Composite plating having a gradient in density of codeposited particles

A process for the codeposition of a composite metallic coating comprising finely divided particulate matter dispersed within metallic matrixes and having a gradient in particle density distribution along the coating thickness. The established gradient ranges from a region of high density of particles to a region of lower density of particles along the coating thickness. The established gradient is affected by the deliberate change(s) in plating parameter(s) during the plating cycle.

BACKGROUND OF THE INVENTION 
The plating of articles with composite coatings bearing finely divided 
particulate matter is well documented. This technology has been widely 
practiced in the field of electroplating and the field of electroless 
plating. The acceptance of these composite coatings stems from recognition 
that the inclusion of finely divided particulate matter within metallic 
matrixes can significantly alter the properties of the coating with 
respect to properties such as wear resistance, corrosion resistance, 
appearance, and lubricity. 
Electroless composite technology is a more recent development as compared 
to electrolytic composite technology. The state of the art can be reviewed 
in a recent text entitled "Electroless Plating Fundamentals and 
Applications," edited by G. Mallory and J. B. Hadju, Chapter 11, published 
by The American Electroplaters Society, 1990. 
The evolution of composite electroless plating dates back to Oderkerken 
U.S. Pat. No. 3,614,183 in which a structure of composite electroless 
nickel with finely divided aluminum oxide was interposed between metallic 
layers for improved corrosion resistance. Thereafter, Metzger et al in 
U.S. Pat. Nos. 3,617,363 and 3,753,667 extended the Oderkerken work to a 
greater variety of particles and miscellaneous electroless plating baths. 
In each of the above cases, the identical condition was maintained 
throughout each test to achieve a composite layer with the finely divided 
particles uniformly codeposited and dispersed within the metallic matrix. 
Christini et al, in Reissue Patent 33,767 further extended composite 
electroless plating to the codeposition of diamond particles. In addition, 
Christini et al demonstrated certain advantages associated with the 
deposition of a barrier layer (strike) prior to the composite layer. 
Yano et al, in U.S. Pat. No. 4,666,786 disclosed the combination of silicon 
carbide with boron nitride which provides with enhanced properties. 
Feldstein in U.S. Pat. Nos. 4,358,922 and 4,358,923, demonstrated the 
advantages of utilizing an overlay layer, above the composite layer. The 
overlay layer is essentially free of any particulate matter. The main 
advantage recognized in these two patents is the ease by which the 
smoothness of hard deposits can be attained in a short duration. Further 
appreciation for the nickel overlay is noted in U.S. Pat. No. 5,164,236. 
Spencer, in U.S. Pat. No. 4,547,407, demonstrated the utility of mixtures 
of dual sizes of particles in achieving smoothness of coating. 
Feldstein et al, in U.S. Pat. Nos. 4,997,686, 5,145,517, and 5,330,330 
demonstrated the utilization of particulate matter stabilizer(s) in the 
deposition of uniform and stable composite electroless plating. 
Henry et al in U.S. Pat. No. 4,830,889 disclosed a composition for the 
codeposition of graphite fluoride. 
Parker, U.S. Pat. Nos. 3,562,000 and 3,723,078, demonstrated the 
codeposition of certain refractory metals and chromium along with 
composite electroless plating. 
Although significant work has been reported in the above cited patent 
literature and publications which are included herein by reference, with 
different objectives and results, there is one common theme in all the 
above references. Specifically, they all demonstrate the practice of 
identical plating conditions throughout the codeposition plating cycle to 
achieve a composite with a uniform density of the particles dispersed 
within the metallic matrix. The prior art has not suggested or recognized 
any advantage(s) associated with composite coating(s) having a gradient of 
particle density within the coating thickness. 
Despite the usefulness of the dual layer (U.S. Pat. Nos. 4,358,922 and 
4,358,923), we have recognized certain practical limitations associated 
with it. It is necessary to use multiple plating tanks, compositions, and 
pre-plate solutions to produce the dual layer, and this not only adds to 
the manufacturing costs but also adds to the costs of waste treatment. In 
addition, the deposition of multiple layers may, at times, lead to poor 
adhesion between the layers and moreover it can not lead to a gradual 
(gradient) change in the percent of particles deposited if required. 
Accordingly, it is highly desirable to achieve the properties of the dual 
layer combination or modifications thereof, but it would be preferable to 
achieve these properties in a single step and from the same plating tank. 
Such an improvement is of special value for articles used in textiles, 
molds, engines, and other applications in which the ease of smoothing or 
break-in time is required. 
SUMMARY OF THE INVENTION 
Generally stated, the present invention accomplishes several of the above 
objectives by providing a novel process for the deposition of composite 
plated articles bearing finely divided particulate matter dispersed within 
metallic matrixes. The finely divided particulate matters may have any of 
several characteristics such as, wear resistance, corrosion resistance, 
and lubricity as well as combinations thereof. 
The present invention provides a composite layer structure wherein the 
finely divided particulate matter is deposited in a non-uniform manner 
which however is in a pre-selected pattern having a gradient in particle 
size density along the deposit thickness. More specifically, the deposited 
gradient density decreases within the metallic layer. The article 
resulting from the present process provides at least the same features as 
the prior art, however it all is incorporated into a single layer and does 
not require multiple steps or layers as taught in U.S. Pat. No. 4,358,922 
and others. An additional benefit associated with the present method is 
the simplicity in the metallization steps, the longer lifetime associated 
with the composite plated articles, and the elimination of multiple 
plating baths. The latter thereby minimize the waste treatment aspect 
required by user. Further advantages of the present article will become 
apparent to those skilled in the art upon consideration of the following 
detailed description. 
In addition, though the main points of the invention are associated with 
composite electroless plating, one skilled in the art will recognize that 
the present invention can be adopted for a composite derived from 
electrolytic plating as well. Accordingly, in the broad sense, this 
invention is applicable to composite plated articles derived from 
electrolytic plating as well. From the prior art it should be recognized 
that a variety of combinations of matrices and particulate matter can be 
codeposited. The inclusion of such combinations can be adapted to the 
present invention and hence their adaption to the present invention will 
fall within the spirit of this invention. 
DETAILED DESCRIPTION OF THE INVENTION 
Plated composites bearing metallic matrixes with finely dispersed 
particulate matter are well known in the art. Many studies have focuses on 
the mechanism of codeposition, particularly in electrodeposition. However, 
the mechanism for codeposition in electroless composite is still not fully 
understood despite the work reported in many publications and issued 
patents. 
There are several known parameters that can affect the density of the 
codeposited insoluble particulate matter. Though we do not wish to be 
bound by theory, in electroless composites, it has been recognized that 
certain plating bath parameters, such as the plating rate, the degree of 
agitation, and the concentration of chemicals can affect the density of 
the codeposition for a specific insoluble particulate matter and specific 
plating bath. The plating rate is generally affected by temperature, pH 
and concentration of chemicals (reactants). 
In all of the prior art, it was generally the objective to yield a uniform 
(even) density of particles throughout the composite layer leading to a 
"regenerative" type coating. 
In the present invention there is a departure from previous practices. 
Specifically, the overall composite layer is plated in a manner that will 
lead to a gradation with respect to the density of particles deposited 
through the metallic coating. It is preferable that the main portion of 
the coating be comprised of a composite with a substantially uniform 
particle density, which then decreases towards a lower density near the 
surface. The density for the insoluble particulate matter nearest the 
surface (or the area adjacent to the interface) will thus be less than 
that of the main portion of the coating. Thus, the matting part in contact 
with the coated machinery part will equilibrate or break-in in a short 
period of time. This feature is particularly useful with codeposits having 
a wear resistance particulate matter. This feature can be achieved with 
great ease by controlling the rotational rate (speed) of the part during 
the plating cycle while immersed in the plating bath. 
The following example demonstrates the process associated with the present 
invention. In this example the rotation was modified as a plating bath 
parameter.

