bournemouth university, school of design, engineering …€¦ · a study on the lapping of ceramic...

A study on the lapping of ceramic balls

J Kang, M. Hadfield

Bournemouth University, School of Design, Engineering and Computing,Tribology Design Research Unit, Studland House,

12 Christchurch Road, Bournemouth, Dorset BH1 SNA, UKEMail: jkang@bournemouth. ac. uk

EMail: [email protected]

Abstract

Ceramic balls are used extensively in hybrid precision bearings which showadvantages in high speed, high temperature, high load and hostile environment.Surface quality generated from the final finishing processes of those balls is criticalto the rolling contact fatigue performance. A brief review on the finishing processof ceramic balls is presented. The design of an experimental eccentric lappingapparatus which allows lapping of 13.4 mm diameter ceramic ball blanks at asmall batch (15 balls each time) is described. Preliminary experimentalinvestigation on the influences of the lapping load and speed, the diamond particlesizes, the different lapping fluid mixtures has been conducted. Finished ballsurfaces are examined by optical microscopy and Scanning Electron Microscopy,and surface quality in relation to different lapping processes is discussed. Theoptimum lapping parameters from this study is proposed and further researchaspects are pointed out.

1 Introduction

Ceramic ball hybrid bearings which are now used extensively in precision machinetools and aerospace engineering have shown advantages because of thermalresistance, corrosion resistance, low density, high elastic modulus and low frictionnature of ceramics [1]. The only restriction that prevents its widespread applicationis the high manufacturing cost of ceramic balls. The manufacturing processes ofceramic elements can be classified into four stages, stage 1 - powder and fibremanufacture, stage 2 - green body forming, stage 3 - transformation to final shape,

Transactions on Engineering Sciences vol 25, © 1999 WIT Press, www.witpress.com, ISSN 1743-3533

390 Surface Treatment

stage 4 - surface finishing [2]. Ceramic balls at stage 3 are made from processeseither Sinter + HIP (Hot Isostatically Pressed), or GPS (Gas Pressure Sintered), orDirect HIP, or SRB (Sintered Reaction Bounded), or HP (Hot Pressed). In anycase, a final surface finishing is needed for the balls to achieve the required surfacequality and geometric accuracy for bearing application.It is estimated that surface finishing of ceramic components for high contact stressapplication constitutes about 50% of the total cost of manufacturing [3]. Thereforefinishing ceramic balls with high precision bearing quality at efficiency and lowcost is critical. Unlike steel balls which can be ground then lapped, due to thehardness of ceramic materials, the grinding of ceramic balls is a low efficiency andhigh cost process seldom chosen by industry. In industry, ceramic balls arefinished through several lapping operations by gradually changing the load,lapping plates and diamond grits sizes in the slurry or suspension. Ceramic ballsurface skin produced in previous manufacturing stages which is compositionallyand microstructurally different from core of the ball has to be removed during thefinal finishing stage. For a 12.7mm ball, 500-800 jam stock in diameter has to beremoved from ball surface.Obviously, the conventional lapping processes of ceramic balls need to beadvanced. Some research has been conducted on the finishing methods ofceramic balls. Magnetic Fluid Grinding (MFG), also called Magnetic FloatGrinding (Polishing), was first developed by Tani and Kawata [4] and improvedsignificantly using a float by Kato and Umehara [5]. Three main groupsinvolved in Magnetic Fluid Grinding of ceramic balls are Kato [6], Childs[7]and Komanduri [8]. Storlaski etc. [3,9] using a four-ball rolling fatigue testmachine as grinding machine by replacing upper ball with a stainless cone andlower with 9 silicon nitride 6.5 mm balls, studied the grinding wear mechanismof silicon nitride in diamond slurry at relatively high speed (3,000 rpm ). Itseems either, or both surface roughness and ball roundness generated from thesemethods have not reached the precision ball bearing requirement yet.As ceramics are brittle materials, their finishing methods can be categorized intoabrasion machining which involves abrasive particles either bounded on thecutting tools(grinding wheels) or in slurry (paste, suspension) mechanicallyscratch or wear against the workpiece to remove stock, and abrasionlessmachining which includes hydrodynamic machining, chemical milling, electricdischarge machining, electron beam machining, laser beam machining and ionbeam machining etc. [10]. Whether or not these abrasionless machining methodscan be applied to ceramic balls need to be studied. In contrast, chemical assistedthree body abrasion (loose abrasion) machining (lapping, polishing) seemsproper and most promising in finishing ceramic balls.Unlike grinding of ceramics, the fundamental mechanism of three body abrasionprocess of ceramics has received less attention. It is a common recognition thatin three body abrasion, plastic deformation and fracture are the mechanisms formaterial removal. They can coexist, the predominant mechanism is dependanton the load on the abrasives in contact with the finishing surface, whether or notthe load exceeds the threshold for fracture[11,12]. Some theoretical models for


