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ORIGINAL REPORTS

Promoting crack self-healing of nanocomposite coating by double slip systemic semi-coherent interface dislocation

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Pages 467-476 | Received 23 Jan 2024, Published online: 08 May 2024

Abstract

A prominent strengthened Y2O3 stabilized t-ZrO2 (YSTZ)/MgO nanocomposite coating is achieved by the plasma electrolytic oxidation (PEO) process and in-situ synthesized YSTZ reinforced phase with a quantitative control approach. The idea of activating double slip systemic semi-coherent interface dislocations in YSTZ/MgO nanocomposite coating to realize crack self-healing is proposed. High dislocation densities are associated with {101} < 101> YSTZ slip and {111} < 101> MgO slip system to coordinate interfacial deformation to stop crack initiation and propagation. This crack propagation path can absorb more fracture energy, providing more opportunities for crack deflection and bridge, which closes crack and realizes crack self-healing.

IMPACT STATEMENT

This paper reveals semi-coherent interface dislocation of double slip systems in nanocomposite coating stop crack initiation and propagation is proposed to realize crack self-healing.

1. Introduction

Ceramics are inherently characterized by brittleness, particularly susceptible to crack initiation during the in-situ synthesis of plasma electrolytic oxidation (PEO) ceramic coating [Citation1,Citation2]. It is widely acknowledged that achieving ceramic crack self-healing based on dislocations is not a feasible proposition [Citation3]. In contrast, the metal can undergo continuous plastic deformation dominated by dislocation after yield strength, with almost no visible crack initiation and propagation [Citation4]. Although a large number of single-crystal ceramics can be deformed by dislocation motion at room temperature, dislocations have little to do with cracks in ceramics [Citation5]. Recent investigations have predominantly focused on elucidating the mechanism of ceramic toughening to mitigate crack propagation during deformation [Citation6,Citation7]. These investigations centered on avoiding a significant crack propagation path that occurs after the crack tip from the applied load. This poses a dilemma in that the ceramics still have the source of crack initiation, and only limited load-carrying capacity and low crack tolerance can be obtained during subsequent deformation.

Recent work has proposed strategies for designing crack self-healing whose inherent dislocations can not only stop cracks but can result in crack healing [Citation8]. Indeed, dislocations in many ceramic materials exhibit mobility even at low temperatures, as evidenced by indentation [Citation9,Citation10]. Once dislocations become mobile, the material's capacity to nucleate dislocations at a crack tip is considered a prerequisite for crack self-healing [Citation11]. While dislocation nucleation at crack tips readily occurs in metals, ensuring the supply of dislocations where needed, this happens to a very limited extent in ceramics [Citation12]. This has contributed to the widespread belief that dislocations do not toughen ceramics, particularly ionic crystal ceramics, which do not allow intergranular cracks to heal due to their high bonding strength and inability to form a dislocation nucleus at the crack tip [Citation13].

The ZrO2 phase, which is commonly employed to fortify ceramic materials, expresses signs of the opposite [Citation14,Citation15]. The two-slip interface between ZrO2 and the ceramic oxide phase displays dislocation activity, especially in the substitutional solid solution formed by Y2O3-stabilized t-ZrO2 (YSTZ) crystal structure, with dislocation pinning occurring at the phase interface [Citation16,Citation17]. Previously, we described that (−112) m-ZrO2 // (111) MgO and (101) t-ZrO2 // (111) MgO semi-coherent interface dislocation adjust interface deformation, and the crack propagation was blocked [Citation18]. This property makes it possible to improve the crack propagation path of the ceramic coating through dislocation [Citation19]. Compared with the dislocation motion at the ZrO2/MgO semi-coherent interface, the continuous dislocation pinning at the special orientations YSTZ/MgO semi-coherent interface is more beneficial to inhibit the crack propagation of ceramic coating [Citation20]. Further investigation by Zhang et al. utilizing atomic simulations explained that the stop of crack propagation in YSTZ ceramics primarily arises from the hindrance of mobile dislocation and dislocation migration [Citation21]. The achievement of crack self-healing is feasible by modulating dislocation densities (DDs) at the YSTZ interface through dislocation pinning and migration to accommodate interface deformation [Citation22,Citation23]. If successful, this approach would offer a promising means to enhance ceramics and significantly broaden their range of applications, particularly in preventing the formation and propagation of cracks.

