TBC Development on Ti-6Al-4V for Aerospace Application

Abstract

The Ti-6Al-4V alloy is widely utilized in the aerospace industry for applications such as turbine blades, where it is valued for its mechanical strength at high temperatures, low specific gravity, and resistance to corrosion and oxidation. This alloy provides crucial protection against oxidation and thermal damage. A thermal barrier coating (TBC) typically consists of a metallic substrate, a bond coating (BC), a thermally grown oxide (TGO), and a topcoat ceramic (TC). This study aimed to investigate laser parameters for forming a TBC with a NiCrAlY bond coating and a zirconia ceramic topcoat, which contains 16.0% equimolar yttria and niobia. The coatings were initially deposited in powder form and then irradiated using a CO₂ laser. The parameters of laser power and beam scanning speed were evaluated using scanning electron microscopy and X-ray diffraction. The results indicated that the optimal laser scanning speed and power for achieving the best metallurgical bonding between the substrate/BC and the BC-TGO/TC layers were 70 mm/s at 100 W and 550 mm/s at 70 W, respectively. Laser-based layer formation has proven to be a promising technique for the application of TBC.

1. Introduction

Ti-6Al-4V is the most extensively used titanium alloy in the aerospace industry, prized for its favorable combination of workability and exceptional properties such as low density, mechanical strength, corrosion resistance, and creep resistance [1]. However, a significant limitation to the service life of titanium alloys is their susceptibility to degradation in high-temperature oxygen-rich environments. This oxidation forms an oxide layer at temperatures exceeding 500 °C, which compromises the alloy’s resistance. To address this limitation, thermal barrier coatings (TBCs) are applied to Ti-6Al-4V alloys. In aerospace applications, such as turbine blades, TBCs provide crucial thermal and chemical protection and have the potential to enhance efficiency and reduce carbon dioxide emissions from aircraft engines [2,3].

The TBC technique is designed to extend the service life of components exposed to aggressive atmospheres and high temperatures while improving thermodynamic and energy performance by enabling turbines to operate at higher temperatures [4,5,6]. The TBC system comprises a surface ceramic layer (Top Coat, TC) based on zirconia, known for its low thermal conductivity, which is adhered to the substrate by a bond coat (BC) rich in aluminum, typically of the MCrAlY type. During high-temperature exposure, a protective intermediate layer, known as the thermally grown oxide (TGO), forms through a diffusion process and is generally composed of alumina [1,3,4].

Among the commonly used ceramic coatings, zirconia ceramic partially stabilized with yttria (7.6 ± 1 mol% YO_1.5, or 7 YSZ) stands out [4]. Research into improving zirconia ceramic coatings with various dopants [7,8,9,10], including yttria and nióbia, is ongoing, with notable studies conducted in Brazil. This research is particularly relevant given Brazil’s significant reserves of zirconium and its status as the world’s largest source of niobium ore, as reported by the National Mining Agency of Brazil [11].

Air plasma spray is the most widely used method for TBC deposition [12,13,14,15]. However, other techniques, such as physical or chemical vapor deposition [16,17] and laser processing [18,19,20,21], are also being explored. Laser processing of coatings aligns with the Industry 4.0 concept, which emphasizes agile, dynamic, precise, and automated manufacturing processes [22]. Laser processing modifies only the targeted area, resulting in a minimal heat-affected zone between layers [22].

Studies have shown that TBCs enhance the creep resistance of titanium alloys [20]. Modifications to the TBC, whether in material composition or processing methods, are being investigated to further improve the lifespan and operating temperature of the metallic alloy.

Laser processing technology for surface treatment has proven to significantly enhance the wear resistance, heat resistance, and mechanical strength of Ti-6Al-4V alloy while also demonstrating robust interlayer adhesion [8,22,23,24]. The parameters involved in laser processing are diverse, with scanning speed and power being among the most commonly studied. As such, further investigation and experimentation on the effects of coating parameters during the coating of Ti-6Al-4V are essential to optimize the process [25]. However, there is a paucity of data in the existing literature on this subject.

This study aims to investigate the deposition parameters for the BC and TC layers using CO₂ laser processing on Ti-6Al-4V substrates in the formation of TBCs. This research is crucial for advancing the technological capabilities of Ti-6Al-4V alloys, particularly for applications in high-temperature and oxidizing environments.

2. Materials and Methods

The Ti-6Al-4V alloy was procured from Realum in its forged condition, then annealed at 190 °C for 6 h and cooled in air. The formation of the thermal barrier coating (TBC) on the titanium alloy was executed in three sequential stages: deposition of the bond coat (BC), formation of the thermally grown oxide (TGO) layer, and application of the topcoat (TC).

