Preparation And Properties Of Nano-TiO₂ Composite Coatings On Magnesium Alloy

As the most potential "green material" in the development, research and application in the 21st century, magnesium and its alloy have been paid more and more attention. Because of low density, high specific tenacity and rigidity, excellent damping and casting property, the strong electromagnetic shielding efficiency and recoverability, "green material" are widely used in the aerospace, automobile manufacture, 3C and many other domains.At present, the amount of casting magnesium alloy products are bigger than the wrought magnesium alloy. But the wrought magnesium alloy may obtain higher intensity, better ductility and diverser mechanics performance, and it may satisfy the different situation as a structural element. Therefore, the development of wrought alloy will be a longer-term trend in the future.But the chemical property of magnesium alloy is extremely active with its low standard electrode potential, so it has a poor corrosion resistance. It will be oxidized even if under the room temperature in the air. So for the magnesium alloy electronic products and the biology medical instrument product, the corrosion of body fluid and the biology also appear very obviously. Moreover the surfaces can breed and disseminate the germ, the virus and other microorganisms which is a threat to people's health. So people are more and more eager to one kind antibacterial material which can both enhance surfaced anti-corrosive performance and have good self-cleaning function on electric appliance, daily product, hospital facility, food processing factory and so on.As a one kind of weak n-semiconducting material, TiO₂ has good chemical stability and photochemical catalysis activity, which can degrade the organic matter, eliminates the dirt, sterilize bacterium and remove smells. In order to solve the problem, we used the method of chemistry composite plating, and chosen the most broadest applicative AZ series magnesium alloy loaded nanometer TiO₂ composite coating.This article determined with the best craft formula by Orthogonal experiment method, discussed the ingredients of AZ series agnesium alloy and other craft parameters, tested the binding force between composite coating and substrate, the microhardness, the corrosion resistance and so on. Using SEM, TEM, XRD and EDS, we observe and analyse the composite coating appearance, the ingredient and the structure. Meanwhile we carried on the discussion to the initial deposition mechanism of composite coating, researched the photochemical catalysis activeness and the antibacterial and sterilized performance. In the process of electroless composite plating, the bath components are various and the plating parameters are complex. But the most important to the chemical plating process is the main salt, reducing agent, complexing agent, temperature, mixing speed and pH value. And this kind of influence is mainly on plating solution stability, the deposition rate and the uniform distribution of nanometer particles. The experiment indicated that, (1) temperature and the mixing speed are important to the dispersion of nanometer particles in the plating solution. When the temperature increases, the deposition rate speeds up, which can be advantageous to dispersition and suspension of the nanometer TiO₂ pellet. But if the temperature is too high, bath stability will be reduce. The suitable agitation may enhance the effective aerosol of nanometer TiO₂ pellet in the plate solution. When mixing speed is excessively quickly, the function of pellet washout and blows rubs aggravates. This article thought that plating temperature is suitable in 87～93℃, the better mixing speed is smaller than 400r/min。(2)The recruitment of nanometer TiO₂ can affect the coating content and the composite coating performance, TiO₂ recruitment scope in 3～9g/L is appropriate. If without stir, 5g/L recruitment is good. If increasing agitation, the recruitment is about 9g/L。(3)Coating surface often exists a few holes, therefore we must take certain measure to seal the holes. The experiment indicated that the TiO₂-SiO₂ sol-gel law with 400℃heat treatment, the TiO₂ sol electrophoresis with 400℃heat treatment and two times composite plating law can all reduce the porosity, also does not affect coating the performance.Under the room temperature Ni-P TiO₂ composite coating is composed of the non-crystal and a few crystal texture. With the heat treatment temperature increasing, the crystal tendency is more obvious. Due to the hindrance of nanometer TiO₂ pellet to the substrate element migration, the anti-local distortion ability is strengthened. So the separation temperature of Ni₃P promotes to 400℃. After the heat treatment of 400℃, the micro-hardness may reach above HV800. Therefore the nanometer composite coating may strengthen wear resisting property.The nanometer composite coating has obviously promoted the substrate electrode potential. The file test and heat shock experiment have proved the union of composite coating and the substrate is excellent, it can cut off effectively the contact between substrate and the external environment. Coating nanometer particle also can reduce the porosity of the Ni-P coating, enhance the compactness and the anti-corrosive performance.The nanometer TiO₂ composite coating has obvious photochemical catalysis performance. Under natural illumination condition, it can cause the methylene blue solution oxidation discoloration. The antibacterial and sterilized experiment to golden yellow Staphylococcus ATCC25923 and Escherichia coli ATCC 25922 indicate that the antibacterial rate of Ni-P+TiO₂ achieve 99.2% and 100% respectively (take the Ni-P coating sample as the comparison). So the coating is effective to resist bacteria and sterilization.The research to nanometer composite plating of AZ series magnesium alloy indicated that because of the potential difference between the alloy beta (Mg₁₇Al₁₂) phase and alpha (Mg) phase, it is possible to form the miniature galvanic battery in plating solution. The electron lost from Mg in the alpha (Mg) phase shifts to beta (Mg₁₇Al₁₂) phase, and the Ni²⁺ in the bath is attracted to the crystal boundary and returns to original state. After this deposition first appears at the crystal boundary. In following process of deposition, on the one hand initial deposited Ni has the autocatalysis function, on the other hand initial deposited Ni also could form the miniature galvanic battery with the beta (Mg₁₇Al₁₂) phase and alpha (Mg) phase, besides the miniature galvanic battery between the beta (Mg₁₇Al₁₂) phase and alpha (Mg) phase. These have all caused the increment of shape nucleus rate, promoted the Ni deposition. About 50 seconds after plating, substrate surface is already completely covered by Ni-P alloy. At this time the Ni autocatalysis activeness is playing a complete role in deposition. Therefore, the initial deposition mechanism can divide three stages. At first, the electrochemistry deposit mechanism appears, then hypophosphite reduction mechanism and electrochemical mechanism are coexisting. After the surface is covered completely by Ni, the mechanisms transform to the hypophosphite reduction mechanism. But in the process of plating, nanometer TiO₂ always stochastically deposits to the substrate surface without the phenomenon of "the segregation". After the nanometer TiO₂ pellet deposits stochastically to the substrate surface, the nanometer TiO₂ pellet is wrapped gradually with the Ni-P, then the new nanometer TiO₂ pellet deposits stochastically to the surface again with wrapping, going round and beginning, one layer upon layer by alveolar coarsening way forms the composite coating with anti-bacteria and sterilization active included the nanometer TiO₂ pellet.