Robotic arm applying a protective coating in an industrial setting.

HVOF Spray Distance: How Does It Impact Coating Quality?

Nico Varela in Tech & Innovation December 2025 • 4 min read.

"Exploring the relationship between spray distance and the properties of WC-12Co coatings created by High-Velocity Oxygen-Fuel (HVOF) spraying."

Thermal-sprayed WC-Co coatings are widely recognized for enhancing the hardness and wear resistance of tools and cutting wheels. Techniques like Detonation Spray Coating (DSC), Air Plasma Spraying (APS), and High-Velocity Oxygen-Fuel (HVOF) spraying are commonly employed to apply these coatings.

Achieving optimal WC-coatings hinges on minimizing WC decarburization and ensuring high coating density. These factors play a pivotal role in determining the final performance and durability of the coated products.

HVOF spraying is favored for its lower flame temperature compared to APS and DSC, yet complete elimination of coating decarburization remains a challenge. The characteristics of thermally sprayed WC coatings are influenced by multiple parameters, including Co content, nozzle size, gas ratio, and, notably, spray distance. Given the significant impact of spray distance on decarburization, this study investigates its effects on the microstructure and properties of WC-12Co coatings.

The Sweet Spot: How Spray Distance Affects Coating Properties

Robotic arm applying a protective coating in an industrial setting.

The study examined WC-12Co coatings applied using HVOF at varying spray distances: 300 mm, 385 mm, and 450 mm. Microscopic analysis revealed a clear trend: increased spray distance led to higher coating porosity. Specifically, porosity values were 0.34% at 300 mm, 0.87% at 385 mm, and 1.68% at 450 mm.

X-ray diffraction patterns indicated the presence of WC, W, and Co, along with W₂C and CoWC phases. These findings suggest that increasing the spray distance accelerates the decarburization process within the coatings.

Porosity: Increases with greater spray distance.
Decarburization: Accelerated at longer spray distances.
Hardness and Fracture Toughness: Both decrease as spray distance increases.

Microhardness tests further demonstrated that hardness and fracture toughness diminished with increased spray distance. This decline isn't solely attributable to porosity; the degree of decarburization also plays a significant role in influencing these mechanical properties.

Optimizing HVOF Spraying: Key Takeaways

This study underscores the critical role of spray distance in determining the microstructure and mechanical behavior of HVOF-sprayed WC-12Co coatings. The key findings highlight a trade-off: while closer spray distances promote denser, harder coatings with better fracture toughness, minimizing decarburization requires careful optimization of the entire spraying process.

Understanding these relationships allows engineers to fine-tune HVOF spraying parameters, tailoring coating properties to specific application requirements. For instance, applications demanding high wear resistance may benefit from coatings sprayed at closer distances, despite the need to control decarburization through other means.

Further research could explore the use of alternative fuel mixtures or modified nozzle designs to mitigate decarburization at closer spray distances, ultimately leading to improved performance and longevity of WC-12Co coatings in demanding industrial environments.

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This article is based on research published under:

DOI-LINK: 10.1166/jnn.2018.14990, Alternate LINK

Title: Effect Of The Spray Distance On The Properties Of High Velocity Oxygen-Fuel (Hvof) Sprayed Wc-12Co Coatings

Subject: Condensed Matter Physics

Journal: Journal of Nanoscience and Nanotechnology

Publisher: American Scientific Publishers

Authors: Seong-Sik Bang, Young-Chul Park, Ji-Woon Lee, Soong-Keun Hyun, Tae-Bum Kim, Jae-Kwon Lee, Jung-Woo Han, Taek-Kyun Jung

Published: 2018-03-01

Everything You Need To Know

What is High-Velocity Oxygen-Fuel (HVOF) spraying?

High-Velocity Oxygen-Fuel (HVOF) spraying is a thermal spray technique used to apply coatings, such as WC-12Co, onto surfaces. It involves propelling heated particles onto a substrate to create a coating. The process is favored due to its lower flame temperature compared to methods like Detonation Spray Coating (DSC) and Air Plasma Spraying (APS), which helps to mitigate certain issues. The choice of HVOF is strategic, particularly when aiming to deposit materials with minimal degradation.

Why is spray distance important in HVOF spraying?

The spray distance in High-Velocity Oxygen-Fuel (HVOF) spraying is the distance between the spray gun nozzle and the surface being coated. It's a crucial parameter because it significantly affects the coating's properties. Varying the spray distance changes the time and conditions under which the particles impact the substrate. This impacts the coating's hardness, porosity, and the extent of decarburization, thereby influencing its performance and durability.

What is decarburization, and why does it matter in WC-12Co coatings?

Decarburization, in the context of WC-12Co coatings created by High-Velocity Oxygen-Fuel (HVOF) spraying, refers to the loss of carbon from the tungsten carbide (WC) phase. This process is detrimental because it weakens the coating. The study shows that longer spray distances increase decarburization, leading to the formation of W₂C and other phases, reducing the coating's hardness and fracture toughness.

What does porosity mean in WC-12Co coatings, and what's its significance?

Porosity in WC-12Co coatings refers to the presence of voids or empty spaces within the coating's structure. Higher porosity, as observed with increased spray distances in HVOF spraying, typically compromises the coating's mechanical properties. The presence of these voids weakens the coating, making it less resistant to wear and reducing its overall durability. The study shows a direct relationship, with longer spray distances increasing porosity.

How can the spray distance be optimized in HVOF spraying to get the best results?

Optimizing the spray distance in High-Velocity Oxygen-Fuel (HVOF) spraying of WC-12Co coatings is essential for achieving the desired coating properties. The research suggests that while shorter spray distances can lead to denser and harder coatings, minimizing decarburization requires careful control. Therefore, the key takeaway is that the spray distance must be optimized to balance coating density, hardness, and the reduction of undesirable phases like W₂C, to ensure the coated product performs well in demanding applications. This often involves finding a sweet spot that provides the best overall results.