ICMCTF2012 Session E3-1/G2-1: Development, Characterization, and Tribology of Coatings for Automotive and Aerospace Applications

The concept of the effectively shaped indenter was introduced by Bolshakov et al in 1995 [1]. It was shown, that this concept is the basis for the so called Oliver and Pharr method [2]. Since then many papers have appeared demonstrating its uses in analyzing not only nanoindentation curves but also much more complex mechanical contact experiments. Among those are also tests on layered materials for normal and multi-axial loading conditions [3, 4].

Within this work the concept will be applied to rough surfaces and time dependent material behavior. It will be shown how the concept of the effectively shaped indenter consistently follows from complex time dependent constitutive material models and how this can be used within new analysis techniques for nanoindentation and physical scratch tests.

[1] A. Bolshakov, W.C. Oliver, and G.M. Pharr, MRS Symp. Proc 356, p 675 (1995)

[2] W. C. Oliver, G. M. Pharr, J. Mat. Res. 7 (1992) 1564-1583.

[3] N. Schwarzer, Phil. Mag. 86(33-35) 21 Nov - 11 Dec 2006 5153 – 5767

[4] N. Schwarzer, J. Mater. Res., Vol. 24, No. 3, March 2009, 1032 - 1036

Maraging alloys are a group of ultra-high strength steels susceptible to be treated by means of thermochemical processes to improve its mechanical properties. In this paper, a series of arc-activated plasma nitriding processes have been applied on maraging steels (grade 300-aged) in order to investigate the modification of their wear resistance and superficial hardness.

The study focuses on how the BIAS potential applied to the substrates and the effect of hydrogen in N₂-Ar plasma during the nitriding process modify the properties of the treated alloys. The samples have been characterized by roughness measurements, wear resistance tests at room temperature and 450 ºC, chemical in-depth profiling, impact tests and Knoop micro-hardness profile tests. The microstructural characterization was realized by optical microscopy, XRD analyses and scanning electron microscopy.

The case layer thickness observed ranged between 20 and 60 microns for BIAS voltages of -200 V and -600 V respectively, thus signaling the effect of the ion energy bombardment. It is shown that the nitriding process increases substantially the hardness on the surface of the substrate and then the wear resistance at room temperature and at 450ºC. On the other hand, the presence of hydrogen in the plasma presents a different effect depending on the applied BIAS voltage. The compatibility of the present plasma nitriding treatments and the aging process characteristics of these alloys are also discussed.

Keywords: maraging steel, nitriding, BIAS, hydrogen, wear resistance, hardness

Diamond-like carbon (DLC) coatings have numerous advantages over their plated counterparts for tribological applications. They have higher wear resistance for a given film thickness, the deposition process does not create an environment where embrittlement is a concern, and they can readily be applied to titanium components. Yet their adoption into the aerospace industry has been quite slow over the past decade.

In early 2000, Boeing specified the use of a DLC coating onto a weapon bay door’s safety-lock pin for one of their platforms. To enable this introduction many important processing parameters had to be met, such as low-temperature deposition (to avoid relaxation of cold-worked components). In addition, adequate QC and QA metrics had to be established and specific processing parameters had to be determined to meet the demands of a “stable and repeatable process;” a key to meeting certification requirements.

Even with these challenging constraints met, there remain two key items which have contributed to this slow incorporation: 1) there has been limited in-service data generated with these coatings, and 2) due to their typically thin profile, they are often not suitable as a drop-in replacement for even the thinnest tribological coatings, such as thin-dense chrome.

In this talk, methods to overcome these barriers will be introduced, key aspects to process control and quality issues will be discussed, and critical coating characteristics will be highlighted.

Magnesium alloys are attracting high attention due to their low density, high specific values of stiffness and strenght, great facility of processing and reasonable cost. These properties make these alloys suitable for application in many sectors and especially in the transportation one. Nevertheless, the use of magnesium alloys is limited because their low resistance to wear.

In this work, as an alternative to improve the wear development of a Mg-Zn alloy (ZE41), Al and Al-SiCp composites coatings were prepared by High Velocity Oxygen Fuel (HVOF) spraying on the magnesium alloy substrates.

The influence of coating parameters- such as reinforcement rate (from 0% to 50% vol. SiCp) size of SiCp and number of layers, as well as HVOF spraying parameters (spraying distance, transversal gun displacement)- in the microstructure and wear resistance of the final coatings was evaluated.

The microstructure of the coatings, the wear tracks, the worn surfaces in the transversal section and the debris formed during the wear tests were characterized by scanning electron microscopy (SEM) and light microscopy. Adhesion pull-off strenght tests (ASTM D4541-02) of the different coatings were performed. Wear tests were carried out under dry sliding conditions using a pin-on-disc tribometer on the ZE41 substrate and on the different sprayed coatings.

The magnesium alloy suffered minor degradation of its microstructure or mechanical properties after the deposition of the coatings with thickness from 100 to 200 µm. The obtained composite layers were almost free of porosity, with high adhesion to the substrate and after optimization of the sprayed conditions a substantial decrease in the specific wear rates were obtained.

Unlike earlier types of MoS₂ coatings, innovative MoS₂ films do not show columnar crystal growth but a dense structure without porosity and a basal orientation of the lattice with the gliding plane parallel to the substrate surface. This paper describes the development of this new type of lubricant film by means of design of experiments (DoE) according to Box-Behnken. For the first time, this methodological approach enables a determination of mutual interactions between deposition process parameters on tribological and mechanical MoS₂ film properties.

