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SiC composites via femtosecond laser
Accepted Manuscript Influence of surface morphology on processing of C/SiC composites via femtosecond laser Zhaoyang Zhai, Wenjun Wang, Jie Zhao, Xuesong Mei, Kedian Wang, Fangcheng Wang, Huizhu Yang PII: DOI: Reference:
S1359-835X(17)30299-3 http://dx.doi.org/10.1016/j.compositesa.2017.07.031 JCOMA 4751
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
18 May 2017 6 July 2017 30 July 2017
Please cite this article as: Zhai, Z., Wang, W., Zhao, J., Mei, X., Wang, K., Wang, F., Yang, H., Influence of surface morphology on processing of C/SiC composites via femtosecond laser, Composites: Part A (2017), doi: http:// dx.doi.org/10.1016/j.compositesa.2017.07.031
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of surface morphology on processing of C/SiC composites via femtosecond laser Zhaoyang Zhai a,b,c, Wenjun Wang a,b,c,*, Jie Zhao a,b,c, Xuesong Mei a,b,c, Kedian Wang a,b,c, Fangcheng Wang a,b,c, Huizhu Yang a,b,c a b
School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China;
State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710054, China; c
Shaanxi Key Laboratory of Intelligent Robots, Xi’an 710049, China
Abstract: Carbon fiber reinforced silicon carbide (C/SiC) was processed with an 800 nm femtosecond laser, and the results were analyzed through theoretical calculations and wave optics simulations. In the ablation experiment, C/SiC morphologies for different parameters such as laser power, defocus distance, and scanning speed were compared. It was found that the roughness prior to processing of the C/SiC surface noticeably affects the ablation effect. Beam waist radius, curvature radius, and electric field intensity of the femtosecond laser were calculated theoretically and the wave optics module was simulated in finite element software. Causes for the different morphologies can be explained directly through the simulation results from the perspective of the electromagnetic field. It was found that the microgroove quality of C/SiC processed subject to the femtosecond laser with high fluence is relatively higher and that the edge oxidation of the processing area can be effectively controlled through argon protection. The comparison between the simulation and the experiment results deepens the understanding of the ablation mechanism, which can provide references for the improvement in processing quality of ceramic matrix composites (CMC) by laser treatment. Keywords: C/SiC, femtosecond laser, ablation effect, oxidation
1. Introduction New high performance materials and advanced composites are produced to meet ever increasing demand for materials in aerospace applications. Ceramic composite materials for continuous fiber toughening are the most important kind of high-temperature structural ceramics, among which CMC-SiC [1-2] (including SiC/SiC and C/SiC) is most commonly used. After long development, remarkable achievements have been made in the production [3], research and application of CMC-SiC materials. Studies on CMC-SiC processing techniques have lagged, comparatively, and are still being explored. To produce high-quality aerospace products, processing is indispensable. However, CMC-SiC is made up of a matrix and fibers, and is hard to process. In addition, SiC itself is a quite hard and brittle material with a hardness between corundum and diamond and a Mohs hardness of 9.5. Therefore, traditional mechanical processing can be easily affected due to the nature of CMC-SiC, which is an anisotropic material [4] that exhibits unevenness and ultrahigh hardness [5]. Within a wide application range, the laser has many advantages for mass production of composite materials [6]. At present, continuous lasers or long-pulse lasers are commonly used in laser processing of composite materials. Due to the strong thermal effect in processing, laser processing can lead to many defects [7-8] such as oxidation, delamination, and cracking. Pulse width is a key factor influencing the interaction between a laser beam and the material. In * Corresponding author. E-mail address: [email protected] (W.J. Wang).
nanosecond-scale pulse processing, absorption, ablation, and heat conduction dominate the processing. Drilling on SiC/SiC has been conducted by Cai et al. using a nanosecond laser [9], and they found that a fused mass appears during the drilling; defects such as delamination and micro-cracks are caused by the heat affected zone (HAZ). Through hole and blind hole [10] of SiC/SiC processing with a picosecond laser has been studied by Hu et al., who also analyzed the processing features. Their study indicated that the wall of the through hole is smooth without a recast layer, which can help to ensure processing accuracy. Wang et al. carried out studies on the technique and mechanism of micro-machining C/SiC using a picosecond laser [11]. Their study showed that with the high power of a picosecond laser, a large amount of vapor-phase substances and a strong shock wave are formed resulting in high recoil pressure so fragments are ejected at high speeds. The technique and mechanism for micro-machining SiC/SiC using a picosecond laser have been studied by Liu et al. [12]. Their results show that parameters of picosecond laser such as energy density, scanning speed, and processing methods have a large influence on the quality and efficiency of the processing. Overall, studying SiC laser processing techniques is of great practical significance and has promising application prospects. However, as CMC-SiC is a new material, studies on laser processing are still in their infancy. Relevant literature has mainly focused on the analysis of parameters on the processing effect, while the causes of defects [13-14] and processing improvements have not been made clear. Particularly, processing hard and brittle materials using a femtosecond laser are just beginning and there has been a lack of systemic theoretical studies. Consequently, femtosecond laser processing has not been applied to CMC parts. Considering the great influence of material on the laser beam transmission and absorption, a model integrating laser transmission, energy absorption, and energy transfer must be established to properly machine materials using a multi-pulse femtosecond laser. In this study, we study the processing of C/SiC using an 800 nm femtosecond laser. The beam waist radius, curvature radius, and electric field intensity of the femtosecond laser beam were theoretically determined; all simulated through wave optics simulation software. The simulation results can help explain the causes for the different morphologies in the results.
