- Email: [email protected]
SiCp composite
Materials Science and Engineering A293 (2000) 146 – 156 www.elsevier.com/locate/msea
Influence of porosity on dry sliding wear behavior in spray deposited Al–6Cu–Mn/SiCp composite Manchang Gui a,b, Suk Bong Kang b,*, Jung Moo Lee b a
National Laboratory of Ad6anced Composites, Beijing Institute of Aeronautical Materials, Beijing 100095, China b Korea Institute of Machinery and Materials, 66 Sangnam, Changwon, Kyungnam 641 -010, South Korea Received 26 July 1999; received in revised form 23 May 2000
Abstract Dry sliding wear behavior of spray deposited Al– 6Cu – Mn/13 vol.% SiCp composites in both as-sprayed and forged states has been studied. Wear mechanisms in different wear regions and the influence of porosity on wear behavior are discussed. Within the applied load range of 5–400 N (corresponding normal stress is 0.1 – 8 MPa), the variation of wear rate with applied load in the as-sprayed composite can be divided into three wear regions, while the as-forged composite four wear regions. A transition load from mild to severe wear could be observed with increasing load. Severe wear was controlled by adhesion mechanism. Mild wear was associated with three different mechanisms: oxidation, delamination and subsurface-cracking assisted adhesion wear. The wear resistance of the as-sprayed composite was similar to that of the as-forged composite at lower loads. However, at higher loads, the as-sprayed composite had a higher wear rate and small transition load, therefore, exhibited inferior wear resistance than the as-forged composite. At lower loads, pores beneath the worn surface of the as-sprayed composite were stable and could not propagate significantly. Therefore, porosity displayed a very small influence on dry sliding wear behavior of the composite. At higher loads, the pores beneath the worn surface became unstable and the cracks originated from these pores could propagate during wear process, resulting in a higher wear rate and smaller transition load. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Sliding wear; Porosity; Metal matrix composite; Wear mechanism; Particle reinforcement
1. Introduction The wear behavior of aluminum matrix composites has, because of their potential tribological applications, been extensively studied during the last 20 years [1–12]. Usually, aluminum matrix composites reinforced with SiC, Al2O3, TiC, TiB2, etc. particles by comparison to monolithic materials display significantly improved wear resistance under abrasive [13,14], lubricated [15,16] and dry sliding conditions [1,2,4,17,18]. The principal factors that influence the sliding wear of the composites include: extrinsic factors such as sliding distance, sliding velocity, applied load, atmosphere temperature, etc. and intrinsic factors such as reinforcement type, shape, size, volume fraction and distribution, interface between reinforcement and ma* Corresponding author. Tel.: +82-551-2803301; fax: + 82-5512803399. E-mail address: [email protected] (S.B. Kang).
trix, matrix material, heat treatment, porosity, etc. The intrinsic factors can be regulated and controlled in the process of material manufacture. It is probable to optimize the wear resistance of the composites by the intrinsic factors. Therefore, the investigations about the influences of intrinsic factors on the wear behavior have received extensive attention. Among them, the intrinsic factors to have been studied are mainly reinforcement size, type and volume fraction. The work in other aspects is limited. Bhansali et al. [19] reported that the 2024/20SiCp composites display an inferior wear resistance than 2024 monolithic alloy when there exists brittle interface between SiC and aluminum causing interfacial reaction of Al4C3. Wang et al. [2] also showed that Ti50Ni25Cu25 particle-reinforced aluminum matrix composite with interfacial reactions exhibits lower wear resistance than the composite without interfacial reaction. Lee et al. [17] studied the effect of sintered porosity in powder metallurgy (PM) 6061 monolithic alloy and 6061/SiCp composites on dry
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 0 5 2 - 2
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156 Table 1 Compositions of Al–6Cu–Mn matrix alloy Element wt.%
Cu 6.07
Mn 1.68
Mg 0.47
Ti 0.24
Zr 0.55
V 0.19
Al Bal.
abrasive wear, indicating that the wear rate of 6061 alloy increases drastically with increasing porosity, while for its composites, the porosity effect is not significant. Spray deposition process is one of several routes to produce particle-reinforced aluminum matrix composites. The composites produced by this process possess fine grain size and uniform particle distribution and reaction between reinforcement particle and aluminum matrix can be avoided because of the short contact time between the melt and the reinforcement. Therefore, the composites produced by spray deposition process possess potential superior mechanical properties. However, studies on the wear behavior of the composites produced by spray deposition process have so far been limited. In addition, similar to PM composites, assprayed composites contain some porosity. It is also worth studying the influence of porosity on sliding wear behavior of the composites. Al–Cu alloys based SiCp-reinforced composites produced by novel manufacturing techniques possess higher strength and stability at elevated temperatures. They often have considerably potential applications in the aircraft-related field where sometimes wear can be quite significant (such as bearing sleeves, valves, etc.). The main objectives of this work are to study the dry sliding wear behavior of a sprayed deposited Al–6Cu– Mn/13 vol.% SiCp composite and to evaluate the effect of porosity on the wear behavior of the composite.
2. Experimental procedures Al–6Cu–Mn alloy based composite reinforced with 13 vol.% SiCp was produced by spray deposition process at Alusuisse-Lonza Services Ltd. in Switzerland. The compositions of the matrix alloy are shown in Table 1. The average size of the SiC particles is 12 mm. To evaluate the influence of porosity on the sliding wear behavior, two state composites, as-sprayed and forged (referred to as as-forged composite), were used. Porosity of the composites was determined by gravimetric method. Theoretical densities of Al – 6Cu–Mn matrix alloy and SiC were taken as 2.88 and 3.2 g cm − 3, respectively. Porosity of the as-sprayed and asforged composites was measured to be 4.6 and 0.8%, respectively. Wear test was carried out in the TE92 type unidirectional pin-on-disc machine (Plint and Partners Ltd.). The wear specimens were machined in the form of
147
cylindrical block with 8 mm diameter and 15 mm height from an as-sprayed ingot and an as-forged rod with a deformation ratio of 13:1. The counterpart was cold work hardened AISI D2 tool steel disc, having a hardness of 730 9 10 VHN. Two pins were mounted during each test and slid against the counterpart with a horizontal contact. The test was repeated, so that at least four specimens were used to obtain each data point. Applied load was varied from 5 to 400 N (corresponding normal stress is 0.1–8 MPa). Sliding velocity was kept constant at 0.8 m s − 1 in all tests. Sliding distance varied in the range of 0–3500 m. As for the wear tests to evaluate the effect of applied load on wear rate, the sliding distance was kept at 1500 m; however, in the case of severe wear, the wear process was ended at the sliding distance of 500 m due to noise and vibration during the tests. All test specimens were subjected to peak-aging heat treatment according to the following process: solution treatment at 530°C for 30 min, quenching in cold water and artificial ageing at 190°C for 4 h. The specimens were ground on 800 grit emery paper before wear test to have a uniform initial surface. Weight loss during wear test was measured using an electronic balance with a resolution of 90.01 mg. The specimens were thoroughly cleaned with acetone in ultrasonic cleaner before and after the wear test. Wear rate was calculated by dividing weight loss by sliding distance. The worn specimens were sectioned parallel to the sliding direction and perpendicular to the sliding surface using a low speed diamond wheel. The cut specimens were mounted and polished and then used for microscopic observations and analyses. The worn surface, subsurface and wear debris were characterized using an optical microscope and a JEOL 8600 type scanning electron microscope (SEM) equipped with an energy dispersive X-ray analysis system (EDX). Phase identification of the wear debris was carried out using a Rigaku X-ray diffractometer (XRD) with Cu Ka radiation at 40 kv and 30 mA.
