Development of a nanocomposite-based strain and force sensors for machining operations

7th International Conference on Virtual Machining Process Technology (VMPT), Hamilton, May 7-9, 2018 Development of a nanocomposite-based strain and force sensors for machining operations A. Sandwell, M. Sanati, H. Mostaghimi, and S. S. Park* Dept. of Mechanical and Manufacturing Engineering University of Calgary, 2500 University Drive N.W. Calgary, T2N 1N4 *simon.park@ucalgary.ca Abstract Sensors provide an interface between mechanical systems and the physical world. Conventional sensors used for monitoring manufacturing processes are often bulky and need complex manufacturing processes. In this study, a novel nanocomposite-based Polyvinylidene fluoride (PVDF) sensor is developed for measuring the strain and force. In addition to PVDF as the piezoelectric polymer matrix, the sensor is comprised of conductive carbon nanotubes creating piezoresisitve behaviour that contributes to increased sensor sensitivity. These sensors are capable of force and strain measurement in a wide frequency band due to their combined piezoelectric and piezoresistive properties. The samples were fabricated using spray coating techniques, and further processing was done to improve their properties. The fabricated sensors were then employed for strain measurement of a cantilever beam under static conditions, free vibration and forced vibration. The obtained results were compared to a reference sensor to verify accuracy. Keywords: Nanocomposite, Strain Sensor, Piezoelectric, Piezoresistive, Machining Operations 1. Introduction Sensors are important devices for monitoring the condition of the mechanical systems. The demand for development of new sensors and monitoring techniques continuously increases as the industry moves towards improving the quality of machined products higher by leveraging increased automation. Different types of sensors, such as piezoresistive, optoelectronic, and piezoelectric, have been developed for the force and strain measurement. These sensors, predominantly, are complex to manufacture which may increase both cost and time. In addition, size and flexibility are other challenges associated with the existing sensors. In the case of piezoelectric based sensors specifically, charge leakage is a serious problem in static and low frequency measurements. The existing strain gauges and piezoresistive sensors are good candidates for low frequency measurements but the drift errors that accumulate over extended operating times are problematic for these transducers [1]. To overcome the challenges associated with existing sensing systems, a new low-cost and flexible polymeric nanocomposite-based sensing system is developed for strain and force measurements over a wide range of operating frequencies. Polymeric nanocomposites (PNCs) are considered a new class of materials, which are made by combining a polymer along with non-organic fillers with characteristic sizes in the nanoscale. A wide range of polymers can be used for fabricating PNCs. For the best results, the base polymer should be compatible with the nanoparticle, and it should have the ability to improve the properties of the resulting nanocomposite. Excellent piezoelectric properties can be achieved by using polyvinylidene fluoride (PVDF) as the polymer matrix [2]. Whereas carbon nanotubes (CNTs) can be used as fillers since they provide favourable mechanical [3], electrical [4] and thermal A. Sandwell, M. Sanati, H. Mostaghimi, S. S. Park 2018

properties [5]. The combination of polymer and CNTs provides nanocomposite materials with unique properties, such as high strength, and high resistance to wear and corrosion. Mechanical properties, piezoresistive properties, and thermal conductivities of a polymer matrix can be improved through dispersing a small amount of CNTs. In addition to piezoresistive characteristics, the inclusion of these nanoparticles within the piezoelectric polymers can enable the PVDF to become polarized at lower voltages [6]. A uniform dispersion and alignment of CNTs inside the polymer matrix are required to obtain desirable physical and mechanical properties of PNCs [7]. To mix CNTs with polymers, different techniques, including solution blending accompanying ultrasonication [8], and melt blending [9] have been used. These techniques can improve the dispersion of CNTs inside the polymer matrix. The main objective of this study is to develop a new sensor for strain and force measurement over a wide frequency range. The developed sensor aims to overcome the limitations and problems of the existing sensors. These sensors are thin and flexible, so that they can be mounted on any surface. The performance of the developed sensor is verified by comparing the experimental results in static and dynamic measurements by those of commercial sensors. The novelties of this study include combining the piezoelectric and piezoresistive properties in a single sensor and covering both static and dynamic measurements. 2. Sensor Fabrication and Experimental Setup Polyvinylidene fluoride (PVDF) is chosen as the polymer matrix due to its piezoelectric properties. A mixture of PVDF and N-N dimethylformamide (DMF) are stirred on a hot plate for 3 hours at 80 C. Concurrently, 0.1 wt% CNTs are mixed with DMF and sonicated for 30 minutes to achieve even dispersion of the nanoparticles. Once the PVDF is fully dissolved, the two mixtures are added together and stirred for an additional hour. The mixture is spray coated onto a glass substrate and heated at 80 C until all volatile compounds have evaporated, and the mixture is solid. The result is a thin nanocomposite polymer film is peeled from the glass substrate. The resulting PVDF film is semi crystalline consisting of four main conformations: α, β, γ, and δ-phases [10]. The C-F bonds of the PVDF are polar and the highest amount of polarization can be obtained from the β-phase where the all dipoles of the polymer are aligned in the same direction. On the other hand, α-phase PVDF provides the dipole moments in random directions and consequently zero net polarization is observed. To improve the piezoelectricity of the prepared samples, the percentage of β-phase crystallites should be maximized. The conversion of α-phase to β-phase is a popular method. There are different ways to achieve this conversion including, high voltage polarization, solution casting, spin coating, and electro spinning [11]. In this study, a two-step approach using mechanical stretching and high voltage polling. It is presented in the literatures that the sequential process of stretching and polling results in enhanced piezoelectric properties in PVDF samples [12]. The prepared samples were first stretched mechanically to 500 % strain in a heated chamber at 80 C. The fourier transformed infrared spectroscopy (FTIR) spectra of the prepared samples provide useful information about the structure of the PVDF polymer and the crystalline forms present. Among different phases of the PVDF polymer, γ and β-phases present similar number of bands mostly in similar wavenumbers due to the similarities in their polymer chain structure. For example, the band at 840 cm -1 is attributed to both γ and β-phases [13]. However, the band at 1279 cm -1 is exclusively attributed to β-phase molecules [13]; consequently, monitoring of the absorbance peak in this wavenumber provides the relative change in the piezoelectric composition A. Sandwell, M. Sanati, H, Mostaghimi, S.S. Park 2018 2

