application of libs technique on the determination of ...18) 7 - 2017.pdf · determination of...

D. F. Mohamed, et al. Arab J. Nucl. Sci. Appl, Vol 50, 4, 142-155 (2017)

142

Determination of Surface Hardness of Ti -based Alloys via Laser Induced

Breakdown Spectroscopy (LIBS)

*O. Aied Nassef and A. Hassan GalmedD. F. Mohamed, National Institute of Laser Enhanced Sciences (NILES), Cairo University, Egypt

Received: 4/10/2016 Accepted: 10/12/2016

ABSTRACT

Laser Induced Breakdown Spectroscopy (LIBS) is a well-known

spectrochemical elemental analysis technique, which has been exploited to

determine the surface hardness of a metallic alloy such as titanium-based alloy

samples (Ti-6Al-4V). It is shown that there is a direct proportionality relation

between the ionic to atomic spectral lines intensity ratios and the surface hardness

values measured mechanically via Vickers hardness test for the Ti-based alloy

samples. In the present study, the proportionality relation has been demonstrated

adopting different ionic and atomic spectral lines ratios from the corresponding

LIBS spectra.

Additionally, the laser induced plasma parameters, namely the electron

density and the plasma excitation temperature have been estimated as a function of

the surface hardness of the adopted alloy samples. The obtained direct

proportionality between the plasma temperature and the samples surface hardness

has been interpreted in view of the increased collisions in the plasma plume due to

the strong repulsive forces for harder targets. On the other hand, such high

repulsive force leads to fast expansion of the laser induced plasma plume and

consequently to a decrease in the electron number density in agreement with the

inverse proportionality obtained between the surface hardness and the electron

density.

The results obtained in the present work demonstrate the potential of using

LIBS in metallurgical and industrial applications for the direct determination of

different alloys surface hardness, saving time and cost adopted by conventional

techniques.

INTRODUCTION

Metal alloys are considered one of the most important materials used in many industrial,

medical, military, art and other applications. Their physical and mechanical properties determine the

applications in which they can be used. For example, especial aluminum alloys, are used in aviation

and missile industry, and also steel alloys are used in cars manufacturing (1).Titanium alloys are

utilized in many industrial applications such as aerospace, marine, chemical processing, connecting

rods in internal combustion engines, as well as in military aircraft and turbines. Moreover, they are

used in the medical field such as surgical implants (bones and teeth). Many metal alloys are

distinguished by superior combination of properties such as low densities that lead to very good

strength to weight ratios allowing lighter and stronger structures, high corrosion and erosion resistance

in many environments and high temperature applications (2,3). Thus, making use of a certain metal

alloy depends on its overall properties and on the easiness of its production.

The non-ferrous alloys can be classified according to their base element such as aluminum,

magnesium, copper, titanium, refractory metals, noble metals and miscellaneous alloys such as those

having nickel, lead, tin, and zinc as base metals. However, they can also be classified according to

their specific characteristics. Ti - 6Al - 4V alloy is stronger than the commercially utilized pure

The Egyptian

Society of Nuclear

Sciences and

Applications

ISSN 1110-0451 (ESNSA) Web site: esnsa-eg.com

Arab Journal of Nuclear Sciences and Applications

Vol 50, 4, (142-155) 2017


143

titanium (cp Ti). Both has the same stiffness and thermal properties, but the thermal conductivity of Ti

- 6Al - 4V is 60 % lower than that of the cp Ti (4). Titanium alloys have a low density, a high strength,

a very high melting point, ease of machining, a high corrosion resistance in diverse atmospheres and

wear out properties. The main limitation of their applications is the titanium chemical reactivity at

high temperatures (1); however they are still the most used alloys in biomedical applications among all

nonferrous alloys (5).

