Flame Thermal Spray Research Articles

Comportamiento a la corrosión electroquímica de aleaciones MgAl con recubrimientos de materiales compuestos Al/SiCp mediante proyección térmica

The corrosion protection of Mg-Al alloys by flame thermal spraying of Al/SiCp composite coatings was evaluated by electrochemical impedance spectroscopy in 3.5 wt.% NaCl solution. The volume fraction of SiC particles (SiCp) varied between 5 and 30%. The as-sprayed Al/SiCp composite coatings revealed a high number of micro-channels, largely in the vicinity of the SiC particles, that facilitated the penetration of the electrolyte and the subsequent galvanic corrosion of the magnesium substrates. The application of a cold-pressing post-treatment reduced the degree of porosity of the coatings and improved the bonding at the coating/substrate and Al/SiC interfaces. This resulted in improved corrosion resistance of the coated specimens. The effectiveness of the coatings slightly decreased with the addition of 5-30 vol.% SiCp compared with the unreinforced thermal spray aluminium coatings.

Oxy-Acetylene Flame Thermal Spray of Al/SiCp Composites with High Fraction of Reinforcements

Aluminum matrix composites reinforced with more that 50 vol.% of SiC particles were fabricated using oxyacetylene thermal spraying. The sprayed material consisted of mixtures of aluminum powder with 60-85 vol.% of SiC particles. To favor the processing of the composite, in some cases, the SiC particles were coated with silica following a sol-gel route. This allowed obtaining as-sprayed samples with thickness above 2 mm and with porosity values below 2%. Post-processing of the samples by hot pressing allowed to reduce further the porosity of the composites and to enhance their microstructural homogeneity. The whole process of spraying and hot pressing has been optimized and the role played by the different spraying parameters and by time length and temperature of hot pressing has been also studied.

Study of microstructure, phases and microhardness of metallic coatings deposited by flame thermal spray

The recharging technique by thermal spraying offers the opportunity of renovating the worn surface parts of a machine element to give it again a new technical life despite its previous degradation in service. This process has consequently interesting economic impacts. In order to improve the adherence between 100Cr6 steel deposits and the substrate material (left worn crankshafts), company SNC ATRA of Béjaïa uses at present a composite formed by (100Cr6 steel/molybdenum bond coat of 0.2 mm thick/crankshaft substrate). As a matter of fact, it is shown in the present work that the molybdenum bond coat is not appropriate since, for the 0.2 mm thickness, lateral cracks are observed in the middle of the bond coat. On the other hand, our experiment is that a deposit of 100Cr6 steel projected directly on the substrate seems more promising since no gaps or cracks were detected at the “deposit/substrate” interface of this two-material composite. Lastly, phase analysis using X-ray diffraction confirmed that during spraying process, a stable α-phase (bcc) of 100Cr6 wire was transformed to a new phase of γ-phase (fcc). The coatings exhibited the higher microhardness which would contribute to increase wear resistance.

Wear behaviour of thermal flame sprayed FeCr coatings on plain carbon steel substrate

The principle aim of this study is to investigate the wear behaviour of FeCr coatings on Ni-based bond deposited plain carbon steel substrate for several applications in power generation plants. For this purpose, FeCr and Ni-based powders were sprayed on plain carbon steel substrates using a thermal flame spray technique. Fabricated layers were characterized by using a X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), microhardness and surface roughness testers. FeCr coatings were subjected to sliding wear against AISI 303 stainless steel counter bodies under dry and acidic environments. A pin-on-plate type of apparatus was used with normal loads of 49 and 101 N and sliding speed of 1 Hz. XRD results revealed that FeCr, Fe, Cr, Fe–Cr–Ni, γ-Fe 2 O 3 and Fe 3 O 4 phases are exist in the coating. In addition, some inhomogenities such as oxides, porosity, cracks, unmelted particles and inclusions were observed by SEM. The surface morphologies of FeCr samples after wear experiments were examined by SEM and EDS. It was found that friction coefficients of the coatings in dry condition are higher than that in acidic environment.

The effect of roughness and pre-heating of the substrate on the morphology of aluminium coatings deposited by thermal spraying

Thermal spraying is a technique to deposit on previously treated surfaces metallic or non-metallic materials whose main adhesion mechanisms are mechanical and chemical–metallurgical anchorage. The preparation of the substrate comprises cleaning, development of a rough surface and sometimes preheating to guarantee mechanical anchorage at microwelding sites. To evaluate splat morphologies, test samples with aluminium coatings deposited by different thermal spray processes, namely, flame spraying, high-velocity oxy-fuel and electric arc spraying, were carried out on substrates with different roughness, with and without preheating. Coating adhesion to the substrate was also evaluated. Different splat morphologies were obtained; the results indicated that coatings on preheated substrates may have lower roughness than that recommended in the literature. Besides, although preheating was essential for the flame spray process, it may be eliminated for the electric arc and high-velocity spraying processes.

Graphite-Ni계 분말의 제조 및 용사 코팅 특성에 미치는 공정변수의 영향

Graphite-Ni composite powders were synthesized by mechanical alloying(MA) and spray drying(SD). Fabricated powders as well as commercial graphite-Ni powders were thermally sprayed on mild steel substrates using high velocity oxygen fuel (HVOF) thermal spray process and flame thermal spray process. The effects of several process parameters on related properties in thermally sprayed coatings have been investigated and correlated with microstructures in this study. The results indicated that the desired properties can be obtained when commercial powders were applied using HVOF process, while coating properties in case of MA powder application were inferior to those in HVOF process in so far. However, it is suggested that property enhancement can be obtained if the fraction of hexagonal graphite phase can be increased in mechanically alloyed powders.

QUATERNARY BASALTS FROM ROMANIA - CHARACTERISTICS AND NON-CONVENTIONAL USES

This paper presents an advanced stage of research work regarding the exploitation of natural volcanic rocks (basalt, andésites, tuffs) that are not to be polished but that can be cut into thin tiles and be cover with different inorganic and organic films. The basalt, andésite and tuff samples from some quarries proved to have very good cutting parameters, linear thermal expansions with a temperature between 5.67 and 8.85 x106C"1 for basalts and andésites and between 0.05 and 3.82 x106 C"1 for tuffs. The classic ceramic coating process used glaze with grinding fineness. The resulting surfaces are glossy, uniformly covered from a decorative aspect and by a variety of colour, suitable for various external and internal applications. The non-conventional coating by oxy-gas flame thermal spraying process on basalt frits with a grinding fineness between 100 and 70 μπι, resulted in rough, dark coloured surfaces, suitable for floors with high compressive strengths.

The influence of process parameters on the degradation of thermally sprayed polymer coatings

Heat transfer theory has been used to predict the influence of process parameters on the decomposition of in-flight particles and deposited layers during the thermal spraying of polymer coatings. The methodology is applicable to all polymers and the generic family of thermal spray processes, including plasma, combustion flame, and high-velocity oxy-fuel spraying. The work shows that polymers are unique amongst engineering materials in developing large temperature gradients, which accelerate the degradation of the surface of the particles and the coating layers. However, the analysis also indicates that the degradation can be limited by the control of the gas composition, the spraying distance and the torch traverse speed. The theoretical analysis has been confirmed by weight loss measurements, wear tests and microstructural observations of plasma-sprayed PMMA coatings. The work shows the existence of a critical traverse speed below which satisfactory coatings cannot be produced. The critical traverse speed is a characteristic parameter for all polymers, and its value is dependent upon the degradation behaviour of the particular material.