101
Fracture toughness determination ceramic and resin-based dental composites
of
Ketil Kvam NIOM, Scandinavian
Institute
of Dental Materials,
Kirkeveien
718, PO Box 70, N-1344 Haslum
A new method has been developed for K,, determinations of brittle materials with precracks introduced by indentations. A reference glass, five ceramic materials, and one resin-based composite were tested. Knoop hardness indentations were made with a load of 49 N in a line from edge to edge vertical to the long axis on one surface of four-point flexure bars, to make a continuous crack under the indentations. Five specimens of each material were fractured in a four-point bend test with the line of indentations placed in the zone of constant and maximum tensile stress. Separate unfractured specimens were ground and polished to expose and measure the preformed continuous crack. The mean of six crack-depth measurements was used together with the fracture load and the dimensions of the bend specimens to calculate the fracture toughness, K,, of each material. The determined K,, value (x + SD) for the reference glass was 0.81 f .24 MPa ml’* and corresponds to previous studies. The resin-based composite material, Silux Plus, had a value of 1.04 + 0.14 MPa m1’2. The K,, values (MPa m”2) were 0.94 + 0.31 for Dicer, 1.41 f 0.18 for Cerestore, 1.50 + 0.29 for NBK-1000, 1.60 ? 0.17 for Vitadur-N and 2.14 +_0.14 for Hi-Ceram. Hi-Ceram had significantly higher K,, values than the other materials. The new method seemed to be of value in determining the fracture toughness of non-metallic dental materials. Keywords: Received
Dental
materials,
22 December
ceramics,
1990; revised
Common methods for fracture toughness measurements are considered inadequate for dental ceramics. The reason is mainly the problems involved in establishing a defined sharp crack. Several methods with various ways to make precracks have been developed to determine the fracture toughness. The equations for calculating Km with these methods include constants that are established empirically in studies of pure ceramics, i.e. non-composite ceramics” ‘. It is likely that parameters will be influenced by the composite structures in dental non-metallic materials. The crack morphology for the composite ceramics may differ considerably from that for the pure materials, and the constants mentioned will probably not be valid for the dental ceramics. Other methods for fracture toughness determination such as the double torsion test and chevron notch bending test are inconvenient because specimens are difficult to prepare. The aim of this study was to evaluate a new method for measuring K,, on ceramic and resin-based dental composites comprising a simple method for production of definite sharp and continuous cracks through one side of the specimen. The method was used to evaluate some dental ceramics and a resin-based dental composite. Correspondence
to Dr K. Kvam.
0 1992 Butterworth-Heinemann 0142-9612/92/020101-04
Ltd
composites,
25 February
fracture
1991; accepted
toughness 26 March
1991
MATERIALS AND METHODS Materials The materials used for the calculation of fracture toughness in this study were a reference glass, one resinbased composite, and five dental ceramic core materials (Table
2).
Table 1 Tested materials. Trade name Soda-lime-silica Silux Plus
Dicer Cerestore NBK-1000 Vitadur-N Hi-Ceram
Manufacturer glass
(reference material) 3M Dental Products Laboratory, St Paul Minnesota, USA DeTrey Dentsply Wiesbaden, Germany Johnson & Johnson Lakewood, Colorado, USA DeTrey Dentsply Dreieich, Germany Vita Zahnfabrik Bad Sackingen, Germany Vita Zahnfabrik
Biomaterials
1992, Vol.
13 No.
2
102
Fracture toughness of dental materials: K. Kvam
A soda-lime-silica glass [73% Si, 16% Na, 6% Ca, 3% Mg-oxides) was tested as a reference material. This material is reported18 ’ to have a fracture toughness of 0.75-0.79 MPa m *” . Silux Plus is a light-cured, microfilled resin-based filling material. NBK-1000 and Vitadur-N are alumina-reinforced porcelains for traditional jacket crowns3. Hi-Ceram is a newly developed high-alumina core materia14. Cerestore is a pressureformed, shrink-free core ceramic consisting of a special blend of alumina and magnesia5. Dicor is a castable glass with added nucleating agents for subsequent crystallization in a heat treatment process”. Specimen
production
Specimens of NBK-1000, Vitadur-N and Hi-Ceram were NIOM in a brass condensed at mould (25 mm X 6 mm X 3 mm)7 before they were fired in accordance with the manufacturer’s recommendations. Wax patterns for the Cerestore and Dicer specimens were made in the same mould. The specimens of these materials were pressure-formed or cast and heat-treated at two commercially licensed dental laboratories in Oslo. Specimens of the resin-based composite were also made in the mould mentioned. Each specimen was ground and polished on the four longest surfaces with diamond particles stepwise from a particle size of 4%lpm. Parallelism between corresponding surfaces and the 90° angles between adjacent surfaces was controlled in a Profile projector [Nikon 6C2, Nippon Kogaku K.K., Tokyo).
