CA1053996A - Chromium oxide densification, hardening and strengthening of formed bodies and coatings having interconnected porosity - Google Patents

Abstract

CHROMIUM OXIDE DENSIFICATION, HARDENING AND STRENGTHENING
OF FORMED BODIES AND COATINGS HAVING INTERCONNECTED POROSITY
ABSTRACT OF THE DISCLOSURE
Chromium oxide densification, bonding, hardening and strengthening of bodies having interconnecting porosity therein by impregnation with a chromium compound convertible to chromium oxide on heating, heating the impregnated body to convert the compound to chromium oxide and repeating the impregnation and heating steps. The body may be of any material composed of an oxide, has an oxide constituent or will form a well adhering oxide on its surface.

Description

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This invention relates to chromium oxide densi~ication~ bonding, hardening and strengthening o~
a body having interconnecting porosity therein by impregnation with a solution o~ a soluble chromium compound conver-tible to chromium oxide on heating, dryin~ and cur:ing the impregnated body by heating same to a temperature su~icient to convert the compound in situ to chromium oxide and repeatin~ the impregna-tion and heating steps. The body may be of any material composed of an oxide, has an oxide constituent or will ~orm a well adhering oxide on its surface. ~he term "body" is intended to cover both an entire article or a coating.
This application discloses and claims subject matter in common with applicants' patents number 911,276, and 953584 issued October 3, 1972 and August 27, 1~7~, respectively, for "Ceramic treating process and product produced thereby".
The basic method described in these earlier applications, and that employed in ^the present application, consists of repeated impregnation-cure cycles o~ a porous body with a soluble chromium compound which is convertible in situ by heat to a chromium oxide. The starting body must have interconnected porosity and may be initially ~ormed by any bonding method such as bisque ~iring (sintering), cold welding, clay binders or other binder materials, or it may be produced

2~ by using a chromium compound "binder" that is converted by heat to form a chromium oxide initial bond between the constituent grains or materials selected ~or the basic body.

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There is disclosed in this applica-tion the fact that a great variety of materials can be bonded, densified, stren~thened and hardened by means of the chromium compound-to chromium oxide, multiple impre~nation-cure cycle method accordin~ to the present invention.
More specifically, we have found that many materials can be chrome oxide bonded provided: (l)it is either composed of an oxide, has an oxide constituent or will form a well adherin~ oxide on its surface; (2)it is not soluble nor adversely reactive to the chromium compound employed as the impregnant; (3)it is inherently stable to at least the minimum heat cure temperature to be employed when converting the soluble chromium compound to a chromium oxide.
It is therefore a principal ob~ect of the present invention to provide an improved method of forming and densifying, hardening, bonding and strengthening bodies of a wide varlety of materials.
A preferred object of the present invention is to provide improved chromium binder compounds for use in the improved process.
Another preferred object of the present inven-tion is to provide an improved method of densifyin~, hardening and strengthening bodies which are bound by glassy bonded systems, are self-bonded, chemically bonded, cold pressed, oxide bonded or chromium oxide bonded.
Examples of materials which can be treated by the process of khe present invention are many of the nitrides~
caxbides, silicides, borides, intermetallics, ferrites, metals and metal alloysl complex oxides and mixtures of any of these including mixtures with oxides. It is well known, for instance that most me-tals form a very thin oxide layer on their surfaces when exposed to air. If not, such a layer wiLl invariably '', ~

be formed with the application of heat in an air or oxidizing atmosphere. The same holds true for silicon carbide, silicon nitride, boron carbide, molybdenum silicide, and the l.ike where oxides of silicon~ ~oron and so on are formed. In fact, most such materials are quite difficult to obtain without these thin protective oxide layers being present.

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Included within the scope of this application are the so-called complex oxides.
As used here, a complex oxide does not mean a mixture of discrete oxides but rather an identifiable chemical compound. Exampies are "~ircon" or zirconium silicate (ZrSiO4 or Zr2 SiO2), Calcium titanate (CaTiO3 or CaO TiO2)f ma~nesium stannate (MgSnO3 or MgO SnO2), cesium zirconate (CeZrO~I or CeO2 ZrO2) etc. These materials, of course, act like oxides insoFar as forming a chromium oxide bond according to the invention.
It should be pointed out that the use of basic porous bodies for this process isnot limited to those formed from finely divided particulate grains or powders. Bodies may also be dénsified, hardened and strengthened, using this chromium oxide bonding processl that are composed of non-particulate materials. Examples are: sintered metal felts; glass or refractory fiber mats or insulation; woven glass, refractory or metal cloth; foamed structures;
particulate bodies into which reinforcing wire, fiber stripsl etc. have been incorporatecl Any non-particulate materials selected must, of course, mee~ the requirements outlinecl earlier in order to prov;de for a strong chrome oxide bond and an ultimate hardness and strength increase durina the multip!e impregnation-cure cycle densification process.
~, The term soluble chromium~compound, as used in this application is intended to mean any of a number of chromium impregnants or "binders" such as water solutions of: chromic anhydride (CrO3)r usually called chromic acid when mixed with water (H2CrO4); chromium chloride (CrC13 xH2O); chromium nitrate (CrNO3)3 ~H20);
chromium acetate (Cr(OAC:)3 4H2~)); chromium sulfate (Cr2(S04)3 ~ lSH20); etc. Also included are a wide variety of dichromates and chromates such as zinc dichromate; mognesium chromate; nnd mixturPs of chromates with chromic ac;d~ A variety of more complex soluble chromium compovnds is aiso included that can perhaps be best categorized by the generalized ormula xCrO3 yCr203 zH200 These are normally prepared by reducing chromic acid with some other chem;cai such as tarclriç acid, carbon, formic acid and the like. A second method is to dissolve Cr203 or Cr j~O3 xH2O or chromium hydroxide in chromic acid.
There is a limit of about 12-15% Cr(lli) from Cr~03 that may be introdueed in this later method clue to the low solubility of Cr203. In some cases with these complex chromium compouncls made using the first method of preparing impregnants there may not be a complete :; -3~
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reaction. For example, a treatment of Formic acid with chromic acid may result in some formate remaining. No quantative analyses have been pPrformed and any remaining organic material will be oxidized and volitized at some point during the heat cure cycle used follow-ing each impre~nation cycle of the porous body.
Some of these binders such as chromic acid are extremely wetting. Others, such as the complex chromium compounds (xCrO3 ~ yCr2O3 o zH~O) can be prepared so as h contain large concentrations of chromium ions in solution.
Others such as the chromates have been founcl useful for achievin~ high hard-ness values in a few impre~nation-cure cycles. These are also useful for filling boclies having 10 a relatively large pore size structure whereas use of a compound such as chromium acetate might require severai impregnat;on cure cycles before achievina a noticeable increase in hardness, Only the acidified soluble chromium~el~compouncls have been founcl to produce extremely hard bodies having improves strength. The basic and neutral solutions made by dissolving chromium binder compounds suc~ as ammonium dichromate, potassium chromate, ancl the like have not been found to produce any significant increase in hardne~s or strength. As a result these appear to be useful only for filling porosity and no bonding of the resultcmt oxide formed upon heating appears to be taking place within the porous body.
While many of these special chromium binders have been found to be very 20 useFul for spccific applicat;ons or for forming initial bonds, the impregrlant preferred for achieving m~ximum harclness and strength is invariably chromic acid~ Chromic acicl has a rr~rked tendency to fc~rm polyacids such as di-chromic,tri-chromic acid, etc. This polymer .~
izafion progresses with time as water is el;minated. We have ~und no noticeable clifferences as ~ar as bonding, hardness and strength is ooncerned whether the acid in use is freshly mixed or is several months old. The term chromic acid as used in this disclosure therefore is also intended to include the polymerizeci forms that may exist in solution.
All of the chromium binder comp~unds are normally used ;n relatively concen-trated lForm ;n order to achieve maximum chromium oxide bonding and dellsific~tion. Dilute solutions may have a tendency to migrate toward the surface of a porolJs part . ausing a 4 ~

