Processing of Spark Plasma Sintered Fe Alloy and Enhancing Its Surface Properties by AlCrN Monolayer Coating by Cathodic Arc Plasma Physical Vapor Deposition Process

: The current investigation observes the outcome of enhancing the surface properties by AlCrN monolayer coating using the cathodic arc plasma method on the Fe–Cu–C–Mo alloys. The compacts were sintered in spark plasma sintering (SPS) with the heat transfer rate of 100 ◦ C / min at 1120 ◦ C for 5 minutes. The Fe–2Cu–0.8C–0.6Mo sample has the highest relative sintered density (97.20%), hardness (96 HRB), and ultimate tensile strength (1000 MPa) compare with the other sintered compacts. AlCrN coating was deposited on Fe, Fe–2Cu, Fe–2Cu–0.8C, Fe–2Cu–0.8C–0.2Mo, Fe–2Cu–0.8C–0.4Mo, and Fe–2Cu–0.8C–0.6Mo samples, using the cathodic arc plasma–physical vapor deposition (CAP-PVD) process. The coated compact samples’ metallography images were examined using a Scanning Electron Microscope (SEM); the Fe–2Cu alloy sintered sample has obtained a uniform structure with high density and a smaller amount of corrosion penetration rate (0.6579 mmpy) as compared to the counterparts. The phase formed in the AlCrN coating was analyzed using the X-ray Di ﬀ raction (XRD). The Fe–2Cu–0.8C–0.6Mo coated compact sample exhibited higher hardness (1134.85 HV 0.3 ) than the other coated compact samples. The Fe–2Cu–0.8C–0.2Mo coated compact sample has proven better corrosion resistance compared to the other coated compact sample.


Introduction
Pure iron is soft and ductile. The alloying elements such as copper, chromium, nickel, graphite (C), and molybdenum are added in iron to enhance the mechanical properties such as strength, corrosion resistance, wear-resistance, and hardness. During the sintering process, copper atoms diffuse slowly in iron powder, form swelling (copper growth), and form a solid solution. The graphite is further added to iron to enhance the strength, hardness by forming a pearlite structure and to control the swelling of copper. The tensile property indicates cup and cone fracture for Fe-Cu and Fe-Cu-C alloys. The production cost is low, and close dimensional accuracy can be maintained for these alloys. Hence it is mostly used in automotive applications [1]. The products produced by the powder metallurgy technique are widely used due to the easy formation of complicated shapes and saving manufacturing time. A heat-treatment process can obtain the high hardness of the sintered The TiCN coating layer was deposited on stainless steel 316L using the PVD process. The corrosion and wear studies were conducted on the coated substrates. The results have proven that the corrosion and wear resistance is higher for TiCN coated samples. The TiCN coated sample phase analysis indicates that the NaCl Cubic FCC (Face Centered Cubic) structure. The TiN and TiC structures were obtained with sufficient adhesion strength with the base material [16]. The two types of chromium coatings such as 1. Cr 2 N (20%) & Cr (80%) and 2. CrN was obtained on S.S. (Stainless steel) 316 L by varying the nitrogen gas flow rate.
The corrosion test was carried for the S.S. 316 L coated samples. The results have shown that the CrN coating obtained on S.S. 316 L has higher corrosion resistance due to lesser interfacial contact resistance than the other coated samples [17]. S. Zhang et al. illustrated that AlCrN coating is better corrosion resistance than the TiN based coatings [18]. The pure Fe, Fe-2Cu, Fe-2Cu-0.8C, Fe-2Cu-0.8C with varying weight percentages (0.2%, 0.4%, and 0.6%) in the present work molybdenum compositions compacts were fabricated by using a spark plasma sintering process. Their mechanical properties for the parent materials were studied and reported. The current literature evidences the significance of AlCrN coating on Fe-Cu alloys to improve the surface properties. In the present study, the cathodic arc plasma-physical vapor deposition (CAP-PVD) process was used to deposit AlCrN coating on the parent materials. The corrosion rate of the sintered compacts before and after the coating was investigated. The characteristics of AlCrN coating were analyzed and assessed for its suitability for automotive applications.

