Issue 78

N. Mohan et alii, Frattura ed Integrità Strutturale, 78 (2026) 129-145; DOI: 10.3221/IGF-ESIS.78.08

coefficient of friction (COF) in dry sliding varied from 0.49 to 0.78, and wear rates ranged from 3.5 to 5.1 × 10 ⁻ 6 mm³/Nm. At 950 °C, wear rates decreased by 25 to 27% because of higher fracture toughness (3.3 to 3.9 MPa √ m). Due to electrolyte lubrication, COF decreased to 0.28-0.49 (16-33% lower) under tribocorrosion. When compared to bare MSS, boronized samples showed noticeably better wear resistance, with wear reduction of 60 80%. Under tribocorrosion, wear mechanisms changed from sticky abrasive in dry sliding to mixed abrasion, adhesion, and tribo-oxidation, controlled by cyclic passivation–depassivation. Boronizing, particularly at 950°C, improves overall tribocorrosion performance despite decreased corrosion resistance by balancing hardness and toughness. K EYWORDS . Martensite stainless steel, Boronizing, Dry sliding wear, Tribocorrosion, Wornout surface

I NTRODUCTION

ydel energy is one of the most sought-after renewable energy sources owing to its long-term sustainability. However, maintenance is challenging as components such as blades undergo severe material loss while operating in a complex environment. Hydro turbine blades typically experience combined effects of wear, cavitation and corrosion making their integrity and longevity highly susceptible [1]. Thus, an in-depth understanding of the impact of these combined effects is essential to enhance the performance and durability of these components. Recent research has focused on developing advanced materials and protective coatings to reduce these effects, thereby extending the service life of hydroturbine components. Due to their excellent mechanical properties, cost-effectiveness and widespread availability, stainless steels are preferred materials for hydroturbine applications. Particularly, 13Cr4Ni and 16Cr5Ni martensite stainless steels (MSS), known for their excellent combination of mechanical strength, toughness, and corrosion resistance, are highly suitable for hydroturbine applications where components are exposed to both wear and corrosive environments [2]. Despite prevalent use of stainless steels, predominant research focus has been concentrated on studies related to their wear characteristics [1, 3]. However, it is crucial to note that freshwater environments often contain dissolved salts, such as chlorides and sulphates, which can significantly influence the corrosion behavior of stainless steels [4]. In hydroturbine environments, the interaction between corrosion and wear is complex and critical. While corrosion leads to weakening of material, making it susceptible to wear; wear can remove protective oxide layers, leading to intensification of corrosion. Hence, understanding tribocorrosion, which is the combined effect of wear and corrosion, is essential. Tribocorrosion studies provide insights into how materials behave under simultaneous mechanical and chemical degradation. To further enhance the properties of these stainless steels, surface coatings are often employed [5]. Various surface treatment methods, including nitriding, carburizing, and boronizing, are used to improve wear and corrosion resistance. Among these, boronizing has shown the most promise. Nitriding and carburizing, while beneficial in increasing surface hardness, do not offer the same level of corrosion resistance as boronizing, due to formation of less dense and less chemically stable surface layers [6]. Boronizing, a thermochemical surface treatment, diffuses boron atoms into the substrate, significantly improving hardness, wear and corrosion resistance [7]. This dual benefit is particularly important in hydroturbine applications, where both mechanical and chemical challenges are present. Powder pack boronizing is one of the most convenient and cost-effective methods employed in boronizing of steels [8]. Boronizing is accomplished using a pack mixture containing boron carbide (5 wt. %), potassium tetra fluoro borate (5 wt%) and silicon carbide (90%) at temperatures between 850- 1050°C [9]. This yields a boronized layer on the substrate material having a composition of Fe 2 B and FeB. However, in the case of high alloy content materials such as stainless steels, additional borides such as CrB, NiB, Cr 2 B, Cr 3 B 4 and Ni 2 B have also been reported [6]. Previous studies have extensively explored the tribocorrosion behavior of boronized stainless steels. For instance, the work of Cetin et al studies boronizing of AISI 904L superaustenitic stainless steel, generally used in corrosive environments. The boronized steel exhibited up to 40 times greater wear resistance but reduced corrosion resistance in 3.5% NaCl, comparable to AISI 316L [10]. The boronized steel exhibited improved wear resistance but reduced corrosion resistance in 3.5% NaCl, comparable to AISI 316L [10]. Campos-Silva et al. investigated the tribocorrosion resistance and cytotoxicity of FeB–Fe ₂ B layers formed on H

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