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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?

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    $\begingroup$ Hardest, not strongest. $\endgroup$ – Mindwin Sep 6 '16 at 12:36
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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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    $\begingroup$ Perfect Answer!! :) $\endgroup$ – Vaishnavi Mar 10 '14 at 13:20
  • $\begingroup$ @mmsser314, So where does graphene fall in? $\endgroup$ – Pacerier Jul 31 '17 at 6:05
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Actually the data presented by You show that iron/steel is more brittle than diamond. Precise tensile strength of diamond is unknown, however values of up to 60000 MPa have been observed. Typical values of tensile strength of iron/steel varies from 100 to 11000 MPa. Therefore diamond can withstand more than iron/steel.

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The data you quote shows that diamond is stronger than steel, but strength isn't the same as brittleness (or, indeed, stiffness). Brittleness is to do with how much energy is needed to break something, and takes into account the amount of deformation it can take as well as its strength. Steel can deform a lot before it breaks, diamond can't and so diamond is more brittle than steel but much stronger.

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A glass rod can have higher tensile strength than iron if it is polished properly. So a diamond rod with a perfect crystal lattice will have great tensile strength. But probably not the greatest, maybe a rolled up graphene rod would do better, as more of the bonds can take up tensile force at the same time. A simple chain of carbon atoms with double bonds between may be a winner. But when crystals break they break, steel is much better that way as it can be ductile and can elongate. The energy needed to pull the material into two pieces may then be higher for some types of steel. Spider silk may also perform well.

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A perfect crystal of diamond etched nanosmooth would have a much higher tensile strength than that. According to https://en.wikipedia.org/wiki/Fracture_mechanics, it's microscopic surface cracks that make brittle solids have a much lower strength because ever tension is applied to it gets concentrated at the tip of the crack. The longer the crack, the less tension is needed to magnify the tension at the tip of the crack to propagate the crack further. Once enough tension is applied to propagate the crack further, tension can be magnified even more easily because the crack is longer so you get the runaway effect of the crack growing at the speed of sound in the material making the object appear to break instantly. Scratching is one way to introduce a crack into a nanosmooth object and it does a good job of that because according to Whit3rd's answer to my question at How does an infinitely hard tip scratch an amorphous brittle material when it slides along it?, the beginning of the scratching action introduces cracks which can then propagate from magnified tension making the surface weak enough to be scratched even deeper, which further weakens the surface and that process keeps going until the scratch is so deep that the tip no longer exerts enough force per area to make the scratch deeper even with the reduced strength as a result of scratching.

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A diamond isn't stronger than steel. It's smoother than steel as it's molecules are much tighter together, making it easier, for instance, for a diamond-tipped drill bit to resist wear and tear, as whatever it's drilling will be slipping around the diamond tip's smoothness. However nowadays safe crackers use a special steel tipped drill bit. They last longer than diamond tips because of how they're made. Steel is far denser than a diamond, each single steel molecule weighing much more than a carbon atom alone.

Smoothness is more or a less a trick of the diamond to make it appear like it's titanium or something. It's not it's just smooth.

Steel is the strongest metal we currently have. Only problem is that steel is made in open air refractories where air molecules and other debris can smelt into the steel. But steel is so strong that it doesn't really matter so much. But yes, make steel as smooth as a diamond and you have superman metal with a greater wear and tear resistance than any other surface known to man.

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