International Journal of Aerospace Engineering

International Journal of Aerospace Engineering / 2021 / Article

Research Article | Open Access

Volume 2021 |Article ID 2791386 | https://doi.org/10.1155/2021/2791386

Mohammed Yunus, Mohammad S. Alsoufi, "Characterization of Thermally Sprayed Coated GT Components Made of 3D Printing Based Selective Laser Melting Processed Inconel Alloy 718", International Journal of Aerospace Engineering, vol. 2021, Article ID 2791386, 14 pages, 2021. https://doi.org/10.1155/2021/2791386

Characterization of Thermally Sprayed Coated GT Components Made of 3D Printing Based Selective Laser Melting Processed Inconel Alloy 718

Academic Editor: Marco Pizzarelli
Received06 Apr 2021
Revised04 May 2021
Accepted24 May 2021
Published22 Jun 2021

Abstract

Functional requirements like higher strength, resistance, and shielding from chemical reactions, degradation, etc., of the base metal are achieved by modifying their exposed surfaces using TBC (thermal barrier coating). Modified outer surface of components in the GT (gas turbine) improves the performance by the increase of inlet temperature, enhancing longer life and providing many interrelated surfaces to withstand severe working conditions. This work is based on characteristic (mechanical, thermal, and tribology) evaluation of TBC-coated GT components made of Inconel 718 manufactured from traditional and 3-D printing-based selective laser melting (SLM) process and compared. TBCs comprising bond coat and top ceramic oxide coat were deposited by High-Velocity Oxy-Fuel and Atmospheric Plasma Spraying techniques, respectively. Characteristics evaluated by varying coating thickness, standoff distance, substrate preheating, Input torch power, etc., revealed that characteristics of substrates manufactured from SLM methods improved significantly except porosity and average surface roughness. These improvements are essential for embedding sensors, thin films, etc., in TBCs for continuous monitoring of outputs.

1. Introduction

Gas turbines used in the propulsion of aircraft are characterised by highly structural integrity, low weight, and high-temperature resistant demand higher thermal efficiency, which is extensively dependent on the GT (gas turbine) inlet temperature for extracting higher levels of energy from combustion [1]. To get enhanced thermal efficiency, several studies have been carried out on using newer materials, variation of operative temperature on the properties of high-temperature corrosion, microstructural changes, creep, etc. Various methods like TBCs, air cooling, and thin films have been followed to enhance the high-temperature characteristics of the GT [2] need special attention due to their versatility and multifunction capability.

TBCs are popular with the well-known practices applied on components subjected to high-temperature performance in GT materials (made of IN718, Waspaloy, Hastelloy, etc.) where these materials had a steep fall in tensile strength beyond 600°C and a gradual fall between 550 and 600°C. TBCs were also used with GT materials to handle problems of a higher range of temperatures such as oxidization, thermal cyclic loading, creep, and thermal corrosion. Otherwise, they would affect life and structural integrity [3]. Generally, TBCs are coated by APS or EB-PVD (electron beam-physical vapour deposition) processes. Compressor, combustion chamber (CC), and turbine form a typical GT in which turbine and the CC are exposed to higher temperatures for along time ensue severe damages and irreparable [4]. TBC consists of different layers of metallic bond and ceramic oxide topcoat on a metallic substrate to offer thermal insulation from an exceedingly high amount of hot gases, which are predominant in the hot region of GT. TBCs also provide various benefits stated above by increasing the durability of the components subjected to high temperatures and stresses. The production of improvised GT enhances life by modifying the surface, which thus helps in reducing the maintenance cost. The turbine blades and the combustion liners have to be refurbished with TBCs [5]. TBCs are deposited onto the GT components, which are exposed to high temperatures for attaining low substrate temperatures and allowing high input/inlet temperature as it increases the thermal efficiency. The major life-limiting weakness of TBCs is the oxidation of the BC [6], and secondly, their liability to be damaged by stress due to higher temperature. The damage mechanisms and ultimate failure of TBCs are yet to be fully understood. Therefore, the TBC material selection is mainly controlled by some of the fundamental prerequisites like melting point, phase transformation, thermal conductivity, thermal expansion, adherences, etc. It is observed that only a handful of materials meet these requirements [7, 8].

