Nanoparticle dispersion-strengthened coatings and electrode materials for electrospark deposition

Thin Solid Films 515 (2006) 1161 – 1165 www.elsevier.com/locate/tsf

Nanoparticle dispersion-strengthened coatings and electrode materials for electrospark deposition E.A. Levashov a,⁎, P.V. Vakaev a , E.I. Zamulaeva a , A.E. Kudryashov a , Yu.S. Pogozhev a , D.V. Shtansky a , A.A. Voevodin b , A. Sanz c a

c

Moscow State Institute of Steel and Alloys, Technological University, Leninsky pr., 4, Moscow 119049, Russia b Air Force Research Laboratory, 2941 Hobson Way, Wright Patterson AFB, OH 45433, United States SKF Engineering and Research Centre, P.O. Box 2350 Kelvinbaan 16, 3430 DT Nieuwegein, 3439 MT Nieuwegein, The Netherlands Available online 16 October 2006

Abstract Advanced electrode compositions were developed using self-propagating high-temperature synthesis (SHS). Electrospark deposition (ESD) was applied to produce tribological coatings which were disperse-strengthened by incorporation of nanosized particles. Nanostructured electrodes of cemented carbides were produced using powder metallurgy technologies. They allow increasing the coatings density, thickness, hardness, Young's modulus and wear resistance. Positive effects of the nanostructure of electrodes on the deposition process and structure and properties of the coatings are discussed. In that case the tungsten carbide phases become predominant in the coatings. A mechanism of the dissolution reaction of WC with Ni at the contact surface of electrode was proposed. It was shown that the formation of the coating structure starts on the electrode and is accomplished on the substrate. © 2006 Elsevier B.V. All rights reserved. Keywords: Electrospark deposition (ESD); Nanosized additives; WC–Co nanostructured electrodes; Microstructure; Coating

1. Introduction Functional nanostructured thin films and coatings strengthened by nanosized particles of refractory metals and compounds with dense morphology, low roughness, tailored tribological properties and extended service life were recently prepared using physical vapour deposition (PVD), electrochemical deposition (ECD), thermal spraying (TS), electrospark deposition (ESD) and others. The results in preparation and studies of coatings strengthened by nanosized particles are described in [1–11]. Significant improvements of ultra fine-grained materials and coating properties were demonstrated by controlling size, shape and distribution of the strengthening phases. ESD electrode materials were produced by the SHS process using Ti–C–Cr–Ni, Ti–B–Al, Ti– C–Ni alloy, Ti–C–steel, Ti–C–Ni–Al, Ti–C–Al, Ti–B, and Ti– Ta–C–steel material systems [6,8,11]. The use of nanosized refractory components was critical for improving the deposition ⁎ Corresponding author. E-mail address: [email protected] (E.A. Levashov). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.07.140

process and coating quality. For example, introduction of a minor amount (3 to 10 vol.%) of nanosized refractory compound as a relatively inert filling material into the green SHS mixture results in decreasing combustion temperature and sintering temperature [6,8,11]. Nanosized powder component as nuclear takes an active part in primary and secondary structure formation of the synthesis products. Solid solutions and new chemical compound can be formed. Introduction of a nanosized component also modifies the grain structure of the produced alloys. In this paper, the influence of nanosized additives of ZrO2, Al2O3, NbC, WC, WC–Co, W and ultra-dispersive diamond (UDD) in electrodes based on TiC–Ni alloy on the electrospark deposition process kinetic and on resulting coating composition, structure, and properties onto Ti-substrate is discussed. Also, this work compares phase composition, structure, and properties of electrospark coatings deposited with micro- and nanostructured electrodes of WC–Co. A special emphasis is made on the role of the phase/structural state of the electrode material on the physical and chemical processes during the coating deposition onto Nisubstrate.

