WO2021239722A2 - Improved electrodeposition - Google Patents

Abstract

A method of preparing an electrolyte solution (107) for electrodeposition is disclosed. The method comprises dispersing nanoparticles (133) in an aqueous solution comprising a surfactant (132), thereby forming a dispersion of nanopart icle-surfactant micellar complexes (135) in solution and adding electrolytes comprising ionic metal (134). The concentration of the surfactant (132) is selected to substantially prevent sedimentation of the nanoparticles (133).

Description

IMPROVED ELECTRODEPOSITION

FIELD OF INVENTION

The present invention relates to improved methods, solutions and apparatus for electrodeposition and to parts formed using such.

BACKGROUND

Electrodeposition is a process that can be used to form metal parts . For example, a part may be formed by electrodeposition on a mandrel configured as a cathode and submerged in an electrolyte. The mandrel may subsequently be removed mechanically or chemically. Such a process may be termed electroforming. The electrolyte comprises metal ions, which are reduced at the cathode and deposited as a metal layer. Nickel is a material that is commonly electroformed, for example to define stencils or moulds.

Electroformed moulds may be used to replicate a master (i.e. the mandrel), for example, to make precision moulded plastic parts (e.g. for microfluidic or optical applications).

Where electroforming is used to create a mould, the layers are typically of relatively high thickness (e.g. 0.5mm to 5mm), and it can take a significant amount of time to electroform the mould (e.g. a day to a week). Moulds are generally required to have a smooth surface finish (i.e. low surface roughness) , high form precision and high replication capability of micro and nano scale structures. It is also important that a mould should be wear resistant. This is a significant challenge in micro-moulding, because micro and nano scale structures can be easily worn.

Nanoparticles may be defined as having at least one dimension on the nanometre scale, in the range 1 to lOOnm. Many nanoparticles have interesting mechanical and tribological properties and can be used to modify the properties of a material that comprises the nanoparticles. For example, a nanocomposite (e.g. a matrix incorporating nanoparticles) may have improved mechanical properties.

Although significant improvements have been made electrodeposition, there remains considerable room for improvement. SUMMARY

According to an aspect of the invention, there is provided a method of preparing an electrolyte solution for electrodeposition, comprising: dispersing nanoparticles in an aqueous solution comprising a surfactant, thereby forming a dispersion of nanoparticle-surfactant micellar complexes in solution; adding electrolytes comprising ionic metal; wherein the concentration of the surfactant is selected to substantially prevent sedimentation of the nanoparticles.

The surfactant may be a non-ionic surfactant. The nanoparticles may be (or comprise) two-dimensional nanoparticles.

The electrolytes may further comprise at least one of: a wetting agent, a pH buffer and/or grain refiners and a flatness agent.

Substantially prevent sedimentation may mean that 90% of the nanoparticles remain suspended in the solution after a 24 hour settling time.

The concentration of the non-ionic surfactant may be selected to inhibit agglomeration of the nanoparticles.

Inhibiting agglomeration may mean that particles size are well under submicron scale and form stable suspensions without sediment, and can be deposited on to the cathode surface with functional properties improvement for coatings or moulds.

The metal ions may comprise nickel ions.

The aqueous solution may comprise nickel sulfamate.

The concentration of nickel sulfamate may be between 200 and 500 grams per litre.

The concentration of the non-ionic surfactant in the aqueous solution may be at least 40% of the critical micelle concentration (CMC) or at least 0.4 g/litre.

The CMC for pluronic F127, for example, is approximately 1000ppm~l g/litre. The mass/volume concentration of the non-ionic surfactant may be between 5 and 20 times the mass/volume concentration of the nanoparticles.

The nanoparticles may comprise graphene oxide nanoplatelets.

The concentration of nanoparticles dispersed in the aqueous solution may be between 0.02 and 0.2 grams per litre.

The non-ionic surfactant comprises a compound selected from: a non-ionic triblock copolymer; a poloxamer (e.g. (polyethylene glycol) -(polyethylene glycol)-(polyethylene glycol), pluronic F127 or P123); a polysorbate 80 (e.g. Tween 80); a polyetholylated stearyl alcohol (Brij 700/S100).

Dispersing the nanoparticles may comprise agitating the electrolyte solution using ultrasound and/or mechanical stirring.

The method may further comprise adding a grain refiner to the aqueous solution.

The grain refiner may comprise saccharin.

The saccharin may be added to a concentration of between 0.02 and 0.5 grams per litre in the aqueous solution.

According to a second aspect, there is provided an electrolyte solution for metal electrodeposition, comprising: metal ions; a stable dispersion of two-dimensional nanoparticles; a surfactant at a concentration selected to substantially prevent sedimentation of the nanoparticles.

In some embodiments, the nano-particles need not be two-dimensional. The two-dimensional nanoparticles may comprise at least one of graphene oxide nanoplates, graphene nanoplatelets, molybdenum disulfate and hexagonal boron nitride

The concentration of the non-ionic surfactant may be at least 40% of the critical micelle concentration, or at least 0.4g/litre.

The metal ions may comprise nickel ions.

The aqueous solution may comprise nickel sulfamate.

The concentration of nickel sulfamate may be between 200 and 500 grams per litre.

