NBSIR 74-569

Influence of Normal Alcohols on the

Abrasive Wear of Glass

M. Wiederhorn and D. E. Roberts

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

July 1974

Interim Report for Period July 1, 1973 through June 30, 1974

Prepared for Department of the Navy

Office of Naval Research

Arlington, Virginia 22217

NBSIR 74-569

INFLUENCE OF NORMAL ALCOHOLS ON THE

ABRASIVE WEAR OF GLASS

So Wiederhorn and D. Eo Roberts

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

July 1974

Interim Report for Period July 1, 1973 through June 30, 1974

Prepared for Department of the Navy

Office of Naval Research

Arlington, Virginia 22217

U. S. DEPARTMENT OF COMMERCE. Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

ABSTRACT

rJorraal alcohols x/ere used as cutting fluids in a study of friction and

wear of soda lime silicate glass against silicon carbide. Abrasion surfaces were bUU grit silicon carbide paper and a roughened plate of hotpressed silicon carbide. As the alcohol chain length increased, the

coefficient of friction decreased. This result was attributed in nart

to better lubrication of the abrading surfaces by the long chain length

alcohols. The v/ear rate and the coefficient of friction were strongly

dependent on pullout of carbide grains from the silicon carbide paper.

\-lear on fresh abrasive paper was independent of alcohol chain length. As

the paper becomes used, long chain length alcohols were more effective cutting fluids. On the silicon carbide plate, the wear rate decreased in a

non-linear fashion as the alcohol chain length increased. For all conditions,

the highest wear rate and coefficient of friction v/ere obtained in v/ater.

Data are explained in terms of classical theories of friction and wear.

i

1. Introduction :

Abrasive finishing is an important fabrication technique that is used

in the manufacture of optical glass and precision ceramic components. In

this technique, fine-grained particles of diamond, aluminum oxide, or silicon

carbide are abraded against the ceramic component to obtain desired dimensions

and surface finish. The technique is used to improve strength and optical

clarity of glass and to obtain the close tolerances necessary in applications

such as gas bearings, ceramic seals, roller bearings and spark plugs. Since

abrasive finishing is an expensive process (often accounting for 90 percent of

the cost of the finished product) there is a great economic incentive to improve

grinding efficiency.

One way of increasing grinding efficiency is by the development of superior

cutting fluids specifically designed to increase grinding rates. Fluid design

for comminution and drilling has been extensively studied and specific cutting fluids have been found to enhance these processes

.

"'' Recently, Westwood

and his colleagues found that the drilling rate of soda lime silicate could

be enhanced by a factor of -20 over that in water by using heptyl alcohol as

a cutting fluid'^, (figure Ic). Although other alcohols also enhanced the

drilling rate, the effect v/as not as dramatic as that obtained in heptyl

alcohol. Considering the effectiveness of heptyl alcohol as a drilling agent,

it was felt that this alcohol might be equally effective as a cutting fluid

1
in abrasive processes. Consequently, a study was conducted to determine

the role of normal alcohols on the abrasive wear of soda lime silicate

glass. The study included friction and wear measurements on silicon carbide

surfaces, a characterization of the wear surfaces by microscopic techniques,

and an analysis of the data in terms of theoretical models of friction and

wear. •

J^

"--- ' ^

'

'

"

2o Experimental Procedure :

>

The coefficient of friction was measured by using a "drag" type of

apparatus (figure 2) . One-quarter inch precision ground soda lime silicate

-5 -4

glass spheres (sphericity 2.5 x 10 inches; diameter ±10 inches) were mounted

rigidly in a brass holder that permitted one end of the sphere to slide on an

abrasive disk rotating at 100 rpm. The holder was mounted on a pivot arm

that supported a fixed mass (either 100 or 375 g) located directly over

the glass sphere. The other end of the pivot arm was attached to a load

cell that was used to measure the friction force. The abrasive surfaces

were 600 grit silicon carbide abrasive paper, or a hot pressed silicon

carbide plate that had been roughly ground using 45 micron diamond paste.

Lateral displacement of the pivot arm by means of a precision screw permitted multiple wear track studies to be conducted.

Rates of wear were determined from the size of the flat that formed

on the glass sphere during each wear experiment. The wear volume was

calculated from equation (1)

2

V = ^

IT (R - y^'R^-r^) (2R + /R^-r^) , (1)

where R is the sphere radius and r is the flat radius.

2
The wear rate (the volume, V, lost after sliding a distance, x, under a load,

L) was calculated from equation (2)

V/xL = V/L7TDtV, (2)

where D is the diameter of the wear track; L is the weight; v is the rotational

frequency of the disk (100 rpm in this study) ; and t is the abrasion time.

The radius of the wear flat was measured either from photomicrographs of

the wear surface, or by using a traveling microscope. VThen the wear flat

was elliptical rather than round (observed mainly on the abrasive papers) an

average of the maximum and minimum radius was used for the calculation.

The friction and wear studies were conducted by the following procedure:

a sphere was placed in the holder, the pivot arm was balanced, and a weight

was placed on the holder. The rotational speed of the disk was set, and the

abrasion surface was flooded with water or a normal alcohol before placing

the sphere on the rotating disk. The disk was continuously flooded during

the entire duration of each experiment (lasting from 30 to 300 seconds).

The alcohols used for the study ranged from raethyl alcohol (carbon chain

length, n, equals 1) to dodecyl alcohol (n = 12), with the exception of

nonyl (n = 9) and unedecyl (n = 11) alcohols. In addition, water alcohol

mixtures were studied for ethyl and butyl alcohol.

3. Experimental Results :

3.1 Friction Measurements—Silicon Carbide Paper

Results from the friction studies on the 600 grit silicon carbide

3
paper are shown in figure 3. The average value of the coefficient of friction

measured at the start of each experiment is given by the closed circles while

the value measured after 30 seconds of abrasion is given by open circles.