Sintering Process
The SEM images of the carbonyl Fe, Copper, Graphite, and Molybdenum powders are shown in Figure 1. The microstructures of the as-sintered samples were shown in Figure 2. The carbonyl Fe has obtained nano circular powder particles with high density and uniform structure. The graphite powder has obtained irregular flake particles, as shown in the SEM image. The characteristics of carbonyl Fe, copper, graphite, and molybdenum powders are shown in Table 1. The powders were then consolidated by employing a DR. Sinter 21,050 SPS furnace. In SPS technology, a conductive die is taken, into which powdered material is loaded. Uniaxial pressure (30 MPa) is applied, and at the same time, a pulsed electric current of voltage (4-20 V) and amperage (0.5-20 kA) is supplied through the die. The sintering temperature was monitored by making use of a thermocouple. The sintering temperature and time were set as 1120 • C and 5 min respectively in a vacuum chamber to prepare the compacts, and then it was kept for furnace cooling. The SPS furnace was maintained with a vacuum pressure of 2 Pa. The spark plasma generated in the furnace was used to increase the powder compact's hotness with a 100 • C/min heating rate. The temperature was measured using a K-type thermocouple. The metallography, tensile, and hardness samples were obtained from solid circular disks formed by the spark plasma sintering process, and the cutting operation is performed using EDM wire-cut.
The Archimedes measurements technique was employed to measure density after sintering. A metallurgical microscope was used to obtain the surface morphology of the sintered samples. Silicon Carbide emery sheets of 220, 320, 500, 800, and 1000 were used to burnish the samples. Cloth polishing techniques was further adopted by making use of a suspension of 0.05 µm Al 2 O 3 mixed with H 2 O. Then, the sample surfaces were subjected to microstructural analysis by employing a DM2500 Leica optical microscope (Leica Microsystems, Mumbai, India). The etching was carried out with a 5% initial solution. For the tensile testing, micro tensile samples are prepared with EDM according to the ASTM-E8 standards [19]. The tensile test is conducted in the universal testing machine (Instron 8801, University Ave Norwood, Norwood, MA, 02062-2643, USA) of full-load 20 kN at an initial strain rate of 3.3 × 10 −4 ·s −1 . To obtain the average value, five sample trails were tested and reported. The Rockwell hardness tester (Technofine Equipment, Model: TRSN-D, Kolkata, India) with a 16-inch ball indenter was used, and a load of 100 kg was applied to obtain the bulk hardness of the sintered compact on an HRB scale. method, three electrodes were used, such as (1) the working electrode (the coated test samples were exposed to 1 cm 2 square area), (2) Platinum counter electrode, and (3) Saturated Calomel Electrode (SCE) as reference electrode was utilized to conduct the electrochemical corrosion testing. The working electrode was immersed for 30 min in the corrosive medium to obtain the open circuit potential (OCP) under steady-state conditions earlier to each electrochemical process. The AlCrN monolayer coated Fe-Cu alloy samples were investigated using a potentiodynamic polarization technique at average room temperature with a 3.5% NaCl solution to obtain the corrosion resistance. The Tafel curve parameters are a scan rate of 10 mV/s, scan area of 1 cm 2 , and potential range of −1.444 to +0.556 V.       The powders' microstructure, tensile fracture surface, and coating characteristics were analyzed using a Scanning Electron Microscope (Model: Research-EVO-18, Carl Zeiss, Jena, Germany) with EDAX. The Fe-Cu coated samples' micro-hardness was measured using the Vickers's micro-hardness tester-Matsuzawa Instruments, (Model: MMT-X, Matsuzawa Co., Ltd. 120-19, Aza-Nanamagaridai, Toshima, Kawabe, Akita-shi, Akita Pref, 019-2611, Japan) of 10 s as dwell time. The XRD analysis was conducted using a Powder X-ray Diffractometer (Model: Advance-D8-BRUKER, Kolkata, India). The corrosion test was studied by employing the potentio-dynamic polarization testing apparatus (Technologies Advanced electrochemical system-IVIUM) (Model: Version 54800, AJ Eindhoven, The Netherlands). In this corrosion testing method, three electrodes were used, such as (1) the working electrode (the coated test samples were exposed to 1 cm 2 square area), (2) Platinum counter electrode, and (3) Saturated Calomel Electrode (SCE) as reference electrode was utilized to conduct the electrochemical corrosion testing. The working electrode was immersed for 30 min in the corrosive medium to obtain the open circuit potential (OCP) under steady-state conditions earlier to each electrochemical process. The AlCrN monolayer coated Fe-Cu alloy samples were investigated using a potentiodynamic polarization technique at average room temperature with a 3.5% NaCl solution to obtain the corrosion resistance. The Tafel curve parameters are a scan rate of 10 mV/s, scan area of 1 cm 2 , and potential range of −1.444 to +0.556 V.

