Achieving enhanced toughness of a nanocomposite coating by lattice distortion at the variable metallic oxide interface

Ceramic coatings are in general a kind of brittle material because they are predominantly made up of ionic crystals that avoid dislocation motion caused by lattice distortion. In this regard, a remarkable toughened ZrO2/MgO nanocomposite coating is obtained by the plasma electrolytic oxidation (PEO) process and in-situ synthesized ZrO2 with quantitative control approach. It is revealed that the toughening behavior of the ZrO2/MgO coating is related to the coordination and diversion of lattice distortion at the metallic oxide interface, which induces distinct dislocation motion at the interface. The semicoherent interface between m-ZrO2 and MgO is verified to act as a buffer to realize toughening of the nanocomposite coating through dislocation slipping induced by lattice coordinated distortion. Simultaneously, significant interfacial lattice distortion transfer and dislocation pinning are discovered at the semicoherent interface between t-ZrO2 and MgO, which are beneficial to toughness enhancement of the nanocomposite coating. The results indicate that the toughening effect occurs along with dislocation slipping and pinning caused by lattice distortion of the ZrO2/MgO semicoherent interface, which enables the toughness of novel nanocomposite coating to reach 2.7 times of the traditional PEO coating.</p

Multiscale exploit the role of copper on the burn resistant behavior of Ti-Cu alloy

“Titanium fire” can cause catastrophic consequences for machinery components made of titanium alloys, especially in the case of aero-engines. As such, there is continuing interests in developing burn resistant titanium alloys. The burn resistant functionality and mechanical strength of titanium alloys rely on their microstructural characteristics that are significantly determined by the alloying components. This work investigated the burn resistant performance of Ti-Cu alloy and analyzed the role of alloying Cu. With sufficient content of Cu in the system, the formation of Cu rich layer cuts off the oxygen transfer pathway and hinders the burning process. Meanwhile, the addition of Cu results in the reduction of chemical kinetics. For the Ti-Cu alloy with insufficient Cu content, the burn resistance is sacrificed by the discontinuous Cu-rich clusters, among which oxygen penetration prolongs the burning process. Apart from the Cu-rich protective layer, various burning processes accompanied with oxidation products are detected via reduced area X-ray diffraction along the burnt cross section. This work provides a comprehensive understanding of the impacts of Cu content on the burn resistant characteristics of Ti-Cu alloy, which offers theoretical foundation of burn resistant mechanism to the design and fabrication of high-performance titanium alloys.</p

One-step plasma electrolytic oxidation with Graphene oxide for Ultra-low porosity Corrosion-resistant TiO2 coatings

The pores formed by molten material ejection from the discharge channel and rapidly solidification during plasma electrolytic oxidation (PEO) act as passageways for corrosive particles. The overall porosity and pore shape, the inevitable features of ceramic coatings, are main factors that determines the corrosion resistance. In this work, we propose a novel approach that utilizes the advantages of graphene oxide (GO) to alter the pore shape and plasma discharge to effectively reduce the overall porosity. Simultaneously, the overall porosity and pore shape were deconstructed by X-ray microscopy. We found that the GO additive not only formed covalent bonds with the metal oxide, limiting the amount and distribution of molten oxide, but also changed the discharge form of the plasma reaction. The obtained coating exhibited an ultra-low surface porosity (1.10%), ultra-low overall porosity (2.11 vol%), and high aspect ratio (0.7–0.8), which are lowered for 94.8%, 90.9%, and increased for 66.9% than that of the traditional coating, respectively. The ultra-low porosity eliminates channels inside the coating and reduces the number of corrosive ions invading the substrate, resulting in superior corrosion resistance.</p

Large lattice mismatch of nanocomposite coating: In-situ establishment of MoS2 by precursor and desulfurization reaction

Constructing large lattice mismatch interfaces, especially two-dimensional materials such as MoS2, is the key to reaching the structural lubricity of heterogeneous structures. Large lattice mismatch prevents the nucleation of reinforcement of composite material, such as in situ synthesized MoS2 during the PEO process, thus affecting the mechanical performance. In this work, a precursor reaction, which can form an amorphous MoS3/TiO2 mixtures, is proposed, and the MoS2/TiO2 noncoherent interface with large mismatch is obtained through solid-state in situ desulfurization of suspended S with the action of reheat. Meanwhile, the precursor reaction is strongly dependent on the sulfur source concentration, which can form the interface with a large number of interfacial dislocations by regulating the concentration of sulfur source. Such edge pining noncoherent interface enables more dislocation and distortion near interface during scratch, which reduces the twist-angle dependence thus further improving the tribological properties. Additionally, these results render the herein presented amorphous precursor reaction strategy generally applicable for exhibiting broad application of in situ synthesis of reinforcement.</p

