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SOLID STATE SYNTHESIS OF LITHIUM-NICKEL-COBALT-MANGANESE MIXED METAL OXIDES FOR USE IN LITHIUM ION BATTERY CATHODE MATERIAL
Field of the Invention
[0001] This invention relates to the preparation of compounds useful as cathodes for lithium-ion batteries.
Background
[0002] Lithium-ion batteries typically include an anode, and electrolyte and a cathode that contains lithium in the form of a lithium-transition metal oxide. Transition metal oxides that have been used include cobalt dioxide, nickel dioxide and manganese dioxide.
Summary of the Invention
[0003] Lithium-transition metal oxide compounds in which cobalt, manganese and nickel are each present in the crystal lattice can be referred to as four metal or quaternary cathode compounds. Single-phase lattices containing appropriate amounts of these metals can provide especially desirable lithium-ion battery cathodes. For example, the quaternary compounds:
are of interest if successfully formed as a single-phase (if multiple phases are present, then battery performance suffers). The equimolar manganese and nickel content in these three compounds is especially desirable and is believed to contribute to formation of a more stable crystal lattice.
[0004] Unfortunately, it can be difficult to form a single-phase quaternary compound containing the transition metals cobalt, manganese and nickel in a lithium-containing crystal lattice. Attainment of a single-phase can be made easier by excluding one or more of the transition metals manganese or nickel (e.g., to make a three metal or ternary system such as LiNi0.8Co0.2o2 or a two metal or binary system such as IiCoO2), but this may
also decrease battery performance or introduce other problems. Attainment of a single-phase quaternary compound may be achieved by coprecipitation of mixed hydroxides as
recommended and employed in U.S. Patent Application No. 2003/0022063 Al (Paulsen et al.) entitled" LITHIATED OXIDE MATERIALS AND METHODS OF MANUFACTURE" and as employed in Examples 19 and 20 of U.S. Patent Application No. 2003/0027048 Al (Lu et al.) entitled "CATHODE COMPOSITIONS FOR LITHIUM-ION BATTERIES". However, coprecipitation requires filtration, repeated washing and drying and thus exhibits relatively limited throughput and high manufacturing costs.
[0005] Paulsen et al. also describes and employs in its Example 6 a high-energy ball milling and sintering process to make certain lithium-transition metal oxide compounds having the formula:
where 0.4
where 0
[0006] We have now found that single-phase lithium-transition metal oxide compounds containing cobalt, manganese and nickel can be prepared by:
a) wet milling cobalt-, manganese-, nickel- and lithium-containing oxides or
oxide precursors to form a finely-divided slurry containing well-distributed
cobalt, manganese, nickel and lithium, and
b) heating the slurry to provide a lithium-transition metal oxide compound
containing cobalt, manganese and nickel and having a substantially single-
phase 03 crystal structure.
Wet milling provides significantly shorter milling times than dry milling and appears to promote formation of single-phase lithium-transition metal oxide compounds. The time savings in the wet milling step more than offsets the time that may be required to dry the slurry during the heating step.
[0007] The invention provides, in another aspect, a process for making a lithium-ion battery cathode comprising the further step of mixing particles of the above-described lithium-transition metal oxide compound with conductive carbon and a binder and coating the resulting mixture onto a supporting substrate.
[0008] The invention provides, in yet another aspect, a process for making a lithium-ion battery comprising placing the above-described cathode, an electrically compatible anode, a separator and an electrolyte into a container.
[0009] The invention provides, in yet another aspect, lithium-transition metal oxide compounds (and a lithium ion battery comprising at least one compound) having the formula:
[0010] The invention provides, in yet another aspect, a lithium-transition metal oxide composition (and a lithium ion battery comprising at least one composition) consisting essentially of a compound selected from the group consisting of the single-phase compounds:
[0011] These and other aspects of the invention will be apparent from the detailed description below. In no event, however, should the above summaries be construed as
limitations on the claimed subject matter, which subject matter is defined solely by the attached claims, as may be amended during prosecution.
Brief Description of the Drawing
[0012] Fig. 1 is a triangular pyramidal plot showing a variety of lithium-transition metal oxide compositions.
