Nickel self-fluxing alloys for coatings

General information on materials and coatings made from self-fluxing alloys

Self-fluxing nickel alloys Ni-B-Si and Ni-Cr-B-Si-С.

These materials are used for deposition and sintering of coatings resistant to corrosion, friction wear and abrasive particles. The coatings resist gas corrosion up to temperatures as high as 700-850 °С, are resistant to fresh- and saltwater, saline solutions, oil-containing environments, ammonia and in other aggressive environments. They are not resistant, or weakly resistant, in inorganic acid solutions. The alloys melt in a range of temperatures typical of materials with eutetics in their structure. The base structural phase of coatings made from these alloys: γ- hard nickel-based super-saturated solution, chromium carbides of the type Cr₂₃C₆ and chromium carboborides; in powders with increased hydrogen content, particles of the stronger carbide Cr₇C₃ are also present. The coatings' hardness and wear-resistance increases as the content of chromium, borium, silicon and carbon. Borium and silicon form low-melting eutectics with nickel, with a melting point of 950-1080 °С, and restores oxide films on the surfaces of substrata with the formation of borosilicate slags (self-fluxing) in the presence of a liquid phase, and improve the substratum's wetability with liquid metals.

The adhesion, cohesion and wear-resistance of surfaces made from alloys with the same composition under dry friction and abrasive actions increases, as a rule, with higher density (reduced porosity) of the coating. Porosity of  unsintered plasma coatings made from self-fluxing alloys can reach 10-12%, and that of gas-sprayed coatings can reach 20%. Reduction of the porosity of coatings and, as a result, a 5-10 times increase in strength is acheived by sintering the coating, which is done either simultaneously with annealment with plasma welding (the PTA process) or with the layer-by-layer deposition of a coating using gas-powder welding by alternating the process of layer deposition with layer fusion. Fusing sprayed layers is also done with a gas-fired burner, a plasmotron flame, through heating in a furnace, or with high-frequency currents. Thanks to their high density (with porosity generally less than 1%), detonation coatings do not require fusion following spraying. Porosity's impact on the quality and workability of coatings is complex. In conjugated surfaces, under semi-dry friction, the optimal porosity—obtained, for example, through plasma spraying without fusion, can promote low wear in a friction assembly. The pores, by accumulating lubricants, can prevent fretting of conjugated surfaces.

Dilatometric studies of sprayed coatings made from the base alloys NiCr13Si2B1, NiCr15Si3B2, NiCr16Si3B3, NiCr17Si4B4 has shown them to be free of phase transmutations when cooling in the range of temperatures from 960 to 20°С. For this reason, phase transmutations occuring in an iron-carbon system with changes in volume can lead to the danger of surface fracturing in steel and cast-iron substrata during cooling. Therefore, it is recommended to cool products with applied coatings made from self-fluxing alloys at a slow pace. The interval of recommended cooling speeds is, for example, for steel 10 – less than 100 100 °С/с; for steel 45 – less than 35 °С/с; for steel 70 – less than 5 °С/с. The general characteristics of Ni-Cr-B-Si-C also include the tendency to maintain hardness and resistance to abrasive wear following drawing-back with heating to 600 °С. The hardness of alloys at high temperatures (“hot” hardness)—for example, at 650 °С— can reach 50-70% of that measured at room temperature.

The field of application of powders made from nickel alloys includes: wear-resistant coatings on parts for metallurgic, mining and oil-extracting, energy, glass and chemical equipment, stamping and pressing instruments, car, train and boat parts, gas-pumping devices, agricultural technology, etc. The alloys are allow used to produce mixtures with other matrials for use in coatings: carbides, intermetallic compounds, and other materials.

Self-fluxing iron-based alloys are represented by the high-carbon alloys AP-Cr4Mn2Si2B4V1, doped with vanadium, chromium and manganese, and the iron-nickel-chromium alloy AP-G14 and the intermediate-carbon alloys FMI with eutectic composition. Coatings made from high-carbon alloy is marked by increased hardness and high resistance to abrasive wear in an aqueous environment, while eutectic alloys (FMI) boast resistance to friction wear at high slipping speeds.

The self-fluxing lead-nickel bronze Cu-Sn-Ni-B-Si is a material for creating wear-resistant coatings for products made from copper, copper alloys, and steel.

Self-fluxing alloys are produced by gas-spraying a molten solution. The resulting polydispersed powders are subjected to screening into fractions that are narrow in terms of particle dimensions for use in various technologies for spraying and welding coatings: detonation and supersonic spraying, gas-powder coating welding, gas-fired and plasma spraying, laser and electrospark welding, and plasma and induction welding.

Particle sizes (base fractions)

Base fractions of powders for various coating technologies:

Based on the client's needs, we can produce powders with other particle sizes. The minimal dimensions of meshes  used to classify powders are: - 40 microns (~400 mesh) and 45 microns (325 mesh). Powders are not screened on meshes of less than 40 microns, since gas-sprayed powders typically contain a small amount of fine particles of less than 15-22 microns.

The shape and structure of powder particles

Gas-sprayed powders are primarily spherical in particle shape, with the structure of a cast material

fig. 1

fig. 2

fig. 3

fig. 4

Dilatometric analysis of self-fluxing alloys, heat content curves include the following characteristics points: the temperature at which the liquid phase appears, Т_0, and the temperature at which the maximum change in heat content is observed, Т_m, (the peak of flowing on a DTA curve). This range of flowing in self-fluxing alloys, the parameter ΔТ = Т₀ – Т_m, plays an important role in selecting the optimal flowing temperature for a coating. It is recommended that the flowing temperature be as close as possible to Т_m, at which the coating become more dense (porosity disappears), and a transitional diffusive layer of the necessary thickness is formed and the maximal strength of the coating and substratum is reached. In actuality, nickel alloys of the system Ni-Cr-B-Si-С that contain nickel borides and silicides, as well as chromium borides and carboborides, flow in a wider range of temperatures than ΔТ, with gradual assimilation during flowing, during heating (dissolution in nickel) of clusters of high-melting compounds.

Powder characteristics and fields of application

Notes to table:
Т_m – melting point (the first peak on the heat content curve of a DTA), reference data;
T_S  - solidus temperature;
T_L  -  liquidus temperature;
f – friction constant in a friction pair with steel, reference data;
The characteristics of materials and coatings provided in the table are for reference only.

Sample coating structures

fig. 5

fig. 6

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