US2983563A - Piston - Google Patents

Description

- quently with a chrome platingiz United States Patent PISTON John O. McLean and Roger H. Hendrick, Henrico County, Va., assignors to Reynolds Metals Company, Richmond, Va,, a corporation of Delaware No Drawing. Filed July 7, 1960, Ser. No. 41,231

Claims. (Cl. 309-4) This invention relates to coated aluminum pistons for internal combustion engines, particularly engines having aluminum alloy cylinder walls.

Aluminum alloy pistons have long been successfully used-in internal combustion engines, including both light and heavy two-cycle and four-cycle gasoline engines, and also in diesel engines. Pistons operate at relatively high temperatures, and reciprocate at high speeds, so that aluminum alloys are particularly useful in this application, because of their high heat conductivity and low weight relative to their strength and volume. Another established use of aluminum alloys is in the heads of internal combustion engines, where the low weight of aluminum alloys is useful, and high heat conductivity is also important. While these applications of aluminum alloys have been helpful in reducing engine weight and in increasing engine efliciency, the application of aluminum alloys to engine blocks has been retarded by difficulties which have been encountered in connection with the cylinder Walls. Aluminum alloy pistons present diflicult problems of scoring and scuffing when used in aluminum alloy cylinder Walls. This problem has been overcome in many commercially accepted engines by inserting cast iron or like cylinder wall liners in an aluminum block. However, this solution of the problem is complicated and expensive and less eflicient in operation, and for many years an extensive search has continued for a practical and economical means for producing an engine, particularly of the type used in conventional automobiles, having aluminum pistons operating successfully over a standard service life in a cast aluminum alloy block having cylinder walls of the same orof a similar alloy, unlined and uncoated.

This object has heretofore been achieved to a large extent, but the one test which has not heretofore been met is repeated cold-starting at 20 degrees Fahrenheit, which is a requirement in some. areas of theUm'ted 'States and other countries. The insertion of special cylinder liners is undesirable for reasons of cost as pointed-out above, and it is not practicable to plate or spray the cylinder walls because of the technical problems of working inside suchsmall spaces orjbecause of the cost ofthe More particularly, we have discovered that a nickel steel alloy can be coated on aluminum alloy pistons to provide a hard, tough coating which has a low coefiicient of friction against aluminum alloy cylinder walls, and which can be applied in such a manner as to give a sufficiently porous surface to retain lubricant for good longwearmg operation against aluminum alloy cylinder walls, even when tested under severe cold-starting conditions. A nickel steel alloy is readily coated on aluminum alloy pistons by flame spraying, after the piston surface to be coated has first been roughened, as by shot blasting, or the like. The roughened surface has an extremely tenacious adherence to molybdenum which must be applied first to the aluminum surface to obtain adherence for the ferrous coating to the aluminum. The use of molybdenum as a pre-coating on aluminum is well known in the metal coating industry.

The portion of the piston which requires coating is the portion below the rings which is in sliding contactwith the cylinder wall, generally referred to as the skirt of the p1ston. For purposes of thisinvention, the coating when it isref erred to will meana ferrous coating containing essentially iron 'plus alloying elements. Some pistons have split skirts, and others have skirts extending solidly all of the way around the piston. Some pistons have special steel struts embedded in them, and others do not. It is conventional to cam-grind pistons to an oval shape, which expands to a generally circular shape to fit the circular cylinder walls after the engine has reached full operating temperature. The ferrous coating when used in conjunction with a thin coating of molybdenumis applicable to any of these piston designs, and, although particularly useful in connection with the close tolerance engines of the kind used in automobiles, is also useful for coating the skirts of aluminum alloy pistons of other internal combustion engines having aluminum alloy cylinder walls. Such cylinder Walls are preferably of aluminum alloy in which the principal alloying element is silicon in high proportions (e.g., up to 25% or more), for good wearing properties, but the invention is also applicable in connection with: cylinder walls of aluminum alloys generally. e i a The particular aluminum alloy of which the piston is made, including the skirt, is notcritical for purposes of the invention, Piston alloys conventionally have silicon as the principal minor alloying element, in relatively plating itself. Various coatings for the pistonshave been i tried, including chrome plating, without success because of poor wear characteristics or adhesion difiiculties, Furmade of specialmaterials, such as cast iron or steel," fre- Since thecoatings tried for the; pistons were not sue cessful, there .was no. assurance whether the problem stemmed solely from the coatingstried on the pistons, or

whether it might be seriously influenced by the operation of the rings.

