Application and wear durability of hybrid thin films on dental steel instruments

Dentistry is one of the few fields today where use of steel instruments operated by hand is acommon practice. These instruments need to possess mechanical properties necessary for theiroperation, good corrosion resistance due to daily autoclaving, and surface related properties, suchas non-stick performance, to shape restorative composite materials. This work investigated howwear resistance, corrosion protection and non-stick properties of dental steel instruments couldbe improved by choosing suitable base steel material, optimal surface treatment and a combinationof thin films. The investigation focused on Gracey curettes and restorative instruments – these aretools for removal of dental calculus and for constructing dental filling, respectively.  Change of base steel material to a highly alloyed high-speed steel (HSS) was shown more effectiveto improve the wear resistance than use of physical vapor deposition (PVD) coatings. Limitedcorrosion resistance of this HSS imposed a need for corrosion protection. Three types of corrosionprotective coatings were investigated: a commercial PVD CrN coating, an in-house depositedatomic layer deposition (ALD) nanolaminate coating consisting of Al2O3/TiO2, and a hybrid coatingconsisting of both of these. Use of plasma pre-treatment was shown critical for the adhesion of theALD film, and its success in corrosion protection.  Superior corrosion protection was shown with the hybrid PVD/ALD coating, with corrosion currentdensities two orders of magnitude lower than with the PVD coating alone. As the corrosionprotection in these hybrid coatings is based on ALD film sealing the pinholes in the PVD coating,the hypotesis in this work was that the corrosion protection should be mostly unaffected even ifthe ALD film is completely worn away from the top of the PVD surface. In this work, it wasshown that the corrosion protection mostly remains after such surface wear, but some of protectionis lost due to new defects opening and thus creating new pathways for the corrosion to attack thesubstrate. These hybrid coatings could be used in applications with heavy abrasive wear, such asGracey curettes.  ALD films were also utilized in multistep fabrication process of omniphboic coatings onrestorative instruments. These coatings consisted of surface structuring by etching in HF:H2O2,surface stabilization by a 7 nm ALD film, and chemical surface functionalization by a self-assembled monolayer of fluorinated organosilicon. This coating greatly reduced the amount ofrestorative materials sticking to the instrument, as shown by contact angle measurements and acustomized dipping procedure developed to simulate dental restorative operation. Some of theperformance with these omniphobic coatings was lost when the surface was worn and autoclaved.