Characterization of W–Cr Metal Matrix Composite Coatings Reinforced with WC Particles Produced on Low-Carbon Steel Using Laser Processing of Precoat

materials

Article Characterization of W–Cr Metal Matrix Composite Coatings Reinforced with WC Particles Produced on Low-Carbon Steel Using Laser Processing of Precoat

Dariusz Bartkowski 1,* , Aneta Bartkowska 2 , Paweł Popielarski 1 , Jakub Hajkowski 1 and Adam Piasecki 2 1 Institute of Materials Technology, Faculty of Mechanical Engineering, Poznan University of Technology, ul. Piotrowo 3, 61-138 Poznan, Poland; [email protected] (P.P.); [email protected] (J.H.) 2 Institute of Materials Science and Engineering, Faculty of Materials Engineering and Technical Physics, Poznan University of Technology, ul. Jana Pawła II 24, 60-965 Poznan, Poland; [email protected] (A.B.); [email protected] (A.P.) * Correspondence: [email protected]; Tel.: +48-616-652-665

Received: 31 October 2020; Accepted: 19 November 2020; Published: 21 November 2020

Abstract: The paper presents the study results of laser processing of precoat applied on C30 steel. The precoat consisted of powder mixtures with a binder in the form of water glass. Tungsten powder, chromium, and tungsten carbide (WC) were used to produce the precoat. The laser processing was carried out using a Yb:YAG disc laser with a rated power of 1 kW. Constant producing parameters (power of laser beam, 600 W; laser beam scanning rate, 400 mm/min) were applied. Chemical composition of the precoat was a variable parameter in coating production. A mixture consisting of 50% W and 50% Cr as a metal matrix was prepared. Subsequently, WC particles in weight ratios of 25%, 50%, and 75% were added to matrix. As a result, W–Cr metal matrix composite coatings reinforced with WC particles were formed. This study focused on investigation of microstructure, microhardness, phase, and chemical composition as well as corrosion and wear resistance, of the newly formed W–Cr/WC coatings. An instrumented nanoindentation test was also used in this study. As a result of laser beam action, the newly formed coatings had an interesting microstructure and good properties which were improved in comparison to substrate material. It is anticipated that the resulting coatings, depending on the treatment parameters (e.g., W–Cr/WC powder mixture) used, can be successfully applied to metal forming or foundry tools.

Keywords: W–Cr coating; tungsten carbide; laser processing; microstructure; microhardness; X-ray diﬀraction (XRD); energy-dispersive spectrometry (EDS); corrosion resistance; wear resistance; nanoindentation technique

1. Introduction In order to characterize the properties of tool materials, the material should be analyzed in terms of its microstructure and a number of physicomechanical properties. When examining the microstructure, special attention should be paid not only to the material core, which must be characterized by both high hardness and high ductility, but also to the condition and properties of the material’s surface layer. Metalworking tools, i.e., tools intended for use in such technologies as machining, founding, or, above all, plastic working, are exposed to intense wear due to friction and very often to dynamic loads, which may cause their cracking. Therefore, the surface layer of the material should be reﬁned with appropriate chemical elements to reduce tool wear. By extending the life of the tools, both the cost of their regeneration and the time needed for their replacement are reduced. Disassembly and assembly

Materials 2020, 13, 5272; doi:10.3390/ma13225272 www.mdpi.com/journal/materials Materials 2020, 13, 5272 2 of 21 of damaged tools and tools for plastic working or foundry generate undesirable downtime and, thus, increase production costs. Improving the operational properties of tools is often associated with an increase in the microhardness, as well as wear resistance, of the surface layer. The improvement of operational properties is possible mainly thanks to methods and techniques of surface engineering [1,2]. It is economically justified as we do not replace the entire tool with one made of more expensive material, but only modify its top layer, which is damaged most quickly. Surface treatment processes lead to the creation of new and often unique properties, and this may positively affect the parameters of technological process. In the literature, there are descriptions of methods and techniques of increasing wear resistance by friction of tools mainly through the use of ceramic materials [3–18], as well as diffusion processes, which may result in the formation of hard and wear-resistant carbides [19] or borides [20]. High-energy methods such as thermal spraying [21] or hardfacing [17,18] are also often used. It is on the methods using high-energy sources that the greatest hopes are currently placed [11,16,22–25]. Among these methods, the most important are plasma spraying [21] and laser cladding [3–16,25–40]. In [25], the authors produced an Ni-based composite coating reinforced with tungsten carbide (WC) particles. The authors formed this coating on a mild steel substrate using a high-power diode laser. They analyzed single-layer coatings contained different quantities of Ni60 matrix powder and W + C reinforcing powder. Additionally, a five-layer coating with different amounts of these materials was produced. The authors found that the multilayer coating was characterized by the highest microhardness among all coatings. The maximum microhardness value was about 3.7-fold greater than seen in the steel substrate material. The obtained coating was also characterized by the absence of pores and cracks. The author of [3] compared wear resistance between conventional hard facing and laser cladding in the coatings of nickel–chromium alloy reinforced with WC particles. The author stated that rapid solidification rate and low dilution obtained by the laser cladding method contributed to better resistance to abrasion. In this study, it was observed that, for metal matrix composite coatings reinforced with WC particles, the laser cladding method contributed to obtaining a finer microstructure and higher WC particle volume content. These features have a significant influence on microhardness and wear resistance. The authors of [27] prepared Fe-based amorphous composite coatings reinforced with a WC phase using laser processing. They stated that additional laser remelting can reduce porosities and cracks of the produced coating, as well as improve corrosion resistance. They noticed that the microhardness of the remelted coating was approximately 1.13-fold higher than without an additional remelting process. Furthermore, in [4], WC–Ni composite coatings obtained using laser processing were described. In this case, Ti-6Al-4V alloy was used as the substrate material. In this study, high-frequency microvibration was used to assist laser cladding of Ni-based composite coatings. The results showed that microvibration characterized by high frequency promoted uniform distribution of WC carbide in the (Ti)Ni eutectic solid solution. The authors observed that an appropriate increase in vibration frequency caused refinement of the grains, resulting in an increase in wear resistance of the composite coating. The addition of WC particles is intended primarily to increase the hardness of the coating and its wear resistance. Many publications focused on laser cladding [3–16,25–40] or laser alloying [41] using hard materials for different applications where high microhardness or wear resistance are required, for example, agriculture [16–18]. The authors of [6] presented the microstructure, microhardness, and wear properties of Fe-based coatings reinforced with various contents of WC particles produced on H13 hot-working die steel using laser cladding technology. The authors found that a small number of the WC particles in the WC-added coatings melted, whereas the unmelted WC particles in the coatings played the role of hard reinforcement. At the same time, the nucleation effect contributed to refinement of coarse grains around WC particles. The increase in WC content in Fe-based coatings led to an improvement of wear resistance and microhardness. In [5], the authors produced the Fe–Cr–W–C surface alloy via application of an in situ reaction during the laser cladding process using WC and AISI 410 stainless-steel precursor powders. The authors observed that the particles no greater than Materials 2020, 13, 5272 3 of 21

12 µm tended to overshoot their boiling temperature and, thus, not make it to the substrate, whereas the particles exceeding 30 µm tended not to achieve their melting point temperature. Such particles survived in the melt pool in solid form. The authors found that the carbide particles were dissolved relatively easily due to the free energy in the Cr–C–W system and formed Cr–C compounds. In [28], the microstructure of Stellite-6/WC coatings obtained via the laser cladding method using a CO2 laser beam on steel was investigated. Powder mixtures with WC contents of 0%, 9%, 18%, 27%, 36%, 45%, 54%, 72%, and 100% were applied via dual powder feeding of Stellite-6 and WC powders using different feeding rates. The authors found two kinds of solidification in the obtained microstructure. The first was characterized by dendrites, as well as interdendritic eutectics (obtained with WC amounts ranging from 0% to 36%). In this case, the WC particles added to the coating were fully melted into the melt pool. The second was characterized by diversely faceted dendrites in flower, block, star, and butterfly shapes. This solidification was obtained for 45% to 100% WC content. It should be mentioned that most of the WC particles were melted, and the microstructure consisted of a resolidified WC phase and Co, as well as various complex carbides (Co–W–C/Fe–W–C). In [6,28,40], the mechanism influencing the microstructure of WC particles was also featured. Recently, there has been increased interest in the use of laser processing methods to produce composite surface layers on steels. Such layers have a composite structure, i.e., they are composed of a metal matrix reinforcing with hard intermetallic phases. The most commonly used matrices involve iron [6,7,17,18,26], nickel [13,14,16,25,26,28,36,38,40], cobalt [33], and alloys containing these elements. These alloys are characterized by good wettability of the reinforcing phase, facilitating adherence of this phase. The most frequently used reinforcing particles are various types of high-melting metal carbides, e.g., tungsten carbide (WC) [3–12,16,21,26–28,30,36,37,40], silicon carbide (SiC) [13,14], boron carbide (B4C) [38,39], or titanium carbide (TiC) [42]. In this work, a study of composite coatings with a matrix of tungsten and chromium (W–Cr) reinforced with hard particles of tungsten carbide (WC) was carried out. The coatings were created on nonalloy carbon steel. The aim of the research was to obtain a layer with increased hardness and wear resistance so that it could be used as a coating to increase tool durability for plastic working and casting.

2. Materials and Methods The W–Cr metal matrix composite coatings reinforced with WC particles were produced on C30 steel substrate. The chemical composition of this material is presented in Table1. Cuboid specimens cut from a square bar with dimensions of 12 mm 12 mm 5 mm were used. The specimens were × × machined (milled and ground) to obtain a surface roughness corresponding to Ra = 1.25.

Table 1. Chemical composition of C30 steel used in study (wt.%).

