Laser-assisted ablation and plasma formation of titanium explored by LIBS,QCM, optical microscopy and SEM analyses along with mechanical modifications under different pressures of Ar and Ne

Maryam RIAZ, Shazia BASHIR, Asma HAYAT and Zarish NAZ

Centre for Advanced Studies in Physics GC University Lahore, Punjab 54000, Pakistan

Abstract This study deals with the investigation of Nd:YAG laser-assisted ablation and plasma formation of Ti at irradiance of 0.85 GW cm-2 under Ar and Ne environment at various pressures ranging from 10-120 Torr.Laser-induced breakdown spectroscopy is used to evaluate plasma parameters, whereas quartz crystal microbalance is used for ablation yield measurements.The crater depth is evaluated by optical microscopy.The surface features are explored by scanning electron microscope (SEM) analysis and the micro-hardness is measured by a Vickers hardness tester.It is observed that the plasma parameters are higher in Ar than in Ne, and are strongly correlated with the ablation yield, ablation depth, surface features and hardness of laser-ablated Ti.These parameters increase with increasing the pressure of environmental gases, attain their maxima at 40 Torr for Ar and at 60 Torr for Ne.Afterwards,they show a decreasing trend up till a maximum pressure of 120 Torr.The maximum value of the electron temperature (Te) is 5480 K, number density (ne) is1.46 × 1018 cm-3 ,ablation depth is184 μm, ablation yield is 3.9 ×1015 atoms/pulse and hardness is 300 HV in the case of Ar atmosphere.SEM analysis reveals the growth of surface features, such as cones, ridges and pores, whose appearance is more distinct in Ar than Ne and is attributed to temperature,pressure and density gradients along with recoil pressure of the Ti plasma.

Keywords: ablation, plasma, electron temperature, electron density, laser-induced breakdown spectroscopy, surface structures

1.Introduction

Laser-assisted ablation and plasma formation are highly beneficial for technologically advanced applications, such as nanoparticle generation, pulsed laser deposition of thin films,micro/nano-structuring, welding, drilling and cutting along with micromachining of materials [1, 2].

Laser-assisted ablation and plasma formation can be identified and analyzed using various techniques including laser-induced breakdown spectroscopy (LIBS) [3] and quartz crystal microbalance(QCM)[4].LIBS provides spectral data for the evaluation of the electron temperature(Te)and electron number density (ne) of plasma [5].QCM is a mass-sensitive instrument that works on the principle of piezoelectric effect and is used to monitor small mass changes for the analysis of ablation yield[6].Ti is a transition metal with a wide range of applications in aerospace, power generation, sporting goods,dental and medical industries [7].

The investigation of a combination of optimum parameters,such as laser irradiance and environmental conditions,is highly important for controlling ablation efficiency,plasma parameters,surface structuring and hardness of materials.The properties of environmental gases, such as their thermal conductivity, ionization potential (E), mass (m), ratio of ionization potential to mass, i.e.E/m, and their pressures are favorable factors for plasma confinement.Their presence at certain pressures restricts the free expansion of metallic plasma in the surroundings, which results in an increase in collisional frequency and greater momentum transfer.This helps in the enhancement of plasma parameters that has been reported by different research groups[5,8].Hermann et al[9]discovered through LIBS analysis that maximum emission intensities of Ti plasma are obtained under 20, 80 and 200 Torr pressures forAr, Ne and He, respectively.Bartolić et al [10] reported that the emission intensities of Ti plasma were slightly higher in vacuum than in air, whereas the number density is smaller in vacuum than in air.Bashir et al[11] reported that the maximum values of Cd plasma parameters are higher in the case of Ar compared to He and air.

Svendsen et al [12] reported that ablation yields of Ag and Ni are higher in Ar than in N2environment.Zhang et al[13] reported that the ablation rate of Au thin film increased monotonically from a few nm to 400 nm with increasing pressure of Ar.Tehniat et al [14] investigated the strong dependency on the nature and pressure of environmental gas on ablation yield and surface structuring of Fe.

