Applications of Nanomaterials in Cemented Carbides

The continuous refinement of WC grains is an important characteristic in the development of cemented carbides. This article discusses the applications of nanomaterials in carbides from four aspects: nanoscale raw materials for carbides, nanocrystalline carbides, nanomaterial-assisted or enhanced ultra-coarse grained carbides, and nanocoating materials for cemented carbides, with a focus on reporting China’s advantages in these areas.

Sandvik, the world’s largest manufacturer of carbides, defines carbides with an average WC grain size of 0.1~0.3μm as nanoseries carbides; while the British Cemented Carbide Association and a German standards organization define those with grains <200nm as nanocrystalline carbides, which has also become an international industry consensus. Of course, there is great controversy over the above definition of nanocrystalline carbides.

Germany has considered renaming “nano” or “near-nano” cemented carbides with an average WC grain size of 0.1~0.2μm as “extra-ultrafine” or “nanoscale” carbides. This article follows the common industry practice and refers to carbides with an average WC grain size <200nm as nanocrystalline carbides. China has taken a leading position in the R&D, production and application of one-dimensional tungsten nanomaterials and nanocrystalline cemented carbides.

Among the various technologies for preparing nanocrystalline cemented carbides, the most important issue is the preparation of nanoscale raw materials. Currently, the advanced technology for preparing nanoneedle violet tungsten, nano-tungsten powder and tungsten carbide powder is the “violet tungsten in-situ reduction technology” adopted by Xiamen Golden Egret Special Alloy Co., Ltd. (abbreviated as Xiamen Golden Egret, GESAC). This pioneering technology was developed in 1997, with the core being the preparation of nanoneedle violet tungsten by traditional processes, followed by in-situ reduction and in-situ carbonization.

Figure 1 is an FESEM image of nanoneedle violet tungsten produced by Xiamen Golden Egret, with the diameter of the violet tungsten nanoneedles ranging from 20 to 50nm. This nanoneedle violet tungsten has a huge specific surface area and Rayleigh instability. Under the action of high-temperature hydrogen, it undergoes in-situ rapid reduction to form beaded nano and ultra-fine tungsten single crystals.

The technology inhibits or weakens “chemical vapor transport”, thereby suppressing the grain growth of nano and ultra-fine tungsten powder; at the same time, using single-phase nano and ultra-fine tungsten powder and carbon black as raw materials, in-situ carbonization is carried out at high temperature to synchronously carbonize the raw materials into nano and ultra-fine tungsten carbide powder.

By adjusting the boat pushing speed, boat loading capacity, reduction (carbonization) temperature and hydrogen flow rate, the average particle size, particle size distribution and carbonization effect of the product can be adjusted.

The particle sizes of nano-tungsten powder and tungsten carbide powder produced by this technology can reach 19nm and 39nm, with BET specific surface areas of 16.05m²·g⁻¹ and 9.97m²·g⁻¹ respectively. This nano-tungsten carbide powder is equiaxed, with uniform particle size distribution, complete particle crystallization morphology and high phase purity, making it a high-quality raw material for manufacturing nanocrystalline and ultra-fine grained cemented carbides. The ultra-fine WC powder product developed alongside this technology has been sold worldwide since 1998, was awarded the National Key New Product, and currently holds a 50% share of the global market.

Another major challenge in nanocrystalline carbides is the abnormal grain growth caused by the high activity of nano-WC during high-temperature sintering. Therefore, even with nanoscale raw materials, it is difficult to prepare nanocrystalline cemented carbides. Using WC raw materials with a grain size of 10nm, the WC grains can grow rapidly to 0.9μm after sintering at 1400°C, a nearly 100-fold increase in particle size, as shown in Figure 2.

Relevant studies have reviewed more than a dozen technologies for sintering carbides using nano-tungsten carbide raw materials, resulting in either porous materials with a relative density of less than 99.9% (insufficient sintering) or grains coarsened into submicron and ultra-fine grained cemented carbides.

In the sintering process of carbides, the presence of the binder phase Co also promotes the rapid growth of nano-WC grains.

Figure 3 shows a comparison of grain growth between 10nm WC and 10nm WC-10%Co (mass fraction) during heating. In the solid-phase sintering stage, nano-WC in WC-10%Co already shows a tendency of rapid growth, while nano-WC without Co does not grow significantly. Since Co preferentially wets the (0001) crystal plane of WC, it will lead to the rearrangement and coalescence of WC grains.

Therefore, during sintering, Co must fully wrap all crystal planes of WC to prevent the rapid abnormal growth of WC grains. Relevant studies believe that in the sintering process of cemented carbides, for WC powder with a particle size of 0.87μm, the mass fraction of Co must be greater than 0.7% to avoid abnormal growth of WC grains; other studies have proven that for 70nm WC powder, the actual minimum filling mass fraction of Co is 12%.

Using 70nm tungsten carbide powder and spherical cobalt powder produced by Xiamen Golden Egret, combined with vacuum sintering and hot isostatic pressing (HIP) treatment, nanocrystalline carbides with a cobalt mass fraction of 12% and an intercept grain size of 130nm have been successfully prepared. The hardness reaches 2002HV30, and the maximum strength exceeds 4500MPa. This alloy has been successfully used in the production of PCB tools, and cemented carbides with finer grains are currently in the laboratory development stage.

