Element Six’s synthetic diamonds

Materials World magazine
,
28 Oct 2019

Graphite-based synthetic diamonds are being used to make cutting tools that help maximise end product performance.

Synthetic diamonds made with graphite can offer strong mechanical properties with readily available raw materials. Element Six, part of the De Beers group, uses chemical vapour deposition (CVD), high pressure and high temperature (HPHT) methods for diamond synthesis, to produce lab-grown diamonds. Element Six takes grey common graphite, one type of carbon, and transforms its structure into diamond. The product’s properties make it the material of choice for applications such as the purification of water, the cooling of semiconductor devices and laser exit windows. Its quantum properties also make it suitable for use in magnetometers and advanced computing.

As demand for higher efficiency and better performing products increases, pressure is being mounted on companies to produce tools which will help their customers deliver this.

Making synthetic diamond

CVD occurs at a low pressure using a plasma reactor. A flat, curved or domed non-diamond substrate is placed inside the chamber. High purity gases are added to the chamber, followed by the addition of microwaves, which heat the gases to generate a plasma with a temperature of more than 3,000°C. Inside the plasma, the chemical species required lab-grown diamond synthesis is created.

HPHT mimics the natural conditions under which diamond naturally forms. High pressure is generated through the concentration of hydraulic force on a small area, reaching a pressure of 55,000 atmospheres. A temperature of 1,500°C – the melting point of steel – is generated by electrical heating. At such high temperatures and pressures, carbon atoms are dissolved in molten metal such as iron, cobalt or nickel and then transformed into micrometre-sized lab-grown diamonds.

These micrometre-sized diamonds can be bonded together by sintering at high pressures and high temperatures into polycrystalline diamond (PCD), a composite material widely used in different industrial applications such as machining, cutting and oil and gas drilling. Diamond has high thermal conductivity and chemical inertness, a low thermal expansion coefficient and thermal shock resistance. When doped with boron, it is also a great electrical insulator and conductor.

The most widely used diamond material in cutting tools is PCD, with a hardness in excess of 50GPa – compared with 20GPa for cemented carbides, the next best class of cutting tool materials. It has been used for decades to transform the productivity of wood machining and aluminium alloy machining for the automotive industry. CVD diamond has a hardness from 90–120GPa. This increase in hardness leads directly to increases in tool life when machining one of the most abrasives materials in industrial use.

In terms of cutting tool materials, diamond shows superior properties for hardness over oxide ceramics, uncoated carbides and micro-grain carbides.

Automotive application

Mobility as a Service (MaaS) has the potential to disrupt the automotive sector. New technologies, enabled by advancements such as 5G networks, artificial intelligence (AI) and electric vehicles, promise the benefits of personal transportation while also being able to watch a film, play a game or work.

As more people relocate to larger cities, local authorities will need to ensure that residents can move around safely and effortlessly, while at the same time tightening the regulations around CO2 emissions and noise pollution. Taking this into consideration, the move to electric vehicles, car rental services and shared transportation becomes ever-more appealing, increasing the pressure on the automotive industry to respond in order to meet changing consumer demands.

Despite advancements leaning towards a more electrical-energy led industry, cars and the internal combustion engine (ICE) are still very much in the picture. With this in mind, one of the primary focal areas for the company is to continue developing materials that help minimise the environmental impact and increase the fuel economy of the internal combustion engine. 

Along with an overall trend towards a reduction in the inefficient use of raw materials, attempts to reduce CO2 will continue via the integration of more lightweight materials and improved combustion.

In the world of manufacturing, companies are grappling with how to satisfy these product demands, incorporate similar technologies into their manufacturing processes and attract the right talent to embrace the changing landscape.

Machining aluminium silicon alloys widely used for engine blocks and cylinder heads in the automotive industry, is a way productivity is improved using diamond. In this case, PCD diamond is used, for example, the milling of locator faces in cylinder heads show dramatic improvements in tool life machining, with 15 times more cylinder heads than cemented carbide in a quarter of the time.

