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Microstructure and properties of Ta coatings on the 3Y-TZP ceramic fabricated by plasma alloying technique
Journal of Alloys and Compounds 805 (2019) 1135e1143
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and properties of Ta coatings on the 3Y-TZP ceramic fabricated by plasma alloying technique Liangliang Li a, Hongjun Hei a, Yongsheng Wang a, Ke Zheng a, Yong Ma a, Jie Gao a, Bing Zhou a, Zhiyong He a, Jie Zong b, Shengwang Yu a, *, Bin Tang a a b
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 February 2019 Received in revised form 18 June 2019 Accepted 15 July 2019 Available online 16 July 2019
Ta coatings on the 3Y-TZP ceramic substrate were successfully fabricated by plasma alloying technique, and its effect on the tetragonal-to-monoclinic phase transformation was investigated. The results indicate that the surface morphology and grain size of Ta coatings were influenced by the deposition temperature. Results of the surface roughness and static contact angle provide further supports. Scratch tests of Ta coatings deposited at 800 C (S2 specimen) show the good adhesion. The Ta coating has a thickness of ~590 nm in S2 specimen. Moreover, the friction properties of these Ta coatings in phosphate buffered saline solution exhibit the low friction coefficient. The Ta coating of S2 specimen exhibits an effectively protection for the phase transformation of the 3Y-TZP ceramic tested by hydrothermal aging. © 2019 Published by Elsevier B.V.
Keywords: 3Y-TZP Ta coating Adhesive strength Hydrothermal aging
1. Introduction In recent years, zirconia ceramics have got a wide spread attentions due to their excellent properties such as high strength, fracture toughness, lower cost, corrosion resistance, excellent cytocompatibility, fewer bacterial adhesions and osteointegration etc. [1e4], providing them a good future in structural and biomedical applications. These advantages of the zirconia ceramic relate with its three crystallographic polymorphs [2,3], the monoclinic phase (m) at room temperature, the tetragonal phase (t) at 1170 C and cubic structure (c) at 2370 C. The metastable tetragonal phase can be got at room temperature by introducing the oxidation into the monoclinic zirconia, such as the tetragonal zirconia polycrystals stabilized with 3 mol % yttria (3Y-TZP) for dental application. The transformation from tetragonal to into monoclinic phase (t/m) would be induced by eternal loading, leading to the increasing resistance to further cracks propagation, named as phase transformation toughening [3]. Many factors such as the load, friction, acid-base environment could promote t/m transformation where zirconia is exposed in vivo [4e6]. However, a high level of m-phase may be detrimental to mechanical properties such
* Corresponding author. E-mail address: [email protected] (S. Yu). https://doi.org/10.1016/j.jallcom.2019.07.155 0925-8388/© 2019 Published by Elsevier B.V.
as strength and toughness, which may impair the long-term clinical success of zirconia-based restorations [1,2,7]. So, approaches to restrain the transformation are really appreciate. Nowadays, some ways have been developed to prevent the early occurrence of t/m phase transformation. Zirconia based composites had been developed by adding the mullite or Al2O3 to alleviate surface degradation in an environment of water or body fluid [1,8,9]. However, the zirconia content and yttria addition in these composites would lower the benefits described above. A further strategy to introduce CeO2 as stabilizer instead of Y2O3 in zirconia can improve significantly the aging resistance [6,10], but simultaneously lower the biaxial strength and hardness. To deposit a protective layer on the surface of 3Y-TZP [11], as a promising alternative way, has attractive more attentions due to the thin protective layer not jeopardize the machined substrate's dimensional accuracy and mechanical properties. Tantalum (Ta) has been developed as a promising metal for biomedical implants or implant coatings owing to its excellent corrosion resistant, bioactive in vivo and fracture toughness [12,13]. Compared with other biomedical metal alloys e.g. CoCr [14], or stainless steel [15], the Ta coating has the similar physical and mechanical properties with 3Y-TZP, e.g. the elastic modulus, melting point and coefficient of thermal expansion [12,13,15,16]. However, Ta coating are hardly to fabricate on the surface of 3YTZP by traditional process such as chemical vapor deposition [17].
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Double glow plasma surface alloys (DGPSA) technique [18], an innovative surface treatment technique, has been developed successfully on the basis of the plasma nitriding and sputtering technology. Researchers has employed this technique to improve the surface properties of metals or alloys, e.g. the wear [19], corrosion and oxidation resistance [20]. In this paper, in order to determine the effect of Ta coating on the t/m phase transformation of the 3Y-TZP ceramic, a group of Ta coatings were successfully fabricated as a protective layer on a 3Y-TZP at various substrate temperature by DGPSA technique. After analyzing the morphology, microstructure, adhering strength and tribological property, a specimen with good combination properties was selected to conduct the hydrothermal aging. The protective effect of the Ta coatings was studied.
2.2. Characterization of Ta coatings The surface and cross-sectional morphologies of Ta coatings were characterized by field emission scanning electron microscopy (FESEM, TESCAN MIRA3 LMH). The surface of the S2 sample was cut by focused ion beam (FIB) and the morphology of the cross section was analyzed. The ion beam parameters in FIB milling mode were 30 kV accelerating voltage and 5 nA beam current. The phase compositions of Ta coatings were identified by X-ray diffraction (XRD, DX-2700 X) using Cu Ka radiation. The average surface roughness was measured by white light scanning profiler (Contour GT-K, Bruker). The static contact angle was measured by sessile drop and the obtained images were analyzed to calculate the contact angle of deionized water for each sample at room temperature. Each reported contact angle is the mean and standard deviation value for at least three independent measurements.
2. Experimental procedure
2.3. Adhesion and wear properties of Ta coatings
2.1. Fabrication of Ta coatings
The adhesion of a Ta coating was tested by WS-2005 scratching tester under a continuous load from 0 N to 100 N and at the loading speed of 50 N/min. The wear resistance in phosphate buffered saline solution was measured against Si3N4 balls with diameter of 5.5 mm by MFT-R4000 reciprocating frication and wear tester with a load of 5 N and sliding duration of 15 min.
