J. BIOMED. MATER. RES.
VOL. 10, PP. 445-453 (1976)
Flame-Sprayed Alumina on Stainless Steel for Possible Prosthetic Application C. M. BALDWIN and J. D. MACKENZIE, School of Engineering and Applied Science, Materials Science Department, University of California, Los Angeles, California 90024
Summary Various methods of roughening type 316 stainless steel substrate surfaces for flame-spraying alumina (A1,O were investigated and tested for the best aluminato-metal bond strength. Best strength values were obtained by means of roughening via anodic polarization pitting of the stainless steel. Subsequent in vitro testing indicated a severe loss in bond strength following exposure to aerated Ringer's solution. It is suggested that the utilization of flame-sprayed devices has potential in orthopedic prostheses, but precautions must be observed.
INTRODUCTION The concept of a physiologically inert ceramic coating such as alumina (AlZO3)attached to a metal shaft has long been considered a viable prosthetic design configurat,ion from the standpoint of combining both the corrosion-resistance of a ceramic with the overall mechanical strength of a metal into one device.' A processing technique which could produce such a configuration is the flame-spraying process. This fabrication process involves the passing of a ceramic powder through a high-temperature oxyacetylene flame (2500°F maximum) whereupon the molten ceramic particles are then sprayed onto a prepared' metal substrate. The resultant metal-ceramic configuration consists of a thin (typically about 30 mil thick), slightly porous ceramic coating bonded to a metal core.2 By controlling the flame-spraying application parameters, the porosity of the coating may be regulated whereby a porous ceramic coating capable of accepting permanent bone or tissue ingrowth can be fabricated. 445 @ 1976 by John Wiley & Sons, Inc.
446
BALD WIN AND MACKENZIE
Clinical studies which have been undertaken to evaluate plasmasprayed* alumina bonded to various substrate meta1s3s4have generally been not too encouraging. Richbourg3 plasma-sprayed 99.9% pure alumina onto the shaft of a stainless steel hip prosthesis and implanted the device into dogs for periods up to 2 years. The prostheses were biocompatible but some coatings were observed to have flaked off. Because of the density of the alumina coating (the average pore size being less than 1 p ) , no tissue ingrowth into the device was observed. In another study, Driskel14 attempted to implant plasma-sprayed alumina devices into the mandibles of monkeys, but technical and operative complications prevented any meaningful results.
EXPERIMENTAL Metco type 101 flame-spray powder (approximately 2.5% TiOz and 97.5% Alz03) was selected as the flame-spray coating material and was applied to type 316 stainless steel via a Metco oxyacetylene flame-spray gun, model 2P. The dimensions of the type 316 stainless steel substrate was in accordance with ASTM standards5 for testing flame-sprayed ceramic-to-metal bond strength; the metal substrate was a 1 in. diameter cylinder by 2 in. in length. The 1 in. diameter metal surface was lathe-polished and acetone-cleaned prior to any substrate roughening preparation for flame-spraying. Several substrate roughening techniques were employed, including 1) conventional alumina grit sandblasting of the metal surface, 2) mechanical roughening using an engraver’s tool bit on a Bridgeport model 541614 milling machine, and 3) anodic polarization roughening where the surface is roughened by electrochemical pitting. In the latter technique, the metal surface was exposed to a 5 N HC1 solution and then anodically polarized a t a constant current density of 87 mA/cm2 for various lengths of time. Following exposure, the samples were rinsed in distilled water and dried. The strength of the resultant flame-sprayed alumina-to-metal bond was measured according to ASTM standards5 by using an Instron model TTCLM 1.6 testing machine. In this test, a viscous, nonpenetrating bonding agent (Devcon “2-ton” epoxy) is coated on the * This process is similar to flame-spraying but produces a much denser ceramic coating. See reference 2 for further details.
