The following disclosure is submitted under 35 U.S.C. 102(b)(1)(A): Kylie E. Van Meter, Tomas F. Babuska, Christopher P. Junk, Kasey L. Campbell, Mark A. Sidebottom, Tomas Grejtak, Andrew B. Kustas, and Brandon A. Krick, “Ultralow Wear Behavior of Iron-Cobalt-Filled PTFE Composites,” Tribology Letter 71 (1), 4 (2023). Published online: 25 Nov. 2022. The subject matter of this disclosure was conceived of or invented by the inventors named in this application.
The present invention relates to polymer composites and, in particular, to ultra-low wear magnetic polymer composites.
Polytetrafluoroethylene (PTFE) is a desirable material for tribological applications due to its low friction coefficient (μ˜0.1), low surface energy, and large temperature operation ranges. See M. M. Renfrew and E. E. Lewis, Ind. Eng. Chem. 38 (9), 870 (1946); C. W. Bunn et al., J. Polym. Sci. 28 (117), 365 (1958); C. W. Bunn and E. R. Howells, Nature 174 (4429), 549 (1954); K. V. Shooter and D. Tabor, Proc. Phys. Soc. Sect. B 65 (9), 661 (1952); and G. J. Puts et al., Chem. Rev. 119 (3), 1763 (2019). During sliding, PTFE adheres to the countersurface, creating a “transfer film” that allows for a low shear-strength interface. See S. Bahadur and D. Tabor, Wear 98 (C), 1 (1984); S. K. Biswas and K. Vijayan, Wear 158 (1-2), 193 (1992); K. R. Makinson and D. Tabor, Nature 201 (4918), 464 (1964); K. Tanaka et al., Wear 23 (2), 153 (1973); and T. A. Blanchet and F. E. Kennedy, Wear 153 (1), 229 (1992). However, unfilled PTFE exhibits high wear rates on the order of 10−4 mm3/Nm due to large-scale delamination wear of the polymer and instability/poor adhesion of the transfer film to the countersurface. Improvements in the wear rate ranging from 100× to 100,000x× have been observed when adding filler materials to PTFE, while still maintaining low friction coefficients (μ<0.2) and many of its desirable material properties. See K. Tanaka, Compos. Mater. Ser. 1 (C), 137 (1986); D. L. Burris and W. G. Sawyer, Wear 260, 915 (2006); B. A. Krick et al., Tribol. Int. 51, 42 (2021); B. A. Krick et al., Tribol. Trans. 57 (6), 1058 (2014); B. A. Krick et al., Tribol. Int. 95, 245 (2016); D. L. Burris et al., Macromol. Mater. Eng. 292 (4), 387 (2007); K. E. Van Meter et al., Macromolecules 55 (10), 3924 (2022); A. A. Pitenis et al., Tribol. Lett. 53 (1), 189 (2013); K. L. Harris et al., ACS Macromol. 48 (11), 3739 (2015); H. S. Khare et al., J. Phys. Chem. C 119 (29), 16518 (2015); D. L. Burris and W. G. Sawyer, Wear 262 (1-2), 220 (2007); D. L. Burris et al., Wear 267, 653 (2009); D. L. Burris and W. G. Sawyer, Wear 261 (3-4), 410 (2006); K. L. Campbell et al., Macromolecules 52 (14), 5268 (2019); and A. A. Pitenis et al., Tribol. Lett. 57 (1), 1 (2015).
The addition of alpha-alumina fillers to PTFE and PTFE-PEEK with concentrations as low as 0.13% have been shown to reduce the wear rates of PTFE-alumina composites to ˜1×10−7 mm3/Nm and PTFE-PEEK-alumina composites to ˜4×10−8. See D. L. Burris et al., Wear 267, 653 (2009); and K. I. Alam et al., Wear 482-483, 203965 (2021). Reduction in wear rate with the addition of alumna nanoparticles has been attributed to the nanoscale aggregate structure of micron-sized particles that can break up due to high-pressure asperity contacts at the sliding surface. See B. A. Krick et al., Tribol. Int. 95, 245 (2016). Nanoscale filler fragments have been shown to accumulate at the sliding interface, creating a mechanically harder, chemically altered, reinforced transfer film and running film. See B. A. Krick et al., Tribol. Trans. 57 (6), 1058 (2014). Furthermore, particle size has been shown to be an important variable as microscale fillers can impart larger improvements in wear rate than their nano counterparts by arresting sub-surface cracks and preventing delamination. See B. A. Krick et al., Tribol. Int. 95, 245 (2016); and S. Bhargava and T. A. Blanchet, J. Tribol. 138 (4)M, 042001 (2016). Particle size needs to be balanced with particle friability, with studies by Krick et al. showing that highly dense alumina microparticles with high hardness can inhibit the formation of the transfer film by abrading the countersurface. See B. A. Krick et al., Tribol. Int. 95, 245 (2016); and S. E. McElwain et al., Tribol. Trans. 51 (3), 247 (2008).
