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Papers - Yan, Jiwang

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What micro-mechanical testing can reveal about machining processes

Axinte D., Huang H., Yan J., Liao Z.

International Journal of Machine Tools and Manufacture (International Journal of Machine Tools and Manufacture) 183 2022.12

ISSN 08906955

For many years, the machining community has dedicated significant efforts to investigate the microscopic scale level phenomena during the material removal process. On one hand much research has been carried out in relation to workpiece surface integrity after machining and the methods for its study. On the other hand, many studies have been dedicated to replicate machining conditions at microscopic scales using high resolution setups. Although these two topics seem to be little related, there is an opportunity of the machining community to take the advantage of the advanced testing/investigation setups that enable these two strands of research to be performed at very high resolution and repeatability, thus giving new pathways for research in this field. Here we are flagging up to the community the research opportunities offered by micro-mechanical testing that can be performed using in-situ scanning electron microscopes (SEM) or other high-resolution imaging instruments. As such, this review paper discusses the recent research advances in using in-situ micro-mechanical testing for: (i) understanding the phenomena occurring in the workpiece (sub) surfaces after machining operation by performing very high resolution micro-mechanical testing (e.g. compression/bending of micro-pillars/beams) within particular zones of machined superficial layers; (ii) studying the material removal mechanisms at micrometric level using common indenters or dedicated edges to understand how the workpiece materials (e.g. groups/single grains) react to cutting conditions. Finally, we comment on possible future research topics using micro-mechanical testing in-situ in high resolution imaging instruments and how this could help to advance the understanding of machining processes.

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Investigating the Combined Effect of Multiple Dent and Bump Faults on the Vibrational Behavior of Ball Bearings

Atef Mahmoud M., Khair-Eldeen Wael, Yan Jiwang, Nassef Mohamed G. A.

Machines (Machines) 10 ( 11 ) 2022.11

The rolling element bearing is a fundamental component of any rotating machinery. During operation, wear debris and lubricant impurities create dents and bumps on the bearing raceway surfaces. Such localized defects produce transient vibration impulses at one of the bearing characteristic frequencies. Having a combination of multiple types of point defects on the raceway results in superimposed vibration patterns, which reduce the ability to recognize these defects’ effects. In this paper, a 6-DOF dynamic model is developed to accurately investigate the vibration characteristic of a ball bearing with a multipoint defect comprising a dent and bump on its raceway surface. The model considers the effects of time-varying contact force produced due to defects, lubricant film damping, bearing preload, and the inertia effect of rolling elements. The simulation results reveal the vibration behavior of multipoint defect bearings. In addition, bearing vibration response is affected by the number of defects, the angle between them, and the type and size of each defect. Furthermore, it is challenging to predict bearing defects parameters such as the numbers, types, sizes, and angles between adjacent defects from acceleration signal analysis without jerk signal analysis. The validation of the model is proved using signals from the Case Western University test setup.

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Surface hardening of Zr-based metallic glass via laser surface alloying with silicon powder

Zhang D., Qian Y., Zhang L., Huang H., Yan J.

Scripta Materialia (Scripta Materialia) 220 2022.11

ISSN 13596462

Increasing the hardness and wear resistance of metallic glasses (MGs) is beneficial to prolong their service life in highly aggressive environments. For the purpose of achieving surface hardening, in this work, laser surface alloying technology was employed to modify Zr-based MG with silicon powder as the pre-placed powder. The experimental results showed that the laser alloyed layer with thickness of several tens of micrometers was generated on the MG substrate. The surface hardness of the laser alloyed layers ranged from 11.34 to 17.97 GPa, which was significantly higher than that of the as-cast MG surface (6.36 GPa). Furthermore, the scratch resistance of the laser alloyed layer was improved compared to the as-cast sample. The chemical composition analysis indicated that the laser alloyed layer was mainly composed of in-situ formed ZrSi2 phase and residual Si, and these hard phases were responsible for the high hardness of the laser alloyed layer.

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Formation, evolution and characterization of nanoporous structures on the Ti6Al4V surface induced by nanosecond pulse laser irradiation

Wang C., Huang H., Zhang Z., Zhang L., Yan J., Ren L.

