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Effect of free mercury on the strengths of dental amalgams
Effect of free mercury on the strengths of dental amalgams
T. Okabe, J. Staman, J. Ferracane, R. Mitchell 1 Dental MaterialsScience Program, Baylor College of Dentistry, Dallas, Texas, 1College of Dentistry, University of Kentucky, Lexington, USA
Okabe T, Staman JW, Ferracane JL, Mitchell RJ. Effect of free mercury on the strengths of dental amalgams. Dent Mater 1985: 1: 180-184. Abstract. - A previous study of several different Ag-Sn, Ag-Cu, and Ag-Sn-Cu alloys revealed that some of these alloys were susceptible to mercury embrittlement. The present study was undertaken to determine the extent to which embrittlement of alloy particles in amalgam affects the strength of the material. Specimens of low copper amalgam (O), high copper admixed amalgam (D), and high copper single composition amalgams (S and T) were condensed into 2 m m x 4 mm x 15 mm molds using a pressure of 14 MPa. After aging the specimens for 7 days at 37~ mercury was coated on each specimen either by electro-plating mercury or by immersion in mercury. In each method, mercury was applied for 15 s or 1 min. A t 30 s after coating, each specimen was placed in a 3-point fracture fixture and immediately loaded at 0.25 and 2.54 mm/min until fracture occurred. As controls, specimens of each amalgam without mercury coating were also tested. Ten specimens per experimental condition were tested. Fractured surfaces were examined using a scanning electron microscope. Strength reductions as high as 60% for O amalgam, 44% for D amalgam, and 31% for T amalgam were found. Two observations suggest that embrittlement of unconsumed alloy particles is primarily responsible for the strength reduction: 1) the amalgams showing susceptibility to mercury embrittlement in the present study contain alloy particles of alloys that were shown to be susceptible to embrittlement in an earlier study, and 2) the amalgam S, which showed the least susceptibility to mercury embrittlement, contains unconsumed alloy particles of an alloy that showed no significant embrittlement in the earlier study. Experimental evidence supports the hypothesis that free mercury released during corrosion or aging of amalgam restorations can contribute to the weakening of these restorations.
The embrittlement of solid metal by liquid metal is a well-known metallurgical phenomenon (1-10). Liquid metal embrittlement is defined as the decrease in strength or ductility of a solid metal or alloy when subjected to stress during or after contact with liquid metals. Mercury embrittlement is a distinct type of brittle fracture, in which mercury weakens atomic bonds at crack tips, thereby decreasing the initial stress for crack propagation. In an earlier study (11), the authors showed that the strengths of many alloys used to make powder for dental amalgam were reduced after plating with mercury. It was suggested that mercury released during aging or corrosion of amalgams in vivo might embrittle unconsumed alloy particles and thereby weaken the amalgam restoration. To test this hypothesis, the strength of mercury-coated amalgams has been measured.
Material and methods
Specimens of low copper amalgam (O)*, admixed amalgam (D)* and single composition amalgams (S and T)* were prepared using trituration procedures recommended by the alloy manufacturers. Each amalgam mass was condensed into a 2 mm x 4 mm x 15 mm steel mold using a pressure of 14 MPa. After aging the specimens for 7 days at 37~ mercury was coated on each specimen, either by electro-plating mercury (12) or by immersion in a mercury bath. Mercury was applied for 15 s or 1 min. Thirty seconds after coating, each specimen was placed in a * O, Optaloy, L.D. Caulk Co., Milford, DE 19963. t D, Dispersalloy, Johnson & Johnson Dental Products, East Windsor, NJ 08520. * S, Sybraloy, Kerr Manufacturing Co., Romulus, MI 48174 and T, Tytin, S.S. White Dental Manufacturing Co., Philadelphia, PA 19102.
Key words: dentalamalgam, mercury, mercury embrittlement. Dr. Toru Okabe, Department of Dental Materials, Baylor College of Dentistry, 3302 Gaston Avenue, Dallas, TX 75246, USA.
Acceptedfor publication June 3, 1985.
three-point fracture fixture and loaded until fracture occurred. For each alloy, tests were performed using two loading speeds, 0.25 ram/rain and 2.54 ram/ rain. In order to ~tetermine if free liquid mercury needed to be present for embrittlement to occur, the flexure strength of amalgams O and D was also tested at the loading rate of 0.25 ram/ rain one week after plating the specimens with mercury for 15 s. Ten specimens of each amalgam were tested for each experimental condition. Multiple comparison tests (Tukey HSD, P = 0.05) and student t-tests (p = 0.05) were used to analyze the data. Fractured surfaces of controls and of 15-s Hg-plated specimens produced at both loading speeds were examined using S E M / X E D A ) The fractured surfaces produced under other experimen~JEOL JSM-35C, Peabody, MA 01960; TN-2000 x-ray analyzer, Tracor Northern, Middleton, WI 53562.
