WO2007147214A1

WO2007147214A1 - Method and apparatus for polishing diamond and diamond composites

Info

Publication number: WO2007147214A1
Application number: PCT/AU2007/000873
Authority: WO
Inventors: Liangchi Zhang; Bruce Oliver; Yiqing Chen; Joseph Alexander Arsecularatne
Original assignee: The University Of Sydney
Priority date: 2006-06-22
Filing date: 2007-06-22
Publication date: 2007-12-27
Also published as: EP2040879A1; IL196050A0; CN101500751A

Abstract

A method and apparatus for polishing a diamond material is disclosed. The method comprises the steps of positioning a sample of the diamond material in relation to a movable metallic surface, and bringing the diamond material sample into contact with the movable metallic surface with simultaneous movement of the metallic surface and rotation of the diamond material sample. Two or more samples of the diamond material can be evenly spaced around a circumference of an imaginary circle centred on an axis of movement of the metallic surface, to be brought into contact with the movable metallic surface whilst moving the metallic surface on its axis. The apparatus can comprise a drive arranged to effect such movements, as well as a lift and place mechanism to constrain sample movements.

Description

METHOD AND APPARATUS FOR POLISHING DIAMOND AND DIAMOND

COMPOSITES

Technical Field Disclosed herein is a method and apparatus for polishing diamond and diamond composites.

Background Art

Polycrystalline diamond (PCD) compacts are used in cutting tools (for machining a wide range of metallic and non-metallic materials) and in many other wear/friction surface applications. PCD 's possess excellent properties such as ultra high hardness, thermal conductivity, strength, and chemical inertness to most corrosive environments. In many of these applications (eg. in precision machining) the PCD must have excellent surface finish and edge sharpness. However, because of the ultra high hardness and chemical inertness of diamond, polishing of a PCD compact to achieve such finish and sharpness has presented a difficult problem.

Some known mechanical abrasive polishing techniques have extremely low polishing rates, of the order of 10 nanometres per hour, and are therefore time- consuming and costly. More recently, physical and chemical means have been explored to polish PCD and diamond films. These have included mechanical polishing, chemically assisted mechanical polishing, thermo-chemical/hot-metal-plate polishing, laser/plasma/ion beam polishing, and dynamic friction polishing. The dynamic friction method has been reported to be the most efficient and cost-effective technique to date. In Dynamic Friction Polishing (DFP), a diamond compact is polished without using abrasives but by pressing it at a predetermined pressure onto a metal disk rotating at a high speed to generate dynamic friction. This process is schematically illustrated in Figure 1. It maybe carried out under atmospheric conditions and utilizes a thermo- chemical reaction induced by dynamic friction between the diamond and the metal disc. The process is intended to provide a highly efficient, abrasive-free polishing of eg. single crystal diamonds and PCD's.

Under the polishing conditions of DFP, the temperature rises dramatically at the interface, and chemical reactions at these elevated temperatures play an important role in the polishing of the PCD. The polishing mechanism has been described as: (a) a conversion of diamond carbon into non-diamond carbon by friction heating and contacting with catalytic metals, which is then removed mechanically; (b) diffusion of carbon atoms into a counterpart metal and a chemical reaction with the metal to form carbides; and (c) oxidization of carbon and evaporation in the form of CO or CO₂ gas. It is understood that the chemical reaction of carbon plays an important role in the material removal during polishing of PCD, wherein the carbon reacts with the metal or oxidizes at the elevated temperatures.

In addition, other components of PCD, such as silicon carbide (SiC) and silicon (Si) may also chemically react and transform to amorphous silicon oxide and/or silicon carbide which are also removed during polishing.

Recent investigations into the DFP process have been concerned with polishing efficiency, the polishing mechanism and the final PCD surface finish. In some cases, experiments were carried out on a milling machine with the metal disk mounted on a machine spindle. In US 6,585,565 and US 6,592,436, the use of various inter-metallic compounds are disclosed (eg. zirconium-nickel, titanium-aluminium, titanium-nickel) for use in dynamic friction and thermo-chemical polishing of PCD' s, single-crystal diamond and diamond films. However, the required tests were carried out on a known milling machine and/or a known surface/lap grinder. It has been discovered that the use of such apparatus makes it impossible to carry out the polishing process efficiently and also makes it difficult to control process parameters such as applied pressure. To date, no economically viable method and apparatus have been developed for the DFP process.

A reference herein to prior art is not an admission that the prior art forms part of the common general knowledge of a skilled person in the art in Australia or elsewhere.

Summary of Disclosure

In a first aspect there is disclosed a method for polishing a diamond material, the method comprising the steps of: - positioning a sample of the diamond material in relation to a movable metallic surface; and

- bringing the diamond material sample into contact with the movable metallic surface with simultaneous movement of the metallic surface and rotation of the diamond material sample.

It has been discovered that simultaneous rotation of both the metallic surface and the diamond material sample significantly increases the polishing process efficiency, to a level whereby DFP becomes an economically viable polishing technique.

In a usual form of the method of the first aspect the metallic surface is rotatable, whereby two or more samples of the diamond material may be positioned in relation to the rotatable metallic surface, and whereby the two or more samples may be evenly spaced around a circumference of an imaginary circle centred on an axis of rotation of the metallic surface.

