Periodic nano-ripples structures fabricated on WC/Co based TiAlN coatings by femtosecond pulsed laser

Preparation of TiAlN coatings

YG6 cemented carbide was selected as substrate, and the main components and mechanical properties are listed in Table 1. Before depositing, the substrate was mirror polished and ultrasonically cleaned in acetone and alcohol progressively, each for 5 min, and dried for approximately 20 min in a pre-vacuum dryer. Pure Ti targets and TiAl composite targets with nitrogen gas (N₂) were introduced into the chamber as a reactive atmosphere to obtain the coatings. The specimens were deposited at 200°C, the dc substrate bias voltage was in the range of −40 to −150 V, Ar and N₂ flow were independently controlled using a mass flow controller, the durations for depositing were 60 min.

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Table 1

Properties of cemented carbide materials

Composition (wt-%)	Density/g cm−3	Hardness/GPa	Flexural strength/MPa	Fracture toughness/MPa m1/2	Thermal conductivity/W m−1 K−1	Thermal expansion coefficient/×10−6 K−1
WC+6 wt-%Co	14·6	16·0	1200	14·8	50·2	5·5

The thickness of TiAlN coatings under above deposition conditions were measured with a scanning electron microscope (SEM, QUANTA FEG 250, USA). In addition, to verify the obtained results, measurements were also made by a surface profilometer (Wyko NT9300). The element compositions of the coatings surfaces were characterised by energy dispersive X-ray spectrum (EDX, X-MAX50, UK). The hardness tests were made on the MH-6 hardness tester at a small load (0·2 N) so as to eliminate the influence of substrate on the tests’ results. Adhesion evaluations of coatings were made through the scratch test using the MFT-3000 device. The diamond stylus radius was 200 μm, and the load increased with the rate of 100 N min⁻¹. The phase compositions were examined by XRD (D/max-2400). All the experiments were repeated three times to avoid any ambiguity.

The surface and cross-section SEM micrographs of TiAlN coatings are shown in Fig. 1. It could be seen from Fig. 1a that the surface of TiAlN coatings exhibits relatively uniform, dense, and fine grained structure. There are some distinct white ‘droplets’ on TiAlN surfaces, which is the typical defect of the cathode arc evaporation technique. The ‘droplets’ were formed due to the evaporation of the metal macro particle which cannot react with nitrogen adequately during coating deposition. The coatings thickness is ∼1·67 μm in Fig. 1b. Figure 2 illustrates the X-ray diffraction analysis of cemented carbide substrates and TiAlN coatings. Properties about the coatings were summarised in Table 2.

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1

a surface and b cross-section morphologies of TiAlN coatings

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2

X-ray diffraction patterns of cemented carbide substrates and TiAlN coatings

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Table 2

Properties of TiAlN coatings

	Composition/at.-%
Coatings	Ti	Al	N	Hardness/GPa	Preferred orientation	Critical load/N	Coating thickness/μm
TiAlN	27·11	21·41	51·48	32·4	{111}{200}	77·8	1·67

Femtosecond laser processing

In the experiment, the femtosecond laser pulse with a pulse width of 120 fs, wavelength of 800 nm and repetition rate of 1000 Hz generated by a Ti:sapphire regenerative amplified laser system (Coherent Inc.) was focused through a lens with focal length of 20 cm to the sample surface. The polarization direction and power of the laser were adjusted by a polarizer and an attenuator, respectively. The femtosecond laser pulses with single pulse energy of 2–3·5 μJ were selected, and the laser spot size was estimated to be 6·5 μm. The sample was placed on a three-dimensional XYZ stage with a precision of 100 nm, the incident angle of the laser beam with respect to the sample surface was near normal. Thus, the laser fluence on samples was estimated to be 2·61–4·56 J cm⁻² respectively. A scanning area of Φ = 300 μm was formed by scanning the focused laser beam onto the samples at the velocity of 100–1000 μm s⁻¹. The scanning spacing was 1–5 μm. All tests were carried out in air condition under atmospheric pressure.

Detailed characterisation of the morphological changes of the samples surfaces after laser irradiation was performed by SEM and atomic force micro-scope (AFM, Nanoscope IIIa, USA). The samples surfaces elemental compositions after laser irradiation were revealed by EDX analysis.

