Engineered surfaces on cemented carbides obtained by tailored sintering techniques

Engineered surfaces on cemented carbides obtained by tailored sintering techniques

Surface & Coatings Technology 258 (2014) 300–309 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsev...

3MB Sizes 0 Downloads 58 Views

Surface & Coatings Technology 258 (2014) 300–309

Contents lists available at ScienceDirect

Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat

Engineered surfaces on cemented carbides obtained by tailored sintering techniques I. Konyashin a,b,⁎, B. Ries a, S. Hlawatschek a a b

Element Six GmbH, Städeweg 12–24, 36151 Burghaun, Germany National University of Science and Technology MISIS, Leninsky Prosp., 4, Moscow 119049, Russia

a r t i c l e

i n f o

Article history: Received 10 June 2014 Accepted in revised form 6 September 2014 Available online 16 September 2014 Keywords: Engineered surfaces Cemented carbides Hardness Wear-resistance Sintering

a b s t r a c t The paper consists of three parts. In the first part, functionally graded cemented carbides comprising hard surface layers with low Co contents are described. Such surface layers form as a result of a tailored sintering technique based on the selective carburization of the near-surface layer of carbide green bodies with original low carbon contents and consequent liquid-phase sintering, which leads to Co drifts from the surface towards the core during sintering. This novel sintering technique allows obtaining Co gradients between the surface layer and the core of carbide articles of up 7 wt.%. As a result of significantly different contraction rates between the near-surface layer and the core of such functionally gradient cemented carbides, high residual compression stresses are created in the carbide near-surface layer. This leads to a dramatically increased combination of both hardness and fracture toughness of the near-surface layer. The presence of the hard and tough surface layers on the novel functionally gradient carbides results in their significantly improved wear-resistance and prolonged tool lifetime in mining applications. In the second part of the paper, functionally graded cemented carbides comprising surface layers with high Co contents are described. They are fabricated by a tailored sintering technique based on the selective de-carburization of the near-surface layer of carbide green bodies with original high carbon contents resulting in Co drifts from the core towards the surface during the consequent stage of liquid-phase sintering. Such functionally graded cemented carbides are successfully employed as substrates for poly-crystalline diamond (PCD) layers. In the third part of the paper, a mechanism of formation of thin Co films on the surface of carbide articles during sintering, which is designated in literature as “Co capping”, is briefly described. On the basis of understanding the mechanism of the Co capping phenomenon, an industrial technology for obtaining such thin Co films on the surface of carbide articles during sintering was developed. The wettability of non-ground carbide articles by braze alloys is dramatically improved by use of the thin Co films, resulting in better quality of various brazed wear parts and tools. © 2014 Elsevier B.V. All rights reserved.

1. Introduction There is a general trend in the cemented carbide industry to employ approaches of surface engineering for improving properties of WC-Co cemented carbides. Besides conventional CVD and PVD coatings, functionally graded cemented carbides comprising engineered surface layers obtained by tailored sintering techniques are being implemented in the carbide industry on a large scale. In many cases, such functionally graded cemented carbides are characterized by a high combination of different properties, such as hardness, wear-resistance, fracture toughness and transverse rupture strength (TRS), which cannot be achieved in conventional bulk WC-Co materials.

⁎ Corresponding author at: Element Six GmbH, Städeweg 12–24, 36151 Burghaun, Germany. Tel.: +49 6652 82412; fax: +49 6652 82390. E-mail address: [email protected] (I. Konyashin).

http://dx.doi.org/10.1016/j.surfcoat.2014.09.009 0257-8972/© 2014 Elsevier B.V. All rights reserved.

The fabrication of functionally graded or functionally gradient WC-Co materials not comprising the η-phase and free carbon has been an objective of intensive research in the last several years [1–3]. First attempts to produce functionally graded cemented carbides were based on pressing carbide articles from two different grade WC-Co powders (see e.g. [4]). However, this approach had a limited practical importance, as Co drifted very fast from a part with a higher Co content into a part with a lower Co content during liquid phase sintering resulting in the homogenization of the Co contents. The formation of significant Co gradients in WC-Co articles can be achieved by carbon gradients within the articles. Fischer at al. disclosed a technology based on the carburization of carbide articles with original low carbon contents during liquid-phase sintering [5,6]. This technology allows the fabrication of functionally graded carbides with a low Co content and consequently high hardness in the near-surface layer. A significant disadvantage of this technology is, however, the presence of a core comprising the brittle η-phase in the carbide articles.