In a second aspect there is provided a method for polishing a diamond material, the method comprising the steps of:

- positioning two or more samples of the diamond material in relation to a movable metallic surface whereby the two or more samples are evenly spaced around a circumference of an imaginary circle centred on an axis of movement of the metallic surface; and

- bringing the two or more diamond material samples into contact with the rotatable metallic surface whilst moving the metallic surface on its axis.

It has been discovered that the even spacing of two or more samples around a circumference of an imaginary circle that is centred on the metallic surface's axis of rotation enables better control of process parameters such as applied pressure, which in turn enables an increase in polishing process efficiency. Also, the even spacing minimises deflection of the metallic surface. Again, these factors further contribute to DFP becoming an economically viable polishing technique. In a usual form of the method of the second aspect, the metallic surface is rotatable about an axis of rotation such that, when the two or more diamond material samples are brought into contact with the rotatable metallic surface, both the metallic surface and the diamond material samples may be rotated. The combination of even spacing and simultaneous rotation can again greatly increase polishing process efficiency.

The terminology "imaginary circle" as employed herein is used to define a geometry assumed by the two or more samples in relation to the metallic surface's axis of rotation. In this regard, the terminology is not intended to imply that there is an actual physical circle defined on the metallic surface, nor is it intended to imply that the metallic surface is itself circular. For example, the metallic surface can define or form part of a plate, disk etc having a variety of peripheral shapes, but which is then rotated on an axis, with the two or more samples thus being spaced evenly with respect to this axis.

The terminology "diamond material" as employed herein includes natural and synthetic diamond, thick/thin diamond films, polycrystalline diamond and diamond composites. Thus the method and apparatus disclosed herein can be applied to any such diamond materials. The reference to a "sample" of the diamond material is intended to include both single and multiple specimens of diamond material, as will become apparent hereafter. In other words, a "diamond material sample" can comprise one or more individual specimens of diamond material of various dimensions.

The terminology "metallic surface" as employed herein includes both metal and metal containing surfaces such as steel and various alloys, especially stainless steel; iron; nickel; chromium; cobalt; titanium; zirconium; and various alloys of these metals etc. The terminology "metallic surface" is thus intended to cover any material comprising a metal that catalyses a reaction with a diamond material.

In the method of both the first and second aspects the average sliding speed of the (or each) diamond material sample with respect to the metallic surface may be in the range of 5 to 60 m/s (more typically in the range of 15 to 30 m/s). For example, an average sliding speed that has been found to be effective is about 21 m/s. This range of sliding speeds has been found to best promote the DFP technique. Such ranges can vary with variations in pressure applied to the sample, and with variations in diamond material surface roughness and thermal properties of the diamond material.

In the method of both the first and second aspects the (or each) diamond material sample may comprise at least two specimens of diamond material that are non- aligned with respect to a rotational axis of the sample. Thus, as a sample is rotated on its rotational axis, the specimens rotate around this axis. For example, the two or more specimens can again be evenly spaced at the sample around a circumference of an imaginary circle, but this time that is centred on the sample rotational axis. This non- alignment of the diamond material specimens also helps to promote an even (eg. flatter) polishing of each specimen and to prevent grooving in the metallic surface. In the method of both the first and second aspects the (or each) sample rotational axis may be parallel to the metallic surface rotational axis. This arrangement of axes makes it easier for set up of the method's geometry.

In the method of both the first and second aspects the (or each) diamond material sample is typically pressured into contact with the metallic surface. For example, an effective pressure on the (or each) diamond material sample to best promote the DFP technique has been found to be in the range of 2-25 MPa, for example, to be around 3.5 MPa. The amount of pressure applied can be varied inversely with the average sliding speed, with the selected speed and pressure being optimised to the type of diamond material to be polished. In this regard, when pressure is increased, average sliding speed may be decreased (and vice versa).

The method of both the first and second aspects may comprise a dynamic friction polishing stage and a subsequent abrasive polishing stage. The dynamic friction polishing stage can have a duration of 1-5 minutes (eg. around 3 minutes) and the abrasive polishing stage can have a duration of 1-20 minutes (eg. around 15 minutes). The abrasive polishing stage may be conducted on an abrasive surface immediately adjacent to the metallic surface.

In a third aspect there is provided apparatus for polishing a diamond material, the apparatus comprising: - a holder for positioning a sample of the diamond material in relation to a movable metallic surface; and

- a drive for bringing the diamond material sample into contact with the movable metallic surface and for causing simultaneous movement of the metallic surface and rotation of the diamond material sample. As with the method of the first aspect, simultaneous rotation of both the metallic surface and the diamond material sample significantly increases the polishing process efficiency.

In a usual form of the apparatus of the third aspect the drive can be arranged to cause rotation of the diamond material sample by rotating the holder, but can also be arranged to cause rotation of the metallic surface.

The apparatus of the third aspect may comprise two or more holders for two or more respective diamond material samples. The holders may be arranged such that the two or more samples are evenly spaced around a circumference of an imaginary circle centred on an axis of rotation of the metallic surface.

In a fourth aspect there is provided apparatus for polishing a diamond material, the apparatus comprising:

- two or more holders for two or more respective samples of the diamond material, the holders being arranged for positioning the samples in relation to a movable metallic surface and such that the two or more samples are evenly spaced around a circumference of an imaginary circle centred on an axis of movement of the metallic surface; and

- a drive for bringing the two or more diamond material samples into contact with the metallic surface whilst moving the metallic surface on its axis.

