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The improved corrosion resistance and anti-wear performance of Zr–xTi alloys by thermal oxidation treatment
Surface & Coatings Technology 283 (2015) 101–107
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
The improved corrosion resistance and anti-wear performance of Zr–xTi alloys by thermal oxidation treatment W.F. Cui a,⁎, C.J. Shao b a b
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110004, China Liaoning Equipment Manufacture College of Vocational Technology, Shenyang 110161, China
a r t i c l e
i n f o
Article history: Received 31 July 2015 Revised 20 October 2015 Accepted in revised form 22 October 2015 Available online 24 October 2015 Keywords: Zr–xTi alloy Oxide coating Bonding force Corrosion Wear
a b s t r a c t By thermal oxidation treatment at 500 °C in air, the monoclinic ZrO2 and orthorhombic ZrTiO4 oxide coatings were in-situ formed on the surfaces of biomedical Zr–20Ti and Zr–40Ti alloys, respectively. The hardness and adhesion strength of the oxide coatings were measured by indentation and scratching tests. The electrochemical corrosion of the Zr–xTi alloys before and after thermal oxidation treatment was performed in the acidified artificial saliva containing 0.1% NaF at 37 °C. The friction and wear performances of the un-oxidized and oxidized Zr–xTi alloys were evaluated by reciprocating ball-on-disc wear tests under the load of 10 N. The results show that the oxide coatings have the hardness of 1420–1480 HV and the adhesion forces of 51 N to the substrates. The oxidized Zr–xTi alloys exhibit the reduced corrosion rates and improved pitting corrosion resistance in comparison with the un-oxidized Zr–xTi alloys. The wear tests demonstrate that the un-oxidized Zr–xTi alloys show the serious adhesive wear and abrasive wear due to the high plasticity and chemical activity. The coefficients of friction and wear rates of the oxidized Zr–xTi alloys decrease 30% and 90%, respectively, which are attributed to the hard oxide coatings with enhanced toughness. Nevertheless, the scabies defects in the oxide coating of Zr–20Ti alloy have a negative effect on the friction and wear property. The oxidized Zr–40Ti alloy has the excellent chemical stability and anti-abrasion performance. It has a great application prospect as dental implant material. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Commercially pure titanium (cp Ti) has been used as the abutment and screw of dental implants due to its excellent corrosion resistance, good biocompatibility and low modulus. However, the low strength of cp Ti increases the risk of small diameter screw fracture (≤3.5 mm). It has been reported that at least 5% fracture in dental implants arises from fatigue over the last decades [1]. Increasing the strength of the materials is an effective approach to prolong the service life of the metal implants. Zirconium is an ideal alloying element for titanium in biomedical applications. Zr–Ti (or Ti–Zr) binary alloys exhibit high strength and good osseointegration in animal and clinical studies [2–6]. Zr–Ti alloys also present other advantages such as lower melting point, lower magnetic susceptibility, lower linear expansion coefficient and less hydrogen absorption than cp Ti and other Ti alloys [7]. Currently, Zr–Ti alloys have become one of the important candidate materials to be used in permanent prosthesis of dental or orthopedic treatments [8]. In oral environment, wear and corrosion frequently occur at interface between metal implant and zirconia abutment [9]. The fine debris particles produced by the poor wear resistance of Ti alloys result in ⁎ Corresponding author. E-mail address: [email protected] (W.F. Cui).
http://dx.doi.org/10.1016/j.surfcoat.2015.10.051 0257-8972/© 2015 Elsevier B.V. All rights reserved.
the aseptic loosening of the implant. At the same time, the dental implants are frequently eroded by fluoride ions from toothpastes, orthodontic gels, dietary supplements, etc. Acidic foods or inflammation in the oral cavity accelerates the damage of fluoride ions to the passive film of titanium metal. The released metal ions stimulate the growth of the macrophage. The rough surfaces caused by corrosion and wear are favorable to the adhesion of various bacteria and the formation of plaque. Therefore, high corrosion resistance and anti-wear performance of dental material are vital for the safe use of the dental implants. The modern surface engineering technologies, such as laser nitridation [10], microarc oxidation [11] and magnetron sputtering [12], provide many approaches preparing oxide or nitride coatings to improve the corrosive wear resistance of titanium and titanium alloys. However, some issues in the use of these technologies, such as the surface finish, uniformity, density and adhesion strength of the coating, are not well resolved simultaneously. Moreover, the expensive equipments, complex process and high energy consumption also increase the cost of the coatings. Recently, a simple thermal oxidation method was suggested to modify the surface properties of biomedical metal materials. It has been reported that the thermally oxidized Ti–6Al–4V and Zr–2.5Nb alloys reduced the adhesive wear tendency and improved the corrosion resistance in simulated body fluid [13–17]. However, the hardness and
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adhesion strength of these oxide coatings formed on Ti alloy or Zr alloy are not high enough. The exfoliation of the oxide layer will increase coefficient of friction and decrease anti-wear performance. Since Ti and Zr metals are easily oxidized by heat treatment in air, the zirconia–titania oxide coatings can rapidly form on the surfaces of the thermally oxidized Zr–Ti alloys. The composite oxides possess higher hardness and toughness than single zirconia or titania oxide [18]. Thus, the thermally oxidized Zr–Ti alloys are expected to have greatly improved corrosive wear resistance. In the present study, the thermal oxidation treatment was performed on Zr–20Ti and Zr–40Ti alloys at 500 °C in air. The phase constitutes, surface morphologies and adhesion strength of the thermally oxidized coatings were examined. The effects of titanium content on the electrochemical corrosion and the wear performance of the un-oxidized and oxidized Zr–xTi alloys were clarified.
2. Experimental Zr–20Ti and Zr–40Ti alloys (wt.%) were prepared using arc melting in argon atmosphere for six times. The ingots were hot forged at 800 °C into the rectangular slabs. After the surface oxide layers were removed, the slabs were heat-treated at 750 °C followed by water cooling. Specimens were machined by electric spark cutting with the dimension of 20 × 10 × 5 (mm3) for wear tests and 10 × 10 × 2 (mm3) for electrochemical tests. All specimens were ground with a SiC abrasive paper up to 3000 grit, and then ultrasonically cleaned in acetone for 10 min. Some of the specimens were thermally oxidized at 500 °C for 2 h in air. The surfaces and cross section morphologies of the thermally oxidized specimens were observed by an optical microscope and a scanning electron microscope (SSX-550, Japan). The phase compositions of the oxide coatings were identified by X-ray diffraction (Smart Lab, Japan) using Cu Kα radiation with the scan rate of 3°/min. The bonding forces of the oxide coatings to the substrates were measured by a microscratch tester (WS-2005, China) with a diamond stylus of 0.2 mm radius. The load increased from 0 to 200 N with a loading rate of 100 N/min. The critical loads at which the oxide started to crack were measured. The Vickers microhardness of the oxidized and un-oxidized specimens were measured by a hardness tester (401 MVDTM, China) with a load
Fig. 2. XRD profiles of the thermally oxidized Zr–20Ti and Zr–40Ti alloys.