For all fluids studied, the coefficient of friction decreased during the

30 second abrasion period. The decrease appeared to be exponential, the coefficient of friction approaching a limit value after a long period of

abrasion. Both the initial and final values of the coefficient of friction

decreased linearly as the alcohol chain length increased. The value for water

was slightly above the projected lines for the alcohols. After 30 seconds

of abrasion, the coefficient of friction was approximately one-half of the

initial value, with the apparent exception of water which was two-thirds of

the Initial value.

3.2 Friction Measurements—Silicon Carbide Plate

The friction data obtained on the silicon carbide plate are summarized in figure 4. The average value of the coefficient of friction j y» is

given by closed circles for the start of each run and by open circles after

three minutes of abrasion. For the alcohols, the coefficient of friction

decreased during the abrasion period. The decrease appeared to be exponential

approaching a constant value for long periods of abrasion. The decrease

in the coefficient of friction during the three minute abrasion period ranged

from approximately 20 to 40 percent of the initial value depending on the

alcohol. In water, the coefficient of friction increased during the three

minute abrasion period. Here too the increase appeared to be exponential

approaching a constant value of long periods of abrasion. The increase was

typically 50 percent of the initial value during the three minute abrasion

period, and severe stick-slip was observed after approximately one minute

of abrasion.

4
In the alcohols, the coefficient of friction depended on the alcohol

chain length, generally decreasing with increasing chain length. However, as

shown in figure 4, the decrease was not linear. The initial value of the coefficient of friction decreased in an irregular manner as the chain length increased from y ~ 0.41 for methyl alcohol to y « 0.21 for dodecyl alcohol. After

three minutes of abrasion, the coefficient of friction still depended on alcohol

chain length, but the dependence was not as great as for the initial values.

In addition, values for ethyl and butyl alcohols were anomalously high.

The amount of water in the alcohols strongly influenced the friction

behavior. By adding water, 1 percent by volume, to either ethyl or butyl

alcohol, the friction behavior could be modified so that the coefficient of

friction neither decreased or increased with abrasion time. The addition of

more than one percent water resulted in a coefficient of friction that increased

with abrasion time, whereas less than 1 percent water resulted in a coefficient

of friction that decreased with time. The coefficient also depended on the

surface condition of the glass and the silicon carbide substrate as could be

demonstrated by changing either the wear track or the glass sphere during the

abrasion period. In either case, the coefficient of friction would change

back to its value at the start of the experiment.

3.3 Wear Measurements—Silicon Carbide Paper

The rate of wear of glass on silicon carbide abrasive paper (figure 5)

depended on the condition of the abrasive paper. As is evident by comparing

figures 5a and 5b, wear on a new track was always greater than wear on a used

track. Depending on the alcohol chain length, the wear rate on a used track

decreased by a factor of from 2 to 10. This decrease in the wear rate was

5
greatest for the short chain length alcohols and least for the water and

long chain length alcohols. As will be discussed in section 4.1, this decrease in wear rate results from grain pullout from the abrasive paper.

The effect of alcohol chain length on the wear rate is shown in figure 5a

"12 2 for fresh abrasive paper. The wear rate in water is 7.*^ x 10 m /I5; the

-12 2

wear rate in the alcohols (~4 x 10 m /W) is apparently independent of chain

length. By contrast, a strong dependence of wear rate on alcohol chain

length is obtained for multiple runs on a single wear track (figure 5b).

Now the wear rate in the short chain length alcohols is approximately

one third that found lor the long chain length alcohols, indicating that

the cutting effectiveness of the abrasive paper is maintained better in

the long chain length alcohols than in the short chain length alcohols.

However, the highest wear rates on a used track are obtained in water,

indicating that the cutting effectiveness of abrasive paper is best in

water.

3.4 Wear Measurements—Silicon Carbide Plate

The wear studies on the silicon carbide plate are depicted in

figure 6. In figure 6a, the wear track was changed every 10 seconds to

approximate a continuous exposure to a fresh wear surface, while in

figure 6b a single wear track was used for a period of three minutes.

Large differences are observed in these two types of studies. In the

multiple wear track study, the wear rate is higher and is strongly dependent

-12 2 on alcohol chain length. The wear rate in water (~2.1 x 10 m /N) is

6
greater than that observed in the alcohols. Ip addition, plateaus at

-12 2 "0.9 X 10 ra /N were observed for the short chain length alcohols (n = 2 -12 2

through 5) and at ~0.4 x 10 m /N for the long chain length alcohols

(n = 8 through 12) . A rapid change in the wear rate was observed between

chain lengths 5 and 8.

In the single wear track study (figure 6b) , the wear rate was

-12 2 again highest in water (~0.86 x 10 m /II). In methyl alcohol the wear rate

-12 2 (-0.45 X 10 m /N) was less than in water, but greater than in the remainder

of the alcohols. For chain lengths ranging from 2 to 10 the wear rate

-12 2 decreased linearly from ~0.24 x 10 m /N for ethyl alcohol (n = 2) to

-0.12 X lO""*"^ m^/M for decyl alcohol (n = 10).

3.5 Microscopic Wear Surfaces

In order to elucidate the mechanisms of wear, the wear surfaces

formed during the abrasion process were studied by optical and scanning

electron microscopy (SEM) . In these studies it was possible to estimate;

the type of surfaces formed; the number of contact points at the abrading

surfaces; the roughness of the abrading surfaces both before and after

abrasion; and the role of plastic flow and fracture in the wear process.

3.5.1 Silicon Carbide Papers

Examination of the fresh silicon carbide paper (figure 7a and 7c)

reveals angular carbide particles imbedded in a plastic matrix. Host of the

particles have sharp Cutting edges which should be effective in cutting the

glass if favorably oriented. During the abrasion process many of these

particles are torn free from the plastic matrix leaving behind holes in the

matrix (figure 7b and 7d) . This loss of cutting points depends on the

lubricant used in the abrasion process. More cutting points are lost when

7
short chain length alcohols are used than when long chain length alcohols

are used. Since the most effective cutting points are expected to be lost

during abrasion (because the force on these points is high) , this observation

qualitatively explains the decrease in the wear rate, and the dependence

of the wear rate on the alcohol chain length (figure 5b) for the used

tracks.

