Andrey A. Voevodin

Supertough wear-resistant coatings with ‘chameleon’ surface adaptation

The chameleons ability to change skin color depending on environment to increase its chances of surviving served as an inspiration in the development of self-adaptive supertough wear-resistant coatings. Surface chemistry, structure and mechanical properties of these thin (0.5 μm) coatings reversibly change with applied load and environment, providing the best wear protection. Coating designs developed in-house are reviewed together with a critical analysis of design reports in the literature. ‘Chameleon’ coatings were prepared using novel nanocomposite structures, consisting of crystalline carbides, diamond-like carbon (DLC), and transition metal dichalcogenides. Various mechanisms were activated to achieve surface self-adaptation and supertough characteristics. They included: transition of mechanical response from hard and rigid to quasi plastic by grain boundary sliding at loads above the elastic limit; friction induced sp3→sp2 phase transition of the DLC phase; re-crystallization and reorientation of the dichalcogenide phase; change of surface chemistry and structure from amorphous carbon in humid air to hexagonal dichalcogenide in dry nitrogen and vacuum; and sealing the dichalcogenide phase to prevent oxidation. These mechanisms were demonstrated using WC/DLC, TiC/DLC, and WC/DLC/WS2 coatings. The hardness of WC/DLC and TiC/DLC composites was between 27–32 GPa and scratch toughness was 4–5 fold above that of nanocrystalline carbides. The WC/DLC/WS2 composites survived millions of sliding cycles in vacuum and air under 500–1000 MPa loading, and exhibited excellent friction recovery in humid↔dry environmental cycling. Their friction coefficients were about 0.1 in humid air, 0.03 in vacuum, and as low as 0.007 in dry nitrogen. The proposed ‘chameleon’ concept can dramatically increase wear-resistant coating applicability, durability, and reliability.

Preparation of amorphous diamond-like carbon by pulsed laser deposition: a critical review

A critical review of the pulsed laser deposition (PLD) of amorphous diamond-like carbon (DLC) films is presented. A short review of the PLD process is followed by a review of various experimental configurations for DLC deposition and a discussion of the influence of process parameters on the composition and energy of ablated carbon plumes. Particular emphasis is given to the relationship between plume properties and film structure and mechanical characteristics. For the first time, a cumulative influence of the laser power density (fluence) and wavelength on the formation and properties of DLC films is shown. The influence of bias, additional auxiliary energy, substrate temperature, and the presence of hydrogen is also discussed. A fluence-wavelength region for DLC formation is proposed and correlated with the kinetic energy of ablated carbon species. It is shown that lower fluences are required to produce DLC films when shorter-wavelength lasers are used. The latest available results on applications of PLD DLC films as protective coatings for reducing friction and wear are also discussed. Methods are proposed to improve film adhesion to steel substrates, so that DLC films can be used in highly loaded friction contacts. Finally, process improvements that are necessary to permit scaling up PLD for growing DLC films are outlined.

Design of a Ti/TiC/DLC functionally gradient coating based on studies of structural transitions in Ti–C thin films

Abstract Development of optimized hard coatings requires a multilayer, functionally gradient approach, where adhesion, load support and low friction regions must be elements of the design architecture. Each of these functional regions should be joined through appropriate transition regions to reach desired coating performance. The design of coatings with hard and lubricious diamond-like carbon (DLC) surfaces requires a study of transitions between adhesive metal, load supporting carbide, and wear-resistant DLC materials. These transitions were investigated on the Ti–C system prepared by a hybrid of magnetron sputtering and pulsed laser deposition. Crystalline α-Ti, TiC and amorphous DLC films were formed at 100 °C substrate temperature by varying film chemical composition. Phase transitions between Ti–TiC–DLC were analyzed with XPS, XRD and Raman spectroscopy. A gradual replacement of hcp α-Ti with fcc TiC, and a two-phase region consisting of crystalline TiC and amorphous carbon (a-C) in transitions from Ti to TiC and from TiC to DLC were found. These transitions were reflected in mechanical properties investigated with nanoindentation. A hardness maximum in the two-phase TiC/a-C region was found. The resulting composition-property maps were used to design a wear-resistant coating with Ti/TiC/DLC transitions and a gradual increase in hardness from a steel substrate to a super-hard (60–70 GPa) self-lubricating DLC layer on the surface. This provided a hard coating with a low friction surface, which also resisted brittle failure in tests with high contact loads.