A commercially available 3Y-TZP ceramic specimens with dimension of 14 mm in diameter and 1.2 mm in thickness were prepared as substrates according to ISO 6872. The as-received surface morphology of the 3Y-TZP ceramic substrate displays the nanocrystalline grains with an average size of 407.6 ± 11.2 nm, as shown in Fig. 1. Ta coatings were deposited on the 3Y-TZP substrate by DGPSA devices described in detail in a previous study under an argon atmosphere (purity of 99.999%) [21]. Here, pure tantalum with purity of 99.99 wt %, as the target material, was cut into the sheet with dimension of 60 mm in diameter and 1.5 mm in thickness. The distance between the upper substrate surface and the lower target surface was set as 18 ± 1 mm. When the argon gas was filled and the two powers were turned on, the sample cathode and the source cathode were surrounded by two glow discharges. Ta atoms sputtered from the target were transferred to the substrate surface, while the substrate was heated by ion bombardments. The working pressure during deposition was maintained at 35 Pa and the Ar flow rate was 60 sccm. The sample cathode voltage range was 500 V and the source cathode voltage range was 750 V. The substrate temperature was maintained using the infrared thermometer. Four specimens of the Ta coating named as S1, S2, S3 and S4 were fabricated by controlling the deposition temperature, which corresponded to 750 C, 800 C, 850 C and 900 C. The deposition time of all coated specimens is of 10 min.
2.4. Hydrothermal aging The protective effect of Ta coatings was investigated by successive hydrothermal aging of the specimens in an autoclave (Type LS-30, BoXun) in water vapor at 134 C and 3 bar. Chemical composition of Ta coatings was determined using a K-Alpha XPS spectrometer (Thermo Fisher Scientific, Waltham). The progress of the t/m phase transformation was observed with five specimens each per group after deal with 0 h, 5 h, 10 h, 20 h, 40 h, 80 h, 100 h, and the aging step was followed by XRD. The cross section of specimens after aging were acquired by FESEM. 3. Results 3.1. Microstructure of Ta coatings Fig. 2 shows the microstructure of Ta coatings and the 3Y-TZP substrate identified by XRD pattern. Clearly, the diffraction peaks appear at diffraction angels of 38.4 , 55.5 , 69.5 and 82.4 (2q) for all coated specimens, corresponding to crystalline plane orientations of (110), (200), (211) and (220) of a-Ta phase with a bodycentered-cubic structure. Additionally, the intensities of these diffraction peaks especially at 69.5 of coated specimens increases with rising of the deposited temperature, meaning an increase of the a-Ta coating with the increasing temperature. Due to the thickness of these Ta coatings less than 1 mm, it is very easy for the X-rays to penetrate through the interlayer to detect the ZrO2. So, the 3Y-TZP substrate diffraction peaks can be observed in all XRD results of coated specimens. On the other hand, no obvious difference of the 3Y-TZP substrate's diffraction peaks between the uncoated and coated specimens suggests that the t/m phase transformation of a 3Y-TZP ceramic did not happen during fabricating these Ta coatings. 3.2. Coss-section and surface morphology
Fig. 1. Surface morphology of the 3Y-TZP ceramic substrate.
Fig. 3 shows the cross-sectional SEM image which was fabricated by FIB. In order to prevent the ion beam from damaging the Ta coating, the Pt coating was prepared. It can be seen obviously that
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Fig. 4(c) and (d). Same trends can be found by comparing the data of the contact angle in Table 1. It indicates that the surface characteristics of these Ta coatings depend on the surface of the deposition temperature. 3.3. Adhesion strength of Ta coatings Fig. 5 shows scratch results including variations of acoustic emission signal and the corresponding friction force for testing the adhesion of these coatings. In the process of loading from 0 to 100 N by a probe, the acoustic emission signals exhibit distinctly peaks of Ta coatings. Typically, the first high-amplitude signal peak of the specimen S1 appears at ~24 N corresponding to the crack in this load, as shown in Fig. 5(a). However, the acoustic emission signal keeps in a very low fluctuation until ~91 N for the specimen S2 during the same loading method, as shown in Fig. 5(b), indicating that the adhesion strength of the specimen S2 is stronger than that of the specimen S1. In comparison, the specimens S3 and S4 display the good adhesion but weaker than that of S2 (Fig. 5(c) and (d)). 3.4. Friction properties of Ta coatings
Fig. 2. XRD patterns of Ta coatings on the 3Y-TZP substrates fabricated at different temperature: S1 750 C, S2 800 C, S3 850 C and S4 900 C.
Fig. 3. The cross-sectional SEM of Ta coating deposited at 800 C.
the thickness of Ta coating is about 590 nm. Fig. 4 shows backscattering electron images of Ta coatings deposited at different temperatures. All Ta coatings consist of nanocrystalline grains with a pyramid-like morphology, and densely cover onto the surface of substrates. Table 1 is the surface roughness of all specimens and the substrate. Clearly, the surface morphology of specimen S1 is in Fig. 4(a). Same defects could be observed on the image at high magnification, as shown in Fig. 4(a1), which may relate with deposition process of Ta coatings. It can be observed that the specimen S2 exhibits the fine and uniform grain with an average size of 66 ± 7.6 nm, as shown in Fig. 4(b) and (b1). With increasing of the deposition temperature, the specimen shows coarser grain on the surface, e.g. specimens S3 and S4 in
On the basis of Fig. 5, the friction properties of Ta coatings in phosphate buffered saline solution were investigated at a load of 5 N. Fig. 6(a1) - (d1) show the friction coefficient curves of a 3Y-TZP ceramic with the Ta coating against Si3N4 balls with diameter of 5.5 mm at room temperature. Clearly, the average friction coefficient dropped slightly with the deposition temperature increasing form 750 Ce800 C, but increased with the further rising temperature to 900 C. The depth and width of the wear tracks exhibited same trends with the rising deposition temperature. It indicates that friction property depends on the Ta coating and 3YTZP substrate. More details of the wear scar surface are investigated in Fig. 7. Clearly, the bare surface and wear tracks of the 3Y-TZP substrate also formed on the worn surface [Fig. 7(a)e(b)], e.g. the bare surface ‘i’ in specimen S1 with a composition of O51.5Zr29.2Ta17.7Si1.3N0.3. A large number of wear debris can be observed on the coarse worn surface of specimens S1 and S2, e.g. debris ‘ii’ in specimen S2 with a composition of Ta38.3O34.0Zr23.2Si4.5 [Fig. 7(c)e(d)]. Moreover, many wear debris particles (‘iii’ in specimen S2 with a composition of O52.3Zr32.8Ta9.1Si5.4N0.4) and grooves (‘i’ in specimen S4 with a composition of O56.3Zr36.4Ta5.0Si2.3) could be seen on the worn surface [Fig. 7(e)e(h)]. The Ta-coatings are easily worn away from 3Y-TZP ceramic substrate by Si3N4 ball due to the difference between tantalum and Si3N4 in Vickers hardness and shear modulus [22]. When the Ta-coatings were worn out, however, the bare surface of the 3Y-TZP substrate wear off against the Si3N4 ball. In comparison, the elastic and shear moduli of Si3N4 and 3Y-TZP ceramic are almost the same [22]. So, many wear debris including Si3N4 and 3Y-TZP ceramic particles would be produced during wear processing. On the other hand, the Ta coating and its fragments would either occur the work-hardening due to the excellent plasticity and ductility, or adhere or stick to the worn region with Si3N4 and 3Y-TZP ceramic particles during wear processing. 3.5. Hydrothermal aging Fig. 8 shows the XPS spectra of the Ta coating specimen deposited at 800 C. The major characteristic peaks consist of oxygen (O1s), Ta (Ta4f, Ta4d, Ta4p and Ta4s) and C (C1s), as shown in Fig. 8(a). All binding energies were calibrated with respect to the C1s characteristic binding energy at 285.0 eV. The presence of carbon may attribute to the environmental contamination. The Ta coating contained a small fraction of oxygen but without Zr signal
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Fig. 4. Surface morphologies and their high magnification images of Ta coatings deposited at different temperatures: (a) S1: 750 C, (b) S2: 800 C, (c) S3: 850 C and (d) S4: 900 C.
