The aim of this study was to evaluate the fracture toughness and surface residual stresses present in various pressable ceramics to zirconia resulting from cooling induced temperature gradients.
Materials and methods
Indentation fracture toughness was used to evaluate the residual stress present in various pressable ceramics (Noritake CZR Press, Vita PM9, Wieland PressXzr and IPS e.max ZirPress) to zirconia when subjected to different cooling regimen. The cooling responses of two ceramics were evaluated by thermocouples embedded in the surface of the porcelains and at the porcelain–zirconia interface.
The effective K c results obtained by indentation tests confirmed the presence of surface residual compressive stress for all-ceramic systems subjected to different cooling procedures. The residual stresses were quantified from the change in the radial crack size and the values compared for each ceramic before pressing, pressed ceramic only and pressed ceramic veneered on zirconia, from fast to slow cooling rates. A significant level of residual stress was found in the materials before pressing. Slow cooling significantly reduced the formation of residual stress for all pressed ceramics. From data produced by the thermocouples it was found that ‘slow cooling’ generated the least temperature difference between inner and outer surfaces of porcelain. A direct relationship was found for the cooling induced temperature difference between the surfaces, and interface thermocouples, and magnitude of the surface residual stresses.
Leucite containing porcelains have higher intrinsic fracture toughness, and for all porcelains fast cooling generated significant residual stress within the veneering porcelain. To reduce development of residual stress, slow cool is recommended on the last heating cycle (e.g. glazing cycle).
Patients’ demands for esthetic restorations have led to an increase in the use of all-ceramic restorations instead of porcelain-fused-to-metal (PFM), full metal crowns and resin composite restorations. This demand has led to application of glass-ceramics for a range of dental restorations. Along with development of glass ceramics, the heat pressing technique for the veneering of all-ceramic core materials is now widely used in dentistry as it offers esthetic outcomes and strength simultaneously . In paper part 1 the strength and adhesion of four pressed ceramic materials for zirconia were investigated. Despite the growing usage of veneering porcelain to zirconia; clinical studies have reported veneering failure; namely chipping as an issue . Moreover, research has shown that differences in failure rate not only exist between the hand veneering and the pressed veneering ceramics, but also between the different pressed ceramic materials . Possible reasons postulated for chipping are: insufficient bond strength, excessive tensile stress due to a coefficient of thermal expansion mismatch with an un-equal thermal contraction between veneer layer and coping, and excessive load due to premature contacts, tensile thermal tempering residual stress introduced during cooling .
Ceramic materials used for the fabrication of all-ceramic restorations are sintered and glazed several hundred degrees above the glass transition temperature ( T g ). At the end of the firing process the dental technician removes the ceramic restoration from the furnace and it is cooled. Once the porcelain is removed from the furnace, its outer surface starts contracting and becomes rigid as it loses heat rapidly, while the interior part is still hot and somewhat viscous. The tensile stresses which would normally develop in an elastic body during such rapid cooling are relaxed while the interior of the porcelain is above the T g , or more specifically above the softening temperature. However, temperature gradient results in the formation of surface compressive stresses and compensating internal tensile stresses, so called tempering stresses in the body. An indication of the magnitude of such stresses has been addressed by Asaoka et al. and Swain for the case of a bilayer material system.
The tensile stresses associated with tempering may cause immediate cracking of the porcelain upon deformation of the restoration, and can increase the probability of fracture during functional loading of the restoration, manifested clinically as chipping of the veneering porcelain . One problem associated with ceramic restoration failure is therefore the development and magnitude of residual stress within the structure. The residual stress developed may also influence the measured nominal fracture toughness of ceramic materials. Marshall and Lawn developed a simple approach using indentation fracture about a Vickers indenter to quantify the magnitude of these residual stresses . Fracture toughness or critical stress intensity ( K 1c ) indicates the ability of a material to resist rapid crack propagation and is an indicator of clinical reliability and serviceability of a ceramic restoration.
The present study thus has three primary objectives, namely; quantifying the fracture toughness of the four pressable veneering ceramic materials; determining the residual stress present in various pressable ceramics bonded to zirconia using the indentation fracture toughness technique and measuring the cooling curves when the ceramics are subjected to different cooling procedures.
