Patent Application
20180056386
This invention relates to a process for applying a low coefficient of friction coating to interacting parts of a mechanical device. The low coefficient coating is comprised of nanoparticles of a metal melting below about 400° C., preferably bismuth. Interacting parts of a mechanical device, prior to assembly of the mechanical device, are submerged in a dispersion of the nanoparticles, then heated to an effective temperature, then cooled, thereby resulting in a coating of the nanoparticles onto the interacting parts.
Friction between surfaces of interacting parts of mechanical devices, particularly devices operating at elevated temperatures, is a major cause of power consumption and wear. The reduction of friction is a goal for improving fuel efficiency and for lowering power consumption and wear. For example, friction resulting from interacting surfaces in automobiles and other lubricated mechanical devices accounts for about one third of the total fuel consumed. Also, for wind turbines, up to one quarter of operating and maintenance costs are due to premature replacement of worn parts of equipment. One approach for reducing friction resulting from interacting surfaces of mechanical devices is the use of low coefficient of friction coatings on interacting surfaces. The application of low coefficient of friction coatings on interacting mechanical device parts during manufacturing of the mechanical device is usually not very successful. Conventional coatings used to provide low coefficients of friction typically have a micron size grain structure as opposed to a nano-size grain structure. One such conventional coating is a diamond coating that is expensive to implement into conventional manufacturing processes. Such coating processes are typically limited in the size of the parts they can coat. In addition, conventional coating processes do not result in coatings that are capable of preserving the designed clearances between interacting surfaces.
Therefore, there is a need in the art for coatings that will provide: a low coefficient of friction between interacting surfaces; will preserve designed clearances between interacting surfaces; have superior wear properties, and that are cost effective to apply.
In accordance with the present invention there is provided a process for applying a low coefficient of friction coating to interacting parts having interacting surfaces, of a mechanical device, prior to assembly of the mechanical device, which process comprises:
i) dispersing about 0.001 wt. % to about 2 wt. % of nanoparticles of one or more metals having a melting point less than about 400° C. in a lubricating oil, thereby forming a dispersion;
ii) placing at least a portion of the interacting surfaces of said interacting parts to be coated into said nanoparticle dispersion for an effective amount of time to enable nanoparticles to be adhered to at least a fraction of the interacting surfaces;
iii) heating said interacting parts to a temperature effective to initiate sintering of said metal nanoparticles thereby resulting in the adhered nanoparticles to form a coating on said interacting surfaces; and
iv) cooling the coated interacting parts thereby resulting in a final coating on the interacting parts ready for assembly into a mechanical device for which the part was designed.
In a preferred embodiment, the metal is selected from the group consisting of bismuth, cadmium, tin, indium, and lead.
In another preferred embodiment the particle size of the metal nanoparticles is from about 2 nm to about 200 nm.
In another preferred embodiment, the interacting parts are manufactured from a material selected from the group consisting of metals, ceramics, and polymeric materials.
In yet another preferred embodiment the mechanical device is selected from engines, motors, turbines, bearings, and transportation vehicle gear boxes and transmissions.
The present invention relates to a method for providing low coefficient of friction coatings having a thickness and grain structure in the nanometer size range and that are capable of preserving the designed clearances between interacting surfaces in mechanical devices requiring lubrication. The present invention is based on a dispersion, preferably a stable colloidal dispersion of nanoparticles of a low melting metal in a lubricant. That is a metal having a melting point less than about 400° C. Such metals include bismuth, cadmium, tin, indium, and lead all of which melt below about 400° C. Bismuth is preferred and as such this application will be written primarily in terms of bismuth. The bismuth nanoparticle oil dispersions of the present invention can be initially be introduced into the oil reservoir of a mechanical device, such as an engine crankcase, gearbox, or a transmission. The mechanical device is then operated under normal operating conditions, preferably under startup conditions, for an effective amount of time to reach the activation temperature of the dispersion. By effective amount of time we mean for at least that amount of time wherein at least an effective percent of the interacting surfaces are at least partially coated with the bismuth nanoparticle coating of the present invention. By at least an effective percent of interacting surfaces coated we mean that at least that amount of coating is applied that will result in at a least 25%, preferably at least a 40%, and more preferably at least a 60% decrease in coefficient of friction compared to the same lubricant but without a dispersion of bismuth nanoparticles.
