An idea of engineering of synthetic substances that are capable of self-repairing and adaptation to changing environments came to the scientific world many years ago. Application of these materials would range from space exploration to civil manufacturing. The objective of this part of the purposed research is to understand a biological mechanism of self-healing phenomena, and based on this knowledge to develop a wide range of selfhealing materials for aerospace structures. The biological findings will be translated into composite materials. A final goal of the project will be a fabrication of a new class of self-healing composites and testing their thermophysical properties and structural behavior.

The concepts of structural polymeric materials with the ability to autonomically heal cracks were developed by some researchers just recently [1]. According to these concepts, the material incorporated a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is then triggered by contact with an embedded catalyst, bonding the crack face. The proposed concept has the disadvantages, such as, high concentration of embedded capsules, weaker mechanical properties of the new material, and non-compatibility of healing and matrix materials.

Another interesting self-healing system is based on utilization of the electrohydrodynamic coagulation ability of particles to close a defect in materials [2]. An electrical field is applied, and a colloidal dispersion of polystyrene or silica particles were used to repair defects that occur in the material when high stress is applied. When a defect occurs in the insulating coating, underneath metal was exposed to create high current density at the damaged site, causing colloidal particles to coagulate around the defect. The disadvantage of this technology is related to difficulties to detect the cracks and damages at the initial stages.

Currently no technology is known for self-healing in metals and metal based composites. The difficulties are high temperature and high pressure during manufacturing (casting, forging, rolling, stamping, etc.) processes of metallic parts. However, incorporation of healing agent must be accomplished without their premature damage during the manufacturing processes.

The addition of Cr to Fe-Al alloys resulted in a remarkable synergistic effect of a great technological importance. This phenomenon has allowed designing of more ductile alloys and coating composites. Compositional effects on oxidation are summarized on the oxide map (where oxidation data are superposed on the ternar y composition triangle) illustrated in Figure 1. These maps are not thermodynamic diagrams but are based upon kinetic and structural processes which take place during the scale development [3-5]. The following three primary regions of oxidation can be identified: (I) Fe₂O₃ /Fe₃O₄ external scales + Cr₂O₃ /Al₂ O₃ internal scales, (II) Cr₂O₃ /Fe(Cr, Al)₂ O₄ external scale + Al₂ O₃ internal scales and (III) external scale of only Al₂ O₃ . The role of the high chromium content in producing TGO Al₂O₃ scales at such lower aluminum content and then in binary Fe-Al alloys can be described by considering the transient oxidation  phenomena for a Fe-45%Cr-4%Al-0.3%La at 1200^oC shown schematically in Figure 2. The initial oxide scale (Figure 2a) contains all the cations of the alloys surface which is composed of the mixture of the nanocrystallite oxides Cr₂O₃ /Al₂O₃ /Fe₂O₃ (Figure 2b). The subscale formation of Al₂O₃ occurs because it is stable at the low oxygen activity defined by the mixture of Cr₂O₃/ Fe(Cr,Al)2O₃ alloy equilibrium and internal oxidation of Al occurs ahead of this front since Al₂O₃ is stable at even lower oxygen activities here. The high chromium content results in a Cr₂O₃ subscale, which may be continuous (Figure 2c) and defines a lower scale-alloy oxygen activity. It reduces the oxygen diffusion, and curtails internal Al₂O₃ formation. Further growth of Cr₂O₃ /Fe(Ce,Al)₂O₄ will be blocked. Eventually, the Al₂O₃ subscale becomes continuous and its rate will be controlled. Thus, on the surface of the alloy specimen with a Beilby layer at 1200^oC, conditions for scale formation even at the initial stages of the oxidation (when the oxide scale thickness does not exceed a few microns) the barrier scale for cation and anion counter diffusion structure (architecture) will be developed. This finally will lead to the formation of slow growing and adherent (due to the alloying with RE elements) TBC which protects metal matrix against a high temperature corrosion.

Figure 1. Compositional Effects on the Oxidation of Fe-Cr-Al Ternary Alloy: (I) Fe₂O₃ /Fe₃O₄ external scales + Cr₂O3 /Al₂O₃ internal scales, (II) Cr₂O₃ /Fe(Cr,Al)₂O₄ external scale + Al O internal scales, and (III) external scale of only Al₂O₃

Figure 2a corresponds to the transformation of Beilby layer into the oxide layer which is a mixture of nanocrystallites of all element oxides composing the alloy, during the first oxidation minutes at 1200^oC. The following schemes (Figures 2b and 2c) illustrate a possibility of the continuous formation of the TGO out of Al₂O₃ on the investigated alloy in spite of the low Al content (<5%) in it. These figures also demonstrate a mechanism of the self-organized TBC with the self healing potential on the alloy surface at high temperatures. The reservoir under TBC supplies Al (as well as Cr) atoms required for the reproduction of the healing agents on the crack surfaces (in case of their appearance in the coating) in the form of Al₂O₃ particles (as well as of Cr₂O₃ , partially). The formation of TBC on the Fe-45%Cr- 5%Al-0.3%La alloy will allow creating a metal/ metal/ceramics composition with a functionally distributed sandwich architecture characterized with the self healing features similar to that of the wound on the skin. Here it is given the schematic of the structure (architecture) on the substrate which is placed together with the cross section of the skin.

