Electronic coatings

The present invention relates to a ceramic coating composition comprising a preceramic material such as silicon oxide precursors, silicon carbonitride precursors, silicon carbide precursors, and silicon nitride precursors and a flux material such as B.sub.2 O.sub.3, PbO.sub.2, P.sub.2 O.sub.5, and Bi.sub.2 O.sub.3. The present invention also relates to a substrate such as an electronic device having said coating applied and ceramified thereon.

BACKGROUND OF THE INVENTION 
This invention pertains to a tamper proof electronic device coating. The 
coating is applied to an integrated circuit device chip at the wafer 
fabrication stage. The use of a flux material in combination with a 
ceramic precursor produces a coating that has improved abrasion resistance 
and toughness. 
The dissection of electronic devices is a major source of information for 
both commercial competitors as well as foreign governments. The devices 
may be analyzed by numerous techniques such as x-rays, cross-sectioning, 
etching and others. Because of the ability to analyze the devices it has 
become desirable to make the electronic devices resistant to the numerous 
analytical techniques. 
The use of certain fillers to improve the tamper-proof characteristics of 
electronic coatings is known in the art. For example, U.S. Pat. No. 
5,258,334 to Lantz, II, U.S. Pat. No. 5,399,441 to Bearinger et al., U.S. 
Pat. No. 5,387,480 to Haluska et al., U.S. Pat. No. 5,436,083 to Haluska 
et al., U.S. Pat. No. 5,436,084 to Haluska et al., U.S. Pat. No. 5,458,912 
to Camilletti et al., U.S. Pat. No. 5,492,958 to Haluska et al. and EP 
0615000 all disclose coating compositions comprising a filler and a silica 
precursor resin, a borosilazane or a polysilazane. The fillers disclosed 
in the above patents impart a variety of properties to the coating such as 
opaqueness, radiopaqueness, or resistance to plasma etching, wet etching 
or cross-sectioning. However, these patents do not specifically teach 
fillers which improve the abrasion resistance and toughness of the 
coatings. 
It is therefore an object of the instant invention to provide a 
tamper-proof electronic coating comprising a flux material in a preceramic 
material. 
SUMMARY OF THE INVENTION 
This invention relates to a coating composition comprising a flux material 
in a preceramic material wherein said coating is applied to a substrate, 
preferably an electronic device, and pyrolyzed to form a ceramic coating 
on at least one surface of the electronic device. This invention also 
relates to an electronic device having said coating applied and ceramified 
thereon. 
THE INVENTION 
This invention relates to a ceramic coatings which are useful as coatings 
for electronic devices. By electronic device it is meant to include 
silicon based devices, gallium arsenide based devices, focal plane arrays, 
opto-electronic devices, photovoltaic cells and optical devices. The 
coating may be applied to other substrate where the tamper proof 
characteristic provided by the coating is desired. 
The preceramic materials useful in this invention include, but are not 
limited to silicon oxide precursors, silicon carbonitride precursors, 
silicon carbide precursors and silicon nitride precursors. 
The preferred preceramic materials to be used in the process of this 
invention are precursors to silicon oxides, especially silica. Such 
silicon oxide precursors include, but are not limited to, hydrogen 
silsesquioxane resin (H-resin); hydrolyzed or partially hydrolyzed 
products of compounds or mixtures of compounds of the formula 
R.sup.1.sub.n Si(OR.sup.1).sub.4-n ; or combinations of the above, in 
which each R.sup.1 is independently an aliphatic, alicyclic or aromatic 
substituent of 1-20 carbon atoms, preferably 1-4, such as an alkyl (e.g. 
methyl, ethyl, propyl), alkenyl (eg. vinyl or allyl), alkynyl (eg. 
ethynyl), cyclopentyl, cyclohexyl, phenyl etc., and n is 0-3, preferably 0 
or 1. 
The hydrogen silsesquioxane resin includes hydridosiloxane resins of the 
formula HSi(OH).sub.x (OR).sub.y O.sub.z/2, in which each R is 
independently an organic group or a substituted organic group which, when 
bonded to silicon through the oxygen atom, forms a hydrolyzable 
substituent, x=0-2, y=0-2, z=1-3, x+y+z=3. Examples of R include alkyls 
such as methyl, ethyl, propyl, butyl, etc., aryls such as phenyl, and 
alkenyls such as allyl or vinyl. These resins may be fully condensed 
(HSiO.sub.3/2).sub.n or they may be only partially hydrolyzed (i.e., 
containing some Si--OR) and/or partially condensed (i.e., containing some 
Si--OH). Although not represented by this structure, these resins may also 
contain a small number (eg., less than about 10%) of silicon atoms which 
have either 0 or 2 hydrogen atoms attached thereto or a small number of 
SiC bonds due to various factors involved in their formation or handling. 
