Effects of Surface Layer Modification on Fatigue, Corrosion, and Wear Behaviour in Metallic Materials

The surface layers (SLs) of metal structural and machine components are the most stressed [1]. SLs are subjected to mechanical, thermal, chemical, and physical influences (including physical fields) and often to combinations of these. Some of these effects persist even during periods when no working load is applied to the metal components. Depending on the influences applied, phenomena such as diffusion, corrosion, heterogeneous catalysis, adhesion, and adsorption can occur in SLs, autonomously or in mutual dependence. Localised damage due to the fatigue, wear, and corrosion of SLs disrupts the normal interaction between components, causing seizure, jamming, shocks, and vibrations. Therefore, the conditions, life, and operation of technical products depend largely on the condition of SLs, which must be processed to perform functions that are different from those performed by the bulk of the material. In other words, the complex properties of these layers, known as surface integrity (SI), must be modified via appropriate finishing.

The techniques and technologies used for modifying SLs fall within the scope of surface engineering (SE), a scientific discipline established in England in the early 1970s. The objectives of SE include the following: (1) improving corrosion resistance through barrier or other protection, (2) improving resistance to oxidation and/or sulfidation, (3) increasing wear resistance, (4) improving lubrication to reduce friction losses, (5) increasing the fatigue limit, (6) increasing microhardness (nanohardness), (7) improving thermal insulation properties, and (8) improving the aesthetic appearance of the components.

Various classifications [1,2,3] of processes within the scope of SE exist. Depending on the potential for changing the chemical composition of SLs, SE processes can be classified into the following three categories: (1) modification of SLs based on the diffusion of new chemical elements into them (thermo-chemical diffusion), (2) modification of SLs by adding new material in the form of coatings or thin films in order to create a barrier between the coated layer and the environment, and (3) modification of SLs without changing the chemical composition of the material. The last of these SE process categories consists of three main approaches—conventional surface heat treatment, hardening through concentrated energy flows (electron or laser beam), and surface cold working based on severe surface plastic deformation—or a combination.

To improve operational behaviour (OB), an integrated approach is taken to studying the following related aspects of SE: (1) finishing (F) processes for components, (2) SI, and (3) OB. Therefore, the purpose of SE is to modify SLs in response to F–SI–OB correlations. Numerous techniques and technologies have been developed for modifying SLs based on the classification of SE processes. The vast majority of research has focused on developing, modelling, and optimising various types of finishing processes and their effects on SI and OB, such as the simple F–SI and F–OB correlations. The SI–OB correlation has been studied to a lesser extent. Experimental studies on OB, particularly on wear and fatigue behaviour, are time-consuming and expensive. The accumulation of sufficient empirical information about the SI–OB correlation will allow synthesis, research, and optimisation in new and promising finishing processes aimed at achieving the desired SI (and hence the desired OB) more quickly and with significantly fewer resources. In this way, the F–SI–OB triangle will be closed.

Several of the works in this Special Issue report investigations into the effects of finishing processes on surface layer modification in terms of SI and the coatings formed in relation to specific applications [4,5,6], i.e., studies focused on the F–SI correlation. Shi et al. [4] studied the microstructure evolution of the GH4151 superalloy under ultrasonic shot peening (USP). The results showed that during the USP process, the plastic deformation of the SL of the superalloy was accompanied by grain refinement and grain rotation, enhancing the randomness of grain orientations. Sun et al. [5] focused on the influence of the powder-to-liquid ratio on the performance characteristics of lost foam casting coatings. The optimal powder-to-liquid ratio, within which coatings demonstrated enhanced uniformity, improved particle distribution, and superior surface morphology, was determined to be between 2.0 and 2.2. Ilievska et al. [6] studied the influence of thickness on the structure and biological response of Cu-O coatings deposited on commercially pure titanium (cpTi) substrates using direct current (DC) magnetron sputtering. Films deposited for 5, 10, and 15 min corresponded to thicknesses of 41, 74, and 125 nm, respectively. The results showed that Cu-O-coated cpTi substrates had 50%–60% higher antimicrobial activity than uncoated substrates. Antimicrobial activity increased with increasing deposition time, suggesting the potential of the deposition process to create antimicrobial coatings on orthopaedic implants.

