Abstract:
This invention is for a polycrystalline diamond composite material comprising diamond particles and a binder phase, the polycrystalline diamond composite material defining a plurality of interstices and the binder phase being distributed in the interstices to form binder pools. The invention is characterised in that there is present in the binder phase a separate tungsten carbide particulate phase in excess of 0.05 total volume %, but not greater than 2 volume %, expressed as a % of the total composite material and that the tungsten carbide particulate phase is homogenously distributed in the composite material in such a manner that the relative standard deviation of the tungsten carbide grain size is less than 1. The invention extends to a method of manufacturing the composite material and to a polycrystalline diamond abrasive compact comprising the diamond composite material for use in cutting or abrading of a substrate or in drilling applications.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to polycrystalline diamond abrasive compacts and a method of producing polycrystalline diamond abrasive compacts. 
         [0002]    Polycrystalline diamond abrasive compacts (PDC) are used extensively in cutting, milling, grinding, drilling and other abrasive operations due to the high abrasion resistance of the polycrystalline diamond component. In particular, they find use as shear cutting elements included in drilling bits used for subterranean drilling. A commonly used PDC is one that comprises a layer of coherently bonded diamond particles or polycrystalline diamond (PCD) bonded to a substrate. The diamond particle content of these layers is typically high and there is generally an extensive amount of direct diamond-to-diamond bonding or contact. Diamond compacts are generally sintered under elevated temperature and pressure conditions at which the diamond particles are crystallographically or thermodynamically stable. 
         [0003]    Examples of composite abrasive compacts can be found described in U.S. Pat. Nos. 3,745,623; 3,767,371 and 3,743,489. 
         [0004]    The PCD layer tends to be relatively brittle and this often limits the lifespan of the tool in application. Hence the PCD layer is generally bonded to a metal backing material, serving as a hard-wearing support for the diamond composite portion. By far the most common form of the resultant body is a disc of polycrystalline diamond bonded to a cylinder of cemented carbide such as WC-Co. Bonding of these two elements is usually achieved in-situ during the sintering of the diamond powder precursor at high pressure and temperature (HpHT). 
         [0005]    The PCD layer of this type of abrasive compact will typically contain a catalyst/solvent or binder phase in addition to the diamond particles. This typically takes the form of a metal binder matrix which is intermingled with the intergrown network of particulate diamond material. This matrix usually comprises a metal exhibiting catalytic or solvating activity towards carbon such as cobalt, nickel, iron or an alloy containing one or more such metals. 
         [0006]    The matrix or binder phase may also contain additional phases. In typical abrasive compacts of the type of this invention, these will constitute less than 10 mass % of the final binder phase. These may take the form of additional separate phases such as metal carbides which are then embedded in the softer metallic matrix, or they may take the form of elements in alloyed form within the dominant metal phase. 
         [0007]    Composite abrasive compacts are generally produced by placing the components necessary to form an abrasive compact, in particulate form, on a cemented carbide substrate. The components may, in addition to ultrahard particles, comprise solvent/catalyst powder, sintering or binder aid material. This unbonded assembly is placed in a reaction capsule which is then placed in the reaction zone of a conventional high pressure/high temperature apparatus. The contents of the reaction capsule are then subjected to suitable conditions of elevated temperature and pressure to enable sintering of the overall structure to occur. 
         [0008]    It is common practice to rely at least partially on binder originating from the cemented carbide as a source of metallic binder material for the sintered polycrystalline diamond. In many cases however, additional metal binder powder is admixed with the diamond powder before sintering. This binder phase metal then functions as the liquid-phase medium for promoting the sintering of the diamond portion under the imposed sintering conditions. 
         [0009]    Under typical high pressure, high temperature sintering conditions, binder metal phase originating from the cemented carbide substrate will also carry with it appreciable levels of dissolved species originating from the carbide layer, as it infiltrates the diamond layer. The amount of dissolved species is strongly affected by the pressure and temperature conditions of sintering—where higher temperatures will typically increase the amount in solution. When the preferred substrate of WC-Co is used, these are W-based species. 
         [0010]    As it infiltrates in to the PCD region, this dissolved tungsten material reacts with carbon from the diamond layer, and can precipitate out as carbide-based phases. Under certain circumstances, this precipitation from the binder occurs on a large and uncontrolled scale. It may therefore manifest as massive WC precipitates of tens and even hundreds of microns in size. They often form on or near the outer periphery of the PDC body during synthesis; and they usually, but not always, tend to be spatially connected with the interface region with the carbide substrate. However, when they do form, the distribution of these precipitates tends to be highly inconsistent across the macroscopic PCD layer. There will be some regions with very little, if any, carbide precipitates present; and certain areas where the relative volume occupied by them is extremely high. 