Bend test The five first specimens of each material were bent to fracture in a four-point bending test with a crosshead speed of 0.5 mm/min in a universal testing machine (Instron Ltd, Bucks, UK). Calculation
of fracture
toughness
This was done using an equation based on Griffith energy criterion for fracture that states that crack extension can occur if the energy to form an incremental crack can be delivered by the system, which includes the energy stored both in the specimen and in the machine*. The equation is mathematically deduced and assumed to have a good reliability for all crack depths, It is formulated as follows: P KIC
=
1.5 $l”z[l
-
A)-3’2
I3 + (wp2
[1.9887 - 1.326A
- (3.49 - 0.68A
1 +
1.35A')A
(l-A)(l+ A)-'] where P is the fracture load in the four-point bending test; B is the width of specimen in the crack plane; W is the length of specimen in the crack propagation direction; A is a/W where a is the crack depth: L is the distance between the upper loading points, or half of the distance between the bearers. See Figure 1.
Precracking
Statistics
On five specimens of each material a Knoop diamond was used to make overlapping indentations in a straight line across one of the narrow sides, perpendicular to the broadest surfaces (Figure 3). A load of 49 N (5 kg) was used to make the hardness indentations on all materials. On one separate specimen of each material, two preformed continuous cracks were made with the same method. These specimens were ground and polished in three steps into the material perpendicular to the indentation lines to expose and measure the depth of the preformed continuous cracks. The cracks were made visible by immersing the specimens in a solution of rhodamine-B in ethanol and measured in a light microscope with the help of light field, dark field, polarized light and interference contrast. Six measurements of each material were thus used as the basis for the statistical calculation of a mean crack depth.
Differences between means were tested for significance by SPSS/PC+, ANOVA-statistics, LSD procedure at P < 0.05 levelg.
RESULTS The results of the fracture toughness evaluation are given in Table 2. The crack depths for the calculations are also given in ‘Able 2. These measurements showed the deepest cracks in the reference glass and Dicer, the highest coefficient of variation for Dicor and the lowest coefficient of variation for Silux-Plus, the latter also showing the smallest crack depth, The fracture toughness of Dicer was not significantly different from that of the reference glass or the resinbased composite. Hi-Ceram had a significantly higher fracture toughness than all other materials in this study.
Table 2
L /L-4
Figure 1
Specimen dimensions and distances between bearers (2Lf and the loading points (L) in the bend test.
Biomaterials
1992, Vol. 13 NO. 2
Measured crack depth and calculated K,, values.
Material
Crack depth (,um)
K,c values (MPa ml’*)
Reference glass Silux Plus Dicor Cerestore NBK-10~ Vitadur-N Hi-Ceram
592 + 171 104, 5 427 zfz158 233 i: 73 154t 35 200 f 62 228 f 67
0.81 1.64 0.94 1.41 1SO 1.60 2.14
+ + f + f t +
0.24 0.14 0.31 0.18 0.29 0.17 0.14
Fracture
toughness
of dental
materials:
K. Kvam
DISCUSSION ‘,.,.
The method used for the fracture toughness evaluation of the various materials in this investigation seems to function well. The variation in KI, values between specimens of each material must be considered moderate when seen in relation to the extremely variable crack depths that may occur in inherently brittle materials with complex structures. The problem of making a continuous sharp crack of limited depth across a ceramic specimen was apparently solved by making a series of hardness indentations with some overlapping with a Knoop diamond indenter (Figure I). Cracks around the indentations may develop either as median cracks, as Palmquist cracks, or in an intermediate situation. A series of overlapping indentations is expected to give a crack morphology as shown in Figure 2. The measured crack depths were in no cases near zero under the indentation line. This supports the assumption that both median and Palmquist cracks have transformed to the morphology as mentioned above, i.e. were continuous through the material under the line. This continuity was also shown fractographically for the reference glass and for Dicer (Figure 3).
