July 30, 2020.
For many decades, the materials, devices, or work pieces’ coatings have been interesting and favorite. The objective of coatings was to increase the tribology properties of the substrate’s surface. The coating will perform as the protective layer of the material or work piece substrate [1,2], resulting in research and development of the coating layer and methods for fabricating various types of coating to be applied in the industry. The thin film protective coating layer may be divided into 2 main techniques which are (1) Chemical Vapor Deposition (CVD)  and (2) Physical Vapor Deposition (PVD) . Nowadays, the technique of physical vapor deposition or PVD coating is an important technique for coating on materials, devices, or work pieces’ surface for the industrial because significantly enhanced production capacity, lifetime, and also reduce production process costs in the production line compared to the chemical vapor deposition.
The development of thin film coatings for many applications that have been focused in the present day however the hard coatings which applied to the surface of various types of materials, devices, or work pieces to increase hardness properties were interested from many researchers and industries. Because it has a high hardness, good wear resistance, chemical stability, good corrosion resistance, and especially high oxidation resistance which helps to prolong the cutting, drilling, milling, turning in the production process for longer and more efficiently. The 2 types of nitride thin film, namely (1) binary nitrides thin Film such as TiN  and CrN  and, (2) ternary nitrides thin films such as TiCrN  and CrAlN , etc were favorable used for tooling.
In general, the nitride compounds thin film hard coating on cutting tools, drilling, milling, and turning were used by most research fields including industries. The toolings were always under a high-temperature environment during the operation. The friction of the toolings results in the accumulation of heat and then a continuous increase in temperatures. The thin film's surface reacts with oxygen in the atmosphere forming an oxide compound layer on the top, known as "thermal oxidation" or most research is known as "oxidation", which is an important reason for the change of deposited thin film until it has reduced hardness . Therefore, the inevitable study of oxidation is important in terms of the study and analysis together with the research and hard coatings used.
The oxidation of nitride compounds thin film was obtained from many techniques. In this article, we focus on the thermal oxidation when the deposited thin film was annealed at high temperature with the specified time to analyze the formation of oxide compounds on the surface of the thin film that is coated on the surface of the substrate. The investigation techniques of (1) Thermo Gravimetric Analysis (TGA), (2) Raman Spectroscopy, (3) Auger Electron Spectroscopy (AES), and (4) Field Emission Scanning Electron Microscopy (FE -SEM) were used.
Oxidation study of the binary nitride thin film
The binary nitride thin film was considered the pioneer or first-generation hard coating. The titanium nitride thin film (TiN) (Figure 1) and chromium nitride thin film (CrN) (Figure 2) were most commonly used. After deposition, the as-deposited films were annealed at various temperatures and then studied the oxidation by FE-SEM technique which is convenient to analyze. A clear investigation of the oxide compounds layer on the deposited film when oxidation and also calculate the rate and the activation energy was obtained from this technique which gives information about the maximum oxidation temperature of the thin film. The limit is approximately 500 oC for TiN thin films [3, 11] and about 600 oC for CrN thin films .
Fig. 1 Compound oxide layer formation on a surface of TiN film .
Fig. 2 Compound oxide layer formation on a surface of CrN film .
Oxidation study of the ternary nitride thin film
The ternary nitride compound thin film was developed from a binary nitride thin film by adding some transition metal elements to the main structure such as aluminum (Al) or chromium (Cr) to the main structure of titanium nitride during the coating resulting in a new nitride thin film as titanium aluminum nitride (TiAlN) or titanium chromium nitride (TiCrN) etc. These this films were more oxidation resistant at higher temperatures compared to the binary nitride thin film. From related research, it was found that the ternary nitride thin films can exhibit the maximum oxidation temperature at 700 oC or more depending on the type and structure of the film obtained from different deposit conditions. For example, TiCrN thin film performed maximum oxidation temperature at 850 oC which oxide compound structure of TiO2 was formed (Figure 3). The CrAlN thin film showed at 600 oC which the oxide structure of Al2O3 was found (Figure 4).
Fig. 3 Compound oxide layer formation on a surface of TiCrN film .
Fig. 4 XRD pattern of compound oxide layer formation on a surface of CrAlN film .
Oxidation study of the nitride hard coating
However, one part of the research at Plasma for Surface Science Research Laboratory at Department of Physics, Faculty of Science Burapha University not only focusing study on vacuum deposition and thin film in term of the preparation process and the analysis of the characteristics of the prepared thin film or the coating layer but also the researcher studied the application of the thin film especially the protective thin film against a substrate, equipment or work pieces. The preparation techniques and characterization of the hard coating thin films for the binary nitrides (TiN, CrN, ZrN) and the ternary nitrides (TiCrN, TiAlN, CrAlN, TiZrN) were focused.
In this paper, the researcher would like to present an example result of TiN and TiCrN thin film oxidation deposited by PVD technique from a research laboratory. It was found that TiN thin film after annealing at various temperatures exhibited maximum oxidation at 600 oC. The structure of TiO2 which is an oxide compound (Figure 5) was investigated. It can clearly see the oxidation obtained from the thin film cross-sectional analysis using FE-SEM. The forming of TiO2 compounds on the TiN film was found (Figure 6) and the thickness of the oxide layer varied with the temperature. Moreover, the oxidation rate was increased when the annealing temperature increased (Table 1). The activation energy was 44.54 kJ/mol.
Fig. 5 XRD pattern of TiN thin films annealing at various temperatures .
Fig. 6 Cross section of TiN thin films annealing at various temperatures , i.e.,
(a) as-deposited TiN thin film
(b) oxidation layer on TiN surface annealing at 600 oC.
Table 1 Oxidation rate of TiN thin films annealing at various temperatures .
In the case, the oxidation of TiCrN thin film was observed at 700 oC. The structure of TiO2 which represents an oxide compound (Figure 7), was also observed. When analyzing the cross-section of thin films using FE-SEM, it was evident that oxidation took place. The porous chromium oxide compound was found to form on the substrate (Figure 8). The thickness of the oxide layer also increased with the annealing temperature. The activation energy was calculated to be 10.98 kJ / mol.
Fig. 7 XRD pattern of TiCrN thin films annealing at various temperatures.
Examples of the oxidation resistance properties of the hard coating thin films of the nitride compounds are the work of the researchers from Plasma for Surface Science Research Laboratory at Department of Physics Faculty of Science, Burapha University including the various types of the nitride compounds thin films prepared by PVD techniques as well as studying the oxidation of thin films. The results of this research can also be used as a guideline for preparation and investigation of oxidation for other nitride compounds thin film in the future. The knowledge is very useful for further understanding of oxidation behavior in order to help determine the limit of appropriate temperature conditions for applying as a hard coating on industrial equipment and work pieces.
Fig. 8 Cross section of TiCrN thin films annealing at various temperatures , i.e.,
(a) as-deposited TiCrN thin film,
(b) oxidation layer on TiCrN surface annealing at 700 oC.
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Asst. Prof. Dr. Adisorn Buranawong E-mail: firstname.lastname@example.org
Asst. Prof. Dr. Nirun Witit-anun E-mail: email@example.com
Plasma for Surface Science Research Laboratory
Department of Physic, Faculty of Science, Burapha University, Chonburi 20131, Thailand