Materials and methods
Materials used are presented in Table 1 in paper part 1.
|Materials used||Manufacturer||CTE (ppm/K)||T g (°C)|
|Vita PM9||Vita Zahnfabrik||9.2||640|
|IPS e.max Zirpress||Ivoclar Vivadent||9.75||530|
|Wieland Xzr||Wieland Dental + Technik||9.3||620|
|Noritake CZR||Noritake Kizai Co.||10.1||615|
|Vita In-Ceram YZ||Vita Zahnfabrik||10.5|
Vita In-Ceram YZ zirconia and the pressed ceramics were prepared to the dimension required for each test following the manufacturers’ instructions as listed in paper 1.
Three groups of specimens were prepared
(1-1) 2 mm zirconia and 2 mm pressed porcelain
(1-2) 2 mm zirconia and 4 mm pressed porcelain
One layer group
(2-1) 2 mm of pressed porcelain ceramic only
(2-2) 2 mm slice from each porcelain ingot before pressing
The bilayered specimens were divided into 3 groups. Before glazing, the pressed ceramic surface of the specimens were all ground and polished to the desired dimension for each test, using 120–4000 grades of abrasive paper (Struers PSA backed Silicon carbide paper) on a metallographic lapping machine (Knuth Rother, Struers, Denmark) for the better observation of the cracks. For group (1) the specimens were self-glazed at the temperature recommended in the manufacturer’s instructions and then removed from the furnace as soon as the support plinth was low enough to remove the samples. The specimens were then force cooled with compressed air (2 bar) from two opposing sources 30 cm apart. Specimens for group (2) were glaze fired and cooled to mimic what dental technicians normally do when building crowns. The specimens were removed from the furnace when the temperature of the furnace drops to the starting temperature of the glazing cycle. Specimens for group (3) were glaze fired and slow cooled by stopping the firing plinth descending from the furnace and waiting until the temperature reduces to 100 °C. The latter took approximately 30–40 min depending on the firing cycle used.
Two groups of specimens were used: (i) 2 mm pressed ceramic only and (ii) a 2 mm slice from an ingot before pressing. Specimens for group (i) were glaze fired and slowly cooled in the furnace to 100 °C to measure the intrinsic fracture toughness without any tempering induced residual stress present.
Indentation cracks on the veneer surface of all specimens were made with a Vickers indenter at a load of 25 N for samples with ceramic only and 35 N for the samples before pressing. Different loads were chosen after making preliminary indentations with loads ranging from 20 to 70 N and the loads mentioned above showed the most suitable crack patterns in terms of size and absence of chipping. Between 5 and 12 indents were made depending on the porosity of the specimen. Indentations were made approximately 3–4 mm apart and cracks were measured directly post indentation. The following equation was used to calculate the effective fracture toughness :
K c = κ E H 0.5 P c 3 / 2
where H is the hardness, P the load, E the elastic modulus, c the length of radial crack, and κ is a constant 0.016.
Indentation cracks were introduced within the veneer surface of all specimens using a Vickers indenter at a load of 50 N for the bi-layered specimens. The high polish achieved on the sample surface minimised the inaccuracies associated with crack terminations due to surface scratches and porosity. An optical microscope with a high resolution digital stage (Nikon, Japan) was used to measure the length of radial cracks and to microscopically inspect the indentation surface of the specimens.
Residual stress determination
The presence of residual stresses is very apparent by the extent of radial crack extension about the residual impression. Marshall and Lawn developed a simple analysis of this problem . The presence of residual or applied tensile or compressive stress in the surface of the material being indented introduces an additional term in the expression for the stress intensity factor as given by Eq. (1) . For the situation where the indentation crack size is small compared with the gradient of stress, the expression for the stress intensity factor is composed of two terms:
K 1 c = K Ind + K App / Res
where K Ind is the indentation stress intensity factor given by Eq. (1) and K App/Res is the stress intensity factor for a crack in the residual or applied stress field and is given by:
K App / Res = Y σ App / Res c
where Y is the shape factor for a half-penny surface crack and σ App/Res is the applied or residual stress present. Combining Eqs. (1)–(3) we have:
K 1 c = κ E H 1 / 2 P c 3 / 2 ± Y σ App / Res c