3. Results
3.1. Microstructure Typical microstructures of the as-sprayed and asforged composites are shown in Fig. 1. In the microstructure of the as-sprayed composite (Fig. 1(a)), some irregular pores with : 40–150 mm in length can be seen. Some SiC particles gathered with these pores and they did not bond with aluminum matrix sufficiently. The other SiC particles have well been wetted by aluminum. As for the as-forged composite, the microstructure was improved significantly (Fig. 1(b))
148
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
3.2. Wear test Fig. 2 shows the variations of wear rate with applied load for both the as-sprayed and as-forged composites in the load range of 5–400 N at sliding velocity 0.8 m s − 1. In the case of the as-forged composite, the variation of wear rate may be divided into four regions. Region I was located in the load range of 5–200 N. In this region, the wear rate increased slowly with increasing load. Region II was located in the load range of 200–275 N and the wear rate hardly changed in this region. Region III was located in the load range of 275–350 N, and exhibited a rapid increase in the wear rate with applied load as compared with regions I and II. When the applied load reached 375 N, the wear rate changed abruptly and largely, and the wear entered region IV. In the case of the as-sprayed composite, the effect of applied load on wear rate was different from that of the as-forged composite. When the applied load was below 100 N, the wear rate was nearly consistent with that of the as-forged composite. However, when the load was beyond 100 N, some difference in the wear rate between the as-sprayed and as-forged composites was displayed. In the load range of 100–200 N, the wear rate increased rapidly with increasing load. The wear rate changed abruptly at 225 N. This transition load was much less than that exhibited in the as-forged composite. Three regions can be concluded in the variation of wear rate with applied load for the as-sprayed composite. These regions were located in the load range of 5–100 N (region I), 100–200 N (region II) and beyond 225 N (region III), respectively. For two materials, the wear rate increased more than one order of
Fig. 1. Typical microstructures of as-sprayed and as-forged composites: (a) as-sprayed, (b) as-forged and (c) high magnification for as-forged composite.
and the pores were eliminated. The SiC particles were randomly distributed in the matrix of both the assprayed and as-forged composites. Besides SiC particles, XRD analysis also indicated that second phases of Al2Cu and MnAl6 existed in the Al matrix. Fig. 1(c) is an optical high-magnification photograph, showing clear Al/SiC interface and no occurrence of interfacial reaction.
Fig. 2. Variations of wear rate with applied load for both as-sprayed and as-forged composites in the load range of 5 – 400 N at sliding velocity 0.8 m s − 1.
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
Fig. 3. Variations of weight loss with sliding distance for both as-sprayed and as-forged composites at several loads at sliding velocity 0.8 m s − 1.
magnitude at the transition load, as had been reported in other composites [20,21]. This load represented the transition load from mild to severe wear. Fig. 3 shows the variations of weight loss with sliding distance for both as-sprayed and as-forged composites at several loads at sliding velocity 0.8 m s − 1. It is found that the variations of weight loss with sliding distance exhibited nearly linear relation. The weight loss of the as-sprayed composite was near to that of the as-forged composite under the conditions of low loads and B 1500 m sliding distance. The difference in the weight loss between two composites increased with increasing sliding distance and applied load.
3.3. Worn surface and wear debris Fig. 4 shows SEM micrographs of the worn surface of the as-sprayed and as-forged composites at several applied loads. Fig. 4(a) and (b) correspond to the as-forged and as-sprayed composite specimens worn at the load of 10 N, respectively. Clearly, the topography of the worn surfaces was identical. It was characterized by a smooth surface with fine abrasion grooves. With increasing load, the grooves on worn surface became wider, and dimples were formed and turned larger. Fig. 4(c) and (d) show typical worn surfaces at the load of 50 N for the as-forged and as-sprayed composites. Few small dimples and larger grooves could be observed. The characteristics of worn surface of the two composite specimens were also consistent. Fig. 4(e) and (f) show the worn surfaces of the as-forged composite at the load of 350 N and as-sprayed composite at the load
149
of 200 N, respectively. The worn surfaces comprised large dimples and wide grooves. Visually, smooth (corresponding to groove) and coarse (dimple) areas could be distinguished. In the severe wear region, the characteristic of the worn surfaces of two composites was distinctly different from that in the other regions. However, the worn surface morphology of both as-sprayed and as-forged composites is the same in this region, as shown in Fig. 4(g) and (h). The worn surface was characterized by a series of stripe like river wave, which indicates a typical adhesive wear. In this case, a great amount of material transfer occurred from the pin to the disc and the wearing track in the disc was covered by the material of pin. Fig. 5 shows SEM micrographs and the corresponding Si dot-mapping images of the worn surfaces of the as-forged composite at the load of 10 and 200 N, respectively. It is found that the worn surface at the high load of 200 N displayed a quite uniform distribution of Si. While, the distribution of Si on the worn surface at the low load of 10 N was inhomogeneous, which indicates that some SiC particles on the worn surface have not undergone fragmentation. These SiC particles had an effect to support the applied load, as had been reported in the literature [4,20]. The debris generated during wear was collected from counterpart disc after wear test and observed by SEM and analyzed by XRD. Fig. 6 shows SEM micrographs of the wear debris generated at different conditions. The debris displayed fine and grey powders for two composites at low loads (about below 50 N). Fig. 6(a) shows the debris with B10 mm powders for the asforged composite at 25 N. With increasing load, the average size of debris increased and platelet debris was formed. In region I, the debris was mainly in the form of powders. For the as-forged composite, the debris was in the form of a mixture of powders and platelets in region II (Fig. 6(b)) and of large platelets plus a small amount of powders in region III. For the assprayed composite, the amount of platelet in wear debris increased more quickly in region II. The debris constituent in the former part of this region was similar to that of the as-forged composite in region II, while in the later part was similar to that of the as-forged composite in region III. In the severe wear range, the debris was in the form of large platelets for two composites (Fig. 6(c) and (d)). XRD was used to analyze the phase constituent of debris. Fig. 7 shows XRD patterns of the wear debris obtained at several loads. The analyses showed that the wear debris obtained in severe wear had the same constituent as that of bulk pin material, while in the other loads, oxides of Al2O3 and Fe2O3 have been detected in wear debris, especially at low loads.
150
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
Fig. 4. SEM micrographs of the worn surface of the as-sprayed and as-forged composites at different applied loads at sliding velocity 0.8 m s − 1: (a) as-forged, 10 N; (b) as-sprayed, 10 N; (c) as-forged, 50 N; (d) as-sprayed, 50 N; (e) as-forged, 350 N; (f) as-sprayed, 200 N; (g) as-forged, 400 N; and (h) as-sprayed, 250 N. Sliding distance is 500 m for the forged composite at 400 N and the sprayed composite at 250 N and the others 1500 m.
