Fabrication And Performance Of Nanostructured Thermal Barrier Coatings On Titanium Alloy Substrate And Surface Laser-Glazing Of Ceramic Coatings

Titanium and its alloys play important roles in new structural materials. The applications of titanium are mainly focused on its high specific strength at room and elevated temperatures, unique corrosion resistance and nonmagnetic properties. This fact has predetermined the wide use of its alloys in aircraft and space industries. However, poor oxidation resistance and oxygen induced embrittlement deteriorate the application of titanium alloy at high temperatures. This poor oxidation resistance results from the formation of a non-protective oxide scale consisting of a heterogeneous mixture of alumina and titania on high temperature exposure. Deposition of protective and thermally insulating coatings is considered as an effective means to reduce the substrate temperature and suppress both oxidation and oxygen induced embrittlement.Thermal barrier coatings (TBCs) have been widely used to provide thermal protection to metallic components from the hot gas stream in gas-turbine engines due to their low thermal conductivity and thermal diffusivity combined with proper chemical stability at high temperatures. The TBC system allows conventional metals to be reliably used at high temperatures because the ceramic layer provides thermal stability to the base metal due to insulation from the heat, while the metallic bond coat provides oxidation resistance, and sufficient toughness. Consequently, plasma spraying thermal barrier coatings on titanium alloy substrates is a shortcut to improve the short-term properties of the titanium alloy at high temperatures.Based on this, a systematic study of nanostructured TBCs on titanium alloy has been carried out. The characterization of nanostructured TBC, the mechanical and thermophysical properties of nanostructured TBCs, heat insulating effect, residual stress in the coating system after plasma spraying, thermal shock behavior and failure mechanism, and surface lazer-glazing have been investigated.Nanostructured TBCs were applied on the titanium alloy by air plasma spraying. Conventional counterparts which were used for comparison were fabricated as well. The microstructure of the titanium alloy inside the substrate keeps unchanged after plasma spraying. Neither interaction nor atomic diffusion evidently takes place at the bond coat/substrate interface. However, there exists a thin layer of plastic deformation zone in the substrate beneath the bond coat/substrate interface and its thickness is non-uniform. The as-sprayed ceramic coatings show a typical lamellar structure in an overlapping and interlocking fashion. Compared to the conventional ceramic coatings, the nanostructured ones have thinner laminas, and which are banded together more tightly. The nanostructure in the ceramic coating is originated from the unmelted powders and the nanostructured columnar grains derived from the melted powders after cooling rapidly. The surface connected porosities in both conventional and nanostructured ceramic coatings present a typical bimodal pore size distribution. The porosity in the nanostructured coating has a lower value than the conventional one. The micropores in the nanostructured coating are smaller and distributed evenly as well. Micro-mechanical property results show that anisotropy in microhardness between the cross section and the top surface of the ceramic coating is examined because of the lamellar structure. The nanostructured ceramic coatings have higher microhardness and adhesive strength compared to the conventional one. Both coatings demonstrate a bimodal distribution of microhardness values, as evidenced by their weibull plots. The nanostructured coating presents a better adhesive strength than the conventional one.Thermophysical properties and heat insulating test are measured. Coefficient of thermal expansion (CTE) for the conventional ceramic coating is 10.4×10^-6 K^-1, while it is 11.3^-6 K^-1 for the nanostructured one. The conventional ceramic coating has the higher thermal conductivity. In general, the thermal conductivities of all the coatings increase slightly with increasing temperatures. The heat insulation test results reveal that all samples reach a state of thermal stability within 180 s, and the nanostructured TBC with smaller micropores presents a better heat insulation effect with a temperature drop of about 130℃. The heat insulation is proportional to the TBC's thickness.An analytical model of distribution of thermal stress in TBCs on the substrate of titanium alloys has been derived based on force, moment balances and classical beam bending theory. The quenching stresses set up due to the progressive deposition process are considered, followed by those due to CTE mismatch during final cooling to room temperature. The calculated values are compared with the measured residual stresses at the ceramic surface, which shows a reasonable agreement. The stresses due to the mismatch of CTE are dominant since the quenching stresses are released largely by generating microcracks. The nanostructured ceramic coatings present lower residual stresses than the conventional counterparts. The absolute values of residual stress at the ceramic surface are increased with the decrease of ceramic coating thickness.The behaviors of the resistance of TBCs on titanium alloy to thermal shock have been studied, and the corresponding failure modes have been put forward. The failure of TBCs on titanium alloy is characterized by spallation and delamination at the ceramic coatings. The weak link exist at the ceramic coating/bond coat interface. During thermal shock cycles, some pores also appear at the substrate/bond coat interface due to the mismatch of CTE. The vertical microcracks are formed firstly near the bond coat, and then propagating in the ceramic coatings. During thermal shock cycling, the undulation of bond coat get amplified, and the horizontal microcracks are generated at the ceramic coating/bond coat interface as well. The horizontal microcracks propagate parallel to the interface in the ceramic coating, and coalesce, which result in the failure of TBCs on titanium alloy. The nanostructured samples have exhibited promising lifetime than the conventional ones. With the increase of temperature, the numbers of thermal shock cycling for both kinds of sample decrease obviously. The lifetime of samples is proportional adversely to the ceramic coating thickness during thermal shock test. After thermal shock test, a diffusion area is developed near the bond coat/substrate interface in the substrate. The diffusion area is concentrated of Ni and Ti elements. Because of the reaction of Ni and Ti, new compounds such as Ti2Ni,TiNi,AlNi2Ti, and Ni3(Al,Ti) have been come into being along the thickness of the diffusion area.Plasma-sprayed conventional and nanostructured ceramic coatings have been subjected to laser-glazing processes which provide a remelting and subsequent solidification of the surface. The results revealed that laser-glazing carried out on plasma-sprayed zirconia coatings has brought in a smooth and dense glazed surface with craters and a network of microcracks. The microcrack network in the conventional coating is sparser than that in the nanostructured coating. For both kinds of ceramic coatings, with the decrease of laser scanning speeds, the microcrack networks turn sparser, and the craters at the surfaces increase. A reduction on surface roughness is achieved after laser processing. The surface roughness increases with the decrease of laser scanning speed for both kinds of coatings. The nanostructured coatings have a deeper and wider molten region than the conventional coatings at the same laser processing conditions. For both kinds of ceramic coatings, the track depths increase with increasing the mean laser energy density in an approximately linearly proportional way; the track widths increase with the increase of mean laser energy density as well. The laser-glazed regions consist of a columnar microstructure. The columnar grain sizes in the nanostructured coating are bigger slightly than that in the conventional coating. With the decrease of scanning speed, for both kinds of coatings, the columnar grain sizes increase, and pores within the column grains is diminished. There are segmentation microcracks in the laser-glazed ceramic coatings. Most of segmentation microcracks don't run through the coatings along thickness. Some of the segmentation microcracks across the densified layers are perpendicular to the surface and start to deviate from the vertical direction within the porous plasma-sprayed coating. The segmentation microcrack density is more dependent on the laser scanning speed, resulting in higher densities of segmentation microcrack with the increase of the scanning speed. The laser-glazed conventional ceramic coatings have lower Ds values than the nanostructured one. The thermal shock resistance of laser-glazed nanostructured ceramic coating under high laser scanning is improved, which has more thermal shock cycling times than that of the plasma–sprayed TBCs on titanium alloy. However, for the samples which are laser-treated at low scanning speed, their thermal shock resistances are deteriorated.