Influence of graphite content on the tribological properties of plasma sprayed alumina-graphite coatings

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Abstract

The main aim of this research was to investigate the influence of graphite as an additive to a plasma-sprayed alumina coating matrix for tribological trends. For the deposition process, pure alumina and alumina with graphite (5%, 10% and 15%) weight percentage powders were used. The graphite-modified alumina coatings were formed on steel substrates by atmospheric plasma spraying using an air-hydrogen plasma. The influence of graphite content on the elemental composition, phase structure, friction coefficient, and specific wear rate was investigated. It was obtained that the addition of graphite enhanced the γ-Al2O3 phase content in the as-sprayed coatings. The highest γ-Al2O3 phase fraction was obtained when the amount of graphite in the feedstock powders was 10 wt.%. The elemental composition results indicated that the concentration of carbon in the alumina-graphite coatings was up to six times lower in comparison to the initial feedstock, and had varied from ∼1.7 to 2.45 wt.%. The incorporation of graphite slightly reduced the friction coefficient of the composite coatings due to the formation of a self-lubricating layer under dry-sliding conditions. The specific wear rates were in the range of ∼10−6 mm3/(Nm) for the alumina-graphite composite coatings, which were similar to the alumina coating, and were an order lower compared to the wear rate of steel.

Introduction

Atmospheric plasma sprayed alumina (Al2O3) coatings are widely used as protective coatings in high temperature applications, for wear resistant utilities, and as corrosion inhibitors. Al2O3 coatings have been used to coat various industrial metal components such as valves, engines in order to increase the lifetime of metal parts [1], [2], [3]. Sliding and abrasive wears as well as a high friction coefficient are serious problems in industries where a two-body contact is present. The friction coefficient of different Al2O3 coatings formed by plasma spraying is in the range of 0.5–0.9 under dry-sliding conditions. The plastic deformation, micro fracture, and splat spallation are the main reasons for the degradation of the Al2O3 coating under the non-lubricated sliding conditions [1], [2], [3], [4], [5], [6], [7], [8], [9].

The researchers are working in order to obtain the optimal experimental conditions and suitable additives to improve the tribological as well as the mechanical or the anti-corrosive properties of alumina coatings. It was demonstrated that various metal oxide materials such as TiO2, ZrO2, Cr2O3, CeO2, etc. are widely used to enhance the tribological properties of Al2O3 coatings deposited by thermal spraying [1,[9], [10], [11], [12], [13], [14], [15], [16], [17], [18]]. By the addition of low quantities of TiO2 (up to 13 wt.%) to Al2O3 coatings the lower friction coefficient, higher wear resistance, lower porosity and an enhancement in fracture toughness was obtained [10,[17], [18]]. The tribological and mechanical properties of aluminum oxide coatings can be improved by adding self-lubricating materials. The improvement of the properties of alumina coatings by addition of various graphite based materials such as graphene [19,20], carbon nanotubes (CNT) [21], [22], [23], [24], graphene oxide (GO) [24], [25], [26] or graphite [4,27] was demonstrated by several authors. It was obtained that the amount and initial properties of the used graphite-based materials has a huge influence on the final tribological properties of the alumina coatings [19], [20], [21], [22], [23], [24], [25], [26], [27]. Y. Li et al. [26] obtained that the alumina coatings reinforced with a low amount (1 and 2 wt.%) of graphene oxide demonstrated up to 31% lower porosity values. Meanwhile, the friction coefficient of the coatings was reduced from 0.52 to 0.44, and the specific wear rate by 80 % under 30 N loads. J.W. Murray et al. [19] demonstrated that alumina reinforced with 1 wt.% of graphene nanoplatelets allows to reduce the friction coefficient from 0.40 to 0.35. However, the friction coefficient (varied from 0.35 to 0.55) and wear rate (was up to 3×10−5 mm3/Nm) of alumina-graphene coatings depended on the load used [19]. The tribological properties of alumina coatings were improved with the addition of CNT due to the increase in hardness, fracture toughness and a reduced porosity [23]. The corrosion resistance of alumina coatings was enhanced with the addition of GO (up to 2 wt.%) into the feedstock powders [25]. A.K. Keshri et al. [21] demonstrated that the splat diameter increased with the reinforcement of alumina with CNT and it could result in higher cohesion values. Our previous studies indicated that the addition of graphite is an effective way to reduce the friction coefficient and enhance the wear resistance of alumina-graphite coatings. However, the tribological properties are greatly influenced by the amount of graphite in the coating, besides the type of alumina used in the feedstock powder [4,27].

Despite the increasing number of studies related to the formation of aluminum oxide – carbon based composite coatings deposited using plasma spraying, and investigation of these coatings properties, there are many unanswered questions. For instance, the challenges in finding the optimal graphite concentration; determining how the graphite concentration in feedstock powder correlates with the graphite amount in the sprayed coating; finding the optimal spraying conditions for the coatings which result in the improvement of tribological properties; identifying the influence of the used load on the tribological properties, etc. are imperative to be addressed.

The main aim was to spray Al2O3-graphite coatings using atmospheric pressure plasma spraying, and determine the influence of graphite concentration on the surface morphology, phase structure, and the tribological properties of alumina-graphite composite coatings.

Section snippets

Experimental setup

Alumina and alumina-graphite coatings were deposited on P265GH steel (dimensions of 40 × 10 × 6 mm) by atmospheric plasma spraying. The coatings were formed using Al2O3 (ALO-101, non-regular shape, size ∼45 μm, Praxair Surface Technologies, USA) and graphite (MOLYDUVAL Fondra NS, size <25 μm (90 %), size <10 μm (10 %)) powders (Fig. 1). The graphite flakes had irregular shape and size of particles was from 5 to 25 μm (Fig. 1b) [27]. The graphite powder was added into the Al2O3 feedstock at

Results and discussions

The surface morphology images of the as-sprayed Al2O3 and Al2O3-graphite coatings are presented in Fig. 2. It can be seen that the surface of all the coatings look homogenous and is composed of fully molten splats and partly melted micrometer size particles (Fig. 2). Spherical and irregular shaped particles of sizes from 1 to 20 µm could be found on the surface of all coatings. The appearance of some pores, micro-size voids, and micro-cracks, could be observed on the surfaces of the as-sprayed

Conclusions

The alumina and graphite-modified alumina coatings were deposited by atmospheric plasma spraying using an air-hydrogen plasma. The EDX measurements indicated that the graphite concentration in the as-sprayed coatings was significantly reduced compared to the feedstock powders due to partial sublimation in the plasma. The graphite concentration was increased from ∼1.7 wt.% to ∼2.5 wt. % with the increase of graphite amount in the feedstock powders from 5 wt.% to 15 wt.%. The increase of alumina

Declaration of Competing Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Acknowledgements

This research was supported by the Research, Development and Innovation Fund of Kaunas University of Technology (project grant No. PP-88G/19) and Internal funding of Lithuanian Energy Institute. The authors would like to acknowledge the contribution of the COST Action CA15107 (MultiComp). The authors also acknowledge the financial support from the Slovenian Research Agency (ARRS), Slovenia (research core funding No. P2-0231).

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