Preparation Parameters of ZnO Piezoelectric Coatings Prepared by Arc Ion Plating
Electrical accidents caused by abnormal pressures in high-voltage and ultra-high-voltage bushings have been frequently re-ported in recent years.The accurate detection of abnormal pressures involved with the axial preload of bolts is of great significance to the stable and safe operation of bushings.The method for detection of preload of bolts by contacted ultrasonic,which can realize rapid in-suit detection without damaging parts,is always the focus of extensive attention.And the acoustic-electric signal conversion method applied in this technology has a great influence on the detection accuracy.Generally speaking,there are three methods for achieving this function:(1)ultrasonic probe and couplant,which is one of the most widely used method,but the detection accuracy remain low and the change of the bolt pre-tightening force in the connection structure cannot be accurately measured;(a)pasting piezoelectric ce-ramic sheets,which is relatively simple to construct,but thickness of the adhesive layer is difficult to be precisely controlled,which significantly deteriorates its detection accuracy;(3)depositing piezoelectric coating on the bolts,which is fast and accurate in the de-tection of preload of bolts,while the deposited coatings are stable and not easy to fall off.In order to improve the accuracy and conve-nience of ultrasonic measurement of abnormal pressure involved with preload of bolts and to meet the urgent need for high-precision measurement of preload of bolts in actual industrial bushings,this paper proposed an approach to prepare permanent thin-film pres-sure sensor(PMTS),through which nano-zinc oxide(ZnO)piezoelectric film deposited on stainless steel substrate can be acquired by pulse arc ion plating.The influence of two key parameters such as the bias voltage and the working current on the crystal structure and microstructure of ZnO piezoelectric films was systematically investigated.The investigation on bias voltage showed that increasing bias voltage noticeably increased the number of large particles deposited on substrate.This was because that increasing bias voltage would change the deposition energy of the particles,which accelerated the movement of Zn particles so that more Zn particles had no suffi-cient time to react with O and directly deposit on the coating surface to form large particles.It was also found that the change of the bias voltage had significant effect on the growth rate of the coating.The bias voltage provided an accelerating electric field,which could ef-fectively increase the speed of Zn particles reaching the substrate,thereby increasing the deposition rate.However,when the bias volt-age increased to a fairly high level,the energy of Zn particles reaching the substrate would be too high that a bombardment effect on the deposited coating would occur and result in decreasing deposition rate.Scanning electron microscopy(SEM)images showed that when the bias voltage was too high,the migration time of particles was shortened so that particles are often confined to non-ideal posi-tions and fail to achieve preferential migration,resulting in multi-directional growth.Therefore,the increase of bias voltage was not preferable to improve the crystalline quality of the(002)orientation of the coating.The investigation on working current showed that when working current increased,the power of the target surface during the arc discharge process would increase,resulting in more large-sized Zn particles evaporated and deposited on the coating surface to form large particle contamination,and such large particles generally reduced the compactness of the coating and deteriorate the properties of the coating.It was also observed that the grain size of the coating increased with increasing working current,while more obvious polygonal structure of the grains could be identified and cluster accumulation was weakened.This was because that the number of high-energy ZnO particles was relatively small when the work-ing current was low,which was not preferable to the migration of particles on the surface of the coating.Therefore,grain growth was limited and fine grains were susceptible to form clusters because of accumulation.In the meantime,when the working current in-creased,a large number of ZnO particles with higher energy were deposited on the substrate and had strong migration ability,which was helpful for the growth of grains and improved the surface quality of the coating.Under the processing parameters designed in this paper,bias voltage of O V and working current of 40 A were found to be beneficial to the preparation of ZnO piezoelectric sensor coat-ings with high resistance and preferrable c-axis preferred orientation.ZnO piezoelectric sensing coatings under the above processing pa-rameters were prepared,and ultrasonic system were used to calibrate loading load and measured sound speed.The experimental data showed that in the load range of 0～64 kN,the sound obtained by ultrasonic measurement had a good linear relationship with the load,and the Pearson's correlation coefficient of the fitting curve reached 0.99984％.The results indicated that ZnO piezoelectric sensing coating with high resistance and high c-axis preferential orientation prepared by the optimal processing parameters in this paper could be applied to the accurate axial load detection of transformer bolts.
high voltage bushingspulse arc ion platingzinc oxideworking currentbias voltage
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