Coating Process
A standard Oerlikon Balzer's coating equipment was used for the cathodic arc plasma-physical vapor deposition (CAP-PVD) process. The outline diagram of the CAP-PVD system is shown in Figure 3. The customized sintered circular targets material of 70 at.% Al and 30 at.% Cr were located inside the compartment during the AlCrN monolayer coating process [20]. The details of the AlCrN monolayer coating process parameters are shown in Table 2. Before AlCrN deposition, all the Fe-Cu alloy samples were cleaned with an ultrasonic cleaning machine and heated with hot air to remove the moisture content. The cleaning process was 1.4 h.   The hard coating was obtained by plasma reaction, assisted by the reactive gases N2 throughout the coating practice. The N2 gas was quickly decomposed, and the reaction occurred in the plasma region, which forms the ionized metal ions, as hard thin films on the substrate surface. The CAP-PVD process is stable without oxidation using the inert environment bypassing the Ar gas into the coating compartment. The Ar and N2 gas pressures are set based on the chamber pressure. The sample temperature was measured before deposition, using K-type thermocouple placed near the sample and maintained at 400 °C with temperature sensors. Throughout the CAP-PVD method, 150 A was set as a substrate current, and 30 V was set as a bias voltage.

Density Analysis
The apparent density is less for graphite, and it is higher for Fe. The Cu and Mo element has obtained moderate density. The effect of varying alloying addition on the densification response of carbonyl iron powder compacts sintered in spark plasma heating furnace was shown in Table 3. The addition of graphite and increasing in Mo element % in Fe increases the sintered compact density. Pure iron has 90.18% relative sintered density; the highest sintered density (97.20%) is observed with the Fe-2Cu-0.8C-0.6Mo sample. With an increase, the Mo content from 0.2%-0.6% in the  The hard coating was obtained by plasma reaction, assisted by the reactive gases N 2 throughout the coating practice. The N 2 gas was quickly decomposed, and the reaction occurred in the plasma region, which forms the ionized metal ions, as hard thin films on the substrate surface. The CAP-PVD process is stable without oxidation using the inert environment bypassing the Ar gas into the coating compartment. The Ar and N 2 gas pressures are set based on the chamber pressure. The sample temperature was measured before deposition, using K-type thermocouple placed near the sample and maintained at 400 • C with temperature sensors. Throughout the CAP-PVD method, 150 A was set as a substrate current, and 30 V was set as a bias voltage.

