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Article

The Effect of Chemical Composition on the Microstructure and Properties of Multicomponent Nickel-Based Boride Layers Produced on C45 Steel by the Hybrid Method

by
Michał Tacikowski
1,
Grzegorz Łukaszewicz
1,*,
Michał Kulka
2,
Ryszard Diduszko
3 and
Tadeusz Wierzchoń
1
1
Faculty of Materials Science and Engineering, Warsaw University of Technology, ul. Wołoska 141, 02-507 Warsaw, Poland
2
Faculty of Materials Engineering and Technical Physics, Poznan University of Technology, Pl. M. Skłodowskiej-Curie 5, 60-965 Poznan, Poland
3
Łukasiewicz Research Network—Institute of Microelectronics and Photonics, al. Lotników 32/46, 02-668 Warsaw, Poland
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(2), 197; https://doi.org/10.3390/coatings14020197
Submission received: 19 December 2023 / Revised: 27 January 2024 / Accepted: 30 January 2024 / Published: 2 February 2024
(This article belongs to the Section Surface Characterization, Deposition and Modification)
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Abstract

:
Layers of iron–nickel Fe-Ni-B-type borides were produced on C45 steel using a new hybrid treatment variant which combines boriding under glow discharge conditions with galvanic nickel precoating. The aim was to investigate whether these layers could constitute an alternative to the previously developed multicomponent Fe-Ni-B-P-type layers produced by a hybrid treatment variant using chemical nickel precoating. The basis for assessing the effects of both alternative treatments was a comparative analysis of the microstructure and performance properties of three model boride layers: iron–nickel boride layers of the Fe-Ni-B and Fe-Ni-B-P types, and reference iron Fe-B-type boride layer. It was demonstrated that the new variant of hybrid treatment produces Fe-Ni-B layers with the highest thickness, slight porosity, the optimal structure of Ni2B boride in the near-surface zone and the best performance properties. These layers show good adhesion, a much higher hardness of 2200 HV0.05 and near-surface compressive stresses of −450 MPa. Fe-Ni-B-P layers show slightly better wear resistance for higher loads, but like Fe-B layers, they are susceptible to spalling. It was demonstrated that Fe-Ni-B layers produced using boriding with nickel galvanic steel precoating could find application in heavy-duty elements of nanobainitic steel processing.

1. Introduction

The production of boride layers on steel through boriding treatment results in a combination of several beneficial properties, which makes boriding, next to carburising and nitriding, one of the three leading, although the least widespread, thermo-chemical treatment technologies used in contemporary industrial practice. Because of their significantly high hardness [1,2,3] and excellent wear resistance [1,4,5], borided layers are produced on surfaces of heavy-duty machine components and tools [6]. Another advantage is good corrosion resistance [6,7,8]. It is also worth noting their high resistance to frictional wear in the absence of lubrication, erosion resistance, heat resistance, and resistance to molten metals [8]. However, their use is limited by brittleness, their major drawback [1]. An effective way must be found to prevent the formation of a hard but relatively brittle, and therefore ultimately unfavourable, phase with FeB stoichiometry in borided steel layers. However, the issue of optimising the phase and chemical compositions of boride layers (especially in terms of eliminating or limiting the content or appropriate arrangement of FeB iron boride in the structure) remains an open challenge.
The improvement in borided layers’ properties has been the purpose of intensive scientific research over the past decades in the context of thermo-chemical treatments of steel and other alloys. Two directions of development can be distinguished here. The first path is to optimise the boriding processes, e.g., by developing new, alternative methods for producing borided layers. Techniques such as boriding under glow discharge conditions [9,10], ion implantation [11], spark plasma sintering [12,13], and boriding in fluidised beds [14,15] should be mentioned here. The second route contains substrate modifications before boriding and various methods of improving the borided layers after they are obtained. Known ways of enhancing the substrate before boriding include mechanical attrition treatment [16,17] and covering it with different metals to produce alloy borides [18,19,20]. In turn, the post-production enhancement of the layers includes, among others, laser modification [21,22,23] and diffusion annealing [24,25]. These types of hybrid technologies involving the combination of various surface treatment processes are the most promising directions in optimising boriding results, as they allow for the creation of diffusion surface layers with unique properties unattainable by other currently used methods. In this way, composite and gradient layers can be produced with full control of the microstructure, the phase and chemical composition, and the residual stress state, and therefore, with specific properties and performance requirements, ensuring increased durability and reliability of the treated products.
One such interesting hybrid solution used for steel is the production of layers of nickel-based alloyed borides by boriding previously nickel-coated substrates [1]. Nickel and its alloys are materials susceptible to boriding, and nickel boride layers have favourable functional properties [1,26,27], including higher plasticity than iron boride layers, which should have a positive effect on the tribological properties of borided nickel alloys.
The inspiration for the present study was our recent work on the application of boriding in the surface engineering of nanobainitic steel [28,29]. Although, as demonstrated, boriding can be effectively combined with advanced steel heat treatments such as nanobainitising, the porosity, the FeB formation-related brittleness, and relatively low wear resistance of the layers based on iron borides limit the achieved improvement effect. Therefore, the idea of modifying the borided layers on nanobainitic steels by replacing iron borides with nickel borides seems to be promising. The search for adequate surface engineering technology became the subject of the present study using three boride model layers systems.
A state-of-the-art literature review shows relatively few works scattered over the past half-century that are devoted to the use of preliminary electrochemical nickel coating of steel substrates subjected to boriding. As mentioned by Krukovich [1], already in early works published in the late 1960s, it was shown that alloyed boron layers obtained by both prior nickel electrochemical and electroless coating of the steel substrate and further boriding exhibited increased plasticity [30,31]. However, no detailed data on this subject are accessible. It should be equally noted that, as reported, the patented method of nickel steel precoating and subsequent gas boriding in 1–5% BCl3 medium [32] allows for the attainment of layers based on nickel–iron borides with favourable stoichiometry of the (Ni, Fe)2B type and reduced surface roughness. In another work, layers built of alloy borides, including nickel borides, were produced in a fluidised bed; these layers show a correlation of boride layers with the degree of alloying [33]. Chinese authors used boriding of previously nickel-plated carbon and high-alloy steel substrates. They observed the formation of iron–nickel borides and multicomponent borides on alloy carbon and steels, which increased the corrosion, wear and fatigue resistance of developed layers [34,35].
Our early source works [36,37] show that multicomponent boride layers based on iron–nickel borides of the Fe-Ni-B-P type can be produced on steel by hybrid treatments combining gas boriding under glow discharge conditions with a previous coating with nickel using an electroless chemical method. The composition of the bath for nickel plating necessitates the presence of nickel phosphides in the boride layers. The multicomponent Fe-Ni-B-P boride layers obtained in this way have favourable tribological properties, significantly higher than those of iron boride layers. Hence, the concept of producing iron–nickel-based boride layers using this hybrid treatment in the surface treatment of nanobainitic alloy steels produced directly on steel substrate without any previous substrate modification was determined. Direct implementation of the cited method is, however, not possible. This is due to the relatively low-temperature stability of the phosphide phases occurring as one of the structure components of the multicomponent boride layer of the Fe-Ni-B-P type, amounting to 970 °C for the Ni3P type phosphide [38]. This excludes the possibility of further heat treatment of borided steel with a Fe-Ni-B-P layer intended to harden the substrate in quenching processes, including nanobainitising, of those steel grades whose austenitising temperature range is higher [38].
Hence, the new concept proposed in the present work is based on producing iron–nickel boride Fe-Ni-B-type layers using a modified, not earlier investigated, variant of hybrid processing. In this variant, the prior nickel coating of steel subjected to boriding under the glow discharge conditions is carried out using a new, alternative galvanic method. Whether such a new hybrid solution allows us to achieve similarly beneficial effects as in the chemical nickel precoating variant remains an open question. The review of the state of the art of nickel-precoated substrate boriding shows insufficient experimental comparative source data to resolve this issue. Our technology combines steel nickel precoating with boriding in glow discharge conditions using a BCl3 medium. It is a novel method, quite different from the boriding methods and their condition used by other authors. One can note a few works referring to the application of plasma boriding in the treatment of bulk steel, nickel alloys and nickel-coated substrate; however, they concern the processes based on the use of different boriding media than BCl3, such as boron trifluoride BF3 [39], trimethyl borate B(OCH3)3 [40] and borax paste [41]. Those works do not report any performance improvements other than hardness, which may provide the context for assessing the new technology’s performance.
Thus, our work aims to compare the different variants of iron–nickel-based boride layers, specifically the newly developed Fe-Ni-B and Fe-Ni-B-P layers [36,37] produced in two alternative hybrid process variants combining glow discharge boriding in BCl3 base atmosphere with the alternative methods of nickel deposition: electrochemical and electroless. The effect of the layers’ chemical composition on their microstructure and the performance properties will be compared to assess the new method’s practical applications utility in the boriding of nanobainitic steels. Comparative research was carried out on three model boride layers: the reference simple iron borides of the Fe-B type and complex two- and multicomponent iron–nickel borides of the Fe-Ni-B and Fe-Ni-B-P types, respectively. The layers’ utility assessment was based on a complex, comprehensive investigation, which included the influence of chemical composition on the microstructure, stress distribution and functional properties, including hardness, resistance to frictional wear, cracking resistance and adhesion of boride layers.

