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Characterization of wear particles for comprehension of wear mechanisms
Wear 265 (2008) 1714–1719
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Characterization of wear particles for comprehension of wear mechanisms Case of PTFE against cast iron C. Kowandy a,∗ , C. Richard a , Y.M. Chen b a b
Laboratoire Roberval, UMR 6253, UTC, Centre de recherche, BP 20529, 60205 Compi`egne Cedex, France CETIM, Avenue F. Louat, BP 80067, 60304 Senlis Cedex, France
a r t i c l e
i n f o
Article history: Received 12 September 2007 Received in revised form 12 March 2008 Accepted 11 April 2008 Available online 26 June 2008 Keywords: Wear particles Debris analysis Polytetrafluoroethylene Cast iron Wear mechanism
a b s t r a c t The aim of this article is to find correlations between the characteristics of the wear particles, the coefficients of friction and the wear rates in order to determine wear mechanisms. In the air compressors, piston rings are made of polytetrafluoroethylene (PTFE) but its wear rate is very high and it must be reinforced. This work is devoted to the influence of temperature with three types of PTFE (pure and filled with glass fibre and carbographite) against flake grey cast iron thanks to a phenomenological study of the contact. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The compressors used for the blowing of the plastic bottles generally consist of a cast iron cylinder and piston and rings which must ensure at the same time the sealing between these parts and the lubrication. Indeed, the air at exit of the compressor must be free from fluid lubricant. This is why the piston rings are made out of materials known as “self-lubricating” such as polytetrafluoroethylene (PTFE). In fact, PTFE is an excellent lubricating solid. It is very largely used for applications of sealing and lubrication. Unfortunately PTFE suffers from a high wear rate which limits its applications. Many filler materials and treatments [1–4] were developed in order to mitigate this disadvantage. Thus, we are interested in the tribological behavior of various types of filled or not PTFE against grey cast iron 250. For that, in addition to the “traditional” techniques such as the measurement of the coefficient of friction, we study the morphology of the generated wear particles in order to understand the wear and friction mechanisms at two temperatures (25 ◦ C and 115 ◦ C) under glass transition. The characterization of the wear particle morphology is important in the study of the wear processes and essential in the evaluation of the good state of tribological systems. Study of wear
∗ Corresponding author. E-mail address: [email protected] (C. Kowandy). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.04.036
particles can be used for preventive maintenance. It may sometimes prevent serious damage in an industrial environment. The wear debris characterization can help to improve the selection and preparation of materials for specific applications. Hamblin and Stachowiak found relationship between the shape of the particles and abrasivity [5,6]. Some writers have become interested in the influence of the observation of particles profile (3D) but this will not be detailed here [7]. Indeed, according to the shape of the particles, we can deduce the type of wear. Certain authors [8–11] classified debris in seven categories, all based on the appearance of the particle, which are as follows: • • • • • • •
Spheres, Distorted smooth ovoids, Chunks and slabs, Platelets and flakes, Curls, spirals and slivers, Rolls, Strands and fibres.
Other authors have used different classifications to describe wear particles [12,13]. They have considered four outline shape attributes and five edge detail attributes (Table 1). This procedure is not always objective because it relies on expert judgement and it can also be expensive. Hence, recent research efforts have enabled the expansion of numerical descriptors [14,15]. Their development seems to be the best way of overcoming these deficiencies. Progress
C. Kowandy et al. / Wear 265 (2008) 1714–1719
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Table 1 Outline shape and edge detail attributes [11,12] and size parameters Regular
Outline shape attributes
Edge detail attributes
Smooth
Apparent area
Size parameters
Irregular
Rough
Circularity =
Factor form =
4 area perimeter2
4 area perimeter2
Straight
Length
in this area has been greatly accelerated with the application of fast computers. In addition, a specific language has been developed around the analysis of wear particles and the characterization of particle shape. A certain number of characteristics are employed but the user must choose the most applicable characteristics according to debris type. There are typically size parameters (for example area, length, perimeter and equivalent circle diameter). The equivalent circle diameter represents the diameter of a circle included in the particle. These size parameters are defined in Table 1. Shape parameters can be used like circularity, form factor and spike parameter (SP) [13]. SP measures the particle angularities by using a measurement of the particle contour with convex triangles. For a smooth particle the SP is zero or about, whereas for a very angular particle it tends towards unity. The formulae for circularity and factor form are given below:
(1)
Circular
Perimeter
Serrated
Elongated
Curved
Equivalent circle diameter
2. Experimental methods 2.1. Tested materials Polytetrafluoroethylene a semi-crystalline polymer, is largely used in tribological applications because of its exceptional thermal stability, good resistance to solvents and low coefficient of friction [16]. In this study, three types of PTFE are used: pure and two filled PTFEs with 25% of carbographite (PTFE + C) and 25% of glass fibre (PTFE + GF). The pure PTFE is the reference material. The percent of 25% of charge was chosen in consultation with industry. Microstructures of PTFE are given in Fig. 1. The counterface material is flake grey cast iron (EN-GJL 250 (NF IN 1561)) and represents the cylinder or piston in the compressor. In this cast iron, graphite has a form of plates [17,18]. Flake grey cast irons are easily manufactured and have good friction resistance. They are generally used in mechanical parts (pumps, valves, etc.), chemical and food industry, armament and oil industry. The principal mechanical characteristics and composition of the flake grey cast iron are given in Table 2. 2.2. Tribometer and tribosystem
(2)
In order to optimize the choice of materials for the compressor rings, friction tests were carried out with tribometer TE 77 (Phoenix Tribology). The tested materials were PTFE, pure or filled, representing segments, against flake grey cast iron, representing cylinder or piston at two temperatures (25 ◦ C and 115 ◦ C, near glass transition of PTFE which is 109 ◦ C). This present work consisted, in addition to the measurements of the friction coefficients and the wear rates, of the characterization of pin surfaces and plates after tests. Then the morphological and geometrical attributes of the wear particles were determined by image analysis techniques. Finally, the mechanisms of wear are proposed by integrating the information given by the morphological analysis of wear particles.
