1. Introduction
Iron-based coatings have become widespread and developed. They have excellent characteristics, such as high hardness and strength, low cost, and easy treatments [
1]. In the last few years, the industry has shown increased interest in iron-based coatings due to the high cost and toxicity of Co- and Ni-based coatings and the need to add a binder phase for stability. There is particular interest in iron-based coatings with refractory crystals of embedding phases—(Ti,Cr)B
2—which have high performance characteristics.
Thermal spraying is considered the most practical method for applying protective iron-based coatings. In thermal spraying, thermal and kinetic energy is used to accelerate the initial powder and it is sprayed onto the substrate. Thermal spraying involves the use of many methods, such as air plasma spraying (APS) [
2,
3], high-velocity oxygen spraying (HVOF) [
4,
5], arc spraying (AS) [
6,
7], vacuum plasma spraying (VPS) [
8,
9], cold spraying [
10,
11], and detonation spraying (DS) [
12,
13]. These methods differ in terms of heat source, the shape of the coating material, and the rate at which the material is applied, resulting in a range of coating properties and applications.
Among the various thermal spraying methods, DS has several advantages due to its simplicity and its ability to produce coatings with high bond strength, wear resistance, hardness, and density [
14,
15,
16,
17]. The high particle velocities achieved via detonation spraying (up to 1200 m/s) ensure the strong adhesion of the coating to the substrate. In addition, the process allows precise control of the coating properties and minimal oxidation due to rapid heating and cooling cycles. However, the major disadvantages of thermal coatings, including detonation coatings, involve porosity, roughness, and relatively poor hardness and corrosion resistance. In addition, in thermal spraying, there is a high density of defects (cracks and pores), the presence of unmelted particles, and greater heterogeneity of the deposited phases. In most cases, an additional post-deposition thermal treatment is required to eliminate metallurgical defects, reduce porosity, and improve inter-plate adhesion as well as adhesion between the coating and the substrate [
18]. After the heat treatment, it is usually possible to obtain a dense coating with high cohesive and adhesion strength [
19]. Generally, conventional heat treatment methods often lead to additional oxidation of the coating due to the low heating rate, which negatively affects the performance of the material [
20,
21]. Therefore, in the era of globalization, product competitiveness is achieved by improving manufacturing processes and developing non-traditional, high-performance surface treatment processes. These include technologies that utilize a concentrated energy flow. One of these technologies is the pulsed plasma treatment. This method involves surface modification using a magnetic field, electric current (or a stream of charged particle flux), and a high-gradient heat flux (plasma) that contains both non-metallic and metallic alloying elements. Pulsed plasma treatment (PPT) provides fast heating (heating time 10
−3~10
−4 s) of the surface layer followed by intensive cooling of it by heat dissipation into the product volume. The high-speed (up to 10
7 K/s) melting and crystallization of surface layers contribute to the formation of a nanodisperse crystal structure and high density of dislocations.
Combined technology, including coating deposition followed by a pulsed plasma treatment, makes it possible, in many cases, to achieve an additional increase in the physical, mechanical, and tribological properties of the material [
22,
23].
Pogrebnyak et al. [
24,
25] investigated surface modification via exposure to a pulsed plasma jet having high velocities of up to 10
7 K/s and a high temperature in the jet. The results demonstrate that surface modification via pulsed plasma flow significantly improves mechanical properties, such as service resistance, hardness, corrosion and abrasion resistance, etc. Tyurin et al. [
26,
27] conducted studies where they treated the surface of metal–ceramic detonation coatings with a pulsed plasma stream. We have carried out studies on pulsed plasma surface modifications of Cr
3C
2-NiCr, Al
2O
3-NiCr, and Ni-Cr-Al coatings [
28,
29,
30]. According to the obtained data, it is possible to say that the modification of coatings using a pulsed plasma flow leads to increased tribological and mechanical properties in comparison with samples not subjected to a pulsed plasma treatment.
Nevertheless, most previous works were limited to the study of metal–ceramic coatings. To date, there are many publications on the preparation of iron-based coatings using various methods, but there is still a lack of work on the effects of pulsed plasma treatments on the structure and properties of iron-based coatings. In the work by Kengesbekov, A. B. et al. [
30] the influence of pulsed plasma treatment on the properties of iron-based coatings was investigated, where it is possible to observe the improvement of coating properties with increasing distance. But in the paper little research was described, i.e., there was no comprehensive study (tribological properties, morphology, erosion resistance, abrasive wear), as well as a further increase in the distance.
In this study, the effects of pulsed plasma surface modification on the properties of Fe-TiB2-CrB2 coatings are investigated.
4. Conclusions
Based on the evaluation and analysis of all the results obtained, the following main conclusions can be drawn from the present research work:
After pulsed plasma treatment, new phase reflexes of (Ti,Cr)B2 are found on the diffraction pattern, which has high wear resistance. In addition, the intensity of α-Fe peaks increased. Pulsed plasma treatment (PPT) contributes to the reduction of the surface roughness value and coating friction coefficient by ~2 times, increasing the hardness of the near-surface layer of Fe-TiB2-CrB2 coatings from ~8.8 GPa (initial) to ~15.6 GPa and wear resistance by a factor of 2 compared to the untreated coating. Pulsed plasma treatment significantly improves the resistance of Fe-TiB2-CrB2 coatings to both abrasive wear and erosion. After pulsed plasma treatment in the subsurface layer of the coating there is a reduction of pores to minimum values, which is due to the melting of the boundary of large defects. Pulsed plasma treatment of detonation coatings provides, in optimal modes, the formation of high-quality coatings from the eutectic alloy system Fe-TiB2-CrB2, including an increase in strength properties and hardness of the matrix without significant degradation and high values of localized defects.
According to the results of the research, the optimal mode of pulsed plasma treatment was selected, which improves the characteristics of detonation coatings of eutectic Fe-TiB2-CrB2 alloy.
This combined coating method, including detonation spraying of eutectic Fe-TiB2-CrB2 alloy coatings and subsequent pulsed plasma treatment, can be considered optimal for protecting the surfaces of parts operating under extreme conditions.