Influence of microstructure on hardness and thermal conductivity of hardmetals

Journal Pre-proof Influence of microstructure on hardness and thermal conductivity of hardmetals

A. Vornberger, J. Pötschke, T. Gestrich, M. Herrmann, A. Michaelis PII:

S0263-4368(19)30461-5

DOI:

https://doi.org/10.1016/j.ijrmhm.2019.105170

Reference:

RMHM 105170

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date:

14 June 2019

Revised date:

29 November 2019

Accepted date:

7 December 2019

Please cite this article as: A. Vornberger, J. Pötschke, T. Gestrich, et al., Influence of microstructure on hardness and thermal conductivity of hardmetals, International Journal of Refractory Metals and Hard Materials(2019), https://doi.org/10.1016/ j.ijrmhm.2019.105170

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© 2019 Published by Elsevier.

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Influence of microstructure on hardness and thermal conductivity of hardmetals A. Vornberger1*, J. Pötschke1, T. Gestrich1, M. Herrmann1, A. Michaelis1 1

Fraunhofer IKTS, Fraunhofer Institute for Ceramic Technologies and Systems, 01277 Dresden, Germany; *corresponding author

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Anne Vornberger: Formal Analysis, Investigation, Visualization, Writing - Original Draft, Writing Review & Editing. Johannes Pötschke: Investigation, Writing - Original Draft, Writing - Review & Editing. Tim Gestrich: Investigation, Resources, Formal analysis. Mathias Herrmann: Investigation, Writing - Original Draft, Writing - Review & Editing, Funding acquisition, Project administration. Alexander Michaelis: Conceptualization , Supervision, Funding acquisition.

Abstract

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Hardmetals or cemented carbides are used in a wide range of applications due to their excellent mechanical properties. WC-Co hardmetals with the same room temperature hardness can be obtained by different combinations of the WC grain size and cobalt content. However, the thermal conductivity of such hardmetal grades is not equal. Applications such as cutting may require a certain combination of hardness and thermal conductivity, which means that a targeted adjustment is desirable. In this study a wide range of hardmetal grades was studied in respect of microstructure, hardness and thermal conductivity in the temperature range between 20 °C and 1000 °C. Results show that thermal conductivity is considerably influenced by Co content, WC grain size and Cr3C2 content. Furthermore, hardmetal grades with the same hardness at room temperature retain hardness very differently at elevated temperatures. For the selection of hardmetal grades for high temperature applications these findings help to choose the right composition in regard to Co content and WC grain size. Keywords Hardmetal, Cemented carbide, Mechanical Properties, Hardness, Thermophysical properties, Thermal conductivity

1

Introduction

Hardmetals or cemented carbides are used in a wide range of applications due to their excellent mechanical properties. The most common composition is tungsten carbide-cobalt (WC-Co). The hardness of these composites can be designed by adjusting the cobalt content and WC grain size dWC . Here, with decreasing Co content and WC grain size the hardness increases [1]. Hardmetals with the same room temperature hardness can thus be obtained by different combinations of these two

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parameters. Many empirical and semi-empirical models have been developed to correlate the measured room temperature hardness with microstructurual parameters. Most models require additional parameters such as contiguity or the mean free binder length dCo. In a previous study [2] it was found that the semi-empirical model from Makhele-Lekala et al. [3] as shown in Eq. 1 predicts the hardness of submicron hardmetals well. 𝐻𝑊𝐶−𝐶𝑜 = 4100 ⋅ (𝑘 ′ ⋅ ( ′

−1

1 2

𝑑𝐶𝑜 2√𝑑𝑊𝐶

) + 1)

− 130 Eq. 1

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with 𝑘 = 22.3 𝑚𝑚

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Some studies have also been conducted on the hot hardness of hardmetals and showed that in general hardness continually decreases with increasing temperature [4–10]. An abrupt change of slope of the hot hardness between 600 °C and 800 °C has been observed in a number of experiments [4–6, 10] which is supposedly caused by a change in the mechanism controlling the hardness [8]. At low temperatures the decrease of hardness of the individual phases of the hardmetal may be the controlling mechanism while at higher temperatures the grain boundary sliding appears to be most dominant. Another explanation of the change in slope is given by Chatfield [10], who argues that the hardness-temperature dependence of the pure WC phase is partially inherited by WC-Co hardmetals. These results showed that coarse grained, binderless WC has a steep decrease in hardness at low temperature and the hot hardness levels off at high temperatures above 500 °C. Fine grained, binderless WC on the other hand has little decrease in hardness at low temperatures, but above 500 °C the hardness loss is increased abruptly. However, also the hot hardness of cobalt based alloys show a change of hot hardness at around 600 °C, hinting at the start of thermal activated plastic deformation of the cobalt phase [4].

