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A graded nano-TiN coating on biomedical Ti alloy: Low friction coefficient, good bonding and biocompatibility
Materials Science and Engineering C 71 (2017) 520–528
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
A graded nano-TiN coating on biomedical Ti alloy: Low friction coefficient, good bonding and biocompatibility Wenfang Cui a, Gaowu Qin a,⁎, Jingzhu Duan b, Huan Wang b a b
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Material Science and Engineering, Northeastern University, Shenyang 110819, China Spinal surgery, Shengjing Hospital, China Medical University, Shenyang 110004, China
a r t i c l e
i n f o
Article history: Received 29 August 2016 Received in revised form 6 October 2016 Accepted 16 October 2016 Available online 18 October 2016 Keywords: Biomedical Ti alloy Nano-TiN coating Mechanical properties Wear Biocompatibility
a b s t r a c t In order to solve wear resistance of Ti alloy biomaterials, the concept of a graded nano-TiN coating has been proposed. The coating was prepared on Ti-6Al-4V bio-alloy by DC reactive magnetron sputtering. The wear performance of the coated specimens was measured in Hank's solution under the load of 10 N, and the biocompatibility was evaluated according to ISO-10993-4 standard. The results show that the gradient coating exhibits a gradual change in compositions and microstructures along the direction of film growth. Nano-TiN with the size of several to dozens nanometers and Ti4N3 − x transitional phase with variable composition form a graded composite structure, which significantly improves adhesion strength (Lc1 = 80 N, Lc2 = 120 N), hardness (21 GPa) and anti-wear performance (6.2 × 10−7 mm3/Nm). The excellent bonding and wear resistance result from a good match of mechanical properties at substrate/coating interface and the strengthening and toughening effects of the nanocrystalline composite. The nano-TiN coating has also been proved to have good biocompatibility through in-vitro cytotoxicity, hemocompatibility and general toxicity tests. And thus, the proposed graded nano-TiN coating is a good candidate improving wear resistance of many implant medical devices. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Titanium and titanium alloys have successfully been used for dental implant, joint prosthesis and spinal fixation owing to their good biocompatibility, corrosion resistance and mechanical properties. However, large amount of fine wear debris caused by poor tribological performance of titanium alloys results in toxicity reaction to the physiological environment surrounding the implants and aseptic loosening of the implants and/or osteolysis [1]. Worn debris of titanium alloy, typically Ti-6Al-4V alloy, also promotes the dissolution of Al and V toxic ions. In order to improve wear resistance of metal biomaterials, TiN coating has been widely studied because of its high hardness, good chemical stability and biocompatibility. It not only improves the tribological performance of titanium alloy, but also prevents the dissolution of metal ions. Up to date, many publications have reported the clinical study of TiN coated Ti alloy or CoCrMo alloy in the aspect of heart, oral and orthopedic implants [2–4]. As an anti-wear coating, on the other hand, the good bonding to the substrate and high toughness are important prerequisites for the safe medical application. The single TiN coating prepared by PVD exhibits the abrupt changes of compositions and mechanical properties. High ⁎ Corresponding author at: School of Material Science and Engineering, Northeastern University, China. E-mail address: [email protected] (G. Qin).
http://dx.doi.org/10.1016/j.msec.2016.10.033 0928-4931/© 2016 Elsevier B.V. All rights reserved.
internal stress leads to early debonding of the coating from the substrate or cracking along the columnar interfaces of the coating [5]. The nano or sub-micro scale multilayer coatings, such as TiN/AlN, TiN/TiCN, and TiN/CrN, are designed to improve fracture toughness [6–9]. But these hard-hard composite coatings on the relatively compliant substrate still behave spallation or high friction of coefficient in many tribological situations [10,11]. The failures are primarily related with high compressive residual stress and large stress gradient within the coatings. The hard-soft coating, such as TiN/Ti multilayer coating, offers a simple approach to control residual stresses, which not only improves adhesion strength, but also produces a toughening response because of the stress buffer effect of Ti interlayer [12]. But the interfacial debonding between TiN and Ti has been found to occur at the surface layer of the coating under the action of sliding contact load. The situation is more serious with the reduction of TiN layer thickness. This causes instable coefficient of friction and degraded anti-wear performance [13]. Recently, the nanocrystalline composite coatings (e.g. ncTiN/a-Si3N4) have attracted more and more attention due to ultrahigh hardness. Unfortunately, they similarly exhibit higher brittleness. Small amount of nickel incorporated in the composite coating can improve the toughness [14,15], but nickel is known to be a toxic element to the living organism. Up to now, the question, how to prepare antiwear coating on implantable medical devices with good adhesion, high toughness and hardness as well as low coefficient of friction, remains open.
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2.2. Microstructure characterization The crystalline structures of the monolayer and gradient coating were analyzed by X-ray diffractometer (Smart Lab, Japan) using Cu Kα radiation at the scanning speed of 8°/min. Cross-sectional morphologies of the coatings were observed by field emission scanning electron microscope (JSM-7001F, Japan). The surface mean square roughness of the coatings were measured using atomic force microscope (EasyScan2,Swizerland) on the area of 30 × 30 (μm2). High resolution transmission electron microscope (JEM-2100, Japan) was used to examine the size and distribution of TiN nanocrystallines in the gradient coating. The concentration change of Ti and N element in the gradient coating was analyzed by electron probe microanalyzer (JEOL JXA8530F, Japan).
2.4. Biocompatibility The biocompatibilities of the graded TiN coating were evaluated through in-vitro cytotoxicity assay, hemocompatibility tests, thermogenic tests and general toxicity tests according to ISO-10993-4 standard. The detailed experimental procedures can be seen in Supplementary methods. 3. Results 3.1. Phase analysis and microstructures The phase structures of the monolayer and the gradient coatings were analyzed by XRD, as shown in Fig. 1. TiN was a main phase of the monolayer and gradient coating. The weak Ti diffraction peak possibly came from Ti transitional layer or Ti alloy substrate. The two coatings contained small amount of variable composition Ti4N3 − x phase in addition to TiN. The diffraction peak of Ti4N3 − x (015) was nearly coincident with Ti (0002), indicating the similarity between Ti4N3 − x and αTi in chemical composition and crystal structure. The superposition of the two diffraction peaks and the crystal defects of non-stoichiometric TiN compound also resulted in the distinct broadening of the diffraction peak at ~ 38°. It is noteworthy that the preferred orientation of TiN changed from (111) plane of the monolayer coating into (200) plane of the gradient coating. The phenomenon has also been reported in nanocrystalline multilayer and nanocomposite coating [16,17]. In the initial stage of the film formation, TiN nucleation prefers to form on (200) plane because of lower surface energy [1,18]. With the growth
monolayer coating graded coating
An in-situ nanomechanical testing system (Hysitron Triboindenter, USA) was used to measure indentation hardness and elastic modulus of the coatings with the maximum load of 8 mN. The adhesion strength of the coatings was tested by a micro-scratch tester (WS-2005, China)
Intensity (cps)
2.3. Mechanical properties
TiN(200)
Titanium target with purity of 99.95% was used for DC reactive sputtering magnetron. The mixed reactive gases consisting of high pure argon (99.99%) and high pure nitrogen (99.99%) were introduced into the chamber for depositing TiN coating. TiN monolayer coating and TiN gradient coating (abbreviated as monolayer and gradient coating, respectively, hereinafter) were deposited on medical grade Ti6Al4V alloy, respectively. The substrate plates were firstly ground up to 2000 grit SiC paper, polished with alumina powder (0.2 μm particles size), and then ultrasonic cleaned in acetone. Prior to deposition, the chamber was pumped down to a pressure of 4 × 10−3 Pa. Titanium target was pre-cleaned via Ar+ bombardment for 10 min in order to remove surface impurities. The working pressure was 0.38 Pa. The substrate plates were preheated to 300 °C. Ti interlayer was firstly deposited on the substrate plate for 10 min to increase the adhesive strength of the coating. The monolayer coating was prepared at constant Ar and N2 gas flow rate. The graded coating was prepared by progressively increasing N2 flow rate while maintaining constant Ar gas flow rate. The details of the deposition parameters are shown in Table 1. The total deposition time was 175 min and the final coating thickness was 5.8–6 μm.
