Al/Graphene/CNT hybrid composites: Hardness and sliding wear studies

Graphene and carbon nanotubes are two carbon based materials known for their unique wear and friction properties. It would be quite interesting to understand the wear behavior of aluminium hybrid composites when these two nanosize reinforcements are incorporated into it. The hybrid composites with varying weight fractions of graphene (1, 2, 3 and 5 wt.%) and fixed CNT content of 2 wt.% were produced using powder metallurgy technique. The effect of varying graphene content on hardness and sliding wear of hybrid composites was studied. The wear tests were done as per ASTM G-99 standard with fixed sliding velocity (2 m/s) and sliding distance (1200 m) but varying applied load (10 - 30 N). Worn surface analysis was conducted using scanning electron microscope to arrive at wear mechanisms responsible for wear of aluminium and its hybrid composites. Increase in graphene content led to increase in bulk hardness with highest value of 61 RHN for hybrid composite with 3 wt.% graphene content. The wear rate of hybrid composites was found to be decreasing with enhancement in graphene content. Lower wear rate in hybrid composites was due to the formation of lubricating layer on the worn surface.

The Effect of Heat-Treatment on the Phase Stability of Fe-28Al-15Si-2Mo Alloy

The structures of Fe-28Al-15Si-2Mo iron aluminide in as cast state and in three states after heat-treatments were investigated for the verification of secondary phases stability. Short-term (at 1000 °C for 24 h and at 1200 °C for 2 h) as well as long-term (at 800 °C for 100 h) annealing were performed. Molybdenum addition enhances the high-temperature mechanical properties due to solid solution strengthening, however the mechanism of hardening could be modified (to solid solution strengthening + strengthening by incoherent precipitates) by another alloying element (f. e. Si or C). The phase compositions of alloys were described by means of scanning electron microscopy equipped with energy dispersive analysis. The complex Fe-Si-Mo carbides were found in the structure. The bulk hardness measurement and image analysis were performed for the verification of secondary phase stability. Particles became coarse with increasing temperature of annealing.

The Microstructure Evolution of Fe-28Al-15Si-0.2Zr Alloy during Different Heat-Treatment

The structures of Fe-28Al-15Si-0.2Zr iron aluminide in the as cast state and in three states after heat-treatments (at 800 °C for 100 hours, at 1000 °C for 24 hours and at 1200 °C for 2 hours) were investigated for verification of secondary phases stability. The type and distribution of precipitates were described by means of light optical microscopy and scanning electron microscopy equipped with an energy dispersive analysis. The presence of complex carbides based on Fe-Si-Zr was shown. The bulk hardness and image analysis of samples was measured for verification of dissolution of secondary phase particles to the matrix. Short-term annealing did not influence distribution and dissolution of secondary particles significantly, while long-term annealing (at 800 °C for 100 hours) leads to the sporadic formation of fine eutectic areas.

Facilitating TiB2 for Filtered Vacuum Cathodic Arc Evaporation

TiB2 is well established as a superhard coating with a high melting point and a low coefficient of friction. The brittle nature of borides means they cannot be utilised with arc evaporation, which is commonly used for the synthesis of hard coatings as it provides a high deposition rate, fully ionised plasma and good adhesion. In this work, TiB2 conical cathodes with non-standard sintering additives (carbon and TiSi2) were produced, and the properties of the base material, such as grain structure, hardness, electrical resistivity and composition, were compared to those of monolithic TiB2. The dependence of the produced cathodes’ electrical resistivity on temperature was evaluated in a furnace with an argon atmosphere. Their arc–evaporation suitability was assessed in terms of arc mobility and stability by visual inspection and by measurements of plasma electrical potential. In addition, shaping the cathode into a cone allowed investigation of the influence of an axial magnetic field on the arc spot. The produced cathodes have a bulk hardness of 23–24 GPa. It has been found that adding 1 wt% of C ensured exceptional arc-spot stability and mobility, and requires lower arc current compared to monolithic TiB2. However, poor cathode utilization has been achieved due to the steady generation of cathode flakes. The TiB2 cathode containing 5 wt% of TiSi2 provided the best balance between arc-spot behaviour and cathode utilisation. Preventing cathode overheating has been identified as a main factor to allow high deposition rate (±1.2 µm/h) from TiB2-C and TiB2-TiSi2 cathodes.

Vanadium Additions to a High-Cr White Iron and its Effects on the Abrasive Wear Behavior.

