NZ227949A - Preparation of chromium-based coatings by electrodeposition from a solution containing a suspension of collodial cluster diamond particles - Google Patents

Abstract

A method of obtaining composite chromium-based coatings consists in the electrochemical deposition from a chromizing electrolyte containing colloidal cluster diamond particles measuring 0.001-0.01 microns at a concentration of 5-40 g/l.

Description

New Zealand Paient Spedficaiion for Paient Number £27949 z 2 7 9 4 9 NO DRAWINGS Priority Date(s): .1.1; f?.\1Sv.

Complete Specification Filed: Class: Publication Date: ....?. A P.O. Journal, No: ..

Patents Form No- 5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION A METHOD OF PREPARING CHROMIUM-BASED COMPOSITE COATINGS X/WE, ALEXANDR IVANOVICH SHEBALIN, Altaisky Krai, Biisk, ulitsa Pribytkova, l/kv.24/ USSR; VALERY DONATIEVICH GUBAREVICH, Altaisky Krai, Biisk, ulitsa Dekabristov, 10/1,kv.69, USSR; JURY NIKOLAEVICH PRIVALKO, Altaisky Krai, Biisk, ulitsa Radischeva, 30,kv.ll, USSR; PETR . MIKHAILOVICH BRYLYAKOV, Altaisky Krai, Biisk, ulitsa Dekabristov, 10/1, kv.24, USSR; VASILY IVANOVICH BESEDIN, Altaisky Krai, Biisk, ulitsa Dekabristov, 13,kv.324, USSR; GENNADY VIKTOROVICH SAKOVICH, Altaisky Krai, Biisk, ulitsa Radischeva, 2/2,kv.35, USSR; ALEXANDR YAKOVLEVICH CHEREMISIN, Altaisky Krai, Biisk, ulitsa Udarnaya, 29,kv.ll4, USSR; ALEXANDR NIKOLAEVICH KOTOV, Altaisky Krai, Biisk, ulitsa Stakhanovskaya 9,kv.23, USSR; (followed by page la) - la - 22 7 9 4 9 tyWE, STANISLAV ALEXEEVICH KOZLOVSKY, Shipilovsky proezd, 63, Korpus 1,kv.33, Moscow, USSR; NAUM BORISOVICH ALTSHULER, Moskovskaya oblast, Mytischi, ulitsa I Krestyanskaya, 1, Korpus 2,kv.55, USSR All ai/e. cl-lit en? o-P^e. US?®. hereby declare the invention, for which X/We pray that a patent may be granted to ipfe/us, and the method by which it is to be performed, to be particularly described in and by the following statement: (followed by page lb) -lb- 22 7 9 4 9 A METHOD OF PREPARING CHROMIUM-BASED COMPOSITE COATINGS The present invention relates to electrodeposition and more particularly to methods of preparing chromium-based composite coatings.

Chromium-based coatings are widely used in various engineering fields, for instance, to increase service life and reliability of moulds, dies, cylinder sleeve^, compression rings of internal combustion engine, and other units operating under great loads with friction, abrasive wear, and cavitation.

Composite coatings represent a metallic matrix made, in this case, of chromium which contains a dispersion phase, in particular superhard particles. Such coatings possess enhanced physico-metallic characteristics and wear resistance. The coatings in which the above characteristics are mainly determined by the dispersion phase and metal serves only for bonding the dispersion particles with one another and with the surface of the article are most widely used.

It is known (E.M.Sokolovsk-aya "Physical chemistry of composite materials" MGU, 197b,. p.2^0) that physico-mechanical properties and wear resistance of composite coatings attain maximum values when the content of the dispersion phase is 5-10 vol.% and the particle size of this phase decreases to 0.5-0.01ym. It ia accepted that a further decrease of the particle size lowers the (followed by page 2) 22 7 9 4 9 content of the particles in the coating and, hence, deteriorates the quality of the coating. Therefore, there are no recommendations on using the dispersion phase with a particle size less than 0.01 jum in the composite coatings.

