*2.1. Materials and Electrolyte*

In this investigation, 3D-printed Inconel 718 samples of size 10 × 10 × 5 mm<sup>3</sup> using the LPBF process were manufactured using SLM<sup>R</sup> 500 machine (SLM Solutions, Wixom, MI, USA) of maximum power 400 W. The deposition parameters for the sample preparation were: Laser power: 165 W, scan speed: 800 mm/s, and layer thickness: 0.4 mm. Detailed manufacturing details are given elsewhere [7]. The porosity of <1% in the LPBF-processed sample has been observed. The microstructure of the as-fabricated cube sample showed large and interconnected directional columnar grains with random columnar grain as discussed in detail in [7]. A wrought Inconel 718 (purchased from McMaster Carr, Elmhurst, IL, USA) has been used as a control material for comparison of the properties being investigated. The chemical composition of both wrought and as LPBF samples qualifies as ASTM B637 for Inconel 718 alloy [17]. The hardness measurement was carried out using a nano-indenter (T1950 Triboindenter, Hysitron, Minneapolis, MN, USA) equipped with a Berkovich indenter. The nanoindentations were performed at different locations using a maximum force of 5 mN. The minimum distance between two indentations was 100 μm. The hardness value of AM Inconel is 5.36 ± 0.30 GPa, which is slightly higher than the hardness value of Wrought Inconel (4.93 ± 0.27 GPa).

The tribocorrosion studies were performed on as LPBF and as purchased wrought samples without any further heat treatment. For this study, square plate samples of Inconel 718 (both the LPBF and wrought) with a 1-inch width and 0.5-inch thickness were used to evaluate its tribocorrosion behavior. The surface of the samples was polished using SiC sandpapers with grit sizes starting from 320 (~46 μm abrasive particles) up to 1200 (~15 μm abrasive particles), followed by fine polishing using three-micron diamond suspension until a surface roughness conditions of *Ra* = 0.1 μm was achieved. The counter material for the tribocorrosion testing was a non-conductive ceramic material, alumina ball of 6.35 mm diameter. The present tribocorrosion study simulates a corrosive lubricated environment according to ASTM B895-99 [18], where 3.5% NaCl solution is used as the electrolyte/lubricant. The test parameters in the present study concentrated on analyzing the tribocorrosion behavior in laboratory conditions. The real applications of AM Inconel 718, such as those in aircrafts may have much more complex boundary conditions, which will include, but may not be limited to, fretting and other vibrations, reciprocating sliding, and other such degradation effects.

### *2.2. Open Circuit Potential (OCP) Measurements*

The OCP measurements were carried out on wrought and AM Inconel 718 surfaces to understand and compare their open circuit kinetics in 3.5% NaCl. The OCP, also known as the corrosion potential is a mixed potential that depends on the rate of the anodic as well as the cathodic reactions. If the corrosion cell involves one dissolution reaction and one cathodic reaction, the corrosion potential will be between the reversible potentials of the two half-reactions. Understanding this evolution of OCP will provide an insight into the kinetics of corrosion without wear. The experiments were conducted using threeelectrode configuration, which consisted of Inconel 718 surface as the working electrode, standard calomel electrode (SCE) as the reference electrode, and 99% pure graphite as the counter electrode. Figure 1 schematically illustrates the tribocorrosion test setup used for the experiments. The OCP was measured using a Gamry reference 3000 potentiostat. All potential in the present study is reported with reference to SCE, which has a potential of 0.250 V vs. SHE (standard hydrogen electrode) at 25 ◦C. A working electrode area of 2.5 cm<sup>2</sup> was exposed to the electrolyte. Each test was carried out three times to ensure repeatability of results.

**Figure 1.** Tribocorrosion test setup and view of the Inconel 718 sample/working electrode.