of the polymer before and after processing. Custom stretching and polling setups were developed for processing the samples. The FTIR results shown at each processing step, i.e. pure PVDF, unstretched PVDF-CNT, stretched PVDF-CNT, and polled PVDF-CNT, are shown in Figure 1. Figure 1. FTIR of the PVDF sample before and after stretching The results show that the unstretched PVDF sample contains absorbance peaks at wavenumbers associated with the α-phase, at 763, 795 and 974 cm -1, and almost no peaks corresponding to the piezoelectric β-phase. The addition of CNTs into the PVDF show an increase in the absorbance at wavenumber 840 cm -1 corresponding to the γ and β-phases; however, there is no considerable absorption peak at 888 and 1234 cm -1 which implies that the polymer still consists mainly of α-phase. Following mechanical stretching, the β-phase peaks at 840 and 1280 cm -1 increase substantially signifying the increase of the polar phase. In addition, the results show that the reheating and polling of the samples after stretching do not have any remarkable effect on the β-phase of the samples, but this step is necessary for electrical alignment of the β-phase. The stretched samples were polled using a custom corona polling device. The schematic of the polling setup used in this study is shown in Figure 2. This process reconfigures the random orientation of the β-phase and aligns the molecules a single direction resulting in an enhanced piezoelectric response. Figure 2. (a) The schematic of the corona polling setup used for polling of the samples; (b) Experimental setup used for static and dynamic tests To obtain the piezoelectric coefficient of the prepared samples experiments using a d33 meter are conducted. The piezoelectric charge coefficients (dij) are defined as the ratios of electric A. Sandwell, M. Sanati, H, Mostaghimi, S.S. Park 2018 3

charges per unit area generated in response to an applied force. Considering that the thickness of the nanocomposite samples is negligible, and the poling occurs in the thickness direction, i.e. in the z direction, the only charge coefficient component which needs to be measured is d33. A significnat improvement was observed in the d33 of the poled samples, from 1.6 Pc/N to 31 Pc/N, showing the improvement of the piezoelectric properties of the samples due to polling. The experimental setup used in this study is shown in Figure 2. The sensor is mounted on an aluminium cantilever. Both sides of the fabricated nanocomposite film are covered with flexible copper electrodes so the generated charge and resistivity changes of the sensor under strain are captured. A charge amplifier and a voltage divider circuit are used for piezoelectric and piezoresistive sensors, respectively. The Macro Fiber Composite (MFC, Smart Naterials) which is a commercial PZT piezoelectric sensor was used to compare the results of the developed sensor under dynamic force. During static testing. the results were verified with an Omega force sensor (DFG 51-50). The results of the developed sensor in strain measurement are compared with those of the MFC sensor and force sensor (Omega force sensor) to verify the performance of the developed sensor. These tests include both static and dynamic experiments, including static bending, free and forced vibrations. 3. Results and Discussion Experiments are performed to validate the accuracy of the developed sensor. The response of the sensor under both static and dynamic loads are collected and compared with commercial sensors. The piezoresistive characteristic of the sensor is used for static measurement. A static force is applied to the free end of the cantilever and measured using the reference force sensor and the strain of the cantilever at the sensor location is measured using the piezoresistive properties of the senor. The obtained results from both our sensor and the commercial sensor are shown in Figure 3. Figure 3. The response of the system under static loading The obtained results prove that there is a good agreement between the results of the nanocomposite sensor and the commercial sensor in static measurements. The above results also show that the developed sensor can detect both tensile and compressive strain. When the force is applied upward at the end of the cantilever, the resistance of the sensor increases due to the increased distances between CNT within the conductive networks which shows that there is a tensile strain in the cantilever. A. Sandwell, M. Sanati, H, Mostaghimi, S.S. Park 2018 4

