- Email: [email protected]
Abrasion and polishing
29 March 1984, The University of Reading, UK
Abrasion and Polishing In the UK at the moment the tribology groups of the IMechE and IoP are active and flourishing. The two groups are not mutually exclusive, as regular attendance at their meetings reveals. No one would deny that each represents closely the interests of its membership and approaches the same subject differently, but this can only be for the good of the subject as a whole engineers and physicists still have much to tell one another. At the University of Reading in March, more than 60 people - divided between the two camps - attended an loP meeting on 'Abrasion and Polishing'. The meeting was chaired and organized by John Lancaster (RAE Famborough) with local assistance from the staff of the Engineering Department who also arranged a tour of the Tribology Laboratory. In planning the meeting, John Lancaster had tried to find 'something for everyone' and from a materials point of view the programme was certainly varied. Metals, glass, rubber and polymers were discussed, with a fifth paper considering multilayer infrared and optical coatings. The first session of the day was devoted entirely to a critical review of the mechanisms of metal polishing, prepared and presented by Dr J.F. Archard. Little of the work reported was his own, he admitted, although the basis of the talk was a consultancy project he had undertaken for NCT Risley. Archard opened his account with an historical review of the theories put forward to explain the polishing of metals. Four main mechanisms have been proposed over the years: fine scale abrasion as propounded by Hooke, elaborated on by Newton, Herschel and Lord Rayleigh; plastic working of the surfaces, (Beilby); surface melting by frictional heating, after Bowden and Hughes; and material removal on an atomic scale (Rabinowicz). Many of the earlier theories hold in some way to the concept of the sub-surface 'Beilby layer', but the idea of this layer with 'special proper-
TRIBOLOGY international
ties' seems to have been well put down by the work of Samuels and his coworkers. It was studying the work of Samuels 'the most complete body of coherent evidence obtained using modern physical techniques' - that persuaded Archard that n o single theory could describe all cases of polishing. Samuels studied the metallurgical polishing of metals by diamond pastes and proposed a mechanism of free scale abrasion. He identified that there was indeed a subsurface region which played a part in subsequent polishing, but found by progressive etching that this was a region of deformation characterized by strain contours which flowed around the scratches. For example, a 0.05/am scratch could be associated with a 0.1/am fragmented layer and a total deformed depth of 0.7/am. Archard concluded that Samuels' view of polishing as small scale abrasion seems to be proved only down to 0.1/am diamond polishing but not for later stages of the metallographic polishing process: "There seems to be a conflict between his view of polishing as abrasion, with resultant sub-surface damage, and his insistence on the excellence of mechanical methods of polishing in metallography". In fact, the finishing process recommended by Samuels - diamond pastes, then MgO and fmally MgO plus an amount of ammonium persulphate led Archard to suggest a mechanism of very mild corrosive film wear for the freest stage of polishing. Turning his attention to the surface melting mechanism, Archard provided the results of his own calculations using a simplified version of the flash temperature theory. Among his assumptions were that the process was small scale - a circular contact radius of 1/am - with maximum contact pressure (for plastic deformation) and all the heat generated being passed to the polished surface. Taking a rubbing speed of 1 m/s and a coefficient of
friction of 0.5 his 'simple sums' indicated that the surface melting theory was not likely to apply to metals or quartz, but was possible for glass or perspex. In conclusion Archard pointed out that the study of wear has brought a general agreement that a range of mechanisms are possible. Sirnilarly, was it not possible that polishing involves a range of mechanisms which occur in different situations? It was probably true that coarser forms of polishing involved micro-abrasion. Surface melting he thought much less likely now that he had made his calculations. Chemical processes were quite likely to play a part in view of the effect of ammonium persulphate in Samuels free polishing technique. Additional evidence for this was also provided in the experience of many of the delegates, he remarked: "Many of you spent a significant proportion of your early life polishing military brass buttons with 'Brasso'. If you ever put the rag to your nose you could not fail to think there was some chemical action involved". In an impromptu addendum to Dr Archard's review, Dr Terry Eyre (Brunel University) gave an example of the significance of metal polishing in an engineering component. He described an investigation at Brunel o n the 'polishing wear' of cylinder bore liners. When bore polishing reaches 25% - usually estimated by the Ford Tornado test - problems arise with lubrication, blowby and even seizure. The surface texture of the liner changes rapidly on running, and the initial value of 0.55/am cla for an unused liner can reduce to 0.40/am (light polishing), 0.20/am (medium polishing) or even 0.08/am (heavy polishing). In the TDC, within a few hours of running, the equivalent to a metallographic 1/am diamond polish can be produced, with all original honing marks removed. Immediately after lunch, Dr Derek Cornish (Sira Ltd) also considered polishing mechanisms, but in his case the material to be polished was glass. The format he adopted was first to show that in normal polishing of glass the only mechanism that operates is the removal of glass down through successive deformed layers to an unstressed region, and then to discuss