The tribological tests were carried out with a TE77tribometer (made by Phoenix Tribology Company) with a plane contact. This tribometer was chosen because it can simulate the real contact configuration. In order to carry out tests under inert atmosphere, a gas enclose was fixed on the machine. Tests were achieved under nitrogen gas with a flow of 5 l/min. The design of the sample holder was modified to improve the surface contact by using ball and socket joint and hemispherical pins. PTFE pins were semi-sphere with a diameter of 18 mm. The cast iron sample was in form of a plate of 58 mm × 38 mm and 5-mm thickness. Its arithmetic roughness (Ra ) was 0.5 m. Reciprocal stroke was 15 mm, frequency was 10 Hz and test duration was 1 h (distance of slip of 1.08 km). The applied normal force was 12 N (0.5 MPa). The temperature of cast iron plates was 25 ◦ C and 115 ◦ C. Tests were carried out without lubricant.
Fig. 1. Microstructures of PTFE.
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C. Kowandy et al. / Wear 265 (2008) 1714–1719
Table 2 Mechanical characteristics and composition of flake grey cast iron EN-FGL 250 Structure
Composition Elements
Pearlite (fine flakes of graphite)
Carbon
These experimental conditions were chosen so as to correspond to the real conditions met in industry. 2.3. Observations and analyses After tests, contact surfaces (of pins and plates) and wear particles were observed and analysed in order to characterize the type of wear. Wear particles were collected at the end of test and were large enough to be separated with a nipper. They were stuck with a carbon disc. Observations were carried out with a Lynx stereo zoom microscope equipped with a Sony UP-2100P video printer. Other observations were made on Philips XL 30 ESEM-FEG scanning electron microscope with environmental control. The analyses were achieved by dispersive X-emission in energy (EDX). An Image Analyser System (IAS) was used to visually examine the debris. In fact, although the human eye has certain acuity and makes it possible to detect meticulously the details, the image analyser has the capacity to provide precise dimensional measurements [15]. The IAS is generally composed: • Means of optically examining the debris (e.g., microscope), • A device to digitize, • The analyser. Three different processes have been developed to identify and analyse individual particle shape features: • Manual process: individual particles are selected using a cursor, • Automatic process: all particles are collectively analysed, • Enhancement process: manual control is used to analyse bad quality samples in the case of overlapped or joined particles. The various pictures were treated using image analysis software Visilog 5.4 developed by Noesis Company. One hundred debris is analysed by type of testing. 3. Results and discussion In this paper, the effect of temperature was studied by performing tests at 25 ◦ C and at 115 ◦ C. The test duration was 1 h which gave slip distance of 1.08 km. These tests are representative of running in. 3.1. Coefficients of friction and wear rates ·
The average friction coefficients and the wear rates (ω) [19], which are calculated for three tests, are given in Table 3. The dimen-
Elastic limit (Mpa)
HB hardness
>250
>165
150/230
Percentage
3.05–3.65
Silicon Manganese Sulfur Phosphorus
Tensile strength (Mpa)
2.10–2.70 0.60–0.70 0.04–0.05 0.170–0.260
sionless wear rate is determined as .