Surface Treatment 391

three body abrasion have been established[ll,13,14]. Lapping of ceramic ballsis a complicated three body abrasion process, the hydrodynamics of lappingfluid, the viscosity change as more and more removed debris added to the fluidwhile lapping going on, the kinematics of balls, the tribochemical reactionbetween the balls, fluid and plates must be taken into account. Systematicresearch on the lapping of ceramic balls are needed both theoretically andexperimentally.The current research project is based on the above scenario. The aim of theproject in to systematically investigate the feasibility of accelerating thefinishing process of ceramic balls through altering various physical and processparameters while maintain high surface quality and long fatigue life. Ceramicball surface generated under different lapping parameters will be examined bySEM, surface profiler, and residual stresses will be measured, then four-ball andfive-ball rolling contact fatigue test will be performed. Thus the relationshipbetween the lapping parameters, surface quality and the fatigue life of the ballwill be established.

2 Design of Experimental Lapping Apparatus

Fig. 1 Overview of experimental lapping apparatus

An experimental lapping apparatus was designed by authors and manufacturedin house. Fig 1 is the picture of this experimental lapping apparatus and Fig 2 isits schematic.A DC motor and gearbox combination (1) through pulleys and belt (2) drivesflange shaft (3) to rotate at setting speed. The lower lapping plate (4) which haseccentric V-groove is mounted on the flange shaft. The top lapping plate (6)which has a flat lapping surface is stationary. Ceramic balls (5) are being lappedbetween the top lapping plate and the V-groove of lower lapping plate. Because



of the eccentricity between the circle centre of V-groove and the rotational axisof the lower plate, there will be an acceleration of ball angular velocity, a skidbetween the balls and lapping plates, and the ball spin angle will change all thetime.

Fig 2 Schematic of experimental lapping apparatus1 DC motor and gearbox combination 2 Pulleys and belt 3 Flange shaft4 Lower lapping plate 5 Ceramic ball being lapped 6 Top lapping plate7 Lapping fluid collection tank 8 Lapping fluid tray 9 Magnetic stirrer10 Lapping fluid mixture 11 Metering pump 12 Spring loading unit13 Backing plate

The lapping fluid outside the V-groove circle on the lower lapping plate willflow to the hub of flange shaft under centrifugal force and drip through a hole tothe lapping fluid tray (8) underneath the flange shaft. From there the fluid goesthrough a pipe to lapping fluid collection tank (7). Lapping fluid is pumped by aProMinent gamma/4 diaphragm-type metering pump (11) at pre-set strokethrough pipeline and hole to the centre of top lapping plate. The lapping fluid isa mixture of diamond paste and lubricant in a container (10) which is mixed andmaintained equal concentration by a magnetic stirrer (9). The amount of lappingfluid applied is controlled by the pre-set stroke number/min of the pump and atimer. The applying time (i.e. 5 min) and time interval (i.e. every 3 hours) arecontrolled by this timer. When time is at the timer's on period, the pump andmagnetic stirrer will activate simultaneously. The load (lapping pressure) isapplied to the top lapping plate by the spring loading unit (12). To ensure thelapping pressure is evenly distributed on all balls, the spring load is appliedthrough a spherical element to the cone surface of a blind countersink on the topcentre of the backing plate (13).Both top and lower lapping plate can be easily removed from the backing plateand flange shaft for change or trim. The lapping plates are made in pair usingdifferent materials (cast iron, steel, and aluminium) and surface treatment. The



eccentricity and angle of V-groove also vary. The change of speed is bydifferent pulley combinations.

3 Preliminary Experimental Results and Discussion

A preliminary experimental lapping investigation has been conducted. Ceramicball blanks (HIPed, and of rolling element bearing quality) from two differentmanufacturers were procured. The average diameter for the ball blanks frommanufacturer A, designated as B. A., is 13.255 mm. Whereas the diameters forball blanks from manufacturer B, designated as B. B., are from 13.460-13.500mm, roundness variation from 0.030-0.075 mm. Surface hardness of these ballblanks were measured at load 10 kg, loading speed 100 Jim/sec., load time 10sec.. The Vikers Hardness Number for B. A. is 1682, for B. B. 1288.

Fig 3 B. B. Ball Blank Surface Before Lapping (XI00)

Before and after each lapping test, balls and lapping plates were cleaned, andball diameters as well as the total weight of the ball batch (14 or 15) weremeasured to 1 |Lim and 1 |ig. The average ball diameter reduced per hour andthe weight lost per ball per hour were deduced and plotted to diagrams.Microscope and SEM observations were also conducted before and afterlapping.Fig 3 is the B. B. ball blank surface as procured from the manufacturer atmagnification of 100. The ball surface is very rough, indicated by many focusedand unfocused areas in the micrograph which show the depth variations. Surfaceimperfection and microstructure variation generated by previous HIP processcan also be identified by different colour in this surface micrograph.