293
the mechanism in more detail. Preliminary stages of surface forming by grinding and smoothing have been well understood for many years as mechanical processes whereby the shape is formed by pitting, fracturing and abrading away glass material on a relatively coarse scale, said Dr Cornish. At this stage the 'greyed' surface of the glass is significantly damaged and considerably stressed. Over the years various theories have been proposed for the mechanisms involved in the ensuing polishing process, he said: mechanical theories where the irregularities are 'microplaned' away, or material is removed from the prominences of a surface and redeposited in cracks and crevices producing the optical surface; a thermal theory whereby the surface is melted under the high pressures at point contacts so as to flow and fill up the irregularities; and more recently a chemical theory of glass removal. As his principal evidence Cornish referred to, and displayed, the results of an extensive study undertaken at
Sirain the 1960s. By marking a 'target' area of surface, several remarkable series of electron micrographs were obtained, showing the same small area of a greyed glass surface as it was gradually polished. These demonstrated effectively that the polishing mechanism was indeed glass removal. The study also showed that this takes place first from the prominences which become worn away. The process continues until the entire subsurface layer of stressed glass is removed to produce an optical finish. During this stage chips of strained glass flake away from the surface and can themselves cause further damage which must also be polished away. If defects are etched out at any stage, then a stress-free surface can be obtained on further polishing. In addition to the electron microscopy, quantitative experiments were carried out to determine the glass removal mechanism. This was shown to be a physical-chemical process involving the glass, defect sites on the polishing particles and water. All effective
14-- 16 May 1985, Munich, FRG
Ion and Plasma Assisted Techniques The fifth international conference in this series will cover both applications and processes, and papers are invited in the following general areas.
Applications tribology, tool coatings, corrosion protection, erosion, thermal barrier coatings, diffusion barriers, mechanical properties • vlsiprocessing, semiconductor devices, opto-electronic devices, electrically conducting layers, optical and magnetic recording materials • optical surfaces, optical waveguides, solar coatings, interference coatings • catalytic surfaces, chemical applications •
Processes •
294
Plasma deposition (PVD, ion beam), plasma enhanced CVD (MOCVD, LPCVD), plasma etching (RIE, anisotropic ion etch-
ing), sputtering,ion implantation (including reactive ion implantation and recoil implantation), ion beam processes (including ionized clusters), molecular beam epitaxy (including ionized beams) Ion plating, arc deposition processes, planar magnetron sputtering, reactive techniques (eg ARE), plasma processing (anodizing, nitriding), characterization and testing of coatings, and techniques for analysing films The conference will also cover associated novel techniques (eg laser glazing, laser annealing and ion exchange) and there will be a specific session on new items and novel concepts in coating equipment. Prospective authors should submit an abstract of 200-300 words to the Secretariat by 31 October 1984. IPAT 85, CEP Consultants Ltd, 26 Albany Street, Edinburgh EH1 3QH, UK
polishing agents are oxides, he stated. Under the high inte~facial pressures developed at point contacts in .polishing, cerium oxide, for example, forms complex bonds with silicon atoms in the surface. This complex formation reduces the activation energy required for hydrolysis of the Si-O-Si bonds of the surface glass matrix. Fully hydrated silica units are then removed by absorption on the polishing agent. Clearly, Cornish concluded, the same mechanism cannot obtain in the polishing of metals, although it may apply in the case of ceramics and semiconductor materials which can be hydrolysed by water under high energy conditions. Dr R.J. King (NPL) discussed work on the abrasion resistance of thin films a study which constitutes just a part of the work at NPL on measuring the optical properties of these films. The abrasion project, running for just two years, covers both optical f'dms and infrared Or) transparent materials. The MoD has a particular interest in ir antireflection coatings for military applications. Coatings often have to provide some degree of protection of the surfaces against abrasion damage, and this can be particularly important for both ir and plastic materials which tend to have poorer resistance than normal optical glasses. The problem that faced the NPL team was the lack of practical methods for testing abrasion resistance (or the adhesion of the film to the substrate). Uncertainties in existing tests were worrying. In the American 'Sebastian' test, for example, where a metal block is cemented to the film and the force to detach the film is measured, what effect does the cement have on the film? King described a machine developed at NPL which produces abrasion with a wiper blade action using a slurry of fine silica particles 'British Standard Dust (Coarse Grade)!' - suspended in distilled water. The wiper blade, an MoD requirement, was 2 cm long and clamped to avoid fexing. Each stroke was 4 cm. At the end of each 'wiping' sequence, the films were examined using a number of techniques: optical examination under 100 W mercury lamp; measurement of the cone of scattered laser light to check uniformity; Nomarski interference microscopy; profilometry using a Rank Taylor Hobson Talystep; transmittance or reflectance measurements.
