Aim and scope of the present work

Aerospace gas turbine engines are designed and operates at temperature very close to their melting points. So current strategies for performance improvements are centered on (thermal barrier coatings) TBCs. This project was designed to obtain a better understanding of the pore morphologies, the surface microstructure, surface densification, phase structures mechanical characteristic, chemical structure, and thermo-physical properties of ceramic coatings in a high temperature corrosive environment and to help designers of advanced aero, industrial and marine gas turbine engines in the area of TBCs. Whilst in aero engines ceramic coatings are primarily temperatures as a means of increasing life.

TBCs are often applied to metallic super alloys to extend the temperature regime in which a component can operate ^{(1 - 6)}. A common failure mechanism of TBCs on thermal cycling is delamination of the ceramic just above the bond coat ceramic interface due to high local stresses and that thermal expansion mismatch between the substrate and ceramic layer is a significant contributor to these stresses ^{(7 - 11)} to reduce these stresses three layer system incorporating intermediate layer of graded material between the bond coat and ceramic have been tried while these three layer coating have proved promising for diesel application ⁽¹²⁾.

Figure: Principle Diagram of the Energy Source Shows the different thermal spraying techniques divided by their principle energy sources:

An ideal coating would exhibit a high uniform, well controlled distribution of pores and micro cracks to both decrease thermal conductivity and allow thermal expansion and contraction without exfoliation. At the same time, the coating must be sealed against penetration of corrosive liquids by a high density surface layer. Two approaches, surface densification by heat treatment and impregnation by thermo - chemical methods have been investigated in this work.

PLASMA - TECHNIK AG M - 1000 apparatus for plasma and auxiliary equipment for the spraying of aerospace components. It comprise : powder source (PT - 800), control console (M - 1000), plasma torch (F4), feed unit (TWIN - 10), water cooler system (T -powder 500) and holding device for the work piece in addition to dust separator (MK - 9) and wet separator (NA - 100).Samples cut out of Nickel base super alloy Ni-Cr 20 Ti Al of DIN designation number is corresponding to 2.4631) of 1.4 mm thickness have been subjected to investigations. First, Ni - 5wt % Al (AMDRY, 956) alloy (bond coat) of 0.4 mm to 0.6 mm thickness and then, Zro₂ - 20wt % Mg O (AMDRY 333) (top coat) of 0.5 mm to 0.8 mm thickness have been deposited by air plasma spraying method upon their surface.

Figure 1: Systamatic diagram of plasma spray M- 1000 (plasma - Technik - AG)

The most important powder parameters are grain size described, in ASTM standard B. 214- 86 using mesh designation number also, in DIN standard DIN 32529 using number to describe the grain size), internal porosity (correlated with apparent density and flow ability (described, in ASTM standard B. 329- 76 and B 213- 83. receptivity). EDX (Electron Diffraction X- ray) was used in chemical analysis. XRD (X- Ray Diffraction) was used in phase identification and quantitative phase analysis studies.

The Vickers Hardness (HV3) was determined using ASTM E- 384 - 73 standard. The Tensile bond strength measurement using ASTM 633 - 96 or DIN 50160 standard. Porosity were tested with (MIP). EDX and XRD were used in chemical and phas identification. The corrosion behavior of specimens was determined using AUTOLAP PGSTAT 30 show in figure (2). Thermal conductivity, λ, was calculated from thermal diffusivity, α, with laser flash apparatus (Theta Industries Inc., Part Washington, NY, USA), specific heat, cp. With DSC 404 C, and density of the free standing coating specimen. Λ = α * Cp * ρ TEC were carried out by dilatometer (Adamel Lhomargy SAS, model DI - 24, France). TCR test was studied in a thermal cycling facility illustrated in figure (3) and table (1).

Figure 2:: Photograph of AUTOLAP PGSTAT 30

Figure 2: Corrosion cell

Figure 3: Illustration of the thermal cycling device, a) photo of the super specimen holder during test, b) position of heating and cooling stations and C) side view of the specimen mounting

Table 1: Thermal cycling test parameter

Result and Discussion

The effect of plasma spray parameters on coating performance, plasma voltage versus H₂ - content % and Metallographic / Image analysis of sprayed Ni - Al bond coat and ZrO₂ – Mg O topcoat with different spray parameters are show in tables (2, 3) and Figure (4, 5), and Figure (6, 7) receptivity.

Table 2: The effect of the plasma gas on the coating performance of Ni - Al bond coat, at plasma current = 450 A, injector angle= 90o, substrate temperature = 140o C, and injection distance = 130 mm

Table 3: The effect of the plasma gas on the coating performance of Zr. O₂ -Mg O top coat at plasma current = 630 A, injection angle = 90°, substrate temperature = 150°C, and spraying distance = 90mm

Figure 4: Effect of H₂ - content % on plasma voltage (plasma voltage versus H2 - content %): Total flow rate = 56.5 L / min, I = 450 for Ni - Al bond coat

Figure 5: Back scattered SEM image for cross section view of (Ni- 5wt% - Al bond coat) coatings sprayed with the optimum conditions (S₀, Nickel - Aluminum (bon coat) coating sprayed with the optimum condition. ), with decreasing of the H₂-gas florate (S₁ H₂ - Content 18.2% plasma voltage 60V porosity percentage 6%)

Figure 6: Effect of H₂-content % on plasma voltage, (plasma voltage versus H₂ -content):Total flow rate = 60 L/min, I = 630A for zirconia (top coat) coating

Figure 7:Back scattered SEM image for cross-section view of zirconia (top coat) coatings on Nickel-Aluminum (bond coat) sprayed with the optimum conditions (/So), without H₂-gas flow rate (/S1)

The average surface roughness was able to be increased from 10μm (by using shut blasting method) up to 20 μ . m (by using NTA with applying LPPS). Figure (8) and table (4) shows installation on LPPS and NTA process parameter.

