This work investigated the behaviour of introducing ZrSi₂ layer between the top coat layer Yttria-stabilized zirconia (YSZ) and the bond coat layer CoNiCrAlY in thermal barrier coatings where ZrSi₂ layer helped self-healing of the cracks. Powder mixtures of ZrSi₂/8YSZ and ZrSi₂ /CoNiCrAlY were used to imitate the real case. At 800 °C under argon, there was no change in the chemical composition of both ZrSi₂ and 8YSZ. ZrSi₂ was oxidized in case of treatment in air atmosphere at 800 °C with no evidence for any self-healing reactions. At 1000 and 1200 °C under argon the formation of ZrSiO₄ phase was detected which was a strong evidence of self-healing reaction. ZrSiO₄ phase was also detected at 1200 °C in air with the appearance of SiO₂ phase. A limited interaction was detected at 1000 °C between ZrSi₂ and CoNiCrAlY under vacuum. Si was detected in the surface of CoNiCrAlY grains and Ni was detected in the composition of ZrSi₂. This behavior could contribute to a chemical bonding between ZrSi₂ and CoNiCrAlY layers. The obtained data were confirmed by XRD, SEM and EDX analyses.

Keywords: Self-healing, zirconium silicide, thermal barrier coatings

Hot components in engines are protected with thermal barrier coatings (TBCs), and its demand has increased as gas turbine engines have been improved [1]. This improvement consists of a higher gas steam working temperature, and this implies the need to protect blades in order to increase their lifetime. TBCs are mainly composed of a ceramic top coat based on yttria partially stabilized zirconia as insulator and an interlayer of MCrAlY (M: Ni, Co) as bond coat (BC). The latter is used to improve the bond between the metallic part and the top coat, and also to provide protection against oxidation and hot corrosion [2]. The coating is also used to protect the superalloy substrate against mechanical damages [3]. During high temperature operations a thermally grown oxide (TGO) layer is developed between the BC and TBC, which acts as a diffusion barrier for oxygen. Rabiei stated that the blade’s lifetime is limited by this oxide scale rather than because of part failure [4].
The interface regions undergo high stresses due to the mismatch of thermal expansion between bond coat and TBC. Additional growth stresses due to the development of TGO and stresses caused by interface roughness are also superimposed. During cooling (thermal cycling) stresses are accumulated, which results in early crack initiation at the bond coat/TGO interface and spallation failure afterwards. Such spallation of the TBC will reduce the lifetime of the coated components in turbine engines [4-6]. It was also reported that during cooling the martensitic phase transformation from tetragonal to monoclinic was developed which may contribute to crack initiation [7].
Researchers have attempted to improve TBC lifetime by employing dense Yttria-stabilized zirconia YSZ coating structures under very low pressure plasma spray (VLPPS), electrodepositing, laser re-melting [8-10] and air plasma spray coating [3]. The dense nanostructure of YSZ layers plays an effective role in reducing oxygen permeation [9], and creates a TBC with favorable thermal shock resistance due to reduced TGO growth in the YSZ/BC interface [10]. Nano-structured SiC  was also used to reinforced TBCs based on zirconia [11].
Recently, the self-healing of cracks comes into the focus as a new repairing concept. Self-healing is the capacity of a system to repair damage by itself so that cracks are sealed [12]. Self-healing materials include polymers [13], metals [14], ceramics [15]  and their composites [16] that when damaged through thermal, mechanical, ballistic, or other means have the ability to restore their own original properties [17]. Self-healing materials are often used to produce anticorrosion coatings [18]. Self-healing can improve the longevity and safety of various structures [19]. In 1970s, ceramic materials’ self-healing was first reported [20] which could heal the cracks in ceramic using heat treatment. Over past few years, many reports had been published about self-healing material of polymers, metal and concrete [21]. The National Aeronautics and Space Administration (NASA) published a detailed report about high temperature lightweight self-healing composites, which included CrSi₂, CrMoSi and MoSi₂ for aircraft engine applications [22]. TiC has been reported as self-generating in situ phase which can improve the oxidation resistance of superalloy [23].
Self-healing of cracks is the most recent trend in solving TBC spallation problems through increasing the lifetime of the gas turbine blades [24-26]. A self-healing concept introduces an additional silicide layer to the TBCs system between the bond coat and the top coat layers. For the component to reach lifetimes of the order of many thousands of hours at high temperature in oxidizing and corrosive environments, as typically required for TBC systems of advanced jet engines, self-healing must be introduced. Few publications were found in the literatures which were related to self-healing of cracks in TBC systems. Disilicide of molybdenum, MoSi₂, was previously investigated as a self-healing agent of TBC systems. Sonoya et al [27] stated that using MoSi₂ as a self-healing agent rose the thermal cycles of TBC from 20 to 60 cycles which meant a three-fold increase in lifetime of the component. Derelioglu [28] showed that the oxidative decomposition of MoSi₂ embedded in a TBC at high temperatures led to the formation of amorphous SiO₂ which flowed into cracks and established direct contact with the crack faces. The wetting of the crack faces was followed by a chemical reaction with the ZrO₂ based TBC coating leading to the formation of solid ZrSiO₄. This chemical reaction generated a strong bonding between healing agent and matrix material and resulted in a complete filling and sealing of the crack [28]. In a parallel study, Frank stated that zircon is observed far away from the MoSi₂ particles and this can be very beneficial for the self-healing mechanism of the composite [29]. TiC was also investigated as a self-healing in TBC systems [26]. TiC was inserted between the top coat and bond coat layers and pre-treated in air atmosphere at 600 °C. TiC acted as a self-healing agent releasing its oxide (TiO₂) by reaction with oxygen in air and heals the cracks/pores present in the coatings [26].
The self-healing of crack depends mainly on the oxidation products of the silicide layer. Silicides usually oxidize and liberate silica and metal oxides (MO) according to the following equation [30].
 
