Patent Application
20150196991
This invention relates to component wear surfaces with embedded wear particles to increase hardness and limit erosion of the surface.
Industrial applications often put tools in cyclic contact with abrasive materials that remove the surface of the tool. Over the service life of the tool the abrasive materials wear away and erode the exposed tool surface until the tool has to be replaced. A harder surface for the tool can extend the service life of the tool by reducing the rate of wear during operation.
Wear tools can be manufactured by casting, powder metallurgy infiltration or other techniques. Methods for modifying the hardness of tool materials include alloying, case hardening and heat treating. Incorporating wear resistant particles or materials into the body of the tool during forming of the part can also limit erosion during operation to provide increased service life.
Material preparations and manufacturing processes that allow the introduction and preferential placement of hard wear particles in cast materials can provide improved wear and service life for tools and other parts exposed to abrasive wear.
Surfaces of wear tools and wear members that are in sliding or impacting contact with other materials are subject to erosion. Examples include ground engaging tools used in extractive mining that penetrate earth and ore to separate material for further processing. Ground engaging tools have very high rates of wear and must be replaced frequently. Examples also include rotating tools such as downhole drill bits which advance boreholes by failing rock. The material of the bore is flushed around the body of the bit which abrades the surface of the bit. Other examples include wear parts over which earthen materials pass such as those attached to chutes, truck trays, etc. While the invention is well suited for tools or parts that engage the ground, it could be used in other abrasive environments to provide greater longevity.
Harder materials are more resistant to abrasion and erosion than softer materials. Many methods of hardening tools and tool surfaces are used to make them more erosion resistant. Material selection, alloying and heat treating provides the broadest hardness properties for the tool. Case hardening can provide additional hardness. Hard particles resistant to wear can also be incorporated into or on the surface of the tool to further limit erosion.
Hard particles incorporated into the tool material can include one or more superabrasives such as boron carbide, vanadium carbide, boron nitride, tungsten carbide, titanium carbide or other compounds. The hardest bulk material is diamond. Practical use of diamond in industrial applications is limited as the diamond structure can break down to graphite in air at temperatures over 700° C. When exposed to molten iron the diamond surface can also chemically react and dissolve into the iron. Metal compounds coating the diamond can provide one or more protective layers that limit contact of the diamond surface with elements that will degrade its structure. Common practice has taught away from embedding multi-coated diamond in ferrous castings as coatings would not be able to protect the diamond from high casting temperatures and aggressive chemical attack.
U.S. Pat. No. 5,224,969 discusses coating diamond particles to improve retention of the diamond particle within a supporting matrix including a resin or phenol formaldehyde. Chrome is deposited as the first layer on the diamond to form a carbide layer followed by deposition of a second layer of a different metal which is then nitrided. A third layer is deposited on the nitrided layer to provide an adhering or bonding layer for the matrix to grip the encapsulated diamond.
The present invention pertains to hard particles also called wear particles that are coated with a metal nitride layer or with metal carbide and metal nitride layers to enable inclusion in or exposure to molten metal and in particular to ferrous based alloys. The present invention enables hard particles (e.g., diamond particles) to be included in or on parts that are cast or produced by other manufacturing processes involving molten metal without undue degradation that would ordinarily prevent their use. This new use of such hard particles can provide a longer useful life for all kinds of products exposed to abrasive wear.
In one embodiment of the invention, a wear particle to be embedded in a ferrous matrix is a diamond particle coated with metal carbide. To protect the metal carbide layer from chemical degradation, a layer of metal nitride is deposited on the carbide layer.
In relation to alloys where the metal nitride coating does not have adequate wetting in relation to a molten ferrous matrix, the nitride layer can be formed as a sub-stoichiometric metal nitride or with an inner portion as a stoichiometric ratio of metal to nitrogen atoms that transitions to an outer sub-stoichiometric layer of metal nitride at the surface. The sub-stoichiometric metal nitride can better interact with the molten matrix so the particle is better retained in the solid matrix.
In another embodiment, a wear particle is coated with a metal nitride. The metal nitride limits degradation of the diamond by exposure to elements of the molten metal. Where the metal nitride coating does not have adequate wetting in relation to a molten ferrous matrix, the wear particle can include a sub-stoichiometric metal nitride coating or a sub-stoichiometric layer of a metal nitride deposited on the nitride coating.
In another embodiment, the encapsulated wear particle is positioned along a casting surface of a mold with a precursor matrix or other means prior to pouring of the molten metal. On introduction of the molten metal into the mold the matrix material is consumed and the encapsulated wear particle disperses in the molten material of the cast part.
