Reaction ofCobalt-Chromium Alloywith Investment

Reaction of Cobalt-Chromium Casting Alloy with Investment

F. C. ALLAN and KAMAL ASGARUniversity of Michigan, School of Dentistry, Ann Arbor, Michigan

SYNOPSIS IN INTERLINGUAREACTION DE ALLIGATO FUSIONAL DE COBALT E CHROMO CON INVESTIMENTO.-Alterationes chimic occur-rente intra le investimento ligate a phosphate durante le prisa e le cocer esseva determinate per technicas adiffraction de radios X. Es postulate le occurrentia de certe reactiones durante le processo. Tests del resisten-tia al compression effectuate a varie temperatures corroborava ille postulates.

Esseva trovate al interfacie inter alligato e investimento tres productos de reaction. Immediatementeadjacente al alligato se formava un scalia de color gris sequite de un intimemente adherents scalia de colorverde. Esseva etiam trovate intra le investimento areas de un composite de color rubiastre-brun. Le sondaelectronic esseva usate pro analysar iste productos.

Cobalt-chromium alloys have been appliedin the field of dentistry as investment cast-ings for partial dentures. These alloys arelight, strong, and corrosion-resistant, butthe metal does not have adequate ductilityfor adjustment purposes. Previous researchwork has shown that one means of obtain-ing this property is to superheat the alloybefore casting.' However, this procedurecauses extensive reaction of the metal withthe mold and a close-adhering green scaleforms. Removal of this scale requires ex-cessive machining, and loss of dimensionalaccuracy results. As part of a project to im-prove cobalt-chromium casting alloys, astudy was made to find means of minimiz-ing this scale formation.To analyze the reaction occurring at high

temperature, the nature and chemistry ofthe investment must be known. Work hasbeen done in the ceramic field on many ofthe compounds present in the investment.Also, in a general way, dental researchershave discussed reactions that may occur.But, the information is scattered and in-complete.

The phosphate-bonded investment wasconsidered in this investigation, since thisinvestment has favorable high-temperatureproperties and had been used in the previousinvestigation.' In the phosphate-bonded in-vestments, a soluble phosphate compound

Presented before the Dental Materials Group of the IADRat Toronto, Canada, July, 1965.

This investigation was supported by U.S.P.H.S. researchgrant DE-02017 from the National Institute of Dental Research,National Institutes of Health, Bethesda, Md.

Received for publication January 7, 1966.

1516

(e.g., NH4H2PO4, Mg(H2PO4)2, or H3P04),which produces a phosphate ion, and a me-tallic oxide (such as MgO) form the bondwhich gives the strength to the investment.

According to Moore and Watts,2 the basicreaction causing setting at room tempera-ture is NH4H2PO4 + MgO- NH4MgPO4 +H20. This is considered a temporary crystal-line bond and is expected to decomposeupon heating. Gilham-Dayton3 has postu-lated that the product formed could be di- ortrimagnesium phosphate or NH4MgPO4*6H20. Kiehl and Hardt4 have shown thatthe compound NH4MgPO4.6H20 forms anintermediate hydrate (NH4MgPO4-H20) atabout 500 C. upon heating. Above this tem-perature, dissociation of the ammoniumsalt begins, so that by 2500 C. Mg2P207forms. Earnshaw5 has found experimentallythat on heating expansion slows to the pointof shrinkage at 1000 C. and attributed thisto a loss of water. He also found, "Copiousevolution of white fumes occurred in thetemperature range 200 260° C.; at the sametime ammonia was also given off." A shrink-age also occurred at temperatures above2800 C., in his opinion due to chemical re-action.

The bond that exists at high temperaturesis attributed to a silicophosphate bond. Tienand Hummel6 have formed SiO2*P205 quitereadily by heating NH4H2PO4 and silicicacid together and holding at 700° C. Mooreand Watts2 have felt that the silicophos-phate bond is formed at the 2800 C. tem-perature.

Another point considered was the green

REACTION OF Co-Cr ALLOY WITH INVESTMENT 1517

TABLE 1

CHEMICAL ANALYSIS OF THE ALLOYS USED

Cr W Fe C Si Co Ni Mn Mo B P S

HS21. 27.43 .... 1.67 .27 .68 Bal. 2.51 .69 5.45 >.001 .... ....

