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In vivo wear properties of ultra-high molecular weight polyethylene used in a total knee prosthesis
Wear, 55 (1979) 1 - 9 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
IN VW0 WEAR PROPERTIES OF ULTRA-HIGH MOLECULAR WEIGHT POLYEhIYLENE USED IN A TOTAL KNEE PROSTHESIS
G. W. HASTINGS Biomedical Engineering Unit, North Staffordshire Polytechnic Health Authority, Medical Institute, Hartshill, Stoke-on-Pent,
and Staffordshire Area ST4 7NY (Ct. Britain)
(Received May 22, 1978)
Summary The wear characteristics of the polyethylene tibial component of a series of geomedic total knee prostheses have been studied. These were removed from patients for various reasons after times of implantation ranging from 4 to 28 months. Visible light microscopy and the scanning electron microscope were used to examine the worn surfaces. Two primary wear processes are proposed. Cold flow leads to smoothing of machine marks and fragmentation with further abrasion resulting from the debris formed. The second process is surface cracking and delamination which may result from physical differences between surface and bulk material. Additional features are galling wear resulting from inhomogeneities in the polymer and the abrasive effects of acrylic bone cement.
1. Introduction Routine examinations of all orthopaedic implants removed from patient8 have been carried out. The aim was two-fold: (a) to obtain information on the in uiuo behaviour of implant materials f’rom a large number of patients and (b) to instruct the younger surgeons in order to improve techniques for internal fixation of fractures and for joint replacement. The programme ~8s not restricted to failed implants, but embraced those removed for a variety of reasons. These included implants used for internal fixation removed routinely following bone union, prostheses removed following infection and failed implants usually associated with loosening or breakage. bhbXid8 received have included metals (mainly stainless steel) and plastics (mainly ultra-high molecular weight polyethylene (UHMWPE). Components of both hip and knee prostheses have been received; thie paper deals only with the latter.
1
2
2. Materials and methodology The knee prosthesis was chosen for examination because a larger number were available at the time and because it represents a different mode of action from the hip. Prostheses examined were of the same type (Geomedic supplied by Howmedica (U.K.) Ltd) and comprise a cobalt chrome-molybdenum condylar component and a tibial plateau component made from UHMWPE (RCH 1000). An exception was the two sets of components, medial and lateral, of a Charnley prosthesis also received and examined. The procedure followed for all implants is outlined in Table 1.
TABLE 1 Methodology for examination of removed surgical implants In operating theatre Implants packed in sealed plastic containers in operating theatre. Each component separate and labelled. In laboratory Using gloved hands, implants immersed in buffered glutaraldehyde solution (Cidexn) for 3 h minimum. Preliminary microscope examination. Gloved hands. J Washing for removal of adherent blood/tissue. Gloved hands. Metals -heat
sterilized in dry air oven.
Plastics - no further sterilization usually. If greatly contaminated further treatment with glutaraldehyde followed by water rinse and air drying. & More detailed examination, e.g. visible light microscopy scanning electron microscopy chemical analysis sectioning for examination of metal structure
Precautions were taken in the handling of all removed implants because of the risk of hepatitis when anything has been contaminated with blood. In some cases samples were sent for bacteriological and histological examination. Patch testing of the skin was also performed in order to determine sensitization to metal ions. Details of prostheses received are given in Table 2.
3 TABLE Details
2 of knee prostheses
examined
Duration of implantation (months)
Reason for removal
13 14 19
20 4 -
Infection Infection -
47
11
Infection
64 95 109
8 Not known 20
Pain Infection
141 162 182 59
28 10 6 14
Infection Infection Infection Loosening
Implant no.
3. Examination
Comments
--
_ Unused tibia1 component rendered unsterile at operation Cement overlap on metal caused wear on plastic Cement overlap movement -
& loosening
-
interfered
with
Bilateral, two prostheses Corrosion pits in metal Charnley prosthesis
of components
Examination by means of visible light microscopy at magnifications of 12.5 X and 50 X showed varying amounts of damage to tie UHMWPE bearing surface. Deep score marks in two cases corresponding to acrylic bone cement overlapping the anterior edge of the steel component were sufficient in one of them to cause separation of the two components during movement. On most bearing surfaces the original machining marks were almost completely worn away and replaced by a random pattern of fine score marks running in an anterior-posterior direction. Severe wear in some regions had produced fragmentation of the surface. Some samples were pitted with craters where polymer had been removed. Isolated “islands” were observed on worn surfaces. These were areas 1 - 2 mm in diameter surrounded by an eroded region and having resisted wear appeared to be harder than surrounding UHMWPE. They were thought to be gel particles containing crosslinked polymer. Approximately spherical regions of increased opacity and similar dimensiohs could be seen’below the surface of the polymer. Bands of whiter more opaque material have been observed in samples of UHMWPE and appear to be a result of the moukling process. Gel particles could also arise during this stage. The bearing surface of one tibial component examined was covered with small (0.1 mm) black particles. The corresponding metal surface contained pits surrounded by brown staining. Metal surfaces were not usually badly marked, although fine score marks were seen running in the direction of motion. Some samples had areas
Fig. 3.
