HARD MATERIAL SEAL MEMBERS HAVING A LOW FRICTION COEFFICIENT

TECHNICAL FIELD

This invention relates to hard material seal members of the type in which the sealing effect is due to precise finish of the sealing mem¬ bers, which are movable in mutual direct contact without interposition of any gasket made of a yielding material. Examples of pairs of seal members of this kind are found in hard material plaques for faucets, in industrial valves, in injection pumps for diesel engines and so on. In the following, reference will be particularly made to the very signifi¬ cant case of hard material plaques for faucets, however it should be un¬ derstood that this does not constitute any limitation to the field of application of the invention.

BACKGROUND OF THE INVENTION The hard material plaques used as flow interception and regulation members in faucets for hydraulic appliances are conventionally made of aluminium oxides or silicates, sintered and then ground to a roughness of about 0.2 to 0.4 microns. Because of these very accurately finished surfaces, said plaques form members capable of effectively intercepting and regulating a flow of water, and due to their hardness, which ranges in the order of 23,000 N/mm, they have a long life and are capable of pushing out hard foreign bodies, such as sand grains which arrive to contact them, without suffering any damage. However, just because of

this very accurate finish, these plaques have a tendency to adhere to one another which, in the case of lack of lubricants, provides a high sliding friction, even higher that that due to the friction coefficient of such materials, which is per se relatively high, approaching 0.12. For a proper operation of faucets including such hard material plaques, the plaques are provided on their cooperating surfaces with a thin layer of silicone lubricant, which reduces the sliding friction to acceptable values. However, in the Long run and with repeated mutual slidings of the plaques, the silicone lubricant is gradually pushed out, and the friction increases to unacceptable levels. In extreme cases, the faucet may become locked.

Attempts have been made to manufacture the hard material plaques for faucets by using wolfram carbide (widia). However the costs of pro¬ duction and machining of this material have proved to be prohibitive. It has also been proposed to manufacture the pairs of plaques for fau¬ cets with a material, equal for both plaques, consisting of silicon carbide (SiC). The silicon carbide, in its ^* known different cry¬ stallographic modifications, provides hardnesses ranging between 28,000 and 35,000 N/mm, which are much higher than those of the materials con- ventionally used for this purpose, while providing a friction coeffi¬ cient approaching 0.05. In addition, this material has a reduced tendency to provide adhesion between machined surfaces having a smooth finish, so that it initially allows to effectively reduce, within certain limits, the friction between the cooperating plaques of a fau- cet. However it has been found that, with a long use, the friction bet¬ ween two plaques both made of silicon carbide greatly increases, until in this case too it reaches unacceptable levels.

Finally, it has further been proposed by the same owner of this Patent to use, ,in a pair of plaques for faucets, two plaques made of ha,rd materials having different characteristics, with at least one of said ^' plaques- made of silicon carbide (SiC). Because of this provision a surprising and' permanent reduction of the friction between the hard

material plaques is observed, even in the absence of any lubrication, whereby the problem mentioned herein may be considered as theoretically solved. However, the manufacture and grinding of silicon carbide pla¬ ques require the use of special machinery and technologies, which are not 'within the reach of all manufacturers. Hence, it was desirable to find a solution of the mentioned problem, which could be of more general use and therefore, in particular cases, even more economical.

Then, requirements similar to those described for the hard material plaques for faucets generally arise for all hard material seal members which cooperate in direct mutual contact.

DISCLOSURE OF INVENTION

Considering these circumstances, the object of this invention is to provide hard material seal members having a Limited cost of production and machining and capable of providing a sufficiently reduced friction coefficient, even in the absence of Lubrication, so as to allow a proper _, operation of the apparatuses for a long period of time without requiring any intervention.

This object is attained, according to the invention, by a pair of hard material seal members in which both said members are made of at least moderately hard materials capable of receiving a precise machin¬ ing, at least one of said members is coated with a thin layer of a high¬ ly hard material obtained by physical or chemical deposition from a gas¬ eous medium, and said two members have cooperating surfaces of different nature or finish.

BEST MODES FOR CARRYING OUT THE INVENTION

Among the highly hard materials which may be deposited for the pur¬ pose of the invention there are particularly the silicon carbide, the carbides and nitrides of metals, especially of the transition metals, metal nitrites, particularly ti-tanium nitrite and chromium nitrite, and the cubic crystallographic lattice carbon.

Various processes of physical or chemical deposition from a gaseous medium are known, which may be used for forming the coating of a highly hard material -as mentioned above. Among these processes there are particularly the CVD and PVD processes developed by the Battelle In- stitute, the surface modification by ion implantation, eventually under laser treatment, the surface modification by ion irradiation, the plas- ma-CVD process, and the magnetic field assisted deposition process by Leybold-Heraeus. Such processes have proved to be of particular ad¬ vantage. Therefore, according to a preferred characteristic of this in- vention, at least one of the seal members is made of an at least mo¬ derately hard material coated with a material of high hardness by means of one of the just mentioned processes.

