Ni-Cr-P composite co-deposition on the surface of 65Mn alloy steel

doi:10.21203/rs.3.rs-3887430/v1

65Mn alloy steel, as a promising raw material for the manufacture of knitting needles, requires in-depth study in order to enhance its surface hardness and wear-corrosion resistance. This study focused on the chemical deposition and properties of Ni-Cr-P composite co-deposition on the surface of 65Mn alloy steel. The effects of plating solution composition, process parameters, and their effects on coating deposition rate, surface hardness, and other indicators were also analyzed. The results showed that, under conditions of 12 g/L CrCl₃·6H₂O, 5 g/L C₆H₈O₇·H₂O, 20 g/L C₃H₆O₃, pH 5.5, and a temperature of 70°C, the coating obtained exhibited optimal performance, with a deposition rate of 3.292 µm/h, a microhardness reaching 627.28 HV_0.2, and P and Cr elemental contents of 9.16 and 0.127 wt%, respectively. The coating structure was dense, with good adhesion and minimal porosity. To further enhance the wear resistance, a Ni-Cr-P coating was prepared using the aforementioned process parameters, followed by heat treatment. The effects of heat treatment temperature on the microstructure, microhardness, X-ray diffraction (XRD) spectra, and other characteristics were analyzed. The results indicated an optimal heat treatment temperature for the coating of 200°C. Following heat treatment, the microhardness of the coating remained relatively unchanged, and there was a tendency for the coating structure to become more crystalline, leading to improved wear and corrosion resistance.

Physical sciences/Materials science/Structural materials/Metals and alloys

Physical sciences/Materials science

Physical sciences/Materials science/Structural materials

Surface strengthening

Chemical deposition

Wear resistance

Corrosion resistance

Knitting needles (hereafter referred to as “needles”) are essential components of modern knitting machines. The raw materials needed for the manufacture of needles often include carbon tool steels such as 70 steel, T9A, T10A, and alloy steels such as 65Mn^[1]. 65Mn alloy steel, with advantages such as a high degree of hardness, good elasticity, and easy processability, is a promising advanced raw material for the manufacture of high-quality needles. In recent years, with the emergence of various high-speed, high-end knitting machines and the widespread application of diverse high-performance yarns, more exacting requirements have been proposed for the performance, especially the surface performance, of needles ^[2]. There is therefore an urgent need for in-depth studies of the surface enhancement of needles.

Feng et al.^[3] employed electroplating technology to obtain a hard chrome coating with a microhardness of approximately 700 HV_0.1 on the surface of commonly used SWP-B guide needles. They investigated the effects of different electroplating temperatures and time gradients on the appearance, adhesion, hardness, microstructure, and other aspects of the chrome-plated layer.

Hong Zikang, Feng Liuxing, Peng Cong, and others^[4–6] conducted chromium electroplating on the hook part of latch needles used in warp knitting (Fig. 1). They studied the effects of heat treatment temperature and electroplating fixtures on the morphology, hardness, wear resistance, appearance, and other aspects of the coating. Their results indicated that the hardness of the chrome-plated layer was approximately 520 HV_0.1, and the microhardness of the chrome-plated layer showed no significant changes when the heat treatment temperature was below 200°C.

During the electroplating process, challenges such as difficulty in controlling coating uniformity, complexity in treating intricate shapes, poor environmental performance, and high energy consumption present themselves. In contrast, chemical plating technology enables the deposition of Ni-P coatings, multi-element alloy coatings, and nano-composite coatings on parts with complex shapes and with high precision. Moreover, this is a more environmentally friendly process and holds greater development potential compared to electroplating. Since the late 1990s, researchers have explored Ni-P-based multi-element alloy chemical plating.

Maozhong et al.^[7] utilized chromic anhydride (CrO₃) as the chromium salt, proposed the use of high concentrations of chloric acid to convert Cr⁶⁺ to Cr³⁺, and, after sufficient complexation of Cr³⁺ with citric acid, prepared a Ni-Cr-P plating solution. They deposited a Ni-Cr-P coating on cold-rolled low-carbon steel sheets, with the coating containing approximately 1% Cr. Xinyi et al.^[8] employed an alkaline chemical plating solution with chromic chloride (CrCl₃·6H₂O) as the chromium salt and obtained a Ni-Cr-P coating on low-carbon steel surfaces with a Cr content of 2.85 wt% and a deposition rate in the range 5–15 µm/h.

Yuwei et al.^[9] plated a Ni-P chemical coating with a microhardness of approximately 450 HV_0.1 on T10A steel sheets. They investigated the effects of plating solution temperature, deposition time, and changes in nickel salt and reducing agent concentrations on the thickness and porosity of the Ni-P coating. The study emphasized the need to control the thickness of the needle coating between 2.5 and 5 µm. It becomes challenging to plate coatings below 2.5 µm thickness uniformly over the plating area of needles, while coatings above 5 µm affect control of the size of the needles.

