The Effect of Crystallization and Phase Transformation on the Mechanical and Electrochemical Corrosion Properties of Ni-P Coatings

coatings Article The Effect of Crystallization and Phase Transformation on the Mechanical and Electrochemical Corrosion Properties of Ni-P Coatings Martin Buchtík1,* , Leoš Doskoˇcil 1, Roman Brescher 1, Pavel Doležal 1,2 , JiˇríMásilko 1and Jaromír Wasserbauer 1   Citation: Buchtík, M.; Doskoˇcil, L.; Brescher, R.; Doležal, P.; Másilko, J.; Wasserbauer, J. The Effect of Crystallization and Phase Transformation on the Mechanical and Electrochemical Corrosion Properties of Ni-P Coatings. Coatings 2021,11, 447. https://doi.org/ 10.3390/coatings11040447 Academic Editor: Kevin Plucknett Received: 2 March 2021 Accepted: 7 April 2021 Published: 13 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Materials Research Centre, Faculty of Chemistry, Brno University of Technology, Purkyˇnova 464/118, 612 00 Brno, Czech Republic; [email protected] (L.D.); [email protected] (R.B.); [email protected].cz (P.D.); [email protected] (J.M.); [email protected] (J.W.) 2Faculty of Mechanical Engineering, Brno University of Technology, Technická2896/2, 602 00 Brno, Czech Republic *Correspondence: [email protected]; Tel.: +42-(073)-644-5019 Abstract: This paper deals with the study of the crystallization and phase transformation of Ni-P coatings deposited on AZ91 magnesium alloy. Prepared samples were characterized in terms of surface morphology and elemental composition by means of scanning electron microscopy with energy-dispersive spectroscopy analysis. The results of X-ray diffraction analysis and differential scanning calorimetry suggested that increasing the phosphorus content caused Ni-P coatings to develop an amorphous character. The crystallization of Ni was observed at 150, 250, and 300 ◦ C for low-, mediumand high-phosphorus coatings, respectively. The Ni crystallite size increased with increasing temperature and decreasing P content. Conversely, the presence of the Ni 3 P phase was observed at a maximum peak of 320 ◦ C for the high-phosphorus coating, whereas the crystallization of the Ni 3 P phase shifted to higher temperatures with decreasing P content. The Ni 3 P crystallite size increased with increasing temperature and increasing P content. An increase in microhardness due to the arrangement of Ni atoms and Ni 3 P precipitation was observed. The deposition of as-deposited Ni-P coatings led to an improvement in the corrosion resistance of AZ91. However, the heat treatment of coatings resulted in a deterioration in corrosion properties due to the formation of microcracks. Keywords: Ni-P coating; AZ91 alloy; phase transformation; crystallization; corrosion behavior 1. Introduction Magnesium and its alloys have unique properties such as low density, a high strength to weight ratio, and good castability, and are thus of great interest in many areas of industry, especially in the automotive industry, aviation, and electrochemistry [ 1 – 3 ]. However, poor corrosion resistance, low hardness, and low wear resistance are their main disadvantages [ 3 , 4 ]. One way of protecting magnesium and its alloys is the application of coatings. Electroless Ni-P coatings deposited on magnesium alloys have great potential for many industrial applications [ 4 , 5 ]. For example, they can improve resistance against external influences, wear, or corrosion, and can also contribute to improving the appearance of the coated part [6–9]. The properties of electroless Ni-P coatings depend on their phosphorus content and the heating process, and, thus, on their microstructure [ 10 , 11 ]. It was reported [ 6 , 12 ] that the hardness and wear resistance of Ni-P coatings decrease with increasing P content. On the other hand, corrosion resistance should increase with increasing P content, as shown in the works of Mainier [13] and Agarwala [14]. It is well known that electroless Ni-P coatings can be distinguished on the basis of whether they are low-phosphorus (LP) Ni-P