The H-resins and methods for their production are known in the art. For 
example, Collins et al. in U.S. Pat. No. 3,615,272, which is incorporated 
herein by reference, teach the production of a nearly fully condensed 
H-resin (which may contain up to 100-300 ppm silanol) by a process 
comprising hydrolyzing trichlorosilane in a benzenesulfonic acid hydrate 
hydrolysis medium and then washing the resultant resin with water or 
aqueous sulfuric acid. Similarly, Bank et al. in U.S. Pat. No. 5,010,159, 
which is hereby incorporated by reference, teach an alternative method 
comprising hydrolyzing hydridosilanes in an arylsulfonic acid hydrate 
hydrolysis medium to form a resin which is then contacted with a 
neutralizing agent. 
Other hydridosiloxane resins, such as those described by Frye et al. in 
U.S. Pat. No. 4,999,397, hereby incorporated by reference, those produced 
by hydrolyzing an alkoxy or acyloxy silane in an acidic, alcoholic 
hydrolysis medium, those described in Kokai Patent Nos. 59-178749, 
60-86017 and 63-107122, or any other equivalent hydridosiloxane, will also 
function herein. 
It is to be noted that in a preferred embodiment of this invention, 
specific molecular weight fractions of the above H-resins may also be used 
in this process. Such fractions and methods for their preparation are 
taught by Hanneman et al. in U.S. Pat. Nos. 5,063,267 and 5,416,190 which 
are hereby incorporated by reference. A preferred fraction comprises 
material wherein at least 75% of the polymeric species have a molecular 
weight above about 1200 and a more preferred fraction comprises material 
wherein at least 75% of the polymeric species have a number average 
molecular weight between about 1200 and about 100,000. 
The hydrogen silsesquioxane resin may contain a platinum, rhodium or copper 
catalyst to increase the rate and extent of conversion to silica. 
Generally, any platinum, rhodium or copper compound or complex which can 
be solubilized will be functional. For instance, platinum acetylacetonate, 
rhodium catalyst RhCl.sub.3 S(CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.3).sub.2 
!.sub.3, obtained from Dow Corning Corporation, Midland, Mich., or cupric 
naphthenate are all within the scope of this invention. These catalysts 
are generally added in an amount of between about 5 to 1000 ppm platinum, 
rhodium or copper based on the weight of hydrogen silsesquioxane resin. 
The hydrogen silsesquioxane resin may also contain an organic or inorganic 
peroxide to increase the rate and extent of conversion to silica. Organic 
and inorganic peroxides useful in the instant invention may be exemplified 
by, but not limited to barium peroxide, strontium peroxide, calcium 
peroxide, .alpha., .alpha.-bis tertiary peroxydiisopropylbenzene, dicumyl 
peroxide, benzoyl peroxide and others. 
A second type of silicon oxide precursor useful herein includes hydrolyzed 
or partially hydrolyzed products of compounds or mixtures of compounds of 
the formula R.sup.1.sub.n Si(OR.sup.1).sub.4-n in which R.sup.1 and n are 
as defined above. Some of these materials are commercially available, for 
example, under the tradename ACCUGLASS (Allied Signal). Specific compounds 
of this type include methyltriethoxysilane, phenyltriethoxysilane, 
diethyldiethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, 
phenyltrimethoxysilane, vinyltrimethoxysilane, tetramethoxysilane, 
tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane. After 
hydrolysis or partial hydrolysis of these compounds, the silicon atoms 
therein may be bonded to C, OH or OR.sup.1 groups, but a substantial 
portion of the material is believed to be condensed in the form of soluble 
Si--O--Si resins. Compounds in which x=2 or 3 are generally not used alone 
as volatile cyclic structures are generated during pyrolysis, but small 
amounts of said compounds may be cohydrolyzed with other silanes to 
prepare useful preceramic materials. 
