US3069758A

US3069758A - Friction composition and method of making

Info

Publication number: US3069758A
Application number: US165835A
Authority: US
Inventors: Wulff John
Original assignee: SK Wellman Corp
Current assignee: SK Wellman Corp
Priority date: 1962-01-12
Filing date: 1962-01-12
Publication date: 1962-12-25
Anticipated expiration: 1979-12-25

Description

3,069,758 FRIcTIoN COMPOSITION AND METHOD oF MAKING Filed Jan. 12, 1962 J. WULFF Dec. 25, 1962 3 Sheets-Sheet l JOHN WULFF BY/MWJMMMMW ATTORNEY Dec. 25, 1962 J. wuLFF 3,069,758

FRICTION COMPOSITION AND METHOD OF MAKING Filed Jan. l2, 1962 3 Sheets-Sheet 2 v oood ooof ooo@ ooQ 08.9 oood. ooof 000.9

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lsd H LsNa J. Nval. IN1/nvm, l S BH'LLdnH BSHBAS JOHN WUI-FF MMM ATTORNEY Dec. 25, 1962 J. wuLFF FRICTION COMPOSITION AND METHOD OF MAKING Filed Jan. 12, 1962 3 Sheets-Sheet 3 MGE 3,959,758 Patented Dec. 25, 1962 tice 3,069,758 FRECTIQN CMPSlTlN AND METHD GF MAKENG Sohn Wulff, Cambridge, Mass., assigner to The S. K. Weliman Company, Bedford, Ohio, a corporation of Ghia Filed dan. 12, 1962, Ser. No. 165,835 11 Claims. (Cl. 29-182.3)

The invention relates to friction articles made from an extremely low density iron powder.

Iron powder has been knownin the past as useful for the ultimate production of sintered predominately metallic friction articles for brakes, clutches, automatic transmissions and the like.

Heretofore, however, iron powders commercially available, at least for friction products, have either exceeded 1.0 gram per cc. apparent density or have been low in transverse rupture strength when compacted, and as available for any purpose (i.e., regardless of expense of production), have exceeded 0.9 gram per cc. apparent density.

The prior iron powders have not been capable of containing a high percent of non-metallic additive as required in many friction articles (e.g., an additive of graphite for low wear, or of grit for high friction, or of M082 for lubrication together with coarse graphite for low rate of friction decay), and the prior art friction articles could not in many cases meet such criteria and still have been pressed at requisite pressure and sintered to form a structural part strong enough for the various applications, nor, as a further example, could they function as friction articles Without backing, and friction facings of sintered predominantly metallic powders have in the past had to be bonded to solid metal backings, the prior art facings being too brittle to stand the force of rivets or otherwise to be regarded as structural parts in and of themselves. rihis is largely due to poor characteristics of the base powders and articles made from them as regards such desiderata as high compression ratio, low molding pressure, good green and sintered strength, high ultimate hardness, proper final density, and resultant protection of the article against breakage and chipping both during manufacture and during use.

Friction materials used for lining or facings in brakes, clutches, automotive transmissions, and the like, must be selected and compounded with many factors in mind if customer acceptance and satisfactory operation are to be assured. These factors include, among others, magnitude of the coeilcient of friction, cost of the materials and of their compounding and assembly, wear of the friction material, wear of the surface which the friction material engages, freedom from grabbing and other erratic operations, noise or quietness of the material in operation, fading or other change in the magnitude of the coeflicient of friction with changes in applied pressure, or in temperature, humidity, or relative speed of the engaging parts or due to the presence of oil, grease or other foreign matter. Of particular importance is the cost of fabrication and this involves consideration of whether or not at solid metal backing material (additional to basic curved or at members as brake shoes and clutch plates) must be provided.

lt is an object of the present invention to provide a novel friction material which satisfies these long-known requirements.

Another object is to provide a powder useable in friction articles and permitting therein higher amounts of filler than heretofore possible without losing requisite strength and providing other advantages as hereafter described.

Other objects will becorne apparent and the invention may be better understood from consideration of the following description taken in connection with the accompanying drawings, in which:

FIG. l is a tabular representation of comparative physical properties of prepared, purchased, and offered iron powders and characteristics of articles made from them, the powders indicated at A and C being according to prior art, and that at B, D, E and F being prepared according to the present invention;

FlG. 2 is a bar graph comparing strengths of articles made of various mixes and with powders of columns A and E of FIG. 1; and

FIG. 3 is a block graph comparison of wear and strength of various mixes.

l prefer to start with mill scale (Fe304), although magnetite or hematite (Fe203) concentrates could be used. The oxide is mixed with ne charcoal in proportions such that the oxide will be reduced and the carbonaceous material will be consumed in the process.

