Proceedings of SerbiaTrib '13

dielectric properties, high resistance to chemicalattack in presence of acids and alkali, high thermalconductivity, high resistance and stiffness, excellentformability and high purity. Due to these propertiesalumina is widely used in technical applications (e.g. automotive industry and in medical implants).The general properties of the two types ofmaterials (Ti6Al4V and Al 2 O 3 ) have leaded to theuse of these in applications where both wear andcorrosion resistant qualities are critical. In theseapplications, especially under relative motion andunder the action of external loads and in activechemical environments, it is mandatory to maintainthe surface integrity state during the service life.In the present paper is presented the evolution ofthe functional integrity state in a comparative wayfor two contact pairs (Ti6Al4V/Al 2 O 3 andTi6Al4V/steel ball) in dry and corrosiveenvironment.2. DRY WEAR AND TRIBOCORROSIONFORMULATIONThe analysis of the functional integrity stateunder dry wear conditions is based on the wellknown Archard's wear law [5]. It says that theamount of the material loss depends on theproperties of the contact surfaces, topographicalcharacteristics of the surfaces and test conditions.The most common form of Archard's equation isVSFn K(1)Hwhere V - the volumetric material loss of the body,K- the wear coefficient (it is dimensionless andalways less than unity), H - the hardness of thesofter body in contact, F n - the applied normal loadand S - the sliding distance.It has been analysed the amonut of material thatwas removed (wear loss) in the wear process. Thesurface analysis of the wear tests was based on theevolution of the depth of the wear track (based onthe material loss) with the applied load. Anotherparameter that was analyzed is the final topographyof the wear track by using the 3D roughnessparameters (Sa – Average Roughness, Sy - Peak-Peak Height, Sq - Root Mean Square Height, Sp -Maximum Peak Height, Sv - Maximum Pit Height).In the case of reciprocating wear tests incorrosive enviroment occur the degradationprocesss of surfaces by tribocorrosion. This processincludes the interaction between mechanical,chemical and electromechanical processes of wearthat lead to loss of weight by adding all theseeffects [6]:Wear=mechanical wear process+electrochemical(and/or chemical response) (2)This process includes the interaction ofcorrosion with: solid corrosive particles (debris),particles resulted due to abrasive processes, frettingprocesses, processes under biological solutionconditions, and triboxidation related to the mutualinteraction process under relative motion conditionsof the surfaces.Generally, oxide layers are formed after thecorrosive attack which protects the material fromfurther corrosive attack. But these oxide films aresusceptible to the tribological processes that willaccelerate the corrosion in these areas. The galvanicactivity that results between the worn and unwornsurfaces under electrochemical conditions [7], leadsto an anodic current I a , [8]:Ia kb1 2 Fn l f H 1 f0i d (3)where: k b – proportionality factor; l – sliding length;f– frequency of the reciprocating motion; F n –normal load; H– surface hardness; i - corrosioncurrent density; τ- time.Equation (3) can be written as [9]:Ip kbVs Fn H 1 2 Qp(4)where: V s – sliding speed; Q p - passivation chargedensity [5].Qpi d(5)0corrOn the other hand, electrochemical wear can bedetermined based on passivation currentIV p t Mn Fmol(6)where: t – time; M – molecular weight; ρ – density;n – valence; I – Faraday's constant.The corrosion rate [10,11] can be determinedbased on linear polarization and on the Stern-Geary’s equation [12] as follows:icorr a c2.31a c R p(7)where: β a şi β c cathodic and anodic Tafel slopes(figure 1); R r – polarisation resistance.Thus the tribocorrosion processes can beanalysed based on the evolution of theelectrochemical parameters β a , β c , I corr , E corr..If in the dry wear conditions the amount of thematerial loss is determined based on the rationF n /H, in the case of wear tests in the corrosionenvironment conditions the wear process isinfluenced by the electrochemical parameters.13 th International Conference on Tribology – Serbiatrib’13 81