**Preface to "Surface Engineering of C/N/O Functionalized Materials"**

Great scientific and technological progress on heat treatment and surface modification has been achieved in the respective material categories to date, but cross references across material categories are barely made, although common technical mechanisms existing therein. The aim of this Special Issue is to present technical synergies, such as characterization and testing methodology, surface reaction mechanism, diffusion mechanism, process–structure–property relationships, etc., between the surface engineering of metals, ceramics, and polymers by evaluating the reaction–diffusion of C/N/O species, and to accelerate scientific discovery in the area of heat treatment and surface engineering. For example, the carburizing and nitriding of metallic materials are vital to enhancing the fatigue life of key base components, such as tools and dies; highly efficient oxygen transport in ceramic oxides is important to accelerate the practical applications of ceramic fuel cells. It appears that metallic materials and ceramic fuel cell materials belong to structural and functional materials, respectively. However, the thermochemical treatment of metallic materials, such as carburization and nitridation, and oxygen transport in ceramic fuel cells have similarities. They have a similar temperature range (400–1000 °C); the element C/N/O penetrates into the material bulk; the elemental processes are the same, i.e., reaction first then diffusion. Therefore, the two categories of materials can be integrated as "C/N/O Functionalized Materials". This Special Issue discusses the latest development of surface engineering of C/N/O functionalized materials, including both experimental and theoretical studies on heat treatment and surface engineering of metals, ceramics, and polymers.

The fundamental understanding of the kinetic reaction–diffusion process in materials depends on robust and precise methods for measurement of the kinetic parameters, such as surface exchange coefficient (*k*) and bulk diffusion coefficient (*D*). A typical method is the so-called "Electrical Conductivity Relaxation (ECR) Measurement". In recent years, the ECR method has been widely used for the mixed-ionic-electronic-conducting materials, such as the electrode materials for solid oxide fuel cells (SOFCs). However, the robustness and accuracy of measurement remains a challenge, since the rate-limiting mechanism and the experimental imperfections are not reflected explicitly in the time-domain ECR data. Yan et al. [1] developed a new method, called "the distribution of characteristic times (DCT)". Using the DCT spectrum, the rate-limiting mechanism and the refection of experimental imperfections are visualized clearly, and the values of k and D can be determined. A strong robustness of the DCT method is verified using noise-containing ECR data. The DCT method is in principle applicable to other materials, such as metals and polymers, by the proper relaxation measurement of the reaction–diffusion process.

For metallic materials, nitriding and carburizing are the best known and most widely applied surface engineering processes (also known as thermochemical treatment) for case hardening. Through the design of processing parameters, the surface layer microstructures can be refined, and therefore the corrosion, wear and fatigue properties of metals can be improved sufficiently. Yan et al. [2] proposed a novel decarburizing–nitriding treatment of low-carbon M50NiL and high-carbon M50-bearing steels. They showed that pre-decarburization reduces the activation energy for nitrogen diffusion and enhances nitrogen diffusivity, and the pre-decarburization can refine the surface-layer microstructure via a spinodal decomposition during plasma nitriding. Liu et al. [3] proposed a composite process of cold rolling and low-temperature plasma nitriding to improve the low hardness and poor wear resistance of the TA2 alloy. They showed that the wear property and hardness of deformed alloy samples can be improved by nitriding due to microstructural refinement. You et al. [4-5] studied the low-temperature plasma nitriding of 3Cr13 steel accelerated by rare-earth block, and the acceleration of plasma nitriding at 550 °C with rare earth on the surface of 38CrMoAl steel. They showed that most of the surface microstructures of the nitrided layer were refined by the addition of La. The presence of La reduces the N content in the modified layer, which accelerates the diffusion of N atoms and thus accelerates the nitriding process, and so the corrosion resistance is improved. Liu et al. [6] studied the effects of transition metal oxides (Ti, Zr, Nb, and Ta) on the mechanical properties and interfaces of B4C ceramics fabricated via pressureless sintering. They showed that the Ta2O5-added sample exhibited better elastic modulus, flexural strength, Vickers hardness, and fracture toughness, and exhibits the best combined properties when the mass fraction of the second phase was around five percent.

For ceramic materials, this Special Issue is restricted to the ceramic oxides for SOFCs, with the functionality (energy conversion from fuel to electricity) given by efficient ionic conduction and high catalytic activity. Ma et al. [7] studied the electrochemical activity of Fe-based perovskite cathodes for SOFCs. A novel cobalt-free perovskite oxide, BaFe1-xYxO3-δ, was evaluated as the oxygen reduction electrode. Through the analysis of the distribution of relaxation times, the oxygen adsorption–dissociation process is determined to be the rate-limiting step at the electrode interface. In addition, a single cell with x=0.10 exhibits a good long-term stability. Yuan et al. [8] studied the effect of NiO addition to La0.99Ca0.01NbO<sup>4</sup> proton-conducting ceramic oxides for SOFCs. The NiO were added by directly mixing or by doping. They showed that both strategies improve the sinterability and conductivity, but the effect of doping is more significant in enhancing both grain growth and conductivity, making it more desirable for practical applications. The optimal doping amount of NiO was shown as 1∼2 wt.%. The origin of the enhanced performance, revealed by first-principle calculations, is the decrease in both oxygen formation energy and hydration energy.