C Mn Si P S Cr Ni Mo Fe 0.32 0.63 0.29 0.03 0.04 0.21 0.27 0.07 base

To produce the coatings described in this work, several types of precoats in the form of paste with a thickness of 100 µm were used. The precoats were applied to the steel surfaces of specimens using a handheld applicator. This applicator was made from two hardened steel plates. The first plate had a slit with a height corresponding to the thickness of the precoat. The second plate (base plate) was equal to the height of the specimen. The specimen was placed in the hole of the base plate. Afterward, the paste was applied to the surface of specimen, and then the paste thickness was evened out using an applicator. The thickness was measured using an ultrasonic sensor. Pastes consisted of powder particles and a binder in the form of water glass. The powder mixture of tungsten and chromium was used as a matrix in a weight proportion of 50%/50%. In order to change the properties of W–Cr coating, 25%, 50%, and 75% WC particles were gradually added. A coating using only WC powder was also Materials 2020, 13, 5272 4 of 21 produced. The precoats were then processed using a Yb:YAG disc laser beam. The size of tungsten and chromium powder particles did not exceed 20 µm, and their shape was irregular, whereas the granularity of the reinforcing phase particles did not exceed 45 µm. Powder mixtures were prepared in a ball mill for 3 h, followed by drying at 100 ◦C for 1h. The specimens with prepared precoats were Materials 2020, 13, x 4 of 22 subjectedprecoats towere laser subjected processing to usinglaser processing a five-axis LaserCellusing a five-axis 3008 laser LaserCell device with3008 alaser TruDisk device Yb:YAG with a laserTruDisk (TRUMPF, Yb:YAG Ditzingen, laser (TRUMPF, Germany). Ditzingen, The rated Germany). power of theThe laserrated used power was of equal the laser to 1 used kW. Thewas laserequal beamto 1 kW. was The characterized laser beamby was a circularcharacterized cross-section by a circular and across-section diameter of and 1.2 mm.a diameter The specimen of 1.2 mm. was The laser-processedspecimen was onlaser-processed the entire surface on the with entire 75% tracksurface overlapping. with 75% track The schemeoverlapping. of laser The processing scheme of laser the precoatprocessing applied of the on precoat C30 steel, applied as well on as C30 the steel, expected as well final as metal the expected matrix composite final metal coating, matrix iscomposite shown incoating, Figure1 is. Theshown laser in processing Figure 1. The parameters laser processing are presented parameters in Table are2 .presented The aim ofin thisTable study 2. The was aim to of determinethis study the was eff toect determine of the chemical the effect composition of the chemical of the precoatcomposition on the of properties the precoat of on obtained the properties coatings. of Theobtained parameters coatings. were The selected parameters on the were basis selected of preliminary on the tests.basis of preliminary tests.

FigureFigure 1. 1. SchemeScheme of laserlaser processing. processing.

Table 2. Parameters of laser processing. Table 2. Parameters of laser processing. Composition of Powder Mixture Parameters of Laser Beam Composition of Powder Mixture Parameters of Laser Beam WW–Cr–Cr WCWC PowerPower of of Laser Laser Beam Beam ScanningScanning Rate Rate of of Laser Laser Beam Beam (%(%)) (%(%)) ((W)W) (mm(mm/min/min) ) 100100 0 0600 600 400400 7575 25 25 600 600 400400 50 50 600 400 50 50 600 400 25 75 600 400 0 25100 75 600 600 400400 0 100 600 400 Microstructure and macroscopic observations were carried out using two different scanning electron microscopes: VEGA 5135 (TESCAN, Brno, Czech Republic) and MIRA3 (TESCAN, Brno, CzechMicrostructure Republic). The and first macroscopic microscope observationswas equipped were with carried a Prism out 2000 using Si(Li) two en diergyfferent-dispersive scanning X- electronray spectrometer microscopes: (Princeton VEGA Gamma 5135 (TESCAN, Tech Instruments, Brno, Czech Princeton, Republic) NJ, andUS) MIRA3and PGT (TESCAN, software Spirit Brno, Czech1.06 software. Republic). The The other first microscope microscope was was equipped equipped with with a Prism an 2000 EDS-UltimMax Si(Li) energy-dispersive energy-dispersive X-ray spectrometerspectrometer (Princeton(Oxford Instruments, Gamma Tech High Instruments, Wycombe, Princeton, UK) and Aztec NJ, US) Energy and PGTLive softwareStandard Spirit software 1.06. software.The microstructure The other microscope was determined was equipped on cross-sections, with an EDS-UltimMax perpendicular energy-dispersive to produced coatings.spectrometer The (Oxfordspecimens Instruments, were first Highsanded Wycombe, on sandpaper UK) and with Aztec grit Energy from 80 Live to 2500 Standard and then software. polished The using microstructure diamond waspaste.determined To reveal the on microstructure cross-sections, of perpendicular produced coatings, to produced etching coatings. by “royal Thewater specimens” solution were (HCl firstand sandedHNO3 in on a sandpaper 3:1 ratio) waswith used. grit from The 80 phase to 2500 analysis and then of W polished–Cr/WC using coatings diamond was performed paste. To reveal on an theEMPYREAN microstructure X-ray ofdiffractometer produced coatings, (PANalytical, etching Malvern, by “royal UK). water” An angle solution range (HCl of and20° to HNO 90° using3 in a 3:1Cu Kα radiation was applied. This kind of lamp has the ability to penetrate material up to 21 μm. A voltage of 45 kV and current of 40 mA were used. X-ray diffraction (XRD) analysis was performed at a temperature of 25 °C. Microhardness tests were carried out on cross-sections of coatings from the surface to the substrate. The microhardness tester FM-810 (Future-Tech, Kawasaki, Japan) equipped with an automatic indentation measuring system (FT-Zero software, Future-Tech, Kawasaki, Japan was used. All measurements were taken using an indentation load of 50 g. The loading time was 15 s. The electrochemical corrosion resistance tests were carried out using an ATLAS 1131 EU&IA device Materials 2020, 13, 5272 5 of 21 ratio) was used. The phase analysis of W–Cr/WC coatings was performed on an EMPYREAN X-ray diffractometer (PANalytical, Malvern, UK). An angle range of 20◦ to 90◦ using Cu Kα radiation was applied. This kind of lamp has the ability to penetrate material up to 21 µm. A voltage of 45 kV and current of 40 mA were used. X-ray diffraction (XRD) analysis was performed at a temperature of 25 ◦C. Microhardness tests were carried out on cross-sections of coatings from the surface to the substrate. The microhardness tester FM-810 (Future-Tech, Kawasaki, Japan) equipped with an automatic indentation measuring system (FT-Zero software, Future-Tech, Kawasaki, Japan was Materialsused. All2020 measurements, 13, x were taken using an indentation load of 50 g. The loading time was5 of 15 22 s. The electrochemical corrosion resistance tests were carried out using an ATLAS 1131 EU&IA device (Atlas(Atlas-Sollich,-Sollich, Rębiechowo, R˛ebiechowo,Poland) Poland) in 5% NaCl aqueous aqueous solution. solution. The The potentiodynamic potentiodynamic method method was was used.used. TheThe corrosion corrosion potential potential and and corrosion corrosion current current of produced of produced composite composite coatings were coatings determined. were determined.These tests wereThese performed tests were atperformed 25 ◦C with at 25 a scanning°C with a rate scanning of 0.5 rate mV /ofs using0.5 mV/s a platinum using a platinum electrode electrodeas an auxiliary as an electrode auxiliary and electrode saturated and calomel saturated electrode calomel as electrode a reference as electrode. a reference The electrode. results wereThe resultsdetermined were on determined the basis of on extrapolation the basis of Tafel extrapolation curves. Wear of Tafel resistance curves. was Wear investigated resistance using was an investigatedAmsler-type using device an under Amsler dry- frictiontype device conditions. under dry A friction friction pair conditions. consisted A of friction the plate-shape pair consisted specimen of thewith plate the- producedshape specimen composite with the coating produced and the composite ring-shape coating counter-specimen. and the ring-shape The counter counter-specimen-specimen. Thewas counter made of-specimen 100Cr6 bearing was made steel of after 100Cr6 hardening. bearing Thesteel hardnessafter hardening of the. counter-specimen The hardness of wasthe counter63 HRC.-specimen Wear resistance was 63 tests HRC were. Wearperformed resistance using the tests following were performed parameters: using load, 400 the N;following rotation parameters:speed of counter-specimen, load, 400 N; rotation 179 speed rpm; timeof counter of friction,-specimen 210, min. 179 rpm; The time mass of loss friction of specimens, 210 min. The was massmeasured loss of using specimens the AS220.R2 was measured analytical using balance the AS220.R2 (RADWAG, analytical Radom, balance Poland) (RADWAG, after every Radom, 40 min Poland)of wear after measured every with40 min an of accuracy wear measured of 0.0001 with g. an In orderaccuracy to determineof 0.0001 g. the In order mechanical to determine properties the mechanicalof coatings, properties a nanoindentation of coatings, technique a nanoindentation was used. This technique technique was enables used. This measuring technique mechanical enables measurpropertiesing mechanical such as the properties modulus of such elasticity as the andmodulus hardness of elasticity (Figure and2). A hardness Fischer (Figure Picodentor 2). A HM Fischer 500 Picodentornanoindenter HM (Helmut 500 nanoindenter Fischer, Sindelfingen, (Helmut Germany)Fischer, Sindelfingen, was employed Germany) in this study. was employed The measurement in this study.stand was The equipped measurement with stand a vibration was equipped isolation plate, with a which vibration dampened isolation vibration plate, which occurring dampened during vibrationwork. In addition,occurring the during measuring work. In device addition, was enclosedthe measuring in a thermoacoustic device was enclosed sheath, in which a thermoacoustic reduced the sheath,effects ofwhich air flow reduced and impact the effects of acoustic of air flow waves. and Theimpact test of was acoustic performed waves. in accordanceThe test was with performed the ISO in14577-1 accordance standard with [43 the]. MeasurementsISO 14577-1 standard were performed [43]. Measurements with a 200 mNwere load performed applied with for 5 s.a 200 In this mN study, load applieda diamond for indenter5 s. In this was study used., a Thediamond parameters indenter from was the used. instrumented The parameters indentation from testthe wereinstrumented based on indentationreadings of test the followingwere based data: on readings indentation of the depth following (h), contact data: depthindentation (hc), elasticdepth displacement(h), contact depth (hs), (hc),projected elastic area displacement (Ap), and surface (hs), projected area (As), area which (Ap) are, and presented surface in area Figure (As),2a. which are presented in Figure 2a.

