Enhancement of Tribological Properties of Cubic and Hexagonal Boron Nitride Nanoparticles Impregnated on Bearing Steel via Vacuum Heat Treatment Method

Abstract

In the current world of coatings and nanomaterials, specifically bearings, zinc, chromium, nickel, diamond-like coatings, and molybdenum disulfide are being used, to name but a few. Boron nitride in various forms has been used to enhance the surface properties, such as hardness, wear resistance, and corrosion resistance of dies, tools, etc. In this paper, a significant focus is being given to the improvement of the surface properties of bearing-steel materials by the impregnation of cubic and hexagonal boron nitride nanoparticles. The vacuum heat treatment method is used for treating the sample pins of material equivalents to EN31. In the design of the experiments, the Taguchi method with L₂₇ orthogonal array is used for the optimization of various parameters, such as the weight % of c-BN and h-BN nanoparticles and the temperature of the vacuum treatment. With the help of preliminary experimentation, the three levels of three parameters are decided. The microhardness analysis shows an improvement from 321 HV0.1 to 766 HV0.1 for a 50 µm case depth of nanoparticle impregnation. The evaluation of the influence of selected factors is also performed using ANOVA and the S/N ratio, and it was revealed that hex boron nitride (h-BN) affects the microhardness value more than the other two factors. The friction and wear testing reveal that the wear properties are improved by approximately 1.6 times, and the frictional force also decreases by approx. 1.4 times. Scanning electron microscope (SEM) analysis shows that the nanoparticles are penetrated by 21.09% and 46.99% atomic weight. In addition, a reduction in the friction coefficient and better wear response were achieved as a result of the heat treatment with nanoparticle impregnation.

1. Introduction

Many researchers are working in the area of surface engineering. A roller bearing is a mechanical part that aids in the movement and mobilization of machinery by supporting rotating shafts. Using rolling bearings, metal components reduce friction and improve adaptability to radial, axial, and thrust stresses. From heavy machinery and equipment to power generation, industry, and aerospace, roller bearings are utilized in a variety of applications. Although inexpensive as a component, their bearing failure [1,2,3] causes hundreds of hours of downtime in various applications.

The life of bearings [1,4,5] can be improved by improving fit and tolerances, material properties, lubrication [3,6,7], etc. The successful option of changing bearing material properties is selected for the current proposal.

To reduce friction and wear, resist chemical deterioration, and increase material durability in challenging environments, surface engineering with thin-layer coatings is an efficient technique. To meet the rising technological demands, it is possible to customize unique physical, mechanical, and tribological qualities by choosing suitable materials and coating designs. Oxidation must be avoided by covering the bearing surface with a fluid film or a wear-resistant coating.

Currently, SAE 52100 steel is being used for roller bearings, which is a high carbon and low alloy steel [4,5,6]. The improvement in bearing steel surface protection is the focus of many researchers [8,9]. Coatings are becoming more popular in bearing applications.

Various materials used in the bearing industry as a coating [8] or externally doped material are black oxide (BO) [9], manganese phosphate (MnPh), zinc phosphate (ZnPh), zinc calcium phosphate (ZnCaPh), galvanic zinc (Zn), zinc iron (ZnFe), zinc nickel (ZnNi), chromium [8,10,11], tin (Sn), diamond-like carbon (DLC) [12,13,14] coatings, and tungsten/molybdenum disulfide (WS₂/MoS₂) [1,15,16].

It was thought that nanoparticles [12,13,14,17] could improve bearing surface properties. The availability, usefulness, cost, feasibility of nanoparticle impregnation, and process must be decided. The various nanoparticles currently used are graphene [15,18,19], titanium dioxide (TiO₂), titanium carbide (TiC), MoS₂, and ZrO₂; nanoparticles are used either as a coating [20] material, or c-BN, h-BN, copper oxide (CuO) nanoparticles are used to reduce friction [21] and wear [22] in either oil or grease of the bearing.