Many quantitative hardness measurement techniques, such as Rockwell, Brinell, Knoop and

Vickers hardness tests, were designed over the years. They provide arbitrary hardness values, not a

fundamental property of the materials. Their hardness measurement values are relative and especial

consideration must be taken when comparing values determined by different techniques. In 2006,

Laser Induced Breakdown Spectroscopy (LIBS) technique has been successfully used to determine the

surface hardness of various solid samples which have different degree of humidity such as concrete

samples (6). Also Abdel Salam et al. (7) used LIBS technique to measure the surface hardness of

calcified tissues. It has been demonstrated that there are a remarkable correlation between the surface

hardness of solid samples and the intensity ratio of ionic to atomic emission spectral lines from the

plasma. In calcified tissues, the intensity ratios of CaII / CaI and MgII / MgI gave a direct correlation

with the samples surface hardness of different types, namely, enamel of human teeth, shells and egg

shells. The results of such measurements reflected that the repulsive force of the laser induced shock

waves is clearly dependent on the surface hardness of the samples. In 2008, Double Pulse Laser

Induced Breakdown Spectroscopy (DPLIBS) method showed that the highest ratio of MgII 280.26 nm

/ MgI 285.22 nm corresponds well with the enamel position in tooth hardness monitoring(8). Abdel

Salam et al. successfully demonstrated a simple method for the qualitative and quantitative estimation

of ferrous metallic alloys surface hardness. The ratio of ZrII/ZrI is found to be proportional to the

target material hardness, depending on the material composition (9). In 2011, the LIBS was applied to

analyze bio ceramic samples (10), the relationship between sample hardness and LIBS plasma

properties was investigated, in comparison with the conventional Vickers hardness measurements.

In this work, we introduce a detailed study of the hardness determination for a non-ferrous

alloy; titanium based alloy samples, Ti-6Al-4V using LIBS technique. This study enabled us to

construct the calibration curves which correlate the surface hardness values of each type of the alloys

which have been measured mechanically and the spectroscopic data (ionic to atomic spectral emission

intensities). Also, an investigation of the physical parameters of the Ti-6Al-4V alloy plasma (electron

density and plasma excitation temperature) is presented.

I. Samples

The standard samples utilized in the present study were mainly titanium based alloys (Ti-6Al-

4V). Their composition is 6% aluminum, 4% vanadium, 0.25% (maximum) iron, 0.2% (maximum)

oxygen, and the remainder is titanium. Table 1 lists the alloy samples used with the corresponding

mechanically measured Vicker’s hardness number (VHN).

Table (1): The Ti-6Al-4V samples with their respective Vicker’s hardness number (VHN)

Ti-6Al-4V VHN

Sample 1 2400

Sample 2 1100

Sample 3 950

Sample 4 900

Sample 5 775

Sample 6 650

II. Experiment

A typical LIBS setup has been used in the present work. This experimental setup utilizes an

Nd:YAG laser (Brio-Quantel, France) delivering 5 ns laser pulses. The maximum energy of each pulse

was 50 mJ at a wavelength of 532 nm as our excitation source. Laser light is focused by a 10-cm focal

length quartz lens onto the surface of the sample. The target was mounted on an X-Y micrometric


144

translational stage. The focused laser pulses of energy of 22 mJ/pulse and a repetition rate of 10 Hz

provide an irradiance of 2.99 × 1011 W/cm2 onto the surface of the Ti-6Al-4V alloy samples. An

oscilloscope coupled to photodiode and Notch filter are used to cut the laser signal and obtain the

integrated light signal of the plasma in each run. The plasma optical emission was collected by a

quartz optical fiber with a diameter of 600 m held at a distance of 2 cm above the plasma at an angle

of 30o with respect to the target surface. The collected plasma emission is then fed via the optical fiber

to the echelle spectrometer (Mechelle 7500, Multichannel, Sweden) coupled to the ICCD camera

(DiCAM PRO-PCO, Computer optics, Germany) for dispersion and detection. The obtained spectra

are displayed and stored on a personal computer for further processing and analysis adopting

GRAM/32 commercial software. All measurements have been performed in air at atmospheric

pressure.