FigureFigure 2. 2. MethodologyMethodology of of the the instrumented instrumented indentation indentation test: test: ( (aa)) scheme scheme measurement; measurement; ( (bb)) example example of of aa typical typical load load–displacement–displacement curve curve obtained during nanoindentation test test.. 3. Results and Discussion 3. Results and Discussion 3.1. Microstructure, Chemical, and Phase Analysis 3.1. Microstructure, Chemical, and Phase Analysis The microstructures of the W–Cr coatings without reinforcing phase produced using laser The microstructures of the W–Cr coatings without reinforcing phase produced using laser processing of the precoat are shown in Figure3. The microstructure of W–Cr coatings consisting of a processing of the precoat are shown in Figure 3. The microstructure of W–Cr coatings consisting of a melted zone obtained by the remelting of steel substrate and precoat could be identified. Below, the heat-affected zone (HAZ) was found. The remaining area shown in Figure 3 was ferrite–pearlite steel substrate. The microstructure of the melted zone consisted of the solid solution of chromium and tungsten in iron (Figures 3 and 4). The W–Cr coating without a reinforcing phase had an average thickness of 430 µm including the heat-affected zone of approximately 170 µm. The coatings were more than four times thicker than the pre-coat, highlighting their high proportion of steel substrate. Produced coatings were characterized by good bonding with the steel substrate. The laser processing parameters used contributed to formation of a parabolic bond line between the coating and the substrate, which is characteristic for this kind of treatment.

Materials 2020, 13, 5272 6 of 21 melted zone obtained by the remelting of steel substrate and precoat could be identified. Below, the heat- affected zone (HAZ) was found. The remaining area shown in Figure3 was ferrite–pearlite steel substrate. The microstructure of the melted zone consisted of the solid solution of chromium and tungsten in iron (Figures3 and4). The W–Cr coating without a reinforcing phase had an average thickness of 430 µm including the heat-affected zone of approximately 170 µm. The coatings were more than four timesMaterials thicker 2020, 1 than3, x the pre-coat, highlighting their high proportion of steel substrate. Produced6 of 22 coatings were characterized by good bonding with the steel substrate. The laser processing parameters used contributed to formation of a parabolic bond line between the coating and the substrate, which is characteristicMaterials 2020, for13, xthis kind of treatment. 6 of 22

Figure 3. Microstructure of W–Cr coating produced using laser processing of precoat applied on steel substrate.

Figure 4b shows the areas of chemical composition testing. The melting of the precoat caused the formation of carbide precipitates (square marked No. 1). In the remaining areas (dark and light), FigureFigureiron 3. 3.Microstructure Microstructurepredominated ,of ofconfirm W W–Cr–Cr coatinging coating its high produced produced content using in using thelaser coating. laser processing processing of precoat of precoat applied applied on steel on steelsubstrate substrate..

Figure 4. MicrostructureFigure 4. Microstructure and the energy-dispersive and the energy-dispersive spectrometry spectrometry (EDS) point (EDS analysis) point of analysis W–Cr coating of W–Cr produced usingcoating laser produced processing using of precoatlaser processing applied of on precoat steel substrate: applied on (a )steel melted substrate zone;: ((ba) magniﬁcationmelted zone; (b) of melted zone.magnification of melted zone.

The microstructures of the W–Cr/WC composite coatings with varying amounts of WC powder, as well as only using WC powder, are shown in Figure 5. As a result of precoat laser processing, composite coatings with a complex microstructure were obtained. The composition of the applied Figure 4. Microstructure and the energy-dispersive spectrometry (EDS) point analysis of W–Cr

coating produced using laser processing of precoat applied on steel substrate: (a) melted zone; (b) magnification of melted zone.

Materials 2020, 13, 5272 7 of 21

Figure4b shows the areas of chemical composition testing. The melting of the precoat caused the formation of carbide precipitates (square marked No. 1). In the remaining areas (dark and light), iron predominated, confirming its high content in the coating. pre-coatThe had microstructures an influence ofon the microstructure W–Cr/WC composite of coatings coatings obtained. with The varying microstructure amounts consisted of WC powder, of the W–Cr matrix and unmelted primary WC particles. The proportion of these particles depended on asMaterials well 2020 as only, 13, x using WC powder, are shown in Figure5. As a result of precoat laser processing,7 of 22 compositetheir amounts coatings in the with precoat. a complex When microstructurethe proportion wereof WC obtained. reinforcing The phase composition was equal of theto 25% applied and pre-coat50%, a very had small an influence amount on of microstructurenonmelted WC of particles coatings in obtained. coatings Thewas microstructureobserved. This consistedproves that of thethe W–Crlaser beam matrix melted and unmelted almost all primary components WC particles. of the initial The proportion coating when of these the particles amount dependedof WC phase ontheir was amountssmaller. Increasing in the precoat. the content When of the reinforcing proportion phase of WC prevented reinforcing larger phase WCwas particles equal from to 25% melting. and 50%, The aprovided very small thermal amount energy of nonmelted was not sufficient WC particles to melt in coatingsall the components was observed. of the This precoat, proves which that theresult lasered beamin the melted formation almost of a all W components–Cr/WC or ofWC the composite initial coating coating. when Laser the amountbeam parameters of WC phase were was constant; smaller. Increasinghowever, due the contentto the change of reinforcing in the chemical phase prevented composition larger of WC the particles precoat, from different melting. geometries The provided of the thermalcoating energycross-sections was not were suffi cient observed. to melt Dendritic all the components microsegregation of the precoat,was a common which resulted feature inof the all formationproduced ofcomposite a W–Cr/ WCcoatings. or WC This composite was the coating. effect of Laser varying beam solidification parameters rates were of constant; the melted however, zone dueover tothe the thickness change of in coatings the chemical from the composition surface to ofthe the steel precoat, substrate. diff Theerent changes geometries in cooling of the rate coating were cross-sectionsmost visible in werethe case observed. of specimens Dendritic with microsegregationthe highest amount was of WC a common reinforcing feature phase of all(Figure produced 5c,d). compositeThe WC particles coatings. became This wasa kind the of e ffchillect of(as varying in a foundry) solidification and accelerated rates of thethe meltedsolidification zone over process the thicknessaround their of coatingsoccurrence. from The the heat surface absorption to the steelby the substrate. high-melting The changescarbides inresulted cooling in ratethe production were most visibleof thinner in the coatings. case of specimens When there with was the highest a lower amount amount of WC of WC reinforcing particles phase in the (Figure precoat,5c,d). heat The WC was particlestransferred became to the a kindsteel ofsubstrate, chill (as inand a foundry)this contributed and accelerated to the melting the solidification of substrate. process This resulted around their in a occurrence.higher proportion The heat of absorption iron in the by coating, the high-melting which in carbidesturn affected resulted the in properties the production of the of produced thinner coatings.coatings. When there was a lower amount of WC particles in the precoat, heat was transferred to the steel substrate, and this contributed to the melting of substrate. This resulted in a higher proportion of iron in the coating, which in turn affected the properties of the produced coatings.

Figure 5. 5. Microstructure of coatings produced using laser processing of precoat applied on steel steel substrate: (a) W–CrW–Cr/25%/25% WC;WC; ((bb)) W–CrW–Cr/50%/50% WC;WC; ((cc)) W–CrW–Cr/75%/75% WC;WC; ( d(d)) 100% 100% WC. WC.

The addition addition of of 25% 25% tungsten tungsten carbide carbide particles particles to the to precoat the precoat contributed contributed to an increase to an in increase thickness in µ ofthickness the melted of the zone melted in the zone obtained in the obtained coating to coating about to 510 aboutm, while510 µm, keeping while keeping the heat-a theff ectedheat-affected zone at µ thezone same at the level, same i.e., level, about i.e., about 170 m 170 (Figure µm (Figure5a). An 5a). unquestionable An unquestionable change change in the in microstructurethe microstructure of of the obtained coatings was the presence of unmelted particles in the WC reinforcing phase. Thanks to this, coatings containing 25% of the WC phase could be called composite coatings. The particles were very rarely identified. It was found that only the largest particles did not melt (Figures 5a and 6). The parameters of the laser beam used did not generate enough heat to melt the WC particles completely. Figure 6a–c show the melting zone along with the areas of chemical composition analysis, marked with squares. It is clearly visible that the unmelted particles were mainly composed of carbon and tungsten, corresponding to the primary WC carbides. On the other hand, the matrix was largely composed of iron with additions of tungsten and chromium. Thus, it can be stated that an Fe–W–Cr Materials 2020, 13, 5272 8 of 21 the obtained coatings was the presence of unmelted particles in the WC reinforcing phase. Thanks to this, coatings containing 25% of the WC phase could be called composite coatings. The particles were very rarely identified. It was found that only the largest particles did not melt (Figures5a and 6). The parameters of the laser beam used did not generate enough heat to melt the WC particles completely. Figure6a–c show the melting zone along with the areas of chemical composition analysis, markedMaterials with squares. 2020, 13, x It is clearly visible that the unmelted particles were mainly composed of carbon 8 of 22 and tungsten, corresponding to the primary WC carbides. On the other hand, the matrix was largely composedmatrix of ironwas with obtained. additions This of proves tungsten the andgreat chromium. influence Thus,of the itsteel can besubstrate stated thaton the an Fe–W–Crproduced metal matrixmatrix was obtained. composite This coating. proves the great influence of the steel substrate on the produced metal matrix composite coating.