As with the method of the second aspect, evenly spacing the two or more samples around a circumference of an imaginary circle centred on an axis of rotation of the metallic surface enables better control of applied pressure and minimises deflection of the metallic surface, which in turn enables an increase in polishing efficiency. Ih a usual form of the apparatus of the fourth aspect the metallic surface can be rotatable about an axis of rotation, whereby the drive can be arranged for causing simultaneous rotation of both the metallic surface and each of the diamond material samples. Again, the drive can cause rotation of each diamond material sample by rotating each holder. In the apparatus of both the third and fourth aspects the drive may comprise a respective motor for each of the sample holder(s) and for the metallic surface. Each motor may be arranged to drivingly rotate the holder or surface about a respective rotational axis. The use of respective motors enables individual control of the rotational speeds of the sample holder(s) and the metallic surface. The apparatus of both the third and fourth aspects may be configured such that a rotational axis of the (or each) sample holder is parallel to a rotational axis of the metallic surface. As with the method of the first and second aspects, this arrangement of axes makes it easier for set up of the apparatus geometry. However, as described below, multi axis apparatus may alternatively be employed. In the apparatus of both the third and fourth aspects the (or each) sample holder may comprise at least two specimen mounting points, each for the mounting of a respective diamond material specimen, the mounting points being offset with respect to the sample rotational axis. The specimen mounting points can be evenly spaced around a circumference of an imaginary circle that is centred, on the sample rotational axis. Again, evenly spacing the specimen mounting points around a circumference of an imaginary circle enables better polishing control and a more even application of pressure, and again minimises deflection of the metallic surface. This in turn leads to more even polishing and an increase in polishing efficiency.

In the apparatus of both the third and fourth aspects the drive may further comprise a ram for urging the sample holder(s) towards and thus urging the (or each) diamond material sample onto the metallic surface. The ram can be pneumatic and can operate between fixed and movable members of the apparatus, with the sample holder(s) being mounted to the movable member. A pneumatic ram provides a simple yet sufficiently robust way of achieving the pressures required for DFP.

As an alternative to a ram, weights can be employed that can be added to or removed from the apparatus to vary the pressure applied to the specimens.

The movable member may form part of a lift and place mechanism for placing the specimens onto the movable metallic surface in a non-deflectable manner (eg. whereby the mechanism prevents lateral deflection during placement and removal of the specimens).

In one form of the lift and place mechanism, opposing ends of the movable member can each be slidably mounted to a respective linear bearing that restrains moveable member movement by the ram to a linear movement towards or away from the metallic surface. A linear bearing is a very effective way of restraining lateral and other uneven movements of the moveable member, thus again promoting more even polishing and an increase in polishing efficiency.

In this form of the lift and place mechanism the fixed member may be defined by an in-use upper cross-beam supported at opposite ends by columns of the apparatus, with the columns extending from an apparatus housing. The movable member may be defined by an in-use lower and sliding cross-beam that is movable down and up with respect to the apparatus housing. The linear bearing can be mounted on each column for apparatus rigidity. The fixed and moveable members, and the columns, can all be fabricated of structural components to provide strength and rigidity to the apparatus.

In another form of the lift and place mechanism, one end of the movable member can be pivotally mounted to a hinge that can again be adapted to restrain moveable member movement to an arcuate movement about the hinge to thus move the specimens towards or away from the metallic surface. In this other form, the fixed member can thus be defined at or by the hinge. The hinge can be mounted to or incorporated into a housing or frame of the apparatus. The arcuate movement can also be effected by a drive (eg. a ram, weights etc) that operates on the movable member. Again, the hinge can provide a simple yet effective way of restraining lateral and other uneven movements of the moveable member.

In the apparatus of both the third and fourth aspects the metallic surface can form part of a metallic disk that is rotated by a part of the drive about a disk central axis. The disk may be in the form of an annulus that defines a dynamic friction polishing surface.

An abrasive polishing surface can then be defined that is located within the annulus. In the apparatus of both the third and fourth aspects the drive can then be adapted to move the (or each) sample holder and thus the (or each) sample from the dynamic friction polishing surface to the abrasive polishing surface. Materials for the abrasive polishing surface can include diamond composites, cubic boron nitride, boron carbide, etc. hi a fifth aspect there is provided apparatus for polishing a diamond material, the apparatus comprising:

- a holder for positioning a sample of the diamond material in relation to a movable metallic surface; and

- a lift and place mechanism arranged for co-operation with the holder or the surface so as to position the sample at the movable metallic surface in a non-deflectable manner.

A lift and place mechanism is a very effective way of restraining lateral and other uneven movements of the sample to thereby promote more even polishing and an increase in polishing efficiency.

Whilst typically the lift and place mechanism is arranged to co-operate with the holder to move it towards or away from the metallic surface, it may be arranged to cooperate with the metallic surface to move it towards or away from the holder, hi yet a further variation, the lift and place mechanism can be arranged to co-operate with both the holder and the metallic surface to move each towards or away from the other. hi the fifth aspect the lift and place mechanism may comprise a member, with the holder or the metallic surface being supported with respect to the member. A drive may then be arranged for operation on the member so as to position the sample at the movable metallic surface in a non-deflectable manner. The drive may be defined by a pneumatic or hydraulic ram, by weights etc.

In one form of the fifth aspect, the holder can be mounted with respect to the member, and opposing ends of the member can each be slidably mounted to a respective linear bearing that restrains member movement, whereby the holder is moved towards or away from the metallic surface in a linear manner.

