Abstract: A heat transfer element (2) for use in boiling a liquid is produced by spraying a substrate, such as a heat exchanger tube, with a plasma of liquified metal particles and concurrently spraying the substrate with liquid carbon dioxide to provide a cooling effect. Preferably the metal particle spray (4) and the carbon dioxide spray (8) are spaced apart, and there is relative movement between the sprays and the substrate, such that a given area of the substrate is precooled by the cold carbon dioxide just prior to liquified metal particles impinging thereon. A shroud (6) may assist in directing the liquid carbon dioxide first onto the substrate, then into the spray of liquified metal particles.