Understanding the effects of filler particle size and mechanical properties on PTFE-alumina composites can give insight into desirable properties of fillers to create other ultralow wear PTFE composites. As sliding-induced shear drives filler particle fragmentation and chain scission of PTFE polymer chains, tribochemistry at the sliding interface driven by bonding of carboxylic end groups to the metallic oxides of the filler material results in ultralow wear rate PTFE-alumina composites. See B. A. Krick et al., Tribol. Trans. 57 (6), 1058 (2014). However, the ultralow wear rates observed in metal-filled PTFE composites suggest that the presence of oxide fillers is not required for ultralow wear. In recent studies by Ullah et al., PTFE filled with titanium, chromium, and manganese microparticles slid against brass exhibited ultralow wear rates (2×10−9-2×10−7 mm3/Nm). See S. Ullah et al., Wear 498-499, 204338 (2022). Bronze is the most commonly used metallic filler in PTFE composites for industrial applications. High weight percent (40-60%) bronze fillers in PTFE are used in linear bearing applications and exhibit wear rates of ˜3×10−7 mm3/Nm. See T. A. Blanchet and F. E. Kennedy, Wear 153 (1), 229 (1992); D. D. Tabor, “Friction, Lubrication, and Wear,” in Mechanical Design Handbook, McGraw-Hill (2006); G. E. Totten ed., ASM Handbook, Volume 18: Friction, Lubrication, and Wear Technology, (2017); B. Aldousiri et al., Adv. Mater. Sci. Eng. (2013); C. A. G. S. Valente et al., Tribol. Trans. 63 (2), 356 (2020); M. Trabelsi et al., Trans. Indian Inst. Met. 69 (5), 1119 (2016); M. J. Khan et al., Int. J. Surf. Sci. Eng. 12 (5-6), 348 (2018); Y. Wang and F. Yan, Wear 262 (7-8), 876 (2007); and H. Unal et al., J. Reinf. Plast. Compos. 29 (14), 2184 (2009). PTFE-bronze has excellent thermal conductivity but low electrical conductivity and has no intrinsic magnetic properties.
However, a need remains for ultralow wear polymer composites made with magnetic or electrically conductive filler materials.
The present invention is directed to a magnetic polymer composite, comprising magnetic alloy particles dispersed in a perfluoropolymer matrix. The composite can comprise between about 1 and about 50 wt. % magnetic alloy particles. The size of the magnetic alloy particles can be less than 100 μm and greater than about 100 nm. For low wear, the particles can be brittle and have a low fracture toughness, with a low yield strength and low strain-to-failure. For example, the magnetic alloy can be an Fe—Co intermetallic alloy and the perfluoropolymer can be polytetrafluoroethylene.
The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers.
The invention is directed to a magnetic polymer composite, comprising magnetic alloy particles dispersed in a perfluoropolymer matrix. The magnetic alloy can be a brittle intermetallic material having a low strain to failure or low fracture toughness. For example, the magnetic alloy can be an iron-cobalt intermetallic alloy. The iron-cobalt alloy can have a range of compositions, for example, from Fe0.3Co0.7 to Fe0.7Co0.3. The Fe—Co alloy can be further alloyed with additional elements, such as vanadium, chromium, niobium, molybdenum, and/or nickel. The size of the magnetic alloy particles can be less than 100 μm and greater than about 100 nm. The composite can comprise between about 1 and about 50 wt. % magnetic alloy. A variety of perfluoropolymers can be used, including PTFE, fluorinated ethylene-propylene (FEP), polychlorotrifluoroethylene (PCTFE), or perfluoroalkoxy alkane (PFA), the latter of which can be injection molded.
