Let me first give you the tensile strength of both substances:
Diamond: 1600 MPa
Steel : 2617 MPa
As you guys should know, tensile strength is how much a material can be stressed or pulled before it breaks. The data above shows that diamond is more brittle than steel. But, I thought a diamond is the strongest natural object in this planet. How can it be more brittle than something which is man-made? Don't the 4 strong covalent bonds in between the carbon atoms resist the pull and carry through huge amount of weight?
Answer
Strength of materials is affected by defects. A perfect crystal of iron would be extremely strong. Once a crack starts, it is not so hard to make it advance one atom deeper. Think of tearing open a plastic bag. Much easier once the tear starts.
Brittle materials can be easier to break because they stretch less. It is easier to tear a sheet of paper than a sheet of rubber. Rubber stretches so the force is distributed through a large region. In paper, it is all at a small region near the tip of the tear. A small region has a smaller cross sectional area. The stress is larger for the same force.
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