Sealed units are protected from liquids, vapors, and fine dust, have excellent grease retention but have increased friction compared to shielded and open bearings. Zirconia offers high strength, wear resistance, and flexibility. It has a superior surface hardness, the highest level of impact toughness for a bearing grade ceramic, corrosion resistance, elastic modulus and thermal expansion similar to bearing steel, and good high-temperature performance. It has low thermal conductivity, is electrically non-conductive and non-magnetizing, making it a suitable material to replace metals in many applications where isolation is essential. One notable weakness is high-temperature steam will degrade Zirconia.