Proceedings of SerbiaTrib '13

alloys in contact with nickel-platedfinish. The testswere conductedin conditions of controlledhumidity and air cleanliness. The dramatic increaseof coefficient of static friction was noticed whenthe normal load reduced to the lowest level. Thiskind of behavior is attributed to the adhesion forceswhich havea significant role and which are moreprominent at low normal loads and smoothsurfaces. Tothe knowledge of authors the studyy ofstatic coefficient of friction under conditionss ofhigh temperature wasnot the topic of moreextensive theoretical and experimental works [14-17]. Behavior of the static and kinematic frictionn ofmaterials coupling at differentvalues oftemperaturee and contact pressures investigatedChaikittiratana, Koetniyom and Lakkam [ 17].Device which is used for performing of theexperiment is specifically constructed for thedetermination of slidingfriction at highertemperatures. It was established that at 100°C thereisn't significant change in the coefficient of frictionwhen the contact pressure varies. However, at atemperaturee of 200°C observed friction coefficientchange with the change of the contact pressure.There was an increase in the coefficient of frictionwhen the contact pressure increasedfrom0.147MPa - 0.252MPa. It is concluded that theincrease in temperaturee significantly increases thecoefficient of friction. One of the most importantconclusionss of this workis that the static s coefficientof friction increases with increasing temperature,which is partly a consequence of increase theplasticity of most contact materials at highertemperatures.The aimof this paper is to determine theinfluence ofpreviously heat treated contact pairss onthe value ofthe static coefficient of f sliding friction,with contact elements made of steel. Experimentalmeasurements were performed on instrumentationdeveloped by the authors, in order to accuratelydetermine the value of the staticcoefficientt ofsliding friction at highertemperatures and relativelylow values of contactt pressures in the variableradius of curvature of contact elements. The resultscould havea significant aplication in industrialapplications.accurately measure very small values of physicalquantities(normaland friction force). f Formeasuring small values of the friction force isnecessary to exist e electronic components in themeasuring chain (sensorss of force and otherelectronic components). Since the forcesensors, forreasons of measurementmreliability, should beoutside the zone of high temperatures,, it becomesimportant ,to the signals associated with smallldisplacement and a force, that they have to betransmitted mechanically from the high temperatureezone (the chamber in which the pair is heated todesired temperature) to thee sensor to quantify thevalue of the friction and normal contact load ofcontact pair. Mechanicaltransmission chain offriction force pulls the corresponding measurementerrors, especially when it comes to measure verysmall values of f force.The authors started from the idea that theprinciple of measuring m the static coefficient offriction over the steep plane have to be upgradeddandenable measurementin conditions of highhtemperatures and low values of contact pressure.Theprinciple of measurement of coefficient offriction over the steep plane (Figure 1) ) is based ongravity. Static coefficient c of friction, ass it is knownnis the ratio of the friction force and the force f normalto the surfacee of contact, where condition forequilibrium on a steep plane is given byexpression ∙sin. In the limiting caseof slidingfriction valid equation is:Fmggsin tg (1)N mggcoswhere are: value of st tatic frictioncoefficient;F - friction force; m - mass of body; g -acceleration off gravity; - steep angle of theplane.2. THEORETICAL CONSIDERATIONSMeasurement of thefriction coefficient at hightemperaturee in contact pairs is associated with anumber ofproblems of physicall and technicalnature. Issuesrelated to the reliability ofmeasurements are especially pronounced in themeasurement of the friction coefficient under lowcontact pressure. Problems related tothe fact that athigh temperatures in the contact pairs must148Figure 1. Thee balance of the body on a steep s planeMeasurement error e of static coefficient of frictionon this principle arising from error of anglemeasurement in i relation to an ideal horizontalposition when the t body which is located on a steepplane crossed from sleep p in a stateof motion(Figure 2), respectively:13 th International Conference C onn Tribology – Serbiatrib’13