Another preferred process of applying a low-melting metal nanoparticle coating onto parts manufactured for use in a mechanical device is to coat the parts before assembly of the device. It is within the scope of this invention that either the entire part can be coated or just the interacting surface(s) of the part. That is, the targeted surface can be either the interacting surface of the part of the entire part. The part can be of a material selected from metal, ceramic, or polymeric. The targeted surface to be coated is preferably immersed in a dispersion comprised of nanoparticles of one or more low-melting metals dispersed in a lubricating oil, preferably a lubricating oil suitable for use in the mechanical device for which the part was intended. The targeted surface is submerged in the nanoparticle oil dispersion for an effective amount of time, which will typically be about one hour or less. By “effective amount of time” we mean for at least that amount of time that will allow nanoparticles to adhere to at least a fraction of the targeted surface of the part being coated. By targeted surface we mean the section(s) of a part that is designed to interact with one or more surfaces of another part designed to be assembled in a mechanical device. It is preferred that the nanoparticles adhere to at least 25%, more preferably at least 40%, and most preferably from about 50 to 100% of the targeted surface of the part being coated. This allows the low-melting metal nanoparticles to adhere to the surface of the part by colloidal or other adhesion mechanism.
The adhesion mechanism plays an important role in the nanoparticles adhering to, and forming, a coating on the targeted surface. For mechanical device parts that will undergo sliding, rolling or other types of surface interactions, the adhesion mechanism without the coating being subjected to an activation temperature, will typically not be strong enough to keep the nanoparticles from being removed/scrapped from the surface during operating conditions of the mechanical device. This unintended removal of nanoparticles will substantially reduce the benefits to be gained by lowering the coefficient of friction. Thus, it is preferred that the temperature of the part, or the temperature of the nanoparticle-containing oil dispersion, or both, be at an effective temperature during or after the adhesion step. The term “effective temperature”, which is sometimes referred to herein as the “activation temperature” is at least that temperature at which the low-melting nanoparticles begin to sinter and become strongly attached to the targeted surface. Since this activation temperature will typically exceed the sintering temperature of the low-melting metal nanoparticles, the nanoparticles will start to bond to each other and to the targeted surfaces they come into contact with. Sintering is the ability of particles to form solid coatings and bodies by the diffusion of atoms across the boundaries of the particles in contact with each other at high enough temperatures. The upper end of the effective temperature will be that temperature wherein the integrity of the nanoparticles, or the coating, begins to fail.
After the targeted surface of the part is coated, the temperature is lowered to below the sintering or melting point of the low-melting metal of the metal nanoparticle. That is, where the low-melting metal of the nanoparticles liquefies. It is more economical to sinter compacted high melt point metal/ceramic particles to solid bodies than to use a more conventional melt and cast procedure.
As the particle size of the nanoparticles decreases, the sintering temperature also decreases. In the nanoparticle size range for bismuth, the sintering temperature drops below the melting point of 274° C. In addition, many metals are soluble in molten bismuth. The bismuth atoms from the nanoparticles in contact with the targeted surface are able to diffuse into the metal surface, and simultaneously dissolving and alloying with the targeted metal surface. At this stage, the bismuth nanoparticles are substantially permanently attached to the surface. The cooling step is preferred to allow the coating structure to fully form. Attachment to ceramic or polymer surfaces is also within the scope of this invention due to the highly reactive bismuth atoms from the nanoparticles diffusing and forming bonds that attach the nanoparticles to such surfaces. The utilization of this technique can be varied in terms of removal or retention of the parts in the oil dispersion while heating to the sintering/activation temperature and is related to the cost of heating the bath/parts, the size of the part, and ability of the bath to be used for more than one immersion of the parts in a batch. The parts can also be placed in a heated oil dispersion at the activation temperature, but care must be taken to prevent the nanoparticles from interacting more with each other than with the targeted surface and forming larger particles that will not attach to the targeted surface.
Any suitable lubricant can be used in the practice of present invention. Preferred lubricants are low volatility lubricating oils. Typical lubricating oils are by necessity low volatility to withstand high operating temperatures. Such oils are prepared from a variety of natural and synthetic base stocks admixed with various additive packages and solvents depending upon their intended application. Modern base stocks for automobile engines typically include mineral oils, polyalphaolefins (PAOs), gas-to-liquid (GTL), silicone oils, phosphate esters, diesters, polyol esters, and the like. Preferred low volatility oils are those that will typically be used as the lubricant for the mechanical devicery to be treated.
