ICMCTF2015 Session E3-1: Tribology of Coatings for Automotive and Aerospace Applications

Thanks to their thermostructural properties, CMCs (Ceramic Matrix Composites) draw interest from aeronautics to partly substitute metals. However, CMC-metal contact induces chemical interactions that call for an environmental barrier to prevent severe degradation. The present study aims at selecting protective ceramic coatings for the CMC and understanding the major mechanical and chemical processes involved at the plane-to-plane ceramic-metal contact under fretting wear.

As a representative contact, a 4-mm-diameter punch made of HS25 (Co-Cr-Ni-W alloy) put on a coated CMC plane is investigated. Different ceramic coatings are submitted to fretting wear tests that model the basic in-flight conditions at 700°C, to both measure wear kinetics and analyse damaging scenarios. Fretting laws are established to quantify wear kinetics as functions of frequency, sliding amplitude and contact pressure that change within an in-flight cycle. As shown for metal-metal interfaces [1], a glaze layer for some ceramic-metal contacts is formed. Depending on the coating, two damaging scenarios are identified: abrasive wear for coatings containing alumina and glaze layer-style wear for ceramics containing metals.

SEM and EDS analyses on polished cross sections of worn surfaces show that for abrasive wear, thin (about 5 µm) and non-continuous sediments are trapped in ceramics’ hollows. By contrast, when formed, the glaze layer is thick (up to 40 µm) and covers the whole contact path [Fig.1]. Glaze layers are formed by mechanical and diffusional (sintering) processes. Even if the sediment layers contain mainly metallic oxides from the punch, the ceramics’ microstructure is crucial as a substrate. Surface energy is dissipated either in the fracture of ceramics and newly formed debris or in a sticky glaze layer. In the first case, fragments are abraded by alumina contained in the ceramic and ejected. In the second case, ceramics are protected and barely worn. However, the glaze layer is only formed above an inherent threshold temperature.

Worn volumes are up to 10 times bigger when no glaze layer is formed. When applied on the ceramic coating, a solid lubricant limits friction during the first 5% of the test before being completely worn. However, worn volumes are much smaller for lubricated ceramics compared to raw coatings. EDS analyses show that after the complete wear of the lubricant layer, particles from the lubricant are blended in sediments, which indicates the double function of the lubricant that firstly reduces friction and secondly keeps wear down.

[1]H. Kato, Severe-mild wear transition by supply of oxide particles on sliding surface, Wear 255 (2003), 426-429

A linerless aluminum (Al) engine block has potential to reduce the weight of an automotive engine and improve the fuel economy. However, the Al cylinder surface of an aluminum engine block is not usually strong enough to withstand the sliding wear against piston rings. A few surface processing technologies are used to protect the surface of cylinders directly. Among them, plasma transferred wire arc (PTWA) thermal spraying coating is already popular. Plasma electrolytic oxidation (PEO) coating is also proposed for increasing the wear resistance of aluminum-silicon alloy (A356) and reducing the friction between the cylinder and piston. In this work, two different PEO coatings with a thickness of around 23 µm were prepared, and a high speed pin-on-disc tribometer was used to study the tribological behavior of the coating at oil lubricant conditions. A cast iron sample was also used to do the same tribological tests for comparison. The coefficient of friction (COF) vs surface roughness (R_a: 0.2 - 0.8 µm) and sliding speeds (up to 6.0 m/s) were particularly studied. The results show that the COF significantly decreased with the increase of sliding speeds, and a smoother coating surface generally exhibited a lower COF and a steeper descent rate of the COF. While such observations were true for both PEO coatings and the cast iron sample, the polished PEO coatings may have a lower COF than cast iron. The study indicates that the Al-Si alloy with PEO coatings could be a feasible solution to reduce the weight and improve the fuel efficiency of an Al engine.

