Effect of Microstructure and Hardness on Cavitation Erosion and Dry Sliding Wear of HVOF Deposited CoNiCrAlY, NiCoCrAlY and NiCrMoNbTa Coatings

Metallic coatings based on cobalt and nickel are promising for elongating the life span of machine components operated in harsh environments. However, reports regarding the ambient temperature tribological performance and cavitation erosion resistance of popular MCrAlY (where M = Co, Ni or Co/Ni) and NiCrMoNbTa coatings are scant. This study comparatively investigates the effects of microstructure and hardness of HVOF deposited CoNiCrAlY, NiCoCrAlY and NiCrMoNbTa coatings on tribological and cavitation erosion performance. The cavitation erosion test was conducted using the vibratory method following the ASTM G32 standard. The tribological examination was done using a ball-on-disc tribometer. Analysis of the chemical composition, microstructure, phase composition and hardness reveal the dry sliding wear and cavitation erosion mechanisms. Coatings present increasing resistance to both sliding wear and cavitation erosion in the following order: NiCoCrAlY < CoNiCrAlY < NiCrMoNbTa. The tribological behaviour of coatings relies on abrasive grooving and oxidation of the wear products. In the case of NiCrMoNbTa coatings, abrasion is followed by the severe adhesive smearing of oxidised wear products which end in the lowest coefficient of friction and wear rate. Cavitation erosion is initiated at microstructure discontinuities and ends with severe surface pitting. CoNiCrAlY and NiCoCrAlY coatings present semi brittle behavior, whereas NiCrMoNbTa presents ductile mode and lesser surface pitting, which improves its anti-cavitation performance. The differences in microstructure of investigated coatings affect the wear and cavitation erosion performance more than the hardness itself.

Effect of Solution Treatment on Cavitation Erosion Resistance of Duplex Stainless Steel Surfacing Layers

In the present work, the possibility of using solution treatment to improve the cavitation erosion resistance of the duplex stainless steel surfacing layers was discussed. The effect of solution treatment on cavitation erosion resistance of duplex stainless steels was investigated. The results showed that the solution treatment can adjust the ratio of ferrite to austenite, reduce the precipitation content, and make the incubation period longer, leading to an increase in the cavitation erosion resistance of the duplex stainless steel surfacing layers. The sample treated at 900°C and water quenching was shown to have the best resistance to the absorption of the energy produced by cavitation erosion, and hence the best cavitation erosion resistance.

Influence of Glow-Discharge Nitrided Temperatures of Cast Stainless Steel for Cavitation-Erosion Enhancement in Seawater

In this work, corrosion resistance and cavitation-erosion characteristics were investigated by applying plasma ion nitriding technique to cast stainless steels used as materials of high speed rotors under seawater environment. Plasma ion nitriding was performed for 10 h at various temperature parameters with 25% N2 and 75% H2 gas ratio. The cavitation-erosion experiment was carried out under vibration amplitude of 30 °C and sea water temperature of 25 °C according to modified ASTM G32-92. The yN phase that improves corrosion resistance and mechanical properties was formed at the all of experimnetal temperatures after plasma ion nitriding treatment. The crystallite size of phases was calculated through the XRD patterns according to Scherrer formula and obtained smallest nano size of yN phase at 450 °C. Cavitation-erosion resistance was enhanced up to 450 °C but was deteriorated at 500 °C.

Cavitation Resistance of Coatings with a Metastable Austenite Structure

The effect of martensitic phase transformation on cavitation erosion resistance for a deposited layer prepared from a Fe-8Cr- C-1.5Al-Ti flux-cored wire of metastable steel was studied. A reference material of AISI 316L stainless steel was used as a substrate. Cavitation tests were performed using a modified ultrasonic tester. X-ray diffraction was used to examine the phase transformation before and after cavitation tests. Also, the eroded surfaces of specimens were investigated by optical microscope (OM), scanning electron microscope (SEM), and 3D optical profilometer. The cavitation results revealed that the deposited layer exhibited a resistance to cavitation erosion approximately 10 times higher than the AISI 316L steel due to the martensitic phase transformation occurring during the cavitation process. The phase transformation plays a main role to minimize the cavitation damage of specimen. This is due to the fact that it contributes to obstructing movement of dislocations and increasing the hardness as a result of the increased hardening on the surface.