Surface morphology

Figures 3–6 present SEM images around the centre of TiAlN coatings surfaces after irradiation with pulse energies of 2–3·5 μJ, scanning speeds ranging from 100 to 1000 μm s⁻¹ and scanning spacing of 5 μm and the EDX surface chemical composition analysis in the selected area. To minimise non-linearity in velocity due to initial acceleration and deceleration, only the centre area of the irradiated samples was analysed. As shown in Fig. 3, following irradiation of the samples with the pulse energy of 2 μJ, HSFLs were created oriented almost to the direction perpendicular to the laser polarization, with the period of 100–140 nm. The SEM analysis (Fig. 3a and b) reveals that coarse, irregular and vague nano-ripples structures were formed on the partial region of the samples irradiated at the relatively high scanning speeds. However, the coarse, irregular and vague region was taken up gradually by arrays of the periodic slender granular ripples with the decreasing scanning speed, as seen in Fig. 3c–f. The results show that at the pulse energy of 2 μJ, with the decrease in scanning speed the change of ripples period was not obvious, but the fine structures were gradually becoming dominant in the laser irradiated region.

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SEM images of ripples evolvement with different scanning speeds a 1000 μm s⁻¹, b 800 μm s⁻¹, c 500 μm s⁻¹, d 300 μm s⁻¹, e 200 μm s⁻¹, f 100 μm s⁻¹ (single pulse energy of 2 μJ, scanning spacing of 5 μm)

Compared with Fig. 3a and b, following irradiation of the samples with the pulse energy of 2·5 μJ, the ripples were formed on the whole TiAlN films surfaces at the scanning speed of 1000 and 800 μm s⁻¹ (Fig. 4a and b), which indicated that the laser pulse energy was important for the formation of periodic ripples. Obviously, the ripples formed at the high scanning speeds were also non-uniform, with some bifurcations. With scanning speed decreasing to 500, 300 and 200 μm s⁻¹, the long, straight and uniform ripples were obtained (Fig. 4c–e). However, at the scanning speed of 100 μm s⁻¹, two distinct regions of femtosecond-LIPSS structures (LSFLs and HSFLs) have been observed on the modified sample surface (Fig. 4f). The EDX surface chemical composition analyses on the two different areas (point 1 and point 2) are illustrated in Fig. 4g and h respectively. The abundant Ti, Al, N elements and negligible W element are identified in point 1, which illustrates that the HSFLs were generated on the TiAlN films. And the existent of negligible W element may be attributed to interdiffusion between the TiAlN films and the cemented carbide substrates due to the effect of heat accumulation within the focal volume of the laser beam irradiating the sample. ¹⁵ However, only the substrate elements (W, C and Co) and O element exist in point 2, which illustrates that part of the TiAlN films on substrates were completely removed by the femtosecond laser with single pulse energy of 2·5 μJ and scanning speed of 100 μm s⁻¹, thus the LSFLs with period of ∼590 nm were formed on the cemented carbide substrates. The orientation of the LSFLs was also almost perpendicular to the laser polarisation, consistent with the classical theory of LIPSSs formation. ¹⁶

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SEM images of ripples evolvement with different scanning speeds a 1000 μm s⁻¹, b 800 μm s⁻¹, c 500 μm s⁻¹, d 300 μm s⁻¹, e 200 μm s⁻¹, f 100 μm s⁻¹ and g, h EDX composition analyses in selected area (point 1 and 2) of f respectively: inserts are local magnified images of every image (single pulse energy of 2·5 μJ, scanning spacing of 5 μm)

