Femtosecond Laser texturing of tungsten carbide

Ashfaq Khan1, Mushtaq Khan

2, Aftab Khan

1, Syed Husain Imran

2, Kamran Shah

1, Mohammad.A

Sheikh2, Lin. Li

2

1University of Engineering and Technology, Peshawar, Pakistan.

2School of Mechanical and Manufacturing Engineering, National University of Sciences and Technology

(NUST), Islamabad, Pakistan. 3School of Mechanical, Aerospace and Civil Engineering, The University of Manchester,

Manchester, M13 9PL, UK

Hard materials, such as Tungsten Carbide, have got immense applications in abrasive and

chip removal processes. Tungsten carbide has been extensively used as tool insert for

machining processes involving chip removal processes. Recent research shows that creating

features on the rake surface of these tools can offer significant advantages in terms of

reduction of the friction and improvement in tribological properties that result in the

extension of tool life. Also, variations in the feature dimensions and shape have effect on the

tribological behaviour of these tools. However, due to the hard nature of the Tungsten carbide

it is a challenge to create custom features on the tool. This research investigates the

generation of custom features on carbide surface using femtosecond laser.

Keywords: surface structuring, carbide, femtosecond laser.

Corresponding author: ashfaq_khann@yahoo.com

1. Introduction

Hard materials have got immense applications in material processing especially as abrasive

tool and in chip removal processes i.e as tool insert [1]. Since these materials are hard to

process these tools are made primarily by single step sintering processes. These single step

processes ensures that there is no further requirement to process these hard material.

However, recent studies have shown that adding custom features (Such as slots and textures)

on the tool rack surface, which is the true contact surface of the tool with the material, the

tribological properties could be altered [2-9] . By these micro structures on the tool rack

surface the frictional forces could be reduced, the tool adhesion could be minimized and

tribological properties improved. All of which can result in the extension of tool life. The

tribological behaviour also varies with the variation in the shape and size of these micro

structures [3, 4]. Thus, by creating features of the optimum size and dimension for a

particular material the frictional forces could be minimized and the tool life could be

substantially enhanced. However, the key challenge is to fabricate these structures on hard

material and also the fabrication process should be flexible enough to rapidly accommodate

for changes in the structure dimensions based on its application.

Laser ablation is a flexible technique that can process hard, wear resistant materials which

would otherwise not be processed by chip removal processes [1]. Lasers can work in air,

water and vacuum, combined with the ability to process at the micro and nano scale without

the need of a mask. It is a non contact process so there are no cutting forces involved to cause

unwanted damage to the material. Additionally, with the availability of state of the art

galvanometer scanners custom features and designs could be generated by simple design

changes on PC Software. Since laser is a heat source it suffers from the drawback of inducing

Heat Affected Zones (HAZ) in the material. These HAZ zones exhibit different properties

than the bulk material and can deteriorate the performance of the tool inserts. However, ultra

short pulsed lasers could be utilized to process materials without creating HAZ.

In laser processing, pulse duration is an important parameter when considering laser ablation.

The interaction between material and laser is greatly affected by the laser pulse duration i.e

the temporal distribution of energy. Pulse duration could be utilized to control HAZ. HAZ

generally reduces with reducing laser pulse duration. During laser processing, electrons are

excited instantly and they transfer the energy to the lattice (positive ions) in a duration of

about 1 ps. When the incident energy is high enough and the pulse is of ultra short duration

(as in the case of femtosecond pulse) the ions break the lattice bonding, thus, causing direct

solid to vapour transition. Since there is no transfer of heat to the crystal lattice there is no

HAZ. Thus, Femtosecond lasers have been utilized for applications that require materials to

be processed without creating HAZ [2, 4, 10-12].

This research investigates the potential of utilizing an ultra short pulsed laser (femtosecond

laser) for the texturing of a hard material (Tungsten carbide) tool insert. The range of

processing parameters is explored and the relationship between the process parameters and

the feature dimensions is established. This research is intended to significantly improve the

utilization of tool insert for chip removal surface by improving the tribological properties and

extending the life of the tool inserts.

2 Materials and Equipment

The experimentation was conducted on a Cermet Turning Insert (Tungsten base uncoated flat

cemented carbide (WC-Co), Sandvik TCMW 16 T308 5015). A Femtosecond laser

(Ti:sapphire femto-second laser, wavelength = λ = 800 nm, max power 1 Watt, spot size 50

µm, repetition rate of up to 1 kHz) was used for texturing. The movement of the laser source

come from a computer controlled galvanometer. Characterization of the samples was

conducted by Scanning Electron Microscope (SEM, Hitachi High Technologies, S-3400N). A

3D optical microscope (Alicona Infinite Focus) and White light interferometer (Wyko

NT1100) were used for measuring the depth of textures.

3. Results and Discussions

3.1 Laser processing

The laser processing was initiated to investigate the generation of slots on the material. The

laser beam was focused to a spot size of 50 µm and laser scanning was conducted by a

computer controlled galvanometer. The scan speed was maintained at 10 mm/s and frequency

of 1 kHz. Under these parameters the pulses overlapped approximately 75%. The overlapping

of the spots was intended to generate slots on the material. All the experimentation was

conducted under room conditions. The schematic of the experimental setup is shown in figure

1.

The scanning of the sample was initiated at 0.5 J/cm2 but with this fluence the sample was not

irradiated despite repeated laser scanning of up to 3 x 103 repetitions. At higher pulse

energies up to 3 J/cm2 the effects were still insignificant. Thus, the scanning on the sample

was conducted at further higher laser fluence.