         [0011]    These WC precipitates have been found to severely compromise the abrasive performance of the compacts, as they reduce mechanical strength by replacing desirable polycrystalline ultrahard material with a lower strength phase. Additionally, these defect regions in the PCD can also act as stress raisers under loading in the application, which then lead to premature fracturing of the PCD material. 
         [0012]    U.S. Pat. No. 6,915,866 discusses the formation of these defects or metal spots and the deleterious effect that they can have on performance of the compact. In this patent, the addition of chromium carbide into the PCD layer is claimed to reduce the formation of these precipitates. However, the use of a foreign species such as chromium carbide, itself represents the introduction of an additional chemical and physical inhomogeneity. It is likely that it too may result in a sub-optimal final structure. There may also be some lessening of the diamond composite&#39;s resistance to thermal degradation due to the presence of chromium carbide. A further drawback to the use of chromium carbide relates to the sinterability of the composite—which is likely to be hindered to some degree at normal sintering temperatures, and therefore may demand higher sintering temperatures than usual in order to achieve an appropriate level of sintering. 
         [0013]    Some success in reducing the occurrence of these large precipitates has been demonstrated through a lowering of the temperatures used in the sintering of the PDC body. However, this is often not always practicable as this will typically result in sub-optimal sintering conditions and hence a less well-sintered PCD. 
         [0014]    A further proposal for reducing the occurrence of large precipitates lies in avoiding any reliance on substrate-originating binder phase. In this case, catalytic material is added exclusively to the PCD powder and infiltration from the carbide substrate is prevented or inhibited. There are however, significant benefits to relying, at least in part, on binder infiltrating from the substrate into the diamond region. 
         [0015]    The use of alternate materials, such as steel, for use in the substrate has also been explored, although these are typically difficult to sinter to the PCD layer and do not give the same performance as the preferred WC-Co substrate. 
         [0016]    The development of an abrasive compact that can achieve optimal properties of impact and wear resistance in the PCD layer is highly desirable. The difficulty lies in that these optimal properties typically occur in a similar sintering environment to that where massive carbide defects in the PCD layer can arise. These carbide defects themselves have a highly detrimental effect on these same required properties. Hence a means of preventing or inhibiting their formation is highly desirable. 
       SUMMARY OF THE INVENTION 
       [0017]    According to a first aspect of the invention, there is provided a polycrystalline diamond composite material comprised of diamond particles and a binder phase; the polycrystalline diamond composite material defining a plurality of interstices and the binder phase being distributed in the interstices to form binder pools, characterised in that there is present in the binder phase a separate tungsten carbide particulate phase in excess of 0.05 volume %, preferably not less than 0.1 volume %, but not greater than 2 volume %, preferably not greater than 1.5%, expressed as a % of the total composite material and the tungsten carbide particulate phase being homogenously distributed in the composite material in such a manner that the relative standard deviation of the WC grain size (expressed as equivalent circle diameter) is preferably less than 1, more preferably less than 0.9 and most preferably less than 0.8. 
         [0018]    The polycrystalline diamond composite material will generally and preferably form a layer bonded to a surface of a cemented carbide substrate forming a polycrystalline diamond abrasive compact. The substrate is preferably a cemented tungsten carbide substrate. 
         [0019]    The polycrystalline diamond composite material of the invention may be made by subjecting a powdered composition of diamond and optionally binder in particulate form to conditions of elevated temperature and pressure suitable for diamond synthesis. The powdered composition is preferably characterised by the presence of finely particulate tungsten carbide particles uniformly distributed in the composition and present in an amount of 0.5 to 5 mass %, preferably 1.0 to 3.0 mass % of the composition. The tungsten carbide particles are finely particulate, having a preferred size of less than 1 μm and more preferably a size of less than 0.75 μm. The preferred concentration of tungsten carbide particles also expressed as the number of tungsten carbide particles per gram of diamond powder mixture is between 10 8  and 10 10 , most preferably of the order of 10 9  particles per gram of diamond. 
         [0020]    The invention extends to the use of the polycrystalline diamond abrasive compacts described above as abrasive cutting elements, for example for cutting or abrading of a substrate or in drilling applications. 
     
