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The effect of accelerated material removal on roundness and residual stresses in ceramic balls
[I
ELSEVIER
WEAR wear 205 (1997) 206-213
The effect of accelerated material removal on roundness and residual stresses in ceramic balls T.A. Stolarski ~'*, S. Tobe b • Brunel O~iversity, Uxbridge, Middlesex. UB8 3PH. UK t, Ashikt ~, ~Institute of Technology. 268 Ohmaecho. Ashikagashi, Tochigiken 326. Japan
Received4 June 1996:acccpled6 September1996
Abstract The results of studies on the influence of the amount of material removed from the surface of silicon nitride ball produced using,the ho~ isostatic pressing (HIP) techniqueon the compressiveresidual stress are presented and discussed.They indicate that the residualcompressive stress decreases as the amount of materia! removed grows. Initially, the rate of stress decrease is quite high hut it decays as the removal of material progresses.The best ~oundnessis achieved when grinding of balls takes place under light load and the presence of small abrasive particles. Keywords: Ceramics;Abrasivewear;Residualstress;Roundness;Siliconnitfide
1. Introduction The benefits of using advanced ceramics have led to their increasing use in industry. A method frequently used to produce high quality ceramic components is hot isostatic pressing (HIP). It combines powder compactions and sintering into a single operation. The powder is hermetically sealed in a flexible, a~rtight, evacuated container and then subjected to a high-temperature, high-pressure environment, generally around 300 MPa and about 1500 °C. Products emerge at full dens~.ty with uniform isotropic properties and high compressive residual stress. In the preparation of ceramic balls, using the hot isostatic pressing technique, the sintering phase results in shrinkage, affecting the roundness of balls, which has to be corrected by grinding during the first stage of the finishing process. The surface skin of 100-200 p.m thickness is compositionally and microstructurally different from the core of the ball due to reactions with the surrounding atmosphere during simering. This region is usually in a state of high compressive stress. The skin has to be removed during the second stage of finishing. Finally, the surface finish suitable for precision engineering applications ( i.¢. rolling contact bearings ) is attained by the third finishing stage. Typically, the high precision bearing ball for machine tool spindles of diameter in the range * Corresponding author. Tel.: 01895 274000, ext. 2716; fax: 01895 256392. 0043-1641s/97/$17.00 © 1997ElsevierScienceS.A.All fightsreserved PI10004 3- 1648 ( 96107 344-9
of 6-14 mm requires Grade 3 or 5 dimensional qual;'#, that is to be round to within 0.08-0.13 p.m and to have surface roughness of 0.012-0.02 p.m R~ [ ! ]. The main objective of the study reported here was to determine the changes in the roundness and the level of compressive residual stress in silicon nitride balls produced by the HIP technique and subjected to accelerated grinding under controlled conditions. In view of the high compressive stress in the surface region of the ball which is a typical feature of the HIP method it was expected that decreasing stress level with progressing material removal would be found. Moreover, it was also anticipated that the geometrical configuration used during grinding experiments should lead to improved roundness of the balls.
2. Experimental apparatus and procedure In order to investigate the change in residual stress level resulting from material removal, model grinding experiments were carried out. After each grinding experiment, conducted at different loads, the amount of material removed was measured. Next, the change in the roundness error of the ball was determined followed by scanning electron microscope (SEM) examinations of its surface. The final stage was residual stress measurement.
TA. Stolarskl, S. Tobe I Wear 205 f1997) 206-213
2.1. Grinding apparatus
(4)
q=KM
All grinding experiments were carried out on a model contact created by the well known four-ball configuration. The details of the apparatus in its normal configuration are described elsewhere [2]. In a normal configuration, the top ball is located in a holder carried by a vertically mounted spindle and is rotated in a loaded contact with three other balls which are free to rotate in a specially shaped cup. In the study reported here, the top ball was replaced by a cone and placed in contact with nine 6.5 mm nominal diameter silicon nitride balls. Thus, each ball was in contact with the cup at two points and with the cone at one point. This configuration called 'three point contact' is shown schematically in Fig. I. "1ne cup and the cone were fabricated from 304 grade stainless steel. The cone together with the spindle was driven by an electronically controlled electric motor,
where
2.2. Residual stress instrument
2.2.2. Measuring equipment
2.2.1. Outline of general principles X-ray stress measurements were used to find the level of residual stress on th~ ball surface after the grinding experiinent. The sin 2y method was employed as being suitable for materials with crystalline structure. When a stress is applied to a material, the atomic distance in the crystal lattice will be, extended or shortened within the elastic limit in proportion. The X-ray diffraction technique is utilized to measure the variations of the inter-planar spacing in the crystal allowing for stress calculation [31. By using Bragg's condition for diffraction nA = 2d sin 0
( I)
strain may tht, n be calculated from ~=~-~=
207
cot 0oA0
K=
Ecot 0o 2(1+o)
and M ffi ~(O) ~(sin2O) The angle of incident X-ray, usually denoted asy, is varied typically from 0 to 40° in intervals of 5 ~. At these settings, the change "n diffraction angle is measured and hence the gradient M evaluated through calculations performed by a computer of the X-ray machine. On the basis of past experience the X-ray elastic constant K was taken to be 1106 MPa/ deg [41.
The measurements of residual stresses were carried out using a Rigaku Rint 2500 machine. A schematic representation of the instrument is shown in Fig. 2. A rotary type Cr target was used with the maximum load of 50 kV and 250 mA. A position sensitive proportional counter, denoted in Fig. 2 as PSPC, with detecting angle of + 12' enabled working in the range of 10 to 159.5". More information about the machine can be found elsewhere [51. A normal procedure of X-ray stress measurement is to select the X-ray tube, its voltage and current. For stress measurements in silicon nitride balls, a Cr K a tube was used. Because the balls had 6.5 mm diameter therefore the X-ray irradiation area, adjusted by a collimator, wasquite small and eau~! to 0.20 mm z (this corresponds to a circle ofO.5 mm diameter). The maximum penetration depth, controlled by
(2)
Strain is calculated from the amount of variation in the Yray diffraction angle (Eq. (2) ). Stress on the surface is then calculated from
[ otOo o
1+ v a(sin~)
.1
L2( 1 + o) a(s'~-n24,)J
(3)
Eq. (3) may be expressed as the product of a constant K and gradient M, as shown in
S~O,m*
ceral, bal~
cup
~atlm
Fig. I. Schematic representation of the contact configuration for grinding experiment.
