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ISSN : 1598-6721(Print)
ISSN : 2288-0771(Online)
The Korean Society of Manufacturing Process Engineers Vol.19 No.2 pp.9-17
DOI : https://doi.org/10.14775/ksmpe.2020.19.02.009

# Laser-Induced Plasma Spectroscopy Measurement on Surface Roughness in Surface Treatment of Titanium Alloys

Ji-Hun Kim*, Joohan Kim*#
*Department of Mechanical Engineering, Seoul National University of Science and Technology
Corresponding Author : joohankim@seoultech.ac.kr Tel: +82-2-970-6314, Fax: +82-2-949-1458
21/11/2019 16/12/2019 21/12/2019

## Abstract

In this study, the surface changes of titanium alloy using laser surface treatment and the surface analysis using laser-induced plasma spectroscopy were carried out. The laser surface treatment induced changes in surface roughness and the diffusion of atmospheric elements. Excessive melting or less melting caused roughness changes, but when moderate levels of energy were applied, a smoother surface could be obtained than the initial surface. In the process, the diffusion of atmospheric elements took place. To analyze the diffusion of atmospheric elements with respect to surface morphology, the surfaces were re-shaped with grinding. In this experimental conditions, the effect of plasma formation by surface roughness was identified. Compensated plasma signals for the material properties were obtained and analysed by removing the background plasma signal.

# 티타늄 합금의 표면 처리에 있어 표면 거칠기에 대한 레이저 유도 플라즈마 분광분석법 측정 적용 연구

김 지훈*, 김 주한*#
*서울과학기술대학교 기계공학과

## 초록

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

The surface quality of metal products is a critical factor determining the overall quality of the products. For this reason, many studies have been conducted to improve the surface properties by processing the surface of metal materials[1,2]. One of the frequently used surface treatment processes involves forming chemicals on the material surface through heat treatment[3]. The chemicals grow from the material surface to the interface layer by diffusion, and the characteristics of the material surface are changed by a structural change of the surface[4-6]. When this process is performed, the reliability of surface diffusion by heat treatment needs to be verified. The reliability of heat treatment includes changes in the element diffusion characteristics caused by the chemical bonding between elements at the diffusion interface and the surface roughness, which is a physical characteristic of the surface.

Laser-induced breakdown spectroscopy (LIBS) is an element analysis technology based on laser spectroscopy that enables qualitative and quantitative analyses of materials. LIBS analyzes element signals using spectroscopy of plasma generated by a laser irradiated to the material surface[7]. The plasma behavior can be changed by material properties, and the properties of the material surface can be quantitatively evaluated by analyzing the changes in plasma behavior. For example, surface roughness analysis[8], hardness analysis[9,10], corrosion analysis [11], and element analysis[12,13] using LIBS have been researched. In these measurements using a laser, the plasma created by a laser is the final result of complex causes, and its reliability is somewhat low. Therefore, the first task is the quantitative analysis of the effects of various factors on the plasma behavior. Among them, surface roughness is selected first, because the energy absorption characteristics can be changed by the surface roughness and the formation of plasma is the result of the absorbed energy.

According to existing studies that analyzed LIBS plasma signals according to surface roughness, the same material can generate different plasma signals depending on the surface structure. The plasma behavior caused by surface roughness was examined until the initial surface was fully removed by laser ablation, and element analysis assuming surface roughness can be performed by applying this[14]. When this is expanded, the chemical changes of homogeneous materials can be analyzed by applying the surface roughness result. This method can simultaneously evaluate the surface roughness and chemical changes caused by the surface treatment of the material.

The metal analyzed in this study is titanium alloy, which has been researched and applied to industries for a long time owing to its excellent mechanical properties[15,16]. Titanium alloy can also be used by applying local surface heat treatment using a laser. In this laser surface treatment, the surface of the alloy reaches a very high temperature locally due to the laser optical energy. It is then liquefied or vaporized, which causes changes in the surface roughness[17]. It has been reported that the diffusion of atmospheric elements occurs simultaneously at high temperatures[18-20]. In this study, the chemical formation by atmospheric diffusion is evaluated using LIBS, and the changes in surface roughness according to laser surface heat treatment are analyzed.

## 2. Background Theory

### 2.1 Diffusion theory

When homogeneous semi-infinite rods with different components are joined together, diffusion occurs at their interface. In one-dimensional diffusion, as shown in Fig. 1, when the position is x, the time is t, and the position of the junction interface is x=0, the initial composition at x<0 is Co (i.e., C(x, 0) = Co) and the composition at x>0 is 0 (i.e., C(x, 0) = 0). $x ^$ is a variable that represents the initial position in the medium. If the composition of the semi-infinite medium at x<0 is not changed, the composition of x=0 at t>0 remains Co . Since the atmosphere has a huge volume, it can be assumed that the composition does not change, and this is applicable to the surfaces of solids exposed to the atmosphere. Therefore, the atmospheric elements are diffused to solids according to Fick's law, and the concentration by position over time can be expressed as Eq. (1) [21].

