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Vacuum brazing of alumina to stainless steel using femtosecond laser patterned periodic surface structure
Author’s Accepted Manuscript Vacuum Brazing of Alumina to Stainless Steel Using Femtosecond Laser Patterned Periodic Surface Structure Y. Zhang, G. Zou, L. Liu, A. Wu, Z. Sun, Y.N. Zhou www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(16)30271-4 http://dx.doi.org/10.1016/j.msea.2016.03.068 MSA33463
To appear in: Materials Science & Engineering A Received date: 8 January 2016 Revised date: 15 March 2016 Accepted date: 15 March 2016 Cite this article as: Y. Zhang, G. Zou, L. Liu, A. Wu, Z. Sun and Y.N. Zhou, Vacuum Brazing of Alumina to Stainless Steel Using Femtosecond Laser Patterned Periodic Surface Structure, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.03.068 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 galley proof before it is published in its final citable 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.
Vacuum Brazing of Alumina to Stainless Steel Using Femtosecond Laser Patterned Periodic Surface Structure Y. Zhang1, G. Zou1, L. Liu1*, A. Wu1, Z. Sun1, Y. N. Zhou1, 2 1
Department of Mechanical Engineering, State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China; 2 Department of Mechanical and Mechatronics Engineering, University of Waterloo, ON, N2L, 3G1, Canada.
*Correspondent mail: [email protected] Abstract Ceramic surface patterning is a promising method to reduce the residual stress of ceramic-metal joints, since it needs no complex multiple interlayer and is cost effective. However, literature shows that surface patterns previously examined for ceramic-metal brazing were at a millimeter scale, which limits the application and the degree of enhancement. In this study, femtosecond laser ablation was used to fabricate microscale periodic surface patterns (grooves) on ceramic surfaces. No defects or damage were induced during the ablation. The groove dimension was tunable by controlling ablation parameters. High temperature vacuum brazing of 304 stainless steel to the Al2O3 ceramic with surface grooves was performed using a commercial AgCuTi brazing filler metal. With properly designed surface grooves, the average joint strength was 66MPa, 2.75 times of that of the flat-interface joints. Finite Element Analysis models combined with fracture path examination indicated that the redistribution of residual stress induced by periodic surface pattern accounted for the joint strength enhancement. Key words: femtosecond laser ablation, ceramic-metal brazing, residual stress, joint strength, surface groove. 1. Introduction Much research has focused on reducing thermal stress in ceramic-metal brazed joints, caused by the coefficient of thermal expansion (CTE) mismatch between ceramic and metal [1, 2]. The technological approaches are mostly from three aspects: advanced process, interface engineering, and ceramic surface patterning. The process advancement is mainly based on reducing total heat input or lowering the brazing temperature. For example, local heat sources, such as defocused laser beam [3] and induction heating [4], have been used to melt filler metal. Low temperature filler metals such as Ti activated Sn based solders have been developed to join ceramic below 300 oC [5]. The interface engineering approach has been studied by many researchers. A hard buffer layer (W, Mo, Kovar) having CTE similar to that of the ceramic can reduce the residual stress in the ceramic side [6]. A ductile interlayer (pure Cu, Ni, Al) can also relax the residual stress through plastic deformation [7]. Multiple interlayers [8], porous cushion layer [9] and composite braze metal [10] have also been developed to reduce the residual stress. Ceramic surface patterning is a promising method since it needs no complex buffer materials and fits both commercial filler metal and traditional brazing equipment and process [11, 12]. According to the results of Xiong et al. [12], a periodic rectangular surface structure could change
the residual stress distribution and lower the peak residual stress on the ceramic side, thus strengthening the braze interface. Shen et al. [11]drilled blind holes on a ceramic surface, resulting in significant increase of joint strength due to the pinning effects of the braze spikes. However, only a few publications in this area are available and there are some limits on the existing techniques. For example, the surface grooves in one study had a dimension of millimeter scale, which was too large to promote capillary flow by the molten braze metal [12]. Mechanical machining cannot fabricate complex patterns and many ceramics, such as SiC, BN are not even machinable [13]. Ultrafast laser micro fabrication is receiving great interest in recent years, since this is known as a precision and “cold process” method [13-15]. Ultrafast lasers can produce microscale and nanoscale complex surface structure in most engineering materials and induce no damage to the base material [16, 17]. In this study, a femtosecond laser was used to fabricate microscale periodic groove structures on ceramic surfaces. The results showed that femtosecond laser patterning obviously increased the joint strength compared with the unprocessed flat surface. Finite Element Analysis (FEA) models were also developed to enhance the understanding of this strength improvement. 2. Experiments 2.1 Materials Commercial Al2O3 ceramic and 304 stainless steel cylinders were used with dimensions of Φ15×10 mm and Φ10×10 mm, respectively. Sample surfaces were polished with #1000 sand paper and ultrasonically cleaned. Active brazing filler foil Ag-35Cu-2Ti was polished to 50μm and ultrasonically cleaned before brazing. Periodic surface groove structures on the ceramic samples were fabricated by an amplified Ti: sapphire laser system (Coherent Inc.). This femtosecond laser had 50 fs pulse duration, 3.8 mJ per pulse at 1 kHz repetition rate. The laser beam was 8 mm in diameter, and was focused by a 25.4 mm-focal-length lens. The grooving experiments were carried out in air. The surface profiles were measured by a confocal laser scanning microscope (Olympus, LEXT OLS 4100). The joints were brazed with consistent parameters in a vacuum oven under 2.0╳10-3 Pa. The temperature profile was: heating to 850 oC at 10 oC/min and holding for 10 minutes; heating to 900 oC at 5 oC/min and holding for 5 minutes; cooling down to 600oC at 10oC/min; and finally furnace cooling to room temperature. The joint strength was examined by shear testing with a constant speed of 1mm/min at room temperature. Each data point was an average of 4-5 samples. Fig. 1 shows a typical brazed joint and the schematic diagram of shear test. The fracture surface and cross section of the fractured sample were examined using an optical microscope (BX51RF, Olympus Inc.).
Fig. 1. (a) Picture of a typical joint, (b) Schematic of shear test, (c) schematic diagram of laser ablation system.
2.2 FEA calculations
Residual stresses of brazed joints were simulated by Finite Element Analysis (FEA) with Abaqus software. Since the actual joint was in mm scale and the grooves were in µm scale, the cross scale simulation would be very time consuming. To reduce simulation time and cost, the groove dimension and the distance between two grooves were set 3 times larger than the actual size. Therefore, in the FEA model, the single groove dimension was set to 200 μm and 800 μm, respectively, and the distance of each parallel grooves was set as 800 μm. A half symmetry model was used in the simulation procedure to further reduce computational cost. The coordinate system and schematic diagram of the model used in simulation are shown in Fig. 2 (a). Fig. 2(b) shows SEM (LEO 1530) images of the cross section of actual joints and the model built in the software.