Fig. 4
with large amounts of scoring. Fine circular scratches have also been observed, presumably remaining from the polishing process. Strongly adherent white deposits were seen on most metal surfaces. These were usually found on the posterior and two internal margins of the condylar surfaces. 4. Scanning electron microscope examination More detailed examination of the surface wear was carried out using a Cambridge Stereoscan 600. Polyethylene surfaces were shadowed with aluminium. Energy dispersive analysis (EDA) was used to obtain a chemical analysis of specific areas particularly for detection of radio-opacifiers from the acrylic cement or titanium from the ethylene polymerization. Typical results are shown in Figs. 1 - 13 and Table 3.
10 pm
10 pm -
I----!
Fig. 6.
Fig. 5.
100
Mm
100 pm
-
t----l
Fig. 7.
Fig. 8.
The first group of results (Figs, 1 - 5) show progressive stages in the wear process. The apparently harder zones, possibly representing inhomogeneities in the material, are in the next group (Figs. 6 - 9) which includes cement particles ground into the surface. Types of debris observed are shown in Figs. 10 and 11 and the final group (Figs. 12 and 13) shows results for the Charnley prosthesis. This is of interest, in that the position of the UHMWF’E is opposite to that in the Geomedic prosthesis. In the Charnley prosthesis the polyethylene is used for the femoral component and moves on a stationary stainless steel tibial plateau. 5. Discussion The relative motions of the two components in the knee prosthesis are quite different from those in the hip. In the knee the femoral condylar im-
1Opm
40 pm + ---.
k----i
Fig. 10.
Fig. 9.
20
100
jltm
Fig. 11.
I.rm
e-----I
I---i
Fig. 12.
plant rolls across the polyethylene of the tibial plateau in a substantially anterior-posterior direction. Superimposed upon this may be varying amounts of rotational and lateral movements, the extent of these depending upon the success of implantation and fixation, Principally unidirectional motion is being obtained in contrast to the hip. The first group of micrographs demonstrates the stages in the wear process. The machined surface is being smoothed out by a process of cold flow under the high loadings to which the joint is subjected. This process continues with further smearing out of asperities and in regions of unusually high applied stress the polymer eventually becomes shredded. When the surface has been scored fine cracks perpendicular to the direction of scoring are observed. This is similar to the fatigue cracking which is observed in plastics. It is observed [ 1] in metals that fretting has an effect upon fatigue and it is possible that there is a relationship between the cold flow effect and fatigue cracking in this case.
200 NIlI
n
Fig. 13. TABLE 3 -. Figure no.
Prosthesis no.