The benefits can be articulated in ways including reduced cost-per-part or increased asset utilisation, but what is clear is that PCD can transform productivity. Aluminium will remain a key material for the future of the automotive market.

Complimenting aluminium could be fibre reinforced polymers (FRP). Carbon-fibre reinforced polymers (CFRP) have been used extensively in high-end applications, such as aerospace and motorsport, to reduce weight. This has led to increased interest in the benefits of FRP in the automotive industry. High specific strength and stiffness, combined with relatively low weight, makes CFRP materials ideal for numerous automotive applications.

One of the key challenges in the adoption of CFRP in aerospace has been its poor machinability. Due to its anisotropic nature and highly abrasive fibres, rapid tool wear often occurs. Diamond, in particular PCD, has been instrumental in the drilling, milling and trimming of CFRP and supporting its widespread adoption in aerospace. Tests show the number of holes drilled with PCD are more than five times than cemented carbide and that they were drilled at twice the speed.

Metal matrix composites

High wear resistance, corrosion resistance and low weight are among the mechanical properties of metal matrix composites (MMC). These materials will be used within the automotive industry for parts like brake discs and callipers, and in the aerospace industry for parts in the airframe, landing gear and fan exit guide vanes.

One of the key challenges in expanding the penetration of MMCs in these industries is the poor machinability, for example the rapid development of severe wear of the cutting tool.

Tool life improvements can be attained by using different classes of diamond tools in the high speed turning of two abrasives types of MMC,

40% silicon carbide, aluminium (SiC-Al) composite 3μm particles, and a 25% SiC-Al composite 20μm particles. Tool life can be more than doubled by the right grade of PCD, and a similar amount again by using CVD diamond.

Electronics and environment

Demand to machine a component to a specific geometrical tolerance as quickly as possible, with maximum tool life, is also seen in the electronics industry. There are some additional requirements in machining extreme aesthetics, including the demand to meet nanometre surface roughness requirements on a smartphone. The company has demonstrated in the high-speed milling of aluminium, a 40nm surface finish can be achieved using synthetic diamond tools.

More than 80% of the entire global tooling market in metal cutting is built on critical and scarce raw materials, such as tungsten and cobalt, according to EU-funded project Flintstone 2020. The project seeks to find alternatives to these materials and diamond-based composites are a key pillar of the alternatives proposed. Along with the scarcity, increasing attention is focusing on lifecycle analysis, where diamond materials such as PCD and PCBN offer favourable alternative solutions to traditional cutting tool materials.

Advanced cutting tool materials are now being focused on the challenges in the machining of lightweight materials including CFRP and MMC, as well as providing the data-sensing capability to reduce the impact of cuttings tools on the environment and utilisation of scarce raw materials.

Digital transformation

Industry 4.0 promises to improve production capacity and efficiency, and to create new business models through the collection and analysis of a large amount of data by networking sensors and devices. Element Six is preparing for Industry 4.0 in its Global Innovation Centre, Oxfordshire, UK, where it has automated cutting tool testing, delivering productivity and resolution previously unachievable.

The company is also working to add intelligence to its materials. In collaboration with a UK-based consortium, Element Six is developing a prototype sensor on a PCD tool that allows a machine operator to determine the condition of a cutting tool without manual inspection. The technological basis of the sensor is the graphitisation phenomenon that occurs within PCD.

At elevated temperatures, diamond converts back to graphite and graphite is electrically conductive. Therefore, a laser can be used to selectively graphitise tracks on a PCD tool surface and the resistance generated within the tool-embedded-sensor can be monitored, for example if resistance increases this would indicate tool wear or breakage. The cutting operations were simulated before trialling the prototype by machining a grade of titanium.

Turning graphite into diamonds upscales the raw material’s value for industrial purposes, particularly when the stone might not be readily available.

The new material’s properties make it suitable for applications in the automotive and electronics industries in forming highly machined end-products.