2. Experimental method The C/SiC sheet was prepared through chemical vapor infiltration (CVI), in which the porosity of C/SiC is smaller and its performance is more desirable. C/SiC mainly consists of SiC settled layer on the outermost side, SiC matrix inside, carbon fiber, and an interface layer of pyrolytic carbon (PyC). There are also many tiny pores in the weave structure of the carbon fiber. the characteristics are given in Tab. 1 and the morphology of the experimental material is shown in Fig. 1. Table 1 Table of material characteristics. Parameter
Value
Diameter of carbon fiber
7 μm
Thickness of PyC layer
0.2 μm
Density
2 g/cm3
Fiber volume fraction
40 %
Porosity
10–20 %
Size
80×30×5 mm3
Fig. 1. C/SiC surface morphology.
A Nd: YLF solid state femtosecond laser device (Spitfire Ace-120F, Spectra-physics Inc) was used in the experiment. A HITACHI SU8010 field emission scanning electron microscope (SEM) was used to observe the C/SiC morphology. A laser scanning confocal microscope (OLS4000, Olympus Corp) was adopted for the measurement size. A micro-CT (YXLON Y. Cheetah) was used to analyze the internal features of the C/SiC. The experiments were divided into two groups to compare the influence of inert gas, but the light paths are the same, as shown in Fig. 2. The laser beam travels from the diaphragm shutter to the beam splitter, which splits the beam into two directions: one to the power meter and the other to the collimating lens and focus lens. It then proceeds to the experimental material clamped on the 3-axis motion stage. The processing parameters are presented in Tab. 2.
Fig. 2. Light paths in the experiment. Table 2 Table of processing parameters. Parameter
Value
Wavelength
800 nm
Pulse width
120 fs
Repetition frequency
1000 Hz
Laser mode
TEM00 (M2<1.3)
Numerical aperture (NA)
0.28
3. Results and discussion 3.1. Ablation threshold estimate
The material removal induced by a femtosecond laser is a complicated physico-chemical process, accompanied by the absorption of laser energy, thermal conduction, avalanche ionization, plasma expansion, liquid phase blasting, and other processes. SiC [15] has a 3.02 eV band gap and the photon energy of a laser pulse at 800 nm is 1.5 eV, so ionization will be achieved only through multi-photon non-linear absorption. Ionization of SiC takes place after absorbing the multi-photon energy, and the number of free electrons will increase by great leaps due to the avalanche effect to produce plasma. The femtosecond laser has a short pulse width, so the influence of plasma shielding can be effectively avoided. Also, because the action time of the laser is much shorter than the relaxation time of the lattice, the heat energy transformed from absorbing photon energy will be conducted inside the lattice without exerting any thermal effect on the surrounding material. However, the accumulative effect means that such processing also has a thermal effect on the surrounding materials, which is seen in the following experimental results. The damage threshold of the C/SiC was estimated using the relationship between the ablation diameter and the pulse intensity. A series of spot ablations on the C/SiC sheet were ablated with laser power ranging from 80 mW to 960 mW at 5000 pulses. The ablation threshold [16] was determined through measuring the diameter of the ablated region. The square of the diameter of the ablated crater is governed by the equation:
D2 202 l n EP 2 02 l nE t h
(1)
In this equation, ω0 is the beam waist radius, EP is the pulse energy and Eth is the pulse energy when the material begins to ablate, where Ep is expressed as:
Ep
P f
(2)
In this expression, f is the pulse repetition frequency and P is the laser average power. The ablation threshold is expressed as:
Pth Wth 2 f 0
(3)
In this equation, Pth is the laser energy when the material begins to ablate. The least square method was used to fit the experimental data with a repetition rate of 1000 Hz and ablation time of 5 s, and the square of diameter of the ablated crater (D2) is in line with the logarithm of the incident laser power Ln (P), as shown in Fig. 3. The power of ablation threshold was the interception on the x-axis obtained through extrapolating the linear fit. The ablation threshold for 5000 pulses was estimated to be 1.63 J/cm2.
Fig. 3. Plot of diameters squared versus logarithmic of peak laser.
The coefficient of determination of the experimental data in linear fitting was calculated with MATLAB, and the coefficient of determination is 0.88. Because the C/SiC sheet was prepared through chemical vapor infiltration, the large roughness of the C/SiC surface noticeably affects the ablation effect, especially when the number of laser pulses is less. The microhole diameter of laser ablation with different powers changes obviously, but the roundness error is generally large. So the experimental data in linear fitting appears to be nonlinear. 3.2. Processing subject to the defocusing condition From previous research, it was found that the laser defocus processing is beneficial to improve the machining efficiency and avoid thermal accumulation [17]. Therefore, we carried out the experiments from the focus processing firstly. A series of experiments with different femtosecond laser parameters were conducted for a detailed study of the processing effect. Major parameters include laser power, defocus distance, and scanning speed. The effects are shown in Fig. 4, from which it can be seen that the microgroove on the C/SiC surface widens as the power rises and defocus distance increases. However, with a quicker scanning speed, the groove gradually narrows. With constant laser power and stable motion stage operation, the morphology of the microgroove exhibits many defects such as curvature and uneven groove widths; both become more noticeable with increased laser power and defocus distance, but a higher scanning speed can improve the quality. The experimental rule observed is not obvious, the reason is that there are some large size convexities on the surface of the material in Fig. 4e and Fig. 4f. As the sizes of convex structure of the C/SiC surface are relatively random, the effects of laser processing are also different. Least envelope zone method was used to calculate straightness error of the microgrooves in the yellow wireframe. The straightness errors are calculated to be 47, 52 and 60 μm with the defocus distances of 10, 50 and 100 μm, respectively.