[0013] Fig* 2 is a triangular plot showing a certain lithium-transition metal oxide compositions from Fig-1.
[0014] Fig. 3 is an exploded perspective view of an electrochemical cell. [0015] Like reference symbols in the various drawings indicate like elements. The elements in the drawing are not to scale.
Detailed Description
[0016] The disclosed lithium-transition metal oxide compounds have particular utility for making lithium-ion battery cathodes. The compounds are formed by wet milling together cobalt-, manganese-, nickel- and lithium-containing oxides or oxide precursors while imparting sufficient energy to the milled ingredients to form them into a finely-divided slurry containing well-distributed cobalt, manganese, nickel and lithium. The oxides or oxide precursors do not need to be mixed together all at once. We have found that by first milling together the lower surface area or larger pariide diameter materials to increase their surface area or reduce their particle size to match the surface area or particle size of the later-added components, a more homogeneous and finely-divided final mixture can be produced using a shorter milling time. Very high surface area components (such as hydroxides) that may tend to agglomerate in a milling vessel can be more homogeneously blended with other components that have already been milled to a similar high surface area. A homogeneous and finely-divided final milled mixture can help promote formation of a single-phase fired product. For example, in a milling scheme that could be referred to as "manganese and nickel first, lithium last", manganese- and nickel-containing oxides or oxide precursors can be wet milled together and formed into a finely divided first slurry containing well-distributed manganese and nickel, followed by addition of a cobalt-containing oxide or oxide precursor to form a finely divided second slurry containing well-distributed cobalt, manganese and nickel, followed by addition of a lithium-containing
oxide or oxide precursor to form a finely divided third slurry containing well-distributed cobalt, manganese, nickel and lithium. A milling scheme that could be described as "cobalt, manganese and nickel first, lithium last" can be used to promote formation of a slurry containing well-distributed cobalt, manganese and nickel prior to addition of lithium. Milling schemes such as "manganese and nickel first, cobalt and lithium last", "manganese and nickel first, cobalt last" (with lithium being added after the manganese and nickel and before the cobalt), "nickel and cobalt first, manganese and lithium last", "lithium and cobalt first, manganese and nickel last" and other permutations that will be apparent to those skilled in the art may also be employed.
[0017] Suitable cobalt-, manganese- and nickel-containing oxides or oxide precursors include cobalt hydroxide (Co(OH)2), cobalt oxides (e.g., CO3O4 and CoO), manganese
carbonate (M112CO3), manganese oxide (MnO), manganese tetroxide (Mn3O4), manganese hydroxide (Mn(0H)2), basic manganese carbonate (Mn2CO3*xMn(OH)2), nickel carbonate (NI2CO3), nickel hydroxide (Ni(OH)2)3 and basic nickel carbonate (Ni2CO3*xNi(OH)2), Preferably at least one of the manganese or nickel precursors is a
carbonate.
[0018] Suitable lithium-containing oxides and oxide precursors include lithium
carbonate (Li2CO3) and lithium hydroxide (LiOH). If desired, hydrates of the precursors
can be employed.
[0019] The amounts of each oxide or oxide precursor typically are selected based on the composition of a targeted final compound. A wide variety of targeted final compounds can be prepared. The plots shown in Fig. 1 and Fig. 2 can assist in selecting a target. Fig. 1 is a triangular pyramidal plot whose vertices A, B, C and D respectively represent the compositions LiCoO2, LiMnO2, IiNiO2 and Li(Li 1/3MN2/3)02. Vertices A, B and C
thus respectively represent maximum cobalt, manganese and nickel contents for binary lithium-transition metal oxide compounds containing these transition metals in the indicated stoichiometry. Point E located midway along edge BC represents the composition LiMn1/2Ni 1/2O2- Points within the plot located above base ABC represent
lithium intercalation compounds. Fig. 2 is a triangular plot representing the plane defined by points A, D and E. The trapezoidal region AEFG in Fig. 2 (but excluding points nearest, e.g., within about 0.01 transition metal mole units, to the vertices A and D)
illustrates an especially preferred set of compositions containing equimolar amounts of manganese and nickel. This preferred set of compositions can be represented by the formula Lia[Cox(Ni1/2 Mn1/2)l-x] O2, where 0≤ a ≤ 1.2 and 0.1≤ x ≤ 0.98. The compounds of Formulas I, II and III are shown as points within region AEFG.