.A..
:I:! ‘.,., :::. .7: ::,: ,._:: .:): ..::. .,,> ::.:: . .. .,. :i.. .;, :: (VI:
(CT. .\ ,:. :, . . ..,
,,o, ,::: .,... ..,. .., :
~
:)>
.
a
Figure4 Comparison between, II, fracture toughness (this study) and, El the flexural strength3 of the ceramics.
_c
b
C
Paimauist
Median
cracks
cracks
Figure 2 Line of overlapping Knoop indentations, a, on the surface and, b, c, expected crack morphology under the indentation Ii&?.
Figure 3
The crosshead speed of 0.5 mm/min was used for the bend test. This speed was chosen as a compromise to reduce the influence of slow crack propagation and to avoid a too-high fracture rate. The equation for calculation is based on Griffith’s theory, and the geometrical relations are mathematically deduced. No empirically established constants are included. It is, therefore, reasonable to assume that this method is reliable as a general method for fracture toughness determination of brittle materials. The fracture toughness value found for the reference glass was in the
Fractographs of the reference glass and Dicer showing the continuous cracks under the indentation lines. Wiomaterials
1992, Vol. 13 No. 2
104
Figure5
Fracture
Microstructure
of, a, Vitadur-N and, b, NBK-1000.
same range as that found in previous studies with traditional indentation methods and supports the reliability. A comparison of the present fracture toughness values of the ceramics with the flexural strength3 shows that Dicer was most influenced by the introduction of surface cracks [Figure 4). Its flexural strength is reported to be the highest of the tested ceramic2, but the K,, value was not significantly different from the reference glass. This might indicate that the crystalline particles produced by the heat treatment process, do not function as crack stoppers. Cerestore had a K,, value in the range of the two traditional jacket crown materials, NBK-1000 and Vitadur-N and NBK-1000 the K,, values in this study of porosity3 and crystalline particles may have subdued the crack displacement, and thereby limited the variation in KI, values. Vitadur-N showed slightly better resistance to crack opening displacement and a lower spreading than NBK-1000. This may be due to the more uniform size and shape of the crystalline particles (Figure 5). For Vitadur-N and NBK-1000 the K, values i this study correlate with the values found by Morena et a1.l using a traditional indentation method. Taira et dl’ tested Cerestore in the same way as Morena et al.‘, but found it impossible to make radial cracks from which a K1, value could be determined. Loading the surface with a Vickers diamond may produce cracks in various directions around and underneath the indentation. The present method will probably produce a continuous crack beneath the indentation line, which is dominating as a fracture initiater in the bending test. The results of this test are therefore not influenced by other cracks that occur. One more factor that may lead to differences in KI, values in different studies using the traditional method is the hardness number. This value may vary with the load because of the variable resistance to the intrusion of the indenter due to the cracks that occur. It seems therefore favourable that the hardness number is excluded when to the present calculating the K1, value according method.
Biomaterials
1992, Vol. 13 No. 2
toughness
of dental
materials:
K. Kvam
The high-alumina core material, Hi-Ceram, had a significantly higher KI, value than the other ceramics. The specimens of this material showed higher values than all specimens of the other materials. This indicates clearly that the high content of alumina particles resisted crack extension and raised the fracture toughness. The resin-based light-cured composite had a low standard deviation for fracture toughness values and particularly for crack depths (Table 2). The indentation method requires a matrix with a sufficient brittleness for cracks to occur around the diamond indenter. This means that the method will function for resin-based composites with a higher glass transition temperature than the testing temperature. The K,, value found for Silux Plus in this study (1.04 MPa ml”) is lower than recently reported in a test with chevron notch specimens (1.18 MPa ml”)ll. This may be due to the sharper cracks that occur with the present method, but the latter value is within the standard deviation obtained for the same material in this study.
