Tribological behavior and mechanism of self-lubricating wear-resistant composite coatings fabricated by one-step plasma electrolytic oxidation

doi:10.1016/j.triboint.2016.01.020

Tribology International

Volume 97, May 2016, Pages 97-107

https://doi.org/10.1016/j.triboint.2016.01.020 Get rights and content

Highlights

•
An alumina–graphite composite coating was prepared through one-step PEO process.
•
It is a self-lubricating wear-resistant composite coating.
•
MD makes up the weakness of conventional experimental method in microscopic study.
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This self-lubricating composite coating will have wider commercial application.

Abstract

The productions of self-lubricating alumina–graphite composite coatings were prepared through one-step plasma electrolytic oxidation in an appropriate graphite-dispersed electrolyte solution, and the microstructure, composition and phase constituents were examined. The friction and wear properties were investigated by sliding the disk samples against Si₃N₄ balls under dry and deionized water conditions, respectively. Molecular dynamics simulations about the interface states of graphite were also carried out to assist in the evaluation of the self-lubricating and wear-resistant mechanisms, making up the weakness of the conventional experimental method in microscopic study. The results indicated that the oxidation coatings greatly improved the wear resistance of pure aluminum. Further improvement in the wear resistance was achieved by self-lubricating of solid lubricant in oxidation coatings. In wear process, the wear debris aggregated together on the worn surface and the graphite particles formed layered structures, which exhibited excellent self-lubricating behavior and decreased the shear stresses generated by the moving stylus.

Introduction

The development trends and application requirements of lightweight for the defense industry, transportation and textile industry become increasingly evident [1]. With respect to the iron and steel materials, aluminum and its alloys have low density, high strength, easy molding and many other advantages, which can lighten weight, reduce the moment of inertia and raise speed. In terms of applicability, aluminum and its alloys have become the second place to the steel metal materials. However, the poor corrosion resistance, abrasion resistance and scratch resistance limited the directly application [2]. Therefore, suitable surface modification should be done [3], [4]. For aluminum and its alloys, the research, development and application about surface treatment technology will provide a good surface protection feature and further expand their field of application. These treated materials have been known as one of the pillars of the 21st century [4], [5].

The plasma electrolytic oxidation (PEO) is considered as an economical and effective surface modification technique of anodic oxidation and primarily applied to aluminum and other light alloys such as magnesium, titanium and zirconium [6], [7], [8]. It promotes the transformation of these metals substrate into hard and compact ceramic coatings of Al₂O₃, MgO, TiO₂, and ZrO₂ oxides by the interaction of anodic oxide growth and microelectric channel shock caused by dielectric breakdown at high voltages, taking place in an aqueous electrolyte [9]. Complex compounds are synthesized between the substrate and the electrolyte through the plasma chemical interactions, and hard oxide ceramic coatings were fabricated on the metal surface simultaneously [8], [9], [10]. Especially, PEO technology can form a continuous hard porous alumina ceramic layer on the surface of aluminum or its alloy, which can effectively improve the friction conditions. High hardness, high impedance and high stability of PEO coatings may be beneficial to meet seawater, high temperature corrosion environment, and also can improve wear resistance to adapt other performance requirements. However, the ceramic coatings tend to have higher friction coefficient. It is easy to result in the wear failure itself, but also will exacerbate the dual material wear. Therefore, in order to further improve the friction and wear performance of PEO coatings, the friction coefficient must be reduced. An effective method is to prepare self-lubricating coating containing solid lubrication. However, the self-lubricating PEO coatings containing lubricating materials were usually fabricated by two-step methods [11], [12].

In this work, the productions of self-lubricating alumina–graphite composite coatings were successfully prepared by one-step PEO process in appropriate graphite-dispersed electrolyte solution. The microstructure, composition and phase constituents were also analyzed. Under dry and deionized water conditions, the tribological properties of the prepared pure aluminum with/without oxide coatings were evaluated using a reciprocating ball-on-disk tribometer at different loads.

