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.

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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 of above-mentione the friction force—thus,d facts. The reduction most widely of the applied friction techniques coefficient andthat consequentlyresult in mainly diminution the reduction of the energyof the friction demands force—thus, for motion sustainability—consistreduction of the friction of thecoefficient application and ofconsequently a lubricant diminution medium between of the energy the two demands conjugated for motion surfaces sustainability—consist [2]. The main categories of the of lubrication,application representativeof a lubricant medium media, and between common the applicationstwo conjugated for eachsurfaces one are[2]. presentedThe main incategories Table1. of lubrication, representative media, and common applications for each one are presented in Table 1.

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Table 1. Main categories of lubrication and representative examples.

Category Representative Lubricants Main Applications Graphite; Phyllomorphous Dies and Molds for Metal Forming Solid-State Lubrication Minerals and Shaping Engines and Tool-Machines for Liquid-State Lubrication Oil-Based; Water-Based Metals Machining Gas-State Lubrication Air Dental Equipment

To ensure their effectiveness, both solid and liquid lubricants should, mainly, exhibit effective adherence to the contact surfaces of the conjugated solids, coherence under the normal and shear stresses and compositional stability at the temperature range to be developed during operation of the tribosystem. During the last decades, several surface modification techniques [4–7] were developed aiming to reduce the wear rate of the material of the main construction by several orders of magnitude, not necessarily leading to a decrease of the friction coefficient. The majority of these techniques are based on the creation of a surface overlayer on the initial material; the former being much more wear resistant than the latter. In this way, the service lifetime of the elements of the tribopair is prolonged and, in some cases, this “protective” surface layer can be easily replaced without any adverse effect on the metal of the main structure. In general, the thickness of such overlayers is smaller compared to the dimensions of the main structure; however, these are efficient to expand significantly the service lifetime of the elements, without crucial dimensional changes of the main components. The main categories of these techniques, representative surface layers, and respective applications are presented in Table2.

Table 2. Main categories of coatings techniques and representative examples.

Representative Wear-Resistant Technique Main Applications Surface Layers Forming, Cutting tools, Dies, Gears, Thermochemical Treatments Nitriding and nitrocarburizing Shafts, Clutches Welding; High-power laser Carbide-reinforced Fe-based composites Heavily-loaded surfaces Oxides (Al O ; TiO , YSZ); Medical implants Thermal Spraying 2 3 2 WC-, Cr3C2 based CerMets Heavily-loaded surfaces Physical or Chemical Vapor TiN; TiC; BN; Diamond; DLC Cutting tools Deposition (PVD or CVD) Metallization/Plating Cr-, Ni- coatings Automotive parts

In particular, the typical coatings obtained via PVD, CVD, and plating (electrolytic or electroless) contribute not only to wear resistance but also to the reduction of the friction coefficient. Despite the fact that both approaches, i.e., lubrication and coatings deposition, contribute significantly to the protection of the metal of the basic construction, their inadequate application or their use under non-recommended operating conditions would lead to the acceleration of degradation of the metallic structure to be protected and to premature catastrophic failure of the elements of the tribosystem. Four relevant case studies that emphasize the degradation of metallic components of representative wear-protected elements due to the failure of the protection are discussed below, the main aim being to present, in a unified and coherent way, the detrimental effects on the performance of tribo-elements that should have exhibited prolonged lifetime after the application of a surface protection technique.

2. Synopsis of Experimental Techniques It is generally admitted that in cases of severe degradation, the root cause of failure could be accurately determined by observing the failed system at low magniﬁcation or even by optical inspection, whilst observations at higher magniﬁcations or complementary diagnostic techniques can be applied in order to further elucidate mechanisms that are unclear, vague, and not widely known. For this purpose Metals 2019, 9, 1057 4 of 17 and on a case-by-case basis, the experimental techniques applied in the following representative examples comprised:

(1) Visual inspection and stereomicroscopic (Leica MS 5,Leica Microsystems GmbH, Wetzlar, Germany) and optical (Olympus BX60, Olympus Corporation, Tokyo, Japan) observations for determining the material’s failure areas that could provide information on the root cause, as well as areas that merit further laboratory examination. (2) Scanning electron microscopy (SEM, JEOL 840A, Jeol Ltd, Tokyo, Japan) coupled with elemental micro-analysis (EDS, OXFORD INCA 300, Oxford Instruments, Abingon, UK) and X-ray diﬀraction (XRD, Siemens D-800, Siemens AG, Munich, Germany) analysis of selected areas, for revealing the material’s ﬂaws or transformations that accelerated failure.

(3) Ball-on-disc tribological measurements (Centre Suiss d0 Electronique et de Microtechnique, CSEM SA, Neuchâtel, Switzerland), for evaluating the performance of wear-resistant protective coatings before service.

3. Case Study I: Failure of Lubricated Rolling Bearings

3.1. Fundamentals of Liquid Lubrication Already since 1902, Stribeck [8] proposed a model for lubricated friction that describes the behavior v of a tribosystem in terms of friction coefficient, taking into account the combined factor η P of the operational parameters, where (η) is the viscosity of the lubricant, (v) the rotation speed, and (P) the load per unit of the projected bearing area. As can be observed in Figure2, depending on the operational parameters, each particular lubricated tribosystem exhibits a wide range of friction coefficient values that correspond to three different lubrication regimes [2]:

Within regime I, also known as boundary lubrication, the friction coefficient value tends to be • similar to that under non-lubricating conditions. The thickness of the lubrication film is very thin, the load is taken on exclusively by the two solids and is transferred from the one to the other trough the contact between their surface asperities; thus, the rheological characteristics of the lubricant do not intervene and the lubrication efficacy depends only on the physicochemical characteristics of the liquid. These exactly “quasi-dry” lubrication conditions result in severe wear of the two solids. Within regime II, also known as mixed or partial elasto-hydrodynamic lubrication, the friction • coefficient drops down to very low values. The thickness of the lubrication film is moderate, and the load is born by both the solid and the liquid film, depending on the thickness of the latter. The consecutive wear is moderate compared to that in regime I. Within regime III, also known as hydrodynamic lubrication, the friction coefficient values are • slightly higher, the thickness of the lubrication film is significant enough to result in complete separation of the two solids and the load is transferred from the one to the other through the lubricant that should take on the entire load; thus, lubrication efficacy is depended only on the rheological characteristics of the liquid. In this case, the wear of the two non-in-contact solids is negligible. Metals 2019, 9, 1057 5 of 17 Metals 2019, 9, x FOR PEER REVIEW 5 of 17

