•Nano REOs increased the integral work function of hardfacing alloys.•The friction coefficients had no direct relation to the integral work functions.•The hardfacing alloy having high integral work function had low wear rate.•Work function can be used to explain the wear behavior of hardfacing alloys.In this paper, work function and dry sliding wear behavior of the hardfacing alloys with different content of nano rare earth oxides (REOs) were studied. The microstructural changes of the hardfacing alloys with and without nano REOs were observed. The integral work function of hardfacing alloys was measured by Scanning Kelvin Probe. The individual work function of the constituent phases was indirectly measured and compared by Atomic Force Microscope. Pin-on-disc wear testing machine was used to conduct dry sliding wear tests. The results showed that nano REOs changed the volume fractions of primary carbide, eutectic carbide and the matrix. Nano REOs increased the integral work function of hardfacing alloys by increasing the volume fraction of primary carbide with high work function. The work functions of primary carbide and eutectic carbide were similar, which were higher than that of the matrix. Nano REOs improved the wear resistance of hardfacing alloys. The variation trends of friction coefficient and integral work function with the content of nano REOs were reversed. The variation trends of wear rate and integral work function with the content of nano REOs were also reversed. The hardfacing alloy having higher integral work function had better wear resistance.

In this paper, the effect of nano-Si3N4 additives and plasma treatment on the wear behavior of Al2O3-8YSZ ceramic coatings was studied. Nano-Al2O3, nano-8YSZ (8 wt.% Y2O3-stabilized ZrO2) and nano-Si3N4 powders were used as raw materials to fabricate four types of sprayable feedstocks. Plasma treatment was used to improve the properties of the feedstocks. The surface morphologies of the ceramic coatings were observed. The mechanical properties of the ceramic coatings were measured. The dry sliding wear behavior of the Al2O3-8YSZ coatings with and without Si3N4 additives was studied. Nano-Si3N4 additives and plasma treatment can improve the morphologies of the coatings by prohibiting the initiation of micro-cracks and reducing the unmelted particles. The hardness and bonding strength of AZSP (Al2O3-18 wt.% 8YSZ-10 wt.% Si3N4-plasma treatment) coating increased by 79.2 and 44% compared to those of AZ (Al2O3-20 wt.% 8YSZ) coating. The porosity of AZSP coating decreased by 85.4% compared to that of AZ coating. The wear test results showed that the addition of nano-Si3N4 and plasma treatment could improve the wear resistance of Al2O3-8YSZ coatings.

•We first prepared ZTA ceramics (20 wt% ZrO2 ) by spark plasma sintering.•The powders experienced ball milling, spray drying, heat treatment and plasma treatment.•The grain size of ZrO2 changed from 250 nm to 160 nm with the addition of nano-SiC.•Wear volume decreased 44% and 14% as the addition of nano-SiC particles at load 5 N and 10 N.•Fine grain, phase transformation toughening and formation of tribofilm improved anti-wear properties of ceramics.In this work, Al2O3–ZrO2 nanocomposite ceramics were fabricated by spark plasma sintering (sps). The effects of nano-sized SiC and plasma treating technology on the tribological behavior were investigated. The experimental results showed that the grain size of ZrO2 changed from 250 nm to 160 nm with the addition of nano-SiC. The highest value of toughness, 6.0 MPa m1/2, was acquired in Al2O3–ZrO2–SiC composite ceramics after plasma treating technology, which was 20% higher than that in Al2O3–ZrO2 ceramics. Wear volume decreased 44% and 14% than Al2O3–ZrO2 ceramics as the addition of nano-SiC particles at load 5 N and 10 N. Fine grain, phase transformation toughening and formation of tribofilm were the main reasons to the improvement of anti-wear properties in composite ceramics.

