Metal-backed layered composite and method of making same

A composite material for use as shock absorber, as bushings or the like having controlled friction characteristics of a layer bonded to a metal substrate is formed by applying a particulate composition of a thermoplastic synthetic resin and carbon fibers having a maximum length of 1 mm to the substrate and heating it to a temperature of 50.degree. to 70.degree. K. above the melting temperature of the synthetic resin.

FIELD OF THE INVENTION 
Our present invention relates to a metal-backed layered composite material 
and to a method of making same. More particularly, our invention relates 
to a bonded material having a metallic support layer and a sliding or 
friction layer (facing layer) containing short carbon fibers as 
reinforcing fibers in a matrix of a synthetic resin which can also include 
various additives for improving the friction or sliding (antifriction) 
properties of the facing layer. 
BACKGROUND OF THE INVENTION 
For improved frictional coupling of two members or for sliding bearing 
(antifriction) purposes, it is known to provide a bonded composite which 
comprises a supporting metal layer and a facing layer of fiber-reinforced 
synthetic resin, the latter defining the sliding (friction or 
antifriction) characteristics of the material. 
In German Pat. No. 30 16 041, for example, a bonded composite of the type 
described whose conventional or facing layer is an organic friction 
material utilizing a matrix of a phenol-formaldehyde resin which contains 
15 to 50% by volume of comminuted carbon fibers with a mean length of 0.2 
to 10 mm and a mean diameter of 3 to 50 microns, 7 to 20% by volume steel 
fibers and 10 to 15% by volume of a filler, generally a particulate solid. 
Friction layers composed of such friction material suffer premature 
fatigue under dynamic loading which will vary depending upon the type of 
synthetic resin matrix and may be traceable at least in part to the 
inclusion of steel fibers in the synthetic resin matrix. 
In German patent publication (open application) No. DE-OS 29 35 205, a 
layered composite and a method of making it are described in which the 
synthetic resin matrix has good sliding characteristics and thus forms a 
highly desirable function layer which also contains short carbon fibers 
together with practically endless carbon fibers, i.e. carbon fibers of 
considerable length. The fabrication method described in this publication 
is complicated and extensive, inter alia, because of the requirement to 
provide adjacent fiber bundles and because of the means for introducing 
them and orienting them. 
Furthermore, when the system of this publication is used to produce 
finished products such as journal bushings or sleeves bearing shells and 
the like, significant variations in quality are recognized. Apparently, 
when such materials are cut to permit the fabrication of the finished 
products, the long fibers suffer from shear breakage. Such shear breakage 
can be discerned upon stamping or other subdidivion of the blanks from 
sheet or strip workpieces. The fiber fragments upon such breakage appear 
to project from edges of the blanks and interfere with the ability to 
effectively use the materials. 
OBJECTS OF THE INVENTION 
It is the principal object of the present invention to provide an improved 
composite material which does not suffer from the disadvantages enumerated 
above and which provides a function layer of long useful life and 
resistance to fatigue, the material being subdividable without the shear 
problems discussed above. 
Another object of this invention is to provide an improved method of making 
a composite material with the advantages described. 
Still another object of the invention is to provide a layered composite 
material having a metal support and a function layer provided with a 
synthetic resin matrix and carbon fiber reinforcement which can be highly 
loaded and especially subjected to considerable thermal loading, which 
possesses a high resistance to fatigue especially under hydropulse 
loading, which has high wear-resistance and bonding stability and yet 
which can be fabricated especially simply. 
Still another object of the invention is to provide a simple and economical 
method of fabricating such a composite material. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter in 
accordance with the present invention in a composite material comprising a 
metallic support and a function layer or facing layer bonded to this 
support and comprising a fiber reinforced synthetic resin matrix. We have 
now discovered, quite surprisingly, that all of the disadvantages 
enumerated above with respect to the prior art systems can be obviated and 
the advantages outlined can be obtained if the sole fiber reinforcement 
for the function layer, i.e. the friction or sliding or antifriction 
layer, consists of short carbon fibers, i.e. carbon fibers having a length 
of at most 10 mm and preferably, as noted, a small fraction thereof, while 
the matrix consists of a thermoplastic synthetic resin or a mixture of two 
or more thermoplastic synthetic resins selected from the group which 
consists of polyarylethers, polyarylketones, polyarylsulfides, 
polyarylsulfones, and polyaryloxides, this function layer being applied to 
a roughened surface of the metallic substrate and including, if desired, 
additives for modifying thermal or sliding characteristics generally in 
the form of particles with a particle size of at most 40 microns. 
It is indeed surprising that a matrix of such thermoplastic synthetic 
resins or two or more such thermoplastic synthetic resins with short 
carbon fibers and particulate modifiers can allow for high thermal loading 
since the matrix in which the short carbon fibers and the particles have 
been dispersed can be applied in a molten form to the previously roughened 
surface of the substrate and bonded to permitting the matrix to harden on 
the surface. It is even more surprising that this mode of operation allows 
the interface between the substrate and the function layer to be 
practically pore free. Furthermore, the distribution of the short fibers 
is especially uniform and the method results in an excellent bonding of 
the short fibers in the synthetic resin matrix. 
