Papers by Keyword: Hardness

Abstract: In this study, hardness and wear resistance of electroless Ni–P and Ni–P/Al₂O₃ composite coating have been investigated. These composite coatings are applied on iron substrate by electroless deposition process and then they were heat treated at 400°C for 1h. Surface and cross-section morphology of composite deposits have been investigated by scanning electron microscopy (SEM) and microstructural changes were evaluated by X-ray diffraction (XRD) analysis. The results showed that the Al₂O₃ particles co-deposited in Ni–P matrix led to an increase in the hardness and improve wear resistance, especially when the heat treated at 400°C will have the maximum hardness and wear resistance.

Abstract: Y₂O₃ has a great application potential at reaction barrier coating of high-temperature composites due to its high thermodynamic stability and high melting point, and the phase structure stability at high temperature and structure dependent mechanical property are key parameters for this application. Y₂O₃ thin films were deposited on silicon (100) wafers by DC magnetron sputtering with various oxygen partial pressure and substrate bias, and then vacuum annealing at 1000°C was performed to investigate the phase structure stability. The microstructure, stress and hardness of as-deposited and annealed Y₂O₃ thin films were explored by X-ray diffraction, transmission electron microscope, and nanoindenter. The result showed that as-60 bias voltage was applied to substrate, cubic-c phase formed regardless of variation of oxygen partial pressure, and the cubic-c phase remains stability and crystallinity became better after annealing at 1000 °C.In addition, the hardness and modulus also just had minor changes as a function of oxygen partial pressure. As oxygen partial pressure was kept at 0.043 Pa, phase transition from cubic-c to monoclinic-b phase took place with increasing substrate bias, accompanying by the increment of hardness and modulus, and 1000 °Chigh-temperature annealing resulted in that as-deposited monoclinic-b phase transforms to cubic-c phase.

Abstract: Tungsten carbide thin films were deposited on silicon (100) substrates by DC reactive magnetron sputtering using CH₄ as a carbon source. The microstructure, compressive stress, hardness and tribological behaviors showed great dependences on the rates of CH₄ flow (F_CH4). Increasing the F_CH4 from 2 to 5 sccm, the film exhibited a phase transition from hexagonal-W₂C to cubic-WC_1-x. Further increasing the F_CH4 larger than 10sccm, the film presented amorphous state. As the F_CH4 increased, the Raman revealed that the films showed a graphitization trend, meanwhile, the surface of the films became smoother and smoother. The hardness of tungsten carbide films first increased, and then decreased after reaching the maximum 38.5GPa (F_CH4=10 sccm). While the sample deposited at 15 sccm obtained the lowest wear rate (2.17×10^-6 mm³/Nm) and low coefficient of friction (CoF, 0.24) and still maintained a high hardness of 32.1 GPa. The lowest wear rate could be ascribed to the highest ratio of H³/E².

Abstract: The cubic-NbN/NbCN multilayers with modulation periodicity (Λ) ranging from 4.2 to 39.1 nm were deposited on Si (100) substrate by reactive magnetron sputtering in a mixture of Ar and N₂ gases. The Λ dependent structural, mechanical and tribological properties for resulting c-NbN/NbCN multilayers were explored. As Λ varied from 4.2 to 39.1 nm, all the films exhibited an obvious modulated structure. Increasing the Λ, the Nb (C,N)(111) peak in XRD gradually shifted to bigger angles and the peak intensity of NbN(111) became stronger. The stress for all multilayers was compressive ranging in between the stress for both NbN and NbCN single layers, and the stress value was stable with increasing Λ. The NbN layer was beneficial to relaxing the compressive stress which induced by NbCN layer. In addition, as Λ increases, the hardness (H) first increased, and then decreased after reaching a maximum value. The obvious enhancement in hardness for multilayers was observed, whose maximum value approaches 43.3 GPa when Λ = 8.4 nm, 37% larger than that obtained by the rule of mixture value. The friction coefficient values of NbN/NbCN multilayers ranging between 0.34 and 0.4 were much lower than that of NbN monolayer but higher than that of NbCN monolayer were.

Abstract: Niobium carbide films was deposited by direct current reactive magnetron sputtering on Si (001) substrates in discharging a mixture of CH₄/Ar gas. The effects of growth temperature (T_s) and methane flow rate (F_CH4) on the phase structure, composition, mechanical and tribological properties for NbC_x films were explored. For the film grown at F_CH4=6 sccm, a phase transition from cubic-NbC phase to hexagonal-Nb₂C phases occurred with increasing the T_s; In contrast, when the film deposited at F_CH4=16 sccm, only the cubic-NbC phase was observed at different T_s. The surface of all the films became rough with increasing the T_s. In addition, when the T_s increased from RT to 600 °C, the films exhibited the compressive stress and kept rising. While as the T_s > 600 °C, the stress partially relaxed both at F_CH4=6 sccm and F_CH4=16 sccm. The hardness (H) for sample grown at F_CH4=6 sccm first increased up to a maximum value, and then decreased with increasing the T_s. And the films grown at F_CH4=16 sccm kept decreasing with the maximum super-hard value of the filmsof 40.5 GPa at F_CH4=6 sccm and 600 °C. The friction coefficient for the film obtained at F_CH4=16 sccm was lower than that at F_CH4=6 sccm, which might be due to the presence more carbon in the film grown at F_CH4=16 sccm.

