Dry sliding wear behavior of cast SiC-reinforced Al MMCs

Dry sliding wear behavior of cast SiC-reinforced Al MMCs

Materials Science and Engineering A360 (2003) 116 /125 www.elsevier.com/locate/msea Dry sliding wear behavior of cast SiC-reinforced Al MMCs Tiejun ...

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Materials Science and Engineering A360 (2003) 116 /125 www.elsevier.com/locate/msea

Dry sliding wear behavior of cast SiC-reinforced Al MMCs Tiejun Ma a, Hideki Yamaura b, Donald A. Koss c,*, Robert C. Voigt d a

Tregaskiss Ltd., 2570 North Talbot Road, Oldcastle, ON, Canada N0R1L0 Moka Works, Hitachi Metals Ltd., 13 Kinugaoka Mohka, Tochigi, Japan c Department of Material Science Engineering, The Pennsylvania State University, 202A Steidle Building, University Park, PA 16802, USA d Department of Industrial Manufacturing Engineering, The Pennsylvania State University, University Park, PA 16802, USA b

Received 14 October 2002; received in revised form 6 May 2003

Abstract Dry sliding block-on-ring wear tests were performed on a squeeze cast A390 Al alloy, a high pressure die cast 20%SiC /Al MMC, and a newly developed as-cast 50%SiC /Al MMC. The testing conditions spanned the transition that control the mild to severe wear for all materials. The results show that the sliding wear resistance increases as SiC particle volume fraction increases. The critical transition temperature, at which wear rates transit from mild to severe, also increases with increasing SiC content. Examination of the wear surfaces, the subsurface characteristics, and the wear debris indicate that a hard ‘mechanically alloyed’ layer, high in SiC content, forms on the sliding surface of the 50%SiC composite. This layer prevents the surface adhesion wear mechanisms active in the A390 alloy, and it inhibits delamination wear mechanisms that control the mild wear of the 20%SiC composite. As a result, mild wear of the 50%SiC composite occurs by an oxidation process. In the 20%SiC material, severe wear occurs as a consequence of material removal by a flow-related extrusion-like process. In contrast, the high SiC content prevents plasticity in the 50%SiC composite, which eventually is susceptible to severe wear at very high temperatures ( :/450 8C) due to a near-brittle cracking processes. # 2003 Elsevier B.V. All rights reserved. Keywords: Metal matrix composites; High volume fraction SiC/Al; Dry sliding wear; Wear mapping

1. Introduction The increased demand for light-weight materials with high specific strengths in the automotive and aerospace industry has accelerated the development and use of MMCs. For discontinuous reinforced aluminum matrix composites (DRA), SiC particles have been found to have excellent compatibility with the aluminum matrix [1] with relatively low cost. The attractive properties of DRA include the comparative ease of processing using conventional techniques [2] and, when compared to conventional aluminum alloys, significantly higher Young’s moduli, and improved wear resistance. A large amount of work has been done on the wear behavior on SiC/Al composites with up to 30% particle content. Alpas, Zhang and other researchers have systematically

* Corresponding author. Tel.: /1-814-865-5447; fax: /1-814-8652917. E-mail address: [email protected] (D.A. Koss). 0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5093(03)00408-8

examined the dry sliding wear behavior of aluminum alloys and DRAs as a function of applied load and sliding velocity [3 /10]. Various wear stages and mechanisms have been identified [11] including running-in wear [12], abrasive wear [6,8], delamination [13,14] and adhesion wear [15,16]. Using wear map schemes for aluminum alloys proposed by Liu [17], Wilson and Alpas composed wear mechanism maps for A356 / 20%SiC composites [18]. Their results showed that the transition from mild to severe wear, (a key wear resistance parameter), shifted toward higher loads and higher velocities with increasing SiC particle content. Increasing particle fraction not only reduced the wear rate, but it also pushed wear transitions to more critical conditions. The transition from mild wear to severe wear was also observed to correlate with the surface temperature of the material during testing. Wear mechanism transitions occurred when the material reaches a certain critical temperature [19,20]. Although the specific temperature value differed from case to case, in general

