Cavitation from Nanosized Pits

The perfect reproducibility of the experiment and the spatial control of nuclei were two important ingredients for the experiment of Borkent et al These authors verified the theoretical nucle-ation threshold of surface bubbles, as developed by Atchley and Prosperetti in 1989. According to this theory, the minimum liquid pressure needed to nucleate a bubble trapped in a cylindrical pit with radius Tc and depth dc is 

Here 6 r is the receding liquid contact angle on the plain surface and 5 (6 r) a geometrical factor. For small pits the third term on the right-hand side of Eq. 7.3 is dominant and thus pm inversely proportional to Tc. Therefore one needs very small cavities to measure large differences in p. For this purpose, Borkent et al. have produced three samples with square arrays containing uniformly sized pits with diameters of 900 nm, 500 nm, and 100 nm, respectively (Fig. 7.9). 

For each array, Borkent et al decreased the minimum pressure of the shockwave applied to the liquid, until all bubbles of the array were nucleated (as observed by the camera). The bubble patterns are shown in Fig. 7.10, and the resulting nucleation phase diagram is shown in Fig. 7.11. In this plot, the line is the theoretical prediction according to Eq. 7.3. It perfectly matches the experimental results. The results of Borkent et al were the first demonstration that nucleation of surface bubbles down to nanometer length scales can be fully controlled and quantitatively understood. 

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Incorporation of nanosized SiC particles in the EL NiP matrix increased the hardness from 627 43 to 704 60HVqqj. However, incorporation of micronsized SiC particles increased the hardness to 1669HVqqj. The cumulative mass loss versus time (see Fig. 8.26) due to cavitation erosion corrosion in 3.5% NaCl is found to be very low for EL Ni-P-nano SiC composite coating compared with its coimterpart incorporating micron-sized SiC particles. The EL Ni-P-nano SiC coated specimen exhibits a smooth and uniform surface after the cavitation erosion corrosion test with no visible pits (see Fig. 8.27). The uniform distribution of the nanosized SiC particles in the EL NiP matrix (R 0.95 0.060 pm) inhibits the formation of pits. However, the higher surface roughness (R 1.72 0.051 pm) promotes bubble formation and causes detachment of the SiC particles, which results in the formation of small cavities on the surface of EL Ni-P-micro SiC coated steel (see Fig. 8.27). Hence, it is evident that incorporation of nanosized particles in EL NiP matrix could provide a better cavitation erosion corrosion resistance and inhibit the onset of erosion damage near surface defects. 

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