Wear and Wear Transition in Silicon Carbide Ceramics during Sliding

Wear and wear transition in silicon carbide ceramics during sliding have been investigated. Three different microstructures, i.e., solid-state-sintered silicon carbide, liquidphase-sintered silicon carbide, and a liquid-phase-sintered SiC-TiB₂ composite, were produced by hot pressing. Wear data and examinations of worn surfaces showed that the wear behavior of these silicon carbide ceramics was significantly different. In the solid-state-sintered silicon carbide, the wear occurred by a grooving process. In the liqudphase-sintered silicon carbide and composite, on the other hand, an abrupt transition in the wear mechanism from an initial grooving process to a grain pullout process occurred during the test. The transition occurred significantly earlier in the composite than in the carbide. The different wear behavior in these silicon carbide ceramics is discussed in relation to the grain or interphase boundary strength.     

Finite Element Analysis of Ceramic Coatings under Spherical Indentation with Metallic Interlayer: Part Ⅰ Uncracked Coatings

Spherical indentation of ceramic coatings with metallic interlayer was performed by means of axisymmetric finite element analysis (FEA). Two typical ceramic coatings with relatively high and low elastic modulus deposited on aluminum alloy and carbon steel were considered. Various combinations of indenter radius-coating thickness ratios and interlayer thickness-coating thickness ratios were used in the modeling. The effects of the interlayer, the coating and the substrate on the indentation behavior, such as the radial stress distribution along the coating surface as well as the coating interface, and the plastic deformation zone evolution in the substrate were investigated in connection with the above mentioned ratios. The coating cracking dominant modes were also discussed within the context of the peak tensile stresses on the coating surface and on the coating interface.     

Tribological behavior of Fe-based bulk metallic glass

The unlubricated tribological characteristics of coating layers containing an Fe-based amorphous matrix have been investigated in air using an alumina ball with a diameter of 3.1 mm. From the ball-on-disk test system, the friction coefficient could be measured for up to 3,000 seconds. The temperature changes on the worn surfaces were also simultaneously measured using an infrared thermometer. Three different types of coating layers having an Fe-based amorphous matrix, an Fe-based amorphous matrix with embedded Ni-based self-fluxing alloy particles, and an Fe-based amorphous matrix with embedded WC particles were prepared through a high velocity oxy-fuel (HVOF) process. Although the coating layers have certain levels of porosity, unique frictional characteristics attributed to the amorphous matrix were observed during the friction tests. Compared with conventional bearing steels such as AISI 521000 (Hv=840), excellent tribological and wear characteristics were obtained, demonstrating that an Fe-based bulk metallic glass powder is a viable engineering material for practical anti-wear coating applications.     

Tribological characteristics of magnetron sputtered MoS2 films in various atmospheric conditions

The friction and wear behaviors of magnetron sputtered MoS₂ films were investigated through the use of a pin and disk type tester. The experiments were performed for two kinds of specimens (ground (Ra 0.5μm) and polished (Ra 0.01 μm) substrates) under the following operating condifions: linear sliding velocities in the range of 22 -66 mm/s(3 types), normal loads varying from 9.8-29.4 N(3 types) and atmospheric conditions of air, medium and high vacuum(3 types). Silicon nitride pin was used as the lower specimen and magnetron sputtered MoS₂ on bearing steel disk was used as the upper specimen. The results showed that low friction property of the MoS₂ films could be identified in high vacuum and the specific wear rate in air was much higher than that in medium and high vacuum due to severe oxidation. It was found that the main wear mechanism in air was oxidation whereas in high vacuum accumulation of plastic flow and adhesion, were the main causes of wear.     

Tribological performance of Al2O3/NiCr coating

The tribological performance of Al₂O₃/NiCr coating deposited on steel (SM45C) was investigated under lubrication. The parameters of sliding wear consist of normal load and coating thickness. Test result showed that there was no evidence of an improved bonding strength in the coating. However, the wear resistance of the Al₂O₃/NiCr coating was significantly greater than that of the Al₂O₃ coating. It was evident that the residual stress for the AI2O3 coating was higher than that of the Al₂O₃/NiCr coating from the scratch test failure of coating. The bond coating played an important role in decreasing the residual stress. Also, it was found that the residual stress had a notable influence on the wear mechanism.     