Rare earth (RE)-modified NiCrAlY powders were prepared by ultrasonic gas atomization and deposited on stainless steel substrate by high-velocity oxygen fuel spraying. The effects of the RE on the microstructure, properties, and thermal shock resistance of the NiCrAlY coatings were investigated. The results showed that the NiCrAlY powders were refined and distributed uniformly after adding RE, while the number of unmelted particles in the coatings was reduced. Moreover, the RE-modified coatings showed improved microhardness and distribution uniformity. The microhardness of the coating reached a maximum after adding 0.9 wt.% RE, being 34.4 % higher than that of coatings without RE. The adhesive strength increased and reached a maximum after adding 0.6 wt.% RE, being 18.8 % higher than that of coatings without RE. Excessive RE decreased the adhesive strength. The thermal cycle life of NiCrAlY coatings increased drastically with RE addition. The coating with 0.9 wt.% RE showed optimum thermal shock resistance, being 21.2 % higher than that of coatings without RE.

Ultra-fine grained WC–Co composites possess higher hardness or strength and wear resistance, compared to their coarse counterparts. However, abnormal grain growth of ultra-fine WC particles often occurs during the traditional pressureless liquid sintering, which substantially impairs the mechanical properties and erosion resistance of the material. Thus, controlling the abnormal grain growth becomes one of the key issues in fabricating ultra-fine grained cemented carbides. In this study, ultra-fine grained WC–Co composites containing different amounts of ceria nano-particles were prepared by spark plasma sintering and effects of the nano-ceria on mechanical properties and erosion behavior of the WC–Co composites were studied. The results demonstrated that trace nano-ceria addition effectively suppressed the abnormal grain growth of WC, leading to uniform and fine microstructures. Such ultra-fine grained WC–Co composites have both improved hardness and fracture toughness, resulting in enhanced resistance to high-speed solid-particle erosion. However, when the added nano-ceria was more than 0.1 wt%, Co pools started to form, which lowered the material density, hardness and toughness and consequently the erosion resistance.

Nine types of trace nano-CeO2-doped WC-11Co cemented carbides designed by orthogonal method were prepared. The variance analysis results show that the relative density increases with increasing sintering temperature in the range of 1400-1450 °C; the hardness is highly dependent on grain size of WC which is closely related to the particle size of the initial WC powder and grain refinement produced by nano-CeO2; the bending strength of WC-11Co cemented carbide with less than 0.15 wt.% of nano-CeO2 is effectively improved because of its effect on the decreasing of porosity and inhibiting the grain growth of WC and martensitic phase transformation of Co; the fracture toughness rises at the beginning then drops later if the particle size of initial WC powder is in the range of 3-11 μm.

Carbidic austempered ductile iron (CADI) is a new type of ductile cast iron containing carbides, providing enhanced wear resistance, compared to conventional ADI. However, the improvement in the wear resistance due to the introduced hard carbides is generally accompanied by a decrease in the impact toughness, which limits its applications. Controlling the morphology, size and quantity of carbides is an approach to achieve the optimal combination of wear resistance and impact toughness. In this work, we modified CADI by adding nano ceria to tailor the morphology, size and quantity of carbides and investigated effects of the added nano ceria on mechanical properties and abrasion resistance of the CADI. It was demonstrated that carbides were refined by the nano ceria, which improved the toughness of the CADI in addition to raised hardness. The abrasive wear resistance of the material was correspondingly enhanced with the change in hardness, largely benefiting from the variations in the volume fraction and size of carbides. However, only a small amount of nano ceria was effective; the benefits of nano ceria diminished if excessive nano ceria was added.

NiCrAlY coatings without and with 0.2 wt.% nano ceria were prepared by high velocity oxygen fuel spraying. The microstructure, mechanical properties, and thermal shock resistance of as-sprayed coatings were investigated. The results showed that in the as-sprayed coatings, the number of un-melted particles was reduced drastically, the microstructure was refined and compact due to the refinement of sprayable powders. Both the hardness and adhesive strength of the NiCrAlY increased due to the refinement of microstructure and the decrease of the defects, such as pores and oxides, after adding nano ceria. The thermal cycle life of NiCrAlY coatings was improved by 15% after adding 0.2 wt.% nano ceria, which is attributed to the low content of spinel NiCr2O4 and high content of Cr2O3 in the thermal cycling, the refined and compact microstructure, and increased interfacial boundary.

