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]. 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