Key Takeaways & Executive Findings
- •• • Coatings with 60–67 wt.% Ni are crack-free and dense, but 70 wt.% Ni induces through-thickness cracks, defining a critical upper limit for Ni content to avoid catastrophic failure in service. • • The 67NiTi coating achieves a hardness of 677.41 HV0.2 (2.05× substrate) due to a maximum Ti2Ni volume fraction of 78.6%, directly correlating with a 65% reduction in wear rate at 20 N (2.74×10⁻⁴ mm³/(N·m)), validating dilution-driven second-phase strengthening. • • Wear rate follows a V-shaped dependence on Ni content, with the minimum at 67 wt.% Ni; this non-monotonic behavior necessitates precise compositional control to avoid the inferior wear performance of both lower (60–65 wt.%) and higher (70 wt.%) Ni coatings. • • The 67NiTi coating exhibits mild abrasive and adhesive wear at 20 N, indicating stable tribological performance under high loads, which is critical for extending the lifespan of TC4 components in aerospace and biomedical implants.
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Abstract
Plasma arc cladding was employed to fabricate NiTi coatings with varying Ni contents (60, 63, 65, 67, and 70 wt.%) on TC4 titanium alloy to enhance surface wear resistance. Microstructural characterization via SEM, OM, and XRD revealed that coatings with 60–67 wt.% Ni were dense and defect-free, whereas the 70 wt.% Ni coating exhibited through-thickness cracks. All coatings exceeded 1.2 mm in thickness and comprised a NiTi toughening phase and Ti2Ni strengthening phase. Increased dilution ratio with higher Ni content reduced actual Ni in the coating, maximizing Ti2Ni fraction (78.6%) in the 67NiTi coating, which achieved a peak hardness of 677.41 HV0.2 (2.05 times that of the TC4 substrate). Tribological testing under 5–20 N loads showed that the average wear rate of all coatings decreased significantly, following a V-shaped trend with Ni content. The 67NiTi coating exhibited the lowest wear rate (2.74×10⁻⁴ mm³/(N·m)) at 20 N, a 65% improvement over the substrate, with wear mechanisms dominated by mild abrasive and adhesive wear. These findings demonstrate that optimized Ni content in plasma-clad NiTi coatings effectively mitigates the poor wear resistance of titanium alloys, offering a viable surface engineering solution for load-bearing applications.
1. Introduction
TC4 titanium alloy is widely used in aerospace and biomedical implants due to its high specific strength and corrosion resistance, but its poor tribological performance—characterized by low hardness (approximately 330 HV) and a tendency for adhesive wear—leads to premature failure in sliding contact applications. Conventional surface treatments such as nitriding or thermal spraying often suffer from insufficient coating thickness, weak interfacial bonding, or process-induced defects, limiting their effectiveness under high loads.
Plasma arc cladding offers a promising route to deposit thick, metallurgically bonded NiTi coatings, but the optimal Ni content for balancing hardness and toughness remains unclear. This study systematically varies Ni content (60–70 wt.%) to elucidate its effect on microstructure, phase evolution, and wear resistance. By identifying a critical threshold at 67 wt.% Ni, where a maximum Ti2Ni fraction yields a 65% wear rate reduction without cracking, the work provides a quantitative framework for designing durable NiTi coatings on titanium alloys.
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ZHANG Qi, YUE Yun, ZHANG Xin, DU Sanming, PING Jingyan, DENG Sier, ZHANG Yongzhen (2026). Effect of Ni Content on the Microstructure and Tribological Properties of NiTi Alloy Coatings. Surface Technology (表面技术). https://doi.org/10.16490/j.cnki.issn.1001-3660.2026.11.003
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Frequently Asked Questions
What is the failure mechanism that limits the Ni content to 67 wt.% in these coatings?
At 70 wt.% Ni, the coating develops through-thickness cracks from the top surface to the fusion line, as observed in SEM. This is attributed to increased thermal stresses and altered cooling rates during cladding, which promote crack initiation. The 67 wt.% Ni coating remains crack-free, with a maximum dilution ratio of 36% and a Ti2Ni fraction of 78.6%, ensuring structural integrity.
How does the wear rate of the optimal coating compare to the TC4 substrate under high load, and what are the dominant wear mechanisms?
Under a 20 N load, the 67NiTi coating exhibits a wear rate of 2.74×10⁻⁴ mm³/(N·m), which is 65% lower than that of the TC4 substrate. The wear mechanisms are predominantly mild abrasive wear and adhesive wear, indicating that the hard Ti2Ni phase effectively resists material removal while the NiTi phase accommodates plastic deformation.
What is the industrial scalability of plasma arc cladding for NiTi coatings on TC4, and are there cost barriers?
Plasma arc cladding is a well-established industrial process capable of depositing thick coatings (>1.2 mm) at high deposition rates. The raw material cost is moderate, as Ni and Ti powders are commercially available. However, precise control of Ni content within ±1 wt.% is critical to avoid the V-shaped wear rate trend; this requires robust powder feeding and dilution management, which may add to capital costs but is offset by extended component life.
Does the dilution ratio affect the actual Ni content in the coating, and how does this influence hardness?
Yes, as the dilution ratio increases with higher Ni in the powder, the actual Ni content in the coating decreases due to Fe and Ti diffusion from the substrate. This promotes the formation of Ti2Ni strengthening phase. The 67NiTi coating achieves the highest Ti2Ni fraction (78.6%) and a hardness of 677.41 HV0.2, which is 2.05 times that of the substrate, directly correlating with improved wear resistance.
What are the long-term stability and potential failure modes of the 67NiTi coating under cyclic loading?
The 67NiTi coating shows no cracks or pores in the as-clad condition, and its wear rate remains the lowest among all compositions. Under cyclic loading, the primary risk is fatigue-induced cracking at the coating-substrate interface due to thermal expansion mismatch. However, the metallurgical bond and the presence of the NiTi toughening phase mitigate stress concentrations. Accelerated wear tests at 20 N indicate stable performance, but further fatigue testing is recommended to validate lifetime predictions.
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