Fig. 2. Diagram of the X-ray apparatus: PSPC= position sensitive proporlion~ counter.
208
T.A. Stolarski, S. Tobe / Wear 205 (1997) 206-213
Table I Silicon nitride X-raystressmeasurementparameters Parameter
Value
Characteristic X-ray Diffractionplane Diffractionangle (°) Inadiationarea (mm-") A(A) Measurementdepth (ttm)
Cr Kot (411 )
126 0.25 2.29 29
the X-ray characteristics, was estimated to be of the order of 29 p.m. Table 1 gives details concerning measurement parameters. Several readings at various locations on the surface of the ball were normally taken and averaged. 2.3. Roundness measurements
The roundness measurements were carried out using a Talydata 2000 instrument which could be. considered as an advanced but standar.'t instrument in a meta'ology laboratory. The signal from the measuring head was sent to a microcomputer to be processed and printed in accordance with the selected menu. 2.4. Experim,.ntalprocedure
The grinding experiments were carried out at a constant rotational speed of the spindle of 3000 rpm. The normal loads on the set of nine balls ranged from 200 to 4000 N. An oilbased diamond slurry with particle size of 15 ttm was used in the majority of the experiments but 1 p.m, 6 ttm and 45 p.m particles were also employed. Usually, 3 ml of the slurry were placed in the cup containing nine balls. The cup was then covered with a lid to prevent spillage. A typical experiment lasted 60 min and was followed by microscopic examinations, material removal rate, roundness and residual stress r~easurements. Every test was repeated three times in the same way in order to determine the variability of results. A satisfactory repeatability was found: however, the number of test repeats did not allow for any statistical analysis of results. The silicon nitride balls (Si3N4), with Vickers hardness of 2200 MPa, Young's modulus of 310 GPa and fracture toughness of 5 MPa m'/2 in the as-received condition, had initial surface roughness of the order of 100-200 p.m Ra and contained various surface imperfections. They were prepared by the hot isostatic pressing technique. The initial value of the roundness error was typically 30-32 p.m. On average, the residual stress level was of the order of 1200 MPa. Fig. 3 shows a SEM micrograph of the initial ball surface. Because of initial surface conditions, each ball before testing was subjected to a run-in process to remove imperfectinng and to bring all balls to nominally the same surface topography. The running-in was conducted under a load of 100 N and speed of 700 rpm for one hour in 3 ml oil-based slurry with 15 p.m particles. After running-in, all balls were cleaned ultrasoni-
Fig. 3. The aPl~aranceof ball surfacebeforethe grindingexpeximent. cally and their diameter measured. On average, the runningin resulted in removal of 100 ttm of the material and decrease in the residual stress level by 320 MPa. Thus, the level of residual stress was around 880 MPa before the proper experimental run. The surface roughness was also improved to approximately 50 tim R,. In order to ensure that the material removal rate measured truly reflects the action of a particular abrasive particle size, a separate cup arid cone were used for every particle size used during the studies. Before the test, the cup as well as the balls and the cone were thoroughly cleaned to ensure that the conditions before each consecutive test were nominally the same. The balls were placed in the cup and a fresh measure of the grinding fluid was added into the cup. The required velocity and load were set and the experiment was run for 60 min. At the end of the experiment, the balls were removed from the cup, ultrasonically cleaned and their diameters measured with a precision micrometer. The average changes in diameter were recorded and the rate of material removal per unit time calculated. Furthermore, the studies involved SEM ball surface examinations, roundness and X-ray residual stress measurements were carried out.
3. Results of grinding experiments and surface studies 3.1. The effect o f load on material removal
Five different loads, namely 200, 800, 1200, 1600, and 4000 N, were used to investigate how material removal changes with the normal load. Fig. 4 shows the material removal, expressed as a linear change in ball diar, eter per unit time, plotted against the applied normal load on a set of nine balls. The first general observation is that the material removal increases with increasing load. However, two clear trends can be distinguished. Firstly, the initial increase in the load does not change the material removal rate significantly
T.A. Stolarski, S. 7 c be / Wear 205 (1997) 206-213
209
that the departure from the initial contact geometry caused by the wear was not a problem. Further details concerning post-test slurry analyses can be found elsewhere [ 6].
6-
3.3. Roundness measurements 4-
3"
~0 L,~dtNI Fig. 4. Material removal rate as a function of load for 15 mm abresive particles. Speed of the conical counterface 3000 rpra; test duration 60 win; grinding fluid 3 wi oil.based diamond slurry.