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
3.4. Worn subsurface Worn subsurface observation and analyses are very important to understand the distinct wear rates of two materials, and the effect of porosity on wear behavior can be also demonstrated clearly in the worn subsurface. Mechanical mixed layer (MML) in subsurface is an important characteristic of the wear behavior of particle-reinforced aluminum matrix composites. It has been reported that the improved wear resistance of the composites is related to its occurrence [12,22]. Fig. 8(a) shows a typical subsurface structure for the as-forged composite at 400 N. Some prows could be seen, which indicates a typical adhesive wear and material removal by shear fracture mechanism [23]. In the as-sprayed composite, the pores located beneath the worn surface could act as crack sources, resulting in shear fracture earlier and more easily (Fig. 8(b), arrow indicates the pore). In severe wear region, no MML was found for both the as-sprayed and as-forged composites. In the other wear regions, MML could be observed. Fig. 9 shows several typical worn subsurface microstructures in these regions, indicating different characteristics of MML and the effect of pore on wear behavior. In general, the MML in the as-sprayed composite specimens exhibited unstable, loose and weak bonding with
151
subsurface deformation layer. On the contrary, the MML in the as-forged composite specimens was more stable, compact and intimate bonding with subsurface deformation layer. With increasing load, the thickness of MML increased in regions I and II and after a maximum decreased in region III. The MML eliminated finally in region IV for the as-forged composite. No certain rule in the change of MML thickness could be concluded for the as-sprayed composite. Fig. 9(a) and (b) correspond to the as-forged composite specimens worn at 50 and 200 N, respectively, which show stable and compact MML and good bonding between the MML and deformation layer. Fig. 9(c) and (d) show the worn subsurfaces obtained at 200 N for the as-sprayed composite. Loose, thin and unstable MML could be seen beneath the worn surface (Fig. 9(c)). In addition, cracks were often observed inside MML or in the boundary between MML and deformation layer (Fig. 9(d)). Fig. 9(e) and (f) illustrate the effect of pore on the removal of material in the worn surface of pin. The pores beneath the worn surface may have been crack sources, which promoted the debris formation by delamination (arrows show the pores in Fig. 9(e)). In addition, the pores near the periphery of pin may have created edge collapse, resulting in the removal of the material from pin (Fig. 9(f)).
Fig. 5. SEM micrographs and the corresponding Si dot-mapping images of the worn surfaces of the as-forged composite at sliding velocity 0.8 m s − 1 and 1500 m sliding distance: (a) and (b) 10 N, (c) and (d) 200 N.
152
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
Fig. 6. SEM micrographs showing wear debris of the as-forged and as-sprayed composites generated at different loads at sliding velocity 0.8 m s − 1. (a – c): 50, 200 and 400 N, as-forged composite; (d) 250 N, as-sprayed composite. Sliding distance is 500 m for the forged composite at 400 N and the sprayed composite at 250 N and the others 1500 m.
4. Discussion
4.1. Wear regime Alpas and Zhang [4] reported that there exist three wear regimes with increasing load for SiC and Al2O3 particle-reinforced aluminum matrix composites. At low loads of below 10 N (regime I), the particles support the applied load and the wear resistance of the composites is at least an order of magnitude better than that of monolithic aluminum alloys. In the intermediate load range (regime II), the wear rate of the composite is comparable to that of monolithic alloys. A transition occurs at the end of regime II and then severe wear occurs (regime III). Zhang et al. [21] showed that two wear regions exist with increasing applied load for SiC and Al2O3 particle-reinforced aluminum composites. The transition between two regions is the critical load from mild to severe wear. As a common rule, for Al–Si alloys or their particle-reinforced aluminum matrix composites, severe wear would be encountered when the applied load reaches a critical value, and the wear rate in severe wear regime increases about one order of magnitude [20,21]. In the present work, there exist four regions for the as-forged composite and three regions
Fig. 7. XRD patterns of the wear debris generated at different loads. (a – c): As-forged composite; (d – f): as-sprayed composite.
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
153
Fig. 8. Optical micrographs of subsurface showing an evident shear fracture: (a) as-forged, 400 N; and (b) as-sprayed, 250 N, arrow indicates a pore as crack source creating shear fracture.
Fig. 9. Worn subsurface microstructures of the worn specimens. (a) As-forged, 50 N; (b) as-forged, 200 N; (c – f): as-sprayed, 200 N; (c) showing loose, thin and unstable MML; (d) showing cracks at MML; (e) arrows show the pores as cracks to create delamination; and (f) pore along periphery of pin creating edge collapse.
for the as-sprayed composite. However, it should be noted that no discontinuity appears between the regions in the former three regions in the case of as-forged
composite (two regions for the as-sprayed composite). The variation of wear rate from region I to III is not significant. Meanwhile, the wear features, such as worn
154
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
surface and wear debris display gradual change from one region to another one. Therefore, these three regions all belong to mild wear. The transition occurs at a load of between region III and IV and the wear rate increases one order of magnitude in this transition. It is clear that the transition is the one from mild to severe wear and after the transition load, severe wear occurs (in region IV). A similar result can be concluded for the as-sprayed composite, i.e. the composite displays mild wear in regions I and II and severe wear in region III.
4.2. Wear mechanism The wear tests showed that the wear rate exhibited some different characteristics in the different regions for two composites. Therefore, the wear process should be controlled by different wear mechanism. In aluminum– silicon alloys and particle-reinforced aluminum matrix composites, oxidative wear has been extensively reported, especially at low applied loads [21,24–27]. In region I, a great amount of Al and Fe oxides have been detected in the wear debris by XRD analysis for both the as-forged and the as-sprayed composites. The wear debris produced in this region is powder-like and dark in color. These reveal that oxidation is the main characteristic. In region I, the wear proceeds mainly by the formation of oxidation layer in the worn surface and its spalling. In addition, at low loads of below 10 N, the SiC particles without fracture on the worn surface can scratch the counterpart surface and act as load-supporting elements (Fig. 5). Therefore, abrasive wear can also play a secondary role at low loads. Furthermore, a small amount of platelets in the wear debris and dimples on the worn surface are found in this region at higher loads. It indicates that delamination wear has occurred and is an assisting process of the material removal. In region I, the oxides, especially iron oxide are known to have a low friction coefficient and pro-
Fig. 10. Microstructure showing the subsurface cracks in the asforged composite pin at 350 N.
duce a lubricating effect [28,29]. It results in a lower wear rate and slow increase of wear rate with applied load. In the case of the as-forged composite, in region II, quite a high amount of platelets are presented in wear debris and the size of dimple on the worn surface has already become larger. Delamination wear becomes the major wear mechanism although there also exists oxidative wear because the oxides can still be detected in the wear debris by XRD analysis. It is well known that delamination is a process of the initiation and propagation of subsurface crack. The wear tests showed that the wear rate in this region hardly changes. This phenomenon was also observed in the SiC particle-reinforced aluminum composite [20]. The reason is that the propagation of subsurface crack needs to meet a certain stress condition, and the crack can keep a relative stability in a certain load range, resulting in a relative stable wear rate. In region III, a lot of large irregular platelets can be observed in the wear debris and large dimples are formed on the worn surface. The shear fracture characteristic (river wave stripe) is clearly displayed on the surfaces of the large dimple (Fig. 4(e)), indicating the occurrence of adhesive wear during the wear process. In this region, the depth of the deformed layer below the worn surface is larger than that in region II, thus the crack is formed in the subsurface more easily, which in turn makes it difficult for any stable MML to form in the subsurface. The removal of large platelet debris in this region is still a process of crack initiation and propagation. It results in limited high wear rate. Fig. 10 gives evidence of subsurface cracks in the as-forged composite pin at 350 N. Therefore, the wear process is controlled mainly by subsurface-cracking assisted adhesive mechanism in region III. For the as-sprayed composite, the amount of platelet in the wear debris increases quickly in region II. The debris in the former part of this region is more similar to that in region II of the as-forged composite, while in the later part is nearly consistent to that in region III of the as-forged composite. Actually, this region in the as-sprayed composite combines the wear characteristics of regions II and III in the as-forged composite. The dominant wear mechanism changes from the delamination in the former part to the subsurface-cracking assisted adhesive wear in the latter part of this region. In region IV, XRD analyses (Fig. 7) indicate that the wear debris has the same constituent as that of bulk pin material for two composites. A large amount of material transfer from the pin to the counterface occurs during wear test. A clean shear fracture (Fig. 4(g) and (h)) can be seen on the worn surface. These evidences show the occurrence of adhesive wear. Thus, adhesive wear is the dominant wear mechanism. During wear process, the removal of material in the worn surface of pin occurs by shear fracture. The shear fracture from
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
subsurface in pin induces the quick detachment of material, resulting in debris in the form of large irregular-shaped platelet and a high wear rate.
