Tool and die makers have always looked for the perfect solution, a hard, tough surface produced from easily workable but wear-resistant material. Perfection is not here yet, but the technique of ion implantation does offer major advantages, using a moderate temperature process to give significant increases in tool life of tools made from low carbon steels (figure 1).
Figure 1. Examples of the types of components that can benefit from ion implantation and increase campaign life.
Ion implantation revolutionised microchip manufacture in the 1970s by allowing controlled levels of specific metallic elements to be introduced into the surface and near-surface layers of semiconductors, using a controllable ion beam in a hard vacuum. Since then, the technique has been developed in a variety of ways, but the most significant development for metallurgists and engineers has been to provide precisely controlled surface and near-surface properties at relatively low processing temperatures, below 200°C. This enables low carbon steels, for example, to have high surface hardness that can extend the lifetimes of slitting, cutting and punching tools by factors of 20 or more. The technique can also be used to change the near-surface properties of aluminium, titanium, ceramics and other non-conductive materials.
Originally developed at the UKAEA's Harwell Laboratory, ion beam implantation is now available as an industrial scale process that allows toolmakers to increase hardness, fatigue life, corrosion resistance and accuracy of form. This extends tool lifetimes, by reducing wear and friction.
Ion implantation takes place within a vacuum chamber. Essentially, the technique involves accelerating ions to speeds of around 103m.s-1 through an electric field (usually around 100kV). The beam of high-speed ions is focused on the workpiece, penetrating the surface and, depending on the type of ion, producing mechanical and chemical changes. These changes can be controlled precisely by varying the intensity of the ion beam. While the process does not require the workpiece to be heated directly, most ion beam processes operate at between 150°C and 200°C, depending on the level of the ion beam flux.
Nitrogen is the most common ion used for metallurgical applications. When the ions penetrate the surface of the workpiece, some of them peg microcracks, some fill lattice spaces in crystalline structures, and some react chemically to form compounds, giving new lattice properties. The best example of this is nitrogen ion implantation of high chrome steels; this causes a substantial increase in hardness by converting chromium in the surface layers to chromium nitride.
Surface Hardening Mechanism
When treated surfaces wear, the atoms trapped interstitially in the metal structures become dislodged and may diffuse deeper into the surface. As this process continues, the atoms continue to close up microcracks. This discourages crack propagation in the workpiece and leads to better abrasion resistance. It can also prevent the ingress of oxygen and other potentially corrosive compounds.
The cumulative effects of the ion implantation produce significant benefits. These benefits have proved particularly useful in tools designed to extrude filled polymers. The filler materials (typically sllica or glass fibre) that are suspended within the liquid plastic cause extensive wear inside the moulds and the adjacent gate areas. While this wear is a problem in itself, it becomes critical where textured moulds are used to produce special surface effects on finished plastic components. A small amount of wear can produce smooth, shiny areas on the moulding, which make the tool useless. Expensive and time consuming re-texturing is then necessary. The frequency of re-texturing can be quartered by nitrogen ion implantation.
High Carbon Steels
Mould steels are usually made from high carbon steels with low temperature tolerance. These materials are adversely affected by high temperature thermal processing, often suffering significant distortions or loss of hardness at temperatures above 200°C. Ion implantation can improve the properties of the original surface with negligible distortion or oxidation effects, and produce surfaces with hardnesses between 1100 and 2000 HV. This results in substantial increases in tool lifetimes - up to a factor of ten in some cases, for chrome-plated tool surfaces. Injection and extrusion screws used to pump molten plastic can also benefit greatly. Here, ion implantation produces a low friction, wear-resistant surface on the screw and allied components.
Since Harwell first developed the ion implantation process as an industrial‑scale metal processing technique, the size of available plant has steadily grown to meet the demands of the commercial market, and a number of commercial operations have taken up licences of the process. Businesses such as Tecvac in Cambridge, a member of the Wallwork Group, provide contract ion implantation services as well as manufacturing capital plant to meet the specialist needs of major engineering and research businesses. A typical ion implanter consists of a large, hard vacuum chamber with a rotating table that moves components under a saddle field ion source. The saddle field source, developed by the Harwell laboratory, consists of a 70mm diameter graphite tube with a 150mm slot, providing an ion beam of 150 x 150mm at a distance of 300mm. This gives a nitrogen ion beam intensity of 3 x 1017ions.cm-2hr-1.
The maximum temperature of the workpiece depends on the ion flux. This is moderated by a computer control system, which controls the gas flow current to supply the specified ion beam dose to the workpiece, while keeping within preset temperature limits.
Commercial ion implantation equipment with the standard half or one metre cube vacuum chamber can use extension tubes to accommodate large workpieces up to two metres long (figure 2). Tecvac designs can include an additional ion source to double processing rates and alternative vacuum systems to provide cryogenic pumping. An electron beam evaporation gun, or a magnetron source to allow ion-assisted coatings or ion-assisted sputtering, increases the machine’s versatility.
Figure 2. An ion implantation device modified to take long workpieces.
Advantages of ion Implantation over Other Coating Procedures
A significant advantage of ion implantation is that the treated surface is an integral part of the workpiece and does not suffer from possible adhesion problems associated with coatings. The moderate heating associated with the process virtually eliminates any risks of distortion or oxidation effects. Ion implantation produces no dimensional changes in the workpiece.
The technique is continuing to develop in the metallurgical areas. Elements other than nitrogen can be used, and rare earth elements such as yttrium can be used to produce high lubricity, corrosion-resistant surfaces. These and other developments of ion implantation will continue to extend the options toolmakers have to use moderate temperature processing methods while still achieving the surface hardness needed for long tool life.