Cavitation Resistance Of Coatings Research Articles

Features of cavitation wear of chrome electrolytic coatings

Cavitation wear is a pervasive phenomenon on water transport. The parts of ship propulsion system as well as the water-cooled surface of cylinder liners of high-speed ship diesels are subjected to cavitation attack. Bright chromium deposits are used to protect the water-cooled surface of liners. Nevertheless, the effect of cracks appearing in the electrolytic chrome coating upon depositing on the cavitation resistance of coating is still an open relevant problem. The goal of the study is to analyze the kinetics and mechanism of the destruction of the bright chrome electrolytic deposit under cavitation attack. The cavitation resistance of chrome electrolytic coatings after grinding and after polishing was studied. The chrome electrolytic coatings were obtained on the plates (90 × 30 × 7 mm in size) made of cast iron SCh21. The chrome was deposited on one side (90 × 30 mm) of the plate placed vertically by flow anodic jet chrome plating in standard electrolyte (kg/m3): chrome anhydride — 250; sulfuric acid — 2.5; trivalent chrome — no more than 5; trivalent iron ions — no more than 10. The electrolyte was prepared using distilled water of a single distillation. The parameters of depositing: electrolyte temperature is 50°C; the current density amounts to 60 A/dm2. Four samples cut from the same plate were used. The coatings on the first two samples were ground using the abrasive paper number 320. The coatings on the second pair of samples were first ground successively on the abrasive paper of different grit: 320, 500, 800, 1000, and 1200, and then polished on the cloth with GOI paste. The cavitation wear tests were carried out on a magnetostrictive vibratory rig in fresh water, the frequency and amplitude of the rig horn vibration was equal to 22 kHz and 28 μm, respectively. The spacing between the chrome-plated sample surface and the horn butt was 0.5 mm. The wear of the samples was evaluated by periodical weighing during testing. The wear of the ground samples turned to be significantly higher, than that of polished samples. The photographs of the surface after different duration of the cavitation attack show that the initial cracks formed on the coating during chrome plating are the centers of cavitation destruction. To reduce the negative impact of initial cracks on the cavitation resistance of the bright chrome electrolytic coatings deposited on the water-cooled surface of the cylinder liners of diesels it is shown expedient to include polishing in the technological process of depositing chrome coatings.

Influence of Particle Size Distribution on the Morphology and Cavitation Resistance of High-Velocity Oxygen Fuel Coatings

The cavitation wear process is one of the major wear mechanisms in turbines and rotors of hydroelectric power plants in Brazil. An effective way to increase the cavitation resistance is the use of coatings, applied by thermal spraying. The high-velocity oxy-fuel process (HVOF) is one of the most used thermal spraying processes, and it is widely adopted for applying coatings for protection against wear and in maintenance components. A FeCrMnSiB experimental alloy was deposited onto SAE 1020 substrate by HVOF process, in order to evaluate the influence of the powder particle size range on the morphology and cavitation resistance of the coatings. The morphology of the coatings showed an increase in oxide content with powder size reduction. The increase in the powder particle size reduced the wettability of the particles, observed by the increase in the quantity of non-melted particles. Higher particle size distribution led to an increase in erosion rate, due to higher presence of non-melted particles in the coatings and consequently reduction of splats adhesion. The cavitation damage was perceived mainly by the mechanism of lamellae detachment; however, part of the damage was also absorbed by strain hardening due to the γ-e martensitic transformation.

Influence of cobalt content and HVOF deposition process on the cavitation erosion resistance of WC-Co coatings

Cavitation is a wear process that occurs in aqueous environments in which water vapor bubbles on the metal surface of a component implode, leading to erosion. The phenomenon can cause significant damage to structural metal alloys, and many components are therefore covered with coatings to protect against this damage. Thermal spray coatings have long been used for this purpose. With the advent of the HVOF (high velocity oxy-fuel) spray process it became possible to produce denser, harder, tougher coatings, which can be used in applications where fracture toughness is more important. Among the materials most frequently used, tungsten carbide-cobalt (WC-Co) cermet coatings offer a combination of high hardness, toughness and adherence and consequent wear resistance. In this work, two WC cermets with different Co (binder) contents were used to investigate the influence of Co content on the fracture toughness and cavitation resistance of WC coatings deposited by the HVOF process using different fuels. The influence of the HVOF process (specifically the type of fuel used) on the residual stresses in these coatings and their microstructure and properties was also studied. An increase in Co content led to an increase in the fracture toughness and cavitation resistance of the coatings. The primary wear mechanism appears to involve carbide particles becoming detached from the Co matrix. The greater compressive residual stress when liquid fuel was used in the HVOF process resulted in improved properties, including fracture toughness, and a concomitant improvement in cavitation resistance.