Effect of angled indentation on mechanical properties

Indentation on a smooth surface, perpendicular to the indenter tip, is critical to obtaining accurate mechanical property values with nanoindentation. However, for some materials, achieving such a scenario may not always be feasible. To investigate the effect this may have, angled indentations were made on flat, sintered hydroxyapatite samples individually mounted so as to produce indentation angles of 10°, 20°, 30°, 40° and 50°, as well as leading contact with either the face or edge of the Berkovich tip used. While significant scatter in results reinforced the importance of perpendicular penetration, two phenomena were found to serve as potential indicators of angled indentation, and hence unreliable data. It is recommended that topographical profiles are obtained on any material of uncertain roughness prior to indentation

Micromechanical properties of single crystal hydroxyapatite by nanoindentation

Knowledge of the intrinsic properties of hydroxyapatite (HAp) single crystals is important for the design of natural systems and will assist further improvements of manufactured biomaterials. Nanoindentation provides a useful tool for determining mechanical properties such as the hardness, elastic modulus and fracture toughness of small samples. A Berkovich indenter was placed on the side and basal planes of a natural single crystal of Durango HAp. The hardness and elastic modulus values obtained revealed higher values for the base (7.1 and 150.4 GPa) compared to the side (6.4 and 143.6 GPa). The cracking threshold, i.e., the load at which cracking initiates, revealed earlier crack formation on the base (at 8 mN) compared to the side (at 11 mN). Fracture toughness was measured as 0.45 ± 0.09 and 0.35 ± 0.06 MPa m1/2 for the side and basal plane, respectively. These results suggest that crystals are less prone to cracking and resist microcrack events better on the side, which is useful in bone, while exposing the base, the hardest face, to minimize mass loss from abrasion with teeth

In vitro testing of plasma-sprayed hydroxyapatite coatings

Hydroxyapatite-coated coupons were tested in various physiological media. Immersion in Ringer's solution showed that heat-treated coatings displayed a weight gain but the as-sprayed coatings underwent a weight loss. Dissolution of the coating was measured by weighing the specimen before and after ageing. Immersion of hydroxyapatite coatings showed the appearance of small spheres that were identified by X-ray diffraction as hydroxyapatite. Changes in coating morphology were detected and the coating degradation mechanisms are discussed. This paper thus looks at the morphology, composition, crystallinity and dissolution of hydroxyapatite coatings aged in Ringer's solution. Hydroxyapatite-coated coupons were tested in various physiological media, immersion in Ringer's solution showed that heat-treated coatings displayed a weight gain but the as-sprayed coatings underwent a weight loss. Dissolution of the coating was measured by weighing the specimen before and after ageing. Immersion of hydroxyapatite coatings showed the appearance of small spheres that were identified by X-ray diffraction as hydroxyapatite. Changes in coating morphology were detected and the coating degradation mechanisms are discussed. This paper thus looks at the morphology, composition, crystallinity and dissolution of hydroxyapatite coatings aged in Ringer's solutions

The use of thermal printing to control the properties of calcium phosphate deposits

The objective of this work was to characterize the deposits of calcium phosphate produced by thermal printing in terms of structure, topography and mechanical properties. Hydroxyapatite was molten and directed to (a) a titanium target in relative motion and (b) stationary titanium substrates preheated to 100 °C and 350 °C. Scanning electron microscopy showed round-like deposits, but high resolution profilometry measured the profile. Micro-Raman spectroscopy and X-ray diffraction characterized the surface for structure, while nanoindentation revealed the hardness and elastic modulus. A symmetrical hemispherical deposit was formed on a surface in slow relative motion, but an off-centre shape formed at a higher relative speed. Deposits on preheated surfaces (100 °C and 350 °C) were identified as amorphous calcium phosphate. Nanoindentation revealed no significant difference in hardness between the amorphous deposits (4.0–4.4 ± 0.3 GPa), but the elastic modulus increased from 65 ± 4 GPa (annealed calcium phosphate reference) to 88 ± 3 GPa (100 °C surface) and then to 98 ± 3 GPa (350 °C substrate). The large change in elastic modulus is thought to arise from the dehydroxylation during thermal printing. Production of functional materials through crystallization is discussed to extend the range of possible microstructures. The characterization and testing approach is useful for hemispherical deposits produced by printing, coatings (laser ablation, thermal spraying, simulated body fluid) and melt extrusion elements in scaffolds

