersive spectroscopy, indicate that the Ag grains for T-s = 500 degrees C are too small to facilitate an effective lubricious surface layer, resulting in a friction coefficient mu = 0.58 and a composite coating wear rate of 3.8 x 10(-6) mm(3)/Nm that are nearly identical to those measured for pure CrN with mu = 0.64 and 3.6 x 10(-6) mm(3)/Nm. The T-s = 600 degrees C coating exhibits a Ag concentration which is 15% higher within than outside the wear track, and acts as a lubricious layer that reduces mu to 0.47 and yields a 16x and 2.4x lower wear rate for coating and counterface, respectively. T-s = 700 degrees C leads to a dramatic increase in surface roughness and an associated increase in friction, mu = 0.85, and wear, 9.9 x 10(-6) mm(3)/Nm. Replacing the steel counterface with an alumina ball results in the lowest mu = 0.50 for T-s = 500 degrees 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 degrees C, which is attributed to a breakdown of the lubricious Ag layer by the harder counterface. The transient friction coefficient mu(t) during experiments with continuously increasing testing temperature T-t = 25-700 degrees 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 degrees C coating, a temperature activated solid lubricant transport yields a lubricious Ag surface layer and a very low mu(t) = 0.05 at T-t similar to 500 degrees C. All coatings exhibit an increasing mu(t) for T-t > 500 degrees C, which is attributed to oxidative degradation. Published by Elsevier B.V.