REFERENCES 1
2
3
4 5
6
7 6
9 10
11
Morena, R., Lockwood, P.E. and Fairhurst, C.W., Fracture toughness of commercial dental porcelains, Dent. Mater. 1986, 2, 58-62 Cook, R.F. and Lawn, B.R., A modified indentation toughness technique, J. Am. Ceram. Sot. 1983,66, C-ZOO Oilo, G., Flexural strength and internal defects of some dental porcelains, Acta Odontol. Stand. 1988, 46, 313-322 Claus, H., Das Hi Ceram-Verfahren. Metallfrue Kronen auf einem keramikgeriist, Dent. Labor. 1987,35,479-482 Starling, L.B., Transfer molded ‘all ceramic crowns’, the Cerestore system, in Dental Ceramics. Proceedings of a Conference on Recent Developments (Eds W.J. O’Brien and R.G. Craig], The American Ceramic Society, Columbus, 1985, pp 41-56 Grossman, D.G., Processing a dental ceramic by casting methods, in Dental Ceramics. Proceedings of a Conference on Recent Developments (Eds W.J. O’Brien and R.G. Craig), The American Ceramic Society, Columbus, 1985, pp 19-40 IS0 6872-1984 Dental ceramic, International Organization for Standardization, Geneva, 1984 Krausse, R.F. Jr. and Fuller, E.R. Jr., Fracture toughness of polymer concrete materials using various chevronnotched configurations, in Chevron-Notched Specimens: Testing and Stress Analysis, ASTM STP 855 (Eds J.H. Underwood, SW. Freiman and F-1. Baratta), American Society for Testing and Materials, Philadelphia, USA, 1984, pp 309-323 Norusis, M.J., SPSS/PC+ for the IBM PC/XT/AT, SPSS Inc., Chicago, USA, 1986 Taira, M., Nomura, Y., Wakasa, K., Yamaki, M. and Matsui, A., Studies of fracture toughness of dental ceramics. J, Oral. Rehab. 1989, 17, 551-563 Mair, L.H. and Vowles, R., The effect of thermal cycling on the fracture toughness of seven composite restorative materials, Dent. Mater. 1989, 5, 23-26
Fracture toughness determination ceramic and resin-based dental composites
of
Ketil Kvam NIOM, Scandinavian
Institute
of Dental Materials,
Kirkeveien
718, PO Box 70, N-1344 Haslum
A new method has been developed for K,, determinations of brittle materials with precracks introduced by indentations. A reference glass, five ceramic materials, and one resin-based composite were tested. Knoop hardness indentations were made with a load of 49 N in a line from edge to edge vertical to the long axis on one surface of four-point flexure bars, to make a continuous crack under the indentations. Five specimens of each material were fractured in a four-point bend test with the line of indentations placed in the zone of constant and maximum tensile stress. Separate unfractured specimens were ground and polished to expose and measure the preformed continuous crack. The mean of six crack-depth measurements was used together with the fracture load and the dimensions of the bend specimens to calculate the fracture toughness, K,, of each material. The determined K,, value (x + SD) for the reference glass was 0.81 f .24 MPa ml’* and corresponds to previous studies. The resin-based composite material, Silux Plus, had a value of 1.04 + 0.14 MPa m1’2. The K,, values (MPa m”2) were 0.94 + 0.31 for Dicer, 1.41 f 0.18 for Cerestore, 1.50 + 0.29 for NBK-1000, 1.60 ? 0.17 for Vitadur-N and 2.14 +_0.14 for Hi-Ceram. Hi-Ceram had significantly higher K,, values than the other materials. The new method seemed to be of value in determining the fracture toughness of non-metallic dental materials. Keywords: Received
Dental
materials,
22 December
ceramics,
1990; revised
Common methods for fracture toughness measurements are considered inadequate for dental ceramics. The reason is mainly the problems involved in establishing a defined sharp crack. Several methods with various ways to make precracks have been developed to determine the fracture toughness. The equations for calculating Km with these methods include constants that are established empirically in studies of pure ceramics, i.e. non-composite ceramics” ‘. It is likely that parameters will be influenced by the composite structures in dental non-metallic materials. The crack morphology for the composite ceramics may differ considerably from that for the pure materials, and the constants mentioned will probably not be valid for the dental ceramics. Other methods for fracture toughness determination such as the double torsion test and chevron notch bending test are inconvenient because specimens are difficult to prepare. The aim of this study was to evaluate a new method for measuring K,, on ceramic and resin-based dental composites comprising a simple method for production of definite sharp and continuous cracks through one side of the specimen. The method was used to evaluate some dental ceramics and a resin-based dental composite. Correspondence
to Dr K. Kvam.