ω (without unit) =
m LA
(3)
where m is the mass loss, the material density, A the apparent contact area and L is the slip distance. The results indicate that at the higher temperature, lower is the friction coefficient except for PTFE + GF. The variation of friction coefficient is more important with PTFE + C (increase of 20%) and about 10% of variation for pure PTFE and PTFE + C. When temperature increases, PTFE is transferred on cast iron. Indeed, the rigidity of polymers decreases with temperature raising and the molecular chains are released and untangled. For PTFE and PTFE + C, when temperature increases, the formation and rupture of interfacial adhesion bonds are made easier and friction coefficient decreases. For PTFE + GF, glass fibres reduce adhesion bonds. The PTFE matrix wears and the contact becomes between harder glass fibres and cast iron. The consequence is an increase of friction coefficient. Concerning the wear rate (Table 3), it decreases when the temperature increases for pure PTFE and PTFE + C and it remains quasi-constant for PTFE + GF. The PTFE does not sufficiently transfer on cast iron because 115 ◦ C is near its glass transition (109 ◦ C). So PTFE is already rigid. The loss of mass can be explained by formation of a transfer film on the cast iron surface and by wear particles production. With pure PTFE, wear rate is very important because of production of large debris. 3.2. Observations of pin facies and plates (film of transfer) After tests, the worn surfaces of the pins (Fig. 2) and of the plates (Fig. 3) were observed. As is well known, the tribological properties of polymers and their composites in sliding condition against a metal under dry conditions are strongly influenced by their ability to form a transfer film on the counterface [4]. It is essential to study the transfer film in order to understand the wear and friction mechanisms. In this work, the morphologies of transfer films on the plates were examined and analysed by SEM and EDX. Table 3 Means of friction coefficient PTFE
PTFE + GF
PTFE + C
25 ◦ C, 1 h Friction coefficient Wear rate
0.18 9.38E−10
0.14 1.42E−11
0.20 4.76E−10
115 ◦ C, 1 h Friction coefficient Wear rate
0.16 1.19E−11
0.16 2.92E−11
0.16 2.06E−11
C. Kowandy et al. / Wear 265 (2008) 1714–1719
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Fig. 2. Micrographies of pins after test (the arrows indicate the direction of the slip).
Table 4 Composition and qualitative characteristics of wear particles PTFE
PTFE + GF
PTFE + C
PTFE et Fe (inclusions) Chunk and platelet Irregular Rough and curved
PTFE et Fe (lot of inclusions) Chunk and platelet Irregular Rough
PTFE, C et Fe (few inclusions) Chunk Irregular Rough and serrated
PTFE et Fe (inclusions) Ovoid Regular Curved
PTFE et Fe (with chunk of Fe) and some glass fibres Platelets and chunk Irregular Curved
PTFE, C et Fe (few inclusions) Platelet and small slab Regular Smooth
◦
25 C, 1 h
Micrographies of wear particles
Composition Type of particles Outiline shape Edge detail 115 ◦ C, 1 h
Micrographie of wear particles
Composition Type of particles Outiline shape Edge detail
The PTFE pins pure and filled with carbographite present a ploughed surface with some iron inclusions which are more important when the temperature increases. In this case, pin wear is abrasive and wear rate are high with PTFE and PTFE + C. Graphite in cast iron plays the role of hard and abrasive particle. Surfaces of pins filled with glass fibres are not completely ploughed. X-rays detect the presence of iron oxide (Fe2 O3 ) and PTFE on their surfaces. Theses oxides cover surface asperities. They can be attributed to a reaction between the pin and the cast iron plate. This wear is tribochemical one.1 The transfer films are similar with PTFE and PTFE + C. There is a PTFE deposit on hollows of rectification strias. Due to the relative motion, molecular chains are broken and the lamellar PTFE structure is destroyed. Actives groups are formed and they react to surface. Adhesive junctions are created between PTFE and cast iron: a transfer film is formed. For the glass fibres filled pins, an iron oxide coating with some PTFE is formed. The reason is that the glass fibres reduce the delamination of PTFE matrix. So the transfer film
1
Tribochemical reactions: formation of new products due to chemical reactions occurring between mating elements and environment during friction [20].
is limited. However, we cannot see a difference of film of transfer between the two test temperatures. Finally it is difficult to observe transfer film because they are very thin. Currently, other techniques to characterize these films are used like glow discharge optical spectrometry (GDOS). 3.3. Characterisation of the wear particles This study was finally supplemented by qualitative and quantitative characterization of the wear particles. Table 5 Quantitative characteristics of wear particles PTFE 25 ◦ C, 1 h Average equivant diameter (m) Average aspect ratio Average spike parameter 115 ◦ C, 1 h Average equivant diameter (m) Average aspect ratio Average spike parameter
PTFE + GF
PTFE + C
362 2.05 0.53
96 1.71 0.48
227
1850 1.63 0.47
70 1.63 0.51
120 1.66 0.54
.93 0.50
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C. Kowandy et al. / Wear 265 (2008) 1714–1719
Fig. 3. Transfer film on cast iron plates.