The nanoparticles may comprise graphene oxide.

The concentration of nanoparticles dispersed in the aqueous solution may be between 0.05 and 1 gram per litre.

The non-ionic surfactant may comprise a compound selected from: a non-ionic triblock copolymer; a poloxamer (e.g. (polyethylene glycol)-(polyethylene glycol)-(polyethylene glycol), pluronic F127 or P123); a polysorbate 80 (e.g. Tween 80); a polyetholylated stearyl alcohol (Brij 700/S 100).

The electrolyte solution may further comprise a grain refiner.

The grain refiner may comprise saccharin.

The saccharin may be at a concentration of between 0.02 and 0.5 grams per litre.

The electrolyte solution may be prepared according to the first aspect.

According to a third aspect, there is provided a method of making a part, comprising: preparing an electrolyte solution in accordance with the first aspect; electrodepositing a metal part from the electrolyte solution . The electrolyte solution may be according to the second aspect, including any optional features thereof.

The part may be a mould.

The method may further comprise agitating the electrolyte solution during electrodeposition. The agitating may comprise ultrasonic agitation.

According to a fourth aspect, there is provided a part formed using the method of the third aspect.

The part may have a thickness of at least 0.5mm.

The part may comprise features with a linewidth of 1mm or less, or 0.5mm or less.

The part may have a Vickers Pyramid number of at least 500.

The part may have a ball-on-disk friction coefficient of less than 0.5.

The part may comprise a mould.

The part may have a surface with a surface roughness R_a of less than 50nm.

According to a fifth aspect, there is provided a method for fabricating a permanent self- lubricating high-performance micro/nano structured mould from a 2D material - reinforced metal nanocomposite using an electroforming process, the method comprising: preparing an aqueous dispersion of self-assembled surfactants and 2D materials via ultrasonic or high-speed shear agitation; preparing a plating bath by mixing the aqueous dispersion of self -assembled surfactants and 2D materials with a metal or metal -alloy electroforming solution and further dispersing the 2D materials for homogenous distribution via ultrasonic agitation; putting a micro/nano structured cathode master obtained from a LIGA or UV - LIGA process and an anode into the plating bath and imposing a current between the cathode master and the anode, thereby electroforming the high-performance micro/nano structured mould on the cathode master.

Optionally: i) the metal or metal-alloy electroforming solution comprises Nickel sulfamate, Nickel sulfate, Copper sulfamate, Copper sulfate, or Nickel sulfate-Cobalt sulfate; ii) the master comprises a line width from 500 micrometers to 10 nanometers; iii) the 2D materials comprise nanoparticles comprising at least one of graphene oxide nanoplatelets, or graphene nanoplatelets, molybdenum disulfate and hexagonal boron nitride 2D materials; iv) the mould has self-lubricating properties without any delamination; v) the dispersion of 2D materials has to be continued throughout electroforming process; vi) the surface of the mould has Vickers Pyramid hardness higher than 500; and/or vii) a working surface of the mould has roughness R_a below 50nm.

Features of each aspect may be combined with the features of other aspects. For example features disclosed with reference to a method of making an electrolyte solution imply that corresponding features may be present in an electrolyte solution. Features described with reference to parts imply that any method of manufacturing the part includes corresponding features.

DETAILED DESCRIPTION

Embodiments will be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 is an example electroforming process for forming a part;

Figure 2 shows micrographs of prior art of injection moulding of plastic part using electroformed moulds, illustrating some of the channel deformation and damage when an electroformed moulds with microscale structures is used for making plastic part;

Figure 3 shows a process for preparing an electrolyte solution according to an embodiment; Figure 4 compares the stability of electrolyte solutions having a non-ionic surfactant with those using other surfactants;

Figure 5 compares the stability of electrolyte solutions with different concentrations of an example non-ionic surfactant;

Figure 6 compares stability of electrolyte solutions comprising saccharin with different types of surfactants;

Figure 7 shows Z-average particle size and zeta-potential for electrolyte solutions comprising different types of surfactant;

Figure 8 shows Z-average particle size and zeta-potential for electrolyte solutions comprising saccharine and different types of surfactant;

Figure 9 illustrates the codeposition mechanism for nanoparticles in a nickel matrix with different types of surfactants;

Figure 10 shows relative texture coefficient and crystallite size for different surfactants and different metal ion concentrations;

Figure 11 shows XRD patterns of pure nickel coatings, nickel/graphene oxide coatings with various saccharine concentration and surfactants;

Figure 12 shows crystallite size for electrolyte comprising saccharine with different surfactants and different metal ion concentrations;

Figure 13 shows electrodeposition rates and efficiencies for different surfactants;

Figure 14 shows microhardness for different surfactants and different metal ion concentrations;

Figure 15 shows microhardness for an electrolyte comprising saccharine with different surfactants and different metal ion concentrations Figure 16 shows coefficient of friction for different surfactants at various electrolyte concentrations;

Figure 17 shows coefficient of friction for different surfactants at various electrolyte concentrations comprising saccharin; and

Figure 18 shows microhardness and friction coefficient for different concentrations of graphene oxide nanoplatelets, stabilised with a non-ionic surfactant;

Figure 19 shows microhardness and crystallite size for different concentrations of graphene oxide nanoplatelets, stabilised with a non-ionic surfactant;

Figure 20 shows microhardness and crystallite size for different ultrasonic power; and

Figure 21 illustrates the mechanisms by which hardness and friction coefficient are improved according to embodiments.

