The Relationship Between Cyclic Multi-Scale Self-Organized Processes

coatings

Article The Relationship between Cyclic Multi-Scale Self-Organized Processes and Wear-Induced Surface Phenomena under Severe Tribological Conditions Associated with Buildup Edge Formation

German Fox-Rabinovich 1,*, Iosif S. Gershman 2 , Edinei Locks 1, Jose M. Paiva 1 , Jose L. Endrino 3, Goulnara Dosbaeva 1 and Stephen Veldhuis 1

1 Department of Mechanical Engineering, McMaster Manufacturing Research Institute (MMRI), McMaster University, Hamilton, ON L8S4L7, Canada; [email protected] (E.L.); [email protected] (J.M.P.); [email protected] (G.D.); [email protected] (S.V.) 2 Joint Stock Company Railway Research Institute, Moscow State Technological University, 127994 Moscow, Russia; [email protected] 3 Basque Center for Materials, Applications and Nanostructures, Bld. Martina Casiano, UPV/EHU Science Park, 48940 Leioa, Spain; [email protected] * Correspondence: [email protected]

Abstract: This paper presents experimental investigations of various interrelated multi-scale cyclic and temporal processes that occur on the frictional surface under severe tribological conditions Citation: Fox-Rabinovich, G.; during cutting with buildup edge formation. The results of the finite element modeling of the Gershman, I.S.; Locks, E.; Paiva, J.M.; stress/temperature profiles on the friction surface are laid out. This study was performed on a Endrino, J.L.; Dosbaeva, G.; Veldhuis, multilayer coating with the top alumina ceramic layer deposited by CVD (chemical vapor deposition) S. The Relationship between Cyclic on a WC/Co carbide substrate. A detailed analysis of the wear process was conducted by 3D Multi-Scale Self-Organized Processes wear evaluation, scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) and Wear-Induced Surface and electron backscattered diffraction (EBSD), as well as X-ray photoelectron spectroscopy (XPS) Phenomena under Severe Tribological methods. The following cyclic phenomena were observed on the surface of the tribo-system during Conditions Associated with Buildup the experiments: a repetitive formation and breakage of buildups (a self-organized critical process) Edge Formation. Coatings 2021, 11, 1002. https://doi.org/10.3390/ and a periodical increase and decrease in the amount of thermal barrier tribo-films with a sapphire coatings11081002 structure (which is a self-organization process). These two processes are interrelated with the accompanying progression of cratering, eventually resulting in the catastrophic failure of the entire Academic Editor: Giorgos Skordaris tribo-system.

Received: 20 July 2021 Keywords: self-organization; self-organized criticality; buildup edge; multi-scale equilibrium and Accepted: 18 August 2021 non-equilibrium processes; alumina CVD coatings; cutting tools Published: 22 August 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- There is significant discussion within the scientific community concerning the distinc- iations. tion and interrelation between self-organization (SO) [1,2] and self-organized critical (SOC) processes [3–8]. This research exhibits real case studies of the underlying relationships between these processes, especially under strong non-equilibrium conditions, which are typical for friction. Copyright: © 2021 by the authors. It was discovered by I. Prigogine that, under certain conditions, non-equilibrium Licensee MDPI, Basel, Switzerland. thermodynamic systems absorb matter and energy from the surrounding environment This article is an open access article in order to make a qualitative jump in complexity by forming dissipative structures [1]. distributed under the terms and This was a focus of our previous research [2] and the current study is a continuation of the conditions of the Creative Commons development of this approach. It is strongly contended by the authors of this study that any Attribution (CC BY) license (https:// advancement in this research direction requires reliable experimental investigations (case creativecommons.org/licenses/by/ 4.0/). studies). The relevant field of applied science and engineering is known as tribology, which

Coatings 2021, 11, 1002. https://doi.org/10.3390/coatings11081002 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 1002 2 of 13