Apparently, following irradiation of the samples with the pulse energy of 3 μJ, the ripples with larger period can be obtained in all cases (see Fig. 5). Meanwhile, it could be seen that the sample with scanning speed of 500 μm s⁻¹ showed long, straight and uniform ripples (Fig. 5c) which were of the best quality among the samples tested with scanning speed of 300–1000 μm s⁻¹. Clearly, according to the above analysis in Fig. 4f, with single pulse energy of 3 μJ and scanning speed of 200 μm s⁻¹ TiAlN films on substrates were also partially removed by femtosecond laser (Fig. 5e). Apparently, in addition to the larger period, the ripples formed on cemented carbide substrates surfaces were clearer, longer, straighter and more uniform than that formed on TiAlN films (Fig. 5g and h), which could be explained by the different intrinsic properties of different materials. ¹⁷ With the scanning speed decreasing to 100 μm s⁻¹, all of the TiAlN films on substrates were ablated, and the ripples formed were all LSFLs having a period of ∼530 nm (Fig. 5f). Figure 6 shows SEM images around the centre of TiAlN coatings surfaces after irradiation with pulse energy of 3·5 μJ and the EDX surface chemical composition analysis in the whole modified zone of SEM images. From the element analysis, it can be deduced that at the scanning speed of 1000, 800 and 500 μm s⁻¹, the periodic ripples with mean period of 340–500 nm were formed on the TiAlN films surfaces, as shown in Fig. 6a–c; while following irradiation of the samples with the scanning speed of 300, 200 and 100 μm s⁻¹, the ripples with period close to the irradiation wavelength were formed on the cemented carbide substrates surfaces, as shown in Fig. 6d–f. It should be noted that the residual coatings elements in Fig. 6d and e may originate from the redeposition of TiAlN films after complete ablation of the coatings. The results show that the scanning speed region for LSFLs formation on cemented carbide substrates surfaces widens with the increasing pulse energy.

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SEM images of ripples evolvement with different scanning speeds a 1000 μm s⁻¹, b 800 μm s⁻¹, c 500 μm s⁻¹, d 300 μm s⁻¹, e 200 μm s⁻¹, f 100 μm s⁻¹ and g, h local magnified images of e: inserts are local magnified images of every image (single pulse energy of 3 μJ, scanning spacing of 5 μm)

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SEM images of ripples evolvement with different scanning speeds a 1000 μm s⁻¹, b 800 μm s⁻¹, c 500 μm s⁻¹, d 300 μm s⁻¹, e 200 μm s⁻¹, f 100 μm s⁻¹: EDX composition analysis images show elemental compositions of whole modified zone at different scanning speeds (single pulse energy of 3·5 μJ, scanning spacing of 5 μm)

Another factor that determines the quality of the formed ripples is scanning spacing of the laser beam. Based on our previous observation, the effective radiation diameter formed by a single scan is about 5 μm under the test conditions. ¹⁸ If the scanning spacing is larger than the radiation diameter, it will cause the discontinuity of ripples formed on the surface. To investigate the effect of overlap spots, we changed the scanning spacing from 1 μm to 5 μm, while keeping pulse energy of 3 μJ and scanning speed of 500 μm s⁻¹. Figure 7 shows the SEM images of the nano-ripples formed on the surfaces of TiAlN coatings by femtosecond laser pulses with different scanning spacings and the EDX surface chemical composition analyses in the selected area. Clearly, the sample with the scanning spacing of 5 μm showed continuous and uniform ripples with large period which were of the best quality among all the samples tested under different scanning spacings (Fig. 7a), while the ones with scanning spacing of 1 and 3 μm showed chaotic granular and non-uniform ripples with small period (Fig. 7b and c). The EDX surface chemical composition analyses on the samples with scanning spacing of 5 and 1 μm (point 1 and 2) are illustrated in Fig. 7d and e respectively. Only the chemical composition of TiAlN coatings was identified on point 1. However, in addition to the chemical composition of TiAlN coatings, W, C and O elements were also identified on point 2. It can be said that if the scanning spacing is much smaller than the radiation diameter, it will cause the repeated scanning on the same area and the overlapping of ripples formed on the surface.

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SEM images of ripples formed on surface of TiAlN coatings by femtosecond laser pulses with different scanning spacings: a 5 μm, b 3 μm, c 1 μm and d, e EDX composition analyses in selected area (point 1 and 2) of a and c respectively (single pulse energy of 3 μJ, scanning speed of 500 μm s⁻¹)