2. Materials and Methods

This research was carried out on three types of model boride layers on C45 steel: simple iron borides of the Fe-B type, and complex two- and multicomponent iron–nickel borides of the Fe-Ni-B and Fe-Ni-B-P types, respectively. These layers were obtained using the hybrid method by subjecting three different substrates to plasma boriding. In order to produce complex iron–nickel boride layers, the steel substrates were modified prior to the boriding process by coating them with nickel using electroless (chemical) or electrochemical (galvanic) methods. Boriding in glow discharge conditions has the advantage of reduced process temperature, down to values as low as 650 °C, combined with the efficient activation of the substrate surface by cathodic sputtering, plus process control with the pressure and electric discharge parameters. The optimal processing parameters for both nickel precoating and boriding were developed in earlier works [36,37,42]. The effect of the chemical composition of the boride layers on the microstructure and the properties of boride layers, including hardness, cracking susceptibility, adhesion, stress distribution, and wear resistance, were the subject of comparative investigations.

2.1. Substrate and Coating Materials

A typical carbon structural steel with a simple chemical composition and properties given in Table 1 and Table 2 was chosen as the substrate material for the model layers’ investigation. The material of the nickel coatings for the galvanic deposition variant was pure nickel, while for the chemical deposition variant, it was an amorphous nickel alloy containing 8 wt% of phosphorus, originating from the deposition process [36]. While the substrates are heated to the boriding temperature, the diffusion of nickel into the substrate and iron from the substrate to the coating transforms galvanic nickel coatings into a diffusional nickel–iron solution layer. For the amorphous chemical nickel coatings, the mutual diffusion of nickel and iron during heating is accompanied by crystallisation processes. This results in its transformation into a diffusion layer composed of nickel-based precipitates and eutectics composed of nickel and nickel–iron phosphide with Ni3P stoichiometry [36]. The thermal eutectic stability at the level of 880 °C [38] was a key criterion for choosing the plasma boriding temperature for the substrate chemically precoated with nickel. The temperature of 850 °C was experimentally proven to be the maximum safe process temperature, which guarantees the elimination of the eutectic melting risk [37,42].

2.2. Coating and Boriding Operations

In the first stage of the hybrid method, specimens of C45 steel were plated with 20 µm nickel coatings using a chemical electroless or galvanic method, a new alternative not investigated earlier. The chemical nickel (with the 8 wt% P content) deposition was conducted in a solution composed of NaH2PO4, NiCl2 and sodium citrate at the temperature of 90 °C for 1 h. The galvanic nickel deposition was performed using Watt’s solution. Specimens were then subjected to plasma boriding under glow discharge conditions at temperatures of 850 °C. The samples were heated in low-temperature glow discharge plasma, and after reaching the assumed process temperature, BCl3 dosing was started. The boriding process was carried out in the prototype of a universal apparatus for thermochemical treatments under glow discharge conditions with a resistance-heated anode. The reactive processing atmosphere was composed of H2 + BCl3 (up to 5 vol.%), the pressure was 5 hPa, and the treatment lasted 2 h.