surface hardening conclition. For certa;n appl;cations, of course, this may be desirable.
While in most cases water is used as the preferred solvent for the soluble chromium compounds, ot5~ers may often be used, such as alcohols, like isopropyl, methyl and the like, N-N, di-methyl formicle ancl the li!<e.
Upon curing at a temperature preferably in e~cess of 600F or higher these - ; soluble chromium compounds will be converted to a chromium oxide. For example, with increasing temperature chromic acid ~H2CrO4) will first lose its water and the chromium anhydride (CrO3~ that remains will then as the temperature is further raised be~in to lose oxygen until at about oOOF and higher, will convert to chromium oxide of the refractory form (Cr203 or Cr203 xH20). The same situation exists for the partially reacted soluble, complex, chromic acid form (x-CrO3 o yCr~03 zH20) discussed earli~r.
Chromium compounds~such as the chlorides, sulfates, acetates, etc. will also convert to Cr203 by heating to a suitable temperature. The chromates ail require a higher temperature to convert to the oxide form ~that is to a chromite or a chromite plus Cr203) than does chromic acid by itself. For the purposes of this disclosure chromites are considered to be a chromium oxide.
Bodies having the required interconnectecl porosity for the multiple, impreg-~; nation~ure cycle, chromium oxide bondina and densification method of this invention may be ~ormed by one of several methods. Keepin~ in mind that clear cut categories are hard to 20 make, these include systems such as the following:
(a) Glassy bonded .systems where the cons~ituent materials of the porous body have been bonded with the aid of a - flux or glass forming material. These require heating of the part - fo a tentperatur2 high enou~3h to form or begin to melt the glassy ~: constituent. Most of the commercial ~rades of ceramic materials, ~`1 inclucling even the 90% grades of alumina can be considered to ~11 in this category of bonding. Nucleated bodies ancl 100% alass bod;es can also be ;ncluded in this group.

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(b~ Seif-bonded skeletal bodies suitable for our process can also be prepared by partially sinterinq or underfiring a formed part to a temperature high enough to begin to establish bonding essen~
tially only at the points of conhct oF the rnaterial s~f which the body is composed. SelF-bonded silicon carbide for example is made in this way. While this is similar to the glassy bonded method, the term self~bonded is generally reserved for and is intended in this application to apply to relatively pure materials when a giass forming - material such as clay, sodium oxide, etc. has not been added.
The sintering temperature (and also pressure in the case of hot pressed bodies) will cletermine the extent oF bonding and the amount of interconnected porosity in the body. This method is also intended to apply to the formation of porous, sinterecl metal bodies and parts.
- (c) Chemically bonded systems where the bond is established by means of an added boncling agent. Examples of bonding agents are sodium silicate, mono~luminum phosphate, a silica such as DuPont Ludo~), etc. Some type of heat is usually required to cure `~ ~ these bonders but the process cannot be calîed sintering in the usual sense oF ceramic art.
2() (d) Cold pressed bodies where self-bonding is also the primaryboncling mechanism. An example is the pressing of certain metal powders, such as aluminum, copper, titani!im, cobalt and the like, where the forcin~ of the materials into close contact wiil cuuse a cold welding action to occurbetween the pieces or particlesp Some refractory grains and other materials such as aluminum oxide and the like will also cold press into a very well knit body ciue to - the interlocking action of the particulate structure. A Few s~f these h~ve been Found to have suffic;ent l'greenll strength without sintering to be subsequently processed by our method withou~ difficulty during the initial impregnation~ure cycle(s).
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(e) Oxide bonded bodias ~here the bonding is accomplish-ed between the consti-tuent materials by means of a natural oxide that forms during a heating cycle. An example is a part or body pressed or slip cast from boron carbide powder. During heating in an air or oxidizing a-tmosphere boron oxide will form on the grains creating a porous but bonded structure. Another example is a body bonded by means of special silicone - binders where a silicon oxide type bond is Eormed at low temperature.
(f) Chromium oxide bonding is a special case of (e) above. Here the initial bonding of the formed body is established by the use of a chromium compound which is converted to a chromium oxide by a heat cure cycle.
This method of bonding is often less expensive and/or more convenient than other methods, especially when considering that the same processing equipment and cycles can normally be used for establishing the :
initial bond as will ~e employed subsequently in the densification, haxdening and strengthening of the body.
It can be seen that the actual method of forming and bonding the initial body is not critical so long as the bonding is not destroyed by the soluble chromium based impregnant to be used during subsequent processing reacting ~herewith and providing the body has suitable interconnected porosity to allow adequate penetration of the impregnant. The bond must, of course, also - be enough to maintain the integrity of the body to at least the temperature employed to convert the chromium hea-~ compound impregnant to a chromium oxide form~ The following sections cover bodies and coatings densified, hardened and strengthened by means of acidic chromium binder impregnants according to a preferred method of the invention.

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1. Chromium Oxide_Processin~ of Preformed Porous Bodies : Some chromium oxide densification test results using porous, self-bonded silicon nitride (Si3N4) rods, 1/4" dia. x 5" long are shown in Table 1. These were pressure ''' ';' .
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impregnated using a concentrated (~1. 7 specific gravity) solution of chromic acid (CrO3 dissolved in H2O) as the impregnating solution. AFter a thorough impregnation, the bars were heated starting at a temperature of 350F and limited to a maximum of 1250F. The en~ire heclting cycle takiny approximately 1 hour. This is a sufFicient range to convert the chromic acid to a refractory chromium oxide (Cr2O3 or Cr2O3 xH2O). These impregnation-cure cycles were repeated for a total of 13 times in order to achieve the hardness values shown. The specifics of the impregnation-cure cycle method used is that identified as Method A as follows:
Method A
(a) Impregnating solution:
soluble chromium solution as specifiecl (b) Solution temperature:
ambient (room temperature) (c) Impregnation cycle:
; ~ 10 min. under solution at 95 psig `~ 20 min. under solution at 0 psig (ambient pressure) . .
10 min. under solution at 95 psig 20 minO under solution at 0 psig (ambient pressure) remove part from solution remove excess solution from part ~s~ ~cl) Cure cycle:

20 min~ at 350F
.~., 20 min. at 850F

20 min. at 1250 F

60 min. cool clown from 1250F to room temperature , (e) Number of impregnction-cure cycles: as specifiecl Thesc silicon nitride rods were so porous prior to densificatlon tha~ it was impossible to measure the h~rdness on the 15NRockwell scclle. The hardrless values after ~, the chromium oxide densification were 9~-97 on the 15-N scale, being in Fact, about as 30 high as miyht be expected for a hot pr~ssed Si3N4 body.

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A m~ximum of 1250F was used for the cure cycle as being sufficiently high to achieve a rapid conversion of the impregnant to chromium oxide.
- Table 11 shows the results of a comparison set of test bars made from recrystal~
ed silicon carbide material. As in the case of the silicon nitride bars just described, these were also^~" diameter x ~" lon~, had a fine interconnected porosity throughout ancl were processed in the same manner. The hclrdness determined on the Rockweil 15N scale prior h the Cr203 densification process was ^~15N-70. After processing, the hardness values ~;~ can be seen toihave reached extremely high values, numely 15N-97 to 98.
Samples of both the densified silicon nitr;de and the silicon carbide bars 10 described above were subsequently run in a sophisticated thermal shock test rig at tempera-tures alternstely cycling between a 2500F oxyacetylene flame and a room temperature air blast alternating at three minut~ intervalsO After 1000 such cycles neither the SiC nor the Si3N4 chrome oxide processed bars had cracked. In addition they showed no decrease in their modulus of rupture strength data as compcred to non-thermal cycled eontrol specimens.
; A large number of additional tests have been mc~de usin~ a variety of porous silicon carbide materials. These inciude self-bonded, oxide bonded and glassy bonded - silicon carbides and even materials made by converting carbon to silicon carbide by chemical-.
thermal conversion means. In nearly every case, a marked increase in hardness and/or stren0th has been observed. Table 111 lists a representative group of these materials that 20 were subsequently measured for hardness and fiexural strength (modulus of rupture). The Dow Corning bars, identified as DC, were ~t" diameter x 5" long while all the others were ,-~" x ~" x 4" in length.