The nanostructured agglomerated feedstock used for plasma spraying was obtained by the nanoparticle reconstituting technique. Nanostructured and conventional ZrO2–8wt%Y2O3 (8YSZ) thermal barrier coatings (TBCs) have been prepared by atmospheric plasma spraying (APS) on 45# steel substrates with the NiCrAlY as the bond-layer. The microstructure and phase composition of feedstocks and corresponding coatings were characterized. The top layer of nanostructured 8YSZ TBCs is denser and has fewer defects than that of conventional TBCs. The elastic modulus, micro-hardness and Vickers hardness of nanostructured 8YSZ TBCs exhibit bimodal distribution while the conventional 8YSZ exhibit mono-modal distribution. The elastic modulus and elastic recoverability were also obtained by the nanoindentation test. The results indicate that the elastic modulus of nanostructured 8YSZ coating is lower than that of conventional 8YSZ coating, but the nanostructured 8YSZ coating has higher elastic recoverability than that of the conventional 8YSZ coating. The prediction of the average elastic modulus was established by the mixture law and weibull distribution according to the fraction of phases with different molten characteristic.Highlights► Nanostructured 8YSZ TBCs. ► Nano-bimodal strucutre. ► Weibull distribution. ► Indentation mechanical characterization.

The defects in materials play very important role on the effective thermal conductivity. Especially, the spatial and geometrical characteristics of pores are significant factors for the thermal insulation behavior of thermal barrier coatings (TBCs). In this paper, finite element method was employed to simulate the thermal transfer behavior of TBCs with different spatial and geometrical characteristic of pores. The simulation results indicate that the thermal insulation effect of TBCs would be enhanced when the pore size, pore volume fraction and pore layers which are perpendicular to the thickness direction increase and the space between the adjacent pores decreases. It is predicted that the effective thermal conductivity is different at different directions for the atmospheric plasma spray (APS) TBCs. A novel method, Computational Micromechanics Method (CMM), was utilized to depict the thermal transferring behavior of actual coatings. At the same time, model with different kinds of defects were established, and the effective thermal conductivity as the function of defect orientation angle, defect volume fraction and defect shape coefficient was discussed in detail. The simulation results will help us to further understand the heat transfer process across highly porous structures and will provide us a powerful guide to design coating with high thermal insulation property.

The nanostructured La2Zr2O7 (LZ) feedstock with high density, suitable size distribution and nearly spherical morphology which can be used for plasma spraying was prepared by spray drying in this study. The spray drying process was discussed. In addition, the formation mechanism of feedstock with hollow shell structure was discussed by finite element method in this paper. The double ceramic layer (DCL) LZ/YSZ (yttria stabilized zirconia) thermal barrier coatings were prepared using the as prepared LZ feedstock. The average grain size computed by Scherrer formulation, the observation of powder size by Transmission Electron Microscope (TEM) and “single splat” deposition experiment indicate that the as prepared LZ feedstock is nanostructured feedstock.In this paper, the La2Zr2O7 feedstock which can be used for plasma spraying was prepared by solid reaction and spray drying. The three factors (flowability, density and size) which determine the thermal spray characteristic of feedstock are discussed in this paper. The formation mechanism was proposed according to the morphology of the feedstock after spray drying. Finite element model was established to analyze the formation process of the feedstock with hollow shell structure.Explosion process of feedstock with the inner gas pore radius increasing and the SEM image of the actual feedstock prepared by spray dryingResearch Highlights► The preparation technique of the nanostructured La2Zr2O7 (LZ) which can be used for plasma spaying was systemically investigated in this study. ► The method of “Single splat” pattern was proposed to indicate that the as prepared LZ feedstock is nanostructured feedstock. ► Finite element simulation was employed to describe the formation mechanism of feedstock with hollow shell structure.