Table 1 The roughness and contact angle of uncoated and Ta coated specimens. Specimens
Uncoated
S1
S2
S3
S4
Roughness/nm Contact angle/
161.1 ± 1.0 41 ± 1.8
138.9 ± 1.8 60 ± 1.5
108.3 ± 6.1 28 ± 0.7
151.6 ± 1.1 43 ± 0.5
165.8 ± 13.1 46 ± 1.1
Fig. 5. Scratch adhesion results of Ta coatings: (a) S1, (b) S2, (c) S3 and (d) S4.
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Fig. 6. Friction coefficient curves of a 3Y-TZP ceramic with the Ta coating as a function of the sliding time and corresponding surface profiles of wear tracks: (a1) and (a2) for S1, (b1) and (b2) for S2, (c1) and (c2) for S3, (d1) and (d2) for S4.
due to the oxidation in air. It indicates that a compact Ta coating was formed and completely covered the 3Y-TZP substrate. To reveal more information of the Ta coating, the high resolution Ta4f spectrum is fitted regard to the 4f7/2 and 4f5/2 peak intensity, as shown in Fig. 7(b). Clearly, the Ta4f core level consists of three peaks. One doublet at binding energies of 21.9 eV and 23.7 eV with the highest intensity are attributed to Ta4f7/2 and Ta4f5/2 of Ta, respectively. The other at binding energies of 24.9 eV is assigned to Ta4f5/2 of Ta2C and 27.0 eV to Ta4f7/2 of Ta2O5. It further indicates that Ta coatings are successfully deposited on the surfaces of ZrO2 substrate but a small amount of Ta2O5 on the surface. In order to evaluate the protecting effect of the Ta coating onto 3Y-TZP ceramic on the t/m phase transformation, the hydrothermal aging was conducted in water steam at 134 C and 3 bar, as shown in Fig. 9. The diffraction peak at 30.1 (2q) corresponds to the tetragonal phase from the crystallographic orientation of (101), but diffraction peaks at 28.2 and 31.3 are the monoclinic phases from the crystallographic orientations of (111) and (111), respectively. 4. Discussion Many literatures have investigated the relationship between the coating and the substrate, such as SiC coating on WC-Co [23], W or Cr coating on diamond [24,25], Ta coating on stainless steel [26] or Ti-6Al-4V [27,28], HfC coating on WC-Co [29] etc. Basically, the effects of these coatings on the properties such as friction property and corrosion resistance etc. are determined by their metallization process, which generally consists of the deposition and annealing process [18,21,30]. Here, the influence of adhesion, friction property and suppressing phase transformation of Ta coatings on the 3Y-TZP ceramic will be discussed on the basis of metallization. 4.1. Deposition and diffusion of metallization When the solid metal target (e.g. W, Ta and Mo etc.) was activated by the glow discharge [21], the metallic particles including atoms and/or ions formed, and deposited on the substrate. Here, due to the constant values of other parameters [such as surface of
the substrate, ratio of (CH4/H2þCH4), substrate voltage etc., as description of II experiment part], the surface morphology, roughness and adhesion were only related with the deposited temperature. At low temperature e.g. 750 C, the substrate surface which may full of pits with size of ~500 nm deposited a thin Ta coating by the fine Ta crystal. Although the Ta coating covered totally the substrate surface, these pits may not be filled up, leading to a high surface roughness, as shown in Fig. 4 and Table 1. Similar results have been reported in previous finding of our group [18,19,21,23e25,29]. The low adhesion contributes to the weak diffusion between the substrate surface and coating [24,31], as shown in Fig. 5 and Table 2. The original pits of the substrate surface would almost be filled up as increasing of the deposited temperature (e.g. 800 C) [24,29], corresponding to the increased thickness of the deposited layer. At the same time, the diffusion layer will increase due to the higher diffusion coefficient at high deposited temperature. So, the surface roughness decreased, but the critical force of the adhesion increased, as shown in Figs. 4 and 5. However, when the deposition temperature further increased, the deposited Ta coating would form itself pyramid morphology on the substrate surface, resulting in the increasing the surface roughness. On the other hand, the cavities may easily emerge at the boundary between the substrate and Ta coating caused by the excessive diffusion, which would worsen the adhesion. It is also found in SiC coating on WC-Co [23], W coating on diamond [24], W coating on C/C composite [30], Ta coating on Ti6Al4V [26]. 4.2. Friction behavior In order to reveal more details, the wear rate of these Ta coatings was calculated by using following equation [32]:
K¼
Lh 3h2 þ 4b2 6PSb
(1)
where K is the wear rate; S and P represent the sliding length and normal load; L, h and b are length, depth and width of wear crack,
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Fig. 7. The SEM images of wear scar surfaces and the corresponding enlarge images: (a) and (b) for S1, (c) and (d) for S2, (e) and (f) for S3, (g) and (h) for S4.
respectively. The wear rate dropped from 4.92*105 mm2 to 3.58*105 mm2 with the rising deposited temperature from 750 C to 800 C, as shown in Fig. 10. However, the value increased with further rising temperature, e.g. 6.88 *105 mm2 when deposited at 900 C. A possible reason is that the original pits on the surface of substrate could be compensated gradually by the Ta coatings during the deposition of metallization process, leading to the dropped wear rate at 800 C. Nevertheless, the higher deposited temperature would promote the surface roughening of the Ta coating, which deteriorates the friction property. Here, the S2 deposited at 800 C shows the good combination properties.