The LIBS experiment for hardness measurements was performed by acquiring fifteen LIBS

spectra of each alloy sample, where five spectra are saved at three different positions on the sample’s

surface. At the same time of acquiring the LIBS spectra, the total integrated plasma, collected by the

fast photo-detector, is also saved via the oscilloscope for each sample at each position. The

identification of the different elements of each sample is initially carried out using each averaged

spectrum. The choice of the suitable spectral lines that are used in the analysis is defined in terms of

intensity, no self-absorption and no interference with other spectral lines. Normalization of the LIBS

spectra to the area under the peak of the oscilloscope’s wave form is performed individually for each

position, then averaged for the three different positions. Construction of the calibration curves, that

relate the TiII/TiI ratios of the specified lines and the corresponding mechanically measured values of

the hardness (VHN), is presented.

III. RESULTS AND DISCUSSION

The LIBS technique was exploited to measure the surface hardness of the Ti-6Al-4V alloy

using five ionic to atomic spectral lines ratios. The results of these tests will be discussed in the

following sections. A typical LIBS spectrum of Ti-6Al-4V alloy is shown in figure (1). It is clear from

this figure that the spectrum is highly crowded with spectral lines (atomic and ionic spectral lines) of

the elements contained in the alloy sample especially the titanium spectral lines. The study of the

surface hardness of these samples requires proper selection of well resolved and intense spectral lines

as shown in figure 2 (some zoomed segments of the spectrum).

250 300 350 400 450 500 550 600

0

200

400

600

800

1000

1200

1400

1600

Inte

nsit

y(a.

u.)

( nm )

Fig (1): Typical LIBS spectrum for Ti-6Al-4V alloy acquired under the adopted

experimental conditions


145

316 317294 295

0

200

400

416 417406398 400 444 445390308 309

Ti II

316.85 nm

Ti I

294.82 nm

Ti II

416.36

Ti I

406.02 nm

Ti I

398.97 nm

(nm)

Ti II

444.38 nm

Inte

nsity

(a.u

.) Al II

390.06

Al I

308.2 nm

Al I

309.3 nm

Fig. (2): Ti-6Al-4V alloy spectral lines used in the data analysis

III.1.a Ti II 316.36 nm/Ti I 294.82 nm for Ti-6Al -4V alloy

The obtained LIBS spectra of Ti-6Al-4V alloy samples of different surface hardness have been

used to plot the relation between the intensity ratios of titanium ionic to atomic spectral lines and the

corresponding mechanically measured Vickers Hardness Number (VHN). The ionic and atomic

spectral lines Ti II 316.85 nm and Ti I 294.82 nm were obtained as a result of focusing the second

harmonic Nd-YAG laser of wavelength 532 nm, are shown in figure( 3).

294 295 315 316 317 318

0

100

200

300

400

500

600(a)

316.85 nm

Ti II

294.82 nm

Ti I

VHN 2400

Inte

nsi

ty (

a.u

.)

(nm)294 315 316 317 318

0

100

200

(b)

VHN 1100

294.82 nm

Ti I

316.85 nm

Ti II

Inte

nsi

ty (

a.u

.)

(nm)

294 295 296 315 316 317 318

0

100

200

(c)

294.82

Ti I 316.85 nm

Ti II

VHN 950

Inte

nsi

ty (

a.u

.)

(nm)

294 295 315 316 317 318

0

50

100

150

200

(d)

VHN 900

316.85 nm

Ti II

294.82

Ti I

Inte

nsi

ty (

a.u

.)

(nm)

Fig. (3): Titanium ionic and atomic spectral lines used in the LIBS data analysis of the

alloy surface hardness of the four samples used

[

T

y

p

e

a

q

u

o

t

e

f

r

o

m

t

h

e


146

From here on, Figure (a) refers to results of raw intensity data while figure (b) presents the

results normalized to the total integrated emission from the plasma plume. Figure (4) shows the ratios

of the above mentioned ionic to atomic spectral lines intensities as a function of VHN where figure

(4.a) is plotted for the raw data and figure (4.b) illustrates the data normalized to the total plasma

emission.

