Development and Research Status of Wear-Resistant Coatings on Copper and Its Alloys: Review

Abstract

Wear-resistant coatings applied to the surface of copper and copper alloys through diverse advanced technologies can substantially enhance their wear resistance and broaden their application spectrum. This paper provides a comprehensive review of the development and current research status of wear-resistant coatings fabricated on copper and its alloys. It presents the research findings on the preparation of wear-resistant coatings using both one-step methods (such as laser cladding, electroplating, thermal spraying, cold spraying, electro-spark deposition, etc.) and two-step methods (chemical plating and heat treatment, electrodeposition and laser cladding, laser cladding and in situ synthesis, etc.). This paper provides an in-depth examination of the characteristics, operating principles, and effects of various coating techniques on enhancing the wear resistance of copper and copper alloys. The advantages and disadvantages of different coating preparation methods are compared and analyzed; meanwhile, a prospective outlook on the future development trends is also offered.

1. Introduction

Copper and its alloys have been extensively utilized in the aerospace, mechanical, electrical, and electronic industries due to their outstanding properties, including high electrical and thermal conductivity, superior tensile strength, excellent malleability, and robust resistance to corrosion in atmospheric and aqueous environments [1,2]. Figure 1 shows various excellent properties of copper and its alloys and their corresponding applications. However, copper and its alloys also exhibit the drawbacks of low hardness and inadequate wear resistance [3], which restrict their application to a certain extent. Therefore, methods aiming to enhance the wear resistance of copper and its alloys hold significant engineering importance. Indicators to determine the wear resistance of materials include the wear rate and friction coefficient (COF), where the equation of wear rate is

$$\mathrm{K}={\displaystyle \frac{\mathrm{V}}{\mathrm{S}\times \mathrm{F}}}$$

(1)

V refers to the wear volume (m³), S indicates the total sliding distance (m), and F represents the load force (N).

The equation of COF is

$$\mathsf{\mu }={\displaystyle \frac{{\mathrm{F}}_{\mathrm{f}}}{{\mathrm{F}}_{\mathrm{n}}}}$$

(2)

${\mathrm{F}}_{\mathrm{f}}$ stands for friction force (N), and ${\mathrm{F}}_{\mathrm{n}}$ stands for normal force (N). The COF depends on various factors, including the nature and roughness of the contact surface, the presence of the lubricant, and the material involved.

Figure 1. Various excellent properties of copper and its alloys and their corresponding applications.

Figure 1. Various excellent properties of copper and its alloys and their corresponding applications.

Currently, there are two primary methods to enhance the wear resistance of copper and its alloys. One involves the alloying of materials [4,5,6,7], while an alternative method consists of the application of protective coatings on them. The incorporation of Mn, Zn, Al, and Ni alloying elements can effectively enhance the wear resistance of copper and its alloys. Nevertheless, the excessive incorporation of alloying elements could potentially have a detrimental impact on other properties and escalate production expenses. Consequently, the utilization of surface protective coatings has emerged as the primary method to enhance their wear resistance. The principal methods for preparing wear-resistant coatings on copper and its alloys include laser cladding [8,9,10] (LCD), electrodeposition (ED) [11,12,13], thermal spraying (THSP) [14,15,16], cold spraying (CS) [17,18,19], and electro-spark deposition (ESD) [20,21]. Furthermore, outstanding wear-resistant coatings can also be achieved using two-step methods, such as electroless plating (EP) and heat treatment (HT), ED and LCD, LCD and in situ synthesis, etc. This paper will present an overview of the development and research status of wear-resistant coatings on copper and its alloys. Through this paper, readers can understand the main preparation methods of copper and its alloy wear-resistant coatings and various factors affecting the wear-resistant properties of the coatings. The paper also puts forward views on future development. It is of great engineering significance to improve the wear-resistant properties of copper and its alloys in some specific fields [22]. At present, there are not many reviews on the preparation of wear-resistant coatings of copper and its alloys. In these articles, only a certain preparation method is introduced [23,24], and this paper, synthesizing various methods to prepare wear-resistant coatings, is more detailed and comprehensive. In this paper, working principles and examples of various methods for preparing wear-resistant coatings of copper and its alloys are introduced in detail, and the factors for improving the wear resistance are analyzed and discussed. The working principles or wear data are presented in pictures or tables, which are conducive to more intuitive and convenient reading and understanding.

2. One-Step Methods

One-step methods have become the preferred technology for preparing wear-resistant coatings on copper and its alloys because of their simplicity and high efficiency.

Table 1. Properties of wear-resistant coatings fabricated on copper and its alloys by one-step method.

