Effect of B₄C Content on Microstructure and Wear Resistance of Laser-Cladding-Enhanced 316 Stainless Steel Coatings

Abstract

This study investigates the effects of B₄C content (2.5, 5, 7.5, and 10 wt.%) on the microstructure and wear resistance of laser cladding 316 stainless steel coatings on a 2Cr12MoV steel substrate. The coating was prepared by laser cladding technology. The phase composition, microstructure evolution, microhardness, and tribological properties of the coating were analyzed. The results show that the decomposition of B₄C particles is complete, and the phase composition of the coating includes Austenite, Fe₂₃ (B₃C₃), Cr₂₃ (B_1.5C_4.5), and a Fe-Ni solid solution. The increase in B₄C content significantly increased the microhardness of the material from 206 HV_0.2 (substrate) to 829 HV_0.2 (10 wt.% B₄C) by 4.02 times. Wear resistance also improved, with the 10 wt.% coating exhibiting the lowest wear rate (10 × 10⁻⁸ mm³/N·m) due to fine-grained and dispersion strengthening mechanisms. However, excessive B₄C (10 wt.%) induced cracks from increased brittleness, resulting in higher friction coefficients. The wear mechanism consists of fatigue wear, adhesive wear, and oxidative wear, and the degree of wear decreases with the increase in B₄C content. This work demonstrates that the addition of B₄C effectively improves the hardness and wear resistance of 316 stainless steel coatings, providing practical insights into surface engineering in high wear applications.

1. Introduction

Surface modification technology, as a key means to improve material properties, covers many processes, such as thermal spraying [1,2], vapor deposition [3,4], electroplating [5,6], and laser cladding [7,8]. Thermal spraying is easy to peel due to mechanical bonding [9]. The coating prepared by vapor deposition has higher purity and density, but the production efficiency is low [10]. The wastewater produced by electroplating will cause environmental pollution and be limited [11], and laser cladding achieves controlled surface quality through high input energy. It is compatible with complex shape repairs and high-performance metal surface strengthening to form a laser cladding layer with metallurgical bonding, which is the ideal choice for manufacturing coating materials with small heat-affected zones, high geometric accuracy, and low dilution ratio [12]. Therefore, it is widely used in aerospace, automobile manufacturing, machinery manufacturing, and ship repairs.

316 stainless steel is an austenitic stainless steel with excellent corrosion resistance and good mechanical properties, strength, and toughness [13,14]. It has strong compatibility with laser cladding technology, is easy to manufacture complex shape parts, has good welding performance, and can form reliable connections with other materials, but has relatively poor wear resistance [15,16]. Ceramic materials (B₄C, TiC, etc.) have the characteristics of high hardness and high wear resistance, which make up for the disadvantages of 316 stainless steel with poor wear resistance [17,18]. Song et al. [19] used 316L stainless steel powder and cast WC powder to repair the surface of 304 stainless steel by laser cladding. The experimental results showed that the microhardness of 24.4 wt.% WC coating was 2.55 times that of the base material, 215.18 HV_0.5, and the wear resistance was significantly higher than that of the base material. Yu et al. [20] prepared a 316L stainless steel coating on the 45 steel surface by laser cladding technology and added 10 wt.% TiC and 10 wt.% NbC ceramic particles inside the coating. The results show that the wear mechanism of TiC and NbC ceramic particles changes from severe adhesive wear and plastic deformation to slight adhesive wear and abrasive wear. Chen et al. [21] used laser cladding h-BN-enhanced 316L stainless steel coating on Q235 steel. The coating with 5 wt.% h-BN achieved an average microhardness of 438.8 HV, which is 2.5 times higher than that of the base material. In addition, h-BN reduced the coefficient of friction when adding 5 wt.% h-BN samples to 45.16% of the wear of the substrate, so the addition of h-BN improved the wear resistance, hardness, and overall properties of the coating.