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C-1 through C-5 and E-17 through E-24 were processed with the chromic acid binder C-1.7 as the impregnant for all cycles.
Samples DC-8 and C-8 used MC-1 as the impregnant for all thirteen cycles.
Similarly Samples E-11 through E-16 used MC-~ as the impregnant for all cycles. Results in all cases using tlle magnesium chromate or di-chromate solutions showed lower flexural strength and hardness than with the chromic acid impregnation method for all cycles~
In Sample G-34, the ZC-8 (zinc chromate~hromic acid) solution was used only as the impregnant for the initial impregnation-cure cycle and was then followed by straight chromic acid for the remaining twelve impregnation~cure cycles. This showed a slight improvement in flexural strength over Sample G-33 where chromic acid was used as the impregnant for all 13 cycles. In other tests, the use of one or two init;al chromate or chromate-chromic acid solution impregnations has often shown improved harclness and/or strength over that of chromic acid along as the impregnant for all cycles. In other cases, It has not. It is believed that the initial pore sizes of the body may play nn important par~ as to the optimum impregnant system. All of the samples in Table 111 were processed using the pressure impregnation and thermal cycle system called Methocl A except tests G-32, G-33 and G-34 which used Method Bo The processing details for Method B are as follows:
Method B
(a) impregnating solution:
soluble chromium solution as specified (b) Solution temperature:
ambient (room temperature) (c) Impregnation cycle:
10 min. under solution at 95 psig 20 min9 under solution at 0 psig (ambient pressure) 10 min. under solution at 95 psig 2~ min. under solu~ion at 0 psig ~ambient pressure) remove part fror!l solution and remove excess so!ution from pnrt , ' -17- .

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(d) Cure cycle:
20 min. at 350F
20 min. at ~00F
20 min. at 850~F
20 min. at 1250F
60 min. cool down from 1250F to room temperature remove any excess oxide build-up on parts after ea h cure cycle (e) Number of impregnation -cure cyc les: as specified Table V covers some comparative strength measurements made with silicon 10 carbide manufactured by the Norton Company. One group of bars was made by the more normal "high-Fired" self-bonding process in which noticeable recrys~allixation occurs. These bars are very similar to the Norton bars covered in Table 111 and which displayed somewhat errat;c strength data upon densification as compared to the Dow Cornin~ or Corning Glass Works material. The other group of samples in Table V used "low-firecl" silicon carbide material. As can be seen from the data, the incre~se in stren~ath between the densified versus non~densitied bars in the Illow-fired" group is very m~rked This test also shows that the Cr~03 densification of the "l~w~ired" m~terial provid~s substantially higher strength than ~or the 'Ihigh-fired" material. The "iow~firecl bars had a very fine and uniform grain structure compared to the 'Ihigh-Fired" ones and showed no obvious recryst~llization. This, 20 coupled with the probability that thf~re is more silicon ox;de present on the silicon carbide arains of the 'llow-fired" matcrial, may account for the improved results.
Table Vl shows test results for some silicon carbide coated graphite rosls in which the carbide co~ting has been densified with the multiple chromic acid impregnatior~-~' cure cycle method of this invention in compur;son with nontreateci rods. Againl the Cr203 densification provides a subst~ntiql increase in strength.
A variety of silicon carbide pwrts includin~ hollow~Gre seal rings andturb;ne bl~des and even glassy bonded abrasive hones huve been densif;ed by the multiple impregnatiorl-cure method of this inventi~r~. Again, the bondi~g and densificati4n that re-5ulll's between the chromium oxide and the silieon carbide provides a very not;ceable inerease ~ '~
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A number of different glass and silica based boclies having interconnected porosity have also been chromium oxide densifi0cl by means of our process. Among these are such materials as Corning Glass Works Vicor~ Foam ~a h;gh purity vitrified silica with very fine interconnected porosity), and Glass rock a~, a commercially available slip c~st fused porous silica. These materials have shown a definite increase in hardness and strength after chromium oxide densification, Table Vll shows test results for two porosity grades of Cercor~ (a (:orning 10 Glass Works' complex oxide product of lithium aluminum silicate and/or magresium alumir-um silicate) and one sample of Corning porous fused silicaO The two 9456 Cercor ~ sslmples (#9 and #10) were originally fired at different temperatures making their pore structure somewhat different from each other. Processing was done in accordance with Method B as set forth above.
Table Vlll shows flexural strength measurements ~or Corning Cercorq~ type 9455 lithium alumina silicate densifi~d in the same manner as the Cercor6D ~56 samples of Table Vll. The average strength in these tests was shown to slightly more than double due to the chromium oxide densification and bonding process, Table IX shows some compressive strength measuremen~s made with chrome ~ oxicle densified Cercor ~, lithium alumina silicate formed into a thin walled, honeycomb, heat exchanger material. In this case, oniy the thin inherently porous Cercor 0 walls of the honeycomb were densified. The excess chromic acid WCI5 carefully removed from the gas c fls~w passageways through the honeycomb structure after each imprea~nation cycle to prevent the p~ssibi3ity of ciosing up any of the intended honeycomb open;ngsO As can be seen, densifying the C:ercor ~ material forming the thin walls of the honeycomb structure greatly improves the compressive strength. The impregnation~ure cycles were per Method B, and `~ again C~1.7 listed as the impregnant signifies a chromlr a~id solutiQn with Cl specif;c gravity of ~1, 7. More recent test results have also shawn a considerable improvement in wear resistance of such chromium oxide densified heat exchangers when sliding against the 30 ~eals used in turbine engine test rigs at el0vated ternperaturesO

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Additional work has been done with heat exchangers made from a natural, mined form of lithium aluminum silicate known as petaliteO The strength increase results are quite comparable to the Cercor (3) material just described. The structure of the petalite honeycombs usecl is also inherently porous and readily accepts the chromic acicl or other soluble chromium compound impregnantO Very recent test r~sults have shown signiFicant wear resistance to turbine enginle seal materia!s at elevated temperatures with even a very few impregnation-cure rycles with the chromium oxide densification methocl.
Sintered metal structures having interconnected poros;ty have been successfully densified, bonded, hardened and strengthened using the process of this inventionO Table X
covers such an example using thre0 grades of a porous sinterecl nickel material. This table shows the buckling strength, hardness, modulus of ruptvre (flexural strength) and compressive strengths measured for non-densified versus chromium oxide densified samples, The increase in all of these listed properties due to the densification process is very significant. Again, the densification process of this invention evidences its ability to form stron~ oxicle type bonds, ~his time with a metal skeletal structure. As explained earlier, it is believed that the bond is established between the chromium oxide and the thin nickel oxide layer that forms on the nickel metal structure. Processing was by Method B with a chromic acid solution having a specific gravity of the order of 1. 7 and a maximum cure temperature of 1250 F for each cycleO
Table Xl shows the same ~eneral type of measurements just described for porous nickel samples of Table X but using instead a porous sintered iron materiai~ Here the processing used was Method A~ and C-l . 7 as the chromic acid impre0nantO Again, it can be seen thut some unusual properties for a metal cerarnic bonded composite does result.
Very little change in the tensile properties occurs while those related to compressive strength are considerably enhclnced over that of the porous metai structure prior to the chromium oxide processing. Values for oommercial, non~hsat treated 1020 steel are included for reference purposes onlyO
~me porous bronze bushings have been clensifie~l with 1. ~S specific gravity chromic acid solutionO So as not to noticeably ox;dize the bronzle alloy/ the maximum cure , , . - ,, . .
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temperature was limited to 9û0F. Except for the change in curing temperature, Method B
was used as the processing method. After nine impregnation-cure cycles, the hardness of the parts read an average of 55.0 on the Rockwell 15T scale. The non~densified bushings averaged a hardness value of 15T-12. The weight of a singie bushing increased from 10.38g to 11.35g after the nine cycles.
A sheet of sintered stainless steel felt material nbout 1/8" thick x 3" x 1~"
was processed in accordance with Method B using 1~6~ specific gravity chromic acid. Aside from the very noticeable increase in stiffness and elastic modulus and decrease in porosity, the Rockwell hardness increased From an average reading of 151-6 prior to processing to l~T-82. !i Gfter thirteen impregnation-heat cure cycles.
A hot pressed boron carbide cylinder about ~" in diameter by ubout 2" in length was obtained for chromium oxide processingO This part had a density of 79-81%.
During hot pressing the sinterina~ temperature, in combinution with the forming pressure, was low enough to obtain some interconnected poros;ty. Processing was accomplished using chromic acid (~1.65 s.g.) using Method B except that the maxjmum cure temperature was limited to 750-800F due to the tendency For B4C to severely oxidize above this range.
Prior to processing, the part weighéd 21.1 grams and after 6 cycles (no further increase in weight through 13 cycles was obtained) the weight had increased to 2307 grams. The hard-ness ch~nged from 45N-22 tc~ 37 with no chromium ox7de processing to 45N~42 to 64 at six impregnation-cure cycles to 45N-63 o9 at thirteen cycles. After the thirteen cycles the variation in hardness readings became much less pronounced indicating that the body had a markecl density (or porosity) non-homogeneity. It should be noted that processing was done in an air atrnosphere oven. Based on other tests with boron carbide bodies initially ohrome oxide bonded (rather than sintered as in the above sample), the use of an inert or reducing atrriosphere shouid have provided much higher hardness vulues by reducing the formation of boron oxide durin~ the h0at cure cycles.
i Only a very limited number of pre-formed non-oxide bodies have been readily available to use commercially, especially w;th a suitable interconnected pore structure. For this reason a fair sized group of nitrides, carbides, silicicles, complex oxi~des, metals, metal alloys, etc~ have been pressed in the laboratory from powders and underfired, cold pressed or chemically bonded into porous bodies suitable for treatmerlt with our chromium oxide process. Weight gains and hardness values have been mesured at different impregnation~ure cycle intervals to demonstrate that a very wide variety of suçh non-oxide pre-formed bodies can be significantly densifiecl, hardened and strengthenedO
Tests oF representative bodies made by underFiring (partial sinterirlg) of complex oxides, carbides, silicides, nitrides, borides and mixtures thereof are shown in Table Xll.
Data listed includes measured weights after 1, 3/ 6, 9 and 13 impregnation-cure cycles along with Rockwell hardness vaiues at 13 cycles. All samples listed were too soft to measure on 10 the Rockwell tester prior to receiving the chromium oxide processing of ~his invention. All samples listed were prPssed in the form o~ small rings about 1 1/8" od.x 5/8" id. with a thickness of about ~". (In some cases only part of a ring was densified accounting for the !ow weight in Table Xll for certain samples). Cure temperatures were limited to 1250F or less as indicated in the Figure. Processing was by means of Method B. Chromic acid with a specific gravity of 1,65 was used in all cases as the impregnant.
Table Xlll shows additional selected porous bodies pre-~rmed by pressing grains that were mixed wl~h a small amount of potassium silicate (Philadelphia Quartz Co.
Kasil ~iD #88 bre~nd). A small amount of SAE20 motor oil and oleic acid were also added as a lubricant to aid in pressiny. The parts were then cured at temperatures oF 1250F to 20 establish the potassium silicate bond. The oil of course burns off long be~ors this temperature is reached. The chromium oxide processing was again by means of Method B. The initial impregnation-cure cycle employed ZC:-5 as the impregnant, Table IV. All other cycies used chromic acid at a specific gravity of ~1.65. It can be seen from the data that these chemically bonded porous bodies can be densified and hardened by means of our process NS
readily as the sintered bodies previously described.
Cold pressed metal bodies densiFicatiori data is covered by Tahle XIVO These parts were made by pressing metal powclers at a pressure high enough tc cause "cold welding' at the contacting grain interfaces. Some degree of interlockîng oF the particulate structvre also probably is contributing to the "as pressed" strengthO As in the case oF the sintered or 30 chemically bonded reFractory grains or the sintered metal bodies clescribed earlier in this ~ , .