Plasma spraying can be beneficial on combustion walls/chambers and vanes due to the low production cost. In the past three decades, TBC technology has developed and found applications in various sectors. It is often spotted that TBCs undergo impulsive failure and this often limits their application range. Aircraft engines using TBC systems consist of a thermally insulating ceramic layer of 120 to 250 μm thicknesses and a 50 to 120 μm metal BC layer between the ceramic oxide TC and the metallic substrate which are generally called duplex TBC often fail due to the mismatching of their coefficients of thermal extension. The BC is an oxidation-resistant layer used to improve the bond strength and to reduce the effect of thermal expansion mismatch between the ceramic and metallic coats [9, 10]. At elevated temperatures, O2 in the CC gets transported from the micro-cracks and voids of TC to the BC. Thus, an oxide film forms on the BC known as TGO (Thermally Grown Oxide) film. This film becomes the reason for the degradation of the TBC system [11].

TGO film depth rises during the oxidation process, which consecutively develops residual stresses at the interface of two coats and leads to delamination of two coats [12, 13]. As TGO film grows, the TBC stresses increase to have larger stress than the thinner films. In previous studies, the major work dealt with the TBC failures caused by TGO film growth [14]. Investigations on TBC systems [15, 16] revealed that Ni with Cr2O2, Al2O3 as coatings produces NiO at the interface of ceramic Al2O3 or YSZ coat with BC of MCrAlY when exposed at a temperature beyond 1000°C. The Ni with Cr2O2, Al2O3 acts as complex oxide causing higher porous within the TGO subfilm forms nucleation spots for crack instigation. These could cause early splitting-up of TC from the BC where carbon gets deposited, and this carbon moves away in carbon ball form can hit the GT blades [17] causing wear and tear of them. Coatings by plasma spraying on a microscale can be treated as a particle-by-particle build approach. Normally, a splat is formed and this occurs when the particles in their molten state or semimolten state strike the surface of the substrate and get adhered and solidified to form a structure known as “splat.” The particles in the plasma stream that do melt are likely to bounce back with a reduction in coating efficiency. It is generally understood that the molten/semimolten particles will be spherical in shape and when impinges onto the substrate at a very high velocity, the particles tend to flatten out into a disk shape; however, a thin sheet of liquid flowing radially turns out to be unstable and collapses to form very fine droplets. This process gets chocked at a stage due to the quick solidification of the droplets. The cooling rate observed at this stage is around 106 K/s [18].

At this stage, it is understood that particles tend to lose heat due to conduction and initiate solidification. Heat transfer will also occur due to convection and radiation also. It is also spotted that voids exist in-between the splats as a result of improper bonding that results in the decrease of adhesion due to residual stress and entrapped gases. The factors that influence the structure of the coating include temperature, particle size, average roughness, and velocity. Due to the parameter’s variation, there exists a difference in the physical, chemical, mechanical, and even electrical properties also. The ceramic TC is used for the following reasons as they exhibit (a) low thermal conductivity, (b) phase stability, and (c) high melting point. The most widely and commonly used material for providing TC to the substrates under consideration is 6 to 8 wt. % YSZ. YSZ has low thermal conductivity and exhibits relatively high toughness [19]. Normally, YSZ coatings are preferred since they are corrosion resistant when compared with Cao, and MgO stabilized Zirconia coatings are used in TBCs [20, 21]. Spallation effect is also less visible with YSZ coatings [58]. The biggest drawback is limited operating temperature, which makes it difficult in long-range applications. YSZ coating’s ability to transport oxygen results in BC oxidation which leads to failure of coatings by spallation is also solved by the TGO film creation. BC role is to provide protection against oxidation and to improve the adhesion with the TC. Normally, NiCrCoAlY alloys are used as BC materials as they provide excellent resistance to oxidation and corrosion. Aluminium acts as a source and reservoir to grow TGO for protection against oxidation [19]. Aluminium is used due to its good adherence and low thermal diffusivity. Chromium safeguards by corrosion and deterioration. In order to avoid diffusion of sulphur into coatings, yttrium can be a choice. TGO film is formed due to BC oxidization. The film is usually formed at the interface of TC and BC.