October 1984 V o l 17 No 5
Some of the results obtained at NPL indicated differences in abrasion resistance when coatings were laid on cold or hot substrates. With simple MgF2 coatings, for example, with a cold substrate no coating remained after only 5000 strokes. With a hot substrate (at approximately 200°C) parts of the coating were lost after 5000 strokes - these were adhesion failures, not scratches - but the coating was not removed completely until 20 000 strokes. The adhesion was much improved by introduction of a three layered coating of Al2 O3 (base layer) - ZrO - MgF2. Abrasion resistance was the same as a simple MgF2 film. This itself had caused problems, however;now, when part of the coating was lost it occasionally dragged glass out from the surface and produced deeper 'scratches' than would ever be achieved through abrasion. For germanium substrates, one commercial coating has given astonishing results. The coating material had a structure somewhere between graphite and diamond and tests at NPL had shown no change in the surface after 80 000 strokes. Roughness measurements were the same and ellipsometry had revealed no differences. Coating of plastics posed many difficult problems. Polycarbonates abrade 20 times more than glass at a tenth the number of strokes. Although some effective non-optical coatings (~ 10/~m thickness) have been identified, little success has been achieved with optical anti-reflection coatings. The substrate heating which could be used for glass was not possible with plastics, and rfsputtering and ion implantation had not proved completely successful.
which are common to tyre wear and other more complicated abrasion processes. In rubber abrasion parallel ridges are produced transverse to the direction of abrasion, and the scale of the pattern increases with the roughness of the opposing rubbing surface. Using the equipment developed for the simple abrasion process it was possible, said Muhr 'to relate the rate of abrasion to pertinent measurements, in particular to the tangential force on the blade'. From the results a theory has been developed which yields the steady-state rate of abrasion from the rate of growth of cracks associated with the abrasion pattern (determined by the tangential force) and the angle the cracks make with the rubber surface (which can be obtained from pattern measurements). The theory is consistent with experiments for unlubricated non-crystallizing gum rubbers, although natural rubber shows departures. In the presence of a lubricant (eg water), however, the rate of abrasion falls by an order of magnitude (for a given normal load on the blade) while the tangential force fails by only 10 or 20 percent. It was suggested that the lubricants might be reducing the tendency of the blade to bend back the 'tongues' or rubber on the surface, compressing them instead. The effect of lubricants is currently being investigated further at MRPRA. Professor A.G. Atkins (University of Reading) discussed the probable role of crack resistance parameters in the mechanics of abrasive and adhesive wear. He illustrated his presentation with references to recent experiments on polymers undertaken in the University's Engineering Department by Mr M.K. Omar.
The last two presentations of the day described fracture mechanics approaches to the abrasion of first rubber and then polymers. Dr A. Muhr (MRPRA) pointed out that although a vast amount of work had been undertaken on the abrasion of rubber (as tyres), little of this was concerned with identifying the mechanism. Like the NPL group, Muhr and his colleagues had built their own test machine - in their case justified because of 'difficulty in interpreting and correlating the multitude of results available'!
