The Influence of Boron Carbide on the Mechanical Properties and Bonding Strength of B₄C/Nickel 63 Coatings of Brake Disc

Abstract

Metal-based ceramic composite laser cladding offers substantial compensations in enhancing brake disc surface characteristics. Laser cladding was utilized to combine B₄C powder (10–40%) with Ni 63 powder to make Boron Carbide (B₄C)/Nickel 63 composite coatings. For the subsequent experiments, the specimens were ground and polished. Bonding strength, fracture toughness, and residual stress were examined with the B₄C content. The fracture morphologies were checked using a scanning electron microscope (SEM). It was observed that the bonding strength of various coatings might approach 175 MPa. Best bonding was observed when the B₄C level was between 15% and 30%. The porousness of the coating continuously raised as B₄C content increased. The coating’s maximum permeability was 5.6% after the B₄C level reached 30%. As the B₄C level in the coating grew, the coating’s compression resistance decreased. The bonding strength was within desirable limits, and compression resistance was consistently strong. The material bending strength increased when the B₄C materials were reduced below 35%; at this level, the bending strength was highest. The bending strength was covered by the optimal range of bonding strength. Good bonding strength and mechanical characteristics were achieved when B₄C content was 20% to 30%. The 20% B₄C coating had the smoothest fracture morphologies and the strongest bonding strength, making it the most stable. For the estimation of total matrix deformation and corresponding coating stress on coated brake discs, Ansys software was utilized to create a static structural model.

1. Introduction

Discs and pads are perhaps the indispensable components in vehicular braking. The braking system’s brittleness releases heat energy that had previously been stored in the vehicle’s kinetic energy [1]. Research on the thermomechanical properties of brake discs has been extensive. Mechanical stress can be examined theoretically, but there is no experimental approach. Modeling thermal stresses and heat transfer in materials with temperature-dependent thermophysical properties is a cumbersome problem [2,3,4]. Reinforcing metals or polymers with nano additives produces nanocomposites with enhanced properties [5,6,7,8,9]. The enhanced mechanical and tribological properties of these nanocomposites make them an excellent choice for different engineering applications [10,11,12]. Ni/B₄C can be added to Ni-based alloy coatings to increase tribological properties. Laser cladding has been utilized to augment the matrix’s properties in a number of different ways [13,14]. Simultaneously, accumulating strengthening particles such as Titanium Carbide and Boron Carbide to metal-based coatings can improve the coating’s mechanical qualities [15]. B₄C was a popular choice for laser cladding because of its high hardness, excellent wear resistance, and good wet ability [16]. As with any composite coating, wear resistance was reliant on the amount of the B₄C reinforced particles in it. The microstructure, microhardness, and wear resistance of the Ni/B₄C-CO coating were all improved [17,18]. Many factors affect the wear resistance of a laser-cladding composite coating, including whether B₄C particles are distributed. To improve the microstructure and wear behavior, laser cladding used in 4Cr14Ni14NW2Mo heat-resistant steel was studied and investigated [19]. According to the results, a thin cladding layer of Ni21 + 20%B₄C + 0.5%CeO₂ had the greatest wear properties at high temperatures (100, 200, 300, and 400 °C). As the amount of B₄C in the coating increased, the microstructure grew more complex [20,21]. Grain boundaries were altered using B₄C, which affected corrosion resistance and the mechanisms involved in the process. Grain refining and microhardness enhancement are facilitated by laser-irradiation increased carbon content. Tungsten Carbide/Ni particles significantly increased the microhardness and wear resistance of Ni-based alloy coatings [22,23]. Adding B₄C particles to traditional nickel-based alloy coatings can improve their performance [24]. Incorporating Tungsten Carbide/Ni particles into Ni-based alloy coatings significantly improved their microhardness and wear resistance. B₄C particles can increase the efficiency of traditional nickel-based alloy coatings [25,26]. To strengthen the Nickel 63 coating corrosion resistance, directed structure Nickel 63 coatings were produced on S45C steel. Tensile properties degraded in proportion to the quantity of B₄C particles in the material [27,28]. These findings indicate that the laser-clad B₄C/Nickel 63 coating’s mechanical characteristics are inadequate for usage in mechanical parts such as brake discs. Coaxial powder feeding fibers were used to cover a QT500-7 ductile cast iron matrix with a range of B₄C/Nickel 63 coatings [28,29]. By optimizing the parameters of laser cladding, the bonding strength and mechanical properties were investigated to explore the improvement of B₄C/Nickel 63 coatings on the performance of brake discs.