2.3. Test Methods

The microstructure of the layers was examined with the classic metallography technique and scanning electron microscopy (SEM) using backscattering imaging mode (BSE) and EDS chemical composition microanalyses. Light microscopy and SEM observations were conducted on the wear test (the rollers) cylindrical samples’ cross-sections. After mechanical grinding and polishing, chemical etching with Nital reagent was applied to reveal the microstructure. Light microscopy observations were performed using a Keyence VHX 7000 light microscope. The thickness of the borided layers was determined based on an average of 10 measurements of boride needles. Microstructural SEM characterisation was performed using a MIRA 3 scanning electron microscope (SEM; TESCAN, Brno, Czech Republic) equipped with an energy dispersive spectrometer (EDS; Oxford Instruments Ultim Max 65, Oxford, UK). EDS X-ray microanalysis of the selected elements was performed. The concentrations of iron, nickel, boron, and phosphorus were measured in the cross-section of the borided layer using line analysis. The chemical composition was also examined in point mode in the selected areas of the layers. Finally, the distribution of the selected element was analysed based on EDS mapping.
The phase composition of the boride layers was determined using X-ray diffraction (XRD). Measurements were carried out on a Rigaku SmartLab 3 kW diffractometer (Tokyo, Japan). For the apparatus, the radiation source was a Cu tube with operating parameters of U = 40 kV and I = 30 mA. The Bragg–Brentano measuring geometry was used with a measuring step of Δ2θ 0.02°.
The Vickers microhardness HV0.05 measurements were performed on mechanically ground and polished borided rollers cross-sections using the Future-Tech FM-810 tester (Kawasaki City, Japan). The layers’ susceptibility to cracking was examined using a scratch test method on CSM Revetest Apparatus, and a microbending type test method was performed on 20 × 5 × 2 mm3 plate-shaped samples. The stress analysis was performed using the Weissman–Philips method in order to examine the stress distribution in boride layers using 70 × 10 × 2 mm3 plate-shaped samples. The wear resistance was examined using a “three rollers–taper” method, in accordance with the Polish standard PN-83/H-04302, in which sliding friction is applied in lubrication conditions between three fixed cylindrical specimens Ø8 × 20 mm (the rollers) and a rotating conical counter body (the taper), the hardness of which is about 30 HRC. The test surface loads were 200 and 400 MPa, and the rotational speed of the taper was 576 rpm.