Ti (101)
2.1. Preparation methods
Ti4N3-x (015) Ti (002)
2. Materials and methods
with a diamond stylus of 0.2 mm radius. The load progressively increased up to 150 N with a loading rate of 100 N/min. The critical loads (Lc) representing adhesion to the substrate were determined by a sudden increase of acoustic emission signal and the initial cracking of the coatings. In order to assure the reproducibility of the results, the nanoindenter and scratch tests were repeated three times on each coated specimen. Linear reciprocating ball-on-disc wear tests were performed using a tribometer (CSM, Switzerland) with Si3N4 ball of 6 mm diameter rubbing against the coated specimens in Hank's solution. The compositions of Hank's solution were: NaCl 8 g/l, KCl 0.4 g/l, MgSO4·7H2O 0.1 g/l, MgCl·6H2O 0.1 g/l, CaCl2 0.14 g/l, NaHPO4 0.154 g/l and KH2PO4 0.06 g/l. The sliding wear started with normal load of 10 N. The sliding stroke was 10 mm at a frequency of 1 Hz. The width and depth of the worn tracks were measured by laser scanning confocal microscope(LSCM 510 system, Germany)and probe-type surface profiler (Dektak 150, USA), respectively.
TiN (111)
In this work, we prepared a novel graded nanocrystalline TiN coating by DC magnetron sputtering deposition, in which the volume fraction of nano-TiN phase gradually increases up to the surface layer of the coating. The gradient coating displays no distinct layer interface. The phenomenon of the interlayer separation that appears in Ti/TiN multilayer coating is thus completely avoided. This study presented a detailed investigation on the microstructures, mechanical properties, tribological performance and biocompatibility of the graded nanocrystalline TiN coating. The strengthening, toughening and anti-wear mechanisms of the coatings were clarified. For comparison, the TiN monolayer coating with the same thickness was investigated in the same conditions.
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Table 1 The design of N2 gas flow rate and deposition time during sputtering process. N2 flow rate (sccm)
1 2 3
4
5
6
7
8
In total
Deposition time (min) Gradient layer 4 8 12 16 20 24 28 63 175 Monolayer – – – – – – – – 175 Note: Ar gas flow rate was kept at 30 sccm. Ti interlayer was deposited for both monolayer and graded coatings.
32
34
36
38
40
42
44
46
2 Theta (deg.) Fig. 1. XRD patterns of the monolayer and gradient coating.
48
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(b)
(a)
5 µm
5 µm
Fig. 2. The cross sectional micrographs of the coatings. (a) columnar crystal structure of the monolayer coating, (b) compact structure of the gradient coating.
growing columnar crystallines sheltered the atoms depositing at a certain inclination angle, as a result, leaving many spaces in these regions. By contrast, the gradient coating exhibited a dense non-columnar crystal structure without distinct layer interface (Fig. 2b). This indicates that more and more new TiN nucleation formed with the increase of N2 gas flow rate during the sputtering deposition, which interrupted the orientated growth of the columnar crystallines. Meanwhile, at lower N2 gas flow rate, Ti atoms were less likely to be collided. Ti atoms with high kinetic energy obtained larger surface mobility, which promoted the horizontal growth of the coating and increased the density of the coating.
of TiN grain, a competitive growth mechanism driven by diffusion and elastic strain energy forces TiN columnar crystal to preferentially grow along (111) orientation. Based on the XRD results, it is deduced that the gradient coating exhibited a noncolumnar crystal structure, which was proved by SEM and TEM observation later. The cross-sectional fracture micrographs of the monolayer and gradient coatings are shown in Fig. 2a and b. The thickness of the two coatings was about 5.8–6 μm. The monolayer coating exhibited columnar crystal structure. The ragged columnar crystallines implied the existence of the voids or nano-gaps in the monolayer coating. These defects arose from the uniform growth rate of columnar crystallines. The fast
(b)
(a)
Fig. 3. The surface AFM images of the monolayer coating (a) and gradient coating (b) showing the mean square roughness of 130 nm and 62 nm, respectively.
20000
N Ti
18000
Substrate
Coating
16000
1 2
Counts
14000
3 4
12000
Coating
Substrate
120 90 60
1 µm
30 0
0.000
0.002
0.004
0.006
0.008
0.010
0.012
WD/mm Fig. 4. Cross sectional image of the gradient coating and EPMA analysis showing Ti and N composition change (along white line).
0.014
W. Cui et al. / Materials Science and Engineering C 71 (2017) 520–528 Table 2 The composition analysis of Ti and N elements in the gradient coating. Locations
1
2
3
4
Ti (at.%) N (at.%)
88 12
75 25
62 38
51 49
The surface roughness of the coating was measured by atomic force microscope (AFM). Fig. 3 gives the three-dimensional surface images of the two coatings. The surface of the monolayer coating looked like upand-down-hills with the mean square roughness of 130 nm. The gradient coating exhibited relatively flat surface on which the fine particles dispersed. The mean square roughness was measured to be only 62 nm. Ti and N element distribution across the gradient coating were analyzed by EPMA, as shown in Fig. 4. It is clearly seen that the gradient coating was divided into two parts. The light-color and dark-color layers represented the “soft” zone with low N content and the “hard” zone with high N content, respectively. Table 2 gives Ti and N concentration at different locations of the coating. In the light-color zone, N content was b25 at.%. In the outer layer, the atomic ratio of Ti and N was close to 1:1. According to Ti-N binary phase diagram, Ti and N can form a series of solid solutions or compounds: Ti4N, Ti3N, Ti2N, TiN, Ti3N4, Ti3N5, Ti5N6 etc. Among them, N content in stable TiN phase is N42 (at.%). Other Ti-N compounds with N contents b32 (at.%) belong to the nitrogen vacancy type transitional phase or solid solution. It is thus believed that Ti4N3 − x transitional phase in the gradient coating formed at low N2 gas flow rate and mainly existed in the inner layer adjacent to the substrate. While non-stoichiometric TiN possibly formed at any N2 gas flow rate due to the strong binding force between N and Ti. The relative proportion of Ti4N3 − x and TiN changed with N2 gas flow rate. Increasing N2 gas flow rate enhanced the volume fraction of TiN until 100% TiN formed in the top layer of the gradient coating. In order to observe the morphologies of TiN and Ti4N3 − x at different level layers of the gradient coating, TEM films were prepared by
(a)
(c)
(220) (200) (111)
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sputtering the coating on copper net at different N2 gas flow rates. Fig. 5 is a group of TEM bright field images of the microstructures and the corresponding selected area diffraction (SAED) patterns. Dispersive nano-scale particles can be clearly seen at various N2 gas flow rates. SAED analysis indicates that all polycrystalline diffraction rings came from TiN phase, which proved the formation of TiN at any N2 gas flow rate. At low N2 gas flow rate, small amount of nano-TiN particles were embedded in Ti4N3 − x transitional phase (Fig. 5a). At high N2 gas flow rate, the volume fraction of nano-TiN increased, at the same time, some nano-TiN particles tended to agglomerate together (Fig. 5d). The average size of nano-TiN also increased with N2 gas flow rate, from 6.5 ± 3.4 nm at 2 sccm N2 flow rate to 11 ± 2.4 nm at 8 sccm N2 flow rate. An interesting phenomenon is that the gray background in Fig. 5a–d becomes more and more clear with the increase of N2 gas flow rate, indicating that strain contrast caused by lattice defects is gradually disappearing. Fig. 6 shows the HR-TEM images of nano-TiN, by which the lattice structure of nanocrystallines can be understood more clearly. At 2 sccm N2 gas flow rate, the two types of nanocrystallines were observed. The one included many lattice defects, such as vacancies, dislocations, stacking faults and lattice distortion, as indicated by white arrows in Fig. 6a. The other exhibited perfect lattice structure. At 8 sccm N2 gas flow rate, few lattice defects were observed in all nanocrystallines. The results are agreement with our analysis mentioned above. The nanocrystallines containing serious defects represented Ti4N3 − x transitional phase because the lack of nitrogen produced a large number of lattice defects. TiN nanocrystallines had relatively perfect crystal structure. Increasing N2 gas flow rate promoted the formation of more nano-TiN in accompanying with the decrease of Ti4N3 − x transitional phase and the disappearance of the lattice defects. This explains why the strain contrast of the background in Fig. 5a–d displayed different change with N2 gas flow rate. Because of the great differences in N content and lattice defects, Ti4N3 − x and TiN behaved different mechanical properties. The graded distribution of the two phases determined the peculiar mechanical behavior of the gradient coating.