Optical studies of the wear surfaces are consistent with the

structure of the silicon carbide paper and the loss of cutting efficiency

as silicon carbide grains are torn from the paper. V/ear surfaces appear to

be formed mainly by a plastic cutting process. During the first few ffdllimeters of motion, plastic flow is indicated by sharply defined grooves left

in the surface of the glass (figure 8a). Sometimes these grooves are

associated with cracks that lay parallel to the grooves. However, most

often the grooves are crack free as evidenced by the fact that etching

the surface with HF gives no indication of general crack formation. Ap2 proximately 60 points (over an area ~0.8 to 3 mm ) were in contact with the

glass surface during abrasion. Thus, the average vertical load on the

carbide grains was ~1.7 g for the 100 g mass used in these experiments.

The groove depths, determined by an optical interferometry

study ranged from between --0.03 and 0.4um; the groove widths

ranged from less than IPm to ~'2ym. Assuming that the grooves were triangular in cross section, the angle between the wear flat and the groove

surface, 6, was estimated from a histogram of the ratio of the groove depth

8
to width. The mean value of 9 was ~18 degrees, which corresponds to

a cutting asperity angle of ~144 degrees. Although this asperity angle

3

is within the range measured by Mulhearn and Samuels, it is considerably

greater than that expected from the appearance of the SEM photomicrographs

in figure 7 or from the way abrasive papers are made (electrostatic fields

are used to align the particles so that sharp ends point out of the paper)

.

As a wear flat develops on the glass spheres, the

grooves that formed during the first few centimeters of motion

overlap, leaving behind a relatively rough surface (figure 9a) with grooves

(~0.5lJm deep) that are similar in appearance to those formed in metals by

abrasive paper. These grooves were formed by multiple passes of individual

cutting points over the glass since each large groove had a jagged cross

section consisting of several finer grooves. After 30 seconds of abrasion, a smoother surface is formed due to the loss of cutting points

from the abrasive paper (figure 9b).

3.5.2 Silicon Carbide Plate

Scanning electron micrographs of the silicon carbide plate

revealed a rough surface containing ~2ym grains (figure 10). As indicated

by a profilimeter trace, and by examination with an optical microscope, the

surface of the plate contained asperities that were separated by distances

that ranged from 50 to 200iJm. The cutting edges of the silicon carbide

grains did not seem to be as sharp as those of the paper. The

Because the resolution of the optical microscope is ~lym, 9 was determined

on scratches that were wider than lym.

9
asperity angle (measured from a profilimeter trace) ranged between 160

and 170 degrees (5° < 6 < 10°).

During the wear experiments, the contact asperities became.

coated with wear debris (figure 11). The amount of debris that

collected depended on the lubricant used for the study.

The greatest amount formed in water; lesser amounts formed in the low

chain length alcohols; and hardly any formed in the long chain length

alcohols. Although the debris could not be removed by scrubbing with a

brush, it could be easily removed by etching with HF or by stripping with

celluloid replicating film. After removal of the debris, wear flats could

be observed on the silicon carbide plate, indicating that even though the

2 hardness of silicon carbide (2500 Kg/mm ) is much greater than that of

2 4 glass (500 Kg/mm ) , mutual wear of the glass and silicon carbide had occurred. The wear flats were readily apparent for the short chain length

alcohols and water, but did not develop in the long chain length, alcohols.

! For multitrack experiments on the silicon carbide plate, a

change in wear mechanism is clearly indicated by a microscopic examination

of the wear surface (figure 12) . In water (figure 12a) , the entire surface

is covered by a network of small cracks and pits suggesting that material

removal is primarily by fracture. In low chain length alcohols (n ^ 5)

the surface is also covered with pits, but the density of pits is -not as

great as in water (figure 12b). This difference in pit density probably

accounts for the lower wear rate obtained in the alcohols. Wear is still

10
primarily by chipping in the alcohol, but not as many chips are formed.

In the long chain length alcohols, the wear surfaces are smooth

and contain only a few large pits (figures 12c and 12d) . xhe smooth appearance

of the wear surface suggests that the wear process has changed from one of chipping

to one of plastic flow. Between alcohols of chain lengths 5 and 7, the change

from a heavily pitted to a relatively smooth surface corresponded to the

observed decrease in wear rate (figure 6a).

For wear studies on a single track, microscopic examination of the

glass surface indicates that the initial stage of wear is by plastic flow, regardless of the test environment. During the first 3 mm of gliding (figure 8b) , shallow grooves v/ere left in the glass sphere. The grooves ranged between 1 and 5|im

in width, and between 0.03 and 0.3Uin in depth. An asperity angle of approximately 175 degrees (6 2.6°) was estimated from measurements of the ratio of

groove depth to width. The number of asperities contacting the glass surface

was ~60 for the silicon carbide plate (the same as for the abrasive paper); the

average load on each point was "6 g for the 375 g mass used in these studies.

The grooves formed in the glass by the plate (figure 8b) were not as smooth as

those formed by the paper. The groove structure resulted from the fact that

Water dissolved in the low chain length alcohols may play an important role in

the chipping process since the addition of only 1 volume percent alcohol to either

ethyl or butyl alcohol results in a wear surface that is indistinguishable from

that formed in water.

11
each asperity contained several grains each of which left a mark in the

glass. Etching of these shallow grooves gave no indication of crack formation during the early stage of the wear process. After about 1 cm of

sliding, the individual grooves overlap to produce a relatively smooth

wear surface. In water , fracture commences after approximately 3 cm of

sliding (figure 13a), and after the glass sphere has traveled 10 cm on the

silicon carbide block, the wear process is almost entirely by chipping.