2. Experimental Materials and Methods

2.1. Preparation of Materials and Samples

The cast iron matrix can be served as the foundation for this investigation. Bonding strength and fracture toughness are measured using a 10 mm × 10 mm × 5 mm thick specimen. The compressive strength test uses a specimen with a dimension of 10 mm × 10 mm × 10 mm, and the bending strength test requires a 50 mm × 5 mm × 5 mm specimen.

Table 1. Chemical properties of ductile cast iron.

Composition	Carbon	Silicon	Manganese	Phosphorous	Sulfur	Iron
Wt (%)	3.58	2.70	0.32	0.31	0.010	Balance

and

Table 2. Chemical properties of Ni63 powder.

Composition	Carbon	Silicon	Boron	Chromium	Iron	Nickel
Wt (%)	0.6–1.2	3.4–5.2	3.2–4.3	15–21	<16	Balance

list the chemical composition of ductile cast iron.

Table 3. Mechanical characteristics of ductile cast iron.

Yield Strength	Percentage of Elongation (%)	Tensile Strength (MPa)	Hardness (HB)
300	8	520	180–200

lists the mechanical properties of ductile cast iron. The laser cladding material was B₄C/Nickel 63 composite powder, with B₄C powder mass fractions of 10, 15, 20, 25, 30, 35, and 40%. B₄C and Nickel 63 powders with particle sizes of 50–150 μm and 75–100 µm, correspondingly, were utilized in the experiment.

2.2. Analysis Methods

This research used a 2000 W, 1.08 µm wavelength, 500 µm fiber diameter, and a 2 mm spot diameter C2000X fiber laser (PRECITEC, Gaggenau, Germany). A six-axis KUKA robot was used to move the laser cladding head into the desired location. The microscratch technique was employed to evaluate the bonding strength of the coatings on the MFT-4000 material surface behavior tester. At a loading rate of 100 Newton per minute, the diamond indenter produced a scratch rate of 5 µm/min. A 4XC metallographic microscope (Laizhou Huayin Testing Instrument Co., Ltd., Laizhou, China). was used to examine and analyze the coating’s microscratch morphology and porosity, selecting six fields and computing the average value based on the gray values evaluated porosity. The fracture toughness of the coating was determined using a Vickers microhardness tester (Wilson Instruments, Carthage, MO, USA). with a load of 100 g and a loading duration of 15 seconds. The residual stress was measured using an X-ray machine. A universal testing machine (CMT5504 electronic universal tester, MTS, Shenzhen, China) was utilized to perform the compression and bending tests.

The compression test used a 100 kN force at a loading speed of 0.03 mm/min, whereas the bending test used a 2 mm/min loading speed. A scanning electron microscope (Merlin Compact, ZEISS, Oberkochen, Germany) using field emission scanning was utilized to examine the coatings for bend fracture morphology.

Table 4. Optimization process parameters.

Process Parameter	Unit	Value
Laser power	W	1300
Scanning speed of laser	m/s	0.004
Feed rate of Powder	g/min	24
Spot diameter	mm	3
Flow rate of Carrier gas	L/min	10
Flow rate of Shielding gas	L/min	9

shows the optimal process parameters observed by the orthogonal test on the coated surface. Figure 1 depicts the cladding process. The coatings containing B₄C content of 10, 15, 20, 25, 30, 35, and 40% had been designated as specimens 1, 2, 3, 4, 5, 6, and 7, respectively.

