Surface engineering of titanium and titanium alloys for dental applications

This thesis features the research work related to thermochemical surface engineering of titanium and titanium alloys for use within the biomedical industry. This work is motivated by the high requirements to the surface appearance and wear properties set for biomedical components. Degradation by wear is considered one of the most significant drawbacks of titanium and titanium alloys. Hitherto, wear mitigating surface modification strategies have entailed high temperatures, thus limiting the geometrical complexity of parts or negatively impacting appearance. For biomedical applications, the appearance of a product is often of utmost importance, and for dental applications, the metallic luster of a product can be
particularly unwanted; examples are the metallic parts of a pin tooth and scan-abutments, where a white surface is highly preferred to a metallic shine.

A new patented gaseous surface engineering process for achieving robust white surfaces on CP titanium was developed. In the first step, gaseous thermochemical oxidation of CP titanium ina low partial pressure atmosphere (pO₂) of CO/CO₂ at 750°C yielded a grey dense oxide layer fully adhering to the substrate. No signs of the micro-spallation usually associated with titanium oxidation were seen. The oxide layer was found to consist of 200-300 nm thick interwoven rutile layers with carbon located in between the individual layers. In the second step, oxidation in an N₂/N₂O atmosphere resulted in the oxide layer turning white without compromising the mechanical integrity. The carbon located between the rutile layers was "retracted" during the second oxidation. X-ray diffraction residual stress analysis of both the grey and the white oxide layer revealed compressive stress throughout the entire layer thickness. The calculated stresses at the surface are relatively low (approximately -130MPa for the grey oxide and -100MPa for the white oxide) and increase towards the oxide–metal interface (approximately -400MPa for the grey oxide and -170MPa for the white oxide). Significant   stress relaxation occurred during the second oxidation step (2.5 times lower close to the interface).

A low-temperature surface hardening process using oxygen (and carbon) was developed and investigated. Oxidizing at low temperatures (<700°C) in a buffered CO/CO₂ gas system leads to a fully adhering oxide layer. This chemically attached oxide functions as an oxygen reservoir during a subsequent vacuum treatment (<10^-4 mbar). Complete dissolution of   the oxide layer was achieved whilst developing a relatively deep (20μm) diffusion zone (oxygen in solid   solution). The surface hardness achieved with this low-temperature process exceeds 700 HV.  Spectrophotometry results showed that the treated components regained all of their metallic  luster. Wear testing using ball-on-disk showed more than 3.5 times wear reduction compared  to untreated parts. Experiments were successfully repeated in an industrial-style retort   furnace, indicating that the process can be scaled to meet industrial needs. The process is patent pending.

A patent pending low-temperature nitriding process was also developed and studied. Gaseous nitriding of CP titanium, Ti6Al4V, Ti15Zr, and Ti13Zr13Nb using a high nitrogen activity NH3 atmosphere at temperatures below 700°C was investigated. The process leads to the formation of a compound layer atop a diffusion zone of nitrogen plus hydrides deeper in the material. The hydrogen is then retracted using a vacuum treatment (< 10^-4 mbar). For CP titanium and Ti6Al4V, results showed dissolution of the formed TiN layer on the surface, leaving a translucent Ti₂N layer with a hardness >900 HV on the surface on top of a 20 μm diffusion zone. A reduction in light reflection was recorded due to the Ti2N present on the surface. The metallic luster was fully regained. For the α-alloy Ti15Zr and the near-β Ti13Zr13Nb alloy, the same nitriding process resulted in a suppression of Ti2N as a result of the presence of  zirconium. Both TiN and ZrN were identified using X-ray diffraction. The diffusion zone was found to be approximately 50 μm in thickness which is 2.5 times thicker than what is seen for CP titanium and Ti6Al4V. A lower vacuum temperature/duration preserves the TiN/ZrN layer whilst still retracting the hydrogen.