(b)
(d)
Fig. 5. TEM images and the corresponding SAED patterns of nano-TiN formed at different N2 gas flow rates. (a) 2 sccm, (b) 4 sccm, (c) 6 sccm, (d) 8 sccm.
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(a)
(b)
TiN (0002)
TiN
TiN
Ti4N3-x
(0002)
TiN
5 nm
5 nm
Fig. 6. HR-TEM images of the nanocrystallines in the gradient coating formed at 2 sccm (a) and 8 sccm (b) N2 gas flow rates.
8000
8000
(a)
(b)
6000
6000
4000
4000
2000
2000
0
0
20
40
60
80
100 120 140 160 180
0
0
20
40
60
80
100
120
140
Fig. 7. The load-displacement curves of the monolayer (a) and graded (b) TiN coating by nanoindentation tests.
3.2. Mechanical properties The nanoindentation hardness and elastic modulus of the coatings were measured by nano-indentation tests. The load-displacement curves of the monolayer and graded TiN coating are shown in Fig. 7. The smooth and continuous curves suggested that no cracks occurred to the monolayer and graded coating during loading, and the substrate had no influence on the deformation behavior of the coatings. The hardness and elastic modulus of the coatings are listed in Table 3. It can be seen that the elastic modulus of the monolayer coating was only 238 ± 3 GPa, lower than the normal value of TiN (~280 GPa). This can be explained by the lower compactness in the outer layer of columnar crystal coating. The elastic modulus of the gradient coating attained 281 ± 5 GPa, also slightly smaller than other TiN coatings [19]. It has been reported that the modulus of non-stoichiometric TiN phase decreases with the decrease of N content because the existence of the vacancies in the lattice reduces the total bond energy [20]. Apparently, Ti4N3 − x transitional phase in the gradient coating was responsible for the reduction of elastic modulus. In spite of this, the hardness of the gradient coating was almost two times higher than the monolayer coating. The compact structure and nanocrystalline strengthening effect
made great contribution to enhancing the hardness of the gradient coating. The adhesion strength of the coating to the substrate is one of the important mechanical properties that determine the service life. Scratch test is widely used for evaluating the adhesion force of the coating. Fig. 8 shows the acoustic emission signal plots during applying load until to 150 N. SEM images of the scratch tracks were superimposed over the plots. A fine tensile stress crack can be seen at the bottom of the scratch tracks. The first sudden change (Lc1) of the signal intensity represented the appearance of the initial crack. The second drastic increase (Lc2) of the signal intensity represented the exfoliation or cracking of the coating. Table 3 shows that both Lc1 and Lc2 values of the gradient coating were much higher than those of the monolayer coating. Zhang et al. [21] suggested that the coating toughness was proportional to Lc1 as well as difference between Lc1 and Lc2. A parameter named “scratch crack propagation resistance” (CPR = Lc1 (Lc2 − Lc1)) was directly used to indicate the coating toughness. According to this argue, the CPR values of the monolayer and gradient coating were 1050 and 3321, respectively. This indicates that the gradient coating exhibited an enhanced resistance to scratch damage. 3.3. Tribological performance
Table 3 The mechanical properties of the monolayer and gradient coatings. Coating
Elastic modulus (GPa)
Hardness (GPa)
Monolayer Gradient
238 ± 3 281 ± 5
10.2 ± 1.2 20.8 ± 1.1
Data are expressed as mean values ± standard deviation.
Bonding force (N) Lc1
Lc2
50 ± 5 81 ± 4
71 ± 4 122 ± 6
The ball-on-disc wear tests were performed on the coated specimens against Si3N4 counterpart ball under the load of 10 N in Hank's solution. The maximum Hertzian contact stress reached 1630 MPa. Fig. 9 shows the change of the coefficient of friction (COF) with time during sliding abrasion. The rapid increase and then decrease of the COFs in the initial stage were caused by the polished process of the primary coarse surface. In comparison with the monolayer coating, the steadystate COF of the gradient coating decreases by 50%. Stolyarov et al.
W. Cui et al. / Materials Science and Engineering C 71 (2017) 520–528
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(b)
Lc1
Signal intensity
Signal intensity
(a)
Lc2
Lc2
Lc1
Load, N
Load, N
Fig. 8. The acoustic emission signals patterns during scratching and the scratch track images showing the critical forces (Lc1 and Lc2) of the coating rupture. (a) Monolayer coating (b) Gradient coating.
[22] suggested that COF can be divided into an adhesion component and a deformation component, which result from the adhesion force and plastic deformation force, respectively. Adhesion force is proportional to the surface energy of the two contacting surfaces [23]. Plastic deformation force depends on the H3/E2 ratio according to the Tsui's expression [24]:
P y ¼ 0:78r 2
H3 E
!
depth and wear track width are shown in Table 4. The wear rate (0.62 × 10−6 mm3/Nm) of the gradient coating is only one-sixth of the monolayer coating. The value is even smaller than that of TiAlN coating (1 × 10−4 mm3/Nm), TiNbN and TiCN coating (5.9 × 10−5 mm3/Nm) in simulated body fluid [19,25], showing the excellent anti-wear performance of the gradient coating. 3.4. Biocompatibility evaluation.
