hardness and fracture toughness of a cemented …€¦ · hardness and fracture toughness tests...

$: HARDNESS AND FRACTURE TOUGHNESS OF A CEMENTED …€¦ · Hardness and fracture toughness tests were performed on metalograficly prepared samples shown in Figure 4. Hardness was measured$
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HARDNESS AND FRACTURE TOUGHNESS OF A CEMENTED CARBIDE

TVRDOĆA I LOMNA ŽILAVOST NANOSTRUKTURIRANOG TVRDOG METALA

Danko Ćorić, Matija Sakoman*, Božo Renić

University of Zagreb Faculty of Mechanical Engineering and Naval Architecture, Zagreb, Croatia

Oral presentation Original scientific paper

Abstract In this paper the hardness and fracture toughness values for nanostructured cemented carbides were tested and analyzed. The experimental part included the testing of three samples: WC with 5% cobalt (WC-5Co), WC with 10% cobalt (WC-10Co) and WC with 15% cobalt (WC-15Co). Hardness was tested by Vickers method (HV30) according to HR EN ISO 6507-1: 2005, and fracture toughness was determined by the Palmqvist method according to ISO 28079: 2009. Palmqvist's method uses the length of cracks that propagate from the tip of the Vickers pyramid imprint to determine fracture toughness. The results show that by increasing the amount of cobalt in cemented carbides hardness values drop, and the fracture toughness increases. They also indicate that using nanostructured cemented carbides increases hardness with a slight change in toughness. Keywords: nanostructured cemented carbides, hardness, fracture toughness *Corresponding author (e-mail address): [email protected]

Sažetak U ovom radu ispitivane su i analizirane vrijednosti tvrdode i lomne žilavosti nanostrukturiranih tvrdih metala. Eksperimentalni dio uključivao je provođenje ispitivanja na tri uzorka: s 5% kobalta (WC-5Co), 10% kobalta (WC-10Co) odnosno 15% kobalta (WC-15Co). Tvrdoda je ispitana metodom po Vickersu (HV30) sukladno normi HR EN ISO 6507-1:2005, a lomna žilavost određena je Palmqvistovom metodom sukladno normi ISO 28079:2009. Kod Palmqvistove metode mjerene su duljine pukotina koje propagiraju iz vrhova otiska. Rezultati pokazuju da se s povedanjem udjela kobalta u tvrdom metalu smanjuje tvrdoda, a lomna žilavost povedava. Također ukazuju da se primjenom nanostrukturiranih tvrdih metala, povedava tvrdoda uz neznatnu promjenu lomne žilavosti. Ključne riječi: nanostrukturirani tvrdi metali, tvrdoća, lomna žilavost

mailto:[email protected]





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INTRODUCTION Cemented carbides are metal composites that represent the most widely-known powder metallurgy product. The microstructure of cemented carbides consists of a higher portion of tungsten carbide (WC) and possibly smaller portions of titanium and tantalum carbides, mutually linked to a binder phase that is cobalt (Co). The cobalt matrix usually provides toughness, and carbides are here to provide high hardness and wear resistance. Their outstanding properties are: high compressive strength and high rigidity, high melting point, satisfactory mechanical properties at elevated temperatures, resistance to thermal shocks, good corrosion resistance and high thermal and electrical conductivity. These materials are most commonly used for the production of cutting tools for metal and stone processing, and tools for the oil and gas drilling industry. Cutting tools made from cemented carbides generally have better properties than high-speed steel ones [1]. With the advancement of industrial methods of consolidation and production of powders, small grain powders are used on a larger scale, which has led to the development of ultrafine (0.2 - 0.5 μm) and nanostructured (<0.2 μm) cemented carbides, Figure 1.

Figure 1. SEM images of WC-Co cemented carbides: (a) submicron and (b) nano-grained WC-Co cemented carbide [2]

Nanoparticle sized powders can nowadays be consolidated into homogeneous microstructures of extremely high strength, hardness and satisfactory fracture toughness, Figure 2.





