Comparative study on stability of boron doped diamond coated titanium and niobium electrodes

Accepted Manuscript Comparative study on stability of boron doped diamond coated titanium and niobium electrodes

Xin-Ru Lu, Ming-Hui Ding, Cong Zhang, Wei-Zhong Tang PII: DOI: Reference:

S0925-9635(18)30730-1 https://doi.org/10.1016/j.diamond.2019.01.010 DIAMAT 7309

To appear in:

Diamond & Related Materials

Received date: Revised date: Accepted date:

24 October 2018 14 January 2019 14 January 2019

Please cite this article as: Xin-Ru Lu, Ming-Hui Ding, Cong Zhang, Wei-Zhong Tang , Comparative study on stability of boron doped diamond coated titanium and niobium electrodes. Diamat (2018), https://doi.org/10.1016/j.diamond.2019.01.010

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ACCEPTED MANUSCRIPT Comparative study on stability of boron doped diamond coated titanium and niobium electrodes Xin-Ru Lu, Ming-Hui Ding, Cong Zhang, Wei-Zhong Tang* Institute for Advanced Materials and Technology, University of Science and

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Technology Beijing, Beijing 100083, China

Abstract

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The service life of boron doped diamond (BDD) coated electrodes is closely

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related to the substrate material and preparation process of the BDD coatings. In this paper, the failure process of BDD coated titanium (Ti/BDD) and niobium (Nb/BDD) electrodes prepared by arc plasma chemical vapor deposition method was

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comparatively studied by accelerated life test. The results showed that the main failure mechanism of the two types of electrodes is delamination of the BDD coatings,

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accompanied by coatings’ corrosion. The delamination of BDD coatings from both the substrates showed an incubation period, with the Ti/BDD electrode having a much shorter incubation period and a higher coating delamination rate than its Nb/BDD

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counterpart. By comparing growth process of diamond coatings and corrosion

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behavior of carbides formed on both substrates, it is found that on a Ti substrate, diamond coating is more liable to incorporate pore like defects, and carbide layer formed beneath the diamond coating is more susceptible to corrosion in electrolysis

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solution. It is believed that these two factors would be responsible for short service

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life for Ti/BDD electrodes as compared with that for Nb/BDD electrodes.

Keywords: BDD electrode, accelerated life test, carbide layer, failure mechanism

1. Introduction Boron-doped diamond (BDD) has been considered as an ideal electrode material with excellent electrochemical properties and high chemical stability [1-3]. BDD coated electrodes could be prepared by chemical vapor deposition (CVD) of thin BDD coatings on substrates of silicon (Si) or refractory metals including tantalum (Ta), titanium (Ti) and niobium (Nb) [4]. But, because of the poor mechanical *

Corresponding author, E-mail address: [email protected]

ACCEPTED MANUSCRIPT performance of Si and high cost of Ta, both of these materials are not suitable to be used as substrates for wide applications. On the other hand, though Nb is also expensive as compared with Ti, BDD coated Nb (Nb/BDD) electrodes possess often a superior service life, and have been already on the market and applied in wastewater treatment applications [4-7]. On the contrary, BDD coated Ti (Ti/BDD) electrodes have ordinarily a short service life and remain as a hot research topic because Ti has

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good mechanical properties and is less expensive [8,9]. To be useful as an electrode material, stability and service life of BDD coated

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electrodes must be considered. Though BDD is known for its high stability, corrosion of BDD coated electrodes could still occur at an appreciable rate under high current

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density and in specific corrosive environment [4,10-12]. Even worse, delamination of BDD coatings seems to be more seriously a problem. Chaplin et al [7] found that the

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primary failure mechanism for BDD coated electrodes on a variety of substrates (Si, Ta, Nb, W and Ti) was delamination of the coatings. They suggested that large differences in thermal expansion of the substrates and their corresponding oxides

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result in delamination of the diamond coatings.

On the other hand, though research has demonstrated that delamination of BDD

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coating is responsible for failure of Ti/BDD coated electrodes [7,13-17], no consistent conclusion has been reached on the reasons for their failure mechanism. Possible

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reasons mentioned include corrosion of titanium carbide (TiC) layer formed between BDD coatings and substrates [17], variation in coating quality and adhesion [13], residual stress of diamond coatings and their corrosion at high potentials [16], etc. On

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the other hand, as compared with that of Ti/BDD electrodes, the failure mechanism of Nb/BDD electrodes have even less been investigated.