The aim of the present work is to evaluate Ti-plasma parameters using LIBS analysis under different pressures of Ar and Ne.The enhancedTeis usually related to enhanced ablation rate of laser-irradiated material.The direct correlation betweenTeand the mass ablation rate of Ti plasma is established for the first time by measuring the ablation yield of Ti after laser ablation under similar conditions.QCM is used for the measurement of ablation yield.Environmental conditions,such as the nature and pressure of ambient gases, influence the various primary and secondary mechanisms responsible for mass removal as well as governing the optical/physical properties of the generated plasmas.Optical microscopy,scanning electron microscope (SEM) analysis and hardness testing have been performed for the study of the ablation depth, surface modification and micro-hardness of irradiated Ti under the same environmental conditions.The maximum ablation rate has been well correlated with the maximum emission intensity,Teandneof Ti plasma as well as with distinct surface features and maximum hardness of ablated Ti.

Figure 1.Schematic of the experimental setup for (a) LIBS analysis of Ti and (b) ablation yield measurements of laser-irradiated Ti using QCM.

2.Experimental details

Commercially available circular-shaped Ti with a diameter of 2.3 cm × 0.4 cm after polishing and grinding was ultrasonically cleaned for 20 min.A Q-switched Nd:YAG laser(Quantel 981 C, 1064 nm, 10 ns, 55 mJ) was employed as an irradiation source for Ti ablation and plasma formation at irradiance of 0.85 GW cm-2.Ti samples were placed in rotating sample holders in a vacuum chamber.The chamber was evacuated to a base pressure of 10-3Torr using a rotary vane pump.Both inert gases Ar and Ne were filled separately in the chamber at pressures of 10, 20, 40, 60, 80, 100 and 120 Torr.The pressure was measured with a pressure gauge and a manometer.The laser was focused with the focusing lens with a focal length 20 cm and the averaged measured focused spot diameter of907μm was analyzed by SEM.

For LIBS analysis of Ti plasma, a spectrometer (LIBS 2500,Ocean Optics USA)was used.The schematic is shown in figure 1(a) and its details are reported in [11].The second set of experiments was performed for the laser-based ablation yield measurements of Ti using QCM (QCM 200 Quartz Crystal Microbalance Digital Controller, SRS.Inc., Sunnyvale, California, USA).Its schematic is shown in figure 1(b).The QCM was placed at an angle of 45° with respect to target surface normal.A quartz crystal with a 5 MHz frequency was placed as a substrate at a distance of 1.3 cm from the Ti target surface.The Ti target was exposed to 100 shots of laser.The ablation yield was then measured as a function of ambient pressure for both Ar and Ne due to the change in frequency of the quartz crystal after deposition from the irradiated targets.

The third set of experiments was performed for the exploration of surface features grown on laser-irradiated Ti using SEM(JEOL JSM-6480 LV)analysis.For this purpose,the Ti targets were also exposed to 100 shots under both environments of Ar and Ne for all seven pressures used in sets 1 and 2.

For the measurement of crater depth, optical emission microscopy (STM-6 Olympus) was used after laser ablation of Ti under different environmental gases.In order to explore the increase in surface hardness and to correlate it with the surface structures and plasma parameters, the hardness of the material was measured with a Vickers hardness tester(Zwick/Roell ZHU-5030).

Figure 2.Emission spectra of Ti plasmas under(a)Ar at 40 Torr pressure,(b)Ne at 120 Torr pressure and the variation in emission intensities of laser-induced Ti plasma at various pressures under ((c) and (d)) Ar and ((e) and (f)) Ne.

3.Results and discussion

3.1.The plasma parameters,ablation yield and crater depth of laser-ablated Ti and their correlation

Figures 2(a) and (b) show the emission spectra of Ti plasma ranging from 450-510 nm obtained under both environments of 40 Torr(maxima)for Ar and 120 Torr(maxima)for Ne.Whereas figures 2(c)-(f) show the variation in emission intensities of selected spectral lines as a function of pressures of Ar and Ne ranging from 10-20 Torr at a fixed irradiance of 0.85 GW cm-2.

It can be clearly seen from the spectra that both pressure and environment play a significant role in the enhancement of the emission intensity of Ti.The emission intensity of Ti plasma at 40 Torr is significantly higher than that at 10 Torr in the case of Ar, whereas the emission intensity at 120 Torr is higher than that at 10 Torr in the case of Ne.

