Slurry Spray Coating Technology with the Aid of Sintering Additive R. Verma1, N.M. Suri3, S. Kant 2, 1
Assistant Prof., Production & Industrial Engg, PEC University University of Technology, Chandigarh Chandigarh E-mail:
[email protected] 2 Associate Prof. Production Production & Industrial Engg, PEC University University of Technology, Chandigarh 3 Assistant Prof. Production & Industrial Engg, PEC University University of Technology, Chandigarh Chandigarh
Abstract
Slurry spray technique (SST) usually known by its low fabrication cost, is a relatively unutilized coating fabrication method which can be used for a variety of sinterable feedstock materials. It utilizes thermal energy corresponding to the sintering temperature for consolidation of the powder constituents, which is substantially lower than their melting temperature. To further lower the fabrication temperatures, use of TiO2 as liquid phase sintering additive is investigated in the present work. Furthermore, the article reports the effect of SST process parameters on response parameters of the produced mullite based environmental barrier coatings. Coating thickness and coating density were measured as the response parameters. The results revealed that coating thickness and coating density increases substantially with increase in sintering time. Moreover, change in sintering temperature drastically affect coating thickness and coating density with incorporation of sintering additive. Keywords: Slurry spray technique, environmental barrier coatings, coating thickness, coating density. 1.
INTRODUCTION
Ceramics are preferable choice and used customaril y as environmental barrier coatings (EBCs) which represent a comparatively thin film of impervious i mpervious material bonded to the substrate. EBC materials are chosen so that they can to protect the metal substrate at elevated temperatures and chemically harsh conditions. Application of EBCs can thus reduce thermal shocks acting on the load carrying structures ranging from the gas turbine, boilers, internal combustion engines and many more. Use of partially stabilised zirconia (PSZ) is prevalent in m anufacturing EBCs; however, research efforts have continually been made towards making use of monolithic mullite (3Al2O3·2SiO2), which is comparatively better alternative and economically viable material in such applications. Functional gradation in the coating structure can help reduce the thermal mismatch between the ceramic and metal when subjected to temperature excursions. This can be done by making use of nickel which closely resembles the thermo-mechanical properties of mullite and ASTM 1018 low carbon steel substrate chosen for the study. These properties are determined by the coefficient of thermal expansion (CTE) and thermal conductivity of these materials. In this present work, SST a novel and unutilized technique was used for manufacturing mullitenickel based functionally graded coatings on ASTM 1018 low carbon/mild steel substrate. The overall aim was to expand the understanding of the mechanism of various fabrication stages and investigate the effect of process deposition parameters of SST on coating compliance. Fly ash emerges as the primary waste generated by combustion of pulverized coal largely in thermal power plants. It comprises of oxides like α-quartz α -quartz (SiO2), mullite (3Al 2O3·2SiO2), hematite (Fe2O3), magnetite (Fe3O4), lime (CaO) and other minor constituents as the loss on ignition (LOI) (White et al., 1990). An effort has been made in the present investigation, to
explore the coating potential of class – F fly ash using SST, to develop EBCs on low carbon steel. Sintering of the green powder compact to form sturdy structure is the critical stage of SST. However, sintering of powdered mullite in its purest form exhibits difficulties due to poor inter diffusions of Si4+ and Al3+ in mullite lattice (Tenzuka et al., 2009). Moreover, sintering at high temperature up to 1600°C poses the threat of defects in the coating as well as the substrate. In response to this, attempts have been made to sinter mullite at lower temperatures with the aid of titanium dioxide (TiO 2) as liquid phase sintering (LPS) additive in the present research. It envisages the processing and characterization of series of slurry sprayed mullite – nickel coating premixed with fly ash and LPS in different proportions. In this research article, SST was studied through one-factor-at-a-time approach. Four control parameters namely fly ash composition, sintering additive, sintering time, and sintering temperature were selected for the study. The experimentation was performed using ASTM 1018 low carbon steel substrate material. Based on extensive review of the literature survey, coating thickness and coating density were chosen as the result of experimentation and analysed to determine the individual effects of the process parameters within the selected range. 2.
