An Introduction to Wear Degradation Mechanisms of Surface-Protected Metallic Components

metals Review An Introduction to Wear Degradation Mechanisms of Surface-Protected Metallic Components Pandora P. Psyllaki Department of Mechanical Engineering, University of West Attica, 250 Thivon avenue & P. Ralli, 12244 Egaleo, Greece; [email protected]; Tel.: +30-210-538-1292 Received: 1 September 2019; Accepted: 25 September 2019; Published: 28 September 2019 Abstract: Despite the fact that ceramics and polymers have found numerous applications in several mechanical systems, metals and metallic alloys still remain the main materials family for manufacturing the bulk of parts and components of engineering assemblies. However, in cases of components that are serving as parts of a tribosystem, the application of surface modification techniques is required to ensure their unhampered function during operation. After a short introduction on fundamental aspects of tribology, this review article delves further into four representative case studies, where the inappropriate application of wear protection techniques has led to acceleration of the degradation of the quasi-protected metallic material. The first deals with the effects of the deficient lubrication of rolling bearings designed to function under oil lubrication conditions; the second is focused on the effects of overloading on sliding bearing surfaces, wear-protected via nitrocarburizing; the third concerns the application of welding techniques for producing hardfacing overlayers intended for the wear protection of heavily loaded, non-lubricated surfaces; the fourth deals with the degradation of thermal-sprayed ceramic coatings, commonly used as wear-resistant layers. Keywords: surface modification techniques; degradation of protective layers; lubrication; nitrocarburizing; hardfacings; thermal-sprayed coatings 1. Introduction The term “tribosystem” or “tribopair” is commonly used to describe the conjugated function of two components that are moving relevant to each other [1]. Although in general, one element of the tribosystem could be in the fluid state, e.g., natural gas moving in a pipeline, in the majority of mechanical systems both elements are in the solid state and their coupled operation serves to transfer motion, power, or mechanical loading; thus, a typical tribosystem is schematically presented in Figure1a. A tribosystem is defined by (a) the geometry of the participating elements, (b) their relevant geometry that defines the contact as either conformal or contra-formal, (c) the microgeometry of the surfaces in contact (roughness, Figure1b), and (d) the construction materials of the two elements and the degree of their hardening. Each such tribopair can perform under various operational parameters that combine the normal load applied (F), the relevant speed (v), and the intervention of a third medium (solid or fluid) at the area between the two in-contact surfaces [1]. Finally, the operation of the tribosystem is carried out in a particular surrounding environment that is characterized by the level of relevant humidity (% RH) and temperature, or even by the application of vacuum. Depending on the type of the relevant motion, the tribosystems in mechanical engineering can be generally divided into rolling or sliding bearings. Metals 2019, 9, 1057; doi:10.3390/met9101057 www.mdpi.com/journal/metals Metals 20192019,, 99,, 1057x FOR PEER REVIEW 22 of 17 Figure 1. (a) Schematic representation of a tribosystem and (b) magnification of the inter-surface area Figure 1. (a) Schematic representation of a tribosystem and (b) magnification of the inter-surface area marked in (a). marked in (a). The resistance to the relevant motion between the two elements is reflected in the friction coefficient valueThe (µ), resistance which is equalto the to relevant the ratio motion of the normalbetween load the appliedtwo elements to the frictionis reflected force in that the appearsfriction μ coefficient= F value ( ), which is equal to the ratio of the normal load applied to the friction force that (µ T ). The higher the friction coefficient, the higher is the energy consumption for keeping the appears ( ). The higher the friction coefficient, the higher is the energy consumption for keeping motion ongoing [2]. Under these conditions, the operation of the tribosystem leads simultaneously tothe material motion removal ongoing from [2]. the Under two conjugated these conditions, surfaces, i.e.,the material operation losses of [3the]. In tribosystem the meantime, leads the applicationsimultaneously of a to normal material load removal at the contactfrom the surface two conjugated leads to a surfaces, distribution i.e., material of applied losses pressure [3]. In that the resultsmeantime, in a distributionthe application of normal of a normal and shear load stressesat