Table 6 Proposed wear mechanisms PTFE First body pin
Uniform abrasive wear
First body plate
PTFE deposit in the level of asperities in cast iron plate adhesive wear High production of wear particles of important size (Part of transfer film) delamination of transfer film
Third body transfer film and wear particles
Wear particles are small, slightly opaque with black inclusions. Examples of wear particles are given in Table 4. The selected qualitative characteristics are outline shapes and edge detail attributes. Higher is the temperature, softer is the edge detail. The particles are generally platelets or chunks which indicate an insufficient lubrication. These types of wear particles are typically characteristic of abrasive wear. For the PTFE filled with glass fibre pins, wear particles have iron inclusions which indicates the severity of the contact. At 115 ◦ C, glass fibres are found in wear particles because the PTFE matrix is softening and glass fibres are released. For the quantitative analysis, the chosen parameters, because they are most applicable to recovered particles, are the equivalent circle diameter (ED), the aspect ratio and the SP. The aspect ratio is the ratio between length and width. The particles resulting from the PTFE pins have always the highest ED compared to filled PTFEs under the same the experimental conditions (Table 5). The fillers reduce the formation of large wear particles because they prevent the delamination of PTFE. At 115 ◦ C, particles recovered with the PTFE pins may correspond to a part of transfer film. Moreover, when the temperature increases, the particles are less stretched out and thus, circular except for PTFE + C. The carbographite is perhaps responsible of the lengthening of debris. Lastly, with filled PTFE, the SP of wear particles decreases and the edge detail are smoother.
3.4. Proposed wear mechanisms 3.4.1. Pin Abrasive wear characterized by strias is observed on pin surface of PTFE and PTFE + C. For PTFE + GF, wear is less abrasive and there is Fe2 O3 oxide in pin asperities. It is characteristic of a tribochemical and adhesive wear of PTFE + GF pins.
PTFE + C
PTFE + GF Nonuniform wear of pins with presence of Fe2 O3 in pin asperities ´ tribochemical wear + adhesive Weak deposit of PTFE with Fe2 O3 adhesive wear and tribochemical
Weak production of small wear particles abrasive wear of transfer film
3.4.2. Plates Whatever the PTFE composition, there is a deposit of PTFE on plate surface. With PTFE + GF, Fe2 O3 oxide is present and formation of the film of transfer is limited. The wear is only adhesive for PTFE and PTFE + C. In case of PTFE + GF, the wear is adhesive and tribochemical. 3.4.3. Third body Finally, with pure PTFE, due to transfer film on plate, slipping is between PTFE and a layer of PTFE and implies a high production of wear particles in platelet form which facilities the slipping. These wear particles were maybe a part of the transfer film because transfer film is separated and products wear particles. For filled PTFE, production of wear particles, which are small, is lowest. The transfer film can be worn by abrasion. The mechanisms, which we propose, are included in Table 6. 4. Conclusion The results can be summarised in the following way: 1. For all temperatures, the lowest wear was obtained with PTFE + GF pins. 2. At 115 ◦ C the average coefficient of friction is 0.16 whatever the type of pin. At 25 ◦ C, it varies from 0.20 to 0.14. 3. The increase of temperature reduces wear and friction coefficient except for PTFE + FV. 4. For wear particles analysis, ED does not seem directly related to the coefficient of friction. But, the size of the wear particles decreases when there are fillers. 5. The wear particles have rather soft shape attributes. These conclusions permit to propose wear mechanisms:
C. Kowandy et al. / Wear 265 (2008) 1714–1719
• For PTFE and PTFE + C, abrasive wear of pins and abrasive and tribochemical wear with PTFE + GF. • Adhesive wear of plates. • Delamination of transfer film for pure PTFE. For the others PTFE, transfer film is subjected to abrasive wear. The morphological analysis of wear particles can be also used for preventive maintenance of mechanical systems. This study has made it possible to highlight the fundamental importance of temperature at which polymers are used and its influence on their structure. But characteristics of polymers, such as their rate of crystallinity and their length of chains, confer particular properties on them. References [1] S. Bahadur, D. Tabor, The wear of filled polytetrafluoroethylene, Wear 98 (1984) 1–13. [2] G. Deli, X. Qunji, W. Hongli, Study of the wear of filled polytetrafluoroethylene, Wear 134 (2) (1989) 283–295. [3] J. Khedkar, I. Negulescu, E.I. Meletis, Sliding wear behavior of PTFE composites, Wear 252 (5–6) (2002) 361–369. [4] S.-Q. Lai, The tribological properties of PTFE filled with thermally treated nanoattapulgite, Tribol. Int. 39 (6) (2006) 541–547. [5] M.G. Hamblin, G.W. Stachowiack, A multi-scale measure and its relation to two-body abrasive wear, Wear 190 (1995) 190–196.
