Functional coatings made of eco-friendly materials

. Thermal barrier coatings protect alloys (for example, in turbine blades) from extreme temperatures. There is a pressing need to find better materials than currently used yttria-stabilized zirconia (i


INTRODUCTION
Thermal barrier coatings (TBCs) protect structural elements (such as turbine blades) from extreme temperatures.There is an urgent need to find new materials that outperform the currently used yttriastabilized zirconia.In this regard, there is a need to search for new TBC materials with a high melting point, high phase stability, low thermal conductivity, a relatively high thermal expansion coefficient, as well as good chemical and mechanical resistance at operating temperatures [1,2].Many refractory oxide materials based on alumina, titanium dioxide, zirconium, hafnium, rare earth elements (e.g.fluorites (Ce, Re)O2x, pyrochlores A23 + B24 + O7, pomegranate (Y3Al5O12, Y3AlxFe5-xO12, Dy3Al5O12), monazite (LaPO4), perovskites A2 + B4 + O3, hexaaluminates (CaAl12O19, LaMgAl11O19) and others are considered as potential TBC materials.Most of these compounds have a symmetrical framework structure, but some of them have a layered structure (BaLa2Ti3O10, Re2SrAl2O7) or chain (Y4Al2O9) structure.Recently, however, the need for environmentally friendly coatings that do not have a toxic effect on the environment during their production has been increasing due to increased attention to the problem of ecology and strict mandatory control.Most of the mentioned promising TBC materials have natural minerals as prototypes.As can be seen, natural or environmentally friendly barrier coatings have demonstrated the potential for bearing wear protection in gas turbines.The main function of environmentally friendly TBC is to protect the alloys from oxidation under high temperature/pressure in the combustion environment.Along with the need to organize thermal protection, the ceramic coating is also selected taking into account its ability to protect the part from corrosion.However, recently, the demand of environmental friendly coatings without toxic effects on the environment during their manufacture grows stronger due to the increasing of the concern in environmental problem and the strong mandatory control.Most of the mentioned promising TBC materials have natural minerals as prototypes.As can be seen, Natural or Environmentally friendly barrier coatings demonstrated potential for the protection of support degradation in gas turbines.The primary function of the natural friendly TBC is to protect alloys to resist oxidation by the high temperature/pressure in the combustion environment.As such, successful natural friendly TBC should be dense and thermochemically stable, highly impervious to oxygen transport, phase-stable over the temperature range of interest, tolerant to thermal strains arising from the cyclic nature of the operation, and resistant to impact from foreign particles.
In this paper, we study the natural mineralgehlenite (Ca2Al2SiO7) for possible use as a material for thermal barrier coatings.