When applying the invention, it is necessary for both seal members to be made of an at least moderately hard material, to be able to recei- ve a precise machining, since only a high degree of finish, such as that given by a precise grinding or lapping, enables the seal members to effectively perform the sealing action, either in a faucet or in any other device. However, unlike the conventional seal members, those of the present invention do not require their bodies to also have the wear resistance and the degree of hardness which are usually required. This is why, in a seal member coated by means of the deposition of a material of high hardness, the material forming the body of the seal member only serves as a support, whilst the cooperation with the other seal member is entirely entrusted to the material of high hardness deposited on at least one of the seal members. Therefore, the seal members according to the invention, or one of them, may also be made of a material which, though being hard and resistant enough to receive an appropriate finish and to maintain in the long run its shape, does not have per se ac¬ ceptable qualities to form a seal member. Among these materials are in¬ cluded for example steatite, ceramic materials of modest quality, metals and some synthetic materials ^' having high hardness, stiffness and stability. Also a material obtained by baking a mixture of a plastic

powder and a hard material powder (socalled tuflite) can be used.

On the other hand, a seal member cooperating with a member coated with a highly hard material and having a high finish does not suffer any damage due to its limited hardness, and a considerable difference in hardness between the two cooperating surfaces turns out to be of ad¬ vantage to reduce the friction coefficient in the absence of lubricat¬ ion. For this reason one of the seal members may lack any coating of highly hard material.

Alternatively, both seal members may be coated with materials of high hardness, but in this case the two cooperating surfaces must have sufficiently different characteristics, which may be obtained either by providing a coating with two different materials, or by applying the coating materials onto surfaces having different degrees of finish, or even by combining these two characteristics with one another. An important advantage which may be suitably exploited in the use, according to this invention, of the hϊghly hard material layers obtained by physical or chemical deposition from a gaseous medium, is that these layers do not substantially modify the degree of finish of the surfaces on which the deposition is carried out. Therefore, the plaque intended to receive the deposition of a material of high hardness may be machined to a high degree of finish, for example by lapping, prior to carrying out the deposition, thus allowing working on a material which is suffi¬ ciently hard for receiving the desired finishing operation, but on the other hand is not so hard as to render difficult and expensive this op- eration or to require special equipments and technologies. This enables obtaining an important reduction in the manufacturing costs and allows the machining to be carried out by means of conventional machinery, by any manufacturer having no special equipments such as those required, for example, for lapping a seal member made of silicon carbide.

The use of pairs of cooperating seal members, one of which, owing to its coating, has a hardness considerably higher than that of the other seal member, and has a high surface finish, alLows to employ for

the other seal member a material of lower hardness, not coated, and hav¬ ing a rougher surface including a high number of microscopic recesses which substantially reduce the bearing surface between the cooperating seal members. Due to this peculiarity a further reduction of the dry sliding friction between the seal members takes place, and further important economies in the manufacturing costs are obtained, inasmuch as the machining of the not coated member is not only less expensive per se, but it may also be limited to a rougher degree of finish.

Particularly, an advantage of the seal members coated by means of a deposition of silicon carbide is the high chemical stability of this compound. In fact, silicon carbide is not subject to corrosions or al¬ terations in any kind of water, and it provides an exceptional resistan¬ ce to oxidation. A similar behaviour is also given by other materials among those of high hardness which may be deposited from a gaseous me- dium, and especially by carbon having a cubic chrystallographic lattice. This material, which could in no way be used to form the body of a seal member, is very advantageous also for its reduced friction coefficient, which approaches the value of 0.02.

Of course, the invention may be used independently of the shape and number of the seal members employed in an apparatus; such seal members may be flat or they can form bodies whose working surfaces are convex or concave, for example as those of a sphere or a cylinder.

The application of this invention allows to ensure the smoothness in the control of the apparatuses which embody such seal members, in- dependently from the presence of a lubricant, for periods of time which are much longer than those obtained hitherto, and this without practi¬ cally increasing the cost of the seal members. In effect, the anyway limited cost of the process of deposition from a gaseous medium is at least approximately counterbalanced by the lower cost of the materials forming the seal members and by a lower cost of the working.

POSSIBLE INDUSTRIAL APPLICATIONS

This invention finds a particularly interesting utilization in val¬ ves and faucets, both in those of the more largely used type, having two substantially disc-shaped plaques, and in those having three substanti- ally disc-shaped plaques, or a single disc-shaped plaque cooperating with sleeve-shaped plaques placed at the inlets. Also the invention can be used both in those valves in which the plaques are crossed in only one direction by the water, which then enters the body of the valve or of a cartridge, and in those valves in which the water flows through the plaques firstly in a direction and then in the opposite direction.

However, the application of the invention to valves and faucets is not exclusive and, as already pointed out above, other possibilites of application of the pairs of seal members according to the invention are for example the industrial valves, particulary those subject to high temperatures and/or to the action of chemical products, the valves and pistons for injection pumps of diesel engines, and so on.