Fig. 4. Surface morphology of C/SiC processed by femtosecond laser subject to the defocusing condition: (a) 100 μm, 0.5 W, (b) 100 μm, 1 W, (c) 50 μm, 0.5 W, (d) 50 μm, 1 W, (e) 10 μm, 0.5 W, and (f) 10 μm, 1 W.
Fig. 5. Profile morphology of C/SiC subject to the defocusing condition: (a) line A, (b) line B.
To visualize the profile morphology of C/SiC processed by the femtosecond laser subject to the defocusing condition, 2D images of C/SiC sample cross sections were generated from non-destructive micro-CT analysis. Fig. 5 shows the images of these cross sections (corresponding to line A and line B in Fig. 4b). The results show that the microgrooves with different processing parameters bend along the optical axis. 3.3. Theoretical analysis In setting up the wave optics simulation of the experiment [18-19], Equation [20] should be used to determine the required parameters. The focal spot size is determined by the wave length of the laser (λ) and the numerical aperture (NA). The beam waist radius (ω0) can be expressed as:
0 M 2 f / s
(4)
In this expression, ωs is the original radius, f is the focal length of the objective lens (f≈ωs/NA), λ is the wavelength of the laser, and M2 is the quality factor representing the difference between a real beam and an ideal Gaussian beam. The value of M2 can be calculated using the following equation:
M D 2
DZ
2 0
D0 1 4 z 2
(5)
In this equation, D0 refers to the light spot diameter at the focal plane (D0=2fλ/πωs), Dz is the light spot diameter at position, z is the distance from the above plane along the optical axis. The expression of the Gaussian beam section radius is:
z 2 0
2
( z ) 0 1
1 2
(6)
According to the above expression, ω(z) is related to z, λ, and ω0. z=0 means that ω0 is the radius where the beam section is the smallest, which is called the beam waist of the Gaussian beam. When the beam spreads in transparent medium, there is a non-lineal relationship between ω(z) and z. The curvature radius can be expressed as:
2 2 R( z ) z 1 0 z
(7)
According to laser resonator diffraction theory, the electric field distribution of a Gaussian beam spreading along z-axis in an isotropic transparent medium is:
E z E0
0 r2 r2 exp 2 exp ik exp i kz ( z ) ( z) ( z ) 2 R ( z )
(8)
Where E0 is the amplitude of electric field, r is the section radius: r2=x2+y2, k is the wave number: k=2π/λ, ω(z) is the section radius of the Gaussian beam, and Φ(z) is the phase factor, which is expressed as:
( z ) arctan
z 02
Fig. 6. Model of femtosecond laser processing simulation of C/SiC material.
(9)
For a direct explanation of the different material morphologies, a wave optics [21-22] module was used to simulate the processing experiment. This module can be used to understand, forecast, and design the electromagnetic wave transmission and resonance effect in optical applications. Through simulation, the electromagnetic field distribution, transmission, reflection, and power loss in the module can be accurately predicted. In addition, it helps evaluate and predict physical features that cannot be measured in experiments. The wave optics simulation includes simulations of frequency domain, time domain, characteristic frequency, and electromagnetic field analysis. Within this framework, the optical physical field is represented by the differential form of Maxwell’s equations and the corresponding initial values and boundary conditions can be worked out. The finite element method (FEM) as well as the boundary-unit discrete method were used, combining with sparse matrices for pre-conditioning and solution. These results are shown in the electromagnetic field, power flow, and loss. The simulation is carried out using COMSOL Multiphysics software with the following steps: define the physical dimensions of the C/SiC and the composition; define the area above the surface of the C/SiC as an air space and the outer side of the space as a perfect matching layer (PML) through frequency-domain analysis in the wave optics module; set the polarization direction of the femtosecond laser to the x-direction; define the electric field density according to Eq. (8); and define finite element meshes and compute the results. The simulation model is shown in Fig. 6. The simulation model of this study was built in the 2D plane. Because the objective lens was used to process the C/SiC, the diameter and height of the convexity were both larger than the focal spot diameter and the Rayleigh length. The laser was not irradiated on a large area of convexities, so the convexity of simulation model was simplified as synthetic surface curvature. We define the area above the surface of the C/SiC as an air space and the outer side of the space as a PML. The wave impedance of the PML is exactly matched with the wave impedance of the adjacent medium, and the incident laser passes through the interface without reflection and enters the air space. Because the C/SiC sheet was prepared through CVI, the surface of C/SiC only consists of SiC settled layer without carbon fiber, so periodic representative volume element/cell was not used in the simulation.
Fig. 7. Simulation of electric field distribution of a Gaussian beam.
As a kind of electromagnetic wave, the electric field and the magnetic field of laser change mutually in the space. It is formed by the the electric vector and the magnetic vector. However, the electric vector is the major factor in substance interaction, so the vector is usually referred to as the light vector. For these reasons, this study focused on the electric vector transmission of the laser. The output of the laser beams abide a Gaussian distribution in the experiments of this study,
so the electric field model for transmission in air is shown in Fig. 7. The simulation is used to analyze and explain why curvature and unevenness appear on the C/SiC surface. Once the femtosecond laser is loaded onto the surface of C/SiC model, the distribution of electric field on the surface of the C/SiC is calculated. The defocus distance was defined to be 10 μm, once the laser is shot on the side surface of convexity (see Fig. 8b), the groove will bend. For larger sizes of the convex structure of the C/SiC surface, the diameter and height of the convexity were both larger than the focal spot diameter and the Rayleigh length, because the objective lens was used to process the C/SiC. The simulation results clearly show that diffraction takes place on the edge of curvature and that the electric field intensity concentrates on one side of the convexity. The corresponding distribution curve shows that, the wave crest on the C/SiC surface shifts almost 2 μm compared to the wave crest of the incident laser. When the laser is shot onto the top surface of convexity (see Fig. 8c), the microgroove narrows. It can be seen from the simulation results that the reflection effect on the convex structure is obvious and the standing wave formed is composed by the incidence laser and the folded light beam of the convex structure. The distribution curve of electric field model shows that the peak value on C/SiC surface is lowered over 30 % than that of the incidence laser. So the cause of microgroove curvature and uneven width lies in the noticeable diffraction and reflection of the femtosecond laser radiating from different positions of the C/SiC surface, and the material on the C/SiC surface was removed in a uniform manner.
