As-sprayed State Research Articles

Superior toughness of thermal barrier coating caused by co-doping of trivalent and pentavalent oxides

The zirconia-based thermal barrier coating co-doped with trivalent and pentavalent oxides was deposited on DD 90 single crystal superalloy using air plasma spraying. The coating is composed of cubic and tetragonal phases denoted by bi-phase solid solution (BPS) ceramic coating. The microstructure, cross-sectional morphology, and cracks evolution of BPS coatings, before and after 1200 °C thermal cycling, were investigated compared with 8YSZ coating. The BPS coating is intact after 250 thermal cycles, and the phase constitution is consistent with as-sprayed state. On the contrary, the brittle monoclinic phase can be observed in the thermal cycled 8YSZ coating, and severe exfoliation occurred. The propagation and splitting of the cracks are inhibited by the randomly distributed nanocrystalline clusters, and the cracks are inclined to grow longitudinally, which are beneficial to improve the toughness and thermal shock resistance of thermal barrier coating.

Bulk-like ductility of cold spray additively manufactured copper in the as-sprayed state

• As-sprayed CSAM Cu can exhibit a simultaneous high ductility and strength. • This is triggered by a specific topology of bimodal grain size. • The deposits have a high thermal conductivity comparable to bulk. • They are perfectly gas-tight with He leakage rate of less than 1 × 10 −7 mbar•l/s. • All these were achieved with 99% efficiency using inexpensive nitrogen. In recent years, cold spray process is increasingly used for additive manufacturing of metallic components, referred to as cold spray additive manufacturing (CSAM). Unlike the fusion-based AM processes, CSAM is achieved in a solid-state, bringing several advantages such as the absence of severe oxidation or phase composition changes. At the moment, the main limitation of CSAM is the generally low ductility of the as-sprayed deposits. In this paper, using Cu as a model material, we demonstrate a way to overcome this limitation. Importantly, a high ductility of the deposits in their as-sprayed state is achieved without a trade-off in mechanical strength. Furthermore, we show that without any post-heat treatment, the properties of CSAM Cu are comparable to bulk, non-AM Cu.

Retardation of superhydrophobicity for the air plasma sprayed ceria coating surface

Ceria has become a potential candidate for the preparation of superhydrophobic surfaces due to the apparent intrinsic hydrophobicity reported in recent years. In the present study, a CeO2 coating with bi-scale structures imitating lotus leaf surface was deposited by air plasma spraying (APS). It was found that the wettability of the coating surface exhibited a special delayed phenomenon, that the surface needed to be stored in the atmosphere for a period of time before it could change from superhydrophilicity in the fresh as-sprayed state to superhydrophobicity. To elucidate the mechanism, the research focused on analyzing the changes in the chemical composition of the surface. The results showed that adsorption effect was the main factor dominated the surface wettability. During atmospheric storage, the coating surface spontaneously adsorbed low surface free energy substances, mainly carboxylates containing alkane chain and alkanes, resulting in the transition of wettability. The bombardment of argon plasma jet to ceria during APS led to the valence state change of Ce atoms, resulting in the generation of coordinatively unsaturated metal cations acted as active adsorption sites, which was the reason for the spontaneous adsorption. Due to the restrictions on the location of the active sites and the limited concentration of airborne adsorbates, the spontaneous adsorption proceeded slowly, which led to the retardation of superhydrophobicity for the coating surface. The study laid a theoretical foundation for the preparation of CeO2 superhydrophobic coatings by APS, and provided further insights into the underlying mechanism of the apparent hydrophobicity of CeO2.

