**3.1 Formation of the MML**

318 Recent Trends in Processing and Degradation of Aluminium Alloys

1. The load acts normal to the surface areas that in contact will exert a compressive stress on the materials, which has a similarity with cold working and is usually concentrated

2. A force exerted by the machine in the direction of motion, overcomes the following

• The friction force, F, that is proportional to the normal load between contacting

• The static coefficient of friction, that is higher at the start of the motion than the

• Adhesion; the tendency of the two mating metals to adhere to each other. It may

result in the surfaces being locally bonded together, forming a junction. • In extreme cases, resistance to motion is caused by abrasive material.

Due to their low density and excellent corrosion resistance, aluminium has become a substitute for steels especially in structures that require high performance and weight reduction. As with most other metals, aluminium reacts with oxygen in air. A submicron thick oxide layer is formed to provide effective corrosion protection. Aluminium is also nonmagnetic and non-toxic, and can be formed by all known metal working processes. The density of aluminium is 2.7g/cm³ or approximately one third the density of steel and aluminium alloys have tensile strengths of between 70 and 700N/mm². At low temperatures the strength of aluminium and its alloys increases without embrittlement in contrast to most steels (Pollack, 1977). Table 1 shows a comparison of the physical characteristics of some of

During the 1980's, about 85% of aluminium was used in the wrought form, that is rolled to sheet, strip or plate, drawn to wire or extruded as rods or tubes (Higgins, 1987 and Polmear, 1989). Some of the alloys may undergo subsequent heat-treatment in order to achieve the desired mechanical properties. The most common methods to increase the strength of

• To disperse any second-phase constituents or elements in solid solution and cold work

• To dissolve the alloying elements in solid solution and re-precipitate them. These are also as heat-treatable or precipitation-hardening alloys (originally known as 'age-

 Al Fe Cu Zn Density (g/cm³) 2.7 7.9 8.9 7.1 Melting Point (YP) (°C) 660 1540 1083 419 Electrical conductivity (%) 63 16 100 30 Specific Heat/Thermal Volume (J/kg, K) 900 450 390 390 Thermal Conductivity (W/m, K) 220 75 390 110 Linear exp. coefficient (10-6/K) 24 12 16 26 Electrical resistance (10-9 ohms/m) 27.5 105 17 58 Young's Modulus, (GPa) 70 220 120 93 Table 1. Physical characteristics of some of the most important construction materials

in the rolling case.

surfaces.

dynamic friction.

**2.2 Wear of aluminium alloys** 

aluminium alloys are:

hardening' alloys).

the most important construction materials.

the alloy; there are known as *non-heat-treatable alloys*.

resistance:

In the case of ductile materials like aluminium alloys, most wear mechanism observed are consistent with Archard adhesive wear characterised by plastic ploughing and transfer of material from the counterface. With respect to friction and wear behaviour, numerous authors (Perrin and Rainforth, 1995, Leonard et al., 1997, Jiang and Tan, 1996, How and Baker, 1997 and Rigney, 1998) have concluded that the tribological behaviour is influenced by the mechanical, physical and chemical properties of these near-surface materials. In all cases, a mechanically mixed layer (MML) was present in most dry worn wrought aluminium alloys due to the repetitive sliding. However, significant differences between the MML of each alloy were observed. Their thickness which varied with loads suggested that the subsurface zones of the materials to the sliding and impact wear consisted of 3 zones (Rice et al., 1981) as indicated in Fig. 1.


 1 The process by which machine parts improve in conformity, surface topography and frictional compatibility during the initial stage of use.