As a rule, the composite coatings with the particles dispersed therein are obtained by the method of deposition from electrolytes containing salts of the metal being deposited and a dispersion phase. The composition of the electrolyte and the properties of the dispersion phase, including the nature thereof and the stability of the particles towards sedimentation and coagulation, determine the quality of the coating being prepared.

It is required that the dispersion particles are chemically stable in the electrolytes being used. Diamond particles are most suitable for strongly agressive strongly acid) electrolytes for chromium-plating.

There is a number of methods for preparing chromium-based composite coatings. Known in the art is a method of preparing such a coating residing in that dispersed silicon dioxide with a particle size from 0.01 to 0.1 Jim is introduced into electrolyte containing a dissolved chromium salt (Saifullin R.S. "Combined electrochemical coatings and materials", Moscow, Khimiya, 1971, p.101). Then anodes and a part are immersed into the electrolyte and the current of the required density is connected. The part being treated serves as a cathode. Aa a result, 227949 ■}- a chromium-based coating is obtained containing the particles of silicon dioxide. The coating possesses an enhanced wear resistance, and strong cohesion with the surface of the part. The coating, however, can be rapidly failed by temperature fluctuations, for instance, upon extrusion or pressing-out of metals. .;iso known in the art is a method of depositing diamond powders with mstals, in particular, with nickel (Ja.L.Prudnikov "Tool with diamond galvanic coating", 1935, Moscow, Mashinostroenie, p.9l)« These coating are only used 33 abrasive and con be used as antifriction and wear resistance coatings only when special treatment is performed residing ir. uh"-t sharp edges of the diamond particles 3 "e dulled to obtain flat are«s on the working surface of the coating which requires great labour con-sunc"i:n and the use of diamond-working tools, i.e. ad-diti-'-al technological operations.

Likewise known in the eri (GB,B,1591001) is a method of preparing composite coatings based on metals, in particular, on chromium. According to this method, the coatings are ^repsred by deposition from elsctrolytj^E the following composition: CrO- 250 g/1 ✓ "-p^O^ 1.25—2.5 g/1, the cstbcaic current density being c.4-10.7 a/dm^. As *1***** a dispersed phase use is made of natural or synthetic" d iamor.d s with 3 particle size of from 0.01 to 50.0 Mm r 22 7 94 9 in amount 10-30 g per 1 of electrolyte. The diamond dispersed particles used in the known method have sharp edges and are, therefore, abrasive. To obtain the coating with antifriction properties in realization of the known method, it is necessary to orient the diamond particles in the coating in such a way that the sharp edges are directed inwards and smooth surfaces outwards which complicates the technology of the process. Besides, to ensure sedimentation stability of diamond suspension in the electrolyte, the diamond particles are preliminary treated in hydrochloric acid, then in sodium hydroxide, in a mixture of sulphuric acid and cumarine, and in a surfactant of the anion type, after which the particles are dried and introduced either into a concentrated solution of a metal salt or 'of an acid for prolonged storage or directly into electrolyte which also complicates the technology of the process. In addition, in realization of this •method the cathodic current density is small which decreases the capacity of the process.

The main object of the invention is to provide a method of preparing chromium-based composite coatings which will ensure the preparation of coatings with enhanced hardness, wear resistance, and high antifriction properties at a low consumption of diamonds by following a simple technology.

This object is accomplished by that a method of preparing a chromium-based composite coating is proposed residing in electrochemical deposition from a chromium-plating electrolyte containing a suspension of diamond particles, wherein, according to the invention, as diamond particles use is made of colloidal cluster particles with a particle size of from 0.001 to 0.01 yim in amount 5-40 g/1. 22 7 9 4 9 Slectrolytes of solid chromium-plating or self-regula-ting (buffer) electrolytas are used as chromium-plating electrolytes.

The cluster diamonds used by the proposed method represent particles with a shape close to spherical or oval one without sharp edges (nonabrasive). Such diamonds form systems stable to sedimentation and coagulations in electrolytes both at working concentration of the components and at enhanced concentrations (in electrolyte c oncentrates).