Nanocrystalline carbide/amorphous carbon composites

Nanocrystalline TiC/amorphous carbon (a-C) composite films were synthesized at near room temperature with a hybrid process combining laser ablation of graphite and magnetron sputtering of titanium. Film microstructure was investigated by x-ray photoelectron spectroscopy, x-ray diffraction analyses, and transmission electron microscopy. Mechanical properties were evaluated from nanoindentation, scratch, and friction tests. The films consisted of 10 nm sized TiC crystallites encapsulated in a sp3 bonded a-C matrix. They had a hardness of about 32 GPa and a remarkable plasticity (40% in indentation deformation) at loads exceeding their elastic limit. They were also found to have a high scratch toughness in addition to a low (about 0.2) friction coefficient. The combination of hardness and ductility was correlated with film phase composition and structural analyses, using concepts of nanocomposite mechanics. The properties of the TiC/a-C composites make them beneficial for surface wear and friction protection.

Nanocomposite tribological coatings for aerospace applications

Challenges in aerospace tribology and composite coatings for aerospace applications are briefly reviewed. Attention is given to nanocomposite coatings made of carbide, diamond-like carbon (DLC) and transition-metal dichalcogenide phases. The preparation of such coatings within the W–C–S material system using a hybrid of magnetron sputtering and pulsed laser deposition is described. Coatings consist of 1–2 nm WC and 5–10 nm WS2 grains embedded in an amorphous DLC matrix. These WC/DLC/WS2 nanocomposites demonstrate low friction and wear in tests performed in high vacuum, dry nitrogen and humid air. Coatings are found to adapt to the test conditions, which results in: (1) crystallization and reorientation of initially nanocrystalline and randomly oriented WS2 grains; (2) graphitization of the initially amorphous DLC matrix; (3) reversible regulation of the composition of the transfer film between WS2 and graphite with environmental cycling from dry to humid; and (4) possible DLC/WS2 synergistic effects, providing friction reduction in oxidizing environments. These adaptive mechanisms achieve low friction coefficients of 0.02–0.05 and an endurance above two million cycles in space simulation tests. This also provides stable coating performance and recovery of low friction in tests simulating ambient/space environmental cycling. Correlations among WC/DLC/WS2 chemistry, structure, hardness, friction and wear are discussed. The tremendous potential of such composites for aerospace tribology is demonstrated.

Friction induced phase transformation of pulsed laser deposited diamond-like carbon

Structural transformations in the sliding friction of hydrogen-free diamond-like carbon (DLC) films prepared by pulsed laser deposition are investigated. Stainless steel disks were coated with 0.5 μm thick DLC films, and ball-on-disk sliding experiments were performed with steel and sapphire balls in humid air, a nitrogen atmosphere, and under vacuum. Friction coefficients of about 0.1 are reported. The low friction is related to a friction induced transformation of the surface into a graphite-like phase and the formation of an adherent transfer film of this material on the counterface. Surface enhanced micro-Raman studies of the wear tracks, wear debris and the transfer film demonstrated that an sp3 to sp2 phase transition has occurred in the wear tracks on the DLC film surface. The formation of a graphite phase after several thousands of cycles caused a humidity sensitive behavior of the DLC films and an increase in the friction coefficient in high vacuum conditions. A lubricating sp2-rich layer on the surface of the hydrogen-free DLC films is proposed as the reason for their extremely low wear rates in ambient environments.

Load-adaptive crystalline-amorphous nanocomposites

Advances in laser-assisted deposition have enabled the production of hard composites consisting of nanocrystalline and amorphous materials. Deposition conditions were selected to produce super-tough coatings, where controlled formation of dislocations, nanocracks and microcracks was permitted as stresses exceeded the elastic limit. This produced a self-adjustment in the composite deformation from hard elastic to quasiplastic, depending on the applied stress, which provided coating compliance and eliminated catastrophic failure typical of hard and brittle materials. The load-adaptive concept was used to design super-tough coatings consisting of nanocrystalline (10–50 nm) TiC grains embedded in an amorphous carbon matrix (about 30 vol%). They were deposited at near room temperature on steel surfaces and studied using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, Raman spectroscopy, nanoindentation and scratch tests. Design concepts were verified using composition–structure–property investigations in the TiC–amorphous carbon (a-C) system. A fourfold increase in the toughness of hard (32 GPa) TiC–a-C composites was achieved in comparison with nanocrystalline single-phase TiC.

