To learn the knowledge of thermal spray technology, it is necessary to master a very comprehensive knowledge. It is far from enough to understand the material alone. To make a good coating, we not only understand the material, but also understand the working conditions of products, equipment and spraying process.

Today we will first talk about the spraying materials we use.

Spray coating materials are also made by powder metallurgy technology, but the performance of the coating sprayed with powder is very different from the performance of pure material science. Here, I will tell a small story.

A customer once requested me to measure the hardness of a tungsten carbide coating using a Rockwell hardness tester, demanding it reach above 70 with full video documentation for verification. This request posed a significant challenge. My supervisor, lacking technical expertise, had already approved the task, insisting I must fulfill the customer's requirements under any circumstances. I argued that measuring this thin coating with a Rockwell hardness tester would never achieve the desired result due to improper methodology. When questioned why, I explained the coating's insufficient thickness for standard testing. The customer then proposed spraying it to 1 mm for measurement, which I deemed inadequate. After further negotiation, I reluctantly sprayed the coating to 2 mm. However, even at this thickness, the measurement still failed to reach HRC70.

The customer's requirements reveal several critical misunderstandings: 1. Tungsten Carbide Hardness: as we know that tungsten steel is indeed extremely hard (typically above HRC80), the customer mistakenly assumed tungsten carbide coatings could easily exceed HRC70. This confusion arises from conflating coating properties with material characteristics. Although tungsten carbide grain size themselves can achieve HV2000 hardness, this doesn't guarantee equivalent performance when combined with coatings. The structural composition of coatings differs fundamentally from alloys. Nevertheless, achieving HRC70+ is achievable. 2. Measurement Methodology: Microhardness testers are the gold standard for coating evaluation, not Rockwell hardness testers. While ultrasonic hardness meters can measure surface hardness, they're less precise than microhardness instruments. The client's flawed methodology inevitably leads to incorrect results. 3. Coating Thickness: We recommend maintaining tungsten carbide coatings under 0.3㎜ thickness. Exceeding this limit increases cracking risks due to internal stress release. However, this doesn't prevent thick coatings. My experimental fixtures with accumulated tungsten carbide coatings up to 32mm never cracked – these cases demonstrate sufficient thickness. Don't dismiss unverified claims based on superficial knowledge.

What I want to emphasize is that our selection of coatings shouldn't be based solely on material knowledge, but rather on an understanding of the coating itself. Coatings are typically porous layered structures composed of individual powder particles stacked together. Their cohesion relies on compressive stress during the spraying process and the inherent fusion of coating materials themselves—a physical bond distinct from molecular or atomic forces.

Commonly used coating materials include carbides, oxides, nickel-based alloys, and cobalt-based alloys. Most of these materials exhibit wear-resistant properties, with selection based on specific operational conditions. However, nickel-based and cobalt-based alloys often get overlooked. Among the numerous products I've developed, I never recommend tungsten carbide to customers. While it's more expensive, its performance remains limited to basic wear resistance. Yet many applications involve not only wear but also high-temperature exposure, corrosion, and oxidation challenges.

I've put together a simple chart:

There are many materials that cannot be listed one by one, details can be consulted by e Zhuzhou Velavision.