The BC deposition utilized NiCrAlY powder from Praxair Surface Technologies, which consists of 57.35% Ni, 31.00% Cr, 11.00% Al, and 0.65% Y by weight. The NiCrAlY powder was suspended in ethyl alcohol to facilitate sedimentation directly onto the surface of the titanium alloy sample. After filtration and evaporation of the liquid, the sample was subjected to CO₂ laser irradiation.

The process was performed with a CO₂ laser beam (Synrad, modelo Ti 100 Series) operating in CW (continuous wave) mode, characterized by wavelength 10.6 µm, Gaussian distribution of energy, and a beam diameter of 100 mm at the focal point. The laser parameters varied and included scanning speeds ranging from 10 mm/s to 140 mm/s, with a fixed power of 100 W and a beam overlap of 50% per pass. The laser parameters were investigated and established based on previous experiments.

The TGO layer formation followed established procedures [26]. For the TC, the raw materials used were monoclinic zirconium oxide (m-ZrO₂) from Stanford Materials Corporation, with a D₅₀ particle size of 1.12 μm and purity greater than 99.95%, and niobium oxide (Nb₂O₅) and yttrium oxide (Y₂O₃) from American Elements, both with a D₅₀ particle size of less than 5 μm. The topcoat composition was a zirconia ceramic with an equimolar addition of 16.0 mol% yttria and niobia (ZrYNb-16), processed using a Fritsch planetary mill, model 05.201.

The deposition of the ceramic powder was carried out by sedimentation of a ZrYNb-16 suspension in distilled water with 4% polyvinyl alcohol (PVal). After filtration and complete evaporation of the liquid, the ceramic powder was uniformly adhered to the sample surface. The sample was then subjected to CO₂ laser irradiation to complete the formation of the TBC coating.

The optimal laser parameters for promoting ceramic layer adhesion were investigated, with laser scanning speeds set at 550 mm/s and beam overlaps at 50%, with power varying between 60 and 100 W. These laser parameters were determined and established based on prior experiments.

Laser irradiation for layer consolidation was performed using a CO₂ laser system (Synrad, model Ti 100 Series) integrated with a deflection head (Raylase, model Miniscan II-14) equipped with a focusing lens of 100 mm focal length. The beam had a focal diameter of 0.162 mm, and the process employed analytical argon (5.0) and industrial nitrogen (N2) as process and optical shielding gases, respectively.

The metallographic preparation was performed by grinding the samples using silicon carbide sandpapers with grits of 320, 400, 600, 800, and 1200 mesh. This was followed by polishing with 6- and 3-micron diamond paste and colloidal silica. Chemical etching was carried out using Kroll’s reagent. The samples were then analyzed using optical microscopy (OM) from Opton, model TNM-07T-PL.

Microstructural characterization was conducted using a scanning electron microscope (SEM) from FEI and Tescan, model VEGA 3 XMU, equipped with energy-dispersive X-ray spectroscopy (EDX) from Oxford Instruments, model x-act, with AZTec software.

X-ray diffraction (XRD) analysis was performed using an X-ray diffractometer (Rigaku, model Ultima IV), operating at 40 kV and 30 mA, with a 2θ angle range from 20° to 80°, an angular step of 0.02°, and a time per step of 10 s.

3. Results and Discussion

3.1. Bond Coating—NiCrAlY

The formation of NiCrAlY layers on the substrate exhibited three distinct behaviors with varying beam scanning speeds, as depicted in Figure 1.

Figure 2 illustrates various microstructures of the NiCrAlY layer on Ti-6Al-4V at laser scanning speeds, categorized as irregular (Figure 2A,B), complete (Figure 2C,D), and incomplete (Figure 2E,F). The formation of the complete layer, characterized by a homogeneous and continuous structure, results from the interaction between the laser beam and the material, which depends on the combined effects of laser power and scanning speed.

The thicknesses of the NiCrAlY layers formed on the Ti-6Al-4V substrate through laser processing are presented in

Table 1. The average thickness of the NiCrAlY layers on the Ti-6Al-4V alloy after CO₂ laser processing.

Sample—CO2 Laser Irradiation Speed (mm/s)	Thicknesses (μm)
10	335.60 ± 6.65
20	223.11 ± 2.77
30	306.81 ± 4.82
40	298.54 ± 8.52
50	230.45 ± 10.66
60	175.80 ± 7.78
70	170.12 ± 4.31
80	168.22 ± 6.26
90	159.74 ± 11.12
100	155.28 ± 7.23
110	152.30 ± 6.95
120	146.90 ± 10.75
130	145.62 ± 5.68
140	150.35 ± 13.23

.