In this paper, the Al2O3-20 wt.%ZrO2 (8 wt.%Y2O3) feedstocks were fabricated and treated by spray drying, calcination, and plasma treatment technology. The morphology of feedstocks was characterized by scanning electron microscope (SEM). The phase structure and grain size were analyzed by X-Ray diffraction (XRD). The flowability and density were measured by Hall Flowmeter and density instrument, respectively. The sphericity and flowability of feedstocks treated by plasma flame increased greatly compared with that of the feedstocks without plasma treatment, and the particle surfaces were very smooth. The optimum flowability was obtained when the critical plasma spray parameter (CPSP) was 363. The compactness also increased greatly with the increment of CPSP, and the maximum value of compactness was got with CPSP of 325. Calcination can make the grain grow and plasma treatment can lead to the decrement of grain size. The phase structure of Al2O3 did not change, which was α-Al2O3 in the composites. The phase structure of ZrO2 (8 wt.%Y2O3) changed from t-phase to c-phase which was affected greatly by plasma treatment.In this paper, the flowability and density of the Al2O3–ZrO2 feedstock are the important properties for the plasma spraying process, which can be affect the mechanical performance of the coatings. The densified coatings can be obtained with high flowability and density. The plasma treatment is used to increase flowability and density with different plasma spraying parameters, showing in following Fig.Research highlights► The nano materials were granulated into micron particles. ► The micron particles keep their nano structures. ► The process of granulation was studied for nano Al2O3 and ZrO2 (8 wt.%Y2O3). ► The effect of plasma treatment parameter on feedstock was investigated deeply. ►The process of plasma treatment was investigated detailedly.

In this paper, the microstructures of Cr–Mo–Cu alloy cast irons with and without nano-additives were characterized by scanning electron microscope (SEM) and energy diffraction spectrum analyser (EDS). The dry sliding wear tests were carried out using a pin-on-disk wear tester against GCr15 steel balls under fixed normal load and rotating velocity. The study aimed to illustrate the effect of graphite morphology on the sliding wear behavior of Cr–Mo–Cu alloy cast irons. The experimental results showed that the graphite morphology of Cr–Mo–Cu alloy cast iron is affected by nano-additives. They are transformed from thick-long flake to thin-short vermicular shape, and the hardness increased notably. The wear test results indicated that the wear rate of Cr–Mo–Cu alloy cast irons decreases with addition of nano-additives while the wear rate of the GCr15 steel ball worn against with the alloy cast iron modified by nano-additives increases. The graphite morphology plays a significant role in the wear performance of the alloy cast iron. The analysis of worn surfaces and wear debris indicated that the dominant wear mechanism for the alloy cast iron without nano-additives is adhesive wear and plough. However, the percent of oxidation wear mechanism for the alloy cast iron with nano-additives increases.Graphical abstractHighlights• The nano materials were used to modify the performance of alloy cast iron. • The hardness of alloy cast iron with nano-additives increased greatly. • The wear resistance of alloy cast iron with nano-additives increased notably. • The grinding efficient of alloy cast iron with nano-additives was dramaticlly better than that of alloy cast iron without nano-additives.

Plasma-sprayed microstructured and nanostructured Al2O3–13 wt.%TiO2 coatings were successfully deposited on Ti–6Al–4V titanium alloy substrates with commercial Metco 130 powder and as-prepared nanostructured feedstock, respectively. The as-sprayed coatings were remelted by a CO2 laser to further enhance their compactness and bonding strength. The effects of laser remelting on the microstructure, phase constituents and mechanical properties of the ceramic coatings were investigated by scanning electron microscope, X-ray diffractometer and Vickers microhardness tester. The results indicate that the laser-remelted coatings exhibit more compact and homogenous structure as well as strong metallurgical bonding to the substrates. The dominating metastable γ-Al2O3 phase in the as-sprayed coatings transforms to stable α-Al2O3 during laser remelting. The microhardness value of the as-sprayed Metco 130 and nanostructured Al2O3–13 wt.%TiO2 coatings is in the range of 700–1000 HV0.3, while the microhardness values of the corresponding remelted coatings are enhanced to 1000–1350 HV0.3 and 1100–1800 HV0.3, respectively. With the decrease of laser scanning velocity, the microhardness is increased.Research highlights▶ Both of the laser-remelted conventional and nanostructured coatings possess a strong metallurgical bonding to the substrates without interface porosity. More compact and homogenous structure is observed in the LRmC, with the elimination of pre-existing defects and lamellae structure of the as-sprayed coatings. ▶ The conventional LRmC exhibit randomly distributed strip-like microstructure, while the nanostructured LRmC possess actinomorphic microstructure. ▶ The microhardness value of both of the as-sprayed microstructured Metco 130 and nanostructured Al2O3–13 wt.%TiO2 coatings is significantly increased.