[0020] A variety of wet milling techniques may be employed including media milling (e.g., ball milling, attritor milling, horizontal milling or vertical milling), medialess milling (e.g., hammer milling, jet milling or high pressure dispersion milling) and other techniques that will adequately pulverize and mix together the cobalt-, manganese- and nickel-containing oxides or oxide precursors. When media milling is employed, suitable media include ceramic media (e.g., ceramic rods or balls). Water is a preferred wet milling liquid but other substances such as low boiling point alcohols, toluene and acetone can be employed if desired. Ball milling should be carried out for a sufficient time and with sufficient vigor so that the final slurry contains well-distributed cobalt, manganese, nickel and lithium. Preferably the slurry is milled until it contains relatively small particles, e.g., with an average particle diameter less than about 0.3 fim, preferably less than about 0.1 fim as measured using scanning electron microscope (SEM) imaging. Perfectly even distribution of the metals throughout the slurry and minimum average particle diameters are not required. However, particles of a given single metal component larger than 0.5 fim preferably are avoided. The extent to which milling is carried out will merely need to be sufficient to provide the desired single-phase iiihiuiii-iiaiisition metal oxide compound at the end of the heating step. Appropriate mixing times (and when used, media) typically will depend in part on factors such as the starting materials and mixing equipment employed. Often some measure of experimentation will help in a given production setting for determining the appropriate milling times or media so that the desired single-phase lithium-transition metal oxide compound can be obtained.
[0021] If desired, other transition metal oxides or oxide precursors can be included in the lithium-transition metal oxide compositions before they are fired to provide the final lithium-transition metal oxide compounds. Representative examples include iron, vanadium, aluminum, copper, zinc, zirconium, molybdenum, niobium, and combinations thereof. These other transition metal oxides or oxide precursors can be added together with the other ingredients used to form the slurry or after the slurry has been formed.
[0022] The slurry is converted to a lithium-transition metal oxide compound by separating the slurry and media (if used) and by firing, baking, sintering or otherwise heating the slurry for a sufficient time and at sufficient temperatures to form the desired single-phase compound. The heating cycle preferably employs a rapid heating rate, e.g., 10 or more ° C per hour. A preferred heating cycle is at least 10° C/min to a temperature of at least 900° C Air is a preferred heating atmosphere but other gases such as oxygen or mixtures of carbon dioxide, carbon monoxide, and hydrogen can be employed if desired. If temperatures above about 1050° C are employed then a "ceramic furnace and longer cooling times may be required. Such higher temperatures can help in obtaining a single-phase lithium-transition metal oxide compound but may also increase capital costs and diminish throughput. If temperatures as high as 1100° C are employed, then lithium ion batteries made using the lithium-transition metal oxide compound may exhibit a slight increase in irreversible first cycle capacity loss. Preferably the maximum heating temperature is less than 1050° C, more preferably less than 1000° C, and most preferably not more than 900° C.
[0023] The resulting lithium-transition metal oxide compound preferably is formed as or converted to finely-divided particles having the desired average particle diameter. For example, the lithium-transition metal oxide compound can be prepared using a feed back mechanism in which the oxide is fired using a rotary calciner or other suitable firing device and sorted by size so that particles larger than desired are wet-milled further (or if desired, dry-milled) and particles that are smaller than desired are fired further in the calciner. In this fashion a suitable particle size distribution can be obtained. [0024] The lithium-transition metal oxide compound may be used alone in the cathode or as a cathodic additive in combination with other cathode materials such as lithium oxides, sulfides, halides, and the like. For example, the lithium-transition metal oxide compound may be combined with conventional cathode materials such as lithium cobalt dioxide or with compounds such as LiMn2O4 spinel and LiFePO4.. The amount of other
cathode material to be added is selected such that the number of moles of lithium available from the other cathode material matches the number of moles of lithium irreversibly consumed by the anode. The number of moles of lithium irreversibly consumed, in turn, is a function of the properties of the individual anode.