Obviously, based on the above experimental observation, with the different processing parameters the surface morphology of irradiated samples was changed, and the quality of the formed ripples depended strongly on the pulse energy, scanning speed, and scanning spacing. HSFLs were only found on the TiAlN films and more pronounced when the sample was irradiated with lower pulse energy and lower scanning speed. Meanwhile, we found that optimum parameters for the formation of clear and regular ripples on TiAlN films were 3 μJ pulse energy, 500 μm s⁻¹ scanning speed and 5 μm scanning spacing, as shown in Fig. 7a. In case of 2 and 2·5 μJ pulse energies, the periodic structures formed were more and more visible with the decreasing scanning speed due to the increasing energies deposited on the samples. However, the nano-structures were highly short range granular, non-uniform and with low period, perhaps due to the energy deposition in these cases was not enough for the formation of good quality ripples. The pulse energy of 3·5 μJ, scanning speeds ranging from 100–300 μm s⁻¹, and pulse energy of 3 μJ, scanning speeds ranging from 100–200 μm s⁻¹ were observed to remove part or all of the TiAlN films on the substrates, due to the energy deposition on samples much more over the target threshold energy for ablation. The other combination of pulse energy and scanning speed also can not generate the clearer, longer, straighter and more uniform ripples on TiAlN films than pulse energy of 3 μJ and scanning speed of 500 μm s⁻¹. In case of 1 μm and 3 μm scanning spacings (3·5 μJ pulse energy and 500 μm s⁻¹ scanning speed), the structures formed on the TiAlN coatings were also irregular and non-uniform.

Ripples dimension

Figure 8 shows the AFM images of two-dimensional surface topographies, and the cross-section images of ripples corresponding to the line after irradiation with pulse energy of 3·5 μJ, scanning speed of 800 μm s⁻¹ and scanning spacing of 5 μm (a and b), pulse energy of 3 μJ, scanning speed of 100 μm s⁻¹ and scanning spacing of 5 μm (c and d). As shown in Fig. 8b, the period of the periodic ripples formed was ∼440 nm, and the depth of the ripples was ∼170 nm, whereas the period of the periodic ripples formed in Fig. 8d was ∼530 nm, and the depth of the ripples was ∼200 nm. The period of the ripples on the visible modified region of TiAlN films surfaces varied with the increasing scanning speed and different pulse energies is plotted in Fig. 9a. Obviously, the period of ripples increased with the increasing pulse energy and increasing scanning speed, and the rate of increase in the period at the relatively low pulse energy and scanning speed was smaller than that at the relatively high pulse energy and scanning speed. Figure 9b shows the period of the ripples formed on the cemented carbide substrates varies with the pulse energies and scanning speeds. With increasing pulse energy, the scanning speed for which the periodic nano-ripples began to be uniformly formed on cemented carbide substrates increased from 100 to 300 μm s⁻¹. From the limited data shown in Fig. 9b, we can deduce faintly that the period of ripples increased with the decreasing pulse energy and increasing scanning speed. Table 3 shows the period of the ripples formed on the surfaces of TiAlN coatings after irradiation with 3 μJ pulse energy and 500 μm s⁻¹ scanning speed at different scanning spacings. It can be seen that, when the scanning spacing is smaller than the radiation diameter, the ripples period increased with the increasing scanning space.

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Table 3

Period of ripples formed on surface of TiAlN coatings with different scanning spacings (single pulse energy was 3 μJ, scanning speed was 500 μm s⁻¹)

Scanning spacing/μm	1	3	5
Ripples period/nm	189	213	240

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AFM images of two-dimensional surface topographies, and cross-section image of ripples corresponding to line after irradiation with a, b 3·5 μJ pulse energy, 800 μm s⁻¹ scanning speed and 5 μm scanning spacing; after irradiation with c, d 3 μJ pulse energy, 100 μm s⁻¹ scanning speed and 5 μm scanning spacing

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9

Period of ripples formed on a TiAlN films and b cemented carbide substrates as function of pulse energy and scanning speed: (scanning spacing of 5 μm)

