Properties of Ni-B/B Composite Coatings Produced by the Electroless Method under Semi-Technical Line Conditions

Abstract

Composite coatings have been successfully fabricated at the laboratory scale in many research centers around the world; however, it is still a major challenge to transfer the positive results of the work to the industrial scale. This paper presents the technology for the production of Ni-B and Ni-B/B composite coatings on a pilot experimental semi-technical line by chemical reduction. A process scheme for the fabrication of Ni-B layers and composite coatings with a nickel–boron matrix and a dispersive phase in the form of boron nanoparticles was developed. All stages of the fabrication process were described in detail. The dispersion phase of the boron particles was characterized, and the performance properties of the Ni-B and Ni-B/B composite coatings produced on a pilot electroplating line were studied. The structure and morphology of the Ni-B/B composite coatings were characterized for comparison with nickel–boron coatings. Their mechanical and tribological properties and adhesion to the substrate were studied. The influence of the dispersion phase of boron particles on the structure and functional properties of the composite coatings was evaluated. In order to improve the performance of the fabricated coatings, a heating process at 400 °C was carried out, and the performance of Ni-B and composite Ni-B/B coatings was studied after the heat treatment operation.

1. Introduction

Ni-B alloy coatings produced by the electroless method are of great interest to scholars from all over the world. There are several factors behind this popularity: the uncomplicated manufacturing technology (temperatures below 100 °C), the ability to coat parts with complex shapes (uniform thickness of deposited coatings), and the properties of this type of material. In a review paper, Barati and Hadavi [1] list industries where Ni-B coatings could find application: chemical, petrochemical, defense, nuclear power, automotive, electronics, hydrogen production. They also mention the possibility of improving the properties of such materials by introducing various types of dispersion phase particles into their matrix, controlling alloy composition and heat treatment. In composite coatings, as a result of the synergistic interaction of the matrix and embedded particles, there is an improvement in properties compared to nickel–boron coatings without embedded particles. The type of particles used is also important and affects the properties. In the paper [2], the effect of boron carbide embedded in the matrix of nickel alloy coatings was studied. The results indicate an increase in the hardness of the composite coatings compared to coatings without embedded particles; however, they also indicate an increase in the coefficient of friction and a deterioration in corrosion properties after the incorporation of B₄C particles. In the work [3], Ni-B/GO composite alloy coatings were the starting point for the production of superhydrophobic coatings for the corrosion protection of AZ91 magnesium alloy. Gul et al. [4] found improved corrosion resistance and tribological properties of ternary Ni-B-P alloy coatings with embedded carbon nanotubes. Better corrosion resistance and improvement in mechanical properties of nickel–boron coatings can be achieved by incorporating α-ZrP particles [5]. The increase in hardness of Ni-B coatings due to built-in hard ceramic particles SiO₂, Al₂O₃, TiO₂ is reported by the authors of [6]. The above findings prove that composite coatings based on nickel–boron alloy have great application potential and can be widely used.

According to the Technology Readiness Level (TRL), which is a widely accepted tool for assessing the maturity of the development of ongoing scientific work [7], work on a given technology can be divided into several stages (Figure 1), beginning with basic observations, moving on to concepts, small-scale trials, and finally the prototype and final implementation of the technology.

To date, research on Ni-B alloy coatings produced electroless has involved laboratory-scale tests in beakers and vessels with capacities ranging from a few hundred cm³ to a few dm³. However, there is a lack of information in the literature on the production of this type of material on an engineering scale in industrial, i.e., close to real, conditions. The challenge at this point becomes shifting the scale from small bath volumes to larger baths with capacities of several tens of liters and providing similar process conditions (temperature, mixing, etc.) to those on the smaller scale. The industrialization of the process of electroless deposition of nickel–boron coatings is also associated with other issues such as bath replenishment, bath cleaning and recycling of used baths. This is mentioned by the authors of the paper [8], and they indicate that research into the industrial application of this type of coating should be the subject of future work in this field.

In the current work, an attempt was made to scale up the deposition process of nickel–boron alloy and Ni-B/B composite coatings under technical conditions (20 dm³) on a plating line. Boron in the coating exists in two forms: as a component of the alloyed Ni-B matrix and as a particulate dispersion phase that is incorporated into it. The team’s previous work [9] on this type of coatings was carried out on a laboratory scale in beakers (0.5–2 dm³), and their main goal was to determine the conditions for proving the deposition process and the concentration of the dispersion phase (boron) in the electrolyte at which the properties of the coatings are best. Laboratory tests showed that for a given type of coatings, the most beneficial tribological, mechanical and corrosion properties were obtained for coatings deposited from baths containing 1 g/dm³ of boron powder as the dispersion phase (TRL 4). Hence, within the framework of the present work, the authors focus on producing and studying the properties of alloyed Ni-B/B composite coatings produced from a bath with a single dispersion phase concentration (1 g/dm³) under semi-technical line conditions in 20 dm³ baths (TRL 6). For comparative purposes, the study also includes a Ni-B alloy coating without boron powder particles. The obtained research results are a step towards the commercial application of this type of coating. An element of novelty within the framework of the present work is the deposition of this type of material under semi-technical line conditions in 20 dm³ baths.

