Tribological behavior of microarc oxidation coatings formed on titanium alloys against steel in dry and solid lubrication sliding

doi:10.1016/j.apsusc.2005.05.009

Applied Surface Science

Volume 252, Issue 8, 15 February 2006, Pages 2989-2998

https://doi.org/10.1016/j.apsusc.2005.05.009 Get rights and content

Abstract

The coatings mainly composed of nanostructured TiO₂ were deposited on Ti6Al4V alloy by microarc oxidation (MAO). The duplex coatings of microarc oxidation combined with spraying graphite process were fabricated for the antifriction purpose. The tribological properties of unpolished, polished and duplex coating against steel under dry friction conditions were examined. It is found that antifriction property of the polished microarc oxidation coating is superior to that of the unpolished one. The improvement is attributed to the low surface roughness and the nanocrystalline structure of coatings. The duplex coating exhibits best antifriction property, registering a lower and steady friction coefficient of ≈0.12 than that of the polished microarc oxidation coating sliding in the similar condition. The good tribological property is attributed to the specially designed duplex structure, the coating adhering strongly to the substrate and serving as the load-supporting underlayer and the graphite layer on top of it working as solid lubricant.

Introduction

Titanium alloys exhibit ever increasing interest in the fields of automotive, medical and offshore engineering, because of their high strength-to-weight, excellent corrosion resistance and biocompatibility. However, the poor tribological behavior caused by high and unstable friction coefficient, severe adhesive wear and a strong tendency to seize have seriously restricted extensive applications of titanium in various industry sectors.

As a surface modification technique, microarc oxidation (MAO) is an effective and economical method to deposit ceramic coatings on titanium alloys for wear resistance protection [1], [2], [3], [4], [5], [6], [7], [8]. In term of energy savings, low friction and low wear loss are desirable for decreasing the frictional resistance between sliding pairs. Therefore, the on-going investigations are focused on fabricating the frictionless coatings on titanium alloy by microarc oxidation method [1], [2], [4]. Coatings applied by microarc oxidation from phosphate baths reduce the friction coefficient in the titanium/titanium couple down to 0.06–0.16 [1], [2], however, the detailed analysis of the sliding process is not available. A friction coefficient of 0.18 is recorded during the testing of softer rutile/anatase films (7 μm thick) formed on Ti6Al4V from a phosphate electrolyte. However, the thin film has poor wear resistance due to the poor load support property [4].

In our previous work [6], a more thicker (up to 40 μm in 30 min treated time) coating composing mainly of rutile and anatase were developed, registering a high nanohardness and elastic modulus (5.5 and 69.1 GPa, respectively) in the compact layer, as well as a high adhesion strength (about 40 MPa) of substrate/coating interface. These coatings possess a porous outer layer, a compact inner layer and the adhesion strength is high, which is determined by the characteristic of microarc oxidation [10].

In this work, the coatings developed in literature [6] are used, further the duplex coatings of microrc oxidation combined with spraying graphite process were fabricated for the frictionless purpose. The tribological properties of unpolished, polished and duplex coatings against a steel ball under dry friction conditions were evaluated using a ball-on-disk wear tester and the possible wear mechanism was analyzed.

Section snippets

Specimen fabrication

Ti6Al4V discs (30 mm diameter and 3 mm thickness) were used as the substrate. The surface of the discs was polished with 600# abrasive papers to a roughness of ≈0.12 μm, and ultrasonically cleaned in distilled water, followed by acetone. The discs were used as anodes, while stainless steel plates were used as cathodes in the electrolytic bath. The samples carefully pretreated as above were microarc anodized for 30 min in accordance with process conditions elsewhere [6]. An aqueous electrolyte was

Microstructure of coating

Surface morphologies of unpolished, polished and graphite treated microarc oxidation coating are shown in Fig. 1. It is very obvious that the surface of the microarc oxidation coating is characterized by micropores of different sizes and shapes, and the porous feature strongly depends on discharging nature involved in the microarc oxidation mechanism. The as obtained coating, with thickness about 30 μm, is compact and uniform in the inner layer, and porous in the outer layer. Typically, the pore

Conclusion

The coatings mainly composed of nanocrystalline TiO₂ were deposited on Ti6Al4V alloy by microarc oxidation (MAO). The duplex coatings of microrc oxidation combined with spraying graphite process were fabricated for the antifriction purpose. The tribological properties of unpolished, polished and duplex coating against steel under dry friction conditions were examined. It is found that antifriction property of polished microarc oxidation coating is superior to that of unpolished one. The

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