Microstructure and wear behaviors of laser clad NiCr/Cr3C2-WS2 high temperature self-lubricating wear-resistant composite coating

The high temperature self-lubricating wear-resistant NiCr/Cr₃C₂-30%WS₂ coating and wear-resistant NiCr/Cr₃C₂ coating were fabricated on 0Cr18Ni9 austenitic stainless steel by laser cladding. Phase constitutions and microstructures were investigated, and the tribological properties were evaluated using a ball-on-disc wear tester under dry sliding condition at room-temperature (17 °C), 300 °C and 600 °C, respectively. Results indicated that the laser clad NiCr/Cr₃C₂ coating consisted of Cr₇C₃ primary phase and γ-(Fe,Ni)/Cr₇C₃ eutectic colony, while the coating added with WS₂ was mainly composed of Cr₇C₃ and (Cr,W)C carbides, with the lubricating WS₂ and CrS sulfides as the minor phases. The wear tests showed that the friction coefficients of two coatings both decrease with the increasing temperature, while the both wear rates increase. The friction coefficient of laser clad NiCr/Cr₃C₂-30%WS₂ is lower than the coating without WS₂ whatever at room-temperature, 300 °C, 600 °C, but its wear rate is only lower at 300 °C. It is considered that the laser clad NiCr/Cr₃C₂-30%WS₂ composite coating has good combination of anti-wear and friction-reducing capabilities at room-temperature up to 300 °C.     

Laser clad Ni-base alloy added nano- and micron-size CeO2 composites

Micron-size Ni-base alloy (NBA) powders are mixed with both 1.5 wt% (%) micron-CeO₂ (m-CeO₂) and also 1.0–3.0% nano-CeO₂ (n-CeO₂) powders. These mixtures are coated on low carbon steel (Q235) by 2.0 kW CO₂ laser cladding. The effects on microstructures, microhardness and wear resistance of the coating by the addition of m- and n-CeO₂ powders to NBA (m- and n-CeO₂/NBA) have been investigated. Addition to the primary phases of γ-Ni, Cr₂₃C₆ and Ni₃B of NBA coating, CeNi₃ shows up both in m- and n-CeO₂/NBA coatings and CeNi₅ appears only in n-CeO₂/NBA coating. Directional dendrite and coarse equiaxed dendrite are grown in m-CeO₂/NBA coating from interface to central zone, whereas multi-oriented dendrite and fine equiaxed dendrite growth by addition of n-CeO₂. The microhardness and wear resistance of coatings are greatly improved by CeO₂ powder addition, and compared to the addition of 1.0% and 3.0%, 1.5% n-CeO₂/NBA is the best. Hardness and wear resistance of the coating improves with decreasing CeO₂ size from micron to nano.     

The comparative study of thermal fatigue behavior of laser deep penetration spot cladding coating and brush plating Ni-W-Co coating

For improving the thermal fatigue behavior of hot work die steel in engineering application, the present work compare the influence of thermal fatigue resistance by the two different surface modified processes, the laser deep penetration spot cladding (LDPSC) and brush plating on the Cr12MoNi hot rolling tool steel. The thermal fatigue tests were fulfilled by heating and quenching in water at a cycle period of 2 min. Before and after thermal fatigue testing, the microhardness distribution profile and microstructure of LDPSC have been investigated. The results show that the LDPSC can be divided to three zones: cladding zone, alloying zone and heat affected zone. The major phases in cladding zone consist of Fe₃C, Cr₇C₃, Cr₂₃C₆ and martensite. The oxidation resistance and thermal stability of brush plating Ni-W-Co coating and reference materials has been determined. The results of thermal fatigue testing show that LDPSC and brush plating Ni-W-Co coating can improve thermal fatigue resistance as compared to the reference material. The brush plating Ni-W-Co coating is more effective than the former owing to its higher thermal stability, oxidation resistance, superfine grain, high-density dislocation in microstructure and combination of strength and ductility.     

