Tool life improvement using a shim with nanostructured composite damping coating synthesized by high power impulse magnetron sputtering

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IN

DEGREE PROJECT MECHANICAL ENGINEERING, SECOND CYCLE, 30 CREDITS

STOCKHOLM SWEDEN 2018,

Tool life improvement using a shim with nanostructured

composite damping coating synthesized by high power impulse magnetron sputtering

GUO SHUAI

KTH ROYAL INSTITUTE OF TECHNOLOGY

SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

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Tool life improvement using a shim with nanostructured composite damping

coating synthesized by high power impulse magnetron sputtering

Guo Shuai

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ABSTRACT

The thesis made an investigation of the damping treatment on cutting inserts for reducing the high frequency vibrations and improving the tool life. The damping coating of Cr:CuCNx has a multiple layer structure, and was synthesized by high power impulse magnetron sputtering (HIPIMS) method.

Comparisons between coated and uncoated shims were carried out under stable turning conditions.

The measured acceleration signals showed that the vibration amplitude on high frequency range between 3000 Hz and 22000 Hz on the tool holder was reduced by 30%-50% by using a shim coated with the nanostructured composite material. The flank wear was 193 μm after 32 minutes tuning process for the insert having the damping coating shim, versus 264 μm flank wear of a conventional shim under the same condition. However, the surface roughness of workpiece showed the only marginal difference between the damping coated shim and the conventional shim.

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SAMMANFATTNING

En undersökning genomfördes, som en del av avhandlingsarbetet, av hur skärplattor kan behandlas genom dämpande beläggning för att minska högfrekventa vibrationer och förbättra verktygets livslängd. Den dämpande beläggningen av Cr: CuCNx har en flerskiktsstruktur, och syntetiserades medelst högeffektimpulsmagnetronsputtering (HIPIMS) -metod. Jämförelser mellan belagda och obelagda underläggsplattor utfördes under stabila bearbetningsförhållanden (svarvning). De uppmätta accelerationssignalerna visade att vibrationsamplituden inom högfrekvensområdet, mellan 3000 Hz och 22000 Hz på verktygshållaren, reducerades med 30 % -50 % genom att använda en underläggsplatta belagd med det nanostrukturerade kompositmaterialet. Fasförslitningen var 193 μm efter 32 minuters svarvning för skärplattan med dämpningsbelagd underläggsplatta, mot 264 μm fasförslitning av ett konventionellt skär under samma villkor. Emellertid visade ytjämnheten hos arbetsstycket endast marginella skillnaden mellan den dämpade belagda plattan och den konventionella plattan.

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1. INTRODUCTION

Generic vibration exists in all metal cutting processes and is affected by the chip segmentation, workpiece material’s grain size and etc. Vibration mitigation on the cutting tools is demonstrated to improve the productivity and reduce the production costs. Tool wear, geometric tolerance and surface quality are significantly related to vibrations in the metal cutting process [1].

A typical cutting tool composes a cutting insert affixed to a cutting tool holder via a shim seat in between. An earlier study by Vladimir A. Rogov et al [2] showed that using shims with high damping capacity could reduce high frequency micro vibrations during the machining processes. The high frequency micro vibrations on tool tip are caused by the forces from the cutting process: 1. the force is applied to the cutting tool during cutting operation and 2. The force originates from the chip deformation process. According to their results, by changing the shim material with different damping capacities, high frequency tool tip vibration can be reduced to obtain a smoother tool-workpiece contact and to produce a better surface finish. The shim made of epoxy granite has the highest damping capacity compared with other shim materials which significantly decreased the amplitude value, resulting in a significant improvement of the surface finish by 20-40%. For this reason, it can be pointed out that the damping property of the shim material has an influence on machining vibration reduction.