FLAME-SPRAYED ALUMINA ON STAINLESS STEEL
447
flame-sprayed ceramic face and a loading fixture is then placed on the epoxy. After hardening, this assembly is loaded in pure tension perpendicular to the face of the ceramic coating. The crosshead speed was held constant at 0.02 in./min. The bond strength u was calculated such that lJ=
Maximum Load Substrate Surface Area
A minimum of six samples prepared under identical conditions were tested for each data point. In vitro tests were conducted in an apparatus similar to one described by Wheeler and James.6 The artificial body fluid was aerated Ringer’s solutions (850 mg NaC1, 33 mg CaC12, 30 mg KC1 per 100 ml distilled water) heated to approximately 110°F. The Ringer’s solution pH was held constant at 7.3 as monitored by a Reckman model 9600 pH meter. The apparatus was designed to simultanously expose up to 12 specimens. Following exposure, the ceramicto-metal bond strengths for all samples were tested according to the aforementioned technique.
RESULTS AND DISCUSSION Bonding Characteristics It is generally acknowledged that the flame- (or plasma-) sprayed alumina-to-metal bond is entirely mechanical and no interfacial chemical reaction ~ c c u r s . ’ ~The ~ gross surface roughness of the substrate therefore dictates the ultimate bond strength of the composite such that, all other processing parameters being equal, the rougher the substrate surface, the stronger the bond. Of the various substrate surface roughening techniques attempted, the maximum ceramic-to-metal bond strengths were obtained from alumina flame-sprayed onto anodic polarization-prepared metal surfaces. The bond strength of this particular configuration was significantly higher than the other specimens and of comparable flamesprayed alumina-titania mixtures sprayed onto stainless steel from the literature (see Table I). As seen in Figure 1, the anodically polarized stainless steel surface is highly pitted, thus enabling the flame-sprayed alumina particles to penetrate into the metal and adhere firmly to the substrate.
448
BALDWIN AND MACKENZIE TABLE I Maximum Bond Strength for Flame-Sprayed Alumina on Stainless Steel
Reference
Surface roughening method
Our studies Our studies Our studies 7 9
Anodic polarization Machine roughening Alumina grit sandblast Alumina grit sandblast Steel grit blast
Bond strength u (max), psi 1066 504 190 500 408
Failure location Ceramic-metal Ceramic-metal Ceramic-metal Ceramic-metal Ceramic-metal
interface interface interface interface interface
The average surface pit diameter D, as measured by scanning electron microscopy was observed to grow with time in the anodically polarized HC1 solution as (see Fig. 2). The corresponding alumina-to-stainless steel bond strength was observed to vary as to.’* (see Fig. 2), indicating a direct correlation between substrate surface roughness (as characterized by the average surface pit diam-
Fig. 1. Scanning electron microscope (225 X ) showing pitted surface of type 316 stainless steel following 4 hr in anodically polarized 5 N HCl with a current density of 87 mA/cm2. The average surface pit diameter is 57 w.
FLAME-SPRAYED ALUMINA ON STAINLESS STEEL 2000
449 200
T 1000
=
100
900 800
80
=- 700
70
I-
60
0 600
3a
5
z
n"
500
5 o K w Iw
n
z
400
" 3n
300
30
w
d v)
3I-
4 0 5-
k W
s
W
s 20
200
10
100 1
2 3 LOG TIME (HOURS)
4
5
Fig. 2. Type 316 stainless steel surface pit diameter (bottom) and the resultant flame-sprayed alumina-to-metal tensile bond strength (top) as a function of the stainless steel substrate anodic polarization time in 5 N HC1 with anodic current density of 87 mA/cm2. A mechanical ceramic-to-metal bond is implied from these curves.
