Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano- and nano-structured WC-Co cemented carbides

doi:10.1016/j.ijrmhm.2015.07.017

International Journal of Refractory Metals and Hard Materials

Volume 54, January 2016, Pages 82-89

$International Journal of Refractory Metals and Hard Materials$

https://doi.org/10.1016/j.ijrmhm.2015.07.017 Get rights and content

Highlights

•
Two powder metallurgy processes; sintering in hydrogen and sinter-HIP were used.
•
Consolidation process, sintering temperature and GGIs influenced the properties.
•
Cr₃C₂ reduced a carbide grain growth, increased hardness and fracture toughness.
•
Relationship between Vickers hardness and fracture toughness is not linear.
•
Larger amounts of GGIs are required in case of sintering in hydrogen atmosphere.

Abstract

In this paper the influence of the consolidation process and sintering temperature on the properties of near nano- and nano-structured cemented carbides was researched. Samples were consolidated from a WC 9-Co mixture by two different powder metallurgy processes; conventional sintering in hydrogen and the sinter-HIP process. Two WC powders with different grain growth inhibitors were selected for the research. Both WC powders used were near nanoscaled and had a grain size of 150 nm and a specific surface area of 2.5 m²/g. Special emphasis was placed on microstructure and mechanical properties; hardness and fracture toughness of sintered samples. Consolidated samples are characterised by different microstructural and mechanical properties with respect to the sintering temperature, the consolidation process used and grain growth inhibitors in starting powders. Increasing sintering temperature leads to microstructure irregularities and inferior hardness, especially for samples sintered in hydrogen. The addition of Cr₃C₂ in the starting powder reduced a carbide grain growth during sintering, improved microstructural characteristics, increased Vickers hardness and fracture toughness. The relationship between hardness and fracture toughness is not linear. Palmqvist toughness does not change with regard to sintering temperature or the change of Vickers hardness.

Introduction

Nanostructured cemented carbides are the most researched powder metallurgy materials but their potential applications have not yet been defined. The range of applications in which nanostructured cemented carbides are used grows rapidly [1]. Nanostructured cemented carbides are characterised by a unique combination of a very fine grained homogenous microstructure and good mechanical properties, which makes them the best choice for many areas of application. Mechanical properties are directly dependent on the developed microstructure in the sintered parts, which is governed by several factors such as WC crystallite size, mean free path of the binding phase, and the contiguity of WC grains [2]. Improvement of hardness and toughness of cemented carbides can be achieved with a decrease in WC grain size to nanoscale, which has industrial importance and a significant influence on the duration and robustness of tools manufactured from cemented carbides [3], [4]. Therefore, considerable efforts have been made over the past years to research nanostructured cemented carbides and to draw certain conclusions about hardness and toughness behaviour. While in conventional cemented carbides fracture toughness decreases with increasing hardness values, the increase of hardness in nanostructured cemented carbides does not decrease their bulk fracture toughness [3]. According to published literature, there is no reduction in fracture toughness versus hardness level when the grain size reaches nanoscale [2] which is the result of a very homogeneous microstructure without grain growth and homogeneous distribution of cobalt between carbide grains.

Nanostructured cemented carbides are produced from near nano- and nanosized WC starting powders. One of the biggest problems of sintering near nano- and nanoscaled powders is the retaining of a small average grain size in the sintered product [5]. Many attempts to achieve nanostructured cemented carbides failed because of very high sintering activity of WC nanopowders. For that reason the addition of grain growth inhibitors, GGIs, is required [6]. Small amounts of GGIs are added to starting powders. The most common ones are vanadium carbide, VC, chromium carbide, Cr₃C₂, tantalum carbide, TaC, titanium carbide, TiC and niobium carbide, NbC. Their primary effect is to retain the particle size of starting powders in the sintered product, meanwhile, not only influencing the mechanical properties; increasing the value of hardness at room temperature, but also affecting the toughness, hardness and creep resistance at elevated temperatures. The combination of VC-Cr₃C₂ (TaC) results in an optimal combination of hardness and toughness [7]. Grain growth inhibitors are dissolved in a Co matrix, thereby forming a solid solution with melting temperature in the range of 1200–1250 °C. The solid solution with a lower melting temperature is saturated with grain growth inhibitors and therefore the dissolution of W and C in the matrix is prevented and grain growth is minimized. In order to obtain nanostructured cemented carbides, nano- and near nanopowders with very low sintering activity with respect to re-crystallisation during liquid phase sintering were developed over the past decade, with positive influences [8].

