Fabrication And Properties Of BN-MgAlON Composite Material From Boron-rich Slag

Boron-rich slag is one of the intermediate products from pyrometallurgy separation of pageit.Boron content and magnesium content in slag are higher than12%and35%respectively. Because oflow activity of boron and magnesium, it is hard to reuse. Using boron-rich slag as raw materials,which is rich of boron oxide and magnesium oxide, BN-MgAlON composite powders weresynthesized by carbothermal reduction and nitridation. And the BN-MgAlON multiphase ceramicswere fabricated by high temperature sintering. Structural properties, mechanical properties andoxidation resistance of the materials were studied.Firstly, phase-equilibrium relationship in the B-C-O-N system was analyzed. BN-MgAlONcomposite powders were synthesized at follow conditions: temperature1480℃, stoichiometric contentof C, x=4.5(Al23O27N5·xMgO), holding time8h and N2flow400mL/min. The sample had extra massloss because of the reduction and volatilization of B2O3and MgO.BN-MgAlON composite ceramics was prepared by hot pressed sintering method. Thedensification behavior during sintering, changes of phase composition, microstructure, mechanicalproperties and anti-friction and wear properties were mainly studied. The results indicate that a certaindegree of texture form inside the hot pressing samples. The optimum hot sintering temperature is1750℃at which the bulk density, porosity, bending strength, fracture toughness and hardness of thematerial are2.79g cm-3,0.3%,283MPa,3.85MPa m1/2and15.33GPa, respectively. The frictioncoefficient of BN-MgAlON/45#steel sliding couple under dry friction condition at the load of50～200N is about0.4. The reducing thickness of material is less than that of wear-resistant steel.Experimental study on the oxidation resistance of BN-MgAlON composite in air shows thatMgAlON is oxidized firstly and BN at last. The starting oxidation temperature of BN-MgAlONcomposite is about988℃. A “protective film” is formed on the surface of the composites afteroxidation at1000℃～1300℃, preventing further oxidation. A parabolic model can well describe theoxidation process. The apparent activation energy for oxidation processes is2.13×105J/mol.