The reliability and performance of SiC shaft seals on multipurpose mechanical pumps are improved by applying a protective coating of ultrananocrystalline diamond (UNCD). UNCD exhibits extreme hardness (97 GPa), low surface roughness (20 nm), low friction (0.1 in air) and outstanding chemical resistance. Consequently, the application of UNCD coatings to multipurpose mechanical pump seals can reduce frictional energy losses and eliminate the downtime and hazardous emissions from seal failure and leakage. In this study, UNCD films were produced by microwave plasma chemical vapor deposition (MPCVD) utilizing an argon/methane gas mixture. Various seeding pretreatments performed prior to UNCD growth, including mechanical seeding, ultrasonic seeding and the carbide-derived carbon (CDC) process, were investigated to optimize the nucleation and growth of the UNCD films on different seal surfaces. The UNCD-coated seals were examined using Raman microanalysis, SEM, EDAX, optical and stylus profilometry and adhesion testing. The coated seals were subjected to friction testing and dynamic wear testing. Reduced friction was observed for the UNCD-coated SiC seals when compared to the uncoated SiC seals. The wear testing was performed at 3000 RPM and 100 psi for up to 10 days during which the seals were periodically removed and inspected. The wear testing revealed that delamination of the UNCD films was prevented when the initial SiC seal surface had a roughness > 0.1 micron. In addition, the UNCD surfaces showed no measurable wear as compared to approximately 0.2 micron of wear for the untreated SiC surfaces. We have successful coated and tested a variety of pump seals including 3" reaction bonded SiC seals, sintered a-SiC 1” automotive seals, sintered a-SiC 2” chemical process pump seals and 2” WC chemical process pump seals. Finally, by achieving the simultaneous coating of multiple mechanical pump seals in the MPCVD system, we have brought this process closer to manufacturability.