Mercury embrittlement of Hg-wetted amalgams
181
11SO S AMALGAM 1[4 0
CONTROL 15 sec. P L A T I N G 1 rain. P L A T I N G
1120
15 sec. I M M E R S I O N I T ~ 1 mln, I M M E R S I O N I ~
1100
80
60
40
20
0 "r
T T
T
,__T
T
T
Fig. 1. Effect of mercury coating condition
Fig. 3. Effect of mercury coating condition
on flexure stress of O amalgam. The error bar on each flexure stress indicates the 95% confidence limit. The bars under the histogram link the groups which were not significantly different (p = 0.05).
on flexure stress of S amalgam. The error bar on each flexure stress indicates the 95% confidence limit. The bars under the histogram link the groups which were not significantly different (p = 0.05).
D AMALGAM
;AM
CONTROL 15
sec. p L A T I N G
1 rain. P L A T I N G 120
15 seo. I M M E R S I O N Q I rain. I M M E R S I O N m
m
=
100
80 60
T
T
T
T
on flexure stress of D amalgam. The error bar on each flexure stress indicates the 95% confidence limit. The bars under the histogram link the groups which were not significantly different (p = 0.05).
Fig. 4. Effect of mercury coating condition on flexure stress of T amalgam. The error bar on each flexure stress indicates the 95% confidence limit. The bars under the histogram link the groups which were not significantly different (p = 0.05).
tal conditions (e.g. 1-min electroplating and immersion in mercury, both for 15 s and 1 min) were covered by amal-
gamation reaction products. Therefore, no microscopic information concerning fracture mode could be obtained.
Fig. 2. Effect of mercury coating condition
Results
The flexure stress for each amalgam at each experimental condition is shown in Figs. 1-4. At the slower strain rate, the flexure stress of all amalgams tested was reduced. For the 15-s Hg plating, reductions in strength were as high as 60% for amalgam O (Fig. 1), 44% for amalgam D (Fig. 2), and 31% for amalgams S (Fig. 3), and T (Fig. 4). At the faster strain rate, significant reductions in" strength were observed only for amalgams O and D. Overall, amalgam S was the least affected by Hg embrittlement. Table 1 contains a ranking of the amalgams at each experimental condition in order of decreasing flexure stress. The flexure stresses for amalgams O and D, tested one week after Hg plating, were 130.4 + 16.5 MPa and 99.0 + 26.0 MPa, respectively. These values were not significantly different from the mean flexure stresses of the nonwetted controls of amalgams O and D. Typical scanning electron micrographs of the control and 15-s Hgplated specimens for each amalgam tested at the loading speed of 0.25 ram/ min are shown at two different magnifications in Figs. 5-12. In amalgam O (Figs. 5, 6), there did not appear to be any difference between the fracture surfaces of the control and Hg-plated specimens even though the strengths were significantly different. In both cases, a majority of the unconsumed alloy particles (area A in Figs. 5b, 6b) fractured and the Y1 matrix (area B in Figs. 5b, 6b) fractured intergranularly. For S and T amalgams (Figs. 7-10), a majority of the unconsumed alloy par-
Table 1. Comparison of the mean flexure stresses of amalgams (MPa). Loading speed 0.25 mm/min Control Optaloy Sybratoy Tytin Dispersalloy
138.7 119.2] 109.7 105.8
15 sec. plating
1 min. plating
Sybraloy 1 0 0 . 5 Tytin 85.811 Optaloy 72.5 Dispersalloy 6 4 . 4
Sybraloy Tytin Dispersalloy Optaloy
80.2 67.211 58.4 52.9
15 sec. immersion
1 min. immersion
Sybraloy Tytin Dispersalloy Optaloy
Sybraloy Tytin Dispersalloy Optaloy
95.9 74.4 61.0 56.4
91.8 66.11 58.2 I 52.0
Loading speed 2.54 mm/min Control
15 sec: plating
1 min. plating
Optaloy 127.5 DispersaUoy 8 9 . 5 1 Tytin 84.9 Sybraloy 83.7
Sybraloy 80.7[ Optaloy 78.21 Tytin 73.2 Dispersalloy 7 3 . 1
Sybraloy Tytin Optaloy Dispersalloy
Means connected by bars are not statistically different (p= .05). 14
D e n t a l M a t e r i a l s 1:5, 1985
71.0 58.71 51.4 50.0
15 sec. immersion
1 rain. immersion
Sybraloy Tytin Dispersalloy Optaloy
Sybraloy Tytin Dispersalloy Optaloy
80.7 80.11] 63.1 62.7
78.9 64.8I 60.0 58.5
182
Okabe et al.
Pig. 5a. A typical fracture of an O amalgam specimen (control). The length of the marker is 10 ~tm.
Fig. 7a. A typical fracture surface of an S amalgam specimen (control). The length of the marker is 10 [*m.
