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Characterization of Thermo-Electrical Properties of Hard Metal

|Extended Abstract|

Ana Cláudia Gomes Teixeira

Abstract

WC-Co cemented carbides are used in applications that require work under extreme temperature gradients

while demanding an excellent wear resistance. The materials final properties can be tuned to the desired

application by varying the amount of the binder phase and the mean particle size of the tungsten carbide

(WC) particles.

This work aims to study the thermal properties of several WC-Co formulations with average size of WC

particles between 0,8 and 8 µm and Co content between 3,5 and 25 %, produced by pressing and sintering.

The microstructure consists of faceted angular particles of and α-WC dispersed in a matrix of α-Co which

volume fraction varies between 4 and 34%.

The hardness increases with decreasing average grain size and decreasing Co content. The influence of Co

content is more significant and is due to the decreasing fraction of the hard phase in the constitution of the

material. The hardness varies from 883 HV1 for a sample with 4 µm grain size and 25 % Co to 2128 HV1 for

the 0,8 µm grain size and 3,5% Co material.

The thermal diffusivity was determined by the laser flash method, between 150 and 800°C, and the

electrical resistivity was determined at room temperature by the four-point technique. Both the thermal

diffusivity (thermal conductivity) and electrical conductivity increase with the average grain size due to the

decrease of the total area of grain boundaries which are regions of high defect concentration, causing

dispersion of the electrons. The effect of the Co content in thermal and electrical conductivity is less clear.

An increase in thermal and electrical conductivity with the Co content is observed in fine grain samples

containing up to 6 % Co, then in the range from 6 to 20 % Co content seems to have no effect and for higher

contents of Co an increase in these properties is observed. This last trend may be explained by the presence

of W and C in solution in Co, however, the behaviour observed for lower Co contents remains unexplained

and requires further study.

[1, 2]

Key-words: Hard Metal; Tungsten Carbide; Thermal and Electrical Properties; Microstructure

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Introduction

Hard metals are composites formed by a uniform dispersion of ceramic particles in a binder matrix. The

straight grade WC-Co cemented carbide, consisting of a large volume fraction of stoichiometric WC particles

dispersed in a cobalt binder phase, is one of the most used materials for the production of tools used in

various industries, including metal cutting tools, mining, oil drilling, tunnelling, wood working and

construction industries. [1-2] Other grades may include different carbides and cobalt alloyed with or

completely replaced by iron, chromium, nickel, molybdenum or alloys with these elements. [3]

In most applications, the WC-Co tools are exposed to severe friction that generates large amounts of heat.

Thus, thermal properties are important parameters for determining the amount of heat conducted through

the material, and therefore important for determining the temperature gradients along the tools, its

maximum working temperatures, thermal stress, product design and wear by diffusional or plastic

deformation processes. [2, 4] Electrical properties are also important for the cutting and finishing

operations that require electrical conductivity of the material, such as electric discharge wire cutting.

The properties and performance of WC-Co are determined by the fraction of the binder phase and the

mean particle size of the carbides. [3] This work aims to study the thermal diffusivity and electrical

conductivity of several formulations of WC-Co and assess the influence of microstructure of these materials,

especially the Co content and WC mean particle size. Understanding how theses parameters affect both

thermal and electrical properties allows the design of WC-Co hard metals with properties tailored to the

application.

Materials and Methods

Samples of WC-Co with several formulations were developed by DURIT – Metalúrgia Portuguesa do

Tungsténio by pressing and sintering. The WC average grain size ranges from 0,8 to 8 µm and cobalt content

from 3 to 25 wt.% (Table 1). The samples were produced by typical powder metallurgy route. WC and Co

powders were blended and dried, uniaxially pressed by a Dorst K50 mechanical press, green machined

considering a contraction factor of 1,25 and sintered in argon or vacuum at temperatures between 1350 ˚C

and 1460 ˚C.

The microstructural analysis of the samples was carried out by X-ray diffraction (XRD), optical microscopy

and scanning electron microscopy (SEM). XRD analysis was performed in a Philips PW 1830 diffractometer,

using Cu-Kα radiation (λ=1,5418 Å), under a voltage of 40 kV and a current of 30 mA, at a scan speed of

0,016 ˚/min. The diffractograms were processed and indexation of peaks was done by comparison with

ICDD charts of the phases present.

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Table 1 – Designation of WC-Co samples tested

The samples microstructure was analysed by optical microscopy and scanning electron microscopy, allowing

the study of the distribution and morphology of the WC particles. Semi-quantitative analyses and element

distribution maps were performed by energy dispersive X-ray spectrometry (EDS).