As the wear process continues (on the same track) the surface pits fill up

with glass debris (figure 13b) which can be easily removed by an etch of

~5 sec in 15% HF. After 2 to 3 minutes of abrasion, wear debris cushions

the glass sphere preventing full contact between the glass sphere and the

silicon carbide plate. The wear mechanism then changes back to a plastic

flow process, resulting in a smooth wear surface (figure 13c) that is largely

unaffected by an HF etch (figure 13d). In the short chain length alcohols,

the development of the wear surface follows a course similar to that of

water.

In the long chain length alcohols the wear process occurs

by plastic flow in both the single and multitrack experiments. Glass wear

surfaces are smooth (figure 14) and surface pitting, due to fracture, occurs

only at isolated points on the wear surface. Plastic flow as a mechanism

of wear is also indicated by the fact that etching in HF gives no evidence

of crack formation on the wear surface.

4. Discussion of Results

4.1 Comparison of Results with those of Westwood et al.

The initial intent of this work was to test normal alcohols as

12
cutting fluids for the abrasive wear of glass on silicon carbide in hopes of

finding a superior fluid for this process. These hopes were based on

earlier work by Westwood and his coworkers who found that drilling rates

in soda lime silicate glass with diamond drills could be increased by a

2

factor of -20 by use of heptyl alcohol as a drilling fluid. The

increased drilling rate was attributed to the effect of this alcohol on the

surface charge of the glass. In heptyl alcohol, the surface charge of the

glass is zero because glass has a zero zeta potential in this alcohol,^

(figure la). Westwood and coworkers proposed a mechanism (involving alkali

ion migration) by which glass attains a maximum hardness in environments of

zero surface charge. Hence drilling in heptyl alcohol occurs more readily

because the glass is harder and is more easily fractured under pressure of

the drill. In water or other alcohols that do not give a zero zeta potential

for the glass, the drilling rate is lower because the glass is softer and does

not fracture so easily. In support of their theory, Westwood and coworkers

showed that the hardness of glass reached a maximum in heptyl alcohol (figure

2 lb, hardness was measured on a pendulum sclerometer) . Evidence of a minimum

in the coefficient of friction in heptyl alcohol (figure Id, sapphire sliding

on soda line silicate glass) was also presented to support their theory. The

minimum in the coefficient of friction for heptyl alcohol was attributed to

increased hardness and consequently to reduced plastic flow during sliding.

The results of the present paper do not display the same anomalous

behavior observed by Westwood and coworkers. Friction on the silicon carbide

This behavior should be reversed if drilling occurs by a flow rather than by

a fracture mechanism.^ Now, the softer material gives a higher drilling rate.

13
paper dropped continuously as the alcohol chain length was increased.

Friction on the silicon carbide block showed anomalous behavior for the

short chain length alcohols, but not for heptyl alcohol. Similarly, wear

studies showed no anomalous behavior for heptyl alcohol, with the possible

exception of the multitrack studies (figure 6a) , which showed a decrease

in the wear rate at heptyl alcohol. This decrease in the wear rate probably

cannot be explained solely by the effect of alcohol on the hardness of glass

because of the accompanying change in wear mechanism from fracture to plastic

flow. These differences in behavior suggest that the friction and wear

mechanisms proposed by Westwood and his coworkers to explain their data

are not directly applicable to the present set of data. Therefore this

- data will be discussed with reference to other more classical theories of

friction and wear.

4,2 Friction of Glass on Silicon Carbide

-

' The friction results of this paper can be explained by the theories

9 proposed by Bowden and Tabor who expressed the frictional force, F, betweentwo bodies as the sum of a shearing force, F , and a ploughing force, F . s p

The shearing force results from adhesive contact between the two abrading

surfaces. To break the adhesive bonds, the softer of the two materials must

* 8

A similar observation has been made by Swain et al. on polycrystalline aluminum

oxide. However, here a minimum in the grinding rate in aqueous solutions and normal

alcohols was correlated v/ith a zero zeta potential in these solutions. This minimum

contrasted with a maximum in the drilling rate for the same solutions. Results

were interpreted in terms of adsorption dependent flow and fracture of alumina.

Differences in the type of surface formed by abrasion were also reported in this

work.

14
flow plastically when relative motion occurs between the two surfaces.

Thus, the shearing force is determined mainly by the plastic shear

strength, CT^ , of the softer of the two materials. The frictional

force, F , can be estimated from this shear stress and the hardness of

s 4

the softer material. The area of real contact between the two materials,

A, is given by A = L/p, where L is the vertical load and p is the hardness

of the softer material. The shearing force is given by = cr^A = O^L/p,

Therefore, the shearing contribution to the coefficient of friction, y , is

given by 5 F^/L = c^^/p, which for many materials equals 1/6. In the

absence of lubrication, the coefficient of friction is usually larger than

this theoretical value because the real area of contact is larger than L/p

once sliding occurs. For very smooth surfaces, the real contact area between the sliding bodies can be large enough to give values of the coefficient

of friction as high as 10. In the presence of lubricants, the value of the

coefficient of friction can be much less than the theoretical value because

lubricants reduce the real area of contact between the sliding surfaces.

Plowing contributes to the coefficient of friction when the harder

of the two sliding surfaces is rough. Then asperities of the harder surface

dig into the softer one and additional force is required to move the

asperities through the soft material. This force depends on the "sharpness"

of. the asperities of the harder material. By assuming that the asperities

can be modelled by a right angle cone having an angle 9 at its base,

4 Rabinowicz has shown that the plowing contribution to the coefficient of

friction is given by y = (l/fr) tan 9. More generally y = (1/k) tan 9,

P P where k is a form factor that depends on the shape of the asperities.

15
k ranges from ~l/2 for triangular pyramids to it for right angle cones.