coatings (approximately 1–5 wt. % of P), mediumphosphorus (MP) Ni-P coatings (6–9 wt. % of P), or high-phosphorus (HP) Ni-P coatings Coatings 2021,11, 447. https://doi.org/10.3390/coatings11040447 https://www.mdpi.com/journal/coatings Coatings 2021,11, 447 2 of 15 (10–13 wt. % of P) [ 12 ]. LP Ni-P coatings are crystalline or microcrystalline, which indicates that the amount of phosphorus atoms in interstitial positions is not sufficient to distort the nickel lattice [ 15 – 17 ]. MP Ni-P coatings are formed by both a mixture of microcrystalline nickel and an amorphous phase, with the fraction of the amorphous phase increasing with increasing phosphorus content in the coating. HP Ni-P coatings are characterized as completely amorphous [ 18 , 19 ]. The amorphous phase is present in the coating because the solubility of phosphorus in nickel is very low (>0.17 % of P) [ 6 , 20 ]. The amorphous phase regions grow due to an increase in lattice distortion caused by phosphorus atoms becoming situated in the interstitial positions of the nickel lattice. As-deposited Ni-P coatings are thermodynamically unstable and the coating tends to become a thermodynamically more stable and energy-efficient equilibrium state [ 11 , 20 , 21 ]. According to some authors, under equilibrium conditions, below the melting point of Ni-P coatings (880 ◦ C), only two phases should be present in the Ni-P coating—an α phase formed by less than 0.17 wt. % of P dissolved in Ni and an intermediate nickel phosphide Ni 3 P phase containing 15 wt. % of P [ 6 , 12 , 22 ]. However, the equilibrium phase diagram can only be used to describe the microstructure of alloys in the equilibrium state, e.g., after the heat treatment. To clarify the description of Ni-P coatings deposited from the electroless plating bath, it is necessary to use a nonequilibrium phase diagram. According to Riedel’s study [ 12 ], the microstructure of N-P coatings can be transformed during heat treatment, resulting in changes in the atomic structure. Both the microcrystalline and amorphous phases undergo the crystallization process, and tetragonal intermediate phase Ni 3 P is formed at the same time [ 11 ]. Apachitei et al. [ 23 ] and Duncan [ 11 ] reported that the metastable β phase and the amorphous γ phase are subject to decomposition reactions, where the stable crystalline α phase and Ni 3 P are formed. Some authors [ 16 , 17 , 24 ] reported that MP and HP coatings can be formed by mixtures of microcrystalline nickel and various crystalline nonequilibrium phases such as Ni 5 P 4 , Ni 12 P 5 , and Ni 5 P 2 created during heat treatment up to 300 ◦ C. Above this temperature, the Ni3P phase is formed from these metastable nonequilibrium phases. This study is focused on the crystallization and phase transformation of deposited Ni-P coatings with various phosphorus contents and attempts comprehensively to describe the relationship between phosphorus content and the microstructural, mechanical, and electrochemical corrosion properties of Ni-P coatings deposited on Mg alloy AZ91. The microstructure and phase transformations of deposited Ni-P coatings were determined during a continuous heating process using X-ray diffractometry (XRD) and differential scanning calorimetry (DSC). Subsequently, the microhardness and electrochemical corrosion properties of the coatings were determined. The results are discussed in terms of the observed microstructural changes and phase transformations. 