ð1Þ
2
where: Py is the yield stress to resist plastic deformation, H is the hardness, E is the elastic modulus, r is the radius of rigid contact ball. Because of smaller mean square roughness and higher hardness, the gradient coating has a lower surface energy and larger H3/E2 ratio than the monolayer coating. It indicates that the gradient coating exhibits low adhesion force and high resistance to plastic deformation. Fig. 9b and c display the wear track morphologies of the two coatings. Many triangular craters and deep plowing grooves can been seen on the wear track of the monolayer coating, which reflect the occurrence of serious adhesion and plastic deformation during sliding wear. In contrast, the gradient coating shows a smoother wear track and a narrower track width. The wear rates of the coatings calculated by measuring wear track
In the cell cytotoxicity tests, the OD values of the cells, which were cultured for 24 h, 48 h and 72 h, respectively, are listed in Table S1. RGRs (%) of three experimental groups are all greater than 90% and show no significant difference one another (p b 0.05). The results of hemocompatibility tests are shown in Table S2. The absorbancy and the hemolysis ratio of the experimental groups and the control groups also show no significant statistical difference (p b 0.05), indicating that the TiN gradient coating presents good blood compatibility. Table S3 shows that the body temperature rise of each SD rat in the experimental group and the control group is below 0.6 °C, and the sum of the body temperature rise of all rats is below 4.5 °C. This implies that the TiN gradient coating is not a thermogenic material. The general toxicity tests indicate that each SD rat in the experimental group and control group exhibits good mental state and normal
(b) 0.7 monolayer coating graded coating
(a) Coefficient of friction
0.6
80µm
0.5
(c) 0.4 0.3 0.2 0.1
80µm
0
1000 2000
3000
4000 5000 6000 7000
Sliding time (s) Fig. 9. (a) COF vs. sliding time curves of the monolayer and gradient coating, (b) the worn track morphologies of the monolayer coating, (c) the worn track morphologies of the gradient coating.
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Table 4 The coefficients of friction and wear rates of the monolayer and gradient coating. Coating
COF
Wear rate (×10−6 mm3/Nm)
Wear track depth (μm)
Wear track width (μm)
Monolayer Gradient
0.49 ± 0.04 0.21 ± 0.02
4.32 ± 0.23 0.62 ± 0.34
0.99 ± 0.12 0.34 ± 0.23
409 ± 11 220 ± 15
Data are expressed as mean values ± standard deviation.
physical performance. There are no significant changes in urine, stool and diet for the rats before and after injection. No toxic symptoms appear, such as blepharoptosis, tremor, collapse and even death. Fig. 10 gives the photographs of histopathological HE stained section (liver, heart and kidney) of SD rats in the experimental group (Group A) and the control group (Group B), which indicate that no inflammatory cells have been found in the visual field; myocardial cells shows no degeneration, necrosis or hypertrophy. The shapes of myocardial fibers are regular, and liver cells show no edema, steatosis, acidophilic change or spotty necrosis, glomeruli are basically normal, and renal tubular epithelial cells shows no edema. According to these basic data, the gradient TiN coating displays a very good biocompatibility. 4. Discussion Hard coating failure on relatively compliant substrate in many tribological situations is generally caused by the coating delaminating from the substrate and the fracture of coating itself. The failure is primarily related to high residual stress gradient at the interface and low toughness of the coating. Evans and Marshall established the relationships
liver
heart
between wear rate W and elastic modulus E, hardness H and fracture toughness KIC of ceramic material [26]: W ¼a
4=5 E 1=2 K H 5=8 H F 1=8
ð2Þ
IC
where a is a constant depending material structure, and F is load. The theory suggests that high anti-wear performance of a hard coating can be obtained through a good match of elastic modulus, hardness and toughness. Unfortunately, the high hardness of the materials is usually accompanied by high elastic modulus and low toughness. The brittleness of the coatings results in early cracking and low adhesion. Actually, the wear rates of the hard or ultra-hard coatings are mostly in the range of 10−6– 10−4 mm3/Nm and the adhesion strength in the range of 20–50 N [27–32]. In this work, the functionally graded nano-TiN coating successfully enhances the load bearing capability as well as the adhesion to the substrate. The gradual increase of nitrogen content within the coating avoids the sharp changes in chemistry, crystal structure and mechanical properties. Refined TiN grains decrease surface roughness and increases hardness. Both the adhesion strength and the tribological performance
kidney
Fig. 10. The hints of pathological HE stained section of the experimental group (Group A) and the control group (Group B). The results indicate that no inflammatory cells were found in the visual field; myocardial cells showed no degeneration, necrosis or hypertrophy; the shapes of myocardial fibers were regular; liver cells showed no edema, steatosis, acidophilic change or spotty necrosis; glomeruli were basically normal, and renal tubular epithelial cells showed no edema.
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of the graded nano-TiN coating are greatly superior to the monolayer TiN coating. Based on the results of XRD analysis and TEM observation, the gradient coating experiences a series of microstructural changes from bottom layer to top layer: Ti adhesion layer (extremely thin film) → Ti4N3 − x transitional phase (major) + nano-TiN (minor) → nano-TiN (major) + Ti4N3 − x transitional phase (minor) → nano-TiN (whole). The size and volume fraction of nano-TiN simultaneously increase with the gradual increase of N2 gas flow rate and deposition time, which determines the graded change of mechanical properties. Ti4N3 − x transitional phase in the inner layer of the coating tends to exhibit metal properties owing to the deficiency of N atoms, e.g. low hardness and elastic modulus, good plasticity and toughness. These properties constitute a good match with Ti alloy substrate, which reduces thermal stress and growth stress of the coating. Moreover, Ti4N3 − x acts as a role of stress buffer and can effectively release in-plane residual stress at the interface. These factors greatly enhance the sustainability of the coating/substrate bonding [33]. Nano-TiN plays an important role in the aspects of enhancing hardness and elastic modulus of the coating. Since the plastic deformation of nanocrystalline materials cannot be realized by dislocation slip because of high resistance to dislocation movement within nano-grains, nanoTiN behaves higher strength and hardness than coarse grain TiN. On the other hand, in the gradient coating, the “hard” TiN nanocrystallines are surrounded by the “soft” Ti4N3 − x transitional phase. The latter is helpful to relax stress concentration at the crack tip through local plastic deformation and consume more energy stopping crack propagation. Therefore, the composite of “hard” and “soft” phases exhibits a good combination of high hardness and toughness. It is an importance assurance that the gradient coating has low friction and high wear resistance. The biological evaluations prove that the biocompatibility of the gradient coating is as good as monolayer coating. It has been reported that the surface nanotopography of the implant plays a great role of increasing the numbers of attached 3T3-L1 fibroblasts [34]. Here, it can be expected that, by optimizing the sputtering deposition routes, the graded nano-TiN coating will comprehensively improve adhesion, tribological performance, biocompatibility and cellular response. The newly developed TiN gradient coating has potential application prospects for the implant medical devices. 5. Conclusions A graded nano-TiN coating was prepared on Ti-6Al-4V biomaterial substrate by controlling N2 gas flow rate during magnetron sputtering deposition. The gradient coating exhibits a gradual change in N composition, microstructure and mechanical properties along film growth direction, which thereby greatly improves adhesion force to the substrate (LC1 = 80 N). The coefficient of friction (0.21) and wear rate (6.2 × 10− 7 mm3/Nm) of the gradient coating are much lower than monolayer TiN coating in physiological environment. Nanocrystalline TiN makes great contribution to increasing the hardness and decreasing the surface roughness of the gradient coating. Meanwhile, the combination of “hard” TiN compound with “soft” Ti4N3 − x transitional phase is helpful to increase the resistance to plastic deformation and cracking. The low friction and high wear resistance of the gradient coating are attributed to the comprehensive improvement of hardness, toughness and surface roughness. A series of biocompatibility evaluations suggest that the newly developed graded nano-TiN coating has good bio-safety and potential application prospect for implant medical devices, especially for movable medical parts. Supplementary data to this article can be found online at doi:10. 1016/j.msec.2016.10.033. Acknowledgement The study was financially supported by Foundation for Key Program of Ministry of Education, China (No. 313014) and National Natural
