BACKGROUND

[0001] Plating or coating of an item or surface is typically performed to provide one or more desirable properties to the surface or item that otherwise lacks that property. For example, a surface may be plated to provide properties of abrasion resistance, corrosion resistance, hardness, conductivity, magnetic properties, and the like, that are otherwise absent from the surface.

[0002] One type of plating process includes electroplating, sometimes also referred to as electrodeposition. There different variations of electroplating, but the process typically includes a conductive item or surface to be metal plated that forms a cathode and an anode that are immersed in an electrolyte containing more than one dissolved metal salt. A battery or a rectifier may supply a direct or pulsed current to the electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] FIG. 1 is a block diagram of an example apparatus of the present disclosure;

[0004] FIG. 2 is a block diagram of an example plating of the present disclosure;

[0005] FIG. 3 is a flow chart of an example method for plating a substrate with a metal matrix coating containing two metals;

[0006] FIG. 4 is an example of X-ray diffraction patterns of Ni and Ni-Bi coatings deposited by DC and PC;

[0007] FIG. 5 is an example of cross-section images of Ni and Ni-Bi coating produced under different plating methods;

[0008] FIG. 6 is an example of intensity of Bi element for different coatings;

[0009] FIG. 7 is an example of micro-hardness of Ni and Ni-Bi coatings with DC and PC deposition;

[0010] FIG. 8 is an example of TEM micrographs of Ni and Ni-Bi coating produced by DC and PC plating methods; [0011] FIG. 9 is an example of TEM images of Ni-Bi coating: (a) Bright field, (b) high-angle annular dark-field (HAAFD) STEM tomography and (c) EDS spectra of the red box region;

[0012] FIG. 10 is an example of selected area diffraction patterns of (a) pure Ni and (b) Ni-Bi coatings;

[0013] FIG. 1 1 is an example of HRTEM images of Ni-Bi coating;

[0014] FIG. 12 is an example of wear volume loss coatings;

[0015] FIG. 13 is an example of potentiodynamic polarization curves of Ni and Ni-Bi coatings deposited by using DC and PC methods;

[0016] FIG. 14 is an example of Nyquist plots of impedance spectra for: (a)

Ni DC, (b) Ni PC, (c) Ni-Bi DC and (d) Ni-Bi PC, all conducted in 3.5 wt. % NaCI solution;

[0017] FIG. 15 is an example of equivalent electrical circuit used for the parameter calculation;

[0018] FIG. 16 is an example of the Bode plots of Ni and Ni-Bi coatings;

[0019] FIG. 17 is an example of (a) XRD pattern of Ag and Ag-Bi alloy coating by incorporation of different amount of Bi, (b) magnified peak Ag of (b) Ag (1 1 1 ) and (c) Ag (200);

[0020] FIG. 18 is an example of ESEM top morphologies of electrodeposited coatings;

[0021] FIG. 19 is an example of ESEM cross-section images of

electrodeposited coatings;

[0022] FIG. 20 is an example of percentage of Bi present in Ag-Bi coatings as a function of nominal amount Bi added to the Ag bath solution;

[0023] FIG. 21 is an example of gradient forward (GF) images of the indentation after tested;

[0024] FIG. 22 is an example of a comparison of load/unload displacement curves of coatings; and

[0025] FIG. 23 is an example of hardness comparison of (a) Ag and Ag alloy coating (b) Ag-0.5 imM Bi, (c) Ag-1 .0 imM Bi, (d) Ag-2.0 imM Bi. SUMMARY

[0026] According to aspects illustrated herein, there are provided a method for plating a substrate with a metal matrix coating containing two metals and a coated substrate. One disclosed feature of the embodiments is a method that provides a plating solution comprising ions of the two metals, wherein a first concentration of a first metal of the two metals and a second concentration of the second metal of the two metals are different, wherein the first metal comprises a transition metal and the second metal comprises a post transition metal or metalloid, heats the plating solution to a pre-defined temperature, inserts the substrate as a cathode and an anode into the plating solution, agitates to the plating solution and applies a constant current to the plating solution for a pre-defined amount of time via a constant current power supply coupled to the cathode and the anode to form the metal matrix coating on the substrate, wherein the metal matrix coating contains nanoparticles of an intermetallic of the first metal and the second metal or an alloy of the first metal and the second metal.

[0027] Another disclosed feature of the embodiments is a coated substrate. The coated substrate includes a metal substrate and a metal matrix coating the substrate to form the coated substrate, wherein the metal matrix coating comprises a first metal that forms a crystalline structure and a plurality of nanoparticles of an intermetallic of the first metal and a second metal, or an alloy of the first metal and the second metal, distributed within the crystalline structure of the first metal that is formed via an electroplating process.

DETAILED DESCRIPTION

[0028] The present disclosure discloses a reusable substrate and a method for plating or coating a metal surface. As discussed above, one type of plating process includes electroplating. Current electroplating methods make it difficult to coat a substrate using a bath that contains ions of two metals that are insoluble in the solid state.

[0029] For example, currently used methods may suspend fine particles of a second metal in a plating bath while the plating bath is agitated vigorously to ensure highly dispersed fine particles of the second metal in the coating. The vigorous agitation may result in a porous coating or create other problems with the metal matrix coating.

[0030] Other methods may require that metals are deposited separately and a post deposition process, such as heat treatment, is used to form intermetallic layers. However, such processes may cause a deterioration or change in the substrate or coating which engenders undesirable properties at the same time as creating the desired intermetallic compounds. In addition, the other methods do not result in uniformly distributed nanoparticles of an intermetallic or an alloy of the metals that engender the desirable properties of the coating as described by the present disclosure.

[0031] Embodiments of the present disclosure provide a method for creating a metal matrix coating that avoids the problems noted above. In one

embodiment, the metal matrix coating contains two metals with different concentrations. One metal forms a substantially crystalline coating, while a combination of the first and second metal creates nanoparticle domains within the crystal matrix.

[0032] The metal matrix coating may be formed in one embodiment using an electrolyte bath containing a combination of ions of the two metals that are insoluble and do not form an alloy. The coating may be deposited using electrodeposition at or near ambient temperatures where ionic co-discharge reduces the ions in the electrolyte onto a substrate to create the coating. The coating may be a crystalline matrix structure of the first metal containing intermetallic nanoparticles of the two metals.