The number of consolidation technique processes is growing rapidly with the rapid development of technology. Also, the consolidation process is becoming more demanding due to high requirements placed on sintered near nano- and nanostructured cemented carbides. During sintering, densification occurs very rapidly due to a large interfacial area between WC and Co [9]. Research showed that the consolidation of WC-10%Co with theoretical density and a grain size of 200 nm can be achieved in only 30 s at sintering temperature of 1400 °C. The densification of nanostructured cemented carbides can be achieved in 15 min if grain growth inhibitors are added [10].

In this paper the study of microstructural characteristics and mechanical properties of nanostructured cemented carbides developed by two different powder metallurgy processes is described. Influences of sintering temperature and grain growth inhibitors on the properties of sintered samples have been analysed.

Section snippets

Material and methods

WC powders used were newly developed tungsten carbide near nanopowders WC DN 2-5 (H.C. Starck). The powders have an average grain size of d_BET = 150 nm and a specific surface area (BET) of 2.5 m²/g. The selected WC powders had an addition of GGIs. As can be seen from Table 1, WC powders with different grain growth inhibitors were selected for research purposes. According to the literature, it is considered that cobalt particles in nanostructured WC/Co mixtures should be much finer because their

Results and analysis

The characteristics of the polished surface of consolidated samples are presented in Table 3.

The degree of porosity of the samples consolidated by sintering in hydrogen is the same for all samples; mostly A02, partially A < 02, mostly B00, partially B02, without uncombined carbon or η-phase. Furthermore, the presence of small cracks was noted on the surface of some samples. The cracks are probably the result of one of the following technological operations; milling, waxing or granulation.

Discussion

The influence of the sintering temperature on the size of the carbide grains of consolidated samples is presented graphically in Fig. 6.

From Fig. 6 it can be seen that the size of WC grains increases with increased sintering temperatures for all mixtures and processes. The largest increase was recorded for mixture SV 2 consolidated by sintering in hydrogen. At the lowest sintering temperature the measured WC grain size is 0.243 μm, which is an increase of nearly 0.1 μm compared to the starting

Conclusion

The conducted research shows that the consolidation process, sintering temperature and characteristics of starting powders (amount and type of GGIs) influence the properties of near nano- and nano-structured cemented carbides. From conducted research it was found:

(i)
The lowest sintering temperature results in a homogenous microstructure and the best mechanical properties. Increasing the sintering temperatures by only 20 °C leads to microstructural irregularities and hardness decrease, especially

References (12)

T. Aleksandrov Fabijanić et al.
Potentials of nanostructured WC-Co cemented carbide as reference material for Vickers hardness
(May 2015)
N. Al-Aqeeli et al.
The synthesis of nanostructured WC-based cemented carbides using mechanical alloying and their direct consolidation
J. Nanomater.
(2014)
K. Jia et al.
Microstructure, hardness and toughness of nanostructured and conventional WC-Co composites
Nanostruct. Mater.
(1998)
Z. Zak Fang et al.
Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide — a review
Int. J. Refract. Met. Hard Mater.
(2008)
V. Richter et al.
Nanoscaled cemented carbides — fiction or reality?
J. Weidow et al.
Effect of V, Cr and Mn additions on the microstructure of WC-Co
(2009)

There are more references available in the full text version of this article.

