Janet B. Quinn

Indentation brittleness of ceramics: a fresh approach

The hardness and brittleness of ceramic materials are interrelated. Hard materials are more apt to fracture in the vicinity of an indentation during a hardness test, while softer materials tend to plastically deform to the indenter shape without fracturing. Measured hardness, in turn, is affected by both specimen deformation and fracture processes. This interrelationship is examined by means of extensive Vickers hardness testing. A new index of brittleness is proposed.

Calcium phosphate cement containing resorbable fibers for short-term reinforcement and macroporosity

Calcium phosphate cement (CPC) sets to form hydroxyapatite and has been used in medical and dental procedures. However, the brittleness and low strength of CPC prohibit its use in many stress-bearing locations, unsupported defects, or reconstruction of thin bones. Recent studies incorporated fibers into CPC to improve its strength. In the present study, a novel methodology was used to combine the reinforcement with macroporosity: large-diameter resorbable fibers were incorporated into CPC to provide short-term strength, then dissolved to create macropores suitable for bone ingrowth. Two types of resorbable fibers with 322 microm diameters were mixed with CPC to a fiber volume fraction of 25%. The set specimens were immersed in saline at 37 degrees C for 1, 7, 14, 28 and 56d, and were then tested in three-point flexure. SEM was used to examine crack-fiber interactions. CPC composite achieved a flexural strength 3 times, and work-of-fracture (toughness) nearly 100 times, greater than unreinforced CPC. The strength and toughness were maintained for 2-4 weeks of immersion, depending on fiber dissolution rate. Macropores or channels were observed in CPC composite after fiber dissolution. In conclusion, incorporating large-diameter resorbable fibers can achieve the needed short-term strength and fracture resistance for CPC while tissue regeneration is occurring, then create macropores suitable for vascular ingrowth when the fibers are dissolved. The reinforcement mechanisms appeared to be crack bridging and fiber pullout, the mechanical properties of the CPC matrix also affected the composite properties.

Strong and macroporous calcium phosphate cement: Effects of porosity and fiber reinforcement on mechanical properties†‡

Because of its excellent osteoconductivity and bone-replacement capability, self-setting calcium phosphate cement (CPC) has been used in a number of clinical procedures. For more rapid resorption and concomitant osseointegration, methods were desired to build macropores into CPC; however, this decreased its mechanical properties. The aims of this study, therefore, were to use fibers to strengthen macroporous CPC and to investigate the effects of the pore volume fraction on its mechanical properties. Water-soluble mannitol crystals were incorporated into CPC paste; the set CPC was then immersed in water to dissolve mannitol, producing macropores. Mannitol/(mannitol + CPC powder) mass fractions of 0, 10, 20, 30, and 40% were used. An aramid fiber volume fraction of 6% was incorporated into the CPC-mannitol specimens, which were set in 3 mm x 4 mm x 25 mm molds and then fractured in three-point flexure to measure the strength, work of fracture, and modulus. The dissolution of mannitol created well-formed macropores, with CPC at 40% mannitol having a total porosity of a 70.8% volume fraction. Increasing the mannitol content significantly decreased the properties of CPC without fibers (analysis of variance; p < 0.001). The strength (mean +/- standard deviation; n = 6) of CPC at 0% mannitol was 15.0 +/- 1.8 MPa; at 40% mannitol, it decreased to 1.4 +/- 0.4 MPa. Fiber reinforcement improved the properties, with the strength increasing threefold at 0% mannitol, sevenfold at 30% mannitol, and nearly fourfold at 40% mannitol. The work of fracture increased by 2 orders of magnitude, but the modulus was not changed as a result of fiber reinforcement. A scanning electron microscopy examination of specimens indicated crack deflection and bridging by fibers, matrix multiple cracking, and frictional pullout of fibers as the reinforcement mechanisms. Macroporous CPCs were substantially strengthened and toughened via fiber reinforcement. This may help extend the use of CPCs with macropores for bony ingrowth to the repair of larger defects in stress-bearing locations.