2. Materials and Methods

The process of chemical deposition of nickel–boron alloy and Ni-B/B composite coatings under semi-technical line conditions consisted of several consecutive operations carried out according to the technological sequence shown in Figure 2.

The process consisted of operations for surface preparation of the workpieces and chemical deposition of Ni-B and Ni-B/B coatings. The surface preparation operations performed on the workpieces were electrochemical degreasing for 6 min in an alkaline bath and activation for 15 s in 10% HCl. After each operation, the workpieces were rinsed in demineralized water to minimize the transfer of bath components in subsequent steps. Compressed air mixing was used. The bath compositions and parameters of the coating deposition operations are given in

Table 1. Bath compositions for deposition of Ni-B and Ni-B/B coatings.

Compounds	Quantity [g/dm3]
	Ni-B	Ni-B/B
NiCl2 * 6H2O	30
C2H8N2	90
NaOH	90
Pb(NO3)2	0.0145
B	-	1.0
NaBH4	1.6

and

Table 2. Ni-B and Ni-B/B coating deposition process parameters.

Parameter	Ni-B	Ni-B/B
Temperature [°C]	90 (±1)
Time [h]	1.5
Bath loading [dm2/dm3]	0.1
Bath volume [dm3]	20
Type of mixing	Static (hydraulic pump) + mechanical (cathodic rail) + compressed air

.

Ni-B and Ni-B/B composite coatings were deposited from a multicomponent electrolyte solution containing the following: nickel chloride (source of nickel ions), ethylenediamine (complexing agent), sodium hydroxide (pH corrector), lead nitrate (stabilizer), sodium borohydride (catalyst, reducing agent) and, in the case of composite coatings, boron powder nanoparticles. The chemical reagents (nickel chloride, ethylenediamine, sodium hydroxide, lead nitrate, acids) sourced for the study from Chempur, Piekary Śląskie, Poland, were of high purity (p.a.); sodium borohydride and boron (Sigma-Aldrich, St. Louis, MO, USA). For scientific papers, various compounds can be used as a reducing agent during the deposition of Ni-B coatings. The most popular are sodium borohydride (NaBH₄) and dimethylene borane (DMAB). The choice of NaBH₄ is due to its higher reducing potential and more favorable functional properties (higher hardness). However, this compound requires an alkaline pH bath [1].

The workstation (Figure 3) was a 30 × 25 × 30 cm bath with a working volume of 20 dm³ equipped with three heating elements of 700 W each. Mixing of the bath was carried out by air using a glass element placed at the bottom of the bath. In addition, closed-loop flow mixing using a pump assisted by mechanical mixing with a cathode rail (up–down movement) was used. The bath temperature was recorded with sensors and controlled on the control panel. Polypropylene spheres were placed in the bath to minimize bath evaporation.

The surface area of one batch was about 2 dm² per 20 dm³ of bath. Such proportions made it possible to produce Ni-B and Ni-B/B coatings in a single process with thicknesses of 48 (±3) µm and 37 (±1) µm, respectively, without the need to refill the bath solution during the process. The coatings were deposited on samples made of carbon steel (S355) (Figure 4).

Various workpiece shapes were used to produce coatings for future research as well as to simulate industrial conditions. Flat specimens with dimensions 60 × 20 × 2 mm were used for morphology, structure and mechanical testing but not tribological testing, for which 1-inch-diameter disks were required.

The boron powder and the surface morphology of the deposited coatings were characterized using a scanning electron microscope JSM-IT100 LA (Jeol, Akishima, Japan). The structure and phase composition of the used boron powder and the deposited coatings were studied by X-ray diffraction (XRD) using an instrument (X-ray diffractometer from Anton Paar—XRDynamic 500, Graz, Austria). Measurements were carried out using a parallel beam of Cu Kα radiation.