[0033] In another embodiment, the electrolyte bath may contain ions of two metals that are minimally soluble in the solid phase and have a limited ability to form an alloy. The relative concentration of the ions may be arranged such that the first metal creates a crystalline metal matrix and an alloy of the first metal and the second metal is deposited as uniformly distributed nanoparticles within the crystalline metal matrix.

[0034] FIG. 1 illustrates an example apparatus 100 for performing the methods described herein. In one embodiment, the apparatus 100 may include a container or vessel 150 that holds a plating solution 102. In one embodiment, the plating solution 102 may be an electrolyte or coating bath.

[0035] In one embodiment, the two different metals may be present as metal ions or metallic salts in the plating solution 102. The type of plating solution 102 that is used may depend on the two different metals that are selected.

Examples of the different plating solutions for different metals are discussed in further detail below.

[0036] In one embodiment, the first metal may be a transition metal.

Examples of the first metal may include iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) or silver (Ag). In one embodiment, the second metal may be a post transition metal or metalloid. Examples of the second metal may include bismuth (Bi), arsenic (As) or tin (Sn).

[0037] In one embodiment, the first metal and the second metal that are selected may be insoluble in a solid phase and do not form an alloy. One example may be where the first metal is Ni and the second metal is Bi.

[0038] In another embodiment, the first metal and the second metal that are selected may be minimally soluble in the solid phase and have a limited ability to form an alloy. However, the concentrations of ions of the second metal may be controlled such that the alloy is not formed in the plating. Rather, the alloy may be deposited via co-ionic discharge as nanoparticles throughout a metal matrix formed by the first metal. One example may be where the first metal is Ag and the second metal is Bi.

[0039] The plating solution 102 may comprise two different metals in different concentrations that are controlled to form a metal matrix or a crystalline metal structure of the first metal having evenly distributed nanoparticles of an intermetallic of the first metal and the second metal, or an alloy of the first metal and the second metal. The differences in concentration between the two different metals may be disproportionate. In other words, a concentration of a first metal of the two different metals may be much larger than a concentration of a second metal of the two different metals.

[0040] In one example using two metals that are insoluble such as Ni as the first metal and Bi as the second metal, the concentration of Ni may be a mass concentration of approximately 250 grams per liter (g/L) and the volumetric concentration of Bi electrolyte may be approximately 5 milliliters per liter of the plating solution (imL/L) to 400 ml/L.

[0041] In another example using two metals that are minimally soluble the concentration of the second metal may be the parameter that controls the characteristics of the plating, or coating. The appropriate amount of the second metallic ionic phase relative to the first metallic ionic phase may depend on the metal ions comprising the first and second phases, the relative ionic charges of the metal ions, the plating current and whether a direct plating current or a pulsed plating current is used.

[0042] The concentration of the second metal may be controlled such that the nanoparticles of an intermetallic of the two metals, or an alloy of the two metals formed by the ionic co-discharge are of a size to produce a combination of dispersion strengthening and grain refinement of the coating. For example, the grain refinement produced by the presence of the nanoparticles may be at least 5% to at least 25% of a grain size for a coating which is a pure alloy of the two metals formed under normal, or previously used, plating conditions.

[0043] In one embodiment, where the two metals are immiscible in the solid phase the percentage of ions of the second metal in the plating solution 102 compared to the ions of the first metal in the plating solution 102 may be approximately less than 1 % to less than 0.1 %. In other words, the

concentration of the ions of the second metal in the plating solution 102 should be low enough that such that the concentration is below the solubility limit of the second metal in the first metal. In one example where Ag is the first metal is Ag and Bi is the second metal, the mass concentration of Ag may be approximately 30 g/L and the volumetric concentration of Bi electrolyte may be approximately 0.5 ml/L of the plating solution to 40 ml/L.

[0044] In one embodiment, the plating solution 102 may be free from any reduction catalyst. In other words, the plating solution 102 does not contain a reduction catalyst or use an autocatalytic process. Unlike traditional

electroplating methods and traditionally used plating conditions, the present disclosure forms a plating on a substrate 106 (also referred to as a cathode 106 interchangeably) using an ionic co-discharge, as discussed in further detail below.

[0045] In one embodiment, the apparatus 100 may include a power supply 108, an agitator 1 10, a heating element 1 12, a heating element control 1 14 and an agitator control 1 16. In one embodiment, the substrate 106 that is to be plated may be used as a cathode that is coupled to a negative terminal of the power supply 108 and an anode 104 may be coupled to a positive terminal of the power supply 108. In one embodiment, the substrate or the cathode 106 may be any type of metal.

[0046] In one embodiment, the substrate 106 may be any type of metal. The substrate 106 may be pre-treated to help the plating deposition. The pre- treating process may include, for example, pre-coating, mechanical polishing, electro-polishing, alkaline washing, acid washing, degreasing, electroactivation, and the like.

[0047] In one embodiment, the anode 104 may be comprised of either of the two different metals that are included as a metal salt or ion in the plating solution 102. In another embodiment, the anode 104 may be an inert substance.

[0048] In one embodiment, the power supply 108 may apply a constant current to the plating solution 102 via the cathode 106 and the anode 104. In one embodiment, the constant current may be supplied as a direct current or a pulsed current. In one embodiment, the amount of current that is applied by the power supply 108 may depend on the metals that are selected for the plating solution 102. In one embodiment, the amount of current may be between an approximate range of 20 milliamps per square centimeter (imA/cm ² ) to 80 imA/cm ² . Specific examples associated with various metals are discussed in further detail below.

[0049] In one embodiment, the plating solution 102 may be heated to approximately a temperature that is between a range of 20 degrees Celsius (°C) to 45 °C. In one embodiment, the temperature is approximately an ambient temperature or slightly above an ambient temperature. The temperature may be controlled via the temperature controller 1 14 and the heating element 1 12.

[0050] In one embodiment, the plating solution 102 may be agitated by the agitator 1 10. The agitator 1 10 may be any type of agitation, such as for example, via air (e.g., bubbling air through the plating solution 102), magnetic stirring, circulatory pumping, vibrating or rocking the container 150, ultrasonic agitation, and the like. In one embodiment, more than one method of agitation may be used (e.g., stirring and rocking).