Figure 2. Synergistic Effects of High Chromium Content on theFormation of Al₂O₃ TGO scales during the transient oxidation of Fe-45%Cr-5%Al-0.3%La alloy at 1200^oC after (a) 10 min, (b) 1hr and ( c) 10hrs

Heterogeneous architectural High Temperature (HT) Thermal Barrier Coating (TBC) systems, and more generally, hybrid materials such as sandwiched metal/metal/ceramic TBCs with thermal activated selfhealing functional surfaces have received special attention in applications such as aerospace jet and gas turbine engines. The existing TBCs do not exhibit any selfhealing capabilities compared to the metallic bond coatings. TBC is made out of a ceramic material and it is a most critical part of the coating system for the lifetime of the coating component. During heating cooling cycles the high stresses will be developed due to the mismatch between the thermal expansion coefficients of the substrate and the different layers of the coating system. A life span of the coating system is set by the development of the crack patterns that coalesce and ultimately lead to the failure. An application of the self-healing TBCs are very attractive as a lifetime of such coatings critically determines a time between overhaul and revision of aerospace jet and gas turbine engines for the specific HT applications.

This work presents the possibility to realize the self healing mechanisms for hetero geneous architectural metal/ceramic high temperature sandwich thermal barrier coating systems on the surfaces refractory metals by analogy of wound healing in the skin. A self-healing of skin is a typical example occurring in nature. Its principles and strategies must be understood and adapted for the engineered heterogeneous architectural metal/metal /ceramic coating systems. A skin protection function was used in our studies in modeling and developing heterogeneous architectural composites with desirable combination of creep resistance and corrosive proof  properties at extreme high temperatures (~1200^oC). A structure of architectural metal/metal/ceramic HT coating system is given in Figure 3. Each of these three regions of this composition has the following functions:

Figure 3. Structure of Architectural metal/ metal/ceramic HT Coating System

• the basis provides the HT creep resistance
• the interlayer of the corrosion resistance alloy serves as a reservoir of the scale forming elements necessary for the existence of the thermally activated self-healing cracks
• TGO layer provides a resistance against HT gas corrosion of the metal matrix.

The SEM–BSE micrographs of the cross-section of EB-PVD Fe-Cr-Al-Y overlay coating at various temperatures are shown in Figures 4 and 5.

Figure 4. SEM–BSE Micrographs of the Cross-Section of EB-PVD Fe-Cr-Al-Y overlay coating deposited at 700^oC on a polycrystalline low alloyed Cr specimen before (a and c) and O after (b and d) specimen exposure at 1200^oC for one hour in air, showing the full healing of the coating growth defects in the form of inter-columnar channels

Figure 5. SEM Images and WDS Spectra of the EB-PVD Fe-45% O Cr-1%Ni-4%Al-0.3%La areas deposited at 650^oC of single crystal substrate of Nb (110): (a) the solidified drop of evaporated alloy splashed on the coating surface; (b) higher magnification of the area indicated on (a) which demonstrates the superfine graininess of coating structure; (c) WDS spectrum of the solidified drop; (d) WDS spectrum of the marked area on the coating

The compositions were obtained on the Nb and Mo monocrystals and low alloyed Cr bulk samples coated with Fe-45%Cr-4%Al-1%Ni-0.3%RE (La, Y) alloy using EBPVD technique. SEM, WDS, AES and LM investigations were carried out. It is shown that:

• The refractory metals base metal/metal composition with an overlay coating of heat resistant alloy of ultra-fine Ograin structure after the pre-oxidation at 1200^oC transforms into the combined metal/metal/ceramic compositional material of the sandwich structure as association the multi-layer architecture of hybrid materials with the distributed different functional behaviors.
• The metallic-oxide hybrid layer formed on the surface of synthesized compositions at high working temperatures has an ability to heal the cracks developed as a result of the mechanical and thermal damages.
• Ultra-fine crystalline structure of both, the overlay coating and the self-organizing TGO protective scale on its surface, provide the relaxation of those stresses caused due to the thermal-expansion mismatch during the codeformation of metallic matrix with nano-scale crystalline layers (minimizing the probability for crack initiation) and promoting a smooth transition between three regions having different physical-mechanical behaviors.
• The realization of considered structural architecture of the different materials combination, generally, for the obtained compositional material is expressed in the possible collective effects such as: increasing in the damage tolerance, the ability of cracks for the thermally activated self-healing, and the optimal combination of high values of heat resistance and heat proof.