Fig. 9a. A typical fracture surface of a T amalgam specimen (control). The length of the marker is 10 [*In.
Fig. 5b. A typical fracture of an O amalgam specimen (control). The length of the marker is 3.3 gin.
Fig. 7b. A typical fracture surface of an S amalgam specimen (control). The length of the marker is 3.3 p.m.
Fig. 9b. A typical fracture surface of a T amalgam specimen (control). The length of the marker is 3.3 gin.
Fig. 6a. A typical fracture surface of an O amalgam specimen (15-s plated). The length of the marker is 10 ~m.
Fig. 8a. A typical fracture surface of an S amalgam specimen (15-s plated). The length of the marker is 10 p,m.
Fig. lOa. A typical fracture surface of a T amalgam specimen (15-s plated). The length of the marker is 10 [*m.
Fig. 6b. A typical fracture surface of an O amalgam specimen (15-s plated). The length of the marker is 3.3 ~tm.
Fig. 8b. A typical fracture surface of an S amalgam specimen (15-s plated). The length of the marker is 3.3 ~tm.
Fig. lob. A typical fracture surface of a T amalgam specimen (15-s plated). The length of the marker is 3.3 ~tm.
ticles (area A in Figs. 7b, 8b, 9b, 10b) fractured intraparticularly in both types of specimens. In S amalgams, a majority of the 7~ grains (area B in Figs. 7b,
8b) had cleaved in both control and Hg-plated specimens. In T amalgams, a considerable n u m b e r of 71 grains had appeared cleaved in the control speci-
mens (B arrows in Fig. 9b); however, there were few cleaved y~ grains in the Hg-plated T specimens (B area in Fig. 10b). Wetting the D specimen with
Mercury embrittlement of Hg-wetted amalgams
Fig. lla. A typical fracture surface of a D amalgam specimen (control). The length of the marker is 10 ~tm.
Fig. 12a. A typical fracture surface of a D amalgam specimen (15-s plated). The length of the marker is 10 ~tm.
Fig. lib. A typical fracture surface of a D amalgam specimen (control). The length of the marker is 3.3 gm.
Fig. 12b. A typical fracture surface of a D amalgam specimen (15-s plated). The length of the marker is 3.3 ~tm.
mercury did not change the fracture mode of the Y1 matrix (intergranular, compare A areas in Figs. 11b, 12b), or of the Ag-Sn particles (intraparticular, compare B areas in Figs. l l a , 12b). However, the fracture mode of Ag-Cu eutectic particles did change. In controls, no Ag-Cu eutectic particles fractured (area C in Fig. l l b ) . However, in mercury-wetted specimens, a majority of Ag-Cu eutectic particles fractured (area C in Fig. 12b). The fracture modes at 0.25 mm/min (shown in Figs. 5-12) seemed to be the same as those found in specimens fractured at a loading rate of 2.54 mm/min.
with mercury. Although the total quantity of mercury released during aging of amalgam in vivo may be low, the concentration may briefly reach high enough levels to cause a localized weakening of the restoration. In spite of the reduced strength of freshly plated amalgams, there was no change in the fracture morphology within alloy particles. This observation is not surprising. In the authors' earlier study, mercury-plating Ag-Sn and Ag-Sn-Cu alloys reduced tensile strength, but did not change fracture morphology (11). Nevertheless, the influence of embrittled alloy particles is clear. The alloys studied earlier had similar compositions to the alloy particles in the present commercial amalgams. When ranked according to percentage strength reduction due to mercury plating, the amalgams ranked in the same order as the alloys from which they were made. Although embrittlement of alloy particles by mercury is probably the dominant reason for the weakening of the amalgams, it is not possible to rule out an effect of mercury on the matrix. Except for amalgam T, none of these amalgams exhibited clear microstructural changes within the matrix after mercury plating. Nevertheless, the obser-
Discussion In the previous study, mercury embrittlement of several different Ag-Sn, Ag-Cu and Ag-Sn-Cu alloys was investigated (11). The study showed that Ag-Sn and Ag-Sn-Cu alloys were susceptible to mercury embrittlement. The present study was undertaken to determine the extent to which embrittlement of alloy particles in amalgam affects the strength of the material. This study revealed that the strength of all amalgams, except amalgams S and T when tested at 2.54 mm/min, was significantly (p = 0.05) lowered by wetting 14"
183
