NiAl Matrix Research Articles

Tribological properties of self-lubricating NiAl/Mo-based composites containing AgVO3 nanowires

Silver vanadate (AgVO3) nanowires were synthesized by hydrothermal method and self-lubricating NiAl/Mo-AgVO3 composites were fabricated by powder metallurgy technique. The composition and microstructure of NiAl/Mo-based composites were characterized and the tribological properties were investigated from room temperature to 900°C. The results showed that NiAl/Mo-based composites were consisted of nanocrystalline B2 ordered NiAl matrix, Al2O3, Mo2C, metallic Ag and vanadium oxide phase. The appearance of metallic Ag and vanadium oxide phase can be attributed to the decomposition of AgVO3 during sintering. Wear testing results confirmed that NiAl/Mo-based composites have excellent tribological properties over a wide temperature range. For example, the friction coefficient and wear rate of NiAl/Mo-based composites containing AgVO3 were significantly lower than the composites containing only metallic Mo or AgVO3 lubricant when the temperature is above 300°C, which can be attributed to the synergistic lubricating action of metallic Mo and AgVO3 lubricants. Furthermore, Raman results indicated that the composition on the worn surface of NiAl-based composites was self-adjusted after wear testing at different temperatures. For example, Ag3VO4 and Fe3O4 lubricants were responsible for the improvement of tribological properties at 500°C, AgVO3, Ag3VO4 and molybdate for 700°C, and AgVO3 and molybdate for 900°C of NiAl-based composites with the addition of metallic Mo and AgVO3.

Morphological Characteristics of W-Nanowires after Selective Etching

An eutectic NiAl–1.5 at.% W alloy prepared by using directionally solidified (DS)was employed as a source for producing W-nanowires. Several growth rate of 8,15,25/s was respectively used at a temperature gradient of ~240 K/cm in a Bridgman-type directional solidification furnace. A combined stability diagram was applied to predict proper conditions for the selective dissolution of NiAl matrix to get W-wires. Etching in a mixture of HCl:H2O2 released parallel aligned W-nanowires with a wire diameter of ~500 nm. Different morphologies, such as nanobelts, lotus-shaped, conical of W-nanowires are obtained at the different conditions.

Microvials with tungsten nanowire arrays

Nanostructured eutectic NiAl–W, fabricated using a Bridgman-type directional solidification facility, combines the advantages of single individual nanowires with the benefit of a conductive macroscopic matrix. Through an electrochemical dissolution step, using conditions derived from the combined Pourbaix diagrams of all three elements involved, the NiAl matrix is selectively dissolved allowing the release of embedded W nanowires. An application of micro-scale electrochemical techniques, such as scanning droplet cell microscopy, facilitates not only selective but moreover local matrix dissolution. Such a local dissolution leads to the formation of cavities on the micro-scale containing arrays of single crystalline W nanowires. In this connection, the depth and volume of fabricated microvials can directly be determined from the charge consumed during potentiostatic dissolution. A subsequent surface functionalisation enhances the hydrophobic behaviour, which is already observed for non-functionalised nanowire arrays, resulting in measured contact angles close to the border to superhydrophobicity.

Growth Behavior of WC Grains in Ni<sub>3</sub>Al Matrix during Liquid Phase Sintering

Growth behavior of WC grains with two different sizes of 10 μm and 4 μm in the same liquid matrix has been studied in the WC–40vol.%Ni3Al system. The samples were sintered under a carbon saturated condition. Surfaces of the coarse WC grains are strongly micro-faceted and stepped. With the increase of sintering time, the average size of the fine WC grain increases continuously and very large plate-like triangular prismatic WC grains appear. The present investigation thus shows that the growth behavior of fine WC grain is affected by lower solubility of W in Ni3Al matrix and the abnormal growth of WC grains is suppressed. However, the growth behavior of coarse WC grain is governed only by the difference in energy between the facets.