Fig. 8. Experimental and simulation comparisons subject to the defocusing condition: (a) SEM image, (b) bending simulation, (c) narrowing simulation.
Fig. 9. Simulation subject to the focusing condition: (a) bending simulation, (b) narrowing simulation.
When we performed the simulation calculations subject to the focusing condition, it can be found that the energy at the laser spot is densely concentrated, which effectively removes the material without serious diffraction or reflection effects (see Fig. 9). The corresponding distribution curve shows that, the wave crest on the C/SiC surface shifts less than 1 μm compared to the wave crest of the incident laser. And the peak value on C/SiC surface is lowered within 20 % than that of the incidence laser. Therefore, the C/SiC processing quality subject to the focusing condition will be significantly improved compared to that subject to the defocusing condition. 3.4. Processing subject to the focusing condition According to the simulation results, it can be inferred that the C/SiC processing quality subject to the focusing condition is apparently improved compared with that subject to the defocusing condition. The microgroove width produced at different scanning speeds changes linearly, just as shown in Fig. 10a. There is no noticeable curvature or uneven width. This is because subject to the focusing condition, the energy at the laser spot is densest and most concentrated, so it can effectively remove the material within the threshold range. However, as the laser spot radius shrinks, the groove will narrow accordingly. On the edge of the ablation region, the laser fluence is near the threshold value and the proportion of O to Si increases remarkably. The thermal-chemical reaction takes place between the oxygen in the air and the material after absorbing the pulse energy, as is shown in Eqns. (10) and (11) [23]. The reaction products CO and CO2 will float into air, leaving white SiO2 particles covering the surface that form an oxide layer. To avoid this SiO2 oxide coating, inert gas shielding can be used during the laser processing. Commonly used inert gases include nitrogen and argon, but the former will produce Si 3N4 with SiC and its oxide through chemical reactions, as is shown in Eqns. (12) and (13) [24]. Therefore, argon was used in the experiment and the results are presented in Fig. 10b. Due to the cooling effect and protection of argon, a high processing quality was ensured and the O on the edge of the processing area was reduced by a large margin so that oxidation was effectively avoided.
2 O2 ( g ) + S i C ( s )2= S i O 2( s ) + C O ( g ) 3O2 (g)+2SiC(s)=2SiO2 (s)+2CO(g) O2 (g)+SiC(s)=SiO(g)+CO(g) 6 S i O ( g ) +2 2 N ( g3 ) =4 S i N ( s2 ) + 3 S i O ( l )
(10) (11) (12) (13)
Fig. 10. Surface morphology of C/SiC subject to the focusing condition: (a) air environment, (b) argon environment.
Fig. 11. Profile morphology of C/SiC subject to the focusing condition: (a) line A, (b) line B.
Fig. 11 shows the micro-CT analysis cross sections (corresponding to line A and line B in Fig. 5a) of the C/SiC samples analysis. The results show that the bending of microgrooves subject to the focusing condition has been apparently improved compared with that subject to the defocusing condition. The width and depth of microgrooves ablated by femtosecond laser under defocusing condition and focusing condition were compared. Results are shown in Fig. 12 (corresponding to Fig. 5a and Fig. 11a). It presents that the width under focusing condition are noticeably smaller than that under defocusing condition. Contrarily, the depth under focusing condition are lager than that under defocusing condition. That is to say, greater aspect ratio can be achieved under focusing condition. Because the spot diameter is small and the laser fluence is high under the focusing
condition, it can effectively remove the material.
Fig. 12. Microgrooves width and depth of laser ablation in different conditions.
4. Conclusions In this study, the processing of C/SiC by femtosecond laser with an 800 nm wavelength was studied. The experimental results were analyzed using theoretical calculations and wave optics simulations. In the ablation experiment on the C/SiC surface, the morphologies for different laser powers, defocus distances, and scanning speeds were compared. The beam waist radius, curvature radius, and electric field intensity of the femtosecond laser beam were determined theoretically all through wave optics simulation software. The simulation results can help explain the causes of different morphologies. The following conclusions were drawn from the experiments and simulations: The roughness prior to processing of the C/SiC surface has a large influence on the ablation effect. Laser radiation works on different locations of the convexities, leading to either narrower microgrooves or curvatures. Both become more noticeable with increasing laser power and defocus distance, but smaller with faster scanning speeds. The major cause of microgroove curvature and uneven width lies in the diffraction and reflection of the femtosecond laser radiating from different positions of the C/SiC surface, resulting in drift or loss of the electric field. Consequently, the material on the C/SiC surface cannot be removed in a uniform manner. The C/SiC processing quality subject to the focusing condition will be significantly improved compared to that subject to the defocusing condition. And argon cooling and protection ensures a high-quality processing and helps avoid oxidation at the edge of processing area by greatly reducing the content of O. To improve the laser processing quality of C/SiC, the diffraction and reflection of the femtosecond laser radiating from the C/SiC surface need to be reduced by steps: firstly the C/SiC surface is polished by laser, and then the substrate is ablated. The method may improve the laser processing quality effectively. Acknowledgments This work was supported by the National Key Research and Development Program of China (grant no. 2016YFB1102502), the National Natural Science Foundation of China (grant nos. 51475361, 91323033 and 51421004), and the Program for Changjiang Scholars and Innovative Research Team in University (grant no. IRT_15R54) .