SLURRY SPRAY TECHNIQUE
The slurry spray technique utilises conventional wet powder spraying method to deposit sinterable slurry materials onto target substrate. The process involves slurry preparation which involves suspending the coating powders within a fluid that can be applied to a surface using conventional gravity feed air pressurised spray gun. Tera sodium pyrophosphate (TSPP) as dispersing agent (Greenwood et al., 1999), hydrosoluble polyvinyl alcohol (PVA) as binder (Roy et al., 2005) and distilled water as solvent was utilized for slurry preparation. Successive layers are then sprayed onto the substrate and dried using varying slurry compositions to produce a functional coating. The multi-layered coatings is left to dry for approximately and hour, depending on ambient conditions (Nguyen, 2007). The loosely held green coating is compressively loaded to form a densified layer. The applied pressure varies depending on the coating thickness and typically ranges between 20 and 40 MPa. Stamping pressure of 30 MPa was held constant throughout the experimentation in this study. The dried green coating layers are subsequently fired in a furnace to the sintering temperature corresponding to the highest melting ingredient which is ceramic. The fabrication in the SST consists of the principle sta ges as explained in Figure 1.
(1) Slurry preparation
(2) Multilayer spraying
Load
(5) Debinding & sintering (4) Pressure stamping
(3) Solvent drying
Figure 1: Stages in slurry spray technique (Verma et al., 2015)
One-factor-at-a-time approach
For the pilot experimentation on the development of mullite-nickel based EBCs, the selected experimental design was according to one-factor-at-a-time (OFAT) approach. This method consists of selecting a starting point or baseline set of levels, for each factor and then successively varying each factor over its ranges with all other factors held constant at the baseline level. After performing all the tests, a series of graphs are usually constructed showing how the response variable is affected by varying each factor with the other factors held constant (Montgomery, 2013). Using these OFAT graphs, one could select the optimal combination of the certain set of considered conditions. However, OFAT is unable to consider any interaction between the variable, which is the major limitation of this method. This is the main reason the OFAT approach is widely used for preliminary experimentations in order to screen out the main process parameters and to decide the range of factors for subsequent detailed and extensive experimentation. The composition of the slurry mixture used for the experimentation is given in Table 1. The selected process parameters and their range for the pilot experimentation are given in Table 2. Table 1: Composition of slurry mixture, (125g) Ceramic to Metal 75-25 50-50 25-75
Mullite
40.8375 27.225 13.6125
Fly-ash (1%) 0.413 0.275 0.138
Nickel
13.75 27.5 41.25
Additive: TiO 2 1% 1.25 1.25 1.25
Binder 2.5% 3.125 3.125 3.125
Dispersant 0.5% 0.625 0.625 0.625
Mix agent 52% 65 65 65
Table 2: Process parameters and their chosen levels considered for the pilot experiments Labels Process Parameter Range Units A Fly-ash composition 1-15 wt.% B Sintering additive 1-7 wt.% C Sintering time 15-45 Minutes D Sintering temperature 800-1100 °C Stamping pressure, 30MPa; Spraying pressure: 4 bar
Level 1 1 1 15 800
Level 2 5 3 25 900
Level 3 10 5 35 1000
Level 4 15 7 45 1100
Coating thickness (µm) and coating density (kg/m 3) were selected as quality characteristics under consideration for pilot experimentation. Coating thickness of each ASTM 1018 low carbon/mild steel coating coupons after the carrier drying stage, and eventually after the sintering stage was carefully noted. For coating density, the weight of the each coating coupons before and after the coating was carefully measured. Each dimension of the uncoated coupons was noted for volume measurement. These measurements were used for calculating coating density as; =
( − )
(Eqn. 1)
The coating coupons were coated via the procedure as per Figure 1. Each experiment was repeated three times, and the responses of respective experiments were recorded (Table 3).
