# Fabrication of Micro-Scale Gratings by Nanosecond Laser and Its Applications for Deformation Measurements

^{1}

^{2}

^{*}

## Abstract

**:**

^{2}with the waist radius of the laser beam 25.7 μm. With the optimal parameters, experimental results indicate that the highest frequency of parallel gratings is about 30 lines/mm, with a line width of 29 μm, and the distance between two adjacent laser pulses being of 10 μm. By performing tensile tests, micro-scale gratings fabricated on specimens are experimentally verified. The verification tests prove that the proposed fabrication method for the micro-scale gratings in GPA measurements is reliable and applicable, and the micro-scale gratings can be fabricated in many areas of interest, such as the crack tip, for deformation measurements. Furthermore, the adhesion between the Al film and the tested sample is strong enough so that the pattern sticks well to the sample.

## 1. Introduction

## 2. Methodology

#### 2.1. Nanosecond Laser Processing and Gratings

_{0}) on the surface of the specimen and its single-pulse energy E is [32],

_{0}is the waist radius of the laser beam (μm). And the etching diameter can be determined as,

_{th}is the threshold of energy fluence (J/cm

^{2}). Substituting Equation (1) into Equation (2) gives,

#### 2.2. GPA

_{f}represents the Fourier component. To describe the variations of contrast and fringe position in the image, the Fourier component A

_{f}is related to the position r,

_{f}is therefore given by,

_{f}(r) is directly related to the component of the displacement field u(r) in the direction of the reciprocal lattice vector f,

## 3. Experimental Setup and Tested Samples

_{4}laser (Continuum, PPII8000, Felles Photonic, Tianjin, China), a quarter-wave plate, a beam expander, a focusing lens, and an X-Y translation stage. The nanosecond laser has a wavelength of 532 nm, pulse width of 500 ns, pulse frequency of 1 kHz, and maximum pulse energy of 1 J. During the experiment, the laser beam first passes through a quarter-wave plate to enable circular polarization and then is expanded by a beam expander before it is reflected by a mirror. Subsequently, the laser beam is guided by a focusing lens to reach the test specimen. The laser frequency and scanning speed are constant, being 1 kHz and 10 mm/s, respectively.

## 4. Experimental Results and Discussion

#### 4.1. Parameter Studying of Single-Line Etching

_{th}) can be obtained when D = 0 based on Equation (3). Thus, the threshold of energy density (I

_{th}) can be calculated according to Equation (1). The calculated results of Si, stainless steel, and Al film are listed in Table 1.

^{2}with the waist radius of the laser beam 25.7 μm). Therefore, the above optimum parameters are recommended for line etching of 2 μm thickness Al film. The line width is 29 μm.

#### 4.2. Gratings

## 5. Applications

#### 5.1. Deformation Measurement during a Tensile Test

#### 5.2. Deformation Measurement of the Crack Tip

## 6. Conclusions

- (1)
- Al film deposition on specimen before laser processing is proposed for fabrication of micro-scale gratings with three main benefits. First, easy operation with the same processing parameters and without a mask. Second, wide applicability to different materials, such as Si, metal, ceramic, composite, etc. Third, high-quality of gratings with high contrast, small HAZ and small roughness.
- (2)
- The energy of laser pulse is optimized for clear line etching on the Al film. The optimal energy of laser pulse is 9.8 μJ, and the optimum fluence is 9.5 J/mm
^{2}with the waist radius of the laser beam 25.7 μm. Parallel gratings are fabricated. The results indicate that gratings of parallel lines fabricated by nanosecond laser will affect each other if the distance between adjacent lines is too small. The highest frequency of parallel gratings is about 30 lines/mm with line width of 29 μm, and the distance between two adjacent laser pulses being of 10 μm. - (3)
- The verification tests prove that the applicability of the proposed fabrication method for the micro-scale gratings in GPA measurements. Moreover, the micro-scale gratings can be fabricated on areas of interest, such as the crack tip, for deformation measurements. The adhesion between the Al film and the tested sample is good enough to ensure that the pattern sticks well to the sample.
- (4)
- The proposed fabrication method of gratings suffers from a few defects. For instance, the minimum line width of gratings is about 10 μm due to the spatial resolution of the nanosecond laser, and thus the frequency of fabricated gratings is relatively low, typically 10–30 lines/mm. If a femtosecond laser is used, higher frequency of gratings can be fabricated since it has higher spatial resolution and pulse energy.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 3.**(

