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Characterization of glass corrosion and durability
Journal of Non-Crystalline Solids 19 (1975) 27-39 © North-Holland Publishing Company
CHARACTERIZATION OF GLASS CORROSION AND DURABILITY L.L. HENCH Department o f Materials Science and Engineering, University o f Florida, Gainesville, Florida 32611, USA New methods for obtaining a quantitative description of surface structural changes in glass are described. Application of the new methods is established through a systematic analysis of a series of glass compositions and environments. Based upon these investigations it is now possible to describe in quantitative detail the surface and bulk structural state of a glass object independent of its processing or environmental history. It is likewise possible to establish changes in bulk or surface structure of a glass due to processing or environmental conditions. Characterization of glass objects, e.g. relating composition, structure and surface to properties, is now possible since extrinsic processing and environmental variables can be described in terms of intrinsic structural features.
1. Introduction Much is now known about the proton-alkali ion reaction that leads to surface deterioration of glasses [ 1 - 3 ] . However, extensive reviews on glass durability [4, 5] show that studies have tended to focus either on (1) ionic changes in an aqueous environment from glass dissolution [ 1 - 3 , 6], or (2) on the development of macroscopic films on bulk glass surfaces [4, 5,7]. During the last several years our efforts have been directed toward combining glass surface and solution approaches. A unified effort is now possible because of increased sensitivity in solution concentration analysis and especially bulk glass surface composition and structural analysis. Changes in the near surface of glass ( 0 - 2 0 A) can be measured with Auger electron spectroscopy (AES), electron spectroscopy for chemical analysis (ESCA), ionscattering spectroscopy (1SS), or secondary-ion mass spectroscopy (SIMS). Coupling these tools with ion-beam milling yields highly detailed compositional profiles of the intermediate glass surface ( 2 0 - 2 0 0 0 A). Measurement of the average composition to the far surface ( 0 - 1 0 000 A) is now routinely available with electron-microprobe analysis (EMP), energy-dispersive X-ray analysis (EDX) in the scanning electron microscope (SEM) or infrared reflection spectroscopy (IRRS). The relative merits of these techniques in glass characterization have been recently reviewed [8]. The objective of this paper is to show how these methods can yield a thorough characterization of glass durability and corrosion when applied throughout a systematic analysis of a series of glass compositions and environments. Based upon these investigations it is now possible to describe in quantitative detail the surface and bulk structural state of a glass object independent of its processing or environmental history. It is 27
L.L. Hench I Glass corrosion and durability
28
likewise possible to establish changes in the bulk or surface structure of a glass due to processing or environmental conditions. Characterization of glass objects and the effects of surface reactions, e.g. relating composition, structure and surface to properties, is now possible since extrinsic processing and environmental variables can be described in terms of intrinsic structural features.
2. Infrared reflection spectroscopy (IRRS) Infrared reflection spectroscopy (IRRS) has been used routinely as a characterization tool throughout this series of studies [9-13] because of several important advantages: (1) it is quantitative; (2) it is non-destructive; (3) it is rapid; (4) it is inexpensive; (5) it requires no vacuum so the surface remains in the as-received condition; (6) it can be used with a standard for high-resolution analysis of surface changes; (7) it can sample various depths of a surface; (8) it can be automated and potentially used for on-line control or large object scanning; and (9) most importantly, analysis can be made on bulk glass surfaces of 1-10 mm 2 areas. The large changes in IRRS spectra due to aqueous surface attack are shown in fig. 1. Three spectra of a 33 L i 2 0 - 6 7 SiO 2 mol% glass (33L) are compared in fig. 1 : (1) a freshly polished (600 grit dry SiC) state; (2) after exposure to air containing 85% relative humidity (RH) for 72 h at 40°C; and (3) after exposure to static 50°C demineralized water for 22 h which had an initial pH of 6.5. Exposure to the static solution reduces the silicon-non-bridging oxygen-alkali (NSX) peak [ 12] as lithium ions are removed from the glass surface. A silica-rich film remains as evidenced by growth of the S peak attributed to S i - O - S i stretching vibrations [ 12]. Recent 60,
S
/-
I
Freshly Abraded 7 2 Hrs. 85% RH 40 ° C ~ ............. 22 Hrs. 3-D H20 50*C
\
50-
2 ......
\ :. :
,,o.
".. :
i
o~
. s
-
/,
... .
i
:
;
i
6)
0 q... 6)
n.-
20-
> izoo
I
I
I
I
iooo
I
I
I
s~o
I
I
I
6~)o'
I
I
WQvenumber ( c m - I ) Fig. 1. Comparison of corrosion reactions on a 30LiO 2 - 7 0 s i O 2 tool% glass using IRRS.
' 400
L.L. Hench / Glass corrosion and durability
29
X-ray diffraction analysis using pair function and disorder distribution function analysis [ 14-15] has shown that the structure of the silica-rich films formed during the corrosion process are more closely equivalent to vitreous SiO 2 produced from the molten state than a hydrated silica structure. Compositional analysis of the film from the IRRS data is possible using the linear relationship of intensity with alkali content recently established [12]. Thej85% RH environment does not permit stable silica-rich film formation as shown by the decrease in intensity of both the S and NSX peaks caused by surface roughening. Because the alkali cannot be removed from the surface after proton exchange in the 85% RH, the mode of attack has to be one of stoichiometric or uniform dissolution (curve 2) rather than the mode of selective dissolution, shown in curve 3, when the alkali ion can diffuse from the surface. Thus IRRS can be used to detect the mode of attack, rate of dealkalization and rate of film formation, as well as composition of the film. Using IRRS at an angle of 25 ° total reflection samples a surface depth of ~ 0.5/~m. Since corrosion may be many micrometers thick it is necessary to have information in addition to IRRS to characterize the film. Microprobe analysis across the interface, or sequential polishing and IRRS measurement, will establish profiles of thick films [10, 16]. However, both procedures are irreversible.