The c-BN [23,24] and h-BN nanoparticles [25,26,27] are used as foreign materials in the present investigation. The reason for selecting these as foreign particles for doping is that, in the periodic table, boron and nitrogen sit next to carbon. Boron and nitrogen have the same number of outer shell electrons, and their atomic radii are similar to carbon.

• Cubic boron nitride(c-BN): only diamond is harder than C-BN, the second-hardest substance known to man. It has qualities including exceptional wear resistance, high thermal conductivity, and chemical inertness. Due to these properties, it has been used in various coatings and machine tools [28].
• Hex boron nitride(h-BN) has excellent lubricating properties due to solid covalent connections holding boron and nitrogen atoms together within each layer. In contrast, weak van der Waals forces hold the layers together. Paints, cosmetics, pencil lead, dental cement, and insulators in the high-temperature furnace are all lubricated with boron nitride in its hexagonal form.

Due to this, it is thought that the surface properties of bearing steel related to wear resistance could be improved by using c-BN and h-BN nanoparticles [5,20,21,23,24,29].

The parent material used was EN31, which is a material equivalent to SAE52100. The purpose was to find the process for impregnating the nanoparticles with the parent material. Depending on the requirement, various coating processes used are ion plating, reactive sputtering, magnetron sputtering [8,13], chemical vapor deposition (CVD) [8,30,31], thermal spraying [8,13,28,32], etc., to coat these materials. Moreover, to select parameters for this impregnation process, the author has worked in the same field using the plasma nitriding process [33]. The boron nitride nanoparticles were impregnated, but not much improvement in case depth microhardness [34] was seen. The scanning electron microscope (SEM) [24] images and EDS analysis revealed that the BN nanoparticles were impregnated by a 20.9% atomic weight.

The primary intention of this research was that the atomic restructuring and durability of the revised material should be more significant. The vacuum heat treatment [13,35,36,37,38,39] process is an appropriate option. The primary purpose is to improve the fatigue life of bearing. Due to the external foreign material impregnation, the bearing steel experiences compressive residual stress (CRS) after the heat treatment or surface improvement. This leads to enhanced tribo-mechanical properties, such as rolling contact fatigue (RCF) [1,5,29] performance. In the case of mechanical products, the h-BN has been used in heat dissipation equipment [40] in epoxy polymer-based composites [37].

An improvement in case depth microhardness, while keeping minor changes to the core hardness of the material, is expected. The impregnation of c-BN and h-BN in various percentages needs to be found. The design of experiments (DOE) [32] was carried out using the Taguchi method. It was proposed to improve the microhardness in the range of 120–150% from 321 HV to 353–480 HV. A microhardness in the range of 120–150% will ensure the substantial enhancement of friction and wear properties.

2. Materials and Characterization Methods

2.1. Making of Pins of EN31 (Material Equivalent of SAE52100)

The pins of EN 31 (the material equivalent of SAE 52100/100 Cr6) are prepared in two sizes. The first pin is of size 10 mm diameter and 32 mm length, and the second is of size 10 mm diameter and 10 mm length. In addition to polishing samples with a rotating linen disc, the diamond paste was applied on a velvet cloth. Then, samples were cooled with white kerosene.

The chemical composition of pins was tested by Spectro analysis at NDT solutions, Bhosari, Pune, as per IS 8811-2018. The details are given in

Table 1. Chemical composition of untreated pins.

Sr. No.	1	2	3	4	5	6
Parameter	C	Si	P	S	Cr	MN
Result	0.97	0.31	0.039	0.048	1.21	0.39

.

Before vacuum treatment, the pin samples were cleaned with acetone [41] and sandblasted [42] with alumina powder. The microhardness of plain pins, i.e., without treatment, is shown in

Table 2. Microhardness of untreated pins.

					Case Depth Hardness in HV0.1
					HV0.1 (50 µm)	HV0.1 (100 µm)	HV0.1 (150 µm)
Plain Pin (without treatment)					321	296	265

.

2.2. Nanoparticles Procurement

The cubic boron nitride and hex boron nitride nanoparticles in nanopowder are used in this experiment. Details are purity 99.9%, APS: 60 nm, high purity c-BN, and h-BN nanopowder. The particles were bought from Nano Shel (Chapel House, UK).