$C ( x , t ) = C o e r f c ( x 2 D t )$
(1)

### 2.2 LIBS measurement method

LIBS is an element analysis method that measures the component elements by focusing a laser beam on materials, thus generating plasma, and analyzing its spectrum. The plasma, which has a high excitation temperature, discharges an energy for a certain time and returns to the ground state again. Element analysis is possible because each element discharges its unique wavelength. When a laser beam is focused on a material, the incident beam is reflected, transmitted, or absorbed. If the beam is not transmitted, the transmission energy is zero, and only absorption and reflection occur. In this case, absorption is significantly affected by the surface condition of the material. If the surface is smooth, single absorption and reflection occur. However, if the surface is rough, the reflected beam is re-absorbed and the absorbed energy increases as a result. The absorbed energy can be expressed as Eqs. (2) and (3):

$E I = E A + E R + E T$
(2)

$E A = E I ( 1 − R ) ∑ n R n − 1 = E I A R a$
(3)

where R is the beam reflectance, n is the number of multiple reflections, and $A R a$ is the absorption rate caused by surface morphology. If the surface is rough, the true area that absorbs energy increases, and the absorbed energy per unit area decreases. When this happens, the plasma signals decrease because the critical energy for generating plasma cannot be exceeded[22]. Therefore, to analyze the surface, the plasma behavior caused by surface roughness needs to be analyzed. When the background signals are removed, the reliability of the plasma signals caused by other surface factors can be improved.

## 3. Experimental Method

Titanium alloy (Ti-6Al-4V) was used in the gas/solid diffusion induction experiment involving laser surface treatment. Table 1 shows the composition of the alloy. The surface roughness of the specimen was Ra 0.27 μm, and its thickness was 1.2 mm. The laser used for surface processing was the pulse laser of yttrium garnet (Nd:YAG) doped with neodymium at a wavelength of 1064 nm. The surface treatment process was performed at the machining interval of 50 μm and the confocal size of 1 mm, as shown in Table 2. To ensure the machining reliability of titanium alloy, a short pulse width of 5 ns was set. The process atmosphere was 25℃ and the oxygen concentration was 20%. To verify the change of physicochemical properties by surface treatment, the surface roughness of the machined alloy was measured, and the changes in surface elements were measured using LIBS. Fig. 2 shows the experimental settings of LIBS. An Nd:YAG pulse laser with a wavelength of 266 nm was used with an energy per pulse of 25 mJ and a pulse width of 5 ns. Since the laser beam irradiated for element analysis also raises the surface temperature of the specimen, the atmospheric elements may be diffused depending on the measurement atmosphere. To prevent this, the experiment was conducted in an inert argon gas atmosphere. The pulse was irradiated 30 times per position. The signals of titanium 368 nm and oxygen 777 nm were used at various spectrum peaks of each element. Table 3 shows the measurement variables of each element. The titanium signals were measured in 1 ms of delay time, and the oxygen signals were measured in 0.1 ms of delay time.

To verify the plasma behavior according to surface roughness, titanium alloys of various surface roughness values were prepared, including an alloy with a smooth surface. The LIBS settings were the same as above. The plasma signals for surface roughness were obtained by irradiating the titanium alloy surface with the laser. These signals were compared with the plasma signals from the laser-surface-treated titanium alloy to examine the chemical changes by surface treatment.

## 4. Results and Discussion

### 4.1 Laser surface treatment and diffusion

When a material surface is irradiated with a pulse laser, the surface temperature rises rapidly, causing melting and transformation. When the surface roughness caused by laser surface treatment was measured, the roughness of the initial specimen was Ra 0.27 μm. Fig. 3 outlines the surface roughness according to the laser surface treatment variables. Not only changes in surface roughness caused by surface treatment but also color changes were observed. This is the result of the diffusion of atmospheric elements into the titanium alloy to form oxides and nitrides and the apparent color change due to optical interference. Increasing the amount of energy irradiated to the surface is expected to increase the thermal effect and cause more atmospheric element diffusion.

The oxygen, nitrogen, and aluminum elements on the surface were examined using the energy dispersive X-ray spectroscopy (EDS) method, and the results are summarized in Fig. 4. According to the EDS results, when the laser energy irradiated to titanium alloy increased, the oxygen signals measured on the surface increased, but the titanium signals decreased. This suggests that an oxide film was formed and grown by the laser surface treatment.

