Nano-additives enabled advanced lubricants

Abstract

The presently disclosed technology relates to a nano-additives to improve the performance of lubricants, oils, and greases. More specifically, the presently disclosed technology relates to applying capped metal oxide nanoparticles, such as capped zirconia nanoparticles, in the lubricants to produce a tribofilms on the lubricating surfaces to provide wear protection to the said surfaces. Also, the interaction of the capped zirconia nanoparticles with other commonly used additives in lubricants may further optimize the performance of the resulting tribofilms.

Claims

1. A method of forming a solid film on a lubricated surface comprising the steps of: (i) providing a lubricant comprised of a dispersion of capped metal oxide nanocrystals, wherein the nanocrystals are capped with a surface capping agent; (ii) introducing the lubricant in a contact region defined by two surfaces in proximity, and (iii) forming the solid film in the contact region defined by the two surfaces in proximity by sliding and/or rolling the two surfaces in proximity so as to produce pressure and/or shear stress on the lubricated surface sufficient to remove the capping agent from the metal oxide nanocrystals and thereby cause metal oxide nanocrystals to bind with each other and to at least one of the two surfaces in proximity to form a solid film of bound metal oxide nanocrystals in the contact region which is adhered to the at least one of the two surfaces in proximity.

2. The method of claim 1, wherein the solid film persists after formation in the absence of the sliding and/or rolling according to step (iii).

3. The method of claim 1, wherein the pressure is in a range of 100 MPa to 5 GPa.

4. The method of claim 1, wherein the shear stress is in a range of 10 MPa 0.5 GPa.

5. The method of claim 1, wherein the capped nanocrystals are present in the lubricant in an amount of 0.01 to 2 percent by weight of the lubricant.

6. The method of claim 1, wherein the temperature in the contact region during the sliding and/or rolling according to step (iii) is in a range of 100 C. to 200 C.

7. The method of claim 1, wherein the lubricant further comprises a zinc dialkyldithiophosphate (ZDDP) additive.

8. The method of claim 1, wherein at least one of the two surfaces comprises a steel composition.

9. The method of claim 8, wherein each of the two surfaces comprises a steel composition.

10. The method of claim 1, wherein the solid film has a hardness of 1 to 20 GPa.

11. The method of claim 1, wherein the solid film has a Young's modulus of 50 to 300 GPa.

12. The method of claim 1, wherein step (iii) is practiced so as to form the solid film to an average thickness of 30 nm to 500 nm.

13. The method of claim 1, wherein step (iii) is practiced so as to induce a shear rate on the lubricant in a range of 10.sup.2 to 10.sup.7 sec.sup.1.

14. The method of claim 1, wherein step (iii) is practiced to form a solid film in the contract region selected from the group consisting of an elasto-hydrodynamic lubricant (EHL) film, a boundary lubricant film and a hydrodynamic lubricant film.

15. The method of claim 1, wherein the lubricant is an oil or a grease.

16. The method of claim 1, wherein the lubricant is a synthetic, mineral or a natural lubricant.

17. The method of claim 1, wherein the lubricant comprises at least one component selected from the group consisting of a synthetic hydrocarbon, an ester, a silicone, a polyglycol and ionic liquid.

18. The method of claim 1, wherein the lubricant is an oil having a viscosity in the range of 2 to 1000 mPAs (cP) at a temperature of 100 C.

19. The method of claim 1, wherein the dispersion comprises zirconia nanocrystals capped with at least one surface capping agent.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1A: is an exemplary illustration of the reciprocating ball-on-flat tester used in Example 1schematic of contact configurationreciprocating ball-on-flat.

(2) FIG. 1B: is an exemplary illustration of the reciprocating ball-on-flat tester used in Example 1schematic of contact configurationreciprocating ring-on-liner.

(3) FIG. 2A: Profilometric images of optical profilometric image a slide-honed cylinder liner surface.

(4) FIG. 2B: Profilometric images of optical profilometric image a top compression ring surface.

