EVALUATION OF INTERNAL COHESION OF MULTIPHASE PLASMA-SPRAYED COATINGS BY CAVITATION TEST: FEASIBILITY STUDY

Mechanical characterization of plasma-sprayed coatings at microscopic level represents a major challenge due to the presence of numerous inherent microstructural features such as cracks, pores, or splat boundaries, which complicate coatings characterization by conventional testing methods. Need for reliable testing of structural integrity of newly developed multiphase plasma-sprayed coatings introduced even more complexity to the testing. In this study, applicability of indirect vibratory cavitation test (adapted from ASTM G32 standard) for such testing was evaluated. Three plasmasprayed coatings having distinctive microstructures were tested: i) conventional alumina coating deposited from coarse powder, ii) hybrid coating deposited by co-spraying of coarse alumina powder and fine yttria-stabilized zirconia (YSZ) suspension, and iii) compact alumina coating deposited from fine ethanol-based suspension. Differences in the coatings internal cohesion were reflected in different failure mechanisms observed within the cavitation crater by scanning electron microscopy and mean erosion rates being i) 280 μm/hour, ii) 97 μm/hour and iii) 14 μm/hour, respectively

MICROSTRUCTURE AND MICROHARDNESS EVALUATION FOR NiCrAlY MATERIALS MANUFACTURED BY SPARK PLASMA SINTERING AND PLASMA SPRAYING

NiCrAlY deposited by different thermal spraying methods is commonly used as the bond coat material in thermal barrier coatings (TBCs). In the present study, two experimental coatings were deposited by hybrid water stabilized plasma (WSP-H) and radio frequency inductively coupled plasma (RF-ICP) using the same feedstock powder. Spark plasma sintering (SPS) was used to manufacture a compact NiCrAlY from the same feedstock powder as a reference material. Microstructure, internal oxidation, phase characterization and quantification of the mechanical behaviour in terms of microhardness were studied. The investigations clearly showed microstructural and mechanical differences between the NiCrAlY samples manufactured by different plasma technologies. The results confirmed that SPS and RF-ICP provide dense structures with no oxides due to the fabrication under protective atmosphere and similar mechanical properties. Thus, RF-ICP may be used for deposition of very dense coatings with microstructure and hardness comparable to compacted materials prepared by SPS

Plazmové stříkání intermetalik na bázi FeAl

Intermetallic materials can provide high corrosion and wear resistance up to elevated temperatures. Their disadvantage is their fragility. In this paper possibility of preparation of the Fe-Al based coating on ductile substrate, as well as their microstructure and fatigue properties are discussed

Suspension plasma spraying of sub-stoichiometric titania by hybrid water/argon stabilized plasma torch

In this study, suspension plasma spraying of sub-stoichiometric titania was attempted using hybrid water/argon stabilized plasma torch (WSP-H). Porous coatings with fine cauliflower-like columnar microstructure were successfully deposited in two separate experiments with different power levels of the plasma torch. In both cases, high solid-load content (40 wt. %) of the water-based suspension resulted in considerable coating thickness increase per deposition cycle. Coating annealing and partial remelting of the surface asperities were also achieved by additional pass of plasma torch in front of the coating surface. According to X-ray diffraction, all coatings consisted dominantly of rutile phase. Detailed microscopic observation of the as-sprayed and annealed deposits showed that the local coloration of the coating (ranging from dark blue to beige) was driven by the local overheating of the rough coating surface which could also promote the oxygen intake. Moreover, sample annealing was also observed to increase the sample reflectivity as observed by UV-VIS-NIR scanning spectrophotometr

Vliv podávání prášku na mechanické vlastnosti plazmových nástřiků mědi a wolframu

Nuclear fusion is considered to be a promising energy source for the future. One of the biggest problems which has to be solved is the development of inner wall material of the fusion reactor. For the inner parts which will be exposed to high levels of heat and particle flux, a combination of tungsten layer on copper parts was proposed. Tungsten is refractory material resistant to high heat and particle flux, while copper can efficiently remove heat due to its high thermal conductivity. But high stress concentration on the materials interface can occur due to the thermal expansion coefficient (CTE) mismatch of both materials when exposed to high temperatures. Therefore plasma spraying is promising technology for this application. One of the critical plasma spraying parameters is the carrier gas flow which has to be optimized to ensure proper particle trajectory along the centerline of the plasma flame. Obtained results are currently being used for the development of FGM coatings

Preparation of multiphase materials with spark plasma sintering

Spark plasma sintering (SPS), also called Field Assisted Sintering Technique (FAST), represents a novel method of preparation of sintered materials from powders. The main advantage of the SPS method is a high achievable heat rate (>200 °C/min) and high sintering temperatures (up to 2200 °C in our laboratory). Combination of high heating rate, rather high pressures (up to 80 MPa) and electric field fluctuations leads to an effective sintering and significant reduction of sintering time for both coarse-grained and nanocrystalline powders. Composite materials may be easily obtained by mixing or layering of different powders. The paper will introduce several examples of multiphase materials sintered by SPS at our institute and the establishment of procedures for routine testing of sub-sized specimens

Deposition of Titania from Solution by Hybrid Water-Stabilized Plasma Torch

Thermal spraying with liquid feedstock presents a novel route for deposition of functional coatings. In this study, possibility of preparation of titania coatings from solution by hybrid water stabilized plasma torch is presented. Coatings were prepared from solution of titanium isopropoxide Ti[OCH(CH3)2]4 in anhydrous ethanol. Fragmentation of feedstock stream in the plasma jet was monitored by shadowgraphy. Deposition was carried out on steel samples mounted to the cooled rotating carousel. Cross-sectional images from SEM microscope showed successful formation of the deposit with dual morphology consisting of fine feather-like features combined with bigger droplets. X-ray diffraction revealed formation of nanometric rutile crystallites

Special methods for observation of failure of plasma-processed materials

This paper presents examples of several methods, which may be used for failure study of materials prepared by plasma processing - plasma spraying or plasma sintering. Microstructure of these materials significantly differs from that of bulk construction materials which limits use of conventional methods. Typically, because not enough volume of material in one piece is available for testing. Aim of this paper is to ilustrate benefits of using combined study of materials changes during loading and their macroscopic propertie

Thermal spraying of suspensions and solutions

Recently beginning to be selected for some applications of thermal spraying interesting administration of nanometric or micrometric powders in suspension, or preparation of particles from a solution directly over zinc deposition. This paper provides basic information about this new route of administration and its benefits for technical practice

Application of SPS technology for preparation of high-tech materials

Spark Plasma Sintering (SPS) is a modern technology, which allows sintering of powder materials in the variable electric field (also FAST technology – Field Assisted Sintering Technique). Heating rate in the range of hundreds °C/minute, high pressure and sintering temperature (more than 2000 °C) enables for example sintering of materials with high melting point or nanometric powders without growth of grain size in the sintered body