Optical, sound and vibration monitoring of plasma electrolytic oxidation process on 15% SiC p /2009 aluminum matrix composite under different voltages

The micro-discharge evolution during bipolar pulsed plasma electrolytic oxidation (PEO) on 15 vol.% SiC p /2009 aluminum matrix composite was monitored by optical emission spectroscopy (OES), sound and vibration non-destructive measurement techniques. OES was detected from a glass window outside the electrolytic bath, meanwhile, the hydrophone, microphone and accelerator were fixed inside the electrolyte, above the electrolyte and on the sample surface exposed in atmosphere environment, respectively to monitor underwater sound, airborne sound and sample vibration signals during the PEO treatment, meanwhile, their three-dimensional time–frequency spectra are evaluated. The variations of optical, airborne sound, underwater sound and sample vibration signals under different voltages were analyzed. It was found that the illumination, spectral line intensity of OES and underwater and airborne sound signals were related to the size and density of all discharge sparks, while the intensity of vibration signals mainly depended on the size of the strong sparks. Moreover, the spike peaks on the intensity curves of sound and vibration signals were also related to strong discharge sparks. With the increase of applied voltage, the intensities of micro-discharges increased, then the frequency response intensities of sound signal within 20 kHz and vibration signal within 10 kHz were enhanced. Sound and vibration signals were mainly attributed to the evolution of bubbles from the plasma discharge. These results confirmed that the PEO discharge process can be effectively monitored by optical, sound and vibration measurement techniques.