POLYCRYSTALLINE MATERIAL, BODIES COMPRISING SAME, TOOLS COMPRISING SAME AND METHOD FOR MAKING SAME

Abstract

Polycrystalline material comprising a plurality of nano-grains of a crystalline phase of an iron group element and a plurality of crystalline grains of material including carbon (C) or nitrogen (N); each nano-grain having a mean size less than 10 nanometres.

Claims

1. A method for making polycrystalline material having a plurality of nano-grains of a crystalline phase of an iron group element and a plurality of crystalline grains of material including carbon (C) or nitrogen (N), wherein each nano-grain has a mean size less than 10 nanometres and a density of the polycrystalline material is at least 98 percent of the maximum theoretical value, the method comprising: providing a precursor structure comprising iron (Fe) and silicon (Si), and a source of carbon (C) or nitrogen (N), in which relative quantities of the Fe, Si and C or N are selected such that the combination of the Fe, Si and C or N has a phase liquidus temperature of at most 1,280 degrees centigrade; heating the precursor structure to a temperature of at least 1,350 degrees centigrade at a mean rate of at least 100 degrees centigrade per second; and cooling the precursor structure to less than 1,000 degrees centigrade at a mean rate of at least 20 degrees per second.

2. The method of claim 1, the method further comprising: combining powder comprising the Fe and powder comprising the Si with polyvinyl compound binder material including a hydroxyl group to provide slurry; and spray drying the slurry to provide a plurality of the precursor structures in a form of granules.

3. The method of claim 1, the method further comprising: providing a plurality of precursor structures; screening the plurality of precursor structures to provide a plurality of screened precursor structures having mean diameter of at least 20 microns and at most 5,000 microns; and selecting at least one precursor structure from the plurality of screened precursor structures.

4. The method of claim 1, the method further comprising: heating the precursor structure at a temperature of at least about 300 degrees centigrade and at most about 1,300 degrees centigrade for at least about 5 minutes in a vacuum or a hydrogen-containing atmosphere or other atmosphere likely to reduce a rate of oxidation; and cooling the precursor structure to below about 300 degrees centigrade prior to including heating the precursor structure to a temperature of at least about 1,350 degrees centigrade at a mean rate of at least about 100 degrees centigrade per second.

5. The method of claim 1, in which the precursor structure comprises Fe, Si, C and chromium (Cr), in which relative quantities of the Fe, Si, C and Cr are selected such that the combination of the Fe, Si, C and Cr has a phase liquidus temperature of at most about 1,280 degrees centigrade.

6. The method of claim 1, in which the precursor structure includes a plurality of carbide material grains, such as WC grains.

7. The method of claim 1, in which the precursor structure includes a plurality of carbide material grains having mean size of at least 0.1 micron and at most 10 microns.

8. The method of claim 1, in which the precursor structure includes a plurality of chromium carbide particles.

9. The method of claim 1, in which the precursor structure contains super-hard material.

10. The method of claim 1, in which the precursor structure comprises at least 13 weight percent WC grains, 0.1-10 weight percent Si, and 0.1-10 weight percent Cr, and the iron group element.

11. The method of claim 1, in which the precursor structure comprises at least about 60 weight percent and at most about 80 weight percent tungsten carbide, at least about 10 weight percent and at most about 20 weight percent Fe, at least about 5 weight per cent chromium carbide grains, and at least about 1.0 weight percent and at most about 5 weight percent Si grains or grains comprising a precursor compound including Si.

12. The method of claim 1, in which the precursor structure has compressive strength of at least 2 MPa.

13. The method of claim 1, in which the precursor structure is granular in form.

14. The method of claim 1, the method further comprising combining a plurality of the precursor structures to provide a unitary structure.

15. The method of claim 1, the method further comprising introducing boron (B) into the precursor structure, in elemental or compound form.

16. The method of claim 1, the method further comprising contacting the precursor structure with a substrate, the precursor structure being at a temperature of at least about 1,350 degrees centigrade while in contact with the substrate, for a sufficient period of time for the precursor structure to fuse with the substrate.