Damping materials can be applied on mechanical structures to dampen out vibrations. Mechanisms of vibration damping of a tooling structure are mainly from material internal damping and structural damping. The commonly used damping material is the viscoelastic material. The advantage of this kind material is its high damping capacity. However, the damping property of the viscoelastic material is sensitive to temperature. Secondly, the loss modulus of the viscoelastic material is relatively low because of its low elastic modulus. Nanostructured composite materials is drawn attention recently with its high elastic modulus and damping property. According to Md. Masud-Ur-Rashid’s work [3], comparisons of damping capacity and dynamic mechanical property have been made between the CNx nanostructured composite coating material and 3M-112 viscoelastic material. The results revealed that CNx nanostructured composite coating material could withstand higher temperature (up to 600°C) without any significant loss in damping performance as well as its mechanical properties compared with 3M-112 viscoelastic material (100°C) [4]. Moreover, the damping property of CNx nanostructured composite coating material was better than 3M-112 viscoelastic material under machining condition since the working temperature is much higher than 100°C. Even though 3M-112 material had higher loss factor than CNx material, the elastic modulus of 3M-112 viscoelastic material was much lower than CNx material which leads to a low loss modulus.

Qilin Fu, et al. [5] applied a novel carbon-based nanocomposite coating produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) method on to a tool holder’s clamping area to investigate the damping effect of the coating material. The results presented that the measured absolute sound level during the process were significantly reduced by about 90% by using the coated tool. Surface roughness measurement of the workpiece showed a decreased Ra values from 3-6 µm to less than 2 µm. According to the recent work of Qilin Fu et al. [6], a multi-layered Cu:CuCNx nanostructured composite material was synthesized by High power impulse magnetron sputtering (HIPIMS) technology. It was reported that the multi-layered Cu:CuCNx composite increased both the loss factor and elastic modulus of the coatings deposited on steel and Si surfaces. They compared the material properties between Cu:CuCNx and commonly used methyl methacrylate material. It was illustrated that Cu:CuCNx material has higher elastic modulus and hardness, and similar loss factor compared with methyl methacrylate polymer.

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To apply the nanostructured composites on a shim used in the machining operation, the adhesion of the nano composite coating to the substrate and its coherence strength need to be high to overcome the high mechanical load in the metal cutting process. The approach of improving adhesion of carbon nitride coatings on steel substrates was studied by Konstantinos D. Bakoglidis et al [7]. It was found that pretreatment by HIPIMS using Cr or W as an interlayer between CNx coating layers and metal substrate could improve the adhesion sufficiently.

In this paper, the multi-layered nanostructure coating material Cr:CuCNx produced by HIPIMS was applied on the shim between the cutting insert and insert seat, and studied in a turning process. The coating with 200 μm thickness was applied on the surface of a shim as a damping material. Tool flank wear, vibration signal and workpiece surface finish are measured and compared between cutting tools with a coated shim and an uncoated shim, under stable machining conditions.

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2. EXPERIMENTAL DETAILS

2.1 Simulation study

A simulation study of the turning cutting tool dynamic behaviours was performed in COMSOL Multiphysics software. A simple cantilever beam model is chosen to represent a simplified cutting tool model which has one fixed end and one free end. In the model, the layer (see in Figure 1) between the cutting insert and the shim has varied elastic modulus and loss factor. The dimensions of the cutting tool are close to the real one which was used under real cutting condition. The simulation calculates the eigenfrequencies and the FRF (frequency response function) of the cutting tool set to investigate the damping effect of adding a damping coating material on the shim.

Figure 1 Schematic view of the set up in simulation study

The frequency response function analysis was performed by computing displacement amplitudes of tool-tip in tangential direction within the specified frequency domain (0-20000Hz). The magnitude of the tangential force is the largest compared with feed force and radial force, and vibration signals in the tangential direction are most sensitive [8]. Thus only the vibration behaviour in tangential direction was considered in this simulation study [8].

An external 1N impulse surface force is applied in the tangential direction on the insert representing forces generated in real cutting condition. Displacement amplitude of the tool-tip in the tangential direction is evaluated as the parameters to verify the cutting tool vibration behaviour.