HS31. 24.81 7.54 1.65 .48 .72 Bal. 10.47 .78 .... ..... .016 .011

scale formed at the interface during casting.Morral* has pointed out that a reaction canbe expected at high temperatures when MgOis used in the investment. Sully7 has alsodiscussed problems of mold reaction en-countered during casting of chromium alloysand has advised using the highest meltingrefractory mold materials (e.g., zirconia orthoria). Chromium was found to oxidizequite readily during casting. Work has beendone by Phalniker, Evans, and Baldwin8 onthe oxidation of Co-Cr alloys. They havefound that the alloy was most resistantwhen 25 per cent chromium was added tothe cobalt, and that selective oxidation ofchromium protected the alloy. CoO wasfound immediately adjacent to the alloy.Chromium compounds, rather than those ofcobalt, occurred at a further distance fromthe metal surface, indicating a higher diffu-sivity of the chromium as compared withcobalt. Felten and Gregg9 have found theinitial oxidation product to be Cr2O3. Littlework other than oxidation of the alloy hasbeen done, and no results have been reportedon interface reaction of Co-Cr alloys withinvestment.

In this investigation, the compoundspresent at the various stages of the mixingand casting procedure were analyzed byx-ray-diffraction techniques. To verify theresults obtained on the green scale reactionproduct, samples were also analyzed by theelectron probe. By combining the presentknowledge with our results from x-ray-dif-fraction and electron-probe analysis, wepropose certain reactions to occur duringthe investing and casting procedures. Com-pressive strength tests were then run to testthe validity of the reactions.

Materials and MethodsPhosphate-bonded investments have the

following approximate composition: quartz* F. R. Morral, Cobalt Information Center, personal com-

munication.

and cristobalite, 90 per cent; MgO, 3 percent; acid phosphate, 7 per cent. The se-lected investment was mixed with liquidand the following stages were selected foranalysis: (1) investment powder, (2) set in-vestment, (3) burned-out investment, (4)green scale formed as reaction product withtwo casting alloys.§

The compositions of the alloys are givenin Table 1.

The powder method of x-ray diffractionwas used for analysis.'0 The samples wereground and passed through a 300-meshscreen. The powder was mixed with ce-ment, formed into a rod-like shape, andmounted in the camera. The high silica con-tent limited the extent to which analysiscould be carried out, and the silica lines pos-sibly obscured lines of minor constituentswhich could be present. Separation methodswere used in stages 2, 3, and 4 and aided inthe analysis. However, some lines were notidentified.To avoid any possible fluorescence, Cu

radiation was chosen as most suitable foranalysis of the investment. A Ni filter wasused. Since any of the metal could be presentin the scale in stage 4, the use of a Cu targetwas questioned. However, very little scatterwas found in the results, which would indi-cate that very little or no Co was presentin the green scale.

Microscopic examination has shown threedifferent reaction products occur. The greenscale is predominant. A reddish-brown com-pound also occurs occasionally as discreetnonadhering crystals. The other scale is agray reaction product, which occurs in avery thin layer (actually, between the greenscale and the metal) immediately adjacent

t Ceramigold Investment, Whip-Mix Corporation, Louis-ville, Ky.

t Ceramigold Liquid, Whip-Mix Corp3ration, Louisville,Ky.

§ Haynes Stellite 31 and 21 alloys, Kokomo, Ind.11 Duco-cement, DuPont, Wilmington, Del.

Vol. 45, No. 5

t5t LL IN tAN 4S [AR f dn f e Octbe 1966

TA\13IJKE 2

(COMPOUNDS FOUND J)URTIXG X RAY IDIFRACTlON AN A.LysIS

1. Ituves/ment powder:a-Quartza-Cristol)alite

2. Set investment:a-( )uartza-CristobalifcMg

3. Ironed-ntl investment:a Quartza-(ristobalileC\lgO

4. lorned as eceleion prodoict:*a-Quartza-CristobaliteMTg (PO0),

MgONH4H2TO(4

NHl-2bP01NH:MgPO4. HO9(g1(g P04) 2 - 4H2)

MIg.21)')7SiPl).()7Mg, (1O.)0)

MNIgCr2O, (MgO*Cr203) (most p)rob)abl1e)Cr101j (CrO*Cr.103) (possible)

*No differences were found in the scales formed by the two different alloys.

to the cast metal surface alnd is intermingledwith the investment.

Investment samples containing the reac-tion products were embedded in miethvlmethacrylate.* These saliples were thenpolished petrographically until a flat surfaceof the desired area was obtained. Two dif-ferent probes were used for the electronprobe analysis. One sample analyzed (Fig.1) contained the green scale (QI) and thereddish-brown compound (B). Scans wererun on these two areas and on the invest-ment inmmediately adjacent to these areas.The other sample analyzed$ (Fig. 2) was ofthe gray reaction product. These resultswere obtained as elemental readout.

