WO2013016779A1

WO2013016779A1 - Methods, systems and compositions for polishing

Info

Publication number: WO2013016779A1
Application number: PCT/AU2012/000930
Authority: WO
Inventors: Li CHANG; Klaus Friedrich; Lin Ye
Original assignee: University of Sydney
Current assignee: University of Sydney
Priority date: 2011-08-03
Filing date: 2012-08-03
Publication date: 2013-02-07
Anticipated expiration: 2014-02-03

Abstract

A method for polishing an article, including the steps of bringing the article into contact with an abrasive shear-thickening fluid, and causing sufficient relative movement of said article with respect to said abrasive shear-thickening fluid to simultaneously shear-thicken said fluid and polish said article.

Description

Methods, systems and compositions for polishing Field of the Invention

[0001] The present invention relates generally to methods, systems and compositions for polishing, and more specifically to such polishing methods, systems and compositions based on the phase transition of shear thickening fluids.

Background of the Invention

[0002] The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of common general knowledge in the field.

[0003] For many years, various techniques have been utilised for cutting and polishing materials such as glass, metals, semiconductors, precious stones, crystals, ceramics, composite materials and the like. In general, the process includes one or more polishing steps in which a polishing pad of a suitable material is applied against the surface to be treated, with a combination of relative motion and pressure (see Figure 1 ). A mechanical polishing formulation, usually in slurry form, is typically interspersed between the polishing pad and the surface to be treated. When pressure is applied in conjunction with a polishing motion, the polishing formulation carried in the slurry can cut, grind, and/or polish the surface to the desired topography or degree of smoothness. However, known processes of this type suffer from several disadvantages.

[0004] For example, this polishing process cannot usually be used to simultaneously polish a number of surfaces of an article, where those surfaces have different heights and/or orientations, or where the surfaces are curved in complex ways. Further, the prior art systems and processes cannot typically be used to polish the interior surfaces of hollow components, particularly where more complex internal shapes or curvatures are involved. Further still, a trained technician, a highly customized machine and/or an elaborate setup procedure is typically required for each type or shape of component, which adds considerably to the capital and/or labour cost. Another problem relates to wear of the polishing pads, which typically require regular inspection and replacement, resulting in additional downtime and maintenance costs. Yet further disadvantages relate to the requirement to remove, treat and store or dispose of the waste slurry materials being cast off the surface of the polishing pad. Further, different slurries are required to effect different polishing finishes, which are typically selected on the basis of the properties of the abrasive particles, such as size and hardness. In a production environment, it is costly and time-consuming to change the polishing media and/or to move the components being polished to different polishing stations. There is also a cost associated with an elaborate inventory of different polishing compounds, polishing pad materials, polishing stations or the like.

[0005] Other prior art polishing techniques use a combination of mechanical and chemical means. One such process is known as chemical-mechanical planarization (CMP), which is a technique typically used in semiconductor fabrication for planarizing semiconductor wafers. This process uses an abrasive and corrosive chemical in conjunction with a polishing pad. The pad and wafer are pressed together by a dynamic polishing head which is rotated with a different axis of rotation compared to the wafer. This removes material and tends to even out any irregular surface topography, making the wafer substantially flat or planar. In this case, the process of material removal is not simply that of abrasive scraping. Rather, the chemicals in the slurry also react with and/or weaken the material to be removed. The abrasive accelerates this weakening process and the polishing pad simultaneously helps to wipe the reacted materials from the polished surface.

[0006] Vapour polishing is another method of polishing used to reduce surface roughness or improve clarity. Typically, in this process, a component is exposed to a chemical vapour causing the surface to flow, thereby improving the surface finish. This method of polishing is frequently used to return transparent materials to an optical quality finish after machining, and the process can also work well in relation to the internal features of components.

[0007] Fluid jet polishing (FJP) is yet another method of contouring and polishing a surface, by aiming a jet of slurry at a component and eroding the surface to create the desired shape. However, this process suffers several disadvantages, such as the overall complexity and the need for a motion system, which is usually computer controlled, to direct the fluid nozzle and optimize the dwell time of the tool pattern on the surface of the component to be polished, in order to achieve the desired final surface topography.