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In our previous work, microstructure evolution during the failure process of a Ti/BDD coated electrode was investigated. We revealed that the failure process of the Ti/BDD electrode could be divided into two stages: in the first stage, defects resulted in the quick cracking and delamination of the BDD coating, and in the second stage, corrosion of BDD grains and dissolution of the intermediate TiC layer occurred accompanied by further coating delamination [18]. To gain deep understanding on failure mechanism of Ti/BDD coated electrodes, a comparative study on the failure process of Ti/ and Nb/BDD electrodes prepared under the same conditions was undertaken in this investigation. Then by comparing diamond growth process and properties of carbide layers formed between diamond

ACCEPTED MANUSCRIPT coatings and Ti and Nb substrates, reasons for the difference in service life of the two types of electrodes were investigated. The results obtained in this study would provide useful clues to the failure mechanism of Ti/BDD coated electrodes as well as possible routes to optimize preparation process and service life of the electrodes.

2. Experimental process

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2.1 Preparation of samples Ti and Nb plates were cut into 18151mm samples, respectively. Surface of the

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samples was mechanically ground with 200#, 400#, 600# and 800# sandpapers successively, and then they were ultrasonically cleaned in ethanol.

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BDD coatings were prepared by using a high current extended direct current arc plasma CVD system [19]. The reactive gas used was a mixture of argon (Ar),

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hydrogen (H2) and methane (CH4).

Two kinds of diamond coated samples were prepared in the experiment. One was

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Ti/BDD and Nb/BDD coating electrodes prepared by depositing BDD coatings on Ti and Nb substrates. In order to facilitate diamond nucleation, pretreatment was given to the substrates by manually grinding them with diamond paste of the size of 0.5μm

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before the CVD process. Deposition of the BDD coatings was carried out in two

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stages. Firstly, during the nucleation stage, a higher methane concentration was used to increase diamond nucleation density on the substrates, and then during the growth stage the methane concentration was lowered so as to improve the quality of the BDD coatings. Trimethyl borate (B(OCH3)3) was used as the precursor for boron doping,

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and it was fed into the deposition chamber through its evaporation. Detailed deposition parameters for these BDD coating electrodes are given in Table 1.

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Microstructure and electrochemical properties of these BDD coated electrodes were characterized and the experimental results will be presented in section 3.1 and 3.2. Table 1 Deposition parameters for preparing BDD coated electrodes Gas flow rate (sccm) * Nucleation stage

Growth stage

Ar\H2\CH4\ B(OCH3)3 Ar\H2\CH4\ B(OCH3)3 1800\100\10\0.4

1800\100\3\0.4

Nucleation/ Growth Time

Temperature /

Pressure

(°C)

(Pa)

750

500

(hours) 2/8

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The second kind of samples was diamond coatings deposited on Ti and Nb substrates without boron doping. The same surface pretreatment method as the preparation of the BDD coated electrodes was adopted before the deposition process. However, the methane concentration remained constant throughout the deposition process. Morphology of deposits was observed after 0.5h, 1.5h, 3h and 6h of

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deposition, respectively. Detailed deposition parameters for these samples are given in Table 2. These diamond coating samples were used to investigate the growth process

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of diamond coatings on different substrates and the results of this study will be

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presented in section 3.3.1.

Table 2 Deposition parameters of studying growth characteristics of diamond

Gas flow rate (sccm)

Growth Time

Temperature (°C)

(Pa)

0.5-6

750

500

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1800\100\6

Pressure

(hours)

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Ar\H2\CH4

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coatings on different substrates

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In order to study the properties of carbide layers formed in BDD coated electrodes, surface carbonized samples of different substrates were also prepared. In these experiments, both Ti and Nb plates were carburized for 1h in the same

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environment as the nucleation stage of the BDD coating deposition, and carbide layers were formed on the surface of these Ti and Nb plates. No pretreatment was

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adopted before the carburization process to facilitate diamond nucleation. The experimental results of these carbonized samples will be presented in section 3.3.2. 2.2 Sample characterization and accelerated life test Morphology of samples was examined by using an FEI Quanta 450 field emission scanning electron microscope (FE-SEM). The phase components of the samples were analyzed by using a D/MAX-2000 X-ray diffractometer (XRD). To assess surface roughness of the samples, a Dektak-150 profiler was used. All electrochemical tests were performed on a CHI660e electrochemical workstation. The measurements were conducted with a classical three-electrode configuration in 0.5mol/L H2SO4 electrolyte at room temperature. The samples to be