2. Materials and Methods Samples of AZ9-cast magnesium alloy with dimensions of 30 mm × 30 mm × 7 mm were used as substrates for the electroless deposition of Ni-P coatings. The chemical composition of the AZ91 alloy is listed in Table 1. The elemental analysis of the magnesium substrate was performed using glow-discharge optical emission spectroscopy (GDOES) on a Spectrumat GDS 750 instrument (Spectruma Analytik GmbH, Hof, Germany). Table 1. Elemental composition of AZ91 magnesium alloy, glow-discharge optical emission spectroscopy (GDOES); Mg balance. Element Al Zn Mn Si Fe Zr Content (wt. %) 8.80 0.81 0.32 0.01 0.004 0.01 The samples of AZ91 alloy with deposited Ni-P coatings were prepared in the same way as reported in previous work [ 25 ]. The Ni 2+ /H 2 PO 2− ratios in the electroless nickel bath were adjusted in order to deposit Ni-P coatings with a high, medium, and low P Coatings 2021,11, 447 3 of 15 content. The deposition time was 4 h and the average thickness of the coatings was approximately 30 µ m. The heat treatment of Ni-P coatings (for the determination of microhardness and electrochemical corrosion properties) was performed in a LAC LM07 muffle furnace (LAC, s.r.o., Židlochovice, Czech Republic) at 400 ◦C for 1 h. The morphology and elemental composition of the deposited Ni-P coatings were analyzed using a Zeiss EVO LS-10 scanning electron microscope (SEM) (Carl Zeiss Ltd., Cambridge, UK) with energy-dispersive spectroscopy (EDS), an Oxford Instruments Xmax 80 mm 2 detector (Oxford Instruments plc, Abingdon, UK), and AZtec software (version 2.4, Oxford Instruments, High Wycombe, UK). The deposited Ni-P coatings were mechanically separated from the magnesium substrates and crushed to a fine powder in an agate mortar. The phase analysis of the Ni-P powder was performed on a Pt pan using an Empyrean X-ray diffraction (XRD) spectrometer (PANalytical, Malvern, UK) with a high-temperature chamber (Anton Paar HTK 16N, Anton Paar, Graz, Austria). The parameter settings were as follows: Cu K α radiation ( λ K α 1 = 0.15406 nm, λ K α 2 = 0.15444 nm); scan range from 25 to 65 ◦ ; scan step size, 0.013 ◦ 2 θ ; time per step, 39 s; generator voltage, 40 kV; and tube current, 30 mA. X-ray diffraction patterns were recorded in the temperature range from 50 to 550 ◦ C with a pattern record step of 50 ◦C. To achieve a more accurate record of phase changes, the pattern record step was set to 10 ◦C for the temperature range from 300 to 400 ◦C. The crystallite size of the Ni and Ni 3 P phases was calculated from the full width half maximum (FWHM) according to the Scherrer equation using HighScore Plus (version 3.0.5, PANalytical B.V., Almelo, The Netherlands) software (Equation (1)): τ= K×λ β1/2 ×cosθ, (1) where τ is the crystallite size [Å], λ is the X-ray wavelength (nm), β1/2 is the peak extension at half the maximum intensity (FWHM), θ is the diffraction Bragg’s angle, and K is the shape factor (Scherrer constant) ranging from 0.62 to 2.08 (usually close to 1). To understand the crystallization and transformation behavior of Ni-P coatings, differential scanning calorimetry (DSC) analysis was performed. Ni-P coatings, separated and crushed to powder, were continually heated at a heating rate of 10 ◦ C · min −1 from 50 to 500 ◦ C using a DSC F1 204 differential scanning calorimeter (Netzsch, Selb, Germany). Samples of about 10 mg were placed into Al pans. An empty pan was used as a reference. The microhardness of Ni-P coatings was measured using an LECO AMH55 (Leco, Saint Joseph, MO, USA) Vickers microhardness tester under an applied load of 25 g and with a dwell time of 10 s. The microhardness was measured ten times on a polished coated-sample cross-section. The electrochemical corrosion properties of as-deposited and heat-treated Ni-P coatings were analyzed by means of a potentiodynamic polarization test in 3.5% NaCl solution using a Bio-Logic VSP-300 potentiostat/galvanostat (BioLogic, Seyssinet-Pariset, France) at room temperature. The solution was boiled before potentiodynamic measurement to remove CO 2 and oxygen. In the case of plain AZ91 magnesium alloy, the specimen surface was ground using SiC paper #1200. It was then cleaned with distilled water and isopropanol and dried with a stream of dry air. Immediately afterward, the measurement was performed. In the case of coated magnesium alloy, the samples were always cleaned before the measurement with distilled water and isopropanol and dried with a stream of dry air. The analyzed area of samples was approximately 1 cm 2 . The measurement was performed using