REACTIVE SYNTHESIS OF ALN TIB2 COMPOSITE

The phase-forming mechanisms in the chemical reaction 2Al + Ti + 2BN = TiB2 + 2AlN have been investigated using X-ray diffraction. It is obs erved that several transient phases (such as TiAl3, TiN and alpha-Al2O 3) exist at different temperatures. It is suggested that the B2O3 in B N powder and oxygen absorbed by Al powder result in the forming of alp ha-Al2O3 at temperatures below 1600 degrees C. This alpha-Al2O3 will b e reduced by the atmosphere in the graphite furnace and disappear at t emperatures higher than 1700 degrees C. Thus combining thermodynamic a nalysis, the synthesis temperature of AlN/TiB2 must be above 1700 degr ees C. Further, the AlN/TiB2 composite has been produced by reactive h ot pressing at 1850 degrees C under 25 MPa for 30 min in Ar atmosphere . The three-point bending strength and SENB fracture toughness are 539 MPa and 5.05 MPa . m(1/2), respectively. The residual stress in the c omposite caused by the considerable difference in thermal expansion be tween the AlN matrix and TiB2 particles has been measured by XRD metho d. Combining crack propagation path analysis by SEM, the toughening me chanism of the composite is suggested to be thermal residual stress to ughening and crack deflection toughening.