I. Konyashin et al. / Surface & Coatings Technology 258 (2014) 300–309

Another technology for the fabrication of WC-Co articles comprising surface layers with a low cobalt content and consequently high hardness but not comprising the η-phase was developed by Guo, Fan, Fang et al. and is described in refs. [2,7,8]. According to this technology WC-Co articles are subjected to the carburization at temperatures, at which WC, liquid cobalt and solid Co coexist according to the W-Co-C phase diagram. However, this technology allows formation of relatively thin near-surface layers with high hardness, the thickness of which usually does not exceed nearly 0.5 mm. Fan, Fang and Guo also developed a modified technology (two-step method) allowing one to produce thick gradient layers of several millimeters in thickness [9,10]. The formation of Co gradients in WC-Co materials consisting of layers with high carbon contents and low carbon contents comprising the η-phase was examined in details (see e.g. [11–14]). However, in all the published works the layers with different carbon contents were produced from WC powders of the same original mean grain size. It is well know that in this case the layer with the low carbon content is characterized by a significantly higher value of coercive force and consequently lower WC mean grain size compared to the layer with the high carbon content [15]. Therefore, besides the various carbon contents, the

301

difference in WC mean grain sizes also plays an important role in the formation of the Co gradients in this case. Recently we elaborated a novel technology for the fabrication of functionally graded or functionally gradient WC-Co cemented carbides [1,16–18]. It is based on (1) the fabrication of WC-Co green bodies with either a low carbon content or a high carbon content, (2) controlled solid-state pre-sintering of the green bodies to obtain a certain gas permeability of their near-surface layer, (3) the carburization of the green bodies with the low carbon content or the de-carburization of the green bodies with the high carbon to a certain depth in the solid state, and (4) the final liquid-phase sintering of the selectively carburized or de-carburized green bodies to obtain Co drifts either from the surface towards the core of the carburized articles or from the core towards the surface of the de-carburized articles. As a result, functionally gradient cemented carbides comprising surface layers with either low Co contents or high Co contents are produced. The novel technology is performed as a single-step technological process. The functionally gradient cemented carbides of the first type can be useful for wear and mining application, where the low-Co hard surface layers play a role of wear-resistant coatings. The functionally gradient cemented

Fig. 1. Microstructures of the functionally gradient carbide for rotary drilling at different distances from the surface: (1) 0.5 mm, (2) 1 mm, (c) 2 mm, (d) 3 mm, (e) 4 mm, and (f) 5 mm.

302

I. Konyashin et al. / Surface & Coatings Technology 258 (2014) 300–309

Fig. 2. Curves indicating (a) the Co distribution and (b) the hardness distribution in the functionally gradient cemented carbide for rotary drilling.

carbides of the second type can be useful as substrates for polycrystalline diamond (PCD) layers, which are produced by press sintering at ultra-high pressures including the infiltration of the PCD layers with cobalt from carbide substrates. Besides the functionally gradient cemented carbides with high-Co layers of several millimeters in thickness mentioned above, thin Co films of several microns in thickness can also be useful for some applications, particularly for the fabrication of carbide articles subjected to brazing. In this case, the wettability of the carbide surface by braze alloys is expected to be significantly improved due to such thin Co films. It is desirable to obtain such Co films during sintering, which