3. Results and Discussion

3.1. Microstructure and Chemical Composition of Layers

The surface morphology and microstructures of the boride layers produced by the hybrid method using boriding under glow discharge conditions carried out at 850 °C for 2 h for three examined substrate variants are shown in Figure 1. As can be seen, the surface morphology, especially its development, depends on the chemical composition of the substrate subjected to the boriding process (C45 steel or C45 steel covered with a nickel coating using a chemical, electroless method, or C45 steel covered with a nickel coating using a galvanic method). The development of the layer surface seems to be the smallest in the case of the Fe-B-type layer produced directly on C45 steel (Figure 1a), and the largest in the case of the Fe-Ni-B-type layer produced on steel previously covered with a galvanic nickel coating (Figure 1e). Surface morphology may significantly impact the course of frictional wear, especially in the running-in period and the initial stages of linear wear. The greater surface development that occurs with a layer of the Fe-Ni-B-P type can promote good lubrication and, therefore, slower wear. It should also be noted that the surface morphology of the boride layer in the Fe-B variant shows the most significant heterogeneity, manifested by the occurrence of dark depression areas, most likely identified with the porosity of the layers. The surface of the layers produced on both previously nickel-plated substrates has a relatively uniform and compact structure. Regardless of the variant, the produced boride layers have a typical needle structure (Figure 1b,d,f). However, they differ clearly, depending on the chemical composition of the borided substrate, both in thickness and structure.
Metallographic observations of layers’ cross-sections prove that the presence of nickel significantly impacts the kinetics of the layers’ growth and structure (Figure 1d,f). They indicate that diffusion processes, and in particular boron diffusion in the presence of nickel, run much faster than in iron, which is supported by the fact that boride layers produced on previously nickel-plated substrates, both chemically (Fe-Ni-B-P type) and galvanically (Fe-Ni-B type), are clearly thicker than iron boride layers, with the thickest layers being formed on a nickel galvanic-plated steel substrate. The estimated thicknesses of the layers produced in the boriding process carried out at a temperature of 850 °C for 2 h, measured by the maximum range of the needles growing into the steel substrate, shown in Table 3, are 53 ± 6 µm, 80 ± 9 µm and 103 ± 5 µm, respectively, for the layers Fe-B, Fe-Ni-B-P and Fe-Ni-B.
It should be noted that in the iron boride layer (the Fe-B type, Figure 1b), the needles growing into the substrate are the finest. For the mixed iron–nickel boride layer formed on previously nickel-plated steel (the Fe-Ni-B type, Figure 1f), the needles are slightly thicker, and they are thickest in a multicomponent layer on steel previously chemically nickel-plated (Ni-Fe-B-P type, Figure 1d). For all tested substrate variants, both precoated with nickel and uncoated, a bright-etching zone, most likely silicon ferrite, can be observed between boride needles and substrate and under the tips of the needles. The darker zones in the steel substrate are composed of a pearlitic and pearlite-ferritic structure. The cause of structural changes in the substrate in the vicinity of the boride layer is most likely local increases in the concentration of silicon and carbon rejected down into the substrate by the growing boride. Such a phenomenon was observed in the borided high alloy nanobainitic steels we recently investigated [28,29]. The formation of the pearlitic zone results from the substrate’s enrichment with carbon due to pushing by the forming layer of borides, which do not dissolve carbon. Similarly, silicon, which is present in the C45 steel and does not dissolve in the layer, accumulates in the vicinity of the borides, forming silicon ferrite. The thickness of the carbon enrichment zone, although uneven, ranges between twice and four times the thickness of the boride layer. Pores are an important characteristic of the microstructure of all tested boride layer variants (Figure 1b,d,f). In addition to the relatively small oval pores located mainly near the surface, large voids are observed in the areas between the boride needles. These zones are located mainly above the transition region between the layer and the substrate. Particularly large voids between the needles, which have a significant share in the volume of the layer, occur in the Fe-B-type layer (Figure 1b).
Iron–nickel boride layers are less porous and more compact in structure. However, in the Fe-Ni-B-P-type layer produced on a chemically nickel-plated substrate, the porosity of a similar nature as in the Fe-B layer is maintained, although reduced (Figure 1b,d). The least porous is the Fe-Ni-B boride layer (Figure 1e). Moreover, the zone of large voids between the needles does not occur in this layer. The details of the microstructure and chemical composition of the tested boride layers are revealed by scanning microscope SEM observations and EDS chemical composition measurements, as shown in Figure 2, Figure 3 and Figure 4.
The differences in contrast visible in the BSE mode show that, in accordance with the theory, the boride layers have a zone structure. For the Fe-B-type layer, two zones can be distinguished. For the Fe-Ni-B and Fe-Ni-B-P types, there are three zones differing in contrast. The internal zones, which, unlike the relatively thin surface zone, constitute the needles, partially penetrate each other in the transition area between the zones. The chemical composition analyses of the elements’ distribution (EDS) in the layers show that in the zones occurring in the boride layers formed on previously nickel-precoated substrates (Figure 3 and Figure 4), there is a chemical gradient resulting from the diffusion of iron from the substrate to the forming layer. Consequently, some of the phases forming these zones are complex nickel–iron boride phases of the general (Fe, Ni)xBy type. The X-ray diffraction patterns of the investigated model layers are shown in Figure 5, Figure 6 and Figure 7. The combined analysis of the XRD (Figure 5, Figure 6 and Figure 7) and EDS (Figure 2, Figure 3 and Figure 4) and WDS [36,37] results of the tested model boride layers shows that hybrid processing combining boriding under glow discharge conditions at 850 °C with prior modification of the substrate by nickel coating deposition results in the formation of the phases indicated in Table 3.
In light of the presented results of the analyses of the chemical and phase composition of the layers (Table 3, Figure 2, Figure 3 and Figure 4), it can be concluded that the thin outer zone of the Fe-B-type layer (Figure 2) is formed by FeB boride, while the needle inner zone is formed by Fe2B boride. In the multicomponent layer of the Fe-Ni-B-P type produced on the electroless, chemically nickel-precoated substrate, the darker outer zone (Figure 3b) is built of complex iron–nickel boride (Fe, Ni)B and, most probably, of Ni4B3 boride (Figure 3b, white precipitates); the intermediate inner zone is formed by needles of iron–nickel boride (Fe, Ni)2B, and the slightly darker needles of iron boride Fe2B penetrating into the substrate. It should be noted that in the multicomponent iron–nickel boride layers produced on previously chemically nickel-coated steel, nickel-based phosphides occur in deeper areas, with a dominant (Ni, Fe)3P type phosphide [36,37]. It is the residue of the prior chemical nickel coating. This type of coating in the as-deposited state has an amorphous structure which crystallises when heated to the boriding process temperature of 850 °C, transforming into a diffusion layer based on nickel phosphide with Ni3P stoichiometry. Due to the simultaneous diffusion of iron from the substrate to the coating, this phosphide is transformed into a mixed nickel–iron phosphide of the type (Ni, Fe)3P. In the structure of the Fe-Ni-B-P layer, this phosphide is located in the upper half of the thickness of the layer below the zone of large voids between boride needles (Figure 3, cyan areas). In the Fe-Ni-B-type layer produced on galvanically nickel precoated substrate (Figure 4), the external zone is formed by nickel base, with nickel–iron boride of the type (Ni, Fe)2B most probably being an almost pure non-alloyed nickel boride near the surface. The inner darker and lighter interpenetrating needle zones are built along the needles of low or iron borides (containing no nickel). The borides are of the (Fe, Ni)2B and Fe2B types, respectively, in the needle bottom and the tip. The lighter zone of the needles directly penetrating into the steel substrate is built of practically iron borides of the FexB (containing no nickel). It is worth emphasising here that the monophase (Ni, Fe)2B type structure of the outer zone of the investigated Fe-Ni-B boride layers free from the brittle FeB boride type obtained in the hybrid process is of crucial importance from the point of view of the layers’ functional properties, in particular, wear resistance improvement and the resulting increase in the layers’ durability.