氧化石墨烯调控GO/TiO2陶瓷膜层自封孔效应研究 (Study on Self-Sealing Pore Effect of GO/TiO2 Coating Controlled by Graphene Oxide)

Self-sealing pore is one of the important technologies to control the pore structure, improve the antifriction and corrosion resistance of micro-arc oxidation coating. In order to solve the problem that the poorer stability of physical sealing pore and expansion of sealing agent impacted the structure of the coating, this paper used the conductivity properties of graphene oxide to prepare GO/TiO2 self-sealing pore ceramic coatings with antifriction effect. The effect of graphene oxide concentration on pore structure and antifriction of ceramic coating was discussed. It is found that the electrochemical balance process of electrolyte is changed by adding graphene oxide, which contributes to controlling the pore structure of the GO/TiO2 coating. When graphene oxide concentration is 5 g/L, the porosity, pore size and average friction coefficient of the self-sealing pore ceramic coating (G5) are 3.6%, 2.5 μm and 0.1, which decrease by 83.2%,78.4% and 87.5%, respectively, compared with the G0 coating. It is believed that the pore structure of micro-arc oxidation coating can be controlled through controlling graphene oxide concentration, which can affect the colloidal deposition and energy release. This provides a new idea for the preparation of antifriction self-sealing pore coating.</p

Morphological evolution of Ti2Cu in Ti-13Cu-Al alloy after cooling from semi-solid state

The mechanical performance of Ti-13Cu-Al alloy is highly determined by its microstructure, which is significantly affected by the cooling process from semi-solid state as it induces peritectic solidification and the subsequent eutectoid transformation of the alloy. This work investigated the effects of cooling rates on the morphology of Ti2Cu phase and mechanical properties of Ti-13Cu-Al alloy after cooling from semi-solid state (β+L phase). Different cooling mediums have been considered, including furnace cooling (0.5 °C/s), air cooling (50 °C/s) and water cooling (150 °C/s), corresponding to a slow, normal and rapid cooling process, respectively. It is found that α+Ti2Cu phases are uniformly distributed in the alloy after different cooling processes, and the morphology of Ti2Cu phase is controlled by the peritectic solidification and eutectoid transformation. Specifically, the thickness and the volume fraction of primary Ti2Cu phase are larger than the sample cooling from β phase field when the cooling rate is the same. The morphology of Ti2Cu phase varies from “laths + blocky + sheaves” to “short laths + blocky + rodlike” as the cooling rate gradually increases. It is further revealed that the β/L interface provides a fast channel for solute atom diffusion, and the peritectic Ti2Cu phase provides a favorable nucleation site for the precipitation of eutectoid Ti2Cu. Based on the tensile property and hardness examinations, the mechanical properties of the Ti-13Cu-Al alloy show a clear dependency with the precipitation of Ti2Cu phase after cooling from semi-solid state, indicating the importance of cooling mode from semi-solid state on the microstructure and mechanical properties of the alloy.</p

Controllable in situ fabrication of self-lubricating nanocomposite coating for light alloys

Intrinsic friction and corrosion issues of light alloys impede their application in tribological and corrosive environments, thus a proper ceramic coating is normally required to overcome these issues for practical engineering deployment. Here, we developed a new approach to fabricate controllable and self-lubricating nanocomposite coating by combining in-situ synthesis of MoS2 and plasma electrolytic oxidation (PEO) process. It is found that the coefficient of friction of this new coating is only about a quarter of the coatings obtained by traditional PEO process due to the self-lubricating characteristic of gradient MoS2. More importantly, this ceramic coating exhibits excellent interfacial strengthen through edge-pinning by noncoherent, which endows excellent tribological and adhesive properties. This facile technique provides a new strategy to fabricate self-lubricating ceramic coating for light alloys, and is believed to have great potential applications in wide engineering sectors and open new avenues for designing novel alloy systems for extreme conditions.</p