Alumina Microstructural Sample Preparation Using a Borate Glaze
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c-54 Communications of the American Ceramic Society March 1986 J. Am. Cerom. SOC., 69 131 C-54 (1986) Alumina Microstructural Sample Preparation Using a Borate Glaze CURTIS SCOTT,* SUSAN HILL,* AND KEVIN HOSMER General Electric Co., Lighting Business Group, Cleveland, Ohio 44112 A method was developed for preparing alumina suqaces for microstructural analysis. Samples were polished using a sodium borate glaze, which produced a smooth suqace with the grain boundaries accented. TECHNIQUES for preparing alumina sam- ples for microstructural ceramographic analysis can take several hours. These tech- niques include cutting, polishing, and then either chemical or thermal etching to en- hance the grain boundaries or to better de- fine the surface artifacts. A method for evaluating alumina surfaces has been de- veloped which greatly reduces preparation time and is applicable for multiple samples. Chemical polishing of alumina, which has been described in in- creases the strength of alumina substrates and the transmittance of arc tubes used in high-pressure sodium lamps. The method consists of a molten bath of sodium borate into which the alumina is immersed. The glass uniformly attacks the alumina sur- face, effectively polishing it. A variation of the chemical polish method was developed for analysis of alumina surfaces which may be described as glaze polishing. A 60 wt% mixture of borax (sodium borate) and water was prepared. The mix required temperatures >70°C to remain fluid. Two types of alumina were evalu- ated: alumina* used in high-pressure so- dium lamps as the arc tube material and aluminat used for substrate applications. The initial surfaces were either as sintered or as cut. Samples were dip-coated into the borax/water mix and dried at room tem- perature. Samples were placed onto Pt foil and glazed by heating in air to 900°C and holding for 30 min. The alumina samples were then cooled to room temperature and the glaze was removed from the surface with a dilute HCl solution. Figure 1 shows the surface of the arc- tube-type alumina sample as cut and after glaze polishing. Little microstructural in- formation can be obtained from the as-cut surface. After treatment with the borax flux, however, the microstructure is easily determined. Typically with this type of alu- mina, mechanical polishing along with thermal etching is used to determine microstructures. With proper selection of glazing conditons, quality micrographs ap- proaching thermal etching were achieved. Figure 2 shows the alumina substrate mate- rial as-fired and after it was polished with the sodium borate flux. The glaze-polished surface is much more representative of the internal microstructure. Firing artifacts on the surface have been removed. During polishing, the glaze removes from 10 to 20 p m of surface alumina. The amount removed was found to depend on the amount of glaze, the time and tem- perature, and the rate of volatilization of the borax. During glaze polishing, it was observed that different crystallographic orientations of the alumina were attacked differently. Well-defined crystals emerge from the surface above the plain of polish- ing (Fig. 1). Porosity and, in some in- stances, lattice defects were also enhanced. Glaze polishing of alumina surfaces lends itself well to rapid metallographic analysis of multiple samples. REFERENCES ‘R. Gmska, R. Mistler, and R. Runk, “Effects of Various Surface Treatments on the Bend Strength of High Alumina Substrates,” Am. Ceram. SOC. Bull., 49 [6] 575-79 (1970). ’A. King, “Chemical Polish and Strength of Alumina”; pp. 529-38 in Materials Science Research, Vol. 3. Edited by W. Kriegel and H. Palmour. Plenum, New York, 1966. ’G. Scott and M. Levenson, “ChemicallyPolished PolyctystallineAluminaMaterial”; U S . Pat. 3 935 495, 1976. 4G. Scott and M. Levenson, “Chemically Polished PolycrystaJhe Alumina Material”; U S . Pat. 4 079 167, 1978. 0 Fig. 1. (A) As-cut surface of arc-tube-type alu- mina and (B) glaze-polished surface of same alu- mina (50X). CONTRIBUTWC EDITOR-R. M. SPRICGS Received October 10, 1985; revised copy received ‘Member, the American Ceramic Society. *Lucalox(R), General Elecmc Co., Willoughhy, November 6 , 1985; approved November 6 , 1985. OH _.. ‘AISiMag-B6O(R), General Electric Ceramics Inc., Laurens, SC. Fig. 2. (A) As-sintered surface of alumina sub- strate and (B) glaze-polished surface of same alumina ( 1000 x ) .