Figure 2. TypicalTypical Stribeck curve for liquid lubrication and associated regimes. Since 1902, this model has been further improved by emphasizing the effect of the surface Since 1902, this model has been further improved by emphasizing the effect of the surface microgeometry (average roughness) of the two conjugated solids [9,10] and the thickness of the microgeometry (average roughness) of the two conjugated solids [9,10] and the thickness of the lubrication film, and the distinction between the three regimes is based on the lambda ratio (λ) that is lubrication film, and the distinction between the three regimes is based on the lambda ratio (λ) that given by Equation (1): is given by Equation (1): hmin λ = q (1) 2ℎ+ 2  Ra,1 Ra,2 (1) 𝑅 𝑅 where hmin is the minimum film thickness for assumed, smooth, bearing surfaces, and Ra is the average roughness of solids (1) and (2). where hmin is the minimum film thickness for assumed smooth bearing surfaces, and Ra is the average Based on this approach, boundary lubrication should be expected for (λ) values lower than one roughness of solids (1) and (2). and hydrodynamic lubrication for (λ) lambda values higher than three [2]. Depending on the load Based on this approach, boundary lubrication should be expected for (λ) values lower than one and the relevant speed of motion applied to the tribosystem, as well as the viscosity of the lubricant and hydrodynamic lubrication for (λ) lambda values higher than three [2]. Depending on the load used, one should have in mind that the low friction coefficient value could suddenly increase to very and the relevant speed of motion applied to the tribosystem, as well as the viscosity of the lubricant high ones and the wear from negligible could turn to a severe one. Special attention should be given used, one should have in mind that the low friction coefficient value could suddenly increase to very to the thermal stability of the rheological characteristics of the liquid that could alter the lubrication high ones and the wear from negligible could turn to a severe one. Special attention should be given mechanisms, as well as its additives that could chemically attack the metallic components, leading to to the thermal stability of the rheological characteristics of the liquid that could alter the lubrication local corrosion [11] that could be the initiation locus of cracking [12]. mechanisms, as well as its additives that could chemically attack the metallic components, leading to 3.2.local Catastrophic corrosion [11] Failure that of could Rolling be Bearingsthe initiation locus of cracking [12].

3.2. CatastrophicThe catastrophic Failure failure of Rolling of a Bearings rolling-element bearing that should operate in the hydrodynamic lubrication regime [13] is presented as an example of metal degradation due to failure of the protectiveThe catastrophic means. failure of a rolling-element bearing that should operate in the hydrodynamic lubricationThe sub-assembly regime [13] failedis presented after six as months an example of continuous of metal operation degradation in a centrifugaldue to failure pump of thatthe wasprotective supporting means. the recirculation of monoethanolamine (MEA) in a petroleum refinery. Unhampered performanceThe sub-assembly of the assembly failed after is crucial six months since suchof contin a cleaninguous operation process ofin severala centrifugal products pump is essentialthat was beforesupporting their furtherthe recirculation treatment; of thus, monoethanolamine interruption of the (ΜΕΑ pump) in operation a petroleum has a negativerefinery. financialUnhampered effect performance of the assembly is crucial since such a cleaning process of several products is essential on the entire production. The temperature at the cleaning unit did not exceed 45 ◦C, whilst the speed wasbefore 2899 their rpm, further ~24% treatment; lower than thus the, interruption maximum value of the recommended pump operation by thehas bearing’a negative s supplier. financial effect on theAfter entire the production. removal of allThe the temperature organic residuals at the cleaning from the unit metallic did not surface, exceed visual 45 °C, inspection whilst the revealed speed thewas catastrophic2899 rpm, ~24% failure lower of than the outer the maximum ring of the va sub-assemblylue recommended (Figure by the3). bearing’ In particular, s supplier. 30% of its peripheryAfter wasthe separatedremoval of into all three the fragmentsorganic residuals that presented from irregularthe metallic fracture surface, surfaces visual (Figure inspection4) and signsrevealed of oxidation the catastrophic on them failure (Figure of5 ).the outer ring of the sub-assembly (Figure 3). In particular, 30% of its periphery was separated into three fragments that presented irregular fracture surfaces (Figure 4) and signs of oxidation on them (Figure 5).

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(a) (b) ((aa)) ((bb)) Figure 3. Rolling bearing as-received (a) and after the removal of organic residuals (b). FigureFigure 3.3. 3. Rolling Rolling bearingbearing as-receivedas-received ( a(a))) andand and afterafter after thethe the removalremoval removal ofof of organicorganic organic residualsresiduals residuals (( b(bb).).).

Figure 4. Stereoscopic images of a fragment of the failed outer ring, exhibiting geometric Figureirregularities.FigureFigure 4. 4.4.Stereoscopic StereoscopicStereoscopic images imagesimages of a fragmentofof aa fragmentfragment of the failed ofof thth outeree failedfailed ring, outer exhibitingouter ring,ring, geometric exexhibitinghibiting irregularities. geometricgeometric irregularities.irregularities.

Figure 5.5. Stereoscopic images of a fragmentfragment of thethe failedfailed outerouter ring,ring, exhibitingexhibiting signssigns ofof oxidationoxidation onon thetheFigureFigure fracturefracture 5.5. Stereoscopic Stereoscopic surface.surface. imagesimages ofof aa fragmentfragment of of thethe failedfailed outerouter ring,ring, exhibitingexhibiting signssigns ofof oxidationoxidation onon thethe fracture fracture surface. surface.

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Metals 2019, 9, x FOR PEER REVIEW 7 of 17 Disassembling of the rolling bearing revealed that failure was not limited to the ring, but it has also aﬀDisassemblingected the rolling of the elements rolling (balls)bearing that revealed exhibited that failure material was removal not limited in the to formthe ring, of cratersbut it has of smaller oralso higher affected dimensions the rolling (Figure elements6). (balls) that exhibited material removal in the form of craters of smallerThe morphologyor higher dimensions of both (Figure the fracture 6). surfaces of the outer ring and the rolling balls advocates for insuTheﬃcient morphology lubrication of both during the fracture operation surfaces that of resulted the outer in ring impact and the loading rolling thatballsled advocates eventually to thefor detrimental insufficient catastrophiclubrication during failure operation of the sub-assemblythat resulted in impact and the loading operation that led interruption eventually to of the the whole cleaningdetrimental unit. catastrophic failure of the sub-assembly and the operation interruption of the whole cleaning unit.