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2. Experimental procedure Electrodes of various compositions of TiC–40% Ni alloy with nanosized additives were synthesized by the force SHSpressing technology according to an established procedure [11]. UDD produced by detonation synthesis (particle size of 40– 60 nm), nanoparticles of stabilized zirconia ZrO2 (30 nm), and Al2O3 (5 nm) produced by sol–gel technology, and plasmachemical synthesized particles of NbC (12 nm), W (170 nm), WC (7 nm) and WC–8% Co (100 nm) were used as additives to the green mixtures. Nanostructured electrodes 92% WC + 8% Co in the shape of rods (4 × 0.4 × 0.4 cm) were produced by hot pressing. The grain size in the sintered material was 0.1 μm, a residual porosity of about 20%. For comparison purposes, 2–3 μm sized structured 92% WC + 8% Co material electrodes (available on the Russian market under a VK8 alloy designation) were used in deposition experiments under identical conditions in Ar atmosphere. A high-frequency electrospark unit “Alier-Metal 2002” was used. ESD pulse parameters were varied in the following ranges: energy (P), 0.01–1.2 J; frequency (F), 100–3000 Hz; current, 150–400 A; duration, 8–100 μs. The frequency of the vibroexciter was normally at 600 Hz. The kinetic of the mass transfer (cathode weight gain ΔK, erosion of anode material ΔA) was measured gravimetrically (VLR-200 balances) with an accuracy of 10− 4 g. Coating phase compositions were determined using X-ray diffraction (XRD) data analysis under monochromatic Co-Kα and Cu-Kα radiation. The fraction of the amorphous phase was estimated upon its imitation with Ni3W3C (structural type E93) with a 1-nm area of coherent scattering. The depth profile of the phase compositions was analyzed by XRD with a grazing-angle beam technique. The thickness of the analyzed layer had a value of 3μ− 1sinα, where α is the angle of grazing of beam incidence and μ is the linear extinction coefficient. In order to observe a fine structure, the samples were subjected to selective ion etching (apparatus FIB Strata 201) with a flux of helium ions (300–1000 pA, accelerating voltage 30 kV). Element composition was determined with a scanning electron microscope JSM-6700F equipped with an accessory JED2300F (JEOL) for energy dispersive spectrometry (EDS). The size of the analyzed volume was about 1 μm3. An image was formed at an accelerating voltage of 15 kV and emission current of 10 mA, giving a resolution of 10 Å. Metallographic analysis was carried out with an optical microscope Neophot-32. Microhardness (Hμ) was measured with a PMT-3 unit at 100-g load. Hardness (H), elasticity modulus (E) and elastic recovery (R) were measured with a Nano-Hardness Tester (CSM Instruments) using Berkovich indenter at a load of 10 mN. The coating refractoriness (T) was evaluated from the sample weight gain after its oxidation in the electrical oven. The temperature of the experiment was equal to 750 °C. The friction coefficient was measured with a “Tribometer” (CSM Instruments) operating in a ball-on-disc mode. Balls made of 94% WC + 6% Co and Al2O3 (3 mm in diameter) were used as a counter body. Coated samples were rotated at a linear

speed of 0.1 m/s under a load of 1 N (for coatings strengthened by nanosized particles) and 5 N (for coatings deposited from nano- and microstructured WC–8% Co electrodes). 3. Results and discussion 3.1. Dispersive strengthening by nanoparticles SHS-electrodes and ESD-coatings onto Ti-substrate The most complete information concerning the influence of nanosized particles additives can be obtained when studying the kinetic dependences of total anode erosion and cathode (substrate) weight increment ∑Δκ. This makes it possible to determine the electrode material transfer rate and estimate the properties of formed coatings. ∑Δκ is a function of electrode material composition, microstructure, residual porosity, thermal and electric conductivity as well as pulse discharge energy and frequency. Electrodes with very small carbide grains allow increasing ∑Δκ and improving coating properties. Fig. 1 demonstrates the kinetic dependences of ∑Δκ on processing time. It can be seen (Table 1) that introducing nanocrystalline additives into electrode composition increases the deposition rate as compared to the electrode without additives. Moreover the ESDcoatings, which were deposited using the SHS-electrodes modified by nanosized powders, have better properties: increased density (S), larger thickness (h), higher microhardness (Hμ), lower friction coefficient (K) and reduced oxidation loss (T). It was established by SEM that a concentration maximum of the nanosized additive was located in the surface layer of the coating and it diminished toward the coating–substrate interface. Modification of the coatings by nanosized additives promotes the wear resistance due to a decrease of the friction coefficient below 0.2. Wear resistance of the coatings with 5% UDD was 3.5–4.0 times above that of the coating with the same composition but without additives. The positive effect of nanoparticles on tribological characteristics can be explained as

Fig. 1. Effect of 7 vol.% nanosized powder additives W (a), Al2O3 (b), WC–Co (c) and WC (d) on the kinetics of TiC–Ni alloy coatings deposition process at P = 0.13 J and F = 3000 Hz. (e) Deposition of TiC–Ni–alloy coating without additives.