In accordance with the present invention, there is added to the reduction mix a small percentage, eg., 0.25% (or less if in solution to 1.5% (of the total of iron oxide and charcoal by weight) of a finely divided carbonate (or mixture of carbonates) of the class of Potassium carbonate (K2CO3) Sodium carbonate (NazCOa) Barium carbonate (BaCO3) Calcium carbonate (CaCO3) Good results have been obtained using for the charge S11/2% of dry scalped mill scale 18% ground charcoal (80% through 325 mesh Tyler Std.) 1/2 of line potassium carbonate powder Permissible ranges for the above depend on time and temperature of reduction, particle size of charcoal and of other ingredients, total thickness and volume of charge, and on whether the gaseous products of reduction are CO rich or CO2 rich which l have found in turn depends on the presence and amount of energizer and temperature.

I have found permissive ranges as follows:

Percent Iron oxide -83. Charcoal 161/2-19. Energizer salt 11-11/2 (with the upper limit dictated mainly by economics).

For iron powder prepared with an energizer, and for friction articles thereafter prepared therefrom, eg., by compounding with graphite with or without other friction agent iller and thereafter compressing and sintering with or without applying to a backing member, mold growth, green strength, sintered strength, work-ability and friction and chatter and Iwear and other properties were all found to be excellent and it was also found that formulation as described produced a curly-whisker-like powder which, compared with any heretofore known, is softer and more compressible, thus giving lower costs for fabrication, longer die life, and permitting longer wearing friction material capable of having a higher and more uniform coeicient of friction and having other advantages apparent to those skilled in the art.

To have the best iron powder for such friction applications it is preferable that the powder have a low cornbined carbon content but any residual carbon not exceeding 1% seems quite tolerable and any oxide content below 5.0% is quite permissible so far as friction and wear properties of the ultimate sintered material are concerned.

If the strength is to be high, this oxygen and carbon content (of powder as reduced) must be low, but still the softer curly-whisker-like iron particles of the present invention permit the compounder to add more graphite, or other filler, for friction, or Wear, or 'non-chatter, or other properties and still have the green and sintered strength ofthe article remain adequate.

Referring now to FIG. 1 which shows comparative physical properties ofV prepared, purchased and offered iron powders, the properties listed in columns A and C are those of iron powders made without energizer additive and of compacts made from such powders more or less according to prior art, those under columns B and D through F are for powders and compacts made according to the present invention, while those under columns G-I are for powders and compacts from the best outside source known to applicant.

Important figures in the table on FIG. l are those for apparent density and those for transverse rupture strength. Apparent density, as referred to in this application, was measured with a Scott owmeter although it could just as Well have been measured with a Hall owmeter, as is known to the art (see A.S.T.M. Standards on Metal Powders B329-58T and 3212-48, or Metal Powders Assn. MPA Standard 4-45).

Apparent density depends not only on particle size and size distribution but also on the shape and internal porosity of the particles. The powder desired is of such structure as to provide low apparent density with relatively coarse particle size, for example with at least 94% above 325 mesh as in the B and E examples of FIG. 3. Finer powders are usually obtained only by grinding, or selective screening. Selective screening is seldom economically feasible without returning coarse rejects for grinding but the more grinding the more the cold work and the harder the powder. Hard powders do not compress easily and will not permit the incorporation of friction agents as well as will soft powders. Thus a tine curly-whisker-like particle of -high plasticity is superior4 for the fabrication of friction material as seen in FIG. 1 where, for both of the powders B and E the compression ratio is over 4.7 (when molded at 12.5 t.s.i.) and the green transverse rupture strength is more than 3500 pounds per square inch which is greater than that of any other powder presently obtainable.

In particular, the softness or high plasticity of applicants powder made with energizer permits the molding of friction material Whose transverse rupture strength is high even at the very moderate (and therefore preferred) molding pressure of 121/2 tons per sq. in.

The ease of pressing powder made according to this application lhas been found useful inthe manufacture of friction facing for large truck brake blocks where the molding equipment is large and expensive and mold depths are such that only low molding pressures are feasible in order to insure econo-mic life of dies, die walls and the press itself.

