  In a protoplanetary disk, dust aggregates in the $\mu$m to mm size range
possess mean collision velocities of 10 to 60 ms$^{-1}$ with respect to dm- to
m-size bodies. We performed laboratory collision experiments to explore this
parameter regime and found a size- and velocity-dependent threshold between
erosion and growth. By using a local Monte Carlo coagulation calculation and
complementary a simple semi-analytical timescale approach, we show that erosion
considerably limits particle growth in protoplanetary disks and leads to a
steady-state dust-size distribution from $\mu$m to dm sized particles.
