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Introduction
The wear and tear of machine parts is an unavoidable reality across every industry. Components are constantly exposed to extreme temperatures, high pressures, friction, corrosion, and continuous production demands. When a critical part fails or wears beyond tolerance, the traditional response has been costly replacement—often involving significant downtime and expense. Thermal spray technology offers a proven alternative. This article explores the role of thermal spray in repairing, reconditioning, and protecting machine parts, covering common applications, typical coating materials, and the parts most suited to this cost-effective approach.
Why Thermal Spray for Machine Parts?
Thermal spray coating is a process that heats coating material—in powder or wire form—to a molten or semi-molten state and propels it onto a receiving component using gas or air. This process can be repeated to build multiple layers to precise specifications, restoring worn surfaces or enhancing their properties.
The benefits are substantial. Thermal spray repair is significantly more cost-effective than replacing components. It can extend the service life of equipment, reducing long-term costs and preventing downtime. The process is also highly precise, treating only the areas that need attention, and highly versatile, compatible with almost any metal and a wide range of coating materials.
For parts that wear quickly and have a high replacement frequency, applying high-hardness, high-durability coatings extends lifespan and reduces maintenance frequency. In some cases, it’s even possible to improve part performance by functionalizing only the surface, without switching to expensive base materials.
Common Applications of Thermal Spray for Machine Parts
Thermal spray repair and enhancement are requested across virtually every industry. Common applications include:
Repair and Reconditioning
This is one of the most requested services. Worn components—including turbine components, engine parts, and shaft journals—can be restored to their original dimensions. For example:
- A worn low carbon steel sleeve was repaired by wire spraying, achieving a proven durability of over 3 years under demanding service conditions.
- An armature shaft’s worn parts were repaired with a 0.3mm thick Molybdenum coating, providing proven endurance of over 5 years.
- Worn crankcases and cylinder heads from internal combustion engines can be rebuilt using a bond layer of Ni-Al alloy followed by a buildup layer of stainless steel, allowing safe reuse and reducing waste.
Wear Protection
For parts subject to abrasion, erosion, or adhesive wear, thermal spray coatings create a durable barrier that prevents further degradation. Representative coatings include WC-based, NiCr-based, and self-fluxing alloy-based materials. Suitable parts include rolls, shafts, sleeves, sliding parts, and powder transport sections.
Corrosion Protection
Materials in corrosive environments degrade over time. Thermal spray applies corrosion-resistant coatings that prolong equipment life. Common materials include zinc, aluminum, stainless steels, and nickel-chromium alloys.
Thermal Barrier and Oxidation Resistance
Components exposed to high temperatures can lose structural integrity. Thermal spray creates thermal barriers to protect against heat damage. For example, machine components in gas turbines can receive coatings combining oxidation-resistant and mechanical wear-resistant materials to reduce erosion and extend longevity.
Anti-Sticking and Functional Surfaces
For parts like rubber machine panels, thermal spray coatings can provide anti-sticking and abrasion resistance, improving performance and service life. Other functional coatings can offer low friction, electrical conductivity, or insulation.
Common Machine Parts Suitable for Thermal Spray
Typical Coating Materials for Machine Parts
Metals and Alloys
- Molybdenum (Mo) – Used for rebuilding worn shafts and providing self-bonding properties.
- 13Cr Stainless Steel – Provides good wear resistance (Hardness HV 420–500) for repair buildup layers.
- Nickel-Chromium (Ni-Cr) – Offers corrosion and oxidation resistance, Hardness HV 250–360.
- Ni-Al Alloys – Often used as bond coats to improve adhesion between the substrate and subsequent buildup layers.
Ceramics
- Alumina (Al2O3) – Hardness HV 750–1,500, excellent wear resistance.
- Chromia (Cr2O3) – Hardness HV 900–1,600, exceptional wear and corrosion resistance, used on sealing surfaces.
- Alumina-Titania (Al2O3-TiO2) – Hardness HV 740–1,180, reduced friction and good wear resistance.
- Titania (TiO2) – Hardness HV 600–750.
- Zirconia (ZrO2) – Hardness HV 400–500, used for thermal barriers.
Cermets and Carbides
- WC-based (Tungsten Carbide) – High hardness and wear resistance, widely used for wear-prone parts.
- Cr3C2-NiCr – Chromium carbide with nickel-chromium binder, high-temperature wear resistance.
Self-Fluxing Alloys
- Ni-Cr-B-Si system – Hard, dense coatings with self-deoxidizing properties, used for trolley wires and wear-resistant applications.
Process Considerations
The application of thermal spray coatings to machine elements is governed by industry standards. The AWS C2.19/C2.19M:2023 Specification defines requirements for OEM and repair applications, including HVOF coatings as an alternative to hard chrome plating. Key aspects include:
- Equipment requirements and surface preparation procedures
- In-process quality control checkpoints
- Qualification procedures and documentation
- Minimum training requirements for operators and inspectors
The DIN EN ISO 14921 standard also outlines procedures for applying thermally sprayed coatings to engineering components, whether to reclaim worn parts or enhance surface properties for specific purposes.
When selecting a coating solution, specifications should be proposed based on usage environment, counterpart materials, and wear conditions. The proper selection of process and materials for the intended purpose is critical.
Conclusion
Thermal spray technology offers a practical, cost-effective means of extending the service life of machine parts—whether by repairing worn components, protecting against future wear, or enhancing surface properties for demanding applications. From simple rebuilds of shafts and sleeves to sophisticated oxidation-resistant coatings for turbine components, the versatility of thermal spray makes it an essential tool for maintenance and manufacturing teams. By understanding the common applications, coating materials, and process standards, industry professionals can make informed decisions to reduce downtime, cut costs, and keep operations running smoothly.
