CVD vs Thermal Spray: Advantages and limitations for harsh environments

When selecting a coating method for critical components in challenging industrial environments, engineers are often presented with two advanced options: Chemical Vapour Deposition (CVD) and thermal spray techniques, such as High Velocity Oxy-Fuel (HVOF). 

Both methods aim to protect components from wear, erosion and corrosion, but they come with distinct advantages and limitations. This article delves into these technologies, offering insights to aid in making informed decisions.

An overview of thermal spray coatings 

Thermal spray processes, like HVOF, involve spraying molten or semi-molten particles at high velocities onto the target surface. The coatings, typically tungsten carbide with a cobalt binder (WC/Co), are widely used in oil and gas, aerospace and other industries requiring robust wear protection.

Strengths

• Material options: Thermal spray can produce thick, hard coatings resistant to abrasion in harsh environments. 
• Established technology: The process is often a reliable choice as it is widely available and well-documented.
• Cost-effectiveness: It is suitable for coating large, simple external surfaces in a cost-effective way.

Limitations

• Geometry restrictions: Thermal spray coatings are line-of-sight processes, making it difficult or impossible to coat internal surfaces or complex shapes​​.
• Porosity and binder wear: HVOF coatings are often porous and rely on cobalt binders, which degrade in corrosive environments. This leads to extremely abrasive surfaces that are described by engineers as a “cheese-grater” effect that increases wear for neighbouring components such as elastomeric seals. 
• Surface finish: Spray coatings produce rough as-applied surfaces and require grinding to achieve smooth finishes. This process is limited to simple geometries like cylinders​​ to get the right surface finish. 

An overview of CVD coatings  

Chemical Vapour Deposition creates coatings by crystallising material from gas-phase precursors directly onto components. Nanostructured tungsten carbide (WC/W) coatings are a prime example, providing unparalleled protection against wear, erosion and corrosion.

Strengths

• Geometry versatility: CVD coatings are non-line-of-sight, enabling the uniform application of the coating on internal surfaces and complex shapes such as valves and pump cylinders​​​.
• Smooth, pore-free coating: The coatings are dense and non-porous, providing superior corrosion resistance without the need for additional sealing​​​.
• Superior performance: CVD coatings greatly extend the lifespan of components in harsh environments with a number of specific types for different applications.
  • Hardness up to 1600 Hv and toughness that resists cracking and spalling under impact​​.
  • Resistance to erosive particles and water droplet impacts, making them ideal for high-stress components​.
  • Up to 12x abrasion resistance compared to alternative coatings like hard chrome, and 6x the wear resistance of HVOF coatings​​​.

• Environmental compliance: Unlike traditional coatings, CVD processes are fully REACH-compliant, aligning with regulations that restrict toxic materials like hexavalent chromium​​.

Limitations

• Initial cost: CVD processes are typically more expensive per component compared to thermal spray, especially for large-scale applications. That said, the cost savings from extended component lifespan and reduced maintenance can more than make up for this initial expenditure. 
• Process time: The deposition process is slower and more complex than thermal spray​. Although once proven, parts can be produced in large batches with often 100’s parts per cycle, resulting in lead times of a week or less. 

Quick coating comparison 

Making the right choice - why Hardide CVD coatings lead the way

Choosing between CVD and thermal spray coatings hinges on the specific demands of the application. While thermal spray is effective for simple, external surfaces, CVD shines in coating complex geometries and environments requiring top-tier performance and environmental compliance. Engineers prioritising long-term durability and regulatory alignment may find CVD the superior option despite its higher initial investment.

At Hardide, we provide CVD coatings that deliver exceptional wear, erosion, and corrosion resistance, even on complex geometries and internal surfaces. Our smooth, seal-friendly finish helps extend component lifespan, and with full compliance to REACH regulations, we offer a high-performance, sustainable solution for industries like aerospace and oil and gas.