ACCEPTED MANUSCRIPT studied (BDD coated electrodes and surface carbonized samples) served as the working electrodes. They were sealed in a plastic electrochemical cell with an exposed area of 1 cm2. A platinum plate was used as the counter electrode. A saturated calomel electrode (SCE) was used as the reference electrode. All potentials in this paper were reported in the SCE scale. Cyclic voltammetry (CV) curves of samples were measured at a scanning rate of 0.1V/s in a potential range between -1.5 and 3V.

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Accelerated life tests (ALT) on BDD coated electrodes were carried out at room temperature in 1mol/L H2SO4 electrolyte by employing a two-electrode system. A

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TRADEX MPS 302 power source was used to provide a current density of 1A/cm2 to the electrolytic cell. A BDD coated electrode and a stainless steel plate were used as

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the anode and cathode, respectively. When the cell voltage rose abruptly, the BDD electrode was considered as failed.

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Both potentiostatic and potentiodynamic polarization were performed in sequence on the carbonized samples. The former was performed on as-prepared samples by polarizing them at 4.5V (near the potential of the BDD coated electrodes

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during ALT), and the latter was performed on these anodized samples at a scanning rate of 5mV/s in a potential range between -0.5 and 6V.

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3. Experimental results and discussions

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3.1. Surface morphology and CV curves of the BDD electrode samples

Fig. 1. Surface and cross-section SEM images of (a, c) Ti/BDD and (b, d)

ACCEPTED MANUSCRIPT Nb/BDD electrode samples. Fig 1 (a) and (b) show surface morphologies of the Ti/BDD and Nb/BDD coating electrode samples. As can be seen from the figures, the surface of the two samples is smooth and compact, and no BDD coating delamination could be noticed. By enlarging the images, it could be seen that the grain size of the two BDD coatings is on the order of 1 μm. The diamond grains are mainly cuboctahedrons with dominant

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{111} faceting. Careful comparison shows that the grain size of the Nb/BDD sample is more uniform, and the surface of this sample is smoother. On the contrary, the

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surface of the Ti/BDD sample is relatively rough, with a high likelihood of existence

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of pore type defects between the BDD grains. Fig.1 (c) and (d) show cross-sectional images of the two BDD coated samples. From the images, we notice that the BDD coatings on both substrates were about 2.6 μm thick. In addition, we see that for the

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Ti/BDD sample, there was an intermediate carbide layer with a thickness of about 1 μm beneath the diamond coating [20]. On the other hand, the carbide layer for the

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Nb/BDD sample was too thin to be discerned.

Fig.2. Raman spectra of the BDD coatings on Ti and Nb substrates

Fig.2 depicts Raman spectra of the BDD coatings on both Ti and Nb substrates. We see that the two spectra exhibited similar features of heavily doped BDD coatings: (1) a wide asymmetric band centered at around 500 cm-1 ascribable to the vibrational mode of boron dimers or boron-carbon bonds [21]; (2) a strong distortion of the diamond one-phonon line (1332 cm-1) into two separate branches leading to an asymmetric band centred at around 1200-1300 cm-1 caused by the Fano resonance effect[22]. The broad band at around 1580 cm-1 was rather weak, indicating that the

ACCEPTED MANUSCRIPT content of graphitic phase in the BDD coatings was very low. From the wide asymmetric band centered at around 500cm-1, boron concentration of the BDD coatings could be estimated as on the order of 1.91021 cm-3, fulfilling the requirement that the boron concentration should be higher than 31020 cm-3 for the BDD coatings

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to possess metallic conductivity and to be useful as electrode materials [23].