The short carbon fibers are preferably graphite fibers, i.e. carbon fibers 
which can be subjected to a combined mechanical and thermal graphitizing 
treatment which increases the modulus of elasticity. Depending upon the 
nature of the composite material desired or the articles to be fabricated 
therefrom, we can incorporate in the function layer graphite fibers with 
especially high modulus of elasticity or graphite fibers with a 
comparatively high ductility or extensibility and/or tensile strength. 
Best results are obtained, in accordance with the invention when the short 
carbon fibers make up 20 to 45% by weight of the function layer and when 
the short carbon fibers have a length of at most 1 mm and a diameter of at 
most 0.1 mm. 
In the best mode embodiment of the invention, the short carbon fibers have 
a length of 0.04 mm to 0.2 mm and a diameter of 0.005 mm to 0.03 mm. 
The thickness of the invention layer is preferably a maximum of 0.1 mm. 
According to a highly advantageous embodiment of the invention, the short 
carbon fibers are dispersed in a polyether-etherketone thermoplastic 
synthetic resin matrix. The polyether referred to herein is a new aromatic 
polyether commercially available from I.C.I. Plastics Division. 
One of the differences between the conventional layer of the invention and 
the material of German open application No. 29 35 205 is that the short 
carbon fibers, preferably graphite fibers, of the invention reduce the 
coefficient of sliding friction of the composite material to the point 
that additives to increase or improve the sliding properties of the 
surface are no longer required. 
Nevertheless we can provide, in accordance with the invention, such 
additives which can include slide lubricants or low friction materials 
such as polytetrafluoroethylene (PTFE), molybdenum disulfide (MoS.sub.2) 
and the like. Other additives which improve the frictional or sliding 
characteristics of the conventional layer can include glass meal, i.e. 
ground glass, and/or mica, preferably synthetic mica. 
The substrate of the composite material of the invention, can be anyone of 
a number of metallic materials, for example, steel, copper, copper alloys 
such as CuSn8, aluminum and aluminum alloys such as AlZn5SiCuPb, and 
corresponding cladded materials such as steel clad with an aluminum or 
aluminum alloy or steel clad with bronze. 
The best mode embodiment of the substrate of the invention is a steel strip 
which is provided with a rough layer in the form of sintered bronze 
particles of a spheroidal shape. This substrate will be described 
hereinafter as a porous sintered spherical bronze coated steel workpiece. 
As noted previously, one of the important advantages of the present 
invention is the especially simple process by which the composite material 
can be formed. 
According to the invention, we provide a mixture of the matrix forming 
synthetic resin particles with the short carbon fibers and, if desired, 
particles of the friction-modifying additives and apply this mixture to 
the previously roughened and cleaned (degreased) surface of the metal 
substrate, the composition being applied in the form of a melt. 
According to the invention, the substrate surface has a minimum roughness, 
i.e. a roughness such that the minimum pit depth is 20 microns 
(hereinafter referred to as a minimum roughness of 50 microns RT) and the 
melt is applied continuously thereto and is formed from particles all of 
which have a particle size of at most 100 microns, the thermoplastic 
synthetic resin particles consisting of one or more of the thermopastic 
synthetic resins mentioned. 
The composition can be doctored onto the surface using a doctor blade, rake 
or like tool which also serves to establish the desired thickness of the 
composition and to orient the fibers. 
In the best mode embodiment of the invention, the composition is applied in 
particulate form and is then melted on the substrate continuously. 
The melting of the composition on the substrate can be effected directly 
after the particular composition has been applied by the doctor blade or 
rake by heating the composition and/or the substrate to a temperature of 
50.degree. to 70.degree. K. above the melting point of the matrix-forming 
resin or resins. 
The doctor blade or rake thus establishes the thickness of the layer in the 
form of the particulate composition and orients the fibers generally in 
the direction of advance of the strip before melting and after melting 
substantially the same thickness and orientation are retained. 
The melting of the composition can be effected in a continuous oven through 
which the strip is passed with the heating being effected from the metal 
band side such that, at least over a portion of the length of the path in 
the oven sufficient to permit escape of all bubbles trapped in the melt, 
the substrate is held at the aforementioned temperature of 50.degree. to 
70.degree. K. above the melting point of the resin or resins. This 
insures, after cooling, a pore-free conventional layer firmly bonded to 
the metal. 
The heating of the substrate or the underside of the composite can be 
effected by radiant heaters disposed therebelow or the substrate can be 
heated inductively. 
To avoid shrinkage cracks in the composite, the latter is slowly cooled 
after leaving the continuous oven, at least initially by the effective 
heated air. 