Synergistic reinforcement of in situ hardening calcium phosphate composite scaffold for bone tissue engineering

Calcium phosphate cement (CPC) hardens in situ to form solid hydroxyapatite, can conform to complex cavity shapes without machining, has excellent osteoconductivity, and is able to be resorbed and replaced by new bone. Therefore, CPC is promising for use in craniofacial and orthopaedic repairs. However, the low strength and lack of macroporosity of CPC limit its use. The aim of the present study was to increase the strength and toughness of CPC while creating macropores suitable for cell infiltration and bone ingrowth, and to investigate the effects of chitosan and mesh reinforcement on the composite properties. Specimens were self-hardened in 3 mm x 4 mm x 25 mm molds, immersed in a physiological solution for 1-84 d, and tested in three-point flexure. After 1d, the unreinforced CPC control had a flexural strength (mean+/-s.d.; n=6) of (3.3+/-0.4)MPa. The incorporation of chitosan or mesh into CPC increased the strength to (11.9+/-0.8) and (21.3+/-2.7)MPa, respectively. The incorporation of both chitosan and mesh synergistically into CPC dramatically increased the strength to (43.2+/-4.1)MPa. The work-of-fracture (WOF) (toughness) was also increased by two orders of magnitude. After 84 d immersion in a simulated physiological solution, the meshes in CPC dissolved and formed interconnected cylindrical macropores. The novel CPC scaffold had a flexural strength 39% higher, and WOF 256% higher than the conventional CPC without macropores. The new composite had an elastic modulus within the range for cortical bone and cancellous bone, and a flexural strength higher than those for cancellous bone and sintered porous hydroxyapatite implants. In conclusion, combining two different reinforcing agents together in self-hardening CPC resulted in superior synergistic strengthening compared to the traditional use of a single reinforcing agent. The strong and macroprous CPC scaffold may be useful in stress-bearing craniofacial and orthopaedic repairs.

Influence of microstructure and chemistry on the fracture toughness of dental ceramics

OBJECTIVES the primary aim of this research was to measure fracture toughness for several groups of dental ceramics, and determine how this property is affected by chemistry and microstructure. METHODS Fracture toughness (KIc) values were obtained using Single Edge Precracked Beam (SEPB) and Single Edge V-Notch Beam (SEVNB) methods. Dynamic Youngs modulus, which often scales with strength and has been used in explaining the microstructure/toughness relationship on a theoretical basis, was also obtained for the three groups of materials comprising this study. The first group, consisting of micaceous glass ceramics, included model materials that varied systematically in microstructure but not in chemistry. The second group, the feldspathic porcelains, varied significantly in microstructure, but little in chemistry. The ceramics comprising the third group were significantly different in both chemistry and microstructure. RESULTS Upper toughness limits for the micaceous glass-ceramics and feldspathic porcelains were significantly raised compared to the base glasses, but remained under 2 MPa m(1/2). The highest toughnesses were associated with high percent crystallinity, large grains and high aspect ratios. The third group KIc values were 2.8 MPa m(1/2) for a lithium disilicate glass-ceramic, 3.1 MPa m(1/2) for a glass-infused alumina, and 4.9 MPa m(1/2) for zirconia. SIGNIFICANCE the correlations between microstructural characteristics and measured properties supports theoretical predictions in the literature. From a practical standpoint, microstructural effects were found to be important, but only within a limited range; the chemistry apparently defined a band of achievable property values. This suggests very large increases in fracture toughness are unlikely to be attained by changes in microstructure alone. A functional relationship determined for the micaceous glass-ceramics enables quantitative predictions of fracture toughness based on the microstructure.