Abstract: The influence of cooling rate and chemical composition on phase transformation and hardness of C70S6 steel were studied by Gleeble-3800 thermal simulation testing machine and box type electric furnace. The results showed that when the cooling rate was between 0.3 and 5 °C/s, the transformation products of two experimental steels were mainly composed of ferrites, pearlite and sorbite. The pearlite content gradually decreased with the cooling rate increasing, while the sorbite content increased and the ferrite content changed little. Both the ferrite and pearlite transformation starting temperature and ending temperature decreased with the cooling rate increasing. Besides, the hardness increased with the cooling rate. At the same cooling rate, the phase transformation temperature increased slightly with the carbon equivalent decreasing, and the pearlite content increased, while the hardness decreased. The hardness of C70S6 steel was reduced by cooling rate decreasing. However, it was difficult to realize the method of decreasing the hardness by adjusting the cooling rate in the case of higher carbon equivalent. Therefore, in order to obtain an appropriate hardness, the Ceq must be controlled. And a Ceq=0.83% was recommended.

Abstract: Special brasses containing Mn and Si possess high wear resistance due to the dispersion of hard Mn₅Si₃ particles. The effect of precipitation hardening on the wear resistance of a Cu–30Zn–3Al–3Mn–0.7Si based brass alloy was investigated. Dry sliding wear test was conducted using a block-on-ring configuration. The results indicated that finely, nanoscale Mn₅Si₃ particles precipitated from the matrix after annealing at 800 °C for 4 h, resulting in the increase of hardness from 240 to 278 HV. Both the wear loss and friction coefficient decreased, indicating the improvement of the wear resistance. From the examination of the worn surfaces, adhesive and abrasive wear were found to be the major wear forms. The adhesion and abrasion decreased after the precipitation-hardening treatment.

Abstract: The deformation behavior and microstructure evolution of LPSO phase of Mg-11Gd-4Y-2Zn-0.5Zr magnesium alloy were investigated. This alloy was deformed by hot compression using Gleeeble 3500 thermal simulation machine at different temperatures. The microstructure was analyzed by optical microscopy (OM), scanning electron microscopy (SEM) with spectroscopy (EDS) and XRD. The results showed that the kink bands of LPSO structure of Mg-11Gd-4Y-2Zn-0.5Zr alloy, after thermal compression processing, aggravated as temperature increasing. The fine lamellar LPSO phase could be observed in the alloy. At 450 °Cand 500 °C, some fine lamellar LPSO phase in the grain had been broken into short rod or small block forms, meanwhile, a new rod-like LPSO structure appeared along the grain boundary. Moreover, the decomposition of LPSO structure was more obvious with the increasing of temperature. During hot compression deformation, the segregation of Y, Zr had eliminated partially and the diffraction peaks of W-phase had disappeared. Moreover, there was an increase in hardness as the dispersion distribution of LPSO phase increased.

Abstract: Copper matrix composites (CMCs) are widely used in electrical equipment and electrical contact materials due to their excellent electrical properties. Al₂O₃ powders are widely used as a reinforcing agent to enhance mechanical properties of MMCs. The xAl₂O₃/Cu (x =0, 0.2, 0.5, 0.7, and 1.0wt. %) composites were prepared via vacuum arc melting method. The mechanical and electrical properties were obtained by measuring the hardness and conductivity. The morphology of copper and Al₂O₃/Cu composites was characterized by optical microscopy (OM) and scanning electron microscopy (SEM). With the addition of Al₂O₃ from 0.2 wt. % to 1.0 wt. %, the relative densities of composites decreased from 98.5% to 97.0%. The hardness of the composites increased with increase in the Al₂O₃ powders content. The hardness of 1.0Al₂O₃/Cu composites was 57.9 HB, which was higher than that of pure Cu by 18.6%.. With the addition of Al₂O₃, the IACS% of Al₂O₃/Cu composites decreased from 88.97 to 86.16.

Abstract: A pollution-free nanocrystalline layer was prepared on the surface of Ti6Al4V by surface mechanical attrition treatment (SMAT). The nanocrystalline samples were vacuum annealed at various temperatures and for different periods of time. The microstructure and thermal stability were characterized by X-ray (XRD), scanning electron microscopy (SEM) and, transmission electron microscopy (TEM). The results showed that the nanocrystalline Ti6Al4V presented a satisfactory thermal stability with the annealed temperature below 650°C. The critical growth temperature for nanocrystalline Ti6Al4V is 100°C higher than that for pure titanium.