[0051] In one embodiment, the amount of agitation may be a function of a minimal mixing that dissipates hydrogen formed at a plating surface of the substrate 106 and that allows the formation of a compact smooth plating or coating. In one embodiment, the agitator 1 10 may be a magnetic stirrer that operates at a speed approximately in the range of 50 rotations per minute (rpm) to 1500 rpm. In one embodiment, where the two metals are minimally soluble (e.g., Ag and Bi), an electrolyte of the second metal (e.g., Bi) may be added under controlled temperature and agitation to provide uniform dispersion of the second metal in the plating solution 102.

[0052] In one embodiment, by using the apparatus 100 to control a plating current, a degree of mixing, and a temperature of the plating solution 102 and controlling concentrations of two different metals and a pH of the plating solution 102 a new plating is created. For example, the substrate 106 may be

suspended in the plating solution 102 for a predetermined amount of time to plate the substrate 106. The amount of time the substrate 106 remains suspended in the plating solution 102 may be a function of a desired thickness of the plating on the substrate 106. Specific examples are discussed in further detail below.

[0053] FIG. 2 illustrates an example of a coated substrate 200 (also referred to as a plated substrate 200). As discussed above, ions of a first metal 202 and a second metal 204 are in the plating solution 102. The constant current applied to the plating solution 102 causes the ions 202 and 204 to be reduced and form a metal matrix 206 having nanoparticles 208 dispersed throughout via an ionic co-discharge.

[0054] As shown in FIG. 2, the coated substrate 200 may comprise the substrate 106 and a metal matrix coating 206. The substrate 106 may be a metal substrate, such as for example, zinc (Zn) coated steel, Cu coated brass, and the like.

[0055] In one embodiment, the metal matrix coating 206 may be a crystalline metal structure formed by the ions of the first metal 202. The metal matrix coating 206 may include nanoparticles 208 formed by an intermetallic of the first metal 202 and the second metal 204 or an alloy of the first metal 202 and the second metal 204. The nanoparticles 208 may be distributed within the crystalline structure of the metal matrix coating 206 that is formed via the electroplating process described herein. The nanoparticles 208 may be uniformly, or evenly dispersed, throughout the metal matrix coating 206. In one embodiment, the metal matrix coating 206 may have a thickness between approximately 20 microns to 600 microns.

[0056] In one embodiment, the nanoparticles 208 may have a particle size that is less than 100 nanometers (nm). In one embodiment, the nanoparticles 208 have a particle size that is less than 20 nm. In one embodiment, the nanoparticles 208 may have a particle size of approximately 10 nm.

[0057] As discussed above, the metal matrix coating 206 is formed via an ionic co-discharge process and not an autocatalytic process. The metal matrix coating 206 having the dispersed nanoparticles 208 may have desirable properties such as, for example, smoothness, hardness, wear resistance, chemical inertness, corrosion resistance, anti-bacterial properties, and the like, that are as good or better than previous coatings and platings that were produced.

[0058] For example, the Vickers microhardness of the metal matrix coating 206 with the nanoparticles 208 formed via the process described herein may be at least 10% to 60% greater than a coating prepared using a different method than the process described herein. In one embodiment, the metal matrix coating 206 having the nanoparticles 208 produced via the process described herein may have a Vickers hardness of approximately 260 kilograms per square millimeter (kg/mm ² ) to about 800 kg/mm ² .

[0059] The grain size of the metal matrix coating 206 produced via the process described herein may be at least 5% to 15% less than the grain size of a coating produced via other methods. It is believed that the intermetallic or alloy nanoparticles 208 in the metal matrix coating 206 formed via the ionic co- discharge of the present disclosure disrupts the grain growth of the metal matrix coating 206. This leads to a reduced grain size compared to coatings prepared via other methods.

[0060] The metal matrix coating 206 produced via the process described herein may have a wear loss volume (as measured according to ASTM G133) of at least about 10% less to 30% less than a coating produced via other methods. The electric resistivity and/or conductivity of the metal matrix coating 206 produced via the process described herein may be within about +/- 6% of a coating produced via other methods.

[0061] FIG. 3 illustrates an example flowchart of a method 300 for plating a substrate with a metal matrix containing two metals. In one example, the method 300 may be performed by the apparatus 100.

[0062] At block 302 the method 300 begins. At block 304, the method 300 provides a plating solution. In one embodiment, the plating solution may comprise ions of two different types of metals. As noted above, a first metal of the two types of metals may be a transition metal such as Fe, Co, Ni, Cu, Ag, and the like. The second metal of the two types of metals may be a post transition metal or metalloid such as Bi, As, Sn, and the like. In one

embodiment, the ions of the second metal may be added to the plating solution as the plating solution is agitated.

[0063] The concentration of ions of the first metal and the concentration of ions of the second metal may be different. The concentrations may be based on the first metal and the second metal that are selected. The concentration of ions of the first metal may be much larger relative to the concentration of ions of the second metal in the plating solution.

[0064] In one example, where the two metals are insoluble in a solid phase and do not form an alloy, the concentration of the first metal may be a mass concentration of approximately 250 g/L and the concentration of the second metal may be a volumetric concentration of approximately 5 imL/L of the plating solution to 400 ml/L. In addition, the plating solution may be a standard Watts Ni bath. [0065] In one example, where the two metals have properties that allow the two metals to be minimally soluble in a solid phase and have a limited ability to form an alloy, the concentration of the ions of the second metal may be controlled. The concentration of the ions of the second metal may be controlled such that the metal matrix coating that is formed from the first metal and nanoparticles of an alloy of the first metal and the second metal are formed and dispersed within the metal matrix coating. The nanoparticles may be uniformly distributed through the metal matrix. In one example, the concentration of the first metal may be a mass concentration of approximately 30 g/L and the concentration of the second metal may be a volumetric concentration of approximately 0.5 imL/L of the plating solution to 40 ml/L. In addition, the plating solution may be a combination of silver cyanide (AgCN), potassium cyanide (KCN) and potassium hydroxide (KOH).

[0066] At block 306, the method 300 heats the plating solution. In one embodiment, the plating solution may be heated to a pre-defined temperature. In one embodiment, the pre-defined temperature may be approximately between 20 °C and 45 °C. In one embodiment, the pre-defined temperature may be approximately an ambient temperature or slightly above an ambient temperature.