9

Bowden and Tabor have noted that the plowing term does not in general depend

on the presence of lubricants at the wear surface, but does depend on the

geometry of the asperities. By combining the shear and plowing components,

the following equation is obtained for the coefficient of friction:

y = a^/p + (1/k) tan 9 (3)

This equation v/ill be used to provide a semiquantitative explanation of the

results of the present paper.

On silicon carbide paper, friction measurements are initially high

because sharp asperities (high 6) contribute a large plowing component to the

friction force. During the wear process, the sharpest grains are torn from

the paper, and the friction coefficient decreases because duller grains are

left behind (thus reducing 0) . This explanation is consistent with the

appearance of the wear surfaces of both the silicon carbide paper and the

glass spheres. KTien sliding starts on a fresh wear track, the coefficient of

friction is probably due to the combined effect of shearing and plowing. If

the decrease in the coefficient of 0.2 (figure 3) can be attributed to a

decrease to zero of the plowing component of the coefficient of friction, then

(assuming k = 1) tan 6 is calculated to be ~0.2 (9-12°) which is within the

3

range estimated in this paper and in the work by Mulhearn and Samuels and

Goddard and Wilman."^^ '

In a detailed analysis of the effect of shape of indenting particle on the

coefficient of friction, Goddard and Wilman^^ showed that the mean value of 1/k

for triangular and square pyramids was ~1. Although their analysis provides a much more complete equation for the coefficient of friction, equation (3) is

adequate for discussing the results of this paper.

16
Coefficients of friction measured on the hot pressed silicon carbide

plate are less than those measured on the abrasive paper because the contact

asperities of the plate are not as large as those of the paper. If the decrease

in the coefficient of friction (~0.07 for the long chain length alcohols in

figure 4) is attributed to plowing, then tan 9 - 0.07 (6 - 4°). This estimate

of tan 9 lies between that estimated from the profilimeter study (tan 9 - 0.13)

and that estimated from wear tracks on the glass (tan 9 ^ 0.046).

The decrease in the coefficient of friction with increasing alcohol

chain length is probably due to lubrication of the wear surfaces by the alcohols.

As the alcohol chain length increases , the alcohols cover the wear surfaces more

effectively, thus reducing the area of real contact between the surfaces. Since

the long chain length alcohols should be more effective lubricants than the

short chain length alcohols, the coefficient of friction is expected to decrease

as the alcohol chain length increases. This decrease in friction coefficient

11 was observed in the present study, in the study by Hardy and Doubleday, and in

the work by Macmillan et al. , confirming the general effect of alcohol lubrication.

In the work by Hardy and Doubleday^^ the coefficient of friction was

measured using polished glass sliders (watch glasses) on polished glass plates.

The coefficient of friction decreased from y=:0.68 in methyl alcohol to u=:0.45 in

* a This estimate of tan o depends on the value selected for k. A variation of k

from -1/2 to ~3 results in a variation in the estimate of tan 9 from 0.1 (9 - 6°)

to 0.6 (9 - 30°). Since k = 1 gives a value of tan 9 that is consistent with

3 10 other measurements of tan 6, ' this value will be assumed for the remainder

of the paper.

17
unedecyl alcohol. This decrease of ~.23 is similar to the decreases measured

in the present study (-0.16 for SiC paper on a new track; -0.13 for SiC paper

after 30 sec wear; -0.21 SiC block in a multitrack experiment) suggesting that

the relative effect of alcohol lubrication is the same for both studies. The

large values of the friction coefficient measured by Hardy and Doubleday may

be due, in part, to the large areas of real contact expected for the smooth surfaces used in their experiments. In the work by Macmillan et al.,^ a sharp minimum in the coefficient of friction at n = 7, was superimposed on the general decrease in friction coefficient (figure Id). In their study the coefficient of

friction decreased from y~0.75 for methyl alcohol to y=0.16 for dodecyl alcohol,

with a value of y^O.lS for heptyl alcohol. The total change in the friction

coefficient, ~0.69, was approximately three times that observed in the present

work or in the work by Hardy and Doubleday. Furthermore, the sharp minimum

observed by Hacmillan et al. for heptyl alcohol was not observed in the present

work (Hardy and Doubleday did not use heptyl alcohol in their studies) . The

reasons for these differences in experimental results is not readily apparent,

but may involve differences in the nature of the glass surfaces studied

(fracture versus abraded surfaces) , or differences in experimental technique

(sapphire sliding on glass versus glass sliding on silicon carbide).

The results of the present study also indicate a dependence of the

coefficient of friction on the condition of the wear surface. For silicon

carbide paper, pullout of the abrasive particles from the paper results in a

large decrease of the coefficient of friction. This decrease is most pronounced in the short chain length alcohols which probably attack the adhesive

bond between the silicon carbide particles and the abrasive paper. For the

18
silicon carbide plate, glass residue accumulation at the cutting asperities

and blunting of the cutting asperities during the abrasive process have a

significant influence on the coefficient of friction. The effect of glass

residue (probably in the form of a gel) is most apparent for the tests conducted in water. Here, the residue probably acted as a buffer between the

glass and the silicon carbide cutting points. The residue must have been

extremely viscous to account for the ~20 percent increase in the coefficient

of friction. In alcohols, the residue did not affect the friction coefficient

in the same way, suggesting a fundamentally different behavior of the friction

process. In alcohols, the coefficient of friction seems to be affected more

by the development of wear flats on the silicon carbide plate than by the accumulation of glass residue.

4.3 Wear of Glass on Silicon Carbide

4.3.1 Mechanisms of Wear

3 4

As discussed by Mulhearn and Samuels and by Rabinowicz,

abrasive wear of materials depends on the presence of asperities on the

abrading surface and on the so-called "attack angle" between the asperities

and the wear surface. Abrasive wear occurs when asperities dig into the

wear surface, plowing the surface and removing material during sliding.

As with friction, the amount of plowing depends on the "sharpness" of the

contact asperities, sharper asperities resulting in a greater amount of wear.

