Characterisation and Application of Nickel Cubic Boron Nitride Coating via Electroless Nickel Co-Deposition

Abstract

The chapter describes the characterisation and application of nickel cubic boron nitride (Ni-CBN) coatings using the electroless nickel co-deposition method. Two different types of substrates were used, that is, high-speed steel (HSS) and carbide. The characterisation of Ni-CBN coating was conducted using Field Emission Scanning Electron Microscope (FESEM) JSM-7800F coupled with Energy-Dispersive X-ray (EDX). As for the application, coated end mill cutting tools were inserted into DMU 50 CNC machine to conduct the machining testing. Cutting speed, feed rate, and depth of cut were chosen for the Taguchi L9 3-level factors. Taguchi analysis was employed to determine the optimal parameters for the Ni-CBN (HSS) surface finish. The ANOVA evaluation was used to identify the most significant effect on surface finish parameters. The FESEM images prove that the nano-CBN powders were embedded in the Ni-CBN coatings and are uniformly distributed. The findings show Ni-CBN-coated tool life is 195 minutes compared to the uncoated is 143 minutes. The surface roughness, Ra values using Ni-CBN-coated tools ranges between 0.251 and 0.787 μm, whereas the uncoated tools Ra values between 0.42 and 1.154 μm. It can be concluded that Ni-CBN HSS cutting tools reduce tool wear and extend tool life. The Taguchi optimum machining condition obtained is 1860 RPM spindle speed, 334 mm/min feed rate, and 2 mm depth of cut.

1. Introduction

Boron is a very useful element that exists in compounds such as borates. Generally, boron is a non-metallic element and can be extracted into pure crystalline boron that is black in colour and conduct electricity at higher temperature and insulator at low temperature. It is as hard as carborundum but too brittle to be used as a tool. Boron is used in medicine, agriculture, decarbonisation purposes and industrial uses.

One of the outstanding compounds of boron is Cubic Boron Nitride (CBN). CBN is a synthetic abrasive material made of Cubic Boron Nitride grains bonded in ceramic material and is commonly known as Borazon™ [1]. CBN is an allotropic crystal of boron nitride (B₄N) and has a hexagonal crystal. It is the second hardest material after diamond but more chemically and thermally stable than diamond and is extensively used in cutting tools [2]. CBN has excellent thermal stability, with oxidation starting at 1000°C and finishing around 1500°C. This is aided by the presence of boron oxide layer, which allows the use of high speed of 30.5–61 ms⁻¹ [3]. Polycrystalline cubic boron nitride (PCBN), an extended version of CBN, is developed for machining, superalloys, and high-temperature alloys. Besides having high-temperature resistance, it has a low coefficient of friction but low fracture toughness [4].

Cubic boron nitride (CBN) is very well known in many machining industries. CBN is man-made material that having a hardness that is second to diamond [5]. Since CBN has hardness after diamond, it has outstanding mechanical and thermal properties, for examples, having high temperatures strength and wear resistance. Multilayer CBN coatings represent a new deposition method that can improve adhesion on metal substrates. Even with high residual stress, this multilayer CBN structure showed outstanding adhesion in atmospheric conditions. A study found that the multilayer CBN films in comparison to monolayer CBN, has lower elastic moduli, but twice as high to their critical loads [6]. In recent years, the performance of CBN tools has been researched [7, 8].

Instead of pure CBN, composite coating of CBN-TiN also being used as machine cutting tools [9]. It is found that, this composite has outstanding CBN-to-TiN as well as the adhesion of composite coating-to-carbide substrate. The characterisation analysis indicates an evenly distributed CBN particles in TiN matrix [10].

1.1 Electroless nickel

Electroless nickel (EN) is an in-situ chemical reaction process where a metallic nickel is deposited onto a surface. This process is different from nickel electroplating that uses an applied current in the electrolytic bath which has effect on the current density, electrolyte composition, pH, bath agitation on the physicochemical and mechanical properties of the deposits [11, 12]. The main ingredients of EN are electroless bath, reducing agents, complexing agents, bath stabilisers and accelerators. Table 1 describes the function and type of each EN ingredients.

