Search

The pursuit of an advanced functional coating that simultaneously combines high hardness, wear resistance, and superior electrical conductivity has remained an elusive goal in the field of copper alloy surface enhancement. Traditional solid solution alloying methods often lead to a significant increase in electron scattering, resulting in a notable reduction in electrical conductivity, making it challenging to achieve a balance between high hardness, wear resistance, and high conductivity. The key lies in identifying a suitable microstructure where dislocation motion is effectively hindered while minimizing the scattering of conductive electrons. In this study, a novel Cu-MoSi2 coating was successfully fabricated on a CuCrZr alloy surface using the coaxial powder feeding high-speed laser cladding technique, with the addition of 10–30% MoSi2 particles. The coating significantly enhances the hardness and wear resistance of the copper substrate while maintaining favorable electrical conductivity. As the quantity of MoSi2 particles increases, the coating's hardness and wear resistance gradually improve, with minimal variance in conductivity. Among the coatings, the Cu-30%MoSi2 coating stands out with the highest hardness (974.5 HV0.5) and the lowest wear amount (0.062 mg/km), approximately 15 times the hardness of the copper base material (65 HV0.5) and only 0.45% of the wear amount (13.71 mg/km). Additionally, the coating exhibits a resistivity of 0.173 × 10−6 Ω·m. The extraordinary hardness and wear resistance of these coatings can be attributed to the dispersion strengthening effect of MoxSiy particles, while the high electrical conductivity is due to the low silicon content dissolved into the copper from the released MoSi2 particles, as well as the rapid cooling rates associated with the high-speed laser cladding process. [ABSTRACT FROM AUTHOR]

To solve the problem of silicide coatings on tantalum substrates failing due to elemental diffusion under high-temperature oxidation environments and to find diffusion barrier materials with excellent effects of impeding Si elemental spreading, TaB2 and TaC coatings were prepared on tantalum substrates by the encapsulation and infiltration methods, respectively. Through orthogonal experimental analysis of the raw material powder ratio and pack cementation temperature, the best experimental parameters for the preparation of TaB2 coatings were selected: powder ratio (NaF:B:Al2O3 = 2.5:1:96.5 (wt.%)) and pack cementation temperature (1050 °C). After diffusion treatment at 1200 °C for 2 h, the thickness change rate of the Si diffusion layer prepared using this process was 30.48%, which is lower than that of non-diffusion coating (36.39%). In addition, the physical and tissue morphological changes of TaC and TaB2 coatings after siliconizing treatment and thermal diffusion treatment were compared. The results prove that TaB2 is a more suitable candidate material for the diffusion barrier layer of silicide coatings on tantalum substrates. [ABSTRACT FROM AUTHOR]

A cold spray–laser cladding composite gradient coating (CLGC) was successfully formed on a Cu substrate. In comparison with traditional laser cladding gradient coatings (LGC), cold spraying the pre-set Ni-Cu alloy's intermediate transition layer not only mitigates the negative impacts due to the high reflectivity of the copper substrate but also helps to minimize the difference in the coefficients of thermal expansion (CTE) between the substrate and coating. This reduces the overall crack sensitivity and improves the cladding quality of the coating. Besides this, the uniform distribution of hard phases in CLGC, such as Ni11Si12 and Mo5Si3, greatly increases its microhardness compared to the Cu substrate, thus resulting in the value of 478.8 HV0.5 being approximately 8 times that of the Cu substrate. The friction coefficient of CLGC is lowered compared to both the Cu substrate and LGC with respective values of 0.28, 0.54, and 0.43, and its wear rate is only one-third of the Cu substrate's. These results suggest CLGC has excellent anti-wear properties. In addition, the wear mechanism was determined from the microscopic morphology and element distribution and was found to be oxidative and abrasive. This approach combines cold spraying and laser cladding to form a nickel-based gradient coating on a Cu substrate without cracks, holes, or other faults, thus improving the wear resistance of the Cu substrate and improving its usability. [ABSTRACT FROM AUTHOR]

A new YSZ (yttria stabilized zirconia) modified silicide coating was prepared on Mo-based alloy through a two-step process, including spraying of Si-Mo-YSZ slurry followed by reaction sintering. Microstructure and oxidation behavior of the coatings with different YSZ content (0, 5, 10 and 15%) were studied. The coating consists of a MoSi 2 -ZrSi 2 -SiO 2 outer layer and a MoSi 2 inner layer. The 10% YSZ modified coating possessed the best oxidation protection at ultra-high temperatures and, for the coating, the mass gain was 0.96 mg/cm 2 after being exposed at 1725 °C in air for 22 h, whereas the uncoated Mo-based alloy completely ablated for 0.35 h. The excellent oxidation resistance of the silicide coating was attributed to the compact and self-healing SiO 2 protective scale, which contained the dispersed ZrO 2 and ZrSiO 4 . [ABSTRACT FROM AUTHOR]

