HVAF Spraying of NiTi Coatings: Microstructure, Phase Transformation and Shape Memory Behavior

Technical Summary: HVAF Spraying of NiTi Coatings

Problem Statement
The fabrication of thick, functional Nickel-Titanium (NiTi) shape memory alloy (SMA) coatings has historically proven difficult. Standard methods such as electroplating and laser cladding are unsuitable for NiTi, while widely used plasma spray techniques (e.g., Atmospheric Plasma Spray) often melt the particles, resulting in chemically inhomogeneous coatings that fail to undergo martensitic transformation. Cold-sprayed NiTi coatings, while avoiding melting, frequently suffer from poor adhesion to substrates. Consequently, there is a lack of robust methods to deposit thick (100–300 µm) NiTi coatings that simultaneously possess strong substrate adhesion and retain the unique functional thermomechanical properties (shape memory and superelasticity) derived from martensitic transformation.

Methodology
The authors employed High-Velocity Air Fuel (HVAF) thermal spraying, a "warm spray" process that operates at lower temperatures than High-Velocity Oxygen Fuel (HVOF) but higher than Cold Spray, to deposit NiTi coatings onto mild steel substrates. The study utilized pre-alloyed, gas-atomized Ni50Ti50 powder (99.5% purity).

Four distinct coating samples were fabricated by varying processing parameters, specifically the nozzle type (M2 vs. M3), chamber configuration, and stand-off distance (SOD). The resulting coatings, ranging from 100 to 300 µm in thickness, were subjected to a comprehensive characterization protocol:

• Microstructural Analysis: Scanning Electron Microscopy (SEM) for porosity and cross-sections; Transmission Electron Microscopy (TEM) for phase identification, grain size, and defect analysis; and Electron Back-Scattered Diffraction (EBSD).
• Chemical Analysis: Energy-Dispersive X-ray Spectroscopy (EDS) line scans to assess macroscopic homogeneity and detect oxidation.
• Phase Transformation: Differential Scanning Calorimetry (DSC) to determine transformation temperatures (Ms, Mf, As, Af) for both as-sprayed and annealed (500 °C for 1 hour) states.
• Mechanical Testing:
  • Detached Samples: Tensile tests and 3-point bending dilatometry on coatings separated from the substrate.
  • In-situ Tests: 3-point bending thermomechanical cycling, progressive-load scratch tests, and multi-cycle nanoindentation performed directly on the coated substrates to evaluate functional properties without detachment.

Key Results

1. Fabrication Success: The study successfully demonstrated the first fabrication of thick (up to 300 µm) NiTi coatings with very good adherence to steel substrates, avoiding the delamination issues common in cold spray.
2. Microstructure and Composition: The as-sprayed coatings exhibited a heterogeneous microstructure containing voids (porosity ranging from 1.6% to 4.7 vol.%), oxide particles (primarily TiO2), and a high density of dislocation defects. While macroscopically homogeneous, the coatings showed local chemical heterogeneity due to internal oxidation, which depleted Titanium from the matrix and enriched the remaining NiTi phase in Nickel.
3. Phase Transformation:
   • As-Sprayed State: The as-sprayed coatings were predominantly austenitic (B2) at room temperature. DSC scans of the as-sprayed state showed no distinct heat flow peaks, likely due to internal stress and chemical heterogeneity suppressing or broadening the transformation.
   • Annealed State: Post-processing annealing at 500 °C for 1 hour was essential to reveal functional properties. Annealed coatings displayed a B2-R-B19' martensitic transformation sequence. The presence of the R-phase was attributed to plastic deformation and compositional changes.
4. Mechanical Properties:
   • Tensile Strength: Detached samples exhibited low tensile strength (96–194 MPa) and low Young's modulus (43–46 GPa), fracturing in the elastic range. The authors attribute this brittleness to porosity, oxides, and residual stress.
   • Functional Behavior: Despite low tensile strength, annealed coatings demonstrated functional thermomechanical properties. 3-point bending tests showed recoverable deformation and shape memory effects. Scratch tests revealed high elastic recovery (57–61%) compared to the steel substrate (30%), with no observable delamination or cracking up to 500 mN loads.
   • Nanoindentation: Multi-cycle nanoindentation confirmed cyclic stability in the recovered depth, distinguishing the functional response of coatings fabricated with different parameters.

Significance and Claims
The paper claims a significant step forward in the field of smart material coatings by proving that HVAF spraying can produce thick, adherent NiTi coatings that retain martensitic transformation capabilities. The authors emphasize that while the current as-sprayed state requires post-processing annealing to function and suffers from limited tensile strength due to porosity and oxidation, the HVAF process itself is viable for creating the necessary composite structure.

The study highlights that HVAF offers a unique balance: it provides sufficient thermal and mechanical energy to compact the coating without melting the particles (preserving the B2 phase and chemical stoichiometry macroscopically), yet it minimizes oxidation compared to HVOF. The authors conclude that while the current quality is not yet optimized for high-stress applications, the successful demonstration of functional thermomechanical properties in thick coatings opens the door for future optimization of HVAF parameters and heat treatments to achieve superior superelasticity and shape memory performance in engineering applications.