DEVELOPMENT AND
CHARACTERISATION OF THERMAL
BARRIER COATINGS ON
CONVENTIONAL AND DIRECT METAL
LASER SINTERED IN718
SUPERALLOYS
S SREENIVAS
Department of Mechanical Engineering, Cambridge Institute of Technology, Bengaluru, Karnataka, India
er_srvs@yahoo.co.in
U CHANDRASEKHAR
Vel Tech University, Avadi, Tamil Nadu
rapidchandra@gmail.com
K HEMACHANDRA REDDY
Department of Mechanical Engineering, JNTU College of Engineering-Anatapur, Anathapuramu, Andhra Pradesh, India
konireddy@gmail.com
B R SRIDHAR
Department of Mechanical Engineering, SEA College of Engineering and Technology
sridhar.br11@gmail.com S SHANKAR
Department of Mechanical Engineering, Cambridge Institute of Technology Bengaluru, Karnataka, India
sanjeevishankar@gmail.com
Abstract
Thermal spraying has been assumed to be a complicated deposition process that deals with a number of interconnected variables / parameters. Variation in these parameters brings about a complete or a partial shift in the particle properties and thus may cause changes in the microstructure of the deposited coatings. Combination of coatings with the substrate surface is a matter of concern as the bonding between the two surfaces involves mechanical, physical and chemical mechanisms. In the present work studies related to the comparison and assessment of mechanical and other related properties of thermal barrier coatings of conventional Inconel 718 (IN718) and Direct Metal Laser Sintered (DMLS) IN718 have been carried out. Comparison between conventional IN718 and DMLS – IN718 superalloy coatings have revealed interesting results.
Keywords: Thermal barrier coatings, Inconel718, DMLS, Microhardness, Adhesion, TGO, superalloy
1. Introduction
additional cooling [2]. The technology of TBC is almost 30 years old and as on today TBCs are not clearly understood. The reason is due to the difficulty in predicting or untangling issues related to mechanical and material phenomena like thermal expansion, diffusion, oxidation, creep, microcracking, sintering, and fatigue. The gas turbines (GTs) are among the most demanded mechanical products produced today. Besides, GTs are dependent on the technical cost and the risk involved in producing them. And they have to be developed at a minimum cost and risk. This calls for its development with continuous improvement in design, materials and fabrication processes to bring down the cost of production [1].
Improvement in GTs manufacturing is incessantly coupled with a rise in working temperature and stresses that arise in the turbine parts. Therefore, researchers have concentrated on the development of more sophisticated cooling systems, advanced materials, thermal barrier coatings (TBC) and other coatings (thin films) that enhance the life of GTs by modifying the surface, which thus helps in reducing the maintenance cost. Often there arises an obligation to carry out surface modifications of the turbine blades of the GTs, the turbine blades and the combustion liners [1]. Therefore, the technology selection to modify the surfaces has to be treated as an essential part in the designing of the engineering components. This requires an understanding of the substrate’s surface in respect of properties like corrosion, erosion, resistance to wear, fatigue, creep, etc., [2]. The base material / substrate used for the coating is a Ni-based superalloy and this has to sustain the harsh environment inside the chamber so as to protect the underlying substrate [3]. Surface modification can be carried out by either thermal or chemical means. Fig. 1 summarizes the classification of surface modification techniques for industrial use.
Fig. 1: Classification of processes used for coating at the industrial level [4]
The present day aero engines [5], gas turbines [6], diesel engines / IC engines [7] use TBCs. TBCs are more often made of Yttria-stabilized zirconia (ZrO2/Y2O3) due to their low thermal conductivity (1 W/mK) and can be used in wide range of applications such as implants like hip and tooth crowns. High hardness and chemical inertness makes it a suitable material for these applications [8]. In oxygen sensors, they are being used as electrolytes [9]. Due to excellent ionic conductivity they are used in solid oxide fuel cells [10, 11]. The TBC’s are produced by plasma spraying or EB-PVD. Major elements and their requirements of a thermal barrier coating system have been shown in the Fig. 2. Kaysser et al. [11], Meier et al. [12], Morrell and Rickerby [13], have reported that EP-PVD proffers better advantages over plasma spraying, regardless of its higher cost and thermal conductivity when used in applications like blade airfoils for obtaining extremely smooth surfaces, excellent adhesion and strain tolerance [14].
