Question

Magnesium fluoride anti-reflective coating on glass

To reduce reflection, eyeglass manufacturers apply anti-reflection coatings on the lenses. A typical coating material is magnesium fluoride with a thermal expansion coefficient of 11.6 x 10^?6 K^-1. Optical glass has a thermal expansion coefficient of 7.6 x 10^?6 K^-1. Manufacturers deposit magnesium fluoride onto glass through a vacuum deposition method whereby they heat the substrate to about 325 ?C. The Young modulus and Poisson ration of magnesium oxide are 138 GPa and 0.27.

At room temperature, is the stress in the coating tensile or compressive?

What is the stress magnitude at room temperature (25 ?C) and at -25 ?C?

Assume a scratch of a = 0.05 mm depth, a critical stress intensity factor of K_Ic = 4.2 MPam^1/2, and a geometry factor Y = 1.1. At what temperature does the crack initiate fracture?

Answer 1