coatings

Article Effect of –Phosphorus and Nickel– Coatings on Electrical Ablation of Small Electromagnetic Rails

Li-Shan Hsu 1, Pao-Chang Huang 2,3,* , Chih-Cheng Chou 2,3, Kung-Hsu Hou 2,3,*, Ming-Der Ger 3,4 and Gao-Liang Wang 5

1 School of Defense Science, Chung Cheng Institute of Technology, National Defense University, Taoyuan 33551, Taiwan; [email protected] 2 Department of Power Vehicle and Systems Engineering, Chung Cheng Institute of Technology, National Defense University, Taoyuan 33551, Taiwan; [email protected] 3 System Engineering and Technology Program, National Chiao Tung University, Hsinchu 30010, Taiwan; [email protected] 4 Department of Chemical and Materials Engineering, Chung Cheng Institute of Technology, National Defense University, Taoyuan 33551, Taiwan 5 Department of Marketing Management, Takming University of Science and Technology, Taipei 11451, Taiwan; [email protected] * Correspondence: [email protected] (P.-C.H.); [email protected] (K.-H.H.); Tel.: +886-3-380-9257 (ext. 262) (P.-C.H.); +886-3-380-9257 (ext. 252) (K.-H.H.)  Received: 27 September 2020; Accepted: 5 November 2020; Published: 10 November 2020 

Abstract: The electromagnetic rail catapult is a device that converts electrical energy into kinetic energy, which means that the strength of electrical energy directly affects the muzzle speed of armature. In addition, the electrical conductivity, electromagnetic rails and armature surface roughness, and the holding force of the rail are influencing factors that cannot be ignored. However, the electric ablation on the surface of the electromagnetic rails caused by high temperatures seriously affects the service life performance of the electromagnetic catapult system. In this study, electrochemically deposited nickel-phosphorus and nickel-molybdenum coatings are plated on the surface of electromagnetic iron rails and their effects on the reduction of ablation are investigated. SEM (scanning electron microscopy) with EDS (energy dispersive spectroscopy) detector, XRD (X-ray diffraction), 3D optical profiler, and Vickers microhardness tester are used. Our results show that the sliding velocity of the armature decreases slightly with the increased roughness of the rail coating surface. On the other hand, the area of electric ablation on the rail surface is inversely related to the hardness of the rail material. The electrically ablated surface areas of the rails are in: annealed nickel–molybdenum < nickel–molybdenum < annealed nickel–phosphorus < nickel–phosphorus < iron material. Heat treatment at 400 and 500 ◦C, respectively for Ni–P and Ni–Mo alloys, significantly increases hardness due to the precipitation of intermetallic compounds such as Ni3P and Ni4Mo phases. Comprehensive data analysis shows that the annealed nickel–molybdenum coating has the best electrical ablation wear resistance. The possible reason for that might be attributed to the high hardness of the heat-treated nickel–molybdenum coating. In addition, the thermal resistance capability of molybdenum is better than that of phosphorus, which might also contribute to the high wear resistance to electric ablation.

Keywords: nickel alloy coatings; electromagnetic rail catapult; electric ablation; wear

Coatings 2020, 10, 1082; doi:10.3390/coatings10111082 www.mdpi.com/journal/coatings Coatings 2020, 10, 1082 2 of 16 Coatings 2020, 10, x FOR PEER REVIEW 2 of 15

1. Introduction Introduction Recently, thethe electromagneticelectromagnetic rail rail catapult catapult has has shown shown its its potential potential to replaceto replace traditional traditional guns guns due dueto the to factthe thatfact that it can it acceleratecan accelerate a projectile a projectile to its to super its super high high speed, speed, has a has simple a simple design, design, and has and good has goodsecurity security [1]. The [1]. electromagnetic The electromag railnetic catapult rail catapult is composed is composed of control of circuits,control pulsecircuits, power pulse supplies, power supplies,metal rails metal arranged rails inarranged parallel, andin parallel, armatures and (including armatures projectiles) (including [2 ].projectiles) A schematic [2]. diagram A schematic of the diagramelectromagnetic of the electromagnetic rail catapult is shownrail catapult in Figure is show1; whenn in interactingFigure 1; when with interacting the currents with passed the throughcurrents passedthe rails through and armature the rails at and that armature time, the magneticat that time, fields the were magnetic generated. fields were The magnetic generated. field The line magnetic formed fieldon a clockwiseline formed circle onaround a clockwise the positive circle railaround and onthe a positive counterclockwise rail and circleon a counterclockwise around the negative circle rail, aroundview from the thenegative leading rail, edge. view The from net the magnetic leading field edge. between The net the magnetic rails is infield the between vertical the direction. rails is So,in thethe Lorentzvertical forcedirection. is directed So, the perpendicularly Lorentz force is to di therected magnetic perpendicularly field and contributes to the magnetic to the direction field and of contributesthe armature to movement.the direction This of the system, armature which moveme does notnt. requireThis system, chemical which energy does propellantsnot require chemical and uses energyelectrical propellants energy as electroand uses power, electric hasal beenenergy valued as electro by many power, countries has been [3–6 ].valued The Lorentz by many force countries will be [3–6].increased The byLorentz increasing force the will current. be increased This means by incr a largeeasing current the current. is needed This to acceleratemeans a large the projectile current tois neededhypersonic to accelerate velocities. the projectile to hypersonic velocities.

Figure 1. SchematicSchematic diagram diagram of of electromagnetic electromagnetic rail catapult.

However, thethe largelarge current current high-speed high-speed electric electric sliding sliding contact contact between between the the armature armature and and the railsthe railscan leadcan lead to the to problemthe problem of rail of rail ablation. ablation. The The rail’s rail’s electrical electrical ablation ablation is is one one ofof thethe mostmost significant significant problems, needing to be solved urgently for rail catapult operation. The The main main reasons reasons are are that high temperature,temperature, large large speed speed and and high high current current lead lead to to change the original smooth surface surface of of the metal railrail [7–11]. [7–11]. Seeking Seeking technology technology that that can can solve solve or or im improveprove the service life of electromagnetic catapult railsrails has become the key to whether electromagne electromagnetictic rail catapult can be practically applied. To endure severe environments such as extrem extremee mechanical wear wear and and high-temperature high-temperature thermal thermal shock during the the high-velocity propulsion, propulsion, rail rail mate materialsrials should should have have high high electrical electrical conductivity, conductivity, high hardness, high thermal conductivity and high resistance to wear [[12].12]. The application of the appropriate coating coating on on rail rail has be beenen reported as an effective effective appr approachoach in reducing the degradation of rail [12–15]. [12–15]. Colombo Colombo et et al. al. reported reported that that the the wear wear and and spark spark erosion erosion of of the the rails had been reducedreduced after after the the rails rails were were coated coated with with TaN TaN and TiN by Plasma Source Ion Implantation and Ion Beam Enhanced Deposition techniques [12]. [12]. Siopis Siopis et et al. al. [13] [13] performed performed a a systematic systematic investigation investigation by by thethe Ashby methodmethod toto selectselect a a rail rail material material that that would would maximize maximize magnetic magnetic energy energy for performancefor performance and anddurability durability for economic for economic viability. viability. Their resultsTheir suggestedresults suggested that a hybrid that a rail hybrid material rail with material an electrically with an electricallyconductive substrateconductive and substrate a damage-resistant and a damage-r surfaceesistant layer consisting surface layer of , consisting chromium, of tungsten, nickel, chromium,or tantalum nickel, can accomplish or tantalum these can two accomplish contradictory thes goals.e two Wattcontradictory electroplated goals. Al Watt on the electroplated surface of UNS Al onC15725 the surface Glidcop of Al-25, UNS anC15725 aluminum Glidcop oxide Al-25, (0.25 an wt.%) aluminum dispersion-hardened oxide (0.25 wt.%) copper dispersion-hardened alloy, and found copperthat Al alloy, coating and can found effectively that Al inhibit coating the can occurrence effectively of gougeinhibit andthe occurrence wear [14]. Dueof gouge to its and high wear hardness, [14]. Duehard to chromium its high hardness, coatings hard were chromium electroplated coatings on the we surfacere electroplated of rail materials on the tosurface reduce of the rail damage materials of tothe reduce rails [ 15the]. damage of the rails [15]. The core component of the pulse power supply in this study is the capacitor stack. Therefore, the life of the capacitor is the key component that determines whether the system is operating normally or not. According to the information provided by the capacitor supplier, the life of the

