Experimental Methods in Tribology

Introduction

Tribology is the study of science and technology about the interactions of surfaces in relative motion and the related subjects and practices that can be explained on the basis of such principles as friction(the part of physics), lubrication (the part of chemistry ), and wear (the part of material science ) (Bhushan 2013). Its main peculiar feature is the possibility to incorporate several scientific and technological disciplines like physics, chemistry, or contact mechanics at the same time. To understand the worth and impact of tribology, it is not enough to know the definitions and use them in practice. It is necessary to comprehend its history and possible economic impact and identify the advantages of studying it. The first signs of tribology could be observed in the activities of Paleolithic people, who introduced the methods of generating fire by banging the stones together.

In fact, the word tribology has Ancient Greek roots. Its root is in the verb trivein meaning “the character formation of privileged children by their home tutors” (Gohar & Rahnejat 2008, p. 1). In 1964, regarding the conditions of the British industrial economy, the term tribology was coined to introduce a scientific nature of studying “the interactions of solid contacting surfaces in relative motion” (Gohar & Rahnejat 2008, p. 1). David Tabor investigated the process of lubrication and the increased friction problems in machines. He needed to introduce a new science that could combine the elements of chemistry, mechanics, and physics. The economic impact of tribology is crucial because the tribological principles can be used to save from 1% to 1,4% of the GPD. The importance of studying tribology and its experiments is explained by the possibility to understand how to reduce the consumption of raw materials, energy, and fuel in an everyday life.

Tribometers

Tribometers for Dry or Partially Lubricated Sliding Contacts

Tribometers for Dry or Partially Lubricated Sliding Contacts
(Stachowiak, Batchelor & Stachowiak 2004).

Atribometer is an instrument that is used in tribology to measure such quantities as the coefficient of friction or wear volume. There are several categories of tribometers, and some of them will be discussed in this paper. For example, one of the largest and most effectively developed classes of tribometers is for dry or partially lubricated sliding contacts. It is usually vested in an apparatus that can help to evaluate lubricants. There are five main types of basic configurations that can be used for dry and partially lubricated sliding contacts: four-ball, Falex, Timken, Pin-on-Disc, and Pin-on-Slab (Stachowiak, Batchelor & Stachowiak 2004).

Four-Ball Tester

The four-ball tester is the instrument that is usually preferred by the users of different lubricants and substances. As it can be understood from the title, there are four balls in this apparatus. Each of them is of 12.7 mm diameter with one that is “held in the chuck rotating at 1770+/- rpm”, and three others are “held rigidly in a pot” (Stachowiak, Batchelor & Stachowiak 2004, p. 28). The upper ball that rotates is dead weight-loaded in comparison to the three balls that perform a supportive function (Bhushan 2013). These three lower balls have the measurements of the wear scars. The size of the scare demonstrates the ability of the chosen lubricant to prevent wear. The process of work is not very complicated. As soon as the pot is filled with the required portion of lubricant, the already fixed balls are pressed against the ball that rotates with an interval of ten seconds. The testing period and characteristics are regulated in accordance with the standard test schedule. A series of ten-second runs is established beforehand in regards to the test load until the process of welding of all four balls occurs. This test aims at checking and developing certain wear preventive and extreme pressure properties, as well as the friction behavior of lubricants.

Falex Tester

The Falex tester is another type of tribometer that can be applied to dry or partially lubricated sliding contacts. The peculiar feature of this tester is the possibility of large contact stresses to be developed without any additional strong mechanical support (Stachowiak, Batchelor & Stachowiak 2004). This kind of tester is frequently used in different types of industrial and laboratory research. The construction of this tool is as follows: four contact forces aim at balancing each other, a V-block closure force, and the torque that helps to drive the pin and measure friction force. Due to the existing high contact pressures, this tester is also preferable for scuffing studies. It represents the information about the value of friction coefficients that can be discovered different lubricant combinations, the limit load that can be observed before the seizure, and the diameter of wear scar after the test takes place. Besides, in a testing process, the Falex tribometer promotes the possibility to change the quality of contact force in case severe wear occurs. This possibility is usually absent in the experiments with other types of tribometers. Therefore, this tester is preferable in many scientific cases.

