Microstructure and oxidation resistance of SiC–MoSi2 multi-phase coating for SiC coated C/C composites

In order to improve the anti-oxidation of C/C composites, a SiC–MoSi2multi-phase coating for SiC coated carbon/carbon composites(C/C)was prepared by low pressure chemical vapor deposition(LPCVD) using methyltrichlorosilane(MTS) as precursor, combined with slurry painting from MoSi2 powder. The phase composition and morphology were analyzed by scanning electron microscope(SEM) and X-ray diffraction(XRD) methods, and the deposition mechanism was discussed. The isothermal oxidation and thermal shock resistance were investigated in a furnace containing air environment at 1500 1C. The results show that the as-prepared SiC–MoSi2coating consists of MoSi2 particles as a dispersing phase and CVD–SiC as a continuous phase. The weight loss of the coated samples is 1.51% after oxidation at 1500 1C for 90 h, and 4.79% after 30 thermal cycles between 1500 1C and room temperature. The penetrable cracks and cavities in the coating served as the diffusion channel of oxygen, resulted in the oxidation of C/C composites, and led to the weight loss in oxidation.     

Microstructure and wear behavior of Mo–60Si–5B coating doped with 0.5 ​at% La by spark plasma sintering

Mo–60Si–5B coating doped with 0.5 ?at% La was prepared on niobium silicon based alloy by spark plasma sintering. The microstructure and wear behavior of the 0.5La–Mo–60Si–5B (0.5La-MSB) coating were investigated. The results show that the mean grain sizes of the Mo–60Si–5B (0La-MSB) and 0.5La-MSB coatings are calculated to be 3.27 ?μm and 2.85 ?μm, respectively. The addition of La plays a role of refining the grains of 0.5La-MSB coating. The specific wear rate of 0.5La-MSB coating is decreased by up to 26.8% at the oscillation frequency of 20 ?Hz and the sliding load of 11 ?N compared to 0La-MSB coating. The anti-friction performance of Mo–60Si–5B coating is modified by the addition of 0.5 ?at.% La. The improvement of anti-friction performance of the Mo–60Si–5B coating is due to the increased hardness and the provided lubrication function by La₂O₃.     

Microstructure and mechanical properties of a hot-extruded Al-based composite reinforced with core–shell-structured Ti/Al_3Ti

An Al-based composite reinforced with core–shell-structured Ti/Al_3Ti was fabricated through a powder metallurgy route followed by hot extrusion and was found to exhibit promising mechanical properties. The ultimate tensile strength and elongation of the composite sintered at 620°C for 5 h and extruded at a mass ratio of 12.75:1 reached 304 MPa and 14%, respectively, and its compressive deformation reached 60%. The promising mechanical properties are due to the core–shell-structured reinforcement, which is mainly composed of Al_3Ti and Ti and is bonded strongly with the Al matrix, and to the reduced crack sensitivity of Al_3Ti. The refined grains after hot extrusion also contribute to the mechanical properties of this composite. The mechanical properties might be further improved through regulating the relative thickness of Al–Ti intermetallics and Ti metal layers by adjusting the sintering time and the subsequent extrusion process.     

The strain ampli tude-controlled cyclic fatigue behavior of Al2O3 fiber reinforced Al–Si alloy composite at elevated temperatures

The strain amplitude-controlled fatigue characteristics of an Al–Si casting alloy and itscomposite reinforced with 17 vol% Al₂O₃ fibers (Al–Si/Al₂O₃) are studied at three different temperatures. Both the alloy and the composite showed different degrees of cyclic softening at elevated temperatures. Increasing the temperature, fatigue damage of either the alloy or the composite occurred with varying mode from brittle fracture of silicon particles to their separation from the aluminum matrix. This is explained by the different thermal expansion coefficients of silicon particles and the aluminum matrix. The reinforcement Al₂O₃ fibers in the composite showed a similar damage behavior with those silicon particles despite temperature variation     

Microstructure and mechanical properties of a hot-extruded Al-based composite reinforced with core-shell-structured Ti/Al3Ti

