materials

Article Application of a Novel CVD on a Biomedical Co–Cr : An Evaluation of Coating Layer and Substrate Characteristics

Si Hoon Song 1, Bong Ki Min 2,*, Min-Ho Hong 3 and Tae-Yub Kwon 4,*

1 Department of Medical & Biological Engineering, Graduate School, Kyungpook National University, Daegu 41940, Korea; [email protected] 2 Center for Research Facilities, Yeungnam University, Gyeongsan 38541, Korea 3 Department of Dental Laboratory Science, College of Health Sciences, Catholic University of Pusan, Busan 46252, Korea; [email protected] 4 Department of Dental Biomaterials, School of Dentistry and Institute for Biomaterials Research & Development, Kyungpook National University, Daegu 41940, Korea * Correspondence: [email protected] (B.K.M.); [email protected] (T.-Y.K.); Tel.: +82-53-810-1835 (B.K.M.); +82-53-660-6891 (T.-Y.K.)  Received: 9 February 2020; Accepted: 2 March 2020; Published: 5 March 2020 

Abstract: (TiN) was deposited on the surface of a cobalt– (Co–Cr) alloy by a hot-wall type chemical vapor deposition (CVD) reactor at 850 ◦C, and the coating characteristics were compared with those of a physical vapor deposition (PVD) TiN coating deposited on the same alloy at 450 ◦C. Neither coating showed any reactions at the interface. The face-centered cubic (fcc) structure of the alloy was changed into a hexagonal close-packed (hcp) phase, and recrystallization occurred over at 10 µm of depth from the surface after CVD coating. Characteristic precipitates were also generated incrementally depending on the depth, unlike the precipitates in the matrix of the as-cast alloy. On the other hand, the microstructure and phase of the PVD-coated alloy did not change. Depth-dependent nano-hardness measurements showed a greater increase in hardness in the recrystallization zone of the CVD-coated alloy than in the bulk center of the alloy. The CVD coating showed superior adhesion to the PVD coating in the progressive scratch test. The as-cast, PVD-coated, and CVD-coated alloys all showed negative cytotoxicity. Within the limitations of this study, CVD TiN coating to biomedical Co–Cr alloy may be considered a promising alternative to PVD technique.

Keywords: cobalt–chromium alloy; titanium nitride; physical vapor deposition; chemical vapor deposition; microstructure

1. Introduction Among various biomaterials, cobalt–chromium (Co–Cr) alloys have been widely used as commercial orthopedic materials due to their excellent biochemical properties, moderate strength, and their resistance to corrosion and abrasion. Among these properties, the biochemical stability of the alloys is known to be due to the passive film on their surface [1,2]. Due to long-term fatigue friction and the subsequent wear of orthopedic implants, however, this passive film may become unstable and, as a result, allow metal ions released from the alloys into the body fluids (serum, urine, etc.) to accumulate between the implant and the tissues, causing an inflammatory reaction in the surrounding tissue [3,4]. Such metal ions and wear debris, concentrated at the implant–tissue interface, may migrate through the tissue, resulting in clinical implant failure, osteolysis, cutaneous allergic reactions, and remote site accumulations [4].

Materials 2020, 13, 1145; doi:10.3390/ma13051145 www.mdpi.com/journal/materials Materials 2020, 13, 1145 2 of 14

Given the increased clinical use of Co–Cr alloys as an implant material, many attempts, including coating, have been made to compensate for these problems. Among these, various physical vapor deposition (PVD) titanium nitride (TiN) coating methods, such as sputtering and arc-ion plating, have been extensively studied and are now commercially available [5,6]. A few microns of TiN coating increases the surface hardness of the implant, improves the mechanical durability and fatigue resistance by reducing the friction coefficient, and effectively prevents metal ions from leaching into the surrounding tissue by creating a dense film resistant to acids and bases [7–12]. Since PVD is a directional deposition method, it may not uniformly cover all surfaces of the implant due to the shadow effect and often creates variations in coating thickness due to poor step coverage. In addition, the method cannot effectively coat the entire inner surface of the implants. The coated layer formed by PVD at low temperature has fundamentally limited adhesion. In particular, the strong stress inevitably generated during the coating procedure may cause peeling and buckling of the coating layer at the sharp edges of the implant [13]. Chemical vapor deposition (CVD) can compensate for the above main disadvantages of PVD technique. CVD can cover all surfaces (inner surfaces as well as outer) of the implant with a relatively uniform thickness. If the coefficient of the implant is greater than that of the coating material, CVD can generate a strong compressive stress on the surface of the implant, enhancing the adhesion. The high purity gaseous raw materials allow high purity to be made. In addition, the coating morphology and specific surface area can be controlled by deposition conditions [14,15]. However, the high coating temperature and some corrosive gas by-products generated during the coating process may cause an undesirable reaction at the implant–coating interface depending on the chemical composition of the implant substrate and its reactivity. Recently, von Fieandt et al. [16] studied the phase content, growth rate, morphology, and microstructure of the CVD of TiN on three different pure transition metal substrates (Fe, Co, and Ni) from a TiCl4-N2-H2 gas system. To date, however, little research has been done on the application of CVD TiN coating to the Co–Cr alloy used as an orthopedic implant material. In this study, TiN was coated on a Co–Cr-based alloy using hot-wall type CVD equipment and the quality of the coating layer as well as potential factors affecting the physical properties of the coating-substrate interface were investigated. CVD coating characteristics were also compared with those of a commercially available PVD coating.

2. Materials and Methods

2.1. Specimen Preparation Disc-shaped Co–Cr alloy specimens (10 mm in diameter and 3 mm in thickness) were prepared using casting. A disc model was designed with computer-aided design (CAD) software (AutoDesk Inventor 2015, Student Version, Autodesk, San Rafael, CA, USA), and wax patterns (VisiJet® M3 Dentcast, 3D Systems, Rock Hill, SC, USA) were prepared from the CAD data using a multi-material 3D Printer (ProJet® 5500X, 3D Systems, Rock Hill, SC, USA) [17]. The patterns were embedded in a phosphate-bonded investment material (Univest Non-Precious, Shofu Inc., Kyoto, Japan), and were then cast using the Co–Cr alloy (StarLoy C, DeguDent, Hanau-Wolfgang, Germany; composition (in wt%): Co 59.4, Cr 24.5, W 10, Nb 2, V 2, Mo 1, Si 1, Fe 0.1) with a centrifugal casting apparatus (Centrifico Casting Machine, Kerr Corp., Orange, CA, USA) [17]. One flat surface of each specimen was mirror-polished with wet silicon paper (up to 2000-grit), with 9, 3, and 1 µm diamond suspensions (Allied High Tech Products, Rancho Dominguez, CA, USA), and finally with 50 nm colloidal silica (Allied High Tech Products) on a polishing cloth using a rotary grinding/polishing machine (M-Prep 3, Allied High Tech Products). All of the polished specimens were cleaned with ethanol in an ultrasonic water bath for 20 min prior to coating. Materials 2020, 13, 1145 3 of 14

2.2. PVD and CVD Coating The disc Co–Cr alloy specimens were subjected to either PVD- or CVD-coating processes (Table1). The PVD coating was performed in a commercial arc-ion plating (AIP) reactor. The TiN coating was deposited at 550 ◦C. After the deposition temperature was reached in an argon atmosphere, the surface was etched to enhance coating adhesion. During deposition of TiN, a 30-volt bias was applied to the specimens using a Ti target and N2 gas for 30 min.