Motivated by this perspective, this work shows that double slip systemic semi-coherent interface dislocation not only stops cracks in coating but also causes cracks to heal themselves through crack bridging and deflection [Citation24]. The crack self-healing is not contingent on mobile dislocation but is closely related to the existing dislocation at the semi-coherent interface [Citation8]. The apparent interfacial dislocation interactions and pinning allow us to smoothly predict dislocation strengthening from high DDs. Consequently, achieving the controlled generation of interfacial high DDs for promoting crack self-healing in YSTZ/MgO nanocomposite coating by the activated two slip systemic dislocations to coordinate interface deformation.

2. Experiment

AZ91 magnesium alloy was rolled by continuous casting with Al: 8.5∼9.5%, Zn: 0.45∼0.9%, Mn: 0.17∼0.4%, Si ≤ 0.05%, Cu ≤ 0.025%, Ni ≤ 0.001%, Fe ≤ 0.004% and marginal Mg (all in wt.%). The specimen size of the substrate was 5 mm ×18 mm x 18 mm, polished until the specimen surface was free of scratches. The polished specimen was degreased with alcohol, cleaned with acetone, and then dried before use.

The main components of Na3PO4, KOH, (NH4)2HPO4, K2ZrF6, and deionized water were selected to form the electrolysis system. On this basis, Y(NO3)3 was added to establish an electrolyte system with four concentration gradients (Without Y3+, 0.4 mol%/L Y3+, 0.8 mol%/L Y3+ and 1.2 mol%/L Y3+), and YSTZ/MgO nanocomposite coating was in-situ synthesized by PEO process. To elucidate the effect of in-situ synthesized YSTZ on improving crack self-healing of YSTZ/MgO nanocomposite coating, the coating prepared by the electrolyte system without Y3+ was called ‘PEO coating’. The PEO power supply and device were used to treat the pretreated AZ91 specimens. The bipolar pulse power supply was 20 kW, the frequency was 500 Hz, the duty cycle was 15%, the current density was 0.2-0.4 A/dm2, and the oxidation time was 10–15 min.

