The aerospace industry makes use of a wide range of materials which are selected due to the characteristics they possess, such as strength, density and rigidity.

Nickel-based super alloys and composites are examples of such materials. However, they are difficult to cut and require specific machining techniques. As a result, diamond is used for applications in which conventional tools cannot be used to reach the final objective of the manufacturing process.

Using Diamond for Longer Tool Life and Improved Cutting Speeds

The success of using diamond tools in the aerospace industry is mainly due to the properties this material possesses, leading to a longer tool life and improved cutting speeds compared to conventional materials.

Diamond has the highest value on the Mohs scale of mineral hardness. This scale allows the classification of the resistance to scratching of a material and has a range comprised between 1 (softest) and 10 (hardest).

Diamond is characterised by a hardness of 10 Mohs, which corresponds to a hardness four times greater than Corundum (Al2O3), which has 9 Mohs.

Diamond tools can be created using different types of the material, depending on the specific application and product requirements. Diamonds can be categorised as either natural or synthetic. Synthetic diamond materials can be classified in different classes: grains and powders, PCD (Polycrystalline Diamond) and CVD (Chemical Vapour Deposition).

Synthetic Diamond Grains

Synthetic diamond grains (grits) and powders are used as abrasives, bonding them to the surface of a tool. This can be achieved in different ways depending on the required process characteristics, such as tool rotational velocity.

Diamond particles can be deposited on the tool surface either using an electroplating technique, using a nickel solution or by sintering them in a tungsten based matrix. Another technique known as braze bonding, works by brazing the diamond particles to the tool metal surface.

Each of these bonding methods have associated benefits and drawbacks related to the tool manufacturing time and the capabilities of the binder in retaining the diamond particles, depending on the final operational characteristics.

PCD (Polycrystalline Diamond) Tools

In PCD tools (or inserts), segments of diamond are bonded, typically using brazing, to a carbide substrate. PCD elements are produced by sintering micron-sized synthetic diamond powders to bond particles together in a process characterized by high temperatures and pressures.

This material is manufactured using cemented carbide baking which provides the metal source necessary, typically cobalt, in order to carry out the sintering process. During the production process the metal from the carbide substrate penetrates between the diamond grains, allowing them to be bonded together.

The metallic content of PCD provides the material with electrical conductivity properties, allowing the use of machining process such wire EDM (Electrical Discharge Machining) in order to cut diamond elements.

CVD Diamond Tools

Another type of synthetic diamond is produced by using a technique known as chemical vapour deposition. CVD diamond is deposited in thin layers on the surface of a tool in a tightly controlled growth condition process.

One of the main advantages in using this kind of synthetic diamond is the possibility to obtain addition geometries and cutting edges, using a material characterised by highly predictable properties.

Diamond in Aerospace Manufacturing Processes

Diamond is currently used in a variety of manufacturing processes in the aerospace industry, mainly due to its high wear resistance and ability to machine difficult-to-cut materials. As previously mentioned, diamond can be produced using different methods, obtaining a variety of products with characteristics specifically tailored to the final tooling application. It can be used in processes which employ tools with defined (turning, milling and drilling) and non-defined (grinding and dressing) cutting edges.

The use of diamond in machining is limited to non-ferrous materials. Diamond is characterized by its strong affinity for iron and nickel, especially at high temperatures. Particular attention needs to be paid in the process maximum temperature, in order to avoid the transformation of diamond in graphite due to the metastable nature of this material.

Specifically, at low pressure, graphitisation of diamond surfaces has been observed at temperature lower than 1700°C, while the conversion temperature can be lowered up to 400°C if iron elements are present.

As a result, carbide tools are currently used when machining titanium alloys, nickel alloys and stainless steels, using defined cutting edges tools. Conversely, in case of abrasive processes, materials like CBN (Cubic Boron Nitride), aluminium oxide and SiC (Silicon Carbide) are used in machining steels, nickel alloys, ceramics and titanium alloys.

Another alternative to diamond in machining ferrous materials is known as PCBN (Polycrystalline Cubic Boron Nitride), a composite material manufactured by sintering micron-sized CBN powders with various ceramics.

The Grinding Process in the Aerospace Industry

One of the many processes employed in the aerospace manufacture is grinding. This operation allows the required surface finish to be obtained as well as geometrical accuracy for specific components, such as turbine blades, by using an abrasive rotary tool with a characteristic profile. Diamond abrasives play a fundamental role in maintaining the grinding wheels performance to optimal levels.

Diamond abrasive grains, both natural and synthetic, are used in a group of abrasive tools known as dressers, used in the preparation of grinding wheels. These tools are used to re-establish the abrasive conditions of a grinding wheel (dressing), removing the clogged debris from its abrasive surface (cleaning) and restoring the required profile (truing).

When talking about conventional grinding tools, such as aluminium oxide wheels, the term ‘dressing’ refers to both the actions of truing and dressing. The two main categories of dressing tools can be distinguished by their movement with respect to the grinding wheel: stationary and rotary dressers.

The rotary dressers are usually used in the manufacture of engine components in the aerospace industry, due to the possibility to dress wheels with complex profiles, such as fir tree root forms in turbine blades.

Diamond in Grinding and Drilling Tools

Diamond, as an abrasive material, has also been used in grinding tools. For example, it has been proven that single-layer electroplated diamond grinding wheels can be used to obtain good results when machining a wide range of disc slot forms and sizes for mounting turbine blades in aerospace engines.

Another application of diamond in the aerospace manufacturing industry is in core drilling tools. Brazed diamond grit cored drills can be in the machining of both CFRP (carbon fibre reinforced polymers) and titanium. These two categories of materials are used to create aerospace components with high strength-to-weight ratios and rigidity, often in combination in order to harness the most favourable characteristics of each material.

The number of aerospace manufacturing processes which make use of diamond is part of an ever growing list. In fact, the high cost of a diamond is justified by the longer tool life and cutting velocities, which allow an increased production rate and lower costs associated with tool replacement.