Journal of Materials Exploration and Findings (JMEF) Journal of Materials Exploration and Findings (JMEF)

: The effect of hard chromium coating on AISI D2's mechanical properties and wear resistance has been investigated using the electrolysis technique with varying current and coating duration. The variations of current used are 0.8 A, 1.2 A, and 1.6 A, while the coating duration used are 300, 600 and 900 seconds. Characterization of the films was conducted using SEM-EDS, and XRD. It can be observed from SEM characterization that the chrome grains resulted from the electroplating process are fine. The XRD identify chrome compund on the surface coating. The highest hardness value was 520.6 HV on a sample with a current of 1.6 A and a coating duration of 900 seconds and the lowest wear value was 2.85x10 -6 mm 3 /mm on a sample with a current of 1.6 A and a coating duration of 900 seconds .


Introduction
The trend of miniaturization in various products has shifted the focus of research studies towards the quality, efficiency, and effectiveness of products in micro sizes. One promising fabrication method for mini-sized metal products is micro/meso-forming. The problem in forming small-sized metal is the occurrence of characteristic deformation changes due to miniaturization. This problem is known as the size effect. The smaller the dimensions of the dies, the greater the coefficient of friction that occurs between the dies and the workpiece. This results in an increase in pressure, causing the material on the dies to wear out quickly. Therefore, the material used for the dies must have high hardness and wear resistance to avoid producing defective products (Sulaiman et al. 2019;Dhaneswara et al. 2022;Hadi et al. 2023;Dhaneswara et al. 2018).
Chromizing or chrome plating is a surface engineering method widely used in industries to obtain a hard metal surface. This method is preferred because it is easier to apply and more economical as it does not require high temperatures, operator skills, vacuum space, and complex equipment. Chrome plating has successfully increased hardness, wear resistance, and corrosion resistance (Yongqing et al. 1998;Aydin et al. 2014;Jagielski et al. 2000). The common method used for coating is electric coating (electroplating), which is a process of coating metals or non-metals by electrolysis using direct current (DC) and a chemical solution (electrolyte) that serves as a supplier of metal ions to form a metal layer on the cathode. Yose Rizal et al. successfully conducted hard chrome plating using the electroplating method by varying the CrO3 electrolyte solution and plating time, with a current of 5 A, an anode-cathode distance of 20 cm, and a plating temperature of 55 0 C (Rizal et al. 2022;Al Hijri et al. 2022). The plating process was carried out based on different time variations, namely 30, 45, and 60 minutes. The results showed an increase in wear resistance of 60%.
In this study, hard chrome plating will be carried out with the aim of determining the optimization of the plating process parameters, namely the current and coating duration. Parameter variations are carried out to produce different chrome deposits on the substrate surface, so that the effect of these parameters on the mechanical and tribological properties of the coating on the substrate surface can be determined.

Research methodology
The AISI D2 material was prepared with a diameter of 15 mm and thickness of 5 mm by cutting using cutting tools. Then, pretreatment was carried out by degreasing with 10% NaOH for 5 minutes, followed by polishing with abrasive paper with grits of 100-5000 mesh. The chemical composition was shown in Table 2.
Then, chrome plating was performed using the electroplating method with an electrolyte solution composition of H2CrO4 and H2SO4 with a concentration of 250 gr/lt and 2.5 gr/lt. Variations in current density and coating duration parameters were given according to Table  1. The plating process was conducted in the metallurgy laboratory of Sultan Ageng Tirtayasa University.
The electroplating process scheme was shown in Figure 1, where there was an anode part which is an inert metal in the form of stainless steel and the cathode part is a AISI D2 substrate. A rectifier is used as a DC current source with options of current of 0.8, 1.2, and 1.6 A. The electrolyte solution used is a mixture of H2CrO4 and H2SO4 with a constant concentration.
The results of the coating were measured before and after plating to determine the weight generated by the hard chrome plating process. With information about the weight generated, the cathode efficiency can be determined for the plating parameters of current density and coating duration. In addition, the layer was characterized using X-ray diffraction (XRD) to determine the compounds formed on the surface. The phase composition of the layers was investigated using a Cu K and PAN analytical AERIS X-ray diffractometer (XRD). Furthermore, scanning electron microscope (SEM-EDS) characterization was performed to observe the morphology and elemental content on the surface and were measured using HITACHI FLEXSEM 100 scanning electron microscopy equipped with an energy dispersive spectroscopy (EDS) which was used to measure the elemental composition of material substrates after the deposition process. Vicker hardness testing was conducted to observe changes in hardness value. The substrate's microhardness was measured using a Matsuzawa MMT-X7 microhardness tester with a 10 gf indenter load. In addition, wear resistance testing was carried out using the Ogoshi method.   Figure 2 shows the results of the hard chrome coating process on the AISI D2 substrate for sample 1, 2, and 3. It can be observed that the coating layer for sample 3 failed because of blisters and peeling that occurred due to the high current density. An increase in current density can raise the temperature around the electrolyte, including the substrate, resulting in a darker coating surface. Moreover, an excess of current density will generate nonhomogeneous deposits on the substrate surface and cause a decrease in adhesion strength.

Cathode Efficiency
Cathode efficiency is the ratio between the weight of the coating that adheres to the substrate and the theoretical weight of the coating (Zarembo & Zarembo 2022). The weight of the coating can be determined by measuring the difference in weight of the substrate before and after the coating process. The theoretical weight can be calculated using Faraday's law. According to Faraday's first law, the amount of substance produced at an electrode during electrolysis is directly proportional to the amount of electric charge passed during electrolysis, according to Eq.1 below: where B is coating weight, l is current, t is coating duration, e is equivalent weight, and F is the faraday constant (96500 C). This means that the weight of Cr deposited on the cathode (AISI D2 substrate) will increase proportionally with the increase in current density and coating duration. Table 3 shows the cathode efficiency of each sample, where the highest cathode efficiency value was obtained from sample 7 with a current density of 1.6 A and a coating duration of 300 seconds, with a value of 51.16%. From the table, it can also be observed that the effect of coating duration on cathode efficiency is inversely proportional. Increasing the coating duration will result in an increase in H2 gas produced from reduction at the cathode, which can disturb the process of ion Cr adhesion to the cathode.