Iron-cobalt (FeCo) alloys are electrically conductive and, depending on the ratio of Fe to Co, have the highest mean atomic moment, and thus magnetization saturation, of any commercially available material soft magnetic alloy. For example, Fe-50Co (referred to hereafter as FeCo) has a high saturation induction (Bmax˜2.4 T) while retaining high permeability (˜8×103) and low coercivity (˜100 A/m), magnetic properties that are well suited for a range of electromagnetic applications, such as motors, transformers, and solenoids. See R. S. Sundar and S. C. Deevi, Int. Mater. Rev. 50 (3), 157 (2005); T. Sourmail, Prog. Mater. Sci. 7, 816 (2005); and A. B. Kustas et al., Addit. Manuf. 21 (1), 41 (2018). However, FeCo alloys, especially at the near-equiatomic composition, are brittle (<5% strain to fracture in tension) due to disorder-order phase transformation, making them difficult to process in bulk form and use in mechanically demanding applications unless alloyed with other elements, like vanadium, chromium, or niobium, among others. See K. Kawahara, J. Mater. Sci. 18 (6), 1709 (1983). The alloying of other elements with FeCo improves the mechanical properties but decreases some of the desirable magnetic properties. For instance, FeCo alloys with a modest 3% niobium addition can exhibit >10% lower saturation magnetization and >600% higher coercivity compared to binary FeCo. See T. Sourmail, Prog. Mater. Sci. 7, 816 (2005). For tribological fillers, however, brittle material properties can be advantageous in the development of transfer films through the breakdown and accumulation of the filler material at the sliding interface. The combination of brittle material properties with the magnetic and electrical properties of FeCo presents an interesting multi-faceted tribological filler material for perfluoropolymer composites.
As an example of the invention, Fe0.5Co0.5 microparticles were used to create a PTFE-FeCo composite. Three polymer composite samples were prepared, PTFE filled with: 5 wt. % FeCo, 5 wt. % Fe, and 5 wt. % Co. Powders consisting of alloyed equiatomic Fe0.5Co0.5 (particle size 45±15 um), Fe (325 mesh, ˜44 um particle size) and Co (1-5 um particle size) were used. To create the composite powder mixture, the dry powder components were combined and isopropyl alcohol (IPA) was added in a 5:1 (IPA to powder) ratio by mass. The solutions were mixed to form a slurry, the slurry was sonicated, and the IPA was allowed to evaporate from the sonicated composite powders.
After drying,10g of composite powder was placed in a 12.7 mm diameter 440 C stainless steel cylindrical mold and compressed to 30 MPa using a hydraulic press at room temperature. The molded cylinder was then wrapped in aluminum foil and free sintered in an oven, ramping from room temperature to 380° C. at 2° C./min, with a dwell at 380° C. for three hours, and cooled at 2° C./min to room temperature. The free sintered cylinder was machined into a 12.7×6.35×6.35 mm polymer pin. Prior to testing, each pin was sonicated in methanol for 30 minutes and allowed to air dry. The countersample used in testing consisted of a 304 stainless steel coupon (0.1875″×1″×1.5″) that was polished to a mirror finish.
The iron (Fe), cobalt (Co), and iron-cobalt powders were imaged using a scanning electron microscope (SEM) with a secondary electron detector to visualize particle size and morphology, as shown in
Laser diffraction results support the particle size and morphology observed in the SEM images, As shown in
The differential volume density distribution of the Fe powder spans across a wide range of particle sizes (1-80 μm) (
The Co powder also has a differential volume density distribution that spans a broad range of particle sizes, from 0.6 to 35 μm (
The FeCo differential volume density distribution includes particles from 12 to 100 μm (
Wear and friction experiments were performed on a bidirectional linearly reciprocating tribometer in a flat-on-flat sample configuration, as shown in
Before testing, each sample was measured with calipers and weighed on a scale to determine material density. After each cycle interval, samples were removed from the tribometer and weighed on the scale. Calculated density (using initial sample dimensions and weight) and subsequent mass loss were used to calculate volume loss of the sample. The wear rate of the sample K was calculated using Eq. 1, where V is the volume loss of sample, FN is the applied load, and d is the sliding distance.
Total wear rate Ktot was calculated using the total lost volume of the sample and the total sliding distance for the entire experiment lifetime. Incremental wear rates Kinc were calculated using the volume loss per test segment and the distance traveled for that test segment. Steady state wear rates Kss and associated uncertainties were calculated through Monte Carlo simulations. See T. L. Schmitz et al., J. Tribol. 126 (4), 802 (2004); and T. L. Schmitz et al., J. Tribol. 127 (3), 673 (2005). All steady-state wear rates reported were calculated in the regions of linearly increasing volume loss, fitting the Monte Carlo simulations to the final four points of volume loss for each sample. Total wear rate demonstrates the overall performance of the sample, while incremental wear rate shows the performance of the sample as a function of incremental sliding cycles. Steady-state wear rate is determined when the sample reaches linear volume loss behavior.
