Laser surface processing of B₄C-TiB₂ eutectic

The formation of a directionally solidified non-oxide eutectic surface layer based on the 75mol% B₄C and 25mol% TiB₂ eutectic composition has been developed using a continuous-wave, high-power, CO₂ laser. To prevent the oxidation of nonoxide compounds and to reduce thermal stress formation, an atmospherically controlled furnace capable of back-heating samples up to 1100°C during the laser processing has been employed. The effect of the laser scan rate on the eutectic microstructure formation is investigated. It is found that at relatively slow laser scan rates, ∼2-4mm/s, the formation of a colony-type eutectic microstructure with submicrometer scale features is observed. At higher heating rates up to 42 mm/s, disordered eutectic grains with nanometer-scale TiB₂ lamellae form. The dependence of the TiB₂ interlamellar spacing on the crystallization rate in the laser solidified B₄ C-TiB₂ eutectic has the same general trend as in the eutectic produced by a floating zone method. However, this method allows the production of much smaller microstructural length scales, on the order of 100nm, as a consequence of the high crystallization velocity.