Machine element and method of making

Machine elements formed which overcome the problems associated with rolling bearings or rolling drive units, which, apart from load under rolling are simultaneously exposed to frictional load, which leads in particular zones to frictional wear. In order to keep this to a minimum the invention teaches to apply on the effected surfaces of such structural parts a coating of a friction bearing material, which, however, according to the findings of the invention must be applied with the PVD method and must have a thickness of the order of magnitude of the surface roughness of the base body; known thicker friction bearing coatings are not suitable. The invention is especially for forming gear wheels, rolling bearings, shafts, compressor screws, and rolling pistons.

FIELD OF THE INVENTION 
Background Of The Invention 
The invention relates to a rolling elements such as roller-bearings, races, 
cylinders, gear elements and the like, which is exposed primarily to 
stress under rolling, which consists of a base material, which resists the 
stress under rolling, and at least one coat of a material, normally 
employed in the friction bearing field applied on it. 
The construction and material selection for machine elements has advanced 
considerably in recent years by practices such as exposing functional 
surfaces of the rolling element substantially to stress under rolling, 
like the gear wheels of gearings or the components of ball and needle 
bearings. Through decades of optimizing geometry, material selection, heat 
treatment, and finishing techniques, wear-resistant products have been 
produced, which can be manufactured within acceptable cost limits. The 
main problem of earlier machine elements, that of fatigue wear, can be 
considered solved. Another problem of rolling body wear, however, could 
not be solved. In all rolling bearings or rolling motions in addition to 
the roll motion, a slide motion also occurs. This leads to friction wear 
in particular zones of the structural parts such as the regions of 
positive and negative slip of gear wheels, slip of shoulders of the race 
and slip of ball bearings. The zones and the stresses occurring there are 
generally known to the builder of conventional machine elements. Such a 
builder will also appreciate many effect factors, which influence this 
microwear, such as: inner stress from heat treatment and processing, the 
kind and extent of surface roughness, and the lubricating conditions. A 
further problem with rolling bodies occurs, if the mean wear problem is 
solved by selecting two very hard components. The relative motion of the 
two hard components is always accompanied by two considerable stored 
elastic deformation forces, which with weak damping, lead to vibrations. 
This is familiar to all users of machines equipped with rolling bearings 
as noise. In some applications, medical technology, passenger vehicles, 
etc.. this noise is disturbing to intolerable. A solution within the frame 
of the state of the art could not be found until now. 
Many attempts have been carried out to solve the known problems by applying 
coatings. For a variety of reasons these attempt have all failed. 
Sometimes, gear parts and gear wheels were galvanically or current-less 
nickel and chromium-plated. Hydrogen embrittlement of the base material is 
the result and the much more decisive resistance against fatigue wear is 
lost. Also coats of copper, indium, lead, and silver applied with PVD 
methods were tried. Unfortunately, these coats of lead, silver, gold and 
indium under conditions of normal use, wear rapidly through 
tribooxydation. In high vacuums lead, silver and gold coats are used, 
however, they are only used for very highly polished surfaces and in 
configurations with very little frictional motion. In that case, very thin 
coats suffice. A solution of the noise problem is, however, not presented 
by these coats. Copper layers are also not suitable for lubricated gears 
because copper abrasions degrade all oils catalytically. 
It has also ben suggested to coat pure friction bearings according to the 
PVD method with different materials (see German patents No. 28 53 724 and 
29 14 618, as well as German Published Application No. 34 04 880). Here 
the PVD technique was used to improve the hot hardness of bearing coats 
with oxide embeddings. Such coats are used today in friction bearings in 
engines (cf. U. Engle, Development and testing of new multilayer materials 
for modern engine bearings, part 2- Copper-lead-three-layer bearings with 
sputtered overly, in SAE Technical Paper Series, Int. Congress and 
exposition, Detroit, Feb. 24 to 28, 1986, pages 76 and 77). In the past, 
however, it has always been shown that such friction bearing material 
coatings are unfit for stress under rolling. In fact, while for decades 
nearly all friction bearings used in technology have had coatings, 
practically no coated rolling bodies are used. 