Effects of Dry Sliding Wear of Wrought Al-Alloys on Mechanical Mixed Layers (MML) 321

**B**

Some MML was very thin and the matrix of the Al alloy almost approached to the top worn surface, is very likely associated with the plastic flow during deformation, where the mixed layer can be replenished with fresh base material by a large plastic flow in the subsurface (Li and Tandon, 1999). Biswas (2000) studied the thickness of the MML appears to be controlled by the depth of crater and abrasion grooves made on the surface. In fact, he further concluded that one of the pre-requisites for the formation of this MML correlates with severe deformation of the top layer of the softer material pair, which in this case is the aluminium alloys. It appears that, when one surface is softer than the other, metal may be transferred from the soft to the hard surface. The material could be transferred back and forth several times during sliding and eventually produce wear debris particles (Heilmann

In agreement with Rice et al. (1981), Heilmann et al. (1983) and Rigney et al. (1984), the mixed layer is composed of a mixture of two mating materials and from this layer, the loose debris were derived. Based on the results of EDS (Figs. 3), the layer had similar structure and composition to the loose debris. Here, presumably some of the wear debris from the counterface, together with the debris from the pin may have been compacted to form the MML. In other words, it can be concluded that such debris are not derived directly from the base material, with an exception for the case of abrasion, in which microcutting and

In the present study, the MML were found to contain Al, Fe (for Al/M2 case) and O (in the form of oxide), which proves the source of element in the MML obviously originated from the counterface. The oxides were found to be coexisted with other phases in the MML and the wear debris, which is an expected phenomenon since the wear system was exposed to air. They could provide microstructural stability as a second phase in the ultrafine grained structure in debris, as proposed by Rigney et al. (1984). The oxides which have been known to form some protective and some destructive (Fischer, 1997 and Ravikiran et al., 1995) were then fractured and comminuted in further sliding process. The crushed oxides can be

Fig. 2. A is an example of A3004 alloys after 10.8km slid against M2 at 140N. B is the

magnified (Normaski) view of a selected area (Ghazali, 2005)

et al., 1983).

**A** 

**3.2 Composition of the MML** 

microploughing are prominent.

c. Zones 3 – is known as tribolayer which forms in-situ, and usually contains chemical species from the counterface and test environment as well as the bulk material.

Fig. 1. An illustration of deformation during dry sliding (Ghazali, 2005)

The mixed layer (zone 3), which is commonly known as the mechanically mixed layer (MML), was formed through the incidental mixing between the two materials that statistically occurs at the contact spots under normal pressures. Crack and void formation were generally associated at the zone 2/3 interface and may dictate the dimensions of the wear debris formed (Suh, 1973). The extent and compositional features of these sub-surface zones were found to depend on the conditions of sliding wear, material and environment. Rice et al. (1981-1982) also indicated that these sub-surface zones developed quickly under dry sliding wear conditions. The present work has confirmed that a MML was formed in the sliding wear of the Al-alloys against both counterfaces. Its particles are recognised to have the same physical structure and chemical composition as those of the base pair (Biswas, 2000). The distinctive morphology of the mixed layer has led to a suggestion that its formation was due to a compression of the transfer material and the entrapped debris, which was followed by mechanical mixing during the sliding process. As highlighted by Heilmann et al. (1983), the MML which develops at an early stage (even before loose debris was obtained), is common in both dry and lubricated sliding wear process. In dry sliding condition, a high compressive pressure and large shear strains in the asperities were produced. Heavy plastic deformation and shear strains in the worn surface give rise to dislocation cells and elongated subgrains, as seen in i.e; Fig. 2, which is consistent with Heilmann et al. (1983), Rigney et al. (1981), Chen (1986), Chen and Rigney (1986) and Kuo and Rigney (1992).

Fig. 2. A is an example of A3004 alloys after 10.8km slid against M2 at 140N. B is the magnified (Normaski) view of a selected area (Ghazali, 2005)

Some MML was very thin and the matrix of the Al alloy almost approached to the top worn surface, is very likely associated with the plastic flow during deformation, where the mixed layer can be replenished with fresh base material by a large plastic flow in the subsurface (Li and Tandon, 1999). Biswas (2000) studied the thickness of the MML appears to be controlled by the depth of crater and abrasion grooves made on the surface. In fact, he further concluded that one of the pre-requisites for the formation of this MML correlates with severe deformation of the top layer of the softer material pair, which in this case is the aluminium alloys. It appears that, when one surface is softer than the other, metal may be transferred from the soft to the hard surface. The material could be transferred back and forth several times during sliding and eventually produce wear debris particles (Heilmann et al., 1983).

### **3.2 Composition of the MML**

320 Recent Trends in Processing and Degradation of Aluminium Alloys

c. Zones 3 – is known as tribolayer which forms in-situ, and usually contains chemical species from the counterface and test environment as well as the bulk material.

