Preparation And Study Of Thermal Barrier Coating Upon Titanium-Based Alloy

Titanium alloy, especiallyα+βtype, due to its excellent specific strength and acceptable thermal tolerance at temperatures under 600oC, is one of the most popular materials used in field of aviation and aerospace. However, its further application in the occasion used for higher temperature and reliability is hindered by two problems. The first is that the outer environment may consume the facial material, and embrittle the near-surface material. The second is that the long exposure at high temperatures may dissolve out some brittle phase, like Ti3X or silicide etc., bring down the plasticity and fracture toughness, and influence the fatigue property of the titanium-alloy. Preparation of a thermal barrier coating (TBC), which is both oxidation- and heat-resistant, on the surface of titanium-alloy supplies an attractive solution for these two problems, so that to expand the service occasion of the Ti-base materials, and raise efficiency and save resources.In this study, we firstly deposited a traditional thermal barrier coating, in which NiCoCrAlY alloy is the bond coating (BC) and 8wt.% yttria-stabilized zirconia (8YSZ) is the thermal-isolation coating, onto the surface of a titanium-alloy TC11. In the base of the knowledge to the thermal behavior and failure mechanisms of that system, we proposed a new method to prepare an oxidation-resistant ternary Ti-Al-Ag alloy coating onto the substrate as a novel BC. Finally, Electron Beam-Physical Vapor Deposition (EB-PVD) was used to deposit 8YSZ onto the ternary BC pre-treated, to construct a novel thermal barrier coating suitable for titanium alloy substrate. The modified ASTM directional pulling, micro indentation, thermal cycling and isothermal exposure tests were used to observe the thermal behavior and study the failure mechanisms. Optical microscopy (OM), SEM, XRD were used as analyses methods, assisted with the finite-element simulation. The main work and conclusions in this study are as the following:EB-PVD and HVOF (High Velocity Oxyfuel Spraying) were used to deposit NiCoCrAlY onto the titanium-alloy substrate and the top ceramic coating was deposited sequentially by EB-PVD in one batch to construct a traditional thermal barrier coating onto the titanium-alloy. TBC with the different properties of the BC were studied:NiCoCrAlY deposited by EB-PVD is denser and more homogeneous, and the bonding to the substrate is metallurgical bonding. Due to"shadow effect", a slim-clustered upper 8YSZ, with good mechanical properties is achieved. This kind of TBC shows better thermally cycling performance at 800oC (longer than 35min× 220), but poor isothermal exposure durability (shorter than 24h). By using HVOF instead of EB-PVD for the preparation of BC, the inhomogeneity and porosity of the BC is increased, resulting in a looser 8YSZ coating having coarse grains and low hardness. The isothermal durability of TBC, however, is extended to longer than 36h at 800oC, but its thermal cycling durability is worsened.Chemical mismatch between the Ni-based BC and the Ti-based substrate is the most essential factor degreading the traditional TBC of the Ti-alloy. It's very easy to diffuse of Ni, Co from BC to the substrate to form a 150~300μm inter-diffusion layer at the substrate near the BC. The main product is Ti2Ni which is very hard at room-temperature but very soft at high temperature near 800oC. Because the coefficient of thermal expansion (CTE) mismatch between the BC and the substrate is great, ~750MPa radial tensile stress and ~154MPa axial tensile stress arise in the system when the sample is cooled down to the room temperature after the preparation. Similarly, tensile stress of about 20~40MPa is formed when the sample is reheated to 800oC. As a result, the NiCoCrAlY BC is vertically cracked and the inter-diffusion layer is horizontally cracked during the thermal service, and the substrate is exposed into the atmosphere and failed.A novel Ti-Al-Ag ternary alloy coating was designed and in-situ prepared by low pressure plasma spray (LPPS) assisted with a sequential inert-gas-protected treatment, and its oxidation-resistance was identified and studied.The commercialγ-TiAl and Ag powders (~75 and 50μm, respectively) are used as the raw materials, and the wet-ball-milling is employed as the mixing and crashing technique. Twenty four hours later the dimension of theγ-TiAl and Ag in the mixture powder are 0.1-10μm and ~10μm, respectively. The mixture powder is spray-dried into the powder suitable for thermal spraying. Then it is deposited onto the surface of the substrate, following by the treatment under the protection of Ar with the pressure of ~1atm. at 820oC for ~5h. It is found that the silver is dissolved into the matrix of theγ-TiAl and a ternary coating is formed with good tightness. The Z phase (Al3Ti5O2) arises obviously in the ternary coating.After oxidized at 700-800oC for 100h, the oxide scale of ternary coating is basically theα-Al2O3 with some local prominence of the TiO2 particles. With the addition of the Ag, the TiO2 particle is changed from subscale-connected to the facial-only above the Al-rich scale, so that the compactness of the scale is higher. The content of the Al2O3 in facial oxide scale is about 3 times of the TiO2. The ternary alloy coating with 4at.% Ag shows the best oxidation-resistance with the 100h-mass-gain of ~0.8mg/cm2 at 700oC and 1.0mg/cm2 at 800oC. It is believed that the addition of the silver decreases the of phase-change energy of the Al2O3, so that increases its velocity of the phase-change fromθtoα. The addition of the Ag is also helpful to elevate the delamination-resistance, smooth the surface of the coating, decrease the inner oxidation, thin the oxide scale of the coating.A novel TBC suitable for titanium-alloy substrate was prepared by the deposition of the 8YSZ onto the oxidation-resistant TiAlAg coating by EB-PVD technology, its thermal behavior and failure mechanisms were studied:After oxidized at 800oC for 100h, the thermal grown oxide (TGO) between the ceramics and ternary coating is still Al-rich. The cracking interface of the isothermal exposed sample resulted from the modified ASTM directly pulling is changed from the location near the TGO/8YSZ interface to the deeper place into the BC, then turn back to the TGO/8YSZ interface, reflecting the changing of the bonding extent between the TGO and the 8YSZ and the growth of the TGO with the isothermal exposure. The thermal cycling vertically cracks the 8YSZ, but the propagation of the cracks are stopped at the 8YSZ/TiAlAg interface because of the better match of the system, as a result the BC/substrate interface is turned into a most stable location of the thermal barrier system.The final failure of the novel TBC should be ascribed to two factors. One is the intrinsic defects on the surface of the TGO induced by the thermal spraying, the other is thermal stresses arise with the severe service environment. The permeation of the outer atmosphere through these defects is slow enough without the presence of the thermal stresses, but the compactness of the TGO is worsened during the thermal cycling. The mixed oxides of Ti and Al are formed at the subscale and deeper zone during the long-term permeation of the outer atmosphere through the enlarged defects at high temperature, therefore, once the TBC is under the out-of-plane tensile stress, it may be failed by the separation around the 8YSZ/TiAlAg interface.