Patent Application
20080226921
The present invention relates generally to the protection of machinery from wear. More particularly, the invention relates to the use of thermoplastic ceramic composite materials as wear surfaces.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Any surface that is in regular contact with another moving surface is subject to degradation over time, no matter how hard the surface may appear to be. For example, archeologically significant sites may have to be closed to tourist traffic to prevent erosion of the stonework from foot traffic. Degradation from wear is especially apparent in machinery and industrial applications where repetitive movements of one surface across another may cause significant erosion in a short time. A common example is the movement of a piston in a cylinder head in a car engine, which will wear out the engine over time. Even a very small apparent force, such as boxes sliding down a packaging chute, may eventually wear out the rails and surface of the chute. In larger applications, where the stresses are higher, wear may occur more quickly. For example, in mining applications, where heavy ore and rocks are being carried by a conveyor belt, the contact of these materials with the side rails of the conveyor belt may necessitate regular replacement of the rails.
It has been recognized since the late 1960s that there is a significant cost to industrial economies from problems caused by wear. Specific studies have indicated that the economy of the United States may lose several billions of dollars each year in replacement of materials and equipment that have been damaged, or “worn out,” by frictional degradation.
Frictional degradation, or wear, may be defined as the process by which interactions of a surface with another surface result in a dimensional loss of the surfaces. The dimensional loss may or may not occur with an actual loss of material. The damaging interaction of the two surfaces is caused by the microscopic roughness of each surface. The high points of each surface are the actual points of contact with the other surface, and are termed “asperites.” As one surface is moved across the other surface, the asperites may deform leading to resistance, or friction, in the movement. Further, as the asperites deform they may break off, causing degradation of the surface. Thus, to decrease wear, it is necessary to either harden the asperites to resist deformation or to decrease the interaction between the asperites of the opposing surfaces.
Hardening a surface may involve manufacturing the surface out of an exotic metal alloy. Examples of such alloys may include chromium steel and alloys of titanium, tungsten, or other hard metals. While these alloys may protect the hardened surface from degradation, a softer opposing surface may actually degrade at a faster rate. For this reason, hardening may often be confined to protecting parts which are harder to replace, such as ball bearings. Further, if the opposing surface is easier to replace, it may deliberately be made from a softer material to lower the wear on the hardened parts.
While effective, hardening a wear surface necessitates creating surfaces or parts out of expensive materials that are, by definition, hard and difficult to work with. For this reason, decreasing the interactions between the two surfaces may be a more economical option.
A primary technique for decreasing surface interactions involves lowering the coefficient of friction between two surfaces by the application of lubricating liquids. For example, the use of oil in car engines decreases friction between the surfaces of the pistons and the cylinders, leading to a longer life span for the engine. Without such lubricants, the engine may overheat or the moving parts may gall, i.e., one surface may lose metal to the other surface, leading to catastrophic failure of the engine. While liquid lubricants may commonly be used in machines having moving parts, such as engines or motors, they may not be practical for use with stationary parts across which other materials slide, such as chutes or side rails for conveyor belts.
For stationary parts, either hardening the surfaces or producing them from low friction materials may be the only practical choices. However, both of these choices may have significant drawbacks. For example, the use of exotic alloys as surface modifiers in low value applications, such as chutes, may be too expensive for many applications. Furthermore, surfaces made from low friction materials may not be durable enough for practical use.
Thermoset plastics may be used to provide more durable low friction coatings for applications in surface protection. However, thermoset plastics are often used in the form of a paste that is spread onto the surface to be protected, forming a permanent bond to the surface. This bonding may limit the reuse of the surface after further wear has occurred, which may force replacement of an entire part. Correct application of a thermoset paste may also require significant time and effort. Furthermore, if the application is performed on the manufacturing site, the equipment may need to be out of service for a significant period of time, further increasing the cost.
Certain aspects commensurate in scope with the originally claimed invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.
An embodiment of the present techniques provides a wear surface that comprises a blend of a thermoplastic matrix and one or more types of ceramic particles. The ceramic particles comprise at least 50% by volume of the blend.
Another embodiment provides a method for protecting a surface from wear. The method comprises blending one or more types of ceramic particles and a thermoplastic matrix. The ceramic particles comprise at least 50% by volume of the blend. A protective surface is formed from the blend.
Another embodiment provides a conveyer belt having a motor driven belt and side rails on each side of the belt. The side rails comprise a blend of one or more types of ceramic particles and a thermoplastic matrix. The ceramic particles comprise at least 50% by volume of the blend.