3. Solution analysis It is also possible to obtain an approximation of the film thickness from the ratio of ions released into the corrosion solution while the film is formed [10]. Ion concentrations, obtained with atomic absorption or atomic emission spectroscopy or colorimetry, used in the following equations yield a the dissolution parameter [17], and e the film parameter [10], for ternary R 2 0 - R O - S i O 2 glasses: ppm SiO2/MW SiO 2
1 - Psi02
(lppm R+/MWR) + ppm R2+/MWR
Psio 2
-
,
e = ppm SiO 2 [(1-o0/a] ,
(1)
(2)
Symbols in the above equations are as follows: Psio2 is the mole fraction of SiO 2 in the glass, ppm R + is parts per million of alkali ions in the corrosion solution, ppm R 2+ is parts per million of alkaline earth ions in the corrosion solution, MW is the molecular weight. When ~ tends toward unity the glass surface is tending toward a mode of uniform dissolution and when a tends toward zero selective leaching is occurring [ 17]. The value of e establishes the amount of SiO 2 available on the surface to form a silica-rich film [10, 13]. It is directly proportional to the film thickness. Consequently, the rate of increase of e with corrosion time is proportional to the rate of film formation. When the rate becomes negative it means the film is being destroyed
30
L.L. Hench / Glass corrosion and durability
at the solution-film interface faster than it can be formed at the film-glass interface. This eventually leads to establishing a uniform dissolution mode of attack when a = 0. Therefore, by measuring the time dependence of e, using solution data, and the IRRS of the glass surface one can obtain composition profiles and the time dependence of formation and destruction of corrosion films. A series of film profiles for three binary alkali silicate glasses is shown in fig. 2. These results are discussed in section 6 and in a recent paper [18].
4. Variables influencing characterization Glass composition, surface state and environment are the primary variables that must be evaluated for complete characterization of corrosion and durability. The importance of bulk compositions will be shown in a series of examples ranging from simple alkali silicate binary glasses to n-component commercial glasses. The importance of a number of surface state variables on glass corrosion has been established in other studies [9-13, 19-21] including: (1) roughness of polishing, (2) powder versus bulk form, (3) proportion of crystal phase, (4) internal or applied stress, (5) phase separation and (6) homogeneity (striae, cords, seeds and stones). The importance of a number of environmental variables established in this series of investigations, includes [9-13, 20, 21]: (1) range of RH, (2) sulfur dioxide and other gas phase reactants, (3) aqueous cation electrolyte type and concentration, (4) aqueous anion electrolyte type and concentration, (5) pH, (6) solution pressure and temperature, (7) ratio of surface volume to corroded area, (8) rate of replenishment of solution and flow rate, (9) time and (10) concentration of buffering ions.
5. Standard state conditions Because of the large number of surface state and environmental variables it is desirable to establish a set of standard state conditions for studying glass corrosion. These conditions are held constant when studying compositional effects or can be varied one by one to study surface or environmental effects. The standard state used in our studies is selected to model conditions of filled container storage, sheet glass in contact or with condensed water films, or conditions at a crack tip. The following variables remain fixed: (1)exposed surfaces are polished to 600 grit dry SiC or finer, (2) static triple-distilled water is used in pre-boiled Teflon* cells with a solution volume to glass area ratio of 1.27 ml/cm 2. Temperature is controlled to -+0.2°C. Time is varied with analyses of solution pH and electrolyte concentrations measured at successive intervals. Surface film analysis with the analytical tools listed earlier is * E.1. Dupont de Nemours.
L.L. Hench / Glass corrosion and durability
31
conducted at the same increments of time. Thus, the dynamics of both the glass surface and the corrosion solution are obtained simultaneously. Using the standard state conditions described above, studies of a number of glass compositional series have been completed. A review of the results obtained follows in section 6.
6. Compositional effects 6.1. Binary alkafi silicates Corrosion of Li20-SiO 2, Na20-SiO 2, and K 2 0 - S i O 2 glasses over a composition range of 10-46% R20 and a temperature range of 25°C-100°C has been studied [10-13, 18]. All glasses show silica-rich film formation. However, fig. 2 shows that
ALKALI S I L I C A T E GLASS CORROSION AT IOO° C H20 K20-SiO2 MOI % SiO2
'°°L
7°I iiiiii i '°eL I0
Mol % SiO2
IO
20
20
30/zm Na20-SiO2
30 Li20-SiO 2
SiO2 70 ~ -.-~....~;..-~. •..-~T...-~. ~..:~ :~
........-'2~.;,~;.-;~...-~.,~.-~. ~
iiiiiiiiiiiiiiiiiii@iiiiiiiiiii r
I0
i
20
t= I min
iii!
i
30~m
t=lO2min
t= 103min
Fig. 2. Schematic profiles of silica-rich films (cross-hatched areas) formed on silicate glasses containing 30 mol%alkali as a function of static aqueous corrosion at 100°C.
32
L.L. Hench / Glass corrosion and durability
the maximum SiO 2 concentration developed on the film depends on the type of alkali in the glass in the order Li20 > Na20 > K20. Consequently, the rate of dissolution of the silica-rich film for K20-containing glasses is many orders of magnitude faster than Li20-containing glasses (fig. 2). The decreasing corrosion resistance of glasses with high alkali content is due to the SiO 2 content of the films developed on the glasses being progressively less as the alkali content increases. As long as a bridging three-dimensional network of the silica-rich film is present, the film remains a barrier to further alkali diffusion through the film. If a combination of film composition or alkaline pH is such that the three-dimensional network is destroyed, the film breaks down, alkali diffusion through the film is increased, and a rapid uniform dissolution mode of attack of the glass occurs. This condition is observed after a few minutes for the 30 mol% K20 glass, but requires in excess of 100 min for an equivalent alkali Na20 glass. Li20 glass still possesses a protective SiO 2 film after this time. Studies of corrosion over the immiscibility range of the binary alkali silicates show no effects of phase separation on the corrosion process [ 18]. 6.2. Mixed alkali silicates
Numerous references report a mixed-alkali effect on glass durability. However, an extensive study in this laboratory shows no evidence of any compositional minimum in corrosion in the L i 2 0 - N a 2 0 - S i O 2 , K 2 0 - N a 2 0 - S i O 2 , L i 2 0 - K 2 0 - S i O 2 or L i 2 0 - N a 2 0 - S i O 2 systems when the alkalis are mixed on a mol% basis [22]. Total alkali contents of 20, 25 and 33 mol% have been studied over a temperature range from 25 to 100°C. All results are similar to those shown in fig. 3, e.g. the surface attack is limited by the better film forming cation up to a ratio of two to one. Consequently, a small percentage of a less soluble alkali ion aids in stabilizing a more soluble glass by promoting a more stable silica-rich film. Previous observations of compositional minima in durability, i.e. a mixed-alkali effect, appear to be due to reporting data on compositional series in terms of weight percent. Therefore, effects of varying SiO 2 content in the bulk glass were additive to the changes in film properties and dissolution rates. A mixed-alkali effect related to long-range mobility of the alkalis, as is the case in electrical properties [23], is not present in glass durability. 6.3. Alkali-alkaline earth silicates
This compositional series includes the commercially vital N a 2 0 - C a O - S i O 2 glasses. A recent study utilizing most of the analytical tools described earlier compares a 20Na20-10CaO-70SiO 2 glass (mol%) with a 20Na20-80SiO 2 glass [24]. Substituting CaO for SiO 2 results in a well-known and remarkable improvement in glass durability. Two reasons for this have been found in our studies. First, the CaO substitution strongly increases the coupling of the vibrational modes of the silicon-
L.L. Hench / Glass corrosion and durability
33
NKS-2S Series 25 'C ,1day IO00C
10O0 Na,K Na
E
tO
0
1o
2500
No20
12.5 12.5 Mole
~o
0025 K20
Fig. 3. Concentration of ions lost from 25 mol% Na20 and K20 alkali silicate glasses after static aqueous corrosion for one day at 25°C.
