Self-lubricating thermoplastic layers containing PTFE additive having a polymodal molecular weight

Abstract

A sliding material for gliding elements includes a thermoplastic matrix material and a PTFE additive. The PTFE additive includes at least two different types of PTFE having different molecular weights.

Claims

1. A sliding element having a metallic support layer, having a metallic porous carrier layer applied to the support layer, and having a sliding material applied to the porous carrier layer or impregnated into the porous carrier layer, which has a thermoplastic matrix material and a PTFE additive, wherein the PTFE additive has at least two different types of PTFE with different molecular weights.

2. The sliding element according to claim 1, wherein the at least two different types of PTFE include high molecular weight PTFE having an average molecular weight of >10.sup.6 g/mol and low molecular weight PTFE having a molecular weight of <10.sup.6 g/mol.

3. The sliding element according to claim 2, wherein the PTFE additive contains 60 to 95% by volume of high molecular weight PTFE.

4. The sliding element according to claim 2, wherein the high molecular weight PTFE has a particle size of <300 ?m.

5. The sliding element according to claim 2, wherein the PTFE additive contains 65-87.5% by volume of high molecular weight PTFE.

6. The sliding element according to claim 2, wherein the PTFE additive contains 12.5 to 35% by volume of low molecular weight PTFE.

7. The sliding element according to claim 2 wherein the PTFE additive contains 5 to 40% by volume of low molecular weight PTFE.

8. The sliding element according to claim 1, characterized in that the total amount of the PTFE additive is 5 to 50% by volume of the sliding material.

9. The sliding element according to claim 1, wherein the thermoplastic matrix material is formed of at least one material selected from the group consisting of PA, PVDF, PFA, ETFE, PPA, LCP, PSU, PEI, PEEK, PPS, and PESU.

10. The sliding element according to claim 1, wherein the thermoplastic matrix material has at least one anti-wear additive selected from the group PBA, PI, PAI, PBI, PPTA and PPSO2.

11. The sliding element according to claim 1, wherein the thermoplastic matrix material is formed by a material selected from the group consisting of PEEK, PPS, PPA and PESU, in combination with at least one anti-wear additive consisting of PPTA or PPSO2.

12. The sliding element according to claim 1, including an additional fraction of at least one of the components: solid lubricants, fibrous materials and hard materials.

13. The sliding element according to claim 12, wherein the additional fraction of anti-wear additives, along with the solid lubricants, fibrous materials and hard materials together, makes up no more than 30% by volume of the sliding material.

14. The sliding element according to claim 12 wherein the solid lubricants contain at least one of the materials selected from the group consisting of MoS2, WS2, hBN, Pb, PbO, ZnS, BaSO4, CaF2 and graphite.

15. The sliding element according to claim 12 wherein the hard materials are at least one of the materials selected from the group consisting of SiC, Si3N4, BC, cBN, phyllosilicates, metal oxides and Fe2O3.

16. A plain bearing comprising one of a sleeve, bearing shell, or thrust washer, formed of at least one sliding element according to claim 1.

17. The sliding element according to claim 1, wherein the low molecular weight PTFE has a molecular weight <10.sup.5 g/mol.

18. The sliding element according to claim 1, wherein the total amount of the PTFE additive is 15 to 45% by volume of the sliding material.

Description

THE DRAWINGS

(1) Further and exemplary embodiments, and other features of the sliding material, the sliding element, and of the radial plain bearing are further illustrated by the following figures,

(2) wherein:

(3) FIG. 1 shows the wear behavior of various inventive sliding material compositions,

(4) FIG. 2 shows the coefficient of friction of various inventive sliding material compositions; and

(5) FIG. 3 is a fragmentary cross-sectional view of a representative sliding element having a support layer, a porous metallic carrier layer and a sliding material.

DETAILED DESCRIPTION

(6) FIGS. 1 and 2 show two graphs, each with two series of measurements carried out on compositions according to the invention, with 30% by volume PTFE and PEEK and/or PESU as the matrix material. The composition of the PTFE varies in each case in such a manner that the total volume of PTFE contains from 0 to 80% by volume low molecular weight PTFE, and the remainder is high molecular weight PTFE.

(7) The following explains the preparation of sliding materials for a sliding element 10 such as that represented in FIG. 3 according to the invention by way of example, the comparative samples for a comparative pin/roller test, and the execution of the tests.

(8) The representative sliding element 10 of FIG. 3 has a metallic support layer 12 onto which is applied a metallic porous carrier layer 14. A sliding material 16 is applied to impregnate, cover, or both, the porous carrier layer 14.

(9) The substances are mixed for 30 seconds at 18,000 rpm in a blender with pulse bladesfor example in a 1,000-mL laboratory blenderto create the compositions of the invention; by way of example, 70 g PEEK, 15 g HMW PTFE, 7.5 g LMW PTFE, and 7.5 g PPTA. This mixture is then raked in the form of a 1 mm-thick layer of powder onto, by way of example, a steel strip with a 300 ?m-thick sintered bronze coating having a pore volume of about 30%, and heated for 5 min at 380? C. After a short cooling of the surface in air, the sample is rolled in such a manner that the still-plastic layer is compressed and pressed into the pores.

(10) For the pin/roller test, a round test blank with a diameter of 10 mm is punched out of the completely cooled sample band. The thickness of the test blank is measured and the test blank is pressed with a defined load of, for example, 20 MPa, onto a dry, degreased steel cylinder rotating at 100 rpm, with a diameter of 100 mm. The roughness of the steel cylinder at the test blank contact surface (shell surface of the steel cylinder) is initially approximately Rz=1 ?m. After completion of the test, the thickness is measured again and the rate of wear is calculated from the difference of thicknesses before and after. The friction coefficients are determined via a torque sensor in the steel cylinder drive.