[0067] At block 308, the method 300 may insert a substrate as a cathode and an anode into the plating solution. For example, the cathode and the anode may be suspended in the plating solution. In one embodiment, the substrate may be any type of metal. In one embodiment, the anode may be comprised of either of the two metals or an inert substance. In one embodiment, the substrate that serves as the cathode may be electroplated or coated with the two metals in the plating solution, as described above with reference to FIG. 2 and discussed in further detail in the examples described below.

[0068] At block 310, the method 300 agitates the plating solution. Any type of agitation may be used, such as for example, via air (e.g., bubbling air through the plating solution 102), magnetic stirring, circulatory pumping, vibrating or rocking the container 150, ultrasonic agitation, and the like. In one embodiment, more than one method of agitation may be used (e.g., stirring and rocking). [0069] At block 312, the method 300 may apply a constant current to the plating solution to form a metal matrix coating containing nanoparticles on the substrate. For example, the cathode and the anode may be coupled to a power supply that supplies the constant current. The constant current may be applied to the plating solution via the cathode and the anode suspended in the plating solution.

[0070] In one embodiment, the constant current may be a direct current or a pulsed current. The constant current may be applied for a pre-defined amount of time that is determined based on a desired thickness of the plating on the substrate. In one embodiment, the amount of constant current may be adjusted to achieve the desired thickness.

[0071] In one embodiment, the metal matrix coating may be formed via a crystalline structure of the first metal. The nanoparticles may be formed via an intermetallic of the first metal and the second metal or an alloy of the first metal and the second metal. The nanoparticles may be uniformly dispersed

throughout the crystalline structure of the metal matrix. An example of the metal matrix coating is illustrated in FIG. 2.

[0072] In one embodiment, the coated substrate or coated cathode may be removed and rinsed. The metal ions in the plating solution may be replenished and the process may be repeated for another substrate. At block 314, the method 300 ends.

Example 1 :

[0073] Example 1 demonstrates the preparation of a metal matrix coating of Ni and Bi on mild steel where the ionic co-discharge process results in the formation of nanoparticles of a Ni-Bi intermetallic and investigates the micro structure, microhardness, wear resistance and corrosion resistance of the coating.

[0074] Electrolyte Preparation:

[0075] Ni coatings were deposited by using standard acidic watt solution. The optimum level of Ni was between 250 g/L to 350 g/L. The Ni containing slat was Ni.SO ₄ .6H ₂ O and/or NiCI.6H ₂ O. The Ni-Bi coating was prepared from a standard Watt's Ni electrolyte by adding with 20 imL/L Bi electrolyte. The Bi electrolyte comprised 0.2 M Bi(NO ₃ ) ₃ -5H ₂ O, 0.2 M tartaric acid and 2.5 M KOH.

[0076] Sample Preparation:

[0077] Zn coated Mild steel sheets coated having a dimension of 20 ^χ 30 ^χ 1 mm ³ were used as substrates. The substrates were first pickled in a mixture solution of 10% v/v sulphuric acid and 1 % v/v Metex XLR8 for 2-3 min in order to strip off the Zn coating. After pickling the substrates were rinsed in distilled water prior to electroplating.

[0078] Electroplating Procedure:

[0079] A pure Ni plate was used as an anode. Both Ni and Ni-Bi coatings were deposited by Direct Current (DC) and Pulsed Current (PC) deposition methods. Both the DC and PC depositions were performed at 45 °C for 30 min while stirring at 300 rpm. For PC deposition, time on (T _on ) and time off (T _off ) were kept constant at 1 milliseconds (ms) and 9 ms, respectively. The average current density of PC deposition was set to 40 imA/cm ² , identical to the DC plating process. A hard compact coating comprising a Ni metal matrix and nanoparticles of an intermetallic form of NiBi with particle dimensions of approximately 15 nm to 20 nm was formed on the substrate.

[0080] Morphology, Elemental Composition, Micro-structure and Chemical Analysis:

[0081] The morphology and elemental composition and phase structure of coatings was characterized by X-ray diffraction (XRD) with Cu K _« radiation (V = 30 kV, l= 15 imA). Cross-section morphology of the deposits were examined by FEI Quanta 200 field emission environmental scanning electron microscope (FESEM) with an energy dispersive spectrometry (EDS) system. Transmission electron microscopy (TEM, Philips CM12) was used to investigate the coating microstructure and the effects of PC plating and Bi content. Chemical analysis was carried out by Laser ablation-inductively coupled plasma-mass

spectrometry (LA-ICP-MS) with a laser power of 40 %, repetition rate of 20 Hz and spot size of 60 μιη.

[0082] Microstructure of coatings:

[0083] The microstructural studies were carried out using high resolution TEM (FEI Tecnai G2 F20, 200 kV) which equipped with an energy dispersive spectroscope (EDS) device. For the TEM sample preparation, the coating was first deposited on a stainless steel. The coating was then peeled off and punched in a 3 mm diameter size using specimen punch (Fischione model 130). After that, the specimen was thinned with a low angle ion milling and polish system (Fischione Instrument model 1010) to obtain extremely thin areas ready for TEM observations.

[0084] Micro-hardness and Wear Resistance Analysis:

[0085] Vickers microhardness was measured by a hardness tester (Leco M400) using a load of 100 gf with a holding time of 15s. Wear resistance was examined by micro-tribometer (Nanovea) under dry sliding condition in air at 25 °C. All tests were conducted under a load of 7 N, a sliding speed of 2 m/min and a contact radius of 6 mm for a total sliding distance of 20 m. The width of wear track was measured using optical microscope.

[0086] Corrosion Resistance Evaluation:

[0087] The corrosion potential of each layer were measured by CHI604D electrochemical workstation using a classical three-electrode cell with platinum as the counter electrode; saturated calomel electrode (SCE) as reference electrode; and the coating samples with 1 cm ² exposed area as the working electrode. All corrosion tests were performed under room temperature using a standard flat cell in 3.5 wt. % NaCI solution. The potentiodynamic curves were measured under a constant scan rate of 0.01 V/s. The electrochemical impedance spectra (EIS) were acquired in the frequency range of 0.01 Hz to 100 kHz with a signal amplitude of 5 mV.