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The exact influence of WC grain size on hot hardness is not completely clear. A study by Milman et al. [6] found that hardmetals with finer WC grain size retain higher hardness values at high temperature compared to coarser grained hardmetals. But two other studies stated that up to a certain temperature fine grained hardmetal grades indeed retain higher hardness compared to coarser grained hardmetal grades. But above this temperature (500 °C according to [9], ≈ 930 °C according to [11]) coarse grained hardmetals retain higher hardness values than fine grained ones. Zunega [9] studied the influence of Co content and WC grain size closely and concluded that the WC grain size is the controlling factor for hot hardness in the low temperature range while in the high temperature range (> 500 °C) the controlling factor is more complex, i.e. not WC grain size or Co content alone. The thermal conductivity λ of hardmetals is – similar to the hardness – mainly determined by WC grain size and Co content. In general thermal conductivity increases with decreasing Co content because the λ of the ceramic phase (WC) is higher compared to the metallic binder phase. For pure WC thermal conductivities between 100 and 200 W/mK were reported while for Co around 70 to 100 W/mK were measured [12–14]. For Co alloys even lower values between 30 and 50 W/mK were observed [15].Typical WC-Co hardmetals have a thermal conductivity in the range of 100 W/mK [16]. The thermal conductivity λ of WC-Co can be calculated based on the thermal conductivity of the components and a term taking the interfacial resistivity into account [17]. Wang et al. [12] has derived Eq. 2

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𝜆 = 𝜆 𝑊𝐶 + (𝜆 𝐶𝑜 − 𝜆 𝑊𝐶 ) ⋅ 𝑓𝐶𝑜 + (𝜆 𝑖𝑛𝑡 − 𝜆 𝑊𝐶 ) ∗ 𝐵/𝑑𝑊𝐶

Eq. 2

where B is a constant. While the volume fraction of Co fCo and the thermal conductivities of WC λWC and Co λCo are known, the thermal conductivity of the WC-Co interface λint is unknown.

Wang et al. [12] used regression analysis to fit the experimental data. The result is shown in Eq. 3. The thermal conductivity of the WC and Co phases were determined to be 155 W/mK and 67 W/mK, respectively. Eq. 3

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𝜆 = 154.989 − 87.7459 ⋅ 𝑓𝐶𝑜 − 65.8092 ⋅ (1/𝑑𝑊𝐶 )

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Defects in the microstructure such as porosity, vacancies, additional components in the lattice or grain boundaries act as scatter centers for both electrons and phonons which both conduct heat in hardmetals [18]. Thus, with increasing defect density thermal conductivity diminishes. This means that with increasing WC grain size (i.e. decreasing number of interfaces per volume) the thermal conductivity increases. So far no clear correlation between carbide contiguity or microstructural inhomogeneities on thermal conductivity could be demonstrated [19]. The amount of dissolved W and C in the metallic binder should change the λ of the metallic binder as well. A decrease of λ with decreasing carbon content is reported in literature [13, 14], but it is not clear whether the change in λ originates in the difference of binder alloy composition or in the effect of carbon content on grain growth and thereby on final WC grain size. A study on Co based alloys showed that solute elements such as Cr or Mo considerably decrease thermal conductivity of the alloys as well [15].

2

Experimental

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Applications such as cutting may require a certain combination of hardness and thermal conductivity, which means that a targeted adjustment is desirable. Thus, the aim of this study is to systematically study both hardness and thermal conductivity at room and elevated temperature. Beside hardmetal grades having the same room temperature hardness, additional grades with constant WC grain size, but increasing cobalt content and vice versa were thus produced as well.