MSi₂ + 1.5O₂ = MO + 2SiO₂
 
When cracks in the TBC propagated and reached the silicide layer, the silicides of metals MSi₂ starts to oxidize due to its interaction with the penetrated oxygen giving rise to SiO₂. The liberated SiO₂ then fills the cracks and chemically bonded to the cracks' sides due to the reaction between silica and zirconia with the formation of zircon: ZrO₂ + SiO₂ = ZrSiO₄. Therefore, the efficiency of healing will depend on the rate of SiO₂ formation during the oxidation of silicide [31].
This work aims at investigating the possibility of introducing ZrSi₂ layer between the top coat layer 8YSZ and the bond coat layer CoNiCrAlY of the thermal barrier system. This aim is organized to be achieved by studying the anticipated interactions of ZrSi₂ with 8YSZ and ZrSi₂ with CoNiCrAlY as powder mixtures at high temperatures. The effect of temperature will be studied and a full concept about the possibility of using ZrSi₂ as intermediate layer for self-healing will be introduced.

The materials used in this study are given in the Table 1.
Both 8YSZ and CoNiCrAlY were provided by Saudi Aramco Company which were used by Aramco in coating processes. The powders were weighed in 1:1 per weight ratio and were mixed in agate mortar and dry blended in a slow rotating mill with zirconia balls for 5 h. The powder mixture was uniaxially pressed without binder at 78 MPa into cylindrical compact of 1.0 cm height and 1.0 cm diameter.
The compact was transferred to a muffle furnace. In some experiment the compacts were treated in a tube furnace in a controlled atmosphere of argon. The temperature of the furnace was then adjusted at the desired value (800 ~ 1,200 °C) and the treatment was carried out for 2 h. In some experiments, the compact was put into a silica tube which then evacuated and sealed.
Different phases of the products were identified by X-ray diffraction analysis using X-ray diffractometer (D8 Advanced Bruker AXS, GMbH, karlsruhe, Germany). Microstructure of specimens was investigated using Scanning Electron Microscope (SEM, Model JSM-5410, JEOL, Tokyo, Japan) equipped with electron dispersive spectroscopy (EDX). The samples were coated with gold to ensure good electrical conductivity of the entire components of the sample. 

Studying the reaction between ZrSi₂ and 8YSZ
This series of experiments was conducted in order to study the reaction between ZrSi₂ and 8YSZ in air and argon atmospheres. According to the geometry of the coatings, the proposed ZrSi₂ layer lays between the top layer (YSZ) and the bond coat one (CoNiCrAlY). Although the top layer completely isolates the ZrSi₂ layer from air, some oxygen from the air can penetrate the top zirconia layer through generated cracks and attacks the ZrSi₂ layer. Therefore, this series of experiments was carried out under argon and air using a blend of 8YSZ and ZrSi₂ powders at different temperatures (800 ~ 1200 °C) for 2 h.
 