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Fig. 3. CV curves of the Ti/BDD and Nb/BDD electrodes in 0.5mol/L H2SO4 electrolyte at a scan rate of 0.1V/s

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Fig. 3 shows CV curves for the two electrodes measured in 0.5mol/L H2SO4

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electrolyte. It is obvious from the figure that both of the electrodes have wide potential windows and low background current. By inferring from Fig. 3, following parameters were derived for the Ti/BDD and the Nb/BDD electrodes: potential

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window of 3.59V and 3.65V, oxygen evolution potential of 2.54V and 2.52V, hydrogen evolution potential of -1.05V and -1.13V, and background current density of

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22μA/cm2 and 11μA/cm2, respectively. It could easily be seen that the difference in potential windows of the two electrodes was mainly a result of the difference in their hydrogen evolution potentials. Previous study [24] has shown that in CV curves of BDD electrodes measured in acid mediums, a peak in potential range between -0.7 and -0.9V could be attributed to O2 reduction, whose intensity may be used as a sensitive diagnostic tool for the presence of sp2 bonded carbon in the diamond coatings. We see from Fig.3 that in the CV curve for the Ti/BDD electrode, this peak was more obvious than for the Nb/BDD electrode, indicating that content of sp2 bonded carbon was higher in the former sample. This conclusion is consistent with the difference observed in hydrogen

ACCEPTED MANUSCRIPT evolution potentials of the two electrodes. Meanwhile, the background current of the Ti/BDD electrode was twice that of the Nb/BDD electrode, which may also be attributed to the relatively higher content of sp2 bonded carbon contained in the Ti/BDD sample. As is generally accepted, grain boundaries are locations where defective sp2 bonded carbon may be concentrated [25,26]. From SEM observations shown in Fig.1,

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it has been noticed that in the Ti/BDD sample the grain size is less uniform and pore type defects is more likely to exist at grain boundaries, though from the Raman

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spectra, no difference in BDD coatings quality could be discerned. Such being the case, the Ti/BDD sample would be more electrochemically active as this

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polycrystalline sample may contain more sp2 bonded carbon along its grain

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3.2 Results of the accelerated life tests

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boundaries.

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Fig. 4. (a) Variation of cell voltage and (b) change in percentage of delaminated area of the BDD coatings vs. electrolysis time for the two BDD

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coating electrodes. The insets in (b) are representative photographs showing coatings delamination during the ALT process

Accelerated life tests (ALT) were performed on the two types of BDD coated electrodes. It could be understandable that under the severe ALT conditions, water electrolysis would occur and numerous gas bubbles could be seen emerging from the surfaces of both the anode and cathode, respectively. Fig. 4(a) shows variation of the cell voltage during the ALT process. As shown in this figure, the overall trend of the cell voltage variation for the two electrodes is quite similar. Firstly, the cell voltage remained stable for a long period of time at around 5.5V. Then, after a short period of

ACCEPTED MANUSCRIPT gradual increase, it increased rapidly, signifying the failure of the electrodes. The results showed that the ALT service life of both electrodes is significantly different, i.e., 127h and 484h for the Ti/BDD and the Nb/BDD electrode, respectively. Fig. 4(b) shows the change in percentage of delaminated area of the BDD coatings during the ALT process for the two types of BDD coated electrodes. As has been described previously [18], during the ALT process, delamination of the BDD

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coatings occurred on both types of the electrodes (see the insets in Fig. 4(b)). It could be seen from the figure that there existed different incubation periods for the two

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BDD coatings to begin delaminating from the electrodes. For the Nb/BDD sample, the incubation period is more than 200h long, while for the Ti/BDD sample, it is less

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than 24h. Also from the slope of curves shown in Fig. (4), different delamination rates may be estimated for the two BDD coating electrodes. The BDD coating delaminated

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around two times as fast in the Ti/BDD electrode as in the Nb/BDD electrode.

Fig. 5. SEM images of the BDD coatings on the two types of electrodes,

observed after different electrolysis times. Ti/BDD: (a) 50h, (b) 100h; Nb/BDD: (c) 50h, (d) 200h. Fig. 5 shows morphologies of BDD coatings of the two electrodes after different electrolysis times. From this figure we see that after 50h of electrolysis, diamond grains of the Ti/BDD electrode have been seriously corroded (Fig. 5(a)), while those of the Nb/BDD electrode were only slightly corroded along grain boundaries (Fig.

ACCEPTED MANUSCRIPT 5(c)). Then after 100h of electrolysis, numerous corrosion pores appeared along grain boundaries of the Ti/BDD electrode (Fig. 5(b)). Similarly, corrosion pores could also be observed for the Nb/BDD electrode after 200h of electrolysis (Fig. 5(d)). Corrosion of the BDD grains appeared much more serious in the case of the Ti/BDD sample than in its Nb/BDD counterpart, as for the Ti/BDD sample, premature coating delamination has occurred, resulting in a higher current density to the remaining BDD

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grains and making these BDD grains more severely corroded [12,27]. Corrosion occurred preferentially along grain boundaries since these are high energy positions

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where both structural defects and impurities would segregate [28,29].