Figure 6. Representative failed rolling balls exhibiting different levels of damage severity. Figure 6. Representative failed rolling balls exhibiting different levels of damage severity. 4. Case Study II: Failure of Nitrocarburized Sliding Bearings 4. Case Study II: Failure of Nitrocarburized Sliding Bearings 4.1. Fundamentals of Nitrocarburizing 4.1. Fundamentals of Nitrocarburizing Several engineering components, such as gears and shafts, are commonly manufactured by steel of differentSeveral grades engineering and hardening components, degrees, such as depending gears and shafts, on the are type commonly and the manufactured level of mechanical by steel loading of different grades and hardening degrees, depending on the type and the level of mechanical loading to which they would be subjected during their final application. In the case of these metals serving to which they would be subjected during their final application. In the case of these metals serving asas parts parts of of a a tribosystem, tribosystem, their surface surface reinforcem reinforcementent against against sliding sliding wear wearis a requirement is a requirement not only not only forfor decreasing decreasing their their wearwear rate, but but also also for for altering altering the the wear wear micro-mechanisms micro-mechanisms that could that lead could to leadthe to the secondarysecondary activation activation ofof severe degradation degradation of ofthe the entire entire structure structure via oxidation via oxidation and/or and crack/or initiation crack initiation andand propagation. propagation. Such a a surface surface modification modification technique technique widely widely applied applied at an atindustrial an industrial scale is scale is nitrocarburizingnitrocarburizing and, and, especially,especially, Tufftriding, Tufftriding, often often referred referred to as to liquid as liquid nitrocarburizing. nitrocarburizing. TuTufftridingfftriding is is a a thermochemical thermochemical technique governed governed by by the the diffusion diffusion of ofnitrogen nitrogen and and carbon carbon in in the ferrousthe ferrous matrix matrix [5,14 [5,14,15].,15]. After After a a first first step step of pr preheatingeheating for for moisture moisture removal, removal, the steel the components steel components areare immersed immersed for forabout about 6 h in in a a bath bath of of cyanide cyanide salts, salts, with with a chemical a chemical composition composition of 60% ofKCN, 60% 24% KCN, 24% KCl, and 16% K2CO3. The treatment temperature remains constant at 580 °C, lower than that of KCl, and 16% K CO . The treatment temperature remains constant at 580 C, lower than that of austenitization; 2thus,3 the technique is characterized as ferritic nitriding. Under these◦ conditions, two austenitization;chemical reactions thus, take the place: technique is characterized as ferritic nitriding. Under these conditions, two chemical reactions take place: 2 → 2KCN2KCN + O+ O 2KCNO2KCNO (2) (2) 2 → 8KCNO → 2K2CO3 + 4KCN + CO2 + (C)Fe + (N)Fe (3) During the first reaction, potassium cyanide is oxidized to potassium oxycyanide, whilst during 8KCNO 2K2CO3 + 4KCN + CO2 + (C)Fe + (N)Fe (3) the second one, atomic nitrogen and→ carbon are produced and adsorbed by the solid surface. The Duringdesirable the firstsurface reaction, layer, enriched potassium in cyanidenitrogen isan oxidizedd carbon, to is potassiumformed via oxycyanide, simultaneous whilst chemical during the secondreaction one, of atomicthe produced nitrogen species and with carbon the ferrous are produced matrix and and nitrogen adsorbed in-depth by the diffusion, solid surface. as described The desirable surfaceby Fick’ layer, s second enriched law: in nitrogen and carbon, is formed via simultaneous chemical reaction of the produced species with the ferrous matrix𝜕𝐶 and nitrogen𝜕𝐶 in-depth diffusion, as described by Fick’ s 𝐷 (4) second law: 𝜕𝑡 𝜕𝑥 ∂C ∂2C After their rinsing with water to remove salt resi= duesD mechanically attached to the steel surface, (4) 2 the components are subjected to a stress relief heat∂t treatment.∂x After their rinsing with water to remove salt residues mechanically attached to the steel surface, the components are subjected to a stress relief heat treatment. Metals 2019, 9, 1057 8 of 17

Metals 2019, 9, x FOR PEER REVIEW 8 of 17 These two simultaneous mechanisms lead to the consecutive formation of two distinct layersThese [16–18 two] on simultaneous the treated surface, mechanisms that both lead consist to the the consecutive so-called caseformation depth (Figureof two 7distinct): layers [16–18] on the treated surface, that both consist the so-called case depth (Figure 7): (a) TheThe outermost outermost one one is is the the product product of the chemical reaction between the diffusing diffusing atoms and the iron of the base steel and consists mainly of ε-carbonitride Fe (C,N). This layer, known as white iron of the base steel and consists mainly of ε-carbonitride Fe2-32-3 (C,N). This layer, known as white oror compound compound layer, layer, is is the the featur featuree that that provides provides high high wear wear resistance resistance to to the the underlying underlying material. material. AlthoughAlthough its thickness doesdoes notnot exceedexceed 10 10µ μm,m, the the compound compound layer layer contributes contributes to to the the decrease decrease of ofspecific specific wear wear even even by by an an order order of of magnitude magnitude [17 [17].]. (b) TheThe layer underneathunderneath is is formed formed via via the the in-depth in-depth nitrogen nitrogen diff usiondiffusion that that leads, leads, primarily, primarily, to α-(Fe,N) to α- (Fe,N)solid solution solid solution with a nitrogenwith a nitrogen content decreasingcontent decreasing with increasing with increasing depth. In previousdepth. In works previous [18], worksit was [18], found it was that found the thickness that the ofthickness this diff usionof this layerdiffusion is strongly layer is a stronglyffected by affected the exact by chemicalthe exact chemicalcomposition composition of the steel of grade,the steel its levelgrade, of its prior level thermal of prior hardening, thermal hardening, or cold working or cold percentage. working percentage.Although this Although layer does this not layer affect does the wear not resistance,affect the itwear results resistance, in a field it of results compressive in a field stresses of compressivethat have a positive stresses e ffthatect onhave the fatiguea positive performance effect on ofthe the fatigue treated performance components. of the treated components