Initially, there is an increase in emission intensity with the increase in pressure up to 40 Torr for Ar and then it decreases up to 120 Torr.However,in the case of Ne there is an increase in emission intensity of Ti plasma up to 120 Torr.From figure 2, it is also clear that all spectral emission intensities of Ti plasma are significantly higher in Ar than in Ne.In the present work, the 5% error bars are included for three repetitions in order to minimize uncertainty.

Figure 3.Variation in electron temperature under (a) Ar and (b) Ne, and number density under (c) Ar and (d) Ne, ablation yield and crater depth under ((a) and (c)) Ar, and ((b) and (d)) Ne of laser-ablated Ti plasma at various pressures.

Table 1.Spectroscopic data for selected spectral lines of Ti [15].

The spectroscopic data of selected Ti lines,obtained from the Atomic Lines List [15] are listed in table 1.

For the evaluation of electron temperature (Te), the plasma is considered to be optically thin under the local thermodynamic equilibrium (LTE) conditions [11].Ti emission lines of wavelengths of 451.224, 485.668, 488.541 and 503.690 nm are selected for the evaluation of electron temperature, whereas, the emission line at wavelength of 453.461 nm is selected for electron number density evaluation of Ti plasma.The Boltzmann plot method[11]is used for the evaluation of electron temperature and the Stark broadening mechanism [16] has been used for electron number density.The observed line shapes have been corrected by subtracting the contribution of the instrumental width [17]:

The instrumental width of the LIBS 2000 spectrometer system is 0.05(2) nm [17, 18].

Figures 3(a)-(d) show the variation in evaluated values ofTeand neof Ti plasma under Ar and Ne as a function of their pressures.For Ar, there is an increase inTeand neof Ti plasma from 3700 to 5480 K and from1.30× 1018to 1.46× 1018cm-3as the pressure increases from 10 to 40 Torr.Whereas for Ne the increase inTeand neof Ti plasma is from 3430 to 4820 K and from1.25× 1018to 1.37 × 1018cm-3for the variation in pressure from 10 to 60 Torr.A further increase in the pressure up to 120 Torr results in a decrease in electron temperature and density of Tiplasma values for both Ar and Ne.

Figure 4.Variation in plasma pressure under different pressures of Ar and Ne.

For plasma to be in LTE the necessary condition of McWhirter’s criterion must be fulfilled which states that[19],

Here,Teshows the temperature of the plasma andΔE shows the bandgap energy.This condition must be fulfilled to confirm that the plasma is in LTE [19].By substituting Teand ΔE in equation (2), the electron density becomes≈1 015cm-3,whereas the experimentally measured electron densities of Ti plasma are in the range of 1.35 ×1018to 1.46 × 1018cm-3, which are significantly higher than the estimated values.Therefore,the plasma is said to be in LTE.

Figures 3(a)-(d) also show the evaluated values of ablation yield as well as crater depth of laser-irradiated Ti under the ambient environments of Ar and Ne at a fixed irradiance of 0.85 GW cm-2.The ablation yield of Ti is measured by QCM using the relation given in [20].All QCM, optical emission spectroscopy and SEM measurements were performed for 100 laser pulses.In the presence of Ar,the increase in ablation yield is from 3.1 ×1014to 3.9 ×1015atom /pulse and crater depth is from 136 to 184 μm up to 40 Torr.Afterwards,it decreases up to 120 Torr.Similar trends are observed in Ne environment with the maxima at 60 Torr.The values of ablation yield vary from 2.4 × 1014to 2.38 ×1015atom/pulse and crater depth varies from 123 to 137 μm up to this pressure and then it decreases to 120 Torr.

The graphs of figures 3(a)-(d) also reveal a direct correlation between ablation yield and the evaluated plasma parameters (Teandne) and crater depth.The increase in ablation yield with pressure produces more dense plasma,which results in an increase in electron temperature and number density.Moreover,the decrease in plasma parameters is attributed to the decrease in ablation yield of Ti under both environments.

Table 2.Some important physical properties of environmental gases of Ar and Ne [23].

3.2.Discussion

The results obtained from LIBS, QCM and optical microscopy show a significant dependence of emission intensities,plasma parameters and ablation yield on the nature and pressure of Ar and Ne.The enhancement in Tein plasma is usually attributed to the enhancement in ablation rate of a solid material [12].The innovation of this present work is to correlate the variation in Tewith the directly measured ablation yield and ablation depth of craters.