[0008] It is an object of the present invention to overcome or ameliorate one or more of the disadvantages of the prior art, or at least to provide a useful alternative.

Summary of the Invention

[0009] According to a first aspect, the invention provides a method for polishing an article, said method including the steps of bringing the article into contact with an abrasive shear-thickening fluid, and causing sufficient relative movement of said article with respect to said abrasive shear-thickening fluid to simultaneously shear- thicken said fluid and polish said article.

[0010] Preferably, the shear-thickening fluid is relatively viscous or thick during said relative movement but relatively fluid at other times. In one embodiment, the shear-thickened fluid is in a substantially solid state. In some embodiments, the fluid preferably has a viscosity of greater than 50, 100, 250, 500, 1000 or 10,000 Pa.s respectively. Preferably, the shear-thickening fluid is caused to become sufficiently solid during the relative movement to provide a substantially solid or sufficiently firm support for the abrasive particles suspended therein to abrade or wear the surface of the article with which the shear-thickened fluid is in contact. Preferably, the relative movement is specifically adapted to apply a high level of shear to the shear- thickening fluid. This relative movement can include rotational, translational, orbital, reciprocating or randomised motion, or any suitable combination of these or other forms of relative movement. The critical shear rate, being the sheer rate at which shear thickening substantially begins, can vary across a wide range, depending upon the specific composition of the fluid, the relative proportion of suspended solids, temperature and other process variables. Preferably, the shear-thickening fluid is selected, designed or formulated to exhibit a critical shear rate of between 2 and around 200 s-1 , and more preferably in some embodiments between 5 and around 50 S-1. [0011] It will be appreciated that to polish a lowermost surface of the article, the lowermost surface will need to be in contact with the fluid. However, if more than just the lowermost surface requires polishing the article may be at least partially immersed into the shear-thickening fluid to a sufficient depth to cover the surfaces requiring polishing. In some embodiments, the article comprises surfaces at different heights or orientations, and/or curved surfaces.

[0012] In one embodiment, the shear-thickening fluid comprises a suspension of polymeric particles in a fluid, such as ethylene glycol. However, those skilled in the art will appreciate the other kinds and varieties of shear-thickening fluids/suspensions which will be appropriate for use in the present invention. ,

[0013] Preferably, the polymeric particles comprise a solid volume fraction of between 20% and around 80% and more preferably between 30% and around 70% of the ethylene glycol. In some embodiments, the shear thickening fluid preferably includes at least 50% by volume of particulates in suspension. It should be appreciated, however, that the optimal proportion will depend upon a number of process variables including the specific composition of the liquid and solid components, the shape a d material composition of the component to be polished, the desired surface finish, the required rate of polishing, and the like.

[0014] In some embodiments, the shear-thickening fluid comprises a base fluid having a sufficient concentration of particles suspended therein such that the resulting composition acts as a shear-thickening fluid, and wherein the suspended particles are themselves abrasive particles, for example, silica or SiC particles suspended in ethylene glycol. In these embodiments, it has been surprisingly found that micro-sized or nano-sized abrasive particles in relatively high concentrations in the base fluid can cause the suspension to act as a shear-thickening fluid and also synergistically provide the desired abrasive effect. In other words, the particles suspended in the base fluid confer, at least to some extent, both shear-thickening and abrasive properties. In other embodiments, the shear-thickening characteristics and the abrasive characteristics the fluid may be conferred by different constituent components are fluid. [0015] Preferably, the volume content of the abrasive suspended in the base fluid is greater than 50 % (v/v). In some embodiments, preferably, the volume content of the abrasive suspended in the base fluid is, respectively, less than or equal to around 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, or 90% (v/v).