E.A. Levashov et al. / Thin Solid Films 515 (2006) 1161–1165 Table 1 Properties of the ESD-coatings strengthened by nanosized particles Coating

Hμ a (GPa)

S (%)

h (μm)

T (g/m2)

K

TiC–Ni alloy—baseline TiC–Ni alloy + ZrO2 nano TiC–Ni alloy + Al2O3 nano TiC–Ni alloy + NbCnano TiC–Ni alloy + Wnano TiC–Ni alloy + UDD

11.7 12.6 14.9 13.3 13.5 13.8

70 100 95 95 98 99

57 104 105 69 102 96

16.1 13.8 14.1 12.0 13.5 13.2

0.26 0.19 0.18 0.15 0.17 0.16

a

− Hμ (Ti-alloy) = 1.9 GPa.

the follows: the grain size of the wear resistant component of the coating is much less when nanoparticles are added to the electrode and coating composition; nanoparticles (even of hard and superhard materials) distributed on interfaces between carbide grains and metallic binder act as a polishing medium as opposed to the abrasion cuts by micron-sized particles produced with conventional electrodes; it is also possible that nanoparticles released on the surface during the wear process are oxidized in the laboratory test environment (high surface area) under formation of lubricious oxides and hydroxides; coatings deposited using electrodes with nanosized additives had higher density and microhardness, providing a good load supporting the foundation for a lubricous surface layer. Although more experiments and wear track analyses are required to unambiguously confirm the above explanation, the tribological improvement with nanosized hard particle addition of different composition is clear and appears to be a general trend. 3.2. Application of WC–Co nanostructured electrodes for ESD onto Ni-substrate In the ESD process the weight gain was always higher for coatings deposited with nanostructured electrodes. The overall gain ∑Δκ grows from 1.6 to 57.7 mg/cm2 with increasing pulse discharge energy P from 0.01 to 1.2 J. Lower values of ∑Δκ for coatings deposited with microstructured electrodes are accompanied by a stronger erosion of substrate and involvement of the latter in formation of the coating. ESD involves both the diffusion of materials and chemical reaction between the constituents. At P = 1.2 J for VK8 electrodes ∑Δκ was negative (− 55.6 mg/cm2) while in the case of nanostructured electrodes it was positive (57.7 mg/cm2). This can be associated with different erosion resistance which is low for the nanostructured material and high for VK8 [12]. The coatings contain not only crystalline phases but also an amorphous phase (which is evidenced by a diffuse halo in the diffraction patterns). Due to short pulse duration (8–100 μs), their local action, and attained high temperatures (5000–11,000 °C), locally heated areas rapidly cool down upon heat transfer into a cold environment. This explains the formation of the amorphous phase (rapid quenching). The amount of amorphous phase is always higher in the coating deposited with a microstructured electrode, a maximum being attained at P = 0.23 J (Fig. 2). The content of carbide phases in these coatings was found to be unexpectedly low, which can be associated with the dissolution reaction of WC in liquid Ni from a substrate, leading to the for-

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mation of the amorphous phase and the Ni-based fcc solid solution Ni1−xWxCy . In coatings deposited with nanostructured electrodes, the carbide phases of WC, WC1−x, and W2C begin to form already at low P, their content increasing with P. The dissolution of WC also takes place, which is evidenced by the formation of W2C, Ni-based solid solution, and amorphous phase. The semi-carbide W2C is known to appear as an intermediate during the formation and dissolution of WC. This can be explained by the low lattice energy of tungsten carbides: 2760 kcal/mol for WC and 2000 kcal/mol for W2C [13]. For any P, the content of W2C is somewhat higher than that of WC. At P = 1.2 J, the difference between the phase composition of the coatings and kinetics of mass transfer is the largest. The coating deposited with a nanostructured electrode was found (Fig. 3a) to contain WC crystallites (0.5–5.0 μm in size) distributed over an amorphous binder. The coating deposited with a microstructured electrode was found (Fig. 3b) to contain fine-grained strata standing out against a background of a predominant “structureless” layer. Essentially, the coating represents an alloyed layer of Ni (53– 68 at.%), the concentration of components being uniform in depth. In order to find out whether the “structureless” component is crystalline or amorphous, XRD analysis with a grazing beam was carried out. The fcc solid solution is formed within a nearsurface layer 1 μm thick, while the amorphous phase is located underneath. This amorphous phase is about 5 μm thick. Therefore, the layers of Ni1−xWxCy alternate with the amorphous layers (Amorph). Apparently, crystallization takes place only in some areas that are seen as local fine-grained strata, while other areas are quenched in their amorphous state. Physical and chemical processes taking place during ESD and formation of a secondary structure on the anode are considered further. The secondary structure is a real object of erosion: the deposited material is not a starting electrode material but a composite formed under the action of electric discharge and reverse mass transfer from the substrate. The secondary structure of nanostructured electrode is a zone of grain growth and was found to be 150 μm long. The EDS data

Fig. 2. X-ray diffraction patterns for the coatings deposited with micro-(1) and nanostructured (2) electrodes (P = 0.23 J, 100 Hz).