The increase in emission intensities,Te,ne,crater depth and ablation yield of laser-ablated Ti with the increase in pressure up to certain values is attributed to the increase in collisional frequencies as well as the increase in plasma pressure.Figure 4 shows the analytically evaluated values of laser-induced Tiplasma pressure (P) using the expression P =TeKBne[21].

The enhancement in collisional effects becomes more pronounced due to the decrease in mean free path between charged species[21].This results in higher cascade growth of plasma species [9].The plasma formed in the presence of ambient gas also transfers a fraction of its energy to the material due to its confinement and may result in more energy transfer while generating laser-induced plasma-assisted ablation effects [12].These effects become more prominent at higher pressures with higher Te,neand ablation yield.

With a further increase in pressures up to certain values(above 40 Torr for Ar and 60 Torr for Ne in the present case),a decrease in emission intensities,Te,neand ablation yield are observed.The decrease at higher pressures refers to the absorption of incoming laser in the shock wave front [22].Hence,more energy is transferred to an ambient environment increasing the conduction power rather than the absorption power ( Pa＜Pc) from the equation Pa= Pv+ PR+Pc.The plasma tends to cool down more rapidly, which in turn decreases the Te, neand ablation yield [21].

While comparing the nature of ambient environments,higher values of spectral intensities,Te,neand ablation yield are observed for Ar than for Ne.The values of ablation as well as plasma parameters of Ti increase with increasing the pressure of environmental gases and achieve their corresponding maxima at a certain pressure (i.e.40 Torr for Ar and 60 Torr for Ne).Afterwards,decreasing trends are observed with further increase in pressures of both environmental gases up to a maximum value of 120 Torr.However,the maximum values for both Ar and Ne were different due to their different nature and their different physical properties.These properties are ionization potential,thermal conductivity, mass and E/m ratio, etc [6].Some important physical properties of gases are listed in table 2 [23].

Figure 5.SEM micrograph of irradiated Ti at angles of (a) 90° and (b) 45° for a single shot of Nd: YAG laser at minimum irradiance of 0.85 GW cm-2.

The ionization potential of Ar is 15 eV, while for Ne it is 21.5 eV.Therefore,the ionization rate of Ar is higher than that of Ne.This breakdown of Ar gas helps to enhance the cascade growth of electrons.Similarly, the smaller E/m ratio of Ar also favors higher cascade growth in Ar compared to Ne environment.

In Ne, with smaller atomic mass of 20 amu, Ti plasma cools down much faster in comparison to Ar, with a higher atomic mass of 40 amu.Similarly, comparing the nature of gases, in Ar greater values of Ti-plasma parameters are obtained since the value of the thermal conductivity of Ar(1 .772 ×10-4W (c m · K)-1) is smaller than that of Ne(4 .93 ×10-4W (c m · K)-1).Therefore, it is found that the nature of the gas also influences LIBS, QCM and ablation depth measurements [4].

3.3.SEM analysis

In order to correlate the plasma parameters with surface structuring, SEM analysis of laser-ablated Ti was performed.

The angles of incidence with respect to the target surface are different in the case of LIBS analysis and ablation yield measurements by QCM.However, the purpose of the present investigation is to explore the effect of the pressure as well as the nature of the gaseous environments on the surface modification,plasma parameters and ablation yield of Ti.For LIBS analysis,all measurements were performed at the same angle, i.e.90° with respect to the target surface.It is then established at which pressure the maximum emission intensity, electron temperature and number density of Ti plasma are achieved.These plasma parameters are then correlated with surface structuring.Similarly,the optimum value of the pressure of environmental gas is explored to establish its maximum ablation yield.This maximum ablation yield is then correlated with plasma parameters and surface structuring.The ablation yield measurements are only possible at an angle of 45° with respect to the target surface.Therefore, the QCM was placed at an angle of 45° with respect to the target surface for the measurement of forward-peaked ablation species.According to our point of view,the angle is not that important for playing a decisive role in controlling plasma parameters and ablation yield when for each measurement the angle of incidence remains the same.Of course,the angle will affect the focused spot diameter on the target surface and consequently the laser irradiance will change.For this purpose, it is shown in figures 5(a)and (b) how the angle of incidence of the laser beam minutely affects the focused spot diameter,and overall the same laser spot diameter is assumed for both measurements.