Figure 8: Installation of LPPS

Table 4: NTA process parameter on the surface of specimens (Ni-super alloy substrate)

An example of the furnace annealing of T.TBCs can be illustrated in table (5).The effect of sintering on ZrO2 - MgO top coating is illustrated in SEM micrographs, Fig (9), by remaining strings of fine pores and closed crack at splate boundaries.

Table 5: The effect of the Furnace treatment on the coating performance of T TBCs

Figure 9: SEM micrographs of annealed and not annealed TBCS which are thermally sprayed at the same optimum conditions illustrating the sintering of the ZrO₂ - MgO top coat structure, string of the fine pores and closed cracks at splat boundaries for annealed coating

The Zr O₂– Mg O coating were sealed with either Al (OH)₂- 85% H₃ PO₄solution at 300⁰ C for 4 hours or pulsed NAG Laser has radiation wave length of 1064 nm. The high magnification SEM micrographs in figure (10) showed no visible reaction layer in the coating / sealant interface. But the quantitative XRD analysis in table (6) , For AlPO sealing showed a clear ZrP₂ O₇peaks with phase transition due to the following reaction

ZrO₂ -(MgO)+ 2AlPO₄→Sealing (300C )ZrP₂O₇ (MgO) +Al₂ O ₃

For laser sealing showed grain refining without phase transition due to the localization nature of laser

Figure 10: Optical micrographs of (a) reference coating, (b) modified Al PO₄ coating based TTBCs

Table 6: EDX result of Al PO₄ sealed ZrO₂ - MgO coatings

TEC of the mixed of ceramic (65 wt % ZrO₂- MgO) with alloy (35 wt % Ni Al) intermediate -6coating was about (12 × 10- -6K ) which is close to that of substrate (18 × 10-1K ), bond coat -6 (14.9 × 10 stress.z-6K ) and top coat (9.5 × 10-1K ), this lead to reduce the generation of residueal Coating failure by thermal cycles depends on the applied stress. SEM after different cycles are presented in figure (11).

Figure 11:X-75 Reference coating after 500 cycles(A),X-75 Reference coating after 400 cycles(B),X-75 Non stabilized ZaO₂ after 280 cycles(C)

Figure 11: X-75 AlPO₄ sealed ZrO₂ - MgO (D),X-75 AlPO₄ sealed ZrO₂ - MgO after 500 cycles (E)

Figure 11: X-75 Laser sealed ZrO₂ – MgO (F),X-75 Laser sealed ZrO₂ - MgO after 500 cycles(G)

Figure 11: X-35 Modified coating of AlPO₄ sealed ZrO₂ -MgO after 950 cycles (Triple Coating) (H),X-35 Modified coating of laser sealed ZrO₂ - MgO after 950 cycle (Triple Coating) (I)

Figure 11:Triple coating after 500 cycles (K)

Pure Zirconia (not stabilized Zirconia) coating was damage after 280 cycles showed visible delamination and craking. Reference (not modified) double layer coatings was showed a horizontal and vertical crack after laser (or Al PO ) sealed ZrO₂- MgO topcoat did not show any change and resisted more than 500 cycles, the coating peeled of near the bond coat after950 cycles. The Calculation of thethermal conductivity λ (T) corresponding to the measured porosity results are present in table (7).

Table 7: Calculated thermal conductivity, λ (T) results for reference material was used in λ (T) calculations also for modified coatings corresponding to the measured porosity.

The electromagnetic parameters derived from linear polarization on experiment and the corresponding Vickers hardness for blank, reference, and modified specimens with the same spray conditions are present in table (8).

Table 8:The electromechanically parameters. Corrosion potential Ecorr, corrosion current Icorr, polarization resistance RP, and corrosion rate CR derived from linear polarization experiments and the corresponding hardness (HV3) for blank specimen, reference and modified coatings with the same spray conditions

Conclusion

Plasma spraying has great advantage over the known galvanizing process in cases where it can be used to coat electrically non- conductive surface with metal. The deviation of plasma spray parameters from the optimum condition lead to the fine microstructure coating became lower in quality i.e. increase of porosity, voids (pull - out effect), un-melted phase, and delamination. The strengthening in phosphate sealing result from two different mechanism, chemical bonding and adhesive binding. Due to chemical reaction and formation of condensed aluminum phosphates in the structure defects of the coating. The sealed top-coat in a few microns (50μm 100μm) were highly density and grain refined with the exception of some closed pores formed in the laser sealing process. Phosphate sealed coating showed excellent wear resistance, 80% lower weight losses than the reference coatings. Laser sealed coating was even more micro hardness and more abrasion resistance than the phosphate sealed coating, only a minor part of the melted top layer was worn away

TBCs, need further development work, both in production technology and characterization Erosion - corrosion resistant, TBCs are needed to prevent overheating of metal part in engines and heat exchangers, for which both coating formulation and deposition methods are needed.Thicker Zirconia coatings and more strain - tolerant coatings are needed for combustion Zones of aero gas turbine engines. Repair of joining techniques for coating materials are always in need of further research. Some turbine blades in service have been re - coated several times, showing the importance of this aspect.

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