Plasma-sprayed nanostructured coatings were successfully deposited on an AZ91D magnesium alloy substrate using the as-prepared nanostructured Al2O3–13 wt%TiO2 feedstock and were subsequently remelted by a CO2 laser. The effects of laser remelting on the microstructure, phase composition and mechanical properties of the ceramic coatings were investigated by scanning electron microscope, X-ray diffractometer and Vickers microhardness tester. The results indicate that the laser remelted coatings exhibit excellent metallurgical bonding to the substrate. Pores and lamellae structures in the as-sprayed coatings have been effectively eliminated and a more compact and homogenous microstructure is achieved after laser remelting. The metastable γ-Al2O3 phase in the as-sprayed coatings was transformed to stable α-Al2O3 during laser remelting. The microhardness of the remelted coatings was enhanced to 1000–1500 HV0.3, which is about 15 times higher than that of the substrate. In addition, with the decrease of laser scanning speeds, the microhardness was increased correspondingly.

The electrochemical corrosion behaviours of the steel substrates coated with three different plasma sprayed Al2O3–13%TiO2 coatings were studied in this paper. The three kinds of Al2O3–13%TiO2 coatings were conventional ME coating, nanostructured NP coating and NS coating. There were micro cracks, laminar splats and straight columnar grains in ME coating. For the two nanostructured coatings, the laminar microstructure and columnar grains were not obvious. The NP coating had the highest hardness and spallation resistance. Electrochemical corrosion behaviour of the three coatings was mainly investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in aqueous Na2SO4 solution.

In situ porous alumina/aluminum titanate ceramic composite was fabricated by spark plasma sintering of nanostructured alumina/titania composite powders. The phase composition and microstructure of the nanostructured composite powders and the fabricated porous bulk ceramic composite were investigated by X-ray diffraction and scanning electron microscopy, respectively. The results indicate that a large number of pores were distributed homogeneously in the porous alumina/aluminum titanate ceramic composite, which possessed excellent mechanical properties.

The conventional Metco 130 coating, densified nanostructured coating and undensified nanostructured coating were deposited by plasma spray. There were typical lamellar splats composed of columnar grains existed in the conventional Metco 130 coating. Whereas, the lamellar structures and columnar grains were not obvious in densified and undensified nanostructured coatings. The densified nanostructured coating had the highest sliding wear and corrosion resistance. Micro fracture, adhesive wear and tribochemical reaction occurred in all three coatings during their sidling against Si3N4 balls. The dominant wear mechanism in Metco 130 coating was micro fracture along splat and columnar grain boundaries. While, in nanostructured coatings the damage was intergranular fracture and grains pull-out. During the corrosion of densified nanostructured coating in 3.5%NaCl solution, the diffusion process of reactant through the coating was a determining factor. The NaCl solution could reach the steel substrate through permeable defects in coatings. The corrosion mainly occurred at the steel substrate near the NiCrAl bond coating/substrate interface.

Nanostructured alumina/titania composite powders were prepared using nanosized alumina and titania doped with nanosized zirconia and ceria through ball-milling, spray drying and heat treating. The nanostructured reconstituted powders were then cool isostatic pressed and pressureless sintered into bulk ceramic composites. The phase constitution and microstructures of as-prepared ceramic composites were characterized by using X-ray diffractometer and scanning electron microscope. The mechanical properties of the ceramic composites were evaluated by Vickers hardness test, flexural strength test and fracture toughness test. The effects of nano-dopants and sintering temperatures on the microstructures and mechanical properties of the composites were investigated. It was found that nano-dopants had the effects of lowering sintering temperature, accelerating densification, reinforcing and toughening the composites. The maximum flexural strength, fracture toughness and Vickers hardness of the composites with nano-dopants were 51, 20 and 56% higher than that of the composites without nano-dopants. The reinforcing and toughening mechanisms are discussed in detail.

The stress field and failure mode of plasma sprayed Al2O3–13%TiO2 coatings under thermal shock were analyzed by experimental and numerical investigations in this paper. Two kinds of nanostructured coatings were derived from reconstituted nanostructured feedstocks and one kind of conventional coating was deposited with commercial fused and crushed feedstock. Testing results showed that the nanostructured coatings exhibited improved thermal shock resistance than the conventional coating. The failure modes of coatings predicted by numerical analysis had a good consistency with the experimental results. The effects of interface morphology and coating defects on thermal stresses were also discussed in this paper. The improved thermal shock resistance of nanostructured coatings was related to their pre-existing spherical pores, reduced cracks and microstructures.