Referring to Figure 1, a schematic electroforming process is shown, in which an anode 14 and a cathode 13 are disposed in an electrolyte solution 17. The electrolyte solution comprises metal ions, which are deposited on the cathode 13 to form a metal part 16. Since the deposited metal part 16 at the cathode 13 conforms to the shape of the cathode, the cathode may be formed as a mandrel which is the inverse of the shape required of the part 16.

In some examples, the cathode/mandrel may be formed by machining a part, or by a lithographic process (defining a pattern, typically on a substrate) and an etching process (masked by the pattern). The substrate may subsequently be chemically and/or physically removed). In certain embodiments the cathode may be formed from a non conducting material (e.g. a glass substrate with a patterned photoresist layer) coated with a conducting layer (e.g. by sputtering, evaporation etc). In other embodiments the cathode may be formed from a conductive material (and may be monolithic).

The anode 14 may be a conductive metal, and may, for example be a sacrificial anode comprising the metal to be deposited, so that the concentration of metal ions in solution is replenished by dissolution of the anode. The anode is connected to a positive electrode of a power supply, and the cathode connected to a negative electrode of a power supply.

In certain embodiments, the metal to be deposited may comprise (or consist substantially entirely of) nickel, and the electrolyte may comprise nickel sulfamate.

One application for parts formed by electrodeposition is for use as a mould, e.g. for injection moulding of micro-parts, or for hot embossing or micro/nanoimprinting. Parts formed by electrodeposition can be distinguished from coatings formed by electrodeposition by the thickness of the layers to be deposited. In electroforming of parts (e.g. moulds) the layer thickness is typically >250 microns, and more typically in the range 0.5mm to 5mm. This is thicker than a typical coating and tends to require a relatively long deposition period, on the order of a day or a week. To achieve more homogenous co-deposition of nanoparticles in an electrodeposited metal matrix, it is desirable for the nanoparticles to remain suspended during the deposition process.

Many moulds used to produce microfluidic devices and functional surfaces are based on nickel moulds produced by lithography and/or etching, electroforming processes, and moulding process, e.g. the so-called LIGA process (or UV-LIGA), owing to its high precision and low surface roughness. However, a nickel mould is much softer and more vulnerable than tool steel, with life time typically less than 10,000 mould cycles, which necessitates replacement of the moulding tool and increases production cost. Such moulds generally cost from €7,000 to €15,000, depending on the pattern design and mould tool insert design. Moreover, to fabricate and commission a new mould tool can take ~2 months, due to the complex fabrication processes and long wait times for carrying out the many semiconductor fabrication steps. Replacing a mould so frequently increases the cost of microfluidic chips, which is undesirable as they are intended to be affordable disposable devices (e.g. for direct use by individual patients).

It may be desirable for a mould to have a smooth surface finish (e.g. a low surface roughness). For example, a moulded microfluidic device may be required to have smooth channels to provide transparency for optical detection or observation, so the roughness may be required to be below 50nm. Small surface structures may be required, on the order of 20 microns to a few hundred microns, and in some applications submicron and nanometer scale. Such features can easily be worn during moulding processes. Demoulding distortion and damage of micro structures during mass production via injection moulding may be caused by friction and adhesion between polymer and mould. This is a common reason for bonding failures of microfluidic chips, which may lead to reduced production yield and increased chip cost. As a result, it may be difficult to commercialise or widen the use of microfluidic point -of-care diagnostic systems. An electroforming process capable of producing a material with high hardness and/or reduced friction may help mitigate some of these problems.

Injection moulded plastic microfluidic channels 16 are shown in Figure 2, in which distortion and damage of a moulded plastic microfluidic channels (a) is shown, along with distortion and damage of honeycomb surface micro patterns (b).

In order to improve the properties of the part (e.g. mechanical and/or tribological) nanoparticles may be included in the electrolyte. Such nanoparticles may comprise one kind or different kinds of two dimensional materials. A two dimensional material may be defined as a material having a thickness that is less than or equal to a threshold size required to exhibit quantum confinement in the thickness direction and a two dimensional electronic density of states. A three dimensional material may be defined as a material having a thickness greater than the threshold size to as to exhibit a three dimensional electronic density of states. The threshold size required to exhibit quantum confinement may be the Fermi wavelength of the charge carriers in the material. The Fermi wavelength is the characteristic scale above which electrons and holes behave as bulk charge carriers. The Fermi wavelength of a two dimensional material is inversely proportional to the Fermi wavevector which in turn is proportional to the square root of the charge carrier density in the material. Examples of two dimensional materials include graphene, graphene oxide, molybdenum disulfate and hexagonal boron nitride.

Two dimensional materials may reduce friction and improve mechanical properties of materials within which they are incorporated. However, it has hitherto been challenging to achieve this promise in the context of electrodeposition. Co -deposition of metals with suspended nanoparticles is not straightforward.