To analyze the dislocations and distortions of atoms at the interface between t-ZrO2/MgO and YSTZ/MgO grains, molecular dynamics (MD) simulation models of t-ZrO2/MgO and YSTZ/MgO were established respectively (see supplementary Figure S1) [Citation21,Citation25]. The three-dimensional model of the two-phase semi-coherent interface was about 78.09Å × 78.09Å × 40.13 Å, containing 22400 atoms, the three-dimensional model of the two-phase interface was about 100Å × 90Å × 105 Å, containing 114,292 atoms. At the same time, for accurately analyzing the crack self-healing of the YSTZ/MgO semi-coherent interface, a three-dimensional model of the (101) YSTZ lattice plane and the (111) MgO lattice plane was established, the model was about 50Å × 50Å × 50 Å, containing 16,768 atoms. A tetragonal zirconia single crystal stabilized by 8.0 mol% Y2O3 was chosen as the standard specimen in this study [Citation26]. To build the atomic model of YSTZ, Zr4+ cations are randomly substituted by Y3+ ions. The substitution of Zr4+ by Y3+ caused the formation of oxygen vacancies to maintain electrical neutrality, which was equal to half of Y3+ ions. The MD simulations were performed by the open-source package LAMMPS [Citation27]. The atomic interactions between Zr4+, O2- and Y3+, Mg2+, and O2- were described by the Buckingham potential function (Buckingham, 1938) [Citation28]. The atomic interactions between Mg2+ and Y3+ were described by the modified embedded-atom method (MEAM) [Citation29]. In addition to the above potential function, the classical 12–6 Lennard-Jones (LJ) potentials were applied to all atomic interactions. The time step of the simulation was set as 0.2 femtoseconds (fs) with periodic boundary conditions applied along all directions. Energy minimization was carried out first, and then the sample was relaxed under the isothermal-isobaric (NPT) ensemble for 40 picoseconds (ps) with zero pressure. The Nose Hoover thermostat was used to maintain the temperature at a low temperature of 10 K, which is a common method for studying the mechanical properties of nanomaterials. When MD simulations were used to evaluate mechanical properties, the loading process was displacement-controlled by applying compressive displacements to the bottom and top atoms. The strain rate in the simulation was set at 5 × 10−7 fs−1, which is slow enough for MD simulations based on our previous research. In addition, a 10 times slower strain rate was selected to test the strain rate effect. The simulation result showed that the mechanical properties of the three-dimensional model such as Young's modulus, strength, yield strain, and dominant deformation mechanism were not affected by the change of strain rate, indicating that the current strain rate of 5 × 10−7 fs−1 was slow enough to produce reliable simulation results [Citation30]. Open Visualization Tool (OVITO) software was used to analyze the results, and the atomic Von Mises local shear strain was tracked to visualize the deformation process [Citation31]. The dislocations in the simulation model were identified by the Dislocation Extraction Algorithm (DXA) and recorded via a Python plugin to calculate dislocation densities (DDs). Due to errors in the thermal fluctuation MD results, the Python processed dislocation data were analyzed using PASW Statistics 18 software, one-way ANOVA was used to determine the level of significance, and post hoc comparisons were made using the Student–Newman–Keuls methodology to determine group-specific differences, which indicates the effectiveness of the simulation result with lower DDs error. It was worth mentioning that the error of crack propagation data of YSTZ/MgO nanocomposite coating was also analyzed by the method.

3. Results

Figure  illustrates the variance in crack distribution within the coating, prepared using different Y3+ ions electrolytes, the raw SEM data of which are detailed in the Supporting Material (see supplementary Figures S2 and S3). Specifically, Figure (a–d) visually presents the surface cracks, while Figure (e–h) delineates the propagation of 3D cracks in the coating by using X-ray microscopy technology (Xradia 610 Versa) [Citation32]. Notably, the PEO coating fabricated without Y3+ ions exhibits evident crack propagation on the surface, whereas the coating prepared with Y3+ ionic electrolyte manifests crack deflection and bridging characteristics (Figure (a–d)) [Citation7]. In contrast to the initially conspicuous penetrating crack, the introduction of Y3+ results in the gradual attenuation of the penetrating crack, confining crack propagation. This results in a significant reduction in surface crack density and 3D crack volume fraction of the PEO coating (Figure (i–j)). However, in electrolyte systems with higher concentrations, the increased solution conductivity inversely leads to a decrease in voltage [Citation33]. In such high-concentration electrolytes, micro-arcs generated at low voltage disrupt the densification of the coating, causing a slight increase in crack density and 3D crack volume.

Figure 1. Crack in YSTZ/MgO nanocomposite coating: (a-d) Surface crack characteristics and (e-h) Internal 3D crack characteristics of YSTZ/MgO nanocomposite coating prepared by electrolytes with different Y3+ content (Without Y3+, 0.4 mol%/L Y3+, 0.8 mol%/L Y3+, and 1.2 mol%/L Y3+), (i) Surface crack density, (j) 3D crack volume fraction.

Figure 1. Crack in YSTZ/MgO nanocomposite coating: (a-d) Surface crack characteristics and (e-h) Internal 3D crack characteristics of YSTZ/MgO nanocomposite coating prepared by electrolytes with different Y3+ content (Without Y3+, 0.4 mol%/L Y3+, 0.8 mol%/L Y3+, and 1.2 mol%/L Y3+), (i) Surface crack density, (j) 3D crack volume fraction.

