CHARACTERIZATION AND ELECTROCHEMICAL CORROSION ANALYSIS OF HEAT-TREATED REINFORCED HA COATINGS DEPOSITED BY VACUUM PLASMA SPRAY TECHNIQUE

Herein, a biomimetic coating of hydroxyapatite (HA)–Al2O3 and HA–ZrO2 was deposited on Ti–6Al–4V-alloy using vacuum plasma spray (VPS) technique. The bond-coat of ZrO2 has been introduced between the substrate and reinforced HA coatings to study the effect of bond-coat on structural, mechanical properties and electrochemical corrosion performance of the developed coatings. In addition, the impact of thermal treatment of coating was investigated on these properties too. Coating characteristics, such as morphology, porosity, surface roughness, and crystallinity were investigated. The corrosion performance of coatings was tested in Hank’s-based salt solution (HBSS). Significant enhancement in crystallinity and surface-hardness has been witnessed after heat treatment; nevertheless, porosity reduced. The electrochemical corrosion study revealed that the corrosion resistance of heat-treated samples was better than the as-sprayed coatings samples. The intensity of XRD peaks of all coatings increased after 24[Formula: see text]h immersion in HBSS for the electrochemical test in comparison to the intensity of peaks before the corrosion test.

MCrAlX (X = Y, Hf and Si) Bond Coats by Cold Spray for High Temperature Applications

MCrAlX powder compositions (M= Ni; Co and X= Y; Hf; Si or combination) are often thermally sprayed (TS) via vacuum plasma spray (VPS); low pressure plasma spray (LPPS) or high velocity oxy-fuel (HVOF) to produce high temperature oxidation and hot corrosion resistant bond coats (BC) for thermal barrier coatings (TBCs). Cold spray (CS) technology is currently considered as a promising alternative to the traditional TS solutions having the advantage of delivering oxide-free and very dense metallic coatings at relatively lower costs compared to VPS and LPPS. Here; we first present high-pressure CS deposition of NiCoCrAlY and NiCoCrAlYHfSi and discuss the influence of feedstock properties on the deposited BCs. CFD numerical simulation is employed to tailor the spray conditions based on the feedstock characteristics. Secondly; we present the laser assisted cold spray (LACS) deposition of NiCoCrAlYHfSi BCs using a low-pressure CS system. We show that LACS can be successfully used to deposit this particular powder while eliminating nozzle erosion and low deposition efficiency disadvantages observed during conventional CS. Lastly; high temperature isothermal oxidation of a TBC architecture having LACS BC is presented.

CHARACTERISATION OF BIOMEDICAL TITANIUM LAYERS DEPOSITED BY A VACUUM PLASMA SPRAY PROCESS

In this paper we will describe the process of the deposition of thick layers of VPS-Ti coating, which is used as a bonding layer for the upper porous Ti coatings on implant substrates. In order to deposit the powder, we used HÖGANÄS Ti powder labelled as AMPERIT 154.086 -63 µm. In order to test the mechanical properties and microstructure of the VPS-Ti coating, the powder was deposited on Č.4171 (X15Cr13 EN10027) steel substrates. Mechanical tests of the microhardness of the coating were performed by the Vickers hardness test method (HV0.3) and tensile strength by measuring the force per unit area (MPa). The microhardness of the coating is 159 HV0.3, which is consistent with the microstructure. The coating was found to have a good bond strength of 68 MPa. The morphology of the powder particles was examined on a scanning electron microscope. The microstructure of the coating, both when deposited and etched, was examined with an optical microscope and a scanning electron microscope. By etching the coating layers, it was found that the structure is homogeneous and that it consists of a mixture of low-temperature and high-temperature titanium phases (α-Ti + β-Ti). Our tests have shown that the deposited layers of Ti coating can be used as a bonding layer for porous Ti coatings in the production of implants.

Characterization of vacuum plasma spray VPS - W coating deposited on stainless steel substrates

In this paper, studied was the melting of W powder particles in plasma, their behavior at oxidation as well as the mechanism of hardening on the surface of the substrate. Tungsten coating layers were deposited with vacuum plasma spray technology (VPS) on the test specimens of steel Č.4171 (X15Cr13 EN10027). VPS technology has advantages over the APS technology due to decreased oxidation of melted powder particles, by producing a coating with a controlled proportion of micro pores and greater uniformity of the deposited layers. Evaluation of mechanical characteristics of the layers was done by examining the microhardness using the HV0.3 method and tensile bond strength by tensile testing. The microstructures of the deposited layers were analyzed by means of optical microscopy (OM) and scanning electron microscopy (SEM). The results obtained showed that the tungsten coating consisted of well melted particles that interconnected and were overlapping, which shows a typical lamellar microstructure. Present in the microstructure are micro pores in very small proportion at the inter-lamellar boundaries. Tests have shown that layers of W coating deposited by plasma spray in inert gas shielding at low pressure have good mechanical properties and microstructure, which in the fully enables its use in exploitation.

Effect of Air Plasma Sprayed Flash Bond Coatings on Furnace Cycle Lifetime of Disks and Rods

Air plasma sprayed (APS) flash coatings on high velocity oxygen fuel (HVOF) bond coatings are well known to extend the lifetime of thermal barrier coatings (TBCs). Recent work compared flash coatings of NiCoCrAlY and NiCoCrAlYHfSi applied to both rods and disk substrates of alloy 247. For rod specimens, 100 h cycles were used at 1100 °C in wet air. Both flash coatings significantly improved the lifetime compared to HVOF-only and vacuum plasma spray (VPS)-only MCrAlY bond coatings with no statistical difference between the two flash coatings. For disk specimens tested in 1 h cycles at 1100 °C in wet air, the NiCoCrAlY flash coating significantly outperformed an HVOF-only NiCoCrAlYHfSi bond coating and a NiCoCrAlYHfSi flash coating. The flash coatings formed a mixed oxide-metal zone that appeared to inhibit crack formation and therefore extend lifetime. In addition to the flash coating increasing the bond coating roughness, the underlying HVOF layer acted as a source of Al for this intermixed zone and prevented the oxide from penetrating deeper into the bond coating. The lower Y+Hf content in the Y-only flash coating appeared to minimize oxidation in the flash layer, thereby increasing the benefit compared to a NiCoCrAlYHfSi flash coating.

An Experimental Study on the Ablation Properties of Carbon/Carbon Composites Using Thermal Plasma

The ablation properties of two carbon/carbon (C/C) composites currently considered as thermal protection materials for radioisotope thermoelectric generators (RTGs) were investigated using a plasma wind tunnel with a heat flux between the range of 2–4 MW/m2. The ablation properties were identified through an analysis of erosion rates and microstructures after testing. During ablation, erosion reactions of the matrix were faster than those of fibers and pores, and the defects and cracks between the fibers and matrix grew. The fibers eroded from their outer surface and ablation led to the formation of sharp tapered tips. Lower density C/C composites appeared to lose more matrix than higher density C/C composites. The erosion rate increased with heat flux. The ablation properties were compared with the test results using a vacuum plasma spray (VPS) facility and an E-beam facility. It was shown that both thermochemical and thermomechanical erosion occurred via oxygen in the plasma flow, and a rapid high-pressure heat stream occurred in tests using the plasma wind tunnel while only thermal ablation occurred in tests using the VPS and E-beam facilities.