Oils of lubricating viscosity useful in the practice of the present invention can be selected from natural lubricating oils, synthetic lubricating oils, mixtures thereof, as well as greases. Natural oils include animal oils and vegetable oils (e.g., castor oil, lard oil); liquid petroleum oils and hydro-refined, solvent-treated or acid-treated mineral oils or the paraffinic naphthenic and mixed paraffinic-naphthenic types. Oils of lubricating viscosity derived from coal or shale also serve as useful base oils. Synthetic lubricating oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as polymerized and interpolymerized olefins, alkylbenzenes; polyphenyls; and alkylated diphenyl ethers and alkylated diphenyl sulfides and derivative, analogs and homologs thereof. Alkylene oxide polymers, and interpolymers and derivatives thereof where the terminal hydroxyl groups have been modified by esterification, etherification, etc., constitute another class of known synthetic lubricating oil. Another suitable class of synthetic lubricating oils suitable for practice of the present invention comprises the esters of dicarboxylic acids with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoether, propylene glycol).
Further, the oil used in the practice of the present invention may comprise a Group I, Group II, Group III, Group IV or Group V oil or blends of the aforementioned oils. The oil may also comprise a blend of one or more Group I oils and one or more of Group II, Group III, Group IV or Group V oil. Definitions for the oils as used herein are the same as those found in the American Petroleum Institute (API) publication “Engine Oil Licensing and Certification System”, Industry Services Department, Fourteenth Edition, December 1996, Addendum 1, December 1998.
As was previously mentioned, the lubricant used in the practice of the present invention can also be a grease. Greases are typically comprised of oil and/or other fluid lubricant that is mixed with a thickener, typically a soap to form a solid or semisolid. Greases are a type of shear-thinning or pseudo-plastic fluid, which means that its viscosity is reduced under shear. After sufficient force to shear the grease has been applied, the viscosity drops and approaches that of the base lubricant, such as a mineral oil. This sudden drop in shear force means that grease is considered a plastic fluid, and the reduction of shear force with time makes it thixotropic. Grease is typically manufactured by first mixing together a mineral oil base stock, a fatty acid or fatty acid ester and an alkali metal salt such as lithium hydroxide. The soap base stock usually contains about 50% of the final oil content of the grease.
Since bismuth nanoparticles of the oil dispersion of the present invention will range in size from about 2 nm to about 200 nm, preferably from about 2 nm to about 100 nm, and more preferably from about 2 nm to about 60 nm. The thickness of the coatings formed will also be in the nano-size range. The coatings of the present invention are substantially superior to conventional coatings intended to reduce the coefficient of friction on interacting surfaces of mechanical device. For example, as previously mentioned, conventional coatings typically have a micron size grain structure whereas the coatings of the present invention have a nanosize grain structure, due to use of the nanoparticles. This nanosize grain structure results in stronger and harder coatings that have superior wear properties compared to conventional micron size grain structure coatings. The coatings of the present invention, because they are substantially thinner than conventional low coefficient of friction coatings, help maintain the designed low clearances between interacting surfaces of mechanical device. Another advantage of the process of the instant invention is that conventional processes for applying low coefficient of friction coatings require that the interacting surfaces of a particular mechanical device be treated with the low coefficient of friction coating prior to assembly of the mechanical device. In contrast, practice of the present invention can treat the same interacting surfaces with a substantially thinner and harder and more wear resistant coating after the equipment has already been assembled and during its normal operating conditions. This can simply be done by replacement of the intended conventional lubricating oil with the novel bismuth nanoparticle lubricating oil dispersion of this invention. The bismuth nanoparticle dispersion of the present invention can be replaced periodically as with conventional lubricating oils. Also, after the removal of the novel bismuth nanoparticle oil dispersion from the mechanical device, a conventional lubricating oil, without the novel bismuth nanoparticle additives of the present invention, can be used in the treated mechanical device and normal operation can continue with reduced friction and wear between the interacting parts because the interacting parts will now have a long-lasting coating of bismuth nanoparticles.