1000 1500 2000 25000.5

1.0

1.5

2.0

2.5

3.0 (a)R = 70

I (

Ti

II )

/ I

( T

i I )

VHN (Hardness)

1000 1500 2000 25000.5

1.0

1.5

2.0

2.5

3.0

(b)R = 96

I (

Ti

II )

/ I

( T

i I

) n

orm

ali

zed

VHN (Hardness)

Fig. (4): The intensity ratio of Ti II 316.85 nm/Ti I 294.82 nm versus the Vickers Hardness Number

(VHN) for Ti-6Al-4V alloy samples (a) raw data of line intensities (b) line intensities

normalized to the plasma total emission

It is clear that the ratio of the intensities of the ionic to atomic spectral lines of Ti- 6Al-4V

alloy(Ti II 316.85 nm/Ti I 294.84 nm) is directly proportional to the corresponding VHN values. This

is because the harder the surface, the stronger the repulsive force of the laser induced shock wave.

This leads to an increase of the collisions between the constituents of the plasma plume and

consequently, increases appreciably the ionic species on the account of the neutral one.

It is also shown that the calibration curve when the line intensities are normalized to the plasma

total emission (Fig.4.b) gives the best results for ionic titanium line at 316.85 nm to neutral line at

294.82 nm intensity ratio which corresponds to the best line regression value (R= 96).

III.1.b Ti II 416.36 nm/Ti I 294.82 nm for Ti-6Al -4V Alloy

Figure (5) illustrates a zoomed part of the LIBS spectrum obtained for the Ti-6Al-4V alloy

samples (a, b, c and d). The spectral lines (Ti II at 416.36 nm and Ti I at 294.82 nm) shown in this

figure are used in verifying the calibration curves which relates the mechanically measured VHN of

the alloy with ionic to atomic spectral lines intensity ratios.


147

294 295 415 416 417

0

100

200

300

400

500(a)

416.36 nm

Ti II294.82 nm

Ti I

VHN 2400

Inte

nsi

ty (

a.u

.)

(nm)

294 415 416 417

0

100

200

(b)VHN 1100

294.82 nm

Ti I

416.36 nm

Ti II

Inte

nsi

ty (

a.u

.)

(nm)

294 295 296 415 416 417

0

50

100

150

(c)

294.82 nm

Ti I 416.36 nm

Ti II

VHN 950

Inte

nsi

ty (

a.u

.)

(nm)

294 295 415 416 417

0

50

100

150

(d)

294.82 nm

Ti I

416.36 nm

Ti II

VHN 775

Inte

nsi

ty (

a.u

.)

(nm)

Fig. (5): Titanium atomic and ionic spectral lines used in LIBS data analysis of the alloy surface

hardness of the four samples used

The ratios of the above mentioned ionic to atomic spectral lines intensities are plotted as a

function of VHN in figure 6. Here also, the raw data are plotted in figure (6.a) and the data normalized

to the total plasma emission are shown in figure (6.b).

1000 1500 2000 25000.5

1.0

1.5

2.0

(a)R = 95

I (

Ti

II )

/ I

( T

i I

)

VHN (Hardness)1000 1500 2000 2500

0.5

1.0

1.5

2.0

(b)R = 94

I (

Ti

II )

/ I

( T

i I

) n

orm

ali

zed

VHN (Hardness)

Fig. (6): The intensity ratio of Ti II 416.36 nm / Ti I 294.82 nm versus the Vickers Hardness Number

(VHN) for Ti-6Al-4V alloy samples (a) raw data of line intensities (b) line intensities

normalized to the plasma total emission


148

It is clear that the best results for the calibration curve is obtained for the non-normalized line

intensities when the intensity ratios of the ionic titanium line at 416.36 nm to the neutral line at 294.82

nm are used.

III.1.c Ti II 316.85 nm/Ti I 406.02 nm for Ti-6Al-4V Alloy

Irradiating the titanium alloy samples by a focused 532 nm Nd-YAG laser enabled the

attainment of different pair of ionic and atomic titanium spectral lines Ti II 316.85 nm and Ti I 406.02

nm. These are used for the spectroscopic estimation of the surface hardness of Ti-6Al-4V alloy are

shown in figure 7(a,b,c and d).

315 316 405 406 407

0

100

200

300

400

500

600

700

800

(a)

316.85 nm

Ti II

406.02 nm

Ti I

VHN 2400

In

ten

sit

y (

a.u

.)