4.50
BALDWIN AND NACKESZIE
eter) and eventual bond strength. The resultant bond strength can therefore be predictably controlled based on the anodic polarization time in acidic solution. A significant drop in tensile bond strength was observed for samples exposed t o the polarized solution for periods longer than 5 hr. This was due to the formation of electrochemical metastable phases, such as Cr(OIT)3 and H 2 C r 0 4 ,which tend to destroy the surface pit morphology.’” The fiame-sprayed alumina surface porosity was analyzed by scanning electron microscopy and was seen to have only 20 pores/cni2 within the pore size range of 15 and SO p. According to published resultsll for the minimum interconnecting pore size range necessary for mineralized bone, osteoid, and fibrous tissue ingrowth into porous materials, this pore size distribution would appear inadequate for mineralized bone ingrowth, although muscle tissue growth into the pores may be assumed. This pore distribution is significantly better than Richbourg’s plasma-sprayed alumina shafts,3 where no tissue fixation of any type was observed or expected based on his porosity studies. In vitro Studies A series of six flame-sprayed samples, prepared by spraying alumina onto anodically polarized, pitted 316 stainless steel t o attain maximum bond strength, were exposed t o the aerated Ringer’s solution for periods of 1, 2, and 3 weeks. Following exposure, the tensile bond strength was measured; the results are plotted in Figure 3 . A significant drop in bond strength is observed with as much as a 70% loss in strength after just 3 weeks. I n addition, “rust spots” (analyzed by x-ray diffraction to be a F e 2 0 3 - H 2 0were ) observed to have formed on the alumina surface of practically all specimens. The saline Ringer’s solution was able to penetrate through the porous flame-sprayed alumina layer and attack the stainless steel surface below. As seen from the literature,12 anodically polarized pit growth in stainless steels exposed t o chlorine environments results in a n electrochemically “active” surface. Thus, continued pit growth will occur upon re-exposure to a chlorine environment (in this case, the Ringer’s solution). The pits may be passivated either by heat treatment or by applying a protection potentia1,I3 but some deterioration would always occur ultimately resulting in loss of bond strength. The rate of strength loss was seen t o vary with time in Ringer’s solution as which corresponds t o published radial pit
FLAME-SPRAYED ALUMINA ON STAINLESS STEEL
45 1
1400
1200
L
h
1000
v
0
: 1I-
800
a
kl
0 2
2
600
w
2
z
v)
I-
400
) 200
0
1
2
3
TIME (WEEKS)
Fig. 3. Flame-sprayed alumina-to-316 stainless steel bond strength following exposure to simulated body environment. Bars represent standard deviation from mean.
growth rates of 18Cr-1ONi-Ti steel in 0.1 N NaC1.14 With this in mind, the severe loss in bond strength would therefore appear to occur a t the ceramic-metal interface such t h a t as the corrosion pit continues t o grow when exposed t o the Ringer’s solution, the metal effectively “pulls away” from the ceramic coating, thus eliminating the interlocking mechanical bond. No conclusions may be drawn for losses in the’bulk strength of the alumina coating following exposure to the Ringer’s solution.
4d2
BALDWIN AND MACKENZIE
CONCLUSIONS In evaluating the potential of flame-sprayed devices, several conclusions may be made based on this research. By orthopedic prosthesis standards, the strength of the flame-sprayed ceramic-to-metal bond is very low, and the device could never be expected to last very long under clinical load-bearing situations. This probably explains Richbourg’s3 poor clinical results inasmuch as plasma-sprayed bond strengths are not much stronger than flame-sprayed bond strengths. Concern should additionally be given to losses in strength due to the breakdown of the ceramic-nietal bond, although the dramatic losses in strength as in this study shouldn’t be expected given the proper precautions for metal substrate passification. Permanent attachment of the prosthesis by tissue ingrowth into the porous alumina coating may occur, but osteoid and mineralized bone growth into the prosthesis is unlikely. Based on these conclusions, it would appear that flame-sprayed alumina prosthesis may have some potential under nonload-bearing situations where muscle tissue fixation into a porous ceramic material is desired. Follow-up clinical research15 into anodically polarized Vitallium with a flame-sprayed alumina coating has been encouraging. The Vitallium substrate after pitting remains “passive,” and subsequent implantation of flame-sprayed alumina on Vitallium into the muscle tissue of rabbits has shown very good attachment with no tissue reaction. The authors are grateful to the National Institute of Health for their financial support of this research under research grant No. AM 16120-01.
