Chapter 1
Properties of Lubricants

Many fluids serve as lubricants in industry. Among them, oil and grease are the most commonly used. Air, water and liquid metals are also used as special lubricants; for example, liquid sodium is often used as a lubricant in nuclear reactors. In some situations, solid lubricants, such as graphite, molybdenum disulfide or polytetrafluoroethylene (PTFE) can also be used. In this first chapter we will discuss the viscosity and density of lubricants, as they are the two important physical properties associated with lubrication.

In lubrication theory, the most important physical property of a lubricant is its viscosity, the most important factor in determining the lubrication film thickness. In hydrodynamic lubrication, the lubricant film thickness is proportional to the viscosity, while in elastohydrodynamic lubrication it is proportional to the viscosity to the powers 0.7. Although in boundary lubrication the viscosity does not directly influence the film thickness, the oil packages formed between peaks and valleys of roughness will carry part of the load. Therefore lubricant viscosity is closely related to its load-carrying capacity.

Furthermore, viscosity is also an important factor influencing the frictional force. A high-viscosity lubricant not only causes a lot of friction loss, but also produces a lot of heat, which make cooling control difficult. Because temperature rise caused by friction can lead to failure of the lubricant film, the surface will be worn increasingly. Therefore, a reasonable viscosity is required for practical lubrication.

The performance of elastohydrodynamic lubrication (EHL) also depends on the rheological characteristics of a lubricant. In point or line contacts, an EHL film is very thin, less than one micro-meter, but the pressure is very high, up to 1 GPa. And, because the contact area is often very small, the shear rate may be higher than 107 s-1 such that the passing time is very short, less than 10-3 s. Therefore, a friction process is always accompanied by high temperature. For such conditions, the properties of a lubricant are quite different from those of a Newtonian fluid. In such cases, therefore, it is necessary to study the rheological properties of lubricants. Experiments show that although the film thickness formula derived from the Newtonian fluid model is usually applied to the elastohydrodynamic lubrication, the frictional force and temperature calculated by a Newtonian fluid model will cause a large error. Therefore, in thermo-elastohydrodynamic lubrication (TEHL), more realistic non-Newtonian fluid models should be used. These belong to a lubricant rheology study which will not only help us understand the lubrication mechanism more deeply but also has major significance in energy conservation and improvement in the life of mechanical elements.

1.1 Lubrication States

The purpose of lubrication is to form a lubricant film to separate the friction surfaces to carry a load with a low shear stress to reduce friction and wear of materials. A lubricant film can be a liquid, a gas or a solid. According to the mechanisms of lubricant film formation, lubrication states can be divided into the following six basic types: (1) hydrodynamic lubrication; (2) hydrostatic lubrication; (3) elastohydrodynamic lubrication; (4) thin film lubrication; (5) boundary lubrication; and (6) dry friction. The features of the lubrication states are listed in Table 1.1.

Table 1.1 Basic features of lubrication states

Lubrication state Typical film thickness Formation method of lubricant film Applications Hydrodynamic lubrication 1-100 µm A relative movement between friction surfaces forms a dynamic lubricant film For surface contacts in high speed situations such as journal bearings Hydrostatic lubrication 1-100 µm Through an external pressure fluid form a lubricant film between friction surfaces For surface contacts in low speed situations such as journal bearings and guides Elastohydrodynamic lubrication 0.1-1 µm Same as hydrodynamic lubrication For point or line contacts in high speed situations, such as gears and rolling bearing Thin film lubrication 10-100 nm Same as hydrodynamic lubrication For point or line contacts in low speed and high precision situations, such as precision rolling contact bearing Boundary lubrication 1-50 nm Physical or chemical reaction such as adsorption between lubricant molecules and metal surfaces For low speed situations, such as journal bearings Dry friction 1-10 nm Surface oxide film, gas adsorbed film, etc. For no lubrication or self-lubricating friction pairs

A lubrication state has its typical film thickness. However, we cannot determine the lubrication state simply and accurately based on the thickness alone because the surface roughness also needs to be considered. Figure 1.1 lists the thickness orders of different lubricant films and roughnesses. Only when a lubricant film thickness is high enough is it possible to form a full film that will completely lubricate to avoid the peaks of the two rough surfaces contacting each other. If several lubrication states exist at the same time, this is known as mixed lubrication, as shown in Figure 1.2.

Figure 1.1 Lubricant film thickness and roughness height.

Figure 1.2 Typical friction coefficients of the lubrication states.

It is often inconvenient to determine a lubrication state based on lubricant film thickness because film thickness measurement is difficult. For convenience, the friction coefficient can also be used to determine a lubrication state. Figure 1.2 presents some typical friction coefficients corresponding to the lubrication states.

With varying working conditions, one lubrication state may transform into another. Figure 1.3 gives a typical Stribeck curve of a journal bearing. The curves indicate the transformation of lubrication states corresponding with the working conditions. Here, the dimensionless bearing parameter (?U/p) reflects the working conditions, where ? is the lubricant viscosity, U is the sliding velocity and p is the average pressure (carrying load per unit area).

Figure 1.3 Stribeck curve of a journal bearing.

It should be noted that methods of studying lubrication states may vary. For hydrodynamic lubrication and hydrostatic lubrication, theories of viscous fluid mechanics and heat transfer are necessarily used to analyze pressure and temperature distributions. As for elastohydrodynamic lubrication, elastic deformation of the contact surfaces and the rheological properties of lubricants must be added, while for boundary lubrication the perspectives of physical and chemical knowledge will help us understand the mechanisms of formation and failure of a boundary film. For dry friction, the main task is to avoid wear and tear. Therefore, its study involves material science, elastic and plastic mechanics, heat transfer, physical chemistry and so on.

Table 1.2 Standard densities of some basic lubricants

Lubricant Density g/cm3 Lubricant Density g/cm3 Triguaiacyl phosphate 0.915-0.937 water-soluble polyalkylene glycol 1.03-1.06 Diphenyl phosphate 0.990 non-water-soluble polyalkylene glycol 0.98-1.00 Hydroxymethyl-phenyl phosphate 1.161 dimethyl silicone oil 0.76-0.97 Hydroxymethyl-phenyl diphenyl phosphate 1.205 ethly-dimethyl silicone oil 0.95 Chlorinated diphenyl 1.226-1.538 phenyl-dimethyl silicone oil 0.99-1.10

1.2 Density of Lubricant

The density is one of the most common physical properties of a lubricant. A liquid lubricant is usually considered to be incompressible, and its thermal expansion is ignored so that the density is considered as a constant. Generally, the density of 20°C is considered the standard. In Table 1.2, the standard densities of some basic lubricants are given.

The density of a lubricant is actually the function of pressure and temperature. Under some conditions, such as in the elastohydrodynamic lubrication state, the density of a lubricant should be considered to be variable.

The volume of lubricant is reduced with increase of pressure, so that its density increases. The relationship of density and pressure can be expressed as follows:

1.1

where C is the compression coefficient; V is the volume of lubricant; M is the mass of lubricant.

The following well-known density equation is available:

1.2

where ?0 and ?p are the densities at pressures p0 and p respectively.

The desirable C can be obtained...