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Tool and die makers have always looked for the perfect solution that includes a hard and tough surface produced from easily workable but wear-resistant material. To this end, ion implantation, which involves the use of a moderate temperature process to significantly improve the surface characteristics of tools comprised of low carbon steels, has shown promising results.
The Development of Ion Implantation
Ion implantation revolutionized microchip manufacturing processes during the 1970s by allowing controlled levels of specific metallic elements to be introduced into the surface and near-surface layers of semiconductors through the use of a controllable ion beam in a hard vacuum. Since then, this technique has been used in a variety of ways, particularly within the semiconductor industry as a method for doping materials. However, the most significant development for metallurgists and engineers has involved using this technique to introduce precisely controlled surface and near-surface properties to materials at relatively low processing temperatures below 200°C. As a result, researchers have developed low carbon steels, for example, that exhibit high surface hardness, as well as extended lifetimes in terms of slitting, cutting and punching tools by factors of 20 or more. Furthermore, this technique has also been used to change the near-surface properties of aluminum, 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 in their products. By reducing unwanted wear and friction, ion beam implantation processes have demonstrated their ability to significantly extend tool lifetimes.
As one of the primary techniques used to introduce foreign elements into a solid, Ion implantation takes place within a vacuum chamber. Essentially, this technique involves accelerating ions to speeds of around 103m.s-1 through an electric field, which is usually around 100kV, in a uniform and controlled manner. The beam of high-speed ions is focused on the workpiece, penetrates the surface and, depending on the type of ion, will produce mechanical and chemical changes. These changes can be precisely controlled by varying the intensity of the ion beam. While the process does not require the workpiece to be directly heated, most ion beam processes will operate within the temperature range of 150°C to 200°C, depending on the level of the ion beam flux.
The Importance of Surface Hardening Procedures
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 ion implantation produce significant benefits. These benefits have proved particularly useful in tools designed to extrude filled polymers. The filler materials, which are typically silica or glass fiber materials, that are suspended within the liquid plastic cause extensive wear inside the molds and the adjacent gate areas. While this wear is a problem in itself, it becomes critical where textured molds are used to produce special surface effects on finished plastic components. A small amount of wear can produce smooth and shiny areas on the molding, which ultimately make the tool useless. To reverse these effects, expensive and time-consuming re-texturing processes are often required. The frequency of re-texturing can be quartered by nitrogen ion implantation.
Nitrogen is the most common ion used for metallurgical applications. When the ions penetrate the surface of the workpiece, some of the ions will peg microcracks, whereas others will either fill lattice spaces in crystalline structures or react chemically to form compounds; regardless of which pathway the ions follow, anew lattice properties will arise. Overall, nitrogen ion implantation has continuously demonstrated its ability to significantly improve hardness and wear resistance properties. In fact, over 90% of the non-electric applications of ion implantation utilize nitrogen ions.
High Carbon Steels
Mold steels are usually made from high carbon steels with low-temperature tolerance. These materials are adversely affected by high-temperature thermal processing, as these materials will often suffer from 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, while also producing surfaces with hardnesses between 1100 and 2000 HV. This results in substantial increases in tool lifetimes, which can reach 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 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. In addition, a large number of commercial operations have taken up licenses 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, which was originally developed by the Harwell laboratory, consists of a 70 millimeter (mm) diameter graphite tube with a 150mm slot that provides an ion beam of 150 x 150 mm at a distance of 300mm. This gives a nitrogen ion beam intensity of 3 x 1017 ions cm-2 hr-1.
The maximum temperature of the workpiece depends on the ion flux. This is moderated by a computer control system that is used to control the gas flow current to supply the specified ion beam dose to the workpiece, as well as maintain the system within the preset temperature limits.
Commercial ion implantation equipment with the standard half or one-meter cube vacuum chamber can use extension tubes to accommodate large workpieces up to two meters long, as demonstrated in Figure 2. Tecvac designs can include an additional ion source to double processing rates, as well as alternative vacuum systems that can be used to provide cryogenic pumping. An electron beam evaporation gun or a magnetron source can also allow ion-assisted coatings or ion-assisted sputtering, both of which increase the machine’s versatility.
Advantages of Ion Implantation over Other Coating Procedures
A significant advantage associated with the use of ion implantation is that the treated surface is an integral part of the workpiece and does not suffer from any possible adhesion problems that may be attributed to the coatings. Furthermore, 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; for example, rare earth elements, such as yttrium, can be used to produce high lubricity and 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.
Several recent studies have investigated the use of plasma immersion ion implantation as a way to improve surface resistance properties of their materials. For example, materials that are used during marine oil extraction procedures, which often causes the surfaces of supermartensitic stainless steels (SMSS) to be eroded by sand particles. By nitriding the SMSS surfaces through plasma immersion ion implantation at high voltages of approximately 10 kV, the researchers found that surface hardness prevailed, elastic recoveries of strained surfaces improved and chromium depletion levels were significantly reduced. These property improvements were attributed to the ability of the ion immersion technique to produce stratified, compact and hard layers in the SMSS.
As previously stated, metallurgic research has become increasingly interested in the possibility of expanding the types of ions used for ion implantation purposes. To this end, a recent study found that the implantation of hexagonal boron nitride with light ions produced a surface layer comprised of nanoparticles. The surface that emerged from this study was found to exhibit significantly improved hardness characteristics following micro-indentation. This study also found that any lion ion, aside from neon that appeared to behave differently, can produce these hardness improvements.
Sources and Further Reading
- Woolley, E. (1997) Materials World, 5(10); 515-16.
- Kurelo, B. C. E. S. de Oliveira, W. R., Serbena, F. C., & de Souza, G. B. (2018). Surface mechanics and wear resistance of supermartensitic stainless steel nitride by plasma immersion ion implantation. Surface & Coatings Technology 353; 199-209. DOI: 10.1016/j.surfcoat.2018.08.079.
- Derry, T. E., Lisema, L. I., Magabe, A. T., Aradi, E., et al. (2018). Allotrope conversion and surface hardness increase in ion implanted boron nitride. Surface & Coatings Technology 355; 61-64. DOI: 10.1016/j.surfcoat.2018.04.005.
This article was updated on the 15th March, 2019.