the investigation of pvd coating, cryogenic lubrication

Jurnal Tribologi 28 (2021) 105-116

Received 6 December 2020; received in revised form 4 January 2021; accepted 12 February 2021.

To cite this article: Sulaiman et al. (2021). The investigation of PVD coating, cryogenic lubrication and ultrasonic

vibration on tool wear and surface integrity in manufacturing processes. Jurnal Tribologi 28, pp.105-116.

The investigation of PVD coating, cryogenic lubrication and ultrasonic vibration on tool wear and surface integrity in manufacturing processes Mohd Hafis Sulaiman *, Natasha A Raof, Aishah Najiah Dahnel

Department of Manufacturing and Materials Engineering, Kulliyyah of Engineering, International Islamic University Malaysia, PO Box 10, 50728 Kuala Lumpur, MALAYSIA. *Corresponding author: [email protected]

KEYWORDS ABSTRACT

Wear DLC/TiAIN coating Lubrication Dry friction Ultrasonic assisted drilling Cryogenic turning

Lubricants are harmful to human health and the environment. However, it is the only effective method to reduce friction and to prevent tool wear, leading to a high-quality surface. This paper presents the development and testing of environmental-friendly tribology systems in manufacturing processes – metal forming and machining. PVD coatings, cryogenic lubrication and ultrasonic vibration were introduced to replace the use of oil lubrication in manufacturing processes, and investigated on experimental works of reciprocating rig, sheet-metal forming, turning and drilling with different process parameters, ie. lubrication conditions and speeds. Combating friction and tool wear with potential tribology systems like the PVD coatings, cryogenic lubrication and ultrasonic-assisted technique have led to positive results to improve lubrication, thereby delay tool wear progression.

1.0 INTRODUCTION

In high-volume manufacturing processes like metal stamping, turning and drilling, the phenomenon of tool wear is undesirable because scoring of workpiece surfaces may appear if lubrication is inadequate. Abundant oils containing Extreme Pressure (EP) and anti-wear additives are applied to avoid tool wear in high-volume manufacturing production. Application of these oils requires additional costs for pre-cleaning, lubrication as well as post-cleaning after manufacturing. These oils pose risks to personnel health and working environment. Insufficient

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post-cleaning promotes hazardous chemical residues on the sheet surface, which may be unacceptable in cases like biomedical and food container products.

A cost-efficient method to prevent the use of such hazardous oils is to perform the manufacturing under dry friction condition. Hard coatings like Diamond-like Coatings (DLC) (Murakawa et al., 1999), (Podgornik et al., 2006) and pure diamond coating (Tsuda et al., 2011) have been reported to function effectively in metal stamping under dry friction conditions or with minimum lubrication of aluminium, stainless steel and titanium materials. Using the diamond coating, the tool wear can further be improved to forming magnesium alloy, but this improvement can only be achieved by applying lubrication with MoS2 (Tsuda et al., 2009). From an industrial perspective, the pure diamond coating is not a cost-effective solution because it can only be deposited on specific tool materials, e.g. tungsten carbide. Furthermore, it requires special surface vibration-assisted polishing technique due to a rough surface of the crystalline diamond.

Tailoring the interaction between the coating and the tool surface can enhance adhesion and promote longer coating life. For instance, multilayer coating structures (Aizawa et al., 2008) segregate multi-coating films with improved internal stresses of each coating layer, while retaining high hardness, good adhesion and wear properties (Vetter, 2014). The multi-coating layers serve to improve hardness and modulus of elasticity of a coating structure, which increases the load-carrying capacity due to improved mechanical properties of the coated tool surface. Adopting an increased surface roughness of the tool substrate before coating (Kataoka, 2008) is a useful technique for improved coating adhesion but it generates a larger surface roughness after coating, which is difficult to polish to a sufficiently low final surface roughness.