4.3. Influence of porosity on wear beha6ior The as-forged composite has a significantly small wear rate at higher loads and high transition load from mild to severe wear, therefore exhibits superior wear resistance than the as-sprayed composite. Clearly, this difference is caused by porosity. Pores in the as-sprayed composite are a kind of serious microstructure defect. Cracks can be precedently created from these pores when an exterior force is applied. Therefore, the pores are equal to crack sources. At loads of B100 N (i.e. in region I of the as-sprayed composite), the wear rate of the as-sprayed composite is near to that of the asforged composite. In this condition, subsurface deformation and strain-induced stress are relatively small. The pores beneath the worn surface remain stable and cannot propagate significantly. Therefore, at lower loads, the porosity in the as-sprayed composite displays very small effects on the wear behavior including wear rate, worn surface and debris characteristic. Especially at the loads of below 10 N, the SiC particles without fracture on the worn surface also have a load-supporting effect against applied load, resulting in the same wear rates for two composites. With increasing load, subsurface deformation and strain-induced stress increase, so the pores beneath the worn surface become unstable and the cracks originated from these pores can propagate during wear process (Fig. 9(e)), which results in material removal from pin by delamination wear. Therefore, the wear process of the as-sprayed composite is developed into delamination and subsurface-cracking assisted adhesive wear from oxidative wear at lower applied loads than the as-forged composite, resulting in a larger wear rate. It is clear that the porosity in the as-sprayed composite would create a significant influence on wear rate when the applied load is large enough that the pores beneath worn surface become unstable. Plastic deformation and fracture have been considered as the main causes of severe wear. The worn surface and subsurface observations indicated that the adhesive wear occurring in severe region proceeds with subsurface shear fracture induced by serious plastic deformation. As the strain-induced shear stress is larger than the shear strength of the material itself in subsurface, the severe wear will occur. Therefore, the occurrence of severe wear depends on the shear strength of pin and the strain-induced shear stress. The presence of pores decreases the shear strength of the as-sprayed composite obviously since the shear fracture takes place directly through these pores. It results in adhesive wear at lower loads, thus the transition load from mild to
155
severe wear for the as-sprayed composite decreases significantly. In Fig. 3, the difference in the weight loss between the as-sprayed and as-forged composites increases with increasing sliding distance and applied load. For the as-sprayed composite, the worn surface may accumulate more defects that are originated from pores with increasing sliding distance. In addition, the increase of applied load can promote to enlarge the size of deformation zone beneath the worn surface, so that more pores can influence the wear process. These may result in a higher weight loss.
5. Conclusions In the load range of 5–400 N (corresponding normal stress is 0.1–8 MPa), the as-sprayed Al–6Cu–Mn based composite reinforced with 13 vol.% SiCp displayed three wear regions, while the as-forged composite four wear regions. A transition load from mild to severe wear could be observed with increasing load. The dominant wear mechanism in severe wear for both the as-sprayed and as-forged composites was adhesive wear by subsurface shear fracture. Mild wear was associated with three different mechanisms. With the increase of applied load, they were successively oxidation, delamination and subsurface-cracking assisted adhesion wear. The wear resistance of the as-sprayed composite was similar to that of the as-forged composite at lower loads (below 100 N). However, at higher loads, the wear rate of the as-sprayed composite increased significantly and the transition load from mild to severe wear decreased largely. The as-sprayed composite exhibited an inferior wear resistance than the as-forged composite. At lower loads, the pores beneath the worn surface of the as-sprayed composite were stable, and could not propagate significantly. Thus, porosity displayed a very small influence on dry sliding wear behavior of the composite. At higher loads, the pores beneath the worn surface became unstable and the cracks originated from these pores could propagate during wear process, which resulted in a higher wear rate and lowered obviously the transition load from mild to severe wear.
References [1] S.Q. Wu, H.G. Zhu, S.C. Tjong, Metall. Trans. A 30A (1999) 243. [2] G. Wang, P. Shi, M. Qi, J.J. Xu, F.X. Chen, D.Z. Yang, Metall. Mater. Trans. A 29A (1998) 1741. [3] S.X. Huang, K. Paxton, JOM 50 (8) (1998) 26. [4] A.T. Alpas, J. Zhang, Metall. Mater. Trans. A 25A (1994) 969. [5] R.A. Saravanan, J.-M. Lee, S.-B. Kang, Metall. Mater. Trans. A 30A (1999) 2523.
156
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
[6] M. Roy, B. Venkataraman, V.V. Bhanuprasad, Y.R. Mahajan, G. Sundararajan, Metall. Trans. A 23A (1992) 2833. [7] A.T. Alpas, J. Zhang, Scr. Metall. Mater. 26 (1992) 505. [8] M.A. Martinez, A. Martin, J. Llorca, Scr. Metall. Mater. 28 (1992) 207. [9] B.N. Pramila Bai, B.S. Ramasesh, M.K. Surappa, Wear 157 (1992) 295. [10] A. Wang, H.J. Rack, Mater. Sci. Eng. 147A (1991) 211. [11] A. Ravikiran, M.K. Surappa, Scr. Mater. 36 (1997) 95. [12] B. Venkataraman, G. Sundararajan, Acta Mater. 44 (2) (1996) 461. [13] M.K. Surappa, S.V. Prasad, P.K. Pohatgi, Wear 77 (1982) 295. [14] A.G. Wang, I.M. Hutchings, Mater. Sci.Technol. 5 (1989) 71. [15] S.V. Prasad, P.K. Rohatgi, J. Met. 11 (1987) 22. [16] C.A. Caracostas, W.A. Chiou, M.E. Fine, H.S. Cheng, Scr. Metall. Mater. 27 (1992) 162. [17] H.L. Lee, W.H. Lu, S.L. Chan, Wear 159 (1992) 223.
.
[18] [19] [20] [21] [22] [23]
[24] [25] [26] [27] [28] [29]
B. Venkataraman, G. Sundararajan, Acta Mater. 44 (1996) 451. K.J. Bhansali, R. Mehrabian, J. Met. 34 (1982) 30. A.T. Alpas, J. Zhang, Wear 155 (1992) 83. Z.F. Zhang, L.C. Zhang, Y.W. Mai, J. Mater. Sci. 30 (1995) 1961. R.L. Deuis, C. Subramanian, J.M. Yellup, Comp. Sci. Technol. 57 (1997) 415. O. Vincsbo, S. Hogmark, in: D.A. Rigney (Ed.), Fundamentals of Friction and Wear of Materials, American Society for Metals, Metals Park, OH, 1981, p. 373. J. Zhang, A.T. Alpas, Acta Mater. 45 (1997) 513. R. Shivanath, P.K. Sengupta, T.S. Eyre, Br. Foundryman 70 (1977) 349. K. Razavizadeh, T.S. Eyre, Wear 79 (1982) 325. K. Razavizadeh, T.S. Eyre, Wear 87 (1983) 261. K.C. Ludema, Wear 100 (1984) 315. S.C. Lim, M.F. Ashby, Acta Metall. 35 (1987) 1.