One-Step Method	Substrate				Coating					Ref.
	Type	Hardness	COF	Wear Rate	Composition	Thickness	Hardness	COF	Wear Rate
LCD	Brass	HV0.2113	-	6.3 mg	Ni solid solution, FeNi3, Cr3Ni2, and Cr23C6, Mo7C3, Cr5B3	-	HV0.2545	-	1.4 mg	[25]
	Copper	-	-	-	Mn5Si2,Mn5Si3,Mn3Si, Ni3Si, Ni2Si, Cu3Si, Mn3Ni2Si and Mn6Ni16Si7	0.8 mm	-	0.1964~0.2582	5.2~12.35 × 10−8 g/N × m	[26]
LCD	Copper	-	-	-	Cr3Si + γ-Ni + Cuss Cr6Ni16Si7 + Ni2Si + Cuss Cr3Ni5Si2 + Cr2Ni3 + Cuss	-	HV0.1400~1000	0.4~0.5	0.2~1.2 × 10−4 mm3/s	[27]
		-	-	-	γ-Fe, Ni), Ni3Si, Cr23C6,Cr5B3 and CrB phases	0.7~1 mm	HV0.1468~775	-	-	[28]
		78 HV	-	4.24 × 10−4 mm3/N × m	FeNi-rich FCC2	-	522~636 HV	-	9.31 × 10−5 mm3/N × m	[29]
		-	-	-	Mo and small amounts of hexagonal Ni3Mo	0.12 mm	600 HV	-	-	[30]
		45 HV0.1	-	-	Cu, Al2O3, Cu0.81Ni0.19 and (Fe, Ni), CFe15.1	-	80~85 HV0.1	-	-	[31]
		60 HV0.5	-	-	γ-Niss	0.55~0.7 mm	260 HV0.5	-	-	[8]
		90.5 HV0.2	-	47 mg	(Fe, Ni) solid solution, Cu3.8Ni Intermetallic Compounds and Cr2Fe14C, Mo α-Co (face-centered cubic structure), CoCx, Fe0.64Ni0.36, Cr23C6 and W2C	1.3 mm	438.6 HV0.2	-	9.3 mg	[32]
		-	-	-	Cr13Ni5Si2 ternary metal silicide and nickel-base solid solution	-	740~780 HV	0.5	3.746~4.527 mm3	[10]
	CuCrZr	80 HV	-	-	FCC and HCP	-	398.6~501.2 HV	0.27	2.16 × 10−15 mm/m	[33]
		98 HV0.1	0.45	-	TiB2,γ-(Ni, Cr), CaF2	About 1.0 mm	810~950 HV0.1	0.24~0.33	-	[34]
Plasma Cladding	Copper	HV0.150	0.22	15.6 mg	γ-(Cu, Fe, Ni), Cr23C6, CrB and Ni3Si	-	HV0.1600	0.12	3.3 mg	[3]
		-	-	15.4 mg	CrB, Cr23C6, γ-(Cu, Fe, Ni), Ni3Si	-	620~650 HV0.1	0.11	3.0~3.9 mg	[35]
		-	-	-	γ-(Cu, Fe, Ni), Cr23C6, CrB, Fe3Ni and Ni3Si	2 mm	-	0.11	2.4~3.1 mg	[36]
ED	Copper	78 HV	-	-	Ni-CO(FCC)	12~15 µm	134 HV	-	1.2~2.1%	[37]
		-	0.45	-	-	20 µm	-	0.29	-	[38]
		122~138 HV0.025	-	-	Ni, Sn	80 µm	498~620 HV0.025	0.05~0.29	1.2 × 10−5 mm3/N × m	[39]
		-	-	-	Ag	-	110~120 HV0.1	0.35~0.5	5.01~6.85 × 10−4 mm3/N × m	[11]
THSP	Copper	115 HV	-	-	Cu, Ti3SiC2, TiCx, TiO2, SiO2	300~450 µm	224 HV	0.39	1.78 × 10−7 mm3/N × m	[40]
		-	0.5	-	silver matrix with ZnO	30~60 µm	1600 MPa	0.3	-	[41]
		-	-	-	Cu, Cu2O	150 µm	112.5~137.5 HV	-	-	[42]
		-	-	-	W	500 µm	850 HV300	-	-	[43]
		-	-	15.4 mg	CrB, Cr23 C6, γ-(Cu, Fe, Ni), Ni3Si	-	600~700 HV0.1	0.11	3.0~3.9 mg	[44]
		84 HV0.3	0.53~0.57	1.52~6.88 × 10−5 mm3/N × m	Cu, SiC, Cu2O Cu3Si, and C phase	-	137~153 HV0.3	0.48~0.56	4.12~1.24 × 10−5 mm3/N × m	[45]
		-	-	-	SiO2, Fe3O4, Al2SiO5, Al6SiO13	-	532.77~659.75 HV	-	60.1~283.74 × 10−4 mm3/N × m	[14]
		-	0.6	-	SiO2, FeTiO3, TiO2, Al2O3, Al2SiO5 and NiMn2O4	280~350 µm	-	0.52	-	[15]
THSP	Tin-Bronze	320 HV0.2	0.9	-	α-Cu,Cu3Al	1 mm	246 HV0.2	0.3	-	[46]
	Brass	-	-	-	-	182.5~207.5 um	614~672 HV0.5	-	-	[47]
	Cu-10 wt% Sn	-	-	-	Cu5.6Sn	-	232.1~242.9 HV0.05	0.19	2.8 × 10−4 mm3/N × m	[48]
CS	CuCrZr	115 HV0.3	-	7.89~291.63 × 10−15 mm3/N × m	CuO	3 mm	150 HV0.3	-	4.24~129.48 × 10−15 mm3/N × m	[49]
	Copper	106 HV0.025	-	1.00	TiB2	1690 µm	151~161 HV0.025	-	2.70	[50]
		119~140.6 HV0.1	0.68	6.2 × 10−3 mm3/N × m	Cu, WC, Co	375 µm	120.3~143.7 HV0.1	0.51	1.1~2.6 × 10−3 mm3/N × m	[51]
		159.55 HV	0.77	-	SiC	-	167.36 HV	0.95	-	[52]
ESD	Aluminum bronze	2.0 GPa	-	-	Cu, Fe3Al, Cu3Al	-	4.1~4.4 GPa	0.303	0.269 mm3	[53]
	Copper	77 HV	0.226	465.28 × 10−6 mm3/N × m	FeAl, Cu, Fe3Al, TiO2	15~30 µm	842 HV	0.190	272.56 × 10−6 mm3/N × m	[54]

outlines the properties of wear-resistant coatings fabricated on copper and its alloys by one-step methods.

2.1. Laser Cladding (LCD)

LCD is a prevalent method utilized for the application of wear-resistant coatings on copper and its alloys [25,26,27,28,29,30,31,32,33,34,55,56,57]. As illustrated in Figure 2 [58], this technology operates on the fundamental principle of utilizing a high-density laser beam to irradiate pre-set or synchronously fed alloy powder onto the surface of a substrate, resulting in simultaneous melting of the powder and substrate. Subsequently, rapid solidification occurs, forming a molten bond layer that achieves metallurgical bonding between the coating and substrate, thereby yielding a surface coating with specific properties. The implementation of this technology not only enhances the surface of the substrate but also distinctly increases its resistance to wear, heat, corrosion, and oxidation [35]. LCD coating preparation technology has been widely used in display, optics, automobile, construction, medical and other fields.

Fe-Co composite coatings were produced on copper using the LCD process [32]. A mold plate measuring 100 mm × 30 mm × 15 mm, comprising 99.0% Cu, 0.8% Cr, and 0.2% Zr, was employed as the matrix. The chemical composition of the alloy powders used to prepare the Fe/Co gradient coatings is outlined in

Table 2. Chemical composition (wt%) of the gradient coating produced on copper using LCD [32].

Coating	Fe	C	B	Si	W	Ni	Cr	Mo	Co
First layer	Bal	1.1	0	1.4	0	21.8	16.0	3.0	0
Second layer	8	1.5	0.6	2.4	4.5	13.0	23.0	0	Bal

. For this process, an Nd: YAG pulse laser (model XL-800) with a wavelength of 1064 nm and a maximum power output of 800 W was used. The cladding layer was formed using a side synchronous powder feeding method, with the parameters for LCD provided in

Table 3. LCD parameters for producing the gradient coating on copper [32].

Coating	Current Intensity/A	Pulse Frequency/Hz	Scanning Speed/(mm·min−1)	Delivery Rate/(g·min−1)	Spot Diameter/mm	Lap Rate/%
First layer	250	22	350	8.8	2.2	40
Second layer	250	22	400	11	2.0	50

.

Yong et al. [25] developed three types of coatings on brass through the LCD method using Ni-based, Fe-based, and Co-based self-fluxing alloy powders, respectively. The experimental results demonstrated that all three coatings exhibit excellent metallurgical bonding with the substrate. The microstructure of the three coatings is characterized by the precipitation of fine dendrites and cell-like intermetallic compounds, with no observable pores or cracks. The average hardness of the coatings is approximately 545 HV_0.2 for the Ni-based, 569 HV_0.2 for the Fe-based, and 372 HV_0.2 for the Co-based coatings, all of which are obviously higher than that of brass (113 HV_0.2). The wear rate of the Co-based coating is 70% that of brass under the same friction testing conditions, whereas the wear rates of the Ni-based and Fe-based coatings are only 25% of that of brass. This indicates that all three coatings effectively enhance the wear resistance of the brass matrix.

Zhang et al. [26,27] developed Ni-Mn-Si and Ni-Cr-Si coatings on copper through LCD. The primary wear mechanisms identified in both types of coatings include abrasive wear and slight adhesive wear. The incorporation of Ni-Si and Mn-Si phases in the Ni-Mn-Si coatings markedly enhances their hardness and wear resistance, with the Ni-Si phase demonstrating a more pronounced effect compared to the Mn-Si phase. Among these coatings, the one with the composition of Ni-20Mn-40Si (atom%) achieved the highest hardness and optimal wear resistance. In the case of Ni-Cr-Si coatings, the configuration of Ni-26Cr-29Si (atom%) displayed the highest hardness, approximately 1050 HV_0.1, alongside superior wear resistance, with a wear rate that is merely one-sixth that of 304 stainless steel. The formation of silicide phases (such as Ni-Si, Mn-Si, and Cr-Si) within the coatings is essential for enhancing their hardness and wear resistance.