. The present inventionfovercomes these difiiculties by providing acoating ,on an aluminum alloy p ston WlllCl'l has proved successful in repeated cold-starts and general j tween about 86 to. about Minor amounts of other.

operating testsjwhen used in a cast aluminum alloy block imetalsjmay be present in .the alloy for convenieneetrof,

high proportions, such as about 8% or. more, but the amount orpresence of silicon is not critical with respect to operation of the coating of the invention. t

t t For the purposes of the llVfillllOll, [molybdenum is essentially pure containing only minor amounts ofo'ther metallic elements as impurities. The thickness of the molybdenum coating may vary from about .001 to .005 inch. The nickel steel wire used to ,obtain the fiame sprayed outer coating may contain from about 2 to. 6%

nickel, from about 1 to.3% manganese, from, about 0.75

to 2% chromium, and Lfro'mabotit l to 3% of molybdenumi Such a nickel steel Wire may be purchased cornmerci all y under the trade name Spraysteel; LSfrpm the Metallizing Company of America, Long Island, New York; Although nickel may be present in amounts up to about 6%,;the preferred range is between 2' and 4%. Similarly, the preferred range for chro-fnium' is "between 75 and 1.5%, and the preferred range for'iron is bemanufacture, but these other metals are not significant for purposes of theinvention, in connection with coating aluminum alloy piston skirts-for improved performance in aluminum alloy cylinders. For example, siliconm'ay be present in amounts up to about .08%.

'The use of nickel 'as an element in the flame sprayed coating 'is considered to be important because it gives to the coating properties which are similar to cast iron. When nickel is used in the range contemplated by this invention, it serves to increase strength and hardness without causing a corresponding reduction in ductility. The flame sprayed nickel steel coating as a whole possesses good bonding and work-hardening properties as well as an improved wear resistance. For example, tests actually' performed on the coatings of this invention showed a Ro'ckwell C hardness of from 20 to 25 and a tensile strength ofabout 33,500 p.s.i.-

The low coefficient of friction of a nickel steel coated aluminnmfalloy piston skirt operating in an aluminum alloy cylinder helps to prevent scoring and scuffing, butanother important feature is the semi-porous nature of the coating when it isflame sprayed onto the skirt. The

degree of porosity depends principally upon the flame" spraying conditions, which can be varied to obtain the desired porosity, as will be understood by-those familiar with conventional flame spraying techniques. The advantageof semi-porosity of the coating is the ability to retain "lubricating oil after the motor has been run up to operating. temperature and then stopped for several hours, when thehot oil has a tendency to drain away from the piston and cylinder walls. The retained oil helps to lubricate the piston and cylinder walls during the next start, before normal circulation of oil is effective. The desired semi-porosity of the nickel steel coating of the invention is not lost during operation of the engine, because the nickel steel is hard and tough enough to retain its shape and hold the pores open for oil retention. Consequently, when the motor is subjected to severe cold-start testing, the combination of lubricant retained on the coating, and the naturally low'coefiicient of friction of the coating, successfully prevent scoring and scuffing of the cylinder wall, and any corresponding injury to the piston skirt.

