Practically all of the major medical implant OEMs are actively pursuing, in one way or another, the viability of manufacturing various common implants from ceramic materials. This is presenting significant challenges when it comes to machining. By Don Graham, Manager of Education & Technical Services, Seco Tools

Ceramics are perfect for implant use. They provide much higher levels of strength, wear resistance, smoothness and biocompatibility when compared with metals and polymers. However, ceramics lack one very important quality – machinability.

Mention ceramics, and most people visualise dinner plates or coffee mugs that easily shatter when dropped on a hard surface. This isn’t the case when it comes to industrial and medical ceramics. Those are much tougher, denser, and therefore not as brittle and, unfortunately, not that easy to machine using conventional methods. Thankfully though, laser beams may offer a remedy.

Currently, a very select few of ceramic implants are being produced. They are simplistic in shape because they are produced using grinding machines with diamond wheels that have limited capabilities when it comes to accessing contours and pockets and other complex part shapes. The grinding process is also slow, making manufacturing costly and, in turn, the implants extremely expensive – so much so that limited numbers of patients opt for them over the more affordable metal implants.

The majority of implants produced today are made from titanium, cobalt chrome or stainless steel. The most common implants are for knee and hip replacements, but femoral, articular and tibular components are also prevalent.

The average lifespan of metal implants depends on use. The more active the implant recipient is, the faster the implant will wear. In some instances, this may only be a short time of about ten years, possibly 25 years for a less active person. This means that, for younger recipients of metal implants, the initial implant would quite possibly have to be replaced one or two additional times in the course of the person’s lifespan. And it should be noted that the rehabilitation for such orthopaedic-type operations as knee and hip replacements is quite painful and extensive.

Now consider ceramic implants, which would last on average 75 years – basically a person’s lifetime. An implant recipient would undergo only one surgery and a single recuperation period. Plus, there would be no implant abrasion to generate foreign particles in the body, as occurs with metal implants when they wear.

On the flip side, none of the benefits of ceramic implants will be realised until the material can be cost-effectively machined, making those implants more readily available and affordable. This is why manufacturing shops, universities and other research facilities have been exploring and testing different approaches to successfully machine ceramics using conventional machine tools. And so far, one technique that involves a laser is generating some very promising results.

Key elements of this cost-effective ceramics machining process are specially designed cutting inserts and a radical use of a laser mounted on a multi-tasking machine tool. The machine precisely positions the laser beam ahead of the cutting insert to plasticise the workpiece material, making it easier to cut.

Developments in cutter technologies that are advancing the cost-effective machining of ceramics include polycrystalline diamond (PCD) and cubic boron nitride (CBN). CBN shows strong potential in several ceramic applications. Additionally, extremely hard carbide tooling has been tested for ceramics.

To date, laser-assisted machining has made it possible to successfully turn, mill and thread ceramic materials such silicon nitride, zirconium and alumina. But most significantly, the system increases cutting tool life and reduces processing times for these materials, while also allowing parts to be produced that were previously impossible to make.

Those entities involved with the development of laser-assisted machining techniques will continue to gain a better overall understanding of the ceramic cutting process, and great strides will be made in the use of ceramics within the medical industry, as well as for other applications such as automotive and aerospace engine components and bearings. Currently, however, there must be more testing to gain a better understanding of cutting tool edge preparations and chemical interactions between cutting tools and specific ceramic materials. Further testing will also help increase efficient use of the laser to heat the ceramic materials faster and improve precision in regard to the portions of a workpiece that need heated.

If progress with the laser-assisted method continues at its current pace, the machining of ceramics could more than likely replace diamond wheel grinding in much the same way that hard turning replaced grinding 20 years ago. And while the method is in its infancy, a major milestone has been passed in the quest to reduce the cost of manufacturing medical implants and components from industrial ceramics.

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