Alumina is a major oxide produced as a result of the oxidization of NiCrCoAlY. Some of the other oxides include spinel, chromia, silica, and nickel oxide. BC oxidation is known to cause failures in TBCs. Wear resistance and fatigue strength of thermally sprayed ceramic coatings were studied [22]. The results of the research indicate that both the wear resistance and fatigue life of the components increased. The main objective was to investigate the effect of various APS factors of different TBC consisting of Zr stabilized with magnesia as TC and NiCrCoAlY as BC as well as their properties with those obtained using reference techniques and the optimal conditions. Various techniques associated with the selection of TBC systems would be based on their ability to sustain the harsh, and corrosive atmosphere without phase changes in the ignition compartment. Moreover, emphasized that α of both the coatings and substrate should match each other [23]. The thermal output of YSZ improved by adding 35 wt. % Al2O3 as it prevents oxygen entry into coatings. APS is widely employed as wear and corrosion-resistant coatings in aerospace, nuclear energy, textiles, chemicals, plastics, and paper industries and a steady increase found in various applications of TBCs.

High-Velocity Oxy-Fuel (HVOF) employs chemical energy (produced by both thermal and kinetic energies) for coating supersonically driven molten particles of NiCrCoAlY as BC on the surface of Inconel substrates from convergent-divergent nozzle to provide an excellent adhesion property with less porous and thick. APS process employs the gas torch (Make Sulzer Metco) with a copper nozzle operating at high gas flow (Argon gas) and current rates with a jet velocity and the temperature in the range of 600 to 800 m/s and 10000 to 18000°C, respectively. Ceramic and metallic powders are fed into the jet to impinge onto the substrate for forming thick coatings. A struck electric arc between the water-cooled electrodes produces extensive heat and velocity from the plasma gas directed onto substrates by feeding coating powders. CoNiCrAlY deposits impinged on Inconel 718 base metals using APS, supersonic APS, D-gun, HVOF, cold gas dynamic spraying techniques showed enhanced characteristics by dimensional variations in TGO layer, microstructural properties, and chemical compatibilities produced with cold gas dynamic technique [24]. Characteristics of hot corrosion and CaO-MgO-Al2O3-SiO2 attack demeanours of single and double-layered TBC systems showed double-layered TBC (La2Zr2O7/YSZ) improved properties in terms of variations in TGO layer, microstructures, and chemical as compared to single-layered (YSZ) [25].

Although TBCs enhance GT efficiency by escalating the combustion temperature [26], it is essential to realize the failure mechanisms of TBCs deposited on various substrates. Therefore, in the present research, preparation of TBCs surfaces for applying sputtered thin films required to embed the sensors for measurement in GT. Therefore, the primary aim of the present investigation was to develop thermal barrier coatings on a Direct Metal Laser Sintered IN718 substrate and to compare their characterization with conventional IN718 substrates. Studies with respect to the porosity, bond strength, hardness, adhesion, erosion, etc., have to be carried out to assess tribo, thermo-mechanical performances so that they can be employed for embedding sensors through sputtered thin films for measurements thereafter. In this research work, various test rigs were custom designed and fabricated to facilitate the experimental work [27].

In the present work, the experimental studies focus on APS-enabled coatings on a typical gas turbine engine material of Inconel 718 or IN718. The role of coatings is in achieving higher gas turbine efficiency on conventionally manufactured and SLM-based additive manufactured substrates made of nickel-based superalloy IN718 so that embedded sensors can be accommodated for continuous monitoring.

2. Materials and Methods

Flat samples () of two substrates made of INCONEL718 (IN718) prepared to the required dimensions by two methods, namely, conventional manufacturing process (Investment casting) and selective laser melting (SLM) from powdered IN718 metal sintered by the scanning of an Yb-fibre laser at 25 to 50 μm layer. SLM method and postprocessing of SLM- IN718 parts carried out using details provided in Table 1. Postprocessing is done by subjecting to series of processes like “solution annealing,” “age hardening” followed by cooling within a furnace, and finally, by argon cooling. Whereas conventional IN718 substrates made of Inconel superalloy sheets were cut using EDM wire cutting at optimized cutting parameters. Both substrates were silica sand blasted having 24 mesh size with 24 openings in 1 in2 mesh area with maximum size (690 μm) sand particle passing through it for generating sufficient surface roughness on the substratum material at pressure (from 2 to 7 bar @ 0.75 bar) for stand-off distance of 75 mm at 45° spraying angle to guarantee a fine sticking of coating onto the substrate as it produces moderate indentations on the surface [28]. After blasting, the substrates clean with acetone ultrasonically and degreased by Freon (Chloro-Fluoro-carbon-113) cleaner solvent to produce a very reactive exterior for instant thermal spraying. Substrate surfaces were first deposited NiCrAlY Bond Coat (BC) and 8% YSZ Topcoat (TC) using HVOF (High-Velocity Oxy-Fuel) and APS (atmospheric Plasma Spraying) processes, respectively, and then cleaned soon after each coating.