Element distribution analysis indicated that the overlapping positions of O, Zr, and Y elements, combined with the crystal plane spacing of 0.295 nm corresponding to the crystal plane index (101), provided evidence for the presence of a YSTZ solid solution in the nanosized coating (Figure (a–d,f)) [Citation34]. Figure (e) confirms the particle size distribution in the coating, which measures approximately 25 nm, further confirming the existence of YSTZ nanoparticles, where cracks present near them are deflected and bridged. Compared to the depolymerization and dissociation that occurs at the MgO/MgO interface, Figure (f–g) displays the YSTZ/MgO semi-coherent interface structure have higher interface cohesion, and the YSTZ/MgO semi-coherent interface lattice distortion induces dislocation to promotes interface interaction, which inhibits crack initiation, while the concomitant emergence of shear stresses bypasses the dislocation node to appear as crack deflect and bridge [Citation3,Citation35]. The Geometric Phase Analysis (GPA) map corresponding to the digital micrograph calculation in Figure (h) depicts the YSTZ/MgO semi-coherent interface strain field at the nanoscale [Citation36]. The strain distribution in the MgO region exhibits a decrease in the green area compared to the YSTZ region, a significant increase in the red area, and a reduction in local strain in the nanocomposite coating after introducing the reinforcing YSTZ phase. This implies that dislocations at the YSTZ/MgO semi-coherent interface enhance the intergranular cracking resistance of ionic crystals and improve interface stability [Citation37].

Figure 2. YSTZ nanoparticles distribution in the nanocomposite coating: (a) Dark-field image of the FIB sample, (b–d) Distribution characteristics of O, Zr, and Y elements in YSTZ/MgO nanocomposite coating, (e) Nanoparticle and crack in YSTZ/MgO nanocomposite coating, (f) Semi-coherent interface structure in YSTZ/MgO nanocomposite coating, (g) IFFT micrograph for the corresponding areas marked in (f), (h) GPA mapping calculated by Digital Micrograph for the corresponding areas marked in (g), showing an overall strain field at nanoscale. The color scale represents the change in strain intensity from 0 (Purple) to 0.4% (Red).

Figure 2. YSTZ nanoparticles distribution in the nanocomposite coating: (a) Dark-field image of the FIB sample, (b–d) Distribution characteristics of O, Zr, and Y elements in YSTZ/MgO nanocomposite coating, (e) Nanoparticle and crack in YSTZ/MgO nanocomposite coating, (f) Semi-coherent interface structure in YSTZ/MgO nanocomposite coating, (g) IFFT micrograph for the corresponding areas marked in (f), (h) GPA mapping calculated by Digital Micrograph for the corresponding areas marked in (g), showing an overall strain field at nanoscale. The color scale represents the change in strain intensity from 0 (Purple) to 0.4% (Red).

The semi-coherent interface strain of the reinforcing phase and matrix phase is investigated in the coating under shear stress by MD simulation [Citation21]. As depicted in Figure (a), in the absence of Y3+ ions stabilizing the t-ZrO2 crystalline structure, distinct localized red strain regions appear in the unstable t-ZrO2/MgO semi-coherent within the PEO coating. Conversely, when YSTZ/MgO semi-coherent form through the replacement of Zr4+ ions with Y3+ ions, no obvious local strain region occurs, with only a weak strain apparent in the MgO (Figure (b)). The stable YSTZ/MgO semi-coherent primarily accounts for the strengthening of nanosized coating. Subsequently, the shear strain process is simulated at the interface of the two heterogeneous phases. The t-ZrO2/MgO semi-coherent interface reveals an increase in localized red strain regions with increasing strain (Figure (c-c2)). This signifies a decrease in interfacial stability, particularly when strain is induced at the phase boundary junction, the interface is destabilized, and the interface fracture phenomenon occurs, which is the essence of the cracking of ionic crystals along the crystals [Citation8]. Simultaneously, the homogeneous interface of the matrix phase in PEO coating is susceptible to fracture, heightening the likelihood of cracking during melt-cooling and solidification (Figure (c-c2 and d-d2)) [Citation38]. On the contrary, the strain occurs at the YSTZ/MgO semi-coherent interface without a clear red localized strain region, and the interface remains stable without interface fracture (Figure (d-d2)). With the increase of strain, the coarsening of the YSTZ/MgO semi-coherent interface becomes more pronounced, particularly near the phase boundary junction (Figure (d2)). Thus, the reinforced and roughened semi-coherent interface can be designed to ameliorate the crack of the ceramic coating, thus stopping the cracks from expanding within the coating [Citation8,Citation39].