Tribological properties of NiAl-based composites containing Ag3VO4 nanoparticles at elevated temperatures

In order to improve the tribological properties of NiAl intermetallic, Ag3VO4 nanoparticles with a size of 100–200nm were synthesized by hydrothermal method and NiAl/Ag3VO4 composites were successfully prepared by mechanical alloying and sintering technique. The composition and microstructure of milled powders and sintered composites were characterized and the tribological properties were evaluated. The results showed that NiAl-based composites were consisted of B2 ordered NiAl matrix and metallic Ag/vanadium oxide precipitates, which were formed by the decomposition of silver vanadate. Wear testing results indicated that NiAl-based composites containing silver vanadate exhibited superior tribological properties at elevated temperatures. Subsequently, Raman results demonstrated that the reproduction of silver vanadate was responsible for the improvement of tribological properties at elevated temperatures.

Formation of friction layer of Ni3Al matrix composites with micro- and nano-structure during sliding friction under different loads

Research on friction layer is needed for mechanical systems and assemblies in order to control failure and wear of contacting materials. This study investigates the formation of friction layer of Ni3Al matrix composites (NMCs) with multi-layer graphene (MLG) under different contact loads. The results show that, under 15 N condition, the friction layer with the ultrafine grain and nanocrystalline structure mainly consists of the thin debris re-embedded layer (DREL) and the matrix refinement layer (MRL). The build up of the DREL is a dynamic equilibrium recovery process. The severe plastic deformation induces the dislocations and twinning, as well as misorientation boundaries, resulting in the formation of the stress dissipative structure in MRL. The low friction coefficient (0.27–0.30) and wear rate (1.2–1.6 × 10−5 mm3N−1 m−1) of NMCs at 15 N load are attributed to the formation of friction layer with micro- and nano-structure during sliding friction.

Effect of counterface balls on the friction layer of Ni3Al matrix composites with 1.5 wt% graphene nanoplatelets

Research on the friction layer is needed to minimize friction- and wear-related mechanical failures in moving mechanical assemblies. Dry sliding tribological tests of Ni3Al matrix composites (NMCs) with 1.5 wt% graphene nanoplatelets (GNPs) sliding against different counterface balls are undertaken at the condition of 10 N–0.234 m s−1 in this study. When sliding against GCr15 steel, a uniform and thick friction layer is formed, resulting in a lower friction coefficient (0.29–0.31) and wear rate (2.0–3.1 × 10−5 mm3 N−1 m−1). While sliding against Al2O3 and Si3N4, the formation and stability of the friction layers are restricted in the severe wear regime, and the NMCs exhibit higher friction coefficients and wear rates. Therefore, various counterface balls have a great effect on the stability and thickness of the friction layer, thus affecting the tribology performance of NMCs. The result also shows that GNPs exhibit enrichment and self-organized microstructures in the friction layer. In addition, the friction layer is also found to be divided into two layers, protecting the subsurface from further damage and reducing shear.

Tribological Behavior of NiAl–1.5 wt% Graphene Composite Under Different Velocities

The progress in aerospace field requires a new NiAl matrix composite that can stand against wear and decrease the energy dissipation through decreasing friction. In this study, the tribological behavior of NiAl–1.5 wt% graphene composite is investigated at room temperature under a constant load of 12 N and different sliding velocities. The results show that the friction coefficient and wear rate increase with increasing sliding velocity from 0.2 to 0.4 m/s due to the adhesion between the sliding bodies and tearing of the graphene layer. The friction coefficient and wear rate tend to decrease at a sliding velocity of 0.6 m/s as a result of severe plastic deformation and grain refinement of the worn surface. However, at 0.8 m/s the friction coefficient reaches a minimum value and the wear rate increases and changes the wear mechanism to fatigue wear. It can be concluded that various wear mechanisms lead to different tribological performance of NiAl–1.5 wt% graphene composite.