Cited by (50)

Study on temperature uniformity of workpieces in multi-layer trays inside vacuum sintering furnace for cemented carbide
2024, International Journal of Thermal Sciences
Numerical simulation has been widely used recently to study temperature field characteristics in sintering furnaces, compensating for the limitations of traditional experimental methods in obtaining temperature field data in high-temperature sintering furnaces. We established a numerical model of the transient temperature field in a vacuum sintering furnace of cemented carbide. Numerical simulation of the transient temperature field of the vacuum heating sintering process is carried out for cemented carbide, and the transient temperature fields at different times in the furnace are obtained. By comparing with the measured data, the maximum error is within 3 %. On this basis, the uniformity of temperature within the different tray layers of the cemented carbide workpieces during the sintering process and at the end moment of sintering is further discussed. The results of the study show that the insulation stage in the sintering process can effectively improve the uniformity of the temperature of the workpieces. The temperature uniformity of the workpieces located in the middle tray layers is significantly worse than that of the workpieces in the upper and lower tray layers. Assuming the middle tray layer as the symmetrical plane, the average, maximum, minimum and coefficient of variation of the temperature of the workpieces among the trays are basically symmetrically distributed. This study provides a reference for improving the local low-temperature and high-temperature zones of the workpieces in the vacuum sintering furnace for cemented carbide.
Ultrafine/nano WC-Co cemented carbide: Overview of preparation and key technologies
2023, Journal of Materials Research and Technology
Ultrafine/nano WC–Co cemented carbide is widely used because of its high hardness, strength, toughness, and other comprehensive properties. However, WC grain growth during preparation is a persistent issue that hinders the development of this type of cemented carbide. In this context, we review current research on the preparation of ultrafine/nano WC–Co composite powder and sintering methods of ultrafine/nano cemented carbide. The correlation between grain growth and densification during sintering is analyzed. The key technologies inhibiting WC grain growth are explored and discussed, and the synergism between transition metal carbides and rare-earth element mixtures is found to effectively inhibit the growth of WC grains and improve the general performance of ultrafine/nano WC–Co cemented carbide. Finally, future research directions and development trends of ultrafine/nano WC–Co cemented carbides are presented.
Novel binder for NbC-based cemented carbides prepared by spark plasma sintering
2022, International Journal of Refractory Metals and Hard Materials
The most advanced alternative to replace cobalt in cemented carbides is alloys based on the Fe-Ni-Co system due to their similar or superior properties to conventional Co-binder. However, recently, successful cermet alloys with Fe-Ni-Nb as a binder have been reported. In this work, cemented carbides NbC-10% wt Fe-Ni-Nb were prepared by solid state pulsed electrical current sintering (PECS), also known as spark plasma sintering (SPS), for 5 min, 40 MPa at temperatures 1280°C, 1300°C and 1350°C. The samples produced were investigated focusing on their structure, mechanical and thermal properties. All samples showed an increase in NbC crystallite size and n-phase formation. The processed NbC-FeNiNb cermet presented good densification and hardness of approximately 1726 HV30, fracture toughness 12.5 ± 0.1 MPa.m^1/2, Young's modulus 385 ± 4 GPa, which is shown as a viable alternative composite in applications cemented carbide such as cutting tools. However, these cermets could have even better properties, eliminating the intermetallic phase present in the matrix and decreasing its particle size. TG and DSC analysis confirmed the formation of a liquid phase and more carbides contributing to the mechanical properties of the composite. The thermal properties showed that the thermal diffusivity and thermal conductivity for NbC-FeNiNb was lower than for WC-Co carbide.
Microstructure and properties of WC-CoCuFeNi composites fabricated by spark plasma sintering
