Wear Resistance of Magnesium Fluoride Films on Glass

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Abstract
The wear resistance of MgF2 films on glass have been examined using a machine fitted with an abrader containing emery powder. The test specimen was rotated as the abrasion head moved over its surface producing a central region of maximum wear. An empirical figure of merit could be assigned to the coating under test by measuring the ratio of the area of coating removed to the total area abraded. It was confirmed that the hardest wearing deposits were formed when the glass specimen had been glow discharge cleaned and raised to 300°C before coating. Burnishing and/or heating the coatings before abrasion testing also increased their wear resistance. The wear resistance of coatings deposited at normal vapor incidence progressively decreased as their thickness increased. The fall in the wear resistance of thick films was attributed to the film deforming under the abrasive action so that particles could be torn from the deposit. Films of constant surface density were deposited at different vapor incidence angles and up to 19° showed a dependence of wear resistance on film thickness, but not on the incidence angle. λ/4-films condensed above 50° had poor wear resistance owing to a porous texture. The wear resistance in this incidence region was a function of both vapor incidence and film thickness. Increasing either the mass deposited or the incidence angle produces a coarse texture because irregularities in the film surface prevent uniform condensation. These results are of practical importance when coating lenses of strong curvature.

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