The crystal structure of h-BN comprises stacks of covalently bound sheets barely held together by weak van der Waals interactions. h-BN is utilized as a primary material in high-pressure, high-temperature super abrasives and for the thermal management of electronics. After diamond, c-BN is the second hardest material. Cutting, milling, and grinding procedures all use c-BN in abrasive applications.

2.3. Vacuum Heat Treatment Process

The vacuum treatment was conducted at Sheetal Vacuum Treat Pvt. Ltd. Bhosari, Pune, India. The vacuum treatment was conducted in a Secco Warwick (Maharashtra, India) make chamber.

The vacuum heat treat chamber is circular in shape. There are graphite rods of circular form, as seen in Figure 1b. The block diagram Figure 1a of the vacuum heat treat chamber is shown for understanding purposes. The allied instruments and the actual photo is also given in Figure 1b. During the basic cycle of vacuum heat treatment, the nitrogen gas is used for cooling/quenching during each soaking time [16].

The vacuum-treated furnace details are maximum load: 600 kg, working dimensions: W × D × H = 600 × 900 × 600 mm, maximum operating temperature: 1300 °C, installed power: 150 KW, temperature uniformity at 500–700 °C: ±5 °C, temperature uniformity over 700 °C: ±3 °C, max vacuum: 5 × 10⁻³ bar, nitrogen cooling pressure: 0–10 bar absolute. The pins underwent three cycles of vacuum heat treatment.

The Flow of the Vacuum Heat Treatment Process

The total cycle is approximately 8 to 10 h. The details related to the vacuum heat treatment process are given in

Table 3. The details of the vacuum heat treat cycle.

Sr. No.	Process	Temp	Time
1	The process of vacuum heat treatment starts	25 °C
2	Pre-heat	650 °C	1 h
3	Soaking time	650 °C	1–1.5 h
4	Temperature increase-Heating	850 °C	20 min
5	Soaking time	850 °C	1–1.5 h
6	Temperature increase-Heating	1050 °C	20 min
7	Soaking time	1050 °C	1–1.5 h
8	Temperature increase-Heating	1200 °C	20 min
9	Soaking time	1200 °C	1–1.5 h
10	Overall Cooling time		30 min
		Total time	Approx 8–10 h

.

Vacuum heat treating is hygienic, manageable, and secure. The product is distortion-free, bright, and is composed of clean parts that only need a small amount of post-treatment finishing and cleaning.

2.4. Taguchi Method

The Taguchi technique is among the best experimental approaches for determining the minimum of trials to run within the acceptable range of variables and levels. In this investigation, the Taguchi method [43,44,45] is used to study the influence of three major parameters, which are cubic boron nitride (c-BN), hex boron nitride (h-BN) weight % for impregnation, and temperature for vacuum treatment. Each parameter has been assigned three levels according to preliminary experimentation.

Table 4. Parameter selection for L27 orthogonal array Taguchi design: DOE.

Parameter	Level 1	Level 2	Level 3
c-BN (weight %)	0	20	50
h-BN (weight %)	50	80	100
Temp (°C)	960	1040	1200

lists the parameters and their levels taken for Taguchi analysis.

In this work, the L₂₇ orthogonal array experimental design was used—the total number of experiments conducted is 27, and microhardness testing l for 27 pins. The response (output) is considered for microhardness values. Once this S/N ratio was studied with the three parameters as input and microhardness as output, it was decided to observe the tribological study related to friction and wear of two-three pins on a sample basis from each group composed of A, B, and C.

2.5. The Scanning Electron Microscope (SEM) and EDS Analysis

The surface-treated samples were divided into sections, mounted on stands, and prepared for metallographic analysis with a scanning electron microscope (SEM). Energy dispersive X-ray spectroscopy (EDS) was conducted. The etched samples were studied using Zeiss FESEM (30 KeV) (model: ultra plus, serial No.4095, equipped with EDS). The sizes of the carbides were also measured in the SEM. High-resolution digital micrographs were taken randomly at different regions of the specimens.

