Copper liners of liquid fuel rocket engines are subjected to high thermomechanical loads which can lead to damage by blanching
[1,2,3] or the so-called doghouse effect
[4,5,6,7,8,9]. To avoid failure, a thermal barrier coating (TBC) may be applied on the inner surface of the combustion chamber. This coating reduces the maximum temperature of the copper liner and protects the surface against oxidation and erosion.
In the past, several different TBC systems for rocket engine application were developed and tested (for a detailed review, see
[10, 11]). These studies show that extreme test conditions are necessary to investigate realistic coating failure. However, in most cases, a detailed elucidation of the observed coating damage is missing.
The main goal of the present research project was to develop an improved coating system based on the findings from the literature and gain a deep understanding of coating failure mechanisms for future coating design. This was done in three steps:
In a first step, state of the art thermal barrier coatings, consisting of a NiCrAlY bond-coat and a zirconia top-coat, were tested to gain a better understanding of the coating failure mechanisms
[12,13,14]. A laser test bed has been set up to test the coatings with a thermal gradient
[13], and a micro model was developed to investigate interface stresses between substrate and coating which led to delaminations during the laser tests
[12].
In a second step, an improved thermal-barrier coating system was developed, consisting of a NiCuCrAl bond-coat and a Ni-based superalloy top-coat
[11, 12, 15, 16].
In a third step, a detailed study of the possible failure mechanisms of the new coating system was performed, and a failure model for coating design and lifetime analysis was set up
[10]. For this purpose, the laser test bed was modified to increase the heat flux density (up to 30 MW/m\(^2\)) and the thermomechanical loads in the coatings
[11, 17]. Although the very high heat fluxes in rocket combustion chambers (up to 150 MW/m\(^2\)) could not be reproduced, the laser experiment goes beyond many other laboratory-scale experiments.
To investigate the coating damage in the laser cycling experiments in more detail, finite element simulations were carried out
[11, 17]. For these simulations, material parameters for the coating system were determined. For this purpose, aluminium substrates were coated and removed in dilute NaOH solution to get free standing coatings
[18]. These free standing coatings were investigated e.g. in tensile and compression tests, vibrating-reed experiments, dilatometric and laser-flash measurements at different temperatures to obtain an extensive set of material parameters
[11, 17, 19].
In the laser cycling experiments, four different damage mechanisms were observed
[10, 11, 20]: Delamination cracks along the substrate/coating interface, diffusion caused interface porosity, large scale buckling, and vertical cracks in the coatings.
Delamination cracks grow due to the different coefficients of thermal expansion of substrate and coating in the roughness profile of the interface
[10]. These cracks were observed after thermal cycling at interface temperatures of 700 \(^\circ \)C and above
[10, 11]. The growth of delamination cracks can be hindered by a diffusion layer between coating and substrate
[11]. A heat treatment of 6 h at 700 \(^\circ \)C is sufficient to avoid delamination in the laser tests even at interface temperatures of 800 \(^\circ \)C
[10].
Interface porosity becomes relevant after long heat exposure: due to the Kirkendall effect
[21,22,23,24], pores can form on a large area at the substrate/coating interface, reducing the layer adhesion. In addition, the heat conductivity through the interface is reduced by the pores, resulting in overheating of the overlying layer. The formation of pores can be avoided (assuming maximum accumulated hot-gas times of <6 h) if the interface temperatures are kept below 750 \(^\circ \)C
[11]. The bond coat/top coat interface is also susceptible to pore formation, but only at temperatures above 1000 \(^\circ \)C.
Buckling of the coatings is caused by the high temperature difference between the hot coating and the cold substrate and thus large thermal compressive strains. If these strains exceed a critical value, the layer bends and buckles. The critical compressive strain or the critical elastically stored energy is massively reduced by small imperfections at the substrate/layer interface. For a detailed discussion see e.g.
[10]. According to the current state of research, it is assumed that the coating system considered here requires massive interface damage to buckle in the laser experiments
[10]. Consequently, to prevent buckling, it is sufficient to prevent damage to the interface (buckling and interface porosity, see above).
Vertical cracks are a result of large cooling stresses near the coating surface. During the heating phase, the high compressive strains in the hot coating can exceed the yield strength. The resulting plastic deformation leads to the formation of tensile strains in the coating during subsequent cooling, which can cause vertical cracks
[10]. At room temperature, the critical elastic strain at which vertical cracks occur is about 0.55%
[10]. At higher temperatures, higher critical strains can be expected as the coatings become increasingly ductile with increasing temperature
[18].
In this article, exemplarily, a coating system was designed for a 1000 kN full scale liquid fuel combustion chamber to demonstrate the benefits of a thermal barrier coating. The mechanical loads in the coatings are quantified by finite element simulations, and possible coating failure is estimated. Furthermore, the coatings were tested in two validation experiments with a hot-gas flow: an arc heated supersonic wind tunnel and a subscale combustion chamber.