Influence of porosity on dry sliding wear behavior in spray deposited Al–6Cu–Mn/SiCp composite Manchang Gui a,b, Suk Bong Kang b,*, Jung Moo Lee b a
National Laboratory of Ad6anced Composites, Beijing Institute of Aeronautical Materials, Beijing 100095, China b Korea Institute of Machinery and Materials, 66 Sangnam, Changwon, Kyungnam 641 -010, South Korea Received 26 July 1999; received in revised form 23 May 2000
Abstract Dry sliding wear behavior of spray deposited Al– 6Cu – Mn/13 vol.% SiCp composites in both as-sprayed and forged states has been studied. Wear mechanisms in different wear regions and the influence of porosity on wear behavior are discussed. Within the applied load range of 5–400 N (corresponding normal stress is 0.1 – 8 MPa), the variation of wear rate with applied load in the as-sprayed composite can be divided into three wear regions, while the as-forged composite four wear regions. A transition load from mild to severe wear could be observed with increasing load. Severe wear was controlled by adhesion mechanism. Mild wear was associated with three different mechanisms: oxidation, delamination and subsurface-cracking assisted adhesion wear. The wear resistance of the as-sprayed composite was similar to that of the as-forged composite at lower loads. However, at higher loads, the as-sprayed composite had a higher wear rate and small transition load, therefore, exhibited inferior wear resistance than the as-forged composite. At lower loads, pores beneath the worn surface of the as-sprayed composite were stable and could not propagate significantly. Therefore, porosity displayed a very small influence on dry sliding wear behavior of the composite. At higher loads, the pores beneath the worn surface became unstable and the cracks originated from these pores could propagate during wear process, resulting in a higher wear rate and smaller transition load. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Sliding wear; Porosity; Metal matrix composite; Wear mechanism; Particle reinforcement
1. Introduction The wear behavior of aluminum matrix composites has, because of their potential tribological applications, been extensively studied during the last 20 years [1–12]. Usually, aluminum matrix composites reinforced with SiC, Al2O3, TiC, TiB2, etc. particles by comparison to monolithic materials display significantly improved wear resistance under abrasive [13,14], lubricated [15,16] and dry sliding conditions [1,2,4,17,18]. The principal factors that influence the sliding wear of the composites include: extrinsic factors such as sliding distance, sliding velocity, applied load, atmosphere temperature, etc. and intrinsic factors such as reinforcement type, shape, size, volume fraction and distribution, interface between reinforcement and ma* Corresponding author. Tel.: +82-551-2803301; fax: + 82-5512803399. E-mail address: [email protected] (S.B. Kang).
trix, matrix material, heat treatment, porosity, etc. The intrinsic factors can be regulated and controlled in the process of material manufacture. It is probable to optimize the wear resistance of the composites by the intrinsic factors. Therefore, the investigations about the influences of intrinsic factors on the wear behavior have received extensive attention. Among them, the intrinsic factors to have been studied are mainly reinforcement size, type and volume fraction. The work in other aspects is limited. Bhansali et al. [19] reported that the 2024/20SiCp composites display an inferior wear resistance than 2024 monolithic alloy when there exists brittle interface between SiC and aluminum causing interfacial reaction of Al4C3. Wang et al. [2] also showed that Ti50Ni25Cu25 particle-reinforced aluminum matrix composite with interfacial reactions exhibits lower wear resistance than the composite without interfacial reaction. Lee et al. [17] studied the effect of sintered porosity in powder metallurgy (PM) 6061 monolithic alloy and 6061/SiCp composites on dry
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 0 5 2 - 2
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156 Table 1 Compositions of Al–6Cu–Mn matrix alloy Element wt.%
Cu 6.07
Mn 1.68
Mg 0.47
Ti 0.24
Zr 0.55
V 0.19
Al Bal.
abrasive wear, indicating that the wear rate of 6061 alloy increases drastically with increasing porosity, while for its composites, the porosity effect is not significant. Spray deposition process is one of several routes to produce particle-reinforced aluminum matrix composites. The composites produced by this process possess fine grain size and uniform particle distribution and reaction between reinforcement particle and aluminum matrix can be avoided because of the short contact time between the melt and the reinforcement. Therefore, the composites produced by spray deposition process possess potential superior mechanical properties. However, studies on the wear behavior of the composites produced by spray deposition process have so far been limited. In addition, similar to PM composites, assprayed composites contain some porosity. It is also worth studying the influence of porosity on sliding wear behavior of the composites. Al–Cu alloys based SiCp-reinforced composites produced by novel manufacturing techniques possess higher strength and stability at elevated temperatures. They often have considerably potential applications in the aircraft-related field where sometimes wear can be quite significant (such as bearing sleeves, valves, etc.). The main objectives of this work are to study the dry sliding wear behavior of a sprayed deposited Al–6Cu– Mn/13 vol.% SiCp composite and to evaluate the effect of porosity on the wear behavior of the composite.
2. Experimental procedures Al–6Cu–Mn alloy based composite reinforced with 13 vol.% SiCp was produced by spray deposition process at Alusuisse-Lonza Services Ltd. in Switzerland. The compositions of the matrix alloy are shown in Table 1. The average size of the SiC particles is 12 mm. To evaluate the influence of porosity on the sliding wear behavior, two state composites, as-sprayed and forged (referred to as as-forged composite), were used. Porosity of the composites was determined by gravimetric method. Theoretical densities of Al – 6Cu–Mn matrix alloy and SiC were taken as 2.88 and 3.2 g cm − 3, respectively. Porosity of the as-sprayed and asforged composites was measured to be 4.6 and 0.8%, respectively. Wear test was carried out in the TE92 type unidirectional pin-on-disc machine (Plint and Partners Ltd.). The wear specimens were machined in the form of
147
cylindrical block with 8 mm diameter and 15 mm height from an as-sprayed ingot and an as-forged rod with a deformation ratio of 13:1. The counterpart was cold work hardened AISI D2 tool steel disc, having a hardness of 730 9 10 VHN. Two pins were mounted during each test and slid against the counterpart with a horizontal contact. The test was repeated, so that at least four specimens were used to obtain each data point. Applied load was varied from 5 to 400 N (corresponding normal stress is 0.1–8 MPa). Sliding velocity was kept constant at 0.8 m s − 1 in all tests. Sliding distance varied in the range of 0–3500 m. As for the wear tests to evaluate the effect of applied load on wear rate, the sliding distance was kept at 1500 m; however, in the case of severe wear, the wear process was ended at the sliding distance of 500 m due to noise and vibration during the tests. All test specimens were subjected to peak-aging heat treatment according to the following process: solution treatment at 530°C for 30 min, quenching in cold water and artificial ageing at 190°C for 4 h. The specimens were ground on 800 grit emery paper before wear test to have a uniform initial surface. Weight loss during wear test was measured using an electronic balance with a resolution of 90.01 mg. The specimens were thoroughly cleaned with acetone in ultrasonic cleaner before and after the wear test. Wear rate was calculated by dividing weight loss by sliding distance. The worn specimens were sectioned parallel to the sliding direction and perpendicular to the sliding surface using a low speed diamond wheel. The cut specimens were mounted and polished and then used for microscopic observations and analyses. The worn surface, subsurface and wear debris were characterized using an optical microscope and a JEOL 8600 type scanning electron microscope (SEM) equipped with an energy dispersive X-ray analysis system (EDX). Phase identification of the wear debris was carried out using a Rigaku X-ray diffractometer (XRD) with Cu Ka radiation at 40 kv and 30 mA.