(5) FIG. 3A: A photo of the Micro-Pitting Rig (MPR) used in the examples.

(6) FIG. 3B: A close-up photo of the MPR used in Example 1 shows the lubricant at rest covering the lower portion of the test rings.

(7) FIG. 4: Provides a schematic of the MPR contact configuration.

(8) FIG. 5A. Optical Images of Tribofilms formed by ball-on-flat test after 1 minute.

(9) FIG. 5B. Optical Images of Tribofilms formed by ball-on-flat test after 5 minutes.

(10) FIG. 5C. Optical Images of Tribofilms formed by ball-on-flat test after 20 minutes.

(11) FIG. 6A. Optical image of ball test scar after room temperature ball-on-flat test using PAO4+1 wt % capped ZrO2 nanoparticles (PAO is poly-alpha-olefins).

(12) FIG. 6B. Optical image of flat test track after room temperature ball-on-flat test using PAO4+1 wt % capped ZrO2 nanoparticles.

(13) FIG. 7A: SEM-EDX (Scanning Electron MicroscopyElectron Dispersion Spectroscopy) spectrum taken outside the flat wear track on the flat formed by 2 wt % capped ZrO2 nanoparticles in PAO oil showing Fe as the dominant element.

(14) FIG. 7B: SEM-EDX spectrum taken inside the flat wear track showing Zr as the dominant element.

(15) FIG. 8A: Optical profilometer image and line scan (solid lines) of a tribofilm formed by 1 wt % capped ZrO2 nanoparticles in PAO at 70 C. on a 52100 flat.

(16) FIG. 8B: Optical profilometer line scan showing approximately 350 nm buildup of tribofilm on the surface of the flat.

(17) FIG. 8C: Region evaluated for the buildup rate of the tribofilm (box).

(18) FIG. 9: An exemplary micrograph showing tribofilm formation on a liner after a test at 100 C. using PAO10+1 wt % capped ZrO2 nanocrystals.

(19) FIG. 10. EDX spectrum performed inside wear track of a flat tested with Mobil 1 10W30 and 1 wt % capped ZrO2 nanoparticles.

(20) FIG. 11: Evolution of tribofilm formation on the ring under for pure sliding during an MPR test.

(21) FIG. 12: Evolution of tribofilm formation on the ring up to 2 hours during an MPR test.

(22) FIG. 13A: SEM image of an area inside the test track on the ring after an MPR test.

(23) FIG. 13B: EDX spectrum of an area inside the test track on the ring after an MPR test.

(24) FIG. 14A: SEM image of an area inside the test track on the ring focused on a groove.

(25) FIG. 14B: EDX spectrum of an area inside the test track on the ring focused on a groove.

(26) FIG. 15: A schematic of the AFM configuration used for generating tribofilms.

(27) FIG. 16A: The tribofilm growth volume as function of mean contact stress, in an AFM set up.

(28) FIG. 16B: The tribofilm growth volume as function normal load, in an AFM set up, demonstrating stress-driven behavior.

(29) FIG. 17A: An exemplary aerial view of the tribofilm generated by an AFM.

(30) FIG. 17B: An exemplary top view of the tribofilm generated by an AFM.

(31) FIG. 17C: An exemplary line scan of the tribofilm generated by an AFM.

(32) FIG. 18: Cross-sectional imaging of the zirconia tribofilms at different magnification showing polycrystalline structure.

(33) FIG. 19A: A cross-sectional TEM image of a tribofilm formed by AFM.

(34) FIG. 19B: Cross-sectional EDX mapping of the same tribofilm showing that zirconia tribofilms are deficient in carbon-containing capping agents and the composition of Fe and Zr formed compositional gradients inside the tribofilm at different depths.

(35) FIG. 20A: Growth rates and cycles to tribofilms nucleation plotted for various sub-ambient test temperatures. Under tested contact conditions, tribofilm growth is observed for all temperatures between 25 C. and 25 C. although some variation in growth rate is observed.