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disclosure, an excellent chromium oxide bonding, dens.iFication and strengthening occurs.. Qbvi.ous.ly, because of the hi:gh metal content of these processed bodies, the Rockwell hardness values are not as great as For most o~ the refractory materials. The chromium oxide proces.sing was identical to that used For the parts just described above in Table XIII.
2 Chromium Oxide Processin of Chromium Oxi.de ~onded Bodi.es.
9 _ _ This section covers a special case o~ the preformed bodies.
described in Section 1 abave, where th.e initial bond is made by means of a chromium compound, con.verted by heat to a chromium oxide. Subsequent chromium oxide processing by means of our multiple impregnati:on-cure cycle method will likewise provide parts with signifi:cantly increased hardness, strength and density in the same way as already shown for the proces.si:ng of preformed bodies bonded by sintering, and other means.
A co-pending patent application descri:bes a large number of refractory oxides, carbides, metals, etc. that can be chrome oxi.de bonded and subsequently chrome oxide densified, hardened and strengthened. The purpose here is to add materials that ~e have now found can be simi:larly processed.
These materials include nitrides, borides, inte.rmetalli:cs, complex oxides, .; ferrites., etc~, as well as. mixtures between thes.e including with oxides, carbides and metals. As stated earlier, we have found -that most any material can be chrome oxi:de bo~ded and suhsequentl~ processed that either is an oxide, has an oxide constituent, or will form a well adhering oxi'de layer on its surfaces. This of course as.sumes that there.is no reaction with the ;mpregnant and that th.e material is. capable o~ withstandi.ng the cure . 25 temperature wsed for the processing~
n addition to slip cas.t, pressed, extruded or other~ise formed ; fine grain parti.culate materials, this chrome oxi:de bonding and densi~icatio~
process can be us.ed wi.th fibrous: and other non-granular or non-powdered materials. For example., glas.s, ceramic or metal fi.bers, ceramic ~hiskers, or Woven g'la~s, cerami:c or metal cloth. lhe di.screte parti:cle~, Fibers, wires, etc. o~ the suitable materi:als to be bonded merely need to come in ' inti.mate (close) contact with each other i.n order to ~orm capillary or i:nters:t;ce small en.ough to retain the soluble chromium compound in position during the impregnation-heat cure cycle while the chrome oxide bond is being ~7~ ~ormed h~ 34 ~6~53~
Table XV siu)ws Rockwell hardness values for a number oF slip cast parts made from silicon nitride (Si3N4) grain. Also included are parts made with silicon nitride mixecl with various amounts and grain sizes of silicon carbide (SiC), aluminum oxide (Al2O3), chromium oxide (Cr2O3), zirconium silicate (ZrSiC)~), and a form of silica (listed as i-12SiO3). These parts were slip cast using the chromium ~Ibinding~ iiquid indicated and include: mclgnesium chromc~tes (MC 1 and MC-2), chromic acid with a specific gravity of 1~7 ~C-107) and a complex chromium compound made by dissolving chromium oxide in chromic acid (C~ These solutions are disclosed in Table IVO Processing was by means of Method B using thirteen impregnation~ure cycles at the maxirnum cure temperutures as listed 10 in the Table XVO The aluminum oxide gruins used in the tests of Table XV are listed as T-61-325 (which is a -325 mesh tabular alumina made by Alcou) and XA-17 and XA-16 SG
(which are reactive gracles of alpha alumina also made by Alcoa). All samples shown were slip cast as either small discs about 1 " in diameter by ~" thick or they were cast as plates and then cut into ~" x ~ll x 4" bars after 2 to 4 impregnation cure cycles. Slip casting was done by making a slurry of the grains with 2 parts wuter and 1 part chromium binder and pouring into metal molds laid on top of a plate of plaster of parisl. The method is identical to that in general use for slip casting ceramic parts except that the chromium binder is adcled to the water normally used. The parts can usually be removed from the plaster within a matter of minutes after the free liquicd has gone into the plaster and the parts placed direc tly 20 in~o a 350f oven for the start of the curing cycleO With the materials used in Table XV l~he initial chrome ox;de bond was strong enough to allow normal handling of the parts during the subsequent chromium oxide dens;fication, hnrdening and strengthenin~ ,orocess. As ccm be seen from the Figure, most of these purts r~ached very high hardness values.
Table XVI shows flexural strengtil ~modulus of rupture~ values obt~ined for some silicon nitride slip cast bodies of the types shown in the preceding table. The proeessing was also identical. 5Ome of these s~mples were also measured fs~r modulus o~ elasticlty using a sonic velocity methodO These results are also listed in the table where rneasured.
- Thermal expansion duta wc~s measured for Samples 002~ 00~ and 069, slip cast m~erials comprised of silicon nitride, sîiicon nitride-aluminum oxide mixture and .

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~5~46 silicon nitride-silicon carbide mixture as shown in Tables XVII, XBIII and XViX, respectively.
As might be expected, the measured expansion rates oF these chrome oxide bonded and densi-~ied mixh~res follow closely the published expansion rates of the constituent materials according to the proportions of each refractory involved.
Table XX shows hardness data measured for a sizeab!e group of nitrides, borides, intermetallics and mixtures thereof including mixture with rarbides, oxicles, complex oxides and meta!s. All samples were made as pressed rings approximately 1-1/8" od~ x 5/8"id. x ~" thick. The chromium binder was thoroughly mixed with the powder or grains to be pressed along with a small amount of SAE 20 non-cietergent motor oil plus oleic acid to act as a lubricant. The lubricant aids in obtaining a fine porosity in the pressed part due to improved compaction. The chromium binders employed in this test series included a con-centrated chromium chloride solution (CC-l) mixed 1:1 by volume with water and zinc di-chromate (ZC-2) also mixed with water 1:1 by volume. See Table IV for specifics on these and other binders. Processing was hy means of Method H using the maximum cure temperature indicated in the table. Method H is the same as Method B except:
(d) Cure cycle:
20 min. at 350F
20 min. at 750F

20 minO at 900F
20 min ~ at 1250 F

60 m;n. cool down from 1250F to room temperature remove any excess oxide build up on parts aFter each cure cycle Weights wsre measured after the initial chromium oxide bonding (labeled "Ox" ;n table) and again after 3, 6, 9 and 13 impregnation-cure cycles when applicable~ 15; N Rock~ell h~rdness values are also shown for these samples either at 13 cycles or after a significant hardness and strength had been achievedO Again, it can be seen from an examinaticn of the data that a w;de variety of ma~rials can be bonded, densifiecl and harclened by means of the chromium oxide bonding process of this invention. The term particulate in this applica-tion is intendecl to mean both finely divided powders and fibers and filaments woven and 30 unwovenu , - .