One challenge to be overcome is the stability of the nanoparticles in solution. Nanoparticles in electrolytic solutions suitable for electrodeposition tend to agglomerate and settle from suspension to form a sediment. The divalent cations (e.g., Ni²⁺, Ca²⁺ and Mg²⁺) typically present in an electrolyte solution can both screen surface charges and also interact with surface functional groups of the 2D materials, leading to cross-linking, which may cause aggregation. Normal and lateral forces may disperse stacked platelets. Normal forces overcome the van der Waals attraction between 2D material platelets, while lateral forces promote relative motion between adjacent platelets via the self-lubricating property of 2D materials in the lateral direction.

Figure 3 illustrates an example method for preparing an electrolyte solution for electrodeposition, in which a metal matrix is co-deposited with nanoparticles (e.g. graphene oxide nanoplatelets).

The process starts with a dispersant 101 comprising water 131 and a surfactant 132. The dispersant 101 is mixed (e.g. by vortex mixing) in step 111, to promote the formation of surfactant micelles, thereby forming a dispersant with micelles 102. At step 112, nanoparticles 133 are added (e.g. 2D platelets such as graphene oxide), to form a dispersion 103 comprising surfactant micelles and nanoparticles 103. Adding the nanoparticles to an aqueous solution of dispersant micelles (without electroplating reagents) means that nanoparticles will form micellar complexes without interference from charged metal ions, which can lead to agglomeration and make subsequent dispersal more difficult.

At step 113, the solution 103 is agitated, for example using sonication, so as to stabilise the nanoparticles by forming micellar complexes 135 comprising nanoparticles 133 and surfactant 132. A dispersion of nanoparticle-surfactant micellar complexes is thereby formed.

At step 115, electroplating reagents are added (e.g. nickel sulfamate) to produce an appropriate concentration of metal ions 134 in solution. The metal ions 134 are screened from interaction with the nanoparticles by the surfactant in each micellar complex 135. At step 116, further agitation (e.g. sonication) may be applied to assist in dispersion of the nanoparticles. The electrolytic solution 107 produced by this process may comprise a stable dispersion of nanoparticles 133, which may improve the properties of a part electrodeposited from the solution 107. In an example, the performance of different surfactants was compared using solutions prepared according to the process outlined in Figure 3. Solutions comprising SDS (sodium dodecyl sulfate), CTAB (Cetyltrimethylammonium Bromide) and PEG (polyethylene glycol) surfactants were prepared (anionic, cationic and non-ionic surfactants respectively), by adding surfactant in 20ml of deionised water and vortex mixing (in step 111) to completely dissolve the surfactant. Subsequently, graphene oxide colloid was added (at step 112) and sonicated for 20 minutes using an ultrasonic probe (step 113) to form a dispersion. The dispersion was added to a nickel sulfamate solution (step 115) and the resulting electrolyte solution subsequently vortex mixed for 10 minutes and then immediately sonicated (using an ultrasonic probe) for 20 minutes (step 116).

In each case, 0.1 g/L of graphene oxide was used with 0.8 g/L of the respective surfactant (SDS/CTAB/PEG). The concentration of the sulfamate solution was 200 g/L, 250 g/L, 300 g/L or 400 g/L. The 400 g/L sulfamate solution comprised different concentrations of saccharine (a grain refiner): without saccharine, 0.05 g/L, 0.1 g/L, 0.2 g/L, and 0.3 g/L. After preparation, the electrolyte solution was stirred for 20 hours using a magnetic stirrer with a rotation speed of 500 rpm. Prior to any electrodeposition, the electrolyte bath was treated with an ultrasonic probe for a further 20 minutes. In this example, the ultrasonic probe used a 24kHz frequency and 180 W power.

Electroplating was carried using the example electrolyte solutions. Both the stability of the electrolyte solutions and the properties of parts formed by electrodeposition from the solutions were investigated. The electrodeposition bath compositions and process parameters are outlined below in table 1 :

Table 1 : Electrodeposition bath composition and parameters

Figure 4 shows example dispersions of graphene oxide dispersed into nickel sulfamate solutions of various concentrations (200, 250, 300 and 400 g/L), with different types and concentrations of surfactants (prepared in accordance with the method outlined in Figure 3). In Figure 4, no saccharine was added, and the concentration of the surfactant (SDS/CTAB/PEG) was 0.8 g/F. Images 201-204, 211-214, 221-224 all show the initial state of the dispersion (substantially free from sediment), and images 251 -254, 261-264, 271-274, 281-284 show the state of the dispersion after 5 hours. It is clear that the non- ionic surfactant PEG (i.e. the non-ionic surfactant) performs markedly better than either the of the ionic surfactants, with the graphene oxide dispersion remaining stable, with no precipitate, after a period of 5 hours for all concentrations of electrolyte (as shown in images 271-274).

Figure 7 shows the Z-average particle size 522 (obtained by dynamic light scattering) and zeta potential 523 for particles in the electrolyte solutions with the different surfactants. The smallest average particle sizes are consistently associated with stabilisation using the non-ionic surfactant (PEG). The positive zeta potential for the solution stabilised with CTAB is consistent with the fact CTAB is an cationic surfactant, therefore forming micellar complexes with positive zeta potential. Fikewise, the negative zeta potential for the solution stabilised with SDS is likewise consistent with the fact that SDS is an anionic surfactant which forms micellar complexes with negative zeta potential. The dispersion of SDS, which relies on electrostatic repulsion of micellar complexes with negative zeta potential, is hindered by the addition of positively charged metal ions, which are attracted to the negative micellar complexes, resulting in neutralisation and aggregation.