of 25 g and a loading time of 30 s. The final hardness was obtained from the average values of the ten testing results. The electrochemical experiments were carried out in the naturally aerated artificial saliva containing 0.1% NaF at 37 °C (pH = 4). The components of the artificial saliva were as follows: NaCl (0.4 g/l), KCl (0.4 g/1), NaH2PO4·2H2O (0.78 g/l), CaCl2·2H2O (0.795 g/l), Na2S·2H2O (0.005 g/l), urea (1.0 g/l) and 1000 ml distilled water. All the chemicals were provided by Guoyao Group Chemical Reagent Co. Ltd. (Shenyang). Three-electrode electrochemical working station (Zennium, Germany) was used with saturated calomel electrode as reference electrode, platinum plate as counter electrode and the specimens as working electrode (1 cm2 exposure area to the solution). Potentiodynamic polarization curves (PPC) were recorded over the potential range of −1.5 V to +1.5 V at a scanning rate of 1 mV/s. The electrochemical impedance spectra (EIS) were measured by applying a sinusoidal potential perturbation of 5 mV with a frequency from 10−2 Hz to 105 Hz in logarithmic increment at the open circuit potential. Before the tests, the specimens were immersed in the electrolyte for 1 h in order to attain stable open
Fig. 1. Oxide coating morphologies of the Zr–20Ti and Zr–40Ti alloys thermally oxidized at 500 °C for 2 h. (a),(b) surface and fracture cross-section images of the oxidized Zr–20Ti alloy showing the scabies defects and uneven top surface, (c),(d) surface and fracture cross-section images of the oxidized Zr–40Ti alloy showing the smooth surface and columnar structure.
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Fig. 4. Potentiodynamic polarization curves of Zr–20Ti and Zr–40Ti alloys before and after the thermal oxidation treatment in 0.1% NaF containing artificial saliva at 37 °C, pH = 4.
the test specimens in air. The sliding wear started with a normal load of 10 N. The reciprocating sliding distance was 20 mm and the number of cycling was 7200 at a frequency of 1 Hz. The surface morphologies of the worn specimens were observed by using SEM (SSX-550) which is equipped with energy dispersive X-ray analysis (EDX). 3. Results 3.1. Morphologies and structural analysis of the oxide coatings
Fig. 3. Acoustic emission signal plots during the scratching and the corresponding scratch track images showing the critical forces of the oxide rupture. (a),(c) Zr–20Ti, (b),(d) Zr–40Ti.
circuit potential. The surface morphologies of the corroded specimens were observed by using SEM (SSX-550). Linear reciprocating ball-on-disc wear testing was performed on the Zr–Ti alloys with and without oxidation treatment by using a tribometer (CSM, Switzerland) with a Si3N4 ball of 6 mm diameter rubbing against Table 1 Hardness of the Zr–xTi alloys before and after oxidization treatment. Specimens
Zr–20Ti
Zr–40Ti
Oxidized Zr–20Ti
Oxidized Zr–40Ti
Hardness (HV0.1)
329 ± 12
340 ± 10
1425 ± 36
1476 ± 55
Note: The values include mean value ± standard deviation.
The surface morphologies of the oxide coatings were observed by an optical microscope, as shown in Fig. 1a and c. The oxidized surface of Zr– 20Ti displayed scabies-like defects. The oxidized surface of Zr–40Ti was smooth and dense. From the cross-section images of the oxide coatings (Fig. 1b and d), it was seen that the thickness of oxide coatings for the oxidized Zr–20Ti and Zr–40Ti was 14 ± 0.5 μm and 11 ± 0.2 μm, respectively, indicating the slower oxidation rate with the increase of titanium content in Zr–Ti alloy. The oxide coatings exhibited a columnar growth structure. The good binding between the coatings and the substrates was seen without cracks or lamination in the coatings. The phase identification of the oxide coatings is shown by XRD patterns in Fig. 2. For Zr–20Ti, the coating mainly consisted of m-ZrO2 with monoclinic structure. The weak diffraction peaks of ZrTiO4 with orthorhombic structure were also detected. But for Zr–40Ti, ZrTiO4 became the main phase of the oxide coating. Small amount of t-ZrO2 with tetragonal structure possibly remained in the coating because the strongest diffraction peak of t-ZrO2 nearly coincides with ZrTiO4 at about 30° diffraction angle. The different morphologies and phase compositions of the oxide coatings indicate that Zr–20Ti and Zr–40Ti alloys exhibited the different oxide formation mechanisms, which can be explained by the oxidation thermodynamics and kinetic theory of alloys. Both Zr and Ti have strong oxygen affinity. They all participate in the oxidation process of the Zr–xTi alloy during thermal oxidation treatment. According to the Ellingham–Richardson diagram, the equilibrium oxygen partial pressure at Zr/ZrO2 interface is smaller than that at Ti/TiO2 interface. Zr in the Zr–xTi alloys is preferentially oxidized into t-ZrO2. TiO2 is subsequently formed in the t-ZrO2 lattice. Although anatase-type TiO2 has the same crystal structure as t-ZrO2, their c/a axis ratios have large difference (2.51 for a-TiO2 and 1.016 for t-ZrO2). In order to reduce the elastic strain energy in the lattice, ZrO2/TiO2 composite oxide transforms into ZrTiO4 compound by the adjustment of the lattice parameters [19,20]. For Zr–40Ti alloy, Zr to Ti atomic ratio approaches 0.8, thus, nearly 100% ZrTiO4 forms in the oxide coating. For Zr–20Ti alloy,
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Table 2 Electrochemical parameters obtained from the potentiodynamic polarization curves. Specimens
Zr20Ti
Zr40Ti
Oxidized Zr20Ti
Oxidized Zr40Ti
Ecorr (V) icorr (μA/cm2)
−1.11 ± 0.03 38 ± 0.7
−0.63 ± 0.06 2.6 ± 0.6
0.04 ± 0.05 0.25 ± 0.08
−0.05 ± 0.04 0.25 ± 0.04
Note: The values include mean values ± standard deviation.
