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Al2O3–ZrO2 debris life cycle during wear: effects of the third body on wear and friction
Wear 208 (1997) 161–168
Al2O3–ZrO2 debris life cycle during wear: effects of the third body on wear and friction K. Cherif, B. Gueroult, M. Rigaud U CIREP, Ecole Polytechnique, 8475 Christophe-Colomb, Montreal, PQ, Canada H2M 2N9
Received 22 July 1996; accepted 6 November 1996
Abstract
Wear debris of alumina–zirconia materials generated during wear by abrasion and chipping (ring on disk) were analyzed in order to study their life cycle and their effects on wear and friction. When generated in the contact, wear debris contributed to redistribute the applied load on more contact points and absorbed a part of the energy provided to the system by microcracking and plastic deformation. The result was a decrease in the wear rate and an increase in the shear stress at the interface and, consequently, in the coefficient of friction. Inversely, removing debris from the contact did increase the wear. T.G.A., D.T.A., temperature measurements at the mid-thickness of the disk and debrismorphology observations allowed us to detect some plastic deformation and a small hydration of the debris. Most of the plastic deformation occurred in zirconia particles since they became amorphous during wear by chipping. Hydration remains a minor phenomenon as the temperature is high at the interface. q Elsevier Science S.A. Keywords:
Debris life cycle; Wear; Friction
1. Introduction
The use of ceramics as wear materials is rapidly gaining an interest. However, an understanding of wear mechanisms is necessary to optimize the wear resistance of such materials. Besides the identification and analysis of mechanisms of material removal [1], the evolution of the third body or wear debris has also to be considered since this material can completely modify wear and friction conditions when it is formed. Indeed, tribo-chemical reactions also often occur in the wear of ceramics. In these processes, chemical reactions that might never normally occur, or only occur at very low rates, are enhanced by the high pressure and high temperature conditions that exist at the interface between the surfaces. Thus, alumina can react with moisture in the atmosphere to form aluminium hydroxide [2], and silicon nitride can be oxidized to form silica or can react with moisture to form ammonia [3]. Tribo-chemical reactions can have a beneficial effect if the resultant reaction product is stable, forming a protective layer that separates the two worn surfaces reducing friction and wear. In some cases, even a liquid compound can be formed due to the high temperature at the interface, the incompatibility of the two materials in contact, and the interU
Corresponding author.
action with the ambient atmosphere [4]. Tribo-chemical reactions can also be deleterious if the reaction rate is high and the reaction product is not coherent. In this case, the reaction product breaks up and is easily removed from the wear system thus giving a high wear rate, and the resulting rough surfaces increase the coefficient of friction. An important factor to consider in the wear behavior of any material is the role of the wear debris commonly called the third body. Only a few studies have been concerned with the exact role of debris when it is generated in a wear system. Through this paper, we tried, at first, to describe the debris life cycle in order to be able to estimate either its positive or negative effects. This paper gives details on the alumina–zirconia debris life cycle and discusses its role in the contact on wear and friction. A friction mechanism is also proposed to explain experimental results. The starting point of this paper is the analysis and the microscopic observation of wear debris. Following this, the nature of the interfacial films formed during wear is discussed in terms of the material composition and the wear test conditions. The discussion concludes with a proposition of a friction mechanism and the description of the formation and evolution of debris and its global role on the wear resistance of the tested material.
0043-1648/97/$17.00 q 1997 Published by Elsevier Science S.A. All rights reserved PII S0043-1648(96)07455-8 /97/$17.00 q 1997 Elsevier Science S.A. All rights reserved PII S0043-1648/97/$17.00 q 1997 Published by Elsevier Science S.A. P I I S0043-1648(96)07455-8 (97)9999-Z
All rights reserved Journal: WEA (Wear)
Article: 7455
162
K. Cherif et al. / Wear 208 (1997) 161–168
2. Experimental procedures
The materials tested in this work were alumina–zirconia composites prepared with A16-SG (ALCOA) alumina powder and a partially-stabilized zirconia with 1.5 M Y2O3. Sample preparation and ring-on-disk wear test conditions are given in Table 1: more details on sample processing and wear apparatus are given in Ref. [1]. During the wear tests, atmospheric moisture had an average value of 40% (relative humidity). A 4=4 Latin square design of experiments (Table 1) was followed. This statistical method was used to study the effect of ZrO2 content, sintering time and normal load on the wear resistance; Fig. 1 is a wear map with the two most influential parameters. The wear-rate analysis facilitates conclusions about the importance of microstructure and debris effects [1]. The experimental procedures focused on the methods to analyze wear debris. Debris generated during low-loading tests was not sufficient to be collected in order to be separately analyzed; it was observed and analyzed on the wear scar. The analysis consisted of microstructural observation using a scanning electron microscope and X-ray analysis in order to detect and measure the amount of tetragonal zirconia [5]. The debris generated by high-loading tests was collected and separately analyzed. As for debris generated at low loading, the analyses consisted of microstructural observation and Xray analysis; thermo-gravimetric analyses (T.G.A.) and differential thermic (D.T.A.) analyses were performed on debris to detect any recrystallization and thepossiblepresence of hydration of debris by atmospheric moisture. T.G.A. and Table 1 Configuration of the latin square (A, B and C are the parameters and 1, 2, 3 and 4 their levels) A1: 24% B2: 100 N C1: 1 h A1: 24% B3: 150 N C3: 4 h A1: 24% B4: 200 N C4: 16 h A1: 24% B1: 50 N C2: 2 h
1
5
9
13
A2: 16% B3: 150 N C2: 2 h A2: 16% B2: 100 N C4: 16 h A2: 16% B1: 50 N C3: 4 h A2: 16% B4: 200 N C1: 1 h
2
6
10
14
A3: 8% B4: 200 N C3: 4 h A3: 8% B1: 50 N C1: 1 h A3: 8% B2: 100 N C2: 2 h A3: 8% B3: 150 N C4: 16 h
3
7
11
15
A4: 0% B1: 50 N C4: 16 h A4: 0% B4: 200 N C2: 2 h A4: 0% B3: 150 N C1: 1 h A4: 0% B2: 100 N C3: 4 h
4
Fig. 1. Wear contour map for ZTA for unlubricated contact (0.26 m sy1 and 300 mm2 contact area). The contour lines represent log wear rates (U unit is mm3 my1).