would allow the fabrication of high-quality carbide articles for brazing without grinding. A reason for the presence or absence of thin Co layers on the surface of WC-Co articles after sintering has been a riddle for a long time. After the sintering process such thin shiny Co layers are sometimes present on the surface of WC-Co articles, and their formation is referred to as “Co capping” in literature. Not much literature is available on the mechanism of the Co capping phenomenon, though the publications of Janisch et al. [19], Guo et al. [20], and Sachet et al. [21] tried to shed light on the matter. All the authors agree on the fact that the migration of liquid Cobalt from the interior to the surface is responsible for the occurrence of Co capping. Janisch et al. [19] claim that if the interior, and not the surface, is first to solidify then liquid cobalt shrinks and thereby squeezes the remaining liquid cobalt outwards toward the surface. Janisch et al. conclude that if cemented carbides have a low carbon content in the interior and a higher carbon content in the surface layer, the interior solidifies first resulting in Co capping. The higher carbon content in the surface layer can be obtained as a result of treatment of carbide articles in a carburizing atmosphere. Janisch et al. make the point that if the surface is first to solidify then the liquid phase would not be able to reach the surface. In contrast to ref. [19], Guo et al. [20] explain the phenomenon of Co capping using the principle of liquid cobalt migration driven by the difference in volume fractions of liquid cobalt between the surface and the interior regions. A de-carburizing atmosphere is recommended for Co capping, as it allows the surface to solidify first (due to the higher melting point associated with the lower carbon content) thereby reducing the amount of liquid cobalt in this region. This in turn creates a gradient between the interior and surface which facilitates the migration of liquid cobalt resulting in Co capping. Sachet et al. [21] believe that the major phenomena leading to Co capping are related to the transport of liquid cobalt towards the surface and high surface tension of liquid cobalt. If the carbide surface layer is slightly de-carburized during sintering, an initial transport of liquid cobalt towards the surface occurs. The high surface tension of liquid cobalt governs the formation of the Co layer. Sachet et al. conclude that fast cooling or cooling under carburizing conditions can prevent the cobalt film formation. Recently we developed a new technology for the fabrication of carbide articles with thin Co films during sintering [22]. It allows the wettability of non-ground carbide articles by braze alloys to be dramatically improved and ensures the elimination of surface defects, e.g. micro-cracks, usually present on the surface of carbide articles after sintering leading to a noticeable increase of the TRS. 2. Experimental details

Fig. 3. Curve indicating different combinations of hardness and fracture toughness for conventional WC-Co cemented carbides with various Co contents and WC mean grain sizes according to ref. [23] (black) and three points (circles) corresponding to different parts of the functionally gradient carbide for rotary drilling.

Functionally gradient cemented carbides were produced according to the procedures described in refs. [15–18]. For the fabrication of the functionally gradient carbides with low-Co, hard surface layers, green articles with original low carbon content but not corresponding to the η-phase formation were pre-sintered at temperatures between 1000 °C and 1200 °C to obtain a certain gas permeability, carburized at 1100 °C in a methane–hydrogen mixture and subjected to liquid phase sintering at 1400 °C. For the production of the functionally gradient carbides with high-Co surface layers, green articles with original high carbon content but not corresponding to the free carbon formation were pre-sintered at temperatures between 1000 °C and 1200 °C, decarburized at 800 °C in hydrogen and subjected to liquid-phase sintering at 1420 °C. Carbide articles with thin Co films were obtained according to the technology described in ref. [22]. Samples of various WC–Co grades were sintered in a laboratory and conventional production Sinter-HIP furnaces at a temperature of 1420 °C for 75 min followed by cooling at tailored rates. The cooling rate was equal to 4.5° per minute between

I. Konyashin et al. / Surface & Coatings Technology 258 (2014) 300–309

303

Table 1 Properties of the near-surface layer of the functionally gradient carbide of 1 mm in thickness in comparison with those of ungraded carbide of nearly the same composition (B20, Element Six GmbH) and standard carbide grade of the same original Co content and WC mean grain size (B25, Element Six GmbH). Grade Near-surface layer Standard carbide grade B20 Standard carbide grade B25

Average Co content, wt.% 7.5 8 10

Specific magnetic saturation (SMS), %

Hc, Oe

Average Vickers hardness, HV10

WC mean grain size, μm

Average Palmqvist fracture toughness MPa m1/2

95 83

105 102

1300 1250

2.8 2.7

18.0 14.6

85

98

1200

2.8

15.0

1420 °C and 1380 °C, 1° per minute between 1380 °C and 1340 °C, and 0.5° per minute between 1340 °C and 1280 °C; afterwards the cooling rate was uncontrolled down to room temperature. Metallurgical cross-sections were made according the standard procedure for cemented carbides and examined on an optical microscope and a high-resolution scanning electron microscope (Philips XL30S). Hardness measurements were carried out according to the DIN ISO 3878 at a load of 300 N. The indentation fracture toughness was measured by the Palmqvist method at a load of 300 N after annealing of the cross-sectional samples in a vacuum at 800 °C for 60 min according to the ISO/DIS28079 standard. The transverse rupture strength (TRS) was examined on non-ground carbide rods of 8 mm in diameter and ground rectangular test pieces of the B type according to the ISO/CD 3327 standard.