however, only small amount of ZrO2/TiO2 transforms into ZrTiO4 due to decreased titanium content. Most of t-ZrO2 grains grow up with the prolonging of oxidation time. Once the grain size reaches the critical value (30.6 nm), a martensite phase transformation (t-ZrO2 → mZrO2) occurs [21]. The phase transformation stress leads to the formation of the scabies defects in the oxide coating, as seen in Fig. 1a [22]. It is worth noting that the oxide coating of Zr–20Ti displayed the strong (200) texture. However, the similar phenomenon did not occur to the oxide coating of Zr–40Ti in spite of the same columnar structure. It is analyzed that (200) texture of m-ZrO2 coating can result in low energy surface, which is favorable to the rapid growth of the columnar crystal. But the oxide coating of Zr–40Ti contains small amount of t-ZrO2. The strong (111) crystal plane diffraction from t-ZrO2 impedes the formation of ZrTiO4 texture. 3.2. Mechanical properties of the oxide coatings The scratching tests were used to assess the adhesion strength of the oxide coatings to the substrates. The critical normal load (Lc) could be detected at which the oxide started to crack by a sudden increase in acoustic emission intensity. Fig. 3a and b shows the adhesion forces of the oxide coatings up to 51 N. The value was twice as much as that of the thermal oxidation coating on Zr alloy or Ti alloy [23–25]. By carefully observing the scratch tracks of the oxidized Zr–20Ti, it was found that there were already some microcracks at the edges of the scratch track before oxide delamination. But in the scratch track of the oxidized Zr40Ti, the uniform plastic deformation was observed until the oxide was completely broken (Fig. 3c and d). These features indicate that
ZrTiO4 oxide exhibited the higher resistance to cracking than m-ZrO2 oxide. The advanced toughness will improve the friction and wear performances of the oxide coating. Table 1 lists the surface hardness of the Zr–xTi alloys before and after thermal oxidation treatment. The hardness of the oxidized Zr–xTi was higher than 1400 HV, which sharply contrasted to the hardness (330–350 HV) of the un-oxidized Zr–xTi. The values are also much higher than those of the other thermally oxidized Ti or Zr-based alloy [26]. The increased surface hardness of the oxidized Zr–xTi alloys was evidently attributed to the thick and compact ZrO2 or ZrTiO4 oxide coatings with columnar structure. 3.3. Electrochemical corrosion behavior The previous studies have reported that the corrosion rates of the titanium alloys were sharply accelerated in fluoride-containing artificial saliva [27–29]. In this study, the electrochemical corrosion behaviors of the Zr–20Ti and Zr–40Ti before and after thermal oxidation treatment were compared in 0.1% NaF containing acidified artificial saliva (pH = 4). Fig. 4 shows the potentiodynamic polarization curves (PPC) of the four specimens. Table 2 gives the correlated electrochemical corrosion parameters. Before the thermal oxidation treatment, the selfcorrosion current density (icorr) and anodic current density (ia) of the Zr–xTi alloys decreased with the increase of titanium content. In spite of this, the lower icorr value was still up to 2.6 ± 0.6 μA/cm2. After the thermal oxidation treatment, the icorr values of the oxidized Zr–xTi alloys decreased to 0.25 μA/cm2. Meanwhile the corrosion potentials (Ecorr) of the thermally oxidized specimens increased to close to zero potential. The surface morphologies of the un-oxidized and oxidized Zr–xTi alloys after anodic corrosion at +1.5 V potential are shown in Fig. 5. It can be clearly seen that the un-oxidized Zr–20Ti displayed local exfoliation due to the serious pitting corrosion, and the un-oxidized Zr–40Ti showed slight pitting corrosion. No visible pitting corrosion was observed on the oxidized Zr–40Ti, but a few pits on the oxidized Zr–20Ti possibly resulted from the scabies defects. The above results indicate that the thermal oxidization treatment greatly improves the
Fig. 5. Surface morphologies of the un-oxidized and oxidized Zr–20Ti and Zr–40Ti after anodic corrosion at +1.5 V potential in 0.1% NaF containing artificial saliva at 37 °C, pH = 4. (a) Zr–20Ti, (b) Zr–40Ti, (c) oxidized Zr–20Ti, (d) oxidized Zr–40Ti.
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activity than the un-oxidized Zr–40Ti, which was reflected by the lower impedance modulus and phase angle in low and medium frequency region. This variation tendency was in agreement with the corrosion current densities in Table 2. After the thermal oxidation treatment, the impedance modulus and phase angles of Zr–20Ti and Zr–40Ti showed a great increase. Among them, the oxidized Zr–40Ti displayed a linear slope of about − 1 in log Zim ~ log f plot and the other features, such as high impedance modulus (4 × 105 Ω·cm2), high phase angle (40–60°) and one time constant. These indicated that the ZrTiO4 oxide coating could be regarded as ideal polarized electrode with low electrode reaction rate at the interface between oxide coating and underneath substrate [30]. The results prove again that the oxide coating of Zr–40Ti alloy has strong protection capability from corrosion even in the rigorous corrosive environment. 3.4. Friction and wear performances
Fig. 6. Bode plots of the Zr–20Ti and Zr–40Ti alloys before and after thermal oxidation treatment in 0.1% NaF containing artificial saliva at 37 °C, pH = 4. (a) Impedance modulus vs. frequency plots, (b) phase angle vs. frequency plots.
anti-pitting corrosion of the Zr–xTi alloys. Among them, the smooth and dense ZrTiO4 oxide coating on Zr–40Ti has the better corrosion resistance in fluoride-containing acidified artificial saliva solution. Fig. 6 shows the electrochemical impedance spectra (EIS) of the unoxidized and oxidized Zr–xTi alloys at open circuit potential in the form of Bode plots. The un-oxidized Zr–20Ti exhibited the higher chemical
The sliding friction tests were performed under a load of 10 N on Zr–20Ti and Zr–40Ti alloys before and after the oxidation treatment. The coefficients of friction (COF) versus sliding time plots of the four specimens are shown in Fig. 7. The COFs of un-oxidized Zr–20Ti and Zr–40Ti gradually increased in the initial sliding stage, which was related to the rupture and removal of the natural passive film, and then stabilized in the range of 0.42–0.47 after a sliding distance of 48 m. The COFs of the oxidized Zr–20Ti and Zr–40Ti at first slowly decreased and then stabilized at 0.36 and 0.31, respectively, after a sliding distance of 30 m. The decrease in the COFs of the oxidized Zr–xTi reflected that the friction and abrasion of the oxide coatings were controlled by different mechanisms from the substrates. Fig. 8 illustrates the SEM images of the worn surfaces of the substrates and the oxide coatings after 150 m sliding distance. Many adhesive craters and deep plowing grooves were observed on the un-oxidized Zr–20Ti and Zr–40Ti. The oxide debris dispersed on the worn surface, as depicted by arrows in Fig. 8a. Clearly, the un-oxidized Zr–20Ti and Zr–40Ti alloys experienced severe wear that was controlled by multiple mechanisms including abrasive, adhesion and oxidation wear. In contrast to the un-oxidized Zr–xTi, no plowing grooves or debris particles were seen in the wear tracks of the oxide coatings, as shown in Fig. 8c and d. The oxide coatings displayed the narrower wear tracks width than the substrates. By EDX analysis, the blackcolored staining on the surface of the coatings contained extremely low concentration of silicon, indicating that very small amount of the counterpart ball powders adhered to the oxide coatings during sliding friction. The fine silicide powders might act as a lubricant, leading to the gradual decrease of the COF with sliding time. Some fatigue cracks were seen near the truncated scabies on the surface of the oxidized Zr–20Ti. Since the protrudent scabies destroyed the continuity of the oxide coating, the contact fatigue wear occurred around the scabies. The propagation and coalescence of the cracks caused the exfoliation of the protuberant scabies, thereafter increased the surface roughness of the coating. The rough surface reversely aggravated the friction between the coating and the counterpart ball. This explains why the oxidized Zr–20Ti had higher COF than the oxidized Zr– 40Ti. The volume wear rates (Wv) of the specimens were calculated according to Eq. (1): Wv ¼
Fig. 7. COF vs. sliding time plots of the Zr–20Ti and Zr–40Ti alloys before and after thermal oxidation treatment, 10 N load.