TableMaterials 1. 2018Main, 11 deposition, x FOR PEERconditions REVIEW of physical vapor deposition (PVD) and chemical vapor deposition3 of 14 (CVD) coatings. was deposited at 550 °C. After the deposition temperature was reached in an argon atmosphere, the surface wasTemperature etched to enhanceCoating coating adhesion.Pressure During depositionBias Voltage of TiN, a 30-volt bias was applied Coating Target Gas to the specimens( ◦usingC) a Ti targetTime (min)and N2 gas for(mbar) 30 min. (V)

PVD 550 30 0.025 60 Ti N2 100% Table 1. Main deposition conditions of physical vapor deposition (PVD) and chemical vapor deposition (CVD) coatings. H2 59.5%; 1 CVD 850 400 150 N/A N/A N2 39.0%; Temperature Coating Pressure Bias Voltage TiCl 1.5% Coating Target Gas 4 (°C) Time (min) 1 Not(mbar) applicable. (V) PVD 550 30 0.025 60 Ti N2 100% H2 59.5%; N2 CVD 850 400 150 N/A 1 N/A 39.0%; TiCl4 1.5% The CVD TiN coating was performed using a hot-wall type CVD reactor in which all gas switchgear 1 Not applicable. and flow rates were controlled by a computer system. Figure1 shows a schematic diagram of the CVD reactor,The CVD in which TiN coating N2 and was H2 performedwere fed intousing the a ho gast-wall mixing type chamberCVD reactor ina in gaseous which all state gas from reservoirsswitchgear (a) and and (b),flow respectively. rates were controlled TiCl4 wasby a computer passed from system. the Figure reservoir 1 shows (c) a toschematic a liquid diagram statevia a liquidof massthe CVD flow reactor, meter, in fedwhich into N2 aand mixer, H2 were vaporized fed into the inside gas mixing the mixing chamber chamber, in a gaseous and state finally from mixed with otherreservoirs gases. (a) and The (b), mixed respectively. gas fed intoTiCl4 thewas furnace passed from was the uniformly reservoir applied (c) to a liquid in the state holes via ofa liquid a rotating graphitemass tube flow (h)meter, located fed into in a a mixer, straight vaporized line from inside the the bottom mixing tochamber, the top and of finally the coating mixed chamber.with other The unreactedgases. gasesThe mixed and gas byproducts fed into the were furnace exhausted was uniformly and collected applied in via the a holes liquefying of a rotating apparatus graphite (i) and tube (h) located in a straight line from the bottom to the top of the coating chamber. The unreacted pump (j). All the inside of the reactor including the graphite fixtures had been pre-coated gases and byproducts were exhausted and collected via a liquefying apparatus (i) and vacuum pump with TiN to eliminate the influence of carbon during the deposition process. The TiN coating was (j). All the inside of the reactor including the graphite fixtures had been pre-coated with TiN to depositedeliminate at 850 the ◦influenceC to minimize of carbon thermal during deformationthe deposition and process. change The inTiN the coating physical wasproperties deposited at of the alloy850 while °C obtainingto minimize a properthermal coatingdeformation thickness. and change The furnacein the physical was heated properties from of room the alloy temperature while to the depositionobtaining a temperature proper coating of 850thickness.◦C in aThe hydrogen furnace atmosphere.was heated from After room that, temperature TiN was deposited to the for 400 mindeposition at a specified temperature gas ratio. of 850 The °C furnacein a hydrogen was then atmosphere. fast-cooled After down that, to TiN 300 was◦C usingdeposited a hydrogen for 400 gas and allowedmin at a tospecified slow-cool gas toratio. room The temperature. furnace was then Excess fast-cooled hydrogen down was to used 300 °C as using a carrier a hydrogen for transporting gas and allowed to slow-cool to room temperature. Excess hydrogen was used as a carrier for other gases and, at the same time, to sequentially dissociate Cl from TiCl4, allowing Ti ions to react transporting other gases and, at the same time, to sequentially dissociate Cl from TiCl4, allowing Ti with N2. In addition, the amount of N2 was determined by considering the reaction rate and activation ions to react with N2. In addition, the amount of N2 was determined by considering the reaction rate energy with TiCl4 [18]. The main reaction scheme of TiN coating in the reactor during deposition is as and activation energy with TiCl4 [18]. The main reaction scheme of TiN coating in the reactor during follows [16]: TiCl4 (g) + 1/2N2 (g) + 2H2 (g) TiN (s) + 4HCl (g). deposition is as follows [16]: TiCl4 (g) + 1/2N→2 (g) + 2H2 (g) → TiN (s) + 4HCl (g).

FigureFigure 1. Schematic 1. Schematic illustration illustration ofof thethe CVD furnace furnace used: used: a: N a:2 feeder; N2 feeder; b: H2 feeder; b: H2 c:feeder; liquid TiCl c: liquid4 TiCl4feeder;feeder; d: d: liquid liquid mass mass flow flow controller controller (LMFC); (LMFC); e: TiCl e: TiCl4 vaporization4 vaporization and gas and mixing gas mixing apparatus; apparatus; f: f: heatingheating element; element; g: g: specimen; specimen; h: h: rotaryrotary type gas gas distribution distribution tube; tube; i: gas i: gas by-product by-product liquefier; liquefier; j: liquid j: liquid rotaryrotary vacuum vacuum pump pump (LRVP); (LRVP); and and k: k: exhaust exhaust of of thethe neutralizedneutralized material. material.

2.3. Characterization For the Co–Cr alloy substrate and either PVD or CVD TiN-coated specimens, phase identification was carried out by X-ray diffractometry (XRD, X’Pert PRO, PANalytical, The Netherlands), using Cu Kα radiation (λ = 0.1541 nm) at an accelerating voltage of 40 kV, a beam

Materials 2020, 13, 1145 4 of 14

2.3. Characterization For the Co–Cr alloy substrate and either PVD or CVD TiN-coated specimens, phase identification was carried out by X-ray diffractometry (XRD, X’Pert PRO, PANalytical, The Netherlands), using Cu Kα radiation (λ = 0.1541 nm) at an accelerating voltage of 40 kV, a beam current of 30 mA, a 2θ-angle scan range of 20◦ to 130◦, a scanning speed of 2◦/min, a sampling pitch of 0.02◦, and a preset time of 0.6 s [19]. The disc specimens before and after coating were cross-sectioned, and the sectioned surfaces were observed using field emission scanning electron microscopy (FE-SEM, Merlin, Carl Zeiss AG, Germany) after mirror polishing as described above. To analyze the elemental composition and gains associated with the microstructure for the region close to the surface of the alloy, the specimens for each group were examined using FE-SEM with energy-dispersive X-ray spectroscopy (EDS, X-MAX, Oxford Instruments, Abingdon, UK) under an accelerating voltage of 20 kV. To determine the crystal phase and crystallographic orientation, electron backscattered diffraction (EBSD) scans were performed on the FE-SEM equipped with a Nordlys Nano EBSD detector (Oxford Instruments). The EBSD data were analyzed using AZtec (Version 3.4, Oxford Instruments) and Channel 5 (Version 5.12.72.0, Oxford Instruments) software. To determine potential change in the mechanical properties of the Co–Cr alloy substrate after coating, the nano-hardness of the region close to the surface in the cross-sections was measured using a table top nano-indentation testing machine (CHQ11-NV10, Nanovea, Irvine, CA, USA). Nano-indentations with an area of 60 60 µm were made at 10, 25, and 45 µm from the surface and also × the bulk center of each specimen. The maximum load was 50 mN, the indentation rate was 0.67 mN/s, and the indentation speed was 33 nm/s. The results were statistically analyzed using two-way analysis of variance (ANOVA), followed by Tukey’s post hoc test at α = 0.05. To evaluate the adhesion between the coating and alloy, a progressive scratch test was performed on the coated alloy surface using a diamond cone (Revetest Scratch Tester, CSM Instruments, Needham, MA, USA). The load was progressively raised from 1 to 30 N with a range of 2 mm (progressive test mode) and a speed of 6 mm/min.

