Atmospheric plasma sprayed thermal barrier coatings with high segmentation crack densities: Spraying process, microstructure and thermal cycling behavior
M. Karger , R. Vaßen, D. Stöver
Surface & Coatings Technology
abstract
Thermal barrier coatings (TBCs) with high strain toleranc e are favorable for application in hot gas sections of
aircraft turbines. To improve the strain tolerance of atmospheric plasm a sprayed (APS) TBCs, 400 μm – 500 μm
thick coatings with very high segmentation crack densities produced with fused and crushed yttria stabilized zirconia (YSZ) were developed. Using a Triplex II plasma gun and an optimized spraying process, coatings with segmentation crack densities up to 8.9 cracks mm-1, and porosity values lower than 6% were obtained.The density of branching cracks was quite low which is inevitable for a good inter-lamellar bonding.Thermal cycling tests yielded promising strain tolerance behav ior for the manufactured coatings. Samples with high segmenta tion crack densities revealed promising lifetime in burner rig tests at rather high surface (1350 °C) and bondcoat temperatures (up to 1085 °C), while coatings with lower crack densities had a reduced performance. Microstructural investigations on cross-sections and fracture surfaces showed that the segmentation crack network was stable during thermal shock testing for different crack densities. The main failure mechanism was delamination and hori zontal cracking within the TBC near the thermal grown oxide layer (TGOs) and the TBC.
Keywords: Plasma spraying ,Segmentation cracks ,Dense vertically cracked,Thermal barrier coatings,Yttria-stabilized zirconia (YSZ)
1. Introduction
Thermal barrier coatings consisting of yttria partially stabilized zirconia are favorably used as protective coatings on hot section components of modern gas turbines [1 – 3] . W ith increasing operation temperatures , long -t erm st abi lity and und er st andi ng the failure mechanisms become more and more important since spallation of the ceramic layer subjects the metallic superalloy to hot gases [4,5]. The a pplication o f atmospheric plas ma spra yed c oating s to components with the highest loading inside a turbine like blades and vanes requires a high thermal shock resistance of the ceramic layer. APS coatings often show spallation due to the strain energy which is stored during thermal cycling processes[6,7]. One approach to reduce the stress accumulation is porous coatings, where micro-cracks and pores absorb part of the stress [8,9]. Another possibility to reduce the stress in fluence is the introduction of vertical cracks with a length of at least half of the coating thickness [10] . The f ail ure mec hanisms of conventi onal APS TBCs c an be understood by regarding the stress levels during thermal shock cycles. At high temperatures, APS coatings are in a tensile stress level due to the difference in the thermal expansion coefficient between substrate and ceramic coating. The tensile stress will be relaxed during isothermal hot periods, and this effect leads to compressive strain after fast cooling down from service temperature to room
temperature. These compressive stress levels are one main reason for crack growing and hence shorter life time [11] . In segmented coatings, the tensile stress levels are on much lower levels during the hot periods, because vertical cracks compensate tensile stress with an opening similar to the columnar microstructure of coatings produced via electron beam physical vapor deposition (EB-PVD)[12] . A lower tensile stress level before cooling gives a lower compressive stress level after cooling down, and the driving force for crack growth is reduced.
Atmo sph eric plas ma spra yed c oatin gs ar e built up fro m the continuous impact of molten particles with a diameter between 5 μm and 50μm form into splats. The microstructure is decisively influenced by a given spray parameter set. One of the most important parameters for different types of microstructures is the substrate surface temper-ature. During the spraying process, it can be adjusted by several process parameters or by using additional cooling nozzles operating with compressed air or other gases.
