WO2008014801A1

WO2008014801A1 - A method for deposition of dispersion-strengthened coatings and composite electrode material for deposition of such coatings

Info

Publication number: WO2008014801A1
Application number: PCT/EP2006/007572
Authority: WO
Inventors: Alejandro Sanz; Evgeny Alexandrovich Levashov; Dmitry Vladimirovich Shtansky; Alexander Evgenievich Kudryashov
Original assignee: Ab Skf; Jsc Scientific Industrial Enterprise Metal
Priority date: 2006-07-31
Filing date: 2006-07-31
Publication date: 2008-02-07

Abstract

A method for deposition of dispersion-strengthened by nanoparticles coatings involves the electrospark processing (ESP) of the surface of a metallic substrate with a composite electrode. ESP is carried out under the following conditions: 0.01 ≤ E ≤ 10 J; 500 ≤ N ≤ 5 400 000; 0.0185 ≤ f/N ≤ 0.1. Where E is the energy of single discharge pulse, f (Hz) is the repetition frequency for discharge pulses, and N is the total number of pulses during ESP. The material of substrate is a Ti- alloy or Ni- alloy or steel. The near-surface layer of substrate is doped with at least one of the following chalcogenides - MoS2, WS2, MoSe2, WSe2 - in amounts up to 30 vol %. Chalcogenide particles fill up all discontinuity flaws on the surface of coating.

Description

A METHOD FOR DEPOSITION OF DISPERSION-STRENGTHENED COATINGS AND COMPOSITE ELECTRODE MATERIAL FOR DEPOSITION OF SUCH COATINGS

TECHNICAL FIELD

The invention concerns surface engineering of metals and alloys for their subsequent use under load in extreme conditions and for strengthening tool and machine parts, including bearings.

BACKGROUND

US 6,336,950 describes the method of TRESS (ThermoReactive

Electrospark Surface Strengthening), which combines Self-Propagating High- Temperature Synthesis (SHS) with Electrospark Deposition (ESD). In this hybrid method exothermal chemical reactions take place between electrode (anode) and substrate (cathode). The method is based on the addition of third inert component (carbides, nitrides, borides, oxides, chalcogenides of transaction metals) in the green mixture. The range of powder particle dimension is of about tens microns. In this method the discharge pulse energy is within the range of 0,01-5 J, it is the case of assisting chemical reactions, that is suitable for the deposition of thick coatings.

SUMMARY An object of the invention is to define a method which overcomes the disadvantages of previously known methods. Unlike TRESS, our new invention is not related with the above described chemical reactions. Unlike the above described method of attaining thick coatings, in our new proposal the range of powder particle dimension of third component is in the nanometric scale. According to the invention we indicate the range of discharge pulse energy 0,01-10 J, which should be kept simultaneously with additional condition 500 ≤ N ≤ 5 400 000, 0.0185 < f/N < 0.1 , where f (Hz) is the repetition frequency for discharge pulses, and N is the total number of pulses during ESP. If all these requirements are fulfilled simultaneously, we can fabricate nanodispersion strengthened coatings on metal substrates with higher physical, mechanical and tribological properties: hardness, continuity, thickness, Young's modulus, wear resistance, surface roughness, thermal resistance, low friction coefficient. Other advantages of this invention will become apparent from the detailed description.

DETAILED DESCRIPTION

The technical task solved by this invention is the deposition of dispersion- strengthened by nanoparticles coatings for operation under load and exhibiting higher physical, mechanical, and service parameters, such as hardness, continuity (density), thickness, Young modulus, wear resistance, heat resistance, and lower friction coefficient and roughness.

Some examples of the process and electrode materials used for deposition of the above coatings are given below.

The method for deposition of dispersion-strengthened by nanoparticles coatings involves the electrospark processing (ESP) of the surface of a metallic substrate with a composite electrode. ESP is carried out under the following conditions:

0.01 < E ≤ 10 J

500 < N ≤ 5400 000

0.0185 < f/N < 0.1 where E is the energy of single discharge pulse, f (Hz) is the repetition frequency for discharge pulses, and N is the total number of pulses during ESP. The material of substrate is a Ti- alloy or Ni- alloy or steel. The near- surface layer of substrate is doped with at least one of the following chalcogenides — MoS₂, WS₂, MoSe₂, WSe₂ — in amounts up to 30 vol %. Chalcogenide particles fill up all discontinuity flaws on the surface of coating.