Density Analysis
The apparent density is less for graphite, and it is higher for Fe. The Cu and Mo element has obtained moderate density. The effect of varying alloying addition on the densification response of carbonyl iron powder compacts sintered in spark plasma heating furnace was shown in Table 3. The addition of graphite and increasing in Mo element % in Fe increases the sintered compact density. Pure iron has 90.18% relative sintered density; the highest sintered density (97.20%) is observed with the Fe-2Cu-0.8C-0.6Mo sample. With an increase, the Mo content from 0.2-0.6% in the Fe-2Cu-0.8C Coatings 2020, 10, 1166 7 of 15 sample relative sintered density gradually increased. For densification, Cu is also playing a pivotal role. Copper melting point (1083 • C) below the sintering temperature, during the sintering copper, melts and penetrates the voids between the Fe-Fe particles and Fe other powder particles; this is attributed to the increasing in the sintered density [21]. If the copper content is high, the copper melt has a transient nature, so there is a chance to form secondary pores. It decreases the density [20], to reduce the swelling behavior of copper graphite is added [22]. However in the case of spark plasma sintering, uniaxial pressure is continuously being applied it is not giving a high chance to expand the copper, so the density is increased. Figure 3 shows the optical microstructure of all the sintered alloys. In Figure 3a,b, pearlite is not formed only in the graphite contain alloys pearlite is formed. Mo acts as the grain refiner, and its influence to increase the tensile strength [23]. Table 3. Effect of varying alloying addition on the densification response of carbonyl iron powder compacts sintered in Spark plasma heating furnace [21].

Hardness Analysis
The Rockwell hardness values of spark plasma sintered alloys were shown in Table 4. Addition of the 2 wt.% copper to the iron showing in the increment (45 to 47 HRB) of hardness due to the increase in the sintered density. Addition of the carbon in the form of graphite showing a high impact on the hardness. The addition of graphite and increasing in Mo element % in Fe improves the sintered compact's hardness. Mo acts as the grain refiner, and it improves the hardness of the material [24]. The highest hardness (96 HRB) is obtained for the Fe-2Cu-0.8C-0.6Mo alloy.

Tensile Test Analysis
The effect of alloying addition on the tensile properties of carbonyl iron compacts was shown in Table 5. The percentage of the elongation is increased with the addition of the Cu to the Fe. Cu has higher slip systems compare with Fe, so it influences the plastic deformation of the material. The addition of graphite and increasing in Mo element % in Fe improves the UTS of the sintered compacts. SEM fractography of the tensile test was shown in Figure 4. In Figure 4a Fe fractography of the surface, two types of fracture surface (ductile and brittle) are observed. In Figure 4b Fe-Cu fracture surface is the maximum ductile fracture surface is observed because of the copper addition effect. With the addition of the graphite to the Fe-Cu dimples and river, patterns are noticed. Not only fracture surface tensile test results also show the elongation decrease with the addition of the C. When Mo is added to the Fe-Cu-C area, the ductile fracture decreases, and the brittle fracture surface area increases. In Figure 4f Fe-2Cu-0.8C-0.6Mo alloy fractography is showing the brittle type of fracture surface. Increasing the Mo content strength of the material increases, it influences the ductile type fracture surface and is converted to the inter-crystalline and trans-crystalline fracture [25]. surface area increases. In Figure 4f Fe-2Cu-0.8C-0.6Mo alloy fractography is showing the brittle type of fracture surface. Increasing the Mo content strength of the material increases, it influences the ductile type fracture surface and is converted to the inter-crystalline and trans-crystalline fracture [25]. Table 5. Effect of alloying addition on the tensile properties of carbonyl iron compacts [21].

Corrosion Analysis
The potentiodynamic polarization corrosion testing machine was used to obtain the corrosion rate following the standard ASTM G59-97 is used [24]. The corrosion current (Icorr) and corrosion potential (Ecorr) are shown in Figure 5, representing the potentiodynamic polarization curves of the carbonyl iron powder compacts sintered samples. Rp (Polarization resistance), Ecorr (Corrosion potential), icorr (Corrosion current density), and C.R. (Corrosion rate) values attained from the potentiodynamic polarization studies for carbonyl iron powder compacts sintered samples were shown in Table 6.
The icorr value (0.0002011 A/cm 2 ) of the Fe-2Cu coated sample indicates a reduced corrosion current than the other samples. A similar type of tendency has been observed in the polarization analysis. The Fe-2Cu sample has attained a reduced corrosion rate of 0.6579 mmpy. The pure Fe sample has obtained more corrosion current (0.000825 A/cm 2 ) and a greater corrosion rate of 2.7 mmpy. The Fe-2Cu sample has shown improved corrosion resistance while compared with the remaining samples. The reduction in icorr indicates the enhancement in the corrosion resistance (corrosion rate declines) [26]. It is confirmed from the icorr and Rp values obtained in Table 6 [27,28].

