Wear is defined as a process of deformation and removal of material on a surface caused by mechanical action of the opposite surface1. A wide range of factors, such as rolling, reciprocating, unidirectional sliding, as well as impact loads, temperature, speed and many others are known to influence wear. Tribology – the study of wear – covers various disciplines, ranging from mechanical engineering and material science to physics and chemistry. Due to the complex nature of wear, isolated studies have to be performed that are focused on particular wear processes or mechanisms, such as surface fatigue, abrasive wear, adhesive wear, erosive wear, and fretting wear2. Yet, "Industrial Wear" often involves multiple wear mechanisms that occur in synergy.
The two extensively used ASTM compliant setups3, 4 are Linear Reciprocating and Rotative (Pin on Disk) wear tests. These tests are used for measuring the materials’ sliding wear behaviors. The wear rate value of any wear test method is generally applied to predict the relative ranking of material combinations, and therefore it is very important to validate the repeatability of the wear rate determined through different test setups. This will allow users to carefully consider the wear rate value described in the literature, which is crucial in understanding the materials’ tribological properties.
Using the linear reciprocating and rotative wear test setups of the Nanovea Tribometer for comparison, the wear rate of an acrylic plate sample is calculated in a controlled and monitored way. The versatility of the Nanovea Tribometer is demonstrated in this study. The device can measure the materials’ wear rate using different setups.
Figure 1. Acrylic sample for the wear tests.
Tribology Measurement Principle
The sample is mounted on a moving stage while simultaneously applying a known force on a ball, or pin, in contact with the surface of the sample to create the wear. When the sample moves in a linear reciprocating (Figure 2a) motion or rotational (pin-on-disk, Figure 2b) motion, the resulting frictional forces between the sample and the pin are measured with the help of a strain gage sensor on the arm. Generally, the wear test is used as a comparative test to examine the materials’ tribological properties.
The coefficient of friction (COF) is recorded in situ. The loss of volume makes it possible to calculate the material’s wear rate. As the action carried out on all samples is identical, the wear rate of the material can be taken as a quantitative comparative value for wear resistance. This is a simple method that allows the determination and analysis of wear behavior and friction of virtually every solid state material combination, with varying time, temperature, velocity, contact pressure, lubrication, humidity and so on.
Figure 2. Schematics of (a) rotative and (b) linear wear tests.
Profilometer Measurement Principle
A white light source is used by the axial chromatism technique, where light travels via an objective lens with a high degree of chromatic aberration. Additionally, the refractive index of the objective lens will differ with respect to the wavelength of the light. In fact, each separate wavelength of the incident white light will again focus at a different distance from the lens (different height). Once the measured sample is within the range of potential heights, a single monochromatic point will be focalized to create the image.
In view of the system’s confocal configuration, only the focused wavelength will travel through the spatial filter with high efficiency, thereby causing all other wavelengths to be out of focus. A diffraction grating is used to perform the spectral analysis. This method deviates each wavelength at a varying position, intercepting a CCD line, which in turn specifies the position of the maximum intensity and enables direct correspondence to the Z height position.
Figure 3. Schematic of axial chromatism technique.
Nanovea optical pens have zero effect from sample reflectivity. Variations do not need sample preparation and have sophisticated ability to determine high surface angles; capable of large Z measurement ranges. Any material can be measured: opaque or transparent, diffusive or specular, rough or polished.
The tribological behavior, for example, wear resistance or COF of the acrylic sample was assessed by Nanovea Tribometer using linear reciprocating and rotative wear test setups for comparison. Next, a SiN ball tip (6 mm dia., Grade 100) was applied against the tested samples. The COF was tracked in situ. Table 1 summarizes the test parameters. Using the formula K=V/(Fxs)=A/(Fxn), the wear rate, K, was evaluated,
V is the worn volume
F is the normal load
s is the sliding distance
A is the cross-sectional area of the wear track
n is the number of strokes
The Nanovea Optical Profilometer was used to evaluate the wear track profiles, and optical microscope was used to examine the wear track morphology.