2.6. The Friction and Wear Testing

The equipment used for friction and wear testing is pin on disc [44,46]. Tester Ducom makes TR-20LE-PHM 400.

The setup is shown in Figure 2.

The pins were tested for dry conditions. Details about the various parameters used for testing are: load: 100 N, speed: 1000 rpm, testing time: 20 min, and track diameter: 130 mm. The counter material for the disc is SAE52100. The pin size required for testing is 12 mm in diameter and 32 mm in height. Pins this size were prepared initially, considering the length needed for testing. Three readings were taken for every test, and the results were averaged.

2.7. X-ray Diffraction (XRD)

Identifying materials based on their diffraction pattern is one of the main applications of XRD analysis. The primary use of traditional X-ray diffraction is to identify major and minor single or multiple phases in the material.

Figure 3 shows the XRD analysis of the A7 pin sample. The phase inclusion behavior is studied. It reveals that ferrous nitride and ferrous boride phases are seen in the treated sample.

Figure 4 and Figure 5 are the phase inclusion of ferrous boride and ferrous nitride phases (standard JCPDS files taken only for reference). Ferrous nitride JCPDS reference file no. 22-1086. Ferrous boride JCPDS reference file no. 38-1364, 39-1314, 32-0463.

3. Results and Discussion

3.1. Taguchi Method-L27 Orthogonal Array Analysis

In this work, the L₂₇ orthogonal array has been taken to study the improvement in the hardness of fabricated pins after nanoparticle impregnation and vacuum heat treatment. The experimental configuration and experimental results are given in

Table 5. Experimental design matrix using L27 orthogonal array with response test results.

L27 Array Analysis before Testing of Vacuum Heat Treatment					Microhardness Testing after Vacuum Heat Treatment (Response)
Sr. No.	cBN (wt%)	hBN (wt%)	Temp (°C)	Symbol	HV0.1 (25 µm)	HV0.1 (50 µm)	HV0.1 (100 µm)	HV0.1 (150 µm)
1	0	50	960	C1	446	455	459	446
2	0	50	960	C2	525	478	468	425
3	0	50	960	C3	413	441	417	468
4	0	80	1040	B1	421	398	390	370
5	0	80	1040	B2	383	380	373	366
6	0	80	1040	B3	450	425	421	417
7	0	100	1200	A7	780	766	740	725
8	0	100	1200	A8	782	761	735	720
9	0	100	1200	A9	776	760	734	722
10	20	50	1040	B4	348	327	309	312
11	20	50	1040	B5	464	425	446	413
12	20	50	1040	B6	413	390	425	441
13	20	80	1200	A1	750	742	660	605
14	20	80	1200	A3	748	739	664	609
15	20	80	1200	A5	381	360	347	379
16	20	100	960	C4	446	401	417	464
17	20	100	960	C5	439	429	398	437
18	20	100	960	C6	627	667	615	649
19	50	50	1200	A2	748	739	664	609
20	50	50	1200	A4	313	343	323	343
21	50	50	1200	A6	286	356	349	325
22	50	80	960	C7	455	464	450	413
23	50	80	960	C8	405	363	380	383
24	50	80	960	C9	762	698	566	673
25	50	100	1040	B7	421	433	387	409
26	50	100	1040	B8	681	579	649	606
27	50	100	1040	B9	376	366	380	363

. The trials were conducted according to this L₂₇ Orthogonal array. The specimen pins are numbered according to temperature-based trials. For 1200 °C, pins are numbered from A1–A9, for 1040 °C, pins are numbered from B1–B9, and, for 960 °C, pins are numbered from C1–C9. The numbering is conducted in the view after testing of pins. In L₂₇ analysis, only microhardness (HV0.1 at 50 µm) is taken as a response, i.e., output for all pin samples.

The graphs are plotted for the three different temperatures of the process for microhardness value vs. case depth, as shown in Figure 6, Figure 7 and Figure 8.