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A variety of glass and ceramic fiber insvlation material (non-bonded type) has been bonded by means of our chromium oxide boncling me~hod. The technique employed consists of saturating the fiber mat with the binder and then centrifuging at c~ high enough r. p. m. to remove the excess bincler From the large spaces between fibers, retaining the binder only where fibers actually cross and touch each other. Curing was clone starting at 350F for about 20 minutes, 700F $or ~0 minu~es, 900F for 20 minutes and finally 1250F
for 20 minutes. Th;s method worked very well and provided a well bonded and significantly stiffened structure. Table XXI lists a variety of such fiber samples that have been chrome oxitle boncled with the process of this invention. Table XXII shows the same irnpregnating and 10 processing method, Method H, applied to non-bonded metal fiber structures. In either the glass, ceramic, or the metal fiber structures listed in Tables XXI or XXil, a microscopic examination showed that a bond did indeed exist at every point where the fibers touched each other. Only a very thin surface coating of fibers could be detected elsewhere. In reality, this is just another variation of the basic process and illustrates that bonding will take place wherever suitable capillaries exist that can retain the solubie chromium compound and establish a chromium oxide bond upon heating. In the case of the aluminum metal fibers the maximum cure temperature was limited to 900F. Although not shown in Tables XXI and XXII, additional impregntion-cure cycles resulted in increased density and strength.
Still another chromium oxicle processing variation was tried involving the 20 bonding, densification and strengthening of flexible, non-bonded, woven S-glass cloth.
Here the bond was primarily established between the fine fibers making up the strands and where the strands of the warp and woclf cross over each of her. Processing was by means of soaking the cloth in impregnant and then draining off excess liquid by placing the specimens on a paper towel. Curing involved 15 minute cycles in a 350F, 700F, 900F and 1250F
oven, respectively. Impregnants employed included magnesiurrl and zinc chromate-chromic acid solutions MC ~, MC-~, MC-6, MC-1~, ZC-2, ZC-4, ZC-8 ancl ZC-10. See Table IV
for det~ils. Impregnation-cure cycles were varied from 3 to 13 times. The test results can be generally summari~ed in that maximum stiFfness after a few impregnation-cure cycles (such as 3-4 cycles) is obtained with high chromate content solutions such as MC ~ to MC-4 ~7-~i ~l ^ ~

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or ZC-2 to ZC~5. Solutions havin~ a higher chromic acid to chromate ratio such as ~C-8 or ~10 or ZC~8 or ~10 will produce a stifferr harder~ more britt]e sample but require more cycles to accomplish this (e.g. 7~13 cycles).
There are also a number of miscellaneous non-oxide materials that can be chrome oxide bonded and densified, hardened and strengthened with our process. Among these are such compounds as ferrites, aluminides, etc. For example barium ferrite grain of the order of -325 mesh size was bonded using a zinc chromate binder, ZC-5 (see Table IV) and pressed into a 1" diameter disc about 3/8" thick using a forming pressure of 5000 psi. After thre~ impregnation-cure cycles using Method B and 1250F maximum cure temperature the hardness measured 15N~67.1, on the Rockwell tester. At this number of cycles the lS disc had adequate strength for its intended purpose as a permanent magnet. (Subsequently placing in a strong magnetic field did in fact cause the disc to become permanently maynetized although the efficiency was only about equivalent to a plastic bonded magnet~.
3. Chromium Oxide Processed Co'atings .. ..... .. .. . _ _ . ........ . .
A co-pending application covers the use of our chromium oxide bonding, densification and hardening process for coatings consisting of oxides, mixtures of oxides and metal or metal alloys. We will now show that a number of other refrac-tories may be used for the particulate materials comprising such coatings.
The slurry type coatings are unique, from the chromium -oxide bonding and densification of solid bodies, in that a chromium oxide bond is established not only be-tween the particulate materials, grains or powders used to form the coating, but also between the coating and the substrate.
When coatings are bonded to non-oxide substra-~es such ~ .

~ - so -as metals, carbides, etc~ ! it is believed that the chromium oxide bond is ac-tually established to a thin oxîde ~ilm that is usually inherent, or at least is subsequently formed, on the substrate during the initial heat-cure cycle.
A number of such coatings bonded to a variety of substrates are shown in Table XXlll, The,se include coatings where the basic grains or powders used in the slurry are carbides, nitrides, borides, silicides, complex oxides and mixtures thereof, includir.g mixtures with oxides and metal powders. These coatings have been applied as a slurry to the ~ - 50a -v) r ~ ~ o 1~ 2 1~ Z
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substrates shown in the figure using a small brush. They could have also been applied usina an airbrush or dipping method depending on the part shape, coating thickness desired, etc.
The slurry is prepared by mixing the powder or grains with water and the chromiom based binder until a relatively thin consistency is achieved.
In Table XXIII the initial binder used with the coating slurry is in all cases ZC-5, a ~inc chromate-chromic acid solution as per Table IV. This has been mixed with water in a ratio of 1:2 by volume. In many cases it has been found that evacuating the slurry prior to application assures that all of the grains are wet with binder ~nd eliminates lumps and air bubbies. The coatings in Table XXIIl were all made using slurries evacuated in this manner. After the coating is applied ;t is leFt to air dry before being placed in a 350F oven for the beginning of the cure cycle. The maximum cure temperature used for each sample is listed in the t~ble. A typical initial bonding cure cycle would be 15-20 minutes at 350F, 700F, 900F and 1250F, respectively.
Following this first cure cycle, when the initial bond is established to the substrate as well as between the grains or particles comprising the coating, further chromium oxide boncling, densification and hardening is accomplishçcl using vur multiple impregnation-cure cycle method. All samples in Table XXIII were processed using Method G which is identical to H except:
Method G
_ .,~
Same as H except:
(c) Impregnation cycle:
5 min. under solution at 95 psig 10 minO under solution at 0 psig (ambient pressure) Because of the heating cycle required to establish a bond between the coat-ing and substrate, it is important that a reasonably good thermal expansion match between the two exist. I:or this reason it is often difficult to apply certain materials to a specific substrate. This can often be overcome in a practical sense by adding a high or a low expan-sion rate material to the slurry formulation. For example, a low expansion rate grain such as siltcon carbide coold be added to a relatively high expansion rate matericll such as -5~-.. . . .. . . . .

~ s~
zirconium oxide in the approximately correct proportions to match a substrate such as titanum metal which has a thermal expansion rate sornewhere between the two. Since most commonly used metals such as steel, stainless steel, bron;~e, aluminum ancl nickel ailoys have a relative-ly high expansion rate compared to mos~ refractory rnaterials, it has often been necessary to find a high expansion rate addi~ive in many practical coating applications. One such additive found to have relatively high thermal expansion properties is a natural mined product sol~l by Central Scientific Company as a "technical grade" silicic acid. X-ray difraction studies have shown that this is an imperfectly crystalized chalcedony or flint. This has been listed QS 1125iO3 in Table XXIII.
Greater coating-to-substrate thermal expansion mismatches can also be tolerated when the slurry has a high percentage of a somewhat ductile material such as a metal powder. Grit blasting the surface of the substrate also often helps in cases where a thermal mismatch is present. Some of the samples in Table XXIII have a grit blasted surFace and others are smooth as indicated under the heacling "Surface Preparation". Specimens 20-1 throu~h ~0-10 were li_htly grit blasted followed by a very thin nickel flash plating. In this case the chrome oxicle bond to the coating is being made to the nickel oxide layer that forms on the plating rather than to the steel substrate.
ln addition to slurry type coatings, a nurnber of other systems have been found to be feasible and very useful. These include the processing of inherently porovs coatings 20 such as oxalate coatings formed on steel, black iron oxide on steel, conversion or electro-lytrically formed coatings on titanium, anodized aluminum and hard chrome plating where the microcracks and porosity can be filled and bonded. In effect, these coating systems are similar to the processitlg of pre~formed bodies initially bonded by other than a chromium oxide bond as described in Section 1. It should be pointed out, however, that the chromium oxicle processing in the case of these pre-formed coatings also greatiy enhance the bond to th~ s~bstrate in addition to the densification, hardening and strengthening of the porous layer itself.
Table XXIV covers test specimens employing oxalate coatin~s on steel discs subsequently chromium oxide clensifiecl, bonded and harclenecl. Discs numbered 1 through 9 ., . . , ,,. . ,~ , . ~

