Study Of Nitriding Of Cast Pure Titanium

Titanium and its alloys are widely used in many areas such as aerospace, marine and biomedical areas due to their outstanding properties such as good mechanical property, chemical stability and biocompatibility. Pure titanium has been used to make crowns and fixed partial denture, denture framework, implants and orthodontic brackets. Titanium is easily passivated and a condense and stable oxide film formed on its surface at room temperature, so it has excellent corrosion resistance. The main problem in using titanium as medical materials is its low hardness and wear resistance. When titanium prosthesis functions in the oral environment, the surface passivation layer is easily destroyed by frictional force, shearing force and other forms of stress. Carries preventive agent such as fluoride-containing toothpaste, gel can also destroy the passivation film to some extent. TiN is known to possess high hardness, good tribological properties, excellent chemical stability and biocompatibility. TiN can be produced either by nitriding on the Ti alloys or by some coating techniques such as physical vapor deposition (PVD), chemical vapor deposition(CVD). In this study plasma nitriding and PVD (arc ion-plating) were used to improve wear and corrosion resistance, surface color of pure cast titanium, the results is expected to improve properties of denture made from titanium.1 Plasma nitriding of pure cast titaniumCP Ti (TA2) was cast in dental titanium casting machine (LZ-2), the specimens were placed into nitriding chamber after being mechanically polished and ultrasonically cleaned and then plasma nitrided in the NH3 atmosphere. The nitriding temperature was 700℃, glow voltage 700V, current intensity 13-15A, chamber pressure 150Pa and nitriding time 1-4h.After plasma nitrided, the titanium specimens appeared dark golden in color, it was not uniform and reduced to the edges due to the"edges effect"of glow discharge. Microhardness, wear resistance and corrosion resistance in artificial saliva improved significantly. During the process of nitriding, a compound nitriding layer containing TiN and Ti₂N formed on the specimen surface, underneath the compound layer was diffusion layer that consisted of an interstitial solution of nitrogen in theα-Ti, the compound layer adhered to the substrate well, the formation of diffusion layer and compound layer increased the wear and corrosion resistance of pure cast titanium greatly.With the increase of nitriding time, microhardness, wear resistance and corrosion resistance in artificial saliva increased gradually. The main reason is that the thickness of compound layer increased with the increase of nitriding time. The main problem of plasma nitriding is that the color is not uniform and treatment time is long. In order to acquire useful nitrided layer, the nitriding time is 1h at least.2 Deposition of TiN film on pure cast titanium by pulsed bias arc ion platingPulsed vacuum arc ion plating was used to deposit TiN film on surface of pure cast titanium. The cast titanium specimens were mechanically polished, ultrasonically cleaned before being placed into vacuum chambers. The base pressure of the vacuum chamber before operation was 8×10-3Pa, the Ar gas was introduced and the specimens were cleaned by Ar ion bombardment for 15min, then deposition of Ti began at a substrate bias of 400V with an arc current of 70A for 3min. After that deposition of TiN began by introducing N2 into the vacuum chamber and the substrate bias was 100V. The deposition lasted for 30min and the arc current was controlled at 70A. The processing parameters were adjusted and the TiN film was characterized deposited at different conditions, the arc current was controlled at 55A, 70A and 90A , the substrate bias at 50V, 100V and 150V, nitrogen pressure at 1.1×10-1, 1.5×10-1, 1.7×10-1 and 1.9×10-1Pa,the substrate temperature was controlled at 400, 500 and 600℃respectively. The film was characterized using a range of analytical and measurement techniques, including scanning electron microscopy (SEM), X-ray diffraction(XRD), XPS, and nanoindentation, etc.After TiN film was deposited on the surface of cast titanium specimens, the specimens appeared bright and golden, microhardness, wear resistance and corrosion resistance improved significantly. Oral environment was simulated by a toothbrushing wear testing machine in which untreated titanium samples and TiN coated samples were toothbrushed with fluoride-containing toothpaste slurry, after being brushed, to the untreated titanium samples, surface roughness increased, surface morphologies changed obviously, discoloration and tarnish could be observed, while surface morphologies and color of TiN coated samples didn't change significantly indicating TiN has good color stability and can protect cast titanium from excessive abrasion.Color of TiN film was different when the N2 pressure was changed, it appeared bright and golden in color when N2 pressure was 1.7×10-1Pa, arc current 55A and substrate bias 100V, the color leaned to red yellow when N2 pressure increased and to light yellow when N2 pressure decreased. Microhardness of TiN increased when N2 pressure increased, the hardness was largest when N2 gas pressure was 1.7×10-1Pa, then the hardness began to decrease when N2 gas pressure continued to increase. The change of hardness is related with the change of phase structure. If the N2 gas pressure was low, the film was mixed with Ti₂N and TiN, the phases changed to TiN structure gradually with the increase of N2 gas pressure. The maximum hardness was attained when N2 pressure was 1.7×10-1Pa, there were plenty of Ti₂N phases at this time and the multiphase structure increased the film hardness.Arc ion plating possess the advantage of evaporation coating(high deposition rate) and sputtering coating(high energy deposition particles), besides it has the advantage of high ioning rate, good film substrate adhesion and . The drawback of ion plating is the formation of macrodroplet during coating process. The use of magnetron filter and pulsed substrate bias instead of direct bias can decrease size and quantity of macroparticle and improve film property. Compared with plasma nitriding, pulsed bias arc ion plating has many advantages including an even TiN film formed on surface of titanium specimens, the temperature of substrate was low and treatment time was short, usually the deposition time was 30min. But the wear resistance of ion-plated TiN film needs improving.Combined with plasma nitriding and pulsed bias arc ion plating is a good way to improve film property, it can overcome the color ununiformity problem of plasma nitriding and improve the wear resistance of ion plated TiN film. Cast titanium is plasma nitrided before TiN film deposition to acquire surface hardening layer which adheres to the substrate well, the layer acts as an interlayer and can support the hard TiN film without plastic deformation when subjected to a high-intensity loading, it can also optimize the stress distribution from the film to the substrate and enhance the film adhesion.After titanium framework of denture was plasma nitrided and TiN coated, its surface was smooth and appeared bright and golden, it had good color stability, its wear and corrosion resistance improved significantly, so the compound surface treatment of pure titanium has major significance in stomatology.