Microstructure and wear resistance of composite layers on a ductile iron with multicarbide by laser surface alloying

Multicarbide reinforced metal matrix composite (MMC) layers on a ductile iron (QT600-3) were fabricated by laser surface alloying (LSA) using two types of laser: a 5 kW continuous wave (CW) CO₂ laser and a 400 W pulsed Nd:YAG laser, respectively. The research indicated that LSA of the ductile iron with multicarbide reinforced MMC layers demonstrates sound alloying layers free of cracks and porosities. The microstructure, phase structure and wear properties of MMC layers were investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD), as well as dry sliding wear testing. The microstructure of the alloyed layer is composed of pre-eutectic austenite, ledeburite, spherical TiC, Cr₇C₃ and Cr₂₃C₆ with various morphologies. TiC particles are dispersed uniformly in the upper region of MMC layers. The average hardness of LSA layers by CO₂ laser and pulsed Nd:YAG laser is 859 HV_0.2 and 727 HV_0.2, respectively. The dry sliding wear testing shows the wear resistance of ductile iron is significantly improved after LSA with multicarbide.     

Microstructure and wear behavior of γ/Al4C3/TiC/CaF2 composite coating on γ-TiAl intermetallic alloy prepared by Nd:YAG laser cladding

As a further step in obtaining high performance elevated temperature self-lubrication anti-wear composite coatings on TiAl alloy, a novel Ni-P electroless plating method was adopted to encapsulate the as-received CaF₂ in the preparation of precursor NiCr-Cr₃C₂-CaF₂ mixed powders with an aim to decrease its mass loss and increase its compatibility with the metal matrix during a Nd:YAG laser cladding. The microstructure of the coating was examined using X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) and the friction and wear behavior of the composite coatings sliding against the hardened 0.45% C steel ring was evaluated using a block-on-ring wear tester at room temperature. It was found that the coating had a unique microstructure consisting of primary dendrites TiC and block Al₄C₃ carbides reinforcement as well as fine isolated spherical CaF₂ solid lubrication particles uniformly dispersed in the NiCrAlTi (γ) matrix. The good friction-reducing and anti-wear abilities of the laser clad composite coating was suggested to the Ni-P electroless plating and the attendant reduction of mass loss of CaF₂ and the increasing of it's wettability with the NiCrAlTi (γ) matrix during the laser cladding process.     

Laser (a pulsed Nd:YAG) cladding of AZ91D magnesium alloy with Al and Al2O3 powders

The laser surface cladding of an AZ91D magnesium alloy with Al and Al₂O₃ powders was investigated using a pulsed Nd:YAG laser. The optimum ratio of Al to Al₂O₃ and the suitable range of laser processing parameters were identified. The resulting microstructure in the modified surface layer was examined and the wear resistance property was evaluated. The results show that the wear resistance of the laser treated samples was much superior to that of the untreated samples.     

Elevated temperature dry sliding wear behavior of nickel-based composite coating on austenitic stainless steel deposited by a novel central hollow laser cladding

In order to improve the high-temperature wear resistance of austenitic stainless steel, a wear resistant composite coating reinforced with hard (Cr,Fe)₇C₃ carbide and toughened by ductile γ-(Ni,Fe)/(Cr,Fe)₇C₃ eutectic matrix was fabricated by a novel central hollow laser cladding technique. The constituent phases and microstructure as well as high-temperature tribological behaviors of the Ni-based coating were investigated, respectively, and the corresponding wear mechanisms were discussed. It has been found that the composite coating exhibits superior wear resistance than substrate either at ambient or high temperatures. The coating shows better sliding wear resistance at 600 °C than 300 °C owing to high-temperature stability of the reinforced carbide and polishing effect as well as formation of continuous lubricious films, which implied it has large potential industrial applications at relatively higher temperatures.     

A study on wear resistance and microcrack of the Ti3Al/TiAl + TiC ceramic layer deposited by laser cladding on Ti-6Al-4V alloy

Laser cladding of the Al + TiC alloy powder on Ti-6Al-4V alloy can form the Ti₃Al/TiAl + TiC ceramic layer. In this study, TiC particle-dispersed Ti₃Al/TiAl matrix ceramic layer on the Ti-6Al-4V alloy by laser cladding has been researched by means of X-ray diffraction, scanning electron microscope, electron probe micro-analyzer, energy dispersive spectrometer. The main difference from the earlier reports is that Ti₃Al/TiAl has been chosen as the matrix of the composite coating. The wear resistance of the Al + 30 wt.% TiC and the Al + 40 wt.% TiC cladding layer was approximately 2 times greater than that of the Ti-6Al-4V substrate due to the reinforcement of the Ti₃Al/TiAl + TiC hard phases. However, when the TiC mass percent was above 40 wt.%, the thermal stress value was greater than the materials yield strength limit in the ceramic layer, the microcrack was present and its wear resistance decreased.     