non-bridging oxygen-modifier bonds to the bridging S i - O - S i network. This is indicated by the increased IRRS intensity in the region between NSX and S peaks for the ternary glass when compared with the binary (fig. 4). Consequently, when aqueous attack occurs the calcia-containing glass loses Na + ions from the lattice more slowly. Secondly, when the Na ÷ ions are lost during corrosion they leave behind a much more stable CaO-SiO 2-rich film, also shown in fig. 4. Without the presence of CaO to stabilize the SiO2-rich film it rapidly breaks down, as shown for the binary glass in fig. 4. Confirmation of the Na + loss and the formation of the CaO-SiO2-rich film has been achieved with electron-micro-probe analaysis using a beam scanning method [16, 24], fig. 5. AES and solution analysis on the same samples have shown additional details of this protective mechanism [24]. Replacement of CaO by SrO in soda-alkaline earth-s~licate glasses produces an even greater resistance to 100% RH attack when the SiO 2 content is < 70 wt% [24]. IRRS data, fig. 6, as a function of time exposed to 100% RH at 25°C, shows that SrO provides a greater resistance to destruction of the protecting calcia-silicate
L.L. Hench / Glass corrosion and durability
34
Corroded 100"C- static aqueous
soln.
8C
= o
60
°_
s a
40
o~
/~2days / / ~ 9 days NSX 20--10--70~-"--/ / ~',~, t/f.o.
20
1400
1200
1000
800
Wavenumber (cm") Fig. 4. Comparison of the IR reflection spectra of a stable CaO-SiO2-rich film formed on a 20Na20-10CaO-70SiO 2 glass (mol%) with the unstable film formed on a 20Na20-80SiO 2 glass, f.a. = freshly abraded.
film on glass. At concentrations o f SiO 2 < 70 wt% the SrO additions are less effective than CaO.
6.4. Mixed-alkali-alkaline earth silicates Although a true mixed-alkali corrosion effect does not appear to exist for bulk glasses (see subsection 6.2), there are indications from the literature that the effect might occur when an alkaline earth oxide is also present in the glass [25]. To test this possibility, two series o f mixed N a 2 0 - K 2 0 - C a O - S i O 2 glasses were studied with 15 and 20 mol% o f total alkali and 10% CaO and 75 or 70% SiO 2. No minima in corrosion as a function of alkali content was observed for either series. Rates o f attack, as determined with the tools cited above, varied monotonically between the compositional end members independent o f whether the glasses were in regimes of film formation, film dissolution or total dissolution.
L.L. Hench / Glass corrosion and durability
35
12000
Si
10000
8000 A
U v
°--
Ca
60110
xl0
g B
4000
0
I
I
2
4
i
I
6
8
10
Corrosion Time (days) Fig. 5. Electron-beam microprobe analysis of the composition of the surface of a 20Na20-IOCaO70SiO 2 glass (mol%) after 100°C static corrosion for nine days.
50
NaloSr2oSim I
4O
I
CaL~oS"tO
•
c-
30
",....."--..,,_~,
c.)
~%oS,,o
~ 2O tr-
Ne2oSr20Si60
IO
0
o
~
4
~
~
b
;'2
,~,
Days Fig. 6. Intensity of the S i - O - S i stretching vibrations (S peak) as a function of 25°C corrosion in 100% RH for CaO- or SrO-containing silicate glasses.
36
L.L. Hench / Glass corrosion and durability
6.5. Alkali-alumina-silicates Samples of glass compositions from the Li20-A1203-SiO 2 series were corroded for various times up to 36 days in a 100°C static aqueous environment [26]. The results reveal that the aluminum ion concentration in the surface layer of the samples increases as a function of corrosion time, while the silica concentration remains relatively constant. AES data from various compositions corroded for a fixed time suggest total dissolution of a sample containing 2.5 mol% A1203 and nearly complete passivation for sample compositions containing 5-11 mol% A1203 . The incorpora; tion of A1203 in the SiO 2-rich surface film formed on the glass makes the film much more resistant to alkali attack. 6. 6. Some multicomponent glasses An extensive investigation of corrosion reactions of an invert soda-lime-silica glass containing P205 (45SIO2, 24.5CAO, 24.5Na20 and 6P205 wt%) has been conducted because of the incredible property of the glass to form a strong and stable bond with living bone [20, 2 7 - 3 2 ] . Nearly all surface characterization methods have been used to understand this phenomena. The secret of the biological success appears to be attributable to the formation of a stable calcium phosphate film on the surface of the material (termed bioglass) when in contact with an aqueous environment. The calcium phosphate film incorporates organic constituents as it grows. Fig. 7 shows a compositional profile of a bioglass implant after extraction from a
IN-VIV0 loo
11" S0
sio2~
ul Q.
O E LLI =-
60
o
411
I~ 211
'~" 2400 4800
I 7200
I
I
I I
9600 12,000
DEPTH IN ANGSTROMS
Fig. 7. Compositional profile of a bioglass surface (24.5Na20 24.5CaO-6P205 45SIO2 bulk composition, wt%) after exposure to a healing rat bone. Obtained by AES and Ar ion milling [32].
L.L. Hench / Glass corrosion and durability
37
rat bone after only 1 h [32]. Sodium ion migration from the surface has already occurred, forming a silica-rich film on the glass surface. The calcium phosphate film develops on the solution side of the silica-rich film and has incorporated organic constituents containing C and N atoms from the body fluids. With time (one to six weeks) the calcium phosphate crystallizes to form a fine-grained polycrystalline apatite mineral phase which bonds the glass surface to the newly growing bone [20]. The presence of the calcium phosphate layer and the silica-rich film protects the bulk glass from further attack. Consequently, this material can serve as a true 'biological glue' even when applied as a thick film coating to other ceramics. Studies of additions to the P2Os-containing glass show that the rate of calcium phosphate film formation can be accelerated with B203 substitution for SiO 2 or reduced and stabilized with CaF 2 substitution for CaO [20]. 6. 7. Commercial glasses Commercial soda-lime-silica glass compositions incorporate nearly all of the compositional features described in subsections 6.1-6.5. Because of the large num-
C o r r o d e d 100°C - static
aqueaus soln.
