Metallic Articles Having Dual Layers of Electroless Metal Coatings Incorporating Particulate Matter

Composite electroless coating is a new generation of composites which can be derived via electroless plating techniques. The following patents and article reflect upon the state of the art, the techniques which are used, as well as those particulate matters which may be incorporated within the electroless plating matrices: U.S. Pat. Nos. 3,617,363; 3,674,447; 3,753,667; U.S. Pat. No. 29,285; R. Barras et al., "Electroless Nickel Coatings-Diamond Containing", Electroless Nickel Conference, Cincinnati, Ohio, November, 1979; and British Pat. No. 1,476,024. These Patents are included herein by reference.

Though electroless plating may be applied to a wide variety of substrates, the coating of metallic substrates is of great technological interest for achieving any of several properties on the initial substrates (e.g., corrosion protection, wear-resistance, etc.). However, plating may be carried forth on non-conductor and semiconductor type substrates as well. Though the mechanism of composite electroless plating is not fully understood, it is believed that the insoluble particulate matter suspended within the electroless plating composition are entrapped during the electroless plating process build-up. For an effective entrapment, the insoluble particle must attach itself to the surface and permit the conventional electroless plating process to proceed without disturbance, and encapsulate the particle(s) without disruption of the plating process.

It is therefore recognized, since the particulate matter does not appear to participate in the actual (basic) mechanism (see (1) R. M. Lukes, Plating, 51, 969 (1964); (2) N. Feldstein et al., J. Electrochem. Soc. 118, 869 (1971); (3) G. Salvago et al., Plating, 59, 665 (1972)) of the conventional electroless plating but rather is entrapped, that it is therefore essential that there be a high probability for the particulate matter to "stick" to the surface and result in fruitful entrapment rather than contacting the surface and falling off the surface into the bulk solution. It is also recognized that the electroless nickel matrix provides "cement" for the entrapment of the particulate matter. Moreover, it is undesirable for the particles to become autocatalytic.

In general, in the present invention, particles in the size range of 0.5 to 75 microns may be contemplated. It is further preferable to select the desired particle size with a narrow particle size distribution. In most applications, generally speaking, the particle size is in the range of 15 to 30% by volume, though is is possible, particularly with higher temperature and/or high bath load concentration, to achieve particle loading within the deposit exceeding 40% by volume.

In the case of diamond particulate matter, especially diamond of a polycrystalline nature manufactured by an explosion process, preferred particles may be selected in the range of 1 to 9 microns in size.

Table 1 provides the hardness of selected materials. The materials demonstrated in Table 1 are diamond, silicon carbide, corundum, tungsten carbide, nitrided steel, hard chrome, etc. It is therefore realized that for wear-resistant application it would be most useful to incorporate particulate matter having a greater hardness value in comparison to the metallic matrix derived via electroless plating or those materials or particles which are higher than the nickel phosphorus alloy which reaches a hardness of approximately 69 Rockwell C units with heat treatment, as is well known in the art.

A process and articles for electroless plating incorporating particulate matter are described. The process and article(s) thereof comprise at least two distinct metallic layers, the first of which comprises the particulate matter and thereafter a metallic layer substantially free of particulate matter.

Articles which are of special concern where the present invention is particularly useful are those articles having a cylindrical or a disc-type geometry.

The articles (e.g., textile machinery parts) which will be used or affected by this process are of those general types known in the textile industry as friction texturing discs, spinning rolls (open end and others) and rotors. Generally, these parts have circular, bell shaped, cylindrical or multicylindrical geometries. They are primarily used in a spinning or rotary fashion with the directional movement of the part about an axis which is perpendicular to the plane of rotation.

Specifically, friction texturing discs used to impart a false twist have a multicylindrical geometry (Textured Yarn Technology/Volume 2, Stretch Yarn Machines, Monsanto Co. 1967, Edited by G. D. Wilkinson ATI, p. 16) with the outside circumference, or that which actively contacts the yarn, slightly rounded or convex. Contact of the yarn is made partly perpendicular to the part's circumference, and parallel to the axis of rotation.

