The Thermal Diffusion (TD) or Thermo-Reactive Diffusion (TRD) coating process was developed by the Toyota Motor Corporation’s R&D Laboratories, Japan. This was specifically invented to solve wear related problems experienced in their metalworking applications. Once a tightly guarded, patented technology held in Japan for its own use, the technology has since spread to use in the USA, Canada, South Africa, China, South Korea, Thailand and several other countries. The TD Coating Centre was fortunate to work closely with Dr Tohru Arai – inventor of the TD process and former Head of Research at Toyota, Japan – to establish the process in South Africa and we provide world-class TD coating.

The TD process produces a smooth, thin, non-porous layer of extremely hard (3200–3800 Hv) Vanadium Carbide (VC) on the surface of steel or sintered tungsten carbide. The thermal diffusion process uses free carbon available in the base material to diffuse and combine with active vanadium atoms available at the surface. This is performed in a high temperature molten salt bath. The TD process is typically performed at the hardening/austenitizing temperature of the steel and is followed by quenching and tempering processes as in conventional heat treatment. Due to TD being a high temperature process, steel selection is very important. See more here here.

TD coatingThe key advantage of the TD process over other coating technologies is the metallurgical bond through the diffusion process which offers unparalleled peeling/flaking resistance. This bond is far greater a pure mechanical bond or electro-plated bond. It doesn’t matter how hard or smooth a coating can be, if it doesn’t stay on the tool, it won’t help under severe load conditions. This property, together with the hardness and low reactivity of Vanadium Carbide, is the key to TD’s success over other competing coatings.

TD’s exceptional properties translate into remarkable extensions in tool life and it is not uncommon to increase the service life of a tool by 10 times or more. 

Properties of TD Coating


Coating Formed Various Carbides, predominantly Vanadium Carbide (VC)
Coating Hardness  3200 – 3800 Hv (~ 90 HRc)
Typical thickness  4 – 6 µm punching, 8 – 10 µm  press tools other, > 10 µm for corrosion applications
Layer Integrity Homogeneous, non-granular, dense and non-porous
Corrosion Resistance High. Can be further enhanced by special coating cycles
Adhesive Wear Very low tendency to adhere to work materials as carbide is stable and has low affinity against many other materials
Abrasive Wear Very hard coating is highly resistant to abrasive wear
Adhesion to substrate Highest bonding strength of any coating due to diffusion bonded layer
Service Temp Max ~ 500 °C (air), higher in non-oxidizing conditions
Co-eff of Friction As low as 0.1 against dry steel (dependent on pre-polish)
Dimensional Stability Coating layer for uniformly however some movement of base material can occur through the thermal cycke. Very tight tolerance components may not be suitable. Seek advice from TD specialist.