A meltable precursor for zirconium carbide ceramics and C/C-ZrC composites

For process simplification and rapid densification of ceramic composites, a meltable single-source ZrC precursor was prepared by condensing zirconium acetylacetonate (Zr(acac)₄) at 190?°C for 40–150?min. The preparation of ZrC precursor and the conversion from precursor to ceramics were investigated by using FTIR and NMR spectroscopies, GPC, DSC-TGA, XRD, SEM, EDS and TEM. The precursor had low viscosity (~ 10?mPa?s) and proper processing window (60?min) for precursor infiltration and pyrolysis (PIP). The ceramic yield at 1650?°C was 29.6%, and EDS revealed that the composition was (ZrC)_0.337(HfC)_0.0025(ZrO₂)_0.044C_0.1865. The ceramics were composed of 0.2–0.5?µm grains which aggregated to form a stacked structure surrounded by amorphous carbon. The preparation processes were designed, and C/C-ZrC composites with the density of 2.45?g/cm³ were successfully fabricated through 11 cycles of PIP with Zr(acac)₄. In conclusion, the synthetic method provides a simple and cheap route for precursors, and allows combined composite preparation with high efficiency.     

Tribological behavior of nanostructured high velocity oxy-fuel (HVOF) thermal sprayed WC-17NiCr coatings

In this research, the nanostructured WC-17NiCr cermet coatings were developed using the high velocity oxy-fuel (HVOF) thermal spraying processes on ACI CD4MCu cast duplex stainless steel substrates, widely used in pump industry for abrasive wear protection of surfaces. The coatings, sprayed by both robotic and manual methods, had two different fuel (methane) to oxygen ratios (FTOR), namely 0.68 and 0.62. Using different analytical and microstructural techniques, the microstructural characteristics of the powder particles and mechanical, microstructural, and tribological properties of the coatings were determined. Different morphologies were assigned to sprayable particles, namely spherical, apple, donut, irregular, and mixed. It was revealed that the rate of WC decarburization had increased with increasing the FTOR. In contrast, the scanning electron microscopy and image analyses showed that the lowest porosity percentage was obtained for the robotically-sprayed coating with 0.68 FTOR. The Vickers microhardness increased along with fracture toughness, which can be attributed to the effect of the ‘duplex structure’ associated with the particle outer coating of Co and is a novelty in the research. The pin-on-disk reciprocal sliding wear tests at various loadings had shown different wear rates in the coatings. It was inferred that the wear performance was improved with the microstructural homogeneity, hardness, and the fracture toughness in the coatings. In all coatings, lower coefficient of friction (COF) was observed at higher loads. Finally, the wear mechanisms involved in the wear processes were identified as deformation and removal of the binder, fracture and pullout of the carbide particles, and delamination and spallation of the splats.     

Design of newly fabricated tribological machine for wear and frictional experiments under dry/wet condition

Nowadays, there is demand to evaluate tribological performance of new engineering materials using different techniques. Various laboratory tribo-machines have been designed and fabricated such as Pin-on-Disc (POD), ASTM G99, Block-on-Ring (BOR), ASTM G77 or G137-953, Dry Sand Rubber Wheel (DSRW), ASTM G655, Wet Sand Rubber Wheel (WSRW), ASTM G105, and sand/steel wheel test under wet/dry conditions (ASTM B611). A concept of integrating more than one tribo-technique at different contact mechanisms (line or area) working simultaneously under same test condition against same material is introduced in a current designed machine. Different wear modes (adhesive, two-body-abrasive, three-body-abrasive, under dry, lubricated, or slurry conditions) can be conducted on the same machine. Results of adhesive wear, friction and interface temperature of glass fibre reinforced polyester composite under wet/dry contact condition are reported at 50 N load for different sliding speeds (2.8–7.8 m/s) using the new machine. Weight loss and friction coefficient of the composite were substantially influenced by introducing water as lubricant. Additionally, the contact condition has the high influence key on the wear and frictional performance of the composite.     

不同环境下AlSiFeMm非晶纳米晶涂层摩擦磨损行为研究

目的研究AlSiFeMm(Mm为镍包混合稀土)非晶纳米晶涂层在干摩擦和3.5%NaCl溶液中的摩擦磨损行为。方法采用Rtec(MFT-3000)往复式磨损试验机测试涂层在干摩擦条件和有腐蚀介质条件下的摩擦磨损性能,使用LEXTOL3000-IR非接触三维表面轮廓仪测定涂层的磨损体积和磨痕的三维形貌,利用扫描电子显微镜对磨痕进行形貌观察和成分分析。结果铝基非晶纳米晶涂层的摩擦系数随着载荷的增加而不断减小。干摩擦条件下,铝基非晶纳米晶涂层的磨损率随着载荷的增加而增大,当磨损速度为10 mm/s、载荷为15 N时,涂层相对耐磨性为6061铝合金的2.5倍,其磨损机制为脆性剥落、磨粒磨损,并伴随氧化磨损。在3.5%NaCl溶液中,涂层的磨损率随着载荷的增加而逐渐降低,当磨损速度为35 mm/s、载荷为30N时,涂层的耐磨性能约为6061铝合金的8倍,其失效机制主要为剥层磨损和腐蚀磨损。结论铝基非晶纳米晶复合涂层在干摩擦和腐蚀介质中均表现出较为优异的耐磨性能,可以作为轻质合金涂层应用于表面防护领域。