vations of the T amalgam suggest that something may also be happening to the matrices of the other amalgams. The increase in intergranular fracture in T suggests that mercury weakens its grain boundaries. This effect is visible in T because of the tendency of grains in the unplated T amalgam to cleave. The fracture mode of the matrix of the other unplated amalgams is entirely intergranular; consequently, embrittlement by mercury cannot increase the percentage of intergranular fracture. The matrix of the D, O and S amalgams may be embrittled even though no change in morphology is detected. There were no significant differences between the strengths of each amalgam after the various mercury wetting procedures. As long as liquid mercury is present, mercury embrittlement occurs in dental amalgams. However, mercury did not reduce the strengths of amalgams O and D tested 7 days after Hg plating. This is understandable, since within a short time after plating, liquid mercury has reacted with unconsumed alloy particles, leaving no free mercury to embrittle the amalgam. As mentioned above, an overall trend in the strength of Hg-wetted amalgams was observed: S > T > D > O. The present results are consistent with the results from the previous study in our laboratory (11). In the previous study, Ag-Cu eutectic, 49% Ag-51% Cu and 40% Ag-30% Sn-30% Cu alloys were not significantly embrittled when tested 5 min after mercury plating. The chemical composition of the last of these alloys was the same as that of the S alloy particles in the present study. The high resistance of the unconsumed S alloy particles to mercury embrittlement explains the relatively good resistance to mercury embrittlement of the S amalgam. Although Ag-Cu alloys exhibited good resistance to mercury embrittlement (11), the strengths of the D amalgam were reduced after wetting with mercury. This reduction in strength is probably due, in part, to the presence of Ag-Sn alloy particles in the D amalgam. The earlier study revealed that the Ag-Sn alloy is severely embrittled by liquid mercury (11). Note that the O amalgam, which also contains Ag-Sn alloy particles, was also severely embrittled by mercury. In the 15-s Hg-plated D amalgams, a majority of the Ag-Cu eutectic particles was found to have been fractured (Fig. 12). Fracture of these particles
184
O k a b e et al.
contributes to the embrittlement of the D amalgam. The fact that mercuryplated Ag-Cu eutectic tensile specimens resisted embrittlement, while Ag-Cu eutectic particles did not, is an interesting anomaly, and may be caused by a difference in the interval between mercury plating and testing. If the interval is short, liquid mercury is likely to be present during testing. As the interval increases, the amount of liquid mercury present decreases at a rate that depends upon: 1) how fast mercury is consumed during the formation of y1 Ag-Hg; or 2) how fast m e r cury diffuses into alloy particles. Diffusion into alloy particles will be especially fast in the eutectic alloy, as these alloys contain a high density of interphase boundaries that act as paths for fast diffusion. Consequently, mercury embrittlement of eutectic alloys may go undetected if testing is not begun immediately after plating, because liquid mercury may have disappeared. The present specimens were fractured within 40 s (at a strain rate of 2.54 mm/ min) of the completion of plating. In the earlier study, tensile tests were not begun until 5 rain after plating. Consequently, the testing interval of the present study is more likely to detect embrittlement. Conclusions
1. The strength of all amalgams, except S, a single composition high copper amalgam, when tested at 2.54 ram/rain, was significantly (Tukey HSD, p = 0.05) lowered by contact with mercury. Strength reductions as high as 60% for O, a low copper amalgam, 44% for D, an admixed high copper amalgam, and 31% for T, a single composition high copper amalgam, have been found.
2. After wetting with mercury, S amalgam was always significantly stronger than O amalgam and D amalgam. 3. An overall trend on the strength of wetted amalgams was observed: S > T >D>O. 4. Controls for the amalgams T, S, and O, respectively, were significantly weaker at 2.54 mm/min than at 0.25 mm/min (T-test, p = 0.05). After each of the mercury-wetting procedures, there was no significant difference in strength of any amalgam at the two loading speeds. 5. There did not appear to be any difference in fracture mode between the control and Hg-plated specimens of O and S amalgams. 6. In D amalgams, intraparticle frac-
ture of silver-copper eutectic particles was more frequently observed in the Hg-plated specimens. Mercury appears to embrittle the eutectic particle, reducing the amalgam strength. 7. In T amalgams, a considerable number of Y1 grains appeared to be cleaved in the control specimens while fracture was mainly intergranular through the matrix for the Hg-plated specimens. Acknowledgements - The present authors
would like to thank those alloy manufacturers who provided the alloys for this investigation. The present study was supported by Grant 5 ROI DE 06539 from the National Institute of Dental Research, National Institute of Health, Department of Health and Human Services, Bethesda, MD 20205.