Whilst in this form the holder is mounted with respect to the member to move therewith, the metallic surface can alternatively or additionally be mounted with respect to the same or another member to move in a manner that is supported and restrained by the linear bearing.

In this form the member may move with respect to a fixed member defined by an in-use upper cross-beam supported at opposite ends by columns of the apparatus, with the columns extending from an apparatus housing. The moving member may then be defined by an in-use lower and sliding cross-beam that is movable down and up with respect to the apparatus housing. The linear bearing can be mounted on each column for apparatus rigidity. The fixed and moving members, and the columns, can all be fabricated of structural components to provide strength and rigidity to the apparatus.

In another form of the fifth aspect, one end of the member can be pivotally mounted to a hinge that can again be adapted to restrain member movement to an arcuate movement about the hinge to thus move the sample towards or away from the metallic surface. In this other form, the member may move with respect to a fixed member defined at or by the hinge. The drive can again operate between the fixed and moving members. Also, the hinge can be mounted to or incorporated into a housing or frame of the apparatus. Again, the hinge can provide a simple yet effective way of restraining lateral and other uneven movements of the moveable member.

The apparatus of the fifth aspect may be otherwise as defined in the third or fourth aspects.

Brief Description of the Drawings Notwithstanding any other forms that may be embraced by the method and apparatus as disclosed in the Summary, specific embodiments of the method and apparatus will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a schematic illustration of dynamic friction polishing; Figure 2 photographically depicts a prototype of a dynamic friction polishing apparatus as disclosed herein;

Figure 3 shows a front, partially sectional schematic view of a dynamic friction polishing apparatus as disclosed herein;

Figure 4 shows two plots (a) & (b) that demonstrate the effect of sliding speed on polishing rate for:

- in plot (a) a Type 1 PCD (consisting of approximately 65% diamond particles of 6 micrometer grain size); - in plot (b) a Type 2 PCD (consisting of approximately 75% diamond particles of 25 micrometer grain size); and

Figure 5 photographically depicts surfaces of PCD specimens: (a) polished by a dynamic friction technique, and (b) as received (before polishing).

Detailed Description of Specific Embodiments

Figure 1 illustrates schematically a dynamic friction polishing DFP process. As can be seen, a polycrystalline diamond compact (PCD) specimen is mounted in a specimen holder. The holder is pressed (Load) to force the specimen at a predetermined pressure onto a special metal disk rotating at a high speed to generate dynamic friction. The DFP process is typically carried out under atmospheric conditions and generates a thermo-chemical reaction (induced by dynamic friction) between the PCD and the metal disk. The process can also be carried out in an oxygen enriched atmosphere to speed up the process (ie. by promoting the oxidising of the carbon). The DFP process enables abrasive-free polishing of eg. a single crystal diamond or a PCD. Referring to Figures 2 and 3, a method and apparatus for carrying out DFP in an efficient and controllable manner will now be described.

In this regard, an apparatus for polishing a diamond material is shown in the form of a polishing machine 10. The machine 10 depicted comprises two specimen holders 12, each for positioning a number of (eg. four) PCD specimens 14 at a rotatable metal disk 16. Of course the machine can readily be adapted to hold more than two specimen holders and each holder can be adapted to hold as little as one or considerably more than four specimens.

The specimen holders 12 are arranged to be evenly spaced around a circumference of an imaginary circle that is centred on a machine main axis A, and on which axis the rotatable metal disk 16 is also centred. The even spacing of the specimen holders 12 around the imaginary circle circumference enables better control of applied pressure and minimises deflection of disk 16, thereby increasing polishing efficiency. The PCD specimens 14 are fastened in mounting collets locatable in the specimen holders 12. These collets are offset from, and are evenly spaced around a circumference of an imaginary circle that is centred on, a specimen holder axis B. The offset and evenly spaced collets enable a flatter surface to be achieved at the polished surface of each specimen and, at the same time, prevent grooves forming in the metal disk 16. The offset also provides for more stable polishing and also enables a more even application of pressure, which again increases polishing efficiency.

The axes B are typically parallel to the main axis A. This arrangement of axes makes it easier for set up of the machine geometry. However, in a multi-axis (eg. a 5- axis) machine (as described below) a deliberate and controlled inclining of axes may be employed that relates to the sample geometry to be polished.

The machine further comprises a drive for bringing the PCD specimens into contact with the rotatable metal disk 16 and for causing simultaneous rotation of both the metallic surface disk and the specimen holders 12.

The drive firstly comprises a respective digitally controlled motor 18 for rotatingly driving each specimen holder 12, with each motor 18 being connected to its specimen holder via a drive shaft 20. Each motor 18 rotates its holder 12 about a respective axis B.

To achieve an optimal average sliding speed between each specimen 14 and the disk 16, a motor having a power rating of 370W was employed, with the power rating being calculated using a coefficient of friction (COF) estimated for the specimens and the disk. In the polishing machine 10 the rotational speed of each holder about its axis B was in the range of 25 to 50 rpm, optimally about 30 rpm. For different specimen pressures, and for different specimen surface roughness, thermal properties and geometries, this rotational speed range would vary. The drive secondly comprises a digitally controlled main motor 22 for rotatingly driving the metal disk 16, with the motor 22 being connected to a drive wheel 24 of the metal disk via a belt drive 25, pulley 26 and a main motor output shaft 28. As shown, the metal disk and main motor components are housed within a machine housing 30.

