Research

I. Surface Modification of Steels

History of the usage of stainless steel has existed for so many years. In our lab, hot-dip galvannealed sheet steel is mainly focused. Hot-dip galcannealed sheet steel is a widely utilized source for automotive body panels and parts due to its excellent corrosion resistance properties.

Although the alloying process and lubrication during working improves the formability of the sheet steel, for large and more complex parts, a more developed lubricant film coating is required during molding and working process. In addition, the Fe-Al interfacial layer is also considered to have some effects.

Surface Coating on Advanced High Strength Steels

When the pre-coating annealing process was conducted at an extremely low dew point, the external selective oxidation prevailed, resulting in various oxides on the steel surface. However, the oxides did not fully cover the steel surface under the reduction atmosphere. Therefore, the molten Zn(Al) reacted locally with the steel surface to form Fe-Al inhibition alloys. Moreover, the Al in the molten Zn(Al) bath effectively removed the oxides by aluminothermic reduction. This Fe-Al inhibition alloy was then gradually transformed to Al-bearing Fe-Zn phases. During the following GA process, the diffusion of Si from the steel substrate to the coating apparently suppressed the forming of the ζ phase, in which the Si is almost indissoluble. As a result, the Fe-Zn alloy formation was largely retarded; specifically, a discontinuous Fe-Zn IMCs layer was formed after 25 seconds of galvannealing at 773 K (500 ℃).

II. Surface Modification of Titanium-based Alloys

Due to the truth of titanium and its alloys have excellent biocompatibility with bone tissues, they are well-selected as implant materials. In this case, a calcium- and phosphorus-containing film on titanium had been proved to further enhance its bioactivity after being implanted in human body.

We recently focused on the microarc oxidation process to have the surface of titanium be modified. Besides, the SBF immersion process were found to be able to increase the contents of calcium and phosphorus in the anodic film.

III. Surface Modification of Magnesium-based Alloys

Magnesium alloys have become a group of the most widely used materials for their outstanding mechanical properties. However, the poor corrosion resistance is a critical weakness of most magnesium alloys. In improving their resistance to corrosion, surface modification is an indispensable process. Among various modification treatments, conversion coating and anodic oxidation are the most commonly used methods.

In the conversion coating for the magnesium alloys, we have worked for several years on various systems, including chromate, phosphate-permanganate, and rare-earth metals. Beside the improvement, our researches especially focus on the microstructure and the mechanism, which are still less disscussed now.

Conversion Coating on Mg Alloys

The formation mechanism of conversion coating on magnesium alloys is a topic of both thermodynamics and kinetics. Through various characterizations, a further understanding of the relations among microstructure, compositions, and electrochemical properties may be acquired, which is of great help in developing conversion coating systems providing sufficient corrosion resistance and mechanical properties.

Permanganate Conversion Coating on AM30 Magnesium Alloys

The hexavalent chromate conversion coating (CCC) developed by the Dow chemical company had been widely adopt to improve the corrosion resistance of magnesium alloys. However, hexavalent chromium is harmful to human health. Consequently, replacing traditional chromate conversion coating with a new developed conversion process becomes increasingly essential.

The TEM image showed that the thickness of permanganate coating is approximately 200 nm, and the coating is mainly composed of manganese dioxide with an amorphous structure. In short, the permanganate conversion coating showed better adhesion and less severe crack by virtue of increasing the chemical reactivity and lowering the immersion time, expecting to replace hexavalent chromate conversion coating and reduce the cost of production.

Cerium Conversion Coating on AZ91 Magnesium Alloys

Cerium conversion coatings were made on AZ91 magnesium plates in cerium nitrate aqueous solution with H₂O₂. H₂O₂ addition increases the reaction rate; however, the blisters on the conversion coating cause poor adhesion and deteriorate the corrosion resistance of the cerium conversion coating. An alternative approach by the addition of sodium metavanadate (NaVO₃) was used to solve the formation of blisters. Microstructure observation shows that complex precipitates are formed on the coating and the amount of blister is significantly reduced. The results suggest that the presence of VO3^- plays an important role in inhibiting the formation of blisters.

(Figure.(a)-(b) Surface morphology of the cerium conversion coating with/without NaVO₃, immersion time 20s; (a) many blisters remains on the cerium conversion coating without NaVO₃ and (b) some precipitates forms on the coating whereas the blisters are eliminated effectively.)

IV. Electrochemical deposition

Electrochemical deposition is a kind of chemical reaction which has been investigated for a long run, and the process applies to lots of fields and industries, such as the automobile industry, cellular phone industry and anti-corrosion field.

In electrochemical process, the main idea is applying an electric field, driving the cations in electrolyte to be reduced on the cathode. All the electrochemical reactions happen on the surface of electrode, hence also regarding as a type of surface science. By means of method, we can get the material we want or modify the surface of the electrode.

CIS/CIGS solar cell

Cu(InGa)Se₂-based solar cells have often been touted as being among the most promising of solar cell technologies for cost-effective power generation. This is partly due to the advantages of thin films for low-cost, high-rate semiconductor deposition over large areas using layers only a few microns thick and for fabrication of monolithically interconnected modules. Perhaps more importantly, very high efficiencies have been demonstrated with Cu(InGa)Se₂ at both the cell and the module levels. Currently, the highest solar cell efficiency is 19.2% with 0.5 cm² total area fabricated by the National Renewable Energy Laboratory (NREL).

The picture above show the plan view and cross-sectional images of Cu(InGa)Se₂-based solar cells which fabricated by the National Renewable Energy Laboratory (NREL). A compact grain structure is observed in cross-section, and faceted grains are visible in plan view.

CIS-TEOS-HNO3-1-4.6-10-annealing-500C-20min-2

V. Sol-Gel process

Sol-gel process Is a method for producing thin films or solid materials. The mechanism of sol-gel process is the combinative chemical reaction of hydrolysis and condensation. The precursor such as alkylsiloxane would be hydrolyzed to the silanol by adding deionized water in proper pH value. Then the silanol was self-condensation by removing water. The products formed to the silica finally. The surface morphology or surface chemical activity could be modified by sol-gel process. It is widely used in powder metallurgy, ferroelectrics, superconducting material, ceramic material, thin film, etc.

Sol-gel coating on the HDG steels

Zinc coating provided the barrier protection and the sacrificial protection over the steel substrate. To further protect the Zn-coated steel against corrosion during its transportation and storage, surface modification is generally adopted. The sol gel coating process was involved the hydrolysis reaction and the condensation reaction of the precursor. This study is aim to produce a functional and environmental friendly coating with an excellent corrosion protection performance on the zinc substrate using a sol-gel process