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This study examined the effects of different carbon nanomaterials on the mechanical and tribological properties of titanium matrix composites. Carbon nanotubes (CNTs), graphene nanoplates (GNPs) and graphene oxide nanoplates (GOs) reinforced Ti–6Al–2Sn–2Zr–3Mo–1Cr–2Nb–Si (TC21) titanium matrix composites were prepared by spark plasma sintering. In the three carbon nanomaterials, CNTs react with the TC21 matrix to form TiC, while GNPs and GOs react with the TC21 matrix to form a TiC coated GNPs/GOs sandwich structure, and the formation of TiC phase promotes the interfacial bonding of the composite. GOs/TC21 composites exhibit better strength and wear resistance than the other two composites. GOs/TC21 composite exhibits excellent mechanical properties, i.e., yield strength of 1043 MPa, the ultimate tensile strength of 1183 MPa and an elongation about of 8%. The wear loss of GOs/TC21 composite decreased by 24.2% compared to that of the TC21 alloy. The superior mechanical and tribological performances of GOs/TC21 composites are attributed to the TC21 matrix strengthening brought by the TiC particles/TiC coated GNPs/GOs sandwich structure and the lubricating properties of the residual GOs, which demonstrates that GOs is an ideal filler for TC21 alloy.

Graphene and its derivates are extensively applied to enhance the mechanical properties of metal matrix nanocomposites. However, their high reactivity with a metal matrix such as titanium and thus the limited strengthening effects are major problems for achieving high-performance graphene-based nanocomposites. Herein, reduced graphene oxide nanosheets decorated with copper or silver (i.e., Cu@rGO and Ag@rGO) nanopowders are introduced into Ti matrix composites using multiple processes of one-step chemical coreduction, hydrothermal synthesis, low-energy ball milling, spark plasma sintering, and hot rolling. The Cu@rGO/Ti and Ag@rGO/Ti nanocomposites exhibit significantly enhanced strength with superior elongation to fracture (846 MPa-11.6 and 900 MPa-8.4%, respectively, basically reaching the level of the commercial Ti-6Al-4V titanium alloy), which are much higher than those of the fabricated Ti (670 MPa-7.0%) and rGO/Ti composites (726 MPa-11.3%). Furthermore, fracture toughness values of the M@rGO/Ti composites are all significantly improved, that is, the highest KIC value is 34.4 MPa·m1/2 for 0.5Cu@rGO/Ti composites, which is 20.28 and 51.5% higher than those of monolithic Ti and 0.5rGO/Ti composites, respectively. The outstanding mechanical properties of Ag@rGO/Ti and Cu@rGO/Ti composites are attributed to the effective load transfer of in situ formed TiC nanoparticles and the formation of interfacial intermetallic compounds between the rGO nanosheets and Ti matrix. This study provides new insights and approach for the fabrication of metal-modified graphene/Ti composites with a high performance.

The influence of αs precipitates on electrochemical behavior and mechanical degradation of Ti-1300 alloy in artificial seawater have been studied. The results show that corrosion resistance and mechanical degradation have been significantly affected by the formation of acicular αs precipitates. The precipitated αs phase with an acicular shape around 40–60 nm in width are uniformly distributed inside β grain. Many αs precipitates are intersected each other and keep a well-defined Burgers orientation relationship with β matrix, which restricts the growth of other αs phases due to pinning effect. Within the electrolyte, the αs phases can form “microgalvanic cells” with their adjacent intergranular β phases, which dramatically deteriorate its corrosion resistance. The mechanical properties of the alloy are also degraded with the increase of immersion time due to the pitting reaction. The precipitated microstructure exhibits an inferior mechanical degradation behavior, and this is mainly because a lot of corrosion cavities are nucleate d and propagated at the interface between αs precipitates and prior β grains.