Studying the reaction between ZrSi₂ and 8YSZ under argon
This series of experiments was devoted for studying the possible reaction between ZrSi₂ and 8YSZ powders at different temperatures (800 ~ 1,200 °C) for 2 h under argon. The blend was pressed into cylindrical compacts of 1.0 cm diameter and 1.0 cm length. Generally, XRD of the samples treated at 800, 1,000 and 1,200 °C, Fig. 1, showed the presence of YSZ and ZrSi₂ as the main phases and traces of monoclinic zirconia. No evidences for the formation of zirconium silicate ZrSiO₄ or silica was observed in the XRD pattern of the sample treated at 800 °C. This indicated that there was no interaction or that it could be a low interaction between ZrSi₂ and YSZ at this temperature. XRD of the sample treated at 1,000 and 1,200 °C showed the appearance of diffraction pattern of (ZrSiO₄) a minor and moderate phase, respectively. The formation of zirconium silicate ZrSiO₄ proved the oxidation of ZrSi₂ to ZrO₂ and SiO₂ with the interaction of the product with YSZ. Although the study was carried out under argon atmosphere, but still there was a chance of some oxygen traces contaminating the argon gas to oxidize ZrSi₂ and produced silica which in turn reacted with YSZ and finally produced ZrSiO₂. The formation of zirconium silicate ZrSiO₄ was a good indication for the occurrence of self-healing as explained at the introduction section [28, 31].
SEM investigations of the sample treated under argon at 1,000 °C and 1,200 °C is given in Fig. 2. It could be seen that there was no big difference between the microstructures of the two samples. 8YSZ was appeared in its spherical shape while ZrSi₂ appeared in irregular particles shapes. Generally, EDX analysis of the irregular shape particles (ZrSi₂) gave information about the oxidation behavior of ZrSi₂ while EDX analysis of spherical shape particles reflected the degree of interaction between the oxidation product of ZrSi₂ and YSZ. EDX analysis of spherical shape particles of the sample treated at 1,000 °C, Fig. 3, showed that the main composition was Zr and O with the presence of small amounts of Si in the compositions. This is a good indication of the formation of ZrSiO₄ on the surface of 8YSZ. This ensured that the interaction between the oxidation product of ZrSi₂ and 8YSZ was occurred at this temperature. The oxidation of ZrSi₂ could be due to the oxygen contamination of the argon gas which was used as inert atmosphere. EDX analysis of the irregular shape particles of the sample treated at 1,000 °C, Fig. 4, showed that these particles were mainly composed of Zr, Si and O. The oxygen was present in appreciable amounts. This also proved that there was a degree of interaction between the ZrSi₂ and the oxygen contamination of the inert gas (argon). At 1,200 °C, there was a noticeable increase in the mass % of Si of the spherical shape particles (21.24 wt %) compared with that (2.22 wt %) of the sample treated at 1,000 °C. This indicated a higher degree of interaction between the oxidation product of ZrSi₂ and YSZ.
 
Studying the reaction between ZrSi₂ and 8YSZ in air
This series of experiments was devoted for studying the possible reaction between 8YSZ and ZrSi₂ powders (1 : 1 wt ratio) at different temperatures (800 ~ 1,200 °C) in air atmosphere for 2 h. XRD of the sample treated in air at 800, 1,000 and 1,200 °C showed the presence of YSZ as the main phase, Fig. 5. The diffraction pattern of ZrSi₂ was completely disappeared. The diffraction pattern of monoclinic zirconia was also detected as minor phase. This meant that ZrSi₂ was completely oxidized at these conditions. No evidence for the formation of zirconium silicate ZrSiO₂ or silica was monitored in the XRD pattern of the sample treated at 800 and 1000 °C in air. The free energies of ZrSiO₄ formation from ZrO₂ and SiO₂ at 800 and 1,000 °C have a negative values (-9.96 and -7.6 Kj/mol respectively) which is good evidence for the reaction occurrence. However, the absence of ZrSiO₄ at these temperatures suggested that the formation reaction proceeded with very low rate. This is in accordance with the previous studies which reported the formation of zircon (ZrSiO₄) from ZrO₂ and SiO₂ at temperatures as low as 1,200 °C [32] and at a thermal treatment at 1,450 ~ 1,500 °C in air [33]. This meant that the oxidation of ZrSi₂ at 800 and 1,000 °C was preceded with the formation of silicon monoxide gas (SiO) instead of silica according to reaction (1):

ZrSi₂ + 2O₂ = ZrO₂ + 2SiO                                        (1)
 
On the other hand XRD of the sample treated in air at 1,200 °C showed the diffraction pattern of silica (SiO₂) and the diffraction pattern of ZrSiO₄ were appeared. The appearance of the diffraction pattern of silica SiO₂ proved the oxidation of ZrSi₂ to ZrO₂ and SiO₂ according to reaction (2):
 
ZrSi₂ + 3O₂ = ZrO₂ + 2SiO₂                                                    (2)
 
The formation of zirconium silicate (ZrSiO₄) indicated the interaction of the produced silica SiO₂ with ZrO₂ content of YSZ according to reaction (3):

ZrO₂ + SiO₂ = ZrSiO₄                                                          (3)
 