Ceramic materials have received a lot of attention because of their high hardness and wear resistance [22,23]. The microhardness and wear resistance of metal-based materials can be significantly increased by incorporating ceramic particles into the material. Therefore, adding ceramic particles to 316 stainless steel in the laser cladding process can greatly improve the microhardness and wear resistance of the coating surface. Common ceramic materials include carbides, oxides, and borides. Among them, B₄C, known as “black diamond”, has high hardness, high wear resistance, and high melting point, which can be used as the reinforcing phase in the laser cladding process to significantly improve the hardness and wear resistance of the cladding layer. Therefore, B₄C has been widely used in other coating materials. Liu et al. [24] introduced B₄C into AlCoCrFeNi high-entropy alloy and found that the microhardness of the high-entropy alloy coating increased by 89%. The synergistic effect of lattice distortion, increased dislocation density, and grain size refinement improved the wear resistance of the coating. Similarly, Guo et al. [25] conducted laser cladding of B₄C-reinforced Inconel 625 coatings on the surface of 20 steel. When the B₄C content was 10 wt.% and the grain size was 10 μm, the maximum microhardness of the coating reached approximately 567 HV0.2, which was 241.2% of that of the Inconel 625 coating. Additionally, when the B₄C content was 5 wt.% and the particle size was 60 μm, the B₄C/Inconel 625 composite coating exhibited the best corrosion resistance. Liu et al. [26] prepared titanium matrix composite (TMC) coatings reinforced with different B₄C contents on Ti6Al4V surfaces by laser cladding technique. The experimental results showed that the TMC coatings exhibited high wear resistance, in which abrasive wear was considered to be the main wear mechanism, and the main reasons for the excellent wear resistance of the TMC coatings with the addition of 7 wt.% and 9 wt.% of B₄C were the symbiotic structure of TiB/TiC, the load-transfer enhancement effect of the primary TiB and the retention of B₄C.

In this study, laser cladding technology was used to fabricate B₄C-reinforced 316 stainless steel coating on the surface of 2Cr12MoV. By incorporating various amounts of B₄C into the laser-cladded layer, significant improvements in both microhardness and wear resistance were achieved. The effects of B₄C content on the phase composition, microstructure, microhardness, and tribological behavior of the coating were investigated. This work provides practical guidance for further enhancing the surface performance of 2Cr12MoV.

2. Materials and Methods

In this experiment, 2Cr12MoV steel with dimensions of 50 mm × 30 mm × 10 mm was selected as the substrate material, and its chemical composition is shown in

Table 1. Chemical composition of 2Cr12MoV stainless steel (wt.%).

Element	C	Si	Mn	Cr	Ni	Mo	V	Fe
Component	0.16–0.18	≤1.00	≤1.00	11.50–14.00	≤0.60	0.80–1.20	≤0.30	Bal.

. The sample surfaces were cleaned with 180-mesh sandpaper to remove oxide scales, roughened, and then washed with anhydrous ethanol to eliminate surface impurities. 316 stainless steel powder was used as the coating material, with its chemical composition listed in

Table 2. Chemical Composition of 316 stainless steel (wt.%).

Element	C	Si	Mn	Mo	Ni	Cr	Fe
Component	0.06	0.92	1.51	2.55	13.23	17.24	Bal.

. The 316 stainless steel powder had a particle size of 25 μm and a purity of 99.99%, while the B₄C powder had a particle size of 0.5 μm and a purity of 99.99%. B₄C powder with contents of 2.5 wt.%, 5 wt.%, 7.5 wt.%, and 10 wt.% were separately added to the 316 stainless steel coating and uniformly mixed using a ball mill, with a ball-to-powder ratio of 4:1. After the ball milling process, the mixed powder was dried for 1 h at 120 °C under an argon space.

In the experiment, the preset powder method was used, and the powder with a height of 0.8 mm was laid on the surface of the substrate. The laser cladding process used a cross-flow CO₂ laser with a wavelength of 10.6 μm. The laser parameters were set as follows: laser power of 1400 W, scanning speed of 360 mm/min, spot diameter of 3 mm, overlap ratio of 30%, and shielding gas flow rate of 6 L/min. A laser cladding experiment was carried out at room temperature. The schematic diagram of the laser cladding process is shown in Figure 1.