The extraordinary mechanical properties of nanocrystalline metals are well documented, though in many ways these materials remain impractical for engineering design. The transition from grain boundary to defect dominated plasticity associated with grain coarsening can be brought about by the addition of even modest amounts of thermal or mechanical energy, dramatically altering their mechanical properties. As recently as 2012 a new class of binary metal alloys was discovered that exhibits intrinsically thermodynamically stable nanocrystallinity. The unusual stability of these alloys was described as a local energetic minimum, where the entropy of segregation exceeds the entropy of mixing for the grain boundary segregated phase, pinning boundaries and mitigating thermally activated grain growth. Modern advances in PVD have similarly enabled the deposition of metal-ceramic nanocomposites with significant thermal and mechanical stability. We present the motivations for, potential impact and results of ongoing investigations of the stability of both nanocrystalline metal matrix composites and binary metal alloys, and explore their practicality as wear resistant tribological coatings.

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

Industrial systems such as gas turbines, air craft compressors will generally experience the problem of blade erosion by solid particles such as dust, soot or scale leading to changes in the blade's shape and surface finish. Therefore, under the dynamic operating conditions, the solid particle erosion gradually reduces turbine efficiency and increases the average fuel consumption rate. The long lasting blades not only reduce the maintenance cost, but also reduce the emission of greenhouse gases. Hence, any measures which can reduce the erosion rate of the blades can be a high value addition for the above sectors. The thin coatings with exceptionally high hardness, toughness and ease of deposition, can be potentially used on numerous components suffering from diverse forms of wear. Therefore, any turbine engine or rotary compressor that suffers from erosion due to airborne particle impact will benefit from such coatings.

In the present study, TiCrN coatings with varying Cr content are deposited on SS 304 and Ti-6Al-4V substrates using cylindrical Cathodic Arc Physical Vapor Deposition (CAPVD) system available in the author’s laboratory (Model: π 300, PLATIT). The coatings were extensively studied for their physical (thickness, adhesion, residual stress and phase composition and micro structure), mechanical (hardness, modulus and toughness) and tribological (sliding and erosion wear) properties. The results obtained were systematically analysed and compared to arrive at the best suitable coating composition for erosion resistance applications. Towards understanding thickness and residual stress effects on erosion properties, coatings with optimized composition were deposited with varying thicknesses (5-20 µm) under continuous and discontinuous modes (with intermediate heat treatment step: T _{Heat treatment} > T _Deposition). Towards benchmarking comparison, the erosion resistance as offered by well known TiN coatings has been comparatively investigated. The mechanism of erosion failures has been studied thoroughly using Focussed Ion Beam and Scanning Electron Microscope.

The results demonstrate that the 20 μm thick TiCrN coating can be a better choice for erosion resistance applications than the TiN coating. The critical coating thickness required to withstand the erosion damage is an important outcome of the present study. For multi-layer and intermittently heat treated films, the predominant material removal mechanism is through chipping of the coating at interfaces and the low density spots like droplets. The results pertaining to the above study will be presented and discussed.

Frictionally engaged joints are widely used in mechanical engineering and require a high static friction. Typical applications are flange joints or shaft-hub joints for example. Dimensions and weight of those joints can be reduced by increasing the static friction coefficient of the relevant friction surfaces. PVD thin films have a high potential to fulfil this requirement.

However, almost all research activities are traditionally focused on the decrease of the dynamic coefficient of friction. In this paper the static coefficient of friction of Nimonic – AlTiN hard coatings and rough ta-C coatings against 42CrMo4 counterparts in torsion experiments was studied. The slipping curves have been determined and a maximum static coefficient of friction μmax up to 0.8 was measured.

Because of the low film thickness in the range of 5 microns the coatings can be used in a great variety of applications and environments. It is a significant addition to existing solutions with thermally sprayed coatings or foils.

In the work, the microstructure and tribological properties of HVOF sprayed WC-CoCr carbide coatings of ultrafine (< 10µm), spherodiz ed and nano structured powders were examined. The spraying was performed with a power gun and powder feeder from Thermico. The tests were conducted with the "ball on disc" method. The counter specimens "ball" were made of various materials: tungsten carbide, silicon carbide, and a carbon-graphite material impregnated with antimony. For the purposes of comparison, a coating sprayed from a powder having a conventional grain size distribution (45µm) and solid ceramics (SiC "disc") were examined. The condition of the top layer of the specimens and counter specimens was determined before and after the friction tests (SEM, EDS, XRD, 3D laser profilometry). The examination revealed that the fine powder coating exhibited less wear. The lowest coefficients of friction were noted for the following friction test pair: tungsten carbide "disc" and counter specimens ("ball") of silicon carbide doped with antimony and resin.