3. Results and discussion 3.1. Functionally gradient cemented carbides comprising surface layers with low co contents The functionally gradient cemented carbides with low-Co surface layers were obtained by the new sintering technology mentioned above comprising the following stages (1) pre-sintering of green carbides articles with the original low carbon content in order to obtain a certain residual open porosity and consequently gas permeability; (2) the carburization of the pre-sintered green articles in methane– hydrogen gas mixtures, and (3) final liquid phase sintering at tailored parameters to obtain the full density and Co drift from the surface towards the core. Fig. 1 shows microstructures of the near-surface layer and core region of a functionally gradient carbide for rotary drilling with the original Co content of 10 wt.% in the grade powder, and Fig. 2 shows curves indicating the hardness and Co distributions in the functionally gradient carbide. As one can see in Fig. 1 the microstructure of the functionally gradient carbide does not comprise any η-phase or free carbon and becomes slightly finer in the core region in comparison with the near-surface region. The Co content shown in Fig. 2a varies from nearly 6.5 wt.% in the surface region to about 14 wt.% in the core region leading to a significant increase of hardness in the near-surface layer compared to the core (Fig. 2b). The difference in hardness between the near-surface layer and core is more than 200 Vickers units. Fig. 3 shows curves indicating different combinations of hardness and fracture toughness for conventional WC-Co cemented carbides with various Co contents and WC mean grain sizes according to ref. [23] and also three points corresponding to different parts of the functionally gradient carbide for rotary drilling. As one can see, the core region is characterized by a similar combination of hardness and fracture toughness as that of conventional WC-Co materials. The region located at a distance of 1 mm from the surface is characterized by a better combination of hardness and fracture toughness compared to that of conventional cemented carbides. The thin surface layer of the functionally gradient carbide has a dramatically improved combination of hardness and fracture toughness in comparison with conventional WC-Co

materials. Table 1 shows properties of the near-surface layer of 1 mm in thickness of the functionally gradient carbide in comparison with those of ungraded carbide of the same composition (WC-8% Co, B20, Element Six GmbH). It can be seen in Table 1 that the near-surface layer of the functionally gradient carbide is characterized by significantly increased combination of hardness and fracture toughness in comparison with the ungraded carbide grade with nearly the same Co content, coercivity and WC mean grain size. This is related to the very high level of residual compression stresses in both the carbide phase (−440 MPa) and the binder phase (− 500 MPa) [24]. It should be mentioned that the hardness, fractures toughness and residual stresses of the surface layer of the functionally gradient carbides were measured on samples annealed at 800 °C for 1 h according to the ISO/DIS28079 standard. Therefore, the high level of hardness, fracture toughness and residual stresses, and consequently significantly improved performance properties of the functionally gradient carbide, which will be described below, are not related to the presence of local stresses forming as a result of grinding or other type of surface finishing. Fig. 4 shows results of laboratory performance tests on percussion drilling of the functionally gradient carbide for rotary drilling. It can be seen that the wear-resistance of functionally gradient carbide designated as B25-GS is nearly as twice as high in comparison with the conventional carbide grade (WC-10% Co) designated as B25 (Element Six GmbH). It was found that the wear-resistance of the functionally gradient carbide containing about 7.5 wt.% Co on average in the surface layer was slightly higher than that of the conventional ungraded WC-8% Co carbide grade (B20, Element Six GmbH) having nearly the same WC mean grain size. Therefore, it is likely that the significantly improved wear-resistance of the functionally gradient carbide is related to both the increased hardness of the near-surface layer and the high residual compressive stress in this layer.

Fig. 4. Results of laboratory performance tests on percussion drilling of the functionally gradient carbide for rotary drilling (B25-GS) in comparison with the conventional ungraded carbide with the same average Co content and similar WC mean grain size for rotary drilling (B25).

304

I. Konyashin et al. / Surface & Coatings Technology 258 (2014) 300–309

Numerous field tests of the functionally gradient carbide for rotary drilling were carried out in drilling of various types of rocks including iron ore, iron quartzite, etc. An increase in wear-resistance of the functionally gradient carbide in many cases was up to 10 times in comparison with the conventional carbide grade with 10% Co for rotary drilling. The average prolongation of tool lifetime of the functionally gradient carbide was found to be about threefold in comparison with the conventional carbide grade. Fig. 5 shows roller cones of a tri-cone bit with inserts of the functionally gradient carbide and conventional carbide after its field test on rotary drilling of iron quartzite. One can see that the wear of the conventional carbide inserts is significantly greater than that of the gradient carbide inserts. It should be noted that the ungraded WC-Co grade with 8 wt.% Co (B20, Element Six GmbH) corresponding to the near-surface layer of the functional gradient carbide cannot be employed in tri-cone bits for rotary drilling because of its insufficient TRS and fracture toughness, therefore, it cannot be used as a control in the field tests. 3.2. Functionally gradient cemented carbides comprising surface layers with high co contents As it was mentioned above, functionally gradient cemented carbides with surface layers containing much more Co than in the core region can be useful for different applications, particularly as substrates for poly-

