Sealed units are protected from liquids, vapors, and fine dust, have excellent grease retention but have increased friction compared to shielded and open bearings. Silicon Carbide is one of the lightest and hardest ceramic materials. It has exceptional thermal conductivity, resistance to acids, and low thermal expansion. It is an excellent material to use if exposure to particle wear is a primary concern; it exhibits excellent erosion and abrasive resistance. Silicon carbide is not attacked by any acids, alkalis, or molten salts up to 800°C. In the air, above 2192 °F (1200 °C), SiC begins to form a silicon oxide coating that is protective of the material but undesirable for precision bearings, limiting the material's maximum use temperature as a bearing. The high thermal conductivity, coupled with low thermal expansion and high strength, provide exceptional resistance to thermal shock. It is a semiconductor. Dielectric performance should be verified if electrical isolation by the bearing is required.