Hardness Testing
Surface hardness is one of the important requirements in machining processes. Hardness testing in this research is conducted using the Vickers method. The testing is carried out at 3 different points on each sample, in the middle, right, and left parts of the sample. In this hardness test, a pressing force of 1000 gf and a pressing time of 10 seconds were used. The hardness of the sample was measured before the coating treatment and a hardness value of 216 VH was obtained. The highest value of hard chrome coating hardness was found in the sample number 9 with a current treatment of 1.6 A for 900 seconds, which was 529 VH. Therefore, the hard chrome coating has successfully increased the hardness value by 144%. Figure 3 shows the graphs of the effect of coating duration and current on hardness value. It can be seen that an increase in coating duration and current can increase the hardness value. The increase in hardness value with coating duration variation is caused by an increase in the amount of Cr ion particles. With a longer coating duration, more Cr ion particles will be deposited onto the substrate, thus increasing the hardness value of the formed coating.
The increase in current given to the sample also results in an increasing hardness value. This is because the reaction that occurs will be faster, so that more Cr ions are formed. It is also true that a larger current produces a larger actual mass. However, a current that is too large will produce a lot of hydrogen gas and hinder the deposition of Cr on the surface of AISI D2 steel.

Wear Resistance
The wear testing in this research uses the Ogoshi method where the test object obtains frictional load from a revolving disc. The contact between the surface of the test object and the revolving disc will produce a surface trace. The size of the surface trace from the rubbed material is used as the basis for determining the level of wear on the material. The larger and deeper the wear trace, the higher the volume of material that is peeled off from the test object (Fatriansyah et al. 2019). Using the Ogoshi method, mass loss is generated which can then be used to calculate the wear rate. The load used in the test is 3.16 kg, the rotation speed is 1.97 m/s, the thickness of the ring is 30 mm, the diameter of the ring is 3 mm, and the sliding distance is 100 m. Before testing the wear resistance of the samples obtained from the coating process, wear resistance measurements were carried out on the as-received samples 86 to determine the effect of the coating results on the wear resistance value. The obtained wear rate of the as-received sample was 5.72 x 10 -6 mm 3 /mm. The lowest wear rate of 2.85 x 10 -6 mm 3 /mm was observed on the sample coated with a current of 1.6 A for 900 seconds for sample number 9. The hard chrome coating treatment has successfully improved the wear resistance significantly. Figure 4(a) shows the relationship between current density and wear rate. It can be seen that an increase in current density results in a lower wear rate. This is because an increase in electrical current enhances the migration of Cr ions from the electrolyte solution to the cathode and produces more chromium on the surface. Figure 4(b) displays the relationship between coating duration and wear rate. It can be observed that an increase in coating duration leads to a lower wear rate. This is because as the coating process prolongs, more Cr ions flow and adhere to the cathode surface. Chromium itself is known for its good wear resistance properties and high hardness.

XRD Characterization
Using XRD (X-Ray Diffraction), the AISI D2 steel with a strong chromium plating was characterized. This was done to identify the compounds formed on the surface of the sample after the hard chrome plating process. Figure 5 shows peaks confirmed to be Cr in the amount of 3, at angles 2θ of 44.4815°, 64.641°, and 81.809°. With crystal orientations of each peak at [110], [200], and [211], the identified crystal structure shape is body-centered cubic (BCC).

Surface Morphology
The SEM (Scanning Electron Microscope) characterization was conducted to analyze the morphology of the hard chrome layer surface. In Figure 6, it can be observed that the chrome grains resulted from the electroplating process are fine, with an average grain size of 3 microns. The hard chrome layer formed on the fine AISI D2 substrate due to the high nucleation rate, resulting in globular formation. The homogeneity of grain size and the distribution of chrome elements on the layer would improve its hardness and wear resistance value To determine the amount of chromium on the sample surface, EDS characterization was performed in Figure 7. EDS has detected an area of the surface, which is represented by Part 002. Figure 7 represents the peaks of the elements on the detected surface. Table 4 shows that the percentage of chrome element on the surface is 94.71%, indicating that the hard chrome plating process was successfully carried out on the surface of the AISI D2 substrate. The carbon and manganese elements found in the EDS characterization result are the base metal content of AISI D2 substrate that moves interstitially to the hard chrome layer and contributes to increasing the hardness and wear resistance of the layer.

Conclusion
Based on the research and discussion conducted, the following conclusions can be drawn. The Hard Chrome layer has been successfully deposited onto AISI D2 substrate, as confirmed by the presence of Chrome peaks in XRD analysis and a Chrome content of 94.71% in EDS characterization. The highest weight of Chrome deposit on the substrate surface was obtained at a coating duration of 900 seconds and a current of 1.2 A, with a value of 0.038 grams. Meanwhile, the highest cathode efficiency was obtained at a coating duration of 300 seconds and a current of 1.6 A, with a value of 51.03%. The highest value of hardness of the coating was found in the condition of 900 seconds and 1.2 A current, which is 520.6 HV. This condition indicates a hardness value of 141.01%. The best wear resistance is also found in the same condition, with a wear value of 2.85 x 10 -6 mm 3 /mm. The wear resistance increased by 50.21% from the wear of the base metal.