Fig. 5. Worn roller cones of a tri-cone bit with inserts of the functionally gradient carbide (a) and conventional carbide (b) after a field test on drilling of iron quartzite.

crystalline diamond (PCD) layers obtained by press-sintering at ultrahigh pressures. The carbide substrates with the PCD layers are usually characterized by the presence of the so-called “Co depleted zone” with significantly reduced Co content adjacent to PCD, as much Co from the carbide substrate infiltrates into the PCD layer during the presssintering process. Therefore, it can be advantageous to produce the carbide substrates with higher Co contents in the near-surface layer to reduce or eliminate the Co depleted zone. The functionally gradient cemented carbides with high-Co surface layers were obtained by the new sintering technology mentioned above comprising the following stages: (1) pre-sintering of green carbides articles with the original medium-high carbon content in order to obtain a certain residual open porosity and consequently gas permeability; (2) the de-carburization of the pre-sintered green articles in pure hydrogen, and (3) final liquid phase sintering to obtain the full density and Co drift from the core towards the surface. Fig. 6a shows the microstructure of the near-surface region, and Fig. 6b shows the microstructure of the core region in the functionally gradient cemented carbide containing 8% Co in the original WC-Co grade powder. Fig. 7 shows curves indicating the Co distribution and hardness distribution in the functionally gradient cemented carbide. One can see that the Co content in the surface layer is equal to nearly 11% and decreases from the surface toward the core. Substrates for poly-crystalline diamond made of the functionally gradient carbide were employed for the PCD press sintering. The functionally gradient carbide substrates with the original low average Co content were

Fig. 6. Microstructure of the functionally gradient cemented carbide with the high-Co surface layer and original Co content of 8 wt.% in the WC-Co grade powder: (a) the nearsurface region, and (b) the core region.

I. Konyashin et al. / Surface & Coatings Technology 258 (2014) 300–309

305

explaining this phenomenon a number of experiments on sintering of carbide articles with different combinations of WC mean grain size and Co content were carried out.

Fig. 7. Curves indicating (a) the Co distribution and (b) the hardness distribution in the functionally gradient cemented carbide with the high-Co surface layer and the original Co content of 8 wt.% in the WC-Co grade powder.

found to be successfully employed in the PCD press-sintering process. It should be noted that the original average Co content in the WC-Co grade powder was significantly lower than that in conventional WC-Co grades used as substrates for PCD, therefore, just the presence of the Co gradient allowed the carbide substrates to be employed for the PCD presssintering process. Some of cobalt from the surface region of the functionally gradient carbide infiltrated into the PCD layer reducing the Co content within a layer of the surface region proximate the interface between the substrate and the PCD layer. As a result, the level of Co contents in the whole carbide substrate became very similar after the PCD press sintering, and almost no Co depleted zone was formed. It should be noted that the employment of the functionally gradient carbide as a substrate for PCD does not result in more effective bonding of PCD to the substrate. Nevertheless, it allows one to use carbide substrates with significantly lower average Co contents and consequently higher hardness, which ensures their noticeably higher wear- and erosionresistance. In some applications, particularly in oil-and-gas drilling and road-planing, the wear- and erosion-resistance of the carbide substrates under the PCD layer can play a very important role affecting the performance of the whole carbide-PCD composite. 3.3. Thin Co films on carbide articles obtained during sintering It is well known that carbide articles are sometimes characterized by a shiny surface and comprise thin Co films, but sometimes they have a dark surface and very little Co after sintering. To establish a mechanism

Fig. 8. Surface of articles of WC-10% Co carbide grades with different WC mean grain sizes: (a) fine-grain grade, the Co content is 76.4 wt.%, and there is a discontinuous Co film on the surface; (b) medium-grain grade, the Co content is 18.7 wt.%, and there are islands of a Co film on the surface; (c) coarse-grain grade, the Co content is 14.1 wt.%, and there is no Co film on the surface.