[0016] In some embodiments, the particles of the abrasive are predominantly nano-sized, i.e. between 1 and around 100 nm. For example, in some preferred embodiments, the particle size is respectively greater than 1 , 10, 20, 30, 40, 50, 60, 70, 80 and 90, and less than 100 nm. In other preferred embodiments, the particles of the abrasive are predominantly micro-sized, i.e. between 0.1 and around 2000 micron, for example in respective embodiments less than 1 , 25, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1500 or 2000 micron. In other embodiments, the abrasive is a mixture of nano-sized and micro-sized abrasive particles. In one particularly preferred embodiment, the abrasive particle is SiC. However, those skilled in the art will appreciate that other materials and material combinations will be suitable for use in the present invention.

[0017] Preferably, the size, shape, and/or volume content of the abrasive particles suspended in the base fluid is chosen such that the viscosity transition at the critical shear rate involves at least a fourfold increase in viscosity. In some embodiments, it is preferred that the viscosity transition involves an increase of between 6 and around 12 times the viscosity at low shear rate. In one embodiment, the increase in viscosity is at least one order of magnitude. In other preferred embodiments, the viscosity increase is greater than two, three, four and five orders of magnitude respectively.

[0018] By way of explanation and example, use of relatively large abrasive particles suspended in the fluid means that the particles do not 'pack' particularly well in the fluid, and the viscosity transition at the critical shear rate is relatively low. However, use of micro-sized, and preferably nano-sized particles provides significantly improved particle packing and particle distribution in the fluid suspension, and gives a relatively sharp viscosity transition at the critical shear rate, meaning a sudden and substantial or "quantum" viscosity increase of at least several multiples and up to several orders of magnitude, relative to the initial viscosity at zero or relatively low shear rates. Such micro-sized or nano-sized particles are preferred since they provide a relatively sharp viscosity transition at the critical shear rate, and by selection of suitable materials, are also capable of abrasion of the article being polished. In some embodiments, the micro-sized or nano-sized abrasive particles are surface treated to control or minimise agglomeration of the abrasive particles. It has been surprisingly found that use of micro-sized or nano-sized particles enhances shear-thickening behaviour while synergistically interacting with one another and with the base fluid to provide an effective polishing medium.

[0019] It will be appreciated that micro-sized or nano-sized abrasive particles are particularly useful for fine polishing of articles of high value, such as jewellery, or for polishing complex shapes ranging from intricate mechanical components to artificial teeth.

[0020] In another embodiment, a controller is preferably employed to detect the resistance to relative movement of the article with respect to the shear-thickening fluid. The controller is preferably further adapted to slow down, speed up or otherwise modify the relative movement in order to optimally control the viscosity of the shear-thickening fluid. In one embodiment, the controller is adapted to maintain a substantially constant viscosity during relative movement. Alternatively or additionally, the controller may be adapted to control the relative movement such that the viscosity is changed over time according to a predetermined viscosity vs time profile. For example, the controller may be adapted to control the relative movement such that the viscosity is initially relatively low, to effect a relatively mild degree of polishing in a first phase, and then the viscosity is changed over time such that the final degree of polish is relatively coarse or 'severe', or vice versa.

[0021] In one further variation, the controller includes a sensor adapted to generate a control signal indicative of the surface finish or typography of the article or component being polished, and a feedback loop whereby the controller, in response to that control signal and/or other control inputs, is adapted to progressively regulate the relative motion, the fluid viscosity or other system parameters until a desired or. predetermined surface finish or surface topography has been obtained.

[0022] According to a second aspect, the invention provides a composition for polishing an article, said composition comprising an abrasive shear-thickening fluid exhibiting a relatively sharp viscosity transition at a critical shear rate. . [0023] According to a third aspect, the invention provides a method for controlling a degree of polish of an article in contact with an abrasive shear-thickening fluid, said method comprising the step of controlling the speed of relative movement of said article with respect to said shear-thickening fluid thereby to control the viscosity of said shear-thickening fluid and hence the degree of polish.