As was mentioned above, it was believed that a dec-crease in particle sizes of the dispersed phase below 0.01jam deteriorates the properties of the coatings.

However, when cluster diamonds with particle sizes less than 0.01 j>im are used, a considerable rise in hardness, cohesion with the part, and wear-resistance of the costing are observed. This is related to the fact that cluster diamonds with above particle size possess low inertia due to which mass transfer of the dispersed phase from the electrolyte to the surface bein^ coated takes place under the most favourable conditions which makes it possible to deposit coatings at high current densities. It was shown that in electrochemical deposition of chromium (as well as in chemical and electrochemical deposition of other metals, for instance, of copper, nickel, silver) cluster diamonds, due to their high physico-chemical 22 7 9 4 9 activity, are sites (nuclei) of crystallization from which crystallisation of the metal beings. Since a great number of particles participate in the process, crystallization is of a mass multinucleous character. The coating being formed has small dimensions of the structural fragments with no long-range order in the crystalline structure. The size of chromium crystallites is close to that of diamond particles,which is confirmed by X-ray structural analysis and electron microscopy. Besides, a combination of almost noninertial mass transfer of the dispersed phase particles and mass crystallization of chromium ensures uniform deposition of coating on equipotential surfaces.

A small size of chromium crystallites (a high degree of structural breaking) ensures a considerable rise (1.5-2.5 times) in microhardness of the coating and an increase (2.5-3.0 times) of wear-resistance as compared with coatings containing diamonds with particle size of from 0.01 to 0.5as a dispersed phase.

In addition, it was shown experimentally that introduction of cluster diamonds into chromium-plating elect-rolytes decreases the energy barrier of Cr —> Cr^ reduction which makes it possible to rule out the operation of initial "working" of the electrolyte for 4-6 hours on any cathodes for the formation of Cr"" ions in the electrolyte. 22 794 9 Thus, the use of cluster diamonds for preparing electrochemical chromium-based composite coatings changes the mechanism of the formation of coatings and substantially improves the properties of the coating, namely: - cluster diamonds form stable dispersions in chromium-plating electrolytes; 6+ - cluster diamond favour the reduction of Cr to Cr^+ which facilitates the preliminary stages of the process and decreases energy consumption for the management of the process; - small mass (low inertia) of diamond clusters ensures ah effective mass transfer of the diamond particles to the surface being coated which makes possible the operation at high current densities; - cluster diamonds, due to their high physico-chemical activity, ensure mass crystallisation of chromium as a. result of which superfinely dispersed structure of the coating is formed with enhanced microhardness and wear resistance; - a small size of diamond clusters and chromium crystallites ensures an exact replica of the surface microrelief which increases the total contact surface area and, as a result, limit loading values of breaking away the coating from the metal matrix; - an improved quality of the coating is attained when the content of diamonds in the coating is low (0.3-1.0 mass %) which makes the process economical; 22 794 9 - composite coatings based on chromium and cluster diamond possess an enhanced corrosion resistance; - a decrease in the energy threshold of chromium reduction, mass crystallization of chromium on cluster diamonds, an effective mass transfer of diamonds to the surface being coated ensures a uniform deposition of coating on equipotential surfaced.

The content of cluster diamonds in chromium-plating electrolytes amounts to 5-^0 g/1. An enhanced content of cluster diamonds (above 40 g/1) results in strong thickening and structurization of electrolytes which hinders gas liberation, electrolyte convection, and passing of the current. A decrease in the content of cluster diamonds in the electrolyte below 5 g/1 deteriorates considerably the quality of the costing. The content of cluster diamonds in electrolyte is determined by the size aid shape of the parts being treated. For instance, in the case of small articles with sharp edges, electrolytes with a content of cluster diamonds equal to 1.5-40 g/1 are more effective. Such articles include thin-blade tools, dentist drills, microsurgical tools. A high concentration of cluster diamonds in electrolyte decreases electric field intensity on sharp edges of the articles and lowers the possibility of dendrite formation. Electrolytes with a concentration of cluster diamonds of from 5 to 15 g/1 make it possible to strengthen effectively the surfaces of the articles of a great size; punches, matrices, hydrocylinder rods, cylinders of internal 227949 combustion engine, guides, reduction gears, gear boxes, and other parts of machines and mechanisms In this case, the electrolyte viscosity is not high and the process is effective under natural heat convection in the electrolyte.