Figure 6. Microstructure and EDS point analysis of W–Cr coating reinforced with 25% WC particles Figure 6. Microstructure and EDS point analysis of W–Cr coating reinforced with 25% WC particles produced using laser processing of the precoat applied on steel substrate: (a) general view of overlapping produced using laser processing of the precoat applied on steel substrate: (a) general view of zone; (b) magniﬁcation of overlapping zone with unmelted WC particles; (c) matrix. overlapping zone; (b) magnification of overlapping zone with unmelted WC particles; (c) matrix. Figures5b and7 show the microstructure of W–Cr /50%WC coatings. Increasing the content of the reinforcingFigures particles 5b and caused 7 show a reduction the microstructure in thickness of of W the–Cr produced /50%WC coatingcoatings. by Increasing 10–20 µm. the At thecontent of same time,the reinforcing increasing theparticles content caused of the a WC reduction reinforcing in thickness phase resulted of the produced in an increase coating in the by amount10–20 µm. of At the unmeltedsame primary time, increasing WC particles. the content In the coating of the WC structure, reinforcing no defects phase such resulted as cracks in anor increase porosity in werethe amount of unmelted primary WC particles. In the coating structure, no defects such as cracks or porosity were found, and the coating was very well bonded to the steel substrate. Figure 7a shows an example of a partially fused particle of the primary WC particle. The formation of new phases with a high content both of tungsten, iron, and carbon can be observed at the boundary of the carbide and the matrix (Figure 7b). These are probably complex secondary carbides of the M7C3 or M2C type. Between the dendrites of the newly formed secondary carbides, the analysis of the chemical composition indicated a high content of both iron (over 55 wt.%) and tungsten (over 30 wt.%). This is the darker area marked with square no. 5. The bright area around the partially melted primary WC carbide (marked with

Materials 2020, 13, 5272 9 of 21 found, and the coating was very well bonded to the steel substrate. Figure7a shows an example of a partially fused particle of the primary WC particle. The formation of new phases with a high content both of tungsten, iron, and carbon can be observed at the boundary of the carbide and the matrix (Figure7b). These are probably complex secondary carbides of the M 7C3 or M2C type. Between the dendrites of the newly formed secondary carbides, the analysis of the chemical composition indicated aMaterials high content 2020, 13,of x both iron (over 55 wt.%) and tungsten (over 30 wt.%). This is the darker area marked9 of 22 with square no. 5. The bright area around the partially melted primary WC carbide (marked with squaresquare no.no. 3) was characterized byby increasedincreased amountsamounts ofof tungstentungsten andand carbon.carbon. The microstructure ofof thethe remainingremaining partpart ofof thethe meltingmelting zonezone waswas clearlyclearly characterizedcharacterized byby aa needleneedleshape shape (Figure(Figure7 c).7c).

Figure 7. Microstructure and EDS point analysis of W–Cr coating reinforced with 50% WC particles Figure 7. Microstructure and EDS point analysis of W–Cr coating reinforced with 50% WC particles produced using laser processing of precoat applied on steel substrate: (a) general view of the melting produced using laser processing of precoat applied on steel substrate: (a) general view of the melting zone; (b) carbide-matrix interface; (c) matrix. zone; (b) carbide-matrix interface; (c) matrix. Figures5c and8 show the microstructure of the W–Cr /75% WC coatings. Increasing the content of WCFigures particles 5c and to 75% 8 show resulted the microstructure in a reduction of in the the W layer–Cr/75% thickness WC coatings. to about Increasing 400 µm and the acontent slight increaseof WC particles in the heat-a to 75%ﬀected resulted zone in to a about reduction 200 µ inm. the The layer amount thickness of unmelted to about primary 400 µm WC and particles a slight increasedincrease in (Figure the heat8a).-affected Figure8b,c zone show to about the boundary 200 µm. of The WC amount carbide of and unmelted the W–Cr primary matrix. WC A change particles in increased (Figure 8a). Figure 8b,c show the boundary of WC carbide and the W–Cr matrix. A change in microstructure at the carbide–matrix interface was clearly visible. This microstructure had a significant impact on the properties of the produced coatings. In dark areas (e.g., marked with square no. 2), a much lower tungsten content was found than in bright areas (e.g., marked with square no. 3). In bright areas, there was a higher carbon content, which may mean that secondary carbides were formed there. The microstructure around large partially melted WC particles was more cellular, and many spherical precipitates of secondary carbides were formed. In each case, a small amount of chromium was found in the studied areas, but this may indicate the binding of chromium into complex carbides. A high content of tungsten and iron was also found in the dark areas of the matrix. This may be comparable to the Fe7W6 phase detected in XRD studies. It is clear from the observation

Materials 2020, 13, 5272 10 of 21 microstructure at the carbide–matrix interface was clearly visible. This microstructure had a signiﬁcant impact on the properties of the produced coatings. In dark areas (e.g., marked with square no. 2), a much lower tungsten content was found than in bright areas (e.g., marked with square no. 3). In bright areas, there was a higher carbon content, which may mean that secondary carbides were formed there. The microstructure around large partially melted WC particles was more cellular, and many spherical precipitates of secondary carbides were formed. In each case, a small amount of chromium was found in the studied areas, but this may indicate the binding of chromium into complex carbides.Materials A high 2020 content, 13, x of tungsten and iron was also found in the dark areas of the matrix. This may 10 of 22 be comparable to the Fe W phase detected in XRD studies. It is clear from the observation of the of the microstructure7 6 that the increase in WC content in the case of W–Cr/WC coatings caused a microstructure that the increase in WC content in the case of W–Cr/WC coatings caused a reduction reduction in the thickness of the produced coatings while maintaining a relatively constant thickness in the thickness of the produced coatings while maintaining a relatively constant thickness of the of the heat-affected zone. heat-aﬀected zone.

Figure 8. Microstructure and EDS point analysis of W–Cr coating reinforced with 75% WC particles Figure 8. Microstructure and EDS point analysis of W–Cr coating reinforced with 75% WC particles produced using laser processing of precoat applied on steel substrate: (a) general view of the melting produced using laser processing of precoat applied on steel substrate: (a) general view of the melting zone; (b) selected carbide; (c) carbide-matrix interface. zone; (b) selected carbide; (c) carbide-matrix interface. The microstructure of the coating produced using only a WC precoat is shown in Figures5d and9 . In this coating,The no microstructure porosity and noof the cracks coating were produced observed using on the only entire a WC cross-section precoat is and shown entire in surface.Figure 5d and Figure 9. In this coating, no porosity and no cracks were observed on the entire cross-section and entire surface. The thickness of the laser-modified surface layer was uniform. Larger-sized tungsten carbides did not melt completely, which resulted in a visible reinforcing WC phase, similar to that seen in the W–Cr/75%WC coating. However, the amount of large WC particles was higher in this case. As already mentioned, these coatings were characterized by the smallest thickness. This may be due to the very high temperatures necessary to melt the tungsten carbides. The heat delivered to the precoat was received by the tungsten carbide particles. Smaller WC particles melted, then mixed with iron, and finally formed a matrix, while larger WC particles received heat and only partially melted (only on the particle surface). The heat accumulated in the WC particles was then transferred to the steel substrate, which resulted in the formation of a fairly deep heat-affected zone, exceeding 220 µm in places. The coating, despite the absence of the W–Cr matrix, did not have cracks. It was

Materials 2020, 13, 5272 11 of 21

The thickness of the laser-modified surface layer was uniform. Larger-sized tungsten carbides did not melt completely, which resulted in a visible reinforcing WC phase, similar to that seen in the W–Cr/75%WC coating. However, the amount of large WC particles was higher in this case. As already mentioned, these coatings were characterized by the smallest thickness. This may be due to the very high temperatures necessary to melt the tungsten carbides. The heat delivered to the precoat was received by the tungsten carbide particles. Smaller WC particles melted, then mixed with iron, and finally formed a matrix, while larger WC particles received heat and only partially melted (only on the particle surface). The heat accumulated in the WC particles was then transferred to the steel substrate, whichMaterials resulted 2020, 13, x in the formation of a fairly deep heat-affected zone, exceeding 220 µm11 inof 22 places. The coating, despite the absence of the W–Cr matrix, did not have cracks. It was characterized by a few porositiescharacterized formed by a atfew the porosities border formed of the at tracks, the border i.e., inof the places tracks, where i.e., in the place materials where wasthe material most heated. Figurewas9 amost shows heated. an exemplary Figure 9a unmelted shows an primary exemplary tungsten unmelted carbide. primary An enlargement tungsten carbide. of this An area is shownenlargement in Figure of9b,c. this Here,area is itshown is clearly in Figure visible 9b,c that. Here, the it surface is clearly of thevisible primary that the WC surface carbide of the primary WC carbide became the site of nucleation and growth of new phases (area marked with became the site of nucleation and growth of new phases (area marked with square no. 5). The chemical square no. 5). The chemical composition corresponded mainly with W2C and M7C3 phases. composition corresponded mainly with W2C and M7C3 phases.

Figure 9. Microstructure and EDS point analysis of coating formed using 100% WC particles produced Figure 9. Microstructure and EDS point analysis of coating formed using 100% WC particles produced using laser processingusing laser processing of precoat of appliedprecoat applied on steel on substrate: steel substrate (a): general(a) general view view of of the the meltingmelting zone; zone; (b) (b) selected carbide;selected carbide; (c) carbide-matrix (c) carbide-matrix interface. interface.