Tungsten-copper (W-Cu) composites are used as high temperature frictional materials under special service conditions for electromagnetic gun rail and precision guide for rolled pieces due to their good ablation resistance and electrical conductivity. However, they have poor wear resistance at elevated temperatures. In this paper, surface carburization method was applied on the W-20 wt%Cu composite to investigate the mechanisms of carburization and its effects on the high temperature friction behavior of composite. Carburization process has been done at a temperature of 1100 °C for 30 h. The obtained results showed that carburizing at 1100 °C with a dwelling time of 30 h resulted into formation of a carburized layer and a dense intermediate sub-layer on the substrate. Also, the surface carburized layer with a thickness of about 70 μm composed of mixed phases of graphite, WC and W2C. The hardness of carburized layer (~HV454) was significantly higher than that of substrate (HV223). Also, bending strength of the carburized W-Cu composites has been significantly improved, although their electrical conductivity and tensile strength was decreased slightly. The carburization mechanism of the W-Cu composites was found to be dominant by carbon atom diffusion through reaction with W atoms and formation of surface liquid copper, which promoted migration and diffusion of tungsten and carbon at high temperatures. Average coefficients of friction and wear rate of carburized W-Cu composites are all lower than these of un-carburized W-Cu composites owing to the presence of surface carburized layer. Also, formation of CuWO4 at high temperatures reduced the friction and wear resistance of the W-Cu composites.

W-based alloys are currently considered promising candidates for high heat flux components in future fusion reactors. In this paper, hot pressed W-20 wt%Cu composites were treated at room temperature using a sliding friction severe deformation (SFD) process, with a moving speed of 0.2 m/s and an applied load of 500 N. Microstructural evolution of composites after the SFD treatment was evaluated and compared with that of the untreated composites. Results showed that there was a gradient structure generated and an obvious refinement in tungsten particles size in the surface layer after the SFD process. The average particle size of tungsten in the SFD treated composites was 2.60 μm, whereas it was 4.5 μm for tungsten in the untreated composites. Fracture surfaces of the composites indicated that the SFD treatment destroyed the W skeleton and changed fracture mode from predominant inter-granular one to trans-granular one due to the decrease in contact area of W-W inter-particles. Yield strength and ultimate tensile strength of composites after the SFD treatment were 308 MPa and 553 MPa, respectively. The treated composites exhibited micro-hardness values with an average reading of about 308 HV. Analysis of the facture microstructures clearly suggested that the tungsten particles in the treated composites are consisted of dislocations and boundaries as well as dislocation tangles. The electrical conductivity of the composites was decreased from 33 IACS% to 28.5 IACS% after the SFD treatment, mainly due to loss or squeezing of copper into the inner surface.

Tungsten copper (W-Cu) composites, as a traditional refractory material, are promising materials for manufacture of electrical contacts and electrodes, heavy duty electronic contacts, welding and electro-forging dies, heat sinks, packaging material, arcing resistance electrodes and thermal management devices owing to their excellent properties. This critical review presents and discusses the current progress of W-Cu composites. Starting with an introduction of the synthesis methods for W-Cu composites, including the conventional and modern preparation approaches. After that we focus on the description of the improvement of mechanical properties and arc-erosion properties by modification techniques. Finally, the advantages of W-Cu composites in applications such as electrical contacts, electronic packaging materials, and heat sinks, as well as military materials, are described, respectively.

Benzotriazole (BTA) and tolyltriazole (TTA) were used to passivate pure copper and chromium bronze alloys for environmental protection. Using the chemical immersion treatment, a dense film of chromium-free passivation layer was formed on surfaces of both the pure copper and chromium bronze alloys. Microstructure, crystalline phases and chemical bonds of the passivation layers were systematically characterized, and the passivation mechanisms were studied. The passivation layer was mainly consisted of CuO, Cu2O, polymers of Cu-BTA-TTA and Cu(I)BTA-TTA. The formation mechanism of the chromium-free passivation layer was identified as combined oxidation and adsorption, and the main passivation processes included oxidation and dissolution of the matrix, adsorption of BTA and its derivatives, self-stabilization process of the adsorbed film, and filling of passivation film’s voids by oxide particles.

Successful applications of WCu alloys in high voltage electrical switches require their high strength and excellent conductivity. Unfortunately, the strategies for increasing their strength such as doping with fine particles and alloying often significantly decrease their conductivity. In this paper, we developed a new pathway for fabricating WCu alloys using spark plasma infiltrating sintering of copper-coated graphene (Cu@Gr) composite powders. Cu@Gr was found to partially prevent the formation of WC after sintering, and graphene was uniformly distributed on the surfaces of network Cu phases. Electrical conductivity of 38.512 M·S/m, thermal conductivity of 264 W·m−1·K−1 and microhardness of 278 HV were achieved for the sintered WCu composites doped with only 0.8 wt.% Cu@Gr powders, which showed 95.3%, 24.3%, 28% enhancement compared with those from the conventional sintering using the undoped WCu powders.