Oxidation kinetics of atmospheric plasma sprayed environmental barrier coatings

Three different Si/Yb-silicate environmental barrier coating systems (EBCs) were atmospheric plasma sprayed using various spray currents (275, 325, 375 A) for Yb-silicate deposition. The EBCs were thermally cycled between room temperature and 1300 °C up to 1000 h in air. Additionally, bare Si coatings were tested under isothermal and thermal cycling conditions in the as-sprayed state and after polishing at 1300 °C in air. Parabolic oxidation kinetics were observed and oxidation protection provided by Yb-silicate was found to be influenced by the spray conditions, i.e. only at 325 A, Yb-silicate was effectively protecting the bond coat. The controlling mechanism was attributed to densification in the Yb-silicate layer during thermal cycling, which was quantified via image analysis. The surface finish of the Si coating was also found to be influencing the oxidation rate. The TGO was thinner and less cracked on polished APS Si coating in comparison with the as-sprayed Si coating surface.

Tuning the microstructure and mechanical properties of additive manufactured aluminum matrix composites by cold spray

Cold spray (CS) has significant technical advantages in additive manufacturing of aluminum matrix composites. In this study, Al with different content of Al2O3 composite deposits was successfully produced by CS. The effect of Al2O3 content and heat treatment temperature on the microstructure and mechanical properties of the deposits was comprehensively studied. The results show that as the content of Al2O3 increases, the hardness, Ultimate Tensile Strength (UTS), and elongation of the deposits gradually improve in the as-sprayed state. Due to the significant hammering effect of Al2O3 particles on Al particles, there is a local metallurgical bond between adjacent Al particles in Al-50 and Al-75 deposits, and dimples appear in the fracture morphology of the deposit. With the increase of the heat treatment temperature, the diffusion of atoms at the particle-particle interface transforms the bonding between Al particles from mechanical interlocking to metallurgical bonding, so that the deposit exhibits improved ductility. However, heat treatment also eliminates the residual compressive stress and introduces grain growth through recovery and recrystallization processes, resulting in the decrease of hardness and UTS of the deposits.

Wetting Behavior of Wear-Resistant WC-Co-Cr Cermet Coatings Produced by HVOF: The Role of Chemical Composition and Surface Roughness

Despite the existence of several methods for production of superhydrophobic coatings from various materials, their application in harsh environments is still a great challenge. In this work, WC-Co-Cr cermet coatings were prepared by means of high velocity oxy-fuel (HVOF) spraying. WC particles dispersed in Co-Cr metallic matrix allowed to form the multi-scale surface roughness and thus to achieve hydrophobicity of the coatings in the as-sprayed state. The additional surface treatment by the silicone oil rendered the coatings superhydrophobic. The WC-Co-Cr coatings were fabricated from three different powder feedstocks: coarse powder with coarse WC particles, coarse powder with ultrafine WC particles, and fine powder with ultrafine WC particles. The investigation of microstructure, phase composition, and surface topography of produced coatings was conducted to study the influence of these factors on the water contact angle and surface free energy, which were obtained by the sessile droplet method. Theoretical models were used to explain the wetting behavior of all the coatings. Finally, preliminary results of the slurry abrasion response test revealed very good robustness of hydrophobicity of the coatings and also pointed to a need for further research on surface modifications for sacrificial applications.

High Velocity Oxygen Liquid-Fuel (HVOLF) Spraying of WC-Based Coatings for Transport Industrial Applications

In this article, we analyse five types of coatings, in terms of their microstructure, hardness, porosity, and wear resistance, in the as-sprayed state. The coatings are WC-based (WC-FeCrAl, WC-WB-Co, and WC-NiMoCrFeCo), alloy-based (Co-MoCrSi), or nanoWC coating-based (nanoWC-CoCr). Two tests were applied to assess the wear resistance of the coatings: a dry-pot wear test with two impact angles and an abrasive test using an abrasive cloth with two grit sizes. Porosity was determined by image analysis. Vickers impression was performed on cross-sections of the coatings, in order to determine their indentation fracture toughness. The highest hardness of the tested coatings was recorded for the nanoWC coating, followed by the rest of the WC-based coatings; meanwhile, the lowest hardness was recorded for the alloy coating. Minimal porosity was achieved by the alloy coating, due to its different nature and the absence of hard particles with a higher melting point. The NanoWC coating and other WC-based coatings had a higher porosity; however, porosity did not exceed 1% for each coating. The best wear resistance was achieved by the nanoWC coating, followed by the other WC-based coatings, with the lowest obtained by the alloy coating. The same tendency was recorded when determining the indentation fracture toughness. From a microscopic point of view, the structure of the evaluated coatings is not compact; nevertheless, their properties are excellent, and they act as compact coatings under load.