Xin et al.^[10] explored the mechanism of Ni-Cr-P chemical plating on 30CrMnSiA steel, indicating that, in acidic chemical plating solutions, hypophosphorous acid struggles to reduce Cr³⁺ to chromium, and the deposition of Cr may be achieved through an ion exchange process with Ni²⁺. A comparison was made between the adhesion, deposition rate, corrosion resistance, and coating thickness of Ni-P and Ni-Cr-P coatings. The results showed that the corrosion resistance of the Ni-Cr-P coating was superior to that of the Ni-P coating, while the deposition rate was relatively reduced compared to the Ni-P coating, with other properties showing minimal differences.

Shashikala et al.^[11] plated a Ni-Cr-P chemical coating on copper foil using an acidic plating solution. They conducted spectral studies on the chemical plating solution and analyzed the surface morphology, elemental composition, and phase composition of the coating using scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX). The results indicated excellent overall performance of the coating.

Yong et al.^[12] deposited a Ni-Cr-P coating on stainless steel and investigated the effect of heat treatment temperature on the crystallization, plating solution coating structure, and corrosion resistance of the coating. The results indicated different trends in microhardness within various temperature ranges, with 400°C serving as the boundary for changes in corrosion resistance. Below 400°C, the corrosion resistance showed minimal variation, while above 400°C, the corrosion resistance gradually decreased with increasing temperature.

Shuo et al.^[13] prepared a Ni-Cr-P coating on Q235 steel, simulating mainly the electrochemical corrosion behavior of the coating in a fuel cell environment. The study demonstrated that the corrosion resistance of the Ni-Cr-P binary alloy coating was significantly superior to that of the Ni-P coating.

The above studies suggest that chemical plating of Ni-Cr-P, as a promising surface treatment technology, can enhance the surface hardness, corrosion resistance, and wear resistance of alloys effectively^[14]. However, current research has focused mainly on preparing Ni-Cr-P coatings on common materials such as carbon steel, cast iron, copper, and stainless steel. There are limited numbers of studies on applying Ni-Cr-P coatings to needle raw materials such as 65Mn alloy steel. Additionally, the understanding of the effect of plating solution composition on coating deposition rate, coating hardness, and P and Cr elemental compositions is as yet insufficient. Improvements in coating surface hardness and wear resistance are still required.

This study focused on the chemical plating process of Ni-Cr-P on 65Mn alloy steel, investigating the effect of plating solution composition, process parameters, and other factors on coating deposition rate, surface hardness, and related indicators. Our study aimed to explore the optimization of Ni-Cr-P chemical plating solution composition. Additionally, the study delved into the thermal treatment process of the coating, examining the impact of heat treatment temperature on microstructure, surface hardness, and wear resistance. The ultimate goal was to further enhance the wear and corrosion resistance properties of the coating.

2.1 Materials

The experiments used a 65Mn alloy steel plate as the substrate for the Ni-Cr-P chemical plating, with its surface treated through bluing, resulting in a hardness of approximately 500 HV_0.2. The chemical composition of the 65Mn alloy steel used is presented in Table 1. A DK7740 wire-cutting machine was employed to cut it into standard specimen blocks with dimensions of 25 × 20 × 1 mm (Fig. 2).

Table 1

Chemical composition of 65Mn alloy steel (wt%).
Element	C	Si	Mn	Cr	Ni	P	S	Fe
Content	0.62–0.70	≤ 0.37	0.9–1.2	≤ 0.025	≤ 0.025	≤ 0.040	≤ 0.040	Margin

2.2 Preparation of the Ni-Cr-P coating

First, the specimens underwent mechanical leveling, alkali cleaning for oil removal, and acid activation pretreatment before being immersed in the chemical plating solution^[15]. Table 2 lists the Ni-Cr-P composite plating solution process parameters.

Table 2

Plating solution process parameters.
Plating solution component/parameter	Parameter range
NiSO₄·6H₂O	10 g/L
CrCl₃·6H₂O	4–16 g/L
NaF	3 g/L
NaH₂PO₂·H₂O	30 g/L
C₆H₈O₇·H₂O	5–20 g/L
C₃H₆O₃	10–25 ml/L
NaC₂H₃O₂·3H₂O	20 g/L
H₂NCSNH₂	1 ppm
C₁₉H₄₂BrN	10 ppm
pH	4–5.5
Temperature	60–90℃

Plating solution configuration steps:

Use an electronic balance to weigh out nickel sulfate, chromium chloride hexahydrate, sodium hypophosphite, citric acid, sodium acetate, and sodium fluoride according to the composition (Table 2) of the Ni-Cr-P chemical plating solution component. Use weighing paper for different chemicals, and change the paper when weighing different substances^[16].

