Yimin Gao

Meta‐analysis of amiodarone versus beta‐blocker as a prophylactic therapy against atrial fibrillation following cardiac surgery

Current guidelines recommend beta‐blocker as the first‐line preventive treatment of atrial fibrillation (AF) after cardiac surgery; if beta‐blocker therapy is contraindicated, then amiodarone is recommended. There is still lack of strong evidence of directly comparing the efficacy of amiodarone and beta‐blocker in preventing postoperative AF (POAF).

Tribological Behavior of Si3N4-hBN Ceramic Materials without Lubrication under Different Test Modes

Unlubricated tribological behaviors of silicon nitride–boron nitride (Si 3 N 4 -hBN) composites were investigated with two test modes in air by using a pin-on-disc tribometer. Under upper-disc-on-bottom-pin test mode, the addition of hBN to Si 3 N 4 resulted in a significant decrease of the friction coefficient, from 0.54 for Si 3 N 4 against Si 3 N 4 to 0.19 for Si 3 N 4 -20% hBN against Si 3 N 4 . The surface analysis indicated that a tribochemical film consisting of SiO2 and H 3 BO 3 was formed on the wear surfaces. The formation of tribochemical film might be attributed to the embedment of wear debris into the spalling pits on the wear surfaces of Si 3 N 4 -hBN specimen. The wear debris reacted with moisture in air, and the resultant tribochemical film lubricated the wear surfaces. Under upper-pin-on-bottom-disc test mode, the wear mechanism was dominated by abrasive wear, and no tribochemical products could be detected on the wear surfaces. A slight decrease of the friction coefficient, from 0.85 for Si 3 N 4 /Si 3 N 4 to 0.56 for Si 3 N 4 /Si 3 N 4 -30% hBN, was obtained, which might be attributed to the layered structure of hBN.

Effect of Ag2Mo2O7 Incorporation on the Tribological Characteristics of Adaptive Ni-Based Composite at Elevated Temperatures

A high-temperature self-lubricating Ni-based composite was prepared with the addition of Ag2Mo2O7 by a powder metallurgy technique. X-ray diffraction (XRD) analysis showed that Ag2Mo2O7 decomposed to Ag and MoO3 during the sintering process. The tribological properties of Ni-based composites were evaluated, and the effects of Ag2Mo2O7 incorporation on the friction and wear characteristics of composites were analyzed. The results showed that the tribological properties of Ni-based composite deteriorated with increasing temperature, and the addition of Ag2Mo2O7 decreased the coefficient of friction and wear rate of Ni-based composite at medium and high temperatures. The coefficient of friction and wear rate of composite containing 20 wt% Ag2Mo2O7 were lowered to 0.33 and 1.51 × 10−5 mm3 N−1 m−1, respectively. The wear mechanism was characterized by plastic deformation and delamination. In addition, the improvement of tribological properties of the Ni-based composite at high temperatures is attributed to synergistic lubricating effect of silver molybdate (reproduce in the rubbing process at high temperatures) and iron oxide (transfer from disk material to the pin).

Interface Structure and Wear Behavior of Cr26 Ferrous Matrix Surface Composites Reinforced with CTCP

Using cast tungsten carbide particles (CTCP) and reduced iron powders as raw materials, the porous ceramic preforms with honeycomb, strip, and layer structure, respectively, were prepared by loose sintering process; then, the CTCP/Cr26 ferrous matrix composites were fabricated by casting infiltration process. The microstructure of the composites was analyzed by SEM, XRD, and EDS. The results show that a sintered shell forms as a result of the reaction of Fe and W2C in the CTCP during loose sintering process; the inner part of the shell around the CTCp consists of WC and Fe3W3C phases, while the outer part between the particles is dominated by Fe3W3C. Therefore, the strength of preforms is enhanced because the particles are connected with each other by sintered shell. During casting infiltration process, a transition layer constituted by WC and Fe3W3C formed at the interface of CTCp and the matrix due to the dissolution and precipitation of the sintered shell in the high-temperature liquid iron. The three-body abrasive wear behavior of the composites was investigated. The result shows the wear resistance of honeycomb structure composite is comparable to that of whole layer (WL) structure composite, which is three times of heat-treated Cr26. However, the honeycomb structure composite has higher performance/cost ratio owing to the lower CTCp volume fraction and higher strength and toughness compared with the WL structure composite.

Three-body abrasive wear resistance of iron matrix composites reinforced with ceramic particles

Pure aluminum oxide (Al2O3), ZrO2 toughened Al2O3 (ZTA; 25–30% ZrO2), and ZTA (55–60% ZrO2) ceramic particles reinforced Cr25 matrix composites are successfully fabricated by the infiltration casting process. The volume fraction of ceramic particles in the composites is 47–55%. The interface structure between ceramic particles and the matrix was analyzed by digital probing microscopy and was found to be continuous mechanical bonding. The wear behavior of the composites was studied by a three-body abrasive wear tester. ZTA (55–60% ZrO2) reinforced Cr25 composite has excellent wear resistance which is approximately six times higher than that of the high chromium cast iron matrix. It is discovered that the wear mechanism of composites is micro-cutting and fatigue stripping. A correlation exists between the wear resistance of composites and the strength of ceramic particles.