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increasing the content of reinforcement particles increased the critical temperatures. In summary, while the wear behavior of a number of SiC-reinforced DRAs has been examined, these studies have all focused on SiC/Al MMCs with 5/30% particulate. Much less is known about the wear behavior of high volume fraction DRAs. Candon et al. observed the wear behavior of a Al /SiC composite containing 65%SiC by rubbing the compacts onto Al2O3 abrasive belts [21] They found that the wear rate depend on both the SiC particle size as well as the Al2O3 abrasive size. Straffelini examined the dry sliding wear of three Al MMCs containing up to 65%SiC by sliding the compacts against a steel counterface at a selected sliding speed and for a given distance. In these tests the wear process was dominated by an oxidation mechanism [22]. The purpose of this study is to contrast the dry sliding wear behavior of two cast SiC-reinforced DRAs to that of an as-cast A390 aluminum alloy. Specifically, we present the wear behavior of a commercial 20%SiC /Al MMC, and a 50%SiC /Al MMC newly developed by one of the authors [23]. Wear testing has been performed under a range of loads (22 /111 N) and sliding speeds (3.3 and 6.0 m s 1) including conditions more severe than were used in many studies. (For example the Straffelini study [22] was performed at sliding speed of 0.6 m s1 */roughly an order of magnitude slower than the highest speed used in this study.) Thus, the wear conditions examined in this study include both mild wear and severe wear, and serve to identify the transition from mild to severe wear as a function of load, sliding speed, and the associated temperature. The dominant failure mechanisms are also identified and contrasted among these three materials.

2. Materials and experiment procedure Table 1 summarizes the materials evaluated in this study. The SAE A390 aluminum alloy was conventionally processed by squeeze casting and evaluated in the as-cast condition. The 20%SiC material was produced by casting and evaluated in the as-cast condition. The 20%SiC material was produced by conventional high pressure die casting. The 50%SiC material was produced by pressure-less infiltration of a pressed SiC compact

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Fig. 1. Microstructures of (a) 20%SiC DRA and (b) 50%SiC materials.

using technique described previously [23]. As shown in Fig. 1, the microstructures of each of each of the composites have a relatively uniform distribution of SiC particles. Dry sliding wear tests were carried out with a blockon-ring type machine, using specimens in the form of 6.35 mm cubes. The wear surfaces were first ground and then polished through 1-mm diamond paste prior to wear testing. The 90 mm diameter counter ring was made of a 52 100 bearing steel hardened to an average hardness of HRC63. The sliding velocities of 3.3 or 6.0 m s 1 were used with applied loads of 22, 45, 67 or 111 N. To estimate the specimen temperature during testing, a 0.4 mm thick slit was cut into the back face of the specimen to enable thermocouple placement. The tip of the thermocouple was located approximately at the body center of the specimen about 3 mm away from the wear surface. A rotating torque meter was also installed to measure the applied torque during the tests so that the coefficient of friction could be determined.

Table 1 Materials tested Designation

Matrix composition

Reinforcement

Density (g cm3)

A390 20%SiC DRAa 50%SiC DRA

Al /17%Si, 4.5%Cu, 0.5%Mg Al /7.5%Si, 2%Cu, 1%Ni Al /12%Si

20 wt.%SiC particles(15 mm) 50 wt.%SiC particles (50 /100 mm)

2.52 2.48 2.72

a

Commercial alloy DuralcanTM F30 20S.

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The specimens were weighed before and after testing, and the weight losses were then transformed to volume losses (assuming the density of 2.52, 2.48 and 2.72 g cm 3 for the A390, 20%DRA and 50%DRA materials, respectively). The wear rates were calculated as volume loss divided by sliding distance. Worn material that was extruded beyond the edge of the specimen during testing (i.e. the mushroom edge) was removed prior to weighing. The wear surface and wear debris were examined using a stereo light microscope and a scanning electron microscope (SEM). The final microstructures and surface condition were also evaluated after wear testing by examining sections parallel and perpendicular to the sliding direction. SEM backscattered electron (BSE) images were also evaluated to identify the spatial distribution of elements in the microstructure.

