  The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) will double
its beam intensity for the needs of High Energy Physics frontier. In order to
ensure coherent stability until the beams are put in collision, the transverse
impedance has to be reduced. As the major portion of the ring impedance is
supplied by its collimation system, several low resistivity jaw materials have
been proposed to lower the collimator impedance and a special collimator has
been built and installed in the machine to study their effect. The results show
a significant reduction of the resistive wall tune shift with novel materials,
in agreement with the impedance model and the bench impedance and resistivity
measurements. The largest improvement is obtained with a 5 {\mu}m Molybdenum
coating of a Molybdenum-Graphite jaw. This coating can lower the machine
impedance by up to 30% and the stabilizing Landau octupole threshold by up to
120 A. The collimators to be upgraded have been chosen based on the improvement
of the octupole threshold, as well as the tolerance to steady state losses and
failure scenarios. A half of the overall improvement can be obtained by coating
the jaws of a subset of 4 out of 11 collimators identified as the highest
contributors to machine impedance. This subset of low-impedance collimators is
being installed during the Long Shutdown 2 in 2019-2020.
