[en] This relates to an anode assembly for rotating X-ray anode tubes and in particular to a composite substrate comprising a graphite member. The substrate comprises a first member to which an anode target is to be affixed, a second member comprising graphite and a single layer of metal joining the two. The metal layer consists of a non-carbide forming material in which the solubility of carbon therein is almost zero up to 1300⁰C but in which from 1-4% of carbon is soluble at the temperature of joining the two members, the metal being soluble in the material of the first member. The invention also provides an anode assembly for rotating X-ray anode tubes using a disc including the composite substrate. Suitable materials for the metal layer are platinum, palladium, rhodium, osmium and ruthenium. Up to 1% of chromium may be included in the platinum. (U.K.)