Fig. 2. (a) Schematic diagram of model and coordinate system adopted in calculation, and (b) the actual joint interfaces and models used in simulation.
Ceramic and stainless steel are assumed separate during the heating cycle. During the cooling cycle after brazing, the FEA calculations evaluated the temperature range from brazing temperature (900 oC) to room temperature (20 oC). Materials properties used in FEA calculations are listed in Table 1. Table 1. Material properties for the thermal residual stresses analysis
Materials Al2O3 ceramic Stainless steel AgCuTi
Expansion coefficient (10-6/oC)/ T (oC)
Young’s modulus (GPa)/ T(oC)
Poisson’s ratio
Yield strength (MPa)/ T (oC)
7.7
304
0.26
---
21.5
160
0.26
16/350, 19.5/410, 20/800, 20.5/900
100/300, 95/600, 90/900
0.36
205/600, 170/750, 120/800, 100/850, 90/900 130/300, 130/410, 125/600, 95/750, 45/900
3. Results and discussion 3.1 Surface groove fabrication and joint microstructure Periodic groove structures on Al2O3 ceramic surfaces were obtained by femtosecond laser ablation. Typical cross sections of grooves are approximate arcs, as shown in Fig. 3. The distance between each parallel groove was set as a constant value, 200 μm. According to Fig.3, the ablated
surface was smoother than the unprocessed surface, and no visible crack was found on the ablated surface. It is well known that a femtosecond laser has ultrahigh power density and ultrashort duration time [18]. In this work, the laser power density at the focal point was far above the ablation threshold of Al2O3. As the ablation reaction took away most of the energy, the heat effect was minimized, which avoided the formation of cracks [14]. Furthermore, the laser pulse width (10-15 s) was far shorter than the heat transfer time (10-12 s), resulting in a precise ablation [19, 20].
Fig.3 (a) Schematic diagram of laser ablation system, and 3D structure of (b) original ceramic surface and the surfaces after (c) 0.3W and (d) 3.4W laser ablation.
Table 2 shows the effects of laser power on groove dimensions. It indicates that both groove width and groove depth increased with laser power up to 2.4W. When the laser power was higher than 2.4W, the neighboring grooves were overlapped. Thus, the groove depth increased with the width remaining unchanged. By changing laser power during the fabrication process, 3 typical joint interfaces were obtained: flat interface (original ceramic surface), sparse periodic groove interface (0.3-1.1 W), and close periodic groove interface (2.4-3.4 W). Table 2. Groove dimensions fabricated by different laser powers.
Laser power (W)
Origin
0.3
1.1
2.4
3.4
Width (μm) Depth (μm)
-----
70 30
110 52
195 77
192 97
Fig. 4 shows the XRD patterns of the ceramic surface before and after laser ablation. The two samples had peaks at almost the same 2θ positions, indicating that laser ablation did not change the phase composition of the ceramic surface. The only difference between them was that the signal intensity of the laser processed surface was lower than that of the original flat surface. This may be because the surface grooves blocked part of the reflected X-rays. Femtosecond laser ablation is a “cold manufacturing” process and has limited heat affects to the base material. Although nanoscale thin amorphous layers have been reported on the ablated Si and Cu surfaces [21, 22], no detectable amorphorization on the Al2O3 ceramic surface was observed in this work after femtosecond laser ablation.
Fig.4 XRD patterns of ceramic surface before (black) and after (red) laser ablation.
The cross sections of ceramic and stainless steel brazed joints are shown in Fig. 5. The grooves were completely filled by braze metal, indicating that the groove patterns had capillary effects on the liquid AgCuTi filler metal. Wetting and spreading spontaneously occurred without special assistance such as ultrasonic vibration or pre-coatings [23], which indicated that the process was simple and reliable. The increase of ceramic surface area depleted more Ti element due to the reaction and formation of interface IMCs. Therefore, less CuTi phase was found in the laser patterned samples because of less Ti concentration in the liquid filler metal. The CuTi intermetallic compounds dispersed homogenously with a round-shape and enhanced the strength of the braze seam.
Fig. 5. The cross section SEM picture of the joints with (a) flat interface, (b) sparse grooves interface, and (c) close grooves interface.
A thin intermetallic compounds (IMC) reaction layer formed during brazing between the ceramic and braze filler metal, which has been well addressed by many researchers [24]. The thickness of this reaction layer slightly increased with the increase of laser power. If the temperature of all samples were the same, the brazing procedure would not affect the interface IMC. Besides temperature and time, the kinetics of the interface reaction between active element Ti and Al2O3 ceramic also depends on Ti concentration, surface residual stress, and density. As the reaction is ongoing, the laser patterned sample has lower Ti concentration in the liquid filler metal than the flat sample, as explained above. However, the interface IMC thickness showed the opposite, slightly thicker in the laser patterned sample. Femtosecond laser patterning may induce
surface residual stress on a material surface, e.g. laser peening process [25]. However, the laser process in this study was done in air and without plasma confinement media, so the residual stress level should be much lower than the laser peening process. Also, the heating cycle would release the original residual stress at 900 oC. Therefore, the residual stress should have little effect on the thickness of the interface IMC layer. It is believed that the thicker IMC layer was mainly caused by the change of density in the grooves which facilitated the reaction rate. The laser ablation lowered the density of the surface material, which can be evidenced by the infiltrating reaction beneath the surface, especially in Fig. 5(c). The average IMC thickness of the close groove sample was 3μm, which was still acceptable [26].
3.2 Mechanical properties of joints Fig. 6 shows the shear strength of the joints with different laser powers. For the periodic linear parallel grooves, shear tests along both parallel and perpendicular directions were conducted, as shown in Fig. 6. The results indicated that the joint strength was highly related to the laser power and the shear direction. The maximum shear strength of the laser patterned joints was 2.7 times larger than the original flat surface joints (24MPa), while the minimum was even lower than the flat joints. For both shear directions, joint strength reached their maximum peaks at 0.3W, and then gradually dropped with the increasing of the laser power. Although the parallel shear strength was lower than the perpendicular one, it was still larger than the flat joints until the laser power was higher than 2.4W. Basically, two failure modes were distinguished by visual observation: ceramic failure through the ceramic and interfacial failure along the braze interface, as shown in Fig. 6 (b) and 6 (c), respectively. The ceramic mode was mostly observed (solid points in Fig. 6a), in which part of the ceramic still attached to the stainless steel cylinder after shear testing (Fig. 6b). The joints, made by 3.4 W laser ablation (open points in Fig. 6 (a)), had close periodic groove surfaces and failed along the braze interface (Fig. 6c), which had the lowest shear strength. Since the thicknesses of the reaction layers were acceptable for all three types of joints, the residual stress distribution evidently played the key role of determining the joint strength.