Figure 3. MD simulation for semi-coherent interface and phase interface in the coating: (a-a3) Strain distribution under shear stress at the t-ZrO2/MgO semi-coherent interface, (b-b3) Strain distribution under shear stress at the YSTZ/MgO semi-coherent interface, (c-c2) Phase interface strain distribution under shear stress in the PEO coating, (d-d2) Phase interface strain distribution under shear stress in the YSTZ/MgO nanocomposite coating.

Figure 3. MD simulation for semi-coherent interface and phase interface in the coating: (a-a3) Strain distribution under shear stress at the t-ZrO2/MgO semi-coherent interface, (b-b3) Strain distribution under shear stress at the YSTZ/MgO semi-coherent interface, (c-c2) Phase interface strain distribution under shear stress in the PEO coating, (d-d2) Phase interface strain distribution under shear stress in the YSTZ/MgO nanocomposite coating.

In addition, the MD model in Figure (c–d) further elucidates the dislocation evolution at the semi-coherent interface, revealing pronounced dislocation line tangles at the YSTZ/MgO semi-coherent interface (see supplementary Figure S4) [Citation40]. Both top-view and front-view observations display severe clogging and entanglement of dislocation lines at the YSTZ/MgO semi-coherent compared to the singular dislocation line motion at the t-ZrO2/MgO semi-coherent (Figure (a–d)). The dislocation line entanglement and blockage that occurs at the YSTZ/MgO semi-coherent interface are caused by the diffusion and migration of interfacial mismatched atoms [Citation35]. The atomic diffusion and migration present near the YSTZ/MgO semi-coherent interface ensures a sustained interfacial lattice distortion. To present the sustained lattice distortion degree of the phase in the coating, the radial distribution function (RDF) g(r) for MgO, t-ZrO2, and YSTZ is calculated, as shown in Figure (e) [Citation41]. The averaged lattice distortion can be evaluated by the full width at half maximum of g(r) at the first nearest neighbor shell [Citation41]. MgO and t-ZrO2 crystal structures are more likely to maintain their original structure with less lattice distortion, while the YSTZ crystal structure is prone to greater lattice distortion under similar conditions, inducing more extensive interfacial dislocation proliferation. Subsequently, in Figure (f), the intrinsic and proliferative DDs in the coating are calculated by the dislocation length (see supplementary Figure S5), where the DDs error is within the allowable range [Citation42]. In the YSTZ/MgO nanosized coating, the DDs increase significantly, where the {101} YSTZ // {111} MgO interface DDs are close to the total DDs (see supplementary Figure S6). It is confirmed that the proliferative dislocation on the {101} YSTZ // {111} MgO slip plane under shear stress plays a dominant role in the process of enhancing and coarsening the interface.

Figure 4. The dislocation evolution in the two simulation models with the different semi-coherent interface: (a) Top-view angle: t-ZrO2/MgO semi-coherent interface dislocation configuration, (b) Top-view angle: YSTZ/MgO semi-coherent interface dislocation configuration, (c) Front-view angle: t-ZrO2/MgO semi-coherent interface dislocation configuration, (d) Front-view angle: YSTZ/MgO semi-coherent interface dislocation configuration, (e) Radial distribution function g(r) for MgO, t-ZrO2 and YSTZ in the coating, (f) Calculated DDs in the two simulation models (Without Y3+ in the PEO electrolyte and 0.8mol%/L Y3+ in the PEO electrolyte).

Figure 4. The dislocation evolution in the two simulation models with the different semi-coherent interface: (a) Top-view angle: t-ZrO2/MgO semi-coherent interface dislocation configuration, (b) Top-view angle: YSTZ/MgO semi-coherent interface dislocation configuration, (c) Front-view angle: t-ZrO2/MgO semi-coherent interface dislocation configuration, (d) Front-view angle: YSTZ/MgO semi-coherent interface dislocation configuration, (e) Radial distribution function g(r) for MgO, t-ZrO2 and YSTZ in the coating, (f) Calculated DDs in the two simulation models (Without Y3+ in the PEO electrolyte and 0.8mol%/L Y3+ in the PEO electrolyte).