Section snippets

Experimental procedures

Pure aluminum samples (denote in this sample as LV1) of size 40 mm×20 mm×4 mm with high strength (560 MPa) and high hardness (HV 144) were used as the substrate in the present work (Table 1), which were prepared by mechanical alloying and press-forming. All pure alloy samples were further ground to 1500 grit with waterproof abrasive papers, followed by acetone and deionized water cleaning.

The PEO process were carried out using 20 kW pulsed bipolar power supply with a frequency of 150 Hz and duty

Properties of the coatings

Fig. 1 displays the surface morphologies of PEO coatings prepared in electrolyte solution without and with graphite particles added. It is presented that the typical PEO porous structures with many micropores and microcracks nonuniformly distributed over the coating surface, which is attributed to the plasma discharge with thermochemical, plasma chemical and physical chemical reactions [16]. Molten aluminum combining with oxygen from the electrolyte formed Al₂O₃, cooled down rapidly by contact

Conclusion

(1)
The densed oxide/graphite composite coating mainly consisted of a combination of γ-Al₂O₃, α-Al₂O₃, graphite and amorphous alumina was successfully fabricated on prepared pure aluminum bulk by one-step PEO process. The hardness of the coated sample was much higher than that of the substrate.
(2)
The friction and wear performances of this pure aluminum bulk and two PEO coatings (with and without graphite addition) were systematically studied. The PEO coating greatly improved the wear resistance of the

Acknowledgments

This work was supported by The National Natural Science Foundation of China (51303188, 51227804). One of the authors (Zhuhui Qiao) gratefully appreciates the support of State Key Laboratory of Rare Earth on Advanced Materials and Valuable Utilization of Resources, Changchun Institute of Applied Chemistry (RERU2014008). The authors gratefully thank Huaguo Tang from Changchun Institute of Applied Chemistry for the aluminum provided.

References (22)

D. Gu et al.
Rapid fabrication of Al-based bulk-form nanocomposites with novel reinforcement and enhanced performance by selective laser melting
Scr Mater
(2015)
R.N. Rao et al.
Effect of heat treatment on the sliding wear behaviour of aluminium alloy (Al–Zn–Mg) hard particle composite
Tribol Int
(2010)
K. Nimura et al.
Surface modification of aluminum alloy to improve fretting wear properties
Tribol Int
(2016)
M.H. Staia et al.
Tribological response of AA 2024-T3 aluminium alloy coated with a DLC duplex coating
Tribol Int
(2015)
J.M. Wheeler et al.
Evaluation of micromechanical behaviour of plasma electrolytic oxidation (PEO) coatings on Ti–6Al–4V
Surf Coat Technol
(2010)
W. Xue et al.
Anti-corrosion microarc oxidation coatings on SiC_P/AZ31 magnesium matrix composite
J Alloy Compd
(2009)
J.A. Curran et al.
Porosity in plasma electrolytic oxide coatings
Acta Mater
(2006)
C. Martini et al.
PEO layers obtained from mixed aluminate-phosphate baths on Ti–6Al–4V: dry sliding behaviour and influence of a PTFE Topcoat
Wear
(2010)
Y.M. Wang et al.
Microarc oxidation and spraying graphite duplex coating formed on titanium alloy for antifriction purpose
Appl Surf Sci
(2005)
A.Y. Toukmaji et al.
Ewald summation techniques in perspective: a survey
Comput Phys Commun
(1996)

A.L. Yerokhin et al.

Characterisation of oxide films produced by plasma electrolytic oxidation of a Ti–6Al–4 V alloy

Surf Coat Technol

(2000)

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Tribological behavior and mechanism of self-lubricating wear-resistant composite coatings fabricated by one-step plasma electrolytic oxidation

Highlights

Abstract

Introduction

Section snippets

Experimental procedures

Properties of the coatings

Conclusion

Acknowledgments

Scr Mater

Tribol Int

Tribol Int

Tribol Int

Surf Coat Technol

J Alloy Compd

Acta Mater

Wear

Appl Surf Sci

Comput Phys Commun

Surf Coat Technol