Δm=d PS
ð1Þ
where Δ m—mass loss of the specimen, g; d—density of alloy or oxide coating, g/cm3; P—load, N; S—sliding distance, m. Table 3 compares the volume wear rates of the Zr–xTi alloys before and after thermal oxidation treatment. The volume wear rates of the oxidized Zr–xTi were only one-tenth that of un-oxidized Zr–xTi. And the volume wear rate of the oxidized Zr–40Ti decreased 50% compared to the oxidized
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Fig. 8. Worn track morphologies of Zr–20Ti and Zr–40Ti alloys before and after thermal oxidation treatment. (a) Zr–20Ti, (b) Zr–40Ti, (c) oxidized Zr–20Ti, (d) oxidized Zr–40Ti.
Zr–20Ti. It is easy to conclude that the smooth, hard and adhesive ZrTiO4 oxide coating contributes much to the improvement of the wear resistance of Zr–40Ti alloy. 4. Discussion As a dental implant material, the chemical stability and mechanical stability are essential prerequisites for the safe use of the prosthesis. It has been realized that the titanium implant has high sensitivity to pitting corrosion in the oral environment containing fluoride. The natural passive film with only 2–3 nm thick on the surface of the metal cannot resist the penetration and erosion of fluoride ions, particularly in the acidic medium. Furthermore, tiny sliding contact inevitably exists between the abutment and the implant as the teeth are chewing food, which leads to the corrosive wear of the metal implants. The present investigation proves that a thick, dense and highly adhesive ZrO2 or ZrTiO4 oxide coating can in-situ rapidly form on the surfaces of Zr– 20Ti and Zr–40Ti alloys by low temperature thermal oxidation treatment. ZrO2 and ZrTiO4 ceramic coatings have high chemical stabilities in fluoride-containing oral environment. They are hardly corroded by the fluoride ions even in the acidic medium, which significantly decreases the risk of the pitting corrosion. In addition to the improved corrosion resistance, the oxide coatings show low friction and good anti-wear properties. Stolyarov et al. [31] divided the coefficient of friction into an adhesion component and a deformation component which results from the adhesion force and deformation force, respectively, during sliding. The coefficient of friction from adhesion component increases with the decrease of hardness. Table 3 Wear rates of the Zr–xTi alloys before and after oxidization treatment.
Specimens
Zr–20Ti
Zr–40Ti
Oxidized Zr–20Ti
Oxidized Zr–40Ti
Wear rates (×10−5mm3/Nm)
45.9 ± 2.4
45.4 ± 2.6
5.18 ± 0.3
2.81 ± 0.4
Note: The values include mean value ± standard deviations.
These arguments explain that the high COFs of the un-oxidized Zr–Ti alloys result from the high chemical activity and high ductility of the substrates. The wear of the un-oxidized Zr–xTi alloys generally experiences a few phases, including plastic deformation → workhardening → brittle fracture. The transferred debris from the worn surface is oxidized in air, which in turn aggravates abrasive wear of the Zr–xTi substrates, as is demonstrated by the abrasive grooves in Fig. 8a and b. Although natural oxide film can rapidly form on the worn surface of Zr–xTi alloy, it is thin, weak and easily removed by the rubbing of the counterpart. The direct ceramic–metallic contact results in the more severe adhesive wear. ZrO2 or ZrTiO4 oxide has much higher hardness compared to the Zr– xTi substrates. The low temperature thermal oxidation treatment is favorable to the formation of dense, adhesive and high toughness ZrO2 or ZrTiO4 oxide coating. The good mechanical properties of the coatings are important factors decreasing the wear rate (W) according to the Evans' expression (2) [32]:
W ¼a
4=5 E 1=2 5=8 H K H F 1=8
ð2Þ
IC
where a—constant depending material structure, F—load, H—hardness, E—elastic modulus, KIC—fracture toughness. In addition to the mechanical properties, the columnar structure of the oxide coating increases the transverse shear resistance as Si3N4 counterpart ball slides on the coating surface under the normal load. This microstructure decreases the tendency of plastic deformation and cracking and increases the abrasive resistance of the oxide coating. Besides, very small contacting area between the coating and the counterpart ball due to the similar hardness and elastic modulus also decreases the adhesive wear of the coating. 5. Conclusions (1) By thermal oxidation treatment in air at 500 °C for 2 h, m-ZrO2 and ZrTiO4 oxide coatings with 11–14 μm thickness can in-situ
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form on the surface of Zr–20Ti and Zr–40Ti alloys, respectively. The oxide coatings exhibit columnar structures with the hardness of 1420–1480 HV and the bonding forces of 51 N to the substrates. The ZrTiO4 oxide coating is smooth and compact, and the m-ZrO2 oxide coating contains scabies defects. (2) In acidified artificial saliva containing 0.1% NaF (pH = 4), the oxidized Zr–xTi alloys exhibited distinctly improved the chemical stabilities, which are shown by the significant decrease of the corrosion current densities and the increase of the electrochemical impedance. The oxidized Zr–40Ti alloy has the higher pitting corrosion resistance than the oxidized Zr–20Ti. (3) During the un-lubricated sliding against Si3N4 counterpart ball under a load of 10 N, the un-oxidized Zr–xTi alloys show the serious adhesive wear and abrasive wear due to the high plasticity and chemical activity. The transferred debris from Zr–xTi alloys becomes oxidized in air, which aggravates the friction and abrasion of the alloys. (4) The anti-wear performances of the thermally oxidized Zr–xTi alloys are greatly improved as compared with the un-oxidized Zr–xTi alloys, which are attributed to the hard and adhesive oxide coatings. The scabies defects in the coating of the oxidized Zr–20Ti alloy slightly increase the coefficient of friction and the wear rate. The smooth and dense oxide coating on the oxidized Zr–40Ti alloy has the excellent wear resistance.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
The improved corrosion resistance and anti-wear performance of Zr–xTi alloys by thermal oxidation treatment W.F. Cui a,⁎, C.J. Shao b a b
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110004, China Liaoning Equipment Manufacture College of Vocational Technology, Shenyang 110161, China
a r t i c l e
i n f o
Article history: Received 31 July 2015 Revised 20 October 2015 Accepted in revised form 22 October 2015 Available online 24 October 2015 Keywords: Zr–xTi alloy Oxide coating Bonding force Corrosion Wear