Ingredients	Functions	Types
		Pure nickel	Acid-based	Alkali-based
EN bath	Provide metallic ion sources	Ni acetate	Nickel sulfate, Nickel chloride	Nickel sulfate, Nickel chloride
Reducing agents	Reduce metallic ion into metal deposit	Hydrazine	Sodium hypophosphite, sodium borohydride, dimethylamine (DMAB)	Sodium hypophosphite, sodium borohydride, dimethylamine (DMAB), hydrazine
Complexing agents	Prevent decomposition of solutions and control reaction onto the catalytic surfaces	Tetrasodium salt, glycolic acid	Citric, lactic, glycolic, propionic acids, sodium citrate, succinic acid	Citric, lactic, glycolic, propionic acids, sodium citrate, sodium acetate, sodium pyrophosphate
Bath stabilisers	Act as inhibitors, increase deposition rate and deposit brightness	—	Thiourea, lead acetate, heavy metal salts, thioorganic compound	Thiourea, lead acetate, heavy metal salts, thioorganic compound, thallium, selenium
Catalyst	Increase the deposition speed and plating rate to be economically high	—	Sodium hydroxide, sulphuric acid	Sodium hydroxide, sulphuric acid, ammonium hydroxide

Table 1.

EN process chemicals and their functions.

Table 1 lists the three types of available EN baths, pure nickel, acid-based and alkali-based chemicals. The pure nickel bath provides pure nickel metallic deposition for semiconductor application purposes. The acid and alkali-based chemicals either produce Ni-P or Ni-B alloy deposition depending on the reducing agent used. The properties of the EN deposits strongly depend on the content of phosphorus or boron in the alloys. As seen in Table 2, the deposit structure changes because the phosphorus or boron content changes. EN bath concentration, temperature, pH, agitation, and bath loading effect the EN process [14].

EN bath type	Reducing agent	Deposit alloys	Phosphorus/Boron content (%)	Structure	Properties
Acid-based	Sodium hypophosphite	Ni-P	3–5	Crystalline	Excellent wear resistance.
			6–9	Mixed Crystalline and amorphous	Good corrosion protection and abrasion resistance.
			10–14	Amorphous	Very ductile and corrosion resistant
	Dimethylamine (DMAB)	Ni-B	0.1–4	Crystalline	High melting point of approx. 1350°C for wear application.
Alkali-based	Sodium hypophosphite	Ni-P	3–6	Crystalline	Good solderability for the electronic industry. However, lower corrosion resistance and lower adhesion to steel. Suitable for plating plastics and non-metals.
	Sodium borohydride	Ni-B	4–7	Mixed Crystalline and amorphous	Low hardness and average wear resistance.
	Dimethylamine (DMAB)	Ni-B	0.2–4	Crystalline	Hardness and superior wear resistance.

Table 2.

Summary of EN baths, reducing agents and their properties [13].

It is known that the EN process provides exceptional standardisation and impenetrable deposition even with a coating thickness of fewer than 10 μm [15]. In manufacturing, EN deposition has been widely used for it provides excellent corrosion, lubricity, ductility, wear and abrasion resistance, high hardness, and electrical properties [16].

1.2 Electroless nickel composite

When incorporated with particles or powders of different materials, EN deposition becomes an EN composite and the process is called EN co-deposition. This incorporation of particles or powders in the EN deposit has remained extensively explored. Similar to the EN deposit, there are two EN composites upon particles incorporation, either Ni-P or Ni-B, depending on the EN reducing agent used. The particles that have been studied include ceramic, polymer and metal particles. Table 3 summarises the particles that have been investigated for various applications. Incorporating ceramic particles into EN deposit produces a composite name cermet, which is the current issue discussed by using CBN particles for cutting tool applications.