For polymer materials, Li et al. [9] studied the mechanical properties for basalt fiber/epoxy resin composites modified with La. To improve the poor interfacial adhesion between basalt fibers and the resin matrix, the modification solution containing different concentrations of Lanthanum ions was synthesized to modify the basalt fiber surfaces. They showed that La in the rare earth modification solution could link active oxygen-containing functional groups to the fibers'surfaces, and thus improve the roughness and the activity of the fiber surfaces, therefore enhancing the bonding between the resin matrix and fibers. In a following study [10], the effect of inorganic reinforced materials (AgNO3, FeCl3·6H2O, nano-graphene) on the mechanical and piezoelectric properties of electrospun PVDF fiber membranes was studied. The results showed that all the three materials can effectively promote the formation of the β-phase and thus enhance the piezoelectric performance. The best mechanical and piezoelectric properties were achieved by the addition of 1.0 wt.% nano-graphene and 0.3 wt.% AgNO3. Ke et al. [11] studied the fabrication process and properties of electrospun and electrosprayed polyethylene glycol/polylactic acid (PEG/PLA) films. PEG was introduced to enhance the cooling performance, due to its lower glass transition and melting temperatures. They showed that, the PEG/PLA film with a PLA content of 35 wt.% has a reduced thermal conductivity of 0.2 Wm-1K-1 and largest elasticity modulus of 378.3±68.5 MPa and tensile strength of 10.5±1.1 MPa.

I would like to express my greatest gratitude to the authors, the reviewers, and the editorial staff members who contributed enthusiastically to this Special Issue.

## **References**

[1] Yan, F.; Wang, Y.; Yang, Y.; Zhu, L.; Hu, H.; Tang, Z.; Zhang, Y.; Yan, M.; Xia, C.; Xu, Y. Distribution of Characteristic Times: A High-Resolution Spectrum Approach for Visualizing Chemical Relaxation and Resolving Kinetic Parameters of Ionic-Electronic Conducting Ceramic Oxides. Coatings 2020, 10(12), 1240.

[2] Yan, F.; Yao, J.; Chen, B.; Yang, Y.; Xu, Y.; Yan, M.; Zhang, Y. A Novel Decarburizing-Nitriding Treatment of Carburized/through-Hardened Bearing Steel towards Enhanced Nitriding Kinetics and Microstructure Refinement. Coatings 2021, 11(2), 112.

[3] Liu, G.; Sun, H.; Wang, E.; Sun, K.; Zhu, X.; Fu, Y. Effect of Deformation on the Microstructure of Cold-Rolled TA2 Alloy after Low-Temperature Nitriding. Coatings 2021, 11(8), 1011.

[4] You, Y.; Li, R.; Yan, M.; Yan, J.; Chen, H.; Wang, C.; Liu, D.; Hong, L.; Han, T. Low-Temperature Plasma Nitriding of 3Cr13 Steel Accelerated by Rare-Earth Block. Coatings 2021, 11(9), 1050.

[5] Liu, D.; You, Y.; Yan, M.; Chen, H.; Li, R.; Hong, L.; Han, T. Acceleration of Plasma Nitriding at 550 °C with Rare Earth on the Surface of 38CrMoAl Steel. Coatings 2021, 11(9), 1122.

[6] Liu, G.; Chen, S.; Zhao, Y.; Fu, Y.; Wang, Y. The Effects of Transition Metal Oxides (Me = Ti, Zr, Nb, and Ta) on the Mechanical Properties and Interfaces of B4C Ceramics Fabricated via Pressureless Sintering. Coatings 2020, 10(12), 1253.

[7] Ma, D.; Gao, J.; Xia, T.; Li, Q.; Sun, L.; Huo, L.; Zhao, H. Insights in to the Electrochemical Activity of Fe-Based Perovskite Cathodes toward Oxygen Reduction Reaction for Solid Oxide Fuel Cells. Coatings 2020, 10(12), 1260.

[8] Yuan, K.; Liu, X.; Bi, L. Exploring the Effect of NiO Addition to La0.99Ca0.01NbO<sup>4</sup> Proton-Conducting Ceramic Oxides. Coatings 2021, 11(5), 562.

[9] Li, C.; Wang, H.; Zhao, X.; Fu, Y.; He, X.; Song, Y. Investigation of Mechanical Properties for Basalt Fiber/Epoxy Resin Composites Modified with La. Coatings 2021, 11(6), 666.

[10] Li, C.; Wang, H.; Yan, X.; Chen, H.; Fu, Y.; Meng, Q. Enhancement Research on Piezoelectric Performance of Electrospun PVDF Fiber Membranes with Inorganic Reinforced Materials. Coatings 2021, 11, 1495.

[11] Ke, W.; Li, X.; Miao, M.; Liu, B.; Zhang, X.; Liu, T. Fabrication and Properties of Electrospun and Electrosprayed Polyethylene Glycol/Polylactic Acid (PEG/PLA) Films. Coatings 2021, 11(7), 790.

> **Yanxiang Zhang** *Editor*

*Article*