The temperature of the electrolyte upon deposition of coatings based on chromium and cluster diamond is chosen depending on the purpose of using the coating. For instance, for friction units, bearings, guides, hydrocylin-ders, and gears which require coatings with a low friction coefficient, the temperature of the electrolyte is chosen from 35 to 50°C. For cutting tools, punches and matrices, compression rings, and camshafts in internal combustion engines, i.e. for those .cases when the coating operates under high "pressure + shear" loadings, the electrolyte temperature is chosen from ?0 to 70°G.

The cathodic current density in electrolytes for deposition of coatings based on chromium and cluster diamonds is established within wide ranges depending on the required coating structure (layered or columnar), optical properties (milky or lustrous), and on hydrodyna-rnic conditions of the process.

For instance, in the case of free convection in the electrolyte the current density is from 40 to 60 a/dm2, Wltsreas for the case of forced delivery of the electrolyte into the zone of applying the coating and mechanical activation of the deposited coating the current density is 200-600 a/dm2. 11 7 9 4 9 Properies of the coating obtained by the proposed method and simplicity of the process make the proposed method competitive with the known methods of applying the coatings such as deposition from a gaseous phase and plasma, ionic and detonation sputtering, deposit by welding, diffusion strengthening of surfaces with nitrogen, boron, carbon, etc.

The coatings based on chromium and cluster diamonds are used for strengthening the working surfaces of the articles of a wide range such as cutting tools (taps, multiflute drills, milling cutters, ssw blades, files, needle files,dentist drills), pressing tools for cold pressing of metallic powders, matrices and punches for deep cold, drawing of metals, parts of machines and mechanisms, for instance, cylinders, piston rings, shafts of gas-distribution mechanisms in internal combustion engine, hydrocylinders; medical tools, blades, knives, dies, etc.

Table 1 presents the data on an increase of the service life of the articles with coatings based on chromium and cluster diamond in comparison with known methods of strengthening the working surfaces.

Table 1 Type of the Known method An increase in service article of strengthe- life of the coating ob- ning tained by the proposed method in comparison with known methods -90 time a ~ of metallic powders 22 7 94 9 1 2 3 2, Matrices and punches for deep cold drawing chromium-pla ting 2.5-4.0 times of metals 3. Saw blades hardening 4.0-8.0 times 4. Screw taps hardening . 4.0-5.0 times . Scr.ew taps titanium nitride 1.3-1.5 times 6. Drills (for glass-reinforced plastic) hardening -30 times 7. Multiflute drills hardening 50 t i me s 8.. Milling cutters for operation on skull hardening 11 t i me s 9. Dentist drills chr omiumr-pla ting -12 times . Gas-distribution shafts of internal chromium-plating 2-2.5 times combustion engines 11. Cylinders of internal combustion engines in sports chromium-pla ting 2-3 t i me s motocycles 12. Matrices and shafts . for drawing high- strong steels from chromium-plating 0.9-1.0 time hard alloys 13. Files and needle f ile s harde ning 2.5-4.0 time A method of preparing composite coatings base on chromium and cluster diamonds is technologically simple and is accomplished as follows. 227949 Aqueous colloid (4-6%) of cluster diamond is introduced into a chromium-plating electrolyte prepared by the known method. The content of cluster diamond in the electrolyte is 5.0-40.0 g/1. The article being treated which serves as a cathode is immersed into a bath with an electrolyte heated preliminary to a preset temperature. The anodes are prepared from lead or from an alloy of lead with antimony. The bath is made of a material resistant to the action of the electrolyte, for instance, titanium, glass, ceramics, or plastic material. Prior to operation, the electrolyte is stirred either mechanically or by blowing gas, for instance, compressed air. Further on no forced agitation is performed. The stability of suspension of cluster diamonds in electrolyte is determined by the properties of cluster diamonds, by liberation of gasee on anode and cathode, and by thermal convection of electrolyte.