3. Results and discussion 3.1. Wear rate behavior Due to the test specimen geometry and surface preparation, most of the specimens exhibited an initial ‘run-in’ wear stage in which the wear rate fluctuated and exhibited unstable behavior. A stable wear rate stage subsequently developed after this run-in sliding distance that depended on materials and testing conditions. For example, the stable stage developed after 1 km of wear

distance for the 20%SiC DRA at 45 N /3.3 m s 1. Data reported in this study were obtained from the tests after initial run-in. Fig. 2 shows wear volumes as a function of sliding distance for each material subjected to a variety of wear loads and wear speeds. At small loads the volume of material removed by wear (i.e. ‘volumetric wear’) is linearly proportional to sliding distance for most conditions, indicating a constant wear rate. Furthermore, most of the low load data in Fig. 2 exhibits increased wear rate with increasing load at a given sliding velocity. However, some of the high-load tests developed wear rates considerably greater than the linear wear trends. Therefore, while the constant wearrate data in Fig. 2 suggest ‘mild’ wear for most test conditions, much higher wear rates, termed ‘severe’ wear, are also evident at higher applied loads. Fig. 3 indicates the dependence of wear rate on applied load for each sliding speed. The ‘mild wear’ conditions shown in Fig. 3 are based on the linear wear behavior shown in Fig. 2. The severe wear rate conditions shown in Fig. 2 can also be clearly seen in the excessively high wear rates in Fig. 3. While the rate/ trend curves indicated in Fig. 3 are not complete enough to rigorously establish wear regions in Fig. 3, these curves are drawn in a manner consistent with that suggested by Wilson and Alpas [19]. They suggest that the wear rates increase gradually with applied load in the mild wear region. However, at a ‘critical load’ that depends on sliding velocity, the wear rate abruptly increases to reflect the condition termed ‘severe’ wear.

Fig. 2. Relationship between sliding distance and volumetric wear at different testing loads (a) A390 Al, (b) 20%SiC DRA, (c) 50%SiC DRA, 3.3 m s 1 (d) 50%SiC DRA, 6.0 m s 1.

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Fig. 3. Dependence of wear rate on testing load at different sliding speeds (a) 3.3 m s 1 (b) 6.0 m s 1.

Fig. 4. Wear rates of the 20%DRA material (this study) displayed on a wear map of A356 /20%SiC MMC from Wilson and Alpas [18].

For these tests, this transition in wear rates from mild to severe wear occurred at a rate of approximately 8 /10 mm3 km 1. From Fig. 3, it is clear that A390 alloy has a higher wear rate than 50%SiC DRA in the mild wear regime, especially at high sliding speeds. In addition, the critical load at which the wear rates transition from mild to severe is lower for the A390 alloy and higher for the 50%SiC DRA composite. Taken as a whole, these data indicated that the 50%SiC DRA composite exhibits the best wear resistance of the three materials examined, and at high sliding speeds, suggest that the increase in SiC content from 20 to 50% provides a significant additional wear resistance. These wear data may also compared to previous results in the form of a wear mechanism map, as first suggested by Tabor [15]. Employing similar testing methods to this study, Wilson and Alpas [18] developed a wear mechanism map for an A356 /20%SiC material which is similar to the 20%SiC DRA evaluated in this study. Fig. 4 shows the 20%SiC DRA wear data from this study together with the Wilson and Alpas-derived wear mechanism map. In Fig. 4, the number beside each data point denotes the actual wear rate expressed in units of 10 4 mm3 m 1 (instead of units of mm3 km 1, as in Fig. 3). Fig. 4 clearly indicates that the results from this study are consistent with those of Wilson and Alpas and, at the same time, extend the wear map to higher sliding speeds.

3.2. Influence of temperature Alpas et al. [14,18/20] have reported that for aluminum alloys and for DRA materials the onset of severe wear occurred within a critical range of temperatures. In this study, the contact surface temperature during wear testing was estimated with a thermocouple placed close to the contact surface (within approximately 3 mm) for the small (B/0.7 g) test specimen, thus the measurement conditions were such that the measured temperature should be similar to that of the contact surface temperature estimated in previous studies. Although the specimen temperatures converged to an almost constant temperature for a given set of test conditions, a fully stable test temperature was not observed. Reported here is the maximum specimen temperature measured during a test run. Fig. 5 shows that severe wear rates occurred when specimen temperatures approached 350 8C for the 20%SiC DRA specimens and about 450 8C for the 50%SiC DRA. Thus, as the SiC particle content increased from 20% to 50%, the transition from mild to severe wear appeared to occur at much higher test specimen temperatures (DT :/100 8C). Though subject to interpretation of Fig. 5, it also appears in that the critical temperature for transition from mild to severe wear decreases somewhat with increasing load at these sliding speeds. Again, these results at the high sliding

