Methods of coating surfaces and in elements comprising such surfaces

219-121*

SR

07-04-72 XR 3*674*544

July 4, 1972 a. grosseau 3,674,544

METHODS OF COATING SURFACES AND IN ELEMENTS COMPRISING SUCH SURFACES

Filed Dec. 24, 1969

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INVENTOR

BY

ATTORNEY

United States Patent Office

3,674,544

Patented July 4, 1972

3,674,544

METHODS OF COATING SURFACES AND IN ELEMENTS COMPRISING SUCH SURFACES

Albert Grosseau, Chaville, France, assignor to Societe

Anonyme Automobiles Citroen, Paris, France 5 Filed Dec· 24,1969, Ser. No. 887,828

Claims priority, application France, Dec. 31, 1968,

182 896

Int. Cl. B05b 5/00, 7/08, 7/14

U.S. Cl. 117—93.1 PF 9 Claims _nn

ABSTRACT OF THE DISCLOSURE

The inner surface of a light alloy crank-case envelope of a rotary piston engine is coated by spraying a wear- 15 resistant layer thereon. Carbon steel and nickel aluminide are sprayed on simultaneously by at least two oxy-acetylene flame or plasma flame guns. Spherical grain particles such as SiC, TiB or BC can be incorporated.

The present invention relates to improvements in methods of coating surfaces and in elements comprising such surfaces. More particularly, because it is in this case that its application seems to offer most advantages, 25 but not exclusively, among these elements, it relates to crank-casing envelopes or stators of aluminium-based light alloy for rotary piston engines.

In the operation of a rotary piston engine, the piston rubs along a narrow contact zone thereby exerting a high 30 pressure against the inner surface of the crank-casing. It is already known to apply, on this surface, coatings constituted by materials which improve the friction and possess good resistance to wear, and which are applied to the surface by methods known as “metallisation.” 35

It is a particular object of the invention, to provide such methods and elements that respond better than hitherto to the various requirements of practice, especially as regards resistance to wear of the rubbing surface and reduction in the coefficient of friction of the member, such 40 as a piston, which co-operates with the above-said element.

According to the invention, a process for coating rubbing surfaces, especially inner surfaces of crank-case envelopes for rotary piston engines constituted of light alloy, comprises projecting onto the said surfaces, simul- 45 taneously, by means of at least two oxy-acetylene flame or plasma flame guns, on one hand, carbon steel and, on the other hand, nickel and aluminium forming, by combination with the oxygen supplied by gun, a composite material comprising nickel aluminides and oxides with 50 substantially ellipsoidal grains, the said composite material, which can also be made to contain, by incorporation on the projection, of spherical grain particles such as silicon carbide and/or titanium boride and/or boron carbide, being alloyed with the carbon steel to form a 55 wear resistant layer, favourable to sliding and averaging a certain minimal thickness, resisting well the high contact pressures developed, according to the method of production preferred, during the operation of the engine, which properties, by reason of the nature of the projected 60 materials, are preserved at the operating temperatures of the engine.

According to another aspect of the invention, there is provided an element with a rubbing surface, said element being arranged so that the said surface is covered 65 with a wear resistant layer constituted by an alloy of carbon steel and a composite material formed principally of nickel aluminides NiAl and Ni₃Al which occur in substantially equal parts with, in smaller proportions, oxides with substantially ellipsoidal grains NiO, A1₂O₃ and 70 NiAI₂O₄, which are formed by reaction during the operation and with which may advantageously be incorporated, in smaller proportion, particles with spherical grains, such as silicon carbide SiC and/or titanium boride TiB₂ and/or boron carbide BC, the wear resistant layer having a thickness advantageously at least equal to 0.3 mm.

Preferably, content of carbon steel is comprised between 0.4% and 1.2% by weight and the proportions, by volume, of the materials to be alloyed may vary from 10% to 90% for carbon steel and from 90% to 10% for the complementary part in which aluminium and nickel are of substantially equal amounts.

Preferably again, the steel is about 0.6% carbon and the proportions, by volume, of the materials to be alloyed are in the neighbourhood of 60% for steel with about 0.6% carbon and 40% for the complementary part in which aluminium and nickel are in substantially equal amounts.

Preferably again, the proportions by volume of the materials to be alloyed are around 60% for steel with about 0.6% carbon and 40% for the complementary part breaking down to 35 to 38% in which aluminium and nickel are in substantially equal amounts and 5% to 2% constituted by spherical grain particles of diameter from 1 to 3/t (microns), of silicon carbide SiC, and/or titanium boride TiB₂ and/or boron carbide BC.

The invention consists, apart from these principal features, in certain other features which are preferably incorporated at the same time and which will be more explicitly described below.

In order that the invention may be more fully understood, a preferred embodiment of the method according to the invention is described below purely by way of illustrative and non-limiting example, with reference to the accompanying drawing, in which the single figure shows, in transverse section, a crank-casing produced according to the invention.

According to the invention, a crank-casing 1 of light alloy based on aluminium obtained by moulding or forging, of cylindrical inner shape and of which a perpendicular cross-section has a shape in the form of a trochoid with two arcs, a wear resistant coating is produced on the inner surface 2 of the envelope 1, in the following manner.