Materials and Design (Materials and Design) 223 2022.11

ISSN 02641275

Manipulating materials at nanoscale is challenging but rewarding. Nanostructures exhibit some unique properties distinguished from macroscopic bulk materials, so fabrication of nanostructures on the material surface is beneficial to broaden their functional applications. In this study, by nanosecond pulse laser irradiation in nitrogen atmosphere, it was found that unique nanoporous structures could be formed on the Ti6Al4V surface. The effects of laser parameters including the laser power, overlap rate, scanning speed, and repetition frequency on the formation and evolution of the nanoporous structures were systematically investigated. By changing the laser parameters, the nanoporous structures with various porosities could be easily achieved on the Ti6Al4V surface. According to the structural evolution and chemical composition, the formation mechanism of the nanoporous structures was discussed. Furthermore, by characterizing the micro-mechanical properties and wettability, it was found that the surface with nanoporous structures exhibited similar surface hardness but improved tribological performances compared to the original Ti6Al4V surface, and the surface wettability could be tuned. This study provides a simple method for fabricating nanoporous structures on the Ti6Al4V surface, which would broaden its functional applications.

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Study on nano-graphitic carbon coating on Si mold insert for precision glass molding

Zhang L., Yan J.

Surface and Coatings Technology (Surface and Coatings Technology) 448 2022.10

ISSN 02578972

Precision glass molding is an efficient and promising technique for large-volume micro-optical manufacturing. Even though significant progress in precision glass molding has been achieved on the anti-adhesive coating, technical challenges and barriers still exist towards tailoring the coating layer to ensure durability and long-term processing consistency in numerous molding cycles. This manuscript presents a novel molding technique utilizing micro-patterned Si mold inserts coated with a newly developed nano-graphitic carbon coating as an isolation layer in high-precision glass molding. The coating is prepared by atmospheric pressure chemical vapor deposition (APCVD), in which the liquid benzene and silicone rubber are used as the carbon source and silicon oxycarbide (SiOC) source, respectively. The nano-morphologies of the nano-graphitic carbon coating with different conditions are characterized and analyzed, illustrating notable differences in surface features with the addition of silicone rubber and various growth durations. In the demonstration, the nano-graphitic carbon-coated Si mold inserts exhibit unique properties, including anti-adhesive and hydrophobic at elevated temperatures. Two kinds of micro-patterned Si mold inserts, a micro-pillar matrix and a micro-lens array, generated by UV lithography and diamond turning, are utilized to demonstrate the feasibility of the anti-adhesive coating in precision glass molding. The obtained results indicate that the micro-optical features on the coated Si mold inserts are successfully transferred to glass substrates with high replication fidelity without notable signs of adhesion or contamination in repetitive molding cycles. This emerging technology makes Si an alternative mold material with low surface adhesion, enhances micro-optical machining efficiency, and replicates high-precision, low-cost micro-optics on large scales that were previously unavailable.

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On the conversion of point-to-linear hierarchical micro/nano-structures on the glassy carbon surface by nanosecond pulsed laser irradiation

Wang C., Zhang H., Huang H., Zhang Z., Zhang L., Yan J.

Applied Surface Science (Applied Surface Science) 599 2022.10

ISSN 01694332

Fabrication of microstructures on the glassy carbon (GC) surface is crucial for its functional applications. However, due to its intrinsic mechanical, physical, and chemical properties, it is still challenging to fabricate hierarchical micro/nano-structures on the GC surface. In this study, by tuning the interval between adjacent points as well as the irradiation time, the conversion from the point to linear hierarchical micro/nano-structures was successfully achieved on the GC surface by nanosecond pulsed laser irradiation. The detailed conversion process and mechanism were explored and discussed. On this basis, the linear hierarchical micro/nano-structures were fabricated by direct line scanning, and the internal nanostructures were always parallel to the laser scanning direction. By repeated line scanning with relatively low laser fluence, the method of direct line scanning was further improved, and well-defined and uniform hierarchical micro/nano-structures were fabricated.

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Towards understanding the mechanism of vibration-assisted cutting of monocrystalline silicon by cyclic nanoindentation

Huang W., Yan J.