Hardmetals in this study were produced using a conventional powder technological route. A total of 13 different powder mixtures were prepared using ball milling in n-heptane. The exact composition and milling time of each mixture is shown in Table 1. Differently sized WC powders from H.C.Starck Tungsten with Fischer Sub Sieve Sizer particle sizes between 0.3 µm and 5.2 µm and cobalt powder from Umicore with 1 µm particle size were used. Particle sizes of used powders are listed in Table 2. Both the particle size calculated from the specific surface via the BET method (dBET) [20] as well as the particle size measured with the Fischer Sub Sieve Sizer (d FSSS) are shown. Grain growth inhibitors (Cr3C2 and VC powders from H.C. Starck) were added when necessary. Samples were uniaxially pressed, debindered in hydrogen atmosphere and finally sintered at 1350 °C in a SinterHIP furnace for 45 min with 60 bar Ar gas pressure. All samples had a density of > 99.8 % of the theoretical density.

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3.1

Results and discussion Properties at room temperature

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Sample characterization included the measurement of the magnetic properties like coercivity Hc according to ISO 3326 and magnetic saturation polarization mS with adjustment of Cr content as described in [21]. Polished cross sections were studied using a field emission scanning microscope (Ultra 55, Carl Zeiss NTS GmbH). The arithmetic average WC grain size dWC was determined using the linear intercept method (ISO 4499-2). Micrographs with appropiate magnification and a number of at least 500 WC grain intercepts were used to determine dWC .The Vickers hardness was measured with a load of 10 kP according to ISO 3878. The hardness was measured between room temperature up to 900 °C. The Laser Flash Analysis method according to DIN EN 821 was used to determine the thermal diffusivity from room temperature up to 1000 °C. The thermal conductivity was calculated by multiplying thermal diffusivity with specific heat capacity and room temperature density. The density was measured according to ISO 3369 using the Archimedes method. Specific heat capacity values were calculated with the software FactSage (ver. 7.0) using the Scientific Group Thermodata Europe 2014 database.

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All produced hardmetal samples were characterized in respect to their magnetic properties and microstructure to ensure that no detrimental phases such as free carbon or eta phase are present. The magnetic properties of all hardmetal grades are listed in Table 3. The magnetic saturation polarization varied between 80 % and 95 % of the theoretical values which indicates that all compositions are in the two phase region WC + Co. Additionally the microstructure was studied using a scanning electron microscope. No free carbon or eta phase was detected. The micrographs were also used to determine the arithmetic average WC grain size dWC , which is shown in Table 3 along with measured hardness and thermal conductivity values at room temperature (RT).

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The hardness and thermal conductivity values at room temperature of all hardmetal grades are shown in Figure 1. Produced hardmetals are divided into four measurement series. Two series with hardmetals having the same RT hardness, but different thermal conductivity due to differences in Co content and WC grain size. Here the two RT hardness levels are 1175 ± 60 HV10 (red circles) and 1540 ± 60 HV10 (black squares). The hardmetals of the other two series have different RT hardness values and also different thermal conductivity. The aim of these series is to separately study the influence of Co content and WC grain size. In one series the Co content is constant, i.e. 10 % (blue triangles), and the WC grain sizes varies between 0.1 µm and 1.5 µm and in the other series the WC grain size is constant, i.e. 0.5 µm (green upside down triangles) while the Co content varies between 5 % and 20 %. Figure 2 shows the thermal conductivity (left diagram) and hardness (right diagram) at room temperature of all investigated hardmetals as a function of the WC grain size. As expected thermal conductivity increases with decreasing Co content and increasing WC grain size while hardness increases with both decreasing Co content and WC grain size. The change of the grain size from 0.1 µm to 1.0 µm with a fixed Co content of 10 % results in a thermal conductivity at RT of 40 W/mK and 130 W/mK, respectively. Further increase in the grain size only slightly changes the thermal conductivity. The hardness varies between 2040 HV10 and 1220 HV10. The difference in conductivity of these samples is 90 W/mK and is larger compared to the hardmetal grades with the same WC grain size of 0.5 µm and varying Co content between 5 % and 20 %. Here the thermal conductivity ranges from 100 W/mK to 60 W/mK. This means

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that a larger variation in thermal conductivity is possible by changing the WC grain size in comparison to changing the cobalt content. The hardness of these grades is between 1810 HV10 and 1190 HV10.