SLM process
Film widthYtterbium () fibre laserPostprocessing
Beam diameterScanning speedCapacityHatch spaceSolution annealingAge hardeningFurnace coolingArgon cooling

0.05 mm0.08 mm1500 m/s0.2 kW0.2 mm950° for 60 min700° for 480 min600° for 60 min600° for 480 min

A heating scheme consisting of a copper block heater, thermocouple, and temperature controller was used to preheat the test surfaces by holding the specimens in it and heating while their top surface was bond coated by the HVOF process. This leads to the preheating of substrates from 35 to 650°C before deposition of bond coats. TBC (Thermal Barrier Coatings) or TCs were deposited via a 3 MB plasma gun (Sulzer Metco made) with 100 kW power input. Preheating was done to reduce moisture present in the 8%YSZ powder. Bond coated (both traditional and SLM Inconel processed) substrates have fixed BC thickness of 100 μm and varying TC thickness from 100 to 500 μm. The purpose of exceeding the total coat thickness of more than 300 μm is to accommodate the embedded sensors in the coatings. The HVOF and APS process factors employed in the experimentation are listed in Table 2.


Spraying processHVOF spraying systemAPS spraying technique

Type of coatNiCrAlYYSZ
Spray distance (mm)200, 225, 250, 275, 30075, 90, 110, 120, 125, 140, 180
Transversal speed (mm/s)16.67
Oxygen pressure (bar)/flow rate (lpm)1.1/220
Hydrogen pressure(bar)/flow rate (lpm)8.6/660
Carrier gas/pressure (bar)/flow rate (lpm)N2/7/36
Principal gas flow (Ar) lpm6.5
Derived gas flow (H2) lpm4.5
Air pressure (bar)/flow rate (lpm)5/330
Powder feed rate (kg/hr)3.62.7
Maximum Mach number2.4
Combustion gas velocity850 m/sec
Voltage, V48
Current, A500

Coated samples were polished by holding in PVC tube in acrylic powder and its solution stirred for ten minutes to do cold setting. Using emery paper, from coarse grit size (220) to fine grit size (2000), was applied on the sample cross-section to carry out polishing and then using diamond paste on velvet cloth having a grit size of 5, 2, and 0.5 microns successively.

2.1. Experiments Conducted

Various experiments conducted for evaluating the characterization of TBC on both types of substrates were explained in this portion. The average surface roughness () of the composite material was measured using a surface roughness measuring device called Taylor Hobson’s surface profilometer. The of the specimen before coating, i.e., on the surface of base metal was measured. Also, after coating with TBC, it was measured.

Microhardness () of polished TC, BC, and base metal was measured by Vickers microhardness tester controlled by computer employing an OSK 14218-1 (OGAWA SEIKI CO., LTD) tester and applied a load of 300 g with diamond indenter of the angle of 136° for 0.2 hour. The hardness of the specimen is measured by making an indentation mark on the coated specimen so that a diagonal is formed as a diamond shape indenter is pressed on the surface of the sample. An average value of overall six readings on each specimen was recorded as the data point.

Porosity is assessed by either direct or digital image of coating from the microscope in this study. But the direct way of measuring porosity is evaluated simply as pore volume which is the result of subtracting the total volume of coating with pore from a total volume without pores which is found by a microscope connected with a CCD camera produces an image of the polished surface of coated samples transmitted to image analysis software [29]. The total area arrested by the microscope and area covered by pores are separately measured by the software.