Commercial Glass
~0
"E 60 =t a
f.a.
40
20
1400
1200
1000
8O0
Wavenumber (am-1 ) Fig. 8. Effect of 100°C static aqueous corrosion on the IR reflection spectra of a commercial soda-lime-silica glass, f.a. = freshly abraded.
38
L.L. Hench / Glass corrosion and durability
ber of components present in the glasses it is difficult to describe specifically the ionic reactions and the film formation and destruction processes that occur. However, the studies to date suggest that commercial soda-lime-silica glasses respond as the Na20-CaO-SiO 2 ternary glass discussed above modified by the percentage of A1203 present in the glass. Thus, the durability of the glass is based upon an A1203 stabilized calcium-silicate-rich film. Diffusion of alkali through this film is very difficult and the film is resistant to conditions of alkaline pH or mechanical abrasion that tend to destroy rapidly the films that are rich only in silica. The extensive resistance to decoupling alkali ions from a typical soda-lime-silica commercial glass even after severe corrosion treatment appears to be responsible for high durability [33] (fig. 8). 7. Conclusions Thus, much of the durability and corrosion resistance essential to the glass industry appears to be dependent on and predicted by the intensity of IRRS vibrations of the glass surface at 950 cm-1. On-line controls, sampling measurements, storage diagnoses, compositional modification, gas phase treatments and other surface-dependent programs might well be based upon such a simple criterion as to maximize the 950 cm- 1 intensity and minimize its rate of change with processing and history. Confirmation of the conclusions obtained by such efforts can mow be made with a wide variety of analytical tools. As shown in the studies reported here, each of these tools is best suited to analyzing a particular type of surface reaction. However, when applied together in a systematic manner they provide a powerful means for achieving improved control over glass durability. Acknowledgement The author gratefully acknowledges a diligent group of coworkers throughout the years of working on this topic including: R.W. Gould, G.Y. Onoda, Jr., D.B. Dove, W.B. Person, D.M. Sanders, C.G. Pantano, A.E. Clark, Jr., D.E. Clark, M.F. Dilmore, E.C. Ethridge, C.V. Gokularathnam, A.W. Smith and S.M. Hill. Financial support given by US Navy Contract No. NO0014-68-A-1073-0006, US Army Contract No. DADA17-70-C-0001, NSF Contract No. 33008 and NIGMS Contract No. GM21056 has been essential. An expression of appreciation for the encouragement and good wishes of Professor Norbert Kreidl over the years is also gratefully extended. References
ll] R.L Charles, J. Appl. Phys. 11 (1958) 1549. [2] M.A. Rana and R.W. Douglas, Phys. Chem. Glasses 2 (1961) 6.
L.L. Hench / Glass corrosion and durability
39
[3] T.M. EIShamy, J. Lewins and R.W. Douglas, Glass Technol. 13 (1972) 81. [4] W.A, Weyl and E.C. Marboe, The Constitution of Glasses: A Dynamic Interpretation, vol. 3, part 2 (Wiley, New York, 1967), Ch. 5. [5] L. Holland, The Properties of Glass Surfaces (Wiley, New York, 1964). [61 F.R. Bacon, Glass Ind. 49 (1968). [7] H.E. Simpson, J. Amer. Ceram. Soc. 42 (1959) 337. [8] L.L. Hench, in: Characterization of Materials in Research, Ceramics and Polymers, eds. V. Weiss and J J . Burke (Syracuse Univ. Press, Syracuse, New York, 1975) pp. 211-251. [9] D.M, Sanders, W.D. Person, and L.L. Hench, Appl. Spectrosc. 26 (5) (1972) 530. 10] D.M. Sanders and L.L. Hench, J. Amer. Ceram. Soc. 54 (7) (1973) 373. 11] D.M. Sanders and L.L. Hench, Bull. Amer. Ceram. Soc. 52 (9) 662. 121 D.M. Sanders, W.B. Person and L.L. Hench, Appl. Spectrosc. 28 (3) (1974) 247. 13] L.L. Hench and D.M. Sanders, Glass Ind. (Feb/Mar. 1974). 14] R.W. Gould and M.S. Hill, Advances in X-ray Analysis, vol. 17 (Plenum, New York, 1973). 15] C.V. Gokularathnam, R.W. Gould and L.L. Hench, Phys. Chem. Glasses 16 (1) (1975) 13. 16] D.E. Clark, L.L. Hench and W.A. Acree, Bull. Ceram. Soc., to be published. 17] Yu. A. Shmidt, Stronenie Stekla, Inst. Khim. Silikatov Akad. Nauk SSSR (Trudy Soveshchaniya, Leningrad, 1953, pub. 1955), pp. 319-321. 18] E.C. Ethridge and L.L. Hench, submitted to J. Amer. Ceram. 19] D.M. Sanders and L.L. Hench, Bull. Amer. Ceram. Soc. 52 (9) (1973) 666. 20] L.L. Hench, J. Amer. Soc., to be published. 21 ] F.R. Rhines, E.D. Verink, L.L. Hench, NSF Report Grant No. GH34550 (1972). 22] M.F. Dilmore and L.L. Hench, submitted to J. Amer. Ceram. Soc. 23] J.O. Isard, J. Non-Crystalline Solids 1 (1969) 235. 24] D.E. Clark, M.F. Dilmore and L.L. Hench, submitted to J. Amer. Ceram. Soc. 25] S. Sen and F.V. Tooley, J. Amer. Ceram. Soc. 38 (5) (1955) 175. 26] M.F. Dilmore and L.L. Hench, to be published. 27] C.G. Pantano, Jr., A.E. Clark, Jr. and L.L. Hench, J. Amer. Ceram. Soc. 57 (9) (1974) 412. 28] L.L. Hench in: Proceedings of Surfaces and Interfaces of Glass and Ceramics, eds. Frechett, LaCourse, Burdeck (Plenum, New York, 1974 pp. 265-283. [29] L.L. Hench and H.A. Paschall, J. Biomed. Mater. Res. (5) part 1 (1974) 49. [30] L.L. Hench, Med. Instrum. 1 (2) (Mar.-Apr. 1973) 136. [31] L.L. Hench, R.J. Splinter, T.K. Greenlee and W.C. Allen, J. Biomed. Mater. Res. (2) part 1 (Nov. 1971) 117. [32] A.E. Clark, Jr., C.G. Pantano, Jr. and L.L. Hench, J. Biomed. Mater. Res. Symp., Materials for Reconstructive Surgery, Clemson Univ. (1975). [33] D.E. Clark and L.L. Hench, to be published.