Particle	Composites	Applications	References
Diamond	Ni-P-C	Cutting tool/Applied to reamers for highly abrasive applications	[17]
	Ni-B-nanodiamond	Wear & friction resistance	[18]
Silicon carbide	Ni-P-SiC	Wear resistance	[19]
Silicon oxide	Ni-P-SiO2	Corrosion resistance	[20]
Silicon nitride	Ni-P-Si3N4	Water lubricated application for corrosion and wear resistance	[21]
Boron carbide	Ni-P-B4C	Magnetic field application	[22]
Boron nitride	Ni-P-BN	Elastic–plastic behaviour	[23]
Alumina	Ni-P-Al2O3	Corrosion resistance	[24]
Cerium	Ni-P-CeO2	Corrosion resistance	[25]
Titanium oxide	Ni-P-TiO2	Surgical instrument	[26]
Iron oxide	Ni–P–Fe3O4	High-temperature oxidation application	[27]
Yttria-stabilised zirconia	Ni-P-YSZ	Cutting tool	[28]
PTFE	Ni-P-PTFE	Dry lubrication of valve for cryogenic applications	[29]
PVP	Ni-P-PVP	Corrosion resistance	[30]

Table 3.

Investigation of various particles for EN composites and their applications.

1.3 Application of Ni-CBN

The coating technology is more demanding due to the increase in productivity rates for industry consumption, especially for cutting tool purposes. It shows the growing market of cutting tools has been developed [31]. The coated tools application is becoming more important in the machining process. These tools are produced using thermal spraying processes such as physical vapour deposition (PVD) and chemical vapour deposition (CVD). Thermal spraying processes are very reliable; however, they are costly, and the high temperature causes materials properties to degrade [32].

In hard milling, the most acceptable significant representation is the cutting tool’s thermal property of the material, such as thermal conductivity. The cutting tool’s function ability can only be estimated via temperature tool measurements. For ferrous materials, cubic boron nitride (CBN) is one of the most demanding cutting tools. Multilayer CBN coatings provide a unique deposition method when applied to metal surfaces. Even under extreme conditions of high residual stress, the adhesion of this multilayer CBN structure was remarkable. Their heavy loads were twice as extraordinary compared to the monolayer CBN coatings, which had lower elastic moduli. It showed that stress relaxation significantly impacts the multilayer CBN structure [33]. This type of cutting tool is essential for cutting ferrous materials in a wide range of industries because of the advantages of suitable coating materials. Some of the most challenging materials to mill, such as aerospace alloys, die steels, and toughened steels, required the employment of CBN cutting tools [34, 35].

The diamond’s remarkable mechanical and thermal capabilities, such as strength at elevated temperatures, abrasion resistance, and hardness, are the second property that the diamond possesses. Thus, numerous sorts of research have been undertaken in the last few years on the performance of CBN tools [36, 37]. The application of CBN as a cutting substance is a suitable method that may affect production. Nonetheless, the presentation of machining, such as progression solidity, tool wear and live performance, and surface finish quality, is significantly affected by differences in high-performance machining, which commonly requires a high material removal rate (MMR) [38, 39]. However, CBN coatings’ application speeds and tool life are still lower than those of some other tools. Certain adjustments and upgrades are required, including raising the coating thickness and a rotational mechanism during the coating process. Hard coatings are typically more fragile and less lasting, whereas reinforced coatings lack strength. For real-world industrial operations, it is more critical to have coatings with a high hardness without sacrificing too much toughness.

Milling is the most common method of cutting metal. There are a variety of milling operations, but the ultimate shape and condition of the raw material dictate which ones are used. Adding features like slots or threaded holes necessitates using a milling machine. The cutting tool quality is directly proportional to the cutting process performance. In order to cut a tough workpiece materials, a harder cutting materials are needed [5]. Due to high process forces and temperatures, the first tool wear occurs in complex machining. The initial tool wear occurs in complex machining due to the high process of forces and temperatures. The machining market offers a wide variety of cutting tools, classified as coated or uncoated. Coated cutting tools typically perform better than uncoated cutting tools. Commercially available coated cutting tools include aluminium nitride (AlN), titanium nitride (TiN), titanium aluminium nitride (TiAlN), and others [28].