The roughening of the substrate surface can be effected in any conventional 
way, e.g. by grinding, wire brushing or blasting, especially by blasting 
with steel particles or silicon carbide (corundum) particles. Preferably 
the mechanically roughened surface of this type is then subjected to an 
alkali or organic solvent degreasing e.g. in perchloroethylene or 
trichloroethylene. 
Of course, the roughening can be in the form of an application of particles 
by sintering to a previously roughened or nonroughened substrate, as 
previously described.

SPECIFIC DESCRIPTION 
From FIG. 1 it will be apparent that a metal band or strip 11, whose upper 
surface is copper-coated and provided with a roughened structure in the 
form of sintered spheroids of bronze bonded to one another and to the 
steel, is advanced between a pair of rollers 12 beneath a powder hopper 13 
from the dry pulverulent composition of the invention is applied to the 
substrate and doctored thereon as shown at 14 to produce a layer 15. 
The powder mixture utilizes a polyether-etherketone thermoplastic synthetic 
resin of a particle size of at most 100 microns, with which short graphite 
fibers having a length below 1.0 mm and a diameter between 0.005 mm and 
0.03 mm are dispersed. These fibers can be coated prior to the dispersion 
with thin films of polyamide and/or polyacrylate compounds to promote 
bonding of the matrix. 
The powder mixture can be prepared by milling the polyether-etherketone in 
liquid nitrogen to the desired particle size range with the carbon fibers 
then being added and blended with these particles and with any solid 
additives for improving the frictional characteristics, before the powder 
mixture is introduced in the hopper. 
The doctor blade 14 can be heated and can be formed as a rake beam. The 
temperature to which this rake beam is heated can be 50.degree. to 
70.degree. K. above the melting point of the matrix forming synthetic 
resin. 
The rake beam serves to uniformly distribute the powder mixture over the 
total breadth of the strip 11 to a uniform thickness and with orientation 
of the fibers in the direction of advance of the strip. The powder layer 
is given a thickness which corresponds to the desired final thickness of 
the facing or functional layer. 
The heated rake 14 partly melts the thermoplastic synthetic resin while 
orienting the fibers so that the fibers are practically prepositioned as 
the strip enters the continuous sintering oven 16 in which the underside 
of the band is heated by radiant heating or the band is inductively heated 
to a temperature of 50.degree. to 70.degree. K. above the melting point of 
the resin. The furnace environment can be held during heating at a 
subatmospheric pressure, i.e. the furnace can be evacuated to minimize the 
trapping of air. 
When the band leaves the oven, it is slowly cooled by contacting it with 
heated air slightly below the temperature in the oven. When the 
temperature is dropped below the melting point of the resin, the composite 
material can pass between the rollers 19 and wound up in a coil 20. 
When the band is not provided with a sintered layer to form the roughened 
surface, it may be composed of copper or aluminum based material and can 
be mechanically roughened as described or by sanding with a sanding belt. 
The mechanical roughening should provide a minimum roughness of 20 microns 
RT. 
FIGS. 2 and 3 illustrate sections through the composite 17 in which the 
substrate 21 of steel has a friction layer or sliding layer 22 penetrating 
within the roughened layer 25 formed of sintered bronze spheroids. The 
steel was copper coated as represented by the layer 24. The matrix is 
shown at 26 to penetrate into the interstices of the sintered structure 25 
and the short graphite fibers can be seen at 27. FIG. 2 shows the graphite 
fibers to be more or less oriented in the longitudinal direction while 
FIG. 3 indicates that they are sectioned transverse to the longitudinal 
direction. 
Instead of the graphite fibers, carbon fibers formed from rayon or bitumen 
can be used and the graphite fibers can be fabricated from 
polyacrylonitrile. The graphite fibers have an advantage over other carbon 
fibers as is well known from the literature that they can withstand higher 
temperatures and mechanical loading and have a high modulus of elasticity. 
Depending upon the graphitization process, the fibers can be generated 
with especially high modulus of elasticity of especially ductility or 
tensile strength. 
Using the composite material formed as described, shock absorber bushings 
and sleeves were fabricated which were able to withstand high loading and 
functioned reliably for longer periods than conventional shock absorber 
sleeves made from composite materials. 
The shock absorber sleeves made by the invention and those of noncomposite 
materials were tested on a hydropulse apparatus under the following 
conditions: 
h.sub.1 =.+-.40 mm at 1 Hz 
h.sub.2 =.+-.8 mm at 12 Hz 
stroke h.sub.1 is superimposed on stroke h.sub.2 
Function: Triangular 
Speed: h.sub.1 =0.16 m/s h.sub.2 =0.40 m/s 
Test duration: 278 hrs. 
Load direction changes: h.sub.1 =1 million strokes h.sub.2 =12 million 
strokes 
Total path length: 320,000 m 
Lateral loading: over 1 min 690 N over 15 s 3,700 N 
The test showed that the shock absorber bushings of the invention had a 
failure rate below 2% while otherwise identical composite materials 
utilizing the longer carbon fibers of the prior art had a failure rate in 
excess of 75%.