W70Cu30(W-30 wt.% Cu) alloys were fabricated using cold pressing and infiltration sintering methods from two types of powders, i.e., mixed copper-tungsten (M-Cu-W) powders and our newly developed copper-coated tungsten composite (Cu@W) powders. Microstructure, mechanical and arc-ablation properties of the W70Cu30 alloys were investigated, and the mechanism of enhanced physical/mechanical properties and arc-erosion resistance of the W70Cu30 alloys was discussed. For the W70Cu30 alloys prepared using the Cu@W powders, their physical properties, including hardness, electrical conductivity and relative density were much better than those prepared from the M-Cu-W powders. The W70Cu30 alloys fabricated from the Cu@W powders were free of cracks, and showed homogenous distributions of W and Cu network structures. Whereas for the alloys prepared from the M-Cu-W powders, segregation of Cu was observed and the segregation size was about 40–100 μm. Characterization of arc-erosion morphologies of the W70Cu30 alloys prepared with the Cu@W powders revealed the occurrence of evaporation of Cu phase; whereas that of W70Cu30 alloys prepared with the M-Cu-W powders revealed the occurrence of the sputtering of Cu. After arc breakdown for 200 times, mass loss of alloys made using the mixed powders was twice as much as those made using the coated composite powders. Based on the experimental results and theoretical analysis, an arc breakdown mechanism of the WCu-C alloys using the composite powders was proposed which is attributed to the formation of a homogeneous Cu-Cu network structure to uniformly disperse arc energy and dissipate the generated heat, thus prolonging the service life of the WCu alloy contacts.

The aim of this work was to join Molybdenum (Mo) and a graphite to obtain laminated structure using a high-temperature and hot press welding method with a Ti-Zr foil as the binder in a vacuum condition. The microstructure and composition changes of the interface were characterized by scanning electron microscopy and X-ray diffraction, respectively. The properties of the interface were studied using shear tests and hardness measurements. The optimal processing conditions for Mo and graphite joints involved a pressure of 0.5–1 MPa, holding time of 10–30 s, and brazing time of 5 min with a melting current of less than 150 A under a 1.3 × 10 −3 Pa vacuum condition. A transition zone with a width of approximately 0.3 mm and an average micro-hardness reaching 565 HV was formed, which is much higher than that of graphite and Mo. The main reason is the formation of different carbide and solid solutions such as MoC, Mo 9 Ti 4 , and Ti 2 C 0.06 . The shear bond strength of the graphite and molybdenum connection is 15.2 MPa, which is higher than that of graphite. The connection mechanism of Mo and graphite was the interlayer reacting with the Mo or graphite substrate at a high temperature to form compounds and solid solutions, respectively, covering the surface of the substrate. Therefore, an effective connection of Mo and graphite was successfully achieved. A model of the Mo and graphite connection structure was introduced.

Titanium alloys have been applied for many lightweight structural components in the fields of aerospace, automobiles and biomedical implants owing to their light-weight, good mechanical properties and biocompatibility. However, poor tribological performance often restricts their wide-range applications. In this study, we synthesized Cu modified Ti-6Al-4 V (TC4) powders with various Cu contents (0, 1, 3, 5, 10 wt%), which was further strengthened with 0.3 wt% graphene oxide nanosheets (GONs) using a powder metallurgy technology. These composite powders were then synthesized into titanium matrix composites using spark plasma sintering. Effects of Cu contents on microstructure evolution, phase composition and tribological properties of Ti matrix composites were systematically investigated. The synthesized composites were consisted of α-Ti, β-Ti, Ti2Cu, in-situ-formed TiC and remained GONs, and showed better tribological properties than those of TC4 alloy. The average coefficient of friction was reduced from 0.168 to a minimum value of 0.120 as the copper content increased from 0 to 3 wt%, meanwhile the wear volume loss was reduced by 49.3%. Whereas further increasing Cu contents resulted in the increases of both coefficients of friction and wear volume loss. These improvements are mainly attributed to the hardness strengthening effects by Ti-Cu intermetallics and TiC@GONs structure, as well as the self-lubricating effect of GONs. Compared with traditional surface modification processes, the new method proposed in this work is cost-effective and promising for improving the tribological performance of titanium alloys in industry applications.