In the caseIn of the the case W–Cr of the coatings, W–Cr coatings, very good very cohesion good cohesion between between the coating the coating and and the the substrate substrate was was found, whichfound, resulted which from resulted a good from mixing a good ofmixing the coating of the coating material material with thewith steel the steel substrate. substrate. In In the the case case of W–Cr/25%WC coatings, a high iron content was found, which also suggests a very good mixing of pre-coat with the substrate and very good metallurgical bonding. In the case of coatings containing 50% and 75% WC particles, a large amount of iron in the matrix and a very small amount of iron in the area of unmelted carbides were found. Coatings produced on the basis of pure WC precoat were characterized by the lowest iron content. This kind of coating was mainly composed of primary and secondary tungsten carbides or complex carbides containing iron.

Materials 2020, 13, 5272 12 of 21 of W–Cr/25%WC coatings, a high iron content was found, which also suggests a very good mixing of pre-coat with the substrate and very good metallurgical bonding. In the case of coatings containing 50% and 75% WC particles, a large amount of iron in the matrix and a very small amount of iron in the area of unmeltedMaterials carbides 2020, 13, x were found. Coatings produced on the basis of pure WC precoat were 12 of 22 characterized by the lowest iron content. This kind of coating was mainly composed of primary and secondary tungstenThe carbides phase or composition complex carbides of the containing produced iron. W–Cr metal matrix composite coatings is shown in The phaseFigure composition 10. The of presence the produced of WC, W–Cr W2C, metal M7C3, matrix and Fe composite7W6 and Feα coatings phases is was shown found. in The coatings produced via laser processing of the precoat containing a greater amount of the WC reinforced phase Figure 10. The presence of WC, W2C, M7C3, and Fe7W6 and Feα phases was found. The coatings produced via laserwere processing characterized of the by precoat a very intense containing peak a of greater WC and amount M7C3 of phases. the WC These reinforced phases phase were only identified in coatings containing a WC reinforcing phase. The M7C3 phase was not identified in the W–Cr were characterized by a very intense peak of WC and M7C3 phases. These phases were only identified coatings. In all coatings, except for the WC coating, the presence of a Fe7W6 phase was found. It can in coatings containing a WC reinforcing phase. The M7C3 phase was not identified in the W–Cr be concluded that the intensity of this phase was comparable in all cases. Very intense iron peaks coatings. In all coatings, except for the WC coating, the presence of a Fe7W6 phase was found. It can be concluded thatwere the found, intensity which of this confirm phaseed was a comparablehigh proportion in all cases.of steel Very substrate intense in iron the peaks coating. were As WC particle found, which confirmedcontent increase a highd proportion, the intensity of steel of the substrate different in carbide the coating. phases As increase WC particled. The content results corresponded increased, the intensityto the EDS of themicroanalysis different carbide presented phases in increased.Figure 4 and The Fig resultsures 6 corresponded–9. to the EDS microanalysis presented in Figures4 and6, Figures7–9.

Figure 10. X-ray diﬀraction of coatings reinforced using laser processing of diﬀerent precoat applied on steel substrate.Figure 10. X-ray diffraction of coatings reinforced using laser processing of different precoat applied on steel substrate. 3.2. Microhardness Results 3.2. Microhardness Results The results of microhardness of the produced W–Cr metal matrix composite coatings reinforced with WC, as well asThe coatings results producedof microhardness using only of the W–Cr produced or WC W precoat,–Cr metal are matrix shown composite in Figure coatings 11. reinforced Measurementswith of microhardness WC, as well as in coatings the melted produced zone were using performed only W–Cr by or omitting WC precoat the larger, are shown and in Figure 11. unmelted WCMeasurements particles. It was of found microhardness that each in coating the melted had a zonemelted were zone performed whose microhardness by omitting the larger and unmelted WC particles. It was found that each coating had a melted zone whose microhardness was no lower than 500 HV0.05. In the case of W–Cr coatings without a reinforced WC phase, a very mild microhardness profile from the surface to the steel substrate was observed. The microhardness gradually decreased from about 600 HV0.05 to about 450 HV0.05 in the heat-affected zone. In the case of W–Cr metal matrix composite coatings reinforced with WC particles, the high microhardness of the melted zone (about 700 HV0.05) resulted from partial melting of WC particles. The addition of a hard reinforcing phase contributed to a microhardness increase in the entire coating. The addition of

Materials 2020, 13, 5272 13 of 21 was no lower than 500 HV0.05. In the case of W–Cr coatings without a reinforced WC phase, a very mild microhardness profile from the surface to the steel substrate was observed. The microhardness gradually decreasedMaterials 2020 from, 13, x about 600 HV0.05 to about 450 HV0.05 in the heat-affected zone. In the case 13 of 22 of W–Cr metal matrix composite coatings reinforced with WC particles, the high microhardness of the 25% WC (W–Cr/25%WC) did not significantly increase the microhardness in comparison to W–Cr melted zone (about 700 HV0.05) resulted from partial melting of WC particles. The addition of a hard coating. In these two cases, the microhardness profiles were very similar. A further increase in the reinforcing phase contributed to a microhardness increase in the entire coating. The addition of 25% content of WC reinforcing phase (to 50% and 75%) in the W–Cr matrix resulted in an increase in WC (W–Cr/25%WC) did not significantly increase the microhardness in comparison to W–Cr coating. microhardness. However, if the unmelted WC particle microhardnesses were also taken into account, In these two cases, the microhardness profiles were very similar. A further increase in the content of the average microhardness of these coatings would be much higher. It should be noted that the iron WC reinforcing phase (to 50% and 75%) in the W–Cr matrix resulted in an increase in microhardness. that came from the substrate also had an influence on microhardness. The steel substrate also melted However, if the unmelted WC particle microhardnesses were also taken into account, the average and mixed with the precoat during the laser action. Therefore, the coatings produced in this way microhardness of these coatings would be much higher. It should be noted that the iron that came from would have a lower hardness than carbide coatings produced by diffusion or, e.g., thermal spraying. the substrate also had an influence on microhardness. The steel substrate also melted and mixed with This also applied to the coating made by laser processing of the 100% WC precoat. However, the the precoat during the laser action. Therefore, the coatings produced in this way would have a lower microhardness profile of all produced coatings was very favorable due to the observed gradual hardness than carbide coatings produced by diffusion or, e.g., thermal spraying. This also applied to decrease in microhardness from the surface to the substrate and an even microhardness in the area the coating made by laser processing of the 100% WC precoat. However, the microhardness profile of of the produced coating. all produced coatings was very favorable due to the observed gradual decrease in microhardness from the surface to the substrate and an even microhardness in the area of the produced coating.

Figure 11. MicrohardnessFigure 11. Microhardness of coatings produced of coatings using produced laser processing using laser of diprocessingfferent precoats of different applied precoat on s applied steel substrateon (omittingsteel substrate the larger (omitting and unmelted the larger WC and particles). unmelted WC particles). 3.3. Corrosion Resistance 3.3. Corrosion Resistance The corrosion resistance test results of the W–Cr metal matrix composite coatings reinforced The corrosion resistance test results of the W–Cr metal matrix composite coatings reinforced with WC particles, without WC particles, and with only WC are shown in Figure 12. The values with WC particles, without WC particles, and with only WC are shown in Figure 12. The values of of the electrochemical parameters (corrosion current and potential) were specified on the basis of the electrochemical parameters (corrosion current and potential) were specified on the basis of Tafel Tafel curves extrapolation and are shown in Table3. Analyzing the data from Table3 and curves curves extrapolation and are shown in Table 3. Analyzing the data from Table 3 and curves presented presented in Figure 12, it is visible that the smallest corrosion current was registered for the W–Cr in Figure 12, it is visible that the smallest corrosion current was registered for the W–Cr coating. This coating. This indicated the high corrosion resistance of this coating. The WC coating was inferior only indicated the high corrosion resistance of this coating. The WC coating was inferior only to the W–Cr to the W–Cr coating taking into account the corrosion resistance. Much worse results in the corrosion coating taking into account the corrosion resistance. Much worse results in the corrosion resistance resistance tests were recorded for the W–Cr metal matrix composite coatings reinforced with WC tests were recorded for the W–Cr metal matrix composite coatings reinforced with WC particles, as particles, as confirmed by the recorded greatest corrosion currents. After comparison of the corrosion confirmed by the recorded greatest corrosion currents. After comparison of the corrosion resistance resistance of all type of coatings, it was found that an increase in WC particle content contributed to a of all type of coatings, it was found that an increase in WC particle content contributed to a decrease decrease in corrosion resistance. in corrosion resistance.

Materials 2020, 13, x 14 of 22

Materials 2020, 13, 5272 14 of 21

Materials 2020, 13, x 14 of 22

Figure 12. Corrosion resistance tests results of coatings produced using laser processing of different precoats applied on steel substrate.