is concerned with the interrelation between physical/chemical phenomena that emerge under the predominantly non-equilibrium conditions in tribological applications [9]. The cutting process constitutes a unique environment in which such phenomena can be studied under severely tribological conditions that are very far from equilibrium, which are rarely observable in any other field. Cutting is a complex tribological process that usually undergoes very high temperatures (within the range of 700–1000 ◦C, and even above [9]), heavy loads (within the range of 1–2 GPa and more) and strongly non-equilibrium states throughout the entire tribo-system [10–12]. Temporal effects strongly associated with self-organization are chiefly considered in chemical, biological and material fields of science [13–20]. A limited amount of scientific literature exists on the temporal wear behavior of materials [21,22]; in particular, the atomic/nano-scale surface tribo-ceramic films that are generated under the outlined conditions [9]. It should be noted that a self-organized critical (SOC) process could also develop under these conditions [23]. Limited information is available in published studies regarding the relationship between temporal self-organization (SO) processes and self-organized criticality (SOC) [24–27]. It is known that strong adhesive bonds are periodically formed and destroyed during cutting, which results in attrition wear [28]. Moreover, the intensive adhesive interaction between the tool and the workpiece during the machining of sticky materials (such as stainless steels) eventually leads to the catastrophic tribological mode of seizure and the generation of large buildups [29]. These buildups are dynamic complex structures [29] driven by a stick–slip phenomenon during friction, which is associated with the self- organized critical (SOC) process [8]. The buildup is structurally similar to a composite “third body” [29], which consists of heavily deformed particles of the machined material as well as various compounds created during cutting by the complex interactions between the tool, the workpiece and the environment [29]. On the one hand, the “ceramic-like” buildup layer provides significant protection to the tool surface [29]. On the other hand, the structural stability of a buildup layer is very poor due to its avalanche-like behavior [8]. An avalanche-like release of energy (dissipation) takes place during the moment at which the buildups become separated from the surface of a cutting tool, which initiates self- organization (via the formation of a tribo-film) in this area. It is known that the system must be in a highly non-equilibrium state in order for self- organization with the formation of dissipative structures to be initiated [30]. Moreover, the system must intensely deviate not only from the equilibrium but also from the stationary state [31]. Thus, the tribo-system needs to be quite heavily loaded, and at the same time, undergo significant dissipation for self-organization, with dissipative structure formation to commence. Since the processes that develop during self-organization (SO) with dissipative structure formation have an intensely negative entropy production, the overall growth of entropy within the system is actually lower. This means that the processes in a tribo- system develop in an avalanche-like manner, thereby absorbing a significant portion of the frictional energy, which would otherwise be expended on the wear process. Conversely, SOC processes begin to aggregate in an avalanche-like manner in order to cause significant energy dissipation and entropy production. As these “avalanches” develop, the SOC significantly increases the instability of the stationary state. This is precisely what is needed to initiate self-organization with the formation of dissipative structures (tribo-films), under which, in contrast to the previous process, the dissipation of energy, entropy production and the ensuing wear rate decrease like an avalanche. Therefore, the SOC increases energy dissipation in an avalanche-like manner, thereby facilitating the transition towards self-organization. Coatings 2021, 11, 1002 3 of 13

This paper investigates the dynamic interactions between various self-organizing processes that coincide with temporal wear processes during cutting with buildup edge formation [5].

2. Finite Element Process Modeling Finite element modeling (FEM) is essential for evaluating the conditions (such as temperature/stress profiles) present at the cutting zone. This step is necessary for a better understanding and control of the wear mechanisms under a machining process with a prevalent buildup edge formation [29]). During metal cutting, the severe plastic deformation of the workpiece material is responsible for the growth of temperature/stress, which results in severe tribological conditions [2]. One of the major challenges of modeling the temperature/stress profiles within the cutting zone during stainless steel machining under wet conditions is presented by the complexity of the machining processes associated with buildup edge formation. It should be noted that this FEM-based approach is quite novel. In this case, simulations of metal machining during turning require a fundamental understanding of the deformation conditions in the relevant deformation zones, as well as the strain rates and frictional conditions at the tool/workpiece interface. Cutting temperature/stress profiles are critical for understanding and controlling a machining process with buildup edge formation [29]). All modeling in this study was conducted by a Third Wave Systems AdvantEdge™ simulation software (version 7.8), which employs advanced finite element models suitable for machining operations. A method of continuous chip formation was developed for a cutting length of 3 mm. The mechanical properties of the workpiece material (an austenitic AISI 304 stainless steel) were obtained using the FEM program database. The material and coolant properties used for the model input, as well as for the friction coefficient, are given in Table1.

Table 1. Materials properties and contact conditions.

Material Properties Property Workpiece Tool Alumina Thermal conductivity (W/m ◦C) 17 58 12 Heat Capacity (J/kg ◦C) 500 205 451 Density (kg/m3) 8000 15,700 3980 Elastic Modulus (GPa) 195 640 413 Poisson Ratio 0.27 0.21 0.33 Coolant Properties Density (kg/m3) 2800 - Heat Transfer Coefﬁcient (W/m2 K) 10,000 - Coolant type Flood - Coolant Initial Temperature (◦C) 20 - Friction Friction Coefﬁcient 0.5 -

The AdvantEdge software combines the Lagrangian method with adaptive remeshing capabilities in order to address the non-linearities generated by high strain rates and plastic deformation inherent to the machining processes. The cutting edge was deﬁned as ideally rigid, in accordance with CNMG 120408 MS Grade KCM25 (Kennametal) inserts used in the experiments. The cutting parameters and tool code geometry used in the simulation are listed in Table2 (see experimental). Coatings 2021, 11, 1002 4 of 13

Table 2. Cutting parameters used in the experiments.