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S1359-835X(17)30299-3 http://dx.doi.org/10.1016/j.compositesa.2017.07.031 JCOMA 4751
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
18 May 2017 6 July 2017 30 July 2017
Please cite this article as: Zhai, Z., Wang, W., Zhao, J., Mei, X., Wang, K., Wang, F., Yang, H., Influence of surface morphology on processing of C/SiC composites via femtosecond laser, Composites: Part A (2017), doi: http:// dx.doi.org/10.1016/j.compositesa.2017.07.031
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of surface morphology on processing of C/SiC composites via femtosecond laser Zhaoyang Zhai a,b,c, Wenjun Wang a,b,c,*, Jie Zhao a,b,c, Xuesong Mei a,b,c, Kedian Wang a,b,c, Fangcheng Wang a,b,c, Huizhu Yang a,b,c a b
School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China;
State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710054, China; c
Shaanxi Key Laboratory of Intelligent Robots, Xi’an 710049, China
Abstract: Carbon fiber reinforced silicon carbide (C/SiC) was processed with an 800 nm femtosecond laser, and the results were analyzed through theoretical calculations and wave optics simulations. In the ablation experiment, C/SiC morphologies for different parameters such as laser power, defocus distance, and scanning speed were compared. It was found that the roughness prior to processing of the C/SiC surface noticeably affects the ablation effect. Beam waist radius, curvature radius, and electric field intensity of the femtosecond laser were calculated theoretically and the wave optics module was simulated in finite element software. Causes for the different morphologies can be explained directly through the simulation results from the perspective of the electromagnetic field. It was found that the microgroove quality of C/SiC processed subject to the femtosecond laser with high fluence is relatively higher and that the edge oxidation of the processing area can be effectively controlled through argon protection. The comparison between the simulation and the experiment results deepens the understanding of the ablation mechanism, which can provide references for the improvement in processing quality of ceramic matrix composites (CMC) by laser treatment. Keywords: C/SiC, femtosecond laser, ablation effect, oxidation
1. Introduction New high performance materials and advanced composites are produced to meet ever increasing demand for materials in aerospace applications. Ceramic composite materials for continuous fiber toughening are the most important kind of high-temperature structural ceramics, among which CMC-SiC [1-2] (including SiC/SiC and C/SiC) is most commonly used. After long development, remarkable achievements have been made in the production [3], research and application of CMC-SiC materials. Studies on CMC-SiC processing techniques have lagged, comparatively, and are still being explored. To produce high-quality aerospace products, processing is indispensable. However, CMC-SiC is made up of a matrix and fibers, and is hard to process. In addition, SiC itself is a quite hard and brittle material with a hardness between corundum and diamond and a Mohs hardness of 9.5. Therefore, traditional mechanical processing can be easily affected due to the nature of CMC-SiC, which is an anisotropic material [4] that exhibits unevenness and ultrahigh hardness [5]. Within a wide application range, the laser has many advantages for mass production of composite materials [6]. At present, continuous lasers or long-pulse lasers are commonly used in laser processing of composite materials. Due to the strong thermal effect in processing, laser processing can lead to many defects [7-8] such as oxidation, delamination, and cracking. Pulse width is a key factor influencing the interaction between a laser beam and the material. In * Corresponding author. E-mail address: [email protected] (W.J. Wang).
nanosecond-scale pulse processing, absorption, ablation, and heat conduction dominate the processing. Drilling on SiC/SiC has been conducted by Cai et al. using a nanosecond laser [9], and they found that a fused mass appears during the drilling; defects such as delamination and micro-cracks are caused by the heat affected zone (HAZ). Through hole and blind hole [10] of SiC/SiC processing with a picosecond laser has been studied by Hu et al., who also analyzed the processing features. Their study indicated that the wall of the through hole is smooth without a recast layer, which can help to ensure processing accuracy. Wang et al. carried out studies on the technique and mechanism of micro-machining C/SiC using a picosecond laser [11]. Their study showed that with the high power of a picosecond laser, a large amount of vapor-phase substances and a strong shock wave are formed resulting in high recoil pressure so fragments are ejected at high speeds. The technique and mechanism for micro-machining SiC/SiC using a picosecond laser have been studied by Liu et al. [12]. Their results show that parameters of picosecond laser such as energy density, scanning speed, and processing methods have a large influence on the quality and efficiency of the processing. Overall, studying SiC laser processing techniques is of great practical significance and has promising application prospects. However, as CMC-SiC is a new material, studies on laser processing are still in their infancy. Relevant literature has mainly focused on the analysis of parameters on the processing effect, while the causes of defects [13-14] and processing improvements have not been made clear. Particularly, processing hard and brittle materials using a femtosecond laser are just beginning and there has been a lack of systemic theoretical studies. Consequently, femtosecond laser processing has not been applied to CMC parts. Considering the great influence of material on the laser beam transmission and absorption, a model integrating laser transmission, energy absorption, and energy transfer must be established to properly machine materials using a multi-pulse femtosecond laser. In this study, we study the processing of C/SiC using an 800 nm femtosecond laser. The beam waist radius, curvature radius, and electric field intensity of the femtosecond laser beam were theoretically determined; all simulated through wave optics simulation software. The simulation results can help explain the causes for the different morphologies in the results.