In another embodiment, a wear resistant surface is deposited on a metal substrate by an arc welding process using welding rod. The welding rod has a metal matrix, core or periphery that incorporates a binder, flux and/or encapsulated hard (e.g., diamond) particles. The encapsulated particle includes a primary layer of encapsulation and a secondary layer of encapsulation to limit degradation of the particle during processing. During welding the particles pass from the binder on the welding rod to the welding pool and are incorporated into or on the surface on solidification.
In another embodiment, a method of incorporating diamond during casting of a wear member comprises depositing a protective coating on the surface of the diamond and depositing a coating on the protective coating.
Many industrial operations involve tools and other parts that are subjected to abrasive materials. For example, tools in mining and drilling operations are quickly worn away by the material contact. Downtime for replacement of worn tools and components during industrial operations can significantly increase operating costs. Increasing the service life of wear members by increasing surface hardness to control erosion can limit downtime and increase operation efficiency.
Incorporating hard particles in a matrix forming the surface of a member can significantly increase wear resistance. Diamond as an extremely hard form of carbon is commonly used as an abrasive in grinding and cutting operations. Distributing diamond particles (or other hard particles) in a ferrous (or other metallic) cast wear member can provide advantageous wear characteristics to the tool or other part, but has in the past not been feasible on account of the degradation of diamond under high heat and/or chemical reaction. Surface wear exposes additional diamond surfaces to provide protection through the service life of the tool when the diamond particles are dispersed through at least some depth along the surface of the wear part.
While the invention is discussed here in terms of diamond as a hard wear-resistant particle, the process can be applied to other hard particles in a matrix that limit wear in an eroding environment. The use of diamond particles in a ferrous-based alloy is one preferred embodiment of the invention as diamond particles are highly wear resistant and these alloys are used in many abrasive environments on account of their economy, strength and durability. Nevertheless, the present invention is suitable for use with diamonds in non-ferrous alloys which similarly degrade diamond or other hard particles when cast or melted in manufacturing processes. In this application, the invention is described in terms of encapsulated diamond used in a ground engaging tool (such as an excavating tooth) solely as an example, i.e., for the purpose of illustration and should not be taken as a limitation. Hard or wear particles useable in this invention also include, for example, ceramic, ceramic fibers, ceramic platelets or metal compounds such as titanium carbide or cubic boron nitride.
Embedding diamond in the surface matrix includes incorporating the diamond particles in molten metal when forming the tool or surfacing the tool. This exposes the diamond to aggressive conditions of chemical reactions and/or heat. In a part manufactured by casting, the diamond can be incorporated into the molten material before it solidifies in a mold. For infiltrated parts the diamond can be packed into a mold and molten matrix metal binds the diamond and any other hard particles in place. Diamond can also be incorporated to a surface by welding.
Diamond can be degraded by contact with molten metals such as iron, copper and nickel as well as other elements during processing. Diamond exposed to oxygen degrades at processing temperatures above 700° C. with the diamond structure converting to graphite which is softer and less wear resistant. In a vacuum or in inert reducing environments degradation can begin above 1500° C. To maintain the structural integrity and material properties of the diamond, each diamond particle can include one or more protective layers. Layers on the diamond surface provide protection against degradation of the diamond at high temperature and from chemical attack by the constituents of the matrix. Protective layers also provide wettability of the diamond surface allowing the molten material to adhere to the coated diamond surface limiting extraction of the coated diamond from the matrix when a portion of the diamond is exposed and contacts impinging materials. This enables the wear particles to be better retained in or on the tool during use of the part such as in a digging operation.
A wear surface 10 with encapsulated wear particles 12 in a matrix 14 are generally shown in
Metal nitride can be difficult to wet with molten metal. If the molten metal does not wet the surface of the encapsulated diamond, the diamond may tend to segregate at a surface, or clump together instead of distributing through all or a portion of the part. Further, insufficient wetting of the hard particle in the molten matrix may lead to the hard particle not being retained on solidification of the molten metal, particularly during use. Adjusting the composition of the nitride layer can allow the surface to better interact with the liquid. A sub-stoichiometric metal nitride composition with the ratio of metal atoms to nitride atoms modified from the most stable form can significantly modify the surface properties to provide preferential wetting of the surface.
