IE59541B1 - Tungsten carbide/cobalt composite having improved frictional properties - Google Patents

Abstract

1. A ceramic/metal composite layer, having improved friction properties, containing tungsten carbide and cobalt, characterized in that it contains free carbon and its relative composition is 30 to 50 atoms of free carbon, 3 to 12 atoms of cobalt, and molecules of tungsten carbide to make up to 100.

Description

The invention relates to a ceramic metal composite layer with improved frictional properties, comprising carbides of tungsten and of cobalt.

Ceramics and ceramic/metal composites or cermets have intrinsic properties of hardness, resistance to elevated temperatures and chemical inertness which make them suitable for certain mechanical applications (layers, abutments, hermetic sealing adjuncts) and especially when the expected temperatures of use (cylinders of thermal motors for example) or the environment (abrasive atmospheres) rule out conventional solutions such as the use of metals resistant to corrosion and suitably lubricated.

Ceramics and cermets, in the expected conditions of use (dry friction or with uncertain lubrication) generally present coefficients of friction considerably greater than 0.1, which is the maximum value acceptable for many applications, since greater values lead to excessive heating by reason of the density of energy brought into play, or more simply lead to high losses which cancel the advantages expected from working at high temperature for example.

The study of the frictional properties of cermets leads to the following conclusions. .

The inferior frictional properties of cermets appear to be ψ due in great part to the metal which constitutes the binder, and which itself presents mediocre or bad tribological properties. With a binder content of 10% there is I often observed a typical metallic behaviour in conditions of friction, with the formation of welds, adhesive wear and transfer of material.

Nevertheless, since the metallic binder has for its premier role the task of reducing the fragility which characterizes ceramic materials, reduction of binder content improves the coefficient of friction and reduces the adhesive type wear, but increases the fragility or brittleness of the layers, with a lessening of the resistance to mechanical wear as a coroll ary.

But the applicant has discovered that by including free carbon in the material of the ceramic metal layer, it is possible practically to suppress the effect of the metallic binder on resistance to wear of the adhesive type, and at the same time to obtain remarkably low coefficients of friction.

In accordance with this discovery, the invention provides a ceramic metal composite layer with improved frictional properties, comprising the carbides of tungsten and of cobalt, characterized in that it comprises free carbon, its proportional composition being from 30 to 50 atoms of free carbon, from 3 to 12 atoms of cobalt, and tungsten carbide molecules to make up the balance of 100.

For preference the proportion of cobalt lies between 3 and 7 atoms per 100 particles, whether atoms or molecules.

The characteristics, advantages and properties of the composite layers of the invention will emerge, moreover, from the following description by way of example, with reference to the attached drawings in which; Figure 1 is a diagram of the resistance to seizing and wear of composite layers as a function of their composition; Figure 2 is a diagram of the measured coefficients of friction of layers as a function of the com10 pressive force applied.

It is pointed out that the layers were obtained by a procedure derived from the conventional preparation of tungsten/cobalt carbide composite layers by pulverization in a plasma arc, and that the carbon present in the free state in the layers originates at least mainly from thermal decomposition of the tungsten carbide.

The frictional experiments were carried out on a tribometric machine, in which a rotating ring is applied to an opposing plate under a pre-determined load, the speed of rotation of the ring being regulated so that the linear sliding speed is 0.5 m/s.

Figure 1 gives the results of the preliminary experiments which were carried out to determine the most preferred compositions of the layers.

The abscissa represents the cobalt content, expressed as the number of atoms per 100 particles, and the ordinate represents a value of the tribological performances, expressed in arbitrary units, proportional to the frictional contact pressure threshold which produces irreparable damage in the layer.

The results of tests carried out on layers which contain carbides of tungsten and of cobalt Co only are located between the curves a^ and Ji.

The results of tests carried out on layers additionally comprising carbon in proportions of 30 to 50% correspond to the band contained between the curves c and dj in this zone there is no appreciable correlation between the dispersion of the performances and the carbon contents.

For layers without carbon the performances are mediocre.

For low concentrations of cobalt, they rise with that concentration to stabilize at above 10 to 12 cobalt atoms per 100 particles. The dispersion of the results, represented by the spacing between the curves ji and hi, remains smal1.

Observation of the samples after tests confirm that, below 10 atoms of Co per 100 particles, wear appears as mechanical abrasion (mechanical and thermal stresses at the point of contact); beyond 10 atoms per 100 particles the phenomenon of adhesive wear (seizing) preponderates.

For layers with 30 to 50 carbon atoms per 100 particles, the performances are very distinctly improved for cobalt contents comprising between 3 and 12 atoms per 100 particles. Moreover, for contents comprising between 3 and 7 atoms of cobalt per 100 particles, where the performance maxima are found, the dispersion of the results, represented by the spacing between the curves c and d_, remains very low. so that the variations in composition of the ternary layers hardly affect the performances at all. The reproducibility of the performances does not imply any very precise control of the composition around the mean value.

Observations of the samples after test show that: - the presence of carbon limits, indeed suppresses, adhesive wear, up to cobalt contents as high as 16 to 18 atoms per 100 particles. This adhesive wear reappears for higher cobalt concentrations.