306

I. Konyashin et al. / Surface & Coatings Technology 258 (2014) 300–309

Figs. 8 and 9 show the surface of carbide grades with various combinations of WC mean grain size and Co content. It was found that the Co content on the surface of medium-grain and coarse-grain grades containing the same Co amount in the bulk significantly decreases after sintering, which is shown in Fig. 8. Fig. 9 shows the surface morphology

Fig. 9. Surface of articles of fine-grain carbide grades: (a) WC-15% Co, the Co content on the surface is 84 wt.%, and there is a continuous Co film on the surface, (b) WC-8% Co, the Co content on the surface is 3.6 wt.%, and (c) WC-7% Co, the Co content on the surface is 3.1%.

of carbide articles with the similar WC grain size of roughly 0.8 μm (ultra-fine grades) but different Co contents. As it can be seen in Fig. 9a the surface of the ultra-fine grade with 15% Co contains 84% Co and is coated with a continuous Co film after sintering. When the Co content in the bulk of the ultra-fine grades is decreased, the Co content on the surface drops, so that the WC-10% Co grade contains roughly 76% Co on the surface and comprises a discontinuous Co film. After further decreasing the Co content in the bulk of the ultra-fine grades down to 8%, the Co content on the surface sharply drops and becomes equal to 3.6% for the WC-8% Co grade and 3.1% for the WC-7% Co grade. Thus, the Co content on the carbide surface after sintering decreases when increasing the WC mean grain size at a certain Co content in the bulk, or when decreasing the Co content in the bulk at a certain WC mean grain size. It is well known that one can significantly increase the Co content on the surface of carbide articles with the aid of slow cooling from sintering temperatures [19–21]. Indeed, as it is clearly seen in Fig. 10 the surface of the fine-grain grade with 10% Co is coated with a continuous Co film as a result of slow cooling after sintering. Nevertheless, the mechanisms explaining this phenomenon proposed in literature are contradictory, so that the reason for the formation of continuous Co films on carbide surfaces as a result of slow cooling has been unclear so far. To explain the phenomena of the absence or presence of Co films on the surface of carbide articles after sintering followed by fast cooling let us take into consideration forces and consequently pressures, at which liquid Co is subjected in narrow channels between WC grains in the

Fig. 10. Surface of articles of fine-grain WC-10% Co carbide grade (a) fast cooling after sintering, the surface is coated by a discontinuous Co film, and (b) slow cooling after sintering, the surface is coated by a continuous Co film.

I. Konyashin et al. / Surface & Coatings Technology 258 (2014) 300–309

Fig. 11. Schematic diagram illustrating the influence of capillary and suction pressures on the surface of liquid Co in a channel between two WC grains on the carbide surface.

carbide near-surface layer during liquid-phase sintering. According to ref. [25] the liquid in a pore or thin channel (capillary) of a composite body is acted upon by the capillarity pressure on the one hand and the migration pressure or suction pressure on the other hand, which is

Fig. 12. Co layers obtained by the new Co capping technology: (a) the Co layer on a metallurgical cross-section, and (b) surface morphology of the Co layer.

307

schematically shown in Fig. 11. The suction pressure tends to return the liquid into the composition body, whereas the capillarity pressure acts on the liquid in such a way that it moves in the capillary toward the surface if the wettability is complete. The capillarity pressure is dependent mainly on the capillary diameter and the wettability of the capillary material by the liquid. The suction pressure strongly depends mainly on the liquid content in the composite body. The presence of contaminations on the surface of WC grains in the carbide nearsurface layer makes their wettability by liquid Co relatively poor; however, the impurities are expected not to incorporate into the carbide article deeply below the surface. In case of the ultra-fine grades the channels between the adjacent WC grains are narrow; therefore, the capillarity pressures acting on the liquid Co towards the surface are high. On the other hand, the liquid Co in the channels is affected by the suction pressures depending on the Co content in the carbide body. The capillarity pressure for the relatively high-Co fine-grain grades (10% Co and more) is presumably higher than the suction pressure, so that the liquid Co moves towards the surface and is “pressed out” or, in other words, “extruded” onto the surface overcoming the repulsion related to the relatively poor wettability of the surface of the contaminated WC grains by liquid Co. This can also lead to the partial dissolution of impurities and contaminations on the surface of the WC grains in the liquid Co extruded onto the surface under the impact of the capillarity pressures. It can be expected that, if the liquid-phase sintering is carried out for a long time, the liquid cobalt can completely

Fig. 13. Palmquist cracks on the surface of the medium-grained WC-6% Co grade (a) before and (b) after performing the Co capping technology.

