TiC-Reinforced NiTi Alloy Plasma Arc Cladding Microstructure and Wear Resistance

Literature Overview and Research Significance

This 2013 study by Li Yong, He Dingyong, Suo Hongli, Zhou Zheng, Jiang Jianmin, and Wang Zhihui from the School of Materials Science and Engineering, Beijing University of Technology, investigates the microstructure and wear resistance of TiC-reinforced NiTi alloy plasma transferred arc (PTA) cladding layers. The research was supported by the Beijing Municipal Education Commission Service Beijing Innovation Talent Team Construction Project. The work addresses a critical need in tribology and surface engineering: developing wear-resistant coatings that combine the toughness of nickel-based alloys with the hardness of ceramic reinforcement.

PTA cladding is a mature technology that uses a plasma arc to melt a carrier gas stream carrying metal or ceramic powder onto a substrate surface. The process offers high deposition rates, precise control of dilution, and the ability to produce dense, crack-free coatings. The combination of NiTi matrix with TiC reinforcement represents a promising system for applications requiring both wear resistance and corrosion resistance, such as pump components, valve seats, and chemical processing equipment.

Material System and Alloy Design

The NiTi alloy matrix, typically a nickel-titanium binary or ternary alloy (Ni-Ti-Cr or Ni-Ti-Al), provides an austenitic or martensitic matrix with good toughness and corrosion resistance. The addition of TiC particles as reinforcement introduces hard ceramic phases that resist abrasive wear through particle pull-out, micro-cutting resistance, and ploughing resistance mechanisms.

Component	Role	Typical Content
Ni	Matrix former, corrosion resistance	50–60 wt%
Ti	Solid solution strengthening, TiC formation	10–20 wt%
Cr	Oxidation resistance, solid solution strengthening	5–15 wt%
TiC (powder)	Hard reinforcement	10–30 wt% of coating
C	Carbon source for TiC in-situ formation	2–5 wt%

Plasma Arc Cladding Process Parameters

The PTA cladding process parameters significantly influence the microstructure and properties of the NiTi-TiC composite coating. Key parameters include plasma arc current, voltage, travel speed, powder feed rate, and shielding gas flow rate.

Parameter	Typical Range	Effect on Coating
Arc current	150–250 A	Higher current increases dilution and melting depth
Arc voltage	20–30 V	Influences arc stability and heat input
Travel speed	200–500 mm/min	Higher speed reduces dilution but may cause porosity
Powder feed rate	200–400 g/min	Must match arc power for complete melting
Shielding gas	Ar or Ar-He mixture	Protects molten pool and coating
Powder particle size	15–45 μm	Smaller particles melt more completely

Dilution Control

Dilution of the coating by the substrate is a critical parameter. For NiTi-TiC coatings on steel substrates, dilution should be controlled below 20% to maintain the intended microstructure and properties. This is achieved by optimizing the travel speed, arc power, and powder feed rate. Excessive dilution introduces iron into the coating, promoting the formation of Fe-rich phases and reducing hardness.

Microstructural Analysis

The microstructure of TiC-reinforced NiTi PTA cladding consists of:

NiTi matrix: Austenitic or martensitic NiTi phase, possibly with intermetallic Ni3Ti or NiTi2 phases depending on composition.
TiC particles: Pre-existing TiC particles that may partially dissolve during the plasma arc melting process, along with in-situ formed TiC from reaction between Ti and C.
Cr carbides: Cr7C3 or Cr23C6 may form if Cr is present in the coating composition.
Matrix microstructure: Fine-grained due to rapid solidification in the PTA process, with possible cellular or dendritic patterns.

The hardness of the NiTi-TiC composite coating typically ranges from HV 800–1200, significantly higher than the base NiTi alloy (HV 300–400) or the substrate steel (HV 200–300). The wear resistance is governed by the volume fraction, size, and distribution of TiC particles within the matrix.

Wear Mechanism and Performance

The wear resistance of TiC-reinforced NiTi coatings is evaluated through pin-on-disc testing, dry sliding wear, or abrasive wear tests. The wear mechanisms observed include:

Abrasive wear: TiC particles resist cutting and ploughing by abrasive particles, reducing material removal rate.
Adhesive wear: The NiTi matrix provides good adhesion resistance due to its corrosion-resistant nature.
Fatigue wear: In cyclic loading conditions, the ductile matrix accommodates stress and prevents crack propagation.
Oxidative wear: At elevated temperatures, the Cr content promotes formation of a protective Cr2O3 scale.

The wear rate of TiC-reinforced NiTi coatings is typically 1–2 orders of magnitude lower than that of uncoated steel substrates under dry sliding conditions. The coefficient of friction is also reduced due to the hard, smooth surface provided by the TiC particles.

Engineering Applications and Considerations

TiC-reinforced NiTi PTA cladding is suitable for applications requiring:

Wear-resistant pump impellers and casing linings in chemical processing
Valve seats and stems in high-pressure, corrosive service
Turbine blade coatings for erosion resistance
Medical implant surfaces requiring both biocompatibility and wear resistance

Process Optimization Recommendations

Use a multi-layer deposition strategy: a first layer for bonding, intermediate layers for building thickness, and a final layer for surface finish.
Maintain interpass temperature below 150 °C to prevent excessive grain growth in the coating.
Consider post-deposition annealing at 800–900 °C for 1 hour to relieve residual stresses and improve toughness.
Perform metallographic examination of cross-sections to verify coating thickness, dilution zone, and absence of defects such as porosity or cracking.

Key Reflections and Study Insights

This research demonstrates the effectiveness of composite cladding in combining the beneficial properties of metallic and ceramic phases. The PTA process is particularly well-suited for depositing ceramic-reinforced coatings due to its ability to melt refractory particles that would be difficult to process by conventional welding methods. The key challenge is balancing hardness (provided by TiC) with toughness (provided by the NiTi matrix) to achieve a coating that resists wear without being prone to spalling or delamination.

For industrial implementation, the following quality control measures should be established:

Verify coating thickness and dilution through metallographic examination.
Perform hardness testing (Vickers, HV0.5 or HV1) across the coating cross-section.
Conduct wear testing on representative samples to confirm performance meets specifications.
Inspect coating surfaces for porosity, cracks, and spatter using visual examination and, if necessary, liquid penetrant testing.

The integration of TiC reinforcement into NiTi matrix coatings represents a significant advancement in surface engineering, offering a versatile solution for wear-resistant applications in corrosive and abrasive environments.