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CVD Tungsten Carbide Coatings to Extend the Life of Tooling Used for Forming Abrasive and Corrosive Materials Jan 12, 2018

CVD Tungsten Carbide Coatings to Extend the Life of Tooling Used for Forming Abrasive and Corrosive Materials

The technological developments in tools and tool-making have been remarkable over recent years and are making an enormous impact on the economy and profitability. Many processes rely on precision tooling to create large numbers of products to very tight specifications over long production cycles.

A tool that fails, or that wears quickly, means an expensive disruption to production. For this reason a great deal of engineering effort has gone into creating better, more accurate and longer lasting tools.

Perhaps the most significant improvement in recent years has been the application of new coating technology to produce a surface that can be engineered to great precision while being extremely hard and tough, yet still remaining economical.

Tool Life Extended by Novel Surface Treatment

The classic engineering trade-off in tool manufacturing has been between these parameters. It is quite straightforward to make a very hard tool – but in conventional metalworking this hardness is accompanied by brittleness.

A brittle tool is prone to sudden catastrophic failure, and in any case a very hard material such as tungsten carbide-cobalt (WC/Co) is difficult and expensive to machine, especially to fine tolerances.

Tougher steel alloys which have high durability may be relatively easy to machine, but are unlikely to hold an edge or stay within tolerance for long periods. This means frequent replacement, interrupting production and reducing profit.

The solution is to create the body of the tool from a tough and durable material, and then on this, build a much harder surface.

Many coating methods have been used, including electro plating (most popularly with chromium), thermal spray coatings, or physical vapour deposition (PVD). Each of these offer benefits but each also has disadvantages.

A chemical vapour deposition (CVD) process from Hardide Coatings has been shown to outperform these alternative techniques, especially when used in difficult manufacturing processes.

Hardide CVD coatings involve the creation of a nano-structured coating of tungsten carbide / tungsten metal composite directly onto the tool substrate, deposited atom by atom to produce remarkable surface properties.

Harder than Steel

Hardide CVD coatings are typically 10-60µm thick and have a Vickers hardness of 1100-1600HV. Typical types of stainless steel, such as 304, have a hardness of around 200HV, while specialist grades may reach 300HV – both much softer than the Hardide coating.

The superior hardness of the CVD surface is matched by its versatility. The coating is crystallised atom by atom from vapour, directly onto the tool surface, having excellent adhesion to stainless steel and tool steel, as well as copper- and cobalt-based alloys. The surface created is extremely hard, pore-free and resistant to corrosion, acids, and aggressive chemicals. Just as important, the coating is created uniformly on even very complex tool shapes.

These last points can be critical in many manufacturing operations. The case of an extrusion die for a PVCu profile manufacturer provided an excellent 'acid test' of the technology. In this process, hot polymer material is extruded through a plate into which has been machined a series of precise cavities. These mould the hot plastic to form the complex cross-sectional shape of the finished product, which is then used to manufacture windows and other products.

Hot PVC is itself quite abrasive, and degrades into a gaseous form of hydrochloric acid, creating an extreme environment in which any damage to the die will be immediately apparent on the surface of the finished plastic section. It has been shown that Hardide coating of the cavities and both faces of the die reduced wear to a large degree, producing more plastic profile within tolerance over a longer period.

A broadly similar challenge in a different industry showed how effective Hardide coatings can be. A wide variety of materials are processed into pellets, including animal feed, fertiliser, and plastics. These pellets are made by forcing a slurry through a die with many holes – often tapered – after which the extruded strings are sliced by a rotating knife into pellets. As in the case with the plastic profile extrusion, the tool is a complex shape and a critical component in the production process.

The Hardide coating was able to successfully treat inside the hundreds of deep holes in the pelletizing die, where chrome plating and a thermal spray coating was unable to be used, and after testing on various products including chemicals, wood chip pellets, and feedstuffs was shown to increase tool life by a factor of three. The treatment was found to be especially useful where consistent pellet dimensions were important.

Another process which subjects tools to extreme abrasion, is the manufacture of pharmaceutical tablets. These are created by compacting powders with a precision hollow punch tool, and the common mode of failure is for the punch to fall out of tolerance due to wear by the highly-abrasive powder. Using a conventional hard steel tool, around 10 to 15 million tablets can be pressed before the punch needs to be replaced.

A Hardide-coated tool was installed on the production line alongside a conventional tool and, after a period of eight months, had produced 36 million tablets with no signs of wear. Meanwhile the uncoated tool had been replaced three times.

Tool life is a critical aspect of manufacturing economy, and in today's increasingly competitive market it makes good financial sense to keep production lines working with as few interruptions as possible.

Applications for Hardide CVD coatings are being found in fields as diverse as oil extraction, pumping abrasive fluids, and ceramics manufacture. This novel coating technology has been shown in all these industries to prolong tool life, typically by a factor of three, facilitating the production of long runs of accurately made goods, and repaying the modest investment many times over.