For relatively low substrate surface temperatures ( b 500 °C) during spraying, splats are more or less loosely bonded. Due to tensile stress levels caused by the large temperature drop between the liquid phase and the substrate surface temperature, the splats form a micro-cracked pattern [13] . These cracks grow individually in each splat and are not connected at the boundaries between other splats. The micro-crack pattern and splat boundaries are possible paths for cracks growth with further temperature loads. This situation is different when higher substrate temperatures are used during the deposition. The high temperature of the coating surface during spraying promotes diffusion and improves contact between layering splats. The strong bonding between the splats leads to larger grains, thus the amount of grain boundaries is decreased. The first systematic induction of vertically cracked coatings was performed by Taylor et al. [14] . The main idea for the development of vertical cracks in a TBC is a temperature gradient between well-bonded splats with the higher temperature at the top surface. This gradient enforces the upper splats to shrink more during cooling. A higher shrinkage leads to more vertical cracks, and micro-cracks from adjacent splats form macroscopic vertical cracks. Segmentation cracks are de fined as vertical cracks running perpendicular to the coating surface and penetrating at least half of the coating thickness. Usually, they initiate from the top edge of the coating, but also cracks obviously starting from inside the coating are possible due to the 2-dimensional cut of the 3-dimensional coating. The segmentation crack density is defined as the amount of vertical cracks across a defined cross section.The coatings produced with this basic concept show segmentation crack densities up to 3.5 c ra c k m m-1 and demonstrated better performance in thermal shock experiments. It is expected that higher segmentation crack densities also enhance the strain tolerance of APS
coatings. Thus, a lot of work was done in this field to increase the amount vertical cracks in the last two decades. Higher crack densities are reported mostly for coatings with a thickness of more than 1 mm [15] . TBCs of components in the hottest regions of modern gas turbines (e.g. first rows of blades and vanes) should not exceed a thickness of
about 500 μm [16,17]. An essential property of plasma spraying ceramic coatings is the rapidity of the solidi fication process, which is in general quite fast with about 106K/s [18,19]. The cooling down of splats is a kind of
directional solidi fication and leads to grain growth with a preferred growth direction perpendicular to the substrate surface and parallel to the direction of heat flux during the spraying process [20] . At high
feeding rates, the mechanism of vertical grain growth accompanied by vertical crack propagation can be continued among several splats. As pointed out and discussed in many other publications, splat deposition at high surface temperatures enables and supports the formation of vertical cracks. On the other hand, a high temperature
gradient between the surface and the impinged material is required. Both contrary demands interact, and it is quite difficult to define clear temperature conditions. A simple method to supply cooling air to the specimen during the spraying process is fr ont -s ide co oli ng, whe re additional cool in g nozzles are mounted beside the plasma torch. In order to increase the segmentation crack density, back-side cooling is proposed. With this technique, the deposition temperature is controlled by cooling air served from the back side of the metallic substrate. The temperature gradient within the ceramic layer should be increased at comparable high deposition temperatures. Thus, a back-side cooling mode might support the propagation of vertical cracks. The substrate thickness (or wall thickness for real components) should be as small as possible in order to maximize the resulting temperature gradient within the ceramic topcoat. Beside the effect on coating microstructure, back-side cooling has two more advantages. One, the heat fl ux from the surface to the backside is enhanced, which might enlarge the size of grains
[21] . Second, the coating process can be transferred to real compo-nents by using already existing cooling systems [22] . It is worth mentioning that a number of factors also affect the stress levels like relaxation processes.
Other than substrate surface temperature during spraying, another important condition for high segmentation cracks is a relatively high pass thickness. The pass thickness for all samples presented here was calculated by dividing the measured coating thickness with the number of coating pass. A high pass thickness can be achieved with a
high powder feeding rate and also with a low movement speed. These conditions and combinations of them lead to a very high heat entry to the substrate and require cooling in order to keep the temperature on an acceptable level. This is also done to prevent the specimen from damages caused by the plasma plume. In our investigations, high pass
thicknesses and high surface temperatures were used as an approach to produce coatings with high segmentation crack densities. A high surface temperature can be adjusted with a short spraying distance, a high plasma power or a low movement speed of the plasma torch. High power for the plasma is necessary in order to melt the injected powder at high feeding rates for high pass thicknesses, the low distance ensures a high heat fl ux to the substrate and prevents particles from cooling down before the impingement.Moreover, it is well known that the use of fused and crushed powders as feedstock for TBC leads to higher segmentation crack densities compared to those coatings fabricated with spray dried powders [23] . Spraying conditions leading to very hot particles give coatings
denser than standard YSZ TBCs with porosity values lower than 5%[24] . A lower porosity gives in general a higher value for the thermal conductivity which might be counterproductive for the improvement of th er ma l ba rri er c oa tings . As a result of the higher thermal conductivity thicker coatings could be used which might fail earlier
than thinner conventional coatings although they show an increased strain tolerance. Coatings with a combination of high segmentation crackd ensities and moderate porosities could be a long- term objective, whereas, for example, suspension plasma sprayed coatings with such properties have already been reported[25] .The main objective of the present work is to achieve very high segmentation crack densities for vertical cracked TBCs with thickness at
most 500μm. With fused and crushed YSZ powder, hot spraying con dition s and modifi ed c oo li n g mo d e, th e a im wa s to ob t ai n 10 cracks mm-1. The thermal shock behavior including the failure mechanism of su ch systems was inves tigated and compared to conventional coatings.