Figures 6 and 7 represent the magnified SEM images of the inner peripheral ablated region of Ti at various pressures of (a)10,(b)20,(c)40,(d)60,(e)80,(f)100 and(g)120 Torr in the presence of Ar (figure 6) and Ne (figure 7).The formation of microstructures,such as ridges,cones and pores can be observed at the inner peripheral boundary under both environments.

The formation of microstructures, such as ridges and cones at the inner periphery, is due to the vaporization and splashing of melted material as a result of thermal stresses induced by the laser irradiation.The movement of molten material away from the center is attributed to the pressure gradients produced by the Gaussian profile of the laser beam[24].Cones are formed due to the impurities, voids and inhomogeneities present in the Ti material.These radiationresistant impurities generally affect the absorption properties of material and consequently the ablation process at preferred lattice sites.Hence, the non-uniform absorption in the material causes heating, melting and hydrodynamical sputtering that result in the formation of conical structures.Similarly,the formation of ridges on the surface is due to incomplete melting and thermal expansion of ablated Ti material away from the center because of the greater recoil pressure [11].

Figure 6.SEM images representing the formation of ridges and cones at the inner peripheral ablated region of Ti targets at various pressures of Ar gas.(a) 10, (b) 20, (c) 40, (d) 60, (e) 80, (f) 100 and (g) 120 Torr.

The surface temperature increases for Ti are estimated using the relation [25],

where T0is the initial surface temperature,R is the reflectivity(at 1064 nm) which is 56%, K is the thermal conductivity(17 W(m·K)-1),a is the thermal diffusivity(0.07 cm2s-1),t is the pulse duration of laser used(10 ns)and I0is the incident laser intensity(0.85 GW cm-2).The incident intensity surface temperature is estimated to be 21108 K.

The growth of cones and ridges can also be attributed to temperature, pressure and density gradients that are ascribed to Kelvin-Helmholtz instability and Raleigh-Taylor instability [16].

Pores formed at higher pressures are due to the presence of diffused gas in the melted material that settles on the surface.When this diffused gas is expelled from the molten material by the laser irradiation,porous structures are formed.This is attributed to the non-equilibrium phase change when the gas is diffused within the material and the pressure drops during re-solidification [24].

Figure 7.SEM images representing the formation of ridges and cones at the inner peripheral ablated region of the ablated Ti targets at various pressures of Ne gas.(a) 10, (b) 20, (c) 40, (d) 60, (e) 80, (f) 100 and (g) 120 Torr.

It can be observed from the figures that there is a maximum number of densities of conical structures along with prominent ridge formation at the minimum pressure of 10 Torr for both Ar and Ne.With further increase in pressure up to 40 Torr of Ar and 60 Torr of Ne, these structures are completely suppressed with pronounced melting.The maximum suppression at these pressures is attributed to maximum plasma recoil pressure due to the maximum values ofTeandneof Ti plasmas evaluated by LIBS analysis and maximum ablation yield by QCM as well as maximum ablation rate.With further increase in ambient pressures, these structures start to re-grow on the surface due to smaller values ofTe,ne,recoil pressure and ablation yield and ablation depth[19,26].

While comparing the nature of ambient environments on the growth of these observed structures, it can be observed that ridges that formed in the presence of Ar are more suppressed.In addition,the number density of cones is greater in the case of Ar than that of Ne.However,in Ar these structures are more prominent compared to Ne.This is attributed to higher evaluated values ofTe,ne,ablation yield and ablation depth of Ti in the presence of Ar than Ne.This can be explained on the basis of greater mass and smaller E/m ratio,higher thermal conductivity as well as smaller ionization potential of Ar compared to Ne.Hence,the laser-generated Ti plasma in Ar environment has more plasma pressure leading to moreTeandnevalues due to less expansion velocity.Thus,more suppression in surface structures can be observed in the presence of Ar than that of Ne [11].

Figure 9.SEM images representing the formation of grains at the outer peripheral regions of laser-ablated Ti at various pressures of Ne of(a)10, (b) 20, (c) 40, (d) 60, (e) 80, (f) 100, and (g) 120 Torr.