3.2.1

Properties at elevated temperatures

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There are two samples with 20 % Co and a WC grain size of approx. 0.5 µm. The exact WC grain size measured via linear intercept method is 0.54 µm (designation WC(0.5µm)-20Co) and 0.48 µm (designation WC(0.4µm)-20Co), respectively. The sample with 0.54 µm WC grain size has a Cr3C2 content of 0.7 % whereas the sample with 0.48 µm has a higher content of this grain growth inhibitor of 1.4 %. This difference in Cr3C2 content and the difference in WC grain size of 0.06 µm results in a difference in hardness of 11 HV10 which is small considering the measurement error. However, the difference in thermal conductivity at room temperature is 17 W/mK and seems to be quite large for this small change in WC grain size of only 60 nm. Thus, the addition of grain growth inhibitors is relevant and further investigation is needed to determine the exact influence of grain growth inhibitors such as Cr 3C2 and VC on thermal conductivity. It is assumed that these additions not only decrease WC grain size but also possibly increase the interface/boundary resistance [17] and the thermal properties of the binder [15] and thus decrease the overall conductivity.

Hardness

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Both hardness as well as thermal diffusivity and conductivity were measured at elevated temperatures in the range between room temperature and 1000 °C. The relative hardness in relation to room temperature of the hardmetal grades with the same RT hardness of 1540 HV10 and 1175 HV10, respectively, is shown in Figure 3. In case of hardmetals with a RT hardness level of 1540 HV10 the decrease in hardness is similar for all four grades between RT and 500 °C. There is a slight tendency that the fine grained grades with high cobalt content (WC(0.3 µm)-15Co und WC(0.1 µm)-20Co) retain higher hardness in this temperature range. Above 500 °C, however, it is clear that the grades with coarser WC grain size and lower Co content retain higher hardness values. This behavior is pronounced at 900 °C. For the set of hardmetals with a lower RT hardness of 1175 HV10 the hardness was measured at additional temperature steps of 400 °C, 600 °C and 800 °C. Due to the lower starting hardness at RT the last temperature step of 900 °C was omitted. The same trend can be observed for these hardmetals as previously described for the hardmetals grades with a hardness level of 1540 HV10. Between RT and 600 °C the fine grained hardmetal with high Co content (WC(0.5 µm-20Co) retains higher hardness compared to the coarser grained hardmetal with lower Co content (WC(1.5 µm-10Co). This trend is again inverted above 600 °C. These findings agree with other publications [9, 10]. Overall these results show clearly that hardmetals with the same RT hardness retain hardness differently at elevated temperatures depending on the temperature range, WC grain size and Co content. Further hardmetal grades were produced to separately study the influence of WC grain size and Co content. Five hardmetals with the same Co content of 10 % and WC grain sizes between 0.1 µm and 1.5 µm and four hardmetals with the same WC grain size of 0.5 µm and Co contents between 5 % and 20 % were studied. The influence of WC grain size and Co content on the hot hardness is shown in Figure 4, where hardness is plotted against the temperature. It can be observed for all hardmetal grades that the decrease in hardness is steeper above 600 °C. But while in the left diagram (varied Co content)

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the curves look overall similar to each other, the curves in the right diagram (varied WC grain size) are noticably different depending on the WC grain size: the nano grained hardmetals have a much steeper drop in hardness above 600 °C compared to the coarser grained hardmetals.

3.2.2

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For further comparison the relative hardness in relation to RT values of these grades is shown in Figure 5. In case of the hardmetals with same WC grain size of 0.5 µm the hardness loss up to 600 °C is similar. Above 600 °C grades with lower Co content (5 % and 10 %) clearly retain higher hardness than grades with high Co content (15 % and 20 %). Thus a low Co content is in general favorable when high hardness values above 600 °C are needed. In case of samples with varying WC grain size and same Co content of 10 % there is a difference in relative hardness visible at already 300 °C. Here the finer grained samples with 0.1 µm and 0.2 µm WC grain size retain higher hardness than the coarser grained samples with 0.4 µm to 1.5 µm grain size. At 700 °C no clear trend is identifiable. But at higher temperatures, i.e. 800 °C and 900 °C, coarser grained samples apparently retain again higher hardness values than the fine grained ones. This resembles the trend that was previously described for the hardmetal grades with the same RT hardness (see Figure 3). Thermophysical properties

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The thermal diffusivity as well as the thermal conductivity between RT and 1000 °C of the hardmetal grades with the same RT hardness of 1540 HV10 and 1175 HV10, respectively, are shown in Figure 6. The difference in thermal diffusivity and conductivity is largest at room temperature, at high temperatures the difference between the grades is smaller. Furthermore, the decrease in thermal diffusivity from RT to 1000 °C is much larger for grades with larger WC grain size (0.4 µm – 1.5 µm), i.e. 50 % to 70 % decrease, compared to grades with very small WC grains (0.1 µm – 0.3 µm) where thermal diffusivity decreases only by 15 % to 20 %.

