Synthesis of hard nano-structured metal matrix composite boride coatings using combined laser and sol-gel technology

摘要

This paper reports the synthesis of nano-structured boride metal matrix composite (MMC) coatings obtained by laser melting of a pre-placed mixture of B_4C + Ti-6Al-4V powders and sol-gel derived nano-particulate Ti(OH)_4. The resulting nano-composite coatings have been characterised in terms of microhardness, microstructure and constituent phase composition. The coatings show evidence of micro-level FeB phases and nano-particulate TiB phases in a (Fe + Fe_2B) matrix. One form of the nano-particulate TiB phase is formed in situ as rosettes as a result of dendritic bending while the other form is of irregular shape. The wear resistance of the hardest coating has been tested at 30 mm/s sliding speed and normal loads of 10, 20, 30 and 40 N under dry conditions. The wear resistance of the coating material shows a 17-fold increase compared to that of the EN43 steel substrate material and comparable or better than most reported work. Reduction of Ti-6Al—4V content in the pre-placed powder mixture does not substantially affect the hardness and wear resistance of the coating.