Thermal Spray Coating Processes

Understanding Thermal Spray Coating Processes in Modern Manufacturing

Thermal spray coating represents one of the most versatile and widely adopted surface engineering technologies in contemporary manufacturing. For industries across Atlantic Canada—from offshore oil and gas operations to marine vessel maintenance and pulp and paper production—these coating processes provide critical protection against wear, corrosion, and thermal degradation. As manufacturing facilities throughout Nova Scotia and the Maritime provinces continue to modernise their operations, understanding the fundamentals and applications of thermal spray technology becomes increasingly essential for engineers and technical managers alike.

At its core, thermal spray coating involves heating a material—whether in powder, wire, or rod form—to a molten or semi-molten state, then propelling it at high velocity onto a prepared substrate surface. Upon impact, the particles flatten, solidify, and mechanically bond to create a protective coating layer. This seemingly straightforward concept encompasses numerous sophisticated processes, each offering distinct advantages for specific applications and operating environments.

Primary Thermal Spray Coating Methods and Their Characteristics

The thermal spray family includes several distinct processes, each characterised by different heat sources, particle velocities, and resulting coating properties. Selecting the appropriate method requires careful consideration of the base material, coating material, intended application, and economic factors.

Flame Spray Coating

Flame spraying represents the oldest and most economical thermal spray process, utilising the combustion of fuel gases—typically acetylene or propane with oxygen—to melt coating materials. Operating at temperatures between 2,500°C and 3,000°C, flame spray systems can deposit a wide range of materials including metals, ceramics, and polymers. Particle velocities typically range from 80 to 100 metres per second, producing coatings with porosity levels of 10-15% and bond strengths of 15-30 MPa.

This method proves particularly suitable for applications requiring corrosion protection, dimensional restoration, and moderate wear resistance. Many Maritime manufacturing facilities employ flame spray coating for rebuilding worn shafts, protecting steel structures in coastal environments, and applying zinc or aluminium coatings for cathodic protection.

Arc Wire Spray

Electric arc spraying uses two consumable wire electrodes that are fed together, creating an electric arc that melts the wire tips. Compressed air or inert gas then atomises and propels the molten material onto the substrate at velocities of 100-150 metres per second. This process achieves deposition rates of 6-40 kilograms per hour, making it exceptionally productive for large-scale applications.

Arc wire spray excels in applying corrosion-resistant coatings to bridges, marine structures, and industrial equipment—applications highly relevant to Nova Scotia's infrastructure needs. The Halifax Harbour and numerous coastal facilities throughout Atlantic Canada benefit from zinc-aluminium pseudo-alloy coatings applied through this efficient process.

Plasma Spray Coating

Plasma spraying generates extremely high temperatures—between 10,000°C and 20,000°C—by ionising an inert gas (typically argon or nitrogen) with an electric arc. These temperatures enable the deposition of virtually any material that melts without decomposing, including high-melting-point ceramics such as zirconia, alumina, and chromium oxide.

Particle velocities in atmospheric plasma spray (APS) systems reach 200-300 metres per second, producing coatings with 3-8% porosity and bond strengths of 20-70 MPa. For applications requiring superior coating quality, vacuum plasma spray (VPS) or low-pressure plasma spray (LPPS) systems operate in controlled atmospheres to minimise oxidation and produce near-theoretical density coatings.

High-Velocity Oxygen Fuel (HVOF) Spraying

HVOF technology represents a significant advancement in thermal spray coating, achieving particle velocities of 500-800 metres per second through the continuous internal combustion of fuel (kerosene, propylene, or hydrogen) with oxygen. These extreme velocities produce exceptionally dense coatings with porosity levels below 1% and bond strengths exceeding 70 MPa.

The lower operating temperatures (2,500°C-3,100°C) compared to plasma spray reduce thermal decomposition of carbide materials, making HVOF ideal for applying tungsten carbide-cobalt (WC-Co) and chromium carbide-nickel chrome (Cr₃C₂-NiCr) coatings. These materials provide outstanding wear resistance for components in mining equipment, hydraulic cylinders, and paper mill rolls—all critical applications in Atlantic Canada's resource-based industries.