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Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
A graded nano-TiN coating on biomedical Ti alloy: Low friction coefficient, good bonding and biocompatibility Wenfang Cui a, Gaowu Qin a,⁎, Jingzhu Duan b, Huan Wang b a b
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Material Science and Engineering, Northeastern University, Shenyang 110819, China Spinal surgery, Shengjing Hospital, China Medical University, Shenyang 110004, China
a r t i c l e
i n f o
Article history: Received 29 August 2016 Received in revised form 6 October 2016 Accepted 16 October 2016 Available online 18 October 2016 Keywords: Biomedical Ti alloy Nano-TiN coating Mechanical properties Wear Biocompatibility
a b s t r a c t In order to solve wear resistance of Ti alloy biomaterials, the concept of a graded nano-TiN coating has been proposed. The coating was prepared on Ti-6Al-4V bio-alloy by DC reactive magnetron sputtering. The wear performance of the coated specimens was measured in Hank's solution under the load of 10 N, and the biocompatibility was evaluated according to ISO-10993-4 standard. The results show that the gradient coating exhibits a gradual change in compositions and microstructures along the direction of film growth. Nano-TiN with the size of several to dozens nanometers and Ti4N3 − x transitional phase with variable composition form a graded composite structure, which significantly improves adhesion strength (Lc1 = 80 N, Lc2 = 120 N), hardness (21 GPa) and anti-wear performance (6.2 × 10−7 mm3/Nm). The excellent bonding and wear resistance result from a good match of mechanical properties at substrate/coating interface and the strengthening and toughening effects of the nanocrystalline composite. The nano-TiN coating has also been proved to have good biocompatibility through in-vitro cytotoxicity, hemocompatibility and general toxicity tests. And thus, the proposed graded nano-TiN coating is a good candidate improving wear resistance of many implant medical devices. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Titanium and titanium alloys have successfully been used for dental implant, joint prosthesis and spinal fixation owing to their good biocompatibility, corrosion resistance and mechanical properties. However, large amount of fine wear debris caused by poor tribological performance of titanium alloys results in toxicity reaction to the physiological environment surrounding the implants and aseptic loosening of the implants and/or osteolysis [1]. Worn debris of titanium alloy, typically Ti-6Al-4V alloy, also promotes the dissolution of Al and V toxic ions. In order to improve wear resistance of metal biomaterials, TiN coating has been widely studied because of its high hardness, good chemical stability and biocompatibility. It not only improves the tribological performance of titanium alloy, but also prevents the dissolution of metal ions. Up to date, many publications have reported the clinical study of TiN coated Ti alloy or CoCrMo alloy in the aspect of heart, oral and orthopedic implants [2–4]. As an anti-wear coating, on the other hand, the good bonding to the substrate and high toughness are important prerequisites for the safe medical application. The single TiN coating prepared by PVD exhibits the abrupt changes of compositions and mechanical properties. High ⁎ Corresponding author at: School of Material Science and Engineering, Northeastern University, China. E-mail address: [email protected] (G. Qin).
http://dx.doi.org/10.1016/j.msec.2016.10.033 0928-4931/© 2016 Elsevier B.V. All rights reserved.
internal stress leads to early debonding of the coating from the substrate or cracking along the columnar interfaces of the coating [5]. The nano or sub-micro scale multilayer coatings, such as TiN/AlN, TiN/TiCN, and TiN/CrN, are designed to improve fracture toughness [6–9]. But these hard-hard composite coatings on the relatively compliant substrate still behave spallation or high friction of coefficient in many tribological situations [10,11]. The failures are primarily related with high compressive residual stress and large stress gradient within the coatings. The hard-soft coating, such as TiN/Ti multilayer coating, offers a simple approach to control residual stresses, which not only improves adhesion strength, but also produces a toughening response because of the stress buffer effect of Ti interlayer [12]. But the interfacial debonding between TiN and Ti has been found to occur at the surface layer of the coating under the action of sliding contact load. The situation is more serious with the reduction of TiN layer thickness. This causes instable coefficient of friction and degraded anti-wear performance [13]. Recently, the nanocrystalline composite coatings (e.g. ncTiN/a-Si3N4) have attracted more and more attention due to ultrahigh hardness. Unfortunately, they similarly exhibit higher brittleness. Small amount of nickel incorporated in the composite coating can improve the toughness [14,15], but nickel is known to be a toxic element to the living organism. Up to now, the question, how to prepare antiwear coating on implantable medical devices with good adhesion, high toughness and hardness as well as low coefficient of friction, remains open.
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2.2. Microstructure characterization The crystalline structures of the monolayer and gradient coating were analyzed by X-ray diffractometer (Smart Lab, Japan) using Cu Kα radiation at the scanning speed of 8°/min. Cross-sectional morphologies of the coatings were observed by field emission scanning electron microscope (JSM-7001F, Japan). The surface mean square roughness of the coatings were measured using atomic force microscope (EasyScan2,Swizerland) on the area of 30 × 30 (μm2). High resolution transmission electron microscope (JEM-2100, Japan) was used to examine the size and distribution of TiN nanocrystallines in the gradient coating. The concentration change of Ti and N element in the gradient coating was analyzed by electron probe microanalyzer (JEOL JXA8530F, Japan).
2.4. Biocompatibility The biocompatibilities of the graded TiN coating were evaluated through in-vitro cytotoxicity assay, hemocompatibility tests, thermogenic tests and general toxicity tests according to ISO-10993-4 standard. The detailed experimental procedures can be seen in Supplementary methods. 3. Results 3.1. Phase analysis and microstructures The phase structures of the monolayer and the gradient coatings were analyzed by XRD, as shown in Fig. 1. TiN was a main phase of the monolayer and gradient coating. The weak Ti diffraction peak possibly came from Ti transitional layer or Ti alloy substrate. The two coatings contained small amount of variable composition Ti4N3 − x phase in addition to TiN. The diffraction peak of Ti4N3 − x (015) was nearly coincident with Ti (0002), indicating the similarity between Ti4N3 − x and αTi in chemical composition and crystal structure. The superposition of the two diffraction peaks and the crystal defects of non-stoichiometric TiN compound also resulted in the distinct broadening of the diffraction peak at ~ 38°. It is noteworthy that the preferred orientation of TiN changed from (111) plane of the monolayer coating into (200) plane of the gradient coating. The phenomenon has also been reported in nanocrystalline multilayer and nanocomposite coating [16,17]. In the initial stage of the film formation, TiN nucleation prefers to form on (200) plane because of lower surface energy [1,18]. With the growth
monolayer coating graded coating
An in-situ nanomechanical testing system (Hysitron Triboindenter, USA) was used to measure indentation hardness and elastic modulus of the coatings with the maximum load of 8 mN. The adhesion strength of the coatings was tested by a micro-scratch tester (WS-2005, China)
Intensity (cps)
2.3. Mechanical properties
TiN(200)
Titanium target with purity of 99.95% was used for DC reactive sputtering magnetron. The mixed reactive gases consisting of high pure argon (99.99%) and high pure nitrogen (99.99%) were introduced into the chamber for depositing TiN coating. TiN monolayer coating and TiN gradient coating (abbreviated as monolayer and gradient coating, respectively, hereinafter) were deposited on medical grade Ti6Al4V alloy, respectively. The substrate plates were firstly ground up to 2000 grit SiC paper, polished with alumina powder (0.2 μm particles size), and then ultrasonic cleaned in acetone. Prior to deposition, the chamber was pumped down to a pressure of 4 × 10−3 Pa. Titanium target was pre-cleaned via Ar+ bombardment for 10 min in order to remove surface impurities. The working pressure was 0.38 Pa. The substrate plates were preheated to 300 °C. Ti interlayer was firstly deposited on the substrate plate for 10 min to increase the adhesive strength of the coating. The monolayer coating was prepared at constant Ar and N2 gas flow rate. The graded coating was prepared by progressively increasing N2 flow rate while maintaining constant Ar gas flow rate. The details of the deposition parameters are shown in Table 1. The total deposition time was 175 min and the final coating thickness was 5.8–6 μm.