308

I. Konyashin et al. / Surface & Coatings Technology 258 (2014) 300–309

dissolve the impurities and contaminations present on the surface of the WC grains according to ref. [26] having a small size in the ultra-fine grades, thus leading to the Co film formation. As a result, the Co film usually forms on the carbide surface in the case of ultra-fine grades with high Co contents. When the Co content in the bulk of the ultrafine grades is decreased the suction pressure starts overcoming the capillary pressure, thus preventing the Co migration towards the surface and consequently the Co film formation. Therefore, no Co film usually forms on ultra-fine grades containing less than 8% Co. On the other hand, when the WC mean grain size and consequently the average diameter of the channels between WC grains in the near-surface layer are increased, the capillarity pressures become much weaker, and the liquid Co cannot be extruded onto the WC grains on the carbide surface. During slow cooling from sintering temperatures the liquid binder starts solidifying in both the near-surface region and core almost simultaneously. In this case, nuclei of solid Co start precipitating on the WC surface in the channels between WC grains. After the original nucleation period a thin layer of solid Co forms on the surface of WC grains in the channels between WC grains in the carbide nearsurface layer. As a result, the diameter of the channels decreases, and the capillary pressure in the channels noticeably increases leading to extruding the liquid Co from the channels onto the surface of the WC grains on the carbide surface. If the WC mean grain size is small or medium, the Co extrusion caused by the growing capillary pressures results in the Co penetration onto the surface of all WC grains and, as a result, the formation of a continuous Co film on the carbide surface. Based on the well understood mechanism of the Co capping phenomenon mentioned above we developed a novel Co capping technology for the fabrication of Co coated carbide articles [22]. The technology is based on stepwise cooling of carbide articles after sintering at tailored rates. Fig. 12 shows Co layers on the carbide surface obtained by use of the technology. There are two major advantages of the Co capping technology for carbide articles not subjected to grinding. The first advantage is related to the fact that the surface of non-ground carbide articles always comprises micro-cracks and other defects after sintering, which can lead to a significant reduction of the TRS and consequently performance properties. The Co films obtained by the Co capping technology can completely “heal” such surface cracks and defects, which can be clearly seen in Fig. 13. The typical Palmqvist cracks forming near the Vickers indentation on the carbide surface shown in Fig. 13a completely disappear as a result of Co capping, which is clearly seen in Fig. 13b. As a result, the TRS of non-ground carbide articles increases by up to 50% and that of ground articles increases by roughly 10%, which can be seen in Table 2. Therefore, the formation of the Co layers on the surface of carbide articles has a similar effect as surface polishing removing all the surface defects, which results in the noticeable increase of TRS. The other major advantage of the Co capping technology is related to significant improvements in the wettability of non-ground carbide surfaces by braze alloys. Fig. 14a shows a typical plate of a finegrained WC-7% (Co,Ni) grade after its wetting by a braze alloy, which indicates the very poor wettability. In contrast, the wettability of the surface of the grade comprising a Co + Ni layer obtained by the Co capping technology is excellent, as one can clearly see in Fig. 14b.

Table 2 TRS of cemented carbides with and without Co layer.

Without Co layer With Co layer

TRS, MPa, WC-6% Co, 0.8 μm, Nonground

TRS, MPa, WC-7% Co/ Ni, 0.6 μm, Non-ground

TRS, MPa, WC-5% Co, 0.8 μm, Ground

1730 2510

1340 1820

2950 3190

Fig. 14. Plates of the fine-grained WC-7% (Co,Ni) grade after wetting by a braze alloy (a) the plate was wetted directly after sintering, and (b) the plate was wetted after sintering followed by the Co capping technology.

4. Conclusions Engineered surfaces containing little Co, much Co or comprising thin Co films can be obtained on WC-Co cemented carbides by tailored sintering techniques. The surface layers with low Co contents are characterized by a dramatically improved combination of hardness and fracture toughness due to the presence of high residual compressive stresses and very effective as hard wear-resistant layers in mining applications. Cemented carbides comprising the surface layers with high Co contents can be successfully employed as low-Co substrates for poly-crystalline diamond (PCD). The thin Co films on carbide articles allow significant improvements in the wettability of carbide surfaces by braze alloys and ensure the TRS of carbide articles to be noticeably increased.