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    The present invention is directed to polycrystalline diamond composite materials, generally as a layer bonded to a cemented tungsten carbide substrate forming a polycrystalline diamond abrasive compact, made under high pressure/high temperature conditions. These composite materials are characterised in that they have a binder phase of such metallurgical nature that a separate precipitated carbide phase is distributed throughout in a homogenous manner. 
         [0022]    The diamond particles may be natural or synthetic in origin. The average grain size of the diamond particles is typically in the range between submicron and tens of microns in size. This invention has particular application where the average diamond grain size is less than 25 μm, more preferably less than about 20 μm and most preferably less than 15 μm. 
         [0023]    To produce the polycrystalline diamond composite material of the invention, a powdered composition as described above will be subjected to known temperature and pressure conditions necessary to produce a diamond abrasive compact. These conditions are typically those required to synthesize the diamond particles themselves. Generally, the pressures used will be in the range 40 to 70 kilobars and the temperature used will be in the range 1300° C. to 1600° C. 
         [0024]    The polycrystalline diamond composite material will generally be bonded as a layer to a cemented carbide support or substrate forming a composite abrasive compact. To produce such a composite abrasive compact, the powdered composition will be placed on a surface of a cemented carbide body before it is subjected to the elevated temperature and pressure conditions necessary for compact manufacture. The cemented carbide support or substrate will be made of cemented tungsten carbide. The binder metal for such carbides may be any known in the art such as nickel, cobalt, iron or an alloy containing one or more of these metals. Typically, this binder will be present in an amount of 10 to 20% by mass in the substrate body, but this may be as low as 6% by mass. Some of the binder metal will generally infiltrate the abrasive compact during compact formation. 
         [0025]    The polycrystalline diamond composite materials of the invention have a binder phase present. This binder phase is preferably a catalyst/solvent for the diamond. Catalyst/solvents for diamond are well known in the art. The binder is preferably cobalt, nickel, iron or an alloy containing one or more of these metals. This binder can be introduced either by infiltration into the mass of abrasive particles during the sintering treatment, or in particulate form as a mixture within the mass of abrasive particles. Infiltration may occur from either a supplied shim or layer of the binder interposed between the substrate and diamond layer, or from the carbide support. Typically a combination of approaches is used. 
         [0026]    During the high pressure, high temperature treatment, the catalyst/solvent material melts and migrates through the diamond particles, acting as a catalyst/solvent and hence causing the diamond particles to bond to one another through the formation of reprecipitated diamond phase. Once manufactured, the composite material comprises a coherent matrix of diamond particles bonded to one another, thereby forming a diamond polycrystalline composite material with many interstices containing binder or solvent/catalyst material as described above. In essence, the final composite material therefore comprises a two-phase composite, where the diamond comprises one phase and the binder the other. 
         [0027]    The applicants have discovered that by introducing finely particulate tungsten carbide into the unsintered diamond mass as a dopant at fairly low mass levels prior to sintering, it is possible to inhibit the subsequent formation of gross carbide-based precipitates within the binder phase on or after sintering. Whilst the additional introduction of an undesirable chemical phase into the system may first appear counter-intuitive, it appears that the well-distributed presence of these initial particulates in the pre-sintered mass significantly inhibits the subsequent uncontrolled formation of gross defects of the same or similar chemical phases where it may occur. Without being bound by theory, it is possible that the doped powder mix behaves as a filter, deliberately drawing out any solute W in a controlled way, and so reducing the overall concentration. This process then prevents uncontrolled precipitation of carbide phases elsewhere in the sintering polycrystalline diamond layer by reducing the available solute for carbide formation. 
         [0028]    The method for generating composite materials of the invention is therefore characterized by the initial addition of finely particulate tungsten carbide to the unsintered diamond abrasive particle mixture that is used. This may take the form of admixed separate particles, or may be introduced by the erosive use of tungsten carbide milling media during diamond powder mix preparation, where the abrasive action of the diamond particles on the tungsten carbide milling balls results in the introduction of the desired levels under fairly strenuous milling conditions. Deposition through chemical or physical means may be used to introduce tungsten carbide into the diamond powder mixture. Sometimes a combination of these methods may be used. 
