Abstract
Objectives
To evaluate the mechanical behavior of a prototype porous ceramic interpenetrating polymer–ceramic material containing 15–20% polymer.
Methods
After sample preparation, elastic modulus, hardness, stress–strain relationship and indentation creep response were measured by a nanoindentation system. Fracture toughness was measured by the single-edge-notched beam (SENB) method. SEM was employed to observe the fractured surface and analyze the fracture mechanisms.
Results
The polymer infiltrated ceramic material has elastic modulus, hardness, and fracture toughness values of 30.14 GPa, 2.59 GPa, and 1.72 MPa m 1/2 , respectively. The material illustrates a significant indentation size effect for elastic modulus and hardness, and has similar indentation creep behavior to human enamel. Manufacturing procedures such as the density of pre-infiltrated porous ceramic and processing pressure influence the final properties of the material.
Significance
This polymer infiltrated ceramic material is anticipated to become a new member of the dental CAD/CAM family.
1
Introduction
Recent trends in esthetic dentistry include the reduction or elimination of metals and increasing use of CAD/CAM technology . These trends have led to increased usage of ceramic and composite materials and have contributed to the growth of the CAD/CAM market. However, although tougher ceramics such as zirconia and alumina have been used in dentistry, all-ceramic systems, especially those with veener porcelains, still experience a relatively high failure rate due to the brittleness of the veneering layer. As a result, all-ceramic restorations have an inferior life span equivalent to that of metal–ceramic restorations . Another potential hazard of current ceramic restorations is excessive wear of the opposing teeth due to the high hardness of the veneering materials . Consequently, the development of novel dental esthetic materials has switched to more polymer based resin-composites. However, traditional dental composites are compromised by their curing shrinkage, low mechanical properties and poor wear resistance. Thus there is an urgent need to develop a new composite system to support the emerging indirect restorative material market. There have been some developments in this area with the release of Paradigm™ MZ100 Block (3 M™) for CEREC ® as a CAD/CAM composite for inlay, onlay, veneer and full crown.
Recently, a new interpenetrating network material in which a porous ceramic is infiltrated by a polymer has been developed by the VITA company (VITA Zahnfabrik, Bad Säckingen, Germany) based on the glass infiltrated ceramic (In-ceram) systems they pioneeered. The aim of this study was to evaluate the mechanical properties of this new material and identify parameters influencing the mechanical properties of this family of materials.
2
Materials and methods
Materials evaluated in this study were prototypes provided by the VITA company (VITA Zahnfabrik, Bad Säckingen, Germany). Porous pre-infiltrated ceramic blocks with two different apparent densities were infiltrated by a polymer and heat polymerized under different pressures to fabricate the polymer infiltrated ceramics. Details of the testing materials are described in Table 1 .
| Material ID | Description |
|---|---|
| P | Pure polymerized resin processed at higher pressure |
| M1 | Pre-infiltrated ceramic with density of 1.66 g/cm 3 |
| M2 | Pre-infiltrated ceramic with density of 1.80 g/cm 3 |
| C1 | Polymer infiltrated M1 under lower pressure |
| C2 | Polymer infiltrated M1 under higher pressure |
| C3 | Polymer infiltrated M2 under higher pressure |
One centimeter thick slices were sectioned from different blocks of the materials listed in Table 1 for nanoindentation tests and 20 mm × 3.5 mm × 4.5 mm beams were cut from different blocks for fracture toughness testing. Cutting of the specimens was done with a high speed cutting machine (Accutom-50, Struers, Copenhagen, Denmark) with a diamond cut-off wheel (331CA, Struers, Copenhagen, Denmark) at a spindle speed of 3000 rpm and feeding speed of 0.05 mm/s under water irrigation. The slices were polished (TegraPol-21, Struers, Copenhagen, Denmark) to 1 μm with diamond polishing paste and the beams were polished to 2400# SiC paper (Struers, Copenhagen, Denmark) to the final dimension of 20 mm × 3 mm × 4 mm.
The beams for fracture toughness test were pre-notched with the high speed cutting machine (Accutom-50, Struers, Copenhagen, Denmark) to a depth of 1.9 mm through the width of 4 mm. Then, a purpose built compressive-air-driven cyclic movement machine with a fixed razor blade and 3 μm diamond paste was employed to sharpen the notch and extend it an additional 100–200 μm. The final length of the notch was measured with an optical microscope (YS2-H, Nikon, Japan) equipped with an objective micrometer (Olympus, Japan) to an accuracy of 1 μm. To minimize the notch blunting effect, the tip of every notch was double checked to ensure the radius of the notch tip was less than 10 μm.
2.1
Nanoindentation tests
The indentation experiments were performed using a nano-based indentation system (Ultra Micro-Indentation System, UMIS-2000, CSIRO, Australia). The polished specimens were mounted on metal bases which contained a strong magnet to ensure adequate contact was obtained with the test base in the UMIS. All the tests were performed under standard laboratory conditions.
The elastic modulus and hardness of the specimens were measured with a calibrated Berkovich indenter. The force applied during the experiments was fixed at 30 and 300 mN. Sixteen indents were made in a 4 × 4 array with the interval of 50 μm between indents at each load value. The maximum load was held for 30 s to minimize the effect of creep on the unloading curve. The Oliver–Pharr analysis method was used by the UMIS software to calculate the elastic modulus ( E ) and hardness ( H ). The first four indents were discarded from the calculation, because of thermal drift influences, and twelve data points were collected for each test.
In the book Hardness of metals , Tabor showed that an equivalence existed between conventional stress–strain curves and that generated by plotting hardness versus contact radius divided by indenter radius ( a / R ) using a spherical tipped indenter. The nanoindentation stress–strain relationship was measured based on the previous described method . Three calibrated spherical tips with the nominal radius of 5, 10 and 20 μm were employed. Contact stress, or hardness ( H ), is calculated by its definition,
H = P A
where P is the maximum load and A is the contact area. The contact strain ( a / R ) is determined from the sin value of the contact angle , which is derived from the indenter tip calibration function a = f ( h p ) and its derivative tan = a ′ = f ′( h p ). As a result, a / R can be expressed as ,
a R = sin = 1 ( tan − 2 + 1 )
With Eqs. (1) and (2) , the nanoindentation stress–strain relationship can be plotted from the results with the different spherical indenters.
For the indentation creep test, the Berkovich indenter was used with a maximum force of 250 mN. One-step loading and unloading method with a maximum-force (250 mN) hold time of 900 s and upon unloading at a minimum-force (5 mN) also with hold time of 900 s was followed . All tests commenced after at least 2 h thermal soak period in the instrument, and thermal drift was deemed to be almost negligible (<0.05 nm/s) before each experiment.
2.2
Fracture toughness test
Standard single-edge-notched beam (SENB) method was used to measure the fracture toughness ( K 1c ) . The prepared samples were loaded in a three-point bending format with the support span of 16 mm at a cross-head speed of 0.5 mm/min till failure in a universal test machine (Model 3369, Instron, Norwood, MA, USA). The following equation was used to calculate the K 1c ,
K 1 c = F L B W 3 / 2 f a W
in which F is the fracture load, L is the lower supporting span, B is the breadth of the beam, W is the width of the beam, and a is the length of the notch. Geometrical factor f ( a / W ) can be calculated by Eq. (4) ,
f a W = 2.9 a W 1 / 2 − 4.6 a W 3 / 2 + 21.8 a W 5 / 2 − 37.6 a W 7 / 2 + 38.7 a W 9 / 2
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