[0025] The cathode can be combined with an anode and an electrolyte to form a lithium-ion battery. Examples of suitable anodes include lithium metal, graphite; hard-carbon, and lithium alloy compositions, e.g., of the type described in U.S. Patent No. 6,203,944 (Turner '944) entitled "ELECTRODE FOR A LITHIUM BATTERY" and PCT Published Patent Application No. WO 00103444 (Turner PCT) entitled "ELECTRODE MATERIAL AND COMPOSITIONS". The electrolyte may be liquid, solid, or a gel. Examples of solid electrolytes include polymeric electrolytes such as polyethylene oxide, polytetrafluoroethylene, fluorine-containing copolymers, and combinations thereof. Examples of liquid electrolytes include ethylene carbonate, diethyl carbonate, propylene carbonate, and combinations thereof. The electrolyte is typically provided with a lithium electrolyte salt. Examples of suitable salts include LiPF6, OBF4, and LiClO4.. Preferably
the battery capacity does not substantially decrease after the battery is charged and discharged between 4.4 and 2.5 volts for at least 100 cycles at a 75 mA/g discharge rate. [0026] The invention is further illustrated in the following illustrative examples, in which all parts and percentages are by weight unless otherwise indicated.
EXAMPLES
X-Ray Diffraction
[0027] A powder x-ray diffraction (XRD) pattern for each sample was collected using a Siemens D500 diffractometer equipped with a copper target X-ray tube and a diffracted beam monochromator. Samples were prepared as flat rectangular powder-beds sufficiently thick and wide that the volume of powder illuminated by the x-ray beam was constant. The data were analyzed using the GS AS version of the Rietveld refinement program as described in A.C. Larson and R.B. Von Dreele, "General Structure Analysis System (GSAS)", Los Alamos National Laboratory Report LAUR 86-748 (2000).. Two
statistics Rp, and Chi2 calculated by the GSAS program were used to determine the quality of fit (expressed as the residual error on fitting for the case of Rp and as the
goodness-of-fit for the case of Chi2) for a model of the intended single-phase crystal-structure to the data. The lower the value for Rp, the better the fit of the model to the data.
The closer Chi2 is to unity (1.000)? the better the fit of the model to the data. Rp and Chi2 are generally higher when an unaccounted-for phase or phases are present. The
lattice constants or dimensions of the unit cell were also calculated using the GSAS program.
Electrochemical Cell Preparation
[0028] The powders were formulated by blending together 2.0 parts of the oxide power, 23 parts of N-methyl pyrrolidinone, 1.1 parts of a solution of 10 wt% KYNAR™ 461 polyvinylidene fluoride (available from Elf Atochem) in N-methyl pyrrolidinone, and 0.11 parts SUPER-P™ conductive carbon (available from MMM Carbon, Belgium). The suspension was stirred at high shear for greater than 1 hour, then coated on aluminum foil with a notch bar to provide a 90% active, 5% polyvinylidene fluoride, 5% conductive carbon coating. The coating was dried under vacuum at 150° C for 4 hrs, then converted into 2325 coin cells (half cells) using a metallic 380 micrometer thick, 17 mm diameter Li foil anode, 2 layers of 50 micrometer thick CELLGARD™ 2400 separator (commercially available from Hoechst-Celanese), and 1 molal LiPFg in a 1:2 by volume mixture of
ethylene carbonate and diethyl carbonate as the electrolyte.
[0029] An exploded perspective view of the electrochemical cell 10 used to evaluate the cathodes is shown in Fig. 3. A stainless steel cap 24 and special oxidation resistant case 26 contain the cell and serve as the negative and positive terminals respectively. The cathode 12 was prepared as described above. The lithium foil anode 14 also functioned as a reference electrode. The cell featured 2325 coin-cell hardware equipped with an aluminum spacer plate 16 behind the cathode and a copper spacer plate 18 behind the anode. The spacers 16 and 18 were selected so that a tightly squeezed stack would be formed when the cell was crimped closed. The separator 20 was wetted with a 1M solution of LiPF dissolved in a 1:2 by volume mixture of ethylene carbonate and diethyl carbonate. A gasket 27 was used as a seal and to separate the two terminals. The cells were cycled at room temperature and a "C/5" (five hour charge and five hour discharge) rate using a constant current cycler.