Coatings 2020, 10, 1082 3 of 16

The core component of the pulse power supply in this study is the capacitor stack. Therefore, the life of the capacitor is the key component that determines whether the system is operating normally or not. According to the information provided by the capacitor supplier, the life of the capacitor is positively related to the operating temperature of the part. For the evaluation of the service life of the capacitor, if the capacitor has been kept operating at a temperature below 75 ◦C, the capacitor can maintain normal operation for at least 5 years (if it operates 1 cycle per day, it is estimated that it can operate at 1800 cycles). Another key component is the electromagnetic rail. If the rail is made of a single metal material, it cannot withstand the damage to the rail surface caused by high voltage (300 DCV) and the rail can be used only one to two times. Therefore, our team applied nickel-based coating processing to the rails, hoping to improve this puzzle. For the deposition of coatings, electroless plating and offer many advantages such as low cost, versatility and the production of a wide range of metallic materials. Among various types of coatings fabricated through electroless plating and electroplating, nickel-based alloy coatings are widely employed owing to their remarkable properties which include good corrosion, low friction coefficient and good wear resistance. Moreover, with subsequent thermal treatment hardness of Ni-based coatings can be even higher than that of hard chromium coatings [16–21]. Ni-based alloy coatings are widely applied in various industries. However, the applications of Ni-based coatings in rail guns have rarely been studied. Based on our previous studies [22–25], electroless plated nickel-phosphorus coatings are characterized by good conductivity, good corrosion resistance, no pinholes, low friction coefficient, stable production and low price and the nickel-molybdenum electrodeposits are known for their high hardness, abrasion resistance and good thermal resistance. Therefore, in this study, nickel-phosphorus and nickel-molybdenum alloy coatings were plating on the surface of the iron rails of the electromagnetic rail and their effects on the reduction of ablation were investigated and compared.

2. Experimental Equipment and Planning This study is divided into two parts, including the preparation of experimental equipment and the electromagnetic rail’s surface improvement plan. The first part is to build a small set of electromagnetic rail catapult experimental equipment to facilitate the valuation of the wear of the contact surface between the armature and the electromagnetic catapult conductive rails. The second part is the electrochemical deposition and modification of the surface of the electromagnetic catapult conductive rails. The nickel–phosphorus and nickel–molybdenum alloy is coated on the surface of the iron rail to evaluate the improvement of the electrical ablation on the conductive rail.

2.1. Preparation of Small Electromagnetic Catapult Experiment Equipment Figure2a is a function diagram of an electromagnetic catapult, which can be electro modulated according to the experimental requirements. This figure shows that the pulse power supply is modulated from 110 ACV to the experiment requires AC potential (0~600 ACV), then rectifies DCV, and then charges the capacitor stack. Regarding the function to provide pulse electricity power for the energizing section of the electromagnetic rail [26–30], in this study, the capacitor stack contained 42 capacitors (2200 µF/450 DCV) and was configured in parallel to obtain a total capacitance of 0.0924 F. In order to respond the electrical limitations of component, the operating voltage set in this experiment was 300 DCV. It can be calculated from Equation (1) that the output energy of the pulse power supply in this experiment is 4158 Joules. CV2 K = (Joules), (1) E 2 where C is the total capacity (unit: Farad) and V is the pulse voltage (unit: Voltage). Coatings 2020, 10, x FOR PEER REVIEW 4 of 15

(Refer to Table 3). Each rail’s size is 0.6 × 0.035 × 0.01 m3. The support structure of the rail is made of bakelite to ensure good electrical insulation and operator safety. In order to have good contact between armature and rails, a U-shaped elastic tail with an outward angle of 3.5°, which was made from elastic steel sheet (used stainless steel model 304: Chromium 18%, Nickel 8%, Manganese 2%), wasCoatings used2020 as, 10 the, 1082 armature carrier. The front arrow weight was made of non-conductive rubber.4 The of 16 armature is displayed in Figure 2b.

FigureFigure 2. ExperimentalExperimental device device ( (aa)) control control circuit; circuit; ( (b)) electromagnetic electromagnetic rail catapult and armature.

DuringThis experimental the experimental device operatio of electromagneticn, the capacitor rail catapultstack was is first shown charged in Figure to 3002b. DCV. The wholeWhen setthe fireof catapult button is length pressed, is 1m it will which provide is composed the mechanical of an acceleration force to push section the armature and anenergizing to generate section. initial speed,The length at the of same the acceleration time, 300 DCV section from is 0.4the m,capacito the mechanicalr stack is connected ejection equipment to the energizing in this section section. is Whenused to the verify armature and compare enters the the metal efficiency rail, the of overall the electromagnetic electrical circuit catapult. is fully The connected muzzle by velocity the physical of the contactarmature between (27 g) obtained the armature by pure and mechanical the rails. The ejection capacitor was about stack 7.6begins m/s whichto release was current measured on usingthe rails an andoptical at the grid same type time speedometer generates (ProChrono Lorentz force, Co., whic Rockford,h promotes IL, USA). the accelerati The substrateon of ofthe electromagnetic armature. The capacityrails are madeof the ofcapacitor medium stack carbon is gradually steel, its electrical discharged conductive until it is resistance completely is 0.224releasedΩ (Refer (capacitor to Table stack 3). Each rail’s size is 0.6 0.035 0.01 m3. The support structure of the rail is made of bakelite to ensure voltage drops to 0 DCV).× × goodThe electrical sliding insulation speed of the and armature operator on safety. the rail In order is measured to have with good an contact electromagnetic between armature field sensor and (PASCOrails, a U-shaped CI-6520A, elastic Roseville, tail with CA, an USA) outward and dedicated angle of 3.5 software◦, which (PASCO was made Capstone, from elastic version: steel 1.12.0). sheet The(used installation stainless steel distance model between 304: Chromium these two 18%, electr Nickelomagnetic 8%, Manganese sensors 2%),is fixed was usedat 0.1 as m. the When armature the armaturecarrier. The enters front the arrow detection weight range was of made the electrom of non-conductiveagnetic sensor, rubber. the screen The armature on dedicated is displayed software in willFigure display2b. a convex wave of the magnetic field strength. The muzzle velocity of the armature is calculatedDuring by the simply experimental dividing operation, the distance the capacitor between stack the wastwo firstelectromagnetic charged to 300 sensors DCV. Whenby the the time fire differencebutton is pressed, measured it willwith provide the electrom the mechanicalagnetic sensors. force toSee push Equation the armature (2). to generate initial speed, at the same time, 300 DCV from the capacitor stack is connected to the energizing section. When the ∆x x -x v = = f i, (2) armature enters the metal rail, the overall electrical∆t circuit∆t is fully connected by the physical contact between the armature and the rails. The capacitor stack begins to release current on the rails and at the The definition of speed is the change in position (∆x, the last position minus the initial position,) same time generates Lorentz force, which promotes the acceleration of the armature. The capacity divided by the time interval (∆t). of the capacitor stack is gradually discharged until it is completely released (capacitor stack voltage 2.2.drops Deposition to 0 DCV). Process of Nickel–Phosphorus and Nickel–Molybdenum Coatings The sliding speed of the armature on the rail is measured with an electromagnetic field sensor (PASCOPrior CI-6520A, to each experiment, Roseville, CA,the rail USA) was and cleaned dedicated with softwareacetone with (PASCO ultrasonic Capstone, agitation version: for 10 1.12.0). min, alkaliThe installation washed in distancea 10% NaOH between solution these of two40 °C electromagnetic for 15 min, pickled sensors with is a fixed50% HCl at 0.1 solution m. When for 30 the s, andarmature washed enters by deionized the detection water. range of the electromagnetic sensor, the screen on dedicated software will displayNickel–phosphorus a convex wave alloy of coatings the magnetic were used field with strength. nickel The sulfate muzzle electrolyte, velocity electroless of the armature plated at is 87calculated °C for 2 byh from simply a bath dividing containing the distance nickel sulfate between hexahydrate, the two electromagnetic sodium lactate, sensorsamino acid by theglycine, time Sodiumdifference hypophosphite, measured with pota thessium electromagnetic iodate and sensors. lead nitrate. See Equation The pH of (2). the plating bath was adjusted to 4.7, and the volume flow rate was 3 L/min. The details of bath compositions can be seen in Table 1. Nickel–molybdenum alloy coatings were∆ xelectroplatedxf xi at 25 °C for 2 h from an electrolyte v = = − , (2) containing nickel sulfate hexahydrate, sodium ∆molybdatet ∆t dihydrate, and sodium citrate. The pH of 2 the platingThe definition was adjusted of speed to 9.5, isthe thechange current indensity position was ( ∆5 xA/dm, the last, and position the volume minus flow the rate initial was position,) 3 L/min. Thedivided details by of the bath time compositions interval (∆t can). be seen in Table 2.