Coating Materials and Their Selection Criteria

The effectiveness of any thermal spray coating depends significantly on appropriate material selection. Engineers must consider the operating environment, mechanical stresses, chemical exposure, and temperature ranges when specifying coating materials.

Metallic Coatings

Metallic thermal spray coatings encompass pure metals, alloys, and composite materials. Common applications include:

• Zinc and aluminium coatings: Provide sacrificial corrosion protection for steel structures, with service lives exceeding 20 years in marine environments when properly applied at thicknesses of 150-250 micrometres

• Nickel-based alloys: Including Inconel and Hastelloy variants, these coatings offer exceptional high-temperature oxidation and corrosion resistance for components operating above 800°C

• Stainless steel coatings: Types 316L and 420 provide economical corrosion resistance for food processing equipment and chemical handling systems

• Bronze and babbit alloys: Used extensively for bearing surfaces and wear-resistant applications requiring conformability

Ceramic Coatings

Ceramic thermal spray coatings provide thermal barriers, electrical insulation, and wear resistance in demanding applications:

• Yttria-stabilised zirconia (YSZ): The predominant thermal barrier coating material, capable of reducing metal surface temperatures by 100-300°C in gas turbine applications

• Aluminium oxide (Al₂O₃): Offers excellent wear resistance and electrical insulation with dielectric strength exceeding 20 kV/mm

• Chromium oxide (Cr₂O₃): Provides exceptional hardness (1,800-2,200 HV) and resistance to fretting wear

• Titanium dioxide (TiO₂): Used in photocatalytic and biomedical applications requiring specific surface chemistry

Cermet and Carbide Coatings

Cermet coatings combine ceramic hardness with metallic toughness, creating materials ideally suited for severe wear applications. Tungsten carbide-cobalt (WC-12Co and WC-17Co) coatings achieve hardness values of 1,000-1,400 HV and are extensively used in Atlantic Canada's mining and forestry equipment. Chromium carbide-nickel chrome coatings maintain their properties at temperatures up to 870°C, making them suitable for high-temperature sliding wear applications in power generation facilities.

Surface Preparation and Quality Control Standards

The success of any thermal spray coating application depends critically on proper surface preparation. Industry standards—including CSA W117.2 in Canada and ISO 2063 internationally—specify detailed requirements for achieving adequate coating adhesion and performance.

Substrate Preparation Requirements

Surface preparation typically involves several sequential steps:

• Degreasing: Removal of oils, greases, and organic contaminants using solvent cleaning or alkaline solutions

• Abrasive blasting: Grit blasting to achieve surface roughness (Ra) values of 3-12 micrometres, depending on coating requirements. Angular aluminium oxide or steel grit of specified mesh sizes creates the mechanical keying necessary for coating adhesion

• Masking: Protection of surfaces not requiring coating using heat-resistant tapes, metal shields, or custom fixtures

• Preheating: Substrate heating to 80-150°C removes moisture and reduces thermal shock during coating application

Quality control measures must address both surface preparation verification and coating inspection. Surface cleanliness standards per SSPC-SP 5 (white metal blast) or SSPC-SP 10 (near-white blast) ensure adequate contamination removal. Surface profile measurements using replica tape or stylus profilometers confirm proper anchor pattern development.

Coating Quality Assessment

Post-application quality verification includes multiple inspection techniques:

• Thickness measurement: Magnetic or eddy current gauges verify coating thickness within specified tolerances, typically ±10% of the nominal value

• Adhesion testing: Pull-off adhesion tests per ASTM C633 or portable adhesion testers confirm bond strength meets minimum requirements

• Microstructural analysis: Metallographic examination reveals porosity, oxide content, and interface characteristics

• Hardness testing: Microhardness measurements verify coating mechanical properties

• Non-destructive examination: Liquid penetrant or ultrasonic testing identifies surface-connected defects or delamination

Industrial Applications in Atlantic Canada

The diverse industrial base throughout Nova Scotia and the Maritime provinces presents numerous opportunities for thermal spray coating applications. Understanding these specific use cases helps engineers specify appropriate coating solutions for their particular challenges.

