What are the most common difficulties found for preparing the samples (roughness, uniformity), for controlling the test parameters, and what should be done to guarantee repeatability?
The sample preparation plays a very important role while performing pin-on-disk wear test.
As mentioned above by prof. Anand pai, geometrical deviation is important. But more deep reason that such kind of data is easy to disperse, exist in new nano-level mechanism in boundary lubrication regime.If you want to know it, I recommend to read abstracts of our publication.
Very good answers, I would only add that humidity can affect friction in certain coatings such as DLC
Effect of temperature and lubrication film thickness is also taking important roll in the pin-on-disk wear test.
Most importantly, the vibrations in the apparatus should be at its minimal. Also, it would be important there are no pre-existing wear particle remains. Furthermore, POD is recommended to be considered only for screening purpose.
Vibration is a real problem that I've found in some test rigs developed by non-experts, Mr. Afnan Haider Khan . About the wear particles, is it common cleaning the sample during the test (vacuum cleaning or air jet, for example) ?
https://iopscience.iop.org/article/10.1088/1757-899X/1114/1/012051/meta
https://iopscience.iop.org/article/10.1088/1757-899X/1059/1/012020/meta
The pin-on-disk characterization process presents characteristics of the strength of the material or thin films applied on its surfaces. To evaluate the affected region, we have to take into account all the deformation generated on the surface. Generally, rougher surfaces present more difficulties in the process, another important point is the definitions of the loads used for the tests.
People. This paper is incredible:
Guicciardi, S., Melandri, C., Lucchini, F., & de Portu, G. (2002). On data dispersion in pin-on-disk wear tests. Wear, 252(11-12), 1001–1006. doi:10.1016/s0043-1648(02)00066-2
https://iopscience.iop.org/article/10.1088/1757-899X/1114/1/012051/meta
you must carefully prepare the surface of your sample. See the distribution of roughness Ra, Rz, Sk ...etc.
and especially apply the same contact pressure to each test.
Make sure the contact type is fixed. You can then control the test by blowing out the wear debris for each test or by leaving the debris formed at the interface of the two friction surfaces. You can also use piezoelectric sensors to measure normal force and tension force to automatically calculate the coefficient of friction.
The environment of your test is also important, for example humidity can play an important role in determining the coefficient of friction.
The applied normal loads have an effect on the wear coefficient.
All the dest
1/ Surface of your sample (should be perfectly polished, perfectly flat, ...),
2/ Shape and surface of your pin/ball (if damaged or not),
3/ Oxydation of sample/pin,
4/ Room humidity and temperature,
5/ Mechanism of tribometer (should be perfectly adjusted),
...
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