(36) FIG. 20B: Growth rates and cycles to tribofilms nucleation plotted for various sub-ambient test temperaturesreducing interfacial temperature reduces the cycles-to-nucleation resulting in a more rapid growth initiation.

(37) FIG. 21: Cross-sectional TEM image of a tribofilm formed by AFM using a PAO4 base oil consisting of 9 wt. % zirconia with 0.8% wt. % ZDDP. Cross-sectional images show that ZDDP restricts grain coalescence and growth normally seen in pure zirconia tribofilms.

(38) FIG. 22: Cross-sectional TEM image of a tribofilm formed by AFM using a PAO4 base oil consisting of 9 wt. % zirconia with 0.8% wt. % ZDDP (left) and EDX analysis performed across the cross-section of this tribofilms (right). EDX confirms the presence of zirconia in the tribofilms, as well as phosphorous, sulfur and zinc, which confirms that these tribofilms consist of a ZDDP phase mixed with zirconia.

(39) The present disclosure provides the following additional embodiments.

EXAMPLES

(40) Test Equipment

(41) Reciprocating Rig

(42) Experiments were performed with two contact configurations (ball-on-flat and ring-on-liner) on the same reciprocating tribometer. The ball-on-flat configuration used 52100 steel counterfaces and 12.7-mm (-in.) diameter balls (Grade 25) sliding against mirror-polished flats (Sq=10 nm). The load of 15.6 N produced an initial peak Hertzian contact pressure of 1 GPa. The ring-on-liner configuration used specimens extracted from components in a commercial heavy-duty diesel engine. During all machining operations to extract test specimens, the original surfaces of the piston rings and cylinder liners were protected in order to retain the original surface roughness and honing pattern. The liners were gray cast iron with a typical honing pattern, and the ring was steel that had been coated with CrN by physical vapor deposition (PVD). The cylinder liner was mounted onto a reciprocating table on the bottom of the test rig, while the piston ring was stationary. The curvature of the ring was adjusted so that a Hertzian contact width of 10 mm was achieved. A load of 200 N produced a contact pressure of approximately 110 MPa, which is similar to the contact pressure experienced by the top compression ring at the top dead center (TDC) position in severe service. Schematics for the two contact configurations are shown in FIG. 1A and FIG. 1B. FIG. 2A and FIG. 2B shows profilometric images of the cylinder liner and top-compression ring surfaces, respectively. Their surface parameters are given in Table 1.

(43) The cylinder liner was mounted onto a reciprocating table on the bottom of the test rig, while the piston ring was stationary. The curvature of the ring was adjusted so that a Hertzian contact width of 10 mm was achieved. A load of 200 N produced a contact pressure of approximately 110 MPa, which is similar to the contact pressure experienced by the top compression ring at TDC in severe service.

(44) A small amount of oil (0.3 ml) was applied at the interface of the test components to create a thin layer at the start of each test. The tests were conducted at 1 Hz reciprocating frequency for 1 hour using a stroke length of 20 mm. Heating elements were embedded into the reciprocating table, and the temperature was controlled by a temperature control unit. Tests were performed at 70 C., 100 C., 130 C., and 160 C. respectively.

(45) Micro-Pitting Rig (MPR)

(46) FIG. 3A is a photo of the Micro-Pitting Rig (MPR) available at ANL. It consists of a center roller in contact with three larger rings. FIG. 3B shows the lubricant at rest covering the lower portion of the test rings. The lubricant is supplied to the contact via splash lubrication. Both the rings and the barrel are uni-directionally satin ground. The contacting area is flat and approximately 1 mm wide. The roughness of a ring is approximately 150 nm. The rotation speed of the rings and roller are independently controlled allowing for a range of slide-to-roll (SRR) speed ratios. The load, speed, temperature, and SRR can all be controlled and set to a condition that is relevant for replicating gear tooth contact. Additionally, the materials and surface roughness of the samples can be tailored to match that of the gear components. During a test, the MPR is capable of measuring the friction force between the roller and the rings, as well as the vibration developed at the contact, indicating the severity of the accumulated surface damage. After a test, the roller and ring samples are analyzed to quantify the amount of surface wear. Further examination of the samples can be used to characterize the protective tribofilm that formed on the surface from the lubricant additives. MPR tests were performed to evaluate the friction and wear (and/or pitting) performance of lubricants formulated with ZrO.sub.2 nanocrystal additives.

