WO1994003281A1

WO1994003281A1 - Method of lubricating a mechanical friction part

Info

Publication number: WO1994003281A1
Application number: PCT/FR1993/000764
Authority: WO
Inventors: Paul Clechet; Nicole Jaffrezic-Renault; Claude Martelet
Original assignee: Ecole Centrale De Lyon
Priority date: 1992-07-29
Filing date: 1993-07-26
Publication date: 1994-02-17
Also published as: FR2694369A1

Abstract

A method of lubricating a mechanical friction part comprises the following essential steps: 1) high density functionalisation of the surface to be treated, preferably to obtain 10?14 to 1015¿ functionalised sites per cm2; 2) impregnation of the functionalysed surface with a liquid consisting of a silane and/or a silane solution; 3) condensation of the silane by heating in a neutral atmosphere or in at least a partial vacuum so that said silane bonds substantially covalently to the functionalised sites of the friction surface(s) to form a substantially monomolecular layer; 4) washing of the surface with solvant, to eliminate excess silane. The method can be applied for the lubrication of a mechanical friction part likely to enter into frictional engagement, and manufactured from a material suitable for chemical functionalisation.

Description

METHOD FOR LUBRICATING A MECHANICAL FRICTION PART

TECHNICAL AREA :

The present invention relates to a method of lubricating a mechanical friction part, capable of being engaged in friction relationships and made of a material suitable for being chemically functionalized.

PRIOR TECHNIQUE:

One of the constant concerns of the general field of mechanics, in which the parts in contact are in movement with respect to each other, is to reduce, as much as possible, the coefficients of friction which are at the origin of wear of parts and significant loss of energy. This problem of limiting friction is particularly crucial in all the recent developments made in the micro-mechanical sector. This cutting-edge sector is based on the use of materials specific to microelectronics, such as semiconductors, for example based on silicon, for the manufacture of moving parts of the micro-actuator type such as motors or turbines, slides, gears, wheels, racks, pivots, ball joints or actuator combs, reproducing, on a scale varying from ten to one hundred micrometers, all the elements of classical mechanics.

The manufacture of such micro-mechanisms has been possible thanks to the appearance of new techniques, such as the sacrificial layer technique, the anisotropic etching method or the various other methods relating to deposition techniques on metallic and dielectric materials. or electrostatic welding.

The scientific and technical literature cites numerous examples of these micro-mechanical achievements, including, in particular, electrostatic micro-motors and silicon micro-turbines, the rotors of which can rotate at speeds amounting to several thousand revolutions per minute. It's clear that such micro-structures are necessarily the seat of very significant phenomena of wear by friction, phenomena which rapidly lead to the deterioration and destruction of these micro-structures.

Thus, it turned out that, failing to find effective means of reducing the coefficients of friction, this advanced sector of micro-mechanics would remain a dead letter.

Various alternatives were then proposed to limit friction, such as, for example, the lifting of moving parts by pressurized gases disclosed by J. B. HUANG, Q. Y. TONG, V. S. MAO in "Gas lubrificated micro-bearing for micro-actuators". PROC. TRANSDUCERS 1991, page 894, or electro-magnetic or electric levitation studied by F. KUMAR, D. CHO, W. CARR in PROC. TRANSDUCERS 1991, page 882, or else the use of materials with improved friction characteristics compared to traditional dielectric materials based on silica or silicon, such as silicon nitride.

These technical proposals are not really satisfactory, either because of their too high level of sophistication, or because of their prohibitive cost, among others.

For obvious dimensional reasons, lubrication by liquid or pulverulent lubricants cannot be transposed from the macroscopic to the microscopic domain. We must not lose sight of the fact that the air gaps of the micro-devices considered vary from ten nanometers to the micrometer.

Still with reference to macro-mechanics, one could imagine taking inspiration from the techniques of lubrication of magnetic read heads, by depositing lubrication coatings made up of one or more layers of compounds of the alkylated silane type.

These techniques are described, in particular by EIJI ANDO et al. in

"Frictional properties of monomolecular layers of silane compounds" - Thin solid films, 180 (1989) 287 - 291 and by M. SUSUKI et al. in "Characterization of monolayer and bilayer (polymers / monolayers) structures for their use as a lubricant" in THIN SOLID FILMS, 160 (1988) 453-462.

These documents report on various known methods of depositing, and not of real and solid fixing, one or more layers of amphiphilic silane compounds.