Fig. 2: The four major elements and their requirements of a thermal barrier coating system.
Present day industries and research establishments have turned their attention towards additive manufactured parts (rapid prototype). Rapid Prototyping (RP) refers to a group of techniques used to produce prototypes swiftly using 3-D computer aided design (CAD) data using layer-by-layer fabrication process [17, 18]. Acceptance of RP began with the use of plastic-based materials for a variety of applications in the latter half of 1980’s thereby initiating the process of replacing many of the conventional metals used at that point of time [19]. Presently, there are several RP techniques that have a specific build style, method and materials and these have been accepted on their own merits and demerits. RP methods include Fused Deposition Modeling (FDM), Laminated Object Modeling (LOM), Selective Laser Sintering (SLS), Stereolithography (SLA), 3D printing, Direct Metal Laser Sintering (DMLS) [17], etc. Among various RP techniques, DMLS (Fig. 3) is considered for variety of applications that includes aerospace, manufacturing, medical, prototyping and tooling.
DMLS is basically a metal prototyping technique. DMLS operates by melting thin layers of metal powder of say (20 - 60µm) using a laser beam of 200W. The technique helps in fabricating parts of any size and shape with high accuracy (±0.05mm) [20]. There are two methods via which DMLS material processing can take place, (1) powder deposition (2) powder bed. Wide range of materials can be developed for DMLS process to deliver superb parts for an extensive variety of uses, and right now a mixture of materials covering the most vital classes of metals for manufacturing (steels, super alloys, light alloys and so on.) have been brought to commercial status. Thus helps in achieving parts with superior finish for extensive variety of applications.
DMLS technology is still relatively new and little understood in the industry and as well as by its users. As the awareness on the use of the technology increases, its benefits also increase. DMLS offers an ideal platform for developing completely new tailored made products and applications that can have varied geometry and internal structures which help in offering fascinating possibilities for designing and optimizing part properties.
In the present research, an attempt has been made to compare and assess the mechanical and other properties of TBC coated conventional IN718 with DMLS-IN718 substrates. The properties studied include porosity, bond strength, hardness, and adhesion.
2. Experimental work
2.1 Preparation of substrates and coating materials 2.1.1. Conventional IN718 Substrates
Nickel base super alloy IN 718 was chosen as a base material. IN 718 was chosen for the reason that it is an industrial accepted material used for aero gas turbine engine parts. Quartz sand of 16-20 mesh was used for roughening the surface of the substrate. An air pressure of about 5 kg/cm2 was used for blasting quartz. A standoff distance of 120-150mm was maintained for blasting. Acetone was used only for cleaning the substrates’ surface by using an ultrasonic cleaner. The average roughness of all the substrates ranged 4.9-5.8 μm. Cleaning was immediately followed by Plasma spraying. The powdered NiCoCrAlY (Particle size: 125+16μm) material was chosen as a bond-coat material for plasma thermal spraying. Yttria-stabilized Zirconia (Particle size: 150+22μm) powder was chosen for production of ceramic coatings. Microstructure and composition analyses were carried out by means of a field emission electron scanning microscope (FESEM, Quanta 200 FEG) with Ultra thin window EDS System (EDAX) using 30 kV of accelerating voltage. The substrate material was cut to the dimensions of 50 x 30mm using wire cut EDM technique. The EDM machine and the optimized cutting parameters utilized in the study are depicted in Fig.4 and Table 1 respectively.
Fig. 4. EDM wire cutting machine
Table 1: Optimized EDM wire cutting parameters
Wire Material Wire Dia., mm Wire Tension, g
Wire feed rate, m.min-1
Resistivity x 104, Ω cm,
Cutting Speed, mm.min-1
Brass 0.25 1300 10 5 4.3
Water flow, L.min-1 Water pressure, bars Water flushing rate, L.min-1
Current, A Voltage, V Dimensional accuracy, µm
10 13 10 52 4.5 5
2.1.2. Plasma spraying
The coatings were applied on to the IN718 substrate strips. The strips were of 50mm x 30mm x 4mm dimensions. Substrate strips were silica grit blasted and bond coated using NiCoCrAlY metallic powder to a thickness of 100µm. The bond coated samples were coated with ZrO2 × 8%Y2O3 (8%Yttria Stabilised Zirconia, YSZ) powder as a top coat. The thickness of top coat was varied in the range of 200µm to 600µm, whereas the bond coat thickness was restricted to 100µm. The parameters of atmospheric plasma spraying (APS) are shown in Table 2.
