Experiments have shown that nonreactive ion-beam sputtering can be used to deposit a transparent film of aluminum oxide on a polycarbonate, silicon, or fused-silica substrate. Similar experiments have shown that reactive ion-beam sputtering can be used to form a transparent film of either aluminum oxide alone or else aluminum oxide filled with a few percent of polytetrafluoroethylene (PTFE). The properties of reactively deposited Al2O3films appear to be superior to those of nonreactively deposited Al2O3 films with respect to the intended applications, as explained below.
These experiments were conducted as parts of continuing efforts to develop hard, adherent, hydrophobic oxide films to protect polymeric windows (e.g., in automobiles and aircraft) against oxidation and abrasion and to enhance their ability to shed water. Earlier efforts along this line of development were reported in "Deposition of Sapphire by Conversion Coating" (LEW-15638), NASA Tech Briefs, Vol. 20, No. 8 (August 1996), page 68.
The deposition apparatus (see figure) was housed in a vacuum chamber and contained two ion sources: one that generated a beam of argon ions at a kinetic energy of 1,000 eV and one that generated an oxygen/argon plasma with an ion kinetic energy of about 170 eV. For nonreactive sputter deposition, the argon-ion beam was aimed at an aluminum oxide sputtering target and the oxygen/argon ion source was not used. For reactive sputter deposition, the argon-ion beam was aimed at an aluminum sputtering target, while the oxygen/argon-ion beam was aimed at the deposition substrate. The addition of PTFE during reactive sputtering was accomplished by placing a wedge of PTFE onto the aluminum sputtering target. In one experiment, a film of PTFE (only) was deposited by use of a PTFE (only) sputtering target.
In each case in which a film was deposited on a fused-silica substrate, part of the substrate was masked with tape to provide a datum surface for measurement of the thickness of the deposit by use of a profilometer. Typical thicknesses of Al2O3 films ranged from 1,600 to 2,700 Å. A deposit of 92 percent Al2O3 and 8 percent PTFE was found to be 3,800 ± 260 Å thick, and a deposit of PTFE alone was found to be 12,300± 1,077 Å thick. Water contact angles were also measured on the fused-silica substrates. In cases in which films were deposited on silicon substrates, the intrinsic stresses in the films were determined by measuring the changes in the surface bows of the substrates. Spectral transmissivities of uncoated and coated fused-silica and polycarbonate substrates were measured. Qualitative tests of resistance to abrasion were performed on two types of polycarbonate substrates: uncoated and one reactively coated with Al2O3 to a thickness of 12,200 Å.
Stresses were found to be slightly lower in reactively deposited than in nonreactively deposited Al2O3 films. Stresses were reduced by incorporation of PTFE. Unexpectedly, films of Al2O3 + PTFE were found to be less hydrophobic than are films of Al2O3 alone an effect that might be attributable to the formation of aluminum fluoride. In comparison with nonreactively deposited Al2O3, reactively deposited Al2O3 proved to be slightly more hydrophobic and more optically transmissive in the ultraviolet. The addition of PTFE exerted little effect on transmittance. The 12,200-Å-thick Al2O3 reactively deposited Al2O3 films on polycarbonate were found to increase the absolute absorptance by only about 1.8 percent while providing some resistance to abrasion by small particles like those typically encountered during cleaning of windows and lenses. Thus, it appears that reactive ion-beam sputtering of Al2O3 is potentially useful as a technique to apply clear, abrasion-resistant films to polymeric windows and possibly other optical components.
This work was done by Sharon K. Rutledge and Bruce A. Banks of Lewis Research Center and Jason Hunt of Ohio Aerospace Institute. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com em> under the Materials category, or circle no. 168 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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