         [0029]    Typically this tungsten carbide addition will be such as to produce in the powdered diamond composition, prior to sintering, a tungsten carbide content in the range of about 0.5 mass % up to about 5 mass % expressed as a percentage of the unsintered powdered composition. It has been found that in polycrystalline diamond materials of the invention with a prevalence for carbide defect formation, levels of tungsten carbide introduced at 0.7 mass % will have positive effects. Typically, however, the more preferred range of addition is from 1.0 to 3 mass %. It should be appreciated however, that the amount of dopant required to prevent runaway precipitation will be characteristic of the polycrystalline diamond composite material being produced. It is therefore anticipated that different composite materials will have differing optimal levels of additive within these wider ranges. It has been found that optimal levels of WC doping for polycrystalline diamond material (PCD of the invention) occur where the number of WC particles is between 10 8  and 10 10  particles per gram of diamond. The most preferred range lies in the order of 10 9  (i.e. between 1×10 9  9.9×10 9  particles per gram of diamond). When the number of particles lies much below approximately 1×10 8  particles per gram of diamond, then the homogenising effect of the doping process is not optimally effective. 
         [0030]    It is also preferred that the tungsten carbide particles are as fine as possible, such that each particle serves as an effective, yet stable, dopant centre without significantly interfering with the diamond sintering process. It is preferred that the average particle size of the WC introduced into the diamond mixture does not exceed 1 μm; and more preferably does not exceed 0.75 μm. It is anticipated that where the particles become too fine in size, the solubility of the WC phase in the molten catalyst/solvent may result in the complete dissolution of significant numbers of the particles. The doping effect would then be substantially compromised. Even in the preferred ranges of the invention, it is anticipated that some of the particles may partially dissolve, although this is mitigated by the fact that the molten catalyst/solvent solution is largely saturated with tungsten from the carbide substrate. 
         [0031]    It is not necessarily required that the carbide particulate be introduced throughout the polycrystalline diamond composite material. Substantial benefits have also been recognised where the composite material only in the region immediately adjacent to the substrate interface has been doped with carbide particles. Thus in this form of the invention, the powdered composition will form a region immediately adjacent to the substrate interface and a layer of diamond, optionally with a binder phase in particulate form, will be placed on the powdered composition. In some cases where the composite material layer is particularly prone to the formation of gross carbide precipitates, however, it may be required that all, or the larger part, of the polycrystalline diamond composite material be doped. For ease of manufacture, it may also be preferred that the entire composite material is doped. 
         [0032]    To distinguish the desired structures of this invention over those typically observed in similar compacts known in the art, it is necessary to consider the homogenising effect of this doping on the overall distribution of the carbide phases in the final sintered microstructure. As previously discussed, the distribution of carbide phases in undoped PCD compacts typically manifests in an uncontrolled and random manner throughout the macroscopic PCD layer. There will be some areas which show little or no visible carbide precipitation; and other areas where large carbide-based gross defects are easily observable. In compacts which are sintered at lower (typically sub-optimal) temperatures, carbide precipitation may not be observable at all. 
         [0033]    The composite material of this invention has a characteristically homogenous or similar-scaled distribution of tungsten carbide phase particulates in the final microstructure. Rather than exhibiting a large extreme in carbide particulate grain size, the size distribution of the carbide phases is characteristically narrow around the average value, which itself tends to be typically fine. The narrow breadth of this distribution can be quantified in statistical terms by the standard deviation, normalised against the overall average or mean value. Composite materials of this invention are therefore characterised in having a standard deviation of the tungsten carbide (WC) phase grain size (expressed as equivalent circle diameter) that is preferably less than 1, more preferably less than 0.9 and most preferably less than 0.8. These values are observed across a range of mean WC phase grain sizes from 0.1 up to 1.5 μm. Typically prior art polycrystalline diamond abrasive compacts with similar average WC grain sizes are observed to have relative standard deviations well in excess of 1.0. 