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3.3 Discussions on service life of Ti/ and Nb/BDD electrodes Above experimental results show that the failure mechanism of Ti/ and Nb/BDD

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electrodes is primarily the delamination of the BDD coatings. The coating delamination problem is much more severe for Ti/BDD samples than for Nb/BDD

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electrodes. To further explore the reasons for the problem, comparative study of diamond growth process and electrochemical behaviors of Ti and Nb carbide layers

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formed on the two substrate materials was conducted.

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3.3.1 Diamond growth process on Ti and Nb substrates

Fig. 6. SEM images of the pretreated (a) Ti and (f) Nb substrates surface, and diamond coatings grown on them. The diamond growth time is (b) 0.5h, (c) 1.5h, (d) 3h, (e) 6h on Ti and (g) 0.5h, (h) 1.5h, (i) 3h, (g) 6h on Nb Fig.6 shows SEM images of pretreated surfaces of and diamond coatings deposited on Ti and Nb substrates with increasing deposition times. Before the diamond deposition, the same surface pretreatment described in section 2.1 has been conducted to both types of the substrates. Fig. 6 (a, f) show SEM images of the pretreated substrate surface. We see that the surface of the pretreated substrates was

ACCEPTED MANUSCRIPT very rough. Measurements showed that the surface roughness (Ra) of the Ti and Nb substrates was about 113nm and 182nm, respectively, measured in 200 m ranges. From Fig. 6, we see that after 0.5h of deposition, diamond nuclei could be observed on the two types of substrates, but the diamond nucleation density on the Nb substrate was much higher than that on the Ti substrate. Also, the distribution of the diamond nuclei was much more uniform on the former than on the latter. After 1.5h of

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deposition, surface of the Nb substrate was completely covered by a coalesced diamond coating. At the same time, only part of the Ti substrate was covered by

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diamond grains, and there remained considerable amount of interstitial space between diamond grains. After 3h of deposition, diamond deposits on both substrates have

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coalesced, though voids could still be found on the Ti substrate. At last after 6h of deposition, diamond grains grew larger, and no obvious voids could be seen on both

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substrates.

Nb and Ti are both strong carbide forming elements. Earlier studies [20,30,31] showed that diamond nucleation occurs on these refractory metals only after forming

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a surface carbide layer, a process related directly to C diffusion rates. It has been suggested that there is a competition between the carbide formation and diamond

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nucleation in the early stage of the deposition. Rapid transport of C into the substrate surface would result in sluggish diamond nucleation. Moreover, due to continuous

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diffusion of C into the substrates, diamond nuclei previously formed could even dissolve again, thus resulting in a lower diamond nucleation density. Our experimental results were consistent with these analyses. Fig.6 proves that under the

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same conditions, diamond is easier to nucleate on Nb substrates than on Ti [32,33]. The reason for this lies in the difference in diffusion coefficients of C in Ti and Nb

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substrates. Near 1000 C, the diffusion coefficients of C in Ti and Nb are on the order of 10-10 and 10-12 m2/s, respectively [34]. This marked difference in diffusion coefficients of C in Ti and Nb substrates may explain the difference in thickness of carbide layers observed in Fig. 1. Because C is more easily diffused into Ti and it is difficult for C to reach a saturation value on the substrate surface, diamond nucleation on Ti substrates would be significantly delayed, and nucleation density is significantly lower [31,35]. Obviously, delayed nucleation would cause the diamond coating formed on Ti substrates more prone to pore type defects.

ACCEPTED MANUSCRIPT 3.3.2 Corrosion behavior of carbonized Ti and Nb samples It has been speculated that dissolution of intermediate TiC layers may be one of the reasons for short service life of the Ti/BDD electrodes [17,18]. Therefore, corrosion behavior of Ti and Nb carbides formed on respective substrates was comparatively studied. To accomplish this, Ti and Nb plates were carburized for 1h in

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the CVD chamber as described in Section 2.1.