Table 2: Optimized parameters of air plasma spraying
TBC Current, A Voltage, V
Primary gas flow (Ar), L.min-1
Secondray gas flow (H2),
L.min-1 Powder feed rate, g.min-1 Spray distance, mm
NiCoCrAiY 450 65 6.5 4.5 120 110
APS was carried out using Metco Sulzer machine of capacity 100 kW power supply. The IN718 substrate was initially preheated using the plasma gun without any particles being injected. Then a bond coat of approximate thickness of 100µm was sprayed on it (constant bond coat thickness was maintained for all samples), these bond coated samples were then top coated with varying thicknesses (200µm, 400µm, 500µm, 600µm) on different samples.
2.1.3 DMLS-IN718 specimen fabrication method
IN718 test specimens (30 mm x 50 mm) are manufactured through DMLS process using layer thickness of 40µm, scan speed of 1500 mm/s, beam diameter of 0.08mm, hatch spacing of 0.19mm and laser power of 195 W. Comparison of the chemical composition of DMLSIN718 parts (Table 3) with that of traditionally manufactured IN718 parts reveals similarity in weight percentages of various metals. Particle size analysis (Table 4) reveals that 90% of the particles lies in the band of 30 to 50 µm. As per the standard post processing procedures, DMLS IN718 parts are subjected to solution annealing and age hardening treatment. The heat treatment is carried in two steps, solution annealing at 980°C for 1 hour followed by and ageing treatment at 720°C for 8 hours. The ageing treatment further involves furnace cooling to 620°C in 2 hours, holding period of 8 hours at 620°C and subsequent argon cooling.
Table 3: Chemical composition (in wt %) of EOS Nickel Alloy IN718 powder
Ni Cr Nb Mo Ti Al Co Cu C Si P B Fe
53 19 5 3 1 0.5 1 0.3 0.08 0.05 0.015 0.006 Balance
Table 4: Particle size distribution of IN718
Particle size (µm) 10 20 30 40 50 60 70 80 90
Percentage 2 2 56 22 12 2 2 1 0
The sintered parts are preheated initially using the plasma spray gun without any particles being injected and then applied with bond coat and top coat. Coatings are deposited using a 3 MB plasma gun with a 100 kW power supply. Prior to deposition of coating, the DMLS parts are grit blasted using silica with a grit mesh size of 24. Powdered NiCoCrAlY with a typical particle size of 125 μm is chosen as the bond coat material for APS. Yttria-stabilized Zirconia with a typical particle size of 150 μm is chosen as the top coat. Process parameters of APS are shown in the Table 2. Microstructure analysis is carried out by means of a field emission electron scanning microscope (FESEM, Quanta 200 FEG) using 30 kV of accelerating voltage [21].
2.2. Characterization of coatings
2.2.1 Determination of Coating Deposition Efficiency
Deposition efficiency is defined as the ratio of the weight of the coating deposited on the substrate to the weight of the expended feedstock. Weighing method is accepted widely to determine this. Each specimen was weighed accurately before and after the coating deposition. The difference of these weights (Gc) measured the deposition of coating material on the substrate. The weight of the expended feed stock (Gp) was determined from the powder feed rate and time period of deposition. The deposition efficiency (η) was then calculated using the following equation (1).
η = (Gc / Gp X 100) % (1)
Weighing of the samples was done using a precision electronic balance with ± 0.1mg accuracy.
2.2.2 Surface roughness measurements
Roughness of Grit blasted conventional IN718 and DMLS-IN718 substrates and as deposited bond and top coatings surface were investigated by contact stylus instrument as per the ASTM standards. Atomic force microscopy (AFM) has also been used to envisage the surface of the coated surface.