Fig. 6. (a) Shearing strength of the joints with surface grooves fabricated by different laser power, and (b) and (c) the fracture surfaces of the joints.
Although the failure modes of the flat joints and the sparse grooved joints were the same (i.e.,
ceramic failure), the sparse grooved joint showed obviously larger shear strength than the flat one. In order to further examine the fracture mechanism, cross sections of the fractured joints were made. Fig. 7 (a) and (b) are the unprocessed flat joint (24 MPa) and perpendicular sheared sparse grooved joint (66 MPa). As shown in the figures, though these two joints were both broken inside the ceramic, the crack paths were quite different. For the flat joint (Fig. 7a), the crack first nucleated at the edge of the joint and then propagated directly into the ceramic. On the other hand, the crack in the sparse grooved joint (Fig. 7b) first propagated along the joint interface for <1 mm, and then turned into the ceramic. The enlarged image in Fig. 7b clearly shows the crack propagation direction changing at the position around the third groove.
Fig. 7. Cross section of the fractured joints with (a) flat interface and (b) sparse grooves interface (0.3W).
The load-time curve shows more evidence (Fig. 8). The shear load of the flat joints increased monotonically with time and sharply dropped once the joints were failed. But for the sparse grooved joints, small peaks appeared before the joints were broken. These peaks indicated a load releasing and bursting process during the shear test, which was caused by a crack-impeding effect. Related to the crack propagation direction changing shown in Fig. 7 (b), the grooves also showed a crack-impeding function.
Fig. 8. Load-time curve of the shearing test.
3.3 Effects of surface patterning on residual stress It is easy to connect the joint strength enhancement to the mechanical pinning effect of the zig-zag interface. However, this would be more convincing if the failure occurred along the braze interface, because the pinning effect has no influence on the ceramic base material. The ceramic
failure mode suggests that mechanical pinning was not the only benefit, or maybe even not the primary reason. It is believed that the change of residual stress by femtosecond laser surface patterning played a critical role in the joint strength enhancement, this will be analyzed according to the simulation results. Our calculation results indicated that normal stress σx and σy are much smaller than the σz, and shear stress τxy and τxz are much smaller than τzy (red solid line), which was also reported in Rao’s study.[27] Therefore, the following analysis only focuses on the largest normal σz and shear stress τzy. Fig. 9 shows the simulation results of the flat joint. Shear stress τzy has a constant value with the direction pointing to the center of the joint. The peripheral part of the interface is subjected to tensile stress, and the center part is subjected to compressive stress. This simulation results match well with the fracture path: crack initiates at the edge of the joint area, since this region is subjected to the maximum tensile and mixed shear stresses [12], then the crack propagates deeper inside the ceramic where there is lower compressive normal stress [27].
Fig. 9. Calculation results of the residual stress at ceramic surface. (a) Residual stress distribution at flat ceramic surface, (b) enlarged picture of residual stress distribution at symmetry plane, and (c) normal stress σz and shear stress τzy distribution of ceramic surface at symmetry plane (red line in (a) and (b)).
The femtosecond laser patterned periodic surface structure totally changes the stress distribution at the ceramic side, as shown in Fig. 10. The vertical grey bars in Fig. 10 (c) mark the groove locations. The simulation results show that the normal stress σz has periodic peaks which correspond to the groove locations. The σz at the first peripheral groove is +150MPa tensile stress. It suddenly changes to around -100 MPa compressive stress at the second peripheral groove. Then the σz stays compressive and the peak values increase towards the joint center. The τzy shows a similar distribution with the σz: peaking at the groove locations and increasing along the direction to the center. When the shear load was applied perpendicular to the grooves, the peripheral area of the joint was the most possible place for crack initiation due to the large tensile and shear residual stress. After initiation, the crack propagated along the braze interface and met the second groove, where the residual normal stress suddenly changes to -100 MPa compressive stress. According to Rao’s study, the compressive stress has impeding effects against crack propagation [27], that is the reason why the crack propagation direction changed at around the position of the second groove. With the shear load increasing, the cracks grew deeper inside ceramic where subjected to lower
compressive stress. When the shear load is applied parallel to the grooves, the compressive stress still exists but is discretely distributed along the interface. Comparing to the perpendicular shear load, the impending effect of parallel shear load is smaller. As a result, the joint strength of parallel shear was relatively lower than the perpendicular shear.
Fig. 10. Residual stress distribution the sparse grooved joint. (a) residual stress distribution at ceramic side, (b) enlarged picture, and (c) normal stress σz and shear stress τzy distribution of ceramic surface at symmetry plane (red line in (a) and (b)). The vertical grey bars mark the groove locations.
The flat joint also has a tensile-to-compressive stress change along the braze interface, but it is different from the sparse groove joint. The stress state in the flat joint is gradually developed, while the stress of the sparse grooved joint has a sharp change from tensile to compressive. So the impeding effect was not obvious. Additionally, the compressive stress value of the flat joint was much smaller than that of the sparse grooved joint. Therefore, the flat joint had only 24 MPa shear strength, while the sparse grooved joint had a higher shear strength of 66 MPa. Fig.11 shows the residual stress distribution of the close grooved joint. The groove ridges are subjected to a high shear and compress stress, and the groove valleys are subjected to a shear and tensile stress. The stress field changes sharply within a small distance, which is catastrophic to a brittle material. Furthermore, the ceramic ridges between grooves are very sharp and thin, which is like brittle knife blades inserted in the braze metal. The cracks generated by the shear stress could easily break through the braze seam, resulting in the lowest strength.
Fig. 11. Residual stress distribution of the close grooved joint. (a) residual stress distribution at ceramic side, (b) enlarged picture, and (c) normal stress σz and shear stress τzy distribution of ceramic surface at symmetry plane (red line in (a) and (b)).