4.3. Improvement of the t/m phase transformation The m-phase fraction, Vm, was determined by following equation [33,34]:
Vm
1:311 I 111 þ I 111 m m ¼ þ I 101 þ I 111 1:311 I 111 m m t
(2)
where Im represents the integral intensity from XRD diffraction peaks of (111) and (111) of the m-phase, It is the integral intensity of the t-phase diffraction peak (101), as shown in Fig. 9. It should be
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Table 2 Parameters of the JMAK equation (Eq. (4)) according to non-linear regression of mphase fraction vs. time. Specimens
Vm0/%
Vmf/%
b/h1
n
Uncoated S2
0 0
79 36
0.035 0.022
0.81 0.74
noticed that the calculated phase content represents an integral value for the near-surface region due to the limited penetration depth of the X-rays caused by the scattering and absorption [35,36]. The m-phase content of the uncoated and coated 3Y-TZP specimens increased with the aging time, but the values of the uncoated specimens are far higher than the corresponding m-phase content of the coated 3Y-TZP specimens, as shown in Fig. 11. After 100 h ageing, the m-phase content of the uncoated 3Y-TZP is of 79%. Similar results have been reported that the m-phase content in the standard 3Y-TZP and other modified zirconia was apt to the saturation level of 70% after several time ageing [3,36,37]. The m-phase percentage of the coated specimen increased with the ageing time, as shown in Fig. 11. However, in comparison, the value of the coated 3Y-TZP is of 36% after 100 h, indicating the strong inhibition of the t/m phase transformation. Moreover, the aging kinetics could be evaluated by exploring relationship between the m-phase and the ageing time to describe the nucleation and growth process, which exhibits a sigmoidal trend as the Mehl-Avrami-Johnson (MAJ) equation [38]:
f¼ Fig. 8. XPS spectra of (a) the as-deposited Ta coating and (b) high resolution Ta4f.
Vm Vm0 ¼ 1 exp ðbtÞn Vmf Vm0
(3)
where Vm and Vmf are the m-phase content and its saturation level, respectively. Vm0 is the m-phase content before ageing. Here Vm0 is zero for the as-received 3Y-TZP ceramic. t is the aging time. The parameters of b and n are related to the apparent activation energy of this transformation and the MAJ exponent. Here, these kinetics parameters b and n of the Ta coated specimens is of 0.022 h1 and 0.74 by fitted the data in Fig. 11, which are lower than the corresponding values of 0.035 h1 and 0.81 of the as-received 3Y-TZP ceramic. This suggested that the Ta coating has the significant effect for retarding the nucleation and growth of the t/m phase transformation. Typically, the chemical adsorption water molecules H2O on the 3Y-TZP ceramic surface could dissociate to form hydroxyls with the oxygen on the surface which may come from the zirconia lattice of Zr-O-Zr bonds [39,40]. Some hydroxyls are brought in the ZrO2 due to the diffusion and migration, leading to increasing the stresses at the surface. The nucleation of the m-phase in the t-phase would be induced by the anion diffusion [40,41]. However, because of the excellent anti-corrosion, the Ta coating as a barrier prevents the direct contacting of the adsorption water from the ZrO2 ceramic surface. So, the nucleation of the m-phase would be prohibited and postponed effectively by the Ta coating where the diffusion and migration of hydroxyls have to pass though firstly. Therefore, the t/m phase transformation is significantly retarded in the Ta coatings. 5. Conclusions
Fig. 9. XRD profiles of uncoated and Ta coated specimens as a function of aging duration.
In summary, a series of Ta coatings on the 3Y-TZP ceramic substrate were successfully fabricated at different deposited temperature by plasma alloying technique. The t/m phase transformation of a 3Y-TZP ceramic did not happen during fabricating these Ta coatings, which was verified by XRD. The morphology and grain size of Ta coatings show the obvious dependence on the deposition temperature. Further results including the surface
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References
Fig. 10. Friction coefficient and wear rate of a 3Y-TZP ceramic with the Ta coatings.
Fig. 11. The monoclinic ZrO2-phase content of uncoated and Ta coated specimens as a function of aging duration.
roughness and static contact angle provide more supports. Ta coatings on the 3Y-TZP ceramic substrate display the good adhesion. Especially, S2 specimen of Ta coating deposited at 800 C has the strongest adhesion strength, corresponding to its fine surface morphology and roughness, and the thickness of Ta coating is ~590 nm. Moreover, the S2 specimen of Ta coatings tested in phosphate buffered saline solution also exhibit the lowest friction coefficient. Specimens of Ta coated (S2) and uncoated were hydrothermally aged in water steam. The result indicates that Ta coating has an effectively protection for the t/m phase transformation of the 3Y-TZP by hydrothermal aging, providing a promising future for its biomedical application.
Acknowledgments This work is supported by National Natural Science Foundation of China (51502193, 51811530058), Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province (2015rst), Shanxi Provincial Key R&D Program (201603D421035), and Shanxi Provincial Natural Science Foundation (201601D021057).