Table 5. The man/hour and cost of the sample.

Specimen	Cost (INR)	Man/h
0	93,720	1.12
1	118,718	1.30
2	125,592	1.26
3	132,393	1.28
4	139,350	1.27
5	146,224	1.23
6	152,773	1.25
7	159,982	1.26

shows the costs and worker hours. The cost of coating samples can be observed to be approximately one to two times the cost of the matrix [30]. When it comes to coating and matrix worker hours, they are incredibly close. As a result, the research is helpful for practical production.

3. Results and Discussion

3.1. Coated Brake Disc Static Structural Simulation

A static structural model was developed by Ansys software to estimate total matrix deformation and equivalent coating stress on coated brake discs, as seen in Figure 2 and Figure 3. The brake disc encounters the most deformation in a circular pattern from inside, outside, or near the friction surface. A fine mesh is carried out, and boundary conditions are set at an angular velocity of 50 rad/sec and hydraulic pressure of 1 MPa. At the coating-friction surface contact, there is substantial distortion [31]. Friction surface deformation and total deformation are more prominent outside the coating, making it more prone to buckling.

The substance inside the disc reduces tension and subsequently decreases it as it moves outward. The innermost layer of the covering has the most stress, and the corresponding abrasion pressure in a coating is smaller than that of friction in a matrix. Since the cladding is so effective, it is all the more remarkable.

Bonding strength analysis

The adhesion strength of the brake disc has a considerable impact on its overall performance. The binding intensity between the coating and the matrix determines a product’s overall performance. Figure 4 indicates the acoustic and friction force indications of various B₄C contents in B₄C/Nickel 63 coatings in microscratch investigations. B₄C content of 5% results in relatively smooth and stable acoustic signal and friction force curves up to 42.82 N, with no perceptible oscillations until the point at which they cross (Figure 4a). When normal force attains 43.6 N, significant variations are seen in both the acoustic and friction force signals. The above experiment results can be used to estimate the critical load of additional B₄C/Nickel 63 coatings. The coating’s adhesion strength was assessed using the given equation.

τ = H/[(πR² H − Wc)/Wc]^0.5

(1)

where,

H = Brinell Hardness of the Substrate;

R = Radius of the indenter (mm);

Wc = Critical Load(N).

There are B₄C/Nickel 63 coatings with varied bonding strengths shown in Figure 5. The coating’s adhesion strength enhances at first and then diminishes with the addition of B₄C. As bonding strength improves, so does durability. When the B₄C content is 20%, the strongest connection is established. The coating–matrix interface is fractured, and Ni (Fe) planar crystals and dendrites promote the formation of a strong metallurgical bond. B₄C/Nickel 63 coatings have bonding strengths exceeding 175 MPa, the coating–matrix interface is fragile, and -Ni (Fe) planar crystals and dendrites promote the formation of a strong metallurgical bond. Various B₄C/Nickel 63 coatings have different microscratch morphologies, as seen in Figure 6. Plastic failure is observed surrounding microscratches in the images of Figure 6a, while microscratches of the same size appear in the images of the microscratches in Figure 6b,c. When the tester’s maximum capacity has been reached, no cracks are formed in the coating, indicating that the bonding force measured using acoustic signals and friction force signals is accurate and trustworthy. The coating–matrix bonding pressure was so tremendous that the indenter could not break any coatings. The surface layer’s coating porosity constantly increases as the B₄C content rises. Permeability of the coating increases by 5.6% once the B₄C content in the formula reaches 30%. Increasing the number of hard particles may cause the coating to develop pores and cracks, which will negatively impact the coating’s quality and performance. Cracks propagated primarily along grain boundaries during the propagation process. Based on a performance guarantee, therefore, the content of B₄C should be practically controlled.