3.
RESULTS AND DISCUSSION
Effect of fly-ash composition on CT and CD
The randomized experiments with process parameters were planned as per Table 3, to study the effect of the OFAT, i.e. fly-ash composition wt.% on the response parameters of coating thickness, CT (µm) and coating density, CD (kg/m 3). Table 3: Results of experimentation - Fly-ash composition
Exp No.
1 2 3 4
Parameter A
1 5 10 15
Response for Coating Thickness, CT (µm)
Response for Coating Density, CD (kg/m3)
R1
R2
R3
Mean CT
R1
R2
R3
Mean CD
227 243 256 192
236 238 267 178
221 261 253 181
228 247 259 184
2598 2115 1768 2828
2648 2128 1875 2847
2612 2132 1892 2869
2619 2125 1845 2848
Stamping pressure, 30MPa; Spraying pressure: 4 bar. R 1, R 2, R 3 represent response value for three repetitions of e ach trial.
The main effect of the process parameter, fly-ash composition (A) on coating thickness, CT and coating density, CD was determined based on the average of the raw response data. The main effect of the Fly-ash composition with all other factors held constant at the baseline level is plotted in Figure 2. ) 300 m µ ( , 250 T C200 s s e n 150 k c i h 100 T g n 50 i t a o C 0
3000 2500 2000 1500 1000 500 1
5
CT
CD
10
15
0
) m / g k ( , D C y t i s n e D g n i t a o C
3
Fly-ash composition (wt.%) Figure 2: Effect of Fly-ash composition on Coating thickness and Coating density in pilot experimentation
The main effect of fly-ash composition shows increasing trend for coating thickness up to 10 wt.%. However, at 15 wt.% there is a drop being noted in coating thickness. This initial increasing trend in coating thickness may be attributed to the liquid phase sintering effect of the fly-ash particles (Dong et al. 2010). The opposite trend of coating thickness and coating density was observed throughout the pilot experimentation for Fly-ash composition studied from 1 – 15 wt.%. Effect of sintering additive on CT and CD
To study the effect of the OFAT, i.e. sintering additive (wt.%) on the response parameters of coating thickness, CT and coating density, CD, the randomized experiments with process parameters were planned as per Table 4.
Table 4: Results of experimentation – Sintering additive (TiO 2) Exp No.
1 2 3 4
Parameter B
1 3 5 7
Response for Coating Thickness, CT (µm) R1 317 267 147 256
R2 321 291 168 244
R3 338 276 159 253
Mean CT 325 278 158 251
Response for Coating Density, CD (kg/m3) R1 2837 3735 2115 1528
R2 2857 3745 2128 1516
R3 2846 2315 2132 1531
Mean CD 2847 3265 2125 1525
Stamping pressure, 30MPa; Spraying pressure: 4 bar. R 1, R 2, R 3 represent response value for three repetitions of each trial.
The main effect of the sintering additive on coating thickness and coating density with all other factors held constant at the baseline level is plotted in Figure 3. The main effect shows a decrease in coating thickness with an increase in sintering additive upto 5wt.%, thereafter there is increase in coating thickness being observed. The decline in coating thickness may be attributed to the increased effect of the low temperature, rapid densification due to liquid phase sintering additive i.e. TiO 2 (Rajeswari et al., 2010). Coating density, however, showed an initi al increase from 1 to 3 wt.%, and beyond 3 wt.% there is a decreasing trend. ) 350 m µ 300 ( , T250 C s s 200 e n k 150 c i h T100 g n i t 50 a o C 0
3500 3000 2500 2000 1500 1000 500 1
3
CT
CD
5
7
0
) m / g k ( , D C y t i s n e D g n i t a o C
3
Sintering additive, TiO 2 (wt.%) Figure 3: Effect of Sintering additive on Coating thickness and Coating density
Effect of sintering time on CT and CD
The main effect of the sintering time on coating thickness and coating density (Table 5), with all other factors held constant at the baseline level is plotted in Figure 4. Table 5: Results of experimentation – Sintering time
Exp No.