Pour the weighed chemicals into different beakers, add deionized water to make up to 50 mL, and then place the beakers in a digitally controlled constant-temperature water bath.

Use a graduated cylinder to sequentially measure out liquid chemicals such as lactic acid, cetyltrimethylammonium bromide, and thiourea. The surfactant and thiourea are pre-prepared solutions with a fixed concentration of 1 ppm/mL.

Mix the complexing agents (lactic acid, citric acid), buffer agent (sodium acetate), and chromium salt (chromium chloride) in order in a 500 mL beaker and place them in a digitally controlled constanttemperature water bath for maturation at 60°C for 10 min.

Slowly pour the nickel salt (nickel sulfate), reducing agent (sodium hypophosphite), sodium fluoride, and the liquid chemicals from the graduated cylinder (lactic acid, cetyltrimethylammonium bromide, thiourea) into the 500 mL beaker mentioned above.

Once the pH meter reading and temperature are stabilized at the set process parameters, immediately immerse the pretreated specimens into the chemical plating solution for deposition.

Chemical plating was conducted using a recirculating constant-temperature water bath with magnetic stirring (Fig. 3). During deposition, the liquid level of the chemical plating solution was maintained 1 cm below the level of the water bath, and the specimens were fully immersed in the plating solution.

2.3 Orthogonal experiment

Using the orthogonal experimental method to investigate the effect of plating solution composition, process parameters, etc., on coating appearance, deposition rate, surface hardness, and other indicators^[17], three plating solution components, namely, chromium chloride (CrCl₃·6H₂O), citric acid (C₆H₈O₇·H₂O), and lactic acid (C₃H₆O₃), and two process parameters, pH and temperature, were selected as experimental variables. The selected composition ranges for the various chemicals were: chromium chloride 4–16 g/L, citric acid 5–20 g/L, and lactic acid 10–25 ml/L, the pH range was 4–5.5, and the temperature range was 60–90°C, with four levels for each variable. Before conducting the orthogonal experiment, extensive preliminary experiments were carried out, determining that a chemical plating time of 1 h was optimal. Beyond 1 h, the plating solution became unstable and prone to decomposition (Fig. 4), where A and B represent the normal and decomposed states of the Ni-P chemical plating solution and C and D represent the normal and decomposed states of the Ni-Cr-P chemical plating solution.

The specific variables and the levels selected are listed in Table 3:

Table 3

Orthogonal experimental variables and their levels.
Variable Level	Chromium chloride (g/L)	Citric acid (g/L)	Lactic acid (g/mL)	pH	Temperature (℃)
1	4	5	10	4	60
2	8	10	15	4.5	70
3	12	15	20	5	80
4	16	20	25	5.5	90

2.4 Single-factor experimental design for heat treatment of Ni-Cr-P coating

In order to enhance the wear resistance of the coating, heat treatment was applied to the Ni-Cr-P coating on the surface of 65Mn alloy steel using an SX2-4-12TP box-type resistance furnace. The four experimental samples were heated to 180°C, 200°C, 220°C, and 240°C, respectively, and kept at that temperature for 1 h. The furnace was then allowed to cool to room temperature, followed by testing of the coating properties.

2.5 Performance testing of the Ni-Cr-P coating

To test the performance of the Ni-Cr-P coating, the following equipment and methods were employed:

(1) Observation using a DSX510 metallographic microscope (Olympus Corporation, Tokyo, Japan) to measure the coating thickness. The chemical deposition rate was then determined from metallographic measurements.

(2) Measurement of the surface hardness of the coating using an Micro Vickers HV1000

hardness tester (Manufacturer, City, Country).

(3) Elemental content analysis of the coating using an EDX-7200 EDX spectrometer (JEOL, Tokyo, Japan).

(4) Determination of coating porosity using the filter paper adhesion method, in accordance with the QB/T 3823 − 1999 standard.

(5) Testing of the bond strength between the coating and substrate according to the GB/T 5270 − 1985 standard.

(6) Friction and wear resistance experiments using a UMT-2 reciprocating friction tester to measure these properties of the coating.

(7) Corrosion resistance testing of the coating using a CH1660E electrochemical workstation (CH Instruments, Inc., Austin, TX, USA).