A first process is that, known as of Langmuir-Blodgett. It consists in treating flat surfaces, for example of porous or non-porous glass, by soaking and removing the support in a bath of hydrophilic liquid having, on its surface, a monomolecular layer of oriented amphiphilic substances, the hydrophilic parts of which are in contact with the liquid and whose lipophilic parts are pushed out of the bath. This deposit may possibly be supplemented with drying by heating, intended to somewhat improve the low solidity of the fixation of the amphiphilic compounds on the support.

By its principle, this process is of application strictly limited to the deposition of a lubrication coating on flat surfaces without crevices. It is therefore totally unsuitable for the lubrication of micromechanical friction parts involving three-dimensional structures of extremely small size.

A second method of depositing a lubricant coating (THIN SOLID FILMS, 160, 1988 - 454-455) consists in depositing, in the vapor phase, alkylated silanes on a flat support (glass).

In a third method, the support to be coated is immersed in a toluene solution containing the lubricating amphiphilic compounds. It is then planned a heat drying treatment (cf. OKAHATA et al. - Langmuir 1986, 2, 538-540, EP-A-0 474 228 among others). The major drawback of all of these prior methods is that the bonds formed between the lubricating amphiphilic silane compounds are of very low energy.

In addition, it is not possible to obtain a dense and coherent chemical network (large number of covalent bonds) between the support and the coating. As a result, the covering is fixed in a weak and unstable manner.

This constitutes a serious handicap, most of the time unacceptable, for lubrication applications.

Furthermore, the last two previous processes mentioned above do not make it possible to control and control the quantities of amphiphilic lubricant compounds applied to the support. This results in deposition inhomogeneities and serious difficulties in forming monomolecular layers, hence, necessarily, a stumbling block in the field of lubrication in micro-mechanics.

It is therefore clear that the techniques for depositing alkylated silane compounds, in particular for the lubrication of magnetic read heads, are also not transferable to the field of micro-mechanics. In addition, they are not entirely satisfactory in macro-mechanics.

It also appears that the prior art does not offer a lubrication process making it possible to obtain mechanical parts, in particular micromechanical parts, with a low coefficient of friction.

In this state of affairs, one of the essential aims of the present invention is to provide a method of lubricating a part capable of being engaged in mechanical friction relationships and, in particular but not limited to, making up micro structures. -dimensional mechanics of the micro-actuator type (motors, turbines), slides, gears, actuator combs ..., which allows to significantly reduce the coefficients of friction, in a durable and effective way, and which also allows to obtain a homogeneous lubrication, whatever the relief and the conformation of the friction surface considered.

Another object of the present invention is to provide a lubrication process which is convenient and economical to carry out.

Quite surprisingly and unexpectedly, the Applicant has discovered that it is possible to lubricate a mechanical part, in particular a micro-mechanical part, by subjecting the friction surface or surfaces of the part in question to functionalization. chemical followed by a fixation treatment of an appropriate lubrication compound. PRESENTATION OF THE INVENTION:

It follows that the present invention relates to a method of lubricating a mechanical friction part consisting, essentially, in chemically functionalizing at least the (or) friction surface (s) of the part in question, then in subjecting the latter a treatment for fixing a silane, the molecule of which comprises at least one hydrophilic end and at least one long lipophilic chain, characterized in that it comprises the following essential steps:

1) high density functionalization of the treatment surface, preferably until 10 ¹⁴ to 10 ¹⁵ functionalized sites per cm ^{2 are} obtained,

2) impregnation of the functionalized surface using a liquid consisting of silane and / or a silane solution,

3) condensation of the silane by heating in a neutral atmosphere or under an at least partial vacuum, so that said silane binds, essentially covalently, to the functionalized sites of the friction surface (s) to form a substantially monomolecular layer,

4) washing the surface with a solvent, so as to remove the excess silane.

Against all expectations, the process according to the invention makes it possible to obtain, over the entire friction surface, a coating composed of a monomolecular layer of a long chain lubrication compound, which turns out to be extremely adherent to the substrate. We are witnessing, in a way, the formation of a chemical continuum between the part to be lubricated and the coating, this continuum being formed of multiple covalent attachments whose solidity is very great. This multiplicity results from the high density of chemical functionalization of the friction surface.

This results in reliable and lasting lubrication which results in remarkable reductions in the friction coefficients, which can, in fact, be divided by at least six. Another remarkable advantage of the process according to the invention is its perfect adaptability to the lubrication of micro-mechanisms made of materials, for example based on silicon, carbon, boron, platinum among others.

It becomes possible to reduce the coefficient of friction in structures comprising air gaps of the order of at least 10 nanometers.

