MIM F75 (Co-Cr-Mo) for high-volume production The impact of sintering conditions on microstructure and properties

The electronic devices industry emerged in the 20th century and is one of the largest industries worldwide today. Society uses a vast array of electronic devices built in automated or semi-automated factories. These devices are now ubiquitous, with billions of people using them in their daily lives.

Communication and computing devices such as smartphones, smart watches, tablets and laptop computers are built with complex combinations of components, many using materials optimised for electronics production. These materials have been the basis for the current age of electronic, information and communication technology and have been a large contributor to worldwide economic growth.

Electromagnetic components based on advanced metallic materials are one of the most significant developments in the modern 3C industry (computer, communication and consumer-electronics). These materials combine excellent mechanical strength with reasonably high corrosion resistance, wear resistance and specific magnetic properties (ferromagnetism or paramagnetism, depending on product design and function). They include stainless steels, cobalt alloys and other leading-edge alloys.

Significant skill and precision engineering are needed to create the components of the devices noted above, and there are many hurdles to overcome. It is important that product designers can find and select appropriate materials quickly and efficiently to keep up with fast-paced developments.

Fig. 2 Examples of MIM components produced by Chenming Electronic Technology Corp. (Courtesy UNEEC)

The attraction of cobalt alloys

Cobalt-base alloys have long been developed for implantable medical devices and have recently been applied to the 3C electronics industry. They exhibit wear-resistant, corrosion-resistant and heat-resistant properties. The most effective use for cobalt-base alloys is in wear-resistant components.

Cobalt is more widely used as an alloying element for heat-resistant applications in nickel-base superalloys, with cobalt tonnages above those used in cobalt-base heat-resistant alloys. Moreover, cobalt-base alloys exhibit excellent resistance to various forms of high-temperature corrosive attack, including oxidation, sulfidation and carburisation reactions.

Many of the commercial cobalt-base alloys that are derived from the Co-Cr-W and Co-Cr-Mo ternaries were first investigated by Elwood Haynes, who discovered the strengthening effect and corrosion resistance imparted to cobalt by chromium in 1907. He later identified tungsten and molybdenum as powerful strengthening agents within the cobalt-chromium system. Co-Cr-Mo alloys, one of the advanced cobalt-base alloys, are widely applied for aircraft engines, medical total hip replacements, dental devices, support structures for heart valves, etc. Co-Cr-Mo alloys are well known for their combination of strong mechanical performance, wear resistance, corrosion resistance and acceptable biocompatibility. However, their main attribute is corrosion resistance in chloride environments.

Besides the previously mentioned applications of Co-Cr-Mo alloys, recently much attention has been paid to their use in the 3C telecommunication industry. For example, smartphone camera bracket components are a promising application for these alloys due to their combination of strength, corrosion resistance, wear performance and non-magnetic properties.

Cobalt-base alloys were introduced to what is now called the superalloy field mainly because of the suitability of the Co-Cr-Mo alloy named ‘Vitallium’ for reproducing complex shapes by precision lost‐wax casting [1]. Many of the properties of the cobalt-base alloys arise from the crystallographic nature of cobalt element. These properties include: the cobalt and solid-solution strengthening effects of chromium, tungsten, and molybdenum; the formation of metal carbides; and the corrosion resistance imparted by chromium. Cobalt-base alloys are strengthened through solid-solution hardening and carbide precipitation hardening by adding carbon, chromium and molybdenum.

Chromium and molybdenum enhance the corrosion resistance of alloys and improve their mechanical properties by reducing abrasive wear and lowering the stacking fault energy. Co-Cr-Mo alloy, an advanced cobalt-base alloy, is widely used in nuclear power plants, aerospace engine vanes and biomedical surgical implants. In the latter case, they are used to make artificial metal-on-metal hip and knee joints. These Co-Cr-Mo alloys are known for their combination of strong mechanical performance, fatigue resistance, low creep, high resistance to wear/corrosion, and biocompatibility, but their main attribute is corrosion resistance in chloride environments. This property is related to their bulk composition (principally the high chromium content) and the formation of a protective surface oxide layer (nominally Cr2O3).

Cobalt-base alloy implants can be conventionally manufactured using wrought or cast techniques. Wrought cobalt alloys are made by forging the material at elevated temperatures under high pressure. Additionally, novel methods for the near-net-shape formation of parts from metal powders via Metal Injection Moulding (MIM) are currently being explored. New applications for MIM components are trending toward smaller, more-complex devices for minimally invasive surgery, especially laparoscopic instruments for grasping tissue, cutting and suturing. Such devices are being designed for greater freedom of movement, which has increased the number of metal components used in the assembly.