4. Conclusions The strength of ceramic-metal brazed joints has been significantly enhanced by femtosecond laser patterned periodic surface structures on the ceramic. The main conclusions are: (1) The dimensions of the periodic surface structures were tunable by adjusting laser parameters. Due to the cold-processing nature of femtosecond laser pulses, no defects or surface amorphorization was induced in the Al2O3 ceramic base material. (2) The joint strength was highly related to the surface patterning. The optimum laser parameters were at 0.3W and 200 μm periodic distance. The maximum strength of the laser patterned joints was 66 MPa, which was 2.7 times that of the original flat joint (24 MPa). (3) The FEA simulations demonstrate that the femtosecond laser patterned periodic surface grooves changed the residual stress. The sparse grooves cause a sudden change of residual stress along the interface, from tensile to compressive state, which could impede crack propagation and enhance joint strength. However, the close grooves induce large stress concentration and drastic fluctuations of residual stress along the interface, which actually weaken the joint strength. (4) The fracture path examination showed a change of crack propagation direction when the crack met the second or third periphery sparse groove. The load-displacement curve indicated the time and position of the crack direction change. Those observations matched well with simulation results and further prove the mechanism of the strength enhancement. Acknowledgments This research was supported by National Natural Science Foundation of China (51405258, 51520105007), State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology (AWJ-M14-05) and by Tsinghua University Initiative Scientific Research Program (No. 2010THZ02-1 2013Z02-1). References [1] Levy A. Thermal Residual Stresses in Ceramic‐to‐Metal Brazed Joints. Journal of the American Ceramic Society. 1991;74:2141-7.
[2] Do Nascimento R, Martinelli A, Buschinelli A. Review article: recent advances in metal-ceramic brazing. Cerâmica. 2003;49:178-98. [3] Rohde M, Südmeyer I, Urbanek A, Torge M. Joining of alumina and steel by a laser supported brazing process. Ceramics International. 2009;35:333-7. [4] Hausner S. Induction Brazing of Alumina and Zirconia with Various Metals.
5th International
Brazing and Soldering Conference: Asm; 2012. [5] Koleňák R, Šebo P, Provazník M, Koleňáková M, Ulrich K. Shear strength and wettability of active Sn3. 5Ag4Ti (Ce, Ga) solder on Al 2 O 3 ceramics. Materials & Design. 2011;32:3997-4003. [6] Bing C, Jihua H, Chuang C, Shuhai C, Xingke Z. Microstructures and mechanical properties of C f/SiC composite and TC4 alloy joints brazed with (Ti-Zr-Cu-Ni)+W composite filler materials. Compos Sci Technol. 2014;97:19-26. [7] Zhou Y, Bao F, Ren J, North T. Interlayer selection and thermal stresses in brazed Si3N4− steel joints. Materials science and technology. 1991;7:863-8. [8] Halbig M, Asthana R, Singh M. Diffusion bonding of SiC fiber-bonded ceramics using Ti/Mo and Ti/Cu interlayers. Ceramics International. 2015;41:2140-9. [9] Shirzadi AA, Zhu Y, Bhadeshia H. Joining ceramics to metals using metallic foam. Mater Sci Eng A-Struct Mater Prop Microstruct Process. 2008;496:501-6. [10] Xiong HP, Mao W, Xie YH, Guo WL, Li XH, Cheng YY. Brazing of SiC to a wrought nickel-based superalloy using CoFeNi(Si, B)CrTi filler metal. Mater Lett. 2007;61:4662-5. [11] Shen Y, Li Z, Hao C, Zhang J. A novel approach to brazing C/C composite to Ni-based superalloy using alumina interlayer. Journal of the European Ceramic Society. 2012;32:1769-74. [12] Xiong JT, Li JL, Zhang FS, Lin X, Huang WD. Direct joining of 2D carbon/carbon composites to Ti–6Al–4V alloy with a rectangular wave interface. Materials Science and Engineering: A. 2008;488:205-13. [13] Wu C, Christensen MS, Savolainen J-M, Balling P, Zhigilei LV. Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target. Physical Review B. 2015;91:035413. [14] Zhang Y, Liu L, Zou G, Chen N, Wu A, Bai H, et al. Femtosecond laser-induced phase transformations in amorphous Cu77Ni6Sn10P7 alloy. Journal of Applied Physics. 2015;117:023109. [15] Liu L, Peng P, Hu A, Zou G, Duley W, Zhou YN. Highly localized heat generation by femtosecond laser induced plasmon excitation in Ag nanowires. Applied Physics Letters. 2013;102:073107. [16] Höhm S, Rosenfeld A, Krüger J, Bonse J. Area dependence of femtosecond laser-induced periodic surface structures for varying band gap materials after double pulse excitation. Applied Surface Science. 2013;278:7-12. [17] Yao J, Zhang C, Liu H, Dai Q, Wu L, Lan S, et al. Selective appearance of several laser-induced periodic surface structure patterns on a metal surface using structural colors produced by femtosecond laser pulses. Applied Surface Science. 2012;258:7625-32. [18] Gattass RR, Mazur E. Femtosecond laser micromachining in transparent materials. Nature photonics. 2008;2:219-25. [19] Ashkenasi D, Rosenfeld A, Varel H, Wähmer M, Campbell E. Laser processing of sapphire with picosecond and sub-picosecond pulses. Applied Surface Science. 1997;120:65-80. [20] Kautek W, Krueger J. Femtosecond pulse laser ablation of metallic, semiconducting, ceramic, and biological materials.
Europto High Power Lasers and Laser Applications V: International Society for
Optics and Photonics; 1994. p. 600-11.
[21] Izawa Y, Izawa Y, Setsuhara Y, Hashida M, Fujita M, Sasaki R, et al. Ultrathin amorphous Si layer formation by femtosecond laser pulse irradiation. Applied physics letters. 2007;90:044107. [22] Hirayama Y, Obara M. Heat-affected zone and ablation rate of copper ablated with femtosecond laser. Journal of applied physics. 2005;97:064903. [23] Wei-yuan Y, Sen-hui L, Xin-ya L, Min-pen L, Wei-gang S. Interface reaction in ultrasonic vibration-assisted brazing of aluminum to graphite using Sn-Ag-Ti solder foil. J Mater Process Technol. 2015;221:285-90. [24] Kozlova O, Voytovych R, Eustathopoulos N. Initial stages of wetting of alumina by reactive CuAgTi alloys. Scripta Materialia. 2011;65:13-6. [25] Zhanga X, Zhangb Y, Lub J, Xuana F, Wanga Z, Tua S. Improvement of fatigue life of Ti–6Al–4V alloy by laser shock peening. Materials Science and Engineering A. 2010;527:3411-5. [26] Hongqi H, Zhihao J, Xiaotian W. The influence of brazing conditions on joint strength in Al2O3/Al2O3 bonding. Journal of materials science. 1994;29:5041-6. [27] Rao MP, Sanchez-Herencia A, Beltz G, McMeeking R, Lange F. Laminar ceramics that exhibit a threshold strength. Science. 1999;286:102-5.