[0088] Results:

[0089] Phase Structure of Coatings:

[0090] The XRD patterns of Ni and Ni-Bi coating prepared by DC and PC electroplating are presented in FIG. 4. It can be seen that all coatings exhibit three peaks at 2Θ= 44°, 51 °, 76°, corresponding to Ni (1 1 1 ), (200) and (220), respectively. According to the PDF cards, the XRD peaks of NiBi (1 10) and NiBi (200) are very close to Ni (1 1 1 ) and Ni (200). Therefore, no obvious NiBi peaks can be observed in the XRD patterns of Ni-Bi coatings due to the relatively low concentration of NiBi phase and the high intensity of Ni peaks.

[0091] Cross-Section of Coatings:

[0092] Cross-sectional ESEM micrographs of Ni and Ni-Bi coatings are shown in FIG. 5. Good interface between coatings and the steel substrate is observed. No abruption or cracks exist at the interfaces of the coatings, evidence of good adhesion of the coatings.

[0093] During the plating process, Ni ²⁺ and Bi ³⁺ ions were co-discharged on the surface of the cathode. NiBi intermetallic phase and Ni atoms were formed in the same time. Ni atoms formed a continuous Ni coating matrix and NiBi intermetallic phase was incorporated in the Ni matrix to form a composite coating.

[0094] The NiBi intermetallic phase was uniformly distributed in the coating as pointed by the arrows in FIGs. 4c and 4d. More NiBi intermetallic compound were shown in the PC Ni-Bi coating, indicating that PC could enhance the Bi incorporation process.

[0095] Ni-Bi coatings exhibit a slightly increased thickness when compared than pure Ni coating. The thickness of Ni-Bi deposited by DC and PC was 12.2±0.3 and 12.6±0.3 μηπ, while the thickness of pure Ni coating was 8.8±0.5 and 8.9±0.2 μιη, respectively. This indicates that the incorporation of Bi ions can increase the deposition rate. However, there is little difference in coating thickness direct and pulsed current depositions.

[0096] Chemical Analysis of Coatings:

[0097] The intensity as a function versus time for mass 209 (Bi) measured using LA-ICP is presented in FIG. 6. A higher Bi intensity can be observed in the pulse electroplated Ni-Bi coating compared to a direct current plated Ni-Bi coating, this indicates a higher content of Bi co-deposited in the coating. An EDS analysis was also carried out to determine the content of Bi in the Ni-Bi coatings. Table 1 shows that the Bi content in the Ni-Bi coatings deposited by DC and PC plating is 4.47 and 5.79 wt. %, respectively. Table 1 : Bi concentration of different Ni-Bi coatings.

Coating sample Bi concentration (wt. %)

Ni-Bi coating prepared under direct 4.47±0.4

current

Ni-Bi coating prepared under pulse 5.79±0.3

current

[0098] Microhardness of Coatings:

[0099] The microhardness of Ni-Bi coatings and pure Ni coatings are presented in FIG. 7. The microhardness of Ni-Bi coatings prepared by DC or PC plating (DC: 758 HV, PC: 767 HV) were much higher than that of Ni coatings (DC: 476.5 HV, PC: 577 HV). However, there was little difference for the Ni-Bi coating that deposited under DC or PC plating.

[00100] The hardness value of coatings depends on a combination of surface morphology, microstructure and quantity and size of the particles of the second phase. The hardness improvement obtained for pure Ni coating using PC plating results from grain refinement according. FIGs. 8a and 8b shows the TEM micrographs of Ni coating deposited under DC and PC plating. It can be clearly seen that the grain size of Ni coating fabricated under pulse current is much finer than which made under direct current.

[00101] In the case of Ni-Bi coatings, the incorporation amount of

particles/second phase plays a more important role. The improvement in microhardness as a function of increasing Bi content of the coating is known to achieve a maxima and then decrease as the Bi ions in the electrolyte increase. PC electroplating increases the percentage of incorporation as proved in FIG. 6. This might be one of the reasons why the microhardness was remained stable. Thus, although grain refinement also occurred in the PC plated coatings as shown in FIGs. 8c and 8d, the microhardness value was not improved.

[00102] Microstructure of Coatings:

[00103] FIG. 9a shows the bright field TEM image of Ni-Bi coating. In order to observe the Ni-Bi compounds clearly, high-angle annular dark-field image (FIG. 9b) and the nanoscale probe EDS analysis (FIG. 9c) are provide. The small white colour grain inside the red box region (FIG. 9b) was confirmed by the EDS results to contain of Ni and Bi. During the electrodeposition, the incorporation of Bi electrolyte into Ni bath solution led to the formation of Ni-Bi intermetallic compounds. Both Ni and the formation Ni-Bi intermetallic are then deposited on the substrate and result in a combination of Ni-Bi intermetallic compound within a crystalline Ni matrix.

[00104] FIGs. 10a and 10b show the SAD patterns of pure Ni and Ni-Bi coating, respectively. Both coatings contained the polycrystalline structure and the diffraction pattern was indexed to the planes indicated in the figure that correspond to hexagonal Ni. The predominant planes of the coatings were Ni (1 1 1 ), Ni (200), Ni (220) and Ni (31 1 ). However, the incorporation of Bi resulted in another four rings in the inner which were not appeared for the pure Ni coatings. These four rings were corresponded to Ni-Bi phase may be seen in the inset to FIG. 10b.

[00105] The HRTEM image of Ni-Bi coating shown in FIG. 1 1 . Where it is clearly evident that there are two parts in the coatings which consist of pure a Ni phase and the combination of Ni and Ni-Bi phase. The d-spacing of pure Ni is about 2.035 A. The Ni and Ni-Bi regions vary in size between 5 and 20 nm.

[00106] Wear Property of Coatings:

[00107] FIG. 12 shows the volume loss and wear rate of pure Ni coating and Ni-Bi coatings under DC and PC plating process. It was found that the Ni-Bi coatings possess a lower wear volume loss and wear rate compared to the pure Ni coatings.

[00108] According to the Archard's law, which can be expressed as:

Q=KW/H=KLN/H (Eq. 1 ) where Q represents the wear rate, W is the total normal load which equals to the product of applied load (Λ/) and total sliding distance (/.), K is the friction coefficient, and H is the hardness of the wear surface.