2. Experimental method The C/SiC sheet was prepared through chemical vapor infiltration (CVI), in which the porosity of C/SiC is smaller and its performance is more desirable. C/SiC mainly consists of SiC settled layer on the outermost side, SiC matrix inside, carbon fiber, and an interface layer of pyrolytic carbon (PyC). There are also many tiny pores in the weave structure of the carbon fiber. the characteristics are given in Tab. 1 and the morphology of the experimental material is shown in Fig. 1. Table 1 Table of material characteristics. Parameter
Value
Diameter of carbon fiber
7 μm
Thickness of PyC layer
0.2 μm
Density
2 g/cm3
Fiber volume fraction
40 %
Porosity
10–20 %
Size
80×30×5 mm3
Fig. 1. C/SiC surface morphology.
A Nd: YLF solid state femtosecond laser device (Spitfire Ace-120F, Spectra-physics Inc) was used in the experiment. A HITACHI SU8010 field emission scanning electron microscope (SEM) was used to observe the C/SiC morphology. A laser scanning confocal microscope (OLS4000, Olympus Corp) was adopted for the measurement size. A micro-CT (YXLON Y. Cheetah) was used to analyze the internal features of the C/SiC. The experiments were divided into two groups to compare the influence of inert gas, but the light paths are the same, as shown in Fig. 2. The laser beam travels from the diaphragm shutter to the beam splitter, which splits the beam into two directions: one to the power meter and the other to the collimating lens and focus lens. It then proceeds to the experimental material clamped on the 3-axis motion stage. The processing parameters are presented in Tab. 2.
Fig. 2. Light paths in the experiment. Table 2 Table of processing parameters. Parameter
Value
Wavelength
800 nm
Pulse width
120 fs
Repetition frequency
1000 Hz
Laser mode
TEM00 (M2<1.3)
Numerical aperture (NA)
0.28
3. Results and discussion 3.1. Ablation threshold estimate
The material removal induced by a femtosecond laser is a complicated physico-chemical process, accompanied by the absorption of laser energy, thermal conduction, avalanche ionization, plasma expansion, liquid phase blasting, and other processes. SiC [15] has a 3.02 eV band gap and the photon energy of a laser pulse at 800 nm is 1.5 eV, so ionization will be achieved only through multi-photon non-linear absorption. Ionization of SiC takes place after absorbing the multi-photon energy, and the number of free electrons will increase by great leaps due to the avalanche effect to produce plasma. The femtosecond laser has a short pulse width, so the influence of plasma shielding can be effectively avoided. Also, because the action time of the laser is much shorter than the relaxation time of the lattice, the heat energy transformed from absorbing photon energy will be conducted inside the lattice without exerting any thermal effect on the surrounding material. However, the accumulative effect means that such processing also has a thermal effect on the surrounding materials, which is seen in the following experimental results. The damage threshold of the C/SiC was estimated using the relationship between the ablation diameter and the pulse intensity. A series of spot ablations on the C/SiC sheet were ablated with laser power ranging from 80 mW to 960 mW at 5000 pulses. The ablation threshold [16] was determined through measuring the diameter of the ablated region. The square of the diameter of the ablated crater is governed by the equation:
D2 202 l n EP 2 02 l nE t h
(1)
In this equation, ω0 is the beam waist radius, EP is the pulse energy and Eth is the pulse energy when the material begins to ablate, where Ep is expressed as:
Ep
P f
(2)
In this expression, f is the pulse repetition frequency and P is the laser average power. The ablation threshold is expressed as:
Pth Wth 2 f 0
(3)
In this equation, Pth is the laser energy when the material begins to ablate. The least square method was used to fit the experimental data with a repetition rate of 1000 Hz and ablation time of 5 s, and the square of diameter of the ablated crater (D2) is in line with the logarithm of the incident laser power Ln (P), as shown in Fig. 3. The power of ablation threshold was the interception on the x-axis obtained through extrapolating the linear fit. The ablation threshold for 5000 pulses was estimated to be 1.63 J/cm2.
Fig. 3. Plot of diameters squared versus logarithmic of peak laser.
The coefficient of determination of the experimental data in linear fitting was calculated with MATLAB, and the coefficient of determination is 0.88. Because the C/SiC sheet was prepared through chemical vapor infiltration, the large roughness of the C/SiC surface noticeably affects the ablation effect, especially when the number of laser pulses is less. The microhole diameter of laser ablation with different powers changes obviously, but the roundness error is generally large. So the experimental data in linear fitting appears to be nonlinear. 3.2. Processing subject to the defocusing condition From previous research, it was found that the laser defocus processing is beneficial to improve the machining efficiency and avoid thermal accumulation [17]. Therefore, we carried out the experiments from the focus processing firstly. A series of experiments with different femtosecond laser parameters were conducted for a detailed study of the processing effect. Major parameters include laser power, defocus distance, and scanning speed. The effects are shown in Fig. 4, from which it can be seen that the microgroove on the C/SiC surface widens as the power rises and defocus distance increases. However, with a quicker scanning speed, the groove gradually narrows. With constant laser power and stable motion stage operation, the morphology of the microgroove exhibits many defects such as curvature and uneven groove widths; both become more noticeable with increased laser power and defocus distance, but a higher scanning speed can improve the quality. The experimental rule observed is not obvious, the reason is that there are some large size convexities on the surface of the material in Fig. 4e and Fig. 4f. As the sizes of convex structure of the C/SiC surface are relatively random, the effects of laser processing are also different. Least envelope zone method was used to calculate straightness error of the microgrooves in the yellow wireframe. The straightness errors are calculated to be 47, 52 and 60 μm with the defocus distances of 10, 50 and 100 μm, respectively.
Fig. 4. Surface morphology of C/SiC processed by femtosecond laser subject to the defocusing condition: (a) 100 μm, 0.5 W, (b) 100 μm, 1 W, (c) 50 μm, 0.5 W, (d) 50 μm, 1 W, (e) 10 μm, 0.5 W, and (f) 10 μm, 1 W.