a standard three-electrode cell: Pt gauze was used as a counter-electrode, a saturated calomel electrode (SCE) as a reference electrode, and a prepared sample as a working electrode. The potential range was set from − 200 mV to +250 mV vs. open circuit potential (OCP) and the scan rate was 1 mV · s −1 . The stabilization time for the samples exposed to the corrosive environment was 10 min. Values of corrosion potential Ecorr and corrosion current density icorr were determined by applying the Tafel analysis. Coatings 2021,11, 447 4 of 15 3. Results and Discussion 3.1. Morphology of Deposited Ni-P Coatings The surface morphology of Ni-P coatings deposited on AZ91 magnesium alloy is shown in Figure 1. The deposited Ni-P coatings had high—(10.8 ± 0.1 wt. % of P), medium—(7.4 ± 0.1 wt. % of P), and low—(5.5 ± 0.1 wt. % of P) phosphorus contents. All deposited coatings show a nodular morphology, which is typical for electroless NiP coatings [ 12 ]. As observed in Figure 1a–c, the nodule size decreases with increasing P content. As reported by Shu [ 26 ], the phosphorus has very low solubility in nickel; therefore, the reduced phosphorus tends to aggregate at the boundaries of the nickel grains during deposition. Hence, the phosphorus inhibits the growth of Ni particles and particles of Ni-P and increases the number of nucleation sites. The same dependence of nodule size on phosphorus content was also observed by Ashtiani et al. [ 10 ]. Some structural defects and associated cracks were rarely observed in MP Ni-P coatings. These microstructural defects (growths, micropores, microcracks) increased the roughness and worsened the corrosion resistance of MP Ni-P coatings. Coatings 2021, 11, x FOR PEER REVIEW 4 of 15 potential (OCP) and the scan rate was 1 mV·s −1 . The stabilization time for the samples exposed to the corrosive environment was 10 min. Values of corrosion potential E corr and corrosion current density i corr were determined by applying the Tafel analysis. 3. Results and Discussion 3.1. Morphology of Deposited Ni-P Coatings The surface morphology of Ni-P coatings deposited on AZ91 magnesium alloy is shown in Figure 1. The deposited Ni-P coatings had high—(10.8 ± 0.1 wt. % of P), medium—(7.4 ± 0.1 wt. % of P), and low—(5.5 ± 0.1 wt. % of P) phosphorus contents. All deposited coatings show a nodular morphology, which is typical for electroless Ni-P coatings [12]. As observed in Figure 1a–c, the nodule size decreases with increasing P content. As reported by Shu [26], the phosphorus has very low solubility in nickel; therefore, the reduced phosphorus tends to aggregate at the boundaries of the nickel grains during deposition. Hence, the phosphorus inhibits the growth of Ni particles and particles of Ni-P and increases the number of nucleation sites. The same dependence of nodule size on phosphorus content was also observed by Ashtiani et al. [10]. Some structural defects and associated cracks were rarely observed in MP Ni-P coatings. These microstructural defects (growths, micropores, microcracks) increased the roughness and worsened the corrosion resistance of MP Ni-P coatings. Figure 1d shows an example of a cross-section of a deposited LP Ni-P coating with an approximate thickness of 30 µm. The thickness of all the deposited coatings was uniform across each entire cross-section and exhibited no obvious defects or inhomogeneities. During the heat treatment, the sizes of the nodules did not change. However, it was evident that the heat treatment led to the cracking of Ni-P coatings (Figure 1e–g) due to the transformation of the as-deposited Ni-P matrix and the precipitation of the Ni 3 P phase. The peeling or the deformation of the coating layer were not observed. The same cracks were observed in our previous research [27]. Figure 1. Cont. Coatings 2021,11, 447 5 of 15 Coatings 2021, 11, x FOR PEER REVIEW 5 of 15 Figure 1. Surface morphology of Ni-P coatings, (a) as-deposited low-phosphorus (LP), (b) as-deposited medium-phosphorus (MP), (c) as-deposited