Details of micrograph
1 2 3 4 5 6 7 8 9 10 11
13 182 162 182 182 182 13 13 182 162 13
12 13
59 59
Machining marks, smearing beginning Severely damaged, shredded region Score marks Lightly worn region showing cracking Fragment partially detached Loose fragment Island in surrounding smeared/shredded area Crater remaining from an island Acrylic cement, individual granules visible Partially detached fragments in crater Needle-shaped particles aligned approx. perpendicularly to wear tracks Charnley prosthesis abraded region Charnley prosthesis fragmented surface
The “islands” and the resultant craters remaining when these are dislodged may represent the presence of harder inhomogeneities, e.g. gel particles. It has been reported [ 21 in studies on composites that when the constituents of a composite respond differently to the environment a lack of cohesion may develop between the polymer matrix and the constituents and cause dislodgement. Carbides in metals lead to a similar observation [3] and this has been referred to as galling wear, (Figs. 7 and 8). It has been suggested that the dislodgements are potential areas for delamination wear. A type of delamination is seen in Figs. 4 and 5 in which initial cracking of the surface is followed by detachment of fragments, one of which is shown in Fig. 6. It does not appear that this particular example of cracking was related to the dislodgements of the “islands” referred to above, but that it is the cracking and elimination of a surface film which is possibly harder than the underlying bulk material. Other results, however,
8
show partially detached fragments around a crater which illustrates a delamination process. Figure 9 shows a particle of acrylic bone cement. The entire surface of this component contained such regions. The individual polymer beads are visible and EDA showed the presence of barium and, compared with the background, excess sulphur from the radio-opacifier barium sulphate which is a component of bone cement. The surface of this sample was covered with black particles but these were not visible underneath the aluminium coating. An attempt was made to recover some after scanning electron microscope examination, and EDA of the recovered regions showed the presence of sulphur and chlorine together with calcium, sodium, potassium and iron; magnesium and silicon were also observed. Further examination of the pitted metal counterface is being carried out. In this prosthesis the tibia1 component was carried on a relatively unsupported thickness of acrylic cement which had cracked and caused movement of the prosthesis. Cement particles may then have become interposed between the joint surfaces. Different types of debris are shown in Figs. 10 and 11. The needleshaped particles were seen on both lateral and medial plateaux, and are also organic. They were aligned both along the score marks and perpendicular to them. It is possible that they are flakes of polymer which have been rolled by the uniaxial motion. Spherical particles have been reported in metals [4]. Figures 12 and 13 show the more severe damage observed in the Charnley prosthesis. In another sample currently under examination the UHMWPE has completely broken up. Applied loadings are more severe when polyethylene is used as the femoral component. It is hoped to report on these in more detail later. Three mechanisms based on the work of Lancaster [ 5,6] are usually presented to explain the wear observed in plastics. Abrasive wear represents the removal of plastic by the action of hard asperities on the metal surface. Fatigue wear is obtained when large deformations and subsequent recovery are obtained and is predominant for elastomers. Adhesive wear is said to result in the transfer of a film of the plastics material to the counterface. It has been suggested [ 61 that adhesive wear is the predominating mechanism for in uiuo wear in the hip joint with the possibility of fatigue cracking occurring after 5 - 10 years of implantation. Bikerman [ 71 has criticized conclusions that adhesion is a factor in frictional phenomena. The presence of absorbed films prevents true molecular contact between opposed faces and therefore inhibits adhesion. A transfer film has been observed during in vitro wear studies of metals and UHMWPE, and this is cited as evidence of adhesion. However, the film formed on the steel surface may result from mechanical transfer by an abrasion process in which the softer polymer is removed by micro-asperities. A transfer film has not been observed during this study. The white deposit around the sides of the metal component referred to above is not yet indentified. EDA shows the presence of sulphur, chlorine, silicon, calcium
9
and potassium and nickel.
in addition
to the constituents
of the metal: iron, chromium
6. Conclusions As a result of the present study it is proposed that the wear process involves the following sequence of events. The major process is that of cold flow produced by the mechanical loadings. This leads to smoothing of residual machining marks and fragmentation of polymer surfaces. A second process is that of delamination. Surface layers are cracked and delaminated from the underlying polymer. This difference in properties may result from machining and sterilization. Radiation sterilization affects molecular order; changes in molecular order and hence mobility affect surface properties [ 81. Abrasive action of debris increases wear. Needle-like particles associated with scoring may be rolled-up flakes of polymer. Two further effects are superimposed upon this pattern. Acrylic bone cement can cause excessive abrasion and can become embedded in the surface. It appears that the abrasive effect of this on the metal counter surface can lead to corrosion pitting of the metal. Galling wear results from inhomogeneities in the polymer which are possibly gel particles. These lead to the formation of islands and eventually to craters.
Acknowledgments The author is greatly indebted to Mrs. V. K. Colclough for technical assistance and to the Research Director of the British Ceramic Research Association who provided facilities for the electron microscopy carried out by Dr. S. N. Ruddlesden and Mrs. P. Fisher.
References 1 2 3 4 5 6 7 8
R. K. Reeves and D. W. Hoeppner, Wear, 40 (1976) 395 - 397. C. Shen and J. H. Dumbleton, Wear, 40 (1976) 351 - 360. R. Komanduri and M. C. Shaw, Wear, 33 (1975) 282 - 292. E. Rabinowicz, Wear, 42 (1977) 149 - 156. J. K. Lancaster, Plast. Polym., 41 (1973) 297. K. J. Brown, J. R. Atkinson, D. Dowson and V. Wright, Wear, 40 (1976) 255 - 264. J. J. Biker-man, Wear, 39 (1976) 1 - 13. V. A. Belyi, V. G. Savkin, A. I. Sviridyonok, V. A. Smurugov and 0. V. Kholidov, Wear, 42 (1977) 91 - 97.