2.2. Deposition Process of Nickel–Phosphorus and Nickel–Molybdenum Coatings

Prior to each experiment, the rail was cleaned with acetone with ultrasonic agitation for 10 min, alkali washed in a 10% NaOH solution of 40 ◦C for 15 min, pickled with a 50% HCl solution for 30 s, and washed by deionized water. Coatings 2020, 10, 1082 5 of 16

Nickel–phosphorus alloy coatings were used with nickel sulfate electrolyte, electroless plated at 87 ◦C for 2 h from a bath containing nickel sulfate hexahydrate, sodium lactate, amino acid glycine, Sodium hypophosphite, potassium iodate and lead nitrate. The pH of the plating bath was adjusted to 4.7, and the volume flow rate was 3 L/min. The details of bath compositions can be seen in Table1.

Table 1. Nickel–phosphorus alloy plating solution composition.

Plating Solution Composition

Nickel Sulfate Hexahydrate (NiSO46H2O) 30 g/L Sodium Lactate (C3H5NaO3) 40 mL/L Amino Acid Glycine (C2H5NO2) 10 g/L Sodium Hypophosphite (NaPO2H2) 30 g/L Potassium Iodate (KIO3) 2 mL/L Lead Nitrate (PbNO3) 0.15 mL/L

Nickel–molybdenum alloy coatings were electroplated at 25 ◦C for 2 h from an electrolyte containing nickel sulfate hexahydrate, sodium molybdate dihydrate, and sodium citrate. The pH of the plating was adjusted to 9.5, the current density was 5 A/dm2, and the volume flow rate was 3 L/min. The details of bath compositions can be seen in Table2.

Table 2. Nickel–molybdenum alloy plating solution composition.

Plating Solution Composition

Nickel Sulfate Hexahydrate (NiSO46H2O) 52.57 g/L Sodium Molybdate dihydrate (MoNa O 2H O) 9.678 g/L 2 4· 2 Sodium Citrate (Na3C6H507) 58.82 g/L

Heat treatment was operated at 400 and 500 ◦C in an argon atmosphere for nickel–phosphorus and nickel–molybdenum-coated rails, respectively. Heat treatment lasted for 1 h and then the samples were cooled to room temperature.

2.3. Characterization of Rails The contact resistance between armature and rails was measured by a Digital Multi-meter 1 (PICOTEST M3500A 6 2 digit, Kaohsiung, Taiwan). The measurement method uses “4-wire” static measurement, sandwiching a copper piece between the armature and the rails, so that the test rod can be stably clamped. The rail after the experiment was cut into test pieces of about 0.01 0.01 m2, which were × placed in acetone and clean water for ultrasonic cleaning and drying. The surface microstructure and composition of the samples were observed using a JEOL JSM-F100 scanning electron microscopy (SEM) (Tokyo, Japan) with an energy dispersive spectroscopy (EDS) detector (Tokyo, Japan). Using Bruker D2 PHASER diffractometer (Karlsruhe, Germany), using CuKα radiation, X-ray diffraction (XRD) structure analysis was performed. The surface roughness was measured using a Chroma 7502 3D optical profiler (Taoyuan, Taiwan). The Digital microhardness tester (HVS-1000) (Taipei, Taiwan) was used to measure the hardness. Load at 300 gf, and calculate the average hardness after 9 tests (result in Table3). Coatings 2020, 10, 1082 6 of 16

Table 3. Material properties of the experimental rails.

Ni–P Coating Ni–Mo Coating Material Ni–P Ni–Mo Iron Coating Heat-Treated Coating Heat-Treated Property at 400 ◦C/1 h at 500 ◦C/1 h Hardness (HV) 215 512 650 720 1045 Electrical Resistance (Ω) 0.224 0.21 0.186 0.226 0.230 R 1.52 R 3.41 R 3.88 R 4.05 R 4.44 Surface Roughness (µm) a a a a a Sa 4.41 Sa 5.01 Sa 5.28 Sa 5.09 Sa 5.39

3. Results and Discussion The main objective of this study is to find coating materials that can reduce or prevent metal rail from ablation. The focus of the analysis is to confirm the properties of the coating materials before

Coatingsthe catapult 2020, 10 experiment., x FOR PEER REVIEW The velocity of the catapult is affected by the electrical energy, armature6 of and 15 rail material characteristics, and the analysis of the area and surface properties of the surface of the rail after electric catapult; in order to explore the optimal coating in the experiment and the possible cause analysis.

3.1. Rail Rail Material/Coating Material/Coating Properties A total total of of five five types types of rail of surface rail surface materials materials was detected was detected in this study, in this including study, iron including rails, nickel–phosphorus-coatediron rails, nickel–phosphorus-coated rails, heat-treated rails, nickel–phosphorus-coated heat-treated nickel–phosphorus-coated rails, nickel–molybdenum- rails, nickel– coatedmolybdenum-coated rails, heat-treated rails, nickel–molybdenum-coated heat-treated nickel–molybdenum-coated rails. The nickel–phosphorus rails. The nickel–phosphorus coating prepared incoating this study prepared has a in thickness this study of has about a thickness 15~20 μm of and about a phosphorus 15~20 µm and content a phosphorus of about content8 to 12 wt.%. of about The 8 nickel-molybdenumto 12 wt.%. The nickel-molybdenum coating has a thickness coating of hasabout a thickness30~35 μm ofand about a molybdenum 30~35 µm and content a molybdenum of about 28 tocontent 31 wt.% of (see about Figure 28 to 3). 31 Table wt.% 3 (see is a Figurelist of 3the). Tableresults3 isobtained a list of for the hardness, results obtained contact resistance for hardness, and surface’scontact resistance average roughness. and surface’s average roughness.

Figure 3. SEMSEM images images of of coating cross-section: ( (aa)) Ni–P Ni–P Coating; Coating; ( (bb)) Ni–Mo Ni–Mo Coating. Coating.