The internal structure of Ni-B and Ni-B/B coatings was studied by SEM on cross sections perpendicular to the surface of a metallographic specimen. The samples were prepared by cutting off 5 mm of the sample and placing it in thermosetting resin (Struers, Ballerup, Denmark), and then the specimens were ground on 120–1200 gradation sandpaper and polished with diamond paste. The metallographic specimens were also used for mechanical property tests using the depth-sensing indentation (DSI) method at a progressive load of 0–300 mN with an indenter indentation rate of 1000 mN/min with a maximum load interval of 15 s. As a result of the DSI test, material parameters of the tested materials were measured, such as Martens hardness (the quotient of the loading force by the indenter surface), indentation hardness (the quotient of the maximum applied force by the indenter surface) and Young’s modulus. Roughness parameters Ra and Rz were measured using a Surftest SJ210 profilometer (Mitutoyo, Kawasaki, Japan). Abrasion resistance was tested using the ball-on-disc method on a T11 device (Łukasiewicz—Institute for Sustainable Technologies, Radom, Poland), using a ceramic ball (Al₂O₃) with a diameter of 6.35 mm at a load of 10 N and speed of 0.1 m/s over a distance of 500 m. The bonding of the produced coatings to the substrate was tested by scratch testing using a CSEM Revetest device (Anton Paar GmbH, Graz, Austria) with a progressive load of 0–100 N for 60 s at a Rockwell indenter travel speed of 10 mm/min. The tests also included heat treatment of the produced Ni-B and Ni-B/B coatings, which was carried out at 400 °C for 30 min in an air atmosphere.

3. Results and Discussion

3.1. Dispersion Phase (Boron)

The characteristics of the dispersion phase used are shown in Figure 5.

The boron powder used was characterized by particles of varying shapes and sizes < 1 µm. Based on XRD studies, it can be concluded that the sample contained a mixture of crystalline phases most likely with τ-B (P1) and β-B (R-3m) type structures with slightly different crystal lattice parameters compared to the entries in PDF4-axiom 2024 (Figure 6c). A description of these types of structures can be found in the paper [10]. However, in another publication [11], this material is also described as amorphous.

3.2. Morphology and Structure of the Produced Ni-B and Ni-B/B Coatings

The results of the surface morphology, structure of cross sections and structure of the deposited Ni-B and Ni-B/B coatings before and after heat treatment are presented in Figure 6 and Figure 7.

The incorporation of boron particles into the Ni-B matrix significantly affects the surface morphology and topography of Ni-B/B composite coatings. Structures similar to cauliflower are turning globular. This is caused by the incorporation of particles and agglomerates of the boron powder phase into the Ni-B alloy matrix. The embedded particles and agglomerates of the boron dispersion phase on scanning electron microscope images can be seen as black points, both on cross sections and on the surface of the produced Ni-B/B composite coatings. They are successively covered by a nickel–boron matrix during the procedure. The Ni-B/B composite coating is characterized by a higher degree of surface development, which is evident in the results of the roughness parameters (

Table 3. Roughness parameters of the studied coatings Ni-B, Ni-B HT, Ni-B/B, Ni-B/B HT.

Coating	Roughness Parameter
	Ra [µm]	Rz [µm]
Ni-B	0.55 ± 0.04	3.99 ± 0.48
Ni-B HT	0.60 ± 0.05	4.39 ± 0.25
Ni-B/B	0.85 ± 0.03	6.03 ± 0.23
Ni-B/B HT	0.87 ± 0.05	6.57 ± 0.56

). The columnar structure of Ni-B and Ni-B/B coating is clearly visible on cross sections.

After the heating operation, the structure changes but still maintains the columnar, compact character, and a porous outer layer appears [12]. After the heating operation, cracks running perpendicular to the substrate are noticeable in both Ni-B and Ni-B/B coatings. XRD studies (Figure 7) of the tested coatings indicate the presence of an amorphous structure in Ni-B and Ni-B/B coatings; small reflections come from the substrate material—alpha iron. In this type of material, the structure is determined by the content of the alloying element (B). For materials with boron content above 5% wt., the structure is amorphous [13,14]. The lack of reflections from the dispersion phase—boron powder—may be due to the small amount embedded in the matrix material. In the case of the samples after heating, crystallization of the primary phase occurred in the tested materials. The samples contain boron phases of the Ni₂B, Ni₃B and Ni₇B₃ types. The presence of pure nickel was not noted.

3.3. Mechanical Properties

The mechanical properties of the substrate and the produced Ni-B and Ni-B/B composite coatings tested by the DSI method are shown in

Table 4. Mechanical properties of produced coatings from DSI tests.

	Microhardness			Depth [nm]	Elasticity Modulus EIT [GPa]
	HIT [MPa]	HM [MPa]	HV
Ni-B	8974 (±315)	5898 (±190)	847 (±30)	1274 (±20)	172 (±7)
Ni-B HT	10,305 (±503)	6625 (±295)	973 (±47)	1203 (±28)	188 (±9)
Ni-B/B	9358 (±375)	6623 (±193)	883 (±35)	1203 (±17)	247 (±7)
Ni-B/B HT	13,072 (±1068)	8081 (±515)	1234 (±101)	1087 (±33)	219 (±10)
substrate	1732 (±48)	1520 (±41)	163 (±5)	2529 (±34)	234 (±11)

and Figure 8.