The tension-based adhesive strength of the TC was calculated separately using a unique form of sample holder on a prepared cylindrical dummy sample () coated. All dummies surfaces were roughened by punching, and each dummy rough surface was then fixed on top of a substrate sample using an “epoxy 900-C” (polymeric adhesive) and subjected to tension after being mounted on the jig (refer to Figure 1). Coated specimens () were placed in a universal-testing machine and pulled having a crosshead speed of 0.02 mm/sec until the two parts separated. The coated specimens were prepared by spraying at 650°C and room temperature after substrate preheating at 650°C could not adhere to the substrate, and so, the adhesion strength could not be measured. Both coatings subjected to tensile test were conducted using at a UTM. The measurement is noted down of load at which the specimen breaks up corresponds to the bond strength of coating [30] performed as per standards of ASTM C633-79. The glued cylinder is assembled in a self-aligning fixture which is mounted on a tensile test machine and pulled in a plane or once rupture occurs, maximum load is recorded (= maximum load/cross-sectional area of the specimen).

The erosion tests were performed at ambient temperature with modified and dark flare spray torch. The coated area () surface was subjected to the jet of Al2O3 particles size varying from 350~500 μm at 20 m/s, 200 g/min feed rate, 90° angle of incidence, 200 mm stand-off distance, and 20 minutes time of erosion to provide the erosion rate measured for every five minutes in terms of grams of coating eroded per kg of erodent.

3. Results and Discussion

Various procedures of coating and trials relating to evaluation of characteristics and their performance of coatings were presented.

3.1. Effect of Substrate’s on Porosity and Microhardness of Coatings

The value of both IN718 and SLM IN718 samples were measured taking an average of at 5 different points of sprayed BC and TC surface by surface profilometer shows a great impact on the superior adhesive performance of TBCs [31] onto the substrate. The results of SLM IN718 specimens showed a high amount of average roughness in comparison with the traditional IN718 samples as produced by SLM IN718 samples exhibited very fine and porous structures. This may increase while producing a variety of shapes (comparing Figures 2 and 3). Figure 2 indicates the influence of growing substrate’s over the roughness of BC and TC surfaces is seen increasing [32]. It also shows that of BC more than substrate and less than TC due to formation of oxide layers, between metallic BC and TC, facilitates the formation of micromechanical interlocks with their corresponding first coats. Same distribution of in SLM IN718 is also seen but higher than IN718 substrates. The of SLMIN718 is higher due to its porous structure which allows good adhesion with lesser interlocking of metal with metal BC at microlevel. The TC is already porous in nature which shows higher Ra than both. The comparison of values for two substrates is shown in Tables 3 and 4.


Substrate roughness ( (μm))BC roughness ( (μm))TC roughness ( (μm))BC microhardness (Hv)TC microhardness (Hv)BC porosity (%)TC porosity (%)

3.54.8788611853.26.5
3.755.12886011653.66.8
45.38.683511483.97.5
4.255.559.282511254.27.8
4.55.869.881511004.48.1
56.110.680510854.78.5
5.256.710.779510604.99.3
5.57.41178510205.110.3
68.21277610005.510.8
6.28.512.5756.59845.712
6.5913743.8962612.6
6.99.513.77319396.313.3
7.21014.37189176.614
7.810.514.85705.5894.46.814.6


Substrate roughness ( (μm))BC roughness ( (μm))TC roughness (Ra (μm))BC microhardness (Hv)TC microhardness (Hv)BC porosity (%)TC porosity (%)

56.289.15860110037.5
5.36.589.684410853.38.6
5.66.929.883610603.89.3
67.1510.28281025410
6.27.5210.882010104.311.8
6.57.9811.258009904.612.8
6.98.2811.75788975513.4
7.29.21137709555.315
7.810.0813.4745915619.2

The decaying stage of TBCs through erosion emerges principally on GT components (blades and vanes) mostly when an aircraft GT works in a harsh environment which can be reduced moderately by the reduction of the porosity [33]. In other words, the increased porosity leads to increased thermal shock resistance (TSR) which generally manufacturers prefer. It is also found that microcracked structure in TBCs at times shows higher resistance to erosion wear jointly with higher TSR. The effect of isothermal heating (1100°C/hr) on the erosion wear resistance of the TBC system has been presented in terms of porosity where the porosity value varies w.r.t. , SOD, substrate temperature, and coating thickness [34].

