Performance evaluation of multilayer thin film coatings under mixed rolling–sliding dry contact conditions

This paper focuses on the application of advanced finite element analysis to simulate the response of complex multilayer coatings when subjected to mixed rolling–sliding dry contact conditions, typical of those experienced by heavily loaded gear tooth flanks. In this study, a versatile model is developed to investigate the response of a variety of complex coatings. Through the investigation of three advanced surface coatings the developed model is shown to offer detailed subsurface stress and strain information, quantifying the benefits provided by the application of each coating.A number of rolling contact fatigue tests were also conducted to complement the computational simulations. An adapted twin disc testing machine was used to investigate the progressive wear rates of the three advanced surface coatings when subjected to mixed rolling–sliding contact at high load and in dry conditions. The result is a clear indication that advanced surface coating techniques can provide significant improvements in wear resistance and surface durability. From the rolling contact fatigue experiments conducted in this study, a multilayer tungsten carbide/carbon coating was shown to provide the most significant increase in surface durability with a 16-fold reduction in measured weight loss compared to the uncoated substrate over the 145,000 cycle test duration investigated.     

Improvement of the tribological properties of Al6061–T6 alloy under dry sliding conditions

The tribological properties of ultrasonic nanocrystalline surface modification (UNSM) treated Al6061–T6 alloy were investigated at various normal loads under dry sliding conditions. Electron backscattering diffraction (EBSD) analysis revealed a microstructure alteration of about 70 μm in thickness generated by the UNSM technique. The friction test results showed that the friction coefficient and wear rate of the UNSM-treated specimen reduced by about 25 and 20% compared to that of the UNSM-free specimen, respectively. X-ray photoelectron spectroscopy (XPS) results showed that the oxide percentage on the worn surface increased, but that of carbon percentage decreased after the UNSM treatment.     

Tribological characterization of electroless Ni–10% P coatings at elevated test temperature under dry conditions

An experimental study of wear characteristics of electroless Ni–10% P coating sliding against hard AISI 52100 steel pin is investigated. Experiments are carried out at room and 550°C temperatures. Heat treatment effects on tribological behavior of this coating are studied. The wear surface and the microstructure of the coatings are analyzed using optical microscopy, scanning electron microscopy, energy dispersion analysis X-ray, and microhardness testing equipment. It is observed that the forming of continuous oxide film on contacting surfaces of pin and disk improves wear resistance and decreases friction coefficient of the Ni–10% P coating. The results indicate that the wear resistance of electroless Ni–10% P coating has improved with heat treatment at room temperature wear test, but it reverses in the wear test at 550°C. In addition, specimens without heat treatment have the highest wear resistance and the lowest friction for wear tests at elevated temperatures.     

Dry sliding wear behavior of TC11 alloy at 500 °C: Influence of laser surface texturing

Six types of micro-textures with different diameters and area densities are produced on the surfaces of the TC11 alloy. The wear characteristics of the textured and untextured surfaces are investigated at 500 °C. It is found that the regular textured surface enhances the amount of oxygen adsorbed on the material surfaces, which promotes the formation of more tribo-oxides on the textured surface compared to the untextured samples, and thus significantly enhances the anti-wear ability of the TC11 alloy. A micro-dimpled structure with relatively small-diameter and high-density is likely to improve the tribological properties of TC11 alloy at 500 °C.     

Tribological behavior of Cu20%Nb and Cu15%Cr in situ composites under dry sliding conditions

The tribological behavior of Cu20%Nb and Cu15%Cr in situ composites was studied on a pin-on-disk tester. Composite pins were slid against a hardened AISI 52100 steel disk under dry ambient conditions. Comparison of coefficient of friction, wear rate and bulk temperature was made between the two composites in terms of effects of normal pressure and sliding speed. Microstructural changes in the composites due to sliding were studied and correlated with the change of tribological behavior. The wear rate increased with increasing normal pressure and decreased with increasing sliding speed in the studied range of normal pressure (0.06–0.56 MPa) and sliding speed (2.50–5.83 m s⁻¹) for both composites. Cu20%Nb showed a much better wear resistance and a lower coefficient of friction than Cu15%Cr. The coefficient of friction slightly increased for Cu20%Nb and slightly decreased for Cu15%Cr with increasing normal pressure. The coefficient of friction for both composites decreased with increasing sliding speed. The bulk temperature was higher and subsurface deformation layer thickness was larger for Cu15%Cr than for Cu20%Nb. Both of them increased with increasing normal pressure and sliding speed. Scanning electron microscopy analysis showed plastic deformation flow on the wear surface at an early stage for both composites. However, the formation of a surface film led Cu20%Nb composite to reach a steady state much sooner and therefore to have a lower wear rate than Cu15%Cr.     