until a certain critical load is reached. In the case ofthe slurry with 15 p.m particles, the critical load seems to he 800 N. Then, the transition is reached when the material removal rate increases two- or even three-fold. Once the transition is re~ched, the material removal rate is again a weak function of the load. Information concerning the influence of other abrasive particle sizes on the rate of material removal can be found elsewhere [61. 3.2. Microstructure studies
Microscopy studies were carried out m examine ball surfaces after the grinding experiment. Fig. 5 shows SEM micrographs of the ball surface after the grinding experiment in 15 p.m slurry and different loads. The first observation is that there are only few surface scratches typical for grinding. Instead, there are isolated spots covering the surface. This feature is characteristic for all loads at which grinding experiments were carried out. Because the spots appear to be isolated from each other a possible hypothesis is that they were created as a result of an indentation either by abrasive p,aticles rolling through the contact zone or by abrasive particles embedded into the cup and upper test piece over which ceramic balls relied. Thus the mechanism responsible for the material removal could be brittle fracture. Examination of cups and cones after the grinding experiments showed that diamond particles are indeed embedded in the surface layer of material. This, however, prevented excessive eear of the cup and cone and, therefore, no significant departure from an initial contact geometry occurred from test to test. Analyses ~f the abrasive slurry after the test were also carried o'at. As expected, it was found that the slurry contains not only silicon nitride particles but also diamond abrasive particles and pieces of stainless steel. Si£ :ificantly, the concentration of stainless steel particles was at its highest after the first two tests with the same cup. Subsequent tests with the same cup produced decreasing amount of stainless steel particles in the glurry. This supports the earlier observation
Results of roundness measurements are shown in Fig. 6. Roundness error represents here deviation from the spherical form. It can be interpreted as the greatest radial distance in any radial plane between a sphere circumscribed around the b~ll surface and any point on the ball surface. These results represent two series of measurements performed on a set of nine balls tested under various conditions of load and abrasive particles size. Averaged change of the roundn~s error with the size of abrasive particles used during grinding experiments carried out at the load of 200 N is given in Fig. 6(a). it is seen that for all abrasive pariicle sizes used a substantial reduction in the roundness error takes place. The most pronouneed decrease takes place for 6 p,m particles and is almost twice of that recorded for I g.m particles. However, the roundness error recorded for 15 p,m and 45 Ixm particles is greater than that found for small particles. The smallest decrease in the roundness error is obtained for 45 ixm particles, although the error is still ~wo times less than that for a blank ball. If the results of Fig. 6(a) are considered in the light of material removal rates [ 4 ] then it is seen that highest material removal rates at the load of 200 N were achieved for i5 ixm particles. Removal rates for 6 Ixm and 45 p.m are almost comparable although the roundness error in the case of 6 p.m particles is five times smaller than that for 45 p,m particles. The material removal rate for ! Ixm particles is ten times smaller than that for !5 p,m particles: however, the corresponding difference in roundness errors is much smaller. Under the load of 200 N the roundness error for I p m particles is only 50% smaller than that for 15 ~ m particles. The question is why the roundness error appears to he smallest for 6 g.m particles. The answer to this question must be sought in the light of the complex motion of balls during the grinding experiments, In order to achieve best possible roundness the balls must not only orbit the cup but also spin about their own axes of symmetry. It is clear then that in the case of larger abrasive particles ( 15 p,m and 45 p,m) the conditions were such that the balls could not orbit and spin freely all the time. hence the increase in the roundness error. If thi,~ argument is accepted O~enit is rather difficult to explain why the decrease in the roundness error is greater for 6 ~ m particles than that for 1 p.m particles. The only sensible explanation for this problem is in the fact that I p.m particles produced very small rates of material removal. It follows then that within the time scak: of grinding experiment (60 rain) I p,m particles were not able to produce roundness comparable to that achieved for 6 p.m particles. Fig. 6(b) gives the change of the roundness error with the load for abrasive particle size of 15 ~m. The results form a rather logical pattern. The largest r~duction in the roundness error is obtained for the lightest load applied. With increasing
210
T.A. Stolarski, S. Tobe lWear 205 (1997) 206-213
Fig 5. Appearanceof the ball surfacea~er grinding experiment: (a) 200, (b) 800, (c) 1200, (d) 1600, (e) 4000 N.
load the reduction in the roundness error decreases substantially and, eventually, the stage is reached where the error is comparable to that for a blank ball. At this load, the material removal rate is at its highest (see Fig. 4).
3.4. Residualstresses The results of residual stress measurements are shown in Fig. 7 in which are reported two series of measurements per-
T.A, Stolarski. $. Tobe I Wear 205 (1997) 206-213
Norroal load = 200 N
~l[-I
blank
I pm
40-
6 ra~ 15 lun Si7~eof abra.si~ ¢ pariicics
45 ~.m~
211
the relationship between the reduction in residual stress per linear unit of material removed and load. This significant reduction in the rate at which residual stresses decrease could he explained by the removal of a layer of material with the highest compressive residual stresses during the first round. The consequence of the above supposition is such that one would expect the level of residual stress to continue decreasing but at a much lower rate. The obvious question is about the level at wh;.ch residual stress stabilizes, if at all. The answer to this question requires a prolonged testing which was not the main objective of the research presented here but will be addressed in the near future.
A b t ~ i ~ c paatclc size = 15 I~m
4. Discussion of the results 30-
i 2o......... -~ N)?g+ .