2.4. In Vitro Cytotoxicity Test The cytotoxicity of the alloys was examined using the agar-diffusion method. A positive control of rubber latex and a negative control of high- polyethylene (HDPE) with the same dimensions as the alloy specimen were also assayed. A total of 10 mL of a 4 104 L929 fibroblast cell suspension × were seeded in 100-mm dishes and incubated at 37 ◦C and 5% CO2 for 48 h. The medium was replaced with 10 mL freshly prepared agar/nutrient medium. A total of 10 mL of neutral-red (Sigma, Madrid, Spain) solution was placed on the agar surface for 10 min in the dark. After the excess dye was removed, the alloy specimens, together with the positive and negative controls, were placed on the agar surface and the dishes were incubated for 24 h. Cytotoxicity was determined by measuring the zone of cell inhibition around the specimens. In addition, the cell cultures around the specimens were then examined under a phase-contrast microscope.

3. Results and Discussion

3.1. Phase Identification Figure2 shows the XRD patterns of the Co–Cr alloy used as the substrate for PVD or CVD coating and TiN-deposited alloys. The matrix phase of the cast Co–Cr alloy is known to be a mainly Co- and Cr-rich γ- (face-centered cubic, fcc) phase and that the precipitate is an ε (hexagonal close-packed, hcp) phase containing W and Nb in the Co–Cr matrix. In this study, typical γ- and ε-phase diffraction patterns were detected in the Co–Cr alloy substrate (Figure2a). All of the di ffraction peaks were indexed by the diffraction peaks of the γ-phase (ICSD: PAN 98-007-2476) and the ε-phase (ICSD: PAN 98-008-7135), additionally taking into account the peak displacements and the indexing data of Materials 2020, 13, 1145 5 of 14

previous XRD studies [17,20,21]. In addition, the peak appearing near 37.7◦ was indexed by the Nb-rich γ-phase (ICSD: PAN 98-008-6919) [17]. The peaks that arbitrarily present without being affected by other peaks at the γ-phase were (111), (200), and (222), the peak with the highest diffraction intensity being (111). The peaks that appeared on the precipitated ε-phase were (010), (011), and (013), the peak

Materialswith the 2018 highest, 11, x FOR intensity PEER REVIEW being(011). 5 of 14

Figure 2.2. X-rayX-ray di diffractometryffractometry (XRD) (XRD) patterns patterns and phasesand ph ofases the mirror-polishedof the mirror-polished Co–Cr alloy Co–Cr (substrate) alloy (substrate)specimen ( aspecimen) and the ( PVDa) and (b )the and PVD CVD (b) ( cand) TiN-deposited CVD (c) TiN-deposited specimens. specimens.

In the PVD specimen (Figure 22b),b), thethe intensityintensity ofof thethe TiNTiN coatingcoating (200)(200) waswas significantlysignificantly higherhigher than that of the matrix, and the intensity ratio of the coating/substrate coating/substrate was much higher than that of the CVD TiN-coated alloy (Figure 22c).c). SuchSuch phenomenaphenomena maymay havehave beenbeen duedue toto thethe highlyhighly texturedtextured one-directional coating and the relatively smooth coating surface. In the the CVD CVD TiN-coated TiN-coated specimen specimen (Figure (Figure 2c),2c), th thee intensity intensity of of the the peaks, peaks, including including the the matrix matrix γ- phaseγ-phase (111) (111) peaks peaks (positioned (positioned on on the the three three vertical vertical dotted dotted lines), lines), was was significantly significantly reduced, reduced, with with only only the ε-phase and TiN (ICDD: 03-065) being detected. The The X-ray diffraction diffraction has a limited penetration depth when diffracted diffracted into a material and, in most cases, presents only the surface properties of the material. The The intensity intensity depends depends on on the the state state of of the the surface, surface, the the density density of of the the material, material, crystallinity, crystallinity, and the the absorption absorption rate rate indicated indicated by by the the wavelength wavelength of the of theX-ray. X-ray. The diffraction The diffraction pattern pattern of the of CVD the TiN-coatedCVD TiN-coated specimen specimen indicates indicates that thatat least at least the thephases phases below below the the TiN-coating TiN-coating layer layer were were mainly mainly composed of the ε-phase, suggesting that γ ε phase transformation or the precipitation of the composed of the ε-phase, suggesting that γ →→ ε phase transformation or the precipitation of the ε- phaseε-phase occurred occurred during during the the CVD CVD coating. coating. 3.2. Microstructure 3.2. Microstructure Figure3 shows the SEM images of the as-cast Co–Cr alloy and its EDS mapping images. The Figure 3 shows the SEM images of the as-cast Co–Cr alloy and its EDS mapping images. The mirror-polished surface of the alloy showed a dendritic structure, composed of the matrix (dark) and mirror-polished surface of the alloy showed a dendritic structure, composed of the matrix (dark) and precipitates (bright), with some small pores (black) [17,22]. The EDS mapping images showed the precipitates (bright), with some small pores (black) [17,22]. The EDS mapping images showed the matrix of the alloy to be mainly composed of Co–Cr. Most of the secondary phase precipitates with a matrix of the alloy to be mainly composed of Co–Cr. Most of the secondary phase precipitates with a dendritic structure consisted of W-Nb-rich Co–Cr. As indicated by the arrows in (Figure3d–f), Nb and dendritic structure consisted of W-Nb-rich Co–Cr. As indicated by the arrows in (e), (e), and (f), Nb V-based precipitates without W were observed as another secondary phase. and V-based precipitates without W were observed as another secondary phase.

Materials 2020, 13, 1145 6 of 14 MaterialsMaterials 20182018,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 66 ofof 1414

FigureFigure 3. 3. Scanning Scanning electron electron microscopy microscopy (SEM) (SEM) micrographs micrographs of of the the mirro mirror-polishedmirror-polishedr-polished as-cast as-cast Co–Cr Co–Cr alloy alloy ((aa)) andand itsitsits energy-dispersiveenergy-dispersiveenergy-dispersive X-ray X-ray spectroscopy spectroscopy (EDS) (EDS) mapping mapping images imagesimages ( ((bb–––fff):):): ( (bb)) Co; Co; ( (cc)) Cr; Cr; ( ((dd)) W; W;W; ((ee)) Nb;Nb; andand ((ff)) V.V. NbNbNb andandand V-basedV-basedV-based precipitatesprecipitatesprecipitates withoutwithoutwithout WWW (arrow(arrow(arrow inin (((dd)))))) werewerewere observedobservedobserved asas anotheranother secondarysecondary phasephase (arrows(arrows inin ((ee)) andandand (((fff)).)).)).