However, the angle between the asperities and the wear surface is also

important. If the attack angle is large, then the wear rate is high because

the contact asperities can cut material away from the wear surface in the

same way that a cutting tool peels metals in lathe operations. If the attack

angle is not large, then the contact asperities merely displace material on

19
the wear surface, and wear rates are not as high. Mulhearn and Samuels

have estimated 90 degrees as the critical attack angle for material removal.

These ideas are expressed quantitatively by the following equation for

the volume, V, lost after sliding a distance, x, under a load, L:

: ^ V/xL = (f tan e)/(pk) (4)

f is the fraction of points that participate in the cutting

process; 6 is the angle between the wear surface and the cutting points;

p is the hardness; and k is a factor that depends on the geometry of the

cutting asperities. As with the plowing component of the coefficient of

friction, k ranges in value from 1/2 for triangular pyramids to tt for right

angle cones.

VJear may also occur by mechanisms other than abrasion, including

adhesion, corrosion, and fretting. In adhesive wear, material is lost from

the wear surface by plastic flow which results when the two sliding surfaces

adhere at contact points. The wear mechanism is identical to that which

accounts for the shearing component of the coefficient of friction.

4 Rabinowicz has derived the following equation for adhesive wear:

V/xL = a/3p (5)

where a is the wear coefficient. Corrosive wear occurs when a corrosive

species attacks the wear surface leaving behind corrosion products that are

removed when the two surfaces rub together. Corrosive wear is believed to

12

be a dominant feature in the preparation of polished glass surfaces. Wear

12 by fretting involves the formation of cracks on the wear surface. Wear

20
occurs by the propagation of these cracks so that wear particles are chipped

from the wear surface.

4.3.2 Abrasive Paper

Abrasive wear was the dominant mechanism for material removal by

the abrasive papers. As with metals, material removal was primarily by plastic

cutting. The wear rate can be estimated from the theory derived by Mulhearn

3 and Samuels (equation 4) . In experiments on cold-drawn steel using a 500 gram

mass, these investigators found that the proportion of abrasive particles, f,

that cue the metal was 0.13 for fresh paper and 0.05 after 50 traverses on an

8 cm radius. The value of p for soda lime silicate glass is ~5 x 10^ N/m^

2 (500 Kg/mm ). For k - 1, f - 0.1 and tan 6 = 0.33, a value of 6.6 x lO""*-^ 2 m /N is calculated for the wear rate, V/xL. This value is close to those

measured in the current study (4-5 x lO""*"^ m^/ll for the 30 second wear study

m — 12 2 the alcohols and 7.9 x 10 m /N for the 30 second wear study in water).

Considering the approximations necessary for this calculation (for f, k and

tan 0) the agreement between the predicted and measured values of the wear

rate is satisfactory.

In the long chain length alcohols, wear on the silicon carbide

plate was approximately 1/10 that on the silicon carbide paper. This decrease

in the wear rate can be attributed to the relatively blunt asperities that

are present on the silicon carbide piate As calculated from groove

The conditions used by Ilulhearn and Samuels were not too different from those

used in this study.

21
measurements, tan 6 for the silicon carbide plate (tan 0 - 0.046) was approximately a factor of 10 less than tan 9 for the silicon

carbide paper (tan 9 - 0.33). Assuming that the fraction of cutting points

-12 2

for the two wear surfaces was the same, a wear rate of ~0.92 x 10 m /IT

is calculated from equation 4. This value is approximately twice that

' * -12 2 measured for the long chain length alcohols, ~0.4 x 10 m /N.

. In the short chain length alcohols, wear on the silicon carbide

plate occurred mainly by fracture. The driving forces for the fracture arose

from adhesive shear forces on the glass sphere. Numerous earlier studies

have shown that, in glass, Hertzian type cracks form as a result of these

12 shear forces. Linking up of these cracks during the abrasion process accounts for the pits observed on the glass wear surface. Since water plays a

13 major role in controlling crack growth in glass, it is probable that the

amount of water in the alcohol governed fracture during abrasion. Furthermore,

since water is relatively soluble in the short chain length alcohols and is

easily absorbed from the air, fracture is also expected as the major wear mechanism in the shore chain length alcohols. In the long chain length alcohols,

fracture is not expected because water is relatively insoluble in these alcohols. Thus, the change in wear mechanism between amyl and heptyl alcohol can

be qualitatively attributed to the amount of water in the alcohols. The detailed mechanism of crack formation and propagation during abrasion (the number

of cracks, size, rate of formation) will not be covered in this paper.

*The estimated value of the wear rate is high probably because the estimated

fraction of cutting points is high. One would expect the fraction of effective

cutting points on the plate to be less than that on the abrasive paper. Agreement between theory and experiment would require that f - 0.05 for the plate.

22
5.. Summary ;

This paper presents the results of a study of the friction and wear

of soda lime silicate glass on silicon carbide. Water and normal alcohols

were used in these studies to determine the relative effectiveness of these

liquids as cutting fluids. Wear surfaces studied were 600 grit silicon

carbide abrasive paper and a hot-pressed SiC plate that had been ground on

45 micron diamond paste. Although the results are complicated, they can be

interpreted in terms of the classical theories of friction and wear.

As expected from lubrication theory, the coefficient of friction for

both the paper and the plate decreased as the chain length of the normal

alcohol increased. Both plowing and adhesion contributed to the coefficient

of friction. Plowing assumed a greater importance for silicon carbide abrasive paper than for the silicon carbide plate because of the sharp, irregular

particles contained in the paper. Generally, as the abrasion time increased,

the coefficient of friction decreased for both the paper and the plate. This

result was attributed to particle pullout for the abrasive paper and to

blunting of the contact asperities of the silicon carbide plate.

The wear rate data on the abrasive paper could be explained by current

theories of abrasive wear. A theoretical estimate of the wear rate of glass

on fresh abrasive paper was in close agreement with experimental measurements

of the wear rate. The wear rate decreased as the abrasive paper was used

because of particle pullout from the paper. This result was most apparent

for the short chain length alcohols.