The heat treatment temperature is selected based on past research experience and literature that recommends the the heat heat treatment treatment temperature temperature of of nick nickel–phosphorusel–phosphorus to to be be set set at at 400 400 °C/1◦C/1 h and the heat treatment temperature of nickel–molybdenum to be set at 500 °C.◦C. It It shows shows from from Table Table 33 thatthat thethe hardness (215 (215 HV) HV) of of the the pristine pristine iron iron substrate substrate is is lowest lowest among among all all tested tested samples. samples. The The hardness hardness of nickel–phosphorus-coatedof nickel–phosphorus-coated and and nickel–molybdenum-coated nickel–molybdenum-coated rail is rail 512 is and 512 720 and HV, 720 respectively. HV, respectively. After annealingAfter annealing at 400 at°C 400 for◦ C1 h, for the 1 h, hardness the hardness of nickel–phosphorus-coated of nickel–phosphorus-coated rail increased rail increased to 650 to HV. 650 HV.On theOn other the other hand, hand, the hardness the hardness of annealing of annealing nickel–mol nickel–molybdenum-coatedybdenum-coated rail increased rail increased to 1045 to HV 1045 after HV heatafter treatment heat treatment at 500 at°C. 500 It is◦C. believed It is believed that heating that heating a Ni–P a coating Ni–P coating at temperatures at temperatures of 400 °C of for 400 1◦ Ch increasesfor 1 h increases hardness hardness which can which be ascribed can be ascribed to the solid to the solution solid solution strengthening strengthening effect as eff aect result as a of result the precipitationof the precipitation of intermetallic of intermetallic compound compound Ni3P during Ni3P during the heat the treatment heat treatment period period [31]. [Similarly,31]. Similarly, the increasethe increase in hardness in hardness of the of the annealed annealed nickel–molybdenum nickel–molybdenum coating coating is is attributed attributed to to the the solid solid solution solution strengthening effect effect of Ni4Mo precipitation [18]. [18]. In catapult processes, the heat conduction between the armature and rails is an important factor for the temperature rise of rails. The ablation and wear caused by the rise of temperature would affect the performance and service life of the rail. A low contact resistance is thus important for selection of rail material. It shows from Table 3 that except for the annealed nickel–phosphorus-coated rail, contact resistance of all rails is slightly higher than 0.2 Ω. For the annealed nickel–phosphorus-coated rail it is about 0.186 Ω, which is lower than 0.2 Ω. The annealed nickel–phosphorus-coated rail has the lowest contact resistance might be ascribed to the fact that the grain size of nickel–phosphorus increases with the heat treatment temperature, resulting in the change of interface volume fraction and its properties [32]. The surface roughness is another factor that will influence the sliding contact between solid armature and rail. As shown in Table 3, the surface roughness of these rails is in the order: iron rail < Ni–P-coated rail < heat-treated Ni–P-coated rail < Ni–Mo-coated rail < heat-treated Ni–Mo-coated rail. The surface roughness is lowest for pristine iron rail (Ra 1.52 μm, Sa 4.41 μm). The surface roughness increases after coating which might be attributed to the surface after the coating is not ground and polished. The slight increase in surface roughness after heat treatment should be due to the effect of grain coarsening.

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In catapult processes, the heat conduction between the armature and rails is an important factor for the temperature rise of rails. The ablation and wear caused by the rise of temperature would affect the performance and service life of the rail. A low contact resistance is thus important for selection of rail material. It shows from Table3 that except for the annealed nickel–phosphorus-coated rail, contact resistance of all rails is slightly higher than 0.2 Ω. For the annealed nickel–phosphorus-coated rail it is about 0.186 Ω, which is lower than 0.2 Ω. The annealed nickel–phosphorus-coated rail has the lowest contact resistance might be ascribed to the fact that the grain size of nickel–phosphorus increases with the heat treatment temperature, resulting in the change of interface volume fraction and its properties [32]. The surface roughness is another factor that will influence the sliding contact between solid armature and rail. As shown in Table3, the surface roughness of these rails is in the order: iron rail < Ni–P-coated rail < heat-treated Ni–P-coated rail < Ni–Mo-coated rail < heat-treated Ni–Mo-coated rail. The surface roughness is lowest for pristine iron rail (Ra 1.52 µm, Sa 4.41 µm). The surface roughness increases after coating which might be attributed to the surface after the coating Coatings 2020, 10, x FOR PEER REVIEW 7 of 15 is not ground and polished. The slight increase in surface roughness after heat treatment should be due to the effect of grain coarsening. 3.2. Influencing Factors of Muzzle Velocity of Armature When3.2. Influencing the electric Factors energy of Muzzle was Velocity fixed of at Armature 300 DCV, the muzzle velocity of the armature was measured,When rails the with electric or wi energythout was different fixed at surface 300 DCV, coatings the muzzle were velocity tested. of theOur armature results was showed measured, that the muzzlerails velocities with or without of the armature different surfacevary from coatings 84 to were 91 m/s tested. depending Our results on the showed rail materials that the muzzle used (See Figurevelocities 4). The ofmuzzle the armature velocity vary of fromthe armature 84 to 91 m obtained/s depending with on different the rail materials rails is in used the (Seefollowing Figure4 order:). iron railThe > muzzle Ni–P-coated velocity rail of the > heat-treated armature obtained Ni–P-coated with diff railerent > Ni–Mo-coated rails is in the following rail > heat-treated order: iron rail Ni–Mo-> coatedNi–P-coated rail. It is rail evident> heat-treated that rail Ni–P-coated material rail will> Ni–Mo-coatedaffect the muzzle rail > heat-treated velocity of Ni–Mo-coated the armature. rail. As It is evident that rail material will affect the muzzle velocity of the armature. As mentioned in the last mentioned in the last section, the contact resistance and surface roughness of rail with different section, the contact resistance and surface roughness of rail with different surface coatings are different. surfaceFigure coatings4 displays are different. the variation Figure of the 4 muzzledisplays velocity the variation of the armature of the muzzle with a velocity surface roughness of the armature of with variousa surface rail roughness materials. Itof appears various that rail the materials. muzzle velocity It appears of armature that the decreases muzzle with velocity the increasing of armature decreasessurface with roughness the increasing of the rails, surface while roughness the contact of resistance the rails, of while the rails the has contact an insignificant resistance e ffofect the on rails has anmuzzle insignificant velocity. effect The high on valuemuzzle of surfacevelocity. roughness The high can value increase of surface the friction roughness leading to can energy increase loss, the frictionand leading as a result, to energy the muzzle loss, velocity and asdecreases. a result, the muzzle velocity decreases.

FigureFigure 4. Surface 4. Surface roughness roughness against against armaturearmature velocity. velocity.

3.3. Wear Analysis of the Rail’s Surface After one shot, the rails were removed. A segment of removed rail was sectioned at a position 0.13 m from the beginning of the energizing section and a wear analysis was performed on this 0.13 m long rail to investigate the effect of rail material on electrical ablation shows the worn surfaces of various rails, as shown in Figure 5, the armature catapult was moving from left to the right.

Figure 5. Surface ablation morphology of different rail. (unit: cm).

Coatings 2020, 10, x FOR PEER REVIEW 7 of 15

3.2. Influencing Factors of Muzzle Velocity of Armature When the electric energy was fixed at 300 DCV, the muzzle velocity of the armature was measured, rails with or without different surface coatings were tested. Our results showed that the muzzle velocities of the armature vary from 84 to 91 m/s depending on the rail materials used (See Figure 4). The muzzle velocity of the armature obtained with different rails is in the following order: iron rail > Ni–P-coated rail > heat-treated Ni–P-coated rail > Ni–Mo-coated rail > heat-treated Ni–Mo- coated rail. It is evident that rail material will affect the muzzle velocity of the armature. As mentioned in the last section, the contact resistance and surface roughness of rail with different surface coatings are different. Figure 4 displays the variation of the muzzle velocity of the armature with a surface roughness of various rail materials. It appears that the muzzle velocity of armature decreases with the increasing surface roughness of the rails, while the contact resistance of the rails has an insignificant effect on muzzle velocity. The high value of surface roughness can increase the friction leading to energy loss, and as a result, the muzzle velocity decreases.

Coatings 2020, 10, 1082 8 of 16 Figure 4. Surface roughness against armature velocity.

3.3. Wear Wear Analysis of the Rail’s Surface After one shot, the rails were removed. A segmen segmentt of removed rail was sectioned at a position 0.13 m from the beginning of thethe energizingenergizing sectionsection andand aa wearwear analysisanalysis waswas performedperformed onon thisthis 0.13 m long rail rail to to investigate investigate the the effect eff ectof rail of railmaterial material on electrical on electrical ablation ablation shows the shows worn the surfaces worn surfaces of various of rails,various as rails,shown as in shown Figure in 5, Figure the armature5, the armature catapult catapult was moving was moving from left from to the left right. to the right.