The hydrolyzed or partially hydrolyzed products may contain a dispersion of 
colloidal silica (SiO.sub.2). Typically the hydrolyzed or partially 
hydrolyzed products contain from 10 to 50 weight percent solids in an 
alcohol-water medium consisting essentially of 10 to 70 weight percent 
colloidal silica and 30 to 90 weight percent of the hydrolyzate or partial 
hydrolyzate. The compositions contain sufficient acid to provide a pH in 
the range of 3.0 to 6.0. These mixtures of colloidal silica and 
hydrolyzate are commercially available as Silvue.RTM. 101 from SDC 
Coatings, Inc., Anaheim, Calif. 
Other ceramic oxide precursors may also be used in combination with any of 
the above silicon oxide precursors. The ceramic oxide precursors 
specifically contemplated herein include compounds of various metals such 
as aluminum, titanium, zirconium, tantalum, niobium and/or vanadium as 
well as various non-metallic compounds such as those of boron or 
phosphorous which may be dissolved in solution, hydrolyzed, and 
subsequently pyrolyzed at relatively low temperatures to form ceramic 
oxides. The use of the ceramic oxide precursors is described in U.S. Pat. 
Nos. 4,753,855 and 4,973,526, herein incorporated by reference. 
The ceramic oxide precursors generally have one or more hydrolyzable groups 
bonded to the metal or non-metal, depending on the valence of the metal. 
The number of hydrolyzable groups to be included in these compounds is not 
critical as long as the compound is soluble or can be dispersed in the 
solvent. Likewise, selection of the exact hydrolyzable substituent is not 
critical since the substituents are either hydrolyzed or pyrolyzed out of 
the system. Typical hydrolyzable groups include, but are not limited to, 
alkoxy, such as methoxy, propoxy, butoxy and hexoxy, acyloxy, such as 
acetoxy, other organic groups bonded to said metal or non-metal through an 
oxygen such as acetylacetonate or an amino groups. Specific compounds, 
therefore, include zirconium tetracetylacetonate, titanium dibutoxy 
diacetylacetonate, aluminum triacetylacetonate, tetraisobutoxy titanium 
and Ti(N(CH.sub.3).sub.2).sub.4. 
When a ceramic oxide precursor is combined with a silicon oxide precursor, 
generally it is used in an amount such that the final ceramic contains 0.1 
to 30 percent by weight ceramic oxide precursor. 
Another example of a preceramic material is silicon carbonitride precursors 
such as hydridopolysilazane (HPZ) resin, methylpolydisilylazane (MPDZ) 
resin and polyborosilazane resin (BHPZ). Processes for the production of 
these materials are described in U.S. Pat. Nos. 4,540,803; 4,340,619 and 
4,910,173, respectively, herein incorporated by reference. Other examples 
of preceramic material include silicon carbide precursors such as 
polycarbosilanes and silicon nitride precursors such as polysilazanes. 
Oxygen can be incorporated into the ceramics resulting from the above 
precursors or the precursors can be converted to silica by pyrolyzing them 
in an oxygen-containing environment. Mixtures of silicon-containing 
preceramic materials may also be used. 
The preceramic material is used in combination with a flux material. By 
"flux materials" it is meant materials which melt and react with the 
preceramic material resulting in a ceramic coating having improved 
adhesion and toughness. Flux materials useful herein may be exemplified 
by, but not limited to B.sub.2 O.sub.3, PbO.sub.2, P.sub.2 O.sub.5, and 
Bi.sub.2 O.sub.3. 
The amount of flux material used can be varied over a wide range depending 
on, for example, the quality and characteristics desired in the final 
coating. Generally the flux materials are used in an amount less than 
about 90 weight percent of the coating composition to insure that there is 
enough preceramic material present to bind the filler. It is preferred 
that the flux material is present in the range of about 1 to 91 volume %, 
more preferred 5 to 80 volume % based on the volume of preceramic material 
and flux material. 
The flux material may be used alone or in combination with one or more 
secondary fillers. By the term "filler" it is meant a finely divided solid 
phase which is distributed within the resin and the final ceramic coating. 
Fillers useful as secondary filler in this invention include, but are not 
limited to, optically opaque fillers, radiopaque fillers, luminescent 
fillers, oxidation resistant fillers, abrasion resistant fillers and 
magnetic fillers. The secondary filler may include various inorganic and 
organic fillers, especially inorganic fillers, in a variety of 
morphologies including, but not limited to powders, particles, flakes, 
microballoons, and the like. 
