International Journal of Refractory Metals and Hard Materials
Effects of ball milling time on the synthesis and consolidation of nanostructured WC–Co composites
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.
Section snippets
Experimental procedure
MA was carried out in a Simoloyer CM01, high energy mill manufactured by Zoz GmbH, Germany. The WC and Co powders (both 99.9% purity, see Table 1) were supplied by William-Rowland Ltd., Sheffield, UK. A WC–10 wt%Co powder composition, together with 5 mm diameter 100 Cr6 balls, was sealed in a 2-l capacity, stainless steel (AISI 304) vessel with a ball-to-powder weight ratio of 10:1. Vacuum pumping of the vessel (10−2 torr) was repeated three times, followed by Ar gas charging. The ball-milling
Powder characterisation
The SEM micrographs of the starting powders are shown in Fig. 1. The WC particles exhibited a rounded, slightly elongated morphology with some agglomeration, whereas the Co particles show a spherical morphology with a narrow particle size distribution. The morphology and size of the MA WC–10Co powders after ball milling for 30, 60, 180 and 300 min are shown in Fig. 2 with Fig. 3 showing the development of the WC–Co composite particles with milling time.
During the MA process, the powders are
Conclusions
In this study, the effects of milling time on the synthesis of WC–10 wt% Co powder were investigated by using a dry, mechanical alloying technique carried out in a horizontal ball mill. Nanostructured WC–10Co powder was mechanically alloyed after 60 min cyclic milling with a WC average domain size (Dv) of 21 nm and a mean square strain value of 0.06%. A fivefold increase in milling time led to an increase in average domain size to 27 nm and a decrease in mean square strain, which may have be due
References (65)
Synthesis of nanostructured materials by mechanical milling: problems and opportunities
Nanostruct Mater
(1997)- et al.
Nanostructured WC–Co alloy prepared by mechanical alloying
J Alloys Comp
(1996) - et al.
Synthesis and characterizations of ball-milled nanocrystalline WC and nanocomposite WC–Co powders and subsequent consolidations
J Alloys Comp
(2000) - et al.
Nanostructured WC/Co composite powder prepared by high energy ball milling
Scripta Mater
(2003) - et al.
Parameters optimization in the planetary ball milling of nanostructured tungsten carbide/cobalt powder
Int J Refract Met Hard Mater
(2008) - et al.
Microstructure and mechanical properties of nanocrystalline WC–12Co consolidated by spark plasma sintering
Int J Refract Met Hard Mater
(2007) - et al.
An experimental study of the sintering of nanocrystalline WC–Co powders
Int J Refract Met Hard Mater
(2005) - et al.
Sintering of nano-sized WC–Co powders produced by a gas reduction–carburization process
J Alloys Comp
(2006) - et al.
Effect of ball milling temperature on the synthesis and consolidation of nanocomposite WC–10Co powders
Int J Refract Met Hard Mater
(2009) - et al.
Effect of mechanical alloying on the morphology, microstructure and properties of aluminium matrix composite powders
Mater Sci Eng A
(2003)
Characterizations of ball-milled nanocrystalline WC–Co composite powders and subsequently rapid hot pressing sintered cermets
Mater Lett
Formation of nanocrystalline Fe–Co powders produced by mechanical alloying
Mater Sci Eng A
Hydrodynamic analysis of the mechanisms of agglomerate dispersion
Powder Tech
Ball temperatures during mechanical alloying in planetary mills
J Alloys Comp
Plastic deformation and its influence on diffusion process during mechanical alloying
Scripta Met Mater
Mechanical alloying and milling
Prog Mater Sci
Nanocrystalline materials and coatings
Mater Sci Eng
The stored energy of cold work
Prog Mater Sci
Nanoparticulate materials densification
Nanostruct Mater
On the mechanics of cold die compaction for powder metallurgy
J Mater Process Tech
Cold compaction of iron powders – relations between powder morphology and mechanical properties. Part I: Powder preparation and compaction
Powder Tech
Effect of nickel particle size on the compaction behavior of rotator mixed and mechanically alloyed nickel and aluminum powders
Mater Sci Eng A
Some considerations on powder compression equations
Powder Tech
A study of a new phenomenological compacting equation
Powder Tech
Tungsten: properties, chemistry, technology of the element, alloys, and chemical compounds
Mechanical solid state mixing for synthesizing of SiCp/Al nanocomposites
J Alloys Comp
Reactive ball mill for solid state synthesis of metal nitrides powders
Mater Sci Forum
Mechanical alloying
Synthesis and structural evolution of tungsten carbide prepared by ball milling
J Mater Sci
Synthesis of WC–10wt.%Co nanocrystalline powders with grain growth inhibitor by MTP
Mater Sci Forum
Attritor milling of WC + 6% Co: effects on powder characteristics and compaction behaviour
Int J Refract Met Hard Mater
Correlating WC grain size analysis techniques with attritor mill monitoring in cemented carbides
Int J Refract Met Hard Mater
Cited by (96)
Friction and wear behaviors of boron-containing high entropy alloy/diamond composites
2024, Diamond and Related MaterialsChallenges and solutions in the synthesis of nano-TiCN: A review
2022, Ceramics InternationalHigh-energy ball milling of WC-10Co: Effect of the milling medium and speed on the mechanical properties
2022, International Journal of Refractory Metals and Hard MaterialsPreparation and characterization of nanodiamond reinforced aluminum matrix composites by hot-press sintering
2021, Diamond and Related MaterialsBoron doped cryptomelane as a highly efficient electrocatalyst for the oxygen evolution reaction
2021, International Journal of Hydrogen Energy