Coatings 2020, 10, x FOR PEER REVIEW 8 of 15 Figure 5. Surface ablation morphology of different rail. (unit: cm). As the armature contactedFigure 5. Surface with rails ablation that morphology produced of a didiscontinuousfferent rail. (unit: and cm). rapid electrical friction and electrical ablation, the worn area, which was enclosed by a colored line, can be clearly seen on As the armature contacted with rails that produced a discontinuous and rapid electrical friction theand surface electrical of all ablation, rails. theFigure worn 6 area,shows which the wasratio enclosed of the byablated a colored area line, to canthe betotal clearly area seen which on thewas estimatedsurface ofusing all rails. image Figure J software6 shows (version: the ratio 1.53a) of the for ablated five areatested to rail the totalmaterials. area which It can was be seen estimated that the ablatedusing ratio image of J softwarethe iron (version:rail is 19.5%, 1.53a) Ni–P-coated for five tested rail rail is materials. 12.9%, annealed It can be seenNi–P-coated that the ablated rail is ratio11.6%, Ni–Mo-coatedof the iron rail rail is 19.5%, is 4.9% Ni–P-coated and annealed rail isNi–Mo-coated 12.9%, annealed rail Ni–P-coated is 1.1%. It railis clear is 11.6%, that Ni–Mo-coatedsome rail materials rail resistis 4.9% ablation and annealed better than Ni–Mo-coated others. It railis known is 1.1%. that It is the clear better that some wear rail resistance materials is resist in accordance ablation better with Archard’sthan others. law Itwherein is known the that wear the better resistance wear resistanceis directly is inproportional accordance withto the Archard’s micro-hardness law wherein [33]. Calculatedthe wear from resistance the surface is directly damage proportional ratio of the to the rail, micro-hardness the ratio of nickel–molybdenum [33]. Calculated from to theiron surface is about 1:17.7damage (refer ratio to Figure of the 6); rail, theref the ratioore, it of is nickel–molybdenum estimated that the nick to ironel–molybdenum is about 1:17.7 rail (refer can to use Figure above6); 15 cycles.therefore, Therefore, it is estimated the result that that the the nickel–molybdenum heat-treated Ni–Mo-coated rail can use aboverail which 15 cycles. has Therefore,the highest the hardness result amongthat theall samples, heat-treated and Ni–Mo-coated is expected to rail have which the highest has the wear highest resistance hardness in among comparison all samples, with the and other is railexpected materials. to have the highest wear resistance in comparison with the other rail materials.

FigureFigure 6. 6. RatioRatio of of ablated ablated area area to to the the rail area afterafter catapultcatapult test. test.

In Figure 7, 3D profiles show the annealed nickel–phosphorous coating rail before and after catapult. Table 4 shows the rail’s surface detected data by the 3D optical profiler. It shows the surface roughness of rails coated with annealed nickel–phosphorus is greater after catapulting (ΔRa = 5.67 μm, ΔSa = 6.32 μm); the surface roughness of annealed nickel–molybdenum-coated rail after catapult is smaller than before catapult. The possible reason for that can be explained as the annealed nickel– molybdenum-coated rail has the highest electrical resistance, resulting in less current appearing in the contact surface.

Figure 7. The 3D profiles for annealed Ni–P plating rail surfaces (a) before catapult; (b) after catapult.

Coatings 2020, 10, x FOR PEER REVIEW 8 of 15

As the armature contacted with rails that produced a discontinuous and rapid electrical friction and electrical ablation, the worn area, which was enclosed by a colored line, can be clearly seen on the surface of all rails. Figure 6 shows the ratio of the ablated area to the total area which was estimated using image J software (version: 1.53a) for five tested rail materials. It can be seen that the ablated ratio of the iron rail is 19.5%, Ni–P-coated rail is 12.9%, annealed Ni–P-coated rail is 11.6%, Ni–Mo-coated rail is 4.9% and annealed Ni–Mo-coated rail is 1.1%. It is clear that some rail materials resist ablation better than others. It is known that the better wear resistance is in accordance with Archard’s law wherein the wear resistance is directly proportional to the micro-hardness [33]. Calculated from the surface damage ratio of the rail, the ratio of nickel–molybdenum to iron is about 1:17.7 (refer to Figure 6); therefore, it is estimated that the nickel–molybdenum rail can use above 15 cycles. Therefore, the result that the heat-treated Ni–Mo-coated rail which has the highest hardness among all samples, and is expected to have the highest wear resistance in comparison with the other rail materials.

Coatings 2020, 10, 1082 Figure 6. Ratio of ablated area to the rail area after catapult test. 9 of 16

In Figure 7, 3D profiles show the annealed nickel–phosphorous coating rail before and after In Figure7, 3D profiles show the annealed nickel–phosphorous coating rail before and after catapult. Table 4 shows the rail’s surface detected data by the 3D optical profiler. It shows the surface catapult. Table4 shows the rail’s surface detected data by the 3D optical profiler. It shows the surface roughness of rails coated with annealed nickel–phosphorus is greater after catapulting (ΔRa = 5.67 μm, roughness of rails coated with annealed nickel–phosphorus is greater after catapulting (∆Ra = 5.67 µm, ΔSa = 6.32 μm); the surface roughness of annealed nickel–molybdenum-coated rail after catapult is ∆Sa = 6.32 µm); the surface roughness of annealed nickel–molybdenum-coated rail after catapult smaller than before catapult. The possible reason for that can be explained as the annealed nickel– is smaller than before catapult. The possible reason for that can be explained as the annealed molybdenum-coated rail has the highest electrical resistance, resulting in less current appearing in nickel–molybdenum-coated rail has the highest electrical resistance, resulting in less current appearing the contact surface. in the contact surface.

FigureFigure 7.7. TheThe 3D3D profilesprofiles forfor annealedannealed Ni–PNi–P platingplating railrail surfacessurfaces ((aa)) beforebefore catapult;catapult; ((bb)) afterafter catapult.catapult. Table 4. The surface roughness of various rails.

Surface Roughness Before Catapult After Catapult

Rail Type Ra (µm) Sa (µm) Ra (µm) Sa (µm) Iron rail 1.52 4.41 4.49 6.59 Ni–P coating rail 3.41 5.00 3.97 7.31 Ni–P HT. coating rail 3.88 5.28 9.55 11.6 Ni–Mo coating rail 4.05 5.09 6.15 7.63 Ni–Mo HT. coating rail 4.44 5.39 4.41 5.26

3.4. Analysis of Surface Morphology, Elements and Structure of the Electrical Ablation Zone In order to analyze the microscopic condition of the rail surface after catapult, the ablated junction area of the rail after the experiment was appropriately sliced and cut into small test pieces of 15 15 × × 5 mm3. SEM, EDS and XRD analyses were performed to observe the ablated rail surface. The ablation zone is divided into three grades from high to low: severe, moderate and light. From the experimental results of ejecting the armature with the tension spring under the non-energized state and the observation and analysis of the surface morphology after catapult, it can be seen that the wear of the rail surface caused by the mechanical sliding friction after one ejection is almost non-existent. This is because the armature in this study is made of elastic spring steel into a U shape (the tail has an external opening angle of about 3.5◦), and the clamping force applied to the armature by the movement of the rail during ejection is just sufficient to support the armature and the rail contact and maintain the ejection until it leaves the rail. Therefore, we can infer that in this study, in any rail surface, the surface wear caused by pure mechanical friction of the armature can be ignored. We infer that the possible mechanism of rail sliding electric friction and wear is as follows: in the energized state of the armature ejection, the armature is pushed by the electromagnetic force and slides through the conductive rail at a large speed; the initial wear of the rail contact surface should be caused by the large current passing through the rail and the armature contact interface, leading to the contact area to be softened by large Joule heat, so that the material can possibly become half-melted-like and fluid as it encounters the frictional counter body and compresses; part of the softened and fluid Coatings 2020, 10, x FOR PEER REVIEW 10 of 15