the contact within surface the upper leads surface to a layersdistribution of the twoof applied solids inpressure contact; that the results former in are a distribution maximized of on normal the contact and shear area, whilestresses the within latter the are upper maximized surface at layers a depth of belowthe two the solids contact in area contact; of the twothe solidsformer [ 1are–4]. maxi This factmized could on resultthe contact in crack area, initiation while and the propagation latter are duringmaximized the operation at a depth of below a tribosystem. the contact area of the two solids [1–4]. This fact could result in crack initiationDepending and propagation on the particular during the application operation of of each a tribosystem. tribo-assembly, the requirement could be for eitherDepending low or high on friction the particular coefficient application values, i.e., of cutting each toolstribo-assembly, or brakes, respectively,the requirement either could low or be high for weareither values, low or i.e., high brakes friction or workpiece,coefficient respectively.values, i.e., cutting In all cases, tools theor brakes, stable operation respectively, of the either tribosystem low or requireshigh wear a constant values, frictioni.e., brakes coeffi orcient workpiece, and uniform respec weartively. rates In during all cases, function, the stable as well operation as normal of andthe sheartribosystem stresses requires lower than a constant the yield friction point ofcoefficient the construction and uniform materials wear of rates the twoduring conjugated function, elements as well andas normal significant and durabilityshear stresses when lower operating than the in the yield fatigue point regime. of the Theconstruction combined materials effect of theof the friction two force,conjugated the normal elements and and shear significant stresses, durability and the adhesion when operating forces developed in the fatigue between regime. the The two combined elements defineeffect of the the wear friction micro-mechanisms force, the normal that and are shea activatedr stresses, on theand contact the adhesion area, leading forces todeveloped material between removal fromthe two both elements conjugated define surfaces the wear [3]. micro-mechanisms that are activated on the contact area, leading to materialThe topography removal from and bo theth mechanical conjugated propertiessurfaces [3]. of the surface are of crucial importance for the operation,The topography since they and determine the mechanical the stress properties distribution, of the the surface ease of are relevant of crucial motion, importance and the for wear the rateoperation, and mechanisms since they determine that affect the surfacestress distributi and sub-surfaceon, the ease crack of initiationrelevant motion, and propagation, and the wear energy rate consumption,and mechanisms and that material affect losses, the surface respectively. and sub-surface In order to crack enhance initiation the performance and propagation, of such bearingenergy surfaces,consumption, several and modification material losses, techniques respectively. are available, In order to involving enhance liquidthe perf orormance solid compounds of such bearing that altersurfaces, the distributionseveral modification of surface techniques stresses, the are relevant available, motion, involving and/ orliquid material or solid removal, compounds but not that the mechanicalalter the distribution properties of thesurface bulk stresses, metal of the the relevant entire structure. motion, Even and/or though material such removal, surface modification but not the techniquesmechanical do properties have a beneficial of the effbulkect onmetal engineering of the surfaces,entire structure. their inappropriate Even though application such orsurface their erroneousmodification selection techniques for a particulardo have a application, beneficial rendereffect on their engineering involvement surfaces, in the tribosystemtheir inappropriate a rather detrimentalapplication or factor their for erroneous the integrity selection of the for entire a partic structure.ular application, render their involvement in the tribosystemIn order a rather to overcome detrimental undesirable factor fo prematurer the integrity failure of the during entire thestructure. operation of a tribosystem consistingIn order of twoto overcome conjugated undesirable metallic solids, premature it is recommended failure during to the modify operation the properties of a tribosystem of their surfacesconsisting exposed of two toconjugated the action metallic of the above-mentioned solids, it is recommended facts. The to most modify widely the appliedproperties techniques of their thatsurfaces result exposed in mainly to the the action reduction of the

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