D.T.A. analyses were performed on 50 mg of debris with a heating rate of 10 8C miny1 from room temperature to 1450 8C with a dwell time of 30 min at 1450 8C. The coefficient of friction m is given by the ratio of the tangential force and the normal force applied on the sample. The friction force was measured using strain gages appropriately fixed on the static part of the wear-testing system [1]. The temperature was measured using a K-type thermocouple fixed by an alumina cement in a hole drilled at the midthickness of the disk. The roughness Ra was measured by a profile sensor ‘‘Talysurf model 4’’ using the integratingmode to directly obtain the arithmetic-average value of the crest to valley distances. Table 2 gives hardness, wear rate and the coefficient of friction of the sixteen samples tested in this work.
8
3. Results and discussion 12
3.1. Debris analysis 16
Parameter description Parameters Levels
A Y-PSZ content (%.wt)
B Normal load (N)
C Sintering time (h)
1 2 3 4
24 16 8 0
50 100 150 200
1 2 4 16
Microstructural observations of pure alumina debris are presented in Fig. 2. For high loading ()150 N), a part of the debris, which had a submicrometer size, formed a layer. It displayed a high level of agglomeration and formed a quite rigid paste that cracked when severely deformed[Fig. 2(b)]. For low-loading (-100 N), all debris had a flake shape 0.2 to 0.3 mm thick that indicated high plastic deformation of the debris before removal from the worn surface. This phenomenon has also been observed for other types of wear debris [6]. The fact that all debris possessed the same flake pattern (shape and size) indicated that the main wear mechanism was plastic deformation. However, for chipping wear
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Table 2 Wear rate, coefficient of friction and hardness of the sixteen samples of the latin square design Hs15.1 GPa ms0.45 Us3.29=10y3 mm3 my1 Hs13.6 GPa ms0.6 Us6.23=10y2 mm3 my1 Hs9.9 GPa ms0.6 Us7.75=10y2 mm3 my1 Hs14.5 GPa ms0.4 Us3.9=10y4 mm3 m y1
1
5
9
13
Hs15.2 GPa ms0.45 Us3.33=10y2 mm3 my1 Hs13.9 GPa ms0.5 Us1.37=10y4 mm3 my1 Hs14.6 GPa ms0.5 Us8.2=10y5 mm3 my 1 Hs14.9 GPa ms0.75 Us6.39=10y2 mm3 my1
2
6
10
14
Hs15.6 GPa ms0.85 Us5.28=10y2 mm3 Hs15.4 GPa ms0.4 Us1.68=10y4 mm3 Hs15.4 GPa ms0.35 Us1.91=10y4 mm3 Hs14.1 GPa ms0.2 Us1.25=10y3 mm3
3
my1 7
my1 11
my1 15
my1
Hs14.1 GPa ms0.15 Us5.22=10y4 mm3 my1 Hs16.2 GPa ms0.9 Us6.16=10y2 mm3 my1 Hs16.4 GPa ms0.8 Us5.7=10y2 mm3 m y1 Hs16.7 GPa ms0.15 Us1.58=10y3 mm3 my1
4
8
12
16
H is the Vicker’s hardness, m is the coefficient of friction and U is the wear rate.
Fig. 3. Alumina cleavage under 200 N normal load and 0.26 m sy1 sliding speed.
Fig. 2. Morphology of wear debris (pure alumina samples): (a) 50 N, (b) 200 N.
(high loading), some debris had a size greater than a micron and an irregular shape which indicated that it had been removed by cleavage or by inter-granular cracking as shown in Fig. 3. Some of the debris was directly removed from the contact area while the rest was ground to a size where plastic deformation occurred which needs less energy than microcracking and gave the result shown in Fig. 2(a).
X-ray analysis of high-loading test debris removed from 24% weight of the zirconia sample, showed the complete spectrum of a-alumina while all zirconia characteristic peaks were missing [Fig. 4(a)]. The expected result was a completely monoclinic zirconia because of the high stress applied on the debris in the contact area. The absence of zirconia diffraction peaks was probably due to the high concentration of dislocation in the grains. In order to prove this assumption, a thermo-differential analysis (D.T.A.) and a thermo-gravimetric analysis (T.G.A.) were performed on the debris in order to detect any exothermic reaction or weight loss due to the formation of hydroxides that had been observed during wear of pure alumina [7]. D.T.A. and T.G.A. were then performed on debris removed from pure alumina and from 24% zirconia samples tested under a normal load of 200 N (apparent contact area of 300 mm2). These analysis (Fig. 5) showed that, for pure alumina debris, there was a weight loss between 112 and 229 8C (points a and b) and between 329 and 646 8C (points c and d) of 0.218 and 0.581% of the total weight, respectively. For alumina–zirconia debris, there was a weight loss between 476 and 699 8C (points e and f) of 0.281% of the total weight, and the D.T.A. indicated the presence of an exothermic reac-
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Fig. 4. X-ray diffraction pattern of: (a) 24% ZrO2 debris under 200 N load; (b) 24% ZrO2 debris annealed at 1100 8C for 1 h; (c) pure alumina debris under 200 N. dhkl values are indicated.
