Studies On The Preparation And Properties Of The Novel Composite Coating For Ni-Ti Dental Burs

According to the problems existing in the clinical treatment of stainless steel dental bur, such as the fracture of substrate, cutting inefficiency and the removement of diamond grit due to the weak bonding between dimaond grits and metallic coatings, we design a new kind of diamond dental bur, in which the biomedical Ni-Ti alloy is used as substrate and novel composite coating as cutting layer. For the novel composite coating, tungsten and phosphor were used as strengthening elements and nanodiamonds as reinforced particles, and the preparation and characteriazation of Ni/nanodiamond, Ni-W/nanodiamond and Ni-P/nanodiamond composite coatings were investigated, and based on the optimum process, the fabrication and performance of Ni-Ti dental bur using diamond embedded in Ni/Ni-P/(Ni-P/nanodiamond) composite coating as cutting layer were investigated.Different mixed acid solutions were used for the pretreatment of Ni-Ti substrate piror to electroplating. The results showed that the pre-treatment in the mixed solutions of HF-H2SO4 and HCl-H2SO4 resulted in the formation of fine needle-like TiH2 phase on the surface of Ni-Ti alloy, which could prevent the oxidation of Ni-Ti subastrate before electroplating and form the metallic bond between Ni-Ti substratrate and electroplated coatings, leading to the good adhesion between Ni-Ti substratrate and electroplated coatings. Howere, due to the severe reacion and cracks formed during pre-treatment in HF-H2SO4 mixed solution, pre-treatment in HCl-H2SO4 mixed solution was selectred as pre-treatment process for Ni-Ti substrate.The optimum process for the preparation of Ni/nanodiamond composite coatings was the current density of 1.5A/dm2, the pH value of 4.0, the stirring speed of 450rpm and the temperature of 30℃The addition of nanodiamond changed the preferred orientiation of nickel from (220) to (111). The maximum hardness and wear resistance was obtained for Ni/nanodiamond composite coatings prepared from the electroplating bath containing 4 g/1 nanodiamond.The addition of nanodiamond did not change the structure of Ni-W/nanodiamond composite coatings, which kept the structure of solid solution of tungsten in nickel. As the prepared current density increased from 8A/dm2 to 20A/dm2, the tungsten content in Ni-W coatings increased from 32.63 wt.% to 34.35 wt.%. The Ni-W coatings with fine and compact crystal structure and possessed good adhesion with Ni-Ti substrate were prepared at 12A/dm2 Ni-W/nanodiamond composite coatings electroplated from the palting bath containing 8 g/1 nanodiamond possessed the best hardness and wear resistance.The addition of nanodiamond lead to the grain refinement of Ni-P coatings, which changed the structure of Ni-P coatings from amorphous to nanocrystalline. Ni-P coatings prepared at the optimum parameters with the current density of 2A/dm2, the pH value of 2.2 and the temperature of 80℃had the amorphous structure and phosphor content up to 10 wt.%. The best hardness and wear resistance of Ni-P/nanodiamond composite coatings was prepared from the electroplating bath containing 8 g/1 nanodiamond.During the preparation of dental bur, the amount and embedding rate of micro diamond paricles increased with increasing the grain-planting current density and post-plating time. Taking account of both the cutting effect of diamond particles and the holding force of metal matrix for diamond parcitles, the grain-planting current density of 1.5A/dm2 and post-plating time of 4-5h were used.Among the above three nanodiamond composite coatings, Ni-P/nanodiamond composite coatings have the best hardness and wear resistance, and Ni-W/nanodiamond composite coatings took the second place, and Ni/nanodiamond composite coatings have the lowest. Abrasive wear and adhesive wear were the main wear mechanism for nanodiamond composite coatings during the dry sliding process.The results of characterization for Ni-Ti dental bur using diamond embedded in Ni/Ni-P/(Ni-P/nanodiamond) composite coating as cutting layer prepared under the above optimum process indicated that diamond particles with proper ebedding rate distributed uniformly in the composite coatings, and the good adhesion between Ni-P/nanodiamond composite coating and diamond particles. The cutting efficiency and service life of Ni-Ti dental bur was about 1.3 and 1.5 times to that of stainless steel dental bur, indicating the property improvement of NiTi dental bur.