Figure 12. Corrosion resistance tests results of coatings produced using laser processing of different Table 3. CorrosionFigure 12.current Corrosion and corrosion resistance potential tests results of composite of coatings coatings produced produced using using laser processing laser of different precoats applied on steel substrate. processing ofprecoat precoats. applied on steel substrate. Table 3. Corrosion current and corrosion potential of composite coatings produced using laser Coating Type Current Icorr (A·cm2) Potential Ecorr (V) processing of precoat.Table 3. Corrosion current and corrosion potential of composite coatings produced using laser processing of precoat. W–Cr 2.72 × 10−6 −0.953 2 Coating Type Current Icorr (A cm ) Potential Ecorr (V) · W–Cr/25% WC Coating Type 5.68 × 10−6 Current Icorr (A·cm2) −1.04 Potential Ecorr (V) W–Cr 2.72 10 6 0.953 × − − W–Cr/50%W–Cr WC/25% WC W–Cr 1.745.68 × 1010−5 6 2.72 × 10−6 1.04−1.05 −0.953 × − − W–Cr/50% WC 1.74 10 5 1.05 −5− −6 W–Cr/75% WC W–Cr/25% WC 2.22 × ×10 5 5.68 × 10 − −1.05 −1.04 W–Cr/75% WC 2.22 10− 1.05 × 6 − WC W–Cr/50% WC 2.31 10−6− 1.74 × 10−5 1.01 −1.05 WC 2.31 × ×10 − −1.01 W–Cr/75% WC 2.22 × 10−5 −1.05 Figure 13 shows the alleged cause of the poor corrosion resistance of W W–Cr–Cr/75%/75% WC coatings. WC 2.31 × 10−6 −1.01 Observing the surface at very highhigh magnification,magnification, a largelarge numbernumber ofof secondarysecondary carbidecarbide precipitatesprecipitates withwith sizes sizes not not exceeding exceeding 1.5 1.5µm μm could could be identified be identified (Figure (Figure 13a). In 13 mosta). In cases, most the cases, size of the the size secondary of the precipitatessecondary precipitates was in theFigure was range in 13 the ofshows 0.3–0.6range the ofµ allegedm.0.3–0.6 This µm.cause created This of thecreate a verypoord a largecorrosion very numberlarge resistance number of corrosion of of corrosion W–Cr/75% cells WC coatings. contributingcells contributing to theObserving to reduction the reduction the in surface corrosion in corrosion at very resistance. high resistance. magnification, The dark The areasdark a large onareas the number on surface the surfaceof were secondary the were places carbidethe precipitates whereplaces corrosionwhere corrosion waswith observed. sizes was not observed. These exceeding areas These were1.5 areas μm much could were less bemuch common identified less in common the (Figure W–Cr in coating13 thea). InW (Figure– mostCr coating cases, 13b). the size of the It(Figure should 13b). be noted It shouldsecondary that be the noted secondaryprecipitates that the carbide secondarywas in precipitates the carbiderange of wereprecipitates 0.3–0.6 not µm. found were This in notcreate this found coating.d a veryin this large coating. number of corrosion cells contributing to the reduction in corrosion resistance. The dark areas on the surface were the places where corrosion was observed. These areas were much less common in the W–Cr coating (Figure 13b). It should be noted that the secondary carbide precipitates were not found in this coating.

Figure 13. Image of surface condition after corrosion tests at large magniﬁcation (10,000 ) for × (a) W–Cr/75% WC coating and (b) W–Cr coating.

Materials 2020, 13, x 15 of 22

Figure 13. Image of surface condition after corrosion tests at large magnification (10,000×) for (a) W– Cr/75% WC coating and (b) W–Cr coating. Materials 2020, 13, 5272 15 of 21

The surface condition and results of EDS mapping for the specimens after corrosion resistance tests Theare surfaceshown in condition Figure 14. and The results most of substantial EDS mapping effects for of the corrosion specimens are after shown corrosion in Figure resistance 14c–e, testswhere are corrosion shown in pits Figure could 14. Thebe seen most on substantial large areas effects of the of corrosion tested coating are shown. In these in Figure cases 14, c–e,the whereentire corrosionsurface of pits the couldcoating be was seen covered on large by areas oxides. of the Furthermore, tested coating. an increased In these cases, chlorine the entirecontent surface was found, of the coatingwhich was was a covered component by oxides. of the NaCl Furthermore, solution anin increasedwhich electrochemical chlorine content tests was were found, carried which out. wasThe aEDS component maps confirmed of the NaCl the obtained solution potentiodynamic in which electrochemical test results. tests Therefore, were carried it could out. be Theconfirmed EDS maps that confirmedthe W–Cr coating the obtained was characterized potentiodynamic by the test highest results. corrosion Therefore, resistance. it could beHere, confirmed the amount that theof oxides W–Cr coatingformed wason the characterized surface was by thesmall. highest The corrosionaddition resistance.of WC particles Here, theto the amount W–Cr of oxidesmatrix formedresulted on in the a surfacereduction was in small. corrosion The addition resistance. of WC In particles the case to of the the W–Cr WC matrix coating, resulted good in corrosion a reduction resistance in corrosion was resistance.obtained, which In the was case most of the likely WC coating,due to the good incorporation corrosion resistance of corrosio wasn-resistant obtained, tungsten which was carbides most likelyinto the due steel to thesubstrate incorporation without of introducing corrosion-resistant other additional tungsten phases carbides that into could the increase steel substrate the possibility without introducingof producing other corrosion additional cells. phases that could increase the possibility of producing corrosion cells.

Figure 14. Macroscopic images of the surface condition after corrosion tests and EDS mapping for the

following coatings: (a) W–Cr; (b) 100% WC; (c) W–Cr/25% WC; (d) W–Cr/50% WC; (e) W–Cr/75% WC. Materials 2020, 13, x 16 of 22

Figure 14. Macroscopic images of the surface condition after corrosion tests and EDS mapping for the following coatings: (a) W–Cr; (b) 100% WC; (c) W–Cr/25% WC; (d) W–Cr/50% WC; (e) W–Cr/75% WC. Materials 2020, 13, 5272 16 of 21 3.4. Wear Resistance 3.4. Wear ResistanceFigure 15 shows the influence of the chemical composition of produced coatings on the wear Figureresistance 15 shows under thedry inﬂuencefriction conditions. of the chemical It was found composition that the ofworst produced wear resistance coatings onwas the characteristic wear resistanceof coatings under drywhich friction did not conditions. have the It reinforcing was found phase that the in worstthe form wear of resistance tungsten wascarbide characteristic particles. Even of coatingsthe addition whichdid of 25% not haveWC particles the reinforcing contribute phased to in a the more form than of tungsten twofold carbidedecrease particles. in the wear Even loss the of the additioncoating. of 25% Each WC successive particles contributed increase in to the a more amount than of twofold WC particles decrease in inthe the W wear–Cr metal loss of matrix the coating. composite Each successivecoatings had increase a positive in the influence amount ofon WC wear particles resistance. in the The W–Cr W– metalCr coating matrix reinforced composite with coatings 75% WC had aparticles positive turnedinﬂuence out on to wear be the resistance. best among The theW–Cr specimens coating reinforced tested. The with coating 75% producedWC particles by laser turnedprocessing out to be theof the best WC among precoat the specimenswas slightly tested. worse. The coating produced by laser processing of the WC precoat was slightly worse.

Figure 15. Wear resistance of produced W–Cr metal matrix composite coatings reinforced with Figure 15. Wear resistance of produced W–Cr metal matrix composite coatings reinforced with WC WC particles. particles. It was found that microcutting was the dominant destruction process in all analyzed coatings It was found that microcutting was the dominant destruction process in all analyzed coatings with WC reinforced particles. This phenomenon occurred when the coating was located in phases with WC reinforced particles. This phenomenon occurred when the coating was located in phases characterized by significant differences in hardness, e.g., hard microparticles or particles interacting characterized by significant differences in hardness, e.g., hard microparticles or particles interacting like an abrasive. Microcutting after wear tests was parallel and was in accordance with the direction of like an abrasive. Microcutting after wear tests was parallel and was in accordance with the direction the counter-specimen movement. In the case of analyzed samples with different percentages of WC, of the counter-specimen movement. In the case of analyzed samples with different percentages of a proportional relationship between the microhardness and weight loss results could be observed. WC, a proportional relationship between the microhardness and weight loss results could be An increase in the coating microhardness contributed to obtaining smaller mass loss. Figure 16 shows observed. An increase in the coating microhardness contributed to obtaining smaller mass loss. the surface of the specimens after wear resistance tests, as well as EDS mapping results for produced Figure 16 shows the surface of the specimens after wear resistance tests, as well as EDS mapping coatings. For the W–Cr coating (Figure 16a), abrasive wear traces with grooving abrasion could be results for produced coatings. For the W–Cr coating (Figure 16a), abrasive wear traces with grooving observed. In these areas, an increased content of oxides was found. Figure 16b–e show the surface abrasion could be observed. In these areas, an increased content of oxides was found. Figure 16b–e condition of the specimens after wear tests and EDS mapping. These figures correspond to Fe–Cr/WC show the surface condition of the specimens after wear tests and EDS mapping. These figures coatings. It can be seen that the coatings reinforced with WC particles were characterized by better wear correspond to Fe–Cr/WC coatings. It can be seen that the coatings reinforced with WC particles were resistance than the W–Cr coatings without a reinforcing phase. Significant signs of wear were observed characterized by better wear resistance than the W–Cr coatings without a reinforcing phase. in WC coatings. As a result of abrasion, significant stresses contributed to cracks. Cracks were not Significant signs of wear were observed in WC coatings. As a result of abrasion, significant stresses observed on the cross-section of coatings under the microscope; thus, they probably arose during wear contributed to cracks. Cracks were not observed on the cross-section of coatings under the tests. All specimens with composite coatings with varying WC contents were characterized by a lower microscope; thus, they probably arose during wear tests. All specimens with composite coatings with content of oxidation products. The oxides formed on the surface were removed by microcutting with hard particles originating from the coating. The surface images after a wear test of WC–Ni laser-cladded coatings without or with vibration were presented in [4] where the authors found that the surface had numerous grooves. They found that the main type of coating damage was adhesive and abrasive Materials 2020, 13, x 17 of 22

varying WC contents were characterized by a lower content of oxidation products. The oxides formed on the surface were removed by microcutting with hard particles originating from the coating. The Materialssurface2020 images, 13, 5272 after a wear test of WC–Ni laser-cladded coatings without or with vibration17 were of 21 presented in [4] where the authors found that the surface had numerous grooves. They found that the main type of coating damage was adhesive and abrasive wear. They also confirmed the formation wear.of deep They grooves also conﬁrmed in the surface the formationafter the wear of deep test. grooves However, in the nosurface chemical after changes the wear were test. shown However, to be nocaused chemical by abrasion. changes wereMoreover, shown in to [ be6], causedthe authors by abrasion. studied Moreover, worn surfaces in [6 ], of the Fe authors-based coatingsstudied wornreinforced surfaces with of different Fe-based contents coatings of reinforced WC particles. with The diﬀ authorserent contents found that of WC there particles. were long, The deep authors, and founduniform that grooves, there were indicating long, deep, that microcutting and uniform was grooves, the main indicating mechanism that microcutting of wear. They was found the main that, mechanismwith increasing of wear. WC They particle found content, that, with the increasingmain wear WC mechanism particle content,of the worn the main surface wear was mechanism abrasive ofwear. the wornThe resulting surface wasgrooves abrasive were wear. shallower The resulting and slender grooveser, and were the shallower adhesive andwear slenderer, degree gradually and the adhesivedecreased. wear degree gradually decreased.