Cheng et al. [28] successfully created a Ni60A coating on copper using an innovative blue diode LCD under the conditions of a preheating temperature of 200 °C and a laser power of 1500 W. The coating demonstrated a robust metallurgical bond with the underlying matrix, and no cracks were identified. The Ni60A coating is primarily composed of an eutectic matrix along with precipitated phases such as C₂₃C₆ and CrB. Its average micro-hardness ranges from 468 HV_0.1 to 775 HV_0.1, which is approximately 8.8 to 14.6 times greater than that of pure copper. Figure 3a,b displays the three-dimensional optical morphology and cross-sectional profile of the wear marks from the initial layer (C1), the subsequent layer (C2), and the copper matrix at temperatures of 25 °C and 300 °C, respectively. It is evident that the width and depth of the wear marks on copper at both temperatures exceed those on the Ni60A coating. Figure 3c illustrates the average friction coefficient (COF) for C1, C2, and the copper matrix at 25 °C and 300 °C. The values and trends align with earlier findings reported by Cheng P et al. [36,44] in prior studies. The trace areas resulting from friction on C1, C2 coatings, and the copper matrix at 25 °C and 300 °C are presented in Figure 3d. The wear resistance of the C1 and C2 coatings, along with the copper matrix, is superior at 25 °C compared to 300 °C. Furthermore, the wear resistance of the coatings and matrix at both temperatures is ranked as follows: C1 coating > C2 coating > copper matrix.

In addition, LCD can also be utilized to produce high-entropy alloy coatings on copper and its alloys to enhance their wear resistance [29,33,55,56,57]. The hardness of the high-entropy alloy coating is primarily influenced by the phase transition between the BCC and FCC phases, with the BCC phase exhibiting greater hardness than the FCC phase. Variations in the hardness of high-entropy alloy coatings will directly impact their wear resistance.

Li et al. [55] prepared AlCrFeNiMn_x (x = 0, 0.5, and 1) high-entropy alloy coatings on copper using LCD. The study found that the addition of Mn element improved the hardness and wear resistance of the coating. The coating had the highest hardness (625 HV) and best wear resistance when x = 0.5, which was attributed to the combined effects of an increase in the BCC phase, solute strengthening, and structure refinement.

Wu et al. [29] fabricated FeCoCrAlCuNi_x (x = 0.5, 1, and 1.5) high-entropy alloy coatings on copper by laser surface alloying techniques, achieving defect-free coatings devoid of cracks and pores. Their study indicated that the micro-hardness of the coatings declines progressively with increasing Ni content. For instance, the micro-hardness of the FeCoCrAlCuNi_0.5 coating measures 636 HV, while that of the FeCoCrAlCuNi_1.5 coating decreases to 522 HV. This reduction can be attributed to two primary factors. Firstly, the addition of Ni triggers a phase transformation from body-centered cubic (BCC) to face-centered cubic (FCC). Secondly, the incorporation of Ni reduces the atomic size disparity within the alloy system, resulting in diminished weak lattice strain micro-hardness. The corrosion resistance of the FeCoCrAlCuNi_x coating is significantly more enhanced than that of pure copper. Notably, the FeCoCrAlCuNi₁ coating demonstrates a wear rate of only 22% that of pure copper, underscoring its outstanding wear resistance. Furthermore, the addition of Al is beneficial, as it enhances both hardness and wear resistance. This improvement is due to the ability of Al to form stronger bonds with transition metals, increase Young’s modulus, and promote a higher proportion of the BCC phase.

Jiang et al. [33] successfully prepared FeMnCoCr high-entropy alloy/TiC/CaF₂ self-lubricating coatings on a Cu–Zr–Cr alloy for a continuous casting mold by LCD for wear-resistance. Through the addition of TiC, the friction coefficient and wear rate were decreased from 0.35 and 3.68 × 10⁻¹⁵ mm³/m to 0.27 and 3.06 × 10⁻¹⁵ mm³/m, respectively. When CaF₂ was added, the friction coefficients and wear rate were decreased to 0.16 and 2.16 × 10⁻¹⁵ mm³/m, respectively, which was 54% lower than the pure FeMnCoCr HEA coating. The main wear mechanism of the FeMnCoCr coating is abrasive wear, while that of the FeMnCoCr/TiC coating is abrasive and adhesion wear. But adhesion wear is dominant for the FeMnCoCr/TiC/CaF₂ coating.

2.2. Electrodeposition (ED)

Presently, ED stands out as one of the most widely utilized methods for creating wear-resistant coatings on copper and its alloys [37,38,39,59,60,61,62,63,64,65,66]. Figure 4 demonstrates a typical setup used for the ED of metals onto a conductive substrate [67]. In this configuration, the battery comprises at least two metal electrodes immersed in a liquid medium: the cathode, where metal deposition occurs, and the anode, which can either be a piece of the same metal or an inert material that has previously undergone ED. Both electrodes are immersed in an electrolyte solution containing metal cations and are connected to a power source, which creates a potential gradient between them. This gradient sets the potential of the cathode (or anode) either below or above the equilibrium potential of the target reaction occurring on the electrode surface. The applied potential facilitates charge transport within the system, driving metal cations toward the cathode and salt anions toward the anode. The fundamental principle of electroplating involves placing the substrate in the electrolyte as the cathode and the target as the anode. Under the influence of the impressed current, a coating forms on the substrate [68]. Furthermore, a reference electrode can be utilized to provide a stable and accurate reference for the potential associated with the cathode. This ensures better control and monitoring of its potential, particularly when a potentiostat is employed as the power source. ED coating preparation technology is mainly used in automotive, electronics, aerospace and other fields, providing anti-corrosion, wear-resistant, insulating, and other properties to enhance product durability and service life.

Malayeri et al. [37] utilized ED to prepare a nickel/graphene oxide coating on a pure copper surface, aiming to enhance its mechanical properties. Their research revealed that several physical and chemical factors can influence the quality of the coating during the ED process, such as ultrasonic stirring, bath type, and solution concentration. Employing ultrasonic stirring at a power of 200 W during ED effectively distributes the graphene oxide layer among the nickel grains and inhibits excessive grain growth. As a result, the coating achieves a hardness of 747 HV, approximately 10 times that of the copper matrix. Additionally, the sample exhibits optimal wear resistance when ED occurs in a chloride bath. An increase in the concentration of graphene oxide within the ED solution raises the graphene oxide content in the coating. This concentration change leads to a decrease in wear loss, as the uniform distribution of nanoparticles hinders and delays dislocation movement, thereby preventing plastic deformation.

Kim K [59] investigated the ED of gold onto a brass surface to mitigate friction and prevent voltage reduction caused by wear debris that arises from relative displacement in electrical connectors. The study revealed that the maximum ratio of tangential force to normal force for self-matched gold-plated brass was approximately 0.55 to 0.56, in contrast to a ratio of 0.84 for self-matched brass without ED. This suggests that the gold deposited on the brass distinctly reduces friction at the contact surface. In a related study, D. Almonti et al. 44 enhanced the wear resistance of a copper substrate by using the ED method to deposit various Cu-GNP (graphite nanosheet) coatings onto it. The main wear mechanisms observed in both the copper base and the coatings were plastic deformation and ductile fracture. The authors proposed that the GNP creates a graphite transfer film between the friction pairs, which safeguards the surface from wear due to its lubricating properties, thereby improving the wear resistance of the matrix.

