Hidenori Era

Thermal-sprayed Fe-10CM3P-7C amorphous coatings possessing excellent corrosion resistance

An alloy of Fe-10Cr-13P-7C was thermally sprayed by three different processes: (1) 80 kW low-pressure plasma spraying (LPPS), (2) high-velocity oxyfuel (HVOF) spraying, and (3) 250 kW high-energy plasma spraying (HPS). The as-sprayed coating obtained by the LPPS process was composed of an amorphous phase. In contrast, the as-sprayed coatings obtained by the HVOF and HPS processes were a mixture of amorphous and crystalline phases. The as-sprayed coatings showed a high hardness of 700 DPN. A very fine structure composed of ferrite, carbide, and phosphide was formed, producing a maximum hardness of greater than 1000 DPN in the LPPS coating just after crystallization on tempering. The corrosion re-sistance of the amorphous coating was superior to a SUS316L stainless steel coating in 1N H2SO4 solution and 1N HC1 solution. Furthermore, the amorphous coating underwent neither general nor pitting corro sion in1NUCI solution and 6% FeCl3 6H2O solution containing 0.05N HCl, whereas the SUS316L stain less steel coating was attacked aggressively.

Properties of Cr3C2-NiCr cermet coating sprayed by high power plasma and high velocity oxy-fuel processes

The structure, hardness, and shear adhesion strength have been investigated for Cr3C2-NiCr cermet coatings sprayed onto a mild steel substrate by 200 kW high power plasma spraying (HPS) and high velocity oxy-fuel (HVOF) processes. Amorphous and supersaturated nickel phases form in both as-sprayed coatings. The hardness of the HVOF coating is higher than that of the HPS coating, because the HVOF coating contains more nonmelted Cr3C2 carbide particles. On heat treating at 873 K, the amorphous phase decomposes and the supersaturated nickel phase precipitates Cr3C2 carbides so that the hardness increases in the HPS coating.The hardness measured under a great load exhibits lower values compared with that measured with a small load because of cracks generated from the indentation. The ratio of the hardnesses measured with different loads can be regarded as an index indicating the coating ductility. The ductility of the HVOF coating is higher than that of the HPS coating. Adhesion strength of the HVOF coating was high compared with the HPS coating. The adhesion of the coatings is enhanced by heat treating at 1073 K, and that of the HVOF coating is over 350 MPa.

Structure and phases in nickel-base self-fluxing alloy coating containing high chromium and boron

The structure of a nickel-base, self-fluxing alloy coating, containing chromium and boron thermal sprayed and fused, was investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). A lumpy M6C carbide, a rodlike M3B2 boride of tetragonal structure, a rodlike M7C3 carbide of hexagonal structure, and a Ni-Ni3B eutectic phase formed in the coating after fusing. Metals of M6C, M3B2, and M7C3 phases are composed of chromium, molybdenum, and nickel; chromium and molybdenum; and mainly chromium, respectively. The nickel phase in the coating has the L12 type superlattice structure.

Characterization of plasma sprayed Fe-10Cr-10Mo-(C,B) amorphous coatings

Alloys of Fe-10Cr-10Mo containing a large amount of carbon and/or boron were plasma sprayed by low-pressure plasma spraying (LPPS) and high-energy plasma spraying (HPS). The as-sprayed coatings obtained by the LPPS process are composed of only an amorphous phase, while as-sprayed coatings obtained by the HPS process are a mixture of amorphous and crystalline phases. The amorphous phase in these coatings crystallizes on tempering at about 773 to 873 K, and the crystallization temperatures depend on the content of carbon and boron. Thermal stability of the amorphous phase containing boron is higher than those phases containing carbon.A very fine mixed structure of ferrite and carbide, borocarbide, or boride is formed by decomposition of the amorphous phase, bringing about a hardness of 1200 to 1400 DPN (Vickers hardness). The coatings containing carbon retain a hardness of more than 1000 DPN, even on tempering at temperatures of 1073 K or higher. The anodic polarization behavior of the coatings exhibits an activation-passivation transition in 1N H2SO4 solution. The active and passive current densities of the as-sprayed amorphous and tempered crystalline coatings containing carbon is lower than the coatings containing boron. The corrosion resistance of the as-sprayed and crystallized coatings containing carbon is superior to a SUS316L stainless steel coating.

Formation of amorphous Fe-Cr-Mo-8P-2C coatings by the high velocity oxy-fuel process

Alloy powders of Fe-10%Cr-8%P-2%C(10Cr), Fe-20%Cr-8%P-2%C(20Cr), and Fe-10%Cr-10%Mo-8%P-2%C(10Mo) compositions (in mass%) were sprayed by the high velocity oxy-fuel (HVOF) process under different conditions. The as-sprayed coatings of 10Mo alloy were composed of only an amorphous phase under all the spray conditions, while the as-sprayed coatings of the 10Cr and 20Cr alloys consisted of an amorphous phase with a small amount of crystalline material. The volume fraction of the crystalline material increased slightly with the rise of the flame temperature. The hardnesses of the as-sprayed coatings of the 10Cr and 20Cr alloys were 600 to 700 DPN, respectively, while the 10Mo coating composed of an amorphous phase revealed 560 DPN. The corrosion resistance of the as-sprayed coating of the 10Mo alloy was the best among three amorphous coatings and also superior to the nickel-base self-fluxing alloy and SUS316L stainless steel coatings in 1N H2SO4 and 1N HCl solutions.