Timken Apparatus

The core of this kind of tribometer is the combination of the test cup and the test block. The test cup is a ring that rotates at 800 rpm and is pressed against the test block (a stationary slab) during the process of lubricant testing. The duration of this test is about ten minutes. It ends successfully as soon as one of the determinants (OK or FAILURE) occurs. The load that has OK in its title signalizes about the maximum load that can be applied without producing scoring marks on the slab (Stachowiak, Batchelor & Stachowiak 2004). This process occurs on the lever arm because it helps to indicate the strength of the lubricating membrane. This type of apparatus is preferred by a number of scientific institutions and industrial organizations. Besides, this tribometer meets the commercial and research needs of organizations. Sometimes, the researchers use different experimental methods at the same investigation in order to clarify how to control contact stress, identify the size of contact areas, and establish the required temperature.

Pin-on-Disc Apparatus

Stachowiak, Batchelor and Stachowiak (2004) underline that pin-on-disc apparatus is one of the most frequently used tools in the world. The construction of this tester is not complicated. It consists of a pin that is pressed against a rotating disc and located on its flat surface or on the circumference of the disc (in this case, the apparatus is called as pin-on-drum). As a rule, the measurements of the disc and a pin on it depend on the type of the experiment that is going to be conducted. When a pin is on the disc, it is easier and more effective to control the conditions of the experiment.

Researchers try to use this particular apparatus even if they have access to other types of tribometers because they enjoy the steady conditions, the absence of systematic variations (so that they can gain control over the procedures), and the period required for the analysis of data. Still, there is one problem in this type of experiment that is connected with the inability to gain control over normal load. When the test begins, the disc surface is smooth. Within a short period, some transfer membrane occurs on the disc surface and influences the process of work. Wear roughening may also lead to the vibrations of the pin, and wear anisotropy (that is frequently found in ceramics) can change the development of the test because one part of the disc may wear sooner than another part of the same disc. The researchers cannot control these changes and have to consider this possible impact carefully.

Pin-on-Slab Apparatus

Regarding the title of this type of tribometer, it is possible to guess that there should be a slab (a kind of a plate) on which a pin could rotate periodically. With the help of this tester, it is possible to study the process of reciprocating sliding. These movements are opposite to those that can be observed on the pin-on-disc apparatus. Stachowiak, Batchelor, and Stachowiak (2004) introduce the Bowden-Leben machine as one of the most popular application that works according to the principles of the pin-on-slab. Many research projects are based on the results that are obtained with the help of this tester. The Cameron-Mills machine is another representative of this type of tribometers that is characterized by the short amplitude.

In the Bowden-Leben tribometer, the amplitude is about 100-150 mm. Such size of the tribometer makes the production of friction and wear effective at the initial stages of the wear and friction process. The pin has to make fewer traverses on the slab, and it turns out to be possible to control surface temperature, sliding speed, and other characteristics of the device. In the Cameron-Mills tribometer, the amplitude is about 1-2 mm. This fact serves as one of the main advantages of this apparatus because a high repetition of sliding contact cycles causes the required portion of friction and wear and leads to the necessary steady state conditions. In general, such apparatus as pin-on-disc and pin-on-slab are more effective in comparison to such tribometers as four-ball, Falex, and Timken because its components may be taken from different materials and not depend on the system that cannot be broken.

Tribometers for Non-Ambivalent Conditions

Tribometers for non-ambivalent conditions create a new group of tribometers that deals with dynamic contacts that occur in non-ambient conditions such as temperature extremes or vacuum. Stachowiak, Batchelor, and Stachowiak (2004) identify it as the method of tribology that is of a high scientific and practical interest. Nowadays, the technological sphere has been dramatically changed and improved so that the work of different mechanical components is possible under different severe environments. These tribometers can be used to promote credible and safe operations in nuclear reactors or space vehicles.

There are several types of tribometers in this category depending on the level of temperature and the conditions under which the procedures take place. Each tribometer of this kind promotes the tribological measurement that should be conducted under different environments. The peculiar feature of these instruments is that the basic mechanism stays unchanged because of non-ambient conditions. Design considerations also depend on this factor regarding such issues as instrumentation, load, observer isolation, and power system (Stachowiak, Batchelor & Stachowiak 2004). In order to ensure the abilities of friction and wear transducers to record data and stay protected from the possible harmful environmental effects, the designers of such tribometers try to choose every detail carefully and describe the peculiarities of a testing process.

High Temperature Tribometers

As it has been mentioned above, many tribological researchers find ceramics as one of the most interesting material to deal with. This substance can be investigated in different environments and under hot and cold conditions and stay conductive. If a researcher needs to understand what temperature range is required for the possibility of ceramics to replace metals or to promote tribological tests, they should make use of a special tribometer that can be used at temperatures higher than 1200oC. High temperature tribometers consist of special data acquisition systems with the help of which it is possible to obtain the required temperature level in a chamber. At the same time, it is possible to an electron beam heater that is placed in an ultrahigh vacuum. Some tribometers of such type are equipped with thermocouple monitors, and some of them have special windows. Such apparatus can measure friction and wear characteristics of ceramics or other substances that reach the point of 1000oC (Stachowiak, Batchelor & Stachowiak 2004). It consists of a water cooled shaft that aims at providing isolation between the hot test chamber and the bearing power system and the pin that is located at the end of the shaft and functions as a cantilever. Such ability turns out to be a very effective contribution to science because people get a possibility to work with the substances under high temperature ranges.