I 0-
~
+~e,%+.,
+ ? :+'+' O"
blank
~
" 200N
8QON 12L~N L, hal dunng grinding
16~X)N
4~N
Fig. 6. Changein roundnesserrorresultingfromgrindingundercontrolled conditions: (top) experimentsunder constant load of 200 N: (bottom) experimentsunderconstantabrasiveparliclesizeof 15 p.m. formed on a set of nine balls subjected to grinding tests lasting 60 min in 15 p-m slurry and at different loads. Typically, a set of nine balls was tested at, for instance, 200 N and an average material removal rate was 2.6 p,m min- ~ (see Fig. 4). Because an experiment lasted 60 min, therefore, total material removal, expressed as a linearchange in the ball diameter, was 156 p-re. Then, residual stress level was measured and the same set of nine balls was subjected again to nominally the same grinding test. As before, the average material removal rate was recorded, the total material removal calculated, and the residual stress level measured. The same procedure was applied to all five different loads used. As a result of that two curves, illustrating the change in residual stress level on ball surface, were obtained. It is seen that after the first round of tests a very substantial decrease, in residual stress level occurred as compared to the residual stress level of 880 MPa recorded after the runningin. This decrease depends on the load; the higher the load the more significant the decrease in residual stress level. Moreover, there is, as expected, a close correlation between decrease in residual stress level and the amount of material removed from the ball surface. It is interesting to note that the second round of tests produced generally much smaller decreases in the level of residual stress as compared to the first round. For example, at the 200 N load, residual stresses were reduced by 3.64 MPa for every 1 p.m of material removed during the first round of testing. A corresponding result for the second round is 0.69 MPa p-m- +. Fig. 8 gives
Results of the studies presented here point to a clear relationship between the amount of material removed from the surface of a silicon nitride ball produced by the HIP method and the level of residual stress. A relatively high initial level of residual stress of 1200 MPa quickly falls to 8go MPa as a result of running-in of the blank ball. A typical rate of residual stress reduction is 3.2 MPa p.m-~. This 27% reduction in residual stress level could he attributed to the fact that the outermost layer of material with highest residual stress level was removed during running-in. Clearly, a substantial reduction in residual stress takes place during removal of the upper surface layer of material from the ceramic ball. Roundness measurements indicate that the best results are achievable for light loads applied. This is certainly true in the case of 15 Izm abrasive particles. Such a trend is clearly compatible with changes in the material removal mechanism taking place when the load is increased. The picture of the relationship between roundness and the size of abrasive particles is more complicated. The best roundness results are obtained for 6 p-m particles but not for I p.m particles--the smallest used. This only deviation from a clear trend can be explained in terms of relative lack of effectiveness of I p-m particles in material removal as well as from the material removal mechanism point of view. In the case of I p-m particles it is reasonable to expect a three-body abrasive material removal mechanism. These particles are small enough to roll through the contact region between the balls and the cup. For 6 p-m particles a two-body abrasive material removal mechanism could he postulated as these particles are too large to pass through the contact region and are embedded in the softer material of the cup. When even larger abrasive particles are used ( 15 p-m and 45 p,m) the two-body abrasive wear is replaced by a brittle fracture type of material removal mechanism and the roundness of the balls deteriotar.es significantly. Another interesting observation is that the rate of residual stress reduction is decreasing with increasing load. Fig. 8 shows that this trend is observed for both rounds of testing but it is more pronounced for the first round. The main reason for that seems to be rather weak dependence of the material removal rate on the load as indicated by Fig. 4. Above a certain load (in this case it is 1600 N) the material removal
212
T.A, Stola rxki. S. Tobe / Wear 205 (1997) 206-213 I000-
%"
800-
.~
SO0"
Norrn,al l~td = g00 N
Normal load= 200N
._e .~, ~
.
21g}"
200°
(a)
(b)
3
2
NOi'rnal Ic~tl= 16~I N
NotraalIt.d = 1200N
(c)
l
2
3
(d)
1
Normal load = 4 ~
2
3
N
=
(e)
t
2
3
Fig. 7. Change in resid',.ml stress in the surface layer of silicon nitride ball: ( I ) denotes initial level of residual stress, ( 2 ) denotes residual stress level after first round of testing, (3) denotes residual stress level after second mend of testing. (a) 200. (b) 800, (¢) 1200, (d) 1600, (e) 4000 N.
rate increases rather slowly. For instance, between 200 and 1600 N the material removal rate increases 0.225 p.m per 100 N increase in the load. In the range of loads from 1600 N to 4000 N the figure is 0.035 tzm per 100 N increase in the load. The reason for this decrease in material removal rate at higher loads is probably the change in operating wear mechanism. This is supported by the images shown in Fig. 5. At low load (200 N) one can see scratches typical for abrasive wear (Fig. 5(a) ) whilst at the high load (4000 N) signs of brittle fracture can be found (Fig. 5 (e) ). Therefore, the supposition that the effective material removal is produced by abrasion seems to be reasonable. Brittle fracture, although involving removal of relatively large fragments of material, is very much a localized process and does not result in an overall high material removal rate.
3,s-
~_
3a
2.52. I
1.5,
,,I-
b
~ 0.5'
t.end {NI Fig. 8. Rate of change in residual stress: (a) denotes first round of testing. (b) denotes second round of testing. The rate of change is exlnessed as a decrease in residual stress per I ttro of reduction in ball diameter.
T.A. Stolarski, $. Tobe / Wear 205 (1997) 206-213
It is rather premature to speculate on the likely pattern of residual stress decrease with the amount of material removed. As indicated earlier, a clear reduction in the residual stress decrease rate was observed during the second round of testing using the same set of balls. This observation points to the possibility of reaching a stable level of compressive stresses within the ball produced by the HIP method. There must he a core of material within the ball where there is no gradient of compressive stress. However, ful:her studies are required to fully support this rather speculative observation.
5. Conclusions Results and observations presented in this paper justify the following conclusions: • residual compressive stress in the silicon nitride ball produced with the HIP method decreases as a result of material removal from the ball surface; • roundness error depends on both the load applied and the size of abrasive particles used during grinding experi-
213
mcnts. The best results are obtained for light loads and small abrasive particles; • the relationship between the rate of residual stress decrease and the amount of material removed is not alinear function. The highest rates are achieved when the first layer of material is removed. Subsequent tests on the same ball result in a much reduced rate of residual stress decay. References [II R.T. Cundill. High precision silicon nitride balls for bearings, SPIE. 1573 ( 1991) 77-86. [2l C.C. Lawrence and T.A. Stolarski. Rolling contact wear of polymers. Wear. 132 (1989) 183--191. [3] G.H. Fmaki. P.H. Markho.and G. Meader, A study of fretting wear with particularreference to me~tgement of residualstres.cesby X-ray • lffracfion. Wear, 148 ( 1991) 249--260. [4] S. K0cJ~ma, g. Akita, i. Mi~wa and S. Tobe, A model for residual stressdistributionin X-rayfractogr~y, Proc. ICR53, ResidualStresses I!1. 1991, 1421-1428. [51 M. Hadfield.G. Fujinawa,T.A. Stolatsk~and S. Tobe, Residual in failed ceramicrolling-contactballs, Ceram. Int.. 9 (1993) 307-313. [6] T.A. Stolat,ski, E. Jisheng. D.T. G ~ aztdS. Panesax,The effect of load and abfaaivepanicle size oft the material removal gale of silicon nitridearlef~,'ts,Cerara. Int.. 21 (1995) 355-366.