Figure 7. Representative cross-section of a tool steel subjected to Tufftriding (optical micrograph). Figure 7. Representative cross-section of a tool steel subjected to Tufftriding (optical micrograph). Compared to other gas nitriding techniques, Tufftriding is a widely applied one, mainly due to its Compared to other gas nitriding techniques, Tufftriding is a widely applied one, mainly due to low cost, easy application, and high reproducibility. Nowadays, the scientific interest of engineers its low cost, easy application, and high reproducibility. Nowadays, the scientific interest of engineers applying this technique is focused on the waste neutralization and their management optimization applying this technique is focused on the waste neutralization and their management optimization strategies, since from a materials point of view, the reliability of the technique is indisputable. strategies, since from a materials point of view, the reliability of the technique is indisputable. 4.2. Degradation of Nitriding Layers and Acceleration of Base Metal Failure 4.2. Degradation of Nitriding Layers and Acceleration of Base Metal Failure In a previous work [19] concerning the comparative study of the same steel grades with and withoutIn asurface previous nitrocarburizing work [19] concerning surface the treatment, compar itative was study demonstrated of the same that steel Tuff gradestriding with prevents and gallingwithout failure surface (Figure nitrocarburizing8), i.e., motion surface suspension treatment, of a it metal–metal was demonstrated tribopair that due Tufftriding to strong adhesion prevents phenomenagalling failure having (Figure occurred 8), i.e., at motion the contact suspension surface duringof a metal–metal dry sliding, tribopair typically due observed to strong in the adhesion case of slidingphenomena bearings. having Moreover, occurred nitrocarburizing at the contact surface protects during the metallicdry sliding, surface typically from observed extensive in oxidation the case (Figureof sliding9a) bearings. that can leadMoreover, to secondary nitrocarburizing tribo-corrosion protects mechanisms the metallic that surface would from increase extensive the wear oxidation rate of the(Figure metallic 9a) that component. can lead to secondary tribo-corrosion mechanisms that would increase the wear rate of the metallic component.

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Figure 8. Evolution of the friction coefficient of a tool steel grade with and without nitrocarburizing Figure 8. Evolution of the friction coefficient of a tool steel grade with and without nitrocarburizing surface treatment. surfaceFigure 8. treatment. Evolution of the friction coefficient of a tool steel grade with and without nitrocarburizing surface treatment. However, overloadingoverloading can can lead lead to to cracking cracking and and exfoliation exfoliation of of the the protective protective white white layer layer (Figure (Figure9b) 9b) creating cavities on the worn surface. In dry sliding under ambient atmosphere, these cavities act creatingHowever, cavities overloading on the worn can surface. lead to In cracking dry sliding and underexfoliation ambient of the atmosphere, protective thesewhite cavities layer (Figure act as as loci of moisture accumulation that promote oxidation of the underlying diffusion zone, leading at loci9b) creating of moisture cavities accumulation on the worn that surface. promote In dry oxidation sliding under of the ambient underlying atmosphere, diffusion these zone, cavities leading act at the same time to extensive internal oxidation [16]. Even if nitrocarburizing has a positive effect on the theas loci same of moisture time to extensive accumulation internal that oxidation promote [oxidation16]. Even of if the nitrocarburizing underlying diffusion has a positive zone, leading effect on at dry sliding performance, a designer should take the operational limits (maximum applied load and the same dry sliding time to performance, extensive internal a designer oxidation should [16]. take Even the if operationalnitrocarburizing limits has (maximum a positive applied effect on load the sliding speed) per steel grade and hardening level into account. In cases of nitrocarburizing of a steel anddry sliding performance, speed) per steel a designer grade and should hardening take the level operational into account. limits In cases(maximum of nitrocarburizing applied load and of a of low hardness, the “quasi-protective” layer would fail under milder operational parameters, steelsliding of speed) low hardness, per steel the grade “quasi-protective” and hardening level layer into would account. fail under In cases milder of nitrocarburizing operational parameters, of a steel accelerating the degradation of the underlying base metal. Since it is rather costly and time- acceleratingof low hardness, the degradation the “quasi-protective” of the underlying layer base wo metal.uld fail Since under it is rathermilder costly operational and time-consuming parameters, consuming to test experimentally a material before proposing it as the suitable one for a specific toaccelerating test experimentally the degradation a material of beforethe underlying proposing itbase as the metal. suitable Since one it for is arather specific costly application, and time- the application, the development of a well-structured artificial neural network (ANN) can provide wear developmentconsuming to of test a well-structured experimentally artificial a material neural before network proposing (ANN) it can as providethe suitable wear one maps for that a specific predict maps that predict the areas of safe application [20] or areas of wear mechanism alteration [21]. theapplication, areas of safethe development application [20 of] ora well-structured areas of wear mechanism artificial neural alteration network [21]. (ANN) can provide wear maps that predict the areas of safe application [20] or areas of wear mechanism alteration [21].

Figure 9. Scanning electron microscopy (SEM) micrographs and EDS point analysis on characteristic Figure 9. Scanning electron microscopy (SEM) micrographs and EDS point analysis on characteristic areas of the worn surfaces before surface treatment (a) and after liquid nitrocarburizing (b). areas of the worn surfaces before surface treatment (a) and after liquid nitrocarburizing (b). Figure 9. Scanning electron microscopy (SEM) micrographs and EDS point analysis on characteristic areas of the worn surfaces before surface treatment (a) and after liquid nitrocarburizing (b).

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5. Case Study III: Failure of Wear-Resistant Hardfacing Overlayers 5. Case Study III: Failure of Wear-Resistant Hardfacing Overlayers