The formation mechanism of ripples has been studied deeply, and it may be different for HSFLs and LSFLs. The origin of HSFLs is still quite controversially discussed in the literature and different theories have been introduced such as second harmonic generation,^19,20 self-organisation, ²¹ excitation of surface plasmon polaritons,^22–24 the interference between the incident beam and the scatter waves ²⁵ and Coulomb explosion. ²⁶ However, the formation of LSFLs on multiple materials which is more common have been generally explained by the surface plasmon polaritons (SPPs) excited in the surface layer by femtosecond laser irradiation. And the period of the ripples Λ is related to the plasmon coupling theory: ²⁴ (1) where λ is the incident light wavelength, η = Re[ϵ/(ϵ+1)]^1/2 is the real part of the effective index of the surface plasmon mode, θ is the angle of incidence, ϵ is the dielectric constant, g is the grating vector and E is the electrical field vector of the incident wave. From equation (1), we can see that the period of the ripples has a direct relationship with the laser wavelength λ, the incidence angle θ, and the effective refractive index. Typically, θ = 0° for normally incident laser light, thus we could obtain the simple relationship: Λ≈λ/η. Obviously, based on the above experimental data, the period of the LSFLs formed on TiAlN films and cemented carbide substrates is smaller than λ, and the Λ increases with the increasing scanning speed, which is in line with the investigations by Vorobyev et al.,^27,28 it can be explained by the change of real part of the effective index of the air sample interface because of surface roughening and structuring induced by femtosecond laser pulse irradiation. It is known that surface roughness causes an increase in the modulus of the surface plasmon wave vector, and this will correspond to an increase in the real part of the refractive index. ²⁷ According to equation (1), an increased real part of the refractive index for propagating surface plasmons will cause a reduced LIPSSs period, and this agrees with our experimental observation. To gain the insight of how initial surface conditions affect the formation of femtosecond-LIPSSs, we carried out a SEM study of the formation of femtosecond-LIPSSs with samples of different surface nanoroughness. We found that the extended LIPSSs were produced with a higher scanning speed when the sample had a greater surface nanoroughness. This phenomenon may be due to that the great surface nanoroughness can develop and affect the propagation of surface plasmons. ²⁸ In addition, a number of studies can be considered to further elucidate the formation mechanism for the ripples using some novel methods. For example, the finite difference time domain (FDTD) method has been used to study the inhomogeneous absorption of linearly polarised laser radiation below a rough surface, which showed the periodicity was not always close to the laser wavelength for the perpendicular direction and suggested that LIPSSs are a signature of the localization of light due to a rough surface. ¹³

Changes in element content of samples

In addition to the surface morphology and ripples dimension analyses, changes in the element content of the samples irradiated in air with the different processing parameters are analysed systematically using the EDX technique. First, the changes of surface composition around the transition region from unmodified zone to modified zone were analysed by line scanning, and the results obtained are given in Fig. 10. It can be seen that Ti, Al and N elements on the modified zone irradiated with pulse energy of 2 μJ, scanning speed of 100 μm s⁻¹ and scanning spacing of 5 μm remained almost unchanged compared to that measured on the unmodified zone, whereas the O element was increasing slightly (Fig. 10a). It indicated that the modification was mild, and slight oxidation occurred. However, the constitutive elements of TiAlN films were decreased abruptly and heavily from the unmodified zone to modified zone irradiated with the pulse energy of 3 μJ, scanning speed of 100 μm s⁻¹ and scanning spacing of 5 μm (Fig. 10b). At the same time, no film cracks or delamination was observed around the transition region. This revealed that severe laser ablation of the TiAlN films on the modified zone occurred without damaging the surrounding TiAlN coatings. The O element was also increasing slightly. Then, the changes in the element content of the whole modified zones irradiated with pulse energies of 2–3·5 μJ at scanning speed of 100 μm s⁻¹ and scanning spacing of 5 μm were analysed. The results obtained are shown in Table 4. Apparently, with the increasing pulse energy Ti, Al, and N elements contents were reduced gradually, and eventually disppeared completely; whereas, W, C, and Co elements contents were increased gradually. Meanwhile, the O element content was increased from 2·56 to 7·89% with the increasing pulse energy from 2 to 3·5 μJ, which illustrated oxidation occurred. The changes in the element content of the samples irradiated indicate the ablation of TiAlN coatings, and the more obvious changes mean that the ablation of TiAlN coatings is severer.

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Line scanning images of Ti, Al, N and O element around transition region from unmodified zone to modified zone on samples irradiated with pulse energy of a 2 μJ and b 3 μJ at scanning speed of 100 μm s⁻¹ and 5 μm scanning spacing

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Table 4

EDX elemental analysis of samples with different pulse energies at scanning speed of 100 μm s⁻¹ and scanning spacing of 5 μm

	Element content/at-%
Energy/μJ	Ti	Al	N	W	C	Co	O	Total
0	27·11	21·41	51·48	…	…	…	…	100
2	26·85	19·56	47·97	0·84	1·60	0·62	2·56	100
2·5	12·28	9·67	23·19	13·89	25·82	10·29	4·86	100
3	2·89	2·29	5·49	23·06	42·87	17·08	6·32	100
3·5	…	…	…	25·58	47·57	18·96	7·89	100