Larry E. McCandlish

Processing and properties of nanostructured WC-Co

Abstract A novel chemical processing method is described for making nanostructured WC-Co powders. Critical to the success of the process is the control of thermodynamics and kinetics of gas-solid reactions in a fluid bed reactor. Of particular importance is the precise control of carbon and oxygen activities in the fluidizing gas stream. The as-synthesized powders have a high surface area and each powder particle is composite in nature. In contrast to conventional WC-Co powders, the extremely good mixing of the ceramic and metal phases in the powder particles, and the interconnected nanoprosity enables these powders to be consolidated by solid state sintering at relatively low temperatures. A low sintering temperature and a short sintering time ensure retention of a nanostructure in the consolidated material. Preliminary evidence indicates that as the scale of the nanostructure is reduced, the mechanical properties, such as hardness, are enhanced, which is in keeping with the well-established trend in properties of conventional WC-Co material.

Chemical processing and properties of nanostructured WC-Co materials

Abstract A new thermochemical processing method for preparing high surface area powders, starting from homogeneous precursor compounds, has been developed. The method has been applied successfully to the synthesis of nanostructured WC-Co powders. These powders have been consolidated by low pressure plasma spraying and by cold compaction and liquid phase sintering. The use of VC as a grain growth inhibitor is essential to mitigate the WC grain growth during liquid phase sintering. Theoretically dense VC-doped WC-Co materials display superior hardness, wear resistance and cracking resistance.

Hot pressing of nanograined Fe-(Fe,Mo)6C composite powders

Prealloyed micron-sized powders, with two volume fractions (0.52 and 0.74) of nanograined dispersions of (Fe, Mo)6C in Fe, were produced by a spray conversion process. The average grain size of both phases in the as-synthesized powders was found to be 50 nm by X-ray line broadening. The carbide grain size of pressureless sintered material was around 0.6-1.3 μm. Hot pressing reduced the consolidation temperature by ∼300°C and the carbide grain size of the hot-pressed material to about 200–300 nm.

Synthesis and Processing of Nanograined Fe - (Fe, Mo) 6 C Composite Powders

Spray Conversion Processing was used to synthesize high volume fractions (0.52 - 0.74) of nanograined (Fe, Mo) 6 C carbide dispersions in iron, starting from water soluble precursors. The essential features of the process are, (1) preparation of a chemically homogeneous precursor powder, and (2) thermochemical conversion of the precursor powder to the desired nanostructured composite powder through controlled gas-solid reactions. The thermodynamic and kinetic features of the gas-solid reactions, and the influence of various processing parameters on the structures developed, are discussed. The powders were consolidated to near theoretical density by pressureless sintering and hot pressing. All the consolidated samples had bicontinuous structures. Compared to M2 high speed tool steels, these high volume fraction carbide strengthened iron alloys display superior hardness values up to 500°C.

Formation and alloying of nanostructured β-W powders

Abstract We have been investigating the reductive decomposition of spray dried ammonium metatungstate (AMT). Reduction of the pyrolysed powder in flowing hydrogen at T ≥ 625° C creates α-W, with bcc structure. Reduction at T ≤ 575° C results in the formation of W3O, with A15 structure. In situ reduction experiments carried out in a controled atmosphere, high temperature X-ray powder diffractometer have shown that W3O transforms to β-W, in which most of the oxygen atoms are absent. Various experiments confirming the existence of WO1−x are described. Initial experiments on its alloying behavior with nitrogen are also discussed.