The adhesive fatigue wear rate of polyethersulfone depends on the parameters of the Paris equation for fatigue crack growth rate and (to a lesser degree) on the static critical stress intensity factor. Marked variations in the mechanical behaviour of polymers may be produced by the action of solvents ,find this has permitted testing of the,theory over a wide range. Throughout his presentation Atkins compared and contrasted their results with related wear behaviour in metals*.
They chose a simple abrasion process, the passing of a blade over a rubber surface. Nevertheless, blade abrasion does produce features, such as the development of an abrasion pattern,
*Some o f the basic ideas he discussed are contained in a short communication published in Wear: Atkins A.G. Toughness in wear and grinding. Wear, 2 June 1980, 61(1), 183-190
TR IBOLOGY international
As mentioned at the beginning of this report, physicists and engineers have much to offer one another. There is always a danger, however, that theories based on over-simplified situations can produce, as John Lancaster puts it, 'a credibility gap'. Professor David Tabor, in commenting on the work of Muhr, also showed concern. " I f you have models of steady-state wear", he remarked, " y o u have removed any interest in the way in which material is actually removed". All4n-all though, an interesting day, and one which delegates from both camps, despite some puzzled frowns and exasperated gestures, seem to have enjoyed. S.J. Snook
Fretting wear seminar The Institution of Mechanical Engineers Tribology Group is holding a seminar entitled 'Fretting Wear' on 2 - 3 April 1985 in Nottingham, UK. Papers to be presented include: fretting in rolling element bearings; coatings; ceramics; orthopaedic implants; and fretting in aqueous environments. Further details from Catherine Nicholson, Engineering Sciences Division, The Institution of Mechanical Engineers, 1 Birdcage Walk, Westminster, London SWl H 9J J, UK
Space tribology Tribology in space presents problems quite different from those experienced on earth. The European Space Tribology Laboratory was established specifically to highlight such problems and hopefully to provide some solutions. A booklet from the European Space Agency provides a general account of some of the phenomena encountered in the simulated thermalvacuum conditions of outer space, and contains descriptions of some of the facilities, services and capabilities available at the Laboratory. The publication comprises 23 pages of text and colour photographs which give the reader some impression of the function of ESTL. ESTL, UKAEA, Risley, Warrington WA3 6AT, UK; ESA, 8 - 1 0 Rue Mario-Nikis, 75738 Paris Cedex 15, France
295
Abrasion and Polishing In the UK at the moment the tribology groups of the IMechE and IoP are active and flourishing. The two groups are not mutually exclusive, as regular attendance at their meetings reveals. No one would deny that each represents closely the interests of its membership and approaches the same subject differently, but this can only be for the good of the subject as a whole engineers and physicists still have much to tell one another. At the University of Reading in March, more than 60 people - divided between the two camps - attended an loP meeting on 'Abrasion and Polishing'. The meeting was chaired and organized by John Lancaster (RAE Famborough) with local assistance from the staff of the Engineering Department who also arranged a tour of the Tribology Laboratory. In planning the meeting, John Lancaster had tried to find 'something for everyone' and from a materials point of view the programme was certainly varied. Metals, glass, rubber and polymers were discussed, with a fifth paper considering multilayer infrared and optical coatings. The first session of the day was devoted entirely to a critical review of the mechanisms of metal polishing, prepared and presented by Dr J.F. Archard. Little of the work reported was his own, he admitted, although the basis of the talk was a consultancy project he had undertaken for NCT Risley. Archard opened his account with an historical review of the theories put forward to explain the polishing of metals. Four main mechanisms have been proposed over the years: fine scale abrasion as propounded by Hooke, elaborated on by Newton, Herschel and Lord Rayleigh; plastic working of the surfaces, (Beilby); surface melting by frictional heating, after Bowden and Hughes; and material removal on an atomic scale (Rabinowicz). Many of the earlier theories hold in some way to the concept of the sub-surface 'Beilby layer', but the idea of this layer with 'special proper-
TRIBOLOGY international