The volume fraction of the -Co matrix was determined by image analysis using Image-J software package

[5] based on SEM images of the microstructure of the materials.

The hardness was determined by Vickers microhardness tests using a load of 1 kgf.

The thermal diffusivity was determined by the laser flash method (FL 5000), in the temperature range 150 -

800 ˚C. Sample preparation included the deposition of a graphite layer on both sides of the 12,6 mm

diameter discs. This technique consists in heating the top surface of the sample with a pulse of laser energy,

and studying the thermal response in its rear face by measuring the temperature rise as a function of time.

The model used for calculating the thermal diffusivity assumes that the sample is homogeneous, isotropic

and thermally insulated. [6] The thermal diffusivity is calculated from experimental data using the equation

proposed by Parker et al. [7] (Eq. 1) that is based on specifying the time that the sample's rear surface takes

to reach half of its maximum temperature (t0,5).

α = 0,1388 𝑒2

𝑡0,5 (1)

Where α is the thermal diffusivity and e is the sample thickness.

The sample's resistance at room temperature was determined by four-point probe method with AC

resistance bridge. The measurement was performed in samples with the dimensions 5 x 1 x 0,5 mm,

mounted in a sample holder coated with gold. The contacts were made by 0,25 µm gold wires welded to

the sample holder with tin and silver glued to the sample. For calculating the samples electrical resistivity

(re), the section of the sample and the distance between the inner contacts were measured. The final value

was obtained using Eq. 2, where R is the resistance, A is the area of the sample's section, and L1 its length.

re = R 𝐴

𝐿1 (2)

Sample Co (wt%) Øm (µm) Sintering

Atmosphere Temperature

BH08 3,50 0,8

Argon 1460 ˚C

GD03 5,45 0,8

GD08 6,00 0,8

GD10 6,00 2,5

GD20 10,00 2,5

GD40 20,00 2,5

Vacuum 1350 ˚C

GD50 25,00 4,0

BD05 6,00 6,0

BD30 12,00 6,0

BD50 20,00 8,0

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Results and Discussion

XRD analysis shows that all samples consist of α-WC and α-Co phases (Fig. 1) and no minor constituent

phases (other carbides) have been detected, either by X-ray diffraction or by progressive etching of samples

with Murakami's reagent. The major constituent is WC and the relative intensity of the Co peaks increases

with the increasing amount of Co in the samples.

Figure 1 - X-ray diffraction patterns for the WC-Co samples.

The samples microstructure is formed of a uniform dispersion of α-WC particles, with faceted and angular

morphology, in a cobalt matrix (Fig. 2). All samples present a relative wide dispersion of WC particle sizes

relative to the average grain size, which is more noticeable in the samples with larger particle sizes. The

samples composition was measured by energy dispersive X-ray spectrometry, confirming the data provided

by DURIT (table 1) and which correspond to volume fractions of Co between 4 and 34 % as the Co content

increase from 3,5 to 25 wt.%.

3,50%

5,45%

6,00%

10,00%

12,00%

20,00%

25,00%

BH03|

GD03|

GD10|

GD20|

BD30|

GD40|

GD50|

|0,8 μm

|0,8 μm

|2,5 μm

|2,5 μm

|6,0 μm

|2,5 μm

|4,0 μm

| %Co | Øm

WC

Co

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Figure 2 - Microstructure of WC-12%Co with 6 µm WC grain size: secondary electrons image and respective EDS element distribution maps.

[8, 9] [3, 10, 9, 11, 12]

The hardness increases with decreasing the grain size and cobalt content in the samples, values ranging

from 883 HV1, for the sample with average grain size of 4 µm and 25 wt.% Co, to 2128 HV1 for the sample

with average grain size of 0,8 µm and 3,5 wt.% Co. Although WC grains are highly anisotropic, their random

distribution and orientation in WC-Co confers the material an isotropic behaviour [8-9]. The hardness values

obtained are consistent with literature data [3, 9-12]. Being a particle reinforced composite material the

hardness of WC-Co is mainly due to the high volume fraction of the hard phase.

Thermal and electrical properties are based on the motion of charge carriers. In thermal properties

(diffusivity or thermal conductivity) heat can be conducted by both electrons and phonons. For electrical

conductivity only electrons carry charge. In WC-Co both Co and WC are good conductors, and therefore,

both electrons and phonons contribute to thermal conductivity (unlike most ceramics where electron

conductivity is negligible). [13]

The results obtained for thermal diffusivity are presented in Fig. 3 at different temperatures for the three

particle sizes analysed as a function of the Co content. Diffusivity decreases with temperature because of

the increase in scattering of electrons due to the higher lattice vibration that occurs as temperature rises.