Characterization of plasma sprayed Fe-17Cr-38Mo-4C amorphous coatings crystallizing at extremely high temperature

A Fe-17Cr-38Mo-4C alloy powder was plasma sprayed by three processes: an 80 kW low-pressure plasma spray (LPPS), a 250 kW high-energy plasma spray (HPS), and a 40 kW conventional plasma spray (APS). The as-sprayed coating obtained by the LPPS process is composed of only amorphous phase. As-sprayed coatings obtained by the HPS and APS processes are a mixture of amorphous and crystalline phases. The three as-sprayed coatings exhibit a high hardness of 1000 to 1100 DPN. The amorphous phase in these coatings crystallizes at a high temperature of about 920 K. A very fine structure composed of hard ϰ-phase and carbides is formed after crystallization. The hardness of the coating obtained by LPPS reaches a maximum of 1450 DPN just after crystallization on tempering and retains a high hardness more than 1300 DPN after tempering at high temperatures of 1173 or 1273 K. The corrosion potential of the amorphous coating is the highest among the three coatings and higher than that of a SUS316L stainless steel coating. The anodic polarization measurements infer that the corrosion resistance of the amorphous coating is superior or comparable to SUS316L stainless steel coating in H2SO4 solution.

A modified shear test for adhesion evaluation of thermal sprayed coating

Abstract A modified shear test has been proposed to evaluate the adhesion strength of thermal sprayed coating. The modified shear test piece was cut to possess a semicircular notch at the corner of the protruded step. The effectiveness of the notch introduction to the test piece was investigated by means of photoelasticity observation using shear test pieces cut out of an epoxy-resin plate, finite element method (FEM) and practical shear testing of thermal sprayed coating. The specimen for the shear testing was prepared from a 75 Cr3C2/25 NiCr (w/w) cermet coating sprayed onto a mild steel by use of a high velocity oxy-fuel process. The photoelasticity observation and the calculation by FEM revealed that the introduction of a notch of a proper size reduced the stress concentration at the corner of the protruded step of a shear test piece. The adhesion strength evaluated from the shear test pieces with a notch increased with increasing the notch radius up to 0.3 mm owing to the reduction of the stress concentration. This means that the introduction of the semicircular notch of a proper size is valid for the evaluation of the adhesion strength of thermal sprayed coatings. The calculation by FEM has shown that the introduction of the notch is more effective for the coatings with larger elastic moduli compared with the substrate.

Structures and tempering behavior of rapidly solidified high-carbon iron alloys

High-carbon iron alloys containing carbide formers of chromium and molybdenum were rapidly solidified by means of a single roller method. In the alloy containing a high level of both chromium and molybdenum (10Cr-5Mo) and a critical carbon content of about 4 pct, the metastable phases,ε phase and austenite, are retained after solidification. Theε phase could contain a large amount of carbon in solid solution so that during tempering at about 900 K, it decomposes to very fine ferrite and carbide, which bring about an enhanced hardness of 1300 DPN. Even after tempering at a high temperature around 1100 K, the hardness hardly deteriorates due to a remarkable dispersion of fine M3C and M7C3 carbides. Thus, coaddition of chromium and molybdenum is effective in obtaining high hardness.

Interface reaction between nickel-base self-fluxing alloy coating and steel substrate

The interface reaction between a nickel-base, self-fluxing alloy coating and a steel substrate has been investigated to examine the formation of “pores,” which are observed along the interface of used boiler tubes. It was found that lumpy precipitates form along the interface instead of pores after heating at high temperatures and that the precipitates are of Fe2B boride. The adhesion strength of the coating is not decreased by the formation of Fe2B precipitates along the interface because of the increase of the adhesion due to interdiffusion.

Improvement of the Adhesion of a Ceramic Coating on a Ceramic Substrate

The structure and adhesion of an alumina coating on a ceramic substrate with NiCrAlY alloy bond coating was investigated by heating at 1573 and 1673 K in the air. Phases of NiO, NiCrO3, NiAl2O4, αAl2O3, and Ni were revealed in a 100 µm thick bond coating on heating at 1573 and 1673 K. A veined structure was also detected in the coating heated at 1573 K. The adhesion strength of the coating was improved and reached approximately 20 MPa on heating at 1573 and 1673 K for 14.4 ks in air although the strength of the as-sprayed coating was only 2 MPa. The improvement of adhesion strength may arise from the formation of NiAl2O4 with a spinel structure at the interfaces of the top coating/bond coating/substrate coating system. The adhesion strength of the coating improved on decreasing the bond coating thickness and reached approximately 45 MPa for a 20 µm thick bond coating which was heated at 1673 K. Only NiAl2O4 oxide was formed in the bond coating.