Another embodiment provides a front end loader comprising a loader bucket. The loader bucket comprises a blend of one or more types of ceramic particles and a thermoplastic matrix. The ceramic particles comprise at least 50% by volume of the blend.
Another embodiment provides a chute for directing gravity propelled objects to a destination. The chute is comprised of a blend of one or more types of ceramic particles and a thermoplastic matrix. The ceramic particles comprise at least 50% by volume of the blend.
Yet another embodiment provides a self-dumping hopper comprised of a blend of one or more types of ceramic particles and a thermoplastic matrix. The ceramic particles comprise at least 50% by volume of the blend.
Advantages of the invention may become apparent upon reading the following detailed description and upon reference to the drawings in which:
One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
The techniques described in detail below include thermoplastic ceramic composites which may be used for the protection of wear surfaces. These thermoplastic ceramic composites may be made from a blend of a thermoplastic matrix and hard ceramic particles. Optionally, the matrix may also contain friction modifying additives to lower the coefficient of friction of the surface. During wear, the hard ceramic particles embedded in the matrix are exposed, which may protect the thermoplastic matrix and slow further wear. These thermoplastic composites may be formed into any number of protective surfaces for specific applications. The parts made from thermoplastic composite materials may be manufactured off-site and purchased as preformed units for specific applications. Further, thermoplastic materials may be melted and reformed, which may be advantageous for resurfacing or recycling of a wear surface.
An example of a thermoplastic ceramic composite 10 that may be used for improved resistance to wear, in accordance with embodiments of the current techniques, is shown in
The thermoplastic matrix 14 used to form the thermoplastic ceramic composite 10 may be selected on the basis of the desired surface friction, hardness, or resistance to wear. Thermoplastics that may be used for the thermoplastic matrix 14 include polyolefins, polycarbonates, poly(phenylene sulfides), poly(phenylene oxides), poly(ether ether ketones), polyamides, or combinations thereof. The choice of materials depends on the combination of properties desired. For example, a polyolefin material may have a lower coefficient of friction than a poly(phenylene sulfide), but may also have a lower resistance to damage from heavy objects. Another parameter that may influence the choice of the thermoplastic matrix 14 is the resistance of the matrix to degradation from heat that may be generated by objects sliding across the surface. Accordingly, in applications where heat may be generated, heat resistant thermoplastics such as poly(phenylene sulfide) or poly (ether ether ketone) may be more appropriate than other thermoplastics.
Although the choice of the material for the thermoplastic matrix 14 is an important consideration, the ceramic particles 16 will be the primary component of the wear surface 12 that is in contact with another surface. Thus, the amount and type of ceramic particles 16 in the thermoplastic ceramic composite 10 may significantly affect the properties and life span of the wear surface 12. In embodiments of the present techniques, the ceramic particles 16 may make up as much as 50% of the total composition by volume. In other embodiments, the ceramic particles 16 may make up at much as 90% by volume of the total composition of the thermoplastic ceramic composite 10.
The ceramic particles 16 may be made from alumina, boron carbide, boron nitride, silicon carbide, silicon nitride, magnesium silicate, magnesium oxide, titanium carbide, titanium oxide, tungsten carbide, zirconia, or combinations of these materials. As the thermoplastic ceramic composite 10 is intended to be low cost and easily replaceable, lower cost ceramic materials may be appropriate. Further, the ceramic particles 16 may be comprised of newly manufactured particles or waste particles, such as used alumina sand blasting dust.
The size of the ceramic particles 16 may range from greater than 50 micrometers to less than 1 millimeter. A wide particle size distribution may improve the wear performance of the panel. For example, while larger particles may be less likely to abrade from the matrix, and, thus, more effective at preventing wear, smaller particles may improve the packing efficiency in the matrix. Improved packing of the ceramic particles 16 may lead to a higher concentration of ceramic particles 16 in the thermoplastic matrix 14. This may increase the number of particles that an opposing surface contacts, which may increase the wear protection of the thermoplastic ceramic composite 10.
The shape of the ceramic particles 16 may also affect the frictional performance of the thermoplastic ceramic composite 10. For example, it may be appropriate to select ceramic particles 16 that are spherical or rod shaped in order to minimize the roughness of the surface of the ceramic particles 16.
Alternatively, the ceramic particles 16 may be flat. These flat particles could be aligned with each other and the wear surface 12 by shear forces during the molding process. The alignment of such flat particles may increase the surface area of ceramic at the wear surface 12, which may increase the wear resistance of the thermoplastic ceramic composite 10.