In tribologically manufacturing processes, PVD coated tools are necessarily important and has been the state of the art to ensure long production runs. However, the generation of high shear stress in the contact surface region resulting in temperature increase, thereby peeling off of the coating from the tool surface (Podgornik et al., 2011). Again, lubrication is a typical solution to solve this issue, The oils are flushed to the coated tool surface before the workpiece material enters the deformation region between the tool/workpiece interface to cool down the temperature increase. All these two PVD coating and liquid lubrication are necessarily important in production. Removing one of them would result in production downtime.

Introduction of cryogenic lubrication in machining (Jawahir et al., 2016), (Muhamad et al., 2019) has been found to provide a consistent cooling effect to cut down the temperature build-up between the tool/workpiece interface (Haron et al., 2019). The cryogenic lubrication technique using Liquid Nitrogen (LN) is feasible to reduce tool wear and power consumption (Gupta et al., 2021), thus improving machining productivity and quality (Shah et al., 2020). Adopting the cryogenically assisted manufacturing processes, literature reports the necessity of the cryogenic lubrication as environmentally-benign, toxic-free, hazardless operations, producing functionally superior products.

Owing to high-strength-to-weight ratio, Carbon Fibre Composite (CFC) and Titanium (Ti) alloys are often used for structuring an aircraft body. Traditional drilling of such difficult cutting materials would result in high cutting temperature and severe tool wear (Uçak & Çiçek, 2018). With the development of the feasibility study on ultrasonic vibration-assisted machining of those difficult cutting materials, literature reports the reduction in cutting force during ultrasonic vibration milling could improve the stability of the processing system, and consequently better surface quality was obtained (Zheng et al., 2018). Furthermore, the excellent chip breakability, superior surface integrity of the drilled hole, enhanced tool cutting ability and prolonged tool life


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were obtained (Li et al., 2017). Experimental evidence proved that the ultrasonic power was the main factor in affecting material removal and tool wear rate (Dhuria et al., 2017).

Improving product quality and preventing production downtime are important issues in industrial manufacturing processes. Traditionally, oils are largely used for lubrication to improve manufacturing performance despite poor ecological and health side effects. Driven by environmental concerns and the enforcement of restrictive legislation to ban the use of hazardous oils has attracted great attention from researchers to conduct extensive studies on environmental-friendly lubrication. Due to this, the present paper highlights some solutions developed by the authors such as development and testing of PVD coating, ultrasonic-assisted technique and cryogenic lubrication for manufacturing processes like sheet-metal forming, turning and drilling.

2.0 SHEET-METAL FORMING: ENGINEERED DLC-COATED TOOL Engineering the tool surface by depositing a very thin layer of Diamond-Like Carbon (DLC)

coating is known to have excellent tribological properties that can eliminate the use of harmful lubricants. But, this introduces a weaker adhesion to the substrate tool material at high loads. A solution to this problem is the introduction of the interface layer, ie TiAlN (Sulaiman et al., 2017a). Two approaches were used; reciprocating rig and strip-reduction test.

The first approach describes a frequently used tribological reciprocating rig that has always been carried out by researchers to investigate the properties of tribo-materials and to characterize friction and wear phenomenon between two sliding bodies in relative motion. Meanwhile, the second approach is process tribology specifically developed for sheet-metal forming operation, known as Strip Reduction test (SRT).

2.1 RECIPROCATING RIG 2.1.1 Materials and Method

The tool surface was engineered to depositing Diamond-Like Carbon (DLC) coating on the tool surface, see Figure 1 (top). The mechanical properties and surface characteristics of the test coating are presented in Table 1, where the uncertainty values represent the observed variations in the investigated areas. Figure 1 shows the experimental setup for both lubricated and dry friction condition for a friction pair consisting of a 100Cr6 steel ball and a Vanadis 4 tool steel flat surface with and without a double-layer a-C:H DLC/TiAlN coating. The test parameters applied in the reciprocating rig experiment are listed in Table 2. A tactile roughness profilometer and a light optical microscope (LOM) were used to investigate wear scar on the DLC coated and uncoated tool steel surfaces after the experiments.