An Al-based composite reinforced with core-shell-structured Ti/Al₃Ti was fabricated through a powder metallurgy route followed by hot extrusion and was found to exhibit promising mechanical properties. The ultimate tensile strength and elongation of the composite sintered at 620℃ for 5h and extruded at a mass ratio of 12.75:1 reached 304 MPa and 14%, respectively, and its compressive deformation reached 60%. The promising mechanical properties are due to the core-shell-structured reinforcement, which is mainly composed of Al₃Ti and Ti and is bonded strongly with the Al matrix, and to the reduced crack sensitivity of Al₃Ti. The refined grains after hot extrusion also contribute to the mechanical properties of this composite. The mechanical properties might be further improved through regulating the relative thickness of Al-Ti intermetallics and Ti metal layers by adjusting the sintering time and the subsequent extrusion process.     

Effect of Zr and Ti on isothermal oxidation behavior of Nb–Si based alloys

The isothermal oxidation behavior of 56Nb-16Si-(20-x)Ti–3Cr–3Al-2Hf-xZr (x ?= ?0, 2, 5, 10 ?at. %) alloys was investigated at 800 ?°C and 1250 ?°C, respectively. The results show that increasing the Zr content evidently increased the oxidation rates at 800 ?°C, accompanied by the obvious occurrence of pesting oxidation. The alloys showed alike linear oxidation kinetics at 1250 ?°C. With the increase of Zr content, the adherence and integrity of oxide scales were improved, but the overall oxidation resistance was slightly deteriorated. The observed oxidation behavior may be attributed to the composition variation of Zr and Ti in the alloys. The oxidation mechanism associated with the composition variation is discussed in this study.     

Microstructure and wear resistance of PTA clad Cr_7C_3/γ-Fe ceramal composite coating

A wear resistant Cr7C3/γ-Fe ceramal composite coating was fabricated on substrate of the hardening and tempering C degree steel by PTA (plasma transferred arc) cladding with (wt%) Fe-25Cr-7C elemental powder blends. Microstructure of the coating was characterized by OM, SEM, XRD and EDS. Wear resistance of the coating was tested under dry sliding wear condition at room temperature. The results indicate that the PTA clad ceramal composite coating has a rapidly solidified fine microstructure consisting of Cr7C3 primary particles uniformly distributed in theγ-Fe matrix and is metallurgically bonded to the C degree steel substrate. The PTA clad Cr7C3/γ-Fe ceramal composite coating has high hardness and excellent wear resistance under dry sliding wear test conditions. The excellent wear resistance of the Cr7C3/γ-Fe ceramal composite coating is attributed to the coating's high hardness, strong covalent atomic bonding and refined microstructure.     

Improvement of oxidation resistance of Nb–Ti–Si based alloys with additions of Al,Cr and B at different temperatures

The effects of Cr,Al and B addition on the microstructure and high-temperature oxidation behaviors(at 1200,1250 and 1300℃) of Nb-Ti-Si based alloys were investigated.The results showed that the addition of Cr stabilized α-Nb₅ Si₃,while Al promoted the formation of β-Nb₅ Si₃ and adding B promoted the formation of γ-Nb₅ Si₃.Among the three elements,Al and Cr were beneficial to oxidation resistance at 1200℃,and B was favorable to the...     

Corrosion behavior of Ni–P/nano-TiC composite coating prepared in electroless baths containing different types of surfactant

In current research, in order to enhance the incorporation of nano-sized TiC particles into electroless Ni–P (EN) coating, different types of surfactant (cationic, anionic, and polymeric) were added to the plating bath. The effects of addition of the surfactants on surface morphology, deposition rate, TiC and P contents of the prepared coatings were investigated. The surface morphology was evaluated by scanning electron microscopy (SEM). It was demonstrated that in the presence of the anionic, polymeric and somehow cationic surfactants, TiC nano-particles were embedded in the matrix which influenced the surface morphology. The effect of surfactant types on the corrosion properties of Ni–P/TiC coated steel was also studied. Corrosion behavior of the coated steel was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) which affected by the incorporation of TiC particles into the Ni–P matrix. The level of corrosion resistance improvement depended largely on the phosphorous and TiC concentration of the applied coating.     

