- Email: [email protected]
Analysis of the composition of a discolored aluminum film vapor deposited onto plastic
Thin Solid Films, 72 (1980) 151-152 © Elsevier Sequoia S.A., Lausanne
Printed in the Netherlands
151
ANALYSIS OF THE COMPOSITION OF A DISCOLORED ALUMINUM FILM VAPOR DEPOSITED ONTO PLASTIC* M. G. LAGALLY, S. R. ANDERSON, N. C. TRAN AND T. HOANG
Department of Metallurgical and Mineral Engineering and Materials Science Center, University of Wisconsin-Madison, Madison, Wisc. 53706 (U.S.A.) K. GILLEO
Northern Engraving Company, Sparta, Wisc.54656 (U.S.A.)
In the evaporation of aluminum onto plastics the rate of deposition and the manner in which a given thickness film is produced may have an important bearing on the purity of the deposit and hence possibly on the color. Thus a condition known as a "burnt shot" resultiffg in a yellow or brownish tint frequently occurs. It has been found in coating plastic parts with marked contours that, in order to obtain sufficient coverage on those areas that have a more glancing incidence to the source, longer evaporation times or higher evaporation rates are necessary, leading frequently to a yellow tint. Auger electron spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements were made as a function of depth through yellow and bright aluminum films. The most notable difference in the spectra was a larger carbon concentration in the yellow sample, with carbon concentrations near the surface of the film reaching 40 at.% compared with approximately 10 at.% for the bright sample. The difference in carbon concentrations was not localized to the surface. Thus at a depth of 200 A the carbon concentration in the yellow sample was still higher than the surface concentration in the bright sample. The carbon concentration varied approximately exponentially in both films, increasing from the polymer-A1 interface to the surface. In addition to the carbon, XPS detected a significant amount of A1203 throughout the layer for both films, the surface ratio being about 5 parts of A1203 to 2 parts of aluminum. As the pressure in the evaporation chamber was known to be constant or to decrease over the firing cycle, a possible cause for the increase in carbon concentration is a heating of the substrate with a consequent rise in vapor pressure, causing an increasing local partial pressure of hydrocarbons at the film surface. The substrate consists of an acrylonitrile-butadiene-styrene (ABS) polymer with a base coating of an air-dried alkyd. The base coating, although designed for good vapor barrier performance, contains some low molecular weight materials that may be volatile under metallizing conditions. Deposition is initiated with samples at room temperature. Taking into account the various sources and sinks of heat at the sample, the temperature rise of the sample was calculated by making simple approximations. The major contribution to the heat load appears to be the heat of condensation of * Abstract of a paper presented at the International Conference on Metallurgical Coatings, San Diego, California, U.S.A., April 21-25, 1980.
152
AUTHORS'ABSTRACTS
the aluminum. Temperature increases from 3 to 20 K were calculated, depending on the input parameters, specifically the rate of deposition, the thickness of the substrate and the thermal properties of the substrates. For most materials a temperature change of 20 K causes an order-of-magnitude increase in vapor pressure. If the initial vapor pressure is already high, as is likely, this increase could easily explain the rise in carbon concentration in the film. Methods of cycling the source to reduce carbon build-up in the film are discussed in terms of the assumptions of the model for the temperature rise of the substrate.
Printed in the Netherlands
151
ANALYSIS OF THE COMPOSITION OF A DISCOLORED ALUMINUM FILM VAPOR DEPOSITED ONTO PLASTIC* M. G. LAGALLY, S. R. ANDERSON, N. C. TRAN AND T. HOANG
Department of Metallurgical and Mineral Engineering and Materials Science Center, University of Wisconsin-Madison, Madison, Wisc. 53706 (U.S.A.) K. GILLEO
Northern Engraving Company, Sparta, Wisc.54656 (U.S.A.)
In the evaporation of aluminum onto plastics the rate of deposition and the manner in which a given thickness film is produced may have an important bearing on the purity of the deposit and hence possibly on the color. Thus a condition known as a "burnt shot" resultiffg in a yellow or brownish tint frequently occurs. It has been found in coating plastic parts with marked contours that, in order to obtain sufficient coverage on those areas that have a more glancing incidence to the source, longer evaporation times or higher evaporation rates are necessary, leading frequently to a yellow tint. Auger electron spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements were made as a function of depth through yellow and bright aluminum films. The most notable difference in the spectra was a larger carbon concentration in the yellow sample, with carbon concentrations near the surface of the film reaching 40 at.% compared with approximately 10 at.% for the bright sample. The difference in carbon concentrations was not localized to the surface. Thus at a depth of 200 A the carbon concentration in the yellow sample was still higher than the surface concentration in the bright sample. The carbon concentration varied approximately exponentially in both films, increasing from the polymer-A1 interface to the surface. In addition to the carbon, XPS detected a significant amount of A1203 throughout the layer for both films, the surface ratio being about 5 parts of A1203 to 2 parts of aluminum. As the pressure in the evaporation chamber was known to be constant or to decrease over the firing cycle, a possible cause for the increase in carbon concentration is a heating of the substrate with a consequent rise in vapor pressure, causing an increasing local partial pressure of hydrocarbons at the film surface. The substrate consists of an acrylonitrile-butadiene-styrene (ABS) polymer with a base coating of an air-dried alkyd. The base coating, although designed for good vapor barrier performance, contains some low molecular weight materials that may be volatile under metallizing conditions. Deposition is initiated with samples at room temperature. Taking into account the various sources and sinks of heat at the sample, the temperature rise of the sample was calculated by making simple approximations. The major contribution to the heat load appears to be the heat of condensation of * Abstract of a paper presented at the International Conference on Metallurgical Coatings, San Diego, California, U.S.A., April 21-25, 1980.
152
AUTHORS'ABSTRACTS
the aluminum. Temperature increases from 3 to 20 K were calculated, depending on the input parameters, specifically the rate of deposition, the thickness of the substrate and the thermal properties of the substrates. For most materials a temperature change of 20 K causes an order-of-magnitude increase in vapor pressure. If the initial vapor pressure is already high, as is likely, this increase could easily explain the rise in carbon concentration in the film. Methods of cycling the source to reduce carbon build-up in the film are discussed in terms of the assumptions of the model for the temperature rise of the substrate.