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Thermal conductivity is calculated by multiplying thermal diffusivity with density and specific heat capacity. The difference in conductivity between RT and 1000 °C is 24 % to 50 % in case of the coarser grained samples. In case of the samples with 0.1 µm and 0.3 µm WC grain size the conductivity even increases by 30 % and 10 %, respectively, because of the fact that thermal diffusivity only slightly decreases but the specific heat capacity steadily increases with increasing temperature, especially when the Co content is high. The thermal conductivity as a function of temperature of hardmetals with constant WC grain size and constant Co content, respectively, is shown in Figure 7. The hardmetal grades with a WC grain size of 0.5 µm and a varying Co content between 5 % and 20 % show a very similar decrease of the thermal conductivitiy with increasing temperature. As described in section 3.1 the thermal conductivity decreases with increasing Co content. The thermal conductivity of the hardmetals with the same Co content of 10 % clearly decreases with increasing temperature in case of the larger WC grain sizes (0.4 µm – 1.5 µm) and slightly increases or stays on the same level in case of the smaller WC grain sizes (0.1 µm – 0.2 µm). Furthermore, it can be seen that the difference in conductivity between samples with a dWC of 0.4 µm and 1.0 µm is quite large (i.e. 48 W/mK at RT) while this difference is considerably smaller between samples with a d WC of 1.0 µm and 1.5 µm (i.e. 3 W/mK). This means that the influence of the WC grain size on conductivity diminishes with increasing WC grain size.

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3.3

Modeling

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As stated in the introduction section, several models exist to predict room temperature hardness from microstructural parameters such as WC grain size and the mean free binder length. The Makhele-Lekala et al. model [3] was used to calculate hardness values from measured microstructural parameters and compared to the actual measured hardness at RT as shown in Figure 8. For most hardmetal grades the result is very accurate and the deviation between calculated and measured value is 1 % to 5 %. For the hardmetal grades with lower hardness values below 1300 HV10 the deviation is higher (up to 13 %). Overall the room temperature hardness of WC-Co hardmetals can be estimated well with microstructural parameters provided that a suitable model is selected. As for the dependence of hardness on temperature it has up to now not been included in any model and seems to be quite complex. Thus , it is unclear if a simple empirical model can be developed.

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In principle it is also possible to calculate the thermal conductivity of WC-Co composites based on the conductivity of the components and the resistivity of the interface as was shown by Wang et al. [12]. If Eq. 3 is applied to the experimental data of this study, however, large deviations between measured and calculated values are apparent as shown in Table 4. For very fine WC grain sizes below 0.5 µm the calculated values are even negative. This fit is therefore not usable for the studied hardmetals. This is due to the fact, that in the study of Wang et al. materials with very large WC grain sizes of up to 9 µm were investigated and the smallest grain size was just 1.0 µm. Most of the hardmetals in this study are very fine grained, i.e. the WC grain size is in the submicron to nano range. Thus a regression analysis was done using the Minitab software as described in [12] and the experimental data measured in this study. If all 13 hardmetal grades are used for the fit, the result is unsatisfactory, because the derived value for the Co phase is then negative (-4 W/mK).

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The modeling of hardmetals with WC grain sizes in the submicron range is more challenging because the thermal conductivity does not linearly decrease with decreasing WC grain size and increasing Co content. Instead the thermal conductivity approaches a certain threshold value. Our data suggests that this threshold is somewhere around 30 W/mK for the tested WC-Co grades. Also the amount of added grain growth inhibitors has to be considered as well as was shown in section 3.1. If the hardmetal grades with dWC below 0.4 µm are excluded from the regression analysis, a reasonable result with small deviations between calculated and measured values is obtained (see Figure 9). The modified formula is shown in Eq. 4 and is only applicable for hardmetals with WC grain sizes above 0.3 µm. The derived value for the WC and Co phase is 185 W/mk and 35 W/mK. For a more detailed prediction of the thermal conductivity the microstructural features have to be taken into account in more detail.