It goes without saying that this process is not limited to this field of application, but can also be used in the field of macro-mechanics, in particular on macro-actuators of the magnetic read head type, for example. In accordance with a preferred characteristic of the invention, the surface chemical functionalization is carried out using a reactive hydroxyl group and / or a group comprising at least one hydroxyl group, such as a carboxyl.

The lubrication compound has the particularity of having at least one hydrophilic end capable of reacting with the functionalized sites of the friction surface to form a strong covalent bond.

Each lubrication molecule forming the monomolecular coating is therefore covalently attached to the mechanical part considered, at least on its friction surface (s). The compound also comprises at least one long lipophilic chain comprising from 5 to 60, preferably from 5 to 30 and, more preferably still, from 10 to 30 carbon atoms.

It is preferably a silane of the following formula: R,

R, - Si - R 4

R ₃ wherein: - R _j is an amino group, alkoxyl or halogen, preferably selected from the following radicals: ^' . (CH ₃ ) ₂ - N—, NH ₂ ,

. CH ₃ - O -, CH ₃ - CH ₂ - O,

. Cl-,

- R ₂ and R ₃ may be identical or different, possibly equal to R ₄ and constituted by an alkyl group of the type C _m H _{2m + 1} , m being between 1 and

30, alkoxylated or halogenated amine, preferably chosen from the following radicals:. CH ₃ -, C ₂ , H ₅ ,

. CH ₃ - O—, CH ₃ - CH ₂ - O—,. Cl-,. NH ₂ -,

- R ₄ = - (CH ₂ ) _n - R ₅ ,

R _s being either an alkyl group, such as methyl, or a halogen group, such as Br or Cl, or a sulphite, carboxylic, hydrocyanic or phosphorus group, such as phosphonates, - and n is between 1 and 30 .

The long alkylated chain - (CH ₂ ) „- may be substituted by unsaturated groups allowing crosslinking by bridging between the chains of the same molecule of lubricating compound and / or between different molecules, or by other groups promoting the lubrication. According to a preferred embodiment of the process according to the invention, the lubrication compound is a silane and the reactive group is a hydroxyl.

In this case, the method comprises the following essential steps:

1) hydroxylation of the treatment surface, preferably until 10 ¹⁴ to 10 ¹⁵ OH sites per cm ^{2 are} obtained,

2) impregnation of the hydroxylated surface using a liquid consisting of silane and / or a silane solution,

3) condensation of the silane by heating in a neutral atmosphere or under an at least partial vacuum, 4) washing of the surface with a solvent, so as to remove the excess silane. Advantageously, the process comprises a step la for washing and drying the hydroxylated surface obtained at the end of step 1 and / or a step 1b for degassing the hydroxylated surface, optionally washed and dried, by heating under vacuum. less partial. According to the invention, preparatory step 1 consists in ensuring the hydroxylation of the friction surface, with a view to preferably obtaining from 10 ¹⁴ to 10 ¹⁵ OH sites per cm ² of surface. This hydroxylation phase can be carried out by pickling or soaking from a basic aqueous solution (pure or diluted) containing, for example, sodium hydroxide at a concentration ideally between 0.1 and 1 M. The duration of this step 1 is preferably between 5 and 30 minutes, with a pickling solution maintained at a temperature preferably between 15 and 40 ° C.

The base can, advantageously, be replaced by an acid used pure or diluted, such as H ₂ SO ₄ . The hydroxylation is optionally carried out in the presence of an oxidizing agent capable of promoting chemical functionalization by hydroxyls of the friction surface. The oxidant can be hydrogen peroxide when using a base and chromium oxide (Cr ₂ O ₃ ) when using an acid.

In the case where the friction surface consists of silica, the hydroxylation consists of hydration in which the friction surface is brought into contact with double-distilled water maintained at a temperature above 40 ° C., preferably at 60 ° C, and more preferably still on the order of 95 ° C, for one to 48 hours and, preferably, for about 12 hours.

Optionally, the friction surface is then washed by soaking or flowing a fluid of distilled water supplied at ambient temperature and drying consisting in placing the substrate to be treated under a flow of inert gas, such as nitrogen, at room temperature.

The optional vacuum degassing step 1b is carried out, preferably, at a pressure between 1 and 5 Pascals and, more preferably still, of the order of 1 Pascal, by heating the substrate to be treated to a temperature between 120 and 160 ° C and preferably equal to about 140 ° C. This phase of vacuum degassing lasts from one to five hours, preferably less than or equal to two hours, so as to obtain a completely dehydrated and degassed surface.