Study on Wear Behaviors of Ni<sub>3</sub>Al/Cr-Carbides Cladding Layer on High Strength Steel Substrate

A cladding layer of Ni3Al matrix composite reinforced with 18 vol. % Cr-carbides was applied by using tungsten inert gas (TIG) welding process on high strength steel substrate. The filler metal as welding wire prepared by horizontal continuous casting process with a composition of Ni-2.5C-30Cr-9Al-0.02B (in wt. %) is used. A bulk material of NAC-alloy composite with 18 vol. % Cr3C2-particles produced by Hot Isostatic Pressing (HIP) was also investigated as a reference material. The composition of NAC-alloy is Ni-9.87Al-11.63Fe-0.50Mn-0.50Ti-0.2B in weight percentage. The pin-on-disk (POD) tribometer tests were carried out at room temperature in dry sliding condition at a pressure of 2.83 MPa (20N). The results revealed that Ni3Al/ Cr-carbides cladded layer by TIG process improved wear properties significantly with very low values of the friction coefficient (0.13 ± 0.02) and the specific wear rate (0.162 x 10-5 mm3/N·m) as compared to the reference HIP processed composite with the friction coefficient (0.68 ± 0.02) and the specific wear rate (0.908 x10-5 mm3/N·m) under the same testing conditions.

Effect of thermal fatigue on the structure and properties of Ni3Al-based alloy single crystals

The effect of thermal fatigue during tests of 〈001〉 and 〈111〉 single crystals according to the schedules 100 ai 850°C, 100 ai 1050°C, 100 ai 1100°C at a peak-to-peak stress Δσtc = 700–1000 MPa (sum of the maximum tensile and compressive stresses in a thermal cycle) on the structure, the fracture, and the fatigue life of an Ni3Al-based VKNA-1V alloy is studied. It is found that, at 103 thermal cycles, the 〈111〉 single crystals have the maximum thermal fatigue resistance at the maximum cycle temperature of 850 and 1050°C, and the properties of the 〈001〉 and 〈111〉 samples are almost the same at the maximum thermal cycle temperature of 1100°C. After thermal cycling at the maximum temperature of 850°C, the γ layers in the two-phase γ′ + γ region in dendrites remain a single-phase structure, as in the as-cast material, and the layer thickness is 100–150 nm. When the maximum thermal cycle temperature increases to 1050 or 1100°C, the discontinuous γ-phase layers in the γ′(Ni3Al) matrix change their morphology and become shorter and wider (their thickness is 300–700 nm). The nickel-based supersaturated solid solution in these layers decomposes with the formation of secondary γ′(Ni3Al)-phase (γ′sec) precipitates in the form of cuboids 50 and 100 nm in size at the maximum cycle temperature of 1050 and 1100°C, respectively. The alternating stresses that appear during thermal cycling cause plastic deformation. As in nickel superalloys, this deformation at the first stage proceeds via the slip of screw dislocations along octahedral {111} planes. Networks of 60° dislocation segments form at γ′/γ interfaces in this case. Fracture begins at the lines of intersection of the slip planes of the {111} octahedron with the sample surface. During fractional, a crack passes from one octahedral plane to another and forms terraces and steps (crystallographic fracture); as a result, the fracture surface bends and becomes curved. In all cases, the fracture surfaces have a mixed brittle-ductile character with a combination of crystallographic and ductile (dimple) fracture elements.

NiAl intermetallics dispersion-strengthened with silica, alumina, and mullite: Synthesis and characterization

Ni-Al intermetallics dispersion-strengthened with silica, alumina, and mullite were prepared by SHS under load and characterized by XRD and SEM/EDS. The observed formation of a transition zone between the embedded mullite particles and NiAl matrix in combustion-synthesized NiAl-mullite composites can be expected to improve the service parameters of thus synthesized material.

Tensile behavior of directionally solidified Ni3Al intermetallics with different Al contents and solidification rates

Despite the excellent high temperature mechanical properties of the Ni3Al intermetallic compound, its application is still limited due to its inherently weak grain boundary. Recent research advances have demonstrated that the tensile ductility can be enhanced by controlling the grain morphology using a directional solidification. In this study, a series of directional solidification experiments were carried out to increase both the tensile ductility and the strength of Ni3Al alloys by arraying either the ductile phase of γ-Ni-rich dendrite fibers or the hard phase of β-NiAl dendrite fibers in the γ′-Ni3Al matrix. The dendrite arm spacing could be controlled by the solidification rate, and the volume fraction of the γ or β phase could be altered by the Al content, ranging from 23 at.% to 27 at.%. With an increasing Al content, the γ dendritic microstructure was transformed into the β dendrite in the γ′ matrix, thereby reducing the tensile ductility by increasing the volume fraction of brittle β dendrites in the γ′ matrix. With an increasing solidification rate, the dendrite arm spacing decreased and the tensile properties of Ni3Al varied in a complex manner. The microstructural evolution affecting the tensile behavior of directionally solidified Ni3Al alloy specimens with different solidification rates and Al contents is discussed.