Figure 16. Macroscopic images of the surface condition after wear resistance tests and EDS mapping Figure 16. Macroscopic images of the surface condition after wear resistance tests and EDS mapping for the following coatings: (a) W–Cr; (b) W–Cr/25% WC; (c) W–Cr/50% WC; (d) W–Cr/75% WC; for the following coatings: (a) W–Cr; (b) W–Cr/25% WC; (c) W–Cr/50% WC; (d) W–Cr/75% WC; (e) (e) 100% WC. 100% WC. 3.5. Instrumented Indentation Test 3.5. Instrumented Indentation Test Example areas of the nanoindentation test are shown in Figure 17, while Figure 18 shows an exampleExample of areas a load–indentation of the nanoindentation depth curve test forare theshown W–Cr in /50%Figure WC 17, coating. while Figure The results18 show ofs allan measurementsexample of a areload presented–indentation in Table depth4. The curve loading–unloading for the W–Cr/50% cycle wasWC made coating. for three The resultsindentations of all formeasurements all studied specimens, are presented and all measurements in Table 4. The were loading repeatable.–unloading It can be cycle seen that was the made area around for three the carbideindentations had increased for all studied hardness. specimens It was, related and all to measurements the occurrence were of secondary repeatable. carbide It can phasesbe seen growing that the atarea the around carbide–matrix the carbide boundary. had increased These hardness. phases had It was an related increased to the tungsten occurrence content, of secondary as confirmed carbide by thephases EDS growing mapping at (Figure the carb 17idec).–matrix The produced boundary. coatings These had phases a higher had an Young’s increased modulus tungsten than content, the steel as substrate.confirmed This by the value EDS was mapping significantly (Figure increased 17c). The by produced the introduced coatings reinforcing had a higher phases, Young whose’s modulus Young’s modulus was threefold greater than the value for steel. This study shows that the coatings were not very susceptible to plastic deformation, as reflected in the wear resistance. The W–Cr coating was characterized by the greatest plastic deformation. As the amount of WC particles in the W–Cr metal MaterialsMaterials 2020 2020, ,13 13, ,x x 1818 of of 22 22 than the steel substrate. This value was significantly increased by the introduced reinforcing phases, whosethan the Young steel ’substrate.s modulus This was value threefold was significantlygreater than increasedthe value by for the steel. introduced This study reinforcing shows that phases, the coatingswhose Young were not’s modulus very susceptible was threefold to plastic greater deformation, than the asvalue reflected for steel. in the This wear study resistance. shows Thethat Wthe– Materials 2020, 13, 5272 18 of 21 Crcoatings coating were was not characterized very susceptible by the to greatest plastic deformation,plastic deformation. as reflected As the in theamount wear ofresistance. WC particles The W in– theCr coating W–Cr was metal characterized matrix composite by the greatest coating plastic increased, deformation. plastic As deformation the amount decreased.of WC particles Plastic in deformationmatrixthe W composite–Cr metalreached coating matrix the minimum increased, composite values plastic coating for deformation coatings increased, produced decreased. plastic by deformation Plastic laser processing deformation decrease of reachedthed . precoat Plastic the containingminimumdeformation values only reache WC for d coatingsparticles. the minimum produced values by laser for processingcoatings produced of the precoat by laser containing processing only of WC the particles. precoat containing only WC particles.

FigureFigure 17. 17. Example Example of of microstructure microstructure within within area area of of nanoindentation: nanoindentation: ( (aa))) WC WC carbide carbide in in W W W–Cr––CrCr matrix; matrix;matrix; ((bb)) thethe boundaryboundary betweenbetween carbidecarbide andand matrix;matrix; ( ((cc))) EDSEDSEDS mapping mappingmapping of ofof nanoindentation nanoindentationnanoindentation area. areaarea..

Figure 18. Example of the instrumented indentation test for W–Cr/50% WC coating; (a) in carbide; FigureFigure 18. 18. Example Example of of the the instrumented instrumented indentation indentation test test for for W W––Cr/50%Cr/50% WC WC coating; coating; ( (aa)) in in carbide; carbide; ( (bb)) (b) boundary between carbide and matrix; (c) matrix. boundaryboundary betweenbetween carbidecarbide andand matrix;matrix; ((cc)) matrixmatrix.. Table 4. Average results of the instrumented indentation test for produced coatings for the following Table 4. Average results of the instrumented indentation test for produced coatings for the following Tableareas: 4. carbide, Average boundary results of between the instrumented carbide and indentation matrix, and test matrix. for produced coatings for the following areas:areas: carbide,carbide, boundaryboundary betweenbetween carbidecarbide andand matrix,matrix, andand matrix.matrix. Vickers Young’s Plastic Type of MartensMartens HardnessVickers Young`s Plastic Type of Designation Martens 2VickersHardness Young`sModulus (E), DeformationPlastic TypeCoating of Designation Hardness(HM), (HM), N/ mm Hardness Modulus (E), Deformation Designation Hardness (HM), Hardness HV Modulus (E),GPa Deformationηplast,% Coating 2 Coating N/mm2 HV GPa ηplast, % 100% W–Cr MatrixN/mm 2154HV 256GPa 186ηplast 85.5, % 100%100% WW–– MatrixMatrix 21542154 13,502 225656 1967 118686 556 8855.562.9.5 CrCr Carbide–boundary W–Cr/25% WC 4889 643 242 76.3 matrix 1313,,502502 11967967 556556 62.962.9 WW––Cr/25Cr/25%% CarbideCarbide–– 4300 520 276 84.4 48848899 646433 242242 76.76.33 WCWC boundaryboundary matrixmatrix 14,048 2164 527 59.1 Carbide–boundary 43004300 552020 272766 84.484.4 W–Cr/50% WC 5506 748 247 72.3 matrix 1414,,048048 22164164 527527 59.59.11 WW––Cr/50Cr/50%% CarbideCarbide–– 4453 525 316 85.5 55065506 748748 247247 72.72.33 WCWC boundaryboundary matrixmatrix 14,692 2259 582 59.6 Carbide–boundary 44534453 525525 316316 85.85.55 W–Cr/75% WC 5775 807 232 70.2 matrix 1414,,696922 22592259 585822 59.659.6 WW––Cr/75Cr/75%% CarbideCarbide–– 4756 589 285 79.6 57755775 807807 232232 70.70.22 WCWC boundaryboundary matrixmatrix 15,342 2385 602 60.4 Carbide–boundary 47564756 589589 282855 79.679.6 100% WC 6349 930 225 63.1 matrix 6027 857 229 67.9

Materials 2020, 13, 5272 19 of 21

4. Conclusions Laser processing enables the production of composite coatings characterized by unique properties in a relatively short time in comparison to diﬀusion methods. According to the results of this study, the following conclusions can be drawn:

increasing the content of WC particles reduces the thickness of the W–Cr/WC coatings; − increasing the content of WC particles contributes to increasing the microhardness of the produced − W–Cr metal matrix composite coatings; increasing the content of WC particles in the W–Cr metal matrix composite coatings contributes − to increasing wear resistance; on the surface of the coating containing 100% WC, cracks were observed after a wear test. These cracks − were probably formed during the friction process, because no cracks were observed during microstructure observation. These cracks led to reducing the wear resistance. Despite this, the wear resistance of 100% WC coatings was quite high; increasing the amount of WC as the reinforcing phase in the W–Cr composite coating reduced − corrosion resistance. This was due to an increase in the number of corrosion cells in the entire coating; as a result of laser processing of precoats consisting of W, Cr, and WC particles, newly formed − coatings rich in primary and secondary tungsten carbides (WC, W2C, M7C3) were obtained. These were carbide phases characterized by high microhardness.