Other methods, such as dry collection on the vacuum chamber walls or on filters, can be utilized, but this often causes undesirable agglomeration of the nanoparticles. If this happens, it is difficult to break down these agglomerates into the smaller more desirable nanoparticles with conventional methods, such as media milling Wet collection in low volatility lubricating oils not only provides a liquid/solids dispersion, but it also quenches the molten nanoparticles in their solid state and preserves the desired nanoparticle size distribution before they are able to form larger particles. In the high temperature environment of the induction furnace, carrier gas temperature scan be between about 100° C. and 200° C. This makes any nanoparticle below about 300 nm zine of the nanodroplet, which needs the lower temperature collection oil to quench and cool them to solid form before larger droplet formation. It also prevents undesired oxidation of the reactive metal nanoparticles. Although other liquid collection methods, such as sparging the nanoparticle gas stream through the low volatility lubricating oil, or contacting with a film of oil, can be used to form a nanoparticle in oil dispersion, spray collection is preferred. This is because spray collection provides a more intimate contact between the lubricating oil and the hot nanoparticles and nanodroplets of molten metal. Without the oil spray cooling process, it is difficult to form a stable nanoparticle particle size distribution containing smaller nanoparticles with low melting points (540° C. and below) without agglomeration occurring.
The heating source for the melting and vaporization of the bismuth, or other suitable metal, can be any source that is capable of providing a relatively constant temperature between about 900° C. and 1800° C., preferably between about 1200 and 1600° C. Non-limiting examples of heating sources that can be used in the practice of the present invention include filament heating and other associated methods, and induction heating. Induction heating is preferred, particularly using a pressed graphite crucible to melt and evaporate the metal. It is also preferred that “bumping” of the melted liquid metal be prevented while the metal is heated in the evaporation crucible. Bumping can lead to the formation of undesirable large micron-size metal droplets. One preferred method to prevent “bumping” is to place a piece of refractory material in the crucible with the melted metal to mitigate and preferably eliminate bumping. The refractory material must be one that will not undergo any chemical or physical changes at the temperatures employed. It is preferred that the refractory material be porous, such as a piece of porous carbon foam. As the metal vapor rises from the crucible during heating, it comes into contact with the inert gas which provides back pressure in the system that results in the formation of nano-droplets and nanoparticles from condensation of the molten metal vapor. The inert gas stream, containing metal nano-droplets and nanoparticles, is passed through a stream of atomized low volatility lubricating oil whose oil droplets come into intimate contact with the newly formed bismuth nanodroplets and nanoparticles. The vapor pressure of the lubricating oil must be low enough so that an undesirable amount does not vaporize in the system and raise the background pressure in the system beyond the capacity of the vacuum pumps. The oil can be heated to insure proper atomization within the system. The oil can also contain one or more non-aqueous stabilizing agents, such as lecithin, in addition to other compounds typically found in lubricating oils to prevent agglomeration, such as magnesium sulfonate, wear, such as tricresyl phosphonate; and oxidation, such amines and phenols. Other lubricant properties, such as pour point and viscosity, can be modified with the addition of polyalkylmethacrylates and polyolefins, respectively.
After formation, the bismuth nanoparticle/ oil dispersion can be filtered through a 200 mesh or greater wire filter to separate out the larger particles and agglomerates that may have formed from molten liquid buildup on the walls and piping of the system. At this point, the oil dispersion can be utilized as a low viscosity lubricant whose performance is enhanced over that of conventional lubricants for the intended mechanical device, even after an initial run time in the mechanical device and after the formation of the a low friction nano-coating on interacting parts. However, it is preferred to remove the bismuth nanoparticle oil dispersion after an initial run time and after the formation of a low friction nano-coatings has formed on the interacting surfaces. The coefficient of friction for the nanoparticle oil dispersion is lower than that of virgin lubricating oil typically used for the system, but it is beneficial to remove the bismuth nanoparticle oil dispersion from the system and replace it with virgin lubricant to prevent larger nanoparticles from having an abrasive effect. The bismuth oil dispersion can also be utilized as a component in various grease formulations.