0 1992 Butterworth-Heinemann 0142-9612/92/020101-04
Ltd
composites,
25 February
fracture
1991; accepted
toughness 26 March
1991
MATERIALS AND METHODS Materials The materials used for the calculation of fracture toughness in this study were a reference glass, one resinbased composite, and five dental ceramic core materials (Table
2).
Table 1 Tested materials. Trade name Soda-lime-silica Silux Plus
Dicer Cerestore NBK-1000 Vitadur-N Hi-Ceram
Manufacturer glass
(reference material) 3M Dental Products Laboratory, St Paul Minnesota, USA DeTrey Dentsply Wiesbaden, Germany Johnson & Johnson Lakewood, Colorado, USA DeTrey Dentsply Dreieich, Germany Vita Zahnfabrik Bad Sackingen, Germany Vita Zahnfabrik
Biomaterials
1992, Vol.
13 No.
2
102
Fracture toughness of dental materials: K. Kvam
A soda-lime-silica glass [73% Si, 16% Na, 6% Ca, 3% Mg-oxides) was tested as a reference material. This material is reported18 ’ to have a fracture toughness of 0.75-0.79 MPa m *” . Silux Plus is a light-cured, microfilled resin-based filling material. NBK-1000 and Vitadur-N are alumina-reinforced porcelains for traditional jacket crowns3. Hi-Ceram is a newly developed high-alumina core materia14. Cerestore is a pressureformed, shrink-free core ceramic consisting of a special blend of alumina and magnesia5. Dicor is a castable glass with added nucleating agents for subsequent crystallization in a heat treatment process”. Specimen
production
Specimens of NBK-1000, Vitadur-N and Hi-Ceram were NIOM in a brass condensed at mould (25 mm X 6 mm X 3 mm)7 before they were fired in accordance with the manufacturer’s recommendations. Wax patterns for the Cerestore and Dicer specimens were made in the same mould. The specimens of these materials were pressure-formed or cast and heat-treated at two commercially licensed dental laboratories in Oslo. Specimens of the resin-based composite were also made in the mould mentioned. Each specimen was ground and polished on the four longest surfaces with diamond particles stepwise from a particle size of 4%lpm. Parallelism between corresponding surfaces and the 90° angles between adjacent surfaces was controlled in a Profile projector [Nikon 6C2, Nippon Kogaku K.K., Tokyo).
Bend test The five first specimens of each material were bent to fracture in a four-point bending test with a crosshead speed of 0.5 mm/min in a universal testing machine (Instron Ltd, Bucks, UK). Calculation
of fracture
toughness
This was done using an equation based on Griffith energy criterion for fracture that states that crack extension can occur if the energy to form an incremental crack can be delivered by the system, which includes the energy stored both in the specimen and in the machine*. The equation is mathematically deduced and assumed to have a good reliability for all crack depths, It is formulated as follows: P KIC
=
1.5 $l”z[l
-
A)-3’2
I3 + (wp2
[1.9887 - 1.326A
- (3.49 - 0.68A
1 +
1.35A')A
(l-A)(l+ A)-'] where P is the fracture load in the four-point bending test; B is the width of specimen in the crack plane; W is the length of specimen in the crack propagation direction; A is a/W where a is the crack depth: L is the distance between the upper loading points, or half of the distance between the bearers. See Figure 1.
Precracking
Statistics
On five specimens of each material a Knoop diamond was used to make overlapping indentations in a straight line across one of the narrow sides, perpendicular to the broadest surfaces (Figure 3). A load of 49 N (5 kg) was used to make the hardness indentations on all materials. On one separate specimen of each material, two preformed continuous cracks were made with the same method. These specimens were ground and polished in three steps into the material perpendicular to the indentation lines to expose and measure the depth of the preformed continuous cracks. The cracks were made visible by immersing the specimens in a solution of rhodamine-B in ethanol and measured in a light microscope with the help of light field, dark field, polarized light and interference contrast. Six measurements of each material were thus used as the basis for the statistical calculation of a mean crack depth.