3.2. Analysis of Residual Stress and Fracture Toughness

Boron Carbide/Nickel 63 coatings have different residual stresses, as seen in Figure 7. Compressive residual stress is formed on the coated surface, whereas modest residual shear stress does not appear to have a significant effect. Laser cladding was used to create the coating because of the higher number of unmelted B₄C particles on its surface. Constraints from the outside are minimal; hence there is a significant amount of stress relief. Since the pores enhance the release of surrounding residual stress, the coated surface is placed under compressive stress. As the B₄C proportion grows, the residual compressive stress rises at first, then reduces. Residual stress is at its highest when the B₄C content is 30% or more. As the B₄C content rises above 30%, residual stress diminishes. Residual compressive stress can increase nanoscale defect shrinking or closure in corrosion-resistant coatings. It is also true that as B₄C content rises, so do stresses such as quenching and residual compressive stress. At 35% B₄C concentration, the energy density required for laser cladding increases significantly. Porosity, compactness, and stress release are enhanced by a rise in the number of unmelted particles. As a result, the compressive stress level begins to fall. Other faults, such as cracks, may be a contributing factor. The fracture toughness was calculated by using the equation below.

K_IC = 0.113HD^0.5/(1 + C_L/2D)^1.5

(2)

where,

H = Rockwell hardness of the coating (HRC);

D = Diagonal length of the Vickers indentation (mm);

C_L = Average length of the crack at the corners of the four diagonals (mm).

The fracture toughness of different Boron carbide/Ni63 coatings with various compositions were given in Figure 8.

As the Boron Carbide particle content increases, so does the fracture toughness, as seen in the graph. The bending of cracks caused by B₄C elements through crack formation in coating increases the material’s fracture toughness. Crack deflection in B₄C/Nickel 63 coatings can considerably increase the path span of crack dispersion, resulting in a significant increase in fracture toughness.

3.3. Strength Analysis

Figure 9 depicts the relationship between compressive stress and strain of ductile iron and a variety of B₄C/Nickel 63 coatings. When the specimens were put through the compression test, they were already in the elastic deformation stage. Compressive strength surpasses 800 MPa for all specimens, even as the applied force is increased over time. The elastic modulus can be estimated from the slant of the stress–strain curve. Figure 9 displays that the 10 % Tungsten Carbide coating has a steeper average slope than the matrix. The more complicated material is, the greater its stiffness and the less efficiently it may bend; it has an elastic modulus. Expandable modulus is a metric used to gauge an object’s confrontation with deformation under the influence of elastic forces.

Consequently, the option coated with 10% B₄C seems to have the most excellent elastic deformation resistance, whereas the substrate has the least elastic deformation resistance among the three options. Figure 10 depicts QT500-7 ductile cast iron compression ratios and various B₄C/Nickel 63 coatings. Comparing B₄C and Co, there is a variation in microstress. The compression ratio of Variant B₄C/Nickel 63 coatings is roughly 35%. Because of its increased compressive strength, the matrix has a compression ratio that is more than 40% higher than the coating. The coating’s compression resistance is finest when the B₄C concentration is 10%. Increasing the B₄C content causes a modest but steady increase in compression rate, indicating that a high concentration of B₄C particles causes the material to become more fragile and has a lower compression resistance. Equation (3) was used to determine the material’s bending strength.

$$\sigma =\frac{3FL}{2B{h}^2}$$

(3)

where,

F = Applied force (N);

L = measuring span (mm);

B = sample width (mm);

h = sample height (mm).

