The effect Etching with Hydrofluoric Acid has on

Surface Flaws in Glass

Introduction

            Glass is used abundantly in everyday life all through the world. It is a versatile and useful material which can be formed into different shapes and sizes very easily. Glass in its purest form is an exceptionally strong material, unfortunately, pure, unflawed glass is extremely hard to produce and is not available on a large scale. What gives glass the brittle properties we know it to have are the surface flaws. These small cracks are caused by chemical or mechanical damage to the surface. These cracks are weak spots and can propagate causing the glass to fail.

            These cracks have a very small tip radius and create high stress concentrations at the tip end. This is calculated by the formula:

1 + 2 (√l/R)

Where  l = depth of the crack

            R = radius.

Experiment Procedure

Involving acid in an experimental procedure meant that safety rules had to be adhered to at all times. These involved the used of rubber protective gloves at all time as Hydrofluoric acid is a highly dangerous and corrosive substance which can injure easily. Spillages need to be made visible by covering the work surfaces in tissue. Any contact with the acid must be washed thoroughly and be treated with magnesium paste and glycerol.

The main aim of the experiment is to see the effect hydrofluoric acid had on the surface flaws on glass. The Glass samples were sectioned into groups of three that would be subjected to the acid for differing intervals. One group was left un-etched, the other groups, etched for five, ten, and fifteen minutes respectively in hydrofluoric acid.

To measure the load the glass can take before breaking, the Hounsfield Tensometer with 1000N and three point loading attachment was set up.

Using a micrometer, the diameters of all the specimens were measured, creating an average for each piece. This was needed in order to work out the tensile stress for each sample.

The experimental procedure was as follows;

• Place group 1 samples in a beaker of 6% Hydrofluoric acid solution for five minutes.
• Remove samples from solution and wash thoroughly in alcohol.
• Dry samples with hot air dryer.
• Repeat process for both the ten minute group, and fifteen minute group.
• Position samples in Hounsfield tensometer three point loading attachment.
• Cover with a sheet of polythene and test the sample to breaking point.

To work out the tensile stress, the following equation is used;

F = (8WL)/( πD^3)

W = Applied maximum load

L = Distance between pair of supports = 30

D = Diameter of sample

Results

Conclusion

            The main conclusion that was found through doing the experiment was that the longer the glass was exposed to the acid, the higher the tensile stress became, This is due to the acid eroding away the tip of the crack to create a larger radius at the end. This process of widening the radius of the tip lowers the stress concentration at the point where the crack would normally propagate, so it takes more to make the crack grow.

            When compared, glass and metals have roughly the same tensile properties, but due to the fact that glass has a very small amount of plastic deformation, when a force is applied, it acts brittle, whereas in metals, the plastic property allows the cracks to propagate much slower and this will only happen when the elastic limit is reached.

            From the graph we can see that there are a lot of scattered results. This is due to the glass samples having many different crack arrangements within them. To have a fair test the amount of samples would have to be vastly increased and then averaged to create a better understanding of how much the Hydrofluoric acid affects the tensile Properties of Glass.

References

Brian S Mitchell : An Introduction to Materials Engineering and Science for Chemical and Materials Engineers

Milton Ohring  : Engineering Materials Science - Page 160