The metal element of the metal nitride and/or the metal carbide can be any of titanium, vanadium, chrome, silicon, boron, tungsten, niobium, tantalum, zirconium, hafnium, molybdenum, aluminum or other metal or alloy. The metal compounds produced can include silicon carbide, boron nitride, titanium nitride, titanium carbon nitride, vanadium nitride, chrome carbide and vanadium carbide. Metal compounds can also include more complex compounds such as titanium Aluminum nitride. The listed elements and compounds are examples and should not be considered a limitation.
Coating of the diamond particles limits degradation of the diamond from chemical reactions at processing temperatures and improves wetting of the surface of the encapsulated diamond particle by the molten metal to improve mobility of the encapsulated diamond within the fluid. The diamond coating can also improve retention of the diamond in the solidified metal so that it is not easily extracted from the matrix during operation.
Encapsulated diamond can be incorporated into the casting by several techniques. In one embodiment, the encapsulated diamond is added to the raw metal of the melt for the casting process. The encapsulated diamond in this process remains at high temperature for a duration of the melting and casting process and has to remain stable and not degrade over a longer period of time. The diamond remains at high temperature through the heating cycle, transfer time moving the melt to the final processing and the pour into the molds. As a result, this process may not be suitable for some operations. Moreover, including the diamond particles in the melt before casting will result in a distribution of the particles throughout the part, which may not be needed in all cases.
Alternatively, encapsulated diamond can be added to the melt after the initial heating process and before being poured into the mold. This method keeps the diamond at high temperature for a shorter period. The melt would ordinarily need to be stirred to distribute diamond evenly through the melt rather than remaining near the surface or unevenly mixed. The encapsulated diamond would be distributed through the entire cast part on solidification.
In another embodiment, the encapsulated diamond is retained at the surface of a mold in advance of pouring in molten metal as shown in
Alternatively, encapsulated diamond can for example be embedded in another sacrificial medium such as a mesh or cloth, a metal ribbon, metal foam or ceramic foam that lines the surface of the mold as shown in
Other placement methods can be used to preferentially distribute the diamonds through the melt or in a particular portion of the part. Encapsulated diamonds can be poured into the mold immediately before the pour or simultaneous with the pour. The method used for including the encapsulated diamonds may be determined by the shape and size of the cast part, the casting process and/or the size of diamond particles.
During operation the working surface of the tool or other part exposed to abrasive conditions continually wears away creating a new surface. Diamond distributed through the working end of the wear tool continually provides new diamond particles over the service life of the tool. Diamond in the mounting end of the tool is not generally effective in increasing the tool service life since the tool is usually replaced before the mounting end becomes a wear surface. Diamond in the mounting end for this kind of tool tends to add unnecessary expense to the tool. Nevertheless, it may be beneficial for some parts to include diamond or other hard particles in the mounting end and/or throughout the part.
Encapsulation of the diamond by the protective layer limits the degradation of the diamond. Where coverage is not optimal, uncoated portions of the diamond can degrade while coated portions of the diamond do not. Nevertheless, this may be suitable for some uses. The thickness of the coating, the temperature of the melt, the time at high temperature, and other factors can affect the coatings needed to limit degradation. Where the diamond degrades at a relatively slow rate at a set temperature and the diamond is subject to the set temperature for a short period, the thickness of the protective layers can be optimized to limit damage to the diamond and minimize processing costs.
Preferably, each layer is no thicker than is required for limiting degradation of the wear particle so it is suitable for the intended use. Deposited layers may be any effective thickness, but are preferably in the range of 1 to 30 microns (μm). The encapsulated wear particle may include residual layers of additional components that are a byproduct of processing or exposure to ambient elements. For example thin oxide layers may develop on exposed surfaces in between processing steps or before or after processing. These layers are not considered to have a significant effect on the physical or chemical properties of the encapsulated diamond.
Layers constituting a metal compound on the diamond can incorporate additional metals in small quantities. For example, titanium carbide could include measurable amounts of silicon. In general, a metal compound can incorporate 5% of a different metal without it being considered a substantial part of the compound or affecting its physical properties.
Layers can be deposited on the diamond surface using any of a number of techniques. The method chosen can depend on the material being deposited and the substrate material it is deposited on. Generally each diamond particle is processed and the protective coatings are applied over the entire surface at a constant thickness, though the thickness and coverage can be dependent on the reactivity of the coating material with the crystal structure of the diamond's surface. Fluidized beds are frequently employed so that the diamond grains are suspended in an aqueous or gaseous flow that allows even deposition of the coating material. Coatings may be applied to the diamond using electroless, electrolytic, chemical vapor deposition (CVD), physical vapor deposition processes (PVD), pre-ceramic polymer pyrolysis or other techniques.
