Surface treatment technology of titanium
Titanium easily reacts with O, H, N and other elements in the air and Si, Al, Mg and other elements in the embedding material at high temperatures, forming a surface contamination layer on the surface of the casting, making its excellent physical and chemical properties worse, and hardness Increase, plasticity and elasticity decrease, and brittleness increases.
The density of titanium is small, so the inertia of titanium liquid is small when it flows, and the fluidity of molten titanium is poor, resulting in low casting flow rate. The difference between the casting temperature and the mold temperature (300℃) is large, the cooling is fast, and the casting is carried out in a protective atmosphere. It is inevitable that there are defects such as pores on the surface and inside of the titanium casting, which have a great impact on the quality of the casting.
Therefore, the surface treatment of titanium castings is more important than other dental alloys. Due to the unique physical and chemical properties of titanium, such as low thermal conductivity, low surface hardness, and low elastic modulus, high viscosity, low conductivity, and easy oxidation This brings great difficulty to the surface treatment of titanium, and it is difficult to achieve the desired effect with conventional surface treatment methods. Special processing methods and operating methods must be used.
The later surface treatment of the casting is not only to obtain a smooth and shiny surface, reduce the accumulation and adhesion of food and plaque, and maintain the patient’s normal oral microecological balance, but also to increase the beauty of the denture; more importantly, through These surface treatment and modification processes improve the surface properties and suitability of castings, and improve the physical and chemical properties of dentures such as wear resistance, corrosion resistance and stress fatigue resistance.
1. Removal of surface reaction layer
The surface reaction layer is the main factor affecting the physical and chemical properties of titanium castings. Before the titanium castings are ground and polished, the surface contamination layer must be completely removed to achieve a satisfactory polishing effect. The surface reaction layer of titanium can be completely removed by pickling after sandblasting.
1. Sandblasting: For the sandblasting of titanium castings, rough blasting of white corundum is generally better. The pressure of sandblasting is lower than that of non-precious metals, and it is generally controlled below 0.45Mpa. Because when the injection pressure is too high, the sand particles impact the titanium surface to produce intense sparks, and the temperature rise can react with the titanium surface, forming secondary pollution and affecting the surface quality. The time is 15~30 seconds, and only the sticky sand, surface sintered layer and part and oxide layer on the surface of the casting can be removed. The remaining surface reaction layer structure should be quickly removed by chemical pickling.
2. Pickling: Pickling can quickly and completely remove the surface reaction layer, and the surface will not be polluted by other elements. Both HF-HCl series and HF-HNO3 series pickling liquids can be used for pickling of titanium, but HF-HCl series pickling liquids have a larger hydrogen absorption capacity, while HF-HNO3 series pickling liquids have small hydrogen absorption and can control HNO3 The concentration of HNO3 can reduce hydrogen absorption, and the surface can be brightened. Generally, the concentration of HF is about 3% to 5%, and the concentration of HNO3 is about 15% to 30%.
2. Treatment of casting defects
Internal pores and shrinkage cavity internal defects: can be removed by hot isostatic pressing, but it will affect the accuracy of the denture. It is best to use X-ray inspection to remove the exposed pores on the surface and use laser repair welding. Surface pore defects can be directly repaired by laser local welding.
Three, grinding and polishing
1. Mechanical grinding: Titanium has high chemical reactivity, low thermal conductivity, high viscosity, low mechanical grinding and grinding ratio, and it is easy to react with abrasives. Ordinary abrasives are not suitable for grinding and polishing titanium. It is better to use good thermal conductivity. For ultra-hard abrasives such as diamond, cubic boron nitride, etc., the polishing linear speed is generally 900~1800m/min. Otherwise, grinding burns and micro-cracks may occur on the titanium surface. compressed spring
2. Ultrasonic grinding: Through the action of ultrasonic vibration, the abrasive grains between the grinding head and the ground surface and the ground surface move relative to each other to achieve the purpose of grinding and polishing. The advantage is that the grooves, dimples and narrow parts that cannot be ground by conventional rotating tools become easier, but the grinding effect of larger castings is not yet satisfactory.
3. Electrolytic mechanical composite grinding: Using conductive abrasive tools, applying electrolyte and voltage between the abrasive tools and the grinding surface, through the combined action of mechanical and electrochemical polishing, reduce surface roughness and improve surface gloss. The electrolyte is 0.9NaCl, the voltage is 5v, and the speed is 3000rpm/min. This method can only grind flat surfaces, and the grinding of complex denture supports is still in the research stage.
