Surface Modification for Improving the Performance of Engineering Components

1. Introduction and Scope

In the manufacturing industry, mechanical and structural elements are essential and are often located in harsh operational environments where they must withstand the synergic effects of wear, corrosion, fatigue, and elevated temperatures [1]. Surface degradation phenomena are responsible for increasing maintenance costs and triggering unplanned industrial downtime, with profound implications for global energy efficiency and resource consumption [2]. To ensure the performance and reliability of machines and devices while extending component service life, various surface modification methods have been developed and continuously refined. Consequently, surface engineering has emerged as a pivotal strategy to enhance material performance by tailoring surface properties while preserving bulk material characteristics.

The landscape of surface modification strategies encompasses a wide number of techniques, ranging from traditional thermal and thermochemical treatments to advanced physical and chemical vapor deposition (PVD, CVD) and thermal spray processes [3,4]. Each technique offers specific advantages depending on the substrate material, desired functional properties, and severity of the operational environment. Hence, selecting an optimal modification route requires a rigorous evaluation of processing parameters, coating–substrate interfacial compatibility, and environmental loading conditions. Recent advances in process control and characterization techniques have enabled more precise optimization of surface properties, fostering the development of components with enhanced tribological, mechanical, and functional characteristics.

Current trends in surface engineering reflect a paradigm shift toward increasingly sophisticated and sustainable solutions. The widespread increase in additive manufacturing techniques has expanded the possibilities for producing complex surface architectures and functionally graded materials [5,6]. In parallel, the integration of circular economy principles through the valorization of industrial waste streams, as demonstrated by the use of recovered materials in coating formulations, is gaining attention [7]. To decode the complexity of these systems and approaches, multi-scale characterization frameworks help to provide a comprehensive understanding of structure–property–performance relationships. Furthermore, there is a growing emphasis on the systematic optimization of post-fabrication heat treatments in order to homogenize microstructures and enhance the mechanical properties of surface-modified components.

Within this framework, this Special Issue showcases five high-quality research papers and one comprehensive review addressing several aspects of surface modification and providing valuable insights into the optimization of processing parameters, microstructural evolution, and functional performance for engineering applications. The contributions span multiple material systems and employ comprehensive characterization methodologies to elucidate the fundamental relationships between processing parameters, microstructure evolution, and resultant properties.

2. Contributions

The first group of papers explores advanced surface treatment techniques applied to lightweight alloys and addresses the critical challenge of enhancing mechanical performance while maintaining the intrinsic advantages of low-density materials for aerospace, automotive, and biomedical applications.

Di Egidio et al. [Contribution 1] investigated the influence of electrochemical oxidation (ECO) on the fatigue and wear resistance of the rare-earth-containing Mg alloy EV31A-T6, comparing it with conventional Plasma Electrolytic Oxidation (PEO) treatment. The ECO-treated alloy demonstrated superior tribological behavior, with a critical load for coating failure one order of magnitude higher and a coefficient of friction approximately 40% lower than PEO. Remarkably, ECO treatment maintained the fatigue strength of the untreated alloy (109 ± 5 MPa), while PEO induced a 15% reduction. The observed performance advantage was attributed to the minimization of the treatment-affected sub-layer in ECO coatings, characterized by higher compactness and lower defectivity resulting from controlled discharge phenomena. The study demonstrates that careful optimization of anodization parameters can simultaneously enhance wear resistance and preserve fatigue performance, a combination rarely achieved with conventional surface treatments on magnesium alloys.

Pezzato et al. [Contribution 2] demonstrated the successful incorporation of TiO₂ powders recovered from photovoltaic waste into PEO coatings on an Al 1050 alloy, addressing both functional performance and circular economy principles. The study investigated TiO₂ concentrations ranging from 5 to 80 g/L in the electrolyte, and the results indicated that higher TiO₂ concentrations (40–80 g/L) facilitated a more homogeneous particle distribution into the coating. The EDS analysis confirmed a Ti incorporation of up to 11.1%. Despite the transformation of anatase and rutile to pure rutile phase, evidence of temperatures exceeding 900–1000 °C during the PEO process, the coatings exhibited photocatalytic activity with methylene blue degradation increasing from 6.78% to 28.81% under UV irradiation. The work exemplifies how surface engineering can simultaneously address environmental sustainability through waste valorization and functional enhancement of materials, contributing to both industrial applicability and reduced environmental impact.