There is an improvement in microhardness for all three temperatures compared to plain pin microhardness. For 1200 °C, a significant improvement from 321 HV0.1 to 766 HV0.1 is seen. Surface microhardness has improved for other temperatures, as well.

One of the research papers compares the hardness values of coatings related to B4C, h-BN, and EKabor [49]. In this research, the Inconel 625 Ni-based alloy borided with nano boron and hexagonal boron nitride powders shows a significant rise in Vickers hardness until 2100 HV0.1.

3.2. Analysis of Signal-to-Noise Ratio

In the Taguchi method [50,51,52], the experimental results are typically further translated into signal-to-noise ratios (S/N) to minimize the experimental mistakes. In this study, more significant, better parameters were chosen because more microhardness reflects better surface layer development performance, as shown in

Table 6. Experiment results for microhardness and associated S/N ratio.

c-BN	h-BN	Temp	HV0.1 (at 50 µm)	SNRA10	STDE10	MEAN10
0	50	960	446	52.0378	58.141	408.333
0	50	960	525	52.0378	58.141	408.333
0	50	960	413	52.0378	58.141	408.333
0	80	1040	421	54.5784	212.467	626.333
0	80	1040	383	54.5784	212.467	626.333
0	80	1040	425	54.5784	212.467	626.333
0	100	1200	780	53.7128	106.579	504.000
0	100	1200	782	53.7128	106.579	504.000
0	100	1200	776	53.7128	106.579	504.000
20	50	1040	348	50.9294	259.293	449.000
20	50	1040	464	50.9294	259.293	449.000
20	50	1040	413	50.9294	259.293	449.000
20	80	1200	750	53.7506	193.304	540.667
20	80	1200	748	53.7506	193.304	540.667
20	80	1200	381	53.7506	193.304	540.667
20	100	960	446	53.0476	164.646	492.667
20	100	960	439	53.0476	164.646	492.667
20	100	960	627	53.0476	164.646	492.667
50	50	1200	748	52.0378	58.141	408.333
50	50	1200	313	52.0378	58.141	408.333
50	50	1200	286	52.0378	58.141	408.333
50	80	960	455	54.5784	212.467	626.333
50	80	960	405	54.5784	212.467	626.333
50	80	960	762	54.5784	212.467	626.333
50	100	1040	421	53.7128	106.579	504.000
50	100	1040	681	53.7128	106.579	504.000
50	100	1040	376	53.7128	106.579	504.000

.

Figure 9 shows the mean S/N ratio curve [53] for microhardness. The mean S/N ratio increases with the vacuum heat treatment temperature, maxing to 1200 °C. The mean S/N ratio for microhardness is shown in Figure 9. This is due to the interstitial dissolution of the boron atoms in the γ-Fe lattice. In the γ-Fe lattice, the theoretical consideration of the boron atoms’ radius and interatomic distances affects the formation.

Additionally, it was found that the difference between the lattice characteristics of pure iron and iron that contains boron gets smaller as the temperature rises. As per S/N ratio analysis, it was observed that, the more the content of hBN by percentage, the more the microhardness is. Additionally, the c-BN rate will not affect microhardness much; the more the temperature of treatment, the more the microhardness.

3.3. Analysis of Variance

The statistical analysis of variance (ANOVA) [54] is applied to find the effect of input parameters on the response, i.e., microhardness. The response table is shown in

Table 7. Response table for signal-to-noise ratios (larger is better).

Level	cBN	hBN	Temp
1	54.40	52.04	53.54
2	53.44	53.52	52.43
3	52.58	54.86	54.45
Delta	1.83	2.83	2.01
Rank	3	1	2

.

As per this analysis, the major affecting factor for microhardness is hBN weight percentage (rank 1), then Temp (rank 2), and, lastly, c-BN weight percentage (rank 3).

3.4. Samples for SEM and EDX Analysis

The surface-treated samples were divided into sections, mounted on stands, and ready for metallographic analysis with a scanning electron microscope (SEM). The pins have shown substantial improvement in microhardness values, and their SEM images are compared with an untreated pin. Figure 10 shows SEM images of untreated pins taken from cross sections. The SEM images in Figure 10 reveal the basic structure of all constituent elements in the base material (parent material).