a b s t r a c t By thermal oxidation treatment at 500 °C in air, the monoclinic ZrO2 and orthorhombic ZrTiO4 oxide coatings were in-situ formed on the surfaces of biomedical Zr–20Ti and Zr–40Ti alloys, respectively. The hardness and adhesion strength of the oxide coatings were measured by indentation and scratching tests. The electrochemical corrosion of the Zr–xTi alloys before and after thermal oxidation treatment was performed in the acidified artificial saliva containing 0.1% NaF at 37 °C. The friction and wear performances of the un-oxidized and oxidized Zr–xTi alloys were evaluated by reciprocating ball-on-disc wear tests under the load of 10 N. The results show that the oxide coatings have the hardness of 1420–1480 HV and the adhesion forces of 51 N to the substrates. The oxidized Zr–xTi alloys exhibit the reduced corrosion rates and improved pitting corrosion resistance in comparison with the un-oxidized Zr–xTi alloys. The wear tests demonstrate that the un-oxidized Zr–xTi alloys show the serious adhesive wear and abrasive wear due to the high plasticity and chemical activity. The coefficients of friction and wear rates of the oxidized Zr–xTi alloys decrease 30% and 90%, respectively, which are attributed to the hard oxide coatings with enhanced toughness. Nevertheless, the scabies defects in the oxide coating of Zr–20Ti alloy have a negative effect on the friction and wear property. The oxidized Zr–40Ti alloy has the excellent chemical stability and anti-abrasion performance. It has a great application prospect as dental implant material. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Commercially pure titanium (cp Ti) has been used as the abutment and screw of dental implants due to its excellent corrosion resistance, good biocompatibility and low modulus. However, the low strength of cp Ti increases the risk of small diameter screw fracture (≤3.5 mm). It has been reported that at least 5% fracture in dental implants arises from fatigue over the last decades [1]. Increasing the strength of the materials is an effective approach to prolong the service life of the metal implants. Zirconium is an ideal alloying element for titanium in biomedical applications. Zr–Ti (or Ti–Zr) binary alloys exhibit high strength and good osseointegration in animal and clinical studies [2–6]. Zr–Ti alloys also present other advantages such as lower melting point, lower magnetic susceptibility, lower linear expansion coefficient and less hydrogen absorption than cp Ti and other Ti alloys [7]. Currently, Zr–Ti alloys have become one of the important candidate materials to be used in permanent prosthesis of dental or orthopedic treatments [8]. In oral environment, wear and corrosion frequently occur at interface between metal implant and zirconia abutment [9]. The fine debris particles produced by the poor wear resistance of Ti alloys result in ⁎ Corresponding author. E-mail address: [email protected] (W.F. Cui).
http://dx.doi.org/10.1016/j.surfcoat.2015.10.051 0257-8972/© 2015 Elsevier B.V. All rights reserved.
the aseptic loosening of the implant. At the same time, the dental implants are frequently eroded by fluoride ions from toothpastes, orthodontic gels, dietary supplements, etc. Acidic foods or inflammation in the oral cavity accelerates the damage of fluoride ions to the passive film of titanium metal. The released metal ions stimulate the growth of the macrophage. The rough surfaces caused by corrosion and wear are favorable to the adhesion of various bacteria and the formation of plaque. Therefore, high corrosion resistance and anti-wear performance of dental material are vital for the safe use of the dental implants. The modern surface engineering technologies, such as laser nitridation [10], microarc oxidation [11] and magnetron sputtering [12], provide many approaches preparing oxide or nitride coatings to improve the corrosive wear resistance of titanium and titanium alloys. However, some issues in the use of these technologies, such as the surface finish, uniformity, density and adhesion strength of the coating, are not well resolved simultaneously. Moreover, the expensive equipments, complex process and high energy consumption also increase the cost of the coatings. Recently, a simple thermal oxidation method was suggested to modify the surface properties of biomedical metal materials. It has been reported that the thermally oxidized Ti–6Al–4V and Zr–2.5Nb alloys reduced the adhesive wear tendency and improved the corrosion resistance in simulated body fluid [13–17]. However, the hardness and
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adhesion strength of these oxide coatings formed on Ti alloy or Zr alloy are not high enough. The exfoliation of the oxide layer will increase coefficient of friction and decrease anti-wear performance. Since Ti and Zr metals are easily oxidized by heat treatment in air, the zirconia–titania oxide coatings can rapidly form on the surfaces of the thermally oxidized Zr–Ti alloys. The composite oxides possess higher hardness and toughness than single zirconia or titania oxide [18]. Thus, the thermally oxidized Zr–Ti alloys are expected to have greatly improved corrosive wear resistance. In the present study, the thermal oxidation treatment was performed on Zr–20Ti and Zr–40Ti alloys at 500 °C in air. The phase constitutes, surface morphologies and adhesion strength of the thermally oxidized coatings were examined. The effects of titanium content on the electrochemical corrosion and the wear performance of the un-oxidized and oxidized Zr–xTi alloys were clarified.
2. Experimental Zr–20Ti and Zr–40Ti alloys (wt.%) were prepared using arc melting in argon atmosphere for six times. The ingots were hot forged at 800 °C into the rectangular slabs. After the surface oxide layers were removed, the slabs were heat-treated at 750 °C followed by water cooling. Specimens were machined by electric spark cutting with the dimension of 20 × 10 × 5 (mm3) for wear tests and 10 × 10 × 2 (mm3) for electrochemical tests. All specimens were ground with a SiC abrasive paper up to 3000 grit, and then ultrasonically cleaned in acetone for 10 min. Some of the specimens were thermally oxidized at 500 °C for 2 h in air. The surfaces and cross section morphologies of the thermally oxidized specimens were observed by an optical microscope and a scanning electron microscope (SSX-550, Japan). The phase compositions of the oxide coatings were identified by X-ray diffraction (Smart Lab, Japan) using Cu Kα radiation with the scan rate of 3°/min. The bonding forces of the oxide coatings to the substrates were measured by a microscratch tester (WS-2005, China) with a diamond stylus of 0.2 mm radius. The load increased from 0 to 200 N with a loading rate of 100 N/min. The critical loads at which the oxide started to crack were measured. The Vickers microhardness of the oxidized and un-oxidized specimens were measured by a hardness tester (401 MVDTM, China) with a load
Fig. 2. XRD profiles of the thermally oxidized Zr–20Ti and Zr–40Ti alloys.