high-phosphorus (HP), (d) cross-sectional micrograph of as-deposited LP coating, (e) heattreated LP, (f) heat-treated MP, and (g) heat-treated HP. 3.2. XRD Analysis The results of the XRD analysis of as-deposited Ni-P coatings showed crystalline, microcrystalline-to-amorphous, and completely amorphous microstructures for LP, MP, and HP coatings, respectively (Figures 2–4). The intensity of Ni {1 1 1} 2θ ≈ 44° for the LP Ni-P coating (Figure 2) increased within the temperature range of 50-150 °C. The peak corresponding to Ni {2 0 0} for this sample was detected near the diffraction angle of 2θ ≈ 51.5° at 200 °C. The precipitation of the Ni3P phase was observed at 350 °C, where the peak corresponding to the Ni3P diffraction plane was detected at 2θ ≈ 41.7°. The distinct crystallization of the Ni and Ni3P phases occurred at 400 °C, when the broad peak of Ni transformed to a sharp crystalline peak and its intensity rapidly increased. The growth of Ni and Ni3P crystallites occurred in the temperature range from 400 to 550 °C. The crystallite coarsening of both Ni and Ni3P was obvious at 550 °C, as indicated by the increase in peak intensities (Figure 2). The presence of NiO was evident between 400 and 550 °C, due to the oxidation of the Ni-P coating. The greatest increase in the intensity of Ni3P was recorded between 350 and 400 °C. For the MP Ni-P coating (Figure 3), the increase in Ni {1 1 1} intensity indicated its crystallization at 250 °C. The presence of Ni3P was detected at a lower temperature (330 °C) but with a higher value of intensity (e.g., Ni3P {3 2 1} at 2θ ≈ 41.7°) compared to the LP coating. The distinct crystallization of the Ni and Ni3P phase was observed at 400 °C. The formation of NiO was simultaneously observed at 400 °C at 2θ ≈ 63°. The intensity of Ni and Ni3P increased with increasing temperature due to the increase in the crystallite size. From the XRD patterns (Figure 3), it is possible to detect peaks at 2θ ≈ 32°, 34°, and 36.5° corresponding to the primary α-Mg phase. The α-Mg phase was also detected in the works of Gu [28] and Hu [29]. Its presence can be explained by the separation of Mg alloy from the Ni-P coating during the XRD sample preparation. Figure 1. Surface morphology of Ni-P coatings, ( a ) as-deposited low-phosphorus (LP), ( b ) as-deposited medium-phosphorus (MP), ( c ) as-deposited high-phosphorus (HP), ( d ) cross-sectional micrograph of as-deposited LP coating, ( e ) heat-treated LP, (f) heat-treated MP, and (g) heat-treated HP. Figure 1d shows an example of a cross-section of a deposited LP Ni-P coating with an approximate thickness of 30 µ m. The thickness of all the deposited coatings was uniform across each entire cross-section and exhibited no obvious defects or inhomogeneities. During the heat treatment, the sizes of the nodules did not change. However, it was evident that the heat treatment led to the cracking of Ni-P coatings (Figure 1e–g) due to the transformation of the as-deposited Ni-P matrix and the precipitation of the Ni 3 P phase. The peeling or the deformation of the coating layer were not observed. The same cracks were observed in our previous research [27]. 3.2. XRD Analysis The results of the XRD analysis of as-deposited Ni-P coatings showed crystalline, microcrystalline-to-amorphous, and completely amorphous microstructures for LP, MP, and HP coatings, respectively (Figures 2–4). The intensity of Ni {1 1 1} 2 θ≈ 44 ◦ for the LP Ni-P coating (Figure 2) increased within the temperature range of 50-150 ◦ C. The peak corresponding to Ni {2 0 0} for this sample was detected near the diffraction angle of 2 θ≈ 51.5 ◦ at 200 ◦ C. The precipitation of the Ni 3 P phase was observed at 350 ◦ C, where the peak corresponding to the Ni 3 P diffraction plane was detected at 2 θ≈ 41.7 ◦ . The distinct crystallization of the Ni and Ni 3 P phases occurred at 400 ◦ C, when the broad peak of Ni transformed to a sharp crystalline peak and its intensity rapidly increased. The growth of Ni and Ni 3 P crystallites occurred in the