Electrochemical behavior of high velocity air fuel (HVAF) WC-Co modified coatings was presented in this article. The commercial powder of WC-Co 83/17 type was used as a basic material. The modification process was relying on mechanical mixing of WC-Co powder with sub-microcrystalline TiC addition in weight ratio from 3 to 10 % wt. In first step of investigations the internal morphology of coatings was characterized. In this area the thickness, porosity, phase composition and overall quality of deposited coatings were analyzed. The second step of investigations consists ofelectrochemical behavior characterization of coated samples at ambient temperature in NaCl, NaOH and H₂SO₄ solutions. To characterize coatings top-surface morphology the scanning electron microscopy with microanalysis of chemical composition, phase constituent by X-ray diffraction and laser profilometer were used. The evident influence of TiC addition on corrosion resistant of coatings has been shown.

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

DLC coatings can combine high hardness with low friction. However, they are often deposited with high levels of intrinsic stress and display low adhesion strength resulting in poor performance in demanding applications. A highly topical challenge is to develop advanced DLC coatings capable of withstanding more demanding applications in the automotive, cutting tools, MEMS and oil and gas sectors.

The results from several nanomechanical and tribological test techniques - nanoindentation, nano-scratch and nano-fretting (nano-wear) – can be used together to aid the design of DLC coating architectures for enhanced durability in specific applications [1-3].

The behaviour of multilayered DLC coatings (Cr/W-C:H/a-C:H, Cr/W-C:H/Si-a-C:H) deposited using industrial scale PECVD Flexicoat 850 system (Hauzer Techno Coating) was compared to that of commercial CrN/a-C:H:W (WC/C, Balinit C Star from Oerliken Balzers). We have previously reported that in nano-wear tests the coatings with higher H/E display greater wear resistance [2]. The stress fields during nano- and micro-scale scratch tests however are different and consequently the behaviour is completely different. Under these conditions the coatings with lower H/E display improved performance. High resolution SEM imaging has been used to investigate this further.

A simple contact model [1] strongly suggests that the position of the maximum in the von Mises stress in relation to the coating-substrate interface is a critical factor in determining the type of wear behaviour observed.

1. Review of recent progress in nanoscratch testing, BD Beake, AJ Harris and TW Liskiewicz, Tribology 7 (2013) 87.

2. Short note on improved integration of mechanical testing in predictive wear models, TW Liskiewicz, BD Beake, N Schwarzer and MI Davies, Surf Coat Technol 237 (2013) 212.

3. Nano-scratch, nanoindentation and fretting tests of 5–80 nm ta-C films on Si(100), BD Beake, MI Davies, TW Liskiewicz, VM Vishnyakov and SR Goodes, Wear, 301 (2013) 575.

Low Hydrogen Embrittling (LHE) Cd and Zn-14wt% Ni coatings are used in close contact conditions for aerospace components such as landing gears and fasteners. Recent regulations have limited the use of Cd due to its carcinogenicity and use of cyanide in the plating process. Intermetallic Zn-14wt% Ni coating has shown promising properties as a suitable replacement for LHE Cd with improved wear and tribological properties. Although LHE Cd coatings are used for last five decades and Zn-14wt% Ni has been studied in the past two decades, open literature available for tribological properties of both coatings is limited or mostly within the industries. This study focuses on evaluating and comparing the tribological properties of LHE Cd and Zn-14wt% Ni coatings under varying relative humidity (RH). A linearly reciprocating pin on disk tribometer was used to study the evolution of coefficient of friction combined with post tribological study of third bodies formed. Piezoelectric sensors were used to measure the lateral force at a sampling rate of 800 Hz and generate friction maps (i.e. triboscopy). For LHE Cd, increasing RH resulted in accelerated wear of the coating. Whereas for the Zn-Ni coating, increase in RH resulted in the formation of Zn oxide and a change of the wear mechanism from mechanical to chemical.