of 25 g and a loading time of 30 s. The final hardness was obtained from the average values of the ten testing results. The electrochemical experiments were carried out in the naturally aerated artificial saliva containing 0.1% NaF at 37 °C (pH = 4). The components of the artificial saliva were as follows: NaCl (0.4 g/l), KCl (0.4 g/1), NaH2PO4·2H2O (0.78 g/l), CaCl2·2H2O (0.795 g/l), Na2S·2H2O (0.005 g/l), urea (1.0 g/l) and 1000 ml distilled water. All the chemicals were provided by Guoyao Group Chemical Reagent Co. Ltd. (Shenyang). Three-electrode electrochemical working station (Zennium, Germany) was used with saturated calomel electrode as reference electrode, platinum plate as counter electrode and the specimens as working electrode (1 cm2 exposure area to the solution). Potentiodynamic polarization curves (PPC) were recorded over the potential range of −1.5 V to +1.5 V at a scanning rate of 1 mV/s. The electrochemical impedance spectra (EIS) were measured by applying a sinusoidal potential perturbation of 5 mV with a frequency from 10−2 Hz to 105 Hz in logarithmic increment at the open circuit potential. Before the tests, the specimens were immersed in the electrolyte for 1 h in order to attain stable open
Fig. 1. Oxide coating morphologies of the Zr–20Ti and Zr–40Ti alloys thermally oxidized at 500 °C for 2 h. (a),(b) surface and fracture cross-section images of the oxidized Zr–20Ti alloy showing the scabies defects and uneven top surface, (c),(d) surface and fracture cross-section images of the oxidized Zr–40Ti alloy showing the smooth surface and columnar structure.
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Fig. 4. Potentiodynamic polarization curves of Zr–20Ti and Zr–40Ti alloys before and after the thermal oxidation treatment in 0.1% NaF containing artificial saliva at 37 °C, pH = 4.
the test specimens in air. The sliding wear started with a normal load of 10 N. The reciprocating sliding distance was 20 mm and the number of cycling was 7200 at a frequency of 1 Hz. The surface morphologies of the worn specimens were observed by using SEM (SSX-550) which is equipped with energy dispersive X-ray analysis (EDX). 3. Results 3.1. Morphologies and structural analysis of the oxide coatings
Fig. 3. Acoustic emission signal plots during the scratching and the corresponding scratch track images showing the critical forces of the oxide rupture. (a),(c) Zr–20Ti, (b),(d) Zr–40Ti.
circuit potential. The surface morphologies of the corroded specimens were observed by using SEM (SSX-550). Linear reciprocating ball-on-disc wear testing was performed on the Zr–Ti alloys with and without oxidation treatment by using a tribometer (CSM, Switzerland) with a Si3N4 ball of 6 mm diameter rubbing against Table 1 Hardness of the Zr–xTi alloys before and after oxidization treatment. Specimens
Zr–20Ti
Zr–40Ti
Oxidized Zr–20Ti
Oxidized Zr–40Ti
Hardness (HV0.1)
329 ± 12
340 ± 10
1425 ± 36
1476 ± 55
Note: The values include mean value ± standard deviation.
The surface morphologies of the oxide coatings were observed by an optical microscope, as shown in Fig. 1a and c. The oxidized surface of Zr– 20Ti displayed scabies-like defects. The oxidized surface of Zr–40Ti was smooth and dense. From the cross-section images of the oxide coatings (Fig. 1b and d), it was seen that the thickness of oxide coatings for the oxidized Zr–20Ti and Zr–40Ti was 14 ± 0.5 μm and 11 ± 0.2 μm, respectively, indicating the slower oxidation rate with the increase of titanium content in Zr–Ti alloy. The oxide coatings exhibited a columnar growth structure. The good binding between the coatings and the substrates was seen without cracks or lamination in the coatings. The phase identification of the oxide coatings is shown by XRD patterns in Fig. 2. For Zr–20Ti, the coating mainly consisted of m-ZrO2 with monoclinic structure. The weak diffraction peaks of ZrTiO4 with orthorhombic structure were also detected. But for Zr–40Ti, ZrTiO4 became the main phase of the oxide coating. Small amount of t-ZrO2 with tetragonal structure possibly remained in the coating because the strongest diffraction peak of t-ZrO2 nearly coincides with ZrTiO4 at about 30° diffraction angle. The different morphologies and phase compositions of the oxide coatings indicate that Zr–20Ti and Zr–40Ti alloys exhibited the different oxide formation mechanisms, which can be explained by the oxidation thermodynamics and kinetic theory of alloys. Both Zr and Ti have strong oxygen affinity. They all participate in the oxidation process of the Zr–xTi alloy during thermal oxidation treatment. According to the Ellingham–Richardson diagram, the equilibrium oxygen partial pressure at Zr/ZrO2 interface is smaller than that at Ti/TiO2 interface. Zr in the Zr–xTi alloys is preferentially oxidized into t-ZrO2. TiO2 is subsequently formed in the t-ZrO2 lattice. Although anatase-type TiO2 has the same crystal structure as t-ZrO2, their c/a axis ratios have large difference (2.51 for a-TiO2 and 1.016 for t-ZrO2). In order to reduce the elastic strain energy in the lattice, ZrO2/TiO2 composite oxide transforms into ZrTiO4 compound by the adjustment of the lattice parameters [19,20]. For Zr–40Ti alloy, Zr to Ti atomic ratio approaches 0.8, thus, nearly 100% ZrTiO4 forms in the oxide coating. For Zr–20Ti alloy,
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Table 2 Electrochemical parameters obtained from the potentiodynamic polarization curves. Specimens
Zr20Ti
Zr40Ti
Oxidized Zr20Ti
Oxidized Zr40Ti
Ecorr (V) icorr (μA/cm2)
−1.11 ± 0.03 38 ± 0.7
−0.63 ± 0.06 2.6 ± 0.6
0.04 ± 0.05 0.25 ± 0.08
−0.05 ± 0.04 0.25 ± 0.04
Note: The values include mean values ± standard deviation.