ties' seems to have been well put down by the work of Samuels and his coworkers. It was studying the work of Samuels 'the most complete body of coherent evidence obtained using modern physical techniques' - that persuaded Archard that n o single theory could describe all cases of polishing. Samuels studied the metallurgical polishing of metals by diamond pastes and proposed a mechanism of free scale abrasion. He identified that there was indeed a subsurface region which played a part in subsequent polishing, but found by progressive etching that this was a region of deformation characterized by strain contours which flowed around the scratches. For example, a 0.05/am scratch could be associated with a 0.1/am fragmented layer and a total deformed depth of 0.7/am. Archard concluded that Samuels' view of polishing as small scale abrasion seems to be proved only down to 0.1/am diamond polishing but not for later stages of the metallographic polishing process: "There seems to be a conflict between his view of polishing as abrasion, with resultant sub-surface damage, and his insistence on the excellence of mechanical methods of polishing in metallography". In fact, the finishing process recommended by Samuels - diamond pastes, then MgO and fmally MgO plus an amount of ammonium persulphate led Archard to suggest a mechanism of very mild corrosive film wear for the freest stage of polishing. Turning his attention to the surface melting mechanism, Archard provided the results of his own calculations using a simplified version of the flash temperature theory. Among his assumptions were that the process was small scale - a circular contact radius of 1/am - with maximum contact pressure (for plastic deformation) and all the heat generated being passed to the polished surface. Taking a rubbing speed of 1 m/s and a coefficient of
friction of 0.5 his 'simple sums' indicated that the surface melting theory was not likely to apply to metals or quartz, but was possible for glass or perspex. In conclusion Archard pointed out that the study of wear has brought a general agreement that a range of mechanisms are possible. Sirnilarly, was it not possible that polishing involves a range of mechanisms which occur in different situations? It was probably true that coarser forms of polishing involved micro-abrasion. Surface melting he thought much less likely now that he had made his calculations. Chemical processes were quite likely to play a part in view of the effect of ammonium persulphate in Samuels free polishing technique. Additional evidence for this was also provided in the experience of many of the delegates, he remarked: "Many of you spent a significant proportion of your early life polishing military brass buttons with 'Brasso'. If you ever put the rag to your nose you could not fail to think there was some chemical action involved". In an impromptu addendum to Dr Archard's review, Dr Terry Eyre (Brunel University) gave an example of the significance of metal polishing in an engineering component. He described an investigation at Brunel o n the 'polishing wear' of cylinder bore liners. When bore polishing reaches 25% - usually estimated by the Ford Tornado test - problems arise with lubrication, blowby and even seizure. The surface texture of the liner changes rapidly on running, and the initial value of 0.55/am cla for an unused liner can reduce to 0.40/am (light polishing), 0.20/am (medium polishing) or even 0.08/am (heavy polishing). In the TDC, within a few hours of running, the equivalent to a metallographic 1/am diamond polish can be produced, with all original honing marks removed. Immediately after lunch, Dr Derek Cornish (Sira Ltd) also considered polishing mechanisms, but in his case the material to be polished was glass. The format he adopted was first to show that in normal polishing of glass the only mechanism that operates is the removal of glass down through successive deformed layers to an unstressed region, and then to discuss
293
the mechanism in more detail. Preliminary stages of surface forming by grinding and smoothing have been well understood for many years as mechanical processes whereby the shape is formed by pitting, fracturing and abrading away glass material on a relatively coarse scale, said Dr Cornish. At this stage the 'greyed' surface of the glass is significantly damaged and considerably stressed. Over the years various theories have been proposed for the mechanisms involved in the ensuing polishing process, he said: mechanical theories where the irregularities are 'microplaned' away, or material is removed from the prominences of a surface and redeposited in cracks and crevices producing the optical surface; a thermal theory whereby the surface is melted under the high pressures at point contacts so as to flow and fill up the irregularities; and more recently a chemical theory of glass removal. As his principal evidence Cornish referred to, and displayed, the results of an extensive study undertaken at
Sirain the 1960s. By marking a 'target' area of surface, several remarkable series of electron micrographs were obtained, showing the same small area of a greyed glass surface as it was gradually polished. These demonstrated effectively that the polishing mechanism was indeed glass removal. The study also showed that this takes place first from the prominences which become worn away. The process continues until the entire subsurface layer of stressed glass is removed to produce an optical finish. During this stage chips of strained glass flake away from the surface and can themselves cause further damage which must also be polished away. If defects are etched out at any stage, then a stress-free surface can be obtained on further polishing. In addition to the electron microscopy, quantitative experiments were carried out to determine the glass removal mechanism. This was shown to be a physical-chemical process involving the glass, defect sites on the polishing particles and water. All effective
14-- 16 May 1985, Munich, FRG
Ion and Plasma Assisted Techniques The fifth international conference in this series will cover both applications and processes, and papers are invited in the following general areas.
