Abrasive wear

Abrasive wear is considered to be the single most effective wear type in causing economic losses in industry [11]. Abrasive wear is a common cause of failure in machine components of earthmoving and transportation vehicles and excavator buckets. High-stress abrasive wear changes the dimensions of the components and weakens the structures as the material thickness is reduced. In pure abrasion, the correlation between volume loss and sliding distance is often linear, which makes pure abrasive wear perhaps more predictable than some other types of wear [10]. However, this is not to say that the prediction of abrasive wear would not be complex, as the wear system is affected by numerous factors of material properties and environmental effects.

Abrasive wear is removal or displacement of material by hard particles or surface protrusions [12]. When another component with protruding asperities or fixed, partially embedded abrasives scratches the surface directly, the wear type is called two-body abrasive wear. In three-body abrasion, on the other hand, wear is induced by the loose abrasives that are free to slide or roll between the surfaces and into which the counterpart is transferring the load [13]. Figure 3 presents a schematic of two- and three-body abrasion. In this work, the term three-body abrasion is used to describe a situation which initially has three active, clearly separate agents affecting the system, and even during the wear process, a clear majority of them remains in their initial role.

Sometimes the difference between two- and three-body abrasion mechanisms is understood so that two-body abrasion produces scratches and three-body abrasion rolling marks. Some authors prefer the use of terms ‘grooving abrasive wear’ and ‘rolling abrasive wear’ for two- and three-body abrasion, respectively [14]. Furthermore, it is possible to divide three-three-body abrasion into open and closed situations: in open three-body abrasion, the two surfaces are far apart from each other, while in the closed situation the abrasive particles are trapped between the closely mated surfaces [15]. Even two-body abrasion can be subcategorized to ‘fixed-particle grooving abrasion’ and ‘free-particle grooving abrasion’ [16]. This demonstrates the complexity of

defining the conditions precisely and the breadth of terminology used in describing the phenomena.

Figure 3. Wear system in a) two-body abrasion and b) three-body abrasion.

The way of classifying abrasion into two- and three-body wear defines the conditions through naming the active bodies participating in the process. However, this kind of oversimplification does not usually appreciate the complexity of real situations, where pure two- or three-body abrasion is rather scarce, and often the two modes occur simultaneously [17]. They can also alternate in the same system, the conditions governing which of the modes is dominant [14].

The division of abrasive processes into two- and three-body situations is more of a description of the initial state than a precise observation of the ongoing process, which is greatly affected by the system in question. For example, the dry sand rubber wheel abrasion test is a three-body abrasion test by its default configuration, but the actual wear occurring in the system can be more towards two-body abrasion, since the sand particles embed in the rubber quite effectively [18]. On the other hand, initially two-body conditions may develop into three-body conditions, if the initially fixed abrasives or existing or forming protrusions are removed from the initial surfaces.

Another classification for specifying the type of abrasive wear is the division to high- and low-stress abrasion. In high-low-stress abrasion, the load induced into the abrasive is so high that it breaks the abrasive, while in low-stress abrasion the abrasive remains intact [5, 12]. Also a division into mild and severe wear has been used [18–20], as it is often difficult to determine the exact conditions present in the interface.

Overall, a common characteristic for the attempts of classifying the wear processes is its complexity and difficulty. In this work, the abrasive wear occurring through scratching contacts can be characterized to consist mostly of high-stress abrasive wear in a closed situation, which is defined initially as a three-body process. The interaction of the surfaces and abrasives leads to both rolling and sliding, as the particles can be partly embedded into the surfaces.

Abrasive wear can further be divided into micromechanisms, which lead to the final outcome.

divide them into cutting, fracture, fatigue by repeated ploughing and grain pull-out [13].

However, grain pull-out is not a generic material removal mechanism, since it can only happen in materials with a grain structure. In another classification, abrasive wear is divided into three different wear modes: cutting, wedge formation and ploughing [21]. In wedge formation, a wedge is formed against the sliding indenter, but some ploughing on the sides of the groove is also occurring.

Figure 4. Schematical presentation of the micromechanisms of abrasive wear[6].