All the Ni-B and Ni-B/B coating variants tested have significantly higher hardness than the steel substrate material. This is well illustrated in Figure 8, where the depths of indentation into the tested materials are shown. For the substrate, the depth is twice that of the Ni-B coatings. The incorporation of dispersion phase (B) particles into the matrix (Ni-B) contributes to the strengthening of the coating material. A coating with embedded particles has higher H_IT, H_M and HV values than a coating without embedded particles. In the case of composite coatings, an increase in hardness is observed especially when hard nanoparticles are introduced into the matrix (Orowan mechanism). Such particles block the movement of dislocations, which results in plastic deformation and an increase in hardness [1] A significant problem with nanoparticles is their high surface energy, which contributes to the formation of agglomerates. The incorporation of large clusters of particles can negatively affect hardness and cause it to decrease. An effective way to counteract this phenomenon is to use a suitable and efficient mixing method. In the present study, in order to counteract the agglomeration phenomenon, mixing with compressed air was used, further reinforced by mixing with a rail with samples attached and using a hydraulic pump. Despite this, embedded agglomerates can be observed on cross sections. The strengthening of Ni-B alloy coatings can also be achieved by heat treatment. As a result of heat treatment, a change in structure from amorphous to crystalline occurs in the coating material. Intermetallic phases of nickel and boron characterized by high hardness are formed [15,16]. The formation of such structures causes an increase in stresses in the coating material and leads to cracks in the direction perpendicular to the substrate (Figure 6b,d).

3.4. Scratch Test

An analysis of the results after scratch testing (Figure 9) shows clear differences in the behavior of coatings before and after heat treatment. Coatings without heat treatment show good bonding to the steel substrate; delamination and chipping of the coating material were not observed. The resulting damage is cohesive in nature. Cohesive cracks are caused by frictional forces between the moving indenter and the surface, the accumulation of stresses upstream and downstream of the indenter, and the movement of the indenter [17]. A similar type of damage was also observed in the work [18], where Ni-B-W alloy coatings with an embedded SiC phase were investigated. In the case of heat-treated coatings, large chipping and destruction of the coating material were observed. Destruction is also evident in frictional force and friction coefficient diagrams, where there are clear fluctuations resulting from the delamination of coatings on the steel substrate. The formation of this type of damage is due to the high hardness of the tested coatings.

3.5. Tribology

The results of the tribological tests in the form of graphs of the friction coefficient as a function of sliding distance and pictures of the sample after the test are shown in Figure 10 and Figure 11 and

Table 5. Values of average wear track width.

Coating	Average Wear Track Width [µm]
Ni-B	338 (±24)
Ni-B/B	303 (±24)
Ni-B HT	307 (±17)
Ni-B/B HT	299 (±24)

.

An analysis of graphs of friction coefficients as a function of distance indicates large fluctuations during the test. This makes it difficult to determine precisely which coating has the best tribological properties. This may be caused by the incorporation of hard boron particles, the surface roughness and the high hardness of the test materials. Similar observations for this type of material were also noted in the work [19]. The authors studied the effect of B concentration as an alloying element on the tribological properties of Ni-B coatings before and after heat treatment. Dilek et al. [20] describe the improvement in tribological properties of Ni-W/TiC composite alloy coatings as being due to the presence of ceramic particles, which strengthen the coating material and reduce direct contact between the matrix and the ball. Based on the measured widths of abrasions during the test, the greatest changes can be observed for the nickel–boron coating without built-in particles and without heat treatment, for which the width of abrasion is the greatest. The damage observations indicate that the dominant degradation mechanism for the studied series of coatings was adhesive and abrasive wear. This can be observed in the form of longitudinal lines inside the traces (Figure 11). The tribological properties are strongly influenced by the hardness and surface roughness of the tested coatings.

4. Conclusions

The presented research was the first attempt to deposit Ni-B alloy matrix composite coatings under industrial conditions. The adaptation of the line to the requirements of the process of deposition of composite coatings by the electroless method can be considered a success for the authors. The produced Ni-B and Ni-B/B composite coatings were characterized by a compact build and good adhesion to the steel substrate. The influence of the incorporation of a boron powder dispersion phase is also observed in the changes in morphology and in the mechanical or tribological properties. The heat treatment of the studied coatings causes the crystallization of the amorphous matrix and significantly increases the hardness. On the other hand, the high hardness of the crystalline nickel boride phases contributes to the high brittleness and cracking of the coatings.

Issues that require further research for this type of material concern the conditions under which the deposition process is carried out with particular attention to uniform conditions throughout the process bath. These conditions include mixing conditions (method, type, speed) and bath temperature. Due to their properties, Ni-B matrix composite coatings are an attractive material as protective and functional coatings. The results obtained in this study represent a major step towards the commercial application of such materials.