temperature range from 400 to 550 ◦ C. The crystallite coarsening of both Ni and Ni 3 P was obvious at 550 ◦ C, as indicated by the increase in peak intensities (Figure 2). The presence of NiO was evident between 400 and 550 ◦ C, due to the oxidation of the Ni-P coating. The greatest increase in the intensity of Ni3P was recorded between 350 and 400 ◦C. Coatings 2021,11, 447 6 of 15 Coatings 2021, 11, x FOR PEER REVIEW 6 of 15 Figure 2. Microstructural XRD analysis of LP Ni-P coating. Figure 3. Microstructural XRD analysis of MP Ni-P coating. Figure 2. Microstructural XRD analysis of LP Ni-P coating. Coatings 2021, 11, x FOR PEER REVIEW 6 of 15 Figure 2. Microstructural XRD analysis of LP Ni-P coating. Figure 3. Microstructural XRD analysis of MP Ni-P coating. Figure 3. Microstructural XRD analysis of MP Ni-P coating. Coatings 2021,11, 447 7 of 15 Coatings 2021, 11, x FOR PEER REVIEW 7 of 15 Figure 4. Microstructural XRD analysis of HP Ni-P coating. As shown in Figure 4, a broad peak 2θ ≈ 40° to 52° at 50 °C indicates the HP Ni-P coating to have a completely amorphous microstructure. Ni crystallization was observed at a lower temperature (300 °C) compared to the Ni-P coatings with lower phosphorus content. However, the onset of precipitation of the Ni 3 P phase was observed at 320 °C. For LP and MP coatings, a significant increase in the crystallite size of Ni and Ni 3 P occurred between 400 and 550 °C. Nickel oxide NiO was again detected between 400 and 550 °C (Figure 4). From the measured data, it is evident that the temperature of Ni crystallization increased and the temperature of Ni 3 P precipitation decreased with increasing P content in the Ni-P coatings (Figure 5). Figure 5. The effect of P content in Ni-P coating on Ni crystallization and Ni 3 P precipitation temperature. Figure 4. Microstructural XRD analysis of HP Ni-P coating. For the MP Ni-P coating (Figure 3), the increase in Ni {1 1 1} intensity indicated its crystallization at 250 ◦ C. The presence of Ni 3 P was detected at a lower temperature (330 ◦ C) but with a higher value of intensity (e.g., Ni 3 P {3 2 1} at 2 θ≈ 41.7 ◦ ) compared to the LP coating. The distinct crystallization of the Ni and Ni 3 P phase was observed at 400 ◦ C. The formation of NiO was simultaneously observed at 400 ◦ C at 2 θ≈ 63 ◦ . The intensity of Ni and Ni 3 P increased with increasing temperature due to the increase in the crystallite size. From the XRD patterns (Figure 3), it is possible to detect peaks at 2θ≈32◦, 34◦, and 36.5◦ corresponding to the primary α -Mg phase. The α -Mg phase was also detected in the works of Gu [ 28 ] and Hu [ 29 ]. Its presence can be explained by the separation of Mg alloy from the Ni-P coating during the XRD sample preparation. As shown in Figure 4, a broad peak 2 θ≈ 40 ◦ to 52 ◦ at 50 ◦ C indicates the HP Ni-P coating to have a completely amorphous microstructure. Ni crystallization was observed at a lower temperature (300 ◦ C) compared to the Ni-P coatings with lower phosphorus content. However, the onset of precipitation of the Ni 3 P phase was observed at 320 ◦ C. For LP and MP coatings, a significant increase in the crystallite size of Ni and Ni 3 P occurred between 400 and 550 ◦ C. Nickel oxide NiO was again detected between 400 and 550 ◦ C (Figure 4). From the measured data, it is evident that the temperature of Ni crystallization increased and the temperature of Ni 3 P precipitation decreased with increasing P content in the Ni-P coatings (Figure 5). Coatings 2021,11, 447 8 of 15 Coatings 2021, 11, x FOR PEER REVIEW 7 of 15 Figure 4. Microstructural XRD analysis of HP Ni-P coating. As shown in Figure 4, a broad peak 2θ ≈ 40° to 52° at 50 °C indicates the HP Ni-P coating to have a completely amorphous microstructure. Ni crystallization was observed at a lower temperature (300 °C) compared to the Ni-P coatings with lower phosphorus content. However, the onset of precipitation of the Ni 3 P phase was observed at 320 °C. For LP and MP coatings, a significant increase in the crystallite