Sharma et al. [39] conducted an experiment investigating the ED of nickel–tin alloy coatings on pure copper, focusing on the friction and wear properties using a circulating ball and dick setup. The nickel–tin alloy coating was primarily composed of β-Sn and Ni. Throughout the experiment, the ED current density varied between 100 and 500 mA/cm². The results indicated that current density obviously influences both the composition and morphology of the coating, thereby affecting its hardness and wear resistance. The micro-hardness of the coating ranged from 498 to 620 HV, approximately 4 to 6 times that of pure copper, with peak hardness achieved at a current density of 300 mA/cm². The wear rate and average COF of the coatings, produced at various current densities and under different loads, are depicted in Figure 5a,b. Figure 5b illustrates the relationship between COF and sliding load distance. In all scenarios, the volume loss, wear rate, and wear factor of the coating were minimal at 100 mA/cm². The wear factor showed further improvement with increased loads but remained relatively stable until reaching 6–8 N at current densities of 200–300 mA/cm². This behavior is likely due to the highest micro-hardness of the film occurring at 300 mA/cm². For comparison, pure copper maintained a constant COF of 0.14 at 10 N, whereas the COF for various coatings demonstrated a continuous increase, ranging from 0.05 to 0.29 (as illustrated in Figure 5b). At the same current density, there was no notable variation in COF at higher loads. Under heavier loads, the predominant wear mechanism of the nickel–tin alloy coating is likely a combination of bonding and oxidation processes.

2.3. Thermal Spraying (THSP)

THSP is a prevalent method for fabricating wear-resistant coatings on copper and its alloys [40,41,42,43,45,46,47,48,69,70,71,72,73,74,75]. As illustrated in Figure 6 [76], the procedure involves the application of a heat source to elevate the temperature of a wire or powder until it attains a molten or semi-molten condition. The heat source, in conjunction with applied compressed air, atomizes these molten droplets into smaller particles, which are then propelled onto the surface of a pretreated workpiece. The atomized particles acquire kinetic energy through the heat source or compressed air, creating a metal jet that impacts the workpiece surface. Due to the considerable temperature difference between the atomized particles and the workpiece, the particles rapidly spread and solidify, forming a layered structure and resulting in a THSP coating of a defined thickness [77]. Common techniques for THSP include plasma spraying, flame spraying, supersonic spraying, and explosive spraying, among others.

Xu et al. [40] developed a Ti₃SiC₂/Cu composite coating with high Ti₃SiC₂/Cu content (20 wt% and 50 wt% Ti₃SiC₂) on a brass surface using plasma spraying technology. The primary phases of the coating include Cu/Cu (Si), Ti₃SiC_2, TiCx, TiO₂, and SiO₂. The microstructure of the 20 wt% Ti₃SiC₂/Cu composite coating is notably dense, characterized by diffusion and interfacial reactions between the ceramic phase and the copper matrix. In contrast, the 50 wt% Ti₃SiC₂/Cu composite coating contains numerous defects, leading to the decomposition of many ceramic particles into nanometer- to sub-micron-sized fragments. The micro-hardness of the 20 wt% Ti₃SiC₂/Cu composite coating is distinctly higher, reaching 224 HV_0.1, which is approximately twice that of brass. Furthermore, the 50 wt% Ti₃SiC₂/Cu coating displays a low COF of 0.39 under dry friction conditions, with the primary wear mechanisms being adhesive wear, abrasive wear, and fatigue wear.

Mana et al. [46] developed a multicomponent aluminum bronze coating on tin bronze using hot flame spraying technology. The microstructure of the coating exhibited a typical layered configuration. The average Vickers hardness of the aluminum bronze composite coating (Cu-10Al-4Fe) was found to be 225 HV_0.5. Although this Vickers micro-hardness is approximately 30% lower than that of tin bronze, both coatings achieve a relatively stable COF after a brief run-in period. Specifically, the average COF for the multicomponent aluminum bronze coating is 0.3, compared to 0.9 for tin bronze, indicating that the COF of the aluminum bronze coating is one-third that of tin bronze. These findings suggest that the multicomponent aluminum bronze coating demonstrates excellent anti-friction performance.

Building upon flame spraying technology, researchers have developed supersonic flame spraying, an advanced method suitable for creating wear-resistant coatings on substrates of copper and its alloys. Zouari et al. [47] employed this supersonic flame spraying technology (HVOF) to apply NiCrBSi alloy and 316L stainless steel coatings on a brass surface. Figure 7 [47] confirms that the 316L stainless steel coating exhibited more defects compared to the NiCrBSi coating, including micro-pores, oxides, and unmelted particles. The adhesion strength of the 316L stainless steel coating was measured at 59.9 ± 2.1 MPa, which surpasses that of the NiCrBSi coating at 46.7 ± 3.1 MPa. However, the average micro-hardness of the NiCrBSi coating was distinctly higher, recorded at 643.14 ± 29 HV_0.5, compared to the 316L stainless steel coating’s 469.13 ± 23 HV_0.5. Both coatings demonstrated micro-hardness levels well above that of brass. The experiments indicate that the HVOF process can yield coatings with a fine microstructure and strong adhesion to the substrate, potentially enhancing the mechanical properties of copper alloy surfaces for industrial applications.

Romanov et al. [41] developed a ZnO-Ag electric explosive coating on copper electrical contacts in the CJ20 electromagnetic starter using an explosive spraying technique. The test results reveal that the average hardness of the electroexplosive coating reaches 1600 MPa, which notably increases the micro-hardness of annealed copper by a factor of 3.8 [78]. Furthermore, the abrasion resistance and COF of the modified layer improved by 1.1 times and 1.3 times, respectively. The application of the ZnO-Ag electroexplosive coating enhances both the strength (nano hardness) and tribological properties (wear resistance) of copper electrical contacts. This enhancement is likely due to the formation of a multi-element (Cu, Ag, and Zn) multiphase submicron–nano structure on the surface layer, as demonstrated in Figure 8. The introduction of this coating could potentially extend the working life of the copper electrical contact by up to two-fold.

Özorak et al. [45] prepared a Cu-SiC/WCCo composite coating on a copper surface by THSP. The intermetallic phases and ceramic particles added to the matrix resulted in increased hardness and wear resistance of the coating layers but decreased electrical conductivity. There were significant increases in hardness by adding ceramic particles to Cu, especially with 20% WCCo additive, and the hardness obtained was 82% higher than the Cu substrate. When the wear properties were examined, both abrasive/adhesive wear types were observed in the copper substrate and coating layers.

2.4. Cold Spraying (CS)

CS presents several advantages over traditional THSP, including reduced process temperatures and minimized thermal impact on raw materials and substrates. This method also accommodates a broader range of sprayable materials and achieves higher deposition efficiency and rates [79]. As depicted in Figure 9 [80], CS typically employs supersonic carrier gas to accelerate solid particles, maintaining them below the melting point of the materials. These accelerated particles create a high-speed, particle-rich jet that strikes the substrate. Upon impact with the matrix, the solid powder undergoes significant plastic deformation, allowing for fusion to achieve deposition. The development of wear-resistant coatings on the surfaces of copper and its alloys through CS has attracted considerable interest [49,50,51,81,82,83,84].