In this study, the graphene was successfully prepared via the reduction of graphene oxide (GO) used thiourea dioxide as a new reductant. The graphene oxide after reduction with thiourea dioxide has been characterized by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Raman photoelectron spectroscopy and transmission electron microscopy (TEM). The factors affecting reduction degree of graphene were discussed in the process of reduction, such as reduction time, temperature and the amount of reducing agent. The result shows that reduction degree of graphene is mainly controlled by temperature and reaction time. Finally, it is founded that the stronger reduction capacity of thiourea dioxide was attributed to the synergistic effect of its hydrolysis products, such as HSO3− and catalyze the ring-opening reaction.

QAl9-4 aluminum bronze and 304 stainless steel composites were fabricated by vacuum smelting-casting at 1150 °C, in a vacuum of 4.0 × 10 −2 Pa. Microstructure and chemical composition were characterized by using transmission electron microscopy (TEM), X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). Properties of interface were studied by shear tests and hardness measurement. Between the QAl9-4 aluminum bronze and 304 stainless steel, a transition zone that the width is about 133 μm and the maximum micro-hardness value is 467 HV was observed. It is much higher than that of QAl9-4 aluminum bronze and 304 stainless steel, the main reason is considered to the formation of the brittle intermetallic compounds such as AlCrFe 2 , Al 4 Cu 9 and AlNi 3 , but these intermetallic compounds are very easy to cause the interfacial fracture. The interfacial shape close to 304 stainless steel is flat, but to QAl9-4 aluminum bronze is zigzag. The shear strength of the composite is 278 MPa. Microstructure formation mechanism of interfacial layer can be mainly considered as Fe, Cr atoms of steel substrate are given a priority in molten copper liquid compared with impurity elements (Mn, Si), eventually forming a good metallurgical bonding state on both sides of the substrate through inter-diffusion of interfacial atoms.

In this paper, arc ablation behavior of tungsten–copper (WCu) alloy contacts was investigated by the measurement of weight-loss, or erosion rate, and microstructures of tungsten–copper alloy contacts were examined by using scanning electron microscopy after electrical breakdown process. Experimental results showed that the arc erosion process started at the tiny sharp edges, then the arc spreaded on the second phases, or copper phase on the surface. Because of electrons/ions bombardment, the rapid evaporation and sputtering of liquid copper droplets were serious. As a result, an even contact surface became rugged, and tungsten skeleton remained because of being less eroded. With the increase of applied breakdown voltage, the ablation of tungsten–copper alloy became more significant. The WCu alloys with a higher content of copper showed more severe erosion rate as compared with that with a higher tungsten content under the same erosion conditions. Furthermore, when the number of breakdown is less, WCu alloys with a high copper content are easier for erosion for the conditions of fewer numbers of breakdowns. Whereas, WCu alloys with a high content of tungsten are easier for ablation for the conditions with many times of arc breakdown.

Nanocomposite powders based on metal-coated graphene were synthesized using an in-situ co-reduction method in order to improve wettability and interfacial bonding between graphene and metal. Graphene oxide (GO) of 2~3 atomic layers was synthesized using the Hummer&rsquo, s method with graphite as a raw material and then dispersed into a dispersing agent solution mixed with N-Methyl pyrrolidone and deionized water to form a homogeneous GO suspension, which was finally added into electroless plating solutions for the reduction process. Copper-coated graphene (Cu@graphene) and nickel-coated graphene (Ni@graphene) were synthesized using this one-step and co-reduction method by mixing salt solutions containing metal ions and GOs into the plating solution. The Cu ions or Ni ions were adsorbed and bonded onto the edges and surfaces of graphene, which was reduced from the GOs using a strong reducing agent of ascorbic acid or sodium borohydride. Crystalline Cu particles with an average size of about 200 nm were formed on the surface of graphene, whereas amorphous or nanocrystalline Ni particles with an average size of 55 nm were formed on the surface of graphene. Distribution of these metal particles on the graphene is homogeneous and highly dispersed, which can effectively improve the sinterability of composite powders. Cohesive energy distribution between graphene and metal interface was analyzed using first-principle calculation method. Formation mechanism of metal coated graphene was identified to be that both the GO and metal ions were simultaneously reduced in the reducing agents and thus a chemical bonding of graphene/metal was formed between the metal particles and graphene.