References 1. S. F. Hulbert, F. A. Young, It. S. Mathews, J. J . Klawitter, C. D. Talbert, arid F. H. Stelling, J . Biomed. Muter. Res., 4, 433 (1970). 2. J. A. Mock, .V!ater. Design Engr., 235, 89 (1966). 3. H. L. Richbourg, hl. S. Thesis, Clemson University School of Engineering, Clemson, S. C., 1973. 4. T . 1). Driskell, M.J. O’Hara, H. 11. Sheers, G. T. Greene, J. R. Hatiella, and J. Armitage, J . Biomed. Muter. Res. Symp. No. 2, 5 , 345 (1971). 5. American Society for Testing arid htaterials. ASTRl Test C633-69. 6. K. R. Wheeler and J. A. James, J . Bzomed. Muter. lies., 5, 257 (1971). 7. S . N. Auk, J . A n w . Perurn. Soc., 40, 69 (1957). 8. S. J. Grisaffe, NASA Techriical Note TN 1)-3113 (1965). 9. M. Levy, G. N. Sklover, and I). H. Sellers, AMRA TR 66-01 (1966).
FLAME-SPRAYED ALUMINA ON STAINLESS STEEL
453
10. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, New York, 1966. 11. J. J. Klawitter and S. F. Hulbert, J . Biomed. Mater. Res. Symp. No. 2, 5, 161 (1971). 12. Z. Szklarska-Smialowska,Corrosion, 27, 223 (1971). 13. J. R. Cahoon, R. Bandyopadhya, and L. Tennese, J . Biomed. Mater. Res., 9, 259 (1975). 14. I. L. Rozenfeld and I. S.Danilov, Protec. Metals, 6 , 10 (1970). 15. B. S. Dunn and M. Reisbeck, paper presented at the 6th Annual Symposium on Biomaterials, Clemson University, Clemson, S. C., 1975.
Received August 24, 1975 Revised October 1, 1975
VOL. 10, PP. 445-453 (1976)
Flame-Sprayed Alumina on Stainless Steel for Possible Prosthetic Application C. M. BALDWIN and J. D. MACKENZIE, School of Engineering and Applied Science, Materials Science Department, University of California, Los Angeles, California 90024
Summary Various methods of roughening type 316 stainless steel substrate surfaces for flame-spraying alumina (A1,O were investigated and tested for the best aluminato-metal bond strength. Best strength values were obtained by means of roughening via anodic polarization pitting of the stainless steel. Subsequent in vitro testing indicated a severe loss in bond strength following exposure to aerated Ringer's solution. It is suggested that the utilization of flame-sprayed devices has potential in orthopedic prostheses, but precautions must be observed.
INTRODUCTION The concept of a physiologically inert ceramic coating such as alumina (AlZO3)attached to a metal shaft has long been considered a viable prosthetic design configurat,ion from the standpoint of combining both the corrosion-resistance of a ceramic with the overall mechanical strength of a metal into one device.' A processing technique which could produce such a configuration is the flame-spraying process. This fabrication process involves the passing of a ceramic powder through a high-temperature oxyacetylene flame (2500°F maximum) whereupon the molten ceramic particles are then sprayed onto a prepared' metal substrate. The resultant metal-ceramic configuration consists of a thin (typically about 30 mil thick), slightly porous ceramic coating bonded to a metal core.2 By controlling the flame-spraying application parameters, the porosity of the coating may be regulated whereby a porous ceramic coating capable of accepting permanent bone or tissue ingrowth can be fabricated. 445 @ 1976 by John Wiley & Sons, Inc.