Processing and Properties of Strong and Non-rigid Calcium Phosphate Cement

A calcium phosphate cement (CPC) sets to form hydroxyapatite and has been used in dental and craniofacial applications. However, when CPC was used in periodontal repair, tooth mobility resulted in the fracture and exfoliation of the brittle implants. The aim of this study was to develop CPC-chitosan lactate composites with higher strength and increased strain before failure. It was hypothesized that the incorporation of chitosan lactate would render CPC non-rigid with improved properties. Two-way ANOVA showed significant effects of chitosan lactate and powder:liquid ratio (p < 0.001) on flexural strength, strain-at-peak-load, work-of-fracture, and elastic modulus. At powder:liquid = 2, the strength (mean +/- SD; n = 6) at 20% chitosan lactate was 15.7 +/- 1.3 MPa, higher than 4.9 +/- 1.4 MPa of CPC without chitosan lactate. At powder:liquid = 1, the strain-at-peak-load was 0.2% for CPC without chitosan lactate; it increased to 15.8% for CPC containing 15% chitosan lactate. The work-of-fracture was increased by more than ten times. The novel strong and non-rigid CPC may provide compliance for tooth mobility without fracturing the implant, and may also extend the use of CPC into the repair of larger defects in stress-bearing locations.

Effects of different whiskers on the reinforcement of dental resin composites.

OBJECTIVE Whiskers were recently used to reinforce dental composites to extend their use to large stress-bearing restorations. The aim of this study was to investigate the effects of different types of whiskers on composite properties. METHODS Silicon nitride and silicon carbide whiskers were each mixed with silica particles at whisker/silica mass ratios of 0:1, 1:5, 1:2, 1:1, 2:1, 5:1, and 1:0, and thermally treated. The composite was heat-cured at 140 degrees C. Strength and fracture toughness were measured in flexure, while elastic modulus and hardness were measured with nano-indentation. RESULTS Both whisker type and whisker/silica ratio had significant effects on composite properties (two-way ANOVA; p<0.001). Silicon nitride whiskers increased the composite strength and toughness more than did silicon carbide. Silicon carbide whiskers increased the modulus and hardness more than silicon nitride did. The silicon nitride whisker composite reached a strength (mean+/-SD; n=6) of 246+/-33 MPa at whisker/silica of 1:1, while the silicon carbide whisker composite reached 210+/-14 MPa at 5:1. Both were significantly higher than 114+/-18 MPa of a prosthetic control and 109+/-23 MPa of an inlay/onlay control (Tukeys multiple comparison test; family confidence coefficient=0.95). Fracture toughness and work-of-fracture were also increased by a factor of two. Higher whisker/silica ratio reduced the composite brittleness to 1/3 that of the inlay/onlay control. SIGNIFICANCE Whisker type and whisker/silica ratio are key microstructural parameters that determine the composite properties. Reinforcement with silica-fused whiskers results in novel dental composites that possess substantially higher strength and fracture toughness, and lower brittleness than the non-whisker control composites.

Fractographic failure analysis of a Procera AllCeram crown using stereo and scanning electron microscopy

OBJECTIVES Presentation of a methodological approach using stereo and scanning electron microscope examination for the failure analysis of an alumina all-ceramic premolar crown (Procera AllCeram). METHODS The recovered part of a fractured Procera alumina crown was examined utilizing first a stereomicroscope and second a scanning electron microscope (SEM). The stereomicroscope analysis was performed at low magnifications with oblique lighting in order to enhance spatial relationships and gross detection of crack features. A preliminary fracture surface map of the stereo observations was drawn and used as a guide for the SEM analysis that followed. Specific sites of interest identified under the stereo microscope were analyzed using the SEM at high magnifications searching for small fracture features such as wake hackle and twist hackle within the veneering ceramic in order to confirm the direction of crack propagation. RESULTS At low magnifications and oblique illumination, the stereomicroscope analysis provided an excellent overview of the fractured topography, showing sites of major interest such as a primary edge chip at a margin, a compression curl indicating the end of the fracture event as well as larger hackle lines distributed over the cracked surface. The greater magnifications with the SEM analysis of the sites of interest showed the presence of wake and twist hackle, indicators of the crack propagation direction. A general map of the fracture events could be reconstructed starting with a primary veneer edge chip at the mesial margin. Hackle and wake hackle of the crack front emanating from this margin arose from hoop stresses and propagated through the full crown thickness towards the distal end of the restoration where the compression curl was located. Additional occlusal surface damage in the form of veneer chipping containing arrest lines and twist hackle running in the opposite direction as the main crack path were observed, but occurred as a secondary event without penetrating the alumina core material. SIGNIFICANCE Stereo and scanning electron microscopy are complementary analysis techniques useful for the mapping and interpretation of the fracture surface. This case examination is intended to guide the clinical researcher in using qualitative (descriptive) fractography as a tool for understanding the failure process in brittle restorative materials, as well as for assessing possible design inadequacies.