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[6] M.G. Hamblin, G.W. Stachowiak, A multi-scale measure of particle abrasivity, Wear 185 (1–2) (1995) 225–233. [7] B.J. Roylance, I.A. Albidewi, M.S. Laghari, A.R. Luxmoore, F. Deravi, Computeraided vision engineering (CAVE)—quantification of wear particle morphology, Lubrification Eng. 150 (1994) 111–116. [8] P. Podsiadlo, G.W. Stachowiak, Characterization of surface topography of wear particles by SEM stereoscopy, Wear 206 (1–2) (1997) 39–52. [9] P. Podsiadlo, G.W. Stachowiak, Evaluation of boundary fractal methods for the characterization of wear particles, Wear 217 (1) (1998) 24–34. [10] S. Raadnui, B.J. Roylance, The classification of wear particle shape, Lubrification Eng. 51 (1995) 432–437. [11] Trevor M. Hunt, Handbook of Wear Debris Analysis and Particles Detection in Liquids, E.a. Science, 1993. [12] B.J. Roylance, Computer-aided vision engineering (CAVE)—quantification of wear particle morphology, Lubrification Eng. 150 (1994) 111–116. [13] B.H. Kaye, Applied fractal geometry and the fine particle specialist. Part I. Rugged boundaries and rough surfaces, Particle Particle Syst. Charact. 10 (1993) 99–110. [14] W. Wieleba, The statistical correlation of the coefficient of friction and wear rate of PTFE composites with steel counterface roughness and hardness, Wear 252 (9–10) (2002) 719–729. ´ [15] C. Souchier, Analyse d’images, Techniques de l’ingenieur, p. 855. ´ ` ´ es, ´ mise en [16] J. Trotignon et al., Precis de Matieres Plastiques: structures–propriet œuvre, Normalisation, AFNOR-Nathan, 1996. ´ ´ ´ 1962. [17] A. Desy, J. Vidts, Traite´ de metallurgie structurale, theorique et appliquee, ´ ´ ´ ` [18] C. Chaussin, G. Hilly, Metallurgie, Tome 1: Alliages Metalliques, Dixieme Edition, Dunod, 1982. [19] K. Friedrich, Friction and Wear of Polymer Composites, Elsevier, Amsterdam, 1985. [20] C. Kadjdas, S.S.K. Harvey, E. Wilusz, Encyclopedia of Tribology, Tribology Series, Elsevier, 1990.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Characterization of wear particles for comprehension of wear mechanisms Case of PTFE against cast iron C. Kowandy a,∗ , C. Richard a , Y.M. Chen b a b
Laboratoire Roberval, UMR 6253, UTC, Centre de recherche, BP 20529, 60205 Compi`egne Cedex, France CETIM, Avenue F. Louat, BP 80067, 60304 Senlis Cedex, France
a r t i c l e
i n f o
Article history: Received 12 September 2007 Received in revised form 12 March 2008 Accepted 11 April 2008 Available online 26 June 2008 Keywords: Wear particles Debris analysis Polytetrafluoroethylene Cast iron Wear mechanism
a b s t r a c t The aim of this article is to find correlations between the characteristics of the wear particles, the coefficients of friction and the wear rates in order to determine wear mechanisms. In the air compressors, piston rings are made of polytetrafluoroethylene (PTFE) but its wear rate is very high and it must be reinforced. This work is devoted to the influence of temperature with three types of PTFE (pure and filled with glass fibre and carbographite) against flake grey cast iron thanks to a phenomenological study of the contact. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The compressors used for the blowing of the plastic bottles generally consist of a cast iron cylinder and piston and rings which must ensure at the same time the sealing between these parts and the lubrication. Indeed, the air at exit of the compressor must be free from fluid lubricant. This is why the piston rings are made out of materials known as “self-lubricating” such as polytetrafluoroethylene (PTFE). In fact, PTFE is an excellent lubricating solid. It is very largely used for applications of sealing and lubrication. Unfortunately PTFE suffers from a high wear rate which limits its applications. Many filler materials and treatments [1–4] were developed in order to mitigate this disadvantage. Thus, we are interested in the tribological behavior of various types of filled or not PTFE against grey cast iron 250. For that, in addition to the “traditional” techniques such as the measurement of the coefficient of friction, we study the morphology of the generated wear particles in order to understand the wear and friction mechanisms at two temperatures (25 ◦ C and 115 ◦ C) under glass transition. The characterization of the wear particle morphology is important in the study of the wear processes and essential in the evaluation of the good state of tribological systems. Study of wear
∗ Corresponding author. E-mail address: [email protected] (C. Kowandy). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.04.036
particles can be used for preventive maintenance. It may sometimes prevent serious damage in an industrial environment. The wear debris characterization can help to improve the selection and preparation of materials for specific applications. Hamblin and Stachowiak found relationship between the shape of the particles and abrasivity [5,6]. Some writers have become interested in the influence of the observation of particles profile (3D) but this will not be detailed here [7]. Indeed, according to the shape of the particles, we can deduce the type of wear. Certain authors [8–11] classified debris in seven categories, all based on the appearance of the particle, which are as follows: • • • • • • •
Spheres, Distorted smooth ovoids, Chunks and slabs, Platelets and flakes, Curls, spirals and slivers, Rolls, Strands and fibres.