Fig. 5. D.T.A. and T.G.A. of: (a) pure alumina debris under 200 N, (b) 24% ZrO2 debris under 200 N.
tion at 942 8C which was probably due to zirconia recrystallization. The weight loss at 476 8C probably indicated the dehydration of boehmite [AlO(OH)] that had been formed by the reaction of alumina debris with atmospheric moisture, while the weight loss at 112 8C corresponded to the removal of the adsorbate water. The low weight losses of debris during the T.G.A. indicated that the amount of hydroxides formed was not enough to allow any detectable X-ray diffraction. The reaction was apparently limited by the high temperature at the interface (low partial pressure of water) which reached an average of 200 8C and a higher temperature at the real contact points. A second X-ray analysis on annealed alumina–zirconia debris at 1100 8C for 1 h was performed in order to be sure that the high concentration of dislocations was the reason for the absence of any X-ray diffraction of the zirconia debris. The result was as expected: a normal spectrum with alumina and zirconia X-ray diffraction peaks [Fig. 4(b)]. However, while the zirconia particles in the matrix were 55% monoclinic, those of the debris were 100% tetragonal which is the unstable phase at room temperature. This condition was due to the size of the zirconia debris which was smaller than the critical size of transformation dc for unconstrained zirconia particles (0.25 mm). The X-ray analysis of pure alumina debris gave a normal diffraction pattern as shown in Fig. 4(c). The scanning electron microscope observations of alumina–zirconia debris showed that they had a very small size (100–150 nm) and were strongly agglomerated [Fig. 6(a)].
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Fig. 6. Morphology of alumina–zirconia wear debris.
They formed spherical agglomerates of about 1–3 mm diameter for alumina and long plate shape for zirconia [Fig. 6(b)]. This difference in shape is probably due to the difference in hardness between the two components.Alumina aggregates were harder and were more resistant to shear stresses, whereas there was an increase in the concentration of dislocations for zirconia leading to a large plastic deformation. 3.2. Debris life cycle
The life cycle of debris represents its behavior from the moment of generation until the particles are eliminated from the contact zone and depends on the applied load and the acting wear mechanism. During abrasive wear, debris was generated by plastic deformation and microcracking and was very small. The particles’ flat shape [Fig. 2(b)] indicates that the plastic deformation was dominant when they were removed from the original material or being ground in the contact. Because of their high reactivity (small size), the alumina debris partially reacted with atmospheric moisture to form a mixture of aluminium hydroxides and alumina. Zirconia debris was transformed to a monoclinic phase under applied stresses as soon
as it was removed from the matrix. The debris agglomerated and gradually formed a continuous layer on the wear scar; the debris decreased the surface porosity, making the wear scar look like a polished surface (the roughness Ra decreased from 1.0 to 0.5 mm). The layer of debris separated the two original surfaces and distributed the load more uniformly, thus avoiding high stress concentration on initial real contact asperities. The debris was removed by cracking and chipping of the debris layer when its thickness increased too much in some areas. However, we were not able to take a sample of the debris in order to analyze it separately. Therefore, microscopic observation and X-ray analysis were performed directly on the wear scar. The X-ray analysis was not very accurate since this layer had numerous gaps and was not thick enough (a few microns). An interfering diffraction from the initial material was also detected. During wear by abrasion, debris had a positive effect since it formed a protective layer separating the two initial surfaces and distributing more homogeneously the stress on the contact. For the high-loading test, the main wear mechanisms were cracking and chipping. The high-wear rates provided enough quantity of debris for different analyses, and the life cycle of this debris is as follows: 1. Debris was initially removed from the material by interor trans-granular cracking [Fig. 2(a)]; the particle size was the same order of magnitude as the grains in the material. 2. Debris was ground in the contact by cleaving to a size where cleaving required more energy than plastic deformation. 3. Zirconia debris, which is softer than alumina fragments, was more plastically deformed. The high temperature enhanced this deformation which increased the concentration of dislocations until no X-ray diffraction pattern was detectable for zirconia. 4. Alumina debris partially reacted with atmospheric moisture to form hydroxides (weight loss during T.G.A. analysis). This debris agglomerated in a spherical shapewhich had a bearing effect in the contact. 5. Finally, debris was removed from the contact area while fresh debris formed. During chipping wear, debris had both positive and negative effects: zirconia, by its plastic deformation, absorbed a large part of the total energy provided to the system which decreased the remaining energy for cracking and debris formation. Alumina, by its spherical shape, distributed the stress in the same way as bearings. The bearing effect was experimentally observed during high-loading tests; wear rates for the initial 5 to 10 min were very high and then decreased to lower and constant levels during the rest of the test (after enough debris was formed). The starting wear rates were about three times higher than the steady-state wear rates. Before debris was formed, the load was localized on few contact points and the load threshold for cracking and chipping was easily reached. Once debris was formed, the number of contact points increased and the local stresses decreased
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since the total applied load was kept constant. However, freshly formed alumina debris had irregular and sharp shapes and acted as free abrasive particles. The global effect of debris seemed to be positive in regard to the wear resistance of a given material for high-loading wear. 3.3. Coefficient of friction and thermal effects
Values of friction coefficients m are listed in Table 2; they vary with both normal loading and zirconia content. For each ZrO2 composition, the coefficients of friction for abrasive wear were smaller than for chipping wear characterized by a high rate of debris formation. The difference between the coefficients of friction of the two wear mechanisms decreased while the content of ZrO2 increased (Fig. 7). During friction, a heating effect takes place at the interface; this phenomenon has to be considered for the analysis of the values of coefficient of friction. The heating effect was detected by measuring the mid-thickness temperature of the disk (the average temperature at the interface was about20 8C higher) while the normal load was varied during the test. Fig. 8 represents the variation of the temperature T with the tangential force ft. The highest temperature (200 8C) recorded indicates that flash temperatures at the contact point were probably a few hundred degrees higher as indicated by Ref. [8].
Fig. 7. Variation of the coefficient of friction with the ZrO2 contentdepending on the normal load.
Fig. 8. Mid-thickness temperature variation with the force of friction.
Fig. 9. Mechanisms of the interaction between surface asperities for ceramic materials.
