Micro-Arc Plasma Cladding of Nickel-Diatomite High-Temperature Sealing Coating for Corrosion and Wear Performance

Literature Overview and Research Background

This study, published in 2017 by researchers from the Rocket Force Engineering University under the National Natural Science Foundation of China (Project No. 51405497), investigates the corrosion and wear characteristics of nickel-diatomite composite coatings deposited via micro-arc plasma cladding (MAPC) technology. The research is particularly relevant to propulsion technology applications where high-temperature sealing surfaces are subjected to aggressive thermal and mechanical environments. The authors — Yuan Xiaojing, Zha Bolin, Chen Xiaohu, Yang Nengjun, and Yao Chunjiang — address a critical engineering challenge: developing overlay coatings that simultaneously resist thermal cycling, chemical corrosion, and abrasive wear in extreme service conditions.

Core Technical Content and Process Parameters

Micro-arc plasma cladding differs fundamentally from conventional plasma transferred arc (PTA) cladding in that it operates at lower current densities with intermittent arc strikes, producing a more dilute but functionally tailored surface layer. The process parameters studied in this work typically fall within the following ranges:

The composite coating consists of nickel as the metallic matrix with diatomite (a natural silica-rich mineral) serving as a reinforcing ceramic phase. The nickel matrix provides ductility and thermal conductivity, while the diatomite particles enhance thermal insulation and reduce thermal expansion mismatch between the coating and substrate. This design philosophy reflects a deliberate trade-off: pure ceramic coatings would exhibit superior thermal barrier properties but suffer from catastrophic brittle failure under thermal cycling, whereas the metallic-ceramic composite achieves a balanced performance envelope.

Corrosion and Wear Mechanism Analysis

The corrosion resistance of the Ni/diatomite coating is attributed to several mechanisms. First, the nickel-rich matrix forms a stable, self-healing oxide layer (NiO/Ni₂O₃) at elevated temperatures, which acts as a diffusion barrier against oxidizing media. Second, the diatomite particles, composed primarily of SiO₂, contribute to the formation of a vitrified glassy phase in the coating microstructure, effectively sealing micro-porosity and blocking corrosive species penetration. Third, the micro-arc plasma process produces a relatively fine-grained, dense microstructure with limited interfacial porosity compared to thermal spray methods.

The wear behavior is governed by the composite nature of the coating. Under dry sliding conditions, the hard diatomite particles (Mohs hardness approximately 6–7) provide abrasive resistance, while the ductile nickel matrix prevents spalling and delamination. The wear mechanisms observed typically include adhesive wear at the metal-metal interface, abrasive wear where ceramic particles act as micro-abrasives against the counterface, and oxidative wear where the protective oxide film is continuously formed and removed. The study likely demonstrates that optimal diatomite content (typically 10–30 wt%) yields the best combined corrosion-wear performance, as excessive ceramic content leads to coating cracking and reduced bonding strength.

Engineering Practice Implications

From a practical standpoint, this research has direct relevance to sealing applications in gas turbine engines, rocket thrust chambers, and high-temperature exhaust systems. The MAPC process offers several advantages over alternative methods: it requires no consumable electrode (unlike ESW or SAW overlay), produces lower dilution with the base material (typically 5–15% compared to 20–40% for arc welding overlay), and allows in-situ composite formation without pre-alloying. However, the process also has limitations — the relatively low deposition rate (typically 0.5–2 kg/h) makes it economically viable only for high-value, small-area applications rather than large-scale industrial cladding.

A key engineering consideration is the residual stress state within the coating. The micro-arc plasma process, due to its intermittent nature, produces alternating heating and cooling cycles that can partially self-relieve residual stresses. Nevertheless, compressive residual stresses in the coating surface are generally beneficial for fatigue and wear performance, and post-weld stress relief heat treatment (typically 400–500°C for nickel-based systems) should be considered for critical applications.

Study Insights and Reflections

This work exemplifies the materials-by-design approach increasingly adopted in advanced overlay engineering. Rather than selecting a single-phase alloy and relying on its inherent properties, the researchers engineer a composite system where each component contributes a specific function. The diatomite, being an abundant and inexpensive natural mineral, represents a cost-effective reinforcement strategy compared to synthetic ceramics such as alumina or zirconia. The micro-arc plasma process complements this philosophy by enabling precise control over the coating microstructure at relatively low energy input.

For engineers working in propulsion or high-temperature sealing applications, the key takeaway is that composite coatings processed via plasma-based methods can achieve multi-functional performance (thermal barrier, corrosion resistance, wear resistance) that exceeds what monolithic coatings can deliver. The challenge lies in optimizing the ceramic volume fraction, particle size distribution, and processing parameters to avoid the trade-off between hardness (from ceramics) and toughness (from the metallic matrix). Future work should address long-term durability under thermal cycling (1000+ cycles between room temperature and 1000°C) and validate performance through component-level testing rather than coupon-level studies alone.

Summary

The micro-arc plasma cladding of nickel-diatomite composite coatings represents a promising approach to solving the multi-functional sealing challenge in high-temperature propulsion systems. The combination of a ductile metallic matrix with hard ceramic reinforcement, processed via a low-dilution plasma method, yields coatings with balanced corrosion and wear resistance. Engineers should note that while the fundamental research is sound, translating coupon-level results to component-scale performance requires careful attention to process consistency, coating thickness uniformity, and post-processing treatments. The economic viability of this approach depends on the specific application value — it is best suited for high-value, safety-critical sealing surfaces where coating failure would result in catastrophic system consequences.