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Xiong, Plasma Surface Metallurgy with Double Glow Discharge Technology-Xu-Tec Process, Science Press, Beijing and Springer Nature Singapore Pte Ltd, 2017. [22] W.F. Gale, T.C. Totemeier, Smithells Metals Reference Book, ButterworthHeinemann, Oxford, 2003. [23] H.J. Hei, J. Ma, X.J. Li, Y.Y. Shen, W.Z. Tang, Preparation and performance of chemical vapor deposition diamond coatings synthesized onto the cemented carbide micro-end mills with a SiC interlayer, Surf. Coat. Tech. 261 (2015) 272e277. [24] J. Gao, H.J. Hei, Y.Y. Shen, X.P. Liu, B. Tang, Z.Y. He, S.W. Yu, Temperature dependence of W metallic coatings synthesized by double glow plasma surface alloying technology on CVD diamond films, Appl. Surf. Sci. 356 (2015) 429e437. [25] D.B. Wei, P.Z. Zhang, Z.J. Yao, W.P. Liang, Q. Miao, Z. Xu, Oxidation of doubleglow plasma chromising coating on TC4 titanium alloys, Corro. Sci. 66 (2013) 43e50. [26] S. Maeng, L. Axe, T.A. Tyson, L. Gladczuk, M. Sosnowski, Corrosion behaviour of magnetron sputtered a- and b-Ta coatings on AISI 4340 steel as a function of coating thickness, Corro. Sci. 48 (8) (2006) 2154e2171. [27] X.M. Yu, L.L. Tan, H.Z. Yang, K. Yang, Surface characterization and preparation of Ta coating on Ti6Al4V alloy, J. Alloy. Comp. 644 (2015) 698e703. [28] A.C. Hee, S.S. Jamali, A. Bendavid, P.J. Martin, C. Kong, Y. Zhao, Corrosion behaviour and adhesion properties of sputtered tantalum coating on Ti6Al4V substrate, Surf. Coat. Tech. 307 (2016) 666e675. [29] J. Gao, H.J. Hei, K. Zheng, R. Wang, Y.Y. Shen, X.P. Liu, B. Tang, Z.Y. He, S.W. Yu, Design and synthesis of diffusion-modified HfC/HfC-SiC bilayer system onto WC-Co substrate for adherent diamond deposition, J. Alloy. Comp. 705 (2017)
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and properties of Ta coatings on the 3Y-TZP ceramic fabricated by plasma alloying technique Liangliang Li a, Hongjun Hei a, Yongsheng Wang a, Ke Zheng a, Yong Ma a, Jie Gao a, Bing Zhou a, Zhiyong He a, Jie Zong b, Shengwang Yu a, *, Bin Tang a a b
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 February 2019 Received in revised form 18 June 2019 Accepted 15 July 2019 Available online 16 July 2019
Ta coatings on the 3Y-TZP ceramic substrate were successfully fabricated by plasma alloying technique, and its effect on the tetragonal-to-monoclinic phase transformation was investigated. The results indicate that the surface morphology and grain size of Ta coatings were influenced by the deposition temperature. Results of the surface roughness and static contact angle provide further supports. Scratch tests of Ta coatings deposited at 800 C (S2 specimen) show the good adhesion. The Ta coating has a thickness of ~590 nm in S2 specimen. Moreover, the friction properties of these Ta coatings in phosphate buffered saline solution exhibit the low friction coefficient. The Ta coating of S2 specimen exhibits an effectively protection for the phase transformation of the 3Y-TZP ceramic tested by hydrothermal aging. © 2019 Published by Elsevier B.V.
Keywords: 3Y-TZP Ta coating Adhesive strength Hydrothermal aging
1. Introduction In recent years, zirconia ceramics have got a wide spread attentions due to their excellent properties such as high strength, fracture toughness, lower cost, corrosion resistance, excellent cytocompatibility, fewer bacterial adhesions and osteointegration etc. [1e4], providing them a good future in structural and biomedical applications. These advantages of the zirconia ceramic relate with its three crystallographic polymorphs [2,3], the monoclinic phase (m) at room temperature, the tetragonal phase (t) at 1170 C and cubic structure (c) at 2370 C. The metastable tetragonal phase can be got at room temperature by introducing the oxidation into the monoclinic zirconia, such as the tetragonal zirconia polycrystals stabilized with 3 mol % yttria (3Y-TZP) for dental application. The transformation from tetragonal to into monoclinic phase (t/m) would be induced by eternal loading, leading to the increasing resistance to further cracks propagation, named as phase transformation toughening [3]. Many factors such as the load, friction, acid-base environment could promote t/m transformation where zirconia is exposed in vivo [4e6]. However, a high level of m-phase may be detrimental to mechanical properties such
* Corresponding author. E-mail address: [email protected] (S. Yu). https://doi.org/10.1016/j.jallcom.2019.07.155 0925-8388/© 2019 Published by Elsevier B.V.
as strength and toughness, which may impair the long-term clinical success of zirconia-based restorations [1,2,7]. So, approaches to restrain the transformation are really appreciate. Nowadays, some ways have been developed to prevent the early occurrence of t/m phase transformation. Zirconia based composites had been developed by adding the mullite or Al2O3 to alleviate surface degradation in an environment of water or body fluid [1,8,9]. However, the zirconia content and yttria addition in these composites would lower the benefits described above. A further strategy to introduce CeO2 as stabilizer instead of Y2O3 in zirconia can improve significantly the aging resistance [6,10], but simultaneously lower the biaxial strength and hardness. To deposit a protective layer on the surface of 3Y-TZP [11], as a promising alternative way, has attractive more attentions due to the thin protective layer not jeopardize the machined substrate's dimensional accuracy and mechanical properties. Tantalum (Ta) has been developed as a promising metal for biomedical implants or implant coatings owing to its excellent corrosion resistant, bioactive in vivo and fracture toughness [12,13]. Compared with other biomedical metal alloys e.g. CoCr [14], or stainless steel [15], the Ta coating has the similar physical and mechanical properties with 3Y-TZP, e.g. the elastic modulus, melting point and coefficient of thermal expansion [12,13,15,16]. However, Ta coating are hardly to fabricate on the surface of 3YTZP by traditional process such as chemical vapor deposition [17].
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Double glow plasma surface alloys (DGPSA) technique [18], an innovative surface treatment technique, has been developed successfully on the basis of the plasma nitriding and sputtering technology. Researchers has employed this technique to improve the surface properties of metals or alloys, e.g. the wear [19], corrosion and oxidation resistance [20]. In this paper, in order to determine the effect of Ta coating on the t/m phase transformation of the 3Y-TZP ceramic, a group of Ta coatings were successfully fabricated as a protective layer on a 3Y-TZP at various substrate temperature by DGPSA technique. After analyzing the morphology, microstructure, adhering strength and tribological property, a specimen with good combination properties was selected to conduct the hydrothermal aging. The protective effect of the Ta coatings was studied.
2.2. Characterization of Ta coatings The surface and cross-sectional morphologies of Ta coatings were characterized by field emission scanning electron microscopy (FESEM, TESCAN MIRA3 LMH). The surface of the S2 sample was cut by focused ion beam (FIB) and the morphology of the cross section was analyzed. The ion beam parameters in FIB milling mode were 30 kV accelerating voltage and 5 nA beam current. The phase compositions of Ta coatings were identified by X-ray diffraction (XRD, DX-2700 X) using Cu Ka radiation. The average surface roughness was measured by white light scanning profiler (Contour GT-K, Bruker). The static contact angle was measured by sessile drop and the obtained images were analyzed to calculate the contact angle of deionized water for each sample at room temperature. Each reported contact angle is the mean and standard deviation value for at least three independent measurements.