ELSEVIER
WEAR wear 205 (1997) 206-213
The effect of accelerated material removal on roundness and residual stresses in ceramic balls T.A. Stolarski ~'*, S. Tobe b • Brunel O~iversity, Uxbridge, Middlesex. UB8 3PH. UK t, Ashikt ~, ~Institute of Technology. 268 Ohmaecho. Ashikagashi, Tochigiken 326. Japan
Received4 June 1996:acccpled6 September1996
Abstract The results of studies on the influence of the amount of material removed from the surface of silicon nitride ball produced using,the ho~ isostatic pressing (HIP) techniqueon the compressiveresidual stress are presented and discussed.They indicate that the residualcompressive stress decreases as the amount of materia! removed grows. Initially, the rate of stress decrease is quite high hut it decays as the removal of material progresses.The best ~oundnessis achieved when grinding of balls takes place under light load and the presence of small abrasive particles. Keywords: Ceramics;Abrasivewear;Residualstress;Roundness;Siliconnitfide
1. Introduction The benefits of using advanced ceramics have led to their increasing use in industry. A method frequently used to produce high quality ceramic components is hot isostatic pressing (HIP). It combines powder compactions and sintering into a single operation. The powder is hermetically sealed in a flexible, a~rtight, evacuated container and then subjected to a high-temperature, high-pressure environment, generally around 300 MPa and about 1500 °C. Products emerge at full dens~.ty with uniform isotropic properties and high compressive residual stress. In the preparation of ceramic balls, using the hot isostatic pressing technique, the sintering phase results in shrinkage, affecting the roundness of balls, which has to be corrected by grinding during the first stage of the finishing process. The surface skin of 100-200 p.m thickness is compositionally and microstructurally different from the core of the ball due to reactions with the surrounding atmosphere during simering. This region is usually in a state of high compressive stress. The skin has to be removed during the second stage of finishing. Finally, the surface finish suitable for precision engineering applications ( i.¢. rolling contact bearings ) is attained by the third finishing stage. Typically, the high precision bearing ball for machine tool spindles of diameter in the range * Corresponding author. Tel.: 01895 274000, ext. 2716; fax: 01895 256392. 0043-1641s/97/$17.00 © 1997ElsevierScienceS.A.All fightsreserved PI10004 3- 1648 ( 96107 344-9
of 6-14 mm requires Grade 3 or 5 dimensional qual;'#, that is to be round to within 0.08-0.13 p.m and to have surface roughness of 0.012-0.02 p.m R~ [ ! ]. The main objective of the study reported here was to determine the changes in the roundness and the level of compressive residual stress in silicon nitride balls produced by the HIP technique and subjected to accelerated grinding under controlled conditions. In view of the high compressive stress in the surface region of the ball which is a typical feature of the HIP method it was expected that decreasing stress level with progressing material removal would be found. Moreover, it was also anticipated that the geometrical configuration used during grinding experiments should lead to improved roundness of the balls.
2. Experimental apparatus and procedure In order to investigate the change in residual stress level resulting from material removal, model grinding experiments were carried out. After each grinding experiment, conducted at different loads, the amount of material removed was measured. Next, the change in the roundness error of the ball was determined followed by scanning electron microscope (SEM) examinations of its surface. The final stage was residual stress measurement.
TA. Stolarskl, S. Tobe I Wear 205 f1997) 206-213
2.1. Grinding apparatus
(4)
q=KM
All grinding experiments were carried out on a model contact created by the well known four-ball configuration. The details of the apparatus in its normal configuration are described elsewhere [2]. In a normal configuration, the top ball is located in a holder carried by a vertically mounted spindle and is rotated in a loaded contact with three other balls which are free to rotate in a specially shaped cup. In the study reported here, the top ball was replaced by a cone and placed in contact with nine 6.5 mm nominal diameter silicon nitride balls. Thus, each ball was in contact with the cup at two points and with the cone at one point. This configuration called 'three point contact' is shown schematically in Fig. I. "1ne cup and the cone were fabricated from 304 grade stainless steel. The cone together with the spindle was driven by an electronically controlled electric motor,
where
2.2. Residual stress instrument
2.2.2. Measuring equipment
2.2.1. Outline of general principles X-ray stress measurements were used to find the level of residual stress on th~ ball surface after the grinding experiinent. The sin 2y method was employed as being suitable for materials with crystalline structure. When a stress is applied to a material, the atomic distance in the crystal lattice will be, extended or shortened within the elastic limit in proportion. The X-ray diffraction technique is utilized to measure the variations of the inter-planar spacing in the crystal allowing for stress calculation [31. By using Bragg's condition for diffraction nA = 2d sin 0
( I)
strain may tht, n be calculated from ~=~-~=
207
cot 0oA0
K=
Ecot 0o 2(1+o)
and M ffi ~(O) ~(sin2O) The angle of incident X-ray, usually denoted asy, is varied typically from 0 to 40° in intervals of 5 ~. At these settings, the change "n diffraction angle is measured and hence the gradient M evaluated through calculations performed by a computer of the X-ray machine. On the basis of past experience the X-ray elastic constant K was taken to be 1106 MPa/ deg [41.
The measurements of residual stresses were carried out using a Rigaku Rint 2500 machine. A schematic representation of the instrument is shown in Fig. 2. A rotary type Cr target was used with the maximum load of 50 kV and 250 mA. A position sensitive proportional counter, denoted in Fig. 2 as PSPC, with detecting angle of + 12' enabled working in the range of 10 to 159.5". More information about the machine can be found elsewhere [51. A normal procedure of X-ray stress measurement is to select the X-ray tube, its voltage and current. For stress measurements in silicon nitride balls, a Cr K a tube was used. Because the balls had 6.5 mm diameter therefore the X-ray irradiation area, adjusted by a collimator, wasquite small and eau~! to 0.20 mm z (this corresponds to a circle ofO.5 mm diameter). The maximum penetration depth, controlled by
(2)
Strain is calculated from the amount of variation in the Yray diffraction angle (Eq. (2) ). Stress on the surface is then calculated from
[ otOo o
1+ v a(sin~)
.1
L2( 1 + o) a(s'~-n24,)J
(3)
Eq. (3) may be expressed as the product of a constant K and gradient M, as shown in
S~O,m*
ceral, bal~
cup
~atlm
Fig. I. Schematic representation of the contact configuration for grinding experiment.