In order to explain the variation of the coefficient of friction with the normal load, we suppose that the interaction between asperities occurred as represented in Fig. 9. Then, the two main interaction phenomena are micro-fracture and localized plastic deformation of one of the two asperities so that the relative motion is not disturbed. The crystallographic orientation (slip systems) of the asperity relative to the sliding direction determines whether fracture or plastic deformation take place (the mechanism which requires minimum energy). Because of the high number of asperities, the two phenomena occur either more or less frequently during friction, which involves a certain tangential force. In the friction model proposed, we have not considered adhesion and the shear stress that is involved during sliding since this kind of interaction is nearly absent due to the type of chemical bonding of ceramics (covalent/ionic) compared to metals. According to the proposed mechanisms for interaction between asperities, the intensity of shear stresses depends on the mechanical properties of the interface and the roughness of the surfaces in contact. When the roughness increases, the energy for asperities fracture or plastic deformation increases as does the coefficient of friction. Experimentally, the roughness Ra, measured by a profile sensor after the test, varied from 0.5"0.1 mm for low loads to 1.7"0.1 mm for high loads (the initial roughness was 1.0"0.1 mm). At low loads, plastic deformation is limited and the heating effect is low, so the adsorbed moisture is not completely eliminated from the surface. This moisture reacts with the matrix to form hydroxides which weaken the surfacemechanical properties so the shear stress at the interface is decreased. Any debris formed has a very small particle size and easily clogs the surface porosity, decreasing the roughness and the coefficient of friction. For this experiment, the friction mechanism of ZTA at low loads is described in Fig. 10. At high normal loads, the friction mechanism is different since plastic deformation is more important and chipping takes place above the cracking threshold. The heating effect limits the moisture adsorption and the formation of hydrox-
Fig. 10. Friction mechanism for low loading conditions.
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Fig. 11. Friction mechanism for high loading conditions.
ides. Chipping produces debris with greater particle size than abrasion and the worn surface is rougher. The high roughness and the absence of hydroxide cause an increase in m. Another important factor increasing m is the energy required for grinding and plastic deformation of debris in the contact zone before its elimination; the plastic deformation is enhanced as temperature increases. For this experiment, the friction mechanism of ZTA at high loads is described in Fig. 11. The variation of m with the zirconia content can now be explained as follows. Since zirconia is softer than alumina, the stress required to initiate plastic deformation of zirconia is lower than that for alumina, and below a certain load, there is only plastic deformation of zirconia particles. Then, for low loads (low energy provided to the system), the higher the amount of zirconia in the matrix, the more important was plastic deformation. The presence of zirconia increased the shear stresses in the contact zone and the coefficient offriction compared to that for pure alumina (limited plastic deformation). However, for high loads, the stress level around the contact areas was high enough to deform plastically both zirconia and alumina. Since the energy consumed by plastic deformation for zirconia was lower than for alumina, the total energy consumed by plastic deformation decreased when the amount of zirconia increased. The result was a lower coefficient of friction compared to that of pure alumina for the same loading conditions. The final result was a lower variation of m for high and low loads when the content of zirconia in the matrix increased. An energy balance of the wear system can be expressed by the following equation: EtotsEplastqEmcqQsFtPV
where Eplast is the energy consumed by plastic deformation, Emc is the energy consumed by microcracking, Q is the dissipated heat, Ft is the force of friction and V is the sliding
speed. The linear variation of the interface temperature T with the force of friction Ft, as observed in Fig. 8, indicates that the value of Q is also proportional to the total energy provided to the system Etot: QsnPEtot with 0-n-1 The surface-temperature estimation can be performed theoretically by supposing that the thermal energy is dissipated at the real contact points [9–11]. However, we have to know the fraction ‘‘n’’ of energy dissipated as heat and the real contact area which is very difficult to measure particularly when there is a sliding motion and wear debris continuously
changing the contact configuration. Methods proposed for calculation of flash temperature [10,11] assume that the total area of contact is determined by plastic deformation. However, the asperity contact is not always plastic for ceramics as it is for metals, microcracking can also occur depending on the applied load. Therefore, even with mathematical modeling of the friction and wear by plastic deformation or chipping, the real contact area which dictates the real contact stresses remains the most difficult value to measure. The real contact area can be measured for transparent or conducting materials, but is not applicable in our case. The presence of debris is also an important parameter to consider for the determination of the surface stresses. 4. Conclusions
The analysis of the debris life cycle offers an understanding of the friction mechanisms for ceramics. The interaction between surface asperities leads either to microcracking or plastic deformation which was clearly observed during the high-loading tests. Debris appeared to promote wear resistance when the normal load exceeded the cracking threshold of the material. The debris uniformly redistributed the contact stresses and consumed a part of the energy provided to the system by plastic deformation. Plastic deformation has an important effect on the coefficient of friction. Even if adhesion is not present for ceramics, as it is in metals, plastic deformation is present and has a major impact on friction and heating effects for ceramic materials. Acknowledgements
This research has been financially supported by the Natural Science and Engineering Research Council of Canada (OGP3520). References
[1] K. Cherif, B. Gueroult, M. Rigaud, Wear behavior of alumina toughened zirconia materials, Wear 199 (1996) 113–121. [2] A.J. Perez-Unzueta, J.H. Beynon, M.G. Gee, The effect of surrounding atmosphere on the sliding wear of alumina, Wear 146 (1991) 179. [3] T.E. Fischer, H. Tomizawa, Interaction of tribochemistry and microfracture in the friction and wear of silicon nitride, Wear 105 (1985) 29–45. [4] A.V. Levy, N. Jee, Unlubricated sliding wear of ceramic materials, Wear 121 (1988) 363–380. [5] P.A. Evans, R. Stevens, J.G.P. Binner, Quantitative X-ray diffraction analysis of polymorphic mixes of pure zirconia, British Ceramic Transactions Journal 83 (1984) 39–43.