3.1 Effect of plating solution composition on deposition rate, hardness, and P and Cr elemental contents

(1) Effect of plating solution composition on deposition rate

The orthogonal experimental results indicated that the deposition rate of chemical plating varied with changes in plating solution composition and process parameters. Table 4 lists the deposition rate records of 16 sets of orthogonal experiments. Due to the introduction of the difficult-to-reduce chromium salt in the Ni-Cr-P chemical plating process, theoretically, it should lead to a significant decrease in deposition rate. The deposition rate was determined from multiple measurements of coating thickness at a fixed magnification using a metallographic microscope, and the average value was calculated for each set^[18]. Table 5 presents the range analysis table for the deposition rate, which utilizes the range values to analyze the impact of each factor on the experimental results. K_i represents the four-level variables investigated in the experiment, aiming to study their impact on the experimental results and determine the primary influencing factors. R represents the range of K_i (K_iMAX–K_iMIN)^[19]. As R increased, the impact of level variations on the reference indicator became more significant, and as R decreased, the impact of level variations became smaller. Through the orthogonal experimental design, it was possible to analyze more effectively the impact of various factors on the experimental results, reduce the number of experiments, and identify the optimal combination of factors.

Table 4

Deposition rates in the Ni-Cr-P chemical plating orthogonal experiment.
Group	1	2	3	4	5	6	7	8
Deposition rate (µm/h)	1.386	3.292	2.773	6.413	0.693	1.560	1.144	0.936

Table 4

contd.
Group	9	10	11	12	13	14	15	16
Deposition rate (µm/h)	3.292	2.599	1.040	3.015	7.843	1.040	1.213	2.416

Table 5

Range analysis of deposition rates in the Ni-Cr-P chemical plating orthogonal experiment.
Group	A	B	C	D	E
Factor Evaluation indicator	Chromium chloride (g/L)	Citric acid (g/L)	Lactic acid (ml/L)	pH	Temperature (℃)
K₁	3.466	3.304	1.612	1.646	1.534
K₂	1.083	2.123	2.053	3.278	2.547
K₃	2.487	1.543	2.010	2.132	3.798
K₄	3.139	3.206	4.500	3.120	2.297
Range R	2.383	1.761	2.888	1.632	2.264
Main factor ranking	CAEBD
Best combination	A₁B₁C₄D₂E₃

From the range analysis of deposition rates in the Ni-Cr-P chemical plating orthogonal experiment, the ranking of the five factors on the deposition rate was found to be in the order: lactic acid (C) > chromium chloride (A) > temperature (E) > citric acid (B) > pH (D). Lactic acid had the most significant impact on the deposition rate, while pH had the least impact. The orthogonal experimental factor and deposition rate contour plot based on the K_i values of the five factors is shown in Fig. 5.

(2) Effect of plating solution composition on surface hardness

The hardness of the coating varied with changes in plating solution composition and process parameters^[20]. Using a microhardness tester, multiple points were measured on the surface of the Ni-Cr-P-coated samples, and the average value was calculated. Table 6 lists the surface hardness records for 16 sets of orthogonal experiments, and Table 7 presents the range analysis table for surface hardness as an evaluation indicator.

Table 6

Surface hardness results for the Ni-Cr-P chemical plating orthogonal experiment.
Group	1	2	3	4	5	6	7	8
Surface hardness (HV_0.2)	585.22	577.63	558.01	601.4	530.53	564.88	581.52	545.73

Table 6

contd.
Group	9	10	11	12	13	14	15	16
Surface hardness (HV_0.2)	627.28	583.18	574.37	583.26	604.47	604.47	569.57	555.36

Table 7

Range analysis of surface hardness for the Ni-Cr-P chemical plating orthogonal experiment.
Group	A	B	C	D	E
Factor Evaluation indicator	Chromium chloride (g/L)	Citric acid (g/L)	Lactic acid (ml/L)	pH	Temperature (℃)
K₁	580.57	586.88	569.96	581.07	570.93
K₂	555.67	574.99	565.25	575.55	585.45
K₃	592.02	570.87	576.33	556.77	577.66
K₄	575.92	571.44	592.64	590.78	570.15
Range R	36.36	16.01	27.40	34.01	15.30
Main factor ranking	A > D > C > B > E
Best combination	A₃B₁C₄D₄E₂

From the range analysis of surface hardness in the Ni-Cr-P chemical plating orthogonal experiment, the ranking of the effect of the five factors on surface hardness was found to be: chromium chloride (A) > pH (D) > lactic acid (C) > citric acid (B) > temperature (E). Chromium chloride had the most significant impact on surface hardness. The contour plot of orthogonal experimental factors and surface hardness, based on the K_i values of the five factors, is shown in Fig. 6.

(3) Effect of plating solution composition on P and Cr elemental contents

The P and Cr elemental contents of the coating were measured using the EDX-7200 EDX spectrometer. Table 8 lists the test results record for P and Cr contents in the 16 sets of orthogonal experiments.