5.1.5.1. Hardfacing Hardfacing Overlayers Overlayers Elaborat Elaborateded via via Melting Melting and and Resolidification Resolidification InIn manymany applications,applications, the the increased increased resistance resistance to surface to surface loading loading is often is achieved often achieved via the deposition via the depositionof composite of composite layers, consisting layers, consisting of a metallic of a matrix metallic reinforced matrix reinforced by ceramic by particulates. ceramic particulates. The latter The are latterusually are oxide usually or carbide oxide or particles carbide that particles are dispersed that are homogeneously dispersed homogeneously in the metal. Suchin the overlayers metal. Such are μ overlayersdeposited withare deposited a thickness with of ~300 a thicknessµm up toof several ~300 mm,m up and to theyseveral exhibit mm, high and hardness they exhibit and ahigh low hardnesswear coe ffiandcient. a low These wear properties coefficient. render These them properti ideales protection render them for surfacesideal protection of conjugated for surfaces elements of conjugatedthat operate elements in tribo-pairs that underoperate severe in tribo-pairs loading in under demanding severe industrial loading environments,in demanding whilst industrial their environments,exact values are whilst defined their by exact both va thelues type are of defined the reinforcing by both particles the typeas of well the reinforcing as their percentage particles and as wellsize distribution.as their percentage and size distribution. TheThe common methodologymethodology forfor depositing depositing composite composit overlayse overlays reinforced reinforced by carbideby carbide particles particles onto ontosteel steel base base metal metal is based is based on the on fundamentals the fundamentals of arc weldingof arc welding and is typicallyand is typically applied applied in filling in cavities filling cavitieson metallic on metallic surfaces surfaces that have that been have subjected been subject to extremeed to extreme abrasive abrasive wear. wear. In this In this particular particular case, case, the theoverlayers overlayers are referredare referred as “hardfacings”—not as “hardfacings”—not simply assimply “surface as coatings”—a“surface coatings”—a term that emphasizesterm that emphasizestheir inherent their properties inherent properties (high hardness, (high hardness, low wear low coe wearfficient), coefficient), beneficial beneficial for applications for applications where wherewear resistance wear resistance is the primeis the prime requirement. requirement. In conventional In conventional arc welding arc welding techniques, techniques, the material the material to be todeposited be deposited is provided is provided as a consumable as a consumable bulk rod bulk that rod contains that contains iron, carbon, iron, carbon, and strongly and strongly carbide-forming carbide- formingelements. elements. During During hardfacing, hardfacing, the feedstock the feedstock material ma atterial its molten at its molten state is state deposited is deposited on the surfaceon the surfaceof the ferrous of the baseferrous metal, base part metal, of which part of is alsowhich subjected is also tosubjected melting. to The melting. relevant The weight relevant percentage weight percentageof the material of the provided material and provided the base and metal, the base when me bothtal, when exist inboth the exist liquid in statethe liquid of total state miscibility, of total miscibility,is called dilution, is called and di itslution, exact and value its isexact depended value is on depended the particular on the welding particular technique welding used technique and the usedoperational and the parametersoperational applied.parameters During applied. cooling, During this cooling, melt is this re-solidified melt is re-solidified leading to leading the desirable to the desirablehard overlayer, hard overlayer, the microstructure the microstructure of which isof governedwhich is governed by the chemical by the chemical composition composition of the melt of [the22] meltand the[22] cooling and the rate cooling imposed. rate This imposed. processing This resultsprocessing in the results dispersion in the of carbides—primarilydispersion of carbides— of the primarilystrongly carbide-forming of the strongly elementscarbide-forming contained elements in the feedstock contained rod—within in the feedstock a Fe-based rod—within matrix, having a Fe- a basedmodified matrix, chemical having composition a modified compared chemical tocomposition the initial one compared of the base to the metal. initial Typical one of examples the base ofmetal. such Typicala structure examples are presented of such ina struct Figureure 10 are. presented in Figure 10.

FigureFigure 10. 10. SEMSEM micrographs micrographs of of primary primary ( (aa)) chromium chromium and and ( (bb)) niobium niobium carbides carbides formed formed on on an an AISI AISI 41304130 steel steel via via arc arc welding welding technique. technique.

In a variation of hardfacing via the arc welding technique known as flux core arc welding (FCAW), In a variation of hardfacing via the arc welding technique known as flux core arc welding the carbide powder comprises the core of a tubular metallic-shell rod, which is used as feeding (FCAW), the carbide powder comprises the core of a tubular metallic-shell rod, which is used as material, so that the carbide particles are introduced in their solid state in the metallic melt and not feeding material, so that the carbide particles are introduced in their solid state in the metallic melt precipitated thereof as primary carbides during solidification, as previously described. In both cases, and not precipitated thereof as primary carbides during solidification, as previously described. In the temperature at the vicinity of the metallic pool should exceed 1550 C, typical of the melting both cases, the temperature at the vicinity of the metallic pool should exceed◦ 1550 °C, typical of the temperature of common steels. Hardfacings deposited via arc welding find wide industrial applications, melting temperature of common steels. Hardfacings deposited via arc welding find wide industrial because of the low cost of the needed facilities and the occupation of non-highly-skilled personnel; applications, because of the low cost of the needed facilities and the occupation of non-highly-skilled personnel; however, the elaborated overlayers often exhibit microstructure flaws due to the rather

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Metals 2019, 9, x FOR PEER REVIEW 11 of 17 however, the elaborated overlayers often exhibit microstructure flaws due to the rather empirical and randomempirical way and of random application. way of These application. flaws, commonlyThese flaws, pores commonly and cracks pores (Figure and cracks 11), are (Figure typical 11), of are all processestypical of basedall processes on the based gradual on solidification the gradual solidification of melts in contact of melts with in non-moltencontact with solids, non-molten deteriorating solids, thedeteriorating elements’ performancethe elements’ and performance reducing their and service reducing life. their Especially service in life. the caseEspecially of manual in the deposition, case of themanual non-precise deposition, control the ofnon-precise the treatment control parameters of the treatment increases theparameters possibility increases of such the flaws. possibility Moreover, of thesuch occasionally flaws. Moreover, prolonged the occasionally exposure of prolonged the workpieces exposure to highof the thermal workpieces loads to resultshigh thermal in extended loads heat-aresultsff ectedin extended zones and heat-affected could cause zones distortion and of could the entire cause structure distortion and /ofor detrimentalthe entire structure transformations and/or ofdetrimental the base metal transformations microstructure. of the base metal microstructure. Besides conventionalconventional arc arc welding, welding, the the plasma plasma transferred transferred arc (PTA)arc (PTA) [23,24 [2] and3,24] laser and beams laser [beams25,26] are[25,26] applied are asapplied non-conventional as non-conventional techniques techniqu for elaboratinges for elaborating hard overlayers hard onoverlayers steel components. on steel Nevertheless,components. Nevertheless, the small diameter the small of the diameter high-energy of the beam high-energy limits the beam melting limits area the to melting a diameter area lower to a thandiameter 3 mm, lower a fact than that 3 inmm, turn a fact imposes that multiple-stepin turn imposes processing multiple-step of larger processing surfaces, of intensifyinglarger surfaces, the negativeintensifying effects the ofnegative high thermal effects loading.of high thermal Recently, loading. concentrated Recently, solar concentrated energy (CSE) solar was energy proven (CSE) [27] anwas attractive proven [27] alternative an attractive for elaborating alternative suchfor elaborating hardfacing such layers hardfacing free of pores layers and free cracks of pores on large and dimensionalcracks on large surfaces. dimensional surfaces. In thethe following following paragraph, paragraph, the casethe ofcase hardfacing of hardfacing TiC-based TiC-based overlayers overlayers deposited deposited via automated via arcautomated melting arc technique melting istechnique presented is aspresented a good practiceas a good example practice ofexample the development of the development of wear-resistant of wear- compositeresistant composite hardfacings. hardfacings.