4. Discussion

Y2O3 and t-ZrO2 are in-situ synthesized by the PEO process with the addition of Y3+ ions in the zirconium salt electrolyte system. Since the radius of Zr4+ ions (0.082 nm) is close to that of Y3+ ions (0.096 nm), Y3+ ions displace the Zr4+ ions to take up the positions of the lattice nodes, enhanced ionic bonding force maintains the t-ZrO2 crystal structure and inhibits lattice torsion, playing a role in stabilizing the t-ZrO2 crystal structure, forming YSTZ substitutional solid solution and constructing YSTZ/MgO semi-coherent interface structure in the coating [Citation43]. Figure (a) displays the (101) YSTZ // (111) MgO slip plane dislocations, revealing significant lattice distortions near the (101) YSTZ // (111) MgO semi-coherent interface, promoting dislocation slip (see supplementary Figure S7) [Citation44]. Dislocations on the (101) YSTZ // (111) MgO slip plane primarily slip along the [−101] direction, with dislocation interaction and pinning occurring at the interface, which increases the interfacial DDs in the coating (Figure (b)) [Citation45]. As shown in Figure (c), the GPA shows the (101) YSTZ // (111) MgO interfacial regional strain field at the nanoscale [Citation36]. The GPA mapping revealed that more dislocations and distortions gathered at the (101) YSTZ // (111) MgO semi-coherent under the action of shear force, indicating that high DDs coordinate localized strains at the (101) YSTZ // (111) MgO interface. The interface atomic motion of the (101) YSTZ // (111) MgO slip plane along the [−101] direction under shear stress by MD simulation (Figure (d–f)) [Citation21,Citation46]. In the straining process, the atoms on the {101} YSTZ slip plane and the {111} MgO slip plane atom move, and obvious atomic diffusion and migration occur, which obvious interfacial lattice distortion. In addition, the semi-coherent property of the YSTZ // MgO interface promotes the migration of lattice distortion at the interface, i.e. the dislocation source [Citation35]. To verify the activation of the interfacial dislocation slip system in the coating, Figure (g) presents the Schmidt factors (SFs) for {101} < 101> YSTZ slip and {111} < 101> MgO slip (see supplementary Figure S8) [Citation4]. The average SFs for the {101} < 101> YSTZ slip and {111} < 101> MgO slip are calculated (Figure (h)). It is clear that the average SFs for two slip systems are distributed in the range of 0.3-0.5, and that dual slip systems are easily activated [Citation4]. It is noteworthy that the average SFs of the {101} YSTZ slip plane is slightly larger than that of the {111} MgO slip plane, which suggests that the {101} YSTZ slip plane is more susceptible to dislocation motion along the <101 > direction. This provides evidence that the {101} < 101> YSTZ slip and {111} < 101> MgO slip interface atomic migration induces dislocation pinning to coordinate interfacial deformation, mitigating crack source initiation (Figure (i)).

Figure 5. Mechanism of crack self-healing: (a-b) IFFT micrograph for the corresponding areas marked in (Figure S6), (c) GPA mapping calculated by Digital Micrograph for the corresponding areas marked in (b), showing an overall strain field at nanoscale, (d-f) Atomic motions of the (101) [−101]YSTZ slip plane and (111) [−101] MgO slip plane under shear stress, (g) SFs of {101} < 101 > YSTZ slip and {111} < 101> MgO slip, (h) Average SFs for {101} < 101 > YSTZ slip and {111} < 101> MgO slip, (i) The interface dislocation schematic diagrams of {101} < 101 > YSTZ slip and {111} < 101> MgO slip.