Effect of the heat treatment on the corrosion behaviour of amorphous Fe-Cr-P-C-Si alloy in 0.5 M H2SO4

The effect of the heat treatment on the corrosion behaviour of amorphous Fe₈₅Cr₅P₆C₃Si alloy in 0.5 M H₂SO₄ has been investigated using electrochemical techniques. Heat treatment was carried out at temperatures varying between 250 and 650 °C at different times 30, 60, 120 and 240 min. The evolution of crystallization processes after annealing was identified by differential thermal analysis (DTA) and by X-ray diffraction (XRD). The diagrams obtained by DTA show that the structure of samples treated at high temperature changes towards a crystalline state. This crystallization phenomenon is confirmed by the analysis with the XRD. The results obtained from the polarization curves reveal that for all the studied temperatures of annealing, Fe-Cr-P-C-Si exhibits a phenomenon of passivation without breakdown of passivity. The best corrosion resistance is obtained at the temperature of annealing 350 °C. For an annealing at higher temperatures, Fe₈₅Cr₅P₆C₃Si becomes less corrosion resistant than same amorphous alloy treated with temperatures lower than 350 °C.     

Preparation and characterization of Fe-based bulk metallic glasses in plate form

Amorphous alloys with composition (at%) Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Gd₂ (alloy A) and Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂ (alloy B) were prepared either using pure elements (A and B1) and a commercial AISI430 steel as a base material (B2). When prepared from pure elements both alloys (A and B1) could be cast in plate form with a fixed thickness of 2 mm and variable lengths between 10 and 20 mm by means of copper-mold injection in air atmosphere. In the case of alloy B2, prepared using commercial grade raw materials, rods of 2 mm diameter were obtained.     

Microstructure and friction and wear behavior of laser boronizing composite coatings on titanium substrate

Three kinds of laser boronizing composite coatings were in situ synthesized on Ti substrate by using powders of B, BN and B₄C as starting materials. Microstructures of the laser boronizing composite coatings were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM); and their worn surface morphologies were also observed by using SEM. Moreover, the friction and wear behavior of the boronizing composite coatings under dry sliding condition were evaluated using a UMT-2MT friction and wear tester. It was found that all the three types of laser boronizing composite coatings had higher microhardness and better wear resistance than pure Ti substrate; and their microstructure and wear resistance varied with varying pre-placed powders of B, BN, and B₄C. Under the same dry sliding test conditions, the wear resistance of the three kinds of laser boronizing composite coatings, i.e., sample 1 prepared from pre-placed B, sample 2 obtained from pre-placed BN, and sample 3 fabricated from pre-placed B₄C, is ranked in an order of sample 1 > sample 2 > sample 3, which, surprisingly, well conforms to their order of hardness and friction coefficients.     

Study of the CO2 decomposition over doped Ni-ferrites

The H₂ reduced NiFe_2−xCr_xO₄ can be used to decompose CO₂ to C repeatedly. A series of nanocrystalline Ni-ferrite doping different contents of Cr³⁺ were synthesized by mixed ions co-precipitation method and characterized by XRD, BET and TEM. The results showed that their crystallite sizes were 1-2 nm and BET surface area changed from 220 to 285 m²/g. The evaluation of the activity and stability indicated that Ni-ferrite with 4 wt% Cr³⁺ dopant could be used repeatedly as many as 60 times and was transformed to Fe_yNi_1−y (0<y<1) alloy and Fe₅C₂ gradually during the cycle decomposition of CO₂ to carbon, especially for no Cr³⁺ sample. After the 60th reaction, although NiFe₂O₄ phase just remained 2.1 wt%, the decomposition activity of Ni-ferrite with 4 wt% Cr³⁺ was still 60% of initial activity. This fact suggests that nanocrystalline Fe_yNi_1−y (0<y<1) alloy from the cycle reaction can contribute to the decomposition of CO₂. The results from scanning electron microscopy (SEM), TEM and XRD show that the deposited carbon from CO₂ decomposition consisted of amorphous, crystallite and carbon nanotubes.     