Cutting Data Machining Workpiece Hardness Speed Feed Depth of Cutting Tool Operation Material HRC m/min mm/rev Cut mm Kennametal Semi-ﬁnish Stainless CNMG 120408 turning, Steel (UNS 20–22 320 0.2 1 Grade KCM25 Wet machining S 30400) turning inserts

The 2D models of temperature/stresses are presented in Figure1. Figure1 presents FEM data (2D stress/temperature proﬁles) at the cutting zone with buildup formation.

Alicona edge cross-sections

Maximum temperatures:

746.3 °C Cutting tool edge with

the buildup

Temperature profile Stress profile

(a) (b)

Figure 1.FigureFEM 1. data FEM (stress/temperature data (stress/temperature proﬁle). profile).

As canAs be can seen be inseen Figure in Figure1, high 1, temperatureshigh temperatures (around (around 740 740◦C) ° andC) and loads loads (around (around 1.5 1.5–2 GPa)– 2 GPa are) generatedare generated on the on cuttingthe cutting edge edge under under analyzed analyzed cutting cutting conditions. conditions. TheseThese severe severe conditions conditions strongly strongly inﬂuence influence the surface the surface phenomena, phenomena, which which unfold unfold duringduring the wear the ofwear the of the the tribo-system. the tribo-system.

3. Materials3. Materials and Methods and Methods The cuttingThe cutting parameters parameters used in used this in work this arework listed are inlisted Table in2 Table. 2. The cemented carbide cutting insert used for the cutting tests was CNMG120408-MP (accordingTable to 2. ISOCutting 1832). parameters The workpiece used in the material experiments. analyzed in this work is an austenitic AISI 304 stainless steel (Table1). Semi-ﬁnish turning cutting tests were performed. A Cutting Data CNC Okuma Crown L1060 lathe was used for the cutting tests. All turning tests were Machining Hardness Speed Feed Depth of Cut conducted at a cuttingCutting speed Tool of 320 Workpiece m/min, feedMaterial rate of 0.2 mm/rev and depth of cut of Operation HRC m/min mm/rev mm 1 mm under wet conditions, using a semi-synthetic coolant with 7% concentration. The Semi-finish Kennametal cutting conditions were based on the industrial practice. In order to remove buildups, turning, CNMG 120408 Stainless Steel (UNS 20–22 320 0.2 1 Aqua RegiaWet etchant wasGrade used. KCM25 S 30400) Cross-sectionmachining studiesturning ofinserts the cutting tools and wear pattern investigations were performed using a Vega 3-TESCAN scanning electron microscope (SEM) (Vega 3-TESCAN, TESCNA, BrnoThe cemented Kohoutovice, carbide Czech cutting Republic). insert used The wearfor the mechanism cutting tests and was morphology CNMG120408 of -MP (according to ISO 1832). The workpiece material analyzed in this work is an austenitic AISI 304 stainless steel (Table 1). Semi-finish turning cutting tests were performed. A CNC Okuma Crown L1060 lathe was used for the cutting tests. All turning tests were conducted at a cutting speed of 320 m/min, feed rate of 0.2 mm/rev and depth of cut of 1 mm under wet conditions, using a semi-synthetic coolant with 7% concentration. The cutting conditions were based on the industrial practice. In order to remove buildups, Aqua Regia etchant was used. Cross-section studies of the cutting tools and wear pattern investigations were performed using a Vega 3-TESCAN scanning electron microscope (SEM) (Vega 3-TESCAN, TESCNA, Brno Kohoutovice, Czech Republic). The wear mechanism and morphology of the worn cutting inserts were also evaluated through the electron backscatter diffraction (EBSD, JSM-7600F Schottky, Jeol Ltd., Tokyo, Japan); Field Emission Scanning Electron Microscope (FE-SEM, JSM-F100, Jeol Ltd., Tokyo, Japan) method. The micro-mechanical