CHARACTERIZATION OF GLASS CORROSION AND DURABILITY L.L. HENCH Department o f Materials Science and Engineering, University o f Florida, Gainesville, Florida 32611, USA New methods for obtaining a quantitative description of surface structural changes in glass are described. Application of the new methods is established through a systematic analysis of a series of glass compositions and environments. Based upon these investigations it is now possible to describe in quantitative detail the surface and bulk structural state of a glass object independent of its processing or environmental history. It is likewise possible to establish changes in bulk or surface structure of a glass due to processing or environmental conditions. Characterization of glass objects, e.g. relating composition, structure and surface to properties, is now possible since extrinsic processing and environmental variables can be described in terms of intrinsic structural features.
1. Introduction Much is now known about the proton-alkali ion reaction that leads to surface deterioration of glasses [ 1 - 3 ] . However, extensive reviews on glass durability [4, 5] show that studies have tended to focus either on (1) ionic changes in an aqueous environment from glass dissolution [ 1 - 3 , 6], or (2) on the development of macroscopic films on bulk glass surfaces [4, 5,7]. During the last several years our efforts have been directed toward combining glass surface and solution approaches. A unified effort is now possible because of increased sensitivity in solution concentration analysis and especially bulk glass surface composition and structural analysis. Changes in the near surface of glass ( 0 - 2 0 A) can be measured with Auger electron spectroscopy (AES), electron spectroscopy for chemical analysis (ESCA), ionscattering spectroscopy (1SS), or secondary-ion mass spectroscopy (SIMS). Coupling these tools with ion-beam milling yields highly detailed compositional profiles of the intermediate glass surface ( 2 0 - 2 0 0 0 A). Measurement of the average composition to the far surface ( 0 - 1 0 000 A) is now routinely available with electron-microprobe analysis (EMP), energy-dispersive X-ray analysis (EDX) in the scanning electron microscope (SEM) or infrared reflection spectroscopy (IRRS). The relative merits of these techniques in glass characterization have been recently reviewed [8]. The objective of this paper is to show how these methods can yield a thorough characterization of glass durability and corrosion when applied throughout a systematic analysis of a series of glass compositions and environments. Based upon these investigations it is now possible to describe in quantitative detail the surface and bulk structural state of a glass object independent of its processing or environmental history. It is 27
L.L. Hench I Glass corrosion and durability
28
likewise possible to establish changes in the bulk or surface structure of a glass due to processing or environmental conditions. Characterization of glass objects and the effects of surface reactions, e.g. relating composition, structure and surface to properties, is now possible since extrinsic processing and environmental variables can be described in terms of intrinsic structural features.
2. Infrared reflection spectroscopy (IRRS) Infrared reflection spectroscopy (IRRS) has been used routinely as a characterization tool throughout this series of studies [9-13] because of several important advantages: (1) it is quantitative; (2) it is non-destructive; (3) it is rapid; (4) it is inexpensive; (5) it requires no vacuum so the surface remains in the as-received condition; (6) it can be used with a standard for high-resolution analysis of surface changes; (7) it can sample various depths of a surface; (8) it can be automated and potentially used for on-line control or large object scanning; and (9) most importantly, analysis can be made on bulk glass surfaces of 1-10 mm 2 areas. The large changes in IRRS spectra due to aqueous surface attack are shown in fig. 1. Three spectra of a 33 L i 2 0 - 6 7 SiO 2 mol% glass (33L) are compared in fig. 1 : (1) a freshly polished (600 grit dry SiC) state; (2) after exposure to air containing 85% relative humidity (RH) for 72 h at 40°C; and (3) after exposure to static 50°C demineralized water for 22 h which had an initial pH of 6.5. Exposure to the static solution reduces the silicon-non-bridging oxygen-alkali (NSX) peak [ 12] as lithium ions are removed from the glass surface. A silica-rich film remains as evidenced by growth of the S peak attributed to S i - O - S i stretching vibrations [ 12]. Recent 60,
S
/-
I
Freshly Abraded 7 2 Hrs. 85% RH 40 ° C ~ ............. 22 Hrs. 3-D H20 50*C
\
50-
2 ......
\ :. :
,,o.
".. :
i
o~
. s
-
/,
... .
i
:
;
i
6)
0 q... 6)
n.-
20-
> izoo
I
I
I
I
iooo
I
I
I
s~o
I
I
I
6~)o'
I
I
WQvenumber ( c m - I ) Fig. 1. Comparison of corrosion reactions on a 30LiO 2 - 7 0 s i O 2 tool% glass using IRRS.
' 400
L.L. Hench / Glass corrosion and durability
29
X-ray diffraction analysis using pair function and disorder distribution function analysis [ 14-15] has shown that the structure of the silica-rich films formed during the corrosion process are more closely equivalent to vitreous SiO 2 produced from the molten state than a hydrated silica structure. Compositional analysis of the film from the IRRS data is possible using the linear relationship of intensity with alkali content recently established [12]. Thej85% RH environment does not permit stable silica-rich film formation as shown by the decrease in intensity of both the S and NSX peaks caused by surface roughening. Because the alkali cannot be removed from the surface after proton exchange in the 85% RH, the mode of attack has to be one of stoichiometric or uniform dissolution (curve 2) rather than the mode of selective dissolution, shown in curve 3, when the alkali ion can diffuse from the surface. Thus IRRS can be used to detect the mode of attack, rate of dealkalization and rate of film formation, as well as composition of the film. Using IRRS at an angle of 25 ° total reflection samples a surface depth of ~ 0.5/~m. Since corrosion may be many micrometers thick it is necessary to have information in addition to IRRS to characterize the film. Microprobe analysis across the interface, or sequential polishing and IRRS measurement, will establish profiles of thick films [10, 16]. However, both procedures are irreversible.