Again, to achieve the optimal average sliding speed, a main motor power rating of 18kW was calculated using the estimated COF. In polishing machine 10 the rotational speed of the disk 16 about its axis A was in the range of 1000 to 1350 rpm, optimally about 1300 rpm. For a different scale of machine, and again for different specimen surface roughness, thermal properties and geometries, this rotational speed range would vary.

The motors 18 and 22 enable simultaneous rotation of each of the specimen holders 12 and the metal disk 16. It has been found that this simultaneous rotation significantly increases the polishing process efficiency and helps to promote the mechanisms (a), (b) and (c) as outlined in the Background. The use of respective motors also enables specific and individual control of the rotational speeds of the specimen holders and the metal disk, to take into account different materials and polishing requirements. The drive thirdly comprises a pneumatic ram 32 for urging the specimen holders 12 towards, and thus urging each PCD specimen 14 into, the metal disk 16. The pneumatic ram provides a simple yet robust way of achieving the pressures required for DFP. In a larger scale-up of the machine a hydraulic ram may be employed, and in a simpler version removable weights may be employed in place of the ram. The ram 32 is mounted to a fixed member in the form of upper cross-beam 34 with its drive rod 36 extending and being connected to a movable member in the form of sliding cross-beam 38. As depicted, the motors 18 are each mounted to the sliding cross-beam 38 via a respective bearing configuration 40, and thus the specimen holders 12 and each PCD specimen 14 are each indirectly mounted to move with the sliding cross-beam 38.

Opposing ends of the sliding cross-beam 38 are each slidably mounted to a respective linear bearing 42 that is in turn mounted on a respective column 44 extending up from the housing 30 to support the upper cross-beam 34 for machine rigidity. The linear bearings 42 can have close tolerances to highly restrain sliding cross-beam movement by the ram 32 to a direct linear movement towards or away from the metal disk 16. A linear bearing has been found to provide a very effective way of restraining lateral and other uneven movements of the sliding cross-beam, thus again promoting more even polishing and an increase in polishing efficiency. A linear bearing is just one type of lift and place mechanism for restraining lateral and other uneven movements of the specimens when positioned at the disk 16. An alternative mechanism to the linear bearing is a hinge-based mechanism, where the hinge is formed as part of or mounted to housing 30, and the beam 38 is mounted at one end to pivot around the hinge. Again, the holders 12 can be affixed in relation to the beam 38 as shown, and can be pivoted down towards or up and away from the disk 16.

The cross-beams 34 and 38, and the columns 44, are typically fabricated of structural components to provide strength, rigidity and robust operation to the machine 10. The metal disk may be a circular plate or an annulus that defines a dynamic friction polishing surface. The disk can be of a metal or metal containing material, and typically comprises just an upper working surface of such material mounted to a less expensive substrate. Suitable disk materials include steel and various alloys, especially stainless steel; iron; nickel; chromium; cobalt; titanium; zirconium; and various alloys of these metals. In fact the metal disk working surface can comprise any metal that catalyses a reaction with the diamond material.

An abrasive polishing surface can then be defined and located within the annulus. Materials for such an abrasive polishing surface can include diamond composites, cubic boron nitride, boron carbide, etc. Further, the drive can be adapted to immediately move the (or each) specimen holder 12 and PCD specimen 14 from the metal disk dynamic friction polishing surface to the abrasive polishing surface, after dynamic friction polishing has taken place. The even spacing of the specimen holders 12 around the imaginary circle circumference can be achieved with two or any number of holders, limited only by the machine geometry (ie. the diameter of the circle and holder size will limit the number of specimen holders that can be arranged in the machine). Similarly, the spacing of the PCD specimens evenly around the specimen holders 12 will be limited by the diameter of the imaginary circle around axis B, which is again limited by the size of each holder, and by the size and number of each specimen. For example, in the apparatus as shown the specimens had a diameter of 12 mm. In commercial applications the diameters may vary from 5 to 75 mm, hence machine geometry may be varied accordingly.

Whilst the method and apparatus that has been described in Figures 2 and 3 shows the polishing of a PCD diamond material specimen, it should be appreciated that it is equally applicable to any diamond material of any size including those sizes disclosed herein.

Also, whilst the method and apparatus that has been described in Figures 2 and 3 shows a set-up for vertical operation, it may be configured to operate at a horizontal orientation (or other inclination). Alternatively, it could be inverted (ie. disk rotating above the specimens).

Examples Non-limiting examples of the DFP process will now be described.

Example 1

The machine of Figure 2 was operated to polish a set of PCD specimens to a specified surface roughness (e.g. 0.06 micrometer Ra) in approximately 18 minutes, that is, 3 minutes of DFP and 15 minutes of abrasive polishing. In conventional abrasive polishing, it was noted that such a process took approximately 3 hours. The machine of Figure 2 thus produced huge time and cost savings.

During the polishing process, controlled rotation of the PCD specimens using the specimen holder drive systems ensured uniform polishing of the PCD's.

The machine set-up of Figure 2 allowed simultaneous polishing of 2 specimens/holder. The specimen holders were easily able to be modified to polish 3, 4, 5, 6, 7, 8, 9, 10 etc specimens simultaneously.