SUMMARY AND OBJECTS OF THE INVENTION 
It is an object of the invention to improve structural parts, in particular 
rolling elements such as roller bearings, races, cylinders, gear elements 
and the like, which primarily are subjected to stress under rolling but 
also frictional stress, that not only withstand rolling wear but also 
frictional wear. 
According to the invention, this task is solved by the fact that, on the 
functional surface of rolling elements subjected to frictional stress at 
least one PVD coating of a friction bearing material is applied, the 
thickness of which is of the same order of magnitude as the mean vertical 
height of surface irregularities of the basic body, it is achieved for the 
first time that at simultaneous stress under rolling and frictional stress 
the wear is kept to a minimum. The inventive tasks, however, cannot-as was 
pointed out-be solved simply through any given frictional bearing material 
coat, such tasks require the specific features of a friction bearing 
material deposited according to the PVD method. The term friction bearing 
material as used in the present specification refers exclusively to 
alloys. The coat thickness of the bearing material deposited must be of 
the same order of magnitude as the mean vertical height of surface 
irregularities since otherwise the carrying ability of the coat for the 
stress under rolling is no longer sufficient. 
The suitability of the friction bearing material coats applied in the PVD 
method is surprising per se and given only when the coat thickness is of 
the same order of magnitude as the surface roughness. In the mentioned 
known friction bearing coats a coat thickness is always used, which is at 
least one order of magnitude above the surface roughness. 
With frictional stress considerable, heat development occurs. Until now 
this was considered one of the main reasons for the . rapid wear of the 
rolling body functional surfaces. Local loss of the mechanical properties 
of the body was associated with such a temperature increase, due to 
exceeding the tempering temperature. If, however, the embedded component 
of the frictional bearing material has a melting point, which lies below 
such critical temperature, for example, the tempering temperature of the 
material, of which the structural part or its surface-hardened edge zone 
consists, the mentioned deterioration of the mechanical properties can be 
avoided. For example as embedded friction materials lead (melting point 
327.4.degree. C.), tin (melting point 231.89.degree. C.), zinc (melting 
point 419.4.degree. C.) or indium (melting point l56.4.degree. C.) or 
their alloys can be used. Aluminum and copper alloys have proven useful as 
a matrix and the matrix may be an alloy including chromium, nickel and 
magnesium. 
It is known that with multiphase friction bearing material coats the 
fineness of the distribution significantly influences the mechanical 
properties of the coat, in particular its hardness. In a particular form 
of the invention for given stresses where particularly high carrying 
ability is demanded, coats, in which the diameter of the particles of the 
embedded material has a statistical normal distribution with a mean of 
x.ltoreq.0.8 .mu.m, are selected. For lesser stresses, as well as with 
rolling bodies, which are not edge zone-hardened, softer, coats have 
proven to be more useful. Developing this degree of freedom is known to 
the expert, optimum hardness curves for edge coats have been published for 
numerous rolling bodies. In some cases the expenditure of generating 
hardness gradients in the friction bearing material cost will be 
worthwhile. The methods for this purpose have in the meantime in principle 
become known to the expert. The composition may be varied, embedded hard 
substances (according to German Patent No. 28 53 724) or for example by 
way of the coating temperature change the average size of the embedded 
particles (according to the Swiss Patent Application No. 02806/86-2). 
Instead of hardness gradients one can also select a sequence of coats with 
different mechanical properties, but one of them should be a friction 
bearing material coat applied according to the PVD method. 
The significance of the choice of the proper coat thickness has already 
been pointed out. The relationship between optimum coat thickness and 
surface roughness depends, of course, on the kind of surface 
irregularities. It is know that for the tribologically relevant 
characterization of surface microgeometries a large number of parameters 
are required. The practitioner would orient himself by the last processing 
step, thus, for example, differentiating between milled, planed, and 
ground gear slopes. He will select the ratio of coat thickness to R.sub.z, 
the irregularities peak; values in the range of 0.2 to 4 times the R.sub.z 
have proven useful. 