2. Experimental
2.1. Spraying details
The powder feedstock was a fused and crushed Yttria-stabilized zirconia with a Y2O3 content of 8 wt.% (8YSZ, d
10=9μm, d 50=23μm,d90=51μm), supplied by Treibacher AG, Austria. Shape and geometry of the feedstock powder are depicted inFig. 1. Typical for this kind of powder is the grain morphology with sharp and jagged edges. For reference coatings, grit-blasted stainless steel with a thickness of 2 mm was used as substrate material, while for the thermal cycling specimens IN738 buttons with a thickness of 3 mm and a diameter of 25 mm were used. To reduce edge effects, the rim of the buttons was rounded. About 150μm NiCoCrAlY bondcoat with composition Ni –21
Co–17 Cr–12Al –1Y ( i n wt .%) (Praxair Surface Technolog ies, Indianapolis, USA) was then vacuum plasma sprayed in a Sulzer Metco spraying chamber. The roughness of the bondcoats was about 8 μ m(Ra), measured with a profilometer M2 (Mahr GmbH, Göttingen, Germany).The ceramic coatings were produced with a three-cathode TriplexII plasma torch (Sulzer Metco AG, Wohlen, Switzerland) mounted on a six-axis robot. The inner diameter of the anode-nozzles was 9 mm, and the plasma jet was a mixture of argon and helium gas. The particles are injected between two of the three lobes of the plasma. These high temperature cores result from the triangular geometry of cathodes inside the nozzle. This arrangement of cathodes and injector affords an optimized particle injection [26] . Further spraying details are listed in Table 1. Prior to the deposition of the ceramic layer, the substrates were pre-heated up to 500 °C with the plasma plume and the TBC was subsequently applied to the hot substrate. Pass thickness was
varied by using different powder feeding rates between 5 g min-1 and 25 g min-1.To get a deeper understanding about the formation of microstruc-tures in segmented coatings and their difference to the microstructure of conv ention al porous APS c oa tings, single spla ts were al so investigated. For these measurements, metallic substrates were used.To obtain single splats, the powder feeding rate was reduced and the velocity of the torch was increased. W ith these modi fi ed conditions, asurface with single splats and in some cases double splats consisting of an underlying splat and a second one covering was obtained.For the thermal cycling tests, two pairs of samples with different
cooling geometries were produced: The first geometry applies the c oo l in g a ir fr om t h e b ac ksi d e o f t he subst r at e . Thi s g i ve s a n additionally high heat flux from the surface to the back side of the specimen. For the second geometry, the cooling nozzles are mounted beside the plasma torch which gives a rather fast cooling of an area
located near to the plume spot on the substrate. Measuring the temperature of the substrate surface via a pyrometer provides two different temperature characteristics as depicted in Fig. 2. Due to the movement of the plasma plume, the temperature in the centre of the surface oscillates. Cooling from the front side gives amplitude in
the range of 100 °C, while cooling from the backside gives slightly hi gher a mpli tude in th e ran ge of 160 °C alth ou gh the absolute temperature is about 200 °C lower. For the same surface temperature of about 950 °C, this gap would be even higher (see Fig. 2 ). Details
about the produced specimen are listed in Table 2. Furthermore,Table 2 contains information about in-house standard and modified TBC systems which will be compared to the coatings presented in this report. Commonly, these coatings were produced with lower surface temperatures ( b 300 °C) and a smaller pass thickness. The porosity is for all standard coatings above 10%. Spraying details for the samples labeled ‘S ’ (standard),‘ SM’ (standard modi fied) and ‘P’ (porous) can be found in [27] .
2.2. Microstructural investigations
The microstructural examinations were performed on the cross-sect ions of sampl es a fte r v acuum impreg nation and po lishing
according to a standard sample preparation process. All sampleswere sawed after the impregnation, which ensures that the creation of cracks during the preparation process is nearly excluded. Images of the coating microstructures were taken with a light microscope, a scanning electron microscope (Ultra 55, Carl Zeiss NTS AG, Germany) or a table top SEM (Phenom, Fei Company, Oregon, USA). For image analysis the software Analysis (Soft Imaging Systems SIS GmbH,Münster, Germany) was used. Measurement of segmentation crack density was repeated atfive well de fined positions on each sample (left, middle left, middle, middle right, and right).