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the worn cutting inserts were also evaluated through the electron backscatter diffraction (EBSD, JSM-7600F Schottky, Jeol Ltd., Tokyo, Japan); Field Emission Scanning Electron Microscopecharacteristics (FE-SEM, (hardness JSM-F100, and Jeol reduced Ltd., Tokyo,elastic modulus) Japan) method. of the coatings The micro-mechanical and corresponding characteristicscarbide substrates (hardness were and reducedmeasured elastic by a modulus)Fischerscope of the HM2000 coatings Hardness and corresponding Tester (Fischer, carbideSindelfingen, substrates Germany) were measured at a load by aof Fischerscope 20 mN. A Vickers HM2000 indenter Hardness geometry Tester (Fischer,was used for Sindelfingen,this analysis. Germany) In order at ato load evaluate of 20 mN.the wear A Vickers performance indenter during geometry the wascutting used tests, for thisprogres- analysis.sive In3D order studies to of evaluate wear volume the wear were performance carried out during with an the Infinite cutting Focus tests, G5 progressive focus variation 3D studiesmicroscope of wear (Optimax, volume Alicona, were carried Austria). out with an Infinite Focus G5 focus variation microscopeThe (Optimax, surface and Alicona, chemical Austria). composition of the formed tribo-films on the surface of Thecutting surface tool were and chemicalanalyzed compositionby X-ray photoelectron of the formed spectroscopy tribo-films (XPS on, theAXIS surface Nova, ofKratos cuttingAnalytical tool were Inc., analyzed Manchester, by X-ray UK), photoelectron AXIS Supra spectroscopyspectrometer (XPS,equipped AXIS with Nova, a hemisp Kratos heri- Analyticalcal energy Inc., Manchester,analyzer and UK), an Al AXIS anode Supra source spectrometer for X-ray generation. equipped with A monochromatic a hemispherical Al K- energyα X analyzer-ray (1486.6 and eV) an Alsource anode was source operated for X-rayat 15 kV. generation. The system A monochromaticbase pressure was Al no K- αhigher X-raythan (1486.6 1.0 eV)× 10 source−9 Torr, waswith operated an operating at 15 kV.pressure The system that did base not pressure exceed was2.0 × no10 higher−8 Torr. The −9 −8 thansamples 1.0 × 10 wereTorr, sputter with- ancleaned operating for 4 pressure min by a that 4 kV did Ar not+ beam exceed prior 2.0 to× the10 collectionTorr. The of any + samplesspectra. were A sputter-cleaned 110 μm beam for was 4 minused by for a 4the kV data Ar collectedbeam prior from to thethe collection samples. ofAll any survey spectra.spectra A 110 wereµm gathered beam was at a used pass for energy the data of 160 collected eV. High from-resolution the samples. data were All surveycollected at spectra were gathered at a pass energy of 160 eV. High-resolution data were collected at a a pass energy of 40 eV. The spectra were obtained at◦ a 90° takeoff angle and Kratos’ charge pass energycompensation of 40 eV. ensured The spectra the neutralization were obtained of atall a samples. 90 takeoff The angle data and were Kratos’ calibrated charge using a compensationC1s signal ensured of C–C the at neutralization 284.8 eV. Da ofta all analysis samples. was The perform data wereed calibratedin Casa XPS using version a C1s signal2.3.18PR1.0 of C–C software at 284.8 eV.(version Data analysis 2.3.24). was performed in Casa XPS version 2.3.18PR1.0 software (version 2.3.24). 4. Results and Discussion 4. Results and Discussion FigureFigure2 shows 2 shows the cross-section the cross-section of the of carbide the carbide tool with tool CVDwith aluminaCVD alumina multilayer multilayer TiN/TiCN/Al O coating. TiN/TiCN/Al2 3 2O3 coating.

(a) (b) CVD 1

FigureFigure 2. Fracture 2. Fracture section section of the of studiedthe studied CVD CVD coating: coating: (a) ( SEMa) SEM image; image; (b )(b EDS) EDS data data (weight (weight %): %): Al—red;Al—red; Ti—green;Ti—green; W— W—yellow;yellow; Co—pink Co—pink color. color.

CuttingCutting edge edge proﬁles profiles were were evaluated evaluated using using an an Alicona Alicona Inﬁnity Infinity Focus Focus white white light light mi- microscope equipped with focus variation and 3D capabilities (Figure3). croscope equipped with focus variation and 3D capabilities (Figure 3).

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Spots for XPS studies

m m 40

BUE heighth, BUE 20

0 200 400 600 800 1000 1200 1400 1600 (a) Cutting length , m

100 90 80 70 60 50 40 30

Amount sapphire, of at. % 20 10 0 200 400 600 800 1000 1200 1400 1600 Cutting length, m (b)