Traction Force

Tangential elastic & plastic

Plastic flow lines Shear crack

Sliding Direction

Debris

displacements

**<sup>F</sup> <sup>F</sup>**

**<sup>R</sup> <sup>N</sup>**

**F**

**T**

**R N T**

**Zone 3 -MML**

Fig. 1. An illustration of deformation during dry sliding (Ghazali, 2005)

The mixed layer (zone 3), which is commonly known as the mechanically mixed layer (MML), was formed through the incidental mixing between the two materials that statistically occurs at the contact spots under normal pressures. Crack and void formation were generally associated at the zone 2/3 interface and may dictate the dimensions of the wear debris formed (Suh, 1973). The extent and compositional features of these sub-surface zones were found to depend on the conditions of sliding wear, material and environment. Rice et al. (1981-1982) also indicated that these sub-surface zones developed quickly under dry sliding wear conditions. The present work has confirmed that a MML was formed in the sliding wear of the Al-alloys against both counterfaces. Its particles are recognised to have the same physical structure and chemical composition as those of the base pair (Biswas, 2000). The distinctive morphology of the mixed layer has led to a suggestion that its formation was due to a compression of the transfer material and the entrapped debris, which was followed by mechanical mixing during the sliding process. As highlighted by Heilmann et al. (1983), the MML which develops at an early stage (even before loose debris was obtained), is common in both dry and lubricated sliding wear process. In dry sliding condition, a high compressive pressure and large shear strains in the asperities were produced. Heavy plastic deformation and shear strains in the worn surface give rise to dislocation cells and elongated subgrains, as seen in i.e; Fig. 2, which is consistent with Heilmann et al. (1983), Rigney et al. (1981), Chen (1986), Chen and Rigney (1986) and Kuo

load

harder

softer

**Zone 2 -Plastic deformation zone**

**Zone 1 -Base material**

Microroughness

Shearing zone

and Rigney (1992).

**F** Resultant Force **F** Normal Force **F** Tangential Force Surface in tension Surface in compression

> In agreement with Rice et al. (1981), Heilmann et al. (1983) and Rigney et al. (1984), the mixed layer is composed of a mixture of two mating materials and from this layer, the loose debris were derived. Based on the results of EDS (Figs. 3), the layer had similar structure and composition to the loose debris. Here, presumably some of the wear debris from the counterface, together with the debris from the pin may have been compacted to form the MML. In other words, it can be concluded that such debris are not derived directly from the base material, with an exception for the case of abrasion, in which microcutting and microploughing are prominent.

> In the present study, the MML were found to contain Al, Fe (for Al/M2 case) and O (in the form of oxide), which proves the source of element in the MML obviously originated from the counterface. The oxides were found to be coexisted with other phases in the MML and the wear debris, which is an expected phenomenon since the wear system was exposed to air. They could provide microstructural stability as a second phase in the ultrafine grained structure in debris, as proposed by Rigney et al. (1984). The oxides which have been known to form some protective and some destructive (Fischer, 1997 and Ravikiran et al., 1995) were then fractured and comminuted in further sliding process. The crushed oxides can be

Effects of Dry Sliding Wear of Wrought Al-Alloys on Mechanical Mixed Layers (MML) 323

0 50 100 150 200 **Wave Rate (mm /m)×10 <sup>3</sup> -4**

(a)

0 20 40 60 80 100 120 140 160

**Load (N)**

 (b) Fig. 4. The relationship between MML thickness and (a) wear rate and (b) load after 10.8km

100

80

60

**MML Thickness (μm)**

40

20

0

120

100

80

60

**MML Thickness (μm)**

40

20

0

against M2 counterface (Ghazali, 2005)

dispersed into the mixed surface layer and act as a pinning source of the grain boundaries in the ultrafine mixture in the MML and in the wear debris (Li and Tandon, 1999).

Fig. 3. Subsurface damage of longitudinal cross sections of A2124 alloys against M2 at 140N after sliding 10.8 km. Black arrows indicate the direction of sliding. A corresponding EDS analysis are shown in A, B and C areas (Ghazali, 2005)