FigureFigure 4 44 showsshows the the low-low- low- andand and high-magnificationhigh-magnification high-magnification SEMSEM SEM imagesimages images ofof TiNTiN of deposited TiNdeposited deposited onon thethe on mirror-mirror- the polishedmirror-polishedpolished Co–CrCo–Cr alloyalloy Co–Cr surfacessurfaces alloy surfacesusingusing PVDPVD using oror CVD.CVD. PVD In orIn thethe CVD. casecase In ofof the PVDPVD case TiN,TiN, of veryvery PVD smallsmall TiN, grains verygrains small (less(less thangrainsthan 100100 (less nm)nm) than werewere 100 depositeddeposited nm) were onon deposited thethe Co–CrCo–Cr on alloyalloy the susu Co–Crrface.rface. OnalloyOn thethe surface. otherother hand,hand, On the thethe other CVDCVD hand,coatingcoating the formedformed CVD acoatinga rougherrougher formed surfacesurface a rougherthanthan thethe surface PVDPVD coating.coating. than the Lenticular-sLenticular-s PVD coating.hapedhaped Lenticular-shaped grainsgrains withwith anan averageaverage grains lengthlength with an ofof average 200200 nmnm andlengthand aa widthwidth of 200 ofof nm 5050 and nmnm agrewgrew width onon of thethe 50 CVD-coatedCVD-coated nm grew on alloyalloy the CVD-coated surface.surface. alloy surface.

FigureFigure 4.4. SEMSEM micrographsmicrographs showing showing TiN TiN depositeddeposited onon theththee mirror-polishedmirror-polished Co–CrCo–Cr alloyalloy surfacessurfaces byby PVDPVD andandand CVD. CVD.CVD. ( a ((a)a) and) andand (b (():bb): PVD): PVDPVD TiN TiNTiN (low (low(low (500 (500×)(500×)) and andand high highhigh (5000 (5000×)(5000×)) magnifications, magnifications,magnifications, respectively). respectively).respectively). (c) and ((cc)) × × (andandd): CVD((dd):): CVDCVD TiN TiN(lowTiN (low(low (500 (500×)(500×)) and and highand highhigh (5000 (5000×)(5000×)) magnifications, magnifications,magnifications, respectively). respectively).respectively). × × TheThe cross-sectionalcross-sectionalcross-sectional SEM SEMSEM images imagesimages (Figure (Figure(Figure5) show 5)5) shsh theowow di thethefference differencedifference between betweenbetween the PVD thethe and PVDPVD CVD-coating andand CVD-CVD- coatinglayerscoating formed layerslayers onformedformed the mirror-polished onon thethe mirror-polishedmirror-polished Co–Cr alloy CoCo–Cr–Cr substrate. alloyalloy substrate.substrate. The CVD TheThe coating CVDCVD coatingcoating formed formed aformed thicker aa (1.5thickerthickerµm) (1.5(1.5 layer μμm)m) than layerlayer the thanthan PVD thethe coating PVDPVD (0.9coatingcoatingµm) (0.9 on(0.9 the μμm)m) substrate. onon thethe substrate.substrate. In the case InIn ofthethe PVD casecase coating ofof PVDPVD methods coatingcoating methodssuchmethods as sputtering suchsuch asas sputteringsputtering or arc-ion oror deposition, arc-ionarc-ion deposition,deposition, surfaces with surfacessurfaces an angle withwith di ananff erent angleangle from differentdifferent the surface fromfrom thethe facing surfacesurface the facingcoatingfacing thethe target coatingcoating may targettarget manifest maymay a manife dimanifefferencestst aa indifferencedifference coating thickness inin coatingcoating due thicknesthicknes to thess straightness duedue toto thethe straightnessstraightness of the deposition ofof thethe depositiondirectiondeposition of directiondirection the coating ofof the material.the coatingcoating The material.material. PVD coating ThThee PVDPVD layer coatingcoating showed layerlayer very showedshowed small columnarveryvery smallsmall grains columnarcolumnar and grainsirregular-shapedgrains andand irregular-shapedirregular-shaped grains, with grains,grains, a clean withwith interface aa cleanclean between interfaceinterface the betweenbetween coating thethe and coatingcoating alloy and devoidand alloyalloy of devoiddevoid chemical ofof chemicalreaction.chemical reaction. Inreaction. the CVD InIn thethe coating, CVDCVD thecoating,coating, growth thethe of growthgrowth columnar-shaped ofof columnar-shapedcolumnar-shaped grains created grainsgrains acreatedcreated very dense aa veryvery coating densedense coatinglayercoating on layerlayer the surface, onon thethe surface,surface, with only withwith a onlyonly few a nano-sizeda fewfew nano-sizednano-sized pores porespores and very andand very smallvery smallsmall secondary secondarysecondary particles particlesparticles at the atat thecoatingthe coating/substratecoating/substrate/substrate interface. interface.interface. One OneOne of the ofof the mainthe mainmain diff erencesdifferencesdifferences between betweenbetween the the twothe twotwo types typestypes of coatings ofof coatingscoatings was waswas the thethe microstructuremicrostructure nearnear thethe surfacesurface ofof thethe alloy.alloy. InIn thethe PVDPVD coating,coating, thethe microstructuremicrostructure ofof thethe alloyalloy

Materials 2020, 13, 1145 7 of 14 Materials 2018, 11, x FOR PEER REVIEW 7 of 14 Materials 2018, 11, x FOR PEER REVIEW 7 of 14 nearmicrostructure the surface near resembled the surface that of the as-cast alloy. In (uncoated) the PVD coating,alloy. On the the microstructure other hand, a oflarge the amount alloy near of smallthenear surface the pit-shaped surface resembled resembled precipitates that of that thewas of as-cast found the as-cast (uncoated)in the (uncoated) alloy alloy. grains alloy. On underneath the On other the other hand, the CVD hand, a large TiN a amountlarge coating amount of layer. small of pit-shapedsmall pit-shaped precipitates precipitates was found was found in the alloyin the grains alloy grains underneath underneath the CVD the TiN CVD coating TiN coating layer. layer.