Example 1
[0030] Metal containing precursors were combined in proportions to yield the final
oxide composition LiNi0.1 Mn0.1Co0.8O2. Accurate batching was achieved by assaying
the precursors. The assays were performed by baking aliquots of the precursors at 600° C
overnight to yield completely water free single phase oxides. Measurements of the weights before and after heating combined with the knowledge of the final phase composition were used to calculate the mass per mole of metal in each precursor. This method allowed batching with at least a +/- 0.1 weight percent precision. The precursors N1CO3 (22.44 parts, available from Spectrum Chemical) and M11CO3 (21.48 parts,
Spectrum Chemical) were placed in a 1 liter high-density polyethylene SWECO™ mill jar (available from Sweco) along with 333 parts ZIRCOA™ 12.7 mm radius end cylinder zirconium oxide media (available from Zircoa, Inc.) and 1000 parts of similar 6.35 mm ZIRCOA zirconium oxide media. 200 parts deionized (DI) water were added to the mill jar and the nickel and manganese carbonates were wet-milled in a SWECO Ml 8-5 mill (available from Sweco) for 24 hours. LI2CO3 (68.12 parts, available from FMC,
Philadelphia, PA), Co(OH)2 (137.97 parts, available from Alfa Aesar) and an additional
100 parts DI water were added to the mill jar, then milled for an additional 4 hours. The resulting wet-milled slurry was poured into a PYREX™ cake pan (available from Corning, Inc.) and air-dried overnight at 70° C. The dried cake was scraped from the pan, separated from the media and granulated through a 25 mesh (707 pun) screen. The resulting screened powder was placed in a clean polyethylene bottle and the lid sealed with tape.
[0031] 15 Parts of the screened powder were placed in an alumina crucible and heated from room temperature to 900° C in oxygen over a one hour period, held at 900° C for 3 hours, and cooled. The resulting fired powder was submitted for XRD analysis using the Rietveld refinement. The observed XRD pattern indicated that the fired powder had a single phase.
[0032] The fired powder was used to form a cathode in an electrochemical cell as described above. The electrochemical cell had a capacity of 146 mAh/g. The irreversible first cycle capacity loss was 5% after charging and discharging the cell to 4.3 volts.
Example 2
[0033] 15 Parts of the wet-milled slurry from Example 1 were heated in oxygen using a "ramp-soak" cycle as follows. The slurry was placed in an alumina crucible and heated in an oven whose temperature was increased from room temperature to 250° C over 20 minutes, held at 250° C for one hour, increased to 750° C over 20 minutes, held at 750° C
for another hour, increased to 900° C over 20 minutes and then held at 900° C for three hours. The fired sample was cooled in the furnace overnight, then submitted for XRD analysis using the Rietveld refinement. The observed XRD pattern of the LiNi0.1Mn0.1Co0.8O2 indicated that the sample had a single phase.
Comparative Example 1
[0034] Powders of Co(OH)2 (7.63 parts, available from Alfa Aesar), NICO3 (1.27
parts, available from Spectrum Chemical) and MnCO3 (1.17 parts, available from
Spectrum Chemical) were combined in a tungsten carbide milling jar having approximately a 100 ml volume and containing one 15 mm ball and seven 6 mm balls of Zircon milling media like that used in Example 2. The components were dry-milled for 30 minutes on a SPEX Model 8000-D Dual Shaker Mixer (available from SPEX CertiPrep Inc.). Lithium was added to the transitional metal mixture in the form of Li2CO3 (3.79
parts, available from FMC). After the lithium addition, further dry-milling was carried out for 15 minutes.
[0035] After milling, the mixture was transferred to alumina crucibles and fired to a temperature of 900° C and held at that temperature for one hour. This yielded a compound of the formula LiNi0.1Mn0.1Co0.8O2 which was found to have at least two phases by XRD analysis.