Because of the extreme hardness of polycrystalline diamond compacts, mono- crystalline diamond and diamond films, as well as excellent surface finish attainable using the DFP method in the machine of Figure 2, diamond parts for a wide range of applications were able to be made at a low cost. These applications were noted to include cutting tools for precision machining, electronic parts, biomedical implants and devices, jewels, optical parts, die, heat sink and wear resistant parts. Example 2

The dynamic friction polishing (DFP) of PCD was carried out on the polishing machine of Figure 2 under atmospheric conditions. The polishing of PCD specimens was achieved by pressing the specimens at a pressure of 3.5 MPa, with the specimen holders rotating at a speed of 30 rpm, onto the metal disk rotating at a high speed of 1300 rpm. The PCD specimens slid on the metal disk at an average sliding speed of 21 m/s. The rotation of the specimen holders and thus the PCD specimens resulted in a uniform polishing of PCD. The polishing rate (or material removal rate) was controlled by varying the metal disk rotational speed and/or pressure applied on the PCD specimens. Stainless steel SUS304 was used as polishing disk material for the following reasons:

(i) the elements in SUS304, in particular, Fe, Cr, Ni and Mn, are major alloying components of catalysts used for the commercial production of synthetic diamond under high pressure, and can also catalyse the conversion of diamond to graphite at low pressures and at temperatures above 700 deg C;

(ii) low carbon content (<0.06%) having advantages of carbon diffusion by thermo- chemical reaction;

(iii) low thermal conductivity which helped to maintain a high temperature of the diamond sample without releasing the heat generated by dynamic friction; (iv) austenitic structure of SUS304 which was also advantageous to the diffusion of carbon.

The components of the polishing machine of Figure 2 and/or their functions were as follows:

(1) Metal (stainless steel) polishing disc. A 400 mm diameter and 20 mm thick SUS304 disk was bolted onto a 400 mm diameter and 80 mm thick mild steel disk so that a rigid disk tool for polishing of diamond was obtained. (2) Metal polishing disk drive assembly that included a main ac motor, pulleys and a toothed belt. The main ac motor power rating was 18 kW with a continuously variable speed in the range 0-2800 rpm. (3) Specimen holders that held up to 4 PCD specimens. Collets made of steel were used for this purpose.

(4) Specimen holder drive assemblies that included 370 W ac motors, couplings, and holder drive shafts.

(5) The sliding cross-beam which carried two specimen holders and their drive assemblies. The sliding cross-beam slid up and down on linear bearings mounted on the two columns which were fixed on flat surfaces made on the housing.

(6) The upper cross-beam which prevented any deflection of the columns under load. The pneumatic cylinder was mounted on this beam. (7) The pneumatic cylinder that controlled the movement of the sliding cross-beam and the load/pressure on specimens.

(8) The housing for the metal disk and its drive assembly which was made of 12.5 mm thick steel plates for rigidity, stability and operator safety. Example 3

Under the set up of Figure 2, the machine allowed simultaneous polishing of 4 PCD specimens. In order to control the speeds of the main and two 370W motors, separate digital control units were used. Based on a predetermined sliding speed for PCD specimens, the metal disk was rotated at the appropriate speed (eg. 1300 -1350 rpm) using the digital controller of the main motor. Using the digital controllers for the 370 W motors, the specimen holders were also rotated at a predetermined speed (eg. 30 rpm) and the specimens were lowered gradually on to the disk. The specimens slid on the disk under full load within a second. The required load was achieved by regulating the air pressure to the pneumatic cylinder. Typical polishing parameters were:

^■ Average sliding speed: approximately 21.2 m/s (main motor drove the metal disk at 1350 rpm while the PCD specimens rotated on the disk at diameters between 300-400 mm). ^■ Pressure: approximately 3.5 MPa on a specimen.

^■ Polishing time: 1-3 minutes. This time depended on average sliding speed, pressure, initial surface roughness of PCD specimens, etc.

The machine of Figure 2 was able to efficiently implement the dynamic friction polishing process for polishing PCD compacts with a required surface finish.

Example 4

Figure 4 shows the effect of the polishing parameters, pressure and speed, on the material removal rate for two types of PCD, where the symbols represent the experimental results, and the lines are the corresponding linear regression fits. Type 1 PCD contained approximately 65% diamond particles of 6 micrometer in grain size

(remainder SiC and Si) with PCD surface roughness before polishing of approximately 0.7 micrometer Ra. Type 2 PCD contained approximately 75% diamond particles of 25 micrometer in grain size (remainder SiC and Si) with a PCD roughness 1.6 micrometer Ra.

When the speed was low (eg. less than 10 m/s), the PCD was only able to be partially polished (or not polished at all). That was because the temperature rise was not high enough to activate the phase transformation and chemical reactions at the polishing interface. At higher sliding speeds (eg. >12 m/s), the polishing rate was observed to increase almost linearly with the increase in sliding speed. In addition, at a given polishing speed, a higher pressure resulted in a higher polishing rate. However, it was noted that cracks could be generated under severe conditions, for example, those lying above the dashed line in Figure 4.

To achieve satisfactory polishing, appropriate polishing parameters, such as pressure sliding speed, etc were selected to generate the required temperatures at the PCD-metal interface. The pressure-speed combinations that were determined for effective polishing are shown in Figure 4. As mentioned above, the dashed line in the Figure 4 indicates an approximate boundary of the safe polishing region, below which polishing was able to be carried out without cracking.