In order to test whether the sulicophos-phate bond does indeed form at 280° C. as

* Castolite, Castolite Compiany, Woodstock, Ill.Noreko AMR/3 Electron Probe Microanalyzer, Philips

electronic Instruments, Mt. Vernon, N.Y.+ AMX, Applied Research Laboratories Glendale, Calif.

proposed by M\Xfoore and Watts, the crushingstrength was determined. Three sampleswere prepared for each temperature. Tem-peratures were chosen in a step-wise fashionto coincide with possible reaction points,and samples vere heated to each of thesetemperatures and kept at that temperaturefor 3 hours before the>- were tested. y3v thismeans, the breakdown of the room-tempera-ture bonds also could be followed.

ResultsTrhe compounds that were identified in

the various stages are listed in Table 2.Typical d-spacing valtles are shown in

m- wtl iERiR;x,: '-'ji:En: :i _ (E\4

BE -gil:E E. ::^ i':

',fi'' :1,.aw i:

tE.tE,

:R :E :e iER,,E''

F1G. 1.--Microstructure of areas in the invest-ment analyzed by microprobe: A, green scale; B,reddish-brown scale.

luG. 2.-Photomicrograph of the investment con-taining the gray scale. Analysis was made along thearea between the two Iparallel lines.

J. denl. Res. September -October 1966


TABLE 3

TYPICAL D-SPACING RESULTS(RUN 1-INVESTMENT POWDER)

a-SiO2 a-Cristobalite

. ..... . .

4.26 (35). . . .. .

..

3.343 (100). . .. ...

....

..

.....

..

..

2.458 (12). . . . ..

..

2.282 (12)2.237 (6)2.128 (9).... .. ... ..

..

..

1.980 (6). . . . . . ..

. ..

1.817 (17). . . .

..

..

1.672 (7)1.659 (3)1. 541 ( 15).... ..... .

.. .

..

..

..

1.435 (3). . . . ..

..

..

..

..

.. ..

. . . . . . . ..

..

4.05 (100). . . . . .. .. .

3.515 (3)..

3.135 (11). . . .. ..

.. .....

2.841 (13).... ..... .

. .. ....

2.485 (20).. .

2.465 (5). .. .. .. ..

. ... ..

2.340 (1). . . . ..

2.118 (5)

2.019 (3). .. ..... .

. . ..

1.929 (5)1.870 (7).. .. ..... .

1.690 (3). .

1.533 (3)1.494 (5)..........

....

.....

...

....

....

NH4H2PO4

5.32 (100).. .. ... .

.. . .

3.75 (64). .

.

3.075 (89)3.065 (75)

2.659 (18)2.651 (15). . . . ..

2.373 (8).. . . . .

..

2.009 (29)2.004 (22). . . . . ..

.

1. 773 (5)

1.685 (5)1.677 (4).. .

1.537 (4). .i.. 7...

1 .473 (41.470 (5)

. . . . .

..

..

..

MgO

. .

2 .431 (10)..

..

..

2.106 (100)..

...

..

1.489 (52). ..

1.270 (4)1.216 (12)1.053 (5).9419 (73).8600 (15).8109 (3)

Table 3 for the run of the investment pow-der. Figures 3 and 4 show the probe resultsobtained on the green area (A) and on theinvestment adjacent. Figures 5 and 6 showthe results for the reddish area (B) and theadjacent investment. A scan was madeacross the line marked in Figure 7 for eachof the elements which could be present in

the area shown in Figure 2. These are pre-sented graphically in Figures 8 and 9. Sinceeach run was set for maximum intensity, theheight of each reading relates the quantityof that element present. Manganese is notincluded in the results, since it did not regis-ter anywhere in this sample.The results for the crushing strength tests

Vol. 45, No. 5

Intensity

Scat MScat MSWide VSScat MScat W

VVWWide VS

MScat MScat M

MScat MScat MScat MSScat WScat MS

WWVWMM

Scat MWMS

Scat M

MWWMSMSVWVW

Scat MScat MScat MScat W

MVVW

Scat MScat W

VVW

WSe- Wlect-edfor WMgOQW

d-Value

5.3244.2644.0443.7863.7033.4473.3573.1433.1023.0652.8492.6612.6412.4912.4782.4632.4392.3802.3372.2852.2442.1302.1222.1102.013

1.9821.9351.8761.8251.8151.7781.6951.6771.6761.6731.6611.5411.5331.4931.4911.474

1.4551.2711.2171.050.9421.8591.8107

30*

A K <, (5 t h |3 4@

340 r K (3rd) fl K (3rd)

386

40o1d

420

44 Men K(14thl44046° S CO K9 (3rd)

48Mn K(4th)

5 00

52054ft_ Cr K,(4th)

MUn at, (5th)

rAS Lc,(lISt)As L(m K()st)

<-500c/s Full Scale >

6

64v

68

SPECTRAL SCAN ON GREEN SCALE

FIG. 3.-Microprobe results for analysis of green scale.