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Fig. 7. XRD patterns for (a) Ti/BDD electrode and carbonized Ti samples, and (b) Nb/BDD electrode and carbonized Nb samples

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In Fig. 7 are shown XRD patterns of the carbonized Ti and Nb samples as

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compared with those of as-prepared Ti/ and Nb/BDD electrodes. As could be seen from the figure, the XRD patterns of both the Ti/BDD and Nb/BDD electrode samples revealed presence of carbide phases, in addition to that of diamond. The carbide phase

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identified for the Ti/BDD sample was TiC, while for the Nb/BDD sample these were NbC and Nb2C phases. On the other hand, the carbide phases present in the

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carbonized Ti and Nb samples were TiC and NbC/Nb2C, respectively, consistent with the result obtained for the corresponding BDD coated electrodes. This certifies the carbonized samples to be used to investigate the behavior of the carbide layers of the BDD coated electrodes. In comparing corrosion behavior of Ti and Nb carbide layers, the carbonized Ti and Nb samples were electrochemically anodized under a constant potential of 4.5V in 0.5mol/L H2SO4 electrolyte. The potential was chosen because both the Ti/BDD and Nb/BDD electrodes have been tested near this potential during their ALT experiments.

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Fig. 8. Current-time responses of carbonized Ti and Nb samples during

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anodization treatment at 4.5V

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Fig. 8 shows current-time responses of both the carbonized Ti and Nb samples during the anodization treatment. It could be seen from the graph that the current

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densities of the two samples decreased rapidly after the beginning of the anodization treatment, stabilizing rapidly to very low values. This indicates that surface of the two

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carbonized samples has rapidly been passivated, and the thickness of the oxidized surface layers increased with anodizing time. On the other hand, though at the beginning, the current density for the carbonized Ti sample decreased faster than that

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for the carbonized Nb sample, it then stabilized at a higher level. After anodization for

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2000 s, the current densities of the carbonized Ti and Nb samples were stabilized at 2.2 and 1.6 ×10-4 A/cm2, respectively. This reveals that though the surface of the carbonized Ti sample was more easily passivated, the passivated film on this sample

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may have either incorporated more defects thus had a high electrical conductivity, or it was relatively unstable and thus had a high dissolution rate. Close observation of

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the figure (inset of Fig. 8) showed that the current density for the carbonized Ti sample fluctuated continuously long after its stabilization. This phenomenon seems to imply that a forming/dissolving process of the passivated film had been occurring under this potential. Similar oscillation in anodic current has been observed experimentally before [36,37], and such a phenomenon has been explained by consecutive formation and repassivation of microsized pits on Ti surface. In contrast, we see from Fig. 8 that no fluctuation in current density appeared for the carbonized Nb sample after the same anodization treatment.

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Fig. 9. Potentiodynamic polarization curves of the carbonized Ti and Nb

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samples at scan rate of 5 mV/s

Potentiodynamic polarization curves of the carbonized Ti and Nb samples which

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had been anodized at 4.5V for 2000s are shown in Fig. 9. It can be seen from the figure that for the carbonized Ti sample, a stable passivating potential range could be

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identified between 0.93 and 1.64V where the current density remained almost constant. When the potential was higher than 1.64V, the current density increased gradually with increasing potential, indicating that the passive film formed on the

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sample became unstable and began to dissolve. For the carbonized Nb sample,

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potential span between 0.9 and 1.7V was the first stable passivating range, after which a current peak appeared implying that the passive film entered an activated state. Then when the potential was higher than 2.8V, a second stable passivating range was

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reached in which a passivating current density of about 3.1  10-4 A/cm2 was observed. By inferring to the two curves shown in Fig. 8, we could speculate that the anodized

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film formed on the carbonized Nb sample is more stable than that on the carbonized Ti sample when potential is higher than 2.8V. This fact implies that the carbide layer of the Ti/BDD coating electrode may be more prone to corrosion than its Nb counterpart in the ALT environment. Previous study has shown that in sulfuric acid, TiC was more active and easily corroded if it was polarized at a potential higher than 1.8V [38]. On the other hand, studies have also shown that though corrosive dissolution could also be observed for Nb carbides, this corrosion was not as severe as for TiC [39,40].