At a fluence of 5 J/cm2, with a repeated scan of up to 1 x 10

3 repetitions the carbide insert

showed little signs of material removal. However at repeated scans of 1.5 x 103 repetitions

the effects began to build up (shown in figure 2). Although there was no distinct depth

indicated by the 3D optical microscope but the SEM images reveal that there is evidence of

ablation and debris formation. The depth of slot increased with the number of repetitions.

With 5 x 103 repetitions the depth and the width of the slots was measured to be 36 ± 5 µm

and 31 ± 5 µm, respectively. The width of the textures is smaller than the laser beam spot size

because the laser had a Gaussian intensity and only the peak intensity of the laser was utilized

for creating textures. This phenomenon is common amongst Gaussian laser and the effect has

been utilized to create textures smaller than the laser beam. Features with lateral dimensions

as small as 100 nm have been generated by utilizing only the peak of the Gaussian intensity

distribution of femtosecond lasers [13]. The processed samples had some micro debris stuck

in the sloth (shown in figure 3a) which was cleaned by sonication in deionised water (shown

in figure 3b). The cross section of the slots is plotted in figure 3c. The relation between the

scan repetition and the depth of the textures is illustrated in figure 3d. It is evident that as the

depth of the slot increases it takes further more repetitions to remove material. Moreover, the

slots have a tapered cross section.

Figure 1: Experimental setup for computer controlled femstosecond laser.

Figure 2: Line textures created after 1.5 x 103 repetitions at 5 J/cm

2.

Figure 3: Line textures created after 5 x 103 repetitions at 5 J/cm

2 (a) before

sonication (b) after sonication (c) cross section of the slots/textures/features (d)

texture depth in relation to the number of repetitions.

3.2 Shape of the texture

The features created on Tungsten carbide are tapered in shape. This is because of the

Gaussian intensity distribution of the femtosecond laser. In the centre of the laser beam spot,

where the higher intensity is available, the material is drilled deeper where as on the

periphery of the beam where the intensity is considerably lower the drilling is comparatively

less. This variation in the intensity and thus the drilling causes the features to be shaped taper.

In this research, the reported values for the width are the highest values measured closer to

the surface of the material.

Figure 4: Line textures created after 1 x 103 repetitions at 5 J/cm

2.

The width of the features varied with the laser scan speed. At a scan speed of 4 mm/s the

width of structure is the maximum. There is a 89% overlap of the laser spots and there is

significant energy available to make the features wide. At scan speed lower than 4 mm/s the

width of features does not increase any further. This is due to the fact that although at lower

scan speeds the pulse overlap percentage does increase but the feature width did not increase

because the maximum energy within Gaussian energy distribution that could possibly be

utilized to ablate the material has been already been utilized. With the increase in the laser

scan speed the width of the feature decreased as shown in figure 4. At a scan speed of 10

mm/s the width of structure is the minimum. At this speed, there is a 75% overlap of the laser

spots but there is just enough overlapping energy to create narrow slots. The peak energy of

the laser Gaussian distribution is utilized for ablation and the narrowest possible features are

created. By increasing the scan speed further the width of features does not decrease any

further. However, eventually at much higher speeds of 50 mm/s the pulses begin to be

separated and distant laser spots create separate features. Figure 5 shows the features created

at 70 mm/s.

Figure 5: Textures created after 2 x 103 repetitions at 5 J/cm

2 and 70 mm/s.

3.2 Mechanism of texture formation

The generation of feature formation is based on the phenomena of ablation. In this case, ultra

short pulse of the femtosecond laser enabled the ablation of material without creating HAZ.

In order to understand the generation of feature the composition of the material needs to be

taken into consideration. The tungsten carbide inserts are made by sintering Carbide

particulates in a cobalt binder at 1350-1500 oC. The carbide particulates provide the tool

insert with the hardness and the cobalt binds the particulates together. The cobalt has a

boiling temperature of about 2926 oC which is about the same as the melting point of the

carbide about 2726 oC. Under the laser irradiation the cobalt with the lower ablation threshold

is evaporated. Although the Tungsten is not ablated but since its binder is evaporated the

tungsten is also removed in the ejected cobalt vapour. Since the femtosecond laser has ultra

short pulse duration, there is not enough time for the electrons absorbing the laser energy to

transfer this energy to the bulk material. Thus, there is no HAZ and the cut quality made by

the femtosecond laser is extremely good [1, 7, 14, 15].

5. Conclusions

In this research, the processing of tungsten carbide for generation of custom structures of

controlled dimensions was experimentally investigated. Relationship between the scan speed,

and pulse repletion was established. A femtosecond laser controlled by a Computer

controlled galvanometer was used to print custom features on the tungsten carbide material. It

was established that for this particular femtosecond laser the pattering of tungsten carbide

was not possible for fluence less than 5 J/cm2. The generated features had a good cut quality

with a little debris which was easily removed by sonication. Moreover, the mechanism for the

formation of textures was also discussed. This research work provides significant

contribution to the scientific knowledge for the processing of Tungsten carbide tool inserts

for the purpose of improving its tribological properties during its utilization in chip removal

processes.

6. Acknowledgements

The corresponding author (A. Khan) gratefully acknowledges the support from the University

of Engineering and Technology (UET), Pakistan. The authors would also like to thank the

staff and members of the Laser Processing Research Centre (LPRC), University of

Manchester, UK, for their support.

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