23
For the silicon carbide plate, two mechanisms of wear were observed:

chipping for the short chain alcohols; and plastic flow for the long chain

alcohols. The chipping was influenced by the presence of water in the

alcohols since the deliberate addition of small amounts of water greatly

enhanced the amount of chipping. The wear rate was also influenced by the

accumulation of wear debris at the contact asperities of the silicon carbide

plate and by blunting of these asperities as a result of the wear process.

In the present study, water was found to be the most effective cutting

fluid for the abrasive wear of glass, regardless of test condition. This

result differs from an earlier one by Westwood and his colleagues who found

that heptyl alcohol was the most effective cutting fluid for drilling soda

lime silicate glass. Other aspects of the present set of data also differ

from those of Westwood et al. These differences suggest that the wear

mechanisms proposed by Westwood et al. are not applicable to the present

set of data. By contrast, the more classical theories of friction and

wear are applicable because the present set of data can be readily interpreted

in terms of these theories. i- .

24
References

1. P. Somasundaran and I. J. Lin, "Effect of Nature of Environment on

Comminution Processes," Ind. Eng. Chem. Process Des. Develop. 11,

321-31 (1972). _ '

2. A. R. C. Westwood, G. H. Parr, Jr., and R. M. Latanison, Amorphous

Materials , John Wiley and Sons, New York, p 153 (1972).

3. T. 0. Mulhearn and L. E. Samuels, "The Abrasion of Metals: A Model

of the Process," Wear 2. 478-98 (1962).

4. E. Rabinowicz, Friction and Wear of Materials , John Wiley and Sons,

New York (1965).

5. A. R. C. Westwood and N. H. Macmillan, The Science of Hardness Testing

Am. Soc. Metals, Cleveland, Ohio, in press (1973).

6. N. H. Macmillan, R. D. Huntington and A. R. C. Westwood, "Chemomechanical

Control of Sliding Friction Behavipr in Nonmetals," J. Mat. Sci. 9_,

697-706 (1974).

7. A. R. C. Westwood and R. M. Latanison, pp 141-153, in The Science of

Ceramic Machining and Surface Finishing, NBS Special Publication 348,

llay 1972.

8. M. V. Swain, R. M. Latanison and A. R. C. Westwood, "EnvironmentSensitive Hardness and Machinability of Alumina," to be presented at

the NSF Hard Materials Workshop, July 1974.

9. F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids,

Part I, Oxford at the Clarendon Press (1950).

25
10. J. Goddard and H. Wilman, "A Theory of Friction and Wear During the

Abrasion of Metals," Wear 5, 114-135 (1962).

11. W. B. Hardy and I. Doubleday, "Boundary Lubrication, The Paraffin Series,"

AlOO, 550 (1922).

12. L. Holland, The Properties of Glass Surfaces, Ch. 1, John Wiley and

Sons, Inc., New York (1964).

13. S. M. Wiederhom, pp. 731-742, in Corrosion Fatigue: Chemistry,

Mechanics and Microstructure, National Association of Corrosion Engineers,

Houston, Texas (1972).

26
Figure Captions

Figure 1. Variation of (a) C potential, (b) pendulum hardness, (c) rate

of penetration with diamond studded bit,^ and (d) coefficient of

sliding friction^ on soda lime silicate glass in water, toluene

and n-alcohol environments. Uq is the number of carbon atoms in

the n-alcohol molecule.

Figure 2,

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.

Figure 12.

Figure 13,

Figure 14,

Friction and wear apparatus

Friction measurements on silicon carbide abrasive paper (600 grit)

Friction measurements on the silicon carbide plate

Wear rate on silicon carbide abrasive paper (600 grit)

Wear rate on silicon carbide plate

Scanning electron micrographs of abrasive paper. (a) and (c) are new papers; (b) and (d) are wear tracks formed in methyl alcohol.

The black areas of (b) and (d) are due to pullout of the silicon

carbide grains.

Wear surfaces formed during the first few millimeters of sliding

(a) abrasive paper and (b) silicon carbide plate

Development of the wear surface during abrasion on silicon carbide

abrasive paper. (a) rough surface formed after ~20 cm of sliding;

(b) smoother surface formed after ~1000 cm of sliding

Scanning electron micrograph of silicon carbide plate

Wear debris formed at the cutting asperities on the silicon carbide

plate (ethyl alcohol as lubricant)

Wear surface of the glass spheres formed by abrasion against the

silicon carbide plate. Track changed every 10 seconds to approximate continuous wear. (a) water, (b) butyl alcohol, (c) heptyl

alcohol, and (d) dodecyl alcohol.

Wear surface formed by abrading the soda lime silicate spheres

against the silicon carbide plate in water. (a) after ~3 cm of

sliding, (b) after ~100 cm of sliding (3 seconds), (c) after ~6000 cm of

sliding (5 minutes), (d) same as (c), but etched with H.F.

Wear surface formed by abrading the soda lime silicate glass spheres

against the silicon carbide plate in decyl alcohol. (a) after ~6 cm

of sliding, (b) after ~100 cm of sliding (3 seconds) , (c) after ~6000

cm of sliding (5 minutes), (d) same as (c) , but etched with H.F. The

large curved crack in (c) and (d) was not formed by the abrasion process.

Cracks of this sort were frequently observed in the as-received spheres.

27
Water loiuene

5 2 , . '^i<3'-ire 1. Variation of (a) C potential, (b) pandulura hardness, (c) rate

of penetration with diamond studded bit,^ and (d) coefficient of

sliding friction^ on soda line silicate glass in water, toluene

/ and n-alcoaol environments. Hq is the number pf carbon atoms in

the n-alcohol molecule.

