Microstructure and Properties of TiB2-Metal Ceramic Coating by Plasma Arc Cladding

Overview of the Topic

Metal ceramic coatings, which combine the hardness and wear resistance of ceramics with the toughness and thermal conductivity of metals, represent a promising class of surface engineering materials. This literature investigates the microstructure, mechanical properties, and wear behavior of TiB2-reinforced metal ceramic coatings produced by plasma transferred arc (PTA) cladding. TiB2 is selected for its exceptional hardness (HV ~3000), thermal stability, and chemical inertness, making it an ideal reinforcement phase for high-temperature wear applications.

Core Technical Content

The study examines PTA cladding of TiB2 particle-reinforced coatings on steel substrates, varying the TiB2 content and particle size to optimize the coating performance. The base matrix alloy is typically a nickel-based or iron-based superalloy that provides a ductile binder phase for the ceramic reinforcement. The following table summarizes the coating compositions investigated:

Coating Designation	TiB2 Content (wt%)	Particle Size (μm)	Matrix Alloy	Process Parameters
Coating A	10	10–30	Ni-20Cr-5Mo	400 A, 32 V, 2 m/min
Coating B	20	10–30	Ni-20Cr-5Mo	400 A, 32 V, 2 m/min
Coating C	30	10–30	Ni-20Cr-5Mo	400 A, 32 V, 2 m/min
Coating D	20	5–15	Ni-20Cr-5Mo	400 A, 32 V, 2 m/min
Coating E	20	10–30	Fe-15Cr-5Ni-3Mo	400 A, 32 V, 2 m/min

Microstructural Analysis

The microstructural examination reveals several key features that govern the coating performance:

Matrix microstructure: The Ni-based matrix exhibits a dendritic structure with primary austenite dendrites and secondary carbide precipitation in the interdendritic regions. The Fe-based matrix shows a martensitic structure with retained austenite.
TiB2 particle distribution: TiB2 particles are uniformly distributed throughout the coating with minimal segregation, indicating good mixing during the PTA process. The particles remain largely intact with limited dissolution, confirming the thermal stability of TiB2 at PTA processing temperatures.
Interface bonding: The TiB2 particles exhibit good metallurgical bonding with the matrix, with no visible gaps or cracks at the interface. However, at higher TiB2 contents (>30 wt%), some particle agglomeration and microcracking at the interface are observed.
Bond line microstructure: The transition zone between the coating and substrate shows a gradient of microstructure with progressive dilution of the coating alloy into the substrate. The dilution rate is typically 15–25% for PTA cladding.

Mechanical Properties and Wear Performance

The mechanical and tribological properties of the coatings are summarized in the following table:

Property	Substrate (42CrMo)	Coating A	Coating B	Coating C	Coating D	Coating E
Hardness HV0.3	280	620	750	820	780	680
Wear rate (mm³/N·m)	45	12	5.2	3.8	4.5	8.0
Bond strength (MPa)	—	320	295	260	305	310
Impact energy (J)	45	18	12	8	15	20
Thermal conductivity (W/m·K)	28	15	12	10	13	14

The results demonstrate that increasing TiB2 content improves hardness and wear resistance but reduces toughness and bond strength. The optimal composition for most applications is Coating B (20 wt% TiB2) with a particle size of 10–30 μm, which provides a good balance of wear resistance and toughness.

Wear Mechanism Analysis

The wear mechanism of the TiB2-reinforced coatings is identified as a combination of abrasive wear, adhesive wear, and delamination wear, with the dominant mechanism depending on the TiB2 content and particle size:

Low TiB2 content (10 wt%): Adhesive wear dominates, with material transfer from the coating to the counterface. The coating is too soft to resist adhesive bonding.
Optimal TiB2 content (20 wt%): Abrasive wear is the primary mechanism, with the hard TiB2 particles providing effective resistance to material removal. The matrix protects against particle pull-out.
High TiB2 content (30 wt%): Delamination wear becomes significant due to reduced matrix ductility and increased internal stresses around the ceramic particles.

Engineering Practice and Application Prospects

The study identifies several practical applications for TiB2-reinforced PTA coatings:

Turbine engine components: Hot section components such as turbine disks and blades benefit from the high-temperature wear resistance and thermal stability of TiB2 coatings.
Mining and construction equipment: Wear parts such as bucket teeth, crusher hammers, and conveyor rollers experience severe abrasive wear that can be mitigated by TiB2 coatings.
Chemical processing: TiB2's chemical inertness makes it suitable for coatings in corrosive and abrasive environments such as slurry pumps and mixers.
Aerospace landing gear: The combination of wear resistance and load-bearing capacity makes TiB2 coatings suitable for landing gear components.

Study Insights and Implications

This literature demonstrates that TiB2-reinforced metal ceramic coatings produced by PTA cladding offer a compelling solution for high-temperature wear applications where conventional hardfacing alloys fall short. The key insight is that the optimal TiB2 content lies in the range of 15–25 wt%, where the coating achieves a hardness exceeding 700 HV while maintaining acceptable toughness and bond strength. Engineers should be aware that the particle size distribution significantly affects coating quality; finer particles (5–15 μm) improve hardness but may increase porosity, while coarser particles (>50 μm) reduce hardness but improve toughness. The future direction points toward nanostructured TiB2 particles and multi-ceramic reinforcement systems (TiB2 + WC + Cr3C2) that could further enhance wear resistance while maintaining structural integrity. The PTA process remains the preferred method for depositing these coatings due to its ability to handle large workpieces and produce thick coatings with acceptable quality.