Due to the availability of suitable coating materials for cutting tools, this ferrous cutting material is indispensable in various industry disciplines. Certain heat-resistant CBN cutting tools are typically used on difficult-to-machine materials, such as aerospace, die steel, or hardened steel [34, 35]. CBN cutting tools have remarkable mechanical and thermal properties, including high-temperature strength, abrasion resistance, and hardness comparable to diamond. Thus, it has been demonstrated recently that CBN instruments produce excellent results in various sorts of research [37, 38]. The use of CBN as a cutting substance is a beneficial strategy that may significantly impact productivity.

CBN-based materials with bonding capabilities are frequently used to improve the machining process, which pushes researchers to continue improving coatings by utilising appropriate materials and procedures. For example, Ni-reinforced vitrified bonds are created in a high magnetic field for CBN grinding wheels. The addition of Ni does not affect the vitrified bond’s refractoriness but enhances its fluidity and bending strength [40].

Additionally, CBN composites have poor machinability characteristics, such as brittleness. One way to mitigate this difficulty is to combine CBN and graphene oxide (GO) composites with the inclusion of Al-SiC at elevated temperatures and a high-pressure sintering procedure, which results in a 27.5% increase in fracture toughness compared to monolithic CBN composites. Besides this, the composites’ bending strength increased from 564.2 MPa to 696.9 MPa [41]. Other studies discovered the use of ultrasonic probe sonication and spark plasma sintering (SPS) to investigate the microstructural, thermomechanical. Tribological properties of low-temperature sintered CBN and Ni-coated CBN reinforced bearing steel composites. It showed that these newly developed CBN and Ni-coated CBN-reinforced conducting steel composites sintered at a temperature of 100⁰ C resulted in increased wear resistance with high wear and fatigue resistance [42].

This study [28] found that an electroless nickel co-deposition technique successfully coated the HSS cutting tool with Ni/YSZ composite. In another study, TiN coated surfaces with mean thickness of 59 μm shows smooth and uniform surface demonstrating consistent surface roughness measurements. For Al/SiC metal matrix composites cutting tool, the surface roughness decreased from 1.3 μm to 0.6 μm m over time when the cutting speed is increased from 300 to 450 mm/min [43].

This study was conducted to investigate the effects of a new electroless Ni-CBN composite evenly coated onto an HSS and carbide substrate. This ceramic-metal surface coating is well-known for its superior resistance to thermal wear [44]. Additionally, the layer was produced using electroless nickel co-deposition, which is more straightforward, requires less energy, and is less expensive than typical thermal spraying procedures [45].

2. Experimental methods

The methodology consists of three sections: Process of EN coating, machining process, and cutting feasibility.

2.1 EN co-deposition

In this experiment, 50 g/l CBN powder was inserted into the bath plus the substrate. Then, suspended particles near the surface were co-deposited onto the substrate surface through the agitation process. It was found that the EN solution pH range was between pH 4.9 and pH 5.4. The bath temperature was maintained at 89 ± 20°C throughout the coating process. The coating time was kept constant at 60 min. Mechanical stirring was performed with a Jenway hot plate equipped with a magnetic stirrer, and the air bubbling was supplied at 1.2 W pressure. The entire coating process is summarised in Figure 1.

Figure 1.

Electroless nickel coating process.

The composite Ni-CBN deposition was carried out on a Carbide and HSS substrate with a dimension standard of ∅10 x 7.8 mm. Chemical etching and mechanical blasting were used to prepare the substrate sample’s surfaces. CBN powder reinforcement ceramic particles were used. CBN powders offer superior heat conductivity and increased surface integrity when it comes to hardened alloys, nickel, cobalt-based superalloys, and tool steels. Figure 2 displays the 7.8 mm diameter sample as a substrate for EN co-deposition.

Figure 2.

Substrates for EN co-deposition: Solid carbide and HSS.

2.2 EN Co-deposition on cutting tools

The end mill cutting tool using carbide and high-speed steel (HSS) with a dimension of 6 mm was used as a substrate of Ni-CBN coating as shown in Figure 3. Before EN co-deposition process done on both cutting tools, chemical etching and mechanical blasting were used to modify the surface of the substrate sample to ensure better substrate-coating bonding.

Figure 3.

End mill cutting tool.