size of Ni and Ni 3 P occurred between 400 and 550 °C. Nickel oxide NiO was again detected between 400 and 550 °C (Figure 4). From the measured data, it is evident that the temperature of Ni crystallization increased and the temperature of Ni 3 P precipitation decreased with increasing P content in the Ni-P coatings (Figure 5). Figure 5. The effect of P content in Ni-P coating on Ni crystallization and Ni 3 P precipitation temperature. Figure 5. The effect of P content in Ni-P coating on Ni crystallization and Ni 3 P precipitation temperature. The shift of Ni crystallization to higher temperatures is caused by the microstructure of the as-deposited Ni-P coatings. LP Ni-P coatings show a crystalline microstructure and the phosphorus atoms present do not cause such a distortion of the Ni lattice [ 15 , 16 ]. Therefore, there is no need for a large amount of energy to arrange Ni atoms into crystalline Ni crystallites, as in the case of Ni-P coatings with higher P content, where the microstructure is more distorted or amorphous [ 30 ]. Hence, the coatings with a higher P content required more energy for atoms to rearrange themselves and form Ni clusters [ 15 , 30 , 31 ]. The lower temperature of the precipitation of Ni 3 P can be explained by the presence of higher P content in the microstructure [ 32 ]. For HP coatings, there is much more frequent interaction between the free phosphorus atom and three atoms of nickel to form the Ni 3 P phase, when compared to MP and LP coatings [11,15,16]. Some authors [ 16 , 30 , 33 ] reported that the microstructure forms and new phases occur after the heating of electroless Ni-P coatings. Hur [ 34 ] reported that LP Ni-P coatings with crystalline or microcrystalline microstructure transform directly into a mixture of crystalline nickel matrix and stable Ni 3 P phase. Coatings with a higher phosphorus content first transform into a mixture of crystalline Ni and metastable Ni 12 P 5 and Ni 5 P 2 phases at lower temperatures (200–300 ◦ C). Then, these metastable phases pass to Ni 3 P when the temperature gradually rises. The stable Ni 3 P phase is only apparent over a temperature of 400 ◦C [16,35]. However, Figures 2–4show that there was no formation of metastable phases (Ni 12 P 5 and Ni 5 P 2 ) in the temperature range of 50–350 ◦ C, and only the crystallization of Ni and Ni3P was observed. The dependence describing Ni crystallite size on the temperature of individual Ni-P coatings is displayed in Figure 6a. As seen from Figure 6a, the Ni crystallization temperature differs depending on the P content, i.e., 150 ◦ C for LP, 250 ◦ C for MP, and 300 ◦ C for HP. A significant increase in Ni crystallite size was observed above 300 ◦ C for all samples. After deposition, the Ni {1 1 1} crystallite size was determined by the Debye–Scherrer method and found to be 38, 23, and 21 Å for LP, MP, and HP, respectively. The Ni crystallite size during the heating process up to 550 ◦ C grew to values of 595, 382, and 287 Å for LP, MP, and HP, respectively (Figure 6a). The Ni crystallite size in MP and HP coatings was lower than the value determined for the LP coating, which is in agreement with findings presented elsewhere [6,19,30]. Coatings 2021,11, 447 9 of 15 Coatings 2021, 11, x FOR PEER REVIEW 9 of 15 Figure 6. Temperature dependence of crystallite size of (a) Ni {1 1 1} and (b) Ni 3 P {3 2 1}. As mentioned in the literature [11,26], elemental phosphorus has a very low solubility in nickel; it inhibits the Ni grain growth and increases the number of Ni nuclei. It can be assumed that during the heat treatment up to ~300 °C, the growth of Ni crystallite is inhibited due to the presence of P in Ni crystallites (Figure 6a). In the temperature range of ~300–400 °C, a significant increase of Ni crystallite occurs due to the migration and rearrangement of P atoms from Ni crystallite, and the formation of Ni 3 P phase. The increase of Ni crystallites size was observed by XRD analysis, which can be explained by the decrease of P in Ni crystallites. Kumar [22] assumes that Ni crystallites are formed from an amorphous Ni-P matrix, and the Ni 3 P phase can formed from both the Ni-P matrix, and from the Ni crystallites formed. The presence of phosphorus in Ni-P coatings increases the number of Ni nuclei. This also results in a slower growth of a larger number of the Ni 3 P crystallites with temperature (Figure 6b). 