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the 50%DRA has the highest coefficient of friction of the materials tested. Additional testing is needed to confirm this apparent trend. 3.4. Tribological characteristics: 20%SiC DRA

Fig. 5. The relationship between the maximum temperature during wear testing and wear rate (a)20% (b) 50%SiC DRA.

speeds for the 20%SiC DRA material are consistent with those of Wilson and Alpas for 20%SiC /A356 Al at lower sliding speeds [18]. Wilson and Alpas also reported a critical mild to severe wear transition temperature of :/338 8C; that value compares favorably with T :/350 8C measured here for the 20%SiC composites. The critical temperature limit resulting in very high ‘catastrophic’ wear is estimated to be approximately 400 8C for the 20%SiC DRA material and 500 8C for the 50%SiC DRA material. These critical temperatures appear to be independent of applied load for the testing range evaluated in this study. For the 50%SiC DRA material, the solidus of the matrix aluminum alloy is approximately 520 8C, and thus the lack of wear resistance at such high contact surface temperatures (even at low loads) can be expected as a consequence of surface melting. 3.3. Coefficient of friction Torque measurements varied considerably during a given wear test, and thus the friction coefficient values estimated from the torque measurements can only be reported as a range of values. The range of friction coefficients for the A390 Al alloy, the 20%SiC DRA material, and the 50%SiC DRA material were 0.4 /0.7, 0.3 /0.7 and 0.5 /1.3, respectively. Despite the uncertainty in the coefficient of friction values, it appears that

Observations of the wear surfaces as well as the microstructure beneath the wear surface indicate significant microstructural changes during wear testing. In this section, we will concentrate on the tribological characteristics of the 20%SiC DRA material compared to the A390 alloy. It has been reported that the wear behavior of SiC-reinforced composites is influenced by the fracture of SiC particles, which can occur even at very low loads [10]. The fractured SiC particles can in turn literally machine through the steel wear substrate, creating elemental iron and iron oxide in the wear debris. Therefore, unlike the A390 alloy, the surface of the 20%DRA material is soon covered by a mixed layer containing fractured SiC particles, iron, and oxides, all within a so-called ‘transferred layer’. The formation of this Fe-rich layer appears to be the cause of the increased wear resistance of SiC DRA composites examined by Hosking [2] and other researchers [3,9,23,24]. Fig. 6 shows a typical wear surface in depth profile for the 20%SiC DRA material tested under mild wear conditions. Three different layers can be distinguished within the wear profile: (1) A plastic deformation layer (labeled 1 in Fig. 6) characterized by deformed/inclined grains near the surface. (2) An oxide layer (labeled 2 in Fig. 6) is present above the plastic deformation layer. It is composed of iron oxide, aluminum matrix, and some small fractured SiC particles. (3) The transfer layer at the surface (labeled 3 in Fig. 6) appears as a dark layer under light microscopy and contains numerous microcracks that extend to the surface. BSE imaging (Fig. 7a) with element mapping (Fig. 7b and c) indicates that the transfer layer has an iron content significantly higher than that of the oxide layer. Iron debris is evident that appears to be a result of microcutting by fractured SiC particles. In the case of the A390 alloy, there is no transfer layer, probably because the silicon particles (HV 1000) in the A390 microstructure are not sufficiently large or sufficiently fracture resistant to induce microcutting on the hardened steel counter ring. The transfer and oxide layers observed on DRA wear specimens contribute to improved high temperature wear resistance by preventing the aluminum matrix from direct contact with the hot steel counter during sliding, hence eliminating adhesion wear. In the case of A390 alloy the plastic deformation layer was thinner than those of DRA materials. Shabel et al. [9] also reported that a DRA