Ti (101)
2.1. Preparation methods
Ti4N3-x (015) Ti (002)
2. Materials and methods
with a diamond stylus of 0.2 mm radius. The load progressively increased up to 150 N with a loading rate of 100 N/min. The critical loads (Lc) representing adhesion to the substrate were determined by a sudden increase of acoustic emission signal and the initial cracking of the coatings. In order to assure the reproducibility of the results, the nanoindenter and scratch tests were repeated three times on each coated specimen. Linear reciprocating ball-on-disc wear tests were performed using a tribometer (CSM, Switzerland) with Si3N4 ball of 6 mm diameter rubbing against the coated specimens in Hank's solution. The compositions of Hank's solution were: NaCl 8 g/l, KCl 0.4 g/l, MgSO4·7H2O 0.1 g/l, MgCl·6H2O 0.1 g/l, CaCl2 0.14 g/l, NaHPO4 0.154 g/l and KH2PO4 0.06 g/l. The sliding wear started with normal load of 10 N. The sliding stroke was 10 mm at a frequency of 1 Hz. The width and depth of the worn tracks were measured by laser scanning confocal microscope(LSCM 510 system, Germany)and probe-type surface profiler (Dektak 150, USA), respectively.
TiN (111)
In this work, we prepared a novel graded nanocrystalline TiN coating by DC magnetron sputtering deposition, in which the volume fraction of nano-TiN phase gradually increases up to the surface layer of the coating. The gradient coating displays no distinct layer interface. The phenomenon of the interlayer separation that appears in Ti/TiN multilayer coating is thus completely avoided. This study presented a detailed investigation on the microstructures, mechanical properties, tribological performance and biocompatibility of the graded nanocrystalline TiN coating. The strengthening, toughening and anti-wear mechanisms of the coatings were clarified. For comparison, the TiN monolayer coating with the same thickness was investigated in the same conditions.
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Table 1 The design of N2 gas flow rate and deposition time during sputtering process. N2 flow rate (sccm)
1 2 3
4
5
6
7
8
In total
Deposition time (min) Gradient layer 4 8 12 16 20 24 28 63 175 Monolayer – – – – – – – – 175 Note: Ar gas flow rate was kept at 30 sccm. Ti interlayer was deposited for both monolayer and graded coatings.
32
34
36
38
40
42
44
46
2 Theta (deg.) Fig. 1. XRD patterns of the monolayer and gradient coating.
48
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(b)
(a)
5 µm
5 µm
Fig. 2. The cross sectional micrographs of the coatings. (a) columnar crystal structure of the monolayer coating, (b) compact structure of the gradient coating.
growing columnar crystallines sheltered the atoms depositing at a certain inclination angle, as a result, leaving many spaces in these regions. By contrast, the gradient coating exhibited a dense non-columnar crystal structure without distinct layer interface (Fig. 2b). This indicates that more and more new TiN nucleation formed with the increase of N2 gas flow rate during the sputtering deposition, which interrupted the orientated growth of the columnar crystallines. Meanwhile, at lower N2 gas flow rate, Ti atoms were less likely to be collided. Ti atoms with high kinetic energy obtained larger surface mobility, which promoted the horizontal growth of the coating and increased the density of the coating.
of TiN grain, a competitive growth mechanism driven by diffusion and elastic strain energy forces TiN columnar crystal to preferentially grow along (111) orientation. Based on the XRD results, it is deduced that the gradient coating exhibited a noncolumnar crystal structure, which was proved by SEM and TEM observation later. The cross-sectional fracture micrographs of the monolayer and gradient coatings are shown in Fig. 2a and b. The thickness of the two coatings was about 5.8–6 μm. The monolayer coating exhibited columnar crystal structure. The ragged columnar crystallines implied the existence of the voids or nano-gaps in the monolayer coating. These defects arose from the uniform growth rate of columnar crystallines. The fast
(b)
(a)
Fig. 3. The surface AFM images of the monolayer coating (a) and gradient coating (b) showing the mean square roughness of 130 nm and 62 nm, respectively.
20000
N Ti
18000
Substrate
Coating
16000
1 2
Counts
14000
3 4
12000
Coating
Substrate
120 90 60
1 µm
30 0
0.000
0.002
0.004
0.006
0.008
0.010
0.012
WD/mm Fig. 4. Cross sectional image of the gradient coating and EPMA analysis showing Ti and N composition change (along white line).
0.014
W. Cui et al. / Materials Science and Engineering C 71 (2017) 520–528 Table 2 The composition analysis of Ti and N elements in the gradient coating. Locations
1
2
3
4
Ti (at.%) N (at.%)
88 12
75 25
62 38
51 49
The surface roughness of the coating was measured by atomic force microscope (AFM). Fig. 3 gives the three-dimensional surface images of the two coatings. The surface of the monolayer coating looked like upand-down-hills with the mean square roughness of 130 nm. The gradient coating exhibited relatively flat surface on which the fine particles dispersed. The mean square roughness was measured to be only 62 nm. Ti and N element distribution across the gradient coating were analyzed by EPMA, as shown in Fig. 4. It is clearly seen that the gradient coating was divided into two parts. The light-color and dark-color layers represented the “soft” zone with low N content and the “hard” zone with high N content, respectively. Table 2 gives Ti and N concentration at different locations of the coating. In the light-color zone, N content was b25 at.%. In the outer layer, the atomic ratio of Ti and N was close to 1:1. According to Ti-N binary phase diagram, Ti and N can form a series of solid solutions or compounds: Ti4N, Ti3N, Ti2N, TiN, Ti3N4, Ti3N5, Ti5N6 etc. Among them, N content in stable TiN phase is N42 (at.%). Other Ti-N compounds with N contents b32 (at.%) belong to the nitrogen vacancy type transitional phase or solid solution. It is thus believed that Ti4N3 − x transitional phase in the gradient coating formed at low N2 gas flow rate and mainly existed in the inner layer adjacent to the substrate. While non-stoichiometric TiN possibly formed at any N2 gas flow rate due to the strong binding force between N and Ti. The relative proportion of Ti4N3 − x and TiN changed with N2 gas flow rate. Increasing N2 gas flow rate enhanced the volume fraction of TiN until 100% TiN formed in the top layer of the gradient coating. In order to observe the morphologies of TiN and Ti4N3 − x at different level layers of the gradient coating, TEM films were prepared by
(a)
(c)
(220) (200) (111)
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sputtering the coating on copper net at different N2 gas flow rates. Fig. 5 is a group of TEM bright field images of the microstructures and the corresponding selected area diffraction (SAED) patterns. Dispersive nano-scale particles can be clearly seen at various N2 gas flow rates. SAED analysis indicates that all polycrystalline diffraction rings came from TiN phase, which proved the formation of TiN at any N2 gas flow rate. At low N2 gas flow rate, small amount of nano-TiN particles were embedded in Ti4N3 − x transitional phase (Fig. 5a). At high N2 gas flow rate, the volume fraction of nano-TiN increased, at the same time, some nano-TiN particles tended to agglomerate together (Fig. 5d). The average size of nano-TiN also increased with N2 gas flow rate, from 6.5 ± 3.4 nm at 2 sccm N2 flow rate to 11 ± 2.4 nm at 8 sccm N2 flow rate. An interesting phenomenon is that the gray background in Fig. 5a–d becomes more and more clear with the increase of N2 gas flow rate, indicating that strain contrast caused by lattice defects is gradually disappearing. Fig. 6 shows the HR-TEM images of nano-TiN, by which the lattice structure of nanocrystallines can be understood more clearly. At 2 sccm N2 gas flow rate, the two types of nanocrystallines were observed. The one included many lattice defects, such as vacancies, dislocations, stacking faults and lattice distortion, as indicated by white arrows in Fig. 6a. The other exhibited perfect lattice structure. At 8 sccm N2 gas flow rate, few lattice defects were observed in all nanocrystallines. The results are agreement with our analysis mentioned above. The nanocrystallines containing serious defects represented Ti4N3 − x transitional phase because the lack of nitrogen produced a large number of lattice defects. TiN nanocrystallines had relatively perfect crystal structure. Increasing N2 gas flow rate promoted the formation of more nano-TiN in accompanying with the decrease of Ti4N3 − x transitional phase and the disappearance of the lattice defects. This explains why the strain contrast of the background in Fig. 5a–d displayed different change with N2 gas flow rate. Because of the great differences in N content and lattice defects, Ti4N3 − x and TiN behaved different mechanical properties. The graded distribution of the two phases determined the peculiar mechanical behavior of the gradient coating.