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Q O ~ J 2 2 were prepared by first etching in a nitric acid bath (6 parts H2O) to 1 part concentrated HNO3 by volume). After thorouahly rinsing they were placed in a boiiing solution of fer-rous oxalate (FeC~O4 2H2O) to which about 1-2% of concentrated chromic acid had been added as an oxidizing agent. An excess of ferrous oxalate was added so that the solution was fully concentrated at all times. The steel discs were suspended on wires in the boiling solution for about 20 minutes after which time a yellowish grey oxalate deposit about .002"
thick had built up on the surfaces. The oxalate coated discs were then placecl in an oven at either 3~0~F or 500F. Heating at 3~0F did not convert the oxalate but did appclrently remove excess water because of a noted color change to yellow. The samples heated to 10 500F turned to a red color indicating a conversion of the iron oxalate coatin~ to iron oxide . . .
of the Fe2O3 form. As can be seen in Table XXIV some of these samples were measured on the 25g Vickers scale and showed that considerable hardness had been achieved. All of the oxalate coatings were quite soft and could be scratchecl with the fingernail prior to the chromium oxide multiple cycle treatment.
Processing was done usin3 Method G, except that the cure cycles were limit-ed to lû minutes at each temperature. These particular samples were only 7/8" diameter x .050" thickness. The maximum cure temperature used is listed in the figure along with tle impregnants employed. These included chromic acid (C-1.65), zinc chromate-chromic acid solutions (ZC-2 and ZC-5) and a compiex chromium compound (CRC-2) which is made by 20 reacting chromic acid with carbon. This latter was usecl mixed with water in the ratio of 3:1 by volume.
Smple 2-6 also included in Table XXIII was prepared in the same general way as Sampies 1-9 above except that the coating was applied using a commercial iron oxalate bath known as Hooker Ferrous Oxalate Bath sold by the Hooker Chemical Division Corporatton .
Samples A and B in Table XXlll were also similar but used a titanium oxalate coating deposited by substituting titanium oxalate for ferrous oxalate in the bath described earlier for Samples 1-9. Sample B in particular showe`cl a high deJree of hardness after the thirteen impregnation~cure cycles. It is assumed that the titanium oxalate coating converted .

to titanium oxide either during the preliminary 500F heating cycle or the subsequent 900~F
cycle when the chromium impregnant was cured.
Tabla X)~V shows a different variation where 1020 steel alloy cliscs have been coated with a commercial black iron oxide (Fe3O4) coatina. Subsequent chromium oxide densification and bonding resultecl in an increase in coating hardness. It should be noted that the b~ack iron oxide coating used could not be applied to a thickness ~3reater than about .00025" so that the 50g Knoop hardness values listed in Table XXV may be lower than actual values because of the influence of the softer substrate directly beneath the very thin coating.
The coated discs were pre-heated prior to processing at temperatures listed in the table to remove any water and convert into a porous coating prior to the chromium oxide clensification and bonding process. The coatings, originally black at room temperature, charged to a bright red at 750F indiçclting a conversion from Fe304 to Fe203. The chromium oxicle processing is therefore being applied to the Fe203 rather than the black Fe30~ coating. Impregnation was by means of CRC-2 and the same Method G process variations as used for the parts just described in Table XXIV.
Another method for forrning coafings involves the acid etching and oxidatîon of the etched surface. This method has been used with steel with excellent success and simply involves etching the surface of the steel with an acid such as nitric or hydrochloric and then plcing the etched parts (after water rinsing) in a furnace and heating ~o a temperature of between 6û0F and 1001~F in an air atmosphere. The heating causes some of the finely etched metal to oxiclize which forms a porous layer highly suitable for the chrome oxide ;lensification and bonding process. ProcessirE1 with multiple impregnation-cure cycles using chromic acid, complex chrom;um compounds (xCrO3 yCr2O3 zH2O), or chromat~-chromic acid mixtures have all providecl very hard and clense coatings after a number of treatments. The same basic method has been used to densify and bond chromiurn oxide eoatings that were ~rmed on a high chromium alloy metal in a controlled atmosphere. Again the coating became very hard aFter a few cycles.
Table XXVI shows 25 gram Vickers hardrless values for some electrolytrically applied titaniurn oxide based coatings applied to titanium metal~ This is a proprietaty coating .

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- : ' : ' system of Pratt and Whitney Div. of Unitecl Aircraft. Again, the chromium oxide processing resulted in a noticeable increase in hardness after several cycles. Best results were found by first henting the coated parts to 12û0F prior to st~rting the multiple çhromium compound impregnation-cure cycl~s. This opens up additional interconnected porosity within the speciaily coated surface allowing more chromium oxide densification and better bonding to the substrate than is possibie without the pre-heating. Two additional types of coatings applied to titanium have been similarly densified with substantial hardness increases. These were nonelectrolytrically applied proprietary coatings prod~ced by Titanium PrGcesssrs, Inc.
and an electroiytically applied coating system developed by Watervliet Arsenal.
Anodized aiuminum coatings are another interesting/ commercially available system that lends itself to improvement in density and hardness by our multiple cycle process.
Some of the hardness results ob~ained after 9 or 13 impregnation-cure cycles are shown in Table XXVII. Processing was again by Method G but the maximum cure temperatures were limited to 900F or less because of the relatively low melting point oF the aO61 aluminum alloy substrc~te. In all cases the hardness values were too low to measure prior to the chrome oxide processi ng .
Best results have been obtained by a pre-treatment of some type to remove water to hydration from the anodized coating. This may consist of pre-heating at an elevated temperature, soaking in concentrated hydrochloric or sulphuric acid or even reagent ~3rade ~0 methonol. For this same reason the anodized coatings should not be boiled in water or otherwise subjected to a so-called "sealing" process as isoften done in commerci~l anodizing as the final processing step.
Impregnating liquids usesl in Table XXVII inclucle the complex chromium compound ~CRC-2) used with water in the ratio of 3:1 by volume and chrornic acici ~C-1.65).
Best results with the densification, bonding and hardening of anodized aluminum have been made using an impregnant such as CRC-2 for at least a few cycles. USinJ a highly acedic solution such as rhromic acid tends ~o cause peeiing of the anodized layer, probably due to an attack of the alurrlinum substrate through the porous anodized layer.

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, ~13 Z 2: Z Z 2 2: z 2 2 o The anodized aluminum surfaces describecl above and in Table XXVII utilize the so-called "hard" anodize method. We have also successfully processed other commereially avaialble anodized aluminum surfaces, namely, those employin~ standard "sulphuric" and "chrome" baths. All three methods cause the formation of an aluminum oxide surface on the aluminum substrate. The coatings From the sulphuric ~nd chrome baths usually are thinner and less hard than those producecl by the "hard" anodize processO
All three of these types of commercial anoclized coatings have been checked for corrosion resistance using both concentrated hyclrochloric acicl and sodium hydroxide (3 Normal) solutions. The test consisted simply of placing drops of the solutions on the surface 10 of the coated samples and observing the length of time required for the formation of gas bubbles indicating an attack of the aluminum metal substrte. For all cases ~he chromium oxide processing significantly ;mproved the resistance to bofh the acid or base. The "hard"
anod;zed surfaces, after 7 more impregnation cure cycles (using CRC-2 as the impregnant) were found to be especially resistant to such chemical attack.
P~ departure from the processing of porous oxide surfaces is the chromium oxide densification of electroplated metai surfaces. Perhaps the system of greatest importance is the application of our process to chromium metal plating, especially to the type of plating sold as "hard" chrome. Chromium platin~ has an inherent problem of porosity. This takes the form of microcracks, often called "chicken wire crazing" that become more pronounced 20 and visible as the plating thickness is increased. We have found that multiple impregnation-cure cycles with our chromium oxide process not only densifies the microcracked plating but also improves the bond tc the substrate, greatly recluces fl~king and in most cases provides increased hardness even ~eneath the surface.
Tabie XXVIII shows test data ~btained from a group of chromium oxide pro-cessed, hard chrome plated steel discs using chromic acid or S:RC-2 as the impregnant.
Piated thickness a~ter finishing was about .002". All samples were heated to 750F prior to processing to remove p!atin~ solution, fin~er prints, etc. from the m;crocrclcks or pores of the plating. The first three samples listed (00,66 and lûl) were measured for hardness at 5, 9 and 13 impregnation-cure cycles wîthout lapping. Samples 10 through 15 were lapp~cl so as ,, 3~

