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36HYBRID SURFACE ENGINEERING TECHNOLOGIES FOR SELF-ADAPTIVE FRICTION AND WEAR BEHAVIOR IN EXTREME ENVIRONMENTS
A. A. Voevodin
Department of Materials Science and Engineering, the University of North Texas, Denton, TX 76203, USA e-mail: andrey.voevodin@unt.edu
DOI: 10.24411/9999-0014A-2019-10186
Self-adaptive low friction and wear resistant coatings, which can re-arrange their structure and chemistry in a response to changes in environment temperature and humidity are of a practical importance for aerospace, automotive and tool industry applications to help reliable operation under cycles of extreme temperatures, mechanical loads and environments. Surface engineering approaches to provide self-adaptive friction and wear behavior were a subject of intensive research. This includes development of hard, abrasion and oxidation resistance ceramic-based coatings with embedded soft and low friction solid lubricants made of graphite and diamondlike carbon (DLC), transition metal dichalcogenides, soft metals and easy to shear oxides. Other approaches include the development of protective coating compositions with the use of transition metals which can react with environment to form low shear moduli oxides at elevated temperatures in air. The resulted self-adaptive coatings have a capability of solid lubricant release as well as self-adjustment of surface chemistry and microstructure in response to temperature and environment variations. These coatings were coined "chameleon" to reflect their ability to self-adjust their surface to operating environments to maintain low friction and wear. The realization of self-adaptive coatings relies on hybrid surface engineering technologies, since hard and oxidation resistance ceramics normally require high temperature and energy parameters for their synthesis, while the inclusions of solid lubricants (temperature and oxidation sensitive) are normally need be processed at reduced temperatures and energies to retain lubricating properties. A review of hybrid surface engineering technologies is presented with a focus on self-adaptive friction and wear protective coatings for extreme environment applications.
Hybrid of magnetron sputtering and pulsed laser deposition
This versatile hybrid physical vapor deposition (PVD) method was used to explore and produce a number of nanocomposite coatings made of hard nano-crystalline carbide, nitride and oxide matrices (ZrO2, AhO3, TiCN, CrN, etc.). These had embedded nano-sized inclusions of solid lubricants (MoS2, WS2, DLC, Ag, Au) and transition metals capable to form low shear modulus oxide for adaptive lubrication in variable humidity and temperatures up to 1000°C [1, 2]. The stored lubricant materials released from nanophase reservoirs facilitate chemistry and structure change of mechanical interfaces to continuously reduce friction and wear in environment cycles (Fig. 1a). Coating compositions were modified to form contact tribofilms made of defected double oxides (e.g. Magnéli phases) as well as ternary oxides, where interlayered metallic and ionic bonding along specific crystallographic orientations provides sliding friction reduction and adaptive lubrication in the challenging mid-temperature regime of 500-700°C, allowing to keep friction coefficient stable over the broad temperature range (Fig. 1b). The nanocomposite "chameleon" structures were also sandwiched between diffusion barrier nitride layers for extended operations and adding temperature cycling capability. In addition, coating nanostructure designs, texture and orientation control were also shown to provide a thermal management functionality, using thermal conductivity anisotropy and phase transitions in tribological contacts [1].
temperature (°C)
(a) (b)
Fig. 1. (a) A design concept of multi-environment self-adaptation for chameleon tribological coating. (b) Performance of a broad temperature range YSZ-Mo-Au-MoS2 chameleon coating with insets showing a dominance of MoS2 lubrication at low temperature and MoO3 lubrication at high temperature [1, 2].
Laser texturing of ceramic surfaces with burnished application of composite solid lubricants
This approach uses laser processing to form micron-sized reservoirs on the surface of hard wear resistant ceramics followed with an application of self-adaptive chameleon compositions to fill these reservoirs and provide solid lubricant replenishment in friction contacts. One example is laser processing of a functionally gradient Ti-TiC-TiC/DLC coatings with laser cut tracks along wear paths filled with MoS2 [3]. Another example is laser machining of arrays of micrometer sized dimple reservoirs on the surface of TiCN coatings, which are filled with chameleon solid lubricants based on MoS2 and graphite applied by burnishing (Fig. 2a) [4]. Such hybrid processed surfaces exhibited environmental adaptation with friction coefficients of 0.15 in humid air and 0.02 in dry or vacuum conditions, while wear life was increased by at least one order of magnitude. The laser-processed micro-reservoirs also helped to renew lubricant supply in multiple cycling between humid air and dry nitrogen, evidenced by repeated change from hexagonal MoS2 to graphitic carbon in the wear track with each dry/humid environment cycle (Fig. 2b).