(47) Characterization Techniques

(48) Surface Profilometry

(49) An interferometric non-contact optical profilometer (Bruker, ContourGT, San Jose, Calif.) was used for measuring roughness, finish, and texture of a surface. Due to optical interference, micrographs of thin transparent films show colors that are a function of film thickness. In order to show the true surface of a tribofilm, the test components were coated with a thin layer of gold.

(50) Microscopy

(51) The wear tracks on the flats and cylinder liners after the tests were examined with an Olympus STM6 optical microscope, an FEI Quanta 400F scanning electron microscope (SEM), a Hitachi S-4700-II SEM, both equipped with energy dispersive x-ray spectroscopy (EDX) capability.

(52) Nano-Indentation

(53) A nanoindenter (Hysitron TI-950 Tribo-Indenter) was used to determine the hardness and modulus of these tribofilms formed on surfaces, under displacement control using a standard Berkovich tip. The same tip was used under scanning probe microscopy (SPM) mode to image the surface topography. The nanoindenter monitors and records the load and displacement of the indenter during indentation with a force resolution of about 1 nN and a displacement resolution of about 0.2 nm. The samples were placed on a magnetic horizontal holder and positioned with the aid of an optical microscope located above the sample. The area function parameters of the tip were calibrated using a fused quartz sample, and tip-shape calibration is based on determining the area function of the indenter tip.

Example 1

(54) Capped nanocrystals can be dispersed into base oil with multiple capping agents at least as high as 10 wt % without significantly affecting the viscosity and appearance of the oil. Concentrations of 0.5 wt. %, 1 wt. %, 2 wt. % and 10%, three different, capping agents, temperature (25 C., 70 C., 130 C., 160 C.), time (5 mins, 20 mins, 60 mins, 4 hrs, 24 hrs), and type of oil were parameters that were investigated.

(55) An important observation is the formation of a unique tribofilm by ZrO.sub.2 nanocrystal additives regardless of temperature. A tribofilm started to form on the flat during the ball-on-flat test only 1 minute after the test started, and a thick and dense (as judged by optical profilometry) tribofilm was fully formed on the flat 20 minutes into the test, as shown in FIG. 5. Due to the relatively long stroke length, the flat experienced much less rubbing than the ball, on which a thick and dense tribofilm was fully formed after 20 minutes.

(56) The formation of a tribofilm was also observed at room temperature using PAO4 as base oil with 1 wt % capped ZrO.sub.2.nanocrystals The ball test scar and flat test track are shown in FIG. 6A and FIG. 6B, respectively.

(57) A prominent zirconium peak in the SEM-EDX spectrum was found in the wear track (FIG. 7B) but it was absent outside the wear track (FIG. 7A), indicating that the tribofilm was zirconium-rich and the tribofilm had indeed originated from the ZrO.sub.2 nanocrystal additives.

(58) The tribofilms were semi-transparent so a thin gold layer was coated on the ball and flat by thermal evaporation to assure the accuracy when examined with optical profilometer. An optical image of a tribofilm obtained by the optical profilometer is shown in FIG. 8A. Instead of a net loss of material characteristic of wear, there was actually a net increase of material on the wear track. Line scans (vertical solid line) across the film revealed that the tribofilm has a height of about 350 nm above the flat surface (FIG. 8C).