(nm)

315 316 317 405 406 407

0

50

100

150

200

(b)

316.85 nm

Ti II

406.02 nm

Ti I

VHN 950

Inte

nsi

ty (

a.u

.)

(nm)

315 316 405 406 4070

50

100

150

200

(c)VHN 900

406.02 nm

Ti I

316.85 nm

Ti II

Inte

nsi

ty (

a.u

.)

(nm)

315 316 405 406 407

0

50

100

150

200(d)

406.02 nm

Ti I

VHN 775

Inte

nsi

ty (

a.u

.)

(nm)

Fig. (7): Titanium ionic and atomic spectral lines used in LIBS data analysis to estimate the alloy

sample surface hardness of the four samples used

The ratios of the spectral lines obtained from Figure (7) are plotted as a function of the VHN

values and shown in Fig.8 where figure 8.a represent the raw data and figure (8.b) shows the

normalized data normalized to the total plasma emission.


149

1000 1500 2000 2500

0.2

0.4

0.6

0.8

1.0

1.2

1.4 (a)R = 86 I

( T

i II

) /

I (

Ti

I )

VHN (Hardness)

1000 1500 2000 2500

0.2

0.4

0.6

0.8

1.0

1.2

1.4 (b)R = 89

I (

Ti I

I )

/ I (

Ti I

) n

orm

aliz

ed

VHN (Hardness)

It is shown that the calibration curve obtained in Fig.(8 a) and b shows almost acceptable

results for the considered spectral line intensity ratio (TiII 316.85 nm / TiI 406.02 nm). III.1.d Ti II 416.36 nm/Ti I 406.02 nm for Ti -6Al -4V Alloy

The ionic and atomic spectral lines Ti II 416.36 nm and Ti I 406.02 nm emitted from the plasma

plume of Ti-6Al-4V alloy after irradiating the sample surface by a focused 532 nm Nd-YAG laser

with irradiance 2.99 × 10 11 W/cm 2 are shown in Fig.( 9).

405 406 415 416 417

0

200

(a)

406.02 nm

Ti I

416.36 nm

Ti II

VHN 1100

Inte

nsit

y (a

.u.)

(nm)

405 406 407 415 416 417

0

50

100

150

200

250

(b)

406.02 nm

Ti I

416.36 nm

Ti II

VHN 950

Inte

nsi

ty (

a.u

.)

(nm)

405 406 415 416 4170

50

100

150

200

(c)VHN 900

416.36 nm

Ti II

406.02 nm

Ti I

Inte

nsi

ty (

a.u

.)

(nm)

405 406 407 415 416 417

0

50

100

150

200

250

300

350

400

(d)

416.36 nm

Ti II406.02 nm

Ti I

VHN 650

Inte

nsi

ty (

a.u

.)

(nm)

The ratios of the ionic and atomic spectral lines Ti II 416.36 nm and Ti I 406.02 nm are plotted

as a function of VHN and shown in figure (10). Figure (10.a) is devoted to the raw intensities data

while the data normalized to the total plasma emission is shown in Fig. (10.b).

Fig. (8): The intensity ratio of Ti II 316.85 nm / Ti I 406.02 nm versus the Vickers

Hardness Number (VHN) for Ti-6Al-4V alloy samples (a) raw data of line

intensities (b) line intensities normalized to the plasma total emission

Fig. (9): Titanium atomic and ionic spectral lines used in the LIBS data analysis of the

alloy surface hardness of the four samples used


150

600 800 1000 12000.4

0.5

0.6

0.7

0.8

0.9

1.0(a)

R = 99 I

( T

i II

) /

I (

Ti

I )

VHN (Hardness)

600 800 1000 12000.4

0.5

0.6

0.7

0.8

0.9

1.0

(b)R = 100

I (

Ti

II )

/ I

( T

i I

) n

orm

aliz

ed

VHN (Hardness)

From this figure, it is clear that both cases a and b give best results for ionic titanium line at 416.36 nm

to neutral line at 406.02 nm intensity ratio where both lines showed a good regression value.