2.3. Thermal cycling details
A gas burner test facility operating with natural gas and oxygen was used for the thermal cycling tests [28] . The flame with a diameter of about 4 cm is adjusted towards the substrate center and gives a homogeneo us tempera ture dis tributi on. The tempera ture on the coating surface is monitored with a pyrometer (Land Uno1, operating
wavelength: 8.0μm –11.5 μm), while the substrate temperature is measured with a thermocouplefixed to a drilled hole in the middle of the substrate. After the sample is placed into the flame automatically,the sample is heated up within 20 s to the surface temperature of 1350 °C, and this temperature is maintained for 280 s. To ensure a
constant temperature of the surface and a constant temperature gradient within the coating, compressed air is applied to the backside.Fo r the coat ings pr esent ed he re, the t empe rat ure m e a sure d i n the mi ddle of the s ubstrate was in t he range between 1010 °C and 1020 °C which gave a calculate d bondcoat t emperature be tween 1040 °C and 1085 °C.The bo ndcoat t emperat ur e of all t he samples was esti mat ed u sing an al gorithm b as ed on the 1 -dimen sional heat diffusion equati on [29] .
The thermal condu c tivity of the pres e nt ed coatings i s set to 1.8 Wm-1K-1. T his re lati vel y hig h value is just ifie d for c oati ngs with porosities be low 10%[30,31]and leads to the above me ntioned bondcoat te mperatures. After heating, the burner is moved automatically away from the sample while an additional cooling nozzle is simultaneously posi-
tioned in front of the specimen. The sample is cooled down from both sides for 2 min with a cooling rate of about 100 K s-1. The number of cycles before failure is determined by two criteria: either surface delamination visible to the naked eye of about 20% or a temperature jump in the central area of about 40 °C as monitored by the pyrometer.
3. Results and discussion
3.1. Microstructure
Micrographs of polished cross-sections of YSZ coatings in the as-
sprayed state corresponding toTable 2are shown in Figs. 3 and 4 . All
coatings exhibit vertical cracks with segmentation crack densities
be twee n 1.8 an d 8. 9 cra cks mm-1. Alt erna te poro us a nd de nse regions are commonly observ ed among the presente d coatings. Alternating regions are caused by the plasma passes. The visibility of these regions is in fluenced by the spray parameters and surface temperature. For coatings produced with a lower surface tempera-
ture, the less dense areas are the sources from which branching cracks or delamination cracks start to form. In some cases, this leads to
partially delaminated areas in the as-sprayed coatings (see arrows inFig. 4).Very low crack density was obtained from coatings with very low
pass thickness, while the high crack densities were achieved with high surface temperature and moderate pass thicknesses.In Fig. 4 , two cross-sections of coatings produced with low substrate temperature are shown. Although the coating temperature is low, segmentation cracks are still formed. In contrast to the coatings sprayed with higher temperatures, more horizontal cracks can be counted. The reason for horizontal cracks in the as-sprayed state is a low bonding strength combined with a high tensile stress level.In most specimens, the vertical cracks run directly through the interface area between the porous and dense coatings zones and show
nearly no interaction like parallel offset. In some cases, a zone with nearly no cracking is located just above the substrate/bondcoat surface. This dense region might be a disadvantage because dense coatings have a higher Young's Modulus and cracks can grow faster[4].Fig. 5 depicts the crack densities depending on the substrate
temperature and pass thickness. Both curves are limited by the appearance of horizontal branching cracks. In contrast to other reports, we observe a high horizontal crack rate for the highest pass thicknesses.Coatings produced with back-side cooling (W1148, W1149) or front-side cooling (W1150, W1151) differ in the number of vertical cracks by a factor of two. This remarkable difference shows that the back-side cooling mode increases the number of vertical cracks. Digital image analysis enables to compare the porosities of the samples. Although it is known that this method for estimation of porosity shows relatively large scatter of results, it is possible to compare the samples among each other in order to bring out tendencies [32] . Using SEM images, pore sizes between 1 μm and 100 μm were estimated. The porosity given by digital analysis was lower than 5%, but segmentation cracks are not considered in these measurements. The porosity given by mercury porosimetry is about 6.0%, including the segmentation cracks. Thus, it could be stated that both methods give similar results in this case.