IN VW0 WEAR PROPERTIES OF ULTRA-HIGH MOLECULAR WEIGHT POLYEhIYLENE USED IN A TOTAL KNEE PROSTHESIS
G. W. HASTINGS Biomedical Engineering Unit, North Staffordshire Polytechnic Health Authority, Medical Institute, Hartshill, Stoke-on-Pent,
and Staffordshire Area ST4 7NY (Ct. Britain)
(Received May 22, 1978)
Summary The wear characteristics of the polyethylene tibial component of a series of geomedic total knee prostheses have been studied. These were removed from patients for various reasons after times of implantation ranging from 4 to 28 months. Visible light microscopy and the scanning electron microscope were used to examine the worn surfaces. Two primary wear processes are proposed. Cold flow leads to smoothing of machine marks and fragmentation with further abrasion resulting from the debris formed. The second process is surface cracking and delamination which may result from physical differences between surface and bulk material. Additional features are galling wear resulting from inhomogeneities in the polymer and the abrasive effects of acrylic bone cement.
1. Introduction Routine examinations of all orthopaedic implants removed from patient8 have been carried out. The aim was two-fold: (a) to obtain information on the in uiuo behaviour of implant materials f’rom a large number of patients and (b) to instruct the younger surgeons in order to improve techniques for internal fixation of fractures and for joint replacement. The programme ~8s not restricted to failed implants, but embraced those removed for a variety of reasons. These included implants used for internal fixation removed routinely following bone union, prostheses removed following infection and failed implants usually associated with loosening or breakage. bhbXid8 received have included metals (mainly stainless steel) and plastics (mainly ultra-high molecular weight polyethylene (UHMWPE). Components of both hip and knee prostheses have been received; thie paper deals only with the latter.
1
2
2. Materials and methodology The knee prosthesis was chosen for examination because a larger number were available at the time and because it represents a different mode of action from the hip. Prostheses examined were of the same type (Geomedic supplied by Howmedica (U.K.) Ltd) and comprise a cobalt chrome-molybdenum condylar component and a tibial plateau component made from UHMWPE (RCH 1000). An exception was the two sets of components, medial and lateral, of a Charnley prosthesis also received and examined. The procedure followed for all implants is outlined in Table 1.
TABLE 1 Methodology for examination of removed surgical implants In operating theatre Implants packed in sealed plastic containers in operating theatre. Each component separate and labelled. In laboratory Using gloved hands, implants immersed in buffered glutaraldehyde solution (Cidexn) for 3 h minimum. Preliminary microscope examination. Gloved hands. J Washing for removal of adherent blood/tissue. Gloved hands. Metals -heat
sterilized in dry air oven.
Plastics - no further sterilization usually. If greatly contaminated further treatment with glutaraldehyde followed by water rinse and air drying. & More detailed examination, e.g. visible light microscopy scanning electron microscopy chemical analysis sectioning for examination of metal structure
Precautions were taken in the handling of all removed implants because of the risk of hepatitis when anything has been contaminated with blood. In some cases samples were sent for bacteriological and histological examination. Patch testing of the skin was also performed in order to determine sensitization to metal ions. Details of prostheses received are given in Table 2.
3 TABLE Details
2 of knee prostheses
examined
Duration of implantation (months)
Reason for removal
13 14 19
20 4 -
Infection Infection -
47
11
Infection
64 95 109
8 Not known 20
Pain Infection
141 162 182 59
28 10 6 14
Infection Infection Infection Loosening
Implant no.
3. Examination
Comments
--
_ Unused tibia1 component rendered unsterile at operation Cement overlap on metal caused wear on plastic Cement overlap movement -
& loosening
-
interfered
with
Bilateral, two prostheses Corrosion pits in metal Charnley prosthesis
of components
Examination by means of visible light microscopy at magnifications of 12.5 X and 50 X showed varying amounts of damage to tie UHMWPE bearing surface. Deep score marks in two cases corresponding to acrylic bone cement overlapping the anterior edge of the steel component were sufficient in one of them to cause separation of the two components during movement. On most bearing surfaces the original machining marks were almost completely worn away and replaced by a random pattern of fine score marks running in an anterior-posterior direction. Severe wear in some regions had produced fragmentation of the surface. Some samples were pitted with craters where polymer had been removed. Isolated “islands” were observed on worn surfaces. These were areas 1 - 2 mm in diameter surrounded by an eroded region and having resisted wear appeared to be harder than surrounding UHMWPE. They were thought to be gel particles containing crosslinked polymer. Approximately spherical regions of increased opacity and similar dimensiohs could be seen’below the surface of the polymer. Bands of whiter more opaque material have been observed in samples of UHMWPE and appear to be a result of the moukling process. Gel particles could also arise during this stage. The bearing surface of one tibial component examined was covered with small (0.1 mm) black particles. The corresponding metal surface contained pits surrounded by brown staining. Metal surfaces were not usually badly marked, although fine score marks were seen running in the direction of motion. Some samples had areas
Fig. 3.