Fig. 2. Diagram of the X-ray apparatus: PSPC= position sensitive proporlion~ counter.
208
T.A. Stolarski, S. Tobe / Wear 205 (1997) 206-213
Table I Silicon nitride X-raystressmeasurementparameters Parameter
Value
Characteristic X-ray Diffractionplane Diffractionangle (°) Inadiationarea (mm-") A(A) Measurementdepth (ttm)
Cr Kot (411 )
126 0.25 2.29 29
the X-ray characteristics, was estimated to be of the order of 29 p.m. Table 1 gives details concerning measurement parameters. Several readings at various locations on the surface of the ball were normally taken and averaged. 2.3. Roundness measurements
The roundness measurements were carried out using a Talydata 2000 instrument which could be. considered as an advanced but standar.'t instrument in a meta'ology laboratory. The signal from the measuring head was sent to a microcomputer to be processed and printed in accordance with the selected menu. 2.4. Experim,.ntalprocedure
The grinding experiments were carried out at a constant rotational speed of the spindle of 3000 rpm. The normal loads on the set of nine balls ranged from 200 to 4000 N. An oilbased diamond slurry with particle size of 15 ttm was used in the majority of the experiments but 1 p.m, 6 ttm and 45 p.m particles were also employed. Usually, 3 ml of the slurry were placed in the cup containing nine balls. The cup was then covered with a lid to prevent spillage. A typical experiment lasted 60 min and was followed by microscopic examinations, material removal rate, roundness and residual stress r~easurements. Every test was repeated three times in the same way in order to determine the variability of results. A satisfactory repeatability was found: however, the number of test repeats did not allow for any statistical analysis of results. The silicon nitride balls (Si3N4), with Vickers hardness of 2200 MPa, Young's modulus of 310 GPa and fracture toughness of 5 MPa m'/2 in the as-received condition, had initial surface roughness of the order of 100-200 p.m Ra and contained various surface imperfections. They were prepared by the hot isostatic pressing technique. The initial value of the roundness error was typically 30-32 p.m. On average, the residual stress level was of the order of 1200 MPa. Fig. 3 shows a SEM micrograph of the initial ball surface. Because of initial surface conditions, each ball before testing was subjected to a run-in process to remove imperfectinng and to bring all balls to nominally the same surface topography. The running-in was conducted under a load of 100 N and speed of 700 rpm for one hour in 3 ml oil-based slurry with 15 p.m particles. After running-in, all balls were cleaned ultrasoni-
Fig. 3. The aPl~aranceof ball surfacebeforethe grindingexpeximent. cally and their diameter measured. On average, the runningin resulted in removal of 100 ttm of the material and decrease in the residual stress level by 320 MPa. Thus, the level of residual stress was around 880 MPa before the proper experimental run. The surface roughness was also improved to approximately 50 tim R,. In order to ensure that the material removal rate measured truly reflects the action of a particular abrasive particle size, a separate cup arid cone were used for every particle size used during the studies. Before the test, the cup as well as the balls and the cone were thoroughly cleaned to ensure that the conditions before each consecutive test were nominally the same. The balls were placed in the cup and a fresh measure of the grinding fluid was added into the cup. The required velocity and load were set and the experiment was run for 60 min. At the end of the experiment, the balls were removed from the cup, ultrasonically cleaned and their diameters measured with a precision micrometer. The average changes in diameter were recorded and the rate of material removal per unit time calculated. Furthermore, the studies involved SEM ball surface examinations, roundness and X-ray residual stress measurements were carried out.
3. Results of grinding experiments and surface studies 3.1. The effect o f load on material removal
Five different loads, namely 200, 800, 1200, 1600, and 4000 N, were used to investigate how material removal changes with the normal load. Fig. 4 shows the material removal, expressed as a linear change in ball diar, eter per unit time, plotted against the applied normal load on a set of nine balls. The first general observation is that the material removal increases with increasing load. However, two clear trends can be distinguished. Firstly, the initial increase in the load does not change the material removal rate significantly
T.A. Stolarski, S. 7 c be / Wear 205 (1997) 206-213
209
that the departure from the initial contact geometry caused by the wear was not a problem. Further details concerning post-test slurry analyses can be found elsewhere [ 6].
6-
3.3. Roundness measurements 4-
3"
~0 L,~dtNI Fig. 4. Material removal rate as a function of load for 15 mm abresive particles. Speed of the conical counterface 3000 rpra; test duration 60 win; grinding fluid 3 wi oil.based diamond slurry.