When computing simulations with an uncoated conventional shim, the coating layers’ material are set to be the same as the material of the shim (tungsten carbide). The coating material properties, i.e.

elastic modulus of material and Loss factor of material, was varied in the simulation study. 6 sets of simulation studies (summarized in Table 1) were conducted to compare the frequency response function of the cutting edge using different types of shims.

Cutting insert Thick coating layer Shim Tool shank

Tangential Cutting force

Fixed constrain

Tangential direction

Feed direction Radial direction

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Table 1 Varying Elastic modulus and Loss factor values of coating materials used in the simulation study

Elastic modulus of material Loss factor of material Uncoated shim 310 GPa (Tungsten carbide) 0.008

Coated shim 40GPa 0.08

Reference group 1 10GPa 0.3

Tool material(A2 tool steel) From Comsol database From Comsol database

2.2 Synthesis of the Cr:CuCNx nanostructured composite coating by HIPIMS method

The detailed HIPIMS theory and process description can be found in Daniel Lundin et al‘s work [9]. A high power supply with pulse control at a low duty factor and low frequency was applied in a magnetron configuration. A higher ionization rate and higher plasma density can be reached through a high power discharge. In HIPIMS discharge, the plasma density was reported up to 10¹⁸m^-3 which is roughly 2 orders of magnitude higher density than a conventional dcMS discharge [9]. An active pulse control enables to remain a low average power but with a high peak power. Pulse control is a benefit to keep a lower substrate temperature without overheating it. A schematic view of HIPIMS chamber can be seen in Figure 2.

HIPIMS CHAMBER

Vacuum Chamber Pump system

Cathode

Target

Substrate table

Substrate bias voltage

Reactive gas

N²and C2H2

Inert gas Ar

Plasma

Coated shim

Figure 2 Schematic view of HIPIMS chamber and process layout

The coating depositions were carried out in a vacuum chamber with unbalanced magnetron cathodes.

Two target plates (pure copper and chromium) were mounted on the two cathodes for CuCNx coating layers and chromium layers deposition respectively. The substrates (conventional shim) were ground down 200μm and then fixed on a metal plate. The metal plate was mounted on a rotating fixture.

Before the deposition process, substrates were pre-treated in order to increase the adhesion between the coating layers and the shim metal substrate. The substrates were cleaned with soap and hot water then followed by ultrasonic bath cleaning. Plasma etching of the substrates was performed in the chamber to activate the surface in order to get a better adhesion.

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2.3 Characterization of nanostructured composite coating material

The available characterization result of a similar nanostructured composite material Cu:CuCNx was presented as a reference to have a general understanding of the structure and properties of the nanostructured composite material. The composite material structure, damping mechanism and material properties of Cr:CuCNx and Cu:CuCNx material were predicted to be similar. The difference between Cr:CuCNx and Cu:CuCNx was the interlayers among CuCNx matrix, which were Cr and Cu interlayers correspondingly.

The microstructure of the coating material was investigated with field emission scanning electron microscopy (FESEM, Zeiss Ultra Plus). For the cross-section preparation, samples were embedded in resin followed by mechanical polishing (from coarser sandpapers to finish final polish with silica nanoparticles).

Mechanical properties were characterized by using a Vickers hardness micro indenter (CSM Instruments, Switzerland). At least 24 indents on the coating surfaces were performed for each sample.

Linear loading conditions were maintained to record Load-displacement curves with a maximum 5N load force. The loading time was 10s at the maximum indentation depth and the loading-unloading rate was 10N min^-1. Oliver & Pharr method was used to characterize the elastic modulus and hardness of the coatings.

2.4 Machining test

Two sets of turning operations were conducted:

1. Turning test with an uncoated conventional shim 2. Turning test with a coated shim

The coating material was the Cr:CuCNx composite material synthesized by HiPIMS method and deposited on one side (contact with insert) of a shim. The shim was ground down by 200 μm before the coating process, and coated with an equally thick layer of the Cr:CuCNx composite material, as shown in Figure 3.