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Fig. 6. Microstructures near the wear surface (a) 20%SiC DRA at 45 N /3.3 m s 1 /0.5 km wear conditions, (b) 20%SiC DRA at 45 N /0.5 km wear conditions showing a transfer layer region, (c) 20%SiC DRA at 45 N /3.3 m s 1 /1.0 km wear conditions, (d) 20%SiC DRA at 45 N /3.3 m s 1 /5.0 km wear (Note: Sliding direction for (c) is from right to left; the others are from left to right).

composite showed evidence of a larger plastic deformation region than a non-reinforced alloy during sliding wear testing. It appears that in the case of A390 under the current testing conditions, adhesion occurred on the surface before the subsurface was deformed enough to promote delamination crack. Close examination of Fig. 6 (see Fig. 6c and d) and similar sections of other specimens under mild wear conditions show that cracks initiated in the transfer layer near the wear surface. Flake-like debris forms as a consequence of the propagation and coalescence of these cracks in the subsurface region. The debris particles are roughly 10 /50 mm in thickness, have a dark appearance, and are composed of oxides and fractured SiC particles.

Thus, the wear debris of 20%SiC DRA at mild wear conditions (up to 67 N at 3.3 m s1 and 45 N at 6.0 m s 1) exhibits typical of delamination wear, consistent with the mild wear region of the wear map for the 20%SiC/A356 composite developed by Wilson and Alpas shown in Fig. 4. As the wear tests specimen temperature increases due to increased applied load and velocity, the matrix, especially the deformed subsurface layer, softens substantially. However the oxide layer and the transfer layer, which contains more iron, iron oxide and small SiC particles, are likely to maintain their high hardness. As a result, plastic deformation within the soft subsurface layer results in material ‘extrusion’, as shown in Fig.

Fig. 7. Back-scattered scanning electron micrographs of mild wear surfaces for 50% DRA material showing the oxide and transfer layers. (a) BSE image (b) Fe elemental distribution (c) Si elemental distribution.

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Fig. 8. (a) Material extrusion due to subsurface flow for 20%SiC DRA /45 N /3.3 m s 1 /2.0 km wear conditions; sliding direction from left to right (a) surface microstructure (b) wear debris for 20%SiC DRA at 111 N /3.3 m s 1 /2.0 km wear conditions (c) material extrusion for 20%SiC DRA /45 N /3.3 m s 1 /2.0 km, sliding direction is from left to right), (c) material extrusion and debris of 20%SiC DRA at 111 N /3.3 m s 1 /2.0 km wear conditions.

8 for the 20%SiC DRA above 45 N at 3.3 m s 1 after 2 km wear distance. This extrusion process forms large debris particles and causes the severe wear at these high specimen temperatures. In summary, delamination wear is the main wear mechanism of the 20%SiC DRA material under mild wear conditions, while subsurface plastic flow and material extrusion is the dominant failure mechanism when the wear rate becomes severe. No adhesion occurred in this material under the current testing conditions because of the presence of the transfer and oxide layers. 3.5. Tribological characteristics: 50%SiC DRA During mild wear conditions the wear rate of the 50%SiC DRA material, is quite low, and the debris consists mostly of fine particles (5/200 mm). The wear surface is covered by a dark oxide film. The debris consists primarily of oxide particles, consistent with a primary oxidation wear mechanism for the 50%SiC DRA material under these wear conditions. The associated wear surface profile (Fig. 9a) includes numerous subsurface voids that formed during sliding. The result is a local void volume fraction that is much higher than the initial porosity fraction in the cast material, which varied from 5 to 7%. A similar observation has been reported by Straffelini [22] for 30%SiC DRA material. It

is likely that fracture of the large (/200 mm) primary silicon particles initiates the voids. A significant difference between the 50%SiC DRA material and the 20%SiC DRA material is that there is no evidence of any subsurface plastic flow in the 50%SiC composite. A key to the good mild wear resistance of the 50%SiC DRA is shown in Fig. 9b, which presents an enlargement of the surface layer region shown in Fig. 9a. Under mild wear conditions, a ‘mechanically alloyed’ layer with a very high volume fraction of SiC particles develops. Despite the presence of extensive porosity just below this mechanically alloyed layer, it remained intact and did not fail during sliding wear at elevated temperatures. Under aggressive wear conditions, the 50%SiC DRA forms very large wear debris particles (:/1/1 /2.5 mm) as a result of near-brittle fracture at very high temperatures. Careful examination of the resulting fracture surface (Fig. 10) shows evidence of alloy sintering as well as the rounding of sharp dimple edges at the fracture surface, close to the contact surface (Fig. 10b and c). These observations are consistent with the measured temperatures in excess of 500 8C that are near the 520 8C solidus temperature of the aluminum matrix. The appearance of the fracture surface progressing away from the wear surface (Fig. 10b /e) clearly shows fracture characteristics of a low toughness material and relatively easy crack growth. Few if any dimples