(59) Quantitative evaluation of the area marked in the FIG. 8B by a solid rectangle showed a net nanocrystal-based tribofilm build-up rate of 62,700 m3 per mm of sliding distance per hour, approximately 1/300 of the total nanocrystal loading included in the amount of oil used in the tests. This indicated that there are significant amounts of nanocrystals left to continue re-generating the tribofilm. The tribofilm was also relatively smooth, the root mean square (RMS) roughness of the tribofilm was measured to be 170 nm while for the mirror polished flat the value was 40 nm.

(60) A tribofilm was also formed on liner segments in ring-on-liner tests at a range of conditions as shown in an exemplary image in FIG. 9.

(61) The modulus and hardness of the tribofilm were also measured using nano-indentation, and exemplary results are shown in Table 2, together with the results of the steel flat. The tribofilm possess very impressive modulus and hardness, only 30% less than 52100 in both cases. A tribofilm that is hard, but slightly softer than the surface material can provide sufficient load bearing capability as a rubbing surface while serving as a protective, regenerative layer if the stress is too high.

(62) A tribofilm also formed by adding capped ZrO.sub.2 nanoparticles in a fully formulated oil (Mobil 1 10W30). The presence of Zr was confirmed with EDX after a test. The result is shown in FIG. 10.

(63) A tribofilm formed under pure rolling conditions in an MPR test, at a load of 200 N, speed of 2 m/s, and a temperature of 70 C., as early as 15 minutes (143,000 cycles), continued to grow over time, and became more uniform throughout the test. The film was maintained up to 24 hours of testing (13.8 million cycles). The evolution of the tribofilm is shown in FIG. 11.

(64) A tribofilm also formed under a combination of rolling and sliding conditions in an MPR using capped ZrO2 nanocrystals loaded mineral oil. The evolution of the tribofilm is shown in FIG. 12.

(65) FIG. 13A showed an SEM image of part of the tribofilm inside the test track on the ring after the MPR test. And EDX analysis was performed and indicated the presence of Zr on the test track on the ring, as shown in FIG. 13B. Also, grooves were observed on the tribofilm and an SEM image of the groove is shown in FIG. 14A, and EDX inside the grooves showed no Zr (FIG. 14B) which means that the grooves are not filled with ZrO.sub.2 nanocrystals.

Example 2

(66) Tribofilms with the capped ZrO.sub.2 nanocrystals were also generated in an atomic force microscope (AFM) at the interface formed by a steel microsphere (ranging between 10 and 100 m in diameter) against either a 52100 steel substrate, or a silicon substrate or a yttria-stabilized zirconia substrate (illustrated in FIG. 15). The contact stress at the sliding contact was varied between 0.1 GPa and 1 GPa. Zirconia tribofilms exhibit a stress-driven growth process where increasing the contact stress increases the thickness of the tribofilms (FIG. 16). Increasing surface roughness increases the rate of tribofilm growth. These tribofilms are strongly bound to the substrate and resist removal during continued sliding with the AFM probe in either base oil or in dry sliding.

(67) Using the AFM, tribofilms with lateral dimensions as small as 2 m and as large as 50 m were generated, with local thickness varying from 10 nm to 200 nm (example shown in FIG. 17).

(68) Tribofilms in the AFM were generated in concentrations of capped zirconia nanoparticles ranging from 0.01 wt. % in PAO4 to 10 wt. % in PAO4. Additionally, tribofilms were generated in other base stocks, including mPAO SYN65.

(69) Using the AFM, tribofilms were generated at temperatures ranging from 25 C. to 130 C. (FIG. 20 shows a range of temperature from 25 C. to 25 C.).

(70) Tribofilm microstructure and chemical composition were analyzed by performing focused-ion beam (FIB) milling to produce a cross-sectional sample of the tribofilm, followed by observation in scanning electron and transmission electron microscopes (SEM/TEM).

(71) Cross-sectional imaging of the tribofilms show a nearly fully dense microstructure with no observable voids. Diffraction analysis confirms that the tribofilms consist of a mostly polycrystalline structure, identified to be zirconia. Through cross sectional imaging, evidence of grain growth and coalescence of individual 5 nm zirconia nanoparticles is also seen, as shown in FIG. 18.