2.1.2 Tool Wear and Surface Integrity in Reciprocating Rig Test

The influence of engineered tool surface by using hard coating is significant. Larger friction coefficient can easily be observed in Figure 2 (left) when no coating is applied to the tool steel during operation. The lowest value of the friction coefficient in steady-state condition was observed for the specimens with a-C:H type DLC/TiAlN coatings in both lubricated and dry friction condition. Owing to the excellent toughness and good wear resistance of the DLC/TiAlN coating, the coating characteristics has shown positive result to reduce the wear of the tool steel, thereby improving the tool lifetime. This is supported by roughness measurement on the wear


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scar profiles with an optical profilometer, in which only minor scratches that is found on the wear track of the DLC/TiAlN coated tool, Figure 2 (midlle). As seen in Figure 2 (right), the DLC layer exhibits a clustery and rough structure, which differs from the large columnar grain structure of the CrN/CrCN interlayer and the fine, smooth morpohology of the TiAlN layer. This combined effects of DLC/TiAlN coating may reduce the sliding-originated surface tensile stresses of the DLC/TiAlN coating, preventing severe wear and extending the tool lifetime (Sulaiman et al., 2019). This explains that the DLC/TiAlN coating is capable to reduce the frictional effects and reached a stable value after long-hour experiment.

Table 1: Properties of the test coating. Parameters Values Thickness (µm) 4 ± 0.40 Hardness (HV) 3,000 Roughness Ra (µm) 0.026

Table 2: Test parameters. Test parameters Values Sliding speed ν 100 mm/s Normal load FN 10 N Stroke length L 16 mm Max. strokes 500 laps Temperature 32 °C

Figure 1: Engineered tool surface by DLC/TiAlN coating (top) and tribological reciprocating rig setup (bottom).

Figure 2: Friction coefficient (left), wear scar profiles after the experiment (middle) and morphology (right) of the DLC/TiAlN coating performance. 2.2 STRIP REDUCTION TEST (SRT)

In sheet-metal forming of tribologically difficult materials like aluminium and stainless steel, problems with lubricant film breakdown and galling are important issues. Simulation of

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industrial production with ironing stainless steel cups of EN1.4307 (Figure 3 right) was carried out using Strip-Reduction Test (SRT), emulating operation 4 – ironing as shown in Figure 3 to the far left. As seen in Figure 3, while the punch return to its original position, the lubricant is scrapped off further when the punch moves upwards. At this stage, galling occurs between the punch and the workpiece surfaces due to contact and extra reduction in thickness during back stroke. As a result, scoring of internal collar surface.

Figure 3: Schematics of strip reduction emulating ironing in a tribo-tester.

2.2.1 Materials and Method

To simulate the industrial production in a laboratory test setup, a Strip-Reduction Test (SRT) was selected to perform an off-line evaluation on a tribology simulator for metal forming (Sulaiman et al., 2019). In this SRT test, the DLC/TiAlN coatings as described in Figure 1 (top) were deposited onto the tool surfaces to overcome galling problem in the industrial production with ironing stainless steel cups of EN1.4307. The test parameters were chosen following the industrial production process: 24% reduction, 50 mm/s drawing speed, idle time between each stroke of 1.8 s, and sliding length 10 mm. The test materials and lubricant are described in Table 3 and Table 4, respectively. The DLC/TiAlN-coated tool was tested under dry friction conditions and lubrication with an environmentally benign mineral oil contained no Extreme Pressure (EP) additives, see Table 4.

Table 3: Test materials for the strip-reduction test.

