Synthesis, processing and characterization of tungsten carbide/cobalt nanophase composites

A new method has been devised for the chemical processing of mixed metal carbide and binary metal/metal carbide composite powders in the Co-W-C ternary system. Experimental quantities of Co/WC powders have been produced with nanoscale microstructures--two orders of magnitude reduction in length scale over that achieved by conventional processing.; The laboratory scale process for synthesizing nanophase WC-Co powders consists of three main steps: (1) preparation and mixing of a starting solution, (2) controlled precipitation from solution to form a homogeneous precursor powder, and (3) thermochemical conversion of the precursor powder to the end product powder.; The thermochemical conversion step is normally carried out in a fixed bed reactor, which is capable of producing only small quantities of powder product. However, adoptation of spray drying and fluid bed reactor technologies to the chemical processing method has enabled the preparation of much larger quantities of powders. This modified and scaleable process involves three coordinated steps: (1) preparation and mixing of starting solutions of the desired composition, (2) spray drying to form homogeneous spherical precursor particles of controlled composition, (3) fluid bed thermochemical conversion into the desired nanophase WC-Co powder.; Powder consolidation has been accomplished by exploiting state-of-the-art technologies for cold compaction and liquid phase sintering, and plasma spraying. Dense, pore free, and ultra-fine grained Co/WC samples have been obtained in sintering times less than 30 seconds. The experiments on coatings produced by plasma spraying of nanophase WC/Co powders have shown reduced friction and lower wear rate, which appears to be due to the formation of a duplex microstructure consisting of a uniform distribution of nanoscale WC particles in an amorphous cobalt-rich matrix phase. The liquid phase sintered WC/Co materials displayed relatively high hardness values, when compared with conventional materials, apparently due to the much finer WC dispersion ({dollar}<{dollar}0.1 {dollar}mu{dollar}m) and the reduced mean free path in cobalt phase. The evidence indicates that theoretically dense nanophase WC-Co will have substantially improved wear properties. Further work is needed on HIP sintering of nanophase composite powders to realize this potential.