2022, International Journal of Refractory Metals and Hard Materials
In this work, a novel low-cost WC-CoCuFeNi cemented carbide has been produced by spark plasma sintering technology. The CoCuFeNi medium-entropy alloy binder has a face center cubic structure with more slip systems and shows good bonding behavior with WC. The newly-developed cemented carbides exhibit a good combination of hardness and fracture toughness. The hardness and fracture toughness of WC-20 wt% CoCuFeNi are up to 1506.7 HV₃₀ and 10.33 MPa·m^1/2, respectively. These results show the WC-CoCuFeNi cemented carbides could be a promising substitute for WC-Co.
Effects of Nb and NbC additives on microstructure and properties of WC-Co-Ni cemented carbides
2022, International Journal of Refractory Metals and Hard Materials
Cemented carbides (WC-Co-Ni) with the addition of Nb and NbC respectively were prepared by the conventional powder metallurgy method. The microstructure and mechanical properties of the sintered samples were analyzed. The results indicated that the addition of Nb or NbC reduced the grain size of WC and Nb had a greater inhibition effect on WC grain growth than NbC. The transverse fracture strength of the samples first increased and then decreased with the increase of the amount of Nb and NbC, and reached 3120 MPa and 2850 MPa respectively when adding 1.0 wt% Nb and NbC. The hardness of cemented carbides showed an upward trend as the content of additives increased. The fracture toughness decreased with the increases of NbC and Nb contents respectively and the fracture toughness of NbC-added samples was lower than that of Nb-added samples. The impact toughness decreased in a mass with the increase of additives.
Refined WC grain size and improved mechanical properties in a hardmetal WC-8Co processed via short-time semi-solid hot pressing
2022, Journal of Alloys and Compounds
Citation Excerpt :
Porosity is reduced by rotation and rearrangement of WC powders and simultaneously by filling of the liquid phase into the remaining pores. In order to achieve full densification, a higher temperature for the sufficient amount of liquid phase and a relatively long holding time (usually a few hours) are required [8–10], resulting in notable WC grain growth relative to WC grain sizes in powders [11,12]. To prohibit WC grain growth, grain-growth inhibitors, e.g., VC, Cr3C2, Mo2C, TaC, etc., are introduced [13–17].
To minimize WC grain growth and avoid WC-WC non-metallurgical contacts in WC-Co hardmetals, we modified conventional solid-state hot pressing into short-time semi-solid hot pressing (STSSHP) for consolidation of WC-Co powders in the present study. STSSHP can produce fully dense WC-Co hardmetals free of WC-WC non-metallurgical contacts at semi-solid temperatures lower than the ideal liquid-phase sintering temperatures within several to several ten seconds, thus minimizing WC grain growth. Herein WC-8 wt%Co processed by STSSHP at 1340 ℃ (the WC-Co eutectic temperature) for only ~15 s achieved a relative density of 99.92% without WC-WC non-metallurgical contacts. Only slight WC grain growth occurred during STSSHP relative to grain sizes in powders. The STSSHP-ed WC-8 wt%Co exhibited much smaller WC grains and thus higher hardness, fracture toughness and bend strength than those in its liquid-phase sintered counterparts. STSSHP provides a novel avenue to retain fine WC grains and hence to improve both hardness and ductility of WC-Co hardmetals.

View all citing articles on Scopus

View full text

Influence of consolidation process and sintering temperature on microstructure and mechanical properties of near nano- and nano-structured WC-Co cemented carbides

Highlights

Abstract

Introduction

Section snippets

Material and methods

Results and analysis

Discussion

Conclusion

Potentials of nanostructured WC-Co cemented carbide as reference material for Vickers hardness

The synthesis of nanostructured WC-based cemented carbides using mechanical alloying and their direct consolidation

J. Nanomater.

Microstructure, hardness and toughness of nanostructured and conventional WC-Co composites

Nanostruct. Mater.

Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide — a review

Int. J. Refract. Met. Hard Mater.

Nanoscaled cemented carbides — fiction or reality?

Effect of V, Cr and Mn additions on the microstructure of WC-Co