Before we get into teeth, note that your dragon will need some really, really strong muscles (and jaw bones) in order to take a bite out of a tank. Perhaps a "how could dragons have strong jaw muscles" question is in order.
According to the comments, depleted uranium is helpful against projectiles, but it won't do much against the compression from dragon teeth. It is usually used with steel or another alloy, however, so now the question becomes what can pierce steel effectively.
Let's use the hardness of steel and a handy unit converter, because for a quick chomp and pierce, tensile strength alone will not matter. While some of these materials may shatter if you put too much pressure on them, they are adequate for puncturing and tearing if you make them sharp enough.
Some of the strongest heat-treated steel will measure at about 444 on the Vickers hardness scale, while tungsten carbide measures in at around 2242! Alternatively, using the Mohs hardness scale, steel is at around a 7, while tungsten carbide measures 9.
It is difficult for a dragon to have tungsten teeth for several reasons:
- Acquiring tungsten in abundance requires a really weird diet
- Processing the elements to create an alloy requires extreme conditions (if they breathe fire, I guess this is plausible)
- Tungsten may be toxic
A possible workaround could include dipping teeth of another material into molten metal, although I doubt molten tungsten carbide exists in nature.
It's hard to find quantitative data to compare the hardness of carbon steel to regular steel, but note that as the carbon content increases, strength increases - so these teeth will likely be stronger than the tank material.
It is difficult for a dragon to have carbon steel teeth because:
- Acquiring steel in abundance requires a really weird diet
- Processing the elements to create an alloy requires extreme conditions (if they breathe fire, this may be plausible)
A possible workaround could include dipping teeth of another material into molten metal, although I doubt molten carbon steel is easy to find in nature.
Chromium has a Vickers hardness of 1060 MPa, compared to steel's reasonable maximum of 444. It would definitely get the job done.
Many yeasts have a high chromium content. While some forms of the element may be toxic, it's reasonable to think that dragons could consume animals that rely on this yeast - or they could cultivate the yeasts themselves.
Titanium has a Vickers number of 830–3420 - higher than that of most steel - so you should be all set.
Organic titanium compounds are found in some natural reactions. These could occur in the prey of your dragons, or their prey's prey; with some more work and possibly some handwaving you could construct titanium teeth.
Diamonds are considered to be one of the strongest natural substances - with a 10 on the Mohs hardness scale, compared to a 6-7 for most steel. These are the most ideal teeth you could have.
No organic processes can create, or have created, diamonds. However, since dragons tend to hoard gemstones (and some consume them) it's reasonable to think that a dragon's body could arrange diamond particles and bind them with an alloy to make semi-diamond teeth.
At a measure of 9 on the Mohs scale, rubies are significantly harder than steel.
Similarly to diamonds, no organic processes can create, or have ever created, rubies. However, since dragons tend to hoard gemstones (and some consume them) it's reasonable to think that a dragon's body could arrange ruby particles and bind them with an alloy to make semi-ruby teeth.