(b)
(d)
Fig. 5. TEM images and the corresponding SAED patterns of nano-TiN formed at different N2 gas flow rates. (a) 2 sccm, (b) 4 sccm, (c) 6 sccm, (d) 8 sccm.
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(a)
(b)
TiN (0002)
TiN
TiN
Ti4N3-x
(0002)
TiN
5 nm
5 nm
Fig. 6. HR-TEM images of the nanocrystallines in the gradient coating formed at 2 sccm (a) and 8 sccm (b) N2 gas flow rates.
8000
8000
(a)
(b)
6000
6000
4000
4000
2000
2000
0
0
20
40
60
80
100 120 140 160 180
0
0
20
40
60
80
100
120
140
Fig. 7. The load-displacement curves of the monolayer (a) and graded (b) TiN coating by nanoindentation tests.
3.2. Mechanical properties The nanoindentation hardness and elastic modulus of the coatings were measured by nano-indentation tests. The load-displacement curves of the monolayer and graded TiN coating are shown in Fig. 7. The smooth and continuous curves suggested that no cracks occurred to the monolayer and graded coating during loading, and the substrate had no influence on the deformation behavior of the coatings. The hardness and elastic modulus of the coatings are listed in Table 3. It can be seen that the elastic modulus of the monolayer coating was only 238 ± 3 GPa, lower than the normal value of TiN (~280 GPa). This can be explained by the lower compactness in the outer layer of columnar crystal coating. The elastic modulus of the gradient coating attained 281 ± 5 GPa, also slightly smaller than other TiN coatings [19]. It has been reported that the modulus of non-stoichiometric TiN phase decreases with the decrease of N content because the existence of the vacancies in the lattice reduces the total bond energy [20]. Apparently, Ti4N3 − x transitional phase in the gradient coating was responsible for the reduction of elastic modulus. In spite of this, the hardness of the gradient coating was almost two times higher than the monolayer coating. The compact structure and nanocrystalline strengthening effect
made great contribution to enhancing the hardness of the gradient coating. The adhesion strength of the coating to the substrate is one of the important mechanical properties that determine the service life. Scratch test is widely used for evaluating the adhesion force of the coating. Fig. 8 shows the acoustic emission signal plots during applying load until to 150 N. SEM images of the scratch tracks were superimposed over the plots. A fine tensile stress crack can be seen at the bottom of the scratch tracks. The first sudden change (Lc1) of the signal intensity represented the appearance of the initial crack. The second drastic increase (Lc2) of the signal intensity represented the exfoliation or cracking of the coating. Table 3 shows that both Lc1 and Lc2 values of the gradient coating were much higher than those of the monolayer coating. Zhang et al. [21] suggested that the coating toughness was proportional to Lc1 as well as difference between Lc1 and Lc2. A parameter named “scratch crack propagation resistance” (CPR = Lc1 (Lc2 − Lc1)) was directly used to indicate the coating toughness. According to this argue, the CPR values of the monolayer and gradient coating were 1050 and 3321, respectively. This indicates that the gradient coating exhibited an enhanced resistance to scratch damage. 3.3. Tribological performance
Table 3 The mechanical properties of the monolayer and gradient coatings. Coating
Elastic modulus (GPa)
Hardness (GPa)
Monolayer Gradient
238 ± 3 281 ± 5
10.2 ± 1.2 20.8 ± 1.1
Data are expressed as mean values ± standard deviation.
Bonding force (N) Lc1
Lc2
50 ± 5 81 ± 4
71 ± 4 122 ± 6
The ball-on-disc wear tests were performed on the coated specimens against Si3N4 counterpart ball under the load of 10 N in Hank's solution. The maximum Hertzian contact stress reached 1630 MPa. Fig. 9 shows the change of the coefficient of friction (COF) with time during sliding abrasion. The rapid increase and then decrease of the COFs in the initial stage were caused by the polished process of the primary coarse surface. In comparison with the monolayer coating, the steadystate COF of the gradient coating decreases by 50%. Stolyarov et al.
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(b)
Lc1
Signal intensity
Signal intensity
(a)
Lc2
Lc2
Lc1
Load, N
Load, N
Fig. 8. The acoustic emission signals patterns during scratching and the scratch track images showing the critical forces (Lc1 and Lc2) of the coating rupture. (a) Monolayer coating (b) Gradient coating.
[22] suggested that COF can be divided into an adhesion component and a deformation component, which result from the adhesion force and plastic deformation force, respectively. Adhesion force is proportional to the surface energy of the two contacting surfaces [23]. Plastic deformation force depends on the H3/E2 ratio according to the Tsui's expression [24]:
P y ¼ 0:78r 2
H3 E
!
depth and wear track width are shown in Table 4. The wear rate (0.62 × 10−6 mm3/Nm) of the gradient coating is only one-sixth of the monolayer coating. The value is even smaller than that of TiAlN coating (1 × 10−4 mm3/Nm), TiNbN and TiCN coating (5.9 × 10−5 mm3/Nm) in simulated body fluid [19,25], showing the excellent anti-wear performance of the gradient coating. 3.4. Biocompatibility evaluation.