𝜆 = (185 ± 5) − (150 ± 18) ⋅ 𝑓𝐶𝑜 − (35 ± 3)(1/𝑑𝑊𝐶 ) 4

Eq. 4

Conclusion

The correlation between microstructure, hardness and thermal conductivity of a wide range of hardmetals was studied in the temperature range between 20 °C and 1000 °C. Specifically the influence of Co content and WC grain size was investigated. Results showed that as expected the hardness of all grades increases with decreasing Co content and decreasing WC grain size. Thermal conductivity increases with decreasing Co content and increasing WC grain size. It was found that a larger variation in thermal conductivity is possible by varying the WC grain size between 0.1 µm and 1.0 µm compared to the variation in Co content between 5 wt% and 20 wt%.

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Hot hardness experiments revealed that hardmetals with the same hardness value at room temperature, but different combinations of Co content and WC grain size, retain hardness at elevated temperatures very differently. Below around 600 °C grades with finer WC grain size retain higher hardness despite a higher Co content while above 600 °C this trend is inverted, the softening of Co is the dominating parameter and that means hardmetals with lower Co content retain higher hardness. Furthermore the hot hardness curves of all tested hardmetal grades had a change in slope at approx. 600 °C. But the grades with very fine WC grain size have a much steeper drop in hardness around this temperature compared to grades with coarser WC grain size. Based on previous results (some more than 50 years old) reasons for the hardness drop at around 600 °C can be named: change of hot hardness of the WC phase [10] or of the alloyed cobalt phase [4] and/or change of hot hardness due to a change of grain boundary strength [6].

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Thermal conductivity decreases with increasing temperature for most tested hardmetal grades. But in case of grades with very fine WC grain size of 0.1 µm or 0.2 µm and Co contents below 20 % thermal conductivity stays nearly constant or even increases with increasing temperature.

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To summarize, results show that hardmetals with same room temperature hardness but different combinations of Co content and WC grain size both have a considerable difference in thermal conductivity as well as significant differences in hot hardness. This will impact the performance of components working at higher temperatures (e.g. 600 °C) strongly. For the selection of hardmetal grades for high temperature applications these findings help to choose the right composition in regard to Co content and WC grain size. Further study is needed to investigate the influence of grain growth inhibitors and binder composition and to verify the origin of the hot hardness drop at 600 °C by doing measurements on binderless WC as well as on cobalt alloys. Future work will also include addition of cubic carbides such as TiC.

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With existing empirical models the RT hardness was calculated from microstructural parameters. However, the influence of temperature has so far not been modeled and should be adressed in further work. For the thermal conductivity at RT a modified formula for hardmetals with WC grain sizes above 0.3 µm and Co contents between 5 and 20 wt% was presented in this work. The expansion to a wider range of hardmetals as well as the inclusion of the temperature dependence of these empirical models will be part of future work. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment Funding: This research was funded by the Deutsche Forschungsgesellschaft (DFG, German Research Foundation), reference number HE 2457/21-1.

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Figure Captions Figure 1: Hardness and thermal conductivity at RT of all hardmetal grades. The two bands with grades with the same RT hardness of 1540 HV10 and 1175 HV10 are marked grey and red, respectively. Figure 2: Thermal conductivity at RT (left) and hardness at RT (right) of hardmetals with different Co contents as a function of WC grain size Figure 3: Relative hardness of hardmetals with the same hardness at RT of 1550 HV10 (left) and 1150 HV10 (right) as a function of temperature

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Figure 4: Hardness of hardmetals with constant WC grain size (left) and constant Co content (right) as a function of temperature

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Figure 5: Relative hardness of hardmetals with constant WC grain size (left) and constant Co content (right) as a function of temperature

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Figure 6: Thermal diffusivity (left) and thermal conductivity (right) as a function of temperature

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Figure 7: Thermal conductivity as a function of temperature of hardmetals with constant WC grain size (left) and constant Co content (right) Figure 8: Measured RT hardness and RT hardness calculated using the Makhele-Lekala et al. model [3]

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References

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Figure 9: Measured RT thermal conductivity and RT thermal conductivity calculated using Eq. 4