Figure 5. Cross-sectional SEM micrographs showing the PVD (a) and CVD (b) TiN coating layers on theFigure mirror-polished 5. Cross-sectional Co–Cr SEM alloy micrographs substrate (10000×). showing the PVD ( a) and CVD ( b) TiN coating layers on the mirror-polished Co–CrCo–Cr alloyalloy substratesubstrate (10000(10000×).). Figure 6 shows the cross-sectional SEM images× of the CVD TiN-coated Co–Cr alloy substrate observedFigure to 6 6a shows showsdepth theofthe 75 cross-sectionalcross-sectional μm from the coating SEMSEM imagesimages interface. ofof thethe In CVDtheCVD CVD-coated TiN-coatedTiN-coated alloy, Co–CrCo–Cr unlike alloyalloy the substratesubstrate as-cast andobserved PVD-coated toto aa depthdepth alloy, of of 75 75 µthe μmm from SEMfrom the theshowed coating coating interface.three interface. different In In the the CVD-coatedtypes CVD-coated (regions) alloy, alloy, of unlike theunlike thecharacteristic as-castthe as-cast and microstructurePVD-coatedand PVD-coated alloy, near thealloy, the SEM interface the showed SEM between threeshowed di theff erent threecoating types different and (regions) the alloytypes of depending the(regions) characteristic onof thethe depth microstructurecharacteristic from the interface:nearmicrostructure the interface (1) a region near between the 12 interface μm the deep coating between showing and the thesmall coat alloy grainsing depending and and the pit-sh alloy on theapeddepending depth irregular from on theprecipitates the depth interface: from inside (1) the a themregioninterface: (Figure 12 µ(1)m a deep6b); region (2) showing a12 region μm small deep 16 grainstoshowing 36 μ andm smalldeep pit-shaped grainshaving irregularand a small pit-sh precipitatesamountaped irregular of insidepit- andprecipitates them string-shaped (Figure inside6b); (2)precipitatesthem a region (Figure 16formed 6b); to 36 (2) µinm aa deepregionlattice having pa16ttern to a36 smallunderneath μm amountdeep having the of (b) pit- regiona and small string-shaped (Figure amount 6c); of and precipitatespit- (3) and a region string-shaped formed 53 to in 73 a μlatticeprecipitatesm deep pattern showing formed underneath a smallin a lattice amount the Figure pattern of 6pit-shapedb underneath region (Figure precip the6itates c);(b) andregion and (3) a a(Figure scratch-like region 6c); 53 toand long 73 (3) patternµ ma region deep extending showing 53 to 73 inaμmsmall one deep direction amount showing of(Figure a pit-shaped small 6d). amount In precipitates particular, of pit-shaped and small a precip scratch-like grainsitates formed and long a inscratch-like pattern the region extending long nearest pattern in onethe extending directioninterface (Figurein one 6 direction6b).d). In particular, (Figure small6d). In grains particular, formed small in the grains region formed nearest in the the interface region nearest (Figure 6theb). interface (Figure 6b).

Figure 6. Low-Low- (1000×, (1000 ,(a)a and)) and higher- higher- (5000×, (5000 b,(–db)– magnificationd)) magnification cross-sectional cross-sectional SEM SEM micrographs micrographs (a) × × showing(Figurea) showing 6. Low-the theCVD (1000×, CVD TiN a TiN) andcoating coating higher- layer layer(5000×, and and b –Cod Co–Cr) –Crmagnification alloy alloy substrate: substrate: cross-sectional approximately approximately SEM micrographs 12 12 μµmm ( (b(ba),),) approximatelyshowing the CVD 16 to 36TiN µμm coating (c), and approximatelylayerapproximately and Co–Cr 5353 to toalloy 7373 µμ mmsubstrate: ((dd)) fromfrom thetheapproximately coatingcoating interface.interface. 12 μ m (b), approximately 16 to 36 μm (c), and approximately 53 to 73 μm (d) from the coating interface. Figure 77 showsshows thethe cross-sectionalcross-sectional SEMSEM imagesimages (up(up toto 2020 μµm from the interface) of the three groupsFigure (as-cast, 7 shows PVD-coated, the cross-sectional and CVD-coated SEM imagesallo alloy)y) and and(up their theirto 20 corresponding corresponding μm from the EDSinterface) EDS mapping mapping of the images. images. three Angroups EDS (as-cast, point analysis PVD-coated, was carried carried and CVD-coated out out to to determine determine alloy) the the and composition composition their corresponding of of the the characteristic characteristic EDS mapping precipitates. precipitates. images. TheAn EDS microstructure point analysis and was precipitate carried out distribution to determine of the the as-cast composition Co–Cr alloy of the and characteristic PVD-coated precipitates. alloy were similar.The microstructure Near the the surface surface and precipitate of of the the CVD CVD distribution TiN-coated TiN-coated of alloy, alloy, the as-cast there Co–Cr was a largealloy distributionand PVD-coated of Cr alloy along were the grainsimilar. boundaries Near the of surface the ε-phase -phaseof the andCVD and also also TiN-coated along along the the alloy, grain grain thereboundaries boundaries was a largebetween between distribution the the dendrite dendrite of Cr and and along matrix. matrix. the Thegrain white boundaries pits and of lattice-patterned the ε-phase and string-shaped also along the grain precipitates boundaries below between the pits thenear dendrite the coating and surface matrix. (seeThe white also Figure pits and 66b,c)b,c) lattice-patterned hadhad aa higherhigher W Wstring-shaped contentcontent thanthan precipitates thethe bulkbulk matrixmatrix below portion.portion. the pits near InIn addition,addition, the coating blackblack surface pits,pits, whose(see also color Figure was 6b,c) similar had to a or higher darker W than content the matrix, than the werewere bulk distributeddistributed matrix portion. aroundaround In thethe addition, whitewhite pits.pits. black pits, whose color was similar to or darker than the matrix, were distributed around the white pits.

Materials 2020, 13, 1145 8 of 14

Table 2. Results of the EDS spot analysis showing the atomic weight (at%) composition ratios for the points marked in images (a), (e), and (i) of Figure7.

EDS Composition (at%) Image Remark Point Co Cr W Mo Nb V 1 63.6 29.0 3.9 0.6 0.5 2.4 Co–Cr (matrix) 2 49.6 20.5 12.9 2.6 12.7 1.7 W-Mo-Nb-rich (a) As-cast Co–Cr (matrix 3 64.6 28.5 3.7 0.6 0.4 2.2 white line) 4 5.0 9.3 N/D 1 N/D 58.9 26.8 Nb-V-rich 1 62.9 29.4 3.9 0.7 0.7 2.4 Co–Cr (matrix) 2 50.4 20.6 12.4 2.5 12.5 1.6 W-Mo-Nb-rich (e) PVD-coated Co–Cr (matrix 3 64.4 28.7 3.7 0.5 0.5 2.3 white line) 4 7.2 9.7 0.8 N/D 56.4 25.8 Nb-V-rich 1 65.7 28.2 3.3 0.5 0.2 2.1 Co–Cr (matrix) 2 51.5 19.4 11.1 2.8 13.7 1.5 W-Mo-Nb-rich Co–Cr (matrix 3 64.8 29.3 2.9 0.5 0.3 2.2 (i) CVD-coated small grain) 4 8.4 10.4 0.3 N/D 56.1 24.9 Nb-V-rich 5 48.3 43.2 3.5 1.2 0.8 3.0 Cr-rich 6 59.9 27.2 7.5 1.4 1.6 2.6 White pit 7 58.7 29.1 3.9 0.7 1.8 5.9 Black pit 1 N/D: not detected. Materials 2018, 11, x FOR PEER REVIEW 8 of 14

Figure 7.7. Cross-sectional SEM micrographs of of the the three three groups groups ( (aa,, ee,,i )and and i) their and correspondingtheir corresponding EDS mappingEDS mapping images images ((b,f ,((j):b, Cr; f, and (c,g j,):k ):Cr; W; (c and), (g), ( dand,h,i ):(k V)): W; of and the Co–Cr(d), (h), alloys and (i (as-cast)): V) of the (a –Co–Crd) and alloys PVD ((as-cast)e–h) and (a CVD–d) and (i– lPVD) TiN-coated (e–h) and alloys CVD ( (upi–l) toTiN-coated 20 µm from alloys the (up interface, to 20 μm 3000 from). the The interface, bright regions 3000×). × seenThe bright at the topregions of images seen at (h the,l) indicate top of images Ti, which (h) isand superimposed (l) indicate Ti, with which V. The is circletssuperimposed seen in with images V. (Thea,e, icirclets) indicate seen the in points images for (a the), (e EDS), and spot (i) analysisindicate (seethe points Table2 for). the EDS spot analysis (see Table 2).