Jiang et al. [49] developed a wear-resistant coating composed of CuCrZr using CS on a copper alloy substrate. The micro-hardness of the coating is measured at 158 HV_0.3, which is 1.4 times greater than that of the substrate, which has a micro-hardness of 115 HV_0.3. This enhancement in hardness may be attributed to the work hardening of CuCrZr particles during the CS process. Figure 10a,b illustrates the COF between the substrate and the D30 coating, achieved with a CS distance of 30 mm. In the initial stages of testing, the selective cutting of the sample’s surface leads to irregular wear marks, causing significant fluctuations in the COF [52,85]. However, after approximately 900 s, the coefficient stabilizes. It is also observed that as the temperature increases, the COF tends to rise. Importantly, at any given temperature, the COF of the coating is lower than that of the substrate. Among all tested coatings, the one applied at 400 °C displays the lowest COF, recorded at 0.52. Figure 10c,d presents the three-dimensional and two-dimensional profiles of the wear marks on both the coating and the substrate at 400 °C. Notably, the wear area demonstrates a significant accumulation of oxides. The matrix exhibits a distinct oxide height of 11 µm along the deep internal grooves, while the oxide height on the coated sample measures only 5.2 µm. Furthermore, the interior of the wear marks appears flatter, as shown in Figure 10c,d. The wear rate results are shown in

Table 4. Wear rate of samples at different ambient temperatures [49].

Experimental Temperature/℃	Wear Rate (10−15 m3 (N⋅m)−1)
	Substrate	Coating
400	7.89 ± 1.25	4.24 ± 0.89
500	157.76 ± 15.33	74.33 ± 23.05
600	291.63 ± 40.10	129.48 ± 24.87

. It is noteworthy that the wear rate of the coating is half that of the substrate when assessed at the same temperature. Furthermore, the layer formed at 400 °C demonstrates the lowest wear rate. The wear marks on the substrate and coating across the three temperatures are covered with flakes and oxide layers, a typical characteristic of layered wear [52].

Pialago et al. [81] developed a ternary Cu-CNT-SiC composite coating on pure copper using CS. Their findings indicated that the composite coating was harder than the pure copper coating, with the version containing SiC exhibiting even greater hardness than its SiC-free counterpart. Several factors may contribute to this enhancement in hardness. First, the increased hardness of pure copper coatings is primarily due to strain hardening resulting from particle deformation during the CS process [86,87]. Second, for composite coatings, the elevated hardness can be attributed to the cumulative strain induced by particle deformation during mechanical alloying (MA) and cold gas dynamic spraying (CGDS) [86,87,88,89]. Finally, SiC composites display the highest hardness values, which can be ascribed to the presence of hard ceramic particles that augment the cumulative strain. Additionally, the hardness of the coating continues to increase as the content of ceramic particles rises [90]. The incorporation of hard SiC particles markedly influences the hardness of the composite coating.

Tazegul et al. [82] employed copper (Cu) and aluminum copper (Al₂Cu) powder as raw materials to develop composite coatings on pure copper using CS. The coatings reinforced with Al₂Cu particles demonstrated a remarkable increase in hardness, as expected. Given that hardness is vital in the wear process, the wear resistance of coatings containing 10% by volume of Al₂Cu was found to be superior to that of those with 5% Al₂Cu. Conversely, coatings without Al₂Cu and those containing 15% by volume of Al₂Cu displayed poor wear resistance, a result attributed to their high porosity (exceeding 2.5 vol.%) and unfavorable wear surface morphology.

Calli et al. [50] employed CS to develop composite coatings on pure copper, investigating the impact of various reinforcement particles, specifically B₄C, TiB₂, and TiC, on the physical and wear properties of these coatings. The intense plastic deformation occurring during the CS process results in a micro-hardness that is distinctly higher than that of pure copper, which measures 106 HV_0.025. The study revealed that the inclusion of reinforcement particles negatively affected the wear resistance of the coatings, which can be attributed to the mechanisms involved in wear. In the case of pure copper coatings, the wear surface is smooth and covered by a layer of copper oxide that acts as a lubricant, thus reducing the wear rate. Conversely, this lubricating effect is absent in composite coatings. Analysis of the worn surfaces of the B₄C and TiC composite coatings indicated uneven oxide formation and increased surface roughness relative to pure copper. This phenomenon likely results from the shedding of ceramic particles during wear, which disrupts the oxide layer in a process known as the third body wear mechanism, leading to an increased wear rate for these coatings. In contrast, the TiB₂ sample displayed a smooth wear surface akin to that of pure copper; however, stratification of the oxide layer was observed on the worn surface of the TiB₂ sample. This stratification further reduces the protective capability of the oxide layer formed during the wear test, contributing to a higher wear rate of the coating.

Wang et al. [51] used Co-coated WC (WC-Co) particles as the reinforcement phase to prepare CMC coatings by CS. With the WC-Co content in the feedstock powder increased up to 20 wt%, the mean free path (MFP) of reinforcement particles in the coating decreases significantly, and the micro-hardness of the CMC coating increases from 129.8 ± 10.8 HV_0.1 (Cu coating) to 155.4 ± 11.7 HV_0.1 (Cu20WC-Co coating). The wear test indicates that the specific wear rate of the Cu20WC-Co coating decreases by 82.3% compared to the Cu coating. The superior wear resistance of CuWC-Co to Cu and CuWC coatings is attributed to their denser structure, the lower MFP of the ceramic particle, and their higher cohesive strength.

2.5. Electro-Spark Deposition (ESD)

ESD is a method employed to deposit electrode materials onto the surface of conductive substrate materials using high-current pulses from a power supply [91]. As depicted in Figure 11 [92], a discharge occurs when the distance between the positive electrode of the power supply and the target substrate, connected to the negative electrode, decreases to a level that produces high-frequency pulses. During this discharge, the energy released melts the electrode material, which is then deposited onto the surface of the base material, creating a strengthening layer and achieving metallurgical bonding. This technique markedly enhances the physical and chemical properties of the substrate material, resulting in improvements in hardness, wear resistance, corrosion resistance, and other critical characteristics [20,21,53,54,93]. ESD coating preparation technology is mainly used in electronic, semiconductor, medical, aerospace, and other fields to prevent electrostatic accumulation and discharge, protect sensitive equipment and components, and improve corrosion and wear resistance.

Zhang et al. [53] developed an aluminum bronze coating on cast aluminum bronze using ESD, followed by ultrasonic rolling (USR) treatment of the coating. The experimental results revealed that the micro-hardness of all coatings surpasses that of the substrate, with supersonic deposition coatings exhibiting greater micro-hardness than electrostatic deposition coatings. The improvements in nano-hardness, static creep resistance, and micro-hardness of the coating can be attributed to work hardening and grain refinement [94,95]. The average COF and wear volume during the steady wear stage for as-cast aluminum bronze, ESD coating, and USR-treated ESD coating under 2.5 MPa static pressure (USR2.5) are depicted in Figure 12. Notably, the COF and wear volume of the ESD coating are obviously higher than those of the cast aluminum bronze. Although the ESD coating is harder, its tribological properties are inferior to those of the cast aluminum bronze, likely due to residual tensile stress and pore defects present in the ESD coating. After the USR2.5 treatment, the average COF in the stable wear stage decreases from 0.367 to 0.303, and the wear volume reduces by approximately 50%, closely resembling the wear characteristics of the as-cast substrate. The wear resistance of the coating improves as pore defects heal, the stress state transitions from residual tensile stress to residual compressive stress, and hardness increases.