Figure 8a shows the ablated area of the iron rail surface. According to the proportion of elements analysisCoatings 2020 by, 10the, 1082 EDX instrument, ref to Table 5, the proportion of iron rail in spectrum 1 is as high10 of as 16 Fe at 95.2 wt.%, and the proportions of other elements (C, Co, Cr, Ni, Mn) are very low, and from the picture from SEM, it is observed in spectrum 1 that there is no obvious electric ablation on the surface ofmaterial the iron peels rail, oonlyff and slight becomes scratches, the thirdwhich body can prov withe metal that the lubricity. surface So,of this during area the is not armature damaged. move In contrast,on the metal the rail,proportion the contact of areaFe in of spectrum the rail surface 2 and would spectrum be subjected 3 decreased to the significantly, interaction a ffandected the of proportioncurrent ablation, of the mechanical remaining frictionelements of (C, the Co, counter Cr, Ni, body, Mn) and increased, the lubrication and it ofis judged the molten from third the bodySEM (debris).pictures that This the results surface in the of micro-morphologythe iron rail is covered formation or sticky of irregular with a third flaking substance. pits, adhesions, It is sufficient and flow to provemarks that in the the wear surface area. of After this area ejection has experiments,obvious electric the ablation EDS detect phenomena, the worn indicating area of the the coating’s degree rail of electricalso shows ablation that inis additionsevere. to the nickel alloy coating’s original material elements, there also exist iron elements;Figure this 8b meansshows thatthe electric the armature ablation iron area material on the becomes nickel–phosphorus-coated the third body and rail transfers after the to thecatapult worn test.surface A large of the number nickel alloy of irregularly coating’s rail.shaped craters on the surface can be clearly seen. As shown in TableIn 5, addition,the EDS analysis from the of visualspectrum field 2 revealed of tribology; that Fe during (95.5 wt.%) the period is the ofmajor electromagnetic element. It looks ejection like thearmature third analysis,substance, part then of thetransferred pulse electric to and energy covered released the surface by the capacitorof nickel–phosphorus-coated stack is disappeared, rail;caused Ni by(40.7 the wt.%) melting and ofFe the(35.9 contact wt.%), metal represents material, major and elements the remaining in the spectrum energy is3 region, used to indicating push the thatarmature this area forward. is only Ifthe partially ability covered of anti-thermal by the resistancesteel from of the the armature. coatings material Most of is the strong, accumulated it would materialbe hard to is softeniron. However, and form athis molten indicates metal that lubricity the degree substance of electric with fluidity,ablation which is moderate. makes the frictional resistanceFigure between 8c is the theelectric armature ablation and area the railof the interface annealed larger, nickel–phosphorus resulting in a decrease plated rail. in the Ref armature to Table 5speed. is EDS This analysis, also canin spectrum be confirmed 2, although by the there results is irregular of the Ni–Mo armature and Fe annealed transfer Ni–Mo and coverage, coating shows rail in thatthis study,the Fe whichin this havearea is a higherless than melting 65 wt.%, point, There so that is less the Fe ejection than the armature nickel–phosphorus speed is slightly rail ablation reduced area,than otherand there rails. are The more damaged C elements area onup theto 22.3 surface wt.%. of This the annealed shows obvious nickel–molybdenum fine molten fragments coating and rail lessis minimized. transfer material accumulation, and a relatively intermediate degree of ablation is moderate. Figure 88d shows shows the the SEM nickel–molybdenum-coated photographs of various rail rail types after of catapult. electric ablationIt belongs areas to light and Tableelectric5 ablation.is the EDX Although instrument many analysis pits ofcan all be types observed of rails surfacefrom this after image, catapult. there All is of still the detectionthird substance points transferredare random. and In covered addition, on for it. convenience,As shown in Table the melting 5 for spectrum substance 5, high of the Fe armature up to 98.6 will wt.% be is called detected the bythird EDS. substance. On the other hand, the presence of abundant Ni (81.4 wt.%), C (10.0 wt.%) and Fe (0.0 wt.%) is indicatedFigure8 withina shows the the spectrum ablated area2 region. of the iron rail surface. According to the proportion of elements analysisFigure by the8e is EDX the instrument, annealed nickel–molybdenum-coated ref to Table5, the proportion ofrail iron after rail catapult. in spectrum This 1 figure is as high shows as Fe that at there95.2 wt.%, are only and a the few proportions pits, most of the other areas elements are very (C, flat, Co, Cr,and Ni, EDS Mn) elemental are very analysis low, and in from Table the 5 pictureshows thatfrom spectrum SEM, it is 1 observedhas Fe (45.1 in spectrumwt.%), Ni (37.0 1 that wt.%), there isMo no (15.0 obvious wt.%), electric and spectrum ablation on2 has the Ni surface (46.9 wt.%), of the Feiron (23.7 rail, onlywt.%), slight Mo scratches,(23.0 wt.%), which C (6.4 can provewt.%). that This the shows surface that of this the area material is not damaged.transferred In from contrast, the armaturethe proportion was evenly of Fe in mixed spectrum with 2 the and nickel–molybdenum spectrum 3 decreased heat-treatment significantly, andrail, theand proportion it is a very of light the ablation,remaining more elements so than (C, the Co, nickel–molybdenum Cr, Ni, Mn) increased, coating. and This it is should judged be from a high the strength SEM pictures rail. It is that easier the tosurface scrape of the the armature iron rail material is covered during or sticky the electric with a friction third substance. process and It leave is suffi itcient on the to plating prove thatfilm, the so thesurface annealed of this nickel–molybdenum area has obvious electric rail is ablation subject phenomena,to the lowest indicating area of electric the degree ablation. of electric This indicates ablation thatis severe. the degree of electric ablation is light.

Figure 8. Cont.

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Figure 8. The SEM photographs of various rail types of electric ablation areas after catapult: ( a)) iron rail; ( b) Ni–P rail; ( c) annealed Ni–P rail; (d) Ni–Mo rail; (e) annealed Ni–MoNi–Mo rail.rail.

Table 5. The EDX instrument analysis of all type rails surface after catapult.

ResultResult EDX AnalysisAnalysis (wt.%) (wt.%) Rail KindKind No.No. FeFe CC CoCo CrCr NiNi MnMn P P Mo Mo SpectrumSpectrum 1 1 95.2 95.2 2.7 2.7 1.7 1.7 0.3 0.3 – – – – – – – – IronIron Rail Rail SpectrumSpectrum 2 2 79.6 79.6 8.1 8.1 0.4 0.4 4.4 4.4 6.6 6.6 1.0 1.0 – – – – Spectrum 3 85.3 2.8 – 3.8 7.0 1.1 – – Spectrum 3 85.3 2.8 – 3.8 7.0 1.1 – – Nickel-Phosphorus Spectrum 2 95.5 – – – – – 4.5 – Spectrum 2 95.5 – – – – – 4.5 – Nickel-PhosphorusCoated Rail Spectrum 3 35.9 – – 14.3 40.7 – 9.0 – Coated Rail annealed SpectrumSpectrum 3 2 35.9 64.2 –22.3 –– 14.3 – 40.713.1 –– 0.4 9.0 – – Nickel–PhosphorusAnnealed Spectrum 2 64.2 22.3 – – 13.1 – 0.4 – Nickel–Phosphorus Spectrum 4 0.2 98.8 – 0.1 0.9 – – – Spectrum 4 0.2 98.8 – 0.1 0.9 – – – CoatedCoated Rail Rail Nickel–Molybdenum Spectrum 2 – 10.0 – 5.7 81.4 – – 2.8 Nickel–Molybdenum Spectrum 2 – 10.0 – 5.7 81.4 – – 2.8 CoatedCoated Rail Rail SpectrumSpectrum 5 5 98.6 98.6 – – – – – – 1.4 1.4 – – – – – – Annealed Spectrum 1 45.1 – – 2.9 37.0 – – 15.0 Annealed Spectrum 1 45.1 – – 2.9 37.0 – – 15.0 Nickel–MolybdenumNickel–Molybdenum SpectrumSpectrum 2 2 23.7 23.7 6.4 6.4 – – – – 46.9 46.9 – – – – 23.0 23.0 CoatedCoated Rail Rail