FigureFigure 3. The 3. The progression progression of buildup of buildup edge edge (BUE) (BUE) vs. length vs. length of cut of cuton the on tool the toolin relation in relation to sapphire to sapphire tribo-film formation on the surface of the CVD-coated tool during the machining of an austenitic AISI 304 stainless steel: (a) numerical data tribo-film formation on the surface of the CVD-coated tool during the machining of an austenitic showing BUE heights with length of cut; (b) quantity of tribo-films formed on the friction surface vs. length of cut (XPS data).AISI 304 stainless steel: (a) numerical data showing BUE heights with length of cut; (b) quantity of tribo-films formed on the friction surface vs. length of cut (XPS data). Investigations were carried out in two stages: Investigations were carried-With outbuildups in two forming stages: in situ; -With buildups forming-With in situ; buildups etched out by chemical etching to prevent the screening effect. -With buildups etchedThis out approach by chemical enables etching the todistinction prevent the of different screening wear effect. mechanisms and the assess- This approach enablesment of the their distinction contribution of different to the entire wear tribo mechanisms-system’s performance and the assess-. ment of their contributionThe to the obtained entire tribo-system’s data also outline performance. the progression of the buildup size with respect to The obtained datawear also time. outline The the size progression of the buildup of the edge buildup (BUE) size tends with to respect strongly to wearfluctuate with a growing time. The size of the builduplength of edge cut. (BUE) Figure tends 3 presents to strongly 3D Alicona fluctuate progressive with a growing wear length studies after each 200 m of cut. Figure3 presentslength 3D Aliconaof cut. Figure progressive 3a depicts wear the studies signific afterant progression each 200 m of length BUE ofwith the length of cut. cut. Figure3a depicts theThe significantBUE instability progression is demonstrated of BUE with by the the periodical length of volatility cut. The BUEof the BUE heights in re- instability is demonstratedlation by to thethe periodicalprevious passes volatility (Figure of the 3a). BUE XPS heights studies in of relation the surface to the in close proximity to previous passes (Figurethe3 a).buildup XPS studieswere performed of the surface after a in similar closeproximity length of cut to. the Typical buildup XPS results at the corresponding wear regions are presented in Figure 3b. Figure 4 shows the typical XPS spectra of the surface of the coated tool after wear. In the initial state, only the corundum (Al2O3)

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were performed after a similar length of cut. Typical XPS results at the corresponding wear regions are presented in Figure3b. Figure4 shows the typical XPS spectra of the surface of the coated tool after wear. In the initial state, only the corundum (Al2O3) phase is present on the surface of the CVD-coated tool, as conﬁrmed by XRD analysis [32]. After wear, the phase is present on the surface of the CVD-coated tool, as confirmed by XRD analysis [32]. sapphire tribo-phase [2] starts to form at nano-scale [9] in the corresponding wear regions After wear, the sapphire tribo-phase [2] starts to form at nano-scale [9] in the correspond- (Figure4). ing wear regions (Figure 4).

Figure 4. Typical XPS spectra (Al 2s) of the worn tool surface with a CVD alumina coating. Figure 4. Typical XPS spectra (Al 2s) of the worn tool surface with a CVD alumina coating. It is interesting to note that the quantity of sapphire tribo-films that form on the surface of theIt is ceramic interesting coating to note exhibits that the the quantity opposite of trend sapphire to BUE tribo formation,-films that with form respect on the to sur- the facelength of the of cutceramic (Figure coating3b). The exhibits formation the opposite of these trend tribo-films to BUE follows formation, an unstable with respect pattern, to thealternating length of betweencut (Figure peaks 3b). and The troughs formation in contrastof these withtribo- thefilms BUE follows height. an Asunstable can be pat- seen tern,in Figure alternating3b the between sapphire peaks tribo-film and contenttroughsreached in contrast 82% with at a cuttingthe BUE length height. of As 200 can m. be As seenthe buildupin Figure height 3b the grew sapphire from tribo 13 to-film 17 µ contentm (Figure reached3a), at 82% a cutting at a cutting length incrementlength of 200 of 200m. Asto the 400 buildup m, the amount height grew of sapphire from 13 tribofilms to 17 μm content(Figure slightly3a), at a decreasedcutting length to 80% increment (Figure3 ofb). 200When to 400 the m, buildup the amount height of had sapphire reached tribofilms 48 µm at acontent 600 m lengthslightly of decreased cut, the sapphire to 80% tribofilm(Figure 3b).content When decreased the buildup to 70%. height It can had be reached concluded 48 μm that at the a decrease600 m length in the of amount cut, the of sapphire sapphire tribofiltribo-filmsm content can be decreased associated to with 70%. a It corresponding can be concluded decrease that the in the decrease thermomechanical in the amount loads of sapphireon the layer tribo- offilms tribo-films. can be associated Consequently, with a the corresponding formation of decrease a buildup in layerthe thermome- assumes a chanicalportion loads of the on surface the layer protective of tribo functions,-films. Consequently, alleviating friction the formation conditions ofon a buildup the tribo-films. layer assumesAt a cutting a portion length of of the 800 surface m, the protective buildup becomes functions, detached alleviating and afriction new buildup conditions layer on begins the triboto form-films. (its At height a cutting is 15 lengthµm). The of 800 detachment m, the buildup of the oldbecomes buildup detached and the and formation a new buildup of a new layersmall begins buildup to increasesform (its theheight thermomechanical is 15 μm). The load detachment on the tribo-film of the old layer. buildup Accordingly, and the as formationa result of of the a new adaptive small response buildupof increases the friction the surface,thermomechanical the amount load of sapphire on the tribo tribo-films-film layer.increases Accordingly, by up to as 88% a result at a of length the adaptive of cut of response 800 m. of At the a lengthfriction of surface, cut of the 1000 amount m, the ofheight sapphire of the tribo new-films buildup increases reaches by 54 upµ m.to 88% Once at again, a length this o buildupf cut of takes800 m. over At a partlength of theof cutsurface of 1000 protective m, the height functions, of the reducingnew buildup the thermomechanicalreaches 54 μm. Once load again, on thethis tribo-filmbuildup takes layer. overAccordingly, a part of the the surface amount protective of sapphire functions, tribofilms reducing was reduced the thermomechanical to 63% at this cutting load on length. the triboAt a-film length layer. of cut Accordingly, of 1200 m, thethe new amount buildup of sapphire layer became tribofilms partially was detached reduced onceto 63% again, at thiswith cutting a consequent length. At partial a length loss of cut itsprotective of 1200 m, functions. the new buildup The thermomechanical layer became partially load on detachedthe tribo-film once layeragain, was with thus a consequent increased. Aspartial a result, loss the of its amount protective of sapphire functions. tribo-films The ther- also momechanicalrose to 70% at load a length on the of tribo cut of-fi 1200lm layer m.Further was thus growth increased. of the As buildup a result, layer’s the amount height of to sapphire60 µm at tribo a length-films of also cut rose of 1400 to 70% m resulted at a length ina of decrease cut of 1200 in the m.thermomechanical Further growth of loadthe buildupon the tribofilmlayer’s height layer, to as 60 well μm asat a length corresponding of cut of reduction1400 m resulted of the in amount a decrease of sapphire in the thermomechanicaltribofilms to 57%. lo Thead on next the partial tribofilm buildup layer, detachmentas well as a corresponding occurred at a cuttingreduction length of the of amount of sapphire tribofilms to 57%. The next partial buildup detachment occurred at a cutting length of 1600 m, resulting in an increase in the thermomechanical load on the tribo-film layer and a consequent growth of the sapphire tribo-phase content to 63%. Figure 5 records the quantitative 3D wear data (Figure 5a) and crater wear volumes (Figure 5b) vs. the time of cut. Figure 5 presents tool wear data with the buildups, which