In accordance with step 2 of the process according to the invention, the friction surface is subjected to an impregnation with a silane-based liquid, optionally pure, but preferably in solution in an inert solvent containing 0.5 5% by weight of silane with fatty chains.

The inert solvent used is preferably an alkane, for example isopentane, hexane, ethane, isopentane being particularly preferred. The silane solution, at a concentration of between 0.5 and 5% by weight, preferably about 2% by weight, is maintained at a temperature between - 10 and - 50 ° C, preferably of the order of - 30 ° C. The impregnation takes place, preferably, under a vacuum between, preferably, 1 and 5 Pascals and, more preferably still, equal to approximately 1 Pascal, for a duration preferably between one to five hours and , more preferably still, at least equal to two hours, to allow complete elimination of the solvent.

Step 3 of the process according to the invention consists in carrying out a condensation of the impregnated silane by subjecting the treated surface to heating, preferably between 120 and 160 ° C., for a period of between 15 and 48 hours, in an enclosure at atmospheric pressure and in the presence of an inert gas, such as argon or nitrogen.

In accordance with an advantageous arrangement of the invention, this step 3 takes place at approximately 140 ° C., for 24 hours.

In accordance with an implementation variant, step 3 can take place, under an at least partial vacuum, in the same way as step 2.

The last step 4 of the process consists in washing the surface with a silane solvent, such as ether, tetrahydrofuran, by soaking or flowing at room temperature and, preferably, renewing the solvent one to four times, so as to remove the excess silane. Advantageously, this step is ended by washing with water.

To perfect the score, it is possible to implement a additional drying step, of the type of that of the step described above.

By proceeding in this way, a covalent grafting of lubrication type silane compounds is obtained on materials capable of being hydroxylated. This covers ceramic materials, semiconductor materials, materials based on silicon, carbon fibers, based on boron, platinum or others. Materials based on silicon are particularly preferred. Mention may be made, in particular, of SiC, SiO ₂ , Si ₃ N ₄ , SiOxNyHz, with:

- x = 0, 1 or 2,

- y = 0, 1, 2 or 3, - z = 0, 1, 2, 3 or 4,

- x + y + z = 4.

Thanks to the hydroxylation, the covalent bonds formed are of the type:

- A - O - If

A belonging to the substrate and which may be constituted by Si. The high bonding energy of these covalent attachments, as well as their considerable number, guarantee an extreme solidity of the fixing of the coating to the substrate. This cohesion is further improved by the formation of organic layers organized by the formation of Van der Waals bonds between the chains.

One can also consider optimizing the grafting characteristics by repeatedly repeating steps 1b, 2, 3, 4 of the process according to the invention, for example from one to six times.

In addition to the excellent cohesion between the coating and the substrate which it provides, the method according to the invention allows perfect penetration of the lubrication coating into three-dimensional structures of reduced sizes. It has been shown that grafting is efficient in porous silicon structures having pores of a few nanometers in diameter. It is thus possible to reach coefficients of friction (μ) less than or equal to 0.1. The present invention also relates to the application of the lubrication method to micromechanical parts or devices such as micro- actuators (motors / turbines), slides, gears, wheels, pivots, ball joints, racks or combs actuators or to parts or macromechanical devices, such as magnetic read heads.

The invention also relates, as a new industrial product, to a lubricated mechanical friction part, the originality of which is that at least its (or its) friction surface (s) is (are) coated (s) of a monomolecular lubrication layer, consisting of molecules of a lubrication compound, each of which comprises at least one hydrophilic end and at least one long lipophilic chain, each molecule being linked, by covalent bond, to the surface (s) friction.

Such a product having coefficients of friction less than or equal to 0.1 does not exist in the prior art.

BEST WAY TO IMPLEMENT THE INVENTION:

The advantages and variants of the invention emerge from the example which follows, of making non-lubricated SiO ₂ test pieces.

EXAMPLE

The mechanical friction parts used in this example are silicon wafers (100) covered with 70 mm of thermal silica.

- LUBRICATION METHODOLOGY FOLLOWING THE INVENTION: The silicon wafers are subjected to a chemical functionalization of hydroxylation consisting of soaking for 10 min in a bath at 70 ° C. and containing:

. 7 volumes of ultra pure H ₂ O,

. 2 volumes of H ₂ O ₂ ,. 1 volume of NH ₄ OH.

We then rinse with water, followed by dehydration and degassing with put under a vacuum of 10 ^"2 Torr.

The next step is that of impregnating the plates with (dimethylamino) octadecyl dimethylsilane liquid and in pure form, at temperature. The ampoules containing the plates are placed under vacuum (10 ² Torr) and sealed for the implementation of this step.