The ceramic particles 16 may include nanospherical particles, e.g., particles having diameters on the order of 50 to 500 nanometers. These very small particles may decrease the microscopic surface roughness of the thermoplastic ceramic composite 10, which may decrease the overall friction seen as another surface moves across the thermoplastic ceramic composite 10. Further, as the very tiny spherical ceramic particles 16 are worn out of the surface and detach from the thermoplastic matrix, they may act as friction modifying additives 20, as discussed in detail below.
The thermoplastic ceramic composite 10 may contain friction modifying additives 20 to lower the frictional interaction between the wear surface 12 and an opposing surface. Without intending to be limiting, such a decrease in the surface friction may be caused by various interactions between the friction modifying additive 20 and the wear surface 12. For example, the nanospherical particles described above may function as tiny ball bearings, allowing the surfaces to roll across one another. Other friction modifying additives 20 may tend to fill in the surface roughness, lowering the number of asperites available to engage asperites on an opposing surface. Still other additives may lower the adhesion between the surfaces, allowing the surfaces to slide over each other more easily. Examples of materials that may be used as friction modifying additives 20 include graphite, boron powder, calcium carbonate, ground sea shells, talc, rock dust, poly(tetrafluoroethylene) powder, molybdenum sulfide, tungsten sulfide, or combinations of these materials.
The nature and value of the opposing surface may also dictate the choice of the friction modifying additive 20. For example, if the thermoplastic ceramic composite 10 is intended for use in an application wherein rock or other heavy ores are coming in contact with the wear surface 12, a lower cost additive may be appropriate. Alternatively, where a high value surface may contact the thermoplastic ceramic composite 10, a higher cost friction modifying additive 20 may be chosen to protect the opposing surface. This situation may arise, for example, when a chute in a manufacturing facility is used for transporting easily scratched packages.
An embodiment of a thermoplastic ceramic composite 10 made in accordance with the procedures described above was tested for resistance to wear. In this embodiment, a polycarbonate, Lexan® EXL 9330 high impact grade from General Electric, was used as the thermoplastic matrix 14. Approximately 50% by volume of ceramic particles 16 made from randomly sized alumina fragments were blended into this matrix. A comparison sample made from an epoxy matrix containing approximately 50% by volume of randomly sized alumina fragments was also tested. For both samples, the alumina fragments had a mesh up to size six, which corresponds to a wide distribution of particle sizes with a maximum size of approximate 0.45 mm. The samples contained no friction modifying additives 20.
The testing was performed using a dry sand rubber wheel test, ASTM number G65. In this test, a sample is placed in a frame that pushes the wear surface 12 against a rotating rubber wheel at a constant force. Dry sand is poured from a hopper in between the wear surface 12 and the rubber wheel and is effectively ground into the surface. The dry sand test is usually run for a single time period of 6,000 revolutions. Measurements of the mass of the sample are taken before and after the test, and the density of the material is used to calculate the volume of the sample that is abraded during the test. Conversion of the material loss from mass to volume compensates for different densities, allowing different materials to be compared.
The test procedure described above was modified to minimize the heat exposure by stopping the run after every 1000 revolutions and weighing the samples. Heating of the samples may have distorted the results by melting the polycarbonate matrix, allowing alumina grains to be extracted from the melt. The results obtained from the tests are shown in Table 1.
The estimated change in volume was calculated on the basis of a composition containing 50% alumina by volume. A density of 3.97 g/ml was used for the alumina, and densities of 1.3 g/ml and 1.2 g/ml were used for the polycarbonate and epoxy, respectively. The testing was stopped after the measured mass dropped below the accurate limits of measurement, around 0.005 g. Accordingly, only two cycles of testing (2000 revolutions) were run on the wear surface 12 having a thermoplastic matrix 14 made from polycarbonate before it dropped below the accurate limits of measurement. In contrast, the epoxy based composite continued to lose a measurable amount of material throughout the test sequence.
The thermoplastic ceramic composites 10 of the present techniques may be used in any number of applications for improvement of wear resistance. Examples of such applications are shown in
As shown in
Another application is shown in
In
It should be understood that the present techniques have been described above by way of example and that such techniques may apply in other situations as well. Indeed, numerous other applications may take advantage of the enhanced wear, low cost, and recyclability of the thermoplastic ceramic composites 10 discussed above. Examples of such applications may include dumpsters, rolling surfaces for bearings, door jambs, entry mats for buildings, and protective mats for high traffic areas of archeologically valuable tourist attractions, among others.
While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and/or described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.