In order to improve the oxidation resistance of the Ta substrate, a novel two-step process including molten salt electrodeposition (Na 2 WO 4 -WO 3 system) and halide activated pack cementation was adopted to prepare a WSi 2 –TaSi 2 coating on tantalum substrate. During the electrodeposition process, dense tungsten coatings were fabricated at current densities of 30 mA/cm2, 40 mA/cm2 and 50 mA/cm2. It was observed that the grain size exhibited a log-normal distribution. When the current density was 40 mA/cm2, the grain size and flattest surface of the tungsten coating reached 9.50 ± 0.23 μm and 6.792 μm, respectively. When performing the static oxidation test, the WSi 2 –TaSi 2 coating could effectively protect the Ta substrate oxidized at 1600 °C for 30 h. This is attributed to the presence of dense SiO 2 and Ta 2 O 5 , which acted as a protective layer and suppressed the further penetration of oxygen. Furthermore, due to the matching thermal expansion coefficient between each layer and the sealing ability of semi-molten SiO 2 , the four-layer SiO 2 –W 5 Si 3 –WSi 2 –Ta 5 Si 3 coating could successfully pass 721 thermal shock tests from 1600 °C to room temperature. [ABSTRACT FROM AUTHOR]

Thermal diffusion process (TD) is an important surface modification technique to prepare carbide coating on steel, which can effectively improve the hardness and wear resistance, as well as prolong the service life of the substrate. However, cracking problems are often encountered in the TD processing of high-speed steel. In this work, TD process was used to prepare vanadium carbide coating on M35 high speed steel. The results indicated that the coarse crack after TD treatment was quenching crack. The mechanism of such failure were attributed to the diffusion of elements (mainly C and V) and the volume expansion caused by martensitic transformation during the TD process. Bainite isothermal quenching and three times tempering process were selected as the way of solving this cracking failure. The microstructure and properties of the coating prepared by the improvement process were studied. The coating was composed of fine equiaxial crystals containing V 6 C 5 and VC. The hardness of the coated sample could achieve 2644 HV, which increased greatly compared with uncoated M35 matrix (750 HV), and the friction/wear performance of the samples treated by TD process increase by 82.8%. Unlabelled Image • VC-V 6 C 5 coating was prepared on M35 steel by thermal diffusion (TD) process. • Further improvement method of bainite isothermal quenching and 3-times tempering was proposed. • The hardness and wear resistance of the treated steel were improved by 252.5% and 82.8%. [ABSTRACT FROM AUTHOR]

• MoSi 2 -NbSi 2 coating and the Ce-modified MoSi 2 -NbSi 2 coating were prepared on Nb alloy by a new SS-HAPC method. • MoSi 2 -NbSi 2 coating and the Ce-modified MoSi 2 -NbSi 2 coating can effectively protect Nb alloy for 24.7 h and 28.5 h at 1600 ℃. • Ce promotes the thickness of the coating and plays a role in grain refinement without changing the structure the coating. MoSi 2 -NbSi 2 coating and Ce-modified MoSi 2 -NbSi 2 coating were prepared on Nb-alloy through a new two-step process, including molybdenum slurry spraying and halide-activated pack cementation. The bare alloy was pulverized after oxidation at 1600 °C for 15 min. The both two coated samples have the effective protection for 24.7 h and 28.5 h with the weight gain rate of 3.578 and 2.947 mg2 cm−4 h−1, respectively. Ce-modified samples show better performance due to Ce has the ability of grain refinement. The reasons for the excellent performance are the good matching between MoSi 2 outer layer and NbSi 2 inner layer and the continuous SiO 2 oxide scale. [ABSTRACT FROM AUTHOR]

A double-layer MoSi 2 thick coating was prepared on Titanium-Zirconium-Molybdenum (TZM) alloy through a new two-step method, including molybdenum slurry vacuum sintering and halide-activated pack cementation. The coating was formed by a porous MoSi 2 outer layer and a dense MoSi 2 inner layer. The ball-milled fine Mo powder can effectively improve the coating density and the high temperature oxidation resistance. The oxidation protection time of the coatings prepared by coarse and fine Mo powders were 1 h and 6 h at 1700 °C, respectively. After oxidation, the coating exhibited multilayer structure consisted of SiO 2 oxide layer, porous MoSi 2 layer, dense MoSi 2 layer and Mo 5 Si 3 layer from the outer to the inner. With oxidation proceeding, the Mo 5 Si 3 layer thickened, while the MoSi 2 layer thinned and eventually transformed into Mo 5 Si 3 completely, leading to coating failure because of the depletion of silicon. Unlabelled Image • A double-layer MoSi 2 thick coating was prepared on Mo alloy through a new two-step method. • The oxidation protection time of the coatings prepared by coarse and fine Mo powders were 1 h and 6 h at 1700°C. • The oxidation evolution of the coating structure is mainly due to the oxidation reaction and the diffusion of silicon. [ABSTRACT FROM AUTHOR]