Comparison Example 2
[0036] Aqueous solutions of nickel, manganese, and cobalt nitrate were combined in a 1:8:1 Ni:Co:Mn molar ratio. The mixture was dripped into a turbulently stirred aqueous solution of 1.6 M LiOH, which was present in 20% excess for the production of Ni0.1Mn0.lCo0.8(OH)2- The resulting slurry was filtered and washed continuously in a basket centrifuge until the residual Li in the wet cake was less than 0.2 atomic percent of the metals present. Next the cake of washed hydroxide material was dried at less than 120° C until brittle and subsequently pulverized to pass a 500 micron sieve. This powder was assayed for metals content. The powder plus Li2CO3 were combined in a 100 ml
tungsten carbide mill (available from Fritsch GmbH) in a 10:1:8:1 Li:Ni:Co:Mn molar ratio. Ten small 5 mm balls of Zircon milling media like those used in Example 2 were
added to the vessel. The vessel was shaken for 10 minutes in a SPEX™ CertiPrep™ mixer/mill (available from SPEX CertiPrep Inc.). The resulting mixture was transferred to an alumina crucible and heat treated for 1 hour at 480° C, 1 hour at 750° C, and finally 1 hour at 900° C. The resulting powder was ground in a mortar and pestle and examined by XRD using Rietveld refinement. The observed XRD pattern indicated that the single-phase compound LiNi0.iMn0.iCo0.802 of Formula I had been obtained. This was the
same product as obtained in Example 1 and Example 2, but required lengthy washing and drying steps that were not needed in Example 1 and Example 2.
[0037] A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.
We claim:
1. A process for making single-phase lithium-transition metal oxide compounds
containing cobalt, manganese and nickel comprising:
a) wet milling cobalt-, manganese-, nickel- and lithium-containing oxides or
oxide precursors to form a finely-divided slurry containing well-distributed
cobalt, manganese, nickel and lithium, and
b) heating the slurry to provide a lithium-transition metal oxide compound
containing cobalt, manganese and nickel and having a substantially single-
phase O3 crystal structure.
2. A process according to claim 1 wherein water is used for wet milling.
3. A process according to claim 1 comprising milling the slurry until it contains
particles having an average particle diameter less than about 0.3 μm.
4. A process according to claim 1 comprising milling the slurry until it contains
particles having an average particle diameter less than about 0.1μm.
5. A process according to claim 1 comprising milling the powders using ceramic
media.
6. A process according to claim 1 wherein the precursors comprise one or more
carbonates.
7. A process according to claim 6 wherein at least one of the precursors comprises
manganese or nickel carbonate.
8. A process according to claim 1 comprising milling together equimolar amounts of
manganese- and nickel-containing oxides or oxide precursors.
9. A process according to claim 1 comprising heating the slurry at a rate of at least
10° C/min to a temperature of at least 800° C.
10. A process according to claim 1 comprising heating the slurry to a temperature at or
below 1050° C.
11. A process according to claim 1 wherein the lithium-transition metal oxide compound is selected from those represented by the formula Lia[Cox(Ni1/2 Mn1/2)l-x] O2,
12 A process according to claim 1 wherein the lithium-transition metal oxide compound has the approximate formula Ii(Co(0.8)Mn0.1Ni0.1)o2.
13. A process according to claim 1 wherein the lithium-transition metal oxide
compound has the approximate formula Li(Co(1/3)Mn(1/3)Ni(1/3))O2.
14. A process according to claim 1 wherein the lithium-transition metal oxide
compound has the approximate formula Li(Li0. 08CO0.15 Mn0.375Ni0.375)02,
15. A process according to claim 1 further comprising mixing particles of the lithium-
transition metal oxide compound with conductive carbon and a binder and coating the
resulting mixture onto a supporting substrate to form a lithium-transition metal oxide
cathode.
16. A process according to claim 15 further comprising placing the cathode, an
electrically compatible anode, a separator and an electrolyte into a container to form a
lithium ion battery.
17. A process according to claim 16 wherein the battery capacity does not substantially
decrease after the battery is charged and discharged between 4.4 and 2.5 volts for at least
100 cycles at a 75 mA/g discharge rate.
