Proceedings of SerbiaTrib '13

4. FRICTION AND WEAR ISSUES RELATEDTO MG ALLOYSTribocorrosion is defined as an irreversibletransformation of a material from concomitantphysicochemical and mechanical surfaceinteractions occurring at tribological contacts [11].In general, metallic implants need to be tested totribocorrosion and wear issues, regardless of theirarea of application. The realised contacts betweenimplant material and either other implant material,or organic body constituent (e.g. bone) andelements of the aggressive physiological bodyenvironment, provoke certain responses (weardebris) that need to be investigated in more depth,especially for newly developed biomaterials. It isproven that even micro contacts within smallregions sometimes influence significant responses,ranging from changes in contact environment up toincreasing deterioration of implant surfaces due towear related processes. Two- or three-body contactsare frequently associated with tribocorrosion [12].Entrapped wear debris acts as an abrasive and isdefined as the third body. The main concernsrelated to the simultaneous action of corrosion andwear in biomedical systems are the ability of thepassive layer to withstand the mechanical stressesarising from wear, the ability of the metal surface torepassivate when the passive film is removed andthe resistance of the new repassivated surface toboth wear and corrosion [12]. Fretting has a biginfluence on the corrosion behavior of orthopedicdevices. The tribocorrosion and fretting corrosionmechanisms have been mainly related to in vitrolaboratory investigations. However, the in vivobehavior of metallic implants under combined wearcorrosion or fretting corrosion has been hardlystudied [12]. Also, the corrosion fatigue ofstructural magnesium alloys has been studied byseveral authors in NaCl and borate-bufferedsolutions, but generally focused on applications inelectronic, automotive and aerospace industries.The fatigue strength of magnesium alloys issignificantly reduced in humid environments, andthe fatigue limit drops drastically in NaCl solution[12]. Even though the corrosion behavior ofbiomedical magnesium-based alloys has beenextensively studied, corrosion fatigue has receivedlittle attention and degradable material mustmaintain appropriate mechanical strength duringthe healing of the fractured bone, to provide safeorthopedic device. Also, fatigue crack propagationbehavior needs to be studied for a wide variety ofbiodegradable Mg alloys [12]. Corrosion and wearresistances are frequently studied in synergy,because of the direct relationship between theseproperties and the biocompatibility of thebiomedical device.Regarding coronary stenting by using metallicimplants, analysis of the fatigue crack growthbehavior is of the utmost importance for its properfunctioning. There are my mathematical models inthe literature, but, in particular, the fatigue crackpropagation approach simulating crevice corrosionconditions in physiological solutions has beenhardly considered for biomedical magnesium alloys[12]. Very important aspect is tribology relatedphenomena during the production of magnesiumbiodegradable vascular stents minitubes, sincemagnesium alloys possess highly limited roomtemperatureformabilities [13]. Highly complexphysical, chemical and mechanical characteristicsof magnesium must be taken into consideration, inorder to keep the magnesium alloy characteristics(microstructure, etc.) unchanged under theinfluence of e.g. severe abrasive friction and wearduring cold drawing [13].Highly important is the understanding of theMg alloy stent behaviour at its positioning duringthe movement through the vasculature, at theinitial interventional cardiovascular treatment.Lubricious coatings have been used for over 20years [14] and the benefits are well established:(1) lower frictional force between the device andthe vessel reduces tissue damage and preventsvasospasm; (2) improved maneuverability aidsnavigation of complex lesions and facilitatesaccess to tortuous vascular sites leading toexpansion of the patient population that canbenefit from these treatments; and (3) reducesthrombogenicity. In addition, reduced frictionbetween the therapy catheters and supportcatheters leads to improved outcomes, reducedprocedure time, and, ultimately, reduced cost [14].The stent is commonly placed at the one fixedposition within a blood vessel, meaning that thereare no further movement between the stentmaterial and surrounding tissue, leading to theconclusion that wear plays no significant role.However, blood flow around the placed stent hasmicro influence and might provoke nano-weardebris, which is not investigated so far. Such tinywear debris represents a form of particulate matterin the vasculature and it is well known that if largeenough and in sufficient quantities, can causeocclusion of blood vessels and lead to tissuehypoxia and, ultimately, necrosis [14]. If alloyingelements of Mg alloys, such as Al, Zn areconsidered as well, it is obvious that this areaneeds further studies. Since Mg alloys stents hasnot been widely tried in clinical practice, there aremany tribology related questions to be addresses.96 13 th International Conference on Tribology – Serbiatrib’13