3.2. Hardness of Layers

The hardness distributions in the near-surface zone of the tested variants of borided C45 steel with different layers are shown in Figure 8. As can be seen, the highest hardness levels, approximately 2200 HV0.05, are achieved for iron–nickel boride layers of the Fe-Ni-B type. The zone with a hardness above 2000 HV0.05 ranges from approximately 25 µm up to 55 µm from the surface. This zone corresponds to the inner needles zone built of nickel–iron and pure iron (Ni, Fe)2B- and FeB-type boride, respectively. The hardness of Fe2B boride may reach up to 2000 HV. Moreover, the iron may bring a hardening effect on the hardness of the Ni2B phase, as reported by other authors [1]. Much lower hardness values are observed in the case of the other two variants of boride layers with a maximum in the surface vicinity zone. For the multicomponent iron–nickel boride layers of the Fe-Ni-B-P type, the hardness reaches approximately 1300 HV0.05. For the iron boride layer Fe-B type, it reaches only near 1000 HV0.05, while the (Fe, Ni)B and FeB borides forming the outside zone should ensure a 2000 HV hardness level. As one can assume, the effect may be related to the presence of small pores in the outer zone of the layers (Figure 1, Figure 2, Figure 3 and Figure 4), which most likely lower the measured hardness values. The low hardness also observed in the outer zone of iron–nickel Fe-Ni-B-type layers next to its porosity is certainly also related to the contribution of the relatively less hard Ni2B-type boride that forms the zone (Table 3). The nature of the hardness distribution in the layers of iron borides Fe-B and iron–nickel borides of the Fe-Ni-B-P type is similar in that it shows an extensive reduced hardness region (Figure 8) situated in the core region of the layer, which can be attributed to the occurrence of an internal zone of larger voids (Figure 2 and Figure 3), especially large in the case of Fe-B-type layer, which certainly affects the hardness measurements’ unambiguity. In the Fe-Ni-B-type boride layer, a similar zone of large voids does not occur, which explains the different hardness distribution. The local hardness increase region is observed deep in the Fe-B and Fe-Ni-B-P layers, with the maximum approximately at a depth of 30 µm (c.a. 800 HV) and at 60 µm (c.a. 1300 HV), respectively. The hardness in these regions is related to the iron Fe2B-type boride occurrence (Figure 2 and Figure 3). However, next to the effect of the voids mentioned above, structural factors cause the hardness of the Fe-B and Fe-Ni-B-P layer variants not to reach as high a hardness level as that of Fe-Ni-B. One of those factors is the presence of silicon ferrite areas separating the boride needles growing into the substrate, as well as the much less hard areas present in the multicomponent layers compared to Fe2B boride (Ni, Fe)3P phosphide (Figure 3).

3.3. Stress Analysis

The stress analysis results in the tested Fe-B, Fe-Ni-B-P and Fe-Ni-B boride layers on C45 steel and the substrate are shown in Figure 9. As can be seen, compressive stresses occur in all layers, although their distribution and values are different. The stress distribution in the Fe-B-type iron boride layers has the most complex oscillatory course, with a maximum of c.a. −700 MPa right at the surface and another maximum of c.a. −550 MPa at a distance of approx. 40 µm in the zone adjacent to the substrate. One should notice that the stress distribution in the voids occurring in the c.a. 25 µm large region below the outside (Fe, Ni)B boride zone (Figure 1a) may be difficult to interpret. A zone of unfavourable tensile stresses appears in the substrate under the Fe-B layer, with the maximum at a distance of c.a 120 µm from the surface. As can be seen, the presence of nickel in the boride layers reduces the value of maximum compressive stresses, with the lowest level (above approx. −150 MPa) being observed in multicomponent boride layers of the Fe-Ni-B-P type. The above observation correlates with the decrease in stress in the boride phases in this layer, as reported by Fillt et al. [43]. The stress level in compressive multicomponent layers is almost three times lower than in Fe-Ni-B layers (max. c.a. −450 MPa), which may be related to the presumed stress relaxation in the phosphide (Ni, Fe)3P areas present in them (Figure 3) that are much less hard than borides. Both variants of boride layers containing nickel show a gradual decrease in compressive stresses deep into the layer and a slight, practically close to zero, level of tensile stresses.

3.4. Wear Resistance

The results of the wear test of the three types of model boride layers: iron borides of the Fe-B type, nickel–iron borides of the Fe-Ni-B type and multicomponent layers based on nickel–iron borides of the Fe-Ni-B-P type produced on C45 steel using the hybrid method are shown in Figure 10. A clear effect of the type of the layer and, thus, of its chemical composition on the tribological properties has been observed. The nickel precoating of the C45 steel substrate, previous to the boriding, for both chemical, electroless and galvanic method variants, improves the performance of the boride layers. The linear wear rate for a 200 MPa load (Figure 10a) significantly decreases compared to uncoated borided C45 steel. This means that the nickel–iron borides exhibit better resistance to wear than simple iron borides. As one can notice, the wear rate is lower for the Fe-Ni-B-type layers compared to the Fe-Ni-B-P layers. This may be related to the much higher hardness level and compressive stress observed in the area just below the surface (Figure 8 and Figure 9). For the Fe-Ni-B-P-type boride layers tested at the load of 400 MPa (Figure 10b), the wear rate increased and became higher than for the Fe-Ni-B-P-type boride layer. In the case of those layers, the nickel electroless precoating gives the essential change in the nature of the wear for 400 MPa load. Observed behaviour suggests the controlling effect related to phosphorus. This effect may be attributed to the phosphide (Ni, Fe)3P type in the multicomponent Fe-Ni-B-P-type boride layers (Figure 3). This phosphide exhibits high wear resistance (Figure 10) and may thus slow down the wear rate of the boride layer. Wear trace SEM analysis (Figure 11) reveals different wear characteristics for the three types of the examined boride layers. The iron boride Fe-B-type layers tend towards decohesion of the superficial needle FeB zone of the layer (Figure 11a). The layer’s outer zone residues are visible at the top right corner of the figure. The relatively smooth area at the bottom left corner seems to be the effect of seizing. The spalling effect of the FeB boride outer zone in the Fe-B-type layer could explain the lack of the linear wear stage and practically immediate transition to accelerated wear and seizing. In the case of Fe-Ni-B layers, on the relatively smooth wear track, some cavities, most probably related to the layers’ porosity, have been observed (Figure 11b). The Fe-Ni-B-P (Figure 11c,d) layers show mixed behaviour—smooth areas situated in the bottom of the ellipsoidal wear test trace seem to suggest good tribological properties in the deeper zone of the layer, which may be related to the highly wear-resistant (Ni, Fe)3P phosphide contribution.

