PEO-Chameleon as a potential protective coating on cast aluminum alloys for high-temperature applications

https://doi.org/10.1016/j.surfcoat.2020.126016Get rights and content

Highlights

  • Hybrid dual-phase coating by burnishing graphite-MoS2-Sb2O3 powder in plasma electrolytic oxidation modified aluminum alloy

  • In-situ Raman analysis suggests high stability of the coating when tribologically tested up to 300 °C.

  • Roughness of the PEO structure significantly affects the tribological properties of the coating.

  • Coefficient of friction for the coating sliding against a silicon nitride ball reduce to 0.02 at 300 °C.

Abstract

Hybrid dual-phase coatings composed of an A356 aluminum alloy modified by plasma electrolytic oxidation (PEO) and burnished with graphite-MoS2-Sb2O3 chameleon solid lubricant powders have been produced. The PEO layer provides high hardness and load support while the solid lubricant powders reduce friction. These hybrid coatings were tribotested against steel and silicon nitride counterparts in air from 25 °C to 300 °C and using variable contact loads. The open porosity and surface roughness of the PEO layers were reduced considerably by the burnishing process. Polishing of the PEO surfaces prior to burnishing significantly reduced the resulting coefficient of friction (COF) of the hybrid coating. In-situ Raman spectroscopy revealed the chemical stability of the dual phase coatings at temperatures up to 300 °C with no signs of oxidation or reduction of the chameleon components. COF values ranged from 0.2 at room temperature down to 0.02 at 300 °C. The observed low friction values were attributed to the synergism between PEO and chameleon layers that promote defect healing and adaptive behavior of the coating. Top view scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM) confirmed that the thermo-mechanical stimulus caused the chameleon coating to fill the voids in the PEO layer. In-situ Raman spectroscopy revealed that the lubricating phases, i.e. MoS2 and graphite, were protected from oxidation by the porous PEO structure. These lubricious phases formed a transfer film in the wear tracks and the counterpart bodies as a result of the contact pressure (up to 1.4 GPa) and thermal energy, which led to an order of magnitude reduction in the COF at high temperatures. The low shear strength of MoS2 and graphite and the good adhesion and integration of the chameleon coating with the PEO sublayer due to high contact pressures during sliding were responsible for the ultra-low friction behavior of the composite coating.

Introduction

The increased need for light-weight, high-strength, corrosion-resistant materials for energy-efficient vehicles in the aerospace and automotive industries has led to many recent studies on advancing aluminum, titanium, and magnesium-based alloys for such applications. One major drawback of using light-weight alloys in mechanical systems is their poor tribological behavior [1] that results in high friction and wear characteristics. Cast Al alloys are widely used as materials for manufacturing engine blocks. To protect Al from the heat and wear, the combustion cylinders of reciprocating engines frequently use steel inserts or liners; however, recently more and more automotive engines use thermally-sprayed coatings on the cylinder bore [2].

Considerable efforts have been dedicated to improving the tribological performance of Al components by modifying their surface structure and composition. Such surface modification approaches include anodizing [3,4], laser-based surface texturing [5,6], and the application of various coatings by physical [7,8] or chemical [9] vapor deposition methods. The use of lubricants is a potentially viable option to enhance the tribological properties of Al-based materials. While liquid lubricants are efficient to reduce friction and wear in metallic surfaces [10,11] [12], normally these are limited to operating temperatures <200 °C. At higher temperatures, oxidation and the chemical evolution of material surfaces during sliding lead to the loss of liquid lubrication and the eventual failure of moving components [13,14]. For temperatures >200 °C, the use of solid lubricants and elimination of liquid lubricants are usually the best choice [[15], [16], [17]]. Soft metals such as lead, gold, or silver were reported to provide shear accommodation during sliding from 200 to about 500 °C [18]. Alternatively, metal oxide coatings with a lamellar crystal structure with weak inter-planar cohesive bonds can be applied for easier shearing but are usually most effective at T > 500 °C [19]. Recently, adaptive coatings [20,21], including self-healing materials [22], were demonstrated to be lubricious over a wide range of temperatures facilitated by thermo-mechanical stimuli that activate the desired lubricant at the surface. The main challenge that remains is the lack of understanding whether these coating materials are suitable for light-weight alloys; i.e., whether they work reliably over a wide range of temperatures, environments, and load conditions similar to those experienced in their potential automotive or aerospace engine applications.

Chameleon coatings were reported to provide optimal performance in variable temperature and humidity conditions [23]. These coatings usually consist of a combination of solid lubricants, such as MoS2 that reduce friction in the absence of water vapors [24] or when the basal plane anisotropy long-range order is provided [25], graphite that is functional at low temperature and in the presence of humidity [26], and boron nitride that tribologically is more effective for high-temperature needs [27]. The environment triggers the appropriate lubricant to be released to the surface of the chameleon coating. Changes in the environment include a controlled ambient temperature [28], the absence of moisture [29,30], or a range of temperatures [31] [32].

