What makes teflon such a strong material?
Teflon (PTFE, polytetrafluoroethylene) is prepared by the polymerization of tetrafluoroethylene as shown below.  If we took polyethylene and replaced all of the hydrogens with fluorines, we would have PTFE.



Here is a Table that compares the strengths of some single bonds involving a methyl group.

\begin{array}\hline
Experimental ~Bond~ Enthalpies for~ CH_3 - X~ (kcal/mole)\\ \hline
\end{array}
\begin{array}{|c|c|c|}\hline
I &amp; 57.6    \\ \hline
Br &amp; 72.1    \\ \hline
Cl &amp; 83.7    \\ \hline
CH_{3} &amp; 90.1    \\ \hline
H &amp; 104.9    \\ \hline
F &amp; 115.0    \\ \hline
\end{array}

The $\ce{C-F}$ bond is one of the strongest bonds to carbon known.  As you add additional fluorine atoms to the carbon, the bond strength increases even further.  While the $\ce{C-F}$ bond is a covalent bond, it has considerable ionic character making the carbon positively polarized and the fluorine negatively polarized.  This bond polarization results in a strong electrostatic attraction between carbon and fluorine which accounts for the high bond strength observed.  It is this very high bond strength that makes fluorocarbons so stable (unreactive).  There are not many conditions that can supply enough energy to break this very strong bond.