Ultrafine grain WC-Co hardmetals have a high reputation for toughness, strength and rigidity [1, 2]. The mechanical properties of WC-Co compounds are dependent on the grain size of carbides, with smaller-grain materials showing excellent mechanical properties [3]. In order to improve the mechanical properties of WC-Co hardmetals can be improved by using materials with microstructures that demonstrate smaller grain sizes. It is necessary to use ultrafine WC powders without aggregates to produce ultrafine grained hardmetals.

The detrimental effects of strong aggregates in ultrafine WC powders on the microstructure evolution during manufacture processing of ultrafine-grained hardmetal have long been recognised, but little quantitative research has been done to determine this important characteristic. Ultrafine or nanoscale WC powders are so fine that most particles will agglomerate during carburisation and subsequent processing. These aggregates will lead to inhomogeneity in the sintered product.

The strength of the aggregates is an important characteristic for ultrafine powders, and a number of investigators [4] have proposed the compact density pressure (CDP) technique to evaluate the strength of aggregates in ceramic powders. The technique is based on interpretation of compaction data plotted as logarithm of compaction pressure vs. relative density.

The unwanted effects of strong aggregates in ultrafine WC powders on microstructure evolution during manufacture processing of ultrafine-grained hardmetal are well-enough known and in this research the team used small angle X-ray scattering and laser diffraction to quantify the extent of aggregate formation. The strength of aggregates was evaluated by CDP and microstructure development during the experiments was demonstrated. The effects of the slenderness ratio and diameter of the steel die on the error of the strength of aggregates were investigated. Compact density-pressure was again used to characterise aggregates during milling.

Ultrafine WC powder provided by Chongyi Zhangyuan Tungsten Co, Ltd and fine WC powder provided by Xiamen Tungsten Co, Ltd (XTC) was used. Aggregate size distribution was measured by (LMS-30) laser diffraction particle size analyser and the primary particle distribution was measured by small angle X-ray scatter with a high-resolution X-ray diffractometer (X’Pert PRO Nano-1 2B). Microstructures of the aggregates were observed by SEM (ZEISS, SUPRA55).

Pressure-compaction data were determined on a tensile compressive strength tester with a 300KN load cell (CMT4305). The powder was continuously compressed in the die, with a punch speed of 0.5mm/min. To minimise frictional effects, different thickness-to-diameter ratios within 1 were used for initial powder charge. The density of the powder was determined by using the volume (calculated from diameter and thickness) and the weight of the disk after compaction. The thickness of the disk during compaction was calculated from the punch moving distance (recorded by computer) and the thickness after compaction. Density corresponding to the applied pressure was determined by using calibration of the disk thickness.

The small primary particles are held together by neck areas formed by diffusion. The aggregates’ diameter for cumulative 10per cent weight (D10) was 624nm and their diameter for cumulative 50 per cent weight (D50) was 1067nm. The primary particle diameter for cumulative 50 per cent weight (d50) is 128.6nm. The scale for this test is limited to 300nm, which means the whole distribution can not be determined, but the distribution is constringent, so the particles larger than 300nm can be neglected. Almost all the primary particles are held together in the form of aggregates because of D10>> d50.

The extent of aggregate formation AF(50) is defined as follows,

AF(50) = D50/ d50 = 1067/128.6 = 8.3

The degree of aggregation in the powder used in this study was 8.3, which is diameter ratio, and it was 572 when it was changed into volume ratio.
The compact density-pressure method was used to evaluate the strength of aggregates in fine and ultrafine WC powder. When plotted as a logarithm of pressure vs. relative density, there is compaction behaviour of fine WC powders. It is more or less a straight line progression. There is no aggregate in this fine WC powder. The compaction mechanism for this powder is believed to be due to particle rearrangement controlled by particle to particle friction. Compaction data for ultrafine WC powder with aggregates can be fitted to a bent line which is composed of two linear portions. There should be two mechanisms for these two linear portions. In the low pressure region, compaction is believed to be aggregate rearrangement and no change in internal aggregate levels. In the high pressure region, compaction is believed to be particle rearrangement due to aggregate breakup or crushing. The pressure intersect point gives a good indication of the commencement of aggregate fragmentation under the pressing conditions, or the strength of WC powder agglomerates.

After compaction at 20MPa (below the break point 36MPa) the sample exhibits the aggregates in the size range of 1-2um. On increasing pressure up to 36MPa (around the break point), the large aggregates disappeared due to breakup. When the pressure is raised to 90MPa (above the break point), the pore size becomes smaller. Thus intersect indicates the strength of powder aggregates at around 36MPa.

To analyse experimental error, a series of powders were compacted in the shape of various diameter and slenderness ratios (determined after the powders were compacted). Under the tested conditions, the errors are no more than five per cent, so this method can be used to evaluate the strength of aggregates.

Ball milling is a normally to break down aggregates, and the compact density-pressure method was used to characterise aggregates during milling. The powder was milled for up to 10minutes by high energy ball mill without a process control agent. A tungsten ball and steel vessel were used, and the ball-to-powder ratio was 5:1. The longer the powder was milled, the higher the pressure was at the intersection and the intersection disappeared when the powder was milled for 10min. There may be two reasons for this phenomenon. One is that aggregates held together weakly can be destroyed easily; the other is that the primary particle (formed from aggregate breakdown during milling) may influence the intersection. Further research is needed. As the powder was milled, the aggregates became fewer and fewer. When the powder had been milled for 10mins, there were only a few aggregates left. This agreed well with compaction behaviour, and it means that compact density-pressure is an effective way to characterise aggregates during milling, and that milling is an effective way to destroy aggregates.

References
1. Schubert W.D., Bock A. and Lux B. General Aspects and Limits of Conventional Ultrafine WC Powder Manufacture and Hard Metal Production. Int.J. of Refractory Metals and Hard Materials. 1995,(13):281~293.
2. Geoffrey E. Sporiggs. A History of Fine Grained Hardmetal. Int.J. of Refractory Metals and Hard Materials. 1995,(13):241~255.
3. Gurland J. The Fracture Strength of Sintered Tungsten Carbide-Cobalt Alloys in Relation to composition and Particle Spacing.Transactions of the Metallugical Society of AIME.1963,(227):1146~1150.
4. Niesz D. E.,Bennett R.B. and Snyder M.J. Srtength Characterization of Powder Aggregates. Cermamic bulletin,1972,51(9):677~680.

The authors
This article was adapted from Characterize the strength of aggregates in ultrafine WC powders, a paper by Cao Ruijun1, Lin Chenguang2, Sun Lan1，and Jia Chengchang1 . It was given at PM Asia 2007 in Shanghai.
1 School of Materials Science and Engineering, University of Science and Technology Beijing , Beijing, 100083, China.
2 Powder Metallurgy and Special Materials Department, Beijing General Research Institute for Nonferrous Metals, Beijing, 100088, China.