[0024] According to a fourth aspect, the present invention provides a system for polishing an article, said system comprising:

an abrasive shear-thickening fluid,

a device for causing relative movement of said article with respect to said shear-thickening fluid, and

a controller for controlling the speed of relative movement of said article with respect to said shear-thickening fluid,

whereby said article is moved with respect to said shear-thickening fluid sufficiently to simultaneously shear-thicken said fluid and polish said article.

[0025] Prior art polishing methods typically require a plurality of polishing steps to achieve a desired degree of polish, with the number of polishing steps increasing significantly in. order to achieve or approach a mirror finish. For example, 2, 4 or 6 polishing steps requiring different sized abrasive particles is common in the industry. The present invention substantially ameliorates this requirement, since the degree of abrasion is dependent upon the speed of relative movement of the article to be polished and the shear-thickening fluid, and/or the viscosity of the shear-thickening fluid. It will be appreciated that one influences the other, i.e. the greater the relative speed of movement the greater the viscosity. Therefore, the present invention provides a significant advance over prior art methods, since only a single shear- thickening fluid/abrasive particle composition may be required to polish an article to a coarse polish, a mirror finish, or any desired finish in between.

[0026] The skilled addressee will understand that the invention comprises the embodiments and features disclosed herein as well as all viable combinations and/or permeations of the disclosed embodiments and features. Definitions

[0027] In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It should also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary skill in the art to which the invention pertains.

[0028] For the purposes herein, a dilatant material is synonymous with a shear- thickening material. A dilatant is a material in which viscosity increases with the rate of shear, and is an example of a non-Newtonian fluid. At low shear, the material is a fluid and the dilatant flows easily, whereas at high shear the material substantially thickens or solidifies.

[0029] Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of 'including, but not limited to'.

[0030] Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term 'about'. It is understood that whether the term 'about' is used explicitly or not, every quantity given herein is meant to refer to the actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred by one of ordinary skill in the art, including approximations due to the experimental and/or measurement conditions for such given value.

[0031] Unless otherwise indicated, !%' will mean 'weight %', 'ratio' will mean 'weight ratio' and 'parts' will mean 'weight parts'.

[0032] The recitation of a numerical range using endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1 , 1 .5, 2, 2.75, 3, 3.80, 4, 5, etc.). [0033] The terms 'preferred' and 'preferably' refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.

[0034] Nano-sized particles are generally considered as particles having a particle diameter less than about 100 nanometers (nm), and micro-sized particles are generally considered as particles having a particle diameter between about 0.1 and 1000 microns.

Brief Description of the Drawings

[0035] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0036] Figure 1 is a schematic of a typical prior art polishing machine;

[0037] Figure 2a is a schematic of a plate-on-plate rheometer;

[0038] Figure 2b is a graph showing typical shear thickening behaviour of a shear- thickening fluid under steady shear conditions;

[0039] Figure 3a is a schematic drawing of one embodiment of the polishing technique of the present invention;

[0040] Figure 3b shows a diagrammatic representation of the polishing mechanism adapted to implement the technique represented in figure 3a;

[0041] Figure 4 depicts micrographs showing the effects of abrasive particle size on (a) the removal rate, and (b) the surface roughness of the finishing surfaces of a steel disk; and

[0042] Figures 5a and 5b show a comparison of the surface profiles of a steel disk respectively before and after the polishing process. Preferred Embodiments of the Invention

[0043] Referring to the drawings, Figure 1 shows a schematic of a typical prior art polishing machine. The specimen to be polished is pressed on the polishing pad, and during the polishing process the lowest part of the specimen is polished by the polishing slurry which contains hard abrasive particles. The polishing pad undergoes wear during the polishing process, and has to be replaced after a certain period of time. The removal rate of the material and the roughness of the finished surface can be controlled by the properties of the abrasive particles, such size and hardness.

[0044] It will be appreciated that this prior art polishing machine cannot readily be used to polish a specimen having surfaces with different heights, orientations or complex curvatures. Another problem is the wear of the polishing pad, which requires routine examination and replacement, resulting in additional maintenance costs.