3. Solution analysis It is also possible to obtain an approximation of the film thickness from the ratio of ions released into the corrosion solution while the film is formed [10]. Ion concentrations, obtained with atomic absorption or atomic emission spectroscopy or colorimetry, used in the following equations yield a the dissolution parameter [17], and e the film parameter [10], for ternary R 2 0 - R O - S i O 2 glasses: ppm SiO2/MW SiO 2
1 - Psi02
(lppm R+/MWR) + ppm R2+/MWR
Psio 2
-
,
e = ppm SiO 2 [(1-o0/a] ,
(1)
(2)
Symbols in the above equations are as follows: Psio2 is the mole fraction of SiO 2 in the glass, ppm R + is parts per million of alkali ions in the corrosion solution, ppm R 2+ is parts per million of alkaline earth ions in the corrosion solution, MW is the molecular weight. When ~ tends toward unity the glass surface is tending toward a mode of uniform dissolution and when a tends toward zero selective leaching is occurring [ 17]. The value of e establishes the amount of SiO 2 available on the surface to form a silica-rich film [10, 13]. It is directly proportional to the film thickness. Consequently, the rate of increase of e with corrosion time is proportional to the rate of film formation. When the rate becomes negative it means the film is being destroyed
30
L.L. Hench / Glass corrosion and durability
at the solution-film interface faster than it can be formed at the film-glass interface. This eventually leads to establishing a uniform dissolution mode of attack when a = 0. Therefore, by measuring the time dependence of e, using solution data, and the IRRS of the glass surface one can obtain composition profiles and the time dependence of formation and destruction of corrosion films. A series of film profiles for three binary alkali silicate glasses is shown in fig. 2. These results are discussed in section 6 and in a recent paper [18].
4. Variables influencing characterization Glass composition, surface state and environment are the primary variables that must be evaluated for complete characterization of corrosion and durability. The importance of bulk compositions will be shown in a series of examples ranging from simple alkali silicate binary glasses to n-component commercial glasses. The importance of a number of surface state variables on glass corrosion has been established in other studies [9-13, 19-21] including: (1) roughness of polishing, (2) powder versus bulk form, (3) proportion of crystal phase, (4) internal or applied stress, (5) phase separation and (6) homogeneity (striae, cords, seeds and stones). The importance of a number of environmental variables established in this series of investigations, includes [9-13, 20, 21]: (1) range of RH, (2) sulfur dioxide and other gas phase reactants, (3) aqueous cation electrolyte type and concentration, (4) aqueous anion electrolyte type and concentration, (5) pH, (6) solution pressure and temperature, (7) ratio of surface volume to corroded area, (8) rate of replenishment of solution and flow rate, (9) time and (10) concentration of buffering ions.
5. Standard state conditions Because of the large number of surface state and environmental variables it is desirable to establish a set of standard state conditions for studying glass corrosion. These conditions are held constant when studying compositional effects or can be varied one by one to study surface or environmental effects. The standard state used in our studies is selected to model conditions of filled container storage, sheet glass in contact or with condensed water films, or conditions at a crack tip. The following variables remain fixed: (1)exposed surfaces are polished to 600 grit dry SiC or finer, (2) static triple-distilled water is used in pre-boiled Teflon* cells with a solution volume to glass area ratio of 1.27 ml/cm 2. Temperature is controlled to -+0.2°C. Time is varied with analyses of solution pH and electrolyte concentrations measured at successive intervals. Surface film analysis with the analytical tools listed earlier is * E.1. Dupont de Nemours.
L.L. Hench / Glass corrosion and durability
31
conducted at the same increments of time. Thus, the dynamics of both the glass surface and the corrosion solution are obtained simultaneously. Using the standard state conditions described above, studies of a number of glass compositional series have been completed. A review of the results obtained follows in section 6.
6. Compositional effects 6.1. Binary alkafi silicates Corrosion of Li20-SiO 2, Na20-SiO 2, and K 2 0 - S i O 2 glasses over a composition range of 10-46% R20 and a temperature range of 25°C-100°C has been studied [10-13, 18]. All glasses show silica-rich film formation. However, fig. 2 shows that
ALKALI S I L I C A T E GLASS CORROSION AT IOO° C H20 K20-SiO2 MOI % SiO2
'°°L
7°I iiiiii i '°eL I0
Mol % SiO2
IO
20
20
30/zm Na20-SiO2
30 Li20-SiO 2
SiO2 70 ~ -.-~....~;..-~. •..-~T...-~. ~..:~ :~
........-'2~.;,~;.-;~...-~.,~.-~. ~
iiiiiiiiiiiiiiiiiii@iiiiiiiiiii r
I0
i
20
t= I min
iii!
i
30~m
t=lO2min
t= 103min
Fig. 2. Schematic profiles of silica-rich films (cross-hatched areas) formed on silicate glasses containing 30 mol%alkali as a function of static aqueous corrosion at 100°C.
32
L.L. Hench / Glass corrosion and durability
the maximum SiO 2 concentration developed on the film depends on the type of alkali in the glass in the order Li20 > Na20 > K20. Consequently, the rate of dissolution of the silica-rich film for K20-containing glasses is many orders of magnitude faster than Li20-containing glasses (fig. 2). The decreasing corrosion resistance of glasses with high alkali content is due to the SiO 2 content of the films developed on the glasses being progressively less as the alkali content increases. As long as a bridging three-dimensional network of the silica-rich film is present, the film remains a barrier to further alkali diffusion through the film. If a combination of film composition or alkaline pH is such that the three-dimensional network is destroyed, the film breaks down, alkali diffusion through the film is increased, and a rapid uniform dissolution mode of attack of the glass occurs. This condition is observed after a few minutes for the 30 mol% K20 glass, but requires in excess of 100 min for an equivalent alkali Na20 glass. Li20 glass still possesses a protective SiO 2 film after this time. Studies of corrosion over the immiscibility range of the binary alkali silicates show no effects of phase separation on the corrosion process [ 18]. 6.2. Mixed alkali silicates
Numerous references report a mixed-alkali effect on glass durability. However, an extensive study in this laboratory shows no evidence of any compositional minimum in corrosion in the L i 2 0 - N a 2 0 - S i O 2 , K 2 0 - N a 2 0 - S i O 2 , L i 2 0 - K 2 0 - S i O 2 or L i 2 0 - N a 2 0 - S i O 2 systems when the alkalis are mixed on a mol% basis [22]. Total alkali contents of 20, 25 and 33 mol% have been studied over a temperature range from 25 to 100°C. All results are similar to those shown in fig. 3, e.g. the surface attack is limited by the better film forming cation up to a ratio of two to one. Consequently, a small percentage of a less soluble alkali ion aids in stabilizing a more soluble glass by promoting a more stable silica-rich film. Previous observations of compositional minima in durability, i.e. a mixed-alkali effect, appear to be due to reporting data on compositional series in terms of weight percent. Therefore, effects of varying SiO 2 content in the bulk glass were additive to the changes in film properties and dissolution rates. A mixed-alkali effect related to long-range mobility of the alkalis, as is the case in electrical properties [23], is not present in glass durability. 6.3. Alkali-alkaline earth silicates
This compositional series includes the commercially vital N a 2 0 - C a O - S i O 2 glasses. A recent study utilizing most of the analytical tools described earlier compares a 20Na20-10CaO-70SiO 2 glass (mol%) with a 20Na20-80SiO 2 glass [24]. Substituting CaO for SiO 2 results in a well-known and remarkable improvement in glass durability. Two reasons for this have been found in our studies. First, the CaO substitution strongly increases the coupling of the vibrational modes of the silicon-
L.L. Hench / Glass corrosion and durability
33
NKS-2S Series 25 'C ,1day IO00C
10O0 Na,K Na
E
tO
0
1o
2500
No20
12.5 12.5 Mole
~o
0025 K20
Fig. 3. Concentration of ions lost from 25 mol% Na20 and K20 alkali silicate glasses after static aqueous corrosion for one day at 25°C.