Reduction–melting behaviors of boron-bearing iron concentrate/carbon composite pellets with addition of CaO

Although the total amount of boron resources in China is high, the grades of these resources are low. The authors have already proposed a new comprehensive utilization process of boron-bearing iron concentrate based on the iron nugget process. The present work describes a further optimization of the conditions used in the previous study. The effects of CaO on the reduction–melting behavior and properties of the boron-rich slag are presented. CaO improved the reduction of boron-bearing iron concentrate/carbon composite pellets when its content was less than 1wt%. Melting separation of the composite pellets became difficult with the CaO content increased. The sulfur content of the iron nugget gradually decreased from 0.16wt% to 0.046wt% as the CaO content of the pellets increased from 1wt% to 5wt%. CaO negatively affected the iron yield and boron extraction efficiency of the boron-rich slag. The mineral phase evolution of the boron-rich slag during the reduction–melting separation of the composite pellets with added CaO was also deduced.     

Effect of Ni content on the wear behavior of Al–Si–Cu–Mg–Ni/SiC particles composites

In recent years, the addition of Ni has been widely acknowledged to be capable of enhancing the mechanical properties of Al–Si alloys. However, the effect of Ni on the wear behaviors of Al–Si alloys and Al matrix composites, particularly at elevated temperatures, remains an understudied area. In this study, Al–Si–Cu–Mg–Ni/20wt% SiC particles(SiCp) composites with varying Ni contents were prepared by using a semisolid stir casting method. The effect of Ni content on the dry sliding wear behavior ...     

Microstructure and wear resistance of PTA clad Cr7C3/γ-Fe ceramal composite coating

A wear resistant Cr₇C₃/γ-Fe ceramal composite coating was fabricated on substrate of the hardening and tempering C degree steel by PTA (plasma transferred arc) cladding with (wt%) Fe-25Cr-7C elemental powder blends. Microstructure of the coating was characterized by OM, SEM, XRD and EDS. Wear resistance of the coating was tested under dry sliding wear condition at room temperature. The results indicate that the PTA clad ceramal composite coating has a rapidly solidified fine microstructure consisting of Cr₇C₃ primary particles uniformly distributed in the γ-Fe matrix and is metallurgically bonded to the C degree steel substrate. The PTA clad Cr₇C₃/γ-Fe ceramal composite coating has high hardness and excellent wear resistance under dry sliding wear test conditions. The excellent wear resistance of the Cr₇C₃/γ-Fe ceramal composite coating is attributed to the coating’s high hardness, strong covalent atomic bonding and refined microstructure.     

Microstructure and wear resistance of PTA clad Cr7C3/γ-Fe ceramal composite coating

A wear resistant Cr₇C₃/γ-Fe ceramal composite coating was fabricated on substrate of the hardening and tempering C degree steel by PTA (plasma transferred arc) cladding with (wt%) Fe-25Cr-7C elemental powder blends. Microstructure of the coating was characterized by OM, SEM, XRD and EDS. Wear resistance of the coating was tested under dry sliding wear condition at room temperature. The results indicate that the PTA clad ceramal composite coating has a rapidly solidified fine microstructure consisting of Cr₇C₃ primary particles uniformly distributed in the γ-Fe matrix and is metallurgically bonded to the C degree steel substrate. The PTA clad Cr₇C₃/γ-Fe ceramal composite coating has high hardness and excellent wear resistance under dry sliding wear test conditions. The excellent wear resistance of the Cr₇C₃/γ-Fe ceramal composite coating is attributed to the coating’s high hardness, strong covalent atomic bonding and refined microstructure.     