1 2 3 4
Parameter C
15 25 35 45
Response for Coating Thickness, CT (µm) R1
R2
R3
Mean CT
Response for Coating Density, CD (kg/m3) R1
185 209 201 198 1965 308 278 292 293 2512 352 368 376 365 2825 275 292 288 285 3190 Stamping pressure, 30MPa; Spraying pressure: 4 bar. R 1, R 2, R 3 represent response value for three repetitions of e ach trial.
R2
R3
Mean CD
1922 2654 3088
1864 2594 2874
1917 2587 2929
3344
3288
3274
) 400 m µ 350 ( , T300 C s s 250 e n k 200 c i h 150 T g 100 n i t 50 a o C 0
3500 3000 2500 2000 1500 1000 500 15
25
CT
CD
35
45
0
) m / g k ( , D C y t i s n e D g n i t a o C
3
Sintering Time, (Minutes) Figure 4: Effect of Sintering time on Coating thickness and Coating density
It was observed during the study, that increase in sintering time increases coating thickness and coating density almost linearly showing the similar trend as shown in figure 4. However, there is a slight decrease in coating thickness being observed from sintering time level 3 to 4 i.e. 35 to 45 minutes. Thus sintering time corresponding to 35 minutes ma y be considered optimal for the given set of experimental conditions, which suggests d ecreased sintering time required for coating densification with the introduction of applied stamping pressure before sintering. Effect of sintering temperature on CT and CD
The main effect of the sintering temperature on coating thickness and coating density (Table 6) deduced from the experiments, with all other factors held constant at the baseline level is plotted in Figure 5. Table 6: Results of experimentation – Sintering temperature
Exp No.
1 2 3 4
Parameter D
800 900 1000 1100
Response for Coating Thickness, CT (µm) R1 302 358 224 198
R2 288 372 230 182
R3 296 380 218 204
Mean CT 295 370 224 195
Response for Coating Density, CD (kg/m3) R1 1822 3190 2895 2038
R2 1918 3328 2820 2078
R3 1865 3282 2892 1981
Mean CD 1868 3267 2869 2032
Stamping pressure, 30MPa; Spraying pressure: 4 bar. R 1, R 2, R 3 represent response value for three repetitions of e ach trial. ) 3500 3
) 400 m µ 350 ( , T 300 C s s 250 e n k 200 c i h 150 T g 100 n i t 50 a o C 0
m /
g 3000 k ( , 2500 D C y t i 1500 s n e 1000 D g n i 500 t a o 0 C
2000
800
900
CT
CD
1000
1100
Sintering Temperature, ( °C) Figure 5: Effect of Sintering temperature on Coating thickness and Coating density
It has been observed that coating thickness initially increases with increase in sintering temperature upto 900°C, and afterwards downfall trend has been observed at the higher temperatures (beyond 900°C). A similar trend is also observed for coating density for the selected range of sintering temperature (i.e. 800 – 1100°C). Increased coating thickness and coating density at 900°C suggest the improved sintering activity, lowering the secondary mullitization temperature, due to TiO2 as well as fly ash. The observed decrease in coating thickness and coating density beyond 900°C may be attributed to the increased shrinkage of mullite due to crystallization of amorphous mullite, which is also observed in mullite coatings by Withey et al., 2007. Surface macrographs of some of the sintered sample during the experiments are shown in Figure 6. Defect free surface of these specimens suggests that the above design settings yielded good results concerning coating integrity and coating thickness obtained. These sintered specimens were ground at one end, showing a clearly distinct substrate and multilayer coating with thickness as shown in Figure 7. Moreover, fly-ash as an alternative to ceramic was tested which showed promising results with intensified coating deposition. So these design parameters can be implemented and could be used for further detailed experiments.