1520

32?> Mn Ks,(3d)3 IW

As K1(55th)

r K (3rd)

40 I -~~~~Si Kojist)42a

44

486

M n K,(4 th)5

52-Cr K (4th)

54P

56 %ll

58, --500 c/s Full Scale->

608

621L Cr Kg(5th) Kn 5th)

64

6 61

_ G~~~~~~~~~r K,,(5th)7 no _" _r- LI AIr

1521

INCLUSION ADJACENT TO GREEN SCALE

FIG. 4.-Microprobe results for analysis of area adjacent to green scale.

I w F*

50'to -A

3 flMn KO(3rd)34.-

Cr Kp(3rd) Mn Kbe3rd)

38.

I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~IO36' 17 -1

42

44

- , ~~~Mn Ka(4th)5

5

5 4 ~~~~~~Cr Kc:(4th)

5

5 Mg Kac|.sf< 500 c/s Full Scale-*

64'6~~~~~~~~~~~~~~~~~~~~~~~~~~

70SPECTRAL SCAN ON REDDISH-BROWN AREA

FIG. 5.-Microprobe results for analysis of reddish-brown scale.

1522

1523

31

3 :

3'

34

31

44

4'

4

4

4

5

5

5

5

5

6

6

6

6

6

INVESTMENT ADJACENT TO REDDISH-BROWN AREA

FIG. 6.-Microprobe results for analysis of area adjacent to reddish-brown scale.

J. dent. Res. September-October 1966

TABLE 4

STRENGTH OF INVESTMENT

Temperature(C.)

Room temperature (2 hours).

Room temperature (6 (lays). .

200................

250 ..................

300.................

450...................

500...................

1,000 ...................

1,300 ...................

1,400 ..................

Compres-sive

Strength(p.S.i.)

Stand-ard

Devia-tion

1,700 40NH4MgPO4' 6H20

2,100 72 1NH4MgPO4*xH2O

2,000 85 j

1,800 10 Mg2P207(2500)

1,700 65 completed

2,100 65

2,300 50

1,150 128

1,600 25133,100+ ... fusing

Remarks

Excess WaterMg3(PO4)- 4H20

Mg,(PO4)2.xHO

Mg3(PO4s2completed

Ifusing

are shown in Table 4. The standard devia-

tions of the compressive-strength values are

shown. This table also correlates the reac-

tions, so that compound formation andbreakdown can be directly related tothe strength at that particular tempera-

ture. The analvsis-of-variance results ofthese strengths at the various testing con-

ditions are shown in Table 5.

DiscussionThe presence of MAgO throughout is at-

tributed to the fact that the compound as

received is impure; the manufacturers adjustthe amount added to the investment to havea certain percentage of reactive MgO.* The

* E. Steinbock, Whip-Mix Corporation, personal communi-cation.

TABLE 5

ANALYSIS OF VARIANCE FOR COMPRESSIVESTRENGTH FOR DIFFERENT

CONDITIONS

Degrees

Source ofVariance

Sum of Fr

Squares dc

Specimens (Columns) 712.64

Experimental Error

(Residual) .......... 15.28

ofee- Meanmrn Square

9 78.07

20 0.764

FIc,. 7.-Back-scatter pattern of same area shownin Figure 2. Line designates path of electron beamduring scans for the elements.

F of the samples =7807/0764 = 102.1. F for 95 per centconfidence at N = 9, N2 = 20, is given in tables (3.96). (A. J.

Duncan, Quality Control and Industrial Statistics, 1955, p. 620.)