SEM investigations of the sample treated at 1,000 °C and 1,200 °C in air are given in Fig. 6(a&b). No significant differences between the EDX analyses of spherical shape particles of the sample treated at 1,000 °C in air compared with that treated under argon. Si was also detected in a little amount (3.7 wt %) in the composition indicating the formation of ZrSiO₄ on the surface of 8YSZ (not detected by XRD). EDX analysis of the irregular shape particles of the sample treated at 1,000 °C in air showed increasing amounts of oxygen (39.7 wt %) compared with that under argon (19.49 wt %) with a correspondence decrease in the Si amounts from 29.77 in case of air to 19.27 wt % in case of argon, Fig. 7. The decrease in silicon amounts could be due to either evaporation from the reaction media as a gas and / or its consumption in the reaction with YSZ. The detection of Si and O in the EDX analysis and the disappearance of silica in XRD investigation suggest that silica was produced in an amorphous phase or silicone monoxide gas was produced and part of it was evaporated from the sample.
EDX analysis of spherical shape particles of the sample treated in air at 1,200 °C showed a very little amount of Si (2.1 wt %) compared with that in case of argon treatment (21.24 wt %), Fig. 8. This can be related to either the very high evaporation rate of the oxidation product of ZrSi₂ or the decomposition of the formed ZrSiO₄. EDX analysis of irregular shape particles of the sample treated in air at 1,200 °C showed a decrease of Si content (14.75 wt %) compared with that in case of argon treatment (30.51 wt %), Fig. 8. This result recommended that the oxidation product of ZrSi₂ was spilt out the sample with a high rate. However, a newly formed phase was detected in case of 1,200 °C. The composition of this phase was free from Zr and contains only silicone and oxygen. This finding was in good accordance with XRD investigation of this sample where SiO₂ was detected at 1,200 °C.

Studying the reaction between ZrSi₂ and bond coat layer CoNiCrAlY
This series of experiments was devoted for studying the possible reaction between ZrSi₂ and CoNiCrAlY powders, 1:1 weight ratio, at 1,000 °C for 2 h. The sample was capsulated into silica tube and treated under vacuum.
XRD of CoNiCrAlY as received powder (without any treatment) is given in Fig. 9. The diffraction pattern was mainly composed of the pattern of Ni₃Al phase and moderate intensity lines of CoAl phases. No other phases were detected. On the other hand, XRD of the sample treated at 1,000 °C showed the presence of ZrSi₂ as the main phases, Fig. 9. Moderate intensity lines were detected for ZrSi phase. The diffraction pattern of Ni₃Al phase was also detected as minor phase. No evidences for the interaction between ZrSi₂ and CoNiCrAlY.
CoNiCrAlY appeared in a spherical shape while the irregular shape particles consisted mainly of Zr and Si according to EDX analysis. EDX analysis of spherical shape particles of the sample treated at 1,000 °C is given in Fig. 10. The spherical shape-particle was composed mainly of Co, Ni, Al, Cr and Si. The presence of Si in the composition of CoNiCrAlY indicated that there was an interaction between CoNiCrAlY and the ZrSi₂ particles. On the other hand, EDX analysis of the irregular shape particles, Fig. 10, illustrated that these particles were mainly composed of Zr and Si. Traces of Ni were also detected. The presence of some Si in composition of CoNiCrAlY and the presence of some Ni in the composition of ZrSi₂ particles proved that there was a kind on interaction between the Si of ZrSi₂ and the Ni of CoNiCrAlY which contributed to an anticipated bonding between the newly introduced layer ZrSi₂ and the bond coat layer CoNiCrAlY.

Powder mixtures of ZrSi₂ with 8YSZ, and ZrSi₂ with CoNiCrAlY were successfully used to imitate the behavior of introducing ZrSi₂ layer into thermal barrier coating system at high temperatures.
At 800 °C under argon, there was no change in the chemical composition of both ZrSi₂ and YSZ.
At 1,000 and 1,200 °C under argon the formation of ZrSiO₄ phase was detected which was a strong evidence of self-healing reaction. ZrSiO₄ phase was only detected in little amounts at 1,200 °C in air with the appearance of SiO₂ phase.
The entire amount of ZrSi₂ was oxidized in case of air treatment at 800, 1,000 and 1,200 °C.
At 1,000 °C under vacuum, Si was detected in the surface of CoNiCrAlY and Ni was detected in the composition of ZrSi₂. This behavior can contribute to an anticipated chemical bonding between ZrSi₂ and CoNiCrAlY layers in the real case of thermal barrier coating.

The authors would like to express their gratitude for Taif University, KSA for funding this project under the grant no. 5237-437.