17. The method of claim 1, the method further comprising: heating a plurality of the precursor structures in granular form to a temperature of at least 1,350 degrees centigrade at a mean rate of at least 100 degrees centigrade per second; depositing the precursor structures onto a substrate while at the temperature; and cooling the precursor structures to less than 1,000 degrees centigrade at a rate of at least 20 degrees per second.

18. The method of claim 1, the method further comprising depositing a plurality of the precursor structures onto a substrate by means of a high temperature spray apparatus.

19. The method of claim 1, the method further comprising depositing a plurality of the precursor structures onto a substrate by means of a plasma torch, laser beam torch or flame torch.

20. The method of claim 1, the method further comprising contacting a plurality of the precursor structures with a substrate such that the precursor structures form a contiguous layer.

21. The method of claim 1, the method further comprising contacting a plurality of the precursor structures with a substrate comprising iron group metal.

22. The method of claim 1, the method further comprising contacting a plurality of the precursor structures with a substrate comprising steel.

23. The method of claim 16, the method further comprising removing at least part of a substrate body.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Non-limiting example arrangements will now be described with reference to the accompanying drawings, of which

[0051] FIG. 1A shows a 500× magnification scanning electron micrographs (SEM) of example polycrystalline material, FIG. 1B shows a 2,000× magnification SEM image of example polycrystalline material, FIG. 1C shows a 4,000× magnification SEM image of example polycrystalline material; and FIG. 1D shows a high resolution transmission electron microscopic (HRTEM) image of the example polycrystalline material, in which nano-grains are indicated by means of a white outline superimposed on the image;

[0052] FIG. 2A, FIG. 2B and FIG. 2C show microstructures of example polycrystalline material in the form of a layer structure fused to a steel substrate, the polycrystalline material having been etched in Murakami reagent for 4 minutes (light microscopy), of which FIG. 2A shows an image of a region proximate the surface of the layer structure, FIG. 2B shows an image of the polycrystalline material etched to a depth of 1.5 millimetres from the surface of the layer structure and FIG. 2C shows an image of the interface with the steel substrate;

[0053] FIG. 3 shows an electron diffraction image of example polycrystalline material;

[0054] FIG. 4A and FIG. 4B show transmission electron microscopic (TEM) images of example polycrystalline material;

[0055] FIG. 5 shows a schematic perspective view of an example pick tool for mining; and

[0056] FIG. 6 shows a schematic side view of an example super-hard pick tool for road milling.

DETAILED DESCRIPTION

[0057] An example method for making example polycrystalline material in the form of a layer structure fused to a steel substrate is described below.

[0058] A powder blend having a mass of 100 kg was prepared, comprising 72.7 weight percent tungsten carbide (WC) powder having mean particle size of 1 micron, 15 weight percent iron (Fe) powder, 10 weight percent chromium carbide (Cr3C2) powder and 2.3 weight percent silicon (Si) powder. The powder blend was milled for about one hour in an attritor mill in water medium using 600 kilograms hard-metal balls and 4 kilograms organic binder based of polyvinyl-containing hydroxyl groups (product KM4034, Szchimmer and Scharz™) to provide a slurry. After milling, the slurry was spray-dried to provide granules which were screened to obtain select granules from about 100 microns to about 180 microns.

[0059] The mean compressive strength of the granules was measured selecting a plurality of the granules at random, measuring the diameter of each and compressing each between two steel plates at forces from 0.1 milli-Newtons to 900 milli-Newtons (for example, by means of an instrument of the Etewe™ GmbH company in Germany). The relationship between the degree of diametric deformation of each grain and the applied forces was measured, and the compressive strength of each grain was calculated on the basis of this measured relationship. The compressive strength of the granules was found to be in the range from about 2 mega-Pascals to about 10 mega-Pascals.