PII: DOI: Reference:
S0921-5093(16)30271-4 http://dx.doi.org/10.1016/j.msea.2016.03.068 MSA33463
To appear in: Materials Science & Engineering A Received date: 8 January 2016 Revised date: 15 March 2016 Accepted date: 15 March 2016 Cite this article as: Y. Zhang, G. Zou, L. Liu, A. Wu, Z. Sun and Y.N. Zhou, Vacuum Brazing of Alumina to Stainless Steel Using Femtosecond Laser Patterned Periodic Surface Structure, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.03.068 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 galley proof before it is published in its final citable 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.
Vacuum Brazing of Alumina to Stainless Steel Using Femtosecond Laser Patterned Periodic Surface Structure Y. Zhang1, G. Zou1, L. Liu1*, A. Wu1, Z. Sun1, Y. N. Zhou1, 2 1
Department of Mechanical Engineering, State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China; 2 Department of Mechanical and Mechatronics Engineering, University of Waterloo, ON, N2L, 3G1, Canada.
*Correspondent mail: [email protected] Abstract Ceramic surface patterning is a promising method to reduce the residual stress of ceramic-metal joints, since it needs no complex multiple interlayer and is cost effective. However, literature shows that surface patterns previously examined for ceramic-metal brazing were at a millimeter scale, which limits the application and the degree of enhancement. In this study, femtosecond laser ablation was used to fabricate microscale periodic surface patterns (grooves) on ceramic surfaces. No defects or damage were induced during the ablation. The groove dimension was tunable by controlling ablation parameters. High temperature vacuum brazing of 304 stainless steel to the Al2O3 ceramic with surface grooves was performed using a commercial AgCuTi brazing filler metal. With properly designed surface grooves, the average joint strength was 66MPa, 2.75 times of that of the flat-interface joints. Finite Element Analysis models combined with fracture path examination indicated that the redistribution of residual stress induced by periodic surface pattern accounted for the joint strength enhancement. Key words: femtosecond laser ablation, ceramic-metal brazing, residual stress, joint strength, surface groove. 1. Introduction Much research has focused on reducing thermal stress in ceramic-metal brazed joints, caused by the coefficient of thermal expansion (CTE) mismatch between ceramic and metal [1, 2]. The technological approaches are mostly from three aspects: advanced process, interface engineering, and ceramic surface patterning. The process advancement is mainly based on reducing total heat input or lowering the brazing temperature. For example, local heat sources, such as defocused laser beam [3] and induction heating [4], have been used to melt filler metal. Low temperature filler metals such as Ti activated Sn based solders have been developed to join ceramic below 300 oC [5]. The interface engineering approach has been studied by many researchers. A hard buffer layer (W, Mo, Kovar) having CTE similar to that of the ceramic can reduce the residual stress in the ceramic side [6]. A ductile interlayer (pure Cu, Ni, Al) can also relax the residual stress through plastic deformation [7]. Multiple interlayers [8], porous cushion layer [9] and composite braze metal [10] have also been developed to reduce the residual stress. Ceramic surface patterning is a promising method since it needs no complex buffer materials and fits both commercial filler metal and traditional brazing equipment and process [11, 12]. According to the results of Xiong et al. [12], a periodic rectangular surface structure could change
the residual stress distribution and lower the peak residual stress on the ceramic side, thus strengthening the braze interface. Shen et al. [11]drilled blind holes on a ceramic surface, resulting in significant increase of joint strength due to the pinning effects of the braze spikes. However, only a few publications in this area are available and there are some limits on the existing techniques. For example, the surface grooves in one study had a dimension of millimeter scale, which was too large to promote capillary flow by the molten braze metal [12]. Mechanical machining cannot fabricate complex patterns and many ceramics, such as SiC, BN are not even machinable [13]. Ultrafast laser micro fabrication is receiving great interest in recent years, since this is known as a precision and “cold process” method [13-15]. Ultrafast lasers can produce microscale and nanoscale complex surface structure in most engineering materials and induce no damage to the base material [16, 17]. In this study, a femtosecond laser was used to fabricate microscale periodic groove structures on ceramic surfaces. The results showed that femtosecond laser patterning obviously increased the joint strength compared with the unprocessed flat surface. Finite Element Analysis (FEA) models were also developed to enhance the understanding of this strength improvement. 2. Experiments 2.1 Materials Commercial Al2O3 ceramic and 304 stainless steel cylinders were used with dimensions of Φ15×10 mm and Φ10×10 mm, respectively. Sample surfaces were polished with #1000 sand paper and ultrasonically cleaned. Active brazing filler foil Ag-35Cu-2Ti was polished to 50μm and ultrasonically cleaned before brazing. Periodic surface groove structures on the ceramic samples were fabricated by an amplified Ti: sapphire laser system (Coherent Inc.). This femtosecond laser had 50 fs pulse duration, 3.8 mJ per pulse at 1 kHz repetition rate. The laser beam was 8 mm in diameter, and was focused by a 25.4 mm-focal-length lens. The grooving experiments were carried out in air. The surface profiles were measured by a confocal laser scanning microscope (Olympus, LEXT OLS 4100). The joints were brazed with consistent parameters in a vacuum oven under 2.0╳10-3 Pa. The temperature profile was: heating to 850 oC at 10 oC/min and holding for 10 minutes; heating to 900 oC at 5 oC/min and holding for 5 minutes; cooling down to 600oC at 10oC/min; and finally furnace cooling to room temperature. The joint strength was examined by shear testing with a constant speed of 1mm/min at room temperature. Each data point was an average of 4-5 samples. Fig. 1 shows a typical brazed joint and the schematic diagram of shear test. The fracture surface and cross section of the fractured sample were examined using an optical microscope (BX51RF, Olympus Inc.).
Fig. 1. (a) Picture of a typical joint, (b) Schematic of shear test, (c) schematic diagram of laser ablation system.
2.2 FEA calculations
Residual stresses of brazed joints were simulated by Finite Element Analysis (FEA) with Abaqus software. Since the actual joint was in mm scale and the grooves were in µm scale, the cross scale simulation would be very time consuming. To reduce simulation time and cost, the groove dimension and the distance between two grooves were set 3 times larger than the actual size. Therefore, in the FEA model, the single groove dimension was set to 200 μm and 800 μm, respectively, and the distance of each parallel grooves was set as 800 μm. A half symmetry model was used in the simulation procedure to further reduce computational cost. The coordinate system and schematic diagram of the model used in simulation are shown in Fig. 2 (a). Fig. 2(b) shows SEM (LEO 1530) images of the cross section of actual joints and the model built in the software.