non-bridging oxygen-modifier bonds to the bridging S i - O - S i network. This is indicated by the increased IRRS intensity in the region between NSX and S peaks for the ternary glass when compared with the binary (fig. 4). Consequently, when aqueous attack occurs the calcia-containing glass loses Na + ions from the lattice more slowly. Secondly, when the Na ÷ ions are lost during corrosion they leave behind a much more stable CaO-SiO 2-rich film, also shown in fig. 4. Without the presence of CaO to stabilize the SiO2-rich film it rapidly breaks down, as shown for the binary glass in fig. 4. Confirmation of the Na + loss and the formation of the CaO-SiO2-rich film has been achieved with electron-micro-probe analaysis using a beam scanning method [16, 24], fig. 5. AES and solution analysis on the same samples have shown additional details of this protective mechanism [24]. Replacement of CaO by SrO in soda-alkaline earth-s~licate glasses produces an even greater resistance to 100% RH attack when the SiO 2 content is < 70 wt% [24]. IRRS data, fig. 6, as a function of time exposed to 100% RH at 25°C, shows that SrO provides a greater resistance to destruction of the protecting calcia-silicate
L.L. Hench / Glass corrosion and durability
34
Corroded 100"C- static aqueous
soln.
8C
= o
60
°_
s a
40
o~
/~2days / / ~ 9 days NSX 20--10--70~-"--/ / ~',~, t/f.o.
20
1400
1200
1000
800
Wavenumber (cm") Fig. 4. Comparison of the IR reflection spectra of a stable CaO-SiO2-rich film formed on a 20Na20-10CaO-70SiO 2 glass (mol%) with the unstable film formed on a 20Na20-80SiO 2 glass, f.a. = freshly abraded.
film on glass. At concentrations o f SiO 2 < 70 wt% the SrO additions are less effective than CaO.
6.4. Mixed-alkali-alkaline earth silicates Although a true mixed-alkali corrosion effect does not appear to exist for bulk glasses (see subsection 6.2), there are indications from the literature that the effect might occur when an alkaline earth oxide is also present in the glass [25]. To test this possibility, two series o f mixed N a 2 0 - K 2 0 - C a O - S i O 2 glasses were studied with 15 and 20 mol% o f total alkali and 10% CaO and 75 or 70% SiO 2. No minima in corrosion as a function of alkali content was observed for either series. Rates o f attack, as determined with the tools cited above, varied monotonically between the compositional end members independent o f whether the glasses were in regimes of film formation, film dissolution or total dissolution.
L.L. Hench / Glass corrosion and durability
35
12000
Si
10000
8000 A
U v
°--
Ca
60110
xl0
g B
4000
0
I
I
2
4
i
I
6
8
10
Corrosion Time (days) Fig. 5. Electron-beam microprobe analysis of the composition of the surface of a 20Na20-IOCaO70SiO 2 glass (mol%) after 100°C static corrosion for nine days.
50
NaloSr2oSim I
4O
I
CaL~oS"tO
•
c-
30
",....."--..,,_~,
c.)
~%oS,,o
~ 2O tr-
Ne2oSr20Si60
IO
0
o
~
4
~
~
b
;'2
,~,
Days Fig. 6. Intensity of the S i - O - S i stretching vibrations (S peak) as a function of 25°C corrosion in 100% RH for CaO- or SrO-containing silicate glasses.
36
L.L. Hench / Glass corrosion and durability
6.5. Alkali-alumina-silicates Samples of glass compositions from the Li20-A1203-SiO 2 series were corroded for various times up to 36 days in a 100°C static aqueous environment [26]. The results reveal that the aluminum ion concentration in the surface layer of the samples increases as a function of corrosion time, while the silica concentration remains relatively constant. AES data from various compositions corroded for a fixed time suggest total dissolution of a sample containing 2.5 mol% A1203 and nearly complete passivation for sample compositions containing 5-11 mol% A1203 . The incorpora; tion of A1203 in the SiO 2-rich surface film formed on the glass makes the film much more resistant to alkali attack. 6. 6. Some multicomponent glasses An extensive investigation of corrosion reactions of an invert soda-lime-silica glass containing P205 (45SIO2, 24.5CAO, 24.5Na20 and 6P205 wt%) has been conducted because of the incredible property of the glass to form a strong and stable bond with living bone [20, 2 7 - 3 2 ] . Nearly all surface characterization methods have been used to understand this phenomena. The secret of the biological success appears to be attributable to the formation of a stable calcium phosphate film on the surface of the material (termed bioglass) when in contact with an aqueous environment. The calcium phosphate film incorporates organic constituents as it grows. Fig. 7 shows a compositional profile of a bioglass implant after extraction from a
IN-VIV0 loo
11" S0
sio2~
ul Q.
O E LLI =-
60
o
411
I~ 211
'~" 2400 4800
I 7200
I
I
I I
9600 12,000
DEPTH IN ANGSTROMS
Fig. 7. Compositional profile of a bioglass surface (24.5Na20 24.5CaO-6P205 45SIO2 bulk composition, wt%) after exposure to a healing rat bone. Obtained by AES and Ar ion milling [32].
L.L. Hench / Glass corrosion and durability
37
rat bone after only 1 h [32]. Sodium ion migration from the surface has already occurred, forming a silica-rich film on the glass surface. The calcium phosphate film develops on the solution side of the silica-rich film and has incorporated organic constituents containing C and N atoms from the body fluids. With time (one to six weeks) the calcium phosphate crystallizes to form a fine-grained polycrystalline apatite mineral phase which bonds the glass surface to the newly growing bone [20]. The presence of the calcium phosphate layer and the silica-rich film protects the bulk glass from further attack. Consequently, this material can serve as a true 'biological glue' even when applied as a thick film coating to other ceramics. Studies of additions to the P2Os-containing glass show that the rate of calcium phosphate film formation can be accelerated with B203 substitution for SiO 2 or reduced and stabilized with CaF 2 substitution for CaO [20]. 6. 7. Commercial glasses Commercial soda-lime-silica glass compositions incorporate nearly all of the compositional features described in subsections 6.1-6.5. Because of the large num-
C o r r o d e d 100°C - static
aqueaus soln.