In order to demonstrate the invention, theengine of a Renault 4 CV automobile, 1956 model, was torn down, and several of the cast iron cylinder sleeves were replaced with aluminum alloy cylinder sleeves (about 15% to 20% silicon). The aluminum pistons Were duplicated with aluminum pistons of the same aluminum alloy (about 8.5% to 10.5% silicon) .and identical construction. .The test pistons were provided with chrome-plated steel piston rings of the same size and shape as the, original rings. All critical dimensions were duplicated as exactly as possible, in each of the replacement parts before they were installed. I a I l :One .set of'pistons' was prepared in accordance with the invention, by turningthem to'v approximately the desired 'finalouter diameter (a little over two inches), and then .grit blasting theni (using No. 30 angular'steel grit supplied by National'carborundum Company) to roughen the outer surface of the flsolid-type skirt of each of the pistons in this set. Each piston was then placed about 3 inches away froma standard fiame'spra-yer which was supplied withiair at 70 p.s.i. and a stream of burning oxygen and acetylene gas,iwhich converted a feed wire qffmolybdenum into fine molten .droplets. These droplets were sprayed against the roughened surface of the pistoniskirt uhtilit hada substantially uniform coating a ar .005,thick all around thepiston. ,Afterthe pistons were given an initial coating of molybdenum, they were Y'theni given aFSecohd coating of Spraysteel LS contain- "ing, on a weig'ht basis, about 3% nickel, 3% manganese,

l% chromium, "2% .mol ybdenum',cand the balance sub 'stantiaIly aIlf-imn applied by the standard flame spraying -technique:. ascdescribed previously; .[In this instance; the

'z-eornhinedimetalliclcoating was about-030 inch thick.

- start test.

coating was then cam-ground down to the final standard dimensions of the piston, and the pistons thus prepared were installed in the aluminum alloy sleeves, and the rebuilt engine was installed in the automobile, which was driven 500 miles at 30 mph. to break in the pistons before the tests began. p

The broken-in rebuilt engine was supplied with the standard amount of oil l030 weight motor oil, Shell Oil Companys X-lOO Multi-Vis). A special oil pan was installed, with means for pumping refrigerant through it, and refrigerant was pumped through the oil pan and engine block until the whole engine had reached a temperature belo'w 0 F. This temperature was maintained for a minimum of three hours at the beginning of each test cycle, and then the engine was started and idled for one minute. It was then driven on the road for ten minutes at 25 mph, and the throttle was then opened wide for about 15 minutes, which caused the automobile to run at about 65 to 70 mph, and the oil temperature to reach about F. The automobile was then stopped and allowed to stand idle at normal temperature for about 3 /2 to 16 hours, which completed the first cycle of the cold- This test cycle was'repeated numerous times, and .then the engine was torn down for inspection of parts. The pistons having the nickel steel coating, and thesurrounding aluminum alloy cylinder walls were found to be in excellent condition, with no evidence of scuffing,

' which refers to wide and deep destruction of the surface finish of the cylinder wall, and no significant scoring, which refers to narrow, shallow lines indented into the surface finish of the cylinder wall.

, For purposes of comparison Babbitt metal was similarly sprayed on the skirts of like pistons, and these pistons were similarly installed and run in the engine. found that the piston skirts became scuffed, and the aluminum alloy cylinder lining became scored and scuffed, after three cycles of cold starts. A set of uncoated pistons caused excessive scufiing and scoring even before the first cold-start cycle was reached. Coatings of a silicone resin on some of the pistons, of a polytetrafluorethylene resin (Teflon of Du Pont de Nemours) on other pistons, and a coating of molybdenum disulfide on still other pistons, applied by other methods than flame spraying, produced excessive scuffing and scoring after three cold-start cycles, in each case. Sets of aluminum alloy pistons plated with chrome were tried and frequently found to scuff excessively during the cold-start tests.