23
LOAD

Figure 2. Friction and wear apparatus
1—I—I—I—I—I—I—I—I

\ \ I r

SILICON CARBIDE ABRASIVE PAPER

0.7 I— • INITIAL VALUE

0.6

^ 05

0.4

SE 0.3

0.2

01

o AFTER 30 sec

I

J

I 1I I

I I I I ,1 I I I I H2O 2 4 6 8 10 12 ALCOLHOL CHAIN LENGTH

Figure 3. Friction measurements on silicon carbide abrasive paper (600 grit)

30
(

r 1 r

0.8

0.7

0.6

^ 0.5

e 0.4

I—

0.3

0.2

0.1

SILICON CARBIDE PLATE

• INITIAL VALUE o AFTER 3 MINUTES

' I I

J

5 5 5

I I I J \ \ L J L_L

H2O 2 4 6 8 10 12 ALCOHOL CHAIN LENGTH

Figure 4. Friction measurements on the silicon carbide plate

31
1—I—

r

10 H

9

8

7

6

5

—1

1 1 1 1

1 1 1—

SILICON CARBIDE ABRASIVE PAPER'

(a) NEW TRACK

'i- 4 la

<LU

I

J I I I J I I i I 1 I I I I rn I r~

(b) USED TRACK

5

H2O 2 4 6 8 10 ALCOLHOL CHAIN LENGTH

1—

r

I I

I I I I I I I I I I 12

Figure 5. Wear rate on silicon carbide abrasive paper (600 grit)

32
2.5

"I—I—I—I—I—I—I—I—I—I—I—I—

2.0

SILICON CARBIDE PLATE

(a) MULTIPLE TRACK

1.5

1.0

0.5

1.0

0.8

0.6 -

04

0.2 —

V1 1

J \ I I L J L

"1 1 1—I—I

1\ 1

|b) SINGLE TRACK

1 r

1 5

I I I 4- hi i + I 4- J_J I I

I L

H20 2 4 6 8 10 12 ALCOHOL CHAIN LENGTH

Figure 6. Wear rate on silicon carbide plate

33
Figure 7. Scanning electron micrographs of abrasive paper. (a) and (c) are

new papers; (b) and (d) are wear tracks formed in methyl alcohol.

The black areas of (b) and (d) are due to pullout of the silicon

carbide grains.

34
35
Figure 9. Development of wear surface during abrasion on silicon carbide

abrasive paper. (a) rough surface formed after "20 cm of sliding;

(b) smoother surface formed after ~1000 cm of sliding

9

36
Figure 10. Scanning electron micrograph of silicon carbide plate

37
Figure 11. Wear debris formed at the cutting asperities on the silicon carbide

plate (ethyl alcohol as lubricant)

38
.,,„re U. wear surface of f-^^^^^r.TJ:^^'^ toZTl^lTZ^Z.

alcohol, and (d) dodecyl alcohol.

39
13. Wear surface formed by abrading the soda lime silicate spheres

against the silicon carbide plate in water. (a) after ~3 cm of

sliding, (b) after ~100 cm of sliding (3 seconds), (c) after ~6000 cm of

sliding (5 minutes), (d) same as (c) , but etched with H.F.

40
Figure 14. Wear surface formed by abrading the soda lime silicate glass spheres

against the silicon carbide plate in decyl alcohol. (a) after ~6 cm

of sliding, (b) after ~100 cm of sliding (3 seconds) , (c) after ~6000 cm of sliding (5 minutes), (d) same as (c) , but etched with H.F. The

large curved crack in (c) and (d) was not formed by the abrasion process.

Cracks of this sort were frequently observed in the as-received spheres.

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NBS-1 14A IREV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. I'lMMK A 1 ION OH KI-POR T NO. MBSIR 74-569

2. (iov't Accession

No.

3. Ri i ipunt's Accession No.

4. Ti ri.l-: AND SUH'l l l l.l' Influence of ilormal Alcohols on the Abrasive Wear of Glass

5. I'uhluaiion IXite Julv vJ cxj,y 1974 X./ / ^

6. Performing Organization ( ode

7. AU rilOK(S) S. M. Wiederhorn and D. E. Roberts

8. Performing Org.m. Report No. NBSIR 74-569

9. P[£RI-ORMINC; ORGANIZATION NAME AND ADDRKSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Proiect/Task/Work Unit No. 3130453

11. ( ontract /Orant No. NR 032-535

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Office of ilaval Research

Code 471

800 N. Quincy St.

Arlington, Virginia 22217

13. Type of Report & Period Covered Interim

7-1-73 thru 6-30-74

14. Sponsoring Agency f ode

15. SUPPI.i: MF.NTARY NOTIiS

To be published in Wear

16. AUSTRAC 1 (A 200-word or less factual summary of most si^ifleant information. If document includes a significant

bibliography or literature survey, mention it here.) Normal alcohols were used as lubricants in a study of friction and wear of soda lime

silicate glass against silicon carbide. Abrasion surfaces were 600 grit silicon carbide paper and a roughened plate of hot-pressed silicon carbide. Generally, as the

abrasion time or the alcohol chain length increased, the coefficient of friction

decreased. This result was attributed to better lubrication of the abrading surfaces

by the long chain length alcohols. The wear rate and the coefficient of friction

were strongly dependent on pullout of carbide grains from the silicon carbide paper.

Wear on fresh abrasive paper was independent of alcohol chain length. As the paper

becomes used, long chain length alcohols were more effective cutting fluids. On the

silicon carbide plate, the wear rate decreased in a non-linear fashion as the alcohol

chain length increased. For all conditions, the highest wear rate and coefficient of

friction were obtained in water. Data are explained in terms of classical theories

of friction and wear.

17. KKY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Alcohol; glass; mechanism; silicon carbide; wear.

18. AVAILABILITY Q_ Unlimited

For Official Distribution. Do Not Release to NTIS

\^~} Order From Sup. of Doc, U.S. Government Printing Office Washington, D.f;. 20402, SD Cat. No. C13

~l Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIF IKD

21. NO. OF PAGES

48

22. Pr

USCOMM-DC 29042-P74