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prepared with 7, 9 or 11 impregnation-cure cycles and the results indicate that a bond is bein~ established within the plating microcracks and the chromiurn oxide clepositecl therein.
Scanning electron microscope studies have been made of sectioned samples that show that the microcracks are indeed being filled with chromium oxicle. The successful use of chromium impregnants for chromium plating processing other than chromic acid or CRC-2 is logically anticipated based on the processing results of other porous bodie!s and coatings described elsewhere in this disclosure.
Finally, some preliminary tests have been made that show that chromium oxide processed plated parts show significantly improved salt water corrosion resistclnce 10 properties as compared to non-processed parts. Processing in one instance involvecl eleven impregnation-cure cycles using chromic acid (~1. 65 s.g. ) as the impregnant. The processing was done following Method G with a maximum cure cycle oF 900F. The parts were com-mercially produced steel centPr punches obtained directly from the plater ancl were from the same plated iot. Plating was approximately .00002" of chrome over .0003" of bright nickel.
The testing was done by supporting the punches about 1 " above an aerated tank of salt water kept at room temperature. This caus~d a constantly changing mist to settle on the surfaces of the parts. After 8 hours rust spots began to show on the non-treated sample. The test was stopped after two weeks at which time there were several badly corroded areas extending through the plating of the non-treated punch and no visible corrosion in the treated part.

Another somewhat different coating system involves the use of substrates without a pre-formed porous layer. In this special case the porous layer is established coin-cidentally with the multip,e cycle clensification and bonding process. More specifically, it has been found that very thin chromium oxide (or chromite-chromium oxicle) layers can be established which, after a sufficient number of impregnation-cure cycles, will become ex-tremely har:l, dense, and well bonded to the substrate. The chromium cornpounds that have been found suitable for this system include chromic acid/ the complex chromium compouncls of the generali~ed form, XCrO3 yC:r~03 zH2C) ancl metal chromate-chromic acid mixtures.
When used to coat metal sur~aces the metal is first cleaned by such means as ~cid etching, electrolytic or chemical cleaning or grit blasting prior to the first impregnation-cure cyc!e.

to remove approximately .0003" of the plated sur4ace. This effectively removed the very thin chromium oxide layer that had built up on the plated surface during the processing.
Sample 15 is a non-processed control disc from the same lot inc:luded for measurement com-parison purposes. Sampl~s 10 through 14 were measured for hardness after nine impregnation-cure cycles and after lapping. A significant increase in hardness over the non-densified Sample 15 is noted for all samples processed at 900F.
Samples 10 and 15, processed to 750F, were also checlced for leakage using a helium leak detector. The measurements were made by clamping the lapped d;sc against a clean, non~reased Vitron ~ "O"-ring seal placecl on the leak detector evacuation plate.
lû These two samples were also cleaned in hot tri-chlorethylene and then heated at 750F for about 2 hours to remove all oil and solvent after lapping. It can be seen frorn the figure that Sample 10, chrome oxide processed for 9 cycles, has a leak rate of less than 10-~ standard cc of heiium per second, while the non-densified control Sample 15 showed "gross leaks". By "gross leaks" it is meant that the leak detector could not be evacuated to a point where helium gas could be introduced into the detector to make a measurement. Adclitional leak tests of non-processed vs. chromiurn oxide processed hard chrome plated samples h~ve con-firmed that the interconnected microcracks are indeed sealed to a marked degree after several impregnation-cure cycles. These have been made with the helium leak detector method described above as well as with other methods. These consisted of a pressurized (200-300 psi) heiium leak method where gas bubbles are detected visually under water and with a hydraulic test where hydraulic fluid leak rates were measured.
Aclditionally, a number of samples were made from .t)30" thick 1020 steei sheet stock, plated with~. 001 " of hard chrome plate, and tested for piating-to~substrate adhesion and/or flaking. The tests were made using a simple bending test or an automatic centerpunch test ~pre-set to ~ g;ven impact level). A microscopic examination of the cracks extending from the overstressecl plating area after bending or denting was then rnade. In all cases a marked lessening of cracks ansl a much less extersîve area of plating rern~aved from the over~tressed areas was noticed in virtuaily ail cases for the chr~mium oxide processed speci-rnens as compared to the non-processed specimens. Processed samples in these tesls were 53~
The first few impregnation cycles are generally made by simply dipping the part in the impreg-nant. After the surface layer begins to build up and becomes more dense it is then often desirable to switch to a pressure imptegnation method such as Method G.
Coatings of this type have been successfully applied to a wide variety of metals and non-metals. Those that have provided extremely hard ~Moh's hardness>9) include:
416, 316 and 17-4PH stainless steel; stellite (both nickel and cobalt based types); Monel;
Inconel; Incramet, naval bronze, and other bronze alloys; aluminum oxide; boron carbicle-silicon carbide alloyed material; pyroceram; etc.

Hardness measurements of these very thin coatings have often been difficult t 10 make, especially where they are applied over relatively soft substrates. Most such measure-ments, therefore have been limited to scràtch tests using tungsten carbide or silicon carbide points. In this case Moh's values of greater than 9 are invariably measured for chromium oxide (or chromate-chromium oxide) coatings prepared in this manner whenever about 10 to 13 impregnation-cure cycles have been employed. Tests with acid over easily attacked substrates also show that the coatings have normally becvme very dense and impervious with such numbers of cure cycles. Some of these codtings have also been measured using the Vickers micro-hardness tester.
Table XXIX shows such Vickers hardness measurements made on coated titanium specimens processed using repeat~d impregnation-cure cycles. Samples 1 through 6 used a ~ 1.65 specific gravity chromic acid (C-1~65) for all cycles. Samples 7 through 12 used a complex chromium compound solution (CRC-2) mixed 3 parts to 1 part by volume with water.
Processing~was by means of Method G and the maximum cure temperature was 1200F.
Another process variation is the densification~ bondin~ and hardening of metal and non oxide refractory (or mixtures thereof) coatings applied to metaiior other sub-strates by means of flame spray, plasma spray, or detonation processes. For example, por-ous plasmo sprayed aluminum nitride coatings ~often used as an undercoating for flame and plasma sprayed cos~tings) has been densiFied using ~1.65 specific gravity chlomic ac;d solution. After 13 impregnation-cure cycles it was found to be very dense and have Rockwell hardness rneasurements greater than 15N-70. Prior to processing the coating was too porous and/or soft to read on the 15N scale.

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Another system that also appears feasible is the chromium oxide pr~ocessing of electrophoretically applied metal and refractory coatings.
From the foregoing disclosures, a wide number of possible coating systems will be obvious to those skilled in the art. The primary criteria is that a surface with inter-connected porosity be first establishecl on the substrate that can then be impregnated with a suitable chromium impregnant without destroying the coating surface and that can be subse-quently heat cured to form a chromium oxide.
In summary it should be stated that only acidic or acidified chromium com-pounds have been found suitable for use in forming well bonded coatings. Densification will 10 of course occur with neutral or basic compounds with pre-bonded coatings (such as flarne and plasma sprayed types) but increased bonding is not noticeable. Also, chromium compounds such as chromium chloride, nitrate, acetate, sulfate, and the like, although admittedly acid-ic in nature, have not been found to be particularly successful in establishing coatings well bonded to the substrate. Only the chromic acid or the complex chromium compounds of the general type xCr203 yCrC)3 zH20) or metal chromate-chromic acid mixtures have been found to make good impregnants where a strong coating~to-substrate bond is desired.
' ' .. .. . .

Claims (51)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of densifying, hardening and strengthening of a body having interconnected porosity which comprises the steps of:
impregnating the porous body with a solution of a soluble chromium compound capable of being converted to chromium oxide on being heated;
drying and curing said impregnated body by heating same to a temperature sufficient to convert the chromium compound in situ to chromium oxide; and, repeating the impregnation and curing steps at least once to densify, harden and strengthen the body, wherein the body consists of a material which is either comprised of an oxide, includes an oxide constituent or has a well adhering oxide on its surface, is insoluble in and non-adversely reactive with the solution of a chromium compound selected as an impregnant, and is inherently temperature stable to at least the minimum heat cure temperature employed in converting the chromium compound impregnant to chromium oxide.