Synthesis and electronic behaviors of TiO2/carbon clusters/Cr2O3 composite materials

Nano-structured TiO₂/carbon clusters/Cr₂O₃ composite material has been successfully obtained by the microwave treatment of a TiO(acac)/Cr(acac)₃/epoxy resin complex. The compositions of the composite materials were determined using ICP, elemental analysis and surface characterization by SEM-EDX, TEM and XRD. ESR spectral examinations suggest the possibility of an electron transfer in the process of TiO₂ → carbon clusters → Cr₂O₃ with an oxidation site at TiO₂ particles and a reduction site at Cr₂O₃ particles. The preliminary experimental results show that the calcined materials could decompose methylene blue under visible-light irradiation.     

Laser induction hybrid rapid cladding of WC particles reinforced NiCrBSi composite coatings

In order to investigate the microstructure characteristics and properties of Ni-based WC composite coatings containing a relatively large amount of WC particles by laser induction hybrid rapid cladding (LIHRC) and compare to the individual laser cladding without preheating, Ni60A + 35 wt.% WC composite coatings are deposited on A3 steel plates by LIHRC and the individual laser cladding without preheating. The composite coating produced by the individual laser cladding without preheating exhibits many cracks and pores, while the smooth composite coating without cracks and pores is obtained by LIHRC. Moreover, the cast WC particles take on the similar dissolution characteristics in Ni60A + 35 wt.% WC composite coatings by LIHRC and the individual laser cladding without preheating. Namely, the completely dissolved WC particles interact with Ni-based alloy solvent to precipitate the blocky and herringbone carbides, while the partially dissolved WC particles still preserve the primary lamellar eutectic structure. A few WC particles are split at the interface of WC and W₂C, and then interact with Ni-based alloy solvent to precipitate the lamellar carbides. Compared with the individual laser cladding without preheating, LIHRC has the relatively lower temperature gradient and the relatively higher laser scanning speed. Therefore, LIHRC can produce the crack-free composite coating with relatively higher microhardness and relatively more homogeneous distribution of WC particles and is successfully applied to strengthen the corrugated roller, showing that LIHRC process has a higher efficiency and good cladding quality.     

Effect of Cr on the wetting in Cu/graphite system

The isotherm wetting and spreading behaviors of the molten Cu-Cr alloys with 0.5, 1.0 and 2.0 at.% Cr on porous graphite substrates were investigated at 1373 K in a flowing Ar atmosphere using a modified sessile drop method. The wettability improves with increasing Cr content in the alloy as a result of phase transition from Cr₃C₂ to more wettable and metallic-like Cr₇C₃ developed at the interface. The spreading kinetics are controlled by the interfacial reaction in the early stage and the diffusion of Cr from the drop bulk to the triple junction in the later stage together with a transition in the intermediate stage.     

Phase composition and tribological properties of Ti-Al coatings produced on pure Ti by laser cladding

Ti-Al coatings with ∼14.7, 18.1, 25.2 and 29.7 at.% Al contents were fabricated on pure Ti substrate by laser cladding. The laser cladding Ti-Al coatings were analyzed with X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS). It was found that with the increase of Al content, the diffraction peaks shifted gradually to higher 2θ values. The laser cladding Ti-Al coatings with 14.7 and 18.1 at.% Al were composed of α-Ti and α₂-Ti₃Al phases, while those with 25.2 and 29.7 at.% Al were composed of α₂-Ti₃Al phase. With the increase of Al content, the cross-sectional hardness increased, while the fracture toughness decreased. For the laser cladding Ti-Al coatings, when the Al content was ≤18.1 at.%, the wear mechanism was adhesive wear and abrasive wear; while when the Al content ≥25.2 at.%, the wear mechanism was adhesive wear, abrasive wear and microfracture. With the increase of Al content, the wear rate of laser cladding Ti-Al coatings decreased under 1 N normal load, while the wear rate firstly decreased and then increased under a normal load of 3 N. Due to its optimized combination of high hardness and high fracture toughness, the laser cladding Ti-Al coating with 18.1 at.% Al showed the best anti-wear properties at higher normal load.     