Author Contributions: Conceptualization, D.B.; methodology, A.B. and D.B.; investigation, A.B., D.B., P.P., J.H., and A.P.; writing—original draft preparation, D.B.; writing—review and editing, D.B. and A.B.; visualization, D.B. and A.B. All authors read and agreed to the published version of the manuscript. Funding: The presented research results were funded by grants for education allocated by the Ministry of Science and Higher Education in Poland. Acknowledgments: The authors would like to thank Piotr Siwak for his help in nanoindentation tests. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Czerwinski, F. (Ed.) Thermochemical treatment of metals. In Heat Treatment—Conventional and Novel Application; InTech Publisher: Rijeka, Croatia, 2012. 2. Davis, J.R. Surface Engineering for Corrosion and Wear Resistance; ASM International: Materials Park, OH, USA, 2001. 3. St-Georges, L. Development and characterization of composite Ni–Cr +WC laser cladding. Wear 2007, 263, 562–566. [CrossRef] 4. Li, C.; Zhang, Q.; Wang, F.; Deng, P.; Lu, Q.; Zhang, Y.; Li, S.; Ma, P.; Li, W.; Wang, Y. Microstructure and wear behaviors of WC-Ni coatings fabricated by laser cladding under high frequency micro-vibration. Appl. Surf. Sci. 2019, 485, 513–519. [CrossRef] 5. Fenech, M.; Grech, M.; Betts, J.C. The in-ﬂight temperature variation and dissolution of WC powder particles producing an Fe–Cr–W–C system by direct laser deposition. Surf. Coat. Technol. 2012, 207, 211–217. [CrossRef] 6. Lu, J.Z.; Cao, J.; Lu, H.F.; Zhang, L.Y.; Luoa, K.Y. Wear properties and microstructural analyses of Fe-based coatings with various WC contents on H13 die steel by laser cladding. Surf. Coat. Technol. 2019, 369, 228–237. [CrossRef] 7. Zhou, S.; Dai, X.; Zheng, H. Microstructure and wear resistance of Fe-based WC coating by multi-track overlapping laser induction hybrid rapid cladding. Opt. Laser Technol. 2012, 44, 190–197. [CrossRef] 8. Zhu, S.M.; Zhang, Y.D. The microstructure and wear-resistant properties of laser cladding Ni-based WC alloy on Q345 steel surface. Appl. Mech. Mater. 2014, 556–562, 189–192. [CrossRef] 9. Guo, C.; Zhou, J.; Chen, J.; Zhao, J.; Yu, Y.; Zhou, H. High temperature wear resistance of laser cladding NiCrBSi and NiCrBSi/WC-Ni composite coatings. Wear 2011, 270, 492–498. [CrossRef] Materials 2020, 13, 5272 20 of 21

10. Sears, J.W.; Costello, A. Laser powder deposition of WC materials for improved wear resistance. In Advances in Powder Metallurgy and Particulate Materials-2007, Proceedings of the 2007 International Conference on Powder Metallurgy and Particulate Materials, PowderMet 2007, Denver, CO, USA, 13–16 May 2007; Metal Powder Industries Federation: Princeton, NJ, USA, 2007; pp. 882–889. [CrossRef] 11. Verwimp, J.; Rombouts, M.; Geerinck, E.; Motmans, F. Applications of laser cladded WC-based wear resistant coatings. Phys. Procedia 2011, 12, 330–337. [CrossRef] 12. Wu, P.; Du, H.M.; Chen, X.L.; Bai, H.L.; Jiang, E.Y. Influence of WC particle behavior on the wear resistance properties of Ni–WC composite coatings. Wear 2004, 257, 142–147. [CrossRef] 13. Abbas, G.; West, D.R.F. Laser surface cladding of stellite and stellite-SiC composite deposits for enhanced hardness and wear. Wear 1991, 143, 353–363. [CrossRef] 14. Abbas, G.; West, D.R.F.; Steen, W.M. Wear studies of variable composition stellite-SiC laser clad deposits. Key Eng. Mater. 1990, 46, 447–454. [CrossRef] 15. Nurminen, J.; Näkki, J.; Vuoristo, P. Microstructure and properties of hard and wear resistant MMC coatings deposited by laser cladding. Int. J. Refract. Met. Hard Mater. 2009, 27, 472–478. [CrossRef] 16. Bartkowski, D.; Bartkowska, A. Wear resistance in the soil of Stellite-6/WC coatings produced using laser cladding method. Int. J. Refract. Met. Hard Mater. 2017, 64, 20–26. [CrossRef] 17. Singh, J.; Chatha, S.S.; Sidhu, B.S. Effect of alloying elements in Fe-based hardfacings to improve the abrasive wear behavior of ploughshares. J. Mech. Eng. Sci. 2020, 234, 4737–4750. [CrossRef] 18. Singh, J.; Chatha, S.S.; Sidhu, B.S. Abrasive wear characteristics and microstructure of Fe-based overlaid ploughshares in different field conditions. Soil Tillage Res. 2021, 205, 104771. [CrossRef] 19. Arai, T.; Fujita, H.; Sugimoto, Y.; Ohta, Y. Diffusion carbide coatings formed in molten borax systems. J. Mater. Eng. 1987, 9, 183–189. [CrossRef] 20. Bartkowska, A.; Swad´zba,R.; Popławski, M.; Bartkowski, D. Microstructure, microhardness, phase

analysis and chemical composition of laser remelted FeB-Fe2B surface layers produced on Vanadis-6 steel. Opt. Laser Technol. 2016, 86, 115–125. [CrossRef] 21. Afzal, M.; Ajmal, M.; Nusair Khan, A.; Hussain, A.; Akhter, R. Surface modification of air plasma spraying WC–12%Co cermet coating. Opt. Laser Technol. 2014, 56, 202–206. [CrossRef] 22. Hajkowski, J.; Popielarski, P.; Ignaszak, Z. Cellular automaton finite element method applied for microstructure prediction of aluminium casting treated by laser beam. Arch. Foundry Eng. 2019, 19, 111–118. 23. Wojciechowski, S.; Przestacki, D.; Chwalczuk, T. The evaluation of surface integrity during machining of Inconel 718 with various laser assistance strategies. MATEC Web Conf. 2017, 136, 01006. [CrossRef] 24. Chwalczuk, T.; Przestacki, D.; Szablewski, P.; Felusiak, A. Microstructure characterisation of Inconel 718 after laser assisted turning. MATEC Web Conf. 2018, 188, 01006. [CrossRef] 25. Baidridge, T.; Poling, G.; Foroozmehr, E.; Kadekar, V.; Gupta, M.C. Laser cladding of Inconel 690 on Inconel 600 superalloy for corrosion protection in nuclear applications. Opt. Lasers Eng. 2013, 51, 180–184. [CrossRef] 26. Shu, D.; Li, Z.; Zhang, K.; Yao, C.; Li, D.; Dai, Z. In situ synthesized high volume fraction WC reinforced Ni-based coating by laser cladding. Mater. Lett. 2017, 195, 178–181. [CrossRef] 27. Zhou, S.; Xu, Y.; Liao, B.; Sun, Y.; Dai, X.; Yang, J.; Li, Z. Effect of laser remelting on microstructure and properties of WC reinforced Fe-based amorphous composite coatings by laser cladding. Opt. Laser Technol. 2018, 103, 8–16. [CrossRef] 28. Zhong, M.; Liu, W.; Yao, K.; Goussain, J.-C.; Mayer, C.; Becker, A. Microstructural evolution in high power laser cladding of Stellite 6+WC layers. Surf. Coat. Technol. 2002, 157, 128–137. [CrossRef] 29. Zhou, S.; Zeng, X. Growth characteristics and mechanism of carbides precipitated in WC–Fe composite coatings by laser induction hybrid rapid cladding. J. Alloys Compd. 2010, 505, 685–691. [CrossRef] 30. Zhou, S.; Huang, Y.; Zeng, X. A study of Ni-based WC composite coatings by laser induction hybrid rapid cladding with elliptical spot. Appl. Surf. Sci. 2008, 254, 3110–3119. [CrossRef] 31. Díaz, E.; Amado, J.M.; Montero, J.; Tobar, M.J.; Yáñez, A. Comparative study of Co-based alloys in repairing low Cr-Mo steel components by laser cladding. Phys. Procedia 2012, 39, 368–375. [CrossRef] 32. Farnia, A.; Malek Ghaini, F.; Ocelík, V.; De Hosson, J.T.M. Microstructural characterization of Co-based coating deposited by low power pulse laser cladding. J. Mater. Sci. 2013, 48, 2714–2723. [CrossRef] 33. Lin, W.C.; Chen, C. Characteristics of thin surface layers of cobalt-based alloys deposited by laser cladding. Surf. Coat. Technol. 2006, 200, 4557–4563. [CrossRef] Materials 2020, 13, 5272 21 of 21

34. Liu, F.; Liu, C.; Chen, S.; Tao, X.; Zhang, Y. Laser cladding Ni–Co duplex coating on copper substrate. Opt. Lasers Eng. 2010, 48, 792–799. [CrossRef] 35. Ocelík, V.; de Oliveira, U.; de Boer, M.; de Hosson, J.T.M. Thick Co-based coating on cast iron by side laser cladding: Analysis of processing conditions and coating properties. Surf. Coat. Technol. 2007, 201, 5875–5883. [CrossRef] 36. Daiß, S.; Bischoﬀ, E.; Grünenwald, B. Metallographic characterisation of stellite/WC composite layers produced by laser cladding. Pract. Metallogr. 1996, 33, 99–111. [CrossRef] 37. Janicki, D. High Power Direct Diode Laser cladding of Stellite 6 + WC coatings. Mach. Technol. Mater. 2012, 7, 27–30. 38. Chao, M.-J.; Niu, X.; Yuan, B.; Liang, E.-J.; Wang, D.-S. Preparation and characterization of in situ synthesized

B4C particulate reinforced nickel composite coatings by laser cladding. Surf. Coat. Technol. 2006, 201, 1102–1108. [CrossRef] 39. Fan, X.H.; Geng, L.; Xu, B.; Li, J. Laser cladding NiCrBSi+2%B4C coating on Ti-6Al-4V. Adv. Mater. Res. 2009, 79–82, 473–476. [CrossRef] 40. Bartkowski, D.; Młynarczak, A.; Piasecki, A.; Dudziak, B.; Go´scia´nski,M.; Bartkowska, A. Microstructure, microhardness and corrosion resistance of Stellite-6 coatings reinforced with WC particles using laser cladding. Opt. Laser Technol. 2015, 68, 191–201. [CrossRef] 41. Bartkowski, D.; Bartkowska, A.; Popławski, M.; Przestacki, D. Microstructure, microhardness, corrosion and wear resistance of B, Si and B–Si coatings produced on C45 steel using laser processing. Metals 2020, 10, 792. [CrossRef] 42. Sampedro, J.; Caecel, B.; Ramos, A.; Amigó, V. Laser Cladding of TiC for Better Titanium Components. Phys. Procedia 2011, 12, 313–322. [CrossRef] 43. ISO 14577-1:2015. Metallic Materials—Instrumented Indentation Test for Hardness and Materials Parameters—Part 1: Test Method; International Organization for Standardization: Geneva, Switzerland, 2015.

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