however, only small amount of ZrO2/TiO2 transforms into ZrTiO4 due to decreased titanium content. Most of t-ZrO2 grains grow up with the prolonging of oxidation time. Once the grain size reaches the critical value (30.6 nm), a martensite phase transformation (t-ZrO2 → mZrO2) occurs [21]. The phase transformation stress leads to the formation of the scabies defects in the oxide coating, as seen in Fig. 1a [22]. It is worth noting that the oxide coating of Zr–20Ti displayed the strong (200) texture. However, the similar phenomenon did not occur to the oxide coating of Zr–40Ti in spite of the same columnar structure. It is analyzed that (200) texture of m-ZrO2 coating can result in low energy surface, which is favorable to the rapid growth of the columnar crystal. But the oxide coating of Zr–40Ti contains small amount of t-ZrO2. The strong (111) crystal plane diffraction from t-ZrO2 impedes the formation of ZrTiO4 texture. 3.2. Mechanical properties of the oxide coatings The scratching tests were used to assess the adhesion strength of the oxide coatings to the substrates. The critical normal load (Lc) could be detected at which the oxide started to crack by a sudden increase in acoustic emission intensity. Fig. 3a and b shows the adhesion forces of the oxide coatings up to 51 N. The value was twice as much as that of the thermal oxidation coating on Zr alloy or Ti alloy [23–25]. By carefully observing the scratch tracks of the oxidized Zr–20Ti, it was found that there were already some microcracks at the edges of the scratch track before oxide delamination. But in the scratch track of the oxidized Zr40Ti, the uniform plastic deformation was observed until the oxide was completely broken (Fig. 3c and d). These features indicate that
ZrTiO4 oxide exhibited the higher resistance to cracking than m-ZrO2 oxide. The advanced toughness will improve the friction and wear performances of the oxide coating. Table 1 lists the surface hardness of the Zr–xTi alloys before and after thermal oxidation treatment. The hardness of the oxidized Zr–xTi was higher than 1400 HV, which sharply contrasted to the hardness (330–350 HV) of the un-oxidized Zr–xTi. The values are also much higher than those of the other thermally oxidized Ti or Zr-based alloy [26]. The increased surface hardness of the oxidized Zr–xTi alloys was evidently attributed to the thick and compact ZrO2 or ZrTiO4 oxide coatings with columnar structure. 3.3. Electrochemical corrosion behavior The previous studies have reported that the corrosion rates of the titanium alloys were sharply accelerated in fluoride-containing artificial saliva [27–29]. In this study, the electrochemical corrosion behaviors of the Zr–20Ti and Zr–40Ti before and after thermal oxidation treatment were compared in 0.1% NaF containing acidified artificial saliva (pH = 4). Fig. 4 shows the potentiodynamic polarization curves (PPC) of the four specimens. Table 2 gives the correlated electrochemical corrosion parameters. Before the thermal oxidation treatment, the selfcorrosion current density (icorr) and anodic current density (ia) of the Zr–xTi alloys decreased with the increase of titanium content. In spite of this, the lower icorr value was still up to 2.6 ± 0.6 μA/cm2. After the thermal oxidation treatment, the icorr values of the oxidized Zr–xTi alloys decreased to 0.25 μA/cm2. Meanwhile the corrosion potentials (Ecorr) of the thermally oxidized specimens increased to close to zero potential. The surface morphologies of the un-oxidized and oxidized Zr–xTi alloys after anodic corrosion at +1.5 V potential are shown in Fig. 5. It can be clearly seen that the un-oxidized Zr–20Ti displayed local exfoliation due to the serious pitting corrosion, and the un-oxidized Zr–40Ti showed slight pitting corrosion. No visible pitting corrosion was observed on the oxidized Zr–40Ti, but a few pits on the oxidized Zr–20Ti possibly resulted from the scabies defects. The above results indicate that the thermal oxidization treatment greatly improves the
Fig. 5. Surface morphologies of the un-oxidized and oxidized Zr–20Ti and Zr–40Ti after anodic corrosion at +1.5 V potential in 0.1% NaF containing artificial saliva at 37 °C, pH = 4. (a) Zr–20Ti, (b) Zr–40Ti, (c) oxidized Zr–20Ti, (d) oxidized Zr–40Ti.
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activity than the un-oxidized Zr–40Ti, which was reflected by the lower impedance modulus and phase angle in low and medium frequency region. This variation tendency was in agreement with the corrosion current densities in Table 2. After the thermal oxidation treatment, the impedance modulus and phase angles of Zr–20Ti and Zr–40Ti showed a great increase. Among them, the oxidized Zr–40Ti displayed a linear slope of about − 1 in log Zim ~ log f plot and the other features, such as high impedance modulus (4 × 105 Ω·cm2), high phase angle (40–60°) and one time constant. These indicated that the ZrTiO4 oxide coating could be regarded as ideal polarized electrode with low electrode reaction rate at the interface between oxide coating and underneath substrate [30]. The results prove again that the oxide coating of Zr–40Ti alloy has strong protection capability from corrosion even in the rigorous corrosive environment. 3.4. Friction and wear performances
Fig. 6. Bode plots of the Zr–20Ti and Zr–40Ti alloys before and after thermal oxidation treatment in 0.1% NaF containing artificial saliva at 37 °C, pH = 4. (a) Impedance modulus vs. frequency plots, (b) phase angle vs. frequency plots.
anti-pitting corrosion of the Zr–xTi alloys. Among them, the smooth and dense ZrTiO4 oxide coating on Zr–40Ti has the better corrosion resistance in fluoride-containing acidified artificial saliva solution. Fig. 6 shows the electrochemical impedance spectra (EIS) of the unoxidized and oxidized Zr–xTi alloys at open circuit potential in the form of Bode plots. The un-oxidized Zr–20Ti exhibited the higher chemical
The sliding friction tests were performed under a load of 10 N on Zr–20Ti and Zr–40Ti alloys before and after the oxidation treatment. The coefficients of friction (COF) versus sliding time plots of the four specimens are shown in Fig. 7. The COFs of un-oxidized Zr–20Ti and Zr–40Ti gradually increased in the initial sliding stage, which was related to the rupture and removal of the natural passive film, and then stabilized in the range of 0.42–0.47 after a sliding distance of 48 m. The COFs of the oxidized Zr–20Ti and Zr–40Ti at first slowly decreased and then stabilized at 0.36 and 0.31, respectively, after a sliding distance of 30 m. The decrease in the COFs of the oxidized Zr–xTi reflected that the friction and abrasion of the oxide coatings were controlled by different mechanisms from the substrates. Fig. 8 illustrates the SEM images of the worn surfaces of the substrates and the oxide coatings after 150 m sliding distance. Many adhesive craters and deep plowing grooves were observed on the un-oxidized Zr–20Ti and Zr–40Ti. The oxide debris dispersed on the worn surface, as depicted by arrows in Fig. 8a. Clearly, the un-oxidized Zr–20Ti and Zr–40Ti alloys experienced severe wear that was controlled by multiple mechanisms including abrasive, adhesion and oxidation wear. In contrast to the un-oxidized Zr–xTi, no plowing grooves or debris particles were seen in the wear tracks of the oxide coatings, as shown in Fig. 8c and d. The oxide coatings displayed the narrower wear tracks width than the substrates. By EDX analysis, the blackcolored staining on the surface of the coatings contained extremely low concentration of silicon, indicating that very small amount of the counterpart ball powders adhered to the oxide coatings during sliding friction. The fine silicide powders might act as a lubricant, leading to the gradual decrease of the COF with sliding time. Some fatigue cracks were seen near the truncated scabies on the surface of the oxidized Zr–20Ti. Since the protrudent scabies destroyed the continuity of the oxide coating, the contact fatigue wear occurred around the scabies. The propagation and coalescence of the cracks caused the exfoliation of the protuberant scabies, thereafter increased the surface roughness of the coating. The rough surface reversely aggravated the friction between the coating and the counterpart ball. This explains why the oxidized Zr–20Ti had higher COF than the oxidized Zr– 40Ti. The volume wear rates (Wv) of the specimens were calculated according to Eq. (1): Wv ¼
Fig. 7. COF vs. sliding time plots of the Zr–20Ti and Zr–40Ti alloys before and after thermal oxidation treatment, 10 N load.