Fig. 2. (a) Schematic diagram of model and coordinate system adopted in calculation, and (b) the actual joint interfaces and models used in simulation.
Ceramic and stainless steel are assumed separate during the heating cycle. During the cooling cycle after brazing, the FEA calculations evaluated the temperature range from brazing temperature (900 oC) to room temperature (20 oC). Materials properties used in FEA calculations are listed in Table 1. Table 1. Material properties for the thermal residual stresses analysis
Materials Al2O3 ceramic Stainless steel AgCuTi
Expansion coefficient (10-6/oC)/ T (oC)
Young’s modulus (GPa)/ T(oC)
Poisson’s ratio
Yield strength (MPa)/ T (oC)
7.7
304
0.26
---
21.5
160
0.26
16/350, 19.5/410, 20/800, 20.5/900
100/300, 95/600, 90/900
0.36
205/600, 170/750, 120/800, 100/850, 90/900 130/300, 130/410, 125/600, 95/750, 45/900
3. Results and discussion 3.1 Surface groove fabrication and joint microstructure Periodic groove structures on Al2O3 ceramic surfaces were obtained by femtosecond laser ablation. Typical cross sections of grooves are approximate arcs, as shown in Fig. 3. The distance between each parallel groove was set as a constant value, 200 μm. According to Fig.3, the ablated
surface was smoother than the unprocessed surface, and no visible crack was found on the ablated surface. It is well known that a femtosecond laser has ultrahigh power density and ultrashort duration time [18]. In this work, the laser power density at the focal point was far above the ablation threshold of Al2O3. As the ablation reaction took away most of the energy, the heat effect was minimized, which avoided the formation of cracks [14]. Furthermore, the laser pulse width (10-15 s) was far shorter than the heat transfer time (10-12 s), resulting in a precise ablation [19, 20].
Fig.3 (a) Schematic diagram of laser ablation system, and 3D structure of (b) original ceramic surface and the surfaces after (c) 0.3W and (d) 3.4W laser ablation.
Table 2 shows the effects of laser power on groove dimensions. It indicates that both groove width and groove depth increased with laser power up to 2.4W. When the laser power was higher than 2.4W, the neighboring grooves were overlapped. Thus, the groove depth increased with the width remaining unchanged. By changing laser power during the fabrication process, 3 typical joint interfaces were obtained: flat interface (original ceramic surface), sparse periodic groove interface (0.3-1.1 W), and close periodic groove interface (2.4-3.4 W). Table 2. Groove dimensions fabricated by different laser powers.
Laser power (W)
Origin
0.3
1.1
2.4
3.4
Width (μm) Depth (μm)
-----
70 30
110 52
195 77
192 97
Fig. 4 shows the XRD patterns of the ceramic surface before and after laser ablation. The two samples had peaks at almost the same 2θ positions, indicating that laser ablation did not change the phase composition of the ceramic surface. The only difference between them was that the signal intensity of the laser processed surface was lower than that of the original flat surface. This may be because the surface grooves blocked part of the reflected X-rays. Femtosecond laser ablation is a “cold manufacturing” process and has limited heat affects to the base material. Although nanoscale thin amorphous layers have been reported on the ablated Si and Cu surfaces [21, 22], no detectable amorphorization on the Al2O3 ceramic surface was observed in this work after femtosecond laser ablation.
Fig.4 XRD patterns of ceramic surface before (black) and after (red) laser ablation.
The cross sections of ceramic and stainless steel brazed joints are shown in Fig. 5. The grooves were completely filled by braze metal, indicating that the groove patterns had capillary effects on the liquid AgCuTi filler metal. Wetting and spreading spontaneously occurred without special assistance such as ultrasonic vibration or pre-coatings [23], which indicated that the process was simple and reliable. The increase of ceramic surface area depleted more Ti element due to the reaction and formation of interface IMCs. Therefore, less CuTi phase was found in the laser patterned samples because of less Ti concentration in the liquid filler metal. The CuTi intermetallic compounds dispersed homogenously with a round-shape and enhanced the strength of the braze seam.
Fig. 5. The cross section SEM picture of the joints with (a) flat interface, (b) sparse grooves interface, and (c) close grooves interface.
A thin intermetallic compounds (IMC) reaction layer formed during brazing between the ceramic and braze filler metal, which has been well addressed by many researchers [24]. The thickness of this reaction layer slightly increased with the increase of laser power. If the temperature of all samples were the same, the brazing procedure would not affect the interface IMC. Besides temperature and time, the kinetics of the interface reaction between active element Ti and Al2O3 ceramic also depends on Ti concentration, surface residual stress, and density. As the reaction is ongoing, the laser patterned sample has lower Ti concentration in the liquid filler metal than the flat sample, as explained above. However, the interface IMC thickness showed the opposite, slightly thicker in the laser patterned sample. Femtosecond laser patterning may induce
surface residual stress on a material surface, e.g. laser peening process [25]. However, the laser process in this study was done in air and without plasma confinement media, so the residual stress level should be much lower than the laser peening process. Also, the heating cycle would release the original residual stress at 900 oC. Therefore, the residual stress should have little effect on the thickness of the interface IMC layer. It is believed that the thicker IMC layer was mainly caused by the change of density in the grooves which facilitated the reaction rate. The laser ablation lowered the density of the surface material, which can be evidenced by the infiltrating reaction beneath the surface, especially in Fig. 5(c). The average IMC thickness of the close groove sample was 3μm, which was still acceptable [26].
3.2 Mechanical properties of joints Fig. 6 shows the shear strength of the joints with different laser powers. For the periodic linear parallel grooves, shear tests along both parallel and perpendicular directions were conducted, as shown in Fig. 6. The results indicated that the joint strength was highly related to the laser power and the shear direction. The maximum shear strength of the laser patterned joints was 2.7 times larger than the original flat surface joints (24MPa), while the minimum was even lower than the flat joints. For both shear directions, joint strength reached their maximum peaks at 0.3W, and then gradually dropped with the increasing of the laser power. Although the parallel shear strength was lower than the perpendicular one, it was still larger than the flat joints until the laser power was higher than 2.4W. Basically, two failure modes were distinguished by visual observation: ceramic failure through the ceramic and interfacial failure along the braze interface, as shown in Fig. 6 (b) and 6 (c), respectively. The ceramic mode was mostly observed (solid points in Fig. 6a), in which part of the ceramic still attached to the stainless steel cylinder after shear testing (Fig. 6b). The joints, made by 3.4 W laser ablation (open points in Fig. 6 (a)), had close periodic groove surfaces and failed along the braze interface (Fig. 6c), which had the lowest shear strength. Since the thicknesses of the reaction layers were acceptable for all three types of joints, the residual stress distribution evidently played the key role of determining the joint strength.