until a certain critical load is reached. In the case ofthe slurry with 15 p.m particles, the critical load seems to he 800 N. Then, the transition is reached when the material removal rate increases two- or even three-fold. Once the transition is re~ched, the material removal rate is again a weak function of the load. Information concerning the influence of other abrasive particle sizes on the rate of material removal can be found elsewhere [61. 3.2. Microstructure studies
Microscopy studies were carried out m examine ball surfaces after the grinding experiment. Fig. 5 shows SEM micrographs of the ball surface after the grinding experiment in 15 p.m slurry and different loads. The first observation is that there are only few surface scratches typical for grinding. Instead, there are isolated spots covering the surface. This feature is characteristic for all loads at which grinding experiments were carried out. Because the spots appear to be isolated from each other a possible hypothesis is that they were created as a result of an indentation either by abrasive p,aticles rolling through the contact zone or by abrasive particles embedded into the cup and upper test piece over which ceramic balls relied. Thus the mechanism responsible for the material removal could be brittle fracture. Examination of cups and cones after the grinding experiments showed that diamond particles are indeed embedded in the surface layer of material. This, however, prevented excessive eear of the cup and cone and, therefore, no significant departure from an initial contact geometry occurred from test to test. Analyses ~f the abrasive slurry after the test were also carried o'at. As expected, it was found that the slurry contains not only silicon nitride particles but also diamond abrasive particles and pieces of stainless steel. Si£ :ificantly, the concentration of stainless steel particles was at its highest after the first two tests with the same cup. Subsequent tests with the same cup produced decreasing amount of stainless steel particles in the glurry. This supports the earlier observation
Results of roundness measurements are shown in Fig. 6. Roundness error represents here deviation from the spherical form. It can be interpreted as the greatest radial distance in any radial plane between a sphere circumscribed around the b~ll surface and any point on the ball surface. These results represent two series of measurements performed on a set of nine balls tested under various conditions of load and abrasive particles size. Averaged change of the roundn~s error with the size of abrasive particles used during grinding experiments carried out at the load of 200 N is given in Fig. 6(a). it is seen that for all abrasive pariicle sizes used a substantial reduction in the roundness error takes place. The most pronouneed decrease takes place for 6 p,m particles and is almost twice of that recorded for I g.m particles. However, the roundness error recorded for 15 p,m and 45 Ixm particles is greater than that found for small particles. The smallest decrease in the roundness error is obtained for 45 ixm particles, although the error is still ~wo times less than that for a blank ball. If the results of Fig. 6(a) are considered in the light of material removal rates [ 4 ] then it is seen that highest material removal rates at the load of 200 N were achieved for i5 ixm particles. Removal rates for 6 Ixm and 45 p.m are almost comparable although the roundness error in the case of 6 p.m particles is five times smaller than that for 45 p,m particles. The material removal rate for ! Ixm particles is ten times smaller than that for !5 p,m particles: however, the corresponding difference in roundness errors is much smaller. Under the load of 200 N the roundness error for I p m particles is only 50% smaller than that for 15 ~ m particles. The question is why the roundness error appears to he smallest for 6 g.m particles. The answer to this question must be sought in the light of the complex motion of balls during the grinding experiments, In order to achieve best possible roundness the balls must not only orbit the cup but also spin about their own axes of symmetry. It is clear then that in the case of larger abrasive particles ( 15 p,m and 45 p,m) the conditions were such that the balls could not orbit and spin freely all the time. hence the increase in the roundness error. If thi,~ argument is accepted O~enit is rather difficult to explain why the decrease in the roundness error is greater for 6 ~ m particles than that for 1 p.m particles. The only sensible explanation for this problem is in the fact that I p.m particles produced very small rates of material removal. It follows then that within the time scak: of grinding experiment (60 rain) I p,m particles were not able to produce roundness comparable to that achieved for 6 p.m particles. Fig. 6(b) gives the change of the roundness error with the load for abrasive particle size of 15 ~m. The results form a rather logical pattern. The largest r~duction in the roundness error is obtained for the lightest load applied. With increasing
210
T.A. Stolarski, S. Tobe lWear 205 (1997) 206-213
Fig 5. Appearanceof the ball surfacea~er grinding experiment: (a) 200, (b) 800, (c) 1200, (d) 1600, (e) 4000 N.
load the reduction in the roundness error decreases substantially and, eventually, the stage is reached where the error is comparable to that for a blank ball. At this load, the material removal rate is at its highest (see Fig. 4).
3.4. Residualstresses The results of residual stress measurements are shown in Fig. 7 in which are reported two series of measurements per-
T.A, Stolarski. $. Tobe I Wear 205 (1997) 206-213
Norroal load = 200 N
~l[-I
blank
I pm
40-
6 ra~ 15 lun Si7~eof abra.si~ ¢ pariicics
45 ~.m~
211
the relationship between the reduction in residual stress per linear unit of material removed and load. This significant reduction in the rate at which residual stresses decrease could he explained by the removal of a layer of material with the highest compressive residual stresses during the first round. The consequence of the above supposition is such that one would expect the level of residual stress to continue decreasing but at a much lower rate. The obvious question is about the level at wh;.ch residual stress stabilizes, if at all. The answer to this question requires a prolonged testing which was not the main objective of the research presented here but will be addressed in the near future.
A b t ~ i ~ c paatclc size = 15 I~m
4. Discussion of the results 30-
i 2o......... -~ N)?g+ .
