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Phase-controlled Tribocorrosion Mechanisms in Heat-treated Electrodeposited Ni–mo–b Coatings

This study demonstrates that heat-treating electrodeposited Ni–Mo–B coatings to form Mo₂NiB₂ boride-rich microstructures significantly enhances hardness and stabilizes tribocorrosion performance in saline environments by shifting the wear mechanism from unstable oxidative–adhesive to stable oxidation-controlled regimes.

Original authors: Svitlana Halaichak, Sergiy Korniy, Vadim Zakiev, Vasyl Vynar, Maksym Danylchuk, Marian Chuchman, Roman Mardarevych, Yuriy Verbovytskyy

Published 2026-07-15

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Svitlana Halaichak, Sergiy Korniy, Vadim Zakiev, Vasyl Vynar, Maksym Danylchuk, Marian Chuchman, Roman Mardarevych, Yuriy Verbovytskyy

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a metal shield, like the armor on a knight's suit, but instead of steel, it's made of a special nickel mixture. This shield is designed to protect machinery from two nasty enemies at once: the grinding friction of moving parts and the biting corrosion of salty water. The researchers in this study decided to see if they could upgrade this shield by giving it a "heat bath" and tweaking its recipe.

The Magic Heat Bath
Think of the original coating as a bowl of mixed ingredients—nickel, molybdenum, and boron—that are just sitting there, not quite working together. When the scientists put these coatings into a furnace at a scorching 950 °C for 2 hours, it was like hitting the "melt and mix" button on a super-charged blender. This heat didn't just warm them up; it triggered a chemical dance where the ingredients fused together to form new, super-tough structures called borides. Specifically, they created Ni₃B and Mo₂NiB₂ phases. It's like turning a pile of loose sand and gravel into a solid, unbreakable concrete wall.

The Secret Ingredient: More Molybdenum
The team tested two versions of this shield: one with 8 wt.% molybdenum and a "super-charged" one with 16 wt.%. Here is the cool part: by doubling the molybdenum, they didn't just get a little bit more of the tough stuff; they got a massive boost. The amount of the super-hard Mo₂NiB₂ phase jumped by a factor of 3.75. Imagine if you had a bag of 4 super-strong bricks, and suddenly you had nearly 15 of them packed into the same space. That's the kind of upgrade they achieved.

The Results: Harder, Stiffer, and Smoother
This new structure made the coating incredibly tough. The researchers measured its hardness and found it reached up to 11.67 GPa, and it became very stiff with an elastic modulus of 292.5 GPa. To put that in perspective, it's like the difference between pressing your finger into a rubber ball versus pressing it into a diamond.

But the real magic happened when they tested how the shield held up in a salty 3% NaCl solution (basically, a bucket of seawater) while being rubbed back and forth by a ceramic ball. This is called "tribocorrosion," a fancy word for "rust and rub."

The 8% Molybdenum Shield: This version was okay, but it had a rough time. As it got rubbed, the friction got unstable, and the surface started to wear down in a messy way, mixing oxidation (rusting) with sticking and tearing (adhesive wear). It was like trying to slide a heavy box across a floor that kept getting sticky and then suddenly slippery.
The 16% Molybdenum Shield: This one was a total game-changer. Because it had so many more of those tough Mo₂NiB₂ bricks, it stayed calm. The friction stayed steady at about 0.2, and the wear track (the scratch left behind) was 46% narrower than the weaker version. Instead of a messy, unstable fight, the surface settled into a stable rhythm where a protective layer formed and kept the damage in check.

What They Ruled Out
The paper makes it clear that the original, unheated coatings were much weaker. In fact, the untreated versions were 18.6 times (for the 8% Mo) and 8.5 times (for the 16% Mo) less resistant to corrosion than the heat-treated ones. So, the heat treatment wasn't just a small tweak; it was the essential step that made the difference between a weak shield and a strong one. The study also showed that simply adding more molybdenum without the heat treatment wouldn't have worked the same way; the heat was required to trigger the formation of those specific, super-hard boride phases.

The Bottom Line
The study suggests that by carefully controlling how much molybdenum is in the mix and then giving it a precise heat treatment, you can engineer a coating that doesn't just survive the harsh combination of rubbing and rusting—it thrives. The 16% molybdenum version, with its high concentration of Mo₂NiB₂, proved to be the champion, offering a stable, low-friction surface that resists wear far better than its weaker cousin. It's a promising step forward for protecting machinery in tough environments like oil rigs or chemical plants, where the battle against wear and corrosion is constant.