**a**) Schematic of the polymethyl methacrylate (PMMA) specimen and (

**b**) the experimental setup including an optical microscope and a tensile loading stage.

**Figure 4.**Scanning electron microscopy (SEM) images of single line etched by nanosecond laser: (

**a**) Si, (

**b**) stainless steel and (

**c**) Al film on quartz glass. The etching energy is 38.6 μJ, 20.0 μJ, and 31.5 μJ.

**Figure 5.**Optical images of straight lines etched by nanosecond laser: (

**a**) Si, (

**b**) stainless steel, and (

**c**) Al film. The etching energy is about 80 μJ, 51 μJ, 39 μJ, and 17 μJ from top to bottom. The arrows indicate the etched lines.

**Figure 6.**Linear fitting of measured data between the square of line width and logarithm of single pulse energy: (

**a**) Si, (

**b**) stainless steel, and (

**c**) Al film.

**Figure 7.**Optical images of straight line etched by nanosecond laser: (

**a**) etching energy is 6.8 μJ, and (

**b**) etching energy is 9.8 μJ.

**Figure 8.**Optical images of gratings fabricated on Al film: (

**a**) d = 50 μm, (

**b**) d = 38.2 μm, (

**c**) d = 28.1 μm, (

**d**) d = 13.9 μm, (

**e**) d = 10 μm, and (

**f**) d = 0 μm. The frequency of gratings is (

**a**) 10 lines/mm, (

**b**) 15 lines/mm, (

**c**) 20 lines/mm, (

**d**) 25 lines/mm, (

**e**) 30 lines/mm, and (

**f**) 35 lines/mm. The energy of laser pulse is 9.8 μJ.

**Figure 11.**Calculated displacement and strain fields in horizontal direction: (

**a**) U and (

**b**) ε

_{x}. One pixel represents 0.55 μm. The tensile load is under 52.9 N.

**Figure 13.**Gratings on the PMMA specimen with a V-shape notch: (

**a**) before load, (

**b**) under 49.0 N load, and (

**c**) under load 73.5 N.

**Figure 14.**Calculated displacement and strain fields in horizontal direction under load 49.0 N: (

**a**) U and (

**b**) ε

_{x}. One pixel represents 0.55 μm.

Specimens | Ω_{0} (μm) | E_{th} (μJ) | I_{th} (J/mm^{2}) |
---|---|---|---|

Si | 52.0 | 8.71 | 2.05 |

Stainless Steel | 75.6 | 17.0 | 1.89 |

Al Film | 25.7 | 6.8 | 6.56 |

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**MDPI and ACS Style**

Yang, G.; He, W.; Zhu, J.; Chen, L. Fabrication of Micro-Scale Gratings by Nanosecond Laser and Its Applications for Deformation Measurements. *Micromachines* **2017**, *8*, 136.
https://doi.org/10.3390/mi8050136

**AMA Style**

Yang G, He W, Zhu J, Chen L. Fabrication of Micro-Scale Gratings by Nanosecond Laser and Its Applications for Deformation Measurements. *Micromachines*. 2017; 8(5):136.
https://doi.org/10.3390/mi8050136

**Chicago/Turabian Style**

Yang, Guanbao, Wei He, Jianguo Zhu, and Lei Chen. 2017. "Fabrication of Micro-Scale Gratings by Nanosecond Laser and Its Applications for Deformation Measurements" *Micromachines* 8, no. 5: 136.
https://doi.org/10.3390/mi8050136