Marine and Offshore Industries

Atlantic Canada's extensive coastline and offshore energy sector demand robust corrosion protection strategies. Thermal spray aluminium (TSA) and thermal spray zinc (TSZ) coatings provide long-term protection for offshore platforms, ship hulls, and harbour infrastructure. These metallic coatings, applied at thicknesses of 200-350 micrometres, offer cathodic protection that actively sacrifices to protect the underlying steel substrate.

Marine propulsion components—including propeller shafts, rudder stocks, and stern tube bearings—benefit from HVOF-applied tungsten carbide coatings that resist the combined effects of seawater corrosion and abrasive wear from suspended sediments.

Pulp and Paper Industry

Nova Scotia's pulp and paper facilities rely extensively on thermal spray coatings for roll restoration and protection. Paper machine rolls, dryer cylinders, and suction rolls experience severe wear from fibrous materials and corrosive process chemicals. Ceramic and cermet coatings extend service intervals from months to years, significantly reducing maintenance costs and production downtime.

Mining and Mineral Processing

The region's mining operations face extreme wear challenges from abrasive materials. Crusher components, conveyor pulleys, and slurry pump impellers benefit from hard-facing coatings that resist gouging abrasion and erosive wear. HVOF-applied chromium carbide and tungsten carbide coatings routinely extend component service life by factors of three to five compared to uncoated alternatives.

Power Generation and Energy

Thermal spray coatings play essential roles in both traditional and renewable energy systems. Boiler tubes in thermal power plants receive chromium carbide coatings to resist high-temperature erosion from fly ash. Wind turbine components—including main bearings and gearbox elements—utilise HVOF coatings to combat the demanding combination of high loads and limited lubrication access.

Economic Considerations and Return on Investment

Implementing thermal spray coating solutions requires careful economic analysis comparing initial costs against lifecycle benefits. While coating application represents an upfront investment, the extended service life and reduced maintenance requirements typically deliver compelling returns.

Consider a typical hydraulic cylinder rod application: an uncoated chrome-plated rod might require replacement every 12-18 months in aggressive service conditions. An HVOF-applied tungsten carbide coating, despite higher initial cost, routinely achieves service lives of 5-7 years—reducing total ownership costs by 40-60% while minimising production interruptions.

For structural corrosion protection, thermal spray metallic coatings applied to bridges and marine structures demonstrate lifecycle costs 30-50% lower than traditional paint systems over 25-year service periods, even accounting for periodic sealer applications.

Emerging Technologies and Future Developments

The thermal spray industry continues evolving through technological advancement and process optimisation. Cold spray technology—which deposits materials at supersonic velocities without melting—enables coating of temperature-sensitive substrates and oxygen-sensitive materials. High-velocity air fuel (HVAF) systems offer HVOF-equivalent performance with reduced operating costs and environmental impact.

Advanced process monitoring systems incorporating real-time particle diagnostics ensure consistent coating quality through automated parameter adjustment. These developments promise improved coating reliability and expanded application possibilities for Atlantic Canadian industries.

Partner with Experienced Engineering Professionals

Successfully implementing thermal spray coating solutions requires comprehensive engineering expertise—from material selection and process specification through quality assurance and performance verification. The technical complexities involved demand experienced guidance to optimise coating performance while managing costs effectively.

Sangster Engineering Ltd., based in Amherst, Nova Scotia, provides professional engineering services to manufacturing and industrial clients throughout Atlantic Canada. Our team understands the unique challenges facing regional industries and delivers practical engineering solutions tailored to Maritime operating conditions. Whether you're developing coating specifications for new equipment, troubleshooting coating failures, or seeking to optimise existing maintenance programmes, we offer the technical expertise and local knowledge to support your success.

Contact Sangster Engineering Ltd. today to discuss how our engineering services can help you leverage thermal spray coating technology for improved equipment reliability and reduced lifecycle costs. Let our experience work for your operation's benefit.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.