  We describe measurements of the X-ray reflectance in the range 2 to 10 keV of
samples representative of coated silicon wafers that are proposed for the
fabrication of the XEUS (X-ray Evolving Universe Spectrometer) mission. We
compare the reflectance of silicon samples coated with bare Pt, with that for
samples with an additional 10nm thick carbon over-coating. We demonstrate a
significant improvement in reflectance in the energy range ~1 to 4 keV, and at
a grazing incidence angle of 10 mrad (0.57 degrees). We consider the resulting
effective area that could be attained with an optimized design of the XEUS
telescope. Typically an improvement of 10 to 60 % in effective area, depending
on photon energy, can be achieved using the carbon overcoat.
