Effects of aerospace-grade steel surface topography and coating on lubricated friction coefficient

Aerospace gear systems prioritize the minimization of frictional losses to maximize drive system efficiency and improve overall fuel economy. In this context, surface roughness, coatings and advanced lubricants play a crucial role to increase tribological performance and reduce power consumption. 

Several studies have shown that surface roughness is a critical parameter, as processes like isotropic superfinishing can reduce frictional losses by approximately 30% compared to standard ground surfaces. However, the differences between the latter and more conventional (and cost-effective) finishing processes such as polishing, lapping or vibro-finishing are still not clear as varying amounts of asperity interactions are involved relative to the initial oil film thickness in the contact. Moreover, it has also been shown that the use of advanced coatings, such as tungsten diamond-like carbon (W-DLC), further enhances performance by lowering the effective friction coefficient and increasing durability in dry or boundary lubrication conditions such as those produced by loss of lubrication. Although the mechanism ruling the traction behavior of such layers has proved to be of multi-physical nature the behavior of coated and lubricated systems in the mixed operating range has not been analyzed, specially when combined  to non-Newtonian and additivated oils typical of aeronautical applications.

This work presents an extensive tribological experimental investigation on the lubricated friction coefficient behaviour of AISI 9310 and 32CDV13 aerospace-quality steels with different surface topographies, hardness and operating conditions compatible with aeronautical gear applications. First, surface topography of test specimens is analysed using an optical interferometer and micro-hardness is measured. Then, traction, Stribeck and dry-running tests are carried out on a commercial ball-on-disc tribometer with a reference lubricant complying with aeronautical MIL-PRF-23699 standard. Several degrees of rolling and sliding speeds, load and oil temperatures are applied which result in different lubrication states ranging from boundary lubrication to full elastohydrodynamic (EHL) conditions. Moreover, the effect of a WC/C coating on such materials is analyzed and compared to base conditions. In all tests, electrical contact resistance and wear depth are monitored along with traction force. Results show that friction coefficient is mainly governed by asperity contacts in non-coated specimens while thermal effects domain in coated ones with varying contributions specially in the mixed lubrication regime. Overall, the combination of low surface roughness and coating has shown to be beneficial in all lubrication states with low friction values in all cases, even in boundary conditions, which leads to high efficiencies in gear applications when subjected to different operating conditions.