Journal of Materials Processing Technology (Journal of Materials Processing Technology) 311 2022.10

ISSN 09240136

Ultrasonic vibration cutting (UVC) is a promising technology that can promote the processing of hard-brittle materials in ductile mode. The feature of intermittent cutting in UVC is considered to promote the ductile responses of brittle materials. However, the effect of cyclic loading/impact on microscopic deformation behavior of the workpiece material in UVC is not clearly understood. This study used cyclic nanoindentation to reveal the UVC mechanism in monocrystalline silicon (Si). The surface morphology and subsurface microstructure of the residual indents were characterized and compared with those of the UVC-machined grooves. This work attempted to correlate the tool-workpiece contacts for cyclic nanoindentation and vibration cutting. Cyclic nanoindentation results show that cyclic loading promotes the extent of silicon amorphization and decreases the threshold for transitioning the β–Sn phase to the bc8 and r8 phases. As the number of indentation cycles increases, the subsurface microstructure changes from the amorphous phase to the bc8 and r8 phases. In UVC, both the critical cutting depth for brittle-to-ductile transition and the degree of amorphization in cutting chips increase in comparison to conventional cutting, which matches the trend of cyclic nanoindentation results. The findings inform the chip formation and subsurface deformation mechanisms of brittle materials during UVC.

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Microstructure and mechanical properties of SiC particle reinforced Zr-based metallic glass surface composite layers produced by laser alloying

Qian Y., Zhang D., Hong J., Zhang L., Jiang M., Huang H., Yan J.

Surface and Coatings Technology (Surface and Coatings Technology) 446 2022.09

ISSN 02578972

Metallic glasses (MGs) are a promising candidate for advanced structural applications due to their superior mechanical properties. Improving their surface mechanical properties would be of great importance for promoting their structural and functional applications. In this study, SiC particles were used as the reinforcement to improve the mechanical properties of Zr-based MG via laser surface alloying. The influences of the average laser power and overlap ratio between neighboring laser processing lines on the microstructure and mechanical properties of the laser-alloyed surface layer were investigated. The experimental results indicated that ZrC and SiC phases were successfully introduced into the MG matrix by laser surface alloying, and the content of these two hard ceramic phases was dependent on the laser processing parameters. The formed laser-alloyed surface layers exhibited a significant improvement in overall hardness compared with the as-cast specimen. At a relatively high overlap ratio of 70%, the average hardness of the MG matrix within the laser-alloyed surface layer reached 28.91 GPa, which was three times higher than that of the as-cast specimen (6.46 GPa). Furthermore, the microstructural characteristics and mechanical properties of the cross-sections of the laser-alloyed samples were characterized. The thickness of the laser-alloyed surface layer reached several tens of microns, and the SiC particles were uniformly dispersed in the whole laser-alloyed surface layer. This study confirms the feasibility for improving the mechanical properties of MGs by laser surface alloying, which is expected to broaden the application of MGs as structural and functional components under harsh severe conditions.

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Achieving Superhydrophobicity of Zr-Based Metallic Glass Surfaces with Tunable Adhesion by Nanosecond Laser Ablation and Annealing

Cui M., Huang H., Wang C., Zhang L., Yan J.

ACS Applied Materials and Interfaces (ACS Applied Materials and Interfaces) 14 ( 34 ) 39567 - 39576 2022.08

ISSN 19448244

Tuning the surface wettability and adhesion of metallic glasses (MGs) is a promising approach to enrich their engineering applications. In this study, using nanosecond laser ablation in air, hierarchical micro/nanostructures were directly fabricated on a Zr-based MG surface. Following subsequent annealing, the surface exhibited superhydrophobicity (maximum contact angle: 166°, minimum sliding angle: 2°). Furthermore, the superhydrophobic surface could be tuned from low to high surface adhesion force by controlling the laser-ablated spot interval. By analyzing the laser-ablated structures and surface chemical compositions, the superhydrophobicity was related to the formation of hierarchical micro/nanostructures and the absorption of organic compounds with low surface free energy in air, and the change in surface adhesion force was attributed to the difference in surface roughness. The experimental results showed that the superhydrophobic surface with low adhesion force could be used in self-cleaning applications, while the superhydrophobic surfaces with different adhesion forces could be used in no-loss liquid transportation. This study provides an efficient and low-cost way to fabricate superhydrophobic MG surfaces with tunable adhesion, which will broaden the functional applications of MGs.

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Visualization of indentation induced sub-surface shear bands of Zr-based metallic glass by nanosecond pulse laser irradiation

Cui M., Huang H., Wang C., Qian Y., Zhang L., Zhang Z., Yan J.