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Transcript of Alumina Microstructural Sample Preparation Using a Borate Glaze
c-54 Communications of the American Ceramic Society March 1986
J. Am. Cerom. SOC., 69 131 C-54 (1986)
Alumina Microstructural Sample Preparation Using a Borate Glaze CURTIS SCOTT,* SUSAN HILL,* AND KEVIN HOSMER General Electric Co., Lighting Business Group, Cleveland, Ohio 44112
A method was developed for preparing alumina suqaces for microstructural analysis. Samples were polished using a sodium borate glaze, which produced a smooth suqace with the grain boundaries accented.
TECHNIQUES for preparing alumina sam- ples for microstructural ceramographic
analysis can take several hours. These tech- niques include cutting, polishing, and then either chemical or thermal etching to en- hance the grain boundaries or to better de- fine the surface artifacts. A method for evaluating alumina surfaces has been de- veloped which greatly reduces preparation time and is applicable for multiple samples.
Chemical polishing of alumina, which has been described in in- creases the strength of alumina substrates and the transmittance of arc tubes used in high-pressure sodium lamps. The method consists of a molten bath of sodium borate
into which the alumina is immersed. The glass uniformly attacks the alumina sur- face, effectively polishing it. A variation of the chemical polish method was developed for analysis of alumina surfaces which may be described as glaze polishing.
A 60 wt% mixture of borax (sodium borate) and water was prepared. The mix required temperatures >70°C to remain fluid. Two types of alumina were evalu- ated: alumina* used in high-pressure so- dium lamps as the arc tube material and aluminat used for substrate applications. The initial surfaces were either as sintered or as cut. Samples were dip-coated into the borax/water mix and dried at room tem- perature. Samples were placed onto Pt foil and glazed by heating in air to 900°C and holding for 30 min. The alumina samples were then cooled to room temperature and the glaze was removed from the surface with a dilute HCl solution.
Figure 1 shows the surface of the arc- tube-type alumina sample as cut and after glaze polishing. Little microstructural in- formation can be obtained from the as-cut surface. After treatment with the borax flux, however, the microstructure is easily determined. Typically with this type of alu- mina, mechanical polishing along with thermal etching is used to determine microstructures. With proper selection of glazing conditons, quality micrographs ap- proaching thermal etching were achieved. Figure 2 shows the alumina substrate mate- rial as-fired and after it was polished with the sodium borate flux. The glaze-polished
surface is much more representative of the internal microstructure. Firing artifacts on the surface have been removed.
During polishing, the glaze removes from 10 to 20 pm of surface alumina. The amount removed was found to depend on the amount of glaze, the time and tem- perature, and the rate of volatilization of the borax. During glaze polishing, it was observed that different crystallographic orientations of the alumina were attacked differently. Well-defined crystals emerge from the surface above the plain of polish- ing (Fig. 1). Porosity and, in some in- stances, lattice defects were also enhanced. Glaze polishing of alumina surfaces lends itself well to rapid metallographic analysis of multiple samples.
REFERENCES ‘R. Gmska, R. Mistler, and R. Runk, “Effects of
Various Surface Treatments on the Bend Strength of High Alumina Substrates,” Am. Ceram. SOC. Bull., 49 [6] 575-79 (1970).
’A. King, “Chemical Polish and Strength of Alumina”; pp. 529-38 in Materials Science Research, Vol. 3. Edited by W. Kriegel and H. Palmour. Plenum, New York, 1966.
’G. Scott and M. Levenson, “Chemically Polished Polyctystalline AluminaMaterial”; U S . Pat. 3 935 495, 1976.
4G. Scott and M. Levenson, “Chemically Polished PolycrystaJhe Alumina Material”; U S . Pat. 4 079 167, 1978. 0
Fig. 1. (A) As-cut surface of arc-tube-type alu- mina and ( B ) glaze-polished surface of same alu- mina (50X).
CONTRIBUTWC EDITOR-R. M. SPRICGS
Received October 10, 1985; revised copy received
‘Member, the American Ceramic Society. *Lucalox(R), General Elecmc Co., Willoughhy,
November 6, 1985; approved November 6, 1985.
OH _.. ‘AISiMag-B6O(R), General Electric Ceramics
Inc., Laurens, SC.
Fig. 2. (A) As-sintered surface of alumina sub- strate and ( B ) glaze-polished surface of same alumina ( 1000 x ) .