Dental resin composites containing silica-fused whiskers—effects of whisker-to-silica ratio on fracture toughness and indentation properties

Dental resin composites need to be strengthened in order to improve their performance in large stress-bearing applications such as crowns and multiple-unit restorations. Recently, silica-fused ceramic whiskers were used to reinforce dental composites, and the whisker-to-silica ratio was found to be a key microstructural parameter that determined the composite strength. The aim of this study was to further investigate the effects of whisker-to-silica ratio on the fracture toughness, elastic modulus, hardness and brittleness of the composite. Silica particles and silicon carbide whiskers were mixed at whisker:silica mass ratios of 0:1, 1:5. 1:2, 1:1, 2:1, 5:1, and 1:0. Each mixture was thermally fused, silanized and combined with a dental resin at a filler mass percentage of 60%. Fracture toughness was measured with a single-edge notched beam method. Elastic modulus and hardness were measured with a nano-indentation system. Whisker:silica ratio had significant effects on composite properties. The composite toughness (mean+/-SD; n = 9) at whisker:silica = 2:1 was (2.47+/-0.28) MPa m(1/2), significantly higher than (1.02+/-0.23) at whisker:silica = 0:1, (1.13+/-0.19) of a prosthetic composite control, and (0.95+/-0.11) of an inlay/onlay composite control (Tukeys at family confidence coefficient = 0.95). Elastic modulus increased monotonically and hardness plateaued with increasing the whisker:silica ratio. Increasing the whisker:silica ratio also decreased the composite brittleness, which became about 1/3 of that of the inlay:onlay control. Electron microscopy revealed relatively flat fracture surfaces for the controls, but much rougher ones for the whisker composites, with fracture steps and whisker pullout contributing to toughness. The whiskers appeared to be well-bonded with the matrix, probably due to the fused silica producing rough whisker surfaces. Reinforcement with silica-fused whiskers resulted in novel dental composites that possessed fracture toughness two times higher than, and brittleness less than half of current dental composites.

Material properties and fractography of an indirect dental resin composite

OBJECTIVES Determination of material and fractographic properties of a dental indirect resin composite material. METHODS A resin composite (Paradigm, 3M-ESPE, MN) was characterized by strength, static elastic modulus, Knoop hardness, fracture toughness and edge toughness. Fractographic analyses of the broken bar surfaces was accomplished with a combination of optical and SEM techniques, and included determination of the type and size of the failure origins, and fracture mirror and branching constants. RESULTS The flexure test mean strength+/-standard deviation was 145+/-17 MPa, and edge toughness, T(e), was 172+/-12N/mm. Knoop hardness was load dependent, with a plateau at 0.99+/-0.02 GPa. Mirrors in the bar specimens were measured with difficulty, resulting in a mirror constant of approximately 2.6 MPa m(1/2). Fracture in the bar specimens initiated at equiaxed material flaws that had different filler concentrations that sometimes were accompanied by partial microcracks. Using the measured flaw sizes, which ranged from 35 to 100 microm in size, and using estimates of the stress intensity shape factors, fracture toughness was estimated to be 1.1+/-0.2 MPa m(1/2). SIGNIFICANCE Coupling the flexure tests with fractographic examination enabled identification of the intrinsic strength limiting flaws. The same techniques could be useful in determining if clinical restorations of similar materials fail from the same causes. The existence of a strong load-dependence of the Knoop hardness of the resin composite is not generally mentioned in the literature, and is important for material comparisons and wear evaluation studies. Finally, the edge toughness test was found promising as a quantitative measure of resistance to edge chipping, an important failure mode in this class of materials.