2. The method of claim 1 wherein the bodies are comprised of materials selected from the group consisting of nitrides, carbides, silicides, borides, intermetallics, ferrites, metals, metal alloys, oxides, complex oxides and mixtures thereof.

3. The method of claim 1 wherein the bodies are comprised of materials selected from the group consisting of silicon carbide, silicon nitride, cesium stannate, cesium titanate, cesium zirconate, magnesium titanate, magnesium stannate, strontium zirconate, chromium carbide, boron silicide, chromium silicide, titanium boride, vanadium boride, aluminum borocarbide, vanadium carbide, titanium nitride, iron nitride, boron carbide, molybdenum silicide, zirconium silicate, calcium titanite, magnesium stannate and cesium zirconate.

4. The methods of claim 1 wherein the chromium compound is a chromic acid.

5. The method of claim 1 wherein the chromium compound is of the general formula xCrO3?yCr2O3?xH2O.

6. The method of claim 1 wherein the soluble chromium compound is an acidic chromium compound.

7. The method of claim 1 wherein the chromium compound is a mixture of chromic acid and a chromate.

8. The method of claim 4 wherein the chromium compound is chromium trio-xide dissolved in water to provide a specific gravity of about 1.65.

9. The method of claim 4 wherein the chromium compound is chromium trio-xide dissolved in water with excess chromium trioxide added to provide a specific gravity of about 1.7.

10. The method of claim 5 wherein the chromium compound is chromic acid reduced by a reducing agent selected from the group consisting of tartaric acid, carbon, formic acid.

11. The method of claim 5 wherein the chromium compound is chromic acid having dissolved therein a compound selected from the group consisting of Cr2O3, Cr2O3 ?
xH2O and chromium hydroxide.

12. The method of claim 10 wherein the chromium compound is chromic acid having an amount of carbon dissolved therein sufficient to provide a specific gravity of about 1.7.

13. The method of claim 11 wherein the chromium compound is chromic acid having chromium oxide dissolved therein to provide a specific gravity of about 1.84 which is then diluted about 3 parts chromium-solution to 1 part water.

14. The method of claim b wherein the chromium compound is a reaction product of chromic acid and an oxide selected from the group consisting of magnesium and zinc oxides.

15. The method of claim 14 wherein the chromium compound is chromic acid having magnesium oxide dissolved therein to provide a specific gravity of about 1.3.

16. The method of claim 14 wherein the chromium compound is chromic acid having magnesium oxide dissolved therein to provide a specific gravity of about 1.65.

17. The method of claim 14 wherein the chromium compound is chromic acid having zinc oxide dissolved therein to provide a specific gravity of about 1.65.

18. The method of claim 12 wherein the carbon is dissolved in the chromic acid with a ratio of about 9 parts by weight of carbon to about 240 parts by weight of chromium trioxide.

19. The method of claim 13 wherein the chromium oxide is dissolved in the chromic acid with a ratio of about 210 parts by weight of chromium oxide to about 1812 parts by weight of chromium trioxide.

20. The method of claim 15 wherein the magnesium oxide is dissolved in the chromic acid with a ratio of about 40.3 parts by weight to about 100 parts by weight of chromium trioxide.

21. The method of claim 16 wherein the magnesium oxide is dissolved in the chromic acid with a ratio of about 40.3 parts by weight to about 200 parts by weight of chromium trioxide.

22. The method of claim 16 wherein the magnesium oxide is dissolved in the chromic acid with a ratio of about 40.3 parts of weight to about 400 parts by weight of chromium trioxide.

23. The method of claim 16 wherein the magnesium oxide is dissolved in the chromic acid with a ratio of about 40.3 parts by weight to about 600 parts by weight of chromium trioxide.

24. The method of claim 16 wherein the magnesium oxide is dissolved in the chromic acid with a ratio of about 40.3 parts by weight to about 1000 parts by weight of chromium trioxide.

25. The method of claim 17 wherein the zinc oxide is dissolved in the chromic acid with a ratio of about 40.7 parts by weight to about 200 parts by weight of chromium trioxide.

26. The method of claim 17 wherein the zinc oxide is dissolved in the chromic acid with a ratio of about 40.7 parts by weight to about 400 parts by weight of chromium trioxide.

27. The method of claim 17 wherein the zinc oxide is dissolved in the chromic acid with a ratio of about 40.7 parts by weight to about 800 parts by weight of chromium trioxide.

28. The method of claim 17 wherein the zinc oxide is dissolved in the chromic acid with a ratio of about 40.7 parts by weight to about 1000 parts by weight of chromium trioxide.

29. The method of forming, densifying, hardening and strengthening of a body having interconnected porosity which comprises the steps of:
forming a porous body of a material which is either comprised of an oxide, includes an oxide constituent or has a well adhereing oxide on its surface; is insoluble in or non-adversely reactive with the solution of chromium compound selected as an impregnant;
and is inherently temperature stable to at least the minimum heat cure temperature employed in converting the chromium compound impregnant to chromium oxide;
impregnating the formed body with a solution of a soluble chromium compound capable of being converted to chromium oxide on being heated;
drying and curing said impregnated body by heating same to a temperature sufficient to convert the chromium compound in situ to chromium oxide; and, repeating the impregnation and curing steps at least once to densify, harden and strengthen the body.

30. The method of claim 29 wherein the bodies are comprised of materials selected from the group consisting of nitrides, carbides, silicides, borides, intermetallics, ferrites, metals, metal alloys, oxides, complex oxides and mixtures thereof.

31. The method of claim 29 wherein the bodies are comprised of materials selected from the group consisting of silicon carbide, silicon nitride, boron carbide, molyb-denum silicide, zirconium silicate, calcium titanite, magnesium stannate and cesium zirconate.

32. The methods of claim 29 wherein the chromium compound is a chromic acid.

33. The method of claim 29 wherein the chromium compound is of the general formula xCrO3 ? yCr2O3 ? zH2O.

34. The method of claim 29 wherein the soluble chromium compound is an acidic chromium compound.

35. The method of claim 29 wherein the chromium compound is a mixture of chromic acid and a chromate.

36. The method of claim 29 wherein the chromium compound is chromic acid reduced by a reducing agent selected from the group consisting of tartaric acid, carbon, formic acid.

37. The method of claim 29 wherein the chromium compound is chromic acid having dissolved therein a compound selected from the group consisting of Cr2O3. x H2O, Cr2O3 and chromium hydroxide.

38. The method of claim 29 wherein the chromium compound is a reaction product of chromic acid and an oxide selected from the group consisting of magnesium and zinc oxides.

39. The method of claim 29 wherein the material from which the body is formed is particulate and has an amount of the chromium compound mixed therewith prior to forming sufficient to provide chromium compound at each point of contact between the particulate material.

40. The method of claim 29 wherein the material from which the body is formed is a slurry of finely divided particles in a liquid which is applied to a substrate to form a coating thereon.

41. The method of claim 40 wherein the slurry is comprised of a mixture of particles of a relatively low expansion rate material and particles of a relatively high expansion rate material in such proportions as to provide a combined expansion rate substantially matching the substrate to which the slurry is applied.

42. The method of claim 40 wherein the slurry includes an amount of a ductile metal powder sufficient to tolerate substantial thermal expansion mismatches between the substrate and the cured coating.

43. The method of claim 29 wherein the porous body is a porous coating formed on a substrate and wherein the porous body comprises an in situ oxide coating on a member selected from the group consisting of stainless steel; stellite; 2/3 nickel 1/3 copper alloy; 78% nickel, 15% chromium, 6% iron alloy; naval bronze and bronze alloys; and boron carbide-silicon carbide alloyed material.

44. The method of claim 43 where the porous coating is electrolytically formed on the substrate.

45. The method of claim 43 wherein the coating is iron oxalate.

46. The method of claim 43 wherein the coating is iron oxide.

47. The method of claim 44 wherein the coating is anodized aluminum.

48. The method of claim 44 wherein the coating is titanium oxide.

49. The method of claim 44 wherein the coating is hard chrome plating.

50. The method of claim 7 wherein the chromates are selected from the group consisting of magnesium and zinc.

51. The method of claim 29 wherein the chromates are selected from the group consisting of magnesium and zinc.