The plates are then subjected to condensation for 5 days at 140 ° C. allowing the covalent grafting of the lubrication compound. Finally, the lubricated plates are recovered and washed with tetrahydrofuran.

- FRICTION TESTS:

The friction experiments were carried out on a pawn-plane tribometer. The pawn is a quartz ball, cleaned with ethyl acetate, then with a torch of radius r = 4.75 mm. Before the experiment, the wafers to be tested are cleaned beforehand with tetrahydrofuran, then with ultrasound. They are then cleaned with a cotton swab soaked in ethanol in an attempt to remove the deposited dust which could distort the measurements. The force applied to the pin is 0.5 N. The tribometer used is of a conventional type (application of a vertical force W, measurement of the drag F and evaluation of the friction coefficient F / W.

- RESULTS:

The results obtained are illustrated in FIGS. 1 and 2 of the attached single plate, which represent the friction coefficient curves (μ) as a function of the number (N) of measurement cycles with W = 0.5 N and a drag distance D = 9.5 mm.

Fig. 1 corresponds to the non-lubricated silicon wafers and FIG. 2 to lubricated wafers according to the invention.

These tests show the remarkable decrease in the coefficient of friction (from 0.6 to 0.1). We note that this reduction continues for a long time (N = 60).

Claims

CLAIMS:

1 - Method of lubrication of a mechanical friction part, consisting essentially of chemically functionalizing at least the (or) surface (s) of friction of the part in question, then subjecting the latter to a fixing treatment of a silane, the molecule of which comprises at least one hydrophilic end and at least one long lipophilic chain, characterized in that it comprises the following essential steps:

3) condensation of the silane by heating in a neutral atmosphere or under an at least partial vacuum, so that said silane binds, in an essentially covalent manner, to the functionalized sites of the friction surface (s), to form a substantially monomolecular layer,

4) washing the surface with a solvent, so as to remove the excess silane. 2 - Process according to claim 1, characterized in that the chemical functionalization is carried out using a reactive hydroxyl group and / or a group comprising at least one hydroxyl group, such as a carboxyl.

3 - Process according to claim 1 and claim 2, characterized in that step 1 is followed by a step la washing and drying of the hydroxylated surface and / or by a step lb of degassing of the hydroxylated surface, optionally washed and dried, by heating under vacuum at least partial.

4 - Process according to any one of claims 1 to 3, characterized in that the hydroxylation treatment of step 1 consists in carrying out a soaking in a base or an acid, optionally in the presence of an oxidant.

5 - Method according to claim 1 and claim 3, characterized in that steps 1b, 2, 3 and 4 are carried out at least twice.

6 - Process according to any one of claims 1 to 5, characterized in that the long chain silane comprises from 5 to 60, preferably from 5 to 30 and, more preferably still, from 10 to 30 carbon atoms. 7 - Method according to any one of claims 1 to 6, characterized in that the mechanical friction part is made from a material based on silicon, carbon, boron or platinum.

8 - Method according to any one of claims 1 to 7, characterized in that the silane has the following formula: R ₂

I

R, - Si - R ₄

IR ₃ in which:

- R, is an amino group, alkoxylated or halogen, preferably chosen from the following radicals:

. (CH ₃ ) ₂ - N-, NH ₂ ,. CH ₃ - O -, CH ₃ - CH ₂ - O,. Cl—,

- R ₂ and R ₃ may be identical or different, optionally equal to Ε ^ and constituted by an alkyl group of the type C _m H _{2m + 1} , m being between 1 and 30, alkoxylated or halogenated amine, preferably chosen from following radicals:

• CH ₃ -, C ₂ , H _S ,. CH ₃ - O—, CH ₃ - CH ₂ - O—,

. Cl-,

• NH ₂ -,

- R ₄ = - (CH ₂ ) _n - R _s ,

R _s being either an alkyl group, such as methyl, or a halogen group, such as Br or CI, or a sulphite, carboxylic, hydrocyanic or phosphorus group, such as phosphonates, - and n is between 1 and 30.

9 - Application of the method according to any one of claims 1 to 8 to the lubrication of parts or devices, such as micro-actuators (motors / turbines), slides, gears, wheels, pivots, ball joints , racks or actuator combs.

10 - Application of the method according to any one of claims 1 to 8 to the lubrication of parts or macromechanical devices, such as magnetic read heads.

11 - Parts or micro or macro mechanical devices lubricated by implementing the method according to any one of claims 1 to 8.