Microstructure and mechanical properties of Mo coating deposited by supersonic plasma spraying

The inherent technological limitations of traditional thermal spraying technologies have the relatively high oxidation degree and porosity of prepared Molybdenum (Mo) coating. Hence, in this study, a Mo coating was fabricated using supersonic plasma spraying technology. Scanning electron microscopy (SEM), X-ray diffraction (XRD), tensile testing machine, high-precision macro/micro-indentation tester and ball-on-disc tribometer were used to investigate the microstructure, phase composition, mechanical and tribological properties of the coating. The results showed that the Mo coating had dense structure and highly diminished oxidation degree. The phase composition was pure Mo. The tensile strength failure occurred within the coating layer. In the meanwhile, the Mo coating exhibited a superior hardness, a high elastic work/total work (W_e/W_t), and excellent tribological properties. The wear mechanisms were fatigue wear and adhesive wear.     

ZnO晶须对激光裂解Ti-Si复合陶瓷涂层结构和摩擦磨损及耐腐蚀性能的影响

采用先驱体转化陶瓷法(PDC法)制备含ZnO晶须的Ti-Si复合陶瓷涂层,将ZnO晶须添加到激光裂解Ti-Si复合陶瓷涂的先驱体中,增强Ti-Si复合陶瓷涂层的防腐蚀性能和减摩耐磨性能。通过扫描电子显微镜(SEM)、X射线衍射仪(XRD)、往复式摩擦磨损测试仪、电化学工作站等手段,分析含不同质量分数ZnO晶须的Ti-Si复合陶瓷涂层的元素组成及存在形式、表面形貌、摩擦磨损性能以及防腐蚀性能。添加ZnO晶须对Ti-Si复合陶瓷涂层的组成和化学价态没有影响,但添加ZnO晶须对Ti-Si复合陶瓷涂层的防腐性能有改善,添加ZnO晶须对Ti-Si复合陶瓷涂层的减摩性能有改善,在较高载荷下添加ZnO晶须可以降低Ti-Si复合陶瓷涂层的摩擦因数,添加ZnO晶须质量分数为10%所得的Ti-Si复合陶瓷涂,载荷为5N和7N时摩擦因数均比45钢低52%。添加不同质量分数ZnO晶须对复合陶瓷涂层表面表面形貌有很大影响,同时可以改善Ti-Si复合陶瓷涂层摩擦磨损性能以及防腐蚀性能。     

High temperature tribology of heavily boron doped diamond films against steel

Despite the fact that Fe, Co, and Ni catalyze the phase transition of diamond into graphite, the question of the applicability of diamond as a functional coating of a metal-cutting tool is still open. For this reason, our work contains investigation of wear and friction of heavily boron doped diamond films against steel at elevated temperature, as well as influence of boron concentration on diamond film oxidation resistance. The obtained data indicated that minimum CoF value is achieved in the temperature range within 570–670 °C and strongly depends on boron concentration in coating (CoF decreases with increasing of boron concentration). Wear rate has the same dependence as the CoF, whereas oxidation resistance decreases with increasing of boron concentration. Besides, the presented results are first obtained for boron doped diamond films synthesized under high B/C ratio conditions (of up to 333 ppb).     

Synthesis of amorphous carbon films by plasma-based ion implantation with simultaneous application of DC and pulse bias

Carbon films were synthesized on a Si wafer by simultaneous application of pulse bias and DC bias by a plasma-based ion implantation system using an electron cyclotron resonance (ECR) plasma source with a mirror field. The relationship between the pulse biasing voltage and the properties of carbon films was investigated. The hardness and tribological properties of the carbon film improved as the pulse bias voltage was decreased from −10 kV to −2 kV. Diamond-like carbon (DLC) films with a low friction coefficient were formed by simultaneous application of a low pulse bias voltage, such as −2 kV, and a DC bias. During the friction test of the DLC film, excellent tribological properties were observed under a high conducted load, such as 20 N, which shows that not only the friction coefficient but also the durability during the friction test was improved. The improvement of the tribological property was attributed to the formation of a mixed layer at the interface between the DLC film and the Si substrate.     

Study of the mechanical properties of tetrahedral amorphous carbon films by nanoindentation and nanowear measurements

Nanoindentation and nanowear measurements, along with the associated analysis suitable for the mechanical characterization of tetrahedral amorphous carbon (ta-C) films are discussed in this paper. Films of approximately 100-nm thick were deposited on silicon substrates at room temperature in a filtered cathodic vacuum arc evaporation system with an improved S-bend filter that yields films with high values of mass density (3.2 g/cm³) and sp³ content (84–88%) when operating in a broad bias voltage range (−20 V to −350 V). Nanoindentation measurements were carried out on the films with a Berkovich diamond indenter applying loads in the 100 μN–2 mN range, leading to maximum penetration depths between 10 and 60 nm. In this measurement range, the ta-C thin-films present a basically elastic behavior with high hardness (45 GPa) and high Young's modulus (340 GPa) values. Due to the low thickness of the films and the shallow penetration depths involved in the measurement, the substrate influence must be taken into account and the area function of the indenter should be accurately calibrated for determination of both hardness and Young's modulus. Moreover, nanowear measurements were performed on the films with a sharp diamond tip using multiple scans over an area of 3 μm², producing a progressive wear crater with well-defined depth which shows an increasing linear dependence with the number of scans. The wear resistance at nanometric scale is found to be a function of the film hardness.     