Friction coefficients and their standard deviations were calculated using methods described in Burris. See D. L. Burris et al., Tribol. Lett. 35 (1), 17 (2009). Friction loops were generated by dividing the measured friction force by the applied normal force throughout each sliding cycle and were used in the calculations for reported friction coefficient.
The steady state wear rate of the PTFE 5 wt. % Fe sample was found to be 1.8×10−4 mm3/Nm with an average friction coefficient of 0.17, as shown in
Wear testing of the PTFE-Fe and PTFE-Co samples was concluded after 50 k sliding cycles (2.5 km distance) due to high wear rates causing excessive volume loss. As shown in
Testing of the PTFE-FeCo sample was carried out for 500 k sliding cycles (50 km), 10× longer than the PTFE-Fe and PTFE-Co samples. After the first 1 k sliding cycles, the PTFE-FeCo sample experienced a high amount of volume loss (˜4 mm3) (
The run-in behavior of the PTFE-FeCo sample can be more closely inspected by plotting the volume loss over FNd, shown in
A relationship between sliding distance and friction coefficient is observed when plotting the friction coefficient over the sliding distance, as shown in
Optical images (
Attenuated total reflectance infrared (ATIR) spectra of the polymer wear surface of the Fe, Co, and FeCo filled PTFE polymer pins were collected and normalized to the 1149 cm−1 peak, the symmetric stretch of CF2 which is a characteristic peak of PTFE (
The chemical composition of the transfer films deposited on the counter sample during sliding was measured using reflectance IR at the conclusion of testing. The spectra was normalized to the 1149 cm−1 CF2 peak (
PTFE filled with 5 wt. % FeCo microparticles exhibited a steady-state wear rate of 2.8×10−7 mm3/Nm, ˜100× lower than PTFE filled with 5 wt. % Co (7.4×10−5 mm3/Nm) and ˜1000× lower than PTFE filled with 5 wt. % Fe (1.8×10−4 mm3/Nm). The ultralow wear rate of PTFE-FeCo coincides with a well-developed transfer film and robust polymer wear surface, including the formation of tribochemical species. The evolution of the sliding interface due to tribochemical changes are observed by the increase in the coefficient of friction (μ˜0.22) at 100 m of sliding (
IR spectroscopy of the polymer wear surfaces shows (
A second explanation for the PTFE-Fe behavior stems from relationships between wear and surface energy. See J. Ye et al., J. Phys. Chem. C 124 (11), 6188 (2020). Surface energy gradients between the polymer pin and the transfer film resulting from iron oxide and carboxylates in the transfer film drive material transfer from the polymer pin (i.e., low to high surface energy). See J. Ye et al., J. Phys. Chem. C 124 (11), 6188 (2020). Though gradients in the surface energy can be beneficial for forming stable transfer films, large gradients can cause excessive wear. The high levels of carboxylates found in the transfer films of the PTFE-Fe sample indicate that there is significant degradation of PTFE chains and subsequent increases in the films surface energy as a result. It is possible that the PTFE-Fe sliding interface has an extremely high surface energy gradient that can result in high material transfer and wear of the pin. In the case of the PTFE-Co sample, the wear rate was ˜2× lower than the PTFE-Fe sample (7.4×10−5 mm3/Nm) yet the PTFE-Co polymer wear surface had the lowest amount of carboxylates present and had relatively small 1360 and 1317 cm−1 PTFE chain shortening peaks. The signal of the IR spectra in the transfer film was exceptionally low, with no observable carboxylates indicating that there was very little material transferred and retained on the counter sample. The lack of tribochemical species and degraded PTFE chains could explain the poor wear rate which is only slightly better than virgin PTFE.
Microparticle fillers for PTFE composites must meet two criteria to achieve ultralow wear rates: (1) particles break up during sliding and accumulate at the sliding interface promoting formation of carboxylates, and (2) sub-surface particles need to arrest crack formation and prevent large-scale delamination events. PTFE filled with iron or cobalt microparticles appear to lack one of the two criteria required for ultralow wear, while PTFE-FeCo meets both.