CTAB, producing micellar complexes with positive zeta potential, is less susceptible to aggregation as a result of attraction of the positive metal ions, but is still prone to aggregation, and the Z average particle size is more sensitive to concentration of the nickel sulfamate than the PEG stabilised preparations.

The non-ionic surfactant (PEG) has a substantially neutral zeta potential, and is therefore substantially not susceptible to influence by electrical fields associated with electrodeposition. The PEG stabilised solutions have the smallest Z-average particle size, and the particle size is only weakly influenced by increasing concentration of nickel sulfamate.

Figure 5 shows the effect of concentration of a non-ionic surfactant, Pluronic F-127 (also known as poloaxamer 407) on long term stability (1 week). The solutions were prepared in accordance with the method outlined in Figure 3. The loading of graphene oxide remained at 0.1 g/L, and the concentration of nickel sulfamate was 400 g/L. Concentrations of over 0.5 g/L of the surfactant are sufficient to substantially prevent sedimentation of graphene oxide from the solution.

Figure 6 shows the effect of different concentrations of saccharin (0.05, 0.1. 0.2 and 0.3 g/L) on the stability of the nanoparticles (graphene oxide) in dispersion, for a 0.1 gram/litre loading of graphene oxide and a 400 gram/litre concentration of nickel sulfamate. Increasing concentrations of saccharine are associated with improvements in stability. Consistent with Figure 4, the non-ionic surfactant PEG performed the best, with CTAB second and SDS third.

Figure 8 shows the Z-average particle size 524 (obtained by dynamic light scattering) and zeta potential 525 for particles in the electrolyte solutions including different concentrations saccharin with the different surfactants. The smallest average particle sizes are consistently associated with stabilisation using the non -ionic surfactant (PEG). Higher concentrations of saccharin are associated with smaller Z-average particle size, consistent with the saccharine improving stability of the nanoparticles in suspension. Similar comments to Figure 7 apply with respect to the zeta potential, and the differing concentration of saccharin did not affect zeta potential.

Figure 9 shows the co-deposition mechanism for nanoparticles (e.g. graphene oxide) with a metal matrix (e.g. nickel) under different types of surfactant (anionic/SDS, cationic/CTAB and non-ionic/PEG). In the bulk electrolyte 501, the surfactant/nanoparticle micelles 514 mix with the metal cations 511.

Where the surfactant is anionic/SDS, complexes 515 of metal ions and surfactant micelles form, which result in aggregation. These aggregates persist through the convection layer 502, diffusion layer 503 and electrical double layer 504, to be included in the deposited coating/part 505 at the cathode. Furthermore, the aggregation may result in sedimentation of the nanoparticles.

Where the surfactant is cationic/CTAB, complexes 512 of metal ions and surfactant micelles tend not to form, and the dispersion is more stable. There is some aggregation of micelles 512.

Where the surfactant is non-ionic/PEG, the micelle complexes 514 remain substantially homogenous and well dispersed in solution, and are not affected by electrical field. As a result, the coating/part 505 incorporates smaller nanoparticles which are more homogeneously dispersed. This tends to improve the properties of the deposited part, as will be shown below.

Figure 10 compares the relative texture coefficient:

where, and Io(hki) ^are the intensity of the (hkl) crystal planes of the coating and the standard nickel power reference in random orientation.

^are both sums of the intensities in all crystal planes from the coating and standard nickel power. The RTC^i^ values for all coatings are linked to the type of surfactant. For example, the PEG functionalised graphene oxide makes a significant change of tecture from [200] to [220] compared to pure nickel, and the influence of ionic surfactants CTAB and SDS is profound and [200] is still the preferred texture.

The average crystallite size D of coatings is calculated as follows: where K is the Scherrer constant (0.89), FWHM is the full-width half maxima at the peak of 20, and L is the wavelength (0.154053nm). For the pure nickel, there is no obvious change in average crystallite size with increasing concentration of nickel sulfamate. However, the introduction of GO (graphene oxide) particles in the deposits overall refines the crystallite size at the low concentration of nickel sulfamate. However, higher concentration of nickel sulfamate tend to suppress the effect of refinement. The coating enhanced by PEG dispersed GO demonstrates the maximum improvement in refined crystallite size, which is 17.6 nm for 200 g/E nickel sulfamate, compared to that of pure nickel with 32.2 nm. This is a 42% reduction in crystallite size. The coating enhanced by PEG-modified GO shows the sensitivity of crystallite size to increasing nickel sulfamate concentration among the three surfactants, particularly at 400 g/L nickel sulfamate concentration. The CTAB has a mild effect on the crystallite refinement and the minimum crystallite size is ~ 25 nm. The SDS functionalized GO shows very limited crystallite refinement effect and the optimal crystallite size is 27.8 nm under the low nickel sulfamate concentration of 200 g/L. It is well known that the hardness and wear resistance of coating are highly associated with its crystallite size, so PEG-modified GO can be expected to have improved properties.