Mertgenc et al. [54] developed TiC-enhanced FeAl intermetallic compound coatings on pure copper using ESD. They assessed the hardness of the copper matrix both before and after the application of the coating. The findings revealed that the hardness of pure copper was 77 HV [96], whereas the hardness of the TiC-reinforced FeAl intermetallic compound coating reached 842 HV, signifying an approximate 11-fold increase in hardness. Additionally, the experimental results demonstrated that the COF and wear rate of the coated copper were signally lower than those of uncoated copper. This reduced COF may be attributed to the influence of aluminides on the inherent low deformation capacity of the WC-Co ball used in the friction and wear testing machine, as well as the lubricating effect of the TiC particles in the coating [97]. While uncoated pure copper exhibits adhesive wear, the coated copper shows evidence of both adhesive and abrasive wear.

To enhance the surface wear resistance of copper alloys, Zhang et al. [20] employed ESD on the surfaces of tin bronze and silver along with copper, Babbitt B83, and graphene oxide (GO) as anti-friction materials to formulate a run-in coating. This run-in coating displays a dense microstructure, fine grain, and uniform distribution, establishing a metallurgical bond with the tin bronze matrix. The COF of the run-in coating is approximately 0.210, representing 64.8% of the substrate’s coefficient of 0.324. To gain insights into the friction and wear mechanisms of the coating, the surface wear marks were analyzed. Figure 13 illustrates the wear marks on the tin bronze substrate both with and without the run-in coating. In the analysis represented in Figure 13a, it was found that the primary wear mechanism of the tin bronze substrate involves obvious plow wear and fatigue delamination. Conversely, as shown in Figure 13b, the run-in coating effectively mitigates fatigue delamination, demonstrating characteristics of plastic deformation, scratches, and mild polishing.

Zheng et al. [21] applied ESD to create a composite coating on tin bronze. This coating alternates between soft materials, specifically silver and Babbitt B83. The results indicate that the composite coating possesses a dense microstructure, fine grain structure, and uniform distribution. Under optimal process parameters, the surface roughness of the composite coating measures 19.43 µm, with a maximum thickness of 80 µm. After the running-in phase, the composite coating demonstrates a minimum COF of approximately 0.177. The primary wear mechanisms observed in the composite coatings prepared under these optimal conditions include plastic deformation and abrasive wear, along with slight polishing.

2.6. Summary

One-step methods are simple and convenient and can be used to effectively prepare wear-resistant coatings to improve the wear-resistant properties of copper and its alloys. For example, a 1.3 mm Fe-Co composite coating was prepared on the surface of copper using LCD technology [32]. Calculations indicate that approximately 200 s is required to prepare a single layer of coating via LCD, while the entire process for creating the gradient coating takes around 7 min. The average micro-hardness of the cladding layer reached 438.6 HV_0.2, which is four times greater than that of the underlying substrate. The carbides formed within the cladding layer observably mitigate the micro-cutting effects of wear particles, resulting in a minimal wear. The cladding layer experiences an average wear rate of only 19.8% compared to the copper matrix. Mana et al. [46] used THSP to prepare a multicomponent aluminum bronze coating on tin bronze. During the THSP process, the wire feeding speed is 0.04 m/s. For tin bronze with a substrate size of 30 mm×30 mm×5 mm, a wear-resistant coating with a thickness of 1 mm can be quickly prepared. The composite aluminum bronze coating has good anti-friction properties and can improve the wear resistance of tin bronze. Mertgenc operates the coating process on a special ESD machine “Shingzhou S3000” [54]. The coating frequency is 500 hz, and the stable voltage output is constant at 42 v, which can quickly prepare the coating. It is found that the COF and wear rate of a TiC-enhanced FeAl intermetallic compound coating is significantly lower than that of pure copper. A TiC-enhanced FeAl intermetallic compound coating prepared by ESD technology on pure copper surface can quickly and effectively improve its wear resistance.

The cost of coating preparation is influenced by materials and equipment. Low-cost methods include CS and ESD. CS features cheap equipment, low maintenance, low material cost, low energy consumption, simple operation, and low labor cost [98]. ESD has affordable equipment, low maintenance, low electrode and working fluid cost, low energy consumption, simple operation, and low labor cost [99]. Medium-cost methods are ED and THSP. ED requires chemicals, metals, high electricity, expensive equipment, water bills, maintenance, and environmental treatment, with a medium comprehensive cost [100]. THSP devices are cheaper than LCD, with a lower material cost than LCD powder, high energy consumption, moderate labor cost due to professional operators, and comprehensive cost between LCD and CS [101]. The most expensive method is LCD, with a high powder cost, high equipment price, variable energy consumption, high labor cost due to the technical personnel required, and high comprehensive cost [102].

The change in wear mechanism before and after coating is one of the important factors affecting the wear resistance of wear-resistant coatings prepared by one-step methods. The wear mechanism of the coating is different from that of the substrate, which may lead to less wear of the coating during friction and improve the wear resistance of the material. For example, Mertgenc found that the COF and wear rate of coated copper were significantly lower than that of uncoated copper [54]. The wear mechanism of uncoated pure copper is adhesive wear, while that of coated copper is adhesive wear and abrasive wear. Zhang et al. [26,27] made the same discovery on Ni-Mn-Si and Ni-Cr-Si coatings prepared by LCD on a pure copper surface. The main wear mechanisms identified in both types of coatings include abrasive wear and slight adhesive wear, which differ from the wear mechanisms of pure copper. After the wear test, the wear resistance of the two coatings is significantly higher than that of pure copper, and the Ni-20Mn-40Si (atom%) coating has the highest hardness and the best wear resistance. Sharma et al. [39] used ED to prepare a nickel–tin alloy coating on pure copper and focused on studying the friction and wear properties of circulating ball and disc devices. It was found that the main wear mechanism of the Ni-Sn alloy coating under heavy load may be the combination of bonding and oxidation processes.

3. Two-Step Methods

Following an extensive investigation into various matrix surface strengthening techniques, researchers concluded that utilizing a single preparation method presents certain limitations. To address these challenges, they implemented a two-step method for coating development, which combines the strengths of different techniques and alleviates their drawbacks. Currently, a range of two-step coating processes has been utilized for the surfaces of copper and its alloys. These processes include EP and HT, ED + LCD and LCD + in situ synthesis.

Table 5. Preparation of wear-resistant coatings by two-step method of copper and its alloys.

Two-Step Method	Substrate				Coating					Ref.
	Type	Hardness	COF	Wear Rate	Composition	Thickness	Hardness	COF	Wear Rate
Chemical Plating + Heat Treatment	C17200	340 HV	-	3.03 × 10−6 mm3/N × m	Ni, Ni3P	20~22 um	998 HV	-	2.04 × 10−6 mm3/N × m	[103]
ED + LCD	Copper	-	-	-	Mo, Ni3Mo, MoO2	600 um	450 HV	-	-	[104]
LCD + In Situ synthesis	Copper	48 HV0.2	-	-	Cu, ZrB2, SiC	1.3~1.9 mm	309 HV0.2	0.2	30 mg/km	[105]

offers detailed insights into the two-step preparation of wear-resistant coatings applied to copper and its alloys’ surfaces, showcasing the enhancements in coating properties relative to the substrates. This integrated approach effectively addresses the limitations inherent in single preparation methods and provides a more comprehensive solution for advancing surface-strengthening technology.