SiO2+P205

ISiP502

breakdownof bond

IU~~fusing

extensivefusing

1524 ALLAN AND ASGA R


MgO that remains is unreactive at mix- 2NH4H2PO4 + 2MgO + 6H20ing temperatures, although reactivity may

> NH4MgPO4*6H20 + NH4H2PO4 (excess)change with temperature increase.By coordinating the literature survey + MgO unreactivee) + H20 .

with the results from this investigationthe following reactions are proposed. At At 1000 C. expansion slows to the point ofroom temperature during mixing of the in- shrinkage, probably due to loss of excessvestment: water in the mix. Also water loss, starting

Si

p

I -1

K -33 ' 2 IEACH MAJOR DIVISION= 20,P

FIG. 8.-Line graphs of investment elements analyzed in Fig ire 7.

I i I-

I-

Vol. 45, No. 5

I I a-1

1526 ALLAN AND ASGAR

at 500 C., occurs as the hexahydrate is re-duced to the monohydrate :4

NH4MgPO4 6H20-* NH4MgPO4*H20 + 5H20.

At the same time, ammonia also beginsto leave the salt. Under equilibrium condi-tions, the monohydrate converts as followsby 2500 C.:

2NH4MgPO4 H20 -+ Mg2P207

+ 2NH34 + 3H20T1.Although not identified in the x-ray-dif-

fraction results, during setting Mg3(P04)2.4H20 should also form. Upon heating, thiscompound also loses water:

Mg3(P04)2 4H20 -> Mg3(PO4)2 + 4H20.

Hydrated magnesium phosphate decom-poses below 5000 C., which would result ina decrease in bond strength in this range.3

At the 200 260° C. range

2NH4H2P04 -> P205 + 2NH3T + 3H20T.

Co

Ni


The results of the crushing strength testsare shown in Table 4.

The results in Table 5 confirm the valid-ity of the compressive-strength values. Anal-ysis of variance is one of the most powerfultools of statistical analysis. This proceduredetermines whether the variations in com-pressive strength were due to actual differ-ences in the specimens or to experimentalerror. As shown in Table 5, the calculatedratio for means square (102.1) is much largerthan the value from the table (3.96); there-fore, the differences in compressive strengthsof the investment at the various conditionsactually occur.The values decrease in the range where

the compounds which give room-tempera-ture strength begin to break down. This iscompleted by 5000 C. The silicophosphatebond appears to form at a higher tempera-ture than the 2800 C. proposed by Mooreand Watts. The strength begins to increaseat about 3000 C. By 1,0000 C. the break-down of this bond is evidenced. By1,3000 C., as fusing of the constituents be-gins, the strength again increases. Thus

I . I I1

4 3i

2 IEACH MAJOR DIVISION = 2 0

FIG. 9.-Line graphs of alloy elements analyzed in Figure 7.

-A-J -


magnesia- and silicophosphate compoundsand unreacted MgO and SiO2 are presentat the casting temperature. The x-ray re-sults indicate that the green scale whichforms is MgCr2O4 (iIgO- Cr2O3) or Cr3O4.The MgCr2O4 more closely correlates withthe x-ray-diffraction lines. The results weresubstantially the same for both castingalloys. Probe analysis substantiates thisresult. Scans of the green scale and of theinvestment adjacent to the green scale areshown in Figures 3 and 4. These results arestrictly qualitative and give only a generalindication of the elements and their amountspresent. Since this was a general scan, phos-phorus, being a light element, was not de-termined.

Almost all the elements that exist in themetal and in the investment appear in thegreen scale analysis. As is shown in Table 1,Cr, Co, and Mn are present in the alloy,whereas Mg and Si are present in the invest-ment. All of these elements are present inthe green scale. Mg and Si would be expect-ed to appear in greater quantity in the in-vestment. However, the concentration ofboth MIg and Si is greater in the green scalethan in the investment immediately adja-cent to the green scale. Evidently this in-vestment was depleted of these elementswhen the green scale formed. Although lessthan in the green scale, Cr, Co, and Mn alsoappear in the investment in noticeable quan-tities. Comparison of the occurrence of thesemetals in Figures 3 and 4 shows that Cr andMn have a higher rate of diffusion into theinvestment than has Co.

The scan on the reddish-brown area didnot show the presence of any Co. This couldbe expected, since none of the thin grayscale has been found adjacent to the reddishareas. Cr and Mn were present in largeamounts. As occurred in the green scale, Crhad a high diffusivity and therefore showedin a high concentration on the adjacent in-vestment. 1\In in this situation does not dif-fuse as readily as in the green-scale situa-tion. The presence of the small amount ofCa is considered as an impurity in the in-vestment. Since this scale appears so sparse-ly, no analysis was made by x-ray diffrac-tion.The scans for the gray scale were run dif-

ferently. Each element was run separately,and the probe was set at maximum inten-sity for that position.