18. Lithium-transition metal oxide compounds having the formula:
19. A lithium-transition metal oxide composition consisting essentially of a compound
selected from the group consisting of the single-phase compounds LiNin iMno.iCoQ gCb,
20. A lithium ion battery comprising at least one lithium-transition metal oxide
compound of claim 18.
21. A lithium ion battery comprising at least one lithium-transition metal oxide
composition of claim 19.
Embedding Ceramic Particle in Composite Chrome deposit and improvements thereon.
Field of the invention
The invention relates to a method of electrolytic deposition of metallic chromium with molybdenum co-deposited simultaneously. This is achieved through a pulse plating process where the chromium-molybdenum layer is opened up with a uniform crack pattern, within which ceramic particles of certain size distribution are deposited.
The end result is a composite plated layer with enhanced hardness, wear resistance and scuff resistance.
State of the art in the field
Many components in engineering applications have to work in an aggressive environment in higher than room temperature calling for a high degree of wear resistance and scuff resistance. Typical applications include internal combustion engines, hydraulic cylinder pumps, compressors, etc.
Various methods have evolved over the time to improve surface hardness, wear resistance and scuff resistance. Some of the existing technologies, that are part of the prior art, that claim to achieve the aforementioned properties are listed below.
The patent WO05108648 relates to production of optionally coated structurized hard chrome layer, used e.g. for decoration, protection or fimctional coating on printing roller or stamping, embossing or deep drawing tool and uses aliphatic sulfonic acid in acid plating bath. The invention discloses a method for producing a structured hard Chromium layer, according to which Chromium from an electrolyte is deposited onto a workpiece, which contains: a) a Cr(VI) compound in a quantity corresponding to between 50 and 300 gA of chromic anhydride; b) 0.5 g/1 to 10 g/1 sulfuric acid, and; c) 5 g/1 to 15 gA aliphatic sulfonic acid having 1 to 6 carbon atoms.
The patent EP0909839 relates to hard Chromium electroplate having layers of different crystalline form. The invention discloses a hard Chromium electroplate, with an anodically widened crack network incorporating solid particles, comprises two or more layers having different crystalline forms. The specification also discloses a method of production of the above electroplate by (a) depositing a first hard Chromium layer onto a cathodically connected workpiece using pulsed d.c. (preferably at 5-250 A/dm2 current density) and a sulfuric acid plating electrolyte containing dispersed solid particles; (b) depositing a hard Chromium coating using constant d.c; (c) connecting the workpiece as an anode so that the crack network of the coating is widened and filled with solid particles; and (d) repeating the above steps until the desired overall electroplate thickness is achieved.
The patent EP0668375 relates to galvanic coating of hard Chromium formed on, e.g. piston rings - comprising continuous layer and succeeding layers containing microcracks with non-metallic particles inside and outside cracks. The invention discloses a process applicable to components subjected to scuffing, such as internal combustion engine piston rings and similar, and wherein, after preferably first depositing a first continuous galvanic layer of Cr in a conventional bath, at least a second layer of Cr is deposited galvanically in a Chromium plating bath in which is dispersed a predetermined concentration of 0.1-20 aem particles of a nonmetal insoluble in the bath, and using a pulsating cathode current varying cyclically in time between a minimum and maximum value, so that the second Cr layer presents microcracks with a predetermined distribution, and the nonmetal particles are included partly inside and partly outside the microcracks, while at the same time limiting inclusion in the layer of the hydrogen developed in the bath.
The patent US4846940 relates to hard Chromium electroplate having cracks containing solid particles to improve properties. The invention discloses an electroplate in which solid particles of primarily hard substances, solid lubricants, ductile metals or their alloys and/or molten polymers are embedded in a network of cracks of hard Chromium coatings to attain improved physical characteristics, primarily to increase wear resistance, sliding behavior, ductility and corrosion resistance. The chrome plating process takes place in a microcrack-forming chrome-plating electrolyte with solid particles dispersed therein and with one-time or repeated current reversal so that, if the workpiece is connected to the anode, the network of microcracks in the Chromium coating is widened and solid particles are embedded within the cracks. Preferred uses are as coatings on the bearing surfaces of piston rings or cylinder bearing sleeves for internal-combustion engines.