Erosion-oxidation of uncoated,aluminized and chromized-aluminized 9Cr–1Mo steels under fluidized-bed conditions at 550–700 °C

Protective coatings, deposited mainly by thermal spraying and diffusion techniques, are considered a solution to extend the lifetime of many components in the energy production sector, such as heat exchangers. In this paper, some results are presented for uncoated, aluminized and chromized-aluminized 9Cr–1Mo steel, subjected to air and to impacts by 200 μm silica particles at angles of 30° and 90° and speeds of 7.0–9.2 m s^?1 at 550 –700 °C, in a laboratory fluidized-bed rig, to determine whether or not aluminized and chromized-aluminized diffusion coatings could protect the steel under such conditions. Erosion-oxidation damage was characterized by measurement of the mean thickness changes using a micrometer and examination of worn surfaces by scanning electron microscopy.Under most conditions, the coatings provided some protection to the substrate: under 30° impacts, up to 650 °C, and under 90° impacts, at 700 °C, both coatings were effective, whereas under 90° impacts, up to 650 °C, only the chromized-aluminized coating gave significant protection. However, for 30° at 700 °C, the oxide scale on the substrate was protective and the coatings were not needed. Explanations for these observations are presented in this paper, in terms of interactions between the erosion and oxidation processes for the materials.     

Experimental and computational investigations of thermal modal parameters for a plate-structure under 1200 °C high temperature environment

When a hypersonic aircraft flies at a high Mach number, the plate-like attitude control structures, such as the wings and rudders, will be exposed to an extremely high-temperature environment. To obtain the thermal modal parameters of a structure that are difficult to measure, a high-temperature transient heating test system and a vibration test system were combined to establish a test system that can perform the thermal/vibration test at 1200 °C. Infrared radiation heating was employed to generate a controlled time-varying high-temperature environment, and an exciter was used to exert vibration excitation on the free end of the cantilever rectangular plate. A self-developed extension configuration of a high-temperature-resistant ceramic pole was used to transfer the vibration signals of the structure to a non-high temperature zone, and the acceleration sensors were applied to identify the vibration signals. The test data were analyzed using a time-frequency joint analysis technique, and next, the key vibration characteristic parameters of structure in a thermal-vibration coupled environment up to 1200 °C (e.g., the modal frequency and modal vibration shape) were experimentally obtained. In addition, the numerical simulation on the thermal modal characteristics of a rectangular plate was performed. The calculated results coincide favorably with the test results, verifying the credibility and effectiveness of the experimental methods. The research results can provide an important basis for the dynamic performance analysis and safety design of structure under high-temperature thermal-vibration conditions for hypersonic flight vehicles.     

The unlubricated reciprocating sliding wear of 316 stainless steel in CO2 at 20–600°C

The friction and wear behaviour of 316 stainless steel in CO₂ has been investigated in the load range 8–50 N from 20 to 600°C. Wear transitions occurred at all temperatures but were load-dependent. At and below 300°C, wear transitions only took place at low loads, whereas above 300°C transitions were observed at all loads. The low temperature wear transition, representing an order of magnitude decrease in wear rate, was associated with a change in friction behaviour. The friction force across the specimen was initially widely fluctuating but after a time, which did not necessarily coincide with the wear transition, became much smoother. The smoother sliding is thought to indicate a trend to oxide-oxide contacts. At higher temperatures wear transitions result in a two orders of magnitude reduction in wear. The corresponding friction transition was similar to the low temperature friction change but also included a marked temporary drop in the coefficient of friction.Pits or troughs up to 450 μm deep were seen in wear scars above 400°C. It is proposed that isolated sections of grooves formed during the initial stages of wear become back-filled with loosely adhering oxide particles. These troughs are then further deepened, possibly by abrasive fretting action of the semi-fluid oxide material.     

Investigation into the wear behaviour of Stellite 6 during rotation as an unlubricated bearing at 600 °C

The wear behaviour of Stellite 6 was studied during rotational sliding in a bespoke bearing rig at 600 °C for times between 2 min and 12 h. Six stages of wear were identified: (i) formation of a mixed oxide ‘glaze’, (ii) cobalt and chromium elemental diffusion to the ‘glaze’ surface forming chromium- and cobalt-dominated oxide layers, (iii) oxygen diffusion into the ‘glaze’ leading to a chromium-dominated oxide layer at the ‘glaze’/substrate interface, (iv) spallation of the ‘glaze’ through chemical failure, (v) re-formation of the ‘glaze’ and (vi) elemental diffusion within the ‘glaze’, again resulting in discrete oxide layer formation.     