Fig. 6. (a) Shearing strength of the joints with surface grooves fabricated by different laser power, and (b) and (c) the fracture surfaces of the joints.
Although the failure modes of the flat joints and the sparse grooved joints were the same (i.e.,
ceramic failure), the sparse grooved joint showed obviously larger shear strength than the flat one. In order to further examine the fracture mechanism, cross sections of the fractured joints were made. Fig. 7 (a) and (b) are the unprocessed flat joint (24 MPa) and perpendicular sheared sparse grooved joint (66 MPa). As shown in the figures, though these two joints were both broken inside the ceramic, the crack paths were quite different. For the flat joint (Fig. 7a), the crack first nucleated at the edge of the joint and then propagated directly into the ceramic. On the other hand, the crack in the sparse grooved joint (Fig. 7b) first propagated along the joint interface for <1 mm, and then turned into the ceramic. The enlarged image in Fig. 7b clearly shows the crack propagation direction changing at the position around the third groove.
Fig. 7. Cross section of the fractured joints with (a) flat interface and (b) sparse grooves interface (0.3W).
The load-time curve shows more evidence (Fig. 8). The shear load of the flat joints increased monotonically with time and sharply dropped once the joints were failed. But for the sparse grooved joints, small peaks appeared before the joints were broken. These peaks indicated a load releasing and bursting process during the shear test, which was caused by a crack-impeding effect. Related to the crack propagation direction changing shown in Fig. 7 (b), the grooves also showed a crack-impeding function.
Fig. 8. Load-time curve of the shearing test.
3.3 Effects of surface patterning on residual stress It is easy to connect the joint strength enhancement to the mechanical pinning effect of the zig-zag interface. However, this would be more convincing if the failure occurred along the braze interface, because the pinning effect has no influence on the ceramic base material. The ceramic
failure mode suggests that mechanical pinning was not the only benefit, or maybe even not the primary reason. It is believed that the change of residual stress by femtosecond laser surface patterning played a critical role in the joint strength enhancement, this will be analyzed according to the simulation results. Our calculation results indicated that normal stress σx and σy are much smaller than the σz, and shear stress τxy and τxz are much smaller than τzy (red solid line), which was also reported in Rao’s study.[27] Therefore, the following analysis only focuses on the largest normal σz and shear stress τzy. Fig. 9 shows the simulation results of the flat joint. Shear stress τzy has a constant value with the direction pointing to the center of the joint. The peripheral part of the interface is subjected to tensile stress, and the center part is subjected to compressive stress. This simulation results match well with the fracture path: crack initiates at the edge of the joint area, since this region is subjected to the maximum tensile and mixed shear stresses [12], then the crack propagates deeper inside the ceramic where there is lower compressive normal stress [27].
Fig. 9. Calculation results of the residual stress at ceramic surface. (a) Residual stress distribution at flat ceramic surface, (b) enlarged picture of residual stress distribution at symmetry plane, and (c) normal stress σz and shear stress τzy distribution of ceramic surface at symmetry plane (red line in (a) and (b)).
The femtosecond laser patterned periodic surface structure totally changes the stress distribution at the ceramic side, as shown in Fig. 10. The vertical grey bars in Fig. 10 (c) mark the groove locations. The simulation results show that the normal stress σz has periodic peaks which correspond to the groove locations. The σz at the first peripheral groove is +150MPa tensile stress. It suddenly changes to around -100 MPa compressive stress at the second peripheral groove. Then the σz stays compressive and the peak values increase towards the joint center. The τzy shows a similar distribution with the σz: peaking at the groove locations and increasing along the direction to the center. When the shear load was applied perpendicular to the grooves, the peripheral area of the joint was the most possible place for crack initiation due to the large tensile and shear residual stress. After initiation, the crack propagated along the braze interface and met the second groove, where the residual normal stress suddenly changes to -100 MPa compressive stress. According to Rao’s study, the compressive stress has impeding effects against crack propagation [27], that is the reason why the crack propagation direction changed at around the position of the second groove. With the shear load increasing, the cracks grew deeper inside ceramic where subjected to lower
compressive stress. When the shear load is applied parallel to the grooves, the compressive stress still exists but is discretely distributed along the interface. Comparing to the perpendicular shear load, the impending effect of parallel shear load is smaller. As a result, the joint strength of parallel shear was relatively lower than the perpendicular shear.
Fig. 10. Residual stress distribution the sparse grooved joint. (a) residual stress distribution at ceramic side, (b) enlarged picture, and (c) normal stress σz and shear stress τzy distribution of ceramic surface at symmetry plane (red line in (a) and (b)). The vertical grey bars mark the groove locations.
The flat joint also has a tensile-to-compressive stress change along the braze interface, but it is different from the sparse groove joint. The stress state in the flat joint is gradually developed, while the stress of the sparse grooved joint has a sharp change from tensile to compressive. So the impeding effect was not obvious. Additionally, the compressive stress value of the flat joint was much smaller than that of the sparse grooved joint. Therefore, the flat joint had only 24 MPa shear strength, while the sparse grooved joint had a higher shear strength of 66 MPa. Fig.11 shows the residual stress distribution of the close grooved joint. The groove ridges are subjected to a high shear and compress stress, and the groove valleys are subjected to a shear and tensile stress. The stress field changes sharply within a small distance, which is catastrophic to a brittle material. Furthermore, the ceramic ridges between grooves are very sharp and thin, which is like brittle knife blades inserted in the braze metal. The cracks generated by the shear stress could easily break through the braze seam, resulting in the lowest strength.
Fig. 11. Residual stress distribution of the close grooved joint. (a) residual stress distribution at ceramic side, (b) enlarged picture, and (c) normal stress σz and shear stress τzy distribution of ceramic surface at symmetry plane (red line in (a) and (b)).