Sensitising the HSS and Carbide cutting tool substrates is needed to activate the surfaces. Because of this, all non-proprietary solutions were produced using AR-grade chemicals and high purity deionised water. The EN co-deposition of Ni-CBN was conducted within 3 hours of the pre-treatment process, as shown in Table 4, to reduce the impacts of chemical degradation [46]. The EN chemicals produced a bright nickel deposit with a mid-phosphorous content between 6 to 9 wt.%. The optimum temperature for electroless nickel solution is at 89°C and was heated using a Jenway hotplate.

Trade name	Soaking time (min)	Temperature (°C)
Coprolite X96DP	15	60
Uniphase PHP Pre-catalyst	15	20
Uniphase PHP Catalyst	15	40
Niplast AT78	15	40
Electroless Nickel SLOTONIP	60	89

Table 4.

EN Co-deposition materials and procedure [46].

2.3 Surface coating characterisation

The composition of the Ni-CBN composite is controlled during deposition to achieve the preferred properties. It is required to obtain a high ceramic-to-metal ratio for erosion, heat, and wear resistance. The influence of process parameters to obtain a high particle ratio was analysed. The surface characterisation and elemental composition of EN co-deposition on the substrates was performed through JSM-7800F Field Emission Scanning Electron Microscope (FESEM) in conjunction with energy dispersive X-rays (EDX) shown in Figure 4.

Figure 4.

Field emission scanning electron microscope (FESEM)—JSM-7800F.

2.4 Surface roughness, tool wear and tool life

Surface roughness was measured every 0.2 mm, and each pocket had a pitch of 0.2 mm. Figure 5 shows the Mitutoyo surface roughness tester SJ-301, a tool used to test surface roughness. The tool wear was measured using the Zeiss Stemi 20,000-C Microscope Profile optical video measuring system, as shown in Figure 6. Tool life is measured by the number of cuts taken by the end mill to reach average flank wear criterion 0.3 mm. All the tools failed primarily on the plank face. For all machining conditions, the machining was stopped when the flank wear land reached about 0.3 mm to ensure that the tool life data is more reliable. The flank wear was measure using Zeiss Stemi 20,000-C Microscope Profile optical video measuring system. The effect of interaction between high cutting speed and feed rate is most significant in shorten tool life. This is claimed by J.P. Urbanski et al. found that tool life decrease drastically as cutting speed is increased because at high cutting speed high temperature will be generated, which accelerates tool wear and consequently shortens tool life [47].

Figure 5.

Mitutoyo surface roughness tester SJ-301.

Figure 6.

Zeiss Stemi 20,000-C microscope profile optical video measuring system.

2.5 Taguchi method

MINITAB 14 software was used to study the influence and range of parameters’ effect on the surface roughness of 7075 Aluminium Alloy. The experiments, based on Taguchi L9, selected spindle speed, depth of cut, and feed rate as the process variables and were conducted at three different levels. The machining parameters are listed in Table 5.

Machining parameters	Levels
	−1	0	1
Spindle speed (RPM)	1860	2650	3450
Feed Rate (mm/min)	180	257	334
Depth of cut (mm)	1	2	3

Table 5.

Level of machining cutting parameters.

Table 6 illustrates the Orthogonal Array (OA) L9 for each substrate was determined using the Taguchi method of experimental design (DOE) with three parameters at three levels. The Ni-CBN HSS coated end mill, and uncoated cutting tools were analysed via 18 tests in this study. The preferences of the end mill manufacturer determined the feed rate and depth of cut and had moved the experiment to the “high cutting speed” category [48, 49].

Experiment number	Spindle speed (rpm)	Feed rate (mm/min)	Depth of cut (mm)
1	1860	180	1
2	1860	257	2
3	1860	334	3
4	2650	180	2
5	2650	257	3
6	2650	334	1
7	3450	180	3
8	3450	257	1
9	3450	334	2

Table 6.

The OA arrangement of the machining process.

2.6 Process of machining

The DMU 50 CNC machine was utilised in the machining process. After coating the HSS end mills with CBN composite material, the Mitotuyo digital micrometre was used to measure the thickness of the cutting tool. The average thickness was determined through the three measures taken from each tooltip.