The present patent application is a continuation-in-part of my'co-pending patent application Serial No. 836,024, led Aug. 25, 1959, having the same title and assigned to the Same assignee as the present application and referring to then co-pending patent application Serial No. 573,708, tiled March 26, 1956, in the name of Samuel K. Wellman, now U.S. Patent No. 2,927,015, assigned to the assignee of the present invention.

Considering columns A, B and C of FIG. 1, the Y powders were all produced in a pit. n

With the A powder, made without an energizer, activator or catalyst, there are still some advantages over the prior art, although recent irons from outside sources' noted in green (molded) transverse rupture strength (column B).

Considering the C, D, E and F columns, the powders were processed in a belt furnace as found advantageous for preparing iron powders to form the main base of mixes used to make friction articles for some applications, where extremely high strength articles are required. With the C powder made without an energizer there is shown no particular advantage over the A powder, except that the strength is minutely better and the reducing time is decreased. When an energizer is used as shown by columns D, E and F the apparent density is lowered and the green transverse rupture strength is increased. The'increase of green transverse rupture strength is reflected in a greatly increased sintered strength as shown by the horizontal tabulation under Sintered Properties.

Columns G, H and I show three different grades of powder believed, to be the best available from outside sources as regards low apparent density and high strength of pressed compacts. The G powder compacts have a strength comparable to the A powder compacts, and thus represent no improvement in strength over the prior art. The iron in column H appears to be the only one vthat is commercially available which offers even a slight strength improvement over the A or C powders but this H powder is of high apparent density and the particle size distribution shows that the powder is too coarse for compounding and pressing friction material as is recognized by those skilled in the art, while the powder in column l shows no improvement in pressed strength over known prior art powder (column A, for example) even though its density is low.

Of great importance is the fact that the new iron powder, because it is so much softer, greatly facilitates Vthe formation of a friction article containing substantial amounts of friction agents. As already mentioned, wherever substantial amounts of a filler such as graphite are required (as for pressed and sintered metallic friction facings) the new processes and powder of the invention are advantageous because they allow such large additions while permitting the use of processing equipment of Yminimum cost and the manufacture of finished articles of requisite strength.

As seen in FIG. 2, graphical results are shown, for exarrple for a mix L or iron, 15% graphite, 10% silicon carbide made with artificial graphite and with the iron being that of FIG. 1 column A for the first -bar graph and that of column E for the second bar graph, all as indicated on the legends ofVFIG. 2. In each case the processes were similar I(rst reducing the iron, then mixingwith the other ingredients, and molding at 15 t.s.i'., and then -sintering at 1800 Fu'while under a pressure of 300 p.S.i. and in a reducing atmosphere) while the mixes differed as indicated, for example mix P being the same as L except that natural graphite was Vsubstituted for the artificial graphite, mix` O being the same as mix L except that the silicon carbidewas coarser'in particle size, etc. In each case the transverse rupture strength was substantially increased when the Vnovel iron of column E was substituted for the A iron. column E iron gave a rbetter matrix strength as is im-v portant in preventing the friction material from deteriorating and crumbling in use or failing structurally particularly at the extreme temperatures of lbrake operation. The SiC perhaps helps prevent such deterioration but it is so operative only if held in place and thus progressively exposedduring usage of thejfriction article. 'I'he superior matrix strengthV of mixes predominately iron made according to the present invention also allows other` ingredients to [be added in amount greater than heretofore. Thus, taking as lan example theA lowest strength mix N, the additives (15% graphite for reducing chatter and providing smooth brake action, 10% SiC for low wearat extreme operating temperatures, and 10% molybdenum disulfide forjminimizing coefficient of friction decay over This is so because, in each case, theV the life of the material) total 35% Iand still with iron rrade according to the invention (second bar graph) the resultant material is still operative, and saleable. Whlle the transverse rupture strength of the prior art iron mix of mix M may appear just as great, with that mix the coetiicient of friction decay was of course much greater, and thus it can tbe seen that with improvements according to the present invention powders can more readily be tailored to specific applications consistent with various vehicle weights, speeds, temperature, and other factors involved.

FIG. 3 is a block graph showing wear and strength data for friction compositions according to prior art and according to the invention with the iron (with or without energizer present) mixed with varying percentage of graphite alone and pressed and sintered and used in a brake block for a highway truck.

In FIG. 3, mix S employed the A iron (of FIG. l) but its strength was almost too low, eld tests having indicated that a range of over 4000 p.s.i. transverse rupture strength is necessary to prevent fracture in severe service and still the wear of mix S (of the friction material) as obtained from dynarrometer tests was relatively high, though not as high as that of prior art asbestos blocks. In other words, with prior art iron powder, strength was at an irreducible minimum. It is known however that going to a higher percentage of graphite in an irongraphite mix will reduce such wear but only at sacrice of strength, but with the S mix the strength could not be less.