Figure 11.11.Microstructure Microstructure ﬂaws flaws in hardfacingin hardfacing overlayers overla depositedyers deposited manually manually via arc welding via arc technique: welding (technique:a) stereoscopic (a) stereoscopic image of top image view, of revealing top view, the revealing development the development of surface poresof surface and cracks;pores and (b) cracks; optical image(b) optical of a cross-section,image of a cross-section, revealing the revealing existence the of a existence sub-surface of a isolated sub-surface macro-pore; isolated and macro-pore; (c) SEM image and of(c) aSEM cross-section image of ata cross-section the vicinity of at the the base vicinity metal, of indicating the base metal, the development indicating the within development the overlayer within of a crackthe overlayer parallel of to a the crack interface. parallel to the interface. 5.2. Performance of TiC-Based Composite Overlayers Produced via FCAW Technique 5.2. Performance of TiC-Based Composite Overlayers Produced via FCAW Technique The microstructure characteristics and the XRD analysis of both the core of the commercial wire The microstructure characteristics and the XRD analysis of both the core of the commercial wire fed for the elaboration of the conventional FCAW hardfacing, as well as of the deposits obtained are fed for the elaboration of the conventional FCAW hardfacing, as well as of the deposits obtained are compared in Figure 12. The particles’ agglomerates (Figure 12a) consist of TiC, which is the compound of interest in the present study, and CaF2 and Si, which are the fluxing agents, whilst traces

Metals 2019, 9, x FOR PEER REVIEW 12 of 17

Metalsof2019 ferrite, 9, 1057 were also detected originating from the wire’s shell (Figure 12b). The deposit obtained12 of 17 Metals (Figure2019, 9, x 12c) FOR is PEER characterized REVIEW by the uniform dispersion of fine TiC particulates in the metallic matrix,12 of 17 with rare presence of small size pores, and consists of only ferrite and TiC (Figure 12d). compared in Figure 12. The particles’ agglomerates (Figure 12a) consist of TiC, which is the compound of ferrite Thewere tribological also detected in-service originating performanc frome thof ethese wire’s FCAW shell deposits(Figure against12b). The an depositAl2O3 ball obtained is (Figureof interestpresented 12c) in is the characterizedin presentFigure 13. study, The by frictionthe and uniform CaF coefficient2 and dispersi Si, reached whichon of areimmediately fine the TiC ﬂuxing particulates the agents, steady-state in whilst the stage metallic traces (Figure of matrix, ferrite withwere rare13a), also presenceacquiring detected aof originating constant small size value frompores, of 0.78 the and ± wire’s 0.02, consists which shell of was (Figure only practically ferrite 12b). and Thethe sameTiC deposit (Figure for both obtained 12d).applied (Figure loads. 12c) is characterized by the uniform dispersion of ﬁne TiC particulates in the metallic matrix, with rare TheThe weartribological coefficient in-service reached the performanc steady-statee conditions of these afterFCAW 20,000 deposits sliding revolutions against an (Figure Al2O 13b),3 ball is × −7 × −7 3 × −1 × −1 presentedpresencetending of in small towardsFigure size 13.(5.45 pores, The ± 0.31) andfriction consists 10 coefficient and of (6.84 only reached ferrite± 0.49) andimmediately 10 TiC mm (Figure N the 12 steady-stated).laps for 5 and stage 10 (FigureN, respectively. 13a), acquiring a constant value of 0.78 ± 0.02, which was practically the same for both applied loads. The wear coefficient reached the steady-state conditions after 20,000 sliding revolutions (Figure 13b), tending towards (5.45 ± 0.31) × 10−7 and (6.84 ± 0.49) × 10−7 mm3 × N−1 × laps−1 for 5 and 10 N, respectively.

FigureFigure 12. (12.a) SEM(a) SEM micrograph micrograph and and ( b(b)) X-ray X-ray didiffractionﬀraction (XRD) (XRD) spectr spectrum,um, respectively, respectively, of the of core the core powderpowder of the of wirethe wire used used for for ﬂux flux core core arc arc welding welding (FCAW) deposits. deposits. (c) ( cSEM) SEM micrograph micrograph and and(d) XRD (d) XRD spectrum, respectively, of the obtained hardfacing layers. spectrum, respectively, of the obtained hardfacing layers.

The tribological in-service performance of these FCAW deposits against an Al2O3 ball is presented in Figure 13. The friction coeﬃcient reached immediately the steady-state stage (Figure 13a), acquiring a constantFigure value12. (a) ofSEM 0.78 micrograph0.02, which and (b was) X-ray practically diffraction the (XRD) same spectr forum, both respectively, applied loads. of the Thecore wear ± coeﬃpowdercient reached of the wire the used steady-state for flux core conditions arc welding after (FCAW) 20,000 deposits. sliding (c revolutions) SEM micrograph (Figure and 13 (db),) XRD tending towardsspectrum, (5.45 respectively,0.31) 10 of7 andthe obtained (6.84 0.49)hardfacing10 layers.7 mm3 N 1 laps 1 for 5 and 10 N, respectively. ± × − ± × − × − × −

Figure 13. Representative ball-on-disk testing results of the FCWA hardfacing layers (counterbody:

Al2O3 ball): evolution of (a) friction coefficient and (b) wear coefficient vs. sliding revolutions.

SEM observations of the worn surface revealed a wear track of ~840 µm width (Figure 14a), within which polishing lines and oxidation areas of the metallic matrix (Figure 14b) can be clearly

Figure 13. RepresentativeRepresentative ball-on-disk ball-on-disk testing testing results results of of the the FCWA FCWA hardfacing layers (counterbody:

Al2OO33 ball):ball): evolution evolution of of (a (a) )friction friction coefficient coeﬃcient and and ( (bb)) wear wear coefficient coeﬃcient vs. vs. sliding sliding revolutions. revolutions.

SEM observations of the worn surface revealed a wear track of ~840 µm width (Figure 14a), within which polishing lines and oxidation areas of the metallic matrix (Figure 14b) can be clearly