This method of analysis, then, showsgraphically the composition of the invest-ment and gray scale along the line where thebeam passed on the specimen. Lines arenumbered correspondingly on Figures 2, 7, 8,and 9. The results showed that the alloyelements are closely approximated in thegray scale and that this scale is a mixture ofthe cast metal and the investment. How-ever, no Mn was found in either the grayscale or the investment. Therefore, the dif-fusibility of the Mn must be so great that itpasses through the thin gray scale into thegreen scale. The Co does not appear in toogreat a quantity considering the high per-centage present in the alloy. Cr and Ni coin-cide with the Mg and must behave in a man-ner similar to Cr in the green scale. Co doesnot occur closely with any of the investmentmaterials but tends to decrease when the in-vestment materials increase. This probablyis the separate oxide CoO mentioned byPhalniker et al.8 and does not combine readi-ly with the investment.

Jackson, Ford, and White"2 have foundthat Cr2O3 wets MgO quite readily andforms a bond easily. In order to test thisconclusion that oxidation product of thealloy (Cr2O3) is reacting with MgO in theinvestment to form an adherent green scale-alloy was cast into a non-MIgO investment(silicate-bonded investment). Although somegreen scale formed, the quantity was not asgreat and did not adhere tightly. The sur-face was much improved.

SummaryA survey was made of the literature to

date in this area. Analysis by x-ray diffrac-tion was used in deriving the reactions oc-curring during setting and burnout. Crush-ing-strength tests at various temperaturesconfirmed these reactions. The reactionproduct of the alloy with the investmentwas shown to be MgCr2O4. It was proposedthat with investment containing MgO, ex-cessive heating of the alloy accelerates bond-ing of the Cr2O3 to the MgO grains, produc-ing a scale which is very difficult to remove.Use of a non-MgO investment substantiatedthis conclusion.The results for the three different reaction

products showed that the thin gray scaleformed immediately adjacent to the castmetal surface and contained most of themetal elements. Mn entirely and Cr par-

Vol. 45, No. 5

1528 ALLAN AND ASGAR

tially diffused rapidly outward and werefound in the green scale. To a lesser extent,Co occurred in the same manner, but not inso great a proportion as is found in the origi-nal alloy. The reddish-brown areas showedas a reaction of the easily diffusing Cr andMn with another constituent of the invest-ment.

References1. ASGAR, K., and PEYTON, F. A. Effect of Casting

Conditions on Some Mechanical Properties of Cobalt-Base Alloys, J. dent. Res., 49:73, 1961.

2. MOORE, T. E., and WATTS, C. H. Investment Material,U.S. Patent 2,479,504. August, 1949.

3. GILHAM-DAYTON, P. A. The Phosphate Bonding ofRefractory Materials, Brit. Cer. Soc. Trans., 62:895,1963.

4. KIEHL, S. J., and HARDT, H. B. The DissociationPressures of Magnesium Ammonium PhosphateHexahydrate and Some Related Substances III.,J. Amer. chem. Soc., 55:605, 1933.


5. EARNSHAW, R. Investments for Casting Co-Cr Alloys,Brit. dent. J., 108:429, 1960.

6. TIEN, T. Y., and HUMMEL, F. A. System SiO2P20,J. Amer. chum. Soc., 45:422, 1962.

7. SULLY, A. H. Chromium, New York, Academic Press,Inc., 1954. pp. 109-24.

8. PHALNIKER, C. A., EVANS, E. B., and BALDWIN,W. M., JR. High Temperature Scaling of Cobalt-Chromium Alloys, J. Electrochem. Soc., 103:429,1956.

9. FELTEN, E. J., and GREGG, R. A. The Physical Metal-lurgy and Oxidation Characteristics of a Cobalt-Base Superalloy, SM-302, A.S.M. Trans. Quart., 57:804, 1964.

10. AZAROFF, L. V., and BUERGER, M. J. The PowderMethod in X-ray Crystallography, New York, McGraw-Hill Book Co., 1958.

11. DUNCAN, A. J. Quality Control and Industrial Sta-tistics, Homewood, Ill., Richard D. Irwin, Inc., 1955,pp. 431-81.

12. JACKSON, B., and FoRD, W. F., and WHITE, J. TheInfluence of Cr2s3 and Fe2O3 on the Wetting ofPericlase Grains by Liquid Silicate, Brit. Cer Soc.Trans., 62:577, 1963.

Documents

Reaction ofCobalt-Chromium Alloywith Investment