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1600 m, resulting in an increase in the thermomechanical load on the tribo-film layer and a consequent growth of the sapphire tribo-phase content to 63%. Figure5 records the quantitative 3D wear data (Figure5a) and crater wear volumes (Figure5b) vs. the time of cut. Figure5 presents tool wear data with the buildups, which have been chemically etched-out. The wear curve consists of the following typical stages: have been chemically etched-out. The wear curve consists of the following typical stages: the initial running-in stage (up to a length of cut of 400 m), the short stable wear stage and the initial running-in stage (up to a length of cut of 400 m), the short stable wear stage and the stage of accelerated wear caused by the intensification of crater wear (at a length of cut the stage of accelerated wear caused by the intensification of crater wear (at a length of exceeding 800 m, Figure5b). Crater wear begins to form after 800 m (Figure5b), mostly due cut exceeding 800 m, Figure 5b). Crater wear begins to form after 800 m (Figure 5b), mostly to the chips sliding along the rake surface of the tool [2]. This process continues to unfold due to the chips sliding along the rake surface of the tool [2]. This process continues to until the end of the tool life (Figure5a,b). The growth of crater wear, in combination with unfold until the end of the tool life (Figure 5a,b). The growth of crater wear, in combina- flank wear, eventually results in the failure of the entire tribo-system (see the 3D image of tion with flank wear, eventually results in the failure of the entire tribo-system (see the 3D the final wear stage presented in Figure5a). image of the final wear stage presented in Figure 5a).

FigureFigure 5.5. QuantitativeQuantitative datadata on on the the 3D 3D wear wear volume volume (a )(a and) and crater crater wear wear (b) ( volumeb) volume of the of the cutting cutting tool vs.tool length vs. length of cut. of cut.

AsAs waswas outlinedoutlined previously,previously, the the process process of of BUE BUE formation formation is governedis governed by by the the stick–slip stick– phenomenaslip phenomena [8]. Corresponding [8]. Corresponding patterns patterns of these of these phenomena phenomena are presented are presented in the in SEM the imagesSEM images of typical of typical buildups buildups in Figure in Figure6. 6.

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(a) (b)

FigureFigure 6. SEMSEM images images of of the the buildups: buildups: ( (aa)) low low intensity intensity (length (length of of cut cut 800 800 m); m); ( (bb)) high high int intensityensity buildup buildup (length (length of of cut, cut, 1000 m).