Among the PVD methods for applying friction bearing material coats thermal 
vaporization, cathode sputtering and light arc vaporization have proven 
particularly useful. Selection, in the individual case, depends on coat 
composition, the shape of the basic body, and the economically justifiable 
expenditures. Extensive literature regarding this topic is available (cf. 
E. Bergmann and J. Vogel: Structural part coating according to the PVD 
method" VDI Bericht 624, 1986). The invention can be used with all machine 
elements, which are primarily exposed to stress under rolling. Mentioned 
in particular are gear wheels, races of ball bearings, bearing surfaces of 
needle bearings, compressor screws, as well as rolling and rotary piston. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which 
preferred embodiments of the invention are illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings in particular, the invention embodied therein, 
comprises a machine element which is adapted to be subjected to stress 
under rolling and also stress under friction. The structural part 1 
includes a body portion having surface irregularities. A friction bearing 
material coating is applied by a PVD method so as to provide a friction 
bearing material coat on the structural part body having a thickness of 
the same order of magnitude as the thickness of the surface irregularities 
of the structural part body. 
In the example of FIG. 1a a gear wheel was produced of casehardened steel 
and subsequently again contour-ground. This generated on the slopes of the 
gear wheel a characteristic surface irregularities as is shown roughly in 
FIG. 1b. The thickness of the applied coats 6 was so selected that it is 
of similar order of magnitude as the greatest peaks 4 of the carburized 
base material 2. FIG. 1c shows an enlarged view of the composite of tough 
base material 2 and the rolling wear edge zone 3 with the embedded primary 
carbides 5 and the overlying AlSn20Cu coat 6 with the extremely finely 
distributed tin droplets 7 in the aluminum alloy matrix 8. Preparation of 
the coat can be carried out in a cathode sputtering arrangement known per 
se, in which an annular dense plasma is concentrated immediately in front 
of the cathode by a magnetic field. The installation shown in the example 
has a cylindrical processing chamber, at the inside of which up to a 
maximum of four sources of 322.6 cm.sup.2 area could each be vertically 
mounted. The substrates to be coated were also placed vertically on a 
carrier, which could be rotated with a drive unit regulatable between 0.2 
and 24.5 rpm (cf. for example BALZERS Product informations BB 800 246 
PD/Aug. 1985, as well as, BB 800 039 RD/July 1985). 
The working surface of the gear wheel shown in FIG. 1b was coated in a 
sputtering installation at a pressure of 1.2 Pa in an argon atmosphere in 
the complete absence of oxygen for 10 minutes. 
As targets during sputtering 3 targets of an aluminum/tin bronze of the 
composition AlSn20Cu were used at a voltage of 800 Volt and driven with a 
current of 20A. At a rotation of the gear wheel at constant rotational 
speed of 15 rotations per minute a coating rate of about 0.6 .mu.m/minute 
corresponding to a coating thickness of about 6 .mu.m was obtained at the 
completion of the treatment process. 
The coating generated in this manner had a weight ratio of Al:Sn:Cu of 
80:20:1 (corresponding to the composition AlSn20Cu) and an oxide content 
of less than 0.2 percent by weight. The average particle diameter was 
about 0.3 .mu.m and the hardness was 113 HV 0.002. 
As a further example FIG. 2a shows the model of a ball bearing inner race 1 
with a coating applied on it according to the invention. In this case the 
race is finely polished with diamond paste center-less which yields the 
surface structure with characteristic uniform roughness 4 shown in 
illustration 2b. Further details can be seen in FIG. 2c. Above the steel 
base material 2 a hard material coat further increasing rolling wear 
resistance, was applied and above a coat of AlSn10Pb10Cu 3, in which the 
particle size of the tin deposits 6, as well as the lead deposits 7 in the 
aluminum matrix 8 increases and, hence, generates a hardness decrease 
toward the surface. 
A fourth example involves the solution of a wear problem in a ball bearing 
and, in particular, an end journal bearing with 60 mm race diameter. It 
consists of 12 balls having a diameter of 9.4 mm and a cage of a nickel 
beryllium alloy and was used under the following conditions: 700 rotations 
per minute, 20 kg axial load, no oil feed. 