Optically opaque fillers are fillers that when mixed with the preceramic 
material render the coating impenetrable to visual light. Optically opaque 
fillers include, but are not limited to, oxides, nitrides and carbides of 
silicon and alumina, metals and inorganic pigments. Preferred optically 
opaque fillers are plasma alumina microballoons having an average size of 
about 6 microns, silica microballoons having an average size of about 5 to 
40 microns, silicon nitride (Si.sub.3 N.sub.4) powder or whiskers; silicon 
carbide (SiC) powder or whiskers, aluminum nitride (AlN) powder and black 
inorganic pigments such as black Ferro.RTM. F6331 having an average size 
of about 0.4 microns. 
Radiopaque fillers are fillers that when mixed with the preceramic material 
render the coating impenetrable to radiation such as x-rays, UV, IR, and 
visible light as well as sound waves. Radiopaque fillers include, but are 
not limited to, heavy metals such as tungsten and lead and insoluble salts 
of heavy metals such as barium, lead, silver, gold, cadmium, antimony, 
tin, palladium, strontium, tungsten and bismuth. The salts can include, 
for example, carbonates, sulfates and oxides. 
Luminescent fillers are fillers that when mixed with the preceramic 
material result in a coating that will absorb energy and emit 
electromagnetic radiation in excess of thermal radiation. The fillers are 
typically phosphors such as sulfides such as zinc sulfide and cadmium 
sulfide; selenides; sulfoselenides; oxysulfides; oxygen dominate phosphors 
such as borates, aluminates, gallates, silicates and the like; and halide 
phosphors such as alkali metal halides, alkaline earth halides and 
oxyhalides. Preferred are sulfides and most preferred is zinc sulfide. The 
phosphor compounds may also be doped with an activator. Activators 
include, but are not limited to, manganese, silver and copper; rare earth 
ions which may be in the form of halides. The activator is generally 
present in amounts of about 0.1 to 10 mol percent based on the phosphor. 
Abrasion resistant fillers are fillers that when mixed with the preceramic 
material render the coating difficult to remove by a frictional means such 
as scraping or polishing without damaging the underlying substrate. 
Abrasion resistant fillers may be exemplified by, but not limited to 
diamond, titanium nitride (TiN), and fibers of graphite, kevlar, nextel, 
soffill, Aluminum oxide (Al.sub.2 O.sub.3), Fiber FP, tungsten carbide, 
tantalum carbide and others. 
Energy resistant fillers are fillers that when mixed with the preceramic 
material render the coating impenetrable and/or reactive to energy sources 
such as ozone, UV-ozone, gaseous free radicals and ions, any vapor or 
liquid borne reactive species and plasmas. Energy resistant fillers may be 
exemplified by, but not limited to heavy metals such as lead, tungsten, 
and others. 
Magnetic fillers are fillers that when mixed with the precerarnic material 
render the coating magnetic (i.e. magnetized by a magnetic field; having 
net magnetic moment). Magnetic fillers may be exemplified by carbon alloys 
ferrites, iron carbonyl, and alloys of metals such as iron manganese, 
cobalt, nickel, copper, titanium, tungsten, vanadium, molybdenum, 
magnesium, aluminum, chromium, zirconium, lead, silicon and zinc such as 
Fe.sub.2 O.sub.3, MnZn, NiZn, CuZn and other ferrite materials. 
Conductive fillers are fillers that when mixed with the preceramic material 
render the coating either electrically or thermally conductive. Conductive 
fillers may be exemplified by gold, silver copper, aluminum, nickel, zinc 
chromium, cobalt and others. 
Other secondary fillers useful herein include synthetic and natural 
materials such as oxides, nitrides, borides and carbides of various metals 
and non-metals such as glass, alumina, silica, titanium dioxide, zinc 
oxide, tungsten oxide and ruthenium oxide; titanates such as potassium 
titanate and barium titanate; niobates such as lithium niobate and lead 
niobate; barium sulfate; calcium carbonate; precipitated diatomite; 
aluminum silicate or other silicates; pigments; metals such as silver, 
aluminum, or copper; wollastonite; mica; kaolin; clay and talc; high 
dielectric constant fillers such as titanate, niobate or tungstate salts 
of metals such as strontium, zirconium, barium, lead, lanthanium, iron, 
zinc and magnesium such as barium titanate, potassium titanate, lead 
niobate, lithium titanate, strontium titanate, barium strontium, lead 
lanthanium zirconium titanate, lead zirconium titanate, and lead 
tungstate. 
