DE102011084608A1 - Plasma spray process - Google Patents

Abstract

The invention relates to a method for producing a coating by thermal spraying, in particular by plasma spraying, in which a component, in particular a cylinder liner, is internally coated with an alloy. It is proposed that, as the transport gas, nitrogen is supplied, wherein a spray additive is a solid alloy wire, which is guided in a plasma stream, and wherein additional powder is coated free.

Description

The present invention relates to a method for producing a coating by thermal spraying, in particular by plasma spraying, in which a component, in particular a cylinder liner of an internal combustion engine, e.g. is made of an aluminum, coated with an alloy, preferably with an iron alloy.

From the EP 1 967 601 A2 It is known that, for example, to coat an aluminum engine block, in particular its cylinder bore with an iron alloy while performing the arc wire spraying. This beats the EP 1 967 601 A2 to use an iron alloy containing inter alia 5 to 25 wt.% Chromium. It is essential in the EP 1 967 601 A2 in that the molten iron is additionally supplied with an additional powder, boron carbide. In the arc wire spraying method of EP 1 967 601 A2 it is the so-called TWAS method in which two wires are fed to a spray head in such a way that the power is transmitted to the wires. When the two wires touch each other, a permanent short circuit causes an arc to melt the wires. Behind the nozzle is a nozzle from which compressed air or an inert gas such as nitrogen escapes. This gas stream atomizes the molten iron alloy and supplies it with the molten borocabid powder to the surface to be coated.

The DE 44 11 296 A1 and DE 44 47 514 A1 are concerned with coatings by means of plasma spraying, wherein, however, a metal powder or a filler wire are melted, and wherein the material mixture nitrogen is supplied by means of metallic nitrogen compounds to harden the coating.

Today's internal combustion engines or their engine blocks can be cast from a metal or aluminum, in particular aluminum blocks have an iron or metal layer on their cylinder bores. The metal layer may be thermally sprayed. As thermal spraying methods, the above-mentioned methods are known.

Also known is the so-called PTWA inner coating process (plasma transfer wire arc). In this method, holes (cylinder bores) can be coated with a wire-shaped spray additive from the inside. Here, therefore, only a single wire-shaped spray additive material is supplied, wherein it is possible to use a filler wire, or to supply also wettable powder. The plasma strikes the preheated wire-shaped spray additive. The plasma gas is usually an argon-hydrogen mixture. As transport gas or atomizing gas, air or compressed air is used in the PTWA process. The layers produced by this process are characterized by a low porosity. The PTWA internal coating process has so far proven itself in the interior coating of cylinder bores.

However, it has been found that the metal or iron coatings of the cylinder bores produced according to the hitherto possible coating methods do not withstand the particular corrosion conditions of ethanol-containing fuels or ethanol fuels. This is particularly observed when the motor vehicle or the internal combustion engine is not used for a long time, which may be the case, for example, when parking during a vacation. Even an alloy containing 17% by weight of chromium had corrosive attack marks on the protective coating.

Based on the recognized corrosion problem of ethanol-containing fuels to metal coatings of cylinder bores, the invention is based on the object to provide a method of the type mentioned above, with which a coating improved in this respect can be produced.

According to the invention the object is achieved by a method having the features of claim 1. Further, particularly advantageous embodiments of the invention disclose the dependent claims.

It should be noted that the features listed individually in the following description can be combined with one another in any technically meaningful manner and show further embodiments of the invention.

According to the invention, a method for producing a coating by thermal spraying, in particular by plasma spraying, in which a component, in particular a cylinder liner of an internal combustion engine made of aluminum or cast, is coated with an alloy, is proposed in which nitrogen is at least is supplied as a transport gas, wherein a Spitzzusatzwerkstoff is a solid alloy wire, which is fed into a plasma stream, wherein additional powder-free or powder-free is coated. It is advantageous if the plasma spraying is a PTWA inner coating (plasma transfer wire arc).

The term "additive powder-free" or "powder-free" in the sense of the invention means that neither a filler wire filled with (metal) powder nor a separately supplied (metal) powder is used. Namely advantageous in the invention only one solid, ie homogeneous or an unfilled spray wire used. A suitable alloy for coating has as alloying element chromium and mainly iron. A preferred metal or iron alloy is disclosed below.

Due to the advantageous use of nitrogen gas as a transport gas instead of air or instead of compressed air, as used for example in the known PTWA method, the conversion or degradation of the chromium is excluded by the oxygen of the air previously used, so that the entire chromium portion of the alloy can be used to form a stable protective layer. Due to the difference in free enthalpy (or Gibbs's free energy) predominantly aluminum nitrides are formed instead of chromium nitrides. These aluminum nitrides replace the previous wear-resistant metal oxides that are produced during the PTWA spraying process with compressed air. As a result, results in the invention not only a wear-resistant coating but also a (corrosion) protective layer, which withstands the corrosive attacks especially ethanol-containing fuels.

