Introduction
Julius Schwarzer, 14-year-old German student, finds a way to improve hip and knee implants during an industrial placement at SIOmec, the Saxonian Institute of Surface Mechanics. We share his findings with all medical device manufacturers in order to safe expensive, uncomfortable and painfull revision surgery thanks to next generation implants. You can also watch his presentation here:
All the projects can be downloaded and opened in the FilmDoctor Viewer package.
State of the art
The implants he looked are using a 6 µm thick hard coating on a titanium substrate.
Best and worst cases
At first he looked at different scenarios and found out that the coating is working well and protects the substrate in most of the cases, especially if the patient is walking normally. In some cases (e.g. missing a stair or stepping into a hole) the contact conditions change dramatically. Now an impact contact situation is created, which increases the contact load and effective counterpart size. These changed conditions can lead to plastic flow within the substrate at the interface to the top coating. This chould lead to micro cracks and over time to an adhesion failure of the coating.
Counterpart size comparison
To detect the worst case scenario he evaluated the stress fields using different counterpart sizes. He found out that a counterpart with a diameter of 200 µm is the most dangerous one for the used coating system.
Optimization
Now he tried to improve the application performance of the coating system for the detected worst case scenario with the 200 µm counterpart.
His solution
As he could see the worst case contact conditions create a very big plastic zone which reaches deep into the substrate. So he added a second layer between the actual coating and the substrate to prevent these high stresses in the substrate.
Because the new coating has a higher yield strength than the substrate is an hold the created stresses without creating microcracks and the substrate is protected.
Improved solution
The solution was a very simple approach and therefore he improved the already working solution furthermore. He added a new 1 µm top coating and reduced the original coating thickness from 6 µm to 5 µm. The thick second layer was also divided into several sublayers. The coating thickness was reduced from 30 µm to 27 µm and three 1 µm thick layers were added. Each layer has a different Young’s modulus to create a nice gradient instead of a jump from 450 GPa to 250 GPa. This leads to a better stress distribution and less shearing stresses at the interfaces.
Finetuned improved solution
In his last step he finetuned the material parameters by dividing the thickest layers into smaller layers and adding intrinsic stresses to these new layers. Instead of 6 layers the implant now is made of 13 layers.