Other authors have used different classifications to describe wear particles [12,13]. They have considered four outline shape attributes and five edge detail attributes (Table 1). This procedure is not always objective because it relies on expert judgement and it can also be expensive. Hence, recent research efforts have enabled the expansion of numerical descriptors [14,15]. Their development seems to be the best way of overcoming these deficiencies. Progress
C. Kowandy et al. / Wear 265 (2008) 1714–1719
1715
Table 1 Outline shape and edge detail attributes [11,12] and size parameters Regular
Outline shape attributes
Edge detail attributes
Smooth
Apparent area
Size parameters
Irregular
Rough
Circularity =
Factor form =
4 area perimeter2
4 area perimeter2
Straight
Length
in this area has been greatly accelerated with the application of fast computers. In addition, a specific language has been developed around the analysis of wear particles and the characterization of particle shape. A certain number of characteristics are employed but the user must choose the most applicable characteristics according to debris type. There are typically size parameters (for example area, length, perimeter and equivalent circle diameter). The equivalent circle diameter represents the diameter of a circle included in the particle. These size parameters are defined in Table 1. Shape parameters can be used like circularity, form factor and spike parameter (SP) [13]. SP measures the particle angularities by using a measurement of the particle contour with convex triangles. For a smooth particle the SP is zero or about, whereas for a very angular particle it tends towards unity. The formulae for circularity and factor form are given below:
(1)
Circular
Perimeter
Serrated
Elongated
Curved
Equivalent circle diameter
2. Experimental methods 2.1. Tested materials Polytetrafluoroethylene a semi-crystalline polymer, is largely used in tribological applications because of its exceptional thermal stability, good resistance to solvents and low coefficient of friction [16]. In this study, three types of PTFE are used: pure and two filled PTFEs with 25% of carbographite (PTFE + C) and 25% of glass fibre (PTFE + GF). The pure PTFE is the reference material. The percent of 25% of charge was chosen in consultation with industry. Microstructures of PTFE are given in Fig. 1. The counterface material is flake grey cast iron (EN-GJL 250 (NF IN 1561)) and represents the cylinder or piston in the compressor. In this cast iron, graphite has a form of plates [17,18]. Flake grey cast irons are easily manufactured and have good friction resistance. They are generally used in mechanical parts (pumps, valves, etc.), chemical and food industry, armament and oil industry. The principal mechanical characteristics and composition of the flake grey cast iron are given in Table 2. 2.2. Tribometer and tribosystem
(2)
In order to optimize the choice of materials for the compressor rings, friction tests were carried out with tribometer TE 77 (Phoenix Tribology). The tested materials were PTFE, pure or filled, representing segments, against flake grey cast iron, representing cylinder or piston at two temperatures (25 ◦ C and 115 ◦ C, near glass transition of PTFE which is 109 ◦ C). This present work consisted, in addition to the measurements of the friction coefficients and the wear rates, of the characterization of pin surfaces and plates after tests. Then the morphological and geometrical attributes of the wear particles were determined by image analysis techniques. Finally, the mechanisms of wear are proposed by integrating the information given by the morphological analysis of wear particles.
The tribological tests were carried out with a TE77tribometer (made by Phoenix Tribology Company) with a plane contact. This tribometer was chosen because it can simulate the real contact configuration. In order to carry out tests under inert atmosphere, a gas enclose was fixed on the machine. Tests were achieved under nitrogen gas with a flow of 5 l/min. The design of the sample holder was modified to improve the surface contact by using ball and socket joint and hemispherical pins. PTFE pins were semi-sphere with a diameter of 18 mm. The cast iron sample was in form of a plate of 58 mm × 38 mm and 5-mm thickness. Its arithmetic roughness (Ra ) was 0.5 m. Reciprocal stroke was 15 mm, frequency was 10 Hz and test duration was 1 h (distance of slip of 1.08 km). The applied normal force was 12 N (0.5 MPa). The temperature of cast iron plates was 25 ◦ C and 115 ◦ C. Tests were carried out without lubricant.
Fig. 1. Microstructures of PTFE.