3. Results
3.1. Microstructure Typical microstructures of the as-sprayed and asforged composites are shown in Fig. 1. In the microstructure of the as-sprayed composite (Fig. 1(a)), some irregular pores with : 40–150 mm in length can be seen. Some SiC particles gathered with these pores and they did not bond with aluminum matrix sufficiently. The other SiC particles have well been wetted by aluminum. As for the as-forged composite, the microstructure was improved significantly (Fig. 1(b))
148
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
3.2. Wear test Fig. 2 shows the variations of wear rate with applied load for both the as-sprayed and as-forged composites in the load range of 5–400 N at sliding velocity 0.8 m s − 1. In the case of the as-forged composite, the variation of wear rate may be divided into four regions. Region I was located in the load range of 5–200 N. In this region, the wear rate increased slowly with increasing load. Region II was located in the load range of 200–275 N and the wear rate hardly changed in this region. Region III was located in the load range of 275–350 N, and exhibited a rapid increase in the wear rate with applied load as compared with regions I and II. When the applied load reached 375 N, the wear rate changed abruptly and largely, and the wear entered region IV. In the case of the as-sprayed composite, the effect of applied load on wear rate was different from that of the as-forged composite. When the applied load was below 100 N, the wear rate was nearly consistent with that of the as-forged composite. However, when the load was beyond 100 N, some difference in the wear rate between the as-sprayed and as-forged composites was displayed. In the load range of 100–200 N, the wear rate increased rapidly with increasing load. The wear rate changed abruptly at 225 N. This transition load was much less than that exhibited in the as-forged composite. Three regions can be concluded in the variation of wear rate with applied load for the as-sprayed composite. These regions were located in the load range of 5–100 N (region I), 100–200 N (region II) and beyond 225 N (region III), respectively. For two materials, the wear rate increased more than one order of
Fig. 1. Typical microstructures of as-sprayed and as-forged composites: (a) as-sprayed, (b) as-forged and (c) high magnification for as-forged composite.
and the pores were eliminated. The SiC particles were randomly distributed in the matrix of both the assprayed and as-forged composites. Besides SiC particles, XRD analysis also indicated that second phases of Al2Cu and MnAl6 existed in the Al matrix. Fig. 1(c) is an optical high-magnification photograph, showing clear Al/SiC interface and no occurrence of interfacial reaction.
Fig. 2. Variations of wear rate with applied load for both as-sprayed and as-forged composites in the load range of 5 – 400 N at sliding velocity 0.8 m s − 1.
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
Fig. 3. Variations of weight loss with sliding distance for both as-sprayed and as-forged composites at several loads at sliding velocity 0.8 m s − 1.
magnitude at the transition load, as had been reported in other composites [20,21]. This load represented the transition load from mild to severe wear. Fig. 3 shows the variations of weight loss with sliding distance for both as-sprayed and as-forged composites at several loads at sliding velocity 0.8 m s − 1. It is found that the variations of weight loss with sliding distance exhibited nearly linear relation. The weight loss of the as-sprayed composite was near to that of the as-forged composite under the conditions of low loads and B 1500 m sliding distance. The difference in the weight loss between two composites increased with increasing sliding distance and applied load.
3.3. Worn surface and wear debris Fig. 4 shows SEM micrographs of the worn surface of the as-sprayed and as-forged composites at several applied loads. Fig. 4(a) and (b) correspond to the as-forged and as-sprayed composite specimens worn at the load of 10 N, respectively. Clearly, the topography of the worn surfaces was identical. It was characterized by a smooth surface with fine abrasion grooves. With increasing load, the grooves on worn surface became wider, and dimples were formed and turned larger. Fig. 4(c) and (d) show typical worn surfaces at the load of 50 N for the as-forged and as-sprayed composites. Few small dimples and larger grooves could be observed. The characteristics of worn surface of the two composite specimens were also consistent. Fig. 4(e) and (f) show the worn surfaces of the as-forged composite at the load of 350 N and as-sprayed composite at the load
149
of 200 N, respectively. The worn surfaces comprised large dimples and wide grooves. Visually, smooth (corresponding to groove) and coarse (dimple) areas could be distinguished. In the severe wear region, the characteristic of the worn surfaces of two composites was distinctly different from that in the other regions. However, the worn surface morphology of both as-sprayed and as-forged composites is the same in this region, as shown in Fig. 4(g) and (h). The worn surface was characterized by a series of stripe like river wave, which indicates a typical adhesive wear. In this case, a great amount of material transfer occurred from the pin to the disc and the wearing track in the disc was covered by the material of pin. Fig. 5 shows SEM micrographs and the corresponding Si dot-mapping images of the worn surfaces of the as-forged composite at the load of 10 and 200 N, respectively. It is found that the worn surface at the high load of 200 N displayed a quite uniform distribution of Si. While, the distribution of Si on the worn surface at the low load of 10 N was inhomogeneous, which indicates that some SiC particles on the worn surface have not undergone fragmentation. These SiC particles had an effect to support the applied load, as had been reported in the literature [4,20]. The debris generated during wear was collected from counterpart disc after wear test and observed by SEM and analyzed by XRD. Fig. 6 shows SEM micrographs of the wear debris generated at different conditions. The debris displayed fine and grey powders for two composites at low loads (about below 50 N). Fig. 6(a) shows the debris with B10 mm powders for the asforged composite at 25 N. With increasing load, the average size of debris increased and platelet debris was formed. In region I, the debris was mainly in the form of powders. For the as-forged composite, the debris was in the form of a mixture of powders and platelets in region II (Fig. 6(b)) and of large platelets plus a small amount of powders in region III. For the assprayed composite, the amount of platelet in wear debris increased more quickly in region II. The debris constituent in the former part of this region was similar to that of the as-forged composite in region II, while in the later part was similar to that of the as-forged composite in region III. In the severe wear range, the debris was in the form of large platelets for two composites (Fig. 6(c) and (d)). XRD was used to analyze the phase constituent of debris. Fig. 7 shows XRD patterns of the wear debris obtained at several loads. The analyses showed that the wear debris obtained in severe wear had the same constituent as that of bulk pin material, while in the other loads, oxides of Al2O3 and Fe2O3 have been detected in wear debris, especially at low loads.
150
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
Fig. 4. SEM micrographs of the worn surface of the as-sprayed and as-forged composites at different applied loads at sliding velocity 0.8 m s − 1: (a) as-forged, 10 N; (b) as-sprayed, 10 N; (c) as-forged, 50 N; (d) as-sprayed, 50 N; (e) as-forged, 350 N; (f) as-sprayed, 200 N; (g) as-forged, 400 N; and (h) as-sprayed, 250 N. Sliding distance is 500 m for the forged composite at 400 N and the sprayed composite at 250 N and the others 1500 m.