III.1.e Ti II 444.38 nm/Ti I 398.97 nm for Ti -6Al -4V alloy

Fig. (11) refers to Ti II 444.38 nm and Ti I 398.97 nm spectral lines used to obtain the

calibration curves which relate the mechanical measured VHN of Ti-6Al-4V alloy samples with ionic

to atomic spectral lines ratio resulted by irradiating the titanium sample by 532 nm Nd-YAG laser

source. These are shown in Fig.(11 a, b, c and d)

398 399 400 443 444 445

0

500

1000

1500

2000

(a)

444.38 nm

Ti II

398.97 nm

TiI

VHN 2400

Inte

nsi

ty (

a.u

.)

nm

398 399 443 444 445

0

100

200

300

400

500

600

(b)VHN 1100

398.97 nm

TiI

444.38 nm

Ti II

Inte

nsi

ty (

a.u

.)

nm

398 399 400 443 444 445

0

100

200

300

400

500

600

(c)

444.38 nm

Ti II

398.97 nm

TiI

VHN 775

Inte

nsi

ty (

a.u

.)

nm

398 399 400 443 444 445

0

200

400

600

(d)398.97 nm

TiI

444.38 nm

Ti II

VHN 650

Inte

nsi

ty (

a.u

.)

nm

Fig. (10): The intensity ratio of Ti II 416.36 nm / Ti I 406.02 nm versus the Vickers

Hardness Number (VHN) for Ti-6Al-4V alloy samples (a) raw data of line

intensities (b) line intensities normalized to the plasma total emission

Fig. (11): Titanium atomic and ionic spectral lines used in LIBS data analysis of the alloy

surface hardness of the four samples used


151

The obtained ratios from the above mentioned ionic to atomic spectral lines intensities are

plotted against VHN and are shown in Fig.(12). Again, the raw data is presented in Fig (12.a) while

the data normalized to the total plasma emission is shown in Fig. (12.b).

1000 1500 2000 25000.4

0.5

0.6

0.7

(a)R = 99

I (

Ti

II )

/ I

( T

i I

)

VHN (Hardness)

1000 1500 2000 25000.4

0.5

0.6

0.7(b)R = 97

I (

Ti

II )

/ I

( T

i I

) n

orm

ali

zed

VHN (Hardness)

Fig. (12): The intensity ratio of Ti II 444.38 nm/Ti I 398.97 nm versus the Vickers Hardness

Number (VHN) for Ti-6Al-4V alloy samples (a) raw data of line intensities (b) line

intensities normalized to the plasma total emission

Following to the R value obtained in Fig. (12.a and b), the best results for ionic titanium line at

444.38 nm to neutral line at 398.97 nm intensity ratio is observed for the calibration curve obtained

when the line intensities are non-normalized.

III.2 Plasma Parameters

The determination of plasma parameters (electron density and plasma temperature) is essential

for the comprehension of the mechanisms underlying the LIBS technique. The local thermodynamic

equilibrium (LTE) approximation is often used for modeling the plasma. When the collisional

processes especially those involving electrons, are the dominating mechanism in plasma, such plasma

is referred to as collisional-dominant plasma (CDP).

A crucial parameter for the establishment of LTE conditions in the CD plasma is its electron

density (ne), since a necessary conditions for the LTE is given by McWhirter criterion (11):

ne (cm-3) ≥ 1.4 × 1014 (kT)1/2 ΔE3 (1)

Where T (in eV) is the plasma temperature and ΔE (in eV) is the energy difference between the upper

and lower levels of the transitions.

III.2.a Determination of the Electron Density

The electron density has been calculated from Hα line at 656.72 nm using LIBS++ software. The

lower limit of Hα is 1.7 × 1017 cm-3, which verifies the LTE criterion. Hydrogen emission is always

present in the LIBS spectra taken in ambient air because of water vapor of the natural humidity of the

air.

The use of the Hα line for the measurement of the electron density has the definite advantage of

providing a result which is not affected by self-absorption, unless the sample itself would contain high

levels of hydrogen. Moreover, the linear Stark effect acting on hydrogen lines results in a large

broadening, which reduces the relative uncertainty of the measurement is compared to the case of lines

emitted by other elements. Figure (13) shows an exponential decrease between electron density and

Vickers hardness number (VHN) for five Ti-6Al-4V alloy samples with 2400, 1100, 950, 775 and 650

VHN.