Fig 4 is the surface of B. B. ball at magnification of 100 after initial lappingunder constant load and different speed, using 45 urn diamond paste and waterbased lubricant mixture. The surface is much smoother than Fig 3, but thesurface imperfection is not fully removed yet (dark spots). Many surfacescratches can be identified, this implies a material removal mechanism byabrasive ploughing of diamond particles.

Fig 4 B. B. Ball Surface After Initial Lapping (XI00)

3.1 Influence of Rotational Speed of Lapping Plate

14

~ 12

"Z210

13

llII.2 =

0 -I-40 60 80 100 120

Lapping Speed (r.p.m)140 160 180

-Diameter Reduced B. A.Diameter Reduced B. B.

-Weight Lost B. A.-Weight Lost B.B.

Fig 5 Material Removal Rate Versus Lapping Speed



Fig 5 shows the effect of rotational speed of lower lapping plate on materialremoval rate (designated as average ball diameter reduced per hour and averageball weight lost per hour). The lapping fluid used is a mixture of 45 Jimdiamond paste and water based lubricant at a ratio Ig : 60 ml. The lapping loadis 1.3 kg/ball. The lapping plate used is a pair of cast iron one with 8 mmeccentricity on lower plate.The material removal rate increases as the lapping speed increasing. Theincreasing is sharp (higher) from low speed to medium speed (8.5 rpm to79rpm) and from medium speed to high speed (140 rpm-169 rpm). The increasingis blunt (dull) within medium speed (from 79 rpm to 140 rpm). The interestingfinding is that although the material removal rates of two kinds ball blanks arevery different, they increase proportionally as speed increasing, B.A. is 3-4 foldof B. B. at all different lapping speed except at very low speed. This illustratesthat the material removal rate is strongly dependent on lapping speed. Actuallythe hardness of B. A. ball blank is 30% higher than B. B. ball blank, so thehardness dependent feature in a previous ceramic ball lapping investigation byGardos[15] does not correlate to this study.

3.2 Influence of Lapping Load

3.5 T

1.15 1.33 1.452Lapping Load (kg / ball)

; 13 Diameter Reduced • Weight Lost |

Fig 6 Material Removal Rate Versus Lapping Load

The influence of lapping load on material removal rate of B. B. ball is illustratedin Fig 6. In this case, the lapping fluid is a mixture of 45 u,m diamond paste andwater based lubricant. The lapping speed is 169 rpm. The lapping plate is thesame pair of cast iron one with 8 mm eccentricity on lower plate.



Obviously, there is an optimum lapping load for material removal. Higher orlower than this will reduce the material removal rate. It is also observed thathigher lapping load will produce balls with less roundness error and willeliminate the ball diameters scatter within the batch lapped. Fig 7 shows theSEM micrograph of B. B. ball surface lapped under load of 1.64 kg/ball, a pairof steel plates with 8.5 mm eccentricity on lower plate. Microcrack can be seenalong the ploughed ditch by diamond particles.

Fig 7 SEM of B. B. ball surface lapped under load of 1.64 kg/ball

3.3 Influence of the Diamond Particle Sizes

10 20 30 40 50Diamond Particle Size (pm)

[̂ Diameter Reduced -*- Weight LosT]

Fig 8 Material Removal Rate Versus Diamond Particle Size



Fig 8 is the experimental results of varying particle sizes in diamond paste.When the diamond particle size is small (l|Lim), the material removal rate is low.

While the diamond particle sizes are from 6 jiim to 45 Jim, the material removal

rate almost remain the same. When the diamond particle size is 60 JLlm, thematerial removal rate increases by 30%. This investigation is conducted on B.A. balls, the lapping speed is 169 rpm and lapping load is 1.3 kg/ball. Beforelapping, O.lg diamond paste was smeared to all balls and 3.6 ml water basedlubricant was filled to the groove in lower lapping plate. The lapping plate usedis a pair of cast iron one with 8 mm eccentricity on lower plate.