2. Experimental procedure
2.3. Adhesion and wear properties of Ta coatings
2.1. Fabrication of Ta coatings
The adhesion of a Ta coating was tested by WS-2005 scratching tester under a continuous load from 0 N to 100 N and at the loading speed of 50 N/min. The wear resistance in phosphate buffered saline solution was measured against Si3N4 balls with diameter of 5.5 mm by MFT-R4000 reciprocating frication and wear tester with a load of 5 N and sliding duration of 15 min.
A commercially available 3Y-TZP ceramic specimens with dimension of 14 mm in diameter and 1.2 mm in thickness were prepared as substrates according to ISO 6872. The as-received surface morphology of the 3Y-TZP ceramic substrate displays the nanocrystalline grains with an average size of 407.6 ± 11.2 nm, as shown in Fig. 1. Ta coatings were deposited on the 3Y-TZP substrate by DGPSA devices described in detail in a previous study under an argon atmosphere (purity of 99.999%) [21]. Here, pure tantalum with purity of 99.99 wt %, as the target material, was cut into the sheet with dimension of 60 mm in diameter and 1.5 mm in thickness. The distance between the upper substrate surface and the lower target surface was set as 18 ± 1 mm. When the argon gas was filled and the two powers were turned on, the sample cathode and the source cathode were surrounded by two glow discharges. Ta atoms sputtered from the target were transferred to the substrate surface, while the substrate was heated by ion bombardments. The working pressure during deposition was maintained at 35 Pa and the Ar flow rate was 60 sccm. The sample cathode voltage range was 500 V and the source cathode voltage range was 750 V. The substrate temperature was maintained using the infrared thermometer. Four specimens of the Ta coating named as S1, S2, S3 and S4 were fabricated by controlling the deposition temperature, which corresponded to 750 C, 800 C, 850 C and 900 C. The deposition time of all coated specimens is of 10 min.
2.4. Hydrothermal aging The protective effect of Ta coatings was investigated by successive hydrothermal aging of the specimens in an autoclave (Type LS-30, BoXun) in water vapor at 134 C and 3 bar. Chemical composition of Ta coatings was determined using a K-Alpha XPS spectrometer (Thermo Fisher Scientific, Waltham). The progress of the t/m phase transformation was observed with five specimens each per group after deal with 0 h, 5 h, 10 h, 20 h, 40 h, 80 h, 100 h, and the aging step was followed by XRD. The cross section of specimens after aging were acquired by FESEM. 3. Results 3.1. Microstructure of Ta coatings Fig. 2 shows the microstructure of Ta coatings and the 3Y-TZP substrate identified by XRD pattern. Clearly, the diffraction peaks appear at diffraction angels of 38.4 , 55.5 , 69.5 and 82.4 (2q) for all coated specimens, corresponding to crystalline plane orientations of (110), (200), (211) and (220) of a-Ta phase with a bodycentered-cubic structure. Additionally, the intensities of these diffraction peaks especially at 69.5 of coated specimens increases with rising of the deposited temperature, meaning an increase of the a-Ta coating with the increasing temperature. Due to the thickness of these Ta coatings less than 1 mm, it is very easy for the X-rays to penetrate through the interlayer to detect the ZrO2. So, the 3Y-TZP substrate diffraction peaks can be observed in all XRD results of coated specimens. On the other hand, no obvious difference of the 3Y-TZP substrate's diffraction peaks between the uncoated and coated specimens suggests that the t/m phase transformation of a 3Y-TZP ceramic did not happen during fabricating these Ta coatings. 3.2. Coss-section and surface morphology
Fig. 1. Surface morphology of the 3Y-TZP ceramic substrate.
Fig. 3 shows the cross-sectional SEM image which was fabricated by FIB. In order to prevent the ion beam from damaging the Ta coating, the Pt coating was prepared. It can be seen obviously that
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Fig. 4(c) and (d). Same trends can be found by comparing the data of the contact angle in Table 1. It indicates that the surface characteristics of these Ta coatings depend on the surface of the deposition temperature. 3.3. Adhesion strength of Ta coatings Fig. 5 shows scratch results including variations of acoustic emission signal and the corresponding friction force for testing the adhesion of these coatings. In the process of loading from 0 to 100 N by a probe, the acoustic emission signals exhibit distinctly peaks of Ta coatings. Typically, the first high-amplitude signal peak of the specimen S1 appears at ~24 N corresponding to the crack in this load, as shown in Fig. 5(a). However, the acoustic emission signal keeps in a very low fluctuation until ~91 N for the specimen S2 during the same loading method, as shown in Fig. 5(b), indicating that the adhesion strength of the specimen S2 is stronger than that of the specimen S1. In comparison, the specimens S3 and S4 display the good adhesion but weaker than that of S2 (Fig. 5(c) and (d)). 3.4. Friction properties of Ta coatings
Fig. 2. XRD patterns of Ta coatings on the 3Y-TZP substrates fabricated at different temperature: S1 750 C, S2 800 C, S3 850 C and S4 900 C.
Fig. 3. The cross-sectional SEM of Ta coating deposited at 800 C.