[0060] A steel substrate was provided and the granules were sprayed onto the substrate by means of a plasma torch apparatus (Plasmastar™) for atmospheric plasma spraying, operated at a current of 100 Amperes in an Ar gas flow. In the process of spraying, the granules were heated at a rate of about 300 degrees centigrade per second to a temperature about 1,400 degrees centigrade. The subsequent cooling rate was about 40 degrees centigrade per second. A contiguous layer formed from the granules was thus provided fused to the substrate, the density of the layer being close to the theoretical density and the mean thickness of the layer being about 3 millimetres. The steel substrates thus coated were heat-treated.

[0061] The Vickers HV10 hardness of the polycrystalline material layer was found to be 1,000 Vickers units and the Palmqvist fracture toughness to be 15 MPa.Math.m.sup.1/2. The Vickers hardness was measured according to the ISO standard ISO 3878 and the Palmqvist fracture toughness was measured according to the ISO standard ISO/DIS 20079. The coated steel substrates were tested by use of the ASTM G65-04 test to measure wear resistance, with uncoated steel substrates being used as controls. The mass loss due to abrasion of the control was about 820 milligrams and that of the coated steel substrate was about 80 milligrams, and the volume loss of the uncoated steel was 105.2 cubic millimetres and that of the coated steel was 6.5 cubic millimetres. This indicates that the wear resistance of the example polycrystalline material was more than 15 times higher than that of the steel substrate.

[0062] Non-limiting, example polycrystalline material provided as described above will now be described.

[0063] Images of the microstructure of the example polycrystalline material are shown in FIG. 1A, FIG. 1B, FIG. 1C and FIG. 1D at various magnifications. The material includes plate-like dendritic or rounded crystallite structures 10 and eta-phase carbide crystallites 12 dispersed in binder matrix 20.

[0064] The nano-hardness of the binder matrix 20 was found to be from about 1.0 GPa to about 1.3 GPa. The nano-hardness is measured by means of a TriboIndenter™ instrument (Hysitron Inc., Minneapolis, Minn., USA) with a Berkovich tip, using maximum loads of 500 micro-N and 2 mN. The loading schedule involved 10 seconds linear loading, 10 seconds hold at constant load, and 10 seconds linear unloading.

[0065] In FIG. 1A, the dendritic structure of the crystallite structures 10 is evident in some instances 10A, while the plate-like or flat aspect of the crystallite structures 10 is evident in other instances 10B that are pictured side-on.

[0066] Tungsten carbide (WC) grains 14 dispersed in the binder matrix 20 are evident in FIG. 1B.

[0067] FIG. 1C shows an electron back-scatter image of the polycrystalline material after removal of a surface layer of about 5 nanometres by means of Ar etching. The compositions of the various micro-structures and materials evident in FIG. 1C were examined by Auger electron spectroscopy (AES). The binder material 20 within which the dendritic crystallites 10, eta-phase crystallites 12 and WC crystallites 14 are embedded comprises two spatially distinct components in distinct regions 201, 202, which are evident as different shades of grey in FIG. 1C. The binder material of the first region 201 includes a plurality of the nano-grains of a crystalline phase of iron. The material of the first region 201 comprises about 24.8 weight percent W, 65.7 weight percent Fe, 3.5 weight percent Cr, 4.3 weight percent C and 1.7 weight percent Si. The binder material of the second region 202 is substantially free of the nano-grains and comprises about 29.8 weight percent W, 57.8 weight percent Fe, 4.9 weight percent Cr, 6.9 weight percent C and 0.6 weight percent Si. The dendritic structures 10 comprise about 62.4 weight percent W, 30.7 weight percent Fe, 1.8 weight percent Cr, weight percent C and 1.6 weight percent Si.

[0068] FIG. 1D shows an high resolution TEM (HRTEM) image of the binder material 20 within the first region 201, in which nano-grains 16 dispersed therein are evident. The nano-grains 16 are indicated by white boundary lines and have a mean size in the range from about 2 nanometres to about 10 nanometres. At least some of the grains 16 appear to be generally elongate and have a mean length of about 7 nanometres and mean width of about 4 nanometres. X-ray diffraction and electron diffraction indicate that the nano-grains 16 comprise crystalline phases of iron, including Fe.sub.3W.sub.3C, ferrite and austenite.

