Y. S. Tian

LASER SURFACE MODIFICATION OF TITANIUM ALLOYS—A REVIEW

Recent developments of laser surface modification of titanium alloys for increasing their corrosion, wear and oxidation resistance are introduced. The effects of laser processing parameters on the resulting surface properties of titanium alloys are reviewed. The problems to be solved and the prospects in the field of laser modification of Ti alloys are discussed. Due to the intrinsic properties, a laser beam can be focused onto the metallic surface to produce a broad range of treatments depending on the input energy. Thus, composite strengthening coatings can be fabricated by the methods of laser alloying, cladding, pulse laser deposition (PLD), etc., which are promising techniques of producing a layer of new materials on the surface of titanium alloys.

RECENT DEVELOPMENTS IN ZIRCONIA THERMAL BARRIER COATINGS

Thermal barrier coatings (TBC) are finding increasing applications in aeronautical, chemical, metallurgy industries, etc. As a ceramic material, ZrO2 is one of the most widely used for TBCs owing to its excellent shock resistance, low thermal conductivity and relatively high coefficient of thermal expansion (CTE). In this paper the thermomechanical properties of ZrO2 and the evolution of ZrO2-TBCs are reviewed. The failure mechanism of TBCs and corresponding methods for lengthening the lifetime of TBCs are discussed. The advantages and disadvantages of graded thermal barrier coatings (GTBC) as well as the problems to be solved in fabricating advanced TBCs are elucidated.

CRACK-FREE WEAR RESISTANCE COATINGS PRODUCED ON PURE TITANIUM AND Ti-6Al-4V BY LASER NITRIDING

Cracks-free wear resistance coatings are fabricated by laser surface nitriding pure titanium and Ti-6Al-4V in an open system. The titanium nitrides which are formed in situ in the top layers of the samples are undeveloped dendritic, and thus reduce the possibility of the crack formation in the coatings during laser processing. The test results show that the nitride layers have higher microhardness and more resistant to sliding wear compared to the as-received sample.

MICROSTRUCTURES AND WEAR PROPERTIES OF THE COMPOSITE COATINGS LASER-FABRICATED ON TITANIUM ALLOY Ti-6Al-4V

Composite coatings containing titanium carbides and borides are produced on the surface of Ti-6Al-4V alloy by laser alloying technique. Microstructural analysis shows that coarse bulk-like and dendritic compounds are formed when alloyed with boron and carbon, separately. However, fine and uniformly distributed dendritic compounds are produced when alloyed with boron and carbon mixed powders. With microhardness of 1450–1600 HV0.1, the coatings have excellent wear resistance compared with as-received sample.

WEAR AND OXIDATION RESISTANCE COATINGS FABRICATED ON Ti-6Al-4V BY LASER SURFACE ALLOYING TECHNIQUE

Composite coatings are produced by laser surface alloying of Ti-6Al-4V with B+Ti and B+Ni+Ti mixed powders. Test results show that the coatings have high hardness, excellent wear and oxidation resistance compared with the as-received sample. In addition, the samples alloyed with B+Ni+Ti have better wear and oxidation resistance than that alloyed with B+Ti.

EFFECT OF LASER PROCESSING PARAMETERS ON THE COATING'S MICROSTRUCTURE AND QUALITY

The effect of laser processing parameters on the microstructure and quality of the coatings fabricated on titanium alloy has been investigated. Results show that the increase of the output power and the decrease of the scanning speed result in the thickness increase and the microstructure coarseness of the coatings. Overlapped fraction of the laser tracks significantly affects the coatings quality. With a lower overlapped fraction, pores and cracks easily occur in the overlapped area due to the inferior metallurgical quality.

Wear and oxidation resistance of composite coatings fabricated on Ti-6Al-4V by laser alloying with nitrogen and silicon

Composite coatings were fabricated by laser alloying of titanium alloy Ti-6Al-4V with nitrogen gas and silicon powder. An XRD spectrum indicates that the coating mainly contains Si3N4, Ti5Si3, Ti2N, etc compounds. EPMA and SEM micrographs show that the compounds in the coatings are not well-developed dendritic with round tips. Compared with laser nitriding of Ti-6Al-4V, the addition of Si not only improves the sliding wear resistance but also impedes the formation of the well-developed dentritic compounds and thus facilitates the reduction of crack formation in the coatings. In addition, the test results also demonstrate that the coatings have better wear and oxidation resistance than the as-received sample and the enhancing mechanism is briefly discussed.

ANALYSIS OF GROWTH MECHANISM OF DENDRITIC TITANIUM CARBIDES PRODUCED BY LASER ALLOYING OF PURE TITANIUM WITH GRAPHITE POWDER

The microstructure of dendritic titanium carbides fabricated by laser surface alloying of pure titanium with graphite powder is investigated using electron probe microanalyzer (EPMA) and high-resolution transmission electron microscope (HRTEM). The growth mechanism of TiC crystals is that crystal grows by the model of continuous growth at the beginning of solidification and then the crystal surfaces that are suited to lateral growth develop fully by the model of spiral growth. So, the final morphology of dendritic TiC looks like it is composed of tiny crystal grains in a linear arrangement.

Self-assembled monolayers of bimetallic Au/Ag nanospheres with superior surface-enhanced Raman scattering activity for ultra-sensitive triphenylmethane dyes detection

The bimetallic Au/Ag self-assembled monolayers (SAMs) were constructed by using mono-dispersed Au/Ag nanospheres (Ag: 4.07%-34.53%) via evaporation-based assembly strategy. The composition-dependent surface-enhanced Raman scattering (SERS) spectroscopy revealed that the Au/Ag (Ag: 16.83%) SAMs provide maximized activity for triphenylmethane dyes detection. With the inter-metallic synergy, the optimized SAMs enable the Raman intensity of crystal violet molecules to be about 223 times higher than that of monometallic Au SAMs. Moreover, the SERS signals with excellent uniformity (<5% variation) are sensitive down to 10-13  M concentrations because of the optimal matching between bimetallic plasmon resonance and the incident laser wavelength.

Photochemical synthesis of ZnO@Au nanorods as an advanced reusable SERS substrate for ultrasensitive detection of light-resistant organic pollutant in wastewater

The prospect of wielding surface-enhanced Raman spectroscopy (SERS) as a powerful technique for ultrasensitive detection of organic molecules in wastewater has received extensive attention in environmental surveillance. Based on ultraviolet (UV, 405 nm) laser irradiation of ZnO nanorods in HAuCl4 solution, ZnO@Au nanorods with controllable Au nanoparticles were successfully fabricated and established as an advanced SERS-based substrate. The reduction of Au ions was driven by the generation of electron-hole pairs via UV laser excitation of semiconductor-based nanomaterials, resulting in the moderate overgrowth of Au nanoparticles on the ZnO nanorods. The Au composition-dependent SERS analysis of crystal violet (CV) molecules revealed that the ZnO@Au nanorods with 16.21% Au contents exhibited optimized SERS activity in comparison with other nano-substrates in this paper. Furthermore, the detection limit of light-resistant methyl blue (MB) dye molecules was achieved at nanomole (nM) level of 10-9 M (0.8 μg/L), providing ultrasensitive detection of organic pollution in wastewater. Even after twenty recycles, the excellent reusability of this novel substrate with 65% original SERS intensity was achieved by subsequently eliminating the residual MB molecules via photocatalytic degradation. Therefore, the as-prepared ZnO@Au nanorods can serve as a cost-effective, clean, reusable and active SERS substrate for ultrasensitive monitoring of light-resistant organic pollutant in natural ecosystems.