The particle size and shape of the above fillers can vary over a wide range 
(i.e., submicron to 1 mil) depending on factors such as the type of 
filler, the desired coating thickness and others. There may be only one 
secondary filler present or a combination of secondary fillers present 
depending on the protection desired. It is preferred to have a combination 
of fillers present to provide a wide range of protection from various 
analytical techniques. 
The secondary fillers are generally used in an amount such that the 
combination of secondary filler and flux material is less than about 90 
weight percent of the coating to insure that there is enough preceramic 
material present to bind the filler. It is preferred that the total amount 
of flux material and filler be present in the range of about 1 to 91 
volume %, more preferred 5 to 80 volume % based on the volume of 
preceramic material, flux material and secondary filler. 
If desired other materials may also be present in the coating composition. 
For example, adhesion promoters, suspending agents and other optional 
components may be added. The adhesion promoters may be exemplified by, but 
not limited to silanes such as glycidoxypropyltrimethoxysilane, 
mercaptopropyltrimethoxysilane, vinyltriacetoxysilane and others. 
The ceramic coating is formed by combining together the preceramic 
material, the flux material and any optional components and applying the 
mixture to the surface of the substrate (i.e. electronic device). 
Preferred substrates are electronic devices, however other substrates such 
as metals, glass, ceramics, etc. may be used herein. The electronic device 
can be bare (i.e. no passivation) or the device can have a primary 
passivation. Such primary passivation can be ceramic coatings such as 
silica, silicon nitride, silicon carbide, silicon oxynitride, silicon 
oxycarbide, PSG, BPSG and others deposited by CVD, PVD, PECVD or sol-gel 
approaches. Primary passivation and methods of depositing are known to 
those skilled in the art. 
The coating composition may be applied in any manner, but a preferred 
method involves producing a solvent dispersion of the preceramic material, 
flux material and any optional components and applying the solvent 
dispersion to the surface of the substrate. Various facilitating means 
such as stirring and/or heating may be used to disperse the preceramic 
material and flux material and create a more uniform application material. 
Solvents which may be used include any agent or mixture of agents which 
will disperse the preceramic material and flux material to form a 
homogeneous liquid mixture without affecting the resultant coating. These 
solvents can include alcohols such as ethyl alcohol or isopropyl alcohol; 
aromatic hydrocarbons such as benzene or toluene; alkanes such as 
n-heptane, dodecane or nonane; ketones; esters; glycol ethers; or cyclic 
dimethylpolysiloxanes. The solvent is present in an amount sufficient to 
dissolve/disperse the above materials to the concentration desired for 
application. Typically the coating composition contains from 5 to 99.9 wt 
% solvent, preferably from 10 to 99 wt %. 
Specific methods for application of the solvent dispersion include, but are 
not limited to spin coating, dip coating, spray coating, flow coating, 
screen printing or others. The solvent is then allowed to evaporate from 
the coated substrate resulting in the deposition of the preceramic 
material and flux material. Any suitable means for evaporation may be used 
such as simple air drying by exposure to an ambient environment, by the 
application of a vacuum, or mild heat (.ltoreq.50.degree.) or during the 
early stages of the curing process. It should be noted that when spin 
coating is used, the additional drying method is minimized as the spinning 
drives off the solvent. 
The preceramic material and flux material are then cured by heating to a 
sufficient temperature to melt the flux material and convert the 
preceramic material into a ceramic matrix. When silicon-containing 
preceramic materials are used the resulting coating is a silica-containing 
ceramic matrix. By silica-containing ceramic matrix it is meant a hard 
coating obtained after heating the preceramic material wherein the coating 
contains both amorphous silica (SiO.sub.2) materials as well as amorphous 
silica-like materials that are not fully free of residual carbon, silanol 
(Si--OH) and/or hydrogen. Generally, the temperature is in the range of 
50.degree. C. to 1000.degree. C. depending on the pyrolysis atmosphere and 
preceramic material. Preferred temperatures are in the range of 50.degree. 
C. to 800.degree. C., more preferred are in the range of 50.degree. C. to 
450.degree. C. Heating is generally conducted for a time sufficient to 
ceramify, generally up to 6 hours with less than 3 hours being preferred. 
The heating may take place at any effective atmospheric pressure from 
vacuum to superatmospheric and under any effective oxidizing or 
non-oxidizing gaseous environment such as an inert gas such as N.sub.2, 
Ar, He and others, air, O.sub.2, ammonia, amines, moisture, N.sub.2 O and 
others. Any method of heating such as the use of a convection oven, rapid 
thermal processing, hot plate, or radiant of microwave energy may be used 
herein. The rate of heating is also not critical but is most practical and 
preferred to heat as rapidly as possible. 
