ICMCTF2014 Session E3-2: Tribology of Coatings for Automotive and Aerospace Applications

Airborne particles ingested by gas turbine engines may cause severe erosion damage to compressor gas-path components, leading to structural and aerodynamic engine performance deterioration and, in extreme cases, causing engine failure. Applying hard coatings on airfoil surfaces is proven to be an effective approach to mitigating erosion damage to engine components. Nanolayered or multilayered coatings, because of their capability of tailoring the properties through modifications in the chemistry and architecture of layer constituents, have been explored as potential candidates for this specific application. In this study, nanolayered CrAlTiN coatings with different modulation periods, along with multilayered CrAlTiN-AlTiN coatings having different number of layers and different thickness of individual layers, were fabricated, characterized and evaluated. All the coatings significantly outperformed the CrN baseline coating in erosion resistance, and their performance was strongly affected by the modulation period of the nanolayered coatings or the layer architectural characteristics of multilayered coatings.

In this paper microstructure and basic mechanical properties of WC-Co coatings obtained by HVOF technique were shown. Two different feedstock powders of WC-Co 83-17 type were used for coatings deposition on a steel substrate. First of them was a standard powder of Amperit 625.074, second one was Inframat from category Infralloy^TM S7400 superfine powder. Four different powders were used to modify coating deposited from Amperit 625.074. Sub-microcrystalline powders applied in order to modify standard WC-Co coating were as follow: WC, Cr₃C₂, B₄C, TiC. The aim of investigations was to compare microstructure and some mechanical properties of coatings deposited from different types of carbides. An influence of sub-microcrystalline additions on basic mechanical properties of coatings was analyzed. the range of investigations included short characterization of feedstock powders by SEM, EDS, EBSD and XRD methods and theirs technological properties as well. Then deposited coatings were characterized. Overall quality, porosity, adhesion of coatings to substrate and theirs tendency to making cracks were analyzed.

Financial support of Structural Funds in the Operational Program - Innovative Economy (IE OP) financed from the European Regional Development Fund - Project No POIG.0101.02-00-015/09 is gratefully acknowledge.

One of the major issues for the use of devices in spacecrafts is the reduction of their mass and lowering power consumption. Harmonic drive gears provide these requirements since the high gear ratio possible, enables the use of small actuator motors (low mass and power). State of the art in harmonic drive technology is the grease lubricated gear. The drawback of these gears in space application is that the used lubricant tends to outgas which causes a reduction of the lubrication ability plus the resulting vapours can contaminate sensible surfaces e.g mirrors, which is even more defacing for the functionality of the respective devices.

The aim of the presented research work here was to apply solid lubrication in the field of harmonic drive gears for vacuum application. The main problem of coatings providing this lubrication is their durability under the high contact stress occurring in theses gears.

After discussing the specifications needed, a multilayer coating of nanoperiod of MoS_x/WC PVD coating with a WC interlayer between substrate and coating was selected. This coating was already tested in space on the ISS in the TriboLAB tribometer and is known for its good tribological properties under space conditions.

The coating was characterized in vacuum pin on disc tests with varying substrate material for pin and discs. This was important since also the condition of the substrate material is important for a successful coating and the components engaged in a harmonic gear, are manufactured out of materials with different properties (e.g. stiffness).

Diamond-like carbon (DLC) coatings exhibit an unusual combination of tribological and mechanical properties: low coefficients of friction and high hardness/high elastic modulus. However, synthesizing a single DLC material to achieve low friction and low wear in all environments (e.g., from ultra-high vacuum to humid air and from ambient to elevated temperatures) is a challenging task. Most commercial DLCs are either doped with metals, hydrogen or synthesized as nanocomposites. This talk will first provide a brief overview of the roles of chemistry and composition of multi-phase DLCs and nanolaminates in imparting environmental robustness. The role of substrate-coating interface reliability, specifically the onset of plastic deformation in the underlying substrate, on the tribological performance of DLCs will be discussed. In this context, finite element modeling was used to predict the contact stresses at which the transition from elastic to plastic deformation in a metallic substrate underneath a hard DLC coating occurs. Experimental data confirmed that accumulated plastic strains at the coating-substrate interface can lead to fracture and subsequent removal of the coating, highlighting the need for incorporating a load bearing nanolaminate underneath the DLC.

The second part of the talk will address the high temperature tribology of DLCs. A tetragonally bonded amorphous DLC and a silica doped diamond like nancomposite (DLN) coating were selected for this study. Friction and wear measurements were made at 300°C in lab air with a RH of 40-50% against a 440C steel ball in reciprocating sliding motion. Results indicate that the silica doped nanocomposite showed low friction (0.05-0.1) and low wear (2E-9 mm3/Nm) both at ambient temperature and at 300°C. Time of flight SIMS and Raman spectroscopy analyses of the interfaces, i.e., wear tracks and transfer films, were used to gain a fundamental understanding of the role of interfacial chemistry on friction and wear mechanisms, and to elucidate the “chameleon” nature of the nanocomposite.

* Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.