Each of the QT500-7-B₄C/Nickel 63-coated samples is illustrated in Figure 11 and Figure 12, respectively, to show how they perform under various loading conditions. The bending strength improves when the B₄C level is less than 35%. When the B₄C concentration is raised to 35%, the bending strength is enhanced, but the brittleness is lowered. The 25% B₄C/Nickel 63 coating, which is 38.74% stronger than the matrix alone, is the strongest of them all in terms of bending strength [32]. According to the requirements, this coating has the lowest bend strength, which lowers by 14.75% compared with an uncoated matrix. Plastic deformation, in which curves have a rapidly decreasing slope, occurs when a material’s strain tolerance is met. A steady reduction in rigidity caused the bending fracture. Load instability and jitter arise in the curve as a result of load mutation points induced by cracks during the bending process. With a quick change in load, the substrate moves more than the covering, suggesting that a more considerable bend causes the cracks. An excessive amount of power is required to break the coating, which is more susceptible to cracking under bending loads. Following the findings of the bending tests, an examination of matrix fracture morphologies and coating–matrix interaction regions is carried out [33]. An example of matrix fracture patterns is shown in Figure 13a, which is beneath 200 microns. Graphite is the substance that gives the spheres their dark color. More graphite is found in the grey spot than in the silver-gray area, which is more evenly distributed. As shown in Figure 13b, at 500, fracture morphology is depicted. Dimples around graphite spheres and matrix in the grey area are generated by the microporous aggregation ductile fracture, which is characterized by large dimples. Fracture morphologies are depicted in Figure 13c at a distance of 1000. A river pattern in a silver-gray area represents the brittle curve.

Figure 14 shows the bonding fracture structure between coating and matrix (500 microns). Figure 14a depicts the covering and matrix fracture morphologies. The coating has no prominent fibers, shear lip areas, or dimples in terms of fracture morphologies. Fractures are shown in Figure 14b with some surface plastic deformation. AB₄C level of 15% is shown in Figure 14c, which depicts the rock-patterned structure. A boost in B₄C concentration weakens the bonding area. Coating and matrix are shown to be joined together by a significant crack (Figure 14e). The brittleness of coatings has dramatically changed. The bonding region between coating and matrix becomes more brittle as the number of B₄C hard particles increases, making large cracks and numerous cracks more likely. Figure 14f shows that the coating–matrix interface is fragile and prone to spalling. Brittle spalling occurs where a big crack extends, minor cracks split around the large crack, and the coated area breaks due to the B₄C’s brittleness. Figure 14g shows a significant amount of fragmentation. There is more unmelted B₄C generated when the coating and matrix have less metallurgical bonding. Different B₄C/Nickel 63-coating fracture morphologies are shown in Figure 15. Fracture morphology is immaculate in the coating with 5% B₄C, as depicted in Figure 15a, which is representative of brittle fractures. Brittle fragments were seen in the coating with 10% B₄C, which were arranged in a step-like pattern. A three-dimensional grain effect is generated when the B₄C concentration in the fracture structure is 15%. The coating in Figure 15d is neat and has good bonding strength. The brittleness of the coating, which has rock-patterned fracture morphology and a 25–30% B₄C concentration, increases as the B₄C particles rise in size. Grain boundaries are shown in Figure 15g with significant cracks, suggesting the presence of several carbides. A large crack appears on the surface of the fracture in Figure 15g, indicating that a large number of carbides gather at the grain boundary. There is a decrease in the strength of the metal-to-metal bond, which causes cracks.

4. Conclusions

The bonding strength of several B₄C/Nickel 63 coatings surpasses 175 MPa, and the coating is modified both constructively and adversely as the B₄C level rises. The concentration of B₄C should be 20% for the tremendous benefits. The optimal bonding strength is achieved when the B₄C concentration is 15% to 30%. The residual stress of a 30% B₄C coating may be clearly seen, and strength to fracture has been rising. There is a slight lag in the rise of compression resistance. High compressive strength is also achieved. The bending strength increases rapidly and remains constant when the B₄C content is 15–25%. The 20% B₄C coating offers the smoothest fracture morphology and the strongest bonding strength, which is consistent. An important practical application is uncovered as a result of the research. There is a wide range of applications for metallic and ceramic coatings. Brake disc efficiency and performance will be improved using these two processes in tandem. In the future, laser additive fabrication will perform on pure ceramic brake disc coatings.