Figure 2 embodies the % of porosity varying w.r.t of a substrate made of traditional IN718 shows BC has less porosity than TC, but the porosity levels increase with the increase in of substrate in both cases.

Figure 3 characterizes the % porosity of BC and TC deposited on SLM IN718 substrates where % porosity rises in both BC and TC with the increase of of substrate. But the % porosity in TC deposited on SLM IN718 substrates were more than that of traditional IN718 due to defect free microstructure develop more porous structure for BC, and in turn, this mismatch of interlocking of metals produces more metal oxides between BC and TC. Therefore, TC retains higher % porosity.

Figure 4 shows the microhardness varying with of substrate in traditional and SLM substrates. With the increasing , the microhardness values were decreased in both BC and TC deposited on both types of base metals. The microhardness values for SLM IN 718 were found to be quite lower than traditional IN718 (refer to Figure 5) in both TC and BC samples and are caused by the porosity in the coating and the development of various phases during and after the TBC coatings [32]. Increasing of of substrate rises both and porosity of BC and TC but relatively decreases the hardness value. The higher microhardness decreases the porosity and the of coating at the low value of of substrate [35].

3.2. Effect of Standoff Distance (SOD) on TBC

The results obtained from experiments revealed that the SOD remained significant parameter affected the TBC’s , microhardness of APS coatings meaningfully (refer to Table 5). Figures 6 and 7 showing the varying of microhardness and % porosity and with respect to SOD. For 75 to 125 mm of SOD, hardness is found lower and higher porosity and found contrary between 140 and 180 mm of SOD. The highest hardness is seen at 125 mm SOD, and lower dense coatings were formed for a high value of SOD. Obviously, it was seen that mostly 8%YSZ TBC particles scattered at a distance of (between nozzle and substrate) 75-110 and 140-180 mm stayed partially or nonmelted. SOD of 120-125 mm formed the least porosity or high dense coatings, and the maximum temperature was reached at this optimum distance (115 to 125 mm) due to complete melting. Beyond this, the particle temperature was kept on reducing. The optimum condition of SOD in APS process formed results into the maximum , the maximum hardness, and low porosity. Less than or more than optimum SOD produced partial or nonmelted coatings with increased average roughness and porosity [15].


Stand-off distance in mmMicrohardness in SLM sampleMicrohardness in IN718 sample% porosity in SLM sample% porosity in IN718 sample in SLM sample (μm) in IN718 samples (μm)

75104511109.57.89.97.7
90105611209.27.59.27.2
100108011358.857.3596.7
110112011908.678.86.4
125115512058.156.88.56.2
140112511608.57.78.76.6
150107011208.98.28.956.85
160103510859.458.99.47.6
180985103511.59.8107.4

Figure 7 depicting the distribution of and % porosity of TC of traditional IN718 and SLM IN718 with respect to SOD shows the coating roughness reaches its least value at an optimum SOD range (115 to 125 mm) because long SODs are attributed by low element impinging velocity forms numerous visible defects like inappropriately bonded sphere-shaped elements and ample porosity. While short SODs make impinging particles with large momentum in a normal direction to the base metal’s surface. Therefore, droplets impinge the base metal at round about 90° by means of no or at very short velocity parallel to the substrate’s surface forms the vertical cracks during cooling. This rises temperature mismatch stress which subsequently cracks to heading towards the thickness.

3.3. Effect of Substrate Temperature

The surface of substrates of both samples was subjected to preheat from ambient (35°C) to elevated temperatures (600°C) at steady-state condition before APS coating. The characteristics such as the hardness, % porosity, and coating roughness were obtained with respect to substrate temperature for both samples are plotted in Figures 8 and 9 for studying their influence on the TBCs at optimum SOD of 120 mm and coating thickness of 100 μm. Results of TBC coated by APS on a preheated sample revealed minimum substrate and TC porosity and maximum microhardness (refer to Table 6). The TC, BC show at 35° utmost porosity due to gaps at the interface of coating and base metal, but at high substrate temperature, this gap disappears. Because that substrate temperature modifies the splat structure. With the increase of substrate temperature, a changeover was detected from a droplet of splash to disk droplets. Droplets of splash at high degree are possible to be collapsed by forming secondary droplets. On contrary, disk droplets show small or no fingering pattern because all the droplets are maintained in the central solidifying liquid. Developed liquid droplets on the base metal aid to reduce the bonding of the coat as voids present at the junction of substrate/BC/TC interface resulted from breaking of molten droplets. Only disk molten droplets bond very well to the base metal in improving the adhesion strength of the coating in the form of tensile strength obtained by improved wettability between them. A large number of disk molten droplets attain sluggish solidifying rate of depositing particles instigated by preheating also increased the tensile adhesion strength. AS was also raised particularly when the base metal was at 500°C without oxide formation, about 2 to 3 times of AS of a nonpreheated surface. Nevertheless, AS even decreased with preheating when base metal was with oxidation. The same behaviour of preheating worked in a variation of microhardness and .