It has been found that when the preceramic material contains a flux 
material the ceramic coating has improved adhesion and increased 
toughness. 
So that those skilled in the art can understand and appreciate this 
invention taught herein, the following examples are presented, it being 
understood that these examples should not be used to limit the scope of 
this invention found in the claims.

EXAMPLE 1 
2.11 g of 1.5% polyborosilazane resin (BHPZ) resin in xylene (57% solids), 
13.5 g tungsten carbide (0.83 .mu.m) Cerac, 0.5 g tungsten (0.6-0.9 .mu.m) 
Strem, 0.5 g lead (5.4 .mu.m) Cerac, 3.5 g diamond dust (4-6 .mu.m) 
Amplex, 0.05 g B.sub.2 O.sub.3 powder Alfa and 1.5 g nonane were mixed for 
four-20 second periods with a sonic probe. A 41/2" square alumina panel 
(40 mils thick, dried for 30 min. at 150.degree. C.) was coated with the 
mixture by using a 2 mil drawdown bar. The coated panel was allowed to air 
dry for 2 hour 35 minutes, then it was pyrolyzed at 400.degree. C. for 1 
hour in air. The coating was found to have no cracks at 1000.times. 
magnification with a microscope. The coating thickness was 32.6 .mu.m. The 
pencil hardness was &gt;9H. 
EXAMPLE 2 
1.7 g of a hydrogen silsesquioxane resin (HSiO.sub.3/2).sub.n, 13.5 g 
tungsten carbide (0.83 .mu.m) Cerac, 0.5 g tungsten (0.6-0.9 .mu.m) Strem, 
0.5 g lead (5.4 pm) Cerac, 3.5 g diamond dust (4-6 .mu.m) Amplex, 0.06 g 
B.sub.2 O.sub.3 powder Alfa and 3.0 g nonane were mixed for four-20 second 
periods with a sonic probe. A 41/2" square alumina panel (40 mils thick, 
dried for 30 min. at 150 .degree. C.) was coated with the mixture by using 
a 2 mil drawdown bar. The coated panel was allowed to air dry for 2 hour 
35 minutes, then it was pyrolyzed at 400 .degree. C. for 1 hour in air. 
The coating was found to have no cracks at 1000.times. magnification with 
a microscope. The coating thickness was 20.0 .mu.m. The pencil hardness 
was &gt;9H. 
EXAMPLE 3 
3.28 g of Silvue.RTM. 101, 13.5 g tungsten carbide (0.83 .mu.m) Cerac, 0.5 
g tungsten (0.6-0.9 .mu.m) Strem, 0.5 g lead (5.4 .mu.m) Cerac, 3.5 g 
diamond dust (4-6 .mu.m) Amplex, 0.06 g B.sub.2 O.sub.3 powder Alfa and 
1.0 g 2-ethoxyethylacetate were mixed for four-20 second periods with a 
sonic probe. A 41/2" square alumina panel (40 mils thick, dried for 30 
min. at 150 .degree. C.) was coated with the mixture by using a 2 mil 
drawdown bar. The coated panel was allowed to air dry for 2 hour 40 
minutes, then it was pyrolyzed at 400 .degree. C. for 1 hour in air. The 
coating was found to have no cracks at 1000.times. magnification with a 
microscope. The coating thickness was 26.8 .mu.m. The pencil hardness was 
5H. 
EXAMPLE 4 
2.4 g of hydridopolysilazane (HPZ) resin in xylene (55.6% solids), 13.5 g 
tungsten carbide (0.83 .mu.m) Cerac, 0.5 g tungsten (0.6-0.9 Em) Strem, 
0.5 g lead (5.4 .mu.m) Cerac, 3.5 g diamond dust (4-6 .mu.m) Amplex, 0.06 
g B.sub.2 O.sub.3 powder Alfa and 1.5 g nonane were mixed for four-20 
second periods with a sonic probe. A 41/2" square alumina panel (40 mils 
thick, dried for 30 min. at 150 .degree. C.) was coated with the mixture 
by using a 2 mil drawdown bar. The coated panel was allowed to air dry for 
1 hour 40 minutes, then it was pyrolyzed at 400 .degree. C. for 1 hour in 
air. The coating was found to have no cracks at 1000.times. magnification 
with a microscope. The coating thickness was 38.3 .mu.m. The pencil 
hardness was &gt;9H.