Δm=d PS
ð1Þ
where Δ m—mass loss of the specimen, g; d—density of alloy or oxide coating, g/cm3; P—load, N; S—sliding distance, m. Table 3 compares the volume wear rates of the Zr–xTi alloys before and after thermal oxidation treatment. The volume wear rates of the oxidized Zr–xTi were only one-tenth that of un-oxidized Zr–xTi. And the volume wear rate of the oxidized Zr–40Ti decreased 50% compared to the oxidized
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Fig. 8. Worn track morphologies of Zr–20Ti and Zr–40Ti alloys before and after thermal oxidation treatment. (a) Zr–20Ti, (b) Zr–40Ti, (c) oxidized Zr–20Ti, (d) oxidized Zr–40Ti.
Zr–20Ti. It is easy to conclude that the smooth, hard and adhesive ZrTiO4 oxide coating contributes much to the improvement of the wear resistance of Zr–40Ti alloy. 4. Discussion As a dental implant material, the chemical stability and mechanical stability are essential prerequisites for the safe use of the prosthesis. It has been realized that the titanium implant has high sensitivity to pitting corrosion in the oral environment containing fluoride. The natural passive film with only 2–3 nm thick on the surface of the metal cannot resist the penetration and erosion of fluoride ions, particularly in the acidic medium. Furthermore, tiny sliding contact inevitably exists between the abutment and the implant as the teeth are chewing food, which leads to the corrosive wear of the metal implants. The present investigation proves that a thick, dense and highly adhesive ZrO2 or ZrTiO4 oxide coating can in-situ rapidly form on the surfaces of Zr– 20Ti and Zr–40Ti alloys by low temperature thermal oxidation treatment. ZrO2 and ZrTiO4 ceramic coatings have high chemical stabilities in fluoride-containing oral environment. They are hardly corroded by the fluoride ions even in the acidic medium, which significantly decreases the risk of the pitting corrosion. In addition to the improved corrosion resistance, the oxide coatings show low friction and good anti-wear properties. Stolyarov et al. [31] divided the coefficient of friction into an adhesion component and a deformation component which results from the adhesion force and deformation force, respectively, during sliding. The coefficient of friction from adhesion component increases with the decrease of hardness. Table 3 Wear rates of the Zr–xTi alloys before and after oxidization treatment.
Specimens
Zr–20Ti
Zr–40Ti
Oxidized Zr–20Ti
Oxidized Zr–40Ti
Wear rates (×10−5mm3/Nm)
45.9 ± 2.4
45.4 ± 2.6
5.18 ± 0.3
2.81 ± 0.4
Note: The values include mean value ± standard deviations.
These arguments explain that the high COFs of the un-oxidized Zr–Ti alloys result from the high chemical activity and high ductility of the substrates. The wear of the un-oxidized Zr–xTi alloys generally experiences a few phases, including plastic deformation → workhardening → brittle fracture. The transferred debris from the worn surface is oxidized in air, which in turn aggravates abrasive wear of the Zr–xTi substrates, as is demonstrated by the abrasive grooves in Fig. 8a and b. Although natural oxide film can rapidly form on the worn surface of Zr–xTi alloy, it is thin, weak and easily removed by the rubbing of the counterpart. The direct ceramic–metallic contact results in the more severe adhesive wear. ZrO2 or ZrTiO4 oxide has much higher hardness compared to the Zr– xTi substrates. The low temperature thermal oxidation treatment is favorable to the formation of dense, adhesive and high toughness ZrO2 or ZrTiO4 oxide coating. The good mechanical properties of the coatings are important factors decreasing the wear rate (W) according to the Evans' expression (2) [32]:
W ¼a
4=5 E 1=2 5=8 H K H F 1=8
ð2Þ
IC
where a—constant depending material structure, F—load, H—hardness, E—elastic modulus, KIC—fracture toughness. In addition to the mechanical properties, the columnar structure of the oxide coating increases the transverse shear resistance as Si3N4 counterpart ball slides on the coating surface under the normal load. This microstructure decreases the tendency of plastic deformation and cracking and increases the abrasive resistance of the oxide coating. Besides, very small contacting area between the coating and the counterpart ball due to the similar hardness and elastic modulus also decreases the adhesive wear of the coating. 5. Conclusions (1) By thermal oxidation treatment in air at 500 °C for 2 h, m-ZrO2 and ZrTiO4 oxide coatings with 11–14 μm thickness can in-situ
W.F. Cui, C.J. Shao / Surface & Coatings Technology 283 (2015) 101–107
form on the surface of Zr–20Ti and Zr–40Ti alloys, respectively. The oxide coatings exhibit columnar structures with the hardness of 1420–1480 HV and the bonding forces of 51 N to the substrates. The ZrTiO4 oxide coating is smooth and compact, and the m-ZrO2 oxide coating contains scabies defects. (2) In acidified artificial saliva containing 0.1% NaF (pH = 4), the oxidized Zr–xTi alloys exhibited distinctly improved the chemical stabilities, which are shown by the significant decrease of the corrosion current densities and the increase of the electrochemical impedance. The oxidized Zr–40Ti alloy has the higher pitting corrosion resistance than the oxidized Zr–20Ti. (3) During the un-lubricated sliding against Si3N4 counterpart ball under a load of 10 N, the un-oxidized Zr–xTi alloys show the serious adhesive wear and abrasive wear due to the high plasticity and chemical activity. The transferred debris from Zr–xTi alloys becomes oxidized in air, which aggravates the friction and abrasion of the alloys. (4) The anti-wear performances of the thermally oxidized Zr–xTi alloys are greatly improved as compared with the un-oxidized Zr–xTi alloys, which are attributed to the hard and adhesive oxide coatings. The scabies defects in the coating of the oxidized Zr–20Ti alloy slightly increase the coefficient of friction and the wear rate. The smooth and dense oxide coating on the oxidized Zr–40Ti alloy has the excellent wear resistance.
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