The oil retaining capacity of-the sprayed nickel steel coating of the invention is illustrated by preparing one 'gro'upof four test specimens of uncoatcd aluminum alloy havingpolished surfaces,and a second group of four specimens of the same alloyroughtened and sprayed first with a molybdenum underlayer and then with the nickel steel coating as described previously, which are polished to the same extent as the coated skirts used group f specimens (regardlessof which aluminum alloy is used) shows -an ayerageloilretention capacity .offil j milligramsper square inch,.asagainst an' average of 35 milligrams per square inch for the specimens coated in accordance with the invention, based on figures obtained p by weighing the testsspecimens dry beforeimmcrsion in .theoil; and weighing'them after being immersed and hungas described' Y i 'It' was also fo'ind that thebspecirnemicoated in ac:- cordance with-the invention, after being washed three times with hexane, retained milligrams square inch;or-about .'7;5% ,of' the .original. oil retentijon'J The j samples-sprayed .with Babbitt metal, on, the other hand,

it was This illustrates the permanent oil retention properties of aluminum alloy piston skirts having a flame sprayed intermediate coating of molybdenum and a flame sprayed exterior coating of a nickel steel.

While present preferred embodiments of the invention, and methods of practicing the same, have been illustrated and described, it will be understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is: g

1. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, an intermediate coating of molybdenum adhering to said skirt, and an exterior coating of a nickel steel adhering to said molybdenum coating.

2. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, an intermediate coating of molybdenum adhering to said skirt, and an exterior coating of a nickel steel adhering to said molybdenum coating, said intermediate and exterior coatings being semi-porous, so that they are capable of absorbing a substantial quantity of oil.

3. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, an intermediate coating of molybdenum adhering to said skirt, an exterior coating of a nickel steel adhering to said molybdenum coating, said intermediate and exterior coatings being semi-porous, so that they are capable of absorbing a substantial quantity of oil, and said molybdenum coating adhering directly to the aluminum alloy of the skirt, with a roughened interface therebetween.

4. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, said aluminum alloy of said skirt having silicon as its principal alloying element, an intermediate coating of molybdenum adhering to said skirt, and an exterior coating of a nickel steel adhering to said molybdenum coating.

5. A piston for an internal. combustion engine, said piston having an aluminum alloy skirt, an intermediate coating of molybdenum adhering to said skirt, and an exterior coating of a nickel steel adhering to said molybdenum coating, said nickel steel coating consisting of from about 2% to 6% nickel, from about 1% to 3% manganese, from about 0.75% to 2% chromium, and from about 1% to 3% molybdenum.

6. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, an intermediate coating of molybdenum adhering to said skirt, and an exterior coating of a nickel steel adhering to said molybdenum coating, said nickel being present in said nickel steel coating in an amount not exceeding 4%.

7. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, an intermediate coating of molybdenum adhering to said skirt, said molybdenum coating having a thickness of from about .001 inch to .005 inch, and an exterior coating of a nickel steel adhering to said molybdenum coating.

8. A piston for an internal combustion engine, said piston having an aluminum alloy skirt, an intermediate coating of molybdenum adhering to said skirt, and an exterior coating of a nickel steel adhering to said molybdenum coating, said nickel steel coating being added to make a composite coating thickness of about .030 inch.

9. An internal combustion engine comprising: an aluminum alloy cylinder wall, and a piston slidable in direct contact with the aluminum alloy surface of said cylinder wall, said piston having an aluminum alloy skirt, an intermediate coating of molybdenum adhering to said skirt, and an exterior coating of a nickel steel adhering to said molybdenum coating.

10. An internal combustion engine comprising: an aluminum alloy cylinder wall, said aluminum alloy cylinder wall having an aluminum alloy surface of which silicon is the principal alloying element, and a piston slidable in direct contact with the aluminum alloy surface of said cylinder wall, said piston having an aluminum alloy skirt, an intermediate coating of molybdenum adhering to said skirt, and an exterior coating of a nickel steel adhering to said molybdenum coating.

No references cited.

Claims (1)

1. A PISTON FOR AN INTERNAL COMBUSTION ENGINE, SAID PISTON HAVING AN ALUMINUM ALLOY SKIRT, AND INTERMEDIATE COATING OF MOLYBDENUM ADHERING TO SAID SKIRT, AND AN EXTERIOR COATING OF A NICKEL STEEL ADHERING TO SAID MOLYBDENUM COATING.