Components Dimension Roughness Ra Surface condition

Tool (Vanadis 4) Ø15×34 mm 0.02 µm Hardened and tempered to 62 HRC Coated with DLC/TiAlN

Workpiece (EN1.4307) W30 × t1.0 mm 0.14 µm “as-received” condition

Table 4: Properties of the test lubricant. Oil type Product name Kinematic viscosity η (cSt @ 40 °C) Mineral oil CR5 Houghton Plunger 660

2.2.2 Tool Wear and Surface Integrity in Strip Reduction Test

Figure 4(a) shows constant drawing load and stable tool rest temperature even after 1,000 strokes, and no sign of pick-up on the tool surface was observed. A steady temperature increase has no significant influence on the lubricant viscosity. This is attributed by open lubrication system in which the new liquid lubricant has always been applied to the workpiece entering the deformation zone. In both lubricated and dry friction conditions, metal-to-metal contact occurred,


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and this causes friction increased due to contact pressure distribution along the tool/workpiece interface. This is verified by the measurement of sheet roughness Ra as shown in Figure 4(b), where both sheet surfaces performed under lubricated and dry friction conditions were lower than the initial workpiece roughness. The results have thus shown that the DLC/TiAlN coating is capable of ironing without lubrication of stainless steel, which is otherwise very prone to galling (Sulaiman et al., 2017b)(Ustunyagiz et al., 2018). Thereby, adopting the double layer coating film by depositing an interlayer metallic coating film like TiAlN in between the DLC film and the tool substrate can therefore improve adhesion strength in the DLC film and even perform well under the extreme test conditions in sheet stamping of stainless steel.

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Figure 4: a) Drawing load and b) sheet roughness Ra after testing with the DLC/TiAlN coating under dry friction with test parameters similar to industrial case under dry condition. 3.0 MACHINING: CRYOGENIC AND ULTRASONIC ASSISTED TECHNIQUE

Machining is important in almost all manufacturing processes. However, the drawback of poor machinability is becoming prevalent in the industry. This is especially pronounced when machining aerospace and automotive materials. This section reported two machining techniques – Ultrasonic Assisted Drilling (UAD) and cryogenic turning.

3.1 ULTRASONIC ASSISTED DRILLING 3.1.1 Materials and Method

Drilling experiments were performed on aerospace materials – 4 mm thick CFC (multidirectional (0˚, 45˚, 90˚, 135˚) carbon fibres with Bismaleimide (BMI) resin) and 4 mm thick titanium alloy Ti6Al4V. Experiments comparing conventional drilling and UAD of CFC/Ti6Al4V stacks were conducted using DMG Ultrasonic 65 Monoblock machine tool and 6.1 mm diameter tungsten carbide 2-flutes twist drills. Figure 5 shows the experimental setup for drilling CFC/Ti6Al4V stacks conventionally (with cutting fluid) and with an ultrasonic using a feed rate of 0.05 mm/rev. The cutting parameters are shown in Table 5.

3.1.2 Tool Wear and Surface Integrity in Ultrasonic Assisted Drilling

Figure 6 compares flank wear rate of the tungsten carbide drills used in conventional drilling and UAD with cutting speeds of 25, 50 and 75 m/min. The graph shows that UAD resulted in lower

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tool wear rate and longer tool life at all cutting speeds used. Based on ISO 3685 standard, tool life ended when the flank wear reached 300 μm (Dahnel et al., 2015). It was observed that titanium adhesion, edge chipping and dull cutting edges were the tool wear features when drilling CFC/Ti6Al4V stacks. With cutting speed of 75 m/min, cutting tool failed after drilling 28 holes conventionally and in UAD, the tool failed after 34 holes since the edge chipping and wear had reached 300 μm. The use of the lower cutting speed of 50 m/min resulted in a longer tool life which was up to 62 holes in conventional drilling and 80 holes in UAD. Concerning tool life, a cutting speed of 25 m/min was seen as the optimum since it resulted in the lowest tool wear rate; with UAD providing longer tool life than conventional drilling. This is supported by the condition of the cutting edges as shown in Figure 7, photo taken by using Scanning Electron Microscope (SEM). The use of higher cutting speeds resulted in more titanium adhesion most likely occurred due to higher cutting temperatures (Dahnel et al., 2016)

Figure 5: UAD experimental setup.

Table 5: Cutting parameters. Drilling type Cutting speed No. of holes

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25 m/min 40 UAD

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Figure 6: Tool wear in different cutting speeds.


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Figure 7: Tool wear after drilling 40 holes.

