Nano- and micro-mechanical properties and corrosion performance of a HVOF sprayed AlCoCrFeNi high-entropy alloy coating

In this work, a gas atomized feedstock was used to fabricate an AlCoCrFeNi HEA coating using the high-velocity oxygen fuel (HVOF) process. The coating's resistance to room temperature surface degradation was evaluated using dry sliding wear and seawater corrosion testing. The coating retained the feedstock phase structure with negligible in-flight oxidation and was composed of a majority BCC phase with a minor B2 phase, resulting in a high micro- and nano-hardness of ~7 GPa. These observed phase compositions were consistent with thermodynamically calculated phase predictions using a CALPHAD model. Microstructure-mechanical property mapping revealed uniform microstructural characteristics. However, the multiscale wear resistance of the coating was critically affected by the presence of the hard BCC/B2 phase composition, which led to severe brittleness. Combinatorial assessment of the worn surface, wear debris and counter body indicated that wear was dictated by a combination of abrasive, surface fatigue, tribo-oxidation and adhesive wear. In addition, the coating exhibited superior general corrosion resistance compared with conventional SS316L, but the selective dissolution of the B2 phase preceded poor localized corrosion resistance, ultimately leading to pitting corrosion

Evaluating the influence of microstructural attributes: Fraction, composition, size and spatial distribution of phases on the oxidation behaviour of high-entropy alloys

Oxidation behaviour of five high entropy alloys (HEAs): AlCoCrFeNi, Al2CoCrFeNi, Al2CoCr1.5FeNi, AlCoCrNi and AlCoCr0.5Ni, has been studied at 1100 °C in the cast-annealed condition. Al2CoCrFeNi and AlCoCr0.5Ni HEAs exhibit the lowest oxidation rates of the order of 10−12 g2 cm−4-s−1. Al-rich nanoscale precipitates are observed to aid protective oxide growth. The Al-lean phase fraction and its Al content also have a major effect on the alloy's oxidation behaviour. A parameter ‘X’ taking the ratio of these two has been introduced, which coupled with CALPHAD predictions can be a potential tool to shortlist HEAs for oxidation resistance applications

Structure property relationship of suspension thermally sprayed WC-Co nanocomposite coatings.

Tribomechanical properties of nanostructured coatings deposited by suspension high velocity oxy-fuel (S-HVOF) and conventional HVOF (Jet Kote) spraying were evaluated. Nanostructured S-HVOF coatings were obtained via ball milling of the agglomerated and sintered WC-12Co feedstock powder, which were deposited via an aqueous-based suspension using modified HVOF (TopGun) process. Microstructural evaluations of these hardmetal coatings included transmission electron microscopy, x-ray diffraction, and scanning electron microscopy equipped with energy dispersive x-ray spectroscopy. The nanohardness and modulus of the coated specimens were investigated using a diamond Berkovich nanoindenter. Sliding wear tests were conducted using a ball-on-flat test rig. Results indicated that low porosity coatings with nanostructured features were obtained. High carbon loss was observed, but coatings showed a high hardness up to 1000 HV2.9N. S-HVOF coatings also showed improved sliding wear and friction behavior, which were attributed to nanosized particles reducing ball wear in three-body abrasion and support of metal matrix due to uniform distribution of nanoparticles in the coating microstructure

Microstructure, multi-scale mechanical and tribological performance of HVAF sprayed AlCoCrFeNi high-entropy alloy coating

Thermal spray high-entropy alloy (HEA) coatings have demonstrated potential for improving the wear resistance of conventional materials used in extreme engineering environments. In the present work, an equiatomic AlCoCrFeNi HEA coating was manufactured using the high velocity air fuel (HVAF) process. The phase and microstructural transformations in gas-atomized (GA) powder during HVAF spraying were analyzed using SEM, EDS and EBSD techniques. The tribological properties of this HEA coating sliding against an Al2O3 ball at both room temperature (RT) and 600 °C were also evaluated. The GA powder was composed of Body Centred Cubic (BCC) + ordered BCC (B2) phases, which transformed to BCC + B2 + minor Face Centred Cubic (FCC) phases during the HVAF coating process, validating the thermodynamic phase prediction projected by the Scheil simulation for non-equilibrium processing conditions. The rapid solidification and high velocity impact-assisted deformation of GA powder resulted in significant grain refinement in the HVAF coating, which ultimately improved the mechanical properties at both micro and nanoscale levels. The wear resistance of the HEA coating at RT was severely impacted by the relatively brittle BCC/B2 phase structure, leading to susceptibility to abrasive wear and surface fatigue. The wear resistance at 600 °C was slightly lower at RT due to the formation of a brittle oxide layer on the worn surface, which induced surface fatigue and aggravated mass loss of the coating