References
1. OLD CF. Micromechanisms of crack growth in liquid metal environments. M a t Sci 1980: 14: 433-440. 2. RO~ERTSONWM. Propagation of a crack filled with liquid metal. Trans Metal Soc Am Inst Min, Metal Petrol Eng 1966: 236: 1478-82. 3. LYNCHSP. A R L Mat Reports No. 101, 102: Melbourne: Dept. of Defense, 1977. 4. STOLOFFNS, JOHNSONTL. Crack propagation in a liquid metal environment. Acta Metall 1963: 11: 251-56. 5. WESTWOODARC, KAMDARMH. Concerning liquid metal embrittlement, particulafly of zinc monocrystals by mercury. Philos Mag 1963: 8: 787-804. 6, GORDONP, AN HH. The mechanisms of crack initiation in metal-induced embrittlement of metals. Met Trans 1982: 13A: 457-472. 7. KRISHTALMA. The formation of dislocations in metals on diffusion of surfaceactive substances in connection with the effect of adsorption embrittlement. Soviet Physics Doklady 1970: 15: 614--617. 8. LYNNJC, WARKEWR, GORDONP. Solid metal-induced embrittlement of steel. Mater Sci Eng 1975: 18: 51-62. 9. KAMDA~MH. Embrittlement by liquid metals. Prog Mat Sci 1973: 15: 289. 10. T~NERNA. A study of fracturing behavior of copper and zinc coated with mercury. Trans Am Inst Metal Eng 1961: 221: 261-265. 11. OKABET, TSENG WW, GALLOWAYS, TIMMERMANB, MITCHELLR. Resistance to mercury embrittlement of some alloys for amalgams. Dent Mater 1985: 1: 19-26.
T. Okabe, J. Staman, J. Ferracane, R. Mitchell 1 Dental MaterialsScience Program, Baylor College of Dentistry, Dallas, Texas, 1College of Dentistry, University of Kentucky, Lexington, USA
Okabe T, Staman JW, Ferracane JL, Mitchell RJ. Effect of free mercury on the strengths of dental amalgams. Dent Mater 1985: 1: 180-184. Abstract. - A previous study of several different Ag-Sn, Ag-Cu, and Ag-Sn-Cu alloys revealed that some of these alloys were susceptible to mercury embrittlement. The present study was undertaken to determine the extent to which embrittlement of alloy particles in amalgam affects the strength of the material. Specimens of low copper amalgam (O), high copper admixed amalgam (D), and high copper single composition amalgams (S and T) were condensed into 2 m m x 4 mm x 15 mm molds using a pressure of 14 MPa. After aging the specimens for 7 days at 37~ mercury was coated on each specimen either by electro-plating mercury or by immersion in mercury. In each method, mercury was applied for 15 s or 1 min. A t 30 s after coating, each specimen was placed in a 3-point fracture fixture and immediately loaded at 0.25 and 2.54 mm/min until fracture occurred. As controls, specimens of each amalgam without mercury coating were also tested. Ten specimens per experimental condition were tested. Fractured surfaces were examined using a scanning electron microscope. Strength reductions as high as 60% for O amalgam, 44% for D amalgam, and 31% for T amalgam were found. Two observations suggest that embrittlement of unconsumed alloy particles is primarily responsible for the strength reduction: 1) the amalgams showing susceptibility to mercury embrittlement in the present study contain alloy particles of alloys that were shown to be susceptible to embrittlement in an earlier study, and 2) the amalgam S, which showed the least susceptibility to mercury embrittlement, contains unconsumed alloy particles of an alloy that showed no significant embrittlement in the earlier study. Experimental evidence supports the hypothesis that free mercury released during corrosion or aging of amalgam restorations can contribute to the weakening of these restorations.
The embrittlement of solid metal by liquid metal is a well-known metallurgical phenomenon (1-10). Liquid metal embrittlement is defined as the decrease in strength or ductility of a solid metal or alloy when subjected to stress during or after contact with liquid metals. Mercury embrittlement is a distinct type of brittle fracture, in which mercury weakens atomic bonds at crack tips, thereby decreasing the initial stress for crack propagation. In an earlier study (11), the authors showed that the strengths of many alloys used to make powder for dental amalgam were reduced after plating with mercury. It was suggested that mercury released during aging or corrosion of amalgams in vivo might embrittle unconsumed alloy particles and thereby weaken the amalgam restoration. To test this hypothesis, the strength of mercury-coated amalgams has been measured.
Material and methods
Specimens of low copper amalgam (O)*, admixed amalgam (D)* and single composition amalgams (S and T)* were prepared using trituration procedures recommended by the alloy manufacturers. Each amalgam mass was condensed into a 2 mm x 4 mm x 15 mm steel mold using a pressure of 14 MPa. After aging the specimens for 7 days at 37~ mercury was coated on each specimen, either by electro-plating mercury (12) or by immersion in a mercury bath. Mercury was applied for 15 s or 1 min. Thirty seconds after coating, each specimen was placed in a * O, Optaloy, L.D. Caulk Co., Milford, DE 19963. t D, Dispersalloy, Johnson & Johnson Dental Products, East Windsor, NJ 08520. * S, Sybraloy, Kerr Manufacturing Co., Romulus, MI 48174 and T, Tytin, S.S. White Dental Manufacturing Co., Philadelphia, PA 19102.
Key words: dentalamalgam, mercury, mercury embrittlement. Dr. Toru Okabe, Department of Dental Materials, Baylor College of Dentistry, 3302 Gaston Avenue, Dallas, TX 75246, USA.