4. Conclusions The strength of ceramic-metal brazed joints has been significantly enhanced by femtosecond laser patterned periodic surface structures on the ceramic. The main conclusions are: (1) The dimensions of the periodic surface structures were tunable by adjusting laser parameters. Due to the cold-processing nature of femtosecond laser pulses, no defects or surface amorphorization was induced in the Al2O3 ceramic base material. (2) The joint strength was highly related to the surface patterning. The optimum laser parameters were at 0.3W and 200 μm periodic distance. The maximum strength of the laser patterned joints was 66 MPa, which was 2.7 times that of the original flat joint (24 MPa). (3) The FEA simulations demonstrate that the femtosecond laser patterned periodic surface grooves changed the residual stress. The sparse grooves cause a sudden change of residual stress along the interface, from tensile to compressive state, which could impede crack propagation and enhance joint strength. However, the close grooves induce large stress concentration and drastic fluctuations of residual stress along the interface, which actually weaken the joint strength. (4) The fracture path examination showed a change of crack propagation direction when the crack met the second or third periphery sparse groove. The load-displacement curve indicated the time and position of the crack direction change. Those observations matched well with simulation results and further prove the mechanism of the strength enhancement. Acknowledgments This research was supported by National Natural Science Foundation of China (51405258, 51520105007), State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology (AWJ-M14-05) and by Tsinghua University Initiative Scientific Research Program (No. 2010THZ02-1 2013Z02-1). References [1] Levy A. Thermal Residual Stresses in Ceramic‐to‐Metal Brazed Joints. Journal of the American Ceramic Society. 1991;74:2141-7.
[2] Do Nascimento R, Martinelli A, Buschinelli A. Review article: recent advances in metal-ceramic brazing. Cerâmica. 2003;49:178-98. [3] Rohde M, Südmeyer I, Urbanek A, Torge M. Joining of alumina and steel by a laser supported brazing process. Ceramics International. 2009;35:333-7. [4] Hausner S. Induction Brazing of Alumina and Zirconia with Various Metals.
5th International
Brazing and Soldering Conference: Asm; 2012. [5] Koleňák R, Šebo P, Provazník M, Koleňáková M, Ulrich K. Shear strength and wettability of active Sn3. 5Ag4Ti (Ce, Ga) solder on Al 2 O 3 ceramics. Materials & Design. 2011;32:3997-4003. [6] Bing C, Jihua H, Chuang C, Shuhai C, Xingke Z. Microstructures and mechanical properties of C f/SiC composite and TC4 alloy joints brazed with (Ti-Zr-Cu-Ni)+W composite filler materials. Compos Sci Technol. 2014;97:19-26. [7] Zhou Y, Bao F, Ren J, North T. Interlayer selection and thermal stresses in brazed Si3N4− steel joints. Materials science and technology. 1991;7:863-8. [8] Halbig M, Asthana R, Singh M. Diffusion bonding of SiC fiber-bonded ceramics using Ti/Mo and Ti/Cu interlayers. Ceramics International. 2015;41:2140-9. [9] Shirzadi AA, Zhu Y, Bhadeshia H. Joining ceramics to metals using metallic foam. Mater Sci Eng A-Struct Mater Prop Microstruct Process. 2008;496:501-6. [10] Xiong HP, Mao W, Xie YH, Guo WL, Li XH, Cheng YY. Brazing of SiC to a wrought nickel-based superalloy using CoFeNi(Si, B)CrTi filler metal. Mater Lett. 2007;61:4662-5. [11] Shen Y, Li Z, Hao C, Zhang J. A novel approach to brazing C/C composite to Ni-based superalloy using alumina interlayer. Journal of the European Ceramic Society. 2012;32:1769-74. [12] Xiong JT, Li JL, Zhang FS, Lin X, Huang WD. Direct joining of 2D carbon/carbon composites to Ti–6Al–4V alloy with a rectangular wave interface. Materials Science and Engineering: A. 2008;488:205-13. [13] Wu C, Christensen MS, Savolainen J-M, Balling P, Zhigilei LV. Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target. Physical Review B. 2015;91:035413. [14] Zhang Y, Liu L, Zou G, Chen N, Wu A, Bai H, et al. Femtosecond laser-induced phase transformations in amorphous Cu77Ni6Sn10P7 alloy. Journal of Applied Physics. 2015;117:023109. [15] Liu L, Peng P, Hu A, Zou G, Duley W, Zhou YN. Highly localized heat generation by femtosecond laser induced plasmon excitation in Ag nanowires. Applied Physics Letters. 2013;102:073107. [16] Höhm S, Rosenfeld A, Krüger J, Bonse J. Area dependence of femtosecond laser-induced periodic surface structures for varying band gap materials after double pulse excitation. Applied Surface Science. 2013;278:7-12. [17] Yao J, Zhang C, Liu H, Dai Q, Wu L, Lan S, et al. Selective appearance of several laser-induced periodic surface structure patterns on a metal surface using structural colors produced by femtosecond laser pulses. Applied Surface Science. 2012;258:7625-32. [18] Gattass RR, Mazur E. Femtosecond laser micromachining in transparent materials. Nature photonics. 2008;2:219-25. [19] Ashkenasi D, Rosenfeld A, Varel H, Wähmer M, Campbell E. Laser processing of sapphire with picosecond and sub-picosecond pulses. Applied Surface Science. 1997;120:65-80. [20] Kautek W, Krueger J. Femtosecond pulse laser ablation of metallic, semiconducting, ceramic, and biological materials.
Europto High Power Lasers and Laser Applications V: International Society for
Optics and Photonics; 1994. p. 600-11.
[21] Izawa Y, Izawa Y, Setsuhara Y, Hashida M, Fujita M, Sasaki R, et al. Ultrathin amorphous Si layer formation by femtosecond laser pulse irradiation. Applied physics letters. 2007;90:044107. [22] Hirayama Y, Obara M. Heat-affected zone and ablation rate of copper ablated with femtosecond laser. Journal of applied physics. 2005;97:064903. [23] Wei-yuan Y, Sen-hui L, Xin-ya L, Min-pen L, Wei-gang S. Interface reaction in ultrasonic vibration-assisted brazing of aluminum to graphite using Sn-Ag-Ti solder foil. J Mater Process Technol. 2015;221:285-90. [24] Kozlova O, Voytovych R, Eustathopoulos N. Initial stages of wetting of alumina by reactive CuAgTi alloys. Scripta Materialia. 2011;65:13-6. [25] Zhanga X, Zhangb Y, Lub J, Xuana F, Wanga Z, Tua S. Improvement of fatigue life of Ti–6Al–4V alloy by laser shock peening. Materials Science and Engineering A. 2010;527:3411-5. [26] Hongqi H, Zhihao J, Xiaotian W. The influence of brazing conditions on joint strength in Al2O3/Al2O3 bonding. Journal of materials science. 1994;29:5041-6. [27] Rao MP, Sanchez-Herencia A, Beltz G, McMeeking R, Lange F. Laminar ceramics that exhibit a threshold strength. Science. 1999;286:102-5.