Commercial Glass
~0
"E 60 =t a
f.a.
40
20
1400
1200
1000
8O0
Wavenumber (am-1 ) Fig. 8. Effect of 100°C static aqueous corrosion on the IR reflection spectra of a commercial soda-lime-silica glass, f.a. = freshly abraded.
38
L.L. Hench / Glass corrosion and durability
ber of components present in the glasses it is difficult to describe specifically the ionic reactions and the film formation and destruction processes that occur. However, the studies to date suggest that commercial soda-lime-silica glasses respond as the Na20-CaO-SiO 2 ternary glass discussed above modified by the percentage of A1203 present in the glass. Thus, the durability of the glass is based upon an A1203 stabilized calcium-silicate-rich film. Diffusion of alkali through this film is very difficult and the film is resistant to conditions of alkaline pH or mechanical abrasion that tend to destroy rapidly the films that are rich only in silica. The extensive resistance to decoupling alkali ions from a typical soda-lime-silica commercial glass even after severe corrosion treatment appears to be responsible for high durability [33] (fig. 8). 7. Conclusions Thus, much of the durability and corrosion resistance essential to the glass industry appears to be dependent on and predicted by the intensity of IRRS vibrations of the glass surface at 950 cm-1. On-line controls, sampling measurements, storage diagnoses, compositional modification, gas phase treatments and other surface-dependent programs might well be based upon such a simple criterion as to maximize the 950 cm- 1 intensity and minimize its rate of change with processing and history. Confirmation of the conclusions obtained by such efforts can mow be made with a wide variety of analytical tools. As shown in the studies reported here, each of these tools is best suited to analyzing a particular type of surface reaction. However, when applied together in a systematic manner they provide a powerful means for achieving improved control over glass durability. Acknowledgement The author gratefully acknowledges a diligent group of coworkers throughout the years of working on this topic including: R.W. Gould, G.Y. Onoda, Jr., D.B. Dove, W.B. Person, D.M. Sanders, C.G. Pantano, A.E. Clark, Jr., D.E. Clark, M.F. Dilmore, E.C. Ethridge, C.V. Gokularathnam, A.W. Smith and S.M. Hill. Financial support given by US Navy Contract No. NO0014-68-A-1073-0006, US Army Contract No. DADA17-70-C-0001, NSF Contract No. 33008 and NIGMS Contract No. GM21056 has been essential. An expression of appreciation for the encouragement and good wishes of Professor Norbert Kreidl over the years is also gratefully extended. References
ll] R.L Charles, J. Appl. Phys. 11 (1958) 1549. [2] M.A. Rana and R.W. Douglas, Phys. Chem. Glasses 2 (1961) 6.
L.L. Hench / Glass corrosion and durability
39
[3] T.M. EIShamy, J. Lewins and R.W. Douglas, Glass Technol. 13 (1972) 81. [4] W.A, Weyl and E.C. Marboe, The Constitution of Glasses: A Dynamic Interpretation, vol. 3, part 2 (Wiley, New York, 1967), Ch. 5. [5] L. Holland, The Properties of Glass Surfaces (Wiley, New York, 1964). [61 F.R. Bacon, Glass Ind. 49 (1968). [7] H.E. Simpson, J. Amer. Ceram. Soc. 42 (1959) 337. [8] L.L. Hench, in: Characterization of Materials in Research, Ceramics and Polymers, eds. V. Weiss and J J . Burke (Syracuse Univ. Press, Syracuse, New York, 1975) pp. 211-251. [9] D.M, Sanders, W.D. Person, and L.L. Hench, Appl. Spectrosc. 26 (5) (1972) 530. 10] D.M. Sanders and L.L. Hench, J. Amer. Ceram. Soc. 54 (7) (1973) 373. 11] D.M. Sanders and L.L. Hench, Bull. Amer. Ceram. Soc. 52 (9) 662. 121 D.M. Sanders, W.B. Person and L.L. Hench, Appl. Spectrosc. 28 (3) (1974) 247. 13] L.L. Hench and D.M. Sanders, Glass Ind. (Feb/Mar. 1974). 14] R.W. Gould and M.S. Hill, Advances in X-ray Analysis, vol. 17 (Plenum, New York, 1973). 15] C.V. Gokularathnam, R.W. Gould and L.L. Hench, Phys. Chem. Glasses 16 (1) (1975) 13. 16] D.E. Clark, L.L. Hench and W.A. Acree, Bull. Ceram. Soc., to be published. 17] Yu. A. Shmidt, Stronenie Stekla, Inst. Khim. Silikatov Akad. Nauk SSSR (Trudy Soveshchaniya, Leningrad, 1953, pub. 1955), pp. 319-321. 18] E.C. Ethridge and L.L. Hench, submitted to J. Amer. Ceram. 19] D.M. Sanders and L.L. Hench, Bull. Amer. Ceram. Soc. 52 (9) (1973) 666. 20] L.L. Hench, J. Amer. Soc., to be published. 21 ] F.R. Rhines, E.D. Verink, L.L. Hench, NSF Report Grant No. GH34550 (1972). 22] M.F. Dilmore and L.L. Hench, submitted to J. Amer. Ceram. Soc. 23] J.O. Isard, J. Non-Crystalline Solids 1 (1969) 235. 24] D.E. Clark, M.F. Dilmore and L.L. Hench, submitted to J. Amer. Ceram. Soc. 25] S. Sen and F.V. Tooley, J. Amer. Ceram. Soc. 38 (5) (1955) 175. 26] M.F. Dilmore and L.L. Hench, to be published. 27] C.G. Pantano, Jr., A.E. Clark, Jr. and L.L. Hench, J. Amer. Ceram. Soc. 57 (9) (1974) 412. 28] L.L. Hench in: Proceedings of Surfaces and Interfaces of Glass and Ceramics, eds. Frechett, LaCourse, Burdeck (Plenum, New York, 1974 pp. 265-283. [29] L.L. Hench and H.A. Paschall, J. Biomed. Mater. Res. (5) part 1 (1974) 49. [30] L.L. Hench, Med. Instrum. 1 (2) (Mar.-Apr. 1973) 136. [31] L.L. Hench, R.J. Splinter, T.K. Greenlee and W.C. Allen, J. Biomed. Mater. Res. (2) part 1 (Nov. 1971) 117. [32] A.E. Clark, Jr., C.G. Pantano, Jr. and L.L. Hench, J. Biomed. Mater. Res. Symp., Materials for Reconstructive Surgery, Clemson Univ. (1975). [33] D.E. Clark and L.L. Hench, to be published.