Acceptedfor publication June 3, 1985.
three-point fracture fixture and loaded until fracture occurred. For each alloy, tests were performed using two loading speeds, 0.25 ram/rain and 2.54 ram/ rain. In order to ~tetermine if free liquid mercury needed to be present for embrittlement to occur, the flexure strength of amalgams O and D was also tested at the loading rate of 0.25 ram/ rain one week after plating the specimens with mercury for 15 s. Ten specimens of each amalgam were tested for each experimental condition. Multiple comparison tests (Tukey HSD, P = 0.05) and student t-tests (p = 0.05) were used to analyze the data. Fractured surfaces of controls and of 15-s Hg-plated specimens produced at both loading speeds were examined using S E M / X E D A ) The fractured surfaces produced under other experimen~JEOL JSM-35C, Peabody, MA 01960; TN-2000 x-ray analyzer, Tracor Northern, Middleton, WI 53562.
Mercury embrittlement of Hg-wetted amalgams
181
11SO S AMALGAM 1[4 0
CONTROL 15 sec. P L A T I N G 1 rain. P L A T I N G
1120
15 sec. I M M E R S I O N I T ~ 1 mln, I M M E R S I O N I ~
1100
80
60
40
20
0 "r
T T
T
,__T
T
T
Fig. 1. Effect of mercury coating condition
Fig. 3. Effect of mercury coating condition
on flexure stress of O amalgam. The error bar on each flexure stress indicates the 95% confidence limit. The bars under the histogram link the groups which were not significantly different (p = 0.05).
on flexure stress of S amalgam. The error bar on each flexure stress indicates the 95% confidence limit. The bars under the histogram link the groups which were not significantly different (p = 0.05).
D AMALGAM
;AM
CONTROL 15
sec. p L A T I N G
1 rain. P L A T I N G 120
15 seo. I M M E R S I O N Q I rain. I M M E R S I O N m
m
=
100
80 60
T
T
T
T
on flexure stress of D amalgam. The error bar on each flexure stress indicates the 95% confidence limit. The bars under the histogram link the groups which were not significantly different (p = 0.05).
Fig. 4. Effect of mercury coating condition on flexure stress of T amalgam. The error bar on each flexure stress indicates the 95% confidence limit. The bars under the histogram link the groups which were not significantly different (p = 0.05).
tal conditions (e.g. 1-min electroplating and immersion in mercury, both for 15 s and 1 min) were covered by amal-
gamation reaction products. Therefore, no microscopic information concerning fracture mode could be obtained.
Fig. 2. Effect of mercury coating condition
Results
The flexure stress for each amalgam at each experimental condition is shown in Figs. 1-4. At the slower strain rate, the flexure stress of all amalgams tested was reduced. For the 15-s Hg plating, reductions in strength were as high as 60% for amalgam O (Fig. 1), 44% for amalgam D (Fig. 2), and 31% for amalgams S (Fig. 3), and T (Fig. 4). At the faster strain rate, significant reductions in" strength were observed only for amalgams O and D. Overall, amalgam S was the least affected by Hg embrittlement. Table 1 contains a ranking of the amalgams at each experimental condition in order of decreasing flexure stress. The flexure stresses for amalgams O and D, tested one week after Hg plating, were 130.4 + 16.5 MPa and 99.0 + 26.0 MPa, respectively. These values were not significantly different from the mean flexure stresses of the nonwetted controls of amalgams O and D. Typical scanning electron micrographs of the control and 15-s Hgplated specimens for each amalgam tested at the loading speed of 0.25 ram/ min are shown at two different magnifications in Figs. 5-12. In amalgam O (Figs. 5, 6), there did not appear to be any difference between the fracture surfaces of the control and Hg-plated specimens even though the strengths were significantly different. In both cases, a majority of the unconsumed alloy particles (area A in Figs. 5b, 6b) fractured and the Y1 matrix (area B in Figs. 5b, 6b) fractured intergranularly. For S and T amalgams (Figs. 7-10), a majority of the unconsumed alloy par-
Table 1. Comparison of the mean flexure stresses of amalgams (MPa). Loading speed 0.25 mm/min Control Optaloy Sybratoy Tytin Dispersalloy
138.7 119.2] 109.7 105.8
15 sec. plating
1 min. plating
Sybraloy 1 0 0 . 5 Tytin 85.811 Optaloy 72.5 Dispersalloy 6 4 . 4
Sybraloy Tytin Dispersalloy Optaloy
80.2 67.211 58.4 52.9
15 sec. immersion
1 min. immersion
Sybraloy Tytin Dispersalloy Optaloy
Sybraloy Tytin Dispersalloy Optaloy
95.9 74.4 61.0 56.4
91.8 66.11 58.2 I 52.0
Loading speed 2.54 mm/min Control
15 sec: plating
1 min. plating
Optaloy 127.5 DispersaUoy 8 9 . 5 1 Tytin 84.9 Sybraloy 83.7
Sybraloy 80.7[ Optaloy 78.21 Tytin 73.2 Dispersalloy 7 3 . 1
Sybraloy Tytin Optaloy Dispersalloy
Means connected by bars are not statistically different (p= .05). 14
D e n t a l M a t e r i a l s 1:5, 1985
71.0 58.71 51.4 50.0
15 sec. immersion
1 rain. immersion
Sybraloy Tytin Dispersalloy Optaloy
Sybraloy Tytin Dispersalloy Optaloy
80.7 80.11] 63.1 62.7
78.9 64.8I 60.0 58.5
182
Okabe et al.