Corrosion Analysis
The potentiodynamic polarization corrosion testing machine was used to obtain the corrosion rate following the standard ASTM G59-97 is used [24]. The corrosion current (I corr ) and corrosion potential (E corr ) are shown in Figure 5, representing the potentiodynamic polarization curves of the carbonyl iron powder compacts sintered samples. R p (Polarization resistance), E corr (Corrosion potential), i corr (Corrosion current density), and C.R. (Corrosion rate) values attained from the potentiodynamic polarization studies for carbonyl iron powder compacts sintered samples were shown in Table 6.
The i corr value (0.0002011 A/cm 2 ) of the Fe-2Cu coated sample indicates a reduced corrosion current than the other samples. A similar type of tendency has been observed in the polarization analysis. The Fe-2Cu sample has attained a reduced corrosion rate of 0.6579 mmpy. The pure Fe sample has obtained more corrosion current (0.000825 A/cm 2 ) and a greater corrosion rate of 2.7 mmpy. The Fe-2Cu sample has shown improved corrosion resistance while compared with the remaining samples. The reduction in i corr indicates the enhancement in the corrosion resistance (corrosion rate declines) [26]. It is confirmed from the i corr and R p values obtained in Table 6 [27,28].

SEM Investigation
The microstructure of the AlCrN monolayer coating deposited on Fe and Fe-2Cu alloys is shown in Figures 6 and 7. The coating thickness obtained for the AlCrN coating was 5 ± 2 μm. The coating's cross-sectional observation confirms that the AlCrN monolayer coating obtained above the substrate, and the SEM photomicrograph shows that the AlCrN monolayer is adequately bonded with the substrates. The SEM photomicrograph of the AlCrN monolayer coating displays a thick structure with an uneven surface with has fewer macroparticles (marked in Figure 7f), tiny pores, and microcavities due to the occurrence of droplets and lumps. Microparticles are the one type of defect linked to the limited technology of the coating process. Micron sized cathode droplet spill off from the cathode spot. The high-pressure collisions between atoms, even before deposition on the substrate surface, promote agglomerates formation [29].
The elemental compositions of the coated samples were shown in Table 7. The thick structure of the coating enhances the hardness of the coated surface [30][31][32].

SEM Investigation
The microstructure of the AlCrN monolayer coating deposited on Fe and Fe-2Cu alloys is shown in Figures 6 and 7. The coating thickness obtained for the AlCrN coating was 5 ± 2 µm. The coating's cross-sectional observation confirms that the AlCrN monolayer coating obtained above the substrate, and the SEM photomicrograph shows that the AlCrN monolayer is adequately bonded with the substrates. The SEM photomicrograph of the AlCrN monolayer coating displays a thick structure with an uneven surface with has fewer macroparticles (marked in Figure 7f), tiny pores, and microcavities due to the occurrence of droplets and lumps. Microparticles are the one type of defect linked to the limited technology of the coating process. Micron sized cathode droplet spill off from the cathode spot. The high-pressure collisions between atoms, even before deposition on the substrate surface, promote agglomerates formation [29].
The elemental compositions of the coated samples were shown in Table 7. The thick structure of the coating enhances the hardness of the coated surface [30][31][32].

Microhardness Analysis
The micro Vickers' indentation load of 0.3 kgf and dwell of 10 s was applied on AlCrN coating. The penetration load was gradually increased after the indenter tip's primary contact on the coated top surface. The test method is IS 1501-2002 [39]. The measurement is taken for eight trials, and their mean values were reported in Table 8. The outcomes specify that the AlCrN monolayer coating has obtained higher hardness (1134.85 Hv 0.3 ) for Fe-2Cu-0.8C-0.6Mo substrate compared to the other samples. Thus, the coating's metallurgical performance strongly influences the grain structure and the hardness [40].

