Sn-containing Si3N4-based composites for adaptive excellent friction and wear in a wide temperature range

Ceramic design based on reducing friction and wear-related failures in moving mechanical systems has gained tremendous attention due to increased demands for durability, reliability and energy conservation. However, only few materials can meet these requirements at high temperatures. Here, we designed and prepared a Sn-containing Si₃N₄-based composite, which displayed excellent tribological properties at high temperatures. The results showed that the friction coefficient and wear rate of the composites were reduced to 0.27 and 4.88 × 10^?6 mm³ N^?1 m^?1 in air at 800 °C. The wear mechanism of the sliding pairs at different temperatures was revealed via detailed analyses of the worn surfaces. In addition, the tribo-driven graphitization was detected on the wear surfaces and in the wear debris, and the carbon phase was identified by SEM, TEM, and Raman spectrum.     

Corrosive wear behavior of HVOF-sprayed micro-nano-structured Cr3C2–NiCr cermet coatings under aqueous media

A novel micro-nano-structured Cr₃C₂–NiCr cermet coating was prepared on 316L stainless steel by high-velocity oxygen fuel spraying technology (HVOF). Cermet coatings with different contents of micro-and nano-sized Cr₃C₂ particles as the hard phase and a NiCr alloy matrix as the bonding phase were prepared and characterized in terms of porosity, microhardness, and corrosive wear resistance in a 3.5% NaCl solution and artificial seawater. Compared to nanostructured coatings, micro-nano-structured coatings avoid decarburization and reduce nanoparticle agglomeration during the spray process, and mechanical and electrochemical properties were improved in comparison with those of conventional coatings. The micro-nano-structured Cr₃C₂–NiCr coating rendered low porosity (≤0.34%) and high microhardness (≥1105.0HV_0.3). The coating comprising 50% nano-sized Cr₃C₂ grains exhibited the best corrosive wear resistance owing to its densest microstructure and highest microhardness. Furthermore, compared to static corrosion, the dynamic corrosion of the coatings led to more severe mechanical wear, because corrosion destroyed the coating surface and ions promoted corrosion to invade coatings through the pores during corrosion wear.     

Exploring the influences of the counterpart materials on friction and wear behaviors of atmospheric plasma-sprayed YSZ coating

Sliding wear behaviors of atmospheric plasma-sprayed Yttria Stabilized Zirconia (YSZ) coating mated with four metallic or ceramic counterparts (Si₃N₄, Al₂O₃, GCr15 and ZrO₂) were investigated. It has been found that YSZ coatings in contact with Si₃N₄ and GCr15 show better tribological performances than the other cases, which is due to the formation of the tribolayer mainly consisting of Si₃N₄ and Fe₂O₃ respectively on the worn surfaces. In the case of YSZ coating-Al₂O₃ and YSZ coating-ZrO₂ tribopairs, the wear debris are more irregular and larger in size, resulting in severe abrasive wear and brittle fracture of debris particles. In particular, the specific wear rate of YSZ coating sliding against GCr15 is negative due to the significant material transfer of the tribo-oxide layer, while that of YSZ coating sliding against ZrO₂ is the highest. Amorphization of the wear particles appears in the four cases due to the repeated mechanical action. It has been demonstrated that the wear of YSZ coating deteriorates with the increased flash temperature between the contact surfaces during rubbing process.     

类富勒烯碳薄膜的结构演变及摩擦学性能研究

通过等离子体增强化学气相沉积技术,以不同沉积时间在硅表面上制备类富勒烯碳薄膜,探究类富勒烯碳薄膜结构演变和摩擦学性能随沉积时间变化规律。利用拉曼光谱和透射电子显微镜,考察类富勒烯碳薄膜微结构和表面形貌随沉积时间的变化。结果表明:碳薄膜内类富勒烯结构含量随沉积时间先增加后保持不变;采用沉积时间为3 h的类富勒烯碳薄膜组成摩擦配伍对,当载荷从8 N增加到14 N时,摩擦因数从0.013降至0.006,即随载荷的增加实现了由低摩擦向超滑的转变。这是因为摩擦诱使类富勒烯碳薄膜发生结构转变,并形成有利于减少摩擦的类球状或外部石墨壳层闭合的纳米颗粒。     

Synthesis and characterization of polycrystalline Mo2BC ceramic

Single-phase polycrystalline Mo₂BC ceramic bulks were synthesized successfully from molybdenum, boron, and graphite powders using the spark plasma sintering method. Herein, it was established that the synthesis temperature of the Mo₂BC ceramic could be as low as 1300 °C. Transmission electron microscopy (TEM) characterization confirmed that the crystal structure of the Mo₂BC ceramic was comparable to that of the MoAlB ceramic. The Vickers hardness of the Mo₂BC ceramic was measured to be 18.1 GPa. Additionally, the compressive strength, flexural strength, and fracture toughness were determined to be 1.74 GPa, 457.72 MPa, and 3.26 MPa· m^1/2, respectively. The Mo₂BC bulk exhibited typical brittle features, in which intergranular and transgranular fractures were the main failure modes.     