[00109] Under the same wear test conditions, the wear rate is inversely proportional to the coating surface hardness and proportional to the friction coefficient. Considering the friction coefficients of coatings studied in this research are very close, the enhancement of wear resistance should be attributed to the improvement of coating hardness.

[00110] Potentiodynamic Polarization Curves of Coatings:

[00111] FIG. 13 shows the comparison of potentiodynamic polarization curves of Ni and Ni-Bi coatings. The corrosion potential (E _CO rr) and current density (/ _∞ΓΤ ) extracted from Tafel plots are summarized in Table 2. The / _CO rr for Ni coating is about 2.1 μΑ/cm ² under DC deposition and the value decreased to 1 .8 μΑ/cm ² under PC deposition. PC deposition showed a better corrosion resistance might be due to the compact and smooth microstructure.

Table 2: Electrochemical results obtained from potentiodynamic polarization curves.

Coatings Ecorr VS. SCE (mV) /corr (μΑ/cm ² )

Ni DC -430 ± 10 2.102 ± 0.5

Ni PC -415 ± 8 1 .788 ± 0.8

Ni-Bi DC -406 ± 5 1 .693 ± 0.4

Ni-Bi PC -386 ± 2 1 .007 ± 0.1

[00112] Ni-Bi coatings also showed a more positive corrosion potential with either DC or PC deposition than the pure Ni coatings. Ni-Bi coatings possessed a lower corrosion current density under PC than DC deposition. Composite Ni- Bi coatings and pulse plating coatings have lower chemical activity than pure Ni coatings. The better electrochemical performances of Ni-Bi coatings may be attributed to the reduction of defect size for the composite coating by the in-situ formation of NiBi intermetallics, which was helpful segregate the corrosive medium and inhibit the corrosive pits from developing.

[00113] EIS of Coatings:

[00114] EIS measurements were performed in 3.5 wt. % NaCI solution.

Nyquist plots of experimental work and simulated model of Ni and Ni-Bi coatings are shown in FIG. 14. After fitting the data from the Nyquist diagrams using the ZsimpWin program, the equivalent circuit is presented in FIG. 15 The fitted results are shown in Table 3. Table 3: Equivalent circuit parameters determined by modelling impedance spectra.

Coatings R _s (Ω-cm ² ) CPEdi (F/cm ) Ret (Ω-cm ² )

Ni DC 15.03 2.63x10 ^"5 5058

Ni PC 29.42 1 .59x10 ^"4 5369

Ni-Bi DC 14.30 3.63x10 ^"5 8262

Ni-Bi PC 15.52 1 .54x10 ^"4 10300

The mathematical expression of the impedance of this circuit in this model written in Eq. 2:

Z = R _S + j- ¹ ——r (Eq. 2)

The impedance of the constant phase element (CPE _d i) is presented by Eq. 3:

Z _C PE _dl = (Eq. 3) where Y ₀ and n are two parameters of CPE _d |. When n=1 , CPE _d i is

corresponding to the capacitance, while n=0, it is a resistance, w is the angular frequency and /=-1 [25].

[00115] All samples (FIGs. 14a-14d) exhibited similar capacitive semicircles i the Nyquist plots. This indicated that the same corrosion mechanism was present in all coatings.

[00116] Although these Nyquist plots have similar shape, they were different in terms of their size. The impedance of Ni-Bi coatings was much higher than the Ni coatings in the frequency range of 10 imHz to 100 kHz. In general, a higher impedance value result in a lower corrosion rate. Furthermore, higher values of the charge transfer resistance (Ret) in the Ni-Bi coatings implies that the Ni-Bi coatings possessed a better corrosion resistance compared to the Ni coatings. The value of R _ct of Ni-Bi coating prepared by PC (1 .03x10 ⁴ Ω-cm ² ) was twice than of a Ni coating produced by DC deposition (5.06x10 ³ Ω-cm ² ). [00117] FIG. 16 shows the Bode diagram of Ni and Ni-Bi coatings deposited by DC and PC. The Ni-Bi coating deposited by either DC or PC electroplating method exhibits a higher impedance and phase angle than pure Ni coating at all frequencies. Incorporation of Bi or deposition by PC, the samples reached the maximum phase angle at the lower frequencies than the Ni coating or coating deposited by DC. Thus, the corrosion resistance of coatings has been improved with the addition of Bi and by PC deposition.

[00118] The improved corrosion resistance of Ni-Bi coatings results from the dispersion of NiBi intermetallic compounds into the Ni matrix. The dispersion of the NiBi intermetallic compounds in the Ni coating may form micro

electrochemical cells that can inhibit localized corrosion.

[00119] Furthermore, the incorporation of Bi into Ni and PC deposition can produce more compact coatings with finer grains. This type of microstructure may have better corrosion resistance. It has been reported that the

incorporation of AI2O3 into Ni coating can enhance the corrosion resistance by slower down the growth of corrosion pits. Our results support this.

Example 2:

[00120] Example 2 demonstrates the preparation of a metal matrix coating of Ag and Bi on brass substrates where the ionic co-discharge process results in the formation of nanoparticles of an Agi-Bi alloy and investigates the micro structure, microhardness, electrical conductivity and antimicrobial properties of the coating.

[00121] Method:

[00122] Pre-Treatment:

[00123] Both Ag and Ag-Bi alloy coatings were electroplated onto the substrate of Ni coated brass plates (20 ^χ 25 ^χ 0.6 mm ³ ). The purpose of the plating a layer of Ni coating on the substrate is to prevent inter-diffusion between the Cu and Ag. The composition of brass is 64 wt.% Cu and 36 wt.% Zn. Prior to electroplating, these specimens were pre-treated in an alkaline solution at 80 °C for 10 s. A short Ag strike coating was then applied for 5 s, using stainless steel as an anode. These specimens were electroplated after cleaning by distilled water.

[00124] Electroplating Procedure:

[00125] The chemical composition and operating conditions for

electrodeposition of Ag and Ag-Bi alloy coatings are presented in Table 4.

Table 4: Bath solution composition and operating conditions of electrodeposited Ag and Ag-Bi alloy coatings.