The thickness of the applied coatings is chosen depending on the purpose of the article. FDr instance, it may be 0.02-0.06 j\,m for blades, 0.5-5.0 y*m for cutting tools, 10.0-50.0 ym for pressing equipment, 80.0-200.0 Ji m for cylinders of internal combustion engine and more than 200.0f*m for the recovered parts of machines and mechanisms.

It is preferable to prepare an electrolyte by preparation of a concentrate and dilution thereof with water 22 7 9 4 9 to the working concentration. The use of a concentrate simplifies the transportation and storage of electrolyte with cluster diamond.

In the course of operation of the bath with cluster diamonds the analysis 3nd required correction of the composition are performed. To apply the coatings on the parts Df complex shapes, use is made of anodes and screens prepared by the known methods. The preparation of the ar-riclea for application of coatings, namely, mechanical treatment, defatting, etching, removal of oxide films is also performed by the known methods.

For a better understanding of the present invention specific examples of realizing thereof are .given herein-below by- way of illustration.

The compositions of the electrolytes do not rule out the possibility of introducing therein different additives, for instance, for decreasing the surface tension of the electrolyte or lowering evaporation. It should be taken into account that such additives interact with cluster diamonds and cause the formation of clusters, coagulation, and sedimentation of diamonds which decreases the efficiency of the process and quality of the coating. Example 1 Chromic anhydride (230,0 g/1) and sulphuric acid (2.5 g/1) dissolved successively in distilled water and 6% aqueous colloid of cluster diamond with a particle 14 127 9 4 9 size of the diamond from 0.001 to O.Ol^um is introduced. The amount of cluster diamond in the electrolyte is 5 g/1. The prepared electrolyte is poured into a bath heated with steam, hot water, or electric heater. The electrolyte is heated to 35°0. The anodes are immersed into the bath and the treated article serves as a cathode. In the case of treating the articles of a complex shape use is made of special anodes and screens.

The article is thoroughly purified, defatted in known chemical and (or) electrochemical baths, washed, and connected to the electrical bus of the cathode.

Use is made of standard sources .of direct current with controlled voltage and current and with variable current polarity.

The articles are placed into the electrolyte, heated to the electrolyte temperature, the 30 a/dm current of an opposite polarity is switched on for 30 s, then the current of the direct polarity with n a magnitude of 60 a/dm is fed for 15 s and then p with a magnitude of 30 a/dm . The deposition rate of the coating, is 1.0-1.1 m/min.

Microhardness of "the coating measured by the method of pressing a pyramid from natural diamond with an p angle at a vertex of 108.9° amounted to 650 kg/mm . 22 794 9 Bxamples 2-5 The coatings are prepared by following the procedure described in Example 1. The amount of cluster diamond in the electrolyte is 10, 20, 30, 40 g/1, respectively. An average microhardness of the obtained coatings is 840, 2 1130, 1200 , and 1030 kg/mm , respectively.

Examples 6-8 The coatings are prepared by following the procedure described in Example 1. The amount of cluster diamond in the electrolyte is 15 g/1. The temperature Df the electrolyte is 45, 55, and 70°C, respectively. An average 2 microhardness of the coating is 1020, 1410, and 1280 kg/mm , r espectively.