Vacuum (Vacuum) 202 2022.08

ISSN 0042207X

Characterizing the shear band of metallic glasses (MGs) is meaningful to understand its plastic deformation mechanism. However, the direct method for detecting the internal shear bands of deformed MGs with large field of view is still lacking. Inspired by the fact that surface defects have the enhanced absorption of laser energy, this study attempted to visualize the indentation induced sub-surface shear bands of Zr-based MG by nanosecond pulse laser irradiation in argon. The principle was first verified by laser irradiation of the surface scratch, and then implemented on the visualization of sub-surface shear bands. The experimental results demonstrated that the indentation induced sub-surface shear bands were successfully visualized by nanosecond pulse laser irradiation, and meanwhile, the laser irradiated surface maintained the amorphous characteristic. This study may open a new window for exploring the shear band characteristics of MGs upon external loads.

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Direct Observation of Discharge Phenomena in Vibration-Assisted Micro EDM of Array Structures

Esser G., Yan J.

Micromachines (Micromachines) 13 ( 8 ) 2022.08

The batch mode electrical discharge machining (EDM) method has been developed to improve the throughput and accuracy in fabricating array structures, but the process suffers from insufficient debris removal caused by the complex electrode geometry. Tool vibration has been used to improve flushing conditions, but to date the underlying mechanism of the tool vibration on the micro EDM of array structures remains unclear. This study aimed to investigate the effect of tool vibration on the machining process by direct observation of the discharge phenomena in the discharge gap by using a high-speed camera. Micro EDM experiments using 9 and 25 array electrodes were performed, and the effect of tool vibration on the discharge uniformity and tool wear was evaluated. It was found that tool vibration improved the uniformity of the discharge distribution, increased the machining efficiency, and suppressed the tool wear. The discharges occurred in periodic intervals, and the intensity increased with the amplitude of tool vibration. The results of this study indicate that the vibration parameters determine the discharge period duration and intensity to achieve optimum stability and efficiency of the machining process.

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A novel method for fabricating micro-dimple arrays with good surface quality on metallic glass substrate by combining laser irradiation and mechanical polishing under wax sealing

Huang H., Qian Y., Zhang L., Jiang M., Yan J.

Journal of Manufacturing Processes (Journal of Manufacturing Processes) 79 911 - 923 2022.07

ISSN 15266125

Surface micro-patterning is an emerging technique to endow a wide range of materials with specific functions, such as superhydrophobicity and anti-reflection, but it is still technically challenging and inordinately expensive via existing manufacturing approaches. Compared with crystalline metals, metallic glasses (MGs) are talented with some unique mechanical and chemical properties due to their isotropic internal structure. Here, a novel two-step method, involving nanosecond pulsed laser irradiation and mechanical polishing, was proposed to fabricate micro-dimple arrays with good surface quality on the MG substrate. In the first step, multi-pulse laser irradiation experiments were performed to study the correlation between the peak laser power intensity and the feature sizes of the micro-dimple (i.e. the depth and diameter of the micro-dimple as well as the height of the pile-up), and then various micro-dimple arrays were fabricated on the MG substrate by adjusting the peak laser power intensity and the interval between adjacent laser shots. Subsequently, aiming at improving the surface quality, the micro-dimple arrays were mechanically polished under wax sealing. The experimental results indicated that after polishing, the pile-ups around the micro-dimples were completely removed, and at the same time, the surface quality of their inner walls was improved. Furthermore, based on experimental results, the possible material removal mechanism during polishing was discussed. This study provides a convenient method to fabricate micro-dimple arrays with good surface quality on the MG substrate, which would be one of the potential alternatives for large-area, low-cost micro/nano-manufacturing.

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Nanometric cutting: Mechanisms, practices and future perspectives

Fang F., Lai M., Wang J., Luo X., Yan J., Yan Y.

International Journal of Machine Tools and Manufacture (International Journal of Machine Tools and Manufacture) 178 2022.07

ISSN 08906955

Nanometric cutting removes material at nanoscale and generates high-quality surfaces with a nanometric finish. In past decades, it has thrived as a mainstream manufacturing technology to produce critical components to advance scientific discovery and promote innovation in various fields, such as astronomy, aerospace, microelectronics, optics and photonics, biology and quantum technology. Therefore, it is timely to develop a review article capturing such developments and establishing directions for future advancement. This article systematically reviews the fundamental issues such as cutting model, material deformation mechanism and tool wear in nanometric cutting. It also presents the working principles of innovative ion implantation-assisted, laser-assisted and ultrasonic vibration-assisted nanometric cutting methods to overcome the challenges of machining difficult-to-cut materials. Practical techniques for the generation of high-quality complex or structured surfaces are also discussed. Finally, challenges and future perspectives of nanometric cutting, as well as the evolution towards atomic and close-to-atomic scale manufacturing, are outlined.