A diamond wear particle can be any form of diamond including engineered diamond from high pressure high temperature synthesis techniques, thermally stable polycrystalline diamond (TSP), CVD diamond, polycrystalline diamond (PCD), recycled PCD tables from cutters, deformed diamond, mono-crystal, synthetic and natural diamond. Diamond may be initially doped with an element such as boron, phosphorous or other element. Doping may be accomplished by implantation or during the manufacture of the diamond and may alter the electrical conductivity of the diamond. The doping element can promote retention of encapsulation layers on the diamond.
In one preferred embodiment, silicon carbide (SiC) is bound to the surface of a diamond particle using chemical vapor deposition in a fluidized bed. Silicon is deposited as a primary layer on the diamond surface and binds to carbon atoms on the surface to form silicon carbide. The silicon carbide on the surface of the diamond limits oxidation and degradation of the diamond in adverse high temperature environments. A layer of titanium nitride (TiN) is deposited on the surface of the silicon carbide also by chemical vapor deposition in a fluidized bed as a secondary layer. The titanium nitride layer limits degradation of the carbide layer by iron (or other metal) in high temperature molten steel operations. Optionally a tertiary layer is deposited on the nitrided layer to improve wetting of the encapsulated diamond by the molten metal. The tertiary layer can be of titanium nitride that is sub-stoichiometric. This can take the form of Ti1-xNx, with the value of x between 0.1 and 0.99, for example TiN0.3. This layer allows the encapsulated diamond to be wetted by, and to mix in, a liquid environment without readily separating.
In an alternative embodiment, a wear resistant particle such as diamond, tungsten carbide, silicon carbide, titanium carbide is coated with an initial layer of metal carbide that can strongly adhere to the hard particle. This initial layer can protect the particle from thermal and oxidative damage resulting in graphitization or degradation. The initial layer of carbide can be a continuous coating which completely covers the hard particle to provide that protection or can be a partial coating that covers more than half the surface of the particle. The first layer of metal carbide can also be a mixture of metal carbides that can enhance fracture toughness compared to a carbide coating of a single metal.
A second layer of metal nitride can adhere to the carbide layer coated hard particle to protect the carbide layer coated hard particle from oxidation and chemical reaction with a molten metal matrix. The second layer can be a carbonitride, such as SiCN and/or Ti(CN) or other carbonitride where the layer will adhere to the carbide layer coated hard particle and provide the particle protection from oxidation and chemical reaction. The carbon chemistry of the carbonitride layer can provide wetting and adhesion in a ferrous based molten metal matrix. Alternatively, the second layer can be an aluminum nitride as in TiAlN, to provide protection from oxidation and chemical reaction. The second layer of metal nitride can be a mixture of metal nitrides, such as Si3N4 with TiN. This Si3N4-TiN composite can have enhanced fracture toughness as compared to either nitride singularly. The second layer material can have a degree of solubility with the carbide initial layer which can promote adhesion of the layers to each other and results in a stronger multilayer coating.
To promote wetting and strong bonding with the molten metal matrix, a third layer is applied to the protective metal nitride second layer. This third layer consists of a substoichiometric metal nitride where there not enough nitrogen atoms to make up the complete crystal structure. In effect the stoichiometry of the metal nitride can be changed to a substoichiometric crystal structure.
Both the materials used as a second layer of metal carbonitride or metal aluminum nitride can be used as a third layer with a substoichiometric chemistry to provide good wetting and adhesion to the metal matrix without sacrificing the protection from chemical reaction and dissolution with the molten metal matrix or oxidation protection.
The third layer of material can be a mixture of materials as previously discussed. These composite materials can have enhanced fracture toughness as compared to either material singularly. This outer coating wettability with the matrix can be varied to increase or reduce adhesion to the metal matrix. In some wear applications,
Sub-stoichiometric compositions can be readily formed during the deposition process. For example, the composition of the deposited layer can be controlled by adjusting the partial pressure of the element supplied during chemical vapor deposition, in this case the nitrogen.
In another alternative embodiment, the processing time for embedding the diamond in a wear member is short and the rate of decomposition of the diamond at temperature is relatively slow so that both the carbide layer and the nitride layer on the carbide layer are not required or can be of limited thickness or coverage. A short processing time of the diamond in this case corresponds to a short exposure to temperatures above graphitization temperatures in the molten metal.