Pig. 5a. A typical fracture of an O amalgam specimen (control). The length of the marker is 10 ~tm.
Fig. 7a. A typical fracture surface of an S amalgam specimen (control). The length of the marker is 10 [*m.
Fig. 9a. A typical fracture surface of a T amalgam specimen (control). The length of the marker is 10 [*In.
Fig. 5b. A typical fracture of an O amalgam specimen (control). The length of the marker is 3.3 gin.
Fig. 7b. A typical fracture surface of an S amalgam specimen (control). The length of the marker is 3.3 p.m.
Fig. 9b. A typical fracture surface of a T amalgam specimen (control). The length of the marker is 3.3 gin.
Fig. 6a. A typical fracture surface of an O amalgam specimen (15-s plated). The length of the marker is 10 ~m.
Fig. 8a. A typical fracture surface of an S amalgam specimen (15-s plated). The length of the marker is 10 p,m.
Fig. lOa. A typical fracture surface of a T amalgam specimen (15-s plated). The length of the marker is 10 [*m.
Fig. 6b. A typical fracture surface of an O amalgam specimen (15-s plated). The length of the marker is 3.3 ~tm.
Fig. 8b. A typical fracture surface of an S amalgam specimen (15-s plated). The length of the marker is 3.3 ~tm.
Fig. lob. A typical fracture surface of a T amalgam specimen (15-s plated). The length of the marker is 3.3 ~tm.
ticles (area A in Figs. 7b, 8b, 9b, 10b) fractured intraparticularly in both types of specimens. In S amalgams, a majority of the 7~ grains (area B in Figs. 7b,
8b) had cleaved in both control and Hg-plated specimens. In T amalgams, a considerable n u m b e r of 71 grains had appeared cleaved in the control speci-
mens (B arrows in Fig. 9b); however, there were few cleaved y~ grains in the Hg-plated T specimens (B area in Fig. 10b). Wetting the D specimen with
Mercury embrittlement of Hg-wetted amalgams
Fig. lla. A typical fracture surface of a D amalgam specimen (control). The length of the marker is 10 ~tm.
Fig. 12a. A typical fracture surface of a D amalgam specimen (15-s plated). The length of the marker is 10 ~tm.
Fig. lib. A typical fracture surface of a D amalgam specimen (control). The length of the marker is 3.3 gm.
Fig. 12b. A typical fracture surface of a D amalgam specimen (15-s plated). The length of the marker is 3.3 ~tm.
mercury did not change the fracture mode of the Y1 matrix (intergranular, compare A areas in Figs. 11b, 12b), or of the Ag-Sn particles (intraparticular, compare B areas in Figs. l l a , 12b). However, the fracture mode of Ag-Cu eutectic particles did change. In controls, no Ag-Cu eutectic particles fractured (area C in Fig. l l b ) . However, in mercury-wetted specimens, a majority of Ag-Cu eutectic particles fractured (area C in Fig. 12b). The fracture modes at 0.25 mm/min (shown in Figs. 5-12) seemed to be the same as those found in specimens fractured at a loading rate of 2.54 mm/min.