3mm
3mm
3mm
Figure 6: Surface macrographs of mullite-nickel based coat ed specimens
3mm
3mm
3mm
Figure 7: Surface macrographs after grinding of coated specimen shows multilayer coatings
4. SEM ANALYSIS
The surface micrograph and the cross-sectional morphology of the as-deposited coating specimen (Table 4: Trial 2 - sintering additive: 3 wt.%) has been shown in Figure 8a & 8b respectively. The fabricated coatings have been deposited b y moving the slurry spray gun onto stationary substrates, and the desired coating thickness is due to varying the number of passes of the distinct functionally graded composition layers (as per Table 1). The as-sprayed splat morphology with the continuous interface is evident from the coating microstructure and suggest a good coating-substrate adherence. Dense inter-splat coating with coarser grain structure has been observed along with the characteristic microstructure features of a thermal spray coating like discontinuities, pores, microcracks, and voids, etc. Inclusions in the form of globular and interlamellar pores, oriented parallel to the substrate surface have been observed in the coatings structure. It can be noted from the cross-section micrographs that the specimen exhibit a little porosity and hence densified structure. The reduced porosity was caused by the sintering mechanism driven by the deduction of surface energies and increa sed grain boundary
areas of constituent particles by fusing to create a continuous medium (Bernard-Granger et al., 2007, Masoumi et al., 2014). Moreover, liquid phase sintering effect imparted by TiO2 caused the constituents to drawn into closer proximity thereby reducing the coating volume.
Figure 8: SEM micrographs of the slurry sprayed mullite-nickel coating on mild steel specimen using SST (a) Surface micrograph, (b) Along the cross-section (Trial 2, Table 4 - Sintering additive: 3 wt.%; Mean coating thickness: 278µm, Mean coating density: 3265 kg/m3)
CONCLUSION
SST which is being identified as new avenue to utilize fly ash as coating ingredient and its low fabrication cost, makes it a suitable choice for further investigation and work towards its large scale use as part of government’s make in India campaign. Pilot experimentation based on OFAT was performed with a focus on experimental studies of the effect of identified control parameters of SST. The selected parameter setting provided a substantial range of coating thickness and coating density, which can be considered as a measure of deposition efficiency of SST. For manufacturing mullite-nickel based EBCs on mild steel substrate, optimum parameters setting (fly ash composition 1-wt.%, sintering additive 3-wt.%, sintering time 35 minutes, and sintering temperature 900°C) was obtained by this experimentation. These parameter setting provided higher coating thickness and coating density values, and delineated good surface integrity free from any visible defects. REFERENCES 1. White SC, & Case ED. Characterization of fly ash from coal-fired power plants. Journal of materials science 1990; 25(12): 5215-5219. DOI: 10.1007/BF00580153. 2. Tenzuka N, Low IM, Davies IJ, et al. Effect of fluoride and oxide additives on the phase transformations in alumina/clay ceramics. Journal of the Australian Ceramic Society Volume 2009; 45 (1): 19-27. 3. Greenwood R and Kendall K. Selection of suitable dispersants for aqueous suspensions of zirconia and titania powders using acoustophoresis. Journal of the European Ceramic Society 1999; 19: 479-88. 4. Roy P, Bertrand G and Coddet C. Spray drying and sintering of zirconia based hollow powders. Powder Technology 2005; 157: 20-26. 5. Nguyen P, Harding S and Ho SY. Experimental studies on slurry based thermal barrier coatings. In: 5th Australasian Congress on Applied Mechanics, Brisbane, Australia, 10-12Dec, 2007, pp.545-50. 6. Verma R, Kant S & Suri NM Adhesion strength optimization of slurry sprayed mullite-based coating using Taguchi method. Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering 2015, DOI: 0954408915595948. 7. Montgomery DC. Design and Analysis of Experiments, 8th ed. New York: Wi1ey, 2013, p.351.
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