152

500 1000 1500 2000 2500-100

0

100

200

300

400

500

600

700

Ele

ctro

n de

nsit

y *1

017 (c

m-3 )

VHN (Hardness )

The electron density is decreased by increasing the surface hardness of the metal alloy due to

the increase in the repulsion force of laser induced shock wave. These results in plasma plume

expansion which in turn lowering the number of electrons per unit volume, hence the electron density

decreases.

III.2.b Determination of the Plasma Temperature

If the LTE is verified in the plasma, the population density of atomic states is described well by

a Boltzmann distribution and the ionization states are populated according to Saha- Boltzmann

equilibrium equation. According to these requirements, the plasma temperature deduced from the

Boltzmann plot method is an important parameter for determining the elemental composition.

In fact, in LTE approximation, the emission intensity of the emitting line is related to the total

density by the Boltzmann law:

Ln [λ Іi / gi Ai] = Ln (N/ZT) i - Ei / k Tex (2)

where the spectroscopic constants: gi, λ and Ai are the ith level degeneracy, the wavelength and the

transition probability, respectively. These relevant spectroscopic constants are tabulated in Table (2), Іi

is the emission intensity of the emitting line; N is the number of atoms or ions, Z (T) is the partition

function of the emitting species, which depends on plasma temperature Tex and k is the Boltzmann

constant. By plotting the left hand side versus Ei the slope of the obtained line is (-1/kT).Therefore, the

plasma temperature can be obtained without knowing the values of N, the total number density, and

Z(T), the partition function.

The special conditions required for thermometric purposes should consider the following; lines

in close spectral proximity, reasonably intense, of known transition probability and with different

upper energy levels. The emission lines of Ti (I) have been used to determine electron temperature

under the condition of local thermodynamic equilibrium (LTE). According to these requirements, the

Ti (I) lines selected to determine the plasma temperature are listed in Table (2) with their required

parameters using the spectra normalized to 461.72 nm atomic titanium line.

Table (2): Spectroscopic data for the wavelengths that was used in the estimation of plasma

temperature for Ti-6Al-4V alloy(11):

(nm) E (eV) Ai gi

263.242 4.71 2.7*10-1 5

295.680 4.20 1.8*10-1 5

372.457 4.83 9.1*10-1 9

375.364 3.32 8.2*10-2 5

398.248 3.11 4.5*10-2 5

418.612 4.46 2.1*10-1 9

441.728 4.69 3.6*10-1 9

Fig. (13): Electron density versus Vickers Hardness Number (VHN) for Ti-6Al-4V alloy


153

The wavelengths listed in Table (2) with their corresponding spectroscopic data were

substituted in equation (2) to calculate the plasma excitation temperature for each sample The

Boltzmann plots for Ti-6Al-4V alloy samples with 1100, 950, 900, 775 and 650 VHN, are shown in

Fig. (14 a-e).

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.02

3

4

5

6

7

8

(a)R = 97

B = -2.0776

T = 5585 K

Ln

( I i /

Aig

i )

E(eV)

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0

2

3

4

5

6

7

8

(b)R = 97

B = - 2.4269

tex

= 4782 K

Ln

( I i /

Aig

i )

E(eV)

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0

2

3

4

5

6

7

8

(c)R = 95

B = -2.4495

Tex

= 4737 K

Ln

( I i /

Aig

i )

E(eV)

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.02

3

4

5

6

7

8

(d)R = 98

B = -2.3471

Tex

= 4944 K

Ln

( I i /

Aig

i )

E(eV)

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.01

2

3

4

5

6

7

8

(e)R = 91.3

B = -2.54493

Tex

= 4560 k

Ln

( I i /

Aig i )

E(eV)

Fig. (14): Boltzmann plots for Ti-6Al-4V alloy sample with (a) 1100, (b) 950, (c) 900, (d) 775, and (e)

650 VHN


154

The linear relation between plasma temperature and hardness for the five Ti-6Al–4V alloy

samples using average spectrum of three spectra normalized to 461.72 nm atomic titanium lines is

shown in Fig.(15). It is noticed that he plasma temperature increases as the hardness of the metallic

alloy target increases. This increase is attributed to the increase of the collision force. This result is

found to be similar to that attained by Cowpe et al.10 when LIBS was applied to the analysis of bio

ceramics samples where they also demonstrated a linear relationship between sample surface hardness

and plasma temperature.