Fig. 4
with large amounts of scoring. Fine circular scratches have also been observed, presumably remaining from the polishing process. Strongly adherent white deposits were seen on most metal surfaces. These were usually found on the posterior and two internal margins of the condylar surfaces. 4. Scanning electron microscope examination More detailed examination of the surface wear was carried out using a Cambridge Stereoscan 600. Polyethylene surfaces were shadowed with aluminium. Energy dispersive analysis (EDA) was used to obtain a chemical analysis of specific areas particularly for detection of radio-opacifiers from the acrylic cement or titanium from the ethylene polymerization. Typical results are shown in Figs. 1 - 13 and Table 3.
10 pm
10 pm -
I----!
Fig. 6.
Fig. 5.
100
Mm
100 pm
-
t----l
Fig. 7.
Fig. 8.
The first group of results (Figs, 1 - 5) show progressive stages in the wear process. The apparently harder zones, possibly representing inhomogeneities in the material, are in the next group (Figs. 6 - 9) which includes cement particles ground into the surface. Types of debris observed are shown in Figs. 10 and 11 and the final group (Figs. 12 and 13) shows results for the Charnley prosthesis. This is of interest, in that the position of the UHMWF’E is opposite to that in the Geomedic prosthesis. In the Charnley prosthesis the polyethylene is used for the femoral component and moves on a stationary stainless steel tibial plateau. 5. Discussion The relative motions of the two components in the knee prosthesis are quite different from those in the hip. In the knee the femoral condylar im-
1Opm
40 pm + ---.
k----i
Fig. 10.
Fig. 9.
20
100
jltm
Fig. 11.
I.rm
e-----I
I---i
Fig. 12.
plant rolls across the polyethylene of the tibial plateau in a substantially anterior-posterior direction. Superimposed upon this may be varying amounts of rotational and lateral movements, the extent of these depending upon the success of implantation and fixation, Principally unidirectional motion is being obtained in contrast to the hip. The first group of micrographs demonstrates the stages in the wear process. The machined surface is being smoothed out by a process of cold flow under the high loadings to which the joint is subjected. This process continues with further smearing out of asperities and in regions of unusually high applied stress the polymer eventually becomes shredded. When the surface has been scored fine cracks perpendicular to the direction of scoring are observed. This is similar to the fatigue cracking which is observed in plastics. It is observed [ 1] in metals that fretting has an effect upon fatigue and it is possible that there is a relationship between the cold flow effect and fatigue cracking in this case.
200 NIlI
n
Fig. 13. TABLE 3 -. Figure no.
Prosthesis no.
Details of micrograph
1 2 3 4 5 6 7 8 9 10 11
13 182 162 182 182 182 13 13 182 162 13
12 13
59 59
Machining marks, smearing beginning Severely damaged, shredded region Score marks Lightly worn region showing cracking Fragment partially detached Loose fragment Island in surrounding smeared/shredded area Crater remaining from an island Acrylic cement, individual granules visible Partially detached fragments in crater Needle-shaped particles aligned approx. perpendicularly to wear tracks Charnley prosthesis abraded region Charnley prosthesis fragmented surface
The “islands” and the resultant craters remaining when these are dislodged may represent the presence of harder inhomogeneities, e.g. gel particles. It has been reported [ 21 in studies on composites that when the constituents of a composite respond differently to the environment a lack of cohesion may develop between the polymer matrix and the constituents and cause dislodgement. Carbides in metals lead to a similar observation [3] and this has been referred to as galling wear, (Figs. 7 and 8). It has been suggested that the dislodgements are potential areas for delamination wear. A type of delamination is seen in Figs. 4 and 5 in which initial cracking of the surface is followed by detachment of fragments, one of which is shown in Fig. 6. It does not appear that this particular example of cracking was related to the dislodgements of the “islands” referred to above, but that it is the cracking and elimination of a surface film which is possibly harder than the underlying bulk material. Other results, however,
8