The bismuth nanoparticle particle size distribution can be tailored to a specific operating temperature range of the intended equipment/mechanical device. For example, every particle size distribution will have an optimum temperature at which a low coefficient of friction nano-coating can be formed on interacting parts. The nano-coating formation occurs by the melting and sintering of the smaller bismuth nanoparticles in the typical bell-shaped particle size distribution curve Nanoparticles smaller than the mean diameter in the particle size distribution will sinter and melt while the larger nanoparticles will remain substantially solid. At effective concentrations, of bismuth nanoparticles in the lubricating oil (typically below 2 wt. %), low coefficient of friction coatings are formed on the interacting surfaces. The concentration of nanoparticles dispersed in the lubricant will range from about 0.001 wt. % to about 2 wt. % based on the total weight of lubricant plus nanoparticles. The entire interacting surfaces will not need to be covered with the coating of the present invention as long as an effective discontinuous coating is formed on the interacting surfaces to provide an effective decrease in coefficient of friction; however total coverage if preferred. An example of nano-coatings formed by practice of the invention is shown in
The formation of the low coefficient of friction nano-coating of the present invention is dependent on such things as the operating temperature of the equipment or mechanical device containing the nanofluid and concentration of bismuth nanoparticles in the nanofluid. The term “nanofluid” is introduced herein to mean the nanoparticle/lubricant dispersion used to coat interacting parts. Below the crucial temperature for a substantially constant temperature where the smaller nanoparticles in the particle size distribution start to sinter and begin to adhere to the interacting surfaces and to other nanoparticles, the nanoparticles will simply roll between the interacting surfaces and act as ball bearings between the surfaces. This will have a small friction reducing effect on the friction between the interacting surfaces. As the temperature increases, a temperature is reached where the smaller nanoparticles melt and sinter and act as a binding agent between the larger, unmelted nanoparticles and the contacting surfaces, forming the nanocoating where the decrease is friction is greatest. As the temperature increases further, the ability of the bismuth nanoparticles to form a low coefficient of friction nano-coatings is compromised and the action of the interacting surfaces results in the formation of larger particle agglomerates of the nanoparticles instead of pressing the melted/unmelted nanoparticles onto the interacting surfaces and forming a nano-coating. To compensate for this effect, a decrease in the concentration bismuth nanoparticles in the lubricating oil, as the temperature increases, allows for the formation of a nano-coating having substantially the same particle size distribution; however, the nano-coatings formed may not be as continuous as at the lower temperature and may not have the same stability. Successive treatments with nanoparticle/lubricating oil dispersion of substantially the same concentration will further decrease the coefficient of friction. Eventually, a coefficient of friction will be obtained which is near, or identical to, that of a pure bismuth coating on the interacting surfaces. The concentration of the bismuth nanoparticles in the lubricating oil must be sufficient to contact each of the interacting surfaces and allow the nanocoatings to form.
The Table below shows the relationship of operating temperature of the mechanical device having interacting surfaces treated in accordance with the present invention versus mean particle size of the nanoparticles. However, due to the dependence of the nano-coating formation on both the concentration of the bismuth nanoparticles and the particle size distribution, there is overlap of the various operating ranges. This indicates the mean particle size range preferred at various temperatures of the operating mechanical device, such as gearboxes, transmissions, engines, etc., can be altered by adjustment of the bismuth nanoparticle concentration. Conversely, the nano-coating formation can occur by the addition of the lower melting particle size distribution to a higher melting particle size distribution to form thicker nano-coatings of bismuth material at temperatures where the higher melting point distribution would not form an effective nanocoating.
The following examples are presented for illustrative purposes only and are not to be taken as limiting the present invention in any way.
The process of the present invention generally involves the evaporation of bismuth metal in an inert gas condensation process under a vacuum.
A mean particle size of about 60 nm is selected for use at 75° C. An initial concentration of about 0.12 wt % with 100 ml of dispersion was selected for use for reducing friction between two thrust washers on a Falex multi-specimen tester with an 88 pound load and a rotation speed of 600 RPM. A heating mantle on the test fixture was used to adjust the temperature to 75° C. Coefficient of friction measurements were taken of a period of about 60 minutes. When 100 ml of the 0.12 wt % bismuth nanoparticle oil dispersion is placed between the two thrust washers and the test load under rotation is applied, the coefficient of friction drops by 50% as compared to the original lubricant oil (Aeroshell 555) at the same test parameters as is shown in
Replacement of the bismuth nanofluid between the two thrust washers after the testing shown in
When the temperature of the heating mantle of the test fixture was raised to 90° C. utilizing the same RPM and load, nanocoating formation and reduction of the coefficient of friction did not occur at the same concentration. However, reduction of the bismuth nanoparticle concentration in the oil does allow the nanocoating formation to occur. For 90° C. with a load of 88 pounds and rotation speed of 600 RPM, the time traces of a 0.06 wt % bismuth nanoparticle oil dispersion is shown in
When the temperature of the heating mantle of the test fixture is lowered to 45° C. and the other test parameters of load and RPM kept at 88 lbs and 600 RPM, the 60 nm mean particle size no longer forms a nanocoating on the contact surfaces of the thrust washers. However, a 30 nm mean particle size will lower the coefficient of friction as shown in
This application is a Continuation-in-Part of application Ser. No. 14/705,934 filed May 6, 2015 which was based on Provisional Application 61/989,480 filed May 6, 2014.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14705934 | May 2015 | US |
| Child | 15805055 | US |