From the present study, vanadium additions up to 6.4% were added to a 14%Cr-3%C white iron, and the effect on the microstructure, hardness and abrasive wear were analysed. The experimental irons were melted in an open induction furnace and cast into sand moulds to obtain bars of 18, 25, and 37 mm thickness. The alloys were characterized by optical and electronic microscopy, and X-ray diffraction. Bulk hardness was measured in the as-cast conditions and after a destabilization heat treatment at 900°C for 45 min. Abrasive wear resistance tests were undertaken for the different irons according to the ASTM G65 standard in both as-cast and heat-treated conditions under a load of 60 N for 1500 m. The results show that, vanadium additions caused a decrease in the carbon content in the alloy and that some carbon is also consumed by forming primary vanadium carbides; thus, decreasing the eutectic M7C3 carbide volume fraction (CVF) from 30% for the base iron to 20% for the iron with 6.4%V;but overall CVF content (M7C3 + VC) is constant at 30%. Wear behaviour was better for the heat-treated alloys and mainly for the 6.4%V iron. Such a behaviour is discussed in terms of the CVF, the amount of vanadium carbides, the amount of martensite/austenite in matrix and the amount of secondary carbides precipitated during the destabilization heat treatment.

Sliding Wear Behavior of Ti-6Al-1.5V-1Mo-0.5Zr-0.1C Alloy Modified with Small Additions of Ru and Different V and Mo Contents.

Titanium alloys have been successfully used in the energy industry due to their stability at high temperature service, good mechanical properties and corrosion resistance. The near-α Ti-6%Al-1.5%V-1.0%Mo-0.5%Zr-0.1%C alloy, has been successfully used in several parts for geothermal energy generation. Several studies have concluded that the wear behavior of Ti alloys is generally poor; however, the specific tribosystem must be analyzed. This work analyzes the additions of 0.3%Ru, and variations of the V and Mo contents on the wear performance of a Ti alloy. Different combinations of α+β and degrees of microstructural refinement were observed depending on the composition. Wear test were undertaken by using a dry sliding block-on-ring configuration under the ASTM G77 standard. Two different loads (7 and 25 N) were used against a M2 hardened steel ring as a counter face with a hardness of 790 HV. Results showed that for the 7N load, the wear behavior is related to the volume fraction and thickness of the α phase; on the other hand, for the 25N load tests, the wear losses are directly proportional to the bulk hardness of the alloys and the α plate thickness, for this condition, best wear performance was achieved by the alloy 3 which contains 1.0wt%V, 1.8wt% Mo and 0.25wt% Ru. From the experimental results of the present study, it has been found that the wear behavior is directly related to the microstructure, e.g. amount of phases, refinement degree, applied load, and, in a lesser extent to the bulk hardness.

Niobium Additions to a 15%Cr–3%C White Iron and Its Effects on the Microstructure and on Abrasive Wear Behavior

From the present study, niobium additions of 1.79% and 3.98% were added to a 15% Cr–3% C white iron, and their effects on the microstructure, hardness and abrasive wear were analyzed. The experimental irons were melted in an open induction furnace and cast into sand molds to obtain bars of 45 mm diameter. The alloys were characterized by optical and electron microscopy, and X-ray diffraction. Bulk hardness was measured in the as-cast conditions and after a destabilization heat treatment at 900 °C for 30 min. Abrasive wear resistance tests were undertaken for the different irons according to the ASTM G65 standard in both as-cast and heat-treated conditions under three loads (58, 75 and 93 N). The results show that niobium additions caused a decrease in the carbon content in the alloy and that some carbon is also consumed by forming niobium carbides at the beginning of the solidification process; thus decreasing the eutectic M7C3 carbide volume fraction (CVF) from 30% for the base iron to 24% for the iron with 3.98% Nb. However, the overall carbide content was constant at 30%; bulk hardness changed from 48 to 55 hardness Rockwell C (HRC) and the wear resistance was found to have an interesting behavior. At the lowest load, wear resistance for the base iron was 50% lower than that for the 3.98% Nb iron, which is attributed to the presence of hard NbC. However, at the highest load, the wear behavior was quite similar for all the irons, and it was attributed to a severe carbide cracking phenomenon, particularly in the as-cast alloys. After the destabilization heat treatment, the wear resistance was higher for the 3.98% Nb iron at any load; however, at the highest load, not much difference in wear resistance was observed. Such a behavior is discussed in terms of the carbide volume fraction (CVF), the amount of niobium carbides, the amount of martensite/austenite in matrix and the amount of secondary carbides precipitated during the destabilization heat treatment.