Figure 3 Cr:CuCNx coated shim and conventional shim

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Turning operations were performed on the lathe SMT Swedturn 300 with coolant. The workpiece material was SS2541-03 with a diameter of 122 mm and hardness of 290HB. Mircona inserts (ALC280 P25/M15/K25 NM7) were used with a nose radius of 0.8 mm. New cutting inserts were used in each test. Carbide shims were used as a conventional shim. Same cutting parameters were processed in each test within recommended parameters range from the manufacturer with a surface speed of 200 mm/min, feed rate of 0.4 mm/rev and depth of cut of 1.2 mm.

Accelerometer with a sensitivity of 10.90 mV/g was mounted on tool holder to measure the acceleration signals in tangential direction during the machining test (Figure 4). The clamping torque of cutting tools was kept the same during the tests and the accelerometer was glued on the tool shank without changing location. The flank wear was measured multiple times during the cutting process, using Nikon Upright Microscope. Surface roughness was measured with a surface roughness measuring instrument (Mitutoyo Surftest SJ-301) at the end of each machining test.

Finishing machining was carried out to generate a better surface finish before measuring the surface roughness. The finishing parameters were: cutting speed of 200 mm/min, depth of cut of 1.2 mm, and feed rate of 0.1 mm/rev.

Figure 4 Schematic view of the accelerometer location

Cutting insert Shim

Accelerometer

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3. RESULTS

3.1 Results of the simulation study

The computed Eigen frequency values of the six simulation studies were summarized in Table 2. All of the mode shapes of vibration contribute to the response of a structure, and only the first three mode shapes and corresponding Eigen frequency were presented.

Table 2 Eigen frequency values from the simulation study

Eigen Frequency

Group

Material 1 Material 2 Material 3 Material 4 Conventional shim

Cr:CuCNx coated shim E=10e⁹

Loss factor=0.03

E=10e⁹ Loss factor=0.3

E=40e⁹ Loss factor=0.08 First Eigen

Frequency 6589.7Hz 6589.9Hz 6592Hz 6592Hz 6573Hz 6591.4Hz SecondEigen

Frequency 6637.7Hz 6638.2Hz 6644.2Hz 6644.2Hz 6624Hz 6642.6Hz Third Eigen

Frequency 12043Hz 12046Hz 12083Hz 12083Hz 12057Hz 12073Hz As can be seen from Table 2, with the increasing elastic modulus of coating material, the Eigen frequency of the whole cutting tool structure increase or decrease slightly. It seems by adding a thick coating layer on a shim, it has an influence on the overall stiffness of the cutting tool structure.

The first, second and third mode shapes of the cutting tool with conventional shim are shown below.

The black colour frames are the original shapes without deformation, and the coloured bodies are the mode shape at a certain frequency. From the figures, it is clear that the first two modes are under a bending deformation whereas the third mode is under a torsional deformation.

Figure 5 First, second and third mode shapes of cutting tool structure

The following figures show the frequency response function analysis of simulation study. Three resonance peaks can be observed in FRF curve which represent the corresponding mode shape mentioned above. The displacement amplitudes for each group were compared in the tangential direction. From Figure 6 to Figure 8, the resonance frequency in a relatively low frequency range (from 6573Hz to 6644.2Hz) for all coated shims slightly shift to a higher frequency level, indicating the enhanced structure stiffness. On the other hand, the variations of displacement amplitudes are too litter to reveal the damping effect of coated shims. Only one group presents a positive damping effect, the one with a lower elastic modulus (10GPa) and high loss factor (0.3). In high frequency range, as can be seen from Figure 9, the coating material with higher elastic modulus (300GPa) result in a higher

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structure stiffness. The coating material with a lower elastic modulus (10GPa) and high loss factor (0.3) displays a better damping effect with a 30% displacement amplitude reduction.