600 800 1000 1200 14004000

4200

4400

4600

4800

5000

5200

5400

5600

5800

6000

R = 76E

xci

tati

on

tem

per

atu

re (

K)

VHN (Hardness)

Fig. (15): The plasma temperature versus VHN (Hardness) for Ti-6Al-4V alloy

CONCLUSION

In this study, it has been proven that the LIBS technique is capable of assessing surface

hardness of different types of metal alloys. This was clearly demonstrated by the calibration curves

obtained for the Ti-6Al-4V alloy samples. The surface hardness of Ti-6Al-4V alloy has been estimated

adopting the ionic to atomic spectral lines intensity ratios of the lines obtained from the LIBS spectra,

namely (a) Ti II 316.85 nm/Ti I 294.82 nm, (b) Ti II 416.36 nm /Ti I 294.82 nm, (c) Ti II 316.85 nm

/Ti I 406.02 nm, (d) Ti II 416.36 nm/Ti I 406.02 nm and (e) Ti II 444.38 nm/Ti I 398.97 nm.

The tudy of the relation between plasma parameters (i.e. electron density and plasma excitation

temperature) and the metal alloy surface hardness revealed a direct relation between the laser-induced

plasma excitation temperature and the surface hardness of the relevant metal alloy. This direct

proportionality relationship was interpreted in view of the increase of collisions in the plasma plume

as a result of the increase of the repulsive force for harder surfaces. On the other hand, the increase in

the repulsive force and consequently in the plasma plume expansion led to an inverse relationship

between the electron density and the sample surface hardness.

In general, the obtained results demonstrate the feasibility of exploiting LIBS, which is well

known as a spectrochemical analytical technique, for fast and precise estimation of different metallic

alloys surface hardness. The fact that LIBS is a quasi-nondestructive technique makes its application

in industry and archaeology appealing, especially when mobile LIBS systems are used for in situ and

real time measurements.

REFERENCES

(1) W. D. Callister, Jr. David, G. Rethwisch, Materials science and engineering an introduction, E 8,

University of Utah, US A , (2007).

(2) R. Boyer, G. Welsh, E.W. Collings, Materials properties Handbook-Titanium alloys, ASM

International, Materials Park, OH, (1994).


155

(3) H. Chandler, Hardness testing, 2 nd edition, ASM international, 06671G (1999).

(4) M. Niinomi, Sci. and Tech. of Advanc Materials, Elsevier; 4, 445 (2003).

(5) K. Tsuyuki, S. Miura, N. Idris, K. Hendrik, T. Jie, K. Kagawa, Appl. Spectrosc.; 60, 61 (2006).

(7) Z. Abdel-Salam, A.H. Galmed, E. Tognoni, M.A. Harith, Spectrochim. Acta B; 62, 1343 (2007).

(8) A. Hrdlicka, L. Prokes, K. Novotny, V. Konecna,V. Kanicky and V. Otruba , Chem. Listy, 102,

1372, (2008).

(9) Z. Abdel-Salam, M. Abdelhamid, S. M. Khalil, M.A. Harith, AIP conf. proc., 49, 1172 (2009).

(10) J.S. Cowpe, R.D. Moorehead, D. Moser, J.S. Astin, S. Karthikeyan, S.H. Kilcoyne, G. Crofts,

R.D. Pilkington, Spectrochem. Acta B; 66, 290, (2011).

(11) R.W. P. McWhirter, In plasma diagnostic techniques, R.H. Huddlestone, S.L. Leonard, (Eds.),

Academic press, New York, 206, (1965).

(12) http:// physics. nist.gov /Phys Ref Data / Handbook / Tables / titanium table . htm.

application of libs technique on the determination of ...18) 7 - 2017.pdf · determination of...

Documents