446
BALD WIN AND MACKENZIE
Clinical studies which have been undertaken to evaluate plasmasprayed* alumina bonded to various substrate meta1s3s4have generally been not too encouraging. Richbourg3 plasma-sprayed 99.9% pure alumina onto the shaft of a stainless steel hip prosthesis and implanted the device into dogs for periods up to 2 years. The prostheses were biocompatible but some coatings were observed to have flaked off. Because of the density of the alumina coating (the average pore size being less than 1 p ) , no tissue ingrowth into the device was observed. In another study, Driskel14 attempted to implant plasma-sprayed alumina devices into the mandibles of monkeys, but technical and operative complications prevented any meaningful results.
EXPERIMENTAL Metco type 101 flame-spray powder (approximately 2.5% TiOz and 97.5% Alz03) was selected as the flame-spray coating material and was applied to type 316 stainless steel via a Metco oxyacetylene flame-spray gun, model 2P. The dimensions of the type 316 stainless steel substrate was in accordance with ASTM standards5 for testing flame-sprayed ceramic-to-metal bond strength; the metal substrate was a 1 in. diameter cylinder by 2 in. in length. The 1 in. diameter metal surface was lathe-polished and acetone-cleaned prior to any substrate roughening preparation for flame-spraying. Several substrate roughening techniques were employed, including 1) conventional alumina grit sandblasting of the metal surface, 2) mechanical roughening using an engraver’s tool bit on a Bridgeport model 541614 milling machine, and 3) anodic polarization roughening where the surface is roughened by electrochemical pitting. In the latter technique, the metal surface was exposed to a 5 N HC1 solution and then anodically polarized a t a constant current density of 87 mA/cm2 for various lengths of time. Following exposure, the samples were rinsed in distilled water and dried. The strength of the resultant flame-sprayed alumina-to-metal bond was measured according to ASTM standards5 by using an Instron model TTCLM 1.6 testing machine. In this test, a viscous, nonpenetrating bonding agent (Devcon “2-ton” epoxy) is coated on the * This process is similar to flame-spraying but produces a much denser ceramic coating. See reference 2 for further details.
FLAME-SPRAYED ALUMINA ON STAINLESS STEEL
447
flame-sprayed ceramic face and a loading fixture is then placed on the epoxy. After hardening, this assembly is loaded in pure tension perpendicular to the face of the ceramic coating. The crosshead speed was held constant at 0.02 in./min. The bond strength u was calculated such that lJ=
Maximum Load Substrate Surface Area
A minimum of six samples prepared under identical conditions were tested for each data point. In vitro tests were conducted in an apparatus similar to one described by Wheeler and James.6 The artificial body fluid was aerated Ringer’s solutions (850 mg NaC1, 33 mg CaC12, 30 mg KC1 per 100 ml distilled water) heated to approximately 110°F. The Ringer’s solution pH was held constant at 7.3 as monitored by a Reckman model 9600 pH meter. The apparatus was designed to simultanously expose up to 12 specimens. Following exposure, the ceramicto-metal bond strengths for all samples were tested according to the aforementioned technique.
RESULTS AND DISCUSSION Bonding Characteristics It is generally acknowledged that the flame- (or plasma-) sprayed alumina-to-metal bond is entirely mechanical and no interfacial chemical reaction ~ c c u r s . ’ ~The ~ gross surface roughness of the substrate therefore dictates the ultimate bond strength of the composite such that, all other processing parameters being equal, the rougher the substrate surface, the stronger the bond. Of the various substrate surface roughening techniques attempted, the maximum ceramic-to-metal bond strengths were obtained from alumina flame-sprayed onto anodic polarization-prepared metal surfaces. The bond strength of this particular configuration was significantly higher than the other specimens and of comparable flamesprayed alumina-titania mixtures sprayed onto stainless steel from the literature (see Table I). As seen in Figure 1, the anodically polarized stainless steel surface is highly pitted, thus enabling the flame-sprayed alumina particles to penetrate into the metal and adhere firmly to the substrate.