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a) b)

Figure 2. (a) Hardness and (b) fracture toughness as a function of the Co content for various WC grain sizes [3]

This allows longer service life of a cutting tool, application at higher cutting speeds and less tolerances of a machined part [4, 5]. Fracture toughness (KIC) is a property that describes the ability of the material to endure the occurrence and spreading of a crack [6]. The properties of these metals are also largely determined by the amount of Co-binder, peculiarly when it comes to mechanical properties such as hardness and fracture toughness. MATERIALS AND METHODS The tests were conducted on cemented carbide samples consolidated by sinter / HIP process as shown in Figure 3. The pre-compacted mixture of cobalt and tungsten carbide powders was sintered in vacuum and pressed by hot isostatic procedure in a protective atmosphere of inert gas (argon). Using this process, samples with 5% Co (WC-5Co), 10% Co (WC-10Co) and 15% Co (WC-15Co) were sintered.





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Figure 3. Sinter/HIP cycle Hardness and fracture toughness of nanostructured cemented carbides requires a carefully prepared test surface to minimize the effect of residual stresses affecting the results [7, 8]. For this reason samples were subjected to metallographic preparation through several grinding and polishing stages:

1. grinding with diamond-abrasive MD-Piano 120 with water cooling, 2. fine grinding with MD-Allegro with a 9 μm granular diamond paste with coolant

lubricant (alcohol and water), 3. fine grinding with MD-Largo with 3 μm granular diamond paste with cooling with

lubricant, 4. polishing with MD-Dac 3 micron granulated diamond paste with coolant lubricant, 5. finial polishing with MD-Chem tile with colloidal paint.

Figure 4. Samples of cemented carbides

WC-5Co WC-10Co WC-15Co

Tem

per

atu

re, ˚

C

Time, h

Pre

ssu

re, m

bar

1350˚C





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Hardness and fracture toughness tests were performed on metalograficly prepared samples shown in Figure 4. Hardness was measured by Vickers method at reference hardness tester (manufactured by Indentec, United Kingdom, type: 5030 TKV) with a load of 294.20 N (HV30). On each sample, a total of 15 measurements were made. The hardness values for Viceks methods were calculated by the following equation:

𝐻𝑉 = 0,1891 ∙𝐹

𝑑𝑠𝑟2 (1)

where:

dsr – mean value of the indentation imprint diagonal, 𝑑𝑠𝑟 =𝑑1+𝑑2

2 [mm],

F – indentation force [N]. Fracture toughness was determined by the indentation technique method. Compared to classical test methods this method has a number of advantages, such as simple and fast measurement that does not require sophisticated measurements of cracks, small sample dimensions, and minimum sample preparation at a low price [9, 10]. Indentation technique is based on measuring the length of cracks extending from the tip of the Vickers pyramid imprint, Figure 5.

Figure 5. Vickers indentation and cracks occuring in cemented carbide materials Nowadays, various mathematical models that describe Vickers indentation fracture toughness, such as the Anstis, Niihari, Casellas or Palmqvist models [11] are known. This paper uses Palmqvist's method of calculating fracture toughness which in the calculation includes the values of hardness, indentation force and the length of the cracks originating from the tip of the Vickers imprint. The crack lengths 𝑙1, 𝑙2, 𝑙3, 𝑙4, were measured using a

l1

l2

l3

l4





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metallographic inverted Olympus GX51F-5 microscope with an integrated DP25 digital camera and an associated image analysis program. The toughness of Palmqvist was determined from the equation [12]:

𝑊𝐾 = 𝐴 ∙ 𝐻𝑉 ∙ 𝑊𝐺 (2)

where: 𝑊𝐾 [MPa m1/2] – fracture toughness, 𝐴 – constant (0,0028), 𝐻𝑉 – Vickers hardness, 𝑊𝐺 [N/mm] – load and crack length ratio T (T = l1 + l2 + l3 +l4). Fracture toughness tests were performed with 15 repetitions on each sample. RESULTS AND DISCUSSION Figure 6 shows the imprints of the Vickers pyramid embedded in the surface of the nanostructured cemented carbide WC-5Co, WC-10Co and WC-15Co after testing. Vickers pyramid diagonals are marked with the yellow colour, and the red colour shows the length of the cracks that extend from the tip of the imprint.

a) b) c)

Figure 6. The Optical micrograph of the Vickers indentation and cracks on the sample:

(a) WC-5Co,(b) WC-10Co,(c) WC-15Co





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Table 1 shows the mean hardness value from 15 measurements together with the measurement uncertainty and standard deviation. The measurement uncertainty is expressed as the half of the maximum and minimum deviation range.