The workpiece is an aerospace material Aluminium Alloy 7075 to determine machining performance. The cutting tools were then examined for their machining capabilities. The profile was machined with 18 pockets and two cutting tools. Both coated and uncoated HSS end mills (Figure 7) were used to machine nine pockets each. Figure 8 shows the machining profile of the machine pockets with 40 mm x 35 mm dimension on the workpiece.

Figure 7.

Cutting tool image for HSS end mill cutting tool (a) uncoated; and (b) coated.

Figure 8.

Machining profile on aluminium alloy 7075 material.

3. Results and discussion

3.1 Coating surface morphology

Figures 9 and 10 shows the surface morphology using Field Emission Scanning Electron Microscope (FESEM) of the Ni-CBN coating captured at different magnifications. Both figures depict microstructure with cauliflower pattern. In Figure 9, the coating does not display micro-cracked, coarse erection and covers the entire exterior of the substrate. HSS has a high thermal shock resistance, making it resistant to sudden and rapid temperature changes [50]. In addition, HSS can withstand large temperature fluctuations.

Figure 9.

Ni-CBN microstructure on HSS substrate (a) 5000X; (b) 10,000X; and (c) 15,000X.

Figure 10.

Ni-CBN microstructure on carbide substrate (a) 5000X; (b) 10,000X; and (c) 15,000X.

Figure 10 illustrates a micro-crack on the surface layer of the carbide substrate coating due to carbide low thermal resistance. High internal stress levels can cause various problems during coating use, including premature disintegration of the part due to substrate fatigue, fracture formation in the coating, and loss of deposit adhesion [50].

Overall, both figures demonstrate rough surface of the coatings. The coating was mainly composed of ceramic CBN powders (white areas), metallic Ni matrix (grey areas), and pores (dark spots). The HSS coating surfaces generally showed a uniform distribution of the ceramic particles compared to the carbide substrate. The carbide substrate shows cracks on the coating surface due to thermal gradient. It is because the roughness of the EN-CBN coatings depends on the roughness of the substrate. It is also due to the growth mechanism of the coating, which forms columns locally perpendicular to the surface. The columns are parallel when the substrate is smooth, and the coating is even softer than the substrate [51].

3.2 Coating elemental composition

The as-deposited Ni-CBN coatings were subjected to energy dispersive X-ray analysis (EDX) to determine the composition of the co-deposited CBN elements in the EN matrix, as shown in Table 7 for HSS substrate and Table 8 for carbide substrate.

Element	B	C	N	O	P	Ni	Totals
Weight (%)	20.69	16.71	9.16	15.01	4.20	34.22	100.00

Table 7.

Elemental composition in weight % of electroless Ni-CBN coating on HSS substrate.

Element	B	C	N	O	P	Ni	Totals
Weight (%)	11.65	21.81	4.40	21.34	7.07	33.73	100.00

Table 8.

Elemental composition in weight % of electroless Ni-CBN coating on carbide substrate.

The EDX spectrum obtained for the Ni-CBN deposited on the HSS and Carbide substrate is depicted in Figures 11 and 12. It displays the peaks corresponding to the CBN, approving the standard deposition of elements in the Ni matrix. There is evidence of significant peak elements of nickel (Ni), boron (B), and phosphorous (P). This proves that metallic nickel and ceramic CBN are exist. The phosphorus element in the composite indicates as one of the most critical elements in the EN hypophosphite-based bath solution [45].

Figure 11.

EDX spectrum of as-deposited electroless Ni-CBN coating on HSS substrate.

Figure 12.

EDX Spectrum of as-deposited electroless Ni-CBN coating on carbide substrate.

3.3 Surface roughness analysis

The most critical factor in improving surface roughness analysis is the quality of the cutting tools. Table 9 compares the Ra results of machined 7075 Aluminium Alloy for coated and uncoated cutting tools. The data indicates Test 8 of HSS coated tools; high level of cutting speed and a medium level of feed rate produced a good surface finish, Ra 0.251 μm. In comparison, the combination of feed rate at high level and cutting speed at low level in Test 3 give a high surface roughness of Ra 1.22 μm. This finding demonstrates the combination of high-value feed rate and spindle obtaining a better surface finish [52, 53]. According to Mohammed [54], the interaction between cutting speed and feed rate will significantly impact the surface finish.