For the purposes of the invention, ethanol-containing fuels for internal combustion engines may contain conventional fossil fuel (for example E5, E10 or E85) as an admixture of ethanol or be used in pure form (E100).

In the previously known PTWA method, it was assumed that an alloy with 17% by weight Cr is particularly suitable for meeting the requirements. By contrast, in the case of the invention, it is expedient to provide for the spray additive wire to have an iron alloy with a chromium content of 12 to 35% by weight. Other alloying constituents may be aluminum (2-10% by weight), silicon (0-1% by weight); Manganese (0-1 wt%), carbon (0-1 wt%) and others such as e.g. Phosphorus (0-1 wt%), Sulfur (0-0.09 wt%), Molybdenum (0-5 wt%), Nickel (0-1 wt%), Copper (0-0.5 wt%), Nitrogen (0-0.5 wt.%), The remainder being iron.

In a particularly preferred embodiment, the sprayed wire according to the invention comprises an iron alloy with 23 wt.% Cr, 5 wt.% Al, less than 0.5 wt.% Si, less than 0.2 wt.% Mn, less than 0.05 Wt% C, the remaining ingredients having an amount less than 2% by weight and the remainder being iron.

Of course, the inventive method can also be used for coating other components.

1 shows a nozzle unit 1 a PTWA inside coating device. The PTWA (Plasma Transferred Wire Arc) coating system is a system for coating bores, in particular cylinders in engine blocks of internal combustion engines. The nozzle unit 1 consists of a cathode 2 , a plasma nozzle 3 , and the electrically conductive alloy wire 4 as anode, perpendicular to the plasma nozzle 3 is supplied. Preference is given as material for the cathode 2 Tungsten used, which may be doped with thorium, for example. The plasma gas 5 , For example, a mixture of argon and hydrogen, is through in the nozzle body 6 mounted, supplied tangentially to the circumferential bores. The cathode holder 7 isolated the cathode 2 opposite the nozzle body 6 , The alloy wire 4 is in the wire feeder 15 rotatably guided and longitudinally displaceable.

The process is controlled by a high voltage discharge, which is the plasma gas 5 between alloy wire 4 , Nozzle body 6 and cathode 2 ionized and dissociated, started. The plasma thus generated flows through the plasma nozzle at high speed 3 , This is the plasma gas 5 to perpendicular to the nozzle 3 continuously supplied alloy wire 4 transported, whereby the electric circuit is closed.

In addition, that is from the pilot nozzle 3 emerging plasma jet 8th a transport gas 9 or a nebulizer gas 9 via feed channels 10 and auxiliary nozzles 11 fed.

The melting and atomization of the alloy wire 4 are influenced by two phenomena. The wire 4 is on the one hand by high currents, typically 65-90 amps, resistance-heated. The impact of the plasma jet 8th on the preheated wire 4 ensures its melting at the wire end 12 , In other words, inside the plasma nozzle 3 generated by high voltage discharge a plasma. A targeted nitrogen gas flow, so the transport gas 9 along the discharge path transports the plasma and the molten spray material 13 to the surface 14 of the workpiece to be coated.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1967601 A2 [0002, 0002, 0002, 0002]
• DE 4411296 A1 [0003]
• DE 4447514 A1 [0003]

Claims (4)

Process for producing a coating by thermal spraying, in particular by plasma spraying, in which a component, in particular a cylinder liner, is coated with an alloy, in which Nitrogen gas is supplied at least as a transport gas, wherein a spray additive is a solid alloy wire, which is guided in a plasma stream, and wherein additional powder is coated free. A method according to claim 1, characterized in that the plasma spraying is a PTWA coating. Method according to claim 1 or 2, characterized in that the sprayed addition wire comprises an iron alloy comprising 12 to 35% by weight of Cr, 2 to 10% by weight of Al, 0 to 1% by weight of Si; 0-1% by weight of Mn, 0-1% by weight of C and others such as e.g. 0-1Gew.% P, 0-0.09Gew.% S, 0-5Gew.% Mo, 0-1Gew.% Ni, 0-0.5Gew.% Cu, 0-0.5Gew.% N, balance Fe , Method according to one of the preceding claims, characterized in that the spray additive wire, an iron alloy with 23 wt.% Cr, 5 wt.% Al, less than 0.5 wt.% Si, less than 0.2 wt.% Mn, less than 0.05% by weight of C, with the remaining ingredients having an amount of less than 2% by weight and the remainder being iron.

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