Novel-structured plasma-sprayed thermal barrier coatings with low thermal conductivity, high sintering resistance and high durability

The microstructure of the ceramic topcoat has a great influence on the service performance of thermal barrier coatings (TBCs). In this study, conventional layered-structure TBCs, nanostructured TBCs, and novel-structured TBCs with a unique microstructure were fabricated by air plasma spraying. The relationship between the microstructure and properties of the three different TBCs was analysed. Their thermal insulation ability, sintering resistance, and durability were systematically evaluated. Additionally, their failure modes after being subjected to two kinds of thermal shock tests were analysed. The results revealed that the novel-structured TBCs had remarkably superior performances in all the examined aspects. The thermal conductivity of the novel-structured TBCs was significantly lower than those of the conventional and nanostructured TBCs both in the as-sprayed state and after thermal treatment for 500 h at 1100 °C. The macroscopic elastic modulus of the novel-structured TBCs after sintering at 1300 °C for 100 h was similar to those of the conventional and nanostructured TBCs in the as-sprayed state. During both a burner rig thermal shock test and a furnace cyclic oxidation test, the thermal shock lifetime of the novel-structured TBCs was much longer than those of the conventional and nanostructured TBCs. This study has demonstrated novel-structured plasma-sprayed TBCs with high thermal insulation ability and high durability.

Evolution mechanism of the microstructure and mechanical properties of plasma-sprayed yttria-stabilized hafnia thermal barrier coating at 1400 °C

Yttria stabilized hafnia (Hf0.84Y0.16O1.92, YSH16) coatings were sprayed by atmospheric plasma spraying (APS). The effects of thermal aging at 1400 °C on the microstructures, mechanical properties and thermal conductivity of the coatings were studied. The results show that the as-sprayed coating was composed of the cubic phase, and the nano-sized monoclinic (M) phase was precipitated in the annealed coating. The presence of M phase effectively constrained the sintering of the coating due to its superior sintering-resistance. The Young's modulus kept at a nearly same level of ~78 GPa even after annealing, and the coating annealed for 6 h yielded a maximum value of hardness but revealed a declining tendency in the Vicker's hardness with prolonged sintering time. The thermal conductivity increased from 0.8-0.95 W m-1 K-1 at as-sprayed state to 1.6 W m-1 K-1 after annealing at 1400 °C for 96 h. The dual-phase coating is promising to serve at temperatures above 1400 °C due to its excellent thermal stability and mechanical properties.

Nanostructuring Behavior of NiCrBSi and CoCrWC Thermal Spray Coatings Formed by Temperature-Controlled Laser Heat Treatment

.For surface hardening of a continuous casting mold component, a thermal spray coating of NiCrBSi (Metco-16C) and CoCrWC (Stellite-1) was performed followed by laser heat treatment of the coatings. To support selective modification of the thermal spray coating, a metallurgically determined surface temperature was maintained during the laser heat treatment, by real-time control of the laser power. In other words, nonhomogeneities in the macrosegregation of certain alloying elements, and voids in the as-sprayed state, could be improved. The main microstructural features of the Metco-16C coating laser-heat-treated at 1423 K were nanosized (100–150 nm) Cr5B3, M7C3, and M23C6 precipitates with a lamellar structure of Ni (FCC) and Ni3Si as the matrix phase. Those of the laser heat-treated Stellite- 1 coating at 1473 K were fine (30–250 nm) precipitates of WC, M7C3, and M23C6 based on a Co (FCC) matrix. The results show that laser heat treatment at 1423 K increased the hardness of the Mecto-16C coating to 1115 HV from the as-sprayed state (754 HV), while treatment at 1473 K increased the hardness of the Stellite-1 coating from 680 HV to 860 HV.