         [0034]    The measurement of the WC phase grain sizes is carried out on the final composite focussing on the PCD layer, by conducting a statistical evaluation of a large number of collected images taken on a scanning electron microscope. The WC phase grains in the final microstructure, which are easily distinguishable from the remainder of the microstructure using electron microscopy, are isolated in these images using conventional image analysis technology. The overall area occupied by WC phase is measured; and this area % is taken to be equivalent to the overall volume % of WC phase(s) present in the microstructure. 
         [0035]    The average value for the volume % of WC present in the structures of this invention is decided by the combination of the WC introduced into the diamond powder mixture as dopant; and the WC originating from the substrate which precipitates near or onto these dopant particles. In prior art cutters, two distinct populations of WC content are typically observable. There are those with little appreciable overall WC content i.e. where the WC content lies below 0.05 volume % or certainly significantly below 0.1 volume %; and those with a WC volume % in excess of this threshold. Typically those with reduced overall WC carbide content will not be optimally sintered; whilst it is those with WC contents in excess of 0.1 volume % that suffer from the mass defect formation previously discussed. Structures of this invention will typically have WC levels in excess of 0.05 volume %, and more typically WC levels in excess of 0.1 volume %. 
         [0036]    The size of the WC grains is measured by estimating a circle equivalent in size or area for each individual grain identified in the microstructure. The collected distribution of these circles is then evaluated statistically. The chosen indicative variable is the diameter of this “equivalent circle”, known as the equivalent circle diameter. An arithmetic average and standard deviation are then determined from the distribution of these diameters. The relative or normalised standard deviation value is calculated by dividing the standard deviation value by the mean value in each case. Typically magnification levels of 1000 times to 2000 times are chosen to characteristically represent PCD structures of interest in this invention, where the average diamond grain size is submicron up to tens of micron in size. 
         [0037]    The invention will now be illustrated by the following non-limiting examples: 
       Example 1 
     Sample 1A—WC Introduced by Admilling 
       [0038]    A multimodal diamond powder with an average grain size of approximately 15 μm was milled under typical diamond powder mix preparation conditions in a planetary ball mill, together with 1% by mass cobalt powder using WC milling balls. The milling conditions were monitored so as to maximise the erosion of the WC milling media allowing the addition of WC to the mixture at an overall level of 0.7 mass % in the final diamond mixture. The size of the WC fragment introduced in this manner was typically less than 0.5 μm. This powder mixture was sintered onto a standard cemented WC substrate under typical pressure and temperature conditions in order to produce a polycrystalline diamond layer ell bonded to the substrate. The resultant sample is designated Sample A in Table 1 below. 
       Sample 1B—WC Introduced by Admixing 
       [0039]    A multimodal diamond powder with an average grain size of approximately 15 μm was prepared under typical diamond powder mix preparation conditions in a high shear mixer, together with 1% by mass cobalt powder in the absence of any WC milling media. Particulate WC powder was added to this mixture to achieve a level of 0.7 mass % in the final diamond mixture. The size of the WC fragment introduced in this manner was typically between 0.35 and 0.7 μm. This powder mixture was sintered onto a standard cemented WC substrate under typical pressure and temperature conditions in order to produce a polycrystalline diamond layer bonded to the substrate. The resultant sample is designated Sample B in Table 1 below. 
       Sample 1C—Comparative Sample Produced by Admixing 
       [0040]    A multimodal diamond powder with an average grain size of approximately 15 μm was prepared under typical diamond powder mix preparation conditions in a high shear mixer, together with 1% by mass cobalt powder in the absence of any WC milling media. This powder mixture was sintered onto a standard cemented WC substrate under typical pressure and temperature conditions in order to produce a polycrystalline diamond layer bonded to the substrate. The resultant sample is designated Sample C in Table 1 below. 
         [0041]    The samples A to C were all subjected to an analysis as described above to determine the homogeneity of the tungsten carbide species in the polycrystalline diamond layer of each sample. The results are set out in Table 1. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Mix preparation details 
                 Final microstructure: 
               
