"A diamond baseball bat would be very easy to break compared to a wood baseball bat." Is this true? If so, why? In a previous question, someone mentioned that a diamond baseball bat would be very easy to break compared to a wood baseball bat. Is this true?
If I could create a baseball bat that is made out of 100% pure, solid diamond without any defects -- would it really be easier to break than wood? If so, why?
 A: There's three things of some relevance here.

*

*The first is tensile strength and compressive strength which is how much force it takes to
break something. High tensile strength makes a material strong: A material that takes a lot of force to break.

*Then there is stiffness which is how much force it takes to achieve
a particular amount of deflection in something.

*With #2 and #3, you get the deflection required to break something. I don't know of a term for this so I will just call it the ultimate deflection in this post.

Then there is hardness which (to me at least) is difficult to quantify. One definition is possibly the resistance to local deformation which makes it abrasion resistant and difficult to scratch. It might sound like stiffness, but it's not.
It is well known diamonds are hard. However, in terms of breaking something, hardness is not involved.
You can get materials with enormous amounts of ultimate deflection. That is, they can deflect a great deal without breaking; Like a rubber band. But obviously, a rubber band has low stiffness because it does not take a lot of force to achieve any particular deflection. Nor does it have very high strength because it does not take a lot of force to break.
Then you have materials like steel which have high strength and stiffness. But, for how much force it takes to bend to break, steel can deflect quite a lot before actually breaking. That makes it tough and able to absorb lots of energy which is different than being able to absorb a lot of force.
Then you have materials like diamond which are even more stiff than steel and take even more force to achieve a particular deflection, but cannot deflect all that much before breaking. It is possible for the reduced ultimate deflection to outweigh stiffness to result in a material with lower breaking strength.
Then you have carbon fiber which takes even more force to break than steel and takes even more force to achieve a particular deflection than steel. Yet, the deflection it breaks at is less than steel. So it is stronger than steel, yet comparatively brittle and less able to absorb large amounts of energy. But still enough toughness to be useful for structural purposes.
Some scenarios are forced limited such as slowly applying force like with diamond anvils. In that case, as long as you load up the material slowly, ensuring to never exceed the strength it can ultimately support a lot more force than a comparable material
However, other scenarios are energy limited, such as impacts. An impact involves changes in momentum and rapid accelerations (or decelerations if you will) and aren't limited by force as much as they are limited by the kinetic energy involved which needs to be dissipated. These scenarios can easily generate forces higher than what can be supported by a material with high strength that deflect enough to absorb the energy. Being able to deflect spreads out the deceleration over time which reduces the peak forces involved in the collisions.
A: Stronger materials are generally more brittle. In stronger materials, locations of stress concentration tend to stay sharp instead of blunting. Specifically, the high strength limits the size of the so-called plastic zone at crack tips, which would otherwise absorb energy.
In brief, Nature says: If I can’t dissipate deformation energy within the material (by generating dislocations, for example), then, well, I’ll put that energy toward creating new surfaces. And so the material shatters.
A: Let's compare the following three cases:
-a steel rod
-a glass rod
-a diamond rod

For the steel let's use the type of steel that is used for the bar of a barbell
When the weighlifter lifts the barbell you see that the bar flexes a little.
A glass rod will bend much less than a steel rod. When a glass rod is subjected to a bending force then at one side, at the very surface, there will be a peak tensile load.
Compare that to the steel rod. As the steel rod is subjected to a bending force the surface at one side will be under the most tensile load, but the steel at that very highest tensile load spot can elongate. The effect of that localized elongation is that the load is distributed over a larger volume of steel. That distribution is key.
In the case of the glass rod:
The glass is extremely rigid. Because of that rigidity I expect that the glass will have almost no opportunity to spread the load. The load will tend to concentrate in one spot, and when that one spot fails the entire glass rod fails.
The diamond rod is even more rigid than the glass rod.