Table2 2shows shows the the results results of of the the EDS EDS point point analysis analysis for for the the points points marked marked in imagesin images (a), (a), (e), (e), and and (i) of(i) Figureof Figure7. The 7. substrate The substrate phase ofphase the as-cast of the and as-cast PVD TiN-coatedand PVD alloysTiN-coated showed alloys similar showed compositions similar incompositions similar microstructure in similar regions,microstructure the scratch-like regions, or the white scratch-like lines showing or white the same lines composition showing the as insame the matrixcomposition phase (pointsas in the 3 of matrix (a) and phase (e)). In (points the CVD-coated 3 of (a) alloyand (e)). (i), the In compositionthe CVD-coated of point alloy 3, showing (i), the acomposition matrix of small of point grains 3, approximatelyshowing a matrix 10 µ ofm small below grains the interface, approximately was not 10 significantly μm below dithefferent interface, from was not significantly different from that of point 1. Although the as-cast alloy also showed some Cr- rich area between the dendrite and matrix as well as in the matrix (see also Figure 3c), the frequency of Cr-rich area was sparse relative to the CVD TiN-coated alloy. A compositional analysis of points 6 and 7 of (i) revealed little difference in the amounts of matrix Co and Cr, while the amount of W and V was relatively high in the white pits and in the black pits, respectively. In this point analysis of the pits, the analysis range of the EDS was larger than the size of the pits, so that the results also reflect the surrounding composition. In fact, the white pits and the black pits looked like intermediate phases of being transformed into W-Nb and Nb-V precipitates, respectively. This is probably because the CVD coating was performed in the aging temperature range where precipitation or phase transformation occurs in Co–Cr alloys (Table 1).

Table 2. Results of the EDS spot analysis showing the atomic weight (at%) composition ratios for the points marked in images (a), (e), and (i) of Figure 7.

EDS Composition (at%) Image Remark Point Co Cr W Mo Nb V 1 63.6 29.0 3.9 0.6 0.5 2.4 Co–Cr (matrix) (a) As- 2 49.6 20.5 12.9 2.6 12.7 1.7 W-Mo-Nb-rich cast 3 64.6 28.5 3.7 0.6 0.4 2.2 Co–Cr (matrix white line) 4 5.0 9.3 N/D 1 N/D 58.9 26.8 Nb-V-rich 1 62.9 29.4 3.9 0.7 0.7 2.4 Co–Cr (matrix) (e) 2 50.4 20.6 12.4 2.5 12.5 1.6 W-Mo-Nb-rich PVD- 3 64.4 28.7 3.7 0.5 0.5 2.3 Co–Cr (matrix white line) coated 4 7.2 9.7 0.8 N/D 56.4 25.8 Nb-V-rich 1 65.7 28.2 3.3 0.5 0.2 2.1 Co–Cr (matrix)

Materials 2020, 13, 1145 9 of 14 that of point 1. Although the as-cast alloy also showed some Cr-rich area between the dendrite and matrix as well as in the matrix (see also Figure3c), the frequency of Cr-rich area was sparse relative to the CVD TiN-coated alloy. A compositional analysis of points 6 and 7 of (i) revealed little difference in the amounts of matrix Co and Cr, while the amount of W and V was relatively high in the white pits and in the black pits, respectively. In this point analysis of the pits, the analysis range of the EDS was larger than the size of the pits, so that the results also reflect the surrounding composition. In fact, the white pits and the black pits looked like intermediate phases of being transformed into W-Nb and Nb-V precipitates, respectively. This is probably because the CVD coating was performed in the aging temperature range where precipitation or phase transformation occurs in Co–Cr alloys (Table1).

3.3. Crystallographic Features Figure8 shows the cross-sectional EBSD images (band contrast (BC) and the phase and inverse pole figure (IPF) maps) of the as-cast, PVD-coated, and CVD-coated alloys, including the coating layer and the alloy interiors. In the BC maps, the as-cast and PVD-coated alloy specimens showed no grain boundaries other than those of the matrix and precipitates. In the CVD-coated alloy, however, very small grains were observed near the surface, together with the grain boundaries of the matrix and the precipitate. In the phase maps of the as-cast and PVD-coated alloy specimens, the matrix (blue) was the γ- (fcc) phase, and the precipitate (red) was the ε- (hcp) phase. In images (Figure8d–f), the PVD-coated alloy showed the same microstructure as the as-cast alloy. In the CVD-coated alloy, however, the phase structure of the precipitate was the same as that of the as-cast alloy, whereas the ε-phase region was continuously formed from the coating surface to a depth of approximately 10 to 20 µm. In addition, scratch-like ε-phase patterns, not extending to the inside of the precipitated W-Nb-rich ε-phase, were observed in the γ-phase region. The length and frequency of the ε-phase patterns suggests a plate-like martensitic transformation [23,24]. Moreover, another Cr-rich ε-phase (yellow), randomly scattered inside the ε-phase, consisted of small grains located near the surface. In the IPF maps of all specimens, the main matrix of the alloy was all single grains within the range of the image (117 87 µm). In addition, the IPF maps of all alloys indicate that the W-Mo-Nb-rich × precipitates shared similar orientations over a large area, suggesting that the precipitation occurred according to the crystal orientation properties of the matrix γ-phase when the cast alloy was formed. In Figure8i, the ε-phase grains concentrated near the coating surface were randomly distributed with no specific orientation, the grain size gradually increasing from the vicinity of the coating surface to the depth direction. The relationship between these phases and microstructures can be seen in more detail in Figure9. In the IPF map of the CVD-coated alloy, the coated TiN maintained homogeneous crystal orientation as it grew into columnar grains. The phase map clearly confirms why the diffraction pattern of the CVD-coated alloy showed mainly the ε-phase (Figure2), which seems to have originated at the interface between the γ-phase matrix and the TiN coating material during CVD coating. The overall microstructural development of the ε-phase observed near the coating surface may be interpreted as the result of phase transformation and recrystallization which simultaneously proceeded in the depth direction from the surface to the body of the alloy. These results are different from the general pearlite- or plate-like martensitic phase transformation ((111) γ//(0001) ε) of Co–Cr alloys when the aging treatment was proceeded [25]. Previous EBSD studies also showed a continuously developed ε-phase near the porcelain-fused-to-metal (Co–Cr alloys) interface [22,26,27]. The Cr-based ε-phase (yellow) in the phase map were indexed as the long ε-phase of the a and c axes (a: 3.7441; c: 12.1493) rather than the crystal lattice values (a: 2.5060; c: 4.0690) of W-Mo-Nb-rich precipitate dendrites. The grains in the phase map exactly overlapped with the EDS mapping image of Cr in Figure9b. Most of the Cr-based ε-phases at the grain boundaries indicate that the diffusion-based phase transformation during the CVD. Materials 2018, 11, x FOR PEER REVIEW 9 of 14

2 51.5 19.4 11.1 2.8 13.7 1.5 W-Mo-Nb-rich 3 64.8 29.3 2.9 0.5 0.3 2.2 Co–Cr (matrix small grain) (i) 4 8.4 10.4 0.3 N/D 56.1 24.9 Nb-V-rich CVD- 5 48.3 43.2 3.5 1.2 0.8 3.0 Cr-rich coated 6 59.9 27.2 7.5 1.4 1.6 2.6 White pit 7 58.7 29.1 3.9 0.7 1.8 5.9 Black pit 1 N/D: not detected.