The interaction time between the laser beam and the material is influenced by the scanning speed. At lower scanning speeds, the extended interaction time allows for sufficient heat transfer, resulting in effective sintering of the NiCrAlY layers and the formation of a thicker coating, as indicated in Figure 2A,B. Conversely, at higher scanning speeds, the reduced interaction time may not provide adequate energy for proper layer formation, leading to thinner and less effective coatings (Figure 2E,F).

An inadequately formed layer can facilitate oxygen permeability into the substrate and increase the risk of delamination, compromising the mechanical strength required for thermal barrier coating (TBC) applications. Delamination often initiates with the formation of cracks perpendicular to the surface caused by thermal stresses during solidification [27]. These issues can be mitigated by optimizing laser processing parameters.

The optimal laser parameters for achieving well-formed NiCrAlY layers on Ti-6Al-4V were found to be scanning speeds between 60 and 80 mm/s. The sample with a scanning speed of 70 mm/s is characterized by energy-dispersive X-ray spectroscopy (EDX), as shown in Figure 3. The elemental analysis reveals the composition of both the NiCrAlY and Ti-6Al-4V layers. An interface region, approximately 20 μm in thickness, is clearly observed.

The EDS analysis of the sample surface map (Figure 3B) reveals the presence of NiCrAlY and oxygen, suggesting the formation of an oxide layer indicative of the formation of the TGO (thermally grown oxide) layer. Phase identification is provided in Figure 4, which presents the X-ray diffraction (XRD) analysis. This analysis identifies the oxides and the main phases in the bond coating (BC) as AlNi and AlNi₃. The AlNi phase is known for its resistance to oxidation, while the AlNi₃ phase facilitates the formation and growth of the Al₂O₃ film, contributing to the thermally grown oxide (TGO) layer at elevated temperatures [28].

The literature studies [29] investigated NiCrAlY and NiCrAlY-Al₂O₃ coatings deposited by plasma and subsequently remelted by CO₂ laser on a GH536 superalloy substrate. These studies reported the formation of a thinner and more homogeneous Al₂O₃ layer in NiCrAlY coatings following laser treatment. The diffusion of aluminum, attributed to its relatively low density (ρ = 2.7 g/cm³) compared to other elements in the material, was noted as a factor in this process.

In another study [30], NiCrAlY coatings deposited on Hastelloy X were found to be free from significant defects. The coating consisted primarily of a solid solution of γ-Ni with a minor presence of Al₅Y₃O₁₂ oxide near the surface. This composition was influenced by the low relative densities of aluminum and yttrium (ρ = 4.5 g/cm³) and the strong chemical affinity between these elements.

In the study [31] on the deposition of NiCrAlY coatings on steels at 1100 °C for varying durations, a dense, stable, and continuous Al₂O₃ oxide layer was obtained. This layer contained inclusions of Ni, Cr, and Y oxides, which preferentially segregated along the grain boundaries of the Al₂O₃. These inclusions help control grain growth and fill voids in the thermally grown oxide (TGO) layer, thereby enhancing the adhesion of the protective layer and impeding the diffusion of oxygen to the substrate [32,33].

The formation of an alumina layer on the surface is crucial for protecting the substrate from oxygen diffusion. The presence of this oxide layer, which forms the TGO layer on the bond coat (BC), improves adhesion between the layers of the thermal barrier coating (TBC) system. It strengthens the bond between the NiCrAlY metallic layer and the ceramic layer [1].

3.2. Top Coating—ZrYNb-16

The parameters of laser processing are dependent on the material being used. The energy absorbed by the material and its interaction with the CO₂ laser beam must be considered to form the desired layers. Consequently, different parameters are required for the formation of the NiCrAlY layer and the ceramic layer. The CO₂ laser exhibits high absorptivity in ceramics, necessitating less interaction time between the laser and the ceramic to form a layer.

Figure 5 illustrates the ceramic layer formed with a laser beam scanning speed of 550 mm/s and power settings ranging from 60 to 100 W.

The thicknesses of the ZrYNb-16 ceramic layer formed on the Ti-6Al-4V through laser processing are presented in

Table 2. The average thickness of the TC (ZrYNb-16) layer on the Ti-6Al-4V alloy after CO₂ laser processing with a scanning speed of 550 mm/s.