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[6] I.A. Cutter, R. McPherson, The structure of MgO abrasive wear debris, Wear 71 (1981) 255–258. [7] M.G. Gee, The formation of aluminium hydroxide in the sliding wear of alumina, Wear 153 (1992) 201–227. [8] A. Blomberg, S. Hogmark, J. Lu, An electron microscopy study of worn ceramic surfaces, Tribology International 26 (6) (1993) 369– 381.
[9] J. Halling, Principles of Tribology, J. Halling (Ed.), Macmillan, London, 1975, pp. 67–71. [10] M.F. Ashby, J. Abulawi, H.S. Kong, Temperature maps of frictional heating in dry sliding, STLE Tribology Transactions 34 (4) (1991) 577–587. [11] E. Rabinowicz, Friction and wear of materials, Wiley, New York,1995, pp. 96–100.
Journal: WEA (Wear)
Article: 7455
Al2O3–ZrO2 debris life cycle during wear: effects of the third body on wear and friction K. Cherif, B. Gueroult, M. Rigaud U CIREP, Ecole Polytechnique, 8475 Christophe-Colomb, Montreal, PQ, Canada H2M 2N9
Received 22 July 1996; accepted 6 November 1996
Abstract
Wear debris of alumina–zirconia materials generated during wear by abrasion and chipping (ring on disk) were analyzed in order to study their life cycle and their effects on wear and friction. When generated in the contact, wear debris contributed to redistribute the applied load on more contact points and absorbed a part of the energy provided to the system by microcracking and plastic deformation. The result was a decrease in the wear rate and an increase in the shear stress at the interface and, consequently, in the coefficient of friction. Inversely, removing debris from the contact did increase the wear. T.G.A., D.T.A., temperature measurements at the mid-thickness of the disk and debrismorphology observations allowed us to detect some plastic deformation and a small hydration of the debris. Most of the plastic deformation occurred in zirconia particles since they became amorphous during wear by chipping. Hydration remains a minor phenomenon as the temperature is high at the interface. q Elsevier Science S.A. Keywords:
Debris life cycle; Wear; Friction
1. Introduction
The use of ceramics as wear materials is rapidly gaining an interest. However, an understanding of wear mechanisms is necessary to optimize the wear resistance of such materials. Besides the identification and analysis of mechanisms of material removal [1], the evolution of the third body or wear debris has also to be considered since this material can completely modify wear and friction conditions when it is formed. Indeed, tribo-chemical reactions also often occur in the wear of ceramics. In these processes, chemical reactions that might never normally occur, or only occur at very low rates, are enhanced by the high pressure and high temperature conditions that exist at the interface between the surfaces. Thus, alumina can react with moisture in the atmosphere to form aluminium hydroxide [2], and silicon nitride can be oxidized to form silica or can react with moisture to form ammonia [3]. Tribo-chemical reactions can have a beneficial effect if the resultant reaction product is stable, forming a protective layer that separates the two worn surfaces reducing friction and wear. In some cases, even a liquid compound can be formed due to the high temperature at the interface, the incompatibility of the two materials in contact, and the interU
Corresponding author.
action with the ambient atmosphere [4]. Tribo-chemical reactions can also be deleterious if the reaction rate is high and the reaction product is not coherent. In this case, the reaction product breaks up and is easily removed from the wear system thus giving a high wear rate, and the resulting rough surfaces increase the coefficient of friction. An important factor to consider in the wear behavior of any material is the role of the wear debris commonly called the third body. Only a few studies have been concerned with the exact role of debris when it is generated in a wear system. Through this paper, we tried, at first, to describe the debris life cycle in order to be able to estimate either its positive or negative effects. This paper gives details on the alumina–zirconia debris life cycle and discusses its role in the contact on wear and friction. A friction mechanism is also proposed to explain experimental results. The starting point of this paper is the analysis and the microscopic observation of wear debris. Following this, the nature of the interfacial films formed during wear is discussed in terms of the material composition and the wear test conditions. The discussion concludes with a proposition of a friction mechanism and the description of the formation and evolution of debris and its global role on the wear resistance of the tested material.
0043-1648/97/$17.00 q 1997 Published by Elsevier Science S.A. All rights reserved PII S0043-1648(96)07455-8 /97/$17.00 q 1997 Elsevier Science S.A. All rights reserved PII S0043-1648/97/$17.00 q 1997 Published by Elsevier Science S.A. P I I S0043-1648(96)07455-8 (97)9999-Z
All rights reserved Journal: WEA (Wear)
Article: 7455
162
K. Cherif et al. / Wear 208 (1997) 161–168
2. Experimental procedures
The materials tested in this work were alumina–zirconia composites prepared with A16-SG (ALCOA) alumina powder and a partially-stabilized zirconia with 1.5 M Y2O3. Sample preparation and ring-on-disk wear test conditions are given in Table 1: more details on sample processing and wear apparatus are given in Ref. [1]. During the wear tests, atmospheric moisture had an average value of 40% (relative humidity). A 4=4 Latin square design of experiments (Table 1) was followed. This statistical method was used to study the effect of ZrO2 content, sintering time and normal load on the wear resistance; Fig. 1 is a wear map with the two most influential parameters. The wear-rate analysis facilitates conclusions about the importance of microstructure and debris effects [1]. The experimental procedures focused on the methods to analyze wear debris. Debris generated during low-loading tests was not sufficient to be collected in order to be separately analyzed; it was observed and analyzed on the wear scar. The analysis consisted of microstructural observation using a scanning electron microscope and X-ray analysis in order to detect and measure the amount of tetragonal zirconia [5]. The debris generated by high-loading tests was collected and separately analyzed. As for debris generated at low loading, the analyses consisted of microstructural observation and Xray analysis; thermo-gravimetric analyses (T.G.A.) and differential thermic (D.T.A.) analyses were performed on debris to detect any recrystallization and thepossiblepresence of hydration of debris by atmospheric moisture. T.G.A. and Table 1 Configuration of the latin square (A, B and C are the parameters and 1, 2, 3 and 4 their levels) A1: 24% B2: 100 N C1: 1 h A1: 24% B3: 150 N C3: 4 h A1: 24% B4: 200 N C4: 16 h A1: 24% B1: 50 N C2: 2 h
1
5
9
13
A2: 16% B3: 150 N C2: 2 h A2: 16% B2: 100 N C4: 16 h A2: 16% B1: 50 N C3: 4 h A2: 16% B4: 200 N C1: 1 h
2
6
10
14
A3: 8% B4: 200 N C3: 4 h A3: 8% B1: 50 N C1: 1 h A3: 8% B2: 100 N C2: 2 h A3: 8% B3: 150 N C4: 16 h
3
7
11
15
A4: 0% B1: 50 N C4: 16 h A4: 0% B4: 200 N C2: 2 h A4: 0% B3: 150 N C1: 1 h A4: 0% B2: 100 N C3: 4 h
4
Fig. 1. Wear contour map for ZTA for unlubricated contact (0.26 m sy1 and 300 mm2 contact area). The contour lines represent log wear rates (U unit is mm3 my1).
