Improved lapping slurries provide for easier and more thorough cleaning of alumina workpieces, as well as inhibit corrosion of the lapping table and provide for easier cleaning of the lapping equipment. The unthickened lapping slurry comprises abrasive grains such as diamond abrasive dispersed in a carrier comprising water, glycerine, and triethanolamine. The thickened lapping slurry comprises abrasive grains such as diamond abrasive dispersed in a carrier comprising water, glycerine, triethanolamine, a water soluble silicate, and acid.

FIELD OF THE INVENTION 
The invention described herein relates generally to lapping compounds and 
more particularly to lapping slurries, compounds or gels which are used in 
industrial, production applications for eliminating or minimizing residues 
on the workpieces and lapping equipment. 
BACKGROUND OF THE INVENTION 
Lapping is a well-known process of abrasion metal-removal or machining for 
smoothing or polishing surfaces to a high degree of refinement or accuracy 
using a loose abrasive lapping compound. The lapping compound is often in 
a liquid suspension or semi-liquid form, and is called lapping slurry. 
The present invention arose out of an effort to improve the cleanliness of 
parts that had been lapped to certain thickness and roughness 
specifications, and subsequently ultrasonically cleaned. In at least one 
instance ceramic wafers of alumina substrates that had been ultrasonically 
cleaned subsequently had metallized microcircuits applied for use as 
automobile sensors. Because of suspected surface contamination from the 
lapping operation, the microcircuits were not adhering properly to the 
contaminated ceramic substrates. Because it was suspected that residual 
lapping swarf or lapping carrier residue was interfering with the 
adherence of the microcircuit, efforts were initiated to more completely 
clean the workpieces after lapping. However, additional cleaning efforts 
were not entirely successful in eliminating the problem, and it was then 
determined that some contaminant adhesive was causing the lapping swarf to 
adhere to the workpieces. It was later determined that the lapping slurry 
in use at the time was a major contributor to the contamination problem. 
The slurry in use consisted of water, triethanolamine, 15-25 um diamond, 
and xanthan gum. 
Xanthan gum is a commonly-used thixotropic component that suspends the 
diamond abrasive until it arrives on the lapping table. Shear forces then 
thin the slurry allowing the diamond to drop out onto the ceramic parts. 
It was found that under the shear forces, the xanthan gum formed into 
balls which were subsequently smeared onto the ceramic surfaces during the 
lapping process. This smeared gum acted as an adhesive for the particulate 
removed from the ceramic surfaces, making cleaning very difficult. 
The lapping slurry in use also contributed to other operating problems 
regarding lapping machine cleanup. When machines are cleaned at the end of 
a work shift, the large cast iron tables are left unprotected overnight. 
In the morning, surface rusting is quite extensive, requiring cleanup 
prior to lapping. The presence of rust on the surface of the ceramic 
wafers is also a reason for rejection. A second problem also involves the 
xanthan gum lapping slurry. When lapping machines are unused during 
weekends, the xanthan gum-alumina swarf sets up like a mortar on the 
lapping table, and this often causes the first set of lapped parts to jam 
the machine on Monday morning. 
In an effort to eliminate the problems described above, various slurry 
replacements were tried. One of the replacements was a thixotropic corn 
starch mixture. While this slurry gave acceptable lapping rates, it left 
intractable deposits on the lapping machine and on the ceramic workpieces. 
Another slurry tried was one made of glycerine, triethanolamine, diamond 
abrasive, and water. This slurry gave excellent lapping times and cleaned 
up superbly. In addition, this slurry facilitated clean-up of the lapping 
table. It did not make "mortar" on the lapping table during off shifts. 
Moreover, residual triethanolamine inhibited surface rusting of the 
lapping tables. The only problem encountered with this slurry was that it 
did not suspend the diamond adequately. It was not thixotropic and had to 
be blended at a low viscosity suitable for lapping. Constant vigorous 
stirring was required to keep the diamond suspended. Silica gel and other 
viscosity enhancers proved to be unacceptable. 
OBJECTS OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a new and 
improved lapping slurry which permits easier and more complete cleaning of 
the workpieces after lapping. 
It is another object to provide a new and improved lapping slurry which 
permits easier and more complete cleaning of the lapping equipment. 
It is yet another object to provide a new and improved lapping slurry which 
inhibits rusting and corrosion of the lapping table. 
Further and other objects of the present invention will become apparent 
from the description contained herein. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, the foregoing and 
other objects are achieved by an unthickened lapping slurry which 
comprises abrasive grains dispersed in a carrier comprising water, 
glycerine, and triethanolamine. 
In accordance with a second aspect of the present invention, the foregoing 
and other objects are achieved by a thickened lapping slurry which 
comprises abrasive grains dispersed in a carrier comprising water, 
glycerine, triethanolamine, a water soluble silicate. 