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Fig. 10. SEM images of surface for carbonized Ti (a, b) and Nb (c, d) samples before (a, c) and after (b, d) the anodization treatment. The insets show

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representative amplified views Fig. 10 shows surface morphologies of the carbonized Ti and Nb samples before and after the anodization treatment. By comparing Fig. 10 (a) and (b), we see that tiny

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corrosion pits appeared on surface of the carbonized Ti sample after anodization

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treatment. This indicates that dissolution has occurred to a certain extent on the surface of the sample during the anodization treatment. On the contrary, as shown in Fig. 10(c) and (d), there are barely corrosion pits on surface of the carbonized Nb

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sample after the same treatment, proving that consistent with the results of Fig. 8 and 9, the carbonized Nb sample was more resistant to corrosion/dissolution under the

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same condition. Messner et al. have indicated that though both TiC and NbC could be dissolved by anodic oxidation, TiC was more prone to corrosion to form a porous structure after anodic oxidation [40]. 3.3.3 Discussions on difference of failure process of Ti and Nb/BDD electrodes Through experiments, we found that the failure mode of Ti/BDD and Nb/BDD electrodes was mainly coating delamination, accompanied by corrosion of BDD coatings in the ALT process. The service life of the Nb/BDD electrode is much longer than that of the Ti/ BDD electrode. Phenomenologically, there exists an incubation period for BDD coatings to delaminate from their substrates, and such an incubation

ACCEPTED MANUSCRIPT period is much short for the Ti/BDD electrode than for its Nb/BDD counterpart. And for the Ti/BDD electrode, the delamination rate of BDD coatings is also higher. Meanwhile, we have shown that diamond growth process on different substrates and electrochemical behavior of carbides of the substrate materials are quite different. Based on these experimental results, it could be inferred that the main reasons for the large difference in the stability and service life of the Ti/BDD and Nb/BDD

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electrodes include probability of pore type defects incorporation in the BDD coatings and the corrosion resistance of the carbides formed on the respective substrates.

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Firstly, the reason for the premature peeling off of the BDD coatings from their Ti substrates is the existence of pore type defects in the BDD coatings, which may lead

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to premature exposure of substrates to corrosive electrolyte. Delayed coating delamination in the Nb/BDD electrodes happens when corrosion pores appear in the

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BDD coatings and the electrolyte invades through these defects. Secondly, the difference in corrosion resistance of the carbides at the interface between the coatings and the substrates affects the delamination rate of BDD coatings. An unstable carbide

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layer would be very detrimental to the service life of the BDD coated electrodes. These analyses indicate that even for void-free BDD coatings, pore like defects would

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develop along grain boundaries as a result of electrochemical corrosion, which would lead to electrolyte invading into the coatings. Once pores in the BDD coatings are

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created that are deep enough for the electrolyte to reach the BDD/substrate interface, corrosive attack of the interface could dissolve the carbide interfacial layer. As a result, the adhesion of the BDD coatings to the substrates would be negatively affected,

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leading ultimately to delamination of the BDD coatings from their substrates. Based on the above analysis, it could be concluded that reducing defect density

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in the BDD coatings, improving the corrosion resistance of both the carbide layers and BDD coatings would be possible ways to improve service life of the BDD coated electrodes.

4. Conclusions Based on our comparative study on stability of BDD coated Ti and Nb electrodes, following conclusions may be drawn: (1) The failure mechanism for the two BDD coated electrodes is mainly delamination of BDD coatings, accompanied by corrosion of the BDD coatings. (2) There exists an incubation period for delamination of the BDD coatings from their

ACCEPTED MANUSCRIPT substrates. Experimentally observed incubation period and the rate of BDD coatings delamination from the two substrates are markedly different. (3) BDD coatings deposited on Ti substrates are more likely to incorporate pore type defects, resulting in short incubation periods for the BDD coatings to delaminate from the Ti/BDD electrodes. (4) TiC present within the Ti/BDD electrodes is less stable than Nb carbides, resulting

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in fast propagation of delamination process in the Ti/BDD electrodes. (5) Differences in both liability of defects incorporation in BDD coatings and service life of the Ti/BDD and Nb/BDD electrodes.

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corrosion resistance of the carbide layers may be used to explain the difference in

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Graphical abstract

ACCEPTED MANUSCRIPT Highlights 1. A comparative study on different BDD coated electrodes was conducted. 2. The main failure mechanism of BDD electrodes is coatings delamination.

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3. Defects in the BDD coatings affect service life of the Ti/BDD electrodes. 4. Corrosion resistance of the carbides affects BDD coating delamination

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rate.