Referring to Figure 11, the addition of saccharin in solution causes the preferred crystal orientation to tend towards [111] and results in broadening of the diffraction peak of coatings (indicative of grain refinement). The surfactant and saccharin have a synergistic effect on the translation of preferred crystal orientation from [200] to [111] and on peak broadening (indicative of grain refinement). Usually, [200] represents the dominated crystal growth orientation for pure nickel coatings with low hardness and high ductility, while the [111] crystal orientation is associated with high -hardness nickel coatings. The diffraction intensity from the [111] crystal plane was significantly increased with increasing concentration of saccharin. This effect was influenced by the surfactant used, with the effect more prominent for non -ionic surfactants (PEG)

Referring to Figure 12, the relationship between crystallite size and saccharin concentration for the three different types of surfactant is shown. The inclusion of the graphene oxide nanoparticles reduces the grain size compared with saccharin without GO nanoparticles and surfactant, especially for the non-ionic surfactant (PEG). The crystallite size was reduced with increasing concentration of saccharin, with the smallest grain size ~9 nm. The pure nickel coating without saccharin had a crystallite size of ~ 35 nm.

Crystalline refinement may therefore be ascribed to the double effect of surfactant modified GO nanoparticles and saccharin. Possible mechanisms for grain refining of nickel/GO composite coatings fabricated with the different surfactants and with saccharin additive may be summarised as: (i) a blocking effect of GO sheets between nickel grains and molecular absorption of surfactant and saccharin, which leads to the evolution of more grain boundaries and nucleation of nickel atoms; (ii) a reduction in thickness of the electrical double diffusion layer reduction induced by additives between the cathode-electrolyte interface, delaying the growth of nickel crystal; (iii) an increase of the cathodic overpotential, indicating the increased nucleation sites and thus grain refining.

Figure 13 shows electrodeposition efficiency for different surfactants, at two different concentrations of nickel sulfamate (200 g/L and 400 g/L). For the lower concentration of nickel sulfamate solution (200 g/L), the SDS/CTAB/PEG representative coatings have an average thickness of 46.29 pm, 33.33 pm, 57.41 pm, respectively, while the thickness of pure nickel coating is 50 pm. This implies that PEG functionalized GO promotes electrodeposition efficiency by up to 74.01%. However, the coatings from SDS and CTAB suppress the electrodeposition efficiency. For CTAB the electrodeposition efficiency is 33.33% with SDS having an electrodeposition efficiency of 46.29%. For the higher concentration of nickel sulfamate solution (400 g/L), the coating thickness and electrodeposition efficiency are significantly increased in comparison to the ones at 200 g/L. The PEG surfactant results in the highest thickness of -74.01 pm and an electrodeposition efficiency of 120.4%. SDS and CTAB result in a deposit thickness of 55.55 pm, 68.52 pm and deposition efficiencies of 90.37% and 111.5%, respectively. Compared to the pure nickel with a thickness of 71 pm, the PEG shows a slight improvement in coating thickness and deposition rate. The electrodeposition efficiencies of more than 100% may be a result of a self-catalytic effect arising from nickel electrodeposition, especially for the high concentration of electroplating bath including CTAB and PEG functionalized GO. Figure 14 shows the effect of concentration of nickel sulfamate solution and surfactant types on hardness (in HV or Vickers Pyramid Number) of nickel coatings. For a pure nickel coating, the hardness does not change very much with the increase of nickel sulfamate concentration. Coatings from PEG functionalized GO nanoparticles show greatly increased hardness, with hardness decreasing with increasing nickel sulfamate concentration. The hardness is increased (compared with pure nickel) by 146% for low nickel sulfamate concentration (200 g/L) and increased by 89% for a high nickel sulfamate concentration (400 g/L). For the coatings from CTAB functionalized GO, the hardness increases by 40% and 14.67% under the low/high nickel sulfamate concentration (200 g/L, 400 g/L), respectively. However, the coatings from SDS functionalised GO nanoparticles increase only by 16% in low concentration of nickel sulfamate, with hardness increase of 4% when concentration is increased to 400 g/L.

Ligure 15 shows the effect of saccharin and different surfactants on the hardness (HV) of nickel deposited from 400 g/L nickel sulfamate. Saccharin significantly enhances hardness for both pure nickel coatings and nickel/GO composite coatings. The addition of 0.2 g/L saccharin provides the optimal improvement of hardness for all of the cases. This is because a critical condition is achieved where the coverage of saccharin molecules on the cathode surface reaches saturation. This provides the highest density of nucleation sites, resulting in maximal hardness. The surfactant modified GO and saccharin have a synergistic effect on the improvement of microhardness of nickel/GO coatings, especially CTAB and PEG. The hardness increased by a maximum of 312% from 165 HV for the pure nickel to 680 HV for the CTAB -GO coating at 0.2 g/L saccharin and increased by 252% for the PEG-GO coating at 0.2 g/L saccharin. The SDS-GO nickel coatings present a similar hardness enhancement trend in the presence of saccharin. The hardness is related to the effectiveness of dispersion of the GO nanoparticles in the nickel electrolyte and thus its homogeneous distribution and incorporation in the deposited coating/part.