D.T.A. analyses were performed on 50 mg of debris with a heating rate of 10 8C miny1 from room temperature to 1450 8C with a dwell time of 30 min at 1450 8C. The coefficient of friction m is given by the ratio of the tangential force and the normal force applied on the sample. The friction force was measured using strain gages appropriately fixed on the static part of the wear-testing system [1]. The temperature was measured using a K-type thermocouple fixed by an alumina cement in a hole drilled at the midthickness of the disk. The roughness Ra was measured by a profile sensor ‘‘Talysurf model 4’’ using the integratingmode to directly obtain the arithmetic-average value of the crest to valley distances. Table 2 gives hardness, wear rate and the coefficient of friction of the sixteen samples tested in this work.
8
3. Results and discussion 12
3.1. Debris analysis 16
Parameter description Parameters Levels
A Y-PSZ content (%.wt)
B Normal load (N)
C Sintering time (h)
1 2 3 4
24 16 8 0
50 100 150 200
1 2 4 16
Microstructural observations of pure alumina debris are presented in Fig. 2. For high loading ()150 N), a part of the debris, which had a submicrometer size, formed a layer. It displayed a high level of agglomeration and formed a quite rigid paste that cracked when severely deformed[Fig. 2(b)]. For low-loading (-100 N), all debris had a flake shape 0.2 to 0.3 mm thick that indicated high plastic deformation of the debris before removal from the worn surface. This phenomenon has also been observed for other types of wear debris [6]. The fact that all debris possessed the same flake pattern (shape and size) indicated that the main wear mechanism was plastic deformation. However, for chipping wear
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Table 2 Wear rate, coefficient of friction and hardness of the sixteen samples of the latin square design Hs15.1 GPa ms0.45 Us3.29=10y3 mm3 my1 Hs13.6 GPa ms0.6 Us6.23=10y2 mm3 my1 Hs9.9 GPa ms0.6 Us7.75=10y2 mm3 my1 Hs14.5 GPa ms0.4 Us3.9=10y4 mm3 m y1
1
5
9
13
Hs15.2 GPa ms0.45 Us3.33=10y2 mm3 my1 Hs13.9 GPa ms0.5 Us1.37=10y4 mm3 my1 Hs14.6 GPa ms0.5 Us8.2=10y5 mm3 my 1 Hs14.9 GPa ms0.75 Us6.39=10y2 mm3 my1
2
6
10
14
Hs15.6 GPa ms0.85 Us5.28=10y2 mm3 Hs15.4 GPa ms0.4 Us1.68=10y4 mm3 Hs15.4 GPa ms0.35 Us1.91=10y4 mm3 Hs14.1 GPa ms0.2 Us1.25=10y3 mm3
3
my1 7
my1 11
my1 15
my1
Hs14.1 GPa ms0.15 Us5.22=10y4 mm3 my1 Hs16.2 GPa ms0.9 Us6.16=10y2 mm3 my1 Hs16.4 GPa ms0.8 Us5.7=10y2 mm3 m y1 Hs16.7 GPa ms0.15 Us1.58=10y3 mm3 my1
4
8
12
16
H is the Vicker’s hardness, m is the coefficient of friction and U is the wear rate.
Fig. 3. Alumina cleavage under 200 N normal load and 0.26 m sy1 sliding speed.
Fig. 2. Morphology of wear debris (pure alumina samples): (a) 50 N, (b) 200 N.
(high loading), some debris had a size greater than a micron and an irregular shape which indicated that it had been removed by cleavage or by inter-granular cracking as shown in Fig. 3. Some of the debris was directly removed from the contact area while the rest was ground to a size where plastic deformation occurred which needs less energy than microcracking and gave the result shown in Fig. 2(a).
X-ray analysis of high-loading test debris removed from 24% weight of the zirconia sample, showed the complete spectrum of a-alumina while all zirconia characteristic peaks were missing [Fig. 4(a)]. The expected result was a completely monoclinic zirconia because of the high stress applied on the debris in the contact area. The absence of zirconia diffraction peaks was probably due to the high concentration of dislocation in the grains. In order to prove this assumption, a thermo-differential analysis (D.T.A.) and a thermo-gravimetric analysis (T.G.A.) were performed on the debris in order to detect any exothermic reaction or weight loss due to the formation of hydroxides that had been observed during wear of pure alumina [7]. D.T.A. and T.G.A. were then performed on debris removed from pure alumina and from 24% zirconia samples tested under a normal load of 200 N (apparent contact area of 300 mm2). These analysis (Fig. 5) showed that, for pure alumina debris, there was a weight loss between 112 and 229 8C (points a and b) and between 329 and 646 8C (points c and d) of 0.218 and 0.581% of the total weight, respectively. For alumina–zirconia debris, there was a weight loss between 476 and 699 8C (points e and f) of 0.281% of the total weight, and the D.T.A. indicated the presence of an exothermic reac-
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Fig. 4. X-ray diffraction pattern of: (a) 24% ZrO2 debris under 200 N load; (b) 24% ZrO2 debris annealed at 1100 8C for 1 h; (c) pure alumina debris under 200 N. dhkl values are indicated.