Microstructure and properties of a wear resistant Al–25Si–4Cu–1Mg coating prepared by supersonic plasma spraying

A high content silicon aluminum alloy(Al–25Si–4 Cu–1Mg) coating was prepared on a 2A12 aluminum alloy by supersonic plasma spraying. The morphology and microstructure of the coating were observed and analyzed. The hardness, elastic modulus, and bonding strength of the coating were measured. The wear resistance of the coating and 2A12 aluminum alloy was studied by friction and wear test. The results indicated that the coating was compact and the porosity was only 1.5%. The phase of the coating was mainly composed of α-Al and β-Si as well as some hard particles(Al_9Si,Al_(3.21)Si_(0.47), and CuAl_2). The average microhardness of the coating was HV 242, which was greater than that of 2 A12 aluminum alloy(HV 110). The wear resistance of the coating was superior to 2A12 aluminum alloy. The wear mechanism of the 2A12 aluminum alloy was primarily adhesive wear, while that of the coating was primarily abrasive wear. Therefore, it is possible to prepare a high content silicon aluminum alloy coating with good wear resistance on an aluminum alloy by supersonic plasma spraying.     

Microstructure and mechanical properties of Nb–Mo–ZrB2 composites prepared by hot-pressing sintering

Nb–Mo–ZrB₂ composites (V(Nb)/V(Mo)=1) with 15vol% or 30vol% of ZrB₂ were fabricated by hot-pressing sintering at 2000℃. The phases, microstructure, and mechanical properties were then investigated. The composites contain Nb-Mo solid solution (denoted as (Nb, Mo)ss hereafter), ZrB, MoB, and NbB phases. Compressive strength test results suggest that the strength of Nb–Mo–ZrB₂ composites increases with increasing ZrB₂ content; Nb–Mo–30vol%ZrB₂ had the highest compressive strength (1905.1 MPa). The improvement in the compressive strength of the Nb–Mo–ZrB₂ composites is mainly attributed to the secondary phase strengthening of the stiffer ZrB phase, solid-solution strengthening of the (Nb, Mo)ss matrix as well as fine-grain strengthening. The fracture toughness decreases with increasing ZrB₂ content. Finally, the fracture modes of the Nb–Mo–ZrB₂ composites are also discussed in detail.     

High-temperature oxidation behaviour of novel Co-Al-W-Ta-B-(Mo,Hf,Nb) alloys with a coherent γ/γ'–dominant microstructure

In this work,2at% Mo,2at% Nb and 2at% Hf were substituted for the same amount of W into a Co-9Al-9W-2Ta-0.02 B alloy(hereafter referred as to 2Mo, 2Nb and 2Hf alloys, respectively, while the original alloy is denoted as 0Me alloy). The effect of the Mo, Hf and Nb additions on the isothermal oxidation resistance, oxide scale evolution and failure mechanism, of the Co-9Al-9W-2Ta-0.02 B alloy when exposed at 800 °C and 900 °C for 100 h was investigated. It was found the Mo, Hf and Nb additions degraded the oxidation resistance of the Co-9Al-9W-2Ta-0.02 B alloy, while the 2Mo alloy always displayed the poorest oxidation resistance, resulted from heavy spallation of the oxide scale. An oxide scale composed of an outer Co_3O_(4+)CoO layer, a middle complex oxide layer enriched with Al, W and Ta, and a γ/needle-like Co_3W zone adhering to the γ/γ' substrate was gradually formed; moreover, a continuous or discontinuous Al_2O_3 layer and dispersive Al_2O_3 dots or slices were observed within the γ/needle-like Co_3W zone, depending on the oxidation temperature and added elements(Mo, Hf and Nb). The formation of volatile MoO_3 in the oxide scale of the 2Mo alloy enhance the exfoliation of the oxide products, resulting in severe spallation and poor oxidation resistance.     