Phase Transformation-Induced Changes in Microstructure and Residual Stresses in Thermally Sprayed MnCoFeO4 Protective Coatings

The contribution comprises the investigation of the microstructure and residual stresses in thermally sprayed Mn1.0Co1.9Fe0.1O4.0 (MCF) protective coatings for interconnectors of SOFC stacks, deposited on ferritic steel Crofer 22 APU via atmospheric plasma spraying (APS). The coatings are designated to prevent Cr evaporation during high operation temperature of the SOFCs. The local microstructure, pore distributions and pore shapes, phase fractions, micro-hardness, Youngs’ modulus and residual stresses through the coating thickness were characterized in as-sprayed state and compared with longtime (10-100 h) heat-treated samples (700 and 850 °C). The results show that the long-term thermal aging treatment causes a successive high sintering of the coatings characterized by a reduction in pore density, by phase transformation from the metastable rock salt structure that gradually transformed to a spinel structure and by a slight relaxation of the process-induced tensile residual stresses in the coating. For SOFC application of the MCF coating, this indicates an improvement in the coatings integrity. During operation, a self-repair proceeds leading to dense and gas-proof coatings, while the mechanical properties are mainly retained.

Improve durability of plasma-splayed thermal barrier coatings by decreasing sintering-induced stiffening in ceramic coatings

In this study, a newly-tailored plasma-sprayed (PS) yttria-stabilized zirconia (YSZ) ceramic coating towards enhanced strain tolerance and sintering resistance was developed to improve the durability of TBCs. The thermal shock life was found to be markedly prolonged by more than four times. Failure mechanisms and sintering behavior of the newly-structured and conventional TBCs were systematically investigated through microstructural and mechanical analyses. Conventional TBCs suffered a premature spallation due to rapid sintering-induced stiffening of the ceramic top coat. In contrast, the new coating exhibits an enhanced sintering resistance whereby preserving a good strain tolerance over time. Specifically, its elastic modulus after thermal exposure remains comparable to the as-sprayed states. The effect of ceramic top coat stiffness on cracking behavior of TBCs was clarified by a corresponding cohesive zone finite element modeling. This study provides a new option for improving TBCs durability and the results could benefit the increased integrity of TBCs.

Microstructure and Oxidation Performance of TiAl-(Cr, Nb, Ta) Coatings Fabricated by Warm Spray and High-Velocity Oxy-Fuel Spraying

To improve the oxidation resistance of near α-titanium alloy IMI834, TiAl-(Cr, Nb, Ta) coatings were deposited by applying high-velocity oxy-fuel (HVOF) and warm spray (WS). Comparison was made in terms of microstructure, surface morphology as well as isothermal and cyclic oxidation behaviors in the air at 750 °C up to 100 h and 100 cycles, respectively. The results show that smoother and less oxidized coatings were deposited by warm spraying. The microstructure of all coatings underwent an appreciable change during the oxidation tests, as in as-sprayed state it occurred in the nonequilibrium state. It was revealed that a small difference in the initial oxidation between the two spraying processes as well as microstructure, the level of porosity and surface roughness significantly influences the oxidation kinetics of the sprayed coatings at high temperature, which should affect the service lifetime as an oxidation-resistant layer for potential applications. After exposure at 750 °C in air, rutile TiO2 was found in addition to α-Al2O3 in the oxide scale formed on the HVOF and warm sprayed coatings. However, isothermal and cyclic oxidation tests of all WS TiAl-(Cr, Nb, Ta) coatings showed improved oxidation resistance of IMI 834 as well as good adherence to the substrate alloy.