In accordance with a third aspect of the present invention, the foregoing 
and other objects are achieved by a method of making a thickened lapping 
slurry comprising abrasive grains dispersed within a carrier, the method 
comprising the steps of: combining liquid ingredients water, glycerine, 
and triethanolamine, to form a first liquid mixture; heating the first 
liquid mixture to a temperature above about 70.degree. C.; dissolving a 
water soluble silicate in the first liquid mixture; combining abrasive 
grains with at least one of said liquid ingredients so that after the 
heating step, the dissolving step, and the combining step are completed, a 
second mixture is formed; and while stirring, combining the second mixture 
with sufficient acid selected from the groups of acids consisting of 
inorganic acids and organic acids to form a thickened lapping slurry 
comprising a carrier and abrasive grains, the carrier having a pH of 
between about 8 and about 10.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention comprises abrasive grains dispersed in a carrier. 
Both thickened and unthickened slurries comprising thickened and 
unthickened carriers are described herein. The present invention may 
comprise any of a variety of abrasive grains, of which a large number are 
well-known to the skilled artisan. Some embodiments described herein 
comprise diamond abrasive between about 1 um and 100 um, but the invention 
can be practiced using any type of abrasive grains. 
An unthickened slurry may comprise abrasive grains dispersed in a carrier 
comprising glycerine in an amount less than 80 wt %; and between about 1 
wt % and about 70 wt % triethanolamine. More often the carrier may 
comprise between about 30 wt % and about 50 wt % glycerine; and between 
about 7 wt % and about 17 wt % triethanolamine. 
A preferred unthickened slurry in this application comprises abrasive 
grains such as diamond abrasive dispersed in a carrier comprising about 40 
wt % glycerine, about 12 wt % triethanolamine, and about 48 wt % water. 
Another, thickened, lapping slurry found to be effective in this 
application comprises 0-80 wt % glycerine 1-70 wt % triethanolamine, 5-100 
carats/liter of 15-25um diamond abrasive, 0-20% sodium metasilicate 
(Na.sub.2 SiO.sub.3.9 H.sub.2 O) or other water soluble silicate, 0-10% 
inorganic or organic acid such as acetic, citric, tartaric, malonic, 
gluconic, or succinic acid, and the remainder water. 
A preferred thickened slurry may comprise abrasive grains dispersed in a 
carrier comprising between about 30 wt % and about 50 wt % glycerine; 
between about 7 wt % and about 17 wt % triethanolamine; between about 0.1 
wt % and about 10 wt % water soluble silicate; and between 0.1 wt % and 
about 5 wt % of the acid. A more preferred thickened slurry may comprise 
abrasive grains dispersed in a carrier comprising about 40 wt % glycerine; 
about 12 wt % triethanolamine; between about 2 wt % and about 4 wt % water 
soluble silicate; and between 0.1 wt % and enough acid to produce a pH of 
between about 8 and about 10 in the carrier. 
To make a thickened slurry, to the unthickened composition add 2-4 wt % 
sodium metasilicate and enough acid to lower the pH to the 8-10 range of 
alkalinity (typically from 0.5 to 3 wt % of the total). The method of 
addition of acid determines the viscosity of the resulting gel. Typically, 
the unthickened formula is heated above 70.degree. C. The sodium 
metasilicate is then dissolved in this heated solution. The acid is added 
gradually with vigorous stirring. Gentle stirring or the absence of 
stirring after addition of the acid results in the formation of a 
well-interconnected gel which is much more viscous than that formed with 
vigorous stirring. 
The thixotropic nature of this gel is shown in FIG. 1. Desirable 
viscosities are in the 10-20 cP range under shear forces. 
Typically, some separation of the water from this gel occurs with time. The 
diamond abrasive remains suspended in the remaining gel and does not 
settle to the bottom. This gel can be easily restored with gentle mixing. 
Gels made with higher concentrations of sodium metasilicate and citric 
acid are less likely to separate over time, but are much too viscous for 
use as a lapping slurry. 
Cleaning of workpieces after lapping with water is simple and complete. The 
formulation incorporating glycerine/TEA components gave the best cleaning 
result. This formulation incorporates a soluble silicate, a commonly-used 
detergent component which enhances cleaning of the workpieces. 
Machine clean-up is also simple. Deposits of this lapping slurry left on 
steel plates remain wet over weekends and are easily removed with a wet 
wipe. Residual triethanolamine and soluble silicate protect the lapping 
tables from corrosion. Citrate ions facilitate cleaning by complexing with 
iron that is removed from the lapping table. 
Another advantage of this lapping slurry is that all components are 
essentially nontoxic. Citric acid not only forms a gel but also reduces 
the alkaline corrosivity of the sodium metasilicate to the safe pH range 
of 9-10. 
Other organic acids may also be used in place of citric acid. These 
include, but are not limited to malonic, gluconic, tartaric, adipic, 
succinic, and ethylenediaminetetraacetic acid. 
In addition, propylene glycol or other polyhydroxy species can be 
substituted for the glycerine. Alternatively, glycerine may be left out of 
the slurry entirely. Glycerine, however, provides for some lubricity on 
the lapping table. Glycerine likewise tends to facilitate clean-up of both 
the workpieces and the lapping table. 
While there has been shown and described what are at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications can be made 
therein without departing from the scope of the inventions defined by the 
appended claims.