Reaction Synthesis Of W₂B₅/C Composites And Their Microstructure And Properties

W₂B₅/C composites were prepared by reaction hot-pressing to develop a new material for dynamic sealing. The influences of sintering temperature and volume fraction of in-situ formed W₂B₅ in the composites on microstructure, room temperature mechanical properties, room temperature friction and wear properties, and high temperature oxidation behavior have been studied in detail by using modern analyzing methods. Mechanisms of strengthening, friction and wear, and oxidation behavior of the composites have also been investigated.The results show that the rout of reaction between B4C and WC is as following: at lower temperature, the B4C is first decomposed into boron atoms and carbon atoms, and boron atoms diffuse towards WC grains under the force of chemical potential gradient and gradually get into WC grains. Once the activity of boron exceeds the solubility limit, the transpositional reaction occurs andαWB and carbon atom are formed. With the temperature increasing, W₂B₅ is formed through the further reaction between boron atom andαWB.Microstructure analysis indicates that W₂B₅ grains with mean size of 1μm are spherical when sintering at 1800℃, whereas when sintering at 1900℃and 2000℃, the W₂B₅ grains with 2⁴μm become polygonal, and some of which present plate-like morphology. With increasing carbon black content, plate-like W₂B₅ grains reduce and the size of W₂B₅ grains decline to 1μm. TEM photographs show that the thickness of the plate-like W₂B₅ grain is about 0.4¹Î¼m. A large amount of stacking faults that parallel to the basal plane {0001} but no pores and microcracks are present in the W₂B₅ grains. HREM micrograph shows that no any reaction phase and amorphous layer can be detected in the interfacial zone of W₂B₅ and C.The mechanical properties results show that the fracture toughness and Vickers hardness of the 70W₂B₅/C composite increase with the sintering temperature. But at 1900℃, the flexural strength of the 70W₂B₅/C composite has a maximum value of 853MPa. The room temperature mechanical properties of the composites are largely improved with increasing W₂B₅ content. The flexural strength, fracture toughness and Vickers hardness of the 70W₂B₅/C composite fabricated at 2000℃reach 786MPa, 8.89MPa?m^1/2 and 12.25GPa respectively.