After the laser cladding experiment, wire-cutting equipment was used to fabricate samples with dimensions of 10 mm × 10 mm × 10 mm, ensuring that the cut was perpendicular to the coating surface. The cross-sectional samples were then ground and polished with 180—2000 mesh sandpaper. Subsequently, the samples underwent ultrasonic cleaning and were naturally dried after thorough alcohol cleaning. The polished cross-sections were etched using aqua regia (HNO₃: HCl = 1:3) for 30 s. After etching, the cross-sections were rinsed with anhydrous ethanol to obtain metallographic samples. A scanning electron microscope (ZEISS Gemini SEM 300, Oberkochen, Germany) was used to observe the microstructure of the coating cross-section, and the elemental distribution of the coating cross-section was analyzed using an accompanying energy spectrometer (OXFORD, XPLORE30, Oxford Instruments, Oxford, UK). The phase composition of the coatings was determined by an X-ray diffractometer (XRD, TD-3500, Dandong Tongda Science & Technology, Dandong, China) at a scanning speed of 0.068°/s with Cu Kα (λ = 1.5406 μm) radiated and generated at a voltage of 40 kV and a current of 30 mA in the range of 20° to 90°. The microhardness of the coating at the polished section was measured using a Vickers microhardness tester (HXD-1000TMC/LCD, KESHIN Automation Technology, Suzhou, China) using a load of 200 gf and a 15 s duration. Three measurements were taken at the same height, and the average microhardness was calculated. In order to study the effect of B₄C content on the wear resistance of 316 stainless steel coating, the tribological behavior of 316 stainless steel coating was studied by a fatigue reciprocating wear testing machine (MGW-02, JiNan Yihua Tribology Testing Technology Laboratories, Jinan, China) at room temperature, and the wear rate was calculated. GCr15 steel balls were used for the counter grinding ball in the wear process, and the experimental parameters were set to 10 Hz frequency, 20 N load, 30 min wear time, and the experimental environment was dry friction wear at room temperature without lubrication. After the wear experiment, the wear surface and the distribution of elements were observed by SEM and EDS.

3. Results

3.1. Phase Composition Analysis of the Coating

Figure 2 shows the phase composition of coatings with different B₄C content. As can be seen from the figure, the coating phase includes Austenite, Fe₂₃(B₃C₃), Cr₂₃(B_1.5C_4.5), and the solid solution Fe-Ni phase. In addition, no diffraction peak of B₄C was detected, indicating that all the B₄C particles have been decomposed. The result obtained through calculation is shown in Figure 3. The content of B₄C did not affect the phase composition of the coating. The diffraction intensities of Fe₂₃(B₃C₃) and Cr₂₃(B_1.5C_4.5) increase with the increase in B₄C content, reaching the maximum when 10 wt.% is added. This is caused by the combination of the B and C elements produced by the decomposition of B₄C with the Fe and Cr elements in 316 stainless steel to produce carbides and borides of iron and chromium. The diffraction intensity of the Fe-Ni solid solution is decreasing continuously.

As the B₄C content increases, the diffraction peak intensity of each coating first decreases and then increases at the diffraction angle ranging from 43° to 44°. The average grain size of the (111) crystal plane was calculated by XRD and is shown in

Table 3. Average grain size of coatings with different B₄C content.

B4C Content	Diffracted Intensity	Half High Width	Grain Size
2.5 wt.%B4C	373	0.656	132
5 wt.%B4C	285	0.721	120
7.5 wt.%B4C	252	0.974	88
10 wt.%B4C	177	0.921	94

. Among them, when 2.5 wt.% B₄C is added, the diffraction intensity is the highest, the peak width at half height is the lowest, and the grain size is the largest. The grain size decreases as the content of B₄C increases. This is because the high-melting-point B₄C particles enhance the solidification rate of the coating, achieving the effect of fine-grain strengthening.

3.2. Microstructure Analysis of the Coating

Figure 4 shows the microstructure of coating cross sections with different B₄C content. It can be seen from the figure that the top structure of the coating is mainly composed of equiaxial crystals, the middle structure is composed of larger equiaxial crystals relative to the top, and the bottom structure is mainly composed of columnar crystals and columnar dendrites. When the content of B₄C reached 10 wt.%, a needle structure appeared on the top of the coating. In the bond between the coating and the substrate, there is a bright ‘white layer’, which is caused by the growth of flat crystals in the micro-melting zone. It is the characteristic of forming a good metallurgical bond between coating and substrate [27,28].