Biomedical application of apatites

Abstract not available

Revealing mechanical properties of a suspension plasma sprayed coating with nanoindentation

Solution and suspension thermal spraying is providing a more economic approach to the production of thin coatings. Advances in this new promising technology require accompanying tools to assess micro and nanosized areas within these deposits. Hydroxyapatite was deposited in an r.f. plasma using a powder and a suspension. The powder feedstock produced a dense, oriented coating, whereas the suspension led to a porous randomly oriented coating. The porosity leads to a decrease in the hardness and elastic modulus of the bulk coating, but site specific indentations on dense areas in the SPS coating revealed greater values (5 ± 0.2 vs 4 ± 0.2 GPa), possibly due to the finer grain size and crystal orientation. Nanoindentation presents a valuable tool for the assessment of the mechanical properties in solid areas of porous materials, and when used with microscopy will be valuable for the further development of SPS coatings

Thermal analysis of amorphous phases in hydroxyapatite coatings

The amorphous phase in hydroxyapatite coatings has been examined by using X-ray diffractometry, Fourier transform infrared spectroscopy, optical microscopy, and thermal analysis methods. The amorphous phase mostly consists of a dehydroxylated calcium phosphate. When heated, crystallization of hydroxyl-rich areas produces hydroxyapatite, followed by diffusion of hydroxyl ions, thus increasing the amount of crystalline phase. Hydroxyl-deficient amorphous areas crystallize to oxyapatite at 700°C. Thus, crystallization occurs over a range of temperatures and is dependent on the hydroxyl content of the amorphous phase and the partial water-vapor pressure. The activation energies of crystallization to hydroxyapatite, diffusion of hydroxyl ions, and crystallization to oxyapatite are 274, 230, and 440 kj/mol, respectively. Shrinkage from these processes leads to a crack network and decreases the mechanical strength of the coating

Optimization of spraying parameters for hydroxyapatite

Abstract not available

The amorphous phase in plasma sprayed hydroxyapatite coatings

Atmospheric plasma spraying of hydroxyapatite was carried out to investigate the influence of the plasma spraying parameters on the formation of the amorphous phase. The amorphous phase was investigated with X-ray diffraction and optical microscopy. The results infer that dehydroxylation of the molten particle and a high cooling rate produce a larger amount of the amorphous phase. Crystalline regions of the coating consist of partially molten particles and elongated recrystallized areas. The amorphous phase is commonly found at the substrate interface. It is shown that the amorphous regions vary throughout the coating which clinically could affect the process of bone deposition and successful implant fixation

Structural changes of plasma sprayed hydroxyapatite coatings during in-vitro testing

The in-vitro behavior of hydroxyapatite (HAp) coatings depends on the thermal history of the coating after thermal spraying. As-sprayed amorphous coatings degrade in Ringer's solution. Heat treatment at 800°C for 2 hours produces a crystalline coating which upon immersion in physiological solution displays a greater stability. Coatings are characterized for surface morphology, composition, crystallinity, roughness, and weight loss. The as-sprayed coating shows changes in the coating morphology with immersion time. Individual lamellae crack and separate from the coating. Those lamellae still intact show evidence of dissolution. After a period of 8 weeks small HAp spheres cover the surface of the as-sprayed and heat treated coatings. These morphological changes are expected to influence the rate of bone bonding. Therefore post-treatment of HAp coatings is seen as a technique to alter or control implant tissue interactions