show partially detached fragments around a crater which illustrates a delamination process. Figure 9 shows a particle of acrylic bone cement. The entire surface of this component contained such regions. The individual polymer beads are visible and EDA showed the presence of barium and, compared with the background, excess sulphur from the radio-opacifier barium sulphate which is a component of bone cement. The surface of this sample was covered with black particles but these were not visible underneath the aluminium coating. An attempt was made to recover some after scanning electron microscope examination, and EDA of the recovered regions showed the presence of sulphur and chlorine together with calcium, sodium, potassium and iron; magnesium and silicon were also observed. Further examination of the pitted metal counterface is being carried out. In this prosthesis the tibia1 component was carried on a relatively unsupported thickness of acrylic cement which had cracked and caused movement of the prosthesis. Cement particles may then have become interposed between the joint surfaces. Different types of debris are shown in Figs. 10 and 11. The needleshaped particles were seen on both lateral and medial plateaux, and are also organic. They were aligned both along the score marks and perpendicular to them. It is possible that they are flakes of polymer which have been rolled by the uniaxial motion. Spherical particles have been reported in metals [4]. Figures 12 and 13 show the more severe damage observed in the Charnley prosthesis. In another sample currently under examination the UHMWPE has completely broken up. Applied loadings are more severe when polyethylene is used as the femoral component. It is hoped to report on these in more detail later. Three mechanisms based on the work of Lancaster [ 5,6] are usually presented to explain the wear observed in plastics. Abrasive wear represents the removal of plastic by the action of hard asperities on the metal surface. Fatigue wear is obtained when large deformations and subsequent recovery are obtained and is predominant for elastomers. Adhesive wear is said to result in the transfer of a film of the plastics material to the counterface. It has been suggested [ 61 that adhesive wear is the predominating mechanism for in uiuo wear in the hip joint with the possibility of fatigue cracking occurring after 5 - 10 years of implantation. Bikerman [ 71 has criticized conclusions that adhesion is a factor in frictional phenomena. The presence of absorbed films prevents true molecular contact between opposed faces and therefore inhibits adhesion. A transfer film has been observed during in vitro wear studies of metals and UHMWPE, and this is cited as evidence of adhesion. However, the film formed on the steel surface may result from mechanical transfer by an abrasion process in which the softer polymer is removed by micro-asperities. A transfer film has not been observed during this study. The white deposit around the sides of the metal component referred to above is not yet indentified. EDA shows the presence of sulphur, chlorine, silicon, calcium
9
and potassium and nickel.
in addition
to the constituents
of the metal: iron, chromium
6. Conclusions As a result of the present study it is proposed that the wear process involves the following sequence of events. The major process is that of cold flow produced by the mechanical loadings. This leads to smoothing of residual machining marks and fragmentation of polymer surfaces. A second process is that of delamination. Surface layers are cracked and delaminated from the underlying polymer. This difference in properties may result from machining and sterilization. Radiation sterilization affects molecular order; changes in molecular order and hence mobility affect surface properties [ 81. Abrasive action of debris increases wear. Needle-like particles associated with scoring may be rolled-up flakes of polymer. Two further effects are superimposed upon this pattern. Acrylic bone cement can cause excessive abrasion and can become embedded in the surface. It appears that the abrasive effect of this on the metal counter surface can lead to corrosion pitting of the metal. Galling wear results from inhomogeneities in the polymer which are possibly gel particles. These lead to the formation of islands and eventually to craters.
Acknowledgments The author is greatly indebted to Mrs. V. K. Colclough for technical assistance and to the Research Director of the British Ceramic Research Association who provided facilities for the electron microscopy carried out by Dr. S. N. Ruddlesden and Mrs. P. Fisher.
References 1 2 3 4 5 6 7 8
R. K. Reeves and D. W. Hoeppner, Wear, 40 (1976) 395 - 397. C. Shen and J. H. Dumbleton, Wear, 40 (1976) 351 - 360. R. Komanduri and M. C. Shaw, Wear, 33 (1975) 282 - 292. E. Rabinowicz, Wear, 42 (1977) 149 - 156. J. K. Lancaster, Plast. Polym., 41 (1973) 297. K. J. Brown, J. R. Atkinson, D. Dowson and V. Wright, Wear, 40 (1976) 255 - 264. J. J. Biker-man, Wear, 39 (1976) 1 - 13. V. A. Belyi, V. G. Savkin, A. I. Sviridyonok, V. A. Smurugov and 0. V. Kholidov, Wear, 42 (1977) 91 - 97.