[0045] The present invention, by contrast, utilises a shear-thickening fluid (STF) which comprises abrasive particles. An STF possesses high frequency and strain- rate dependent characteristics, as expressed by a recoverable phase transition between liquid and solid or semi-solid. Figure 2b shows the typical shear thickening behaviour of an STF tested with a plate-on-plate rheometer of the type shown in figure 2a.

[0046] As will be apparent from Figure 2a, the STF can transition from a liquid state to a thickened or substantially solid state when the shear rate reaches a critical value. Due to the dilatancy effect, the STF also results in a considerable normal force on the plates which confine its expansion.

[0047] Figure 3a shows a schematic drawing of the novel polishing technique of the present invention. Under static conditions, the STF remains in its liquid state and the specimen can be fully immersed therein. Abrasive particles are added to the STF in order to act as the abrasive medium. When the rotational speed of the specimen reaches the critical value, the surrounding STF substantially solidifies, which firmly holds the abrasive particles in place. Meanwhile, a normal force is also generated due to the shear-thickening behaviour of the STF (see Figure 2b). As a result, the abrasive particles can polish all the surfaces of the specimen in contact with the STF simultaneously, including surfaces A and B disposed at different heights (see Figure 3a), which cannot be simultaneously polished by using the prior art polishing technique shown in Figure 1.

[0048] To verify the novel polishing technique of the invention by using an abrasive STF, polishing tests were conducted with the experimental setup shown in Figure 3a. In particular, the materials include:

(i) shear thickening fluid: Ethylene Glycol based STF filled with 60 wt% polymeric particles (BASF AG, Ludwigshafen, Germany);

(ii) abrasive particles: SiC with the sizes of 45, 250, 500, 1000 prn; and

(Hi) specimen: a steel disk made of 100Cr6.

[0049] During the tests, the surface of the steel disk was positioned as the 'Surface A' shown in Figure 3. The polishing results are summarized in Figures 4 and 5. These results verify that the STF having abrasive particles can be used to polish the surfaces of metallic materials at different heights. The surface roughness can be controlled by selecting the kind, size and quantity of the abrasive particles as well as the speed of relative movement.

[0050] This represents a significant departure from the prior art, in that the present invention allows an article to be polished without the need for a polishing pad. However, more importantly, the technique enables polishing of complex structures which have curved surfaces or surfaces at different heights or orientations, such that these surfaces are polished evenly and simultaneously. Furthermore, the technique potentially enables even the inner surfaces of an article to be polished, and potentially simultaneously with the exterior surfaces. In addition to these advantages, the process can substantially increase production rates by reducing the number of process steps, while reducing process downtime and maintenance costs, by avoiding the need for polishing pads to be periodically inspected and replaced. The invention also potentially reduces the extent of operator training required. Further, there is reduced requirement to treat, store or dispose of the waste shear-thickening fluid, since relatively less fluid is required compared to some prior art polishing techniques. Further still, the present invention means that a single shear-thickening slurry can be used to effect different polishing finishes by controlling certain process parameters. This contrasts with the prior art processes, wherein different slurries need to be prepared with different sizes /or types of abrasive particles. Further advantages of the invention, at least in some preferred embodiments, are that no expensive and complicated computer system is required to control the polishing of multi-faceted articles. In these and other respects, the invention represents a practical and commercially significant improvement over the prior art.

[0051] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. In particular, features of any one of the various described examples may be provided in any combination with any of the other described features or examples.

Claims

Claims:

1. A method for polishing an article, said method including the steps of bringing the article into contact with an abrasive shear-thickening fluid, and causing sufficient relative movement of said article with respect to said abrasive shear- thickening fluid to simultaneously shear-thicken said fluid and polish said article.

2. A method according to claim 1 , wherein the shear-thickening fluid is relatively viscous during said relative movement and relatively fluid at other times.

3. A method according to claim 1 or claim 2, wherein the shear-thickening fluid is in a substantially solid state when shear-thickened in response to said relative movement.

4. A method according to any one of the preceding claims, wherein the fluid has a viscosity above a threshold level, selected from a set of threshold viscosity levels comprising: 50, 100, 250, 500, 1000 and 10,000 Pa.s.