RESEARCH METHODOLOGY
For experimental study we choose natural gehlenite (ref. FMM_1_41299) from collection of Fersman Mineralogical Museum (Moscow, Russia).This sample originally was collected in 1930th in skarns of Vesuvianite hills near Kedabek mine (Azerbaijan).The rock is consisting of light-grey fine-grained gehlenite, which is single-phase according to X-ray powder diffraction (XPRD) data.Gehlenite Ca2Al[AlSiO7] is the Al-rich member of melilite group of minerals, which forms series of solid solution with åkermanite Ca2Mg[Si2O7], alumoåkermanite NaCaAl[Si2O7] and Fe-bearing end members [9].The crystal structure of gehlenite is tetragonal (a = 7.684, с = 5.065 Å, P 421 m, Z = 4) [3,4] and contains the layers parallel to (001), which consist of corner-sharing AlO4 and (Al0.5Si0.5)O4tetrahedra connected in five-membered rings.The Ca 2+ cations are in eightfold coordination (Thomson cubes) E3S Web of Conferences 459, 09002 (2023) https://doi.org/10.1051/e3sconf/202345909002XXXIX Siberian Thermophysical Seminar between the tetrahedral layers (Fig. 1).Pure and doped crystals and ceramics of gehlenite were considered as laser hosts, high temperature piezoelectric sensors, pigments, dielectric materials, bone regeneration scaffolds [4][5][6][7][8].Available data are scarce, but do indicate the high promise of gehlenite for TBC applications: it undergoes no solid-solid phase transitions at ambient pressure, has relatively high congruent melting point (1590 °C) [9,10] and volumetric CTE (28.3•10 -6 K -1 ) [9], low thermal conductivity (1.5 W m -1 K -1 at 30 °C) [10], good mechanical, and anti-hydration properties [11,12].Interestingly, mesoporous C2AS ceramics sintered at 90-1450 °C has a very low thermal conductivity of 0.3-0.42W m -1 K -1 at 1000 °C [12].To study the properties of the material, gehlenite crystals with a particle size of ~2-5 mm were ground to 40-60 μm in a steel grinding jar using a vibro-grinding stand ВИ-4х350 followed by annealing for 3 h in air at 1273K.The morphology of the particles of the starting materials and the resulting composites was studied using a scanning electron microscope (SEM) Hitachi ТМ-3000, equipped with an energy dispersive analyzer, at an accelerating voltage of 15 kV.Microprobe analysis was also carried out by Camebax-microbeam (France) with an energy-dispersive Si (Li) detector and the INCA Energy Oxford analysis system; accelerating voltage was 20 kV, beam current -30 nA.XPRD analysis was carried out using a powder diffractometer «STOE-STADI MP» (Germany) equipped with a curved Ge(111) monochromator which gives strictly monochromatic CoK-1 radiation (λ = 1.78897Å).The data were collected in a regime of sequential overlapping of scanned regions using a linear positionsensitive detector with a 2 scanning angle of 5 and a channel width of 0.02.The unit-cell parameters were calculated with WinXPow software package (WinXPow Software STOE&CIE GmbH 2002).Calculation was carried out according to the XPowINDEX program, which includes: Werner`s TREOR, Visser`s ITO and Louer`s DICVOL.
Thermogravimetric studies were carried out on a NETZSCH STA 449F thermal analyzer in air in the temperature range 303-1473K (20 mg sample, heating rate 10 deg/min).Thermal cycling tests were carried out on a setup that allows periodic changes in the given thermal load of the test sample at a given rate.To control the position of the object under test, a PLLM-11-500 linear movement module was used, with a PL57H76-D8 bipolar stepper motor installed.To control the positioning system, a programmable controller of stepper motors SMSD 4.2 was used.The temperature at the control points was recorded using chromel-alumel thermocouples and an OPTRIS CT pyrometer.Gehlenite coating was applied to steel plates 70 × 50 × 2 mm in size.