Tribological properties of in situ Fe3Al-20 wt% Al2O3 composites

The tribological properties of Fe3Al-20 wt% Al2O3 composites, prepared by means of mechanical alloying and plasma-activated sintering, were investigated under air atmosphere against GCr15 bearing steels from room temperature to 500 °C. It was found that the friction coefficient of the composites (at 200 °C/350 °C) decreased with increasing temperature and the value was lower than that of Fe3Al alloys at the same temperature, and the friction coefficient increased again at 500 °C. Moreover, the wear rates of all the pins and discs were low at the medium/high temperatures. The dominant wear mechanism of the composites is microplowing and a little oxidation wear at room temperature. However, at elevated temperatures, oxidation wear and a little microplowing are found to be the main wear mechanism. There is also spalling induced by microfracture at 500 °C. The in situ Fe3Al-20 wt% Al2O3 composites have excellent wear resistance at a wide temperature range.

Structural, bonding, anisotropic mechanical and thermal properties of Al4SiC4 and Al4Si2C5 by first-principles investigations

Abstract The structural, bonding, electronic, mechanical and thermal properties of ternary aluminum silicon carbides Al4SiC4 and Al4Si2C5 are investigated by first-principles calculations combined with the Debye quasi-harmonic approximation. All the calculated mechanical constants like bulk, shear and Young’s modulus are in good agreement with experimental values. Both compounds show distinct anisotropic elastic properties along different crystalline directions, and the intrinsic brittleness of both compounds is also confirmed. The elastic anisotropy of both aluminum silicon carbides originates from their bonding structures. The calculated band gap is obtained as 1.12 and 1.04 eV for Al4SiC4 and Al4Si2C5 respectively. From the total electron density distribution map, the obvious covalent bonds exist between Al and C atoms. A distinct electron density deficiency sits between Al–C bond along c axis among Al4SiC4, which leads to its limited tensile strength. Meanwhile, the anisotropy of acoustic velocities for both compounds is also calculated and discussed.

Cr3C2–20%Ni cermets prepared by high energy milling and reactive sintering, and their mechanical properties

Cr3C2–20%Ni cermets were fabricated with Cr, C and Ni mixed-powder by high energy milling and reactive carburising sintering. The elemental powders of Cr, C and Ni were mixed with proper ratio (Cr/C = 3:2 atomic ratio) and milled to the nanometre crystallite sizes (about 20–30 nm). Specimens were cold-isostatically pressed at 200 MPa and sintered at 1453, 1503 and 1553 K with vacuum atmosphere for 1 h, respectively. It was shown that the relative density of the sintered specimens increases first and then decreases slightly with increasing sintering temperature. The maximum values of the hardness (86.7HRA) and bending strength (1140 MPa) were achieved at 1503 K.

Influence of Sliding Speed on the Tribological Characteristics of Si3N4-hBN Ceramic Materials

The influence of sliding speed on the unlubricated tribological behaviors of silicon nitride–boron nitride (Si3N4-hBN) composites was investigated with two modes in air by a pin-on-disc tribometer. Using the upper disc–on–bottom pin test mode, as the sliding speed increased, the friction coefficient of the sliding pairs showed an upward trend; for example, from 0.18 at the sliding speed of 0.40 m/s to 0.54 at the sliding speed of 1.31 m/s for the Si3N4/Si3N4–20% hBN pair. The surface analysis indicated that a tribochemical film consisting of SiO2 and H3BO3 formed on the wear surfaces of the Si3N4/Si3N4–20% hBN sliding pair at sliding speeds of 0.40 and 0.66 m/s. Moreover, the formation of this film lubricated the wear surfaces. At the sliding speed of 1.31 m/s, no tribochemical film formed on the wear surfaces, most likely due to the increase in surface temperature. In the upper pin–on–bottom disc test mode, the wear mechanism was dominated by abrasive wear, and no tribochemical products could be detected on the wear surfaces. The increase in sliding speed weakened the degree of abrasive wear, leading to a decrease in the friction coefficients.

Effect of Hot Forging on Microstructure and Abrasion Resistance of Fe-B Alloy

The effect of hot forging on the microstructure and abrasion resistance of Fe-B alloy was studied. The results showed that boride networks are broken down by hot forging. After hot forging, the hardness of Fe-B alloy increased marginally, and the toughness increased considerably. In the two-body abrasion test, unforged Fe-B alloy exhibited excellent wear resistance and soft abrasive tended to show higher wear resistance. When alloys were tested against very hard abrasives, the wear resistance of forged Fe-B alloy was similar to that of unforged Fe-B alloy, but in the case of soft abrasives the wear resistance of forged Fe-B alloy was lower than that of unforged Fe-B alloy.