Metals 2019, 9, 1057 13 of 17 Metals 2019, 9, x FOR PEER REVIEW 13 of 17 observed,SEM observationstogether with of the the emerged worn surface fine carbides revealed dispersion a wear track (Figure of ~840 14c).µm Compared width (Figure to the 14a), deposits within whichobtained polishing via the linesnovel and CSE oxidation techniqu arease previously of the metallic mentioned matrix [27], (Figure the friction14b) can coefficient be clearly values, observed, as welltogether as the with wear the emergedcoefficient fine and carbides the associated dispersion mi (Figurecro-mechanisms, 14c). Compared exhibit to significant the deposits differences obtained viaoriginating the novel from CSE the technique size of the previously reinforcing mentioned TiC particles [27], and the frictiontheir dispersion coefficient in values,the matrix: as well as the wear(a) The coe fficonventionalcient and the FCAW associated deposits, micro-mechanisms, enhanced by exhibitTiC particles significant of difinerfferences size, originatingexhibit a rather from the sizemetallic of the nature, reinforcing with TiChigh particles friction coefficient and their dispersion values, tending in the towards matrix: those of a typical tool steel [17] and the relevant wear micro-mechanisms, involving polishing and oxidation of the metallic (a) The conventional FCAW deposits, enhanced by TiC particles of finer size, exhibit a rather metallic matrix. nature, with high friction coefficient values, tending towards those of a typical tool steel [17] and (b) The solar surface layers, enhanced by TiC particles of larger size, exhibit the typical nature of a the relevant wear micro-mechanisms, involving polishing and oxidation of the metallic matrix. ceramic-particle-reinforced metal (CPRM) composite, with the ceramic phase playing the (b) The solar surface layers, enhanced by TiC particles of larger size, exhibit the typical nature predominant role in the low friction coefficient values. The wear micro-mechanism is a clear of a ceramic-particle-reinforced metal (CPRM) composite, with the ceramic phase playing the superposition of the plastic deformation of the metallic matrix and the micro-fragmentation of predominant role in the low friction coefficient values. The wear micro-mechanism is a clear the carbides, leading finally to exfoliation (Figure 15). superposition of the plastic deformation of the metallic matrix and the micro-fragmentation of The comparative results of the tribological performance of the FCAW deposits and those the carbides, leading finally to exfoliation (Figure 15). concerning non-conventional CSE-obtained hardfacing layers are summarized in Table 3.

Figure 14. Top view SEM micrographs of the worn surface of the FCAW hardfacing layers, after Figureball-on-disk 14. Top testing view for SEM 200,000 micrographs sliding revolutions,of the worn applyingsurface of a the normal FCAW load hardfacing of 10 N: ( alayers,) entire after width ball- of on-diskthe wear testing track, for (b) 200,000 and (c) successivesliding revolution magniﬁcationss, applying within a normal the wear load track, of 10 demonstratingN: (a) entire width extensive of the wearoxidation track, and (b abrasion) and (c) scarssuccessive along themagnifications sliding direction. within the wear track, demonstrating extensive oxidation and abrasion scars along the sliding direction.

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Figure 15. TopTop view view SEM SEM micrographs micrographs of ofthe the worn worn surface surface of the of the“solar” “solar” hardfacing hardfacing layers, layers, after afterball- ball-on-diskon-disk testing testing for 200,000 for 200,000 sliding sliding revoluti revolutions,ons, applying applying a normal a normal load load of 10 of N. 10 N. The comparative results of the tribological performance of the FCAW deposits and those concerning Table 3. Ball-on-disk tribological performance of conventional FCAW deposits, compared to CSE non-conventional CSE-obtained hardfacing layers are summarized in Table3. overlayers [27].

Table 3. Ball-on-disk tribologicalFriction Coefficient, performance ofµ conventional Wear FCAW Coefficient, deposits, k compared(mm−3.N−1 to.laps CSE−1) Applied Load overlayers [27]. FCAW Deposits CSE Overlayers FCAW Deposits CSE Overlayers × -7 × -7 5 N 0.78 ± 0.02 0.43 ± 0.02 (5.45 ± 0.31) 10 (3.26 ±3 0.41)1 101 Friction Coefficient, µ Wear Coefficient, k (mm− .N− .laps− ) Applied10 LoadN 0.78 ± 0.02 0.46 ± 0.02 (6.84 ± 0.49) × 10-7 (4.18 ± 0.42) × 10-7 FCAW Deposits CSE Overlayers FCAW Deposits CSE Overlayers 5 N 0.78 0.02 0.43 0.02 (5.45 0.31) 10-7 (3.26 0.41) 10-7 6. Case Study IV: Failure of± Wear-Resistant Thermal-Sprayed± Ceramic± × Coatings ± × 10 N 0.78 0.02 0.46 0.02 (6.84 0.49) 10-7 (4.18 0.42) 10-7 The last surface modification± family of techniques± widely applied± ×at an industrial± scale× for the wear protection of metallic components is thermal spraying [7,28]. Ceramic particles or bulk metallic 6. Case Study IV: Failure of Wear-Resistant Thermal-Sprayed Ceramic Coatings material in the form of rods are introduced in an area of high temperature, where they are heated evenThe to their last melting surface modificationtemperatures familyand are of accelera techniquested toward widely the applied surface at to an be industrial coated. These scale fortotally the wearor partially protection molten of metallic particles components arriving at isthe thermal target sprayingtransmit [their7,28]. thermal Ceramic and particles kinetic or energy bulk metallic almost materialinstantaneously in the form and ofare rods solidified are introduced rapidly inproviding an areaof the high desirable temperature, coating. where Nowadays, they are several heated eventhermal to theirspraying melting techniques temperatures are available, and are acceleratedeach one characterized toward the surface by the toratio be coated.of thermal These to kinetic totally orenergy partially attributed molten to particles the material arriving to at be the deposi targetted, transmit the means theirthermal implemented and kinetic to create energy the almost high instantaneouslytemperature area, and and are the solidified atmosphere rapidly surrounding providing the accelerating desirable coating. particles. Nowadays, Ceramic several materials, thermal due sprayingto their high techniques tribological are available,performance each and one their characterized characteristic by themechanical ratio of thermalresistance to to kinetic compressive energy attributedloading [29], to also the materialattract interest to be deposited, for their applicat the meansion as implemented protective coatings. to create Typically, the high temperatureatmospheric area,plasma and spraying the atmosphere (APS) is surrounding applied to deposit the accelerating oxide layers particles. [30,31], Ceramic whilst materials, the high-velocity due to their oxyfuel high tribological(HVOF) technique performance is used and to form their carbide characteristic [32,33] mechanicalwear-resistant resistance layers. to compressive loading [29], also attractDue to interest the peculiarities for their application of the deposition as protective techni coatings.que, ceramic Typically, plasma-sprayed atmospheric coatings, plasma sprayinghaving a (APS)thickness is applied of 200–400 to deposit µm, exhibit oxide layersstratified [30 ,struct31], whilsture with the high-velocityinterfacial roughness, oxyfuel (HVOF)which promotes technique the is usedformation to form of carbideporosity [32 (Figure,33] wear-resistant 16a) and mechanical layers. anchorage to the metallic surface. In order to enhanceDue tothe the latter peculiarities prior to ofthermal the deposition spraying, technique, it is necessary ceramic plasma-sprayedto prepare the metallic coatings, solid having via a thicknesssandblasting. of 200–400 This pre-processingµm, exhibit stratified could result structure in sand with residues interfacial (Figure roughness, 16b) on the which surface promotes leading the to formationpoor adhesion of porosity to the metallic (Figure substrate. 16a) and In mechanical particular in anchorage the case of to carbide the metallic coatings, surface. their deposition In order tounder enhance non-recommended the latter prior toparameters thermal spraying, of spra itying is necessary technique to preparecauses thetheir metallic in-flight solid viadecomposition/decarburization sandblasting. This pre-processing [34–39]. could result in sand residues (Figure 16b) on the surface leading to poor adhesion to the metallic substrate. In particular in the case of carbide coatings, their deposition under non-recommended parameters of spraying technique causes their in-flight decomposition/decarburization [34–39].