Magnetocaloric effect in high-energy ball-milled Gd5Si2Ge2 and Gd5Si2Ge2/Fe nanopowders

Evolution of structure and magnetocaloric properties in ball-milled Gd₅Si₂Ge₂ and Gd₅Si₂Ge₂/0.1 wt% Fe nanostructured powders were investigated. The high-energy ball-milled powders were composed of very fine grains (70–80 nm). Magnetization decreased with milling time due to decrease in the grain size and randomization of the magnetic moments at the surface. The magnetic entropy change (ΔS_M) was calculated from the isothermal magnetization curves and a maximum value of 0.45 J/kg K was obtained for 32 h milled Gd₅Si₂Ge₂ alloy powder for a magnetic field change of 2 T while it was still low in Fe-contained alloy powders. The thermo-magnetic measurements revealed that the milled powders display distribution of magnetic transitions, which is desirable for practical magnetic refrigerant to cover a wide temperature span.     

Investigations on the growing, cracking and spalling of oxides scales of powder metallurgy Rene95 nickel-based superalloy

The surface and cross-sectional morphologies of powder metallurgy (PM) Rene95 nickel-based superalloy after 100 h oxidation in the temperature range of 700-1100 °C were investigated. It is shown that oxides nucleate first on the surface of the alloy and form an oxides scale. Afterwards, oxides scale endures decohesion, rumpling, cracking and finally spalling owing to the weak cohesive strength of the scale/alloy interface. The XRD and EDS analyses confirmed that the oxides scale of PM Rene95 superalloy is mainly composed by Cr₂O₃ at 800 °C and NiCr₂O₄ is the main spinel at 1100 °C. The subsequent analysis of internal stress verified that cracking and spalling are caused by growth stress and promoted by thermal stress. On these bases, improvement of the cohesive strength of the scale/alloy interface is considered to be the main way to increase the oxidation resistance of PM Rene95 superalloy.     

The formation model of Ni-Cr oxides on NiCoCrAlY-sprayed coating

The atomic arrangement and distribution of oxides (Cr₂O₃, NiCr₂O₄ and NiO) on the sprayed-NiCoCrAlY coating after oxidation are analyzed. The formation and the growth model of Ni-Cr oxide phases are discussed according to the matching relationship between atoms. The outline character and a scale of spinel NiCr₂O₄ are discussed. The results show that Cr atoms can form two close-packed arrangements in the crystal plane of Cr₂O₃ perpendicular to 〈0 0 1〉 orientation. The atomic spacing in the first arrangement corresponds to double that of Ni/Ni₃Al in {1 1 1} crystal face. This suggests that Ni/Ni₃Al is the substrate for Cr₂O₃ to grow along 〈0 0 1〉 direction. The lattice mismatch between Cr₂O₃ and Ni/Ni₃Al is less than that of Al₂O₃, which indicates that Cr₂O₃ is easier to form than Al₂O₃ during the oxidation process. The atomic spacing in another close-packed arrangement of Cr₂O₃ perpendicular to 〈0 0 1〉 orientation is approximately equal to that of Ni or Cr in the plane of NiCr₂O₄ and NiO perpendicular to 〈1 1 1〉 orientation. So Cr₂O₃ can be the substrate for NiCr₂O₄ and NiO to grow in the 〈0 0 1〉 direction. NiCr₂O₄ and NiO can grow directly along the 〈1 1 1〉 orientation on each other. NiCr₂O₄ can grow outward in the planes of Cr₂O₃ perpendicular to 〈0 0 1〉 and grow inward along 〈1 1 1〉 orientation of NiO.     

Preparation and high-temperature properties of Au nano-particles doped α-Al2O3 composite coating on TiAl-based alloy

Au nano-particles doped α-Al₂O₃ composite coatings were successfully prepared on TiAl-based alloy by electrodeposition, using the Al₂O₃ sols with minor addition of HAuCl₄ solution. The even distribution of Au nano-particles (<2.0 wt.%) in the α-Al₂O₃ matrix has been observed. Isothermal oxidation tests of the samples coated with the as-prepared novel coatings at 900 °C in static air for 200 h shown that the oxygen inward diffusion can be effectively suppressed to a low level. The results of high-temperature cyclic oxidation test at 900 °C in air revealed that the oxidation and spallation resistance of TiAl-based alloy were improved significantly under thermal cycling. In the as-prepared coatings, cracks were shielded by means of crack bridging and the fracture resistance of the formed scales can be improved by toughening effects of the composite structure. Surface scratching tests after the cyclic oxidation exhibited that the adhesion of the formed composite scale on TiAl-based alloy was remarkably improved by the Au nano-particles doped α-Al₂O₃ composite coating.