Behaviour of ferritic–martensitic steel and aluminium base coatings under demanding elevated temperature conditions

Abstract

Ferritic–martensitic steels are found in many elevated temperature applications due to their excellent strength properties and thermal conductivity. However, their resistance to elevated temperature corrosion, wear and combination of these is typically not at a desired level. One solution is to improve the surface properties by the application of a coating. In this study, aluminium diffusion coatings were deposited on 9Cr–1Mo steel and characterised in terms of microstructure and elevated temperature corrosion and erosion–corrosion behaviours. The two behaviours are then compared to those of an uncoated steel. The results from the tests indicate that aluminising shows great potential under the studied demanding elevated temperature conditions. The benefits and challenges of the deposition and use of aluminised coatings are discussed in this paper.     

Adhesive wear of stir cast hypereutectic Al–Si–Mg alloy under reciprocating sliding conditions

This paper describes an attempt to enhance the wear properties of hypereutectic cast aluminium–silicon alloys produced by semi-solid metal (SSM) processing technique. The rheological experiments on SSM slurries were performed under continuous cooling condition from liquidus temperature. Wear characteristics of alloy under investigation were studied using pin on flat wear system over a range of normal load (10–40 N) at constant average sliding speed (0.2 m/s) against cast iron and stainless steel counter surface. Stir cast alloy showed lesser weight loss compared to conventional cast alloy. Stir cast and conventional cast alloys showed higher weight loss against the stainless steel as compared to that against cast iron counter surface. Optical microscopy of the conventional cast and stir cast hypereutectic alloy has shown that stir casting causes refinement of primary silicon particles and modification of eutectic silicon compared to conventional cast alloy. The scanning electron microscopy of wear surfaces was carried out to investigate the mode of wear.     

A study on the tribological characteristics of duplex-treated Ti–6Al–4V alloy under oil-lubricated sliding conditions

The tribological characteristics of the polished, dimpled and over-coated dimpled specimens were investigated. Dimples were produced on a Ti–6Al–4V alloy specimen using a laser surface texturing (LST). A Cr-doped diamond-like carbon (DLC) film was deposited on a dimpled specimen using an unbalanced magnetron sputtering (UBMS). The effects of dimples and over-coated Cr-doped DLC film on the tribological characteristics were investigated by performing the friction tests against a Cr-plated steel pin. The test results showed that the over-coated dimpled specimen exhibited a lower friction coefficient and wear compared to those of the polished and dimpled specimens, which may be attributed to the storage of wear debris and high hardness. A model of the wear reduction mechanism of the specimens was discussed.     

Tribological behaviour of SiC?TiC?TiB2 composites under unlubricated sliding conditions

The friction and wear behaviour of eight different SiC TiC TiB₂ composite materials, with a practically constant SiC:TiC ratio of 1 and an increasing amount of TiB₂, was determined comparatively in oscillating sliding tests at room temperature under unlubricated conditions. The influence of the relative humidity (RH) of the surrounding air was investigated in tests in dry, normal, and moist air. All tests were performed against SiC balls and Al₂O₃ balls as counterbodies. The friction was affected by RH but barely at all by the composition of the composites. The wear resistance of the composites was found to be improved considerably by addition of TiB₂ in the range 20–60%. The highest wear resistance of the system was found when Al₂O₃ was used as the counterbody material.     

A new method to investigate the sliding wear behaviour of materials based on energy dissipation: W–25 wt%Cu composite

A new method is proposed to explain and monitor wear behaviour based on energy dissipation. The wear of a W–25 wt%Cu composite against 52100 steel was used to demonstrate this approach with pin-on-disc tests conducted under three normal loads. An energy-dependent criterion, namely, specific wear volume (wear volume/dissipated energy (mm³/J)), was defined to evaluate the wear of the composite. The specific wear volume can be used as a substitute for the traditional wear rate due to the simultaneous expression of several wear parameters and because of its strong dependence on the wear mode. The specific wear volume appears to be constant in any particular “wear mode” regardless of the active “wear processes”. In the wear of this composite, processes such as particle pull-out, mechanically mixed layer (MML) formation, crack propagation and delamination were observed. But, combination of these processes in each test had identical specific wear volumes. Thus, all of these wear processes were considered to be consecutive stages of the same wear mode: fatigue wear. The amount of dissipated energy and the volumetric loss increased with increasing normal load. Also, changing the normal load changed the rate of energy dissipation per unit sliding distance.     

Analysis of dry sliding friction and wear behaviour of AA6351–SiC–B4C composites using grey relational analysis

Abstract

This paper describes the multifactor based experiments that are applied to investigate the dry sliding wear system of aluminium matrix alloy (AA6351) with 5 wt-% silicon carbide (SiC), 5 wt-% and 10 wt-% of boron carbide (B₄C) reinforced metal matrix composites (MMCs). Stir casting route was adopted to prepare the composites and the tribological experiments were carried out on pin-on-disc type wear machine. The effects of parameters like applied load, sliding velocity, wt-% of B₄C on the dry sliding wear and frictional coefficient of aluminium MMCs using grey relational analysis (GRA) are reported. The orthogonal array with L9 layout and analysis of variance were used to investigate the influence of the parameters. It is observed that the dry sliding friction and wear behaviour of the composites are influenced by the applied load, sliding velocity and wt-% of B₄C with a contribution of 60·82%, 21·72% and 14·28% respectively. The optimal design parameters were found by grey relational grade and a good agreement was observed for 95% level of confidence.     

