*2.2.2 Ceramic surfaces*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

other, the friction between the mating surfaces converts the kinetic energy in to heat or thermal energy. Generally, the term used to describe the friction is the coefficient of friction. It is denoted with dimensionless scalar value (μ) and explaining the ratio of the friction between two mating surfaces (F) and the applied force on

*Elasto-plastic (a) complete plastic, (b) brittle-type contact, and (c) between plane and a sphere.*

In a sliding condition, it is normally required to monitor or calculate the frictional behavior during the experiment. The changes in friction with wear data normally offer the beneficial data regarding modeling and mechanisms. The friction is generally classified as two categories: (a) static friction and (b) dynamic friction. In the case of two objects is not sliding each other is called as static friction. In another hand, both the mating objects sliding relatively to each other is called as dynamic friction [10]. **Table 1** shows the required parameters which need to be considered

Generally, the asperities contact in metal surfaces is normally plastic. The metals such as titanium, cobalt, magnesium with hcp (hexagonal closed packed) crystal lattice provides the coefficient of friction nearly 0.5 while sliding against themselves. In addition to that the hcp metals possess a reduced ability to deform

Metal: Nickel alloy, iron, stainless steel, cobalt alloy and titanium alloy

Material Nonmetal: composites, polymers, ceramics and bio glass

Geometry ball-on-socket, plane-on-plane and point-on-plane Motion type Rotation motion, unidirectional, freeing, multidirectional

Atmosphere Room temperature, high temperature and corrosive medium

Lubrication Oil, saliva, synovial fluid and blood

Loading type Cyclic loading, constant loading, etc.

*Necessary input parameters to be considered for wear experiment.*

F F =µ( <sup>n</sup> ) (1)

them (Fn). It is described in Eq. (1):

while performing wear experiments.

**Parameter/condition Example**

*2.2.1 Metal surfaces*

**Figure 3.**

**188**

**Table 1.**

In ceramics material the contact at the asperities is normally mixed. This may be fully in elastic while the surface roughness shows with less [6]. Else, if the surface roughness is high, it may be shows with plastic stage. Generally, the coefficient of friction must be independent for the normal applied load in elastic contacts. For example, the alumina balls sliding on the alumina surface and the friction showed around 0.4. It can be noted that the friction coefficient in ceramics material with dry atmosphere is lesser around 0.3–0.7 while applying the minimum load with less than 200°C temperature. The obtained frictional values are relatively similar to metal alloys and this may seem to be quite surprise. In case, the ceramics material is characterized with higher hardness and elastic modulus, and lesser values of the surface energy [12].

Besides, the surface energy of ceramics material is reduced through the surface reactions with water vapor and the presence of other substances on the working atmosphere. Hence, the lesser frictional force can be expected from the sliding wear experiment. Though, while continuous sliding with real contact area is notably increases with increase in friction. Further, the applied load is considered as major input parameter in the ceramic material. If the applied load is increased, the brittle contact may establish, and this will increase the coefficient of friction as much as high around 0.8. Specifically, the brittle contact can be occurred while the tangential stresses owing to higher friction due to the occurrence of critical microcracks on the surfaces. This type of microcracks can be seen on the ceramics surfaces normally and producing the defects such as porosity, flaws and inclusions. The cracks can be initiated from the development of asperity because of continuous applied load during the tribological study. **Figure 4** represents the coefficient of friction and applied load for alumina sliding on the alumina.