Microstructural analysis and hot corrosion behavior of HVOF-sprayed Ni–22Cr–10Al–1Y and Ni–22Cr–10Al–1Y–SiC(N) coatings on ASTM-SA213-T22 steel

The present paper deals with the investigation of microstructure and high-temperature hot corrosion behavior of high-velocity oxy fuel(HVOF)-produced coatings. Two powder coating compositions, namely, Ni22Cr10Al1Y alloy powder and Ni22Cr10Al1Y(80 wt%; microsized)–silicon carbide(SiC)(20 wt%; nano(N)) powder, were deposited on a T-22 boiler tube steel. The hot corrosion behavior of bare and coated steels was tested at 900°C for 50 cycles in Na_2SO_4–60 wt%V_2O_5 molten-salt environment. The kinetics of corrosion was established with weight change measurements after each cycle. The microporosity and microhardness of the as-coated samples have been reported. The X-ray diffraction,field emission-scanning electron microscopy/energy dispersive spectroscopy, and X-ray mapping characterization techniques have been utilized for structural analysis of the as-coated and hot-corroded samples. The results showed that both coatings were deposited with a porosity less than2%. Both coated samples revealed the development of harder surfaces than the substrate. During hot corrosion testing, the bare T22 steel showed an accelerated corrosion in comparison with its coated counterparts. The HVOF-sprayed coatings were befitted effectively by maintaining their adherence during testing. The Ni22Cr10Al1Y–20 wt%SiC(N) composite coating was more effective than the Ni–22Cr–10Al–1Y coating against corrosion in the high-temperature fluxing process.     

Effects of minor B additions on tensile strength,fracture toughness and oxidation resistance of Nb–Si based alloys

The effects of minor additions of B on microstructure, ambient tensile strength, fracture toughness and high-temperature oxidation resistance of Nb–Si based alloys were investigated. The added contents were designed as 0.05, 0.10 and 0.20 ?at.%. The results show constituent phases in 0.05B and 0.20B alloys were Nbss, α-Nb₅Si₃; while only 0.10B alloy consisted of γ-Nb₅Si₃. Minor B was prioritized into Nbss and redundant one dissolved into silicide enhanced volume fraction of the silicide. With increasing concentrations of boron, the microstructure was refined at first and then coarsened, while the tensile strength was enhanced remarkably. Compared with other two alloys, the 0.10B alloy containing γ-Nb₅Si₃ shows both the best fracture toughness and oxidation resistance. The important roles of γ-Nb₅Si₃ in balancing overall properties are emphasized.     

Effect of Y_2O_3 content in the pack mixtures on microstructure and oxidation resistance of B–Y modified silicide coating

B–Y modified silicide coatings were prepared on Nb–Si based alloy by pack cementation at 1300 ℃ for 10 h. The effect of Y_2O_3 content in the pack mixtures on microstructure and oxidation resistance of the coatings was investigated. The results show that the four coatings have similar structures, which possess a(Nb,X)Si_2 outer layer and a(Nb,X)_5Si_3 transitional layer. Y_2O_3 content in the pack mixtures has an obvious effect on the Si content in the coating. The mass gains of the coatings prepared with 0.5, 1, 2 and 3 wt% Y_2O_3 in pack mixtures are 2.33, 1.96, 2.05 and 2.86 mg/cm~2 after oxidation at 1250 ℃ for 100 h, respectively. The coating prepared with 1 wt% Y_2O_3 exhibits the best oxidation resistance due to the formation of a dense glass-like borosilicate scale.     

Microstructural evolution and mechanical behavior of metastable β-type Ti–30Nb–1Mo–4Sn alloy with low modulus and high strength

A metastable P-type Ti-30Nb-lMo-4Sn alloy with ultralow elastic modulus and high strength was fabricated.Under the solution treatment state,the Ti-30Nb-1Mo-4Sn alloy possesses low yield strength of about 130 MPa owing to the presence of the coarse α " martensitic laths.Upon a cold rolling and annealing process,the martensitic transformation from β to α" is significantly retarded due to the inhibitory effect of grain boundaries and dislocations.As a result,the metastable β phase with low total amount of β-stabilizers is retained to room temperature,giving rise to a low modulus of 45 GPa.Meanwhile,nano-sized a precipitates and dislocation tangles play a key role in strengthening the Ti-30Nb-1Mo-4Sn alloy,resulting in a high tensile strength of ~ 1000 MPa.With low elastic modulus and high strength,the metastable P-type Ti-30Nb-1Mo-4Sn alloy could be a potential candidate for biomedical materials.