with mercury. Although the total quantity of mercury released during aging of amalgam in vivo may be low, the concentration may briefly reach high enough levels to cause a localized weakening of the restoration. In spite of the reduced strength of freshly plated amalgams, there was no change in the fracture morphology within alloy particles. This observation is not surprising. In the authors' earlier study, mercury-plating Ag-Sn and Ag-Sn-Cu alloys reduced tensile strength, but did not change fracture morphology (11). Nevertheless, the influence of embrittled alloy particles is clear. The alloys studied earlier had similar compositions to the alloy particles in the present commercial amalgams. When ranked according to percentage strength reduction due to mercury plating, the amalgams ranked in the same order as the alloys from which they were made. Although embrittlement of alloy particles by mercury is probably the dominant reason for the weakening of the amalgams, it is not possible to rule out an effect of mercury on the matrix. Except for amalgam T, none of these amalgams exhibited clear microstructural changes within the matrix after mercury plating. Nevertheless, the obser-
Discussion In the previous study, mercury embrittlement of several different Ag-Sn, Ag-Cu and Ag-Sn-Cu alloys was investigated (11). The study showed that Ag-Sn and Ag-Sn-Cu alloys were susceptible to mercury embrittlement. The present study was undertaken to determine the extent to which embrittlement of alloy particles in amalgam affects the strength of the material. This study revealed that the strength of all amalgams, except amalgams S and T when tested at 2.54 mm/min, was significantly (p = 0.05) lowered by wetting 14"
183
vations of the T amalgam suggest that something may also be happening to the matrices of the other amalgams. The increase in intergranular fracture in T suggests that mercury weakens its grain boundaries. This effect is visible in T because of the tendency of grains in the unplated T amalgam to cleave. The fracture mode of the matrix of the other unplated amalgams is entirely intergranular; consequently, embrittlement by mercury cannot increase the percentage of intergranular fracture. The matrix of the D, O and S amalgams may be embrittled even though no change in morphology is detected. There were no significant differences between the strengths of each amalgam after the various mercury wetting procedures. As long as liquid mercury is present, mercury embrittlement occurs in dental amalgams. However, mercury did not reduce the strengths of amalgams O and D tested 7 days after Hg plating. This is understandable, since within a short time after plating, liquid mercury has reacted with unconsumed alloy particles, leaving no free mercury to embrittle the amalgam. As mentioned above, an overall trend in the strength of Hg-wetted amalgams was observed: S > T > D > O. The present results are consistent with the results from the previous study in our laboratory (11). In the previous study, Ag-Cu eutectic, 49% Ag-51% Cu and 40% Ag-30% Sn-30% Cu alloys were not significantly embrittled when tested 5 min after mercury plating. The chemical composition of the last of these alloys was the same as that of the S alloy particles in the present study. The high resistance of the unconsumed S alloy particles to mercury embrittlement explains the relatively good resistance to mercury embrittlement of the S amalgam. Although Ag-Cu alloys exhibited good resistance to mercury embrittlement (11), the strengths of the D amalgam were reduced after wetting with mercury. This reduction in strength is probably due, in part, to the presence of Ag-Sn alloy particles in the D amalgam. The earlier study revealed that the Ag-Sn alloy is severely embrittled by liquid mercury (11). Note that the O amalgam, which also contains Ag-Sn alloy particles, was also severely embrittled by mercury. In the 15-s Hg-plated D amalgams, a majority of the Ag-Cu eutectic particles was found to have been fractured (Fig. 12). Fracture of these particles
184
O k a b e et al.
contributes to the embrittlement of the D amalgam. The fact that mercuryplated Ag-Cu eutectic tensile specimens resisted embrittlement, while Ag-Cu eutectic particles did not, is an interesting anomaly, and may be caused by a difference in the interval between mercury plating and testing. If the interval is short, liquid mercury is likely to be present during testing. As the interval increases, the amount of liquid mercury present decreases at a rate that depends upon: 1) how fast mercury is consumed during the formation of y1 Ag-Hg; or 2) how fast m e r cury diffuses into alloy particles. Diffusion into alloy particles will be especially fast in the eutectic alloy, as these alloys contain a high density of interphase boundaries that act as paths for fast diffusion. Consequently, mercury embrittlement of eutectic alloys may go undetected if testing is not begun immediately after plating, because liquid mercury may have disappeared. The present specimens were fractured within 40 s (at a strain rate of 2.54 mm/ min) of the completion of plating. In the earlier study, tensile tests were not begun until 5 rain after plating. Consequently, the testing interval of the present study is more likely to detect embrittlement. Conclusions
1. The strength of all amalgams, except S, a single composition high copper amalgam, when tested at 2.54 ram/rain, was significantly (Tukey HSD, p = 0.05) lowered by contact with mercury. Strength reductions as high as 60% for O, a low copper amalgam, 44% for D, an admixed high copper amalgam, and 31% for T, a single composition high copper amalgam, have been found.
2. After wetting with mercury, S amalgam was always significantly stronger than O amalgam and D amalgam. 3. An overall trend on the strength of wetted amalgams was observed: S > T >D>O. 4. Controls for the amalgams T, S, and O, respectively, were significantly weaker at 2.54 mm/min than at 0.25 mm/min (T-test, p = 0.05). After each of the mercury-wetting procedures, there was no significant difference in strength of any amalgam at the two loading speeds. 5. There did not appear to be any difference in fracture mode between the control and Hg-plated specimens of O and S amalgams. 6. In D amalgams, intraparticle frac-
ture of silver-copper eutectic particles was more frequently observed in the Hg-plated specimens. Mercury appears to embrittle the eutectic particle, reducing the amalgam strength. 7. In T amalgams, a considerable number of Y1 grains appeared to be cleaved in the control specimens while fracture was mainly intergranular through the matrix for the Hg-plated specimens. Acknowledgements - The present authors
would like to thank those alloy manufacturers who provided the alloys for this investigation. The present study was supported by Grant 5 ROI DE 06539 from the National Institute of Dental Research, National Institute of Health, Department of Health and Human Services, Bethesda, MD 20205.
References
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