FigureFigure 66aa depicts a a BUE BUE during during the the initial initial running running-in-in stage stage and and Figure Figure 6b6 showsb shows the progressionthe progression of BUE of throughout BUE throughout the consequent the consequent wear stages wear (see stages Figure (see 5a). Figure It is interesting5a). It is tointeresting note that, toat notethe very that, beginning at the very of beginning wear, during of wear, the running during-in the stage running-in (see Figure stage 3 and (see FigureFigures 5a),3 and the5 risea), theof intensive rise of intensive thermomechanical thermomechanical processes processes (due to the (due adhesive to the adhesive interac- interactions at the tool/chip interface) results in heavy frictional loads and temperatures tions at the tool/chip interface) results in heavy frictional loads and temperatures within within the cutting zone. This is a consequence of the stick–slip phenomenon (Figure6a), the cutting zone. This is a consequence of the stick–slip phenomenon (Figure 6a), which which is directly related to the self-organized critical process [3]. The resulting growth is directly related to the self-organized critical process [3]. The resulting growth of entropy of entropy production [3,4] immediately prompts an adaptive response from the surface production [3,4] immediately prompts an adaptive response from the surface engineered engineered layer to the external stimuli and directly leads to the intensification of energy layer to the external stimuli and directly leads to the intensification of energy dissipation dissipation (self-organization) [1]. The tribo-films are generated at a highly rapid rate at (self-organization) [1]. The tribo-films are generated at a highly rapid rate at the initial the initial stage of friction (Figure3b). A similar trend was already presented in [9]. stage of friction (Figure 3b). A similar trend was already presented in [9]. SEM images of the chip undersurface, as well as cutting force data, confirm these SEM images of the chip undersurface, as well as cutting force data, confirm these phenomena (Figure7). phenomena (Figure 7). The stick–slip phenomenon [8] occurs on the chip/tool surface at the moment of the most intensive buildup edge formation (wave-like patterns indicated by arrows, at a length of cut of 1000 m, see Figure7a). The most intensive spikes of frictional forces occur at the same length of cut (Figure7a). They strongly diminish at the next length of cut of 1200 m (Figure7b). Cutting force spikes become less intense (Figure7b) and wave-like patterns on the chips undersurface are practically eliminated at the next moment of cut (Figure7b) . This outcome can be attributed to the generation of thermal barrier tribo-films on the friction surface at the corresponding moment of wear (Figure3b). Another micro-scale phenomenon known as deformation twinning [33], occurring on the alumina coating layer during various stages of wear, was caused by the repetitive shock loading that arose from the periodical formation and breakage of buildups (Figure8).

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Figure 7. Typical chip undersurface (1000×) and cutting force data: (a) stick–slip phenomenon; intensive cutting force Figure 7. Typical chip undersurface (1000×) and cutting force data: (a) stick–slip phenomenon; intensive cutting force fluctuations at the moment of buildup edge formation; (b) predominant slip phenomenon at the moment of buildup de- fluctuationstachment. at the moment of buildup edge formation; (b) predominant slip phenomenon at the moment of buildup detachment. The stick–slip phenomenon [8] occurs on the chip/tool surface at the moment of the Figure8 presents the SEM and EBSD images of the coated tool surface (in the re- most intensive buildup edge formation (wave-like patterns indicated by arrows, at a gion close to the tip) during the initial state (Figure8a) and after wear (Figure8b) with length of cut of 1000 m, see Figure 7a). The most intensive spikes of frictional forces occur chemically etched buildup. The SEM image of the surface morphology at the initial state at the same length of cut (Figure 7a). They strongly diminish at the next length of cut of indicates the presence of alveoli all over the surface (Figure8a). The electron backscattered 1200 m (Figure 7b). Cutting force spikes become less intense (Figure 7b) and wave-like diffraction (EBSD) images of the initial state (Figure8a) depict the crystalline structure of patterns on the chips undersurface are practically eliminated at the next moment of cut the as-deposited surface coating layer. The crystal orientation map images indicate the (Figure 7b). This outcome can be attributed to the generation of thermal barrier tribo-films absence of any plastic deformation and a clear grain orientation distribution function (ODF) on the friction surface at the corresponding moment of wear (Figure 3b). of (0 0 1), (2 1 0) and (1 2 0) (see the pole figures in Figure8a). The pole figures indicate a (0 Another micro-scale phenomenon known as deformation twinning [33], occurring 0 1) preferred orientation in the range of 0–20 degrees, predominantly perpendicular to on the alumina coating layer during various stages of wear, was caused by the repetitive the surface, which is typical for a CVD alpha-alumina orientation structure after deposi- shock loading that arose from the periodical formation and breakage of buildups (Figure tion [33–36]. SEM images of the post-wear surface morphology (Figure8b) show that the 8). alveoli disappear due to the intensive metal flow at the chip/tool interface. This leads to the formation of twinning patterns (Figure8b) caused by the plastic deformation of the surface layer. The crystal orientation map image also reveals intensive plastic deformation on the tool surface during cutting. FEM data indicate that grain refinement of the alumina

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coating layer could be caused by heavy load/high-temperature conditions at the cutting edge, (Figure1). Surface deformation is more intense near the tool tip, decreasing with distance from the edge (see red arrow, Figure8b). The reduction of grain elongation in this direction can be clearly seen. The preferred crystal (0 0 1) orientation is still there, but it is "blurred" and the (0 0 1) texture is within a range of 30–60 degrees in the direction perpendicular to the surface. Such a change in crystallographic orientation on the surface layer can be attributed to the aforementioned severe plastic deformation on the friction surface, which is conﬁrmed by the change in the preferred crystal orientation to (2 1 0) and (1 2 0).

