  The hard phase model describes a relativistic barotropic irrotational fluid
with sound speed equal to the speed of light. In this paper, we prove the local
well-posedness for this model in the Minkowski background with free boundary.
Moreover, we show that as the speed of light tends to infinity, the solution of
this model converges to the solution of the corresponding Newtonian free
boundary problem for incompressible fluids. In the appendix we explain how to
extend our proof to the general barotropic fluid free boundary problem.