Fig. 5. Profile morphology of C/SiC subject to the defocusing condition: (a) line A, (b) line B.
To visualize the profile morphology of C/SiC processed by the femtosecond laser subject to the defocusing condition, 2D images of C/SiC sample cross sections were generated from non-destructive micro-CT analysis. Fig. 5 shows the images of these cross sections (corresponding to line A and line B in Fig. 4b). The results show that the microgrooves with different processing parameters bend along the optical axis. 3.3. Theoretical analysis In setting up the wave optics simulation of the experiment [18-19], Equation [20] should be used to determine the required parameters. The focal spot size is determined by the wave length of the laser (λ) and the numerical aperture (NA). The beam waist radius (ω0) can be expressed as:
0 M 2 f / s
(4)
In this expression, ωs is the original radius, f is the focal length of the objective lens (f≈ωs/NA), λ is the wavelength of the laser, and M2 is the quality factor representing the difference between a real beam and an ideal Gaussian beam. The value of M2 can be calculated using the following equation:
M D 2
DZ
2 0
D0 1 4 z 2
(5)
In this equation, D0 refers to the light spot diameter at the focal plane (D0=2fλ/πωs), Dz is the light spot diameter at position, z is the distance from the above plane along the optical axis. The expression of the Gaussian beam section radius is:
z 2 0
2
( z ) 0 1
1 2
(6)
According to the above expression, ω(z) is related to z, λ, and ω0. z=0 means that ω0 is the radius where the beam section is the smallest, which is called the beam waist of the Gaussian beam. When the beam spreads in transparent medium, there is a non-lineal relationship between ω(z) and z. The curvature radius can be expressed as:
2 2 R( z ) z 1 0 z
(7)
According to laser resonator diffraction theory, the electric field distribution of a Gaussian beam spreading along z-axis in an isotropic transparent medium is:
E z E0
0 r2 r2 exp 2 exp ik exp i kz ( z ) ( z) ( z ) 2 R ( z )
(8)
Where E0 is the amplitude of electric field, r is the section radius: r2=x2+y2, k is the wave number: k=2π/λ, ω(z) is the section radius of the Gaussian beam, and Φ(z) is the phase factor, which is expressed as:
( z ) arctan
z 02
Fig. 6. Model of femtosecond laser processing simulation of C/SiC material.
(9)
For a direct explanation of the different material morphologies, a wave optics [21-22] module was used to simulate the processing experiment. This module can be used to understand, forecast, and design the electromagnetic wave transmission and resonance effect in optical applications. Through simulation, the electromagnetic field distribution, transmission, reflection, and power loss in the module can be accurately predicted. In addition, it helps evaluate and predict physical features that cannot be measured in experiments. The wave optics simulation includes simulations of frequency domain, time domain, characteristic frequency, and electromagnetic field analysis. Within this framework, the optical physical field is represented by the differential form of Maxwell’s equations and the corresponding initial values and boundary conditions can be worked out. The finite element method (FEM) as well as the boundary-unit discrete method were used, combining with sparse matrices for pre-conditioning and solution. These results are shown in the electromagnetic field, power flow, and loss. The simulation is carried out using COMSOL Multiphysics software with the following steps: define the physical dimensions of the C/SiC and the composition; define the area above the surface of the C/SiC as an air space and the outer side of the space as a perfect matching layer (PML) through frequency-domain analysis in the wave optics module; set the polarization direction of the femtosecond laser to the x-direction; define the electric field density according to Eq. (8); and define finite element meshes and compute the results. The simulation model is shown in Fig. 6. The simulation model of this study was built in the 2D plane. Because the objective lens was used to process the C/SiC, the diameter and height of the convexity were both larger than the focal spot diameter and the Rayleigh length. The laser was not irradiated on a large area of convexities, so the convexity of simulation model was simplified as synthetic surface curvature. We define the area above the surface of the C/SiC as an air space and the outer side of the space as a PML. The wave impedance of the PML is exactly matched with the wave impedance of the adjacent medium, and the incident laser passes through the interface without reflection and enters the air space. Because the C/SiC sheet was prepared through CVI, the surface of C/SiC only consists of SiC settled layer without carbon fiber, so periodic representative volume element/cell was not used in the simulation.
Fig. 7. Simulation of electric field distribution of a Gaussian beam.
As a kind of electromagnetic wave, the electric field and the magnetic field of laser change mutually in the space. It is formed by the the electric vector and the magnetic vector. However, the electric vector is the major factor in substance interaction, so the vector is usually referred to as the light vector. For these reasons, this study focused on the electric vector transmission of the laser. The output of the laser beams abide a Gaussian distribution in the experiments of this study,
so the electric field model for transmission in air is shown in Fig. 7. The simulation is used to analyze and explain why curvature and unevenness appear on the C/SiC surface. Once the femtosecond laser is loaded onto the surface of C/SiC model, the distribution of electric field on the surface of the C/SiC is calculated. The defocus distance was defined to be 10 μm, once the laser is shot on the side surface of convexity (see Fig. 8b), the groove will bend. For larger sizes of the convex structure of the C/SiC surface, the diameter and height of the convexity were both larger than the focal spot diameter and the Rayleigh length, because the objective lens was used to process the C/SiC. The simulation results clearly show that diffraction takes place on the edge of curvature and that the electric field intensity concentrates on one side of the convexity. The corresponding distribution curve shows that, the wave crest on the C/SiC surface shifts almost 2 μm compared to the wave crest of the incident laser. When the laser is shot onto the top surface of convexity (see Fig. 8c), the microgroove narrows. It can be seen from the simulation results that the reflection effect on the convex structure is obvious and the standing wave formed is composed by the incidence laser and the folded light beam of the convex structure. The distribution curve of electric field model shows that the peak value on C/SiC surface is lowered over 30 % than that of the incidence laser. So the cause of microgroove curvature and uneven width lies in the noticeable diffraction and reflection of the femtosecond laser radiating from different positions of the C/SiC surface, and the material on the C/SiC surface was removed in a uniform manner.