Surface roughness (Ra)
Test No.	Spindle speed (rpm)	Feed rate (mm/min)	Depth of cut (mm)	Coated (μm)	Uncoated (μm)
1	1860	180	1	0.576	0.695
2	1860	257	2	0.787	1.154
3	1860	334	3	0.890	1.220
4	2560	180	2	0.481	0.534
5	2560	257	3	0.301	0.586
6	2560	334	1	0.412	0.619
7	3450	180	3	0.296	0.485
8	3450	257	1	0.251	0.421
9	3450	334	2	0.527	0.729

Table 9.

Data of design experiment and surface roughness.

3.4 Tool wear and tool life analysis

Tool wear for every 0.2 mm of machining was examined using Zeiss Stemi 20,000-C Profile Optical. In accordance to ISO 8688-21:1989, the end mill cutting tool with the lowest tool wear is the best and most durable. Figure 13 shows the tool wear on the cutting tool before and after the machining process.

Figure 13.

Flank wear on HSS cutting tool on machining (test 9): (a) before; and (b) after.

Comparing the flank wear trends in Figures 14 and 15, the coated cutting end mill tool performed better in terms of both cutting time and tool life. Test 3 and Test 5 produced the most extended tool life, 195 min. Figure 15 depicts an uncoated end mill’s cutting time-based flank wear trend. The substrates performed better than the coated end mills in terms of scattering. Test 3 yielded the most extended tool life for the uncoated tools at 143 min. High-value of feed rate, spindle speed and depth of cut and cutting time will cause significant tool wear. The previous studies found that the cutting speed and feed rate interaction is significantly affecting the tool wear [52, 55, 56].

Figure 14.

Flank wear versus cutting time of uncoated HSS.

Figure 15.

Flank wear versus cutting time of coated HSS.

3.5 Taguchi analysis

The Taguchi L9 (3³) Orthogonal Array (OA) was applied. The OA was generated by Minitab 14 consists of 9 runs with 3 factors at 3 levels. Table 10 shows the Orthogonal Array (OA) of the coated HSS end mills experiment and the combinations of conditions for each control factor (A-C).

3.5.1 Regression equation

Surface roughness equations were generated using machining parameters such as spindle speed, feed rate, and depth of cut. Eq. (1) outline the main effects of surface roughness and Ra response. Figure 16 shows the normal probability plot for Ra response based on Eq. (1).

Figure 16.

Normal probability plot for Ra response.

$$Ra=0.654-0.197\mathit{spindle}\mathit{speed}+0.0793\mathit{Feed}\mathit{Rate}+0.0413\mathit{Depth}\mathit{of}\mathit{Cut}$$E1

3.5.2 Analysis of variance (ANOVA)

The OA L9 (3³) contains nine tests of ANOVA investigation that identify the effects of the different parameters on the response variables. A significance level of 95% was chosen in the ANOVA analysis, and the factor was considered adequate if the P-value was less than 0.05 [53]. In this study, the relation of spindle speed (A), feed rate (B), and depth of cut (C) factors on the surface roughness Ra responses are identified using ANOVA analysis. The model was formulated for a 95% confidence level. The P-value shows that the model is significant and has no influence on noise. The experiment result of surface roughness (Ra) formed the first-order model using the Minitab software.

The ANOVA results depicted in Table 11 is the estimation for machining parameters, with a selected 𝛼-level of 0.05. The outcomes show that the spindle speed factor has the lowest p-value. This reveals that the consequence of spindle speed is significant as p-value factors that are above 0.05 are considered as insignificant [57].

	Parameters			Response
Test No.	Spindle speed (rpm) (A)	Feed rate (mm/min) (B)	Depth of cut (mm) (C)	Surface roughness Ra (μm)	S/N ratio, d/B
1	1860	180	1	0.576	4.7916
2	1860	257	2	0.787	2.0805
3	1860	334	3	0.890	1.0122
4	2650	180	2	0.481	6.3571
5	2650	257	3	0.301	10.4287
6	2650	334	1	0.412	7.7021
7	3450	180	3	0.296	10.5742
8	3450	257	1	0.251	12.0065
9	3450	334	2	0.527	5.5638

Table 10.