             
          
           
               
                   
                 Amount 
                   
                 WC character 
               
             
          
           
               
                   
                 WC 
                 Average 
                   
                 Relative 
               
             
          
           
               
                 ID 
                 Description 
                 (mass %) 
                 size 
                 Volume % 
                 sd 
               
               
                   
               
             
          
           
               
                 1A 
                 WC 
                 0.7 
                 &lt;0.5 μm 
                 0.16 
                 0.84 
               
               
                   
                 (admilled) 
               
               
                 1B 
                 WC 
                 0.7 
                 0.35- 
                 0.31 
                 0.55 
               
               
                   
                 (admixed) 
                   
                 0.7 μm 
               
               
                 1C 
                 Undoped 
                 — 
                 — 
                 0.26 
                 1.2 
               
               
                   
               
             
          
         
       
     
         [0042]    It will be noted from the above, that relative standard deviation of WC grain size for Samples A and B, according to the invention, was far less than that of Sample C, produced using a method of the prior art. 
         [0043]    When bulk quantities of PCD materials were then generated following the compositions of samples 1A, 1B and 1C; a very significant decrease in the number of carbide precipitate defects was observed in the materials generated from compositions 1A and 1B. For the same synthesis conditions, the levels of the defect in the undoped sample C type materials were five times higher than in those of this invention (sample A and B type materials). The defects were additionally of much larger scale in the undoped materials. 
       Example 2 
     Sample 2A—WC Introduced by Admilling 
       [0044]    A multimodal diamond powder with an average grain size of approximately 6 μm was milled under typical diamond powder mix preparation conditions in a planetary ball mill, together with 1% by mass cobalt powder using WC milling balls. The milling conditions were monitored so as to maximise the erosion of the WC milling media allowing the addition of WC to the mixture at an overall level of 1.5 mass % in the final diamond mixture. The size of the WC fragment introduced in this manner was typically less than 0.5 μm. This powder mixture was sintered onto a standard cemented WC substrate under typical pressure and temperature conditions in order to produce a polycrystalline diamond layer ell bonded to the substrate. The resultant sample is designated Sample 2A in Table 2 below. 
       Sample 2C—Comparative Sample Produced by Admixing 
       [0045]    A multimodal diamond powder with an average grain size of approximately 6 μm was prepared under typical diamond powder mix preparation conditions in a high shear mixer, together with 1% by mass cobalt powder in the absence of any WC milling media. This powder mixture was sintered onto a standard cemented WC substrate under typical pressure and temperature conditions in order to produce a polycrystalline diamond layer bonded to the substrate. The resultant sample is designated Sample 2C in Table 2 below. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Mix preparation details 
                 Final microstructure: 
               
             
          
           
               
                   
                 Amount 
                   
                 WC character 
               
             
          
           
               
                   
                 WC 
                 Average 
                   
                 Relative 
               
             
          
           
               
                 ID 
                 Description 
                 (mass %) 
                 size 
                 Volume % 
                 sd 
               
               
                   
               
             
          
           
               
                 2A 
                 WC 
                 1.5 
                 &lt;0.5 μm 
                 0.54 
                 0.62 
               
               
                   
                 (admilled) 
               
               
                 2C 
                 Undoped 
                 — 
                 — 
                 0.47 
                 1.3 
               
               
                   
               
             
          
         
       
     
         [0046]    When bulk quantities of PCD materials were then generated following the compositions of samples 2A, and 2C; a significant decrease in the number of carbide precipitate defects was observed in the materials generated from composition 2A. For the same synthesis conditions, the levels of the defect in the undoped sample 2C type materials were at least double those that occurred in materials of this invention (sample 2A type material).