Differences between means were tested for significance by SPSS/PC+, ANOVA-statistics, LSD procedure at P < 0.05 levelg.
RESULTS The results of the fracture toughness evaluation are given in Table 2. The crack depths for the calculations are also given in ‘Able 2. These measurements showed the deepest cracks in the reference glass and Dicer, the highest coefficient of variation for Dicor and the lowest coefficient of variation for Silux-Plus, the latter also showing the smallest crack depth, The fracture toughness of Dicer was not significantly different from that of the reference glass or the resinbased composite. Hi-Ceram had a significantly higher fracture toughness than all other materials in this study.
Table 2
L /L-4
Figure 1
Specimen dimensions and distances between bearers (2Lf and the loading points (L) in the bend test.
Biomaterials
1992, Vol. 13 NO. 2
Measured crack depth and calculated K,, values.
Material
Crack depth (,um)
K,c values (MPa ml’*)
Reference glass Silux Plus Dicor Cerestore NBK-10~ Vitadur-N Hi-Ceram
592 + 171 104, 5 427 zfz158 233 i: 73 154t 35 200 f 62 228 f 67
0.81 1.64 0.94 1.41 1SO 1.60 2.14
+ + f + f t +
0.24 0.14 0.31 0.18 0.29 0.17 0.14
Fracture
toughness
of dental
materials:
K. Kvam
DISCUSSION ‘,.,.
The method used for the fracture toughness evaluation of the various materials in this investigation seems to function well. The variation in KI, values between specimens of each material must be considered moderate when seen in relation to the extremely variable crack depths that may occur in inherently brittle materials with complex structures. The problem of making a continuous sharp crack of limited depth across a ceramic specimen was apparently solved by making a series of hardness indentations with some overlapping with a Knoop diamond indenter (Figure I). Cracks around the indentations may develop either as median cracks, as Palmquist cracks, or in an intermediate situation. A series of overlapping indentations is expected to give a crack morphology as shown in Figure 2. The measured crack depths were in no cases near zero under the indentation line. This supports the assumption that both median and Palmquist cracks have transformed to the morphology as mentioned above, i.e. were continuous through the material under the line. This continuity was also shown fractographically for the reference glass and for Dicer (Figure 3).
.A..
:I:! ‘.,., :::. .7: ::,: ,._:: .:): ..::. .,,> ::.:: . .. .,. :i.. .;, :: (VI:
(CT. .\ ,:. :, . . ..,
,,o, ,::: .,... ..,. .., :
~
:)>
.
a
Figure4 Comparison between, II, fracture toughness (this study) and, El the flexural strength3 of the ceramics.
_c
b
C
Paimauist
Median
cracks
cracks
Figure 2 Line of overlapping Knoop indentations, a, on the surface and, b, c, expected crack morphology under the indentation Ii&?.
Figure 3
The crosshead speed of 0.5 mm/min was used for the bend test. This speed was chosen as a compromise to reduce the influence of slow crack propagation and to avoid a too-high fracture rate. The equation for calculation is based on Griffith’s theory, and the geometrical relations are mathematically deduced. No empirically established constants are included. It is, therefore, reasonable to assume that this method is reliable as a general method for fracture toughness determination of brittle materials. The fracture toughness value found for the reference glass was in the
Fractographs of the reference glass and Dicer showing the continuous cracks under the indentation lines. Wiomaterials
1992, Vol. 13 No. 2
104
Figure5
Fracture
Microstructure
of, a, Vitadur-N and, b, NBK-1000.