2.2.3 Microhardness testing
Vickers microhardness was determined on all the coated samples of aluminides and alumina by using a LEITZ microhardness tester equipped with a monitor and a microprocessor based controller, under a load of 0.493N and a loading time of 20seconds.Ten readings were taken on each sample and the average value is reported as the data point.
2.2.4 Tensile Adhesion Test (TAT)
tension after being mounted on the jig. The coating pull out test was carried out using the set up Instron 1195 at a crosshead speed of 1mm/min. The moment the coating got torn off from the specimen, the reading (of the load), which corresponds to the bond strength of coating, was recorded [22]. The test is performed as per ASTM C-633.
2.2.5 Porosity Measurement
Porosity of the coatings was measured using a microscope (Neomate) equipped with a CCD camera (JVC, TK 870E).This system is used to obtain a digital image of the object [23]. The digitized image of the polished surface (coated samples) is transmitted to a computer fitted with VOIS image analysis software. The total area captured by the objective of the microscope or a fraction thereof can be accurately measured by the software. Hence the total area and the area covered by the pores are separately measured and the porosity of the surface under examination is determined.
3. Results and discussions
3.1 Coating deposition efficiency
Coating thickness was measured with an optical microscope and vernier calipers. The bond coat thickness (average) was about 100 μm for Inconel718 (IN718) substrate and about 97 μm for DMLS-IN718 substrate as seen from Fig. 5. The coating (Top coat) thickness via plasma spraying obtained at different power levels on DMLS-IN718 and conventional IN718 has been presented in Fig. 6. Each value is the average of three measurements.
The techno-economics of the process is determined by the factor known as deposition efficiency (DE). The DE was determined by simple gravimetric method. Also there are many factors on which the ‘DE’ depends viz., input power, properties of the materials (melting point, grain size, heat capacity, standoff distance). Torch power appears to be a vital factor for a given material that has specific particle size and a standoff distance with the defined ‘DE’. The ‘DE’ is usually defined as the measure of the fraction of the powder that has melted without vapourizing or decomposing into gaseous products.
Fig. 7 depicts the variation of ‘DE’ with variation in torch input power (TIP). It is to be noted that the ‘DE’ has increased in all the cases with the increase in ‘TIP’. Also, one more observation reveals that DMLS substrate face a setback when compared to conventionally processed IN718 substrates. The reason for this variation could be attributed to more uneven surface of the DMLS-IN718 substrates.
Fig. 6: Variation of Top coat (YSZ) Thickness with Torch Input Power.
The ‘DE’ of the plasma spray of the said materials in all probabilities depends on (i) melting point (ii) thermal heat capacity (iii) particle size of the powder. When the input power is low, the temperature of the plasma jet is not good enough to melt the powder particles that get into the plasma jet. With the increase in power, the temperature of the plasma rises to a stage that will melt a larger portion of the powder. However, literatures suggest that beyond a certain point, the temperature of the plasma would be so high that it starts to melt the entire powder mass and thus causes a decrease in ‘DE’ and this could also be a cause of reduced ‘DE’ in case of DMLS-IN718 substrates. Plasma power exceeding which there is decrease in efficiency depends on the particle size and also on the chemical nature of the powder. The present investigation reveals that the ‘DE’ increased from 26% to 59% on conventional IN718 substrate and by 20% to 46% on DMLS processed IN718 substrates (with input power to plasma torch increasing from 12 kW to 22 kW).
3.2 Surface roughness measurements
Grit blasted conventional IN718 and DMLS-IN718 substrates and as deposited bond and top coating surface roughness values were investigated by contact stylus instrument as per the ASTM standards. The average surface roughness values of the substrate and the as-sprayed coatings are as shown in the Table 5. Also atomic force microscopy (AFM) has been used to envisage the surface of the coated surface. AFM is a quantitative measurement tool and has been used for the measurement of the surface texture of top coated (600µm) samples (conventional and DMLS) (Fig. 8).
The surface roughness of bond coat and top coat has a significant influence on performance of TBCs [24]. It has been pointed out that an appropriate roughness is essential for the bond coat and hence for the top coat for adhesion to the substrate.