ð1Þ
2
where: Py is the yield stress to resist plastic deformation, H is the hardness, E is the elastic modulus, r is the radius of rigid contact ball. Because of smaller mean square roughness and higher hardness, the gradient coating has a lower surface energy and larger H3/E2 ratio than the monolayer coating. It indicates that the gradient coating exhibits low adhesion force and high resistance to plastic deformation. Fig. 9b and c display the wear track morphologies of the two coatings. Many triangular craters and deep plowing grooves can been seen on the wear track of the monolayer coating, which reflect the occurrence of serious adhesion and plastic deformation during sliding wear. In contrast, the gradient coating shows a smoother wear track and a narrower track width. The wear rates of the coatings calculated by measuring wear track
In the cell cytotoxicity tests, the OD values of the cells, which were cultured for 24 h, 48 h and 72 h, respectively, are listed in Table S1. RGRs (%) of three experimental groups are all greater than 90% and show no significant difference one another (p b 0.05). The results of hemocompatibility tests are shown in Table S2. The absorbancy and the hemolysis ratio of the experimental groups and the control groups also show no significant statistical difference (p b 0.05), indicating that the TiN gradient coating presents good blood compatibility. Table S3 shows that the body temperature rise of each SD rat in the experimental group and the control group is below 0.6 °C, and the sum of the body temperature rise of all rats is below 4.5 °C. This implies that the TiN gradient coating is not a thermogenic material. The general toxicity tests indicate that each SD rat in the experimental group and control group exhibits good mental state and normal
(b) 0.7 monolayer coating graded coating
(a) Coefficient of friction
0.6
80µm
0.5
(c) 0.4 0.3 0.2 0.1
80µm
0
1000 2000
3000
4000 5000 6000 7000
Sliding time (s) Fig. 9. (a) COF vs. sliding time curves of the monolayer and gradient coating, (b) the worn track morphologies of the monolayer coating, (c) the worn track morphologies of the gradient coating.
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Table 4 The coefficients of friction and wear rates of the monolayer and gradient coating. Coating
COF
Wear rate (×10−6 mm3/Nm)
Wear track depth (μm)
Wear track width (μm)
Monolayer Gradient
0.49 ± 0.04 0.21 ± 0.02
4.32 ± 0.23 0.62 ± 0.34
0.99 ± 0.12 0.34 ± 0.23
409 ± 11 220 ± 15
Data are expressed as mean values ± standard deviation.
physical performance. There are no significant changes in urine, stool and diet for the rats before and after injection. No toxic symptoms appear, such as blepharoptosis, tremor, collapse and even death. Fig. 10 gives the photographs of histopathological HE stained section (liver, heart and kidney) of SD rats in the experimental group (Group A) and the control group (Group B), which indicate that no inflammatory cells have been found in the visual field; myocardial cells shows no degeneration, necrosis or hypertrophy. The shapes of myocardial fibers are regular, and liver cells show no edema, steatosis, acidophilic change or spotty necrosis, glomeruli are basically normal, and renal tubular epithelial cells shows no edema. According to these basic data, the gradient TiN coating displays a very good biocompatibility. 4. Discussion Hard coating failure on relatively compliant substrate in many tribological situations is generally caused by the coating delaminating from the substrate and the fracture of coating itself. The failure is primarily related to high residual stress gradient at the interface and low toughness of the coating. Evans and Marshall established the relationships
liver
heart
between wear rate W and elastic modulus E, hardness H and fracture toughness KIC of ceramic material [26]: W ¼a
4=5 E 1=2 K H 5=8 H F 1=8
ð2Þ
IC
where a is a constant depending material structure, and F is load. The theory suggests that high anti-wear performance of a hard coating can be obtained through a good match of elastic modulus, hardness and toughness. Unfortunately, the high hardness of the materials is usually accompanied by high elastic modulus and low toughness. The brittleness of the coatings results in early cracking and low adhesion. Actually, the wear rates of the hard or ultra-hard coatings are mostly in the range of 10−6– 10−4 mm3/Nm and the adhesion strength in the range of 20–50 N [27–32]. In this work, the functionally graded nano-TiN coating successfully enhances the load bearing capability as well as the adhesion to the substrate. The gradual increase of nitrogen content within the coating avoids the sharp changes in chemistry, crystal structure and mechanical properties. Refined TiN grains decrease surface roughness and increases hardness. Both the adhesion strength and the tribological performance
kidney
Fig. 10. The hints of pathological HE stained section of the experimental group (Group A) and the control group (Group B). The results indicate that no inflammatory cells were found in the visual field; myocardial cells showed no degeneration, necrosis or hypertrophy; the shapes of myocardial fibers were regular; liver cells showed no edema, steatosis, acidophilic change or spotty necrosis; glomeruli were basically normal, and renal tubular epithelial cells showed no edema.
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of the graded nano-TiN coating are greatly superior to the monolayer TiN coating. Based on the results of XRD analysis and TEM observation, the gradient coating experiences a series of microstructural changes from bottom layer to top layer: Ti adhesion layer (extremely thin film) → Ti4N3 − x transitional phase (major) + nano-TiN (minor) → nano-TiN (major) + Ti4N3 − x transitional phase (minor) → nano-TiN (whole). The size and volume fraction of nano-TiN simultaneously increase with the gradual increase of N2 gas flow rate and deposition time, which determines the graded change of mechanical properties. Ti4N3 − x transitional phase in the inner layer of the coating tends to exhibit metal properties owing to the deficiency of N atoms, e.g. low hardness and elastic modulus, good plasticity and toughness. These properties constitute a good match with Ti alloy substrate, which reduces thermal stress and growth stress of the coating. Moreover, Ti4N3 − x acts as a role of stress buffer and can effectively release in-plane residual stress at the interface. These factors greatly enhance the sustainability of the coating/substrate bonding [33]. Nano-TiN plays an important role in the aspects of enhancing hardness and elastic modulus of the coating. Since the plastic deformation of nanocrystalline materials cannot be realized by dislocation slip because of high resistance to dislocation movement within nano-grains, nanoTiN behaves higher strength and hardness than coarse grain TiN. On the other hand, in the gradient coating, the “hard” TiN nanocrystallines are surrounded by the “soft” Ti4N3 − x transitional phase. The latter is helpful to relax stress concentration at the crack tip through local plastic deformation and consume more energy stopping crack propagation. Therefore, the composite of “hard” and “soft” phases exhibits a good combination of high hardness and toughness. It is an importance assurance that the gradient coating has low friction and high wear resistance. The biological evaluations prove that the biocompatibility of the gradient coating is as good as monolayer coating. It has been reported that the surface nanotopography of the implant plays a great role of increasing the numbers of attached 3T3-L1 fibroblasts [34]. Here, it can be expected that, by optimizing the sputtering deposition routes, the graded nano-TiN coating will comprehensively improve adhesion, tribological performance, biocompatibility and cellular response. The newly developed TiN gradient coating has potential application prospects for the implant medical devices. 5. Conclusions A graded nano-TiN coating was prepared on Ti-6Al-4V biomaterial substrate by controlling N2 gas flow rate during magnetron sputtering deposition. The gradient coating exhibits a gradual change in N composition, microstructure and mechanical properties along film growth direction, which thereby greatly improves adhesion force to the substrate (LC1 = 80 N). The coefficient of friction (0.21) and wear rate (6.2 × 10− 7 mm3/Nm) of the gradient coating are much lower than monolayer TiN coating in physiological environment. Nanocrystalline TiN makes great contribution to increasing the hardness and decreasing the surface roughness of the gradient coating. Meanwhile, the combination of “hard” TiN compound with “soft” Ti4N3 − x transitional phase is helpful to increase the resistance to plastic deformation and cracking. The low friction and high wear resistance of the gradient coating are attributed to the comprehensive improvement of hardness, toughness and surface roughness. A series of biocompatibility evaluations suggest that the newly developed graded nano-TiN coating has good bio-safety and potential application prospect for implant medical devices, especially for movable medical parts. Supplementary data to this article can be found online at doi:10. 1016/j.msec.2016.10.033. Acknowledgement The study was financially supported by Foundation for Key Program of Ministry of Education, China (No. 313014) and National Natural
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