From simulation result analysis it can be concluded that the higher loss factor of the coated material, the better damping effect of the cutting tool structure. On the contrary, a coating material with a relatively lower elastic modulus has a better performance.

Elastic=1E10 Loss=0.3 Elastic=3E11 Loss=0.3 Elastic=1E10 Loss=0.03 Elastic=3E11 Loss=0.03 Elastic=3.1E11 Loss=0.008 Elastic=4E10 Loss=0.08

Figure 6 Holistic view of simulated Frequency Response Function from 0-20000Hz

Figure 7 First and second peaks from 6530 Hz to 6660 Hz (Zoomed in view from the blue box of Figure 6)

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Figure 8 First peak from 6585.5 Hz to 6594 Hz (Zoomed in view from the red box of Figure 6)

Figure 9 Third peak from 11900 Hz to 12250 Hz (Zoomed in view from the green box of Figure 6)

3.2 Structure and material properties of coatings

In this section, the available characterization result of a similar Cu:CuCNx material was reviewed according to the result from Qilin Fu et al [6].

To obtain Cu:CuCNx composite coatings, CuCNx matrix were deposited by using the mixture of Ar, N2

and C2H2 gases while sputtering off a copper target. Cu interlayers were introduced into CuCNx matrix by switching off N2 and C2H2 while only using Ar. In the case of depositing Cr:CuCNx composite coatings, a chromium target was sputtered in order to add Cr interlayers to the CuCNx matrix.

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A cross-sectional view of the multi-layered structure of Cu:CuCNx composite coatings was clearly illustrated in Figure 10 [6] through field emission scanning electron microscopy. The multi-layered structure was the consequence of the process control and periodic deposition conditions. The thickness of the Cu layers and CuCNx composite layers was altered by the deposition time. The total thickness of the Cu:CuCNx sample presented here was 110 μm, where the average thickness of a single Cu and CuCNx interlayer were 0.6 ± 0.1 μm and 1.6 ± 0.1 μm respectively. In addition, a multi-layered structure could be observed within the CuCNx composite layer which the thickness of a single layer was approximately 60 nm.

Figure 10 SEM cross-section for Cu:CuCNx microstructure [6]. The first image shows the multi-layered structure, where the brighter colours are Cu layers while the darker colours are CuCNx layers. The second image shows the boundaries among Cu and CuCNx layers. The third image shows the high magnification imaging of CuCNx nanostructured multi-layers.

To Cr:CuCNx composite material, the total thickness of the coating was 200 μm. The microstructure of the Cr:CuCNx material can be predicted to be similar to the Cu:CuCNx material, the difference was that the Cu interlayers were replaced by Cr interlayers, the CuCNx layers kept the same. However, the thickness of a single Cr layer and a CuCNx layer in Cr:CuCNx material, and the total number of the different layers were unknown.

The mechanical property of multi-layered composite material was highly influenced by the microstructure of material, grain size, metallic inclusion as well as the ratio of elements. From the creep tests result from Qilin Fu et al. [6], the reported best performing coating of the Cu:CuCNx sample has a hardness value of 2.4 GPa, an elastic modulus of 34 GPa and calculated loss factor of 0.077.

3.3 Machining test results

3.3.1 Results of flank wear measurement

Results of flank wear measurement in each test can be seen in Table 3. The flank wear was measured in intervals 16 minutes, 26 minutes and 32 minutes. It is apparent that after 32 minutes of machining, the flank wear of the cutting insert with damping coated shim is minimal which is only 193 μm. From 16 minutes to 32 minutes, flank wear increased slightly with a coated shim. It must be noticed that a measurement error occurred at 26 minutes, flank wear dropped from 176 μm to 156 μm. The measurement error occurs may due to the insert positioning error when placing the insert on the measurement table with angular differences. On the other side, flank wear with conventional shim reached 264 μm after 32 minutes, this is close to the end of useful tool life of 300 μm according to ISO 3685 standard. To sum it up, tool life was significantly prolonged by the use of damping coated shim.