Valkov et al. [Contribution 3] developed Ti/TiC composite surface layers on commercially pure titanium through a two-step electron-beam modification process, combining carbon powder alloying and directional remelting. The optimized processing conditions, i.e., 5 mm/s beam scanning speed, produced a ~55 µm thick layer with fine TiC particles homogeneously distributed in an α-Ti matrix, achieving a hardness of 510 HV, 2.5 times that of the substrate. The substantial hardness improvement was attributed to the combined Hall–Petch strengthening from grain refinement and Orowan strengthening from TiC particle dispersion. Increasing the remelting speed to 15 mm/s resulted in a coarser microstructure and reduced layer thickness (~30 µm), highlighting the critical role of thermal gradients and Marangoni convection in controlling microstructural features. The study demonstrates the potential of electron-beam surface modification for producing hard, wear-resistant layers on titanium components.

This group of contributions highlights that advanced electrochemical methods and high-energy beam processing can strongly improve the surface properties of lightweight alloys, enabling their use in increasingly demanding service conditions. Furthermore, the integration of sustainable material recovery strategies underscores a pivotal shift toward environmentally conscious surface engineering without compromising operational properties.

The second group of papers focuses on the fundamental relationships between microstructure and tribological behavior, spanning from multi-scale characterization of dual-phase steels to the optimization of cemented carbide coatings produced via additive manufacturing.

Penfornis et al. [Contribution 4] conducted a comprehensive investigation into the friction and wear behavior of dual-phase microstructures with identical macrohardness and composition but varying grain sizes, providing fundamental insights into tribological mechanisms at the microscale. Through systematic heat treatments, eleven samples with martensite fractions ranging from 50% to 100% were produced, exhibiting grain sizes from 1.45 to 27.95 µm. Nanoindentation and microscale scratch tests revealed that ferrite hardness and friction follow Hall–Petch relationships with grain size, while martensite properties depend linearly on carbon content. The study validated predictive models distinguishing between Equal Wear and Equal Pressure modes, demonstrating that the transition depends on the ratio between contact size and grain size. This work provides a quantitative framework for optimizing dual-phase steel microstructures based on specific tribological loading conditions, advancing our fundamental understanding of multi-phase material behavior under contact loading.

Morales et al. [Contribution 5] investigated the effect of post-fabrication heat treatments (PFHTs) on the microstructure and mechanical properties of WC-12Co cemented carbide coatings produced via Laser-Directed Energy Deposition (L-DED). The as-deposited three-layer coatings exhibited high microstructural heterogeneity, including coarse and fine WC particles, brittle η-phase structures, and inhomogeneous Co distribution, with hardness values (849–1010 HV10) lower than expected for bulk cemented carbides. Systematic PFHT at temperatures ranging from 400 to 700 °C and holding times from 30 to 180 min revealed that optimal conditions (500 °C for 180 min followed by air cooling) achieved a hardness of 1030 ± 95 HV10 through improved microstructural homogeneity and reduced η-phase content. The improvement was attributed to a balance between thermal stress relaxation and phase evolution mechanisms. The study demonstrates that while PFHTs can partially mitigate the heterogeneity inherent to layer-by-layer deposition, thermal stress-induced cracking resulting from substrate–coating thermal expansion mismatch remains a fundamental challenge requiring further process optimization in the additive manufacturing of cemented carbides.

These contributions underscore the role of multi-scale microstructural characterization in understanding and enhancing tribological performance, whether through fundamental studies of phase interactions or through systematic process parameter optimization in new-generation manufacturing routes.

This Special Issue also includes a comprehensive review examining surface modification strategies for metallic nanoparticles in targeted drug delivery applications [Contribution 6], illustrating how surface engineering principles extend across scales and disciplines, from macro-components to nano-carriers. The review provides an in-depth examination of surface modification strategies, including polymer coating, functional group modification, and bio-conjugation with targeting ligands, discussing their application in cancer therapeutics, vaccines, gene delivery, and blood–brain barrier penetration.

3. Concluding Remarks and Outlook

The research compiled in this Special Issue effectively demonstrates the multifaceted nature of modern surface engineering for mechanical and structural components. The contributions encompass a broad spectrum of material systems, from lightweight alloys to cemented carbides and steels. Surface properties were systematically enhanced through diverse processing techniques, ranging from advanced electrochemical treatments and electron-beam modification to additive manufacturing. These modifications were subsequently evaluated using a multi-scale characterization framework, spanning from nano-indentation to macroscale mechanical testing and from optical to scanning electron microscopy. This breadth reflects the field’s capacity to address diverse industrial challenges through tailored surface modification strategies.

The contributions to this Special Issue offer valuable insights for material engineers and researchers designing and optimizing surface-modified components for demanding industrial applications. By integrating fundamental understanding of structure–property relationships with practical optimization strategies, these studies collectively advance the field toward more sustainable, efficient, and high-performance surface engineering solutions.