EBSD EBSD analysis analysis

(a)

EBSD analysis Twinning

(b)

EDS analysis

FigureFigure 8. 8.SEM SEM and and EBSD EBSD imagesimages of the coated coated tool tool surface surface during during (a () ainitial) initial state state and and (b) (afterb) after wear. wear.

ThisFigure performance 8 presents the affects SEM the and tool’s EBSD underlying images of the surface coated area tool and surface causes (in twinning the region in the aluminaclose to the coating tip) during layer (Figure the initial8b). state Twins (Figure are possible 8a) and after channels wear for (Figure decreasing 8b) with the chemi- system’s entropycally etched [34]. buildup. This process The SEM could image also of bethe associated surface morphology with yet at another the initial self-organization state indi- processcates the that presence develops of alveoli within all the over tribo-system the surface at (Figure the micro-scale 8a). The electron [33–36]. backscattered diffractionEnergy (EBSD) dissipation images occurs of the on initial three state levels: (Figure the macro-scale8a) depict the (formation crystalline andstructure detachment of the as-deposited surface coating layer. The crystal orientation map images indicate the of buildup on a scale of tens of microns), the nano-scale (tribo-ﬁlm formation through absence of any plastic deformation and a clear grain orientation distribution function a strongly non-equilibrium process) and the micro-scale (twinning on the surface of the (ODF) of (0 0 1), (2 1 0) and (1 2 0) (see the pole figures in Figure 8a). The pole figures several-microns-thick ceramic layer). These multi-scale processes occur both in a state of indicate a (0 0 1) preferred orientation in the range of 0–20 degrees, predominantly per- equilibrium and in a strongly non-equilibrium state. pendicular to the surface, which is typical for a CVD alpha-alumina orientation structure after deposition [33–36]. SEM images of the post-wear surface morphology (Figure 8b) show that the alveoli disappear due to the intensive metal flow at the chip/tool interface. This leads to the formation of twinning patterns (Figure 8b) caused by the plastic deformation of the surface layer. The crystal orientation map image also reveals intensive plastic deformation on the tool surface during cutting. FEM data indicate that grain refinement of the alumina coating layer could be caused by heavy load/high-temperature conditions at the cutting edge, (Figure 1). Surface deformation is more intense near the tool tip, decreasing with distance from the edge (see red arrow, Figure 8b). The reduction of grain elongation in this direction can be clearly seen. The preferred crystal (0 0 1) orientation is still there, but it is "blurred" and the (0 0 1) texture is within a range of 30–60 degrees in

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5. Conclusions This study demonstrates the relationship between multi-scale self-organizing processes and wear-induced surface phenomena under severe tribological conditions associated with the formation of buildups. The following phenomena were shown to have taken place: (1) the periodical formation and breakage of buildups (a process which occurs at a scale of tens of microns and is associated with self-organized criticality (SOC); (2) a periodical increase and decrease in the quantity of sapphire tribo-ﬁlms, (SO: self-organization at the nano-scale), which was initiated by the SOC process; (3) the development of an additional micro-scale self-organization process in the form of twinning, which contributes to the complexity of the overall studied phenomena; combined with (4) thermal-mechanical processes (cratering) and ﬂank wear. This eventually results in the failure of the entire tribo-system. All of these multi-scale equilibrium and non-equilibrium processes are tem- porally interrelated in a complex way. A thorough analysis of the temporal progress of these processes enables a comprehensive evaluation of the wear performance of the entire tribo-system.

Author Contributions: G.F.-R.: conceptualization and writing—original draft preparation; I.S.G.: writing—review and editing; E.L.: data providing and modeling; J.M.P.: data interpretation and editing; J.L.E.: data providing and interpretation; G.D.: data providing, writing and editing; S.V.: funding acquisition and general supervision. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Natural Sciences and Engineering Research Council of Canada NSERC Grant NETGP 479639-15. Partial financial support was also provided by the grant from the Russian Science Foundation (project no. 21-79-30058) and Basque Government Industry Department (ELKARTEK Intool2), Spain. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors also acknowledge the McMaster Manufacturing Research Institute (MMRI) for the use of its facilities. Critical proofreading of the manuscript by Michael Dosbaev is acknowledged. Conflicts of Interest: The authors declare no conflict of interest.

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