In one embodiment as shown in
Alternatively, encapsulation can comprise one primary layer such as a carbide that serves as a bonding layer rather than a protective layer and can bind to both the diamond and the nitride layer. A secondary sub-stoichiometric nitride layer can provide for interaction of the surface with the liquid. The layers deposited on the diamond surface preferably range from, for example, one micrometer to one millimeter but could be smaller or larger depending on the intended purpose. Diamond particles preferably range from nanometer sized up to 5 millimeters, but other sizes could be used.
Encapsulated diamond can be advantageous in many applications other than wear components. Embedded encapsulated diamond surfaces can be used in applications including gun barrel linings, armor plate, cutting tools, pump vane surfaces, bearings and biomedical implants.
Wear components have also been produced with powder metallurgy infiltration techniques. Infiltration combines materials with contrasting properties that have limited solubility and will generally not form an alloy. Distributing hard particles in a matrix of softer material binds the hard particles in place. Drill bits for oil and gas are typically made by packing tungsten carbide particles in a mold. The charged mold may be sintered to bind the tungsten carbide particles together. Encapsulated diamond can be included with the tungsten carbide particles for additional wear protection. A molten matrix material is then flowed into the sintered tungsten carbide and diamond so that it fills the interstices between the hard particles binding the grains together.
Matrix materials used to infiltrate the sintered tungsten carbide include copper, aluminum, iron and nickel or alloys of these and other materials. The matrix materials are heated to melting temperature to flow into the sintered tungsten carbide. Other wear particles than tungsten carbide can be used in infiltration applications including cubic boron nitride, titanium carbide or other hard particles. Encapsulated diamond wear particles can be applied to other applications such as creating and restoring points on wear members as disclosed in US Patent Publication 20120258273 which is incorporated herein by reference in its entirety.
Infiltration provides a very hard primary material that is resistant to wear embedded in the softer matrix material. The matrix material holding the primary material in place wears away exposing the tungsten carbide and diamond particles as the wear surface.
While infiltration processing temperatures are generally lower than the casting process, on the order of the melting temperature of the matrix materials, chemical interaction with the infiltration materials can degrade the diamond structure. Coating the diamonds limits the chemical interaction and diamond degradation.
Encapsulated diamond can also be compatible with arc welding processes used to apply a hardfacing surface to a softer body. Welding rod generally comprises a metal rod with an overlay that can include flux and/or oxygen-excluding materials such as sodium silicate. In this alternative embodiment, the overlay around the metal core of the welding rod can include encapsulated diamond particles. During welding the diamond passes into the molten weld pool with the core metals as the rod is consumed. The pool solidifies with the diamond as a component of the hardfacing of the tool.
Coated diamond and/or other hard particles can be incorporated in a tube such as copper and used as a welding rod. Additional flux materials can be incorporated in the tube with the hard particles. Alternatively, coated diamond can be placed in a mold configured as a trough and binding materials and/or flux can be poured over the particles to bind them together and form welding rod.
The weld process can be much less challenging than molten metal casting as the diamond is at a high temperature for a much shorter period of time. The diamond coating can protect the diamond from chemical attack and graphitization during the welding process. The welding rod can incorporate other hard particles instead of, or in addition to, the diamond that are incorporated in the weld pool and the hardfacing.
Encapsulated diamond can be similarly used with plasma transferred arc welding (PTAW), electroslag surfacing, plasma spray or other surfacing techniques. Alternatively, the diamond can be introduced into the molten weld pool separately from the welding rod. For example, diamond particles can be blown or poured in the molten weld pool.
It should be appreciated that although selected embodiments of the representative encapsulated wear particles are disclosed herein, numerous variations of these embodiments may be envisioned by one of ordinary skill that do not deviate from the scope of the present disclosure. The presently disclosed methods and configurations for encapsulated diamond lend themselves to use for many types of wear particles, and the resulting hardened surfaces are well suited to a variety of applications beyond wear members.
It is believed that the disclosure set forth herein encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. Each example defines an embodiment disclosed in the foregoing disclosure, but any one example does not necessarily encompass all features or combinations that may be eventually claimed. Where the description recites “a” or “a first” element or the equivalent thereof, such description includes one or more such elements, neither requiring nor excluding two or more such elements. Further, ordinal indicators, such as first, second or third, for identified elements are used to distinguish between the elements, and do not indicate a required or limited number of such elements, and do not indicate a particular position or order of such elements unless otherwise specifically stated.
| Number | Date | Country | |
|---|---|---|---|
| 61926080 | Jan 2014 | US |
