Effects of ball milling time on the synthesis and consolidation of nanostructured WC–Co composites

Abstract

The effects of ball milling time on the synthesis and consolidation of WC–10 wt%Co powder were investigated by high energy milling in a horizontal ball mill. Nanostructured powder was mechanically alloyed after 60 min cyclic milling with a WC average domain size of 21 nm. The number of nanosize (<0.2 μm) particles increased with milling time. Contamination by Fe increased with milling time, reaching almost 3 wt% after 300 min milling. The onset of the WC–Co eutectic was lowered to 1312 °C through an increase in milling time. The density of the compacted powders increased with the compaction pressure but decreased with milling time achieving 61.7% after 300 min milling compared to 64.4% for 30 min. The compressibility behaviour of the milled powders was determined using a compaction equation. Densification and hardness reached optimum levels for the 60 min milled powder after both pressureless sintering and sinter-HIP.

Introduction

The unique properties accessible for the transition-metal carbides are of great importance for several industrial applications, due to their excellent high temperature strength and good corrosion and fracture resistance, whilst being chemically and thermally very stable even at high temperatures. Among the hard alloys and refractory carbides, hardmetals find a wide range of industrial applications, being used extensively in commercial applications such as tips for cutting and drilling tools, extrusion and pressing dies, and wear-resistant surfaces in many types of machines. The need for hardmetals with improved properties, particularly increased hardness and strength coupled with increased toughness, has focussed attention on the development of increasingly finer-grained hardmetals [1].

Synthesis of nanopowders involves precursors from the solid, liquid, or gaseous state. The more conventional solid-state synthesis or “top-down” approach is to bring the solid precursors into close contact through controlled, mechanical attrition and to subsequently heat treat this mixture at high temperatures to facilitate diffusion of atoms or ions. To realise the potential of these materials, cost effective preparation and retention of the nanostructure into bulk materials remains a key challenge. Mechanical alloying (MA) of powder particles has been developed as a versatile alternative to other processing routes in preparing nanostructured materials with a broad range of chemical compositions. Although there are numerous techniques to produce nanostructured materials, MA has become a popular method to fabricate nanocrystalline materials due to its simplicity, relatively inexpensive equipment and its potential for large-scale production [2], [3].

The MA process is greatly affected by a number of factors that play very important roles in the fabrication of homogeneous materials. The properties of the milled powders, such as the particle size distribution, degree of disorder or amorphisation, and the final stoichiometry, depend on the milling conditions and, as such, the more complete the control and monitoring of the milling conditions then the better the end product [4].

Milling time and speed are two of the most important variables to be considered. Generally, milling time is chosen to achieve a steady state between the fracturing and cold welding mechanisms. The time required will vary according to the type of mill used, milling intensity, ball-to-powder ratio (BPR), milling temperature, and the powder system. Very low rotational speeds lead to increased periods of milling, thereby inducing a large inhomogeneity in the powder due to inadequate kinetic energy input and insufficient localised heat input for alloying [5]. Conversely, very high speeds can lead to excessive heating of the vessel, high wear of the balls causing increased contamination, and lower powder yields. The level of contamination increases if the powder is milled for longer times than required, for example, 20 at.% Fe has been found in a W–C mixture milled for 310 h and 33 at.% Fe in pure W milled for 50 h in a SPEX mill [6].

There are a number of different types of mills for conducting MA and they differ in their capacity, speed of operation, and their ability to control the operation. Examples include attritor, shaker, drum, and planetary mills. The motion of the milling medium and the charge varies with respect to the movement and trajectories of individual balls, the movement of the mass of balls, and the degree of energy applied to impact, shear attrition, and compressive forces acting on powder particles. The majority of reported studies of ball milled WC–Co have utilised planetary [7], [8], [9], [10], [11] or attritor [12], [13], [14], [15], [16], [17], [18] type mills. The present study incorporates a more efficient, horizontal ball milling process that can deliver up to three times higher relative velocity than conventional mills [19] and will establish the effects of milling time on the synthesis of WC–10Co.