The results reveal that DMLS samples have higher roughness compared with the conventional samples and the same has been ascertained via AFM (Fig. 8). The AFM studies indicate that the average roughness (Ra) values are 1.1 nm and 2 nm for conventional and DMLS samples (600 µm top coat) respectively. Surface roughness for S1 – S4 of IN718 with 200, 400, 500 and 600 µm top coatings is given in Table 5. (S5 – S8) are corresponding values of surface roughness for DMLS samples (Table 5).
(a) Conventional IN718 600 µm top coated sample
(b) DMLS-IN718 600 µm top coated sample
Fig. 8: AFM images of conventional and DMLS (IN718) 600 µm samples subjected to surface roughness studies. Table 5. Average surface roughness values of the substrate and the as-sprayed coatings
Conventional IN718 Ra (µm) DMLS-IN718 Ra (µm)
S1 S2 S3 S4 S5 S6 S7 S8
Grit blasted 6.14 6.12 6.13 6.16 Grit Blasted 7.12 7.15 7.14 7.16 Bond Coated 4.57 4.55 4.50 4.59 Bond Coated 6.32 6.35 6.41 6.40 Top Coated 5.24 5.29 5.31 5.35 Top Coated 6.98 6.75 6.89 7.0
3.3 Microhardness
Vicker’s microhardness measurements were carried on polished cross section of the coatings. Machining followed by lapping is done to reduce the surface roughness of the samples from 8µm to 1-2µm to enable hardness measurement.
Variation of hardness values may be due to the formation of different phases before and after the coatings. The results obtained are comparable with the results obtained by Satrughna Das [25], Krishnamurthy and Anil Kumar C [26].
Results indicate that DMLS substrates possess high hardness thereby showing that surface grinding is not a viable choice.
Solution annealing, results in reduction of hardness (Fig.10). Solution annealed samples subjected to age hardening reveal an increase in hardness. A significant leap in hardness from 680HV to 800 HV was noticed after coating. Bond strength noted in the present study is around 16 MPa and this is quite close to the findings of Andi et al. (14.5 MPa) [27]. The obtained bond strength is quite lower than that (52 MPa and 34 MPa) reported by Iwamoto et al. [28] and Shankar et al. [29] for stainless steel and mild steel substrates. Low bond strength presently observed could possibly a result of pre-existing residual tensile stress during deposition and the same is influenced by initial substrate temperature and thickness of the coating [27, 30, 31]. And this needs to be corroborated by further work.
Fig. 9:Microhardness of the conventional IN718 specimens before and after heat treatment.
3.4 Evaluation of coating-substrate adhesion strength
Literature suggests that adhesion is to be considered from the microscopic point of view. Adhesion is determined by physic-chemical surface forces (such as Vanderwall, covalent and ionic) existing at the substrate and coating interface [32]. In the present investigation, adhesion strength was determined by coating pull out method as per ASTM C-633 standard. It was observed that the samples fractured at the Coating / Substrate interface. The results of the adhesion measurements on top coat (TC) and bond coat (BC) for conventional and DMLS processed IN718 substrates have been shown in Tables 6 and 7 respectively.
Several researchers have pointed out that adhesion, if considered from the mechanical point of view, is to be measured as a result of the force related with interfacial fracture and is basically macroscopic in nature. Usually, if the fracture takes place at the coating / substrate interface, then the fracture mode is treated to be adhesive and this depends absolutely on the surface characteristics of the substrate and the adhering phase [33, 34]. Fig. 11 illustrates the variation of adhesion strength of TC with reference to different input power level and as well as at various torch to base distances (TBD). Maximum adhesion strength of BC was observed to be 7 MPa and 5 MPa in conventional and DMLS processed substrates respectively (for power input level of 10 kW and TBD of 100 mm).
Similarly, the adhesion strength of top coats (200, 400, 500 and 600 µm) were also measured. The observed values were 12 MPa, 15 MPa, 17 MPa and 20 MPa for conventional processed IN718 materials and 10 MPa, 13 MPa, 14 MPa and 17 MPa for DMLS processed IN718 materials (TBD of 100 mm and a power level of 12 kW). Tables 6 and 7 highlight the observations made with respect to (12 kW and 22 kW input power) conventional and DMLS processed coated materials with varying ‘TBD’. The DMLS processed materials have shown reduced adhesion. It has been observed that samples coating strength increases with the increase in the torch input power (TIP). On the contrary, the adhesion strength has been found to decrease with the increase in ‘TBD’ (Tables 6 and 7).