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E.A. Levashov et al. / Thin Solid Films 515 (2006) 1161–1165 Table 2 Mechanical properties of coatings on the Ni-alloy substrate Coating

Hμ (GPa)

H (GPa)

E (GPa)

R (%)

Deposited with nanostructured electrode Deposited with microstructured electrode Ni-substrate

16.9

27.2

481

49

3.8

8.6

202

25

1.2

3.4

234

9

II—with a fine-grained structure in which the dispersion of WC grains takes place; III—each WC grain is isolated in order to migrate the Ni from a substrate leading to an increase in the volume fraction of the Ni + Co binder; IV—starting material. Tentatively, the mechanism of the dissolution reaction of WC in the presence of Ni at the contact surface of electrode proceeds by the scheme: WC þ Niliquid YWC1−x þ W2 C þ Ni1−x Wx Cy þ C þ Niliquid YNi1−x Wx Cy þ C þ Niliquid

Fig. 3. Microstructure (after selective ion etching in apparatus FIB Strata 201) of the coatings deposited with nano- (a) and microstructured (b) electrodes (P = 1.2 J, F = 100 Hz).

shows that Ni penetrates into the electrode material up to 9 μm deep. The longitudinal cross section of the conventional cemented carbide can be subdivided into four regions (Fig. 4): I— the near-surface region with an ultra-finely grained structure;

The Ni content is about 20 at.% and 18 μm in length. According to the metallographic data, the density of the coatings deposited at P = 1.2 J with nanostructured electrodes was 98%, while it was 90% for those with microstructured electrodes. The thickness of deposited layers was 40 μm for ESD with nanostructured electrodes and 30 μm for ESD with microstructured electrodes. Coatings deposited with microstructured electrodes were formed by overlapping coarse drops while that deposited with nanostructured electrodes by significantly smaller particulates. The mechanical characteristics of the coatings are summarized in Table 2. The hardness H of coatings deposited with nanostructured electrodes was higher by a factor of 3; the elasticity modulus E by a factor of 2.4; while the elastic recovery R by a factor of 2. The values of microhardness Hμ are lower compared to those of H. This can be associated with the fact that the H values refer to a thin near-surface layer (125–320 nm thick) where the material was modified to a greater extent. Interestingly, the difference in microstructure and properties of both coatings did not cause any significant difference in their tribological behavior. After 600 m, the friction coefficients were identical near 0.4, while the wear of counter body (Al2O3) was

Fig. 4. Longitudinal cross section of microstructured electrode after its operation at P = 1.2 J and F = 100 Hz. Point “0” corresponds to the contact surface of electrode.

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different: 7.6 · 10− 7 mm3 N− 1 m− 1 for a ball in contact with a coating deposited with nanostructured electrode and 2.9 · 10− 7 mm3 N− 1 m− 1 for a ball in contact with a coating deposited with microstructured electrode. Comparing this observation with the nanosized additions of different composition to TiC–Ni alloys, one can suggest a critical role of the tribosurface chemistry modification by varying electrode composition. This will be studied in the follow-up research.

Comparative analysis of the phase composition of electrodes (at the contact surface) and coatings allows to conclude that the formation of coating structure is started on the electrode surface and finished on the substrate. Nanosized electrodes enable to shift this process toward WC formation with a uniform distribution of small-sized crystallites.

4. Conclusions

The work has been supported in part by CRDF award RUE11506-MO-05, The Netherlands Program for Cooperation with Countries in Eastern Europe (PSO), and the European FP6 Program under the Contract Number NMP3-CT-2005-515703 Project EXCELL.

Positive effects of nanosized powder additives on microstructure and properties of ESD-coatings were established. Wear- and heat-resistant coatings on the base of TiC–Ni alloy system on titanium alloy substrates with low friction coefficient were deposited. Nanosized powders of ZrO2, Al2O3, NbC, WC, WC–Co, W and UDD can be recommended for applications requiring an optimum combination of high hardness, oxidation resistance, and low friction coefficient. Nano- and micro-sized structures of WC–Co electrodes were used to emphasize the sintering powder size effects on the ESD process and produced coating properties without influence of the electrode chemical composition. It was found that a low strength of intergrain and interphase boundaries in nanostructured electrodes and their reduced residual porosity facilitates intense mass transfer, while a contribution from the reverse mass transfer (from substrate onto electrode) is relatively low. In view of a high surface energy of nanosized carbide grains, their sintering, recrystallization, and growth can start to occur on the contact surface of the electrodes. By varying pulse energy, tungsten carbide phases can be made predominant with crystallites being uniformly distributed over the binder. Nanostructured electrodes allow increasing the coatings density, thickness, strength, Young's modulus, and wear resistance. Microstructured electrodes exhibited a higher erosion resistance. This facilitated the reverse mass transfer from a substrate with a formation of fine-grained near-surface structure due to the dissolution reaction and fatigue failure of WC grains in Ni-melt. Coatings deposited with microstructured electrodes had alternating layers of the amorphous phase and the Ni-based fcc solid solution.

Acknowledgments

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