Figure 11 shows the EDS analysis of untreated pins. EDS displays the elemental analysis of materials for untreated pins. Carbon, iron, silicon, chromium, and oxygen are seen in the EDS analysis.

Figure 12 shows SEM images [20,24] for treated pins taken from cross sections. The diffusion zones under FeB and Fe₂B are seen in Figure 12.

The coating element boron and another diffusion element can be seen in Figure 12 from the EDS examination of the specimen.

Figure 13, an EDS analysis of treated pins, shows that the boron is impregnated in the treated pins by 21.09% and 46.99% atomic weight. Boron is the 5th element in the periodic table, having an atomic number of 10.81. Charge accumulation occurs due to the non-conduction of electrons, which is why it shows white color there. If it is conducting electrons, it shows the material’s original color. Because boron is not an electron conductor, the white color in SEM is reflected due to the presence of boron.

Table 8. The table of EDS report of a treated pin.

Element	Weight%	Atomic%
B K	21.09	46.99
C K	8.44	16.92
N K	4.41	7.58
Cr K	0.75	0.35
Fe K	65.31	28.16

of the EDS analysis of the treated pin reveals that boron is impregnated by 21.09% weight and 46.99% by atomic weight.

3.5. Friction and Wear Analysis

It is assumed that the microhardness improvement will help improve wear based on the assumption that some of the carbide, nitride, and boride elements are better at resisting stress than other materials. The pins which have shown improvement in hardness values are tested for friction and wear. The samples of pins are tested with “Pin-on-Disc Tester -TR -20 LE -PHM 400” equipment. The standards used for testing are ASTM G99 [55].

The plain pins without vacuum heat treatment are initially tested to compare with heat-treated pin performance. Two/three pins from Group A/B/C are selected to test treatment performance for friction and wear [8,32].

Figure 14 shows the coefficient of friction against various samples tested. There is improvement in treated pins as compared to plain pin performance.

Figure 15 shows the wear against various samples tested. From the above graphs, it can be concluded that the boron nitride particles, after vacuum heat treatment, give better friction and wear test results. The microhardness tested for C9 samples has shown marginal improvement compared to A8 and A9 samples, but, in friction and wear, C9 models have shown better values than A8 and A9 samples. The more significant, the better results, and the optimized combination as per S/N ratio analysis is for A7, A8, and A9 pins. This also holds concerning the friction and wear results of A8 and A9 pins.

It can be stated that this wear type is observed in samples with low surface hardness and manifests itself as parallel scratches on the surface. In coated samples, it can be mentioned that the wear mechanism is fracture type, and there are fractured areas [56].

3.6. XRD Analysis

Figure 2 illustrates a representative XRD pattern, where the peaks correspond to characteristic intensities associated with ferrous boride, nitride, and carbide phases. The patterns also show a slight shift in peak position towards lower d-spacing values with increasing boron and nitrogen content in ferrous stages. The various steps of FexNy and FexBy are revealed [48,54].

4. Conclusions

The effect of impregnation and vacuum-treated process parameters (including cubic boron nitride (c-BN) and hex boron nitride (h-BN) content for impregnation and temperature for vacuum treatment) on the microhardness of pin samples was studied using the Taguchi method based on a L27 orthogonal array. The main conclusions are as follows:

(1)

The optimal process parameters were confirmed as a c-BN percentage of 0%, a h-BN percentage of 100%, and temperature of the vacuum treatment as 1200 °C.

(2)

The most influential parameter on surface microhardness is the h-BN weight percentage.

(3)

The microhardness of surface treated pins reduces from surface to material (viz. 782-720 HV0.1 (25 µm) reduces to 464-401 HV0.1 (middle of the hook)). As the temperature of the vacuum treatment increases, it increases the microhardness.

(4)

The wear and friction tribological properties also show better performance after the modification of the surface layer by CBN and hBN nanoparticle impregnation.