Spark plasma sintering opens up potential to wider range of industries
Thermal Technology’s commercial-scale spark plasma sintering (SPS) system is opening up the current and future potential of this production process to an even wider range of industries, from aerospace to nuclear fuel. Adrian Goodbrand (CEng IOM3), Product Sales Manager at VFE, the exclusive distributor and service provider for Thermal Technology systems throughout Europe, explores the process and its benefits.
Based in Santa Rosa, California, Thermal Technology (TT) has been providing advanced thermal processing solutions for over 70 years. Supplying over 3,000 furnaces worldwide TT is widely regarded as a global leader in the vacuum furnace market. Working in partnership, Thermal Technology and VFE, the UK’s leading provider of vacuum furnace products, can offer custom design solutions to meet specialist requirements for high temperature materials processing, as well as planned and emergency maintenance solutions.
Pioneer
TT has helped pioneer the use of innovative advanced materials in industries such as medical, lighting and engineered ceramics. Always at the forefront of new developments, TT is now seeking to take SPS technology to the next level with a large-scale system which is available for proof of concept testing and operational cost analysis for industrial applications. Designed from the ground up using state-of-the-art components, the DCS 200 Series has a 250 metric tons maximum force, 40,000A of heating power and maximum operational temperature of 2400˚C. The specification also offers advanced mixed material recipes and diffusion bonding.
Process
Although Spark Plasma Sintering is probably the most commonly accepted term for this technology it is also known as Direct Current Sintering (DCS) and is sometimes called Field Assisted Sintering, which uses a pulsed direct current. For the purposes of this article I will refer to the process as SPS/DCS, which is TT’s preferred description. In general terms, SPS/DCS is a high-speed powder consolidation/sintering technology capable of processing conductive and non-conductive materials. It is carried out in a vacuum chamber under low-pressure conditions. Powder is placed in a die with a matching punch and the powder is compressed in the die using a hydraulic ram. A direct current is then applied through the ram, giving rise to Joule heating in the charge. The combination of pressure and temperature sinters the particles of powder together and consolidates it into a single piece of material. For non-conductive materials such as ceramics the direct current can be passed through the tooling (punch and die) and so indirectly heating the powder. Tooling is typically silicon carbide (SiC) or graphite but other conductive materials can be used. With graphite tooling temperatures up to 2400˚C, necessary for processing ceramics, can be achieved.
Benefits
SPS/DCS’ operational or ‘monitored’ temperatures (200-2400˚C) are commonly significantly lower than with conventional sintering. Material can be heated at very high rates – at up to 1500˚C a minute compared with just 20 to 40˚C a minute – thus enabling a shorter cycle time. This brings major commercial benefits, not only in terms of faster processing speed but also minimising grain growth. The entire process, from powder to finished bulk sample, is completed quickly, with high uniformity and without changing the characteristics of the particles. Additionally, being a lower temperature sintering technology, the power consumption of SPS/DCS is 20% to 33% of conventional technologies, making it an economic and eco-friendly production solution.
Applications
There are a lot of processes involving the production of parts from powder where SPS/DCS offers benefits both in manufacturing efficiency and material properties. For example, hard metals and materials, such as tungsten carbide used in tools and abrasives, can be processed from powder to component in just one step with no need for a binder/wax. This saves time, reduces costs and waste material. What’s more, the heating rate achieved using this process means there is no need for post-heat treatment to achieve the final hardness. In fact, it can achieve Vickers hardness values that are even higher than those achieved with post-heat treatment. With the world tungsten market currently standing at around 80,000 tonnes per year, this could be a considerable market opportunity for SPS/DCS.
Nuclear fuel
Perhaps one of the largest growth markets for SPS/DCS at the moment is in the production of uranium dioxide nuclear fuel pellets. Current production
relies on sintering the ceramic pellets in a resistance-heated hydrogen furnace, which takes a long time to heat up. Compared with conventional methods, SPS/DCS can reduce total sinter time from 1-10 hours to under an hour. This makes a massive difference to operational costs and improves safety because there is no flammable gas atmosphere. Controlled grain growth and the ability to produce high-density, accurately shaped pellets are further advantages for this specialist application.
Minimal grain growth is also a benefit in the production of ‘nanophase’ structures within other specialised industries as the ultra thin layers stay separate rather than diffusing into each other.
Future markets
Another area with great potential for SPS/DCS in the future could be in thermoelectric materials which have the ability to convert heat into electricity and vice versa. This application utilises ‘exotic’ materials with a high rate of thermal conductivity so the rate of heating and cooling has to be quick to keep the nanophase layers separate. One possible application for this innovative material could be in a car engine to convert waste heat into electricity and reduce fuel consumption.
As well as the production of parts, SPS/DCS can be used instead of PVD and CVD (Physical and Chemical Vapour Deposition) to produce very thin coatings to protect against wear and friction, for example. This can result in big savings in energy costs and cycle times and also provides a safer process.
Whilst SPS/DCS does have some limitations, such as the range of shapes and sizes, improvements are being made in these areas all the time. For its part, TT will continue to invest in the research and development of SPS/DCS in order to bring the advantages of this technology to many more applications – some that haven’t even been thought of yet! With a commercial-scale system now available for critical production trials, TT could hold the key to unlocking the huge untapped potential for SPS/DCS.