Microstructure evolution and nanohardness of nanostructured TiAlN coating under N+ ion irradiation
Introduction
Transition metal nitrides with the sodium chloride (B1) structure exhibit high hardness, melting point, chemistry inertness, excellent wear and corrosion resistance [1], [2], [3]. These properties enable them to be good protective coating materials which have been used widely in the fields of industry, electrical engineering, optical engineering and medical materials [4], [5], [6]. Among them, titanium nitride (TiN) has early on received a great deal of attention. The hardness and elastic modulus of TiN film have been reported up to 33.58 GPa and 407 GPa, respectively [7]. Depending on its stoichiometry, TiN can maintain its chemical inertness up to 500 °C in air. However, it will be subjected to intensive oxidation at higher temperature with the formation of brittle TiO2. The ternary TiAlN system, a thermal stable phase by alloying Al into TiN, shows the superior oxidation resistant and mechanical properties over conventional binary TiN coatings, and now has been of the most promise hard coatings for high-speed cutting and dry machining [8]. In recent years, TiN-based coatings like TiN and TiAlN came out as superior and attracted in nuclear systems, such as diffusion barrier coating for nuclear fuel-cladding [9], [10], protective coating against coolant corrosion in Light-Water Reactors and Lead Fast Reactors [11], [12], [13], [14]. Besides, intrinsic high hardness of TiN-based coatings is also important to protect the structural materials from fretting damage for the entire period of employment [15], [16], [17].
Despite TiN-based coating materials have shown a promising application in nuclear reactor, the extreme neutron irradiation environment in a nuclear reactor should not be ignored. As is well known, long-term exposure to neutron radiation will cause a significant change of material properties (often degraded). So it is important to evaluate the irradiation response of these coating materials, prior to its employ in reactors.
The previous reports [18], [19] have found that the TiN-based binary or ternary nitride coatings with nanostructure have greater resistance to radiation damage than other materials, because numerous crystallite boundaries serve as defects sinks, suppressing the radiation-induced damage. Wang et al. [20] reported that nanostructural TiN with mean grain size of 8–100 nm exhibits higher radiation resistance than its coarse grained counterpart, as evidenced by the diminishing damage zone subjected to He ion irradiation.
The TiN-based coatings were firstly invented as hard and wear resistant coating on high speed and dry cutting tools. Hardness is considered as one of the important indicators of coating properties. So in early stages, the studies on the mechanical properties of hard coatings under irradiation conditions focused on the search of process parameters that lead to the increase of the hardness of the irradiated coating. Considerable work has been conducted on, and it is generally observed that noble-gas ions irradiations, such as Ar, Kr and Xe, induce an increasing in hardness of the TiN coating, and there are threshold fluences beyond which the hardness will decrease [21], [22], [23].
Though the irradiations of carbon, nitrogen or metal ions may improve the tribological properties, oxidation behavior and hardness of the hard coatings [24], [25], [26], there are many other studies that reported a reduction in hardness (i.e., softening effect) in TiN-based or other nitride coating materials under irradiations [27], [28], [29], [30]. These studies suggest that the hardness reduction should be attributed to crystal amorphization of materials occurred under irradiation, and it can be regarded as an amorphization level indicator of radiation damage. However, compared with a lot of research on irradiation response of other TiN-based coating materials, there are only a few works about the TiAlN coatings. Konstatinov et al. [31] studied the irradiation resistance of TiAlN coating under 200 keV Ar+ ion irradiation, and the authors attributed the hardness decreasing of TiAlN coating at the fluence of 2.0 × 1017 ions/cm2 to the spinodal phase segregation. Yet, this is far from enough. It is still necessary to carry out more research on the changes of TiAlN coating properties after irradiation to clarify whether it can be applied in the nuclear industry in the future.
In this work, the TiAlN coatings were deposited by cathodic arc ion plating method. The Ti-Al-N system was selected as an isostructure model system, where solid solutions with B1-structure (NaCl type) can be stabilized in the whole compositional range. The N ion irradiation experiments were carried out at various temperatures with different fluences. The irradiation induced nanohardness and microstructure evolution were investigated, and the relationship between them was discussed.
Section snippets
Experimental
TiAlN coatings were prepared by a cathodic arc ion plating method, which was equipped with one Cr target and single Ti0.35Al0.65 target. WFeNi plates with size of 10 × 10 × 4 mm3 were used as the substrates, which were grinded by diamond sand papers with grits increasing up to P2000, and then polished with diamond pastes down to 0.25 μm. The polished substrates were ultrasonically cleaned by acetone for 15 min. Prior to the coating deposition, Ar+ ion bombardment was performed at 2 × 10−2 Pa to
GIXRD investigation
GIXRD spectra (as shown in Fig. 3) were used to analyze the phase structure of as-deposited and irradiated TiAlN coatings. For the as-deposited coating, the GIXRD pattern shows the typical cubic B1 NaCl-type (fcc) structure; three characteristic peaks located at 37.22o, 43.13o and 63.07o are corresponding to (111), (200) and (220), respectively. It also shows a strong preferred orientation of (200) texture. The peaks from the substrate (W) were also detected because x rays penetrated in the
Microstructure evolutions after irradiation
Based on the characterizations of TEM and GIXRD, the as-deposited TiAlN coating shows a dense unconspicuous columnar nanostructure with an average grain size about 10 nm. Despite the high irradiation damage levels, ~10 dpa at RT and high temperatures, no amorphization or phase transformation was observed in each of the irradiated samples, which reveals the nanostructural TiAlN coating has an excellent irradiation resistance in phase stability and amorphization. It is well known that the sorts
Conclusions
The coating of pure stoichiometric TiAlN B1 NaCl cubic phase on WFeNi substrate was prepared by the cathodic arc ion plating method and the irradiation to TiAlN coatings was conducted under 1.4 MeV N+ with different irradiation fluences (from 2.0 × 1016 ion/cm2 to 4.0 × 1016 ion/cm2) and temperatures (RT, 300 °C and 500 °C). It is found that there are no amorphization or phase transformation observed even under damage level of ~10 dpa at RT and high temperatures, indicating that the prepared
CRediT authorship contribution statement
Pengfei Tai: Conceptualization, Methodology, Writing-Original draft preparation. Lilong Pang: Conceptualization, Methodology, Funding acquisition, Writing- Reviewing and Editing. Tielong Shen: Supervision, Project administration. Zhiguang Wang: Conceptualization, Methodology, Funding acquisition. Peng Jin: Experiment, Investigation. Sihao Huang: Experiment, Validation. Hailong Chang: Experiment, Validation. Kongfang Wei: Writing-Reviewing and Editing, Software. Minghuan Cui: Data curation,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Natural Science Foundation of Guangdong Province, China (Grant No. HNY20301GJT), the National Natural Science Foundation of China (Grant Nos. 12075292, U1832206, Y505030GJO). The authors are grateful for ions irradiation experiments to the staff of the 320 kV multi-discipline research platform.
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