Furthermore, the experiments show that the tribological properties of dense coatings without columnar structure are sensitive to residual stress. It turned out that compressive residual stress which can be controlled by the process parameters is crucial for film durability due to its influence on adhesion and hardness. The results indicate that the optimum is depending on the loading conditions. Therefore, low wear rates in ball-on-disc tests combined with high durability of the coating-substrate bond were achieved at moderate amounts of internal stress. Deposition induced residual stress of each sample was analyzed by the substrate curvature method. Interactions of residual stress and process parameters are discussed in terms of the kinetic energy of sputtered particles.

The XRD analysis was used to validate the highly basal oriented lattice. A shift in the spectra of the (002) reflex to slightly lower angles was observed. This shift is already known in literature, but has not yet been entirely clarified. In pursuing annealing experiments a minor effect of residual stress on this shift was observed. Film adhesion was evaluated by a modified scratch test. Film growth, surface topography and failure mechanisms where analyzed by SEM imaging.

Additionally this paper gives advice to select and apply methods suitable for deposition processes from a broad range of methods in the statistical design of experiments (DoE).

In this work, a series of finite element analyses were conducted to analyze the stresses in thin film-coated piston rings under contact loads. The actual normal and tangential pressure observed during a complete four-stroke gasoline engine cycle (720^o) was used as input to load an axisymmetric film/substrate mesh. Four values of coating thickness were analyzed (from 20 to 100 μm) and, for each one, five values of Young´s modulus (from 144 to 578 Gpa) were considered. The systems were compared based on the stress distribution, particularly in terms of the intensity and position of the peak stresses in the film and at the film/substrate interface. Results show that the stiffer (or thicker) the film, the higher the stresses at the interface immediately under the film-sleeve contact, and the higher the compressive stresses around this region. The range of thickness and Young’s modulus values considered in this work did not provide significant changes in terms of substrate stresses.

Transition metal dichalcogenides (TMD) belong to one of the most developed class of materials for solid lubrication. However, one of the main drawbacks of most of the self-lubricating coatings is their low load-bearing capacity, particularly in terrestrial atmospheres. In previous works, alloying TMD thin films based on tungsten disulfide with non-metallic interstitial elements, such as carbon or nitrogen, has been studied in order to improve tribological performance in different environments. Excellent results were reached having the deposited coatings hardness, in some cases, more than one order of magnitude higher than single W-S films.

In this work, W-S-C films were alloyed with Cr by-co sputtering chromium and composite WS2-C targets. Cr-contents in the range [0-25 at.%] were selected for sliding tests (pin-on-disc, 100Cr6 steel ball as a counterpart) under different conditions (dry or humid air, increasing load, etc.) Besides the usual physical, chemical and mechanical characterization, including the evaluation of the chemical composition, the structure, the morphology, the hardness and the cohesion/adhesion, special attention was paid to the friction and wear analyses. The surfaces in the contact were analysed by X-ray photoelectron spectroscopy (bonding), Auger emission spectroscopy (surface chemical composition), and scanning electron microscopy (SEM). Selected ball wear scars and wear tracks were investigated by transmission electron microscopy (TEM); the samples for them were prepared by focus ion beam (FIB).

Surface and sub-surface structural modification of the coating and composition of the transferred tribolayer are discussed in detail. The main aim of this study is to shed light on following issues: i) the role of Cr in microstructural transformation, ii) the role of Cr in sliding process, iii) the formation of low-friction tribolayer based on tungsten disulfide.

Minimization of the friction losses is a major concern for Internal Combustion Engine designers. The strategy often followed, to reduce losses, is to modify the topography of the rubbing surfaces of the cylinder bores and the piston rings. This strategy aims to reduce metallic friction and to allow less oil consumption and high operational reliability. The surface modification of cylinder bore with improved sliding properties is often produced at industrial level by honing process. Abrasive stones are hence loaded against the bore and simultaneously rotated and oscillated. To guarantee a robust surface production of cylinder bore of specific shape with acceptable dimensional accuracy and surface quality, the stone dynamic effects in continuous balanced contact with the workpiece are of primary importance. This paper addresses these effects on honed surface structure of cylinder bores. The stone dynamic behavior while bore honing was studied at conventional regime ranged from 0,5g to 1,5g as often used in mass bore production. Under these dynamic conditions of bore processing, the dimensional accuracy holds by opposition to the surface appearance. However, the highly accelerated regime in honing up to 2,5g promotes simultaneously the form quality (especially straightness) and reduces the cycle time. The reported results are the first demonstration that the bore surface finish can be dynamically controlled when honing. This technology is enabled by a microscale regeneration mechanism of abrasive stones.

Diamond-like carbon (DLC) coatings exhibit low coefficient of friction (COF) and good adhesion mitigating properties when placed in sliding contact against aluminum but their tribological properties are very sensitive to the environmental conditions. This study examines tribological properties of 20 at % Si and 12 at % F containing DLC coatings (with 14 at % O and 18 at % H) tested against 1100 aluminum under a vacuum atmosphere (0.01 Pa) and an ambient air (39% relative humidity) condition. Pin-on-disk type sliding test showed a COF of 0.08 under both conditions. The changes in contact surface tribo-chemistry and material transfer processes were studied. Carbonaceous transfer layers that incorporated F, Si and O compounds formed on aluminum contact surface under both testing conditions. The details of the composition and microstructure of the transferred layers were investigated using cross-sectional focused ion beam (FIB), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) methods. The F concentration was the highest at the aluminum/transfer layer interface and formation of an AlF₃ compound was observed. The top surfaces of the transfer layer in contact with DLC were richer in Si and O. Partial pressure variations of residual gases monitored during the vacuum test indicated desorption of H₂O molecules. The observation of low COF values and atmosphere independent tribological properties observed in this coating were interpreted in terms of hydration/de-hydration and passivation mechanisms that operate on contact surfaces.