EFFECTS OF EXPOSURE HISTORY ON THE SURFACE MECHANICAL PROPERTIES OF FLOAT GLASS

Open Access
Author:
Kolluru, Pavan Venkata Narayana
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 19, 2008
Committee Members:
  • Christopher L Muhlstein, Thesis Advisor
  • David John Green, Thesis Advisor
  • Carlo G Pantano, Thesis Advisor
Keywords:
  • Float glass
  • tin diffusion
  • aging
  • weathering
  • leaching
  • surface cleaning
  • pH
  • nanoindentation
  • reduced elastic modulus
  • hardness
Abstract:
Commercial float glasses are technologically important materials that are commonly used in architectural, automotive and structural applications. The surface layers of these glasses are usually affected by their exposure to corrosive environments leading to structural modifications at the atomic scale. Such variations in surface mechanical properties of these glasses are expected to control a wide range of glass characteristics such as the flaw generation and propagation, glass strength and adhesion of coatings. The current study evaluated the mechanical property variations in commercial float glasses on a length scale equal to the thickness of the corrosion modified surface layer using nanoindentation. This provided an insight into the influence of surface cleaning treatments and corrosion on elastic and plastic response of the float glass surfaces. Variations in the reduced elastic modulus and hardness were evaluated for fresh soda-lime-silica float glass surfaces that were subjected to controlled surface cleaning treatments in pH 0.9 hydrochloric acid, pH 7.1 reverse osmosis water and pH 9.5 Alconox® solutions. Further, controlled experiments were performed to evaluate the effects of short-term corrosive exposure conditions such as weathering and leaching in de-ionized water on the surface mechanical properties of fresh float glass. Reduced elastic modulus variations in the range of 0.5 – 9% and hardness variations from 2 – 35% were observed due to these controlled exposure conditions. Effects of corrosion on the surface mechanical properties of the tin-enriched side of fresh float glass were found to be very different from the effects on the air side. The relative hardness of the tin side compared to the air side of the float glass was dependent on the exposure history of the glass, which may be the dominant factor that controls float glass surface mechanical properties. Nanoindentation experiments were also performed on 2, 6 and 8 mm thick aged commercial soda-lime-silica float glasses with known tin concentration profiles that had been exposed to laboratory air over a long period of 16 years. The indentation penetration depths lay between 50 and 225 nm, the region with the largest concentration and the steepest gradient of diffused tin. Attempts to correlate the trends in reduced elastic modulus and hardness of these surfaces with their tin concentration profiles indicated that the tin diffusion into float glass was not the most dominant parameter affecting their mechanical properties. Upon comparing the reduced elastic modulus and hardness results of the aged 2 mm thick float glass with those from the 2 mm thick fresh float glass that was subjected to different controlled exposure conditions, it appeared that the aged float glasses were also being affected by their exposure history. The reduced elastic modulus and hardness of the tin side of the fresh float glass were found to nearly invariant after exposing to corrosive environments, indicating that the tin side was more resistant to corrosion. Further, the less resistant air side underwent reduction in hardness and reduced modulus after corrosion due to which the corresponding tin sides were harder and had a higher modulus. While an earlier study had indicated that the tin side may be harder than the tin side, a few others had reported that the possibilities of tin side possessing the same or lower hardness and elastic modulus as compared to the air side. Based on the results from both fresh and aged float glasses, these contradictory observations in the published literature are attributed to the exposure history dependence of float glass. The fact that even the short duration surface cleaning treatments significantly affected the reduced elastic modulus of these float glass surfaces raises serious questions about the use of silicate glasses to calibrate nanoindentation systems.