Mixes T, U and V employed the B iron with varying percentagesof graphite, for T, 22.5% for U and 25% for V. `From the results it is seen that while the iron of the invention greatly increased matrix strength (for 20% graphite mix) it also greatly increased the wear (more than doubled it for same percent of graphite) but this could be more than overcome by increasing the graphite until at graphite the strength was still quite adequate and the wear was at the unusually low value of 8 mils (based on a dynamometer procedure including one hundred and fifty 60-rnile-an-hour stops, which procedure of course was the same in each case as were other factors inciuding the type of rrating surface, actually of cast iron).

Mix W was made from the second strongest outside source iron G (since H wouldnt hold together even during manufacture possibly because of the coarseness of the iron). For the W mix, 80% of G iron was mixed with 20% graphite. With this mix the wear was fairly satisfactory but the article was quite unuseable because the transverse rupture strength was much too low, as could be expected due to the low green strength of this iron powder as noted in FIG. l.

Thus it was found that as the strength increased the wear increased in general, but at the same molding pressure the use of applicants iron made with an energizer permitted the use of more filler. The filler is graphite in a the case of FIG. 3, and in FIG. 2 is graphite and SiC, and in some cases M082, also. In other cases the filler might be another metal such as lead and/or other friction tailoring agents.

Thus iron powder, and/or permissible friction articles, according to the invention, are superior. Time and temperature of reduction may be of i"portance, but I have found that mill scale can be reduced (at a given temperature) in considerably less time using an energizer according to the invention, or, alternatively, time may be kept the same and temperature reduced to greatly increase equipment life when an energizer is employed.

In accordance with one aspect of the invention, energizer is not removed during or after making the metal powder. In fact it is advantageous to leave it in, or if not present previously to add it, for inclusion in the friction article, where its presence presents numerous advantages. For one thing, presence of the energizer in the friction material mix results in substantially total reduction of residual oxides during sintering of pressed compacts. When such energizer is not present the graphite and entrapped air or oxide do not serve as an adequate source of the reducing agent CO, but with the energizer present (say, 0.5% in mill scale, charcoal and energizer mix `becorres 0.75% in reduced iron which, in turn, becomes 0.56%

in a 65% iron friction mix) such residual oxides are gone and this insures ybetter ibonds at metal particle contacts, permitting better bonding in the matrix and also between friction material and any solid steel back-up material used 'because here again the residual oxides will have been reduced (on steel backing as well as in the iron powder), and a preferable metal to metal contact is achieved for superior bonding.

Tests have been conducted to determine bond strength both within sintered predominately metallic friction material itself and strength of bond between such material and solid backing material to which it was bonded during sintering. With tests made under identical conditions and with samples prepared in exactly the same manner, except with one set made without purposeful addition of energizer per FIG. l, column A, and the other made with 0.5% addition of energizer to the original mill scale and charcoal according to FIG. l, column B, it was found that twice as great a force was required to shear sintered friction composition made with E iron as compared with the force which would shear sintered friction composition made with the A iron, a similar ratio of shearing forces obtaining w-hether considering strength within the sintered material itself or considering strength of sinter-bond of such material to a solid steel backing.

I have found, further, that an energizer, if none were available previously within or added to the raw materials, can be introduced (e.g. 0.4 to 2.4%) for the first time when mixing iron powder with one or more friction tailoring agents including graphite. Even this will somewhat increase the strength and provide better bonds. But this is not the preferred practice for, of course, it will not make the iron soft nor provide it with curly whiskers and thus will not provide anywhere near all the advantages of mixing the energizer with the ore to be reduced in the first place, and then leaving it in after reduction, and duringsubsequent sintering.

I have found that most commercial charcoal contains in its ash a certain amount of akali and alkaline earth compounds. But no more than very random and unusual lots of any commercial grade of charcoal has the energizer content necessary for practice of the present invention, and for the most part the charcoal must be fortified by purposeful addition (e.g., of 1A. wt. percent of the combined weight of the ironoxidecharcoal mix to have about 0.38% in reduced iron and a final 0.2 to 0.28 wt. percent of energizer in the later iron-graphite, or other, friction mix).