Chemicals Composition Plating Value

Parameters

AgCN Solution 30 g/L Temperature 30 °C

Silver salt

Free KCN 120 g/L Current density 10 imA/cm ²

KOH 10 g/L Agitation speed 200 rpm

Organic brightening trade secret Plating time 30 min agents (optional) Bi electrolyte 0.5-2.0mM Bi

Bi Electrolyte

Bi(NO ₃ ) ₃ -5H ₂ O 0.2 M

tartaric acid 0.2 M

KOH 2.5M

[00126] Morphology, Elemental Composition and Micro-Hardness Analysis:

[00127] The crystal structure of the coatings was characterized by X-ray diffraction (XRD) with Cu Kcc radiation (D2 Phaser Bruker, V=30 KV, /= 10 mA). Diffraction patterns were recorded in the 2 theta range from 35° to 85° at the scanning step if 0.01 °.

[00128] The surface morphology and composition of coatings were analysed using a field emission scanning electron microscope (FESEM) with an energy dispersive spectroscopy (EDS) system.

[00129] The nano-indentation test was conducted on a Nanoindenter

(Hysitron Tl 950 Triobolndenter). The indentation was carried out using a Berkovich diamond indenter tip with a maximum loading of 5000 μΝ. In order to prevent the substrate and roughness effects, the specimens were polished to have a mirror-like surface, and the indention was made on the cross-section samples.

[00130] Electrical Resistivity:

[00131] The electrical resistivity of the samples was measured by four point probe method coupled with Keithley 2602 System Source Meter, interfaced with a Lab view Tracer software. The current was passed through the two outer probes and the potential was measured between two inner probes.

[00132] Antimicrobial Activity:

[00133] For the evolution of antibacterial activity, an inhibition percentage test was carried out. Escherichia coli {E. coli ATCC25922) were cultured in 100 imL of Tryptic Soy Broth (TSB) overnight at the temperature of 37 °C. Then 1 imL of E. coli containing TSB medium was inoculated into a 100 mL fresh TSB medium in sterile conic flasks. Sterile conic flasks containing a 100 mL diluted E. coli solution and coatings samples were incubated at 37 °C for 18 h. 100 mL of E. coli solution was also incubated without coating samples and acted as a control sample. After incubation, the optical density

of E. coli solutions was measured with a UV-Vis spectrophotometer at the wavelength of 600 nm. All experiments were conducted in triplicate.

[00134] Results:

[00135] Surface and Cross-Section Morphologies of Coatings:

[00136] FIG. 17 shows the comparison of XRD spectra of Ag coating and Ag- Bi alloy coatings with the variation addition amount of Bi into Ag metal matrix. It can be observed that there are five prominent peaks of the coatings. There are Ag (1 1 1 ), Ag (200), Ag (220), Ag (31 1 ) and Ag (222). The same XRD pattern on a small 2Θ range (37.5-39.0°) and (43.0-46.0°) as a function of addition of different content of Bi is presented in Fig. 12b and c.

[00137] No Bi peak or metastable phase of AgBi2 could be observed in the XRD spectra. This may be due to both the low quantity of Bi and the solubility at the levels used of Bi in Ag. Ag-Bi alloys with the Bi content less than 3 wt.% are microscopically homogeneous. With increasing Bi content, the peaks are slightly shifted to the lower diffraction angle. This might be due to the formation of solid solution. [00138] The grain size measurement was conducted based on the Scherrer's formula, = 0.9/1/ βοοεθ , where λ is the wavelength of the radiation (0.154 nm), β is the full width at half maximum (FWHM) after subtracting the

instrumental line broadening, in radians and Θ is the Bragg angle. Based on this calculation, all the grain sizes of the coatings are in nanometre scale. The grain size of pure Ag coating was about 40 nm. Ag-Bi generally showed a slightly smaller grain size (27-34 nm) compared to the pure Ag coating. It was expected that the alloying of Bi increased the nucleation sites and thus lowered the grain size.

[00139] FIG. 18 shows the surface morphology of Ag and Ag-Bi coatings with addition of different content of Bi. All the coatings show a typical nodular structure. As increasing the content of Bi, the grain appears clearer and more nucleation has been formed than in the pure silver coating.

[00140] The cross-sectional images of pure Ag and Ag-Bi alloy coating were deposited using similar electrodeposition parameters are shown in FIG. 19. A clear boundary between the brass substrate, Ni layer and Ag or Ag-Bi coatings was observed. There were no visible cracks or abruptions at the interface of the coating. All the coatings were uniform and homogeneous. Interestingly, the thickness of the Ag-Bi coatings (-10 μιη) were slightly higher than the pure Ag (~8 μιη) when deposited using identical electrodeposition conditions indicating an improvement in bath efficiency. The thickness of Ag-Bi coating was almost the same and independent of its Bi content.

[00141] FIG. 20 shows the actual percentage of Bi in the Ag-Bi coating as determined by EDS after the electrodeposition. It is seen that the % of Bi that deposited in the Ag-Bi coating increases as the content of Bi content in the bath solution increases. However, the maximum of the Bi content detected is about 2.35 wt% which is lowered than the solubility limit in the solid state Ag. Thus, it is expected Ag-Bi alloy coating is deposited by controlling the content of Bi added in the Ag bath solution.13.

[00142] Nano Indentation:

[00143] FIG. 21 illustrates the gradient forward images of the indentation at a maximum loading of 5000 μΝ. In order to prevent the substrate or Ni layer effects, the indentations were made on the cross-section. This imposed a limitation on the indentation depth which should exceed 10 % of the coating thickness. The hardness was determined by using the Oliver Pharr method as described by the Eq. 4:

H = ?≡≡ Eq. (4)

where P _max is the peak of indentation load and A is the projected contact area of hardness. In this experiment, Berkovich indenter was used, and thus the mean contact pressure (or hardness) is:

H = Eq. (5)

24.5h _p ^{z ;}

where h _p is the measure of the "plastic" depth of penetration.

[00144] The typical of load-displacement curves and the hardness results are shown in FIG. 22 and FIG. 23, respectively. The pure Ag coating has an average hardness of 1 .87±0.1 GPa. However, alloying the Bi into Ag coatings has resulted in gradually improvement in hardness. The value of hardness has increased from 1 .87 GPa to 2.66 GPa by alloying 0.5 imM Bi into Ag. The improvement of about 42 % was achieved. However, further increasing the content of Bi led to a slightly decrease the hardness value. Overall, the hardness value is increasing with the alloying of Bi.