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FigureFigure 16.16. Microstructure flawsflaws developeddeveloped duringduring plasma spray deposition of ((aa)) aa titaniatitania coatingcoating Figure 16. Microstructure flaws developed during plasma spray deposition of (a) a titania coating (optical(optical micrograph),micrograph), wherewhere aa longitudinallongitudinal crackcrack propagatedpropagated parallelparallel toto the coatingcoating/substrate/substrate interfaceinterface (optical micrograph), where a longitudinal crack propagated parallel to the coating/substrate interface andand ((bb)) aa WC-CoWC-Co coatingcoating (SEM(SEM micrograph)micrograph) ofof poorpoor adhesionadhesion toto thethe metallicmetallic substrate,substrate, duedue toto thethe and (b) a WC-Co coating (SEM micrograph) of poor adhesion to the metallic substrate, due to the presencepresence ofof residuesresidues ofof thethe pre-depositionpre-deposition sandblastingsandblasting preparationpreparation of of the the metallic metallic surface. surface. presence of residues of the pre-deposition sandblasting preparation of the metallic surface. DespiteDespite allall thethe aboveabove microstructuremicrostructure flawsflaws possiblypossibly presentpresent inin aa thermal-sprayedthermal-sprayed coating,coating, theirtheir Despite all the above microstructure flaws possibly present in a thermal-sprayed coating, their tribologicaltribological performanceperformance could could be be acceptable acceptable when when operating operating under under mild mild conditions conditions [40]. Under[40]. Under more tribological performance could be acceptable when operating under mild conditions [40]. Under severemore severe service service conditions, conditions, these structure these structure flaws accelerate flaws accelerate their degradation their degradation (Figure 17(Figurea) via 17a) vertical via more severe service conditions, these structure flaws accelerate their degradation (Figure 17a) via crackingvertical cracking (Figure 17(Figureb), providing 17b), providing the route the to theroute moisture to the moisture or corrosive or corrosive gases, to penetrategases, to penetrate until the vertical cracking (Figure 17b), providing the route to the moisture or corrosive gases, to penetrate metallicuntil the surface metallic that surface would that be would hence oxidized.be hence oxidized. until the metallic surface that would be hence oxidized.

Figure 17. Degradation of a composite plasma-sprayed coating: (a) after tribological service a Figure 17. Degradation of a composite plasma-sprayed coating: (a) after tribological service a through-depthFigure 17. Degradation crack was of developed a composite below plasma-sprayed the contact area coating: and (b) propagated(a) after tribological through the service carbide a through-depth crack was developed below the contact area and (b) propagated through the carbide reinforcingthrough-depth particles crack resulting was develope in theird below multi-rupture. the contact area and (b) propagated through the carbide reinforcing particles resulting in their multi-rupture. 7. Epiloguereinforcing particles resulting in their multi-rupture. 7. Epilogue 7. EpilogueIn general, the operation of metallic parts and components as elements of a tribosystem requires theirIn further general, surface the operation enhancement. of metallic Several parts techniques and components are available as elements for facilitating of a tribosystem relevant requires motion In general, the operation of metallic parts and components as elements of a tribosystem requires oftheir two further solids surface in contact, enhancement. which is reﬂected Several techniques in low friction are available coeﬃcient for values, facilitating and /relevantor for prolonging motion of their further surface enhancement. Several techniques are available for facilitating relevant motion of theirtwo solids lifetime in contact, by using which surface is reflected layers of in lower low friction wear rates.coefficient In this values, review, and/or four for cases prolonging of common their two solids in contact, which is reflected in low friction coefficient values, and/or for prolonging their wearlifetime protective by using techniques surface layers were presentedof lower wear in order rates. to emphasizeIn this review, that thefour wrong cases applicationof commonof wear the lifetime by using surface layers of lower wear rates. In this review, four cases of common wear “quasi-protective”protective techniques layer were on thepresented contact in surfaces order to does emphasize not prevent that butthe ratherwrong accelerate application the of degradation the “quasi- protective techniques were presented in order to emphasize that the wrong application of the “quasi- ofprotective” the metallic layer component. on the contact surfaces does not prevent but rather accelerate the degradation of the protective” layer on the contact surfaces does not prevent but rather accelerate the degradation of the metallicWhen component. using an oil lubricant, a strict schedule of periodic inspection of the sum-assembly should metallic component. be followedWhen using in parallel an oil lubricant, to the periodic a strict oil schedule drain (oil of periodic discharge), inspection while its of selection the sum-assembly should be should made When using an oil lubricant, a strict schedule of periodic inspection of the sum-assembly should takingbe followed into account in parallel the compatibilityto the periodic of oil the drain lubricant’ (oil discharge), s additives towhile the metalits selection of the solid.should be made be followed in parallel to the periodic oil drain (oil discharge), while its selection should be made taking into account the compatibility of the lubricant’ s additives to the metal of the solid. taking into account the compatibility of the lubricant’ s additives to the metal of the solid.

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In the case of implemented diffusion coatings, as the nitrocarburized ones, the bulk hardness of the protected component should be carefully selected, since it affects the good performance of the wear-resistant white layer. In the case of hardfacing overlayers elaborated via arc welding techniques, the formation of structure flaws associated to solidification of liquid metals (pores and cracks) could be prevented by the proper selection of processing parameters and, if possible, of an automated technique. Finally, in the case of thermal-sprayed ceramic coatings the selection of the proper technique is of crucial importance to avoid structural flaws that eventually lead to the rapid degradation of the underlying metallic part.

Funding: This research received no external funding. Acknowledgments: The author would like to thank all the under-graduate and post-graduate students as well as the Ph.D. candidates having carried/carrying out their theses in the Tribology Laboratory of our Department; their work allowed collecting the experimental knowledge briefly presented in this review article. Conflicts of Interest: The author declares no conflict of interest.

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