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C. Kowandy et al. / Wear 265 (2008) 1714–1719
Table 2 Mechanical characteristics and composition of flake grey cast iron EN-FGL 250 Structure
Composition Elements
Pearlite (fine flakes of graphite)
Carbon
These experimental conditions were chosen so as to correspond to the real conditions met in industry. 2.3. Observations and analyses After tests, contact surfaces (of pins and plates) and wear particles were observed and analysed in order to characterize the type of wear. Wear particles were collected at the end of test and were large enough to be separated with a nipper. They were stuck with a carbon disc. Observations were carried out with a Lynx stereo zoom microscope equipped with a Sony UP-2100P video printer. Other observations were made on Philips XL 30 ESEM-FEG scanning electron microscope with environmental control. The analyses were achieved by dispersive X-emission in energy (EDX). An Image Analyser System (IAS) was used to visually examine the debris. In fact, although the human eye has certain acuity and makes it possible to detect meticulously the details, the image analyser has the capacity to provide precise dimensional measurements [15]. The IAS is generally composed: • Means of optically examining the debris (e.g., microscope), • A device to digitize, • The analyser. Three different processes have been developed to identify and analyse individual particle shape features: • Manual process: individual particles are selected using a cursor, • Automatic process: all particles are collectively analysed, • Enhancement process: manual control is used to analyse bad quality samples in the case of overlapped or joined particles. The various pictures were treated using image analysis software Visilog 5.4 developed by Noesis Company. One hundred debris is analysed by type of testing. 3. Results and discussion In this paper, the effect of temperature was studied by performing tests at 25 ◦ C and at 115 ◦ C. The test duration was 1 h which gave slip distance of 1.08 km. These tests are representative of running in. 3.1. Coefficients of friction and wear rates ·
The average friction coefficients and the wear rates (ω) [19], which are calculated for three tests, are given in Table 3. The dimen-
Elastic limit (Mpa)
HB hardness
>250
>165
150/230
Percentage
3.05–3.65
Silicon Manganese Sulfur Phosphorus
Tensile strength (Mpa)
2.10–2.70 0.60–0.70 0.04–0.05 0.170–0.260
sionless wear rate is determined as .
ω (without unit) =
m LA
(3)
where m is the mass loss, the material density, A the apparent contact area and L is the slip distance. The results indicate that at the higher temperature, lower is the friction coefficient except for PTFE + GF. The variation of friction coefficient is more important with PTFE + C (increase of 20%) and about 10% of variation for pure PTFE and PTFE + C. When temperature increases, PTFE is transferred on cast iron. Indeed, the rigidity of polymers decreases with temperature raising and the molecular chains are released and untangled. For PTFE and PTFE + C, when temperature increases, the formation and rupture of interfacial adhesion bonds are made easier and friction coefficient decreases. For PTFE + GF, glass fibres reduce adhesion bonds. The PTFE matrix wears and the contact becomes between harder glass fibres and cast iron. The consequence is an increase of friction coefficient. Concerning the wear rate (Table 3), it decreases when the temperature increases for pure PTFE and PTFE + C and it remains quasi-constant for PTFE + GF. The PTFE does not sufficiently transfer on cast iron because 115 ◦ C is near its glass transition (109 ◦ C). So PTFE is already rigid. The loss of mass can be explained by formation of a transfer film on the cast iron surface and by wear particles production. With pure PTFE, wear rate is very important because of production of large debris. 3.2. Observations of pin facies and plates (film of transfer) After tests, the worn surfaces of the pins (Fig. 2) and of the plates (Fig. 3) were observed. As is well known, the tribological properties of polymers and their composites in sliding condition against a metal under dry conditions are strongly influenced by their ability to form a transfer film on the counterface [4]. It is essential to study the transfer film in order to understand the wear and friction mechanisms. In this work, the morphologies of transfer films on the plates were examined and analysed by SEM and EDX. Table 3 Means of friction coefficient PTFE
PTFE + GF
PTFE + C
25 ◦ C, 1 h Friction coefficient Wear rate
0.18 9.38E−10
0.14 1.42E−11
0.20 4.76E−10
115 ◦ C, 1 h Friction coefficient Wear rate
0.16 1.19E−11
0.16 2.92E−11
0.16 2.06E−11
C. Kowandy et al. / Wear 265 (2008) 1714–1719
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Fig. 2. Micrographies of pins after test (the arrows indicate the direction of the slip).
Table 4 Composition and qualitative characteristics of wear particles PTFE
PTFE + GF
PTFE + C
PTFE et Fe (inclusions) Chunk and platelet Irregular Rough and curved
PTFE et Fe (lot of inclusions) Chunk and platelet Irregular Rough
PTFE, C et Fe (few inclusions) Chunk Irregular Rough and serrated
PTFE et Fe (inclusions) Ovoid Regular Curved
PTFE et Fe (with chunk of Fe) and some glass fibres Platelets and chunk Irregular Curved
PTFE, C et Fe (few inclusions) Platelet and small slab Regular Smooth
◦
25 C, 1 h
Micrographies of wear particles
Composition Type of particles Outiline shape Edge detail 115 ◦ C, 1 h
Micrographie of wear particles
Composition Type of particles Outiline shape Edge detail
The PTFE pins pure and filled with carbographite present a ploughed surface with some iron inclusions which are more important when the temperature increases. In this case, pin wear is abrasive and wear rate are high with PTFE and PTFE + C. Graphite in cast iron plays the role of hard and abrasive particle. Surfaces of pins filled with glass fibres are not completely ploughed. X-rays detect the presence of iron oxide (Fe2 O3 ) and PTFE on their surfaces. Theses oxides cover surface asperities. They can be attributed to a reaction between the pin and the cast iron plate. This wear is tribochemical one.1 The transfer films are similar with PTFE and PTFE + C. There is a PTFE deposit on hollows of rectification strias. Due to the relative motion, molecular chains are broken and the lamellar PTFE structure is destroyed. Actives groups are formed and they react to surface. Adhesive junctions are created between PTFE and cast iron: a transfer film is formed. For the glass fibres filled pins, an iron oxide coating with some PTFE is formed. The reason is that the glass fibres reduce the delamination of PTFE matrix. So the transfer film
1
Tribochemical reactions: formation of new products due to chemical reactions occurring between mating elements and environment during friction [20].