3.1. Electroless Plating (EP) + Heat Treatment (HT)

EP is a highly effective technique for modifying material surfaces. Its fundamental principle relies on using strong reducing agents to convert metal ions from a solution into elemental metals, which are then deposited onto the surfaces of base materials to create a dense coating [106,107,108]. This process obviously enhances various properties, including the wear resistance, corrosion resistance, electrical conductivity, and magnetic performance of the base material. Currently, research into EP of copper and its alloys predominantly centers on a nickel–phosphorus alloy (Ni-P), celebrated for its outstanding wear resistance [103,109,110,111].

Reis et al. [103] enhanced the wear resistance of a copper–beryllium alloy by applying an electroless Ni-P coating in conjunction with heat treatment. Their research revealed that the surface hardness of the specimen increased dramatically from 340 HV without any coating to 997 HV after EP and heat treatment, reflecting a roughly threefold improvement. The average hardness of the Ni-P coating before heat treatment is measured at 527 HV. This substantial increase in coating hardness is linked to alterations in the crystal structure that occur during the heat treatment process. The treatment specifically causes the precipitation of Ni₃P intermetallic compounds, which form a very hard phase within a ductile nickel matrix [112,113]. Furthermore, the heat treatment creates a diffusion zone between the coating and the substrate, which enhances the interaction between the two, thereby further improving the wear resistance [114].

The wear coefficients for the various samples were measured as follows: the coating with heat treatment exhibited a wear coefficient of 2.04 × 10⁻⁶ mm³ N⁻¹ mm⁻¹, the coating without heat treatment recorded a coefficient of 2.43 × 10⁻⁶ mm³ N⁻¹ mm⁻¹, and the Cu-Be alloy sample had a coefficient of 3.03 × 10⁻⁶ mm³ N⁻¹ mm⁻¹. Notably, the wear coefficients for the coatings are distinctly lower than that of the copper–beryllium alloy. Additionally, the wear coefficient for the heat-treated coating is less than that of the non-heat-treated coating, confirming that heat treatment enhances wear resistance. Analysis of the wear coefficients alongside micro-hardness results revealed that samples with higher hardness displayed lower wear coefficients, corroborating observations from other studies [112,113,114,115]. Figure 14 [103] showcases optical profile measurements and FESEM images of the worn surfaces following micro-scale abrasive wear tests. The depth of the wear pits observed in the optical profiles, depicted in Figure 14a–c, indicates that the copper–beryllium alloy experienced the deepest wear, followed by the non-heat-treated coating, while the heat-treated coating showed the shallowest wear depth. This illustrates that the copper–beryllium alloy has the least wear resistance, whereas the heat-treated coating provides the best wear resistance. Figure 14d–f further details the characteristics of the worn surfaces. Micro-indentation is apparent within the crater areas, a feature typical of rolling abrasive wear, alongside grooves indicative of groove abrasive wear. This evidence confirms that the friction system operates under a mixed wear mechanism, characterized by both rolling and groove wear.

3.2. ED + LCD

The ED + LCD method entails the formation of a coating on a substrate through the application of ED. Subsequently, this coating is melted and adhered to the substrate using LCD.

Zhou et al. [104] focused on the low hardness problem of copper alloy current-carrying friction pairs and adopted LCD to prepare various types of Mo coatings on dispersion-strengthened copper. First, the researchers applied nickel ED treatment to the copper surface. This step was essential for reducing the high reflectivity of the copper alloy to infrared lasers and overcoming solubility issues between copper and molybdenum. The surface morphology of the Mo coating, prepared by optimizing process parameters, exhibited excellent quality with no detectable defects. The main components of the coating included Mo, Ni₃Mo, MoO₂, and Cu_0.81Ni_0.19. As depicted in Figure 15, all three coatings established a robust metallurgical bond with the substrate, marked by a distinct fusion line and a dense bonding structure within the coating. During the preparation process, distinct cracks developed in the middle layers of coatings A and B; however, these cracks were absent in the surface layer. This phenomenon can be attributed to unmelted particles acting as a hard phase within the coating, which may lead to cracking under internal stress. Nevertheless, when preparing the surface layer, appropriate heat input induced a remelting effect in the transition layer, effectively repairing these cracks. The unmelted particles were likely molybdenum, which inhibits grain growth and contributes to improved hardness. In contrast, coating C exhibited no noticeable defects in its middle and surface layers. This outcome was primarily due to the effective decomposition of agglomerated powder during the cladding process, resulting in optimal melting.

The hardness of the coatings exceeds that of the substrate, with the triaxial spheroidized powder coating (A) demonstrating the highest hardness, followed by the ring spheroidized powder coating (B), and finally the ring’s broken agglomerated powder coating (C). This difference in hardness is attributed to the composite coating created from plasma spheroidized powder, which features a denser distribution of dendrites. The unfused spheroidized particles function as a hardening phase within the coating, distinctly enhancing its hardness. Regarding microstructure and composition, coatings A and C exhibit a uniform distribution along the cross-section, resulting in minimal fluctuations in hardness. In contrast, coating B experiences considerable hardness variation due to an uneven distribution of hard particles. As the dilution rate increases, a greater amount of copper-rich phases form near the coating’s interface, leading to a decrease in hardness. In conclusion, the hardness of the pulverized agglomerated powder coating is notably inferior to that of the plasma spheroidized powder coating. The presence of unmelted spherical Mo particles contributes to a uniform interaction with the dendrites within the coating, and as the content of these unmelted particles rises, a positive correlation is observed between their presence and the hardness of the coating.

3.3. LCD + in Situ Synthesis

In recent years, LCD + in situ synthesis technology has emerged as a pivotal approach for surface modification of materials, offering distinct advantages such as enhanced interface cleanliness, refined microstructures, and denser coatings when compared to conventional coating methodologies [105,116,117,118,119].

Lv et al. [116] successfully fabricated a ZrB₂-ZrC reinforced copper matrix composite coating on pure copper utilizing LCD and self-propagating high-temperature synthesis (SHS) reactions. The micro-hardness distribution was examined across the cross-section of the composite coating, as depicted in Figure 16. It was found that micro-hardness gradually decreased from the surface to the copper substrate, primarily due to the gradient distribution of ceramic reinforcement within the composite. This micro-hardness distribution can be divided into three distinct regions: the reinforcement region, the transition region, and the substrate. In the RZ, micro-hardness starts at a value of 490 HV_0.2 at the surface and decreases to 305 HV_0.2 at greater depths. In the TZ, there is a marked reduction in micro-hardness, accompanied by a notable increase in micro indentation size. The substrate region exhibits the lowest micro-hardness, with a measurement of 70 HV_0.2. On average, the micro-hardness of the composite coating reaches 410 HV_0.2, which is nearly 6 times higher than that of pure copper.

The wear volume loss of the composite coating is a mere 0.015 mm³, which is distinctly less than the 0.101 mm³ observed in pure copper. This clearly demonstrates a substantial improvement in the wear resistance of the composite coating. The coating’s high hardness provides excellent resistance to plastic deformation during friction and under external loads. The robust bond between the in situ synthesized ceramics and the copper matrix enables the composite to effectively withstand most external loads during the reciprocating sliding wear process, thereby preventing notable plastic deformation of the copper matrix. Furthermore, the steady-state COF of the composite coating is notably lower at 0.4257, compared to 0.5901 for the substrate, highlighting its exceptional wear resistance. In summary, the in situ synthesis of the ZrB₂-ZrC reinforced copper matrix composite coating demonstrates impressive performance in reducing the COF and enhancing wear resistance.