3.3. Crystallographic Features Figure 8 shows the cross-sectional EBSD images (band contrast (BC) and the phase and inverse pole figure (IPF) maps) of the as-cast, PVD-coated, and CVD-coated alloys, including the coating layer and the alloy interiors. In the BC maps, the as-cast and PVD-coated alloy specimens showed no grain boundaries other than those of the matrix and precipitates. In the CVD-coated alloy, however, very small grains were observed near the surface, together with the grain boundaries of the matrix and the precipitate. In the phase maps of the as-cast and PVD-coated alloy specimens, the matrix (blue) was the γ- (fcc) phase, and the precipitate (red) was the ε- (hcp) phase. In images (d–f), the PVD-coated alloy showed the same microstructure as the as-cast alloy. In the CVD-coated alloy, however, the phase structure of the precipitate was the same as that of the as-cast alloy, whereas the ε-phase region was continuously formed from the coating surface to a depth of approximately 10 to 20 μm. In addition, scratch-like ε-phase patterns, not extending to the inside of the precipitated W- Nb-rich ε-phase, were observed in the γ-phase region. The length and frequency of the ε-phase patterns suggests a plate-like martensitic transformation [23,24]. Moreover, another Cr-rich ε-phase (yellow), randomly scattered inside the ε-phase, consisted of small grains located near the surface. In the IPF maps of all specimens, the main matrix of the alloy was all single grains within the range of the image (117 × 87 μm). In addition, the IPF maps of all alloys indicate that the W-Mo-Nb-rich precipitates shared similar orientations over a large area, suggesting that the precipitation occurred according to the crystal orientation properties of the matrix γ-phase when the cast alloy was formed. In Figure 8i, the ε-phase grains concentrated near the coating surface were randomly distributed with noMaterials specific2020 ,orientation,13, 1145 the grain size gradually increasing from the vicinity of the coating surface10 of to 14 the depth direction.

Materials 2018, 11, x FOR PEER REVIEW 10 of 14

Figure 8. Cross-sectional electron backscattered diffraction (EBSD) images of the as-cast (a–c), PVD- coated (d–f), and CVD-coated (g–i) Co–Cr alloys: (a), (d), and (g) band contrast (BC) maps; (b), (e), and (h) phase maps; and (c), (f), and (i) inverse pole figure (IPF) maps.

The relationship between these phases and microstructures can be seen in more detail in Figure 9. In the IPF map of the CVD-coated alloy, the coated TiN maintained homogeneous crystal orientation as it grew into columnar grains. The phase map clearly confirms why the diffraction pattern of the CVD-coated alloy showed mainly the ε-phase (Figure 2), which seems to have originated at the interface between the γ-phase matrix and the TiN coating material during CVD coating. The overall microstructural development of the ε-phase observed near the coating surface may be interpreted as the result of phase transformation and recrystallization which simultaneously proceeded in the depth direction from the surface to the body of the alloy. These results are different from the general pearlite- or plate-like martensitic phase transformation ((111) γ//(0001) ε) of Co–Cr alloys when the aging treatment was proceeded [25]. Previous EBSD studies also showed a continuously developed ε-phase near the porcelain-fused-to-metal (Co–Cr alloys) interface [22,26,27]. The Cr-based ε-phase (yellow) in the phase map were indexed as the long ε-phase of the a and c axes (a: 3.7441; c: 12.1493) rather than the crystal lattice values (a: 2.5060; c: 4.0690) of W-Mo-Nb-rich Figure 8. Cross-sectional electron backscattered diffraction (EBSD) images of the as-cast (a–c), precipitatePVD-coated dendrites. (d–f), andThe CVD-coated grains in the (g– phasei) Co–Cr map alloys: exactly (a,d, goverlapped) band contrast with (BC) the maps; EDS (mappingb,e,h) phase image of Crmaps; in Figure and (9b.c,f,i )Most inverse of polethe Cr-based figure (IPF) ε-phases maps. at the grain boundaries indicate that the diffusion- based phase transformation during the CVD.

Figure 9. Cross-sectional high-magnifica high-magnificationtion images images of of the CVD TiN- TiN-coatedcoated Co–Cr alloy analyzed using EBSD and EDS simultaneously: (a) BC map; ( b) EDS map of Cr; ( c) phase phase map; and ( d) IPF IPF map. map.

3.4. Vickers Vickers Hardness Figure 1010 shows shows the the Vickers Vickers hardness hardness (Hv) (Hv) values va convertedlues converted from the from nano-indentation the nano-indentation hardness µ values.hardness The values. size ofThe the size indentations of the indentations at 50 mN at was 50 aboutmN was 3.5 about to 4.5 3.5m to in 4.5 length, μm in enabling length, hardnessenabling hardnessmeasurement measurement of the phase of transformation the phase transformation region of the CVDregion TiN-coated of the CVD alloy. TiN-coated All indentations alloy. were All µ indentationsmade outside were the W-Mo-Nb-richmade outside theε-phase W-Mo-Nb-rich precipitates, ε-phase with aprecipitates, parallel interval with ofa parallel about 10 intervalm. The of abouthardness 10 μ valuesm. The of hardness the CVD-coated values of alloy the CVD-coated were higher allo thany were those higher of the as-castthan those and of PVD-TiN-coated the as-cast and PVD-TiN-coated alloy at the three depths (p < 0.05), except at the bulk center of the specimen (p > 0.05). In particular, at a depth of 10 μm, the average Hv value of the CVD TiN-coated alloy (Hv 972) was much higher than that of the as-cast alloy (Hv 618), probably because all the γ-phases were transformed into ε-phases with high hardness and the grain size was reduced by recrystallization. In addition, the CVD TiN-coated alloy showed a significantly higher hardness value than those of the

Materials 2020, 13, 1145 11 of 14 alloy at the three depths (p < 0.05), except at the bulk center of the specimen (p > 0.05). In particular, at a depth of 10 µm, the average Hv value of the CVD TiN-coated alloy (Hv 972) was much higher than that

Materialsof the as-cast 2018, 11 alloy, x FOR (Hv PEER 618), REVIEW probably because all the γ-phases were transformed into ε-phases11 withof 14 high hardness and the grain size was reduced by recrystallization. In addition, the CVD TiN-coated as-castalloy showed and PVD-coated a significantly alloys, higher even hardnessat a depth value of 45 thanμm. thoseThis is of probably the as-cast due and to the PVD-coated development alloys, of theeven plate-shaped at a depth of ε 45-phaseµm. by This the is local probably martensitic due to thetransforma developmenttion during of the plate-shapedthe coating processε-phase and by the smalllocal martensiticintermediate transformation phase grains precipitated during the coating in the form process of pits. and The the smallhardness intermediate (Hv 588) of phase the W-Mo- grains Nb-richprecipitated ε-phase in the precipitates form of pits. in Thethe as-cas hardnesst alloy (Hv (points 588) of 2 the of W-Mo-Nb-richFigure 7a,e andε i)-phase was much precipitates higher in than the thatas-cast (Hv alloy 478) (points of the γ 2-phase. of Figure 7a,e,i) was much higher than that (Hv 478) of the γ-phase.