Figure 5. Mechanism of crack self-healing: (a-b) IFFT micrograph for the corresponding areas marked in (Figure S6), (c) GPA mapping calculated by Digital Micrograph for the corresponding areas marked in (b), showing an overall strain field at nanoscale, (d-f) Atomic motions of the (101) [−101]YSTZ slip plane and (111) [−101] MgO slip plane under shear stress, (g) SFs of {101} < 101 > YSTZ slip and {111} < 101> MgO slip, (h) Average SFs for {101} < 101 > YSTZ slip and {111} < 101> MgO slip, (i) The interface dislocation schematic diagrams of {101} < 101 > YSTZ slip and {111} < 101> MgO slip.

Consequently, during coating formation, cracks in PEO coating expand and merge to form more complete crack propagation [Citation7]. In contrast, in YSTZ/MgO nanocomposite coating, cracks start to spread in different directions [Citation3]. The in-situ synthesized YSTZ effectively inhibits cracks and disperses the energy extension at the crack tip during the coating formation process, stopping the emergence and propagation of cracks in the coating [Citation3,Citation47]. Migration and diffusion of interfacial atoms along the <101 > direction induced dislocation interaction and pinning at the YSTZ/MgO semi-coherent interface, while the concomitant emergence of shear stresses bypasses the dislocation node to appear as crack deflect and bridge [Citation22]. The semi-coherent interface dislocation pinning of {101} < 101> YSTZ slip and {111} < 101> MgO slip is beneficial to crack closure [Citation47]. For YSTZ/MgO nanocomposite coating, the decrease in crack density and 3D crack volume fraction by 70% and 60% (Figure (i,j)), respectively, is primarily attributed to the increased interface DDs in the coating by activating dislocation. Therefore, the macroscopic experimental results regarding cracking characteristics can be elucidated by the reinforcement of special orientations YSTZ/MgO semi-coherent dislocations at the microscopic level [Citation48]. The mobile dislocation also existed in the PEO coating but with a lower effect than in the YSTZ/MgO coating [Citation15]. The interfacial DDs of {101} < 101> YSTZ slip and {111} < 101> MgO slip in YSTZ/MgO nanocomposite coating are approximately 2 times in PEO coating. High DDs are more likely to interact with crack tips, coordinating the interface deformation caused by dislocation interactions and pinning, providing more opportunities for deflection and bridging. Such a crack propagation path can absorb more fracture energy, which closes the crack and achieves crack self-healing in ceramic coating [Citation46].

5. Conclusions

We introduce the idea of strengthening ceramic coating by designing high DDs of two slip systems interface with special orientations and thus crack self-healing. The identified double slip systems, {101} < 101> YSTZ slip and {111} < 101> MgO slip, activate interfacial dislocations, raising DDs to coordinate interfacial deformation. This is verified by MD simulations that the continuous lattice distortion maintained by the atomic diffusion and migration activates the dislocation of the slip system at a semi-coherent interface. The results demonstrate a 70% decrease in crack density and a 60% decrease in 3D crack volume fraction due to the high interfacial DDs of the two specific slip systems. Hence, it is crucial to in-situ synthesize YSTZ by the PEO process and quantitatively control methods to efficiently introduce high dislocation densities at semi-coherent interfaces. This has the potential to greatly enhance the phase interface strength of ceramics, stop crack propagation, and achieve crack self-healing.

Supplemental material

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Disclosure statement

No potential conflict of interest was reported by the author(s).

CRediT authorship contribution statement

Conceptualization: Zhen Zhang and Bingkun Ning; funding acquisition: Yongnan Chen and Shaopeng Wang; methodology: Weifeng Qian, Shuang Wang and Nan Wang; writing-original draft: Zhen Zhang; writing-review and editing: Yao Li, Qinyang Zhao and Hongzhan Li.

Additional information

Funding

This study was supported by National Natural Science Foundation of China (52271051), Technology Major Project of Shaanxi Province (2020-zdzx04-01-02), National Key Laboratory Foundation of Science and Technology on Materials under Shock and Impact (6142902220202), Special project for development of key industrial of Shaanxi Province (2112-610162-04-02-405567) and QINCHUANGYUAN PLATFORM Team Construction Project (2023KXJ-272).

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