TheFigure XRD8b showspatterns the corresponding electric ablation to nickel–phosp area on the nickel–phosphorus-coatedhorus and annealed nickel–phosphorus-coated rail after the catapult railtest. after A large the catapult number test of irregularly are provided shaped in Figure craters 9. onAs theseen surface in Figure can 9a, be the clearly peak seen. corresponding As shown to in theTable fcc5 ,nickel the EDS crystal analysis lattice of (111) spectrum can be 2 observed revealed at that 2θ Fe= 44° (95.5 for wt.%)both Ni–P is the coatings major element.with and Itwithout looks annealing.like the third In substance,addition to then the transferreddiffraction toof andNi, coveredthere was the clear surface diffraction of nickel–phosphorus-coated around 2θ angle of 43.5° rail; correspondingNi (40.7 wt.%) andto Ni Fe3P (35.9 for wt.%),both coatings. represents Duncan major elements [34] indicated in the spectrumthe nickel 3 region,converted indicating to Ni3P that at 310~330 °C. This implies that the sliding contact between armature and rail causes the temperature of the ablation zone to rise to 310 °C or above. Thus, the melting of iron from armature material and its final deposition on the rail surface takes place easily when Ni–P-coated rail is used. From patterns

Coatings 2020, 10, 1082 12 of 16 this area is only partially covered by the steel from the armature. Most of the accumulated material is iron. However, this indicates that the degree of electric ablation is moderate. Figure8c is the electric ablation area of the annealed nickel–phosphorus plated rail. Ref to Table5 is EDS analysis, in spectrum 2, although there is irregular armature Fe transfer and coverage, shows that the Fe in this area is less than 65 wt.%, There is less Fe than the nickel–phosphorus rail ablation area, and there are more C elements up to 22.3 wt.%. This shows obvious fine molten fragments and less transfer material accumulation, and a relatively intermediate degree of ablation is moderate. Figure8d shows the nickel–molybdenum-coated rail after catapult. It belongs to light electric ablation. Although many pits can be observed from this image, there is still third substance transferred and covered on it. As shown in Table5 for spectrum 5, high Fe up to 98.6 wt.% is detected by EDS. On the other hand, the presence of abundant Ni (81.4 wt.%), C (10.0 wt.%) and Fe (0.0 wt.%) is indicated within the spectrum 2 region. Figure8e is the annealed nickel–molybdenum-coated rail after catapult. This figure shows that there are only a few pits, most of the areas are very flat, and EDS elemental analysis in Table5 shows that spectrum 1 has Fe (45.1 wt.%), Ni (37.0 wt.%), Mo (15.0 wt.%), and spectrum 2 has Ni (46.9 wt.%), Fe (23.7 wt.%), Mo (23.0 wt.%), C (6.4 wt.%). This shows that the material transferred from the armature was evenly mixed with the nickel–molybdenum heat-treatment rail, and it is a very light ablation, more so than the nickel–molybdenum coating. This should be a high strength rail. It is easier to scrape the armature material during the electric friction process and leave it on the plating film, so the annealed nickel–molybdenum rail is subject to the lowest area of electric ablation. This indicates that the degree of electric ablation is light. The XRD patterns corresponding to nickel–phosphorus and annealed nickel–phosphorus-coated rail after the catapult test are provided in Figure9. As seen in Figure9a, the peak corresponding to the fcc nickel crystal lattice (111) can be observed at 2θ = 44◦ for both Ni–P coatings with and without annealing. In addition to the diffraction of Ni, there was clear diffraction around 2θ angle of 43.5◦ corresponding to Ni3P for both coatings. Duncan [34] indicated the nickel converted to Ni3P at 310~330 ◦C. This implies that the sliding contact between armature and rail causes the temperature of the ablation zone to rise to 310 ◦C or above. Thus, the melting of iron from armature material and its finalCoatings deposition 2020, 10, x on FOR the PEER rail REVIEW surface takes place easily when Ni–P-coated rail is used. From12 patterns of 15 shown in Figure 10a, it is evident that in the case of Ni–P coating, the peak intensity of iron (2θ = 52 ) shown in Figure 10a, it is evident that in the case of Ni–P coating, the peak intensity of iron (2θ = 52°) is ◦ is much higher when compared to the annealed Ni–P coating. This indicates that the electrical ablation much higher when compared to the annealed Ni–P coating. This indicates that the electrical ablation ofof annealed annealed Ni–P-coated Ni–P-coated rail rail is is less less thanthan thatthat ofof Ni–P-coatedNi–P-coated rail. rail. Figure Figure 9b9b is is the the X-ray X-ray curve curve pattern pattern ofof the the ablation ablation area area of of moderate moderate wear,wear, showingshowing that the relative relative intensity intensity of of the the diffraction diffraction peak peak of of NiNi3P3 isP higheris higher than than that that of the of mainthe main peak ofpeak nickel, of nick andel, the and relative the relative intensity intensity of iron is of also iron significantly is also reduced;significantly this may reduced; be due this to may the be energy due to of the the energy passing of currentthe passing is used current to heat is used the to nickel–phosphorus heat the nickel– coating,phosphorus and only coating, a smaller and only part a is smaller used to part melt is used the armature. to melt the armature.

(a) (b)

FigureFigure 9. 9.X-ray X-ray didiffractionffraction (XRD) patterns patterns of of worn worn surf surfacesaces on onNi–P Ni–P and andannealed annealed Ni–P Ni–Pcoatings: coatings: (a) (a)Ni–P Ni–P wear wear region; region; (b ()b annealed) annealed Ni–P Ni–P wear wear region. region.

The XRD patterns corresponding to nickel–molybdenum and annealed nickel–molybdenum- coated rail after the catapult test are provided in Figure 10. Figure 10a displays the XRD patterns from the light ablation region. As seen in Figure 10a, the intensity of the peak corresponding to iron is much higher than that of Ni for both Ni–Mo coating with and without heat treatment, indicating that more iron from the armature was melted by the current and transferred to the surface coating of rails during the sliding friction. Comparing the two XRD patterns in Figure 10a, it can be strongly suggested that more iron covered the surface of Ni–Mo coating than annealed Ni–Mo coating. However, the XRD patterns displayed in Figure 10b show that the intensity of peak corresponding to Ni crystal is higher than that of iron when the ablation is light for both Ni–Mo and annealed Ni–Mo coatings. Moreover, the Ni4Mo phase in the XRD profile can be seen at 2θ = 75° even for Ni–Mo coating without heat treatment. It has been reported that the precipitation of Ni4Mo initiates at a heat treatment temperature of 500 °C [24]. It is reasonable to think that most of the current passing through this ablation zone was used to heat the nickel–molybdenum coating to a temperature of 500 °C, and only a smaller part of heat was used to melt the armature. As a result, the obvious precipitation of the Ni4Mo phase is observed in the XRD patterns. Through the above experimental analysis results, it can be clearly understood that part of the pulsed electrical energy released by the capacitor stack during the catapult experiment is consumed by the melting of the armature material, and the rest of the energy is used to propel the armature forward. The nickel–molybdenum and annealed nickel–molybdenum-coated rails used in this study have a higher melting point than nickel, iron, and phosphorus, resulting in the smallest area of electrical ablation on the surface of the rail. Although the armature catapult velocity is slightly reduced, the damaged area of the rail coated with annealed nickel–molybdenum is minimal. Reducing the surface roughness of the nickel–molybdenum coating will be an effective way to enhance the speed of armature ejection.

