CLADDING TECHNOLOGY SHANXI CO., LTDStudy Notes on Microstructure and Properties of TiB2-Metallic Ceramic Coatings by Plasma Arc Cladding
Overview and Technical Significance
Titanium diboride (TiB2) is one of the most promising hard phases for plasma arc transferred (PTA) cladding applications due to its exceptional hardness exceeding 3000 HV, high thermal stability, and excellent corrosion resistance. The integration of TiB2 into metallic ceramic coatings combines the toughness of a metallic matrix with the hardness and wear resistance of a ceramic reinforcement phase. This hybrid approach addresses the fundamental trade-off between hardness and toughness that limits purely ceramic coatings in practical engineering applications.
The study examines the microstructural evolution and mechanical properties of TiB2-metallic ceramic coatings deposited on various substrate materials using the plasma arc cladding process. The plasma arc provides a narrow heat-affected zone and rapid cooling rates that favor the formation of fine microstructures and suppress the coarsening of ceramic particles.
Microstructural Characteristics
The microstructure of TiB2-metallic ceramic coatings is characterized by a complex interaction between the TiB2 ceramic particles and the metallic matrix. During the plasma arc melting process, TiB2 particles can undergo partial dissolution, reaction with the matrix, and re-solidification, leading to a range of microstructural features.
| Microstructural Feature | Description | Effect on Properties |
|---|---|---|
| Intact TiB2 particles | Retained original morphology | High hardness contribution |
| Dissolved TiB2 | Partially dissolved into matrix | Solid solution strengthening |
| TiB2-derived carbides | Reaction products with carbon | Hard phase dispersion |
| Matrix phase | Metallic binder (Fe, Ni, Co based) | Toughness and ductility |
| Interface reaction zone | Chemical interaction at particle-matrix boundary | Bond strength enhancement |
The degree of TiB2 particle dissolution is strongly dependent on the plasma arc current, powder feed rate, and travel speed. Higher plasma currents increase the thermal input, promoting greater particle dissolution and a more homogeneous microstructure. However, excessive dissolution can lead to the formation of coarse borides and carbides that reduce the overall hardness of the coating.
Phase Evolution During Solidification
The solidification behavior of TiB2-metallic ceramic coatings follows a sequence of phase transformations that is critical to the final properties. The rapid cooling rates achievable with PTA cladding (typically 10-100 K/s) favor the formation of fine-grained microstructures. The TiB2 particles act as heterogeneous nucleation sites, refining the matrix grain size and increasing the volume fraction of fine carbides and borides.
The presence of carbon in the powder blend leads to the formation of TiC and TiB2 carbide-boride complexes at the particle-matrix interface. These interfacial phases enhance the bond strength between the ceramic reinforcement and the metallic matrix, reducing the likelihood of particle pull-out during wear.
Mechanical Properties and Performance
The hardness of TiB2-metallic ceramic coatings typically ranges from 1200 to 2500 HV, depending on the TiB2 content, particle size, and matrix composition. The wear resistance is characterized by a significant improvement over conventional metallic overlay coatings, particularly in dry sliding and abrasive wear conditions.
| Property | TiB2-Metallic Ceramic | Conventional Metallic Overlay | Improvement Factor |
|---|---|---|---|
| Hardness (HV) | 1500-2500 | 600-900 | 2-3x |
| Wear rate (mm3/Nm) | 1-5 x 10^-7 | 10-50 x 10^-7 | 5-10x |
| Friction coefficient | 0.3-0.5 | 0.5-0.8 | 40% reduction |
| Corrosion resistance | Excellent | Good | Enhanced |
| Thermal conductivity | Moderate | High | Reduced |
The improved wear resistance is attributed to the combined effects of the hard TiB2 particles resisting abrasive penetration, the refined microstructure inhibiting crack propagation, and the formation of protective oxide films during sliding. In corrosive-abrasive environments, the TiB2 phase provides additional chemical stability that reduces the synergistic degradation observed in purely metallic coatings.
Process Parameters and Quality Control
The plasma arc cladding parameters must be carefully optimized to achieve the desired balance between coating thickness, dilution, and microstructural quality. Key parameters include plasma current, powder feed rate, travel speed, arc voltage, and shielding gas flow rate.
| Parameter | Typical Range | Effect on Coating |
|---|---|---|
| Plasma current | 100-300 A | Controls melting depth and dilution |
| Powder feed rate | 200-600 g/min | Controls coating thickness per pass |
| Travel speed | 200-800 mm/min | Controls heat input and cooling rate |
| Arc voltage | 20-35 V | Controls arc stability and penetration |
| Shielding gas (Ar) | 8-15 L/min | Prevents oxidation |
Dilution control is a critical aspect of PTA cladding quality. Excessive dilution reduces the TiB2 content in the coating and introduces base metal elements that can form undesirable phases. A dilution rate of 10-20% is typically targeted for TiB2-metallic ceramic coatings. Multi-pass cladding with progressive dilution reduction in subsequent passes is a common practice to achieve uniform composition throughout the coating thickness.
Engineering Practice Insights
The application of TiB2-metallic ceramic coatings is particularly beneficial for components operating in severe abrasive and corrosive environments, such as pump impellers, valve seats, and extrusion dies. The coating thickness is typically limited to 0.2-0.5 mm per pass, with total coating thicknesses of 0.5-2.0 mm achievable through multi-pass deposition.
A key challenge in engineering practice is maintaining consistent powder quality and feed stability. TiB2 particles are dense and can cause feeding inconsistencies in standard powder feeders. The use of centrifugal powder feeders with optimized powder blend particle size distribution (typically 15-45 micrometers) is recommended to ensure uniform coating composition.
The study highlights that the optimal TiB2 content in the powder blend is approximately 30-50 wt%. Below this range, the wear resistance improvement is marginal. Above this range, the coating becomes brittle and susceptible to spalling under impact loading. The selection of the matrix composition (Ni-based, Co-based, or Fe-based) should be matched to the operating environment and the base substrate material to ensure metallurgical compatibility.