The patent US4774393 relates to wear-resistant sliding material production by forming hard plating layer on mother material, and irradiating with convergent high density thermal energy beam. The invention discloses a method comprising applying a hard layer, e.g., Chromium, onto a parent material, irradiating the hard layer with a high energy density laser beam onto the plating under conditions such that concavities having a softened layer are formed thereon, the concavities being used to keep oil during the contact with the other member. The hard layer is finished grinded after irradiation.
The patent US3595590 relates to piston ring. The invention discloses a piston ring where the outer surface of a support ring is first provided with a protrusion, for example by grinding down protrusion-adjacent parts of the surface. Then the entire outer surface is coated with a hard chrome layer of substantially uniform thickness. Finally, depressions in the chrome are filled with Molybdenum. The sliding-friction resistant properties of Molybdenum are thus combined with the wear-resistant properties of Chromium into a mechanical unit capable of withstanding rough usage.
The improved coating which is special of this invention provides improved hardness and wear resistance along with improved scuff resistance to parts that are coated. The invention overcomes one of the usual deficiencies in obtaining harder coatings for better wear resistance which invariably decreases the scuff resistance. The presence of
molybdenum as a co-deposited metal provides better scuff resistance than conventional coatings. The embedment of ceramic particles enhances wear resistance. The use of Boronized layers further enhances the wear resistance. The presence of multiple cracks using pulse current enhances lubrication of the mating surfaces.
Object of the invention
Various methods that have been proposed to improve the wear resistance to increase the life of parts in mechanical systems, especially those that work under high temperatures. The various methods proposed in the prior art references provided often lead to decreased scuff resistance.
The primary object of the present invention is to provide improved surface hardness along with good scuff resistance through electrolytic deposition of hard metallic Chromium-Molybdenum with pulse current.
Another object of the present invention is to provide improved wear resistance by providing a Boronized layer.
Yet another object of the present invention is to provide lubrication through multiple crack formation achieved by pulse current plating.
A further object of the present invention is to provide enhanced surface hardness by embedding ceramic particles of particular size within the Chromium-Molybdenum layer.
Statement of invention
Accordingly, the present invention describes a process that provides embedment of hard ceramic particles with additional Boronised layer in a composite coating of Chromium and Molybdenum.
Chromium and Molybdenum are co-deposited by a electrolytic plating process where the bath composition is controlled to provide the required relative optimal ratio of Chromium to Molybdenum. The micro hardness values of the alloy obtained under optimum conditions are higher than those of either chrome or molybdenum. Alloy deposition offers considerable possibilities of coatings with unique characteristics with very good adhesion to the basic metal.
The pulse plating process helps in creating a crack network on the layers of the Composite Coating. Current densities and timings are chosen with change of polarity that help in achieving uniformity of crack network with desired channel width and depth. Into these cracks are deposited ceramic particles such as Aluminium Oxide, of the correct size distribution.
Brief description of the drawings
Picture 1 shows the creation of a network of cracks in composite plating layer of chrome and molybdenum, achieved by pulse plating method.
Picture 2 shows embedded particles of Alumina in the cracks so developed as seen in a Scanning Electron Microscope under 500 magnification. Similar successive layers are applied one over the other over an initial coating till the required coating thickness is achieved. Boric acid addition to the bath improves the hardness and wear resistance of the composite coating.
Detailed description of the invention
The composite coating is produced in known micro crack forming electrolyte with Chromic acid, Alkyl Sulphonic acid, Anionic molybdic salt. And this electrolyte is also mixed with Ceramic particles of micron grids such as Aluminium oxide.
The first electrolyte consists of following:
A second option of the electrolyte involves addition of Boric acid to the above mentioned bath compositionas under:
Scope of the invention
Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined.
The invention can be applied to the peripheral or side wearing surfaces of components that work under the high temperatures and loads in dynamically operating equipment or appliances. Piston rings of an internal combustion engine, bore surfaces of cylinder liners or engine blocks, skirt surfaces or ferrous reciprocating pistons working in prime movers and similar engineering components requiring wear and scuff resistant coatings are typical examples for such an application described in the invention.
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