3.5. Layers’ Cracking Susceptibility

The scratch tests were used to examine the resistance to crack initiation of the three investigated Fe-B-, Fe-Ni-B- and Fe-Ni-B-P-type boride layers. The data analysis was based on the visual identification of the first cracks or other damage effects on the scratch test trace combined with corresponding values of scratch distance and force determination (Table 4). As can be seen for the multicomponent Fe-Ni-B-P-type iron–nickel boride layers, the first cracks were observed at the lowest scratch distance and force values. This behaviour may be attributed to the (Ni, Fe)3P phosphide occurrence that is supposed to weaken the boride layer structure. In the case of the Fe-Ni-B-type iron–nickel boride layers, the critical parameters of the cracking initiation are almost twice as high. One should notice that what may seem surprising is that the highest values are registered for the iron boride Fe-B-type layers.
In addition, to investigate the susceptibility to cracking of the examined model boride layers, the bending tests combined with the acoustic measurement of cracking effects and corresponding critical forces and deflection values were carried out. The bending test results confirm the observation obtained through the scratch test, which shows that the Fe-Ni-B-P type layers are most susceptible to crack formation, occurring at relatively low deformation of samples, as shown in Table 4.
The tested samples were also subject to SEM observations (Figure 11). The observations show that the iron boride Fe-B-type layers and the multicomponent Fe-Ni-B-P-type nickel boride layers exhibit a visible tendency for spalling (Figure 11a,b). The spalling effect is particularly extensive in the iron boride layers’ case (Figure 11a). In the case of the multicomponent boride layer, a slightly lower surface area seems to be damaged (Figure 11c). The observed spalling effect in both cases is probably related to the low cohesion in the outside boride zone of the layer, which is composed of the most harmful brittle FeB or (Fe, Ni)B-type phase separated from the substrate by the large voids zone between boride needles. The decohesion of the superficial, wear-resistant layer of the boride layer exposes the deeper zones of the layer susceptible to seizing observed in the wear test (Figure 11). The Fe-Ni boride layers also crack during the bending test (Figure 12c); however, this process does not lead to the layers’ fragments’ decohesion (Figure 12c).
The analysis of the layers’ microstructure, chemical composition, and properties described above leads to the following comprehensive summary. The test results of wear resistance tests of the three types of model boride layers are as follows: iron borides of the Fe-B type, iron–nickel borides of the Fe-Ni-B type, and multicomponent layers based on iron–nickel borides of the Fe-Ni-B-P type produced on C45 steel using the hybrid method combining boriding processes under glow discharge conditions with prior nickel coating of steel, respectively, by galvanic or chemical methods, show that regardless of the nickel coating variant, the modification of the steel substrate with a nickel coating prior to boriding processes, leading to the formation of iron–nickel complex boride layers on the steel, results in a radical improvement in wear resistance. The modification of the borided substrate with nickel in a hybrid process using boriding under glow discharge conditions in accordance with the literature [1] seems to be a potentially advantageous application solution enabling the elimination of known operational problems resulting from the brittleness of iron boride layers, especially those with FeB stoichiometry. However, the comparative analysis of the investigation results of both iron–nickel boride layers produced using the hybrid method proves that the microstructure and properties of Fe-Ni-B and Fe-Ni-B-P layers show significant differences. A critical evaluation seems essential from the point of view of the application suitability of the tested hybrid solutions. This analysis, in a comprehensive assessment, leads to the conclusion that, despite certain limitations, the optimal solution from the point of view of practical use seems to be a solution based on the initial modification of the substrate with nickel using the galvanic coating method. This solution, although it improves the resistance to wear of boride layers to a slightly lower extent than the perceived until now as the most prospective solution using chemical modification [36,37], allows for the formation of iron–nickel boride layers of the Fe-Ni-B type, which exhibit several key advantageous functional features. These layers are thicker and more compact than multicomponent layers of the Fe-Ni-B-P type. Their porosity is significantly lower, and most importantly, these layers are free of large voids between the boride needles, which are a source of susceptibility to spalling of the outer zone of the boride layer. What is extremely important is that Fe-Ni-B-type layers, unlike Fe-Ni-B-P-type layers, are only composed of Ni2B- and Fe2B-type borides that are less susceptible to cracking; therefore, they have the expected monophase structure, free of unfavourable FeB-type borides. The outer zone of the layers is a favourable, relatively plastic nickel boride of the Ni2B type [1] with a relatively small iron content, in contrast to the Fe-Ni-B-P-type layers, in which in the outer zone is formed by an undesirable iron–nickel boride with a stoichiometry of the type (Fe, Ni)B. In terms of performance properties, Fe-Ni-B layers exhibit the highest hardness, up to almost 1000 HV0.05 higher than the hardness recorded in the cross-section of Fe-Ni-B-P layers. These layers are also much more resistant to cracking and do not show the tendency to spalling of the outer zone during the wear resistance tests. Moreover, among the tested layer variants in the case of the Fe-Ni-B-type layers, the highest level of beneficial compressive stresses in the outer zone is recorded, while in multicomponent Fe-Ni-B-P layers, it is the lowest.
One should also notice that the newly developed iron–nickel Fe-Ni-B-type layers are free from an additional drawback manifested by multicomponent Fe-Ni-B-P-type boride layers, which is their relatively low-temperature stability (970 °C) related to the phosphides present in the boride layers structure. This limitation excludes any steel’s core hardening heat treatment subsequent to boriding for those steel grades which are austenitised over 1000 °C [29]. On the contrary, the iron–nickel Fe-Ni-B-type boride layers produced in the developed hybrid process variant are composed exclusively of boride phases (Table 3), which are stable up to at least c.a. 1100 °C [38].
In the conclusion of the above analysis, it seems justified to consider optimal from the application point of view a hybrid solution that allows for the production of iron–nickel borides of the Fe-Ni-B type, combining boriding under glow discharge conditions with a prior modification of the substrate with a nickel coating produced by the galvanic method. In the perspective of the development and potential application of this solution in the surface processing of advanced high-strength nanobainitic steels, further investigations of the novel hybrid method may strongly strengthen their structures.
A prospective direction of further development of the new hybrid method is plasma and gas boriding technologies using BCl3-based reactive atmospheres, which ensure the precise control of boriding parameters during processing and thus of the forming boride layers’ phase composition and structure. However, in view of the environmental and safety aspects, the most interesting issue to be studied would be the modified hybrid treatment in which the plasma boriding process would use metalorganic-compound-based reactive media such as, e.g., trimethyl borate, B(OCH3)3, as investigated by other authors [40].

