Abstract: A heat transfer surface for boiling liquid refrigerant 11 and its equivalents and method for producing the surface. The surface comprises a porous, open-cell coating at least 15 mils thick, of oxidized metallic particles which are flame-sprayed onto a metal substrate. This surface includes a substantial number of nucleate boiling cavities having an equivalent radius in the range of 1.5 to 4 mils, which are the result of one or more of the following conditions in the flame-spraying process: (1) The flame-spraying nozzle is oriented so that the metallic particles impact the metal substrate at an angle in the range of 30.degree. to 75.degree.; (2) The metal substrate is preheated to a temperature which is at least 600.degree. F., but below the melting point of the substrate; and (3) The flame-spraying apparatus is positioned such that the metallic particles travel from 3 to 6 inches before impacting the substrate.