Element Analysis of Ceramic Coatings under Spherical Indentation with Metallic Interlayer：Part Ⅱ Ring Crack

Spherical indentation of ceramic coatings with metallic interlayer was performed by means of axisymmetric finite element analysis （FEA）. Two typical ceramic coatings with relatively high and low elastic modulus deposited on aluminum alloy and carbon steel were considered. The fracture mechanics of the ceramic coatings mechanisms due to occurrence of surface ring cracks extending traverse the coating thickness under spherical indentation are investigated within the framework of linear fracture mechanics. The J-integral associated to such cracks was computed. The evolution of J-integral vs the crack length and the indentation depth was studied. The effects of the interlayer, the coating and the substrate on the J-integral evolution were discussed. The results show that a suitable metallic interlayer can improve the fracture resistance of the coating systems under the same indentation conditions through reducing the J-integral.     

Antioxidation Synergism Between ZnDTC and ZnDDP in Mineral Oil

Antioxidation synergistic effects between ZnDTC and ZnDDP in a mineral base oil were investigated by the Rotary Bomb Oxidation Test (RBOT) and the Indiana Stirring Oxidation Test (ISOT). The results indicate that there is strong antioxidation synergism between the two additives. Fourier transmission infrared spectroscopy (FTIR) analyses show that the oxidation products in the tested oils simultaneously containing the two additives are much less than those in the tested oils containing a single additive and the base oil.     

Element Analysis of Instrumented Sharp Indentations into Pressure-sensitive Materials

Finite element analysis was carried out to investigate the conical indentation response of elastic-plastic solids within the framework of the hydrostatic pressure dependence and the power law strain hardening. A large number of 40 difierent combinations of elasto-plastic properties with n ranging from 0 to 0.5 and σy/E ranging from 0.0014 to 0.03 were used in the computations. The loading curvature C and the average contact pressure Pave were considered within the concept of representative strains and the dimensional analysis.Dimensionless functions associated with these two parameters were formulated for each studied value of the pressure sensitivity. The results for pressure sensitive materials lie between those for Von Mises materials and the elastic model.     

Sliding wear behavior of Fe-based bulk metallic glass at high temperature

In the past decade Fe-based bulk metallic glasses (BMGs) have attracted increasing attention due to their beneficial properties, including high glass forming ability (GFA), high strength and hardness and high fracture toughness in both fundamental science and engineering application. Most research using these materials has been conducted at room temperature environment, and research that assesses their behavior especially at high temperature has been scarce. We present the results of high temperature effect on the friction and wear behavior of Fe-based bulk metallic glass (BMG), and we tested that this material may satisfy wear and oxidation resistance at high temperature as well as to explore the high temperature wear mechanism of the Fe-based BMG. The dry sliding tribological behaviors of Febased BMG against Si₃N₄ ceramic were conducted with a pin-on-disc friction and wear tribometer. The morphology of the worn surfaces of Fe-based BMG was examined by scanning electron microscopy (SEM) and the chemical composition characterized with energy dispersive spectroscopy (EDS) to observe the wear characteristics and investigate the wear mechanisms. The overall average friction coefficient value generally decreased with increasing temperature, and the glass transition and the formation of protective oxide film played an important role in the tribological behavior of BMG. The wear resistance of Fe-based BMG was not only from their hardness but also from the formation protective oxide layer. Analysis of the worn surface revealed abrasion, plastic deformation and oxidation during sliding test.     

Tribological Performance of Alternating-Layered Si-DLC/DLC Films Under Humid Conditions

The tribological behavior of alternating-layered diamond-like carbon (DLC) films was examined under a variety of humid conditions. Alternating deposited layers with Si-incorporated DLC (Si-DLC) and DLC films were prepared using a hybrid coating system. The residual stress of the alternating-layered films was reduced while the hardness was relatively less dependent on the number of alternating-layered sets. A ball-on-disk type tribological test was carried out under the following humid conditions: dry, 50% and 90% relative humidity. The friction coefficient for higher number of alternating-layered sets decreased with increasing humidity conditions but there was no dependency on the wear rate.