Substrate temperature in °CMicrohardness in SLM sampleMicrohardness in IN718 sample% porosity in SLM sample% porosity in IN718 sample in SLM sample (μm) in IN718 samples (μm)

35110010009.96.810.27.9
100113010259.56.5107.8
200116510708.969.87.5
300122011008.25.89.77.4
400128011407.45.59.57.3
500134011806.94.99.47.1
600140012806.14.29.26.8

In this section, various characteristics such as hardness, roughness, and porosity of substrates were studied with the variation of coating thicknesses are shown in Table 7. The microhardness of the APS used for TC is seen as reducing with the increasing value of coating thickness due to rise in porosity of TBC. Figure 10 depicts the relationship of hardness with varying coating thickness (100 to 600 μm) when TC deposited at SOD of 120 mm resulted into decreasing hardness with increasing coating thickness.


Coating thickness (μm)Microhardness in SLM sampleMicrohardness in IN718 sample% porosity in SLM sample% porosity in IN718 sample in SLM sample (μm) in IN718 samples (μm)

100116012008.16.69.26.5
200111011759.57.59.87
3001090113011911.28
400105010901211.612.58.4
500100010401312149.2
600900950141314.69.6

The changes in porosity level with varying coating thickness are plotted in Figure 11 whereby increasing coating thickness, porosity level also increased due to increasing coating roughness .

3.4. Evaluation of Coating-Substrate Adhesion Strength (AS)

In this section, AS of TBCs onto the substrate in respect of BC is limited to a constant thickness value of 100 μm for all both types of substrate surfaces and TC with different coating thicknesses (100 to 600 μm). The BC was produced with HVOF technique using an optimum level of parameters as specified in Table 1. Table 8 designates the AS of BC on traditional and SLM substrate materials resulting into constant AS of BC showing the maximum of 51 MPa for traditional IN718 and minimum of 42 MPa for SLM IN718 when it was coated to a thickness of 100 μm [16].


Substrate materialAS (MPa)

Traditional IN71851
SLM IN71842

AS of TC also called as TBCs were developed using APS process (refer to Table 9) for different coating thicknesses, torch power input to APS process, and SOD as shown in Figures 1214.


Power in kWAS of SLM sampleAS of IN718 sampleCoating thickness in μmAS of SLM sampleAS of IN718 sampleSODAS of SLM sampleAS of IN718 sample

10182210028331002833
12202515030401252629
14222720032411502423
16273030022361752021
18303340016312001517
202127.550010232251215
241823600413

The AS increases with increasing value of torch input power up to 18 kW and beyond this power decreases (refer to Figure 12) due to increased thermal stresses produced in the coating will remain as residual stresses after developing coatings at increasing power. The residual stresses in the coating decrease the AS of the TBC. The AS of TC shows the maximum value when thickness lies in the range of 150 to 200 μm (refer to Figure 13), and beyond this range, as coating thickness increases, AS decreases due to increasing residual stresses in the coating with the increase of thickness.

As it is seen AS reducing with increasing SOD value on both the substrates but SLM IN718 substrates exhibited lower values at all APS process factors (SOD, torch input power, and coating thickness) compared to traditional IN718 when the AS was found out by the coating pull out test based on ASTM C-633 standard. To understand the clear difference between the substrates of traditionally prepared and SLM processed, the maximum AS is compared at 18 kW of torch power which is about 40 MPa for traditional and 30 MPa for SLM substrates when the thickness of coating is 200 μm and good AS was achieved for TC (for coating thickness of 150-200 μm) over BC.