RESULT AND DISCUSSION
On fig. 2 is an electron microscope (SEM) image of a natural gehlenite powder after grinding and fractionation.It can be seen that it is a mixture of irregularly shaped particles with sharp edges with an average size of 40-60 µm.According to elemental dispersion analysis (EDX), the composition of the studied powder coincides with the composition of gehlenite (Table 1).As a result of calcination at 1273 K, the composition of the gehlenite samples does not change in all cases, which is confirmed by the XRD and DSC data (See also figs.3, 4).Thus, the diffraction pattern (Fig. 3) shows the absence of reflections other than gehlenite, although at small angles there is a slight increase in the background, probably due to the small size of some of the crystallites of the sample under study.The TG curve of gehlenite powder calcined in air at 1273 K for 3 h (Fig. 4) shows that the sample does not undergo phase transformations with a slight weight loss of 0.2%.The maximum thermal effect observed at 819 K is apparently associated with the oxidation of carboncontaining impurities present in the sample.Thus, it can be concluded that gehlenite is the main phase for further research.To obtain detailed information about the thermophysical properties of the deposited coating based on gehlenite, we carried out a series of thermal cycling tests of the obtained samples of gehlenite coatings of various thicknesses deposited on steel substrates.On fig. 5 shows coated specimens before and after thermal cycling.It is seen that for samples with a coating thickness of 150-250 μm, even after 50 cycles  of thermal cycling (on a hot surface (coating) tmax = 1123 K, on a cooled surface (metal) tmax = 1033 K), no visible cracks and delaminations are observed.At the same time, for samples with a coating thickness of 300-460 μm, almost complete destruction of the protective layer based on gehlenite is observed already after 5 test cycles (on a hot surface (coating) tmax = 1323 K, on a cooled surface (metal) tmax = 1121 K).Based on the obtained data on thermal cycling of model coating samples, the optimal conditions for obtaining TBC based on helenite deposited on heat-resistant alloys of the Incone type used as a substrate were selected.To compensate for the difference in thermal expansion coefficients (CTE) between Gehlenite and Inconel, a NiAl layer and a YSZ-Nd or ZrO2-Nd layer with a thickness of 40-60 µm and 20-25 µm, respectively, were deposited on the alloy surface.The thickness of the layer based on gehlenite in all cases was 40-50 µm.The study of the structure of the resulting coating was carried out by optical and electron microscopy on transverse sections of samples of composite materials (Fig. 6).The analysis of optical and electron microscopic images shows that the deposited coatings are dense and lowporous.At the micron and submicron level, there are no interfaces between the deposited layers.At the same time, for all samples, an intermediate (interface) layer is detected at the "substrate-coating" interface with a thickness of 5-7 μm.The presence of an interface layer is probably associated with the formation of metal oxides as a result of cyclic thermal treatments carried out in an oxidizing atmosphere.At high temperature, the MCrAlY bond coat oxidizes to form a thermally grown oxide layer (TGO).This layer can slow down further oxidation of the bond coat by acting as a diffusion barrier.The main component of TGO is Al2O3 and a small amount of Cr and Co oxides at the boundary of the interface layer.The ideal TGO component is homogeneous, defect-free -Al2O3, which has a very low diffusion coefficient of oxygen ions and good adhesion.As the oxide layer grows to a thickness of ~10 µm, TGO becomes the main source of stresses that triggers the formation of cracks in the coating.At this stage of "aging" of the coating, the coefficient of linear thermal expansion is no longer so critical.

CONCLUSION
The experimental data on thermal cycling of gehlenite coatings based on the natural mineral are encouraging.The temperature of layer destruction (1600 K) is high enough.Other parameters of gehlenite, such as its high melting point (1863 K), absence of phase transitions up to 1273 K, together with the already mentioned low thermal conductivity and high CTE necessitate a more detailed study of this material as a thermal barrier coating.The gehlenite coating reacts with metals at relatively low temperatures, which suggests the need for a protective barrier layer.In our experiments, a 5-10 µm thick protective YSZ layer allowed us to significantly increase the temperature of coating degradation.No interaction of gehlenite with YSZ was observed.One of the advantages is that gehlenite contains Ca, Mg, Al, Si, O, i.e. the same elements that cause calciummagnesium-aluminosilicate (CMAS) corrosion, a critical factor provoking the destruction of thermal barrier coatings, and should not be susceptible to the CMAS corrosion.Gehlenite does not suffer from CMAS corrosion.This work is still a preliminary study and a model to estimate the properties of gehlenite as a possible TBC material, while its other important and useful properties are yet to be calculated and measured in the subsequent works.
The work was supported by the Russian Science Foundation grant № 23-13-00117.

Fig. 3 .
Fig. 3. X-ray diffraction pattern of crushed powder of gehlenite (a) calcined at 1273 K. Figure (b) also shows the diffraction pattern of gehlenite.

Fig. 6 .
Fig. 6.Electron microscopic and optical images of the surface and cross sections of gehlenite layers: (a), (d), (g) -view of gehlenite deposited on inconel substrates; (b), (e), (h) corresponding optical micrographs of transverse sections of composite material samples; (c), (f), (i) -electron microscopic images of transverse sections of composite material samples.

Table 1 .
The composition of natural and crushed gehlenite samples.