4. Conclusions

1. The comprehensive comparative investigations carried out on three types of model boride layers, i.e., iron borides of the Fe-B type, iron–nickel borides of the Fe-Ni-B type and multicomponent layers of the Fe-Ni-B-P type produced on C45 steel, showed that the modification in the chemical composition of iron boride layers based on a hybrid process combining boriding under glow discharge conditions with prior nickel coating of the steel substrate, both in the chemical, electroless deposition variant and the new galvanic variant, results in relatively thicker iron–nickel boride layers. These layers are characterised by a more favourable, more compact microstructure and significantly higher functional properties than those of iron boride layers obtained in the standard process of direct boriding of uncoated steel substrate. The main disadvantages of the standard process are porosity and brittleness associated with the formation of an external zone of the FeB phase, resulting in the seizing effect in the relatively severe three roller–taper method of testing wear resistance. Iron–nickel boride layers eliminate this critical phenomenon, which can be determined to be their key application advantage.
2. The novelty demonstrated in this work is that layers of iron–nickel borides of the Fe-Ni-B type, produced in a new variant of hybrid processing combining boriding under glow discharge conditions with its prior nickel coating steel using the galvanic method, are characterised by a structure and performance properties significantly more favourable than the reference multicomponent Fe-Ni-B-P layers obtained through the originally developed method of boriding using previous nickel coating of steel in a chemical, electroless variant. These layers, formed in the outer zone of nickel-based borides of the type (Ni, Fe)2B, which are less susceptible to cracking, and of Fe2B-type borides in the inner zone, are characterised by relatively small, residual porosity, with a thickness up to approximately 100 µm. Boride layers of the Fe-Ni-B type exhibit relatively high hardness, up to 2200 HV0.05, favourable compressive stress state in the near-surface zone of approximately −450 MPa, high resistance to frictional wear, good adhesion to the substrate, and lower susceptibility to cracking. The alternative reference Fe-Ni-B-P layers show a hardness almost half as high, a value of near-surface compressive stresses approximately three times lower, and a value of the critical forces generating cracking in the scratch test approximately four times lower.
3. A prospective application of the new variant of boriding under glow discharge conditions with previous galvanic nickel coating of steel substrates may be its use in surface treatment processes of heavy-duty machine parts, for example, gear mechanisms made of high-strength nanobainitic steels, working under high tribological exposure. The expected effect of the treatment is the elimination of the disadvantages of classic steel boriding by producing a relatively thicker layer of hard and tribologically resistant nickel–iron borides without the porous and the brittle FeB boride phase, optimally of the type (Ni, Fe)2B in the outer zone of the layer, thus increasing the operational durability of the processed elements. The unquestionable advantage of the new boriding method would be the elimination of expensive finishing operations, such as grinding the surface of borided elements to remove the porous and brittle external zone, which would otherwise be necessary to increase the performance properties and durability of the borided steel machine parts.
4. The most interesting perspective issue to be studied to further develop the new hybrid method based on the combination of nickel galvanic steel precoating with boriding in glow discharge conditions is the modified hybrid treatment using more environmentally friendly, non-toxic metalorganic-compound-based reactive media for plasma boriding.