Among the conditions tested, the appropriate parameters for the Type 1 PCD (Figure 4 (a)) were: pressure = 2.7 MPa, sliding speed = 20 to 25 m/s; (2) pressure = 3.1 MPa, sliding speed = 16 to 25 m/s; and (3) pressure = 3.8 MPa, sliding speed = 16 m/s. The polishing time for these pressure/speed combinations was 2 minutes. The appropriate polishing parameters for Type 2 PCD (Figure 4 (b)) were: (1) pressure = 2.7 MPa, sliding speed = 25 m/s; (2) pressure = 3.1 MPa, sliding speed = 18 to 25 m/s; (3) pressure = 3.8 MPa, sliding speed = 16 m/s; and (4) pressure = 5 MPa, sliding speed = 12 m/s. The polishing time for these pressure/speed combinations was 3 minutes. Example 5

In some cases it was found that a thin layer of steel and/or metal oxide adhered on to the polished PCD specimen surface that required mechanical removal. Research was commenced to determine the polishing conditions under which the adherence of steel and/or metal oxide did not occur. Mechanical means in the form of an abrasive polishing step to remove this steel and/or metal oxide was incorporated into the polishing machine, as described below. Additionally, such abrasive polishing was also found to improve the final surface roughness of the PCD.

In order to obtain a polished PCD surface roughness of 0.06 micrometer Ra (which is normally required for commercial PCD cutting tools), abrasive polishing using a diamond wheel on another machine was performed after DFP. When the abrasive polishing process was in-built to the machine of Figure 2 for example, an abrasive wheel was placed within the steel disc (now in the form of an annulus). This eliminated a second set-up step and allowed efficient completion of the entire PCD polishing process on one machine.

In addition, research was commenced to incorporate components of the polishing device machine into a multi-axis CNC machining centre. For example, a 5- axis (such as up/down/lateral/front/back/disk-tilt) machining centre can be used to perform free surface polishing. The numerical control of a multi-axis machine was noted to allow for some very advanced polishing of more complex sample geometries to take place (ie. free surface polishing).

Comparative Example Comparative tests were performed to compare surface roughness and polishing times for polished specimens using a known abrasive method as compared with a dynamic friction polishing (DFP) technique using the apparatus of Figure 2. The results are presented in Table 1.

Table 1. Surface roughness and polishing time results for PCD compacts using abrasive polishing and DFP.

Polishing technique Abrasive Present Apparatus/Method

Surface roughness Ra

0.06 0.06

(micrometer)"

18 minutes (3 min DFP and Polishing time 3 hours

15 min abrasive)

Surface roughness tester: Mitutoyo Surftest 402 and Surftest analyser

A polished PCD specimen surface using the present apparatus/method is shown in Figure 5 (a). Compared to the as-received specimen (Figure 5 (b)), an optical surface finish has been achieved in 18 minutes of polishing. The above results clearly show that the polishing apparatus can considerably reduce the diamond polishing time.

Whilst a number of embodiments of method and apparatus for polishing a diamond material have been described, it will be appreciated that the method and apparatus can be embodied in many other forms.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense (ie. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments).

Claims

1. A method for polishing a diamond material, the method comprising the steps of:

- positioning a sample of the diamond material in relation to a movable metallic surface; and - bringing the diamond material sample into contact with the movable metallic surface with simultaneous movement of the metallic surface and rotation of the diamond material sample.

2. A method as claimed in claim 1 wherein the metallic surface is rotatable, whereby two or more samples of the diamond material are positioned in relation to the rotatable metallic surface, and whereby the two or more samples are evenly spaced around a circumference of an imaginary circle centred on an axis of rotation of the metallic surface.

3. A method for polishing a diamond material, the method comprising the steps of:

4. A method as claimed in claim 3 wherein the metallic surface is rotatable about an axis of rotation such that, when the two or more diamond material samples are brought into contact with the rotating metallic surface, both the metallic surface and the diamond material samples are rotated.

5. A method as claimed in claim 2 or 4 wherein the (or each) diamond material sample is rotated about a sample rotational axis.

6. A method as claimed in claim 5 wherein the (or each) diamond material sample comprises at least two specimens of diamond material that are non-aligned with respect to the sample rotational axis such that the specimens rotate around this axis.

7. A method as claimed in claim 6 wherein the two or more specimens are evenly spaced at the sample around a circumference of an imaginary circle that is centred on the sample rotational axis.

8. A method as claimed in any one of claims 5 to 7 wherein the (or each) sample rotational axis is parallel to the metallic surface rotational axis.

9. A method as claimed in any one of the preceding claims wherein the average sliding speed of the (or each) diamond material sample with respect to the metallic surface is in the range of 5 to 60 m/s.

10. A method as claimed in claim 9 wherein the average sliding speed is about 21 m/s.

11. A method as claimed in any one of the preceding claims wherein the (or each) diamond material sample is pressured onto the metallic surface.

12. A method as claimed in claim 11 wherein the pressure on the (or each) diamond material sample is in the range of 2.5 to 25 MPa.

13. A method as claimed in claim 12 wherein the pressure is about 3.5MPa.

14. A method as claimed in any one of the preceding claims comprising a dynamic friction polishing stage and a subsequent abrasive polishing stage.

15. A method as claimed in claim 14 wherein the dynamic friction polishing stage has a duration of 1-5 minutes.

16. A method as claimed in claim 14 or 17 wherein the abrasive polishing stage has a duration of 1 -20 minutes.

17. A method as claimed in any one of claims 14 to 16 wherein the abrasive polishing stage is conducted on an abrasive surface immediately adjacent to the metallic surface.

18. A method for polishing a diamond material as herein described with reference to the Examples and the accompanying drawings.