448
BALDWIN AND MACKENZIE TABLE I Maximum Bond Strength for Flame-Sprayed Alumina on Stainless Steel
Reference
Surface roughening method
Our studies Our studies Our studies 7 9
Anodic polarization Machine roughening Alumina grit sandblast Alumina grit sandblast Steel grit blast
Bond strength u (max), psi 1066 504 190 500 408
Failure location Ceramic-metal Ceramic-metal Ceramic-metal Ceramic-metal Ceramic-metal
interface interface interface interface interface
The average surface pit diameter D, as measured by scanning electron microscopy was observed to grow with time in the anodically polarized HC1 solution as (see Fig. 2). The corresponding alumina-to-stainless steel bond strength was observed to vary as to.’* (see Fig. 2), indicating a direct correlation between substrate surface roughness (as characterized by the average surface pit diam-
Fig. 1. Scanning electron microscope (225 X ) showing pitted surface of type 316 stainless steel following 4 hr in anodically polarized 5 N HCl with a current density of 87 mA/cm2. The average surface pit diameter is 57 w.
FLAME-SPRAYED ALUMINA ON STAINLESS STEEL 2000
449 200
T 1000
=
100
900 800
80
=- 700
70
I-
60
0 600
3a
5
z
n"
500
5 o K w Iw
n
z
400
" 3n
300
30
w
d v)
3I-
4 0 5-
k W
s
W
s 20
200
10
100 1
2 3 LOG TIME (HOURS)
4
5
Fig. 2. Type 316 stainless steel surface pit diameter (bottom) and the resultant flame-sprayed alumina-to-metal tensile bond strength (top) as a function of the stainless steel substrate anodic polarization time in 5 N HC1 with anodic current density of 87 mA/cm2. A mechanical ceramic-to-metal bond is implied from these curves.
4.50
BALDWIN AND NACKESZIE
eter) and eventual bond strength. The resultant bond strength can therefore be predictably controlled based on the anodic polarization time in acidic solution. A significant drop in tensile bond strength was observed for samples exposed t o the polarized solution for periods longer than 5 hr. This was due to the formation of electrochemical metastable phases, such as Cr(OIT)3 and H 2 C r 0 4 ,which tend to destroy the surface pit morphology.’” The fiame-sprayed alumina surface porosity was analyzed by scanning electron microscopy and was seen to have only 20 pores/cni2 within the pore size range of 15 and SO p. According to published resultsll for the minimum interconnecting pore size range necessary for mineralized bone, osteoid, and fibrous tissue ingrowth into porous materials, this pore size distribution would appear inadequate for mineralized bone ingrowth, although muscle tissue growth into the pores may be assumed. This pore distribution is significantly better than Richbourg’s plasma-sprayed alumina shafts,3 where no tissue fixation of any type was observed or expected based on his porosity studies. In vitro Studies A series of six flame-sprayed samples, prepared by spraying alumina onto anodically polarized, pitted 316 stainless steel t o attain maximum bond strength, were exposed t o the aerated Ringer’s solution for periods of 1, 2, and 3 weeks. Following exposure, the tensile bond strength was measured; the results are plotted in Figure 3 . A significant drop in bond strength is observed with as much as a 70% loss in strength after just 3 weeks. I n addition, “rust spots” (analyzed by x-ray diffraction to be a F e 2 0 3 - H 2 0were ) observed to have formed on the alumina surface of practically all specimens. The saline Ringer’s solution was able to penetrate through the porous flame-sprayed alumina layer and attack the stainless steel surface below. As seen from the literature,12 anodically polarized pit growth in stainless steels exposed t o chlorine environments results in a n electrochemically “active” surface. Thus, continued pit growth will occur upon re-exposure to a chlorine environment (in this case, the Ringer’s solution). The pits may be passivated either by heat treatment or by applying a protection potentia1,I3 but some deterioration would always occur ultimately resulting in loss of bond strength. The rate of strength loss was seen t o vary with time in Ringer’s solution as which corresponds t o published radial pit
FLAME-SPRAYED ALUMINA ON STAINLESS STEEL
45 1
1400
1200
L
h
1000
v
0
: 1I-
800
a
kl
0 2
2
600
w
2
z
v)
I-
400
) 200
0
1
2
3
TIME (WEEKS)
Fig. 3. Flame-sprayed alumina-to-316 stainless steel bond strength following exposure to simulated body environment. Bars represent standard deviation from mean.