Dynamic excitations including the free and forced vibrations are applied to the free end of the cantilever and the strain of the beam at the sensor location is measured using the piezoelectric properties of the sensor. The results of the free and forced vibrations for both our sensor and the commercial sensor are shown in Figure 4. The dynamic results from our sensor and the reference sensor verifies the performance of the developed sensor in dynamic measurements; however, the frequency bandwidth of the nanocomposite sensor and performance at higher excitation frequencies is still needed. Figure 4. Experimental results: (a) Free Vibration, (b) Forced Vibration One of the main limitation observed in this study is that the effect of polling on improving the piezoelectricity decreases after adding nanoparticles. Another limitation of this study is that we tested the piezoelectric and piezoresistive properties, separately. In order to obtain improved information of the piezoelectric and piezoresistive nanocomposite sensors and achieve higher frequency bandwidth, the output signals of each sensor need to be fused to generate a more accurate sensor overall. 4. Conclusions A new nanocomposite-based sensor was developed in this study for force and strain measurements. This sensor can be used for measuring the strain of the tool and then indirect measurement of cutting forces in machining operations. This sensor which combines piezoelectric and piezoresisitve properties, is flexible, and can be mounted on any surface. The piezoelectricity of the sensor using sequential stretching and corona polling was improved. The performance of the developed sensor in strain measurement was investigated through static and dynamic experiments. The accuracy of the sensor was verified after comparing the experimental results of A. Sandwell, M. Sanati, H, Mostaghimi, S.S. Park 2018 5

the sensor with those of the commercial sensors. The static results showed that the developed senor can measure static forces beside detecting the sign of the strain applied to the system. Also, the dynamic results prove that this sensor can be used for free and forced vibration measurement after being calibrated with a reference sensor. However, the sensitivity of the sensor needs further improvement. As future work, the authors of this work would like to combine the piezoelectric and piezoresistive properties. In addition, further study of different fabrication methods along with different nanoparticle concentrations and polling conditions to improve the sensitivity of the sensor are needed. Acknowledgements The authors would like to thank NSERC CANRIMT, and Pratt Whitney Canada for their funding of this project. References [1] Park, S.S., et al., Polymeric carbon nanotube nanocomposite-based force sensors, CIRP Annals-Manufacturing Technology, 65(1), 361-364, 2016. [2] Ueberschlag, P., PVDF piezoelectric polymer. Sensor review, 21(2), pp.118-126, 2001. [3] Salvetat, et al., Mechanical properties of carbon nanotubes, Appl. Phys. A, 69(3), pp. 255 260, 1999. [4] Yao, Z., et al., High-Field electrical transport in single wall carbon nanotubes, Phys. Rev. Lett., 84(13), pp. 2941 2944, 2000. [5] Ruoff R. S., Lorents D. C., Mechanical and thermal properties of carbon nanotubes, Carbon, 33, pp. 925 930, 1995. [6] Kim, J., et al. Piezoelectric polymeric thin films tuned by carbon nanotube fillers, Proceeding in Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, 6932, San Diego, California, USA, 2008. [7] Moniruzzaman M., Winey K. I., Polymer nanocomposites containing carbon nanotubes, Macromolecules, 39 (16), pp. 5194-5205, 2006. [8] Barrau S., et al., Effect of Palmitic Acid on the Electrical Conductivity of Carbon Nanotubes Epoxy Resin Composites, Macromolecules, 36(26), pp. 9678 9680, 2003. [9] Bhattacharya A. R., et al., Crystallization and orientation studies in polypropylene/single wall carbon nanotube composite Polymer, 44, pp. 2373-2377, 2003. [10] Fontananova, E., et al., From hydrophobic to hydrophilic polyvinylidenefluoride (PVDF) membranes by gaining new insight into material's properties, RSC Advances, 5(69), pp. 56219-31, 2015. [11] Dhakras, D., et al., Enhanced piezoresponse of electrospun PVDF mats with a touch of nickel chloride hexahydrate salt, Nanoscale, 4(3), pp. 752-756, 2012. [12] Mahadeva, S.K., et al., Effect of poling time and grid voltage on phase transition and piezoelectricity of poly(vinyledene fluoride) thin films using corona poling, Journal of Physics D: Applied Physics, 46(28), p. 285305, 2013. [13] Martins, P., et al., Electroactive phases of poly(vinylidene fluoride): Determination, processing and applications, Progress in Polymer Science, 39, 683 706, 2014 A. Sandwell, M. Sanati, H, Mostaghimi, S.S. Park 2018 6