Sample—CO2 Laser Irradiation Speed (mm/s)	Thicknesses (μm)
60	2.15 ± 2.78
70	20.72 ± 6.48
80	18.52 ± 13.26
90	21.36 ± 13.98
100	20.89 ± 15.23

.

Figure 5B displays a uniform ceramic layer that is well-adhered to the substrate.

Figure 6 presents an EDX map analysis of the sample cross-section to confirm the formation of the layers. The elemental identification indicates the formation of the TBC. In the region closest to the surface, the presence of Zr, Nb, and Y is detected, which are indicative of the ceramic layer. This is followed by the identification of Ni and Cr, the predominant elements by weight in the composition of the BC, NiCrAlY. Additionally, the elements characterizing the substrate, Ti, Al, and V, are also observed.

The analysis concluded that the optimal parameters for forming the ZrYNb-16 ceramic layer, which demonstrated the best performance among the layers in the TBC system, were a CO₂ laser scanning speed of 550 mm/s and a power setting of 70 W.

Figure 7 presents the X-ray diffraction (XRD) analysis for the ZrYNb-16 ceramic powder mixture both before and after laser processing for the thermal barrier coating (TBC). The analysis detected the following phases: monoclinic (m-ZrO₂), tetragonal (t-ZrO₂), and cubic (c-ZrO₂).

In Figure 7, X-ray diffraction (XRD) analysis reveals that, for the ZrYNb-16 ceramic powder, only the monoclinic phase (m-ZrO₂) of zirconia was identified. However, after laser processing, the XRD peaks indicate the presence of multiple zirconia phases: monoclinic (m-ZrO₂), tetragonal (t-ZrO₂, pseudocubic), and cubic (c-ZrO₂), with some overlapping of peaks.

The literature [7] on the microstructural characterization of zirconia partially stabilized with yttria (YSZ) through XRD has typically identified only the cubic phase. However, this does not exclude the presence of other phases, as overlapping peaks from the tetragonal phase can create pseudocubic diffraction patterns similar to the cubic phase. The tetragonal phase (t-ZrO₂, pseudocubic) exhibits lattice parameters close to those of the cubic phase and is often referred to as ferroelastic tetragonal.

In a study [10], after processing ZrO₂-YO_1.5-NbO_2.5 ceramic with a 16% equimolar composition using a planetary mill, the peaks for monoclinic (m-ZrO₂), tetragonal (t-ZrO₂), and cubic (c-ZrO₂) phases were observed. The Rietveld analysis from this study revealed varying concentrations of the monoclinic phase and a predominance of the tetragonal phase.

Additionally, XRD analysis detected titanium oxide on the TBC surface. This indicates that titanium from the substrate not only diffused into the metal layer but also migrated into the TBC layer during the laser processing due to the thermal energy imparted by the laser.

4. Conclusions

The following conclusions can be drawn from this study on the optimization of deposition parameters of the BC and TC layers by CO₂ laser on the Ti-6Al-4V alloy substrate in the formation of TBC:

• The thickness of the NiCrAlY layer decreased as the scanning speed of the CO₂ laser beam increased despite maintaining a constant power of 100 W. This reduction is attributed to the shorter interaction time between the laser and the material, which affects the energy transfer necessary for layer densification.
• In the consolidation of the NiCrAlY layer on Ti-6Al-4V, the most suitable deposition parameters were identified as a scanning speed of 70 mm/s, a laser power of 100 W, and a beam overlap of 50% per track. These conditions resulted in an average layer thickness of 170.1 μm, characterized by a uniformly homogeneous surface, minimal large cracks, and no significant porosity. The X-ray diffraction (XRD) analysis revealed that the predominant phases in the layer were β-NiAl and γ’-Ni₃Al.
• After laser irradiation, a TGO layer was formed on the substrate. This layer, composed of aluminum and yttrium oxides, was uniformly distributed across the entire surface of the top layer, contributing to the thermal protection and bonding of the TBC system.
• The ZrYNb-16 ceramic layer was optimally consolidated with a laser process speed of 550 mm/s, a laser power of 70 W, and a beam overlap of 50% per track. The resulting layer had the best average thickness of 20.72 μm compared to other parameters. XRD analysis identified the presence of monoclinic (m-ZrO₂), tetragonal (t-ZrO₂, pseudocubic), and cubic (c-ZrO₂) zirconia phases in the ceramic layer.
• Laser processing can effectively consolidate layers to thicknesses suitable for TBC applications in a precise, efficient, and rapid manner while minimizing the heat-affected zone.