ICMCTF2013 Session E3-1+G: Tribology of Coatings for Automotive and Aerospace Applications

Stainless steels are widely used in the aeronautical field. Among these steels, the 15-5 PH martensitic stainless steel (X5CrNiCu15-5), known for high strength and good corrosion resistance, presents poor tribological behavior, particularly in high temperature environment. Moreover, in the field of surface treatments and coatings, recent environmental regulations encourage working on new non polluting process.

In this study, a process was developed to improve the tribological behavior of the martensitic stainless steel until 500°C. Organic-inorganic hybrid coatings were prepared via sol-gel route and deposited onto stainless steel by dip-coating technique.

The effects of an additional thermal treatment, under different temperatures and in air or in nitrogen atmospheres was then studied. Combined analysis was conducted by thermal analysis (DTA/TGA), Nuclear Magnetic Resonance (of ²⁹Si and ¹³C) and Raman Spectroscopy on the xerogel and on the coating.

The mechanical properties and the tribological behavior (evaluated with a pin-on-disk tribometer) of the coated samples were also studied.

In aeronautics tribology, mechanical parts are required to operate with increasing temperature. The increased functioning temperature (up to 700°C) of the contacts prone to friction and wear (such as bearings and other structural parts) is a direct consequence of the increasing power of jet engines. In the case of ball bearings, the substrate materials as well as the coating durability are affected by temperature. There is then a pressing need to introduce new coatings demonstrating effective tribological behavior at high temperature (e.g. 600°C instead of 150°C for some aeronautical joints), especially in order to replace silver coating used at normal temperature but that cannot work at higher temperature. In partnership with Airbus Aerospace and SKF Aerospace, the durability of ball bearings functioning under extreme conditions in terms of temperature and sliding friction was studied.

First, SEM/EDX analyses were done on the silver coating deposited in the contact between the ball bearings rings, in order to acknowledge the actual bearings surface damage. Tribological testing of the silver coating was then performed in a standard cylinder-on-flat configuration in order to compare in situ and experimental damages.

More than ten different coatings (both soft and hard coatings, with solid lubrication properties and/or wear resistance properties) were then tested in the same configuration and conditions as the silver coating. These tests allowed us to identify the best suitable coatings for the application. An original test rig was also designed in the lab, in order to better simulate the bearings functioning conditions. This tribometer makes it possible to perform tests in ring-on-flat configuration (close contact) and to simultaneously apply a normal force up to 50 kN and a reciprocating rotating motion. Using this tribometer, the final tests were conducted on the best candidates among the different coatings, in a configuration as close to reality as possible, with a hard coating covered ring and a soft coating covered flat, the contact between the two being greased. From these tests, mechanisms of solid lubrication were examined and typical tribological damages such as adhesion and abrasion were observed. Finally, several possible solutions were determined, for two different functioning temperatures: 200°C and 600°C.

Solid Particle Erosion (SPE) degradation of engine components in aircraft operating in harsh environments is a well known issue causing severe maintenance and reliability problems. In order to enhance the lifetime of engine components many different hard protective coatings have been developed over the last two decades. While these coating systems are mostly based on TiN/Ti multilayer microstructures, they present varying material removal mechanisms depending on the phase constitution and microstructure of the target, and on the erodent characteristics (particle size, speed, composition and shape). Studying these mechanisms in detail is quite challenging given that SPE testing is notoriously inaccurate due to its aggressive nature and its many methodological uncertainties. In this presentation we will outline the work recently performed at Polytechnique Montreal on the methodological aspects of gas-blast SPE testing of hard protective coatings.

We will first present our work with respect to “traditional” erosion testing by evaluating the volume loss from the tested samples in order to accurately compare the SPE performance of different coatings without the need of obtaining coating density which is rarely accurately known. In the second part of the presentation, we will demonstrate a new in-situ real time erosion testing methodology using a quartz crystal microbalance in order to study the SPE of different hard protective coating systems. Using the previous results by volume loss measurement, we validate and discuss the advantages and challenges related to such a method. Finally, this time-resolved technique enables us to discuss some transient events present during SPE testing of hard coating systems leading to new insights into the erosion process.