Acknowledgements The authors gratefully acknowledge the support from the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of MISIS (grant No. К2-2014-012).

I. Konyashin et al. / Surface & Coatings Technology 258 (2014) 300–309

References [1] I. Konyashin, S. Hlawatschek, et al., Int. J. Refract. Met. Hard Mater. 28 (2010) 228–237. [2] J. Guo, F. Wang, et al., Adv. Powder Metall. Part. Mater. (2010) 8/29–8/39. [3] Z. Li, W. Yuan-jie, et al., Int. J. Refract. Met. Hard Mater. 26 (2008) 195–300. [4] C. Colin, L. Durant, N. Favrot, et al., Int. J. Refract. Met. Hard Mater. 12 (1993–1994) 145–152. [5] Fischer U, Waldenstrom M, Hartzell T. Cemented carbide body with increased wear resistance. US Patent 5,856,626, 1999. [6] Fischer U, Hartzell E, Akerman J. Cemented carbide body used preferably for rock drilling and mineral cutting. US Patent 4,743, 515, 1988. [7] J. Guo, P. Fan, Z. Fang, in: L. Sigl, P. Rödhammer, H. Wildner (Eds.), Proc. 17th Int. Plansee Seminar, v. 2, Reutte, 2009, pp. 50/1–50/6. [8] Fang Z, Fan P, Guo J. Functionally graded cemented carbide with engineered hard surface and the methos for making the same. US Patent Application US2010/0101368A1 (2010). [9] P. Fan, Z. Fang, J. Guo, Int. J. Refract. Met. Hard Mater. 36 (2013) 2–9. [10] Fang Z, Fan P, Guo J. Functionally graded cemented tungsten carbide with engineered hard surface and the method for making the same. US Patent Application, 20110116963 (2011). [11] O. Eso, Z. Fnag, A. Griffo, Int. J. Refract. Met. Hard Mater. 23 (2005) 233–241. [12] O. Eso, P. Fang, Z. Fang, Int. J. Refract. Met. Hard Mater. 26 (2) (2008) 91–97. [13] O. Eso, P. Fan, Z. Fang, Int. J. Refract. Met. Hard Mater. 25 (2007) 286–292. [14] Y. Liu, H. Wang, J. Yang, et al., J. Mater. Sci. 39 (2004) 4397–4399.

309

[15] I. Konyashin, S. Hlawatschek, et al., Int. J. Refract. Met. Hard Mater. 27 (2009) 234–243. [16] I. Konyashin, S. Hlawatschek, et al., in: L. Sigl, P. Rödhammer, H. Wildner (Eds.), Proc. 17th Int. Plansee Seminar, v. 2, Reutte, 2009, pp. 6/1–6/12. [17] Konyashin I, Hlawatschek S, Ries B, Lachmann F. A hard-metal body. PCT Patent Application WO2010/097784A1. [18] Konyashin I, Hlawatschek S, Ries B, Lachmann F. A superhard element, a tool comprising same and methods for make such superhard element. PCT Patent Application WO2010/103418A1. [19] D.S. Janisch, W. Lengauer, K. Rödiger, K. Dreyer, H. van den Berg, Int. J. Refract. Met. Hard Mater. 28 (2010) 466–471. [20] J. Guo, P. Fan, X. Wang, Z. Fang, Int. J. Refract. Met. Hard Mater. 28 (2010) 317–323. [21] E. Sachet, W.D. Schubert, G. Mühlbauer, J. Yukimura, Y. Kubo, Int. J. Refract. Met. Hard Mater. 31 (2012) 96–108. [22] I. Konyashin, B. Ries, F. Lachmann, Cemented carbide article and method for making same. WO2012/098102A1, 2012. [23] B. Roebuck, M.G. Gee, R. Morrell, in: G. Kneringer, P. Rödhammer, H. Wildner (Eds.), Proceedings of the 15th International Plansee Seminar, vol. 4, Reutte, 2001, pp. 245–266. [24] I. Konyashin, B. Ries, F. Lachmann, A.T. Fry, Int. J. Refract. Met. Hard Mater. 36 (2013) 10–21. [25] A. Lisovsky, Metall. Mater. Trans. A 25A (1993) 733–740. [26] I. Konyashin, S. Hlawatschek, B. Ries, F. Lachmann, M. Vukovic, Int. J. Refract. Met. Hard Mater. 42 (2014) 142–150.