3.2 CRYOGENIC TURNING 3.2.1 Materials and Method

The test material was AISI 4340 alloy steel in a quenched and tempered state, with a 100 mm diameter and 317 HB hardness. The initial microstructure of the test material shown in Figure 8 has a typical lath-martensitic structure. The test material was turned on a CNC lathe machine with a CVD coated (TiCN and Al2O3) carbide insert. Cutting parameters are listed in Table 6. For cryogenic flushing, a flexible hose was connected to the Liquid Nitrogen (LN) tank and a copper pipe was used as a nozzle pointed to the clearance face of the insert.

Figure 8: Initial microstructure of test materials.

Table 6: Cutting parameters.

Parameters Description Cutting speed (m/min) 160, 200, 240 Feed rate (mm/rev) 0.3 Depth of cut (mm) 1.0 Coolant Dry and cryogenic (LN)

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3.2.2 Tool Wear and Surface Integrity in Cryogenic Turning Figure 9 shows the measured cutting temperatures during turning in dry and cryogenic

flushing. The application of cryogenic LN during turning has effectively reduced the cutting temperature by 35% - 55% compared to dry turning, especially at higher cutting speeds. The lower temperatures in cryogenic machining are advantageous to reduce the machined surface alteration caused by thermal activity. However, the differences in the roughness Ra values of the work material at different cutting speeds were not that significant and apparent in cryogenic turning. The positive result of cryogenic cooling effect to control the heat generated due to the cutting speeds were found to be more effective at higher speeds. This is associated with the chip morphology formed during the cutting (Raof et al., 2019b). At higher speeds, the chip becomes thinner and curlier. Moreover, the chips become harder with improved breakability due to sudden cooling by the cryogenic. This makes the penetration of LN into the chip-tool interface easier, hence reducing the cutting temperature (Yıldırım, 2020).

The machining induced two surface layers that have different characteristics: the refined grain layer (RL) and transition layer (TL) as shown in Figure 10. The increased hardness of the work material at the surface (RL) and subsurface (TL) is presented in Figure 11. It is noted that the machined surface is harder compared to the bulk and the hardness is lower with increasing profile depth. Cryogenically machined surfaces show a superior value of hardness compared to that of dry machined surfaces in all speed variations. This is due to the reduction of the thermal softening effect and the strain hardening of the work material at low temperatures, which resulted in a higher density of refined carbide particles in cryogenically machined surfaces (Raof et al., 2019a). This investigation has revealed that excessive heat caused by high cutting speeds is undesirable (Muhamad et al., 2019), (Haron et al., 2019). This will result in thermal damage at the machined surface and subsurface influencing surface quality, increase of tensile residual stresses, and reducing dimensional accuracy of the work material.

Figure 9: Cutting temperature and roughness Ra of the work materials.

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Figure 10: Subsurface layer and hardness profile.

Figure 11: Hardness distribution of machined surface and subsurface of work material.

4.0 CONCLUSION

The present investigations on the tribological performance of tool wear and surface integrity in manufacturing processes have provided feasible solutions to dry friction manufacturing operation in order to replace the use of hazardous lubrication systems with less harmful lubrication techniques. The tool wear can be reduced significantly through some tribological systems such as the development and testing of new tool coating, ultrasonic-assisted technique, and cryogenic lubrication. In the sheet-metal forming, the use of DLC as the top layer coating and TiAlN as an interlayer coating revealed their combined excellent tribological properties, including low-friction, anti-wear, high toughness and good adhesion strength, and therefore providing an environmentally-friendly alternative to outperform conventional approaches which utilizing hazardous lubricants in production. Meanwhile, in the machining process, the positive results of using cryogenic turning and ultrasonic-assisted drilling have proven that these methods are capable of replacing the conventional flooded machining. From these investigations performed on manufacturing processes, the tribology knowledge plays an important role for understanding


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on what happens at the tool/workpiece interface, and design and reliability of the new tribosystems need to find the promising environmental-friendly lubrication.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support from the Research Management

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the investigation of pvd coating, cryogenic lubrication

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