The fretting wear behavior of conventional and nanostructured Al2O3–13 wt%TiO2 coatings fabricated by plasma spray was studied in this paper. The conventional coatings were deposited with commercial Metco 130 feedstock, and the nanostructured coatings were deposited with agglomerated feedstock with nanostructure. There were typical lamellar structures existing in conventional coating, however, those were not obviously observed in nanostructured coating. Amorphous phases, nanosized grains and some submicron grains existed in nanostructured coating. In fretting wear tests, the coatings wear against 52100 steel ball. In all of three conditions tests, the fretting maintained in gross slip regime for both nanostructured and conventional coatings. The coefficient of friction (COF) ranged from 0.7 to 0.9 in the fretting wear test. There was a transfer iron oxide layer formed on the worn coating surface. Fretting cracks propagate along the splat boundary in conventional coatings but propagate at random in nanostructured coatings. Test results showed that nanostructured coatings exhibited much better fretting wear resistance than conventional coating. The improved fretting wear resistance of nanostructured coatings was attributed to the nanosized grains, reduced lamellar structures and amorphous phases.

The effects of ceria addition to thermal sprayed NiAl intermetallic coatings were investigated through micro-indentation, thermal shock testing and microstructural analysis techniques (scanning electron microscopy with energy-dispersive X-ray spectrometry and X-ray diffraction analysis). It has been found that the addition of CeO2 to NiAl coatings reduces the tendency of brittle peeling during thermal spraying. This reduction in peeling is presumably due to the improved wetting of the substrate by the molten coating material, which leads to better coating adhesion. The addition of CeO2 resulted in higher coating hardness and elastic modulus. The coatings containing CeO2 also exhibited significant increases in thermal shock resistance compared with that of the pure NiAl coating. The NiAl coating containing 2 wt.%CeO2 had the highest hardness, elastic modulus and thermal shock resistance of the four NiAl-based coatings tested. The possible mechanisms responsible for the improvement of the properties upon addition of CeO2 are addressed.

Electrophoretic deposition (EPD) was applied to the fabrication of nanostructured Ni–WC–Co composite coatings on nickel-plated stainless steel substrates. The micro-hardness and ball-on-disk sliding wear of the sintered composite coatings were tested for, and the results correlated to those of the microstructure analysis, which was determined by scanning electron microscopy (SEM) and X-ray diffraction (XRD).Sintering after EPD on the substrates and high-pressure pressing resulted in dense composite coatings containing high volume fractional of nanostructured WC or WC–Co particles. Compared with pure nickel coatings or stainless steel, the coatings containing nanostructured WC–Co particles showed a significant increase in hardness and wear resistance. The remarkable improvement in hardness and wear resistance is attributed to the dispersion hardening of nanostructured WC–Co particles in the composite coatings.

•The DCL TBCs have better high temperature oxidation resistance.•The DCL structure has obvious advantage in thermal insulation aspect.•Rare-earth doping is a promising method for the advanced TBCs.Thermal barrier coatings (TBCs) for more efficient gas turbine engines require higher operating temperature. Adopting pyrochlore structure materials such as rare-earth zirconate and double-ceramic-layer (DCL) structure can well satisfy service demands of TBCs at elevated temperature. The nanostructured single-ceramic-layer (SCL) 8YSZ, DCL Sm2Zr2O7 (SZ)/8YSZ and doping 8 wt% CeO2 nanoparticles in SZ (8CSZ)/8YSZ TBCs were prepared by atmospheric plasma spraying (APS) in this work. The high temperature oxidation behavior and thermal insulation property of the three kinds of as-sprayed TBCs were investigated systematically at 1000 °C and 1200 °C. The results indicate that the nanostructured DCL 8CSZ/8YSZ TBCs have the best oxidation resistance and thermal insulation property at different temperatures among others. The influence of microstructure on thermal insulation property of the coating has been discussed in detail.