The Thermal Diffusion or Thermo-Reactive Diffusion coating process was developed by the Toyota Motor Corporation's Central R&D Laboratories in Japan specifically to solve wear related problems.

The TD process produces a smooth, thin, non-porous layer of extremely hard Vanadium Carbide (VC) on the surface of a steel. The main advantage of the TD process is that, unlike other conventional coatings that are merely deposited onto the surface, the TD coating is metallurgically diffusion bonded onto the steel substrate.

This diffusion bonding offers unparalleled peeling resistance and adhesion of the TD coating. It doesn’t matter how hard or smooth a coating can be, if it doesn’t stay on the tool, it won’t help. This is the key to TD’s success over other hard coatings.

TD coating, which is actually Vanadium Carbide, creates a surface layer of 5 - 10 µm with a surface hardness between 3200 and 3800 Vickers (approximately 90 HRc). This surface hardness is far superior to many other surface hardening techniques as shown left (click to enlarge). High hardness resists abrasion directly and prevents the work material from ploughing or cutting the surface of the steel. Vanadium is added to many high performance tool steels and powder metallurgical steels to form fine vanadium carbide particles held in the steel. With TD Coating, a pure layer of Vanadium Carbide is formed on the surface .... exactly where you need it.

When TD Coating is applied to the surface of a highly polished tool, not only does its high hardness protect the highly finished surface, promoting very low coefficient of friction, the simple fact that the coating is made of an inert carbide, means that the adhesive tendencies are vastly reduced. Cold welding of work material to tool steel is a thing of the past due to the carbide layer between the two reducing the chemical affinity for cold welding to occur. This means an end to galling, scoring, pick-up and seizure.
The properties of the TD coating impart substantial improvement to all wear related applications, e.g. stamping dies, forming dies, forging dies, pipe, tube and wire manufacturing, roll forming, rubber forming, and aluminium die-casting, to name a few.
.... see applications below

TD coating your tools has both direct benefits and leveraged benefits that make it invaluable:
• extremely hard and inert coating resists galling and metal pickup
• improvement in product quality due to improved surface finish, appearance and dimensional tolerance on the work piece
• saving of dies though life improvement
• improved tooling reliability
• savings in lubrication or complete disuse thereof in special cases
• reduced labour costs for re-polishing, sharpening and maintenance of dies
• reduction in labour and time for finishing or reworking product
• ability to produce better components from problematic materials such as stainless steel, aluminium, high strength and plated steels
• difficult shapes can more readily be formed due to less galling, metal pickup and scoring.
• less downtime experienced
• exceptional increase in tool performance
• great corrosion resistance (this benefit can be specially enhanced with a special treatment for corrosion specific applications)
“Tool coatings are a lever to achieve quality and production efficiencies. If you do not value quality or efficiency, you don’t need coating. But if you do, then can you afford not to coat your tooling?”
If you wish to compare the differences between TD Coating and PVD coating please click here

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The TD process is very similar to a conventional heat treating process, with the coating stage included at the austenising temperature of the steel. The actual Thermal Diffusion process involves fully immersing parts in a molten salt bath mostly in the temperature range between 950 and 1050 °C. TD process salts are a special formulation and have a syrup-like consistency unlike conventional heat treatment salts. Active vanadium dispersed in the salt bath combines with free carbon atoms contained in the steel substrate to form the vanadium carbide layer. This high temperature reaction also creates the extremely high bond strength. Due to the temperature ranges concerned with the diffusion process , this means some steels are not suitable for the TD process. Click here to see a table of suitable tool steels for TD Coating or other coating options for your grade.
TD Coating through the salt bath method means that the coating reaction occurs wherever the salt comes in contact with the steel. This means that coating of difficult geometries is easily possible due to the full submersion of parts in the salt bath. Also the high density of the salt helps to displace the weight of the tool through Archimedes principle which reduces distortion of tools under their own weight at high temperature.
The thickness of the TD coating can be precisely controlled through adjusting the processing time, bath temperature and accounting for the composition of the substrate. Parts can also readily be re-coated. Re-coating of up to nine times have been reported, however this depends on application, and practically a new tool is required after approximately four to five re-coats.
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The TD coating works extremely well when an adhesive and/or abrasive types of wear are present. The coating can be applied to dies used in both short and long production runs and can be considered as a unique "problem solver" to remedy specific manufacturing problems. Notably TD performs well in high volume production and also when using thicker gauge materials, High Strength Low Alloy (HSLA) steels, plated steels, stainless steels and aluminium. Although every application has different dynamics acting on the tool, more than 20 times increase in part life is common. Typical applications in which exceptional performances have been obtained is given in the accompanying table. The TD coating is extremely hard and virtually any application that requires exceptional wear resistance, and resistance to seizure or galling can benefit from the coating. The coating has very high seizure resistance even compared to cemented carbides and it has a very low coefficient of friction. The very hard surface layer also does not reduce the toughness of the underlying steel. The TD coating is harder than aluminium oxide, and coated cores, pins and inserts used in the aluminium die casting industry show excellent performance, reducing maintenance and downtime. It is important to note that the TD coating performs best when mated with the correct steel in the correctly heat treated condition. This compatibility is best discussed with the TD Coating Centre

The TD coating also performs very well on cutting blades and knives for the metal, paper, plastic and rubber industries, guides in the textile and metal industries, pump components and machine parts such as cams, plungers, sleeves, bushes, nozzles and valves. TD has a friction co-efficient better than hard chrome and offers excellent sliding properties. In the correct application, TD will outperform any other coating or surface modification.

Typical TD Applications
Press dies, forming dies, deep drawing dies, bending dies, extrusion dies, swaging, crimping, pierce and notch punches, extrusion punches, bolt forming tooling, hex trim punches, blanking tools, shear blades, cropping dies, slitters, cut-off blades, wood chipper knives, mandrels, non-return valves, drill bushes, collets, casting core pins and inserts, traveller rings, rollers, locating pins, cam plates, high pressure pump pistons etc.

Several South African industries have already performed trials with the TD coating and achieved excellent results. These include companies from the following areas :
• Automotive - pressing, forming, bending, punching
• Metal Pressing - deep drawing, forming, punching, blanking
• Tube Forming - draw dies, bending, expanding (ferrous and non-ferrous applications)
• Mining - applications requiring wear and corrosion resistance
• Polymers - die inserts, rotating cores, pelletizing, applications requiring anti-seize properties
• Aluminium Casting - aluminium cores, inserts, ejector pins
• General wear resistance - plungers, guides, pistons, knives