Figure 5 shows the distribution of elements in the middle of coatings with different B₄C content. It can be seen that Fe and Ni elements are mainly enriched in the grain, and the XRD results show that the grain is mainly composed of Fe-Ni solid solution. There are mainly Cr elements, C elements, B elements, and Fe elements at the grain boundaries. Combined with the XRD results, they show that the grain boundaries are mainly composed of Fe₂₃(B₃C₃), Cr₂₃(B_1.5C_4.5), and Austenite. By comparing the distribution of Fe elements with different contents, it can be seen that with the increase in B₄C content, Fe elements gradually diffuse from the inner grain to the grain boundary. In addition, lamellar structures were observed in the grain boundary region. It is speculated that due to the enrichment of C and B elements at the grain boundary in the late solidification period, the remaining liquid may undergo eutectic reaction and form a lamellae structure with austenite as the matrix and hard carbides or borides.

EDS analysis was performed on the top structure of the coating with 10 wt.% B₄C content added, as shown in Figure 6a. It can also be seen from Figure 6b that there is a continuous network structure beside the acicular structure. The EDS results show that the distribution of each element is relatively uniform. These needle-like structures may be martensitic phase transitions induced by rapid cooling. Although 316 stainless steel is an austenitic stable phase under normal conditions, during the extremely fast cooling process of laser cladding, local areas may be transformed into high-carbon martensite, especially after the addition of B₄C particles. B₄C decomposition releases more C elements and forms high-carbon martensite. The continuous network structure may be intergranular precipitates. Combined with the XRD results, we analyzed that the B and C elements released after the decomposition of B₄C will form Fe₂₃(B₃C₃) and Cr₂₃(B_1.5C_4.5) with Cr and Fe elements in the matrix.

3.3. Microhardness Analysis of Coatings

Figure 7 shows the microhardness distribution of 316 stainless steel coatings with different B₄C content. As seen in the figure, the microhardness distribution can be divided into three regions, which correspond to the cladding layer, the heat-affected zone, and the substrate. The microhardness of the cladding layer is significantly higher than that of the substrate. The maximum microhardness of the cladding layer can reach 829 HV_0.2, which is about 4.02 times that of the matrix (206 HV_0.2). As the content of B₄C increases, the microhardness of the coating shows a gradual increase. This is because the addition of B₄C increases the solidification rate of the coating, thereby achieving the effect of fine-grain strengthening. B₄C and Cr elements are combined into borides and carbides, which are dispersed in the coating structure and play a role in dispersion strengthening.

3.4. Tribological Analysis of Coatings

The friction coefficients of coatings with different B₄C content are shown in Figure 8. The wear stages include the initial wear stage and the stable wear stage. When dry friction is first applied to the surface, the actual contact area is small and the number of contact points is small due to the roughness of the coating surface, and the friction factor is unstable, which leads to a faster wear rate of the coating surface. With the wear process continues, the convex points of the wear surface are gradually worn away, the surface roughness is reduced, the contact points and contact surfaces are increased, the surface wear gradually becomes stable, and the wear speed of the coating is also reduced.

The average friction coefficient of the coatings with different B₄C content in the stable wear stage is shown in Figure 9. The average friction coefficients with the addition of 2.5 wt.%, 5 wt.%, 7.5 wt.% and 10 wt.% B₄C coatings were 0.44, 0.31, 0.28, and 0.42, respectively. When 2.5 wt.%, 5 wt.% and 7.5 wt.% of B₄C were added, the average friction coefficient of the coating was reduced, but the average friction coefficient of the coating was increased when 10 wt.% of B₄C was added. The friction coefficient showed a trend of first rising and then decreasing.