Example 9 Chromic anhydride (250 g/1) and sulphuric acid (0.5 g/1) are dissolved in distilled water after which barium sulphate (6 g/1), potassium fluosilicate (20 g/1), and aqueous colloid (4%) of cluster diamond are introduced. The amount of cluster diamond is 15 g/1* The prepared electrolyte is heated to 55-2°C and stirred for 15 min according to the known methods for passing into solution a part of poorly soluble component9, namely, potassium fluosilicate and barium sulphate. All further operations are accomplished by- following the procedures described in Example 1. The cathodic current 2 density is 60 a/dm and the current of direct polarity p 90 a/dm . The rate of depositing the coating amounts to 22 7949 1.1-1.3ytun/min. The properties of the coating are given in Table 2, Examples 10-14 The coatings are obtained as described in Example 9« The content of cluster diamonds in the electrolyte is 2, 4, , 20, and 40 g/1, respectively. The properties of the coating are listed in Table 2.

Table 2 Comparative properties of the coatings Costing Diamond Wear resistance Friction Micro- Temperature content coating, coun- coeffi- hard- in the fric- in elec- terbody cient ness tion zone,°C trolyte, turn , _ , 2 g/1 J J — kg/mm'1 3 Y Cromium without a d ispersed phase - 20.2 27.0 Cr omium with diamond 0.01- 0.5 fxi 15 5.6 47.0 Chromium with clus- 2 ter diamonds (example 8.9 29.0 ) (example 4.8 26.5 11) (example 12) 0.15 610 (example 9) (example 13) 40 (example 14) 2.9 25.4 2.0 24.0 .3 30.4 7.7 27.2 0.24 0.14 0.10 0.09 0.09 0.13 0.14 790 710 920 1480 2100 1900 1630 185 170 165 165 160 155 175 180 22 7 9 4 9 The wear-resistance tests of the coatings were performed on a friction machines by the "block-roller" scheme with a dr-opwise supply of a low-viscoua oil.

The friction surface area was 1 cm and the loading on the friction surface was 100 kgs.

The duration of the test was 100 hrs and the sliding velocity at the contact 0.8 m/a.

The block and the roller were made of steel with a chromium content of 4 mass %. The coatings are applied on the block and the roller serves as a counterbody.

The use of cluster diamonds in accordance with the above examples decreases abrasive wear-resistance of the counterbody, lowers friction coefficient and temperature in the friction zone.

Example 15 Chromic anhydride (225 g/1) is dissolved in distilled water. Then strontium sulphate (6 g/1), potassium fluosilicate (20 g/1), and cluster diamonds are introduced similarly to Bxample 9. The parameters of the process are also similar to those in example 9« The prepared electrolyte possesses weaker corrosion action as compared with electrolytes prepared as described in Examples 1 and 9* It is predominantly used for articles from aluminium and the alloys thereof, as well as for articles of small sizes and articles with sharp edges, for instance, blades needles, thin drills, and dentist drills. Microhardness

Claims (3)

and wear resistance of the coatings are equivalent to those in Bxample 9. WHAT ilWE CLAIM IS:- - 19- 227949

1. A method of preparing chromium-based composite coatings comprising electrochemical deposition from a chromium-plating electrolyte containing a suspension of diamond particles/ wherein/ said diamond particles are colloidal cluster particles with a particle size of from 0.001 to 0.01 um in an amount of 5-40 g/1.

2. A method according to claim 1 and substantially as described in this specification with reference to the examples.

3. A chromium-based composite coating whenever prepared by a process according to either claim 1 or 2. ALEXANDR IVANOVICH SHEBALIN/ VALERY DONATIEVICH GUBAREVICH, JURY NIKOLAEVICH PRIVALKO, PETR MIKHAILOVICH BRYLYAKOV, VASILY IVANOVICH BESEDIN, GENNADY VIKTOROVICH SAKOVICH, ALEXANDR YAKOVLEVICH CHEREMISIN, ALEXANDR NIKOLAEVICH KOTOV, STANISLAV ALEXEEVICH KOZLOVSKY, NAUM BORISOVICH ALTSHULER OLCSW3LK. . By Their Attorneys BALDWIN, SON & CAREY •T V >3 NOV 1990'