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
3.4. Worn subsurface Worn subsurface observation and analyses are very important to understand the distinct wear rates of two materials, and the effect of porosity on wear behavior can be also demonstrated clearly in the worn subsurface. Mechanical mixed layer (MML) in subsurface is an important characteristic of the wear behavior of particle-reinforced aluminum matrix composites. It has been reported that the improved wear resistance of the composites is related to its occurrence [12,22]. Fig. 8(a) shows a typical subsurface structure for the as-forged composite at 400 N. Some prows could be seen, which indicates a typical adhesive wear and material removal by shear fracture mechanism [23]. In the as-sprayed composite, the pores located beneath the worn surface could act as crack sources, resulting in shear fracture earlier and more easily (Fig. 8(b), arrow indicates the pore). In severe wear region, no MML was found for both the as-sprayed and as-forged composites. In the other wear regions, MML could be observed. Fig. 9 shows several typical worn subsurface microstructures in these regions, indicating different characteristics of MML and the effect of pore on wear behavior. In general, the MML in the as-sprayed composite specimens exhibited unstable, loose and weak bonding with
151
subsurface deformation layer. On the contrary, the MML in the as-forged composite specimens was more stable, compact and intimate bonding with subsurface deformation layer. With increasing load, the thickness of MML increased in regions I and II and after a maximum decreased in region III. The MML eliminated finally in region IV for the as-forged composite. No certain rule in the change of MML thickness could be concluded for the as-sprayed composite. Fig. 9(a) and (b) correspond to the as-forged composite specimens worn at 50 and 200 N, respectively, which show stable and compact MML and good bonding between the MML and deformation layer. Fig. 9(c) and (d) show the worn subsurfaces obtained at 200 N for the as-sprayed composite. Loose, thin and unstable MML could be seen beneath the worn surface (Fig. 9(c)). In addition, cracks were often observed inside MML or in the boundary between MML and deformation layer (Fig. 9(d)). Fig. 9(e) and (f) illustrate the effect of pore on the removal of material in the worn surface of pin. The pores beneath the worn surface may have been crack sources, which promoted the debris formation by delamination (arrows show the pores in Fig. 9(e)). In addition, the pores near the periphery of pin may have created edge collapse, resulting in the removal of the material from pin (Fig. 9(f)).
Fig. 5. SEM micrographs and the corresponding Si dot-mapping images of the worn surfaces of the as-forged composite at sliding velocity 0.8 m s − 1 and 1500 m sliding distance: (a) and (b) 10 N, (c) and (d) 200 N.
152
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
Fig. 6. SEM micrographs showing wear debris of the as-forged and as-sprayed composites generated at different loads at sliding velocity 0.8 m s − 1. (a – c): 50, 200 and 400 N, as-forged composite; (d) 250 N, as-sprayed composite. Sliding distance is 500 m for the forged composite at 400 N and the sprayed composite at 250 N and the others 1500 m.
4. Discussion
4.1. Wear regime Alpas and Zhang [4] reported that there exist three wear regimes with increasing load for SiC and Al2O3 particle-reinforced aluminum matrix composites. At low loads of below 10 N (regime I), the particles support the applied load and the wear resistance of the composites is at least an order of magnitude better than that of monolithic aluminum alloys. In the intermediate load range (regime II), the wear rate of the composite is comparable to that of monolithic alloys. A transition occurs at the end of regime II and then severe wear occurs (regime III). Zhang et al. [21] showed that two wear regions exist with increasing applied load for SiC and Al2O3 particle-reinforced aluminum composites. The transition between two regions is the critical load from mild to severe wear. As a common rule, for Al–Si alloys or their particle-reinforced aluminum matrix composites, severe wear would be encountered when the applied load reaches a critical value, and the wear rate in severe wear regime increases about one order of magnitude [20,21]. In the present work, there exist four regions for the as-forged composite and three regions
Fig. 7. XRD patterns of the wear debris generated at different loads. (a – c): As-forged composite; (d – f): as-sprayed composite.
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
153
Fig. 8. Optical micrographs of subsurface showing an evident shear fracture: (a) as-forged, 400 N; and (b) as-sprayed, 250 N, arrow indicates a pore as crack source creating shear fracture.
Fig. 9. Worn subsurface microstructures of the worn specimens. (a) As-forged, 50 N; (b) as-forged, 200 N; (c – f): as-sprayed, 200 N; (c) showing loose, thin and unstable MML; (d) showing cracks at MML; (e) arrows show the pores as cracks to create delamination; and (f) pore along periphery of pin creating edge collapse.
for the as-sprayed composite. However, it should be noted that no discontinuity appears between the regions in the former three regions in the case of as-forged
composite (two regions for the as-sprayed composite). The variation of wear rate from region I to III is not significant. Meanwhile, the wear features, such as worn
154
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
surface and wear debris display gradual change from one region to another one. Therefore, these three regions all belong to mild wear. The transition occurs at a load of between region III and IV and the wear rate increases one order of magnitude in this transition. It is clear that the transition is the one from mild to severe wear and after the transition load, severe wear occurs (in region IV). A similar result can be concluded for the as-sprayed composite, i.e. the composite displays mild wear in regions I and II and severe wear in region III.
4.2. Wear mechanism The wear tests showed that the wear rate exhibited some different characteristics in the different regions for two composites. Therefore, the wear process should be controlled by different wear mechanism. In aluminum– silicon alloys and particle-reinforced aluminum matrix composites, oxidative wear has been extensively reported, especially at low applied loads [21,24–27]. In region I, a great amount of Al and Fe oxides have been detected in the wear debris by XRD analysis for both the as-forged and the as-sprayed composites. The wear debris produced in this region is powder-like and dark in color. These reveal that oxidation is the main characteristic. In region I, the wear proceeds mainly by the formation of oxidation layer in the worn surface and its spalling. In addition, at low loads of below 10 N, the SiC particles without fracture on the worn surface can scratch the counterpart surface and act as load-supporting elements (Fig. 5). Therefore, abrasive wear can also play a secondary role at low loads. Furthermore, a small amount of platelets in the wear debris and dimples on the worn surface are found in this region at higher loads. It indicates that delamination wear has occurred and is an assisting process of the material removal. In region I, the oxides, especially iron oxide are known to have a low friction coefficient and pro-
Fig. 10. Microstructure showing the subsurface cracks in the asforged composite pin at 350 N.
duce a lubricating effect [28,29]. It results in a lower wear rate and slow increase of wear rate with applied load. In the case of the as-forged composite, in region II, quite a high amount of platelets are presented in wear debris and the size of dimple on the worn surface has already become larger. Delamination wear becomes the major wear mechanism although there also exists oxidative wear because the oxides can still be detected in the wear debris by XRD analysis. It is well known that delamination is a process of the initiation and propagation of subsurface crack. The wear tests showed that the wear rate in this region hardly changes. This phenomenon was also observed in the SiC particle-reinforced aluminum composite [20]. The reason is that the propagation of subsurface crack needs to meet a certain stress condition, and the crack can keep a relative stability in a certain load range, resulting in a relative stable wear rate. In region III, a lot of large irregular platelets can be observed in the wear debris and large dimples are formed on the worn surface. The shear fracture characteristic (river wave stripe) is clearly displayed on the surfaces of the large dimple (Fig. 4(e)), indicating the occurrence of adhesive wear during the wear process. In this region, the depth of the deformed layer below the worn surface is larger than that in region II, thus the crack is formed in the subsurface more easily, which in turn makes it difficult for any stable MML to form in the subsurface. The removal of large platelet debris in this region is still a process of crack initiation and propagation. It results in limited high wear rate. Fig. 10 gives evidence of subsurface cracks in the as-forged composite pin at 350 N. Therefore, the wear process is controlled mainly by subsurface-cracking assisted adhesive mechanism in region III. For the as-sprayed composite, the amount of platelet in the wear debris increases quickly in region II. The debris in the former part of this region is more similar to that in region II of the as-forged composite, while in the later part is nearly consistent to that in region III of the as-forged composite. Actually, this region in the as-sprayed composite combines the wear characteristics of regions II and III in the as-forged composite. The dominant wear mechanism changes from the delamination in the former part to the subsurface-cracking assisted adhesive wear in the latter part of this region. In region IV, XRD analyses (Fig. 7) indicate that the wear debris has the same constituent as that of bulk pin material for two composites. A large amount of material transfer from the pin to the counterface occurs during wear test. A clean shear fracture (Fig. 4(g) and (h)) can be seen on the worn surface. These evidences show the occurrence of adhesive wear. Thus, adhesive wear is the dominant wear mechanism. During wear process, the removal of material in the worn surface of pin occurs by shear fracture. The shear fracture from
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
subsurface in pin induces the quick detachment of material, resulting in debris in the form of large irregular-shaped platelet and a high wear rate.
