ICMCTF2016 Session E3: Tribology of Coatings for Automotive and Aerospace Applications

Coatings are well established to solve tribological problems in automotive applications. Higher operating temperatures, new lubricants / additives, new materials or material combinations, and so on make it necessary to continuously update the coating portfolio.

Problem solving in automotive applications is usually a competition between many different approaches. In the end, the compromise with the lowest overall costs will succeed, even if there are solutions which in some aspects give a superior performance. The most popular carbon based coatings, usually called DLC for Diamond-Like Carbon, are a-C:H coatings which means amorphous carbon with a certain amount of hydrogen. These coatings are relatively simple to manufacture in high volumes at an acceptable price. The application of these coatings is limited by the maximum operation temperature which can be as low as 200°C depending on the applied pressure. In state of the art engines these temperatures are reached during normal operation, at least as transient localized flash-temperatures in the frictional contact.

Molybdenum nitride based coatings could be an alternative for DLC in lubricated systems. MoN coatings were produced by arc-evaporation. This high-energy deposition process gives some advantages with respect to the microstructure.

A significant drawback is the largely increased surface roughness by droplets; a dedicated post treatment is mandatory. The post treatment is basically a brushing operation. This process looks rather simple, but is difficult to control. Some methods for the efficiency assessment were evaluated. The counter body wear of a bronze material was used as a benchmark. Several crystalline phases of MoN could be identified. In particular hexagonal MoN seems to be favorable. A detailed analysis showed that the coating structure can be modified by adaptation of process parameters. Piston pins were coated with a single phase MoN coating for series production.

Besides a “simple” MoN coating also doped modifications or MoN as constituent of a multi-layer system were made for different applications. Of particular interest was the compatibility of MoN with lubricants and additives. It is well known that carbon based coatings usually don’t work too well with additives. It could be shown that MoN behaves quite similar to steel surface in most of the cases.

The planetary roller screw (PRS) mechanism is used in the aeronautics industry for electro-mechanical actuators application. It transforms a rotation motion into a translation motion, and it is designed for heavy loads. The main components are made of martensitic stainless steel, and lubricated with grease. Like most usual rolling mechanisms, smearing and jamming can occur before the theoretical fatigue lifetime, especially in poor lubricating conditions.

The aim of this work is to investigate the tribological behavior of a WC/C coating for different operating and design parameters such as load, speed, slip ratio, lubrication… on a laboratory tribometer, representing the conditions of a single contact in the PRS. In a PRS, a single contact is mainly subjected to a rolling contact pressure, together with sliding, perpendicular to the rolling direction. The main parameters are then the normal load (leading to maximum contact pressure from 2 to 4 GPa), the rolling speed (from 0.5 to 2 m/s), and the slip ratio (ratio between the sliding speed and the rolling speed, from 0 to 10 %). A specific test rig has been developed to simulate a simplified configuration by using a torus (free to roll) and a flat rotating disc, which represent respectively the roller and the screw profiles.

Textured surface with low friction have been prepared in air by using CO₂ laser. The textured surface friction performance and their chemical composition were explored. For textured surface production, a layer of graphite and molybdenum disulfide powder were placed separately onto titanium alloy (Ti-6Al-4V) substrate and a CO₂ laser beam (CO₂LB) was irradiated on the covered surface. The CO₂LB produced graphite and molybdenum disulfide integration on titanium surface, at the same time, changed graphite and MoS₂ chemical structure. Textured surface showed low friction coefficient in air environment. The results showed that textured surfaces are good candidates to be used as lubricated devices in aircraft, aerospace and automotive industry. The CO₂LB can be used to write or draw textured lines, or letters producing lubricated surfaces. Also can be used to prepare lubricated areas to reducing fretting during satellite launching and avoiding cold welding in many satellite devices in satellite orbit.

Despite tremendous development in gas and steam turbine materials, erosion still remains as a challenge due to the progressive loss of material that results from solid particles entrained in the air ingested by the engine, in particular during landing and take-off in dust intensive environments. The main problems associated with erosion are pitting and cutting of the blades leading and trailing edges and forming burrs thus affecting the overall performance of an engine. Thus, coating of blades to improve engine performance and to lower the maintenance cost is found to be a possible solution against erosion. The use of single layer nitride coatings (TiN) has been a known method for protection against erosion due to their hardness. In contrast, TiAlN coatings with comparatively higher hardness have been devised as a better alternative to TiN coatings.

In this particular work, monolayer PVD coatings were designed and applied on different substrate materials. The behavior of these coatings against solid particle erosion (SPE), water jet erosion and cavitation erosion at room temperature was tested. The influence of design on adhesion, hardness, residual stress was analyzed and correlated with erosion rate and the fatigue debit of the substrate material. SPE (GE specifications E50TF121) was used to evaluate the erosion durability at an angle of 20°, using abrasive particles of Al₂O₃. Water jet erosion test was carried out using different angles, 12°, 30°,60°,90°, and at room temperature. The resistance of coatings to cavitation erosion has been evaluated. All tested PVD coatings proved to have much higher erosion resistance than the substrate material. The different erosion test methods enabled us to study the erosion durability of coatings under various conditions.