Therefore, the present work reveals that increasing the torch input power, the interface bond strength between substrate, BC, and TC enhances and hence the AS increases. Typically, AS varies as a function of SOD and TIP can be associated with the thermal stage of the impinging particles on to the substrate. Because low amount of power produces the plasma jet stream whose temperature is inadequate to bring all the particles into molten state (partial melting) which only causes only the decrement in the coating strength. Required amount of high plasma temperature is used to complete melting for improving AS of the TBCs.

3.5. Erosion Rate on as Sprayed and Isothermally Heat-Treated Coatings

Results of erosion rate (in gm/Kg) of both samples during as sprayed condition as well as heat treated under isothermal condition (refer Table 10). To explore it, initially, the erosion rate for as sprayed without heat treatment is studied with respect to TBC porosity level which shows erosion rate increasing in both samples coated with TBCs with increasing porosity, but higher erosion rate is found in SLM substrates than traditional IN718 ones.


No.% porosity for traditionalErosion g/kg for traditional% porosity for SLMErosion g/kg for SLM

16.85.938.110
27.279.113
3889.714
48.61010.516
591112.418
69.51213.620
7101414.221
811.216.11622
9141820.424

Figure 15 showing the erosion rate in as sprayed and after heat treatment in isothermal condition at 1100°C for about 120 minutes was reducing from 18 to 6 g/kg for traditional IN 718 and from 24 to 10 g/kg of wearing down (abrasive) material for SLM IN718. Figures 16 and 17 showing the results of microhardness increased and porosity decreased for heat-treated samples revealing that heat treatment enhanced their resistance against erosive capacity due to sintering [36]. The results of the erosion rate, porosity level, and microhardness in as sprayed and heat-treated samples of both types were tabulated in Table 10 indicating improved resistance with isothermal heating at 1100°C/hr.

4. Conclusions

The of GT predominantly depends on inlet/input temperature and by its increased value will lead to high-value efficiency but restricts the working temperatures of GT components, excessive corrosive environment, and changes in microstructure and creep. Selective Laser Melting (SLM) with reference to rapid prototyping GT components used for studying the characterization of samples prepared by sintering of fine powder with high power lasers to compare with conventional IN718 specimens were fabricated by EDM wire cutting. BC and TC were prepared with HVOF and APS, respectively. Characterization of TBC is carried out by (1) coatings with respect to various input factors such as SOD and coating powder feed rate; (2) porosity, hardness, surface quality, and erosion are discussed. Experimental results of various characteristics of TBCs on Inconel and SLM Inconel 718 helped to draw the subsequent conclusions: (i)Surface roughness () and porosity of TBCs for the deposition of multilayer architecture shown value of both IN718 and SLM IN718 samples of as sprayed BC and TC surface having a great impact on the superior adhesive performance. The influence of substrate (before coating) over the of BC and TC surfaces (after coating) in which the of both BC and TC surfaces increased as a function of substrate surface such that of of and Ra of SLM IN718 of IN718 substrate where higher of BC and TC of SLM substrates(ii)The efficiency of deposition of SLM IN718 samples is found lesser than IN718(iii)For BC prepared by HVOF spraying process, the is found increased as it could be attributed to high temperatures in the CC by presence of higher quantity of oxygen is increasing with powder feed rate(iv)For TC APS sprayed, w.r.t. input torch power, standoff distance, and powder supplying rate shown low value in case of SLM IN718 than that of TBCs coated on traditional IN 718 substrates(v)Erosion rate of heat processed . And erosion rate of TBC coated on SLM on IN718(vi)TBC on SLM IN718 substrates exhibited good thermal drop verified the use of Selective Laser Manufacturing technique for high-temperature application

The above work may be extended for other types of composite coatings especially for thin coatings. Nanoindentation study above coatings would help in estimating modulus, Poisson ratio, etc., along with hardness for its mechanical strength characterization.

Data Availability

Data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors have no conflicts of interest.

Acknowledgments

The authors would like to thank the Deanship of Scientific Research at Umm Al-Qura University for supporting this work. The authors received financial support for the research and/or for the publication of this article by the Deanship of Scientific Research at Umm Al-Qura University with Grant Code (20UQU073DSR).

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Copyright © 2021 Mohammed Yunus and Mohammad S. Alsoufi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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