Table 1. Test parameters of the wear measurement.
||60 rpm / 1 Hz for reciprocating wear
|Number of strokes
|Duration of test
||1 h for rotative wear
0.5 h for linear reciprocating wear
Results and Discussion
Figure 4 shows the COFs recorded in situ. It must be remembered that one revolution in the linear reciprocating wear test comprises of back and forth two strokes on the wear track, when compared to one stroke per revolution for the rotative setup. The sample that was tested using linear reciprocating and rotative setups shows similar COF of ~0.6 to 0.65 all through the measurements following the run-in period, which is denoted by the progressive increase of COF in the first few hundreds of cycles.
The Nanovea non-contact optical profilometer was used to measure the wear track profiles of the acrylic sample following linear reciprocating and pin-on-disk tests, as illustrated in Figure 5. In Figure 6, the corresponding wear rate and wear track depth are compared. Interestingly, after the same number of strokes, the acrylic sample displays considerably different wear rate in the rotative and reciprocating wear tests. During the reciprocating wear test, the wear rate is ~5.6 x 10-5 mm3/Nm, four times compared to that during rotative wear test (~1.4 x 10-5 mm3/Nm).
Next, the wear tracks of the acrylic sample following the linear reciprocating and rotative wear tests were inspected and compared under optical microscope as shown in Figure 7. After linear reciprocating wear test, the acrylic sample exhibits a much deeper wear track. On the contrary, the wear track post the rotative wear test exhibits an entirely different morphology where features of one directional plastic deformation can be seen. Such a major change in the wear behaviors in two standard wear test setups could be attributed to local fatigue wear under plastic contact occurring in the linear reciprocating wear setup5.
During the process of repeated sliding in two opposite directions, the plastic flow wear particles and asperities grow and project in the wear track of the acrylic sample. The movement of the SiN ball creates back and forth shear stress that weakens the cohesion of the formed asperities with the base material and coarsens the surface of the wear track, which in turn accelerates the mass loss by removal of the asperities and debris.
The significantly different wear resistance determined in two commonly used wear test setups highlights the significance of comparing the materials’ wear resistance using identical experimental setup and test conditions. It shows that when small changes in testing conditions are introduced into the tribosystem, wear can change dramatically. The versatility of the Nanovea Tribometer enables measuring the linear reciprocating and rotative wears in a single system under different conditions, such as lubrication, high temperature, tribocorrosion, etc., making it a perfect tool for research/testing laboratories tackling a wide range of materials applied in various tribological conditions.
Figure 4. Coefficient of friction during (a) linear reciprocating and (b) rotative wear tests.
Figure 5. Wear track profiles of the acrylic plate after reciprocating and rotative wear tests.
Figure 6. Wear rate and wear track depth after reciprocating and rotative wear tests.
Figure 7. Wear track images of acrylic sample after reciprocating and rotative wear tests.
Based on the detailed tribological analysis in this study, it was shown that the acrylic sample has considerably different wear rate determined under linear reciprocating and rotative pin on disk setups. This highlights the significance of caution when it comes to comparing the wear rate established using different types of test setups or reported in the literature.
Using ASTM and ISO compliant linear and rotative modes, the Nanovea Tribometer provides precise and repeatable friction and wear testing. The device offers optional lubrication, high temperature wear and tribocorrosion modules that are available in a single pre-integrated system. Such versatility enables users to better replicate the actual application environment and thus enhance the fundamental understanding of the tribological properties and wear mechanism of a variety of materials. An optional 3D non-contact profiler is also provided for high resolution 3D imaging of wear track, aside from other surface measurements, for example roughness.
1 Rabinowicz, E. (1995). Friction and Wear of Materials. New York, John Wiley and Sons.
2 Jones, M., H., and D. Scott, Eds. (1983). Industrial Tribology: the practical aspects of friction, lubrication, and wear. New York, Elsevier Scientific Publishing Company.
3 ASTM G133 - 05(2010). Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear.
4 ASTM G99 - 05(2010). Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus.
5 Bhushan, B. (2001). Modern Tribology Handbook, CRC Press.
This information has been sourced, reviewed and adapted from materials provided by Nanovea.
For more information on this source, please visit Nanovea.