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Micro Electrical Discharge Machining of Ultrafine Particle Type Tungsten Carbide Using Dielectrics Mixed with Various Powders

Sai Dutta Gattu and Jiwang Yan

Micromachines ({MDPI} {AG}) 13 ( 7 ) 998 2022.06

Electrical discharge machining (EDM) is widely used to machine hard materials, such as tungsten carbide; however, the machining rate and surface quality are low. In this research, the effects of mixing electrically conductive carbon nanofiber (CnF), semiconductive silicon (Si) powder, and insulative alumina powder (Al2O3) at different concentrations in a dielectric fluid were studied by observing single discharge craters and hole machining performance in the EDM of ultrafine particle type tungsten carbide. Craters obtained using carbon nanofiber and alumina were much smaller than in oil-only conditions. In contrast, The results show that adding CnF significantly improved the material removal rate under all conditions. Si and Al2O3 powders only improved the machining performance at a high discharge energy of 110 V. Furthermore, improvement in surface roughness was observed prominently at high voltages for all the powders. Among the three powders, alumina was found to improve the surface roughness the most.

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Crack Propagation Behavior of Fused Silica during Cyclic Indentation under Incremental Loads

Kosai K., Zhao Y., Yan J.

Applied Sciences (Applied Sciences (Switzerland)) 12 ( 13 ) 2022.06

Fused silica is an important optical material with important applications, where the surface must be precisely machined without subsurface damage. In this study, multi-cyclic indentations under incremental loads were performed on fused silica using two kinds of indenters to clarify the mechanisms of crack generation and propagation induced by precision grinding. It was found that incremental loading cyclic nanoindentation induced various patterns of subsurface cracking and surface spalling. Four kinds of surface spalling were identified at different locations around an indent, the temporal formation mechanisms of which were clarified by microscopic observation and topographical measurement. Load–displacement curve analysis demonstrated that incremental propagation of lateral cracks during early indentation cycles caused large-scale brittle fractures during later cycles. Compared with a Berkovich indenter, a cube-corner indenter caused more significant brittle fractures and surface spalling. The findings in this study will deepen the understanding of subsurface damaging mechanism of fused silica and other brittle solids caused by cyclic tool-workpiece interactions in grinding and other mechanical machining processes.

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Microstructures and mechanical properties of Zr-based metallic glass ablated by nanosecond pulsed laser in various gas atmospheres

Y Qian, M Jiang, Z Zhang, H Huang, J Hong, J Yan

Journal of Alloys and Compounds, 163717 (Journal of Alloys and Compounds) 901 2022.04

ISSN 09258388

The surface with micro-convex prepared by laser ablation is beneficial to improving the functional properties of metallic glasses (MGs). However, accompanied by the formation of micro-convex, surface hardness of the ablated area generally decreases due to the softening effect caused by laser thermal shock. Previous studies have shown that the mechanical properties of laser-ablated MG surface are significantly different when using different gas atmospheres. In this study, a comparative investigation was performed to analyze the influence of gas atmosphere (i.e. argon, nitrogen, and air) on the formation of micro-convex as well as its surface characteristics and mechanical properties. Experimental results showed that the atmosphere type did not affect the formation of micro-convex, but significantly affected the surface morphology and element composition of the laser-ablated area. The element analysis and Raman spectral measurements indicated that laser ablation in nitrogen or air atmosphere resulted in local nitridation or oxidation. The results of nanoindentation tests demonstrated that laser ablation in nitrogen or air atmosphere also resulted in surface hardness inhomogeneity, i.e., softening and hardening effects coexist in the ablated area, which could be attributed to the combined influence of laser thermal shock, the introduction of secondary phase as well as laser ablation induced loose structure.