The increase in thermal diffusivity with the particle size is due to the decrease of the total area of grain

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boundaries and the consequent decrease in scattering of electrons by the these high defect concentration

regions as suggested by Wang et al. [2]. This trend is not so pronounced in WC-Co with high Co contents.

The dependence of diffusivity on the Co content is less clear. There is a significant increase in diffusivity

with the Co content up to 6 % in the fine grain materials, followed by a region where the increase of Co

content from 6 to 20% seems to have no effect on diffusivity and for the high Co content materials there is

a decrease in diffusivity.

T= 150 ˚C

T= 200 ˚C

T= 300 ˚C

T= 400 ˚C

T= 500 ˚C T= 600 ˚C

T= 700 ˚C

T= 800 ˚C

T2,5 T0,8

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Figure 3 - Variation of thermal diffusivity with Co content for temperatures ranging from 150 to 800 ˚C, for the samples with grain sizes T0.8, T2.5 and T6.0 (0.8, 2.5 and 6 µm).

[12] [14] [15, 16, 17]

Faria et al. [12] report a thermal conductivity in WC-15%Co higher than that for a WC-6%Co obtained by

Miranzo [14], but provide no explanation. Other authors report the trend generally observed for thermal

conductivity to decrease with the Co content based on two explanations: the decrease in conductivity is

caused by the presence of W and C in solution in the Co phase [15-17] occurring during sintering, or the

breaking of the carbides skeleton (substitution of WC/WC interfaces for WC/Co) [18]). However, no

explanation is possible to provide for the variation observed in thermal diffusivity in samples with fine and

intermediate grain sizes and Co contents under 20 wt.%. Further investigation on this topic involving a

larger number of samples is necessary to enable an understanding of the phenomena controlling the

variations in thermal properties observed.

T= 150 ˚C

T= 200 ˚C

T= 300 ˚C

T= 400 ˚C

T= 500 ˚C

T= 600 ˚C

T= 700 ˚C

T= 800 ˚C

T6,0 T0,8

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Resistivity values measured were converted to electric conductivity and are presented in Fig. 4. This

property varies in a similar way to the thermal diffusivity with grain size and Co content. An increase in the

electrical conductivity with the increase in grain size is observed and may be explained by the mechanism of

grain boundaries area reduction previously proposed. In what concerns the influence of Co content on the

electrical conductivity, the same evolution reported for diffusivity is observed. Sinha et al. [19] studied the

electrical properties of WC-Co containing 6 to 14 vol% Co and reported and significant increase in electrical

conductivity at Co content above 10 vol%, attributing this fact to the percolation effect of the Co matrix. A

percolation limit of 10 vol% seems a value too low to ensure continuity of the conducting phase.

Nevertheless, this may be a possible way for explaining the results obtained and surely deserves further

investigation.

Figure 4 –Variation of electrical conductivity with grain size and Co content.

Conclusions

In this study samples of various WC-Co with average WC particle sizes between 0,8 and 8 µm and Co

contents between 3,5 and 25 wt.% were studied. Their microstructure consists of faceted angular particles

of α-WC dispersed in a matrix of α-Co which volume fraction varies between 4 and 34%.

The hardness increases with decreasing average grain size and Co content. The influence of Co content is

more significant and is due to the decreasing fraction of the hard phase in the constitution of the material.

35

40

45

50

55

60

65

0 5 10 15 20 25 30

The

rmal

Co

nd

uct

ivit

y ((

µΩ

.cm

)-1 x

10

-3)

Co Content (wt%)

T0,8 T2,5 T6,0

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The hardness varies from 883 HV1 for a sample with 4 µm grain size and 25 % Co to 2128 HV1 for the 0,8

µm grain size and 3,5% Co material.

Both the thermal diffusivity (thermal conductivity) and electrical conductivity increase with the average

grain size due to the decrease of the total area of grain boundaries which are regions of high defect

concentration, causing dispersion of the electrons.

The effect of the Co content in thermal and electrical conductivity is not fully understood. An increase in

thermal and electrical conductivity with the Co content is observed in fine grain samples containing up to

6 % Co, then in the range of 6 to 20 % Co content seems to have no effect and for higher contents of Co an

increase in these properties is observed. While the latter trend may be explained by the presence of W and

C in solution in Co, the behavior observed for lower Co contents remains unexplained and requires further

study.

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