Table 8

P and Cr elemental contents in the Ni-Cr-P chemical plating orthogonal experiment.
Group	1	2	3	4	5	6	7	8
P content (wt%)	7.108	8.495	9.694	9.772	8.243	9.331	8.16	6.886
Cr content (wt%)	0.193	0.102	0.13	0.182	0.187	0.196	0.181	0.231

Table 8

contd.
Group	9	10	11	12	13	14	15	16
P content (wt%)	9.16	7.687	8.652	9.562	9.905	7.835	8.69	9.037
Cr content (wt%)	0.127	0.16	0.218	0.137	0.034	0.221	0.181	0.143

In all 16 sets of orthogonal experiments, the P content in the coating of all samples ranged from 7 to 9 wt%, indicating that the Ni-Cr-P coating deposited from this plating solution composition had a medium-to-high phosphorus coating with an amorphous structure. The elevation of P content contributed to the improvement in coating hardness^[21]. Range analysis of P content revealed the ranking of the five factors on the P content in the coating to have been in the order: temperature (E) > pH (D) > chromium chloride (A) > citric acid (B) > lactic acid (C).

In all 16 sets of orthogonal experiments, the Cr content in the coating of all samples ranged from 0.1 to 0.2 wt%. Within this range, the elevation of Cr content contributed to the improvement in corrosion resistance of the coating^[22]. The difficulty in reducing Cr content resulted in a relatively low Cr content in the coating. The difficulty in reducing Cr content was due to ion exchange between Cr³⁺ and Ni during the deposition process^[23]. Range analysis of the Cr content revealed the ranking of the five factors on the Cr content in the coating to have been in the order: temperature (E) > chromium chloride (A) > lactic acid (C) > citric acid (B) > pH (D). Of the five factors, temperature had the most significant impact on the P and Cr elemental contents of the coating. This was because temperature, as the direct energy source for the Ni-Cr-P chemical plating reaction, directly affected the breaking and recombination of chemical bonds in the reactants and products of the reduction chemical equations for P and Cr. Increase in temperature significantly promoted the forward progress of the reaction.

3.2 Analysis of coating surface appearance, porosity, and adhesion

(1) Coating surface appearance

Careful observation of the macroscopic appearance of the 65Mn alloy steel following the Ni-Cr-P chemical plating quickly provided a preliminary judgment as to the quality of the Ni-Cr-P coating so obtained. The presence of phenomena such as missed plating, yellow spots, peeling, etc., on the sample surface indicated poor coating quality^[24]. Following observation, except for the presence of yellow spots on the surface of the coating in Group 7, the appearance of the coatings in the other 15 experimental groups was good. In Fig. 7, sample A had a bright and smooth surface, with no missed plating or peeling, presenting a good macroscopic appearance. In contrast, sample B showed the presence of yellow spots, indicating poor coating quality.

(2) Coating porosity

The coating porosity was determined using the filter paper adhesion method, in accordance with the QB/T 3823 − 1999 standard^[25]. A test solution was prepared containing 20 g/L K₃Fe(CN)₆ and 20 g/L NaCl. Filter paper was immersed in the solution so as to thoroughly absorb it and was then applied to the coating surface for 1 min. The number of blue spots on the filter paper was then recorded.

The results of the porosity measurements for the 16 sets of samples were categorized into three levels: F (few) with 1–2 spots, N (normal) with 3–5 spots, and M (many) with > 5 spots. The complete set of test results are listed in Table 9.

Table 9

Coating porosity test results.
Group	1	2	3	4	5	6	7	8
Result	M	F	F	N	F	F	F	N

Table 9

contd.
Group	9	10	11	12	13	14	15	16
Result	F	M	M	F	N	N	F	F

From Table 9, it can be seen that the coated samples from Groups 1, 10, and 11 all had > 5 porosity spots. The coated samples from Groups 4, 8, 13, and 14 had porosity spots ranging from 3 to 5. In contrast, the coated samples from Groups 2, 3, 5, 6, 7, 9, 12, 15, and 16 had only 1 to 2 porosity spots.

(3) Coating adhesion

For thin coatings, the adhesive strength between the coating and the substrate was tested using the scratch method according to the GB/T 5270 − 1985 standard^[26] (Fig. 8). Measurement results indicated that there was no significant peeling of the coating along the edges of the scratches for all experimental groups. This suggested that the adhesion of the coatings in all 16 experimental groups was good.

3.3 Determination of the optimal Ni-Cr-P preparation process

Based on the analysis of porosity spots in the Ni-Cr-P chemical plating orthogonal experiment, Groups 2, 3, 5, 6, 7, 9, 12, 15, and 16 exhibited favorable porosity levels.

According to the macroscopic appearance of the Ni-Cr-P-coated samples, Group 7 showed the presence of yellow spots, while Groups 2, 3, 5, 6, 9, 12, 15, and 16 had a good macroscopic appearance.