As a result of analyzing the laser-surface-treated titanium alloy using LIBS, when the laser fluence increased in surface treatment, the signals of oxygen generated from the initial pulse increased. Furthermore, as the fluence increased, the signals were generated more continuously. Fig. 5 summarizes the behaviors of oxygen plasma signals relative to fluence and pulse order. Although the effect of surface roughness on oxygen signals was small, when diffusion was induced by a local temperature increase, the signals generated from the initial pulse increased approximately 13-fold. This suggests that oxides were formed by the surface diffusion of titanium alloy.

### 4.2 LIBS surface element analysis for roughness

In the case of a smooth surface in LIBS, the initial surface was removed by only one pulse irradiation, but a rough surface required multiple pulse irradiations to remove the initial surface. The titanium plasma signals generated from this process were analyzed for surface roughness. In the present experimental conditions, the titanium plasma signals tended to decrease as the surface roughness increased. Fig. 6 shows the titanium plasma signals according to the pulse order, and Fig. 7 shows the titanium plasma signals according to the surface roughness. Although multiple absorptions occurred by surface morphology, it is presumed that the plasma intensity decreased because the increase in area had a greater effect. The plasma signals for roughness can be calibrated using this data. Furthermore, in the case of oxygen, oxygen plasma signals were generated because an oxidation layer was formed by natural diffusion in atmospheric conditions. In the oxygen signal measuring condition, plasma signals increased with the surface roughness, but their effect was very small. This suggests that the effect of oxygen on the plasma signals to be calibrated is limited.

In the case of titanium signals, because a valid plasma behavior caused by surface roughness was verified, the signals were analyzed after removing the surface roughness background signals. Consequently, the amplitude of signals generated from the initial pulse increased as the laser fluence in the titanium signals increased. Fig. 8 summarizes the calibrated titanium plasma signals relative to fluence. The signals were calibrated using three laser fluences. For each laser fluence, the roughness values in the relationship between surface roughness and laser fluence in Fig. 3 can be inferred by applying the laser spot size and laser scanning speed. The calibration factor was determined by substituting the inferred surface roughness value in the titanium plasma signals relative to roughness. Fig. 8 shows the graph obtained by applying this calibration factor. The maximum signal of the titanium plasma increased 1.45-fold when the laser fluence increased 1.40-fold from 0.91 J/mm2 to 1.28 J/mm2 and 2.18-fold when the fluence increased 2.34-fold to 2.13 J/mm2. This is an interesting result of surface analysis using LIBS. Surface analysis using LIBS has low reliability due to various factors. When the background signals caused by surface roughness are removed, plasma signals calibrated by other factors can be obtained. These experiments using LIBS allowed for more accurate evaluation of the surface roughness of materials and the generation of chemicals by atmospheric diffusion.

## 5. Conclusion

The changes in the titanium alloy surface by laser surface treatment were analyzed using LIBS. The changes in surface roughness and the diffusion of atmospheric elements were induced by laser surface treatment and were measured and analyzed using LIBS. When the surface was treated with the initial surface roughness of Ra 0.27 μm, changes in surface roughness by melting and solidification were observed. Rough surfaces of 0.71 μm at the maximum to smooth surfaces of 0.15 μm at the minimum were obtained. In this process, the atmospheric elements were diffused, and the physicochemical changes of the surface were measured. The diffusion of the atmospheric elements was analyzed to verify the incidental effects of the surface treatment process. To examine the differences in LIBS measurements by roughness, surfaces with different roughness values were formed by artificial polishing, and the plasma signals generated from them were analyzed. In the present experimental conditions, titanium plasma signals tended to decrease with increasing surface roughness, and accurate plasma signals could be obtained by removing the background signals. These signals increased with the surface treatment laser fluence, and this was similar to the result of the surface analysis through EDS. These experimental results demonstrated that the plasma formation was affected by the surface roughness of the specimen. In addition, the plasma behavior resulting from the thermal changes of diffusion or microstructural changes could be clearly identified by removing the background signals related to surface roughness.

## Acknowledgement

This study was supported by the Research Program funded by the SeoulTech(Seoul National University of Science and Technology).

## Figure

Gas/Metal diffusion model
Schematic of LIBS setup for gas/solid interface analysis
Ra surface roughness respect to laser surface treatment parameters
EDS element profiles by laser surface treatments: (a)EDS image, (b) Oxygen, (c) Titanium, (d) Nitrogen, (e)Aluminium
Oxygen 777nm plasma signal for the different laser surface treatment fluence as a function of the number of pulses
Titanium 368nm plasma signal for the different surface roughness as a function of the number of pulses
Titanium 368nm plasma signal relative to surface roughness
Calibrated titanium 368nm plasma signal obtained by removing surface roughness background signal

## Table

Composition of Ti-6Al-4V specimen (wt%)
parameters of laser surface treatment
LIBS analysis setup by target

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