According to the invention, there is projected onto the inner surface 2 of the envelope 1, simultaneously, by means of two oxy-acetylene flame or plasma flame guns, on one hand, carbon steel and, on the other hand, nickel and aluminium forming, by combination with the oxygen supplied by the gun, a composite material comprising nickel aluminide NiAl and Ni₃Al and oxides with substantially ellipsoidal grains, NiO, A1₂O₃ and NiAl₂O₄, the said composite material and the carbon steel being thus alloyed to form a wear resistant layer 3.

The inner surface 2 is previously machined to dimensions in the neighbourhood of (but slightly greater than) those of the surface 4 which must be obtained for the finished casing 1. The two surfaces 2 and 4 are substantially parallel and their mutual distance 5 is equal to the desired thickness for the wear resistant layer.

To prepare the surface 2 for spraying with fused metal (or metallisation) and to facilitate the attachment of the metallic layer sprayed onto the surface 2, it is advantageous to effect a sanding operation in which there is projected violently against the said surface an abrasive jet, for example of corundum, so as to render the surface 2 rugged and adapted to ensure good adhesion.

The envelope 1 is then brought to a temperature of about 180° C. so as to produce, as explained below, a final prestressing.

The metallisation proper is then carried out by means of two guns adjusted to spray on the same spot, respectively, carbon steel, on one hand, and aluminium and

3,674,544 nickel, on the other hand, the materials being supplied to the guns in the form of wire and/or powder. The discharge rates of the guns are adjustable and means are provided to ensure relative rotational and translational movements between the envelope and the assembly of guns so that the whole surface 2 passes before the guns and can be metallised.

A first passage is advantageously effected during which only the aluminium and nickel gun discharges, so as to deposit a thin keying film of composite material formed of aluminides of nickel and of oxides (of thickness generally comprised between 0.02 and 0.05 mm.), over the whole of the surface 2.

The two guns are then simultaneously discharged to form the wear resistant layer proper whose attachment is improved by the above-said thin film. The ratio of the discharges by volume of the two guns is equal to the ratio of the proportions by volume desired for the carbon steel, aluminium and nickel, if desired supplemented with particles of other material.

These proportions by volume may vary between 10% and 90% of carbon steel of which the content of carbon is between 0.4% and 1.2% by weight. Complementarily, the proportion to be followed for aluminium and nickel may vary between 90% and 10%, the aluminium and the nickel occurring in substantially equal amounts. Preferably, a steel with 0.6% approximately of carbon is selected and a proportion, by volume, in lie region of 60% of steel for 40% of complementary part in which aluminium and nickel are in substantially equal amounts. Preferably again, the proportions, by volume of the materials to be alloyed are in the neighbourhood of 60% steel with about 0.6% carbon and 40% of complementary part comprising 35% to 38% of which aluminium and nickel are in substantially equal amounts and 5% to 2% of particles, with spherical grains of diameter of 1 to 3μ (microns or thousandths of a millimeter), of silicon carbide SiC, and/or titanium boride TiB₂, and/or boron carbide BC.

The two guns are made to discharge during as many passes as is necessary to obtain a layer 3 of thickness at least equal to 0.3 mm. This thickness confers on the said layer good resistance to contact pressures exerted by the piston.

The wear resistant layer thus obtained is uniform and has a fine surface porosity which may be put to profit for hot impregnation, in an oven, of the said layer with lubricating products such as, for example, mixtures of graphite and boron nitrite, of tungsten bisulphide and/or of molybdenum bisulphide. Clogging of the pores is thus ensured by products favourable to sliding of the piston.

The envelope 1 is then allowed to cool to ambient temperature, which has the effect of placing under compression the layer 3 which has been deposited on the previously heated envelope 1, hence expanded. A pre-stressing is thus created in the layer 3 of which the mechanical holding will be better on operation of the engine, the temperature of preliminary heating being adapted to the temperature of this operation.

The layer 3 is subjected to a truing operation and, finally to an operation of grinding and/or of rolling for finishing of the geometry and of the surf ace. condition.

It has been experimentally observed that the wear resistant layer 3 thus obtained with an alloy of’carbon steel, of aluminium and nickel if necessary supplemented with particles, and more particularly with the alloy corresponding to the preferred composition, not only resisted wear remarkably but was favourable to sliding on itself of the piston or, more exactly, of the segments, constituted preferably of grey pig iron, of the said piston.

As one hand, the projection of the wear resistant layer 3 is effected very rapilly and, on the other hand, the materials used are relatively cheap in comparison with other materials tried and which gave worse results, the method of coating and the envelope obtained by the invention have been found to be very economical.

Consequently, there is obtained a coating of the inner surface of the crank-case envelope for a rotary piston engine which well meets the desired object.

As goes without saying, and as results besides already from the preceding description, the invention is in no way limited to those of its modes of application, nor to those of is methods of production of its various parts, which have been more particularly envisaged; it embraces, on the contrary, all variations.