EXAMPLE 1 
Steel rods 51/2" in length and 3/8" in diameter were used. The plating 
composition was a CDC electroless nickel plating bath manufactured and 
sold by Surface Technology, Inc., Trenton, N.J. This bath comprised a 
nickel salt and sodium hypophosphite as the reducer. Diamond dust having a 
mean particle size of approximately 1.7 micron was used with a loading of 
approximately 3.5 g/l of plating bath. The bath was operated at 
188.degree. F. with a pH of 4.6. The rods were submerged within the 
plating composition and plated according to the following schedule: 
11/4 hours with a rotation of 9.3 rpm. 
11/4 hours with a rotation of 168 rpm. 
After plating, photomicrographs of a cross sectional cut at 400.times. and 
1,000.times. magnification were taken of the plated rods with the 
following observations: 
Corresponding to the first rotational speed, a dense layer with diamond was 
deposited with an overall thickness of about 19 microns. Thereafter a 
layer without any particles and a thickness of about 15 microns was 
observed. 
In this experiment the gradient reflects an extreme case, it is obvious 
that other gradients may be derived based and controlled by the rotational 
speeds imposed (changes) during the plating cycle. 
Further experimentation at varied rotational speeds revealed intermediate 
diamond densities. There appears to be a linear relationship between the 
diamond density codeposited vs. the rotational speed when plotting a 
function related to diamond density vs. the rotational speed. Similar 
observations were noted with silicon carbide, aluminum oxide, and boron 
nitride particulate matter, though having different sensitivities, all 
however, having a negative slope. 
Also, in this example modification(s) of rotation speed was illustrated and 
it is recognized that other plating bath parameters may be used in 
practicing the present invention. 
From the above, it should be obvious that the present invention is not 
limited to the nature of the particles used nor the plating bath or 
substrate used. 
In the current process, by adjusting the selected parameter(s), the use for 
the plating bath can be made for repeated uses. From the above example, 
readjusting the rotational speed to 9.3 rpm results in a coating 
substantially the same as the starting point, provided that the chemical 
ingredients are at the set concentration and other parameters are held 
constant.