5. A method according to any one of the preceding claims, wherein the shear- thickening fluid is caused to become sufficiently solid during the relative movement to provide a substantially solid or sufficiently firm support for the abrasive particles suspended therein to abrade or wear the surface of the article with which the shear- thickened fluid is in contact.

6. A method according to any one of the preceding claims, wherein the relative movement is adapted to apply a high level of shear to the shear-thickening fluid.

7. A method according to any one of the preceding claims, wherein the relative movement includes rotational, translational, orbital, reciprocating or randomised motion, or any suitable combination of these modes of relative movement.

8. A method according to any one of the preceding claims, wherein the shear- thickening fluid exhibits a critical shear rate of between 2 and around 200 s-1.

9. A method according to any one of the preceding claims, wherein the shear- thickening fluid exhibits a critical shear rate of between 5 and around 50 s-1.

10. A method according to any one of the preceding claims, including the step of immersing the article in the shear-thickening fluid to a sufficient depth so substantially to cover the surfaces requiring polishing.

1 1. A method according to any one of the preceding claims, wherein the shear- thickening fluid comprises a suspension of polymeric particles in a fluid.

12. A method according to claim 11 , wherein the fluid includes ethylene glycol.

13. A method according to claim 12, wherein the polymeric particles comprise a solid volume fraction of between 20% and around 80% the ethylene glycol.

14. A method according to claim 13, wherein the polymeric particles comprise a solid volume fraction of between 30% and around 70% of the ethylene glycol.

15. A method according to any one of the preceding claims, wherein the shear thickening fluid includes at least 50% by volume of particulates in suspension.

16. A method according to any one of the preceding claims, wherein the shear- thickening fluid comprises a base fluid having a sufficient concentration of particles suspended therein such that the resulting composition acts as a shear-thickening fluid, and wherein the suspended particles are abrasive particles.

17. A method according to any one of the preceding claims, wherein the shear thickening fluid includes silica or SiC particles suspended in ethylene glycol.

18. A method according to any one of the preceding claims, wherein the shear thickening fluid includes micro-sized or nano-sized abrasive particles suspended in a relatively high concentration in a base fluid, such that a resultant suspension acts as a shear-thickening fluid and also performs an effective abrasive function.

19. A method according to any one of the preceding claims, wherein the shear thickening fluid comprises a volume content of abrasive particulate material suspended in a base fluid, and wherein the volume content is greater than 50 % (v/v).

20. A method according to any one of the preceding claims, wherein the shear thickening fluid includes abrasive particles that are predominantly nano-sized, ranging between 1 and around 100 nm.

21. A method according to any one of claims 1 to 19, wherein the shear thickening fluid includes abrasive particles that are predominantly micro-sized, ranging between 0.1 and around 2000 micron.

22. A method according to any one of the preceding claims, wherein the shear thickening fluid includes a mixture of nano-sized and micro-sized abrasive particles.

23. A method according to any one of claims 18 to 22, wherein the abrasive particles are predominantly, or at least include a substantial proportion of SiC.

24. A method according to any one of the preceding claims, wherein the size, shape, and/or volume content of abrasive particles suspended in a base fluid forming the shear thickening fluid are selected such that a viscosity transition at the critical shear rate involves at least a fourfold increase in viscosity. ¹

25. A method according to any. one of the preceding claims, wherein the size, shape, and/or volume content of abrasive particles suspended in a base fluid forming the shear thickening fluid are selected such that a viscosity transition at the critical shear rate involves an increase in viscosity of at least one order of magnitude.

26. A method according to any one of the preceding claims, including the step of surface-treating abrasive particles in the shear thickening fluid sites to control or minimise agglomeration of the abrasive particles.

27. A method according to any one of the preceding claims, including the step of using a sensor to detect resistance to relative movement of the article with respect to the shear-thickening fluid.

28. A method according to claim 27, including the step of using a controller, responsive to the sensor, to slow down, speed up or otherwise modify the relative movement in order to control the viscosity of the shear-thickening fluid.