Applications tribology, tool coatings, corrosion protection, erosion, thermal barrier coatings, diffusion barriers, mechanical properties • vlsiprocessing, semiconductor devices, opto-electronic devices, electrically conducting layers, optical and magnetic recording materials • optical surfaces, optical waveguides, solar coatings, interference coatings • catalytic surfaces, chemical applications •
Processes •
294
Plasma deposition (PVD, ion beam), plasma enhanced CVD (MOCVD, LPCVD), plasma etching (RIE, anisotropic ion etch-
ing), sputtering,ion implantation (including reactive ion implantation and recoil implantation), ion beam processes (including ionized clusters), molecular beam epitaxy (including ionized beams) Ion plating, arc deposition processes, planar magnetron sputtering, reactive techniques (eg ARE), plasma processing (anodizing, nitriding), characterization and testing of coatings, and techniques for analysing films The conference will also cover associated novel techniques (eg laser glazing, laser annealing and ion exchange) and there will be a specific session on new items and novel concepts in coating equipment. Prospective authors should submit an abstract of 200-300 words to the Secretariat by 31 October 1984. IPAT 85, CEP Consultants Ltd, 26 Albany Street, Edinburgh EH1 3QH, UK
polishing agents are oxides, he stated. Under the high inte~facial pressures developed at point contacts in .polishing, cerium oxide, for example, forms complex bonds with silicon atoms in the surface. This complex formation reduces the activation energy required for hydrolysis of the Si-O-Si bonds of the surface glass matrix. Fully hydrated silica units are then removed by absorption on the polishing agent. Clearly, Cornish concluded, the same mechanism cannot obtain in the polishing of metals, although it may apply in the case of ceramics and semiconductor materials which can be hydrolysed by water under high energy conditions. Dr R.J. King (NPL) discussed work on the abrasion resistance of thin films a study which constitutes just a part of the work at NPL on measuring the optical properties of these films. The abrasion project, running for just two years, covers both optical f'dms and infrared Or) transparent materials. The MoD has a particular interest in ir antireflection coatings for military applications. Coatings often have to provide some degree of protection of the surfaces against abrasion damage, and this can be particularly important for both ir and plastic materials which tend to have poorer resistance than normal optical glasses. The problem that faced the NPL team was the lack of practical methods for testing abrasion resistance (or the adhesion of the film to the substrate). Uncertainties in existing tests were worrying. In the American 'Sebastian' test, for example, where a metal block is cemented to the film and the force to detach the film is measured, what effect does the cement have on the film? King described a machine developed at NPL which produces abrasion with a wiper blade action using a slurry of fine silica particles 'British Standard Dust (Coarse Grade)!' - suspended in distilled water. The wiper blade, an MoD requirement, was 2 cm long and clamped to avoid fexing. Each stroke was 4 cm. At the end of each 'wiping' sequence, the films were examined using a number of techniques: optical examination under 100 W mercury lamp; measurement of the cone of scattered laser light to check uniformity; Nomarski interference microscopy; profilometry using a Rank Taylor Hobson Talystep; transmittance or reflectance measurements.
October 1984 V o l 17 No 5
Some of the results obtained at NPL indicated differences in abrasion resistance when coatings were laid on cold or hot substrates. With simple MgF2 coatings, for example, with a cold substrate no coating remained after only 5000 strokes. With a hot substrate (at approximately 200°C) parts of the coating were lost after 5000 strokes - these were adhesion failures, not scratches - but the coating was not removed completely until 20 000 strokes. The adhesion was much improved by introduction of a three layered coating of Al2 O3 (base layer) - ZrO - MgF2. Abrasion resistance was the same as a simple MgF2 film. This itself had caused problems, however;now, when part of the coating was lost it occasionally dragged glass out from the surface and produced deeper 'scratches' than would ever be achieved through abrasion. For germanium substrates, one commercial coating has given astonishing results. The coating material had a structure somewhere between graphite and diamond and tests at NPL had shown no change in the surface after 80 000 strokes. Roughness measurements were the same and ellipsometry had revealed no differences. Coating of plastics posed many difficult problems. Polycarbonates abrade 20 times more than glass at a tenth the number of strokes. Although some effective non-optical coatings (~ 10/~m thickness) have been identified, little success has been achieved with optical anti-reflection coatings. The substrate heating which could be used for glass was not possible with plastics, and rfsputtering and ion implantation had not proved completely successful.