[0069] With reference to FIG. 2A, FIG. 2B and FIG. 2C, the dendritic crystallites 10 appear brown in colour after etching in Murakami reagent for about 5 seconds. The dendritic crystallites 10 comprise eta-phase carbide. Some of the dendritic crystallites 10 become black and some remain brown after further etching in the Murakami reagent for about 4 minutes WC crystallite grains 14 are also evident in example polycrystalline material.

[0070] The electron diffraction image shown in FIG. 3 indicates that the binder matrix has a nano-crystalline structure with very little amorphous phase present.

[0071] As shown in FIG. 4A and FIG. 4B, transmission electron microscopy (TEM) indicates the presence of nano-sized grains 30 of eta-phase carbide in the binder matrix 20, having the general form of nano-plates, nano-rods or nano-spheres, which are embedded in the binder matrix 20.

[0072] With reference to FIG. 5, an example pick tool 50 for mining comprises a steel base 55 and a hard face structure 56 fused to the steel substrate 55. The pick tool 50 comprises a cemented carbide tip 52 having a strike point 54 and joined to the steel base 55. In some examples the tip 52 may comprise diamond material such as PCD material or silicon carbide-bonded diamond material. The hard face structure 56 is arranged around the cemented carbide tip 52 to protect the steel substrate 55 from abrasive wear in use. In use breaking up a rock formation comprising coal or potash, for example, rock material may abrade the steel base 55 leading to premature failure of the pick tool 50. The hard face structure 56 comprises or consists essentially of example polycrystalline material according to this disclosure.

[0073] With reference to FIG. 6, an example pick tool 60 for a road pavement milling comprises a steel holder 65 provided with a bore, and a strike tip 64 joined to a cemented carbide base 62 that is shrink fit or press fit into the bore. A hard face structure 66 comprising polycrystalline material according to this disclosure is fused to the steel holder 65, arranged around the bore to protect the steel holder body 65 from wear in use. The strike tip 64 may comprise a PCD structure joined to a cemented tungsten carbide substrate. The holder 65 comprises a shank 68 for coupling to a base block (not shown) attached to a road milling drum (not shown).

[0074] As used herein in relation to grains comprised in polycrystalline material, the term “grain size” d refers to the sizes of the grains measured as follows. A surface of a body comprising the hard-metal material is prepared by polishing for investigation by means of electron backscatter diffraction (EBSD) and EBSD images of the surface are obtained by means of a high-resolution scanning electron microscope (HRSEM). Images of the surface in which the individual grains can be discerned are produced by this method and can be further analysed to provide the number distribution of the sizes d of the grains, for example. As used herein, no correction (e.g. Saltykov correction) is applied to correct the grain sizes to account for the fact that they were obtained from a two dimensional image in this way. The grain size is expressed in terms of equivalent circle diameter (ECD) according to the ISO FDIS 13067 standard. The ECD is obtained by measuring of the area A of individual grains exposed at the surface and calculating the diameter of a circle that would have the same area A, according to the equation d=square root of (4×A/π). The method is described further in section 3.3.2 of ISO FDIS 13067 entitled “Microbeam analysis—Electron Backscatter Diffraction—Measurement of average grain size” (International Standards Organisation, Geneva, Switzerland, 2011). The mean grain size D of WC grains in cemented WC material is obtained by calculating the number average of the WC grain sizes d as obtained from the EBSD images of the surface. The EBSD method of measuring the sizes of the grains has the significant advantage that each individual grain can be discerned, in contrast to certain other methods in which it may be difficult or impossible to discern individual grains from agglomerations of grains. In other words, certain other methods may be likely to give false higher values for grain size measurements.

[0075] Certain terms and concepts as used herein are briefly explained below.

[0076] As used herein, a hard face structure is a structure such as, but not limited to, a layer joined to a substrate to protect the substrate from wear. The hard face structure exhibits a substantially greater wear resistance than does the substrate.