3.2. Fracture surfaces
Fig. 6 compares fracture surfaces of the as-sprayed state of an in- house standard coating and a highly segmented one at different
magnifications. In general, an APS coating consists of splats with ahorizontal dimension up to a hundred micrometers and a thicknessin the range of a few micrometers. The typical micrograph of the standard coating ( Fig. 6 a, b) shows that the bonding between the splats is poor in many areas and most of the splats are still visible as
single splats. Only a few larger grains with bonding over more than two splats can be detected. The fracture surface of the highly segmented coating shows several different structures. One important difference to a standard APS coating is the induced segmentation cracks marked with‘A’ in Fig. 6c. The segmentation crack penetrates
the who le coating where its propagation direction i s nearl y independent of the present microstructure. In the area between two
passes of the plasma torch (B), overspray and re-solidified particles could be found. Particles of this type can be detected in the standard
coating between all splats. The coatings produced with high surface t emp erat u res show qui te l arge g rains and th ere fore st rong ly connected splats. The coating presented in Fig. 6c and d was sprayed with a pass thickness of 60μm, hence one pass consists of more than
ten splats. Two other types of cracks are visible: branching cracks (D) running from the segmentation cracks running parallel to the surface and some intra-splat cracks (C) formed within the splats. Cracks in- between s pl ats are al so presen t in th e stan da rd coati ng ( C). Altogether, the microstructure is dominated by vertical grains which are strongly bonded to each other (F). The widths of these grains are about 1.5 μm and lengths of about 50 μm which is nearly the pass thickness. Areas without cracks show quite a low amount of pores (E).
3.3. Single splat investigations
In general, single splats are investigated using polished substrate surfaces. This approach ensures characterization of splats without any
interference from the surface roughness [33] . To have a more realistic view of the splat surface microstructure after solidifi cation on a substrate, the grit-blasted surface of a stainless steel substrate was used for single splat investigations. Although the blasted surface of the substrate in fl uences the shape of the splats, this approach gives an overview of crack patterns.
As depicted in Fig. 7 both standard (Fig. 7 a, b) and segmented( Fig. 7c, d), types of coatings show a network of cracks crossing the complete splat. For the splats forming a segmented coating, the den-sity of intra-splat cracks is about twice of the density in the standard coating. This high amount of cracks is caused by a higher tensile st re ss lev el gi ve n by a hi gh er su bs tr ate te m pe ra tu re . Al so th e flattening of the splats is higher due to the higher spraying surface temperature. Cracks in the underlying splats are mostly continued to the overlying splat but for the standard coatings, these cracks end in some cases in-between the second splat (see arrows in Fig. 7a). This behavior for the highly segmented coatings is caused by the good bonding between the layers which enables crack continuation from one splat to another. For standard coatings, a lower bonding strength between splats forms pores where crack propagation is almost stopped.
At higher magnifications, the diameter of the single grains can be estimated. For the standard coatings, the grains have an average diameter of 200 nm and for the segmented coatings this value is around 350 nm. Thus, the grains in the presented highly segmented coatings exhibit nearly twice the diameter of grains in standard coatings.3.4. Thermal cycling results Fig. 8 a shows a TBC specimen with segmentation cracks after thermal cycling. A large area of the TBC has spallen-off exposing the bare bondcoat where specs of the TBC still left bonded. This failure pattern is similar to all cycled samples and typical for TBCs tested at temperature regions where the coating material is still stable (~ 1250 °C). The failure of the coating is caused by the growth of the TGO primarily which coef ficient of thermal expansion largely differs from that of the TBC. Fig. 8 b depicts an overview cross section of a cycled sample. The network of vertical segmentation cracks is still present, but the heat treatment seems to densify the coating. Large delamination cracks are running parallel to the TGO in the TBC as shown inFig. 8c. The cracks also penetrate the TGO, but the main crack path runs in a TBC region close to the interface between coating and TGO ( Fig. 8d). Such a crack pattern is mostly observed for TBC systems cycled at relatively high temperatures. In general, the growth of the TGO layer is enabled by oxygen transport through the TBC during the hot periods. The transport is mainly affected by two different mechanisms: namely the diffusion of oxygen ions and a more or less pronounced open and interconnected porosity that allows the ingress of oxygen from the environment. Additionally, it was discussed that segmentation cracks would be a channel for high oxygen transport from hot gases [34] . This might enable a higher local oxidation rate as even the opening of cracks is larger at higher temperatures. In contrast to this, regions with a higher TGO thickness or other anomalies in fluenced by a vertical crack cannot be found in the highly segmented coatings. Fig. 8c and d shows the region near to a segmentation crack ending close to the TGO. The appearance of the TGO in this area does not differ from regions where the TGO has no direct contact to a segmentation crack. Measurements of the TGO thickness for the cycled samples yielded values between 4.0 μm and 5.8 μ m. As reported in[1,35] , the TGO thickness for the given lifetime is lower as compared to standard types of coatings. The lower value indicates an earlier failure for a given temperature. The reason for this behavior is not clear, but two possibilities could be that the dense microstructure decreases the growth rate of the TGO or the stress levels in the TBC are responsible for early failure. The horizontal cracks or porous regions of the layered microstructure could not be taken as a reason for spallation because most of the cracks are running through the dense parts of the coatings and not through the porous regions with lower density. In some cases we observe sintering of the branching cracks which might be an additional effect to avoid crack propagation along the porous and horizontal cracked regions in the coating (see Fig. 9a). Segmentation cracks with a low opening show small sintering effects after thermal cycling, while cracks with larger diameters are not related to this effect (Fig. 9b). Altogether, sintering does not play a large role for hot time given by the chosen cycling conditions. X-ray diffraction measurements also show that phase decomposition has not reached a detectable limit. The cubic YSZ phase gave nearly no signal in X-ray diffraction measurements.
