You should look at certain temperature where you have a healing process. We call it solid tribofilm formation for certain lubricant. At that moment you have a reduction in friction and thus you have an improved wear characteristics.

There is a device called Nancrush which you can measure the friction coefficient . There is furnace beside as you can put your samples in it and then put the samples into the Nanocrush and you will get the desired results within minutes.

Temperature has been conclusively proved to have a definite effect on the frictional behaviors of steel contact in either dry or lubrication condition. Rising in temperature possibly induces oxidation of the steel tribopairs (the degree of oxidation is a function of temperature), thus directly changes the chemical and mechanical properties of the tribo-interfaces. In term of lubrication, increase in temperature results in oxidation of the lubricant which is typically associated with degrading friction-reducing performance. Again, it depends very much on whether or not your lubricant contains any anti-friction additive which is prone to be oxidized at elevated temperatures.

Hope it helps.

Thanks Bach

Under unlubricated conditions, friction and wear of steel contacts (in air atmosphere) are generally observed to decrease with temperature due to the formation of the so-called „glaze” layer consisting of sintered iron oxide wear particles. The formation of this layer which works as a protective coating is promoted at higher temperatures – see: “Pearson et al., Wear 303, 2013, pp. 622-631” (https://doi.org/10.1016/j.wear.2013.03.048). However, this behaviour was typically reported for fretting conditions or sliding under low contact pressures: in our study: "Velkavrh et al., Tribol Int 98 , pp. 155-171 ,2016" (https://doi.org/10.1016/j.triboint.2016.02.022), under high normal loads, friction and wear of unlubricated steel contacts have increased at higher temperature. We presume this is because the “glaze” layer cannot withstand the severe contact conditions and due to the thermal degradation of material at elevated temperatures.

It is possible, that in your tests, the "glaze" layer started to efficiently form first at 80°C, while at 40°C it hasn´t yet formed to such an extent that it could provide any wear-protective properties. Therefore, thermal degradation was dominating the wear behaviour. At 80°C, "glaze" layer could already be formed and has provided some wear protection. Maybe you should try conducting additional tests at T > 80°C (e.g. 100°C or higher) to verify this hypothesis.

Note that in his study, Pearson has reported that the transition temperature, where the "glaze" layer has started to form was 85°C.

Der Gagandeep:

Regarding your questions:

1. ¿ Have you ever seen an increase in friction with an increase of temperature during dry or lubricated conditions by using plint off tribometer(reciprocating sliding contact) or any friction testing techniques.?

My answer: It depends. I suppose both tribopairs are made of stainless steel, if that is the case, under unlubricated conditions (dry) an increase in friction coefficient is observed by adhesion in areas that are not protected by surface films. In most sliding systems, surface of the contacting bodies are protected against strong adhesion by several layers that vary in chemical and physical nature, even in the case of stainless steel there will always be a thin protective layer caused by the atmosphere (air). One of the most accepted hypothesis is that adhesion during sliding increases when the rate of protective films destruction exceeds that of formation of protected films. The rate of destruction increases with temperature in the form of an Arrhenius-type function.

2. ¿Why does the friction reduce instead of increasing during dry conditions after 40 celsius?

My answer: ¿Are your values after 40 C plotted as the average friction coefficient per experiment?. In my opinion, under unlubricated conditions, it does not make much sense, even in the case of boundary lubrication, you will expect friction to decrease with temperature due a decrease in viscosity (lower friction force to shear the fluid). Please check carefully the data and the conditions of the experiment.

I do not think temperature has a totally negative effect on properties thus increasing friction. we notice that temperature will have a positive effect sometimes depending on the lubricant used. I totally agree with the oxidation criteria, but this will develop mostly at the onset of rising temperature; afterword wear debris and reaction temperature with lubricant will enhance the formation of solid tribofilm and thus reduce friction for long duration tests and this will contribute positively to the reduction in wear. Review several Nehme et al papers.