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Tool Path Generation and Optimization for Freeform Surface Diamond Turning Based on an Independently Controlled Fast Tool Servo

Y Sato, J Yan

International Journal of Extreme Manufacturing (International Journal of Extreme Manufacturing) 4 ( 2 ) 025102 2022.03

ISSN 26318644

Diamond turning based on a fast tool servo (FTS) is widely used in freeform optics fabrication due to its high accuracy and machining efficiency. As a new trend, recently developed high-frequency and long-stroke FTS units are independently driven by a separate control system from the machine tool controller. However, the tool path generation strategy for the independently controlled FTS is far from complete. This study aims to establish methods for optimizing tool path for the independent control FTS to reduce form errors in a single step of machining. Different from the conventional integrated FTS control system, where control points are distributed in a spiral pattern, in this study, the tool path for the independent FTS controller is generated by the ring method and the mesh method, respectively. The machined surface profile is predicted by simulation and the parameters for the control point generation are optimized by minimizing the deviation between the predicted and the designed surfaces. To demonstrate the feasibility of the proposed tool path generation strategies, cutting tests of a two-dimensional sinewave and a micro-lens array were conducted and the results were compared. As a result, after tool path optimization, the peak-to-valley form error of the machined surface was reduced from 429 nm to 56 nm for the two-dimensional sinewave by using the ring method, and from 191 nm to 103 nm for the micro-lens array by using the mesh method, respectively.

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Generation of Si@ C/SiC@ C core–shell nanoparticles by laser irradiation of silicon grinding waste

K Minami, K Kobinata, J Yan

Nano Select (Wiley) 3 ( 7 ) 1192 - 1200 2022.03

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Flexible fabrication of fresnel micro-lens array by off-spindle-axis diamond turning and precision glass molding

L Zhang, YY Allen, J Yan

Precision Engineering (Elsevier {BV}) 74 186 - 194 2022.03

Research paper (scientific journal), Joint Work, Accepted, ISSN 01416359

Fresnel micro-lens arrays with close-to-wavelength features have significant advantages in minimization of optical systems. However, in the fabrication of a Fresnel micro-lens array by conventional diamond turning, the discontinuous features of the surface profile cause interference between the tool flank face and the finished surface. In this manuscript, a novel off-spindle-axis diamond turning method is proposed to fabricate Fresnel micro-lens array mold insert and a toolpath generation algorithm is developed. The generation of each lenslet is treated as an independent turning operation, and the whole fabrication of a Fresnel micro-lens array is completed by a succession of repetitive turning operations. With this approach, the interference between the tool and the finished surface is avoided, and the form accuracy and surface integrity are improved for each lenslet. Then, the Fresnel micro-lens array mold insert is further utilized in precision glass molding to replicate the micro-lens structures onto transparent polymer substrates. The experimental results indicate that both the diamond-turned and molded Fresnel micro-lens arrays are achieved with homogeneous quality. As an application, a compact imaging system based on the molded Fresnel micro-lens array is demonstrated. The proposed machining method in this study can be employed in the fabrication of Fresnel micro-lens arrays and other micro-optics with discontinuous profiles with high accuracy and machining efficiency.

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Development of High-Sensitivity Electrically Conductive Composite Elements by Press Molding of Polymer and Carbon Nanofibers

S Aikawa, Y Zhao, J Yan

Micromachines 13 (2), 170 ({MDPI} {AG}) 13 ( 2 ) 170 2022.01

Research paper (scientific journal), Joint Work, Accepted

Carbon nanofibers (CNFs) have various excellent properties, such as high tensile strength, electric conductivity and current density resistance, and thus have great application potential in electrical sensor development. In this research, electrically conductive composite elements using CNFs sandwiched by thermoplastic olefin (TPO) substrates were developed by press molding. The metal mold used for press molding was processed by a femtosecond laser to generate laser-induced periodic surface structures (LIPSS) on the mold surface. The aggregate of CNFs was then flexibly fixed by the LIPSSs imprinted on the TPO substrate surface to produce a wavy conductive path of CNFs. The developed composite elements exhibited a sharp increase in electrical resistance as strain increased. A high gauge factor of over 47 was achieved, which demonstrates high sensitivity against strain when the composite element is used as a strain gauge. Scanning electron microscope observation revealed that the TPO filled the spaces in the aggregate of CNFs after press molding, and the conductive path was extended by the tensile strain. The strain-induced dynamic changes of contact states of CNFs and CNFs networks are discussed based on the electrical performance measurement and cross-sectional observation of the elements. This research provides a new approach to the production of flexible and high sensitivity strain sensors.

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