3.4. Two-Step Method Summary

Compared with one-step methods, two-step coating preparation processes are more complicated. Only when the two-step process is fully applied can a coating with high wear resistance be prepared. For example, the coating prepared by EP on a copper beryllium alloy does not have high wear resistance, and the wear resistance of the coating will be greatly improved after HT [103]. The average hardness of the Ni-P coating is 527 HV before heat treatment and 997 HV after heat treatment. The increase in coating hardness is related to the change in crystal structure during heat treatment. In particular, this treatment results in the precipitation of Ni₃P intermetallic compounds, forming very hard phases in the malleable nickel matrix. In addition, the heat treatment forms a diffusion zone between the coating and the substrate, which enhances the interaction between the two, thus further improving the wear resistance. In the process of ED + LCD, the ED treatment of copper surface can reduce the high reflectivity of copper alloys to infrared laser and overcome the problems of material solubility, thus affecting the wear resistance of the coating. In addition, in the process of LCD + in situ synthesis, the materials synthesized in situ may have various strengthening effects on the coating, thus improving the wear resistance of the coating. In the study of Lv, the micro-hardness of the composite coating is increased because when the ZrB₂-ZrC reinforced copper matrix composite is synthesized in situ, nickel dendrites are coated with fine acicular ZrB₂ to form fiber reinforcement [116]. This innovative composite structure facilitates effective load transfer from the copper matrix to the reinforcement. ZrC functions similarly to particle reinforcements by hindering dislocation motion, which leads to plastic deformation of the matrix. The synergistic effect of these two strengthening mechanisms significantly increases the micro-hardness of the composite coating, thereby enhancing its wear resistance.

The cost of two-step wear-resistant coating preparation is higher than one-step methods due to the complexity. In terms of equipment cost, LCD is the most expensive [120], while ED + LCD and LCD + in situ synthesis need high-power lasers. EP equipment is simple and low-cost [121]. EP + HT has a high cost due to the use of multiple chemical reagents and a protective atmosphere; ED + LCD electroplating’s cost is low; the LCD powder material cost depends on the specific material; and LCD+ in situ synthesis is costly due to the addition of reactants or catalysts. In terms of energy cost, ED + LCD and LCD + in situ synthesis consume more energy, while EP + HT consumes less. In terms of labor cost, LCD + in situ synthesis is the highest, followed by ED + LCD, and EP + HT is lower. LCD + in situ synthesis offers excellent performance for high-demand applications. EP + HT and ED + LCD balance cost and performance, suitable for scenarios with cost and performance requirements.

4. Summary and Perspectives

At present, copper and its alloys are predominantly utilized in various sectors, including mechanical manufacturing, aerospace, electrical engineering, and electronic engineering. As industries such as aerospace, oil and gas, and automotive evolve, there has been a notable increase in the demand for copper and its alloy components that exhibit high wear resistance, stability, strength, and load-carrying capacity. These technological advances present both challenges and opportunities for the production and application of copper and its alloys. The objective is to achieve prominent improvements in surface hardness and wear resistance. Current surface technologies designed to improve the friction and wear characteristics of copper and its alloys can be categorized into one-step methods (including LCD, ED, THSP, CS, and ESD) and two-step methods (such as EP and compound methods).

This review presents an analysis of the current research regarding one-step and two-step methods for the preparation of wear-resistant coatings on copper and its alloys. The coatings prepared by all methods can improve the wear resistance of the base material to varying degrees. The friction and wear properties of copper and copper alloys with and without coatings under different friction modes are shown in

Table 6. Friction and wear properties of copper and copper alloys with and without coatings.

Friction Mode	Wear Rate Without Coating	Wear Rate with Coating	Percentage Decrease in Wear Rate	COF Without Coating	COF with Coating	Percentage Decrease in COF	Ref.
Pin disc friction	6.3 mg	1.4 mg	77.8%	-	-	-	[103]
	-	-	-	0.45	0.24~0.33	26.7~46.7%	[32]
	-	-	-	0.5	0.3	40.0%	[41]
Ball disc friction	465.28 × 10−6 mm3/N × m	272.56 × 10−6 mm3/N × m	41.4%	0.226	0.190	15.9%	[54]
	4.24 × 10−4 mm3/N × m	9.31 × 10−5 mm3/N × m	78.1%	-	-	-	[29]
	7.89~291.63 × 10−15 mm3/N × m	4.24~129.48 × 10−15 mm3/N × m	46.3~55.6%	-	-	-	[49]
Rotary reciprocating wear test	47 mg	9.3 mg	80.2%	-	-	-	[32]
Transverse sliding friction	15.6 mg	3.3 mg	78.8%	0.22	0.12	45.5%	[3]
Rolling contact fatigue friction	15.4 mg	3.0~3.9 mg	74.7~80.6%	-	0.11	-	[35]
Ball flat reciprocating friction	-	-	-	0.45	0.29	35.6%	[38]
	1.52~6.88 × 10−5 mm3/N × m	1.24~4.12 × 10−5 mm3/N × m	18.4~40.2%	0.53~0.57	0.48~0.56	9.4~15.8%	[45]
	-	-	-	0.9	0.3	66.7%	[46]
	6.2 × 10−3 mm3/N × m	1.1~2.6 × 10−3 mm3/N × m	58.1~82.3%	0.68	0.51	25.0%	[51]
	3.03 × 10−6 mm3/N × m	2.04 × 10−6 mm3/N × m	32.7%	-	-	-	[103]
	0.101 mm3	0.015 mm3	85.1%	0.5901	0.4257	27.9%	[116]

. For example, TiC-enhanced FeAl intermetallic compound coatings were prepared on the surface of pure copper by ESD, and it was found that the wear rate and COF of the coating were reduced by 41.4% and 15.9%, respectively [54]. In addition, FeCoCrAlCuNi_x (x = 0.5, 1 and 1.5) high-entropy alloy coatings were prepared by LCD on a copper surface, and it was found that the wear rate was reduced by 78.1% after the ball–disc friction test [29]. This will provide a theoretical basis for expanding its application range and increasing its service life. These advancements have substantial implications for the practical utilization of copper and its alloys. The one-step preparation process is characterized by its simplicity, requiring less complex equipment, and it offers a wider range of applications in everyday scenarios. This method effectively enhances the wear resistance of copper and its alloys at a lower cost, subsequently improving material utilization. Conversely, two-step preparation methods allow for finer corrections of specific deficiencies associated with coatings produced via the one-step method. Two-step methods are particularly beneficial in applications demanding high material standards by yielding coatings with fewer defects and superior overall performance. This approach ensures that the wear-resistant properties of copper and its alloys meet stringent requirements for various uses.

In the future, it is imperative to focus on surface strengthening technologies that enhance these materials while preserving their inherent high thermal and electrical conductivity. However, further investigation is required to elucidate the relationships among the various technologies within the two-step method. It is essential to identify the key factors that enhance the efficiency of wear-resistant coating preparations and those that facilitate the co-optimization of coating quality and wear resistance. Given that the properties of copper and its alloys may limit the application of certain surface strengthening technologies, it is crucial to consider these limitations. For instance, the high electrical conductivity of these materials can diminish the effectiveness of micro-arc oxidation, while the elevated laser reflectivity can impede the success of LCD. Future research should concentrate on surface strengthening technologies that are more suitable for copper and its alloys, as well as auxiliary methods for producing wear-resistant coatings. Additionally, there should be a focus on integrating pre- and post-treatment processes to enhance overall performance.