2.3.7 Physical Film Formation
Physical film formation is similar to the oxidative wear mechanism in that a layer or film is developed. In oxidative wear the film is formed chemically, while physical film formation involves physical processes such as smearing. The film formed can be a transferred film, a third-body film, or contain some of each. Factors which influence the formation of this film include the roughness, geometry, load, sliding speed, temperature, and adhesion/materials properties of the rubbing surfaces. Similar to oxide film formation and removal, a physically formed film can change composition as wear progresses. It is often made from a combination of the two original sliding surfaces, oxides which have formed, and pieces of debris which may have accumulated and then gotten redistributed in this film. The actual film formed is unique to the materials involved, the testing conditions (or operating conditions in the case of actual machinery), and the overall tribosystem.
Physically formed films can be very efficient in controlling wear and there are even instances where adding a lubricant will increase the wear rate of a given system which is known to rely on physical film formation to control wear. More is discussed on this physical film formation in terms of a "fine-grained mechanically mixed layer" in Section 2.3.9 -- Wear Mechanism Overview. Many automobile brake pad researchers believe that physical film formation (more of a glaze due to heating effects and the materials involved) between the brake pad and brake rotor/drum is the controlling factor in the performance of brake systems.
2.3.8 Mild vs. Severe Wear
Many tribologists like to refer to wear as being mild or severe. While the distinction has been made quantitatively in some wear equations, there are differences of opinion as to where one draws the line in these quantitative equations. I will only discuss these two regimes qualitatively.
Mild wear has a much lower wear rate than severe wear and the wear scar generally has fine features. Severe wear has much higher wear rates and the wear scars generally have coarser features. Most materials can show both mild and severe wear depending on the tribosystem. The tribosystem consists of many factors including the geometry, loading conditions, materials used, sliding speed, and chemical environment. In addition, the wear mode can change from mild to severe or even from severe to mild.
A transition from mild to severe wear could be caused by many things including an increase in the sliding speed, load, or a combination of these. These will both increase the temperature at the interface which can then affect things such as adhesion. An increased load might change the wear mode from oxidative wear to adhesive wear as the oxide film is ruptured, thus allowing metallic bonding between asperity contacts to occur. Many believe that transitions in wear rate are related to the formation or breaking through of surface films, particularly oxides [28].
Perhaps more important in the transition from mild to severe wear is the effect of hardening. Akagaki and Rigney found that for a pin-on-disk configuration, when the disk was harder than the pin, mild wear with smooth friction traces resulted. However, if the pin work hardened and became harder than the disk (as a general case involving two different materials or in the case of a self-mated system) this often led to a transition to severe wear with much rougher friction traces. It should be pointed out that a pin which was harder than the disk to begin with will also often lead to severe wear with rough friction traces. The time required for this transition to occur is very much dependent on all processes giving hardening [34].
Others such as Bowden and Tabor have found slightly different disk/pin hardness ratios (Akagaki's and Rigney's study found a critical ratio of 1) at which this transition occurs [35]. The point is that the relative hardness of the pin and disk can change during a test and thus affect the transition from mild to severe wear. Perhaps self-mated wear tests often lead to high wear rates and high friction coefficients because the pin material work hardens much more quickly than the disk material. This is because of more contact and deformation in the pin when compared to the disk.
In the case where abrasive wear is dominant, a transition from mild to severe wear might occur due to an increase in the hardness of the abrasives. For example, the abrasives might work harden with continued sliding. Abrasive size, which can change with sliding, also affects wear. Ultrafine abrasives tend to polish while coarse abrasives lead to severe wear and create a rough wear scar with coarse grooves. Martensite formation might also cause a transition from mild to severe wear. In many systems or applications, one wants to try to avoid severe wear as it involves much higher maintenance and shorter service lives of equipment.
2.3.9 Wear Mechanism Overview/Debris Generation
The five wear mechanisms described were categorized for convenience. They should not be considered on an individual basis, but rather the ideas behind them should be used in conjunction with each other since the wear process is quite complex. It is the reasoning, chemistry, physics, mechanics, past data, prior tribotest results, and other evidence which led to their existence in the first place which is important. Nevertheless, five wear mechanisms which may be applicable to this research have been outlined. They are: 1. adhesive, 2. abrasive, 3. fatigue, 4. oxidative, and 5. physical film formation. This is far from a complete list of wear mechanisms proposed and some tribologists do not even agree with all of them, and instead suggest variations or combinations of them. Nevertheless, these 5 mechanisms cover a range of wear behavior appropriate for this research. To say all these wear mechanisms were definitely observed in this research would require a much more detailed study. This type of study would need many more samples studied in detail at intermediate sliding distances. Each tribological variable would need to be controlled with extreme care.
Of the five wear mechanisms suggested, oxidative wear and physical film formation are best viewed as "wear modifiers" while the other three can be considered "basic" wear mechanisms. The "wear modifiers" can be thought to "affect or influence" the "basic" wear mechanisms. Considering the basic wear mechanisms of adhesion, abrasion, and fatigue wear, abrasive wear is generally the most severe. Thus it is favorable to try to avoid abrasive wear. One way this might be done is by using smoother surface finishes.
It should be emphasized that while all the mechanisms were listed separately, in reality many or all (and there are many others which were not even mentioned) occur simultaneously. The wear process is quite complex and to view it as a simplified process can be misleading. Debris generation, for example, often involves local contacts, large plastic strains, changes in near-surface microstructure, localized shear, adhesion, mechanical mixing, transfer, deformation, fracture, blending, and the formation of a fine-grained mechanically mixed layer [24].
Models have been proposed which suggest that flake debris are generated when a critical layer thickness of this mechanically mixed layer is reached [36]. Flake debris usually are generated when the hardness of the fine-grained layer is less than the hardness of the "highly deformed" base material. In this case, the fine-grained layer is not able to press into the deformed layer and elevated plateaus are formed. It is from these elevated plateaus that the flakes are generated. This generally leads to a noisy friction trace and a rough wear scar [20].
When the hardness of the fine-grained layer produced on the surface is greater than the hardness of the "highly deformed" base material, more irregular (generally lamellar) debris can delaminate. The fine-grained layer should be hard enough to enable it to be pressed into the substrate for this irregular, non-flake debris to be generated [37]. This generally leads to a smoother friction trace and a smoother wear scar [20].
As this mechanically mixed layer is often the source of debris particles, the wear debris (except when microcutting is dominant) often have the same chemical composition, the same mix of phases, the same microstructure, and the same hardness as this mechanically mixed material on the sliding surface [29]. Any combination or even all of the wear mechanisms mentioned help create this mechanically mixed layer in the first place.
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