Spinning rolls are of a cylindrical geometry in which yarn contacts the outside circumference in a parallel or unidirectional fashion, or perpendicular to the axis of rotation. The outside or active circumference of this part usually contains pins or teeth which actually perform the necessary functions.

Rotors (see U.S. Pat. No. 3,439,487) are bell shaped articles in which the yarn contacts the bell or open end of the piece in a direction which is parallel to the axis of rotation.

Many of these materials are of aluminum, aluminum alloy, or plastics due to their light weight, cost, and relative ease of shaping. Other metallic substrates which may be used with the present invention, aside from aluminum alloys, may be carbon steel alloy and tooled steel, 400 series stainless steel, high speed steel, 300 series stainless steel, brass, copper and its alloys, nickel and its alloys, and combinations thereof.

For achieving the maximum hardness (e.g., 69 R.sub.c) for typical nickel phosphorus coating, heat treatment at a temperature of about 750.degree. F. should be carried forth. This heat treatment step, aside from providing improved adhesion of the metallic layers to the substrate, provides the well known matrix hardening for nickel phosphorous or nickel boron type alloys.

In the metallization of textile machinery parts, the final roughness of the coating is an important parameter in order to insure no damage to the contacted yarn. At the same time, the inclusion of the particulate matter to the wear-resistant coating leads to increased surface roughness. The resulting surface roughness is proportional to various parameters such as particle size, concentration of particles in the coating, the thickness plated, as well as the initial roughness of the substrate.

In order to meet the "smoothness" requirement for textile applications, particularly in texturing operations, smoothing of the final coating bearing the metallic matrix and particles has been an accepted procedure. In so doing, e.g., by brushing, loosening and knocking off of outer particles has taken place. This procedure is time consuming and costly since the wear resistant particles are extremely hard (e.g., diamond). It is thus highly desirable to provide with coated articles with which the brushing time would be reduced and the resulting product be useful in wear resistant application.

In demonstrating the present invention, aluminum friction texturing discs were utilized. The discs were cleaned, zincated for activation, and thereafter were plated in a commercial electroless plating bath (Enplate 415, product of Enthone, Inc.) to which 7 g/l of 6 micron polycrystalline diamonds (explosion type product) were included. The bath was operated at 82.degree. C. and plating to about 19 microns in thickness was undertaken. Thereafter, several discs were plated in an electroless plating bath which was substantially devoid of particulate matter. In the actual experiment, the electroless bath was Niculoy 22 (sold by the Shipley Company) without any particulate matter. Parts were plated under the conditions recommended by the vendor, from zero to 2 hours of plating time. To insure good adhesion, a heat treatment step to yield a matrix hardness of about 69 R.sub.c was undertaken. It is noted that heat treatment does not change the surface roughness of the coating. Table 2 provides the resulting roughness with varied brushing time. It is noted that though some of the coated parts have a higher surface roughness (as-plated) in comparison to the control, their surface roughness decreased more rapidly when subjected to equal brushing time(s).

Though there are many electroless plating formulations which may be deposited as the final layer, bright type deposits are preferred (see "Modern Electroplating", Third Edition, R. Lowenheim, editor, Chapter 31, and references therein which are included herein by reference).

It is thus recognized that the articles produced by the present invention comprise at least two layers, specifically, a first layer which is a composite containing the particulate matter and a second layer which is substantially devoid of particulate matter. In general, it is preferable that the second layer comprise the same composition as the composite layer, however, in the absence of particulate matter. Moreover, due to the regenerative type nature of the coatings in this invention, any wear of the outer layer will still yield useful articles capable of withstanding wear during usage.

Though in the above illustration diamond has been used, it would be obvious to substitute other particulate matter for both wear resistance and/or lubrication purposes. Such substitution falls within the spirit of this invention.