Coatings 2020, 10, 1082 13 of 16 Coatings 2020, 10, x FOR PEER REVIEW 13 of 15

(a) (b)

FigureFigure 10. 10.XRD XRD patterns patterns of of worn worn surfacessurfaces on Ni–Mo and and annealed annealed Ni–Mo Ni–Mo coatings: coatings: (a) ( aNi–Mo) Ni–Mo wear wear region;region; (b ()b annealed) annealed Ni–Mo Ni–Mo wear wearregion. region.

4. ConclusionsThe XRD patterns corresponding to nickel–molybdenum and annealed nickel–molybdenum- coated rail after the catapult test are provided in Figure 10. Figure 10a displays the XRD patterns from This study tested the rail surface materials of the electromagnetic rail catapult system, including thefive light types ablation of coatings—iron region. As seen rail, in nickel–phosphoru Figure 10a, the intensitys coating of rail, the nickel–phosphorus peak corresponding heat to irontreatment is much highercoating than rail, that nickel–molybdenum of Ni for both Ni–Mo coating coating rail withand nickel–molybdenum and without heat treatment, heat treatment indicating coating that rail. more ironThis from research the armature obtained was the meltedfollowing by results: the current and transferred to the surface coating of rails during the sliding friction. Comparing the two XRD patterns in Figure 10a, it can be strongly suggested that 1. The hardness of these five tested materials is 215, 512, 650, 720 and 1045 HV for iron, nickel– more iron covered the surface of Ni–Mo coating than annealed Ni–Mo coating. However, the XRD phosphorus, annealed nickel–phosphorus, nickel–molybdenum, and annealed nickel– patterns displayed in Figure 10b show that the intensity of peak corresponding to Ni crystal is higher molybdenum, respectively. Heat treatment increases the hardness of the Ni–P and Ni–Mo than that of iron when the ablation is light for both Ni–Mo and annealed Ni–Mo coatings. Moreover, coatings significantly, which can be ascribed to the precipitation of intermetallic compounds such the Ni4Mo phase in the XRD profile can be seen at 2θ = 75◦ even for Ni–Mo coating without heat as Ni3P precipitation for Ni–P and Ni4Mo precipitation for Ni–Mo coating. treatment.2. The contact It has been resistance reported of the that five the rail-type precipitation materials of Ni 4isMo slightly initiates higher at a than heat treatment0.2 Ω except temperature for the of 500annealed◦C[24]. nickel–phospho It is reasonablerus-coated to think that rail (0.186 most ofΩ). the However, current its passing impact through on the armature this ablation catapult zone was usedvelocity to heat is not the as nickel–molybdenum obvious as the surface coating roughness to a of temperature the rail. The of sliding 500 ◦C, velocity and only of the a smallerarmature part of heatdecreases was used slightly to melt with the the armature. increase Asin the a result, surface the roughness obvious of precipitation the rail. of the Ni4Mo phase is observed3. During in the the XRD catapult patterns. process, discontinuous and rapid electrical ablation on the rail surface was Throughobserved. the The above ratio of experimental ablated area analysisto the total results, area of itthese can rails be clearly is: ironunderstood rail (19.5%) > that Ni–P-coated part of the pulsedrail electrical (12.9%) energy> annealed released Ni–P-coated by the capacitor rail (11.6%) stack > Ni–Mo-coated during the catapult rail (4.9%) experiment > annealed is consumedNi–Mo- by thecoated melting rail of (1.1%). the armature Our results material, show that and the the hardne rest ofss theof the energy rail material is used is to an propel important the armature factor forward.for the The resistance nickel–molybdenum of electrical ablation. and annealed nickel–molybdenum-coated rails used in this study have4. aEDS higher analysis melting shows point that than there nickel, is more iron, iron and in phosphorus,the ablation area resulting of nickel–phosphorus in the smallest areaand of electricalannealed ablation nickel–phosphoru on the surface ofs-coated the rail. rails, Although indicating the armaturethe iron from catapult the armature velocity ismaterial slightly melted reduced, the damagedand transferred area of to the nickel–phosphorus rail coated with annealedcoating easi nickel–molybdenumly. On the contrary, the isminimal. average value Reducing of iron the surfacein roughness the ablation of zone the nickel–molybdenum of annealed nickel–molybdenum coating will coating be an e ffisective relatively way small. to enhance the speed of 3 armature5. XRD ejection. analysis of the ablation zone identifies the presence of Ni P precipitation for nickel– phosphorus and annealed nickel–phosphorus-coated rails and Ni4Mo phase is observed for 4. Conclusionsnickel–molybdenum and annealed nickel–molybdenum-coated rails. Therefore, it can be deduced that the rail surface of Ni–P and Ni–Mo systems can reach a high temperature that can Thisinduce study thetested second the phase rail precipitation. surface materials Most of of the the electromagneticpassing current is rail used catapult for heating system, up includingthe rail five typesmaterial of coatings—iron and less energy rail, can nickel–phosphorus be used to melt the coating armature, rail, resulting nickel–phosphorus in a decrease heat in electrical treatment coatingablation. rail, nickel–molybdenum Thus, the thermal conductivity coating rail should and nickel–molybdenum also be an important factor heat treatment for the rail coating material rail. This researchto withstand obtained the electrical the following ablation. results:

1. The hardness of these five tested materials is 215, 512, 650, 720 and 1045 HV for iron, nickel–phosphorus, annealed nickel–phosphorus, nickel–molybdenum, and annealed nickel– molybdenum, respectively. Heat treatment increases the hardness of the Ni–P and Ni–Mo coatings significantly, which can be ascribed to the precipitation of intermetallic compounds such as Ni3P precipitation for Ni–P and Ni4Mo precipitation for Ni–Mo coating.

Coatings 2020, 10, 1082 14 of 16

2. The contact resistance of the five rail-type materials is slightly higher than 0.2 Ω except for the annealed nickel–phosphorus-coated rail (0.186 Ω). However, its impact on the armature catapult velocity is not as obvious as the surface roughness of the rail. The sliding velocity of the armature decreases slightly with the increase in the surface roughness of the rail. 3. During the catapult process, discontinuous and rapid electrical ablation on the rail surface was observed. The ratio of ablated area to the total area of these rails is: iron rail (19.5%) > Ni–P-coated rail (12.9%) > annealed Ni–P-coated rail (11.6%) > Ni–Mo-coated rail (4.9%) > annealed Ni–Mo-coated rail (1.1%). Our results show that the hardness of the rail material is an important factor for the resistance of electrical ablation. 4. EDS analysis shows that there is more iron in the ablation area of nickel–phosphorus and annealed nickel–phosphorus-coated rails, indicating the iron from the armature material melted and transferred to nickel–phosphorus coating easily. On the contrary, the average value of iron in the ablation zone of annealed nickel–molybdenum coating is relatively small.

5. XRD analysis of the ablation zone identifies the presence of Ni3P precipitation for nickel– phosphorus and annealed nickel–phosphorus-coated rails and Ni4Mo phase is observed for nickel–molybdenum and annealed nickel–molybdenum-coated rails. Therefore, it can be deduced that the rail surface of Ni–P and Ni–Mo systems can reach a high temperature that can induce the second phase precipitation. Most of the passing current is used for heating up the rail material and less energy can be used to melt the armature, resulting in a decrease in electrical ablation. Thus, the thermal conductivity should also be an important factor for the rail material to withstand the electrical ablation.

Author Contributions: Methodology, L.-S.H., P.-C.H., and C.-C.C.; investigation, L.-S.H., P.-C.H., and C.-C.C.; resources, K.-H.H. and M.-D.G.; writing—original draft preparation, L.-S.H.; writing—review and editing, P.-C.H., K.-H.H., and G.-L.W.; supervision, M.-D.G., and G.-L.W.; project administration, K.-H.H., M.-D.G., and G.-L.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Ministry of Science and Technology, Taiwan, Grant No. 108-2623-E-606-004-D. Acknowledgments: The supports of this work from MOST No. 108-2623-E-606-004-D is appreciated. Conflicts of Interest: The authors declare no conflict of interest.

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