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[1] Exner H, Gurland J. A Review of Parameters Influencing Some Mechanical Properties of Tungsten Carbide-Cobalt Alloys. Powder Metallurgy 1970;13(Nr. 25):14–31. [2] Vornberger A, Pötschke J, Gestrich T, Herrmann M. Mechanical and Thermophysical Properties of Hardmetals at Room and Elevated Temperatures. In: Euro PM2018 Congress & Exhibition Euro PM2018 proceedings 14-18 October 2018, Bilbao Exhibition Centre (BEC), Bilbao, Spain. Shrewsbury, United Kingdom: European Powder Metallurgy Association (EPMA); 2018. [3] Makhele-Lekala L, Luyckx S, Nabarro FRN. Semi-empirical relationship between the hardness, grain size and mean free path of WC-Co. Int. J. Refract. Met. H. 2001;19:245–9. [4] Dawihl W, Altmeyer G, Mal MK. Das Verformungsverhalten von Wolframkarbid-Kobalt-Legierungen bei Temperaturen bis zu 1000°C. Zeitschrift für Metallkunde 1963;54(2):66–72. [5] Lee M. High temperature hardness of tungsten carbide. Metallurgical Transactions A 1983;14(8):1625–9. [6] Milman Y, Luyckx S, Northrop IT. Influence of temperature, grain size and cobalt content on the hardness of WC-Co alloys. Int. J. Refract. Met. H. 1999;17:39–44. [7] Milman Y, Chugunova S, Goncharuck V. On the dependence of the low and high temperature hardness of WC-Co alloys on cobalt content and grain size 1998. [8] Milman Y, Chugunova S, Goncharuck V, Luyckx S, Northrop IT. Low and high temperature hardness of WC-6 wt%Co alloys. Int. J. Refract. Met. H. 1997;15(1-3):97–101. [9] Zunega JC. High temperature indentation of WC/Co hardmetals. Dissertation; 2013.

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[10] Chatfield C. The Influence of Carbide Grain Size on the Hot-Hardness of Polycrystalline Tungsten Carbide and WC-Co Cemented Carbides. Powder Metallurgy International 1985;17(3):113–5. [11] Mari D, Bolognini S, Feusier G, Viatte T, Benoit W. Experimental strategy of study the mechanical behaviour of cermets for cutting tools. Int. J. Refract. Met. H. 1998;17(1-3):209–25. [12] Wang H, Webb T, Bitler JW. Study of thermal expansion and thermal conductivity of cemented WC: Co composite. Int. J. Refract. Met. H. 2015;49:170–7. [13] Shinohara K, Ueda F, Tanase T. Thermal Expansion Coefficient and Thermal Conductivity of WC Based Cemented Carbides. J. Jpn. Soc. Powder Powder Metallurgy 1993;40(1):29–32. [14] Perecherla A, Williams WS. Room-Temperature Thermal Conductivity of Cemented TransitionMetal Carbides. J Am Ceram Soc 1988;71(12):1130–3. [15] Terada Y, Ohkubo K, Mohri T, Suzuki T. Thermal conductivity of cobalt-base alloys. Metallurgical and Materials Transactions A 2003;34(9):2026–8. [16] Roebuck B, Gee MG, Bennett EG, Morrell R. Mechanical tests for hardmetals; 2009. [17] Hasselman DPH, Johnson LF. Effective Thermal Conductivity of Composites with Interfacial Thermal Barrier Resistance. Journal of Composite Materials 1987;21(6):508–15. [18] Antonov M, Pirso J. Thermal Shock Resistance of Chromium Carbide-Based Cermets. In: Hetnarski RB, editor. Encyclopedia of thermal stresses. Dordrecht: Springer Reference; 2014, 5128–5135. [19] Ono A, Okada K, Homma H, Nakanishi Y. Influence of microstructure on the thermal conductivity of cemented carbide. In: Proceedings 19th International Plansee Seminar. Reutte, Austria: Plansee SE; 2017, HM18. [20] Brunauer S, Emmett P-H, Teller E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc. 1938;60(2):309–19. [21] Pötschke J, Säuberlich T, Vornberger A, Meese-Marktscheffel JA. Solid state sintered nanoscaled hardmetals and their properties. Int. J. Refract. Met. H. 2018;72:45–50.

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Table 1: Powder mixtures Mixture designation