SUPERHARD, FUNCTIONALLY GRADIENT, NANOLAYERED AND NANOCOMPOSITE DIAMOND-LIKE CARBON COATINGS FOR WEAR PROTECTION

Abstract Recent progress in the development of advanced carbon-based tribological coatings is reviewed. Pulsed laser deposition (PLD) was used to produce superhard (60–70 GPa) self-lubricating diamond-like carbon (DLC) with low friction and low wear rate. Attention was given to the improvement of coating toughness. A hybrid of magnetron sputtering and PLD was used to deposit coatings with architectures designed to withstand 1–10 GPa contact stress. This included: (1) composite coatings combining hydrogen-free and hydrogenated DLC; (2) functionally gradient metal-ceramic-DLC and nanolayered coatings with crystalline metal and carbide interlayers (about 10 nm thick) between DLC layers (about 60 nm thick); and (3) nanocrystalline-amorphous composite coatings with 10–50 nm diameter carbide particles encapsulated in a DLC matrix. The research led to the fabrication of thin (2–3 μm) DLC-based coatings for steel substrates, which could maintain friction coefficients of about 0.1 for several million cycles of unlubricated sliding at contact pressures above 1 GPa. Their scratch resistance exeeded that of conventional ceramic (TiN, TiC) coatings.

Architecture of multilayer nanocomposite coatings with super-hard diamond-like carbon layers for wear protection at high contact loads

Abstract Super-hard and low-friction diamond-like carbon (DLC) coatings deposited at low temperatures are currently of great interest for wear protection and friction reduction. However, their high hardness (50–80 GPa), intrinsic stresses, and poor adhesion limit their use to applications where contact pressures are below I GPa and the coating thickness is below 0.5 μm to prevent cracking and delamination. These negative effects are especially pronounced when the coatings are applied to relatively soft substrates, such as steels. The limitations were removed by a multilayer design, where metal and ceramic layers were used to increase the load support capability, improve the adhesion strength, and increase the thickness of DLC layers. Existing approaches to the design of tough multilayer coatings were considered critically and a coating architecture was suggested using the following concepts: (i) formation of a load support and adhesion promoting underlayer with mechanical characteristics varied gradually from the substrate to the DLC layer; (ii) separation of hard DLC layers with interlayers of softer material to reduce stresses and brake cracks; (iii) use of crystalline interlayers with thickness permitting operation of dislocation sources for stress relaxation and deflection of cross-sectional cracks. The development of these concepts is discussed sequentially from bilayer ceramic/DLC coatings to functionally gradient metal/carbide/DLC coatings, and finally to nanocomposite coatings consisting of stacks of Ti/DLC, TiC/DLC, and CN/DLC layers with individual thickness within 10–60 nm deposited onto a gradient TiTiC-DLC underlayer. The coatings were deposited onto stainless steel substrates by a hybrid of magnetron sputtering and pulsed laser deposition. They had a 1–2 μm total thickness of super-hard (60–70 GPa) DLC layers and resisted delamination to 50–80 N loads in scratch adhesion tests. In ball-on-disk sliding tests, these coatings supported Hertzian contact pressures above 2 GPa and had friction coefficients around 0.1. Their wear lives exceed 10 6 cycles under initial contact pressures of 1.4 GPa. The conceptual architecture of multilayer nanocomposite coatings presented extends the range of wear resistant applications for super-hard DLC materials.

Characterization of wear protective AlSiO coatings formed on Al-based alloys by micro-arc discharge treatment

Abstract The wear life of components manufactured from Al-based alloys can be drastically increased by the application of ceramic coatings. However, coatings deposited by conventional methods such as vacuum deposition or plasma spray have either insufficient adhesion to Al-based materials or the deposition process causes the component to overheat. A recently developed micro-arc discharge oxidizing (MDO) technique allows for the formation 100–200 μm thick AlSiO coating on the surface of Al alloys. A composite Al2O3SiO2 coating is formed at room temperature as a result of a reactive process between Al in the alloy itself and O and Si supplied by an electrolyte. AlSiO coatings were investigated with XPS, Vickers and nanoindentation hardness tests, ball-on-disk, and block-on-ring friction and wear tests. Coatings were found to consist of at least two phases: a hard Al2O3 phase and a softer aluminasilicate phase. A maximum hardness of 17 GPa was found for coatings with highest content of Al2O3 phase. The tribological properties of AlSiO coatings with different composition are discussed. The lowest friction coefficient was found for the Al0.26Si0.08O0.66 coating and was measured around 0.15–0.25 depending on the test environment. The application of this coating decreased the wear rate of components fabricated from an Al-based alloy by several orders of magnitude and permitted operation of coated friction pairs at 1 GPa contact load.