High temperature tribology influences operational efficiency in industries like metals, energy, mining, and transportation. Figure 1 shows the different operations like engine turbine blades that enhance fuel efficiency, brake pads that allow shorter braking distance, boiler tubes that lower downtime cost in coal power plant and metal forming tools that ensure high quality of surface finish are affected by wear of materials at high temperature (T > 600 °C).
Figure 1. Represents different operations that require a thorough knowledge of tribology.
As shown in Figure 2, various factors like elastic modulus, thermal conductivity, hardness, thermo-mechanical fatigue, diffusion, grain structure, and oxidation can largely influence friction and wear at high temperature thus making it a multicomponent problem. Laboratory scale test instruments (also called as Tribometers) are often employed to study the influence of these factors on wear of materials. Such an understanding is frequently used in ranking of materials for industrial applications. However, the selection of tribometers for the test is still an important challenge.
Figure 2. Wear process at environment temperature above 600 °C
In our experience, the screening of materials and its selection for high temperature applications will rely mostly on two vital qualities in a tribometer:
- Tribometer design that can reach and sustain high temperatures in addition to incurring extremely low maintenance cost
- Tribometer features that can reproduce the friction and wear seen during industrial applications
It is defined depending upon the type of heating methods and heat resistant materials used in the construction of tribometer. Typically, either an open heating system – Induction and Ohmic heat or a closed heating system – Radiation heat (see Figure 3) is used. Table 1 lists the advantages and disadvantages of these heating systems.
Table 1. Importance of heat control methods on maintenance cost, heating time, control and compatibility.
Closed heating system
(flow of gas or vacuum)
|Induction and Ohmic
Open heating system
Figure 3. Ducom Rotary Tribometer with pin on disc configuration is ready for test at 900 °C.
Despite the fact that these heating methods offer temperature (specimen and environment) above 1000 °C; however, it is the yield strength of the materials used in the tribometer that are a potential threat. Poor yield strength of materials at high temperature will result in plastic welding of specimen holders (fasteners), bursting of rotating spindle at high speeds and an increase in tolerance of nozzle affecting the particle flow rate. Thus, selection of materials for constructing tribometers has to be based on the operating temperature, as described in Table 2. Ultimately, the design should assure a safe and economical use of a tribometer.
Table 2. List of materials used in construction of tribometer as a function of temperature.
|RT to 500
||Stainless Steel (SS 304, SS 316L, etc.)
|500 to 950
||Different grades of Inconel (IN 700, IN 725, etc.)
||K-type Thermocouple, Pyrometer
|950 to 1200
(see Figure 4)
|Combination of Al2O3 and SiO2 coatings on Inconel base and other cladding materials
||R-type Thermocouple, Pyrometer
Figure 4. Pre-testing of particles and air heating element in Ducom Air Jet Erosion Tester (1200 °C).
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The friction coefficient, wear rate, and wear phenomenon are the three significant tribological parameters used to select materials with variety of surface and bulk properties. These parameters can be reproduced in the lab if the tribometers can emulate the basic field operating parameters, such as speed, motion, pressure and temperature. It should be noted that a tribometer alone cannot reproduce the entire wear phenomenon. For instance, high temperature erosion and abrasion are two different wear process that can be studied using the Ducom Air Jet Erosion Tester (see Figure 5) and Ducom Rotary Tribometer (see Figure 6) / Hot Strip Tribometer (see Figure 7), respectively. Table 3 explains some of the functional features of high temperature tribometers that is widely recognized by industries.
Table 3. Specifications of high temperature tribometers used in tribology research.
|Air Jet Erosion Tester
Particle velocity < 200 m/s, Flow rate < 300 g/min, Impact angle > 15°, ASTM G76, ASTM G211-14
||Ohmic and Radiation
||Energy (Boiler tubes in Powerplants)
Transportation (Turbine blades in Engine)
Speed < 2000 rpm, Force < 1 kN, rotation & oscillation, ASTM G99, DIN 50324
||Induction or Radiation
||Transportation (Brakes in vehicles/ tram)
Mining (Drill bit)
|Hot Strip Tribometer
Punching force < 5 kN, Stroke length < 1200 mm
||Metals (Metal forming tools)
Figure 5. Image of the Ducom High Temperature Air Jet Erosion Tester
Figure 6. Image of the Ducom High Temperature Rotary Tribometer (Closed Heating System)
Figure 7. Image of the Ducom Hot Strip Tribometer
This information has been sourced, reviewed and adapted from materials provided by Ducom.
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