THE EFFECT OF TiB₂ PARTICLE SIZE ON STEEL-TiB₂ COMPOSITES HARDNESS

Metal matrix composites with ceramic reinforcements, such as TiB₂ particles are promising candidates for structural and armor applications. Moreover, the composites with a steel matrix may be heat treated to improve their mechanical properties. In the present work the effects of TiB₂/steel ratio, TiB₂ particle size and thermal treatments on the hardness of steel-TiB₂ composites, fabricated by free infiltration were studied. Two types of TiB₂ powders with average particle sizes of 2 and 43 μm were used. TiB₂ preforms with various porosity (from 45 to 25 vol.%) were fabricated using the spark plasma sintering apparatus. Prior the infiltration process, a set of wetting experiments with molten 1070 steel (0.7 wt.% C) and TiB₂ substrate at 1450°C was conducted. It was found that the waiting angle (~ 20°) at this temperature is adequate for free infiltration of TiB₂ preforms. The infiltration process was conducted at 1450°C in a vacuum (10^-4 torr).

The hardness of the composites, fabricated from the fine TiB₂ powder, increased from 1250 to 1600 HV, with the decreasing of the metal fraction from 45 to 25%. The hardness values of the composites fabricated from the coarse TiB­₂ powder were slightly lower. To increase hardness, the infiltrated composites were heat-treated at 900°C and quenched in oil. The quenched samples show a significantly increased hardness values. For instance, the hardness value of the composites with 25% of metal phase after the heat treatment was about 2100HV. This effect was attributed to the microstructural changes in the steel matrix and the formation of novel ceramic phases.