(72) Through these cross-sectional images and accompanying chemical spectroscopy (such as EDX, EELS and FTIR), it is confirmed that zirconia tribofilms are deficient in carbon, indicating that tribological stresses during sliding result in the removal of capping agents prior to tribofilms formation (FIG. 19).

(73) The mechanism of tribofilms growth as deduced from these images is as follows: nanoparticles undergo selective removal of surface ligands, i.e. capping agents, at the sliding contact due to tribological stresses. In the absence of dispersing ligands, the nanoparticles interact strongly with the substrate and each other and tribological stresses cause the nanoparticles to bind strongly to the substrate and to each other, resulting in the nucleation and growth of a compact tribofilm. As the film grows, stress-driven grain coarsening occurs. Tribofilms generated in the sliding contact of the AFM show superior mechanical properties. The modulus and hardness of these films was measured to be about 160 GPa and 7.3 GPa, respectively. These values approach known literature values of bulk zirconia.

(74) Tribofilms in the AFM were also generated with a mixture of capped zirconia nanoparticles mixed with zinc dialkyldithiophosphates (ZDDP) anti-wear additives. In these measurements, zirconia was added to a PAO4 base oil in either 9 wt. %, 1 wt. %, 0.1 wt. % or 0.01 wt. %, and mixed with 0.8 wt. % ZDDP. With this oil containing both ZDDP and capped zirconia nanoparticles, measurements were made at a variety of temperatures including 25 C., 15 C., 5 C. and 5 C. Other parameters for these AFM tests (load, speed, etc.) were similar to those indicated in example 2. For all tested temperatures, and for all concentrations of capped zirconia mixed with ZDDP additive, a tribofilm growth and formation was observed in the AFM. Similar results are expected for lower temperatures, such as 15 C. and 25 C. These zirconia-ZDDP tribofilms were morphologically similar to pure zirconia tribofilms. However, for identical test conditions and durations, the zirconia-ZDDP tribofilms generated had a significantly higher thickness (i.e. volume) compared to pure zirconia tribofilms. In addition, tribofilms formed within the AFM with zirconia-ZDDP mixed in PAO4 were found to nucleate on the surface much more rapidly in comparison to pure zirconia tribofilms, which resulted in a significantly rapid tribofilms growth initiation.

(75) Cross-sectional imaging of tribofilms formed in oils containing both zirconia and ZDDP exhibit zirconia nanocrystal sizes of 5 nm, which indicate that ZDDP is effective in inhibiting grain growth and coalescence as is seen in pure zirconia tribofilms (FIG. 21). Chemical spectroscopy of FIB/SEM cross-sections of these ZDDP-zirconia tribofilms indicate the presence of both zirconia, as well as zinc, phosphorous and sulfur, and a relative high concentration of carbon, which confirm that these tribofilms consist of a distinct zirconia phase as well as a distinct ZDDP phase (FIG. 22).

(76) TABLE-US-00001 TABLE 1 Surface parameters of the samples used in Example 1. Liner Liner (slide-honed) (slide-honed) PVD CrN Ring PVD CrN Ring 10 0.55 50 1.0 10 0.55 50 1.0 Sa (m) 0.662 0.175 0.822 0.208 Sq (m) 0.936 0.224 1.451 0.268 Ssk () 2.132 0.655 0.991 0.77 Sku () 10.238 4.867 41.51 20.88 Sp (m) 2.951 1.363 36.524 10.297 Sv (m) 10.932 1.869 33.041 3.407 Sz (m) 13.883 3.231 69.565 13.703

(77) TABLE-US-00002 TABLE 2 Exemplary Modulus and Hardness Measurement Results of the Tribofilm Surface Modulus (GPa) Hardness (GPa) Tribofilm 148.40 7.04 52100 steel 216.83 11.48
The contents of all references referred to herein are incorporated in their entirety in this disclosure.