The 1.5% upper limit of energizer (in reduction mix) is dictated by economics as previously mentioned, particularly since the use of a higher percentage increases the difficulty of processing and use of products made from the powder, and this 1.5% becomes an upper limit of about 2.4% in reduced iron or 1.5% in a final irongraphite, or other, friction mix. It is, of course, dicult to assure accuracy to more than the nearest one-tenth percent by weight, and the appended claims have been written with this in mind.

By virtue of the possibly Whisker like powder (low apparent density with relatively coarse sieve size) according to one aspect of the invention, a stronger compact can be made, and/0r (depending on when energizer is added) by virtue of the fact that energizer is present, according to another aspect of the invention, better sintering of matrix is provided to hold friction tailoring agents and provide better bond to any backing.

While I have described particular embodiments, various modifications may obviously be made without departing 7 from the true spirit and scope of my invention which I intend to have dened only by appended claims taken with all reasonable equivalents. Y

claim:

l. An iron powder having an apparent density of not more than 1.0 gram per cubic centimeter, and a particle size such that at least 94 percent remains on a 325 mesh screen, and having a plasticity and particle shape such that when pressed at 12.5 t.s.i. the compact green transverse rupture strength is greater than 3000 p.s.i.

2. An iron powder having an apparent density of not more than 0.7 gram per cubic centimeter and a particle size such that at least 95% remains on a 325 mesh screen, and with a green transverse rupture strength greater Ythan 3000 p.s.i. when pressed at 12.5 t.s.i. Y

3. An iron powder containing at least 0.4% while not more than 2.4% residual energizer said energizer being of the group consisting of potassium carbonate, sodium carbonate, barium carbonate and calcium carbonate, and said iron powder having an apparent density of not more than 0.7 gram per cubic centimeter and a particle shape such that at least 95 percent remains on a 325 mesh screen and with a green transverse rupture strength greater than 3000 p.s.i. when pressed at 12.5 t.s.i.

4. A sintered friction composition product being an article formed of from 65 to 80 percent an iron powder, the remainder being friction tailoring agents and 0.2 to 1.5% energizer of the group consisting of potassium carbonate, sodium carbonate, barium carbonate and calcium carbonate, said iron powder having an apparent density of not more than 0.7 gram per cubic centimeter with a particle size such that at ieast 95% remains on a 325 mesh screen, whereby said article is characterized by great structural rigidity and desirable friction and'wear properties.

5. A friction composition product which is a compacted and sintered mix of graphite powder and iron powder and containing at least 0.2 wt. percent While not more than 1.5 Wt. percent energizer of the group consisting of potassium carbonate, sodium carbonate, barium carboi.: nate and calcium carbonate, with the iron powder having an apparent'density of not more than 1 gram per cc.

6. A friction composition product comprising a pressed and sintered mixture of by Weight more than iron, and more than 20% friction agent ller, and containing 0.2 to 1.5% of energizer of the group consisting of potassium, sodium, barium and calcium carbonates.

7. Method of producing a friction composition product including heating a charge of mill scale and charcoal and at least 1t% metal carbonate energizer, maintaining the heating temperature for a period of time sufficient to effect reduction of the mill scale, removing the charge, mixing the resultant iron and energizer wit-h at least one friction agent and pressing and sintering said mix to provide a product characterized by maximum strength for minimum percent iron present.

8. A friction member comprising a reinforcing metal backing vmember and sintered and bonded thereto a compacted friction facing, said facing comprising a maior proportion of iron, a minor proportion of a non-metallic friction affecting material, and 0.2% to 1.5% by weight of the facing a metal carbonate energizer.

9 A friction ,member as in claim 8 further characterized by the reinforcing metal backing member being of solid steel.

10. A friction member as in claim 9 further characterized by the major proportion of iron being more than by weight of the facing.

11. A friction member as in claim l0 further characterized by the minor'proportion of non-metallic friction affecting material including graphite.

References Cited in the le of this patent UNITED STATES PATENTS 2,863,211 Wellman Dec. 9, 1958 2,974,039 Deventer et al. Mar. 7, 1961 2,978,323 Schmeckenbecher Apr. 4, 1961 FOREIGN PATENTS 731,504 Great Britain June 8, 1955

Claims

8. A FRICTION MEMBER COMPRISING A REINFORCING METAL BACKING MEMBER AND SINTERED AND BONDED THERETO A COMPACTED FRICTION FACING, SAID FACING COMPRISING A MAJOR PROPORTION OF IRON, A MINOR PROPORTION OF A NON-METALLIC FRICTION AFFECTING MATERIAL, AND 0.2% TO 1.5% BY WEIGHT OF THE FACING A METAL CARBONATE ENERGIZER.