Improving the corrosion resistance of a-C:H film by a top a-C:H:Si:O layer

During the deposition of a-C:H film, defects (pinholes or discontinuities) caused by excessive stress will inevitably appear, which will reduce the corrosion resistance of the a-C:H film. In this study, top a-C:H:Si:O layers (thickness of approximately 0.3 μm) on the surface of a-C:H films were deposited on a large scale by PACVD technology using acetylene (C₂H₂) and/or hexamethyldisiloxane (HMDSO) as reactants, to improve the corrosion resistance of a-C:H films while ensuring the appropriate overall hardness of the films. The corrosion behaviors of the films were studied by electrochemical impedance spectroscopy (EIS) and Tafel polarization. We found that the a-C:H/a-C:H:Si:O films possess a lower electrolyte penetration rate due to their stronger capacitance characteristics. In addition, the corrosion current density of the a-C:H/a-C:H:Si:O films (10^?10 A cm^?2) were reduced by 2 orders of magnitude compared to the a-C:H film (10^?8 A cm^?2), and by 3 orders of magnitude compared to 316 stainless steel (10^?7 A cm^?2). The impedance results obtained by EIS were simulated using appropriate equivalent circuits, and the corresponding electrical parameters were used to further verify the electrochemical protection behavior of the top a-C:H:Si:O layer.     

Ammonia borane assisted solid exfoliation of graphite fluoride for facile preparation of fluorinated graphene nanosheets

Fluorinated graphene, which combines the unique properties of graphite fluoride and graphene, has attracted considerable attention in recent years. Here, we developed a facile, efficient, and scalable method for high-yield exfoliation of graphite fluoride into fluorinated graphene (fluorographene) nanosheets. The exfoliation approach consists of solid ball milling of graphite fluoride with ammonia borane and followed washing with ethanol to get rid of ammonia borane from the products. The majority of the as-synthesized fluorographene nanosheets consist of 1–6 atomic layers with grain sizes in the range of 0.3–1 μm. X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy demonstrated that fluorographene has the same structure as pristine graphite fluoride.     

The influence of dehydration on the interfacial bonding,microstructure and mechanical properties of poly(vinyl alcohol)/graphene oxide nanocomposites

The relationship between the interfacial bonding, microstructure and mechanical properties of the poly(vinyl alcohol)/graphene oxide nanocomposites (PVA/GO) has been investigated by controlling the water content through a dehydration process. The interfacial bonding in PVA/GO was predominantly by hydrogen bonds which were strongly affected by the dehydration process. Micro-voids in the microstructure formed after dehydration due to the shrinkage of the fibrils. A variety of hydrogen bonds including water–water, water–GO and water–PVA can be replaced with the strong PVA–GO interfacial bond resulting in a transition from ductile to brittle fracture. The tensile modulus and strength properties of the PVA and PVA/GO increased as the amount of residual water reduced, while the fracture strain was decreased. The surface mechanical properties of PVA/GO measured by nanoindentation showed broadly similar trends with water content as the bulk mechanical properties. However, there was a threshold value of approximately 3 wt.% water below which the surface mechanical properties decrease slightly. The indentation modulus was higher than the tensile modulus by a factor of at least three. The combined influence of the microstructure and the distribution of water in the nanocomposite is considered to be responsible for this.     

Alloying behavior and novel properties of CoCrFeNiMn high-entropy alloy fabricated by mechanical alloying and spark plasma sintering

An equiatomic CoCrFeNiMn high-entropy alloy was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). During MA, a solid solution with refined microstructure of 10 nm which consists of a FCC phase and a BCC phase was formed. After SPS consolidation, only one FCC phase can be detected in the HEA bulks. The as-sintered bulks exhibit high compressive strength of 1987 MPa. An interesting magnetic transition associated with the structure coarsening and phase transformation was observed during SPS process.