A discernible distinction in the adhesion strength of the coating has been observed in the case of conventional and DMLS processed substrates. Adhesion strength is more in case of conventional than DMLS processed substrate. Maximum adhesion strength of 30 MPa and 28.1 MPa was observed at 22 kW power in conventional and DMLS IN718 coated (600 µm) substrate. This reveals that higher the coating thickness better the adhesion strength. Fig. 11 and 12 illustrate the variation of torch input power and adhesion strength for varying coating thickness of the YSZ coat on conventional IN718 substrates. It is observed that higher the input power and higher the coating thickness, higher is the adhesion strength. Similar observations were made with DMLS-IN718 substrates also.
Table 6: Adhesion strength of YSZ coating of conventional IN718 coatings
Power (kW) TBD (mm) Adhesion strength of Top coat (MPa) 200 µm 400 µm 500 µm 600 µm
12 100 12 15 17 20
12 200 10.9 12.2 14.6 17.2
12 300 7 8 11.5 12.6
22 100 16 20 24 30
22 200 14 17.4 19.1 26.7
22 300 11.9 14.8 16.3 21.5
Table 7: Adhesion strength of YSZ coating of DMLS IN718 coatings
Power (kW) TBD (mm) Adhesion strength of Top coat (MPa) 200 µm 400 µm 500 µm 600 µm
12 100 10 13 14 17
12 200 8.1 11.2 12.3 14.1
12 300 6.9 7.1 10.6 11
22 100 15 18.9 22.8 28.1
22 200 13.1 15.1 17 14.2
Fig. 11: Adhesion Strength of YSZ Coating on NiCoCrAlY Bond coat for varying thickness and torch input power on conventional IN718 substrate.
4.5 Coating porosity
Image analysis technique was used for measuring porosity of the coated samples. Optical microscope outfitted with a CCD camera was used to study the polished interfaces of the coatings. VOIS image analysis software was used for obtaining the digitized image and thus to determine the coating porosity. Fig. 13 shows the variation in the porosity level with the increase in torch input power (TIP).
Tables 8 and 9 indicates the variation in porosity level with the increase in ‘TIP’ (?).The results indicate that the porosity volume fraction of YSZ coatings lie in the range of 2.7 to 5 % for conventional IN-718 samples (600 µm) and 4 to 15 % in case of DMLS – IN718 samples. Porosity has been observed to be more in coatings made at lower power levels. A similar trend has been observed for DMLS coating. With increase in coating thickness porosity level is found to be decreasing for both materials. Thus the results obtained in the study on the coatings are found to be similar with the observations made by other researchers [35].
Fig. 13: Coating Porosity in top coat Vs Power for DMLS-IN718 samples Table 8: Coating porosity in top coat for conventional IN718 samples
Table 9: Coating Porosity in top coat for DMLS-IN718 samples
Power (kW) Porosity (Vol. %)
200 µm 400 µm 500 µm 600 µm
12 16 15.1 14.7 14
16 13.9 13.1 12.6 12
18 10.9 10.2 9.8 9
22 10.6 9.3 8.9 8
Power (kW) Porosity (Vol. %)
200 µm 400 µm 500 µm 600 µm
12 8 7.1 6.4 5
16 7 6.2 5.2 4
18 5.9 4.9 4.5 3.9
4. Conclusions
• Present work successfully compares coating on conventional IN718 with that on the upcoming Direct Metal Laser Sintered (DMLS) IN718.
• With increase in the torch input power (TIP), the interface bond strength increases and this enhances the adhesion strength.
• With increase in the surface roughness of the sample, surface roughness as well as the porosity level of the coating increase. This is accompanied by lowering of the hardness of the coatings.
• Thick coatings have enhanced mechanical strength and higher load bearing capacity owing to low porosity level.
• Coated properties on DMLS material are superior to those on conventional material.
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