is limited. However, we cannot see a difference of film of transfer between the two test temperatures. Finally it is difficult to observe transfer film because they are very thin. Currently, other techniques to characterize these films are used like glow discharge optical spectrometry (GDOS). 3.3. Characterisation of the wear particles This study was finally supplemented by qualitative and quantitative characterization of the wear particles. Table 5 Quantitative characteristics of wear particles PTFE 25 ◦ C, 1 h Average equivant diameter (m) Average aspect ratio Average spike parameter 115 ◦ C, 1 h Average equivant diameter (m) Average aspect ratio Average spike parameter
PTFE + GF
PTFE + C
362 2.05 0.53
96 1.71 0.48
227
1850 1.63 0.47
70 1.63 0.51
120 1.66 0.54
.93 0.50
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Fig. 3. Transfer film on cast iron plates.
Table 6 Proposed wear mechanisms PTFE First body pin
Uniform abrasive wear
First body plate
PTFE deposit in the level of asperities in cast iron plate adhesive wear High production of wear particles of important size (Part of transfer film) delamination of transfer film
Third body transfer film and wear particles
Wear particles are small, slightly opaque with black inclusions. Examples of wear particles are given in Table 4. The selected qualitative characteristics are outline shapes and edge detail attributes. Higher is the temperature, softer is the edge detail. The particles are generally platelets or chunks which indicate an insufficient lubrication. These types of wear particles are typically characteristic of abrasive wear. For the PTFE filled with glass fibre pins, wear particles have iron inclusions which indicates the severity of the contact. At 115 ◦ C, glass fibres are found in wear particles because the PTFE matrix is softening and glass fibres are released. For the quantitative analysis, the chosen parameters, because they are most applicable to recovered particles, are the equivalent circle diameter (ED), the aspect ratio and the SP. The aspect ratio is the ratio between length and width. The particles resulting from the PTFE pins have always the highest ED compared to filled PTFEs under the same the experimental conditions (Table 5). The fillers reduce the formation of large wear particles because they prevent the delamination of PTFE. At 115 ◦ C, particles recovered with the PTFE pins may correspond to a part of transfer film. Moreover, when the temperature increases, the particles are less stretched out and thus, circular except for PTFE + C. The carbographite is perhaps responsible of the lengthening of debris. Lastly, with filled PTFE, the SP of wear particles decreases and the edge detail are smoother.
3.4. Proposed wear mechanisms 3.4.1. Pin Abrasive wear characterized by strias is observed on pin surface of PTFE and PTFE + C. For PTFE + GF, wear is less abrasive and there is Fe2 O3 oxide in pin asperities. It is characteristic of a tribochemical and adhesive wear of PTFE + GF pins.
PTFE + C
PTFE + GF Nonuniform wear of pins with presence of Fe2 O3 in pin asperities ´ tribochemical wear + adhesive Weak deposit of PTFE with Fe2 O3 adhesive wear and tribochemical
Weak production of small wear particles abrasive wear of transfer film
3.4.2. Plates Whatever the PTFE composition, there is a deposit of PTFE on plate surface. With PTFE + GF, Fe2 O3 oxide is present and formation of the film of transfer is limited. The wear is only adhesive for PTFE and PTFE + C. In case of PTFE + GF, the wear is adhesive and tribochemical. 3.4.3. Third body Finally, with pure PTFE, due to transfer film on plate, slipping is between PTFE and a layer of PTFE and implies a high production of wear particles in platelet form which facilities the slipping. These wear particles were maybe a part of the transfer film because transfer film is separated and products wear particles. For filled PTFE, production of wear particles, which are small, is lowest. The transfer film can be worn by abrasion. The mechanisms, which we propose, are included in Table 6. 4. Conclusion The results can be summarised in the following way: 1. For all temperatures, the lowest wear was obtained with PTFE + GF pins. 2. At 115 ◦ C the average coefficient of friction is 0.16 whatever the type of pin. At 25 ◦ C, it varies from 0.20 to 0.14. 3. The increase of temperature reduces wear and friction coefficient except for PTFE + FV. 4. For wear particles analysis, ED does not seem directly related to the coefficient of friction. But, the size of the wear particles decreases when there are fillers. 5. The wear particles have rather soft shape attributes. These conclusions permit to propose wear mechanisms:
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