which are common to tyre wear and other more complicated abrasion processes. In rubber abrasion parallel ridges are produced transverse to the direction of abrasion, and the scale of the pattern increases with the roughness of the opposing rubbing surface. Using the equipment developed for the simple abrasion process it was possible, said Muhr 'to relate the rate of abrasion to pertinent measurements, in particular to the tangential force on the blade'. From the results a theory has been developed which yields the steady-state rate of abrasion from the rate of growth of cracks associated with the abrasion pattern (determined by the tangential force) and the angle the cracks make with the rubber surface (which can be obtained from pattern measurements). The theory is consistent with experiments for unlubricated non-crystallizing gum rubbers, although natural rubber shows departures. In the presence of a lubricant (eg water), however, the rate of abrasion falls by an order of magnitude (for a given normal load on the blade) while the tangential force fails by only 10 or 20 percent. It was suggested that the lubricants might be reducing the tendency of the blade to bend back the 'tongues' or rubber on the surface, compressing them instead. The effect of lubricants is currently being investigated further at MRPRA. Professor A.G. Atkins (University of Reading) discussed the probable role of crack resistance parameters in the mechanics of abrasive and adhesive wear. He illustrated his presentation with references to recent experiments on polymers undertaken in the University's Engineering Department by Mr M.K. Omar.
The last two presentations of the day described fracture mechanics approaches to the abrasion of first rubber and then polymers. Dr A. Muhr (MRPRA) pointed out that although a vast amount of work had been undertaken on the abrasion of rubber (as tyres), little of this was concerned with identifying the mechanism. Like the NPL group, Muhr and his colleagues had built their own test machine - in their case justified because of 'difficulty in interpreting and correlating the multitude of results available'!
The adhesive fatigue wear rate of polyethersulfone depends on the parameters of the Paris equation for fatigue crack growth rate and (to a lesser degree) on the static critical stress intensity factor. Marked variations in the mechanical behaviour of polymers may be produced by the action of solvents ,find this has permitted testing of the,theory over a wide range. Throughout his presentation Atkins compared and contrasted their results with related wear behaviour in metals*.
They chose a simple abrasion process, the passing of a blade over a rubber surface. Nevertheless, blade abrasion does produce features, such as the development of an abrasion pattern,
*Some o f the basic ideas he discussed are contained in a short communication published in Wear: Atkins A.G. Toughness in wear and grinding. Wear, 2 June 1980, 61(1), 183-190
TR IBOLOGY international
As mentioned at the beginning of this report, physicists and engineers have much to offer one another. There is always a danger, however, that theories based on over-simplified situations can produce, as John Lancaster puts it, 'a credibility gap'. Professor David Tabor, in commenting on the work of Muhr, also showed concern. " I f you have models of steady-state wear", he remarked, " y o u have removed any interest in the way in which material is actually removed". All4n-all though, an interesting day, and one which delegates from both camps, despite some puzzled frowns and exasperated gestures, seem to have enjoyed. S.J. Snook
Fretting wear seminar The Institution of Mechanical Engineers Tribology Group is holding a seminar entitled 'Fretting Wear' on 2 - 3 April 1985 in Nottingham, UK. Papers to be presented include: fretting in rolling element bearings; coatings; ceramics; orthopaedic implants; and fretting in aqueous environments. Further details from Catherine Nicholson, Engineering Sciences Division, The Institution of Mechanical Engineers, 1 Birdcage Walk, Westminster, London SWl H 9J J, UK
Space tribology Tribology in space presents problems quite different from those experienced on earth. The European Space Tribology Laboratory was established specifically to highlight such problems and hopefully to provide some solutions. A booklet from the European Space Agency provides a general account of some of the phenomena encountered in the simulated thermalvacuum conditions of outer space, and contains descriptions of some of the facilities, services and capabilities available at the Laboratory. The publication comprises 23 pages of text and colour photographs which give the reader some impression of the function of ESTL. ESTL, UKAEA, Risley, Warrington WA3 6AT, UK; ESA, 8 - 1 0 Rue Mario-Nikis, 75738 Paris Cedex 15, France
295