The ESP under the following conditions: 0.01 < E ≤ 10 J

500 < N < 5 400 000 0.0185 < f/N < 0.1 ensures stable processing, deposition of pore- and crack-free coatings exhibiting high mechanical and service parameters.

When E < 0.01 J, the deposition process is unstable, while the deposited coatings have a low thickness and density and hence low Young modulus, heat resistance, wear resistance and also high roughness and friction coefficient.

When E > 10 J, the process is accompanied by overheating of electrode material, so that the eroded material forms a flow of coarse fragments of electrode material. The above fragments can not strongly link up to neither a substrate material or to other fragments, so that the density of resultant coatings turns out to be low. Moreover, during subsequent processing (runs of electrode over the surface) some of the above fragments are removed from the surface, thus diminishing a thickness of coating and increasing the number of defects (pores, cracks), which finally leads to lower mechanical and service parameters. The ESP for E > 10 J results in deposition of non- uniform high-roughness coatings with high internal stress.

The ESP for N < 500 results in formation of coatings that exhibit a low density and thickness, while that for N > 5 400 000 is accompanied by overheating of the electrode material, so that the eroded material represents a flow of coarse fragments of electrode material. All this leads to formation of highly strained coatings with numerous defects (cracks). Also, long-term processing affords high-roughness coatings with a high friction coefficient, in addition to low thickness, density, Young modulus, wear resistance, and heat resistance.

When the ESP is carried out for f/N>0.1 , resultant coatings turn out to be thin and non-uniform, while the process becomes low productive. When the ESP is carried out for f/N<0.0185, resultant coating are thin and exhibit weak adhesion to a substrate because of low E and short pulse duration.

Doping the surface layer of coatings with chalcogenide particles in amounts up to 30 vol % leads to a decrease in friction coefficient, density, and thickness of deposited coatings. Doping the surface layer of coatings with chalcogenide particles in amounts below 30 vol % is insufficient for filling all non-continuities on the surface, so that for the above reason the friction coefficient cannot be markedly reduced.

One of the composites suitable for deposition of dispersion-strengthened by nanoparticles coatings is the nanostructured WC-Co alloy with the Co content of 3-25 wt %. In this alloy, the size of WC grains is 2-120 nm at a residual porosity of 8-40%. This alloy can be prepared by sintering in vacuum or by HIP- process.

Another type of composite electrode materials suitable for deposition of dispersion-strengthened by nanoparticles coatings is alloy consist of carbides of one of IV-VI group transition metals and/or complex carbide of IV-V group transition metals and/or complex carbide of V-Vl group transition metals and/or borides and/or nitrides and/or suicides of IV-VI group transition metals and metal binder dispersion-strengthened by nanoparticles. The size of nanoparticles is 2-170 nm, their content being 3.5-40 vol %. The metal binder content of alloy is 5-40 vol %. Nanosized powders of ZrO₂, AI₂O₃, WC, NbC, TiN, TiC, MgO, Y₂O₃, Si₃N₄, and W are being used as nanoparticles. Metals or their alloys or steels or intermetallides TiAI, TiNi, NiAI, Ni₃AI are being used as a metal binder. Nanoparticles are uniformly distributed over the metal binder and over grain boundaries of a refractory compound. Nanoparticles are uniformly distributed over the bulk of refractory grains, metal binder, and over grain boundaries of a refractory compound.