Considering the analysis of deposition rates, the optimal range for Ni-Cr-P coating deposition rate was in the range 2.5–5 µm/h. Below 2.5 µm/h, it was challenging to uniformly cover the plating area of the weaving needle, and a rate above 5 µm/h could impact the size control of the weaving needle. Within this range, only the deposition rates of Groups 2 and 9 met the requirements.

Based on the analysis of surface hardness, the Ni-Cr-P coating in Group 2 had a surface hardness of 577.63 HV_0.2, and the coating in Group 9 had a surface hardness of 627.28 HV_0.2. Ultimately, it was determined that the composition and process parameters of Group 9 from the orthogonal experiment were the optimal ones. The complete set of optimal composition and process parameters are listed in Table 10.

Table 10

Optimal compositions and process parameter ranges from the orthogonal experiment.
Plating solution component/parameter	Parameter range
NiSO₄·6H₂O	10 g/L
CrCl₃·6H₂O	12 g/L
NaF	3 g/L
NaH₂PO₂·H₂O	30 g/L
C₆H₈O₇·H₂O	5 g/L
C₃H₆O₃	20 ml/L
NaC₂H₃O₂·3H₂O	20 g/L
H₂NCSNH₂	1 ppm
C₁₉H₄₂BrN	10 ppm
pH	5.5
Temperature	70℃

3.4 Surface hardness of the Ni-Cr-P coating following heat treatment

To investigate the impact of heat treatment on the substrate and Ni-Cr-P coating, the latter was heated to different temperatures, specifically 180°C, 200°C, 220°C, and 240°C, each maintained for 1 h, followed by cooling to room temperature. Figure 9 shows the surface hardness bar chart of the 65Mn alloy steel substrate, the untreated Ni-Cr-P coating, and the Ni-Cr-P-coated samples following heat treatment at different temperatures.

The surface hardness of the Ni-Cr-P-coated samples was significantly higher than that of the substrate, increasing from 495 to 627 HV_0.2. Within the heat treatment temperature range of 180°C to 240°C, the surface hardness of the Ni-Cr-P-coated samples gradually decreased following an initial increase. At 180°C, the surface hardness reached a minimum of 548 HV_0.2, while at 220°C, the surface hardness peaked at 618 HV_0.2. Although the surface hardness of the sample was relatively high at 220°C, it is essential to consider that, with increasing temperature, the grain size of the sample’s metallographic structure also increased, leading to a decrease in material toughness and strength^[27].

In contrast, at 200°C, the surface hardness reached 613 HV_0.2, and was also more conducive to suppressing the growth of metallographic structure grains. Considering these factors, it was determined that the optimal heat treatment temperature for 65Mn alloy steel with a Ni-Cr-P coating is 200°C.

3.5 Microscopic morphology of the Ni-Cr-P coating following heat treatment

Four 65Mn alloy steel samples with the Ni-Cr-P coating under optimal process conditions were subjected to heat treatment at temperatures of 180°C, 200°C, 220°C, and 240°C, respectively, each maintained for 1 h, followed by cooling to room temperature. Figure 10 presents the microscopic morphology of the untreated Ni-Cr-P coating on the 65Mn alloy steel substrate under a metallographic microscope, both at the surface and in cross-section, with magnifications of × 693 and × 1040, respectively.

In Fig. 10a and b, the surface microscopic morphology of the coating is shown. Due to the thinness of the coating, it was challenging to completely cover the original polishing marks on the substrate^[28]. Overall, the coating appeared to evenly cover the substrate, but there were still a few microscopic pores and water droplet-shaped nodular protrusions. Figure 10c and d depicts the cross-sectional microscopic morphology of the coating, revealing a uniform and smooth white appearance, tightly bonded to the substrate.

Figure 11 shows the microscopic morphology of the Ni-Cr-P-coated samples following heat treatment at four different temperatures for 1 h, with magnifications of ×2000 (left) and ×1000 (right). From the images, it can be seen that, at the highest heat treatment temperature of 240°C, the surface morphology of the Ni-Cr-P coating shows minimal changes compared to before heat treatment. The coating structure consists of accumulated, unevenly sized, closely arranged cell-like units. The surface microscopic structure is dense, with a few microscopic pores and water droplet-shaped nodular protrusions.

3.6 XRD spectra of the Ni-Cr-P coating following heat treatment

Figure 12 displays the XRD spectra of the four samples of Ni-Cr-P coating following 1 h of heat treatment at different temperatures, which, from the graphs, can be seen to be essentially identical. Two distinct characteristic peaks are present at around 2θ = 45° and 65°, corresponding roughly to the characteristic peaks of the Ni(111) crystal plane at 44.5°, the Cr(110) crystal plane at 44.5°, and the Cr(200) crystal plane at 64.5°, according to the standard card.

