Processing And Biomedical Properties Of Commonly Used Dental Alloys

In this study, the techniques of laser welding, chemical polishing and surface bioactivating treatment were adopted to process the commonly used dental alloys, including CoCr, NiCr, NiTi and pure titanium. The analyzing methods of XRD, SEM, EDS and AAS were used to investigate the microstructure, mechanical properties and corrosion resistance of the samples processed by above technologies. In vivo and in vitro tests, as well as clinical application, were chosen to evaluate biocompatibility and bioactivity of the processed samples. The results are as follows:1. The suggested laser welding parameters is electric voltage 280 V, pulse time 10 ms, spot diameter 0.6 mm. Under this condition, the tensile strength (565.42±60.02 MPa) and elongation (6.24±0.38%) of laser-welded CoCr-NiCr alloys could meet common requirements of dental prostheses. The microstructures of the fusion zone were dense and fine. Metallurgical bonding between the fusion zone and both of the parent metals was observed. In the side of NiCr alloy close to the fusion zone, the presence of inter-grain corrosion was found. Heat treatment and heating in porcelain processing led to decreasing this tendency. The atoms of element beside the CoCr alloy in the fusion zone diffused evenly, and heat treatment resulted in more even element distribution, rather than heating in porcelain processing. After laser welding, the joint of dissimilar alloys showed good corrosion resistance. During electrochemical corrosion, CoCr alloy acted as anode and protected NiCr alloy from corrosion, which contributed to reducing the release of Ni²⁺ ion from NiCr alloy. Concerning laser welded CoCr-NiCr dissimilar alloys, both the in vivo and in vitro experiments indicated that, there was no cytotoxicity reaction, the pathohistological reaction was grading zero, there were no histomorphologic changes after long-term contact with oral mucosa, the RGR (relative growth rate) was 93.69%, the reaction degree was grade one, and the acute toxicity of whole body was conform to the regulations. Clinical application and follow-up visit observation demonstrated that there were no signs of crack, deformation, corrosion and discoloration in welding zone of laser welded CoCr-NiCr dissimilar alloys, the dental prostheses could be inserted and taken off smoothly. And the dentures were also felt comfortable with the excellent retention and masticatory efficiency.2. The chemical polishing liquid for dental casting pure titanium was developed, which was consisted of hydrofluoric acid, nitric acid and lactic acid in volume ratio of 1:1:3. After 1min of chemical polishing, the Ra value of titanium casting samples was 0.092μm, the weight-lost rate was 2.768%, and the thickness of the superficial oxidized film was about 5μm. Under adequate condition, the chemical polishing technique merely affected the mechanical properties of the alloys, and the size of the polished samples met the clinical precision requirement of the dental prostheses. Clinical application verified that chemical polishing of dental titanium prostheses was simple and convenient, excellent polishing effect could be achieved, the polished surface maintained a lasting luster after a long-term use, and there was no apparent discoloration or getting darkness.3. The chemical bioactivating treatment was used to coat the surface of NiTi alloy with biomimetic HA. The composition and structure of NiTi alloy coated with HA were similar to the mineral material within natural bone. The measurement of the crystal grains was at nano level. After long-term (2 years) use of the implant embedded in the thighbone of rabbit, the result showed the complete osseointegration between the implant and the bone. Compared with matched control group without any treatment on the surface, the NiTi alloy coated with biomimetic HA developed more mature bone structures including osseous lamellas and cement line at the bone reconstruction zone, which manifested that good biocompatibility and long term biological safety of NiTi alloy implant could be realized by means of the technique of chemical activating treatment.