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Thick-layered heat-protection coatings of the composition “ZrO2-Y2O3-ceramic fiber” for structural alloys protection

https://doi.org/10.38013/2542-0542-2018-4-46-51

Abstract

The paper introduces a technology for producing heat-protection ceramic coatings ZrO2-Y2O3, including the reinforcement of ceramics with ceramic fiber and metal substrates using nickel-chromium spirals. The microstructure, strength and heat resistance of the coating have been studied

For citation:


Zakirov I.F., Nikulin A.D., Obabkov N.V. Thick-layered heat-protection coatings of the composition “ZrO2-Y2O3-ceramic fiber” for structural alloys protection. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2018;(4):46-51. https://doi.org/10.38013/2542-0542-2018-4-46-51

Introduction

For producing heat-protection coatings, oxide ce­ramics ZrO2-Y2O3 are used [1]. These materials can operate effectively when they have adequate cohesion with the base surface. With increase of ceramic layer thickness the heat protection effi­ciency improves, but under conditions of ther­mal shocks and heat cycling, coating durability decreases. Thick-layered oxide coatings on metals are distinguished, as a rule, by low heat re­sistance, because heating produces considerable thermal stresses at the border between base and coating [2]. Low plasticity of ceramics does not facilitate relaxation of those stresses, but rather leads to formation of cracks at the base - coating interface. Insufficient adhesion of ceramic coating to the metal base, as a rule, does not prevent spreading of cracks at the interface between them and results in coating peeling off. In the authors’ opinion, thick-layered coatings are those with thickness over 1 mm [3].

To increase thickness of oxide layers while at the same time meeting the heat resistance re­quirements, multi-layered structures are used. The most common solutions are, for instance, an outer layer of ZrO2-Y2O3 ceramics and a sub-layer of Ni-Cr-Al-Y, as well as gradient coatings with al­ternating content of sub-layer and ceramics com­ponents [4].

Technology for producing thick-layered zirconia- based composite coatings

The authors have developed a technology for producing heat-resistant coatings on ZrO2-Y2O3 base, with thickness over 5 mm, having satisfac­tory adhesion to a metal substrate of stainless steel 12Kh18N10T

Obtaining zirconia-based composite for ap­plication of coating. For producing heat-resistant ceramic coatings, powders of the composition ZrO2 - 7 % Y2O3 were used, synthesised by dif­ferent methods:

  • joint ammonia-induced precipitation of hydroxides from solutions of zirconium and yttrium nitrates;
  • thermal hydrolysis of zirconium and yttrium nitrates solution at 60 °Сin the presence of sulphuric acid;
  • hydrothermal treatment of zirconium and yttrium nitrates solution at pH= 8.

To improve heat resistance of the coating material, it was reinforced with discrete ceramic fibres obtained from fibre material of composition ZrO2 - 14-17 %; Al2O3 - 50-56 %; SiO2 - 27-36 % by milling them in aqueous medium with the use of a blender. It was determined that in order to obtain fibres about 200 μm long, the milling time must not exceed 15-25 s (Fig. 1).

Fig. 1. Dependence of fibre mean size on milling time: у = 0,0881х2 - 8,3595х + 289,14; R2 = 0,9521

The composite material “ZrO2 - 7 % Y2O3-ceramic fibre” was obtained using slip tech­nology. For that purpose, the initial powders ZrO2 - 7 % Y2O3, after milling in a ball mill, were mixed in aqueous medium with the obtained discrete fi­bres and dried to a constant mass. The fibres were introduced in the amount of 1-10 mass %. Then a thick slip was prepared from the mixture, using paraffin as a binding substance.

Shown in Fig. 2 are microstructures of the coating materials made from powders synthe­sised by different methods. The studies demon­strated that the best characteristics were those of the coating material obtained from powder synthesised with hydrothermal treatment. The material structure is more homogeneous, with the minimum number of defects. Such powders were subsequently used for producing thick- layered coatings.