29. A method according to claim 28, wherein the controller is adapted to maintain a substantially constant viscosity during the relative movement.

30. A method according to claim 28 or claim 29, wherein the controller is adapted to control the relative movement such that the viscosity is changed over time according to a predetermined viscosity vs time profile.

31. A method according to any one of claims 28 to 30, wherein the controller includes a sensor adapted to generate a control signal indicative of the surface finish or typography of the article or component being polished, and a feedback loop whereby the controller, in response to that control signal and/or other control inputs, is adapted to progressively regulate the relative motion, the fluid viscosity or other system parameters until a desired or predetermined surface finish or surface topography has been obtained.

32. A composition for polishing an article, said composition comprising an abrasive shear-thickening fluid exhibiting a relatively sharp viscosity transition at a critical shear rate.

33. A composition according to claim 32, wherein the shear-thickening fluid exhibits a critical shear rate of between 2 and around 200 s-1.

34. A composition according to claim 32 or claim 33, wherein the shear- thickening fluid exhibits a critical shear rate of between 5 and around 50 s-1.

35. A composition according to any one of claims 32 to 34, wherein the shear- thickening fluid comprises a suspension of polymeric particles in a base fluid.

36. A composition according to claim 35, wherein the base fluid includes ethylene glycol.

37. A composition according to claim 36, wherein the polymeric particles comprise a solid volume fraction of between 20% and around 80% of the ethylene glycol.

38. A composition according to claim 37, wherein the polymeric particles comprise a solid volume fraction of between 30% and around 70% of the ethylene glycol.

39. A composition according to any one of claims 32 to 38, wherein the shear thickening fluid includes at least 50% by volume of particulates in suspension.

40. A composition according to any one of claims 35 to 39, wherein the suspended particles are abrasive particles.

41. A composition according to any one of claims 32 to 40, wherein the shear thickening fluid includes silica or SiC particles suspended in ethylene glycol.

42. A composition according to any one of claims 32 to 41 , wherein the shear thickening fluid comprises a volume content of abrasive particulate material suspended in a base fluid, and wherein the volume content is greater than 50 % (v/v).

43. A composition according to any one of claims 32 to 42, wherein the shear thickening fluid includes abrasive particles that are predominantly nano-sized, ranging between 1 and around 100 nm.

44. A composition according to any one of claims 32 to 42, wherein the shear thickening fluid includes abrasive particles that are predominantly micro-sized, ranging between 0.1 and around 2000 micron.

45. A composition according to any one of claims 32 to 44, wherein the shear thickening fluid includes a mixture of nano-sized and micro-sized abrasive particles.

46. A composition according to any one of claims 32 to 45, wherein the abrasive particles are predominantly, or at least include a substantial proportion of SiC.

47. A composition according to any one of claims 35 to 46, wherein the size, shape, and/or volume content of the abrasive particles suspended in the base fluid forming the shear thickening fluid are selected such that the viscosity transition at the critical shear rate involves at least a fourfold increase in viscosity.

48. A composition according to any one of claims 35 to 47, wherein the size, shape, and/or volume content of abrasive particles suspended in the base fluid forming the shear thickening fluid are selected such that the viscosity transition at the critical shear rate involves an increase in viscosity of at least one order of magnitude.

49. A composition according to any one of claims 35 to 48, wherein the suspended particles in the shear thickening fluid are surface treated to minimise agglomeration.

50. A method for controlling a degree of polish of an article in contact with an abrasive shear-thickening fluid in accordance with the method for polishing defined in any one of claims 1 to 31 , said method including the further step of: controlling the speed of relative movement of said article with respect to said shear-thickening fluid thereby to control the viscosity of said shear-thickening fluid and hence the degree of polish.

51. A system for polishing an article in accordance with the method defined in any one of claims 1 to 31 , said system comprising:

an abrasive shear-thickening fluid,

a drive mechanism for causing relative movement of said article with respect to said shear-thickening fluid, and