Technical Summary: Phase-Controlled Tribocorrosion Mechanisms in Heat-Treated Electrodeposited Ni–Mo–B Coatings

Problem Statement

Industrial equipment operating in aggressive environments faces combined mechanical tribological loads and corrosive agents, leading to accelerated wear, functional degradation, and premature failure. While nickel-based coatings offer promise due to their ability to be alloyed for specific requirements, electrodeposited Ni–Mo–B coatings often suffer from non-uniform phase composition and limited hardness, reducing their effectiveness under severe industrial conditions. Although heat treatment is known to improve adhesion and reduce residual stresses, the specific relationship between thermally induced phase transformations, tribochemical film formation, and tribocorrosive wear mechanisms in these coatings remains insufficiently studied. There is a need to elucidate how boride phases control friction, wear rates, and electrochemical stability under combined loading.

Methodology

The study utilized low-alloy S355J2 steel substrates prepared via mechanical polishing, ultrasonic cleaning, and chemical activation. Ni–Mo–B coatings were electrodeposited from a citrate-based electrolyte containing amorphous boron suspension at current densities of 2.5 and 5.0 A/dm², resulting in coatings with molybdenum (Mo) contents of approximately 8 wt.% and 16 wt.%.

The coatings underwent isothermal annealing at 950 °C for 2 hours in an inert argon atmosphere to induce diffusion-driven phase transformations. Characterization included:

Structural Analysis: Scanning Electron Microscopy (SEM) with Energy-Dispersive Spectroscopy (EDS) for morphology and composition; X-ray Diffraction (XRD) with Rietveld refinement for phase quantification.
Micromechanical Testing: Dynamic nanoindentation (Oliver-Pharr method) to determine microhardness ( $H_v$ ), elastic modulus ( $E$ ), and plastic/elastic work of deformation.
Corrosion Evaluation: Anodic polarization in 3% NaCl solution under argon bubbling to assess passivation behavior.
Tribocorrosion Testing: Reciprocating "plane–ball" friction tests (Al₂O₃ ball, 10 N load) in 3% NaCl solution to measure friction coefficients, wear track dimensions, and tribopotential evolution.

Key Contributions and Results

1. Phase Evolution and Microstructure

Heat treatment at 950 °C triggered a diffusion-driven transformation of amorphous boron within the Ni–Mo matrix. This resulted in a composite microstructure reinforced by nickel boride ( $Ni_3B$ ) and the ternary boride $Mo_2NiB_2$ .

Increasing Mo content from 8 to 16 wt.% significantly altered the phase balance: the fraction of the strengthening $Mo_2NiB_2$ phase increased by a factor of 3.75 (from 4% to 15%), while the total boride content rose from 71% to 92%.
The microstructure featured a diffusion zone at the substrate interface with no observed cracks or pores. Higher Mo content led to finer, more uniformly distributed hard phases.

2. Micromechanical Properties

The evolution of the phase composition directly enhanced mechanical performance:

Hardness: Increased from 9.555 GPa (8 wt.% Mo) to 11.67 GPa (16 wt.% Mo).
Elastic Modulus: Increased from 285.3 GPa to 292.5 GPa.
Deformation Behavior: The 16 wt.% Mo coating exhibited reduced plastic work of deformation (~16% decrease) and improved elastic recovery, indicating effective dispersion strengthening by the boride phases.

3. Corrosion and Tribocorrosion Performance

Corrosion Resistance: Heat-treated coatings demonstrated significantly higher corrosion resistance compared to their as-deposited counterparts. In 3% NaCl, they formed stable passive films that limited chloride ion diffusion. The 8 wt.% and 16 wt.% Mo coatings showed comparable corrosion potentials, though the 8 wt.% variant exhibited a slightly higher active dissolution rate initially.
Tribocorrosion Mechanisms:
- 8 wt.% Mo: The wear mechanism was characterized as unstable oxidative–adhesive wear, marked by fluctuating friction coefficients and surface damage (micro-galling/scoring).
- 16 wt.% Mo: The system transitioned to a stable, oxidation-dominated regime. This coating maintained a stable friction coefficient ( $\mu \approx 0.2$ ) with minimal fluctuations.
Wear Resistance: The 16 wt.% Mo coating demonstrated superior resistance to combined mechanical–corrosive degradation, evidenced by a 46% reduction in wear track width (420 µm vs. 780 µm) compared to the 8 wt.% coating.

Significance and Claims

The paper posits that the fraction of the $Mo_2NiB_2$ ternary boride phase is the governing factor in the transition of wear mechanisms and the stability of tribocorrosion processes. The study concludes that heat treatment at 950 °C is a critical processing step that enables the formation of a reinforced structural-phase state.

The authors claim that Ni–Mo–B coatings with 16 wt.% Mo, when heat-treated, offer a scientifically grounded solution for enhancing wear and corrosion resistance in aggressive environments. Specifically, these coatings are presented as promising candidates for protective applications in oil, gas, and chemical engineering systems subjected to severe combined wear–corrosion loading, where the stabilization of tribofilms and reduction of synergistic degradation are essential. The work highlights phase engineering as a key strategy for optimizing the performance of boride-strengthened electrodeposited coatings.