Figure 10.Comparison of micro-hardness of untreated and lasertreated Ti in different environmental gases of Ar and Ne at different pressures from 10-120 Torr.

Figures 8 and 9 represent the SEM images of the outer peripheral boundaries of irradiated Ti at various pressures of(a)10,(b)20,(c)40,(d)60,(e)80,(f)100 and(g)120 Torr in the presence of Ar (figure 8) and Ne (figure 9).Distinct grains are observed under both Ar and Ne at the outer peripheral boundary.The grain boundaries become more distinct with the increase in pressure of both gases.The electron density, electron temperature and ablation yield of Ti-ablated plasma are maximum under 40 Torr pressure of Ar, whereas for Ne the value of optimum pressure is 60 Torr.These optimum pressures of environmental gases along with maximum values of plasma and ablation parameters are considered to be responsible for the growth of most well-defined and distinct grain boundaries on the surface of Ti explored by SEM analysis.Afterwards,the density of grains decreases up to 120 Torr with the decrease in plasma parameters, ablation yield and ablation rate.Rapid heating, cooling and recrystallization are responsible for the formation of granular morphology.When comparing the results as a function of the nature of the gas,Ne is responsible for more distinct grain growth on the Ti surface compared to Ar.In the presence of Ne,compared to Ar,the higher thermal conductivity of Ne (4.93 × 1 0-4W (c m · K)-1) compared to Ar (1.772 × 1 0-4W (c m · K)-1) is responsible for the rapid heat dissipation and causes sudden cooling of Ti plasma.This supports the growth of more distinct grain boundaries on the Ti surface in the case of Ne than Ar [22].

3.4.Micro-hardness test of laser-irradiated Ti

Figure 10 shows the variation in micro-hardness of laser-irradiated Ti under Ar and Ne environments for different pressures ranging from 10-120 Torr.The micro-hardness of the laser-ablated Ti samples increases in both environments compared to an untreated sample having micro-hardness of 177 HV.The graph represents the increase in hardness from 195 to 300 HV by increasing the pressure from 10 to 40 Torr in Ar, and it decreases from 300 to 230 HV with increasing pressure up to 120 Torr.Similarly, the hardness of laser-ablated Ti increases from 191 to 232 HV under Ne environment for a pressure range of 10-60 Torr.Later, it decreases up to 180 HV by a further increase in pressure up to a maximum of 120 Torr.The micro-hardness of laser-ablated Ti is greater in the case of Ar compared to Ne environment.The variation in values of micro-hardness is attributed to lattice disorder and generation of defects in the material[6].These changes are produced due to thermal compressive stresses and structural modifications due to the laser-induced heating.The results of micro-hardness are well correlated with ablation yield, plasma parameters and SEM analyses.The maximum values of ablation yield and plasma parameters are accountable for maximum surface and mechanical modifications in both environments [4].

4.Conclusion

The significant effects of the nature and pressure of environmental gasses on plasma parameters, ablation yield, ablation depth, surface morphology and mechanical modification of laser-irradiated Ti have been observed.LIBS is used to evaluate plasma parameters and QCM is used to evaluate ablation yield measurements.The material depth is evaluated by optical microscopy, surface features are explored by SEM analysis and Vickers hardness test is used to measure the micro-hardness of laser-irradiated material.In addition, SEM is employed to study the surface morphology.The experiment is performed under two non-reactive gasses, i.e.Ar and Ne.Inert gasses and their pressure play a significant role in the enhancement of plasma parameters (Teandne) as well as ablation yield of laser-ablated material.All parameters increase with increasing pressures,achieve their maxima and then reduce at higher pressures.However, all evaluated values of plasma parameters,ablation yield,ablation depth,surface structures at the inner peripheral boundaries and micro-hardness are higher in Ar than in Ne.Whereas, the grain growth at the outer peripheral boundary is more distinct in the case of Ne than in Ar.By controlling environmental conditions, plasma parameters and ablation yield can be controlled,which contributes significantly to surface and mechanical modifications of Ti.Ti, after the formation of surface structures and improved hardness, is highly beneficial for industry and has biocompatible material with enhanced friction, hardness and reduced reactivity.