Corrosive chemical, electrochemical, and microbiological degradation of metals causes significant economic losses, which is why F–SI–corrosion behaviour correlations have been examined in several studies [7,8,9,10,11,12]. Sun et al. [7] reviewed corrosion mechanisms, protection strategies, and monitoring technologies for Fe and Al metals and their alloys used in power equipment, emphasising the important role of corrosion in power supply reliability. Telegdi [8] reviewed multifunctional inhibitors (chemicals or coatings) that limit corrosion by aggressive materials and reduce microbial adhesion. Sanchez et al. [9] modified the surface of an AM60 magnesium alloy with an Al nanocoating 65.62 nm thick, using DC magnetron sputtering to enhance the coating’s resistance to degradation in aggressive marine environments. Immersion tests conducted in a simulated marine environment confirmed that the sputtered Al nanocoating mitigated the surface degradation of Mg–Al alloys in aggressive saline marine environments. Yuan et al. [10] used environmentally friendly arc ion plating to deposit TiBN, CrAlN, and nano-multilayer CrAlN/TiBN coatings. Measurements revealed that the CrAlN/TiBN coating had the lowest friction coefficient (0.489), followed by CrAlN (0.491) and TiBN (0.642). Electrochemical tests conducted in artificial seawater confirmed that TiBN-based nano-multilayer coatings offered the greatest potential for corrosion protection. Boshkova et al. [11] studied the effects of the thickness of a zinc alloy sublayer on the corrosion resistance and stability of three types of bi-layer systems composed of Co- or Ni-modified zinc coatings (both as sublayers) and a top sol–gel ZrO₂ film in a 5% NaCl solution. The alloy sublayers were electrodeposited at three different thicknesses (1, 5, and 10 µm, respectively) on a low-carbon steel substrate. The results confirmed the increased corrosion resistance of the protective system, which contained an electrodeposited sublayer of Zn–Co alloy with a 10 µm thickness in a corrosive test medium. Chliveros et al. [12] proposed a segmentation module to identify areas of corrosion and segment pixels in regions of inspection interest for corrosion detection in marine vessels. The module was based on Eigen tree decomposition and information-based decision criteria to identify specific corroded spots as regions of interest. A comparison with several state-of-the-art deep learning architectures showed that the method achieved higher accuracy and precision while maintaining a consistent significance score across the entire dataset.

Two studies were focused on F–SI–wear resistance correlations [13,14]. One proved the advantages of an ultrasonic surface-rolling process (USRP) and a hybrid of USRP with electropulsing treatment (EP–USRP) in modifying SLs in 30CrNi2MoVA steel in comparison with turning [13]. The USRP and EP–USRP treatments produced refined microstructures with fine-grain depths of 60 and 100 μm, respectively. The lowest roughness values of 0.035 and 0.040 μm obtained for USRP and EP–USRP samples, respectively, were approximately 12 times lower than that of the turning surface roughness of 0.421 μm. Wear tests showed that the friction coefficient was the lowest after USRP, which was correlated with the lowest roughness of 0.035 μm and the highest surface hardness of 360 HV. Wang et al. [14] prepared a brake cylinder coating consisting of a composite of mixed Fe₃Al and Cr₃C₂ powders by adding laser cladding onto carbon structural steel. The influences of different processes on the morphology of the carbide-strengthening phase were found to be relatively small. Using pin-on-disc wear tests under dry friction, seven groups of cladding layers were examined in terms of their friction coefficient curves and wear properties. The wear resistance of a Cr₃C₂-reinforced Fe₃Al laser cladding material was found to be far superior to that of a commonly used wear-resistant material, vermicular cast iron.

An in-depth, multifaceted study of F–SI–OB correlations with a focus on fatigue behaviour was conducted by Argirov et al. [15], who investigated the evolution of SI characteristics in relation to the fatigue and wear behaviour of diamond-burnished and fine-turned AISI 304 steel specimens after prolonged exposure to natural seawater. All types of diamond-burnished specimen showed lower mass loss (indicating higher corrosion resistance), higher fatigue strength, greater plasticity, and greater wear resistance than the corresponding fine-turned specimens.

Most studies to date have focused on F–SI–corrosion behaviour correlations. Very few studies have examined F–SI–wear and fatigue behaviour correlations.

The research conducted so far confirms the enormous potential of SE processes for significantly improving the SI and OB of a wide range of materials and structural components. The findings of this research form the basis for future work on the following subjects:

• Developing new finishing processes based on combinations of known approaches to achieve synergistic effects;
• Addressing the gap in research on the SI–OB correlation;
• Closing the F–SI–OB triangle;
• Studying the evolution of SI (including the SL microstructure) under extreme conditions (high temperatures and highly aggressive environments, including seawater);
• Modifying the SLs of advanced alloys (including titanium, nickel, and chromium alloys, as well as light alloys, such as aluminium and magnesium alloys) and composite materials;
• Developing new cost-effective autonomous and combined processes with beneficial ecological influences on the environment (mainly burnishing and electron beam/laser surface hardening).