Fig. 2. Microstructure of coating material: а – joint precipitation; b – thermal hydrolysis; c – hydrothermal treatment

Application of reinforced material on a substrate of steel 12Kh18N10T. To ensure co­hesion of thick-layered ceramic coating on a metal substrate, the surface of the latter was prepared by applying reinforcement. Use of a soldered-on wire mesh for coatings thicker than 5 mm is ineffective, as under conditions of heat cycling, formation of cracks at the ceramics - metal interface occurs, resulting in premature destruction and peeling of the heat protection coating [5].

The authors of the paper have developed a technology for metal substrate reinforcement with the help of metal spirals. For that purpose, a layer of HMP solder VPr-11-40N was applied on the surface of substrate of stainless steel 12Kh18N10T Further, nickel-chromium spirals were arranged on this layer in a certain order and soldered in vacuum at a temperature of 1150 °С for 30 min. Then the upper ridges of spirals were cut, and the ‘whiskers’ produced this way were straightened out perpendicularly to the substrate surface (Fig. 3). It was determined that diameter (d) of the coils of nickel-chromium spirals depends on coating thickness (h) and must be

 

Fig. 3. Diagram of reinforcing elements attachment to metal substrate: 1 – soldering points; 2 – reinforcing elements; 3 – substrate

Pitch 5 of the spiral coils must be equal to 3 mm. At 5 " 2, ceramic layer defectiveness in­creases considerably and its application becomes more difficult technologically, and at 5 > 3 mm, the efficiency of metal substrate surface reinforce­ment decreases. The recommended distance be­tween spirals is b ≤ 2d.

Next, a sub-layer of slip consisting of a mix­ture of nickel and aluminium powders, with the content of the latter making 10-15 mass %, was applied on the reinforced stainless steel substrate (Fig. 4). After that, a layer of thick slip “ZrO2 - 7 % Y2O3-ceramic fibre” with paraffin was ap­plied. The obtained composition was compacted by repressing, dried, and annealed in vacuum at a temperature of 1200 °С. In this way, coatings with thickness of 5-10 mm can be produced. A microstructure of the “coating – substrate” interface area is shown in Fig. 5. 

Fig. 4. Reinforced substrate of stainless steel 12Kh18N10Т: а – before application of coating; b – after application of coating

 

Fig. 5. Microstructure of composite coating «ZrO2 - 7 % Y2O3-ceramic fibre” on stainless steel 12Kh18N10Т

One of the drawbacks of the developed tech­nology is low melting point (~1150 °С) of the sol­dered seam restraining reinforcement elements of the metal substrate. The soldered seam material is a low-melting eutectic in the Ni-Fe-B-Si system. Apparently, to increase the melting temperature, it is necessary to reduce boron concentration in the soldered layer. For that purpose, a diffusion annealing of the substrate with the soldered layer was performed at a temperature of 1200 °С for 1 and 5 hours. Fig. 6 shows the results of measu­ring microhardness in the area of “soldered layer - substrate” interface. A substantial decrease of hardness near this interface is indicative of consi­derable diffusion of Si and В into the substrate and soldered layer stabilisation at a high temperature.

In the applied sub-layer of a mixture of nickel and aluminium powders, in the process of vacuum annealing at a temperature of 1200 °С and due to exothermic reaction between nickel and aluminium, formation of nickel aluminide Ni3Al occurs, as well as, partially, of ferrum aluminides at the interface with the substrate. It facilitates ad­ditional strengthening of the ceramic layer with metal substrate.

It was established that the content of alu­minium in the mixture had to be 10-15 mass %, since at its lower content strengthening is insuffi­cient, and when it is over 15 mass %, the sub-layer would turn brittle, which provoked formation of a large number of microfractures in the interaction zone. It was also established that in the process of repressing, partial bending deformation of the reinforcing elements of the surface of ‘whiskers’ occurred, which plays a positive role. Due to this, the ceramic layer is more firmly held down to the substrate surface, and the ‘whiskers’ act as ‘timber dogs’, additionally fixing the coating.