the thickness of Ta coating is about 590 nm. Fig. 4 shows backscattering electron images of Ta coatings deposited at different temperatures. All Ta coatings consist of nanocrystalline grains with a pyramid-like morphology, and densely cover onto the surface of substrates. Table 1 is the surface roughness of all specimens and the substrate. Clearly, the surface morphology of specimen S1 is in Fig. 4(a). Same defects could be observed on the image at high magnification, as shown in Fig. 4(a1), which may relate with deposition process of Ta coatings. It can be observed that the specimen S2 exhibits the fine and uniform grain with an average size of 66 ± 7.6 nm, as shown in Fig. 4(b) and (b1). With increasing of the deposition temperature, the specimen shows coarser grain on the surface, e.g. specimens S3 and S4 in
On the basis of Fig. 5, the friction properties of Ta coatings in phosphate buffered saline solution were investigated at a load of 5 N. Fig. 6(a1) - (d1) show the friction coefficient curves of a 3Y-TZP ceramic with the Ta coating against Si3N4 balls with diameter of 5.5 mm at room temperature. Clearly, the average friction coefficient dropped slightly with the deposition temperature increasing form 750 Ce800 C, but increased with the further rising temperature to 900 C. The depth and width of the wear tracks exhibited same trends with the rising deposition temperature. It indicates that friction property depends on the Ta coating and 3YTZP substrate. More details of the wear scar surface are investigated in Fig. 7. Clearly, the bare surface and wear tracks of the 3Y-TZP substrate also formed on the worn surface [Fig. 7(a)e(b)], e.g. the bare surface ‘i’ in specimen S1 with a composition of O51.5Zr29.2Ta17.7Si1.3N0.3. A large number of wear debris can be observed on the coarse worn surface of specimens S1 and S2, e.g. debris ‘ii’ in specimen S2 with a composition of Ta38.3O34.0Zr23.2Si4.5 [Fig. 7(c)e(d)]. Moreover, many wear debris particles (‘iii’ in specimen S2 with a composition of O52.3Zr32.8Ta9.1Si5.4N0.4) and grooves (‘i’ in specimen S4 with a composition of O56.3Zr36.4Ta5.0Si2.3) could be seen on the worn surface [Fig. 7(e)e(h)]. The Ta-coatings are easily worn away from 3Y-TZP ceramic substrate by Si3N4 ball due to the difference between tantalum and Si3N4 in Vickers hardness and shear modulus [22]. When the Ta-coatings were worn out, however, the bare surface of the 3Y-TZP substrate wear off against the Si3N4 ball. In comparison, the elastic and shear moduli of Si3N4 and 3Y-TZP ceramic are almost the same [22]. So, many wear debris including Si3N4 and 3Y-TZP ceramic particles would be produced during wear processing. On the other hand, the Ta coating and its fragments would either occur the work-hardening due to the excellent plasticity and ductility, or adhere or stick to the worn region with Si3N4 and 3Y-TZP ceramic particles during wear processing. 3.5. Hydrothermal aging Fig. 8 shows the XPS spectra of the Ta coating specimen deposited at 800 C. The major characteristic peaks consist of oxygen (O1s), Ta (Ta4f, Ta4d, Ta4p and Ta4s) and C (C1s), as shown in Fig. 8(a). All binding energies were calibrated with respect to the C1s characteristic binding energy at 285.0 eV. The presence of carbon may attribute to the environmental contamination. The Ta coating contained a small fraction of oxygen but without Zr signal
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Fig. 4. Surface morphologies and their high magnification images of Ta coatings deposited at different temperatures: (a) S1: 750 C, (b) S2: 800 C, (c) S3: 850 C and (d) S4: 900 C.
Table 1 The roughness and contact angle of uncoated and Ta coated specimens. Specimens
Uncoated
S1
S2
S3
S4
Roughness/nm Contact angle/
161.1 ± 1.0 41 ± 1.8
138.9 ± 1.8 60 ± 1.5
108.3 ± 6.1 28 ± 0.7
151.6 ± 1.1 43 ± 0.5
165.8 ± 13.1 46 ± 1.1
Fig. 5. Scratch adhesion results of Ta coatings: (a) S1, (b) S2, (c) S3 and (d) S4.
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Fig. 6. Friction coefficient curves of a 3Y-TZP ceramic with the Ta coating as a function of the sliding time and corresponding surface profiles of wear tracks: (a1) and (a2) for S1, (b1) and (b2) for S2, (c1) and (c2) for S3, (d1) and (d2) for S4.
due to the oxidation in air. It indicates that a compact Ta coating was formed and completely covered the 3Y-TZP substrate. To reveal more information of the Ta coating, the high resolution Ta4f spectrum is fitted regard to the 4f7/2 and 4f5/2 peak intensity, as shown in Fig. 7(b). Clearly, the Ta4f core level consists of three peaks. One doublet at binding energies of 21.9 eV and 23.7 eV with the highest intensity are attributed to Ta4f7/2 and Ta4f5/2 of Ta, respectively. The other at binding energies of 24.9 eV is assigned to Ta4f5/2 of Ta2C and 27.0 eV to Ta4f7/2 of Ta2O5. It further indicates that Ta coatings are successfully deposited on the surfaces of ZrO2 substrate but a small amount of Ta2O5 on the surface. In order to evaluate the protecting effect of the Ta coating onto 3Y-TZP ceramic on the t/m phase transformation, the hydrothermal aging was conducted in water steam at 134 C and 3 bar, as shown in Fig. 9. The diffraction peak at 30.1 (2q) corresponds to the tetragonal phase from the crystallographic orientation of (101), but diffraction peaks at 28.2 and 31.3 are the monoclinic phases from the crystallographic orientations of (111) and (111), respectively. 4. Discussion Many literatures have investigated the relationship between the coating and the substrate, such as SiC coating on WC-Co [23], W or Cr coating on diamond [24,25], Ta coating on stainless steel [26] or Ti-6Al-4V [27,28], HfC coating on WC-Co [29] etc. Basically, the effects of these coatings on the properties such as friction property and corrosion resistance etc. are determined by their metallization process, which generally consists of the deposition and annealing process [18,21,30]. Here, the influence of adhesion, friction property and suppressing phase transformation of Ta coatings on the 3Y-TZP ceramic will be discussed on the basis of metallization. 4.1. Deposition and diffusion of metallization When the solid metal target (e.g. W, Ta and Mo etc.) was activated by the glow discharge [21], the metallic particles including atoms and/or ions formed, and deposited on the substrate. Here, due to the constant values of other parameters [such as surface of
the substrate, ratio of (CH4/H2þCH4), substrate voltage etc., as description of II experiment part], the surface morphology, roughness and adhesion were only related with the deposited temperature. At low temperature e.g. 750 C, the substrate surface which may full of pits with size of ~500 nm deposited a thin Ta coating by the fine Ta crystal. Although the Ta coating covered totally the substrate surface, these pits may not be filled up, leading to a high surface roughness, as shown in Fig. 4 and Table 1. Similar results have been reported in previous finding of our group [18,19,21,23e25,29]. The low adhesion contributes to the weak diffusion between the substrate surface and coating [24,31], as shown in Fig. 5 and Table 2. The original pits of the substrate surface would almost be filled up as increasing of the deposited temperature (e.g. 800 C) [24,29], corresponding to the increased thickness of the deposited layer. At the same time, the diffusion layer will increase due to the higher diffusion coefficient at high deposited temperature. So, the surface roughness decreased, but the critical force of the adhesion increased, as shown in Figs. 4 and 5. However, when the deposition temperature further increased, the deposited Ta coating would form itself pyramid morphology on the substrate surface, resulting in the increasing the surface roughness. On the other hand, the cavities may easily emerge at the boundary between the substrate and Ta coating caused by the excessive diffusion, which would worsen the adhesion. It is also found in SiC coating on WC-Co [23], W coating on diamond [24], W coating on C/C composite [30], Ta coating on Ti6Al4V [26]. 4.2. Friction behavior In order to reveal more details, the wear rate of these Ta coatings was calculated by using following equation [32]:
K¼
Lh 3h2 þ 4b2 6PSb
(1)
where K is the wear rate; S and P represent the sliding length and normal load; L, h and b are length, depth and width of wear crack,
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Fig. 7. The SEM images of wear scar surfaces and the corresponding enlarge images: (a) and (b) for S1, (c) and (d) for S2, (e) and (f) for S3, (g) and (h) for S4.
