Tribological properties of coatings obtained by electro-spark alloying C45 steel surfaces

Highlights

• Coated material distribution persists even after wear tests of ESD coatings.

• Wear behaviour is proportional to applied loads.

• Friction is more sensitive to load.

• Molybdenum coat has the best tribological properties.

• Bronze coat is efficient at lower loading.

Abstract

Various coating methods have become an essential way for surface strengthening; therefore new materials and coating technologies are being extensively studied. Bronze, molybdenum, and chromium, as well as a complex coating of T15K6 + bronze, were investigated in present study. The block-on-roll tribometer was employed to measure their tribological properties under boundary lubricating conditions. The results show that the molybdenum coating is the most stable, while the bronze coating has a low coefficient of friction in low-load applications. The observed tribological properties are closely related with mechanical properties. The surface characterisation and mechanical properties are in the line with results obtained by other scientists.

Introduction

Coatings are an essential way to strengthen and regenerate the surfaces of machine elements subjected to extensive wear. Several high-energy coating technologies have been used to produce amorphous alloy coatings, including laser cladding, vacuum plasma spraying, magnetron sputtering, arc metallization, chemical and physical vapour deposition, etc. All these methods have their own advantages and disadvantages and are used in specific applications. The electro-spark deposition (ESD) or electro-spark alloying technology has been investigated since the 1980s, and it has clear advantages over those coating technologies, because ESD is easy to implement using simple equipment that does not require high qualifications for its use; material and energy consumption is significantly lower compared to other coating technologies [1], [2]. The biggest advantage of ESD is extremely good coating adhesion to the substrate (it competes only with diffusion coatings in that regard), because it forms the coating through intensive mixing of the molten materials of the processing electrode (anode) and the substrate (cathode). The advantage of ESD compared to other methods is that it is unnecessary to prepare the surface prior to treatment, because the discharge torches clean any contaminants from the coating area. The equipment has good portability, because electrical pulse generators have a weight in the range of several kilograms and they can be used anywhere where single-phase current is available [1], [2], [3].

The wear-resistant coating on metallic surfaces is formed during a large number of short-time arcs when small particles of electrode material are melted, accelerated in the arc and impacted against the substrate [3], [4], [5]. Typically, the duration of the arc is very short (1–10 μs) relative to the interval between pulses. Therefore, with the ESD method, thin layers of deposited material can be made with minimum thermal impact on the substrate. This minimizes changes in the microstructure and mechanical properties of the substrate [2]. Management of the surface roughness for ESD coatings can be achieved by controlling the parameter selection of the pulses and their sequence [6].

There has been a great deal of research into the deposition and mechanical properties of coatings. Z. Chen and Y. Zhou introduced a comprehensive study on hard-layer deposition on a resistance-welding electrode. They found that the affinity between the coating and the substrate should be taken into account in order to get high-quality coatings [2]. The structure and mechanical strength of ESD coatings, such as titanium carbonitride (TiCN) and tungsten carbide (WC) were compared by X-ray, SEM, TEM, AFM microscopies and other surface investigation methods. It was found that the coatings produced void- and impurity-free interfaces, but that the interfaces are drastically different, which could be explained based on the different melting points of the materials [7]. Another study of a WC92–Co8 coat on titanium substrate shows that a thick and dense coating layer with rare cracks is obtained using electro-spark coating [8].

A study was also carried out on strengthening the ESD coating by introducing nanoparticles [9]. The cracking of the thicker ESD coating was found to be a disadvantage of this method. Laser treatment after deposition was suggested in a few studies [2], [10]. The deposition of amorphous layers of various metallic materials shows the versatility of ESD [11]. The investigation of a hard chromium carbide ESD coating shows the increased wear resistance of such a coated surface at dry friction conditions due to significantly higher hardness compared to a steel surface [12].

Electro-spark alloying has become more and more popular in surface processing technology. ESD coatings are frequently applied in industry, for instance, to produce and strengthen implants, biological cell development at substrate surface, cutting tool inserts or friction pairs of piezoelectric actuators [10], [13], [14], [15].

However, there is a lack of information on the tribological properties that are essential for application definition. The aim of present study is to evaluate the tribological properties of selected ESD coatings in boundary lubrication conditions. For this study, coating materials were selected that can form a wear-resistant hard surface (molybdenum, chromium and T15K6 carbide) and a metal which has good anti-frictional properties (bronze).