Fig. 8. Experimental and simulation comparisons subject to the defocusing condition: (a) SEM image, (b) bending simulation, (c) narrowing simulation.
Fig. 9. Simulation subject to the focusing condition: (a) bending simulation, (b) narrowing simulation.
When we performed the simulation calculations subject to the focusing condition, it can be found that the energy at the laser spot is densely concentrated, which effectively removes the material without serious diffraction or reflection effects (see Fig. 9). The corresponding distribution curve shows that, the wave crest on the C/SiC surface shifts less than 1 μm compared to the wave crest of the incident laser. And the peak value on C/SiC surface is lowered within 20 % than that of the incidence laser. Therefore, the C/SiC processing quality subject to the focusing condition will be significantly improved compared to that subject to the defocusing condition. 3.4. Processing subject to the focusing condition According to the simulation results, it can be inferred that the C/SiC processing quality subject to the focusing condition is apparently improved compared with that subject to the defocusing condition. The microgroove width produced at different scanning speeds changes linearly, just as shown in Fig. 10a. There is no noticeable curvature or uneven width. This is because subject to the focusing condition, the energy at the laser spot is densest and most concentrated, so it can effectively remove the material within the threshold range. However, as the laser spot radius shrinks, the groove will narrow accordingly. On the edge of the ablation region, the laser fluence is near the threshold value and the proportion of O to Si increases remarkably. The thermal-chemical reaction takes place between the oxygen in the air and the material after absorbing the pulse energy, as is shown in Eqns. (10) and (11) [23]. The reaction products CO and CO2 will float into air, leaving white SiO2 particles covering the surface that form an oxide layer. To avoid this SiO2 oxide coating, inert gas shielding can be used during the laser processing. Commonly used inert gases include nitrogen and argon, but the former will produce Si 3N4 with SiC and its oxide through chemical reactions, as is shown in Eqns. (12) and (13) [24]. Therefore, argon was used in the experiment and the results are presented in Fig. 10b. Due to the cooling effect and protection of argon, a high processing quality was ensured and the O on the edge of the processing area was reduced by a large margin so that oxidation was effectively avoided.
2 O2 ( g ) + S i C ( s )2= S i O 2( s ) + C O ( g ) 3O2 (g)+2SiC(s)=2SiO2 (s)+2CO(g) O2 (g)+SiC(s)=SiO(g)+CO(g) 6 S i O ( g ) +2 2 N ( g3 ) =4 S i N ( s2 ) + 3 S i O ( l )
(10) (11) (12) (13)
Fig. 10. Surface morphology of C/SiC subject to the focusing condition: (a) air environment, (b) argon environment.
Fig. 11. Profile morphology of C/SiC subject to the focusing condition: (a) line A, (b) line B.
Fig. 11 shows the micro-CT analysis cross sections (corresponding to line A and line B in Fig. 5a) of the C/SiC samples analysis. The results show that the bending of microgrooves subject to the focusing condition has been apparently improved compared with that subject to the defocusing condition. The width and depth of microgrooves ablated by femtosecond laser under defocusing condition and focusing condition were compared. Results are shown in Fig. 12 (corresponding to Fig. 5a and Fig. 11a). It presents that the width under focusing condition are noticeably smaller than that under defocusing condition. Contrarily, the depth under focusing condition are lager than that under defocusing condition. That is to say, greater aspect ratio can be achieved under focusing condition. Because the spot diameter is small and the laser fluence is high under the focusing
condition, it can effectively remove the material.
Fig. 12. Microgrooves width and depth of laser ablation in different conditions.
4. Conclusions In this study, the processing of C/SiC by femtosecond laser with an 800 nm wavelength was studied. The experimental results were analyzed using theoretical calculations and wave optics simulations. In the ablation experiment on the C/SiC surface, the morphologies for different laser powers, defocus distances, and scanning speeds were compared. The beam waist radius, curvature radius, and electric field intensity of the femtosecond laser beam were determined theoretically all through wave optics simulation software. The simulation results can help explain the causes of different morphologies. The following conclusions were drawn from the experiments and simulations: The roughness prior to processing of the C/SiC surface has a large influence on the ablation effect. Laser radiation works on different locations of the convexities, leading to either narrower microgrooves or curvatures. Both become more noticeable with increasing laser power and defocus distance, but smaller with faster scanning speeds. The major cause of microgroove curvature and uneven width lies in the diffraction and reflection of the femtosecond laser radiating from different positions of the C/SiC surface, resulting in drift or loss of the electric field. Consequently, the material on the C/SiC surface cannot be removed in a uniform manner. The C/SiC processing quality subject to the focusing condition will be significantly improved compared to that subject to the defocusing condition. And argon cooling and protection ensures a high-quality processing and helps avoid oxidation at the edge of processing area by greatly reducing the content of O. To improve the laser processing quality of C/SiC, the diffraction and reflection of the femtosecond laser radiating from the C/SiC surface need to be reduced by steps: firstly the C/SiC surface is polished by laser, and then the substrate is ablated. The method may improve the laser processing quality effectively. Acknowledgments This work was supported by the National Key Research and Development Program of China (grant no. 2016YFB1102502), the National Natural Science Foundation of China (grant nos. 51475361, 91323033 and 51421004), and the Program for Changjiang Scholars and Innovative Research Team in University (grant no. IRT_15R54) .
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