The numerical analysis of stress and temperature distribution in a double-ceramic-layer (DCL) type of La2Zr2O7/8YSZ thermal barrier coatings (TBC) in as-sprayed state

The numerical analysis of stress and temperature distribution in a double-ceramic-layer (DCL) type of La2Zr2O7/8YSZ thermal barrier coatings (TBC) in as-sprayed state

In-situ Sip/A380 alloy nano/micro composite formation through cold spray additive manufacturing and subsequent hot rolling treatment: Microstructure and mechanical properties

Cold gas dynamic spray (cold spraying, CS) has emerged as a promising solid state manufacturing technique to fabricate thick metallic deposits with limited risk of oxidation, phase transformations and residual thermal stresses. As-sprayed deposits usually exhibit poor mechanical properties and often require post-spray heat treatments to reduce inter-splat defects. In most of the cases, traditional heat treatments offer limited benefits to rejuvenate the mechanical properties of the deposit. This preliminary work presents an effective way to modify the microstructure and improve the mechanical properties of cold sprayed A380 aluminum alloy. As-sprayed alloy deposits were subjected to hot rolling treatments with different thickness reductions. Results suggest that the hot rolled samples show better strength and ductility compared with their as-sprayed state. Microstructural evolution and resulting mechanical properties, at different processing conditions, were analyzed through optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), high resolution transmission electron microscopy (HRTEM) and tensile tests. Sample with thickness reduction of 40% displayed highest values of ultimate strength (UTS, 420 ± 2.1 MPa) and elongation (Elf, 5 ± 2.3%) compared with the corresponding values of as-sprayed (100 ± 13.4 MPa, 0%) and conventionally heat treated (186 ± 17 MPa, 0.93 ± 0.05%) samples. The improvement in mechanical properties of the rolled samples were mainly attributed to the progressive elimination/reduction of inter-splat defects together with in-situ composite microstructure formation. EBSD and HRTEM revealed that in-situ composite structure was formed through progressive refinement and uniform distribution of Si particles in α-Al matrix containing coherent θ′ precipitates.

Variation of Thermal Barrier Coating Lifetime Characteristics with Thermal Durability Evaluation Methods

The thermal durability of thermal barrier coating systems (TBCs) obtained using feedstock powders with different purity and phase content was investigated by thermal shock testing with different cycle times, including the effects on the sintering and phase transformation behaviors. Four 8 wt.% yttria-stabilized zirconia powders, with regular purity (TC1), high purity (TC2 and TC3), and without monoclinic phase (TC4), were employed to prepare the topcoat of TBC by atmospheric plasma spray on a NiCoCrAlY bondcoat deposited by high velocity oxy-fuel. The microstructure and phase stability of the topcoats affected the TBCs’ lifetime in the short-term (1 h) and long-term (24 h) furnace cyclic test (FCT) at 1100 °C and jet engine thermal shock (JETS) test. In the short-term FCT and JETS tests, in which coatings are severely subjected to thermal stress, the TBCs’ lifetime is most affected by the microstructure of the topcoat. The coating layer with the lowest monoclinic phase in the as-sprayed state showed the lowest phase-transformation characteristics in the isothermal oxidation test (1400 °C). These properties resulted in the best lifetime in the long-term FCT. Therefore, the coating material and evaluating methods of TBCs’ life should be selected depending on the usage environment.

Wear Behavior of Plasma Spray Deposited and Post Heat-Treated Hydroxyapatite (HA)-Based Composite Coating on Titanium Alloy (Ti-6Al-4V) Substrate

The present study concerns a detailed evaluation of wear resistance property of plasma spray deposited composite hydroxyapatite (HA)-based (HA-50 wt pct TiO2 and HA-10 wt pct ZrO2) bioactive coatings developed on Ti-6Al-4V substrate and studying the effect of heat treatment on it. Heat treatment of plasma spray deposited samples has been carried out at 650 °C for 2 hours (for HA-50 wt pct TiO2 coating) and at 750 °C for 2 hours (for HA-10 wt pct ZrO2 coating). There is significant deterioration in wear resistance for HA-50 wt pctTiO2 coating and a marginal deterioration in wear resistance for HA-10 wt pct ZrO2 coating in as-sprayed state (as compared to as-received Ti-6Al-4V) which is, however, improved after heat treatment. The coefficient of friction is marginally increased for both HA-50 wt pct TiO2 and HA-10 wt pct ZrO2 coatings in as-sprayed condition as compared to Ti-6Al-4V substrate. However, coefficient of friction is decreased for both HA-50 wt pct TiO2 and HA-10 wt pct ZrO2 coatings after heat-treated condition as compared to Ti-6Al-4V substrate. The maximum improvement in wear resistance property is, however, observed for HA-10 wt pct ZrO2 sample after heat treatment. The mechanism of wear has been investigated.