[0077] As used herein, a wear part is a part or component that is subjected, or intended to be subjected to wearing stress in application. There are various kinds of wearing stress to which wear parts may typically be subjected such as abrasion, erosion, corrosion and other forms of chemical wear. Wear parts may comprise any of a wide variety of materials, depending on the nature and intensity of wear that the wear part is expected to endure and constraints of cost, size and mass. For example, cemented tungsten carbide is highly resistant to abrasion but due to its high density and cost is typically used only as the primary constituent of relatively small parts, such as drill bit inserts, chisels, cutting tips and the like. Larger wear parts may be used in excavation, drill bit bodies, hoppers and carriers of abrasive materials and are typically made of hard steels which are much more economical than cemented carbides in certain applications.

[0078] Pick tools may be used for breaking, degrading or boring into bodies, such as rock, asphalt, coal or concrete, for example, and may be used in applications such as mining, construction and road reconditioning. In some applications, for example road reconditioning, a plurality of pick tools may be mounted on a rotatable drum and driven against the body to be degraded as the drum is rotated against the body. Pick tools may comprise a working tip of a super-hard material, for example polycrystalline diamond (PCD), which comprises a mass of substantially inter-grown diamond grains forming a skeletal mass defining interstices between the diamond grains.

[0079] Synthetic and natural diamond, polycrystalline diamond (PCD), cubic boron nitride (cBN), polycrystalline cBN (PCBN) and silicon carbide bonded diamond (ScD) material are examples of super-hard materials. As used herein, synthetic diamond, which is also called man-made diamond, is diamond material that has been manufactured. As used herein, polycrystalline diamond (PCD) material comprises a mass (an aggregation of a plurality) of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume percent of the material. Interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst material for synthetic diamond, or they may be substantially empty. As used herein, a catalyst material for synthetic diamond is capable of promoting the growth of synthetic diamond grains and or the direct inter-growth of synthetic or natural diamond grains at a temperature and pressure at which synthetic or natural diamond is thermodynamically stable. Examples of catalyst materials for diamond are Fe, Ni, Co and Mn, and certain alloys including these. Bodies comprising PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains. As used herein, PCBN material comprises grains of cubic boron nitride (cBN) dispersed within a matrix comprising metal or ceramic material.

[0080] Other examples of superhard materials include certain composite materials comprising diamond or cBN grains held together by a matrix comprising ceramic material, such as silicon carbide (SiC), or cemented carbide material, such as Co-bonded WC material (for example, as described in U.S. Pat. No. 5,453,105 or 6,919,040). For example, certain SiC-bonded diamond materials may comprise at least about 30 volume percent diamond grains dispersed in a SiC matrix (which may contain a minor amount of Si in a form other than SiC). Examples of SiC-bonded diamond materials are described in U.S. Pat. Nos. 7,008,672; 6,709,747; 6,179,886; 6,447,852; and International Application publication number WO2009/013713).

[0081] A grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material.

[0082] Several phases comprising tungsten (W), cobalt (Co) and carbon (C) are known and are typically designated by Greek letters. An eta-phase composition is understood herein to mean a carbide compound having the general formula Mx M′y C2, where M is at least one element selected from the group consisting of W, Mo, Ti, Cr, V, Ta, Hf, Zr, and Nb; M′ is at least one element selected from the group consisting of Fe, Co, Ni, and C is carbon. Where M is tungsten (W) and M′ is cobalt (Co), as is the most typical combination, then eta-phase is understood herein to mean C03W3C (eta-1) or Co6WeC (eta-2), as well as fractional sub- and super-stochiometric variations thereof. There are also some other phases in the W—Co—C system, such as theta-phases C03W6C2, Co4W4C and Co2W4C, as well as kappa-phases Co3WgC4 and CoW3C (these phases are sometimes grouped in the literature within a broader designation of eta-phase).

[0083] As used herein, the phase “consists essentially of” means “consist of, apart from practically unavoidable impurities”.

[0084] As used herein, the term “about” in relation to a value is understood to indicate a range of values explicitly including the stated value, the limits of the range being understood to be informed by the number of significant figures evident in the value as stated.