3.5. Lifetime evaluation
Fig. 10 compares the life time and bondcoat/surface temperatures between samples prepared with segmentation cracks and standard APS samples previously reported [27] . Comparisons between samples with different spraying routes and feedstock are vague, but using the same powder material (8YSZ) and roughly the same thickness, some conclusions regarding the thermal cycling life time can be made. Table 3compares some properties of the segmented coatings to those of the standard ones. For the segmented coatings, the TGO layer is slightly thinner compared to porous coatings exposed to nearly the same number of hours at high temperature. Smaller TGO thickness also results in a thinner depletion zone of theβ-phase in the bondcoat. The relationship between lifetime and cycling temperature for coatings cycled at low substrate temperatures (~ 950 °C) and high substrate temperatures (~ 1070°) is shown in Fig. 10 wh ere b oth c urve s drop down at a surface temperature of about 1400 °C. The lifetime of the samples cycled with hot surface temperatures is enlarged compared to the other coatings. The real advantage of the highly segmented coatings is a larger temperature drop across the coating at comparable loading conditions and lifetime results. As shown, the surface temperature of the highly segmented coatings can reach 1350 °C with a lifetime com pa-rable to standard coatings. The surface temperature of the highly segmented TBC is about 80 °C higher, which indicates an important improvement of coating performance. The enhanced performance in thermal c ycling e xp eri m ent i s associated w ith a hig h t hermal conductivity. The value for the thermal conductivity for the segmented coatings is estimated close to that for EB-PVD coatings. Comparing the cycles to failure for the three pairs of tested coatings implies that the coatings produced with cooling from the back side survive significantly longer than the coatings sprayed with front-side cooling. Thinner coatings also have a shorter lifetime. The longer lifetime of the back-side cooled samples is obviously caused by higher segmentation crack density resulting in an enhanced strain tolerance.
4. Conclusions
With optimized plasma spraying conditions and fused and crushed YSZ powder feedstock, segmented thermal barrier coatings with a thickness of 500 μ m and segmentation crack densities abov e 8 cracks mm-1 were successfully produced. These coatings show a good performance in thermal cycling tests due to their increased strain tolerance. Sintering effects did not play a role in the performed thermal cycling experiments. Both characteristics can be explained by the microstructure with a high segmentation crack density and a high rate of well-bonded splats. The good contact between the splats enables large vertical grains which support these properties. In thermal cycling tests, they show a performance comparable or even better than the standard porous coatings at similar testing conditions. Furthermore, they allow a larger temperature gradient in the coating.
5. Outlook
The use of vertically cracked coatings with an alternation of more and less dense regions shows promising performance in thermal cycling tests. To increase the capability of high temperature exposure, further developments based on the presented results are proposed. To achieve a lower thermal conductivity and thus a lower bondcoat temperature, one approach could be an improved porosity without decreasing the density of segmentation cracks. First spraying trials show that this approach has the potential to give porosities of up to 10% with acceptable high crack densities. A second way for increasing strain tolerance would be even higher amount of vertical cracks.
本站文章未经允许不得转载；如欲转载请注明出处，北京桑尧科技开发有限公司网址：http://www.sunspraying.com/
|