[00145] The enhancement of the mechanical properties of Ag-Bi alloy coatings may be explained by the grain refinement strengthening. The grain refinement strengthening from the Hall-Petch relationship can be described as Eq. 6: o _y= o ₀ + kd ^~1/2 Eq. (6) where o _y and σ ₀ is the yield stress and friction stress, respectively, k is a constant and d is the grain size. This equation is usually empirically related to the hardness through Eq.7:

HV = 3a _y Eq. (7)

[00146] Based on Eq. (6) and Eq. (7), the hardness is increased when the grains are refined. As mentioned earlier, the grain size of the Ag-Bi coating was reduced with the alloying of Bi.

[00147] Electrical Resistivity:

[00148] Due to the thickness of coating (t) is much smaller than the length of the samples, the electrical resistivity ( ) can be calculated by Eq. 8.

» = (Τ!)(7) Χ ^£

[00149] The electrical conductivity was obtained by using the Eq. 9

[00150] The electrical resistivity, the conductivity value and the conductivity in terms value of % IACS is listed in Table 5. IACS is an international annealed copper standard that commonly used for metals and alloys relative to the standard annealed copper. A 100% conductivity sample has a resistivity of 1 .7241 x 10 ^"8 Ω- m (or equal to the conductivity of 5.80x 10 ⁷ S/m) at the 20 °C.

[00151] The electrical resistivity and electrical conductivity of pure Ag coating are 1 .78±0.02 (x10 ^"8 Ω-m) and 97.1 ±1 .2 (%IACS), respectively. Pure Ag metal has the highest electrical conductivity among all metals. It may be noted that the thin Ag solid film also could exhibit nearly the similar desire properties, depending on the deposition conditions. Table 5: Resistivity and electrical conductivity of electroplating Ag and Ag-Bi alloy coatings.

Resistivity Conductivity Conductivity

Coating

(x10 ^"8 Ω-m) (x10 ⁷ S/m) %IACS

Ag 1 .78±0.02 5.63±0.07 97.1 ±1 .2

Ag-0.5 imM Bi 1 .88±0.02 5.31 ±0.06 91 .5±1 .0

Ag-1 .0 mM Bi 1 .92±0.01 5.22±0.03 89.9±0.4

Ag-2.0 mM Bi 1 .99±0.02 5.04±0.04 86.8±0.7

[00152] It is observed that the electrical resistivity slightly increases with the amount of Bi added to Ag bath. Although the electrical conductivity of Ag-Bi coatings shows a slight decrease with the additional of Bi content, the

conductivity maintains essentially the same magnitude.

[00153] The resistivity of a two -phase material is proportional to the volume fractions of the two phases. In this experiment, the maximum of content Bi deposited in the Ag-Bi coatings was just 2.35 wt.%. This volume fraction was very low and below the solubility limit of solid state Ag. Thus, the electrical conductivity of Ag-Bi coatings did not show any significant influenced by alloying.

[00154] A small reduction in electrical conductivity might be attributed to increase of scattering effects. The grain size was reduced with the alloying of Bi and with more grain boundaries formed there tends to be a decrease in electrical conductivity.

[00155] Antimicrobial Activity:

[00156] The antimicrobial activity of the Ag and Ag-Bi coatings were investigated using E. coli ATCC 25922. The percentage inhibition of the coatings was calculated by using the Eq. 10:

_{I % =} ( ^con i8-cono)-(samp ₁₈ -sam _Po ) _d00 Eq. (10)

(con ₁₈ -con ₀ ) where I is the percentage inhibition of growth, con-i ₈ and con ₀ are the optical density at 600 nm of E. coli ^' m TSB as the control of the organism at 18 and Oh, respectively. Sampi ₈ and samp ₀ are the optical density at 600 nm of E. coli ^' m TSB with the presence of samples at 18 and Oh, respectively.

[00157] Based on the Eq.10, the inhibition of coatings after bacteria contacting with coating for 18 h are calculated and listed in Table 6. The bare substrate of the Ni coated on brass was also tested for comparison purpose. The pure Ag coating inhibited the E. coli growth by 76.6 %, while Ag-0.5 imM Bi coatings showed an inhibition of 76.2%. However, further additional of Bi content, the inhibition of bacteria growth was slightly decreased.

Table 6: The percentage inhibition of growth for the coatings.

Percentage of

Coating

Inhibition (%)

Bare substrate (Ni coated 31 .2

brass) 76.6

Ag

Ag-0.5 imM Bi 76.2

Ag-1 .0 mM Bi 72.8

Ag-2.0 mM Bi 71 .6

[00158] There are a few factors that influenced the antibacterial properties of a material such as concentration of the ion release, surface morphology, grain size, etc. The antimicrobial activity of silver greatly influenced by silver ion release, the higher the level of ion release the more efficient the antibacterial properties. For the use in the medical devices, the level of silver ion release should be in range of 0.1 -1 .6 ppm. In the present study, it was found out there was no significant difference for the Ag ion release after 18 h of incubation. All the samples exhibited ion release in the range of 0.1 0-0.12 ppm.

[00159] Furthermore the morphology and roughness of the surfaces may also exert a greater influence on the degree of bacterial attachment and thus influence the antibacterial properties. Smoother surfaces generally enhance the antibacterial property. The smaller grain size may also enhance the inhibition rate and more Ag ions could be released. In this study, the grain size of the coating was slightly decreased with the alloying of Bi content. However, the ion release rate was almost remained the same for all the samples. Thus, the percentage inhibition of bacterial of the samples was also equal. [00160] It should be noted that reference to ranges or numbers disclosed herein (for example 1 to 10) also incorporates reference to all rational numbers within that range (e.g., 1 , 1 .1 , 2, 3, 3.9, 4, 5, 6, 6.5, and so forth) and to any range of rational numbers within that range (e.g., 2 to 8, 1 .5 to 5.5, 3.1 to 4.7, and the like). In other words, all possible combinations of numerical values between a lowest value and a highest value enumerated as part of a range are to be considered to be expressly stated.

[00161] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, or variations, therein may be subsequently made which are also intended to be encompassed by the following claims.