Cavitation Erosion and Sliding Wear of MCrAlY and NiCrMo Coatings Deposited by HVOF Thermal Spraying

The investigation into wear resistance is an up-to-date problem from the point of view of both scientific and engineering practice. In this study, HVOF coatings such as MCrAlY (CoNiCrAlY and NiCoCrAlY) and NiCrMo were deposited on AISI 310 (X15CrNi25-20) stainless steel substrates. The microstructural properties and surface morphology of the as-sprayed coatings were examined. Cavitation erosion tests were conducted using the vibratory method in accordance with the ASTM G32 standard. Sliding wear was examined with the use of a ball-on-disc tribometer, and friction coefficients were measured. The sliding and cavitation wear mechanisms were identified with the SEM-EDS method. In comparison to the NiCrMo coating, the MCrAlY coatings have lower wear resistance. The cavitation erosion resistance of the as-sprayed M(Co,Ni)CrAlY coatings is almost two times lower than that of the as-sprayed NiCrMoFeCo deposit. Moreover, the sliding wear resistance increases with increasing the nickel content as follows: CoNiCrAlY < NiCoCrAlY < NiCrMoFeCo. The mean friction coefficient of CoNiCrAlY coating equals of 0.873, which almost 50% exceed those reported for coating NiCrMoFeCo of 0.573. The as-sprayed NiCrMoFeCo coating presents superior sliding wear and cavitation erosion resistance to the as-sprayed MCrAlY (CoNiCrAlY and NiCoCrAlY) coatings.

Influence of HVOF deposition thickness on adhesion strength of WC–CrC–Ni coatings

In this paper, adhesion strength of WC–CrC–Ni coatings obtained by HVOF thermal spraying process of WOKA7504 powder has been studied in correlation with deposition thickness, chemical and structural properties. Besides the parameters strictly related to the process, deposition thickness was found to be the most significant factor affecting the adhesion strength. In the frame of experimental program 304L stainless steel and S235JR structural steel were used as substrate. Mechanical tests, SEM and EDX analysis, have been performed in order to assess structural and mechanical properties, phase composition of the deposited layers. Maximum average adhesion strength has been experimentally found of 23.4 N/mm2 for stainless steel and of 35.8 N/mm2 for structural steel, for a deposition thickness of 220 µm, achieved in two layers. Once the coatings’ thickness increases, the adhesion strength decreases constantly and over thickness of 400 to 650 µm, the existing coating imperfections and elongated carbides placed in the interlayer interface area could fail due to the thermal stresses during the HVOF thermal spraying process, since they are brittle stress concentrators and thus leading tocracking of coatings at small loads, since the coating layershave high hardness of 990 to 1151 HV 1.

Microstructure and selected properties of Ni-Cr-Re coatings deposited by means of HVOF thermal spraying

The article presents selected results of testing Ni-Cr-Re coatings deposited by means of HVOF supersonic spray. The substrate made of 16Mo3 chromium molybdenum boiler steel was sprayed with a powder material of Oerlikon Amdry 4535 80% Ni, 20% Cr, 20÷45 μm, to which 1% of metallic rhenium was added using the high energy milling method in a ball mill. The Rhenium is an alloying additive that improves the heat resistance of alloys, creep and high temperature oxidation resistance. Alloys with the addition of rhenium are widely used in the aerospace industry (nickel superalloys) and in power industry. Metallographic microscopic examinations, microhardness tests and surface profilometry were carried out.

Cavitation Erosion Behaviour of Cooper Base Layers Deposited by HVOF Thermal Spraying

The recent development in enhancing the corrosion resistance of materials by covering with powdered layers open the way to use the procedure also for cavitation erosion protection, phenomenon always present in hydraulic machinery runners as well as ship propellers. The present research analyzes the behavior of four different types copper layers, deposited with high velocity flames, HVOF upon specimens realized from cast steel for general use 270-480 W (equivalent with OT500-3 used in Romania), to cavitation erosion in a Laboratory device. Even if the powder density and the layers thickness have close values, the behavior to cavitation erosion is different and depend primarily on the powder chemical composition and the microscopic structure.

The High Temperature Creep Properties of a Thermally Sprayed CoNiCrAlY Coating via Small Punch Creep Testing

Thermal barrier coatings (TBC’s) protect superalloy components from excessively high temperatures in gas turbines. TBC’s comprise of a ceramic top coat, a metallic bond coat and a thermally grown oxide (TGO). The creep behaviour of the MCrAlY bond coat, which is sensitive to the composition and the method of deposition, has a significant effect on the life of the TBC. High velocity oxy-fuel (HVOF) thermal spraying is a popular deposition method for MCrAlY bond coats however the creep properties of HVOF MCrAlY coatings are not well documented. The creep behaviour of a HVOF thermally sprayed CoNiCrAlY coating has been determined by small punch creep (SPC) testing. Tests were conducted between an equivalent uniaxial stress range of 37-80 MPa at 750 °C on two different SPC rigs and between 30-49 MPa at 850 °C on a single SPC rig. The measured steady-state creep deformation rates at 750 °C were consistent across the two rigs. A comparison with previous work demonstrated that the creep behaviour of HVOF CoNiCrAlY coatings is not sensitive to the manufacturing variability associated with HVOF thermal spraying. The CoNiCrAlY coating exhibited typical SPC deformation at 750 °C. At 850 °C the CoNiCrAlY coating showed significantly different creep behaviour which could be attributed to the onset of superplasticity.