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Table 3 Flank wear measurement results

Flank wear of insert with a conventional

shim

Flank wear of insert with a coated shim

16minutes 183 μm 176 μm

3.3.2 Results of vibration signals from machining test measured by accelerometer

The power spectrum density (PSD) of measured acceleration data in tangential direction during machining process was summarized as shown below. Firstly, the vibration amplitude signals were measured in the time domain at each cutting run (the moment from the insert plough into the workpiece to the insert run out of the workpiece) during machining, then transferred to PSD through Fast Fourier Transform. The power spectrum density graphs show that the vibration energy is mostly concentrated in a high frequency range from 3000Hz to 16000Hz for all of the measured machining operations. A number of resonance peaks occur within the frequency range. In addition, the maximum peak values in all cutting runs can be observed between 7500Hz and 8000Hz. There is no obvious resonant frequency shift, but only distinct peak amplitude reduction was obtained. This can be explained that the high damping capacity of the coating material has a beneficial effect to reduce harmful vibrations while retaining the stiffness of the structure. It is evident that the maximum value of the power spectrum density is much lower when the cutting tool with a coated shim. From the frequency band between 7500 Hz to 8000 Hz where the maximum peak occurs, the maximum amplitude of the tool with a conventional shim has a larger range of variation (compared from the beginning to the end of machining, seen from Figure 11 to Figure 14). For the tool with a coated shim, the maximum amplitude increase slightly with the tool wear increase as the machining proceeding.

This indicates that a coated shim not only reduce the vibration during machining but also provide a much more stable machining condition.

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Figure 11 The comparison of power spectrum density of measured acceleration data in the tangential direction of cutting run 1 from 0-2minutes of cutting (the green line was coated shim and the red line was conventional shim)

Figure 12 The comparison of power spectrum density of measured acceleration data in the tangential direction of cutting run 10 from 15minutes to 16 minutes 30seconds of cutting

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Figure 13 The comparison of power spectrum density of measured acceleration data in the tangential direction of cutting run 14 from 20 minutes to 21 minutes 20 seconds of cutting

.

Figure 14 The comparison of power spectrum density of measured acceleration data in the tangential direction of cutting run 18 from 26 minutes to 27minutes 20 seconds of cutting

Figure 15 displays the vibration amplitude signals plotted in the time domain for the cutting tool with a conventional shim (red), and the coated shim (green) at cutting run 18 (from 26 minutes to 27 minutes 20 seconds of cutting) in each cutting condition. It is shown that during the machining process,

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the cutting tool with coated shim has less vibration amplitude which indicates a more stable cutting process.

Figure 15 The comparison of measured acceleration data in the tangential direction from 26minutes to 27minutes 20 seconds of cutting. (The red line was conventional shim and the green line was coated shim)

3.3.3 Results of machined workpiece surface finish

The surface finish (Ra) of the machined workpieces were measured at the end of each machining test (after 32 minutes). Three measuring points were selected along the workpiece bar (the midpoint of the workpiece and both ends). At each measuring point, three measurements were taken. The surface roughness measurements for the cutting tool with a conventional shim and a coated shim are summarized in Table 4.

Table 4 Summary of the surface roughness measurement of the machined workpiece in each condition after 32 minutes machining

From the table above it can be observed that the cutting tool with a coated shim has a slightly lower average surface roughness compared with the cutting tool with a conventional shim. However, the difference between the average Ra values is marginal which can be neglected in this situation. It can be concluded that damping coated shim has no significant impact on surface finish improvement under a finishing operation with a low feed rate of 0.1 mm/rev.