An investigation via L9 OA of Ni-CBN HSS coated end mills.

Parameters	DOF	Sum of squares	Mean square	F-value	P-value
Spindle speed	2	0.280658	0.140329	35.11	0.028
Feed rate	2	0.051875	0.025937	6.49	0.134
Depth of cut	2	0.051723	0.025861	6.47	0.134
Residual error	2	0.007993	0.0033996	—	—
Total	8	0.392248		—	—

Table 11.

ANOVA table for Ra response.

3.5.3 Factor level combination and determination of optimum parameter

Based on the rank in Table 12, spindle speed ranks first, followed by the depth of cut and feed rate. This demonstrates spindle speed as the significant factor that affects surface roughness. Spindle speed is the most critical machining parameter affecting surface roughness because it is substantially influenced [56]. The table also represents the Taguchi response to determine the optimal factors affecting surface roughness. According to Signal to Noise (smaller is better), the optimum machining settings are 1860 RPM for spindle speed, 334 mm/min for feed rate, and 2 mm for depth of cut are. The experiment was confirmed through the S/N ratio using the optimum parameter level A1B3C2.

S. No	Level	Spindle speed (A)	Feed rate (B)	Depth of cut (C)
1	1	2.628	7.241	8.167
2	2	8.163	8.172	4.667
3	3	9.381	4.759	7.338
4	Delta	6.753	3.413	3.500
5	Rank	1	3	2

Table 12.

Response table for S/N ratio (smaller is better).

The surface finish was the most important influence on spindle speed and feed rate, as shown in Figures 17 and 18. The slope between the horizontal line and spindle speed is more pronounced than the depth of cut and feed. The changes in spindle speed significantly affect the surface roughness [56]. The optimum machining settings are determined at spindle speed value of 1860 RPM, feed rate of 334 mm/min, and depth of cut of 2 mm.

Figure 17.

Main effects plot for SN ratios.

Figure 18.

Main effects plot for means.

Figure 19 shows the interaction plot for surface roughness, Ra in the machining process. When the lines are more non-parallel, an interaction occurs, resulting in higher strength of the interaction. The factors of spindle speed affect the surface roughness more than other factors for machining Aluminium Alloy 7075 with a Ni- CBN HSS coated end mill.

Figure 19.

Interaction plot for Ra.

4. Conclusions

This study investigates the process of electroless and machinability of Ni-CBN on HSS and Carbide substrate. The electroless Ni-CBN coating has been successfully performed on the substrate and proven using the EDX Analysis. The EDX analysis revealed the presence of major peak for nickel (Ni), carbon (C), oxygen (O), boron (B), and phosphorous (P) elements on the HSS carbide substrate. According to the stability of the coating, 6 mm diameter HSS end mill was chosen. The coated HSS end mill thickness is 15 μm on average.

For machinability, Taguchi L9 (3³) was used in this research to produce a Design of Experiment (DOE) using 18 runs number of experiments with three factors and three levels. The factors were spindle speed, feed rate, and depth of cut. The outcome of machining for surface roughness, tool wear and tool life was analysed by comparing the results between HSS coated and uncoated end mill. The comparison showed Ni-CBN HSS end mill produce good performance on the surface finish and is able to slightly reduce the tool wear and extend tool life.

Analysis of variance (ANOVA) was used for the optimisation parameters of the Ni-CBN HSS end mill tool. The Spindle speed is a significant factor compared to the other factors as it had the lowest P-value, that is below 0.05. For determination of optimum parameters, 1860 RPM for spindle speed, 334 mm/min for feed rate, and 2 mm for depth of cut were identified as the optimum machining settings. The experiment was validated through the S/N ratio using the optimal parameter level A1B3C2.

Acknowledgments

The authors would like to acknowledge University College TATI for financially support the research through the UC TATI Short Term Grant (STG) 9001-1808 under the Advanced Manufacturing Cluster.