Corrosion Analysis
The potentiodynamic polarization curves obtained for AlCrN monolayer coated samples were shown in Figure 9. The potentiodynamic polarization data obtained from the electrochemical corrosion test is shown in Table 9. The i corr value is obtained as 0.00002539 A/cm 2 for Fe-2Cu-0.8C-0.2Mo coated sample shows the reduced corrosion current than the other coated samples. The corrosion rate of 0.08306 mmpy was observed for Fe-2Cu-0.8C-0.2Mo coated sample. The Fe-2Cu-0.8C-0.6Mo coated substrate has shown a more significant corrosion current (0.00008557 A/cm 2 ) and a larger corrosion rate of 0.28 mmpy. The Fe-2Cu-0.8C-0.2Mo coated sample has evidenced better corrosion resistance while related to the remaining coated substrates. It was evident from the polarization resistance and corrosion current density according to the results attained in Table 8. The decline in i corr permits the corrosion resistance (corrosion rate decreases) [28,40].

Microhardness Analysis
The micro Vickers' indentation load of 0.3 kgf and dwell of 10 s was applied on AlCrN coating. The penetration load was gradually increased after the indenter tip's primary contact on the coated top surface. The test method is IS 1501-2002 [39]. The measurement is taken for eight trials, and their mean values were reported in Table 8. The outcomes specify that the AlCrN monolayer coating has obtained higher hardness (1134.85 Hv0.3) for Fe-2Cu-0.8C-0.6Mo substrate compared to the other samples. Thus, the coating's metallurgical performance strongly influences the grain structure and the hardness [40].

Corrosion Analysis
The potentiodynamic polarization curves obtained for AlCrN monolayer coated samples were shown in Figure 9. The potentiodynamic polarization data obtained from the electrochemical corrosion test is shown in Table 9. The icorr value is obtained as 0.00002539 A/cm 2 for Fe-2Cu-0.8C-0.2Mo coated sample shows the reduced corrosion current than the other coated samples. The corrosion rate of 0.08306 mmpy was observed for Fe-2Cu-0.8C-0.2Mo coated sample. The Fe-2Cu-0.8C-0.6Mo coated substrate has shown a more significant corrosion current (0.00008557 A/cm 2 ) and a larger corrosion rate of 0.28 mmpy. The Fe-2Cu-0.8C-0.2Mo coated sample has evidenced better corrosion resistance while related to the remaining coated substrates. It was evident from the polarization resistance and corrosion current density according to the results attained in Table 8. The decline in icorr permits the corrosion resistance (corrosion rate decreases) [28,40].

Conclusions
The Fe with varying the weight percentage of the copper (Cu), graphite (C), and molybdenum (Mo) 6 alloys were prepared using a spark plasma sintering process, and the mechanical properties were studied and reported. AlCrN monolayer coating was deposited on the sintered compact samples, using the CAP-PVD process, electrochemical corrosion resistance of the Fe alloys with and without coating was investigated, the following conclusions are made: • A Fe-2Cu-0.8C-0.6Mo compact sample has obtained a higher relative sintering density of 97.20%, high hardness of 96 HRB, and high ultimate tensile strength of 1000 MPa compared with the other sintered compacts. The SEM fractography shows both ductile and brittle type fracture surfaces.

•
The Fe-2Cu compact has obtained a lower corrosion rate of 0.657 mmpy while related to the other sintered compacts.

•
The SEM images clearly show that the interlayer diffusion of the AlCrN coating with the Fe-2Cu alloy sintered compacts.

•
The XRD analysis shows that the high peaks obtained for the AlN and CrN patterns and the average crystallite size from 38 to 58 nm was obtained.

•
The microhardness testing shows that the Fe-2Cu-0.8C-0.6Mo coated compact sample has obtained a high hardness of 1134.85 HV.