To quantify the surface quality of deposited pure nickel coatings and nickel/GO composite coatings, the surface roughness in terms of Sa (arithmetical mean height variation from the mean surface) was measured, as shown in table 2, below. Lor pure coatings, the Sa of 350 nm and 370 nm are obtained from 200 g/L and 400 g/L nickel sulfamate, respectively. The addition of surfactant and high nickel sulfamate concentrations increase surface roughness. The PEG functionalized GO has the most significant effect and coating roughness can be up to ~3 pm at 400 g/L nickel sulfamate solution, which is 4 times higher with Sa 575 pm from 200 g/L nickel sulfamate bath. The change in roughness is agreed with the surface morphology observation from SEM analysis. The rough surface arises from the production of protrusions or bulge structures in the coating surface where the GO sheets are involved in deposits. Such microstructural development of the coating surface can be beneficial to its wear resistance.

Table 2: Surface roughness with different surfactants The surface roughness (Sa) of nickel/GO composite coatings, deposited from the different surfactant-rich electrolytes with the different concentrations of saccharin, are summarized in table 3, below (for 400 g/L nickel sulfamate solution). The surface roughness is significantly affected by the GO nanoparticles in the coating, and the different surfactant types and saccharin concentrations have a significant effect on the surface roughness of nickel/GO composite coatings. The PEG-GO coatings have the biggest Sa, followed by CTAB and SDS. This is related to the GO incorporation in coatings, since the embedded GO sheets tend to roughen the coating surface, producing protruding structures (which were observed from SEM analyses). The addition of saccharin in the electrolyte decreased the Sa to some degree. This effect can be ascribed to a synergistic crystal refinement mechanism with GO.

Surface roughness with different saccharin concentration Referring to Figure 16, the coefficients of friction for deposits formed using different surfactants are compared. For many applications (such as bio-implants, medical devices, and moulds, low surface friction is desirable). The ball-on-disk test was performed to study the friction properties of the coatings. The friction coefficient is related to surface micro structures and chemical properties. The incorporation of GO nanoparticles into the nickel matrix was therefore expected to have a significant impact on the tribological properties of the coatings. Figure 16 shows the relationship between the average friction coefficient of pure nickel coatings and nickel/GO composite coatings with respect to the nickel sulfamate concentration, as well for different the surfactant types. As concentration of nickel sulfamate increases, the trend for friction coefficient of coatings is rising, due to the tendency of GO to agglomerate under high concentrations of nickel ions. Pure nickel coatings show no evident change in friction coefficient at -0.55 over the concentrations investigated.

The nickel/GO composite coating from PEG has a dramatic reduction in friction coefficient, at -0.15, representing a 72% reduction compared with pure nickel (at the lowest concentration of 200 g/L). The non-ionic surfactant provides a dramatically reduced coefficient of friction compared with the pure nickel, and also compared with deposits using ionic surfactants.

Figure 17 shows the coefficient of friction obtained from deposits obtained from solutions including saccharin. Again, the coatings including nanoparticles functionalised with non-ionic surfactant exhibit dramatically reduced coefficients of friction, when compared with either pure nickel with saccharin or compared with nickel with saccharin, nanoparticles and ionic surfactants.

Figure 18 shows microhardness (HV) and friction coefficient for nickel deposited with various concentrations of graphene oxide with Pluronic-F127 surfactant (from a 400 g/L concentration nickel sulfamate solution). The results show even higher microhardness than PEG at GO concentration of 0.1 g/L, of -450 HV, compared with -325 HV for PEG (see Figure 15). The coefficient of friction (at 0.1 g/L GO) is slightly higher for Pluronic than for PEG, at -0.35 compared with -0.25 respectively.

Increasing concentration of GO is associated with increased microhardness and a trend of slightly decreasing friction coefficient. Very high microhardness of > 1050 are achieved at GO concentrations of 0.4 g/L, accompanied by a low friction coefficient of <0.35.

Figure 19 shows the same microhardness data as Figure 18, plotted with the crystallite size. Decreasing crystallite size is associated with increased hardness, and smaller crystallites are seen with increasing concentrations of GO nanoparticles.

Figure 20 shows ultrasonic power vs microhardness and crystallite size for different ultrasonic powers with concentration of GO nanoparticles O. lg/L, showing that increased hardness and decreased crystallite size are associated with increasing ultrasonic power.

Figure 21 explains the mechanism by which hardness is increased, and friction decreased, in coatings according to certain embodiments. Where the nanoparticles comprise a two-dimensional material, such as graphene or graphene oxide, the nanoparticle may have very high strength but stacked layers of the nanoparticles may shear easily due to weak van der Waals interactions between layers and the atomically smooth surface. This interlayer shearing enables reduced friction coefficient. Initially, the embedded 2D material sheets in 601 a nickel matrix 603 are exposed to the ball at the rubbing interface, The amount of these sheets increases with scar extension. Completely exposed multi-layered sheets slip against each other along the sliding direction to reduce friction, or they form a lubricating film which effectively reduces the direct contact area (with the underlying nickel), thus decreasing friction.