Fig. 5. D.T.A. and T.G.A. of: (a) pure alumina debris under 200 N, (b) 24% ZrO2 debris under 200 N.
tion at 942 8C which was probably due to zirconia recrystallization. The weight loss at 476 8C probably indicated the dehydration of boehmite [AlO(OH)] that had been formed by the reaction of alumina debris with atmospheric moisture, while the weight loss at 112 8C corresponded to the removal of the adsorbate water. The low weight losses of debris during the T.G.A. indicated that the amount of hydroxides formed was not enough to allow any detectable X-ray diffraction. The reaction was apparently limited by the high temperature at the interface (low partial pressure of water) which reached an average of 200 8C and a higher temperature at the real contact points. A second X-ray analysis on annealed alumina–zirconia debris at 1100 8C for 1 h was performed in order to be sure that the high concentration of dislocations was the reason for the absence of any X-ray diffraction of the zirconia debris. The result was as expected: a normal spectrum with alumina and zirconia X-ray diffraction peaks [Fig. 4(b)]. However, while the zirconia particles in the matrix were 55% monoclinic, those of the debris were 100% tetragonal which is the unstable phase at room temperature. This condition was due to the size of the zirconia debris which was smaller than the critical size of transformation dc for unconstrained zirconia particles (0.25 mm). The X-ray analysis of pure alumina debris gave a normal diffraction pattern as shown in Fig. 4(c). The scanning electron microscope observations of alumina–zirconia debris showed that they had a very small size (100–150 nm) and were strongly agglomerated [Fig. 6(a)].
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Fig. 6. Morphology of alumina–zirconia wear debris.
They formed spherical agglomerates of about 1–3 mm diameter for alumina and long plate shape for zirconia [Fig. 6(b)]. This difference in shape is probably due to the difference in hardness between the two components.Alumina aggregates were harder and were more resistant to shear stresses, whereas there was an increase in the concentration of dislocations for zirconia leading to a large plastic deformation. 3.2. Debris life cycle
The life cycle of debris represents its behavior from the moment of generation until the particles are eliminated from the contact zone and depends on the applied load and the acting wear mechanism. During abrasive wear, debris was generated by plastic deformation and microcracking and was very small. The particles’ flat shape [Fig. 2(b)] indicates that the plastic deformation was dominant when they were removed from the original material or being ground in the contact. Because of their high reactivity (small size), the alumina debris partially reacted with atmospheric moisture to form a mixture of aluminium hydroxides and alumina. Zirconia debris was transformed to a monoclinic phase under applied stresses as soon
as it was removed from the matrix. The debris agglomerated and gradually formed a continuous layer on the wear scar; the debris decreased the surface porosity, making the wear scar look like a polished surface (the roughness Ra decreased from 1.0 to 0.5 mm). The layer of debris separated the two original surfaces and distributed the load more uniformly, thus avoiding high stress concentration on initial real contact asperities. The debris was removed by cracking and chipping of the debris layer when its thickness increased too much in some areas. However, we were not able to take a sample of the debris in order to analyze it separately. Therefore, microscopic observation and X-ray analysis were performed directly on the wear scar. The X-ray analysis was not very accurate since this layer had numerous gaps and was not thick enough (a few microns). An interfering diffraction from the initial material was also detected. During wear by abrasion, debris had a positive effect since it formed a protective layer separating the two initial surfaces and distributing more homogeneously the stress on the contact. For the high-loading test, the main wear mechanisms were cracking and chipping. The high-wear rates provided enough quantity of debris for different analyses, and the life cycle of this debris is as follows: 1. Debris was initially removed from the material by interor trans-granular cracking [Fig. 2(a)]; the particle size was the same order of magnitude as the grains in the material. 2. Debris was ground in the contact by cleaving to a size where cleaving required more energy than plastic deformation. 3. Zirconia debris, which is softer than alumina fragments, was more plastically deformed. The high temperature enhanced this deformation which increased the concentration of dislocations until no X-ray diffraction pattern was detectable for zirconia. 4. Alumina debris partially reacted with atmospheric moisture to form hydroxides (weight loss during T.G.A. analysis). This debris agglomerated in a spherical shapewhich had a bearing effect in the contact. 5. Finally, debris was removed from the contact area while fresh debris formed. During chipping wear, debris had both positive and negative effects: zirconia, by its plastic deformation, absorbed a large part of the total energy provided to the system which decreased the remaining energy for cracking and debris formation. Alumina, by its spherical shape, distributed the stress in the same way as bearings. The bearing effect was experimentally observed during high-loading tests; wear rates for the initial 5 to 10 min were very high and then decreased to lower and constant levels during the rest of the test (after enough debris was formed). The starting wear rates were about three times higher than the steady-state wear rates. Before debris was formed, the load was localized on few contact points and the load threshold for cracking and chipping was easily reached. Once debris was formed, the number of contact points increased and the local stresses decreased
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since the total applied load was kept constant. However, freshly formed alumina debris had irregular and sharp shapes and acted as free abrasive particles. The global effect of debris seemed to be positive in regard to the wear resistance of a given material for high-loading wear. 3.3. Coefficient of friction and thermal effects
Values of friction coefficients m are listed in Table 2; they vary with both normal loading and zirconia content. For each ZrO2 composition, the coefficients of friction for abrasive wear were smaller than for chipping wear characterized by a high rate of debris formation. The difference between the coefficients of friction of the two wear mechanisms decreased while the content of ZrO2 increased (Fig. 7). During friction, a heating effect takes place at the interface; this phenomenon has to be considered for the analysis of the values of coefficient of friction. The heating effect was detected by measuring the mid-thickness temperature of the disk (the average temperature at the interface was about20 8C higher) while the normal load was varied during the test. Fig. 8 represents the variation of the temperature T with the tangential force ft. The highest temperature (200 8C) recorded indicates that flash temperatures at the contact point were probably a few hundred degrees higher as indicated by Ref. [8].