Figure 4 shows the FESEM image and flexural strength distribution of this 130nm nanocrystalline cemented carbide. In Figure 4, the WC grains are small with a narrow particle size distribution, and there are no abnormally grown grains; the material strength is stable, with an average of 4200MPa.

In 2013, German researcher Richter also prepared nanocrystalline carbides with a grain size of 100~200nm using tungsten carbide powder from HCStark and low-pressure sintering technology (SinterHIP). The flexural strength exceeds 5000MPa, the hardness is greater than 2000HV10, and the hardness of grades with low Co content is close to 2900HV10.

Figure 5 shows the test results of milling Nimonic 80A (NiCr20TiAl) nickel alloy with nanocrystalline carbide end mills. It can be seen from Figure 5 that compared with submicron and ultra-fine grained carbides, nanocrystalline cemented carbide tools achieve the best milling results both in finish machining and rough machining.

The application of nanomaterials in ultra-coarse grained carbides is mainly realized through the “nanoparticle dissolution method”. Specifically, the overall ultra-coarsening of WC grains in carbides is completed through the dissolution-precipitation process during liquid-phase sintering.

According to the Thomson-Freundlich equation, the solubility of fine tungsten carbide powder is significantly higher than that of coarse tungsten carbide powder, and the greater the particle size difference, the larger the solubility difference.

The preparation of ultra-coarse grained cemented carbides by adding nano-tungsten carbide powder to the mixture utilizes this principle; during the liquid-phase sintering stage of carbides, nano-tungsten carbide powder will first dissolve into the Co binder phase.

The higher the addition amount, the higher the supersaturation degree of W and C atoms in the liquid Co, which can better inhibit the dissolution of coarse tungsten carbide powder and promote the growth of coarse WC grains through re-precipitation. The ultra-coarse grained cemented carbides prepared by this method can have a grain size of up to 12μm with a uniform grain size distribution.

(Figure 6 is a metallographic image of ultra-coarse grained carbide prepared by the National Engineering Research Center for Tungsten Materials). Among them, the 12μm WC-10%Co (mass fraction) ultra-coarse grained cemented carbide has a fracture toughness of up to 27.7MPa·m¹/² and exhibits obvious plastic deformation behavior during compression.

In 2005, German researcher Konyashin et al. reported an ultra-coarse grained carbide named “MASTER GRADES”, which consists of round WC grains and a Co binder phase containing nano η-phase (Co₃W₃C) grains.

Figure 7 shows the HRTEM image and electron diffraction pattern of nano Co₃W₃C grains in the fcc-Co phase of MASTER GRADES alloy, indicating that the diameter of Co₃W₃C grains is about 2~3nm and their crystal lattice has a good match with fcc-Co. Figure 8 shows the appearance photos of the alloy product before and after use.

It can be seen from Figure 8 that under the same service conditions and time, compared with ordinary ultra-coarse grained alloy products, this nano-enhanced alloy has significantly less wear and its service life is increased by 2~3 times. Since its launch, it has received high praise and is regarded as a major breakthrough. However, this type of carbide for mining and engineering applications has not been widely promoted in the international market.

The nanonization of coating materials is a development trend of carbide tools. Different cutting conditions have different requirements for coating performance. Multilayer composite coatings utilize combinations of different coatings to better exert the superior performance of each coating.

Currently, coating technology has developed from single-layer coatings to multilayer composite coatings, even up to thousands of layers, with a single-layer thickness reaching the nanoscale.

For example, the AC105G series tools developed by Sumitomo Electric feature a TiN/AIN nanocomposite coating with 2000 layers, each layer about 1nm thick; when coating materials are nanonized, the coating surface roughness decreases while the hardness increases.

For instance, when the TiAlN crystal size is less than 10nm, dislocation sources are difficult to initiate in the nanocrystalline structure, and the amorphous phase can prevent the migration of crystal dislocations.

Even under high stress, dislocations cannot cross the amorphous grain boundaries. The Vickers hardness of this coating can reach 5000kgf·mm⁻², the oxidation resistance temperature is above 800°C, and the elastic modulus can reach 500GPa; the nanocomposite film composed of AlTiN grains and amorphous Si₃N₄ nanocomponents also has a hardness of up to 45GPa, and the film’s stability and oxidation resistance can reach 1000°C; in addition, nanocrystalline coatings and nano-multilayer coatings not only have improved hardness and wear resistance but also possess crack propagation resistance, which further improves the service life of carbide tools.

Figure 9 is a schematic diagram of crack propagation in three types of diamond coatings produced by CemeCon. It can be seen from Figure 9 that compared with polycrystalline diamond coatings, the crack propagation path in nanocrystalline diamond coatings is significantly tortuous and longer; when cracks cross the interface of multilayer diamond coatings, they will deflect or even stop propagating.

Nanoscale materials have been widely used in the production and application of carbides. China has taken a leading position in related fields, with technologies represented by the “violet tungsten in-situ reduction technology” leading the development of the industry. With the advancement of nanotechnology, the performance of nanocrystalline cemented carbides, nanocomposite coatings and related products will continue to be optimized and upgraded.