The best performances arise with a cobalt content of about I to 5 cobalt atoms per 100 particles, ie for a concentration at which, in the absence of carbon, the layers are significantly affected by mechanical wear (due to their sensitivity to mechanical and thermal stresses).

It has been demonstrated by micrographic tests, that the friction of the covered ring on the opposing plate creates, at the interface, a superficial film or transfer film, having a high carbon content, which limits the intrinsic action of the metallic binders. These transfer films benefit from the superior mechanical qualities of the substrate of tungsten carbide (hardness and Young's modulus) which are not affected by the high temperatures developed at the interface by the friction. These transfer films possess excellent resistance to mechanical stresses, in compression and in shear.

It will be seen that, after a running-in friction has established the transfer film, the cobalt present in the composite layer can undergo physicochemical changes without adverse repercuss ions on function and longevity. It even seems, on the contrary, that the formation of cobalt compound could be beneficial, because these compounds in general have better tribological properties.

In addition it has been established that at less than 30 carbon atoms per 100 particles, the transfer films are insufficiently developed, although above 50 carbon atoms per 100 particles, the composite layer has a reduced cohesion.

The tests on composite layers have subsequently given information on the determination of coefficient of friction r under increasing loads.

The tests were carried out on a tribometric machine under the following conditions: - a cover was applied to the rotating ring I - the opposing plate was of steel 35 CD4 quench-tempered - sliding speed 0.5 m/s - dry friction without use of an extraneous lubricant - load increasing in steps of 100 N from 100 N up to o 1000 N, which corresponds to 100-340 N/mm in hertz contact pressure.

The composition of the sample I of the invention was Co: atoms per 100 particles, C: 50 atoms per 100 particles. Comparative tests were made with coverings of compositions located outside the scope of the invention.

Sample II: Co 10 atoms per 100 particles; C 33 atoms per 100 particles Sample III: Co 20 atoms per 100 particles; C 30 atoms per 100 particles.

The results are given in Figure 2, each curve bearing the reference of the corresponding sample.

Sample I has a coefficient of friction of about 0.1 for a load of 100 newtons reducing to 0.07 under 400 N and 0.06 under 1000 N.

At the end of the test the wear was negligible. It is inferred from these results that the acceptable specific pressure is greater than 340 N/mm (this value representing the maximum load of the machine used).

By comparison sample II exhibits under 100 N a coefficient of friction of 0.15, which increases more than linearly with load, to reach 0.18 under 500 N.

Sample III exhibits a coefficient of friction of 0.2 under 100 N, which increases very rapidly with load, to exceed 0.4 under 200 N.

A third series of tests was carried out to estimate the influence of the opposing or antagonist frictional surface. The rotating ring was in every case of the same composition as in sample I.

Test IV was done on a plate provided with a coating of the same composition as the ring.

Test V was done with a steel plate provided with a coating of titanium nitride.

Test VI on a plate of steel Z30 C 13.

Test VII on a plate of sintered alumina.

The results are given in the following table- Table - Coefficients of friction Test number Load P (N) 1100 1200 1300 |400 |500 |600 |700 |800 |900 |1000| I I I I I I I I I I I IV 10.1011.10|0.08|0.07|0.06|0.06|0.06|0.06|0.06|0.06 | V |0.15|0.12|0.08|0.08|0.06|0.05|0.05|0.05|0.05|0.05| VI |0.1510.1210.11|0.12|0.08|0.05|0.05|0.05|0.04|0.04| VII 10.1610.12|0.10|0.09|0.08)0.0810.08|0.07|0.07|0.07 | I I I I I I I I I I 1 It follows from this table that, if correction is made for values related to the running-in period, all the coefficients of friction are of the same order of magnitude, whatever the nature of the antagonist layer of coating according to the invention. Particular note should be made of the advantageous behaviour of composite layers of the invention in friction against sintered alumina.

It should be recalled that tests I and IV-VII were carried out with increasing load. It will be observed that if, subsequently, the load is caused to decrease, substantially the same coefficients of friction are obtained for the same loads; in particular, in the case where the load is caused to decrease, an increase of the coefficient of friction in the range 500-300 N is observed.

The invention, of course, is not limited to the examples described, but includes all embodiments within the scope of the claims.

Claims (5)

1. A ceramic metal composite layer with improved fricI tional properties, comprising carbides of tungsten ahd of cobalt, and containing free carbon, its proportional 05 composition being from 30 to 50 atoms of free carbon, from 3 to 12 atoms of cobalt, and molecules of tungsten carbide to make up the balance of 100.

2. A composite layer as claimed in claim 1, wherein the proportional composition comprises from 3 to 7 atoms of 10 cobalt.

3. A composite layer as claimed in claim 2, wherein the proportional composition substantially comprises 50 atoms of free carbon.

4. Atoms of cobalt and 46 molecules of tungsten carbide. 15 4. A composite layer as claimed in any of claims 1 to 3, '-wherein the free carbon content arises, at least to its greater extent, from the decomposition of tungsten carbide.

5. \a ceramic metal (cermet) composite layer with improved frictional properties, as claimed in claim 1, substantially as herein described.