Additionally, nanoparticles (including 2D materials) can be excellent filler materials to improve the mechanical and thermal properties of a metal matrix. As nanoparticles (and particularly 2D nanoparticles) are easily embedded into a nickel matrix, they can wrap and/or bridge nickel grains, thereby preventing free movement of nickel grains, enhancing nickel strength and extending tool life.

Ultrasonic vibration (i.e. sonication) may induce localized shearing and ultrasonic cavitation to break and separate aggregations of nanoparticles. The prevention of such aggregations enables the maximum advantages associated with codeposition of nanoparticles during electrodeposition. Additionally, ultrasonic vibration can promote the mass transport of electrolyte, and therefore alter the electrodeposition rate and the electrochemical reaction behaviours, reducing porosity from hydrogen formation. As a result, ultrasonic vibration can enhance electrodeposition quality in both mechanical performance and appearance. Although examples of the invention have been described, the skilled person will understand that variations are possible, within the scope of the appended claims. For example, although a number of specific non-ionic surfactants have been discussed, other non-ionic surfactants can be used to achieve similar results, such as Tween 80, Brij 700, Pluronic® P-123, etc.

Claims

1. A method of preparing an electrolyte solution for electrodeposition, comprising: dispersing nanoparticles in an aqueous solution comprising a surfactant, thereby forming a dispersion of nanoparticle-surfactant micellar complexes in solution; adding electrolytes comprising ionic metal; wherein the concentration of the surfactant is selected to substantially prevent sedimentation of the nanoparticles.

2. The method of claim 1, wherein the metal ions comprise nickel ions.

3. The method of claim 2, wherein the aqueous solution comprises nickel sulfamate.

4. The method of claim 3, wherein the concentration of nickel sulfamate is between 200 and 500 grams per litre.

5. The method of any preceding claim, wherein the concentration of the non -ionic surfactant in the aqueous solution is at least 40% of the critical micelle concentration or at least 0.4 g/litre.

6. The method of any preceding claim, wherein the nanoparticles comprise two dimensional nanoparticles.

7. The method of any preceding claim, wherein the concentration of nanoparticles dispersed in the aqueous solution is between 0.02 and 0.2 grams per litre.

8. The method of any preceding claim, wherein the surfactant is non-ionic

9 The method of claim 8, wherein the surfactant comprises at least one of: a non-ionic triblock copolymer; a poloxamer; a polysorbate 80; a polyetholylated stearyl alcohol.

10. The method of any preceding claim, wherein dispersing the nanoparticles comprises agitating the electrolyte solution using ultrasound and/or mechanical stirring.

11. The method of any preceding claim, further comprising adding a grain refiner to the aqueous solution.

12. The method of claim 11, wherein the grain refiner comprises saccharin.

13. The method of claim 12, wherein the saccharin is added to a concentration of between 0.02 and 0.5 grams per litre in the aqueous solution.

14. A method of making a part, comprising: preparing an electrolyte solution in accordance with any preceding claim; electrodepositing a metal part from the electrolyte solution.

15. The method of claim 14, further comprising agitating the electrolyte solution during electrodeposition.

16. The method of claim 15, wherein the agitating comprises ultrasonic agitation.

17. A part formed using the method of any of claims 14 to 16.

18. The part of claim 17, wherein the part has a thickness of at least 0.5mm.

19. The part of claim 17 or 18, wherein the part comprises features with a linewidth of 1mm or less, or 0.5mm or less.

20. The part of any of claims 17 to 19, wherein the part has a Vickers Pyramid number of at least 500.

21. The part of any of claims 17 to 20, wherein the part has a ball-on-disk friction coefficient of less than 0.5.

22. The part of any of claims 17 to 21, wherein the part comprises a mould.

23. The part of any of claims 17 to 21, wherein the part has a surface roughness R_a of less than 50nm.

24. A method for fabricating a permanent self-lubricating high-performance micro/nano structured mould from a 2D material-reinforced metal nanocomposite using an electroforming process, the method comprising: preparing an aqueous dispersion of self-assembled surfactants and 2D materials via ultrasonic or high-speed shear agitation; preparing a plating bath by mixing the aqueous dispersion of self -assembled surfactants and 2D materials with a metal or metal -alloy electroforming solution and further dispersing the 2D materials for homogenous distribution via ultrasonic agitation; putting a micro/nano structured cathode master obtained from a LIGA or UV - LIGA process and an anode into the plating bath and imposing a current between the cathode master and the anode, thereby electroforming the high-performance micro/nano structured mould on the cathode master.

25. The method of claim 24, wherein: i) the metal or metal-alloy electroforming solution comprises Nickel sulfamate, Nickel sulfate, Copper sulfamate, Copper sulfate, or Nickel sulfate -Cobalt sulfate; ii) the master comprises a line width from 500 micrometers to 10 nanometers; iii) the 2D materials comprise nanoparticles comprising at least one of graphene oxide nanoplatelets, or graphene nanoplatelets, molybdenum disulfate and hexagonal boron nitride 2D materials; iv) the mould has self-lubricating properties without any delamination; v) the dispersion of 2D materials has to be continued throughout electroforming process; vi) the surface of the mould has Vickers Pyramid hardness higher than 500; and/or vii) a working surface of the mould has roughness R_a below 50nm.