Effect of Mo and Ag on the friction and wear behavior of ZrO2 (Y2O3)–Ag–CaF2–Mo composites from 20 °C to 1000 °C

In this paper, a series of ZrO₂ matrix high-temperature self-lubricating composites were prepared by hot-press technique. The effect of Mo and Ag on the friction and wear behavior of the ZrO₂(Y₂O₃)–Ag–CaF₂–Mo composites in a wide temperature range was investigated. The XRD results showed that CaMoO₄ formed on the worn surface above 400 °C. The excellent lubrication performance of CaMoO₄ endowed the low coefficient of friction of the ZrO₂(Y₂O₃)–Ag–CaF₂–Mo composites at high temperatures. The ZrO₂(Y₂O₃)–10Ag–10CaF₂–10Mo composites showed favorable wear resistance at all the tested temperatures which was attributed to the combined action of hardness and phase transformation.     

The effect of air humidity on tribological behaviours of W–C:H coatings with different tungsten contents sliding against bearing steel

Friction and wear behaviors of W–C:H coatings with different tungsten contents sliding against bearing steel balls at different air humidities were investigated. The worn out surfaces of steel balls and coatings were analyzed with the aid of scanning electron microscopy (SEM) and Raman spectroscopy. A tribolayer composed of a graphite-like material mixed with tungsten and iron oxides was observed on the friction surfaces of the steel balls. The chemical and phase compositions of the tribolayer, which depend both on the tungsten content in coatings and air humidity, determine the tribological properties of the W–C:H coating in a frictional contact with bearing steel. At average air humidity (50%), those coatings that contain less than 10 at% of tungsten in a frictional contact with steel exhibit favorable tribological properties. The friction coefficient of frictional contacts under test reaches a low value (f～0.01) at a low air humidity and increases with humidity of up to ca. 0.2. The best tribological properties in a wide range of air humidity (5–90%) have been found for W–C:H coatings with the tungsten content between 2 and 5 at%.     

The Performance of PS400 Subjected to Sliding Contact at Temperatures from 260 to 927°C

Adequate high-temperature lubrication between loaded surfaces in sliding contact can be one of the most challenging tribological problems confronting today's designers. In an attempt to provide a possible solution a test program was initiated to evaluate PS400, a recently patented, high-temperature solid lubricant coating. Made from nickel–molybdenum–aluminum, chrome oxide, silver, and barium fluoride–calcium fluoride, PS400 is a variant of the earlier coating, PS304, but is formulated for higher density, smoother surface texture, and greater dimensional stability. It was initially developed to minimize the start–stop wear in foil air bearings but is expected to perform well in other high-temperature applications where sliding friction and wear are a concern, such as variable inlet guide vanes and process control valve stems. To better define its operational capabilities, a series of tests was conducted to study the behavior of PS400 under reciprocating sliding contact at temperatures from 260 to 927°C. The tests were performed on stationary, uncoated cobalt-based superalloy bushings loaded against reciprocating PS400-coated shaft specimens in a flat-on-cylinder configuration at Hertz contact pressures from 14.1 to 20.1 MPa. For tests conducted below 927°C, friction coefficients ranged from 0.37 to 0.84 with wear factors on the order of 10^?5 and 10^?6 at the high temperatures but substantially increased at the lowest temperature. Data collected at 927°C were limited because the coating was found to be dimensionally unstable at this temperature.     

Development of Surface Coatings for Air-Lubricated,Compliant Journal Bearings to 650°C

Surface coatings for an air-lubricated, compliant journal for an automotive gas turbine engine were tested to find those capable of withstanding temperatures of either 540°C (1000°F) or 650°C (1200°F). Also, the coatings have to be capable of surviving the start-stop sliding contact cycles prior to rotor lift-off and at touchdown. Selected coating combinations were tested in start-stop tests at 14 kPa (2 psi) loading for 2000 cycles at room and maximum temperatures. Specific coating recommendations are: Cdo and graphite on foil versus chrome carbide on journal up to 370°C (700°F); NASA PS-120 (Tribaloy 400, silver, and CaF₂) on journal versus uncoated foil up to 540°C (1000°F); and chemcially adherent Cr₂O₃ on journal and foil up to 650°C (1200°F). The chemically adherent Cr₂O₃ coating system was further tested successfully at 35 kPa (5 psi) loading for 2000 start-stop cycles.