Ti matrix composites reinforced with 0.6 wt% reduced graphene oxide (rGO) sheets were fabricated using spark plasma sintering (SPS) technology at different sintering temperatures from 800 °C to 1100 °C. Effects of SPS sintering temperature on microstructural evolution and mechanical properties of rGO/Ti composites were studied. Results showed that with an increase in the sintering temperature, the relative density and densification of the composites were improved. The Ti grains were apparently refined owing to the presence of rGO. The optimum sintering temperature was found to be 1000 °C with a duration of 5 min under a pressure of 45 MPa in vacuum, and the structure of rGO was retained. At the same time, the reaction between Ti matrix and rGO at such high sintering temperatures resulted in uniform distribution of micro/nano TiC particle inside the rGO/Ti composites. The sintered rGO/Ti composites exhibited the best mechanical properties at the sintering temperature of 1000 °C, obtaining the values of micro-hardness, ultimate tensile strength, 0.2% yield strength of 224 HV, 535 MPa and 446 MPa, respectively. These are much higher than the composites sintered at the temperature of 900 °C. The fracture mode of the composites was found to change from a predominate trans-granular mode at low sintering temperatures to a ductile fracture mode with quasi-cleavage at higher temperatures, which is consistent with the theoretical calculations.

Copper-aluminum (Cu-Al) based lamellar composites were prepared using a solid-liquid compound casting (SLCC) technology. Characterization results showed that the Cu-Al composites were fully-sintered at 700 °C under an argon atmosphere using the SLCC technology. Cu-Al interfacial bonding was uniform with a well-defined transitional and inter-diffusion region. Intermetallic compounds and solid solutions of CuAl2, CuAl, Cu9Al4, CuAl3 and Cu3Al2 were detected at the interfacial region. With the increase of annealing temperature, the width of the Cu-Al interfacial region was increased, and the interfacial bonding strength was also increased, whereas the types of the intermediate phases were changed. With the increase of dwelling time at a given annealing temperature, the width of Cu-Al interfacial region was increased, the interfacial bonding strength was decreased and the mesophases were changed. The bonding strength of the as-prepared composite was 30 MPa, whereas those of specimens annealed at 200 °C for 2 h, 300 °C for 2 h, 400 °C for 2 h, 300 °C for 30 min and 300 °C for 1 h were 59, 39, 74, 56, and 49 MPa, respectively. The Cu-Al interfacial bonding mechanisms were identified to be rapid inter-diffusion of copper and aluminum and formation of interfacial and graded microstructures. The formation of copper-aluminum interface is a combined result of inter-atomic diffusion and interfacial chemical reactions, the latter of which is more dominant in the diffusion process.

Al 2 O 3 /Cu (with 30 wt% of Cu) composites were prepared using a combined liquid infiltration and spark plasma sintering (SPS) method using pre-processed composite powders. Crystalline structures, morphology and physical/mechanical properties of the sintered composites were studied and compared with those obtained from similar composites prepared using a standard liquid infiltration process without any external pressure. Results showed that densities of the Al 2 O 3 /Cu composites prepared without applying pressure were quite low. Whereas the composites sintered using the SPS (with a high pressure during sintering in 10 min) showed dense structures, and Cu phases were homogenously infiltrated and dispersed with a network from inside the Al 2 O 3 skeleton structures. Fracture toughness of Al 2 O 3 /Cu composites prepared without using external pressure (with a sintering time of 1.5 h) was 4.2 MPa m 1/2 , whereas that using the SPS process was 6.5 MPa m 1/2 . These toughness readings were increased by 18% and 82%, respectively, compared with that of pure alumina. Hardness, density and electrical resistivity of the samples prepared without pressure were 693 HV, 82.5% and 0.01 Ω m, whereas those using the SPS process were 842 HV, 99.1%, 0.002 Ω m, respectively. The enhancement in these properties using the SPS process are mainly due to the efficient pressurized infiltration of Cu phases into the network of Al 2 O 3 skeleton structures, and also due to high intensity discharge plasma which produces fully densified composites in a short time.