Microstructural characterization of Ni-based self-fluxing alloy after selective surface-engineering using diode laser

The microstructural characterization of thermal-sprayed Ni-based self-fluxing alloy (Metco-16C®) after laser-assisted homogenization treatment was performed. To this end, a high-power diode laser system was used. This supported the real-time control of the target homogenization temperature at the substrate surface. Non-homogeneities of the macrosegregation of certain elements (C and Cu) and the local concentration of Cr-based carbides and borides in certain regions in the as-sprayed state could be enhanced with the application of homogenization. After homogenization at 1423 K, the hardness of the thermal-sprayed layer was found to have increased by 1280 HV from the as-sprayed state (750 HV). At this homogenization temperature, the microstructure of the thermal-sprayed layer consisted of a lamellar structuring of the matrix phase (austenite and Ni3Si) with fine (<5 μm) carbides and borides (the rod-like phase of Cr5B3, the lumpy phase of M23C6, and the extra-fine phase of M7C3). Despite the formation of several kinds of carbides and borides during homogenization at 1473 K, the lowest hardness level was found to be less than that of the as-sprayed state, because of the liquid-state homogenization treatment without formation of lamellar structuring between austenite and Ni3Si.

Cold spraying SiC/Al metal matrix composites: effects of SiC contents and heat treatment on microstructure, thermophysical and flexural properties

In this paper, cold spray was used as an additive manufacturing method to fabricate 5 mm thick SiC/Al metal matrix composites with various SiC contents. The effects of SiC contents and heat treatment on the microstructure, thermophysical and flexural properties were investigated. Additionally, the composites were characterized for retention of SiC particulates, splat size, surface roughness and the progressive understanding of strengthening, toughening and cracking mechanisms. Mechanical properties were investigated via three-point bending test, thermophysical analysis, and hardness test. In the as-sprayed state, flexural strength increased from 95.3 MPa to 133.5 MPa, an appreciation of 40% as the SiC contents increased, and the main toughening and strengthening mechanisms were zigzag crack propagation and high retention of SiC particulates respectively. In the heat treatment conditions, flexural strength appreciated significantly compared to the as-sprayed condition and this was as a result of coarsening of pure Al splat. Crack branching, crack deflection and interface delamination were considered as the main toughening mechanisms at the heat treatment conditions. Experimental results were consistent with the measured CTE, hardness, porosity and flexural modulus.

Effect of Heat Treatment on Microstructure and Mechanical Properties of A380 Aluminum Alloy Deposited by Cold Spray

The microstructure and mechanical properties of cold-sprayed bulk A380 alloy were investigated after heat treatment at various conditions, using optical and electron microscopy and tensile and hardness tests, respectively. The results revealed that heat treatment increased the strength and ductility of the cold-sprayed A380 alloy deposits compared with as-sprayed state. Heat treatment showed two different effects on the mechanical properties of the deposits. On the one hand, it resulted in effective diffusion at interparticle boundaries that altered the particle bonding mechanism from pure mechanical interlocking to metallurgical bonding. Thus, the strength and ductility of the material were greatly enhanced. On the other hand, interparticle diffusion during high-temperature heat treatment resulted in growth of the Si phase and pores, which ultimately reduced the strength and elongation of the alloy. This observation was consistent with the hardness results, which showed a decreasing trend with increase of the heat treatment temperature.