Femtosecond Laser texturing of tungsten carbide Ashfaq Khan 1 , Mushtaq Khan 2 , Aftab Khan 1 , Syed Husain Imran 2 , Kamran Shah 1 , Mohammad.A Sheikh 2 , Lin. Li 2 1 University of Engineering and Technology, Peshawar, Pakistan. 2 School of Mechanical and Manufacturing Engineering, National University of Sciences and Technology (NUST), Islamabad, Pakistan. 3 School 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: [email protected]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
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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.
(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.
Dr Kamran Shah
Sticky Note
slot
Dr Kamran Shah
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remove more
Dr Kamran Shah
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remove "a"
Dr Kamran Shah
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cross sections.
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.
Dr Kamran Shah
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Divide this Figure into two. 1. Line texture before and after sonication. 2. Cross section of slot and the texture depth relationship with no. of repetition.
Dr Kamran Shah
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You used the word ablation before and now drilling....I think you should use ablation depth instead of drilling.
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.
Dr Kamran Shah
Sticky Note
This figure should go after the following paragraph.
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.
References
1. Dumitru, G., Romano, V., Weber, H.P., Gerbig, Y., Haefke, H., Bruneau, S., Hermann, J. and
Sentis, M., "Femtosecond laser ablation of cemented carbides: properties and tribological