Co / wt%

Cr3C2 / wt%

VC / wt%

WC powder

Milling time / h

WC(0.5 µm)-5Co WC(1.0 µm)-5 Co

5 5

0.35 -

-

WC DS80 WC DS250

12 12

WC(0.1 WC(0.2 WC(0.4 WC(1.0 WC(1.5

µm)-10Co µm)-10Co µm)-10Co µm)-10Co µm)-10Co

10 10 10 10 10

0.90 0.50 0.70 -

0.6 0.2 -

WC DN4.0 WC DN4.0 WC DS80 WC DS250 WC MAS500

24 24 12 12 12

WC(0.3 µm)-15Co WC(0.5 µm)-15Co WC(1.0 µm)-15Co

15 15 15

0.80 1.05 -

0.3 -

WC DN4.0 WC DS80 WC DS250

24 12 12

WC(0.1 µm)-20Co WC(0.5 µm)-20Co WC(0.4 µm)-20Co

20 20 20

1.80 0.70 1.40

1.2 -

WC DN4.0 WC DS80 WC DS80

24 12 12

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-

Specific surface BET / m²/g

H.C. Starck H.C. Starck H.C. Starck H.C. Starck Umicore H.C. Starck H.C. Starck

Tungsten Tungsten Tungsten Tungsten

Particle size (dBET) / µm

4.0 1.3 0.4 0.15 3.3 2.0 3.0

Particle size (dFSSS) / µm

0,10 0.29 0.95 2.55 0.21 0.45 0.35

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WC DN 4.0 WC DS80 WC DS250 WC MAS 500 Co Half Micron Cr3C2 160 VC HV160

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Powder designation

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Table 2: Powder properties

0.3 0.8 2.9 5.2 0.7 1.8 1.2

Designation WC(0.5 µm)-5Co WC(1.0 µm)-5 Co

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Table 3: Measured properties of all studied hardmetal grades dWC / µm

mS / µTm³kg-1

mS / % theo.

Hc / kA/m

Hardness at RT / HV10

Λ at RT / W/mK

0.49 ± 0.01 1.02 ± 0.02

9 8

95 80

24 13

1810 1548

98 142

0.14 0.21 0.44 1.06 1.52

± 0.01 ± 0.01 ± 0.01 ± 0.02 ± 0.02

15 16 16 18 18

83 84 84 88 91

44 30 18 10 8

2036 1791 1578 1314 1223

41 42 81 129 132

WC(0.3 µm)-15Co WC(0.5 µm)-15Co WC(1.0 µm)-15Co

0.29 ± 0.01 0.49 ± 0.01 1.05 ± 0.02

24 25 28

84 88 91

21 14 8

1516 1364 1131

49 71 117

WC(0.1 µm)-20Co WC(0.5 µm)-20Co WC(0.4 µm)-20Co

0.14 ± 0.01 0.54 ± 0.01 0.48 ± 0.01

33 36 34

88 92 91

28 11 11

1514 1175 1186

35 78 61

WC(0.1 WC(0.2 WC(0.4 WC(1.0 WC(1.5

µm)-10Co µm)-10Co µm)-10Co µm)-10Co µm)-10Co

dWC-arithmetic average WC grain size, mS-magnetic saturation, Hc-coercivity, λ-thermal conductivity

Journal Pre-proof

Table 4: Measured and calculated thermal conductivites at RT using the d WC and nominal binder volume fraction (Co and Cr3C2) Calculated Λ in Calculated Λ in Measured Λ at RT W/mK using Eq. W/mK using Eq. / W/mK c 3 4 98 12.6 99.9

Mixture designation

dWC / µm

Binder volume fraction / vol%

WC(0.5 µm)-5Co

0.49

0.09

WC(1.0 µm)-5 Co

1.02

0.08

142

83.0

138.0

WC(0.1 µm)-10Co

0.14

0.19

41

-332.2

-

WC(0.2 µm)-10Co

0.21

0.18

42

-174.0

-

WC(0.4 µm)-10Co

0.44

0.18

81

-10.2

79.0

0.16

129

78.5

127.5

1.52

0.16

132

97.3

137.4

WC(0.3 µm)-15Co

0.29

0.26

49

WC(0.5 µm)-15Co

0.49

0.26

WC(1.0 µm)-15Co

1.05

0.24

WC(0.1 µm)-20Co

0.14

0.36

WC(0.5 µm)-20Co

0.54

0.32

WC(0.4 µm)-20Co

0.48

0.33

26.0

-1.8

75.3

117

71.5

116.1

35

-346.4

-

78

5.3

72.7

61

-11.0

62.9

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71

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1.06

WC(1.5 µm)-10Co

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WC(1.0 µm)-10Co

Journal Pre-proof

Highlights

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Hardness and thermal conductivity of hardmetals was studied between 20 °C to 1000 °C Correlation between these properties and microstructure is discussed Hot hardness changes at 600 °C drastically Thermal conductivity decreases for coarse hardmetal grades but not for fine grades

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9