Mechanical seals are generally two flat faces in contact with one another, blocking fluid flow in a centrifugal pump. They must be extremely smooth, wear resistant, and able to dissipate heat. Silicon carbide (SiC) is the most common seal material, but seal failure remains a leading cause of downtime in these pumps, which handle most industrial fluids and slurries. Ultrananocrystalline Diamond (UNCD) Mechanical Seals, researched and developed by Argonne National Laboratory, Ill., Advanced Diamond Technologies Inc., Romeoville, Ill., and John Crane Inc., Morton Grove, Ill., stands out as competition for SiC. The seal gets its name from its grain size, which at 2-5 nm and 20 carbon atoms in diameter, is much finer than microcrystalline diamond films. In 2002, a large-area deposition system (winner of a 2003 R&D 100 Award) was created for applying UNCD to semiconductor wafers, but the potential for making mechanical seals with it was underestimated at the time. Now that a proprietary process has been developed, UNCD mechanical seals are available in many shapes and sizes from John Crane Inc. The grain boundaries are a mixture of diamond- and graphite-bonded carbon, giving a film hardness of 97 GPa. No mechanical polishing is required, and the coefficient of friction is just 0.02 in air.