3.7 Friction and wear performance analysis of the Ni-Cr-P coating following heat treatment

The UMT-2 friction test machine was employed to conduct reciprocating friction and wear experiments on the Ni-Cr-P coating. The tests were performed on the 65Mn alloy steel substrate, the Ni-Cr-P coating substrate, and the Ni-Cr-P coating substrate following heat treatment at 200°C. Figure 13(a), (b), and (c) shows the microscopic morphology of wear scars obtained from the friction and wear experiments on the 65Mn alloy steel substrate, the Ni-Cr-P coating substrate, and the Ni-Cr-P coating following heat treatment at 200°C, respectively

From Fig. 13(a), it can be seen that the wear scar exhibits a comet-like tail from left to right, with rust-colored ends and noticeable traces of oxidation. The widest part of the wear scar measured 1025.18 µm. A large amount of wear debris was generated during the friction process, and frictional heat oxidized it. Numerous quantities of wear debris were found adhering to the substrate on the local wear scars, indicating significant adhesive wear. The main reason for the comet-like tail of the wear scar was the poor wear resistance of the substrate^[29]. As the GCr15 steel ball continuously penetrated the 65Mn alloy steel sample, the resistance of reciprocating friction increased, and the stroke gradually decreased.

In Fig. 13(b), the overall shape of the wear scar appears as a long rod with circular ends. The upper edge of the wear scar shows a white coloration, with only the central part exhibiting the color of oxidized iron filings and a sky blue coloration. This indicates that the coating had been partially worn, and the oxidized wear debris from coating delamination appears sky blue, while the oxidized wear debris from substrate wear appears rust-colored. The widest part of the wear scar measured 1008.58 µm, with some adhering wear debris in the local area, indicating adhesive wear.

In Fig. 13(c), the main body of the wear scar also presents a long rod shape with circular ends. The overall wear scar appears white, with sky blue wear debris in the left half, suggesting that the coating has not been worn through. The wear debris acts as a lubricant during reciprocating friction, continuously moving back and forth and eventually adhering to the left half of the wear scar. The widest part of the wear scar measured 930.10 µm. Under identical experimental conditions, a wider maximum width of the wear scar in reciprocating friction experiments indicated poorer wear resistance. In summary, the wear resistance of the Ni-Cr-P coating following heat treatment was superior to that of the untreated Ni-Cr-P coating and significantly better than the 65Mn alloy steel substrate.

3.8 Friction coefficient analysis of the Ni-Cr-P coating following heat treatment

Figure 14 shows the friction coefficient plots for the 65Mn alloy steel substrate, Ni-Cr-P-coated sample, and the Ni-Cr-P-coated sample following heat treatment at 200°C. From the graphs, it can be seen that the friction coefficient of the substrate rapidly jumped from 0.1 to 0.35 within a short time and then slowly increased over time, stabilizing around a value of 0.5. The friction coefficient of the Ni-Cr-P coating sample quickly jumped from 0.1 to 0.5 within an extremely short time and fluctuated thereafter. For the Ni-Cr-P-coated sample following heat treatment, the friction coefficient rapidly jumped to 0.4 within a very short time, then slightly decreased before gradually climbing, roughly stabilizing around a value of 0.4.

In terms of friction coefficient, the Ni-Cr-P coating following heat treatment exhibited better wear resistance compared to the untreated Ni-Cr-P coating, and was also superior to the 65Mn alloy steel substrate.

The above experimental results indicate that the surface hardness of the Ni-Cr-P coating significantly increased following heat treatment at 200°C compared to the substrate, approaching the surface hardness of the untreated Ni-Cr-P coating. However, in terms of wear resistance and friction coefficient, the Ni-Cr-P coating following heat treatment outperformed both the substrate and the untreated Ni-Cr-P coating.

3.9 Corrosion resistance performance comparison and analysis of the Ni-Cr-P coating following heat treatment

The corrosion resistances of the 65Mn alloy steel substrate, the Ni-Cr-P-coated sample, and the Ni-Cr-P-coated sample following heat treatment at 200°C were evaluated using the CH1660E electrochemical workstation. Figure 15 presents the polarization curves for these three samples, and Table 10 provides the electrochemical corrosion parameters obtained after fitting the polarization curves. E_corr and I_corr represent the corrosion potential and current density, respectively. Cat slp and Ano slp represent the reciprocals of the absolute values of the cathodic and anodic slopes, while R_p represents the linear polarization resistance after fitting^[30]. A more positive corrosion potential indicates a lower tendency for corrosion, a smaller corrosion current density suggests a slower corrosion rate, and a higher polarization resistance reflects better corrosion resistance.

A more positive corrosion potential, smaller corrosion current density, and higher polarization resistance for the Ni-Cr-P coating following heat treatment would indicate improved corrosion resistance compared to the untreated Ni-Cr-P coating and the substrate.