growth rates of 18Cr-1ONi-Ti steel in 0.1 N NaC1.14 With this in mind, the severe loss in bond strength would therefore appear to occur a t the ceramic-metal interface such t h a t as the corrosion pit continues t o grow when exposed t o the Ringer’s solution, the metal effectively “pulls away” from the ceramic coating, thus eliminating the interlocking mechanical bond. No conclusions may be drawn for losses in the’bulk strength of the alumina coating following exposure to the Ringer’s solution.
4d2
BALDWIN AND MACKENZIE
CONCLUSIONS In evaluating the potential of flame-sprayed devices, several conclusions may be made based on this research. By orthopedic prosthesis standards, the strength of the flame-sprayed ceramic-to-metal bond is very low, and the device could never be expected to last very long under clinical load-bearing situations. This probably explains Richbourg’s3 poor clinical results inasmuch as plasma-sprayed bond strengths are not much stronger than flame-sprayed bond strengths. Concern should additionally be given to losses in strength due to the breakdown of the ceramic-nietal bond, although the dramatic losses in strength as in this study shouldn’t be expected given the proper precautions for metal substrate passification. Permanent attachment of the prosthesis by tissue ingrowth into the porous alumina coating may occur, but osteoid and mineralized bone growth into the prosthesis is unlikely. Based on these conclusions, it would appear that flame-sprayed alumina prosthesis may have some potential under nonload-bearing situations where muscle tissue fixation into a porous ceramic material is desired. Follow-up clinical research15 into anodically polarized Vitallium with a flame-sprayed alumina coating has been encouraging. The Vitallium substrate after pitting remains “passive,” and subsequent implantation of flame-sprayed alumina on Vitallium into the muscle tissue of rabbits has shown very good attachment with no tissue reaction. The authors are grateful to the National Institute of Health for their financial support of this research under research grant No. AM 16120-01.
References 1. S. F. Hulbert, F. A. Young, It. S. Mathews, J. J . Klawitter, C. D. Talbert, arid F. H. Stelling, J . Biomed. Muter. Res., 4, 433 (1970). 2. J. A. Mock, .V!ater. Design Engr., 235, 89 (1966). 3. H. L. Richbourg, hl. S. Thesis, Clemson University School of Engineering, Clemson, S. C., 1973. 4. T . 1). Driskell, M.J. O’Hara, H. 11. Sheers, G. T. Greene, J. R. Hatiella, and J. Armitage, J . Biomed. Muter. Res. Symp. No. 2, 5 , 345 (1971). 5. American Society for Testing arid htaterials. ASTRl Test C633-69. 6. K. R. Wheeler and J. A. James, J . Bzomed. Muter. lies., 5, 257 (1971). 7. S . N. Auk, J . A n w . Perurn. Soc., 40, 69 (1957). 8. S. J. Grisaffe, NASA Techriical Note TN 1)-3113 (1965). 9. M. Levy, G. N. Sklover, and I). H. Sellers, AMRA TR 66-01 (1966).
FLAME-SPRAYED ALUMINA ON STAINLESS STEEL
453
10. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, New York, 1966. 11. J. J. Klawitter and S. F. Hulbert, J . Biomed. Mater. Res. Symp. No. 2, 5, 161 (1971). 12. Z. Szklarska-Smialowska,Corrosion, 27, 223 (1971). 13. J. R. Cahoon, R. Bandyopadhya, and L. Tennese, J . Biomed. Mater. Res., 9, 259 (1975). 14. I. L. Rozenfeld and I. S.Danilov, Protec. Metals, 6 , 10 (1970). 15. B. S. Dunn and M. Reisbeck, paper presented at the 6th Annual Symposium on Biomaterials, Clemson University, Clemson, S. C., 1975.
Received August 24, 1975 Revised October 1, 1975