The races of this ball bearing were manufactured of 100 Cr6, hardened, and 
tempered at 190.degree. C. In a last step the running surface was 
polished, and, in particular, to a mean roughness depth of 0.2 .mu.m. 
Subsequently the races were coated in a method similar to example 1. As 
alloy for the targets, however, AlSn10Pb10 was used and the coating time 
was only 1 minute. This was sufficient in order to deposit on the ball 
bearing rolling surface a coat of 0.3 .mu.m. The parts were additionally 
heated during the coating so, that the temperature during the coating 
increased from 8.degree. C. to 170.degree. C. The raster electron 
microscope showed a particle size of embedded lead and tin, which was 
smaller in the region of the steel surface by 0.05 .mu.m while in the 
region near the surface a marked segregation was visible. From comparison 
measurements on test bodies it is known that the hardness of the coat near 
the steel surface had to be in the range of 140 Hv and the sink in the 
region near the surface to 30 Hv. While uncoated ball bearings showed 
already after 20 minutes heavy wear in the test, which became noticeable 
in the form of noise and strong vibrations, coated ball bearings run for 
24 hours without any increase in the noise level. 
A fifth example pertains to the problem of the manner in which the 
unbearable noise development of passenger car gears at high speeds can be 
reduced. In a given gear the sound emission under strong load in fourth 
gear was 18 decibels. The gear was manufactured of case-hardened 16 Mn 
Cr5. The toothing was milled and had on it slopes a surface irregularities 
with an R.sub.z of 10 .mu.m. Coating by cathode sputtering was out of the 
question for reasons of cost. Therefore, an ion plating method was chosen 
as it is mentioned, for example, in Swiss Patent No. 64 51 37 but with two 
electron beam vaporizers. Onto the tooth slopes of the wheels a coating of 
18 .mu.m was applied. The vaporizer power in the crucible with tin was 
selected relative to the crucible with aluminum so that on the parts, 
which were on a carousel, a coating with a mass ratio of aluminum to tin 
of approximately 4:1 was obtained. For the remainder, the characteristics 
corresponded to the coating of example 1, except that, due to the 
increased plasma density, a temperature of 120.degree. C. could be 
permitted. In a gear of the type in question, in which only the drive 
wheel of the fourth gear was coated, a decrease of the noise level to 16 
decibel could be achieved. 
In a sixth example, the issue again was a ball bearing wear problem, which 
occurred in a high-speed ball bearing, which is used in the control 
gyroscopes aircraft. In order to avoid wear in these ball bearings, balls 
of hard metal are used, and specifically of a tungsten carbide/colbalt 
alloy, which in general is not subject to wear. The wear of the rolling 
surface also, in general, is within the range of tolerance. In 1% of the 
bearings in operation, however, premature failure occurred. Damage 
analysis established that be traced back to friction wear, the cause of 
which lies in strong accelerations. The balls of these bearings had 
surface irregularities R.sub.z of 0.02 .mu.m. They were coated similarly 
to example 1, with the exception that the installation was equipped with 
copper, one lead, and one tin target. The balls were not cooled. After 
three minutes a cost of 0.06 .mu.m had formed on them which in terms of 
composition corresponded to CuZn20Pb10. Investigations under the 
transmission electron microscope showed that the coat consisted of very 
fine embedded lead/tin particles and a brass matrix. Ball bearings 
equipped with balls coated in manner showed no failure at all. 
The following seventh example shows how-with the composite material 
according to the invention-the problem of a rolling piston pump, which 
must be driven unoiled in order to transport pure gases, can be solved. 