Table 11

Electrochemical corrosion test results.
Sample	−E_corr/mV	R_p/Ω	I_corr/µA	Cat slp (1/V)	Ano slp (1/V)
Substrate	569	531.6	61.4	5.311	8.006
Ni-Cr-P coating	445	939.2	42.24	4.873	6.087
Ni-Cr-P coating following heat treatment at 200°C	389	3197.8	14.48	5.540	3.852

From the curves in Fig. 15, it can be seen that the anodic resistances of the 65Mn alloy steel substrate and Ni-Cr-P-coated samples were greater than the cathodic resistance, while the cathodic resistance of the Ni-Cr-P coating sample following heat treatment was greater than the anodic resistance. This indicates that the former two rely on the anodic protection mechanism^[31], while the latter shows a relatively constant corrosion current in the range − 0.1–0.2 V, suggesting the occurrence of significant passivation. The corrosion resistance depends on whether the sample can quickly form a passivation film in the corrosive medium, preventing further corrosion^[32]. From Table 11, it can be seen that the self-corrosion potentials of the three samples were 569, 445, and 389 mV, respectively, and the corrosion currents were 61.4, 42.24, and 14.48 µA, respectively. Both the self-corrosion potential and self-corrosion current showed a gradual decreasing trend. The polarization resistances of the three samples were 531.6, 939.2, and 3197.8 Ω, respectively, showing a gradual increasing trend. The corrosion current density was the lowest, and the corrosion potential was relatively the most positive for the Ni-Cr-P coating treated at 200°C. Under the given potential, the Ni-Cr-P coating following 200°C heat treatment exhibited the lowest corrosion rate. These results indicate that the Ni-Cr-P coating following 200°C heat treatment had better corrosion resistance compared to the other two samples.

This study used an orthogonal experimental method to prepare alloy coatings with varying factors such as chromium chloride, lactic acid, and citric acid plating solution composition concentrations, pH, and temperature. The experimental group coatings were characterized for bath stability, macroscopic appearance, deposition rate, microhardness, porosity, adhesion, and the P and Cr elemental contents in the coating. The optimal preparation parameters for the Ni-Cr-P coating under the chosen experimental conditions were determined. The results indicated that, under the chosen experimental conditions, the optimal preparation parameters for Ni-Cr-P coating were as follows: NiSO₄·6H₂O at 10 g/L, CrCl₃·6H₂O at 12 g/L, NaH₂PO₂·H₂O at 30 g/L, NaF at 3 g/L, C₆H₈O₇·H₂O at 5 g/L, C₃H₆O₃ at 20 g/L, NaC₂H₃O₂·3H₂O at 20 g/L, CH₄N₂S content at 1 ppm, C₁₉H₄₂BrN content at 1 ppm, a pH value at 5.5, and a temperature at 70°C. The structure of the Ni-Cr-P coating obtained exhibited a cellular structure with cells of varying sizes, closely packed, with good adhesion, low porosity, and a deposition rate of 3.292 µm/h. It showed a microhardness value of 627.28 HV_0.2, significantly higher than that of the substrate. The P and Cr elemental contents of the coating were 9.16 and 0.127 wt%, respectively.

The effects of the five factors (A: chromium chloride, B: lactic acid, C: citric acid, D: pH, E: temperature) on the deposition rate were ranked in the order: C > E > A > B > D, on the microhardness as A > D > C > B > E, on the P content in the coating as E > D > A > B > C, and on the Cr content in the coating as E > A > C > B > D.

The various impacts of different heat treatment temperatures (180°C, 200°C, 220°C, 240°C) on the microstructure, microhardness, and organizational structure of the Ni-Cr-P coating were also investigated. The results showed that the microhardness of the Ni-Cr-P coating was optimal following heat treatment at 200°C, reaching a value of 613.17 HV_0.2, which is 118.48 HV_0.2 higher than that of the 65Mn alloy steel substrate. The wear resistance of the coating significantly improved following heat treatment, with the friction coefficient stabilizing at a value of approximately 0.4. The corrosion resistance of the Ni-Cr-P coating following 200°C heat treatment was markedly better than that of the untreated coating.

Data availibility

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Author contributions

M and Y together wrote the main part of the paper manuscript, Z and Z completed the experimental section, and B composed the charts section. All authors reviewed the manuscript.

Funding

This work was supported by The National Key Research and Development Program, and Innovation Base (SQ2023YFB4600241,111HTE2022002),During the investigation and manuscript preparation, J. Yang received support from the Chinese Government Scholarship, funded by the Ministry of Education of the People's Republic of China (Scholarship Certificate Number: 202210810001).

Competing interests

The authors declare no competing interests.

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No competing interests reported.

Ni-Cr-P composite co-deposition on the surface of 65Mn alloy steel

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials and methods