Proceedings of SerbiaTrib '13

Model of the phenomena that occurs in microzonesof contact during friction is shown in Figure1 [2]. One can distinguish three main stages ofinteraction between contact surfaces during frictionStage I. At this stage contact occurs as well asmechanical action between contact surfaces whichare initially covered with oxide layers, whereby theamount of oxide depends largely on the type ofprocesses (metal forming of hot or cold), andsusceptibility to oxidation of the metals that formcontact pairs. The dominant phenomenon at thisstage of contact is the plastic deformation, not onlyof the surface roughness but also, and in theconsiderable volume of the material.Stage IStage IIStage IIIFigure 1. Model of the phenomena that occur in the micro-areas of contact during friction [1]Stage II. As a result of molecular interaction,adhesive joints (compounds) are formed. Thequantity of joints depends largely on the geometryof the contact and the specific pressure.Stage III. This stage of the contact surfaceinteraction includes the destruction of adhesionjoints which were formed during the relativedisplacement of contact pair metals. Failuremechanism of contact joints can be very complex.In the first stage of joint destruction micro-slip willsurely occur, and therefore the complex phenomenaof movements and mutually dependent movementof dislocations. As a result of these phenomenasurface defects such as micro-cracks and micronotchescan be created.As a result of repeated displacement ofdeformed metals in regards to the surface of thetool, the effect of friction and the associatedforming and destruction of adhesive joints, toolwear occurs.In the case of metal forming, process ischaracterized by the fact that multiple repeatedoperations (forging or ironing) causes a gradualincrease in thickness of glued layer, which meansthat the sum of the individual joints goes into acontinuous layer.As a result of predominance of adhesive forceover resistance to plastic flow in the glued layer, infurther stage glued particles suffer from plasticdeformation until they are torn off and smeared.When a sufficiently large thickness of gluedlayers is formed as a result of repeated process ofplastic deformation (in a series of passages whichleads to increasing and decreasing of mutualinteraction), process begins which leads to theseparation of adhesive joint from tool surface bypeeling (shear) or tearing. This leads to thesignificant damage of the tool surface layers, andtherefore to the increase in surface roughness. Theoccurrence of peeling or tearing depends on thetype of formed joint.In case of contact pairs with higher chemicalaffinity, the strength of diffusion produced joints(solid solutions) may be greater than strength ofmaterial of contact pairs, causing the destruction ofjoint to occur in depth of less strong and nofortified material, which means it occurs at such adepths at which there is no more squeezing.In the case of diffusion-less joints, as well as forthe occurrence of brittle inter-metallic phases, thedestruction of joint will be based mainly on layeredpeeling of metal with less strength. Tearing off ofglued particles also occurs. Further relativedisplacement of contact elements (the work piece isplastically deformed in relation to the tool), makesthese glued particles to reappear on the surface ofthe friction. Afterwards they are compressed on thetool surface, which results in creation of groovingin "partner" made of material with less hardness(usually plastically deformed work piece). The typeand intensity of this secondary effect depends onthe hardness of the plastically deformed andstrengthened adhesive joints.266 13 th International Conference on Tribology – Serbiatrib’13