Effect of Load and Sliding Distance on Tribological Properties of Aisi 1040 Steel by Magnetron Sputtered Ti-6Al-4V Coating

In the present study, Ti-6Al-4V-2B4C coatings are deposited using a DC magnetron sputtering system on AISI 1040 steel substrates. The composition and elemental confirmation of the coating was explored by SEM, XRD and EDS. Wear and friction tests were trained to study the influence of sliding distance and load on tribological behaviour of Ti-6Al-4V-2B4C coated AISI 1040 steel for machine elements. Wear tests were performed on coated AISI 1040 steel substrates counter to E-52100 steel balls by using a ball-on-disc wear testing apparatus under 2N and 3N normal loads at room temperature. The different sliding distances (63, 78, 94 and 110 m) has taken for wear test under room temperature and low coefficient of friction obtained for higher sliding distance and load. Ti-6Al-4V-2B4C coated AISI 1040 steel reduces the frictional coefficient from 0.65 to 0.23 and improves the wear resistance. Specific wear rate of the coating reaches the maximum value with higher sliding distance. On the outset, it is clearly showing that the formation of oxide layers of Ti-6Al-4V-2B4C coatings mainly affects its tribological characteristics.


INTRODUCTION
Aeronautical and medical fields are focusing on Ti-6Al-4V alloy (titanium grade-5) in line for its solitary strength-to-weight proportion [1]. Industrial machinery and components are habitually failed during working due to surface damage [2]. It is also has been rectified through surface modifications techniques especially by wear-resistive coatings. High-quality films with a good vary of industrially necessary have been developed through magnetron sputtering deposition technique [3,4]. One of the familiar materials in the engineering field is carbon steel AISI 1040 and properties enhancement is achieved by various hard coatings [5].
Nitrides and carbides with higher hardness take on a vital role in the surface designing field. Carbide coatings deliver higher hardness than nitride coatings which exhibit low wear nature on steel [6]. Applications of boron carbide coating are boundless; such as fusion reactors and coatings on nozzles etc. Higher hardness has been achieved through B 4 C coatings on steel and it is often used as fusion reactors [7,8]. In this study, wear characteristics of Ti-6Al-4V-2B 4 C coated AISI 1040 steel is studied by a ball-on-disc wear test machine. The sliding distance and load were selected with a suitable range of working environments for genuine machine components. Figure 1: SEM of (a) Pure AISI 1040 steel (b) Ti-6Al-4V-2B 4 C Green compact Fig. 1(a) shows the surface morphology of the pure AISI 1040 steel. The target is the prepared forms of pure Ti, Al, V and B 4 C powders. Pre-compacted Ti, Al, V and B 4 C raw powders exhibit an irregular shape is shown in Figure 1 (b). AISI 1040 steel is coated through the magnetron sputtering process with prepared Ti-6Al-4V-2B 4 C target. Substrates of 3 mm thicknesses with 60 mm diameter AISI 1040 steel plates are taken for coating. Before sputtering the chamber was vacuumed to 1x10 -5 bar for a free flow of atoms in the chamber.

MATERIALS AND METHODS
The temperature of 100 ± 20 °C was kept up during the coating with 0.5h and 1h coating time duration has been fixed for all coatings. Gas pressure range of (Ar) 0.001 to 0.1 bar has been maintained as during sputtering coating. Wear test carried out by ball-on-disc wear tester on

Frictional behavior
The present discussion focused on the effect of B 4 C reinforcement on frictional behavior coatings. In sliding contacts, CoF varies with sliding distance and loads; the reinforcement plays a major role in reducing the CoF. At the initial stage, the contact between the surfaces of a specimen in the ball and disc led to low frictional force, so that CoF decreases due to the ploughing effect. The reinforcement from the coating is protruding out at results in the dropping of CoF. It shows that CoF decreases gradually due to the development of an oxide layer in between the disc and ball during sliding. During sliding, heat is generated by friction between contact surfaces in addition to the adhesion of flat and spherical surfaces of sliding. Due to the relative motion of the counter surface, the continuous generation of frictional heat and low heat dissipation rate occur due to time lacking. Again, as a result of the superior flow ability of the coating surface and more sliding action; frictional heating is reduced and subsequently, the coefficient of friction is also reduced. Based on load and sliding distance increment; the formation of the transfer layer increases gradually. The transfer layers were found in between the sliding surfaces for sliding distance higher than 63 m. Besides, when the sliding distance increases more than 63 m wear rate also increases accordingly. The formation of the transfer layer controls the wear behaviour of sliding surfaces due to its lubricating nature. Figure 8 (a) illustrates the worn surface analysis of Ti-6Al-4V-2B 4 C coated AISI 1040 steel by SEM which includes transferred oxide layers and Figure.

CONCLUSIONS
Wear behaviour of Ti-6Al-6V-2B 4 C coating was investigated and results are drawn.
 The Ti-6Al-6V-2B 4 C coatings are not ruptured up to 110 m sliding distance under the normal atmospheric test conditions.  The lubricating film is created between coating and E-52100 ball during sliding by developed oxide layer which supports to improve the wear resistance also gradually decrease the CoF for higher loads and sliding distances.
 The Ti-6Al-6V-2B 4 C coatings delivers better wear performance even for higher loads and sliding distances; thus E-52100 steel ball shows higher wear rates.  As an outset, Ti-6Al-6V-2B 4 C coated AISI 1040 steel is a good replacement for engineering applications such as bearings, tappets, gudgeon pins, etc.