(a) (b)
Fig. 2. (a) Laser textured TiCN surface before applying a top burnished chameleon MoS2/graphite/Sb2O3 and (b) variation of the friction coefficient (solid line) in sliding tests a cycled environment humidity (dotted line). Insets show Raman spectra taken from wear tracks [4].
Cold spray deposited metal matrix coatings with addition of hard carbide phase This approach allows fabrication of relatively thick coatings with adaptive lubrication by using metal matrices capable to form low shear strength binary and ternary oxides. As an example, tribological properties of cold sprayed Ni-WC metal matrix composite coatings were
investigated under dry sliding conditions from room temperature to 400°C, including thermal cycling to explore their temperature adaptive friction and wear behavior [5]. The adaptive tribo-oxide formation mechanisms were used to control friction and wear, where both friction coefficients and wear rates were reduced by easy to shear NiO and NiWO4 oxide formations on sliding contact surfaces. The coating also exhibits low friction behavior during thermal cycling by restoring the lubricious oxides inside the wear track at 400°C temperature intervals [5].
Duplex technology combining plasma electrolytic oxidation (PEO) with burnished nanocomposite adaptive solid lubricant
Plasma electrolytic oxidation (PEO) is an attractive technology for improving wear resistance, temperature and corrosive environment protection of aluminum and titanium alloys [6]. PEO results in the hard and well-adhered ceramic coatings which morphology is graded from a dense region near the substrate interface to a porous outside region (Fig. 3a). Such properties provide PEO as an ideal underlying layer for the application of solid lubricants, which can be entrapped in outside reservoirs for the self-adaptive lubrication. In one example, a PEO produced Al-Si-O surface was over-coated with a MoS2/Sb2O3/C chameleon layer (Fig. 3b) for self-adaptive friction behavior and fretting wear reduction [7]. The tests demonstrated low friction coefficients in multiple environments (Fig. 3c) and also considerable reduction in a critical amplitude for the stick-slip transition for fatigue wear mitigation of aluminum and titanium alloy substrates.
(a) (b) (c)
Fig. 3. Surface topography (a) Al-Si-O PEO coating and (b) PEO/chameleon duplex coating. Coefficient of friction evolution of PEO and PEO/chameleon coatings against steel in fretting wear with about 1.6 GPa contact pressures and 100 p,m stroke amplitude [7].
Self-propagating high temperature synthesis (SHS) of sputtering targets and electro spark deposition (ESD) electrodes for surface engineering of tribological coatings
SHS is versatile technology allowing for the synthesis of complex composition ceramic materials leading to virtually unlimited combinations of hard oxide, borides, nitrides and carbides [8], which makes it attractive for engineering adaptive wear protective surfaces. SHS was used extensively to manufacture a number multicomponent targets for hybrid magnetron sputtering and ion beam depositions, including for the hybrid PVD growth of self-adaptive low friction and wear coatings, such as TiAlSiCN/MoSeC [9], TiCrBN/WSex [10] and other.
SHS was also widely used for fabrication of electrodes for ESD processes, which produce hard, low wear and oxidation resistant protective coatings with an excellent adhesion to metal alloy substrates due to metallurgical and chemical mixing of deposited electrode material with substrate surfaces in spark discharges. Nanostructured ESD electrodes made of cemented carbides (WC, TiC) and Ni matrices provide adaptive tribological coatings, capable for easy to shear oxide formations, and additions of refractory nano-powders (ZrO2, AhO3, NbC, Si3N4, W, Mo, WC, WC-Co, and diamond) into SHS powders lead to further improvements of wear and friction reducing ESD coatings [8, 11].
(a) (b)
Fig. 4. ESD + MS duplex coating cross section and results of sliding friction tests against steel counterparts for coatings prepared by ESD, magnetron sputtering, and duplex MS-ESD [12].
Exothermic SHS reactions can be initiated by electric discharges within an inter-electrode space to provide coatings with high density, mechanical strength and adhesion [8]. The ESD technology was also combined with magnetron sputtering (MS). In this duplex MS-ESD technology an underlying relatively thick Ti-C-Ni-Fe layer is obtained by ESD for load support and high toughness, whereas a top Ti-C-Ni-Al layer is fabricated by sputtering to improve coating wear, corrosion resistance, and provide a stable and low friction coefficient behavior (Fig. 4) [12].
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