Author Contributions

Conceptualisation, M.T. and T.W.; methodology, M.T., M.K. and T.W.; validation, M.T.; formal analysis, M.T.; investigation, G.Ł. and R.D.; resources, M.T. and T.W.; data curation, G.Ł. and R.D.; writing—original draft preparation, M.T.; writing—review and editing, M.T., G.Ł., M.K., R.D. and T.W.; visualisation, G.Ł.; supervision, M.T. and T.W.; project administration M.T. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Faculty of Materials Science and Engineering of the Warsaw University of Technology.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data included in this study are available upon request by contacting the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Surface morphology created by the hybrid method of layers of the type Fe-B (a), Fe-Ni-B-P (c), Fe-Ni-B (e) and corresponding light microscopy microstructures (b,d,f).
Figure 1. Surface morphology created by the hybrid method of layers of the type Fe-B (a), Fe-Ni-B-P (c), Fe-Ni-B (e) and corresponding light microscopy microstructures (b,d,f).
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Figure 2. SEM image of Fe-B-type layer in SE mode combined with EDS mapping (a), the image in BSE mode with the linescan track (b), EDS microanalysis linescan (c). The colours used for the linescan graph correspond to those used on the EDS mapping: green for iron and yellow for boron.
Figure 2. SEM image of Fe-B-type layer in SE mode combined with EDS mapping (a), the image in BSE mode with the linescan track (b), EDS microanalysis linescan (c). The colours used for the linescan graph correspond to those used on the EDS mapping: green for iron and yellow for boron.
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Figure 3. SEM image of Fe-Ni-B-P-type layer in SE mode combined with EDS map (a), the image in BSE mode with the linescan track (b) and phosphorus intensity map (c), EDS microanalysis linescan (d). The colours used for the linescan graph correspond to those used on the EDS map: green for iron, red for nickel, yellow for boron and turquoise for phosphorus.
Figure 3. SEM image of Fe-Ni-B-P-type layer in SE mode combined with EDS map (a), the image in BSE mode with the linescan track (b) and phosphorus intensity map (c), EDS microanalysis linescan (d). The colours used for the linescan graph correspond to those used on the EDS map: green for iron, red for nickel, yellow for boron and turquoise for phosphorus.
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Figure 4. SEM image of Fe-Ni-B-type layer in SE mode combined with EDS mapping (a), the image in BSE mode with the linescan track (b), EDS microanalysis linescan (c). The colours used for the linescan graph correspond to those used on the EDS mapping: green for iron, red for nickel and yellow for boron.
Figure 4. SEM image of Fe-Ni-B-type layer in SE mode combined with EDS mapping (a), the image in BSE mode with the linescan track (b), EDS microanalysis linescan (c). The colours used for the linescan graph correspond to those used on the EDS mapping: green for iron, red for nickel and yellow for boron.
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Figure 5. X-ray diffraction patterns of the Fe-B-type borided layer.
Figure 5. X-ray diffraction patterns of the Fe-B-type borided layer.
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Figure 6. X-ray diffraction patterns of the Fe-Ni-B-P-type borided layer.
Figure 6. X-ray diffraction patterns of the Fe-Ni-B-P-type borided layer.
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Figure 7. X-ray diffraction patterns of the Fe-Ni-B-type borided layer.
Figure 7. X-ray diffraction patterns of the Fe-Ni-B-type borided layer.
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Figure 8. The microhardness distributions of HV0.05 in the near-surface zone of the Fe-B, Fe-Ni-B and Fe-Ni-B-P variants of borided C45 and, for comparison, the distribution for the (Fe-Ni-P) layers based on nickel–iron type (Ni, Fe)3P phosphide.
Figure 8. The microhardness distributions of HV0.05 in the near-surface zone of the Fe-B, Fe-Ni-B and Fe-Ni-B-P variants of borided C45 and, for comparison, the distribution for the (Fe-Ni-P) layers based on nickel–iron type (Ni, Fe)3P phosphide.
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Figure 9. The effect of the chemical composition of the boride layers on the stress distribution in the surface zone: Fe-B, Fe-Ni-B and Fe-Ni-B-P layers.
Figure 9. The effect of the chemical composition of the boride layers on the stress distribution in the surface zone: Fe-B, Fe-Ni-B and Fe-Ni-B-P layers.
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Figure 10. The effect of composition on the linear wear of the boride layers for Fe-B, Fe-Ni-B and Fe-Ni-B-P and unborided Fe-Ni-P layer annealed at 850 °C for 2 h: the load 200 MPa (a), the load 400 MPa (b).
Figure 10. The effect of composition on the linear wear of the boride layers for Fe-B, Fe-Ni-B and Fe-Ni-B-P and unborided Fe-Ni-P layer annealed at 850 °C for 2 h: the load 200 MPa (a), the load 400 MPa (b).
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Figure 11. The wear traces SEM images for the examined Fe-B, Fe-Ni-B and Fe-Ni-B-P boride layers; three rollers–taper wear resistance test.
Figure 11. The wear traces SEM images for the examined Fe-B, Fe-Ni-B and Fe-Ni-B-P boride layers; three rollers–taper wear resistance test.
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Figure 12. The surface view of the bending tested samples with the Fe-B (a), Fe-Ni-B-P (b) and Fe-Ni-B (c) boride layers.
Figure 12. The surface view of the bending tested samples with the Fe-B (a), Fe-Ni-B-P (b) and Fe-Ni-B (c) boride layers.
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Table 1. Chemical composition of C45 steel according to PN EN 10083-2 standard (in wt%). Cr + Mo + Ni ≤ 0.63.
Table 1. Chemical composition of C45 steel according to PN EN 10083-2 standard (in wt%). Cr + Mo + Ni ≤ 0.63.
ElementCSiMnPSCrMoNi
min0.42 0.50
max0.500.400.800.0450.0450.400.100.40
Table 2. Yield stress (YS), tensile strength (TS) and elongation after fracture (A5) of C45 steel according to PN EN 10083-2 standard.
Table 2. Yield stress (YS), tensile strength (TS) and elongation after fracture (A5) of C45 steel according to PN EN 10083-2 standard.
StateYS [MPa]TS [MPa]A5 [%]
normalised≥340≥620≥14
quenched and tempered≥490700–850≥14
Table 3. Thickness and phase composition of the investigated boride layers produced in the boriding process at 850 °C for 2 h (XRD, EDS and WDS [37,42] phase identification for outer and inner zones analyses, respectively).
Table 3. Thickness and phase composition of the investigated boride layers produced in the boriding process at 850 °C for 2 h (XRD, EDS and WDS [37,42] phase identification for outer and inner zones analyses, respectively).
Layer VariantThickness [µm]Phase Composition
Fe-B53 ± 6FeB, Fe2B
Ni-Fe-B-P80 ± 9(Fe, Ni)B, Ni3B4, (Fe, Ni)2B, Fe2B, (Ni, Fe)3P
Ni-Fe-B103 ± 5Ni2B, (Ni, Fe)2B, FeB, Fe2B
Table 4. First crack apparition critical parameters in 1—scratch test: critical scratch distance XSCR and force FSCR (visual detection); 2—bending test: critical deflection vector V and force F (acoustic detection).
Table 4. First crack apparition critical parameters in 1—scratch test: critical scratch distance XSCR and force FSCR (visual detection); 2—bending test: critical deflection vector V and force F (acoustic detection).
ParameterFe-BFe-Ni-B-PFe-Ni-B
XSCR [mm]3.981.462.39
FSCR [N]40.9315.7326.06
V [mm]0.460.10.41
F [N]650630600
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MDPI and ACS Style

Tacikowski, M.; Łukaszewicz, G.; Kulka, M.; Diduszko, R.; Wierzchoń, T. The Effect of Chemical Composition on the Microstructure and Properties of Multicomponent Nickel-Based Boride Layers Produced on C45 Steel by the Hybrid Method. Coatings 2024, 14, 197. https://doi.org/10.3390/coatings14020197

AMA Style

Tacikowski M, Łukaszewicz G, Kulka M, Diduszko R, Wierzchoń T. The Effect of Chemical Composition on the Microstructure and Properties of Multicomponent Nickel-Based Boride Layers Produced on C45 Steel by the Hybrid Method. Coatings. 2024; 14(2):197. https://doi.org/10.3390/coatings14020197

Chicago/Turabian Style

Tacikowski, Michał, Grzegorz Łukaszewicz, Michał Kulka, Ryszard Diduszko, and Tadeusz Wierzchoń. 2024. "The Effect of Chemical Composition on the Microstructure and Properties of Multicomponent Nickel-Based Boride Layers Produced on C45 Steel by the Hybrid Method" Coatings 14, no. 2: 197. https://doi.org/10.3390/coatings14020197

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