Peterson 
APPENDIX 5B 
APPLICATION OF COATINGS TO THE HEADFORM 
As a result of a previous study at NSRDC, [22] » it was 
concluded that through the use of the principles of surface chemistry, 
the number of cavity nucleation sites on the solid surface could be 
significantly reduced. In order to assist in evaluating the role of 
free stream originating nuclei, it was necessary to determine whe- 
ther or not surface nucleation sites were contributing to acousti- 
cally measured cavitation inception on the headform, 
The underlying objective of the surface coating procedure was 
to enhance the wettability of the solid on a microscopic scale. The 
coating selected was one formed by the application of colloidal silica, 
As is well known, [4 1] » amorphous silica has a very low interfa- 
cial surface energy in contact with water. This is basically because 
the atomic structure of water is quite similar to silica. When silica 
dissolves in water, the process involves simultaneous hydration of 
the Si0, surface and depolymerization, This leads to the formation 
of monosilicic acid, 
(SiO, ) ts 2m (H,0) = m Si(0H), 
From monosilicic acid colloidal particles of silica can be produced. 
On the surface of each particle a monolayer of water is chemisorbed 
that can only be removed.by heating a dried surface to 600°C. Itis 
also known that these particles will have a negative charge in an 
alkaline medium. 
The concept of the coating process is to utilize this negative 
charge and the colloidal particle dimensions to microscopically coat 
a solid surface. The surface selected was gold that was recently 
plated on the headform. The headform was flushed with spectro- 
scopically pure acetone and then cleaned in a special chamber with 
steam produced from a potassium permanganate solution, Following 
cleaning, a 1 percent solution of colloidal silica* was put in the 
chamber and a positive potential applied to the headform. The head- 
* Ludox SM, Colloidal Silica, manufactured by E.I. Dupont de 
Nemours and Co., Wilmington, Delaware. 
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