In modern materials science, improvement of materials (alloys) is often achieved upon modification of their structure. A current trend is the use of nanostructured materials. For instance, nanostructured WC-Co alloys are known to exhibit better mechanical properties (hardness, strength) compared to those of the same microstructured alloys. The use of WC-Co nanosized powders in technologies of thermal spraying (gas-flame and plasma deposition) of coatings leads to a higher quality of deposited coatings (bending strength, hardness, tribological behavior) compared to those deposited from microsized powders. The use of nanostructured WC-Co with the Co content below 3 wt % as an electrode material does not ensure a sufficiently high quality of deposited coatings because of a low erosivity of this material. The use of nanostructured WC-Co with the Co content above 25 wt % as an electrode material leads to formation of thin coatings with low strength, wear resistance, heat resistance and elevated friction coefficient and roughness. The ESP with composite WC-Co electrodes with the WC grains below 2 nm in size leads to formation of defect-rich coatings (due to high residual porosity of material, up to 40%) with a low density and reduced mechanical and service live parameters. The ESP with composite WC-Co electrodes with the WC grains above 120 nm in size and porosity below 7% also produces insufficiently good coatings (low thickness and density) largely because of low erosivity of the material. Nanostructured WC-Co composite electrodes can be prepared by sintering in vacuum or by HIP- process.

Introduction of nanoparticles to the composition of electrode material (based on carbides of one of IV-VI group transition metals and/or complex carbide of IV-V group transition metals and/or complex carbide of V-Vl group transition metals and/or borides and/or nitrides and/or suicides of IV-VI group transition metals) improve the service parameters of coatings doped with refractory nanoparticles (wear resistance, corrosion resistance, heat resistance, hardness, tribological properties, etc.). Carbides of one of IV-VI group transition metals and complex carbide of IV-V group transition metals and complex carbide of V-Vl group transition metals are known for their high hardness, melting point, and chemical resistance. Nitrides of IV-VI group transition metals are known for their high heat resistance, resistance to the action of mineral acids, relatively high melting point and corrosion resistance. Borides of IV-VI group transition metals are known for their high heat resistance, high-temperature strength, hardness, and corrosion resistance. Suicides of IV-VI group transition metals are known for their high resistance to the action of mineral acids and relatively high heat resistance. The ESP with electrodes doped with refractory nanoparticles can be expected to result in deposition of coatings also doped with refractory nanoparticles and exhibiting improved mechanical and service parameters. In this case, the electrode material contains a metal binder (5-40 vol %) and is doped with nanoparticles (ZrO₂, AI₂O₃, WC, NbC, TiN, TiC, MgO, Y₂O₃, Si₃N₄, W) 3.5-40 vol % in amount and 2-170 nm in size. The ESP with composite electrode containing below 5 vol % metal binder is accompanied by the erosion of largely refractory particles, so that deposited coatings exhibit reduced mechanical and service parameters. The ESP with composite electrode containing a larger amount of metal binder (above 40 vol %) results in deposition of coatings with reduced hardness, hardness, strength, wear resistance, heat resistance, and elevated friction coefficient and surface roughness. When a composite electrode is doped with nanoparticles in amounts below 3.5 vol %, the resultant coatings are characterized by a nonuniform distribution of nanoparticles, which results in the anisotropy of their properties. When the amount of added nanoparticles is above 40 vol %, the resultant coatings exhibit an elevated porosity, which reduces the service parameters of such coatings. Composite electrodes doped with the nanoparticles below 2 nm in their size are non-uniform and exhibit elevated porosity. As a result, the coatings deposited with such electrodes exhibit low mechanical parameters. When the size of added nanoparticles is above 170 nm, the resultant coatings are highly deficient (cracks, pores), its surface is rough, along with high friction coefficient and low service parameters (low continuity, hardness, strength, wear resistance, heat resistance).

In order to ensure high performance of dispersion-strengthened coatings, added nanoparticles should be uniformly distributed either over the bulk of metal binder and over the boundary of refractory grains, or over the bulk of refractory grains, over the bulk of metal binder, and over the boundaries of refractory grains simultaneously.

Some variants of practical realization of the invention are illustrated in Tables 1-3. Table 1 shows the conditions for ESP of the Ni- alloy NiMoI 6Cr16Ti (DIN 17444) aiming at deposition of coatings strengthened by nanoparticles. The nanostructured WC-Co composite was used as an electrode. Table 1 also shows the properties of deposited coatings. In the table K_f - friction coefficient, R_t - roughness of coating measured as maximum profile altitude.

Table 2 shows the practical realization of the invention at the following parameters of ESP: E = 0.15J

N = 144 000 f/N = 0.02 f = 3 000 Hz

The Ti- alloy TiAI6V4 (DIN 17851 ) was used as a substrate. Table 2 presents also the composition of dispersion-strengthened refractory composites used as an electrode and the properties of deposited coatings.