In microploughing, no actual removal of material takes place but the material is only displaced to the sides of the scratch. Microcutting, instead, leads to the removal of material, as it is cut away from the surface like a chip. In microfatigue, small pieces of repetitively deformed material become detached from the surface, while in microcracking the material is removed through crack formation and propagation, especially in brittle materials. [6]

2.1.1 Role of abrasives

In abrasive wear, the abrasives are in an essential role in determining the wear process. It is generally accepted that in order for a scratch to form, the hardness of the abrasive has to be at least 1.2 times the hardness of the material to be scratched [22, 23]. Some other abrasive properties affecting wear are the crushability [Publication IV], abrasive size [19, 24–31] and angularity or shape of the abrasives [32–37].

Some of these properties, such as hardness and angularity, have a direct effect on how the abrasive is able to penetrate the material. The attack angle can also determine the more specific wear mechanism: with a low attack angle, the abrasive is more likely to cause ploughing, whereas with a high angle, cutting is more probable [20, 38]. On the other hand, some other properties such as crushability determine the behavior of the abrasive in the system, and thus have a more indirect effect on wear. As an example, an abrasive with high crushability produces a larger quantity of small abrasive particles, which are freshly ground and have high angularity.

Larger abrasives are more blunt and have lower attack angles, thus causing less cutting [30]. If the crushing of particles happens to a large extent, it can mask the effects of abrasive angularity [39]. Moreover, according to Gåhlin and Jacobson [40], if the abrasives are ideally sharp, the size effect does not apply to them.

For relatively small abrasive sizes, it has been found that increasing abrasive size also increases wear. This observation is often called the particle size effect. However, the particle size effect is valid only for small particle sizes, up to 80-150 µm [27, 41, 42], above which the increase in particle size does not increase the damage at the same rate. There are several theories of the reasons for the abrasive size effect. To name some, Misra and Finnie [27] concluded the effect to be caused by the physical size of the abrasive in contrast to the depth of the hardened surface layer. Coronado and Sinatora [30] suggested that the critical size, after which the wear rate changes, is originating from the transition from microcutting with small abrasives to microploughing with larger abrasives, but the occurrence of this phenomenon was dependent on the studied material.

The effect of particle size on abrasive wear is not fully clear in larger scales. For macroscale abrasives of millimeters in size, the size effect remains generally undefined. For particles with a size of several hundreds of micrometers or above, there are findings stating both increased [42]

and decreased [43, 44] wear rate with increasing particle size. Elementally, it is a question of the conditions in the tribosystem. For example, if the machine or a wear tester is adjusted in such a way that the abrasives below a certain size can move freely between the surfaces without being loaded, the small particles do not interact with the surface at similar loads as the larger particles.

In tunneling and mining, it is common to define the abrasiveness of the abrasive for making predictions of the service life of the wear parts and for preparing maintenance schedules.

Abrasiveness indicates the ability of the rock to cause wear. Abrasiveness can be measured with a number of different procedures, such as thin section analysis [45], Cerchar test [45–53], LCPC test [45, 49, 51–55], Schimazek index test [49, 51], Sievers C-value test [45], Böhme grinding test [45], the brittleness value test, Sievers J-value test, and abrasion value and abrasion value cutter steel test [56]. The LCPC test and Cerchar abrasivity index appear to be the most used tests in Europe recently [45]. The idea behind these tests is quite different: the LCPC test measures the mass loss of a standardized steel block worn with a batch of certain size gravel in impact-abrasive conditions [49, 53, 55, 57], whereas for measuring the Cerchar index a steel pin is sliding against a block of rock [47, 49, 52]. As the methods and the wear mechanisms they produce differ widely, the values obtained by the tests are not comparable. However, some empirical dependence between them has been reported [55].

2.1.2 Role of counterparts

In three-body abrasion, the properties of the counterpart material affect the wear system, thus impacting the wear besides the abrasives. Axén et al. [58] reported that the wear mechanism in abrasive conditions can vary markedly depending on the hardness ratio of the sample material and the counterpart. A softer material (whether it is the sample or the counterpart) is more likely

to become into contact with each other during the test, which adds the question of how the contacts between the surfaces affect the situation, as opposed to the interactions taking place only through abrasives. This would lead to a difference in the active wear mechanism between counterparts of different hardness, and possibly to higher wear rates in the harder sample. In some cases, it could lead to a situation where increased hardness increases wear [59].