3.4 Influences of Different Lapping Fluid and Mixture

Type

Type

Type

Type

Type

Type

A

B

C

D

E

F

45

45

45

45

45

45SUJ

Jim diamond

Jim diamond

Jim diamond

Jim diamond

Jim diamond

Jim diamond;pension (wat

paste, water based

paste, brake fluid

suspension (water

paste, oil based lul

paste, water based

paste, water baseder based)

lubricant,

based)

Dricant,

lubricant,

lubricant,

distilled water

45 Jim diamond

Table 1 Different Lapping Fluid and Mixture Used

C DLapping Fluid Type

JBDiameter Reduced OWeight Lost j

Fig 9 Material Removal Rate Versus Different Lapping Fluid



Six types of different lapping fluid and mixture are used to investigate theirtribochemical reactions with silicon nitride. Table 1 is the summery of thesefluid mixtures. The test is on B. B. Balls, the lapping speed is 169 rpm andlapping load is 1.3 kg/ball, The lapping plate used is still the pair of cast ironone with 8 mm eccentricity on lower plate.The lowest material removal rate is found on type B fluid, a mixture of 45 jimdiamond paste and brake fluid. Oil based lubricant (type D) is better than waterbased lubricant (type E). Adding distilled water in the mixture of water basedlubricant and 45 |im diamond paste (type A) will increase the material removalrate, but the consequence is severe surface pitting. The highest material removalrate is on type F, a mixture of 45 jam diamond paste, water based lubricant, 45|Lim water based diamond suspension, with no surface pitting.

4 Summery

Preliminary investigation on the lapping of ceramic balls has shown that thematerial removal rate is strongly dependent on the lapping speed, there is anoptimum lapping load, material removal rate increases by 30% when using 60fim diamond paste, and different lapping fluid and mixture has great influenceon material removal rate. Lapping has great potential to increase its efficiency,could be one of the most effective processes in finishing ceramic components.Further research will be emphasised on establishing a model in which the spin ofthe balls, the hydrodynamics of lapping fluid, the kinematics of diamondparticles, the tribochemical reactions must be taken into account.

Acknowledgement

The authors would like to thank the financial support of the project from SKFEngineering & Research Centre B. V. in the Netherlands, in particular thetechnical advice from Dr Robin Cundill. Thanks also due to F. J. Engineeringin Milford on Sea, Dorset (UK) for manufacturing the experimental lappingapparatus and to ProMinent Fluid Control (UK) Ltd, for contributing aGamma/4 diaphragm-type metering pump.

Reference

1. Cundill, R. T., Light-weight material for the rolling elements of aircraftbearings, Ball Bearing Journal, 216, pp. 33-36, 1983.

2. McColm, I. J. & Clark, N. J., Forming, Shaping and Working of High-Performace Ceramics Blackie and Son Ltd, 1988.

3. Jisheng, E., Stolarski, T. A. & Gawne, D. T., Tribochemically assisted wearof silicon nitride ball, Journal of the European Ceramic Society, 16, pp.



25-34, 1996.

4. Tani, Y. & Kawata, K., Development of High-Efficient Fine FinishingProcess Using Magnetic Fluid, CIRP Annals, 33, pp. 217-220, 1984.

5. Umehara, N. & Kato, K., Study on magnetic fluid grinding (1st report, Theeffect of the floating pad on removal rate of S1//3N//4 balls), Nippon KikaiGakkai Ronbunshu, C Hen/Transactions of the Japan Society ofMechanical Engineers, Part C, 54, pp. 1599-1604, 1988.

6. Umehara, N. & Kato, K., Magnetic fluid grinding of advanced ceramicballs, Wear, 200, pp. 148-153, 1996.

7. Childs, T. H. C., Mahmood, S. & Yoon, H. J., Magnetic fluid grinding ofceramic balls, Tribology International, 28, pp. 341-348, 1995.

8. Jiang, M. & Komanduri, R., On the finishing of Si3N4 balls for bearingapplications, Wear, 215, pp. 267-278, 1998.

9. Stolarski, T. A. & Jisheng, E., Effect of water in oil based slurry on wear ofsilicon nitride, British Ceramic Transactions, 97, pp. 61-67, 1998.

10. Firestone, R. F., Abrasionless Machining Methods for Ceramics, Thescience of ceramic machining and surface finishing II : proceedings of asymposium held at the National Bureau of Standards, Gaithersburg,

-7J, 7P7&, pp. 261-281, .

11. Buijs, M. & Korpelvanhouten, K., 3-Body Abrasion of Brittle Materials AsStudied By Lapping, Wear, 166, pp. 237-245, 1993.

12. Chandrasekar, S., Kokini, K. & Bhushan, B., Influence of abrasiveproperties on residual stresses in lapped ferrite and alumina, Journal of theAmerican Ceramic Society, 73, pp. 1907-1911, 1990.

13. Bulsara, V. H., Ahn, Y., Chandrasekar, S. & Farris, T. N., Mechanics ofpolishing, Journal of Applied Mechanics-Transactions of the Asme, 65, pp.410-416, 1998.

14. Ahn, Y. & Park, S. S., Surface roughness and material removal rate oflapping process on ceramics, Ksme International Journal, 11, pp. 494-504,1997.

15. Gardos, M. N. & Hardisty, R. G., Fracture Toughness-Dependent andHardness-Dependent Polishing Wear of Silicon-Nitride Ceramics,Tribology Transactions, 36, pp. 652-660, 1993.


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