Mechanical properties of NiAl-Y2O3-based powdered alloys produced by directional recrystallization

The mechanical properties of NiAl-Y2O3-based powdered composite alloys (0.5–7.5 vol %), including those with an NiAl intermetallic matrix alloyed with 0.5 wt % Fe and 0.1 wt % La have been studied. Structures with various aspect ratios (AR, the ratio of the grain length to the grain diameter) are formed using deformation and subsequent annealing. A combination of the optimum amount of strengthening phase (2.5 vol % Y2O3) and a quasi-single-crystalline structure with a sharp axial texture with the (100) main orientation and AR ≈ 20–40 provides the maximum short-term strength and life at temperatures up to 1400–1500°C. An NiAl-Y2O3 alloy (2.5 vol %) has the best strength properties among all known nickel superalloys at temperatures higher than 1200°C and can operate under moderate loads at temperatures higher than the working temperatures of nickel superalloys (by 100–400°C) and their melting points. Additional alloying with 10 wt % Co and 2 wt % Nb makes it possible to increase the ultimate tensile strength of an intermetallic NiAl matrix at 1100°C by a factor of 1.3–1.4.

Effect of Ti3SiC2 Content on Tribological Behavior of Ni3Al Matrix Self-Lubricating Composites from 25 to 800 °C

The self-lubricating composites of Ni3Al-Ti3SiC2-TiC-C (NMC) with varying Ti3SiC2 contents were fabricated by spark plasma sintering technique. Dry sliding pin-on-disc friction and wear tests of NMC against Si3N4 ceramic ball were undertaken at 25, 200, 400, 600, and 800 °C in air, respectively. The results showed that NMC with 15 wt.% Ti3SiC2 lubricant owned the excellent tribological properties over a wide temperature range from 25 to 800 °C, whose friction coefficients and wear rates were about 0.17-0.58 and 0.31-4.2 × 10−5 mm3/N/m, respectively. A possible explanation for these results was that the subsurface microstructure self-refinement and the special stratification morphology of the tribo-layer were beneficial to the reduction of friction coefficient. Meanwhile, the protective action of the tribo-layer for the frictional surface could also decrease the wear rate.

Tribological properties of WC and CeO2 particles reinforced in-situ synthesized NiAl matrix composite coatings at elevated temperature

WC and CeO2 particles reinforced in-situ synthesized NiAl matrix (WC/CeO2/NiAl) composite coatings were prepared on aluminum alloys by laser surface alloying. The microstructure, microhardness and fracture toughness as well as the tribological properties at 25°C and 400°C of the composite coatings were investigated. The results show that the WC/CeO2/NiAl composite coating contains the phases of NiAl, WC, W2C, γ-(Fe,Ni), CrB, Cr23C6, Al3Ni2 and CeNi5. Due to the synergistic strengthening effects of WC and CeO2 particles, the average grain size of the composite coating is reduced, and the microhardness and fracture toughness are increased. The friction coefficient and wear loss of the composite coatings rise in ascending order with the temperature, which are smaller than those of the WC/NiAl composite coatings, the NiAl coatings and the substrates. As the temperature increases from 25°C to 400°C, the wear mechanisms of the composite coatings change from micro-cutting wear into multi-plastic deformation wear and slight oxidative wear.

Grain refinement: A mechanism for graphene nanoplatelets to reduce friction and wear of Ni3Al matrix self-lubricating composites

Investigations in graphene of controlling friction and wear of Ni3Al matrix self-lubricating composites (NMSC) are needed for moving mechanical assemblies. The friction and wear behaviors of NMSC with graphene nanoplatelets (GNPs) against Si3N4 ball are tested using a constant load of 10N and a constant speed of 0.2m/s from room temperature (RT) to 600°C. Tribological test results have revealed that small amounts of GNPs in the NMSC are able to drastically reduce the friction coefficients and wear rates over the effective operating range (RT-400°C). A possible explanation for these results is that the refinement of grains accompanying the slippage of laminated sheets between GNPs could provide a source of the stress dissipation and form the GNP protective layer during sliding process, leading to the reduction of wear rates as well as the friction coefficients. It is concluded that GNPs hold great potential applications as an effective solid lubricant for moderate loads and stress, and can be easily used for the preparation of the self-lubricating composites in the future.