(11) For a lubricated test, the test track is additionally continuously wetted by wick with hydraulic shock absorber oil.

(12) The diagram in FIG. 1 presents the wear in ?m of two different sliding materials according to the invention (30% by volume PTFE in the matrix material PEEK, and 30% by volume PTFE in the matrix material PESU, wherein in each case the composition of the PTFE is varied so that the total volume of PTFE contains from 0 to 80% by volume of low molecular weight PTFE, and the rest is high molecular weight PTFE), as a function of the ratio of the proportion of low molecular weight PTFE to the total volume of the PTFE. This data was obtained in the pin/roller test described above, at a load of 20 MPa and a relative speed of 0.5 m/s without lubrication.

(13) It can be seen that the wear is the greatest without low molecular weight PTFE (corresponding to 100% by volume high molecular weight PTFE; at the coordinate origin of the x-axis). As the proportion increases, and therefore the fraction of low molecular weight PTFE increases, the wear is reduced in the range A according to the invention (5 to 40% by volume of low molecular weight PTFE), such that it is significantly below the PTFE types used alone (as only high molecular weight or low molecular weight PTFE). In the preferred range A (12.5 to 35% by volume of low molecular weight PTFE), the wear values are positioned at a further significantly lower level. The wear increases as the proportion of low molecular weight PTFE continues to increase, until it reaches a plateau starting at about 45% by volume. This result applies for both the PEEK and the PESU matrix.

(14) The diagram in FIG. 2 presents the coefficient of friction of the same sliding materials according to the invention (30% by volume PTFE in the matrix material PEEK, and 30% by volume PTFE in the matrix material PESU, wherein in each case the composition of the PTFE is varied so that the total volume of PTFE contains between 0 and 80% by volume of low molecular weight PTFE) in FIG. 1, as a function of the ratio of the low molecular weight PTFE to the total volume of the PTFE, determined in the pin/roller test described above at 20 MPa and 0.5 m/s, with lubrication.

(15) Here it can be seen that the coefficient of friction is also the greatest without low molecular weight PTFE (at the origin of the x-axis). As the proportion increases, and therefore the fraction of low molecular weight PTFE increases, the coefficient of friction is also reduced, such that in the range A according to the invention (5 to 40% by volume of low molecular weight PTFE) it is significantly below the PTFE types used alone (as only high molecular weight or low molecular weight PTFE). In the preferred range A (12.5 to 35% by volume of low molecular weight PTFE), the coefficients of friction are positioned at a further significantly lower level. As the proportion of low molecular weight PTFE increases further, the coefficient of friction increases until it reaches a plateau starting at about 45% by volume.

(16) The following table provides a summary of sliding material compositions according to the invention, by way of example:

(17) TABLE-US-00001 Hard Wear High-temp. Solid Lubricant material Fibers LMW PTFE HMW PTFE Friction Coeff. Dry # Matrix Plastic % by volume Lubricated [?m] 1 prior art PEEK 20 0.081 52 2 prior art PEEK 20 0.085 59 3 invention PEEK 5 15 0.063 40 4 prior art PEEK 7 Graphite 7 C-fiber 7 0.100 52 5 prior art PEEK 7 Graphite 7 C-fiber 7 0.110 63 6 invention PEEK 7 Graphite 7 C-fiber 2 5 0.084 31 7 prior art PVDF 20 0.058 55 8 prior art PVDF 20 0.066 57 9 invention PVDF 5 15 0.049 37 10 prior art PPS 5 PPTA 24 0.049 41 11 prior art PPS 5 PPTA 24 0.054 35 12 invention PPS 5 PPTA 6 18 0.044 24 13 prior art PPS 1 Fe.sub.20.sub.3 28 0.047 42 14 prior art PPS 1 Fe.sub.20.sub.3 28 0.054 39 15 invention PPS 1 Fe.sub.20.sub.3 7 21 0.044 27 16 prior art PESU 8 PPS0.sub.2 22 0.053 34 17 prior art PESU 8 PPS0.sub.2 22 0.048 40 18 invention PESU 8 PPS0.sub.2 5.5 16.5 0.042 24 19 prior art PESU 5 WS.sub.2 5 C-fiber 20 0.074 52 20 prior art PESU 5 WS.sub.2 5 C-fiber 20 0.079 52 21 invention PESU 5 WS.sub.2 5 C-fiber 5 15 0.066 31 22 prior art PESU 5 h-BN 2 SiC 20 0.072 64 23 prior art PESU 5 h-BN 2 SiC 20 0.068 51 24 invention PESU 5 h-BN 2 SiC 5 15 0.066 42 25 prior art PESU 11 PAI 4 C-fiber 20 0.070 32 26 prior art PESU 11 PAI 4 C-fiber 20 0.069 36 27 invention PESU 11 PAI 4 C-fiber 5 15 0.052 21

(18) The table shows different embodiments of the invention of the sliding material which have been tested by means of the pin/roller test at 20 MPa and 0.5 m/s, with and without lubrication. Two comparative tests are shown for each example according to the invention listed in the table. In each of these, compositions outside of the invention were tested with only one of the two types of PTFE. Compositions were tested with a variety of matrix materials, such as PEEK, PVDF, PPS or PESU, various solid lubricants or hard materials, and fibers. The proportions of LMW and HMW PTFE of the composition according to the invention varied in this case between 2:5; 5:15; 5.5:16.5: and up to 6:18 and then 7:21% by volume.

(19) All sliding materials constituted according to the invention demonstrate lower wear without lubrication, and at the same time a lower coefficient of friction with lubrication, than sliding materials with the same PTFE content of one type, and an otherwise identical composition, independently of the matrix material or the additional solid lubricants, hard materials or fibers.