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Fig. 9. (a) The profile of the wear surface of 50%SiC DRA at 45 N /6.0 m s 1 /5.0 km showing subsurface voids; (sliding direction from right to left. ‘A’ denotes fracture of Si particles) (b) the enlargement of the ‘indicated’ region from (a) showing the very high SiC volume fraction on the surface layer.

Fig. 10. SEM images of fracture surfaces at selected locations; 50%SiC DRA at 111 N /6 m s 1 and 1.4 km sliding distance.

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or ductile tear features are evident. In some instances, un-wetted particle-matrix interfaces and fractured brittle SiC or silicon particles were evident. The results described above suggest that the superior mild wear resistance of the 50%SiC DRA material results from the formation of a hard ‘mechanically alloyed’ surface layer with a very high volume fraction of SiC particles. When compared to the 20%SiC DRA, the high transition temperature from mild to severe wear of the 50%SiC DRA appears to be a consequence of the combined effects of the hard surface layer remaining intact and the high temperature strength of the composite substrate. Subsurface plastic flow is restricted and material extrusion is eliminated in this high volume fraction SiC-reinforced composite. Thus, the severe wear process of the 50%SiC DRA material involves a near-brittle crack initiation/propagation process associated with the formation of a significant subsurface void population. As a result, crack growth causes the removal of large blocks of material. The presence of porosity and large brittle SiC particles (50 /100 mm) as well as large primary silicon particles (200 mm) all contribute to easy crack growth once a crack is initiated.

fraction of SiC particles that wears very slowly as a result of oxidation wear. (4) A principal mechanism of severe sliding wear for A390 alloy is seizure and galling, while for the 20%SiC DRA material, the principal mechanism is subsurface flow resulting in material loss by an extrusion-like process. In the case of the 50%SiC DRA where plastic deformation is restricted even at an elevated temperatures, the severe wear process is dominated by a nearbrittle crack initiation/propagation process promoted by subsurface voids. The high volume fraction of large brittle particles (the primary silicon phase as well as SiC) contributes to easy crack growth once a crack is initiated. Under the most severe wear condition, the temperature of 50%DRA specimens at 111 N load and 6.0 m s1 sliding velocity exceeded 500 8C and alloy sintering was evident on the fracture surfaces.

Acknowledgements This research was supported by Hitachi Metals Ltd. The authors also would like to thank Professor Joe Conway, Penn State University, for his advice during wear testing.

4. Conclusions The following conclusions can be made from this study in: (1) Both the 20%SiC and 50%SiC DRAs show better dry sliding wear resistance than the A390 alloy. Under the relatively high sliding speeds employed in this study (3 and 6 m s 1), the 50%SiC DRA exhibits the lowest wear rates. The transition from mild to severe wear also shifts to increased loads and velocities with increasing SiC content. (2) The wear behavior of all of the materials is temperature sensitive such that the transition to severe wear initiates at higher critical temperatures as SiC content increases. Thus, while the A390 alloy undergoes a transition to severe wear at roughly 180 8C, this transition occurs at about 350 8C for the 20%SiC DRA and 450 8C for the 50%SiC DRA. These critical temperatures also tend to decrease somewhat with increasing applied loads. Moreover, the transition temperature for catastrophic wear occurs at approximately 400 8C for the 20%SiC DRA and 500 8C for the 50%SiC DRA. (3) Under mild sliding wear conditions, oxide and transfer layers form on the wear surface of the 20%SiC DRA, preventing adhesion wear and contributing to improved wear resistance compared to the A390 alloy. Delamination wear mechanisms appears to control mild wear of the 20%SiC DRA. In contrast, during mild wear, the 50%SiC DRA forms a hard ‘mechanically alloyed’ surface layer comprised of a high volume

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