Table 1. Hardness measurement results

Sample Hardness mean

value, HV Measurement uncertenty, HV

Standard deviation, HV

WC-5Co 2263 28.0 15.2

WC-10Co 2014 25.6 12.3

WC-15Co 1779 12.6 6.0

It is visible that the hardness values are significantly decreasing with the increase in cobalt content. The difference in hardness of the sample with the 5% and 15% Co is around 500 HV. The dissipation of the hardness measurement results is very small which indicates a relatively homogeneous microstructure of the tested samples. The average hardness value as a cobalt share function is graphically shown in Figure 7.

Figure 7. Hardness as a function of the Co content

The regression curve can be described by the exponential equation Y=1.52·X2-80.04·X+26438 with a high coefficient of correlation (R2 = 0.9982). If the curve was theoretically prolonged to 0% Co, the result would be the hardness of a WC consistent with

Har

dn

ess,

HV

% Co by weight





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the literature data [13]. By comparing the measured hardness values shown in Figure 2a, the investigated samples are harder than the extra-fine and ultra-fine cemented carbides, which goes in favour of the results carried out earlier that indicate a nanostructured material. Table 2 contains the mean values of the fracture toughness with the associated measurement uncertainty and standard deviations for all three samples.

Table 2. Fracture toughness measurement results

Sample Fracture toughness mean

value, MPa·m1/2

Measurement uncertainty,

MPa·m1/2

Standard deviation, MPa·m1/2

WC-5Co 8.39 0.15 0.10

WC-10Co 9.03 0.16 0.07

WC-15Co 9.35 0.15 0.09

Results clearly show that the values of fracture toughness increase with increasing cobalt content. The dependence of the fracture toughness and cobalt content is shown in Figure 8.

Figure 8. Fracture toughness as a function of the Co content

The obtained regression curve with high correlation coefficient (R2 = 0.999) shows the trend of fracture toughness dependence on the cobalt content in a cemented carbide from 5% to

% Co by weight

Frac

ture

to

ugh

nes

s, M

Pa·

m1

/2





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15% cobalt. Regression equation can be successfully applied to the lower contents of Co since the starting point of the curve point is 7.63 which corresponds to the fracture toughness of the WC which, according to the available literature, is approximately 7.6 MPa·m1/2 [13]. When comparing the fracture toughness of extra-fine and ultra-fine cemented carbides with the same Co content (see Figure 2b), the existence of extremely small nano-sized grain is confirmed.

CONCLUSION Cemented carbide consolidation by sinter / HIP process has proven to be very successful in the production of nanostructured WC-Co composites of exceptional mechanical properties. Thanks to the simultaneous application of high pressure at high temperature, sintered cemented carbides have much better properties than those obtained from classic compacting and subsequent sintering processes. All the results obtained on the composite WC-Co system indicate the importance of the Co binder, which means that the matrix share significantly influences the hardness and fracture toughness of the WC-Co material. The test results showed that the hardness of the test material was significantly reduced by the increase in cobalt content. With 1% of cobalt percentage reduction, the reduction in hardness is approximately 50 HV. As opposed to that at higher Co contents the fracture toughness increases, and this is more visible at lower Co amount, while at a higher contents the trend the milder. Based on the above it can be concluded that the hardness and fracture toughness properties of the tested samples are inversely proportional, and harder WC-Co materials are more likely to form cracks that are will unstably spread in the surface. WC-Co systems with more cobalt content are tougher and thus more resistant to initiation of cracks, but have a lower hardness. Since cemented carbide cutting tools often require high hardness and high fraction toughness, it is necessary to find a compromise between these two properties and choose the appropriate cobalt content that provides the optimal combination of toughness and hardness for a particular application. Alongside the chemical composition, cemented carbide properties are largely determined by the size of the WC grain. From the results presented and comparing with already known hardness and fracture toughness of ultra-fine and extra-fine cemented carbides, it can be concluded that the investigated materials are nanostructured materials. This is also confirmed by the results of earlier metallographic investigations. The nano size of the grain in these samples provides high hardness, higher than that of large grained cemented carbide. High hardness with good fracture toughness are two dominant features of nanostructured cemented carbides especially when it comes to their application for cutting tools because they allow for faster, cheaper and higher quality production.