Interestingly, the particle morphology and size analysis of the Fe, Co, and FeCo particles do not correspond to the working mechanistic hypothesis for other ultralow wear materials like PTFE-alumina. In the PTFE-alumina wear system, the lowest wear particles are microscale particles with nanoscale primary particles or features, and are porous enough to break down during sliding. See B. A. Krick et al., Tribol. Int 95, 245 (2016). Microscale, dense particles used as filler materials in PTFE tend to abrade the countersurface and disrupt transfer film formation. See B. A. Krick et al., Tribol. Int. 95, 245 (2016); D. L. Burris et al., Macromol. Mater. Eng. 292 (4), 387 (2007); and S. Bahadur, Wear 245 (1-2), 92 (2000). Nanoscale particles lack the ability to reinforce the bulk polymer, and do not result in ultralow wear. See B. A. Krick et al., Tribol. Int. 95, 245 (2016). In the case of the Fe, Co, and FeCo powders, the particle size and apparent agglomerate morphology of the Fe and Co particles (
The unexpected wear behavior of the Fe, Co, and FeCo microparticles indicates that particle size and morphology are not always the dominating factor in promoting ultralow wear. Particle friability and chemical interactions with reactive elements appears to dominate. While Fe promotes the formation of carboxylates and PTFE chain shortening, the particles cannot prevent sub-surface cracking and delamination, driving PTFE-Fe to be high wear. It seems that Fe microparticles can react with PTFE, causing excessive degradation at the sliding interface. Cobalt microparticles improved the wear rates of PTFE by reinforcing the bulk polymer and arresting sub-surface cracking and delamination events. Hard micro-scale particles have been shown to incrementally improve the wear rate of PTFE. See B. A. Krick et al., Tribol. Int. 95, 245 (2016); and W. G. Sawyer et al., Wear 254 (5-6), 573 (2003). Limited improvements in wear rates are a result of hard particles not breaking into smaller particles that accumulate at the surface during sliding but instead scratch the countersurface (
The superior wear performance of the PTFE-FeCo sample is attributed to the same low-wear promoting mechanisms present in low filler wt. % composites like PTFE-alumina, but with vastly different particle morphology and mechanical properties. Micro-scale FeCo particles reinforce the PTFE matrix and prevent sub-surface cracking and delamination. The inherent brittleness of FeCo alloys is a benefit for PTFE composites as it allows for the FeCo particles to break down into smaller particle sizes and accumulate during sliding, despite the dense, microscale state of the particles. Binary Fe-50Co is characterized by low yield strength (200-300 MPa) and low strain-to-failure (0-6%) in tension, depending on the alloy processing thermal history. See T. Sourmail,Prog. Mater. Sci. 7, 816 (2005); and E. P. George et al., Mater. Sci. Eng. 329-331, 325 (2002). Specifically, the mechanical properties are significantly influenced by a characteristic phase transformation at ˜730 ° C. in which a chemically disordered bcc lattice transitions to a chemically ordered B2 structure, impeding dislocation accommodation mechanisms during plastic deformation. See R. S. Sundar et al., Int. Mater. Rev. 50 (3), 157 (2005); T. Sourmail, Prog. Mater. Sci. 7, 816 (2005). The high-temperature of the disorder-order transition leads to high-atomic mobility, making it difficult to avoid the ordered phase through conventional processing and can only be suppressed through rapid quenching from the high-temperature bcc phase region at rates in excess of 1000° C./sec. See D. W. Clegg and R. A. Buckley, Met. Sci. J. 7 (1), 48 (1973). FeCo was utilized as a gas atomized powder for a filler material and likely possesses some extent of the high-temperature disordered bcc phase due to rapid solidification associated with the powder processing. Note that cooling and solidification rates in gas atomization are unknown for the particular FeCo powder evaluated in this study, but previous literature has suggested cooling rates between 102 and 108° C./sec for gas atomization powder processing, which is sufficiently rapid to promote at least partial, if not full, chemical disorder in FeCo. See D. W. Clegg and R. A. Buckley, Met. Sci. J. 7 (1), 48 (1973); and A. M. Mullis et al., Mater. Sci. Metall. Mater. Trans. B 44 (4), 992 (2013). Nonetheless, the powder filler itself is anticipated to have very limited ductility. Therefore, the intrinsically brittle nature of the FeCo alloy is likely enabling the particles to break down as a result of sliding, subsequently accumulating in and reinforcing developing tribofilms. Without the brittle nature of the FeCo alloy, these filler particles would be expected to only promote marginal improvements in the wear of PTFE while also abrading the countersurface and tribofilms, inhibiting ultralow wear behavior.
The present invention has been described as an ultra-low wear magnetic polymer composite. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.
This application claims the benefit of U.S. Provisional Application No. 63/330,194, filed Apr. 12, 2022, which is incorporated herein by reference.
This invention was made with Government support under Contract No. DE-NA0003525 awarded by the United States Department of Energy/National Nuclear Security Administration. The Government has certain rights in the invention.
Number | Date | Country | |
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63330194 | Apr 2022 | US |