Effect of fibre content in composites on their properties

Fibre content, mass %

Density, g/cm3

Porosity, %

Bending strength, MPa

After sintering

After 10 thermal cycles

1

4,8

14

49

2

2

4,2

24

39

4

3

4,1

26

30

5

5

3,8

29

27

7

7,5

3,3

35

22

8

10

3,0

40

20

10

It has been experimentally determined that the optimal pitch of spirals is 3 mm. With a smaller pitch, ceramic layer defectiveness increases con­siderably and its application becomes more dif­ficult technologically; it is also impossible to ensure tight adherence of coating to the base, with wide gaps forming between the coating and the metal substrate, which is detrimental to the strength properties. In the tests, coating sep­aration occurs over the substrate - ceramics interface, and adhesion strength is provided by the presence of reinforcing spirals only.

Adhesive strength of the coatings was stu­died by the gluing method. The tests demonstrated that samples with spiral pitch of 3 mm sustained cohesive separation across the coating body at a tension of about 10 MPa, which testifies to ade­quate adhesion of the produced coatings.

The basic characteristics of coatings were determined, namely: density, open porosity, heat resistance. The best results were shown by the composite “ZrO2 - 7 % Y2O3 - 10 %-ceramic fibre”. This material withstood 10 thermal cycles (heating up to 1100 °C, cooling in water) without visible destruction; its porosity was 40 %, density - 3.0 g/cm3. The bending strength was 20 MPa, decreasing to 10 MPa after 10 thermal cycles.

Heat resistance of the coating materials was assessed by the value of coating material strength properties decline under thermal cycling condi­tions: heating up to 1100 °C and cooling in water. It was determined that entering ceramic fibres in amount of 10 mass % leads to a decline of ceramic composite strength properties, which is presuma­bly caused by significant increase of ceramic layer porosity, up to 40 %. However, increasing the content of fibres is instrumental in obtaining more resistant materials working under thermal cycling conditions. With higher concentrations of fibres, a non-uniform distribution of the matrix com­ponent across the composite volume can be observed. For example, composites with fibre content over 13 mass % would practically not sin­ter at a temperature of 1200 °C and would disin­tegrate after unloading from the oven. The study results are given in the table.

Tests were performed on ceramic coatings 5 mm thick, applied on stainless steel 12Kh- 18N10T, obtained according to the developed technology under thermal cycling conditions: heating with gas jet 300 K → 2200 K for 5 s, cooling by air blow-off 2200 K → 300 K for 15 s. The test results showed that the coatings with­stood over 5-10 thermal cycles without visible deterioration.

Conclusion

A technology has been developed for producing thick-layered composite coatings “ZrO2 - 7 % Y2O3-ceramic fibre” with the property of high heat resistance under thermal cycling conditions.

Optimal compositions of the composite ma­terials for thick-layered heat resistant coatings were experimentally determined.

The basic characteristics of coatings were determined: density and porosity, bending strength, heat resistance, and adhesive properties.

The obtained composite coatings can be re­commended for use as heat-protection materials.

About the Authors

I. F. Zakirov
Federal State Autonomous Educational Institution of Higher Education “Ural Federal University named after the First President of Russia B. N. Yeltsin”
Russian Federation


A. D. Nikulin
Federal State Autonomous Educational Institution of Higher Education “Ural Federal University named after the First President of Russia B. N. Yeltsin”
Russian Federation


N. V. Obabkov
Federal State Autonomous Educational Institution of Higher Education “Ural Federal University named after the First President of Russia B. N. Yeltsin”
Russian Federation


For citation:


Zakirov I.F., Nikulin A.D., Obabkov N.V. Thick-layered heat-protection coatings of the composition “ZrO2-Y2O3-ceramic fiber” for structural alloys protection. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2018;(4):46-51. https://doi.org/10.38013/2542-0542-2018-4-46-51

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