same range as that found in previous studies with traditional indentation methods and supports the reliability. A comparison of the present fracture toughness values of the ceramics with the flexural strength3 shows that Dicer was most influenced by the introduction of surface cracks [Figure 4). Its flexural strength is reported to be the highest of the tested ceramic2, but the K,, value was not significantly different from the reference glass. This might indicate that the crystalline particles produced by the heat treatment process, do not function as crack stoppers. Cerestore had a K,, value in the range of the two traditional jacket crown materials, NBK-1000 and Vitadur-N and NBK-1000 the K,, values in this study of porosity3 and crystalline particles may have subdued the crack displacement, and thereby limited the variation in KI, values. Vitadur-N showed slightly better resistance to crack opening displacement and a lower spreading than NBK-1000. This may be due to the more uniform size and shape of the crystalline particles (Figure 5). For Vitadur-N and NBK-1000 the K, values i this study correlate with the values found by Morena et a1.l using a traditional indentation method. Taira et dl’ tested Cerestore in the same way as Morena et al.‘, but found it impossible to make radial cracks from which a K1, value could be determined. Loading the surface with a Vickers diamond may produce cracks in various directions around and underneath the indentation. The present method will probably produce a continuous crack beneath the indentation line, which is dominating as a fracture initiater in the bending test. The results of this test are therefore not influenced by other cracks that occur. One more factor that may lead to differences in KI, values in different studies using the traditional method is the hardness number. This value may vary with the load because of the variable resistance to the intrusion of the indenter due to the cracks that occur. It seems therefore favourable that the hardness number is excluded when to the present calculating the K1, value according method.
Biomaterials
1992, Vol. 13 No. 2
toughness
of dental
materials:
K. Kvam
The high-alumina core material, Hi-Ceram, had a significantly higher KI, value than the other ceramics. The specimens of this material showed higher values than all specimens of the other materials. This indicates clearly that the high content of alumina particles resisted crack extension and raised the fracture toughness. The resin-based light-cured composite had a low standard deviation for fracture toughness values and particularly for crack depths (Table 2). The indentation method requires a matrix with a sufficient brittleness for cracks to occur around the diamond indenter. This means that the method will function for resin-based composites with a higher glass transition temperature than the testing temperature. The K,, value found for Silux Plus in this study (1.04 MPa ml”) is lower than recently reported in a test with chevron notch specimens (1.18 MPa ml”)ll. This may be due to the sharper cracks that occur with the present method, but the latter value is within the standard deviation obtained for the same material in this study.
REFERENCES 1
2
3
4 5
6
7 6
9 10
11
Morena, R., Lockwood, P.E. and Fairhurst, C.W., Fracture toughness of commercial dental porcelains, Dent. Mater. 1986, 2, 58-62 Cook, R.F. and Lawn, B.R., A modified indentation toughness technique, J. Am. Ceram. Sot. 1983,66, C-ZOO Oilo, G., Flexural strength and internal defects of some dental porcelains, Acta Odontol. Stand. 1988, 46, 313-322 Claus, H., Das Hi Ceram-Verfahren. Metallfrue Kronen auf einem keramikgeriist, Dent. Labor. 1987,35,479-482 Starling, L.B., Transfer molded ‘all ceramic crowns’, the Cerestore system, in Dental Ceramics. Proceedings of a Conference on Recent Developments (Eds W.J. O’Brien and R.G. Craig], The American Ceramic Society, Columbus, 1985, pp 41-56 Grossman, D.G., Processing a dental ceramic by casting methods, in Dental Ceramics. Proceedings of a Conference on Recent Developments (Eds W.J. O’Brien and R.G. Craig), The American Ceramic Society, Columbus, 1985, pp 19-40 IS0 6872-1984 Dental ceramic, International Organization for Standardization, Geneva, 1984 Krausse, R.F. Jr. and Fuller, E.R. Jr., Fracture toughness of polymer concrete materials using various chevronnotched configurations, in Chevron-Notched Specimens: Testing and Stress Analysis, ASTM STP 855 (Eds J.H. Underwood, SW. Freiman and F-1. Baratta), American Society for Testing and Materials, Philadelphia, USA, 1984, pp 309-323 Norusis, M.J., SPSS/PC+ for the IBM PC/XT/AT, SPSS Inc., Chicago, USA, 1986 Taira, M., Nomura, Y., Wakasa, K., Yamaki, M. and Matsui, A., Studies of fracture toughness of dental ceramics. J, Oral. Rehab. 1989, 17, 551-563 Mair, L.H. and Vowles, R., The effect of thermal cycling on the fracture toughness of seven composite restorative materials, Dent. Mater. 1989, 5, 23-26