respectively. The wear rate dropped from 4.92*105 mm2 to 3.58*105 mm2 with the rising deposited temperature from 750 C to 800 C, as shown in Fig. 10. However, the value increased with further rising temperature, e.g. 6.88 *105 mm2 when deposited at 900 C. A possible reason is that the original pits on the surface of substrate could be compensated gradually by the Ta coatings during the deposition of metallization process, leading to the dropped wear rate at 800 C. Nevertheless, the higher deposited temperature would promote the surface roughening of the Ta coating, which deteriorates the friction property. Here, the S2 deposited at 800 C shows the good combination properties.
4.3. Improvement of the t/m phase transformation The m-phase fraction, Vm, was determined by following equation [33,34]:
Vm
1:311 I 111 þ I 111 m m ¼ þ I 101 þ I 111 1:311 I 111 m m t
(2)
where Im represents the integral intensity from XRD diffraction peaks of (111) and (111) of the m-phase, It is the integral intensity of the t-phase diffraction peak (101), as shown in Fig. 9. It should be
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Table 2 Parameters of the JMAK equation (Eq. (4)) according to non-linear regression of mphase fraction vs. time. Specimens
Vm0/%
Vmf/%
b/h1
n
Uncoated S2
0 0
79 36
0.035 0.022
0.81 0.74
noticed that the calculated phase content represents an integral value for the near-surface region due to the limited penetration depth of the X-rays caused by the scattering and absorption [35,36]. The m-phase content of the uncoated and coated 3Y-TZP specimens increased with the aging time, but the values of the uncoated specimens are far higher than the corresponding m-phase content of the coated 3Y-TZP specimens, as shown in Fig. 11. After 100 h ageing, the m-phase content of the uncoated 3Y-TZP is of 79%. Similar results have been reported that the m-phase content in the standard 3Y-TZP and other modified zirconia was apt to the saturation level of 70% after several time ageing [3,36,37]. The m-phase percentage of the coated specimen increased with the ageing time, as shown in Fig. 11. However, in comparison, the value of the coated 3Y-TZP is of 36% after 100 h, indicating the strong inhibition of the t/m phase transformation. Moreover, the aging kinetics could be evaluated by exploring relationship between the m-phase and the ageing time to describe the nucleation and growth process, which exhibits a sigmoidal trend as the Mehl-Avrami-Johnson (MAJ) equation [38]:
f¼ Fig. 8. XPS spectra of (a) the as-deposited Ta coating and (b) high resolution Ta4f.
Vm Vm0 ¼ 1 exp ðbtÞn Vmf Vm0
(3)
where Vm and Vmf are the m-phase content and its saturation level, respectively. Vm0 is the m-phase content before ageing. Here Vm0 is zero for the as-received 3Y-TZP ceramic. t is the aging time. The parameters of b and n are related to the apparent activation energy of this transformation and the MAJ exponent. Here, these kinetics parameters b and n of the Ta coated specimens is of 0.022 h1 and 0.74 by fitted the data in Fig. 11, which are lower than the corresponding values of 0.035 h1 and 0.81 of the as-received 3Y-TZP ceramic. This suggested that the Ta coating has the significant effect for retarding the nucleation and growth of the t/m phase transformation. Typically, the chemical adsorption water molecules H2O on the 3Y-TZP ceramic surface could dissociate to form hydroxyls with the oxygen on the surface which may come from the zirconia lattice of Zr-O-Zr bonds [39,40]. Some hydroxyls are brought in the ZrO2 due to the diffusion and migration, leading to increasing the stresses at the surface. The nucleation of the m-phase in the t-phase would be induced by the anion diffusion [40,41]. However, because of the excellent anti-corrosion, the Ta coating as a barrier prevents the direct contacting of the adsorption water from the ZrO2 ceramic surface. So, the nucleation of the m-phase would be prohibited and postponed effectively by the Ta coating where the diffusion and migration of hydroxyls have to pass though firstly. Therefore, the t/m phase transformation is significantly retarded in the Ta coatings. 5. Conclusions
Fig. 9. XRD profiles of uncoated and Ta coated specimens as a function of aging duration.
In summary, a series of Ta coatings on the 3Y-TZP ceramic substrate were successfully fabricated at different deposited temperature by plasma alloying technique. The t/m phase transformation of a 3Y-TZP ceramic did not happen during fabricating these Ta coatings, which was verified by XRD. The morphology and grain size of Ta coatings show the obvious dependence on the deposition temperature. Further results including the surface
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References
Fig. 10. Friction coefficient and wear rate of a 3Y-TZP ceramic with the Ta coatings.
Fig. 11. The monoclinic ZrO2-phase content of uncoated and Ta coated specimens as a function of aging duration.
roughness and static contact angle provide more supports. Ta coatings on the 3Y-TZP ceramic substrate display the good adhesion. Especially, S2 specimen of Ta coating deposited at 800 C has the strongest adhesion strength, corresponding to its fine surface morphology and roughness, and the thickness of Ta coating is ~590 nm. Moreover, the S2 specimen of Ta coatings tested in phosphate buffered saline solution also exhibit the lowest friction coefficient. Specimens of Ta coated (S2) and uncoated were hydrothermally aged in water steam. The result indicates that Ta coating has an effectively protection for the t/m phase transformation of the 3Y-TZP by hydrothermal aging, providing a promising future for its biomedical application.
Acknowledgments This work is supported by National Natural Science Foundation of China (51502193, 51811530058), Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province (2015rst), Shanxi Provincial Key R&D Program (201603D421035), and Shanxi Provincial Natural Science Foundation (201601D021057).
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