Effect of graphene nanoplate addition on the tribological performance of Ni3Al matrix composites

The main objective of this work has been the characterization of wear behavior of the graphene nanoplates (GNPs) in Ni3Al matrix composites (NMC). The friction and wear behaviors of NMC with the addition of 1 wt.% GNPs against Si3N4 ball are tested under different loads using a constant speed of 0.2 m/s. Tribological test results have revealed that small amounts of GNPs are able to drastically improve the antifriction and antiwear properties of NMC. A possible explanation for these results is that, the GNPs not only provide an enhanced effect for NMC to produce better wear resistance, but also form a local protective layer on the contact surfaces to reduce the friction. The investigation shows that GNPs hold great potential applications as an effective solid lubricant for Ni3Al matrix composites and possibly other alloys.

Deformation and heat treatment of NiAl-Y2O3-based powder alloys: III. Directional recrystallization

The effect of recrystallization annealing of various sections produced by twofold extrusion with a total reduction λ = 28 and blade profiles produced by isothermal stamping on the formation of a directional structure with a low fraction of transverse boundaries has been studied in NiAl-Y2O3 (2.5 and 7.5 vol %) alloys. Annealing at temperatures of (0.89–0.95)Tm of NiAl is performed under isothermal conditions or in a temperature field with a temperature gradient along the product length. It is shown that Y2O3 particles of irregular shapes introduced during mechanical activation into the NiAl powder prepared by hydride-calcium reduction take a rounded shape during deformation and heat treatment. As a result of mechanical activation, extrusion, and subsequent directional recrystallization (DR), complex oxides form. The crystallographic texture of the extruded and recrystallized rods and profiles is studied. It is shown that hot extrusion and subsequent DR result in the formation of a quasi-single-crystal structure in a NiAl matrix with coarse elongated recrystallized grains.

Synergetic lubricating effect of MoS2 and Ti3SiC2 on tribological properties of NiAl matrix self-lubricating composites over a wide temperature range

NiAl matrix self-lubricating composites with MoS2 and Ti3SiC2 lubricants were prepared by spark plasma sintering. The tribological behaviors of the NiAl–Ti3SiC2–MoS2 composites against Si3N4 were investigated from room temperature to 800°C. The results showed that the composites exhibited excellent self-lubricating and anti-wear properties over a wide temperature range. At 400°C, the composites containing 5Ti3SiC2–5MoS2 (wt.%) had a very low friction coefficient of about 0.13 and a low wear rate of 4.5×10−5mm3N−1m−1. MoS2 played the main role of lubrication at low temperatures, while Ti3SiC2 was responsible for low friction at high temperatures. Ti3SiC2 and MoS2 lubricants in the NiAl–Ti3SiC2–MoS2 composites showed the excellent synergetic lubricating effect over a wide temperature range from room temperature to 800°C.

Tribological behavior of Ni3Al matrix self-lubricating composites containing WS2, Ag and hBN tested from room temperature to 800 °C

Ni3Al matrix self-lubricating composites (NMSC) containing varied amounts of WS2, Ag and hBN (WAh) with weight ratio of 1:1:1 were fabricated by in situ technique using spark plasma sintering. The friction and wear properties of NMSC against the commercial Si3N4 ceramic ball at the load of 10N and sliding speed of 0.234m/s for 80min from room temperature to 800°C were investigated. The results showed that the tribological properties of NMSC strongly depended on the addition content of WAh. Moreover, NMSC with 15wt.%WAh and 5wt.% TiC exhibited the relatively lower friction coefficients and the less wear rates from RT to 800°C. The excellent tribological behavior of NMSC with 15wt.%WAh and 5wt.% TiC was attributed to the synergetic action of composite lubricants of WAh and reinforced phase of TiC.