4. Bucket grinding: Use the centrifugal force generated by the revolution and rotation of the grinding barrel to make the denture and the abrasive in the barrel frictionally move to reduce the surface roughness. The grinding is automatic and efficient, but it can only reduce the surface roughness but not increase the surface gloss. The grinding accuracy is poor, and it can be used for deburring and rough grinding before denture fine polishing.
5. Chemical polishing: Chemical polishing is the purpose of leveling and polishing through the oxidation-reduction reaction of metal in a chemical medium. The advantage is that chemical polishing has nothing to do with the hardness of the metal, the polishing area and the shape of the structure. All parts in contact with the polishing liquid are polished, no special complex equipment is required, and the operation is simple. It is more suitable for polishing complex structure titanium denture stents. However, the process parameters of chemical polishing are difficult to control, and it is required to have a good polishing effect on the denture without affecting the accuracy of the denture. A better titanium chemical polishing solution is HF and HNO3 prepared in a certain proportion. HF is a reducing agent, which can dissolve titanium metal and have a leveling effect. The concentration is less than 10%. HNO3 has an oxidizing effect to prevent excessive dissolution of titanium and hydrogen absorption. , At the same time can produce bright effect. Titanium polishing solution requires high concentration, low temperature and short polishing time (1~2min.).
6. Electrolytic polishing: also known as electrochemical polishing or anodic dissolution polishing. Due to the low conductivity of titanium, the oxidation performance is very strong. The use of acidic electrolytes such as HF-H3PO4 and HF-H2SO can hardly affect titanium. After polishing and applying external voltage, the titanium anode will be oxidized immediately, making the anode dissolution impossible. However, the use of anhydrous chloride electrolyte at low voltage has a good polishing effect on titanium. Small specimens can be mirror polished, but for complex restorations, complete polishing can not be achieved. Maybe change the cathode shape and add cathode The method that can solve this problem needs further research. Extension spring
Fourth, the surface modification of titanium
1. Nitriding: chemical heat treatment techniques such as plasma nitriding, multi-arc ion plating, ion implantation and laser nitriding are used to form a golden yellow TiN infiltration coating on the surface of the titanium denture, thereby improving the wear resistance and corrosion resistance of titanium And fatigue resistance. However, the technology is complicated and the equipment is expensive, and it is difficult for the surface modification of titanium dentures to achieve clinical practicality.
2. Anodizing: Titanium anodizing technology is relatively easy. In some oxidizing media, under the action of applied voltage, titanium anode can form a thicker oxide film, thereby improving its corrosion resistance, wear resistance and weather resistance. The electrolyte for anodic oxidation generally uses H2SO4, H3PO4 and organic acid aqueous solutions.
3. Atmospheric oxidation: Titanium can form a thick and firm anhydrous oxide film in the high temperature atmosphere, which is effective for overall corrosion and crevice corrosion of titanium, and the method is relatively simple.
In order to increase the beauty of titanium dentures and prevent the discoloration of titanium dentures from continuous oxidation under natural conditions, surface nitriding treatment, atmospheric oxidation and anodic oxidation surface coloring treatments can be used to form light yellow or golden yellow on the surface to improve the titanium denture’s Beauty. The anodic oxidation method uses the interference effect of the titanium oxide film on the light to produce natural color, which can form colorful colors on the titanium surface by changing the cell voltage. Torsion spring
6. Other surface treatment
1: Surface roughening: In order to improve the bonding performance of titanium and facing resin, the surface of titanium must be roughened to increase its bonding area. In clinical practice, sandblasting is often used for roughening treatment, but sandblasting will cause contamination of aluminum oxide on the titanium surface. We use oxalic acid etching to obtain a good roughening effect. The surface roughness (Ra) can reach 1.50 after 1h of etching. ±0.30μm, 2h etching Ra is 2.99±0.57μm, which is more than double the Ra (1.42±0.14μm) of sandblasting alone, and its bond strength is increased by 30%.
2: Surface treatment for high temperature oxidation resistance: In order to prevent the rapid oxidation of titanium at high temperatures, titanium silicon compounds and titanium aluminum compounds are formed on the surface of titanium to prevent oxidation of titanium at temperatures above 700°C. This kind of surface treatment is very effective for the high temperature oxidation of titanium. Perhaps this kind of compound coated on the titanium surface is beneficial to the combination of titanium and porcelain, and further research is needed.