I 0-
~
+~e,%+.,
+ ? :+'+' O"
blank
~
" 200N
8QON 12L~N L, hal dunng grinding
16~X)N
4~N
Fig. 6. Changein roundnesserrorresultingfromgrindingundercontrolled conditions: (top) experimentsunder constant load of 200 N: (bottom) experimentsunderconstantabrasiveparliclesizeof 15 p.m. formed on a set of nine balls subjected to grinding tests lasting 60 min in 15 p-m slurry and at different loads. Typically, a set of nine balls was tested at, for instance, 200 N and an average material removal rate was 2.6 p,m min- ~ (see Fig. 4). Because an experiment lasted 60 min, therefore, total material removal, expressed as a linearchange in the ball diameter, was 156 p-re. Then, residual stress level was measured and the same set of nine balls was subjected again to nominally the same grinding test. As before, the average material removal rate was recorded, the total material removal calculated, and the residual stress level measured. The same procedure was applied to all five different loads used. As a result of that two curves, illustrating the change in residual stress level on ball surface, were obtained. It is seen that after the first round of tests a very substantial decrease, in residual stress level occurred as compared to the residual stress level of 880 MPa recorded after the runningin. This decrease depends on the load; the higher the load the more significant the decrease in residual stress level. Moreover, there is, as expected, a close correlation between decrease in residual stress level and the amount of material removed from the ball surface. It is interesting to note that the second round of tests produced generally much smaller decreases in the level of residual stress as compared to the first round. For example, at the 200 N load, residual stresses were reduced by 3.64 MPa for every 1 p.m of material removed during the first round of testing. A corresponding result for the second round is 0.69 MPa p-m- +. Fig. 8 gives
Results of the studies presented here point to a clear relationship between the amount of material removed from the surface of a silicon nitride ball produced by the HIP method and the level of residual stress. A relatively high initial level of residual stress of 1200 MPa quickly falls to 8go MPa as a result of running-in of the blank ball. A typical rate of residual stress reduction is 3.2 MPa p.m-~. This 27% reduction in residual stress level could he attributed to the fact that the outermost layer of material with highest residual stress level was removed during running-in. Clearly, a substantial reduction in residual stress takes place during removal of the upper surface layer of material from the ceramic ball. Roundness measurements indicate that the best results are achievable for light loads applied. This is certainly true in the case of 15 Izm abrasive particles. Such a trend is clearly compatible with changes in the material removal mechanism taking place when the load is increased. The picture of the relationship between roundness and the size of abrasive particles is more complicated. The best roundness results are obtained for 6 p-m particles but not for I p.m particles--the smallest used. This only deviation from a clear trend can be explained in terms of relative lack of effectiveness of I p-m particles in material removal as well as from the material removal mechanism point of view. In the case of I p-m particles it is reasonable to expect a three-body abrasive material removal mechanism. These particles are small enough to roll through the contact region between the balls and the cup. For 6 p-m particles a two-body abrasive material removal mechanism could he postulated as these particles are too large to pass through the contact region and are embedded in the softer material of the cup. When even larger abrasive particles are used ( 15 p-m and 45 p,m) the two-body abrasive wear is replaced by a brittle fracture type of material removal mechanism and the roundness of the balls deteriotar.es significantly. Another interesting observation is that the rate of residual stress reduction is decreasing with increasing load. Fig. 8 shows that this trend is observed for both rounds of testing but it is more pronounced for the first round. The main reason for that seems to be rather weak dependence of the material removal rate on the load as indicated by Fig. 4. Above a certain load (in this case it is 1600 N) the material removal
212
T.A, Stola rxki. S. Tobe / Wear 205 (1997) 206-213 I000-
%"
800-
.~
SO0"
Norrn,al l~td = g00 N
Normal load= 200N
._e .~, ~
.
21g}"
200°
(a)
(b)
3
2
NOi'rnal Ic~tl= 16~I N
NotraalIt.d = 1200N
(c)
l
2
3
(d)
1
Normal load = 4 ~
2
3
N
=
(e)
t
2
3
Fig. 7. Change in resid',.ml stress in the surface layer of silicon nitride ball: ( I ) denotes initial level of residual stress, ( 2 ) denotes residual stress level after first round of testing, (3) denotes residual stress level after second mend of testing. (a) 200. (b) 800, (¢) 1200, (d) 1600, (e) 4000 N.
rate increases rather slowly. For instance, between 200 and 1600 N the material removal rate increases 0.225 p.m per 100 N increase in the load. In the range of loads from 1600 N to 4000 N the figure is 0.035 tzm per 100 N increase in the load. The reason for this decrease in material removal rate at higher loads is probably the change in operating wear mechanism. This is supported by the images shown in Fig. 5. At low load (200 N) one can see scratches typical for abrasive wear (Fig. 5(a) ) whilst at the high load (4000 N) signs of brittle fracture can be found (Fig. 5 (e) ). Therefore, the supposition that the effective material removal is produced by abrasion seems to be reasonable. Brittle fracture, although involving removal of relatively large fragments of material, is very much a localized process and does not result in an overall high material removal rate.
3,s-
~_
3a
2.52. I
1.5,
,,I-
b
~ 0.5'
t.end {NI Fig. 8. Rate of change in residual stress: (a) denotes first round of testing. (b) denotes second round of testing. The rate of change is exlnessed as a decrease in residual stress per I ttro of reduction in ball diameter.
T.A. Stolarski, $. Tobe / Wear 205 (1997) 206-213
It is rather premature to speculate on the likely pattern of residual stress decrease with the amount of material removed. As indicated earlier, a clear reduction in the residual stress decrease rate was observed during the second round of testing using the same set of balls. This observation points to the possibility of reaching a stable level of compressive stresses within the ball produced by the HIP method. There must he a core of material within the ball where there is no gradient of compressive stress. However, ful:her studies are required to fully support this rather speculative observation.
5. Conclusions Results and observations presented in this paper justify the following conclusions: • residual compressive stress in the silicon nitride ball produced with the HIP method decreases as a result of material removal from the ball surface; • roundness error depends on both the load applied and the size of abrasive particles used during grinding experi-
213
mcnts. The best results are obtained for light loads and small abrasive particles; • the relationship between the rate of residual stress decrease and the amount of material removed is not alinear function. The highest rates are achieved when the first layer of material is removed. Subsequent tests on the same ball result in a much reduced rate of residual stress decay. References [II R.T. Cundill. High precision silicon nitride balls for bearings, SPIE. 1573 ( 1991) 77-86. [2l C.C. Lawrence and T.A. Stolarski. Rolling contact wear of polymers. Wear. 132 (1989) 183--191. [3] G.H. Fmaki. P.H. Markho.and G. Meader, A study of fretting wear with particularreference to me~tgement of residualstres.cesby X-ray • lffracfion. Wear, 148 ( 1991) 249--260. [4] S. K0cJ~ma, g. Akita, i. Mi~wa and S. Tobe, A model for residual stressdistributionin X-rayfractogr~y, Proc. ICR53, ResidualStresses I!1. 1991, 1421-1428. [51 M. Hadfield.G. Fujinawa,T.A. Stolatsk~and S. Tobe, Residual in failed ceramicrolling-contactballs, Ceram. Int.. 9 (1993) 307-313. [6] T.A. Stolat,ski, E. Jisheng. D.T. G ~ aztdS. Panesax,The effect of load and abfaaivepanicle size oft the material removal gale of silicon nitridearlef~,'ts,Cerara. Int.. 21 (1995) 355-366.