Wear developes rapidly in these pumps on the rolling surfaces, which leads 
to scoring. In order to eliminate the problem, the following solution was 
chosen: 
The rolling surfaces of the piston were nickel-plated according to a method 
customary in the trade, and, in particular, with a coat thickness of 16 
.mu.m. Following the nickel-plating, which brought about slight 
improvement but not a solution of the problem, the rolling surfaces had 
surface irregularities of R.sub.z =2 .mu.m with buckled structure 
characteristic for chemical nickel. The coating method of choice was again 
cathode sputtering and the process was carried out in a matter similar to 
example 1. In this application, however, two targets of aluminum and two 
targets of tin were chosen. The discharge currents on the targets were 
changed continuously during the coating, and specifically as follows: 
______________________________________ 
Total Discharge Current 
Total Discharge Current 
Time Aluminum Targets 
Tin Targets 
______________________________________ 
0-5 minutes 
40 A 0 
5-40 minutes 
40 A increasing from 
0 to 30 A 
______________________________________ 
The obtained coat thickness was 6 .mu.m. The temperature of the parts was 
maintained at below 50.degree. C. by cooling so that again an extremely 
fine distribution of tin droplets in an aluminum matrix could be observed, 
except the thickness of the tin particles increased toward the surface. 
The mean diameter also increased somewhat, and, specifically, from 0.4 
.mu.m to 0.9 .mu.m. Rolling piston pumps with functional surfaces treated 
in this manner could be operated over longer periods of time without 
problems of scoring occurring. 
In an eighth example the problem is the wear of the shaft on a bicycle. The 
suspension of the front wheel in this case is of the nature that the balls 
run on the shaft held fixedly by the forks. The outer race of this ball 
bearing forms hub of the bicycle. This suspension is often under load a 
strong incline, which leads to a deflection of the balls from the central 
race track and effects a superimposed friction motion under high Hertzian 
stress, which exceeds the carrying ability of the grease. Mixed friction 
results and, consequently, visible adhesion wear on the shaft. In the 
model, the problem of which was solved with the coating, these shafts of 
turned 100 Cr6 were hardened and tempered at 180.degree. C. Turning leaves 
slight denting which on the generating surface of the cylinder corresponds 
to a surface roughness of R.sub.z= 6 .mu.m, while on the circumference an 
Ra of 0.6 .mu.m is measured. These shafts were coated on all sides 
according to the cathode sputtering method with a 2.1 .mu.m thick coating 
of the following composition: copper 73%, lead 23%, tin 4%. 
These are percentage weights. The substrate temperature in this coating was 
not controlled. It may have been in the range of 60.degree. C. to 
120.degree. C. The coats were checked under the optical microscope at 200 
times magnification, and no lead deposits of any kind were detected as 
they are otherwise characteristic for this material in sintered or cast 
form. In operation, it was found that in shafts coated in this manner the 
rolling friction coefficient does not increase even after 2000 km. 
Lastly, a ninth example shows the solution of the problem of wear of the 
rotors of screw compressors, so-called compressor screws. These 
compressors are of two different types, synchronized as well as driven. In 
fully synchronized screws a nearly pure rolling motion occurs. In driven 
ones a friction motion is so provided that through this sliding with the 
corresponding friction the force transmission necessary for driving is 
intended to take place. The advantage of this system are the low costs; 
the disadvantage: the wear tied to the friction in the slop region of the 
slopes. The two rotors of such a compressor were produced of 100 Cr6. In 
dry operation, which is necessary for the compression of pure gases, 
without coating an operating life of 16 hours is given. The surfaces of 
these rotors were contourground, so that a surface roughness of R.sub.z= 
0.8 .mu.m was obtained. As the method of coating cathode sputtering was 
chosen. The structure of the installation and arrangements has already 
been described (Dr. E. Bergmann, Dr. J. Vogel; J. Vac. Sc. Techn. A5 
(1987) page 70). 
As cathode material an aluminum bronze of the following composition in 
percentage weight was used: 
Aluminum 78.8%, tin 20%, silicon 1.2%. 
The coating conditions were as follows: installation pressure 1.2 Pa, 
sputtering power 4.times.18 kW, partial pressure of oxygen 0.06 Pa. 
Through the addition of oxygen the hardness was doubled to HV.sub.0.1 =300. 
The compressor screws coated in this manner showed an operating life of 40 
hours under the same conditions as the uncoated ones. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.