Ra( μm ) Measuring point 1 Measuring point 2 Measuring point 3

1 2 3 1 2 3 1 2 3 Average Ra( μm )

Conventional

shim 0.84 0.95 0.92 0.83 0.91 0.92 0.08 1.02 0.79 0.8866 Coated shim 0.85 0.85 0.85 0.82 0.80 0.83 0.99 0.76 0.99 0.8600

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4. DISCUSSIONS

4.1 The observation of simulation study

From the measurement of Eigenfrequency and frequency response functions, we observed the insignificant difference between cutting tool with a conventional shim and a coated shim. Even though the elastic modulus of coating material increased from 10e⁹to 300e⁹, the stiffness of the cutting tool structure changed slightly. No significant amplitude reduction was observed in the FRF curve. Machine tool system and machining process are complex as all components with joint interface interact each other. A simplified simulation for a certain region from machine tool system might not be able to reflect the real characteristics of a complex system.

4.2 The vibration signal measurement, tool life and surface roughness in machining

From the vibration signal measurement, it can be seen that the damping effect of a coated shim has an apparent influence on cutting tool vibration in the tangential direction, which differs from the results of simulation study. As seen from acceleration measurement in the time domain, the vibration amplitude of cutting tool with a coated shim is 22% less than the one with a conventional shim. From the power spectrum density graph in Figure 11 to Figure 14, the vibration amplitude with a coated shim was reduced 30%-50% throughout the frequency band. The result demonstrates that Cr:CuCNx composite coating improved the damping capacity of cutting tool under practical machining conditions.

The similar improvement was also realized by Vladimir A. Rogov et al [2]. With a shim made of epoxy granite material, the magnitude value of FRF was adequately reduced compared with a standard shim.

The tool wear measurement result shows that cutting tool with a coated shim with a reduced vibration amplitude has less flank wear (193 μm) after 32minutes of machining compared with that of a conventional shim (264 μm). This proves that the tool wear in machining process is strongly related to cutting tool vibration.

The surface finish resulted from a coated shim (Ra 0.86μm) is similar to a conventional shim (Ra 0.8866μm) after finishing. Since feed rate is regarded as a predominant factor in the generation of good surface finish under turning operations, here we may predict that the damping coating has no sufficient effect on vibration damping of the cutting tool in feed direction.

The machining tests of the cutting tool with a coated shim and an uncoated shim were performed on two same workpieces independently. However, the clamping conditions of workpiece and workpiece material in each test might have minimal differences which can be regarded as variables resulting in a small margin of error. The measurement results might be subject to disturbance of these errors. With respect to this concern, similar tests were repeated and all of the results were accordant with the results obtained in this paper that indicating the results were reliable and the tests were repeatable.

4.3 Cutting tool vibration caused by chip segmentation

One of the vibrations generated in machining operation originates from chip formation process.

Discontinues chip type was observed and the segmentation distance of chip was measured. From the Equation 1 below, the chip segmentation frequency can be calculated.

Equation 1 Chip segmentation frequency calculation

L Vc

s /

F 

Where Fs is the frequency of chip segmentation, Vc is the cutting speed (200m/min-3.33m/s) and L is the segmentation distance on the chip (ranging from 150 μm to 190 μm). The resulting segmentation

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frequency is calculated in the range between 17368 Hz and 22000 Hz. From the measured vibration signal in section 3.3.2, it can be clearly observed that on the frequency band from 18000 Hz to 22000 Hz, the vibration amplitude with a coated shim is lower than a conventional shim. Even though the relative amplitude at that frequency range is much lower than in low frequency range, but the high frequency micro vibration acted on cutting insert is extremely harmful to tool life. It can be proved that the Cr:CuCNx damping material coated onto a shim has a beneficial effect when it comes to reducing high frequency micro vibration caused by chip segmentation.