Low Temperature Tribometers

However, there are also the materials the properties of which can be tested regarding low temperatures only. There are special polar conditions that can help to identify the tribological properties of lubricants. They cannot be neglected because the tribological behavior of substances is a crucial point for consideration regarding the opportunities available with such type of tribometer. The tool consists of liquid nitrogen and helium that help to cool the contact that exists between the tested materials. One of the main tasks is the necessity to isolate nitrogen and helium from the moving parts and control the dynamic contact. The idea to use long shafts to analyze and drive the work of the wear specimen and the stationary specimen is offered. There are certain distance requirements that can be used to the positioning of the vessel that contains the liquid nitrogen or liquid helium and the tools that help to stabilize temperatures of the shaft end, the cryogenic fluid, another end that has a direct connection with the drive system, and the friction transducer (Stachowiak, Batchelor & Stachowiak 2004). This design of a low temperature tribometer is not the only one, and the cryogenic fluid can be replaced by gas that can be chilled before the contact itself.

Tribometers for Operation in Vacuum

Operations in a vacuum are of crucial importance for science in general and space vehicle technologies in particular. There are several tribometers that can be used to test the processes of lubrication, friction, and wear that take place under vacuum. As a rule, they are small and compact, and researcher can use them under different conditions. Due to the small size, these tribometers can fit into such apparatuses as the Scanning Electron Microscope, Auger Spectrometer, and Electron Spectrometer for Chemical Analysis (Stachowiak, Batchelor & Stachowiak 2004). There are several important details in this process.

First, a vacuum chamber that is used in the test should be properly chosen to contain the dynamic contact, a drive system, and vacuum pumps. Second, the shaft that can drive the dynamic contact should pass through the wall of the chamber. Finally, there is a shutter in the chamber that unites its parts with an electric monitor. Friction and wear transducers can be located in or outside the vacuum system. It may happen that researchers add some heating devices to this type of tribometers in order to continue research under severe environments and succeed in observing wear mechanisms and evaluating the nature of membrane that occurs in a result of friction. In a vacuum, friction coefficients are unstable and can be changed in a short period. That is why a certain attention is paid to the developed of a strong support structure.

Tribometers for Operation in Corrosive Liquids

Researchers also demand the development of the tribometers that can be used for operation in corrosive liquids. Usually, such apparatus are used for environmental purposes and simulate wear under certain conditions. Different pollutants influence the quality of wear of seals, and it is very important to gain strict control over each operation taken. There is a tub that contains a corrosive fluid that can isolate the fluid from the instrumentation and other driving or loading systems (Stachowiak, Batchelor & Stachowiak 2004). This tub is forced against the shaft that, in its turn, drives a disc. Wear can be measured with the help of the displacement levels of the tub, and friction depends on the torque measurements that happen between the motor and the shaft. As well as the tribometers that are used for operations in a vacuum, the tribometers for operation in corrosive liquids can be improved with a special heating system with the help of which it is possible to investigate the substances under different temperatures.

Special Types of Tribometers

In addition to the already discussed tribometers and their worth in tribology and science, there are a number of other special types of apparatus that can be offered to researchers. For example, the tribometers for fretting studies help to control and simulate fretting movements and contact. An electrodynamic exciter is used to drive a microsliding contact by means of frequent reciprocating moments that are characterized by small amplitudes. The tribometers for extremely high sliding speeds help to control imbalanced forces with a more than 100 m/s limit. This tool was firstly designed by Bowden and Freitan. Its peculiar feature is a steel ball that is located above the stationary wear specimens with the help of magnets. Still, the list of useful tribometers is far from being full because each new construction requires the presence of new tribometers, and the field of tribology undergoes considerable improvements from time to time.

Reference List

Bhushan, B 2013, Introduction to tribology, John Wiley & Sons, New York.

Gohar, R & Rahnejat, H 2008, Fundamentals of tribology, Imperial College Press, Hackensack, New Jersey.

Stachowiak, G, Batchelor, AW & Stachowiak, GB 2004, Experimental methods in tribology, Elsevier, San Diego, California.