3.3. DSC Analysis All DSC curves (Figure 7) contain a prominent exothermic peak corresponding to Ni 3 P transformation from the Ni-P matrix [20,36]. DSC analysis confirmed that the temperature of Ni 3 P transformation shifts to lower values with increasing P content in the coating and more energy is released (Figure 7a–c). The DSC curve corresponding to the LP Ni-P coating (Figure 7a) shows that Ni 3 P transformation reached its maximum at 398 °C and the energy evolved was calculated to be 347.4 mJ·mol −1 . DSC peaks for MP and HP Ni-P coatings (Figure 7b,c) were observed at temperatures of 389 and 355 °C, with corresponding evolved energies of 390.6 and 507.9 mJ·mol −1 , respectively. The temperature determined correlates with the findings from XRD (Figure 2). Figure 6. Temperature dependence of crystallite size of (a) Ni {1 1 1} and (b) Ni3P {3 2 1}. The results also show that with increasing P content in the Ni-P coating, a smaller size of Ni crystallite was achieved. This is related to Ni {1 1 1} diffraction reflections in XRD spectra (Figure 6). From Figure 6b it is evident that the precipitation of Ni 3 P occurs at lower temperatures with increasing P content. In the case of HP coatings, precipitation occurred at 320 ◦ C, whereas precipitation occurred at 350 and 330 ◦ C for LP and MP coatings, respectively. The presence of this phase was not detected up to these temperatures. The temperature dependence of Ni 3 P crystallite size was almost linear, whereas the growth rate of the Ni 3 P phase was the highest for LP Ni-P coatings and the lowest for HP Ni-P coatings. The Ni 3 P crystallite sizes {3 2 1} at 550 ◦ C were 441, 256, and 179 Å for LP, MP and HP coatings, respectively. Figure 6b also shows that a comparable Ni 3 P crystallite size (~250 Å) occurred in the narrow temperature range from 390 to 400 ◦C. As mentioned in the literature [ 11 , 26 ], elemental phosphorus has a very low solubility in nickel; it inhibits the Ni grain growth and increases the number of Ni nuclei. It can be assumed that during the heat treatment up to ~300 ◦ C, the growth of Ni crystallite is inhibited due to the presence of P in Ni crystallites (Figure 6a). In the temperature range of ~300–400 ◦ C, a significant increase of Ni crystallite occurs due to the migration and rearrangement of P atoms from Ni crystallite, and the formation of Ni 3 P phase. The increase of Ni crystallites size was observed by XRD analysis, which can be explained by the decrease of P in Ni crystallites. Kumar [ 22 ] assumes that Ni crystallites are formed from an amorphous Ni-P matrix, and the Ni 3 P phase can formed from both the Ni-P matrix, and from the Ni crystallites formed. The presence of phosphorus in Ni-P coatings increases the number of Ni nuclei. This also results in a slower growth of a larger number of the Ni 3 P crystallites with temperature (Figure 6b). 3.3. DSC Analysis All DSC curves (Figure 7) contain a prominent exothermic peak corresponding to Ni 3 P transformation from the Ni-P matrix [ 20 , 36 ]. DSC analysis confirmed that the temperature of Ni 3 P transformation shifts to lower values with increasing P content in the coating and more energy is released (Figure 7a–c). The DSC curve corresponding to the LP Ni-P coating (Figure 7a) shows that Ni 3 P transformation reached its maximum at 398 ◦ C and the energy evolved was calculated to be 347.4 mJ · mol −1 . DSC peaks for MP and HP Ni-P coatings (Figure 7b,c) were observed at temperatures of 389 and 355 ◦ C, with corresponding evolved energies of 390.6 and 507.9 mJ · mol −1 , respectively. The temperature determined correlates with the findings from XRD (Figure 2).