Fig. 7. Variation of the coefficient of friction with the ZrO2 contentdepending on the normal load.
Fig. 8. Mid-thickness temperature variation with the force of friction.
Fig. 9. Mechanisms of the interaction between surface asperities for ceramic materials.
In order to explain the variation of the coefficient of friction with the normal load, we suppose that the interaction between asperities occurred as represented in Fig. 9. Then, the two main interaction phenomena are micro-fracture and localized plastic deformation of one of the two asperities so that the relative motion is not disturbed. The crystallographic orientation (slip systems) of the asperity relative to the sliding direction determines whether fracture or plastic deformation take place (the mechanism which requires minimum energy). Because of the high number of asperities, the two phenomena occur either more or less frequently during friction, which involves a certain tangential force. In the friction model proposed, we have not considered adhesion and the shear stress that is involved during sliding since this kind of interaction is nearly absent due to the type of chemical bonding of ceramics (covalent/ionic) compared to metals. According to the proposed mechanisms for interaction between asperities, the intensity of shear stresses depends on the mechanical properties of the interface and the roughness of the surfaces in contact. When the roughness increases, the energy for asperities fracture or plastic deformation increases as does the coefficient of friction. Experimentally, the roughness Ra, measured by a profile sensor after the test, varied from 0.5"0.1 mm for low loads to 1.7"0.1 mm for high loads (the initial roughness was 1.0"0.1 mm). At low loads, plastic deformation is limited and the heating effect is low, so the adsorbed moisture is not completely eliminated from the surface. This moisture reacts with the matrix to form hydroxides which weaken the surfacemechanical properties so the shear stress at the interface is decreased. Any debris formed has a very small particle size and easily clogs the surface porosity, decreasing the roughness and the coefficient of friction. For this experiment, the friction mechanism of ZTA at low loads is described in Fig. 10. At high normal loads, the friction mechanism is different since plastic deformation is more important and chipping takes place above the cracking threshold. The heating effect limits the moisture adsorption and the formation of hydrox-
Fig. 10. Friction mechanism for low loading conditions.
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Fig. 11. Friction mechanism for high loading conditions.
ides. Chipping produces debris with greater particle size than abrasion and the worn surface is rougher. The high roughness and the absence of hydroxide cause an increase in m. Another important factor increasing m is the energy required for grinding and plastic deformation of debris in the contact zone before its elimination; the plastic deformation is enhanced as temperature increases. For this experiment, the friction mechanism of ZTA at high loads is described in Fig. 11. The variation of m with the zirconia content can now be explained as follows. Since zirconia is softer than alumina, the stress required to initiate plastic deformation of zirconia is lower than that for alumina, and below a certain load, there is only plastic deformation of zirconia particles. Then, for low loads (low energy provided to the system), the higher the amount of zirconia in the matrix, the more important was plastic deformation. The presence of zirconia increased the shear stresses in the contact zone and the coefficient offriction compared to that for pure alumina (limited plastic deformation). However, for high loads, the stress level around the contact areas was high enough to deform plastically both zirconia and alumina. Since the energy consumed by plastic deformation for zirconia was lower than for alumina, the total energy consumed by plastic deformation decreased when the amount of zirconia increased. The result was a lower coefficient of friction compared to that of pure alumina for the same loading conditions. The final result was a lower variation of m for high and low loads when the content of zirconia in the matrix increased. An energy balance of the wear system can be expressed by the following equation: EtotsEplastqEmcqQsFtPV
where Eplast is the energy consumed by plastic deformation, Emc is the energy consumed by microcracking, Q is the dissipated heat, Ft is the force of friction and V is the sliding
speed. The linear variation of the interface temperature T with the force of friction Ft, as observed in Fig. 8, indicates that the value of Q is also proportional to the total energy provided to the system Etot: QsnPEtot with 0-n-1 The surface-temperature estimation can be performed theoretically by supposing that the thermal energy is dissipated at the real contact points [9–11]. However, we have to know the fraction ‘‘n’’ of energy dissipated as heat and the real contact area which is very difficult to measure particularly when there is a sliding motion and wear debris continuously
changing the contact configuration. Methods proposed for calculation of flash temperature [10,11] assume that the total area of contact is determined by plastic deformation. However, the asperity contact is not always plastic for ceramics as it is for metals, microcracking can also occur depending on the applied load. Therefore, even with mathematical modeling of the friction and wear by plastic deformation or chipping, the real contact area which dictates the real contact stresses remains the most difficult value to measure. The real contact area can be measured for transparent or conducting materials, but is not applicable in our case. The presence of debris is also an important parameter to consider for the determination of the surface stresses. 4. Conclusions
The analysis of the debris life cycle offers an understanding of the friction mechanisms for ceramics. The interaction between surface asperities leads either to microcracking or plastic deformation which was clearly observed during the high-loading tests. Debris appeared to promote wear resistance when the normal load exceeded the cracking threshold of the material. The debris uniformly redistributed the contact stresses and consumed a part of the energy provided to the system by plastic deformation. Plastic deformation has an important effect on the coefficient of friction. Even if adhesion is not present for ceramics, as it is in metals, plastic deformation is present and has a major impact on friction and heating effects for ceramic materials. Acknowledgements
This research has been financially supported by the Natural Science and Engineering Research Council of Canada (OGP3520). References
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