The major limitation of chameleon coatings is being soft which reduces their wear protective effect on Al alloy surfaces [33]. To overcome such a potential drawback, a promising solution is a modification of the surface of the aluminum alloy material to form a hard wear protective oxide layer and to embed the chameleon coating material inside this layer. The cast aluminum alloy substrate surface can be successfully transformed into hard alumina-based layers of an order of a few 100-micrometer thickness range using the plasma electrolytic oxidation (PEO) approach [34]. PEO coatings are usually grown anodically, in silicate alkaline electrolytes, above breakdown voltage of the formed oxide layer. This results in plasma-assisted electrochemical conversion of the Al alloy surface, which provides metallurgical bonding to the metal substrate leading to surface structures comprising a dense and hard alumina inner layer which gradually transitions to a softer and more porous Al-Si-O surface layer. The application of PEO coatings on wrought Al alloys was shown to provide considerably extended abrasion and adhesive wear resistance for such treated surfaces [35,36]. Variations in processing parameters allow controlling surface porosity and roughness, as well as the overall thickness of PEO coatings [37]. However, for cast Alsingle bondSi alloy substrates, the major problem is presented by a coarse heterogeneous structure of the alloy containing Alsingle bondSi eutectic which is preferentially oxidized, leading to the formation of a non-uniform porous surface structure with inferior mechanical and tribological properties [[38], [39], [40]]. Specifically, anodic oxidation of Al is hindered by the formation of the anodic alumina layer, whereas oxidation of Si in the eutectic phase yields soluble silicate anions and the grains will remain electrochemically active until either all Si in it is consumed or a solubility limit of silicate ions in the electrolyte is reached and they would start precipitating back to the surface [[38], [39], [40]]. Attempts have been made to resolve this problem by refining alloy microstructure [41], modifying electrolyte composition [42,43], and/or applying sequential treatments in different electrolytes [44]. Since these approaches often led to more complex and laborious manufacturing routes, or coatings with insufficient thickness and mechanical properties to be used in structural applications, the studies of PEO coating tribology on cast Al alloys are rather scarce and those concerned with high-temperature tribology are virtually non-existent.

Despite that, surface morphology and residual porosity of PEO coatings generally provide ideal conditions for adherence of solid lubricant chameleon compositions, creating reservoirs of adaptive solid lubricant in hard wear-resistant and relatively thick protective oxide layers. A chameleon coating composition based on graphite, MoS2, and Sb2O3 mixture [45] can be applied to the surface of the PEO coatings by a simple, low-cost burnishing process. Such a PEO-Chameleon coating combination was demonstrated by recent fretting wear studies to maintain the lubricity and to extend the lifetime of the coating on a rough AA6082 Al alloy substrate [33,39]. These studies were focused on testing the PEO-Chameleon coatings over a range of humidity conditions against steel and ceramic counterparts at room temperatures.

In the current study, we expand this approach to cast Al alloy substrates to further evaluate the tribological potential of PEO-Chameleon coatings upon their exposure to high-temperature conditions. We use in-situ Raman analysis [46] and SEM to evaluate the origin of high-temperature lubricity up to 300 °C. Our results indicate that the roughness of the PEO matrix is important in defining the wear resistance of the coating. At high temperatures, shear-assisted healing of microcracks allows the reduction in friction and wear and provides good thermal stability. We show that the PEO matrix also provides good thermal barrier characteristics essential for engine applications which protect the underlying aluminum substrate from exposure to heat and prevent the reduction of yield strength at elevated temperature of the material.

Section snippets

Coating

PEO-Chameleon coating was produced on an aluminum A356 substrate (Table S1), frequently selected for high-temperature automotive applications. The alloy was machined in 50 × 50 mm2 coupons and hardened using the T6 heat treatment, which is an artificial aging treatment procedure. The PEO coatings were grown using the equipment described elsewhere [47]; a pulsed bipolar current waveform was applied at 1 kHz frequency, with anodic and cathodic current densities set at 100 and 130 mA/cm2,

Results and discussion

The PEO-Chameleon coating schematic is shown in Fig. 1a. PEO-Chameleon coating shows up to 5 times reduction in thermal conductivity as compared to the bulk Al alloy (reduction from 158 to 33–40 W/mK, Table 1). This lower conductivity is due to intrinsically low thermal conductivity of Al-Si-O coatings, as linked to granular crystalline oxides, amorphous phases, and voids in the outer coating layers [39]. The cross-sectional and surface morphology micrographs of a typical PEO layer before

Conclusions

In this study, PEO-Chameleon coatings were produced on an aluminum alloy (A365) substrate. A porous morphology, especially in the surface region of the PEO layer, was found to be practically responsible for reduced thermal conductivity and contributes to the changes in surface roughness upon polishing. The application of the chameleon top layer reduced surface roughness significantly by filling in surface valleys and voids with the MoS2-graphite-Sb2O3 powder. The residual surface roughness and

CRediT authorship contribution statement

Asghar Shirani:Investigation, Data curation, Writing - original draft, Writing - review & editing.Tasha Joy:Data curation.Aleksey Rogov:Resources.Mengyu Lin:Resources.Aleksey Yerokhin:Resources, Writing - original draft, Writing - review & editing.Jon-Erik Mogonye:Writing - original draft, Writing - review & editing.Andras Korenyi-Both:Resources.Samir M. Aouadi:Writing - original draft, Writing - review & editing.Andrey A. Voevodin:Supervision, Conceptualization, Writing - original draft,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Parts of this research were supported by the U.S. CCDC Army Research Laboratory (Award No. W911NF-19-2-0281) and the European Research Council under the ERC Advanced Grant (320879 ‘IMPUNEP’) and ERC-2018-PoC programme (825122 - 3D Cer-Met). This work was performed in part at the University of North Texas' Materials Research Facility. Support from Advanced Materials and Manufacturing Processes Institute (AMMPI) at the University of North Texas is acknowledged. The authors acknowledge Dr. Alam

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