Key Technology Progress and Applications in Surfacing and Thermal Spraying

Literature Overview

This comprehensive review by Huang Zhiquan and colleagues, published in "China Surface Engineering" (Vol. 39, No. 1, 2026), provides a state-of-the-art assessment of surfacing and thermal spraying technologies over the past decade. The authors from the Zhengzhou Mechanical Research Institute, Xi'an Jiaotong University, Beijing University of Technology, the Chinese Mechanical Engineering Society, and the Zhejiang Baton Welding Technology Research Institute offer a panoramic view of materials development, process innovation, equipment advancement, and industrial applications. This paper serves as an authoritative reference for engineers seeking to understand current capabilities and future directions in surface engineering.

Surfacing Technology Development

Arc Surfacing

Arc surfacing remains the most widely used industrial surfacing method due to its flexibility, relatively low equipment cost, and ability to deposit thick layers. Key developments include:

Technology	Key Feature	Deposition Rate	Typical Application
Conventional GMAW surfacing	Wire + shielding gas	5–15 kg/h	General wear/corrosion protection
Flux-cored wire surfacing	Self-shielded or gas-shielded flux-cored wire	8–20 kg/h	Heavy-duty wear applications
Submerged arc surfacing (SAW)	Flux-covered, high deposition	20–60 kg/h	Large structural components
Pulsed arc surfacing	Current pulsing for HAZ control	5–15 kg/h	Heat-sensitive substrates
Multi-wire surfacing	Multiple wire feeders	20–50 kg/h	High-productivity applications

The trend toward pulsed arc surfacing is notable, as it enables better control of heat input and dilution, resulting in improved microstructural control and reduced distortion. Multi-wire systems offer significant productivity gains for large-scale industrial applications.

High-Energy-Beam Surfacing

Laser cladding and electron beam surfacing represent the premium segment of surfacing technology:

Laser cladding: Uses a focused laser beam (typically 1–10 kW fiber laser) with powder or wire feed. Key advantages include extremely low dilution (1–5%), precise geometric control, and the ability to deposit high-performance alloys. Recent advances include dual-laser systems for large cross-section deposits and in-situ monitoring using optical pyrometry and acoustic emission.
Electron beam surfacing: Operates in vacuum or controlled atmosphere, offering very high energy density and minimal oxidation. Suitable for reactive metals and high-purity applications.

Special Surfacing Technologies

The review highlights several emerging surfacing approaches:

Technology	Principle	Advantage	Limitation
Friction stir surfacing	Plastic deformation + frictional heating	No melting, low dilution	Limited to ductile substrates
Cold metal transfer (CMT)	Low-energy pulsed arc	Very low heat input	Low deposition rate
Electromagnetic stir-assisted surfacing	EM stirring of molten pool	Homogeneous composition	Special equipment required
Hybrid laser-arc surfacing	Laser + arc combined	High rate + low dilution	Complex equipment

Thermal Spraying Technology Progress

Cold Spray

Cold spray is a solid-state thermal spray process where high-velocity particles (typically 300–1200 m/s) are accelerated through a convergent-divergent nozzle and deposited onto a substrate through plastic deformation. Recent advances include:

Parameter	Typical Range	Impact on Coating
Particle velocity	300–1200 m/s	Higher velocity → better bonding
Substrate temperature	20–200 °C	Elevated T improves bonding
Gas temperature	1000–3000 °C	Affects particle heating and velocity
Spray distance	20–50 mm	Affects particle velocity at impact

Key advantages of cold spray include: no melting (preserves bulk material properties), low residual stress, high coating density (>99%), and the ability to deposit reactive metals (Ti, Al, Mg) without oxidation. Applications include aerospace components, electronics, and high-performance structural repairs.

Plasma Spraying Variants

Variant	Feature	Application
Conventional plasma spraying	Atmospheric plasma, 10–40 kW	General thermal barrier coatings
Low-pressure plasma spraying (LPPS)	Vacuum chamber, reduced oxidation	High-quality TBCs for turbine blades
Suspension plasma spraying	Nanoparticle-containing slurry	Nanocomposite coatings
Atmospheric plasma PVD (APPVD)	Hybrid plasma + PVD	Ultra-dense, high-bond-strength coatings

The suspension plasma spraying (SPS) technique is particularly noteworthy for depositing nanocomposite coatings with enhanced properties. By dispersing nanoparticles (e.g., WC, TiC, Al₂O₃, SiC) in a liquid carrier and atomizing the suspension into the plasma jet, coatings with controlled nanostructure and improved tribological or thermal properties can be achieved.

Exploded Spraying

Exploded spraying (also known as detonation spraying) produces very dense coatings (>99.5%) with excellent adhesion and low porosity. Recent developments include:

Improved detonation gun designs for higher deposition rates
Application to large structural components (pipelines, pressure vessels)
Development of multi-component coatings (e.g., WC-Co-Cr, NiCrAlY)

Application Overview

The review categorizes surface engineering applications by industry:

Industry	Typical Application	Process	Key Requirement
Oil & gas	Pipeline corrosion protection	HVOF, SAW	Low porosity, high adhesion
Power generation	Turbine blade TBC	LPPS, APS	Thermal cycling resistance
Mining	Excavator bucket wear protection	SAW, GMAW	Abrasion resistance
Aerospace	Structural repair	Cold spray, laser cladding	Property retention, NDT compatibility
Automotive	Cylinder bore restoration	HVOF, cold spray	Dimensional accuracy, low stress
Chemical	Reactor liner corrosion protection	Plasma surfacing, HVOF	Chemical inertness
Marine	Propeller cavitation erosion	GMAW, cold spray	Fatigue resistance

Future Development Directions

The authors identify several strategic directions for the field:

High-end equipment surfacing materials: Development of new alloy systems for extreme environments (ultra-high temperature, ultra-high pressure, multi-coupled loading). This includes high-entropy alloys, functionally graded materials, and self-healing coatings.
Composite surfacing technology: Multi-layer and multi-component surfacing systems that combine different functional layers (e.g., bonding layer + transition layer + functional layer) to achieve synergistic property combinations.
Automation, intelligence, and flexibility: The surfacing process is moving toward fully automated systems with real-time monitoring and adaptive control. This includes:

In-situ process monitoring (temperature, dilution, defect detection)
Robot-integrated surfacing systems for complex geometries
Digital twin models for process optimization and quality prediction
data analysis-based parameter optimization

New materials and processes for thermal spraying: Focus on improving coating density, reducing porosity, and developing novel coating systems for emerging applications such as hydrogen energy equipment, nuclear fusion components, and space exploration hardware.
Enhancing coating density: The development of new spraying parameters, substrate preparation methods, and post-treatment techniques to achieve denser, more reliable coatings, particularly for critical applications where porosity can lead to premature failure.

Study Reflection and Implications

This review provides an excellent roadmap for surface engineering professionals. The breadth of coverage—from fundamental materials science to industrial applications—makes it a valuable reference for both researchers and practicing engineers. One key insight is the convergence of surfacing and thermal spraying technologies: both are moving toward similar goals of low dilution, high deposition quality, and intelligent process control. The emphasis on automation and digitalization reflects the broader Industry 4.0 transformation in manufacturing. For engineering practice, the practical implications are clear: engineers must stay current with rapidly evolving process capabilities, and material selection must be based on comprehensive understanding of the service environment rather than traditional empirical approaches. The development of high-entropy alloys and functionally graded coatings represents a paradigm shift that could unlock entirely new application domains in the coming decade.