Adding an additional element to an existing alloy system to fine tune its properties is a well-established technique. The additional element being added affects the packing of unit cells in a massive way, thereby bringing changes in the structural, morphological and chemical nature of the resultant alloy. This can be crudely visualized by using the example of small grains of sand being used to fill the gaps between rocks inside a jar. Normally the doping element is chosen from the proximity in the periodic table of the existing elements in an alloy system to closely mimic rest of the property. In our next phase of experimentation to develop super hard biocompatible coatings, we aim to develop thin films of Ti3Au doped with small quantities of Ag in varying concentrations. We can recall that my deposition system is only capable of handling two targets at one time. Therefore, to enable us to dope with a third element, we designed a custom made ternary target in which holes are drilled across the racetrack region of Ti target. A calculated number of ternary element plugs are inserted into these holes to perform simultaneous sputtering of a third element along with Ti. These custom made targets have been manufactured for us by Pi-Kem Ltd, UK.
Along with being a noble metal, Ag is also well known for its antibacterial properties and for its electrical conductance. Therefore, addition of Ag to Ti-Au matrix is sure to bring these added benefits to this alloy system. It has been found in studies that greater than 3% atomic concentration of Ag in Ti matrix can bring an antibacterial response better than 98% i.e. 98% of bacterial colonies are terminated. Ti-Ag alloys have also registered improved hardness and elongation values when compared to commercially pure (cp) Ti. The peak for hardness is achieved for Ag concentration between 10-25% and further increase in Ag decrease these values but not below that of cp-Ti. The increase in the mechanical properties is due to three phenomenon: solid solution hardening, precipitated intermetallic compounds and eutectic structures (beyond the scope of a simple blog article, but readers are welcome to read a future journal publication on this subject). However at higher Ag content, Ag from the intermetallic begins to agglomerate and segregate out. This segregation appears as uniformly distributed white balls in in the SEM imaging. This leads to embrittlement of Ti-Ag structures reducing hardness. Increasing concentration of Ag also helps in increasing the electrical conductivity as expected because of high electrical conductivity of metallic Ag.
Therefore, it can be expected that addition of small quantities of Ag will improve the electrical, mechanical and biomedical properties of Ti-Au thin films. But how about Copper, which belongs to the same group as Ag and Au and is known to exhibit similar antimicrobial and electrical properties. Stay tuned to read our article on the effect of Cu in Ti based thin films.
Comments