References:

1. Tabakoff, W. Surface and Coatings Technology, 1999, 120, 542

2. Iwai, Y et al. Wear, 2001, 251, 861

3. Antonov, M et al. Wear, 2009, 267, 898

In tribological contact, not only the topography but also the subsurface microstructure is altered by the friction forces. The present study focusses on the influence of subsurface microstructure of AlSi disks on their running-in behavior. The tribological behavior of disks with different subsurface microstructures but comparable topographies was characterized with a pin-on-disk tribometer using a radioactive tracer technique to monitor wear in situ with a high resolution. The subsurface plastic deformation was characterized using different markers of gallium and platinum placed with a focused ion beam. Analysis of the markers showed together with micropillar testing results, that differences in hardening behavior due to the different microstructures can explain the observed differences in the wear behavior. Moreover, results of the marker analysis give insight in the initial stages of third body formation.

Boron carbide (B₄C) is a potential tool coating for machining of titanium alloys because of its high hardness, high temperature stability and low sliding COF against titanium. Boric acid could be formed on the surface of B₄C coatings after annealed above 600 °C, and this boric layer can work as lubricant leading to low sliding COF. In this paper, without annealing, a lubrication mechanism consisting of graphitization and passivation of graphitized carbon was established based on wear tests performed in various gas and liquid environments. Ball-on-disk (B₄C) type sliding tests were performed in dry argon, and nitrogen (with 0%RH) atmospheres, and air with humidity levels varying from 0% to 85% RH. B₄C samples were also tested while they were submerged in ethanol (C₂H₅OH) and iso-propyl alcohol (C₃H₇OH). The B₄C coatings exhibited high coefficient of friction (COF) values of 0.65, 0.59, and 0.59 in dry argon, nitrogen and dry air with 0.5% RH. Testing under an air atmosphere containing 70% RH reduced the steady state COF to 0.25 and further reduction COF to 0.20 was recorded in air with 85% RH. A COF of 0.15 was measured when the tests were carried out in ethanol. The observed reductions in COF were attributed to H and OH passivation of the C rich transfer layer as confirmed by the Raman, Fourier transform and X-ray photoelectron spectroscopy techniques. Iso-propyl alcohol provided the most effective OH passivation, as evident from the lowest COF of 0.07.

Ultra-thick (up to 558 µm) TiSiCN nanocomposite coatings were prepared on H-13 steel using a plasma enhanced magnetron sputtering (PEMS) process. In the deposition process, Ti was sputtered from Ti targets in a mixed gas environment of Ar, N₂ and TMS (trimethylsilane). In addition to the magnetron plasma, in the PEMS process a global plasma was produced from thermionic emission of tungsten filaments for enhanced ion bombardment of the substrates to achieve dense structure, good adhesion, and low internal stress. Two circular magnetrons of 170 mm in diameter, with the sample fixture rotating between them, were used to prepare single layered TiSiCN coatings and multi-layered TiSiCN/Ti coatings to achieve the coating thickness up to 188 µm. To further increase the coating thickness, one magnetron was used, while the samples faced to the magnetron without rotation, to achieve a single layer of 490 µm and a multi-layered coating of 558 µm.

Ceramic coatings raise interest from aeronautic industry as a protective barrier for new generation composite structures. They are being tested to protect turbine blade root from fretting wear damage against the metallic disk in high temperature stages of turbojet motors.

Former work [1] showed that for some ceramic coatings, a solid lubricant layer is decisive to create a protective third body in a simplified blade/disk contact. This third body, called “glaze layer”, prevents the interface from wear. The present study aims at describing the physicochemical properties of such tribofilm for another ceramic-metallic contact where no lubricant aggregates the glaze layer. This gives clues about the role of a ceramic substrate in the glaze layer formation that is known for fully metallic interfaces [2].

The experimental interface consists in a plane to plane contact between a Haynes 25 (cobalt-based alloy) punch and a ceramic flat, submitted to fretting solicitations. The glaze layer formed is stable above 500°C and provides both low friction and wear in the contact. An adhesive wear mechanism is observed on the flat, as a prominent transfer. Wear kinetics shows that this transfer is made of early lubricious debris that are progressively compacted and sintered to form the final 20 µm thick third body, which also sticks on the punch.

To answer the questions how and why the glaze layer protects the interface, both mechanical and chemical analyses are carried on. On one hand, the inner mechanics can be better understood by indentation tests performed under glaze layer formation conditions. On the other hand, it was evidenced by EDX and XPS that its chemical composition mainly consists in oxidized metals from the punch whereas particles from the ceramic flat are erratic. As the ceramic is not necessary to create a glaze layer, it is crucial for its adhesion in this particular contact. This suggests that mechanical and chemical contributions may be closely linked.

[1] Viat, A., Fouvry, S., De Barros Bouchet, M.-I., & Pin, L. (2015). Influence of carbon-based solid lubricant on fretting wear response for alumina-based ceramics versus cobalt superalloy contact. Surface and Coatings Technology.