The wear rates of coatings with different B₄C contents calculated according to Formula (1) are shown in Figure 10. The calculation formula of the wear rate is shown in Formula (1). With the increase in B₄C content, the wear rate of the coating surface showed a decreasing trend. This is proportional to the microhardness of the coating. According to Formula (2) [29], the microhardness of the coating is proportional to the wear resistance, so the coating with 10 wt.% B₄C has good wear resistance. The wear rate of the coating is reduced to 10 × 10⁻⁸ mm³/N·m.

$$\omega ={\displaystyle \frac{\mathrm{v}}{F·\mathrm{S}}}$$

(1)

where ω is the wear rate (mm³/N·m), v is the wear volume loss (mm³), F is the normal load applied (N), and S is the sliding distance (m).

$$V=K{\displaystyle \frac{PL}{H_V}}$$

(2)

where V is the wear amount, K is the wear coefficient, P is the positive load, L is the sliding distance, and Hv is the value reflecting the overall hardness. According to the calculation results of the above formula, the increase in hardness reduces the penetration depth of the counter grinding ball on the coating surface, weakens the plow behavior of the grinding ball on the coating surface, and thus reduces the wear of the grinding ball on the coating surface. In addition, Fe₂₃(B₃C₃) and Cr₂₃(B_1.5C_4.5) phases are dispersed at the grain boundaries, and play a nailing effect as the reinforcing phase, resisting the coating peeling caused by the adhesion point, and preventing the abrasive particles penetrating into the surface of the coating from plowing the coating tissue during the friction process, preventing the plastic deformation of the coating, and thus improving the wear resistance of the coating.

The wear surfaces of coatings with different B₄C content are shown in Figure 11. There are spalling pits on the worn surface, which is a typical characteristic of fatigue wear. With the increase in B₄C content, the area and number of spalling pits are gradually reduced, which indicates that the addition of B₄C reduces the fatigue wear of the coating. It is observed that there are adhesive wear chips on the wear surface, which is due to the “cold welding” phenomenon of wear chips on the wear surface during the wear process, which makes the wear chips adhere to the wear surface of the coating. Therefore, the wear mechanism of the coating also includes adhesive wear. In addition, when 10 wt.% B₄C is added, and cracks on the wear surface are observed, which is due to the high microhardness of the coating. During the wear process, the normal force exerted on the grinding ball on the wear surface produces local high pressure, which causes the stress concentration on the wear surface and leads to brittle fracture. The appearance of cracks increases the roughness of the worn surface, which is also the reason why the friction coefficient increases after adding 10 wt.% B₄C.

Figure 10. Wear rate of coatings with different B₄C content.

Figure 10. Wear rate of coatings with different B₄C content.

The element distribution of the wear surface of the coating with different B₄C content is shown in Figure 12. The distribution area of Cr, Ni, and Mo elements is opposite to that of O elements, so Cr, Ni, and Mo elements are not oxidized during wear. At the same time, the Fe element is evenly distributed, so the main component of the grinding chips is iron oxide. In addition, with the gradual increase in B₄C content, the residual debris on the wear surface is gradually reduced. In particular, when 10 wt.% B₄C was added, and no debris was observed on the wear surface. Therefore, it can be concluded that the addition of B₄C reduces the degree of oxidative wear and adhesive wear.

4. Conclusions

The B₄C-reinforced 316 stainless steel coating was successfully prepared on the surface of 2Cr12MoV by laser cladding technology. The effects of B₄C content on phase composition, microstructure, microhardness, and wear resistance of 316 stainless steel coating were evaluated, and the wear mechanism of the coating was analyzed. The phase composition of the coating includes Austenite, Fe₂₃(B₃C₃), Cr₂₃(B_1.5C_4.5), and the solid solution Fe-Ni phase, and the B₄C particles have been completely decomposed in the laser cladding process. The top of the coating is fine equiaxed crystal, the middle equiaxed crystal is thicker, and the bottom is dominated by columnar crystal and columnar dendrite. The addition of B₄C particles refines the coating grains. The increase in content of B₄C makes the microhardness and wear resistance of the coating increase continuously. With the increase in B₄C content, the microhardness and wear resistance of the coating continue to increase. When the B₄C content reaches 10 wt.%, the coating hardness and wear resistance reach the optimal values, which are 829 Hv_0.2 and 10 × 10⁻⁸ mm³/N·m, respectively. The wear mechanism of the coating mainly includes fatigue wear, adhesive wear, and oxidation wear, and the degree of these three wear mechanisms decreases with the increase in B₄C content.