Figure 10. Nano-indentationNano-indentation hardness hardness values values meas measuredured at at depths depths of of 10, 10, 25, 25, and and 45 45 μµm and at the center of the cross-sectioned surface of the as-cast,as-cast, PVD-coated,PVD-coated, and CVD-coatedCVD-coated alloys. Each value was converted into Vickers hardness (Hv) values. The horizontal bars connect statistically equivalent values among the three coating conditions withinwithin eacheach depthdepth conditioncondition ((pp >> 0.05). The same small case letters above the bars bars indicate indicate statistically statistically similar means among the four depth conditions within each coating ( p > 0.05).0.05).

3.5. Adhesion of the Coating 3.5. Adhesion of the Coating Figure 11 shows the results of the progressive scratch test (load ranged from 1 to 30 N) performed Figure 11 shows the results of the progressive scratch test (load ranged from 1 to 30 N) to evaluate the adhesion of the coating on the alloy. On the PVD-coated alloy (Figure 11a), surface performed to evaluate the adhesion of the coating on the alloy. On the PVD-coated alloy (a), surface cracks started from a 10 N load, and a progressive increment of the load increased the crack formation. cracks started from a 10 N load, and a progressive increment of the load increased the crack The peeling and breakage of the coating grew more severe near the scratch mark as the load was formation. The peeling and breakage of the coating grew more severe near the scratch mark as the progressively increased. On the CVD coating (Figure 11b), in contrast, no cracking and peeling load was progressively increased. On the CVD coating (b), in contrast, no cracking and peeling phenomenon was found and only plastic deformation of the coated substrate was noted. The results phenomenon was found and only plastic deformation of the coated substrate was noted. The results show that the adhesion of the CVD coating was much superior to the PVD coating, even though the show that the adhesion of the CVD coating was much superior to the PVD coating, even though the CVD produced a rougher surface than the PVD (Figure5). The high temperature coating conditions CVD produced a rougher surface than the PVD (Figure 5). The high temperature coating conditions and chemicalchemical reactionsreactions at at the the coating coating and and alloy alloy interfaces interfaces (Figure (Figure1), as well1), as as thewell gradient as the ingradient hardness in hardnessnear the interface near the (Figureinterface 10 (Figure), may have10), may enhanced have enhanced interfacial interfacial adhesion ofadhesion the CVD of coating.the CVD coating.

Figure 11. ProgressiveProgressive scratch scratch test test marks marks of of the the PVD-coated ( (aa)) and and CVD-coated ( (b)) Co–Cr alloy (maximum load of 30 N and length ofof 22 mm).mm).

3.6. In Vitro Cytotoxicity In the agar-diffusion test, no cell inhibition zones were seen around the negative control (HDPE) and the three alloys specimens. A clear inhibition zone was found only around the positive control (rubber latex). Moreover, cell cultures around the negative control (Figure 12b) and alloy specimens

Materials 2020, 13, 1145 12 of 14

3.6. In Vitro Cytotoxicity In the agar-diffusion test, no cell inhibition zones were seen around the negative control (HDPE) and the three alloys specimens. A clear inhibition zone was found only around the positive control Materials 2018, 11, x FOR PEER REVIEW 12 of 14 (rubber latex). Moreover, cell cultures around the negative control (Figure 12b) and alloy specimens showed neutralneutral red-stained red-staine livingd living cells cells (Figure (Figure 12c–e). 12c–e). These These findings findings suggest suggest that neither that theneither negative the controlnegative nor control any test nor specimens any test sp (includingecimens (including the CVD TiN-coatedthe CVD TiN-coated alloy) were alloy) cytotoxic. were cytotoxic.

Figure 12.12. Morphology of L929 cellscells aroundaround thethe testtest specimensspecimens (100(100×):): (a) positivepositive controlcontrol (rubber(rubber × latex);latex); ((bb)) negativenegative control control (high-density (high-density polyethylene polyethylene (HDPE)), (HDPE)), (c) as-cast(c) as-cast alloy, alloy, (d) PVD-coated (d) PVD-coated alloy, andalloy, (e )and CVD-coated (e) CVD-coated alloy. alloy.

4. Conclusions 4. Conclusions InIn thisthis study,study, forfor thethe firstfirst timetime toto ourour knowledge,knowledge, TiNTiN waswas coatedcoated toto aa commercialcommercial biomedicalbiomedical Co–Cr alloy byby aa hot-wallhot-wall typetype CVDCVD apparatus.apparatus. The characteristics of the coating layer, interfacialinterfacial layer,layer, andand substratesubstrate werewere comparedcompared withwith thosethose ofof as-castas-cast andand PVDPVD TiN-coatedTiN-coated alloys.alloys. TheThe deposition of TiNTiN onon thethe Co–CrCo–Cr alloyalloy byby CVDCVD createdcreated aa uniformuniform coatingcoating layer,layer, developeddeveloped phase-transformedphase-transformed and recrystallized smallsmall grainsgrains nearnear thethe alloyalloy surface,surface, andand increasedincreased thethe hardnesshardness ofof thethe area.area. On the other hand,hand, thethe PVD-coatedPVD-coated alloy alloy did did not not show show any any microstructural microstructural or or phase phase changes. changes. The The findings findings of thisof this study study suggest suggest that that CVD CVD TiN TiN coating coating to to biomedical biomedical Co–Cr Co–Cr alloy alloy may may be be consideredconsidered aa promisingpromising alternative to PVD techniques, though it should be further optimized in order to avoid corrosion or unwanted phases.phases. FurtherFurther researchresearch on on CVD CVD TiN TiN coating coating to to biomedical biomedical alloys alloys is neededis needed to confirm to confirm the cytocompatibilitythe cytocompatibility and and efficacy efficacy of the of novelthe novel coating coating method. method. Author Contributions: Conceptualization, S.H.S., B.K.M. and T.-Y.K.; Methodology, B.K.M., M.-H.H. and T.-Y.K..;Author FormalContributions: Analysis, Conceptualization, B.K.M. and T.-Y.K.; S.H.S., Investigation, B.K.M. and S.H.S. T.-Y and.K.; M.-H.H.;Methodology, Resources, B.K.M., S.H.S. M.-H.H. and B.K.M.;and T.- Writing-OriginalY.K..; Formal Analysis, Draft Preparation, B.K.M. and S.H.S.;T.-Y.K.; Writing-Review Investigation, S.H.S. & Editing, and B.K.M.M.-H.H.; and Resources, T.-Y.K.; Visualization, S.H.S. and B.K.M.; S.H.S., M.-H.H.Writing-Original and T.-Y.K.; Draft Funding Preparation, Acquisition, S.H.S.; T.-Y.K.Writing-Review All authors & haveEditing, read B.K.M. and agreed and T.-Y.K.; to the publishedVisualization, version S.H.S., of theM.-H.H. manuscript. and T.-Y.K.; Funding Acquisition, T.-Y.K. Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2017R1A5A2015391). Korea government (MSIT) (2017R1A5A2015391). Acknowledgments: The authors thank the Core Research Support Center for Natural Products and Medical MaterialsAcknowledgments: (CRCNM) forThe technical authors supportthank the regarding Core Research the nano-indentation Support Center test. for Natural Products and Medical Materials (CRCNM) for technical support regarding the nano-indentation test. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study;Conflicts in the of collection,Interest: The analyses, authors or declare interpretation no conflict of data; of interest. in the writing The funders of the manuscript, had no role and in the in the design decision of the to publish the results. study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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