Table 3 illustrates practical realization of the invention at different parameters of pulsed discharge in case of the 100Cr2 steel (DIN EN ISO 683-17) used as a substrate. Nanostructured WC-Co alloys as well as other composite electrodes strengthened with added nanoparticles were used as an electrode. In this case, the amount of chalcogenide (M0S2, WS2, MoSe2, WSe₂) particles in deposited coatings attained a value up to 30 vol %. Table 3 also shows the properties of deposited coatings. Electrospark deposition was carried out in an "Alier-Metal" trade mark set. The mechanical and tribological properties of the coatings deposited with composite electrodes were measured by using the following precise equipment: field emission scanning microscope JSM-6700F equipped with a JED-2300F JEOL accessory for energy-dispersive spectrometry, microhardness tester PMT-3 (load 50 N), nano-hardness tester (CSM Instruments), tribometer (CSM Instruments), Neophot-32 and Axiovert 25 CA optical microscopes (Zeiss), friction apparatus SMTs-1 (Russia) (operating in a shaft-pad mode) [friction pair: diamond-containing shaft surface (0 40 x 12 mm; diamond grains 40- 60 μm in size, 50 vol %, bonze M1 as a binder) and electrospark-deposited coating; slip velocity 1 m/s; load 1 kg)], electric furnace SNOL 1.1.6/12-MZ (T = 750°C, 35 h), and optical profilometer WYKO NT 1100 (Veeco).

The invention is not restricted to the above-described embodiments, but may be varied within the scope of the following claims.

Table 1

Table 2

W

Table 3

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Claims

1. A method for producing coatings strengthened with nanoparticles including electrospark processing of a surface of a metallic substrate with a composite electrode carried out under the following conditions:

0.01 < E < 10J 500 < N < 5 400 000

0.0185 < f/N < 0.1 where E is the energy of single pulsed discharge, f (Hz) is the repetition frequency for discharge pulses, and N is the total number of pulses during the electrospark deposition

2. The method according to claim 1.characterized in that a titanium alloy or nickel alloy or steel is used as the metallic substrate.

3. The method according to claim 1 , characterized in that the surface layer of a dispersion-strengthened coating is doped with at least one of the

M0S₂, WS₂, MoSβ2, WSe₂ chalcogenides in an amount up to 30 vol %unit.

4. The method according to claim 1 , characterized in that the chalcogenide particles cover all non-continuities on the surface of deposited coating.

5. A material of composite electrode used for deposition of dispersion- strengthened by nanoparticles coating based on the nanostructured WC-Co alloy with the Co content 3-25 wt %, the size of WC grains 2-120 nm, and the residual porosity 8-40%.

6. The material according to claim 5, characterized in that the nanostructured alloy is prepared by sintering in vacuum or HIP

7. A material of composite electrode used for deposition of dispersion- strengthened by nanoparticles coatings of a dispersion-strengthened refractory compound — carbide of one of IV-VI group transition metals and/or complex carbide of IV-V group transition metals and/or complex carbide of V- Vl group transition metals and/or boride and/or nitride and/or suicide of at least one of IV-VI group transition metals — and a metal binder with the size of nanoparticles 2-170 nm, the nanoparticles¹ content of alloy 3.5-40 vol %, and the content of metal binder of alloy 5-40 vol %.

8. The material according to claim 7, characterized in that the nanosized powder of at least one of the ZrO₂, AI₂O₃, WC, NbC, TiN, TiC, MgO, Y₂O₃, Si₃N₄, W refractory compounds is used as nanoparticles added to an alloy.

9. The material according to claim 7, characterized in that a Ni- alloy or Ti alloy or steel or one of the intermetallides TiAI, TiNi, NiAI, Ni₃AI is used as the metal binder.

10. The material according to claim 7, characterized in that the nanoparticles are uniformly distributed over the bulk of metal binder and over the boundaries of refractory grains.

11. The material according to claim 7, characterized in that the nanoparticles are uniformly distributed over the bulk of refractory grains, metal binder, and over the boundaries of refractory grains simultaneously.