The role of compound formation in reactive wetting: the Cu/SiC system

Wettability and reactivity in the liquid Cu/SiC couple are studied by the sessile drop technique in high vacuum or purified helium using monocrystalline or sintered α-SiC. This couple exhibits a very unusual wetting behaviour. Particularly, during spreading, an amazing drop shape is established and, for specific conditions, “hexagonal wetting” is observed. We show that these observations can be correctly explained using the concepts of reactive wetting developed recently.     

Enhancement of oxidation resistance at 1000–1400 °C of low carbon Al2O3–C refractories with pre-synthesized SiCnw/Al2O3

The oxidation resistance of low carbon Al₂O₃–C refractories with the addition of SiC_nw/Al₂O₃ composite powders and the enhancement mechanisms were investigated. The oxidation resistance was evaluated by oxidation index (O.I.) and oxidation rate constant (k). The enhancement mechanisms of SiC_nw/Al₂O₃ on oxidation resistance were analyzed based on the phases and microstructures. The results showed that the SiC_nw/Al₂O₃ can improve the oxidation resistance of Al₂O₃–C refractories, the O.I. and k of A6 (6 wt% SiC_nw/Al₂O₃ addition) were 26.0% and 34.5% lower than those of reference sample A0, respectively. The oxidation resistance of refractories was improved in a range of 1000–1400 °C due to the introduction of SiC_nw/Al₂O₃. The enhancement mechanisms can be explained that SiC_nw is more susceptible to be oxidized due to its high specific surface area, which expanded the action temperature range of other antioxidants and itself. The mullite and dense protective layer generated during oxidation is also beneficial to impede the diffusion of O₂.     

中碳钢水下湿法激光焊接焊缝成形行为与性能

为了探索水下湿法激光焊接的可行性,使用光纤激光器探讨了激光功率、离焦量、水深等因素对45钢湿法焊缝成形性的影响,对比研究了空气中焊接和水下焊接焊缝的组织性能。结果表明,水下湿法激光焊接过程中,在水中入射激光和工件表面之间形成了一个“激光通道”,这个通道能否稳定存在影响着焊接的稳定性。不同的激光功率对应不同的水深阈值,当水深超过此阈值后,水会对激光产生强烈的屏蔽作用,导致焊接无法进行。激光功率、离焦量和水深影响湿法焊接焊缝的成形性,激光功率增加有利于形成外观良好的焊缝,适当的负离焦有利于形成具有较大深宽比的焊缝,水深增加不利于焊缝成形。空气中焊接焊缝中心主要是珠光体+铁素体,而水下焊接的焊缝主要是马氏体和少量残余奥氏体。水下焊接焊缝硬度高于空气中焊接,拉伸强度达606 MPa,达到基体拉伸强度的94.8%,断后伸长率降低至3.1%。     

空间环境因素对涂层型自润滑关节轴承磨损寿命影响规律及机理

目的 探究典型空间环境因素对涂层型自润滑关节轴承寿命的影响。方法 通过非平衡磁控溅射技术在GE17型关节轴承表面沉积了H-DLC(含氢类金刚石)薄膜,并使用自主研发的关节轴承磨损试验机,分别在大气、真空、原子氧侵蚀、紫外辐照4种环境下,对关节轴承进行了全寿命试验,通过扫描电子显微镜和白光三维轮廓仪等仪器,对关节轴承内外圈磨痕进行了表征。结果 在不同环境下关节轴承磨损过程中的摩擦扭矩信号和摩擦面温度信号都可以作为轴承磨损失效的物理信号,但是温度信号的突变点要早于扭矩信号;在大气、真空、原子氧侵蚀、紫外辐照4种环境下,关节轴承平均磨损寿命分别为87.48、30.55、17.06、29.37 h,轴承在4种环境下都主要发生黏着磨损和磨粒磨损,在原子氧侵蚀后的轴承还存在一定的氧化磨损。此外,轴承内圈磨损比轴承外圈更加严重,轴承外圈更容易吸附大量球状磨屑。结论 真空环境导致轴承内外圈产生的摩擦热不能快速排出,因此轴承摩擦接触面温度较高,H-DLC薄膜性能下降,导致轴承磨损寿命大幅度降低,原子氧侵蚀会导致H-DLC薄膜部分氧化分解,在原子氧侵蚀后轴承磨损寿命进一步降低,而紫外辐照对轴承磨损寿命影响不大。     

Effects of modification of hBN by nickel plating on coating structure and properties of supersonic plasma spraying NiCr-Cr3C2-hBN@Ni coatings

Hexagonal boron nitride (hBN) with excellent self-lubrication performance is expected to relieve the friction resistance and wear of NiCr–Cr₃C₂ coatings. However, the poor wettability of hBN with most materials makes it difficult to fabricate NiCr–Cr₃C₂-hBN composite coating with good cohesion strength. In this study, hBN was firstly pretreated through magnetron-sputtering aided Ni plating to form hBN@Ni particles. Then, NiCr–Cr₃C₂-hBN@Ni powder was prepared by spray granulation. Next, corresponding coatings were prepared through supersonic atmosphere plasma spraying. It was found that in comparison with NiCr–Cr₃C₂-hBN coating, the NiCr–Cr₃C₂-hBN@Ni coating exhibited a decreased porosity (from 3.6% to 0.3%), elevated cohesion (from 52.78 N to 62.11 N), and the wear rate decreased by an order of magnitude. It was concluded that hBN@Ni can effectively improve the component interface inside powder, enhance the cohesion of molten in-flight particles, and make the internal structure of the coating denser.