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REFERENCES [1] D. Ćorid, Special Metallic Materials - Part III, textbook, Faculty of Mechanical

Engineering and Naval Architecture University of Zagreb, Zagreb, 2017. [2] M. Brieseck, W. Lengauer, B. Gneiß, K. Wagner, S. Wagner, A straightforward method

for analysing the grain-size distribution in tungsten carbide - cobalt hardmetals, Microchimica Acta, 168 (2010) 3-4, pp. 309-316.

[3] Sandvik Hard Materials, Understanding Cemented Carbide, Available online: http://allaboutmetallurgy.com/wp/wp- content/uploads/2016/12/UnderstandCementedCarbide.pdf (Downloads 12.03. 2018.)

[4] V. Bonache, M. D. Salvador, D. Busquets, P. Burguete, E. Martinez, F. Sapina, E. Sanchez, Synthesis and processing of nanocrystalline tungsten carbide: Toward cemented carbides with optimal mechanical properties, Int. J. Refract. Met. Hard Mater., 29 (2011) pp. 78-84.

[5] T. Aleksandrov Fabijanid, Ž. Alar, D. Ćorid, Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near-nano and nanostructured WC-Co Cemented Carbides, Int. J. Refract. Met. Hard Mater., 54 (2015) pp. 82-89.

[6] D. Ćorid, Ž. Alar, Selected chapters from the mechanical properties of the material, textbook, Faculty of Mechanical Engineering and Naval Architecture University of Zagreb, Zagreb, 2016.

[7] D. K. Shetty, I. G. Wright, P. N. Mincer, A. H. Clauer, Indentation fracture toughness of WC-Co cermets, J. Mater. Sci., 20 (1985) pp. 1873-1882.

[8] M. W. Barsoum, Series in Material Science and Engineering Fundamentals of Ceramics, Taylor & Francis: Abingdon, UK, 2003.

[9] F. Sergejev, M. Antonov, Comparative study on indentation fracture toughness measurements on cemented carbides, Proc. Estonian Acad., Sci. Eng., 12 (2006) pp. 388-398.

[10] S. Sheikh, R. M’Saoubi, P. Flasar, M. Schwind, T. Persson, J. Yang, L. Llanes, Fracture toughness of cemented carbides: Testing method and microstructural effect, Int. J. Refract. Met. Hard Mater., 49 (2015) pp. 153-160.

[11] G. D. Quinn, Fracture toughness of ceramics by the Vickers indentation crack lenght method: A critical review, National Institute of Standards and Technology, Gaithersburg, 2006.

[12] International Standard Organisation. ISO 28079:2009: Hardmetals-Palmqvist Toughness Test. Available online: https://www.iso.org/obp/ui/#iso:std:iso:28079:ed-1:v1:en (Downloads 15.12.2017).

[13] R. W. Armstrong, The Hardness and Strength Properties of WC-Co Composites, Materials, 4 (2011) pp. 1287-1308.

https://link.springer.com/journal/604

http://allaboutmetallurgy.com/wp/wp-%20content/uploads/2016/12/UnderstandCementedCarbide.pdf

http://allaboutmetallurgy.com/wp/wp-%20content/uploads/2016/12/UnderstandCementedCarbide.pdf

https://www.iso.org/obp/ui/#iso:std:iso:28079:ed-1:v1:en

https://www.iso.org/obp/ui/#iso:std:iso:28079:ed-1:v1:en

hardness and fracture toughness of a cemented …€¦ · hardness and fracture toughness tests...

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