4.4 Effect of elastic modulus and damping property of shim material on vibration damping in the machining process

In this current study, only one type of damping coating was tested under real machining test. It is not possible to find out whether elastic modulus or damping capacity of coating material has a major impact on vibration damping on cutting insert. Vladimir A. Rogov et al [2] compared the damping effect of shim material on vibration damping in turning operation. 6 shims made of different materials were compared under machining test to investigate the damping effect. The elastic modulus of shim materials following the sequence from high to low which is: ceramic > chlorite schist > granite > silty micaceous sandstone > epoxy granite. Their results indicated that shim made of epoxy granite and silty micaceous sandstone have the best performance on vibration reduction and the generation of better surface finish. Even though the elastic modulus of epoxy granite and silty micaceous sandstone were much lower (25-40GPa) compared with other materials (up to hundreds of GPa), the resulting structural stiffness of cutting tool has no significant change which still remaining as rigid as before.

Hence, as long as the elastic modulus of coating material is sufficient enough to support the cutting tool structure without a tremendous loss of structural rigidity, the damping property of coating material plays an important role.

4.5 Damping mechanism of the coated shim

The high frequency vibrations on tool tip are caused by the forces applied to the cutting tool during machining and the force originates from the chip formation process. The vibratory energy acted on cutting insert could lead to the debonding of neighbouring particles from insert which results in the loss of small particles (tool wear). The existence of damping coating under the cutting insert could effectively absorb vibratory energy and dissipate it into other forms of energy (mostly heat). From the result, we could observe that the coated shim is able to reduce vibrations from 3000 Hz to 22000 Hz.

Amir abdulllah [9] found that ultrasonic vibration (25000Hz) of the tool was more effective in attaining a high material removal rate when machining cemented tungsten carbide. This explains the high frequency aggressive energy can separate tungsten carbide grains, which have lost the neighbouring binder. In our test, the cutting insert was made of tungsten carbide. According to Amir abdulllah’s [10]

report, the high frequency vibrations could accelerate the loss of the tungsten carbide insert particles during machining leading to a fast tool wear. Hence, the high frequency vibration reduction through a Cr:CuCNx coated shim is beneficial to reduce the tool wear of tungsten carbide insert.

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5. CONCLUSIONS

In this work, a nano-structured composite coating (Cr:CuCNx) was synthesized by high power impulse magnetron sputtering (HIPIMS) method. The coating was applied on a standard shim and tested under stable turning conditions. Comparisons between an uncoated standard shim and a coated shim were carried out to investigate their cutting performances. Experiments have shown that:

 The cutting tool with a nano-structured damping coated shim has high damping capacity to suppress high frequency vibrations.

 The coating material supports the cutting insert reliably to retain the rigidity of the cutting tool system.

 It was concluded that the measured vibration signal in tangential direction during machining process could be reduced by 30%-50% by using the coated shim.

 As a consequence of vibration reduction, the flank wear of the cutting tool was reduced by 27%

with a coated shim after 32 minutes machining.

 The improvement of tool life was achieved by using a nano-structured coated shim. The use of a coated shim had minimal effect on surface finish improvement.

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6. ACKNOWLEDGEMENT

The author wishes to thank Dr. Qilin Fu and Professor Cornel-Mihai Nicolescu for their invaluable guidance, teaching and support. I also would like to thank XPRES (Excellence in production research) platform funded by Swedish government through Vinnova, and HiCut-Nanostructured Vibration- Damping Coatings for High-Performance Cutting Technologies (Eureka Eurostars 11 509) for the support.

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7. REFERENCES

1 M.E.R Bonifacio and A.E Diniz, Correlating tool wear, tool life, surface roughness and tool vibration in finish turning with coated carbide tools, 1993, Wear, Volume 173, Issues 1–2, April 1994, Pages 137-144.

2 Vladimir A. Rogov et al, Improvement of cutting tool performance during machining process by using different shim, Archives of Civil and Mechanical Engineering, Volume 17, Issue 3, May 2017, Pages 694-710

3 Md. Masud-Ur-Rashid, Characterization of Dynamic Elastic Modulus and Damping Property of CNx Coating Material by Experimental Modal Analysis and Finite Element Approach, December, 2012, Master’s Thesis, KTH Royal Institute of Technology.

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