19. Apparatus for polishing a diamond material, the apparatus comprising:

- a drive for bringing the diamond material sample into contact with the movable metallic surface and for causing simultaneous movement of the metallic surface and rotation of the diamond material sample.

20. Apparatus as claimed in claim 19 wherein the drive is arranged to cause rotation of the diamond material sample by rotating the holder, and is arranged to also cause rotation of the metallic surface.

21. Apparatus as claimed in claim 20 comprising two or more holders for two or more respective diamond material samples, the holders being arranged such that the two or more samples are evenly spaced around a circumference of an imaginary circle centred on an axis of rotation of the metallic surface.

22. Apparatus for polishing a diamond material, the apparatus comprising: - two or more holders for two or more respective samples of the diamond material, the holders being arranged for positioning the samples in relation to a movable metallic surface and such that the two or more samples are evenly spaced around a circumference of an imaginary circle centred on an axis of movement of the metallic surface; and

23. Apparatus as claimed in claim 22 wherein the metallic surface is rotatable about an axis of rotation, whereby the drive is arranged for causing simultaneous rotation of both the metallic surface and each of the diamond material samples.

24. Apparatus as claimed in claim 23 wherein the drive causes rotation of each diamond material sample by rotating each holder.

25. Apparatus as claimed in any one of claims 20, 21, 23 or 24 wherein the drive comprises a respective motor for each of the sample holder(s) and the metallic surface, each motor being arranged to rotate the holder or surface about a respective rotational axis.

26. Apparatus as claimed in claim 25, the apparatus being configured such that the (or each) sample rotational axis is parallel to the metallic surface axis.

27. Apparatus as claimed in claim 25 or 26 wherein the (or each) sample holder comprises at least two specimen mounting points, each for the mounting of a respective diamond material specimen, the mounting points being offset with respect to the sample rotational axis.

28. Apparatus as claimed in claim 27 wherein the specimen mounting points are evenly spaced around a circumference of an imaginary circle that is centred on the sample rotational axis.

29. Apparatus as claimed in any one of claims 19 to 28 wherein the drive further comprises a ram for urging the sample holder(s) towards and thus urging the (or each) diamond material sample into the metallic surface.

30. Apparatus as claimed in claim 29 wherein the ram is pneumatic and operates between fixed and movable members of the apparatus, with the sample holder(s) being mounted with respect to the movable member.

31. Apparatus as claimed in claim 30 wherein the movable member forms part of a lift and place mechanism for placing the sample onto the metallic surface in a non- deflectable manner.

32. Apparatus as claimed in claim 31 wherein opposing ends of the movable member are each slidably mounted to a respective linear bearing that restrains moveable member movement by the ram to a linear movement towards or away from the metallic surface, with the fixed member being defined by an in-use upper cross-beam supported at opposite ends by columns of the apparatus, with the columns extending from an apparatus housing, and with the movable member being defined by an in-use lower and sliding cross-beam that is movable down and up with respect to the apparatus housing.

33. Apparatus as claimed in claim 32 wherein the linear bearing is mounted on each column.

34. Apparatus as claimed in claim 31 wherein one end of the movable member is pivotally mounted to a hinge that is adapted to restrain moveable member movement to an arcuate movement about the hinge to thus move the sample towards or away from the metallic surface, with the fixed member being defined at or by the hinge, and the hinge being mounted to or incorporated into a housing or frame of the apparatus.

35. Apparatus as claimed in any one of claims 19 to 34 wherein the metallic surface forms part of a metallic disk that is rotated by a part of the drive about a disk central axis.

36. Apparatus as claimed in claim 35 wherein the disk is an annulus that defines a dynamic friction polishing surface, with an abrasive polishing surface being defined and located within the annulus.

37. Apparatus as claimed in claim 36 wherein the drive is adapted to move the (or each) sample holder and thus the (or each) sample from the dynamic friction polishing surface to the abrasive polishing surface.

38. Apparatus for polishing a diamond material, the apparatus comprising:

39. Apparatus as claimed in claim 38 wherein the lift and place mechanism comprises a member, with the holder or the metallic surface being supported with respect to the member.

40. Apparatus as claimed in claim 39 further comprising a drive arranged for operation on the member so as to position the sample at the movable metallic surface in a non- deflectable manner.

41. Apparatus as claimed in any one of claims 38 to 40 wherein the holder is mounted with respect to the member, and opposing ends of the member are each slidably mounted to a respective linear bearing that restrains member movement, whereby the holder is moved towards or away from the metallic surface in a linear manner.

42. Apparatus as claimed in claim 41 wherein the member moves with respect to a fixed member defined by an in-use upper cross-beam supported at opposite ends by columns of the apparatus, with the columns extending from an apparatus housing, and with the moving member being defined by an in-use lower and sliding cross-beam that is movable down and up with respect to the apparatus housing on a linear bearing mounted on each column.

43. Apparatus as claimed in any one of claims 38 to 40 wherein one end of the member is pivotally mounted to a hinge that is adapted to restrain member movement to an arcuate movement about the hinge to thus move the sample towards or away from the metallic surface.

44. Apparatus as claimed in claim 43 wherein the hinge is mounted to or incorporated into a housing or frame of the apparatus.

45. Apparatus as claimed in any one of claims 38 to 44 which is otherwise as defined in any one of claims 19 to 37.

46. Apparatus for polishing a diamond material as herein described with reference to the accompanying drawings.