4.3. Influence of porosity on wear beha6ior The as-forged composite has a significantly small wear rate at higher loads and high transition load from mild to severe wear, therefore exhibits superior wear resistance than the as-sprayed composite. Clearly, this difference is caused by porosity. Pores in the as-sprayed composite are a kind of serious microstructure defect. Cracks can be precedently created from these pores when an exterior force is applied. Therefore, the pores are equal to crack sources. At loads of B100 N (i.e. in region I of the as-sprayed composite), the wear rate of the as-sprayed composite is near to that of the asforged composite. In this condition, subsurface deformation and strain-induced stress are relatively small. The pores beneath the worn surface remain stable and cannot propagate significantly. Therefore, at lower loads, the porosity in the as-sprayed composite displays very small effects on the wear behavior including wear rate, worn surface and debris characteristic. Especially at the loads of below 10 N, the SiC particles without fracture on the worn surface also have a load-supporting effect against applied load, resulting in the same wear rates for two composites. With increasing load, subsurface deformation and strain-induced stress increase, so the pores beneath the worn surface become unstable and the cracks originated from these pores can propagate during wear process (Fig. 9(e)), which results in material removal from pin by delamination wear. Therefore, the wear process of the as-sprayed composite is developed into delamination and subsurface-cracking assisted adhesive wear from oxidative wear at lower applied loads than the as-forged composite, resulting in a larger wear rate. It is clear that the porosity in the as-sprayed composite would create a significant influence on wear rate when the applied load is large enough that the pores beneath worn surface become unstable. Plastic deformation and fracture have been considered as the main causes of severe wear. The worn surface and subsurface observations indicated that the adhesive wear occurring in severe region proceeds with subsurface shear fracture induced by serious plastic deformation. As the strain-induced shear stress is larger than the shear strength of the material itself in subsurface, the severe wear will occur. Therefore, the occurrence of severe wear depends on the shear strength of pin and the strain-induced shear stress. The presence of pores decreases the shear strength of the as-sprayed composite obviously since the shear fracture takes place directly through these pores. It results in adhesive wear at lower loads, thus the transition load from mild to
155
severe wear for the as-sprayed composite decreases significantly. In Fig. 3, the difference in the weight loss between the as-sprayed and as-forged composites increases with increasing sliding distance and applied load. For the as-sprayed composite, the worn surface may accumulate more defects that are originated from pores with increasing sliding distance. In addition, the increase of applied load can promote to enlarge the size of deformation zone beneath the worn surface, so that more pores can influence the wear process. These may result in a higher weight loss.
5. Conclusions In the load range of 5–400 N (corresponding normal stress is 0.1–8 MPa), the as-sprayed Al–6Cu–Mn based composite reinforced with 13 vol.% SiCp displayed three wear regions, while the as-forged composite four wear regions. A transition load from mild to severe wear could be observed with increasing load. The dominant wear mechanism in severe wear for both the as-sprayed and as-forged composites was adhesive wear by subsurface shear fracture. Mild wear was associated with three different mechanisms. With the increase of applied load, they were successively oxidation, delamination and subsurface-cracking assisted adhesion wear. The wear resistance of the as-sprayed composite was similar to that of the as-forged composite at lower loads (below 100 N). However, at higher loads, the wear rate of the as-sprayed composite increased significantly and the transition load from mild to severe wear decreased largely. The as-sprayed composite exhibited an inferior wear resistance than the as-forged composite. At lower loads, the pores beneath the worn surface of the as-sprayed composite were stable, and could not propagate significantly. Thus, porosity displayed a very small influence on dry sliding wear behavior of the composite. At higher loads, the pores beneath the worn surface became unstable and the cracks originated from these pores could propagate during wear process, which resulted in a higher wear rate and lowered obviously the transition load from mild to severe wear.
References [1] S.Q. Wu, H.G. Zhu, S.C. Tjong, Metall. Trans. A 30A (1999) 243. [2] G. Wang, P. Shi, M. Qi, J.J. Xu, F.X. Chen, D.Z. Yang, Metall. Mater. Trans. A 29A (1998) 1741. [3] S.X. Huang, K. Paxton, JOM 50 (8) (1998) 26. [4] A.T. Alpas, J. Zhang, Metall. Mater. Trans. A 25A (1994) 969. [5] R.A. Saravanan, J.-M. Lee, S.-B. Kang, Metall. Mater. Trans. A 30A (1999) 2523.
156
M. Gui et al. / Materials Science and Engineering A293 (2000) 146–156
[6] M. Roy, B. Venkataraman, V.V. Bhanuprasad, Y.R. Mahajan, G. Sundararajan, Metall. Trans. A 23A (1992) 2833. [7] A.T. Alpas, J. Zhang, Scr. Metall. Mater. 26 (1992) 505. [8] M.A. Martinez, A. Martin, J. Llorca, Scr. Metall. Mater. 28 (1992) 207. [9] B.N. Pramila Bai, B.S. Ramasesh, M.K. Surappa, Wear 157 (1992) 295. [10] A. Wang, H.J. Rack, Mater. Sci. Eng. 147A (1991) 211. [11] A. Ravikiran, M.K. Surappa, Scr. Mater. 36 (1997) 95. [12] B. Venkataraman, G. Sundararajan, Acta Mater. 44 (2) (1996) 461. [13] M.K. Surappa, S.V. Prasad, P.K. Pohatgi, Wear 77 (1982) 295. [14] A.G. Wang, I.M. Hutchings, Mater. Sci.Technol. 5 (1989) 71. [15] S.V. Prasad, P.K. Rohatgi, J. Met. 11 (1987) 22. [16] C.A. Caracostas, W.A. Chiou, M.E. Fine, H.S. Cheng, Scr. Metall. Mater. 27 (1992) 162. [17] H.L. Lee, W.H. Lu, S.L. Chan, Wear 159 (1992) 223.
.
[18] [19] [20] [21] [22] [23]
[24] [25] [26] [27] [28] [29]
B. Venkataraman, G. Sundararajan, Acta Mater. 44 (1996) 451. K.J. Bhansali, R. Mehrabian, J. Met. 34 (1982) 30. A.T. Alpas, J. Zhang, Wear 155 (1992) 83. Z.F. Zhang, L.C. Zhang, Y.W. Mai, J. Mater. Sci. 30 (1995) 1961. R.L. Deuis, C. Subramanian, J.M. Yellup, Comp. Sci. Technol. 57 (1997) 415. O. Vincsbo, S. Hogmark, in: D.A. Rigney (Ed.), Fundamentals of Friction and Wear of Materials, American Society for Metals, Metals Park, OH, 1981, p. 373. J. Zhang, A.T. Alpas, Acta Mater. 45 (1997) 513. R. Shivanath, P.K. Sengupta, T.S. Eyre, Br. Foundryman 70 (1977) 349. K. Razavizadeh, T.S. Eyre, Wear 79 (1982) 325. K. Razavizadeh, T.S. Eyre, Wear 87 (1983) 261. K.C. Ludema, Wear 100 (1984) 315. S.C. Lim, M.F. Ashby, Acta Metall. 35 (1987) 1.